Al concentration dependent oxidation behavior of Ti1-yAlyB2 and Ti1-xAlxN coatings
Final Report Abstract
Resistance against oxidation is one of the critical design criteria for protective coatings used in cutting and forming applications as well as in energy conversion. As the research in this project was initiated, several publications seeking to address the Al concentration dependent oxidation behaviour of (Ti,Al)B2 have been published. However, as the Al concentration was varied the B concentration was affected and covered a range from over stoichiometric to under stoichiometric. Hence, independent control of the Al and B concentrations was never achieved. Therefore, the causality between oxidation behaviour and Al concentration could not be reported as all previously published data was affected by varying B concentrations. The major objectives reached within this project were: • We have developed a synthesis approach that allows for independent control of the Al and B concentrations. • This processing approach enabled – for the first time – the systematic investigation of the influence of the boron-to-metal ratio (B/M) at constant Al concentration on the oxidation behavior. Furthermore, the influence of the systematically varied Al concentration could be investigated for stoichiometric coatings, also for the first time. • It was shown that addition of Al to TiB2 improved the oxidation resistance of the metastable solid solution coatings. From the comparison of scale thicknesses obtained after oxidation at 700, 800 and 900 °C for up to 8 hours it was evident that coatings with ≤ 15 at.% of Al exhibit the formation of non-passive scales, while coatings containing ≥ 21 at. % of Al form passive scales. • Also, the passive scales are thinner than the scales formed on (Ti,Al)N coatings with an equivalent Al concentration. Likely this difference in oxidation resistance lies in the scale formation and may be related to the presence of B in the scale forming on (Ti,Al)B2 with ≥ 21 at. % of Al compared to the B free scales forming on TiAlN. • The thermal stability of Ti0.12Al0.21B0.67 thin film (forming passivating scales during oxidation) was vacuum annealed at 1000 °C for up to 3 h and segregation into Al- and Tirich (Ti,Al)B2 domains was observed consistent with spinodal decomposition and the formation of AlB12 with a concomitant reduction in Al concentration from 20.9 to 12.5 at. %, likely by evaporation. • The here identified thermal stability limit, revealed with spatially resolved structure and composition probes, confines the application temperature range of Ti0.12Al0.21B0.67 in vacuum to temperatures < 1000 °C and underlines that thermal stability investigations solely based on XRD data result in an overestimated thermal stability.
Publications
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Synthesis and oxidation behavior of Ti0.35Al0.65By (y = 1.7–2.4) coatings. Surface and Coatings Technology, 442, 128190.
Navidi, Kashani Amir Hossein; Mráz, Stanislav; Holzapfel, Damian M.; Hans, Marcus; Löfler, Lukas; Ondračka, Pavel; Primetzhofer, Daniel & Schneider, Jochen M.
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Morphology, mechanical properties, and oxidation behavior of stoichiometric Ti0.33-xAlxB0.67 coatings (x = 0.04, 0.15, 0.21, and 0.28). Acta Materialia, 270, 119829.
Navidi, Kashani Amir Hossein; Hans, Marcus; Lellig, Sebastian; Holzapfel, Damian M.; Löfler, Lukas; Mráz, Stanislav; Primetzhofer, Daniel; Michler, Johann & Schneider, Jochen M.
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Temporally-resolved decomposition of Ti0.12Al0.21B0.67 thin films at 1000 °C. Surface and Coatings Technology, 487, 131026.
Navidi, Kashani Amir Hossein; Lellig, Sebastian; Hans, Marcus; Löfler, Lukas; Mráz, Stanislav; Schweizer, Peter; Müller, Arnold; Primetzhofer, Daniel; Michler, Johann & Schneider, Jochen M.
