Correlation between the Film Forming Ability and Rheological Properties of New and Aged Low Sulfated Ash, Phosphorus and Sulfur (Low SAPS) Automotive Lubricants

Abstract

The rheological behavior of fresh and aged diesel low sulfated ash, phosphorus and sulfer (Low SAPS) SAE 5W-30 lubricants was studied with an elastohydrodynamic (EHD) tribometer and with low shear and high shear viscometers. The film thickness in the EHD contact, which varied between 10 and 150 nm, displayed two types of lubrication mechanisms according to rolling speed. On the one hand, a classical EHD behavior, controlled by viscosity, was observed above some critical film thickness. On the other hand, a steep drop of film thickness is an indicator of the inability of the viscosity improver polymer to form a thin film within the contact, as well as of the influence of the particle sizes contained in the lubricants. Indeed, there was a correlation between the critical film thickness and particle size. Thus, the aged lubricant implies a critical film thickness higher than that of the new lubricant. Furthermore, the lubricant friction properties are related to this critical film thickness in some conditions.

A characteristic of the tested lubricants was their strong non-Newtonian behavior, which was highlighted in the tribological experiments. These observations are connected to rheological measurements carried out at shear rates ranging from 10 to 10⁷s⁻¹ and temperatures between 25 and 150°C. A rheological modeling, based on the Cross model coupled to a dimensionless approach, was developed to calculate the relevant rheological parameters such as polymer relaxation time and to study the temperature dependence of the non-Newtonian effect.

KEY WORDS:

• Automotive
• Diesel Engine Oils
• Oil Degradation
• EHL with Non-Newtonian Lubricants
• Rheology
• Non-Newtonian Behavior
• Friction

ACKNOWLEDGMENTS

The authors are deeply grateful to PSA Peugeot Citroën and TOTAL companies for their financial and technical support. We would also like to acknowledge PCS Instruments for having performed high-temperature and high shear rate viscosity measurements.

Review led by Selda Gunsel