Superlubricity: Sliding with very low stress

Although introductory physics classes often imagine frictionless surfaces, in reality, surfaces that slide past one another almost always radiate or dissipate some of their energy away into sound or heat.   For many materials, the friction force, F_f, increases linearly with the amount of microscopic area that is actually in contact.  (If the microscopic contact area increases linearly with the applied normal load, F_N, then the system obeys Amontons’ famous law, F_f = μ F_N where μ is the constant friction coefficient.)  However, theoretical work [1] predicts that the friction force between incommensurate lattices grows as a fractional power of area, such that the friction force per area tends to zero.  This suggests that contacting solids could be protected from significant shear stresses by specially-designed coatings.

However, the theoretical predictions of zero friction stress relied on several simplifying assumptions.  These include neglecting the presence of free boundaries, contact shape, and the 3D elasticity of the solids.  We developed fast elastic computational methods that seamlessly interface with molecular dynamics simulations to explore this phenomenon [2]. This allowed us to find the breakdown of the effect in realistic geometry at the atomic scale [3].  The breakdown occurs when the incommensurate surfaces are able to elastically deform to approximate similar commensurate surfaces.  This happens essentially via the mechanism of introducing lattice dislocations from the boundaries.  We describe how the breakdown depends on the contact area, the atomic interactions between the solids, and the similarity of the incommensurate configuration to a lower-energy commensurate configuration.

[1] Hirano, Motohisa and Kazumasa Shinjo “Dynamics of friction: superlubric state.” Surface Science 283.1 (1993)
http://www.sciencedirect.com/science/article/pii/003960289391022H

[2] Pastewka, Lars, Tristan A. Sharp, and Mark O. Robbins. “Seamless elastic boundaries for atomistic calculations.” Physical Review B 86.7 (2012): 075459.
https://journals.aps.org/prb/abstract/10.1103/PhysRevB.86.075459

[3] Sharp, Tristan A., Lars Pastewka, and Mark O. Robbins. “Elasticity limits structural superlubricity in large contacts.” Physical Review B Rapid Communication 93 (2016): 121402(R)
https://journals.aps.org/prb/abstract/10.1103/PhysRevB.93.121402