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B. Kohlhauser, M. Ripoll, H. Riedl, C.M. Koller, N. Koutná, G Ramirez, A. Erdemir, C. Gachot, P.H. Mayrhofer:
"On the In-situ Formation of Transition Metal Disulphides in Lubricated WN or WC Coating Contacts";
Talk: 46th International Conference on Metallurgical Coatings and Thin Films, San Diego; 2019-05-19 - 2019-05-24.

In recent years, solid lubricants such as transition metal dichalcogenides have attracted attention in form of nanoparticle additives for conventional oil-based lubricants. Those nanoparticle additives entail several disadvantages, including the need to compose a stable dispersion. An in-situ formation of lubricating solid compounds via a tribochemical reaction path between a coating and conventional S containing extreme pressure additives could prove to be much more advantageous.
While MoS2 can easily be formed on metallic Mo surfaces, the conditions in lubricated tungsten contacts are not severe enough to produce WS2. A new generation of functional coatings is helping to overcome this energetic barrier, which is linked to the formation of intermediate tungsten oxide phases. WN and WC based coatings are able to facilitate this in-situ formation of WS2 and combine excellent mechanical properties with an optimization of friction and wear via the formation of a low friction tribofilms.
The coatings were prepared by magnetron sputtering and their mechanical, structural and chemical properties were investigated with nanoindentation, energy dispersive x-ray spectroscopy, X-ray diffraction, and transmission electron microscopy.
The tribofilms, resulting from lubricated pin on disk tests, were studied in detail and consist of a fraction of amorphous WS2 and C based oil derived debris, but also of nanocrystalline WS2 domains along the surface. These lubricating WS2 sheets lead to a low coefficient of friction around 0.05 and the tribofilm protects coating and counter body from any wear, even after 100 h of testing.