Superlubricity Mechanisms of Graphene Oxide-Ionic Liquid Solution at Metal Interfaces

Lithium salt (LiPF₆) and polyalkylene glycol (50-HB-260, PAG260) were mixed to prepare ionic liquids (ILs, Li(PAG)PF₆). Thereafter, graphene oxide (GO) nanosheets were added into aqueous solution of ILs (IL(aq)) as additives to formulate IL (aq)+GO solution. Rotational sliding friction tests were carried out using the universal micro-tribometer-3 (UMT-3) with ball on disc mode. Both the balls (Φ7.938 mm, R_a ≈ 10 nm) and discs (R_a≈25 nm) were made of bearing steel (GCr15). The normal loads ranged from 2~4 N and the sliding speed was 100 mm/s. All tests were carried out in an atmospheric environment with a relative humidity of 10%~30%. Testing results showed that a super low friction coefficient (μ≈0.009), corresponding to the superlubricity level, was achieved at steel interfaces with the lubrication of IL(aq)+GO. Based on the analysis, the lubrication process included the wearing-in and superlubricity period. The wearing-in period was dominated by boundary lubrication. The collision and wear between solid asperities on the contact surface led to the growth in contact area and reduction in contact pressure. In addition, GO nanosheets were adsorbed on the steel surfaces via the oxygen-containing functional groups to form GO adsorption layer, and the anions of ILs (PF₆^-) reacted with steel surfaces to form tribochemical layer. The effect of rubbing motion, GO adsorption layer and tribochemical layer resulted in friction reduction. Meanwhile, the reduction in contact pressure allowed the formation of fluid layer. The wearing-in period was thereafter completed and the lubrication process entered the superlubricity period. According to the Hamrock−Dowson theory, the thickness of fluid layer at the interfaces during the superlubricity period was approximately 270 nm. Combined the film thickness with worn surface roughness of the ball (approximately 85 nm) and disc (approximately 44 nm), the lubrication state during the superlubricity period was located at the mixed lubrication state, indicating the existence of GO adsorption layer, tribochemical layer and fluid layer at the interfaces. The GO adsorbed at the interfaces undertook a part of the normal load, making the fluid layer stably and continuously existed at the interfaces, which contributed to the friction reduction and relatively stable lubrication state. Testing results proved that the tribochemical layer and fluid layer provided low friction resistance, but not reached the superlubricity level. Simultaneously, the GO adsorption layer transformed the shearing interface from the collision of solid asperities to the extremely-low shearing stress of GO interlayers, further reducing friction resistance and facilitating the realization of superlubricity. Therefore, the superlubricity mechanism of IL(aq)+GO at the bearing steel interfaces was attributed to the combined effect of the GO adsorption layer, tribochemical layer and fluid layer. This work showed that it was feasible to realize superlubricity at steel interfaces by designing lubricants possessing superlubricity properties. Although other factors were supposed to be considered in industrial lubrication, such as friction pair materials, thermal stability, high speed and load, etc., this research proved that the GO nanosheets could be used as superlubricity additives at steel interfaces. It showed the potential of two-dimensional materials represented by GO as industrial lubricating additives and provides a research basis for the application of superlubricity.