Atomic Scale Mechanisms of Contact Ageing

• Applicant: Dr. Dirk Dietzel
• Subject Area: Experimental Condensed Matter Physics
• Term: from 2018 to 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 403024866

The term contact ageing describes a fundamental tribological effect, where significant changes in friction are associated with the contact time between substrate and slider. Contact ageing can manifest itself by an increase of static friction with time, while at the same time also changes in sliding friction can be linked to contact ageing by considering a dynamic contact time depending on the sliding velocity. Both effects can be important for technological and ecological systems. Pronounced static friction peaks resulting from continuous contact can lead to increased wear and also the alternating static and dynamic phases of seismic activity can be correlated to contact ageing effects.As a consequence, scientists and engineers are have long since been trying to incorporate contact ageing into tribological models. In most cases, these models associate contact ageing with a continuous increase of the effective contact area, that is formed by connecting surface asperities. However, it has to be taken into account that ageing processes are not necessarily restricted to an increasing overall contact area. Instead, contact strengthening of existing contacts can also play an important role. Here, especially atomic scale processes are relevant, like e.g. the formation of chemical bonds or structural relaxation on the atomic level. However, these processes not yet well explored or understood and therefore contribute to the problem, that contact ageing usually needs to be described by phenomenological models.The main goal of this project now lies in experimentally analyzing the atomic scale mechanisms of contact ageing. Based on theoretical considerations we can anticipate major influences by different key parameters like temperature, load or shear stress. By systematic variation of these parameters for a variety of different material combinations, we will be able to identify the different ageing channels and assess their relevance for technologically relevant systems. In doing so, this project also approaches the fundamental questions if atomic scale ageing processes are compatible with the established concepts of thermal activation and how mechanical stress can influence the interface evolution on the atomic level. All these questions will be approached by experimental techniques based on scanning probe microscopy, where a novel approach, that was developed especially for this project, allows the analysis of contact ageing without the need to consider the complex processes during contact rupture.A second part of the project then deals with active control of nanoscale contact ageing. Based on the results of the first part, different means of mechanical actuation will be applied to the interface in order to increase or suppress contact ageing. Both aspects are highly relevant for nano- or microelectromechanical systems (NEMS, MEMS), but have not yet been analyzed systematically and must therefore be considered unsolved technological problems.

• DFG Programme: Research Grants