Morphological and kinetic aspects of thermal oxidation of SiC whisker reinforced Al 2 0 3

The thermal oxidation susceptibility of a 30 vol.-%SiC whisker reinforced Al₂0₃ composite in air at 1300–1500°C has been investigated. It was found that the morphological changes associated with scale evolution lead to the formation of mullite and a glassy phase. In particular, the scale cross-section consisted of a porous external scale (white), and an internal black layer in contact with the unreacted matrix. The black scale region was relatively thin (0–6 μm), and contained partially oxidized SiC whiskers as well as glassy phases. In some instances, the inner black scale/matrix interface exhibited intergranular cracking. The presence of these cracks was attributed to internal stresses induced by volume increases associated with the oxidation reaction. Additionally, the rates of scale thickening and of weight gain exhibited parabolic behaviour. At 1500°C parabolic rate constants K_p and K_m of 1·18 × 10⁻¹⁴ m² S⁻¹ and 2·23 × 10⁻⁹ kg² m⁻⁴ S⁻¹, respectively werefound. From the rate constant data, activation energies of 577 and 542 kJ mol⁻¹ were determined on the basis of scale thickness and weight gain, respectively. Apparently, the oxidation rates in the black subzone were somewhat high to be rate limiting. Hence, the exhibited activation energies were ascribed to the diffusion of oxidant across the white scale region.

MST/2013