Effect of Mo on microstructure and oxidation of NiCoCrAlY coatings on high Nb containing TiAl alloys

In order to improve the oxidation resistance of high Nb containing TiAl alloys, the microstructure and oxidation behavior of NiCoCrAlY(Mo) coatings were investigated at 900 °C for 100 h by isothermal oxidation tests. The oxidation resistance of TiAlNbCr alloys was significantly improved after adding coatings and the oxidation resistance of NiCoCrAlY coating was slightly better than that of NiCoCrAlYMo coating. A continuous and dense Al2O3 film was formed on the surface of NiCoCrAlY coating. Mo has negative influence on the oxidation resistance of NiCoCrAlY coating. It is found that Mo was oxidized to form MoO3, meanwhile the evaporation of MoO3 resulted in the spallation of oxide film and severe internal oxidation. However, Mo was beneficial to enhance the adhesion between coating and substrate due to the formation of Al2O3 pegs. Elemental interdiffusion at the interface of coating-alloy was inhibited by the MoO3 and Al2O3 diffusion barriers effectively.