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E. Aschauer, P. Felfer, M. Arndt, P. Polcik, H. Riedl, P.H. Mayrhofer:
"Oxygen Diffusion Pathways in High Temperature Oxidation Resistant Ti-Al-N/Mo-Si-B Multilayer Coating";
Vortrag: 45th International Conference on Metallurgical Coatings and Thin Films 2018, San Diego; 23.04.2018 - 27.04.2018.

In high temperature oxygen containing atmospheres, the oxidation resistance of a protective coating is mainly based on the ability of the participating metals to form adherent and continuous oxide scales separating the reactants from the oxidizing atmosphere. Corundum type Al2O3 or Cr2O3 represents such highly stable oxide structures, whereby the formation of Al-rich scales was crucial in the success story of Ti1-xAlxN. Nevertheless, oxide scales only constitutes semipermeable barriers slowing down scale growth, where the kinetic of the growing scale is determined by the fastest species - e.g. metal or oxygen ion outward and inward diffusion, respectively. With respect to the temperature (T < 0.6 Tm), the dominant transport mechanism in growing scales is along fast-diffusion pathways such as dislocations, voids, or especially column boundaries rather through the bulk crystal lattice. To further enhance the oxidation resistance of e.g. Ti1-xAlxN, a fundamental understanding of the oxidation process and a distinct knowledge on the phase evolution and present diffusion pathways in the atomic scale range is highly desired.
Therefore, we used atom probe tomography (APT) to analyse the diffusion pathways in our high-temperature oxidation resistant Ti-Al-N/Mo-Si-B multilayer coatings. The repeated incorporation of very thin sputtered Mo-Si-B layers (λ ≈ 25 nm) in arc-evaporated Ti-Al-N (λI ≈ 100 nm) leads to an interrupted growth of the V-shaped Ti-Al-N columns, and hence to highly distinct areas in 3D chemical mapping. To overcome the difficulty of mass/charge peak overlap during APT investigations, the samples were annealed in O18 for 60 min at 900 °C. For optimized volume analysis, three tips parallel to the layered structure, next to the growing scale, were prepared by a standard FIB lift out technique - uniformly distributed from top to bottom in the unaffected coating region. These results were correlated with a detailed analysis on the phase evolution applying nano-beam X-ray diffraction as well as morphology by using high-resolution transmission electron microscopy in the as deposited and thermal treated state.

Projektleitung Paul Heinz Mayrhofer:
Christian Doppler Labors für anwendungsorientierte Schichtentwicklung