Effect of microstructural coarsening on Hertzian contact damage in silicon nitride

The critical role of grain size in determining the nature of damage accumulation in silicon-nitride ceramics was evaluated using Hertzian contact testing. Single-cycle tests were conducted on materials of two grain sizes, 0.5 μm (fine) and 2.0 μm (coarse). Damage patterns for these two materials were compared using a special bonded-interface specimen to investigate the subsurface regions. Optical and thermal wave-imaging techniques provided complementary pictures of the damage patterns: whereas the optical image mapped elements of both deformation and fracture, the thermal wave image mapped only the fracture. Taken together, these two imaging methods disclosed a fundamental transition in the mechanical response in the two silicon nitrides, from cone-crack-dominated in the fine material to distributed-microcrack-dominated in the coarse material.