Film Forming Capability of Polymer-Base Oil Lubricants in Elastohydrodynamic and Very Thin Film Regimes

The development of high-performance lubricants to insure the separation of mechanical parts, to reduce engine friction and consequently fuel consumption remains a major challenge for oil manufacturers. Polymer additives called viscosity index improvers (VII) have been extensively used to reduce the dependency of the lubricant's viscosity on temperature, to maintain an acceptable hydrodynamic lubrication in extreme conditions. This work focuses on understanding the role of VII in engine lubricants to bridge their tribological response with their rheological behavior. Simplified lubricants are studied, composed of polymers of different chemistries, molecular weights and conformations added to a mineral base oil. Film thickness measurements are carried out in pure rolling and isothermal conditions to quantify and clarify the effect of polymer addition on the lubricants behavior in the elastohydrodynamic (EHD) and very thin film (VTF) regimes. They are compared with accurate analytical predictions based on rheological models which lead to a good agreement with measured film thicknesses in the EHD regime, whereas some polymer solutions present a significant deviation to EHD predictions at lower thickness, i.e. in the VTF regime. Several explanations are considered, among them the non-Newtonian behavior of the lubricants and the adsorption of some polymers on the surface of the solid bodies.