Oxidation Behavior of Chemically-Vapor-Deposited Silicon Carbide and Silicon Nitride from 1200° to 1600°C

The isothermal oxidation of pure CVD SiC and Si₃N₄ has been studied for 100 h in dry, flowing oxygen from 1200° to 1600°C in an alumina tube furnace. Adherent oxide formed at temperatures to 1550°C. The major crystalline phase in the resulting silica scales was alpha-cristobalite. Parabolic rate constants for SiC were within an order of magnitude of literature values. The oxidation kinetics of Si₃N₄ in this study were not statistically different from that of SiC. Measured activation energies were 190 kJ/mol for SiC and 186 kJ/mol for Si₃N₄. Silicon oxynitride did not appear to play a role in the oxidation of Si₃N₄ under the conditions herein. This is thought to be derived from the presence of ppm levels of sodium impurities in the alumina furnace tube. It is proposed that sodium modifies the silicon oxynitride, rendering it ineffective as a diffusion barrier. Material recession as a function of oxide thickness was calculated and found to be low. Oxidation behavior at 1600°C differed from the lower temperatures in that silica spallation occurred after exposure.