Experimental advances in superlubricity

Abstract

Superlubricity, or structural lubricity, is a state that has two contacting surfaces exhibiting no resistance to sliding. This effect has been theoretically described to be possible between two completely clean single crystalline solid surfaces. However, experimental observations of superlubricity were limited to nanoscale and under high vacuum or inert gas environments even after twenty years since the concept of superlubricity has been suggested in 1990. In the last two years, remarkable advances have been achieved in experimental observations of superlubricity ranging from micro-scale to centimeters and in ambient environment. This study aims to report a comprehensive understanding of the superlubricity phenomenon.

Stiffness-dependent interlayer friction of graphene

“…This is consistent with the previous studies on superlubricity in graphite or graphene [6,13,14,17,18,20,22] which showed that superlubricity may occur in incommensurate contacts. However, for a very soft substrate (e.g., with a support stiffness of k=0.05 nN/nm, nearly suspended),…”

“…This means that a soft substrate (e.g., with a support stiffness less than 0.05 nN/nm) may impede superlubricity, further indicating that, besides the commensurability [13,14,18,20,22,23], normal load [21], flake size [18,21], etc., the stiffness too is a dominant parameter for superlubricity. The physical mechanism of the friction in the commensurate case higher than that in the incommensurate case is correlated with the stick-slip behavior of the flake on the substrate.…”

“…In recent years, the friction properties of graphene and its allotrope, carbon nanotubes, have been extensively investigated [19,20]. These studies showed that the friction between graphene layers is significantly influenced by the normal load [16,21], commensurability [12-18, 22, 23], contact edge [21,[24][25][26][27], surface conditions [28][29][30][31][32][33], and environmental conditions such as humidity [34], etc.…”

“…The friction force generally increases with increasing the normal load because it reduces the interfacial spacing and thus increases the interfacial shear resistance [16,21]. When two graphene sheets incommensurately contact with each other, the interlayer friction is substantially lower than those in the commensurate contact [12][13][14][15][16][17][18], even achieving superlubricity, due to the fact that the lattice mismatch can lead to an ultrasmooth potential energy surface and cancellation of the shear forces [20,22]. The edge barriers induced by the contact edge may enhance friction energy dissipation, which consequently results in an edge effect on the nanoscale friction -it depends not only on the contact area [18,21] but also on the contact edge [21,[24][25][26][27].…”

Ballistic Diffusion in Polyaromatic Hydrocarbons on Graphite

“…[1][2][3][4] The high economical impact of wear, friction (energy dissipation) and lubrication in industry and transport 5 has fostered intensive research in friction free or super-diffusive systems. 6 Super-diffusivity has been observed in systems involving graphitic compounds: fullerenes, 7,8 metallic clusters 9 or graphene flakes adsorbed on graphite. 10,11 The existence of quadrupole or higher order interactions may cause the super-diffusivity between graphene and graphite observed with atomic force microscopy (AFM).…”