Static Friction of Smooth Clean Vulcanized Rubber

Abstract

Abstract This picture of the origin of static friction for smooth rubber indicates that it is governed by the purely elastic consideration of the strain necessary to produce surface instability. The interfacial adhesion energy required to detach the rubber, which will be the true thermodynamic value at zero peel rate, does not appear to be the limiting factor. The rate effects observed, which persist to surprisingly low rates, are due to the viscoelastic behavior of the rubber. Once sliding has begun, the energy is lost in the peeling process, and this is then dominated by interfacial peel adhesion scaled by viscoelastic characteristics. At zero rate the value found here of F8/G=0.09 implies a static friction coefficient of about 0.6. However, if the normal load were reduced to zero, a finite contact area would remain, at least for smooth surfaces, due to surface attraction, and to express static friction as a coefficient would be meaningless in this case. It is believed that this investigation demonstrates the existence of a true static friction for smooth clean rubber and suggests that its magnitude is determined by the elastic deformation and consequent instability of the rubber.