Investigation on tribological behaviour of boron doped diamond coated cemented tungsten carbide for cutting tool applications

In this paper, tribological performance of boron doped diamond (BDD) films and boron doped graded layer diamond thin films (BDD/transition layer/NCD) were studied in detail. The widely used cemented tungsten carbide (WC-6 wt%Co) was selected as a substrate material for diamond coating. Diamond films were deposited on WC-Co by Hot-filament CVD reactor (HFCVD) setup. Tribology experiment was conducted using a reciprocating tribometer with a normal load of 30 N and a sliding velocity of 10 mm/s for a constant stroke length of 3 mm. Silicon nitride (Si3N4) ball was used as a counterpart to study the friction and wear behaviour of diamond films. The surface morphology, topography & roughness of the diamond films were analysed using scanning electron microscope and atomic force microscope respectively. The hardness of the thin diamond films was measured by using Berkovich nano indentation test method. In MCD coating (spa C-C) the wear rate increases with increase in hardness. On the contrary for NCD (sp(2) graphite), wear rate decreases with decrease in hardness because of the corresponding phases. The test results found that BDD and boron doped graded layer shows a stable lowest friction coefficient values of 0.004 and 0.003 compared with conventional microcrystalline diamond films (0.007). On the other hand, the wear diameter of the silicon nitride ball for BDD and boron doped graded layer are found to be 620 pm and 785 inn, relatively lower in comparison with microcrystalline diamond films (897 pm). The wear track width was measured by scanning electron microscope and shows that BDD and boron doped graded layer indicates lower wear track width 564 pm and 596 pm compared with microcrystalline diamond films (712 gm). Raman mapping was conducted on the wear track of the diamond films to know about the phase pure diamond (spa) and partly graphite phases (sp(2)) which in turn contributes for the distinct residual stresses in thin films. The obtained lower friction coefficient for BDD on WC-Co cutting tool can be suitable for machining of aluminium based metal matrix composites effectively.