Growth of diamond coatings on functionally graded cemented carbides

Cemented carbides with diamond coatings have been widely used for cutting tools. However, the reaction of Co with the diamond coating during the deposition leads to the deterioration of the coating structure and the bonding strength. In this work, functionally graded cemented carbides (FGCCs), with a WC-rich surface, Co-rich intermediate layer and η phase-containing central part, were prepared. Diamond coatings were prepared on the FGCC by the hot filament chemical vapor deposition (HFCVD), a method of catalytic decomposition of methane and hydrogen mixtures at a heated filament. The crystal growth and the bonding behavior of diamonds to different layers in FGCCs were investigated. Results indicated that the different regions of FGCCs show apparently different grain sizes and morphologies of diamond coatings. The Co-poor layer has a high crystallinity and purity of diamond, and the η phase-containing region is beneficial for the formation of fine diamond grains. The Co-rich layer has a diamond coating of low bonding strength due to the formation of graphite and porosity, while the η phase-containing region has a high bonding strength. By the combined effects of the three different layers, FGCC shows generally a high bonding strength, and is promising for using as a substrate for the diamond coating.