Verschleiß von Polymeren und polymeren Verbundstoffen unter Rollkontakt
Final Report Abstract
The results of this project can be summarized as follows: 1) Four different wear testing machines were built or modified in order to examine the rolling wear behaviour of various polymer types. Each test rig has a different test configuration and gives a contribution to the understanding of rolling wear phenomena. The machines produce reproducible wear results which allow the comparison of polymer materials from different points of interest, i.e. the specific testing configuration, the kind of motion, the load and rolling velocity. Necessarily, an appropriate evaluation approach was established in order to determine the rolling wear amount. 2) The comparison of neat polymer materials, i.e. PA 6, POM, PEEK, with their reinforced types shows in all rolling wear experiments that the particle or fiber filled polymers decrease the wear resistance enormously. From the "classical" reinforcements no improvements of the rolling wear behaviour can be confirmed. 3) The rolling wear mechanism which causes the deterioration of the materials can be described by the aid of SEM pictures. The non-conformal geometry between the moving contact partners leads to Hertzian stress maxima beneath the surface. The subsequently failure of material areas generates micro cracks which grows together to bigger cracks. When the cracks break then the surface and connect with adjacent ones, volume parts of the material are isolated and detached in a following step. 4) The FEM simulations show in the case of linear BoP experiments a good correlation with the results of the wear tests. The temperature during a 600 N wear examination rises up to 42 °C which is presented by the modeling results in an appropriate agreement. 5) In the 2-Disk rolling contact, two ranges of wear behaviour could be identified. During moderate test conditions, i.e. speed and load, are applied on the wear experiment, no deterioration of the polymer has been observed for more than 40 hours. But only a doubling of stress parameters leads after only a few minutes to a total failure of the material. The polymer melts approximately 9 mm beneath the surface which gives an assumption of more than 200°C in this region. This fact observed follows Hertzian law that describes the maximum stress of non-conformal contacts beneath the surface. 6) The incorporation of micro-scale Piezo crystals into the polymer matrix enhances considerably the rolling wear resistance. However, an addition of filler and fibers to the testing material like in the aforementioned polymer composites results in a rise of wear. Thus, the reinforcement of polymer materials with Piezo crystals shows a new and success promising possibility to improve the rolling wear resistance. Furthermore, the addition of carbon black exhibits also an enhancement of the wear behaviour. The final conclusion of this project is the fact that commonly used methods to improve wear properties, especially in the case of sliding wear, can not be utilized for materials in components stressed under rolling motion.
Publications
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M. Harrass, D. Felhös, K. Väradi, K. Friedrich, A.K. Schlarb: Polymers under Rolling Stress. IVW-Kolloquium 2006, 14.-15. November 2006, IVW-Schriftenreihe Band 68, 166
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M. Harrass, K. Väradi and K. Friedrich: Tribological Investigation of Cast Polyamide (PA12G) and Polyoxymethylene (POM) on a Rolling Wear Testing Machine, Proc. 8th Int. Conf. On Tribology, 3rd and 4th July 2004, Veszprem, Hungary.
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M. Harraß, T. Peifer, K. Friedrich: Tribologische Untersuchungen an Polyoxymethylen (POM)- und Polyamid (PA6)-Materialien auf einem Rollverschleiß-Prüfstand. Tribologie- Fachtagung der GfT 2005, 26. - 28. September 2005, Göttingen
