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 windingvery 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 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, 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 reinforce￾ments 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 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 txm) and inflexible which presents difficulty in shaping the preform using mass production methods such as filament winding. Composite Materials 403