Preprint Article Version 1 Preserved in Portico This version is not peer-reviewed

THz-TDS Reflection Measurement of Coating Thicknesses at Non-Perpendicular Incidence: Experiment and Simulation

Version 1 : Received: 15 April 2021 / Approved: 16 April 2021 / Online: 16 April 2021 (17:09:19 CEST)

A peer-reviewed article of this Preprint also exists.

Burger, R.; Frisch, J.; Hübner, M.; Goldammer, M.; Peters, O.; Rönneberg, E.; Wu, D. THz-TDS Reflection Measurement of Coating Thicknesses at Non-Perpendicular Incidence: Experiment and Simulation. Sensors 2021, 21, 3473. Burger, R.; Frisch, J.; Hübner, M.; Goldammer, M.; Peters, O.; Rönneberg, E.; Wu, D. THz-TDS Reflection Measurement of Coating Thicknesses at Non-Perpendicular Incidence: Experiment and Simulation. Sensors 2021, 21, 3473.

Journal reference: Sensors 2021, 21, 3473
DOI: 10.3390/s21103473

Abstract

Time-domain spectroscopy (TDS) in the Terahertz (THz) frequency range is gaining in importance in nondestructive testing of dielectric materials. One application is the layer thickness measurement of a coating layer. To determine the thickness from the measurement data, the refractive index of the coating layer must be known in the surveyed frequency range. For perpendicular incidence of the radiation, methods exist to extract the refractive index from the measurement data itself without prior knowledge. This paper extends these methods for non-perpendicular incidence, where the polarization of the radiation becomes important. Furthermore, modifications considering effects of surface roughness of the coating are introduced. The new methods are verified using measurement data of a sample of Inconel steel coated with yttria-stabilized zirconia (YSZ) and with COMSOL simulations of the measurement setup. To validate the thickness measurements, scanning electron microscopy (SEM) images of the layer structure are used. The results show good agreement with an average error of 1% for the simulation data and under 4% for the experimental data compared to reference measurements.

Subject Areas

nondestructive evaluation; THz time-domain spectroscopy; layer thickness measurement; thermal barrier coatings; surface roughness; porosity; polarization; yttria-stabilized zirconia

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