6 Tailoring Composite Materials The techniques that are used to tailor composite materials in order to achieve improved properties-as needed for a variety of applications-are covered in this chapter.These techniques include the selection and modification of the compo- nents and the engineering of the interfaces in the composite.An example of an interface is that between the reinforcement and the matrix.Interfaces can greatly affect the properties of a composite. 6.1 Tailoring by Component Selection 6.1.1 Polymer-Matrix Composites Epoxy is by far the most widely used polymer matrix for structural composites. This is due to the strong adhesiveness of epoxy,in addition to the long history of its use in composites.Tradenames of epoxy include Epon,Epi-rez,and Araldite Epoxy displays an excellent combination of mechanical properties and corrosion resistance,is dimensionally stable,exhibits good adhesion,and is relatively in- expensive.Moreover,the low molecular weight of uncured epoxide resin in the liquid state results in exceptionally high molecular mobility during processing. This mobility helps the resin to quickly spread on the surface of carbon fiber,for example. Epoxy resins are characterized by having two or more epoxide groups per molecule.The chemical structure of an epoxide group is shown in Fig.6.1. An epoxy is a thermosetting polymer that cures upon mixing with a catalyst (also known as a hardener).This curing process is a reaction that involves poly- merization and crosslinking. H Figure 6.1.Chemical structure of an epoxide group 157
6 Tailoring Composite Materials The techniques that are used to tailor composite materials in order to achieve improved properties – as needed for a variety of applications – are covered in this chapter. These techniques include the selection and modification of the components and the engineering of the interfaces in the composite. An example of an interface is that between the reinforcement and the matrix. Interfaces can greatly affect the properties of a composite. 6.1 Tailoring by Component Selection 6.1.1 Polymer-Matrix Composites Epoxy is by far the most widely used polymer matrix for structural composites. This is due to the strong adhesiveness of epoxy, in addition to the long history of its use in composites. Tradenames of epoxy include Epon, Epi-rez, and Araldite. Epoxy displays an excellent combination of mechanical properties and corrosion resistance, is dimensionally stable, exhibits good adhesion, and is relatively inexpensive. Moreover, the low molecular weight of uncured epoxide resin in the liquid state results in exceptionally high molecular mobility during processing. This mobility helps the resin to quickly spread on the surface of carbon fiber, for example. Epoxy resins are characterized by having two or more epoxide groups per molecule. The chemical structure of an epoxide group is shown in Fig. 6.1. An epoxy is a thermosetting polymer that cures upon mixing with a catalyst (also known as a hardener). This curing process is a reaction that involves polymerization and crosslinking. O CH2 C | H Figure 6.1. Chemical structure of an epoxide group 157
158 6 Tailoring Composite Materials 0 PI PEEK PPS PES ● H CH PEI CH, Figure 6.2.The mers(repeating units)of thermoplastic polymers typically used in structural composites There are many types of epoxy.The most common epoxy resin is produced by a reaction between epichlorohydrin and bisphenol A.The mers(repeating units)of thermoplastic polymers that are typically used in structural composites are shown in Fig.6.2. The properties of these thermoplastics are listed in Table 6.1.In contrast,epoxies have a tensile strength of 103 MPa,an elastic modulus of 3.4 GPa,a ductility (elon- gation at break)of 6%,and a density of 1.25g/cm3[1].Thus,epoxies are stronger, stiffer and more brittle than most thermoplastic polymers.Another major differ- ence between thermoplastics and epoxies is the higher processing temperatures of thermoplastics(300-400C). 6.1.2 Cement-Matrix Composites Component selection for cement-matrix composites involves the use ofadmixtures, which are additives included in the cement mix.These additives serve various functions,as described below:
158 6 Tailoring Composite Materials Figure 6.2. The mers (repeating units) of thermoplastic polymers typically used in structural composites There are many types of epoxy. The most common epoxy resin is produced by a reaction between epichlorohydrin and bisphenol A. The mers (repeating units) of thermoplastic polymers that are typically used in structural composites are shown in Fig. 6.2. The properties of these thermoplastics are listed in Table 6.1. In contrast, epoxies have a tensile strength of 103MPa, an elastic modulus of 3.4GPa, a ductility (elongation at break) of 6%, and a density of 1.25g/cm3 [1]. Thus, epoxies are stronger, stiffer and more brittle than most thermoplastic polymers. Another major difference between thermoplastics and epoxies is the higher processing temperatures of thermoplastics (300–400°C). 6.1.2 Cement-Matrix Composites Componentselectionforcement-matrixcompositesinvolvestheuseofadmixtures, which are additives included in the cement mix. These additives serve various functions, as described below:
6.1 Tailoring by Component Selection 159 Table 6.1.Properties of thermoplastics PES PEEK PEI PPS PI T:(C) 2302 1703 2251 86 256 Decomposition temperature(C) 5502 590 5554 5271 550- Processing temperature(C) 3502 3803 3503 316 304 Tensile strength(MPa) 84d 70d 105e 66d 1170 Modulus of elasticity(GPa) 2.4d 3.8d 3.0° 33d 2.1d Ductility (elongation) 80 150d 50-65 1 Izod impact(ft lb/in.) 1.6d 1.6d 1 0.5d 1.5d Density(g/cm) 1.37 131d 1.27 1.30d 139d Data from:[2];b [3];[4];d [5] Water reducing agent-a minor additive to increase the workability of the mix Polymer(such as latex)-to decrease liquid permeability and bond strength Fine particles (such as silica fume)-to decrease liquid permeability,bond strength and drying shrinkage,and to increase the modulus and the abrasion resistance Short fiber(such as steel fiber)-to increase the flexural toughness. Continuous fibers are not suitable for inclusion in a cement mix,although they can be applied prior to cement pouring and can serve as a reinforcement.Both processing and material costs are high.In addition,penetration of the cement mix into the small spaces between microfibers is difficult.On the other hand, macroscopic steel rebars are similar in shape to continuous microfibers and are commonly used to reinforce concrete. 6.1.2.1 Polymers in Cement-Matrix Composites Polymer particles used as admixtures can take the form of a dry powder or an aqueous dispersion of particles.The latter form is more common.The inclusion of either form as an admixture results in improved joining of the mix constituents (e.g.,sand),due to the presence of interweaving polymer films.The improved joining leads to superior mechanical and durability characteristics.Aqueous dis- persions of polymer particles are more effective than dry polymer powder for the development and uniform distribution of polymer films.The most common form of polymer in aqueous dispersions is latex,particularly butadiene-styrene copolymer.The dispersions are stabilized by the use of surfactants. In polymer-modified cement-based material,polymer particles are partitioned between the interiors of hydrates and the surfaces of anhydrous cement grains.The presence of the polymer results in an improved pore structure,thereby decreased porosity.Furthermore,the workability is enhanced and the water absorption is decreased.This enhanced workability allows the use of lower values of the wa- ter/cement ratio. The rate of hydration is reduced by the presence of the polymer.The addition of a polymer tends to increase the flexural strength and toughness,but lower the
6.1 Tailoring by Component Selection 159 Table 6.1. Properties of thermoplastics PES PEEK PEI PPS PI Tg (°C) 230a 170a 225a 86a 256b Decomposition temperature (°C) 550a 590a 555a 527a 550b Processing temperature (°C) 350a 380a 350a 316a 304b Tensile strength (MPa) 84d 70d 105c 66d 117d Modulus of elasticity (GPa) 2.4d 3.8d 3.0c 3.3d 2.1d Ductility (% elongation) 80d 150d 50–65c 2d 10d Izod impact (ft lb/in.) 1.6d 1.6d 1c 0.5d 1.5d Density (g/cm3) 1.37d 1.31d 1.27c 1.30d 1.39d Data from: a [2]; b [3]; c [4]; d [5] Water reducing agent – a minor additive to increase the workability of the mix Polymer (such as latex) – to decrease liquid permeability and bond strength Fine particles (such as silica fume) – to decrease liquid permeability, bond strength and drying shrinkage, and to increase the modulus and the abrasion resistance Short fiber (such as steel fiber) – to increase the flexural toughness. Continuous fibers are not suitable for inclusion in a cement mix, although they can be applied prior to cement pouring and can serve as a reinforcement. Both processing and material costs are high. In addition, penetration of the cement mix into the small spaces between microfibers is difficult. On the other hand, macroscopic steel rebars are similar in shape to continuous microfibers and are commonly used to reinforce concrete. 6.1.2.1 Polymers in Cement-Matrix Composites Polymer particles used as admixtures can take the form of a dry powder or an aqueous dispersion of particles. The latter form is more common. The inclusion of either form as an admixture results in improved joining of the mix constituents (e.g., sand), due to the presence of interweaving polymer films. The improved joining leads to superior mechanical and durability characteristics. Aqueous dispersions of polymer particles are more effective than dry polymer powder for the development and uniform distribution of polymer films. The most common form of polymer in aqueous dispersions is latex, particularly butadiene-styrene copolymer. The dispersions are stabilized by the use of surfactants. In polymer-modified cement-based material, polymer particles are partitioned between the interiors of hydrates and the surfaces of anhydrous cement grains. The presence of the polymer results in an improved pore structure, thereby decreased porosity. Furthermore, the workability is enhanced and the water absorption is decreased. This enhanced workability allows the use of lower values of the water/cement ratio. The rate of hydration is reduced by the presence of the polymer. The addition of a polymer tends to increase the flexural strength and toughness, but lower the
160 6 Tailoring Composite Materials compressive strength,modulus of elasticity,and hardness.Furthermore,polymer addition is effective at enhancing the vibration damping capacity,the frost re- sistance,and the resistance to biogenic sulfuric acid corrosion(relevant to sewer systems).In addition,polymer addition imparts stability and thixotropy to grouts and enables control over the rheology and the stabilization of the cement slurry against segregation.Dry polymer particles used as an admixture can be water- redispersible polymer particles,such as those obtained by spray drying aqueous dispersions.Examples are acrylic and poly(ethylenevinyl acetate).Redispersibility may be attained through the use of functional monomers.The effectiveness of redispersible polymer particles depends on the cement used.One special cate- gory of polymer particles is superabsorbent particles (hydrogel),which serve to provide the controlled formation of water-filled macropore inclusions(i.e.,wa- ter entrainment)in the fresh concrete.The consequence of this is control over self-dessication.Another kind of superabsorbent polymer barely absorbs alkaline water in fresh/hardened concrete,but absorbs a great deal ofneutral/acid water and creates a gel.Thus,when neutral water is poured onto the concrete after setting, the concrete is coated with the gel and can thus be kept without drying. Organic liquid admixtures can be polymer solutions(involving water-soluble polymers such as methylcellulose,polyvinyl alcohol and polyacrylamide)or resins (such as epoxy and unsaturated polyester resin).The liquid form is attractive due to the ease with which it can be uniformly spatially distributed,and hence its effectiveness in even small proportions.In contrast to polymer solutions,particles (including particle dispersions)tend to require a higher proportion in order to be comparably effective.Polymer solutions used as admixtures can serve to op- timize the air void distribution and rheology of the wet mix,thereby improving workability with low air contents. Short fibers rather than continuous ones are used because they can be incorpo- rated in the cement mix,thereby facilitating processing in the field.Furthermore, short fibers are less expensive than continuous ones.Polypropylene,polyethylene and acrylic fibers are particularly common due to the requirements of low cost and resistance to the alkaline environment in cement-based materials.Compared to carbon,glass and steel fibers,polymer fibers are attractive due to their high ductility,which results in high flexural toughness in the cement-based material. The combined use of short polymer fibers and a polymer particle dispersion(e.g., latex)results in superior strength(tensile,compressive,and flexural)and flexural toughness compared to the use of fibers without a polymer particle dispersion. 6.1.2.2 Silica Fume in Cement-Matrix Composites Silica fume is very fine noncrystalline silica produced by electric arc furnaces as a by-product of the production of metallic silicon or ferrosilicon alloys.It is a powder with particles that have diameters that a hundredfold smaller than those of anhydrous Portland cement particles (i.e.,the mean particle size is between 0.1 and 0.2 um).The SiO2 content ranges from 85 to 98%.Silica fume is pozzolanic- it has a limited ability to serve as a cementitious binder
160 6 Tailoring Composite Materials compressive strength, modulus of elasticity, and hardness. Furthermore, polymer addition is effective at enhancing the vibration damping capacity, the frost resistance, and the resistance to biogenic sulfuric acid corrosion (relevant to sewer systems). In addition, polymer addition imparts stability and thixotropy to grouts and enables control over the rheology and the stabilization of the cement slurry against segregation. Dry polymer particles used as an admixture can be waterredispersible polymer particles, such as those obtained by spray drying aqueous dispersions. Examples are acrylic and poly(ethylenevinyl acetate). Redispersibility may be attained through the use of functional monomers. The effectiveness of redispersible polymer particles depends on the cement used. One special category of polymer particles is superabsorbent particles (hydrogel), which serve to provide the controlled formation of water-filled macropore inclusions (i.e., water entrainment) in the fresh concrete. The consequence of this is control over self-dessication. Another kind of superabsorbent polymer barely absorbs alkaline water in fresh/hardened concrete, but absorbs a great deal of neutral/acid water and creates a gel. Thus, when neutral water is poured onto the concrete after setting, the concrete is coated with the gel and can thus be kept without drying. Organic liquid admixtures can be polymer solutions (involving water-soluble polymers such as methylcellulose, polyvinyl alcohol and polyacrylamide) or resins (such as epoxy and unsaturated polyester resin). The liquid form is attractive due to the ease with which it can be uniformly spatially distributed, and hence its effectiveness in even small proportions. In contrast to polymer solutions, particles (including particle dispersions) tend to require a higher proportion in order to be comparably effective. Polymer solutions used as admixtures can serve to optimize the air void distribution and rheology of the wet mix, thereby improving workability with low air contents. Short fibers rather than continuous ones are used because they can be incorporated in the cement mix, thereby facilitating processing in the field. Furthermore, short fibers are less expensive than continuous ones. Polypropylene, polyethylene and acrylic fibers are particularly common due to the requirements of low cost and resistance to the alkaline environment in cement-based materials. Compared to carbon, glass and steel fibers, polymer fibers are attractive due to their high ductility, which results in high flexural toughness in the cement-based material. The combined use of short polymer fibers and a polymer particle dispersion (e.g., latex) results in superior strength (tensile, compressive, and flexural) and flexural toughness compared to the use of fibers without a polymer particle dispersion. 6.1.2.2 Silica Fume in Cement-Matrix Composites Silica fume is very fine noncrystalline silica produced by electric arc furnaces as a by-product of the production of metallic silicon or ferrosilicon alloys. It is a powder with particles that have diameters that a hundredfold smaller than those of anhydrous Portland cement particles (i.e., the mean particle size is between 0.1 and 0.2μm). The SiO2 content ranges from 85 to 98%. Silica fume is pozzolanic – it has a limited ability to serve as a cementitious binder
6.1 Tailoring by Component Selection 161 Silica fume used as an admixture in a concrete mix has significant positive effects on the properties of the resulting material.These effects pertain to the strength, modulus,ductility,vibration damping capacity,sound absorption,abrasion resis- tance,air void content,shrinkage,bonding strength with reinforcing steel,per- meability,chemical attack resistance,alkali-silica reactivity reduction,corrosion resistance of embedded steel reinforcement,freeze-thaw durability,creep rate, coefficient of thermal expansion(CTE),specific heat,thermal conductivity,defect dynamics,dielectric constant,and degree of fiber dispersion in mixes containing short microfibers.However,silica fume addition degrades the workability of the mix.This problem can be alleviated by using more water-reducing agent or by treating the surfaces of the silica fume particles with silane. 6.1.2.3 Short Fibers in Cement-Matrix Composites Short fibers are used as admixtures in cement-based materials in order to decrease the drying shrinkage,increase the flexural toughness,and in some cases to increase the flexural strength too.When the fibers are electrically conductive,they may also provide nonstructural functions,such as self-sensing(sensing the strain,damage, or temperature),self-heating (for deicing),and electromagnetic reflection (for electromagnetic interference shielding;i.e.,EMI shielding). Although continuous fibers are more effective than short fibers when used as a reinforcement,they are not amenable to incorporation in a concrete mix and they are relatively expensive.Low cost is critical to the practical viability of cement- based materials. Although macroscopic steel fibers that are around 1 mm in diameter are used,the most effective fibers are usually microfibers with diameters ranging from 5 to 100 um.For example,carbon fibers are typically around 10 um in diameter.Nanofibers with diameters that are typically around 0.1 um are less effective than microfibers as a reinforcement,although they are more effective than microfibers at providing EMI shielding(due to their small diameters and the skin effect,which refers to the phenomenon in which high-frequency electromagnetic radiation only interacts with the near-surface region of an electrical conductor).In general,the smaller the fiber diameter(and thus the higher the aspect ratio),the more difficult it is to disperse the fibers.Similarly,the smaller the fiber length(which relates to a lower aspect ratio),the easier it is to disperse the fibers.This is due to the tendency for fibers with small diameters or longlengths to cling to one another.The effectiveness of a fiber admixture at improving the structural or functional properties of cement- based materials is greatly affected by the degree of fiber dispersion.The attainment of a high degree of fiber dispersion is particularly critical when the fiber volume fraction is low.A low fiber volume fraction is usually preferred because the material cost increases,the workability decreases,the air void content increases,and the compressive strength decreases as the fiber content increases.The fiber dispersion is enhanced by improving the hydrophilicity(i.e.,the wettability by water)of the fibers,as the cement mix is water-based.The hydrophilicity can be controlled by treating the surfaces of the fibers prior to incorporating the fibers into the cement mix.Furthermore,the fiber dispersion is affected by the admixtures that may
6.1 Tailoring by Component Selection 161 Silica fume used as an admixture in a concrete mix has significant positive effects on the properties of the resulting material. These effects pertain to the strength, modulus, ductility, vibration damping capacity, sound absorption, abrasion resistance, air void content, shrinkage, bonding strength with reinforcing steel, permeability, chemical attack resistance, alkali-silica reactivity reduction, corrosion resistance of embedded steel reinforcement, freeze–thaw durability, creep rate, coefficient of thermal expansion (CTE), specific heat, thermal conductivity, defect dynamics, dielectric constant, and degree of fiber dispersion in mixes containing short microfibers. However, silica fume addition degrades the workability of the mix. This problem can be alleviated by using more water-reducing agent or by treating the surfaces of the silica fume particles with silane. 6.1.2.3 Short Fibers in Cement-Matrix Composites Short fibers are used as admixtures in cement-based materials in order to decrease the drying shrinkage, increase the flexural toughness, and in some cases to increase the flexural strength too. When the fibers are electrically conductive, they may also provide nonstructural functions, such as self-sensing (sensing the strain, damage, or temperature), self-heating (for deicing), and electromagnetic reflection (for electromagnetic interference shielding; i.e., EMI shielding). Although continuous fibers are more effective than short fibers when used as a reinforcement, they are not amenable to incorporation in a concrete mix and they are relatively expensive. Low cost is critical to the practical viability of cementbased materials. Although macroscopic steelfibersthat arearound1mmindiameter areused,the most effective fibers are usually microfibers with diameters ranging from 5 to 100 μm. For example, carbon fibers are typically around 10μm in diameter. Nanofibers with diameters that are typically around 0.1μm are less effective than microfibers as a reinforcement, although they are more effective than microfibers at providing EMI shielding (due to their small diameters and the skin effect, which refers to the phenomenon in which high-frequency electromagnetic radiation only interacts with the near-surface region of an electrical conductor). In general, the smaller the fiber diameter (and thus the higher the aspect ratio), the more difficult it is to disperse the fibers. Similarly, the smaller the fiber length (which relates to a lower aspect ratio), the easier it is to disperse the fibers. This is due to the tendency for fiberswithsmalldiametersorlonglengthstoclingtooneanother.Theeffectiveness of a fiber admixture at improving the structural or functional properties of cementbased materials is greatly affected by the degree of fiber dispersion. The attainment of a high degree of fiber dispersion is particularly critical when the fiber volume fraction is low. A low fiber volume fraction is usually preferred because the material cost increases, the workability decreases, the air void content increases, and the compressive strength decreases as the fiber content increases. The fiber dispersion is enhanced by improving the hydrophilicity (i.e., the wettability by water) of the fibers, as the cement mix is water-based. The hydrophilicity can be controlled by treating the surfaces of the fibers prior to incorporating the fibers into the cement mix. Furthermore, the fiber dispersion is affected by the admixtures that may
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