OXIDATION BEHAVIOR OF CHEMICALLY-VAPOR-DEPOSITED SILICON-CARBIDE AND SILICON-NITRIDE FROM 1200-DEGREES TO 1600-DEGREES-C

The isothermal oxidation of pure CVD SiC and Si3N4 has been studied fo r 100 h in dry, flowing oxygen from 1200 degrees to 1600 degrees C in an alumina tube furnace. Adherent oxide formed at temperatures to 1550 degrees 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 f or Si3N4. Silicon oxynitride did not appear to play a role in the oxid ation of Si3N4 under the conditions herein. This is thought to be deri ved from the presence of ppm levels of sodium impurities in the alumin a furnace tube. It is proposed that sodium modifies the silicon oxynit ride, rendering it ineffective as a diffusion barrier. Material recess ion as a function of oxide thickness was calculated and found to be lo w. Oxidation behavior at 1600 degrees C differed from the lower temper atures in that silica spallation occurred after exposure.