Energy research is not just about developing better, cheaper, and cleaner energy sources. It also deals with reducing unnecessary energy losses to maximize energy efficiency. Friction and wear, for example, not only can cause damage to engines and machines, but are responsible for significant energy consumption as well.
Motor oils are widely used as lubricants in transportation vehicles, wind turbines, and nuclear power plants, and because friction or hydrodynamic drag in an internal combustion engine is proportional to the viscosity of motor oils, researchers have been working on reducing the viscosity, which, on the other hand, inevitably increases the risk of wear.
Now, Jun Qu and his team at Oak Ridge National Laboratory and Shell Global Solutions (both in the United States) have found an additive formula that could well balance between lowering friction and reducing wear. The new formula consists of an oil-soluble phosphonium alkylphosphate, ionic liquid (IL, [Pxxxx][DEHP]) and a classic anti-wear additive zinc dialkyldithiophosphate (ZDDP) working synergistically to reduce both friction and wear.
The authors first test the performance of a gas-to-liquid 4 cSt base oil added with about 1 wt% of equimolar [Pxxxx][DEHP] and ZDDP combination. They find that, in the reciprocating sliding tribo-test with a steel ball against a cast iron flat, it reduces the steady-state friction coefficient and the wear volume by about 30% and over 70%, respectively, compared to the base oil with about 1 wt% of [Pxxxx][DEHP] or ZDDP only.
Such synergy is further studied by several advanced microscopic and spectroscopic techniques. It turns out that the active elements in [Pxxxx][DEHP] and ZDDP are not uniformly distributed in the oil, and their concentrations are about 30 to 70 times higher on the oil surface than the nominal, which is believed to be responsible for the improved lubricating behavior. Interestingly, such a synergistic effect is only observed between phosphonium-alkylphosphate ILs and ZDDP, but not for other groups of ILs such as phosphonium-alkylphosphinate or ammonium-alkylphosphate salts, indicating both the cation and the anion are indispensable.
The authors therefore propose an anion-exchange mechanism to explain the extraordinary tribological performance of this particular additive combination. They envision that one of the two dithiphosphate anions in ZDDP may possibly be replaced by the alkylphosphate anion in [Pxxxx][DEHP] as an energetically favorable process, yielding zinc alkylphosphate alkyldithiophosphate (ZOTP). While this is backed by chemical analysis, it remains unclear why ZOTP is enriched on the oil surface.
With this new discovery, the authors now plan to further investigate and develop the novel class of hybrid lubricant additives. One focus of their future fundamental studies is to reveal the driving force behind the order of magnitude higher concentrations of active agents at the oil surface, and to verify such ‘hyper-surfactant’ phenomena at oil-solid interfaces. On the development side, they will seek direct synthesis of the new ZOTP anti-wear compounds, optimize the performance via molecular structure tailoring, and investigate the compatibility with other additives in engine lubricants.
