Impact of Water Content on the Superlubricity of Ethylene Glycol Solutions

Abstract

Aqueous solutions of water and ethylene glycol (EG) are prevalently employed in braking, heat transfer, and lubrication systems. However, the precise mechanism through which water content affects the lubricative attributes of EG solutions remains elusive. This research systematically examines the tribological characteristics of EG solutions at varying concentrations using a ceramic–TiAlN friction-pair system. As the concentration of EG increases, the sequential transformation of the associated molecular complex structure in the lubricating medium can be described as follows: [H2O]m·EG → [H2O]m·[EG]n → H2O·[EG]n. Among them, the stoichiometric coefficients “m” and “n” are the simplest mole ratio of H2O and EG in the molecular complex structure, respectively. The most favorable EG concentration was determined to be 50 wt.%. At this concentration, a flexible molecular complex adsorption structure ([H2O]m·[EG]n) with a significant bearing capacity (due to intense hydrogen bonding) forms on the surface of the friction pair, which results in a reduction in the running-in duration and facilitates the achievement of superlubricity, and the coefficient of friction (COF) is about 0.0047. Solutions containing 50 wt.% EG enhance the load-bearing ability and hydrophilicity of the lubricating medium. Moreover, they minimize the roughness of the worn region and curtail the adhesive forces and shear stress at the frictional interface, enabling the realization of superlubricity. Consequently, this research offers valuable insights into the optimal water-to-EG ratio, revealing the mechanism of a superlubricity system that possesses exceptional tribological attributes and holds significant potential for practical applications.