Quality Assurance Evaluation of Thermal Barrier Coatings by Electrochemical Impedance Spectroscopy
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Quality Assurance Evaluation of Thermal Barrier Coatings by Electrochemical Impedance Spectroscopy Jianqi Zhang, Danyash Tamboli and Vimal Desai Advanced Materials Processing & Analysis Center, University of Central Florida, Orlando, FL 32816-2450, U. S. A. ABSTRACT The technique of electrochemical impedance spectroscopy (EIS) was used to examine the behavior of intact thermal barrier coatings (TBC) at ambient temperature. Cross-sectional morphological examination of TBC was conducted by scanning electron microscope (SEM). By correlating the SEM visual examination with EIS data, TBC was characterized non-destructively. A model of EIS alternative current (AC) equivalent circuit was proposed to establish the relationship between the EIS elemental parameters in the circuit and the microstructural characteristics of TBC. A linear relationship was found to exist between the electrical impedance of TBC topcoat and the thickness of the topcoat. The porosity of TBC top coat showed a linear relationship with the capacitance of ceramic TBC, and the pore shape in the TBC topcoat was represented by the value of the electrical impedance of the pore. The result in the study has demonstrated that EIS can be used as a non-destructive evaluation (NDE) technique for quality assurance of TBC.
INTRADUCTION Advanced turbine system engines are slated to operate at turbine inlet temperatures of nearly 27500F so as to achieve 60% and higher overall efficiency. No currently used superalloys can withstand such high temperatures so that use of TBC is essential to reduce the substrate skin temperatures to acceptable level [1]. The successful application of TBC in land based gas turbines is a greater technical challenge compared to aeroengine applications because there are significant differences regarding the size/time/environment/stress duty cycles between these two systems. Land based gas turbine systems are expected to give inspection free long-term service while incurring harsh corrosive environment. Therefore, the long-term reliability of TBC is a far more critical concern in industrial gas turbines. There are lots of disadvantages in currently used detection techniques that restrict their application. The techniques in characterizing the porosity or detecting the defects of TBC such as density, hardness and surface area measurements, optical microscopy, and mercury intrusion porosimetry are destructive. Conventional off-line NDE techniques such as radiography, eddy current and ultrasound to detect hidden flaws or debonding of TBC are much time-consuming. Small angle neutron scattering (SANS) [2] and nuclear magnetic resonance (NMR) imaging [3] techniques recently arouse great interest in detecting pores and cracks of TBC. However, the interpretation of SANS and NMR data is highly speculative. Reports show that thermal wave imaging is a rapid non-contact inspection technique capable of locating defects near the surface of 1~2 mm thick M8.6.1
samples by monitoring surface temperatures. Acoustic microscopy imaging can detect defects in s
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