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In this presentation tribological investigations on steels, polymers and coatings in gaseous and liquid hydrogen are shown. For steels the results of show that although martensitic steel may suffer from hydrogen embrittlement, application e.g. in roller bearings is possible. On the other hand, martensitic transformation of austenitic steels may cause problems in the application of these materials. Furthermore, the tribological performance of polymer composites with and without solid lubricant fillers in gaseous and liquid hydrogen was investigated. In particular, PEEK- and PTFE-matrix compounds showed low friction and sufficient low wear under these conditions. Among the solid lubricants, carbon-based materials are appropriate for hydrogen applications. Several variants of DLC and ta-C materials were tested. Beside the results for hydrogen, also a comparison with CH4-environment is shown.
The magnetic plasma confinement system of the nuclear fusion experiment “Wendelstein 7-X”, which recently produced the first plasma, consists of 20 planar and 50 non-planar superconducting coils. To keep them precisely in position, these coils contact each other at special support elements. The supports are operated in high vacuum at a temperature of about 4 K, and when the magnetic field is ramped up, forces up to 1500 kN and sliding motion of some mm occurs. Because mechanical disturbances may cause severe failures, only low friction without any stick-slip behaviour is tolerable. In pre-tests a PVD-MoS2-coating had proven to be the optimum solution. It was tested with downsized samples in reciprocating motion in liquid nitrogen (T = 77 K) and liquid helium (T = 4,2 K). In long-term tests in liquid nitrogen this coating showed stable sliding for up to 14.900 friction cycles without coating failure. However, in liquid helium an unexpected stick-slip effect occurred, whose origin could not be clarified so far.
In a first test series, the friction and wear properties of 6 polymeric candidate materials for application in LNG tribosystems were tested. The most favorable properties showed PTFE composite, filled with carbon fibers and PEEK, which showed low and stable friction and a wear coefficient in the order of 10-7 mm3N-1m-1. PTFE containing PEEK composites seem to be also appropriate for LNG applications. Admixtures of graphite as solid lubricant result in lower wear. Contrary, a lubricating mechanism of MoS2 was not observed. The PI-materials showed comparatively high friction and medium wear.
The talk gives a historical introduction and introduces some of the biggest and most important research institutes in Germany dealing with tribology at Universities, scientific societies and companies. Some examples for research projects and focus areas are given. As an example for a research institute with a long history in tribological research the activities of BAM in Berlin are presented.