High temperature oxidation resistance of TNM alloy coated with/without 8YSZ/NiCoCrAlY thermal barrier coatings

• Ti-Al-N and (Nb, Mo)-rich layers suppress the diffusion of air and metal ions. • 8YSZ/NiCoCrAlY coating enhances the oxidation resistance of TiAl alloy at 950 °C. • The Nb, Mo elements hinder diffusion of Ni, Co, and Cr elements to the TiAl alloy. • Relationship between diffusion distance of Ni, Co and oxidation time is revealed. • Thermal growth oxide evolution model considering the elements diffusion is proposed. The oxidation resistance of the TNM alloy (Ti-44Al-4Nb-1.5Mo (B, Y) (at. %)) coated with/without 8YSZ/NiCoCrAlY thermal barrier coating (TBC) was conducted through cyclic oxidation at 950 ℃. The formation mechanism of oxide scale, interface interdiffusion mechanism, and evolution law of thermal growth oxide (TGO) were systematically investigated. Results show that the weight gains of the TNM alloy coated with/without TBC are 2.45 mg/cm 2 and 6.98 mg/cm 2 , respectively. The Ti-Al-N layer and the (Nb, Mo)-rich layer near the surface play a role in blocking the internal diffusion of air and the external diffusion of metal ions. After a comprehensive analysis of EPMA and SEM results, it is concluded that the diffusion rate of elements in NiCoCrAlY is Ni>Co>Cr. At the same time, the relationship between the interface diffusion layer thickness and oxidation time is built based on Fick’s first law. In addition, as the oxidation time increases, TGO evolves from a dense Al 2 O 3 layer to CS (chromia, spinel oxide) layer, and CSN (chromia, spinel, and nickel oxide) cluster oxides. The mixed oxides induce crack nucleation and growth, thereby reducing TBC lifetime.