Abstract
TEM and HREM investigations were performed on two types of silicon nitride ceramics: A - GPS Si₃N₄ + 9 wt.% Y₂O₃ + 1 wt.% Al₂O₃ in which the intergranular phase was present in thick layers; and B - HP Si₃N₄ + 5 wt.% Yb₂O₃, with very thin layers of the glassy phase (< 10 nm). The results were related to those obtained from the creep tests and the high temperature deformation mechanisms were identified. In the case of the first material an intensive cavitation causing a low creep resistance took place, while for the high creep resistance of the second material the mechanisms of diffusion were probably decisive.

Key words
TEM and HREM investigations, silicon nitride ceramics, microstructural behaviour

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