High-Temperature Oxidation Behavior of Chemical Vapor Deposited Silicon Carbide.

Abstract

The transition behavior from active to passive oxidation and from passive oxidation to bubble formation for CVD SiC was studied by thermogravimetry at wide-ranged oxygen partial pressures in O2-Ar, CO2-Ar and CO-CO2 atmospheres at temperatures from 1600 to 2000 K. In O2-Ar, the mass loss rate (i.e., active oxidation rate) increased with increasing PO2, and at a critical PO2 the active-to-passive transition (i.e., mass loss to mass gain) was clearly observed. In CO-CO2, the mass loss was significant at a low PCO2/PCO ratio and then the mass loss decreased with increasing PCO2/PCO ratio. The transition from mass loss to mass gain was gradual, being contrast to that in O2-Ar. The transition mechanism in O2-Ar and CO-CO2 was explained by Wagner model, volatility diagram and numerical thermodynamic calculation.The transition from passive oxidation to bubble formation was observed at more than 1985K in O2-Ar at PO2>5kPa. At PO2<5kPa the transition temperature decreased with decreasing PO2 in O2-Ar and CO2-Ar. The bubble formation might be initiated by the stoichiometric reaction between SiO2 and SiC forming SiO and CO vapors at the SiO2/SiC interface. The transition temperature could be calculated from the condition that the equilibrium SiO and CO vapor pressures exceed the ambient pressure.