The sliding friction of thermoplastic polymer composites tested at low speeds

Abstract An investigation of the tribology of three commercialized thermoplastic polymer composites based on polytetrafluorethylene, polyethylene terephthalate and polyamide, which are considered to be used as sliding bearings in nanopositioning, is carried out by a microtribometer in a speed range of 5–1000 μm/s. Deionized water and acetone plus methanol are used in surface cleaning. By using a steel sphere as counterface, the reciprocating sliding friction is measured in dry and with water as lubricant for a variety of running conditions and correlated with surface analysis techniques including scanning electron microscopy and energy-dispersive spectroscopy. The tribological performance of polymer composites is highly depending on the surface conditions. When they are cleaned with water, the friction is independent of materials and a master curve of friction force versus normal load is observed. After cleaning with acetone plus methanol, the friction turns materials dependent. The high Young's modulus is found to be beneficial for the formation of a thin transfer film, which is crucial to obtain a low and stable coefficient of friction. The low yielding strength results in a thick transfer film, which causes large fluctuations in the friction coefficient. Two kinds of velocity weakening of friction are observed from all composites, the one with a mild linear decrease of friction with logarithmic increase of speed in lubricated sliding and the other with a sharp decrease of friction in the speed range of 20–100 μm/s in dry sliding. The results are discussed with the potential applications of these polymer composites in nanopositioning.

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