Polyoxometalate-ionic liquids (POM-ILs) – a new type of ionic liquid additive family for lubricants†

Abstract

The focus on energy efficiency to move towards a more sustainable use of resources has intensified efforts to minimize friction and wear in mechanical systems, which account for 23% of the world's energy consumption. In this study, polyoxometalate ionic liquids (POM-ILs) are introduced as environmentally acceptable lubricant additives, for their potential friction-reducing and anti-wear (AW) properties. These compounds, characterized by their complex structures and tunable properties, have been investigated for their tribological performance across base fluids of varying polarities. Their performance has been compared against zinc dialkyldithiophosphate (ZDDP), a standard AW additive. Our findings demonstrate that POM-ILs exhibit promising friction-reducing and AW capabilities, comparable to traditional additives. The efficacy of POM-ILs was found to be highly dependent on the base fluid used, with significant variations observed in their ability to interact with stainless steel surfaces. Adsorption studies confirmed strong adsorption of POM-ILs onto stainless steel, with notable influence from the base fluid. Advanced characterization techniques revealed the formation of a predominantly oxide-rich tribofilm on the metal surface, as a result of POM-IL decomposition and reaction with the metal. POM-ILs show significant potential as lubricant additives, with their structural versatility offering a promising path for future greener developments in tribology.