Environmental protection of γ-TiAl based alloy Ti-45Al-8Nb by CrAlYN thin films and thermal barrier coatings

The oxidation behaviour of CrAlN + 2 mol.% YN thin films deposited on γ-TiAl based alloy Ti–45Al–8Nb (at.%) was investigated at 850 and 900 °C under cyclic oxidation conditions in air. At both temperatures the coated samples exhibited significantly reduced mass gains compared to the bare substrate material. Slow oxide growth rates were observed for up to 2000 cycles of 1-h dwell time at high temperature, indicating effective oxidation protection provided by the CrAlYN coating to the γ-TiAl alloy. Microstructural examinations of coated specimens after high temperature exposure revealed an outer mixed scale of chromia and alumina grown on the degraded nitride film. Beneath the coating, TiN and Ti2AlN layers formed. When thermally cycled at 900 °C in air, thermal barrier coatings of yttria partially stabilized zirconia deposited on γ-TiAl samples coated with CrAlN + 2 mol.% YN thin films exhibited lifetimes exceeding the maximum exposure time period of 1000 1-h cycles. No spallation of the thermal barrier coating system was observed. The zirconia topcoats produced by electron-beam physical vapour deposition were well adherent to the mixture of chromium and aluminium oxides formed on the nitride coating.