Metal matrix, fibre–metal and ceramic matrix composites for aerospace applications
Metal matrix composites (MMCs) are lightweight structural materials used in a small number of aircraft, helicopters and spacecraft. MMC materials consist of hard reinforcing particles embedded within a metal matrix phase. The matrix of MMCs is usually a low density metal alloy (e.g. aluminium, magnesium or titanium). The metal alloys used in aircraft structures, such as 2024 Al, 7075 Al and Ti–6Al–4 V, are popular matrix materials for many MMCs. Nickel superalloys may be used as the matrix phase in MMCs for high-temperature applications.
The metal matrix phase is strengthened using ceramic or metal oxide in the form of continuous fibres, whiskers or particles. Boron (or borsic, a SiC-coated boron), carbon and silicon carbide (SiC) are often used as continuous fibre reinforcement, and these are distributed through the matrix phase. Silicon carbide, alumina (Al2O3) and boron carbide (B4C) are popular particle reinforcements. The maximum volume content of reinforcement in MMCs is usually below 30%, which is lower than the fibre content of aerospace carbon–epoxy composites (55–65% by volume). Reinforcement contents above about 30% are not often used because of the difficulty in processing, forming and machining of the MMC owing to high hardness and low ductility.
MMCs offer a number of advantages compared with their base metal, including higher elastic modulus and strength, lower coefficient of thermal expansion, and superior elevated temperature properties such as improved creep resistance and rupture strength. Some MMCs also have better fatigue performance and wear resistance than the base metal.
Lower density is an attractive property of MMCs made using a metal matrix having a higher specific gravity than the ceramic reinforcement. The density of most ceramic materials is moderately low (generally under 3 g cm− 3) and, when used in combination with a denser metal, there is an overall reduction in weight. Titanium, steel and nickel matrix composites, for example, have lower densities than their base metal which translates into a weight saving. However, aluminium and magnesium alloys, which have a lower or similar density to the ceramic reinforcement, may incur a weight penalty. Figure 16.1 shows the percentage density change of several aerospace alloys with increasing volume content of silicon carbide reinforcement. The weight saving is an incentive for heavy metals such as nickel-based superalloys, provided this is done without degrading important structural properties such as toughness and creep resistance.
MMCs are characterised by high stiffness, strength and (in most materials) fatigue resistance. The improvement in these properties is controlled by the stiffness, strength, volume content and shape of the reinforcement. This control allows the properties of MMCs to be tailored to an application requiring a combination of high properties. The properties of MMC reinforced with continuous fibres are anisotropic, with their mechanical properties such as stiffness and strength being highest in the fibre direction. The modulus is isotropic in MMCs containing whiskers that are randomly aligned or particles that are evenly dispersed through the metal matrix phase. Figure 16.2 shows the effect of increasing volume content of continuous fibres of alumina on the elastic modulus and yield strength of an aluminium–lithium alloy in the parallel and transverse (anti-fibre) directions. The longitudinal and transverse properties increase linearly with the fibre content, and it is possible to increase the stiffness and strength by 50–100% compared with the base metal. The greatest improvement in longitudinal stiffness and strength is achieved using continuous fibres, followed by whiskers and then particles. The fatigue performance of metals can also be improved by ceramic reinforcement. For example, Fig. 16.3 shows a large increase in the fatigue life of an aluminium alloy when reinforced with silicon carbide particles. Improvements in fatigue are generally the result of higher modulus and work-hardening rates of the composite compared with the base material. However, there are occasions when the fatigue life is degraded by ceramic reinforcement.
MMCs are characterised by low fracture toughness and ductility, which are problems when used in damage-tolerant structures. The toughness and ductility of MMCs depends on several factors: composition and microstructure of the matrix alloy; type, size and orientation of the reinforcement; and the processing conditions. Figure 16.4 shows reductions to the fracture toughness and ductility of aluminium when the reinforcement content is increased to 30–40%, which are typical concentrations used for high-strength applications. Reductions in toughness of over 30% and ductility to under 2–3% are common with many MMC materials.
Aerospace applications of MMCs are few, and their use is not expected to grow significantly in the foreseeable future owing to problems with high manufacturing costs and low toughness. The current applications are confined to structural components where the design and certification issues are straightforward and there is low risk of failure. MMCs are currently not used on civil airliners and only rarely in military aircraft. One notable application is the two ventral fins on the F-16 Fighting Falcon, which are located on the fuselage just behind the wings (Fig. 16.5). The original ventral fins were made of 2024-T4 aluminium. The fins are subjected to turbulent aerodynamic buffeting which causes fatigue cracking in the aluminium alloy. Replacement ventral fins made using a ceramic particle-reinforced aluminium matrix composite (6092Al–17.5% SiC) are fitted to the F-16 to alleviate the fatigue problem. This MMC increased the specific stiffness of the fins by 40% over the baseline design, which reduced the tip deflections by 50% and lowered the torsion loads induced by buffeting. The use of MMC is expected to extend the service life of the fins by about 400%; with the reduced maintenance, downtime and inspections costs saving the USAF an estimated $26 million over the aircraft life. The 6092Al–17.5% SiC particle composite is also used in fuel access door covers on F-16 aircraft. Similar to the ventral fins, the higher stiffness, strength and fatigue life of the MMC eliminated cracking problems experienced with the original aluminium alloy used in door covers.
MMCs are used in the main rotor blade sleeve of the Eurocopter EC120 and N4 helicopters. The sleeve is a critical rotating component because failure results in total loss of the main rotor blade. Sleeve materials require an infinite fatigue life under the operating stresses of the rotor blades, together with high specific stiffness and good fracture toughness. Titanium alloy is normally used for the sleeve, but to reduce cost and weight while maintaining high fatigue performance, strength and toughness, the sleeves for the EC120 and N4 are fabricated using a particle-reinforced aluminium composite (2009Al–15% SiC).
The first successful application of MMCs reinforced using continuous fibres was in the space shuttle orbiter. Struts used to stiffen the mid-fuselage (payload) section of the orbiter are made using aluminium alloy reinforced with 60% boron fibres. This composite is also used in the landing gear drag line of the orbiter. The continuous boron fibres are aligned along the axis of the tubular struts and drag line to provide high longitudinal specific stiffness. About 300 MMC struts are used as frame and rib truss members to form the load-bearing skeleton of the orbiter cargo bay, which provides a 45% weight saving over conventional aluminium construction.
Another space application of continuous-fibre MMCs is the high gain antenna boom for the Hubble space telescope (Fig. 16.6). The boom, which is 3.6 m long, is a lightweight structure requiring high axial stiffness and low coefficient of thermal expansion to maintain the position of the antenna during space manoeuvres. The boom also provides a wave guide function and therefore needs good electrical conductivity to transmit signals between the spacecraft and antenna dish. The boom is made using 6061 aluminium reinforced with continuous carbon fibres. The material provides a 30% weight saving compared with previous designs based on monolithic aluminium or carbon–epoxy composite.
There are many potential applications for MMCs in aircraft gas turbine engines and scram jet engines owing to their low weight, high-temperature stability and excellent creep resistance. Titanium matrix composites may replace heavier nickel-based superalloys in high-pressure turbine blades and compressor discs for jet engines, although much more development work is needed before this is achieved.
The engine and structural applications for MMCs is limited owing to a number of technological problems that are difficult to resolve. MMCs are expensive materials to process into finished components because of high costs in manufacturing, shaping and machining. MMCs are difficult to plastically form using conventional plastic forming processes such as rolling or extrusion owing to their low ductility and high hardening rate. Relatively low levels of plastic forming can cause micro-cracking in the forged MMC component. MMCs are also difficult to machine by milling, routing, drilling and other material removal processes because of their high hardness, which causes rapid tool wear.
MMCs also have poor ductility and low toughness, which is a major concern for the aerospace applications where damage tolerance is a key design consideration for many structural and engine components. Lastly, aerospace engineers are not familiar with MMCs and the large database of technical information required for aircraft certification is lacking. Until the technical issues are resolved, the technology reaches maturity, and aircraft designers become familiar and confident with MMCs, it is likely that the applications for these materials will remain limited despite their high specific stiffness and strength.
Fibre–metal laminates (FMLs) are lightweight structural materials consisting of alternating thin layers of metal and fibre–polymer composite (Fig. 16.7). FML is made using thin sheets (0.2–0.4 mm) of lightweight metal, such as aluminium or titanium, bonded to thin layers of prepreg composite, with the outer surfaces being metal. The most common FML used in aerospace structures is GLARE® (glass-reinforced fibre–metal laminate). GLARE® consists of thin sheets of aluminium alloy bonded to thin layers of high strength glass–epoxy prepreg composite. GLARE® is used in the upper fuselage and leading edges of the vertical fin and horizontal stabilisers of the Airbus 380 aircraft. GLARE® is also used in the cargo doors of the C-17 Globemaster III. Another FML is ARALL® (aramid aluminium laminate) that is made of alternating sheets of aluminium alloy and aramid–epoxy. ARALL® has exceptional impact resistance and damage tolerance, but its use in aerospace structures is limited because of moisture ingress problems into the aramid–epoxy layers, which reduces material integrity.
• Low weight. FMLs are lighter than the equivalent monolithic metal owing to the lower density layers of prepreg composite. For example, GLARE® has a lower density (~ 2.0 g cm− 3) than monolithic aluminium alloy (2.7 g cm− 3), which results in a significant weight saving in the A380.
• Mechanical properties. FMLs have higher tensile strength, damage tolerance, impact strength and fatigue resistance than the monolithic metal. These are key reasons for the use of GLARE® in the A380 fuselage. Under flight-loading conditions, the rate of fatigue crack growth in GLARE® is 10–100 times slower than monolithic aluminium alloy because the cracks are stopped or deflected at the metal–composite interfaces.
• Tailored properties. It is possible to tailor the structural properties of FMLs by adjusting the number, type and alignment of the prepreg layers to suit local stresses and shapes through the aircraft.
• Corrosion resistance. Through-the-thickness corrosion is prevented in FML owing to the barrier role played by the composite layers. This limits corrosion damage to the surface metal layer and the internal metal sheets are protected by the composite. Therefore, the incidence of corrosion damage such as pitting and stress corrosion cracking, which are major problems for monolithic aluminium aircraft structures (see chapter 21), is reduced with GLARE®.
• Fire. FML has high fire resistance owing to its low thermal conductivity. GLARE® has lower through-thickness thermal conductivity than monolithic aluminium which results in slower heat flow through the fuselage structure in the event of post-crash fire. FML has better resistance to burn-through in the event of fire and can potentially substitute for titanium in firewalls.
Despite the many benefits derived from using FMLs such as GLARE®, the future of these materials in aircraft structures is uncertain because of their high cost. FMLs are typically 7–10 times more expensive than the monolithic metal, and the high cost is a major impediment to their use in aircraft.
Ceramics are used mainly in aerospace for their outstanding thermal stability; which includes high melting temperature, low thermal conductivity, and high modulus, compressive strength and creep resistant properties at high temperature. There are many types of ceramics, with the most important for aerospace being polycrystalline materials and glass–ceramics. A characteristic of ceramic materials is high strength under compression loading, but low strength and toughness under tension. The compressive strength of most polycrystalline ceramics is in the range 4000–10 000 MPa, whereas the tensile strength is only 50–200 MPa. The fracture toughness of most ceramics is 10–50 times lower than high-strength aluminium alloy. Ceramics have low tolerance to tiny voids and cracks that occur during fabrication or in service, and these defects cause brittle fracture under low tensile loads. For this reason, the strength, ductility and toughness properties are too low for ceramics to be used in aerospace structures.
Ceramic matrix composites (CMCs) are used to increase the tensile strength and toughness of conventional ceramic material while retaining such properties as lightness, high stiffness, corrosion resistance, wear resistance and thermal stability. CMCs consist of a ceramic matrix phase reinforced with ceramic fibres or whiskers. The reinforcing fibres and whiskers greatly increase the breaking strength and toughness of ceramics, as shown in Table 16.1. Improvements in the strength and toughness of over 100% are gained when ceramics are reinforced with continuous fibres. Although the mechanical properties are improved by the reinforcement, the strength and toughness are still too low for their use in primary aerospace structures. However, the properties are sufficient for the use of CMCs in lightly loaded aerospace components requiring thermal stability.
|Material||Flexural strength (MPa)||Fracture toughness (MPa m− 1/2)|
CMCs also have high resistance against thermal shock, which is the resistance to cracking and failure when rapidly heated and cooled (often in the presence of high pressure). Rapid cooling of the surface of a hot material is accompanied by surface tensile stresses. The surface contracts more than the interior, which is still relatively hot. As a result, the surface ‘pushes’ the interior into compression and is itself ‘pulled’ into tension. Surface tensile stress creates the potential for brittle fracture. The high thermal shock resistance of CMCs is the result of their high strength at high temperature combined with their low coefficient of thermal expansion. The excellent thermal shock resistance and thermal stability allows CMCs to be used at temperatures many hundreds of degrees higher than the melting point of metal alloys. Current metals-based technology can produce alloys stable to about 1000 °C, and nickel superalloys can be used at slightly higher temperatures when insulated with a thermal barrier coating and when cooling systems are included. CMCs can survive for long periods of time at temperatures well above the melting point of superalloys. CMCs retain their stiffness, strength and toughness properties to temperatures close to their melting temperature, which can exceed 3000 °C.
The CMC materials most often used in aerospace are silicon carbide-silicon carbide (SiC–SiC) composite, glass–ceramic composite, and carbon–carbon composite. All three types are used in aerospace thermal protection systems. SiC–SiC composites are only used in a few niche applications, such as convergent–divergent engine nozzles to fighter aircraft where the temperatures reach ~ 1400 °C. Several types of glass–ceramic composites are used in heat shields for re-entry spacecraft such as the space shuttle orbiter. The most important ceramic matrix composite is carbon–carbon, which is used in heat shields, rocket engines and aircraft brake pads.
Carbon–carbon composites consist of carbon fibres embedded in a carbon matrix. Continuous carbon fibres (rather than short whiskers) are used as the reinforcement to maximise strength and toughness. Similarly to polymer matrix laminates, the tailored arrangement of carbon fibres in selected orientations within a continuous carbon matrix is used to achieved the desired properties. The fibre architectures include unidirectional, bidirectional, three-dimensional orthogonal weaves, or multidirectional weaves and braids.
Carbon–carbon composites are expensive to fabricate and costly to machine into the final product shape. A major drawback of these materials is their high cost which makes them prohibitively expensive for many aerospace applications where otherwise they are well suited. The manufacturing process involves impregnating fibrous carbon fabric with an organic resin such as phenolic. The material is pyrolysed (i.e. heated in the absence of oxygen) at high temperature to convert the resin matrix to carbon. The material, which is soft and porous, is impregnated with more resin and pyrolysed several more times until it forms a compact, stiff and strong composite.
Carbon–carbon composites have many thermal and mechanical properties required by aerospace materials that must operate at high temperature. These properties include stable mechanical properties to temperatures approaching 3000 °C; high ratios of stiffness-to-weight and strength-to-weight; low thermal expansion; and good resistance to thermal shock, corrosion and creep. The material properties of carbon–carbon composites, as for fibre–polymer composites, are anisotropic. The stiffness, strength and thermal conductivity is highest along the fibre axis and lowest normal to the fibre direction. The properties also depend on the fibre fraction, type of carbon fibre, fibre architecture and processing cycle.
Table 16.2 shows the improvement in properties achieved by reinforcing polycrystalline carbon with continuous carbon fibres. The carbon fibres improve the properties of monolithic carbon by ten times or more, with no increase in weight. A unique feature of carbon–carbon composite is that its strength can increase with temperature. Figure 16.8 shows the strength properties of several carbon–carbon composites and other aerospace materials over a wide temperature range. The strength of advanced types of carbon–carbon increase with the temperature up to at least 2000 °C, which is an obvious benefit when used at high temperature. The improvement in strength is caused by the closing of microcracks in the interface between the carbon matrix and the fibre matrix when the temperature is raised.
|Property||Polycrystalline (monolithic) carbon||Reinforced carbon–carbon composite|
|Elastic modulus (GPa)||10–15||40–100|
|Tensile strength (MPa)||40–60||200–350|
|Compressive strength (MPa)||110–200||150–200|
|Fracture toughness (MPa m− 1/2)||0.07–0.09||5–10|
16.8 Dependence of specific strength on temperature for carbon–carbon composites and nickel superalloy. (adapted from D. R. Askeland, The science and engineering of materials, Stanley Thornes (Publishers) Ltd., 1996)
Carbon–carbon composite suffers severe high-temperature oxidation which causes rapid degradation and erosion. Carbon becomes susceptible to oxidation when heated above 350 °C and the oxidation rate rises rapidly with temperature. As a result, there is carbon–carbon breakdown in the presence of air. It is necessary to protect carbon–carbon composites using an oxidation-resistant surface coating. The carbon–carbon tiles on the space shuttle orbiter, for example, are coated with silicon carbide to provide oxidation protection to about 1200 °C.
Heat shields of re-entry space vehicles such as the space shuttle orbiter are made using carbon–carbon composite. This material is used in the nose cone and leading edges of the orbiter where the temperature reaches 1600 °C during re-entry. These materials are also used in the nose cones of intercontinental ballistic missiles. Carbon–carbon is also used in rocket engines and nozzles for its high thermal stability and thermal shock resistance. During blast-off, the thrusters generate temperatures ranging from 1500–3500 °C at heating rates of 1000–5000 °C s− 1 and produce combustion pressures approaching 300 atmospheres. This generates extreme thermal shock, a rapid rise in temperature and pressure, which shatters most materials. The excellent thermal shock resistance of carbon–carbon composite ensures their survival in these harsh conditions.
Another space application for carbon–carbon composites is in thermal doublers, which are used to remove heat from a spacecraft (usually generated by internal electronics equipment) and then radiate that heat into space. These composites have the thermal properties and low weight required for thermal doublers on satellites. It is also possible to use carbon–carbon composite in aircraft heat exchangers, which are used to cool hot gases and liquids such as hydraulic and engine fluids. Heat exchangers require materials with high thermal conductivity, corrosion resistance, stiffness, strength, low permeability and high temperature stability; carbon–carbon composite is one of the few materials with this unique combination of properties.
Carbon–carbon composite is used in aircraft brake pads because of its low weight, high wear resistance and high-temperature properties. The friction between aircraft brake discs can generate average temperatures of around 1500 °C, with transient hot-spot temperatures of up to 3000 °C. The disc material must therefore have high temperature strength, thermal shock resistance and high thermal conductivity to rapidly remove heat from the contact zone between the disc surfaces. Carbon–carbon is one of the few materials with this combination of properties required for brakes discs. The weight of aircraft brakes is also significant. A large civil airliner usually has eight sets of brakes with a combined weight of over 1000 kg when made using heat-resistant steel. An equivalent set of brakes made of carbon–carbon weigh less than 700 kg. Carbon–carbon brakes are used in most fighter aircraft and increasingly in civil airliners. The Boeing 767 and 777 airliners and several Airbus aircraft are equipped with carbon–carbon brake discs.
Although not currently used, CMCs such as carbon-carbon have potential applications in jet engines because of their ability to operate uncooled at temperatures beyond the reach of metals. Cycle efficiency improvements, from reducing cooling air to turbine aerofoils and seals, lead to significant fuel consumption benefits. For example, it has been estimated that the replacement of metal seals by advanced CMCs technology would yield a saving of $250 000 on the Boeing 777 over 15 years.
Section 16.7 at the end of the chapter presents a case study of the use of ceramic matrix composites in the space shuttle orbiter.
Metal matrix composites offer several improved properties compared with the monolithic base metal, including higher stiffness, strength, wear resistance and, in some materials, fatigue performance and reduced weight. However, there are also drawbacks with MMCs, such as higher manufacturing and machining costs together with lower toughness and ductility.
The aerospace applications of MMCs are limited because of their high cost and low toughness. MMCs have been used in military fighter aircraft and helicopter rotor blades to reduce fatigue problems, fuselage struts in the space shuttle orbiter, and selected components on the Hubble space telescope.
Fibre–metal laminates are lightweight structural materials consisting of thin sheets of metal and fibre–polymer composite. The most popular fibre–metal laminate used is GLARE® that consists of alternating layers of high-strength aluminium alloy and glass fibre–epoxy. The future of fibre–metal laminates is uncertain because of their high manufacturing cost.
GLARE® is used in the A380 fuselage because of its lower weight, higher static and impact strengths, better damage tolerance, longer fatigue life and superior corrosion resistance compared with monolithic aluminium alloy.
Ceramic matrix composites are used for high-temperature applications. The materials are characterised by low weight, high stiffness and compressive strength, and excellent thermal stability and thermal shock resistance.
Thermal shock.: Cracking of materials (usually brittle solids such as ceramics) caused by a sudden drop or rise in temperature. Thermal shock occurs when a sharp thermal gradient causes different parts of an object to expand by different amounts, thus generating a stress gradient. At some point, the thermal stress overcomes the strength of the material, causing a crack to form.
Dogan, C.P. Properties and performance of ceramic–matrix and carbon–carbon composites. In: Miracle D.B., Donaldson S.L., eds. ASM handbook volume 21: composites. Material Park, OH: ASM International; 2001:859–868.
The space shuttle orbiter is the world’s first, and so far only, reusable spacecraft. (The Russian built Buran was developed in the 1980s as a reuseable vehicle similar to the space shuttle which performed just one unmanned space mission). The construction of the orbiter is similar to a conventional airliner; the body is built mostly of high-strength aluminium alloys and the payload doors are fibre–polymer sandwich composite material. Although speciality materials are used in highly stressed components of the mainframe, such as titanium and metal matrix composite, most of the structure is built with the same aluminium alloys used in civil and military aircraft. Chapter 3 provides more information on the structural materials used in the orbiter.
The orbiter is covered with ceramic tiles that protect the vehicle, payload and crew from the extreme heat generated during re-entry into the Earth’s atmosphere. Figure 16.9 shows the temperature profile over the orbiter surface during re-entry; the nose and leading edges are heated to 1000–1400 °C whereas the maximum temperatures of other sections are 200–1000 °C. The properties of aluminium and polymer matrix composite demand that the orbiter’s structure is kept below 150–200 °C, and therefore the heat insulation tiles are essential.
The orbiter is covered with over 25 000 reusable ceramic matrix composite tiles (Fig. 16.10). All the tiles are brittle and can crack when stressed or impacted, as tragically proven when the Columbia broke up during re-entry on flight STS-107 (February 2003). Chapter 18 describes the impact fracture of the carbon–carbon tiles that caused the Columbia incident. Because the aluminium structure of the orbiter expands and contracts owing to temperature changes over the course of a flight mission, the tiles are not mounted directly onto the skin. Instead, compliant adhesive felt pads are used to bond the tiles to the aluminium.
16.10 Heat insulation tiles on the space shuttle orbiter where the black regions are carbon–carbon composite and white regions are mostly high-purity silica ceramic. RCC is reinforced carbon–carbon composite; HRSI is high-temperature reusable surface insulation; LRSI is low-temperature reusable surface insulation; FRSI is fibrous reusable surface insulation.
The forward nose cap and leading edges of the wings are covered with carbon–carbon composite tiles. These tiles are coated with black silicon carbide for oxidation resistance, with the black colour helping to radiate heat during re-entry. Carbon–carbon tiles are used in the hottest regions where the temperature exceeds about 800 °C. The cooler regions, where the temperature is 200–800 °C are covered mostly with white ceramic tiles that reflect solar radiation to keep the space shuttle cool. The tiles consist of porous high-purity silica ceramic covered with borosilicate glass. Two other types of silica-based tiles are used on the orbiter: fibrous refractory composite insulation (FRCI) or toughened unipiece fibrous insulation (TUFI). FRCI is used in a few selected regions whereas the TUFI is applied over the extreme back of the orbiter near the engines.