very small lattice spacing near the inner regions of the CNT. Carbon nanotubes(CNTs) have some remarkable properties, such as better electrical conductivity than copper, exceptional mechanical strength, and very high flexibility( with futuristic potential for use in even earthquake- resistant buildings and crash-resistant cars). There is already considerable interest in industry in using CNTs in chemical sensors, field emission elements, electronic interconnects in integrated nanotube circuits, hydrogen storage devices, temperature sensors and thermometers, and others Because of the exceptional properties of CNTs(e. g, Youngs modulus of CNT'is 1-4 TeraPascals, TP Pa), there has been some interest in incorporating CNTs in polymers, ceramics, and metals Owing to CNT's metallic or semiconducting character, incorporating CNT in polymer matrice permits attainment of an electrical conductivity sufficient to provide an electrostatic discharge at very low CNT concentrations. Similarly, extremely hard/and wear-resistant metal-matrix composites and tough ceramic-matrix composites are being developed. Since the discovery of CNTs in 1991, similar nanostructures were formed in other layered compounds such as BN, BCN and WS2, etc. For example, whereas CNTs are either metallic or semiconducting(depending on the shell helicity and diameter), bn nanotubes are insulating and could possibly serve as nanoshields for nanoconductors. Also, bn nanotubes are thermally more stable in oxidizing atmospheres than are CNTs and have comparable modulus. The strength of nanotubular materials be increased by assembling them in the form of ropes, as has been done with CNt and Bn anotubes, with ropes made from single-walled CNTs being the strongest known material. The spacing between the individual nanotube strands in such a rope will be in the subnanometer range; for example, this spacing is 0. 34 nm in a rope made from multiwalled BN nanotubes which is on the order of the(0001)lattice spacing in the hexagonal BN cell Organic Fibers. Because the covalent C-C bond is very strong, linear-chain polymers such as olyethylene can be made very strong and stiff by fully extending their molecular chains. A wide range of physical and mechanical properties can be attained by controlling the orientation of these lymer chains along the fiber axis and their order or crystallinity. Allied Corporations Spectra 00 and Du Pont' s aramid fiber Kevlar are two successful organic fibers widely used for com posite strengthening. Aramid is an abbreviated name of a class of synthetic organic fibers that are aromatic polyamide compounds. Nylon is a generic name for any long-chain polyamide. Many highly sophisticated manufacturing techniques have been developed to fabricate the organic fibers for use in composites. These techniques include: tensile drawing, die drawing, hydrostatic extrusion, and gel spinning. A wide range of useful engineering properties is achieved in organic fibers depending on the chemical nature of the polymeric material, processing technique, and the control of process parameters. For example, high modulus polyethylene fibers with a modulus of 200 GPa, and Kevlar fibers with a modulus of 65-125 GPa and tensile strength of 2.8 GPa have been developed Kevlar fibers have poor compression strength and should be used under compressive loading only as a hybrid fiber mixture, that is, as a combination of carbon fiber and Kevlar. One limitation of most organic fibers is that they degrade (lose color and strength) when exposed to visible or ultraviolet radiation, and a coating of a light-absorbing material is used overcome this problem. Metallic Fibers. Metals such as beryllium, tungsten, titanium, tantalum, and molybdenum, and alloys such as steels in the form of wires or fibers have high and very consistent tensile strength values as well as other attractive properties. Beryllium has a high modulus(300 GPa) and low density (1.8 g/cc)but also low strength(1300 MPa). Fine(0. 1-mm)diameter steel wires with a high carbon(0.9%)content have very high strength(5 GPa). Tungsten fibers have a 402 MATERIALS PROCESSING AND MANUFACTURING SCIENCE
very small lattice spacing near the inner regions of the CNT. Carbon nanotubes (CNTs) have some remarkable properties, such as better electrical conductivity than copper, exceptional mechanical strength, and very high flexibility (with futuristic potential for use in even earthquakeresistant buildings and crash-resistant cars). There is already considerable interest in industry in using CNTs in chemical sensors, field emission elements, electronic interconnects in integrated nanotube circuits, hydrogen storage devices, temperature sensors and thermometers, and others. Because of the exceptional properties of CNTs (e.g., Young's modulus of CNT is 1-4 TeraPascals, TPa), there has been some interest in incorporating CNTs in polymers, ceramics, and metals. Owing to CNT's metallic or semiconducting character, incorporating CNT in polymer matrices permits attainment of an electrical conductivity sufficient to provide an electrostatic discharge at very low CNT concentrations. Similarly, extremely hard/and wear-resistant metal-matrix composites and tough ceramic-matrix composites are being developed. Since the discovery of CNTs in 1991, similar nanostructures were formed in other layered compounds such as BN, BCN, and WS2, etc. For example, whereas CNTs are either metallic or semiconducting (depending on the shell helicity and diameter), BN nanotubes are insulating and could possibly serve as nanoshields for nanoconductors. Also, BN nanotubes are thermally more stable in oxidizing atmospheres than are CNTs and have comparable modulus. The strength of nanotubular materials can be increased by assembling them in the form of ropes, as has been done with CNT and BN nanotubes, with ropes made from single-walled CNTs being the strongest known material. The spacing between the individual nanotube strands in such a rope will be in the subnanometer range; for example, this spacing is --~0.34 nm in a rope made from multiwalled BN nanotubes, which is on the order of the (0001) lattice spacing in the hexagonal BN cell. Organic Fibers. Because the covalent C-C bond is very strong, linear-chain polymers such as polyethylene can be made very strong and stiff by fully extending their molecular chains. A wide range of physical and mechanical properties can be attained by controlling the orientation of these polymer chains along the fiber axis and their order or crystallinity. Allied Corporation's Spectra 900 and Du Pont's aramid fiber Kevlar are two successful organic fibers widely used for composite strengthening. Aramid is an abbreviated name of a class of synthetic organic fibers that are aromatic polyamide compounds. Nylon is a genetic name for any long-chain polyamide. Many highly sophisticated manufacturing techniques have been developed to fabricate the organic fibers for use in composites. These techniques include: tensile drawing, die drawing, hydrostatic extrusion, and gel spinning. A wide range of useful engineering properties is achieved in organic fibers depending on the chemical nature of the polymeric material, processing technique, and the control of process parameters. For example, high modulus polyethylene fibers with a modulus of 200 GPa, and Kevlar fibers with a modulus of 65-125 GPa and tensile strength of 2.8 GPa have been developed Kevlar fibers have poor compression strength and should be used under compressive loading only as a hybrid fiber mixture, that is, as a combination of carbon fiber and Kevlar. One limitation of most organic fibers is that they degrade (lose color and strength) when exposed to visible or ultraviolet radiation, and a coating of a light-absorbing material is used to overcome this problem. Metallic Fibers. Metals such as beryllium, tungsten, titanium, tantalum, and molybdenum, and alloys such as steels in the form of wires or fibers have high and very consistent tensile strength values as well as other attractive properties. Beryllium has a high modulus (300 GPa) and low density (1.8 g/cc) but also low strength (1300 MPa). Fine (0.1-mm) diameter steel wires with a high carbon (0.9%) content have very high strength (~5 GPa). Tungsten fibers have a 402 MATERIALS PROCESSING AND MANUFACTURING SCIENCE
ery high melting point(3400Cand are suited for heat-resistant applications. These various metallic fibers have been used as reinforcements in composite matrices based on metals (e.g copper), concrete and polymers. For example, tungsten(density 19.3 g/cc)has been used as a reinforcement in advanced Ni- and Co-base superalloys for heat-resistant applications, and in Cu alloys for electrical contact applications. Similarly, steel wire is used to reinforce concrete and polymers(e. g, in steel belted tires). Other metallic reinforcements used in composite applications include ribbons and wires of rapidly quenched amorphous metallic alloys such Fego B20 and Fe60 Cr] Mo] B 28 having improved physical and mechanical properties. Ceramic Fibers. Ceramic fibers such as single crystal sapphire, polycrystalline Al2O3, SiC, Si3 N4, B.C and others have high strength at room- and elevated temperature, high modu lus, excellent heat-resistance, and superior chemical stability against environmental attack. Both polymer pyrolysis and sol-gel techniques make use of organometallic compounds to grow ceramic fibers. Pyrolysis of polymers containing silicon, carbon, nitrogen, and boron under controlled conditions has been used to produce heat-resistant ceramic fibers such as SiC, Al2O3 Si3N4, BN, B4C and several others The commercial alumina fibers have a Youngs modulus of 152-300 GPa and a tensile strength of 1.7 to 2.6 GPa. Alumina fibers are manufactured by companies such as Du Pont(fiber FP), Sumitomo Chemical (alumina-silica), and ICI(Saffil, 8-alumina phase). Fiber FP is made by dry-spinning an aqueous slurry of fine alumina particles containing additives. The dry-spun yarn density of a-alumina. A thin silica coating is generally applied to heal the surface flaws, giving higher tensile strength than uncoated fiber. The polymer pyrolysis route to make Al2 O3 fibers akes use of dry-spinning of an organoaluminum compound to produce the ceramic precursor, ollowed by calcining of this precursor to obtain the final fiber. 3M Company uses a sol-gel route to synthesize an alumina fiber(containing silica and boria), called Nextel 312. The technique ses hydrolysis of a metal alkoxide, that is, a compound of the type M(OR)n where M is the metal, R is an organic compound, and n is the metal valence. The process breaks the M-OR bond and establishes the MO-R to give the desired oxide Hydrolysis of metal alkoxides creates sols that are spun and gelled. The gelled fiber is then densified at intermediate temperatures. The high surface energy of the fine pores of the gelled fiber permits low-temperature densification Silicon carbide fibers, whiskers and particulates are among the most widely used reinforce- nents in composites. SiC fiber is made using the CVd process. a dense coating of SiC is vapor-deposited on a tungsten or carbon filament heated to about 1300C The deposition process involves high-temperature gaseous reduction of alkyl silanes(e.g CH3 SiCl3)by hydrogen. Typically, a gaseous mixture consisting of 70% H2 and 30% silanes is introduced in the CVd reactor along with a 10-13 um diameter tungsten or carbon filament. The Sic-coated filament is wound on a spool, and the exhaust gases are passed through a condensersystem to recover unused silanes. The CVD-coated SiC monofilament(100-150 ur diameter)is mainly B-SiC with some a-SiC on the tungsten core. The SCS-6 fiber of AvcO Specialty Materials Company is a CVD SiC fiber with a gradient structure that is produced from the reaction of silicon-and carbon-containing compounds over a heated pyrolytic graphite coated carbon core. The SCS-6 fiber is designed to have a carbon-rich outer surface that acts as a buffer layer between the fiber and the matrix metal in a composite, and the subsurface structure is graded to have stoichiometric SiC a few micrometers from the surface The Sic fiber obtained via the Cvd process is thick(140 um) and inflexible which prese difficulty in shaping the preform using mass production methods such as filament winding
very high melting point (3400~ and are suited for heat-resistant applications. These various metallic fibers have been used as reinforcements in composite matrices based on metals (e.g., copper), concrete and polymers. For example, tungsten (density 19.3 g/cc) has been used as a reinforcement in advanced Ni- and Co-base superalloys for heat-resistant applications, and in Cu alloys for electrical contact applications. Similarly, steel wire is used to reinforce concrete and polymers (e.g., in steel belted tires). Other metallic reinforcements used in composite applications include ribbons and wires of rapidly quenched amorphous metallic alloys such as Fe80B20 and Fe60Cr6Mo6B28 having improved physical and mechanical properties. Ceramic Fibers. Ceramic fibers such as single crystal sapphire, polycrystalline A1203, SiC, Si3N4, B4C and others have high strength at room- and elevated temperature, high modulus, excellent heat-resistance, and superior chemical stability against environmental attack. Both polymer pyrolysis and sol-gel techniques make use of organometallic compounds to grow ceramic fibers. Pyrolysis of polymers containing silicon, carbon, nitrogen, and boron under controlled conditions has been used to produce heat-resistant ceramic fibers such as SiC, A1203, Si3N4, BN, B4C and several others. The commercial alumina fibers have a Young's modulus of 152-300 GPa and a tensile strength of 1.7 to 2.6 GPa. Alumina fibers are manufactured by companies such as Du Pont (fiber FP), Sumitomo Chemical (alumina-silica), and ICI (Saffil, g-alumina phase). Fiber FP is made by dry-spinning an aqueous slurry of fine alumina particles containing additives. The dry-spun yarn is subjected to two-step firing: low firing to control the shrinkage and flame-firing to improve the density of c~-alumina. A thin silica coating is generally applied to heal the surface flaws, giving higher tensile strength than uncoated fiber. The polymer pyrolysis route to make A1203 fibers makes use of dry-spinning of an organoaluminum compound to produce the ceramic precursor, followed by calcining of this precursor to obtain the final fiber. 3M Company uses a sol-gel route to synthesize an alumina fiber (containing silica and boria), called Nextel 312. The technique uses hydrolysis of a metal alkoxide, that is, a compound of the type M(OR)n where M is the metal, R is an organic compound, and n is the metal valence. The process breaks the M-OR bond and establishes the MO-R to give the desired oxide. Hydrolysis of metal alkoxides creates sols that are spun and gelled. The gelled fiber is then densified at intermediate temperatures. The high surface energy of the fine pores of the gelled fiber permits low-temperature densification. Silicon carbide fibers, whiskers and particulates are among the most widely used reinforcements in composites. SiC fiber is made using the CVD process. A dense coating of SiC is vapor-deposited on a tungsten or carbon filament heated to about 1300 ~ The deposition process involves high-temperature gaseous reduction of alkyl silanes (e.g., CH3SiC13) by hydrogen. Typically, a gaseous mixture consisting of 70% H2 and 30% silanes is introduced in the CVD reactor along with a 10-13 Ixm diameter tungsten or carbon filament. The SiC-coated filament is wound on a spool, and the exhaust gases are passed through a condenser system to recover unused silanes. The CVD-coated SiC monofilament (--~ 100-150 txm diameter) is mainly/3-SIC with some u-SiC on the tungsten core. The SCS-6 fiber of AVCO Specialty Materials Company is a CVD SiC fiber with a gradient structure that is produced from the reaction of silicon- and carbon-containing compounds over a heated pyrolytic graphitecoated carbon core. The SCS-6 fiber is designed to have a carbon-rich outer surface that acts as a buffer layer between the fiber and the matrix metal in a composite, and the subsurface structure is graded to have stoichiometric SiC a few micrometers from the surface. The SiC fiber obtained via the CVD process is thick (140 txm) and inflexible which presents difficulty in shaping the preform using mass production methods such as filament winding. Composite Materials 403
A method, developed in Japan, to make fine and flexible continuous SiC fibers(Nicalon fibers) es melt-spinning under N2 gas of a silicon-based polymer such as polycarbosilane into precursor fiber. This is followed by curing of the precursor fiber at 1000"C under N2 to cross- link the molecular chains, making the precursor infusible during the subsequent pyrolysis at 300C in N2 under mechanical stretch. This treatment converts the precursor into the inorganic SiC fiber. Nicalon fibers, produced using the above process, have high modulus(180-420 GPa) and high strength(2 GPa) Besides the SiC and Al2O3 fibers described in the preceding paragraphs, silicon nitride, boron bide, and boron nitride are other useful ceramic fiber materials. Si3N4 fibers are produced by CVD using SiCla and NH3 as reactant gases, and forming the fiber as a coating onto a carbon or tungsten filament. In polymer-based synthesis of silicon nitride fibers, an organosilazane compound (i.e, a compound that has Si-NH-Si bonds)is pyrolyzed to give both SiC and Si3N4 Fibers of the oxidation-resistant material boron nitride are produced by melt-spinning a boric oxide precursor, followed by a nitriding treatment with ammonia that yields the BN fiber. A final thermal treatment eliminates residual oxides and stabilizes the high-purity bN phase. Boron carbide(B4 C)fibers are produced by the Cvd process via the reaction of carbon yarn with BCl3 and H2 at high temperatures in a CVD reactor. In addition to the use of long and continuous fibers of different ceramic materials in composite matrices, vapor-phase grown ceramic whiskers have also been extensively used in composite materials, whiskers are monocrystalline short ceramic fibers(aspect ratio 50-10, 000)having extremely high fracture strength values that approach the theoretical fracture strength of the material Figure 6-3 compares the room-temperature stress versus strain behavior of boron, Kevlar, and glass fibers; high-modulus graphite(HMG)fiber; and ceramic whiskers. The figure shows that whiskers are by far the strongest reinforcement, because of the absence of structural faws, which results in their strength approaching the material,s theoretical strength. Usually, however, there is considerable scatter in the strength properties of whiskers, and this becomes prob lematic in synthesizing composites with a narrow spread in their properties. Selected thermal and mechanical properties of some commercially available fibers are summarized in Table 6-1 Whiskers Boron Kevlar FIGURE 6-3 Schematic comparison of stress-strain diagrams for common reinforcing fibers and whiskers(HMG, high-modulus graphite fiber).(A. Kelly, ed, Concise Encyclopedia of Composite Materials, Elsevier, 1994, p. 312). Reprinted with permission from Elsevier. 404 MATERIALS PROCESSING AND MANUFACTURING SCIENCE
A method, developed in Japan, to make fine and flexible continuous SiC fibers (Nicalon fibers) uses melt-spinning under N2 gas of a silicon-based polymer such as polycarbosilane into a precursor fiber. This is followed by curing of the precursor fiber at 1000~ under N2 to crosslink the molecular chains, making the precursor infusible during the subsequent pyrolysis at 1300~ in N2 under mechanical stretch. This treatment converts the precursor into the inorganic SiC fiber. Nicalon fibers, produced using the above process, have high modulus (180-420 GPa) and high strength (~2 GPa). Besides the SiC and A1203 fibers described in the preceding paragraphs, silicon nitride, boron carbide, and boron nitride are other useful ceramic fiber materials. Si3N4 fibers are produced by CVD using SIC14 and NH3 as reactant gases, and forming the fiber as a coating onto a carbon or tungsten filament. In polymer-based synthesis of silicon nitride fibers, an organosilazane compound (i.e., a compound that has Si-NH-Si bonds) is pyrolyzed to give both SiC and Si3N4. Fibers of the oxidation-resistant material boron nitride are produced by melt-spinning a boric oxide precursor, followed by a nitriding treatment with ammonia that yields the BN fiber. A final thermal treatment eliminates residual oxides and stabilizes the high-purity BN phase. Boron carbide (B4C) fibers are produced by the CVD process via the reaction of carbon yarn with BC13 and H2 at high temperatures in a CVD reactor. In addition to the use of long and continuous fibers of different ceramic materials in composite matrices, vapor-phase grown ceramic whiskers have also been extensively used in composite materials. Whiskers are monocrystalline short ceramic fibers (aspect ratio ~50-10,000) having extremely high fracture strength values that approach the theoretical fracture strength of the material. Figure 6-3 compares the room-temperature stress versus strain behavior of boron, Kevlar, and glass fibers; high-modulus graphite (HMG) fiber; and ceramic whiskers. The figure shows that whiskers are by far the strongest reinforcement, because of the absence of structural flaws, which results in their strength approaching the material's theoretical strength. Usually, however, there is considerable scatter in the strength properties of whiskers, and this becomes problematic in synthesizing composites with a narrow spread in their properties. Selected thermal and mechanical properties of some commercially available fibers are summarized in Table 6-1. 21 c~ 13- 14 r ffl e" ~ 7 I--- n I I 0 10 20 Elongation (%) 30 FIGURE 6-3 Schematic comparison of stress-strain diagrams for common reinforcing fibers and whiskers (HMG, high-modulus graphite fiber). (A. Kelly, ed., Concise Encyclopedia of Composite Materials, Elsevier, 1994, p. 312). Reprinted with permission from Elsevier. 404 MATERIALS PROCESSING AND MANUFACTURING SCIENCE
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The mechanical and physical properties such as elastic modulus(E)and coefficient of thermal expansion( CTE)of fibers are strongly orientation dependent, and usually exhibit significant disf- erences in magnitude along the fiber axis and transverse to it. The high-temperature strength of some commercial silicon carbide fibers is compared in Figure 6. 4. It can be noted that the fiber tains high strength to fairly high temperatures; for example, NLP 101 fiber retains a strength of 500 MPa at 1300C, which is comparable to the room-temperature tensile strength of some high-strength, low-alloy steels In addition to the synthetic fibers and whiskers, numerous low-cost, discontinuous fillers ave been used in composites to conserve precious matrix materials at little expense to their engineering properties. These fillers include mica, sand, clay, talc, rice husk ash, fly ash, natural fibers(e.g, lingo-cellulosic fibers), recycled glass, and many others, including environmentally conscious biomorphic ceramics based on silicon carbide and silicon dioxide obtained from pyrolysis of natural wood. These various fillers and reinforcements permit a range of composite microstructures to be created that have a wide range of strength, stiffness, wear resistance, and other characteristics. Figure 6-5 shows the porous structure of pyrolyzed wood that has been used as a preform for impregnation with molten metals to create ceramic- or metal-matrix composites Interface Interfaces in composites are regions of finite dimensions at the boundary between the fiber and the matrix where compositional and structural discontinuities can occur over distances varying from an atomic monolayer to over five orders of magnitude in thickness. Composite fabrication processes create interfaces between inherently dissimilar materials(e. g, ceramic fibers and a 0200400600800100012001400 FIGURE 6-4 High-temperature strength of some SiC fibers plotted as applied stress versus test mperature.(B S. Mitchell, An Introduction to Materials Engineering and Science for Chemical Materials Engineers, Wiley Interscience, Hoboken, N), 2004) 06 MATERIALS PROCESSING AND MANUFACTURING SCIENCE
The mechanical and physical properties such as elastic modulus (E) and coefficient of thermal expansion (CTE) of fibers are strongly orientation dependent, and usually exhibit significant disfferences in magnitude along the fiber axis and transverse to it. The high-temperature strength of some commercial silicon carbide fibers is compared in Figure 6.4. It can be noted that the fiber retains high strength to fairly high temperatures; for example, NLP 101 fiber retains a strength of 500 MPa at 1300~ which is comparable to the room-temperature tensile strength of some high-strength, low-alloy steels. In addition to the synthetic fibers and whiskers, numerous low-cost, discontinuous fillers have been used in composites to conserve precious matrix materials at little expense to their engineering properties. These fillers include mica, sand, clay, talc, rice husk ash, fly ash, natural fibers (e.g., lingo-cellulosic fibers), recycled glass, and many others, including environmentally conscious biomorphic ceramics based on silicon carbide and silicon dioxide obtained from pyrolysis of natural wood. These various fillers and reinforcements permit a range of composite microstructures to be created that have a wide range of strength, stiffness, wear resistance, and other characteristics. Figure 6-5 shows the porous structure of pyrolyzed wood that has been used as a preform for impregnation with molten metals to create ceramic- or metal-matrix composites. Interface Interfaces in composites are regions of finite dimensions at the boundary between the fiber and the matrix where compositional and structural discontinuities can occur over distances varying from an atomic monolayer to over five orders of magnitude in thickness. Composite fabrication processes create interfaces between inherently dissimilar materials (e.g., ceramic fibers and 1"6I 9 1.4 r"- 1.2 13.. (.9 "6" 1 (/) (/) m 0.8 (/) "o a. 0.6 Q. < 0.4 ~ i 0 o NLP 101 9 NLM 102 9 N LM 202 I 200 I I I I I I 400 600 800 1000 1200 1400 Temperature (~ FIGURE 6-4 High-temperature strength of some SiC fibers plotted as applied stress versus test temperature. (B. S. Mitchell, An Introduction to Materials Engineering and Science for Chemical and Materials Engineers, Wiley Interscience, Hoboken, NJ, 2004). 406 MATERIALS PROCESSING AND MANUFACTURING SCIENCE