Sequential morphology of cobalt from cemented tungsten carbide in microcrystalline and nanocrystalline diamond films by HF-CVD

Polycrystalline diamond films are used as protective films on cutting tools for carbon fibre-reinforced plastic, aluminium alloy or graphite because of their excellent hardness and high thermal conductivity. As a base material for tools, cemented tungsten carbide with a Co binder (WC-Co) is usually employed. However, the Co binder in the WC substrate causes the non-uniform formation of diamond films. Herein, the mechanism of non-uniform film formation during diamond deposition on WC-Co substrates using hot filament chemical vapor deposition (HF-CVD) method was investigated. The surface and cross-section of the substrates were observed at each process, from substrate preparation to the formation of the microcrystalline and nanocrystalline diamond (MCD and NCD, respectively) films. During the filament carburization process before diamond film deposition, Co particles were segregated on the substrate surface, and some WC grains were uplifted from the substrate surface. In the formation of MCD films, non-uniform diamond films were formed because of the Co particles remaining on the film surface, as reported in previous studies. Meanwhile, non-uniform NCD films were formed because of uplift of the WC grains on the substrate surface caused by the generation of the carbon filament extending from the Co particle to the substrate. In addition, the Co particles were covered with a carbon shell. In the deposition of MCD and NCD films, the Co binder segregated to the substrate surface before diamond nucleation. Owing to the negative effect of Co, the segregation of Co in the initial stage should be prevented to form a good-quality uniform film.