Torque and Twist against Superlubricity

Abstract

Superlubricity between incommensurate surfaces provides a desired low-friction state essential for the function of small-scale machines. Here we demonstrate experimentally and theoretically that superlubricity in contacts lubricated by lamellar solids might be eliminated due to torque-induced reorientation coupled to lateral motion. We find that the possibility of reorientation always leads to stabilization of a high frictional state which corresponds to a commensurate configuration. DOI: 10.1103/PhysRevLett.100.046102 PACS numbers: 68.35.Af, 07.79.Sp, 46.55.+d, 68.37.Ps The practical importance and the relevance to basic scientific questions have motivated studies towards understanding the conditions which lead to superlow friction. A mechanism for superlow dry friction, which arises from the structural incompatibility of two contacting solids, was first suggested by Hirano and Shinjo [1,2]. This phenomenon is also referred to as superlubricity. However, incommensurability of the interfaces results in a cancellation of only one of the channels of energy dissipation, which originates from a stick-slip instability. Other dissipative processes, such as electronic and phononic friction, etc., still persist, and therefore even in the case of complete incommensurability the net friction force will not be identical to zero. Nonetheless, typical to the superlubricity state is a reduction of the friction force by orders of magnitude.Detailed experimental studies of superlubricity have been performed recently by , who measured friction between a graphite flake attached to the tip of a frictional force microscope (FFM) and an atomically flat graphite surface. Superlow friction forces (<50 pN) have been found for most relative orientations of the flake and the substrate, for which the contacting surfaces find themselves in incommensurate states. For narrow ranges of orientations corresponding to commensurate contacts, stick-slip motion was observed and friction was high (typically 250 pN). A few earlier experiments [6,7] also provided indications of the superlubricity phenomenon in dry friction.Measuring friction between graphite flakes and an underlying graphite surface we found that the ''lifetime'' of a superlubric state can be finite and therefore, superlow friction does not persist. We demonstrate that the dominating contribution to such residual friction in contacts lubricated by lamellar solids may stem from torque-induced reorientation of the flakes attached to the tip. Dynamics of the flake reorientation result in irregular sharp peaks of the lateral force shown in Fig. 1, which are observed during the restoration of the high friction state. The lifetime may depend on the structure of the tip and the density of defects on the graphite surface. Our calculations show that the possibility of flake rotation always stabilizes the high frictional state which corresponds to the commensurate configuration of the contacting surfaces in relative motion. Reorganization of a sliding layer leading to a locking in a ''comme...

Microscale Motion Control through Ferromagnetic Films

“…Specifically at the nanoscale, peculiar means to control friction would be available, for instance the commensurability of the crystal lattices of the sliding nano-objects and the onset of a superlubric state [25]. However, once the materials have been chosen, it is hard to modify the contact geometry and no efficient friction control can be achieved [12,13]. One possibility to circumvent this difficulty is to tune the material properties through some external parameter.…”

Observation of kinks and antikinks in colloidal monolayers driven across ordered surfaces

“…In contrast, at incommensurate interfaces, atomic friction studies revealed a superlubrication regime, where the friction coefficient vanishes 15,16 . This behaviour can be explained by simple mechanical models such as the FrenkelKontorova model, a generalized Prandtl-Tomlinson scheme or the double-chain model [17][18][19][20] . In the Frenkel-Kontorova approach the interface between two solids is described by a monolayer of elastically interacting beads on a periodic substrate potential [5][6][7][8] .…”

Current trends in the physics of nanoscale friction

“…Superlubricity is experimentally rare. 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.…”