Combined rotating and linear motion effects on the wear of polymers

Abstract

The unusually low friction of polytetrafluoroethylene, PFTE, is utilised in many commercial applications¹. Unfortunately this low friction is often accompanied by rather high wear, and it is generally necessary to modify the polymer, usually by the incorporation of solid inorganic filler particles, to reduce the wear to acceptable levels^1,2. Although several attempts have been made to establish the mechanism of this improved wear behaviour, the optimisation of filler type, size and concentration remains an empirical exercise, and several relatively simple wear testing machines have been developed to assess prospective materials^3,4. These tests attempt to reproduce the sliding conditions encountered in practice. A widely used test is the pin-on-disk configuration where a fixed pin of polymer is loaded against a rotating disk. The load and relative velocity are fixed and the pin describes a circular path over the disk surface for a pre-determined period, after which the wear is measured, generally by weighing the polymer pin. Important additional variables are the environment, the surface topography of the disk, the temperature and the geometry of the pin⁵. In the process of conducting such a series of measurements we have identified and report here an additional variable which may be of general importance: the radius, R, of the path described by the pin on the disk.