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Mechanical properties of Si3N4/SiC-whisker-reinforced Si3N4 three-layer composites

The relationship between the mechanical properties of three-layer composites which consist of surface Si3N4 layers and an inner SiC-whisker-reinforced Si3N4 layer and the surface residual compressive stress was investigated. It was found that the mechanical properties, such as Vickers hardness, fracture toughness, and bending strength decreased with the thickness of the surface Si3N4 layers. On the other hand, the surface residual stress was measured and calculated by the finite element method (FEM). It was found that the surface compressive residual stress decreased with the thickness of the surface layers and also increased with the content of SiC whiskers. Based on comparison of the behaviors of the mechanical properties and surface compressive stress of layered composites to the surface layer thickness, it was thought that the improvements in the mechanical properties of the layered composites were caused by the surface compressive stress resulting from mismatch in the thermal expansion.