Layered silicon nitride ceramics with improved flaw tolerance

In situ-toughening of silicon nitride can result in dramatic improvements in toughness and R-curve properties. However, the improved toughness is offset by a notable reduction in strength. This problem can be overcome through the fabrication of a layered ceramics. By placing a thin layer of high-strength material on the surface of a high-toughness body, the toughness and flaw tolerance of the whole body can be maintained without sacrificing strength. In this study, α-Si₃N₄ and β-Si₃N₄ seeded α-Si₃N₄ were used for the surface layer and inner layer, respectively. The inner layer was formed by isostatic pressing and subsequent annealing, and the surface layer was coated on it by controlled dipping. The surface layer consisted of relatively fine β-grains with duplex microstructure, while the inner layer consisted of relatively coarse β-grains with duplex microstructure. The layered material showed stronger R-curve behavior and better flaw tolerance than monolithic materials. The processing and mechanical properties of these materials were discussed.