Tribological Properties of Borate-Based Protic Ionic Liquids as Neat Lubricants and Biolubricant Additives for Steel-Steel Contact

Abstract

A series of four novel halogen-free borate-based protic ionic liquids were synthesized with identical organoborate anions but dissimilar ammonium cations, to allow systematic discussion of the effects of cation alkyl chain group length on their physicochemical and tribological properties. The ionic liquids (ILs) studied showed up to a 30% friction reduction compared to a biolubricant and even displayed negligible wear when tested as neat lubricants. Blends of 1 wt.% ionic liquid and biolubricant were also investigated, leading up to maximum friction and wear reductions of 25% and 96%, respectively, compared to the base oil. Interestingly, the ionic liquid that performed least effectively as a neat lubricant due to the solidification of the fluid achieved the best tribological response as a lubricant additive. The tribological improvements were attributed to the formation of a self-assembled adsorbed layer that separated the frictional surfaces. This theory was supported by rheological studies and elemental analysis.