Abstract
Despite its fundamental importance, physical mechanisms that govern friction are poorly understood. While a state of ultra-low friction, termed structural lubricity, is expected for any clean, atomically flat interface consisting of two different materials with incommensurate structures, some associated predictions could only be quantitatively confirmed under ultra-high vacuum (UHV) conditions so far. Here, we report structurally lubric sliding under ambient conditions at mesoscopic (∼4,000–130,000 nm2) interfaces formed by gold islands on graphite. Ab initio calculations reveal that the gold–graphite interface is expected to remain largely free from contaminant molecules, leading to structurally lubric sliding. The experiments reported here demonstrate the potential for practical lubrication schemes for micro- and nano-electromechanical systems, which would mainly rely on an atomic-scale structural mismatch between the slider and substrate components, via the utilization of material systems featuring clean, atomically flat interfaces under ambient conditions.
Highlights
Despite its fundamental importance, physical mechanisms that govern friction are poorly understood
While the predominantly straight facets exhibited by the gold islands are indicative of crystalline structure, we have performed transmission electron microscopy (TEM) experiments to directly confirm the crystalline character of the islands (Fig. 1c)
The recorded values of Fl remain below 1 nN during the entire scan line, in very good agreement with manipulation experiments performed on gold islands on graphite under UHV11, and about three orders of magnitude smaller than the results reported for antimony islands under ambient conditions[25]
Summary
Physical mechanisms that govern friction are poorly understood. While a state of ultra-low friction, termed structural lubricity, is expected for any clean, atomically flat interface consisting of two different materials with incommensurate structures, some associated predictions could only be quantitatively confirmed under ultra-high vacuum (UHV) conditions so far. Any rigid interface formed by two atomically flat, incommensurate surfaces that is free from contaminant molecules is expected to undergo sliding with ultralow friction, characterized by a sub-linear relationship between friction force and contact area[7,8,9,10]. Under vacuum conditions, there have been reports of ultra-low friction sliding at very small contacts formed by scanning probe microscopy tips[18] and the absence of static friction for adsorbed monolayers of, for example, Kr on gold surfaces as measured by quartz crystal microbalance experiments[19]
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