Terahertz Thickness Measurement Method for Thermal Barrier Coatings Based on Physical Constraints and Micro Variations

Thermal barrier coatings (TBCs) are insulation coatings for turbine blades in aero-engines, and the thickness of topcoat (TC) is the key factor in determining the insulation performance. Consequently, determining the thickness of the ceramic TC is a pressing issue in TBC research. The uneven microstructure of TBCs leads to the change of refractive index, which makes the time-of-flight (TOF) thickness measurement method is inaccurate. Thickness measurement method based on model inversion does not require prior information of the sample, and it has higher accuracy than the TOF method. However, it currently suffers from issues of instability in the results. This article presents a thickness measurement method based on the terahertz (THz) technique, utilizing a physics-constrained and micro variation (MV) algorithm. Since refractive index can be converted through the TOF of sample’s THz experimental signal, it is feasible to calculate the refractive index using thickness and TOF, achieving parameter dimensionality reduction. This eliminates some local optima, enhancing the algorithm’s optimization efficiency and precision. It can prevent the algorithm falling into local optimal solution by monitoring the population radius and introducing “micro-variation” perturbations to increase population diversity when it is insufficient. Experiments show that this method provides stable thickness measurement results, with an average relative error rate of less than 1%, demonstrating high thickness measurement accuracy.