Bridging high pressure rheology and film-forming capacity of polymer-thickened lubricants in EHL
Résumé
Over the past decades, many technological breakthroughs have been made by both automobile and lubricant manufacturers to reduce friction and wear in engines in order to decrease pollution, extend the lifetime of engines and improve their performance. One of the key parameters controlling the lubrication is the film thickness which must be optimal to fulfil its role. Indeed, the fluid film must not be too thick to avoid viscous friction and not too thin to ensure a sufficient thickness to separate the moving surfaces. The prediction of the film thickness in EHD contacts is thus of main interest.
The objective of this research is to provide an accurate knowledge of the thickness of the lubricant film under severe conditions of temperature, pressure and shear stress. Film thicknesses are measured and compared with both analytical and numerical solutions in which rheological models are implemented.
The present work focuses on the tribological study of simplified automotive lubricants. A base oil and three polymers (with different physico-chemistries and conformations) mixed in solutions are characterized. Central and minimal film thickness measurements are run on a ball-on-disc tribometer. The principle of the method is based on white light optical interferometry. Then, experiments are compared with Hamrock-Dowson analytical models and a finite element numerical solver that features, in a full-system approach, the coupled resolution of the EHL equations and the physical characteristics of the fluid (compressibility and rheology).
The Newtonian predictions are in good agreement with the experimental data for the base oil and one of the polymer solutions. However, the comparisons witness a deviation between the Newtonian solution and the non-Newtonian case for the two others polymer solutions. The overestimation of the real film thickness considering a Newtonian fluid reveals the importance of using a reliable rheological characterization under extreme conditions.
The rheology is thereby, sharply influencing the film-forming capacity. Since the physico-chemistry of lubricants is strongly correlated to rheology, by extension the tribology is thus linked to the molecular scale. This multi-scale analysis is relevant for oil makers to improve the matching of the lubricant additives to the engine specifications.