Environmental and thermal protection of high Nb-containing gamma titanium aluminides

For use at elevated temperatures, gamma-TiAl alloys require protection against the oxidizing environment. In the present study nitride coatings based on Ti-Al-Cr-Y-N, either monolithically grown or with superlattice structure, as well as metallic Ti-Al-Cr and Ti-Al-Ag coatings were tested in air at 750degreesC up to 3000 h. Particularly the nitride coatings provided excellent oxidation protection under isothermal and cyclic conditions. Microstructure investigations revealed formation of protective oxide scales and long-term stability of the nitrides. Moreover, thermal barrier coatings deposited by EB-PVD demonstrated very promising lifetime results at 850 and 900degreesC, while at 950degreesC rapid oxidation of the Ti-45Al-8Nb base alloy led to early spallation of the ceramic top coating. Oxidation of the base alloy at all temperatures leads to a fairly complex setup of the oxide scale. Adjacent to the metal/oxide interface nitrides form that might affect oxide scale formation.