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Triboactive Low-Friction Coatings Based on Sulfides and Carbides
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Inorganic Chemistry.ORCID iD: 0000-0002-2535-3107
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

For sustainable development, it is highly important to limit the loss of energy and materials in machines used for transportation, manufacturing, and other purposes. Large improvements can be achieved by reducing friction and wear in machine elements, for example by the application of coatings. This work is focused on triboactive coatings, for which the outermost layer changes in tribological contacts to form so-called tribofilms. The coatings are deposited by magnetron sputtering (a physical vapor deposition method) and thoroughly chemically and structurally characterized, often theoretically modelled, and tribologically evaluated, to study the connection between the composition, structure and tribological performance of the coatings.

Tungsten disulfide, WS2, is a layered material with the possibility of ultra-low friction. This work presents a number of nanocomposite or amorphous coatings based on WS2, which combine the low friction with improved mechanical properties. Addition of N can give amorphous coatings consisting of a network of W, S and N with N2 molecules in nanometer-sized pockets, or lead to the formation of a metastable cubic tungsten nitride. Co-deposition with C can also give amorphous coatings, or nanocomposites with WSx grains in an amorphous C-based matrix. Further increase in coating hardness is achieved by adding both C and Ti, forming titanium carbide. All the WS2-based materials can provide very low friction (down to µ<0.02) by the formation of WS2 tribofilms, but the performance is dependent on the atmosphere as O2 and H2O can be detrimental to the tribofilm functionality.

Another possibility is to form low-friction tribofilms by tribochemical reactions between the two surfaces in contact. Addition of S to TiC/a-C nanocomposite coatings leads to the formation of a metastable S-doped carbide phase, TiCxSy, from which S can be released. This enables the formation of low-friction WS2 tribofilms when a Ti-C-S coating is run against a W counter-surface. Reduced friction, at a moderate level, also occurs for steel counter-surfaces, likely due to formation of beneficial iron sulfide tribofilms.

The studied coatings, whether based on WS2 or TiC, are thus triboactive, with the ability to form low-friction tribofilms in a sliding contact.

 

 

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2014. , 86 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1183
Keyword [en]
coatings, thin films, tribology, tungsten disulfide, transition metal dichalcogenide, nanocomposite, TiC/a-C, tribofilms, PVD, XPS, HAXPES, XRD, SEM, TEM, Raman spectroscopy, nanoindentation
National Category
Inorganic Chemistry
Research subject
Chemistry with specialization in Inorganic Chemistry
Identifiers
URN: urn:nbn:se:uu:diva-230989ISBN: 978-91-554-9041-6 (print)OAI: oai:DiVA.org:uu-230989DiVA: diva2:746440
Public defence
2014-10-31, Häggsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 09:15 (English)
Opponent
Supervisors
Available from: 2014-10-09 Created: 2014-09-02 Last updated: 2015-01-23Bibliographically approved
List of papers
1. Understanding the effects of sputter damage in W–S thin films by HAXPES
Open this publication in new window or tab >>Understanding the effects of sputter damage in W–S thin films by HAXPES
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2014 (English)In: Applied Surface Science, ISSN 0169-4332, E-ISSN 1873-5584, Vol. 305, 203-213 p.Article in journal (Refereed) Published
Abstract [en]

WS2 is an excellent solid lubricant in dry conditions, and can be applied as thin films. The analysis of WS2 and WS2-based films by x-ray photoelectron spectroscopy (XPS) can be challenging, due to contaminationand oxidized material on the surface. The investigations have traditionally therefore included sputter etching by ion bombardment, which however leads to changes of the remaining material. In this study, hard x-ray photoelectron spectroscopy (HAXPES) has been used to study W–S films deposited bymagnetron sputtering. High-resolution reference measurements for crystalline WS2 and metallic W are also presented. The W–S films were analyzed before and after sputter cleaning by Ar+ ion bombardment, using photon energies of 3 and 6 keV. The as-deposited films were found to consist mainly of a WSx phase,similar to WS2 but with a broader range of chemical states. It is shown that ion bombardment of the surface not only removes the outermost oxidized material, but also leads to preferential sputtering of sulfur and the formation of metallic tungsten. The results are of strong interest for the analysis of WS2-based materials, as they demonstrate that spectra from sputter-cleaned films include effects of sputter damage,and may not be representative of the original sample.

Place, publisher, year, edition, pages
Elsevier, 2014
National Category
Inorganic Chemistry Materials Chemistry Condensed Matter Physics
Research subject
Chemistry with specialization in Inorganic Chemistry; Physics with spec. in Atomic, Molecular and Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-224513 (URN)10.1016/j.apsusc.2014.03.038 (DOI)000336525400028 ()
Funder
Swedish Foundation for Strategic Research
Available from: 2014-05-13 Created: 2014-05-13 Last updated: 2017-12-05Bibliographically approved
2. Amorphous W-S-N thin films: the atomic structure behind ultra-low friction
Open this publication in new window or tab >>Amorphous W-S-N thin films: the atomic structure behind ultra-low friction
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2015 (English)In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 82, 84-93 p.Article in journal (Refereed) Published
Abstract [en]

Amorphous W–S–N in the form of thin films has been identified experimentally as an ultra-low friction material, enabling easy sliding by the formation of a WS2 tribofilm. However, the atomic-level structure and bonding arrangements in amorphous W–S–N, which give such optimum conditions for WS2 formation and ultra-low friction, are not known. In this study, amorphous thin films with up to 37 at.% N are deposited, and experimental as well as state-of-the-art ab initio techniques are employed to reveal the complex structure of W–S–N at the atomic level. Excellent agreement between experimental and calculated coordination numbers and bond distances is demonstrated. Furthermore, the simulated structures are found to contain N bonded in molecular form, i.e. N2, which is experimentally confirmed by near edge X-ray absorption fine structure and X-ray photoelectron spectroscopy analysis. Such N2 units are located in cages in the material, where they are coordinated mainly by S atoms. Thus this ultra-low friction material is shown to be a complex amorphous network of W, S and N atoms, with easy access to W and S for continuous formation of WS2 in the contact region, and with the possibility of swift removal of excess nitrogen present as N2 molecules.

National Category
Inorganic Chemistry Materials Chemistry Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-230988 (URN)10.1016/j.actamat.2014.08.043 (DOI)000347017800008 ()
Available from: 2014-09-02 Created: 2014-09-02 Last updated: 2017-12-05Bibliographically approved
3. Influence of composition, structure and testing atmosphere on the tribological performance of W-S-N coatings
Open this publication in new window or tab >>Influence of composition, structure and testing atmosphere on the tribological performance of W-S-N coatings
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2014 (English)In: Surface & Coatings Technology, ISSN 0257-8972, E-ISSN 1879-3347, Vol. 258, 86-94 p.Article in journal (Refereed) Published
Abstract [en]

W-S-N coatings deposited by reactive magnetron sputtering offer the possibility of ultra-low friction in unlubricated sliding. In this work, W-S-N coatings of different composition and structure have been deposited, characterised and evaluated with respect to the tribological performance and tribofilm formation. The composition was varied by changing the flow of N-2 into the deposition chamber, leading to N contents ranging from 0 to 47 at.%. W-S-N coatings deposited without substrate heating are amorphous, while substrate heating results in coatings containing nanocystalline tungsten sulphide (WSx) for low N contents, and nanocrystalline tungsten nitride (WyN) at a high N content. The coatings were tribologically tested against steel balls in four different atmospheres dry N-2, dry air, humid N-2 and humid air to study the effects of atmospheric O-2 and H2O both separately and simultaneously. In dry N-2, all coatings exhibited an excellent performance with very low friction (mu approximate to 0.02) and wear. Notably, this included the N-richest and hardest coating, containing nanocrystalline WyN and only 13 at.% of S. The friction and wear increased on changing the atmosphere, in the order of dry air-humid N-2-humid air. In these three non-inert atmospheres, the friction and wear also increased with increasing N content of the coating. It is thus concluded that the presence of O-2, the presence of H2O, and a high N content (i.e., low Wand S contents) are three factors increasing the risk of high friction and wear, especially when occurring together. Raman spectroscopy mapping of the contact surfaces on the coatings and the balls showed that low friction and wear is connected to the presence of WS2 tribofilms in the contact, and that the three previously mentioned factors affect the formation and function of this tribofilm.

Place, publisher, year, edition, pages
Elsevier, 2014
National Category
Tribology Inorganic Chemistry Materials Chemistry
Research subject
Chemistry with specialization in Inorganic Chemistry; Engineering Science with specialization in Tribo Materials
Identifiers
urn:nbn:se:uu:diva-230987 (URN)10.1016/j.surfcoat.2014.09.061 (DOI)000346895000008 ()
Available from: 2014-09-02 Created: 2014-09-02 Last updated: 2017-12-05Bibliographically approved
4. Influence of Ti addition on the structure and properties of low-friction W–S–C coatings
Open this publication in new window or tab >>Influence of Ti addition on the structure and properties of low-friction W–S–C coatings
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2013 (English)In: Surface & Coatings Technology, ISSN 0257-8972, E-ISSN 1879-3347, Vol. 232, 340-348 p.Article in journal (Refereed) Published
Abstract [en]

Transition metal dichalcogenides, such as WS2 and MoS2, are known for their layered structure and lubricating properties. When deposited as thin coatings, however, their use as solid lubricants is limited by their low hardness and load-bearing capacity. The addition of another element, such as carbon, can improve the mechanical properties, although the hardness of for example W-S-C coatings is still rather low. In this work, Ti has been added to W-S-C coatings in order to further increase the hardness by carbide formation. W-S-C and W-S-C-Ti coatings were deposited by non-reactive magnetron sputtering, and characterized with regard to composition, structure and mechanical and tribological properties. It was found that the addition of Ti leads to the formation of a new carbide phase, and a significant increase in hardness for coatings with moderate carbon contents. The friction properties of W-S-C-Ti coatings were found to be comparable to that of W-S-C coatings, with friction coefficients down to mu approximate to 0.02 and similar wear rates against steel in a dry atmosphere. Formation of WS2 in the wear track of W-S-C-Ti was confirmed by transmission electron microscopy. It has thus been shown that the addition of Ti to W-S-C coatings can increase the hardness, while still maintaining WS2 lubrication.

Place, publisher, year, edition, pages
Elsevier, 2013
National Category
Inorganic Chemistry Tribology
Research subject
Inorganic Chemistry; Engineering Science with specialization in Materials Science; Engineering Science with specialization in Electronics; Engineering Science with specialization in Tribo Materials
Identifiers
urn:nbn:se:uu:diva-202062 (URN)10.1016/j.surfcoat.2013.05.032 (DOI)000327691300045 ()
Funder
Swedish Foundation for Strategic Research
Available from: 2013-06-19 Created: 2013-06-19 Last updated: 2017-12-06
5. Extreme friction reductions during inital running-in of W-S-C-Ti low-friction coatings
Open this publication in new window or tab >>Extreme friction reductions during inital running-in of W-S-C-Ti low-friction coatings
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2013 (English)In: Wear, ISSN 0043-1648, E-ISSN 1873-2577, Vol. 302, no 1-2 SI, 987-997 p.Article in journal (Refereed) Published
Abstract [en]

The disulphides of tungsten and molybdenum are known for their low friction properties when used as solid lubricants. Due to their low hardness, their load bearing capacity when used as thin films is poor. When carbon is added to a WS2 coating, both of these shortcomings are improved, and a structure consisting of nanocrystals of WS2, and possibly tungsten carbide, in a matrix of amorphous carbon is formed. In this study, an attempt is made for further increasing the hardness of such coatings, by addition of Ti, a strong carbide former. A number of W–S–C(–Ti) coatings were deposited using magnetron co-sputtering, and characterised with regard to chemical composition, structure and tribological properties. It was seen that addition of Ti significantly increased the hardness of the coatings, while maintaining their excellent low friction properties in dry atmosphere. However, the coatings with Ti showed extremely high initial friction, a feature not seen for the coatings without Ti. The mechanisms behind this running-in behaviour were investigated by studying surfaces at early stages of wear. It was observed that tribofilms formed during sliding for the coatings containing Ti consisted mainly of TiO2, with platelets of WS2 appearing in the contact only after prolonged sliding. For the pure W–S–C coatings, WS2 was observed in the sliding interface almost instantly at the onset of sliding.

Place, publisher, year, edition, pages
Elsevier, 2013
National Category
Nano Technology Tribology
Research subject
Engineering Science with specialization in Tribo Materials; Engineering Science with specialization in Materials Science; Engineering Science with specialization in Electronics
Identifiers
urn:nbn:se:uu:diva-191211 (URN)10.1016/j.wear.2013.01.065 (DOI)000322682800018 ()
Conference
19th International Conference on Wear of Materials 2013; 14-18 April 2013; Portland, OR, USA
Funder
Swedish Foundation for Strategic Research
Available from: 2013-01-09 Created: 2013-01-09 Last updated: 2017-12-06Bibliographically approved
6. Tribochemically Active Ti–C–S Nanocomposite Coatings
Open this publication in new window or tab >>Tribochemically Active Ti–C–S Nanocomposite Coatings
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2013 (English)In: Materials Research Letters, ISSN 2166-3831, Vol. 1, no 3, 148-155 p.Article in journal (Refereed) Published
Abstract [en]

We demonstrate a new concept of self-adaptive materials, where sulphur is incorporated into TiC/a-C coatings and may be released in, for example, a tribological contact. By reactive sputtering with H2S, sulphur goes into the carbide to form a TiC x S y phase in an amorphous carbon matrix. The addition of sulphur lowers the friction against steel. Significantly lower friction is obtained against a tungsten counter-surface, as WS2 is generated in the contact. Annealing experiments and formation energy calculations confirm that sulphur can be released from TiC x S y . Ti–C–S coatings are thus chemically active in tribological contacts, creating possibilities of new low-friction systems.

Place, publisher, year, edition, pages
Taylor & Francis, 2013
National Category
Inorganic Chemistry Tribology
Research subject
Inorganic Chemistry; Engineering Science with specialization in Materials Science; Engineering Science with specialization in Electronics
Identifiers
urn:nbn:se:uu:diva-202064 (URN)10.1080/21663831.2013.802262 (DOI)000209767500005 ()
Funder
Swedish Foundation for Strategic Research Swedish Research CouncilKnut and Alice Wallenberg FoundationEU, European Research Council, 247062 - ASD
Available from: 2013-06-19 Created: 2013-06-19 Last updated: 2017-02-15Bibliographically approved
7. Tribochemical formation of sulphide tribofilms from a Ti-C-S coating sliding against different counter surfaces
Open this publication in new window or tab >>Tribochemical formation of sulphide tribofilms from a Ti-C-S coating sliding against different counter surfaces
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2014 (English)In: Tribology letters, ISSN 1023-8883, E-ISSN 1573-2711, Vol. 56, no 3, 563-572 p.Article in journal (Refereed) Published
Abstract [en]

Tribochemically active Ti-C-S coatings are nanocomposite coatings containing a S-doped titanium carbide, from which S can be released in a tribological contact. This work studies tribochemical reactions between a Ti-C-S coating and various counter surface materials, and their effect on the tribological performance. Tribological tests were performed in a ball-on-disc set-up, using balls of five different materials as sliding partners for the coating: 100Cr6 steel, pure W, WC, 316-L steel and Al2O3. For W balls, a WS2 tribofilm was formed, leading to low friction (down to A mu = 0.06). Furthermore, increasing normal load on the W balls was found to lead to a strong decrease in A mu and earlier formation of the low-friction WS2 tribofilm. Similar WS2 and MoS2 tribofilms were, however, not formed from WC- and Mo-containing 316-L balls. The performance when using WC and Al2O3 balls was significantly worse than for the two steel balls. It is suggested that this is due to sulphide formation from Fe, analogous to formation of anti-seizure tribofilms from S-containing extreme pressure additives and steel surfaces. The tribochemical activity of Ti-C-S coatings, with the possibility of S release, is thus beneficial not only for pure W counter surfaces, but also for Fe-based sliding partners.

Place, publisher, year, edition, pages
Springer Science+Business Media B.V., 2014
National Category
Tribology Materials Chemistry Inorganic Chemistry
Identifiers
urn:nbn:se:uu:diva-231928 (URN)10.1007/s11249-014-0437-6 (DOI)000345339300017 ()
Available from: 2014-09-11 Created: 2014-09-11 Last updated: 2017-12-05Bibliographically approved
8. Formation of 2D transition metal dichalcogenides on TiC1-xAx surfaces (A=S, Se, Te): A theoretical study
Open this publication in new window or tab >>Formation of 2D transition metal dichalcogenides on TiC1-xAx surfaces (A=S, Se, Te): A theoretical study
2014 (English)In: Journal of Materials Research, ISSN 0884-2914, E-ISSN 2044-5326, Vol. 29, no 2, 207-214 p.Article in journal (Refereed) Published
Abstract [en]

Using first principle density functional calculations, we study the formation of 2D transition metaldichalcogenides (TMDs) on TiC1-xAx, (A = S, Se, and Te) surfaces. We examine the structural misfits between chalcogen-containing TiC and different TMDs and demonstrate that the conditions for formation of TMDs are fulfilled in TiC1-xAx. We also demonstrate the influence of chalcogens on the cohesive properties and electronic structure of the carbides. We find that they react with W and form W-dichalcogenides. In the experimentally reported Ti–C–S nanocomposite coatings, the carbide grains are embedded in an amorphous carbon matrix. We discuss here the role ofthis matrix in the reaction. We propose that TiC1-xTex and TiC1-xSex are the favorable sources fordichalcogenide formation and suggest an alternative way to produce 2D materials in general. Furthermore, we argue that using Ti–C–Te or Ti–C–Se in nanocomposite coatings may be more advantageous for tribological applications than that of Ti–C–S.

National Category
Condensed Matter Physics Inorganic Chemistry Materials Chemistry Tribology
Research subject
Materials Science; Chemistry with specialization in Inorganic Chemistry; Physics of Matter
Identifiers
urn:nbn:se:uu:diva-213666 (URN)10.1557/jmr.2013.350 (DOI)000332933800005 ()
Funder
Swedish Foundation for Strategic Research Swedish Research CouncileSSENCE - An eScience CollaborationKnut and Alice Wallenberg FoundationEU, European Research Council, 247062-ASD
Available from: 2014-01-02 Created: 2014-01-02 Last updated: 2017-12-06Bibliographically approved

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