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O Hudak:
"Development of γ-TiAl-based oxidation resistant physical vapor deposited coatings";
Supervisor: H. Riedl, P.H. Mayrhofer; Institute of Materials Science and Technology/E308-01, 2019; final examination: 2019-08-01.

In order to lower production and repair expenses, as well as reduce carbon emissions whilst improving in-service fuel efficiencies, industries tied to modern aerospace and aviation have heavily invested in the development of lighter and more durable high-temperature materials. Considering that diminishing natural resources will demand a considerable shift towards environmentally conservative resource-management, γ-TiAl based alloys have entered the arena competing against conventional and heavier nickel-alloys. Furthermore, TiAl-alloys have shown great prospect due to their low densities (4.1 g/cm3), distinguished creep and fatigue resistance properties, as well as thermal shock resistance at high temperatures. However, γ-based TiAl alloys only exhibit adequate oxidation resistance up to 780 °C, being still a bending scientific issue. This has encouraged researchers to design and tailor oxidation resistive coatings on the basis of Al2O3, a naturally growing oxide, when TiAl is oxidized, in order to prolong the lifetime of the underlying γ-TiAl based components. In this regard, surface technologies provide two general approaches: (i) a direct deposition of Al2O3 onto the γ-TiAl based alloy, or (ii) a thermo-chemical conversion, of the base material that would self-generate a protective Al2O3 top-coating. This project aimed to combine both approaches to produce an improved oxidation resistive material system for exposure temperatures above 800 °C. Based on an experimentally investigation, this thesis successfully developed oxidation resistant -TiAl coatings on an already industrially established γ-TiAl based titanium aluminide (TNM, Titanium-Aluminum-Niobium-Molybdenum based alloy) utilizing a state-of the-art semi-industrial scale magnetron sputtering system. An over-stoichiometric γ-TiAl target was used for depositing Al-rich γ-TiAl coatings, obtaining similar thermal expansion coefficient as the substrate materials, as well as facilitate the necessary Al-content for an improved Al2O3 scale formation without jeopardizing the chemical composition of the γ-TiAl structure. Firstly, this project dealt with studying the effect of co-sputtered Al-contents during the PVD process, in relation to a thermally grown Al2O3 protective oxide scale. The Al-contents have been varied between around 62 at. % to a maximum of around 70 at. % by co-sputtering Al. To simulate application near thermal and oxidative stress states, all coated samples were oxidized in a chamber furnaces at 850 °C and 900°C for a maximum duration of 100 h. Subsequent analytical techniques such as SEM, TEM, XRD, EDX, nanoindentation, and white light interferometry were used for determining morphology, adhesion properties, elemental and crystal phase composition, as well as mechanical properties in the as deposited state. Results evidently showed that γ-TiAl based coatings with additionally co-sputtered Al-contents did not improve the oxidation resistive behavior. In all cases, significantly inferior oxidation resistivity and weaker mechanical properties were recorded compared to the TiAl coating, produced with the intermetallic γ-TiAl target without co-sputtering of Al. The second part of this thesis addressed diffusional processes that arise during oxidation of γ-TiAl based coatings. The diffusion of O2 into the coating material, as well as the Al-inter diffusion between coating and underlying substrate material was traced using EDX and investigated in conjunction with varying coating-thicknesses and different oxidation temperatures. Diffusion coefficients were calculated in accordance with Fick's 1st and 2nd Laws of diffusion that made subsequent predictions of diffusion pathways well beyond collected experimental data possible. Conclusively, the γ-TiAl coating on top of a TNM bulk material, joint via PVD based magnetron sputtering presented throughout this thesis pose a viable alternative for currently used oxidation resistive coating concepts.

Project Head Paul Heinz Mayrhofer:
MTU DA V