Investigation of the Rheological Behaviour of Three Industrial Lubricants at High Shear Rates and Pressures

This paper aims to investigate the rheological behaviour of industrial lubricants at high shear and high pressure. A twin-disk rheometer based on a standard UMT apparatus is used to measure the rheological features and film thickness of three lubricants, namely, 150N, UB-3, and 15W/40, with the shear rate ranging from 0 s⁻¹ to 10⁷ s⁻¹ and the pressure at GPa. A semiempirical rheological model that considers the influence of heat, shear, and fluidic plasticity was proposed to adequately fit the experimental data of three organic lubricants. The rheology of the lubricants has a linear to nonlinear relationship with increasing shear rate, indicating shear thinning, which is then followed by a sharp decrease at approximately 10⁶ s⁻¹ because of thermal effects. At a higher shear rate, the shear stress saturates to a critical value. Moreover, the critical traction coefficients in the saturation region show similar changes in pressure and temperature for the three lubricants. The coefficients are greater at 1 GPa but decrease and saturate above 1.45 GPa, probably because the molecular-free volume is compressed by the constraint. The coefficients change little with varying inlet temperature at 1.45 GPa. This research sheds light on the complex rheological behaviour of three lubricants at high shear rates and high pressures and attempts to explain them theoretically.