High-Resolution Terahertz Imaging of Debonding Defects in Thermal Barrier Coatings Using an Optimal Wavelet Transform

The debonding defects in the thermal barrier coatings (TBCs) are tiny and thin, yielding severely overlapped terahertz (THz) echoes. Hence, we introduce a powerful signal processing method using a customized wavelet basis-based wavelet transform. To begin with, an analytical model of THz signals in TBCs is constructed. Then, THz echoes and typically established wavelet basis are measured in similarity, and the Gaussian wavelet basis is selected to separate overlapped THz pulses due to the largest similarity. To further improve the performance of Gaussian wavelet transform, a novel firefly algorithm (FA) is proposed to search the optimal wavelet parameters. The superiority of the novel FA lies in dynamically adjusting step size by monitoring the change of attractiveness, allowing for jumping out of local optima. Finally, experiments were carried out, and extensive comparison was made with multiple reported methods. The results demonstrate that the proposed method enables high resolution and accuracy of THz imaging for debonding defects in TBCs when temporal resolution of THz time-domain spectrometer is hardware limited.