According to the International Air Transport Association (IATA), the demand for air travel rose by 7.4% in 2018 from 2017 levels. Air passengers accounted for 81.9% of the load factor on aircrafts, while freight load was 49.3%, the IATA data reveals. However, the COVID-19 pandemic has created turmoil in the global economy. One of the worst-hit industries by this pandemic is the aerospace & defense industry, with lockdowns and trade restrictions choking supply-chains worldwide.
Rising deployment of airplanes for passenger and cargo transport necessitates the utilization of efficient materials in building aircraft components. Any laxity shown in the quality of materials used will endanger the lives of thousands of passengers and put at risk highly valuable cargo. Thus, increasing air travel has escalated the demand for next-gen aerospace & defense materials.
Aerospace Materials have evolved from wooden frames to fiber-reinforced composite polymers and self-cleaning and self-healing materials, with primary focus on weight reduction, improving damage tolerance, fatigue and corrosion resistance. Weight has been an important consideration in military and commercial aircraft design since the beginning of manned flight. It affects directly the amount of lift required to fly, which in turn affects the drag on the aircraft and therefore the thrust required to achieve the desired performance. Weight also has indirect impacts on the cost of the aircraft. Aerospace has the highest carbon footprint per tonne-km over any other mode of transportation and regulatory demands and economic advantages mean that saving any weight is a constant target.
The demand for advanced materials is principally driven by the need for lighter, stronger and durable materials. Traditionally aluminum dominated the aerospace industry, it was considered to be lightweight, inexpensive, and state-of-the-art. In fact, as much as 70% of an aircraft was once made of aluminum. In the aerospace domain, although aluminium is still widely used, advanced materials such as carbon-fibre composites and superalloys are gaining popularity among engineers aiming to improve efficiency and performance, while reducing weight.
Significant investments towards R&D has resulted in the development of sophisticated fabrication processes for advanced aerospace materials. For instance, in April 2018, Toray Composite Materials America, Inc. announced successfully developing a new fabrication technology for Carbon Fiber Reinforced Plastics (CFRPs) that enables both improved dimensional accuracy and energy savings for aircraft manufacturing. Dispersal of several new procurement contracts for aerial assets is envisioned in the future, driving the North America segment of the market in focus during the forecast period.
Challenges in development of advanced materials
It is the wear at the contact surface due to small amplitude (less than about 100 µm) oscillatory relative movement between the surfaces. It creates the crack initiation which slowly develops into fringes, and eventually causes rupture. No material is free from fretting wear, and the challenge lies in developing techniques to minimize it. To further complicate the problem, the general theory that identifies the fretting behavior and the prevention of fretting is still unclear. However, experiments have shown that the modification of surface hardness and adhesion can greatly influence fretting wear.
Reaction with the surrounding causes chemical change or degradation in the material, known as corrosion. Corrosion can be uniform corrosion, pitting corrosion, crevice corrosion, or galvanic corrosion. It is one of the major cause of structural collapse. In the United States alone, metal corrosion causes a loss of $276 billion per year, which is much greater than the loss caused by the natural disasters. Based on statistical predictions, only 30% of the corrosion loss can be prevented by the prevention methods. Further, in aerospace, not all the high corrosion resistant materials can be used. Therefore, it is a great challenge to protect aircrafts from corrosion by developing new materials or by innovating much effective techniques.
Materials for Aircraft Structural Applications
The airframe materials provide shape, structural rigidity, and long term support to static (time independent constant loads, eg. self weight) and dynamic loads (time varying loads especially during service) occurring in the aircraft. They, further, have to withstand the highly varying environmental conditions. So, it is desired that they be of minimum weight and have the appropriate mechanical and thermal properties.
Consider the aircraft wing. It is greatly subjected to bending. The wings’ upper surface is under compression during flight due to net upward lift force, and at tension during taxiing (movement of aircraft on the ground) due to downward acting weight. The bottom surface in under the opposite loading. Therefore, the wing materials should provide both high tensile strength and high compressive strength. Likewise, the fuselage should withstand high cabin pressurization and shear forces, and requires materials with high tensile and shear strength.
At present, aluminium alloys, polymer matrix composites (PMCs) and to some extent titanium alloys constitute the airframes of aircrafts. In the next few decades, the developments in lightweight materials for use as structural members in airframes will most likely continue to be centered upon these three classes of materials. In case of aluminium alloys, minor addition of scandium to conventional aluminium alloys has resulted in unexpected increase in strength, corrosion resistance and weldability. However, the high price of scandium is an inhibiting factor. The availability of cheaper scandium will brighten its use for bulk applications.
Al-Li alloys are also making a comeback with damage tolerant variants of Al-Li alloys having higher fracture toughness and fatigue crack growth resistance being developed but achieving higher strength levels is to be addressed. High-strength aluminium alloys having non-equilibrium phases – amorphous and quasi-crystalline phase are also being investigated. The possibility of using aluminium alloys processed through powder metallurgy (P/M) route is being evaluated to meet the demands of high temperatures (up to 450°C) for high speed vehicles. While there is confidence on the thermal stability of these alloys at 450°C and ambient temperature properties, mechanisms for retaining these at elevated temperatures is being explored. The challenge with P/M technology would be assuring improved levels of properties coupled with assured reliability and reproducibility.
Titanium alloys due to their higher strength to weight ratio, better stiffness and temperature resistant ability are presently used in the airframe structures of most modern fighter aircrafts with speeds in excess of Mach 2.5. At such speeds, the structures invariably experience temperatures well in excess of 150°C, the temperature capability limit of most aluminium alloys in use today. Titanium alloys being more compatible with composites than aluminium alloys, are thus an ideal choice for airframe substructures that are part metal and part composite.
While Ti-6Al-4V has been predominantly used in airframe applications, Ti-6-22 a newer alloy with improved strength is finding greater applications. Alloy 54M is also being explored as an easily machinable and cheaper replacement for Ti-6Al-4V. Newer processes like ‘meltless titanium’ is also gaining importance making alloys with compositions impossible through melting route a reality.
Alpha titanium alloys
These are the titanium alloys with hexagonal close packed (HCP) crystal structure. Alpha titanium alloys have a lower density, higher creep resistance and better corrosion resistance than beta titanium alloys. Therefore alpha titanium alloys like Cp-Ti, Ti-3Al-2.5V, etc. are used in compressor blades in aircraft engines. However, their functionality is highly limited at high temperatures. Al is widely used to improve their high temperature performance.
Beta titanium alloys
These are the titanium alloys with body centered cubic (BCC) crystal structure. They have higher tensile and fatigue strength than alpha titanium alloys, and are easier to fabricate. Beta stabilizers like V, Mo, Nb, and Cr can reduce the binding energy of the beta-Ti atomic cluster, resulting in stronger bonds between Ti and alloying atoms, thereby increasing the strength of the alloy. For eg. beta titanium alloy, Ti-3Al-8V-6Cr-4Mo-4Zr, has ultimate tensile strength of 1240 MPa and is used in high-stress regions of aircraft like landing gears and springs.
Alpha-beta titanium alloy
Combining the properties from both alpha and beta titanium alloys, alpha-beta titanium alloys boast the excellent strength, fracture toughness, ductility and corrosion resistance. They are the most widely used Ti-based alloys and comprise 70 % share of the U.S. titanium market. Alpha-beta alloys like Ti-6Al-4V and others are used in fuselage, landing gear, floor support structures, nacelles and compressor discs
Composite materials are produced from two or more dissimilar materials, which are combined to give the properties unlike the individual constituent. The use of composite materials is booming in aerospace industry with them constituting more than 25% of the Airbus A380 and 50% of Boeing 787. Composite materials have low density, and thus higher specific strength and better corrosion and fatigue resistance than most metals.
Ceramic matrix composites (CMC)
CMCs consist of ceramic fibers embedded in a ceramic matrix. CMCs like silicon carbide(SiC), alumina(Al2O3), silicon nitride(Si3N4) etc. have excellent high-temperature stability(even at 1400 °C), high hardness, and high corrosion resistance. Therefore, they are usually used in high temperature regions like exhaust nozzle.
However, ceramic matrix composites have poor fracture toughness. Recent studies have focused on the use of carbon nanotubes(CNT) and graphene nanaoplatelets(GNP) to improve CMCs fracture toughness. Scientific experments have shown that fracture toughness of Si3N4 can increase by 235% by adding 1.5 vol% of graphene.
Metal matrix composites (MMC)
MMCs consist of reinforcing materials dispersed in a metal matrix. MMCs have higher yield strength, fracture toughness, low thermal expansion, and suitable wear resistance.
Metals like Al, Mg, Ti, Cu and Ni are used as matrix. Carbon fibers and glass fibers are commonly used reinforcing materials. However, these fibers have already offered the maximum limit of their ability. So, new materials like carbon nanotubes (CNTs) and graphene nanosheets are being studied for improved mechanical properties. It has been found that the mechanical properties ( like tensile strength, yield strength, hardness) of Mg, Ti, Al, Cu, and Ni matrix composites will improve with the controlled addition of CNTs.
PMCs are composed of short or continuous fibers bound together by an organic polymer matrix. Fiberglass, graphite and aramid are commonly used fibers. Depending upon the characteristics of polymer matrix, PMCs are classified as thermoset (cannot be heat remolded) and thermoplastic(can be heat remolded). Thermoset matrices like epoxy, polyurethanes, polyamides, etc. are widely used to develop PMCs for aerospace industry which are used in ailerons, flaps, and landing gear doors.
PMCs are known for their higher specific strengths and specific modulus. For eg. as compared to the Al-based alloys, the density of carbon fiber reinforced in an epoxy matrix binder is half, and the tensile strength and elastic modulus are three times and two times higher, respectively. However, the carbon fibers are prone to stress concentration due to their brittleness. So natural fibers (like flax, hemp, banana, bambo, etc.), carbon nanotubes, graphene, and basalt are currently being investigated as reinforcing materials for polymer matrix.
PMCs based on epoxy resin matrices and carbon fiber reinforcements constitute 30-40% of the weight of present day fighter aircrafts. To enhance their performance the focus has been to improve the properties of the matrix like toughness, ease of fabrication and hydrophobic properties (for moisture resistance) as well as increase their temperature capabilities. PMC systems would remain the designers’ choice in the future as its manufacturing technology permits embedding functional materials like sensors, actuators and control systems into the structure required for building smart structures of future for shape control and vibration damping. Advanced polymers based on bismaleimides, cynate esters, benzoxazines and pthalonitriles having higher temperature capabilities (~350°C) will be used as epoxy based polymers can withstand up to 130°C.
The fifth generation fighter aircrafts have adopted bismaleimide matrices for more than 50% of the composite parts owing to their superior temperature capabilities up to 220°C for long-term use. Advanced polymers for long time use beyond 350°C will be most likely tailored using molecular engineering. The carbon fibers used as reinforcement currently possess strength and modulus that are far lower than their theoretical values. However, carbon fibers with elastic modulus near their theoretical limit, have been made for other applications but with reduced strengths. Fine carbon fibers with double the tensile strength (~ 7 GPa) than the commonly used fibers have also been made. But to attain C fibres with a combination of high stiffness and strength (both tension and compression) will require novel efforts. Obtaining proper interfacial properties is also an important area. Carbon nanotube fibres will also be explored due to their superior properties.
Recently, there is a renewed interest to use magnesium alloys for many applications including as major structural material. The major problem of corrosion is being overcome with a better understanding of the effects of iron, copper and nickel impurities (in ppm amounts) towards promoting corrosion in magnesium. Apart from being lightweight, its other advantages are high vibration dampening capability, ease of machining, electromagnetic shielding and its easy recyclability compared with polymers. The properties can be significantly enhanced by incorporating micro- and nano- particles to form metal matrix composites. With concerted and rigorous research efforts being pursued worldwide, magnesium based materials are poised to be one of the major structural material in the future.
Military Aircraft materials
Advanced materials have the ability to significantly improve operational effectiveness in military missions. Military aircraft should be strong, light weighted, able to fly faster, heat resistant, with increased agility and heavier payloads. Survivability is a design parameter for all military aircraft but a primary driver only for fighters, which sometimes depend on low observability to get to the target. Fifth-generation technology aircraft, made of advanced composite materials, and stealth technology are emerging trends for military aircraft, which will drive its demand in the market.
Defense aircraft material are lightweight materials known to deliver high performance. They are selected depending upon various properties such as strength, thermal shock resistance or expansion, resistance, flammability, stealth, and fuel efficiency. There is an increase in the requirement of aircraft materials owing to the rise in the demand for fuel-efficient aircraft. Military forces around the world prefer lightweight aircrafts, which provide fuel efficiency and higher strength. This leads to the use of different types of materials that can be used instead of the conventional ones to create efficient and advanced aircrafts.
The different types of materials used in defense aircrafts are aluminum alloys, super alloys, steel alloys, composite materials, and titanium alloys among others. The aluminum alloys segment dominated the market in the past years, but the increasing use of composites is replacing the aluminum alloys from the aerospace industry. However, due to low fatigue and high strength capabilities, aluminum alloys will still be preferred over composite materials.
A modern military aircraft is composed of several metal alloys and composite structures joined to create the specific structural profile of airframe and other constituent parts and components that fulfill the specified operational requirements of the aircraft.
Advanced materials can be used for a wide range of applications in defence, including camouflage and protection for platforms as well as soldiers. They provide the advantages of improved functionality, survivability and low life cycle cost. Materials such as metamaterial- can even make military systems invisible.
Despite all the benefits and positive results, such aircrafts materials have high cost in terms of maintenance. This in turn limits the defense aircraft material market growth. However, the inclination of countries toward increase in defense power and newer technology presents a good opportunity for both defense aircraft material industry and defense aircraft manufacturers.
Military air systems will have enhanced functionality, operational profile and capability in the future. The Unmanned Aerial Vehicles (UAVs) of varying sizes flying at hypersonic speeds will become more common with capability to perform different tasks including ISR for long durations. Greater emphasis will be placed on materials to reduce structural weight, operate at higher temperatures and evade detection.
There is also a growing interest in developing ultrahigh temperature materials (UHTM). These are materials with temperature capability greater than 1650 °C and able to withstand extreme erosive / corrosive environments. UHTM should possess high strength at high temperatures, oxidation resistance, ablation resistance, thermal shock resistance. These materials will be required for hypersonic air breathing vehicles, hyper speed cruise missile, hypersonic aircrafts, re-usable launch vehicles to protect leading edges and nose cones that experience very high temperatures (> 2000 °C). Refractory diboride composites like ZrB2, HfB2 etc and multilayer coatings of HfC, SiC on C-C composites are most promising UHTM candidates.
Materials for aircraft engines
Thrust improvement and engine weight reduction are two major scopes of designing aircraft engines. So, the engine materials should have low density, good high temperature mechanical properties, and excellent corrosion resistance. The engine generally consists of two sections: cold and hot.
Cold section, consisting of fan, compressor and casing, requires material with high specific strength (strength per unit mass), and corrosion resistance. Titanium based alloys (eg. Ti-6Al-2Sn-4Zr-6Mo) are generally used in this section because of their high strength (Yield Strength = 640 MPa) at high temperature (450 °C) and excellent corrosion resistance.
Hot section, consisting of combustion chamber and turbine, requires materials with high specific strength, creep resistance (resistance to deformation over the long period of exposure to high level of stress), hot corrosion resistance, and high temperature resistance. Since the operating temperatures (1400 °C – 1500 °C) greatly exceed the limit of Ti based alloys, Nickel based super alloys (eg. Ni-14.5Zr-3.2Mo) are used in this section.
Ni-based superalloys have very high strength even at high operating temperatures. Therefore, they are widely used in combustion chamber and turbine section of the aircraft engine where the operating temperatures range between 1100 and 1250 °C. Inconel (composed of Ni >72%, Cr 14%-17%, Fe 6%-10%, and little Mn, Cu, Si, S, C) and Nimonic (composed of Ni 54%, Cr 18–21% ,and some Co, Ti, Al) are the most popular Ni-based superalloys. Al is added to nickel-based superalloys to improve their oxidation resistance.
The advancements in materials technology including process technology will play a major role in the progress of future gas turbine engine to power both manned and unmanned aircrafts. Materials and technologies that are under consideration are:
Advanced composites based on polymer and metal matrix with engineered nanoparticles like CNT, BNNT etc are found to enhance strength and modulus at room and elevated temperature, respectively. These are considered for fan blade and casing applications. For higher temperatures, hollow titanium fan blades with stiffer cores like beryllium alloys are promising. A viable solution for negating the toxicity of beryllium is also being explored. Nano-grained components realized through consolidating cryomilled powders or subjecting materials to severe plastic deformation which promise large increments in tensile strength and fatigue properties will be explored for use in initial stages of compressor. New design concept of using bladed rings (blings) that results in weight reduction of 40% compared to bladed disks (blisks) has been demonstrated using SiC reinforced titanium metal matrix composite. Replacement of titanium blisks with blings and subsequent use of SiC reinforced Ti MMC for blings is very much a possibility to achieve weight reduction. The light weight gamma titanium aluminides in cast/wrought forms is gradually replacing titanium and superalloy parts. Titanium aluminide composites based upon the Ti2AlNb reinforced with continuous SiC monofilaments are being considered for blisk, bling and impeller applications at temperatures up to 700°C.
Melt infiltrated SiCf/SiC composites as combustion chamber liners that do not require air cooling is expected to replace the heavier nickel base superalloys. This will also reduce weight, reduce NOx emissions and make higher burn temperatures possible. However, environmental barrier coatings required for SiC based CMCs, durability issues etc. will have to be addressed. Discs with tailored chemistry and microstructures to provide creep resistance at rim and fatigue resistance at bore through newer powder metallurgy routes are being explored. Studies so far have shown that in addition to increasing component life, such discs could offer an increased temperature capability of about 30°C in existing alloys, though issues like appropriate constitutive model for the transition zone is yet to be fully addressed.
To allow the engine to operate at higher turbine entry temperatures (TET), Zirconia based thermal barrier coatings stabilized with Gadolinium Zirconium Oxide & Gadolinium Oxide instead of Yittria is being explored for reducing the thermal conductivity by at least half. These coatings in nanostructured form produced through processing routes like solution precursor plasma spraying are promising. Possibility of dramatic decrease of thermal conductivity using layered deposition of nanostructured YSZ and Al2O3 has been reported.
For blade and vane materials capable of withstanding a turbine inlet temperature exceeding 1800°C, many newer class of materials are being explored at laboratory scale. Many metallic /intermetallic based on platinum aluminides, iridium, silicides and borosilicides of refractory metals like niobium and molybdenum as well as CMC systems are being evaluated worldwide. However, issues related to fracture toughness and long term surface stability need to be addressed before they could be used for critical rotating blades.
With concept of more-electric engine (MEE) and more-electric aircraft assuming greater significance, magnetic materials which will retain their magnetic properties at high temperatures (> 200 C) will be needed. High temperature magnetic materials will also be required for magnetic bearings. With the move to the MEE, control software and the electronics in which they are embedded will also shift in to the engines. Silicon carbide based electronics systems capable of functioning at temperatures of the order of 400°C as against 100°C capability of silicon based semiconductor systems will become important.
Materials for Stealth
It is imperative that all military air vehicles of future possess adequate stealth depending on their role during service. Futuristic technologies will address methods to reduce overall visual, acoustic, magnetic, infra-red and radio frequency signatures. Stealth technologies are evolving continuously out pacing the detection capabilities. The application of radar absorbing materials (RAM) to reduce the radar cross section due to reflective edges and cavities such as inlets have evolved as important strategies to achieve stealth. Many RAMs engineered by loading dielectric materials with conductive, reflective and magnetic elements are being developed to soak up microwave energy. The present day low weight honey comb RAMs are innovative Radar Absorbing Structures (RAS). Frequency selective fabric composites (FSFCs) produced by weaving glass and carbon fibres together in particular patterns are also promising as they can be both load bearing and RF energy absorbing.
Active radar absorbers in which electromagnetic properties of one or more of the layers is varied in response to an electrical or optical signal) and smart / adaptive, dynamic RAMs based on conductive polymers where resonant frequency can be tuned by changing the resistive and capacitive elements of the material are areas where more research will progress. Such materials will permit adaptive control of radar cross sections of air vehicles which could be changed from a high value during peace-time operations to a low value in combat situations which could eventually eliminate the concept of exclusive stealth air vehicles. Plasma based solutions will also be seriously considered. This technology involves absorption of radar waves by the free electrons in the plasma and transforming it to other forms, such as heat dramatically reducing the radar cross section of air vehicles. Issues related to creating and maintaining a cold plasma cloak in flight will be intensely researched in future for possible application in stealthy aircraft. New generation engines with supercruising capabilities without afterburner and active cooling of leading edges to cut down the IR signatures from hot locations is also becoming important.
Visual and acoustic signature reduction will also be important in future, especially for low flying unmanned air vehicles. Active visual camouflage / stealth using thin sheets of light-emitting “electrochromic” polymers that glow and change colors depending on the voltage used for charging is under study. This may lead to photosensitive receptors mounted at various locations on the vehicle that will change the color, brightness, and texture of the skin to match the sky above the plane or the terrain below it depending on the inputs about ambient light and color of the sky and ground.
Smart materials and structures
With the technological advancement marching forward, self- cleaning polymerics and self-healing materials have a huge potential as the aerospace materials of the future.
Self-cleaning materials can be found in the natural environment, with lotus leaves being an example. If water is poured over these leaves, it rolls of as beads and cleans the dirt on the leaves. There are two known mechanisms for self cleaning surfaces.
a. Based on wettability of surface as in lotus leaves
b. Based on photocatalytic feature, where light is captured by the photoactive surface, which then produces oxidative radicals that can mineralize and absorb organic molecules.
Future self-cleaning materials can be developed based on these properties, and can be applied to fabric of seats and carpets, and who knows, maybe even to the skin of the future aircraft.
Many smart materials and structures will also become an integral part of airframes of future air vehicles. Major areas where smart materials will be used in structures are:
(A) Health & Usage Monitoring (HUM) for identification or validation of damage in structures. Smart materials embedded in structures will act as sensors to diagnose the state of the structure. It will be part of ‘Damage Prognosis’ which essentially would combine the information from the SHM sub-systems and the usage monitoring sub-systems and apply damage mechanics and behavioural laws to determine the prognosis (residual life) and finally provide inputs for overall health management of the structure i.e., maintenance, repair etc. Implementing damage prognosis concepts will require basic research that would include (i) exploring sensitive materials for sensors and actuators (ii) developing technologies to miniaturize sensors and actuators and to embed them without degradation of the parent structures and (iii) deterministic and probabilistic predictive modeling capabilities, as well as the ability to quantify the uncertainty in these predictions – all of which are at the embryonic stage of development.
(B) Active and adaptive structures for shape control and vibration damping. Smart materials like shape memory polymers, piezoelectric and magnetostrictive materials will play a significant role in the sensing and actuation of wings to change the shape in case of morphing aircraft.
(C) Smart skins which carry load and have integrated avionics (antennae) which can be either active, adaptive or even intelligent structures.
(C) Self-healing of structures that will prevent propagation of cracks in matrix. Many bio-inspired methods of self-healing polymers are being researched that includes microencapsulation of self-healing compounds in polymer matrices and microvascular material systems capable of carrying out multiple healing cycles, even to an area healed earlier. Repair of reinforcing fibers, though not in focus at this point of time, will also assume prominence in the overall approach towards self-healing structures.
Self-healing or self repairing materials are another scope of study which will significantly contribute to keep the functioning intact even after corrosion or mechanical damage. They can de divided into two categories:
a. Intrinsic self-healing materials: These materials can restore back to their structural integrity without any outside help. In these materials, local increase in mobility of the polymeric chains allows crack to be repaired on their own. For eg. wolverine fabric.
b. Extrinsic self-healing materials: These materials use healing agent stored in the material in advance, which is released when triggered by cracks.
Self healing materials containing Boron can be used in aircraft engines, which through the formation of B2O3 can seal the matrix cracks at high temperature. Likewise, self-healing epoxy composites which can produce coatings can be used to protect aircraft structures from corrosion.