WEAR OF HARD-METALS IN ROCK DRILLING: A SURVEY OF THE LITERATURE

Abstract

The literature concerning the wear of tungsten carbide-cobalt alloys used as tool bits in rock drilling is surveyed. The possible mechanisms of rock breakage and of tool wear are briefly discussed. Wear takes place as a result of shock impact or impact-fatigue spalling, by abrasion-mainly from the quartz grains in the rock, and also by thermal fatigue. The mechanism that dominates in any given conditions depends on the method of drilling and on the strength and abrasiveness of the rock. For rotary-percussive drilling, impact-fatigue wear and abrasion operate simultaneously,though essentially independently.

Published data on the relations between the wear of WC-Co alloys in rock drilling and their structure and properties are critically discussed. It appears that the resistance to impact wear is a direct function of the bulk compressive or transverse rupture strength and is related to the WC grain size and the Co mean free path. It is directly proportional to the blow energy in percussion. The results indicate that abrasion of WC-Co alloys by quartz is more complicated than abrasion of ‘simpler’ metals by either hard or relatively soft abrasives. The wear in abrasion during running-in is quite high and becomes greater with larger WC grain size, while the steady-state wear rate becomes less. The abrasion-resistance increases with rise in hardness and with decrease in WC grain size and cobalt content, but not in a simple fashion. It is proposed that abrasive wear takes place both by microfracture at the point of abrasive/metal contact and by preferential removal of cobalt. The former factor dominates for hard, brittle alloys and when the abrasive grains have a high resistance to fracture. The latter dominates for softer, more cobalt-rich alloys and when the abrasives are friable.

The considerable need for further research on all aspects of the wear behaviour of these alloys is stressed.