24th International Colloquium Tribology - January 2024 89 Atomistic Insights into the Behavior of Solid Lubricants Under Tribological Load Andreas Klemenz 1* , Michael Moseler 1,2 1 Fraunhofer Institute for Mechanics of Materials IWM, Micro Tribology Center µTC, Wöhlerstraße 11, 79108 Freiburg, Germany 2 Institute of Physics, University of Freiburg, Hermann-Herder-Straße 3, 79104 Freiburg, Germany * Corresponding author: andreas.klemenz@iwm.fraunhofer.de 1. Introduction The optimization of technical lubrication systems often follows the trial-and-error approach, which frequently reaches its limits. A fundamental understanding of the underlying physical processes can simplify this task or even open up completely new perspectives. Atomistic simulations have proven to be a valuable tool in this regard over the last decades. This contribution presents two examples of the investigation of fundamental tribological processes using a combination of experiments and molecular dynamics simulations. 2. Mechanisms of Graphite Lubrication Graphite is one of the oldest dry lubricants in technical use. A popular explanation for its lubricating properties is based on the lamellar structure of graphite. Since the layers interact with each other only by weak van der Waals forces, it is assumed that they can move against each other like in a deck of playing cards. This model provides a simple explanation for the lubricating properties of graphite. However, it has been known since the 1930s that graphite loses these properties in dry environments. Various subsequent approaches to explain this behavior, such as the saturation of dangling bonds on graphite planes or the increase of the interlayer distance due to water intercalation have failed to provide a satisfactory explanation, and the mechanisms underlying graphite lubrication are still not completely understood. Tight-Binding Molecular Dynamics simulations are employed to investigate the influence of water on the lubricating properties of graphite. In an extensive parameter study, the amount of water in the gap and the pressure on the systems are systematically varied. The behavior of the system is dominated by two different effects: When there are only small quantities of water in the gap, the surfaces tend to cold weld, resulting in high friction (Fig. 1). When the amount of water is sufficiently high, the system behavior changes and continuous water films form between the surfaces, accompanied by low friction. An estimate of the typical amount of water present on the surfaces due to condensation from ambient humidity suggests that graphite will be predominantly in the low friction regime under normal laboratory conditions. Occasionally, the contacts may run dry and cold weld locally. These results explain the good lubricating properties of graphite under normal laboratory conditions. The results also provide an explanation for experimentally observed structures of graphite layers subjected to tribological load. TEM investigations revealed the formation of layers of turbostratic carbon at the sliding interfaces, which are not included in conventional models of graphite lubrication and can be explained by the occasional cold welding of the surfaces. Fig. 1: Typical atomic scale structures of graphite under tribological load. Under high pressure and with a low amount of water in the gap, the surfaces typically cold-weld. With increasing amounts of water, aromatic structures can form at the contact, leading to a drastic decrease of friction forces. If the amount of water becomes large enough, continuous water films can form, separate the surfaces and lead to low friction. 3. Degradation of Carbon Nanotubes Under Tribological Load Carbon nanotubes (CNTs) are not among the typical dry lubricants. However, by coating surfaces with CNT films, a reduction in friction can be achieved in many cases. In tribometer experiments with CNT coated iron surfaces and subsequent TEM investigations, degradation of the CNTs can be observed after a short period of time. Since CNTs are mechanically very stable, the question arises by which mechanisms this degradation takes place and what influences it. Fig. 2: SEM images of CNT coated iron surfaces. (Images taken from Ref. [2]) 90 24th International Colloquium Tribology - January 2024 Atomistic Insights into the Behavior of Solid Lubricants Under Tribological Load To investigate these questions, layers of CNTs were simulated under tribological load using classical molecular dynamics. Both, the structure of the CNTs (diameter, number of inner walls) and the structure of the coatings are varied. The degradation of CNTs clearly depends on their structure. CNTs with a large number of inner walls are more resistant to mechanical deformation than CNTs with a small number of walls, which is reflected in higher pressures required to induce damage in a layer of CNTs. In addition to the structure of the individual CNTs, the structure of the film as a whole also influences its stability under tribological load. Experimental CNT coatings often have irregular structures with randomly oriented CNT axes and a significant amount of empty space between the tubes (Fig. 2). Different CNTs therefore only contact each other at a small number of points. The contact pressures at these points are considerably higher than the macroscopic pressures applied to the coatings. Therefore, the pressures necessary to induce the first damages in the CNTs depend on the structure of the coatings and the resulting density of CNT contact points. Atomistic simulations with model systems for the coating structures show that these pressures can be easily reduced by more than one order of magnitude compared to a close packing of CNTs (Fig. 3). Fig. 3: Atomistic simulations show that a reduced density of contact points between CNTs can result in a decrease of the pressures necessary to damage the CNTs by more than one order of magnitude. The results obtained from the simulations are used to develop an analytical model that describes the onset of film degradation as a function of the CNT radius and the number of inner walls. Comparison of the model with experimental data shows good agreement between the model and the degradation of CNT films in tribometer experiments. References [1] C.E. Morstein, A. Klemenz, M. Dienwiebel, M. Moseler, Nature Communications 13, 5958 (2022) [2] T. MacLucas, A. Klemenz et al., ACS Applied Nano Materials 6, 1755 (2023)