Influence of Ultrasonic In-Plane Oscillations on Static and Sliding Friction and Intrinsic Length Scale of Dry Friction Processes

The force of friction between plates of different materials (steel, brass, copper, titanium, glass, aluminum, rubber, and Teflon, among others) and a steel sample oscillating in the sliding plane at a frequency of 40-70 kHz has been studied. The measured friction coefficient as a function of sliding velocity and velocity oscillation amplitude fits well with theoretical predictions based on the simple Coulomb friction law at sliding velocities larger than the actuation velocity. However, the friction coefficient tends to a finite value at small sliding velocities, which is contrary to the theoretical prediction. The static limit has been studied in detail. A strong decrease in the static friction force takes place at oscillation amplitudes of 20-60 nm. Such amplitudes are enough to control the friction coefficient. The experimental data for both static and sliding friction are interpreted within the framework of a microscopic model and a phenomenological macroscopic model. The notion of intrinsic friction slip length is introduced.