Effect of polymers on lubricant rheology in an elastohydrodynamic contact

Abstract

Understanding the behaviour of fluids in a tribological contact is crucial to design better lubricants and enhance energy efficiency through friction reduction. Obtaining local and in situ insights into tribological contacts is a significant challenge. All the more for complex fluids such as polymer solutions, which are likely to exhibit non-Newtonian and unpredictable behaviours in a tribological contact in the elastohydrodynamic lubrication regime.

In order to gain greater knowledge on the link between the molecular properties of polymers, their mechanisms at the microscale and the rheology of polymer-containing lubricants, new in situ techniques were developed. Using phosphorescence imaging velocimetry, Raman spectroscopy, fluorescence anisotropy, and fluorescence lifetime, the behaviour of polymer solutions was investigated in a ball-on-disc contact in the elastohydrodynamic regime, under high-pressure and high-shear.

The flow behaviour of polymer and copolymer solutions deviates from a Couette flow, their velocity profiles exhibit slip and plugs. Quantification of the adsorption supports the hypothesis that the adsorption of polymer molecules on the contact surfaces promotes deviations from a Couette flow.

Raman spectroscopy was designed for the first time in situ in an elastohydrodynamic contact to evaluate the presence of polymer molecules in the contact. No change in polymer concentration was observed at the inlet, in the contact, and at the outlet. These results indicate that, for the polymer solutions investigated, an unexpected film thickness is not caused by aggregation or depletion of polymer molecules at the inlet.

Preliminary work using fluorescence anisotropy to generate viscosity maps of a tribological contact was carried out. Investigation up to a viscosity of 5 Pa·s found a very promising performance of the molecular rotor Bodipy, as a viscosity sensor.

Analytical models, based on Barus- and Eyring-like forms of the viscosity, were developed to estimate the viscosity-pressure relationship of fluids, from friction measurements. Compared to high-pressure viscometry data, a good agreement was found up to 1 GPa. This powerful method is of special interest when no viscometer is available.