Normal contact and friction of rubber with model randomly rough surfaces

TL;DR

This study investigates the normal contact and friction behavior of rubber with textured surfaces, validating multi-asperity models and revealing the influence of elastic interactions and shear stress variations at different scales.

Contribution

It provides experimental validation of extended Greenwood-Williamson models accounting for elastic interactions and explores the scale-dependent nature of shear stresses in rubber contacts.

Findings

Contact area follows a sub-linear power law with load.

Long-range elastic interactions dominate the non-linearity of contact area.

Frictional shear stress at asperity scale exceeds macroscopic shear stress.

Abstract

We report on normal contact and friction measurements of model multicontact interfaces formed between smooth surfaces and substrates textured with a statistical distribution of spherical micro-asperities. Contacts are either formed between a rigid textured lens and a smooth rubber, or a flat textured rubber and a smooth rigid lens. Measurements of the real area of contact $A$ versus normal load $P$ are performed by imaging the light transmitted at the microcontacts. For both interfaces, $A(P)$ is found to be sub-linear with a power law behavior. Comparison to two multi-asperity contact models, which extend Greenwood-Williamson (J. Greenwood, J. Williamson, \textit{Proc. Royal Soc. London Ser. A} \textbf{295}, 300 (1966)) model by taking into account the elastic interaction between asperities at different length scales, is performed, and allows their validation for the first time. We find that long range elastic interactions arising from the curvature of the nominal surfaces are the main source of the non-linearity of $A(P)$. At a shorter range, and except for very low pressures, the pressure dependence of both density and area of micro-contacts remains well described by Greenwood-Williamson's model, which neglects any interaction between asperities. In addition, in steady sliding, friction measurements reveal that the mean shear stress at the scale of the asperities is systematically larger than that found for a macroscopic contact between a smooth lens and a rubber. This suggests that frictional stresses measured at macroscopic length scales may not be simply transposed to microscopic multicontact interfaces.