Microstructural Evolution of Silica on Single-Crystal Silicon Carbide. Part I: Devitrification and Oxidation Rates

Dry thermal oxidation of single-crystal silicon carbide (6H-SiC) at 1400 degrees C in low-water oxygen using an alumina tube furnace initially yields a predominantly vitreous oxide scale. After 28 h, approximately one quarter of the oxide scale's surface becomes crystalline, with disk-like cristobalite aggregates (radialites) statistically distributed within the vitreous matrix. Crystalline areas were found to be thinner than vitreous regions using optical microscopy and atomic force microscopy/hydrofluoric acid (AFM/HF)-etching analysis, providing evidence for different oxidation rates of SiC covered by vitreous silica and cristobalite. As the bulk oxide scale continues to grow during devitrification, the radialites assume their characteristic morphology with a deepened center. Line profiles of the oxide scale's thickness across radialites obtained from AFM/HF etching were used to determine the oxidation rate of SiC covered by crystalline silica and the crystallisation rate applying a two-stage parabolic equation. As a result, it was found that the parabolic rate-constant B(vitreous) is approximate to 4.2 times larger than the corresponding rate-constant in crystalline areas (B(cristobalite)), suggesting similar differences in effective oxygen diffusion coefficients. For the crystallization rate nu, we determined a value of 1.5 +/- 0.1 mu m/h.