The influence of normal load on the tribological performance of electrophoretic deposition prepared graphene coating on micro-crystalline diamond surface

In the present study, the friction and wear behaviors of a graphene/diamond coating are examined under a variety of normal loads ranging from 4 N to 10 N, which correspond to contact pressures of 0.77-1.04 GPa on the sliding interface. The graphene coating is deposited on the surface of a microcrystalline diamond (MCD) film by electrophoretic deposition (EPD) method. Friction tests are carried out on a ball-on-plate reciprocating Universal Micro-Tribotester (UMT-3, CETR) in ambient air condition (60% RH). Silicon nitride balls (Φ 4 mm) are used as the counterpart materials. The results show that a steady-state coefficient of friction is maintained in the range of 0.07-0.09, showing no discernable dependence on the applied normal load. The highly similar equilibrium sliding interfaces formed under varying normal loads are supposed to attribute to this effect largely. Mixtures of graphene flakes and wear particles (Si3N4 or SiO2) produced during the sliding promote the transition from two-body abrasion to three-body abrasion. Meanwhile, the exposed diamond crystallites on the sliding interface are protected from being polished and smoothened thanks to the existence of residual graphene flakes, even though only a portion of the wear track is covered. After the sliding tests, the remaining graphene flakes within the wear track on the Gr/MCD film present no structural damage or degradation, but become thicker sheets by overlapping with each other under the relatively high pressure.