Rubber friction: The contribution from the area of real contact

There are two contributions to the friction force when a rubber block is sliding on a hard and rough substrate surface, namely, a contribution F_ad = τ_f A from the area of real contact A and a viscoelastic contribution F_visc from the pulsating forces exerted by the substrate asperities on the rubber block. Here we present experimental results obtained at different sliding speeds and temperatures, and we show that the temperature dependency of the shear stress τ_f, for temperatures above the rubber glass transition temperature T_g, is weaker than that of the bulk viscoelastic modulus. The physical origin of τ_f for T > T_g is discussed, and we propose that its temperature dependency is determined by the rubber molecule segment mobility at the sliding interface, which is higher than in the bulk because of increased free-volume effect due to the short-wavelength surface roughness. This is consistent with the often observed reduction in the glass transition temperature in nanometer-thick surface layers of glassy polymers. For temperatures T < T_g, the shear stress τ_f is nearly velocity independent and of similar magnitude as observed for glassy polymers such as PMMA or polyethylene. In this case, the rubber undergoes plastic deformations in the asperity contact regions and the contact area is determined by the rubber penetration hardness. For this case, we propose that the frictional shear stress is due to slip at the interface between the rubber and a transfer film adsorbed on the concrete surface.