THE OXIDATION-KINETICS OF BETA-SIALON CERAMICS

The oxidation behaviour of beta-sialons (Si6-zAlzOzN8-z) with the composition z = 0.5 and 3.8, prepared without and with an addition of 3 and 6 wt% yttria, has been investigated isothermally during 20 h in a TG unit at temperatures between 1200 and 1450-degrees-C. A comparison is also given of the oxidation behaviour of beta-sialons prepared from two Si3N4 powders with different amounts of impurities; and finally the oxidation behaviour of single-phase beta-Si3N4. Only a few of the obtained oxidation curves followed the parabolic rate law during the entire experiment. Most of the curves could, however, be interpreted with the modified rate law, (DELTAw/A0) = a arctan (bt)1/2 + c(t)1/2 + d either exhaustively or in combination with the parabolic rate law. The parabolic rate constants and activation energies for the oxidation process could be determined and were found to increase with increasing amounts of impurities. The parabolic rate constants were also found to be lower for the beta-sialons prepared without yttria than for 'pure' Si3N4, but the activation energy was about the same. The oxidation kinetics of the materials with z = 0.5 and 3 wt% yttria could not be conclusively determined, but the evolution of the microstructure of the oxide scale showed that its crystallization had an important influence on the oxidation rate.