Nucleation and Adhesion of Diamond Films on Co Cemented Tungsten Carbide

Diamond deposits were grown using hot filament chemical vapor deposition (CVD) on pretreated Co cemented tungsten carbide (WC-Co) substrates with an average grain size of 6 μm. Depositions were performed with 0.5 or 1.0% methane concentration and with substrate temperatures ranging from 750 to 1000°C. Diamond nucleation densities were measured by scanning electron microscopy. Scratched and bias-enhanced nucleation pretreated substrates showed the larger nucleation densities. Etching of the WC performed by Murakami's reagent, followed by surface-Co dissolution (MP pretreatment), led to a roughened but scarcely nucleating surface. The performance of a scratching prior to the MP pretreatment allowed one to increase the nucleation density, due scratching-induced defects, confined in the outermost layer of WC grains, which act as nucleation sites. Smaller nucleation densities were observed with increasing the substrate temperature and reducing the methane concentration, confirming that diamond nucleates via a heterogeneous process. The adhesion of continuous films was evaluated by the reciprocal of the slope of crack radius-indentation load functions. The substrate pretreatments mainly affected the film adhesion, while the influence of CVD process conditions was minor. The two main factors that improve the diamond film adhesion are the coating-substrate contact area and the surface-Co removal.