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 excessively depends on lubricating oil viscosity and rubbing pairs materials. So, establishing a new route is of scientific significance and application value. Here, a novel strategy, i.e., the assembly of dialkyl-dithiophosphoric-modified copper nanoparticles (DDP-Cu NPs) lubricating additive and polyetheretherketone (PEEK)-GCr15 friction pairs, is reported to realize oil-based superlubricity in poly-alpha olefin oil (PAO40, a synthetic base oil with a 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/Nm level (lower than that published in literature by 1 ~ 2 order). Further, such superlubricity occurs without running-in period and remains during the whole sliding process. It is found that these results are mainly attributed to the rolling effect of sphere Cu NPs, the formation of a high-performance carbon-based tribofilm at sliding counterface, and in-operando formation of some few (even mono-)-layer graphene in the lubricant (the tribocatalysis effect of DDP-Cu NPs). Namely, DDP-Cu NPs exerts in-operando tribocatalysis to promote the decomposition of PEEK matrix and PAO40 and generate some carbon species, including amorphous carbon, 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.