Accurate measurement of TBC thickness with specific wavelength multimodal SAW induced by grating laser

•Development of a calibration-exempt method using a grating laser for topcoat thickness measurement in TBCs.•The proposed method can provide an accurate and convenient measurement of topcoat thickness.•Successful validation through theoretical fitting, FEM-based model simulations, and experimental results, showcasing convincing measurement accuracy within typical topcoat thickness ranges. The topcoat thickness is critical in assessing the performance of thermal barrier coatings (TBCs). A calibration-exempt method utilizing grating-laser-induced multimodal surface acoustic wave (SAW) with a specific wavelength is proposed to bridge the deficiency in topcoat thickness evaluation, enabling accurate and convenient measurement. Optically detected the first and second mode signals of SAW with a well-defined wavelength are converted directly into local acoustic velocities, which carry the thickness information. In this work, deriving the dispersion equation of SAW in a multi-layer structure, dispersion curves are obtained numerically. With the given wavelengths, the theoretical relationship between multimodal SAW velocities and the topcoat thicknesses is established and linearly fitted by the least squares method. All data is confined within the prediction error band to determine the optimal linear range. Further, a simulation program based on the finite element method (FEM) is developed to confirm the feasibility of the proposed method. Ultimately, experiments utilizing fabricated TBC samples are undertaken. The experimental results illustrate that multimodal of SAWs have convincing measurement accuracy within the typical topcoat thickness, which validates the application potential of the proposed method.