Synergistic Effect Between Phosphonium-Based Ionic Liquid and Three Oxide Nanoparticles as Hybrid Lubricant Additives

Abstract The tribological properties of ionic liquid (IL) trihexyltetradecylphosphonium bis(2,4,4-trimethylpentyl) phosphinate along with Al2O3, CuO, and SiO2 nanoparticles (NPs) have been investigated as a lubricant additive in a group 1 mineral base oil. About 0.5 wt% concentration of additives were added in base oil, and tribological tests were conducted at mild (stipulated) and severe (ASTM D 4172D) working conditions to assess the synergy between IL and NPs. This study shows the excellent synergy between IL, Al2O3, and CuO NPs in improving tribological and extreme pressure (EP) properties. Al2O3 and CuO hybrid nanolubricants decreased friction by 19% and 24%, whereas wear by 32% and 36%, respectively, at ASTM test conditions. IL displayed very good EP properties with a total improvement of 19%, and the highest load-bearing capacity was observed for Al2O3 and CuO hybrid nanolubricants with an improvement of 30% and 34%, respectively. No conclusive evidence of synergy has been observed between IL and SiO2 NPs. Surface characterization techniques, such as scanning electron microscope, energy dispersive X-ray spectrometer, and Raman spectra, demonstrated the formation of a tribofilm rich in phosphate and tribosintered NPs on the worn surface responsible for improved triboperformances.