Debonding defect imaging of thermal barrier coating with grating laser acoustic spectroscopy

Thermal barrier coatings (TBCs) are crucial for protecting high-temperature components in gas turbines and aeroengines. However, conventional non-destructive testing (NDT) methods, such as infrared thermography, face challenges in precisely detecting and quantifying debonding defects due to limitations in imaging contrast and sensitivity to surface states. In this work, a novel scheme for imaging debonding defects in TBCs using grating laser acoustic spectroscopy is proposed. The scheme, validated by numerical simulations and experimental results, is the first to introduce the mode conversion mechanism of Rayleigh and Lamb waves in the field of TBCs, achieving noncontact photoacoustic imaging of debonding defects for the first report. The alloptical laser ultrasonics system generates velocity maps with enhanced contrast and immunity to surface states, outperforming infrared thermography. Threshold segmentation precisely outlines defect contours, enabling precise quantification of defect size with an error below 5%. This work demonstrates a noncontact, precise, high-contrast imaging and surface-state-immune technique for evaluating the integrity and durability of TBCs, with potential applications in engineering.