An energetical approach to study the structural degradation of traditional and biogenic lubricating greases

Abstract

In a tribological contact, the friction processes lead to irreversible effects associated with wear, which are the result of frictional energy input to a tribological system. In general, various types of lubricants are applied to machine elements in order to reduce friction and wear. In the last decades, environmental damages caused by chemicals and/or end-use products, such as lubricants based on petrochemically refined mineral oils, have driven the demand of natural components in a wide variety of manufactured products. In this sense, some lubricant industries have promoted the development of their products with the replacement of non-renewable raw materials by natural resources in order to avoid direct and indirect environmental pollution caused by their products. With this purpose, first, vegetable oils instead of mineral oils, and then, completely biogenic lubricating greases, which are obtained using vegetable oils, as well as by substituting traditionally used thickener agents with biogenic thickeners, have been introduced into a variety of industrial and consumer applications. In this sense, the target of this work is to investigate the tribological and rheological response of new completely biogenic lubricating greases, two reference semi-biogenic lubricating greases, and some traditional lubricating grease samples, emphasizing the energetic aspects of the friction and wear processes. With this aim, completely biogenic lubricating grease samples, which were formulated with different biogenic oils, e.g., high-oleic sunflower oil (HOSO), glycerol, castor oil, and a combination of HOSO and castor oil, and different biodegradable thickener agents, e.g., beeswax, corncob grits, natural cellulose, and lignin, have been tribologically examined with a nanotribometer and a tribometer using a material combination of a steel ball on a steel disc and compared with traditional lithium-12-hydroxystearate/HOSO and lithium-calcium soap/synthetic ester greases. In this way, it was gained a deeper understanding of the influence of the composition of these model greases on the friction and wear processes in a tribological contact. In addition, using the energy-based wear model, the friction and wear processes could be interpreted as a cause-effect sequence. In addition, rolling bearing tests were performed to investigate the tribological behavior of some selected biogenic greases during real machine element contact and to assess their lubrication performance. These real application tests allowed for the quantification of the friction torque of the full bearing at different operating conditions and the wear produced through lubricant analysis procedures, i.e., ferrometry and ferrography. Moreover, the model greases were rheologically investigated by means of rotational transient flow measurements 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. Furthermore, correlations between the activation energy and structural degradation of some traditional lubricating grease samples under different shear situations were studied by means of rotational transient flow and oscillation amplitude sweep tests. This experimental work provided useful information for a better understanding of the shear-induced structural degradation process in greases which can facilitate the improvement of the service life of lubricating greases. Overall, a number of new biogenic lubricating greases completely based on renewable raw materials, two reference semi-biogenic lubricating greases, and some traditional lubricating grease samples were investigated for a better understanding of grease lubrication mechanisms in tribological contacts. All results were also analyzed from an energetic point of view which can help to qualify these biogenic lubricating greases as real alternatives to conventional lubricating greases.