Tribological performance of greases – different tribological test modes Ameneh Schneider, Optimol Instruments Prüftechnik GmbH, Munich/Germany Mathias Woydt, Matrilub, MATRILUB Materials Ι Tribology Ι Lubrication, Berlin/Germany
Abstract: Lubricating greases can be used in a wide range of applications and conditions, and they are gaining more importance for e-mobility. Lubricating greases and lubricating oils behave differently in tribo¬logical contacts. The complex tribological characteristics of greases have been discussed through the last decades; the effect of thickeners on tribological performance has been the core of many publications. The aim of this study is to investigate the influence of test parameters and modes on the tribological response of differently formulated greases. Challenges such as modeling the operating conditions in the field with the test environment are discussed. Special emphasis is placed on types of motion, velocity, and temperature as well as test geometry. In addition to the coefficient of friction, the associated wear volumes are taken into consideration. 1. Introduction Greases are the most versatile and functionalized lubricants, and they can be used in a wide range of applications and conditions. There are various lubricating grease types with complex behavior depending on the type and amount of thickener, additives, and base oil viscosities. Lubricating greases and lubricating oils behave differently in tribological contacts. The complex tribology of greases has been discussed through the last decades. In particular, the effect of thickeners on the tribological performance has been emphasized [1,2,3]. It has also been concluded that the condition of full lubrication cannot be expected in highly stressed contacts, and the formation of the film differs depending on the type of thickener (e.g., with or without fibrous portions). J. Wang et al. [2] explained that the mean and minimal film thicknesses stabilize after hundreds of cycles, independent from the total speed. They also explained that film formation by greases in rolling and sliding contacts can be influenced by temperature and surface roughness of materials. K. Kanazawa et al. [3] suggested that for slowly running conditions, the grease behavior is influenced by the thickener, while for fast running conditions, the base oil is more critical than the thickener. By adding solids (e.g., graphite, Mo2S, or PTFE) to greases to improve the tribological properties, the rheological properties, especially the value of the shear stress, are influenced. The extent of this influence depends, again, on the type of grease thickener [4]. Patzer explained results from a tribological study using the translatory [translational] oscillation tribometer SRV® (Schwing (oscillation), Reib (friction), Verschleiß (wear)) and different standardized test methods to evaluate greases that meet application, economic, and ecological targets [5]. Furthermore, investigations of wear scars and comparisons of effective coefficients of friction (COFs) for different formulations can elucidate a deeper understanding of the tribological complexity of lubricating greases. Fathi-Najafi et al. [6] investigated the impact of high viscosity naphthenic base oil on different thickeners by using an SRV® tribometer [6]. Weber et al. discussed the importance of test parameters when using DIN 51834, part 2 and ASTM D7594 (fretting tests) to develop highperformance (HP) greases [7]. The future of grease development is going towards energy efficient greases. By optimizing the lubrication of the bearings, theoretical improvements of energy efficiency are predicted. -8NLGI Spokesman | VOLUME 85, NUMBER 5 | November/December 2021
