Coating thickness and interlayer effects on CVD-diamond film adhesion to cobalt-cemented tungsten carbides

Abstract In this study, diamond coating adhesion on cobalt-cemented tungsten-carbide (WC–Co) substrates was investigated using scratch testing. In particular, a methodology was applied to evaluate the effects of the coating thickness and the interlayer on diamond coating delaminations. In the coating thickness effect research, substrate surface preparations, prior to diamond depositions, were common chemical etching using Murakami solutions. On the other hand, to study the interlayer effect, Cr/CrN/Cr and Ti/TiN/Ti, were deposited to WC–Co substrate surfaces (no chemical etching) by using a commercial physical vapor deposition (PVD) system in a thickness architecture of 200 nm/1.5 μm/1.5 μm, respectively. Diamond films were synthesized by using a hot-filament chemical vapor deposition (HFCVD) reactor at a fixed gas mixture with varied deposition times. Scratch testing was conducted on the fabricated specimens using a commercial instrument. It is noted that the onset of coating delamination can be clearly identified by high-intensity acoustic emission (AE) signals and high tangential force fluctuations when such events occur, which can be used to determine the critical load for coating delaminations. Scratched track morphology was also characterized by scanning electron microscopy and white light interferometry. The results show that the adhesion of diamond coatings on WC–Co substrates increases with the increased coating thickness, with a nearly linear relation. The trend is consistent with the findings from a previous study that experimentally evaluated the coating thickness effect on the diamond-coated tool performance. Scratched tracks were characterized by diamond coating cracking and coating delaminations once the adhesion critical load is reached. For the two types of interlayer materials tested, neither of them seems to be effective and the diamond coating with the Ti-interlayer shows poor adhesion compared to the Cr-interlayer coatings.