Whole course oil-based superlubricity for polyetheretherketone-bearing steel sliding pair by introducing tribocatalytically active dialkyl-dithiophosphoric-modified copper nanoparticles in poly-alpha olefin oil

At present, achieving oil-based superlubricity depends on lubricating oil viscosity and rubbing pair materials excessively. Therefore, establishing a new route is of scientific significance and application value. Here, a novel strategy, i.e., the assembly of a dialkyl-dithiophosphoric-modified copper nanoparticle (DDP-Cu NP) lubricating additive and polyetheretherketone (PEEK)-GCr15 friction pairs, is reported to achieve oil-based superlubricity in polyalpha olefin oil (PAO40), a synthetic base oil with high viscosity. To our surprise, when the concentration of DDP-Cu is only 0.25 wt%, the lowest friction coefficient is 0.002, and the wear rate reaches 10⁻⁹ mm³/(N·m) level (which is lower than that published in the literature by 1–2 orders). Furthermore, such superlubricity occurs without a running-in period and remains during the whole sliding process. These results are attributed mainly to the rolling effect of the spherical Cu NPs, the formation of a high-performance carbon-based tribofilm at the sliding counterface, and the in operando formation of some few (even mono)-layered graphene layers in the lubricant (the tribocatalytic effect of the DDP-Cu NPs). Specifically, DDP-Cu NPs perform in operando tribocatalysis to promote the decomposition of the PEEK matrix and PAO40 and generate some carbon species, including amorphous carbon and even graphene, all of which are responsible for the surprising oil-based superlubricity. The findings will contribute to enriching relevant tribological theory and promoting the exploration and application of oil-based superlubricity.