Nondestructive Evaluation of Thermal Barrier Coating Thickness Based on Terahertz Time-Domain Spectroscopy

Abstract

Thermal barrier coatings (TBCs) are critical for the thermal protection of aeroengine components, necessitating precise nondestructive evaluation of their thickness. While terahertz time-domain spectroscopy (THz-TDS) is a promising technique, traditional time of flight measurements are often compromised by echo overlap and noise interference due to the complex porous structure of TBCs. This paper proposes an enhanced thickness measurement method using normalized cross-correlation (NCC) time-frequency fusion. Using a reflective THz-TDS system on atmospheric plasma-sprayed samples, the NCC algorithm was employed to amplify weak interface reflections and suppress noise. By iteratively reconstructing and stripping reflected pulses, the method effectively separates overlapping echoes, allowing for the precise extraction of transit time. The refractive index was simultaneously determined via frequency-domain analysis. Comparative experiments with scanning electron microscopy (SEM) measurements demonstrated that the proposed method achieved a relative error of only 3.38%, significantly lower than the 9.10% observed with the traditional time of flight method. These results confirm that the NCC-based approach provides superior accuracy and reliability for TBC thickness characterization.

Keywords: thermal barrier coating, terahertz time-domain spectroscopy, nondestructive evaluation, thickness measurement, normalized cross-correlation

1. Introduction

Thermal barrier coatings (TBCs) serve as critical thermal protection systems in advanced gas turbine engines, enabling significant improvements in engine performance and component longevity. These multilayer ceramic-metallic coating systems typically consist of a metallic bond coat (BC) and a ceramic top coat (TC), deposited on nickel-based superalloy substrates. The accurate measurement of TBC thickness is essential for ensuring coating quality, predicting service life, and maintaining the structural integrity of aeroengine components.