Grain-boundary Cavitation and Creep Fracture of Ceramics

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The high-temperature mechanicalproperties of fine and dense ceramic materials were investigated. In fine grained mullites, which were fabricated using a fine grained sol-gel powder and have high resistance to creep deformation, static fatigue due to subcritical crack growth (SCG) at relatively higher stresses and creep fracture due to grain-boundary cavitation at lower stresses were observed at temperatures<greater than or equal>1200ﾟC.The similar transition of fracture mechanism with stress and temperature, which is important from the viewpoint of life assessment, should take place in other ceramics (alumina, magnesia, zirconia and their composites), therefore the experimental study is carried out in the process of the research. The creep deformation and creep fracture of fine grained alumina and zironia (Y-TZP), both with grain sizes<1mum, were studied. Y-TZP deforms superplastically, showing nonniform deformation with materials transport (diffusion) near the grain boundaries. The grain boundary in Y-TZP seems to be stable at high temperatures ; the grain growth during deformation, or dynamic grain growth, is limited. On the other hand, fine-grained alumina deforms by diffusional (Coble) creep at high strain rates, limited by interface reaction, followed by strain hardening due to dynamic grain growth, causing intergranular cavitation. Thus the grain boundary in alumina is relatively unstable in spite of small addition of magnesia. These results obtained in the research suggests that the grain boundary plays an important role in deformation and fracture at high temp