ACTIVE-TO-PASSIVE TRANSITION AND BUBBLE FORMATION FOR HIGH-TEMPERATURE OXIDATION OF CHEMICALLY VAPOR-DEPOSITED SILICON-CARBIDE IN CO-CO2 ATMOSPHERE

Oxidation behavior of chemically vapor-deposited SiC in CO-CO2 atmosph eres (0.1 MPa) was investigated using a thermogravimetric technique at temperatures from 1823 to 1923 K. Active or passive oxidation was obs erved depending on temperature and CO2/CO partial pressure ratio (P(CO 2)/P(CO)). The critical P(CO2)/P(CO) value for the transition was 10(2 ) times as large as a theoretical value calculated from the Wagner mod el. In the passive oxidation above 1873 K, SiO2 bubbles were grown. Th e expansion and rupture of bubbles caused cyclic rapid mass gain and m ass loss.