The influence of dynamic loading on sliding friction

Abstract

THERE have been many studies of the rheology of thin polymeric films in sliding contacts^1–4, and many of the results have been obtained with a particular experimental arrangement which involves studying the motion of a glass hemisphere traversing a thin film deposited on a smooth glass flat^3–5 (Fig. 1). The development of this subject has concentrated on an important quantity of practical interest, τ, the interfacial shear strength, which may be considered to be the energy dissipated per unit distance per unit contact area. These dissipative processes occurring in a particular film are not generally known. The interfacial shear strength has been found, for a wide range of organic polymers, to depend on many parameters, such as contact pressure, temperature and strain rate, for which simple empirical relations have been found⁵. Providing the real area of contact can be estimated, these relationships are of value in predicting the friction of certain lubricated contacts. These relationships are broadly similar to those obtained from the study of the variation of the bulk yield stress of polymers and in certain cases the magnitude of the fitting coefficients (for example, τ₀ and α below) are similar for a given polymer, in bulk and interfacial shear tests. Direct comparison is difficult because interfacial shear measurements generally correspond to much higher pressures and effective strains and rates of strain. However, it seems that τ may be considered a measure of the bulk shear yield or rupture stress of the organic layer in relatively intense deformation conditions. Using a modification of the basic experimental method we have now obtained detailed information on the effect of velocity or contact time on τ.