Boundary Slip of Oil Molecules at MoS2 Homojunctions Governing Superlubricity.

Molybdenum disulfide (MoS2) nanoflakes are widely used as nano-additives in oil for the excellent lubrication performance. However, the molecular mechanism of MoS2 nanoflakes in oil governing the friction properties remains elusive. In this study, MoS2 homojunctions were constructed by combining the fabricated MoS2 probe and MoS2 crystal with an atomic force microscope (AFM), and the superlubricity with an ultralow friction coefficient of approximately 0.003 at MoS2 homojunctions was attained after the formation of a confined oil layer, exhibiting a 67% reduction of the friction coefficient in comparison to that under a nitrogen atmosphere. The boundary slip of oil molecules on the MoS2 crystal with a small energy barrier was observed, causing the shear to occur at the interface of oil/MoS2 crystal with an extremely low shear strength, which contributes to the achievement of superlubricity. This boundary slip of oil molecules at MoS2 homojunctions can be extended to the macroscale for friction reduction, supplying a fundamental insight into the lubrication mechanism of MoS2 nanoflakes in oil, which has potential applications for designing an efficient lubrication system with nano-additives.