Material transfer of POM in sliding contact

Material transfer is an important factor in the sliding behaviour of polymers, as it governs the development of friction and wear in the course of time. However, little is known about the first stages of material transfer. Therefore, measurements were performed with polymers sliding onto steel. The first stages of material transfer were studied with a slider-on-sheet apparatus. The results of these measurements are compared with the results of pin-on-disc measurements. In the latter case, polymer sliding onto polymer was also measured. Polyoxymethylene (POM)-C was chosen as polymer material. Scanning electron microscopy showed that polymer transfer is initiated by mechanical interlocking of metal asperities into the polymer. The resulting debris particles are smeared out during further passages in slider-on-sheet measurements. Pin-on-disc experiments resulted in a less dense transfer layer. Metal asperities can still plough through the polymer surface in pin-on-disc experiments, while ploughing is hindered in slider-on-sheet experiments. The friction coefficients found during pin-on-disc and slider-on-sheet experiments were comparable within the experimental error. The friction coefficient of POM-C sliding against stainless steel starts at a relatively low friction level of about 0.2 and rises to higher values after 20 h of sliding due to material transfer. Such a relatively high friction value was also found when POM-C was slid against POM-C, confirming that the increase in friction found during the experiments against steel was due to material transfer. Pin-on-disc experiments with POM-C sliding against POM-C showed rather large differences in wear rate of the polymer pin.