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Tribology of Metal-Graphite Composites: A Study of Sliding Electrical Contact Surfaces
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Materials Sciences. (Tribomaterial, Tribomaterials)
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. , p. 75
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1565
Keywords [en]
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: urn:nbn:se:uu:diva-329860ISBN: 978-91-513-0079-5 (print)OAI: oai:DiVA.org:uu-329860DiVA, id: diva2:1143844
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
List of papers
1. Influence of mechanical and electrical load on a copper/copper-graphite sliding electrical contact
Open this publication in new window or tab >>Influence of mechanical and electrical load on a copper/copper-graphite sliding electrical contact
2018 (English)In: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464, Vol. 121, p. 1-9Article in journal (Refereed) Published
Abstract [en]

For sliding electrical contacts a combination of satisfactory current transfer and good tribological performance is desired. A complete tribo-electrical study is presented where a mechanical and an electrical load are applied and tribological and electrical properties are evaluated. Stationary copper plated spring steel wires are tested against rotating copper-graphite rings at varying mechanical and electrical load. The focus is on surface modifications of the copper and correlations to the electrical performance are discussed. It is concluded that the tribological action is dominating over any influence from current, resulting in a mechanically mixed layer of Cu20 and Cu, for all currents as well as for the pure mechanical case. A graphitic layer is observed on the very surface of the wire.

Keywords
Wear; Contact resistance; Copper-graphite; Mechanical mixing
National Category
Materials Engineering
Research subject
Engineering Science with specialization in Tribo Materials
Identifiers
urn:nbn:se:uu:diva-329857 (URN)10.1016/j.triboint.2018.01.004 (DOI)000428605700001 ()
Available from: 2017-09-21 Created: 2017-09-21 Last updated: 2018-06-13Bibliographically approved
2. Wear phenomena and tribofilm formation of copper/copper-graphite sliding electrical contact materials
Open this publication in new window or tab >>Wear phenomena and tribofilm formation of copper/copper-graphite sliding electrical contact materials
2018 (English)In: Wear, ISSN 0043-1648, E-ISSN 1873-2577, Vol. 398-399, p. 227-235Article in journal (Refereed) Published
Abstract [en]

Copper-graphite composites of different compositions sliding against copper are investigated with regards to friction, wear and contact resistance. We find that comprehensive studies including the tribological as well as electrical aspects are rarely found in the literature. We combine friction, wear and contact resistance measurements with thorough surface analysis to complete the understanding. We find that including graphite in the composite greatly reduce the coefficient of friction. However, increasing the vol.% of graphite above 74 will not continue to reduce the friction. We conclude that the coefficient of friction is slightly lower when testing is performed without current than at 5 A current. A comparison of the specific wear rate for the tested composites shows that the one with 74 vol.% of graphite gives the lowest wear rate. In general, the wear rate is higher when tests are performed without current. Chemical analysis of the tribofilm that forms on the copper surface shows that it consists of graphite as well as Cu2O. It is also shown that the amount of graphite in the tribofilm correlates to the composition of the copper-graphite. Novel cross-section images of the tribofilm contribute to a deeper understanding of how sliding electrical contact surfaces are affected by current and sliding motion.

Place, publisher, year, edition, pages
Elsevier, 2018
Keywords
Tribofilm, electrical contact, copper-graphite, contact resistance, friction
National Category
Materials Engineering
Research subject
Engineering Science with specialization in Tribo Materials
Identifiers
urn:nbn:se:uu:diva-329858 (URN)10.1016/j.wear.2017.12.012 (DOI)000424367800023 ()
Available from: 2017-09-21 Created: 2017-09-21 Last updated: 2018-03-22Bibliographically approved
3. Friction, wear and tribofilm formation on electrical contact materials in reciprocating sliding against silver-graphite
Open this publication in new window or tab >>Friction, wear and tribofilm formation on electrical contact materials in reciprocating sliding against silver-graphite
2013 (English)In: Wear, ISSN 0043-1648, E-ISSN 1873-2577, Vol. 302, no 1-2 SI, p. 1481-1491Article in journal (Refereed) Published
Abstract [en]

In this study nanocomposite coatings mating silver-graphite were investigated for sliding electrical contact applications with the aim to optimize tribological and electrical properties. Apart from two different Ti–Ni–C nanocomposite coatings, brass, steel and TiN were also tested against a commercial silver-graphite at varying load and current. Friction, wear and contact resistance were measured in reciprocating sliding in ambient air. It was concluded that the wear of the silver-graphite was increased by current, for TiN and steel as much as four times, at a 5 N load. A tribofilm, with properties differing from the silver-graphite, formed on the coating/metal surface in all cases. This resulted in a very similar coefficient of friction, 0.3, for all mating materials. However, different load and current gave rise to slightly different thickness and morphology. A too low load was detrimental, as the coating became damaged, while a too high load was not favorable for tribofilm formation. In tests with varying current, a specific current could be identified that best governed the build-up of a well conducting and stable tribofilm. The largest differences were observed in the initial stages of testing, since once the tribofilm was built up, the contact resistance approached that of self mated silver-graphite, 40 mΩ. Experiments showed that the load and the current can be optimized to minimize wear of the silver-graphite which in turn would allow for lower maintenance costs.

Place, publisher, year, edition, pages
Elsevier, 2013
Keywords
Electrical contacts, Tribofilms, Nanocomposite, Ti–Ni–C, Contact resistance, Silver-graphite
National Category
Engineering and Technology
Research subject
Engineering science with specialization in Applied Mechanics; Engineering Science with specialization in Materials Science
Identifiers
urn:nbn:se:uu:diva-202032 (URN)10.1016/j.wear.2013.02.007 (DOI)000322682800074 ()
Funder
Vinnova, 2008-03401
Available from: 2013-06-26 Created: 2013-06-19 Last updated: 2017-12-06Bibliographically approved
4. Ti-Ni-C nanocomposite coatings evaluated in a sliding electrical contact application
Open this publication in new window or tab >>Ti-Ni-C nanocomposite coatings evaluated in a sliding electrical contact application
Show others...
2015 (English)In: Surface & Coatings Technology, ISSN 0257-8972, E-ISSN 1879-3347, Vol. 276, p. 210-218Article in journal (Refereed) Published
Abstract [en]

Nanocomposite Ti-Ni-C coatings, with nanosized carbide grains in an amorphous carbon (a-C) matrix have been suggested to have low friction and low contact resistance making them suitable for sliding electrical contacts. In this study we investigate further the previously observed influence of the amount of amorphous carbon, in a test set-up simulating instrumentation and control applications. The tribological and electrical performance is evaluated at high speed and continuous sliding against silver-graphite, where the mechanical load and current are fairly low. It is shown that under these circumstances there is no significant influence from the amount of a-C on neither the contact resistance nor the amount of wear of the silver-graphite. The reason for this is suggested to be that similar tribofilms are formed on the surface of the coatings, regardless of the amount of a-C phase. Degradation of the nanocomposite coatings is observed under electrical load, even though they are both much harder than the silver-graphite counter surface.

Keywords
Nanocomposite, Electrical contacts, Tribofilm, Silver-graphite, Wear, a-C
National Category
Inorganic Chemistry Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-263455 (URN)10.1016/j.surfcoat.2015.06.016 (DOI)000360594600026 ()
Available from: 2015-10-06 Created: 2015-09-30 Last updated: 2017-12-01Bibliographically approved
5. Wear and electrical performance of a slip-ring system with silver-graphite in continuous sliding against PVD coated wires
Open this publication in new window or tab >>Wear and electrical performance of a slip-ring system with silver-graphite in continuous sliding against PVD coated wires
2016 (English)In: Wear, ISSN 0043-1648, E-ISSN 1873-2577, Vol. 348, p. 138-147Article in journal (Refereed) Published
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.

Keywords
Sliding wear, Electrical contacts, Silver-graphite, PVD coatings, Tribofilms
National Category
Materials Engineering
Research subject
Engineering Science with specialization in Tribo Materials
Identifiers
urn:nbn:se:uu:diva-279558 (URN)10.1016/j.wear.2015.12.002 (DOI)000369372800014 ()
Funder
VINNOVA, 2008-03401Swedish Foundation for Strategic Research
Available from: 2016-03-02 Created: 2016-03-02 Last updated: 2017-12-15
6. A wear tolerant slip-ring assembly, individual spring-wire brushes in a v-grooved metal-graphite ring
Open this publication in new window or tab >>A wear tolerant slip-ring assembly, individual spring-wire brushes in a v-grooved metal-graphite ring
2014 (English)In: Proceedings of the 27th International Conference on Electrical Contacts, VDE Verlag GMBH Berlin Offenbach , 2014, p. 237-243Conference paper, Published paper (Refereed)
Place, publisher, year, edition, pages
VDE Verlag GMBH Berlin Offenbach, 2014
National Category
Engineering and Technology
Research subject
Engineering Science with specialization in Tribo Materials
Identifiers
urn:nbn:se:uu:diva-239221 (URN)
Conference
27th International Conference on Electrical Contacts, Dresden, Germany, June 22-26, 2014
Available from: 2014-12-19 Created: 2014-12-19 Last updated: 2017-12-15

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