Abstract
In this work, a terahertz time-domain spectroscopy (THz-TDS) system was used to measure the thickness of thermal barrier coatings (TBCs) and characterize the interface morphology of TBCs after erosion. Reflection mode, with an angle of incidence of 0, was used for inspection before and after erosion. The refractive index, thickness, and internal structure evolution tendency of the yttria-stabilized zirconia (YSZ) top coat were estimated under consideration of the interaction between the pulsed THz waves and the TBCs. The surface roughness of the top coat surface was considered for the errors analysis in the refractive index and thickness measurement. To reduce the errors introduced by the refractive index change after erosion, two mathematical models were built to assess the thickness loss. Then, the thickness loss was compared with results estimated by the micrometer inspection method. Finally, the basic erosion sample profile with Ra roughness was obtained, and the broadening of THz pulses were suggested as a possible measure for the top coat porosity change, showing that THz waves can be a novel online non-destructive and non-contact evaluation method that can be widely utilized to evaluate the integrity of TBCs applied to gas turbine blades.
Highlights
Thermal barrier coatings (TBCs) are widely applied to the metallic surfaces on the gas-turbine or aero-engine parts operating at elevated temperatures, enabling the engines to operate at gas temperatures well above the melting temperature of the superalloy, thereby improving engine efficiency and performance [1]
A THz time-domain spectroscopy (THz-TDS) system of the reflection mode was used to measure the thickness of thermal barrier coatings (TBCs) and characterize the interface morphology of TBCs after erosion
For the interface morphology characterization of the eroded sample, the interaction between THz waves and the TBCs was discussed with respect to the intact and erosion areas, and two different mathematical models were used to estimate the thickness before and after the erosion test
Summary
Thermal barrier coatings (TBCs) are widely applied to the metallic surfaces on the gas-turbine or aero-engine parts operating at elevated temperatures, enabling the engines to operate at gas temperatures well above the melting temperature of the superalloy, thereby improving engine efficiency and performance [1]. Various NDT techniques have been developed to examine the TBCs up to now, such as ultrasonic waves testing, eddy current testing, rare earth luminescence examination, and X-ray imaging, etc. The eddy current test signal is susceptible, and the measurement of non-metallic materials was not suitable for the evaluation of complex components as it requires the lift-off operation step, which generates large noise [13,14,15,16]. NDT method to characterize the interface morphology of TBCs. The frequency range of terahertz waves is generally considered to be between 0.1 and 10 THz, which lies between the infrared waves and microwaves. A THz time-domain spectroscopy (THz-TDS) system of the reflection mode was used to measure the thickness of TBCs and characterize the interface morphology of TBCs after erosion
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