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.

Titanium dioxide (TiO₂) nanocrystals have garnered significant interest in nanofluid research field due to their controllable morphology, eco-friendly nature, and superior lubrication properties. However, the long-term dispersion stability of TiO₂ nanocrystals remains a significant challenge. This study demonstrates that the morphology, {001} facet exposure, and consequent surface modifier adsorption density of TiO₂ nanocrystals are synergistically controlled by modulating the facet-specific adsorption of oleic acid (OA), oleylamine (OM), and dioleamide during solvothermal synthesis. This control mechanism directly governs both dispersion stability and interfacial lubrication behavior. Crucially, the high-density modifier layer establishes a steric stabilization barrier, which ensures colloidal stability. The optimized TiO₂ nanocrystals (TiO₂-OAOM) exhibit colloidal stability for over 5 months and enhance the anti-wear properties of the base oil by 10-fold. Furthermore, synergistic interactions between TiO₂-OAOM and zinc dialkyl dithiophosphate (ZDDP) are confirmed in multiple friction pairs. These interactions promote the hardness and wear resistance of the tribo-film, thereby improving lubrication performance and suppressing metal transfer. This study provides a novel strategy for the co-optimization of dispersion stability and tribological properties of nanocrystals through surface engineering, offering novel insights into the design of nanofluids and a potent solution for lubrication challenges on low-hardness metals.