Enhancement on the tribological performance of diamond films by utilizing graphene coating as a solid lubricant

The present study reports the significantly enhanced tribological performance of the microcrystalline diamond (MCD) film by utilizing the graphene coating on its surface as a solid lubricant. The graphene coating is fabricated by the electrophoretic deposition (EPD) method. A typical EPD processed graphene coating with compact and continuous surface morphology could be obtained with the applied voltage of 15 V or higher, and it is constructed by horizontally stacked micron-sized graphene single-sheets with randomly-orientated nano-sized multi-layered graphene (MLG) sheets embedded between them. The friction tests show that the EPD graphene coating could remarkably reduce the coefficient of friction (COF) of MCD film from ~ 0.08 to ~ 0.04-0.05 under the normal load of 1 N (0.48 GPa), decreased by more than 40%. Its wear rate is measured as 4.67 × 10− 6 mm3/N·m. Furthermore, in a durability friction test adopting a higher normal load of 4 N (0.8 GPa) and a much longer duration of 36,000 sliding cycles, the stable COF of such graphene/MCD film are measured as ~ 0.06 and 1.32 × 10− 6 mm3/N·m, respectively. This dramatically friction reducing effect is attributed to the realization of three-body abrasion regime by the residual graphene sheets wrapping around the Si3N4 particles in the sliding interface and acting as wear particles. Although a large portion of graphene coating is worn out as the sliding proceeds, the friction reducing effect maintains well and no sign of structural degradation in the residual graphene flakes is detected. The present study provides an economic and effective approach of utilizing graphene as a solid lubricant, combining with CVD diamond film, for a wide range of applications.