Low temperature tribometers and the behaviour of ADLC coatings in cryogenic environment
Introduction
In a cryogenic environment components with interacting surfaces in relative motion (tribosystems) like bearings, seals and valves cannot be lubricated conventionally by using oils and greases because the cryogenic temperature range is far below the pour point of any base oil. Thus, such components are critical in respect to wear and frictional heat generation. For these applications solid lubricants like MoS2, hard wear resistant coatings like TiN, amorphous carbon, or material combinations with low friction and wear in dry sliding like polymers vs steel are used. An overview on early measurements of tribosystems at low temperatures is given by Kragelsky [1].
Hard carbon coatings are available in a broad variety of modifications depending on the deposition method and parameters. Their high hardness, low friction and in some cases nearly undetectable wear makes them ideal candidates for tribological applications. Therefore, these coatings have been investigated intensively in the past decades and data of their tribological properties at room temperature are available [2]. However, for a cryogenic environment only very few results have been published [3].
Section snippets
Experimental
To extend the experimental capacity of the tribology division of BAM to temperatures down to 4.2 K special test devices (cryotribometers) were constructed. For a study of a broad variety of materials for cryogenic tribosystems various methods for cooling, loading, and measurement in liquid and gaseous environment are employed [4], [5]. The setups of the cryotribometers are shown in Fig. 1, Fig. 2, Fig. 3 and their technical data are given in Table 1.
The sample chambers of all cryotribometers
Results
Eleven different commercially available and some experimental amorphous diamond like carbon (ADLC)-coatings were tested at 10, 35, 77 K and room temperature. Depending on the composition and coating method they can be devided into four groups:
A. Si–C:H: silicon containing physical-chemical vapour deposition (PCVD) coating
B. Me–C:H: three types of metal containing PCVD-coatings, single- or multilayer type for improvement of the adherence
C. Me–C:H: four variants of ion beam assisted deposition
Conclusion
Three devices for friction and wear tests at low temperatures in various cryogenic media have been constructed. Their operation field ranges from rapid material survey to tests under very special conditions like hydrogen environment at the critical point.
Tests with hard carbon coatings show that they are in principle appropriate for tribosystems in a cryogenic environment. Under all tested conditions contacts with only one coated surface need a stable transfer film of carbon wear debris for low
Acknowledgements
The authors would like to thank Mr R. Döring, Mr O. Berndes and Mr M. Heidrich for preparation and performance of the experiments. The investigations on the carbon coatings were supported by the Deutsche Forschungsgemeinschaft, Project-No.: Gr 1002/3-1. The samples were DYNAMANT-coatings provided by IKOS GmbH, Hösbach, OMT, Lübeck, and MAT, Dresden (all in Germany).
References (5)
Wear
(1993)- et al.
Friction behaviour and wear resistance of diamond-like carbon films under cryogenic temperatures
Diamond Rel. Mater.
(1994)