Effect of high temperature heat treatment in vacuum on microstructure and bending properties of SiCf/SiC composites prepared by CVI

Thermal stabilities of SiCf/SiC composites prepared with chemical vapor infiltration have been examined. Three types of SiC fibers of commercial grade Nicalon. Hi-Nicalon and stoichiometric Hi-Nicalon Type-S with oxygen concentrations of respectively 11.7%, 0.5% and 0.2% were used as preforms. After heat treatment at 1273-2273 K in vacuum, changes in oxygen content of the composites were measured. Bending properties such as flexural strength and fracture toughness were also examined at room temperature. Deoxidation was clearly observed for the commercial grade Nicalon SiCf/SiC composite (SiCf(CG)/SiC) at temperatures above around 1700 K. Furthermore, decomposition of fibers completely occurred above 1973 K for SiCf(CG)/SiC. A slight degradation was also observed for Hi-Nicalon SiCf/SiC composite. On the other hand, Hi-Nicalon Type-S SiCf/SiC did not show an apparent degradation even after heat treating at 2200 K. The decomposition of the commercial grade Nicalon SiC fibers can be mainly explained by the sublimation of SiO which is formed according to the reaction SiCxO3→ SiO+SiC. (orig.)