Coefficient of Friction of High Polymers as a Function of Pressure

Abstract

In a previous paper, we described a method for calculating the coefficient of friction μ for thin solid films deposited on harder substrates. In order to use this method, the shear strength of the film material as a function of pressure must be known. A major limitation of the earlier study was the paucity of such data for materials of interest as dry film lubricants. Recently, data have become available for five high polymers: low-density polyethylene, high-density polyethylene, polypropylene, polyvinylidenefluoride, and polytetrafluoroethylene. The frictional behavior of these same materials in addition to a copolymer of ethylene and tetrafluoroethylene and a copolymer of tetrafluoroethylene and hexafluoropropylene, both in bulk form and as thin films, forms the substance of this report. The experimental values of μ for each polymer agreed well with the values calculated from the pressure-shear-strength data, even when these data were extrapolated to higher or lower pressures. It is concluded that μ as a function of pressure can be determined from shear measurements and, conversely, shear strength as a function of pressure can be estimated from friction measurements of thin films. In general, for thin polymer films, where pressures are usually high, μ will be determined almost entirely by the slope of the shear-strength-vs-pressure curve. For bulk polymer specimens, μ will be greatly influenced by the intercept on the shear axis of that curve and the hardness of the polymer. This has led to an explanation that accounts for the large difference in the frictional properties between bulk specimens of high- and low-density polyethylene.