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Formation and Function of Low-Friction Tribofilms
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Applied Materials Sciences. (Tribomaterialgruppen, Tribomaterials group)
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The use of low-friction coatings on machine elements is steadily increasing, and they are expected to play an important role in the reduction of fuel consumption of future motorized vehicles. Many low-friction coatings function by transformation of the outermost coating layer into tribofilms, which then cover the coating surface and its counter surface. It is within these tribofilms that sliding takes place, and their properties largely determine the performance. The role of the coating is then not to provide low friction, but to supply support and constituents for the tribofilm.

In this thesis, the formation of such tribofilms has been studied for a number of different low-friction coatings. The sensitivity of the tribofilm formation towards changes in the tribological system, such as increased surface roughness, varied surrounding atmosphere and reduced availability of the tribofilm constituents has been given special attention.

For TaC/aC coatings, the formation of a functioning tribofilm was found to be a multi-step process, where wear fragments are formed, agglomerated, compacted and eventually stabilized into a dense film of fine grains. This formation is delayed by a moderate roughening of the coated surface.

Coatings based on tungsten disulphide (WS2) are often able to provide exceptionally low friction, but their use is restricted by their poor mechanical properties and sensitivity to humidity. Large improvements in the mechanical properties can be achieved by addition of for example carbon, but the achievable hardness is still limited. When titanium was added to W-S-C coatings, a carbidic hard phase was formed, causing drastically increased hardness, with retained low friction. Titanium oxides in the tribofilms however caused the friction to be high initially and unstable in the long term. In a study of W-S-N coatings, the effects of humidity and oxygen were studied separately, and it was found that the detrimental role of oxygen is larger than often assumed.

Low friction tribofilms may form by rearrangement of coating material, but also by tribochemical reactions between constituents of the coating and its counter surface. This was observed for Ti-C-S coatings, which formed WS2 tribofilms when sliding against tungsten counter surfaces, leading to dramatic friction reductions.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2014. , 76 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1187
Keyword [en]
tribology, tribofilm, PVD, coating, low-friction, tungsten disulphide, transition metal dichalcogenide, tribochemistry
National Category
Tribology
Research subject
Engineering Science with specialization in Tribo Materials
Identifiers
URN: urn:nbn:se:uu:diva-233712ISBN: 978-91-554-9065-2 (print)OAI: oai:DiVA.org:uu-233712DiVA: diva2:754210
Public defence
2014-11-28, Polhemsalen, Lägerhyddsvägen 1, Uppsala, 10:15 (English)
Opponent
Supervisors
Available from: 2014-11-05 Created: 2014-10-08 Last updated: 2015-01-23
List of papers
1. On the influence from micro topography of PVD coatings on friction behaviour, material transfer and tribofilm formation
Open this publication in new window or tab >>On the influence from micro topography of PVD coatings on friction behaviour, material transfer and tribofilm formation
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2011 (English)In: Wear, ISSN 0043-1648, E-ISSN 1873-2577, Vol. 271, no 9-10, 204-2057 p.Article in journal (Refereed) Published
Abstract [en]

PVD coatings based on amorphous carbon with metal-carbides are currently gaining a high interest for use on machine elements due to their potential to give low friction and low wear of the counter surface. However, the performance varies significantly between the various types of such coatings and the causes of this are not clear. One factor is the micro topography of the coating surface. This topography may influence the friction in many ways; by changing the state of lubrication, by causing scratching of the counter surface, by modifying the topography of the counter surface the material transfer, the tribofilm formation, etc.

TaC/a-C coatings, produced by co-sputtering of carbon and tantalum in an argon atmosphere, were deposited on high speed steel substrates exposed to varying degrees of etching to produce a range of surface roughnesses. Ball-on-disc experiments were used to evaluate the tribological properties of the coatings in dry condition against a ball bearing steel ball. The surfaces were analysed using various advanced techniques, including, SEM, XPS, Raman, EDS and AFM, all both prior to and after the testing.

It was shown that the resulting surface topography of the coating is affected even by very small protrusions on the substrate. The coefficient of friction decreased during use to a stable level, due to a complex process including tribofilm build-up on the sliding ball. Surfaces with lower protrusions exhibited a faster friction decrease, i.e. a faster running in.

Place, publisher, year, edition, pages
Elsevier, 2011
Keyword
PVD coating, Wear, Tribofilm, Topography, TaC/a-C
National Category
Materials Engineering Engineering and Technology
Research subject
Engineering Science with specialization in Tribo Materials; Engineering Science with specialization in Materials Science
Identifiers
urn:nbn:se:uu:diva-156788 (URN)10.1016/j.wear.2010.12.036 (DOI)000294590200106 ()
Available from: 2011-08-09 Created: 2011-08-09 Last updated: 2017-12-08
2. Design of low-friction PVD coating systems with enhanced running-in performance - carbon overcoats on TaC/aC coatings
Open this publication in new window or tab >>Design of low-friction PVD coating systems with enhanced running-in performance - carbon overcoats on TaC/aC coatings
2013 (English)In: Surface & Coatings Technology, ISSN 0257-8972, E-ISSN 1879-3347, Vol. 222, 48-54 p.Article in journal (Refereed) Published
Abstract [en]

The widespread use of low friction PVD coatings on machine elements is limited by the high costs associated with fulfilling the demands on the surface quality of both the supporting substrate and the counter surface. In this work, an attempt is made at lowering these demands, by adding a sacrificial carbon overcoat to a TaC/aC low friction coating. Both coatings were deposited by planar magnetron DC sputtering, as separate steps in a single PVD-process. Coatings were deposited on substrates of two different surface roughnesses, in order to test the ability of this coating system to function on rougher substrates. Reciprocating ball on disc tests was performed, using balls with two different surface roughnesses. The worn surfaces were investigated using 3-D profilometry and SEM. The ability of the different overcoats to initially reduce the roughness of both the coated surface and the counter surface and to produce stable, low-friction conditions was examined for the different initial roughnesses. The implications for design of efficient run-in coatings for various systems are discussed.

Keyword
Low friction coatings, Surface roughness, Running-in, PVD DLC
National Category
Engineering and Technology
Research subject
Engineering Science with specialization in Materials Science
Identifiers
urn:nbn:se:uu:diva-200653 (URN)10.1016/j.surfcoat.2013.02.003 (DOI)000318135100008 ()
Available from: 2013-06-03 Created: 2013-06-03 Last updated: 2017-12-06Bibliographically approved
3. 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
4. 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
5. 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
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

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