ABRASION OF HOT FLAME-DEPOSITED DIAMOND COATINGS

Diamond coatings were deposited on cemented carbide rock drilling bits using the hot flame method. The samples were tested in sliding contact with SiC, Al2O3, and flint abrasive papers. The coating wear and failure mechanisms were investigated using light optical microscopy, scanning electron microscopy, energy-dispersive X-ray spectroscopy, Raman spectroscopy and finite element calculations. The wear of the coatings occurs by fragmentation of protruding diamond grains. This process continues until the surface has a hat appearance and thereafter the wear diminishes to very low rate. Instead the failure of the coatings occurs by interfacial spalling. It is seen that the lifetime of the coatings is controlled by the coating thickness. All the obtained lifetime curves display the following features. (i) There exists a coating thickness below which the lifetime is zero. (ii) There is a coating thickness above which the lifetime seems to be infinite. (iii) Between these values the lifetime follows a power distribution. A model for the coating failure process is proposed. It involves fatigue induced slow crack growth at the interface. The model correlates well with the experimental data.