Reaction Sintering Process and Mechanical Properties of Silicon Oxynitride

Abstract

Silicon oxynitride ceramics were prepared by hot-pressing an equimolar Si3N4/SiO2 mixture with a small amount of CeO2. The formation of Si2N2O 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. CeO2 additive gave a liquid phase with a portion of SiO2 and Si3N4 above the eutectic temperature. The liquid phase promoted the formation of Si2N2O with increasing temperature. SiO2 content in the liquid phase increased with increasing temperature and time, until residual SiO2 particles dissolved completely in the liquid. After all of the residual SiO2 particles dissolved in the liquid, SiO2 component in the liquid was consumed for the formation of silicon oxynitride, and the Ce2O3/SiO2 ratio in the liquid increased. When the ratio reached a certain value, the liquid crystallized during cooling. Fracture toughness KIC of hot-pressed specimens increased by increasing soaking time and the amount of CeO2. Since the intergranular phase in the specimen with a high KIC 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 CeO2. This result was also attributed to the difference in thermal and mechanical properties (e.g. softening temperature) of intergranular phases.