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Tribology | technical article


Figure 5: Images of wear scar after wear tests with PA66 ball


was observed for C-Therm graphite compared to KS44. Against steel, the compound containing C-Therm exhibits both lower friction coefficient and higher PV limit compared to KS44. The low friction coefficient is likely due to the high aspect ratio and high specific surface area of C-Therm, which allows it to easily form a lubricating tribofilm on the steel. Against PA66, the friction coefficient of PS containing C-Therm is similar to KS44, but this friction coefficient can be maintained up to much higher forces (higher PV limit). In order to better understand the cause of this


difference, thermal conductivity in the through-plane direction was measured by Laserflash method (Netzsch LFA447). As expected, thermal conductivity was seen to increase with increasing graphite content, while C-Therm showed much higher thermal conductivity compared to KS44 (Figure 6). It is interesting to note that the thermal conductivity of PS containing 10wt% C-Therm is the same as PS with 30wt% KS44, and that the friction curve of these two compounds against PA66 ball is overlapping (Figure 3). It seems, therefore, that the PV limit is controlled by the thermal conductivity of the polymer compound. While thermal conductivity is surely not the only parameter that controls friction coefficient and wear, a high thermal conductivity certainly contributes to better dissipation of frictional heat, therefore increasing the PV limit of the polymer compound. Other graphite proper- ties such as aspect ratio, crystallinity and the like, can affect the formation of a tribofilm and influence the friction coefficient and wear resistance of the compound.


Conclusions The results of these tests show that graphite is an effective solid lubricant for PS and can extend the PV-limit to much higher values compared to virgin PS. The tribological performance of graphite-filled com- pounds is shown to strongly depend on the counterpart material (for example, the friction coefficient and wear against steel is higher compared to PA66 while the PV limit is higher for steel compared to PA66). Tribological behaviour is also influenced by graphite loading level and graphite type. In particular, the special C-Therm


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Figure 6: Through-plane thermal conductivity of graphite-filled PS


high conductivity graphite outperformed synthetic KS44 graphite in all test conditions. Graphite can play an important role in the development of improved self- lubricating polymer compounds that can match the requirements for metal replacement.


References 1. R. Gilardi, D. Bonacchi, M.E. Spahr, “Graphitic Carbon Powders for Polymer Applications”, Polymers and Polymeric Composites: A Reference Series, Springer- Verlag (2016) 2. J. Bijwe, “Potential of fibres and solid lubricants to enhance the tribo-utility of PEEK in adverse operating conditions”, Industrial Lubricants and Technology 59/4 (2007), 156 3. R. Gilardi, “Tribology of Graphite-Filled Polystyrene”, Lubricants 4 (2016) 20 4. D. Bonacchi, “The carbon approach to thermal conductivity”, Compounding World (February 2016), 57


About the author: Raffaele Gilardi is R&D Leader, Carbon Technology and Polymer Applications at Imerys Graphite and Carbon. ❙ www.imerys-graphite-and-carbon.com


November 2016 | COMPOUNDING WORLD 43


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