Tribological and Rheological Characterization of New Completely Biogenic Lubricating Greases: A Comparative Experimental Investigation

Against the background of raw material shortage and the ever-expanding environmental consciousness, the use of biodegradable greases becomes more and more important. The aim of this experimental work is to investigate the tribological response of completely biodegradable greases. Complete biodegradable lubricating greases were formulated with high-oleic sunflower oil (HOSO) and/or castor oil, and different biodegradable thickener agents such as natural cellulose fibers of different lengths and some derivatives, as well as glyceryl and sorbitan stearates. To investigate the friction process, the model greases were tribologically examined with a nanotribometer at a normal force of 200 mN using a material combination of a steel ball on a steel disc. All frictional results, along with the volumes of wear tracks and micrographs of the main contacting area on the steel plate, are presented and discussed. In addition to this, rotational transient flow measurements were carried out on a rheometer at different temperatures to monitor the evolution of the shear stress with time at a constant shear rate, and to characterize the internal friction behavior by quantifying the energy density. All results were also analyzed from an energetic point of view