Effect of WC grain growth inhibitors on the adhesion of chemical vapor deposition diamond films on WC-Co cemented carbide

Two sets of Co-cemented tungsten carbide (WC-Co) cutting inserts were sintered using WC powders having different average sized particles (1 and 6 μ). Fine grained WC-Co inserts contained 5.8 wt.% Co and were doped by 0.2 wt.% VC and 0.2 wt.% TaC, which acted as grain growth inhibitors in the liquid-phase sintering. Coarse substrates contained 6 wt% Co and no dopants. Prior to deposition, the inserts were etched using Murakami reagent and then with an acid solution of hydrogen peroxide. The substrates were coated by 31-33-μm diamond films using hot filament chemical vapor deposition (HFCVD) in an atmosphere of 1.5% methane in hydrogen for 14 h, at a substrate temperature of 950 °C. Upon cooling from CVD temperature, only films deposited onto coarse grained inserts were adherent, while films grown on fine grained substrates underwent spontaneous delamination. This fact was due to the presence of a layer of graphitic carbon at the interface between the diamond film and fine grained substrates only. The formation of this sp²-carbon layer correlated well with the observed huge segregation of grain growth inhibitors at the interface between diamond and fine grained substrates.