Oxidation Transitions for SiC Part II. Passive-to-Active Transitions


Author to whom correspondence should be addressed. e-mail: bryan.harder@nasa.gov


Oxidation of SiC can occur in a passive mode, where a protective film is generated, or in an active mode, where a volatile suboxide is generated. The transitions from active-to-passive and passive-to-active are particularly important to understand as they occur via different mechanisms. In Part II of this article, the passive-to-active transition is explored. Three different types of SiC are examined—Si-rich SiC, stoichiometric SiC, and C-rich SiC. In addition to an in situ transition from passive-to-active, the effect of a preformed film on all three types of SiC is explored. It appears that the passive-to-active transition occurs when the SiO2 scale begins to react with the SiC substrate. This reaction generates SiO(g) and CO(g), which build pressure beneath the SiO2 scale, eventually causing the oxide to rupture. In addition, the SiO(g) can oxidize a distance away from the surface leading to the formation of SiO2 needles and further promoting this SiO2/SiC reaction. Thermodynamic and kinetic data are used to predict transition pressures of oxygen, which show reasonable agreement with those measured.