Variation of the oxidation rate of silicon carbide with water-vapor pressure

Chemically vapor deposited silicon carbide (CVD SiC) was oxidized at temperatures of 1000 degrees-1400 degrees C in H2O/O-2 gas mixtures with compositions of 10-90 vol% water vapor at a total pressure of 1 atm. Additional experiments were conducted in H2O/argon mixtures at a temperature of 1100 degrees C. Experiments were designed to minimize impurity and volatility effects, so that only intrinsic water-vapor effects were observed. The oxidation kinetics increased as the water-vapor content increased. The parabolic oxidation rates in the range of 10-90 vol% water vapor (the balance being oxygen) were approximately one order of magnitude higher than the rates that were observed in dry oxygen for temperatures of 1200 degrees-1400 degrees C. The power-law dependence of the parabolic oxidation rate on the partial pressure of water vapor at all temperatures of the study indicated that the molecular species was not the sole rate-limiting oxidant. The determination of an activation energy for diffusion was complicated by variations in the oxidation mechanism and oxide-scale morphology with the partial pressure of water vapor and the temperature.