Interdiffusion Behavior and Microstructure Change of Al-, Ti-, or Ta-doped NiCoCrAlY Bond Coat on IN 792 Superalloy

During power generation, the turbine inlet temperature of an industrial gas turbine is increasing continuously. As a high inlet temperature results in changes to microstructure of superalloys along with mechanical degradation, a highly reliable thermal barrier coating is typically developed by doping an additional element on barrier coating. To investigate the effect of such doping on barrier coating and interdiffusion behavior, Al-, Ti-, and Ta-doped NiCoCrAlY bond coat – IN 792 superalloy diffusion couples were fabricated and assessed in an isothermal oxidation test performed at 1000 °C. The results showed that the addition of Al content in the NiCoCrAlY bond coat did not suppress the change in the microstructure of the IN792. The addition of Ti in the bond coat retarded the diffusion of Ti in IN 792 superalloy, while the formation of the γʹ depletion layer of IN 792 superalloy was suppressed; however, the γ phase under the interface was instead depleted, and topologically close packed phase (TCP) phase was formed. Meanwhile, the addition of Ta in the bond coat forms (Ta, Ti)C at the interface. The carbides partially suppress the Ti diffusion of the IN792 superalloys, thus causing a reduction in the γʹ depletion layer. The experimental results indicate that the main factor for microstructure change is the Ti diffusion of IN 792 superalloys. The addition of Ta in the NiCoCrAlY bond coat is expected to improve the lifetime of the turbine blade by suppressing the change in the microstructure of the IN 792 superalloy without the formation of TCP phase.

Graphical Abstract