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25 | https://www.frontiersin.org/journals/materials/articles/10.3389/fmats.2021.575530/full | MINI REVIEW article
Front. Mater., 23 April 2021
Sec. Structural Materials
Volume 8 - 2021 | https://doi.org/10.3389/fmats.2021.575530
This article is part of the Research Topic Latest Developments in the Field of Magnesium Alloys and their Applications View all 17 articles
Novel Magnesium Based Materials: Are They Reliable Drone Construction Materials? A Mini Review
\r\nDaniel Hoche,*Daniel Höche1,2*Wolfgang E. WeberWolfgang E. Weber3Eugen Gazenbiller,Eugen Gazenbiller1,2Sarkis Gavras,Sarkis Gavras2,4Norbert Hort,Norbert Hort2,4Hajo Dieringa,Hajo Dieringa2,5
1Institute of Surface Science, HELMHOLTZ-Zentrum Geesthacht, Zentrum für Material-und Küstenforschung GmbH, Geesthacht, Germany
2MagIC - Magnesium Innovation Centre, HELMHOLTZ-Zentrum Geesthacht, Zentrum für Material-und Küstenforschung GmbH, Geesthacht, Germany
3Chair of Structural Analysis, Faculty of Mechanical Engineering, HELMUT-SCHMIDT-University/University of the Federal Armed Forces, Hamburg, Germany
4Institute of Metallic Biomaterials, HELMHOLTZ-Zentrum Geesthacht, Zentrum für Material-und Küstenforschung GmbH, Geesthacht, Germany
5Institute of Materials and Process Design, HELMHOLTZ-Zentrum Geesthacht, Zentrum für Material-und Küstenforschung GmbH, Geesthacht, Germany
Novel magnesium-based materials are ideal candidates for use in future aviation vehicles because they are extremely light and can therefore significantly increase the range of these vehicles. They show very good castability, are easy to machine and can be shaped into profiles or forgings to be used as components for next generation aerial vehicle construction. In the case of a large number of identical components, high-pressure die casting of magnesium alloys is clearly superior to high-pressure die casting of aluminum alloys. This is due to the lower solubility of iron in magnesium and thus tool/casting life is significantly longer. In addition, the die filling times for magnesium high-pressure die casting are approximately 30% shorter. This is due to the lower density: aluminum alloys are approximately 50% heavier than magnesium alloys, which is a significant disadvantage for aluminum alloys especially in the aerospace industry. There are cost-effective novel die casting alloys, besides AZ91 or AM50/60 such as DieMag633 or MRI230D, which show very good specific strength at room and elevated temperatures. In the case of magnesium-based wrought alloys, the choice is smaller, a typical representative of these materials is AZ31, but some new alloys based on Mg-Zn-Ca are currently being developed which show improved formability. However, magnesium alloys are susceptible to environmental influences, which can be eliminated by suitable coatings. Novel corrosion protection concepts for classical aerial vehicles currently under development might suitable but may need adaption to the construction constraints or to vehicle dependent exposure scenarios. Within this mini-review a paradigm change due to utilization of new magnesium materials as drone construction material is briefly introduced and future fields of applications within next-generation aerial vehicles, manned or unmanned, are discussed. Possible research topics will be addressed.
Lightweight materials such as magnesium and its alloys are of great interest for the industrial sector. Potential applications can be found in the automobile industry and in civil engineering as structural components (Dieringa and Kainer, 2013), in batteries as anode material (Deng et al., 2018; Höche et al., 2018) and in medical engineering as biocompatible, resolvable implants (Kirkland, 2012; Luthringer et al., 2014). The aspects of applying magnesium based materials in vehicle concepts have been widely discussed (Dieringa et al., 2007; Dieringa and Kainer, 2009; Dieringa and Bohlen, 2016) with the result that functionalizations of magnesium alloys are feasible (Xianhua et al., 2016). Thus, it is reasonable to think about application of novel magnesium based materials to construct additional vehicle components e.g., for a quadcopter or other next generation aerial vehicles, if they meet technical and economic constraints.
Typically, drones are classified according to weight or flight range (Weibel and Hansman, 2004; Arjomandi et al., 2006; Brooke-Holland, 2012; Dalamagkidis, 2015). Of these weight or flight range classifications, improvements to ultra-light-weight drones is an ultimate goal to achieve. Presently drone materials are very often composites as sandwich panels applying fiberglass, graphite fiber or aramid based systems as skin and reinforcement materials, and foams mostly based on polymers as panel core materials (Fahlstrom and Gleason, 2012). Such systems are already established within aviation sector and just need to be applied. For example, the City-Airbus, the Lilium-Jet (Lilium, 2018) or Volocopter aiming to conquer the air taxi sector construct their vehicles with CFRP concepts. Generally lightweight vehicle constructions require a high strength to weight ratio with the ability to adapt shapes onto aero- and flight dynamical requirements. In this context, morphing materials are of great interest and might bring constructional aspects to the limit (Sun et al., 2016; Goh et al., 2017). Despite this trend there is a lot of space to use new magnesium alloys, such as DieMag633, MRI230D (Gavras et al., 2019; Tu et al., 2019), Mg-Zn-Ca based (Pan et al., 2016; You et al., 2017; Tu et al., 2019), magnesium based foams (Kucharczyk et al., 2017) or high strength AM60 + 1AlN nanocomposites (Dieringa et al., 2017; Malaki et al., 2019). Particularly with regards to manned aerial vehicles (MAV’s) or even air-taxis which might become very expensive and not environmental benign if they are conventionally designed. The application of magnesium in recent aerial vehicles is a first step. For example, the commercially available DJI Inspire 2 unmanned aerial vehicle (UAV), has an AZ91 housing. The DJI Mavic Air applies brackets based on AZ91. The Phantom 4 Pro V2.0 quadcopter applies a titanium magnesium hybrid structure to achieve maximal rigidity of the airframe for excellent maneuverability1. Obviously, based on this existing experience, there is a lot of potential in conceptional design approaches using magnesium materials. Possibilities going beyond this need to be further explored.
Design of unmanned aerial vehicles and manned aerial vehicles follow the well-established construction criteria of aircraft industry. However, this does not mean that there is no space for innovation and novel thinking. Considering the price developments (Q4/2020 at 2.55–2.70 $/kg Source: Platts Metals Week) but also enhanced manufacturing capabilities Mg based components might replace vehicle components and attachments (e.g., camera housings) but also on structural level.
The structure of a typical UAV consists of basic subsystems. All those components typically do not mainly differ in their function from conventional aircraft concepts but might be adapted. The most important are listed:
- The fuselage including stringers, the skin (coated), longerons (longitudinal structural members reinforcing the skin), and some bulkheads can be made from many lightweight engineering materials.
This can be impregnated fabrics, standard AA2024 structures or many other composites (CFRP, GLARE, etc.). Components for the fuselage can be made of magnesium alloys. These are, for example, sheets for the outer skin made of AZ31 or aluminum-free ZE10. The latter shows excellent forming properties and a reduced texture. Frames and cross members can be made of AM60 by high-pressure die casting. Any further fairings (and filets) can still be made of Al alloys or respective foils.
- The wings including the spars (mono and multi), ribs, stiffeners, and the skin in 2, 3, or 4 rotor systems (sheathed and unsheathed) are structural parts as well and critical loads are transferred to the fuselage by a topological designed beam or truss structure. Materials in use must be light but also stiff and limit vibrations. However, using current lightweight materials such as CFRP leads to structures being more flexible than desired. Thus, additional effort is necessary to limit the deformations—one example for that are truss braced wings (see e.g., Scott et al., 2016). Instead, extruded profiles of wrought magnesium alloys could be used here for the load-bearing structures; they show yield strengths of up to 300 MPa. Cast components for wing and spar construction made of AZ91 or AM60 could be used as ultralight structural components here as well.
- The tails (horizontal and vertical) very often are made of carbon- or glass fiber materials applying resins, wax and hardeners with foam cores.
- The rotor blades are mostly made of composite materials involving impregnated fabrics. Especially for UAV’s which are used for civil applications in urban areas, an outer ring housing the rotor blades is necessary in order to prevent people and animals from accidental collisions and to reduce noise. This ring might be made of magnesium or magnesium alloys because this material may better withstand collisions with both the built environment and moving objects such as other UAV’s. Additionally, the damping properties of such Mg based systems are excellent.
- The engine/powertrain group consisting of for example pylons, inlet, supports, etc. does not follow the classical turbine design with Ti- or Ni based alloys. For UAV’s like quadcopters electrical engines depending on battery power are required, where it should be mentioned that battery mass is a problem. Casings or heads of piston engines are often made of cast aluminum but might be replaced by Mg based materials. Ti alloys or steels are other options if the situation (temperature, etc.) requires countermeasures. Magnesium alloys for higher temperatures are available, as well, although they are often more costly (AE-series, DieMag-series, AJ-series) (Gavras et al., 2019).
- The landing gear (and if required, its bay door) can also be part of the structure especially for MAV’s. It has to take up high loads and can already be made of carbon composites or simply AISI 300M steels.
- It should be noted that the dies for applying the forming processes mentioned above may also be made from ultra-high performance concrete (UHPC) (see e.g., Kleiner et al., 2008), which may lead to economic gains. This holds especially for low and medium contoured part shapes. For producing parts with small radii, the UHPC dies should be additionally confined or reinforced (see also Kleiner et al., 2008).
Material selection especially for aerospace vehicles has been introduced by Arnold et al. (2012) based on Ashby (2010). The structure and its materials have to ensure the functionality of the drone. They have to keep the aerodynamics of the UAV and carry the occurring loads at any time. Thus, structural components are constructed following the lightweight design rules (e.g., high Youngs modulus/density ratio E/ρ, etc.; Arnold et al., 2012) as shown in Figure 1, whereas safe design in c) has special emphasis for manned vehicles. Here the most common material is aluminum, followed by composite materials, such as glass/epoxy and carbon fiber. Surprisingly, magnesium based materials such as magnesium based metal matrix nanocomposites (MMNC’s) are yet not in use despite their very good potential. Some of these Mg-MMNC’s show excellent properties caused by only small additions of ceramic nanoparticles. The great advantage of these materials is that the nanoparticles not only increase mechanical strength (by grain refinement or Orowan strengthening), but also increase ductility (Dieringa, 2011; Dieringa et al., 2017).
Figure 1. Materials selection following Ashby for aerospace applications (based on Granta-design data). (A) Stiffness limited, (B) strength limited, and (C) safe design as proposed by standard structural design concepts, whereas the bottom-right diagram (D) shows the product of previous mechanical parameters vs. the product of non-physical properties like the material price, the carbon dioxide footprint and the reciprocal of recycling fraction.
The structural members are designed to carry the flight loads or to handle stress without failure. In designing the structure, the entirety of the wing and fuselage must be considered in relation to the physical characteristics of the material of which it is made. Every part of the structure is engineered to carry the load which is applied on it. The structural designer has to determine flight loads (including bird strikes) and other loads, calculate stresses, and design the structural elements such as to allow the UAV components to perform their aerodynamic functions and durability requirements efficiently. This goal will be considered simultaneously with the objective of the lowest structural weight.
The entire airframe and its components are joined by rivets, bolts, screws, and other fasteners. Welding, adhesives, and special bonding techniques are also employed. In order to enhance the delamination toughness of e.g., CFRP laminates, techniques such as z-pinning are investigated (cf. Mouritz, 2007). Additionally, composite materials with enhanced delamination toughness such as GLARE are of scientific interest (see Rittmeier et al., 2018). The most common design of UAV structure is semi- monocoque (single shell) which implies that the respective skin is stressed and thus needs to be reinforced.
Structural parts of UAV’s are subjected to: (1) tension, (2) compression, (3) torsion, (4) shear, and (5) bending. Typically, a single member of the structure is often subjected to a combination of the resulting stresses. It is worth mentioning that the UAV is subjected to time-harmonic loads (i.e., due to vibrations induced by the engine) and impact loads (e.g., during the landing phase, due to bird strikes or impacts such as the Hudson River landing in 2009). Additionally, fluttering as well as limit-cycle oscillations due to wind loads is a major issue. Hence, the design principles known from e.g., aeronautical applications or wind power plants are a good starting point when designing UAV’s. A major issue in designing lightweight-structures from composite materials is that these structures tend to delaminate. Although there are substantial contributions to prevent structural elements from delamination, this problem is not yet solved. Promising approaches to increase delamination toughness can be found in the literature (e.g., Mouritz, 2007; Rittmeier et al., 2018). However, by suggesting magnesium based construction materials for e.g., quadcopter drones, the authors believe that the potential problem of delamination can be circumvented. This advantage holds for the drone’s skin and bulkhead, which are also subjected to lateral loads. Structural members such as frames and longerons are mainly loaded longitudinally and consequently can be treated as rods in most cases (cf., Rothert and Gensichen, 1987). Using lightweight configurations made of composite materials mentioned above has another effect: the respective structures are getting more flexible. Thus, scientific effort is necessary in order to reduce the resulting deformations. For example, this led to the development of the so-called truss braced wing (see Scott et al., 2016). Again, the authors of the current contribution believe that the rigidity of structural members made of magnesium or magnesium alloys is sufficient to ensure moderate deformations and might improve acoustic emission profiles. Concerns about corrosion issues are not appropriate due to the existence of novel corrosion protection concepts (Lamaka et al., 2016; Dieringa et al., 2018) and the expected uncritical corrosive in-service exposure conditions.
Besides these mechanical and design aspects, the ecological footprint (Ehrenberger, 2020) and the social acceptance of the materials used in drone design is also of interest. Many contributions show the positive effect on fuel/energy consumption when using CFRP for MAV’s (see Timmis et al., 2015). The authors expect that similar conclusions hold for UAV’s and other lightweight materials such as magnesium or magnesium alloys. However, recycling of lightweight materials such as CFRP is challenging (Dong et al., 2018). In comparison, recycling of magnesium and magnesium alloys is quite well-understood. Additionally, magnesium and magnesium alloys can be separated easily from the other materials which is not the case for carbon in CFRP. Thus, from an ecological but also from an economical point of view the use of magnesium and magnesium alloys seems to be promising since the price and CO2 fingerprint are on the same level as aluminum. In a life cycle assessment study (Ehrenberger et al., 2013; Ehrenberger, 2020), it was shown that magnesium, which is primarily produced by electrolysis, has a better carbon footprint than aluminum from the production stage. Over the service life of a component, this advantage increases continuously due to its lower weight. In a comparison of aircraft components made of magnesium or aluminum, the slightly higher greenhouse gas emissions caused by the production of magnesium are compensated for after only a few flights by the fuel savings during flight.
The utilization of Mg based materials generally launches a fire risk discussion. A dedicated study on flammability of the material within aircraft fire tests (Marker, 2013) indicates that for example the behavior of the Mg alloy E21 does not significantly differ to aluminum alloys. Nowadays the SAE standard AS8049D paves the way toward utilization of novel Mg based materials (Czerwinski, 2014) at commercial cabin interiors.
The use of magnesium in the aircraft industry has been an important topic since the beginning of aircraft construction. Even before the Second World War, a large number of magnesium parts were used in aircraft construction, including propellers and sheet metal for the outer skin (Reed, 1925; Beck, 1939; Hallion, 2017). During the Second World War, however, a great amount of magnesium was used to build aircrafts in Germany. Both the Messerschmidt Bf 110 and the Junkers Ju 88 used both die-cast and forged magnesium parts (small bell cranks, engine cowl flaps, large dive brake forgings) (DOW, 1941). The first all-magnesium aircraft was described in 1954 (American Aviation, 1955). The F-80C was characterized by a simpler construction of the wings, because considerably fewer components were needed, the number was reduced from 1,644 to only 508, which corresponds to a decrease of 69%. The lighter construction led to an increase of the maximum speed by 5 mph. After the war, the United States built the Convair B-36 bomber, an aircraft that was made of 10% magnesium (Jenkins, 2001) large parts of the outer skin and the control parts were made of magnesium alloys. In 1950 the Sikorsky S-56 Helicopter from Westland Aircraft Ltd., applied 115 kg magnesium. Furthermore, in the Sovjet Union in 1963 the TU-134 from Tupolev already had 1,325 parts made of Mg (Ostrowsky and Henn, 2007).
The operating flight loads limits on a UAV are usually presented in the form of a V-n diagram (airspeed and load variation diagram). Structural designers selecting Mg based materials will construct this diagram with the cooperation of the flight dynamics group. The diagram will determine the failure areas, and area of structural damage/failure. The UAV should not be flown out of the flight envelope, since it is not safe for the structures. The UAV structural design is out of scope of this mini-review. Detailed information can be found elsewhere (Megson, 2016).
Figure 2 shows the principles and an idea to design a hypothetical quadcopter mainly based on Mg-materials. The different requirements for the respective construction parts can be fulfilled by the materials in the boxes. They are:
Figure 2. Possible Mg-based alloys/hybrid materials for application in aerial vehicle construction like a quadcopter. Exemplified sources for further information about the materials are a: Naghdi et al. (2016), b: Dieringa et al. (2004), c: Gavras et al. (2019), d: Dieringa et al. (2017).
• Rigid—such as MgZnCa wrought alloys with 306 MPa YS and 11% elongation (You et al., 2017).
• Vibration limiting—AM60 based nano composites with high ductility of 15.4% at RT (Dieringa et al., 2017).
• Creep resistant—such as novel DieMag alloys without rare-earth elements showing better creep resistance compared to commercial alloys and better high temperature yield strength than even aluminum alloy A380 (Gavras et al., 2019).
• High strength and thermally resistant—such as carbon fiber reinforced Mg MMCs up to 300°C (Dieringa et al., 2004).
Considering production and design aspects, it becomes obvious that besides efficient manufacturing- and surface technologies (computational based), topological design but also efficient joining technologies for Mg based parts are key enablers for this technology. It will require a concerted research effort and might also be a future market for additive manufactured parts made out of Magnesium.
Magnesium based materials can become an essential part of design concepts of UAV’s, especially if flight maneuvers require high rigidity and noise pollution become critical decision criteria. Also crash-worthiness is an aspect where magnesium has benefits. The authors are convinced that the economic advantages of using magnesium based materials in drone construction may be further increased if promising techniques such as e.g., performing sheet metal hydroforming by means of UHPC dies are applied. As compared to CFRP and related composite materials, magnesium based materials can be easier separated and recycled once the UAV’s reached the end of its service-life.
With respect to the title of this contribution and based on the precedent sections the authors thus are convinced that: Magnesium is—A drone construction material!
DH wrote the main part of the manuscript. EG performed the visualization. HD and SG contributed to the historical aspects. WW and NH contributed to the mechanical and historical aspects. All authors contributed to the manuscript revision, read and approved the submitted version.
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
This article is a personal acknowledgement to K. U. Kainer.
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Keywords: ultra-lightweight construction, hybrid design, magnesium alloy, aerial vehicle, urbane mobility
Citation: Höche D, Weber WE, Gazenbiller E, Gavras S, Hort N and Dieringa H (2021) Novel Magnesium Based Materials: Are They Reliable Drone Construction Materials? A Mini Review. Front. Mater. 8:575530. doi: 10.3389/fmats.2021.575530
Received: 23 June 2020; Accepted: 30 March 2021;Published: 23 April 2021.
Edited by:
Reviewed by:
Copyright © 2021 Höche, Weber, Gazenbiller, Gavras, Hort and Dieringa. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: Daniel Höche, [email protected]
Disclaimer:
All claims expressed in this article are solely those of the authors and
do not necessarily represent those of their affiliated organizations, or
those of the publisher, the editors and the reviewers. Any product that
may be evaluated in this article or claim that may be made by its
manufacturer is not guaranteed or endorsed by the publisher. |
26 | https://www2.deloitte.com/us/en/insights/industry/aerospace-defense/aerospace-and-defense-industry-outlook.html |
2024 aerospace and defense industry outlook
Aerospace and defense companies should embrace new technologies and innovation to help navigate upcoming challenges and capitalize on growth opportunities.
Article
•
19-min read
•
28 November 2023
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Deloitte Research Center for Energy & Industrials
Lindsey Berckman
Lindsey Berckman
United States
Kate Hardin
Kate Hardin
United States
Matt Sloane
Matt Sloane
United States
Tarun Dronamraju
Tarun Dronamraju
India
In 2023, the aerospace and defense (A&D) industry witnessed a revival in product demand. In the aerospace sector, domestic commercial aviation revenue passenger kilometers surpassed prepandemic levels in most countries.1 This surge in air travel led to an increased demand for new aircraft and aftermarket products and services. In the US defense sector, new geopolitical challenges, along with the prioritization of modernizing the military, drove robust demand in 2023, particularly for weapons and next-generation capabilities.2
The demand for A&D products and services is expected to continue into 2024. On the commercial side, travel is likely to continue its upward trajectory. In the defense segment, demand for products is expected to continue to increase as geopolitical instability grows. Furthermore, companies in emerging markets, such as advanced air mobility, are expected to advance testing and certification as they prepare for commercialization.
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While these trends are likely to drive both domestic and international spending, the increased demand may cause A&D companies to deal with new challenges as they grapple with ongoing ones such as supply chain issues, longer lead times, and a talent shortage.
To address these challenges, A&D companies may further embrace digitalization and adopt emerging, advanced technologies and, thus, could achieve profitability by both addressing their cost challenges and initiating the development of novel revenue streams. Such technologies may be foundational for A&D companies in creating a more resilient supply chain, mitigating logistical issues, attracting new talent, and rapidly creating new products.
As A&D companies prepare for the year ahead, there are some key trends they may consider focusing on to take on the challenges and capitalize on emerging opportunities, with digitalization being the unifying theme across the trends:
Talent: A&D companies are addressing workforce challenges amid rising demand and changing workforce expectations
Supply chain: Global supply chain complexity is driving a multitiered solution
Digital transformation: The industry can unlock growth and efficiency through digital technologies
New product introduction: Evolving customer preferences and growing emphasis on sustainability are fueling product innovations
Defense and commercial spending: Industry spending is helping drive growth and innovation
Table of contents
Talent
Supply chain
Digital transformation
Product introductions
Spending
The future
1. Talent: A&D companies are addressing workforce challenges amid rising demand and changing workforce expectations
The A&D industry is facing a new talent landscape that is mainly driven by increasing salary levels, increased employee mobility, a reevaluation of employee relationships with the workplace, and an intensely competitive job market. Evolving employee expectations, which have only amplified since the pandemic, have often made it challenging for A&D companies to attract, retain, and develop a skilled workforce. These workforce issues could prove to be a significant pain point for A&D companies as they begin to scale production operations to meet growing demand across the industry.
Attracting talent
The US A&D industry has surpassed prepandemic levels in terms of employment. In 2022, US-based A&D companies increased their workforce by 101,700 to 2.2 million, marking a 4.87% increase from 2021 and surpassing prepandemic 2019 levels of 2.18 million.3
Despite the rise in workforce numbers, A&D companies are likely to face talent shortages. These shortages may hamper production demands resulting from domestic travel increases and nations increasing their armament stockpiles. Currently, the industry is struggling to attract talent, ranging from technicians and manufacturers to engineers. One leading Asia-based engineering company noted that the shortage of technicians may lead to increased maintenance costs and impact fleet expansion forecasts.4 Furthermore, in the National Association of Manufacturers’ (NAM’s) 2023 third quarter manufacturers’ outlook survey, about three out of four surveyed US manufacturers stated that attracting and retaining a quality workforce is their primary business challenge.5
The manufacturing industry, including A&D manufacturing, is increasingly competing with other industries for skilled talent.6 A&D companies recognize this, as top talent with digital skills can be drawn to digital jobs within a multitude of areas, including automotive (for example, the battery electric vehicle and autonomous vehicle segments) and life sciences (for example, medical technology, and biotechnology).
A&D companies may need to expand their strategies beyond compensation to accommodate the changing needs of the workforce. The US A&D industry has an annual average salary of US$108,900 which is approximately 55% above the national average.7 That said, many Gen Z employees are keen to have careers with broad impact and a purpose.8 These companies may attract talent with mission-focused marketing campaigns and undertake more strategic recruitment to ensure that the values of the employees and the company align.
Retaining talent
Despite a significant number of new hires, job openings in the manufacturing industry were still 604,000 in August 2023.9 Additionally, quits accounted for about 68% of total separations. To compete in a tightening labor market, A&D companies will likely address both talent attraction and, potentially more significantly, talent retention issues. The A&D industry has high requirements and expectations for its employees, including specific educational criteria and security clearances. This could necessitate a focus on retaining talent, especially with the increasing number of retirees.
Historically, these companies have succeeded with a workforce that possesses deep experience and a strong connection to the company culture. Therefore, given the competitive state of the job market, it is important for the industry to retain highly qualified workers.
A&D companies are using compensation and flexible work arrangements to enhance retention and turn the tide of employee attrition. This approach has been adopted by the manufacturing segment of the A&D industry where, according to Deloitte’s 2024 manufacturing industry outlook, there was a rise of 4% in the average hourly earnings of employees between the first quarter of fiscal year 2022 and the first quarter of fiscal year 2023. Additionally, the manufacturing industry has undergone a significant shift toward remote work over the last two years, during which there has been a 19% reduction in the average number of quits.10
The retention levels of A&D companies could plateau at a certain point if they continue to only offer flexibility benefits. According to the NAM’s 2023 second quarter manufacturers’ outlook survey, inflexible production, shift needs, and the creation of differences between workforce populations are some major challenges manufacturers face when offering such flexibility to workers.11 The industry will likely explore new avenues to both attract and retain talent.
Pursuing and implementing emerging technology can help not only address profit margins, but also invigorate a workforce. Regardless of an employee’s job description, A&D companies may begin to consider initiating new programs that allow employees to engage with advanced technologies, which may include tech-based rotational programs or designated time to pursue passion projects within the company.
Developing talent
With an increasing number of retirements and an aging workforce (about 26% of the A&D workforce is over the age of 55),12 the industry should train younger professionals for long-term sustainable operations and security. A&D companies can upskill their workforce by developing more learning avenues in alignment with the technological evolution within the industry. These avenues may include:
Working closely with colleges to create novel training and workforce development programs;
Expanding internships, apprenticeships, and other career development initiatives; and
Furthering cooperative research agreements with tier one research universities.
Employees in the A&D sector, across all career levels, should be educated on the importance of cybersecurity and should possess a clear understanding of their specific responsibilities in upholding a secure environment.13 And in line with that, A&D companies’ workforce development strategies will not only attract and retain talent but can also help ensure a secure environment for them.
Pursuing and implementing emerging technology can help not only address profit margins, but also invigorate a workforce. Regardless of an employee’s job description, A&D companies may begin to consider initiating new programs that allow employees to engage with advanced technologies.
2. Supply chain: Global supply chain complexity is driving a multitiered solution
A&D companies should expect to see continued fragility of and disruption in the global supply chain, which may lead to production delays, delivery delays, and increased pricing for both raw materials and components. From raw material and tier three suppliers through to original equipment manufacturers and maintenance, repair, and overhaul providers, the issues across the supply chain include shortages of skilled labor, labor attrition, unavailability of materials and parts, and measurable inflation. Since the pandemic, these factors have been causing an increase in material lead times and supplier decommits, resulting in increased volatility in operating, program, and financial performance.14
The manufacturing industry has seen only a slight recovery in lead times―the delivery times for production materials reached 87 days in August 2023 from its peak of 100 days in July 2022; however, the average lead time has yet to reach the prepandemic level.15 Despite this improvement in 2023, the continuing shortage of raw materials, semiconductors, microelectronics, and other key components or parts (e.g., engine casting or forgings) will likely remain a key issue for A&D companies in 2024 as production demand increases across the board.
Raw material sourcing, especially for “critical” minerals used to support defense, poses a unique challenge for A&D supply chains. Current production and reserves of such minerals is minimal in the United States. For instance, the United States was 100% reliant on net imports of gallium from countries such as China, the United Kingdom, Germany, and others.16 At present, China leads in the export of a majority of the 13 major critical minerals,17 with a market share of 50% to 70% for each, while the United States leads in only one.18 Reliance on these minerals may be a complicating factor for A&D companies, as replacing them is most likely unrealistic in the short term.19 As geopolitical tensions often lead to increasing trade barriers, affecting both raw materials and advanced technologies, A&D companies are likely to see increased restrictions on the availability of key imports as well as their ability to export sensitive items. A&D companies reliant on the most advanced material technologies, such as space technology, electronics, and semiconductors, are likely to monitor geopolitical events closely in the coming year and may consider maintaining strategic reserves of these critical minerals, such as gallium and germanium.20
The challenge of limited or sole sourcing for parts continues within the industry. The A&D industry has been slow in developing alternate sources of supply, as meeting industry standards and requirements through the qualification and certification of production lines is an arduous process. As such, companies often must rely on current production for the supply of both new and aftermarket parts. These companies will likely bolster their strategic inventory to build additional stock for critical items and maintain a flexible supply chain as they continue to develop new production options.
In the United States, production options include cross-border manufacturing and the continued exploration of friendshoring. The US government is continuing to work with international partners to develop its defense production capabilities. Additionally, A&D companies may further invest in novel manufacturing technologies to create an eventual alternate source of supply and avoid sole-source manufacturing, long lead times, and obsolescence issues.
Finally, it can be expected that A&D companies might continue turning to supply chain digitalization and automation options in 2024. In a recent Deloitte supply chain study, 78% of survey respondents agreed that digital solutions would enhance visibility and transparency throughout the supply network.21 A&D companies may develop digital twins of the supply chain to obtain a complete view of the procurement, production, and delivery processes.
The A&D supply chain is a complex, globalized ecosystem of customers and original equipment manufacturers; multiple tiers of suppliers; and maintenance, repair, and overhaul providers. This complexity makes implementing diversification and transparency across the value chain extremely difficult, but imperative. By maintaining strategic raw material reserves, committing to bulk buying of long lead time items, exploring alternate sources of supply, and digitizing operations, A&D companies may position themselves well to handle any continued fragility across the entire supply chain.
In a recent Deloitte supply chain study, 78% of survey respondents agreed that digital solutions would enhance visibility and transparency throughout the supply network. A&D companies may develop digital twins of the supply chain to obtain a complete view of the procurement, production, and delivery processes.
3. Digital transformation: A&D companies can unlock growth and efficiency by embracing digital technologies
A&D companies are continuing to advance in their digital transformation journey and can accelerate it further in 2024. The frontrunners in the industry are leading the charge on digital technologies such as model-based enterprise and digital twins. Digitalization can enhance product development, improve operational efficiencies, and help capitalize on growth opportunities. However, before adopting advanced technologies, A&D manufacturers might consider modernizing their processes, technologies, and tools. This may allow them to increase throughput with existing infrastructure and to effectively manage demand fluctuations and costs, despite persistent labor and supply chain challenges.
Digital transformation of the production process
A&D companies operate in a highly complex environment that is defined by regulatory requirements and specific customer, contract, and product specifications. Modernizing and integrating processes and enabling technologies are some significant steps for the A&D industry to improve production throughput and cost efficiency. Embracing digital transformation can reshape A&D production processes at all stages, significantly reducing industrialization cycle times, improving efficiencies, increasing production yield, and elevating quality standards.
At the design stage, model-based systems engineering22 approaches can allow for the virtual design, analysis, and verification and validation of systems, potentially shortening the time to qualify and certify a new manufacturing line. Similarly, employing Industrial Internet of Things (IIoT) technologies and integrating both operational technology (OT)23 and information technology (IT)24 systems can help generate better insights in the manufacturing environment. These types of end-to-end visibility solutions can help improve production yield and elevate product quality.
The implications of digital transformations and emerging digital technologies, such as digital twins, do not end with producing a final product. A&D companies can implement digital twin technology to track parts throughout their life span to improve maintenance protocols. It can also open opportunities for aftermarket service provider readiness. By leveraging the vast amounts of data produced by A&D systems, aftermarket companies may implement AI techniques to predict how and when an aircraft or system will need to be serviced. This data, including information on the parts and labor required, could reduce repair times, improve turnaround times, and create competitive advantage.
The pace of digital transformation among A&D companies varies and is often dependent on a company’s unique requirements, priorities, and available resources. Nonetheless, digitalization is expected to be imperative in the coming year as A&D companies continue to compete for business.
Digital enhancement
The demand for sustainability, product innovations, and wartime defense is expected to result in rapid technological evolution. A&D companies are exploring and applying digital technologies, specifically artificial intelligence and generative AI (a subset of AI), in an array of scenarios.
AI can streamline operations, enhance productivity, enable real-time data synchronization, and simplify customization processes. AI solutions have benefited the A&D industry for various applications, from improving cockpit avionics technology for surveillance and decision-making to optimizing maintenance and defect monitoring for manufacturers. Generative AI, through generative design, can be critical in delivering more opportunities for creating energy-efficient designs and low-carbon products.
Beyond product development, generative AI–enabled virtual field assistants can support engineers by improving their problem-solving capabilities and productivity.25 Such a use case of AI technology may have further workforce implications, as worker productivity enhancements may prove to be one approach for A&D companies to ease the effects of a talent shortage.
As A&D companies continue to enhance their digital capabilities, they will likely move to promote trust in their technologies, manage risks associated with data security, and monitor potential federal or state regulatory activity. To promote trust in their digital technologies, such as AI, A&D companies will likely strive to ensure that they are using the proper data and possess a complete understanding of the implemented algorithms.26 To protect sensitive data against growing cyber risks, A&D companies will likely make substantial investments in robust cybersecurity risk management measures and strengthened digital infrastructure systems. Finally, to ensure lawful practices, A&D companies may more closely monitor the increasing regulatory focus on the responsible use of these digital technologies, including national security implications.
A&D companies should continue exploring the major challenges impacting digital transformation and move from viewing it as optional to treating it as a requirement for achieving a competitive advantage and long-term success.
Generative AI, through generative design, can be critical in delivering more opportunities for creating energy-efficient designs and low-carbon products. Beyond product development, generative AI–enabled virtual field assistants can support engineers by improving their problem-solving capabilities and productivity.
4. New product introductions: Evolving customer preferences and growing emphasis on sustainability are fueling product innovations
Developing a new product in the A&D industry can, by itself, be a difficult task. And when you add in the complex operating environment, various regulatory requirements, and extensive testing demands associated with product certification, the task may seem more daunting. Despite these hurdles, evolving consumer demands for enhanced technology, greater sustainability, reduced emissions, higher performance systems, and lower costs are driving the A&D industry toward new product innovations and introductions in 2024.
In the coming year, A&D companies are likely to continue developing environmentally friendly propulsion alternatives to reduce emissions and prepare for any future emissions regulations. Concurrently, commercial and government entities will likely continue to push for development in supersonic and hypersonic flight, respectively. While these are major areas for product innovation, research and development is likely to continue in an array of areas in the A&D industry, such as materials development and advanced manufacturing incorporation.
Sustainability and emissions reductions
With the goal of achieving net-zero CO2 emissions by 2050 (since the United States is a signatory of the Paris Agreement),27 the aerospace industry is actively developing innovative designs and sustainable propulsion technologies. A&D companies are setting targets to reduce greenhouse gas emissions, water waste, and energy use as they progress toward meeting their interim 2030 sustainability targets. A&D companies are even more likely to continue their push for sustainability and reduced emissions as they acknowledge the broader implications of California’s Climate Accountability Package and await the release of the final version of the US Securities and Exchange Commission’s climate disclosure rule.
The demand for reduced and zero-emission aviation is likely to drive the growth momentum for electric, hybrid, hydrogen, and even solar-powered propulsion technologies in the coming year. For instance, a prominent US defense contractor has strategically invested in an electric propulsion solutions provider to drive the development of electric aviation solutions.28 The A&D industry should anticipate further opportunities to apply these technologies in the defense sector, as well as in maritime and aviation applications.
Beyond traditional aircraft, the advanced air mobility (AAM) industry is trending upward. The AAM industry is expected to advance design, testing, and certification in 2024 while preparing for commercialization in 2025. The industry has witnessed substantial orders from airlines for operating AAM aircraft in 2024 and beyond.29
In the short term, AAM manufacturers are focused on establishing robust manufacturing capabilities, transitioning from the proof-of-concept phase to scaled production. One leader in the AAM industry has already revealed their plans to construct manufacturing plants for large-scale aircraft production.30
Currently, the AAM industry seems to be considering electric propulsion for five- to six-seater aircraft. Moving forward, as the A&D industry moves to develop sustainable aircraft with 20 to 100 seats, companies may begin to experiment with and further develop hybrid electric or hydrogen fuel systems.
Regardless of the application, the development of these propulsion technologies takes time and can demand significant changes in aircraft design. The A&D industry will likely continue to see sustainable aviation fuel as an interim option to bridge the gap between the propulsion systems of the past and the future. The International Air Transport Association (IATA) anticipates that sustainable aviation fuel could contribute 62% of the carbon mitigation required to achieve the 2050 emissions reduction goals.31
Finally, as A&D companies seek an edge in sustainability, both aircraft and engines are expected to undergo significant redesigns. These companies are actively engaged in producing lightweight aircraft components using advanced materials. This can help not only reduce fuel consumption but also bolster the overall structural strength of an aircraft. In commercial aviation, engine manufacturers are dedicated to developing engines that incorporate cutting-edge technologies. Such integrations are aimed at enhancing fuel efficiency, reducing noise emissions, and minimizing environmental impact. On the defense front, the US Air Force is making strides in developing new generation fighter aircraft as part of the Next Generation Air Dominance program. This program is poised to leverage adaptive engine technology, which has been under development as part of the Next Generation Adaptive Propulsion (NGAP) program. Under the Air Force’s proposed fiscal year 2024 budget, the NGAP is fully funded with US$595 million.32
Supersonic and hypersonic technologies
NASA’s X-59 QueSST program aims to demonstrate quiet supersonic travel by reducing sonic boom intensity.33 Despite technological evolution, significant hurdles remain. Manufacturers claim that it is possible to develop sustainable and economically viable supersonic aircraft, but it is still at a nascent stage of product development. That said, a leading supersonic aircraft maker is securing investments and a collaboration with one of the major airlines in the United States, thereby increasing optimism in the overall promise of such a technology.34
Beyond supersonic aircraft, there is a growing demand for both offensive and defensive hypersonic technology in 2024. In fiscal year 2024, the US Department of Defense (DOD) and NASA have allocated budgets for developing and testing hypersonic capabilities.35 With the influx of funding from the DOD in the hypersonic space, A&D companies have been and will likely continue to work toward operationalizing hypersonic weapons, including glide vehicles and cruise missiles.
The A&D industry invests substantially in R&D to create new products, enhance existing ones, and drive technological progress. Each of these programs will likely rely on speed to market with a qualified and certified system. To truly reap the benefits of the most cutting-edge technologies, A&D companies are expected to prioritize the digital transformation of their entire supply chain and production processes. The digital thread—digital tools and representations that connect engineering, supply chain, manufacturing, and aftermarket activities and services to enable a model-based enterprise—will likely play a pivotal role in new product introductions throughout the industry.
The A&D industry invests substantially in R&D to create new products, enhance existing ones, and drive technological progress. Each of these programs will likely rely on speed to market with a qualified and certified system. … The digital thread … will likely play a pivotal role in new product introduction throughout the industry.
5. Defense and commercial spending: Industry spending is helping drive growth and innovation
The A&D industry should expect an increase in spending, from both the defense and commercial segments, in the year ahead. Defense spending will likely increase to meet the demand generated by escalating geopolitical tensions and the necessity for future capabilities. Commercial spending will likely stem from the increased demand for passenger flights and advancements in emerging markets.
Defense spending
The outlook for the global defense sector remains robust, with defense expenditures surpassing US$2.24 trillion in 2022 (most recent data at the time of publication).36 Geopolitical events across the world have driven, and are likely to continue to drive, an increase in defense funding. The US DOD requested a US$842 billion budget for fiscal year 2024, representing a 3.2% increase compared to fiscal year 2023 at the enacted base level. This budget will prioritize fostering innovation and countering potential adversaries from other countries.37
In 2024, the rise in military spending can drive innovation and digital infrastructure by implementing commercial technologies in defense applications and maximizing the use of private capital for emerging defense technologies and manufacturing. The US DOD has requested approximately US$1.8 billion for AI applications (a 63.6% increase in comparison with FY23), US$9.3 billion for advanced technologies, and US$0.69 billion for the experimentation and evaluation of advanced technologies for joint warfare in fiscal year 2024.38
The lion’s share of funding on technology is expected to focus on enhancing capabilities and improving readiness. To do this, the United States is concentrating its investments on procuring next-generation air, ground, and naval vehicles and modernizing existing ones to help meet defense requirements.39 Additionally, the US DOD is committed to investing in defense supply chain resiliency to help ensure the stability of the industrial base and meet future demand. Additionally, they are embracing and emphasizing AI as a technology that can support the United States in maintaining its strategic advantage in future conflicts.40 The DOD is also investing in several technology initiatives such as microelectronics, advanced propulsion, quantum technologies, and advanced computing.41
In response to global advancements, the US DOD is funding development from an array of entities. The DoD has established various mechanisms to engage the private sector (particularly small businesses and startups) and academia, promote innovation, and accelerate the integration of emerging technologies. These include, but are not limited to, the Defense Innovation Unit (DIU), Accelerating the Procurement and Fielding of Innovative Technologies (APFIT) program, National Security Innovation Network (NSIN), and AFWERX.42
US spending in the space sector has increased to keep pace with the growing demand for space capabilities. According to the Space Foundation, total government spending on space programs increased by 8% in 2022, with the United States holding a majority of the total spending share at approximately 60%.43 The United States Space Force (USSF) has requested funding of US$30.1 billion for fiscal year 2024,44 a 15% increase from fiscal year 2023 at the enacted level. The USSF is focused on building resilient, ready, combat-credible space forces, with an emphasis on cybersecurity, by using modern talent management processes. For this purpose, the USSF has allocated about 60% of the total budget for research, development, testing, and evaluation (RDT&E).45 Furthermore, NASA has requested a budget of US$27.1 billion for fiscal year 2024, a 7% increase from fiscal year 2023, with a focus on continued research on the International Space Station (ISS) and space exploration to create scientific and economic opportunities.46
Despite the rise in defense spending, growing inflation rates could be causing concerns among defense authorities. In real terms, the growth in defense spending is not substantial—the defense budget has increased year-over-year by 3.18% which is significantly lower than the current inflation rate of 6%.47 Also, the US defense spending as a percentage of GDP has decreased over the years and currently stands at 3.1%. This percentage is expected to decrease further to 2.8% by 2033.48 Such a situation could lead to future challenges for the US DOD in executing planned activities, as inflation may limit flexibility in launching new missions or require fund reallocation in order to prioritize essential activities. Nevertheless, in 2024, A&D companies can still capitalize on the DOD’s interest in R&D of cutting-edge defense equipment and advanced technologies.
Commercial spending
The international and domestic revenue passenger kilometers are expected to recover to prepandemic levels in 2024. This could result in increased demand for new aircraft orders. Furthermore, continued investments from emerging markets like AAM and space are expected to drive increased commercial spending. This spending may become a strategic capability that can aid the sustainable growth of A&D companies and shape new business opportunities. Commercial spending is likely to be focused on digitalization, new product development, and more broadly on future-facing subsectors such as AAM and space.
The AAM sector is poised for continued spending in 2024 to establish full-scale manufacturing and operational capabilities. AAM aircraft manufacturers are working to achieve type certification and are anticipated to enter into service and commercialization phases in 2025. Substantial investments are likely required to create facilities capable of accommodating future aircraft demand. To support operations, the AAM industry is expected to invest in developing new vertiports, modernizing underutilized airports, establishing maintenance, repair, and overhaul facilities, expanding charging and battery infrastructure, building pilot training facilities, and fostering public awareness and acceptance. AAM manufacturers face an interesting road ahead as they continue to scale for commercialization while carefully managing spending.
The commercial space industry witnessed a shift in funding dynamics in 2023. While government investments contributed to the space industry’s growth, albeit at a slower pace than in 2022, commercial investment experienced a notable decline in 2023.49 In 2024, it is anticipated that there will be an increase in acquisition and consolidation within the industry. Commercial investors are likely to exercise more caution as the revenue projections of previous Special Purpose Acquisition Companies (SPACs) for space companies are not being met.50 As a result, although the space industry is expected to grow, the investments and growth observed in recent years are not likely to return. Growth is expected to align with more conservative trends in space activities.
The future: Technology and innovation will likely drive growth in the A&D industry
In 2024, the A&D industry may be shaped by various factors, from geopolitical challenges to innovation and market demands, to technological leaps. As the industry navigates this uncertain landscape, it may need to prioritize adaptability and agility and commit to embracing technological advancements. The A&D industry is often characterized by long product development cycles and complex supply chains, which can demand constant innovation, technology integration, and continuous improvement. A&D companies that can leverage these areas may thrive in an ever-evolving global landscape.
The speed-to-market factor remains a significant challenge for the development of new products. Quick prototyping, certification, and commercialization can be crucial. Therefore, A&D companies should consider adopting digital technologies, such as the digital thread, to help accelerate product development by connecting value chain phases and yielding results in less time.
Finally, increased demand from both the defense and commercial markets could lead to stronger revenue streams for A&D companies in 2024. To capitalize on the potential growth opportunities in the industry, A&D companies may consider positioning themselves strategically in the following areas:
Customer demands: A&D companies should remain agile and innovative in a growing number of areas to meet evolving customer needs while upholding their standard of delivering high-quality products and services.
Technological advancements: The top A&D companies will likely strive to stay at the forefront of technological advancements in an effort to maintain a competitive edge, enhance business and operational processes, and attract top talent.
Cyber resiliency: As the A&D industry becomes more digitally interconnected, cyberattack risk grows. Thus, establishing a robust security culture and investing in cyber-resiliency measures may prove to be vital in ensuring a secure digital future.
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27 | https://www.techbriefs.com/component/content/article/33914-lightweighting-in-aerospace-component-and-system-design | Lightweighting in Aerospace Component and System Design
Lightweighting design is an extensively explored and utilized concept in many industries, especially in aerospace applications, and is associated with the green aviation concept. The contribution of aviation to global warming phenomena and environmental pollution has led to ongoing efforts for the reduction of aviation emissions. Approaches to achieve this target include increasing energy efficiency. An effective way to increase energy efficiency and reduce fuel consumption is reducing the mass of aircraft, as a lower mass requires less lift force and thrust during flight. For example, for the Boeing 787, a 20% weight savings resulted in 10 to 12% improvement in fuel efficiency. In addition to reduction of carbon footprint, flight performance improvements such as better acceleration, higher structural strength and stiffness, and better safety performance could also be achieved by lightweight design.
Lightweighting optimization of a solar-powered unmanned aerial vehicle (UAV) is an example of using both clean energy and lightweight structures to achieve green aviation operation. Current solar-powered UAV designs face challenges such as insufficient energy density and wing stiffness. Lightweight design is essential for ultralight aviation, enabling longer flight duration.
The principle of lightweight design is to use less material with lower density while ensuring the same or enhanced technical performance. A typical approach to achieve lightweight design for aerospace components is to apply advanced lightweight materials on numerically optimized structures, which can be fabricated with appropriate manufacturing methods. As such, the application of lightweight materials can effectively achieve both weight reduction and performance improvement. Although metal materials — especially aluminum alloys — are still the dominant materials in aerospace application, composite materials have received increasing interest and compete with aluminum alloys in many new aircraft applications.
Structural optimization is another effective way to achieve lightweighting, by distributing materials to reduce materials use, and enhance the structural performance such as higher strength and stiffness and better vibration performance. Conventional structural optimization methods are size, shape, and topology. Manufacturability is a crucial constraint in both material selection and structural optimization. The development of advanced manufacturing technologies such as additive manufacturing, foam metal, and advanced metal forming not only enable the application of advanced materials, but relax constraints, enhancing the flexibility of multiscale structural optimization.
Many examples of lightweight design have been successfully applied in the design of lightweight aircraft. Figure 1(a) illustrates the SAW Revo concept aircraft (produced by Orange Aircraft), which is an ultralight aerobatic airplane with carbon fiber-reinforced composite wings and a topologically optimized truss-like fuselage. The empty weight of this 6-meter-wingspan aircraft is 177 kg. Figure 1(b) shows a high-altitude, pseudo-satellite, solar-powered UAV from Airbus. The Zephyr 7 currently holds the world record for the longest absolute flight duration (336 hours, 22 minutes, 8 seconds) and highest flight altitude (21,562 m) for UAVs, partly from increased energy efficiency by lightweighting. Figure 1(c) shows a model of a future concept lightweight airplane for 2050 from Airbus, inspired by a bird skeleton. Figure 1(d) demonstrates a concept of a box wing aircraft where shape optimization is employed in the wing design. Structural efficiency could be increased by using a box wing structure; higher stiffness and lower induced drag force result from the box wing compared with conventional wing structures.
The selection of aerospace materials is crucial in aerospace component design since it affects many aspects of aircraft performance, from the design phase to disposal, including structural efficiency, flight performance, payload, energy consumption, safety and reliability, lifecycle cost, recyclability, and disposability. Critical requirements for aerospace structural materials include mechanical, physical, and chemical properties such as high strength, stiffness, fatigue durability, damage tolerance, low density, high thermal stability, high corrosion and oxide resistance, and commercial criteria such as cost, servicing, and manufacturability. Studies have indicated that the most effective way to improve structural efficiency is reducing density (around 3 to 5 times more effective compared with increasing stiffness or strength), i.e. using lightweight materials.
The most commonly used commercial aerospace structural materials are aluminum alloys, titanium alloys, high-strength steels, and composites, generally accounting for more than 90% of the weight of airframes. From the 1920s until the end of the century, metal — because of its high strength and stiffness, especially aluminum alloy — has been the dominant material in airframe fabrication, with safety and other flight performance measures driving aircraft design decisions. Lightweight aluminum alloys were the leading aviation structural materials — accounting for 70%–80% of the weight of most civil aircraft airframes before 2000 — and still play an important role. Since the mid-1960s and 1970s, the proportion of composites used in aerospace structures has increased due to the development of high-performance composites. Figure 2 illustrates the materials distributions for some Boeing products.
Aluminum Alloys. Although high-performance composites such as carbon fiber are receiving increasing interest, aluminum alloys still make up a significant proportion of aerospace structural weight. The relatively high specific strength and stiffness, good ductility and corrosion resistance, low price, and excellent manufacturability and reliability make advanced aluminum alloys a popular choice of lightweight materials in many aerospace structural applications, e.g. fuselage skin, upper and lower wing skins, and wing stringers. The development of heat-treatment technology provides high-strength aluminum alloys that remain competitive with advanced composites in many aerospace applications. Aluminum alloys can offer a wide range of material properties meeting diverse application requirements, by adjusting compositions and heat treatment methods.
Titanium Alloys. Titanium alloys have many advantages over other metals, such as high specific strength, heat resistance, cryogenic embrittlement resistance, and low thermal expansion. These advantages make titanium alloys an excellent alternative to steels and aluminum alloys in airframe and engine applications; however, the poor manufacturability and high cost (usually about 8 times higher than commercial aluminum alloys) result in the restriction of titanium alloys being used extensively. Hence, titanium alloys are used where high strength is required but limited space is available, as well as where high corrosion resistance is required. The current applications of titanium alloys in aerospace are mainly in airframe and engine components, overall comprising 7% and 36% of the weight, respectively.
High-Strength Steel. Steel is the most commonly used structural material in many industry applications due to good manufacturability and availability, extremely high strength and stiffness in the form of high-strength steels, good dimensional properties at high temperatures as well as the lowest cost among commercial aerospace materials. But high density and other disadvantages, such as relatively high susceptibility to corrosion and embrittlement, restrict the application of high-strength steels in aerospace components and systems. Steel normally accounts for approximately 5% to 15% of structural weight of commercial airplanes, with the percentage steadily decreasing. Despite the limitations, high-strength steels are still the choice for safety-critical components where extremely high strength and stiffness are required. The major applications for high-strength steels in aerospace are gearing, bearings, and undercarriage applications.
Aerospace Composites. High-performance composites such as fiber reinforced polymer and fiber metal laminates (FML) have received increased attention in aerospace applications, competing with the major lightweight aerospace materials such as aluminum alloys. In general, aerospace composites have higher specific strength and specific stiffness than most metals at moderate temperatures. Other advantages of composites include improved fatigue resistance, corrosion resistance, and moisture resistance as well as the ability to tailor layups for optimal strength and stiffness in required directions; however, the higher cost of composites in comparison to metals is one of the major obstacles for the application of composites.
Carbon fiber reinforced polymer (CFRP) represents the most extensively used aerospace structural material apart from aluminum alloys, with the major applications being structural components of the wing box, empennage, and fuselage as well as control surfaces (e.g. rudder, elevator, and ailerons). Glass fiber reinforced polymer (GFRP) is used in radomes and semi-structural components such as fairings. Aramid fiber polymers are used where high impact resistance is required. Fiber metal laminates, especially glass fiber reinforced aluminum (GLARE), are other types of composites that have applications in aerospace (especially in the Airbus A380) due to enhanced mechanical properties such as reduced density, high strength, stiffness, and fatigue resistance compared with monolithic metals. The main applications of GLARE are the fuselage skin and empennage.
Shape memory polymer composites (SMPC) are smart materials that can change their form as a result of a certain stimulus such as change of temperature, an electric or magnetic field, particular light wavelengths, etc. by releasing the internal stress stored in the material. The applications of SMPCs in aerospace components and systems include the wing skin of morphing-wing aircraft, and the solar array and reflector antenna of satellites. The advantages of SMPCs over shape memory alloys (SMAs) includes lower density, higher shape deformability and recoverability, better processing, and lower relative cost.
The development of nanotechnology provides an opportunity to improve multifunctional properties (physical, chemical, mechanical properties, etc.) at the nanoscale. Unlike conventional composites, nanocomposites offer the opportunity to improve properties without too much tradeoff of density increase by only adding a small amount of nanoparticles (e.g. layered silicate, functionalized carbon nanotubes (CNTs), and graphite flakes). To increase the oxidation resistance of composites, for example, nanoparticles could be included such as silicate, CNTs, or polyhedral oligomeric silsesquioxane (POSS) that could form passivation layers.
The addition of CNTs, silica, and layered silicate into composite matrix could promote energy dissipation on structural failure, increasing the toughness of the composite and resulting in the potential application to high-damage-tolerance structures. In addition to high modulus, high-strength nanoparticles such as continuous CNT could improve the stiffness and strength of the composite.
The development of nanocomposites offers the opportunity for redundancy elimination and weight reduction, which provides significant potential in promoting the properties of aerospace components, especially in lightweighting.
Manufacturability is a crucial constraint throughout the design process, governing the possibility of whether a design can be fabricated into a real product. Manufacturing constraints must be taken into consideration during materials selection, structure design, and optimization. Topological optimized designs tend to result in a complex geometry that cannot be fabricated by conventional manufacturing methods, such as casting and forming, without modification. Hence, manufacturing methods have significant effect on lightweighting design.
The development of advanced manufacturing technology, such as additive manufacturing (AM), foam metal manufacturing, and advanced metal forming, could significantly expand the flexibility of lightweighting design, both in material selection and in structural optimization.
AM was initially developed to produce prototypes rapidly and has now become a standard manufacturing tool. Although the advantages of AM attract much attention, challenges exist for AM to compete with conventional manufacturing methods, including quality of fabricated components, time-consuming processes, relatively expensive raw materials, and establishment of standards, qualification requirements, and certification.
Selection of materials for an aerospace system is based on the operating conditions of the specific component or system — such as loading conditions, operating temperatures, moisture, corrosion conditions, and noise — in combination with economic and regulatory factors; for example, wings mainly sustain bending during service as well as tension, torsion, vibration, and fatigue. Hence, the main constraints for wing materials are stiffness, tensile strength, compressive strength, buckling strength, and vibration. Composites such as CFRPs and GLAREs usually have much higher specific strength and stiffness than metals, which makes composites an attractive choice for lightweighting design for many aerospace components and systems; however, metals have the advantages of ease of manufacture and availability as well as much lower cost, making them still extensively used in many aerospace applications.
Lightweighting represents an effective way to achieve energy consumption reduction and performance enhancement. This concept has been well accepted and utilized in many industries, especially in aerospace component and system design. Lightweighting design involves the use of advanced lightweight material and numerical structural optimization, enabled by advanced manufacturing methods.
This article was written by L. Zhu, N. Li, and P.R.N. Childs of the Imperial College London, UK. Learn more here .
This article first appeared in the March, 2019 issue of Tech Briefs Magazine.
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28 | https://www.sciencedirect.com/science/article/abs/pii/S1526612523009180 |
An analytical model for pressure distribution in automated fiber placement on irregular surfaces and its application in aeronautical manufacturing
Author links open overlay panelLei Miao a b, Weidong Zhu a b, Yingjie Guo a b
, Wei Liang a b, Xiaokang Xu a b, Shubin Zhao c, Yinglin Ke a b
https://doi.org/10.1016/j.jmapro.2023.09.057
Automated fiber placement (AFP) is a key technology in the fabrication of composite aeronautical components, but the layup defects associated with uneven and insufficient contact pressure limit its application on irregular surfaces. Existing methods for pressure optimization in AFP are mostly based on extensive compaction experiments, which lacks theoretical basis and is not applicable to complex surfaces. In this paper, an analytical model of pressure distribution in AFP is proposed, which is suitable for any irregular surfaces. The influence of mechanical properties of contact materials and the mold curvatures on the calculation of contact pressure is taken into consideration. The Hertz theory is also extended to the three-dimensional contact area to suit the practical needs. Furthermore, an algorithm for pressure calculation along any layup path is proposed to rapid construct the pressure field. Then three sets of experiments are carried out on two aeronautical molds to validate the proposed model. The results show that the model can accurately calculate the contact pressure distribution in AFP, and compared with the previous model, the comprehensive prediction accuracy of the peak pressure is improved by 27.25 %. Combined with the layup path information, this model can be used to analyze the mechanical mechanism of layup defects, which provides a theoretical basis for the optimization of AFP parameters based on pressure distribution.
Introduction
Carbon fiber reinforced composites are widely used in modern aviation manufacturing because of their light weight and high strength [[1], [2], [3], [4]]. With the demand of excellent performance, the application of composites has gradually changed from secondary load-bearing components to main load-bearing components, which puts forward a higher standard for the forming quality of composite components. Compared with traditional labor-intensive methods, automated fiber placement (AFP) is more efficient and accurate, which is suitable for forming large and complex aeronautical components [5]. There are many kinds of AFP machines and each has its own characteristics. The flexibility of the robotic one (Fig. 1. (a)) is suitable for complex parts but the stiffness is poor [6,7]. The accuracy of horizontal one (Fig. 1. (b)) is relatively high but the processing range is limited. The gantry type (Fig. 1. (c)) has high accuracy and wide processing range, which is suitable for laying large aeronautical components such as wings. Despite the variety, the technological processes of AFP are basically the same: several resin-based carbon fibers are dragged separately with a controlled tension and pressed against the mold or the previous layer at appropriate pressure by a rubber-covered roller, the heat source works simultaneously to enhance the adhesion of the prepreg fibers, as shown in Fig. 1. (d). When layup at a constant speed, tension and heating power are constant, the layup quality is closely related to contact pressure. Sufficient and even contact pressure is beneficial to reduce many layup defects such as bulges and bridgings [8,9]. For thermoplastic prepreg, the increasing contact pressure is conducive to eliminate the voids in and between layers and improve the mechanical properties of composite components [[10], [11], [12]]. Therefore, it is essential to control the contact pressure of AFP within a reasonable range for the improvement of layup quality.
The contact pressure is generated by the compaction force applied to the rigid shaft of the roller. The bearing seats at both ends of the rigid shaft are usually equipped with two force sensors, and the contact pressure can be controlled by maintaining the resultant force on the shaft through an output cylinder. For easy control, the resultant force in the process of AFP is constant, which is obviously unreasonable. For the same resultant force, the pressure distribution on the plane, convex surface and concave surface is different, and its influence on the layup quality is also different. Therefore, in order to find the appropriate compaction force and contact pressure, it is often necessary to conduct a large number of compaction tests [[13], [14], [15]]. Bakhshi et al. [16] studied the influence of the roller hardness and geometric parameters on the contact pressure and layup quality through a method of controlling variables. The mold analyzed in this research is plane, which is different from the irregular mold commonly used in practice. Considering the influence of mold curvature on contact pressure, Qu et al. [17] explored the contact mechanical properties of plane, convex and concave molds, defined the placement suitability based on the contact state, and used it for path optimization before AFP. The curved surface molds used in their research were simplified into fixed curvature molds, which is closer to the actual mold than the plane one, but the accuracy is not enough. The data obtained from extensive experiments provide valuable references for the optimization of AFP process parameters under similar working conditions. Due to the limitations of experimental methods, a few scholars began to study the general AFP contact mechanics model which can accurately reflect the contact pressure distribution. Jiang et al. [18] established a model of pressure distribution for laying irregular curved mold, which is based on the relationship between the combined deformation and the contact pressure. This numerical model can accurately predict the contact pressure distribution on irregular surfaces. The complexity of this method lies in that it requires a lot of information of point positions on the mold to calculate the combined deformation, which is not convenient for engineering application. In order to realize the rapid calculation of contact pressure distribution with high precision, the analytical model based on classical contact theory is an important breakthrough point.
In practical applications, analytical modeling of rolling contact is a challenging problem [19,20]. Complex boundary conditions such as multi-material contact, nonlinear mechanical properties of materials and geometric constraints of irregular surfaces increase the difficulty of analysis. Hertz theory, as a classical theory of contact mechanics, is often used to analyze the contact pressure of elastomers under small deformation conditions [[21], [22], [23], [24]]. Generally, the hardness of the rubber-covered roller in AFP is about 35 HA (shore hardness), the contact pressure is not more than 0.6 MPa, and the deformation of the roller along the radius is within 3 mm [18]. This deformation is much smaller than the radius of the roller and the mold surface. Therefore, Hertz theory can be used to approximate the solution of the contact pressure in AFP [25]. It should be noted that Hertz contact is usually presented in the form of two-dimensional solutions [26], which is not applicable to the uneven pressure distribution caused by axial deformation in AFP [27,28], so it needs to be extended in three dimensions. In addition, the mechanical properties of roller and prepregs, as well as the variation of mold curvatures during AFP cannot be ignored in the calculation of pressure distribution.
In the present work, a 3D analytical model of the contact pressure during AFP is established for irregular curved molds. Based on Hertz theory, this model unifies the constitutive equations of prepreg layers and rubber-covered roller in the calculation of pressure distribution. A discretization method is proposed to realize the rapid extraction of mold geometric information, and an algorithm is further proposed for pressure calculation along any layup path. Several contact points on two aeronautical molds are selected to validate the model. Additional AFP experiment is carried out to demonstrate the validity of the model in predicting layup defects. The purpose of this work is to provide an accurate analytical model for the rapid calculation of pressure distribution on irregular surfaces during AFP process. The model can predict layup defects with the cooperation of layup path information, which has not only theoretical value but also engineering application prospect.
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In this section, a novel modeling approach of contact pressure is proposed, which can be applied to the fiber placement of curved components. Firstly, the mechanical properties of two contact materials, rubber and resin-based prepreg fiber, are investigated. The influence of mold curvatures and end-effector pose on the contact pressure calculation is simultaneously analyzed. The model is based on the classical contact mechanics theory and integrates the constitutive equations of different
Pressure calculation along the layup path
Automated fiber placement is carried out path by path and layer by layer according to the preset procedure. Each path is composed of a series of discrete path points, each containing information about the position, orientation, and speed of the end-effector during fiber placement. In order to analyze the pressure distribution within the contact area, discretization of the contact area is performed as a prerequisite step. Taking the i-th path point on the current path as the reference, m points
Experimental setup
In this study, the six-axis gantry AFP machine independently developed by Zhejiang University was used as the experimental execution equipment, as shown in Fig. 8(a). The pressure distribution of typical positions in the fiber placement of winglet mold (Fig. 8(b)) and hyperboloid mold (Fig. 8(c)) was taken as the analysis object. By comparing and analyzing the calculated results with the experimental results, the proposed model can be validated. The prepreg used in the experiment was supplied
Conclusion
In this paper, a theoretical model of pressure distribution for fiber placement on irregular curved surfaces is proposed by considering the mechanical properties of the contact objects and the mold curvatures. The elastic moduli and the constitutive equations of the roller and the prepreg fibers are obtained by finite element simulation and compaction tests. According to the discretization method proposed in this paper, the mold curvatures around different path points are extracted. Combing
CRediT authorship contribution statement
Lei Miao: Conceptualization, Methodology, Investigation, Data curation, Formal analysis, Visualization, Writing - Original draft. Weidong Zhu: Writing - Review and Editing, Project administration, Resources, Supervision. Yingjie Guo: Writing - Review and Editing, Data curation, Validation, Resources, Supervision. Wei Liang: Experiments, data analysis. Xiaokang Xu: Experiments, data analysis. Shubin Zhao: Experiments, data analysis. Yinglin Ke: Resources, Supervision.
Declaration of competing interest
We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled.
Acknowledgment
This research was supported by the “Pioneer” and “Leading Goose” R&D Program of Zhejiang (No. 2022C01134), the National Natural Science Foundation of China (No. 52105535) and the Major Research Plan of the National Natural Science Foundation of China (No. 91948301).
Lei Miao received the M.S. degree from China University of Mining and Technology, Xuzhou, China in 2020. He is currently working toward the Ph.D. degree in the School of Mechanical Engineering, Zhejiang University. His current research interests include automated fiber placement in aeronautical manufacturing, and robot processing technology.
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29 | https://www.mdpi.com/2075-4701/9/6/662 |
Alloys for Aeronautic Applications: State of the Art and Perspectives
by Antonio Gloria
1, Roberto Montanari
2,* [ORCID] , Maria Richetta
2 [ORCID] and Alessandra Varone
Institute of Polymers, Composites and Biomaterials, National Research Council of Italy, V.le J.F. Kennedy 54-Mostra d’Oltremare Pad. 20, 80125 Naples, Italy
2
Department of Industrial Engineering, University of Rome “Tor Vergata”, 00133 Rome, Italy
Author to whom correspondence should be addressed.
Metals 2019, 9(6), 662; https://doi.org/10.3390/met9060662
Submission received: 16 May 2019 / Revised: 3 June 2019 / Accepted: 4 June 2019 / Published: 6 June 2019
(This article belongs to the Special Issue Processing-Structure-Property Relationships in Metals)
Abstract
In recent years, a great effort has been devoted to developing a new generation of materials for aeronautic applications. The driving force behind this effort is the reduction of costs, by extending the service life of aircraft parts (structural and engine components) and increasing fuel efficiency, load capacity and flight range. The present paper examines the most important classes of metallic materials including Al alloys, Ti alloys, Mg alloys, steels, Ni superalloys and metal matrix composites (MMC), with the scope to provide an overview of recent advancements and to highlight current problems and perspectives related to metals for aeronautics.
Keywords:
alloys; aeronautic applications; mechanical properties; corrosion resistance
1. Introduction
The strong competition in the industrial aeronautic sector pushes towards the production of aircrafts with reduced operating costs, namely, extended service life, better fuel efficiency, increased payload and flight range. From this perspective, the development of new materials and/or materials with improved characteristics is one of the key factors; the principal targets are weight reduction and service life extension of aircraft components and structures [1]. In addition, to reduce the weight, advanced materials should guarantee improved fatigue and wear behavior, damage tolerance and corrosion resistance [2,3,4].
In the last decade, a lot of research work has been devoted to materials for aeronautic applications and relevant results have been achieved in preparing structural and engine metal alloys with optimized properties.
The choice of the material depends on the type of component, owing to specific stress conditions, geometric limits, environment, production and maintenance. Table 1 reports the typical load conditions of structural sections of a transport aircraft and the specific engineering property requirements, such as elastic modulus, compressive yield strength, tensile strength, damage tolerance (fatigue, fatigue crack growth, fracture toughness) and corrosion resistance.
Table 1. Typical load conditions and required engineering properties for the main structural sections in an aircraft. Elastic modulus (E); compressive yield strength (CYS); tensile strength (YS); damage tolerance (DT); corrosion resistance (CR).
Table
This work describes the state of the art and perspectives on aeronautic structural and engine materials.
Structural materials must bear the static weight of the aircraft and the additional loads related to taxing, take-off, landing, manoeuvres, turbulence etc. They should have relatively low densities for weight reduction and adequate mechanical properties for the specific application. Another important requirement is the damage tolerance to withstand extreme conditions of temperature, humidity and ultraviolet radiation [5].
Figure 1 shows a transport aircraft (Boeing 747), and Table 1 lists the typical load conditions together with the required engineering properties for its main structural sections.
Figure 1. The transport aircraft (Boeing 747) and its main structural sections.
As shown in Figure 2, engines consist of cold (fan, compressor and casing) and hot (combustion chamber and turbine) sections. The material choice depends on the working temperature. The components of cold sections require materials with high specific strength and corrosion resistance. Ti and Al alloys are very good for these applications. For instance, the working temperature of the compressor is in the range of 500–600 °C, and the Ti-6Al-2Sn-4Zr-6Mo alloy (YS = 640 MPa at 450 °C; excellent corrosion resistance) is the most commonly used material.
Figure 2. Schematic view of a turbofan engine.
For the hot sections, materials with good creep resistance, mechanical properties at high temperature and high-temperature corrosion resistance are required, and Ni-base superalloys are the optimal choice.
2. Aluminum Alloys
For many years, Al alloys have been the most widely used materials in aeronautics; however, the scenario is rapidly evolving, as shown by Table 2, which reports the approximate primary structure materials used by weight in Boeing aircrafts. From these data, it is evident that an increasing role is being played by composites [4].
Table 2. Materials used in Boeing aircrafts (weight %). The term “Others” refers to materials present in very small amounts, including metal alloys (Mg, refractory metals etc.) and carbon.
Table
Anyway, in spite of the rising use of composites, Al alloys still remain materials of fundamental importance for structural applications owing to their light weight, workability and relative low cost, and relevant improvements have been achieved especially for 2XXX, 7XXX and Al-Li alloys. In general, the 2XXX series alloys are used for fatigue critical applications because they are highly damage tolerant; those of the 7000 series are used where strength is the main requirement, while Al–Li alloys are chosen for components which need high stiffness and very low density.
2.1. 2XXX Series—(Al-Cu)
Where damage tolerance is the primary criterion for structural applications, Al-Cu alloys (2XXX series) are the most used materials. The alloys of the 2XXX series containing Mg have: (i) higher strength due to the precipitation of the Al2Cu and Al2CuMg phases; (ii) better resistance to damage; (iii) better resistance to fatigue crack growth compared to other series of Al alloys. For these reasons, 2024-T3 is still one of the most widely used alloys in fuselage construction.
Nevertheless, it is worth noting that the 2XXX series alloys present some drawbacks: (i) the relatively low YS limits their use in components subject to very high stresses; (ii) the phase Al2CuMg can act as an anodic site, drastically reducing the corrosion resistance.
Improvements can be achieved by a suitable tailoring of the composition and a strict control of the impurities. In particular, the addition of some alloying elements such as Sn, In, Cd and Ag can be useful to refine the microstructure, thus improving the mechanical properties, e.g., an increase in hardness, YS and UTS was found by increasing Sn content up to 0.06 wt% [6].
A further increase in mechanical properties can be obtained by controlling the level of impurities such as Fe and Si. For example, the alloy 2024-T39, which has a content of Fe+Si equal to 0.22 wt%, much lower than that of the 2024 alloy (0.50 wt%), exhibits an ultimate tensile strength (UTS) value of 476 MPa, while that of a conventional 2024 alloy is 428 MPa.
2.2. 7XXX Series—(Al-Zn)
Among all metals, Zn has the highest solubility in Al, and the strength results improved by increasing Zn content. The 7XXX series alloys represent the strongest Al alloys, and are used for high-stressed aeronautic components; for example, upper wing skins, stringers and stabilizers are manufactured with the alloy 7075 (YS = 510 MPa).
Mg and Cu are often used in combination with Zn to form MgZn2, Al2CuMg and AlCuMgZn precipitates which lead to a significant strengthening of the alloy [7].
However, there are also some drawbacks to the 7XXX series. Specifically, the low fracture toughness, damage tolerance and corrosion resistance limit the use of the 7075 alloy in the aeronautic industry. Anyway, the composition can be varied to improve their properties.
The optimal properties of the 7XXX series are obtained when the Zn/Mg and Zn/Cu ratios are approximately equal to 3 and 4, respectively. Alloy 7085 is a possible alternative to 7075 for aerospace applications due to its excellent mechanical properties (YS = 504 MPa, elongation = 14%) and better damage tolerance (44 MPa m1/2). Zr and Mn can be added up to 1% as they refine the grain and consequently improve the mechanical properties.
Another important issue related to the specific applications of 7XXX series alloys is the fatigue behavior, and a lot of work has been devoted to the matter, taking into consideration different parameters [8,9,10,11,12,13].
Material discontinuities are often associated with crack nucleation. On a micro-scale, roughness and precipitate particles may act as preferred nucleation sites; however, the most serious problems arise at macro-scale level. Coating layers due to cladding and/or anodizing, and defects (machining marks, scratches etc.) induced by the manufacturing process have been found to be the principal sources of failure [12]. The fatigue performance of the 7075-T6 alloy is significantly reduced by the anodic oxidation process, and the degrading effect of the oxidation increases with the coating layer thickness. Such a detrimental effect is mainly ascribed to deep micro-cracks which form during the anodizing process. Moreover, the brittle nature of the oxide layer and the irregularities beneath the coating contribute to degradation [13].
Components with complex geometrical shapes, made of Al alloys, are usually obtained by closed-die forging of a billet, and are manufactured to obtain a good combination of strength, fatigue resistance and toughness. Some forging experiments have been performed on the 7050 alloy in agreement with AMS4333 requirements, and an alternative process [14], involving an intermediate warm deformation step at 200 °C between the quenching and ageing steps, showed the possibility to improve fracture toughness without effects on YS and UTS. Results showed a more homogeneous and finer grain structure, after warm deformation, which can explain the increase in fracture toughness.
2.3. Al-Li Alloys
The density of Li is very low (0.54 g/cm3); thus, it reduces that of Al alloys (~3% for every 1% of Li added). Moreover, Li is the unique alloying element that determines a drastic increase in the elastic modulus (~6% for every 1% of added Li).
Al alloys containing Li can be hardened by aging, and Cu is often used in combination with Li to form Al2CuLi and improve the mechanical properties [15]. In ternary Al-Cu-Li alloys, six ternary compounds have been identified; the most important among them are T1 (Al2CuLi), T2 (Al6CuLi3), and TB (Al15Cu8Li3). The phases precipitating from the supersaturated solid solution depend on the Cu/Li ratio [16,17]; the precipitation sequence has been described in ref. [18].
Al-Li alloys exhibit lower density and better specific mechanical properties than those of the 2XXX and 7XXX series; thus, they are excellent materials for aeronautic applications [19,20]. For example, the use of the 2060-T8 Al-Li alloy for fuselage panels and wing upper skin results in 7% and 14% weight reduction if compared to the more conventional 2524 and 2014 alloys, respectively.
However, Li content higher than 1.8 wt% results in a strong anisotropy of mechanical properties resulting from texture, grain shape, grain size, and precipitates [21]. This was a serious drawback in the first two generations (GEN1 and (GEN2) of Al-Li alloys, which also had low toughness and corrosion resistance.
Al-Li alloys were first developed in the 1920s, and the 2020 alloy (GEN1) started to be produced in 1958 for the wing skins and empennage of the Northrop RA-5C Vigilante aircraft. The deep understanding of the relations between the microstructure and mechanical characteristics of these materials matured much later in the 1990s, leading to the production of the third generation (GEN3), a family of alloys with an outstanding combination of properties for aeronautic applications. The former generations of Al-Li alloys had a higher Li content and a lower density than GEN3 alloys, but suffered from high anisotropy associated with the precipitation of coarse Li phases, such as T2 [22,23].
Anisotropy has been partially reduced in GEN2 alloys through a suitable recrystallization texture and the tailoring of composition [24]. In GEN3 alloys with Li content between 1 and 1.8 wt%, the anisotropy problem has been substantially overcome. These materials exhibit excellent mechanical properties; in particular, the specific stiffness ranges from 28.9 to 31.2 GPa g−1 cm3 and is much better than that of the 2XXX (26.1–27.1) and 7XXX (25.9–26.4) series. The phase δ’(Al3Li) is not present, and strengthening is mainly due to the precipitation of the T1 phase forming platelets on {111} Al planes [25,26,27].
The typical morphology of T1 precipitates is shown in Figure 3. In the first stage of precipitation up to the aging peak, T1 platelets have a constant thickness (one unit cell), then, it increases with a consequent decrease in mechanical performances [26]. Although some years ago T1 precipitates were believed to be unshearable by dislocations, more recent investigations through high-resolution electron microscopy evidenced sheared precipitates in deformed samples [27,28]. T1 precipitates are sheared in a single-step shearing event. The transition between shearing and by-passing is progressive, connected to the increase in T1 plate thickness, and takes place after peak ageing. The by-passing mechanism favours the homogenization of plasticity up to the macroscopic scale. Strain localization within the matrix can be minimized by changing the deformation mode from dislocation shearing to dislocation by-passing of the precipitates.
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Figure 3. Typical morphology of T1 precipitates in Al-Cu-Li alloys.
A great variety of T1 microstructures can be been obtained, operating under different conditions of deformation and aging. The parameters of the T1 precipitate distributions have been systematically characterized and modelled by Dorin et al. [29].
In the conventional manufacturing route for producing aeronautic plates, stretching is carried out after solution heat treatment for relieving residual stresses due to quenching. The operation, which involves a plastic strain of about 5%, also allows the obtainment of a homogeneous distribution of T1 precipitates after aging, since dislocations represent preferred sites for precipitate nucleation. Such homogeneous distribution is the key factor for the excellent mechanical properties of Al-Li alloys. Increasing the pre-strain induces a higher density of dislocations, i.e., the preferred T1 nucleation sites; thus, the average diffusion distance of alloying elements is reduced and the aging kinetics is accelerated. The benefits of stretching in Al-Cu-Li alloys saturate at pre-strains of 6–9% [25].
Moreover, stretching prior to ageing is connected to a relevant technological problem: today, advances in rolling technology enable the production of plates with desired thickness, which is of great interest for manufacturing near-net-shape sections (e.g., tapered wing skins). The stretching of a tapered plate leads to a strain gradient, and it is necessary to know the maximum strain that can be achieved without fracture. Recently, Rodgers and Prangnell [30] have investigated the effect of increasing the pre-stretching of the Al-Cu-Li alloy AA2195 to higher levels than those currently used in industrial practice, focussing the attention on the behavior of the T1 phase. At the maximum pre-strain level before plastic instability (15%), YS increased to ~670 MPa and ductility decreased to 7.5% in the T8 condition. In fact, increasing the pre-strain prior to ageing leads to a reduction in the strengthening provided by the T1 phase, in favour of an increase in the strain hardening contribution.
In recent years, Al-Li alloys have experienced a great development, mainly based on the tailoring of composition and the knowledge/control of the precipitation sequence of stable and metastable phases [19,31,32,33,34,35,36]. In the alloy compositions of major interest, Cu content is around 3 wt%, Li is always below 1.8 wt% (in most recent alloys it does not exceeds 1.5 wt%), Mg content varies in an extended range, and other elements, in particular Ag and Zn, can be also added. Of course, the precipitation sequence depends on the specific composition. For example, a high Li content favors the formation of the metastable δ’ phase [27], while Mg leads to the precipitates typically present in the Al-Cu-Mg alloys, namely Guinier–Preston–Bagaryatsky (GPB) zones, S’/S [37,38]. In spite of strength increase, the presence of the δ’ phase is generally undesired because it is prone to shear localization, leading to poor toughness and ductility.
In a recent paper, Deschamps et al. [36] described the microstructural and strength evolution during long-term ageing (3000 h at 85 °C) of Al-Cu-Mg alloys with different contents of Cu, Li and Mg. They found that T1 is always the dominating phase in T8 condition, S phase is also present and, in the case of a high Li content, δ’ precipitates are observed. The examined alloys exhibit a very different level of microstructural stability during long-term ageing. Although the high Li alloy originally (T8 condition) has the lowest strength, its evolution leads to mechanical properties comparable with those of the other alloys after 3000 h of treatment. This is due to the precipitation of an additional fraction (~10%) of δ’ precipitates, whereas the two other alloys form a limited amount of metastable phases.
Another significant drawback to the GEN2 alloys is poor fracture toughness and ductility. Delamination cracking, which is a complex fracture mechanism involving initial transverse cracks with length comparable to grain size, is of great relevance in the fracture process. Delamination cracks along grain boundaries have been observed and described by many investigators [39,40,41,42,43]; Kalyanam et al. [44] systematically investigated the phenomenon in the 2099-T87 alloy, described the locations, sizes and shapes of delamination cracks and the extension of the primary macro-crack, and found that an isotropic hardening model with an anisotropic yield surface describes the constitutive behavior of the alloy.
In conclusion, GEN3 alloys have low density, excellent corrosion resistance, an optimal combination of fatigue strength and toughness, and are also advantageous in terms of cost in comparison to Carbon Fiber Reinforced Polymers (CFRP), which are considered as competitor materials to replace the traditional alloys of the 2XXX and 7XXX series in the design of new aircrafts.
2.4. Aluminum Composites
Composites with a metal, ceramic and polymer matrix are increasingly used in the aeronautic industry, replacing other materials (see Table 2). They are of relevant interest for applications in both structural and engine parts of aircrafts.
Metal matrix composites of light alloys (Al, Ti, Mg) are usually reinforced by ceramics (SiC, Al2O3, TiC, B4C), in the form of long fibers, short fibers, whiskers or particles. Typically, these composites are prepared using SiC or Al2O3 particles instead of fibers, which are used only for special applications, such as some parts of Space Shuttle Orbiter [45]. In addition, the nature of reinforcement is a relevant factor to the production costs, and whiskers and ceramic particles seem to be a good compromise in terms of mechanical properties and costs [46,47].
Al matrix composites, prepared with SiC and Al2O3 particle reinforcement, exhibit higher specific strength and modulus, fracture toughness, fatigue behavior, wear and corrosion resistance than the corresponding monolithic alloys. To further improve their mechanical properties, other types of reinforcements such as carbon nanotubes (CNTs) and graphene nano-sheets have been recently investigated [48,49,50]. If compared to the conventional reinforcements, CNTs and graphene are stronger and provide better damping and lower thermal expansion. A critical aspect is the optimization of reinforcement content because the properties of Al matrix composites strongly depend on such a parameter. For example, Liao et al. [49] found that the best characteristics are achieved with 0.5 wt% of multi-walled nanotubes.
In addition to high mechanical properties, good corrosion resistance is a requirement of Al composites. The topic has been extensively investigated for many years (e.g., see [51,52,53]); however, it is not yet completely clear how the presence of reinforcing phases influences the corrosion resistance and mechanisms. It is a common opinion that galvanic corrosion may take place due to the contact between reinforcement particles and the matrix: galvanic coupling between Al and ceramic particles has been detected, with the reinforcement acting as an inert electrode upon which O2 and/or H+ reductions occur [54].
Anyway, composites are more susceptible to pitting corrosion than the corresponding monolithic alloys, and preferential attack occurs at the reinforcement–matrix interface [55,56]. The phenomenon is enhanced by the presence of precipitates, in particular when they are located at the junction between the reinforcement particles and the metal matrix [57].
2.5. Advanced Joining Techniques for Aluminum Alloys
The development of innovative joining techniques is a relevant aspect for the aeronautic applications of Al alloys. Recently, Friction Stir Welding (FSW) gained increasing attention in the aerospace industry (e.g., airframes, wings, fuselages, fuel tanks), and a lot of research efforts have been devoted to one of its variants, namely, Friction Stir Spot Welding (FSSW) [58,59,60,61,62,63,64,65]. This method is an alternative to resistance welding, riveting, and adhesive bonding in the fabrication of aircraft structures, and allows the joining of components made of Al alloys with lower costs and better strength than conventional techniques. Welding time, tool rotation speed, tool delve depth, tool plunge speed and tool exit time are crucial parameters which should be properly optimized [63,64,65].
A serious problem is represented by the hole resulting from the welding process, which strongly weakens the joint strength. A novel technique, Refill Friction Stir Spot Welding (RFSSW) [66,67], allows us to overcome this drawback through the filling of the hole. RFSSW employs a tool made of a pin and a sleeve, and its procedure is described in detail in the paper of Kluz et al. [68].
RFSSW is very useful for joining materials whose microstructure can be remarkably changed by conventional welding processes, especially the alloys of 2XXX and 7XXX series.
Many advantages are related to spot welding, causing a decline in the riveting and gluing of Al alloys: (i) the drilling of parts and the use of rivets as additional fasteners are not required; (ii) a great resistance to corrosion can be achieved for welded joints; (iii) the possibility to perform simple repairs of joints; (iv) no part of the joint extends beyond the surface of the joined elements. The optimization of the RFSSW parameters to get the best mechanical performances of joints has been studied by many authors [66,67,68,69,70].
3. Titanium Alloys
Owing to their excellent specific strength and corrosion resistance, Ti alloys are increasingly used for manufacturing structural parts of aircrafts. They are also employed in engine sections operating at intermediate temperature (500–600 °C).
Ti alloys can be divided into three main classes (α, β and α-β). Independently of the specific class, the mechanical properties of Ti alloys depend on O and N in solid solution [71,72]. The solubility of these interstitial elements in both α and β phases is high, increases with temperature and the part of the gas absorbed at high temperature remains entrapped in the metal after cooling, causing lattice distortion. In addition to modifying the mechanical properties, this phenomenon plays a role also in manufacturing processes and stress relieving heat treatments. X-ray diffraction experiments on the Ti6Al4V alloy carried out up to 600 °C in vacuum or different atmospheres demonstrated that the effects of O and N are synergic with the intrinsic anisotropic thermal expansion in determining the distortion of the hexagonal lattice [73,74,75,76].
The surface integrity of machined aeronautical components made of Ti alloys [77,78,79] also represents a critical problem. The cutting of Ti alloys generates an enormous amount of heat at the chip–tool interface, which is not suitably dissipated owing to the low thermal conductivity; this causes surface damage and residual stresses [80].
3.1. α-Ti
In general, α-Ti alloys have better creep behavior and corrosion resistance than β-Ti alloys [81], therefore, some of them (e.g., Ti-3Al-2.5V, Cp-Ti, Ti-5-2.5, Ti-8-1-1, Ti-6-2-4-2S, IMI829) are commonly used to make compressor disks and blades of aeronautic engines.
In order to improve the microstructural stability of α-Ti alloys at increasing temperature, and, consequently, their mechanical performances, different compositions have been studied by adding Al, Sn, Zr and Si. The results are not completely satisfying because the achievement of some advantages is often accompanied by drawbacks. For instance, Jiang et al. [82] modified the composition of a Ti-25Zr alloy by adding Al up to 15% and found that the YS increase is accompanied by a reduction of ductility.
3.2. β-Ti
β-Ti alloys exhibit higher strength and fatigue behavior than the α-Ti alloys, thus they are employed for high-stressed aircraft components, e.g., landing gear and springs are currently manufactured using Ti-15V-3Cr-3Al-3Sn and Ti-3Al-8V-6Cr-4Mo-4Zr alloys [83], while Ti-10V-2Fe-3Al, Ti-15Mo-2.7Nb-3Al-0.2Si, Ti-5Al5V5Mo3Cr0.5Fe and Ti-35V-35Cr are applied in airframe parts [84].
A drawback of these materials is the relatively low ductility, which can be mitigated through tailoring the composition (Ti-1300 [85]) and suitable heat treatments (Ti–6Al–2Sn–2Zr–2Cr–2Mo–Si [86]).
3.3. α-β–Ti
Ti-6Al-4V is the most used Ti alloy owing to its excellent combination of mechanical properties (strength, fracture toughness and ductility) and corrosion resistance [87]. Moreover, Zr addition further improves its strength through the solid solution hardening mechanism; Jing et al. [88] showed that hardness is increased to 420 HV and YS to 1317MPa by adding 20 wt % Zr at the expense of ductility (elongation ratio drops to ~8%).
Ti-6Al-2Zr-2Sn-3Mo-1Cr-2Nb, Ti-6Al-2Sn-2Zr-2Cr-2Mo-Si and ATI 425 are other α-β–Ti alloys widely used for manufacturing aircraft parts such as fuselage, landing gear and compressor disks.
3.4. Ti Composites Reinforced with SiC Fibers
Ti composites are materials of great interest for aeronautic applications and, in particular, attention has been focused on those reinforced with long ceramic fibers [89,90,91,92,93,94,95,96,97,98,99,100,101,102]. Among them, the Ti6Al4V-SiCf composite is a promising material for turbine components and structural high-stressed parts.
Figure 4a shows the stratified structure of the SiC fibers: a C layer of about 3 µm separates the SiC fiber from the Ti6Al4V matrix. The composite is commonly prepared by Hot Isostatic Pressing (HIP) or Roll Diffusion Bonding (RDB) of Ti6Al4V sheets alternated with SiC fiber layers [99,102]; the resulting structure is displayed in Figure 4b.
Figure 4. Stratified structure of SiC fibers (a). Typical morphology of Ti6Al4V-SiCf composite (b).
The Ti6Al4V-SiCf composite is a promising material for mechanical components operating at medium temperatures, especially turbine blades and structural high-stressed parts of aeronautic engines. The performances mainly depend on the fiber–matrix interface and chemical reactions occurring during the manufacturing process and in-service life, when it is exposed for a long time to temperatures around 600 °C.
Direct contact of the Ti6Al4V matrix with SiC induces the formation of brittle compounds like Ti5Si3, which deteriorate the mechanical behavior of the composite [103,104], therefore the fibers are coated with a thin C layer. This coating hinders chemical reactions, preserves the fiber integrity, reduces the interfacial debonding and deflects the propagation of micro-cracks along the fiber. However, when the composite is operating for a long time at medium-high temperatures, C diffuses into the matrix, forming TiC. The TEM micrograph in Figure 5a displays TiC particles of ~200 nm forming an irregular layer around a fiber which has grown during the fabrication process at high temperature.
Figure 5. The TEM micrograph shows TiC particles forming an irregular layer around a fiber (a). AFM (Atomic Force Microscopy) evidences a groove all around the fiber, in correspondence to the C coating after a treatment of 1 h at 600 °C in air (b). The depth profile measured along the line in (b) is displayed in (c). AFM observations were carried out using a Multimode III of Digital Instruments in contact mode and a Si3N4 tip.
X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES) and scanning photoemission microscopy (SPEM) analyses, carried out before and after heat treatments at 600 °C of up to 1000 h, evidenced the stability of the fiber–matrix interface [94,95,96,97,101]. The fiber–matrix interface is also stable after prolonged heat treatments because a thin TiC layer forms all around the C coating during the fabrication process, hindering further C diffusion towards the matrix and delaying interface degradation. This is due to the fact that C diffusion through TiC is much slower than through Ti (for C in Ti, the diffusion parameters are D0 = 5.1 × 10−4 m2 s−1 and Q = 182 kJ mole−1 [105], whereas for C in TiC, D0 = 4.1 × 10−8 m2 s−1 and Q = 207 kJ mole−1 [106]).
If the material is heated in air, C atoms present in the core and the coating of fibers may react with O, forming CO and CO2; in this case, a groove is observed all around the fibers (Figure 5b,c) and in the fibers’ core. Therefore, the surface of the composite during in-service life must be protected to avoid direct contact with O in the air.
The stability of the fiber–matrix interface leads to the stability of the mechanical properties. YS, UTS and Young’s modulus E, determined from tensile tests carried out at room temperature on the Ti6Al4V-SiCf composite in as-manufactured condition and after different heat treatments, are reported in Figure 6a [96]. It is clear that the mechanical properties are scarcely affected by heat treatments, even in the most severe conditions. Moreover, fracture surfaces show plastic deformation of the matrix and pull-out of the fibers, i.e., a correct load transfer from the matrix to the fibers (Figure 6b).
Figure 6. Yield stress (YS), ultimate tensile strength (UTS) and Young’s modulus E determined from tensile tests at room temperature carried out on the Ti6Al4V-SiCf composite in as-manufactured condition and after the indicated heat treatments (a). Figure 6a was redrawn from data reported in [94]. Fracture surface of a sample exposed at 600 °C for 1000 h (b).
Figure 7 compares the results obtained from fatigue tests carried out on the material in as-manufactured condition and after 1000 h at 600 °C. Also, these data confirm the good stability of composite mechanical properties after long-term exposure to high temperature.
Figure 7. Curves determined from fatigue tests carried out on the material in as-manufactured condition and after 1000 h at 600 °C.
4. Magnesium Alloys
Mg is the lightest metal used in structural applications and exhibits excellent castability [107], with good fluidity and less susceptibility to hydrogen porosity than other cast metals such as Al alloys [108]. In fact, wrought Mg alloys have better mechanical properties than casting alloys; however, the higher asymmetry in plastic deformation represents a serious problem [109]. For this reason, casting is the principal way for manufacturing Mg components, and various processes are currently used for producing castings (a literature overview can be found in ref. [107]). Other relevant advantages of Mg are its abundance and recyclability [110].
On the other hand, the poor mechanical properties and low corrosion resistance of Mg alloys limit their use in manufacturing parts of aircrafts, even if some alloys (AZ91, ZE41, WE43A and ZE41) are commonly used for gear boxes of helicopters. For commercial casting alloys, the tensile yield strength is in the range 100–250 MPa and the ductility at room temperature is limited (elongation in the range 2–8%) [111,112].
The strategies for strengthening Mg alloys mainly rely on: (i) grain refinement, (ii) precipitation of second phases, and (iii) control of microstructural features on a nano-scale.
The first approach is based on techniques for obtaining ultrafine grains, smaller than 1 μm [113,114,115]. The numerous grain boundaries represent obstacles for dislocation motion, thus Mg alloys can reach YS values of about 400 MPa, but strength is reduced when grain growth occurs at relatively low temperature (0.32 Tm) [114]. Another drawback is the fact that grain refining tends to suppress deformation twinning, which is an important strengthening mechanism together with dislocation slip [116,117,118,119].
The precipitation of second phases involves the composition tailoring of Mg alloys by adding elements like Al, Zn, Zr and rare earths. Moreover, the increase in Al content also remarkably improves the corrosion resistance [120,121].
As shown in Figure 8, YS and UTS of Mg alloys increase with Zn content up to 4 wt%, while for higher values they remain constant or slightly decrease [122]. The precipitation of MgxZny phases induces relevant hardening and guarantees an interesting combination of good strength and ductility. However, for the corrosion resistance of Mg–Zn based curves determined from fatigue tests carried out on the material in as-manufactured condition and after 1000 h at 600 °C, the alloy decreases as Zn content in the alloy increases [123] due to the cathodic effect of the MgxZny phases, whose volume increases with Zn.
Figure 8. YS and UTS of Mg alloys vs. Zn content. Data are taken from [122].
Homma et al. [124] developed Mg–6Zn–0.2Ca–0.8Zr alloys, and found that Zr addition is beneficial to refine the grain size as well as to disperse fine and dense MgZn2 precipitates containing Ca and Zr. Mechanical properties are enhanced through the combination of texture strengthening, grain size refinement and precipitation strengthening.
Mg alloys containing rare earths can exhibit high strength at both room and elevated temperatures; the effects of Y on mechanical properties have been extensively investigated by Xu et al. [125], who observed that Mg–Zn–Y phases are formed at the grain boundaries. The phases vary with Y content: when Y is 1.08 wt.%, the alloy mainly contains I-phase, whereas for higher Y content (1.97–3.08 wt.%) W-phase is also present. The alloy with 1.08 wt.% of Y has the highest strength because I-phase is closely bonded with the Mg matrix and retards the basal slip. Since W-phase easily cracks under deformation, Y contents in the range 1.97–3.08 wt.% induce the degradation of mechanical properties.
An effective method to produce ultra-strong Mg alloys (YS = 575 MPa, UTS = 600 MPA) with uniform elongation (~5.2%) has been proposed by Jian et al. [126], and is based on stacking faults (SFs) with nanometric spacings, induced by hot rolling. These investigators studied a T4 treated Mg–8.5Gd–2.3Y–1.8Ag–0.4Zr (wt%) alloy which was subjected to increasing deformation up to 88% of reduction and observed that the mean distance between SFs decreases with thickness reduction. Since SFs act as barriers for dislocation motion, a higher SFs density leads to an increase in strength.
In addition to grain refinement, the precipitation of second phases and the control of microstructural features on a nano-scale, non-traditional approaches have been also considered to obtain high strength in Mg alloys, for instance rapid solidification and powder metallurgy achieved YS ≅ 600 MPa in a Mg–Zn–Y alloy with uniform distribution of ordered structures [127]. However, the process involves a significant loss of ductility and is difficult to transfer from a lab scale to an industrial scale.
5. Steels
Ultra-High Strength Steels (UHSS) are commonly used for manufacturing aircraft parts such as landing gears, airframes, turbine components, fasteners, shafts, springs, bolts, propeller cones and axles. Some of them exhibit very high YS values, e.g., 300M (1689 MPa), AERMET100 (1700 MPa), 4340 (2020 MPa); however, there is a tendency to progressively replace these materials by composites. The reason is related to their low specific strength and corrosion resistance. Moreover, UHSS are weakened by H atoms, which favor crack growth and micro-void formation with consequent localized deformation and failure [128].
Recently, Oxide Dispersion Strengthened (ODS) steels have attracted the attention of aeronautic industries. A lot of scientific work has been devoted to ODS ferritic steels, because they are promising candidate materials for applications in nuclear reactors [129,130]. ODS steels are strengthened through a uniform dispersion of fine (1–50 nm) oxide particles which hinder dislocation motion and inhibit recrystallization. High-temperature performances are also improved by refining the ferritic grain in combination with oxide dispersion strengthening [131,132].
Usually, ODS steels are prepared by high-energy mechanical alloying (MA) of steel powders mixed with Y2O3 particles, followed by hot isostatic pressing (HIP) or hot extrusion (HE) [133,134] and annealing at ~1100 °C for 1–2 h. A drawback of the aforesaid procedure is that the final high temperature annealing causes the equiaxed nanometric grains obtained by MA to transform into grains with a bimodal grain size distribution, involving anisotropic mechanical properties and a remarkable decrease in hardness, YS and UTS [135]. Nano-ODS steels were also produced via Spark Plasma Sintering (SPS), by exploiting the high heating rate, low sintering temperature and short isothermal time at sintering temperature [136,137]; however, the difficulty in manufacturing large mechanical parts by SPS is a clear shortcoming of the technique.
Some of present authors [138,139] prepared a nano-ODS steel by means of low-energy MA without the annealing stage at high temperature, obtaining a microstructure of fine equiaxed grains. As shown in Figure 9 taken from ref. [138], ODS steel exhibits higher YS and UTS than the unreinforced one, even if the difference progressively decreases above 400 °C because dislocations can easier get free from nano-precipitates. Mechanical properties are also better up to 500 °C than those of ODS steel prepared through the conventional route. The data of a conventional ODS steel (ODS*) reported in Figure 9 are taken from [140]; L and T indicate samples taken along longitudinal and transverse direction, respectively. Since precipitate distribution is not homogeneous in ODS steel prepared by low-energy MA, YS and UTS remarkably decrease when the strengthening role played by precipitates becomes dominant (above 500 °C).
Figure 9. YS (a) and UTS (b) of ODS steel prepared by low-energy MA are compared with those of the unreinforced steel (steel matrix) and of another material (ODS*), prepared through the conventional route (high-energy MA, hot extrusion at 1100 °C, annealing of 1.5 h at 1050 °C). L and T indicate samples taken along longitudinal and transverse direction, respectively. The figure is taken from [138].
6. Ni-Based Superalloys
Ni-based superalloys with a biphasic structure (γ + γ‘) are usually employed to manufacture parts of aeronautic engines such as blades and rotors operating in the highest temperature range (1100–1250 °C). Three topics of great industrial relevance will be discussed: (i) microstructural stability; (ii) manufacturing parts of complex geometry; (iii) welding of superalloys.
6.1. Microstructural Stability
In order to increase the working efficiency of aero-engines, they must operate at higher temperature, thus the high temperature properties of superalloys are very important, especially the microstructural and mechanical stability.
Recently, some authors have evidenced an early stage of microstructural instability in both single crystal (PWA1482) [141,142] and directionally solidified (IN792 DS) [143] Ni-based superalloys, connected to the re-arrangement of dislocation structures induced by heating to moderate temperature (~500 °C). Dislocation cells present in the precipitate free (PF) zones of the matrix (Figure 10a,b) grow to form cells of larger size; the process proceeds by steps modifying dislocation density and average distance of pinning points; finally the growth stops when cells reach a size comparable to that of the corresponding PF zone.
Figure 10. PWA 1483 superalloy. Precipitate free zones (PFZ) are indicated by red circles (a). The TEM micrograph in (b) displays a network of dislocations inside a PFZ. Figure is taken from [142].
In general, the coarsening of the ordered γ‘ phase and changes in its morphology (rafting) are the most relevant phenomena leading to the degradation of mechanical performances at high temperature. As shown in Figure 11a,b, at high temperature and under an applied stress, the γ’ particles, which usually have a cuboidal shape (a), tend to coalesce, forming layers known as rafts (b). At very high temperatures (above 1050 °C), rafting takes place during the initial part (1–3%) of the creep life, while at lower temperature (~900 °C) it only completely develops during the tertiary creep.
Figure 11. The typical morphology of the γ’ phase in Ni-based superalloys (a). Results changed by rafting (b).
At the beginning of creep, dislocations are forced to bow in the narrow matrix channels where all the plastic strain occurs, while γ’ phase deforms elastically [144]. The progressive increase in plastic deformation in the γ phase enhances internal stresses, leading to dislocation shearing of γ’ particles during the tertiary creep. Of course, γ’ particle coarsening involves the degradation of creep properties.
Refractory elements, such as Re, Ta, Ru and W, are today added to Ni-based superalloys to improve their high temperature properties [145,146,147,148]. These elements provide good creep strength because their low atomic mobility retards dislocation climb in both γ and γ’ phases. Re concentrates mostly in the γ matrix, forming nanometric atomic clusters with short-range order, which reduce rafting during creep and hinder dislocation movement. Moreover, Re promotes the precipitation of topologically close-packed phases (TCP) [149]. The partition of refractory metals between the γ and γ’ phases occurs and is dependent on their relative contents in the alloy composition. For instance, the amount of Re in the γ’ phase increases by increasing W content in the alloy.
In general, these alloys have more than seven alloying elements in their composition, and the addition of further elements may strongly alter segregation profiles in casting, thus solidification has been extensively investigated, focusing the attention on the partition of elements in solid and liquid during cooling [150,151,152]. Guan et al. [151] reported that liquidus and solidus decrease by increasing Cr in Re-containing alloys, and changes of these critical lines induced by Ru, were observed by Zheng et al. [153].
The addition of B and N to superalloys containing refractory metals affects solidification defects. For instance, N has been proven to increase the micro-porosity [154], while B retards grain boundary cracking and reduces the size of carbides with consequent improvement in mechanical properties [155].
Today, grain boundary engineering (GBE) represents an interesting field of research that could contribute to the reduction of inter-crystalline damage to superalloys and, in general, to the improvement of their mechanical properties [156]. Annealing twin boundaries are very important for GBE owing to their low energy. Recently, Jin et al. [157] reported an interesting result about the correlation of the annealing twin density in Inconel 718 with grain size and annealing temperature. These investigators showed that twin density mainly depends on the original one in the growing grains, but not on the temperature at which they grow, namely no new twin boundaries form during the grain growth process.
6.2. Manufacturing Parts of Complex Geometry
An aspect of relevant importance for these materials is the possibility of manufacturing aeronautic components of complex geometry. Owing to their high hardness and poor thermal conductivity, the machining of superalloys is challenging and novel techniques (e.g., see [158,159,160,161,162]) have been investigated. For example, laser drilling and electrical discharge machining are used to produce effusion cooling holes in turbines blades and nozzle guide vanes [158]. Even though the description of such novel techniques goes beyond the scope of the current paper, it is worth noting that recently there is an increasing interest of aeronautic industry in the use of Additive Manufacturing (AM) for the production of Ni-based high-temperature components. Among the different AM technologies selective laser melting (SLM) and selective electron beam melting (SEBM) are the most interesting as they enable the preparation of almost fully dense metal parts of complex shape, starting from a computer-aided design (CAD) model [163,164,165,166,167,168,169].
Components manufactured through SLM exhibit excellent mechanical properties and a strong anisotropy. The directional heat flow during the process leads to columnar grain growth with consequent crystalline texture, which especially affects creep resistance and fatigue life [170,171,172,173,174]. The (001) crystallographic direction has the lowest stiffness involving better creep resistance and longer fatigue lives, thus it is optimal for the upward direction in gas turbine blades.
Recent experiments by Popovich et al. [174] on Inconel 718 demonstrated that suitable SLM process parameters and laser sources allow material anisotropy to be controlled with great design freedom (either single component texture or random oriented grains, or a combination of both of them in a specific gradient). The same approach can be also applied to design functional gradients with selected properties and/or heterogeneous composition depending on the specific application.
SEBM is characterized by very high solidification rates and thermal gradients, leading to relevant microstructure refinement with primary dendrite arm spacings two orders of magnitude smaller than as-cast single crystals. Moreover, in samples of the CMSX-4 superalloy prepared with high cooling rates, Parsa et al. [175] observed a high dislocation density, indicating the presence of internal stresses which could lead to crack formation.
6.3. Welding of Superalloys
Cracks may form in Ni-based superalloys during both the production process and service life under severe conditions of high temperature and stress in an extremely aggressive environment. Such defects are generally repaired through welding [176], with significant economic saving.
Welding should preserve, as far as possible, the original microstructure without relevant residual stresses in the molten (MZ) and heat affected (HAZ) zones, and chemical segregation changing the composition of γ and γ’ phases. During the solidification the microstructure, the MZ is affected by dendritic growth and solute partitioning, with the consequent formation of metallic compounds such as carbides, borides etc. Another critical aspect is connected to the presence of low melting compounds which could lead to micro-cracks after post-welding heat treatments (PWHTs) [177] and local residual stresses in the MZ [178,179].
Some welding technologies are already mature, such as Transient Liquid Phase (TLP) bonding, developed by Pratt & Whitney Aircraft and based on the spread with Ni-Cr-B or Ni-Cr-B-Si fillers; Activated Diffusion Bonding (ADB) developed by General Electric with fillers of composition close to that of the reference superalloy and with the addition of B and/or B+Si; Brazing Diffusion Re-metalling (BDR) developed by SNEMECA with fillers with two components: one of a composition close to that of the alloy, and the other, in small quantities, containing elements such as B and Si which lower the melting point. The advantage of BDR is the slow isothermal solidification that makes the interdiffusion of the elements easier, and guarantees a composition of the joint similar to that of the bulk superalloy. Unfortunately, the costs of the above techniques are very high, particularly BDR.
In recent years, research has been focused on high energy density welding techniques such as Laser Welding (LW) [180,181,182] and Electron Beam Welding (EBW) [183,184,185,186], which provide greater penetration depth, reduced HAZ and minimal distortion, if carried out with a high speed of passes. These techniques seem to be promising, as they represent simpler and cheaper solutions for repairing cracks in Ni-based superalloys. Thanks to a reduced thermal input, high energy density welding techniques, can realize joints with narrower seams and HAZ. By using LW and EBW techniques, the superalloy microstructure is changed at little extent, so that residual stresses, micro-cracks, porosity and other defects in the junction are limited. In addition, for each welded superalloy, the optimization of the process parameters, such as pass speed and pre-heating of the workpiece, clearly plays a crucial role (e.g., see ref. [186,187,188]).
7. Conclusions
The work provides an overview of recent advances in alloys for aeronautic applications, describing current problems and perspectives.
The needs of the aeronautic industry stem from the strong competition to manufacture aircrafts with improved technical features and reduced costs (i.e., extended service life, better fuel efficiency, increased payload). In this challenge, materials play a crucial role. Advanced structural materials should guarantee reduced weight, improved fatigue and wear behaviour, damage tolerance and corrosion resistance, while for the hot engine sections alloys with better creep resistance, mechanical properties at high temperature and high-temperature corrosion resistance are required.
A critical analysis has been carried out on different kinds of materials including Al alloys, Ti alloys, Mg alloys, steels, Ni superalloys and metal matrix composites (MMC), emphasizing the structure–property relationships.
The development of new materials involves new technologies, and some of great relevance for the aeronautic sector have been briefly examined. The attention has been focused on Refill Friction Stir Spot Welding (RFSSW) for joining structural parts made of Al alloys, high energy density techniques for welding Ni superalloys, and Additive Manufacturing (AM) for fabricating components of complex geometry.
In the future, the microstructural and mechanical stability of Ni superalloys will be further investigated and improved through careful tailoring of the composition to get higher operative temperatures of aero-engines. Important advancements in structural materials are also expected. The competition with Carbon Fiber Reinforced Polymers (CFRP) will drive new efforts to improve the mechanical properties, especially fatigue strength and toughness, and corrosion resistance of Al alloys. In the case of Ti alloys, the focus will be on the high-temperature resistance to pursue through the control of phases and thermo-mechanical processing. The availability of new types of reinforcing particles and fibers will be exploited to enhance the properties of MMC. Finally, the development of strengthening methods for producing very strong Mg alloys opens new horizons for the aeronautic applications of these materials which are excellent for weight reduction.
Author Contributions
All the authors contributed to examining the literature and writing the manuscript.
Funding
This research received no external funding.
Conflicts of Interest
The authors declare no conflict of interest.
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30 | https://ns.afahc.ro/ro/afases/2013/eng_mech/Tanasa_Zanoaga.pdf | “HENRI COANDA” “GENERAL M.R. STEFANIK” AIR FORCE ACADEMY ARMED FORCES ACADEMY ROMANIA SLOVAK REPUBLIC
INTERNATIONAL CONFERENCE of SCIENTIFIC PAPER
AFASES 2013
Brasov, 23-25 May 2013
FIBER-REINFORCED POLYMER COMPOSITES AS STRUCTURAL MATERIALS FOR AERONAUTICS
Fulga TANASA, Madalina ZANOAGA
”Petru Poni” Institute of Macromolecular Chemitry, Iasi, Romania
Abstract: Composite materials based on fiber-reinforced polymers are becoming preferred materials for aircraft and spacecraft construction. Their use as structural materials in recent years has proved their advantages. This paper offers an overview of several applications in aeronautics, but it focuses on composites application as structural materials, due to the fact that they have recorded a significantly increased use. The nature of composite materials and their behavior under specific stress, special problems in design and preparation, as well as issues connected with their impact damage and damage tolerance, environmental degradation and long-term stability, are presented in this report.
Keywords: composites, structural materials, aeronautics
1. INTRODUCTION
Aeronautics engineering is changing [1,2,3]. Planes have traditionally been made out of metals – usually, aluminum and its alloys, steel, and titanium alloys. Nowadays, engineers are increasingly working with carbon fiber composites. Fiber-reinforced composite materials were originally used in small amounts in military aircrafts during the 1960s, and within civil aviation from the 1970s. By the 1980s, polymer composites were being used by civil aircraft manufacturers for a variety of secondary wing and tail components (such as rudder and wing trailing edge panels).
The latest generation of airliners, such as the Airbus A380, the world’s largest passenger aircraft, shows that these composite materials have been employed extensively in the primary load carrying structure: A380 uses composite materials in its wings, which helps enable a 17% lower fuel use per passenger than other comparable aircraft [2], whereas the
newest Boeing, the 787 Dreamliner, has the highest content of composites (Figure 1) 50% (http://www.newairplane.com/787).
Figure 1. Use of composites in the structure of Boeing 787 Dreamliner (www.boeing.com)
Fiber-reinforced polymer composites can provide a much better strength-to-weight ratio than metals, sometimes by 20%. The lower weight results in lower fuel consumption and emissions, enhanced aerodynamic efficiency, lower manufacturing costs. The aviation industry was the first interested in such benefits and it was the manufacturers of military aircrafts who initially seized the opportunity to use composites characteristics
to improve the speed and maneuverability of their products.
In the last four decades, the aerospace sector underwent a series of changes in terms of composites implementation in aircraft and helicopter production. Fiberglass composites were the first to be used by the aerospace industry, followed by two other composites, i.e. carbon fiber and aramid fiber, added in the early 1970s. The main applications of composite materials for helicopter and aircraft interior design include the fabrication of instrument panels, fuselage skin panels, and fuselage fairing panels. Some of the main advantages of using composite materials are the relatively low fabrication and installation costs, as well as lower toxicity and increased resistance to fire.
One great innovation in the field of composite materials for aeronautics is the ability to produce complex parts in one piece, particularly through thermoforming, which will enable reduced costs related to machining and to component assembly. Research provided a reliable database for the development of composites, so that enabled parts to be designed with unique physical and chemical properties for specific use and to meet the specific needs of the industry. Besides offering the opportunity to design durable and resistant parts, fiber-reinforced composites have the advantage of providing excellent resistance to corrosion.
2. COMPOSITES AS STRUCTURAL MATERIALS IN AERONAUTICS
2.1 Aeronautics features. Whether it is a single engine private plane, a giant commercial airliner, or a supersonic fighter plane, aircrafts are the work of engineers. Specific structures in aeronautics have to meet characteristic requirements, such as safety standards (special demands of fire-retardancy [4] and crashworthiness [5-7]), fuel sealing, easy access for equipments maintenance; vacuum, radiation and thermal cycling has to be considered and special materials are required to be developed for durability.
Two major directions of research in this field had a significant influence on the
development of new generations of materials and, hence, aircrafts: advances in the computational sciences, generating powerful computational tools, as well as CAD modelling and computer interfaces in manufacturing, and the progress of the composites technology using fibre-reinforced polymeric materials as structural materials for aeronautics. Some requirements of an aircraft structure are presented in Table 1, as well as design demands arising from them [8].
Table 1. Aircraft requirements and subsequent design demands
Requirement
Design demands
Obs.
Low weight
Semi-monocoque
construction
Thin-walled-box or
stiffened structures
Use of low density
materials: wood, Al-alloys, composites
High strength/weight ratio, high stiffness/ weight ratio
Application area: all
aerospace
programs
High
reliability
Strict quality control
Extensive testing for
reliable data
Certification: proof of
design
Application area: all
aerospace
programs
Passenger
safety
Use of fire retardant
materials and coatings
Extensive testing:
crashworthiness
Application area:
passengers transport
Aerodynamic performance
Highly complex loading Thin flexible wings and control surfaces
Deformed shape: aero
elasticity, dynamics
Complex contoured shapes: processability, machining, moulding
Application area: all
aerospace
programs
Stealth
Specific surface and shape of aircraft
Stealth coatings
Application in military programs
All-weather
operation
Lightning protection,
erosion/corrosion resistance Corrosion prevention
schemes
Issues of damage and safe life, life extension
Extensive testing for
required environment
Thin materials with high integrity
Application area: all
aerospace
programs
2.2 Specific requirements for polymer composites used in aeronautics. The use of advanced fiber-reinforced polymer composites in aeronautics has been conditional upon their properties given by both fillers (carbon or aramid fibers) and matrices [9]. Their combined characteristics granted them lightweight, due to high specific strength and stiffness, fatigue and corrosion resistance, availability towards optimization (e. g., tailoring the directional strength and stiffness), enhanced processability (ability to mould large complex shapes in short time cycles, reducing part count and assembly times), time and place stability, low dielectric loss, achievable low radar profile and stealth availability, etc.
Still, despite all these advantages, these composites have a few flaws: some of the corresponding laminates display weak interfaces adhesion, yielding in poor resistance to out-of-plane tensile loads; susceptibility to impact-damage and strong possibility of some internal damages evolving unnoticed; moisture absorption and consequent degradation; occurrence of possible manufacturing defects.
Nowadays, the use of advanced composite materials has been extended to a large number of aircraft components, both structural and non-structural, based on various factors. Some details from civilian and military aviation [10] are presented in Table 2 and 3.
A realistic approach indicates that estimated benefits, especially when it comes to the new generations of composites [11], are significant and almost all aerospace programs use increasing amounts of composites. Hence, it is necessary to take into consideration the complex behavior of these materials, since they are anisotropic and inhomogeneous, have different fabrication and processing requirements, and need different control methods, new and complex analysis protocols for quality assurance. Moreover, their behavior is not always predictable which makes reliance on several expensive and time consuming tests mandatory.
Table 2. Use of composites in
Airbus and Boeing series
Aircraft
type
Parts and components
(%)
Airbus
Radome, fin leading edge and tip, fin
5
A300B2/
B4
trailing edge panels, cabin and cargo hold furnishings.
Fairing -pylon, wing/ fuselage rear.
Airbus
A310-300
Rudder, elevator, vertical stabilizer, spoilers, cowl (inlet & fan), thrust reverser, main & nose landing gear door of wing leading & trailing edge panels, nacelles.
Fairings -Ion, flap track, win fuselage.
7
Airbus
A320/A31 9/A321
Aileron, horizontal and vertical stabilizer, elevator, rudder, spoilers, flaps, engine cowl, radome, landing gear doors (main & nose), floor panels, wing panels (leading & trailing edge), other access panels, nacelles.
Fairings-flap track, wing/fuselage (forward & rear), main landing gear leg.
15
Airbus
A330
Ailerons, rudder, flaps, spoilers, elevator, horizontal and vertical stabilizer, wing panels (leading & trailing edge), landing gear doors (main & nose), nacelles. Fairings -flap track, wing/fuselage (forward & rear).
12
Airbus
A340
Ailerons, rudder, flaps, spoilers, elevator, horizontal and vertical stabilizer, wing panels (leading & trailing edge), landing gear doors (main & nose), nacelles. Fairings -flap track, wing/fuselage (forward & rear).
12
Boeing
737 (200, 300, 400)
Spoilers and horizontal stabilizer (both limited production), trailing edge flaps. Aileron, elevator, rudder, nacelles. Aileron, elevator, rudder, nacelles.
<1
3
Boeing
747-400
CFRP winglets and main deck floor panels. CFRP and AFRP used in cabin fittings engine nacelles.
3
Boeing
757
Aileron, elevator, rudder, spoilers, flaps (in-board & outboard), fairings and nacelles.
3
Boeing
767
Ailerons, elevator, rudder, spoilers, landing gear doors (nose & main), fairings and nacelles.
3
Boeing
777
Ailerons, elevator, rudder, spoilers, flaps (in-board & outboard), floor beams, landing gear doors (nose & main), fairings and nacelles.
10
These challenges can be met by using the advances in computer technology and analysis methods to implement schemes based on computer aided design, computer aided engineering, finite element methods of analysis.
Table 3. Use of composites in military
airplanes and helicopters
Aircraft type
Parts and components
F-14
Doors, horizontal tail and fairings
F-15
Rudder, vertical tail, horizontal tail, speed brake
F-16
Vertical tail, horizontal tail
F-18
Doors, vertical and horizontal tail, fairings, wing box, speed brake
B-1
Doors, vertical/horizontal tail, flaps, slats
AV-8B
Doors, rudder, vertical tail, horizontal tail, aileron, flaps, wing box, body and fairings
Typhoon
Wing, fin, rudder, in-board aileron, fuselage
LCA
Wing, fin, rudder, control surfaces, radome
MBB BK
117
Main rotor blades, tail rotor blades, horizontal stabilizer, vertical stabilizer
Bell 206L
Vertical stabilizer
Bell 402
Main rotor blades
Dauphin
Main rotor blades, vertical stabilizer
McDonnell
Douglas
MD 520N
Main rotor blades, tail boom
McDonnell
Douglas
MD 900
Main rotor blades, fuselage mid section, tail boom, canopy frame, internal fuselage, horizontal stabilizer, vertical stabilizer
ALH
Main & tail rotor blades, rotor hub, nose cone, crew & passenger doors, cowling, most of the tail unit, lower rear tail boom, cock it section
Thus, the entire process is computer assisted, from design and analysis up to manufacturing, enabling the fast transfer of information and accurate analysis methods for a reasonable prediction of composites complex behavioral patterns [8].
2.3 Reliability and safety issues. Fiber reinforced polymer composites used in aeronautics have to meet reliability and safety issues, which requires testing at all stages (design and development, proving and certification, in-service inspection and repairs), due to the composites complex behavior and difficulty in creating predicting models. Additional operations are reflected in increased final costs.
Safety issues - risk-based approaches and tools have been developed by the aeronautic communities, especially by the military, to ensure aircrafts availability and to reduce costs while maintaining structural safety [12-15].
Impact damage and damage tolerance – some composites laminates made of fiber reinforced polymers are characterized by weak interfacial interactions (due mostly to a certain incompatibility between matrix and filler) and this favors phenomena as delamination or debonding under stress [16].
Figure 2. In many types of composite structures (e.g. aircraft, marine, etc.), delaminations are the most common form of defect/damage
Even more, when discontinuous plies (made to create thickness changes) or sharp bends (required in stiffening pieces) are used for structural features, these phenomena are more intense, especially when it comes to damages at impact because they might not be evident in initial stages, but worsen under prolonged stress. This behavior occurs in case of impact with blunt objects at low to medium velocity (accidental dropping, hail, debris, shocks even before the aircraft assembly, or even a bullet impact which, in the case of a fuel tank, will cause a hydraulic ram effect in the fuel, leading to explosion). These flaws may occur not only in 2-D, but they can propagate through the entire thickness, mainly when micro-cracks emerge in back plies or other hidden stress concentrators, detectable by ultrasonic C-scan method. It is possible to limit effects of these damages by combining various approaches: (1) design (structures with alternate load paths) [17], (2) setting lower allowable stress values and (3) defining new inspection intervals and protocols. Damage tolerant structures are designed to sustain cracks before failure occurs, so that the defect is detected in and the damaged part is repaired or replaced (Figure 3). In addition, damage tolerance takes into account initial material or manufacturing flaws by assuming an initial crack, which the fail-safe principle does not do [18].
Humidity is causing weight gain in most fiber-reinforced composites, no matter whether the matrix is a thermoset or thermoplastic polymer. Under the normal operating conditions, the maximum equilibrium moisture gain in an aircraft component can be 1.0-1.4%.
Figure 3. Theoretical damage tolerance inspection regime to detect cracks before they become critical
This may cause swelling and dimensional changes, lowering the glass transition temperature (Tg) of the matrix, as well as a decrease of shear and compression strength.
The diminution of the shear and compressive strength is a major concern in aircraft structures, mainly at high temperatures close to Tg, because polymers Tg is decreasing due to the moisture sorption. Therefore, the design of a structural component proceeds, generally, by reducing allowables for moisture degradation. As a general observation, the dimensional changes and weight gain are not significant in many aircraft structures, but they may be of considerable significance where extreme precision is required, such as in antennae panels and in satellite structures.
Significant issues relate to the UV degradation and radiation effects in long term exploitation, especially for spacecrafts structures. Current studies have provided some solutions.
3. FIBER-REINFORCES POLYMER COMPOSITES USED IN AEROSPACE INDUSTRY
3.1 Background. Advanced research enables scientists and engineers a better understanding of how to use fiber-reinforced polymer composites as structural materials for aerospace industry, but studies also encompass the interactions of the structure with the aircraft system as a whole. Aero-elastic tailoring is one example of such interaction. On another hand, by developing standard tools to test the potential performance of composites, there are possibilities to increase the use of composites in other leading industries.
3.2 Fibers as reinforcement. Carbon fiber reinforced plastics (CFRPs) – used for the first time during the 1960s - owe their high structural performance to the exceptional properties (low density, high thermal conductivity and excellent mechanical properties at elevated temperatures) of the individual strands. By way of comparison, the ultimate strength of aerospace grade aluminum alloys is 450MPa, whilst that of a carbon fiber would be five times higher. Glass, aramid and boron (far superior to carbon fibers, but 6 times more expensive) fibers are also used, but it seems carbon fibers have the best strength/cost ratio for primary load-bearing structure. The carbon fibers technology continues to improve by valorizing the versatility of carbon fibers and new varieties with improved modulus and strength are available. A comparison between fibers used as reinforcement in aeronautics is presented in Table 4 and a synthetic review of aramid fibers commercially available is shown in Table 5.
Two directions of development seem to be concerned:
(1) aircraft applications - higher strength (>5 GPa) concurrent with improvements in modulus to moderate levels (>300 GPa) and
(2) space applications - high modulus (>500GPa) along with moderate strength (~3.5 GPa). The development in aramid fibers is also aiming at higher modulus concurrent with increased strength.
Table 4. Reinforcing fibers used in aerospace industry
Fibers
Density
(g/cm3)
Modulus
(GPa)
Strength
(GPa)
Application
Glass
E-glass
2.55
65-75
2.2-2.6
Small passenger aircraft parts, radomes, rocket motor casings
S-glass
2.47
85-95
4.4-4.8
Highly loaded parts
Aramid
(modulus)
Low
1.44
80-85
2.7-2.8
Fairings; unloaded bearing parts
Interme
diate
1.44
120-128
2.7-2.8
Radomes, some structural parts; rocket motor casings
High
1.48
160-170
2.3-2.4
Highly loaded parts
Carbon
(modulus)
Standard
1.77-1.80
220-240
3.0-3.5
Widely used for almost all types of parts, satellites, antenna dishes, missiles, etc.
Interme
diate
1.77-1.81
270-300
5.4-5.7
Primary structural parts in high performance fighters
High
1.77-1.80
390-450
2.8-3.0
4.0-4.5
Space structures, control surfaces
Ultra-high
1.80-1.82
290-310
7.0-7.5
Primary structural parts in high performance fighters, spacecrafts
Boron
3-; 4-; 5.6-
mil Boron
2.38-2.54
380-400
3.6-4.0
Structural reinforcement; thermal and radiative deflectors
Table 5. Aramid fibers used in aerospace industry
Name
Structure
Applications
VECTRAN
Advanced composite materials used by NASA's Extravehicular Mobility Unit and for all of the airbag landings on Mars: Mars Pathfinder in 1997 and on the twin Mars Exploration Rovers Spirit and Opportunity missions in 2004, as well as for NASA's 2011 Mars Science Laboratory in the bridle cables.
TWARON
Reinforcement in composite parts such as fairings and airfreight containers, containment belts used in turbine engines to protect the passenger compartment in case of engine failure.
ZYLON
Zylon is used by NASA in long-duration, high altitude data collection. Braided Zylon strands maintain the structure of polyethylene superpressure balloons.
TECHNORA
Suspension cords for the strongest and largest supersonic parachute used by NASA for Curiosity Rover.
However, the major improvement for composite reinforcements is the multidirectional weaving. Several processes (weaving, knitting, braiding) have been developed for this purpose and prepregs with multidirectionally woven fibers have been obtained. Significant advances based on the translation of high fibers properties into high performance composites are envisaged, but costs reduction and environmental protection are also aimed.
3.3 Polymer matrices. A remarkable effort in improving composites is focused on improving matrix polymers. The two major concerns mentioned earlier, impact damage tolerance and hydrothermal degradation,
provide the main motivation. A major direction of improvement appears to be in the toughness which should result in higher resistance to delamination and impact. High failure strain of matrix polymer would help in translating the higher performance of the improved fiber to the entire composite. Higher shear modulus polymers will achieve better transfer of load from fiber to matrix and again to fiber, therefore improving the compression strength. It is possible for polymeric materials to achieve moduli of approx. 5 GPa, since the current matrices have shear modulus values of about 2 GPa. As far as hygrothermal degradation is considered, newer systems based on cyanate esters look very promising and some of these have already found application.
Another route being investigated is the use of thermoplastic polymers [26-31] and their blends. Poly-ether-ether-ketone (PEEK) has been considered very promising, but the industry needs to resolve the problems associated with high temperature (> 350°C) processing of the material. Other promising new matrices are temperature-resistant polymers, such as polypropylene (PP) [28,29], polyphenylene sulphide (PPS) [30], polymethacrylimide (PMI) [30], polyvinyl chloride [30] and their derivatives and blends.
A polymer with excellent properties is PrimoSpire® SRP (self reinforced polypheny lene) by Solvay [32]: tensile properties that are comparable to those of many reinforced plastics (Figure 4), lighter weight and no loss of ductility, high compressive strength – one of the highest among plastics.
Figure 4. Tensile properties of PrimoSpire® SRP
Due to these characteristics, it is an excellent candidate for weight-sensitive applications that also require superior mechanical performance.
Current approaches appear to be directed towards producing polymeric systems which can be processed in conventional ways. Two promising classes of such materials are under development:
(1) polymerizable liquid crystalline monomers that should result in thermoset
resins having high fracture toughness and Tg=170°C, and with high degree of retention (=90%) under hot-wet conditions; compared to thermoplastic PEEK, such matrix will have almost similar fracture toughness along with the advantage of conventional processing. The approach for the development of these new polymers is to synthesize, first, controlled molecular weight backbones consisting of aromatic ethers, esters or rigid aliculic systems [8] with hydroxyl end groups and then to end cap them with reactive end groups like cyanate ester group or glycidyl ethers.
(2) phthalonitrile resins for high temperature applications which can be cured in conventional manner (at 180-200°C), but can be also post-cured, albeit in inert atmosphere, at high temperatures up to 600°C. Compared with the current resins synthesized by polymerization of monomer reactants for high temperature (250-350°C) applications, the new resins will have better processability, good fracture resistance, better strength and modulus, and very low moisture sorption.
The other area of advances in matrices research is the of low-loss polymers, especially for radomes which use high performance radars. Different low-loss polyesters and cyanate esters are under study.
4. FUTURE DEVELOPMENT
4.1 Nanoparticles. Fiber-reinforced polymer composites for aeronautics may be further developed by the use nanofillers such as electrospun carbon nanotubes and nanofibers, electrospun silica nanofibres. Adding small amounts of carbon nanotubes (CNTs) (0.15%) to a tetraethyl orthosilicate matrix, obtained via a sol-gel process, will yield in composites with CNTs binding
directly the adjacent layers. The electrically charged nanoparticles would bind directly to adjacent plies, each given an electrical charge in advance, to allow binding of the oppositely charged nanoparticles. This would create a “velcro” effect which will reduce reliance on the binding properties of the matrix, producing composites with enhanced strength, more impact resistant and lighter than those known today.
Great opportunities for carbon nanofibre patches are envisaged for advanced repair of composite structures. Such patches would increase the contact surface available for bonding and reduce the need to delaminate additional areas in order to repair a delaminated zone of the structure.
4.2 Ceramic matrix composites and metal matrix composites are two other groups of composites able to respond to aerospace industry demands. Ceramic matrix composites (CMCs) made of silicon carbide fibers in a silicon carbide matrix are of great interest for low-pressure turbine blades, pre
combustion mixer of engines and, potentially, the high-pressure core of the engine. In he case of metal matrix composites (MMCs), the research is still in the beginning, but it is already predicted that these materials, which will use carbon or metal nanotubes to strengthen metal matrices, will have twice the strength of comparable existing metal structures, but only 2/3 of their weight. Such materials would be ideal for engine tie rod struts which transfer the engine thrust into the airframe.
Other directions for the future development of the domain: study of properties of composites with basalt or clay particles, electrospun silica nanotubes, etc.
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31 | https://www.sciencedirect.com/science/article/pii/S1000936123002339 | Chinese Journal of Aeronautics
Volume 36, Issue 10, October 2023, Pages 1-23
Chinese Journal of Aeronautics
Research progress on construction strategy and technical evaluation of aircraft icing accretion protection system
Author links open overlay panelQiang HE a b c, Kangshuai LI a, Zehua XU a, Jiwen WANG a, Xiaosen WANG a, Anling LI a
https://doi.org/10.1016/j.cja.2023.07.003
Abstract
The impact of unstable supercooled water droplets suspended in the cloud on the solid will cause its surface to freeze, and the flight safety of the aircraft will be seriously affected when flying in this environment. Aircraft icing protection system is an important device to reduce icing accidents and improve aircraft safety performance, which is of great significance to ensure flight safety. Based on the energy source, this paper proposes a general strategy for constructing an aircraft icing protection system, including Active Anti-icing and De-icing (AAD) system, Passive Anti-icing and De-icing (PAD) system and Composite Anti-icing and De-icing (CAD) system. The principle, scope of application, advantages and disadvantages of aircraft anti-icing and de-icing technologies such as electric pulse de-icing, low-frequency piezoelectric de-icing, and hydrophobic material anti-icing are explored in detail, and the corresponding improvement measures are proposed. The future development of aircraft anti-icing and de-icing technology is prospected, and some new ideas are provided for the improvement of aircraft anti-icing and de-icing technology.
When an aircraft encounters a cloud layer containing supercooled water droplets with a temperature of 0 °C to −20 °C during flight, the water droplets will accumulate on the body in the form of ice, thereby affecting the aerodynamic characteristics of the aircraft and jeopardizing flight safety.1 Ice accretion usually occurs on the exposed front surface of the aircraft, and the parts seriously affected by icing include wings, 2, 3 rotors,4, 5 engines,6, 7 etc. According to the analysis of the causes of aircraft accidents in recent years, aircraft icing is the main external cause of aircraft accidents, and the resulting flight accidents have attracted more and more attention, especially the launch of some major aircraft projects, making the research related to aircraft icing a hot spot.8, 9, 10 At present, the research on aircraft icing mainly focuses on icing on the wings, because it can have serious adverse consequences. When the wing freezes, the change in the airfoil shape can lead to a decrease in lift and an increase in drag, which may cause flight accidents. 11, 12 Therefore, the research of aircraft ice accretion protection system is a problem that must be considered in aircraft research and development.
With the rapid development of modern aircraft technology, in order to ensure the flight safety of the aircraft in the cloud layer containing supercooled water droplets, researchers have used numerical model simulation,13, 14 icing wind tunnel experimental verification15, 16 and other technologies to develop some effective ice protection systems. According to their different construction strategies, ice protection systems are divided into Active Anti-icing and De-icing (AAD) system, Passive Anti-icing and De-icing (PAD) system and Composite Anti-icing and De-icing (CAD) system. Current technologies for aircraft anti-icing and de-icing systems include electrical pulse de-icing,17 low-frequency piezoelectric de-icing,18 pneumatic de-icing,19 ultrasonic de-icing,20, 21 hot air anti-icing and de-icing,22 electro-thermal anti-icing and de-icing,23, 24 anti-icing of hydrophobic material,25 plasma anti-icing,26, 27 synthetic jet anti-icing,28 memory alloy de-icing,29 electro-thermal hydrophobic material anti-icing and de-icing,30 etc. With the deepening of the research on the existing aircraft icing protection system, the defects of its complex testing equipment and low efficiency are gradually revealed. Therefore, it is particularly important to develop a simple, high-efficiency, green, low-energy-consumption, multi-functional and stable aircraft anti-icing and de-icing system.
Based on the existing aircraft icing protection system, this paper expounds the overall strategy of constructing the aircraft icing protection system in detail, focuses on the research progress of the current aircraft commonly used anti-icing and de-icing technology, summarizes the advantages and disadvantages of the existing aircraft anti-icing and de-icing technology research, and prospects the future development direction of aircraft anti-icing and de-icing technology. The purpose is to let relevant scholars have a comprehensive and rapid understanding of aircraft anti-icing and de-icing technology, and provide ideas and methods for the development of new aircraft anti-icing and de-icing systems.
2. Overall strategy for building an aircraft icing protection system
When the aircraft is flying under the cloud layer of unstable supercooled water droplets, water droplets accumulate on the surface of the aircraft in the form of ice. Table 1 shows the common aircraft types of ice accretion and its meteorological causes and main locations.31 To prevent some parts of the aircraft from freezing, it is necessary to adopt an appropriate aircraft icing protection system to ensure the flight safety of the aircraft. The existing aircraft icing protection systems can be divided into anti-icing systems and de-icing systems according to whether there is an ice layer on the surface of the aircraft. The anti-icing system refers to the use of heating or hydrophobic coating to prevent aircraft surface parts from icing; the de-icing system allows a small amount of ice to form on the aircraft surface and then remove it before the icing has a significant adverse effect.
Table 1. Several common types of icing on aircraft, their meteorological causes and main locations. 31
Category Characteristic Occurrence site
Soft rime Large particle/Rough surface/Low adhesion Low speed flight/The surface of a parked aircraft
Frost Rough surface/Opacification/Low density and strength High thermal radiation surfaces for parking aircraft
Rime White opacity/Rough patches/Low density and strength The windward surface on which an aircraft flies
Clear ice Smooth/transparent/Close texture/Slow freezing Wing/Wing leading edge
Mixed ice Rough surface/Out-of-shape/Solid The windward surface on which an aircraft flies
Rime ice Thin depth/Caducous Cockpit windshield/Wing
Due to the precision of the aircraft structure, the design of the aircraft icing protection system should meet the anti-icing and de-icing requirements of the entire aircraft. When designing an aircraft icing protection system, some typical status points are usually selected, and the load and energy consumption of the aircraft anti-icing and de-icing system are determined according to these typical status points. The state point of the anti-icing system includes the flight status of the aircraft and the icing meteorological conditions. By selecting the typical flight state of the aircraft, and according to the meteorological design specification of anti-icing and de-icing system, the icy meteorological conditions of aircraft are determined. Finally, the minimum energy required and the overall design of the anti-de-icing system are further determined according to the limiting conditions.32 Meier and Scholz33 took Boeing 787 as an example to estimate the power of Boeing 787 by deducing the comprehensive technical parameters of the electric de-icing system, which greatly simplifies the calculation process of the load of the anti-icing and de-icing system. Hann et al.34 carried out anti-icing and de-icing experiments on the electrothermal anti-icing and de-icing system in the icing wind tunnel, and determined the most energy-saving anti-de-icing method by studying the parameters of melting time. In addition, the severity of icing on different parts of the aircraft surface is different, and the degree of influence on the aerodynamic performance and stability of the aircraft is also different. A reasonable design of the protection area can greatly reduce the energy consumption of the system and increase the load rate of the engine. Therefore, it is very important to develop a multi-functional, stable and fast-response ice accretion protection system. This paper summarizes in detail three existing general strategies for constructing aircraft icing protection systems: AAD system, PAD system, and CAD system.
2.1. AAD system
AAD systems use energy from external systems for the purpose of keeping the aircraft safe. Thermal methods are the most widely used method in AAD systems, which use heat to raise surface temperatures to prevent icing and to melt the ice. The heat can come from hot air from the engine 35 or electric heaters embedded under the surface of the wing. 36 The method ensures the flight safety of the aircraft in icing conditions, but energy consumption has been the main topic of discussion. Due to the relatively high cost of thermal energy methods and the limitation of experimental conditions, researchers usually choose numerical simulation studies.35Wright 37 calculated a representative value of the heat transfer coefficient and applied it to a piccolo tube system with temperature data. The experimental results showed that the method overestimated the surface temperature, resulting in a different shape of the ice remnant than the experimental shape. Zhou et al.38 proposed a method for predicting the surface temperature and backflow ice of a 3D thermal gas anti-icing and de-icing system. Based on this method, it was concluded that the influence of liquid water content and Mach number is much larger than that of the external flow temperature.
At present, the substandard experimental parameters of different numerical simulation studies lead to large errors in the experimental simulation results. Therefore, it is necessary to improve the numerical simulation software of aircraft icing and to divide the experimental conditions more precisely to achieve the accuracy of system performance prediction. Papadakis and Wang 39 used the Navier-Stokes equation to calculate and study the effects of diffuser geometry, hot air temperature and mass flow on system performance. In addition, the team developed and designed an optimization simulation tool for the hot air anti-icing and de-icing protection system, and based on this, the influence of hot air mass flow and hot air temperature on the wing skin temperature was studied.40 Pourbagian and Habashi41 first studied the effects of parameters such as surface temperature, ambient temperature, airspeed and angle of attack on the energy consumption of aircraft anti-icing and de-icing systems. In addition, the team conducted a parameter sensitivity analysis to conduct a parametric study of the energy consumption of the aircraft anti-icing and de-icing system. Zhang et al. 42 studied the influence of structural parameter uncertainty on the thermal efficiency of the anti-icing cavity of aircraft wings, and found that the height of the double-layer channel and the diameter of the jet hole are the core factors affecting the functional reliability of the anti-icing cavity. In addition to optimizing the icing protection system on the wing, researchers also optimize the icing protection system on the engine by improving techniques such as numerical simulation software.43, 44, 45, 46
Mechanical force is one of the common methods used in AAD systems, where the ice is broken up and removed by applying mechanical force on the accumulated ice. One of the representative mechanical force anti-icing and de-icing methods is pneumatic de-icing, which works by setting many expandable rubber tubes under the surface of the leading edge of the wing. When the surface of the aircraft freezes, the rubber tubes inflate and expand to break the ice, and then use the air flow to blow the ice away. Helicopters use pneumatic de-icing due to their poor load-bearing capacity and limited power sources. 47 Compared with the thermal energy method, the system has the advantages of light weight, high practicability and low cost. In addition, since pneumatic de-icing can only remove the ice accretion after the ice accretion thickness is greater than the threshold, it has the disadvantage of low efficiency and cannot be removed when the ice accretion is thin. The electromechanical pulse de-icing technology uses the electric pulse generated by the internal coil of the wing to excite the high-frequency oscillation of the wing skin to remove the ice, which solves the problem of thin ice layer during the flight of the aircraft. Möhle et al.48 used finite element simulation to analyze the magnetic field and structure coupling of the electric pulse de-icing process. Based on this method, the de-icing process of the electric pulse system could be simulated accurately, and the de-icing conditions under different icing thickness could be predicted accurately. With the deepening of the research, the researchers' research on the electric pulse system is gradually refined, and the factors considered are more comprehensive. Many researchers 49, 50 have studied the coil problem and analyzed in detail the influence of the number of coils, diameter of coils, arrangement mode and start-up time on the de-icing device, providing a basic idea for the future research and design of electric pulse system.
In addition to thermal energy method and mechanical force method, chemical method is also one of the important technologies of AAD system. Chemical methods reduce ice adhesion by exploiting the chemical properties of substances to avoid ice on aircraft surfaces. Corsi et al.51 tested the biochemical oxygen demand and chemical oxygen demand of icing inhibitors, and tested the degradation rate of different de-icing formulations in seawater. While chemical methods have been used in anti-icing and de-icing technology, these chemicals often have corrosive properties that can destroy the aerodynamic properties of aircraft surfaces and even cause irreversible damage to the environment. Therefore, the search for a green and pollution-free chemical substance is a field worthy of attention52.Since benzotriazoles have the properties of inhibiting metal corrosion, researchers have focused on benzotriazoles in the study of the environmental impact of icing inhibitors.53 Wolschke et al.54 studied the distribution of benzotriazoles and other organic compounds in different estuaries in Central Europe and the North Sea, and tested their pollution characteristics to rivers.
The AAD system uses thermal energy method and mechanical force method to remove the ice layer in time after finding the icing. It has quick response and can eliminate the icing problem in different degrees to ensure flight safety. However, under certain conditions, the melted ice layer will cause secondary icing and lead to the formation of nodular ice, which has a great impact on the aerodynamic performance of the aircraft.
2.2. PAD system
PAD systems reduce the ice adhesion on the aircraft surface by changing the hydrophobicity of the aircraft surface, or use the properties of memory alloys to deform the ice layer to achieve the purpose of anti-icing/de-icing. Li et al. 55 studied the self-cleaning and delayed icing properties of the superhydrophobic composite membrane, and the experimental results showed that the composite membrane had excellent delayed icing performance. Although the development of hydrophobic materials has been studied for decades, the study of the durability of hydrophobic coating surfaces is particularly important due to the instability of the surface microstructure of hydrophobic materials. 56, 57, 58, 59 Xue et al. 60 placed the hydrophobic sample flat on the sandpaper, and then used a weight of 100 g to conduct the sandpaper abrasion resistance test on its surface. The test process and results are shown in Fig. 1(a). In addition, durability experiments such as ultraviolet irradiation, chemical etching and artificial friction were also carried out on the hydrophobic samples. The experimental results show that the hydrophobic material has a strong ability to resist harsh environments. Zhuang et al. 61 conducted water droplet impact experiments on hydrophobic composites, and used contact angle combined with SEM to analyze the changes in surface hydrophobicity after impact (Fig. 1(b)). In addition, atmospheric durability tests such as abrasion resistance test, tape peeling test and ultraviolet resistance test were also carried out, simulating the real environment of the aircraft flying in the air. The experimental results show that the hydrophobic composite exhibits excellent stability. Zhu et al. 62 fabricated a transparent superhydrophobic coating with mechanochemical stability and reversible wettability, verified the mechanical stability of the coating with adhesive tape, and tested its hydrophobicity in extreme pH environment. The transparent superhydrophobic coatings prepared based on this method are resistant to various mechanical and chemical attacks and exhibit excellent performance in anti-icing and de-icing and self-cleaning. Li et al. 63 prepared a strong and flexible superhydrophobic film, and performed a stretching cycle experiment on the superhydrophobic film, and used SEM to characterize the surface morphology change during the stretching process (Fig. 1(c)). In addition, the superhydrophobic film prepared based on this method can quickly recover the damaged superhydrophobic surface by sanding without using a healing agent, which greatly improves the service life of the superhydrophobic film in aircraft anti-icing and de-icing applications.
Fig. 1. (a) Friction photo of sandpaper and hydrophobicity after abrasion; 60 (b) Change of hydrophobicity with drop height and surface morphology in water impact and sand abrasion test; 61 (c) Photographs, surface morphology and contact angle changes of polydimethylsiloxane (PDMS) /SiO2 porous films during stretching. 63
In addition, due to the excellent liquid storage inside the porous surface, injecting a smooth lubricant into the porous surface to prepare a hydrophobic coating can save the preparation cost, which has received extensive attention from researchers. 64, 65 The hydrophobic coating acts as a lubricant between the ice and the surface of the aircraft to inhibit ice formation on the surface of the aircraft. Yin et al. 66 obtained a self-lubricating photothermal coating with anti-icing and de-icing function by infiltrating a smooth liquid into the porous surface to obtain a durable lubricating effect, and using the photothermal effect provided by Fe3O4 nanoparticles. However, because the lubricant is depleted by evaporation or consumption, the lifespan of the porous hydrophobic coating infused with the lubricant is short. To improve durability, Wang et al. 67 reported a solid organic gel material with renewable sacrificial alkane surface layer, which has excellent durability and still maintains good anti-icing and de-icing performance after 20 anti-icing and de-icing cycles or sandpaper wear.
The hydrophobic surface mentioned above can significantly reduce the adhesion of ice to achieve the purpose of anti-icing and de-icing. However, some scholars have found that the factors affecting icing adhesion include not only external environmental factors, but also internal factors such as the properties of icing carriers and their surface roughness. And different conclusions were obtained by studying the physical principles of ice-phobia on hydrophobic surfaces and the relationship between ice-phobia and ice adhesion. Varanasi et al. 68 studied the effect of frosting on the ice-phobic properties of hydrophobic surfaces, and concluded that the formation of frost on hydrophobic surfaces can increase the adhesion of ice, and frost condensation can occur in all areas of the hydrophobic surface texture, making its hydrophobic properties loss, thereby affecting the effectiveness of the hydrophobic surface in reducing ice adhesion. And Varanasi's point of view has also been supported by other research groups. 69 Liu et al. 70 studied the effect of frosting process on the ice adhesion strength on the surface of micro-nanostructure formed by femtosecond laser. The results show that the ice adhesion strength of micro-nanostructure surface will be seriously affected by the accumulation of frost on the surface.
The PAD system restrains icing by using chemical or physical properties, which does not require additional energy consumption during anti-icing and de-icing, and the water droplets on the aircraft surface with hydrophobic materials are very easy to roll off under the action of external forces. Thus, the water flow before freezing is greatly reduced and the formation of icing on the aircraft surface is restrained. However, when there is accumulation of frost on the surface of the hydrophobic coating, the adhesion of ice will be greatly increased, resulting in a serious decline in the anti-icing performance of the hydrophobic coating. In addition, the PAD system is mostly in the experimental research stage, and has not been applied in aircraft, and the hydrophobic coating will be damaged by the collision of some particles when the aircraft is moving at high speed, which will seriously affect its anti-icing and de-icing performance.
2.3. CAD system
Hydrophobic material is an ideal anti-icing technology because of its low energy consumption. Although various hydrophobic materials have been developed, none of the coatings have been proven suitable for aerospace applications. 71 In the process of passive anti-icing using hydrophobic materials, the CAD system is combined with electric heating and external light energy to improve the surface temperature of the substrate, so as to achieve excellent anti-icing and de-icing performance. Morita et al. 72 proposed a hybrid anti-icing and de-icing system combining hydrophobic coating and electrical heating, which was validated and demonstrated in an icing wind tunnel using an icing wind tunnel. The experimental results show that, compared with the existing thermal anti-icing and de-icing system, the combination of thermal method and hydrophobic coating uses only 30%–70% of the power consumption of the thermal method itself, which significantly reduces the power consumption. Pommier-Budinger et al. 73 proposed a simple-structured analytical model for evaluating the power and voltage required to obtain ice accretion separation at the ice interface. In addition, the team studied the effect of combining different hydrophobic coatings with a low-frequency piezoelectric de-icing system, and experimentally measured the ice adhesion of different hydrophobic coatings. The experimental results show that for a given shear stress, a high resonant frequency can reduce the tensile stress entering the PZT material and improve the de-icing efficiency of the system.
Compared with AAD system and PAD system, CAD system has the advantages of green, high efficiency and strong response. However, the key parameters affecting the comprehensive properties of the CAD system and the practical application of the coating need to be further studied.
3. Technical evaluation of aircraft anti-icing and de-icing system
From the previous section, we have learned about three strategies for building an aircraft icing protection system. This section will provide a more detailed breakdown of aircraft icing protection systems and group them into specific technologies or methods. Several typical aircraft anti-icing methods are summarized, such as electric pulse de-icing, electrothermal anti-icing and de-icing, hydrophobic material anti-icing and electro-thermal hydrophobic material anti-icing and de-icing.
3.1. Electrical pulse de-icing
The electric pulse de-icing technology originated from the design of the electric pulse system published by Dr. Levin. 74 The basic principle is to use the capacitor bank to discharge to the coil to generate a strong magnetic field from the coil and a mechanical force with high amplitude and short duration on the aircraft skin to make the ice break and fall off. Electric pulse de-icing has great advantages. On the one hand, it reduces the energy required for de-icing, and has no obvious negative effect on the parameters of the engine. On the other hand, it expands the air temperature range for de-icing, which can reach −50 °C. and the system is also easily ground tested. The successful application of Dr. Levin in aircraft has set off an upsurge in the research of electric pulse de-icing technology.
In subsequent studies, more scholars optimized the de-icing efficiency of the system based on theoretical algorithms and finite element software analysis. Zhang et al. 75 proposed an improved de-icing criterion, emphasizing the de-icing effect of shear stress, and used an explicit central difference algorithm to model the electrical impulse de-icing process of the leading-edge structure. The external circuit generates pulses of different sizes under the action of the instantaneous pulse force F on the cross section of the pulse coil, and finally tests the relationship between force and time (Fig. 2(a)). Jiang and Wang 50 used Solidworks software to model the pulse coil-aluminum plate, and then imported it into the finite element analysis software (Fig. 2(b)). Then the electric pulse de-icing system of the side double coil (Fig. 2(c)) and the head single coil (Fig. 2(d)) were simulated respectively. The experimental results show that the electric pulse de-icing system with side double coils can greatly improve the de-icing effect of the electric pulse de-icing system and the safety and durability of the leading-edge structure. A simplified model of the leading-edge structure that ignores all rivets is used for modeling (Fig. 2(e)). Based on this method, when the required time is 100 μs, the pulse force reaches the peak pressure to achieve the maximum de-icing efficiency.
Fig. 2. (a) Simplified model of leading-edge structure; 75 (b) Electric simulation model; 50 (c) Simulation results of side dual-coil electrical pulse de-icing system with simplified model; 75 (d) Simplified model and simulation results of single-coil electric pulse de-icing system of machine head; 75 (e) Relationship between impact force and time. 50
Compared with the research that emphasizes theoretical algorithms and finite element analysis under specific models, researchers prefer to simulate the real icing environment to intuitively describe the de-icing process of the electrical pulse system. Wang and Jiang 76 used circuit parameters such as voltage, inductance, and capacitance obtained by optimization calculations to construct an electrical pulse de-icing system at a natural icing station (Fig. 3(a)). Under the condition of voltage U0 = 500 V and charging capacitor C = 1000 μF, the de-icing results are observed and recorded after each pulse (Fig. 3(a)). Endres et al. 77 applied short, high-current pulses to coils placed under the aluminum skin of the leading edge of the wing, causing the pulses to generate opposing time-varying magnetic fields around the coils and the shell. The resulting magnetic forces repel each other, causing the aluminum skin to oscillate for de-icing. In addition, the team tested the electrical pulse de-icing system at −10 °C and −20 °C respectively. The experimental results show that the ice layer on the leading edge of the wing falls off obviously after the electric pulse system is started (Fig. 3(b)). Sommerwerk et al. 78 placed two coils powered by a pulse generator in an aluminum shell leading-edge deicer, and the coils deformed the shell by inducing magnetic force to cause the ice layer to fall off. In addition, the de-icing process was recorded in detail by a high-speed camera (Fig. 3(c)). The de-icing results show that the electric pulse de-icing system has good de-icing performance. Tian et al. 79 adopted the dielectric barrier discharge plasma drive technology, installed two kinds of plasma brakes around the leading edge of the model, and adjusted the geometry of the medium in the actuator to reduce the influence of the model shape on the experimental results (Fig. 3 (d)). The experimental results show that the power consumption of the de-icing device during the de-icing process is lower than that of the existing de-icing methods and it is a promising de-icing technology. Sommerwerk et al. 80 carried out the de-icing experiment of the electric pulse system in the icing wind tunnel. With the increase of the number of pulses, the residual ice on the wing gradually decreased, until after the seventh pulse, 5% of the initial icing area was left on the wing (Fig. 3 (e)), showing a good de-icing effect.
Fig. 3. (a) Comparison of prediction results and icing test of natural ice station; 76 (b) Ice removal after 8 minutes of icing at −10 °C and −20 °C; 77 (c) Different de-icing process under the number of pulses; 78 (d) Schematic diagram of experimental system; 79 (e) Comparison of numerical and experimental de-icing results of continuous pulses. 80
At present, electric pulse de-icing is mainly researched on circuit analysis model and pulse coil design. Although the finite element analysis method can basically simulate the electric pulse de-icing process, it still needs a refined analysis model and magnetic induction intensity test experiments to simulate the de-icing process more accurately. Besides, in the context of today's big data era, more of those who focus on the research on electrical pulse de-icing technology still need to do basic research work. In addition, how to combine intelligent algorithms to develop an intelligent-response electrical pulse de-icing system, so that the key technologies of electrical pulse de-icing can be innovated, is the primary problem that scholars need to face in the future.
3.2. Low-frequency voltage de-icing
Low-frequency piezoelectric de-icing systems utilize the principle of inverse piezoelectricity to install piezoelectric drivers where de-icing protection is required on the aircraft. By applying an electric field to the surface of the piezoelectric material, the relative displacement of the positive and negative charges inside the material is caused, which makes the material deform and achieves the purpose of removing the ice layer. With the development of piezoelectric technology, low-frequency piezoelectric de-icing has attracted more and more attention because of its low power loss, and micro-nano vibration does not change the airfoil (compared to pneumatic de-icing) and many other advantages.
The researchers determined the optimal positions of the corresponding device components by means of a combination of finite element theory and experiments, combined with the relevant calculation formula of the shear stress when the ice layer fell off, greatly improving the de-icing efficiency of the de-icing system and reducing the power consumption during the research process. Villeneuve et al. 81 used finite element software and a numerical model of the blade to analyze the positioning and dimensions of the actuator array (Fig. 4(a)), and to simulate the drive excitation resonant frequency and corresponding vibration mode to determine the optimal frequency model. The team then validated the model's numerical predictions (Fig. 4(e)) with a laser vibrometer by locating the actuator in the appropriate location (Fig. 4(c)). Based on this method, the arrangement of the driver is optimized, the power required by the low-frequency piezoelectric de-icing system is reduced, and the de-icing efficiency is improved. Bai et al. 82 deduced a shear model for the bonding between the ice layer and the substructure, then calculated the shear stress at the interface of the ice plate using the finite element method, and compared the shear model and the shear stress obtained by the finite element method (Fig. 4(f)). Based on this method, the low-frequency piezoelectric de-icing system is obtained in the process of vibration de-icing, and the initial peeling of ice occurs at the edge. In addition, the team carried out experimental verification (Fig. 4(d)), and the experimental results show that although there is a certain error with the finite element simulation results (Fig. 4(b)), the overall agreement is good. Volat et al. 83 developed a finite element numerical model suitable for the new blade geometry and selected frequencies from 0 kHz to 5 kHz for modal analysis. The team then designed a piezoelectric de-icing system applied to a small rotating blade. After the test, the blade achieved de-icing, and the power consumption was reduced by 25% compared with the electric heating system. Pommier-Budinger et al. 84 proposed a method for calculating the voltage and current of the piezoelectric de-icing system by estimating the voltage, current and power of ice stripping. Based on this method, two actuators and a sensor were placed on the leading-edge structure and then tested in an ice wind tunnel. The correctness of the numerical calculation results is verified by comparing the numerical calculation results with the experimental results. Zhu and Li 85 carried out modal analysis and simple harmonic analysis through finite element software, calculated the shear stress and mode shape of the experimental surface model, and theoretically proved the feasibility of piezoelectric de-icing. Then, piezoelectric de-icing experiments were carried out in a refrigerator at −15 °C, and finally the successful de-icing was achieved at a voltage of 650 V and a frequency of 1530 Hz.
Fig. 4. (a) Finite element analysis of resonance modes of Bell206 wing leading-edge structure; 81 (b) Structural response calculated by experimental measurements and harmonic analysis; 82 (c) Positional distribution of actuator inside leading edge; 81 (d) Schematic diagram of experimental setup; 82 (e) Numerical (left) and experimental model (right) comparison example; 81 (f) Calculation of shear stress at the interface of ice plate by shear model and finite element method. 82
Compared with the combination of theory and experiment, the size and structure design, placement design, applied voltage and frequency of the piezoelectric actuator, and the bonding of the actuator also have a great influence on the improvement of the de-icing effect. Song et al. 86 arranged 49 piezoelectric plates and 16 piezoelectric plates uniformly on the bottom surface of the substrate respectively, and carried out experiments with different freezing positions and sizes. Fig. 5(b) and Fig. 5(c) show the active mode control results of 49 piezoelectric sheets on square and rectangular ice cubes. In addition, the team conducted 16 active mode control experiments of piezoelectric materials (Fig. 5(a)). It can be seen that although the effect can make the real vibration mode close to the hypothetical de-icing mode, the control effect is not as good as that of 49 piezoelectric sheets. The above simulation results show that the number and placement of the piezoelectric sheets greatly affect the de-icing efficiency, and the place without ice hardly vibrates, making the structure of the piezoelectric de-icing system more stable. Shi and Jia 87 simulated the different vibration modes of the composite rectangular sample in the finite element method (Fig. 5 (e)). It can be seen that when the third mode of excitation is applied, the torsional shear stress can achieve better de-icing effect. Moreover, the team performed finite element mode shape simulations of higher-order modes, and along the length of the composite plate, multiple shear stress peaks were observed (Fig. 5(d)). The experimental results show that better de-icing effect can be induced under smaller excitation amplitude, which lays a foundation for low-power de-icing and light de-icing of aircraft in flight in the future. Venna et al. 88, 89, 90, 91 provided a method to use a single piezoelectric element to excite shear stress and impact force for de-icing, and studied the excitation effect of different sizes of piezoelectric elements on the leading-edge structure of the wing. And through modeling and finite element analysis, the optimal placement of piezoelectric elements and the structural vibration effect under the optimal applied voltage are studied.
Fig. 5. (a) Active mode control results under 16 piezoelectric patches; 86 (b) Active mode control results of non-fixed square ice cubes; 86 (c) Active mode control results of non-fixed rectangular ice cubes; 86 (d) Shear stress simulation of piezoelectric actuators with higher order modes; 87 (e) Shear stress simulation of piezoelectric actuators with different shear stresses in different modes. 87
The low-frequency piezoelectric de-icing system has the advantages of simple structure, low noise and good maintainability. It meets the high requirements of modern aircraft for low energy consumption, light weight and no pollution, and has a good development prospect. However, there are still some deficiencies. Due to the limitations of the experimental conditions, the low-frequency piezoelectric de-icing system has only been tested in a cold environment on the ground. There is a big gap between the influence of airflow and the actual flight environment. In the future, scholars need to find a suitable ice wind tunnel or a natural environment with a relatively cold climate to achieve a more realistic experimental effect.
3.3. Ultrasonic de-icing
Ultrasonic de-icing technology uses the thermal effect and mechanical effect of ultrasonic waves to remove ice. The thermal effect of ultrasonic de-icing is due to the fact that in the process of wave propagation, the sound energy of the medium will be converted into thermal energy. On the other hand, when the mechanical vibration of the tool head hits the ice, a large amount of mechanical energy is converted into thermal energy, resulting in the melting of the ice. The mechanical effect of ultrasonic waves is that the mechanical vibration of ultrasonic waves causes the vibration of related particles. When the vibration exceeds a certain limit, the purpose of destroying the ice layer can be achieved.
In the application of ultrasonic waves, by calculating and analyzing the mode shape and natural frequency of the elastic solid, it is ensured that the vibration system works at the resonant frequency and achieves the best power conversion. Through the numerical calculation of finite element, the system is divided into enough suitable units to obtain the optimal frequency of the ultrasonic de-icing system. Wang et al. 92 carried out icing and de-icing tests in a refrigerator at 15 °C by fixing the side of the aluminum substrate (Fig. 6 (a)) using a fixture and adhering two PZT transducers to the aluminum plate through epoxy resin (Fig. 6 (c)). The experimental results of ultrasonic de-icing show that ultrasonic de-icing is a continuous process (Fig. 6(b)), and the system achieves the best de-icing effect when the frequency is 34 kHz, which is consistent with the optimal frequency of 37.5 kHz predicted by the model. Zeng and Song 93 studied the numerical simulation and experimental test of the sandwich transducer ultrasonic de-icing system. The team used a special adhesive to stick the sandwich transducer on a thin aluminum plate (Fig. 6(e)), by placing the aluminum plate in a freezer at −16 °C for up to 44 min, until the surface of the thin aluminum plate was iced up to 2 mm thick layer, followed by ultrasonic de-icing experiments (Fig. 6(d)). The results of the de-icing experiments showed that the two sandwich transducers removed a 2 mm thick layer of ice from the surface of the thin aluminum plate in less than one minute.
Fig. 6. (a) Experimental setup for ice detachment; 92 (b) De-icing process of ultrasonic de-icing system; 92 (c) Fixed position of PZT transducer on aluminum plate; 92 (d) Ultrasonic wave wiring diagram of de-icing experiment; 93 (e) Way of pasting two sandwich transducers on thin aluminum plate.93
In order to verify the feasibility of ultrasonic de-icing technology on aircraft wings, Yin et al. 94 connected the transducer made of PZT-4 to an arbitrary waveform generator through an amplifier. Subsequently, the input signal was amplified to the ultrasonic transducer with an amplifier, and after applying a voltage of 50 V, the de-icing experiment was performed in a refrigerator at −15 °C. The experimental results show (Fig. 7(a)) that the higher the frequency, the better the de-icing effect. In addition, the ultrasonic de-icing time is much shorter than the freezing time. When the ultrasonic de-icing system works continuously under icing weather conditions, the surface of the aircraft wing will always remain ice-free. Wang et al. 95 used two piezoelectric actuators with a natural frequency of 40 kHz for de-icing research. The de-icing experimental setup is shown in Fig. 7(c). A video camera was used to record the ice fragmentation process by placing the deiced specimen and ultrasonic transducer in a freezer at −15 °C for 2 h, and then clamping the composite sheet. The results of the ultrasonic de-icing experiment show that the de-icing effect is the most obvious when the ultrasonic frequency is 34.8 kHz, and the main body of the ice layer falls off within 3 min and 6 s. Daniliuk et al. 96 froze the plate specimen attached with the sensor at −15 °C for one hour, then applied high-frequency alternating current to the transducer using an ultrasonic generator with a maximum output power of 440 W and an output frequency continuously adjusted from 20 kHz to 150 kHz, and finally used a camera to capture the de-icing process (Fig. 7 (b)). The experimental results show that the simulation results are consistent with the experimental results, which proves the feasibility of ultrasonic de-icing.
Fig. 7. (a) Changing curve of de-icing thickness under different ultrasonic frequencies at −15 °C with time; 94 (b) Actual ultrasonic de-icing equipment (left) and schematic diagram of de-icing experiment (right); 96 (c) Schematic diagram of ultrasonic de-icing experimental setup; 95 (d) Cross-sectional view of lithium niobate transducer. 97
Because lithium niobate has strong piezoelectric effect and weak adhesion with ice matrix, using lithium niobate as piezoelectric energy exchange material can greatly improve the efficiency of de-icing and reduce power consumption. The ultrasonic de-icing technology can be better applied to aircraft wings and has a good application prospect. Wang et al. 97 designed a lithium niobate transducer for ultrasonic de-icing of aircraft wings (Fig. 7(d)). In addition, the team studied the relationship between de-icing thickness and time at different ultrasonic frequencies under the experimental conditions of −15 °C and 50 V sinusoidal voltage. The experimental results show that when the frequency is increased to 140 kHz, a single transducer can remove the ice layer with a thickness of 3 mm and an area of 260 mm2 in 151 s, which proves the feasibility of the lithium niobate transducer to remove the ice layer on the surface of the wing. In the follow-up study, Wang et al. 98 used lead-free lithium niobate compounds to fabricate light-weight ultrasonic transducers. In addition, a single transducer was used for shear de-icing and pulse de-icing, and 441.2 kHz was given as the best de-icing frequency.
At present, reducing the output frequency of the system is an important design parameter for designing an ultrasonic de-icing system. Further research is needed on how future workers can perform grid analysis on the structures in the ultrasonic system, and how to ensure that the ultrasonic generator maintains a stable frequency during operation and better matches the impedance of the transducer.
3.4. Pneumatic de-icing
Pneumatic de-icing system is the earliest mechanical de-icing system applied to aircraft. Its working principle is to install a layer of aerodynamic cover composed of expansion tubes on the surface of the wings. When de-icing is required, the system flushes into the expansion tubes. Compressed air is injected to make it expand, and a shear force is applied to the ice layer covering the aerodynamic cover to break the ice layer, and the ice layer gradually separates from the surface of the aircraft under the action of the airflow. In addition to the expansion tube, the pressure regulating source and the vacuum source are also important modules of the pneumatic de-icing system. The pressure regulating source controls the amount of compressed air entering, thereby controlling the expansion degree of the expansion tube to cope with ice layers of different thicknesses. The function of the vacuum source is to control the effective contraction of the expansion tube, to ensure that the surface of the expansion tube is smooth without freezing, and to affect the aerodynamic characteristics of the aircraft as little as possible.
There are two main ways to install the expansion tube in the aerodynamic hood, one is along the span direction, and the other is along the chord direction. Although the resistance of the chordwise arrangement of the conduit is lower than that of the spanwise arrangement of the conduit, the manufacturing difficulty will be increased. At the same time, the installation of the aerodynamic cover on the wing will change the shape of the wing, which will seriously affect the aerodynamic characteristics of the aircraft during flight. Bowden 99 carried out aerodynamic de-icing experiments on the airfoil of NACA0011, and tested the effects of pneumatic de-icing arrangement along the span direction and chord direction on lift, drag and pitching moment. The results show that when arranged along the spreading direction, the resistance of the pneumatic cover increases from 7% to 37%, and the angle of attack increases from 0° to 4.6°. Broeren et al. 100 studied the effect of increased icing on the aerodynamic performance caused by the cyclic operation of the aircraft aerodynamic de-icing system by conducting icing wind tunnel experiments on the NACA3012 airfoil. The experimental results show that the interstitial ice accumulation during the cycle can lead to a significant decrease in the aerodynamic performance of the aircraft, and the maximum lift coefficient is reduced by 60%. Palacios et al. 19 proposed a novel aerodynamic method for protecting helicopter rotor blades from ice accretion by installing six aerodynamic diaphragms below the leading edge of a titanium alloy. When the thickness of the ice accumulation reaches a critical value, the diaphragm begins to expand, causing the leading edge of the wing to deform, which promotes the shedding of the ice layer.
In the pneumatic de-icing system, the life problem is always a difficult point of the pneumatic de-icing system. Since the moisture in the compressed air source will accumulate in the pneumatic hood, it will have a serious impact on the performance of the hose. Goodrich has done research on this situation, designed a pneumatic de-icing system with an exhaust valve, and applied for a patent in the United States and Europe 101. The connected path facilitates the discharge of moisture in the compressed air, which greatly increases the reliability and service life of the pneumatic de-icing system. The main components of the pneumatic de-icing system are the input port and the drain valve, which are mutual.
Pneumatic de-icing system has the advantages of light weight and less modification to the aircraft, but when the pneumatic de-icing system works, it will destroy the aerodynamic shape of the aircraft surface, resulting in increased resistance of the aircraft during flight, and poor applicability to high-speed aircraft.
3.5. Anti-icing of hydrophobic materials
The anti-icing technology of hydrophobic materials utilizes the non-wetting phenomenon of objects in nature to prevent ice. The so-called hydrophobic material refers to the material whose surface contact angle with water is greater than 150° and the rolling angle is less than 10°. 102, 103, 104 Compared with traditional anti-icing technologies that rely on electricity or heat, small aircraft usually cannot meet their operating power consumption. However, the use of hydrophobic materials can delay icing to achieve anti-icing effect without additional energy consumption, and is currently considered to be the most effective solution to the problem of anti-icing and de-icing system power consumption.
In the past few years, a great deal of research has been devoted to hydrophobic materials capable of retarding ice formation or reducing the adhesive strength during ice formation. 105 The chemical composition and roughness of hydrophobic materials are two important factors that determine their surface hydrophobicity. 106 Therefore, to prepare a surface with superhydrophobic properties, the following two principles should be followed: one is to construct a micro-nano rough structure on the hydrophobic surface, and the other is to modify the rough surface with low surface energy substances. 107, 108 Ren et al. 109 proposed a simple chemical etching and modification method to produce super hydrophobic metal rubber with lotus-like hierarchical structure on the surface of stainless-steel wire by two-step chemical etching process. At the same time, the team carried out waterproof performance test (Fig. 8(a)) and oil–water separation test (Fig. 8(b)). The experimental results show that the prepared superhydrophobic metal rubber can keep its surface dry when taken out of water, and the superhydrophobic Metal Rubber (MR) has better oilwater separation performance than the pristine Metal Rubber (MR). Chen et al. 110 introduced an efficient and practical method for surface modification of silicone rubber with nanofiber laser to make it superhydrophobic. After nanofiber laser treatment, a micro-nano rough structure was formed on the surface of the silicone rubber, and its surface had strong hydrophobic properties (Fig. 8(d)). Maghsoudi et al. 111 used compression molding system to directly copy the surface of silicone rubber with micro-nano-rough structure, and prepared a superhydrophobic surface with a contact angle greater than 160° and a rolling angle less than 5°.
Fig. 8. (a) Waterproof performance test;109 (b) Oil-water separation experiment;109 (c) Schematic diagram of preparation process of hydrophobic coating; 62 (d) State of water droplets on the surface of silicone rubber, unprocessed surface (orange), and superhydrophobic surface (white); 110 (e) Durability test of hydrophobic coating.62
The preparation of hydrophobic materials has a relatively complete process, but due to the instability of the surface microstructure of hydrophobic materials, it is particularly important to study the durability of the surface of hydrophobic coatings. Researchers achieve delayed icing of materials through high temperature 112 or complex fabrication techniques to improve the durability of hydrophobic materials. Although these techniques can produce controllable micro-nano rough structures, the preparation time is long and the efficiency is low. Asadollahi et al. 113 studied an atmospheric pressure plasma jet technique to build a microporous structure by placing an aluminum sample at an extremely short jet-to-substrate distance for multiple air plasma treatments. Meanwhile, the team conducted multiple icing and de-icing cycle experiments to explore the effects on the chemical composition, surface morphology, and wetting behavior of microporous alumina-based surfaces. Zhu et al. 62 proposed a hydrophobic coating consisting of polydimethylsiloxane (PDMS) nanoparticles (NPs) and PDMS microparticles (MPs) functional NPs through a combination of thermal treatment and spray treatment (Fig. 8(e)). The hydrophobic coating was subsequently subjected to a series of durability experiments such as sand impact test, abrasion test and tape peeling (Fig. 8(c)). The experimental results show that the hydrophobic coating has good durability and is expected to be used in aircraft windshield anti-icing to improve cockpit visibility.
The advantage of the hydrophobic material anti-icing technology is that icing is fundamentally avoided, no energy is consumed in the process, and very little additional volume or mass is required. However, since the micro-rough structure on the surface of hydrophobic materials is easily destroyed, the hydrophobic properties of the materials decrease sharply or even fail. Therefore, how to maintain the durability of the material for a long time is the key to the research of this technology.
3.6. Electro-thermal anti-icing and de-icing
The electro-thermal anti-icing and de-icing system is essentially an electric heater. By converting electrical energy into thermal energy, the heat is transferred to the surface to be protected through the heating element. Through continuous heating, the bottom ice layer melts, which reduces the adhesion between the ice layer and the outer surface of the aircraft, so that the ice layer is removed under the action of centrifugal force or aerodynamic force.
Electro-thermal anti-icing and de-icing technology is one of the traditional aircraft anti-icing and de-icing systems, and a lot of research has been done at home and abroad since the last century. Since the system needs to consume a lot of electricity, the power of the electro-thermal anti-icing and de-icing system largely determines the weight of the aircraft. Due to the limitation of aircraft design standards, the power load should be kept as low as possible. In order to reduce the power of the system, researchers have studied the power characteristics of the electro-thermal anti-icing and de-icing system surface heat transfer mechanism and model optimization. Pourbagian et al. 114 provided optimization formulas for various constraint problems of the wing electro-thermal anti-icing and de-icing system in the wetting and evaporation states, and conducted numerical optimization simulation of the system by solving the conjugate heat transfer problem between the fluid and solid domains. The optimization results are compared with the experimental data, and the results show that the optimization results significantly reduce the power demand. Targui and Habashi 115 embedded the simulation results of conjugate heat transfer provided by 3D finite element Navier-Stokes analysis package-ICE code into a special rotor aircraft simulation toolkit. Through reduced-order modeling, the timeliness evaluation of objectives and constraint functions at each iteration is provided. The proposed method optimizes the heating range and power of the heater. Raj and Myong 116 used partial differential equations to analyze and calculate air flow, droplet collision, ice accumulation and coupled heat transfer, and predicted the anti-icing and de-icing effect of electro-thermal anti-icing and de-icing technology. The data are in good agreement, and the minimum power consumption of the aircraft anti-icing and de-icing system is analyzed and calculated based on this method. Wu et al. 117 used a genetic algorithm to optimize the distribution of heating power on the surface of the component's electro-thermal anti-icing and de-icing system. Compared with the case where the surface temperature was uniformly distributed, the total heating power was greatly reduced. In addition, the team found that the total heating power was minimal when the water film length was just close to the droplet impact limit.
Compared with model optimization to reduce power to ensure the endurance and safety of the aircraft, the required de-icing threshold heat flux density can be accurately predicted by coupling the internal thermal analysis and calculating the temperature field through simulation. 118 Bu et al. 119 established a tightly coupled simulation method for aircraft electrical-thermal anti-icing and de-icing system under icing conditions by using a tightly coupled mass transfer and heat transfer model of backflow water in a virtual thin wall. Considering the influence of the surface temperature distribution on the air convective heat transfer coefficient, the convective heat transfer coefficient under the dry air condition was solved by the tightly coupled simulation method, and compared with the result under the isothermal wall boundary condition (Fig. 9(a)). The experimental results show that the tightly coupled simulation method successfully captures the effect of surface temperature on the convective heat transfer coefficient and predicts a higher temperature on the surface of the electrothermal anti-icing and de-icing system at a lower rate of decline. Shu et al. 120 proposed a numerical method to determine threshold de-icing power density by calculating loosely coupled fluid and temperature fields. In order to provide the necessary ice shape and corresponding validation for the model, the team carried out icing and threshold de-icing experiments on custom-built small wind turbines (Fig. 9(c)). The experimental results show that the numerical prediction is in good agreement with the experimental value, and the maximum score error is 9.3%.
Fig. 9. (a) Convective heat transfer coefficients at different surface temperatures; 119 (b) Thermal mapping vs time of graphene-polymer composites and thermal power density at different power values; 127 (c) Threshold division calculation and experimental results of ice heat flux and ambient temperature; 120 (d) Preparation process of carbon fiber/ceramic composites. 128
The heating element of the traditional electro-thermal anti-icing and de-icing system is usually made of metal, which has poor flexibility and is prone to breakage when it is attached to the surface of the wing for a long time. 121 In addition, the uneven heating of metal components will cause the local temperature to be too high and burn out the internal circuits of the electrical-thermal anti-icing and de-icing system. Due to the advantages of high temperature resistance, shock absorption and fatigue resistance, 122 composite materials are widely used in the manufacture of aircraft airframes. Yao et al. 123 fabricated highly aligned carbon nanotube networks by chemical vapor deposition and subsequently inserted them between pre-cured layers of unidirectional carbon fiber reinforced polymer. The electrical conductivity and thermal conductivity were tested under different curing conditions and when carbon nanotube fibers with different layers were sandwiched. The experimental results show that the composite material exhibits good electrical conductivity, and the thermal conductivity does not change significantly due to different conduction modes. Li et al. 124 fabricated samples using carbon fiber reinforced composites, glass fiber prepregs, and copper meshes, and then embedded sprayable metal films as heating elements in them to prepare composites. The team then tested the effect of thermal cycling and mechanical cycling on the electro-thermal anti-icing and de-icing system, measuring the real-time resistance of the sprayable metal film and the number of cycles to understand the effect of cyclic loading on the heating performance and fatigue life of the system.
Graphene is a perfect two-dimensional material with extremely high electrical conductivity, thermal conductivity and good flexibility, as opposed to adding different media into the polymer matrix. By adding graphene into the polymer, matrix can perfectly solve the defects of low heat transfer efficiency and short life of traditional composite materials. 125, 126 Ba et al. 127 obtained graphene-polymer composites in surface coating with conductive thin graphene-based polymer matrix. Besides, the team analyzed the thermal mapping of the composite as a function of time and the thermal power density of the composite at different power values (Fig. 9(b)). The experimental results show that the as-prepared composites have fast heating response and excellent performance, and can be used as light-duty and low-power electro-thermal anti-icing and de-icing systems. Xiong et al. 128 prepared graphene-coated carbon fiber/ceramic composites with rapid temperature response through a simple one-step firing method (Fig. 9(d)). Based on this method, the team investigated the crystal structure, morphological characteristics, electrical properties, and electro-thermal behavior of modified carbon fibers and carbon fiber/ceramic composites. The experimental results show that the composite material has good electrothermal and mechanical properties. Glover et al. 129 proved the feasibility of heatable multi-functional graphene surface in de-icing application, and studied the electrical properties, microstructure, heat transfer and electrical response of the material and the shape of the composite structure with the change of temperature.
Over the years, scholars have never stopped research on electrothermal anti-icing technology, but currently only the Boeing 787 is actually applied to its civil aircraft. In the latest civil aircraft Boeing 787, its structural design subverts the previous design ideas. By canceling the engine bleed air system and adopting a multi-electric environmental control system, the stability of the aircraft during the de-icing process is ensured. 130 In addition, the electric heating anti-icing and de-icing system on the Boeing 787 aircraft adopts heating pads with partition design to prevent the aircraft from icing, and adopts symmetrical logic control on both sides of the aircraft wings to ensure the aerodynamic stability of the aircraft. Even if the wing heating pad on one side of the aircraft fails, the system will automatically power off the symmetrical wings to prevent asymmetrical icing of the wings on both sides due to icing weather conditions, ensuring the flight safety of the aircraft.
With the development trend of electric aircraft, various countries have carried out research on the power optimization of electro-thermal anti-icing and de-icing system, which will surely promote the development and rise of the research field of electro-thermal anti-icing and de-icing system. Traditional electro-thermal anti-icing and de-icing systems usually use metal resistance wire as the electric heating element of the ice accretion protection system. Metal heating elements have defects such as heating unevenness and low heating efficiency. New composite materials with electrical conductivity have become one of the research directions of electro-thermal anti-icing and de-icing systems. In the future, an electro-thermal anti-icing and de-icing system with low energy consumption and high reliability needs to be designed, and how to make multiple anti-icing and de-icing systems work together for anti-icing and de-icing will still be a huge challenge.
3.7. Hot air anti-icing and de-icing
The hot air anti-icing and de-icing system is a flute-shaped tube in which the heating air is distributed to the leading edge of the anti-icing parts through the air supply pipeline. The flute-shaped pipe heats the leading edge of the skin by means of a small hole impinging jet to achieve the purpose of anti-icing and de-icing. 131
In order to accurately control the experimental conditions and reduce the influence of climatic conditions and time, the researchers placed the aircraft model in the wind tunnel to study the interaction between the gas flow and the aircraft model in the hot air anti-icing and de-icing system. Dong et al. 132 studied a hot air film heating method for a small aero-engine cone anti-icing and de-icing system. In their research work, the team conducted experiments in icing wind tunnels that can simulate the various icing environments that aircraft encounter under actual flight conditions. To maintain the desired hot air flow and velocity, a 10 kW electric air heater is used, the hot air temperature is controlled by a programmable logic controller system with an accuracy of ± 2 °C, and the flow rate is measured using a flowmeter with an accuracy of 1%. The experimental results show that the hot air anti-icing and de-icing system can continuously act on the ice layer on the leading edge of the small aero-engine cone, and the de-icing effect is obvious.
At present, due to the continuous reduction of the cost of computer hardware, the numerical simulation method has a huge economic advantage, so the numerical simulation method has gradually become an important means to study the problem of aircraft hot air anti-icing and de-icing. 133, 134, 135 Yu et al. 136 used the three-dimensional internal and external robust heat transfer method to calculate and check the performance of the engine nacelle hot air anti-icing and de-icing system. The Euler method was used to calculate the impact characteristics of water droplets, and the inverse distance interpolation method was used to interact with the internal and external flow field data, and the results of the nacelle surface temperature and overflow water were obtained. In addition, the team used the three-dimensional internal and external coupling method to simulate and calculate the outer surface temperature and surface overflow water of the skin under the condition of the total pressure of the hot air vent of the hot gas anti-icing and de-icing system of 211313 Pa and the total temperature of 493 K (Fig. 10(a)). The simulation results show that the engine nacelle hot air anti-icing and de-icing system meets the anti-icing and de-icing performance requirements. Saeed et al. 22 used CFD software to conduct a two-dimensional numerical simulation study on the heat transfer of a hot air jet hitting a curved surface like the leading edge of a wing or a wing plate. The team found that the heat transfer efficiency increased by 20% when moving from the flat to the curved model, and the smaller nozzle spacing resulted in better heat transfer at smaller channel heights. In addition, with the increase of the channel inlet position angle, the distance from the jet inlet to the channel increases, and the heat transfer coefficient decreases. The simulation results show that the performance of the hot air anti-icing and de-icing system can be greatly improved by designing the leading-edge surface with reasonable geometry and flow characteristics. Zhang et al. 137 studied the design and performance calculation of the wing hot air anti-icing and de-icing system, and established a simulation model of the airflow distribution of the hot air anti-icing and de-icing system using the one-dimensional thermal fluid simulation platform Flowmaster. In addition, the team obtained the results of stable surface temperature and return water distribution of the anti-icing and de-icing system through the energy conservation equation (Fig. 10(b)). Based on the simulation results of the simulation model, the team verified the performance of the wing hot air anti-icing and de-icing system. The experimental results show that the designed hot air anti-icing and de-icing system can well meet the anti-icing requirements.
Fig. 10. (a) Contour of surface temperature of engine nacelle and contour of surface overflow water flow; 136 (b) Experimental simulation results of hot gas anti-icing system. 137
The purpose of the hot air anti-icing and de-icing system is to keep the temperature of the aircraft surface above freezing, thereby avoiding icing. It can directly heat the skin with hot air, and has the advantages of simple and reliable structure. However, the hot air anti-icing and de-icing system also has the problems of high energy consumption and poor utilization rate, and the hot air cycle de-icing system is rarely used at present, which easily leads to the formation of ice slips behind the heating area, which seriously affects the flight safety. In addition, there are many simplifications and assumptions of key parameters in the currently used anti-icing and de-icing calculation model, which is quite different from the results of icing wind tunnel experiments or flight experiments. In the future, a more realistic anti-icing and de-icing calculation model needs to be designed, and a lot of research is needed to make technical breakthroughs.
3.8. Electro-thermal and hydrophobic material CAD
The traditional electric heating system is prone to breakage when it is attached to the surface of the wing for a long time, which reduces the mechanical strength and makes maintenance difficult. Therefore, directly heating the ice interface is the most effective way to save a lot of energy during heat transfer. But resistance wires and other traditional heating elements are not easy to implement, and the method of CAD by using hydrophobic materials for Joule heating and de-icing is increasingly favored by researchers.
Zhao et al. 30 prepared a combined electric heating coating based on Multi-Walled Carbon Nanotubes (MWCNTs) and a top superhydrophobic coating on Glass Fiber Reinforced Polymer (GFRP) substrates by spraying method. In order to further investigate the electrical conductivity and anti-icing and de-icing performance of the combined electric heating coating, the electric heating performance test (Fig. 11(a)) and de-icing performance test (Fig. 11(d)) were carried out. The experimental results show that the combined electric heating coating has strong damage resistance and anti-icing and de-icing performance. Fig. 11(b) shows the automatic replenishment mechanism of lubricating oil in microchannel. 138 Fig. 11 (c) shows the surface preparation of self-lubricating liquid water layer. 139 Liu et al. 140 directly constructed closely-arranged micropores on the multi-walled carbon nanotube/polymer-based electric heating coating, and then injected silicone oil into the micropores to obtain synovial fluid injection with good ultra-low ice adhesion Porous Electrically Heated Coating (PEHC) (Fig. 11(e)). Then, the power consumption and de-icing efficiency were analyzed through static de-icing experiments and dynamic de-icing experiments. The experimental results show that the porous electric heating coating can significantly shorten the de-icing time, thereby reducing the energy consumption of electric heating by up to 53%. Morita et al. 72 developed a hybrid anti-icing and de-icing system combining hydrophobic coating and electric heating, and then conducted anti-icing and de-icing tests in a large-scale icing wind tunnel. The experimental results show that the surface covered with the hydrophobic coating reduces the power consumption by 30% to 70% during anti-icing and de-icing experiments. Ibrahim et al. 141 used 3D printing technology to develop a continuous wire polymer composite heater by combining the continuous wire network with the polymer matrix, and sprayed a hydrophobic coating on its surface to obtain a panel that can be heated with hydrophobic properties. The team then tested the panel in harsh environments to examine the effectiveness of its anti-icing and de-icing system in icing conditions. Experimental results show that the heatable and hydrophobic panel can greatly reduce its power consumption under icing conditions.
Fig. 11. (a) Electric heating performance test before and after coating damage; 30 (b) Schematic diagram of automatic replenishment mechanism of lubricating oil in microchannel; 138 (c) Schematic diagram of surface preparation of self-lubricating liquid water layer; 139 (d) Schematic diagram of de-icing experiment; 30 (e) Preparation of porous electrothermal coatings with different functions. 140
The above studies show that the anti-icing and de-icing technology of electrothermal hydrophobic material is the most promising aircraft anti-icing technology. Through the combination of AAD and PAD systems, it makes up for the high energy consumption of electric heating de-icing and the poor anti-icing effect of hydrophobic coating, shortens de-icing time and reduces de-icing energy consumption. However, the actual anti-icing and de-icing performance and damage resistance are the challenges for their large-scale applications. In the future, scholars should focus on the practical significance of CAD of electrothermal hydrophobic materials.
3.9. Other methods
In addition to the above-mentioned common methods, there are some other aircraft anti-icing and de-icing technologies, but most of them are not systematic and have poor comparability or similar inspections for reference, such as plasma anti-icing, synthetic jet anti-icing, chemical anti-icing, liquid lubricating layer anti-icing, memory alloy de-icing, etc. This section summarizes these unique methods.
As an active anti-icing technology, plasma anti-icing technology has the advantages of simple structure, fast structure, easy automation and so on. In recent years, as one of the common means of active plasma flow control, dielectric barrier discharge has a wide range of applications in aircraft anti-icing, drag reduction and lift increase.142, 143 Wei et al.144 proposed a nanosecond pulsed SDBD-based “flow-to-plasma hot knife” structure, and tested the de-icing performance of the structure under two typical icing conditions. Excellent anti-icing performance was achieved in both icing conditions. Linder et al.145 fabricated a miniaturized dielectric barrier discharge plasma actuator using microelectromechanical systems. And by using a flexible inorganic zirconia substrate to make the actuator sample stretchable, it can be well applied to the aircraft anti-icing system. In addition, the icing wind tunnel experiments show that the plasma brake has a good anti-icing effect.
As an active flow control technology, synthetic jet technology has the advantages of simple structure, easy control, and no need for external air source. Nagappan et al.146 used the finite element analysis software ANSY-FLUENT and FENSAP-ICE to study the influence of synthetic jet actuator driving on icing with or without jet heating. And the use of heated synthetic jet exciter to achieve the anti-icing effect of aircraft is proposed for the first time. Liu et al. 147 proposed an impingement rod type plasma synthetic jet for de-icing, and conducted a series of de-icing experiments on it. The experimental results show that compared with the traditional plasma synthetic jet brake, the impact rod type plasma synthetic jet brake has better de-icing performance and can better ensure the flight safety of the aircraft.
The chemical liquid anti-icing technology is to spray anti-icing liquid on the surface of the wing to reduce the freezing point of supercooled water droplets and prevent the surface of the wing from freezing. In the early days of the study, the researchers tried to coat the surface of the wing with a layer of salt. When supercooled water droplets were collected on the surface of the wing, the salt dissolved in the water and lowered the freezing point of the water without freezing on the surface. However, salt corrodes the metal skin, so this method has not been applied in practice. The use of anti-icing solution instead of salt can meet the needs of anti-icing. The main components of anti-icing solution are ethylene glycol, propylene glycol and other polyols, which can dissolve the ice and snow on the wing surface and prevent secondary icing. Besides, the anti-icing liquid also contains surfactants and corrosion inhibitors, which can improve the hydrophobicity of aircraft surface, improve the efficiency of de-icing, and prevent aircraft surface corrosion. 148
The anti-icing technology of the liquid lubricating layer refers to injecting lubricant into the micro-channels with interweaving. With the help of the natural oil reservoir and container characteristics of the micro-channels, the lubricant is firmly locked and stored in the grid, resulting in an effective hydrophobic performance and achieving delayed icing. Chen et al. 139 prepared a robust anti-icing coating with a Self-lubricating Liquid Water Layer (SLWL) by grafting cross-linked hygroscopic polymers in the micropores of the silicon wafer surface, and when the top of the self-lubricating liquid water layer freezes, the ice will fall off under the action of natural wind (Fig. 11(c)). In addition, the surface of the self-lubricating liquid water layer also exhibits excellent mechanical durability and self-healing ability. Li et al. 138 designed a network surface structure with interconnected microchannels and cross-linked nanosheets, and the team performed anti-icing and de-icing experiments and stability tests on the structure. In addition, to reduce the influence of the surface lubricating oil layer consumed during the de-icing process on the anti-icing performance of the structure, the team investigated how the lubricating oil stored in the microchannels is automatically replenished (Fig. 11(b)).
The memory alloy de-icing method utilizes the unique metallurgical composition of the shape memory alloy, and at a relatively narrow phase transition temperature, the ice surface has a large shape and size change, thereby deforming the ice layer and achieving the effect of removing the ice layer. Liu et al. 149 studied the de-icing performance of shape memory alloys on non-rotating structures of aircraft and aero-engines, and carried out numerical analysis on the characteristics and influencing factors of shape memory alloy deicers. The advantages of the memory alloy de-icing method are small size, small mass, simple structure, good durability and low cost, but the deformation of memory alloy with temperature changes is difficult to control. The current research work mainly focuses on optimizing the strain of shape memory alloys, increasing the strain output and reducing the temperature hysteresis range.
As mentioned above, scientific researchers have adopted a large number of anti-icing and de-icing technologies such as electrical pulse de-icing, ultrasonic de-icing, anti-icing of hydrophobic materials, pneumatic de-icing and other general methods to remove the ice on the surface of the aircraft, so as to ensure the flight safety of the aircraft in icing climate conditions. In addition, researchers conducted additional tests on the device characteristics of specific technologies, such as durability and impact resistance tests for hydrophobic materials, voltage stability and power loss tests for piezoelectric de-icing. However, when researchers conduct aircraft anti-icing and de-icing tests, most of the experimental devices they use are made by themselves, and the experimental test standards are different, and most of them are simulated experiments on computers, which are rarely used on actual aircraft surfaces. This greatly reduces the comparability of the reported test results, which further hinders the practical application of aircraft icing accretion protection systems. In the future, the special equipment for icing tests under special meteorological conditions needs to be further improved.
4. Conclusions and future work
In short, the development of aircraft has brought great convenience to people's production and life. However, the icing problem of aircraft under special weather conditions seriously threatens the safety of flight. Starting from the hazards of aircraft icing, this paper analyzes the current progress, principles, advantages and disadvantages of typical aircraft anti-icing and de-icing technologies. Although aircraft anti-icing and de-icing technology has been widely used, there are still some shortcomings, and the main conclusions are obtained as follows.
(1)
At present, for the construction of aircraft icing protection systems, a variety of aircraft anti-icing and de-icing systems have been developed to achieve the suppression of aircraft surface ice, and the removal of aircraft surface ice. However, in various reports, the test materials used in the aircraft anti-icing and de-icing system are different, the experimental devices are different, and the voltage and frequency used in the de-icing process are not unified and standardized, which reduces the reliability of the anti-icing and de-icing effect. Therefore, the standardization of experimental devices and test materials will be the primary problem faced by aircraft icing protection systems.
(2)
Although the electric pulse de-icing technology has obvious advantages such as simple structure, high efficiency and convenient maintenance, the electric pulse de-icing system will cause fatigue damage to the skin, and an electromagnetic field will be generated during the de-icing process, and the interference will seriously affect the safety of the aircraft.
(3)
The pneumatic de-icing technology has a simple system structure and consumes little air flow, but when the system is de-icing, the expansion tube will protrude from the skin surface of the aircraft, destroying the original aerodynamic shape of the aircraft.
(4)
Although the anti-icing technology of hydrophobic materials basically avoids the generation of icing and does not consume energy in the process, the rough structure existing on the surface is very easy to be damaged, resulting in a significant decrease in the hydrophobic performance of the material or even failure, and it is difficult to achieve long-term aircraft anti-icing.
(5)
The hot air anti-icing and de-icing technology realizes the direct introduction of a large amount of high-temperature hot gas from the compressor section of the engine, and achieves the advantages of simple and reliable structure, but it consumes a lot of valuable energy and the structural stress problem caused by the temperature gradient of the aircraft surface. Although the electro-thermal anti-icing and de-icing technology has the advantages of high reliability and easy control, the metal heating elements used in it have the defects of poor flexibility, uneven heating and low heating efficiency.
The next research focus of aircraft icing protection system should mainly focus on icing physics, exploring new principles, reducing energy consumption, developing new technologies and applying new materials. In the future, the exploration and application of new principles, new technologies and new materials will improve the efficiency of aircraft icing protection systems and reduce energy consumption. Therefore, it will remain a huge challenge for the aircraft icing protection system to become more environmentally friendly, reliable and efficient in the future.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This study was co-supported by the Open Fund of Key Laboratory of Power Research of China and the National Natural Science Foundation of China (No. 2018YFC0809500), the Sichuan Science and Technology Plan Project, China (No. 23NSFSC1923), the Laboratory of Icing and Anti/De-icing of CARDC, China (No. IADL20220406), the Key R&D Special Projects in Henan Province, China (No. 221111321000), the Basic Scientific Research Business Expenses of Central Universities, China (No. J2023-033).
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32 | https://www.mdpi.com/2571-6131/6/1/12 |
Review of Ceramic Composites in Aeronautics and Aerospace: A Multifunctional Approach for TPS, TBC and DBD Applications
by Kateryna O. Shvydyuk
1 [ORCID] , João Nunes-Pereira
1,2 [ORCID] , Frederico F. Rodrigues
1 [ORCID] and Abílio P. Silva
1
Centre for Mechanical and Aerospace Science and Technologies (C-MAST), Universidade da Beira Interior, Rua Marquês d’Ávila e Bolama, 6201-001 Covilhã, Portugal
2
Centro de Física das Universidades do Minho e do Porto (CF-UM-UP), Campus Gualtar, 4710-057 Braga, Portugal
Author to whom correspondence should be addressed.
Ceramics 2023, 6(1), 195-230; https://doi.org/10.3390/ceramics6010012
Submission received: 18 October 2022 / Revised: 21 December 2022 / Accepted: 4 January 2023 / Published: 10 January 2023
Abstract
The quest for increased performance in the aeronautical and aerospace industries has provided the driving force and motivation for the research, investigation, and development of advanced ceramics. Special emphasis is therefore attributed to the ability of fine ceramics to fulfill an attractive, extreme, and distinguishing combination of application requirements. This is impelled by ensuring a suitable arrangement of thermomechanical, thermoelectric, and electromechanical properties. As a result, the reliability, durability, and useful lifetime extension of a critical structure or system are expected. In this context, engineered ceramic appliances consist of three main purposes in aeronautical and aerospace fields: thermal protection systems (TPS), thermal protection barriers (TBC), and dielectric barrier discharge (DBD) plasma actuators. Consequently, this research provides an extensive discussion and review of the referred applications, i.e., TPS, TBC, and DBD, and discusses the concept of multifunctional advanced ceramics for future engineering needs and perspectives.
Keywords:
thermal protection systems; thermal barrier coatings; dielectric barrier discharge; structural ceramics; multifunctional ceramics
1. Introduction
Advanced ceramics are inorganic, nonmetallic solids, basically crystalline materials of rigorously controlled composition and raw materials that are prepared from powdered materials and fabricated into products through the application of heat, which display properties such as hardness, strength, low electrical conductivity, and brittleness [1,2,3]. In this way, the term advanced ceramics refers to high-performance, high-tech, engineered, fine, or technical ceramics, i.e., materials with highly specialized and unique properties capable of outstanding performance under the most extreme conditions and, consequently, able to solve today’s challenges in research, manufacturing, and use.
Concerning high-performance ceramics, a distinction is made between structural and functional ceramics. Briefly, advanced structural ceramics are conventionally best suited in mechanical, structural, tribological, thermal, or chemical load applications, owing to their chemically inert nature, high compression, flexural strength, and toughness, in addition to their high corrosion, wear, and thermal shock resistance. In contrast, advanced functional ceramic applications are based on their functional capabilities ruled by microstructural effects that involve semiconducting, piezoelectric, ferroelectric, pyroelectric, and superconducting properties. Nevertheless, from a chemical composition perspective, two classes of fine ceramics may be identified: oxide and non-oxide ceramics. Oxide ceramics are recognized for properties such as oxide resistance, chemical inertness, thermal non-conductivity, and electrical insulation with a slightly complex manufacturing process. Conversely, non-oxide ceramics are characterized by low oxide resistance, being extremely hard, chemically inert, highly thermally and electrically conductive (due to their covalent bonding), highly energy-dependent to manufacture, and quite expensive [1,2,3].
Considering the versatility of advanced ceramics, this sector comprises different fields inscribed with new challenges in exploring the concept of multifunctional ceramics, which are materials still with unexplored potential, namely [3]:
-
Structural ceramics where enhancement of the mechanical properties (based on affordable raw materials, optimized technologies, and simulations of the complete process chain) as well as exploration of the reliability of the materials (by auxiliary sensor integration for structural health control or even self-healing ceramics) are mandatory.
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Miniaturization and integration density of devices and systems. To this aim, better understanding and control of corresponding changes in specific properties of materials, new testing, and measurement methods are crucial.
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Modeling is a sensitive issue of uplift since complete production chains and faithful multi-scale modeling (digital twins) must be matured for new materials and devices with higher emphasis in cases of coupled (multifunctional) properties.
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Functional ceramics in which defective structure (atomic and electronic) dissemination should be achieved to take advantage of full temperature dependence.
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Functional ceramics and property enhancement allow investigation of multifunctional ceramics exhibiting additive effects, based on the coupling of their properties. These effects are little explored, yet they promise to provide and stimulate scientific and technological advancements henceforward.
To leverage the entire innovative potential of advanced ceramics, new lines of research are needed to guarantee the sustainable development and growth of the advanced ceramic materials market using accessible raw materials and preferably with optimized energy costs. Thus, it is essential to know the properties required for a component subject to multiple functions.
The quest for increased performance in the aeronautical and aerospace industries has provided the driving force for the development of high-temperature ceramics with attractive combinations of thermomechanical properties, oxidation resistance, as well as low-to-moderate density [4]. One of the most common and well-known uses for high-performance ceramics in aviation, rocketry, and space technologies is as part of thermal protection systems. This application of ceramics protects the intended components against hazardous aerothermal environments. Examples of thermal protection systems can be encountered in coatings of various heat-resisting materials for aircraft engine nacelle, thrust reverser fire protection, helicopter cowlings, gas turbine engines, satellites, rockets, and re-entry vehicles [5]. In addition to thermal protection itself, the coating enables higher operating temperatures, consequently increasing, for example, an engine’s combustion efficiency, which in turn reduces consumption and harmful residual emissions. Additionally, another application is shielding against foreign objects. Examples of shielding purposes based on advanced ceramic materials include the conservation and safety of propulsion components from existing particles in the surroundings or residuals resulting from poor combustion processes, space debris, or micrometeoroid particles in the case of a spacecraft or rocket [6].
Lastly, an important field of application of high-tech ceramics is electroceramics, a specific category in which materials are combined with specific characteristics, such as piezoelectric and dielectric properties and corrosion and thermal resistance, for use in aircraft instrumentation and control systems, such as missile guidance systems, satellite positioning equipment, ignition systems, instrument display, and engine monitoring equipment [7].
The presented review is specifically focused on the needs of the aerospace and aeronautical industries and research performed to date to find solutions for three current functions: thermal protection system (TPS), thermal barrier coating (TBC), and dielectric barrier discharge (BDB). In this context, the subsequent three subsections aim to provide an in-depth introduction, contextualization, delineation, and analysis of the role of state-of-the-art advanced ceramics in these three domains.
2. Advanced Ceramics in Aerospace and Aeronautical Engineering
2.1. Thermal Protection System (TPS)
Thermal protection systems (TPSs) play a crucial role in the aerospace and aeronautical industries as they are single point of failure systems that work above all as thermal shields, i.e., a subsystem that protects structures, aerodynamic surfaces, payload of probes, missiles, warheads, and space vehicles from severe aerothermodynamic heating. Accordingly, an effective TPS system must uphold a reliable shield against aerothermal loads, without adding significant weight penalties or compromising the structure of the vehicle. Nonetheless, TPSs also work as structural components and aerodynamic bodies [8,9,10].
The idea of using a protection layer to prevent damage to interior parts of a vehicle dates back to 1920 and is attributed to Robert Goddart, who developed the concept of a heat shield after observing the behavior of meteors entering the Earth’s atmosphere. However, the origin of modern protection systems, as they are known nowadays, can be traced back to the period of World War II. This period, considered the golden age of space flight, ushered countries to invest in developing long-range missiles and rockets capable of leaving the earth’s atmosphere and subsequently reenter to deliver payloads. Several studies were conducted during this period, and it was soon concluded that the vehicles had no capacity for reentering the earth’s atmosphere due to high heat loads and high reentry speeds, as well as a lack of suitable TPS materials. From the mid-twentieth century to date, various TPS technologies have been developed and tested with the aim of ensuring the safety of space vehicles [11].
2.2. TPS Classification
The type of protection on any space-venturing vehicle or, more precisely, on any given area of a vehicle, depends largely on the magnitude and duration of the heat load as well as various operational considerations. In the broadest sense, thermal protection systems can be categorized into three major classes—passive methods, semi-passive methods, and active methods—based on their physicomechanical working principle for thermal management, which can be insulation, ablation, dissipation, or cooling, as shown in Figure 1 [11,12,13]. Uyanna and Najafi (2020) [11] presented a review gathering information regarding TPS methods and materials employed in different space missions throughout the years since the 1950s. Notably, the authors presented a timeframe graph illustrating in-depth the tendencies and preferences of TPSs during the last decades since their conceptualization. More specifically, passive thermal protection systems are the simplest TPS and, as the name itself suggests, have no moving parts. Examples of passive TPS are heat sinks, hot structures, or insulated structures. In turn, semi-passive methods that have been explored and tested for TPS applications, including heat pipes and ablative surfaces. Lastly, convective, film, and transpiration cooling are three different active TPS technologies widely investigated in applications such as rockets and hypersonic vehicle engines. All in all, the correct selection of a TPS includes considering first and foremost the propulsion system of the vehicle, its geometry, and the amount of heat flux on the surface, as well as the time of exposure [8,11,12,14,15,16].
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Figure 1. TPS classification based on their working principle and developed structures of each method.
Despite the provided classification, it is important to emphasize that some authors suggest a different grouping of TPSs. Based on the properties and nature of the application of TPS, a distinction is hence made between a TPS that is reusable (also designated insulative or radiative TPS, i.e., non–ablative TPS) and ablative TPS [13,17,18,19,20,21]. It should be noted that this type of classification is a simplification since dissipation and cooling mechanisms are put aside [8]. To cover every aspect, reusable insulative systems are usually relegated to parts of the vehicle that experience less intense heating during reentry. Reusable insulative systems consist of materials that are mechanically or chemically unchanged by flight mission—no mass variation or composition of the materials occurs during their exposure to the hazard environments—and can be relatively safely flown a number of times, with or without service. In contrast, when the vehicle is subjected to very high heat fluxes, ablative TPS can withstand much higher heat loads through the processes of phase change and mass loss [17,22,23]. Ablative forms of TPS include organic polymers and composites, as well as inorganic polymers/oxides and metals.
The insatiable aspirations of the aerospace industry are the source of intense demand for more efficient and powerful vehicle–structure thermal protection systems. To put it simply, severe operating conditions, including higher temperatures, faster speeds, higher stress, and hostile environments require the constant investigation and improvement of available materials in conjunction with cooling systems for TPS applications [23]. Therefore, the TPS of next-generation hypersonic and reentry space vehicles must offer a combination of suitable properties [22], including high melting point (>3000 °C), high softening temperature, low areal density, low recession rate, high impact resistance, high ablation resistance, ability to withstand radiative heating, superior oxidation resistance, thermal shock resistance, high fracture toughness, high-temperature strength, and low-to-moderate thermal conductivity (depending on the area of application).
2.3. Ceramic Materials for TPS Systems
Advanced structural ceramics play a key role in addressing these challenges considering the vast range of improvements they offer, such as weight reduction, longer lifetime, and thus cost savings. Intuitively, oxide ceramics, such as alumina, zirconia, and mullite, appear to be ideal candidates for high-temperature structural applications due to their high-temperature stability, high hardness, and good corrosion and erosion resistance, together with comparatively low costs. Nevertheless, relatively poor mechanical properties, videlicet, creep, fatigue, fracture toughness, large volume change (generated by phase transformation), and significant grain growth above 1000 °C severely limit oxide ceramics as structural components in high–temperature applications [22]. In contrast, non-oxide ceramics, such as nitrides, carbides, and borides, can achieve high strength and excellent creep resistance at elevated temperatures due to their predominant covalent bonding. Unfortunately, the fundamental drawback of these materials is their susceptibility to oxidation [24].
Thus, to overcome the problems associated with conventional ceramics, ceramic matrix composites (CMCs) were developed to achieve damage tolerance and favorable failure behavior. CMCs are called “inverse composites” because, unlike polymeric or metallic matrix composites, the failure strain of the matrix is lower than the failure strain of the fibers. Hence, under load, the matrix fails first. Overall, long-term high-temperature stability, creep resistance, and oxidation stability properties are sought. In essence, CMCs consist of ceramic fibers or whiskers in a ceramic matrix and interphase generally provided by a fiber coating. Both the fibers and matrix can be made of any ceramic material. The choice of systems with similar matrices and fibers is mainly justified by the need to minimize the residual stress associated with a mismatch between the thermal expansion coefficients of the matrix and reinforcement material. Nevertheless, by carefully combining different ceramic matrix materials with especially suitable fibers, new properties can be created and tailored [25,26].
The most commonly used CMCs are non–oxide CMCs, namely carbon/carbon (C/C), carbon/silicon carbide (C/SiC), and silicon carbide/silicon carbide (SiC/SiC). Hybrid composites and composites with nanostructured reinforcements, such as carbon nanotubes (CNTs) and graphene, have paved the way for further investigations. Cho et al. (2009) [27] reviewed the status of the research and development of CNT-loaded ceramic matrix composite materials, whilst Porwal et al. (2014) [28] provided a comprehensive overview of graphene ceramic matrix composite (GCMC) in comparison to polymer composites. Nevertheless, carbon fiber–reinforced silicon carbide (C/SiC) CMCs are among the most famous composites for high–temperature structural applications.
Wei et al. (2018) [29] focused their research on integrated thermal protection systems (ITPS) comprised of a cellular core sandwich panel and filling insulative material in the core. Compared to using metal sandwich panels, ITPS incorporating CMC sandwich panels gave notable advantages of high-temperature resistance up to 1600 °C and areal density of 17.22–30.56 kg/m2, which was much lower than that of reported ITPSs (23.66–88.84 kg/m2). Notably, Heidenreich et al. (2021) [30] studied the shear properties of C/C–SiC sandwich structure samples based on different core types. The results showed that sandwich samples with fold–cores were preferred because they offered higher specific stiffness and effective shear modulus of up to Geff = 6.4 GPa/(g/cm3) compared to sandwich samples based on grid–cores (Geff = 4.2 GPa/(g/cm3)).
Interestingly enough, Huang et al. (2022) [31] investigated a novel SiC coating with a relatively high crack resistance property, in addition to outstanding thermal shock resistance achieved by means of the pack cementation technique. The improvements verified in the microstructure resulted in superior mechanical capabilities, antioxidation performance (900 °C), and thermal shock resistance (up to 1500 °C).
Despite this broad investigation of CMC materials, ultra-high temperature ceramic (UHTC) materials have been the focus of intensive research in recent years in order to extend the temperature range capabilities of state-of-the-art materials in addition to developing components able to withstand larger and multiple aerothermal–chemical loads. Fundamentally, UHTCs encompass carbides, nitrides, and borides of transition metals, e.g., zirconium diboride (ZrB2), hafnium diboride (HfB2), titanium diboride (TiB2), zirconium carbide (ZrC), hafnium carbide (HfC), and tantalum carbide (TaC), that are characterized by melting points above 3000 °C, high temperature strength, and excellent oxidation ablation resistance. This portends that they can maintain non–ablative properties and structural integrity in hazardous environments above 1800 °C for long periods [6,30].
Among UHTC materials, ZrB2 and HfB2 are the most widely investigated. Opila et al. (2004) [32] reported that the addition of SiC up to 30 vol.% improved both the oxidation resistance and mechanical properties of sintered ZrB2–SiC and HfB2–SiC composites. Likewise, Chamberlain et al. (2004) [33] investigated zirconium diboride ZrB2 composites containing 10, 20, and 30 vol.% of either SiC or molybdenum disilicide (MoSi2) prepared by hot pressing. The results exhibited an improvement in the strength of ZrB2, reaching a maximum of ≈1 GPa at 30 vol.% additives. In particular, SiC additives increased the fracture toughness to 5.25 MPa·m1/2. Overall, the addition of MoSi2 and SiC decreased the oxidation rate when compared to monolithic ZrB2. Later, Zhang et al. (2019) [34] proposed a novel eutectic engineered microstructural design of ZrB2–SiC UHTCs to improve oxidation resistance by means of directional solidification.
Notwithstanding the advances in the TPS materials already obtained, continuous investigation led to follow-up research on the strengthening and toughening of UHTCs. By combining the unique properties of UHTCs with the concepts of CMCs, a new class of materials known as fiber-reinforced UHTCMCs (ultra-high temperature ceramic matrix composite) was developed. This class of materials focused on overcoming the inherent brittleness and poor mechanical resistance of bulk UHTCs. Therefore, UHTCMCs are very promising for applications in extreme conditions and are considered the best candidates for a new generation of high-thermal protection materials [35].
The EU-funded project C3HARME aspires to combine the best features of CMCs and UHTCs to design, develop, manufacture, and test UHTCMCs with self-healing capabilities to be achieved in situ by nanosized ceramic dopants. Within this line of reasoning, Sciti et al. (2018) [36] reported that the preferred matrix was essentially based on ZrB2 enriched with secondary phases and different functionalities.
It is worth noting that hybrid technology, i.e., TPSs combined with TBCs, has paved the way for continuous investigations in this domain in the research community.
In 1998, Gary B. Merrill and Thomas B. Jackson released a method of ceramic high-temperature insulation for ceramic matrix composites under high-temperature and high-heat flux environments. The authors explained the thin thickness drawback of TBCs, as well as their thermal and dimensional instability, dictated by conventional application methods, i.e., air plasma spray and physical vapor deposition. Additionally, the inventors highlighted the prolonged high-temperature exposure and cooling constraints of CMCs. Therefore, ceramic compositions comprising a plurality of hollow oxide-based spheres of various dimensions, a phosphate binder, and at least one oxide filler powder were proposed to insulate CMCs and provide them with erosion and thermal shock resistance [37].
As a follow-up in 2007, Gary B. Merrill and Thomas B. Jackson disseminated a method describing the application of an outer thermal barrier coating to a ceramic matrix composite to offer components and/or structures with the high-temperature stability of ceramics without the characteristic brittleness of monolithic ceramics [38].
In parallel, Zhu (2018) extensively reviewed NASA’s evolution of thermal and environmental coatings technologies. Special attention was given to the application of EBC layers to protect the SiC-based ceramic components in gas turbine engines for high-pressure and high-temperature section components and exhaust nozzles. The author described the core problem of nickel-based superalloys reaching their upper temperature limit and how SiC fiber-reinforced SiC/SiC CMCs are perceived as an alternative next-generation turbine engine hot-section material. Typically, silicon-based ceramics and composites, such as SiC/SiC, are selected for this purpose due to their low density, high-temperature creep strength, and oxidation resistance in dry oxidizing environments. However, EBCs are necessary to prevent the SiC/SiC ceramic matrix composite from water vapor attack in the engine combustion process, originated by the volatilization of the protective SiO2 scales on SiC when reacting with water vapor [39].
Additionally, a study by Zhu et al. (2002) investigating the thermal gradient cycle behavior of thermal and environmental barrier coatings on SiC/SiC CMSs was conducted to develop high-performance ceramic coating systems as well as to simulate coating operation temperatures and stress conditions [40].
Carbon fiber-reinforced UHTC composites, consisting of carbon fibers embedded in a UHTC matrix or C–SiC–UHTC matrix, are also a promising class of materials for surpassing monolithic UHTC materials in terms of fracture toughness and thermal shock resistance. Tang et al. (2016) [41] reviewed this topic, including the design, preparation, and properties of such materials for aerospace applications.
Carbon-reinforced ultra-high temperature ceramic matrix (C/UHTC) composite fabrication processes—hot pressing (HT), chemical vapor infiltration (CVI), polymer impregnation and pyrolysis (PIP), and melt infiltration (MI)—were reviewed by Arai et al. (2019) [42]. In detail, the fracture toughness, thermal conductivity, and recession behavior in an oxidizing atmosphere of C/UHTC were evaluated. It was concluded that Zr- and Hf-based mechanical behavior and thermal conductivity can be tailored by varying their fiber volume fraction and by the formation of a “weak” interface using fiber coatings. Further, MI was pointed out as an efficient approach for the preparation of C/UHTC composites.
An extensive aerothermodynamic characterization of UHTCMCs produced by sintering technology, including ZrB2–SiC matrix reinforced with short random or continuous fiber. was performed by Mungiguerra et al. (2022) [43]. This study tested conditions aiming to reproduce the typical heat fluxes (around 2 MW/m2) and stagnation pressure (around 70 kPa) of a reference re-entry mission with a high amount of dissociated oxygen, i.e., approximately 22 wt.%. All of the materials successfully passed the base qualification and cycling exposure three times, achieving temperatures of approximately 2000–2500 K. The materials developed and tested were ZS–SF and ZSY (53 vol.% ZrB2–SiC matrix, 45 vol.% chopped carbon fibers, with porosity below 2%. The difference between the two samples was the SiC content with respect to the UHTC matrix); ZSY–LF (45 vol.% ZrB2–SiC matrix, 50 vol.% continuous carbon fibers (0°/90° configuration) with porosity of 5%); and CS (baseline C–SiC material loaded with 10 vol.% ZrB2 phase and porosity of approximately 10%). Altogether, the capability of these novel UHTCMCs to maintain their functionality and structural integrity after repeated exposure was confirmed, making them extremely appealing for future reusable TPS applications.
3. Thermal Barrier Coating (TBC)
Thermal barrier coating (TBC) systems are generally explored to enhance energy durability and therefore the efficiency of hot components of aero-engines, gas turbines, and parts for combustion power plants. Thus, TBCs protect the substrate structure by preventing them from experiencing high temperatures and harsh environmental degradation. Consequently, thermal barrier coating, as a surface modification technique, provides resistance to wear, oxidation, thermal shock, and corrosion for prolonged service times and thermal cycles without failure, increasing both the efficiency and lifetime of the desired components [44,45,46]. Ultimately, TBCs are multifunctional systems that provide a wide range of benefits, such as [47]:
shielding of metallic structure,
decreased thermal conductivity,
high thermomechanical stability,
increased exhaust gas temperature,
increased engine power efficiency,
decreased fuel consumption, and
increased lifespan of parts through decreased fatigue and stress.
The concept of “thermal barrier coating” is believed to have been first introduced by the National Advisory Committee for Aeronautics (NACA) and the National Bureau of Standards (NBS) with the publication of the earliest turbine blade-oriented ceramic coatings research entitled “Review of an Investigation of Ceramic Coatings for Metallic Turbine Parts and Other High–Temperature Applications” by W.N. Harrison, D.G. Moore, and J.C. Richmond in 1947 [48,49,50]. The pioneer ceramic coatings for aerospace applications were frit enamels used in aircraft engines throughout the 1950s [50]. With the development of the flame-sprayed ceramic coating technique, further applications included the protection of sheet metal in jet engines and rocket engine thrust chambers. With regard to the materials appraised for TBC purposes, flame-sprayed zirconia-calcia coatings were widely applied to the regeneratively cooled XLR99 thrust chamber for the X-15 experimental rocket planes. In addition, “modern” plasma-sprayed TBCs began to be employed on hot section transition ducts and other hot section sheet metal components in commercial gas turbines in 1970 [51]. Most recently, the microstructure of thermal barrier coatings, the materials applied, the coating preparation technologies, and the failure mechanisms, as well as lifetime prediction models, have all been part of the different branches of extensive investigation [48].
3.1. TBC Structure, Fabrication Techniques, and Failure Mechanisms
A great deal of effort has been devoted over the past few decades so that TBCs systems could enable higher operating temperatures and reduce cooling systems costs, thus improving the overall capabilities and effectiveness of components [47]. Irrespective of the evolution achieved, the stability of TBC systems continues to be a major concern of the scientific community. During operating service, TBCs are exposed to complex phenomena, such as thermomechanical stress, corrosion by foreign objects, erosion, diffusion, oxidation, phase transformation, and sintering [46]. In essence, the TBC is a complex, multilayered, and multi-material coating system composed of (1) a top coat, (2) a metallic bond coat, (3) a thin thermally grown oxide (TGO) layer, and (4) a superalloy substrate (structure), as depicted in Figure 2 [45,47].
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Figure 2. Schematic illustration of a traditional TBC system (not to scale).
The ceramic top coat also referred to in the literature as the “TBC layer”, is generally, as the name itself implies, a ceramic material layer that provides, more importantly, thermal protection to the substrate, but also strain tolerance and thermal shock resistance for components through reduction of heat transfer. Consequently, to decrease the temperature of the metal substrate, this top coat should have essentially low thermal conductivity [44,45,47]. A state-of-the-art TBC top coat material is yttria-stabilized zirconia (YSZ) composed of ZrO2 with 7–8 wt.% Y2O3 because it has excellent thermomechanical properties, such as [44,45,52]:
very high mechanical strength,
very high wear resistance,
very high erosion resistance,
high impact resistance,
high corrosion resistance,
high chemical resistance,
very low thermal conductivity, and
relatively high coefficient of thermal expansion when compared to other ceramics.
Following this, the TGO layer is created via the diffusion of oxygen from the bond coat through the top coat of metallic elements during manufacturing and operation processes. The TGO layer acts as a protective layer to retard further thermal and oxidation diffusion. Nonetheless, the TGO layer may increase the internal stress in the TBC system, hence potentially originating cracking at the interface between the bond and top coats. This phenomenon may eventually lead to the unwanted spallation or delamination of the top coat in service [46,47,53].
Lastly, the metallic bond coat acts as a precoating interface between the substrate and top coat, aiming to protect the superalloy substrate from oxidation and corrosion, increasing the adhesion between the layers, and ultimately guaranteeing the structural integrity of the coating by matching the thermal properties and stress between the substrate and ceramic coating [46,47]. Two types of metallic bond coats are common. The first-generation bond coat of platinum (Pt)-modified aluminide is recognized for having good stabilization and adhesive strength of the coating by reducing inter-diffusion between the coating and substrate layers. Unfortunately, Pt is an expensive component and it does not possess desirable mechanical robustness at high temperatures [48,54]. Another second-generation bond coat consists of the MCrAlY coatings. These include NiCrAlY, CoCrAlY, and NiCoCrAlY, which have good oxidation and hot corrosion resistance [44,48,55]. Moreover, these compositions can be enhanced by adding Ta (Tantalum), Nb (Niobium), Re (Rhenium), Hf (Hafnium), Zr (Zirconium), and/or other components to improve the high-temperature performance, extend the lifespan, and match specific requirements [44,48].
Concerning the fabrication techniques of TBCs, several different methods are known and ready to be used, e.g., atmospheric plasma spraying (APS) and electron beam physical vapor deposition (EB-PVD), whereas others are promising candidates to achieve better results in the near future, such as plasma spray physical deposition and vapor deposition technology. Plasma-sprayed (PS) TBCs were proposed in the 1960s. Subsequently, several different variations of this technique appeared, including APS, low-pressure plasma spraying (LPPS), solution precursor plasma spraying (SPPS), vacuum plasma spraying (VPS), and protective atmosphere plasma spraying (PAPS).
Among them, APS and LPPS are the two main methods utilized in TBC deposition since they are characterized by low cost, rapid deposition rate, high efficiency, and easy management [48]. APS and LPPS are distinguished by their complex horizontal laminated structure [55]. Substantially, the acceptable porosity of APS TBCs lies in the range of 10–15%, which is essential for high strain compliance and effectively further reduces thermal conductivity. Despite the advantages that these methodologies offer, inter-lamellar pores, microcracks, and microstructural defects give way to the possibility of delamination and spallation. Therefore, APS and SPPS are useful for structures with large volumes and weaker mechanical properties needed, i.e., combustion chambers and stator vanes [47,48,56,57].
In the 1980s, the focus shifted to TBC deposition techniques by EB-PVD, which was quickly popularized with the advent of low-cost EB-PVD equipment in the 1990s. In 1994, Thomas E. Strangman disclosed a methodology to provide a superalloy substrate with a TBC coating that included a ceramic layer resistant to sintering during high-temperature gas exposure. Additionally, the protective ceramic layer was shown to have a columnar microstructure due to the electron beam physical vapor deposition procedure [58]. Overall, EB-PVD coatings exhibit excellent aerodynamic properties, with better surface roughness, and they do not block fine cooling holes. EB-PVD coatings exhibit a columnar morphology within randomly distributed multi-scale porosity, as well as a thin layer in the form of equiaxed grains near the interface between the bond and ceramic top coat. This microstructure improves the TBC system’s strain tolerance and thermal shock resistance, in addition to relaxing the thermal expansion mismatch stress. Nonetheless, EB-PVD TBCs unfortunately have higher thermal conductivity and lower thermal insulation than APS TBCs [47,48,56,57].
Figure 3 illustrates the contrast in the coating produced by the EB-PVD process which, as mentioned, exhibits a columnar morphology whereas the coating deposited via APS exhibits a lamellar morphology.
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Figure 3. Photographs of (a) APS TBC showing a laminar morphology and (b) EB-PVD TBC showing a columnar morphology (Adapted from Beele et al. (1999) based on ref. [59] with the permission of Elsevier).
For the sake of completeness, as explained, a critical property of solid materials is surface morphology. The microstructural properties are an important link between material processing and their performance. Therefore, microstructure quality control is essential for all material processing routes [60].
Apart from their morphology, the functional properties of materials are governed by the composition considered [61]. Therefore, an increasingly growing development in the literature of functional coatings is based on a layered, also known as graded, functional coating architecture, since one individual layer or the complete thin film system may simultaneously fulfill several functions, among them mechanical, thermal, and electrical. Consequently, it is increasingly important to assess and optimize the TBC layer characteristics not only individually, but in their complexity, with respect to their metallic compatibility and designed applications [62].
In view of reducing the weight of structural elements, Kaczmarek et al. (2013) studied the deposition of carbon coatings at room temperature by pulsed laser deposition, which is a promising methodology for mono- and multilayer coatings even at temperatures below 100 K. The authors presented a study on the influence of pressure and composition of the processing atmosphere parameters on the deposition of carbon coatings with a titanium interlayer on an aluminum alloy 7075 substrate [63].
Within the same framework of thought, Deng et al. focused on the design, fabrication, and characterization of deposited graded thermal barrier coatings. More specifically, the authors focused on the assessment of adhesion strength and thermal conductivity of functionally graded YSZ coating by fabricating 2 μm thick lab-scale YSZ coatings. A continuously varying composition profile was produced using the dual-beam pulsed laser deposition method on stainless steel 316 L [64].
TBC failure can occur in a multitude of ways depending on the TBC system and the service conditions due to the sheer complexity of the interactions between the three primary layers described. It is important to note that all of these layers have distinct physical, mechanical, and thermal properties. When the word “failure” is applied in the TBC context, it implies that the coating is no longer capable of fulfilling its functional requirements. Simply put, when the top coat spalls due to, for example, fatigue, corrosion, or erosion, the TBC is considered unfit and has “failed” [65,66].
Generally, damage in TBCs may result from thermal shock and gradients, sintering, phase transformation, oxidation, external mechanical damage, calcium–magnesium–aluminosilicate (CMAS) attack, corrosion, as well as environment-induced erosion. Nevertheless, the different existing failure types of TBC structures, such as thermal fatigue, corrosion, and erosion, are the basic damage mechanisms [65,66]. Thermal fatigue-based failure mechanisms are related to impairment of the TBC structure motivated by cyclic thermal stresses due to temperature oscillations; corrosion-based failure mechanisms are responsible for destabilization of the coating by the acceleration of oxidation and/or mechanical damage; whilst erosion-based failure mechanisms of the top coat may happen thanks to the impact of abrasive particles existing in the environment on the coating surface. Smaller particles are normally the origin of erosion, whilst, in contrast, larger particles are the cause of so-called foreign object damage. Figure 4 presents a schematic illustration of the major drives of material failure in TBC structures when subject to harsh environments.
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Figure 4. Different damage types that occur to TBCs in hazardous environments.
3.2. Ceramic Materials for TBC Systems
The selection of effective materials for TBC applications is highly restricted by many desirable properties, such as [54,67,68,69]:
high melting point,
crystalline phase stability in the operating temperature range,
chemical inertness,
low thermal conductivity,
low thermal diffusivity,
thermal shock resistance,
no oxygen transparency (i.e., impermeable),
good adherence to the metallic substrate,
low sintering rate of the porous microstructures,
thermal expansion matches with the metallic substrate.
Table 1 summarizes the major material requirements for thermal barrier coatings and briefly explains their importance [70].
Table 1. Material requirements for the ceramic top coat of thermal barrier coatings [70].
Table
Consequently, the number of materials that can be used as TBCs is very limited. To date, only a few ceramics have been found to satisfy the majority of these requirements. Naturally, a single compound ceramic can hardly meet all the requirements for TBC applications; therefore, the combination of two or more ceramic materials becomes mandatory. Among the properties referred, special attention should be paid to thermochemical stability, thermal conductivity, as well as the thermal expansion coefficient.
Yttria-stabilized zirconia, YSZ, is the most successful top coat ceramic material and considered an industry standard. The development of YSZ started way back in the 1970s and continues to dominate the TBC field. The main reason behind this truth is that YSZ has a considerable number of features that make it an attractive top coat, including relatively low density, high strain tolerance, high fracture toughness, a high coefficient of thermal expansion, and low thermal conductivity attributed to its high concentration of point defects, ability to relax stress caused by compatible CTE, and high resistance to thermal shock when compared to other ceramic top coats, as well as thermochemically compatible with the protective TGO. Hence, a superior successor to YSZ has not yet been developed [54,67,68,69].
Better performance of YSZ is typically achieved by varying the Y2O3 content from 6 to 8 wt.% in ZrO2. This improves both the thermal and mechanical properties, i.e., high melting point, low thermal conductivity, and a high thermal expansion coefficient are obtained. Fundamentally, yttria is added to zirconia to stabilize its phase at high temperatures. Pure zirconia is allotropic; it exhibits a monoclinic structure up to 1170 °C, a tetragonal structure in the temperature range of 1170–2370 °C, and a cubic structure up to its melting point at 2690 °C. The phase transformation of zirconia from tetragonal to monoclinic is martensitic and leads to a significant volume expansion of approximately 4–6%. This is sufficient to damage the mechanical integrity of the coating, which is a serious concern, due to fatigue failure when the coating is subjected to repeated thermal cycle and thermal expansion mismatch. Yttria, when added to zirconia in the range of 7–8 wt.%, forms a non-transformable tetragonal prime (t’
) phase. This phase is stable up to 1200 °C, above which a phase transition causes catastrophic delamination of the top coat [65,68,69].
For this reason, YSZ has a functional operation limit of approximately 1200 °C, which means that YSZ barrier coatings are unreliable for long-term use at temperatures of over 1200 °C due to their catastrophic phase transformation, which in turn escalates thermal conductivity and boosts spallation in the TBCs. In addition, sensitivity to hot corrosion, sinterability, and accelerated TGO formation caused by extremely high ionic oxygen diffusion in ZrO2–based ceramics also restricts YSZ usage. Early studies suggested a double layer coating design of the top coat to minimize the delamination phenomenon by compensating for thermal expansion mismatch, whereas some authors claimed that changing the stabilizing oxide or adding components in YSZ, such as aluminum oxide, calcium oxide (CaO), titanium dioxide (TiO2) and magnesium oxide (MgO), was critical in overcoming the limited phase transition of YSZ [65,68].
As a result, many parallel investigations have been carried out exploring alternative materials to YSZ. Substantially, two approaches may be considered: (1) development of new structural coating materials with higher temperature resistance, i.e., advanced multicomponent oxide-doped ZrO2 and HfO2 (hafnium dioxide or hafnia) solid solution-based ceramics; and (2) multi-element modification and optimization of YSZ materials. From the literature, it is generally accepted that defect cluster TBCs, crystal structures of perovskite, pyrochlore, and lanthanum compounds, are regarded as potential materials for advanced TBCs. Some other materials, such as mullite, silicates, and garnet, were also considered candidate materials through the years, but, regrettably, their typical low CTE precludes the likelihood of their implementation [48,54,67,68,69,71].
3.3. Defect Cluster TBCs
As mentioned, many new TBC materials have been proposed to achieve the low thermal conductivity and high-temperature capability mandatory for a coating system. In particular, ZrO2 and HfO2 solid solution-based ceramic coatings co-doped with Y2O3 as a primary stabilizer and additional paired rare-earth (RE) cluster oxides were suggested to offer significantly better features. Defect cluster TBCs possess much lower thermal conductivity, and, consequently, confer better thermal stability to the material since the resulting point defects are thermodynamically stable. In addition, they have better sintering resistance at high temperatures than the state-of-the-art YSZ due to the reduction of effective defect concentration and the increase in activation energy by clustering [72].
Zhu et al. [73] studied the conventional oxidation behavior of advanced multicomponent oxide-doped zirconia–based TBCs designed based on the defect cluster concept. The aim was to report the furnace cyclic oxidation performance of plasma-sprayed multicomponent rare-earth (RE2O3) oxide-doped zirconia thermal barrier coatings as a function of dopant concentration and processing variation. The analyzed coatings were ZrO2-based oxides, stabilized with the primary yttria, Y2O3, dopant, and/or paired Group A and Group B RE oxide co-dopants. Group A dopants consisted of neodymium(III) oxide (Nd2O3), gadolinium(III) oxide (Gd2O3), and samarium(III) oxide (Sm2O3), whilst Group B dopants were ytterbium(III) oxide (Yb2O3) and scandium(III) oxide, also known as scandia (Sc2O3). The results showed that the tested multiphase coatings had significantly lower thermal conductivities and better thermal stability, mainly in the lower total dopant concentration (which varied between 4.5 and 52.5 mol.%) compared with ZrO2–8 wt.% Y2O3 coating. The defect cluster coatings consisted of a 180–250 μm
thick ceramic top coat, a 120 μm thick NiCoCrAlY or NiCrAlY bond coat, and finally, a 3.2 mm
thick nickel superalloy. It was proven that the oxide defect cluster had the potential to achieve better cyclic performance than the binary ZrO2–Y2O3 coatings owing to their high-temperature stability, reduced grain growth, and increased toughness. Nevertheless, it was pointed out by the authors that the cyclic lifespan of the ceramic coating generally decreased as the dopant concentration increased due to the reduced fraction of the tetragonal phase and increased fraction of the cubic phase. The fully stabilized cubic phase normally showed an enhanced grain growth behavior and also lacked the additional grain-refining and toughening mechanism of the tetragonal to monoclinic phase transformation, which was present in a partially stabilized tetragonal phase. Therefore, in the high-dopant-concentration coating, a very low toughness of the coating structure was distinguished.
In subsequent research, Zhu et al. (2005) [74] similarly proposed advanced alternative oxide ceramic compounds, a low-conductivity and high-stability TBC based on an oxide defect-clustering design approach, but obtained this time by applying a laser high-heat-flux thermal conductivity technique. The laser test approach emphasized real-time monitoring of the coating conductivity at high temperatures to evaluate its performance under engine-like heat-flux and thermal gradients. Briefly, the advanced oxide coatings were designed by incorporating multicomponent, paired-cluster rare–earth oxide dopants into conventional zirconia– and hafnia–yttria oxide systems. The dopant oxides were selected by considering their interatomic and chemical potentials, lattice elastic strain energy, polarization, as well as electroneutrality within the oxides. Selected oxide cluster TBC systems, including ZrO2–Y2O3–Nd2O3(Gd2O3Sm2O3)–Yb2O3(Sc2O3), were synthesized and their conductivity and sintering behavior were investigated. The tests aimed to essentially promote the production of thermodynamically stable, highly defective lattice structures and/or nanoscale ordered phases that would in turn reduce the oxide coating thermal conductivity and improve the coating sintering resistance. The study highlighted the conclusion that despite a similar trend between the advanced oxide cluster coatings and the binary ZrO2–Y2O3 coatings in the furnace cyclic behavior (where, as discussed, the cyclic lifespan generally decreased with the increase in total dopant concentration), the oxide cluster coatings showed promise to have significantly better cyclic durability (comparable to that of zirconia—4.55 mol.% yttria) than the binary ZrO2–Y2O3 coatings with equivalent dopant concentrations.
Further improvements are expected in defect cluster TBCs by utilizing advanced coating architecture design, dopant type and composition optimization, and improved processing techniques [72,73,74].
3.4. Perovskites
Perovskite oxides are a class of ABO3 crystal structure that can accommodate a wide variety of ions in a solid solution, including ions with large atomic mass. Perovskite-type oxides have been favored by researchers because of their enthusiastic structure features and properties, especially ABO3 (A = Ca [calcium], Sr [strontium], Ba [barium]; B = Zr, Ti [titanium], Ce [cerium]) perovskites. The major advantage of using perovskite oxides as thermal barrier coatings is their 20% lower thermal conductivity than YSZ, which provides good thermal stability at high temperatures [68,75,76,77,78].
In greater depth, materials exhibiting a perovskite structure have attracted much attention as YSZ replacements mainly due to their high melting point (higher than 1800 °C), high thermal expansion coefficient (higher than 8.5×10−6 K−1
), relatively low thermal conductivity (lower than 2.2 W/mK
), and low Young’s modulus (approximately 210 GPa). The biggest drawbacks of materials exhibiting a simple perovskite structure are mainly their inferior fracture-related mechanical properties, as well as the partial evaporation of constituents of the perovskite phase during plasma spraying process. This leads to impurity phases in the coating, that, in turn, often have detrimental effects on the coating performance [76,79]. Perovskites offer the possibility of extensive substitution of ions at the A or/and B site, thus allowing their properties to be tailored towards specific requirements [80]. The well-known and studied simple perovskite oxide materials for TBC applications include strontium zirconate (SrZrO3), barium zirconate (BaZrO3), and calcium zirconate (CaZrO3) [81].
The early candidate for TBC applications, BaZrO3, first attracted attention due to its high melting temperature of 2600 °C; however, both a relatively poor thermal expansion coefficient and interior chemical stability induced the coating to failure in the course of thermal cycle tests, minimizing the TBC service lifetime [77]. Contrarily, SrZrO3 exhibits better performance on these cyclic tests with surface temperatures above 1250 °C with respect to its high melting temperature (2800 °C), relatively low thermal conductivity, and high thermal expansion coefficient of ≈11×10−6 K−1
(30–1000 °C). Therewith, both the sintering rate and Young’s modulus of SrZrO3 are lower than those of YSZ, which is of assistance to favorable mechanical responses [77,80,82]. Unfortunately, this perovskite has been reported to suffer temperature-induced phase transformation that has a detrimental effect on the performance of this type of TBC material. Some studies highlighted that such transformation could be suppressed by doping gadolinium oxide (Gd2O3) or ytterbium(III) oxide (Yb2O3) in addition to enhancing the thermophysical properties of the coatings [82].
CaZrO3, on the other hand, is the latest material to be considered for TBC application in this group. Although its melting temperature is lower than that of YSZ, it has an encouraging thermal conductivity of approximately 2 W/mK and excellent mechanical properties [78,82,83]. For instance, Garcia et al. (2008) [83] carried out a comparative study of CaZrO3 coatings prepared by air plasma and flame spray processes. The results showed that the two techniques produced coatings with different microstructures and thus properties. All of the coatings were porous, but the flame-sprayed coatings exhibited interplast cracks whereas the atmospheric plasma-sprayed coatings had larger round pores. Nevertheless, all of the CaZrO3 coatings showed very low thermal conductivity.
Generally speaking, perovskites may be subdivided into the discussed zirconates and complex forms. Under the concept of compositional control of properties, complex substituted structures have also been a focus of studies as YSZ substitutes. In particular, complex forms with an A(B’1/3B’’2/3)O3 structure, such as BaLa2Ti3O10, Ba(Mg1/3Ta2/3)O3, and La(Al1/4Mg1/2Ta1/4)O3, have promising bulk properties for TBC applications [69,77,82]. However, the thermal expansion coefficients of these materials remain lower than those of substrates and bond coats, leading to thermal stresses in TBC systems. Moreover, relatively low toughness values are observed. As a result, the thermal cycling properties are poorer than those of YSZ coatings and further improvements are necessary [71]. The application of complex perovskites as TBCs was considered by Jarligo et al. (2009) [76], concluding that these compounds show promising TBC performance since the means to control the propagation of interfacial cracks from TGO along the interface of the coatings was proven possible.
3.5. Pyrochlores
The group of pyrochlore-structured oxide compounds have gained gradual importance as advantageous ceramic top coats to replace the state-of-the-art YSZ. The features that have drawn special attention is their distinctive arrangement of ions and vacancies within the AxBxOz
(or also sometimes found in the literature as A2B2O7) compositional structure where the first metal cation A is a rare earth element, typically a lanthanide such as lanthanum (La), gadolinium (Gd), neodymium (Nd), yttrium (Y), etc., and the second metal cation is Zr, Hf, titanium (Ti), or molybdenum (Mo). The vacancies at the A3+, B4+ and O2- sites make the composition flexible to achieve attractive material properties by incorporating other RE elements [84]. Materials with pyrochlore structures show excellent thermophysical properties, i.e., high melting point, stable phase conditions and morphology at temperatures up to 1400 °C, a relatively high coefficient of thermal expansion, low thermal conductivity, and pronounced CMAS resistance, making them suitable for applications as high-temperature thermal barrier coatings. Conversely, the lower thermal expansion coefficient (9–10×10−6 K−1) than that of YSZ (10–11×10−6 K−1) may lead to higher thermal stresses in the TBC system as both substrate and bond coat have higher thermal expansion coefficients (approximately 15×10−6 K−1
) [84,85,86,87].
Among pyrochlore materials, lanthanum zirconate, LZ (La2Zr2O7), gadolinium zirconate, GZ (Gd2Zr2O7), cerium zirconium oxide (Ce2Zr2O7), samarium dititanium oxide (Sm2Ti2O7), dilanthanum dihafnate (IV) (La2Hf2O7), and neodymium zirconate (Nd2Zr2O7) are especially interesting candidates [87].
Specifically, La2Zr2O7 seems to be one of the most promising pyrochlores for TBC application due to its outstanding bulk properties compared to standard YSZ, with a high thermal stability up to 2000 °C
, low thermal conductivity of 1.56 W/m K, and eminent sintering resistance. A major drawback, however, is the relatively low thermal expansion coefficient of approximately 9×10−6 K−1 compared to YSZ with 10–11×106 K−1, which leads to higher thermal stress from thermal expansion mismatch and poor toughness that lowers the thermal cycling lifetime of LZ as a TBC. In this regard, the higher thermal expansion coefficient of Gd2Zr2O7 (1.1×106 K−1
) is advantageous [84].
To overcome the thermal cycling lifetime issue of LZ pyrochlore, Vaßen et el. (2004) [87] suggested a pyrochlore/YSZ double layer systems based on La2Zr2O7 and Gd2Zr2O7 pyrochlores. The results showed similar performances to YSZ coatings at temperatures below approximately 1300 °C. At higher temperatures, however, the double-layer system coatings revealed excellent thermal cycling behavior, i.e., at the highest test conditions, the lifetime was orders of magnitude better than that of YSZ coatings. In another study, Bansal et al. (2007) [88] focused on lowering even further the thermal conductivity of pyrochlore oxide compounds. An oxide doping approach was used where part of cation A was substituted by other cations, e.g., A2−xMxB2O7
(where x = 0–0.5 and M
= RE or other cation) in the pyrochlore materials. Pyrochlore oxide powders of various compositions were synthesized using the sol-gel process and hot-pressed into 2.54 cm diameter discs, whereas the thermal conductivity was measured using a steady-state laser heat flux test. It was concluded that the performed investigation was successful since doping with RE cations at the A sites in the La2Zr2O7 (A2B2O7) pyrochlore greatly reduced the thermal conductivity.
Yang et al. (2018) [89] investigated and synthesized the pyrochlore-related Sm2FeTaO7 compound as a potential material for TBC top coat with low thermal conductivity, better mechanical properties, and high-temperature phase stability. It was concluded that the compound had low thermal conductivity (approximately half of YSZ) due to a complex and distorted crystal lattice, high concentration of defects, and large differences in the atomic masses of cations. Lastly, Che et al. (2021) [90] studied the sintering behavior of nanostructured pyrochlore-type La2(Zr0.7Ce0.3)2O7, designated as LZ7C3. It was proven that the novel LZ7C3 compound exhibited significantly higher sintering resistance than the host La2Zr2O7 and typical 8YSZ at temperatures up to 1773 K.
3.6. Hexaaluminates
Two important thermophysical properties influence the lifespan of TBC materials: thermocycling and thermal shock resistance. These parameters are mainly influenced by the microstructure, the coefficient of thermal expansion, and the aging behavior of the TBC material [91]. Lanthanum hexaaluminate (LHA) with a magnetoplumbite structure has proven to be a promising competitor to the state-of-the-art yttria stabilized zirconia as a TBC material, especially bearing in mind that most zirconia-based coatings age significantly due to thermal loads and thus include undesired densification at temperatures exceeding 1200 °C. Fortunately, in contrast to zirconia, lanthanum hexaaluminate permits operating at high temperatures owing to its high-temperature thermal stability (up to 1600 °C) and electrical insulating properties. In addition to these features, LHA particularly possesses a high melting point, high thermal expansion, low thermal conductivity, high fracture toughness, and outstanding sintering resistance since such kinds of oxides usually crystallize in hexagonal platelet-like grains [91,92]. LHA materials have both superior thermochemical and thermophysical characteristics, which grants them an attractive thermal cycling lifespan and makes them a sublime candidate material for TBC application. As a last remark, oxides with a magnetoplumbite structure have a nominal composition of LnMAl11O19 (Ln = La to Gd; M = Mg [magnesium], Mn [manganese] to Zn [zinc], Cr [chromium] or Sm [samarium]) [93]. Some specific examples of investigated LHA materials for TBC applications include LaMgAl11O19, LaZnAl11O19, and LaTi2Al9O19 [94].
Among LHA materials, LaMgAl11O19 (also known as LaMA or LMA) has been the most widely studied during the last decades. LaMA with a magnetoplumbite-type structure displays like its analogues a high thermochemical stability, superior sintering resistance, and high fracture toughness [95]. Furthermore, LaMA single-layered coating exhibits a thermal cycling lifetime similar to the traditional YSZ coating. However, the relatively lower CTE than traditional YSZ in combination with the recrystallization behavior that reduces the bond strength between the ceramic coating and bond coat results in the single LaMA coating being less durable under higher service temperatures. The recrystallization issue forces LaMA to have some shortcomings [96]. On the one hand, the ED-PVD LaMA coating, usually characterized by the columnar structure with a superior strain tolerance and service lifetime, has shown difficulties in being successfully prepared, whereas partial decomposition of LaMA oxide usually occurs in the APS LaMA coating, originating a large amount of amorphous phase due to rapid quenching from the molten state. Therefore, subsequent recrystallization of the coating during high temperature service may compromise the reliability of the LaMA layer, in terms of variation of the heat capacity of the material, that consequently will have a strong influence on the thermal conductivity and CTE, giving rise to the formation of residual stress [68].
Chen et al. (2011) [97] approached this problem and investigated the thermal aging behavior of plasma-sprayed LaMgAl11O19 thermal barrier coatings. LaMA powders were synthesized using a solid-state reaction method. La2O3, γ-Al2O3, and MgO were selected to be the starting materials. The results showed that the recrystallization and grain growth rates could be significantly accelerated when LaMA coating was isothermally aged at temperatures above 1173 K. The well-crystallized LaMA coating exhibited improved properties, such as reduced microhardness, with consequently enhanced strain tolerance and thermal shock resistance, as well as CTE and heat capacity close to its bulk counterpart.
Furthermore, to overcome the mentioned drawbacks of LaMgAl11O19—relative lower CTE than traditional YSZ in combination with the recrystallization behavior—double ceramic top coat TBCs based on the LaMA/YSZ system were studied. For instance, Chen et al. (2011) [98] evaluated the thermal cycling failure of LaMgAl11O19/YSZ double ceramic top coat material, and the weak bond strength at the interface of LaMA and YSZ were addressed with the help of two different types of LaMA/YSZ composite coatings. The results exhibited improved strain tolerance and thermal cycling lifetime in comparison to single layer YSZ and LaMA coatings. It was noted that specific crystal chemistry in addition to the nano-crystallization of the LaMA coating induced by recrystallization during thermal cycling also made contributions to further enhance the LaMA layer-containing LaMA/YSZ double ceramic TBCs.
On top of that, functionally graded thermal barrier coatings systems based LaMgAl11O19 and YSZ designed and prepared by APS were introduced to improve the durability and temperature capability of LaMA top coat materials. Thus, Chen et al. (2012) [99] prepared a new five-layer quasi-gradient functionally graded thermal barrier coating based on LaMgAl11O19/YSZ, of which the microstructure, thermal, and mechanical properties were investigated. It was proven that the burner-rig thermal cycling lifetime increased by approximately 50% in comparison with the double-layered TBCs of the same ceramics. More recently, and to further understand the factors related to thermal cycling lifetime, Chen et al. (2020) [100] analyzed three multilayered TBCs similarly based on LaMA/YSZ, but this time with different variations in composition and thickness of the intermediate gradient layers.
Table 2 provides as a summary of the properties, discriminated as “advantages” and “disadvantages”, of the materials and categories of materials discussed throughout this last subsection.
Table 2. Summary of parallel research on material alternatives to YSZ that could be used as TBCs based on ref. [68,69].
Table
4. Dielectric Barrier Discharge (DBD)
The dielectric barrier discharge (DBD) mechanism was introduced more than a century and a half ago and its research continues to suffer ongoing technological developments as well as industrial exploration on a large scale [101,102,103].
Dielectric barrier discharge (also known as barrier discharge, silent discharge, or ozonizer discharge) is a simple device that is ignited by applying a high voltage—both at low and atmospheric pressure—between two electrodes wherein at least one of the electrodes is insulated by a dielectric [102,104,105]. Typically, dielectric materials of low dielectric loss and high breakdown strength are used, including glass, quartz, ceramics, enamel, mica, plastics, silicon rubber, Teflon, or Kapton [102,103,106].
By using an insulator, which works in the same manner as a capacitor, many fine plasma filaments are usually formed between the electrodes, and the formation of a spark or an arc discharge is thus prevented [104,105,107,108]. Specifically, with increasing pressure and neutral gas density, gas discharge has the tendency to become non-uniform, unstable, and constricted. Consequently, a glow-to-spark/arc transition occurs. Thereby, it is fundamental to accurately design and control some parameters, such as the use of special geometries, electrode arrangements, excitation methods, and other techniques to obtain non-equilibrium plasmas at elevated pressures [109]. It is worth emphasizing that these filaments (also called microdischarges) have a random distribution over the dielectric surface, as well as a very short lifetime, more precisely in the range of a few nanoseconds. This occurs due to the accumulation of charge carriers on the dielectric surface, which generates an opposite field to the externally applied voltage, so that the discharge disappears again. Additionally, in DBDs, microdischarges are observed in every half cycle of the applied voltage [110,111].
The main aspects that influence the general performance of dielectric barrier discharge plasma actuators, are (a) geometry (configuration), (b) dimensions of the electrodes, (c) gap between the electrodes, (d) dielectric thickness, (e) dielectric material, (f) applied voltage, (g) voltage waveform, and (h) AC frequency. The influence of these parameters is highly nonlinear and interdependent, which regrettably makes it more difficult to design, optimize, and mathematically model these devices. For this reason, the continuous investigation of dielectric barrier discharge plasma actuators is crucial to guide their future implementation for several applications. Despite the above mentioned parameters, the most important characteristic of DBD devices is that non-equilibrium plasma conditions can be provided more simply than those based on other existing alternatives—for instance, low-pressure discharges, fast pulsed high-pressure discharges, or electron beam injections. The flexibility of DBD concerning geometrical configurations, operating medium, and operating parameters is unprecedented [112]. Therefore, advantages such as low costs associated with the construction of reactors, the low-frequency power supply needed, as well as easy scalability by numbering-up, make dielectric barrier discharge an attractive and easily adaptable technology for particular desired applications [104,106].
The literature indicates that DBDs are predestined for a large volume of applications, including ozone and UV generation, plasma display panels of large-area flat television screens, pollution control by air and wastewater treatment, sterilization of packing and food, as well as activation, cleaning, etching, and coating of surfaces [104,111,112,113].
Figure 5 summarizes and highlights some of the main aspects that influence the general performance of DBD plasma actuators.
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Figure 5. Surface DBD plasma actuator impact performance parameters [114,115,116,117,118,119,120,121,122,123,124,125,126,127,128,129,130,131,132,133,134,135,136].
4.1. DBD Actuator Classification System
Based on the described general working principle of the DBD mechanism, as well as the different existing applications, dielectric barrier discharge designs may be divided into two main categories. It is noteworthy that the classification of configurations is distinguished by the presence and usage of the insulating material in the discharge path [102,106].
If the space between the electrodes includes both a dielectric and discharge gap, the plasma is therefore ignited in the volume existing between the two electrodes. In that case, the DBD is considered a volume dielectric barrier discharge, or VDBD geometry.
Otherwise, if the space between the electrodes is completely filled by a dielectric, the plasma is consequently ignited on the surface of the dielectric exposed to the gas volume. In that case, the DBD is named a surface dielectric barrier discharge, or SDBD geometry.
It is important to highlight that SDBDs, as shown in Figure 6, have been considered for space applications for decades. Since the 1990s, surface dielectric barrier discharge actuators—which are non-equilibrium plasma devices capable of generating forces in air without any moving parts—have been considered an engaging technology in the aerospace sector.
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Figure 6. Schematic representation of surface DBD (SDBD) plasma actuator configuration.
In 1993, Roth et al. disclosed methods and an apparatus for generating low-power density glow discharge plasmas at atmospheric pressure. To generate the plasma discharge, the inventors used a pair of electrically insulated metal plate electrodes mounted face-to-face in parallel or uniformly spaced alignments covered with dielectric insulation. Although this invention considered an arrangement similar to the conventional plasma actuator configuration, it did not disclose the possibility of performing active flow control operations [137].
Only in 2004, Enloe et al. disclosed the conventional plasma actuator configuration and explained that this device enabled partial ionization of gases using one or more electrode pairs, each having one electrically encapsulated electrode and one air stream exposed electrode that was energized by a high-voltage alternating current waveform. They also mentioned that this invention was particularly appropriate for use in airfoils for aerodynamic purposes, such as drag reduction, stall elimination, and airfoil efficiency improvement [138].
Overall, the engaging features of DBDs, such as their mechanical simplicity, light weight, planar and low drag-structure, as well as relatively low-performance power level consumption make them a great choice for, for example, aerodynamic flow control and aircraft propulsion investigations [113]. It is noted that various adaptations of DBDs to different problems have directed this technology towards the adoption of several special configurations. Just to name a few, DBD-based plasma jet actuators, multiple encapsulated electrode plasma actuators, nanosecond-DBD (i.e., NS-DBD), sliding discharges, and capillary plasma electrode discharges have been demonstrated and applied in numerous studies over the last few years, with special focus on aeronautical and aerospace purposes [102,139].
4.2. DBD Technology in Aerospace and Aeronautical Sectors
The origin of the dielectric barrier discharge is attributed to Ernst Werner von Siemens in 1857 [101,102,103,112]. The first experimental investigations performed by Siemens et al. [140] were focused on the generation of ozone and the experiment discharge apparatus design featured many novel traits, including electrodes positioned outside the discharge chamber that were not in contact with the plasma. For this reason, DBDs were considered ozone discharges for a long time [101,103,112,141]. In 1860, Andrew and Tait [142] named the system silent discharge because of its quiet and silent discharge process. Such nomenclature is still frequently used in English, German, and French scientific literature [101,103,141]. Later, K. Buss [143] made a notable contribution to characterizing the discharge in 1932 by reporting that the breakdown of atmospheric pressure air between the planar parallel electrodes covered by dielectrics always gave origin to a large number of tiny short-lived current channels. The first photographic traces—Lichtenberg figures—of these channels (or microdischarges) and oscilloscope recordings of current, in addition to the voltage applied, were obtained [103,112]. Further investigation of channels (or filaments) in more detail was conducted by several research groups, including Klemenc et al. (1937) [144], Suzuki (1950) [145], Honda and Naito (1955) [146], and later by Gobrecht et al. (1964) [147], Bagirov et al. (1971) [148], Tanaka et al. (1978) [149], Hirth (1981) [150] and Heuser (1984) [151]. Despite the ongoing research through the years, another key contribution was made by T.C. Manley in 1943 [152] who proposed a method for determining the dissipated power in DBDs using closed voltage/charge Lissajous figures and derived an equation that became known as the power formula for ozonizers [102,141].
Over the last years, plasma actuators based on the dielectric barrier discharge mechanism have attracted much attention for aerospace and aeronautical applications [153]. Broadly speaking, DBD actuators may be applied in research investigating aerodynamic active flow control and heat transfer [139].
4.3. Plasma Actuators for Aerodynamic Flow Control and Drag Reduction
Active flow control is an important subject of study since it allows for improving the efficiency of several mechanical systems by enhancing their performance through both fuel consumption and environmental impact reduction [154,155]. Thus, the ability to manipulate a flow field is crucial for the scientific community worldwide. In particular, dielectric barrier discharge plasma actuators are a technology with great characteristics for this aim, since it is characterized by easy implementation (i.e., simple construction), absence of moving parts, extremely low mass, robustness, low power requirements, and fast response to electrical signals [154,155,156,157]. From a practical point of view, this type of actuator allows modification of the airflow owing to the electrokinetic conversion mechanism, which is called the electrohydrodynamic (EHD) phenomenon. The exploited electrohydrodynamic force originating from the electrohydrodynamic phenomenon is produced due to momentum transfer from charged species accelerated by an electrical field to neutral molecules by collision [154,155]. On the whole, research regarding plasma actuators for active flow control includes turbulent boundary-layer separation control, steady airfoil leading-edge separation control, oscillating airfoil dynamic stall control, and circular cylinder wake control [154].
4.4. Plasma Actuators for Heat Transfer
Apart from the main described aerodynamic applications, dielectric barrier discharge plasma actuator devices can be considered and used within the field of heat transfer, for example, for film cooling of gas turbine blades and heat generation for de-icing or anti-icing objectives [157,158]. In essence, the thermal behavior of DBD plasma actuators has great importance for these types of applications. Nevertheless, there have been a relatively limited number of studies reporting these applications [158]. The concept of using plasma actuators for active flow control in the case of film cooling enhancement was introduced by Roy and Wang in 2008 [159], and it was shown by numerical simulations that the application of plasma discharges could improve the film cooling efficiency up to 26% [139,160]. Other studies were conducted over the years; however, the general conclusion is the same: despite different boundary conditions or geometries of components to be cooled, plasma aerodynamic actuation improves the overall efficiency of the film cooling process by enhancing the adherence of the coolant working fluid (also called “coolant jet”) [139,161]. Lastly, and regarding the application of plasma actuators in aircraft icing mitigation, Van den Broecke [162] was the first to conduct research on the feasibility and effectiveness of using DBD plasma actuators to remove ice accretion from a stationary flat plate. Currently, more studies have been carried out on this topic and plasma actuation has gained great attention concerning icing mitigation due to its unique features [163,164]. SDBD was described by Jia et al. (2022) [165] as a “novel anti-icing method featuring low energy consumption, geometrical simplicity, and rapid heating effect” and both nanosecond pulse SDBD (nSDBD) and alternating-current SDBD (AC-SDBD), depending on the driving waveform, have been verified through experiments for anti-icing purposes.
4.5. Ceramic Materials for DBD Systems
The performance of DBD plasma actuators may be predominantly considered in terms of their three major features, i.e., their electrical parameters, the geometry chosen, and the material properties of particular interest (dielectric barrier materials). Despite the several advantageous features of DBD plasma actuators, one of the major weaknesses of these devices is their longevity.
Moreau (2007) [166], Corke et al. (2009) [167], and Bernard and Moreau (2014) [168] have elaborated comprehensive reviews of the physics, modeling, experiments, and applications of plasma actuators [169,170]. In addition, the properties of dielectric materials have been described by Bian et al. (2017) [170] and Rodrigues (2019) [139]. For instance, parameters such as dielectric thickness and its influence on DBD plasma actuator performance, the relationship between the concentration of discharge filaments and the consequent dielectric breakdown, as well as the surface temperature of the dielectric and its impact on the transition from glow to filamentary discharge have all been addressed. Furthermore, both novel dielectric barrier materials and material modifications, focusing mainly on polymers—due to their simplicity of use—have also been part of the current research regarding DBD devices [167]. Nevertheless, polymers have been reported to be very vulnerable to ion bombardment, radical species, and ultraviolet radiation emitted by plasma filaments in air at atmospheric pressure, thus making them extremely susceptible to material degradation [139,171].
As a result, ceramics appear to be a suitable substitute for the widely used polymers, since this type of material offers several superior and favorable traits, such as corrosion resistance, high- and low-temperature resistance, excellent dielectric properties, and heat conduction—which clearly highlights the possibility of ceramics being a good dielectric barrier in the years to come [170]. In spite of today’s advancements in many technical and scientific fields, extensive research exploring dielectric barrier layers in standard applications of dielectric barrier discharge is lacking in the literature [172]. This is particularly concerning since physical properties and plasma-chemical efforts are highly dependent on the material of the dielectric barrier. In other words, both surface and electrical properties of the DBD actuator are particularly influenced by the chemical composition of the dielectric barrier. These features are of utmost importance since they affect charge accumulation, charge traps, and electric field distribution in the vicinity of the dielectric surface [173,174].
In an attempt to emphasize the most prominent investigations on dielectric barrier materials found in the literature, some studies will be summarized below.
Pons et al. (2008) [171] analyzed the surface degradation of two types of polymers as a dielectric barrier on a DBD actuator, i.e., polymethyl methacrylate (PMMA) and polyvinyl chloride (PVC) materials, and compared them with borosilicate glass. Images captured of both polymers showed clear degradation after operation in terms of roughness and burning, as well as color changing. Contrary to the evaluated polymers, images captured of borosilicate glass plates presented no obvious modification of the surface. It was concluded that this material is indeed more robust to chemical and radiation exposure.
Silica glass (ceramic) dielectric barriers have been successfully utilized in many DBD experiments and have demonstrated improved resilience over organic materials, including epoxies and polyimide film (Kapton tape) [170,175]. Zito et al. (2013) [175] fabricated microscale dielectric barrier discharge plasma actuators and experimentally characterized silicon dioxide for the dielectric barrier. It was stated that by using SiO2 as a dielectric barrier, the lifetime of the actuators was extended when compared with the first generation of DBD actuators, i.e., polymer dielectric material. Furthermore, Fine and Brickner (2010) [176] proposed the addition of a heterogeneous catalyst on the surface of the dielectric exposed to the plasma as an approach to increase actuator thrust. It was reported, according to the results obtained, that the use of titania (TiO2) as a plasma catalyst increased the actuator thrust by 120% compared to a catalyst-free actuator.
In addition, in order to determine the time-resolved body force induced by a DBD plasma actuator and a correlation between induced body force, flow behavior, and phase of the dielectric discharge, Neumann et al. (2012) [177] performed flow studies captured with high spatial and temporal resolution. A ceramic dielectric manufactured using low temperature co-fired ceramics technology (LTCC) was applied which, according to the authors, allowed the use of a very durable and lasting ceramic, possibly enabling the future application of this material in harsh environments, such as turbomachines. In turn, Segawa et al. (2007) [128] reported the characteristics of a DBD actuator under elevated temperatures—up to 600 °C. In their study, the authors developed a DBD plasma actuator with ceramic and quartz insulators and verified the performance deterioration with increasing temperature.
On top of all the investigations described, to obtain a homogeneous DBD in air at atmospheric pressure, many methods—including different types of barrier materials, power supply, and electrode arrangements—have been explored [178].
Ran et al. (2018) [178] focused their work on the factors that influence the formation of homogeneous discharges at atmospheric pressure in air with a greater focus on dielectric properties on discharge modes. The experimental set-up featured plane-parallel electrodes which were covered with quartz plates of 0.5 to 1 mm thick or Al2O3 ceramic plates of 0.5 to 3.25 mm thick. It was found that the dielectrics played a crucial role in the formation of atmospheric pressure Townsend discharge (APTD) in the open air. Briefly, the Townsend discharge consists of a gas ionization process where an initially small amount of free electrons, accelerated by a sufficiently strong electric field, gives rise to electrical conduction through a gas by avalanche multiplication. Once the number of free charges drops or the electric field weakens, the phenomenon ceases [179]. Three dielectric characteristics were distinguished of major importance: type of dielectric material, thickness of the dielectric barrier, and surface roughness of the dielectric barrier. Nevertheless, it was highlighted by the authors that the rougher the dielectric material, the greater number of shallow traps, so more electrons can be provided for the next half-cycle discharge of the DBD. In addition, it was shown that the surface roughness of the dielectric also reduced the breakdown electric field due to its uneven surface. The ceramics used in this study had more shallow traps than quartz glass, which explained the difficulties quartz has in generating homogeneous DBD in air.
As a follow-up, Ran et al. (2020) [180] approached the factors that affect the transition of discharge mode for obtaining a homogeneous atmospheric pressure discharge in air. The surface morphology of different dielectric materials—quartz glass and ceramic—was given special attention. Once again, it was concluded that the surface morphology of different applied dielectrics has a remarkable influence on the discharge mode and emission spectrum of the discharge.
Particularly interesting about DBD actuators is the efficiency of the DBD plasma chemical reaction. In fact, this efficiency is expected to increase by increasing the permittivity of the barrier material, since the transported charge of the plasma reaction is proportional to the permittivity of the dielectric material. Ceramics with high permittivity tend to break by supplying a high voltage thanks to their modest dielectric strength, and therefore SiO2, which has a low permittivity, is generally used as a dielectric material [181]. Nevertheless, MTiO3 (M = Ca, Sr, Ba) ceramics are actually recognized as a typical dielectric material possessing a variety of dielectric properties. Li et al. (2004) [181] investigated the sinterability and mechanical and dielectric properties of Ca0.7Sr0.3TiO3 using Li2Si2O5 as a sintering additive. The produced ceramic was applied as a dielectric barrier for the decomposition of CO2. For comparison purposes, alumina and silica glass were also used as dielectric barriers. The results indicated that the permittivities of the three types of ceramics at 100 °C and 10 MHz were in the order of Ca0.7Sr0.3TiO3 (207) >> alumina (10.4) > silica glass (4.6); and the CO2 conversions greatly changed depending on the barrier materials in the same order as the permittivity, i.e., Ca0.7Sr0.3TiO3 >> alumina > silica glass.
In the same train of thought, Song et al. (2016) [174] evaluated the performance of Ca0.8Sr0.2TiO3 ceramics as a dielectric barrier, based on different amounts of glass addition, in the decomposition of carbon dioxide at atmospheric pressure. Several conclusions were reached, including the feasibility of using Ca0.8Sr0.2TiO3 for the decomposition of CO2, since both the conversion rate and efficiency increased with increasing glass content.
Moreover, the literature states that alumina ceramics are widely applied as ceramic dielectric barriers, since they are considered sufficient for DBD actuator applications due to their advantageous features, such as high mechanical and dielectric strength, high resistivity, and small dielectric losses [173]. Regarding the influence of the DBD actuator above the construction threshold on the two-dimensional subsonic boundary layer, Moralev et al. (2018) [182] used an actuator design with a two-layer underlying electrode to stabilize the position of the filaments and 1 mm thick dielectric plates of alumina ceramic were placed between these two electrode layers, as well as above them. Recently, Kelar et al. (2020) [173] determined the ignition and quenching voltage of the DBD regarding the effect of adding several types of oxides to a pure alumina ceramic. The aim was to determine the impact of the chemical composition of the dielectric barrier. Overall, the final findings showed that the addition of small amounts of oxide dopant into pure alumina ceramic affected both the chemical composition and surface structure of the ceramic, which in turn influenced the plasma parameters. Similarly, the work carried out by Pribyl et al. (2020) [173] consisted of a complex study on alumina-based ceramic barriers doped with spinel, i.e., MgAl2O4. It was concluded that the change in the sample composition resulted in a nonlinear response of the physical properties for coplanar DBD (CDBD). It was remarked that the determination of bulk and surface properties is necessary for complex analysis of the suitability of materials for use as a dielectric barrier for CDBD; however, based on the knowledge and experience already acquired, alumina-based ceramics with a small addition of MgAl2O4 are promising materials for effective cold nonthermal plasma generation.
Lastly, a remarkable investigation was carried out by Bian et al. (2017) [170] in which the material characterization and performance evolution of an AlN ceramic-based DBD plasma actuator was reported. A conventional Al2O3 ceramic was also investigated as a control. Many conclusions were extrapolated, but, in general, the authors highlighted that the AlN-based actuator produced a more uniform discharge whilst the discharge of the Al2O3 actuator easily became filamentary. The latter condition unfortunately leads to higher power consumption and earlier failure due to electrode oxidation.
5. Multifunctional Advanced Ceramics
The function of an engineered ceramic material may be defined as the specific purpose for which it is used in a particular application. Moreover, multifunctional ceramic systems composed of different materials—each offering primarily a single function—are well known. Nevertheless, sometimes even for a monofunctional application, a fine ceramic is frequently able to fulfill a set of secondary purposes based on their secondary properties [183]. Taking into consideration the three aforementioned and reviewed applications of advanced ceramics in aerospace and aeronautical engineering fields, i.e., thermal protection systems, thermal barrier coatings, and dielectric barrier discharges—Figure 7, in addition to the materials assessed in each subsection, the following three chemical compositions of ceramic systems carefully studied follows: MgO-doped aluminum oxide, MgO-doped calcium zirconate oxide, and yttria-stabilized zirconia.
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Figure 7. Examples of engineering applications of (a) TPS woven material (adapted from Uyanna et al. (2020) based on ref. [11] with the permission of Elsevier), (b) TBC space shuttle tile, and (c) DBD plasma actuators during discharge phenomenon.
5.1. MgO-Doped Aluminum Oxide
Aluminum oxide, commonly referred to as alumina, is one of the most widely used, cost-effective materials in the family of fine ceramics. With an excellent combination of properties and reasonably priced, available raw materials, fine-grain technical alumina has a very wide range of applications. In detail, this material possesses strong ionic interatomic bonding, which consequently gives rise to its desirable key properties, such as:
high-temperature stability,
excellent size and shape molding capabilities,
high strength, stiffness, hardness, and wear resistance,
good corrosion and erosion resistance,
resistant to strong acid and alkali attacks at elevated temperatures,
high dielectric strength and small dielectric losses, and
commercial availability in purity ranges from 94% to 99.8% for the most demanding high-temperature applications.
Moreover, the listed characteristics make alumina-based ceramics the material of choice for a wide range of applications, including high temperature and aggressive environments, wear and corrosion resistance, metal cutting tools, microwave components, and electrical insulation.
Furthermore, authors such as Pribyl et al. [173], Mollá et al. [184], and Ramírez González et al. [185] evaluated the addition of magnesium-based dopants in standard alumina ceramics. It was reported and highlighted that Mg-based dopant components are usually used as a sintering aid in the alumina fabrication process since they inhibit grain growth and increase the final density of the material. Accordingly, and bearing in mind two crucial considerations described in Section 2.1 and Section 2.3, respectively, that (a) oxide ceramics are intuitively good candidates for passive TPS application, (b) alumina is a state-of-the-art ceramic dielectric barrier material of DBD actuators.
From the TBC point of view, alumina oxide is a very stable phase with very high hardness and chemical inertness. However, the plasma-sprayed coating of alumina contains mainly unstable phases, such as gamma- and delta-Al2O3. These unstable phases will transform into alfa-Al2O3 during thermal cycling, accompanied by a significant volume change (≈15%), which results in microcrack formation in the coating [67,186,187].
Nevertheless, in hexaaluminates, MgO played an important role in recrystallization and grain growth rates that promote coatings with improved properties, such as reduced microhardness, greater strain tolerance and thermal shock resistance, as well as better CTE, as described by Chen et al. (2011) [98]. The MgO-doped alumina increased the densification and grain size reduction, thus improving the mechanical properties of the alumina coating, such as coating hardness and substrate oxidation resistance. Furthermore, the formation of MgAl2O4 spinel phase can promote cracking-healing behavior within the coating [188].
5.2. MgO-Doped Calcium Zirconate Oxide
Calcium zirconate oxide, also named calcium zirconate (CZ), is reported to be a potential candidate for many purposes in mechanical, coating, and electrical applications. As described in Section 2.2, calcium zirconate has appealing and exciting thermal properties, rendering this ceramic material a convenient candidate for thermal barrier applications. In addition, CZ is a material with a perovskite structure that is of fundamental significance for its electrical properties. All in all, some of the attractive properties of calcium zirconate oxide include:
excellent mechanical properties,
low thermal conductivity,
high thermal and chemical stabilities,
good thermal shock resistance,
high melting point, and
excellent dielectric properties, i.e., high dielectric constant, low loss factor, and both of qualities are stable between 1 kHz and 1 MHz.
Calcium zirconate has been applied in different sectors, for example, as a sensor material in aluminum melts, refractory material for titanium metallurgy, and microwave dielectric ceramic in modern communication systems. Nonetheless, it is considered to be an alternative material to YSZ in thermal barrier coatings in aeronautical and aerospace fields, as pointed out by Ma et al. [82], Garcia. et al. [83], and explained in Section 4.4 (perovskites).
Additionally, the literature indicates that simple perovskite structure materials are of essential significance for their electrical properties, including ferroelectricity, piezoelectricity, and superconductivity. Studies showed that perovskite ceramics increased the efficiency of the dielectric barrier discharge process. Li et al. [181] and Song et al. [174] focused their studies on the family of CaTiO3-based compositions, however, these ceramics had the drawback of lower temperature stability when compared to CaZrO3.
In the literature, studies proposing CaZrO3 for TPS are limited; however, in some previous works [189,190,191,192,193], research was carried out on the behavior, mechanical properties (flexural resistance, compression, wear, hardness, and toughness), and thermomechanical characteristics (diffusivity, heat transfer, thermal conductivity, and CTE) of the CaZrO3–MgO system, showing that it is a valid option for structural applications at high temperatures.
Taking the above aspects into account and the densification and inhibition of grain growth during the sintering process, the MgO-doped CaZrO3 ceramic system is also considered to be a reasonable and suitable material for TPS, TBC, and DBD applications.
5.3. Yttria-Stabilized Zirconia
As described in Section 4.2 (Ceramic Materials for TBC Systems), yttria-stabilized zirconia has been widely considered and adopted for thermal barrier coatings on gas turbine blades typically made of a Ni-based superalloy because of its attractive properties, such as high thermal stability, low thermal conductivity, and a relatively large thermal expansion coefficient, which is close to that of the metal substrate. In addition, it is known that zirconia or ZrO2 has a very high melting point (3053 K), as well as high-temperature, wear, and corrosion resistance. Nonetheless, pure zirconium dioxide undergoes a phase transformation from monoclinic (stable at room temperature) to tetragonal (at approximately 1170 °C) and then to cubic (at about 2370 °C). Therefore, in order to obtain stable zirconia ceramic products, stabilized zirconia has been developed and studied by doping ZrO2. Particularly, by adding yttrium oxide or yttria (Y2O3), which has excellent chemical inertness and high corrosion resistance, it is possible to obtain a fully stabilized zirconia. Precisely, ZrO2 with 7–8 wt.% Y2O3 composition has been studied for years for TBC applications due to its unique properties enumerated in Section 3 on Thermal Barrier Coatings, and enumerated once again:
very high mechanical strength and wear resistance,
very high erosion resistance,
high impact resistance,
high corrosion resistance,
high chemical resistance,
very low thermal conductivity, and
relatively high coefficient of thermal expansion when compared to other ceramics.
Recent MSc work by Balça (2021) [194] specifically studied the optimization of multiphase composites of zirconium oxide for thermomechanical aeronautical applications by applying DoE analysis. On the whole, the research consisted of the fabrication and microstructural, physical, mechanical, and thermal characterization of seven multiphase distinct ceramic compositions in which pure (monoclinic phase, mZ), tetragonal phase (3YSZ, tZ), and cubic (8YSZ, cZ) zirconias served as the base materials. After performing a DoE study, the composition (wt.%) of 1:3 cZr, 1:3 tZ, and 1:3 cZ (1:3 all YSZ) was selected, based on the mechanical and thermal results, as the best fit for a thermal barrier coating and passive thermal protection system.
It is important to emphasize that the choice was mainly influenced by the material’s high mechanical resistance and notably low thermal conductivity, which is a crucial parameter for the two applications referred. To sum up, Figure 8 outlines the three ceramic systems chosen to be investigated, i.e., MgO-doped Al2O3, MgO-doped CaZrO3, and 1:3 of all yttria-stabilized zirconia, in addition to their intended applications based on the aeronautical and aerospace implementations exploited (TPS, TBC, and DBD) illustrated in Figure 7.
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Figure 8. Ceramic systems proposed by the literature and likely possible applications in aeronautical and aerospace fields.
The selections were made in the sense that, when possible, one of the functions of each candidate ceramic system should be based on their usual state-of-the-art application. For example, yttria-stabilized zirconia is a state-of-the-art material for TBCs, whereas alumina ceramic is used for DBD. The second application should be an alternative application according to the literature reviewed. The third and remaining application, which is always opposite to the material considered in the diagram, serves as a suggestion and, therefore, further studies are expected to determine its suitability, such as MgO-doped calcium zirconate as a TPS, MgO-doped alumina for TBC usage, and lastly, yttria-stabilized zirconia as a dielectric barrier for DBD actuators.
6. Conclusions
The premise that materials permeate the technological innovations that contribute to our social well-being and impact our daily lives is well accepted, so we have two major challenges: (i) increase energy savings and consequently increase the energy efficiency and durability of products; (ii) access more economical and local raw materials, thus minimizing transport and geopolitical constraints. Therefore, continuous research and enhancing our understanding of materials allow us to optimize new products by improving existing materials, adapting them to new manufacturing processes, and developing new functions for them. Hence, novel opportunities arise. Advanced ceramics can be designed to add value to forms of current manufacturing and may provide innovative alternative solutions to current problems.
This review presents solutions with multifunctional ceramic composites for fundamental applications in aerospace and aeronautics, including thermal protection systems (TPS) and thermal barrier coatings (TBC). In this context, TPS and TBC are mature systems with valid industrial solutions. However, new proposals and solutions are welcome with the objective of improving energy efficiency and increasing durability. The integration of new functionalities, such as dielectric barrier discharge (DBD) requires the use of reliable ceramic composites with predictable properties and well-known manufacturing processes that allow their adaptation to new designs and specifics.
Thus, the proposed ceramic composites are valid solutions for some functions and still little known for other uses. For example, MgO-doped aluminum oxide is proposed for passive TPS application and in dielectric barrier discharge; however, like TBC, there are other alternatives. Nevertheless, their application in view of a multifunctional response is a potential solution. The ceramic MgO-doped CaZrO3 system used in TBC is a good candidate for DBD due to its electrical properties, and its proposed use in TPS is a reliable solution. Yttria-stabilized zirconia (YSZ) is a widely adopted material for TBC and TPS functions. The proposal of mixing zirconias (1:3 monoclinic, 1:3 tetragonal, and 1:3 cubic phases) as a base ceramic composite may enhance thermal barrier coatings and passive thermal protection system applications. The multiphase microstructure of this ceramic creates the fundamental electrical potential for DBD function.
Author Contributions
Conceptualization, J.N.-P., F.F.R. and A.P.S.; investigation, K.O.S.; Writing—Original draft preparation, K.O.S.; Writing—Review and editing, J.N.-P., F.F.R. and A.P.S.; Supervision, J.N.-P., F.F.R. and A.P.S.; Project administration, A.P.S.; Funding acquisition, A.P.S. All authors have read and agreed to the published version of the manuscript.
Funding
This work was supported by the following projects and organisms: Portuguese Foundation for Science and Technology, I.P. (FCT, I.P.) FCT/MCTES through national funds (PIDDAC), under the R&D Unit C-MAST/Center for Mechanical and Aerospace Science and Technologies (Project UIDB/00151/2020) and under the R&D Unit CF-UM-UP/Centro de Física das Universidades do Minho e do Porto (Project UID/FIS/04650/2020). JNP also thanks FCT, I.P., European Social Fund (ESF), European Union (EU), and Regional Operational Programme Centro 2020 and Norte 2020 for the grant SFRH/BPD/117838/2016.
Data Availability Statement
Not applicable.
Acknowledgments
Portuguese Foundation for Science and Technology, I.P. (FCT, I.P.) FCT/MCTES through national funds (PIDDAC), under the R&D Unit C-MAST/Center for Mechanical and Aerospace Science and Technologies (Project UIDB/00151/2020) and under the R&D Unit CF-UM-UP/Centro de Física das Universidades do Minho e do Porto (Project UID/FIS/04650/2020). JNP also thanks FCT, I.P., European Social Fund (ESF), European Union (EU), and Regional Operational Programme Centro 2020 and Norte 2020 for the grant SFRH/BPD/117838/2016.
Conflicts of Interest
The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.
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33 | https://www.prnewswire.com/news-releases/boom-supersonic-completes-construction-of-overture-superfactory-302173677.html | Boom Supersonic Completes Construction of Overture Superfactory
(PRNewsfoto/Boom Supersonic)
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Jun 17, 2024, 11:15 ET
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First supersonic airliner factory in the U.S. strengthens next-generation American leadership in aerospace manufacturing
Overture aircraft produced at Superfactory will bring vital innovation to aviation and set a new standard for global air travel
Hundreds of millions of passengers will fly supersonic on Overture airliners
GREENSBORO, N.C. and DENVER, June 17, 2024 /PRNewswire/ -- Boom Supersonic, the company building the world's fastest airliner, today held a ribbon-cutting ceremony at the Overture Superfactory, celebrating the completion of construction. Located at the Piedmont Triad International Airport in Greensboro, North Carolina, the Overture Superfactory is the first supersonic airliner factory in the United States. Overture is Boom's supersonic airliner, capable of flying twice as fast as today's commercial planes on up to 100% sustainable aviation fuel (SAF).
Located in Greensboro, North Carolina, the Overture Superfactory is the first supersonic airliner factory to be built in the United States.
Located in Greensboro, North Carolina, the Overture Superfactory is the first supersonic airliner factory to be built in the United States.
Less than 17 months after breaking ground on the site, construction on the 179,000 square foot Overture Superfactory is complete.
Less than 17 months after breaking ground on the site, construction on the 179,000 square foot Overture Superfactory is complete.
Construction of the Overture Superfactory represents a major milestone toward ensuring the United States' continued leadership in aerospace manufacturing, said Blake Scholl, founder and CEO of Boom Supersonic. Supersonic flight will transform air travel, and Overture provides a much-needed innovative alternative for airlines across the globe.
Hundreds of millions of passengers will fly supersonic on aircraft produced at the Overture Superfactory. This first assembly line has the capacity to produce 33 Overture aircraft per year, valued at more than $6 billion. Boom plans to build an additional assembly line, scaling to produce 66 supersonic airliners annually. The Overture Superfactory campus will also include a delivery center where airlines including United Airlines, American Airlines, and Japan Airlines will receive their supersonic aircraft.
As the state that was first in flight, North Carolina is excited to see Boom's progress toward delivering the world's first sustainable supersonic airliner, said Governor Roy Cooper. Our state has the skilled workforce, infrastructure and perfect location to help Boom revolutionize air travel.
North Carolina economists estimate that the full Boom manufacturing program will grow the state's economy by at least $32.3 billion over 20 years, with the Superfactory directly adding more than 2,400 jobs.
The Piedmont Triad region is an economic powerhouse, helping solidify North Carolina's place as the top state for business. Boom's Overture Superfactory, which will produce a paradigm-changing airliner, showcases the new and innovative projects that are bringing jobs and investment here, said North Carolina Senate President Pro Tempore Phil Berger.
Built by BE&K Building Group and designed by BRPH, the Overture Superfactory will be LEED certified and is expected to be at least 40% more energy efficient compared to similar manufacturing facilities.
With the building now complete, Boom will focus on operationalizing the production floor. In partnership with tooling supplier Advanced Integration Technology (AIT), Boom will begin procuring and installing tooling into the Superfactory, beginning with an advanced test cell unit. As the first major piece of equipment to be installed, the test cell will be used to develop manufacturing processes, optimize the flow of the assembly line, and prepare staff for Overture production.
We look forward to Boom's success in its mission to bring back commercial supersonic flight and help the U.S. maintain its leadership in aviation innovation, said Paul Mengert, Chair of the Piedmont Triad Airport Authority. We're grateful for our partnership with the Economic Development Partnership of North Carolina, elected leaders, and the local community who will continue to give Boom a warm welcome to the Triad.
Today's construction completion event comes shortly after the inaugural flight of XB-1, the supersonic demonstrator aircraft for Overture, which represents a major milestone toward the return of supersonic air travel. XB-1 has received a first-of-its-kind Special Flight Authorization (SFA) to Exceed Mach 1 from the Federal Aviation Administration (FAA). The XB-1 flight test program continues to progress in Mojave, California, and will confirm the aircraft's performance and handling qualities up to and through supersonic speeds.
About Boom Supersonic
Boom Supersonic's mission is to make the world dramatically more accessible through flights that are faster, more affordable, more convenient, and more sustainable.
Boom is developing Overture, the world's fastest airliner – optimized for speed, safety, and sustainability. Overture will fly at twice the speed of today's airliners and is optimized to run on up to 100% sustainable aviation fuel (SAF). Overture's order book stands at 130 aircraft, including orders and pre-orders from American Airlines, United Airlines, and Japan Airlines. Boom is working with Northrop Grumman for government and defense applications of Overture. Suppliers and partners collaborating with Boom on the Overture program include Aernnova, Aciturri, Collins Aerospace, Eaton, Honeywell, Latecoere, Leonardo, Safran Landing Systems, and the United States Air Force.
Symphony™ is the propulsion system that will power Overture, a Boom-developed engine with world-class suppliers including Florida Turbine Technologies (FTT), a business unit of Kratos Defense & Security Solutions, Inc., Colibrium Additive – a GE Aerospace Company, and StandardAero.
XB-1 is Boom's technology demonstrator aircraft and the world's first independently developed supersonic jet. The aircraft first took flight in Mojave, CA in March 2024. For more information, visit https://boomsupersonic.com.
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34 | https://www.aerospacetestinginternational.com/news/flight-testing/deutsche-aircraft-delays-d328eco-program-by-two-years.html | Deutsche Aircraft Delays Entry Into Service For D328eco Regional Turboprop To 2027
Deutsche Aircraft has pushed back the entry into service date of its upcoming D328eco plane by two years, with the sustainable turboprop now delayed until the end of 2027. The company stated that the reasons behind its decision are multifaceted, but the longer timeline would allow it to implement additional features on the aircraft.
D328eco entry delayed by two years
According to a statement from the company, the upcoming 40-seater turboprop is now penciled in for a Q4 2027 entry into service (EIS) following changes to the certification and compliance process. Deutsche Aircraft arrived at the decision to push back the timeline by two years after a detailed internal review and discussions with potential customers and other stakeholders.
Dave Jackson, CEO of Deutsche Aircraft, commented,
While we have had to realign the EIS for our D328eco, we are taking this opportunity to investigate further product enhancements and satisfied by the tremendous progress of the programme to date.
The company remains in close coordination with customers and supply partners to meet changing regulations. It has already achieved some important milestones in the program, such as the start of construction of the plane's Final Assembly Line (FAL) in Leipzig.
STOL enhancements
The company says it is evaluating several new features and capabilities, including advanced avionics compatible with the plane's Garmin avionics suite, and improvements to the aircraft's Short Take-off and Landing (STOL) performance specifically in non-benign operational environments.
The changes will made in the context of changing regulatory requirements and allow the D328eco to meet evolving market demands. The company added that the turboprop continues to attract strong interest from different segments across the market.
Progress continues
Certification with the European Union Aviation Space Agency (EASA) continues, with the company claiming it is advancing successfully. Along with progress on its assembly line in Leipzig, Deutsche Aircraft has cemented over 95% of supply partners for the D328eco. It promises to reveal additional program milestones at the upcoming Farnborough International Airshow (FIA).
Deutsche Aircraft announced the D328eco back in 2020 and hopes to revolutionize the regional air travel market with a highly efficient and sustainable aircraft. The design builds on the popular Dornier 328, of which over 200 were produced, and will feature PW127XT-S engines capable of flying on 100% sustainable aviation fuel (SAF).
D328eco 10-min
Photo: Deutsche Aircraft
The turboprop's low operating and maintenance costs will be ideal for future operators - the D328eco can fly up to 30% cheaper than other 50-seat aircraft and will be 50% cheaper to maintain. It will also be capable of multi-role missions with a large in-flight operable door enabling medical or firefighting capability.
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35 | https://www.airwaysmag.com/new-post/eva-air-first-asian-airline-aeroshark | EVA Air: First Asian Airline to Adopt AeroSHARK Tech
DALLAS — Taiwanese airline EVA Air (BR) becomes the first carrier in Asia to implement AeroSHARK technology, the innovative fuel-saving solution developed by Lufthansa Technik in collaboration with BASF.
The airline will equip its nine Boeing 777F cargo aircraft fleet with drag-reducing riblet films to enhance fuel efficiency and reduce carbon emissions.
The first BR aircraft, identified as B-16786, has already been modified with AeroSHARK at Taipei Taoyuan International Airport (TPE). The work is being carried out by the airline's affiliate, Evergreen Aviation Technologies Corporation (EGAT). According to BR, the modified aircraft will return to service in early September.
AeroSHARK technology mimics the structure of sharkskin, featuring tiny riblets that reduce the aircraft's frictional resistance. When applied to the fuselage and engine nacelles, these films can reduce fuel consumption by approximately one percent.
For BR's fleet, this translates to annual savings of over 2,500 metric tons of kerosene and a reduction of more than 7,800 metric tons of CO2 emissions.
Albert Liao, EVA Air’s Executive Vice President of Corporate Planning, emphasized the airline's commitment to achieving net-zero carbon emissions by 2050.
He expressed enthusiasm for the partnership with Lufthansa Technik, noting that adopting AeroSHARK technology cuts fuel consumption and contributes to the airline's ongoing efforts to minimize its environmental impact.
At the start of the week, Lufthansa Technik announced its plans to implement the innovative AeroSHARK surface film on the Boeing 777-200ER variant. Four Austrian Airlines (OS) aircraft will be modified using this groundbreaking technology. This innovation utilizes riblets that mimic the friction-reducing properties found on shark skin.
Currently, the Lufthansa Group of airlines has equipped 17 aircraft with AeroSHARK. This includes a Lufthansa-operated Boeing 747-400, 12 SWISS-operated Boeing 777-300ER aircraft, and four Lufthansa Cargo-operated Boeing 777F cargo aircraft.
EVA Air plans to closely monitor the technology's fuel-saving benefits and may consider applying it to additional aircraft in the future. |
36 | https://www.aerospacetestinginternational.com/news/industry-news/airbus-restructures-manufacturing-business-to-create-new-subsidiary.html | Airbus restructures manufacturing business to create new subsidiary
Ben SampsonBy Ben Sampson5th January 2022
3 Mins Read
Airbus has reorganized its manufacturing business to create a new subsidiary called Airbus Atlantic, which is supplying so-called “plug-and-fly” aerostructures to itself and other OEMs.
Airbus Atlantic, which was officially established on January 1 consolidates Airbus’ manufacturing sites in Nantes and Montoir-de-Bretagne in France with an existing subsidiary called Stelia Aerospace and its 10 production facilities around the world.
The move, which was initially announced in April 2021 is intended to strengthen the aerostructure assembly business within Airbus by increasing its competitiveness and thereby the level of innovation and quality of its products.
Airbus Atlantic employs 13,000 people in five countries and has an estimated business volume of around €3.5 billion (US$3.9 billion). The company supports a global supply chain of more than 2,500 companies.
The set up of Airbus Atlantic creates the second largest Tier One aerostructures supplier in the world after Spirit Aerosystems, which was formed in a similar way in 2005 by Boeing from its aerostructures manufacturing sites.
As well as the development, test and manufacturing of complex composite and metallic aerostructure components, Airbus Atlantic also supplies pilot seats, ducting and pipework. The company is working with customers in both the civil and military sectors including Bombardier, ATR, Dassault Aviation and Embraer.
The newly formed company inherits participation in several Airbus programs including the A220, A320, A330, A350, ATR 42/72, the Beluga XL and A400M as well as the Dassault Falcon 10X business jet and the ATR42 and ATR72 turboprop aircraft.
The fully equipped sections the company can produce include mechanical, hydraulic and electrical systems which have been functionally tested for things such as pressure and continuity before delivery, so OEMs can perform “plug and fly” assembly. For example, the central fuselage section it supplies to Bombardier for its GLobal 7500 business jet is equipped with electrical and hydraulic systems and the deal for the wingsets it supplies to ATR includes the supply of the final test benches for fuel and flight controls.
Cédric Gautier, CEO of Airbus Atlantic said, “Airbus Atlantic aims at meeting the great challenges linked to a sustainable aviation industry, pioneering new technologies. Our first mission will be to ensure the satisfaction of all our customers and to establish new standards of excellence in terms of quality and operational efficiency.
“I have full confidence in the talent, enthusiasm and commitment of the Airbus Atlantic teams to write this new chapter of our history with success.”
The seating the company supplies will continue to be marketed under the Stelia brand. |