High oxidation resistance of hot pressed silicon nitride containing yttriaand lanthania

Oxidation tests were carried out on Si3N4-La2O3-Y2O3 hot pressed ceramics u p to 1500 degrees C. morphological and analytical characterizations were pe rformed on surfaces and reaction scales after oxidation and correlated with the oxidation kinetics. (Near)-parabolic behaviour was observed at tempera tures <1450 degrees C for short periods, while for higher temperatures and longer exposures rite kinetics shifted to a linear behaviour. Moreover the excellent oxidation resistance (as demonstrated by extremely low weight gai ns), particularly zip to 1450 degrees C, was related to the high refractori ness of the grain boundary phases in this additive system. Strength degrada tion after oxidation at several temperatures was also studied and discussed . (C) 1999 Elsevier Science Limited. All rights reserved.