Super hard

Carbides / Nitrides

Hard chrome

Coefficient of friction

Illustration 12: Hardness and tribological properties of different coating materials against steel (without lubricant, medium humidity, source: Schultrich 2002)

In high vacuum a strongly decreased effect from intermediate layers is noticed, because no fluids lubricants can be applied, so that the coverage layers (e.g. water or hydrocarbons) can not be regenerated. Beyond that, a very broad range of application conditions exist in space regarding mechanical impact, temperature variations or floating speeds. Therefore a direct transfer of approved terrestrial tribological systems to space is not feasible. Thus it is noticed that the coefficient of friction of ta-C increases strongly with the transition from humid air to ultra high vacuum, whereas strongly decreases for a-C:H (> 40 % H). An explanation for this is that in the vacuum, in case of a-C:H, the high hydrogen content can supply the needed hydrogen for the saturation of free surface bindings originating from the tribological contact (Schultrich 2002).

For space applications nanocomposites appear promising which combine the tribological properties of different material systems. For example the Tribological layers based material system ta-C/WC/W2S, which can be manufatured by means of on nanocomposites are pulse laser ablation of graphite targets and magnetron sputtern of WS2- promising for space targets, has been evaluated for space applications (Voevodin et al. 1999). applications

In this case the high hardness and the mechanical stability of ta-C and tungsten carbide are combined with the low friction coefficient of WS2 in vacuum. Application potentials in space are conceivable for example for low-friction and lubricant free bearings, cryogenic coolers for liquid hydrogen or thermal control layers in nanosatellites.

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