Negative Friction Coefficients in Superlubric Graphite–Hexagonal Boron Nitride Heterojunctions

Abstract

Negative friction coefficient, where friction is reduced upon increasing normal load, is predicted for superlubric graphite/hexagonal boron nitride heterojunctions. The origin of this counterintuitive behavior lies in the load-induced suppression of the moiré superstructure out-of-plane distortions leading to a less dissipative interfacial dynamics. Thermal induced enhancement of the out-of-plane fluctuations leads to unusual increase of friction with temperature. The highlighted frictional mechanism is of general nature and is expected to appear in many layered materials heterojunctions.Energy dissipation, wear, and the ensuing failure of moving components are problems encountered in many human activities. Reducing friction is of particular importance in microscopic and nanoscopic mechanical devices [1] , where the damaging consequences of local heating are amplified by the large surface-to-volume ratio. Standard lubrication schemes may fail in these extremely confined conditions, which calls for novel alternative solutions [2], such as dry solid coatings. In this approach, frictional forces are reduced thanks to the effective cancellation of lateral interactions occurring between incommensurate rigid crystalline surfaces. This phenomenon, often termed structural superlubricity, was first proposed theoretically a few decades ago [3] as a way to achieve extremely low friction coefficients [4][5][6][7]. Despite the fact that structural superlubricity has been observed in different material contacts [8][9][10][11][12], its implementation in practical solid/solid lubrication schemes remains a challenging task. Nevertheless, recent breakthrough demonstrations [13][14][15][16][17], based on van-der-Waals heterojunctions, suggest that this goal may be within our reach.Among the family of layered compounds, junctions formed between graphite and hexagonal boron nitride (h-BN) have been predicted as promising candidates to achieve robust superlubricity [18,19]. Figure 1: (a) The model heterojunction: a graphene monolayer aligned over a four-layers thick h-BN substrate with rigid bottom layer. Carbon atoms are colored according to their vertical position with respect to the average basal plane of the graphene layer, where blue and red correspond to the maximum and minimum values attained, respectively. The scale bar is the same as in panel c. (b) Colored map of the average carbon-carbon distance (in units of the equilibrium bond-length) after relaxation of the graphene layer over h-BN at zero normal load. (c) The corresponding colored map of the out-ofplane distortions of graphene. (d) Schematics of the simulations setup: the graphene layer is attached to a rigid stage moving at fixed velocity of stage along the x direction. Driving forces are exerted via identical springs of elastic constant ∥ acting only parallel to the substrate. No forces are exerted by the springs in the normal direction. The perpendicular position of the stage is shifted vertically for clarity of the presentation. Carbon, boron, and nitrogen atoms ar...

Negative friction coefficient in microscale graphite/mica layered heterojunctions

“…Very recently, NFC is found in superlubric graphite/h-BN heterojunctions using MD simulations. It originates from the load-induced suppression of the out-of-plane distortions in Moiré superstructure, leading to a less dissipative interfacial dynamics (7). However, in this case, the friction increases as temperature increases, which is qualitatively different from our experimental observations showing negative dependence.…”

Structural lubricity in soft and hard matter systems

“…If contact pressure may in general favor interface locking and friction, particularly acting on the compliant edge boundaries of a finite-size system, on the other hand, an increase in normal load can also suppress out-of-plane surface deformations, especially relevant for 2D layered materials, thereby inhibiting a dissipation channel and leading to a negative friction coefficient 98 . Most tribological systems show a logarithmic dependence of friction on the sliding velocity, which results from thermally activated crossing of sliding potential barriers 3 .…”

Dramatic effect of a transverse electric field on frictional properties of graphene

“…This electrostatic attraction phenomenon has been observed in . It is known that the adhesive contact normal force can lead to a negative coefficient of friction [38], which has recently been reported by experiments [39,40] and simulations [41] as a remarkable microscopic phenomenon. For a mult-layered graphitic system, the effect of a transverse electric field can be expected to be more pronounced due to further induced opposite-signed charges in the layers beneath the surface.…”