Early stages of diamond-film formation on cobalt-cemented tungsten carbide

The surface composition of cemented tungsten carbide (WC-5.8 wt% Co) was st udied by X-ray photoelectron spectroscopy (XPS), during the early stages of diamond-film deposition, by hot-filament chemical vapor deposition (HFCVD) , The nucleated diamond films were analyzed by scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS),and automatic image analysis ( AIA). The evolution of the surface composition of cemented tungsten carbide during the early stages of diamond-film deposition was strongly dependent on the substrate temperature, At relatively Low temperature (750 degrees C) , cobalt-rich particles started to segregate at the substrate surface after a few minutes of diamond deposition. The conspicuous seg regation of the b inder partly inhibited the formation of stable diamond nuclei, through inte nse carbon dissolution or carbon segregation at the binder surface, but did not affect nucleic growth. At higher temperatures (940 degrees C), no coba lt-rich particles formed at the substrate surface, even after 2 h of deposi tion. However, XPS results demonstrated the presence of cobalt in a surface layer, although in a lower amount than at 750 degrees C. Nevertheless, the nucleation density of diamond at 940 degrees C was much lower than at 750 degrees C, Gaps between WC grains formed within 10 mins. Therefore, intergr anular cobalt was removed at 940 degrees C, a finding attributed to the etc hing performed by monohydrogen, rather than to binder evaporation. The time evolution of the substrate area fraction covered by diamond islands, S(t), was well described by Avrami kinetics for two-dimensional phase transforma tions, suggesting that dia mend formation took place via a heterogeneous nu cleation process. The S(t) functions exhibited a similar trend at 750 degre es and 940 degrees C, because the higher growth rate of diamond crystallite s at higher temperature counteracted the slower nucleation rate at the high er temperature.