Grain refinement induced friction reduction and anti-wear performances of electrodeposited graphene/Ni composites with low content reduced graphene oxide

Graphene and its derivatives have attained a considerable amount of popularity as effective reinforcements in the electrodeposited nickel (Ni) matrix composites for lubrication. However, the composites suffer from certain challenges, such as the agglomeration of graphene nanosheets in the electrodeposition process and the uncertain role of graphene on the friction reduction and wear resistance during sliding. By using a polyvinylpyrrolidone assisted reduction method to improve the dispersity of reduced graphene oxide (RGO) in the electrolyte, we prepare the bulk RGO/Ni composites with different RGO adding amount and perform a complete research on the tribological behaviours. Our work reveals that although an extremely low amount of RGO nanosheets (carbon content below 0.018%) have been incorporated into the Ni matrix, it induces significant grain refinement and friction reduction effects. Compared with the RGO-free Ni deposit, the friction coefficient and wear rate of the composite is reduced by 25.6% and 27.5%, respectively. The improved tribological properties are ascribed to the fine-grain strengthening effect and the formation of a continuous easy-shear nickel oxide film on the contact surface. This finding offers a new view on the wear mechanism of graphene/Ni composites with low graphene content, for which the fine-grain strengthening rather than the formation of carbon-rich transfer layer will dominate the lubricating and anti-wear performances. (C) 2020 Elsevier B.V. All rights reserved.