14 COMPOSITE MATERIALS FOR AIRCRAFT STRUCTURES Table 1.5 Candidate Continuous Fiber MMCs Compared with PMCs ·Promising Systems boron/aluminium alloy;silicon carbide/aluminum;alumina/aluminum -silicon carbide/titanium;silicon carbide/titanium aluminide -carbon/aluminum;carbon/magnesium (only for space applications) ·Advantages -higher temperature capability,particularly titanium and titanium aluminide -higher through-thickness strength,impact damage resistant -higher compressive strength -resistant to impact damage -high electrical and thermal conductivity Disadvantages -limited and costly fabrication technology difficult and inefficient joining technology -limited in temperature capability by fiber/matrix chemical incompatibility -prone to thermal fatigue:fiber/matrix expansion mismatch problem -prone to corrosion,particularly with conducting fibers applications of the MMCs include engine components,such as fan and compressor blades,shafts,and possibly discs,airframe components,such as spars and skins,and undercarriage components,such as tubes and struts. Carbon/aluminum alloy and carbon/magnesium alloy composites are particularly attractive for satellite applications,including aerials and general structures.These MMCs combine the high specific properties and low,thermal expansion coefficients exhibited by the PMCs together with the advantages indicated in Table 1.5.For example,high conductivity serves to minimize thermal gradients,and therefore distortion,when a space structure is subjected to directional solar heating. However,MMCs based on carbon fibers,although potentially low-cost,suffer several drawbacks for non-space applications.These include oxidation of carbon fibers from their exposed ends at elevated temperature and corrosion of the metal- matrix in wet environments due to galvanic action with exposed fibers.Other potential non-structural applications of carbon/metal composites include 1)carbon/ lead and carbon/copper-tin alloys for bearings,2)carbon/copper for high-strength conductors and marine applications,and 3)carbon/lead for battery electrodes. The earliest developed and probably still the most exploited aluminum matrix MMC is boron/aluminum,based on CVD boron filaments.This MMC is used in the tubular structure in the Space Shuttle.In the future,boron/aluminum may be superseded by CVD silicon carbide/aluminum (or silicon carbide coated boron), which has the advantage of much greater resistance to attack by liquid aluminum. The increased resistance simplifies composite fabrication and improves fiber/ matrix compatibility at elevated temperature
14 COMPOSITE MATERIALS FOR AIRCRAFT STRUCTURES Table 1.5 Candidate Continuous Fiber MMCs Compared with PMCs • Promising Systems - boron/aluminium alloy; silicon carbide/aluminum; alumina/aluminum - silicon carbide/titanium; silicon carbide/titanium aluminide - carbon/aluminum; carbon/magnesium (only for space applications) • Advantages - higher temperature capability, particularly titanium and titanium aluminide - higher through-thickness strength, impact damage resistant - higher compressive strength - resistant to impact damage - high electrical and thermal conductivity • Disadvantages - limited and costly fabrication technology - difficult and inefficient joining technology - limited in temperature capability by fiber/matrix chemical incompatibility - prone to thermal fatigue: fiber/matrix expansion mismatch problem - prone to corrosion, particularly with conducting fibers applications of the MMCs include engine components, such as fan and compressor blades, shafts, and possibly discs, airframe components, such as spars and skins, and undercarriage components, such as tubes and struts. Carbon/aluminum alloy and carbon/magnesium alloy composites are particularly attractive for satellite applications, including aerials and general structures. These MMCs combine the high specific properties and low, thermal expansion coefficients exhibited by the PMCs together with the advantages indicated in Table 1.5. For example, high conductivity serves to minimize thermal gradients, and therefore distortion, when a space structure is subjected to directional solar heating. However, MMCs based on carbon fibers, although potentially low-cost, suffer several drawbacks for non-space applications. These include oxidation of carbon fibers from their exposed ends at elevated temperature and corrosion of the metalmatrix in wet environments due to galvanic action with exposed fibers. Other potential non-structural applications of carbon/metal composites include 1) carbon/ lead and carbon/copper-tin alloys for bearings, 2) carbon/copper for high-strength conductors and marine applications, and 3) carbon/lead for battery electrodes. The earliest developed and probably still the most exploited aluminum matrix MMC is boron/aluminum, based on CVD boron filaments. This MMC is used in the tubular structure in the Space Shuttle. In the future, boron/aluminum may be superseded by CVD silicon carbide/aluminum (or silicon carbide coated boron), which has the advantage of much greater resistance to attack by liquid aluminum. The increased resistance simplifies composite fabrication and improves fiber/ matrix compatibility at elevated temperature
INTRODUCTION AND OVERVIEW 15 (a) b Fig.1.6 a)Boron-fiber/aluminum composite,showing boron fibers around 125 um in diameter (see Chapter 3 for fiber details);b)carbon-fiber/aluminum composite produced from aluminum-coated carbon fibers(fibers around 8 um in diameter). A typical microstructure of a boron/aluminum composite is shown in Figure 1.6a,whereas,for comparison,Figure 1.6b shows the microstructure of a typical carbon/aluminum composite. Current aluminum matrix MMCs do not offer a significant increased temperature capability over PMCs based on high-temperature matrices such as BMIs and polyimides.Thus,unless some other properties are required,such as thermal conductivity,aluminum MMCs generally have no major advantage over PMCs and are far more expensive. In contrast,titanium alloy and titanium aluminide MMCs,based on CVD silicon-carbide-fiber reinforcement,have a large margin on temperature capability over PMCs.They also have excellent mechanical properties compared with conventional titanium alloys(100%increase in stiffness and 50%increase in strength);however,they cannot match PMCs in terms of moderate temperature properties and are much more expensive. Titanium-based MMCs are damage tolerant,and so in addition to high- temperature applications in high-speed transport and gas-turbine engines,they are also being evaluated as a replacement for steel undercarriage components where they could prove to be cost-effective.Titanium MMCs lend themselves very well to selective reinforcement(where reinforcement is applied only in high-stress areas), as titanium is readily diffusion bonded.For example,layers of titanium/silicon carbide can be used to reinforce a high-temperature compressor disk with a 70% weight saving.The large weight saving results from the elimination of much of the inner material of the disk.The resulting construction is a titanium MMC- reinforced ring.If the disk has integral blades,it is called a bling.Blings provide marked improvements in the performance of military gas-turbine engines.Titanium MMCs can also be used to reinforce titanium-skinned fan blades or for the face skins of a sandwich panel with a super-plastically formed core. MMCs capable of operation to temperatures over 800C are also keenly sought for gas-turbine applications.Unfortunately,the use of available
INTRODUCTION AND OVERVIEW 15 (a) (b) Fig. 1.6 a) Boron-fiber/aluminum composite, showing boron fibers around 125 p~m in diameter (see Chapter 3 for fiber details); b) carbon-fiber/aluminum composite produced from aluminum-coated carbon fibers (fibers around 8 pLm in diameter). A typical microstructure of a boron/aluminum composite is shown in Figure 1.6a, whereas, for comparison, Figure 1.6b shows the microstructure of a typical carbon/aluminum composite. Current aluminum matrix MMCs do not offer a significant increased temperature capability over PMCs based on high-temperature matrices such as BMIs and polyimides. Thus, unless some other properties are required, such as thermal conductivity, aluminum MMCs generally have no major advantage over PMCs and are far more expensive. In contrast, titanium alloy and titanium aluminide MMCs, based on CVD silicon-carbide-fiber reinforcement, have a large margin on temperature capability over PMCs. They also have excellent mechanical properties compared with conventional titanium alloys (100% increase in stiffness and 50% increase in strength); however, they cannot match PMCs in terms of moderate temperature properties and are much more expensive. Titanium-based MMCs are damage tolerant, and so in addition to hightemperature applications in high-speed transport and gas-turbine engines, they are also being evaluated as a replacement for steel undercarriage components where they could prove to be cost-effective. Titanium MMCs lend themselves very well to selective reinforcement (where reinforcement is applied only in high-stress areas), as titanium is readily diffusion bonded. For example, layers of titanium/silicon carbide can be used to reinforce a high-temperature compressor disk 9 with a 70% weight saving. The large weight saving results from the elimination of much of the inner material of the disk. The resulting construction is a titanium MMCreinforced ring. If the disk has integral blades, it is called a bling. Blings provide marked improvements in the performance of military gas-turbine engines. Titanium MMCs can also be used to reinforce titanium-skinned fan blades or for the face skins of a sandwich panel with a super-plastically formed core. MMCs capable of operation to temperatures over 800°C are also keenly sought for gas-turbine applications. Unfortunately, the use of available
16 COMPOSITE MATERIALS FOR AIRCRAFT STRUCTURES high-performance carbon or ceramic fibers is not feasible with high-temperature alloy matrices because of severe compatibility problems.Attempts to use barrier layers on fibers,such as metal oxides or carbides,to prevent chemical reaction have been unsuccessful.In addition,due to the high temperatures and mismatch in coefficients of thermal expansion,thermal fatigue would be a serious problem with these composites.A practical,but not very attractive solution because of the poor specific properties,is the use of refractory metal wire as the reinforcement. This approach has the potential to produce turbine blade materials with an additional 100C capability over conventional superalloys.A promising composite is based on tungsten alloy wires (W-1%ThO2 or W-Hf-C type)in an iron-based(Fe-Cr-Al-Y)matrix.This alloy has relatively high ductility and excellent oxidation resistance requiring no protective coating.However,a coating such as TiC or TiN may be needed on the fibers to avoid attack by the matrix. Costs of the continuous fiber MMCs are(and almost certainly will continue to be)very high compared with PMCs,and the range of sizes and shapes that can be produced is much more limited.As mentioned previously,MMCs based on aluminum alloy matrices will be strongly challenged for most elevated temperature applications by current and emerging PMCs. An alternative to the use of"artificial"fiber reinforcement to produce high- temperature MMCs is to use directionally solidified eutectics.Here the reinforcing phase,produced by eutectic (or eutectoid)decomposition,is in the form of aligned platelets or fibers.These "natural"composites have a great advantage in that the matrix and reinforcement are in chemical equilibrium. However,surface energetics can cause the fibers or laminates to form spherical particles over long periods at elevated temperature,destroying the reinforcing effect.In addition,thermal fatigue can cause internal cracking as well as accelerating spheroidizing of the microstructure.Promising systems studied in the past include Co-Ta-C and Ni-Ta-C. 1.7.2 Particulate MMCs Particulate MMCs should be mentioned in this overview because they may have extensive aerospace applications10 as structural materials.In these composites,aluminum or titanium alloy-matrices are reinforced with ceramic particles,generally silicon carbide or alumina in the micron range.Because reinforcement is not directional as with fiber-reinforced MMCs,properties are essentially isotropic.The specific stiffness of aluminum silicon-carbide particulate MMCs (Al/SiCp.where the subcript p refers to particulate)can exceed conventional aluminum alloys by around 50%at a 20%particle volume fraction.For comparison,an MMC with inclusion of silicon-carbide fibers at a similar volume fraction will increase its specific stiffness increased by around 100%
16 COMPOSITE MATERIALS FOR AIRCRAFT STRUCTURES high-performance carbon or ceramic fibers is not feasible with high-temperature alloy matrices because of severe compatibility problems. Attempts to use barrier layers on fibers, such as metal oxides or carbides, to prevent chemical reaction have been unsuccessful. In addition, due to the high temperatures and mismatch in coefficients of thermal expansion, thermal fatigue would be a serious problem with these composites. A practical, but not very attractive solution because of the poor specific properties, is the use of refractory metal wire as the reinforcement. This approach has the potential to produce turbine blade materials with an additional 100°C capability over conventional superalloys. A promising composite is based on tungsten alloy wires (W-l% ThO2 or W-Hf-C type) in an iron-based (Fe-Cr-A1-Y) matrix. This alloy has relatively high ductility and excellent oxidation resistance requiring no protective coating. However, a coating such as TiC or TiN may be needed on the fibers to avoid attack by the matrix. Costs of the continuous fiber MMCs are (and almost certainly will continue to be) very high compared with PMCs, and the range of sizes and shapes that can be produced is much more limited. As mentioned previously, MMCs based on aluminum alloy matrices will be strongly challenged for most elevated temperature applications by current and emerging PMCs. An alternative to the use of "artificial" fiber reinforcement to produce hightemperature MMCs is to use directionally solidified eutectics. Here the reinforcing phase, produced by eutectic (or eutectoid) decomposition, is in the form of aligned platelets or fibers. These "natural" composites have a great advantage in that the matrix and reinforcement are in chemical equilibrium. However, surface energetics can cause the fibers or laminates to form spherical particles over long periods at elevated temperature, destroying the reinforcing effect. In addition, thermal fatigue can cause internal cracking as well as accelerating spheroidizing of the microstructure. Promising systems studied in the past include Co-Ta-C and Ni-Ta-C. 1.7.2 Particulate MMCs Particulate MMCs should be mentioned in this overview because they may have extensive aerospace applications 1° as structural materials. In these composites, aluminum or titanium alloy-matrices are reinforced with ceramic particles, generally silicon carbide or alumina in the micron range. Because reinforcement is not directional as with fiber-reinforced MMCs, properties are essentially isotropic. The specific stiffness of aluminum silicon-carbide particulate MMCs (A1/SiCp, where the subcript p refers to particulate) can exceed conventional aluminum alloys by around 50% at a 20% particle volume fraction. For comparison, an MMC with inclusion of silicon-carbide fibers at a similar volume fraction will increase its specific stiffness increased by around 100%
INTRODUCTION AND OVERVIEW 17 The primary fabrication techniques are rapid-liquid-metal processes such as squeeze casting or solid-state powder processes based on hot-pressing. Particulate MMCs also have the considerable cost advantage of being formable by conventional metal-working techniques and possibly super-plastic forming and diffusion bonding in the case of titanium-matrix systems.However,because of their high wear resistance,special tools such as diamond-coated drills and diamond-impregnated grinding wheels are required for machining. When fabricated using clean high-grade particles with low porosity and mode- rate particulate volume fraction,particulate MMCs have high strength,acceptable fracture toughness,and good resistance to fatigue crack propagation. The MMCs also have high stiffness and wear resistance compared with conventional alloys.They are therefore suited to small components requiring high stiffness combined with fatigue and wear resistance. 1.7.3 Ceramic-Matrix Composites Ceramic-matrix composites(CMCs)summarized in Table 1.6,offer the main long-term promise for high-temperature applications in gas turbine engines and for high-temperature airframe structures,although there are formidable problems to be overcome.The main requirement is for lightweight blades able to operate uncooled in environments around 1400C. The main limitation is the unavailability of fibers with high-elastic moduli and strength,chemical stability,and oxidation resistance at elevated temperatures. For suitable reinforcement of ceramic matrices (such as alumina and silicon carbide or silicon nitride),the fiber must have high oxidation resistance at high Table 1.6 Candidate Matrix Composites-Advantages and Disadvantages Compared with PMCs ·Systems silicon carbide/glass;silicon carbide silicon nitride carbon/carbon;carbon/glass -alumina/glass ●Advantages -high to very high temperature capability (500-1500C) resistant to moisture problems low conductivity low thermal expansion resistant to aggressive environments ●Disadvantages fabrication can be costly and difficult joining difficult -relatively low toughness matrix microcracks at low strain levels
INTRODUCTION AND OVERVIEW 17 The primary fabrication techniques are rapid-liquid-metal processes such as squeeze casting or solid-state powder processes based on hot-pressing. Particulate MMCs also have the considerable cost advantage of being formable by conventional metal-working techniques and possibly super-plastic forming and diffusion bonding in the case of titanium-matrix systems. However, because of their high wear resistance, special tools such as diamond-coated drills and diamond-impregnated grinding wheels are required for machining. When fabricated using clean high-grade particles with low porosity and moderate particulate volume fraction, particulate MMCs have high strength, acceptable fracture toughness, and good resistance to fatigue crack propagation. The MMCs also have high stiffness and wear resistance compared with conventional alloys. They are therefore suited to small components requiring high stiffness combined with fatigue and wear resistance. 1.7.3 Ceramic-Matrix Composites Ceramic-matrix composites (CMCs) 6 summarized in Table 1.6, offer the main long-term promise for high-temperature applications in gas turbine engines and for high-temperature airframe structures, although there are formidable problems to be overcome. The main requirement is for lightweight blades able to operate uncooled in environments around 1400°C. The main limitation is the unavailability of fibers with high-elastic moduli and strength, chemical stability, and oxidation resistance at elevated temperatures. For suitable reinforcement of ceramic matrices (such as alumina and silicon carbide or silicon nitride), the fiber must have high oxidation resistance at high Table 1.6 Candidate Matrix Composites---Advantages and Disadvantages Compared with PMCs • Systems - silicon carbide/glass; silicon carbide silicon nitride - carbon/carbon; carbon/glass - alumina/glass • Advantages - high to very high temperature capability (500-1500 °C ) - resistant to moisture problems - low conductivity - low thermal expansion - resistant to aggressive environments • Disadvantages - fabrication can be costly and difficult - joining difficult - relatively low toughness - matrix microcracks at low strain levels
18 COMPOSITE MATERIALS FOR AIRCRAFT STRUCTURES temperature because microcracking of the ceramic allows contact between the fibers and the external environment.The fiber must also be chemically compatible with the matrix and must closely match it in its coefficient of thermal expansion.Thus,the use of similar materials for both components appears to offer the most promise,for example,silicon-carbide-fibers/silicon-carbide- matrix or alumina fibers/alumina matrix.1 Unfortunately,available fibers either do not maintain strength at high enough temperatures or(in the case of carbon fibers,for example)have adequate oxidation resistance to provide anywhere near the full exploitation of the potential benefits. CMCs are sometimes based on three-dimentional fiber architectures because in many (but not all)applications,the fibers are required to provide toughness, including through-thickness toughness,rather than stiffness as required in other classes of composites.Thus,for some CMCs,the relatively low fiber volume fraction resulting from this form of construction is not a major limitation. Glass and glass-ceramic matrices are promising for applications at temperatures around 500C because of their excellent mechanical properties and relative ease of fabrication.In contrast to CMCs based on conventional ceramics,such as silicon carbide,the low modulus matrix can be effectively stiffened by suitable fibers and relatively high toughness achieved(typically,an increase of over 30 times the matrix glass alone).Because the matrix does not microcrack at relatively modest strain levels and temperatures,carbon fibers can be used.However,for higher-temperature applications more oxidation resistant fibers such as silicon carbide fibers must be used. Carbon/carbon composites2 have no significant chemical or thermal expansion compatibility problems.However,unless protected,they are also prone to rapid attack atelevated temperature in an oxidizing environment.Even where oxidationis a problem,the composites can be used where short exposures to severe applications at temperatures over 2000C are experienced,for example,in rocket nose-cones, nozzles,and leading edges on hypersonic wings.In the presence of oxygen-reducing conditions,for example with a hypersonic engine running slightly rich on hydrogen fuel,operations for prolonged periods can be maintained.Carbon/carbon composites could be used for prolonged periods at elevated temperature,above 1600C,if effective oxidation-preventative barrier coatings were available.This is a topic of considerable research interest because this composite has the best structural capability of any material at the highest operation temperatures when compared on a specific strength,creep-resistance,or stiffness basis. Some oxidation barriers include silicon carbide or silicon nitride coatings. which provide an oxidation-resistant outer layer over an inner glass layer;the glass can flow into cracks to seal the coating against oxygen penetration.This approach is called self-healing.An inner oxidation-resistant layer may also be used under the glass layer.The refractory layers are applied by CVD or by dip coating,from a liquid or sol-gel precursor.This coating is applied after producing,in the case of the inner layer,a thin tie (coating anchor)layer on the surface of the composite by reaction with,for example,boron or silicon
18 COMPOSITE MATERIALS FOR AIRCRAFT STRUCTURES temperature because microcracking of the ceramic allows contact between the fibers and the external environment. The fiber must also be chemically compatible with the matrix and must closely match it in its coefficient of thermal expansion. Thus, the use of similar materials for both components appears to offer the most promise, for example, silicon-carbide-fibers/silicon-carbidematrix or alumina fibers/alumina matrix. 11 Unfortunately, available fibers either do not maintain strength at high enough temperatures or (in the case of carbon fibers, for example) have adequate oxidation resistance to provide anywhere near the full exploitation of the potential benefits. CMCs are sometimes based on three-dimentional fiber architectures because in many (but not all) applications, the fibers are required to provide toughness, including through-thickness toughness, rather than stiffness as required in other classes of composites. Thus, for some CMCs, the relatively low fiber volume fraction resulting from this form of construction is not a major limitation. Glass and glass-ceramic matrices are promising for applications at temperatures around 500°C because of their excellent mechanical properties and relative ease of fabrication. In contrast to CMCs based on conventional ceramics, such as silicon carbide, the low modulus matrix can be effectively stiffened by suitable fibers and relatively high toughness achieved (typically, an increase of over 30 times the matrix glass alone). Because the matrix does not microcrack at relatively modest strain levels and temperatures, carbon fibers can be used. However, for higher-temperature applications more oxidation resistant fibers such as silicon carbide fibers must be used. Carbon/carbon composites 12 have no significant chemical or thermal expansion compatibility problems. However, unless protected, they are also prone to rapid attack at elevated temperature in an oxidizing environment. Even where oxidation is a problem, the composites can be used where short exposures to severe applications at temperatures over 2000°C are experienced, for example, in rocket nose-cones, nozzles, and leading edges on hypersonic wings. In the presence of oxygen-reducing conditions, for example with a hypersonic engine running slightly rich on hydrogen fuel, operations for prolonged periods can be maintained. Carbon/carbon composites could be used for prolonged periods at elevated temperature, above 1600 °C, if effective oxidation-preventative barrier coatings were available. This is a topic of considerable research interest because this composite has the best structural capability of any material at the highest operation temperatures when compared on a specific strength, creep-resistance, or stiffness basis. Some oxidation barriers include silicon carbide or silicon nitride coatings, which provide an oxidation-resistant outer layer over an inner glass layer; the glass can flow into cracks to seal the coating against oxygen penetration. This approach is called self-healing. An inner oxidation-resistant layer may also be used under the glass layer. The refractory layers are applied by CVD or by dip coating, from a liquid or sol-gel precursor. This coating is applied after producing, in the case of the inner layer, a thin tie (coating anchor) layer on the surface of the composite by reaction with, for example, boron or silicon