Advances in the Use of Ionic Liquids as Smart Lubricants and Additives for Tribological Applications

Ionic liquids have emerged as a promising class of lubricants and lubricant additives due to their unique physicochemical properties, including negligible vapor pressure, high thermal stability, tunable molecular structure, and strong surface affinity. This review summarizes recent advances in the application of ionic liquids in tribology, with emphasis on molecular design, lubrication mechanisms, rheological behavior, material compatibility, and industrial applicability. The role of cation–anion combinations in governing adsorption, boundary film formation, and tribochemical reactivity is critically discussed. Particular attention is given to ionic liquids used as neat lubricants and as additives in conventional mineral and synthetic oils, where synergistic interactions and concentration-dependent effects are evaluated. The formation of protective tribofilms, confinement-induced structuring, and rheological characteristics under high pressure are analyzed as key contributors to friction and wear reduction. Challenges related to corrosion, miscibility, viscosity control, cost, and large-scale synthesis are examined in the context of practical implementation. Finally, future research directions are outlined, including data-driven molecular design, computational screening, and sustainable synthesis strategies. Although ionic liquids offer significant advantages under boundary and extreme operating conditions, their broader industrial deployment requires systematic optimization balancing performance, compatibility, environmental safety, and economic feasibility.