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Frictional behavior of self-adaptive nanostructural Mo-Se-C coatings in different sliding conditions
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Materials Sciences. (Ångströms Tribomaterialgrupp)
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Materials Sciences. (Tribomaterial)
2013 (English)In: Wear, ISSN 0043-1648, E-ISSN 1873-2577, Vol. 303, no 1-2, 286-296 p.Article in journal (Refereed) Published
Abstract [en]

Sliding properties of Mo-Se-C coatings with two different carbon content deposited by magnetron sputtering were investigated in different sliding environments (argon, nitrogen, dry and humid air). Both coatings had a structure that was identified as randomly oriented structures of MoSe2 embedded into amorphous carbon matrix. The worn surfaces, i.e. the wear tracks and the wear scars of the balls, were analyzed by optical microscopy, Raman spectroscopy and scanning electron microscopy. The material transferred to the ball steel surfaces was almost exclusively MoSe2, whereas the wear tracks on the coatings were more complex, with areas rich in MoSe2 and areas similar to that of as-deposited coatings. The friction was lowest in argon (0.012 at a load of 10 N) and highest in humid air, but still remarkably low; as best 0.05 at 10 N load; however, the exceptionally low wear rate was almost identical. Thus, we focused our detailed analysis on these two examples to understand the mechanisms responsible for the difference between the friction coefficients. SEM, EDX, XPS, Raman and TEM with EELS and EDX were applied to investigate the composition and structure of localized spots of interest on the tested surfaces. In both cases, we observed well-ordered MoSe2 tribofilms with negligible amount of oxides. Carbon was not present in the sliding interfaces, although large amount of carbon was found outside the contacts on both surfaces. Based on our investigations, we suggest the increase in friction of Mg-Se-C in humid air is primarily due to the increase in shear strength of the MoSe2 structure by the presence of water molecules in the sliding interface

Place, publisher, year, edition, pages
2013. Vol. 303, no 1-2, 286-296 p.
National Category
Materials Engineering Nano Technology Tribology
Research subject
Engineering Science with specialization in Materials Science
URN: urn:nbn:se:uu:diva-191179DOI: 10.1016/j.wear.2013.03.032ISI: 000322422500033OAI: oai:DiVA.org:uu-191179DiVA: diva2:585044
Swedish Foundation for Strategic Research Swedish Research Council
Available from: 2013-01-09 Created: 2013-01-09 Last updated: 2014-01-31Bibliographically approved
In thesis
1. Triboactive Component Coatings: Tribological Testing and Microanalysis of Low-Friction Tribofilms
Open this publication in new window or tab >>Triboactive Component Coatings: Tribological Testing and Microanalysis of Low-Friction Tribofilms
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Coatings are often used on critical components in machines and engines to reduce wear and to provide low friction in order to reduce energy losses and the environmental impact.

A triboactive coating not only provides this desired performance, it also actively maintains the low friction by a structural or chemical change in a very thin top layer of these already micrometer thin coatings. This so-called tribofilm is often 5-50 nm thick and can be formed either from the coating itself or by a reaction with the counter surface or the surrounding atmosphere, i.e. gas, fuel, oil, etc. The tribofilm will maintain the wanted performance for as long as the system is not chemically disturbed.

This thesis provides a detailed overview of the functionality of triboactive low-friction coatings, in many different systems. The majority of the tribofilms discussed, formed in very different environments, are built up by tungsten disulfide (WS2), which is a material similar to graphite, with a lamellar structure where strongly bonded atomic planes may slip over each other almost without resistance. The major difference is that WS2 is an intrinsically triboactive material, while graphite is not. However, graphite and other carbon-based materials can be made triboactive in certain atmospheres or by addition of other elements, such as hydrogen.

The remarkable affinity and driving force to form such WS2 low-friction tribofilms, regardless of the initial states of the sulfur and tungsten, and even when the forming elements are present only at ppm levels, is a recurrent observation in the thesis.

Addition of an alloying element to sputtered coatings of WS2 can improve its mechanical and frictional properties significantly. Several promising attempts have been made to find good candidates, out of which a few important ones are investigated in this thesis. Their achievable potential in friction reductions is demonstrated.

By reducing friction, energy losses can be avoided, which also results in lower particle and exhaust emissions, which directly reduces the environmental impact. Triboactive coatings are shown to be a promising route to significantly improve tribological applications and allow more environmental friendly and energy efficient vehicles.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2013. 98 p.
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1011
tribofilms, low-friction coatings, tungsten disulfide, TEM
National Category
Materials Engineering Nano Technology
Research subject
Materials Science
urn:nbn:se:uu:diva-191223 (URN)978-91-554-8576-4 (ISBN)
Public defence
2013-02-22, Siegbahnsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 10:15 (English)
Swedish Research CouncilSwedish Foundation for Strategic Research
Available from: 2013-02-01 Created: 2013-01-09 Last updated: 2013-02-11Bibliographically approved

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