Evaluation of tribological behavior of a circulation oil with ionic liquid and hybrid additives

In this study, the synergistic effects of methyltrioctylammonium bis(trifluoromethylsulfonyl)imide [N1888] [NTf2] ionic liquid (IL) with zinc dialkyldithiophosphate (ZDDP) and zinc oxide (ZnO) nanoparticles (NPs) as hybrid additives in a circulation oil for steel–steel contacts at different temperatures. The wear test results indicated that the additions of single additives (IL, ZDDP, and ZnO NPs) could enhance the tribological performance of the circulation oil. Among these additives, the IL exhibited the most effective at the same weight concentration blended into the tested oil. The mixture of IL and ZDDP showed superior friction-reducing and wear-reducing properties compared to the IL + ZnO formulation. The hybrid additive formulation consisting of 0.5 wt% IL, 0.25 wt% ZDDP, and 0.25 wt% ZnO NPs exhibited excellent tribological properties at higher temperatures in the boundary lubrication regime. Analysis using scanning electron microscopy/energy dispersive X-ray reveals that all single additives contribute to the formation of a tribofilm wear mechanism. However, the role of ZnO NPs in the hybrid additive conditions was changed from the most likely tribosintering effect to the most likely nano bearing effect at 100 °C. The interactions among IL, ZDDP, and NPs examined in this study can provide fundamental insights for the development of future lubricants.