Progress in terahertz nondestructive testing of thermal barrier coatings: a review

Thermal barrier coatings (TBCs) are critical thermal protection systems for the high-temperature components of aero-engines, and their performance evaluation is essential. Terahertz nondestructive testing (THz-NDT) technology, known for its unique advantages of non-contact application and high penetration capability, has demonstrated significant potential in evaluating TBCs. This paper presents an overview of recent advances in THz-NDT applications for TBCs, with a focus on detecting optical parameters, thickness, porosity, and thermal growth oxide of the coatings. Terahertz time-domain spectroscopy enables an in-depth analysis of the physical properties of TBCs through two detection methods: transmission and reflection. Moreover, advancements in signal processing algorithms, image processing technologies, and machine learning have substantially enhanced the sensitivity, spatial resolution, and real-time performance of THz-NDT. The integration of these technologies has improved the analysis of complex coating structures and broadened the application range of terahertz technology. However, terahertz technology still encounters various challenges in practical applications, including signal attenuation in high-temperature environments and processing difficulties due to complex coating structures. In the future, as terahertz technology continues to advance and new algorithms are integrated, THz-NDT is expected to play an increasingly vital role in the quality assessment, life prediction, and health monitoring of high-temperature materials.