Initiation of sliding of an elastic contact at a nanometer scale under a scanning force microscope probe

A contact between a scanning force microscope Si3N4 probe and a flat surface of similar material is created. The low roughness of surfaces and their mechanical properties allow generating a nanoscopic elastic contact governs by an extended Hertz theory (DMT theory). The behavior of the initiation of sliding is investigated by submitting the contact to lateral sinusoidal displacements whose amplitude increases from zero to a few nanometers. The lateral force generated by the displacement is analyzed by a lock-in amplifier and the in-phase and out-of-phase components are recorded as a function of the displacement amplitude. Experimental results are compared to the Mindlin and Savkoor theories, which describe the initiation of sliding of macroscopic elastic contact. A relatively good agreement between our experiments and these theories is observed. For our particular experimental conditions, i.e., Si3N4 probe sliding on a similar material, the Mindlin's model gives a slightly better agreement than the Savkoor's model. This study shows that macroscopic concepts remain valid at the nanoscale, at least for the particular case studied here.