Modelling of mixed hydrodynamic and boundary lubrication: Layered structures and shear thinning

Abstract

This paper is concerned with an intermediate regime between boundary and hydrodynamic lubrication. With high molecular weight polymer lubricants, it is accepted that polymeric molecules stick to the boundary surfaces in the form of immobile solid-like layers whose thickness may be of the order of 50 nm. Such layers may comprise about half of the minimum film thickness under severe sliding conditions. In addition, shear thinning may occur in a central region of the film, away from the thickened layers. From this physical picture, a modified Reynolds equation is developed which depicts an immobile layer model representing molecular microstrucrure and a shear thinning effect away from the surface. Compared to the Newtonian case, the molecular microstructure causes much higher pressure levels. This result is consistent with the physical idea that slow sliding speeds can apparently generate sufficient forces to separate surfaces in boundary lubrication conditions, although the classical hydrodynamic theory would predict otherwise.