Friction is a fundamental mechanical phenomenon present in all technological applications that involve moving parts ranging from bearings in wind turbines down to accelerometers in mobile phones. While friction is well described from an engineering point of view on the macroscopic scale, microscopic friction is still subject to extended investigations. In particular the sliding of atomically flat interfaces is of interest, since these interfaces can show the effect of almost vanishing friction, also coined superlubricity or structural lubricity. The effect arises from an aperiodic relationship of the interfaces and was demonstrated previously for several materials including graphite  and gold . However the microstructure evolution during sliding as well as effects like contact aging in these interfaces are still not fully understood yet.
In this work we present a method based on in situ scanning electron microscopy (SEM) mechanical testing that allows for the measurement of friction forces in micrometer-scale atomically flat contacts. To generate these interfaces we are using a layered van-der-Waals crystal (VSe¬2), which is structured using focused ion beam (FIB) milling. The prepared pillar structures are subsequently tested using a micromanipulator as shown in figure 1 (a). The Force is measured by putting the sample on a deflectable spring table and tracking the displacement via digital image correlation. By inducing a rotation between both parts of the interface, incommensurate configurations showing superlubric behavior were measured. In this configuration the kinetic friction force becomes much lower than the restoring van-der-Waals force also enabling the calculation of the fundamental adhesion energy of VSe2 layers (0.18 J/m²). Additionally contact aging was observed which causes a drastic increase in static friction after a sufficient waiting time between experiments as demonstrated in figure 1 (b).
By using a FIB lift-out technique we are able to extract the sliding interface after testing and investigate it using transmission electron microscopy (TEM) as shown in figure 1 (c). With TEM the interface can be analyzed regarding defects and misfit between the two sides of the interface. First in situ STEM experiments have also been conducted showing the interface not only after, but also during sliding.
 M. Dienwiebel, G. S. Verhoeven, N. Pradeep, J. W. M. Frenken, J. A. Heimberg, H. W. Zandbergen: “Superlubricity of Graphite“, Physical Review Letters, 92 (12), 2004
 D. Dietzel, M. Feldmann, U. D. Schwarz, H. Fuchs, A. Schirmeisen: “Scaling Laws of Structural Lubricity“, Physical Review Letters, 111, 2013
|Category||Short file description||File description||File Size|
|Manuskript||Figure corresponding to the Abstract||Figure 1: a) SEM image taken during an in situ experiment showing the testing principle. The arrow denotes the direction of movement. b) Force-displacement curves measured for an incommensurate interface showing the effect of increasing friction after a waiting period (blue curve). c) Plan-view TEM image of a lamella prepared from an interface after testing used to determine the slip direction and orientation with the help of electron diffraction (inset) as well as analyze defects.||509 KB||Download|