Chemical Stability Limits of Epoxy Resin Coatings in Solid-Oil Hybrid Lubrication: From Sustained Superlubricity to Failure

Frictional energy dissipation accounts for ~30% of global primary energy consumption, highlighting the critical need for superlubricating materials (COF < 0.01). We previously reported that solid-oil hybrid lubrication using epoxy resin coatings enables superlubrication with different viscous oils. However, the effect of chemical structure of the epoxy resin coatings on the solid-oil hybrid lubrication behavior remains unclear. This study investigates the influence of chemical structure of the coatings which is induced by annealing treatments at different temperature on their solid-oil hybrid superlubrication performance. The results show that the mechanical properties, crosslinking structure and chemical composition of the epoxy resin coating were significantly changed by heat treatment. Under the lubrication of nonpolar oil (150 N mineral oil), the epoxy resin coating treated at high temperature (> 300°C) fail to achieve superlubrication. This failure is attributed to two primary factors. On the one hand, the coating undergoes thermal degradation, the number of polar functional groups such as hydroxyl and epoxy groups decreases, and it is difficult to form surface hydroxylation at the friction interface. On the other hand, the degree of curing is strengthened, which inhibits the formation of EP transfer layer on the surface of the grinding ball.