Reduction of friction by normal oscillations. II. In-plane system dynamics

TL;DR

This study investigates how in-plane normal oscillations influence friction, emphasizing the importance of system dynamics and identifying key parameters, including resonance effects, which align with experimental observations.

Contribution

It extends previous models by incorporating system stiffness and dynamics, revealing a resonant case that affects friction reduction without a critical sliding velocity.

Findings

Resonant oscillations can eliminate the critical sliding velocity for friction reduction.

System stiffness significantly influences the effectiveness of oscillation-induced friction reduction.

Experimental results qualitatively support the theoretical predictions in the resonant case.

Abstract

The influence of out-of-plane oscillations on friction is a well-known phenomenon that has been studied extensively with various experimental methods, e.g. pin-on-disk tribometers. However, existing theoretical models have yet achieved only qualitative correspondence with experiment. Here we argue that this may be due to the system dynamics (mass and tangential stiffness) of the pin or other system components being neglected. This paper builds on the results of a previous study (Popov M. et al. Friction, 2016, submitted) by taking the stiffness and resulting dynamics of the system into account. The main governing parameters determining macroscopic friction, including a dimensionless oscillation amplitude, a dimensionless sliding velocity and the relation between three characteristic frequencies (that of externally excited oscillation and two natural oscillation frequencies associated with the contact stiffness and the system stiffness) are identified. In the limiting cases of a very soft system and a very stiff system, our results reproduce the results of previous studies. In between these two limiting cases there is also a resonant case, which is studied here for the first time. The resonant case is notable in that it lacks a critical sliding velocity, above which oscillations no longer reduce friction. Results obtained for the resonant case are qualitatively supported by experiments.