Structural lubricity under ambient conditions

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.

Chemical effects on the sliding friction of Ag and Au(111)

“…In Ref. [9] the sliding of gold nano-island deposited on graphite was reported to yield similar friction values as in UHV-conditions, thus pointing at the importance of chemical inertness of the tribological couple and the abesence of contamination at the sliding interface for superlubricity. The effect of tip chemistry has been inferred as the coefficients of friction measured on Pt(111) with a SiO x AFM-tip or a diamond AFM-tip differed by an order of magnitude [10].…”

“…The growth of thin metallic films onto silicon devices for operation in reciproc motion appears to be a feasible alternative. Recently, the nanoscale friction and wear behavior of metals has been investigated [5][6][7][8][9][10]. In the low load regime where adhesive force prevails, friction has been reported to vanish and the sliding of a slingle asperity onto single crystalline metallic surfaces was characterized by atomic stick-slip [5].…”

“…Further, the authors demonstrated the role of passivating selfassembled monolayer in suppressing the formation of such a neck and the switching of nanoscale friction on Au(111) by electrochemical control. Moreover, the nanotribology of thermanlly deposited gold on graphite was studied [8,9]. In contrast to single-asperity friction measurements performed in ultra-high vacuum (UHV)-conditions in Refs.…”

Current trends in the physics of nanoscale friction

“…Until recently, it has been demonstrated or implied in a relatively small number of cases [29,[42][43][44][45][46]. There are now more evidences of superlubric behavior in cluster nanomanipulation [32,33,47], sliding colloidal layers [48][49][50], and inertially driven rare-gas adsorbates [51,52] (see Fig. 2).…”

“…on the contact area was carried out recently for nanosized metal clusters on graphite in ultraclean and even atmospheric conditions [32,33], whose results indicate a size dependence of kinetic friction which is scattered. For certain clusters a regular linear scaling with area is observed, while others can be grouped in sets compatible with sublinear scaling, which match the expectations for structural lubricity [34][35][36][37][38][39], sometimes also called superlubricity.…”