Experimental Study on the Expended Energy on Structural Degradation of Lubricating Greases

Abstract

One of the keystones of tribological studies is the energetical approach to the lubrication process. In the particular case of lubricating greases, part of the lubrication process's energy dissipates due to a shear-induced structural rearrangement of the 3D network of the thickening agent dispersed in the base oil. This fact confers them a particular consistency, mechanical stability, rheological and tribological behaviour. In this research work, we investigate the mechanical structural degradation induced by shear stress applied in rheological tests (rotational and oscillation mode) and the influence of thickener (type and composition) and base oil on both the degradation process and the expended mechanical energies. For this purpose, lithium, calcium and polyurea-based greases of NLGI grade 2 were used. These greases have been manufactured with a different base oil (mineral, synthetic and vegetable oils) and kinematic viscosity of 48 or 240 mm2/s. Some biogenic greases were also included in this research. The optical microscopy analysis revealed thickener particles-based agglomerates with different shapes and sizes that reduced notably, if not almost completely destroyed, after stress. Due to the thickener particles-based agglomerates distribution, significant differences in the shear-induced frictional energy inside the bulk grease during the shear process were detected. The size of agglomerates depended on both the thickener content and the base oil viscosity and not the type of base oil.