Atomic scale investigations of thermally treated nano-structured Ti-Al-N/Mo-Si-B multilayers

Mo-Si-B alloys came into the focus of research activities as an extremely creep resistant material for high temperature oxidative environments. Here, we studied the mechanical, microstructural, and chemical changes of physical vapour deposited protective Ti-Al-N/Mo-Si-B multilayer coatings during annealing in inert and oxidative atmosphere (up to 1600 °C). We focused our investigation on morphological and structural changes of the coating architecture, by applying a set of high-resolution techniques (e.g., high-resolution transmission electron microscopy as well as atom probe tomography), in order to separate temperature or oxygen-induced effects on the protective behaviour. The 31 nm thick Ti-Al-N layers crystallise in the preferred face centred cubic structure, whereas the alternating Mo-Si-B layers are X-ray amorphous. Their thermomechanical properties, with a hardness of at least 31 GPa of the film up to vacuum annealing temperatures of 1200 °C, are clearly superior to homogeneously grown Ti-Al-N coatings. Applying X-ray powder diffraction, we observed that annealing in inert atmosphere leads to the formation of the Mo-rich intermetallics T1-Mo5Si3 (A15, cP8,Cr3 Si-prototype) and T2-Mo5SiB2 (D8l, tl32,Cr5B3-prototype), accompanied by the spinodal decomposition of Ti-Al-N. In contrast, exposing this multi-layered coating to oxygen causes the formation of a rutile-TiO2 and α-Al2O3 oxide scale. The presence of Mo-Si-B is evident up to temperatures of 900 °C, leading to a retarded oxidation process, where only 200 nm out of the 3.0 μm Ti-Al-N/Mo-Si-B multilayer is oxidised.