Significant friction and wear reduction using mandelic acid–cyclic amidine protic ionic liquids in polyalphaolefin lubricants

In this work, three novel mandelic acid–cyclic amidine-based protic ionic liquids (PILs) were synthesized through a straightforward proton-transfer reaction and further investigated as lubricant additives for PAO10. The results showed that these P and S-free PIL additives, featuring short alkyl chains, exhibited good solubility and tribological performance in PAO10 at only 0.5 wt% addition. Notably, compared to the performance of PAO10, the addition of mandelic acid – 1,8-diazabicyclo[5.4.0] undec-7-ene (DBU)-derived PIL could achieve a 60% reduction in the coefficient of friction and an 85% decrease in wear volume. Further analysis revealed that the unique a-hydroxyl acid and cyclic amidine structures produce multiple interactions, including proton acceptance, hydrogen bonding, electrostatic adsorption, and tribochemical reactivity; thus, formation of a powerful tribofilm including organic carbonaceous species and an inorganic iron oxides. Moreover, the renewable nature of mandelic acid, combined with the environmental-friendly and good tribological performance of these PILs, positions this work as a reference for the development of high-performance lubricant additives.