A Unified Theory of Rubber Friction

Since the 1940s, it has been common to apply the constant coefficient-of-friction model to rubber, using such coefficients in practice in the same fashion as those obtained from friction testing of two metals. While the constant coefficient is an inherent material property of metals, and thus has a theoretical foundation for its use, the rubber coefficient of friction has had no verified theoretical basis and is not constant under varying loads. A new theory of rubber friction has recently been proposed, one in which individual forces, rather than coefficients, are considered. The theory incorporates the three dynamic rubber friction forces currently recognized and accepted: (1) adhesion; (2) macrohysteresis generated by bulk deformation of sliding rubber on a harder, macroscopically rough material; and (3), cohesion losses arising from physical wear of the contacted rubber. With these the proposed theory includes a previously posited fourth rubber friction force, surface deformation hysteresis, developed from adhesive interaction of rubber with the microtexture of the paired surface. Analysis of numerous experimentally determined rubber friction data sets obtained by others has revealed evidence for the existence of the microhysteretic force. This paper presents the results of rubber friction testing performed on a pin-on-disk tribometer over a range of applied normal loads. The results provide experimental validation of the theoretical model, demonstrate the importance of considering each rubber friction force individually, and indicate the usefulness of the testing methodology. Practical applications of the findings are discussed.