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 interfaces 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.
抄録全体を表示