Non-destructive evaluation of thermal barrier coating thickness using IR thermography in chemical removal processes

Active thermography is a non-destructive testing (NDT) technique with great potential for evaluating the integrity, thickness, homogeneity, and structure of thermal barrier coatings (TBCs), which are widely used in the hot gas path of thermoelectric power plants. In this study, active thermography was used as a non-destructive analysis technique to measure the thickness of the YSZ ceramic layer in thermal barrier coatings subjected to a chemical removal process as a function of exposure time to the agent. The TBC samples analyzed consisted of ZrO2-7 wt% Y2O3 atmospheric plasma sprayed with NiCoCrAlY bond coat in nickel-based superalloy. The ablation process consisted of complete immersion of the sample in an aqueous solution of ammonium bifluoride (0 min, 90 min, 240 min, 330 and 420 min), maintaining a temperature of about 60 °C to about 68 °C and subjecting it to an ultrasonic bath. Thermographic evaluation was performed at room temperature using two 600 W flash lamps, one 500 W halogen lamp, and a long-wave infrared camera with an acquisition rate of 30 Hz. Samples were taken for microstructural analysis and thickness measurement using SEM microscopy and eddy currents. Thermographic information was processed using thermal signal reconstruction algorithms and correlated with microstructural characteristics and thickness as a function of immersion time. The results show differences in both the heating and cooling curves of the samples, which can be correlated to differences in the thickness of the coatings. In this study, active thermography was used as a non-destructive analytical technique to measure the thickness of the YSZ ceramic layer in thermal barrier coatings subjected to chemical removal. ZrO2-7 wt% Y2O3 coatings were immersed in an aqueous solution of ammonium bifluoride. Thermographic information was acquired using a 30 Hz infrared camera and two 600 W flash lamps. The results show differences in their heating and cooling curves, which allow them to be correlated with the coating thickness measured by SEM microscopy and eddy currents.