Transmission electron microscope observation of creep-deformed Al₂O₃, SiC and Si₃N₄ ceramics

Microstructural observation using transmission electron microscope were conducted on creep-deformed SiC, Al₂O₃ and Si₃N₄ ceramics in order to analyze microscopic deformation mechanism in these ceramics directly. Thin foils were prepared from the tensile surface of the four-point bending bars after creep test at various conditions. In SiC ceramics crept at 1250°C, very sharp microcracks propagated relatively straight and transgranularly. Microcracks also branched and some of them extended along grain boundary. In SiC crept at 1480°C, grain boundary cavities and dislocations were also observed. In Si₃N₄ crept at around 1200°C, cavities along grain boundary were frequently observed. Dislocations were sometimes observed. No transgranular cracks were happened. In Al₂O₃ ceramics crept at 1000°C, sharp microcracks ran through both grain boundary and inside grains with small number of very tiny grain boundary cavities. Grain boundary glassy phase was partly crystallized. Based on the observed microstructure, creep mechanisms of these ceramics were discussed in relation with creep ductility.