Tribological performance and mechanism of two sulfur-based anti-wear additives in low-viscosity gear oil

Purpose The purpose of this study is to select two types of ester–polysulfide hybrid additives with distinct molecular polarities and investigate their tribological performance in low-viscosity gear oil. Design/methodology/approach Low-viscosity HVIP2 base oil was adopted as the lubricating medium to formulate the candidate oils with sulfurized olefin and sulfurized triglyceride, respectively. Tribological performance was evaluated via the standard four-ball tribotest. The chemical composition and elemental distribution of the transfer film on the worn surface were characterized by scanning electron microscopy, energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy (XPS). Furthermore, molecular dynamics (MD) simulation was performed to quantify the adsorption energy and shear behavior of the two additives. Findings The results show that sulfur-based additives exhibit optimal anti-wear performance at a dosage of 1.5 Wt.%. The friction coefficients of sulfurized olefin and sulfurized triglyceride are 0.058 and 0.05, with the corresponding wear scar diameters of 0.848 and 0.489 mm, respectively. XPS analysis reveals that sulfurized triglyceride forms a protective tribofilm dominated by FeS2 and Fe2(SO4)3, whereas sulfurized olefin mainly generates an FeS film with abundant residual thiol groups remaining. MD simulation results demonstrate that sulfurized triglyceride possesses significantly higher adsorption energy on the iron surface (603.45 kcal/mol) than sulfurized olefin (95.9 kcal/mol). Originality/value This study compares the tribological performances and mechanisms of sulfur-based additives to satisfy the requirements of low-viscosity gear oil. The intrinsic discrepancies in interfacial adsorption, shear stability and tribofilm formation pathways are clearly distinguished. Peer review The peer review history for this article is available at: Link to the website of publons.