Synergistic effects of oleic acid-modified TiO₂ 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 enhancing 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 approximately 1.4 μm, demonstrating excellent load-bearing capabilities of 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) revealed 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 for the in situ wear generated by extreme shear. Cross-sectional analysis of EP wear scars revealed 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. This 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.