Micro-scratch evaluation of rock types—a means to comprehend rock drill wear

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Drilling, transportation and handling of rock always result in wear of the equipment that comes into contact with the rock and rock fragments. Rock is normally considered rather hard, and the contact leads to abrasion, erosion and point fatigue. However, the wear mechanisms of the tools are often complex and vary in character depending on the rock type. To understand the wear mechanisms of the cemented carbide used in rock drill bits, it is central to understand how different rock types respond to severe scratching from hard tips. A cemented carbide tip with radius 10 μm was used in a scratch tester at progressive loads of 0 to 20 N and at constant loads of 10 N. The tested rock types were calcite, two types of magnetite, hematite, leptite, mica schist, granite, sandstone and quartz spanning average hardness values from 190 up to 1220 HV. The scratches were investigated in SEM and the friction coefficient between rock and the tip was measured. In addition, the wear volume of the cemented carbide tip is measured. The rock types differed significantly with respect to damage mechanism and critical load for transition from a mild to a more severe damage. The friction behaviour correspondingly shifted from rather smooth to very fluctuant. The wear of the tip was found to be correlated to the hardness of the rocks, but was also influenced by the grain size, the quartz content and isotropy. The implications from the present results on full-scale rock drill wear are discussed.