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 ␣ grains; medium (M), bimodal with mostly ␤ grains of intermediate size; and coarse (C), with almost exclusively elongated ␤ 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.

The evolution of deformation-microfracture damage below Hertzian contacts in a coarse-grain Ti 3 SiC 2 is studied. The Hertzian indentation stress-strain response deviates strongly from linearity beyond a well-defined maximum, with pronounced strain-softening, indicating exceptional deformability in this otherwise (elastically) stiff ceramic. Surface and subsurface ceramographic observations reveal extensive quasi-plastic microdamage zones at the contact sites. These damage zones are made up of multiple intragrain slip and intergrain shear failures, with attendant microfracture at high strains. No ring cracks or other macroscopic cracks are observed on or below the indented surfaces. The results suggest that Ti 3 SiC 2 may be ideally suited to contact applications where high strains and energy absorption prior to failure are required.