Synergistic friction-reducing and anti-wear behaviors of graphene with micro- and nano-crystalline diamond films

In present study, a layer of continuous and compact graphene coating was deposited on the surface of microcrystalline diamond (MCD) and nanocrystalline diamond (NCD) film by electrophoretic deposition (EPD) method. The tribological performance of as-deposited films, referred as Gr/MCD and Gr/NCD respectively, was investigated in a set of ball-on-plate reciprocating friction tests in ambient atmosphere (60% RH). Under a normal load of 4 N (corresponding to a contact pressure of 770 MPa), the stable COF of Gr/MCD film reduced by 60% compared with that of MCD film, which decreased from 0.166 to 0.068; in contrast, the stable COF of Gr/NCD film was comparable with that of NCD film, which were measured as 0.069 and 0.074 respectively. For the Gr/MCD film, the unique surface morphology of underneath MCD film was supposed to play a crucial role on its exceptional friction performance. It produced a plenty of wear particles during the run-in sliding stage and a large portion of these partides were trapped in the sliding interface. During the following stable sliding stage, graphene scrolls formed by graphene sheets wrapping around the wear particles they encountered and attributed to the reduction in friction by decreasing the interfacial contact area and forming an incommensurable contact between the graphene scrolls and counterpart surface. For the Gr/NCD film, in contrast, the absence of wear particles on its sliding interface and structural degradation in its top-layered graphene sheets was supposed to result in its failure in reducing friction. Different from the Gr/MCD film, the residual graphene flakes on the worn surface of Gr/NCD film were transformed to disordered graphitic structures; meanwhile, increased defects and severe oxidation effect were detected in them. Furthermore, due to the exceptional protection effect of graphene coating, the Gr/MCD and Gr/NCD film produced severer wear on the counterface than the MCD and NCD film. (C) 2016 Elsevier B.V. All rights reserved.