High Oxidation Resistance of Hot Pressed Silicon Nitride Containing Yttria and Lanthania

Oxidation tests were carried out on Si3N4±La2O3±Y2O3 hot pressed ceramics up to 1500C. Morphological and analytical characterizations were performed on surfaces and reaction scales after oxidation and correlated with the oxidation kinetics. (Near)-parabolic behaviour was observed at tem- peratures <1450C for short periods, while for higher temperatures and longer exposures the kinetics shifted to a linear behaviour. Moreover the excellent oxidation resistance (as demonstrated by extremely low weight gains), particularly up to 1450C, was related to the high refractoriness of the grain boundary phases in this additive system. Strength degradation after oxidation at several temperatures was also studied and discussed.