Robust vibration-activated lubricity

Abstract Friction can be reduced or eliminated when the contact interface is subjected to an external vibration; we refer to this phenomenon here as vibration-activated lubricity. According to prior literature, vibration-activated lubricity is limited to oscillation amplitudes and frequencies that depend strongly on case-specific experimental variables such as the instrument resonance frequency, sliding speed, and slip length of the tribo-pair. This paper aims to overcome these limitations and clarify their origins. Specifically, we used a quartz crystal microbalance (QCM) to directly oscillate the sample at a fixed frequency and at oscillation speeds that exceeded the sliding speed by orders of magnitude. Under these direct oscillation conditions, vibration-activated lubricity persisted for alumina probes ranging from 50-1500 µm in diameter, loads from 20 µN − 5 mN, speeds from 5 µm/s − 1 mm/s, gold and single crystal molybdenum disulfide samples, and two instruments – a custom microtribometer and a commercial atomic force microscope. Under all conditions, vibration-activated lubricity was characterized by very small but non-vanishing friction coefficients (~ 0.01–0.05). Our findings suggest that the following criteria satisfy the conditions for robust vibration-activated lubricity: (1) direct coupling between the oscillator and the sample; (2) probe inertial or spring forces > > available friction; (3) oscillation speed > > sliding speed; (4) oscillation amplitude ≅ or > the slip length.