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Thermographic imaging under steady-state heat flow was used to nondestructively detect the defects in thermal barrier coatings (TBCs). The finite element method (FEM) analyses and the experimental thermography observations were performed using artificially grooved ZrO2 plates and the indentation-tested TBC-metal specimens. The FEM results show that: (1) Defects (nonuniformity or internal cracks) of the TBC can be effectively detected by the thermographic imaging method; (2) The apparent thermal images would be far greater than the real sizes of the defects, but the half-height sizes of the temperature profile were found to give good estimates for the latter; and (3) The higher value of heat flow would contribute to the detections. Besides these results, the influence of the defect situations (morphology, size and position) on the thermal images was also predicted by the FEM analysis. Although, due to some difficulties in preparing the test specimens, the quantitative comparison between each FEM result and that of the actual measurement was not performed, however, the experimental results of the grooved ZrO2 plates were found to be in good agreement with the FEM predictions. For the thermographic experiments of the indentation-tested specimens, both the internal-cracks and the thickness-nonuniformity of the TBCs were successfully observed, and the smallest cracks detectable had a diameter far less than 1 mm.

Detection of Defects in Thermal Barrier Coatings by Thermography Analyses