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In this study, the tribological behaviors of graphene as a lubricant additive for steel/copper and steel/steel friction pairs were compared. For the steel/copper friction pair, the graphene sheets remarkably decreased the coefficient of friction and wear scar depth under low loads, but these slightly increased under high loads. The steel/steel friction pair showed excellent tribological properties even under high loads. Severe plastic deformation on the copper surface reduced the stability of the graphene tribofilm because of a rough copper transfer film on the steel during the running-in period. The results provide a better understanding of the mechanism of graphene as a lubricant additive.

High-pressure axial piston pumps operate in high-speed and high-pressure environments. The contact state of the slipper against the swashplate can easily change from an oil film lubrication to a mixed oil film/asperity contact, or even dry friction. To improve the dry friction performance of slipper pairs and to avoid their potentially rapid failure, this study examined the effects of material matching on the dry friction performance of the slipper pair for high-pressure axial piston pumps. A FAIAX6 friction and wear tester was developed, and the dry friction coefficients of the slipper pairs matched with different materials were studied using this tester. Based on the thermo-mechanical coupling of the slipper pair with the working process, the contact surface temperatures of the slipper pairs matched with different materials were calculated and analyzed for the same working conditions. Following this, the effects of the material properties on the temperature increase at the slipper sliding contact surfaces were revealed. The reliabilities of the temperature calculations and analysis results were verified through orthogonal tests of slipper pairs matched with different materials. The results indicate that the influence of the material density on the friction coefficient is greater than that of the Poisson’s ratio or the elastic modulus, and that the slipper material chosen should have a high thermal conductivity, low density, and low specific heat, whereas the swashplate material should be high in specific heat, density, and thermal conductivity; in addition, the slipper pair should be a type of hard material to match the type of soft material applied; that is, the hardness of the swashplate material should be greater than that of the slipper material.