Molecular Structure–Tribological Performance Relationship in Tungsten-Based Polyoxometalate Ionic Liquid Lubricant Additives

Abstract

Polyoxometalate ionic liquids (POM-ILs) represent a class of multifunctional materials combining the structural tunability of polyoxometalates (POMs) with the versatile properties of ionic liquids (ILs). In this work, a series of POM-ILs were designed, synthesized, and evaluated as lubricant additives and as plain lubricants to demonstrate the relationship between their molecular structures and tribological performance. The influence of both the cationic moiety (ammonium vs phosphonium) and the anion structure (SiW11 vs SiW9) was investigated on two metallic substrates, AISI 316L stainless steel and AISI 52100 bearing steel, under boundary lubrication conditions. Comprehensive characterization combining friction and wear measurements, QCM-D adsorption analysis, and surface and subsurface chemical and structural analysis (XPS, SEM, and FIB-SEM) revealed that the POM-IL molecular structure critically influences friction and wear behavior. Increasing the lacunarity and negative charge density of the POM enhanced antiwear performance and reduced friction, likely through the formation of viscoelastic layers that promote surface separation. In addition, the base lubricant (PAO8) was found to be detrimental to the POM-IL surface interactions, reducing their efficiency as additives compared to their plain form. The results obtained in this work demonstrate that POM-ILs offer an effective alternative to conventional antiwear additives such as ZDDP, combining high molecular designability, strong substrate activity, and enhanced antiwear and friction-reducing behavior. These insights establish a structure-to-function relationship and highlight the potential of molecularly engineered POM-ILs for next-generation lubrication systems.