Thermal stability of polycarbosilane-derived silicon carbide fibers under reduced pressures

Three types of polycarbosilane-derived SiC fibers-Nicalon Hi-Nicalon, and H i-Nicalon S-were exposed at temperatures of 1573-1773 K under a reduced pre ssure of 1.3 Pa. The thermal stability of the fibers was investigated throu gh examinations of the gas evolution, grain growth, specific resistivity, f iber morphology, and tensile strength. The thermal decomposition of the sil icon oxycarbide phase began at 1523 K; then, active oxidation of the beta - SiC crystallites occurred at >1673 K. The active oxidation caused serious d amage to the fiber structure, which resulted in significant degradation of the fiber strength. Hi-Nicalon had a tensile strength of similar to0.5 GPa after exposure at 1773 K, although Nicalon and Hi-Nicalon S fibers complete ly lost their strength, el en after exposure at 1673 K, Hi-Nicalon fiber ha d relatively good thermal stability under reduced pressure.