Tertiary Amine-Terminated Carbon Dots with Reversible CO2 Switchable Amphiphilicity as the Versatile Lubricant Additives

Tertiary amine-terminated carbon dots (CDs-TA) with reversible CO2 switchable amphiphilicity were synthesized as the versatile lubricant additives. First, the same batch of CDs-TA was used as the additives of various base lubricants including polyalphaolefin (PAO4), polyethylene glycol (PEG200), and H2O by the facile CO2 regulation. At the optimum adding concentrations, CDs-TA made the friction coefficients and wear volumes of the above lubricants reduce by 28.1–40.4 and 62.1–70.4%, respectively, exhibiting the excellent friction reduction and wear resistance functions. The wear scar surface analysis results revealed that CDs-TA as additives of PEG200 triggered the formation of a boundary lubrication film, which was in fact a heavy tribofilm with a thickness of more than 680 nm. Second, the recovery of CDs-TA from the discarded PAO4 was conveniently achieved by alternately bubbling of CO2 and N2 with the relatively favorable recovery rate. More importantly, the tribological performance of recovered CDs-TA was stable during six cycles of the recovery process, establishing the foundation for the recycle of CDs-TA as additives of PAO. This work is hopeful to open the door for broadening the application range and reducing the cost of CD-based additives in the lubrication field.