| 1. |
- Kindlund, Hanna, 1984-
(författare)
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Toughness Enhancement in Hard Single-Crystal Transition-Metal Nitrides : V-Mo-N and V-W-N Alloys
- 2014
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Transition-metal nitrides are known for their high hardness, good wear resistance, high-temperature stability, and chemical inertness. Because of these properties, they are extensively used in many industrial applications, notably as protective wear, erosion, and scratch resistant coatings, which are often subjected to high thermo-mechanical stresses. While high hardness is essential, most applications also require high ductility, to avoid brittle failure due to cracking. However, transitionmetal nitrides, as most ceramics, generally exhibit low ductility and hence poor toughness.Improving toughness, the combination of hardness and ductility, of ceramic materials requires suppression of crack initiation and/or propagation, both of which depend on the microstructure, electronic structure, and bonding nature of the coating material. This, however, is an extremely challenging task that requires a fundamental understanding of the mechanical behavior of materials. Theoretical studies, for example, ab initio calculations and simulations are therefore useful in the design of “unbreakable” materials by providing information about the electronic origins of hardness and ductility. Recent density functional theory calculations predicted that alloying can increase toughness in a certain family of transition-metal nitrides such as V-Mo-N and V-W-N alloys. Toughness enhancement in these alloys arises from a near optimal filling of the metallic d-t2g states, due to their high valence electron concentrations, leading to an orbital overlap which favors ductility during shearing.This thesis focuses on the growth and characterization of V1-xMoxNy (0 ≤ x ≤ 0.7, 0.55 ≤ y ≤ 1.03) and V1-xWxNy (0 ≤ x ≤ 0.83, 0.75 ≤ y ≤ 1.13) cubic alloy thin films. I show that alloying VN with WN increases the alloy hardness and reduces the elastic modulus, an indication of enhanced toughness. I investigated the growth, nanostructure, and atomic ordering of as-deposited V1-xWxNy(001)/MgO(001) thin films. In addition, I studied the growth, structural and mechanical properties, and electronic structure of V1-xMoxNy(001)/MgO(001) and V0.5Mo0.5Ny(111)/Al2O3(0001) thin films. I demonstrate that these alloys exhibit not only higher hardness than the parent binary compound, VN, but also dramatically increased ductility. V0.5Mo0.5N hardness is more than 25% higher than that of VN. Using nanoindentation I show that while VN and TiN reference samples undergo severe cracking typical of brittle ceramics, V0.5Mo0.5N films do not crack. Instead, they exhibit material pile-up around nanoindents, characteristic of plastic flow in ductile materials. Furthermore, the wear resistance of V0.5Mo0.5N is significantly higher than that of VN. I also show, for the first time, anion-vacancyinduced toughening of single-crystal V0.5Mo0.5Ny/MgO(001) films. Nanoindentation hardness of these alloys increases with the introduction of N-vacancies, while the elastic modulus remains essentially constant. In addition, typical scanning electron micrographs of nanoindents show no cracks, which demonstrate that N-vacancies lead to toughness enhancement in these alloys. Valence band x-ray photoelectron spectroscopy analyses show that vacancy-induced toughening is due to a higher electron density of d-t2g(Metal) – d-t2g(Metal) orbitals with increasing N-vacancy concentration, and essentially equally dense p(N) – d-eg(Metal) first neighbor bonds.Overall, I demonstrate that it is possible to design and deposit hard and ductile transition-metal nitride coatings. My research results thus provide a pathway toward the development of new tough materials.
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| 2. |
- Höglund, Carina, 1981-
(författare)
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Growth and Phase Stability Studies of Epitaxial Sc-Al-N and Ti-Al-N Thin Films
- 2010
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- ¨This Thesis treats the growth and characterization of ternary transition metal nitride thin films. The aim is to probe deep into the Ti-Al-N system and to explore novel Sc-Al-N compounds. Thin films were epitaxially grown by reactive dual magnetron sputtering from elemental targets onto single-crystal substrates. Ion beam analyses were used for compositional analysis and depth profiling. Different X-ray diffraction techniques were employed, ex situ using Cu radiation and in situ during deposition using synchrotron radiation, to achieve information about phases, texture, and thickness of films, and to follow roughness evolution of layers during and after growth. Transmission electron microscopy was used for overview and lattice imaging, and to obtain lattice structure information by electron diffraction.In the Sc-Al-N system, the perovskite Sc3AlN was for the first time synthesized as a thin film and in single phase, with a unit cell of 4.40 Å. The hardness was found to be 14.2 GPa, the elastic modulus 21 GPa, and the room temperature resistivity 41.2 μΩcm. Cubic solid solutions of Sc1-xAlxN can be synthesized with AlN molar fraction up to ~60%. Higher AlN contents yield three different epitaxial relations to ScN(111), namely, #1 Sc1-xAlxN(0001) || ScN(111) with Sc1-xAlxN[11210] || ScN[110], #2 Sc1-xAlxN(1011) || ScN(110) with Sc1-xAlxN[1210] || ScN[110], and #3 Sc1-xAlxN(1011) || ScN(113). An in situ deposition and annealing study of cubic Sc0.57Al0.43N films showed volume induced phase separation into ScN and wurtzite structure AlN, via nucleation and growth at the domain boundaries. The first indications for phase separation are visible at 1000 °C, and the topotaxial relationship between the binaries after phase separation is AlN(0001) || ScN(001) and AlN<01ɸ10> || ScN <1ɸ10>. This is compared with Ti1-xAlxN, for which an electronic structure driving force leads to spinodal decomposition into isostructural TiN and AlN already at 800 °C. First principles calculations explain the results on a fundamental physics level. Up to ~22% ScN can under the employed deposition conditions be dissolved into wurtzite Sc1-xAlxN films, while retaining a single-crystal structure and with lattice parameters matching calculated values.In the Ti-Al-N system, the Ti2AlN phase was synthesized epitaxially by solid state reaction during interdiffusion between sequentially deposited layers of AlN(0001) and Ti(0001). When annealing the sample, N and Al diffused into the Ti layer, forming Ti3AlN(111) at 400 ºC and Ti2AlN(0001) at 500 ºC. The Ti2AlN formation temperature is 175 ºC lower than earlier reported results. Another way of forming Ti2AlN phase is by depositing understoichiometric TiNx at 800 °C onto Al2O3(0001). An epitaxial Ti2Al(O,N) (0001) oxynitride forms close to the interface between film and substrate through a solid state reaction. Ti4AlN3 was, however, not possible to synthesize when depositing films with a Ti:Al:N ratio of 4:1:3 due to competing reactions. A substrate temperature of 600 ºC yielded an irregularly stacked Tin+1AlNn layered structure because of the low mobility of Al ad-atoms. An increased temperature led to Al deficiency due to outdiffusion of Al atoms, and formation of the Ti2AlN phase and a Ti1-xAlxN cubic solid solution.
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| 3. |
- Kerdsongpanya, Sit, 1985-
(författare)
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Design of Transition-Metal Nitride Thin Films for Thermoelectrics
- 2015
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Thermoelectric devices are one of the promising energy harvesting technologies, because of their ability to convert heat (temperature gradient) to electricity by the Seebeck effect. Furthermore, thermoelectric devices can be used for cooling or heating by the inverse effect (Peltier effect). Since this conversion process is clean, with no emission of greenhouse gases during the process, this technology is attractive for recovering waste heat in automobiles or industries into usable electricity. However, the conversion efficiency of such devices is rather low due to fundamental materials limitations manifested through the thermoelectric figure of merit (ZT). Thus, there is high demand on finding materials with high ZT or strategies to improve ZT of materials.In this thesis, I discuss the basics of thermoelectrics and how to improve ZT of materials, including present-day strategies. Based on these ideas, I propose a new class of materials for thermoelectric applications: transition-metal nitrides, mainly ScN, CrN and their solid solutions. Here, I employed both experimental and theoretical methods to synthesize and study their thermoelectric properties. My study envisages ways for improving the thermoelectric figure of merit of ScN and possible new materials for thermoelectric applications.The results of my studies show that ScN is a promising thermoelectric material since it exhibits high thermoelectric power factor 2.5x10-3 Wm-1K-2 at 800 K, due to low metallic-like electrical resistivity while retained relatively large Seebeck coefficient. My studies on thermal conductivity of ScN also suggest a possibility to control thermal conductivity by tailoring the microstructure of ScN thin films. Furthermore, my theoretical studies on effects of impurities and stoichiometry on the electronic structure of ScN suggest the possibly to improve ScN ZT by stoichiometry tuning and doping. For CrN and Cr1-xScxN solid solution thin films, the results show that the power factor of CrN (8x10-4 Wm-1K-2 at 770 K) can be retained for the solid solution Cr0.92Sc0.08N. Finally, density functional theory was used to enable a systematic predictionbased strategy for optimizing ScN thermoelectric properties via phase stability of solid solutions. Sc1-xGdxN and Sc1-xLuxN are stabilized as disordered solid solutions, while in the Sc-Nb-N and Sc-Ta-N systems, the inherently layered ternary structures ScNbN2 and ScTaN2 are stable.
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