Delamination Defect Detection in Thermal Barrier Coatings Based on Hemispherical Near-Field Focused Annular Slit Leakage Wave (ASLW) Antennas

This paper utilizes a hemispherical near-field focused annular slit leakage wave (ASLW) antenna in tandem with dual-frequency focusing technology to detect delamination defects in thermal barrier coatings. It commences by shedding light on the principles of microwave near-field focusing and the fundamentals of the ASLW antenna. Subsequently, it progresses to elucidating the antenna parameters based on the cross-projection method, optimizing and determining the parameters for dual-frequency point detection. Simulation analyses are performed to evaluate the effectiveness of the proposed detection method for delamination defects in FR4 internal ceramic coatings. The antenna is then fabricated, followed by experimental testing. Simulation and testing results indicate that the proposed antenna can achieve a detection distance of up to 4 cm with a favorable focusing effect (focal spot diameter of only 5 mm). For delamination defects in the FR4 internal layer, 2 mm deep and 80 µm thick, the amplitude variation of the low-frequency resonant point is 4.32 dB, whereas the frequency offset variation of the high-frequency resonant point is 90 MHz. Furthermore, ceramic sheets (Al2O3) of varying thicknesses (10 mm × 10 mm × \(dn\), where \(dn\) = 0 µm, 40 µm, 60 µm, and 80 µm) embedded within the FR4 material were scanned using the low-frequency band. Conversely, ceramic sheets of a constant thickness (\(dn\)=80 µm) but with varying lengths (\(L\) = 0 mm, 5 mm, 8 mm, 10 mm) underwent scanning via the high-frequency band. The results indicate that the high-frequency band is sensitive to the size of delamination defects, allowing for rapid localization, while the low-frequency band can quantitatively detect the thickness of the defects. Compared to other focusing antennas, the two resonant points of this antenna respond independently to thickness and defect parameters, demonstrating superior performance.