FINE-DIAMETER POLYCRYSTALLINE SIC FIBERS

Various organosilicon polymers have been converted to small-diameter, polycrystalline silicon carbide fibers by melt-spinning, cross-linking and pyrolyzing to high-temperature in argon. Several wt% boron was doped into the fibers before pyrolysis. Use of polycarbosilane precursor gave 8-10 mum diameter fibers having up to 2.6 GPa tensile strength, 450 GPa elastic modulus, 3.1-3.2 g/cm3 density. The microstructure consists of >95 wt% beta-SiC crystallites of 30-40 nm average crystallite size. Stoichiometric fibers or fibers having excess carbon content have been prepared. Fiber has been thermally aged under inert conditions at 1800-degrees-C for 12 h with minimal strength and microstructural change. Stoichiometric fiber maintains higher strength after oxidative aging at 1370-degrees-C. Current processing efforts are aimed at preparing the fiber in continuous tow form.