Nondestructive Evaluation of Thermal Barrier Coatings Using Impedance Spectroscopy

Abstract

This article reviews previous studies on nondestructive evaluation of thermal barrier coatings (TBCs) using impedance spectroscopy (IS). IS or electrochemical impedance spectroscopy has been widely used to measure the electrical properties of materials and electrochemical behavior at electrode/electrolyte interfaces. TBCs, which comprise metallic and ceramic multilayers, have been widely used in the hot section of aeroturbine engines to increase turbine efficiency and to extend the life of metallic components. Since 1999, IS has been developed to examine degradation of the TBCs as a nondestructive evaluation tool, which is critical for prediction of TBCs lifetime during service. IS has been used both at high temperature in dry environments and in aqueous solutions. Impedance spectra of TBCs reflect change in TBC thickness, porosity, cracks, sintering, and yttria-stabilized zirconia phase transformation. Meanwhile, impedance measurements indicate the thermally grown oxide growth and the failure in TBCs. In addition, the thermal conductivity of TBCs can be correlated to impedance measurement results.

Impedance spectroscopy: A useful technique to unveil the mechanism of a CO oxidation reaction

“…This technique provides useful information for the application of these materials as energy sources, coatings, sensors, and in other applications. [11][12][13][14][15] Despite being a powerful technique, few papers are found aligning IS with the study of reaction kinetics and mechanisms. For instance, it can be used in investigating methanol oxidation, [16][17][18] membrane reactors, [19][20][21][22] and electrocatalytic processes like water splitting 23 and CO oxidation.…”

“…Additionally, it has been employed to study surface processes, such as catalytic reactions, adsorption processes, and mass transfer. This technique provides useful information for the application of these materials as energy sources, coatings, sensors, and in other applications 11–15 …”

Progression in Non-Destructive Spallation Prediction and Elevated Temperature Mechanical Property Evaluation of Thermal Barrier Coating Systems by Use of a Spherical Micro-Indentation Method

“…An electric field with varying frequency is then applied and directed through the TBC system. Electrical responses from the various components of the TBC system (YSZ, TGO and BC) reflect in differing frequency domains[131].Due to this, the electrical response from the multiple layers can be separated in the impedance spectra and thus the microstructure of the TBC can be correlated to these impedance spectroscopy parameters. Variations in the TBC microstructure along thermal exposure lead to changes in electrical material properties and thereby variations in impedance spectra.Highly temperature dependent, this electrical signal is highly affected by temperature, moisture and dopants in the material.…”