System temperature and tribological behaviour of polymer-metal sliding couples in pin-on-disc model testing

Abstract

Pin-on-disc model tests from −100 to +250°C show that the physical state of the polymer is important for the tribological behaviour of polymer-metal sliding couples. Comparison of semicrystalline thermoplastics with transition regions at different temperatures (e.g. poly(butylenterephthlate) and poly(tetrafluoroethylene)) shows that a maximum in the friction is to be expected at each transition region because of great mechanical losses in cyclical deformation. The transition of the polymer state may be caused by tempering (heating, cooling) of the system or by frictional heating during the running in of untempered sliding couples. Above a certain low sliding velocity, which depends on the polymer structure, the system-dependent frictional heating causes a non-negligible difference between the local and the measurable temperatures of the polymer surface. Only up to this speed is the maximum in friction is allocated to the right temperature. When the sliding velocity is increased, the maximum values of the friction are shifted to higher temperatures and decreased because of local heating. The shift may also be explained to some extent by the time-temperature equivalence principle. The minimum in the wear rate, which is measured in the stationary state at the glass transition for low speeds, is influenced in the same way as the maximum in the friction. At second-order transitions the maximum friction may be accompanied by a maximum wear rate. Investigations into the polymer surface and the shape of wear particles show that wear patterns are characteristically related to the polymer state and by this to the level of friction and wear.