Hardness properties across multiscales of applied loads and material structures

Abstract

Indentation hardness measurements determined from continuous loading tests are compared on a hardness stress–strain basis previously developed to show the relationship of different test results. In such a manner, the elastic, plastic and cracking behaviours are described here for NaCl, oxidiser AP, MgO and explosive RDX crystals. The measurements cover a very large range of impression sizes spread over macro- to nano-scale test results. In addition, connection is made with compiled measurements of applied forces against residual indentation sizes and crack sizes for MgO, RDX and Si crystals also over a very large range of loads and deformations. Loading rate influences are shown for MgO crystal hardness and cracking behaviours, in the latter case relating to indentation fracture mechanics (FM) prediction. For internal structural relationship, first, slip system and dislocation pile-up influences are demonstrated for MgO crystal results. Exceptional subgrain boundary and grain boundary hardness measurements for Nb crystals and Nb and Mo polycrystals are then presented. Such results relate to a Hall–Petch type inverse square root of grain diameter dependencies of hardness shown for the following cases: (i) historical measurements on α-brass; (ii) wide ranging grain size coverage in polycrystal Cr, α-Ti and W materials; and (iii) particle size and binder mean free path dependencies for WC–Co polyphase composites. For the WC–Co system, there is connection with the contiguity parameter for WC to WC particle contacts and to the FM behaviour.