Abstract
The influence of heat-treatment on high-temperature creep and sub-critical crack growth in hot-pressed β′ Si-Al-O-N ceramics has been analyzed from microstructural evidence and determination of stress exponents and activation energies. The most significant change is the suppression of cavitation during creep and of the cavity-interlinkage mechanism for slow crack propagation. A creep mechanism of grain-boundary diffusion is characterized by stress exponent n=1 and unusually high activation energy >820 kJ mol−1. The microstructural origin of the transformation in grain-boundary dominated properties is mainly the removal of triple-junction glassy residues within which cavities are nucleated. This is caused by grain-boundary diffusion of metallic impurities (Mg, Mn, Ca) into a surface silica oxidation layer, and consequent crystallization of the remaining glass components as β′. There is a continued improvement in grain-boundary cohesion and increased difficulty of grain-boundary diffusion following the stage at which triple-junction glass is removed. The resultant ceramics, in addition to superior mechanical behaviour, have an increased temperature for application due to a marked reduction in susceptibility to dissociation above 1400° C.
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