Recent Advances in Nanotribology of Ionic Liquids

Background: Ionic liquids (ILs) have recently attracted considerable attention in tribology owing to their unique physico-chemical properties and promising lubrication performance when used in a wide range of material pairs. Objective: The aim of this review article is to summarize recent advances in our knowledge related to the lubrication mechanisms of neat ILs, with a particular focus on nanoscale studies dealing with the behavior of ILs in the boundary lubrication regime. Methods: We first discuss the current state-of-the-art concerning the normal pressure-dependent lubrication mechanism of ILs and then focus on the dynamic behavior of ILs upon nanoconfinment. Finally, we summarize recent research efforts aiming to control the tribological response of ILs by changing the surface charge density, evaluate the effects of impurities on the lubricity of ILs, and shed light on the IL tribochemistry at small length scales. Results: While the field of IL-mediated lubrication has made significant progress, several open questions still remain, including the effects of temperature, impurities, and surface roughness on the friction response and dynamic behaviors of nanoconfined ILs. Additionally, a mechanistic understanding of the tribochemical reactivity of ILs is still lacking. Conclusions: To harness the full potential of ILs for tribological applications, significant work is still required to establish links between the IL structure, lubrication mechanism(s), and performance. These advancements will be instrumental for the predictive design, development, and implementation of ILs with enhanced tribological properties in next-generation lubricants for a variety of applications across several sectors, including manufacturing and transportation.