Achieving oil-based superlubricity with near-zero wear via synergistic effect between PEEK-PTFE and PAO40 containing DDP-Cu nanoparticles

Oil-based superlubricity introduces a novel approach to energy conservation. Facing two main obstacles at present, i.e., higher viscosity of lubricating oil and special tribopairs, a novel solid-liquid combined system is built in this work, which is mainly based on excellent tribological property of oil-soluble dialkyl-dithiophosphoric-modified Cu nanoparticles (DDP-Cu NPs) and superior self-lubricating capability of polymeric tribopairs. In this system, polyetheretherketone (PEEK) is coupled with polytetrafluoroethylene (PTFE) to form a similar/dissimilar tribopair; and neat PAO40 (a real serving lubricating oil with high viscosity in poly-alpha olefin oil family), or PAO40 containing oil-soluble DDP-Cu NPs (PAO40 + Cu), is used as a lubricant during the sliding test. The results show that only dissimilar tribopair, i.e., PEEK-PTFE, can achieve oil-based superlubricity under the selected sliding conditions, which can be attributed to the synergistic effect between PEEK-PTFE tribopair and PAO40/PAO40 + 0.25 Cu. Furthermore, due to lower cohesive energy density (CED), PTFE is more likely to be transferred onto PEEK together with some tribochemical reactions, finally generating a high-performance tribofilm rich in carboxylated PTFE. In addition, it is concluded that, when lubricated with PAO40, the above synergistic effect is a key to achieve superlubricity; and for PAO40 + 0.25 Cu, the carboxylated PTFE and the exposed Cu core will create chelate effect, which can not only further enhance this synergistic effect, but also improve the wear-resistance of the system reaching near-zero wear. This work not only provides a novel idea for designing oil-based superlubricity system, but broadens the type of tribopair materials achieving oil-based superlubricity to some extent.