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 SCIENCEA 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 cross￾link 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 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. 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