Effect of microstructural coarsening on Hertzian contact damage in silicon nitride

Abstract

The critical role of grain size in determining the nature of damage accumulation in silicon-nitride ceramics is evaluated using Hertzian contact testing. Single-cycle tests are conducted on materials of two grain sizes, 0.5 Μm (fine) and 2.0 Μm (coarse). Damage patterns for these two materials are compared and contrasted using a special bonded-interface specimen to investigate the subsurface regions. Optical and thermal wave-imaging techniques provide complementary pictures of the damage patterns: whereas the optical image maps elements of both deformation and fracture, the thermal wave image maps only the fracture. Taken together, these two imaging methods disclose 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. Scanning electron microscopy (SEM) confirms the incidence of microfracture in the latter case. Thermal-wave measurements also allow a quantitative evaluation of the microfracture damage. Multiple-cycle tests on the coarse material show a build up of subsurface damage with increasing number of cycles, indicating mechanical fatigue. The results are discussed in terms of a shear-fault model, in which subsurface microcracks initiate from intrinsic planes of shear weakness in the microstructure. Implications concerning the microstructural design of silicon nitride ceramics for strength and wear applications are briefly considered, with reference to countervailing resisting and driving forces in the long-crack and short-crack toughness characteristics.