Macroscale superlubricity of viscous base oils achieved on soft epoxy resin coatings: The effects of molecular structure of oils containing hydroxyl groups

It still is a challenge for the most of base oils to reach the superlubricity at macroscale because of their intrinsic viscosity. Hybrid lubrication that combines the polymer coatings with liquid lubricants, has proven to be a successful approach to achieve superlubricity for viscous oils. However, the molecule structural effects of oil lubricants on the hybrid lubrication performance as well as the synergistic lubrication mechanisms of the oils on polymer coatings still are unclear. In this work, the molecular structural effects on the superlubricity behaviors on epoxy resin coatings were investigated by the comparative lubrication performance of linear and branched polar oils. The results showed that linear polar oils provided superior hybrid lubrication performance on epoxy resin coatings compared to the branched polar oils, even achieving robust superlubricity at the high rotary speeds. The superlubricity disappearance of branched oils at low rotary speeds could be related to the absence of epoxy resin transfer layers and weak hydroxylation of the counter ball surface. While the adsorption behaviors of the different polar oils on the epoxy resin coatings, investigated through molecular dynamics simulations, are the main responsibility for the lubrication failure of branched oils at high rotary speeds.