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Molybdenum disulfide (MoS₂) is a well-known solid lubricant for tribosystems running in vacuum or dry gases. Problems arise due to its sensitivity to humidity, which is a drawback for its application under ambient conditions. However, by using a physical vapor deposition (PVD) process, deposition parameters can be optimized not only to gain a coatings structure with favorable frictional properties but also to minimize the sensitivity to attack by water molecules. Therefore, an improved tribological behavior even under moist conditions can be achieved. MoS₂coatings are also candidates for being applied at cryogenic temperatures. They already have proven their suitability, e.g., for sliding support elements between superconducting magnets of the nuclear fusion-experiment Wendelstein 7-X. However, these coatings were exclusively produced for this particular application and the utilization for more common tribosystems may be precluded due to cost considerations. In view of a wider range of applications, pure and Cr containing PVD-MoS₂ coatings with an optimized structure were tested under varying environments including hydrogen gas and cryogenic temperatures. Results of the most promising variant are presented in this paper.
In order to obtain a better understanding of the role of tribofilms during automotive braking, their structures were investigated and the essential features identified.
Next, different ingredient combinations were studied by preparing some model materials with simpler compositions than real tribofilms. A test method was developed for verifying the tribological properties of the model materials and for comparison with results obtained with numerical sliding simulations of such structures. Prerequisites of good brake Performance properties were identified. Although the coefficient of friction could be varied in a wide ränge, smooth sliding conditions could only be achieved with values smaller than 0.4. Tests with artificial third body powders turned out to be useful as screening method for the selection of raw materials for brake pad formulations.
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.
This paper presents investigations on the tribological behaviour of PTFE composites against steel at cryogenic temperatures. The results showed that the friction coefficient decreases with temperature down to 77 K, but did not follow a linear evolution further down to extreme low temperatures. It can be stated that the cryogenic environment has a significant influence on the tribological performance of the polymer composites. The effect of low temperatures was more clearly detected at low sliding speed, where friction heat is reduced. A change in wear mechanism from adhesive to abrasive was observed in this case. SEM and AFM analyses showed that the PTFE matrix composites investigated under these experimental conditions have transferred material onto the disc down to very low temperatures. Chemical analyses indicate the presence of iron fluorides.
Friction systems running in cryogenic environment are critical in respect to wear and frictional heat generation because in this temperature range, conventional lubricants can't be applied. In order to test the friction and wear behaviour of appropriate solid lubricants and wear resistant material combinations in cryogenic environment, including liquid hydrogen, special test rigs have been constructed. These devices provide test conditions for tribological investigations at temperatures ranging from 4.2K to room temperature in liquid or gaseous environment at pressures between 10-3 mbar and 20 bar. Investigations on polymers, composites and metals with and without coatings show that there is a broad variety of appropriate materials for cryogenic friction units, but the behaviour of austenitic steels in such systems gives an example of non sufficient knowledge in this field.