7 Metal,Ceramic,and Carbon Matrix Composites In the earlier chapters of this book,we considered the performance,manufac- turing,and design issues pertaining to polymer matrix composites.In this chapter,we review the thermomechanical properties of metal,ceramic,and carbon matrix composites and a few important manufacturing methods used in producing such composites. The history of development of metal,ceramic,and carbon matrix compos- ites is much more recent than that of the polymer matrix composites.Initial research on the metal and ceramic matrix composites was based on continuous carbon or boron fibers,but there were difficulties in producing good quality composites due to adverse chemical reaction between these fibers and the matrix.With the development of newer fibers,such as silicon carbide or aluminum oxide,in the early 1980s,there has been a renewed interest and an accelerated research activity in developing the technology of both metal and ceramic matrix composites.The initial impetus for this development has come from the military and aerospace industries,where there is a great need for materials with high strength-to-weight ratios or high modulus-to-weight ratios that can also withstand severe high temperature or corrosive environments. Presently,these materials are very expensive and their use is limited to appli- cations that can use their special characteristics,such as high temperature resistance or high wear resistance.With developments of lower cost fibers and more cost-effective manufacturing techniques,it is conceivable that both metal and ceramic matrix composites will find commercial applications in automobiles,electronic packages,sporting goods,and others. The carbon matrix composites are more commonly known as carbon- carbon composites,since they use carbon fibers as the reinforcement for carbon matrix.The resulting composite has a lower density,higher modulus and strength,lower coefficient of thermal expansion,and higher thermal shock resistance than conventional graphite.The carbon matrix composites have been used as thermal protection materials in the nose cap and the leading edges of the wing of space shuttles.They are also used in rocket nozzles,exit cones,and aircraft brakes,and their potential applications include pistons in 2007 by Taylor&Francis Group.LLC
7 Metal, Ceramic, and Carbon Matrix Composites In the earlier chapters of this book, we considered the performance, manufacturing, and design issues pertaining to polymer matrix composites. In this chapter, we review the thermomechanical properties of metal, ceramic, and carbon matrix composites and a few important manufacturing methods used in producing such composites. The history of development of metal, ceramic, and carbon matrix composites is much more recent than that of the polymer matrix composites. Initial research on the metal and ceramic matrix composites was based on continuous carbon or boron fibers, but there were difficulties in producing good quality composites due to adverse chemical reaction between these fibers and the matrix. With the development of newer fibers, such as silicon carbide or aluminum oxide, in the early 1980s, there has been a renewed interest and an accelerated research activity in developing the technology of both metal and ceramic matrix composites. The initial impetus for this development has come from the military and aerospace industries, where there is a great need for materials with high strength-to-weight ratios or high modulus-to-weight ratios that can also withstand severe high temperature or corrosive environments. Presently, these materials are very expensive and their use is limited to applications that can use their special characteristics, such as high temperature resistance or high wear resistance. With developments of lower cost fibers and more cost-effective manufacturing techniques, it is conceivable that both metal and ceramic matrix composites will find commercial applications in automobiles, electronic packages, sporting goods, and others. The carbon matrix composites are more commonly known as carbon– carbon composites, since they use carbon fibers as the reinforcement for carbon matrix. The resulting composite has a lower density, higher modulus and strength, lower coefficient of thermal expansion, and higher thermal shock resistance than conventional graphite. The carbon matrix composites have been used as thermal protection materials in the nose cap and the leading edges of the wing of space shuttles. They are also used in rocket nozzles, exit cones, and aircraft brakes, and their potential applications include pistons in 2007 by Taylor & Francis Group, LLC
internal combustion engines,gas turbine components,heat exchangers,and biomedical implants. 7.1 METAL MATRIX COMPOSITES The metal matrix composites(MMC)can be divided into four general categories: 1.Fiber-reinforced MMC containing either continuous or discontinuous fiber reinforcements;the latter are in the form of whiskers with approxi- mately 0.1-0.5 um in diameter and have a length-to-diameter ratio up to200. 2.Particulate-reinforced MMC containing either particles or platelets that range in size from 0.5 to 100 um.The particulates can be incorporated into the metal matrix to higher volume fractions than the whiskers. 3.Dispersion-strengthened MMC containing particles that are <0.1 um in diameter. 4.In situ MMC,such as directionally solidified eutectic alloys. In this chapter,we focus our attention on the first two categories,more specifically on whisker-and particulate-reinforced MMCs.More detailed infor- mations on MMC can be found in Refs.[1-4]. Continuous carbon or boron fiber-reinforced MMCs have been under development for >20 years;however,they have found limited use due to problems in controlling the chemical reaction between the fibers and the molten metal at the high processing temperatures used for such composites.The result of this chemical reaction is a brittle interphase that reduces the mechanical properties of the composite.Fiber surface treatments developed to reduce this problem increase the cost of the fiber.Additionally,the manufacturing cost of continuous carbon or boron fiber-reinforced MMC is also high,which makes them less attractive for many applications.Much of the recent work on MMC is based on silicon carbide whiskers (SiCw)or silicon carbide particulates (SiCp).SiC is less prone to oxidative reactions at the processing temperatures used.Furthermore,not only they are less expensive than carbon or boron fibers,but also they can be incorporated into metal matrices using common manufacturing techniques,such as powder metallurgy and casting. 7.1.1 MECHANICAL PROPERTIES In Chapter 2,we discussed simple micromechanical models in relation to polymer matrix composites in which fibers carry the major portion of the composite load by virtue of their high modulus compared with the polymer matrix,such as epoxy.The same micromechanical models can be applied to MMC with some modifications.The modulus of metals is an order of magni- tude higher than that of polymers (Table 7.1).Many metals are capable of 2007 by Taylor Francis Group,LLC
internal combustion engines, gas turbine components, heat exchangers, and biomedical implants. 7.1 METAL MATRIX COMPOSITES The metal matrix composites(MMC) can be divided into four general categories: 1. Fiber-reinforced MMC containing either continuous or discontinuous fiber reinforcements; the latter are in the form of whiskers with approximately 0.10.5 mm in diameter and have a length-to-diameter ratio up to 200. 2. Particulate-reinforced MMC containing either particles or platelets that range in size from 0.5 to 100 mm. The particulates can be incorporated into the metal matrix to higher volume fractions than the whiskers. 3. Dispersion-strengthened MMC containing particles that are <0.1 mm in diameter. 4. In situ MMC, such as directionally solidified eutectic alloys. In this chapter, we focus our attention on the first two categories, more specifically on whisker- and particulate-reinforced MMCs. More detailed informations on MMC can be found in Refs. [1–4]. Continuous carbon or boron fiber-reinforced MMCs have been under development for >20 years; however, they have found limited use due to problems in controlling the chemical reaction between the fibers and the molten metal at the high processing temperatures used for such composites. The result of this chemical reaction is a brittle interphase that reduces the mechanical properties of the composite. Fiber surface treatments developed to reduce this problem increase the cost of the fiber. Additionally, the manufacturing cost of continuous carbon or boron fiber-reinforced MMC is also high, which makes them less attractive for many applications. Much of the recent work on MMC is based on silicon carbide whiskers (SiCw) or silicon carbide particulates (SiCp). SiC is less prone to oxidative reactions at the processing temperatures used. Furthermore, not only they are less expensive than carbon or boron fibers, but also they can be incorporated into metal matrices using common manufacturing techniques, such as powder metallurgy and casting. 7.1.1 MECHANICAL PROPERTIES In Chapter 2, we discussed simple micromechanical models in relation to polymer matrix composites in which fibers carry the major portion of the composite load by virtue of their high modulus compared with the polymer matrix, such as epoxy. The same micromechanical models can be applied to MMC with some modifications. The modulus of metals is an order of magnitude higher than that of polymers (Table 7.1). Many metals are capable of 2007 by Taylor & Francis Group, LLC
Taykr Francis Group. TABLE 7.1 Properties of Some Metal Alloys Used in Metal Matrix Composites Tensile Density, Modulus, YS, UTS,MPa Failure CTE 10-6 Melting Material g/cm3 GPa(Gsi) MPa (ksi) (ksi) Strain,% per℃ Point,℃ Aluminum alloy 2024-T6 2.78 70(10.1) 468.9(68) 579.3(84) 11 23.2 6061-T6 2.70 70(10.1) 275.9(40) 310.3(45) 17 23.6 7075-T6 2.80 70(10.1) 503.5(73) 572.4(83) 23.6 8009 2.92 88(12.7 407(59) 448(64.9)】 17 23.5 380(As cast) 2.71 70(10.1) 165.5(24) 331(48) 540 Titanium alloy Ti-6A1-4V 4.43 110(16) 1068(155) 1171(170) 9.5 1650 (Solution-treated and aged) Magnesium alloy AZ91A 1.81 45(6.5) 158.623) 234.5(34) 26 650 Zinc-aluminum alloy ZA-27(Pressure die-cast) 5 78(11.3) 370(53.6) 425(61.6) 3 26 375
TABLE 7.1 Properties of Some Metal Alloys Used in Metal Matrix Composites Material Density, g=cm3 Tensile Modulus, GPa (Gsi) YS, MPa (ksi) UTS, MPa (ksi) Failure Strain, % CTE 106 per 8C Melting Point, 8C Aluminum alloy 2024-T6 2.78 70 (10.1) 468.9 (68) 579.3 (84) 11 23.2 6061-T6 2.70 70 (10.1) 275.9 (40) 310.3 (45) 17 23.6 7075-T6 2.80 70 (10.1) 503.5 (73) 572.4 (83) 11 23.6 8009 2.92 88 (12.7) 407 (59) 448 (64.9) 17 23.5 380 (As cast) 2.71 70 (10.1) 165.5 (24) 331 (48) 4 — 540 Titanium alloy Ti-6A1-4V (Solution-treated and aged) 4.43 110 (16) 1068 (155) 1171 (170) 8 9.5 1650 Magnesium alloy AZ91A 1.81 45 (6.5) 158.6 (23) 234.5 (34) 3 26 650 Zinc–aluminum alloy ZA-27 (Pressure die-cast) 5 78 (11.3) 370 (53.6) 425 (61.6) 3 26 375 2007 by Taylor & Francis Group, LLC
undergoing large plastic deformations and strain hardening after yielding.In general,they exhibit higher strain-to-failure and fracture toughness than poly- mers.Furthermore,since the processing temperature for MMCs is very high, the difference in thermal contraction between the fibers and the matrix during cooling can lead to relatively high residual stresses.In some cases,the matrix may yield under the influence of these residual stresses,which can affect the stress-strain characteristics as well as the strength of the composite. 7.1.1.1 Continuous-Fiber MMC Consider an MMC containing unidirectional continuous fibers subjected to a tensile load in the fiber direction.Assume that the matrix yield strain is lower than the fiber failure strain.Initially,both fibers and matrix deform elastically The longitudinal elastic modulus of the composite is given by the rule of mixtures: EL EfVf EmVm. (7.1) After the matrix reaches its yield strain,it begins to deform plastically,but the fiber remains elastic.At this point,the stress-strain diagram begins to deviate from its initial slope (Figure 7.1)and exhibits a new longitudinal modulus, which is given by: do EL=Evf十 Vm? (7.2) /m do where is slope of the stress-strain curve of the matrix at the composite strain se.The stress-strain diagram of the composite in this region is not elastic. In addition,it may not be linear if the matrix has a nonuniform strain- hardening rate. For brittle fiber MMCs,such as SiC fiber-reinforced aluminum alloys,the composite strength is limited by fiber fracture,and the MMCs fail as the com- posite strain becomes equal to the fiber failure strain.For ductile fiber MMCs, such as tungsten fiber-reinforced copper alloys [5]and beryllium fiber-reinforced aluminum alloys [6],the fiber also yields and plastically deforms along with the matrix.In addition,the composite strength is limited by the fiber failure strain, unless the fibers fail by necking.If the fibers exhibit necking before failure and its failure strain is lower than that of the matrix,the strain at the ultimate tensile stress of the composite will be greater than that at the ultimate tensile stress of the fiber alone. If the composite failure is controlled by the fiber failure strain,the longitudinal composite strength is given by OLm OfuVf +om(1 -Vf), (7.3) 2007 by Taylor Francis Group,LLC
undergoing large plastic deformations and strain hardening after yielding. In general, they exhibit higher strain-to-failure and fracture toughness than polymers. Furthermore, since the processing temperature for MMCs is very high, the difference in thermal contraction between the fibers and the matrix during cooling can lead to relatively high residual stresses. In some cases, the matrix may yield under the influence of these residual stresses, which can affect the stress–strain characteristics as well as the strength of the composite. 7.1.1.1 Continuous-Fiber MMC Consider an MMC containing unidirectional continuous fibers subjected to a tensile load in the fiber direction. Assume that the matrix yield strain is lower than the fiber failure strain. Initially, both fibers and matrix deform elastically. The longitudinal elastic modulus of the composite is given by the rule of mixtures: EL ¼ Efvf þ Emvm: (7:1) After the matrix reaches its yield strain, it begins to deform plastically, but the fiber remains elastic. At this point, the stress–strain diagram begins to deviate from its initial slope (Figure 7.1) and exhibits a new longitudinal modulus, which is given by: EL ¼ Efvf þ ds d« m vm, (7:2) where ds d« m is slope of the stress–strain curve of the matrix at the composite strain «c. The stress–strain diagram of the composite in this region is not elastic. In addition, it may not be linear if the matrix has a nonuniform strainhardening rate. For brittle fiber MMCs, such as SiC fiber-reinforced aluminum alloys, the composite strength is limited by fiber fracture, and the MMCs fail as the composite strain becomes equal to the fiber failure strain. For ductile fiber MMCs, such as tungsten fiber-reinforced copper alloys [5] and beryllium fiber-reinforced aluminum alloys [6], the fiber also yields and plastically deforms along with the matrix. In addition, the composite strength is limited by the fiber failure strain, unless the fibers fail by necking. If the fibers exhibit necking before failure and its failure strain is lower than that of the matrix, the strain at the ultimate tensile stress of the composite will be greater than that at the ultimate tensile stress of the fiber alone. If the composite failure is controlled by the fiber failure strain, the longitudinal composite strength is given by sLtu ¼ sfuvf þ sm 0 (1 vf), (7:3) 2007 by Taylor & Francis Group, LLC.
Brittle fiber Ductile fiber Ductile fiber ssens composite Brittle fiber composite Secondary modulus Primary modulus Matrix Strain FIGURE 7.1 Schematic representation of longitudinal tensile stress-strain diagram of a unidirectional continuous fiber-reinforced MMC. where om is the matrix flow stress at the ultimate fiber strain that is determined from the matrix stress-strain diagram. Equation 7.3 appears to fit the experimental strength values for a number of MMCs,such as copper matrix composites(Figure 7.2)containing either brittle or ductile tungsten fibers [5].In general,they are valid for MMCs in which (1) there is no adverse interfacial reaction between the fibers and the matrix that produces a brittle interphase,(2)there is a good bond between the fibers and the matrix,and(3)the thermal residual stresses at or near the interface are low. The longitudinal tensile strength predicted by Equation 7.3 is higher than the experimental values for carbon fiber-reinforced aluminum alloys.In these systems,unless the carbon fibers are coated with protective surface coating,a brittle Al4C3 interphase is formed.Cracks initiated in this interphase cause the fibers to fail at strains that are lower than their ultimate strains.In some cases, the interfacial reaction is so severe that it weakens the fibers,which fail at very low strains compared with the unreacted fibers [7].If the matrix continues to carry the load,the longitudinal tensile strength of the composite will be OLtu Omu(1-Vf), (7.4) 2007 by Taylor&Franeis Group.LLC
where sm 0 is the matrix flow stress at the ultimate fiber strain that is determined from the matrix stress–strain diagram. Equation 7.3 appears to fit the experimental strength values for a number of MMCs, such as copper matrix composites (Figure 7.2) containing either brittle or ductile tungsten fibers [5]. In general, they are valid for MMCs in which (1) there is no adverse interfacial reaction between the fibers and the matrix that produces a brittle interphase, (2) there is a good bond between the fibers and the matrix, and (3) the thermal residual stresses at or near the interface are low. The longitudinal tensile strength predicted by Equation 7.3 is higher than the experimental values for carbon fiber-reinforced aluminum alloys. In these systems, unless the carbon fibers are coated with protective surface coating, a brittle Al4C3 interphase is formed. Cracks initiated in this interphase cause the fibers to fail at strains that are lower than their ultimate strains. In some cases, the interfacial reaction is so severe that it weakens the fibers, which fail at very low strains compared with the unreacted fibers [7]. If the matrix continues to carry the load, the longitudinal tensile strength of the composite will be sLtu ¼ smu(1 vf), (7:4) Brittle fiber Ductile fiber Ductile fiber composite Brittle fiber composite Secondary modulus Primary modulus Matrix Strain Stress FIGURE 7.1 Schematic representation of longitudinal tensile stress–strain diagram of a unidirectional continuous fiber-reinforced MMC. 2007 by Taylor & Francis Group, LLC