Tungsten-Based Polyoxometalate-Ionic Liquid as Lubricant Additive for Low-Viscosity PAO: Effect of Steel Composition and Microstructure on the Boundary Lubricating Performance

Abstract The development of environmentally acceptable lubricants and lubricant additives has become a focal point within tribology due to increasing regulatory and sustainability demands. In this context, low-viscosity lubricants are gaining attention for their potential to reduce energy losses. However, their performance under a boundary lubrication regime, where thinner oil film build-up is present, requires more efficient boundary additives. This work evaluates a polyoxometalate-ionic liquid (POM-IL) as a multifunctional boundary additive in a low-viscosity polyalphaolefin-based lubricant, comparing its performance to zinc dialkyldithiophosphate (ZDDP) and a halogen-containing ionic liquid (IL). Tribological tests on AISI 316L stainless steel and AISI 52100 bearing steel revealed that while ZDDP showed substrate-independent adsorption and tribological performance, the IL-based additives had substrate-dependent behaviour. Strong chemisorption was consistent for both IL-based additives, yet their anti-wear and friction-reducing properties differed, showing evidence for the presence of a combined mechanism that includes both strong adsorption and tribochemical reactions. Additionally, the interaction between POM-ILs’ negatively charged surfaces, W atoms, and Cr(III) in 316L was identified as a key factor in their performance. Notably, significant work-hardening was observed in 316L lubricated with POM-IL-containing blends, further enhancing its anti-wear properties. These findings emphasize the role of substrate chemistry in boundary lubricant additive performance in low-viscosity lubricants, offering insights for the development of more efficient multifunctional boundary lubrication solutions.