An investigation of film formation and pressure-viscosity relationship of water-based lubricants in elastohydrodynamic contacts

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.