Effects of Ag/RGO composites as lubricant additives on the tribological properties of lubricating oil

Ag-coated graphene (Ag/RGO) composite additives were successfully synthesized by the chemical reduction method, the phase composition and microstructure of the Ag/RGO composites were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively, and the tribological properties of the Ag/RGO composites as the lubricant additives were evaluated by UMT-2 friction and wear tester. The results show that, the Ag nanoparticles with the average size of about 200 nm are well-dispersedly attached to the RGO sheets. It is shown that, during the whole wear process, the hard micro-bulges between the counterpart and the worn surface are in physical contact using the pure lubricating oil, which leads to the poor lubricating effect. However, using the lubricating oil with Ag/RGO composite additives, the hard micro-bulges between the counterpart and the worn surface are in physical contact only at the initial wear process; as the wear time increases, a lubricating film forms due to the addition of Ag/RGO composites, which can avoid the direct contact between the worn surface and counterparts, and can further generate the boundary lubrication. Moreover, some of the silver nanoparticles in the Ag/RGO composite additives act as the micro bearings and make the lubricating effect better.