Erosive wear behaviour of thick chemical vapour deposited diamond coatings

This paper reports work carried out on 60-200 mu m thick CVD diamond coatin gs deposited on tungsten and cemented tungsten carbide substrates. The eros ive wear behaviour of these coatings relative to cemented tungsten carbide is described. Erosion tests used quartz silica sand, on average 194 mu m in diameter, in air at a velocity of 268 ms(-1). The erosion rates and micro- mechanisms, and their effect on coating life are presented as a function of coating thickness and the surface conditions of as-grown or lapped coating s. The eroded surfaces were studied by scanning electron microscopy (SEM) a nd surface profilometry. Ultrasonic imaging and taper polishing of tested s amples were also performed to reveal sub-surface damage and to elucidate it s contribution to coating degradation. The results suggest that the samples erode by a gradual chipping of grains in the early stages followed by the accumulation of damage at the coating-substrate interface. It is this latte r feature which eventually leads to catastrophic failure of the coating alo ng the interface. These features are discussed in the context of the classi cal erosion damage features normally exhibited by brittle materials as well as the coating microstructure. The propensity for coating debonding would suggest that improved coating adhesion would further enhance erosive wear b ehaviour of thick CVD diamond coatings. (C) 1999 Elsevier Science S.A. All rights reserved.