Ultimate Behavior of Confined Fluids Under Very High Pressure and Shear Stress
Résumé
Lubricants in the EHL regime are submitted to severe conditions, namely very high pressures, slide-to-roll ratios (SRR) and confinement, which can lead to a peculiar behavior of the lubricant itself.
Indeed, at a given high pressure, the shear stress experienced by a lubricant can reach a maximum value, the socalled Limiting Shear Stress (LSS). The LSS origin is still not clear, which limits the proper modeling of EHD contacts. Previous researches attribute this LSS to either a modification of the lubricant state (liquid/glassy/solid) or a specific velocity field in the contact (wall slip and shear localization). However, these scenarios most of time assumed under extreme experimental conditions make very difficult to validate one of them.
Our approach aims to explore the origins and consequences of the LSS by measuring in situ the evolution of the microstructural state of a lubricant when the LSS initiates.
Lubricants that exhibit both a strong Raman response and a LSS behavior are a prerequesite for this study. After being calibrated, friction experiments are performed in a lubricated contact submitted to high contact pressures and low SRR. Next step is to make a correlation between the lubricants microstructure and friction measurements. Finally, these measurements are compared to pressure-velocity mapping of the lubricants behavior from the literature.