Beyond scratching the surface : intrinsic tribological performance of polymers

Abstract

Quantifying and understanding friction and wear behaviour of any type of material remains a challenge to this day. This is also true for polymers that are used frequently in sliding applications. This thesis focuses on the development of quantitative measurement techniques that can be used to understand friction and wear of polymers. To understand the influence of material properties on friction and wear behaviour it is necessary to zoom in on the relevant processes in a sliding contact. A macroscopic contact between two surfaces typically consists of multiple contacts between roughness peaks. These micro-contacts make up the real contact area which is usually a small fraction of the apparent contact area, and which depends on the mechanical properties of both surfaces as well as on the loading conditions. The friction force measured in experiments is the product of this real contact area and an average effective shear stress. Because the real contact area is difficult to control and measure for macroscopic contacts such contacts are not very useful in separating the contributions to the friction force. In contrast, single asperity techniques offer the possibility to control independently the contact area and normal load and therefore offer a way forward to a critical interpretation of measured friction forces. In the work described in this thesis microscopic tribological single–asperity experiments are used to study structure-property relations. These single asperity experiments are performed using the Lateral Force Apparatus that was drastically modified to better suit this purpose. A new driving system was developed that allows friction measurements in which the sliding velocity may be varied across 5 orders of magnitude with accurate position control. This combination makes it possible to perform single–asperity measurements at widely differing speeds which are shown to be important for the interpretation of sliding friction on polymers. Accurate position control is shown to be crucial in developing advanced wear measurement techniques. In sliding friction distinction between the contribution of contact area and effective xi xii SUMMARY shear stress to the friction force is a key issue. Depending on mechanical properties and loading conditions, all materials exhibit creep on a characteristic time scale. In polymers creep is especially relevant since the associated timescales are relatively short. In single asperity friction the asperity radius and sliding speed set a contact time, during which the contact area may evolve by creep. It is shown that the contributions of contact area and effective shear stress can be distinguished from one another using single–asperity measurements at widely differing sliding velocities. In the study of wear the interpretation of measurements on macroscopic multi– asperity contacts also pose problems since they consist of a collection of microcontacts between deformed asperities. Since the strain at failure of a polymer is expected to be an important factor in determining the wear of polymers the unambiguous strain distribution of a single asperity contact is an advantage in the study of structure-wear relations. In this thesis a novel single–asperity technique to measure wear rate is developed. In this method the wear rate is measured in real time. The method is fast, uses very little material, and yet gives good statistics and a strong correlation with macroscopically measured wear rates. In a study on PE it is found that the wear rate is related to the molecular weight. Quantitative single–asperity measurements are a critical step in understanding structure-tribology relations. While macroscopic tribological experiments can only scratch the surface of structure-tribology relations, single–asperity techniques probe the material properties lying underneath.