Stable macroscopic liquid superlubricity induced by asymmetric contact of a mixture of unequal-diameter nanosphere additives

Solid additives can effectively overcome/alleviate the deficiencies of pure liquid lubricants themselves. However, the additive morphology involved in most of the reported solid additive/liquid lubricant systems is mostly irregular. They take more into account species differences and ignore the potential advantages of the sizes of unequal particles for the purpose of superlubricity. Herein, a mixed unequal-diameter sphere additive/ionic liquid analogue (Uneq-SA/ILA) model system is designed from the perspective of asymmetric contact between unequal-diameter spheres. It can easily enter a stable macroscopic superlubricated state (coefficient of friction as low as 0.0046) with negligible wear relative to an equal-diameter sphere additive/ILA system. Such excellent lubrication performance is attributed to the synergy of asymmetric contact between unequal-diameter spheres and low-shear-viscosity ILA; the ILA adsorbed on the sphere additive makes asymmetric slip interface between the spheres mainly concentrated inside the ILA, avoiding direct point-to-surface contact between the spheres. The film thickness of 948 μm implies that unequal-diameter spheres in the ILA have a normal accumulation space during contact, and the lubrication mechanism is dominated by hydrodynamic lubrication. This study deepens the understanding of asymmetric contact function of solid additives and provides new insights for the design of next-generation solid additive/liquid superlubrication systems.