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Complex frictional analysis of self-lubricant W-S-C/Cr coating
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Materials Sciences.
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Materials Sciences.
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Materials Sciences.
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2012 (English)In: Faraday discussions (Online), ISSN 1364-5498, E-ISSN 1359-6640, Vol. 156, 383-401 p.Article in journal (Refereed) Published
Abstract [en]

Transition metal dichalcogenides belong to one of the most developed classes of materials for solid lubrication. However, one of the main drawbacks of most of the self-lubricating coatings is their low load-bearing capacity, particularly in terrestrial atmospheres. In our previous work, alloying thin films based on tungsten disulfide with non-metallic interstitial elements, such as carbon or nitrogen, has been studied in order to improve tribological performance in different environments. Excellent results were reached with the deposited coatings hardness, in some cases, more than one order of magnitude higher than single W-S films. In this work, W-S-C films were deposited with increasing Cr contents by co-sputtering chromium and composite WS2-C and targets. Two films were prepared with approx. 7 and 13 at.% of Cr. Alloying with chromium led to dense films with amorphous microstructure; the hardness and adhesion was improved. Sliding tests were carried out in dry and humid air using a pin-on-disc tribometer with 100Cr6 steel balls as a counterpart. To analyse the sliding process, the surfaces in the contact were investigated by X-ray photoelectron spectroscopy (bonding), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Raman spectroscopy. Surface and sub-surface structural modification of the coating and composition of the transferred tribolayer are discussed in detail. High friction in humid air was attributed to the absence of a well-ordered WS2 sliding interface. On the other hand, the existence of such an interface explained the very low friction observed in dry air.

Place, publisher, year, edition, pages
2012. Vol. 156, 383-401 p.
National Category
Engineering and Technology Tribology
Research subject
Engineering Science with specialization in Materials Science
URN: urn:nbn:se:uu:diva-178152DOI: 10.1039/c2fd00003bISI: 000305892200025OAI: oai:DiVA.org:uu-178152DiVA: diva2:542148
Available from: 2012-07-30 Created: 2012-07-30 Last updated: 2013-02-11Bibliographically 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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