Interface debonding defect detection of thermal barrier coatings with laser lock-in thermography based on edge sharpness optimization

Abstract

Micro-defects created during the manufacturing of coatings would develop into interface defects during the service process and eventually the coating would fall off, it is crucial to detect and visualize minor debonding defects in coatings at an early stage. Lock-in thermography (LIT) is a non-destructive evaluation method capable of inspecting defects in composites by calculating differences of the thermal amplitude and phase between sound and defective regions. For debonding defects of different diameters, different optimal excitation frequencies need to be used to obtain phase characteristics. In order to adapt to the detection of debonding defects of multiple diameters and to improve the edge definition of the defects, a multi-frequency fusion LIT method is proposed in this paper. Multiple frequencies were used to excite the specimen to obtain optimal excitation frequencies for different diameters, and the effects of thermal diffusion length with amplitude and phase difference at different excitation frequencies were also discussed; then, multiple optimal excitation frequencies were used to extract phase features. Finally, fusion images comprising debonding defects of different diameters were constructed using principal component analysis. The method was also compared with other conventional methods, and the experimental results showed that the method could implement high signal-to-noise ratio and sharpened debonding defect extraction. In addition, the method realizes the extraction of debonding defects with a minimum diameter of 1 mm, which surpasses the detection limit of minute debonding defects at the coating interface.