6.1 Tailoring by Component Selection 167 15 12 11 10 10 20 304050 60 70 TiB,(vol % Figure 6.6.Effect of TiB2 volume fraction on the coefficient of thermal expansion (CTE)of copper-matrix composites. Circles-coated filler method of powder metallurgy,squares-admixture method of powder metallurgy.(From [7]) volume fraction (Fig.6.7).The electrical resistivity increases monotonically with increasing TiBz volume fraction(Fig.6.8).Both the compressive yield strength (Fig.6.9)and the hardness(Fig.6.10)increase with increasing TiB2 volume fraction up to a certain TiBz volume fraction,beyond which they decrease with increasing TiB2 volume fraction.This behavior of the compressive yield strength and hardness is due to the porosity(Fig.6.11),which increases abruptly with increasing TiBz volume fraction when this volume fraction exceeds a certain value.The coated filler method gives a slightly lower CTE,a higher thermal conductivity,a lower electrical resistivity,a higher compressive yield strength,a higher hardness,and a lower porosity than the admixture method for the same TiBz volume fraction. In general,the difficulty involved in getting molten metal to wet the surface of a ceramic or carbon reinforcement complicates the fabrication of metal-matrix composites.This difficulty is particularly severe for high-modulus carbon fibers (e.g.,Amoco's Thornel P-100)which have graphite planes that are mostly aligned parallel to the fiber surface.The edges of the graphite planes are more reactive with the molten metal than the graphite planes themselves,so low-modulus carbon fibers are more reactive and are thus wetted more easily by the molten metals. Although this reaction between the fibers and the metal aids the wetting,it produces a brittle carbide and degrades the strength of the fibers. The wetting of a reinforcement by molten metal can be improved by adding alloying elements to the molten metal.When aluminum is used as the matrix and carbon fiber as the reinforcement,effective alloying elements include Mg,Cu, and Fe
6.1 Tailoring by Component Selection 167 Figure 6.6. Effect of TiB2 volume fraction on the coefficient of thermal expansion (CTE) of copper-matrix composites. Circles – coated filler method of powder metallurgy; squares – admixture method of powder metallurgy. (From [7]) volume fraction (Fig. 6.7). The electrical resistivity increases monotonically with increasing TiB2 volume fraction (Fig. 6.8). Both the compressive yield strength (Fig. 6.9) and the hardness (Fig. 6.10) increase with increasing TiB2 volume fraction up to a certain TiB2 volume fraction, beyond which they decrease with increasing TiB2 volume fraction. This behavior of the compressive yield strength and hardness is due to the porosity (Fig. 6.11), which increases abruptly with increasing TiB2 volume fraction when this volume fraction exceeds a certain value. The coated filler method gives a slightly lower CTE, a higher thermal conductivity, a lower electrical resistivity, a higher compressive yield strength, a higher hardness, and a lower porosity than the admixture method for the same TiB2 volume fraction. In general, the difficulty involved in getting molten metal to wet the surface of a ceramic or carbon reinforcement complicates the fabrication of metal-matrix composites. This difficulty is particularly severe for high-modulus carbon fibers (e.g., Amoco’s Thornel P-100) which have graphite planes that are mostly aligned parallel to the fiber surface. The edges of the graphite planes are more reactive with the molten metal than the graphite planes themselves, so low-modulus carbon fibers are more reactive and are thus wetted more easily by the molten metals. Althoughthisreactionbetweenthefibersandthemetalaidsthewetting,itproduces a brittle carbide and degrades the strength of the fibers. The wetting of a reinforcement by molten metal can be improved by adding alloying elements to the molten metal. When aluminum is used as the matrix and carbon fiber as the reinforcement, effective alloying elements include Mg, Cu, and Fe
168 6 Tailoring Composite Materials 400 350 300 250 200 150 100 50 10 20 30 4050 6070 TiB,(vol % Figure 6.7.Effect of TiB2 volume fraction on the thermal conductivity of copper-matrix composites.Circles-coated filler method of powder metallurgy:squares-admixture method of powder metallurgy.(From [7]) 0 30 20 10 10 20 3040.50 60 70 TiB,(vol % Figure 6.8.Effect of TiB2 volume fraction on the electrical resistivity of copper-matrix composites.Circles-coated filler method of powder metallurgy;squares-admixture method of powder metallurgy.(From [7])
168 6 Tailoring Composite Materials Figure 6.7. Effect of TiB2 volume fraction on the thermal conductivity of copper-matrix composites. Circles – coated filler method of powder metallurgy; squares – admixture method of powder metallurgy. (From [7]) Figure 6.8. Effect of TiB2 volume fraction on the electrical resistivity of copper-matrix composites. Circles – coated filler method of powder metallurgy; squares – admixture method of powder metallurgy. (From [7])
6.1 Tailoring by Component Selection 169 700 600 500 400 300 200 10 10203040506070 TiB,(vol % Figure 6.9.Effect of TiB2 volume fraction on the compressive yield strength of copper-matrix composites.Cirdles-coated filler method of powder metallurgy:squares-admixture method of powder metallurgy.(From [7]) 240 220 200 180 160 120 100 80 6 10 20 30 40506070 TiBz (vol%) Figure 6.10.Effect of TiB2 volume fraction on the hardness of copper-matrix composites.Circes-coated filler method of powder metallurgy;squares-admixture method of powder metallurgy.(From [7])
6.1 Tailoring by Component Selection 169 Figure 6.9. Effect of TiB2 volume fraction on the compressive yield strength of copper-matrix composites. Circles – coated filler method of powder metallurgy; squares – admixture method of powder metallurgy. (From [7]) Figure 6.10. Effect of TiB2 volume fraction on the hardness of copper-matrix composites. Circles – coated filler method of powder metallurgy; squares – admixture method of powder metallurgy. (From [7])
170 6 Tailoring Composite Materials 20 15 10 0 0 203040506070 TiB,(vol%) Figure 6.11.Effect of TiB volume fraction on the porosity of copper-matrix composites.Cirdles-coated filler method of powder metallurgy:squares-admixture method of powder metallurgy.(From [7]) 6.2 Tailoring by Interface Modification 6.2.1 Interface Bond Modification 6.2.1.1 General Concepts Effective reinforcement requires good bonding between the filler and the ma- trix,especially for short fibers.For an ideally unidirectional composite (i.e.,one containing continuous fibers all aligned in the same direction)containing fibers with a modulus that is much higher than that of the matrix,the longitudinal tensile strength is quite independent of the fiber-matrix bonding,but the trans- verse tensile strength and the flexural strength(for bending in the longitudinal or transverse directions)increase with increasing fiber-matrix bonding.On the other hand,excessive fiber-matrix bonding can cause a composite with a brittle matrix (e.g.,carbon and ceramics)to become more brittle,as the strong fiber-matrix bonding causes cracks to propagate linearly in the direction perpendicular to the fiber-matrix interface without being deflected to propagate along this interface.In the case of a composite with a ductile matrix (e.g.,metals and polymers),a crack initiating in the brittle fiber tends to be blunted when it reaches the ductile matrix, even when the fiber-matrix bonding is strong.Therefore,an optimum degree of fiber-matrix bonding (i.e.,not too strong and not too weak)is needed for brittle- matrix composites,whereas a high degree of fiber-matrix bonding is preferred
170 6 Tailoring Composite Materials Figure 6.11. Effect of TiB2 volume fraction on the porosity of copper-matrix composites. Circles – coated filler method of powder metallurgy; squares – admixture method of powder metallurgy. (From [7]) 6.2 Tailoring by Interface Modification 6.2.1 Interface Bond Modification 6.2.1.1 General Concepts Effective reinforcement requires good bonding between the filler and the matrix, especially for short fibers. For an ideally unidirectional composite (i.e., one containing continuous fibers all aligned in the same direction) containing fibers with a modulus that is much higher than that of the matrix, the longitudinal tensile strength is quite independent of the fiber–matrix bonding, but the transverse tensile strength and the flexural strength (for bending in the longitudinal or transverse directions) increase with increasing fiber–matrix bonding. On the other hand, excessive fiber–matrix bonding can cause a composite with a brittle matrix (e.g., carbon and ceramics) to become more brittle, as the strong fiber–matrix bonding causes cracks to propagate linearly in the direction perpendicular to the fiber–matrix interface without being deflected to propagate along this interface. In the case of a composite with a ductile matrix (e.g., metals and polymers), a crack initiating in the brittle fiber tends to be blunted when it reaches the ductile matrix, even when the fiber–matrix bonding is strong. Therefore, an optimum degree of fiber–matrix bonding (i.e., not too strong and not too weak) is needed for brittlematrix composites, whereas a high degree of fiber–matrix bonding is preferred
6.2 Tailoring by Interface Modification 171 for ductile-matrix composites.In particular,the fiber-matrix bond strength in carbon-carbon composites must be optimal.If the bond strength is too high,the resulting composite may be extremely brittle,exhibiting catastrophic failure and poor strength.If it is too low,the composites fail in pure shear,with poor transfer of the load to the fiber. The mechanisms involved in filler-matrix bonding include chemical bonding, interdiffusion,van der Waals bonding,and mechanical interlocking.Chemical bonding gives a relatively large bonding force provided that the density of chemical bonds across the filler-matrix interface is sufficiently high and that a brittle reaction product is absent from the filler-matrix interface.The density of chemical bonds can be increased by (i)chemically treating the filler,(ii)using a suitable sizing (coating)on the filler,and (iii)using a molecular coupling agent.Interdiffusion at the filler-matrix interface also results in bonding,though its occurrence requires the interface to be rather clean.Mechanical interlocking between the fibers and the matrix is an important contribution to the bonding if the fibers form a three- dimensional network.Otherwise,the filler should have a rough surface in order to allow a small degree of mechanical interlocking to take place. Chemical bonding,interdiffusion and van der Waals bonding require the filler to be in intimate contact with the matrix.For intimate contact to take place,the matrix or matrix precursor must be able to wet the surfaces of the filler during the infiltration of the matrix or matrix precursor into the filler preform.Wetting is governed by the surface energies.Chemical treatments and coatings can be applied to the fibers to enhance wetting through their effects on the surface energies.The choice of treatment or coating depends on the matrix.A related method involves adding a wetting agent to the matrix or matrix precursor before infiltration.As the wettability may vary with temperature,the infiltration temperature can be chosen to enhance wetting.Although wetting is governed by thermodynamics,it is strongly affected by kinetics.Thus,yet another way to enhance wetting is to use a high pressure during infiltration. The occurrence of a reaction between the filler and the matrix aids the wetting and bonding between the filler and the matrix.However,an excessive reaction degrades the filler,and the reaction product(s)may have an undesirable effect on the mechanical,thermal,or moisture resistance properties of the composite. Therefore,an optimum amount of reaction is preferred. An example relates to the reaction between silicon carbide and aluminum during the fabrication of a silicon carbide aluminum-matrix composite by infiltrating molten aluminum into a heated SiC preform.The reaction is 4A1+3SiC→Al4C3+3Si, (6.1) which consumes a part of the SiC,produces silicon(Si)that dissolves in the molten aluminum,and also produces the AlC;compound at the interface between the SiC and the aluminum matrix.The fraction of SiC consumed increases with decreasing SiC volume fraction in the composite.The amount of silicon produced by the reaction increases with increasing SiC volume fraction.The extent of the reaction increases with increasing infiltration temperature.The fraction of SiC whiskers
6.2 Tailoring by Interface Modification 171 for ductile-matrix composites. In particular, the fiber–matrix bond strength in carbon–carbon composites must be optimal. If the bond strength is too high, the resulting composite may be extremely brittle, exhibiting catastrophic failure and poor strength. If it is too low, the composites fail in pure shear, with poor transfer of the load to the fiber. The mechanisms involved in filler–matrix bonding include chemical bonding, interdiffusion, van der Waals bonding, and mechanical interlocking. Chemical bonding gives a relatively large bonding force provided that the density of chemical bondsacrossthefiller–matrixinterfaceissufficientlyhighandthatabrittlereaction product is absent from the filler–matrix interface. The density of chemical bonds can be increased by (i) chemically treating the filler, (ii) using a suitable sizing (coating) on the filler, and (iii) using a molecular coupling agent. Interdiffusion at the filler–matrix interface also results in bonding, though its occurrence requires the interface to be rather clean. Mechanical interlocking between the fibers and the matrix is an important contribution to the bonding if the fibers form a threedimensional network. Otherwise, the filler should have a rough surface in order to allow a small degree of mechanical interlocking to take place. Chemical bonding, interdiffusion and van der Waals bonding require the filler to be in intimate contact with the matrix. For intimate contact to take place, the matrix or matrix precursor must be able to wet the surfaces of the filler during the infiltration of the matrix or matrix precursor into the filler preform. Wetting is governed by the surface energies. Chemical treatments and coatings can be applied to the fibers to enhance wetting through their effects on the surface energies. The choice of treatment or coating depends on the matrix. A related method involves adding a wetting agent to the matrix or matrix precursor before infiltration. As the wettability may vary with temperature, the infiltration temperature can be chosen to enhance wetting. Although wetting is governed by thermodynamics, it is strongly affected by kinetics. Thus, yet another way to enhance wetting is to use a high pressure during infiltration. The occurrence of a reaction between the filler and the matrix aids the wetting and bonding between the filler and the matrix. However, an excessive reaction degrades the filler, and the reaction product(s) may have an undesirable effect on the mechanical, thermal, or moisture resistance properties of the composite. Therefore, an optimum amount of reaction is preferred. An example relates to the reaction between silicon carbide and aluminum during the fabrication of a silicon carbide aluminum-matrix composite by infiltrating molten aluminum into a heated SiC preform. The reaction is 4Al + 3SiC → Al4C3 + 3Si , (6.1) which consumes a part of the SiC, produces silicon (Si) that dissolves in the molten aluminum, and also produces the Al4C3 compound at the interface between the SiC and the aluminum matrix. The fraction of SiC consumed increases with decreasing SiC volume fraction in the composite. The amount of silicon produced by the reaction increases with increasing SiC volume fraction. The extent of the reaction increases with increasing infiltration temperature. The fraction of SiC whiskers