FACTORS AFFECTING THE FRACTURE-RESISTANCE OF SILICON-NITRIDE CERAMICS

Ln 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 hipp ing procedure. By comparing the crack lengths, produced by indentation on the boundary, along and perpendicular to the grain boundary, the f racture energy relative to that of the bulk material, G(gb)/G(M), is o btained. This is found to have a value of ca. 0.3, which is higher tha n that considered favorable for interfacial fracture and fiber debondi ng in composites. This is consistent with observations of crack trajec tories in the material, which indicate little crack deflection at the fibrous beta-Si3N4, and indentation measurements, which indicate almos t no R-curve behavior. The indentation measurements also indicate that there is a significant residual stress between the silicon nitride an d the intergranular glass phase in the NC132 ceramic, with the glass p hase being in tension.