Micro scale hardness distribution of rock types related to rock drill wear

All rock handling requires tool materials with special qualities. In crushing and screening, speciality steels are used to resist abrasion and other major wear mechanisms. In drilling and rotary crushing, WC/Co cemented carbides are used. On top of the extremely tough conditions for the material, one of the most difficult problems is the shifting character of the rock. Even within a single mineral ore, the character often shifts significantly. To optimise a cemented carbide grade for wear life in rock drilling, a suitable characterisation of the rock is required. To perform this characterisation of the rock, very low load hardness of calcite, magnetite, hematite, leptite, quartz, mica schist, granite and sandstone have been measured using nanoindentation. This fixed depth hardness was measured using 1 μm indentation. To get a high resolution mapping of the local hardness values, 3×160 closely spaced indentations were placed, covering an area of 3.2 mm×0.075 mm. This study elucidates the importance of scale in hardness measurements of rock, especially for rocks of multi-mineral character. The hardness distributions achieved at 1 μm fixed depth indentations also clearly indicated that micro hardness measurements at 500 g load is too blunt a tool for a representative rock characterisation. For minerals with wide distributions, such as hematite and mica schist, the micro hardness value could be as low as 1/5 of the top hardness values measured at 1 μm fixed depth.