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Performance and Tribofilm Formation of a Low-Friction Coating Incorporating Inorganic Fullerene Like Nano-Particles
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.
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Materials Sciences.
2012 (English)In: Surface & Coatings Technology, ISSN 0257-8972, E-ISSN 1879-3347, Vol. 206, no 8-9, 2325-2329 p.Article in journal (Refereed) Published
Abstract [en]

A new tribological coating with potential as a coating for components for low-friction applications is tested and compared to three, state of the art, commercial low-friction PVD coatings. The new coating is an electrodeposited coating composed by a Ni-P matrix incorporating fullerene like nanoparticles of WS2. The performance of the new coating is compared with three reference coatings in a ball-on-disc setup. The tribological tests involved ball bearing steel balls slid on coated discs, under different conditions of humidity and also with or without oil lubrication. Both mating surfaces were closely investigated in a scanning electron microscope and the low-friction tribofilms formed were further analysed. The correlations found between coefficient of friction, surface roughness of the wear scar, tribofilm formation and contact conditions are discussed. The new coating exhibits a very low coefficient of friction at low humidity, a behaviour closely resembling that of the MoS2-based commercial reference, whereas the carbon based PVD coatings tested exhibit the reversed dependence of humidity. The low-friction behaviour of the new coating is correlated to the formation of a WS2 tribofilm with a superficial alignment of the basal planes parallel to the sliding direction. When lubricated with a base oil, the beneficial tribofilm is not formed and the coefficient of friction is much higher. The results show that even though the coating is an electrodeposited coating it can compete with state of the art commercial PVD coatings.

Place, publisher, year, edition, pages
2012. Vol. 206, no 8-9, 2325-2329 p.
Keyword [en]
Friction, Wear, Tungsten disulphide, Nanoparticles, Fullerene, Tribofilm
National Category
Materials Engineering
Research subject
Engineering Science with specialization in Tribo Materials; Engineering Science with specialization in Materials Science
Identifiers
URN: urn:nbn:se:uu:diva-160806DOI: 10.1016/j.surfcoat.2011.10.012ISI: 000300458500038OAI: oai:DiVA.org:uu-160806DiVA: diva2:453055
Available from: 2011-10-31 Created: 2011-10-31 Last updated: 2017-12-08
In thesis
1. Nanocomposites for Use in Sliding Electrical Contacts
Open this publication in new window or tab >>Nanocomposites for Use in Sliding Electrical Contacts
2011 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In this thesis nanocomposite materials for use in high performance electrical contacts are tested. Self mating silver as coatings on cupper substrates are the most used material combination in power connectors today. In this work two new concepts were tested. The first one was to change one of the mating surfaces to a hard thin coating and keep the other surface made of silver. Tested coatings were nanocomposites with hard carbides in a matrix of amorphous carbon. TiC/a-C and  Ti-Ni-C/a-C were tested both electrically and tribologically. The total amount of carbon and the amount of carbon matrix was important, both for the electrical and the tribological properties. The Ti-Ni-C coating also showed that substituting Ti in TiC with the weak carbide former Ni changed the stability of the carbides. The substitution resulted in more a-C matrix and less C in the carbides. Thin coatings of nc-TiC/a-C and  Ti-Ni-C/a-C showed high potential as material candidates for use in electrical contacts.

The other tested concept was to modify the used silver instead of replacing it. This was done by embedding nanoparticles of solid lubricant IF-WS2 in the silver. The results from reciprocating sliding displayed low friction and high wear resistance. The modified silver surfaces lasted for 8000 strokes with a friction of about 0.3 while at the same time allowing for a low contact resistance. The results for surfaces of pure silver coating displayed a friction of 0.8-1.2 and that the silver was worn through already after 300 strokes.

A new method to investigate inherent hardness and residual stress of thin coatings, on complex geometries or in small areas, was also developed. An ion beam was used to create stress free coating as free standing micro pillars. Hardness measured on the pillars and on as-deposited coating were then used to calculate the residual stress in the coatings.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2011. 49 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 879
Keyword
tribology, materials science
National Category
Materials Engineering Other Materials Engineering Composite Science and Engineering
Research subject
Materials Science; Engineering Science with specialization in Materials Science; Engineering Science with specialization in Tribo Materials
Identifiers
urn:nbn:se:uu:diva-160809 (URN)978-91-554-8219-0 (ISBN)
Public defence
2011-12-09, Siegbahnsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Polacksbacken, Uppsala, 10:15 (Swedish)
Opponent
Supervisors
Available from: 2011-11-18 Created: 2011-11-01 Last updated: 2011-11-23Bibliographically approved
2. 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.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1011
Keyword
tribofilms, low-friction coatings, tungsten disulfide, TEM
National Category
Materials Engineering Nano Technology
Research subject
Materials Science
Identifiers
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)
Opponent
Supervisors
Funder
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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André, BennyGustavsson, FredrikSvahn, FredrikJacobson, Staffan

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