Microstructure evolution of thermal spray WC–Co interlayer during hot filament chemical vapor deposition of diamond thin films

Abstract

In this paper, diamond film was deposited on WC–Co coated steel substrate by hot filament chemical vapor deposition (HFCVD). This method can not only prepare diamond film on the sample, but also can be used as a new way of WC–Co coating heat treatment. Brittle phases, such as η phase, which were produced during ordinary heat treatment process, will be suppressed in HFCVD at high temperature. This paper systematically studied the microstructure (defects and phase) transition of WC–Co interlayer in the process of depositing diamond film. The mechanism of phase transition was discussed. The results showed that at high deposition temperature (e.g. 800°C), the transition of binder phase was fast. After 5h of deposition, the phase composition of WC–Co coating was almost the same as sintered WC–Co block. At low deposition temperature (e.g. 700°C), the phase transition of interlayer slowed down and there was a large amount of nano-phases in the interlayer even after 12h of deposition.Another notable feature is that phase transition of the surface part of interlayer was the fastest, followed by the inner part and the middle part was the slowest. The phase transition rate of the interlayer and the binder phase is closely related to the depth of the coating. In the surface part of the coating, Co will evaporate and diffuse into diamond film. Also, activated carbon atoms will diffuse into the interlayer, but the depth of diffusion layer was limited. Fe and Co will diffuse into each other at the interface between interlayer and steel substrate. In contrast, the middle part of interlayer was not affected by diffusion of elements.In sum, the results suggest that the change in element content (i.e. concentration diffusion) is the main driving force of the phase transition, while temperature mainly affects phase transition rate (i.e. the rate of diffusion).