Synergistic effects of oleic acid-modified TiO ₂ and alkyl polysulfide in forming extreme pressure tribo-films: A mechanistic study

The synergistic effect of nanoparticles (NPs) with alkyl polysulfides represents a promising avenue for the enhancement of extreme pressure (EP) performance in lubricants, potentially reducing the reliance on sulfur-containing additives. However, the underlying synergistic mechanism remains unclear. In this study, oleic acid-modified titanium dioxide (TiO₂-OA) NPs and dialkyl pentasulfide (DPS) formed a mechanically interlocked EP tribo-film with a maximum thickness of about 1.4 μm, demonstrating excellent load-bearing capabilities up to 7,256 N (6.1 GPa). Under conditions of EP friction, the addition of TiO₂-OA NPs facilitates the development of a robust tribo-film of substantial thickness, which in turn bolsters the load-bearing capability of the lubricating oil. Surface analysis of EP wear scars using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS) showed that TiO₂-OA NPs were rapidly deposited and enriched at the friction interface with increasing load (3,530-7,256 N) and friction time (0-10 s), which in turn compensated in-situ the wear generated by extreme shear. Cross-sectional analysis of EP wear scars indicates that the preferential formation of ferrous sulfide (FeS) and subsequent rapid enrichment and mechanical interlocking of TiO₂-OA NPs are essential for preventing welding and improving EP performance. The study presents novel insights into the synergistic mechanisms of NPs with sulfur-containing additives, advancing the widespread implementation and progress of NPs in the field of tribology.