Correlation Between Film Forming Ability and Rheological Properties of New and Aged Lowsaps Automotive Lubricants

The rheological behavior of fresh and aged diesel LowSaps 5W30 lubricant, is studied with both ElastoHydroDynamic tribometer, and low shear and high shear viscometers. The evolution of the film thickness in EHD contact, that varies between 1nm and 150 nm, displays two types of lubrication process according to rolling speed. On the one hand, a classical EHD behaviour, controlled by viscosity, may be observed beyond 35 nm with a significant shear thinning effect in the range of temperatures. On the other hand, below 35nm, a steep drop of film thickness indicates the unability of the viscosity improver polymer to form a thin film within the contact. These observations are closely related to rheological measurements carried out in a shear rate range from 10s⁻¹ to 10⁷s⁻¹, and temperatures between 25°C and 150°C. Indeed, analysis of two rheological parameters, polymer relaxation time and hydrodynamic radius, may explain non newtonian and thin film effects in the EHD experiments. A rheological modelling based on the Cross model is developed to calculate these parameters. Then, the principle of time-temperature equivalence is used to map the characteristics of lubricants. Finally, the approach coupling EHD tribometer\Viscometer is applied to lubricants which have been degraded during a light duty diesel engine test.