Oxidation and fatigue behaviour of gamma titanium aluminides coated with yttrium or zirconium containing intermetallic Ti–Al–Cr layers and thermal barrier coating

Abstract

Intermetallic Ti–60Al–13Cr based coatings with additions of 1 at.-% yttrium or 4 at.-% zirconium were deposited on a γ-TiAl based Ti–45Al–8Nb (at.-%) TNB alloy using magnetron sputtering. The Zr containing layer was also used as bond coat for a thermal barrier coating (TBC) of yttria partially stabilised zirconia produced by electron beam physical vapour deposition. Cyclic oxidation tests at 850°C in laboratory air revealed excellent oxidation behaviour of the Y containing coating up to more than 2000 one hour cycles. Similarly high oxidation resistance was also observed for the Zr containing bond coat beneath the TBC. However, without ceramic topcoat, the Ti–60Al–13Cr–4Zr coating degraded after 400 one hour cycles due to the occurrence of cracks in the brittle intermetallic coating. Furthermore, fatigue tests of coated tension specimens were performed after an isothermal exposure at 850°C for 300 h. The deposition of the intermetallic Ti–Al–Cr based coatings reduced the fatigue strength by more than 50% in comparison to the bare substrate material; it was further deteriorated by the TBC.

Keywords:

• Titanium aluminides
• Intermetallic Ti–Al–Cr coatings
• Reactive element effect
• Laves phase
• Thermal barrier coatings
• Fatigue strength

Acknowledgements

The authors would like to thank Mr J. Brien for manufacture the intermetallic Ti–Al–Cr–Y and Ti–Al–Cr–Zr coatings and Mr D. Peters for the deposition of the ceramic TBC using EB-PVD. Furthermore the authors thank Mr D. Lütz and Mrs J. Wischek for carrying out the fatigue tests.