Reibung und Kontaktalterung auf Metalloberflächen
Experimentelle Physik der kondensierten Materie
Zusammenfassung der Projektergebnisse
Friction, wear, and lubrication are processes of enormous ecologic and economic impact, given the loss of energy due to friction and the reduction of machine lifetime due to wear. Their fundamental understanding requires scientific investigation of microscopic mechanisms at the sliding interface. Force microscopy (AFM) has evolved into a key method to study microscopic friction processes for nanometer-scale contacts. In this project, we have applied high-resolution friction force microscopy to metal surfaces under well-controlled conditions, namely in a vacuum chamber. Here, we report two key results of the project, the excellent lubrication properties of graphene layers on a platinum surfaces and the complex interplay of friction, wear, and chemistry in sliding contacts of silicon and gold. Graphene is a single atomic layer of carbon atoms, whose strong bonds in the layer make graphene an extremely tough material. We have grown graphene on platinum surfaces and discovered that the friction is reduced to very low values and that platinum is protected by graphene against cutting by the sharp tip of the AFM. Graphene is so tough that it supports the sliding AFM tip under high loads, even for loads at which the metal starts to deform. The underlying mechanisms have been fully understood in collaboration with modeling experts at the IWM Fraunhofer Institute. With the ever increasing computing power available, our experiment at the nanometer scale could be simulated atom by atom, revealing all dissipation, deformation, and rupture mechanisms. We have also explored the directional dependence of friction on graphene and found preferential sliding on one of the crystalline directions. Finally, we studied graphene grown in different orientations with respect to the platinum crystal. Atomic-scale friction force microscopy is sensitive enough to visualize the atomic lattice of graphene and the orientation of the platinum surface underneath. We found that friction is lower for those orientations of graphene which have a stronger interaction with the platinum substrate. Contact ageing is an important phenomenon in tribology, with impact for example in geology where earth quakes are observed to be stronger if the sliding rocks had been longer in contact without sliding. Material creep is a macroscopic mechanism leading to contact ageing. With this project we have joined other research groups who recently started to investigate mechanisms of contact ageing at the interface by friction force microscopy. To our surprise, we did not find any signs of contact ageing, i.e. stronger static friction after longer contact in rest, in our experiments with oxidized silicon and gold in vacuum. We concluded that structural relaxations in the contact occur faster than we can observe by force microscopy and that in vacuum no chemical reactions contribute to contact ageing because of passivating oxide layers. Previous studies have shown that in air the oxide surfaces are chemically activated by water molecules. We have activated our surfaces in vacuum by removal of the oxide layer from silicon and started a complex interplay of friction, wear, and chemistry. These tribochemical processes tend to reduce friction by forming an amorphous material layer at the interface. Other experiments revealed material transfer to activated surfaces. We need to learn how to control tribochemical reactions before reproducible contact ageing experiments become feasible. Our results contribute to a growing number of tribological studies which find that the development of nanostructured layers at the interface is a key mechanism in friction reduction.
Projektbezogene Publikationen (Auswahl)
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Atomic Scale Mechanisms of Friction Reduction and Wear Protection by Graphene. Nano Lett. 2014, 14 (12), 7145-7152
Klemenz, A.; Pastewka, L.; Balakrishna, S. G.; Caron, A.; Bennewitz, R.; Moseler, M.
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Preferential sliding directions on graphite. Phys. Rev. B 2014, 89 (24)
Balakrishna, S. G.; de Wijn, A. S.; Bennewitz, R.
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Contrast in nanoscale friction between rotational domains of graphene on Pt(111). Carbon 2017, 113, 132-138
Chan, N.; Balakrishna, S. G.; Klemenz, A.; Moseler, M.; Egberts, P.; Bennewitz, R.
