Oxidation behaviour of TiAlYN/CrN and CrAlYN/CrN nanoscale multilayer coatings with Al2O3topcoat deposited on γ-TiAl alloys

Abstract

TiAlYN/CrN and CrAlYN/CrN nanoscale multilayer coatings were deposited on γ-TiAl specimens using magnetron sputtering techniques. The nitride layers were manufactured by unbalanced magnetron sputtering (UBM) and high power impulse magnetron sputtering (HIPIMS). The CrAlYN/CrN coatings had an oxy-nitride overcoat. On some of the coated samples an additional alumina topcoat was deposited. The oxidation behaviour of the different coatings was investigated at 750 and 850°C performing quasi-isothermal oxidation tests in laboratory air. Mass change data were measured during exposure up to failure or the maximum exposure length of 2500 h. When exposed to air at 750°C, the Ti-based nitride films exhibited higher oxidation resistance than the Ti – 45Al –8Nb substrate material. The alumina topcoat enhanced the oxidation protection of this coating system, acting as diffusion barrier to oxygen penetration. At 850°C, the TiAlYN/CrN films exhibited poor stability and rapidly oxidised, and therefore were not applicable for long-term protective coatings on γ-TiAl alloys. The beneficial effect of the additional Al2O3 layer was less pronounced at this exposure temperature. The Cr-based nitride films exhibited high oxidation resistance during exposure at 850°C. HIPIMS deposition improved the oxidation behaviour of the CrAlYN/CrN nanoscale multilayer coatings in comparison to UBM coatings. For these coatings, the decomposed and partially oxidised nitride films were an effective barrier to oxygen inward diffusion. The alumina topcoat did not significantly increase the oxidation resistance of the γ-TiAl alloy coated with Cr-based nitride films.