Atomic Force Microscopy and Thermo-Rheological Characterisation of Lubricating Greases

In this work, non-perturbed microstructures of several commercial and model lubricating greases, differing in nature and concentration of the thickener agent, were examined using the atomic force microscopy (AFM) technique. Grease microstructure mainly depends on the nature of the thickener employed and, also, on thickener concentration and viscosity of the base oil. Thermal-induced changes in the viscoelastic response of lubricating greases have been investigated by using different rheological techniques in a temperature range of 0-175 °C. Small-amplitude oscillatory shear (SAOS) measurements were carried out to determine the mechanical spectra of the different samples studied. Lubricating grease rheological thermal susceptibility was analysed by following the evolution of the plateau modulus with temperature. SAOS functions dramatically decrease, in most cases, above a characteristic temperature, which depends on nature and/or concentration of the thickener used and therefore on grease microstructure. The thermo-mechanical reversibility of grease microstructure has been studied by carrying out triple-step-shear stress tests (shear stresses inside and outside of the linear viscoelasticity range), at different temperatures. The degree of lubricating grease non-reversible structural breakdown, which increases with temperature, depends on the shear stress applied above the linear viscoelasticity limits.