Bryan Harder

Active oxidation of silicon carbide

Abstract The active oxidation of SiC has been studied at 1390 and 1490°C, paying particular attention to the active-to-passive and passive-to-active transition points. First the active-to-passive transition for pure silicon was studied at 1290°C. The beginning of passivity is characterized by micron-sized SiO2 rod formation on the surface due to the oxidation of SiO(g), consistent with other investigators. These rods were not observed in the active-to-passive transition for SiC; but they were observed in the passive-to-active transition for SiC. This type of microstructure yields information about the breakdown of the passive film. Unlike pure silicon, at a fixed temperature a substantial difference in the transition oxygen pressure for the active-to-passive and passive-to-active transitions was not observed for SiC. This is due to the fact that both processes are controlled by SiCySiO2 interfacial reactions. Studies were also conducted on active oxidation of SiC with a pre-formed SiO2 scale in order to understand the breakdown of the passive film.

Hysteresis in the Active Oxidation of SiC

Si and SiC show both passive oxidation behavior where a protective film of SiO2 forms and active oxidation behavior where a volatile suboxide SiO(g) forms. The active-to-passive and passive-to-active oxidation transitions are explored for both Si and SiC. Si shows a dramatic difference between the P(O2) for the two transitions of ~10-4 bar. The active-to-passive transition is controlled by the condition for SiO2/Si equilibrium and the passive-to-active transition is controlled by the decomposition of SiO2. In the case of SiC, the P(O2) for these transitions are much closer. The active-to-passive transition appears to be controlled by the condition for SiO2/SiC equilibrium. The passive-to-active transition appears to be controlled by the interfacial reaction of SiC and SiO2 and subsequent generation of gases at the interface which leads to scale breakdown.