162 6 Tailoring Composite Materials be used along with the fibers.These admixtures may be fine particles (such as silica fume,which has a typical particle size of around 0.1 um),the presence of which helps the fibers to break loose from one another as mixing occurs.Other admixtures may be polymers such as latex particle dispersions,which help fiber- cement bonding as well as fiber dispersion. 6.1.3 Metal-Matrix Composites Aluminum is the most common matrix material used in metal-matrix composites because of(i)its low melting temperature,which allows casting to be conducted at relatively low temperatures,and(ii)its low density.Copper's high density makes it unattractive for lightweight composites,but its high thermal conductivity and low electrical resistivity make it attractive for electronic applications(Table 6.2). Metals and ceramics tend to have very different in their properties,as shown in Table 6.2.Metals are electrically and thermally conductive.The thermal conduc- tivities of aluminum and copper(Table 6.2)are higher than those of any of the ceramics listed,while the electrical resistivities of aluminum and copper are much lower (by many orders of magnitude)than those of any of the ceramics listed. However,most metals exhibit a high coefficient of thermal expansion(CTE)and a low elastic modulus compared to ceramics.The CTE values of aluminum and copper are higher than those of any of the ceramics listed,and the elastic moduli of aluminum and copper are lower than those of any of the ceramics listed.The density of aluminum is comparable to those of ceramics,but the density of copper is higher than those of ceramics. Among the metals,molybdenum,tungsten,Kovar(Fe-Ni29-Col7)and Invar (Fe-Ni36)have exceptionally low CTE values (Table 6.2).Their CTE values are comparable to those of ceramics;indeed,the CTE of Invar is even lower than those of ceramics.Thus,Invar is used for precision instruments,such as clocks, physical laboratory devices,seismic creep gauges,and valves in motors.Guillaume received the Nobel Prize in Physics in 1920 for discovering Invar(which means "invariability in relation to the essential absence of thermal expansion").However, all of these metals/alloys have low values of thermal conductivity.In addition, their densities are high.Furthermore,molybdenum and tungsten are refractory metals(i.e.,metals that are extraordinarily resistant to heat and wear;a group that also includes niobium,tantalum,and rhenium).For example,the melting point of tungsten is 3,410C.Such high melting temperatures make metal processing that involves casting(melting and subsequent solidification)difficult. Carbon fibers exhibit slightly negative CTE values,so they are highly effective CTE-reducing fillers.The electrical resistivities of carbon fibers (all grades)are lower than those of metals,but their thermal conductivities can be lower or higher than those of metals,depending on the fiber grade.High-modulus carbon fiber can be more thermally conductive than metals(and even more thermally conductive than copper for the grade of high-modulus carbon fiber shown in Table 6.2), whereas high-strength carbon fiber is less thermally conductive than metals. The incorporation of a carbon or ceramic filler into a metal to form a metal- matrix composite is attractive for attaining a low CTE(although not as low as that
162 6 Tailoring Composite Materials be used along with the fibers. These admixtures may be fine particles (such as silica fume, which has a typical particle size of around 0.1μm), the presence of which helps the fibers to break loose from one another as mixing occurs. Other admixtures may be polymers such as latex particle dispersions, which help fiber– cement bonding as well as fiber dispersion. 6.1.3 Metal-Matrix Composites Aluminum is the most common matrix material used in metal-matrix composites because of (i) its low melting temperature, which allows casting to be conducted at relatively low temperatures, and (ii) its low density. Copper’s high density makes it unattractive for lightweight composites, but its high thermal conductivity and low electrical resistivity make it attractive for electronic applications (Table 6.2). Metals and ceramics tend to have very different in their properties, as shown in Table 6.2. Metals are electrically and thermally conductive. The thermal conductivities of aluminum and copper (Table 6.2) are higher than those of any of the ceramics listed, while the electrical resistivities of aluminum and copper are much lower (by many orders of magnitude) than those of any of the ceramics listed. However, most metals exhibit a high coefficient of thermal expansion (CTE) and a low elastic modulus compared to ceramics. The CTE values of aluminum and copper are higher than those of any of the ceramics listed, and the elastic moduli of aluminum and copper are lower than those of any of the ceramics listed. The density of aluminum is comparable to those of ceramics, but the density of copper is higher than those of ceramics. Among the metals, molybdenum, tungsten, Kovar (Fe-Ni29-Co17) and Invar (Fe-Ni36) have exceptionally low CTE values (Table 6.2). Their CTE values are comparable to those of ceramics; indeed, the CTE of Invar is even lower than those of ceramics. Thus, Invar is used for precision instruments, such as clocks, physical laboratory devices, seismic creep gauges, and valves in motors. Guillaume received the Nobel Prize in Physics in 1920 for discovering Invar (which means “invariability in relation to the essential absence of thermal expansion”). However, all of these metals/alloys have low values of thermal conductivity. In addition, their densities are high. Furthermore, molybdenum and tungsten are refractory metals (i.e., metals that are extraordinarily resistant to heat and wear; a group that also includes niobium, tantalum, and rhenium). For example, the melting point of tungsten is 3,410°C. Such high melting temperatures make metal processing that involves casting (melting and subsequent solidification) difficult. Carbon fibers exhibit slightly negative CTE values, so they are highly effective CTE-reducing fillers. The electrical resistivities of carbon fibers (all grades) are lower than those of metals, but their thermal conductivities can be lower or higher than those of metals, depending on the fiber grade. High-modulus carbon fiber can be more thermally conductive than metals (and even more thermally conductive than copper for the grade of high-modulus carbon fiber shown in Table 6.2), whereas high-strength carbon fiber is less thermally conductive than metals. The incorporation of a carbon or ceramic filler into a metal to form a metalmatrix composite is attractive for attaining a low CTE (although not as low as that
6.1 Tailoring by Component Selection 163 Table 6.2.Properties of metals,carbons and ceramics Material Density Thermal Electrical Elastic CTE (g/cm3) conductivity resistivity modulus (10-6/K) (w/(mK)) (2cm) (GPa) Aluminum 2.70 237 2.65×10-6 70 23.1 Copper 8.96 401 1.68×10-6 110-128 16.5 Molybdenum* 10.22 142 5.2×10-6 320 4.9 Tungsten* 19.3 155 5.3×10-6 400 4.5 Kovar 8.35 17 49×10-5 159 5.2 (Fe-Ni29-Col7) Invar 8.05 10.5 8.2×10-5 141 12 (Fe-Ni36) Carbon fiberb 1.76 P 1.8×10-3 231 -0.60 (high strength) Carbon fiber 2.17 640 2.2×10-4 827 -1.45 (high modulus) Silicon 3.1 120 102-106 410 4.0 carbide (SiC) Silicon nitride 3.29 30 310 3.3 (SisN4) Aluminum 3.26 140-180 >1014 330 4.5 nitride (AIN) Aluminum 3.89 35 >1014 375 8.4 oxide(Al2O3) Boron nitride 1.9 121 >1014 -0.46 (hexagonal) Titanium 4.50 96 10-5 565 6.4 diboride(TiB2) Zirconium 6 >1010 200 10.3 oxide(ZrO2), Y2O3 stabilized a Metal;b carbon;ceramic of the carbon or ceramic filler)and a high elastic modulus(although not as high as that of the carbon or ceramic filler),in addition to high thermal and electrical conductivities(although not as high as those of the metal matrix).When copper is used as the matrix,the composite also allows a reduction in density (although the density does not become as low as that of the carbon or ceramic filler). The combination of low CTE and high thermal conductivity is particularly attractive for electronic packaging,such as heat sinks,housings,substrates,lids, etc.The combination of high electrical and thermal conductivity and hardness is particularly attractive for welding electrodes,motor brushes,and sliding contacts. Among the ceramic fillers listed in Table 6.2,titanium diboride and silicon carbide are most attractive due to their high elastic moduli.This is an important factor for strengthening the composite.Among the ceramic fillers listed,aluminum nitride is most attractive due to its high thermal conductivity,although silicon carbide and hexagonal boron nitride have quite high thermal conductivities.One
6.1 Tailoring by Component Selection 163 Table 6.2. Properties of metals, carbons and ceramics Material Density Thermal Electrical Elastic CTE (g/cm3) conductivity resistivity modulus (10−6/K) (W/(m K)) (Ωcm) (GPa) Aluminuma 2.70 237 2.65 × 10−6 70 23.1 Coppera 8.96 401 1.68 × 10−6 110–128 16.5 Molybdenuma 10.22 142 5.2 × 10−6 320 4.9 Tungstena 19.3 155 5.3 × 10−6 400 4.5 Kovara 8.35 17 4.9 × 10−5 159 5.2 (Fe-Ni29-Co17) Invara 8.05 10.5 8.2 × 10−5 141 1.2 (Fe-Ni36) Carbon fiberb 1.76 8 1.8 × 10−3 231 −0.60 (high strength) Carbon fiberb 2.17 640 2.2 × 10−4 827 −1.45 (high modulus) Silicon 3.1 120 102–106 410 4.0 carbide (SiC)c Silicon nitride 3.29 30 / 310 3.3 (Si3N4) c Aluminum 3.26 140–180 > 1014 330 4.5 nitride (AlN)c Aluminum 3.89 35 > 1014 375 8.4 oxide (Al2O3) c Boron nitridec 1.9 121 > 1014 / −0.46 (hexagonal) Titanium 4.50 96 10−5 565 6.4 diboride (TiB2)c Zirconium 6 2 > 1010 200 10.3 oxide (ZrO2)c, Y2O3 stabilized a Metal; b carbon; c ceramic of the carbon or ceramic filler) and a high elastic modulus (although not as high as that of the carbon or ceramic filler), in addition to high thermal and electrical conductivities (although not as high as those of the metal matrix). When copper is used as the matrix, the composite also allows a reduction in density (although the density does not become as low as that of the carbon or ceramic filler). The combination of low CTE and high thermal conductivity is particularly attractive for electronic packaging, such as heat sinks, housings, substrates, lids, etc. The combination of high electrical and thermal conductivity and hardness is particularly attractive for welding electrodes, motor brushes, and sliding contacts. Among the ceramic fillers listed in Table 6.2, titanium diboride and silicon carbide are most attractive due to their high elastic moduli. This is an important factor for strengthening the composite. Among the ceramic fillers listed, aluminum nitride is most attractive due to its high thermal conductivity, although silicon carbide and hexagonal boron nitride have quite high thermal conductivities. One
164 6 Tailoring Composite Materials drawback of aluminum nitride is its reactivity with water to form aluminum oxynitride,which has a much lower thermal conductivity than aluminum nitride Aluminum oxide and zirconium oxide have particularly low thermal conduc- tivities. Among the ceramic fillers,titanium diboride is most attractive because of its low electrical resistivity,which allows it to be used as an anode material for aluminum smelting (the extraction of aluminum from its oxide,alumina)and to be machined by electrical discharge machining (abbreviated to EDM,and also called spark machining;this refers to the removal of material using electric arcing discharges between an electrode,which is the cutting tool,and the workpiece in the presence of an energetic electric field).EDM requires that the workpiece is electrically conductive. Due to its low cost and high elastic modulus,silicon carbide is the filler most commonly used to reinforce metals.SiC is also used as an abrasive(e.g.,in sandpa- per).There are numerous polymorphs of SiC,but the most common polymorph is a-SiC,which has a hexagonal crystal structure(similar to wurtzite).Aless common polymorph is B-SiC,which exhibits the zinc blende crystal structure. Silicon carbide is available in particle and whisker forms.A whisker is a short fiber that can be essentially a single crystal.The SiC particle is typically a-SiC,with a size of 1-10 um.The SiC whisker is typically B-SiC,with a diameter of about 1 um and a length ofabout 20 um.Figure 6.3 shows an SEM photograph ofB-SiC whiskers of diameter 1.4 um and length 18.6 um.Figure 6.4 shows SEM photographs of an aluminum-matrix composite containing 10 vol%SiC whiskers of the type shown in Fig.6.3.The composite is fabricated by liquid metal infiltration at an infiltration pressure of 13.8 MPa.The porosity in the composite is 0.5%. 10 um Figure 6.3.SEM photograph of silicon carbide whiskers without a matrix (from [6])
164 6 Tailoring Composite Materials drawback of aluminum nitride is its reactivity with water to form aluminum oxynitride, which has a much lower thermal conductivity than aluminum nitride. Aluminum oxide and zirconium oxide have particularly low thermal conductivities. Among the ceramic fillers, titanium diboride is most attractive because of its low electrical resistivity, which allows it to be used as an anode material for aluminum smelting (the extraction of aluminum from its oxide, alumina) and to be machined by electrical discharge machining (abbreviated to EDM, and also called spark machining; this refers to the removal of material using electric arcing discharges between an electrode, which is the cutting tool, and the workpiece in the presence of an energetic electric field). EDM requires that the workpiece is electrically conductive. Due to its low cost and high elastic modulus, silicon carbide is the filler most commonly used to reinforce metals. SiC is also used as an abrasive (e.g., in sandpaper). There are numerous polymorphs of SiC, but the most common polymorph is α-SiC, which has a hexagonal crystal structure (similar to wurtzite). A less common polymorph is β-SiC, which exhibits the zinc blende crystal structure. Silicon carbide is available in particle and whisker forms. A whisker is a short fiber that can be essentially a single crystal. The SiC particle is typically α-SiC, with a size of 1–10μm. The SiC whisker is typically β-SiC, with a diameter of about 1μm and a length of about 20μm. Figure 6.3 shows an SEM photograph of β-SiC whiskers of diameter 1.4μm and length 18.6μm. Figure 6.4 shows SEM photographs of an aluminum-matrix composite containing 10vol% SiC whiskers of the type shown in Fig. 6.3. The composite is fabricated by liquid metal infiltration at an infiltration pressure of 13.8MPa. The porosity in the composite is < 0.5%. Figure 6.3. SEM photograph of silicon carbide whiskers without a matrix (from [6])
6.1 Tailoring by Component Selection 165 10μm 50μm b Figure6.4.SEMphotographs of mechanically polished sections of aluminum-matrix compositescontaining 10 vol%silicon carbide whiskers.a High-magnification view,b low-magnification view.The whiskers are essentially randomly oriented; the whisker diameter is 1.4um and the whisker length is 18.6um.(From [61) Compared to silicon carbide,titanium diboride has a higher modulus but a lower thermal conductivity (Table 6.2).The high modulus makes titanium diboride a highly effective reinforcing material.The addition of TiBz to a metal greatly increases the stiffness,hardness and wear resistance and decreases the CTE,while it reduces the electrical and thermal conductivity much less than the addition of
6.1 Tailoring by Component Selection 165 Figure6.4. SEMphotographsofmechanicallypolishedsectionsofaluminum-matrixcompositescontaining10vol%silicon carbide whiskers. a High-magnification view, b low-magnification view. The whiskers are essentially randomly oriented; the whisker diameter is 1.4μm and the whisker length is 18.6μm. (From [6]) Compared to silicon carbide, titanium diboride has a higher modulus but a lower thermal conductivity (Table 6.2). The high modulus makes titanium diboride a highly effective reinforcing material. The addition of TiB2 to a metal greatly increases the stiffness, hardness and wear resistance and decreases the CTE, while it reduces the electrical and thermal conductivity much less than the addition of
166 6 Tailoring Composite Materials 10μm 10μm b Figure6.5.Optical microscopephotographs of a copper-matrix composite containing:a 15vol%TiB2 platelets;b60vol% TiB2 platelets.(From [7]) most other ceramic fillers.Figure 6.5 shows optical microscope photographs of copper-matrix composites containing TiB2 platelets with diameters 3-5um and aspect ratios of about 3.The composites are made by the coated filler method (Fig.1.9)of powder metallurgy.The CTE decreases monotonically with increasing TiBz volume fraction (Fig.6.6)such that the coated filler method gives slightly lower CTE than the admixture method (Fig.1.7)for the same TiB,volume fraction. However,even for the coated filler method,a high TiB,volume fraction of 60% is needed in order to reduce the CTE of copper from 17 x 10-6 to 8.5 x 10-6/C (Fig.6.6).The thermal conductivity decreases monotonically with increasing TiBz
166 6 Tailoring Composite Materials Figure 6.5. Optical microscope photographs of a copper-matrix composite containing: a 15vol% TiB2 platelets; b 60vol% TiB2 platelets. (From [7]) most other ceramic fillers. Figure 6.5 shows optical microscope photographs of copper-matrix composites containing TiB2 platelets with diameters 3–5μm and aspect ratios of about 3. The composites are made by the coated filler method (Fig. 1.9) of powder metallurgy. The CTE decreases monotonically with increasing TiB2 volume fraction (Fig. 6.6) such that the coated filler method gives slightly lower CTE than the admixture method (Fig. 1.7) for the same TiB2 volume fraction. However, even for the coated filler method, a high TiB2 volume fraction of 60% is needed in order to reduce the CTE of copper from 17 × 10−6 to 8.5 × 10−6/°C (Fig. 6.6). The thermal conductivity decreases monotonically with increasing TiB2