FORMATION OF BETA-SIALON AS AN INTERMEDIATE OXIDATION-PRODUCT OF SIC-ALN CERAMICS

The oxidation mechanisms in SiC-AIN solid solutions, exposed at 1370 degrees C for 200 h in air, have been investigated by scanning electron microscopy (SEM), analytical and high-resolution electron microscopy (HREM), and X-ray diffraction techniques, Five hot-pressed compositions, including pure SiC and pure AlN, were examined, Assuming parabolic rate behavior, the values of the parabolic rate constants based on oxide layer thickness increased by over 2 orders of magnitude by increasing AIN contents from 0% to 62%, A continuous amorphous phase containing cristobalite and mullite crystallites was present at the oxide layer in 13% AlN and 27% AlN samples, Two reaction layers mere found in the oxidized SiC-62% AlN solid solution. The inner layer consisted of a mixture of beta-SiAlON and graphite phases, while the outer layer consisted of mullite as a major phase and cristobalite as a minor phase, The oxidation proceeded in two stages: a partial oxidation of SIC-AIN to beta-SiAlON and carbon (graphite), and a final oxidation of beta-SiAlON to mullite + SiO2.