Ulrik Beste

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

Abstract 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.

Surface damage on cemented carbide rock drill buttons

Abstract Wear phenomena on cemented carbide drill buttons tested in percussive drilling of Kiruna-magnetite under realistic conditions have been studied with scanning electron microscopy, light optical microscopy and energy dispersive X-ray spectroscopy. In addition, a comprehensive literature study has been performed. The aim has been to chart wear phenomena, such as crack formation, occurrence of reptile skin and various wear mechanisms. It is shown that surface cracks form shortly after drilling has started. Also the valleys and plateaus characterising the reptile skin pattern were found after only short drilling periods. It appears that the formation of the initial surface cracks and the reptile skin pattern are separate mechanisms, which presumably interact. In addition to the formation of cracks, there is also a continuous wear of the buttons. This is assumed to be caused by wear fracturing and pullout of tungsten carbide (WC) grains and wear of cobalt (Co) by abrasion, possibly assisted by corrosion. A wear mechanism table, based on a comprehensive literature review and on experience from real drilling conditions, is suggested.

The influence of pH on sliding wear of WC-based materials

Cemented carbides with various binder metals and a binderless carbide have been evaluated in a self-mated lubricated pin-on-disc test. The pH of the water-based lubricants was varied between pH 1 and 14. The influence of pH on both friction and wear was measured. In order to clarify the changes in surface chemistry with pH, corrosion tests were also performed. The corroded and worn surfaces were studied with SEM, LOM, XRD, TEM, XPS and surface profilometry. The results showed a strong dependence between pH of the lubricant and wear rate. This was especially evident for the binderless carbide which showed a low wear rate in acidic environments and a high wear rate in high pH.