Friction dynamics: Displacement fluctuations during sliding friction

Abstract

We investigated fluctuations (noise) in the positions of rectangular blocks made from rubber or polymethyl methacrylate (PMMA), sliding on various substrates under constant driving forces. For all systems, the noise power spectra exhibit large low-frequency regions with power laws, $\omega^{-\gamma}$, where $\gamma$ ranges between 4 and 5. The experimental results are compared from three interfacial interaction models: a spring-block model, an asperity-force model, and a wear-particle model. The spring-block model, where small sub-blocks are connected to a larger block via viscoelastic springs, yields $\gamma = 4$, consistent with experiments without wear particles. The asperity-force model assumes that the force acting on the block fluctuates in time due to variations in the number and size of contact regions and predicts $\gamma = 6$, which overestimates the exponent due to neglect of load redistribution. The wear-particle model considers the irregular dynamics of wear particles of varying sizes moving at the interface and can reproduce $\gamma = 5$ (which corresponds to $1/f$ noise in the friction force as observed in some cases) when suitable trapping–release probability distributions are used. This work introduces a new approach for investigating sliding friction via distance fluctuations and provides new insight into the underlying mechanisms.