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technical article | Tribology


Figure 4: Friction coefficient (left) and wear scar width (right) at the end of wear test with PA66 ball (10 minutes at 10 N / 500 rpm)


around 0.1 up to the final value shown in Figure 4. Only formulations with 30wt% KS44 and 20wt% C-Therm could maintain a low and stable friction coefficient for the whole test duration. After the wear tests, wear scar images were analysed using a digital optical microscope – the measured width of the wear scar is shown in Figure 4 and visual scar appearance shown in Figure 5. As expected, samples with a higher final friction coefficient show larger wear scars. In the case of unfilled PS, the wear scar is extremely large and it was observed that molten polymer was transported from the ball on the side of the wear scar (meaning a local temperature of more than 200°C had been reached). Wear scar size is strongly reduced with increasing amount of KS44, whereas C-Therm is the best perform- ing graphite with the lowest wear.


Discussion of results The experiments reveal two key findings. Firstly, it was observed that the tribological behavior of graphite filled-PS against metal is very different to that against


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EXTRAORDINARY TECHNOLOGIES FOR TODAY‘S PLASTICS


plastic. In particular tests, the steel ball gave a higher friction coefficient and higher wear, but also a higher PV limit compared to a PA66 ball. This apparent contradiction can be explained by the higher thermal conductivity of steel compared to PA66 (16.2 W/mK against 0.25 W/m.K). As a consequence, the steel ball can dissipate some of the frictional heat, while in the case of PA66 most of the heat has to be dissipated by the graphite-filled PS plates. This means that the limiting force in the case of the steel ball is higher than the PA66 ball despite the higher friction coefficient (and related frictional heat). Moreover, for the tests with the PA66 ball, the friction


coefficient at low forces is about the same (around 0.1) for all samples, whiles there is a big difference in the PV limit depending on the graphite loading and graphite type. In other words, while there is a correlation between friction coefficient and wear (a lower friction coefficient resulting in lower wear) there is no direct correlation between friction coefficient and PV limit. Secondly, a much better tribological performance


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