6 Composite Materials Definition and classification Composite materials are material systems that consist of a discrete constituent(the rein- forcement) distributed in a continuous phase(the matrix)and that derive their distinguishin characteristics from the properties and behavior of their constituents, from the geometry and arrangement of the constituents, and from the properties of the boundaries(interfaces)between the constituents. Composites are classified either on the basis of the nature of the continu ous(matrix) phase(polymer-matrix, metal-matrix, ceramic-matrix, and intermetallic-matrix composites), or on the basis of the nature of the reinforcing phase(particle reinforced, fiber reinforced, dispersion strengthened, laminated, etc. ) The properties of the composite can be tailored, and new combinations of properties can be achieved. For example, inherently brittle ceramics can be toughened by combining different types of ceramics in a ceramic-matrix com posite, and inherently ductile metals can be made strong and stiff by incorporating a ceramic reinforcement It is usually sufficient, and often desirable, to achieve a certain minimum level of reinforce nent content in a composite. Thus, in creep-resistant dispersion-strengthened composites, the reinforcement volume fraction is maintained below 15% in order to preserve many of the useful properties of the matrix. Other factors, such as the shape, size, distribution of the reinforcement, and properties of the interface, are also important. The shape, size, amount, and type of the rein- forcing phase to be used are dictated by the combination of properties desired in the composite For example, applications requiring anisotropic mechanical properties(high strength and high stiffness along one particular direction)employ directionally aligned, high-strength continuous fibers, whereas for applications where strength anisotropy is not critical and strength require ments are moderate, relatively inexpensive particulates can be used as the reinforcing phase. fIgure 6-1 shows some examples of continuous and discontinuous reinforcements developed Fi
6 Composite Materials Definition and Classification Composite materials are material systems that consist of a discrete constituent (the reinforcement) distributed in a continuous phase (the matrix) and that derive their distinguishing characteristics from the properties and behavior of their constituents, from the geometry and arrangement of the constituents, and from the properties of the boundaries (interfaces) between the constituents. Composites are classified either on the basis of the nature of the continuous (matrix) phase (polymer-matrix, metal-matrix, ceramic-matrix, and intermetallic-matrix composites), or on the basis of the nature of the reinforcing phase (particle reinforced, fiber reinforced, dispersion strengthened, laminated, etc.). The properties of the composite can be tailored, and new combinations of properties can be achieved. For example, inherently brittle ceramics can be toughened by combining different types of ceramics in a ceramic-matrix composite, and inherently ductile metals can be made strong and stiff by incorporating a ceramic reinforcement. It is usually sufficient, and often desirable, to achieve a certain minimum level of reinforcement content in a composite. Thus, in creep-resistant dispersion-strengthened composites, the reinforcement volume fraction is maintained below 15% in order to preserve many of the useful properties of the matrix. Other factors, such as the shape, size, distribution of the reinforcement, and properties of the interface, are also important. The shape, size, amount, and type of the reinforcing phase to be used are dictated by the combination of properties desired in the composite. For example, applications requiring anisotropic mechanical properties (high strength and high stiffness along one particular direction) employ directionally aligned, high-strength continuous fibers, whereas for applications where strength anisotropy is not critical and strength requirements are moderate, relatively inexpensive particulates can be used as the reinforcing phase. Figure 6-1 shows some examples of continuous and discontinuous reinforcements developed for use in modem engineered composites. 397
1 9m m 10y250 FIGURE 6-1 ( a) Scanning electron photomicrograph of sintered Tio2 fiber. ( D. French and R.b.caSs,DEvelopingiNnovativeCeramicFibers,www.ceramicbulletin.orgMay1998,pp.61-65) (b)SEM photomicrograph of an individua/ PZT filament of 25 um diameter. ( D. French and R.b.cAss,DevelopinginnovativeCeramicFibers,www.ceramicbulletin.orgMay1998,pp.61-65) (c) PZT fiber weave for a smart structure composite French and R. B. Cass, Developing InnovativeCeramicFibers,www.ceramicbulletin.orgMay1998,pp.61-65).(d)single-crystalSic platelets(nominal size, 150 um) Fibers Long, continuous fibers with a large aspect ratio (i.e, length-to-diameter ratio)of metals, ceram- ics, glasses and polymers are used to reinforce various types of matrices. A hard and strong naterial such as a ceramic in a fibrous form will have fewer strength-limiting flaws than the same material in a bulk form. As preexisting cracks lower the fracture strength of brittle ceramics, reducing the size and/or probability of occurrence of cracks will diminish the extent of streng loss in the ceramic, thus allowing the actual strength to approach the theoretical fracture strength in the absence of cracks, which is 0.1 E. where e is the elastic modulus If the fiber diameter scales with the grain size of the material, then the fracture strength will be high. In other words 398 MATERIALS PROCESSING AND MANUFACTURING SCIENCE
(a) (c) (b) (d) FIGURE 6-1 (a) Scanning electron photomicrograph of sintered -1702 fiber. (J. D. French and R. B. Cass, Developing Innovative Ceramic Fibers, www.ceramicbulletin.org, May 1998, pp. 61-65). (b) SEM photomicrograph of an individual PZT filament of 25 i.tm diameter. (J. D. French and R. B. Cass, Developing Innovative Ceramic Fibers, www.ceramicbulletin.org, May 1998, pp. 61-65). (c) PZT fiber weave for a smart structure composite. (J. D. French and R. B. Cass, Developing Innovative Ceramic Fibers, www.ceramicbulletin.org, May 1998, pp. 61-65). (d) Single-crystal SiC platelets (nominal size, 150 pm). Fibers Long, continuous fibers with a large aspect ratio (i.e., length-to-diameter ratio) of metals, ceramics, glasses and polymers are used to reinforce various types of matrices. A hard and strong material such as a ceramic in a fibrous form will have fewer strength-limiting flaws than the same material in a bulk form. As preexisting cracks lower the fracture strength of brittle ceramics, reducing the size and/or probability of occurrence of cracks will diminish the extent of strength loss in the ceramic, thus allowing the actual strength to approach the theoretical fracture strength in the absence of cracks, which is ~0.1 E, where E is the elastic modulus. If the fiber diameter scales with the grain size of the material, then the fracture strength will be high. In other words, 398 MATERIALS PROCESSING AND MANUFACTURING SCIENCE
smaller the fiber diameter, greater is its fracture strength. In the case of continuous fibers, a critical minimum aspect ratio of the fiber is needed to transfer the applied load from the weaker matrix to the stronger fiber. Furthermore, a small diameter allows a stiff fiber to be bent for shaping a preform that is used as a precursor in composite fabrication. Many commercial fibers are flexible, and permit filament winding and weaving techniques to be used for making a pre- form ers are, however, shaped into preforms by using a fugitive binder materia For example, an organic compound that cements the fibers in the desired preform shape may be used. The binder decomposes and is eliminated when the matrix material is combined with the preform to provide it support and rigidity. Selected examples of fibers used in composite matrices are briefly described below. For more details, the reader is referred to the book by Chawla referenced at the end of the chapte Glass. Glass is a generic name for a family of ceramic fibers containing 50-60% silica(a glass former)in a solid solution that contains several other oxides such as Al2O3, CaO, MgO, K2O Na2O, and B2O3, etc. Commercial glass fibers are classified as E-glass (for high electrical resistivity), S-glass(for high silica that imparts excellent high-temperature stability), and C-glass (for corosion resistance Glass fiber is manufactured by melting the oxide ingredients in a furnace and then transferring the molten glass into a hot platinum crucible with a few hundred fine holes at its base. Molten glass flows through these holes and on cooling forms fine continuous filaments. The final fiber diameter is a function of the hole diameter in the platinum crucible, the viscosity of molten glass, and the liquid head in the crucible. The filaments are gathered into a strand, and a sizing is applied before the strand is wound on a drum. Glass is a brittle solid, and its strength is lowered by minute surface defects. The sizing protects the surface of glass filaments and also binds them into a strand. A common type of sizing contains polyvinyl acetate and a coupling agent that makes the strand compatible with various polymer matrices. The final fiber diameter is a function of the hole diameter in the platinum crucible, the viscosity of molten glass, and the liquid head in the reservoir. Another method to grow glass fibers makes use of a sol-gel-type chemical precipitation process. A sol containing fine colloidal particles is used as the precursor: due to their fine size, the particles remain suspended in the liquid vehicle and are stabilized against flocculation through ionic charge adsorption on the surface. The sol is gelled via pH adjustments, i. e, the liquid vehicle in the gel behaves as a highly viscous liquid, thus the physical characteristics of a solid. The gelling action occurs at room temperature. The gel is then drawn into fibers at high temperatures, that are lower than the temperatures used in onventional manufacture of glass fiber by melting. The Nextel fiber manufactured by the 3M company is a sol-gel-derived silica-based fiber Moisture decreases the strength of glass fibers. They are also prone to static fatigue; that is, they cannot withstand loads for long periods of time. Glass fiber-reinforced plastics(GRPs)are widely used in the construction industry Boron. Boron fibers are produced by vapor depositing boron on a fine filament, usually made from tugsten, carbon, or carbon-coated glass fiber. In one type of vapor deposition process a boron hydride compound is thermally decomposed, and the boron vapor heterogeneously ucleates on the filament, thus forming a film, Such fibers are, however, not very strong or dense, owing to trapped vapor or gas that causes porosity and weakens the fiber. In an improved chemical vapor deposition( CVD)process, a halogen compound of boron is reduced by hydrogen gas at high temperatures, via the reaction 2BX3+3H2- 2B+6HX(X=Cl, Br, or I).Because of the high depe temperatures involved, the precursor filament is usually tungsten. Fibers of Composite Materials 399
smaller the fiber diameter, greater is its fracture strength. In the case of continuous fibers, a critical minimum aspect ratio of the fiber is needed to transfer the applied load from the weaker matrix to the stronger fiber. Furthermore, a small diameter allows a stiff fiber to be bent for shaping a preform that is used as a precursor in composite fabrication. Many commercial fibers are flexible, and permit filament winding and weaving techniques to be used for making a preform. Very stiff fibers are, however, shaped into preforms by using a fugitive binder material. For example, an organic compound that cements the fibers in the desired preform shape may be used. The binder decomposes and is eliminated when the matrix material is combined with the preform to provide it support and rigidity. Selected examples of fibers used in composite matrices are briefly described below. For more details, the reader is referred to the book by Chawla referenced at the end of the chapter. Glass. Glass is a generic name for a family of ceramic fibers containing 50-60% silica (a glass former) in a solid solution that contains several other oxides such as A1203, CaO, MgO, K20. Na20, and B203, etc. Commercial glass fibers are classified as E-glass (for high electrical resistivity), S-glass (for high silica that imparts excellent high-temperature stability), and C-glass (for corrosion resistance). Glass fiber is manufactured by melting the oxide ingredients in a furnace and then transferring the molten glass into a hot platinum crucible with a few hundred fine holes at its base. Molten glass flows through these holes and on cooling forms fine continuous filaments. The final fiber diameter is a function of the hole diameter in the platinum crucible, the viscosity of molten glass, and the liquid head in the crucible. The filaments are gathered into a strand, and a sizing is applied before the strand is wound on a drum. Glass is a brittle solid, and its strength is lowered by minute surface defects. The sizing protects the surface of glass filaments and also binds them into a strand. A common type of sizing contains polyvinyl acetate and a coupling agent that makes the strand compatible with various polymer matrices. The final fiber diameter is a function of the hole diameter in the platinum crucible, the viscosity of molten glass, and the liquid head in the reservoir. Another method to grow glass fibers makes use of a sol-gel-type chemical precipitation process. A sol containing fine colloidal particles is used as the precursor; due to their fine size, the particles remain suspended in the liquid vehicle and are stabilized against flocculation through ionic charge adsorption on the surface. The sol is gelled via pH adjustments, i.e., the liquid vehicle in the gel behaves as a highly viscous liquid, thus acquiring the physical characteristics of a solid. The gelling action occurs at room temperature. The gel is then drawn into fibers at high temperatures, that are lower than the temperatures used in conventional manufacture of glass fiber by melting. The Nextel fiber manufactured by the 3M company is a sol-gel-derived silica-based fiber. Moisture decreases the strength of glass fibers. They are also prone to static fatigue; that is, they cannot withstand loads for long periods of time. Glass fiber-reinforced plastics (GRPs) are widely used in the construction industry. Boron. Boron fibers are produced by vapor depositing boron on a fine filament, usually made from tugsten, carbon, or carbon-coated glass fiber. In one type of vapor deposition process, a boron hydride compound is thermally decomposed, and the boron vapor heterogeneously nucleates on the filament, thus forming a film. Such fibers are, however, not very strong or dense, owing to trapped vapor or gas that causes porosity and weakens the fiber. In an improved chemical vapor deposition (CVD) process, a halogen compound of boron is reduced by hydrogen gas at high temperatures, via the reaction 2BX3 + 3H2 ~ 2B + 6HX (X = C1, Br, or I). Because of the high deposition temperatures involved, the precursor filament is usually tungsten. Fibers of Composite Materials 399
consistently high quality are produced by this process, although the relatively high density of w filament slightly increases the fiber density In the halide reduction process using BCl3, a 10- to 12-um-diameter W wire is pulled in a reaction chamber at one end through a mercury seal and out at the other end through another mercury seal. The mercury seals act as electrical contacts for resistance heating of the substrate wire when gases( BCl3+H2) pass through the reaction chamber and react on the incandescent wire substrate to deposit boron coatings. The conversion of BCl3 to B coating is only Boron is also deposited on carbon monofilaments. a pyrolytic carbon coating is first applied to the carbon filament to accommodate the growth strains that result during boron deposition. There is a critical temperature for obtaining a boron fiber with optimum properties and tructure. The desirable amorphous(actually, microcrystalline with grain size of just a few nm) form of boron occurs below this critical temperature, whereas above this temperature there also occur crystalline forms of boron, which are undesirable from a mechanical properties view point. Larger crystallites lower the mechanical strength of the fiber. Because of high deposition temperatures in CVD, diffusional processes are rapid, and this partially transforms the core region from pure w to a variety of boride phases such as W2 B, WB, WB4, and others. As ron diffuses into the tungsten substrate to form borides, the core expands as much as 40%0 by volume, which results in an increase in the fiber diameter. This expansion generates resid ual stresses that can cause radial cracks and stress concentration in the fiber, thus lowering the fracture strength of the fiber. The average tensile strength of commercial boron fibers is about 3-4 GPa, and the modulus is 380-400 GPa, Usually a SiC coating is vapor-deposited onto the fiber to prevent any adverse reactions between B and the matrix such as al at high Carbon Fiber. Carbon, which can exist in a variety of crystalline forms, is a light material (density: 2.268 g/cc). The graphitic form of carbon is of primary interest in making fibers. The other form of carbon is diamond, a covalent solid, with little flexibility and little scope to grow diamond fibers, although microcrystalline diamond coatings can be vapor-deposited on a fiberous substrate to grow coated diamond fibers. Carbon atoms in graphite are arranged in the form of hexagonal layers, which are attached to similar layers via van der Waals forces. The graphitic form is highly anisotropic, with widely different elastic modulus in the layer plane and along the c-axis of the unit cell (i.e, very high in-plane modulus and very low transverse modulus) The high-strength covalent bonds between carbon atoms in the hexagonal layer plane result in an extremely high modulus(1000 GPa in single crystal) whereas the weak van der Waals bond between the neighboring layers results in a lower modulus(about one-half the modulus of pure Al)in that direction. In order to grow high-strength and high-modulus carbon fiber, a very high degree of preferred orientation of hexagonal planes along the fiber axis is needed The name carbon fiber is a generic one and represents a family of fibers all derived from carbonaceous precursors, and differing from one another in the size of the hexagonal sheets of arbon atoms, their stacking height, and the resulting crystalline orientations. These structural variations result in a wide range of physical and mechanical properties. For example, the axial tensile modulus can vary from 25 to 820 GPa, axial tensile strength from 500 to 5,000 MPa, nd thermal conductivity from 4 to 1100 W/m K, respectively. Carbon fibers of extremely high modulus are made by carbonization of organic precursor fibers followed by graphitization at high temperatures. The organic precursor fiber is generally a special long-chain polymer-based textile fiber(polyacrylonitrile or PAN and rayon, a thermosetting polymer) that can be car- bonized without melting. Such fibers generally have poor mechanical properties because of a 400 MATERIALS PROCESSING AND MAN NG SCIENCE
consistently high quality are produced by this process, although the relatively high density of W filament slightly increases the fiber density. In the halide reduction process using BC13, a 10- to 12-1xm-diameter W wire is pulled in a reaction chamber at one end through a mercury seal and out at the other end through another mercury seal. The mercury seals act as electrical contacts for resistance heating of the substrate wire when gases (BC13+H2) pass through the reaction chamber and react on the incandescent wire substrate to deposit boron coatings. The conversion efficiency of BC13 to B coating is only about 10%, and reuse of unreacted gas is important. Boron is also deposited on carbon monofilaments. A pyrolytic carbon coating is first applied to the carbon filament to accommodate the growth strains that result during boron deposition. There is a critical temperature for obtaining a boron fiber with optimum properties and structure. The desirable amorphous (actually, microcrystalline with grain size of just a few nm) form of boron occurs below this critical temperature, whereas above this temperature there also occur crystalline forms of boron, which are undesirable from a mechanical properties viewpoint. Larger crystallites lower the mechanical strength of the fiber. Because of high deposition temperatures in CVD, diffusional processes are rapid, and this partially transforms the core region from pure W to a variety of boride phases such as W2B, WB, WB4, and others. As boron diffuses into the tungsten substrate to form borides, the core expands as much as 40% by volume, which results in an increase in the fiber diameter. This expansion generates residual stresses that can cause radial cracks and stress concentration in the fiber, thus lowering the fracture strength of the fiber. The average tensile strength of commercial boron fibers is about 3-4 GPa, and the modulus is 380-400 GPa. Usually a SiC coating is vapor-deposited onto the fiber to prevent any adverse reactions between B and the matrix such as A1 at high temperatures. Carbon Fiber. Carbon, which can exist in a variety of crystalline forms, is a light material (density: 2.268 g/cc). The graphitic form of carbon is of primary interest in making fibers. The other form of carbon is diamond, a covalent solid, with little flexibility and little scope to grow diamond fibers, although microcrystalline diamond coatings can be vapor-deposited on a fiberous substrate to grow coated diamond fibers. Carbon atoms in graphite are arranged in the form of hexagonal layers, which are attached to similar layers via van der Waals forces. The graphitic form is highly anisotropic, with widely different elastic modulus in the layer plane and along the c-axis of the unit cell (i.e., very high in-plane modulus and very low transverse modulus). The high-strength covalent bonds between carbon atoms in the hexagonal layer plane result in an extremely high modulus (~ 1000 GPa in single crystal) whereas the weak van der Waals bond between the neighboring layers results in a lower modulus (about one-half the modulus of pure A1) in that direction. In order to grow high-strength and high-modulus carbon fiber, a very high degree of preferred orientation of hexagonal planes along the fiber axis is needed. The name carbon fiber is a generic one and represents a family of fibers all derived from carbonaceous precursors, and differing from one another in the size of the hexagonal sheets of carbon atoms, their stacking height, and the resulting crystalline orientations. These structural variations result in a wide range of physical and mechanical properties. For example, the axial tensile modulus can vary from 25 to 820 GPa, axial tensile strength from 500 to 5,000 MPa, and thermal conductivity from 4 to 1100 W/m.K, respectively. Carbon fibers of extremely high modulus are made by carbonization of organic precursor fibers followed by graphitization at high temperatures. The organic precursor fiber is generally a special long-chain polymer-based textile fiber (polyacrylonitrile or PAN and rayon, a thermosetting polymer) that can be carbonized without melting. Such fibers generally have poor mechanical properties because of a 400 MATERIALS PROCESSING AND MANUFACTURING SCIENCE
high degree of molecular disorder in polymer chains. Most processes of carbon fiber fabrication involve the following steps:(1)a stabilizing treatment(essentially an oxidation process)that enhances the thermal stability of the fibers and prevents the fiber from melting in the subsequen high-temperature treatment, and (2)a thermal treatment at 1000-1500 C called carbonization that removes noncarbonelements(e. g, N2 and H2). An optional thermal treatment called graphi- tization may be done at 3000C to further improve the mechanical properties of the carbon fiber by enabling the hexagonal crystalline sheets of graphite to increase their ordering To pro- duce high-modulus fiber, the orientation of the graphitic crystals or lamellae is improved by graphitization which consists of thermal and stretching treatments under rigorously controlled conditions. Besides the PAN and cellulosic(e.g, rayon precursors, pitch is also used as a raw material to grow carbon fibers. Commercial pitches are mixtures of various organic compounds with an average molecular weight between 400 and 600. There are various sources of pitch; the three most commonly used are polyvinyl chloride(PVC), petroleum asphalt, and coal tar. The same processing steps(stabilization, carbonization, and optional graphitization) are involved in converting the pitch-based precursor into carbon fiber. Pitch-based raw materials are generally cheap, and the carbon fiber yield from pitch-based precursors is relatively high A recent innovation in carbon-based materials has been carbon nanotubes (CNt). Carb nanotubes are relatively new materials-discovered in 1991--as a minor by-product of the carbon-arc process that is used to synthesize carbons fullerene molecules. They present exciting possibilities for research and use. CNTs are a variant of their predecessor, fullerene carbon (with a geodesic dome arrangement of 60, 70, or even a few hundred C atoms in a molecule) Figure 6-2 shows a photograph of CNT. Single-walled CNT have been grown to an aspect ratio of 10, with a length of about 100 um, and therefore, from a composite mechanics standpoint, they can be considered as long, continuous fibers. The multiwalled CNT has an onion- like"layered structure and is under extremely high internal stress, as evident from FIGURE 6-2 Photograph of carbon nanotubes and polyhedral nanoparticles during fullerene pro- duction(R, Malhotra, R, S. Ruoff and D. C. Lorents, "Fullerene Materials, " Advanced materials rocesses, April 1995 p. 30). Reprinted with permission from ASM International, Materials Park, Oh(www.asminternational.org Composite Materials 401
high degree of molecular disorder in polymer chains. Most processes of carbon fiber fabrication involve the following steps: (1) a stabilizing treatment (essentially an oxidation process) that enhances the thermal stability of the fibers and prevents the fiber from melting in the subsequent high-temperature treatment, and (2) a thermal treatment at 1000-1500~ called carbonization that removes noncarbon elements (e.g., N2 and H2). An optional thermal treatment called graphitization may be done at ~3000~ to further improve the mechanical properties of the carbon fiber by enabling the hexagonal crystalline sheets of graphite to increase their ordering. To produce high-modulus fiber, the orientation of the graphitic crystals or lamellae is improved by graphitization which consists of thermal and stretching treatments under rigorously controlled conditions. Besides the PAN and cellulosic (e.g., rayon) precursors, pitch is also used as a raw material to grow carbon fibers. Commercial pitches are mixtures of various organic compounds with an average molecular weight between 400 and 600. There are various sources of pitch; the three most commonly used are polyvinyl chloride (PVC), petroleum asphalt, and coal tar. The same processing steps (stabilization, carbonization, and optional graphitization) are involved in converting the pitch-based precursor into carbon fiber. Pitch-based raw materials are generally cheap, and the carbon fiber yield from pitch-based precursors is relatively high. A recent innovation in carbon-based materials has been carbon nanotubes (CNT). Carbon nanotubes are relatively new materials--discovered in 1991--as a minor by-product of the carbon-arc process that is used to synthesize carbon's fullerene molecules. They present exciting possibilities for research and use. CNTs are a variant of their predecessor, fullerene carbon (with a geodesic dome arrangement of 60, 70, or even a few hundred C atoms in a molecule). Figure 6-2 shows a photograph of CNT. Single-walled CNT have been grown to an aspect ratio of ~105, with a length of about 100 Ixm, and therefore, from a composite mechanics standpoint, they can be considered as long, continuous fibers. The multiwalled CNT has an "onion-like" layered structure and is under extremely high internal stress, as evident from FIGURE 6-2 Photograph of carbon nanotubes and polyhedral nanoparticles during fullerene production (R. Malhotra, R. S. Ruoff and D. C. Lorents, "Fullerene Materials," Advanced Materials & Processes, April 1995 p. 30). Reprinted with permission from ASM International, Materials Park, OH (www.asminternational.org). Composite Materials 401