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- 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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| 2. |
- Alfredsson, Sara, et al.
(författare)
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Tribological conditions of curling - the ultimate friction sport?
- 2011
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Ingår i: 18th International Conference on Wear of Materials, Philadelphia, USA, April 3-7, 2011.
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Konferensbidrag (refereegranskat)abstract
- Curling is an Olympic winter sport in which 8 players forming two teams slide stones across a sheet of ice towards a target area. The two teams have eight stones each and take turns to slide the stones over to the target area, some 28 m away from the release line. After being released, the stone is only affected by the sliding friction. However, this friction may be somewhat modified by sweeping the ice just in front of the sliding stone, using special curling brooms. Further, the trajectory of the stone becomes slightly curled. By slowly turning the stone clockwise when it is released, it will turn to the right, and vice versa. The best team in each round of 16 stones score one point for each stone resting closer to the target than the best stone from the opponent team.The game makes up a very interesting tribological system, presenting a number of challenging problems. These problems include understanding exactly:what determines the level of friction, and how it is affected by the sweeping,how the roughness of the stone influences the friction, and how the sliding surface should best be prepared to give a stable and repeatable friction,how the intentionally bumpy "pebbled" ice structure influences the friction,the size and distribution of the contact spots between the rough stone and the pebbled ice,the mechanism causing the curl of the stone - "Why does the stone curl?".This poster is based on an experimental project, and presents this very intriguing tribological system. It offers some ready explanations and challenges the visiting tribologists to contribute their insights.
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| 3. |
- Nyberg, Vanja E, 1941-, et al.
(författare)
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Swedish quality registry for pain rehabilitation: : purpose, design, implementation and characteristics of referred patients
- 2011
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Ingår i: Journal of Rehabilitation Medicine. - : Foundation for Rehabilitation Information. - 1650-1977 .- 1651-2081. ; 43:1, s. 50-57
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Tidskriftsartikel (refereegranskat)abstract
- Objective: In order to facilitate comparisons of pain rehabilitationprogrammes in Sweden and to enable audit spirals for single programmes as well as outcome studies, the Swedish Association for Rehabilitation Medicine initiated a national quality registry in 1995. Patients: Referred for rehabilitation due to pain-related disability. Methods: The registry collects standardized self-reports before assessment, after rehabilitation and one year later, covering demographic, educational and psychometric data, pain intensity, physical disability and life satisfaction. Sick leave data are collected from the National Insurance Board before and up to two years after rehabilitation. At each programme unit self-reports are processed into individual assessment profiles relevant to plan rehabilitation. Data are sent annually to the central registry for analysis and compared with“return to work” data. Each unit can compare its results with national means. Results and conclusion: The organization of the registry is described. Data indicating that contextual factors, but not pain characteristics, depression or activity limitations vary between patients referred to different centres, makes comparisons difficult. As of 2007, data from the multidisciplinary assessment of 19,833 patients have been collected. A total of 7289 patients attended a rehabilitation programme, generating two more self-reports. A limitation of the study is a lack of follow-up data from some units.
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| 4. |
- Olofsson, Johanna, 1981-, et al.
(författare)
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On the influence from micro topography of PVD coatings on friction behaviour, material transfer and tribofilm formation
- 2011
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Ingår i: Wear. - : Elsevier. - 0043-1648 .- 1873-2577. ; 271:9-10, s. 204-2057
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Konferensbidrag (refereegranskat)abstract
- 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.
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