Processing-structure-microscale properties of silicon nitride.

This paper presents an overview of recent works on microscale mechanical and other properties of silicon nitride (Si 3 N 4) determined by microcantilever bending tests and their relationships with the processing and microstructures. We first focus on deformation behaviors and fracture strength of Si 3 N 4 single crystals. β-Si 3 N 4 single crystals are plastically deformed at room temperature under high bending stress, and the yield stress depends on the crystal orientation. The critical resolved share stress of the primary slip system is determined to be below 1.5 GPa from the yield stress. Next, we address microscale mechanical properties of Si 3 N 4 polycrystalline ceramics. Emphasis is placed on their grain-boundary strength or toughness in conjunction with intergranular glassy film (IGF) which is determined by processing parameters such as sintering additives. Assessment is made on two cases of fractures at the IGF-grain interface and that within the IGF, and in each of the cases, effects of rare earth oxide additives are discussed. In Si 3 N 4 ceramics doped with Al 2 O 3 , β-SiAlON layer forming on β-Si 3 N 4 grains enhances the microscale grain-boundary strength. Finally, we shed light on microscale property-degradation behaviors of Si 3 N 4 ceramics, including the deterioration due to the contact with molten Al and the corrosion in sulfuric acid solution. The variation of the microscale properties appears in very short periods compared to the macroscale approaches, demonstrating the advantage in terms of rapid assessment of time-dependent degradation behaviors. [ABSTRACT FROM AUTHOR]