Shortlisted boronising of cemented carbides for improved diamond layer adhesion

Abstract

As with a number of other CVD processes, the presence of cobalt has a negative influence on the coating of cemented carbide substrates with CVD diamond films. The cobalt binder phase is therefore usually removed in the near surface region by chemical etching prior to deposition. However the removal of the cobalt binder phase by etching may weaken the near surface region, resulting in a decrease in the performance of the coated cemented carbide tool. An alternative to selective etching of the cobalt binder phase could be the conversion of cobalt into chemically inactive cobalt containing intermetallic phases. The present paper focuses on the formation of intermetallic B–Co phases in the near surface region of pure cobalt substrates and cemented carbide insert tips. One method investigated was coating of substrates with boron by PVD with subsequent tempering. Another method tested was powder pack cementation. Phase analyses were performed by grazing incidence X-ray diffraction. The suitability of pretreatment methods for deposition of diamond films by the hot filament CVD process on cemented carbide insert tips was investigated by WDX linescans and element mappings, SEM, and wear tests. The analyses revealed in all cases the formation of intermetallic B–Co phases after boronising of the substrates. In particular, boronising by a powder pack cementation process led to significantly enhanced wear resistance of the diamond films. It could also be shown that much better adhesion was produced by converting cobalt into intermetallic B–Co phases than by removing the cobalt binder phase in the near surface region.