Vacuum pressure dependence of the tribological behaviors of sintered polycrystalline diamond sliding against silicon nitride

The friction and wear behaviors of sintered polycrystalline diamond (PCD) were investigated under various vacuum pressures using a ball-on-disk space tribometer. The wear surface features of PCD and Si3N4 mating balls were examined by scanning electron microscope (SEM). Energy dispersive X-ray spectrum (EDS), X-ray diffraction (XRD) and Raman spectroscopy were performed to detect the chemical composition of the worn surfaces formed on PCD discs and mating balls. The results proposed that the tribological behaviors of PCD are significantly affected by the vacuum pressure. The coefficient of friction (COF) presented low stable values under high pressures, while COFs were much higher and fluctuated in lower pressures. It might be contributed to different degrees of passivation of σ bonding on the surface of PCD. In high pressure, the occurrence of mild abrasive results in slight wear for both PCD and Si3N4 mating balls. Inversely, much more severe adhesive wear takes place in lower pressure. It demonstrated that a high friction-induced graphitization and strong adhesive force lead to the shedding of diamond grains, resulting in the serious wear of PCD in low pressures. These results proposed that the tribological behaviors of PCD were significantly affected by vacuum pressure.