Local rheology of lubricants in the elastohydrodynamic regime

Abstract

Numerous models have been developed to describe the viscosity and rheology of lubricants in elastohydrodynamic (EHD) lubrication, but little experimental and theoretical work has been done on the flow of lubricants. Due to the high pressures in a tribological contact it is likely that lubricants may undergo structural changes, which would significantly affect their flow.

Photobleached-fluorescence imaging velocimetry was applied to a glass-glass EHD contact, lubricated with the oligomer polybutene, which was doped with fluorescent dye. The technique involved tagging a volume inside the contact, by making it dark compared to its surroundings. A model was developed to solve for the through-thickness velocity profile using the experimental data and the technique was validated experimentally using a parallel plate Couette setup. Velocity profiles of polybutene in an EHD contact were measured under various conditions. Three distinct rheological responses could be observed. At low pressures, the velocity profile was mostly linear. At a critical pressure, a low shear rate plug formed in the centre of the film, possibly due to pressure-induced glass transition of the lubricant. The application of a low surface energy coating caused the lubricant to slip at the interface, depending on the applied pressure.

The velocimetry studies were supported by film thickness and friction measurements. Laser-induced fluorescence was used to measure the film thickness, showing that the plug flow of polybutene coincided with an anomalous increase in film thickness, while the occurrence of boundary slip resulted in reduced film thickness. Friction measurements showed that plug flow had negligible effects on friction. Boundary slip however caused a decrease in friction (up to 70 %).

Results suggest that lubricant flow in an elastohydrodynamic contact is non-trivial and deserves more consideration than is typically given. Direct flow measurements could be useful to elucidate the complex relationship between film thickness, friction and flow.