How does friction affect sliding contact mechanics?

Abstract

Dealing with semi-infinite solids, interfacial friction is usually neglected, as normal and tangential displacements fields are independent on each other (unless material dissimilarity occurs). However, contacts involving a sufficiently thin layer, do not stick to such a simplified assumption, as thickness related normal/tangential coupling occurs, and surface frictional shear stresses do matter. It is the case, for instance, of classical rotary seals in boundary lubrication regimes, where rough frictional contacts between thin polymeric sealing lips and rotating shafts occur. Also, functional coatings to control the interface (adhesive, frictional, chemical, etc.) behavior, may be very thin and compliant, and usually experience frictional sliding during operation. All the same, these examples indicates that conditions exists where elastic coupling between in plane and out of plane displacements cannot be neglected and must be considered, instead. Here, we present our results on the rough contact mechanics of elastic and viscoelastic thin layers. We assume sliding conditions and friction at the interface, and we investigate the contact problem in the framework of linear (visco)elasticity, by relying on the Green’s functions approach. We show that, due to the friction and coupling, the presence of interfacial friction may lead to a significant increase of the contact area (up to 10%), compared to the frictionless case, which may affect specific functional response of the interface, such as electrical and thermal conductivity. Since the normal gap distribution is also affected by coupling and friction, the leak rate at the interface turns out significantly altered too. Coupling and friction also affect the contact pressure, which presents a certain degree of asymmetry leading (even for purely elastic materials) to an additional interlocking contribution to the tangential force opposing the relative motion at the interface. Therefore, the overall macroscale friction cannot be predicted by summing-up the local friction contributions occurring at microscale as commonly expected; it should instead include an additional coupled-induced term. The surface stress tensor is also affected, as due to friction and coupling very high tensile stresses are localized at the contact trailing edge, which are likely to induce material failure. In conclusion, we show that the common practice to neglect in-plane interactions in contact mechanics may lead to misleading tribological predictions.

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