From the first friction quantitative experiments by Leonardo da Vinci (1452-1519) and most notably from our everyday life experience, it is known that there is an inevitable force opposing to the initiation of motion of one body on the surface of another. This force has to be exceeded, in order for the motion to start. Recently, however, it was found, that this inevitable force sometimes may not be that much inevitable and may disappear. This effect is called superlubricity.
On the other hand, the exact definition of the superlubricity is not an easy problem, as discussed by Dr. Muser in his chapter of the book (Fundamentals of Friction and Wear on the Nanoscale, the chapter is available for download from Research Gate). In superfluids, the internal friction (viscosity) goes to zero, in superconductors, the resistance to electrical current vanishes, but in superlubric states, nothing happens to the properties of the bodies. Nothing goes to zero, nothing vanishes, no fundamental changes in the solids properties can be observed.
Therefore, as concluded by Dr. Muser. it is necessary to use an ad hoc definition of superlubricity. As it was proposed, the superlubricity state can be defined as the state with very low kinetic friction coefficient, less than 0.001. At the same time, there must be a certain limitation for the speed of motion, lets say, the minimum speed must be in the order of 0.01 m/s.
More details can be found in the original article of Dr. Muser.
Credit for the image: www.newtonsapple.org.uk.
Administration of the project