Monitoring damage evolution in thermal barrier coatings with thermal wave imaging

Thermal wave imaging (TWI) is a promising non-destructive evaluation (NDE) technique with the ability to detect integrity and thickness of thermal barrier coatings (TBCs) extensively used for advanced gas turbine engines. Development of a robust NDE technique is essential for quality control, life assessment, and health monitoring of TBCs for applications, maintenance, and prevention of catastrophic failure. In this study, TWI was employed as an NDE technique to examine as-coated TBCs with varying thicknesses, and thermally cycled TBCs for initiation and progression of subcritical–subsurface damage as a function of thermal cycling. TBC specimens examined consisted of air plasma sprayed ZrO2–7 wt.% Y2O3, NiCoCrAlY bond coats and Haynes 230 superalloy. Thermal cycling was carried out in air with 30-min heat-up, 10-h dwell at 1150 °C, 30-min air-quench and 1-h hold at room temperature. During thermal cycling, TBC specimens were evaluated non-destructively with TWI at room temperature every 10 to 20 thermal cycles, and selected specimens were removed from thermal cycling for microstructural analysis by scanning electron microscopy (SEM). TWI analysis was correlated to the microstructural characteristics and damage progression of TBCs based on phenomenological expressions of thermal diffusion. Higher thermal response amplitude associated with disrupted heat transfer was observed where localized spallation at or near the YSZ/TGO interface occurred.