Unusual Competitive and Synergistic Effects of Graphite Nanoplates in Engine Oil on the Tribofilm Formation

To exploit the improved efficiency from formulated oil modified with graphite nanoplates (GNP), it is important to understand how GNP behave alongside conventional additives such as zinc dialkyl dithiophosphates (ZDDP). The results in this work demonstrate unusual tribological responses of engine oil due to GNP additions, which cannot be explained by the traditionally low shearing mechanism between graphene layers in GNP. A competitive and synergistic effect of GNP on tribofilm formation is proposed for this unusual friction and wear behavior. The presence of GNP modifies the tribofilm formation and spontaneously creates alternate hard and soft regions from the microscopic view of the surface. At low concentrations (≤0.05 wt%), graphene is formed by shear-assisted exfoliation of GNP and intermixed with intermediate oxide region. Due to its random orientation, graphene produces mechanical reinforcement of the tribofilm, but low shear yielding is not achieved. This gives rise to high friction but low wear conditions. At higher concentrations (>0.05 wt%), GNP align with their hexagonal sheets parallel to the surface to promote low-shear sliding, but wear increases. This is due to GNP clumping together which makes them less effective at forming graphene and providing stable reinforcement of the tribofilm.