Macro- to Nano-indentation Hardness Stress–Strain Aspects of Crystal Elastic/Plastic/Cracking Behaviors

Single crystal KCl and MgO indentation hardness test results spanning macroscopic, microstructural, and nanoscale measurements are brought together in a ball-indenter-based stress–strain description. For a significant range of hardness measurements made on MgO (001) crystal surfaces, increasingly greater plastic flow stresses are determined at smaller loads applied to smaller effective ball sizes at the rounded tips of Berkovich indenters; and, at the smallest ball diameters of 3,200 nm and 400 nm, the plastic flow stresses are shown to approach the predicted Hertzian elastic loading stresses. The resultant hardness stress–strain description, that is extended to cover the elastic, plastic, and cracking behaviors of MgO crystals, is usefully applied also to a comparison of indentation test results reported for NaCl and RDX crystals. In general, the dislocation-induced cracking stress measurements are shown for MgO and RDX crystals to be lower than cracking stresses evaluated on an (elastic) indentation fracture mechanics (IFM) basis. Comparison is made with metal nanoindentation hardness test results.