It is well known that two ideally confident surfaces should give the effect of superlubricity, e.g. should slide without friction. In principle, the superlubricity deals with absence of energy dissipation mechanism. If we consider interatomic interactions, we see that the number of atoms, which resist sliding is equal to the number of atoms that push slider. In the case of noncontact quantum friction interacting surfaces are divided by some spatial interval. This sliding can take place in probe (atomic force or scanning tunneling) microscopy. However, experiments usually show nonzero friction force in this case.
Nowadays there are several mechanisms of the noncontact friction. According to all of these models the noncontact friction arises from photons momentum transfer between surfaces.
But there is much more efficient mechanism of noncontact friction dealing with electron tunneling. Two tunneling electron flows or tunneling currents between close conductive surfaces transmit momentum from a moving body (slider) to the fixed one (substrate) and at the same time in backward direction.
At the thermodynamic equilibrium conditions these two counter-flows are equal. We have calculated these flows. Two different approaches have been applied — quantum mechanical and quasi-classical ones. The complex shape of the sliding surface have been taken into account by introduction of special function for the distribution of the tunneling gap width. In this model, noncontact friction is similar to Newtonian viscous friction in the fluid.
Friction force has been calculated for both variants. Numerical evaluations according to both formulas have shown rather similar results. It has been found that in both cases friction force is proportional to the slider’s speed and exponentially decreases with increase of the tunneling gap.
In addition, the friction force disappears at zero temperature. The tangential stresses have been obtained from numerical calculations for different surfaces with different roughness and for the atomically smooth surfaces. These values are close to macroscopic friction stress in experiments.