Rheological Behavior of Lubricating Greases at High Temperatures

Abstract

The rheological behavior of lubricating greases was studied in the temperature range from room temperature to about 250°C by using a concentric cylinder-type viscometer, which is more convenient for measurements at high temperatures than the widely used ASTM viscometer of capillary type.
The following results were obtained:
(1) Lubricating greases exhibited the Ostwald flow; that is, the viscosity was independent of the rate of shear in both regions of extremely low shear rates and extremely high shear rates.
(2) The rate dependence of viscosity was observed within a rather narrow shear rate range at high temperatures.
(3) The relative viscosity of each soap-thickened grease started to decrease as the temperature was increased beyond a certain value. This behavior was attributable mainly to the rupture of structural linkage of thickener micells or to the rupture of micells themselves, judging from the observation of electron micrographs.