Effect of long-term oxidation on creep and failure of Si3N4 and Si3N4/SiC nanocomposites

The high-temperature mechanical behaviour of an Si3N4/SiC nanocomposite and its monolithic Si3N4 reference material was studied after long-term oxidat ion treatments intended to simulate future operating conditions in a severe environment. Creep and failure at elevated temperature were significantly affected, in the direction of increased brittleness. The transition stress between the ductile range present at low stresses and the brittle range exi sting at high stresses was shifted to distinctly lower values. The creep re sistance in the low-stress range was increased by the oxidation treatment. The failure rime under a given stress was drastically reduced; this was att ributed to an increased sensitivity to subcritical crack growth. The failur e stress for a given failure time was decreased by about half. The phenomen a are explained in terms of a purification of the intergranular phase and b y the formation of surface defects and of a uniformly distributed pore popu lation. (C) 1998 Elsevier Science Limited. All rights reserved.