酸窒化ケイ素の反応焼結過程と機械的性質

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Silicon oxynitride ceramics were prepared by hot-pressing an equimolar Si_3N_4/SiO_2 mixture with a small amount of CeO_2. The formation of Si_2N_2O occurred not in the solid state but via the liquid phase in which the reactants dissolve. The chemical composition of the intergranular phase ( liquid phase) changed during the reaction. CeO_2 additive gave a liquid phase with a portion of SiO_2 and Si_3N_4 above the eutectic temperature. The liquid phase promoted the formation of Si_2N_2O with increasing temperature. SiO_2 content in the liquid phase increased with increasing temperature and time, until residual SiO_2 particles dissolved completely in the liquid. After all of the residual SiO_2 particles dissolved in the liquid, SiO_2 component in the liquid was consumed for the formation of silicon oxynitride, and the Ce_2O_3/SiO_2 ratio in the liquid increased. When the ratio reached a certain value, the liquid crystallized during cooling. Fracture toughness K_<IC> of hot-pressed specimens increased by increasing soaking time and the amount of CeO_2. Since the intergranular phase in the specimen with a high K_<IC> value was crystalline and silicon oxynitride grains were larger, probably the residual strains and microcracking were induced on inter faces between the silicon oxynitride grain and the intergranular crystalline phase due to thermal expansion mismatch, resulting in interfacial bonding weakened to some extent. Consequently, crack deflection by rod-shaped silicon oxynitride grains efficiently occurred in the specimen. Flexural strength and its temperature dependence also changed by increasing the soaking time and the amount of CeO_2. This result was also attributed to the difference in thermal and mechanical properties ( e. g. softening temperature ) of intergranular phases.
社団法人日本セラミックス協会の論文
1988-11-01

酸窒化ケイ素の反応焼結過程と機械的性質

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