Three types of polycarbosilane-derived SiC fibers-Nicalon, Hi-Nicalon, and Hi-Nicalon S-were exposed at temperatures of 1573-1773 K under a reduced pressure of 1.3 Pa. The thermal stability of the fibers was investigated through examinations of the gas evolution, grain growth, specific resistivity, fiber morphology, and tensile strength. The thermal decomposition of the silicon oxycarbide phase began at 1523 K; then, active oxidation of the ␤-SiC crystallites occurred at >1673 K. The active oxidation caused serious damage to the fiber structure, which resulted in significant degradation of the fiber strength. Hi-Nicalon had a tensile strength of ϳ0.5 GPa after exposure at 1773 K, although Nicalon and Hi-Nicalon S fibers completely lost their strength, even after exposure at 1673 K. Hi-Nicalon fiber had relatively good thermal stability under reduced pressure.

SiC fibers were synthesized from polycarbosilane (PCS) fibers by heat treatment after electron beam irradiation curing. The pyrolysis reaction mechanisms from the organic PCS to ceramic SiC were investigated by the analysis of gases evolved during heat treatment. There were two steps in the major reaction: the first step was at 800–1200 K where H2 and CH4 evolved by scission of Si‐CH3 and Si‐H and by rearrangement reactions, and the second step was at 1000–1700 K where H2 evolved by reactions related to C atoms in the PCS main chain. H2 evolution in the first step was reduced with increasing oxygen content in the cured PCS fibers.

Low-oxygen silicon carbide fibers (Hi-Nicalon) were oxidized at temperatures from 1073 to 1773 K under an oxygen partial pressure of 0.25 atm. The strength of the unoxidized core was practically unaffected by the oxidation temperatures. The strength of the as-oxidized fibers with an SiO 2 film decreased markedly with increasing oxidation temperature. When exposed subsequently to 1773 K in argon, the fibers with a SiO 2 film of 0.3-0.5 m thickness had the best thermal stability.

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