Molecular Origins of Low-Friction Sliding in Ethyl Lactate and Its Regulation by Ethylene Glycol

Abstract Ethyl lactate (EL) is a promising green lubricant additive; however, its tribochemical mechanisms and performance limitations remain unclear. ReaxFF molecular dynamics simulations were performed to investigate the tribological behavior of pure EL and EL/ethylene glycol (EG) mixtures confined between α-Fe(100) surfaces. For pure EL, pressure-induced ester-bond cleavage governs the low-friction mechanism. Increasing normal load enhances molecular reactivity and accelerates ester-bond cleavage, producing strongly anchored fragments. The coupled effects of mechanical compression and tribochemical reactions transform a disordered interface into an ordered, H-terminated tribofilm with a layered structure that enables stable shear. However, the tribological performance of pure EL is sensitive to operating conditions. Under lubricant-deficient conditions, bridge-bond formation and interfacial disorder become pronounced, leading to increased friction and wear. In contrast, EG modifies the interfacial evolution pathway by occupying surface active sites and regulating molecular distribution. EG suppresses intermolecular cross-linking and reduces EL-derived bridge bonds while forming a uniform short-chain adsorption layer. This regulation prevents premature interfacial deterioration and sustains the tribochemical evolution of EL. Consequently, the EL/EG mixture retains the favorable tribofilm-forming capability of EL while significantly improving tolerance to variations in lubricant concentration, enabling stable friction reduction over a broader operating range.