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Dissertationen (eigene und begutachtete):

E. Aschauer:
"Mo-Si-B based coatings: Compositional and architectural approaches for high temperature environments";
Betreuer/in(nen), Begutachter/in(nen): P.H. Mayrhofer, H. Riedl, C. Eisenmenger-Sittner, P. Felfer; E308, 2018; Rigorosum: 12.12.2018.



Kurzfassung englisch:
Molybdenum silicon boron based alloys are promising candidates for next generation high temperature materials, since they offer a high melting point, an outstanding creep resistance as well as an excellent oxidation resistance up to 1600 °C. As in many other systems, the materials properties highly depend on the prevalent phase composition, morphology, chemical composition as well as structural constitution, whereby all of them need to be adjusted carefully. PVD offers the possibility to enhance the limited properties of bulk materials, for example by applying very stiff or oxidation resistant coatings onto ductile materials in order to extend the field of application of the so formed compound. Nevertheless, due to fundamentally changed kinetics during PVD based synthesis, a comprehensive understanding is essential for utilising Mo Si B also as thin film material for ultra high temperature environments.
Therefore, the thermomechanical properties and the phase evolution at elevated temperature of homogeneously grown Mo Si B and Mo Ti Si B coatings were studied in detailed. The synthesised coatings show very promising results. However, the mechanical properties of the annealed coatings ask for further improvements to tailor the material for applications, which require high thermal stability and exceptional wear resistance during mechanical loading. Based on these results, the developed high temperature resistant Mo Si B thin films are implemented in a Ti1-xAlxN matrix to form a multilayer arrangement, in order to combine excellent mechanical properties with outstanding oxidation resistance. The concept is based on the interruption of the prevalent continuous column boundaries of
Ti1-xAlxN, by repeatedly embedding nanometre-thin Mo Si B layers, which consequently impedes the oxygen inward diffusion. Additionally, this approach allows for advanced tribological properties due to the formation of MoOx based Magneli phases, which are known for in situ self lubrication.
For a fundamental understanding of the multilayer system, the as deposited as well as annealed state was examined in detail, with respect to the chemical composition, the phase constitution as well as the prevalent oxidation mechanism. Detailed structural, morphological, and chemical analyses of Ti0.57Al0.43N/Mo0.58Si0.28B0.14 multilayers - carried out by transmission electron microscopy - reveal the nanocrystalline or even amorphous character for the Mo0.58Si0.28B0.14 interlayers. Furthermore, high resolution imaging gives evidence for the successful interruption of the columnar V shaped grain morphology of the Ti0.57Al0.43N layers by nanometre scaled Mo0.58Si0.28B0.14 interlayers. This was additionally proven by atom probe tomography, confirming the absence of continuous diffusion channels through the coating material.
Due to the highly specific morphology of the multilayer architectures, phase transformation processes are significantly retarded. In detail, the characteristic spinodal decomposition of Ti0.57Al0.43N is delayed for around 300 °C to 1200 °C. Spatially resolved synchrotron X ray diffraction strain/stress analyses also confirmed the effect of decelerated phase transformations for smaller bilayer periods. Furthermore, it has been shown that small bilayer periods not only reduce the stress in the as deposited state, but additionally supress the formation of hexagonal closed packed AlN, which would tremendously decrease the mechanical properties. The initially amorphous Mo Si B interlayers also transform into Mo based intermetallic phases, T1-Mo5Si3 and T2 Mo5SiB2, known for their high potential to enhance the oxidation resistance. The following oxidation tests proofed a superior oxidation as compared to homogeneously grown Ti0.57Al0.43N, forming dense and adherent oxide scales up to 1000 °C.
During application near hot forming tests of Al-Si coated steel sheets, the material showed its high potential for applications including high mechanical loads and wear demanding characteristics. Especially, the adhesive wear formation on coated forming tools when processing Al Si coated steel sheets was significantly reduced. Furthermore, the tests allowed for a fundamental understanding of adhesive wear formation, showing a strong dependency of the diffusivity of Al and Fe at the coating/build up interface in relation to the formed build-up material.

In summary, the study clearly highlights the high potential of Mo Si B based thin films as protective coating material but also as feature in smart coating architectures, allowing for superior high-temperature characteristics - involving oxidation as well as mechanical properties - up to around 1100 °C.

Erstellt aus der Publikationsdatenbank der Technischen Universität Wien.