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Microstructural design of self-lubricating laser claddings for use in high temperature sliding applications
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements. AC2T Research GmbH.ORCID iD: 0000-0003-1542-5872
Saarland University, Department of Materials Science and Engineering.
Technische Universität Wien, Department of Engineering Design and Logistics Engineering, Vienna.
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.ORCID iD: 0000-0003-1454-1118
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2018 (English)In: Surface & Coatings Technology, ISSN 0257-8972, E-ISSN 1879-3347, Vol. 337, p. 24-34Article in journal (Refereed) Published
Abstract [en]

Nickel-based self-lubricating claddings with the addition of Ag and MoS2 were prepared by means of laser cladding on stainless steel substrates, aiming at their implementation in metal forming applications involving demanding tribological conditions at high temperatures. The novelty of this approach is the addition of MoS2 with the aim to achieve a uniform silver distribution within the resulting cladding by means of an encapsulation mechanism. This prevents it from floating to the surface during the deposition process and thus being subsequently lost during surface preparation. The role of Ag and MoS2 concentration on the encapsulation process is discussed in terms of phase composition and resulting microstructures. The tribological behaviour of the resulting laser claddings was evaluated at up to 600 °C under unidirectional sliding. The encapsulation of Ag leads to outstanding tribological properties while keeping the concentration of used Ag low, thus increasing the economic viability of the claddings. An improvement in terms of both friction and wear was observed for the self-lubricating claddings compared to the nickel-based reference alloy, thus making them good candidates for use in high temperature applications such as hot metal forming.

Place, publisher, year, edition, pages
Elsevier, 2018. Vol. 337, p. 24-34
National Category
Tribology (Interacting Surfaces including Friction, Lubrication and Wear)
Research subject
Machine Elements
Identifiers
URN: urn:nbn:se:ltu:diva-67203DOI: 10.1016/j.surfcoat.2017.12.060ISI: 000430643900003Scopus ID: 2-s2.0-85039709492OAI: oai:DiVA.org:ltu-67203DiVA, id: diva2:1172249
Note

Validerad;2018;Nivå 2;2018-01-09 (andbra)

Available from: 2018-01-09 Created: 2018-01-09 Last updated: 2018-12-21Bibliographically approved
In thesis
1. Self-Lubricating Laser Claddings in the Context of Hot Metal Forming
Open this publication in new window or tab >>Self-Lubricating Laser Claddings in the Context of Hot Metal Forming
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Laser cladding is a coating technique with significant advantages like the high quality of the resulting layers, their excellent metallurgical bonding to the substrate or the possibility to repair/rework high-value mechanical components. In recent years, the incorporation of solid lubricants to the base powder in order to produce self-lubricating claddings has been shown in the literature to be possible, with several of the described coatings being able to operate at high temperatures with low friction and wear. This has been considered to hold a great potential for industrial applications involving high temperature work pieces like hot metal forming.

In recent years, the hot stamping of ultra-high strength steel has become increasingly popular due to the enhanced ductility of the work piece and the possibility to achieve a fully martensitic microstructure, to the point that this forming technique has become widespread in the automotive industry. However, the use of Al-Si-based protective coatings on the work piece in order to prevent oxidation and decarburisation is the source of a poor tribological behaviour due to the formation of Al-Fe intermetallics by diffusion from the steel substrate. This can lead to significant material transfer to the tool in addition to a decreased quality of the finished product due to surface damage.

In an attempt to improve the tribological contact in high temperature metal forming applications while at the same time decreasing the need for lubrication, nickel- and iron-based self-lubricating coatings have been prepared by means of laser cladding, featuring the incorporation of different combinations of solid lubricants including soft metals like silver and copper in addition to transition metal dichalcogenides like MoS2 and WS2. The resulting laser claddings were thoroughly characterised, including their microstructure, oxidational properties and their tribological behaviour at high temperatures under different contact configurations and counter bodies.

During the present study, it has been observed that the addition of sulfur-containing precursors to the base powder used for coating preparation leads to the encapsulation of silver, preventing it from floating to the melt pool surface during the cladding process and thus allowing for a uniform distribution of the soft metal across the whole thickness of the coating.

Additionally, it has been observed that the chromium sulfides resulting from the thermal degradation of transition metal dichalcogenides during laser cladding are effective solid lubricants at high temperatures, while silver also contributes to decreased friction at room temperature. Thus, the addition of Ag and MoS2 to nickel-based self-lubricating claddings has been considered optimum in terms of the resulting tribological behaviour, as it leads to decreased friction up to temperatures of 600°C. Additionally, it has been found that the addition of solid lubricants like MoS2 to the nickel-based claddings leads to negligible counter body wear at high temperatures, coupled to the formation of a protective tribolayer on the counter body composed of oxidised nickel, chromium and sulfur. This behaviour has been consistently observed under different testing configurations, like reciprocating against both steel- and aluminium-based counter bodies, in addition to high temperature sliding tests against Al-Si-coated boron steel, and it is expected to protect the surface of the work piece during hot metal forming processes.

Place, publisher, year, edition, pages
Lulea: Luleå University of Technology, 2019
Series
Doctoral thesis / Luleå University of Technology 1 jan 1997 → …, ISSN 1402-1544
Keywords
Tribology, High Temperature, Laser Claddings, Self-Lubricating, Hot Stamping
National Category
Metallurgy and Metallic Materials Tribology (Interacting Surfaces including Friction, Lubrication and Wear)
Research subject
Machine Elements
Identifiers
urn:nbn:se:ltu:diva-72367 (URN)978-91-7790-290-4 (ISBN)978-91-7790-291-1 (ISBN)
Public defence
2019-03-12, Lulea, 09:00 (English)
Supervisors
Note

This work was funded by the Austrian COMET Programme (Project K2 XTribology, Grant No. 849109) and has been carried out within the “Austrian Excellence Center for Tribology” (AC2T research GmbH)

Available from: 2018-12-27 Created: 2018-12-21 Last updated: 2019-01-30Bibliographically approved

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