| 1. |
- Barrirero, Jenifer, 1981-
(författare)
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Eutectic Modification of Al-Si casting alloys
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Aluminum alloys with silicon as the major alloying element are the most widely used aluminum casting alloys. The eutectic phase in these alloys is formed by hard and brittle silicon plates in an aluminum matrix. Such silicon plates can act as crack propagation paths deteriorating the toughness of the material. To enhance ductility, silicon can be modified to a coral-like microstructure by addition of a modifying agent. Amongst the elements proposed as modifiers, only strontium, sodium and europium induce a plate-tocoral transition, while others such as ytterbium, only refine the silicon plates. The exact mechanism for the remarkable plate-to-coral change, and the reason why certain elements only refine the structure, is still not completely understood.In this investigation, atom probe tomography and transmission electron microscopy were used to analyze and compare the crystal structure and the distribution of solute atoms in silicon at the atomic level. An unmodified alloy and alloys modified by strontium, sodium, europium and ytterbium were studied. Elements inducing silicon plate-to-coral transition were found to contain nanometer sized clusters at the defects in silicon with stoichiometries corresponding to compounds formed at the ternary eutectic reaction of each system. In contrast, the addition of ytterbium, that only refines the silicon plates, is unable to form clusters in silicon. We propose that the formation of ternary compound clusters AlSiNa, Al2Si2Sr and Al2Si2Eu at the silicon / liquid interface during solidification restrict silicon growth. The formation of clusters on silicon facets create growth steps and increase growth direction diversity. The incorporation of clusters in silicon explains the high density of crystallographic defects and the structural modification from plates to corals.The parallel lattice plane-normals 011Si // 0001Al2Si2Eu, 011Si // 610Al2Si2Eu and 111Si // 610Al2Si2Eu were found between Al2Si2Eu and silicon, and absent between Al2Si2Yb and silicon. We propose a favorable heterogeneous formation of Al2Si2Eu on silicon. The misfit between 011Si and 0002Al2Si2X interplanar spacings shows a consistent trend with the potency of modification for several elements such as strontium, sodium, europium, calcium, barium, ytterbium and yttrium.
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| 2. |
- Engberg, David
(författare)
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Atom Probe Tomography of TiSiN Thin Films
- 2015
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis concerns the wear resistant coating TiSiN and the development of the analysis technique atom probe tomography (APT) applied to this materials system. The technique delivers compositional information through time-of-flight mass spectrometry, with sub-nanometer precision in 3D for a small volume of the sample. It is thus a powerful technique for imaging the local distribution of elements in micro and nanostructures. To gain the full benefits of the technique for the materials system in question, I have developed a method that combines APT with isotopic substitution, here demonstrated by substitution of natN with 15N. This alters the time-of-flight of ions with of one or more N and will thereby enable the differentiation of the otherwise inseparable isotopes 14N and 28Si. Signs of small-scale fluctuations in the data led the development of an algorithm needed to properly visualize these fluctuations. A method to identify the best sampling parameter for visualization of small-scale compositional fluctuations was added to an algorithm originally designed to find the best sampling parameters for measuring and visualizing strong compositional variations. With the identified sampling parameters, the nano-scale compositional fluctuations of Si in the metal/metalloid sub-lattice could be visualized. The existence and size of these fluctuations were corroborated by radial distribution functions, a technique independent of the previously determined sampling parameter. The radial distribution function algorithm was also developed further to ease in the interpretation. The number of curves could thereby be reduced by showing elements, rather than single and molecular ions (of which there were several different kinds). The improvement of the algorithm also allowed interpretation of signs regarding the stoichiometry of SiNy. With a combination of analytical transmission electron microscopy and APT we show Si segregation on the nanometer scale in arc-deposited Ti0.92Si0.0815N and Ti0.81Si0.1915N thin films. APT composition maps and proximity histograms generated from Ti-rich domains show that the TiN contain at least ~2 at. % Si for Ti0.92Si0.08N and ~5 at. % Si for Ti0.81Si0.19N, thus confirming the formation of solid solutions. The formation of relatively pure SiNy domains in the Ti0.81Si0.19N films is tied to pockets between microstructured, columnar features in the film. Finer SiNy enrichments seen in APT possibly correspond to tissue layers around TiN crystallites, thus effectively hindering growth of TiN crystallites, causing TiN renucleation and thus explaining the featherlike nanostructure within the columns of these films.
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| 3. |
- Johnson, Lars, 1983-
(författare)
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Inside The Miscibility Gap : Nanostructuring and Phase Transformations in Hard Nitride Coatings
- 2012
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis is concerned with self-organization phenomena in hard and wear resistant transition-metal nitride coatings, both during growth and during post deposition thermal annealing. The uniting physical principle in the studied systems is the immiscibility of their constituent parts, which leads, under certain conditions, to structural variations on the nanoscale. The study of such structures is challenging, and during this work atom probe tomography (apt) was developed as a viable tool for their study. Ti0.33Al0.67N was observed to undergo spinodal decomposition upon annealing to 900 °C, by the use of apt in combination with electron microscopy. The addition of C to TiSiN was found to promote and refine the feather-like microstructure common in the system, with an ensuing decrease in thermal stability. An age-hardening of 36 % was measured in arc evaporated Zr0.44Al0.56N1.20, which was a nanocomposite of cubic, hexagonal, and amorphous phases. Magnetron sputtering of Zr0.64Al0.36N at 900 °C resulted in a self-organized and highly ordered growth of a two-dimensional two-phase labyrinthine structure of cubic ZrN and wurtzite AlN.The structure was analyzed and recovered by apt, although the ZrN phase suffered from severe trajectory aberrations, rendering only the Al signal useable.The initiation of the organized growth was found to occur by local nucleation at 5-8 nm from the substrate, before which random fluctuations in Al/Zr content increased steadily from the substrate. Finally, the decomposition of solid-solution TiB0.33N0.67 was found, by apt, to progress through the nucleation of TiB0.5N0.5 and TiN, followed by the transformation of the former into hexagonal TiB2.
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| 4. |
- Moreno, Maiara, 1993-
(författare)
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Wear behavior of Ti1-xAlxN-based coatings during turning
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Ti1-xAlxN coatings are extensively used on cutting tools used for metal cutting. In this thesis, the wear behavior of TiN, TiAlN with different Al-content, and TiAlWN and TiAlMoN coatings is investigated after turning, using electron microscopy and X-ray absorption spectroscopy techniques. An in operando study using high-energy X-ray diffraction during turning is also carried out, to understand the strain and phase evolution of TiAlN coatings during machining. The main wear mechanisms when cutting stainless steel 316L with cutting speeds from 60 m/min up to 220 m/min are investigated. The results show a difference in wear behavior with cutting speed. At low cutting speeds, adhesive wear is the main mechanism that generates coating failure, causing fracture of the coating. The higher Al-content coatings (x ≥ 0.53) perform better compared to lower Al-content coatings, likely due to a better fracture resistance. At higher cutting speeds, 220 m/min, crater wear due to abrasive wear and chemical reactions between coating and workpiece material occurs. In this case, the high temperatures achieved during turning results in formation of hexagonal (h)-AlN in Ti0.38Al0.62N coatings, which lowers their wear resistance. For TiAlMoN and TiAlWN, an improved wear behavior is observed compared to pure TiAlN, because it retards spinodal decomposition and the subsequent formation of h-AlN. Investigations of the sliding area using TEM, EDS and XANES spectra from the Ti 1s-edge reveals that there are differences in level of spinodal decomposition, thus differences in temperature, in different regions of the tool. During in operando orthogonal turning of alloy steel, spinodal decomposition was observed to take place after only 10 s of turning for the highest Al-content coatings. Decomposition occurs where the temperature of the rake face is the highest. In summary, the results achieved lead to a better understanding of the interactions between tool and workpiece material and the different wear mechanisms which may expand the application envelope for these coatings.
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| 5. |
- Rogström, Lina, 1983-
(författare)
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High temperature behavior of arc evaporated ZrAlN and TiAlN thin films
- 2012
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Hard coatings can extend the life time of a tool substantially and enable higher cutting speeds which increase the productivity in the cutting application. The aim with this thesis is to extend the understanding on how the microstructure and mechanical properties are affected by high temperatures similar to what a cutting tool can reach during operation.Thin films of ZrAlN and TiAlN have been deposited using cathodic arc-evaporation. The microstructure of as-deposited and annealed films has been studied using electron microscopy and x-ray scattering. The thermal stability has been characterized by calorimetry and thermogravity and the mechanical properties have been investigated by nanoindentation.The microstructure of Zr1−xAlxN thin films was studied as a function of composition, deposition conditions, and annealing temperature. The structure was found to depend on the Al content where a low (x < 0.38) Al-content results in cubic-structured ZrAlN while for x > 0.70 the structure is hexagonal. For intermediate Al contents (0.38 < x < 0.70), a nanocomposite structure with a mixture of cubic, hexagonal and amorphous phases is obtained.The cubic ZrAlN phase transforms by nucleation and growth of hexagonal AlN when annealed above 900 ◦C. Annealing of hexagonal ZrAlN thin films (x > 0.70) above 900 ◦C causes formation of AlN and ZrN rich domains within the hexagonal lattice. Annealing of nanocomposite ZrAlN thin films results in formation of cubic ZrN and hexagonal AlN. The transformation is initiated by nucleation and growth of cubic ZrN at temperatures of 1100 ◦C while the AlN-rich domains are still amorphous or nanocrystalline. Growth of hexagonal AlN is suppressed by the high nitrogen content of the films and takes place at annealing temperatures of 1400 ◦C.In the more well known TiAlN system, the initial stage of decomposition is spinodal with formation of cubic structured domains enriched in TiN and AlN. By a combination of in-situ xray scattering techniques during annealing and phase field simulations, both the microstructure that evolves during decomposition and the decomposition rate are found to depend on the composition. The results further show that early formation of hexagonal AlN domains during decomposition can cause formation of strains in the cubic TiAlN phase.
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| 6. |
- Salamania, Janella, 1992-
(författare)
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Defects in Titanium Aluminum Nitride-Based Thin Films
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Coatings and thin films inherently contain several types of defects. This thesis aims to enhance the understanding of the relationship of defects on the growth, structure, stability, and properties of titanium aluminum nitride films synthesized by physical vapor deposition techniques.Heteroepitaxial cubic and wurtzite films in the Ti-Al-N system grown by reactive magnetron sputtering were studied in relation to their defect structures. The dislocation structures of heteroepitaxial TiN and Ti1-xAlxNy films were analyzed by high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM). Together with atomistic simulations, it was revealed that the presence of different dislocation types in TiN enhances the metal-metal bonds which locally weakens the directionally covalent metal-N bonds. In epitaxial cubic Ti1-xAlxN films, microstrain analysis shows that increasing N-vacancies influences the strain and compositional fluctuations in as-deposited states. During spinodal decomposition induced by annealing to high temperatures, the delay in coarsening and strain correlates with the amount of N vacancies. Detailed characterization of the decomposing domains exposed the formation of stacking faults and partial dislocations as a strain-relieving mechanism which also facilitates the known cubic-to-wurtzite transformation in Ti-Al-N.Cathodic arc deposited Ti1-xAlxN films were grown by applying a low duty cycle pulsed-substrate bias and high nitrogen pressures. This resulted into films with coarse grains and low lattice defects within them, indicating a kinetically controlled route to modify the defect structures in arc-deposited films. Applying the same technique on single crystalline TiN seed layer films kinetically stabilizes a pseudomorphic growth, allowing to form a highly textured, pseudo epitaxial wurtzite Ti1-xAlxN films by arc deposition. In combination with theoretical calculations, it was revealed that w-Ti1-xAlxN films also exhibit a miscibility gap which enables spinodal decomposition and thus age hardening when annealed. Finally, magnetron sputtered nitrogen-deficient w-Ti1-xAlxNy heteroepitaxial films were shown to exhibit a decomposition route that involves the formation of coherent intermediate MAX-like phases before transforming to pure c-TiN and w-AlN phases, which results to continued age hardening up to 1200°C.The findings in this work increase the fundamental understanding of the role of defects in Ti-Al-N films and open new routes for defect-based engineering strategies.
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| 7. |
- Shulumba, Nina, 1988-
(författare)
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Vibrations in solids : From first principles lattice dynamics to high temperature phase stability
- 2015
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this thesis I introduce a new method for calculating the temperature dependent vibrational contribution to the free energy of a substitutionally disordered alloy that accounts for anharmonicity at high temperatures. This method exploits the underlying crystal symmetries in an alloy to make the calculations tractable. The validity of this approach is demonstrated by constructing the phase diagram via direct minimization of the Gibbs free energy of a notoriously awkward and technologically important system, Ti1-xAlxN. The vibrational entropy including anharmonic effects is shown to be large and comparable to the configurational entropy at high temperatures, and with its inclusion, the theoretical miscibility gap of Ti1-xAlxN is reduced from 6560 K to 2860 K, in line with atom probe experiments. A similar treatment of Zr1-xAlxN and Hf1-xAlxN alloys suggests that mass disorder has a minimal effect on phase stability compared with chemical ordering. My method is also capable of demonstrating that Hf1-xAlxN, which is dynamically unstable at room temperature, is stabilised at high temperatures. Moreover I develop a new method of computing temperature dependent elastic constants for alloys from their phonon spectra, and show that for Ti1-xAlxN, the elastic anisotropy is found to increase with temperature, helping to explain the spinodal decomposition.The effects of lattice dynamics on phase stability, mechanical, magnetic and transport properties on other materials are also examined. Four specific systems are discussed in detail. Firstly, in the case of CrN, lattice vibrations are shown to decrease the antiferromagnetic to paramagnetic phase transition temperature from 500 K to 380 K, in line with experimental evidence. Secondly, a temperature/pressure induced phase transition in AlN becomes much more facile than in the quasiharmonic approximation, and the thermal conductivity of the rocksalt phase is shown to be much lower than that of the wurtzite phase, as a result of the increased anharmonicity in the rocksalt structure. Thirdly, the temperature dependence of elastic constants of TiN becomes more isotropic as the temperature increases. Finally, iron carbides are evaluated as potentially important phases at the Earth’s core; specifically, calculating the Gibbs free energy of a recently discovered orthorhombic phase of Fe7C3 demonstrates that it is not stable relative to the known hexagonal phase at extreme pressure and temperatures.
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| 8. |
- Sveen, Susanne
(författare)
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Wear of coated and uncoated PCBN cutting tool used in turning and milling
- 2014
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This licentiate thesis has the main focus on evaluation of the wear of coated and uncoated polycrystalline cubic boron nitride cutting tool used in cutting operations against hardened steel. And to exam the surface finish and integrity of the work material used. Harder work material, higher cutting speed and cost reductions result in the development of harder and more wear resistance cutting tools. Although PCBN cutting tools have been used in over 30 years, little work have been done on PVD coated PCBN cutting tools. Therefore hard turning and hard milling experiments with PVD coated and uncoated cutting tools have been performed and evaluated. The coatings used in the present study are TiSiN and TiAlN. The wear scar and surface integrity have been examined with help of several different characterization techniques, for example scanning electron microscopy and Auger electron spectroscopy. The results showed that the PCBN cutting tools used displayed crater wear, flank wear and edge micro chipping. While the influence of the coating on the crater and flank wear was very small and the coating showed a high tendency to spalling. Scratch testing of coated PCBN showed that, the TiAlN coating resulted in major adhesive fractures. This displays the importance of understanding the effect of different types of lapping/grinding processes in the pre-treatment of hard and super hard substrate materials and the amount and type of damage that they can create. For the cutting tools used in turning, patches of a adhered layer, mainly consisting of FexOy were shown at both the crater and flank. And for the cutting tools used in milling a tribofilm consisting of SixOy covered the crater. A combination of tribochemical reactions, adhesive wear and mild abrasive wear is believed to control the flank and crater wear of the PCBN cutting tools. On a microscopic scale the difference phases of the PCBN cutting tool used in turning showed different wear characteristics. The machined surface of the work material showed a smooth surface with a Ra-value in the range of 100-200 nm for the turned surface and 100-150 nm for the milled surface. With increasing crater and flank wear in combination with edge chipping the machined surface becomes rougher and showed a higher Ra-value. For the cutting tools used in milling the tendency to micro edge chipping was significant higher when milling the tools steels showing a higher hard phase content and a lower heat conductivity resulting in higher mechanical and thermal stresses at the cutting edge.
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| 9. |
- Wu, Zhixing, 1990-, et al.
(författare)
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Conducting Polymer‐Based e‐Refinery for Sustainable Hydrogen Peroxide Production
- 2023
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Ingår i: Energy & Environmental Materials. - : Wiley-Blackwell. - 2575-0356.
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Tidskriftsartikel (refereegranskat)abstract
- Electrocatalysis enables the industrial transition to sustainable production of chemicals using abundant precursors and electricity from renewable sources. De-centralized production of hydrogen peroxide (H2O2) from water and oxygen of air is highly desirable for daily life and industry. We report an effective electrochemical refinery (e-refinery) for H2O2 by means of electrocatalysis-controlled comproportionation reaction (2(H)O + O -> 2(HO)), feeding pure water and oxygen only. Mesoporous nickel (II) oxide (NiO) was used as electrocatalyst for oxygen evolution reaction (OER), producing oxygen at the anode. Conducting polymer poly(3,4-ethylenedioxythiophene): poly(styrene sulfonate) (PEDOT:PSS) drove the oxygen reduction reaction (ORR), forming H2O2 on the cathode. The reactions were evaluated in both half-cell and device configurations. The performance of the H2O2 e-refinery, assembled on anion-exchange solid electrolyte and fed with pure water, was limited by the unbalanced ionic transport. Optimization of the operation conditions allowed a conversion efficiency of 80%.
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| 10. |
- Wu, Zhixing, 1990-
(författare)
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Mesoporous transition metal oxides for oxygen electrocatalysis in energy conversion technologies
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Electrocatalysis, the foundation of electrical to chemical energy transformation, enables the mitigation of the electrical energy losses during reactions and the control of selectivity of the process to certain chemical products. The slow rate and the multi-step complexity of oxygen-associated reactions, namely oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), motivate the use of platinum group metals (PGM) catalysts, which significantly increase the price of the technologies due to the cost and scarcity of PGM-based materials. This thesis aims to fundamentally understand the electrocatalytical aspects of oxygen-associated reactions and their relevance to sustainable technologies by development of cheap and abundant materials.In this work, hydrothermal treatment routes are developed for synthesis of mesoporous MOx (M = Cr, Fe, Co, Ni, Ce) and NiCo2O4 as water-processable oxygen electrocatalysts. Firstly, anionic surfactant templated mesoporous NiO shows the lowest voltage loss with the highest turnover frequency for OER in consequence of the most accessible active sites among of nanoporous nickel (II) oxide (Paper I). It is observed that nickel and cobalt oxides are efficient bifunctional oxygen electrocatalysts compared to other investigated metal oxides. This stems from the lower voltage loss and by the presence of surface adsorbed hydroxyl species. In situ quantification shows that hydrogen peroxide is either the terminal product or the intermediate for ORR on meso-Cr2O3 and on other electrocatalysts, respectively (Paper II). In Paper IV, mesoporous NiO and NiCo2O4 are synthesized by using a template-free hydrothermal route, and NiCo2O4 performs more efficient bifunctional oxygen catalysts compared to NiO. It is found that ORR on mesoporous NiO and NiCo2O4 follow (2+1)e- and 4e-ORR path, with hydroxyl radical and hydroxyl ion as terminal products, respectively.Integrating the ORR and OER in electrochemical cells enables the study and development of energy conversion technologies. The bifunctional oxygen activity of meso-NiO is demonstrated in a PGM-free oxygen pump fed with air and water, resulting in a low faradic efficiency due to limited triple reaction points (Paper II). The performance of the oxygen pump has been significantly improved by exchanging the catalyst to mesoporous NiCo2O4 and the anolyte to concentrated KOH. The same setup is used for synthesis of the hydroxyl radical using mesoporous NiO. The hydroxyl radical is identified using degradation of rhodamine B, and a degradation rate of 0.034 min−1 is obtained in Paper IV. Additionally, two effective 2e-ORR electrocatalysts of porous organic conducting polymer poly(3,4-ethylenedioxythiophene): poly (styrene sulfonate) (PEDOT: PSS) (Paper III) and mesoporous chromium (Paper V) have been studied for electrochemical refinery H2O2 by electrocatalysis-controlled comproportionation reaction (2?2? + ?2 → 2?2?2). It is observed that the hydrogen peroxide as terminal product of oxygen reduction shows ~70% Faradic efficiency on these two materials. The optimization of operation conditions on PEDOT: PSS-based hydrogen peroxide electrolyzer allows the conversion efficiency of 80% below 1V cell voltage. The optimized meso-Cr2O3-based hydrogen peroxide electrolyzer enables the conversion efficiency up to 90% that can be assigned to the suppressed of deterioration of catalyst.To summarize, this thesis has developed mesoporous metal oxides use as PGM-free electrocatalysts for investigating oxygen-associated reactions in the alkaline condition. Furthermore, the work has explored the energy conversion applications using the functionality of the developed oxygen electrocatalysts.
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