Active Oxidation of SiC

Silicon carbide (SiC) forms a protective condensed-phase oxide (SiO2) in passive oxidation and a volatile sub-oxide (SiO(g)) in active oxidation. The transition between these two modes of oxidation and the rates of active oxidation are critical issues. A literature review indicates that impurity effects, the difference between active-to-passive and passive-to-active transitions, and the effect of total pressure on these transitions remain unexplored for SiC. Measurements were made in a thermogravimetric apparatus (TGA) by changing oxygen potentials either by blending O-2/Ar mixtures or changing total pressures in a pure oxygen gas stream to the point where a transition occurs. Specimens were examined with standard optical and electron-optical techniques. Active-to-passive and passive-to-active transitions were measured and found to be similar for SiC, which is in contrast to pure Si. The similarity in SiC is attributed to SiC/SiO2 interfacial reactions producing the necessary conditions for passive scale formation (active-to-passive) or passive scale breakdown (passive-to-active). Comparable results were obtained in both the O-2/Ar and reduced total O-2 pressure cases for SiC.