Lubricant greases: How rheology affects tribology
Lubricant greases play an important role in the conception as well as the service life of lubricated components. The current study focuses on evaluation of the visco-elastic, as well as frictional behavior of greases and establishing a correlation between the same. The tests were carried out on a rheometer-tribometer combination equipped with a highly sensitive air-bearing. This facilitates an in-depth investigation of the greases, all the way from their behavior in the static friction regime, through boundary and mixed friction, to their film forming properties in the fluid friction regime.
In order to investigate the impact of visco-elastic flow properties on the tribological behavior of greases, model-scale tribometric tests were carried out with ball-on-three-plates test configuration. It is shown here that the oscillatory amplitude or torque sweeps that are well suited to investigate the visco-elastic behavior and consistency of lubrication greases, are also quite suitable to determine the static friction properties of a tribosystem with grease as a lubricant. Valuable information on the visco-elastic behavior, i.e. the storage and the loss moduli as well as on the stress values at the flow point/break-away is obtained. Furthermore, the shear dependent impact of greases on film formation in a model tribosystem is investigated with the help of Stribeck curves. The investigations aided in establishing a strong correlation between the rheological and tribological properties of greases.