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F. Radermacher:
"Development of multilayered Alumina/Mo-Si-B coatings for enhanced oxidation resistance";
Supervisor: H. Riedl, P.H. Mayrhofer; E308, 2018; final examination: 2018-01-25.

Due to the continuous growth of air traffic (doubling by 2036) and reduced fossil resources the development of efficient and high-performance turbines became a central theme for the aviation industry. To improve the thermal efficiency, the applied materials are increasingly affected by higher pressures and temperatures. A lot of bulk materials cannot be used as they exhibit not proper resistance against oxidation in high temperature atmospheres. This is a major reason for the utilization of protective coatings, which should inhibit material degradation based on environmental effects, such as oxygen containing atmospheres at high temperatures. The aim of this study is the development and optimization of a multilayered protective coating system based on (Al,W)2O3 and Mo Si B to improve the oxidation resistance of γ TiAl. Furthermore, a self-healing effect based on the formation of partly viscous (B,Si)2O3 phases should be investigated in detail. The formation of these partly viscous phases is related to the Pesting phenomena, which is well known for Molybdenum based alloys. For refractory metals the alloying of Silicon and Boron inhibits the formation of volatile phases, e.g. MoO3, due to the formation of dense and partly viscous borosilicate based oxide scales. The depositions were carried out on a lab-scaled UHV unbalanced magnetron sputter system. By varying the bilayer period from 20 to 500 mm the hardness, growth morphology and thermal stability can be adjusted. The influences of different process parameters on the coating properties are examined using electron microscopy (SEM), X ray spectroscopy (EDX), X ray diffraction (XRD), nanoindentation and profilometry. To evaluate the proposed self-healing effects, defined cracks by Vickers indentation are generated on the surface of the deposited coatings. To initiate self healing thermal treatments at 900 °C for 1, 10, 30 and 100 h were conducted. Based on our results we can conclude that the deposition of a multi-layer coating based on Al2O3 and Mo Si B is generally possible. The proposed self healing effect could not be proofed within the investigated temperature range, due to a pronounced formation of volatile MoO3 suggesting a not perfectly adjusted Si to B ratio, especially with respect to the glass transition temperature. Nevertheless, all multilayered coatings investigated exhibited a strongly retarded oxidation kinetic and a minor or even no oxygen diffusion towards γ-TiAl.