Detection of Debonding Defects in Thermal Barrier Coatings by Long Pulse and Vibro-Thermography

Thermal barrier coatings (TBCs) are widely used in turbine blade for protection of superalloy substrate. Interfacial debonding will lead to serious safety accidents of aircrafts and should be detected in advance. Vibrothermography (VT) and long pulse thermography (LPT) are two kinds of effective non-destructive techniques. In this paper, the two methods were used for defect detection in TBCs. Artificial debonding defects with diameter from 5 mm to 2 mm were fabricated. The results show that both VT and LPT can be successfully used to identify defects with diameter larger than 2 mm. The thermal contrast of the two methods reduces with the decreasing size of debonding defect, and there is a higher thermal contrast in VT than that in LPT, when a proper ultrasonic excitation frequency is selected in VT. Measurement accuracy is investigated by comparing the defect area, which shows that the average value of the relative error is reasonable except for defects with small size. The lateral thermal diffusion effect and imperfect manufacturing process of artificial debonding may be the main causes for the relative error. In addition, parameters of each method should be optimized for better defect detection in TBCs.