| 1. |
- Kubart, Tomas, et al.
(författare)
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Modelling of Magnetron Sputtering of Tungsten Oxide with Reactive Gas Pulsing
- 2007
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Ingår i: Plasma Processes and Polymers. - : Wiley. - 1612-8850 .- 1612-8869. ; 4, s. S522-S526
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Tidskriftsartikel (refereegranskat)abstract
- Reactive sputtering is one of the most commonly employed processes for the deposition of thin films. However, the range of applications is limited by inherent instabilities, which necessitates the use of a complex feedback control of reactive gas (RG) partial pressure. Recently pulsing of the RG has been suggested as a possible alternative. In this report, the concept of periodically switching the RG flow between two different values is applied to the deposition of tungsten oxide. The trends in the measured time dependent RG pressure and discharge voltage are reproduced by a dynamical model developed for this process. Furthermore, the model predicts the compositional depth profile of the deposited film reasonably well, and in particular helps to understand the formation of the interfaces in the resulting multi-layer film.
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| 2. |
- 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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| 3. |
- 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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| 4. |
- 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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| 5. |
- 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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| 6. |
- Särhammar, Erik, et al.
(författare)
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Mechanisms responsible for compositional variations of films sputtered from a WS2 target
- 2012
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Ingår i: International Conference on Metallurgical Coatings and Thin films (ICMCTF) 23-27/04 2012, San Diego, abstract number:428.
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Konferensbidrag (refereegranskat)abstract
- Transition metal dichalcogenides (TMDs) such as WS2 are well-known for their layered structure and solid lubricant properties. However, beside low friction, a solid lubricant coating must also have a long wear life in order to perform well in a tribological situation. Thus, by adding carbon to the material the mechanical properties can be improved. However, when using a magnetron sputtering process, the resulting thin films are found to be sub-stoichiometric with respect to sulphur. This is due to a number of different effects; take-off angle, scattering, different sticking coefficients and energetic particle bombardment of the substrate.In this work we have used a non-reactive magnetron sputtering process to see how these effects affect the resulting film stoichiometry, and hence the tribological properties. This was done by changing the process pressure, DC-RF power, the location of the substrate (in and off axes) and by adding carbon to the material. Also, a newly developed Monte Carlo computer model is presented which makes it possible to simulate and predict how these changes will affect the resulting film stoichiometry.Simulations and experiments alike show that by reducing the energetic particle bombardment of the substrate, the S/W ratio increases. Tribological evaluation of the films concludes that an increasing S/W ratio is beneficial as it decreases the coefficient of friction of the films.
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| 7. |
- Austgen, M, et al.
(författare)
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Sputter yield amplification by tungsten doping of Al(2)O(3) employing reactive serial co-sputtering : process characteristics and resulting film properties
- 2011
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Ingår i: Journal of Physics D. - : IOP Publishing. - 0022-3727 .- 1361-6463. ; 44:34, s. 345501-
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Tidskriftsartikel (refereegranskat)abstract
- The deposition rate of reactively sputtered Al(2)O(3) coatings is demonstrated to increase by 80% upon tungsten doping of the used aluminium target. This effect is based on the recoil of the sputtering species at implanted dopants below the target surface and is termed sputter yield amplification. For the investigation of this effect, a novel type of magnetron sputter deposition system is employed that facilitates serial co-sputtering. In this technique doping of the elementary target is enabled by a dynamic sputtering process from an auxiliary cathode. In our case, the rotating aluminium target is dynamically coated with tungsten from this auxiliary cathode. Since the primary target rotates, the auxiliary cathode is placed in series with the primary erosion zone. The deposition rate of Al(2)O(3) can be considerably increased in this process already for very low concentrations of approximately 1% of tungsten in the resulting film. A characterization of the dynamics of reactive sputtering as a function of target rotation speed is performed.
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| 8. |
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| 9. |
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| 10. |
- Gao, Xindong, et al.
(författare)
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Epitaxy of Ultrathin NiSi2 Films with Predetermined Thickness
- 2011
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Ingår i: Electrochemical and solid-state letters. - : The Electrochemical Society. - 1099-0062 .- 1944-8775. ; 14:7, s. H268-H270
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Tidskriftsartikel (refereegranskat)abstract
- This letter presents a proof-of-concept process for tunable, self-limiting growth of ultrathin epitaxial NiSi2 films on Si (100). The process starts with metal sputter-deposition, followed by wet etching and then silicidation. By ionizing a fraction of the sputtered Ni atoms and biasing the Si substrate, the amount of Ni atoms incorporated in the substrate after wet etching can be controlled. As a result, the thickness of the NiSi2 films is increased from 4.7 to 7.2 nm by changing the nominal substrate bias from 0 to 600 V. The NiSi2 films are characterized by a specific resistivity around 50 mu Omega cm.
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