| 1. |
- Nyberg, Harald, et al.
(författare)
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Extreme friction reductions during inital running-in of W-S-C-Ti low-friction coatings
- 2013
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Ingår i: Wear. - : Elsevier. - 0043-1648 .- 1873-2577. ; 302:1-2 SI, s. 987-997
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 2. |
- Nyberg, Harald, et al.
(författare)
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Tribochemical formation of sulphide tribofilms from a Ti-C-S coating sliding against different counter surfaces
- 2014
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Ingår i: Tribology letters. - : Springer Science+Business Media B.V.. - 1023-8883 .- 1573-2711. ; 56:3, s. 563-572
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 3. |
- Sundberg, Jill, 1986-, et al.
(författare)
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Influence of composition, structure and testing atmosphere on the tribological performance of W-S-N coatings
- 2014
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Ingår i: Surface & Coatings Technology. - : Elsevier. - 0257-8972 .- 1879-3347. ; 258, s. 86-94
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 4. |
- Sundberg, Jill, et al.
(författare)
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Influence of Ti addition on the structure and properties of low-friction W–S–C coatings
- 2013
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Ingår i: Surface & Coatings Technology. - : Elsevier. - 0257-8972 .- 1879-3347. ; 232, s. 340-348
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 5. |
- Sundberg, Jill, 1986-, et al.
(författare)
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Quaternary W-S-C-Ti films for tribological applications
- 2011
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Konferensbidrag (refereegranskat)abstract
- IntroductionTransition metal dichalcogenides such as WS2 are well-known for their layered structure and solid lubricant properties. The addition of another element, such as carbon, can improve the mechanical properties of the material, such as the hardness, while still maintaining the solid lubrication.1,2 Different theories regarding the friction mechanisms in W-S-C have been proposed: the low friction could be solely due to the WS2 phase2 or both the WS2 and the carbon phase could be responsible.1 Despite the hardness increase compared to pure WS2, W-S-C films still exhibit a quite low hardness. One route to increasing the hardness is to add a fourth element, which is a strong carbide-former (e. g. titanium), to form a hard carbide phase. In this work, W-S-C-Ti films have been deposited by magnetron sputtering and characterized with a variety of techniques. The mechanical and tribological properties have been studied and related to the composition.Experimental ProceduresThe films were deposited by non-reactive DC magnetron sputtering using two targets: graphitic carbon and WS2, with a ring-shaped titanium component mounted on the latter. The titanium content was varied by the size of the metal component, while the carbon content was varied by the carbon target power. Four series of films were deposited at room temperature and at 300°C.The micro- and nanostructure of the films was investigated by SEM and TEM, and XRD was used to study the presence of crystalline phases. The composition was determined by EDS, and the chemical bonding was studied by XPS and Raman spectroscopy. Nanoindentation was used to probe the mechanical properties of the different films, and ball-on-disc tests were performed in order to evaluate the tribological properties.Results and DiscussionPrevious studies on W-S-C suggest that the material consists of WS2 nanocrystallites embedded in an amorphous matrix. Also in this study, the only phase detected with XRD is WS2, with the typical WS2 peaks becoming broader with the addition of carbon indicating a decrease in crystallinity. TEM shows WS2 nanocrystallites embedded in an amorphous matrix. However, our results indicate that the composition of the matrix is more complex than what has previously been suggested. Chemical information from XPS suggests that the matrix is not based on carbon alone, but that it also includes a carbidic component. Furthermore, the S/W ratio in the samples is approximately constant but significantly lower than 2; such substochiometry in WS2 films is well known and we will discuss possible mechanisms for this behaviour.By adding titanium to W-S-C, the chemical bonding in the material is changed. XPS indicates the presence of Ti-C bonds even when no crystalline TiC grains are observed by XRD. For high titanium and carbon contents, a crystalline phase with the sodium chloride structure is observed, which has a cell parameter significantly larger than TiC. Furthermore, the added titanium changes the mechanical properties of the films, and an increase in hardness up to 100% from 6 GPa to 12 GPa can be observed. The effect of titanium addition, however, is dependent on the film composition and the deposition temperature.Tribological testing show friction coefficients down to approximately 0.02 in ball-on-disc tests using a steel ball in dry atmosphere for W-S-C films. The effect of titanium addition varies with the composition; high titanium contents combined with suitable carbon levels yields films that exhibit low and stable friction coefficients well under 0.02 under the aforementioned conditions. Thus, it is possible to tune the mechanical properties of W-S-C films, while still obtaining low friction, by the addition of titanium.References[1] A.A. Voevodin, J.S. Zabinski, Thin Solid Films 370, 223-231 (2000)[2] T. Polcar, M. Evaristo, A. Cavaleiro, Plasma Process. Polym. 6, 417-424 (2009)
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| 6. |
- Sundberg, Jill, 1986-, et al.
(författare)
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Sulfur-doping of nc-TiC/a-C films by reactive sputtering
- 2012
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Ingår i: Thirteenth International Conference on Plasma Surface Engineering, Garmisch-Partenkirchen, Germany, 10-14 September 2012.
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Konferensbidrag (refereegranskat)abstract
- Nanocomposite thin films with carbide grains in a matrix of amorphous carbon have been found interesting for various mechanical and electrical applications. An important advantage of these materials is the possibility to tune the properties by varying the composition and the microstructure. A well-known example is the nc-TiC/a-C system, which is interesting for its tribological as well as its electrical and chemically protective properties. One way to modify the material is doping with a third element. Usually, another metal or a p-element such as oxygen or nitrogen is considered. In this work, however, Ti-C films have been doped with sulfur. The doping was performed by introduction of increasing amounts of H2S to the chamber during DC magnetron sputtering from elemental Ti and C targets.An increased flow of H2S during deposition leads to an increase in the S content of the films, as well as a slight decrease in the C:Ti ratio. Pure TiC/a-C films were proved by GI-XRD and XPS to contain crystalline TiC with the NaCl structure in a matrix of amorphous carbon. The introduction of S leads to a significant and gradual increase of the cell parameter of the carbide phase – from 4.3 Å up to more than 4.8 Å for coatings with approximately 20 at-% of S. This clearly indicates that the S atoms enter the carbide phase, forming a previously unknown Ti-C-S solid solution. The addition of S also affects the mechanical properties, such as the hardness which was seen to decrease from 8 GPa for pure TiC in an amorphous carbon matrix, to 5 GPa when doped with sulfur.Thus, the introduction of S is shown to have effects on the chemistry as well as the properties of nc-TiC/a-C thin films. In the current work, the effect of S doping on the structure, chemical bonding and mechanical properties as well as tribological performance will be presented.
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| 7. |
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| 8. |
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| 9. |
- Sundberg, Jill, et al.
(författare)
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Tribochemically Active Ti–C–S Nanocomposite Coatings
- 2013
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Ingår i: Materials Research Letters. - : Taylor & Francis. - 2166-3831. ; 1:3, s. 148-155
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 10. |
- Särhammar, Erik, et al.
(författare)
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Mechanisms for compositional variations of coatings sputtered from a WS2 target
- 2014
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Ingår i: Surface & Coatings Technology. - : Elsevier BV. - 0257-8972 .- 1879-3347. ; 252, s. 186-190
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Tidskriftsartikel (refereegranskat)abstract
- Magnetron sputtering fromcompound targets is widely used for the deposition of compound films since it is easyto scale up and it exhibits a high reproducibility. Controlling film stoichiometry is crucial for obtaining filmswithdesired properties. However, the process is rather complex and sputtering from a compound target frequentlyresults in film compositions that deviate significantly from that of the target. This is due to a number of effectsrelated to the nature of the sputtering process which include preferential re-sputtering due to energetic particlebombardment at the substrate, different take-off angles, scattering in the gas phase, and different stickingcoefficients.In this work, we have investigated how sputtering from aWS2 target results in different film compositions as afunction of the position in the chamber, for different processing conditions. Hence, the films have not been characterizedwith respect to structural or morphological properties. A Monte-Carlo based software, accounting fordifferent take-off angles and scattering in the gas phase, was developed to simulate the compositional variationsat various positions in the chamber. Further, a number of experimentswere performed by varying the target voltage,process pressure, as well as the location of the substrate (on and off axis). Simulations and experiments revealsignificant compositional variations for different processing conditions. Experiments show that thesevariations are only slightly affected by the target voltage, while the most significant variations result from theprocessing pressure and position on the chamber. From the qualitatively good agreement between experimentsand simulations it is clear that gas phase scattering must be taken into account to explain the observed compositionaltrends, while the other effects are less important and sticking coefficients effectsmay even be negligible.It is therefore concluded that themajor effect responsible for the compositional variation of the film is the differentscattering behaviour of S andWin the gas phase.
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