High temperature oxidation of refractory-grade SiC was carried out in O2 and Ar atmospheres mixed with various concentrations of water vapor, and its oxidation behavior was followed mainly by using a mass-spectrometer. The results were as follows,
(1) In the dry and wet O2 atmospheres, main gaseous products in the temperature-raising process up to 1400°C were CO and CO2 formed from the free carbon. Water vapor accelerated the oxidation of free carbon. On the other hand, in the successive soaking process at 1400°C, CO2 formed from SiC was a dominant product. The evolution behavior of CO2 during the soaking process was well fitted with the parabolic law and a positive tendency was observed between the parabolic rate constant and the water vapor content.
(2) In the wet Ar atmospheres, CO, CO2 and H2 were evolved due to oxidation of the free carbon by water vapor in the temperature-raising process. The oxidation behavior of SiC in the soaking process changed with the water vapor content. At high water vapor contents, the passive oxidation took place, obeying the parabolic law and the parabolic rate constant increased in proportion to the content of water vapor. At the water vapor content as low as 0.6vol% H2O, however, the initial part of the reaction proceeded according to the active oxidation mechanism obeying the linear law. White spots observed on the surface of the sample after the active oxidation were composed of SiO2 fiber, which was presumably formed by the vapor phase reaction among SiO, CO and H2O.