Thermal barrier coatings on γ-TiAl protected by the halogen effect

S. Friedle, N. Nießen, R. Braun, M. Schütze

Establishing the formation of a thin alumina scale, fluorine-treatment of the surface of γ-TiAl alloys can function as a bond coat in thermal barrier coating (TBC) systems, eliminating problems associated with thickness and chemical compatibility of conventional bond coat types. In the present work, yttria partially stabilized zirconia (YSZ) coatings were deposited on two recently developed γ-TiAl alloys, two-phase TNB-V5 and β-stabilized TNM-B1, using electron-beam physical vapor deposition (EB-PVD). The deposition temperature was 900 °C and 1000 °C. Prior to TBC deposition, the substrate materials were treated with fluorine by dipping in dilute hydrofluoric acid or spraying of an organic fluorine-containing compound, followed, in general, by a pre-oxidation step at 900 °C for 24 h in air. With some of the samples, the latter pre-oxidation step was omitted. As revealed by scanning electron microscopy, the as-deposited zirconia topcoats were tightly adherent to both fluorine treated alloys, regardless of the fluorination technique, the TBC deposition temperature and the pre-oxidation treatment. The oxidation behavior of the coated specimens was determined under cyclic oxidation conditions at 900 °C in air. Cyclic short-term oxidation tests revealed that the pre-oxidation step was crucial to maintain the oxidation protection by the halogen effect after EB-PVD TBC application. Whereas a continuous alumina layer was found beneath the zirconia topcoat on pre-oxidized samples, a mixed layered oxide scale of TiO2 and Al2O3 formed on specimens without pre-oxidation. When thermally cycled at 900 °C, the TBC system on substrates fluorine-treated by hydrofluoric acid exhibited lifetimes exceeding 500 cycles of 1 h dwell time at high temperature. No spallation of the zirconia topcoat was observed for both TNB-V5 and TNM-B1 alloys. Therefore, YSZ TBCs offer a promising concept to thermally protect γ-TiAl based alloys in combination with the fluorine effect.

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