Role of Microstructure in Hertzian Contact Damage in Silicon Nitride: I, Mechanical Characterization

In this Part I of a two‐part study of Hertzian indentation in silicon nitride we characterize irreversible contact damage as a function of microstructure. Three controlled silicon nitride microstructures are examined, representing a progression toward greater long‐crack toughness: fine (F), bimodal with predominantly equiaxed α grains; medium (M), bimodal with mostly β grains of intermediate size; and coarse (C), with almost exclusively elongated β grains. An effect of increasing the microstructural heterogeneity in this sequence is to suppress ring cracking around the indenter, ultimately to a degree beyond that expected from increased toughness alone. Along with the crack suppression is a parallel tendency to enhanced damage accumulation beneath the indenter, such that the contact in the coarsest microstructure is predominantly quasi‐plastic. The characterization of damage includes the following: determination of indentation stress‐strain curves, to measure the level of quasi‐plasticity; measurement of threshold loads for the initiation of ring cracking and subsurface yield, to quantify the competing damage processes; and measurement of characteristic dimensions of the ensuing cracks and deformation zones in their well‐developed stages. These quantitative results are considered in terms of formal contact mechanics, along with finite element modeling to generate the essentially elastic‐plastic fields in the different silicon nitride structures. This contact mechanics description serves also as the basis for subsequent analysis of strength degradation in Part II. Implications concerning microstructural design of silicon nitride ceramics for specific applications, notably bearings, are considered.