Oxidation and Microstructure of Sintered Silicon Nitride
Three sintered silicon nitrides with different kinds of grain boundary phase were oxidized and the changes in microstructure were examined. The material with glassy grain boundary phase exhibited the highest oxidation rate. The additive cations in the grain boundary migrated to the surface oxide scale, and beneath the scale, the decrease of glass phase were observed. The material with crystallized grain boundary phase of Mg-sialon and Mg2SiO4 also showed the high oxidation rate. The decomposition of Mg-sialon was observed in the subscale and its constituents migrated to the scale. The material with crystallized grain boundary phase of N-diopside and small amount of N-apatite produced the lowest oxidation rate. The migration of grain boundary constituents to the scale was very slow. It is considered that the oxidation resistance of sintered silicon nitride is determined by the diffusion flux of the additive cations to the scale.
KeywordsBoundary Phase Oxidation Rate Reaction Zone Oxidation Resistance Glass Phase
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