Shearing of adsorbed polymer layers in an elastohydrodynamic contact in pure sliding
Résumé
The valve train wear of automotive diesel engines mainly depends on the kinematics of the contact, the metallurgy of the rubbing surfaces and the lubricant [1]. The presence of soot in the oil causes a dramatic increase in wear of the cam/tappet contact [2-3]. It has been shown, in this case, that the interactions between the soot particles and the rubbing solid surfaces are very important in the understanding of the wear and lubrication process. These interactions are partially governed by the dispersant polymer in solution in the automotive lubricant. Then, the tribological and the rheological behaviour of the boundary films anchored on to the surfaces must be considered. This paper focusses on the shearing of polymer films inside an elastohydrodynamic contact. The application of low speed alternative motions of the contact with successive periods of pure rolling (vanishing sliding speed) and pure sliding (vanishing entrainment speed) leads to the formation of film accumulations that are always entrained by the lubricant flow. The shearing and the geometry of these aggregates strongly depend on their adherence to the solid surfaces. The agglomeration process is stopped when the adsorbed layers of polymers have been consumed and totally peeled off from the solids. A numerical simulation including the peculiar kinematics of the contact in a pure sliding elastohydrodynamic lubrication regime gives the evolution of the oil film thickness versus time. The results of this model are compared to the experimental results. The consumption and the adherence of these layers to the substrates can be related to the wear process of the valve train.