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Spacers and supports belong to the critical elements in the structure of a nuclear fusion device, particularly, when they are installed in the vacuum or superconducting magnet system of the reactor. These supports have to sustain high loads while allowing a certain amount of tangential movement with low friction and without any stick-slip motion. Vacuum and cryogenic environments are extremely hostile for smooth sliding systems, because conventional liquid lubricants cannot be employed, and only a very limited number of materials for dry sliding systems under these conditions are available. In order to test the friction behaviour of support elements in the field coils of the fusion experiment WENDELSTEIN 7-X, a test rig was build which allows testing in liquid helium environment at 4.2 K, with a normal force up to 150 kN, and a sliding velocity of 0.1 mm/s in oscillating motion. First tests have shown that the apparatus is appropriate for simulating the sliding behaviour of support elements. As an example, a result of an earlier model test of an MoS2-coating is shown.
The solid lubricant molybdenum disulfide is a well known material for extreme conditions where conventional oil or grease lubrication is not feasible (vacuum, high pressures). However, under reactive environments e.g. high humidity or hydrogen MoS2 is prone to premature failure. By using a PVD process, deposition parameters can be optimized to gain coatings with a more textured structure orienting the basal planes parallel to the Substrate surface, A similar concept has been used by Koch et al. for coatings developed to withstand the extreme operational conditions of the WENDELSTEIN W7-X fusion experiment. Therefore, an improved tribological behavior even under adverse conditions can be achieved.
In a future energy supply system based on renewable sources hydrogen technology will play a key role. Because the amount of energy from renewable sources, such as wind or solar power, differs seasonally and regionally, an energy storage method is necessary.
Hydrogen, as an environmentally friendly energy carrier, can fill this gap in an ideal way, in particular for mobile applications (Wurster et al., 2009). Already today, in Germany the amount of hydrogen as a byproduct in chemical industry is enough for fuelling about 1 Mio passenger cars1. Excess electrical power can be used to produce hydrogen by electrolysis. On demand, this hydrogen can be used for mobile or stationary fuel cells. Beside this new developing technology, hydrogen is used as fuel for rocket engines and in chemical industry since a long time. Table 1 comprises some physical parameters of hydrogen. It can be seen that hydrogen gas has a very low density which makes storage at high pressure or in liquid form (LH2) necessary.
Supporting elements between large superconducting magnets like the field coils in WENDELSTEIN 7-X have to sustain high loads while allowing a certain amount of tangential movement without any stick-slip motion. In model tests with reciprocating friction in liquid helium and vacuum a PVD-MoS2-coating has proven its suitability for these components. It showed a very stable sliding behaviour with a friction coefficient below 0.1 without coating failure under all test conditions.
For tribological components working under extreme
conditions, such as vacuum, or broad temperature
ranges, it is often a problem to achieve acceptable
durability of their friction parts. Thus, new material
requirements are involved for these tribo-systems, in
particular regarding operability and reliability. At BAM,
several projects were conducted in the recent years,
dealing with the tribological properties of friction
couples at cryogenic temperature and in vacuum
environment. This paper describes the vacuum and ultra
high vacuum tribometers constructed at BAM and
presents experiments carried out with polymer
composites. The experiments presented were performed
with PEEK, filled with solid lubricants, against 304
steel with a pin-on-disc configuration in oscillating
sliding. Friction tests were carried out in vacuum in the
temperature range between -40°C and +160°C and in
ultra high vacuum at room temperature.
Cryogenic solid lubrication
(2013)
Vakuumtribologie
(2010)
Tieftemperaturtribologie
(2010)