Around 23% of the world’s energy consumption results from rubbing contacts, in which 20% is used to overcome friction and 3% is due to wear and the consequent failure. The implementation of lubricants and effective lubricant additives are indispensable to reduce friction and wear of rubbing materials. Protic Ionic Liquids (PILs), which are easily obtained by proton transfer from a Brønsted acid to a Brønsted base, have shown great potential to be used as lubricants due to their structures and tunable properties. In this study, two kinds of novel PILs, bis(2-hydroxyethylammonium) succinate (DSu) and tri-[bis(2-hydroxyethylammonium)] citrate (DCi), were synthesized and tested as lubricant additives. The tribological behavior of the two PILs is studied as additives in 1 wt. % to a base synthetic lubricant (PAO) for steel-steel contact under different temperatures (room temperature and 100°C) and normal loads (3 N and 4 N) using a ball-on-flat reciprocating tribometer. When 1 wt. % of any PILs is added into PAO, friction is reduced compared to that with neat PAO at all temperatures and loads studied, and good anti-wear performance is also obtained under the higher temperature studied. Particularly, 1 wt. % DCi +PAO shows the best high-temperature tribological behavior under 4N, with friction and wear reductions of 33% and 35%, respectively.

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