Effect of boron doping gradient on cemented carbide diamond coatings

To improve the binding properties of diamond coating on carbide tools, three micrometer crystal diamond coatings were successfully deposited on YG8 carbide substrate via hot-filament chemical vapor deposition, whose boron gradient ranged from high (HGBMCD) to low (LGBMCD) and zero (MCD). The effect of the gradient size of the decreasing concentration of boron doping method on the nucleation and growth properties of diamond coatings during deposition was investigated. The results showed that the nucleation density of diamond increased with the doping of boron and that the diamond grains became smaller and more uniform after six-hour growth. Among them, the grain size of LGBMCD was mostly in the range of 2 to 3 μm. In addition, the graphite phase in the gradient boron-doped diamond coating was inhibited throughout the growth process, and IDia/IG was up to 14.65 in HGBMCD. The concentrations of the boron and cobalt compounds (i.e., Co2B and CoB) increased as the boron doping gradient decreased. Meanwhile, the residual stress in the diamond coatings gradually changed from compressive stress to tensile stress due to the doping of gradient boron, and the calculated residual stress decreased first and then increased, with the minimum residual stress of –0.255 GPa. Rockwell indentation showed that the gradient doping of boron improved the binding properties of diamond coatings, and that the optimal binding properties were observed at the LGBMCD, which was up to HF2 level at 1 470 N. Therefore, a proper boron doping gradient was demonstrated to improve the quality and binding performance of diamond coatings.