Role of Microstructure in Hertzian Contact Damage in Silicon Nitride: II, Strength Degradation

Abstract

In this Part II of a two‐part study of the role of microstructure on Hertzian contact damage in silicon nitride we determine strength degradation properties. As previously, three microstructures are investigated: fine (F), medium (M) and coarse (C), representing a progressive transition from brittle to quasi‐plastic damage. In both the F and M materials, failures originate from cone cracks (although limited quasi‐plasticity is evident in the latter material). These two materials show abrupt losses in strength at the critical contact loads for cone crack initiation, and steady falloff thereafter at higher contact loads. In the C material, failures occur from critical shear faults within the damage “yield” zones. The strengths in this material fall off much more gradually, without abrupt drop, above about twice the critical load for the onset of yield. Fracture mechanics models for each type of failure mode provide explicit relations for the degraded strength as a function of contact load. These models account for all the essential features in the observed strength degradation data. Particular attention is given to indenter size effects in the strength responses. Sphere radius has a profound influence on the critical loads for the onset of degradation, but relatively little influence on the degraded strengths at higher loads. Implications of the results concerning contact fatigue and wear are briefly considered.