Transition Between Mixed Lubrication and Elastohydrodynamic Lubrication with Randomly Rough Surfaces

We are interested in understanding the effect of roughness on friction in mixed lubrication (ML) and elastohydrodynamic lubrication (EHL), notably in the case of realistic surfaces. Rolling-sliding experiments in piezoviscous base oil are conducted between rough steels obtained with different machining, polishing and coating processes in order to represent realistic surface conditions found typically in a car engine. The Reynolds number based on the maximum lubricant film thickness in our experiments is below R-e = 4 . 10(-3). Our experiments at moderate pressure (<1 GPa) and speeds (mm/s -> m/s) allow to reach EHL for all surface roughnesses which range from the nanometre up to the micron. These surfaces have a complex and multi-scale texture and require a statistical description. Using the most raw and simple filter on large topographic measurements, we correlate the most probable standard deviation of surfaces to friction during Stribeck experiments occuring both in ML and EHL without measurable wear. In order to predict the conditions leading to high friction in ML, we propose a friction-based definition of ML and describe the role played by pressure and roughness on the onset of ML.