The Role of Grease Composition and Rheology in Elastohydrodynamic Lubrication

Abstract

Grease lubrication is inherently complicated due to its non-Newtonian flow dynamics. The flow behavior of grease is difficult to determine under tribological shear rates as these shear rates are often extremely high, and therefore, are not accessible on a regular rheometer. In this work, we demonstrate the use of shear rate-concentration superposition method to extrapolate the data measured with a rheometer to tribologically relevant shear rates. This method is similar to the other superposition methods routinely employed by rheologists for predicting flow behavior beyond the measurement range of rheometer. In this method, a data master curve, which extends over a broad range of shear rates, is constructed by superposing individual flow curves for greases with different thickener concentrations. The master curve is utilized to determine infinite shear viscosity of grease which is then used for the film thickness estimation. Furthermore, the grease samples are tribologically tested using a four-ball tester to identify the individual and interactive roles of the thickener type, concentration and base oil in lubrication behavior. A change in thickener concentration or type influences frictional torque and wear scar size through modification in infinite shear viscosity and structural stability of grease. The SEM images show deposition of thickener particles on the contact surface, which also apparently affect friction and wear during the test. The lubrication performance is strongly influenced by base oil, which points towards the importance of its role in dictating flow resistance and film-forming behavior of grease inside elastohydrodynamic contact.