Environmental protection of γ-TiAl alloys coated with CrAlYN/CrN nanoscale multilayer coatings and EB-PVD thermal barrier coatings

The oxidation protection capability of CrAlYN/CrN nanoscale multilayer coatings for gamma titanium aluminides was studied at 750 and 850 degrees C in air. Nitride coatings of about 5 mu m thickness overlayed with an oxy-nitride top coat were deposited on gamma-TiAl specimens using unbalanced magnetron sputtering. The samples were pre-treated in highly ionised plasma generated by a high power impulse magnetron sputtering discharge using different targets. On some of the samples thermal barrier coatings (TBCs) were deposited by electron-beam physical vapour deposition. The oxidation resistance of the different coating systems was studied performing cyclic oxidation tests for tip to 2000 1-h cycles. Cross-sections of oxidised samples were examined using scanning electron microscopy and energy-dispersive X-ray spectroscopy. CrAlYN/CrN + CrAlYON coatings exhibited excellent oxidation resistance at 750 degrees C. Beneath the coating an similar to 2 mu m thick TiN layer formed followed by a thin zone enriched with aluminium and niobium. The nanoscale multilayer nitride coatings provided also good oxidation protection at 850 degrees C, as indicated by a noticeably lower mass gain compared to the uncoated substrate material. No spallation of the TBC systems was observed during 2000 cycles to 850 degrees C. The thermal barrier coating exhibited excellent adhesion to the oxy-nitride top coat and to protrusions of an outer oxide scale with a columnar structure formed above cracks in the nitride coating.