Tribological Properties of Nanoparticles: from Superlubricity to Contact AgeingTuesday (27.09.2016) 11:30 - 11:45 Part of:
In an effort to reduce the friction between sliding components scientists and engineers have developed a multitude of lubrication schemes. In the field of dry lubrication the idea of superlubricity, i.e. the state of (almost) vanishing friction, has received widespread attention lately. One of the most intriguing concepts in this framework is referred to as `structural lubricity', where flat surfaces are thought to slide past each other virtually frictionless if their atomic structures are incommensurate. Here, we analyze the fundamental mechanisms that govern the area-dependence of friction in extended but atomically flat contacts. The resulting sublinear power laws, which link mesoscopic friction to atomic principles, are then confirmed by measuring the sliding resistance of gold and antimony particles on graphite . These findings suggest that engineering surfaces with very low friction can be realized up to mesoscopic contact areas. Furthermore, it is shown that nanoparticles can co-exist in two frictional states, exhibiting ‘frictional duality’: Some particles show linear scaling with contact area reminiscent of Amonton’s friction law while others remain in the state of structural lubricity . This duality is explained by a model of partial interface contamination. Lastly, we investigated contact ageing, detrimental in the field of earthquake modelling: The shear strength of tectonic plates is believed to increase logarithmic in time, leading to the infamous strong sudden energy dissipation events, i.e. earthquakes. Interestingly, we find similar ageing dynamics for nanoparticles, as evidenced by stick-slip movements of those objects. A complex interplay of ageing dynamics with thermally activated stick-slip friction explains the commonly observed friction peak at low temperatures . These examples demonstrate how nanoparticle manipulation by atomic force microscopy techniques can contribute to the understanding of fundamental friction processes of atomically defined interfaces .
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