Effect of PAO Base Oils with Different Viscosities on Rheological and Tribological Properties of Lithium Greases

This study employed four polyalphaolefin (PAO) synthetic base oils with a viscosity gradient. A series of lithium greases were prepared via the preformed soap method to investigate the influence of PAO base oil viscosity on the physicochemical properties, microstructure, rheological behavior, and tribological performance of lithium greases, along with their underlying interrelationships. The results demonstrated that greases formulated with low-viscosity PAO exhibit finer thickener fiber diameters and a more porous network structure. Conversely, greases prepared with high-viscosity PAO display coarser thickener fibers and a denser fiber network, leading to lower porosity. These microstructural distinctions imparted superior thixotropic properties to high-viscosity PAO greases, while low-viscosity PAO greases possessed higher structural strength. Thickening significantly enhanced the anti-friction and anti-wear properties of the base oils. Greases synthesized with high-viscosity PAO exhibited superior tribological performance (exhibiting average friction coefficients of 0.072 and 0.062, respectively), representing reductions of 37% and 24% compared to their respective base oils. Wear volumes were concurrently reduced by 29% and 19%. Under elastohydrodynamic lubrication (EHL) conditions, the film thickness of low-viscosity PAO greases was predominantly governed by residual thickener fiber clusters within the contact zone. High-viscosity PAO greases maintained a higher film thickness, relying on a dimpled oil film formed by squeeze action. Owing to their higher porosity and larger pore size, low-viscosity PAO greases demonstrated a higher oil bleeding rate. This facilitated rapid replenishment of the lubricant reservoir within the contact zone, ultimately resulting in a larger recovered reservoir size. This research provided a robust theoretical foundation and technical guidance for the selection of PAO base oils in lubricating grease formulation and application.