Thermal stability of SiO2-coated SiC fiber (Hi-Nicalon) under argon atmosphere

Thermal stability of low-oxygen SiC fiber (Hi-Nicalon) coated with SiO2 fil m was investigated. The SiO2 film of same thickness but different crystal s tructure was formed by heating low-oxygen SiC fiber (Hi-Nicalon) under diff erent oxidation conditions. The oxidation treatment and the subsequent expo sure at 1773 K in argon caused very little loss of strength for unoxidized core. For as-oxidized fiber with SiO2 film which contained imperfections, f urther loss of strength was caused after exposure in argon. There was littl e degradation of core strength on being exposed repeatedly at rapid heating and cooling rate in argon. The fiber oxidized at 1773 K kept high level of strength even after exposure at 1823 K. This is because the change in crys tal structure of SiO2 film before and after exposure in argon, which was th e controlling factor in the degradation of strength, was diminished with in creasing oxidation temperature. (C) 2000 Kluwer Academic Publishers.