Oxidation kinetics and mechanical property of stoichiometric SiC fibers (Hi-Nicalon-S)

The oxidation behavior of stoichiometric SiC fiber (Hi-Nicalon-S) was inves tigated and compared with those of Nicalon and Hi-Nicalon fibers. Hi-Nicalo n-S fiber was oxidized in Ar-O-2 gas mixtures with 25-100%O-2 at 1213-1773 K. The oxidation rates were measured with thermobalance experiments. The th ickening in SiO2 film produced a large decrease in the strength of the as-o xidized fiber. The SiO2 film-removed fiber (unoxidized core) retained high strength independent of the SiO2 film thickness. The oxidation rate obeyed the diffusion-controlled contracting-disc formula. The rate constant was pr oportional to the oxygen partial pressure of Ar-O-2 gas mixture. The activa tion energy was 79 kJ/mol. The oxidation rate was considered to be controll ed by the diffusion of oxygen molecule through the micropores in SiO2 him. Compared to Nicalon and Hi-Nicalon, Hi-Nicalon-S is resistant to oxidation and has good strength in the oxidized state.