Quantitative Determination of Porosity in Thermal Barrier Coatings Using Terahertz Reflectance Spectrum: Case Study of Atmospheric-Plasma-Sprayed YSZ Coatings

Microstructures of yttria-stabilized zirconia (YSZ) atmospheric-plasma-sprayed (APS) thermal barrier coatings (TBCs) and their terahertz dielectric properties were investigated to quantitatively determine porosities by effective medium theory (EMT). Various samples with different porosities ranged from 9.15% to 21.58% were prepared by different particle size powders and spray power. Both the theoretical and experimental results showed that the interferential oscillation period of reflectance spectra of APS YSZ TBCs samples depends on optical thickness of ceramic coatings, and optical thickness depends on permittivity for known geometric thickness. The real part of permittivity ε' decreased as the porosity increased, the depolarization factor ζ of APS YSZ TBCs pores, which was estimated by comparing the experimental value with the simulated value obtained by the Maxwell-Garnett EMT model and Bruggeman EMT model, approached to a fixed value, owing to the typical lamellar structure of APS-TBCs, and was 0.7 and 0.6 in respective models. Samples of 185-459 μm thickness verified that the technique varied from scanning electron microscope statistics results by <; 15%. Finally, a novel, convenient, online, nondestructive, and noncontact porosity evaluation method is proposed that can potentially be utilized on the health monitoring of TBCs in gas turbine blades.