Characterisation and Friction Reduction Mechanisms of Diamond-Like Carbon Graphene Nanoplatelet Nanocomposites for Lubricated Tribological Applications

This thesis investigates the combination of Diamond-like carbon (DLC) with graphene nanoplatelets (GNP) to unveil the mechanisms that lead to a low-friction nanocomposite film in boundary lubrication for the first time. The manufacturing of DLC-GNP nanocomposites is provided along with the crucial optimisation parameters (GNP coverage / DLC thickness) that provide increased tribological performance. The motivation of the composite is to create a low friction and wear composite which does not rely on environmentally damaging additives common to commercial engine oils. The DLC-GNP coatings in this thesis were evaluated by tribological testing using a reciprocating pin at elevated temperatures in the boundary regime to replicate a demanding cam-follower environment. A base-oil was used to ensure that only the mechanism between the cast iron counter-body and the DLC-GNP coating was observed. The results show that an optimum GNP coverage (4.5%) reduced the coefficient of friction (~0.03) and wear (< 1.3 x 10-19m2/Nm), beyond this coverage, the tribological performance decreased, and a breakdown of the coating was observed. The DLC thickness was a crucial factor in friction and wear reduction. A minimum DLC thickness of 1.2μm is required to ensure GNP are not removed during the tribo-tests. Thick DLC >3μm was found to isolate the GNP from the counter-body and not contribute to friction reduction. The formation of a highly graphitic transfer film on the counter-body was determined to be the primary mechanism for friction and wear reduction. The GNP provided a lubricant reservoir for high-quality graphitic transfer film, which reduced adhesive forces during sliding wear, leading to low friction. The reduced adhesive forces suppressed the graphitisation of the DLC matrix and resulted in lower wear.