| 1. |
- 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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| 2. |
- 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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| 3. |
- 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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| 4. |
- 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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| 5. |
- Nyberg, Harald, 1985-, et al.
(författare)
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Calculated trajectories of curling stones sliding under asymmetrical friction
- 2012
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Ingår i: Nordtrib 2012, 15th Nordic Symposium on Tribology.
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Konferensbidrag (refereegranskat)abstract
- In the sport of curling, stones are slid across an ice sheet, aimed towards a target area. A sliding stone does not move in a straight line, but follows a curled trajectory, deviating in a direction determined by its rotation. As of yet, no satisfactory explanation for this motion has been presented, although many attempts have been made. In many of them, the curling motion has been attributed to an asymmetrical distribution of the friction force acting on the sliding stone, typically such that the friction on the rear of the stone (as seen in the direction of motion) is higher than that on the front. In this paper, the motion of a rotating curling stone sliding over ice is calculated, for different assumed distributions of the coefficient of friction in the contact between stone and ice, using a numerical method. It is shown that no redistribution of the friction, no matter how extreme, can explain the observed motion of a real curling stone.
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| 6. |
- Nyberg, Harald, 1985-, et al.
(författare)
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Design of low-friction PVD coating systems with enhanced running-in performance - carbon overcoats on TaC/aC coatings
- 2012
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Ingår i: Nordtrib 2012, 15th Nordic Symposium on Tribology.
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Konferensbidrag (refereegranskat)abstract
- 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 were 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.
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| 7. |
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| 8. |
- Nyberg, Harald, 1985-, et al.
(författare)
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On the influence from micro topography on the structure and growth of low friction amorphous carbon PVD coatings
- 2010
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Ingår i: Twelfth International Conference on Plasma Surface Engineering (PSE 2010), September 13-17, 2010, Garmisch-Partenkirchen.
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Konferensbidrag (refereegranskat)abstract
- The interest in metal carbide doped amorphous carbon coatings produced by physical vapour deposition (PVD) for use on heavily stressed machine elements is currently increasing, mainly due to their ability to achieve low friction and reduced wear of the counter surface. The tribological properties of these types of coatings have however been found to vary strongly between seemingly similar coatings. A potential source of these differences could be the micro topography of the coated surfaces. Argon ion etch cleaning of the substrates is a common process step in the production of PVD coatings and is usually performed as the final cleaning step before coating deposition. For some materials, the etching process may result in a roughening of the substrate surface, due to differences in the etch rates of the different parts of the material. In high speed steels, carbides typically etch slower than the metallic phase, resulting in a surface covered by protruding carbides. In the current study, it was examined how varying amounts of argon ion etching of highly polished high speed steel substrates prior to coating influences the micro topography of the substrates and the final coatings. Tantalum carbide doped amorphous carbon coatings (TaC:C) were produced by co-sputtering of carbon and tantalum, in an argon atmosphere. The impact of the substrate micro topography on the growth and structure of the coatings was studied, using high resolution scanning electron microscopy (SEM) of superficial coating cross sections produced with a focused ion beam (FIB). Special attention was paid to coating growth in the immediate vicinity of protruding carbides, as well as to the structure of the coating in these regions.
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| 9. |
- Nyberg, Harald, 1985-, et al.
(författare)
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The asymmetrical friction mechanism that puts the curl in the curling stone
- 2013
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Ingår i: Wear. - : Elsevier. - 0043-1648 .- 1873-2577. ; 301:1-2, s. 583-589
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Tidskriftsartikel (refereegranskat)abstract
- Curling is an Olympic winter sport in which two teams slide stones across a sheet of ice towards a target area, some 28 m away from the release line. The sport has its name from the fact that the trajectory of a rotating stone becomes slightly curled, a fact used to reach open spots or take out opponent stones behind hindering “guarding” stones, etc. By slowly turning the stone clockwise when it is released, it will curl to the right, and vice versa. The resulting sideward deviation is typically slightly more than a metre. This intriguing tribological phenomenon has so far lacked a satisfactory explanation, although many attempts have been presented. In many of them, the curling motion has been attributed to an asymmetrical distribution of the friction force acting on the sliding stone, such that the friction on the rear of the stone (as seen in the direction of motion) is higher than that on the front. In a recent paper, we could show that no such redistribution of the friction, no matter how extreme, can explain the magnitude of the observed motion of a real curling stone. The present work presents an alternative asymmetrical mechanism that actually is strong enough to account for the observed motion. Further, in contrast to previous models, it satisfies other observed phenomena, including the independence of rotational speed of the stone and the strong dependence of the roughness of the stone. The model is backed up by experimental evidence and is based on the specific tribological conditions presented by the contact between a scratched curling stone and a pebbled ice sheet.
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| 10. |
- Skiöld Nyberg, Harald, 1985-
(författare)
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Formation and Function of Low-Friction Tribofilms
- 2014
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- 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.
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