Effect of a Long Alkyl Chain in Protic Ionic Liquids Molecules on Tribological Properties at High Temperature

In this work, three protic ionic liquids (PILs) were newly synthesized to prepare thermally stable lubricants. The thermal stability of PILs increases with increasing ΔpKa, defined as the difference in the logarithm of the acid dissociation constant (pKa) between a Brønsted acid and a Brønsted base. Long alkyl chains in oiliness additives are effective for reducing friction, and thus PILs with a high ΔpKa and an octadecyl group were expected to reduce friction at high temperatures. To verify this design concept experimentally, the friction and wear properties of PILs that have high ΔpKa values with and without an octadecyl group were evaluated using pin-on-disk tribotester with high-frequency induction heating in comparison to the conventional lubricant metallocene polyalphaolefin (mPAO; molecular weight: 5,400). During tribotests at 200°C, the PIL without an octadecyl group afforded a high friction coefficient, whereas both the PILs containing octadecyl groups and mPAO exhibited relatively low friction coefficients. At 300°C, the friction coefficient for mPAO abruptly increased and stick–slip noise was observed because the lubricant evaporated from the disk. In contrast, the friction coefficients of the PILs with an octadecyl groups remained low during the friction tests, even at 300°C