Compositional dependence of creep behaviour in silicon nitride ceramics

The compressive creep behaviour of undoped and Ba-doped silicon nitride ceramics were studied at 1400°C in air. Precise density meaurements were used to determine cavitation during creep. Microstructural evolution was examined by means of transmission electron microscopy. The Ba-doped material gave a stress exponent of 2.6 and exhibited extensive cavitation, plus strain whorls at grain boundaries. The undoped material, however, exhibited no cavitation or strain whorls and had stress exponent of about 1. These results can be explained by the difference in composition of the grain-boundary glass phase.