On the shear-induced structural degradation of lubricating greases and associated activation energy: An experimental rheological study

Abstract

Abstract The stability of the thickener microstructure in lubricating greases influences their ability to reduce friction and protect the lubricated surfaces in a tribological system. Shear-induced degradation of the thickener network in lubricating greases that occurs as a result of reduced particle interactions can be considered a process associated with exceeding a critical energy level, i.e. the activation energy. The aim of this experimental study was to investigate the relationship between the activation energy, which represents the amount of energy required to produce shear-induced structural degradation, and the extent of the structural degradation that occurs under different shearing conditions. To this end, rotational transient flow tests were carried out in a rheometer at varying temperatures (25–70 °C) and a constant shear rate to quantify the activation energy of six model greases. The model greases differed in the nature and concentration of the thickener used, and the activation energy was calculated by fitting the rheological energy density to the Arrhenius equation. The activation energy was found to depend on the nature and concentration of the thickener in the greases; higher thickener concentrations resulted in lower activation energies. In addition, oscillation amplitude sweep tests were performed to obtain information about the early stages of shear-induced structural degradation. Lower activation energies were found to result in an earlier onset of shear-induced structural degradation. Finally, correlations between the activation energy and structural degradation of the model greases under different shear situations were presented and discussed. This enhanced understanding of the shear-induced degradation will facilitate the improvement of the service life of lubricating greases.