A low-viscosity ionic liquid demonstrating superior lubricating performance from mixed to boundary lubrication

This study reports on an ionic liquid (IL) 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide as a promising engine lubricant candidate This IL has a relatively low viscosity of 4.7cSt (similar to 0W-10 racing engine oil), a low viscosity-pressure (V-P) coefficient of 6.5GPa^-1 at 100°C, a high viscosity index of 159, and a high onset decomposition temperature of 472°C. The ionic liquid delivers substantially lower friction in mixed lubrication (ML) and provides better scuffing protection in boundary lubrication (BL) than those of hydrocarbon lubricants with similar viscosities. The low ML friction is possibly attributed to its low V-P coefficient and a hypothetical multi-layer boundary film at the interface. In BL, the tribochemical reactions between the IL and the contact surfaces to form a protective tribo-film are believed to be responsible for the IL's superior anti-scuffing behavior. Microstructural examination and chemical composition analysis were conducted on the wear scars from both the top surface and the cross section to study the wear mode and characterize the tribo-film and near-surface structure.