Interactions of hard metal substrates during diamond deposition

Abstract

Deposition and adhesion of diamond coatings on hard metal substrates depend sensitively on the substrate composition. In particular during deposition on low-carbon hard metal substrates carbon is picked up by the substrate from the carbon-containing environment and the formation of a diamond layer onto the hard metal is retarded. The process runs over a long period of deposition and can lead to a partial dissolution of the diamond coating at the diamond/hard metal interface, thereby weakening the interface adhesion. To investigate this interaction during deposition, experiments were carried out on carbon deficient substrates, containing small amounts of η-carbides (Co 3W 3C). Using these model alloys it is easy to demonstrate the carbon diffusion into the substrate by observing the migration of the Co 3W 3C+C⇒Co(C, W)+WC reaction front from the surface into the bulk material. It is shown, that at 900 °C both the C and W diffusion rate as well as the reaction rate are high enough to transform all η-carbides into Co(C, W) and WC. At 20 h deposition time the reaction front was progressed already about 0.9 mm into the bulk of the substrate. The carbon saturation of the cobalt binder during deposition and the partial dissolution of the diamond layer can be furthermore demonstrated by lattice parameter measurements performed in different depths segments of the substrate and SIMS measurements across the interface.