Tribological investigation of the structural degradation of lubricating greases from an energy point of view

Abstract

Studying lubricating greases encompasses various aspects essential for understanding their behaviour and performance. Lubricating greases, such as viscoelastic lubricants, are subject to shear-induced structural degradation, defined in this thesis as “grease wear” when subjected to mechanical stress during friction. The barely understood mechanisms underlying this phenomenon are the focus of this thesis. This research work aims to achieve a quantifiable differentiation of the grease wear behaviour of pure biogenic and non-biogenic model greases to investigate, model and analytically describe these mechanisms. Various greases were investigated, including lithium, calcium, and polyurea greases, which had different base oils and viscosities. A key element of this tribological study is the energetic consideration of the friction process, which is particularly relevant for lubricating greases. Thus, one part of this thesis deals with the expended energy during the friction process regarding the shear-induced structural rearrangement of the three-dimensional network of thickeners in the base oil. In this context, a grease wear parameter (Rtee) was developed. It provided decisive insights into the structural degradation of lubricating greases due to mechanical stress. The higher the parameter value, the lower the structural degradation of the stressed grease. Consequently, greases with a higher thickener content had lower Rtee-values, which indicates more significant structural degradation in the stressed lubricating grease. Another part analyses the mechanical structural degradation induced by shear stress in rheological tests, considering the influence of thickener type, thickener composition and base oil properties. Different optical measurements were used to quantify further the structural degradation caused by the internal friction process of lubricating greases. By analysing eleven model greases under stressed and unstressed conditions using transmitted light microscopy, SEM and AFM, images showed significant differences in the structure of the lubricating greases due to the shear-induced frictional energy compared to unstressed samples. Furthermore, it was found that the type of thickener exerted a significant influence on the internal frictional energy. Lithium and calcium greases exhibited lower mechanical energy dissipation during shearing than polyurea or isolated pure biogenic greases. The differences were due to the distribution of agglomerates based on thickener particles, the size of which depended on both the thickener content and the base oil viscosity. The last part of this thesis investigates the internal friction-dependent temperature changes in the bulk of all greases studied under controlled environmental conditions with high shear stress using two measurement setups. A conventional rheometer-measuring cell and a newly developed temperature-measuring cell (Calidus) were used. At the same time, the temperature changes were linked to the lubricating grease components and the structural degradation. In particular, greases formulated with lithium and calcium soaps show the highest internal temperature profiles and drastic temperature drops with time, indicating significant shear-induced structural degradation. In contrast, a grease with a lower base oil viscosity shows the lowest maximum internal frictiondependent temperature changes and a moderate variation during the test. Within the group of pure biogenic greases, the biogenic grease B3 shows the slightest temperature changes. In summary, this thesis contributes to a comprehensive understanding of the frictional behaviour of lubricating greases, covering aspects such as grease wear behaviour, shear-induced microstructural changes, mechanical structural degradation, and internal friction-dependent temperature changes. The investigations aimed to provide insight into the frictional behaviour of different types of lubricating greases under different conditions, including comparing biogenic and non-biogenic compositions.