Understanding the onset of sliding is the key to understanding mechanisms of friction as a fundamental dissipative surface phenomenon. Most recent studies of the abrupt stick-to-slip transition concentrate on either nanotribological or fracture mechanics analysis. Here, we investigate ultraslow sliding friction, which is intermediate between the static and dynamic friction. During ultraslow friction, the sliding velocity is comparable with the relaxation rate of the material. Friction between two very smooth steel samples was studied at the sliding velocity of 37 nm/s. The force-displacement dependencies suggest that the onset of sliding is a gradual transition between two regimes rather than an abrupt irreversible transition from static to kinetic friction observed at higher sliding velocities.

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