Non-contact characterization of thermal barrier coating plates using laser-based zero group velocity modes

The service life of thermal barrier coatings (TBCs) in aircraft engines is limited by high-temperature degradation and mechanical wear, necessitating non-destructive assessment to monitor their condition. To address the complexity of guided wave dispersion in coated structures, a zero group velocity (ZGV) Lamb wave-based approach is introduced for thickness measurement. This study proposes a method for characterizing thermal barrier coating specimens using ZGV Lamb modes, based on an all-laser ultrasonic excitation and detection system. Finite element simulations were performed to extract ZGV-related parameters, which were fitted to the coating thickness using least squares and multivariate regression analysis. Simple and multiple regression models were developed and validated through laser ultrasonic experiments. The results demonstrate a robust, linear relationship between selected ZGV parameters and coating thickness, allowing accurate estimation of layer thickness. The average relative errors of TBC and substrate thickness estimation are less than 7.71% and 1.55%, respectively. The combination of Lamb wave mode analysis and model-based regression offers a viable and non-contact method for the quantitative evaluation of ceramic coating thickness.