Post-Exposure Evaluation of Thermal Barrier Coatings by Electrochemical Impedance Spectroscopy
- PDF / 63,781 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 40 Downloads / 193 Views
Post-Exposure Evaluation of Thermal Barrier Coatings by Electrochemical Impedance Spectroscopy Jianqi Zhang and Vimal Desai Advanced Materials Processing & Analysis Center, University of Central Florida, Orlando, FL 32816-2450, U. S. A. ABSTRACT High temperature behavior of air plasma sprayed (APS) thermal barrier coatings (TBC) was investigated in this study by electrochemical impedance spectroscopy (EIS). Scanning electron microscope (SEM) was used to examine cross-sectional morphology of exposed TBC. It was found that characteristic EIS spectra were obtained effectively to differentiate variant post-exposed TBC such as isothermal oxidation and cyclic oxidation. In addition, the type and length of high temperature exposure were qualitatively discernible from the EIS data. A model of an EIS alternative current (AC) equivalent circuit was proposed quantitatively to relate the EIS parameters to the morphological properties of the exposed TBC and functional relationships have been established between them. From the results, EIS has been identified capable of monitoring the microstructure of post-exposed TBC and evaluating TBC damage.
INTRODUCTION The application of thermal barrier coatings (TBC) is one of the most critical approaches in the development of advanced turbine systems. It allows an increase in the turbine inlet temperature without an increase in the skin temperature of the substrate superalloys, thereby efficiently increasing the overall efficiency. However, there is insufficient durability/reliability information to assure that the coating will remain protective during the long-term continuous operation with far in-between inspections. Thus the evaluation of the TBC service performance becomes a paramount issue. Typically, a conventional TBC consists of two coats: The outer ceramic topcoat, almost universally based on ZrO2 partially stabilized with ~7wt.%Y2O3, possesses very low thermal conductivity thereby offering thermal insulation to the substrate; the inner bond coat is generally of a Pt-modified aluminide or MCrAlY overlay type coat to provide oxidation or/and hot corrosion resistance to the substrate. A layer of thermally grown oxide (TGO), typically a-Al2O3 less than 1mm thick is normally formed between the bond coat and topcoat during the topcoat processing. There are a number of reasons responsible for TBC failure at high temperatures: 1. Sintering of TBC topcoat can result in an increase in TBC thermal conductivity, stiffness and internal residue stresses, thereby loss of thermal insulation or/and spallation occurs just above the interface between the topcoat and TGO. Such cases typically happened in the air plasma sprayed TBC [1]; 2. The growth of TGO causes an increase in stresses at the interface of TGO/substrate, resulting in spallation of TBC at the interface after TGO grows up to a critical thickness. Such cases occurred in the electron beam physical vapor sprayed TBC [2]. M9.2.1
The service reliability of industrial TBC has been still a critical concern, thereby various NDE methods ha
Data Loading...
