Millimeter Wave Thickness Evaluation of Thermal Barrier Coatings (TBCs) Using Open-Ended Waveguide Probes

ABSTRACT Nondestructive testing (NDT) of thermal barrier coatings (TBCs) is a critical and ongoing topic of research and development. In particular, inspection techniques that determine the thickness of ceramic topcoat and thermally grown oxide (TGO) are of interest. This work investigates the utility of open-ended rectangular waveguide probes in the millimeter wave frequency range of 26.5–110 GHz for evaluation of topcoat and TGO thicknesses through a compressive set of electromagnetic (EM) simulations. In addition, these EM simulations are used to illustrate the influence of probe size and TBC substrate curvature on the complex reflection coefficient properties and the subsequent thickness estimation. The impact of volumetric porosity level on the same is also investigated. A standing-wave probe at V-band (50–75 GHz) is constructed and used to measure the topcoat thickness on three button-type TBC samples. This probe eliminates the need for using expensive and bulky vector network analyzers (VNA), which is quite desirous from a practical point-of-view. The experimental results indicate the capability of estimating the topcoat thickness to within ±15 μm (0.6 mils).