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technical article | Tribology


loading is insufficient to impart self-lubricating properties to the compound. At 20wt%, however, KS44 the friction coefficient is


decreased to around 0.2 and is stable up to arond 40N. Above 45N the friction coefficient starts to increase, indicating that the tribological stress limit has been reached (the so-called limiting force or PV limit). By further increasing the loading of KS44 to 30wt%, a further slight decrease in friction coefficient (to around 0.16) and higher limiting force (50N) was observed. The KS44 results show that the loading level of


Figure 1: Friction coefficient as a function of normal force for steel ball at fixed rotational speed (500 rpm) for different graphite-filled compounds


graphite is very important in achieving the desired friction coefficient and PV limit against steel. However, the effect of graphite loading is not linear - at 10wt% there is no difference compared to virgin PS, whereas the results at 30wt% are only slightly better than at 20wt%. The results show that the PS compound with 20wt% C-Therm graphite displays the best tribological behavior - the friction coefficient is extremely low (around 0.12) and the limiting force is larger than 50 N (the maximum force of the tribometer). Wear tests have been performed at fixed force (30N)


and fixed speed (500 rpm) and the results are shown in Figure 2. The wear rate of virgin PS is seen to be very high (due to the high friction coefficient). As expected, the wear rate decreases with an increasing amount of graphite KS44 but the compound containing 20wt% C-Therm showed the best performance in terms of wear resistance (better also compared to the compound containing 30wt% KS44).


Figure 2: Wear rate of graphite-filled PS compounds at fixed velocity (500rpm) and force (30N) against steel ball (Inset: Images of wear scars for PS+20wt% KS44 and PS+20wt% C-Therm)


Figure 3: Friction coefficient as a function of normal force for PA66 ball at fixed rotational speed (500 rpm) for different graphite-filled compounds


40 COMPOUNDING WORLD | November 2016


Graphite-filled PS against polyamide ball Similar experiments have been performed using a PA66 ball in place of the steel ball and have revealed a dramatic shift in the test results. Unfilled PS can withstand higher forces against PA66 than against steel, with a sharp increase in friction coefficient only above 10N (Figure 3). With the PA66 ball, 10wt% KS44 is sufficient to stabilise the friction coefficient at around 0.1 up to a force of 13N. By adding more graphite, the limiting force is shifted to higher values (around 20N for 30wt% KS44) while friction coefficient remains around 0.1. A PS compound containing C-Therm graphite shows a similar friction coefficient as for KS44 but a much larger limiting force (around 30N for 20% C-Therm). The curve with 10wt% C-Therm overlaps with that for 30wt% KS44, indicating that only one third of the loading level of this special graphite is required to achieve the same tribological performance as KS44 graphite. Wear tests performed at fixed force (10N) and fixed speed (500 rpm) showed that, for most of the samples, the friction coefficient is not stable during the duration of the test (10 minutes) and gradually increases from


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