Effects of High Water‐Vapor Pressure on Oxidation of Silicon Carbide at 1200°C

Abstract

The oxidation of SiC at 1200°C in a slowly flowing gas mixture of either air or air + 15 vol% H 2 O at 10 atm (1 MPa) was studied for extended times to examine the effects of elevated water‐vapor pressure on oxidation rates and microstructural development. At a water‐vapor pressure of 1.5 atm (150 kPa), distinct SiO 2 scale structures were observed on the SiC; thick, porous, nonprotective cristobalite scales formed above a thin, nearly dense vitreous SiO 2 layer, which remained constant in thickness with time as the crystalline SiO 2 continued to grow. The pore morphology of the cristobalite layer differed depending on the type of SiC on which it was grown. The crystallization and growth rates of the cristobalite layer were significantly accelerated in the presence of the high water‐vapor pressure and resulted in rapid rates of SiC surface recession that were on the order of what is observed when SiO 2 volatility is rate controlling at high gas‐flow velocities (30 m/s). The recession process can be described by a paralinear kinetic model controlled by the conversion of dense vitreous SiO 2 to porous, nonprotective SiO 2 .