Factors affecting the fracture resistance of silicon nitride ceramics

In order to separate the intrinsic grain boundary fracture resistance of a hot-pressed silicon nitride ceramic (NC132) from microstructural contributions, such as crack deflection and crack bridging, synthetic grain boundaries were fabricated by a novel diffusion bonding and hipping procedure. By comparing the crack lengths, produced by indentation on the boundary, along and perpendicular to the grain boundary, the fracture energy relative to that of the bulk material, GgbGM, is obtained. This is found to have a value of ca. 0.3, which is higher than that considered favorable for interfacial fracture and fiber debonding in composites. This is consistent with observations of crack trajectories in the material, which indicate little crack deflection at the fibrous β-Si3N4, and indentation measurements, which indicate almost no R-curve behavior. The indentation measurements also indicate that there is a significant residual stress between the silicon nitride and the intergranular glass phase in the NC132 ceramic, with the glass phase being in tension.