| 1. |
- von Fieandt, Linus
(författare)
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Cutting Edge Titanium-based CVD Hard Coatings
- 2018
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Modern tools for metal cutting applications, such as turning or milling, are typically improved with a thin protective coating. Despite being only a few microns thick, the coating can increase the lifetime of the tool by more than 100 times compared to an uncoated tool. Two different types of techniques are normally used to deposit the coatings, i.e. chemical vapor deposition (CVD) or physical vapor deposition (PVD). A CVD coated tool often includes several different layers. TiN-Ti(C,N)-Al2O3-TiN is a common combination. The research in this thesis has focused on deposition, characterization, and optimization of TiN and Ti(C,N) layers. CVD has been used to deposit all coatings studied in this thesis. They were characterized with a variety of techniques such as: X-ray diffraction, electron microscopy and X-ray photoelectron spectroscopy.TiN was deposited on three different substrates, Co, Fe and Ni. It was found that the TiN coating was strongly affected by the substrate. TiN deposited on Fe substrates resulted in a porous interface caused by substrate etching by the reaction gas mixture. CVD of TiN on Ni substrates resulted in an unwanted intermetallic phase (Ni3Ti) in addition to TiN. Etching or corrosion of the Fe substrates could be reduced by lowering the deposition temperature. In addition, the formation of (Ni3Ti) could be significantly reduced by adjusting the partial pressure of the reactant gases. This shows that CVD of TiN on cutting tools with Fe or Ni as a binder phase needs to be optimized with respect to the process parameters.Thermodynamic calculations of the Ti(C,N) CVD process indicates that the major growth species using CH3CN, TiCl4 and H2 as precursors, was HCN and TiCl3. They were formed in the gas phase by homogeneous reactions. Furthermore, it was found that by adjusting the composition of the reaction gas mixture, the preferred orientation, morphology, and micro-structure of the Ti(C,N) coatings could be tailored. As a result, the tribological/mechanical properties of the Ti(C,N) coatings could be significantly improved. A hardness of 40 GPa, i.e. close to super hard could for instance be achieved. The origin of the mechanical improvements was attributed to a more ordered crystallographic orientation in the <111> direction as well as a high defect density close to the coating surface. In addition to the excellent mechanical properties, the Ti(C,N) coatings were also found to have a high corrosion resistance in sea water, thanks to a formation of a passivating surface layer (TiO2).
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| 2. |
- Särhammar, Erik, 1982-
(författare)
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Sputtering and Characterization of Complex Multi-element Coatings
- 2014
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The thin film technology is of great importance in modern society and is a key technology in wide spread applications from electronics and solar cells to hard protective coatings on cutting tools and diffusion barriers in food packaging. This thesis deals with various aspects of thin film processing and the aim of the work is twofold; firstly, to obtain a fundamental understanding of the sputter deposition and the reactive sputter deposition processes, and secondly, to evaluate sputter deposition of specific material systems with low friction properties and to improve their performance.From studies of the reactive sputtering process, two new methods of eliminating the problematic and undesirable hysteresis effect were found. In the first method it was demonstrated that an increased process pressure caused a reduction and, in some cases, even elimination of the hysteresis. In the second method it was shown that sufficiently high oxide content in the target will eliminate the hysteresis.Further studies of non-reactive magnetron sputtering of multi-element targets at different pressures resulted in huge pressure dependent compositional gradients over the chamber due to different gas phase scattering of the elements. This has been qualitatively known for a long time but the results presented here now enable a quantitative estimation of such effects. For example, by taking gas phase scattering into consideration during sputtering from a WS2 target it was possible to deposit WSx films with a sulphur content going from sub-stoichiometric to over-stoichiometric composition depending on the substrate position relative the target.By alloying tungsten disulphide (WS2) with carbon and titanium (W-S-C-Ti) its hardness was significantly increased due to the formation of a new titanium carbide phase (TiCxSy). The best sample increased its hardness to 18 GPa (compared to 4 GPa for the corresponding W-S-C coating) while still maintaining a low friction (µ=0.02) due to the formation of easily sheared WS2 planes in the wear track.
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| 3. |
- Fritze, Stefan
(författare)
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Microstructure and Mechanical Properties of Magnetron Sputtered Refractory Metal Thin Films
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The design and development of new multifunctional materials that exhibit a combination of high hardness and ductility, as well as a high corrosion resistance and thermal stability, is one of the key challenges in the field of material science. The focus of this thesis is on the development of novel multifunctional magnetron sputtered CrNbTaTiW–C based thin films. Carbon was selected as an alloying element to investigate if it could modify the microstructure (via grain refinement) and improve the properties (e.g. the hardness and ductility).TaW-rich and near-equimolar high entropy alloys in the CrNbTaTiW system were selected as starting points for this study. The latter alloys were predicted, based on empirical design rules, to form a single-phase solid solution. In contrast, thermodynamic calculations showed that the films at equilibrium should be composed of a mixture of several phases at temperatures below 1100 °C. Experimentally, however, a single-phase bcc structure was observed for the deposited films and it was concluded that the films were kinetically and not entropy stabilised. A hypothesis is that the kinetics during sputtering allow a ’direct’ phase selection by tuning the process parameters and evidence of this was found in the HfNbTiVZr alloy system.The CrNbTaTiW–C system is, however, complex and additional studies were carried out on the W–C and TaW–C systems. All metallic films crystallised in a bcc structure with a <110> texture and the column width of these films varied between 25 nm and 80 nm. The films were very hard (~ 13 GPa), which was explained by the small grain size. A single-phase bcc structure was also obtained upon the addition of 5-10 at.% carbon for all compositions except the near-equimolar CrNbTaTiW. X-ray diffraction indicated a unit cell expansion, which was attributed to the formation of a supersaturated solid solution. Additional atom probe tomography (APT) studies on selected samples confirmed the formation of such solid solutions. The supersaturated solid solution is not thermodynamically stable and an annealing study showed that heat treatment yielded segregation and clustering of carbon at the grain boundaries. The addition of carbon had a grain refining effect in the W–C system and the multicomponent CrNbTaTiW–C system. In general, the addition of carbon increased the hardness, which was mainly caused by a reduced grain size in line with the Hall-Petch relationship. Excellent mechanical properties of carbon supersaturated films were further confirmed in pillar tests on W–C films, which showed very high yield strength (~ 9 GPa) and no brittle fracture. The results show that carbon can be used as a chemical approach to control the grain size and properties of these films. Multicomponent carbides with a B1 structure were formed at high carbon concentrations (~ 40 at.%). The microstructure of these films depended strongly on the process parameters and a higher deposition temperature was found to increase the film density and hardness. The TaW-rich carbide exhibited a very high hardness of ~ 35 GPa and excellent corrosion resistance.
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| 4. |
- Lewin, Erik, 1979-
(författare)
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Design of carbide-based nanocomposite coatings
- 2009
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this thesis research on synthesis, microstructure and properties of carbide-based coatings is reported. These coatings are electrically conducting, and can be tailored for high hardness, low friction and wear, along with load-adaptive behaviour. Tailoring these properties is achieved by controlling the relative phase content of the material. Coatings have been synthesised by dc magnetron sputtering, and their structures have been characterised, mainly by X-ray photoelectron spectroscopy and X-ray diffraction.It has been shown that nanocomposites comprising of a nanocrystalline transition metal carbide (nc-MeCx, Me = Ti, Nb or V) and an amorphous carbon (a-C) matrix can result in low contact resistance in electrical contacts. Such materials also exhibit low friction and high resistance to wear, making them especially suitable for application in sliding contacts. The lowest contact resistance is attained for small amounts of the amorphous carbon phase.It has been shown that specific bonding structures are present in the interface between nc-TiCx and the a-C phases in the nanocomposite. It was found in particular that Ti3d and C2p states are involved, and that considerable charge transfer occurs across the interface, thereby influencing the structure of the carbide.Further design possibilities were demonstrated for TiCx-based nanocomposites by alloying them with weakly carbide-forming metals, i.e., Me = Ni, Cu or Pt. Metastable supersaturated solid solution carbides, (T1-xMex)Cy, were identified to result from this alloying process. The destabilisation of the TiCx-phase leads to changes in the phase distribution in the deposited nanocomposites, thus providing further control over the amount of carbon phase formed. Additional design possibilities became available through the decomposition of the metastable (Ti1-xMex)Cy phase through an appropriate choice of annealing conditions, yielding either more carbon phase or a new metallic phase involving Me. This alloying concept was also studied theoretically for all 3d transition metals using DFT techniques.It has also been demonstrated that Ar-ion etching (commonly used in the analysis of carbide based nanocomposites) can seriously influence the result of the analysis, especially for materials containing metastable phases. This implies that more sophisticated methods, or considerable care are needed in making these analyses, and that many of the earlier published results could well be in error.
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| 5. |
- Pettersson, Robert
(författare)
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Sami tourism in Northern Sweden : Supply, demand and interaction
- 2004
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Indigenous tourism is an expansive sector in the growing tourism industry. The Sami people living in Sápmi in northern Europe have started to engage in tourism, particularly in view of the rationalised and modernised methods of reindeer herding. Sami tourism offers job opportunities and enables the spreading of information. On the other hand, Sami tourism may jeopardise the indigenous culture and harm the sensitive environment in which the Sami live. The aim of this thesis is to analyse the supply and demand of Sami tourism in northern Sweden. This is presented in four articles. The first article analyses the potential of the emerging Sami tourism in Sweden, with special emphasis on the access to Sami tourism products. The study shows that there is a growing supply of tourism activities related to the Swedish Sami. The development of tourism is, however, restricted by factors such as the peripheral location and the lack of traditions of entrepreneurship. The second article analyse which factors influence tourists when they make their decisions about Sami tourism. In the article the respondents are requested to answer a number of hypothetical questions, ranking their preferences regarding supply, price and access. The study indicates that tourism related to the Sami and Sami culture has a considerable future potential, but also that there is a gap between supply and demand. In the third article the analysis shows that the festival in Jokkmokk, thanks to continuously added attractions, has been able to retain a rather high level of popularity, despite its peripheral location. Finally, the fourth article analyses to what extent the winter festival in Jokkmokk is a genuinely indigenous event, and to what extent it is staged. It is argued that the indigenous culture presented at the festival and in media is highly staged, although backstage experiences are available for the Sami and for the tourists who show a special interest.
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