consistently high quality are produced by this process, although the relatively high density of w filament slightly increases the fiber density In the halide reduction process using BCl3, a 10- to 12-um-diameter W wire is pulled in a reaction chamber at one end through a mercury seal and out at the other end through another mercury seal. The mercury seals act as electrical contacts for resistance heating of the substrate wire when gases( BCl3+H2) pass through the reaction chamber and react on the incandescent wire substrate to deposit boron coatings. The conversion of BCl3 to B coating is only Boron is also deposited on carbon monofilaments. a pyrolytic carbon coating is first applied to the carbon filament to accommodate the growth strains that result during boron deposition. There is a critical temperature for obtaining a boron fiber with optimum properties and tructure. The desirable amorphous(actually, microcrystalline with grain size of just a few nm) form of boron occurs below this critical temperature, whereas above this temperature there also occur crystalline forms of boron, which are undesirable from a mechanical properties view point. Larger crystallites lower the mechanical strength of the fiber. Because of high deposition temperatures in CVD, diffusional processes are rapid, and this partially transforms the core region from pure w to a variety of boride phases such as W2 B, WB, WB4, and others. As ron diffuses into the tungsten substrate to form borides, the core expands as much as 40%0 by volume, which results in an increase in the fiber diameter. This expansion generates resid ual stresses that can cause radial cracks and stress concentration in the fiber, thus lowering the fracture strength of the fiber. The average tensile strength of commercial boron fibers is about 3-4 GPa, and the modulus is 380-400 GPa, Usually a SiC coating is vapor-deposited onto the fiber to prevent any adverse reactions between B and the matrix such as al at high Carbon Fiber. Carbon, which can exist in a variety of crystalline forms, is a light material (density: 2.268 g/cc). The graphitic form of carbon is of primary interest in making fibers. The other form of carbon is diamond, a covalent solid, with little flexibility and little scope to grow diamond fibers, although microcrystalline diamond coatings can be vapor-deposited on a fiberous substrate to grow coated diamond fibers. Carbon atoms in graphite are arranged in the form of hexagonal layers, which are attached to similar layers via van der Waals forces. The graphitic form is highly anisotropic, with widely different elastic modulus in the layer plane and along the c-axis of the unit cell (i.e, very high in-plane modulus and very low transverse modulus) The high-strength covalent bonds between carbon atoms in the hexagonal layer plane result in an extremely high modulus(1000 GPa in single crystal) whereas the weak van der Waals bond between the neighboring layers results in a lower modulus(about one-half the modulus of pure Al)in that direction. In order to grow high-strength and high-modulus carbon fiber, a very high degree of preferred orientation of hexagonal planes along the fiber axis is needed The name carbon fiber is a generic one and represents a family of fibers all derived from carbonaceous precursors, and differing from one another in the size of the hexagonal sheets of arbon atoms, their stacking height, and the resulting crystalline orientations. These structural variations result in a wide range of physical and mechanical properties. For example, the axial tensile modulus can vary from 25 to 820 GPa, axial tensile strength from 500 to 5,000 MPa, nd thermal conductivity from 4 to 1100 W/m K, respectively. Carbon fibers of extremely high modulus are made by carbonization of organic precursor fibers followed by graphitization at high temperatures. The organic precursor fiber is generally a special long-chain polymer-based textile fiber(polyacrylonitrile or PAN and rayon, a thermosetting polymer) that can be car- bonized without melting. Such fibers generally have poor mechanical properties because of a 400 MATERIALS PROCESSING AND MAN NG SCIENCEconsistently high quality are produced by this process, although the relatively high density of W filament slightly increases the fiber density. In the halide reduction process using BC13, a 10- to 12-1xm-diameter W wire is pulled in a reaction chamber at one end through a mercury seal and out at the other end through another mercury seal. The mercury seals act as electrical contacts for resistance heating of the substrate wire when gases (BC13+H2) pass through the reaction chamber and react on the incandescent wire substrate to deposit boron coatings. The conversion efficiency of BC13 to B coating is only about 10%, and reuse of unreacted gas is important. Boron is also deposited on carbon monofilaments. A pyrolytic carbon coating is first applied to the carbon filament to accommodate the growth strains that result during boron deposition. There is a critical temperature for obtaining a boron fiber with optimum properties and structure. The desirable amorphous (actually, microcrystalline with grain size of just a few nm) form of boron occurs below this critical temperature, whereas above this temperature there also occur crystalline forms of boron, which are undesirable from a mechanical properties view￾point. Larger crystallites lower the mechanical strength of the fiber. Because of high deposition temperatures in CVD, diffusional processes are rapid, and this partially transforms the core region from pure W to a variety of boride phases such as W2B, WB, WB4, and others. As boron diffuses into the tungsten substrate to form borides, the core expands as much as 40% by volume, which results in an increase in the fiber diameter. This expansion generates resid￾ual stresses that can cause radial cracks and stress concentration in the fiber, thus lowering the fracture strength of the fiber. The average tensile strength of commercial boron fibers is about 3-4 GPa, and the modulus is 380-400 GPa. Usually a SiC coating is vapor-deposited onto the fiber to prevent any adverse reactions between B and the matrix such as A1 at high temperatures. Carbon Fiber. Carbon, which can exist in a variety of crystalline forms, is a light material (density: 2.268 g/cc). The graphitic form of carbon is of primary interest in making fibers. The other form of carbon is diamond, a covalent solid, with little flexibility and little scope to grow diamond fibers, although microcrystalline diamond coatings can be vapor-deposited on a fiberous substrate to grow coated diamond fibers. Carbon atoms in graphite are arranged in the form of hexagonal layers, which are attached to similar layers via van der Waals forces. The graphitic form is highly anisotropic, with widely different elastic modulus in the layer plane and along the c-axis of the unit cell (i.e., very high in-plane modulus and very low transverse modulus). The high-strength covalent bonds between carbon atoms in the hexagonal layer plane result in an extremely high modulus (~ 1000 GPa in single crystal) whereas the weak van der Waals bond between the neighboring layers results in a lower modulus (about one-half the modulus of pure A1) in that direction. In order to grow high-strength and high-modulus carbon fiber, a very high degree of preferred orientation of hexagonal planes along the fiber axis is needed. The name carbon fiber is a generic one and represents a family of fibers all derived from carbonaceous precursors, and differing from one another in the size of the hexagonal sheets of carbon atoms, their stacking height, and the resulting crystalline orientations. These structural variations result in a wide range of physical and mechanical properties. For example, the axial tensile modulus can vary from 25 to 820 GPa, axial tensile strength from 500 to 5,000 MPa, and thermal conductivity from 4 to 1100 W/m.K, respectively. Carbon fibers of extremely high modulus are made by carbonization of organic precursor fibers followed by graphitization at high temperatures. The organic precursor fiber is generally a special long-chain polymer-based textile fiber (polyacrylonitrile or PAN and rayon, a thermosetting polymer) that can be car￾bonized without melting. Such fibers generally have poor mechanical properties because of a 400 MATERIALS PROCESSING AND MANUFACTURING SCIENCE