Solid-on-solid sliding: superlubricity, dissipation, and the role of lattice mismatch
Nicola MANINI
(Università degli Studi di Milano)
Friction at the nanoscale often involves the contact of crystalline surfaces, which, depending on the experimental conditions, can be either dry or lubricated.
In the case of lattice-matched identical surfaces, dry contact leads to very high static and kinetic friction associated to atomic interlocking. In the more common condition of lattice-mismatched crystals, atomic interlocking is frustrated, with a resulting important reduction in friction. In controlled experimental conditions null static friction (superlubricity) can be realized. Kinetic friction is always nonzero, but it is usually dramatically reduced, especially when the contacting materials are very stiff. Dissipation is promoted by the displacement of the moire' pattern of solitonic lines at the contacting surface.
In a simulated model for lubricated nanofriction, we discovered a peculiar dynamical sliding state with the lubricant advancing at a fixed fraction of the sliding speed, depending uniquely on a ratio of lattice spacings. The lubricant speed remains constant even when the values of several mechanical parameters are varied within a broad range. This fixed-speed "quantized" sliding state is understood as due to the motion of solitonic structures, arising in the lubricant due to the mismatch with one of the sliders, being pinned and dragged along by the other slider.
