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Wear and electrical performance of a slip-ring system with silver-graphite in continuous sliding against PVD coated wires
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.
2016 (English)In: Wear, ISSN 0043-1648, E-ISSN 1873-2577, Vol. 348, 138-147 p.Article in journal (Refereed) Published
Resource type
Text
Abstract [en]

Sliding electrical contacts transferring current between stationary and rotating components are also tribological systems. Although low contact resistance and noise are prioritised, lower wear rates reduce material usage, and lower friction reduce energy loss. In this paper a slip-ring assembly with wires contacting a silver-graphite ring is investigated with the aim to optimize the wire material to displace all wear to the ring. Uncoated wires and wires coated with nanocomposite Ti-Ni-C or TiN are tested at 100 mA current. Tribofilms, consisting mainly of silver and carbon, form on the wires and a contact resistance of around 0.5 Omega is measured for all wire materials. The properties of the tribofilms control the overall performance and the similarity between them, regardless of wire material, is the reason for the similar contact resistance. The Ti-Ni-C coating wear least on the silver-graphite. Both coatings degrade and wear off during testing, exposing the steel substrate. The steel itself also wears, although not at a rate excluding it as a possible wire material. None of the three surfaces fully displaces wear to the ring only. Considering the performance of the uncoated steel wire, coatings cannot be motivated on behalf of either improved electrical performance or wire protection.

Place, publisher, year, edition, pages
2016. Vol. 348, 138-147 p.
Keyword [en]
Sliding wear, Electrical contacts, Silver-graphite, PVD coatings, Tribofilms
National Category
Materials Engineering
Identifiers
URN: urn:nbn:se:uu:diva-279558DOI: 10.1016/j.wear.2015.12.002ISI: 000369372800014OAI: oai:DiVA.org:uu-279558DiVA: diva2:908446
Funder
VINNOVA, 2008-03401Swedish Foundation for Strategic Research
Available from: 2016-03-02 Created: 2016-03-02 Last updated: 2017-11-30Bibliographically approved
In thesis
1. Tribology of Metal-Graphite Composites: A Study of Sliding Electrical Contact Surfaces
Open this publication in new window or tab >>Tribology of Metal-Graphite Composites: A Study of Sliding Electrical Contact Surfaces
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

An environmentally sustainable production of electrical power is important for preserving the earth’s natural resources. In order to utilize this power as efficiently as possible, it is of great importance to minimize the losses, for example in sliding electrical contacts. A sliding electrical contact is where current is transferred from one rotating to one stationary component and power is lost due to friction and contact resistance. Also in some signal applications, high performance sliding contacts are crucial to ensure stable signal transfer with low noise. Although sliding electrical contacts are primarily designed for good electrical performance, the system will benefit also from optimization of the tribological properties.

The aim of this thesis is to increase the fundamental knowledge of the tribological and electrical performance of metal-graphite composite materials for sliding electrical contacts. The influence of mechanical and electrical load was investigated. Different stationary materials, from pure copper to nanocomposite coatings, were tested against copper- and silver-graphites. Two complementary test setups were used, one with reciprocating and one with unidirectional sliding. Surface analysis was essential to gain deepened understanding of the influence of the interaction on the surfaces. Especially my novel imaging of cross-sections has advanced the level on knowledge in this research field.

On the stationary material surface, a tribofilm forms with constituents from the metal-graphite and the surrounding atmosphere. Cross-sectioning reveals a material flow that indicates turbulence. Furthermore, the presence of oxides in the tribofilm is not necessarily detrimental for the contact resistance as long as there is also pure metal available. The presence of graphite is vital for low friction and wear. It is shown that the tribological and electrical behaviour of this system is only marginally influenced by the material selection of the stationary contact. Increasing the metal content in the composite, on the other hand, greatly reduces the contact resistance while there is no significant impact on friction and wear. The mechanical load has to be optimized to compromise between low wear (achieved with low load) and low contact resistance (achieved with high load). Pure mechanical tests show a lower friction and higher wear rate in comparison to tests with a five ampere current.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2017. 75 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1565
Keyword
tribology; tribofilm; surface analysis; wear; friction; electrical contacts; metal-graphite; contact resistance; cross-section; nanocomposite coating; copper; silver
National Category
Materials Engineering
Research subject
Engineering Science with specialization in Tribo Materials
Identifiers
urn:nbn:se:uu:diva-329860 (URN)978-91-513-0079-5 (ISBN)
Public defence
2017-11-10, Häggsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 13:15 (English)
Opponent
Supervisors
Available from: 2017-10-20 Created: 2017-09-22 Last updated: 2017-10-30

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Grandin, MartinaWiklund, Urban

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