Understanding elastohydrodynamic (EHL) film formation and the pressure-viscosity response of lubricants is necessary for designing rolling/sliding tribological contacts. This article investigates the EHL behaviour of four formulated water-based lubricants (glycerol-water, glycol-water, and ionic liquid-water) and one reference oil under moderately high pressures, typical in gears and bearings applications. A ball-on-disc tribometer with optical interferometry was employed to measure the film thickness of the water-based lubricants. The results highlight the sensitivity of film formation to entrainment speed, slide-to-roll ratio (SRR), temperature, and lubricant composition. Water loss due to evaporation significantly impacts film formation at high temperatures. Additionally, an unusual increase in film thickness was observed for the glycol-water solution, likely due to complex tribological conditions. The limitations of the classical Hamrock-Dowson film thickness equation for water-based lubricants are also discussed. Furthermore, pressure-viscosity coefficients of the water-based lubricants were estimated using both optical interferometry and high-pressure viscometer methods. The effect of water content on the pressure-viscosity coefficient was also examined, revealing that higher water content leads to reduced pressure and temperature dependence of viscosity.

This study evaluates the tribological performance of fully formulated water-based lubricants (WBLs) under rolling/sliding conditions. The WBLs are composed of water-soluble glycols and glycerol with a functional proportion of water. They exhibited significantly lower friction across all lubrication regimes, with near superlubricity achieved under certain conditions. Unlike conventional oils, WBLs showed minimal shear heating effects and exhibited a low shear response with increasing slide-to-roll ratios. Long-duration tests confirmed consistently lower friction for the WBLs. Post-test analysis showed comparable wear between selected WBLs and the reference oil at 40 °C but greater surface modification for WBLs at higher temperatures. Observed wear mechanisms included abrasive and adhesive wear, with occasional instances of spalling after prolonged tests. X-ray photoelectron spectroscopy (XPS) analysis suggested the presence of chemical species on wear scars, attributed to additive-surface interactions. These findings highlight the potential of fully formulated WBLs in rolling/sliding contacts and their current state in comparison to a conventional gear oil.