Tribological Testing and Analysis of Ionic Liquids as Candidate Anti-Wear Additives for Next-Generation Engine Lubricants

In this work, fourteen ionic liquids (ILs) were assayed as potential next-generation engine oil additives. After screening for corrosion, thermal stability and oil solubility, candidate additives were subjected to friction and wear tests in both boundary and mixed regime lubrication. While each IL demonstrated friction and wear reduction compared to base oil without any additives, oil miscible ILs tetraoctylphosphonium bis 2-ethylhexyl phosphate ([P8888][DEHP]) and trioctylammonium bis 2-ethylhexylphosphate ([N888H][DEHP]) were the best performers in bench tests with a XX% and XX% improvement in wear over the base oil respectively. Each of these ILs excellent solubility and superior performance was attributed to their symmetric nature and hydrogen bonding in the case of [N888H][DEHP]. The worn specimens generated using these two ionic liquids were chosen for detailed surface analysis. Each showed the presence of an amorphous protective tribofilm dominated by phosphates and iron oxides. Each of these ILs performed well in experimentally formulated oils as well. [P8888][DEHP] was chosen for full-scale engine fuel economy testing due to its synergy with the common anti-wear additive, zinc dialkyldithiophosphate (ZDDP). The [P8888][DEHP]-containing formulated oil demonstrated better fuel economy compared to an oil of the same viscosity containing only ZDDP as the anti-wear additive. Its superior performance was well-correlated with friction and wear bench tests. These results demonstrate the effectiveness of using ILs as engine oil additives and provide a scientific basis for further work towards implementing and optimizing additive packages containing ILs.