Decoupling the Effects of Shear Thinning and Thermal Softening on EHL Film Thickness and Friction

In elastohydrodynamic lubrication (EHL) contacts, lubricant viscosity plays an important role in friction and film formation. Previous studies have established that shear thinning and thermal softening influence both friction and film thickness. However, the distinct effects of temperature- and shear-rate-dependent viscosity on local cross-film lubricant flow and their relationship to changes in film thickness at high slide-to-roll ratios (SRRs) remain insufficiently resolved. A validated full Navier–Stokes thermo-EHL line-contact model with three viscosity formulations was employed to distinguish the effects of shear thinning and thermal softening on friction, film thickness, and local lubricant flow: (i) a thermal shear-thinning (non-Newtonian) formulation, (ii) a thermal Newtonian formulation, and (iii) a thermally uncoupled shear-thinning (isothermal non-Newtonian) formulation. Two parameter sweeps were performed: the slide-to-roll ratio (SRR) was varied at a fixed entrainment velocity, and the entrainment velocity was varied at a fixed SRR. With increasing sliding speed, thermal softening becomes more important for friction behaviour relative to shear thinning, resulting in the transition from shear-thinning-dominated to thermal-softening-dominated friction behaviour. Similarly, thermal softening has a greater influence on film thickness than shear thinning. Shear thinning primarily reduces viscosity and wall shear stress while maintaining a nearly linear Couette-type velocity profile, with only a minor effect on film thickness. In contrast, an important new phenomenon is that thermal softening has a distinct effect on film thickness. When the contacting surfaces move at different velocities, thermal softening produces a non-uniform cross-film viscosity distribution, which redistributes the velocity gradient and increases the mean lubricant velocity. For a given mass flow, this increase in mean velocity reduces film thickness, consistent with the conservation of mass.