Degradation and life prediction of gas-turbine duplex coatings

The high-temperature degradation mechanisms in GT29+ and GT33+ coatings on GTD-111 substrate were characterized by performing cyclic oxidation tests at maximum temperatures of 1010 degreesC and 1066 degreesC. Weight change data were measured as a function of thermal cycles. Metallographic examinations were performed on sectioned specimens to determine changes in microstructure, aluminum content, and volume fraction of beta phase in the coatings. These experimental results were analyzed in terms of a coating life model to assess the dominant degradation mechanism affecting the coating life and the appropriate characterizing parameters. Comparison of model calculation and experimental data indicates that conservatively, the useful life of duplex coatings can be predicted solely based on the Al content in the top coat. The predicted value of the coating life is increased by a factor of 1.4 to 2 if MCrAlY is included in the lifetime estimation.