Coating Life Prediction for Combustion Turbine Blades

Abstract

A life prediction method for combustion turbine blade coatings has been developed by modeling coating degradation mechanisms including oxidation, spallation, and aluminum loss due to inward diffusion. Using this model, the influence of cycle time on coating life is predicted for GTD-111 coated with an MCrAlY, PtAl, or aluminide coating. The results are used to construct a coating life diagram that depicts failure and safe regions for the coating in a log-log Plot of number of startup cycles versus cycle time. The regime where failure by oxidation, spallation, and inward diffusion dominates is identified and delineated from that dominated by oxidation and inward diffusion only. A procedure for predicting the remaining life of a coating is developed. The utility of the coating life diagram for predicting the failure and useful life of MCrAlY, aluminide, or PtAl coatings on the GTD-111 substrate is illustrated and compared against experimental data.

Predicting Coating Degradation Under Variable Peak Temperatures

“…The computer model, named COATLIFE, has been evaluated against laboratory data of MCrA1Y, aluminide, and PtAJ-modified aluminide coatings with promising results. Agreement between model predictions and field data of PtAl coating is also very encouraging (Chan et al, 1997(Chan et al, , 1998.…”

“…The main features of the coating life model are: (I) oxidation kinetics, (2) spallation kinetics of oxides formed on the coating, (3) outward and inward diffusion, (4) overall kinetics of Al loss from the coating due to oxidation, spallation, and inward diffusion, and (5) a life prediction scheme based on a critical value of Al content that corresponds to the complete depletion of 13 in the microstructure. Since the coating life model has been published, the details of the model will not be repeated here but they are available in earlier publications (Chan, 1997;Chan et al, 1997Chan et al, , 1998.…”

“…The remaining life. RL is defined based on the assumption that the useful life of the coating is zero when the volume fraction of the phase is zero, leading to (Chan et al, 1997(Chan et al, , 1998 RL-X Ar(n)-X 1 for 4 1 X , 41 (n) Al 0 Schematics of the coating life model (Chan et al, 1997(Chan et al, , 1998: (a) degradation mechanisms treated in the model, and (b) life prediction scheme based on a critical Al content.…”

“…A life prediction methodology has been developed by the present authors for combustion turbine coatings (Chan, 1997;Chan et al, 1997Chan et al, , 1998. This methodology is based on a computer model that treats coating degradation mechanisms such as oxidation, spallation, and diffusion.…”

“…The cyclic oxidation behavior of a CoCrAlY coating under variable temperature conditions has been characterized by performing multi-step temperature cyclic oxidation tests at peak temperatures of 954°C, I010°C, and 1066°C (1750°F, I850°F, and I950°F). The multi-step temperature cyclic oxidation data are used in conjunction with single-step temperature cycling data obtained in previous investigations (Chan et al, 1997(Chan et al, , 1998 to evaluate the coating life model. In particular, the COATLIFE model is used to predict the response of the coaling under both single and multi-step temperature conditions.…”