| 1. |
- Carlsson, Tobias, et al.
(author)
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Off-equilibrium response of grafted polymer chains subject to a variable rate of compression
- 2011
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In: Physical Chemistry, Chemical Physics - PCCP. - : Royal Society of Chemistry (RSC). - 1463-9076 .- 1463-9084. ; 13:24, s. 11757-11765
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Journal article (peer-reviewed)abstract
- We present Brownian dynamics simulations of single grafted semiflexible chains (i.e., "polymer mushrooms'') with varying persistence lengths, intra-chain interactions, and subject to confinement. The results from different rates of compression are presented in the cases of an approaching infinite plane and a paraboloid tip. We discuss the different behaviour observed for grafted chains with strong and weak self-attraction (i.e., "hard'' and "soft'' polymer mushrooms). In both cases the effect on the size and shape is more pronounced for a slow compression rate, especially for "hard mushrooms''. We have also studied the relaxation of the chain while the compressing plane is maintained, and when it is removed suddenly. We find that the response depends strongly on the time allowed for relaxation in the compressed state. When using instead a paraboloid tip, the overall effects are similar yet less pronounced because the chain can dodge the confining object via an "escape transition.''
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| 2. |
- Freitas de Abreu, Marcio, et al.
(author)
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Morphology and mechanisms of cavitation damage on lamellar gray iron surfaces
- 2020
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In: Wear. - : Elsevier BV. - 0043-1648 .- 1873-2577. ; 456-457, s. 203324-203324
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Journal article (peer-reviewed)abstract
- Engine parts in contact with liquids may suffer cavitation erosion damage. Understanding its mechanisms in realistic operating environments is necessary for improvements in the service life and durability of materials for heavy-duty diesel engines. This work illustrates the cavitation erosion behavior of a cylinder liner material with a follow-up of detailed, high-magnification SEM images of damaged sites on the surface. The cylinder liner encloses the piston and combustion chamber in the engine of large trucks and its material of choice is usually a lamellar gray cast iron, its microstructure consisting of flake-shaped graphite, a pearlitic matrix and some steadite. Testing was carried out using an ultrasonic vibratory apparatus, and the liquid of choice was a commercial engine coolant composed of water, glycol and inhibitors. Based on observations of tested surfaces, a sequence of damage patterns is proposed as an explanation of the material’s cavitation erosion behavior. Initiation consists of: chipping at graphite cluster centers, graphite flake removal, pitting along graphite flakes and direct matrix pitting. Development consists of: evolution of chipped spots into matrix-damaging pits, radial pit expansion, pit merging and surface roughening. It can be concluded that presence and morphology of graphite are critical to the cavitation erosion behavior of LGI.
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| 3. |
- Grandin, Martina, et al.
(author)
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Ti-Ni-C nanocomposite coatings evaluated in a sliding electrical contact application
- 2015
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In: Surface & Coatings Technology. - : Elsevier BV. - 0257-8972 .- 1879-3347. ; 276, s. 210-218
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Journal article (peer-reviewed)abstract
- 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.
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| 4. |
- Isaeva, Leyla, et al.
(author)
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Amorphous W-S-N thin films: the atomic structure behind ultra-low friction
- 2015
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In: Acta Materialia. - : Elsevier BV. - 1359-6454 .- 1873-2453. ; 82, s. 84-93
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Journal article (peer-reviewed)abstract
- 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.
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| 5. |
- Kádas, Krisztina, et al.
(author)
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Formation of 2D transition metal dichalcogenides on TiC1-xAx surfaces (A=S, Se, Te) : A theoretical study
- 2014
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In: Journal of Materials Research. - : Springer Science and Business Media LLC. - 0884-2914 .- 2044-5326. ; 29:2, s. 207-214
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Journal article (peer-reviewed)abstract
- 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.
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| 6. |
- Nyberg, Harald, et al.
(author)
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Extreme friction reductions during inital running-in of W-S-C-Ti low-friction coatings
- 2013
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In: Wear. - : Elsevier. - 0043-1648 .- 1873-2577. ; 302:1-2 SI, s. 987-997
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Journal article (peer-reviewed)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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| 7. |
- Nyberg, Harald, et al.
(author)
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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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In: Tribology letters. - : Springer Science+Business Media B.V.. - 1023-8883 .- 1573-2711. ; 56:3, s. 563-572
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Journal article (peer-reviewed)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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| 8. |
- Sundberg, Frida, et al.
(author)
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Managing diabetes in preschool children.
- 2017
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In: Pediatric diabetes. - : Hindawi Limited. - 1399-5448 .- 1399-543X. ; 18:7, s. 499-517
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Journal article (peer-reviewed)
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| 9. |
- Sundberg, Jill, 1986-, et al.
(author)
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Influence of composition, structure and testing atmosphere on the tribological performance of W-S-N coatings
- 2014
-
In: Surface & Coatings Technology. - : Elsevier. - 0257-8972 .- 1879-3347. ; 258, s. 86-94
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Journal article (peer-reviewed)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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| 10. |
- Sundberg, Jill, et al.
(author)
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Influence of Ti addition on the structure and properties of low-friction W–S–C coatings
- 2013
-
In: Surface & Coatings Technology. - : Elsevier. - 0257-8972 .- 1879-3347. ; 232, s. 340-348
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Journal article (peer-reviewed)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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