Development of measuring method for the thickness of TBC oxide scale by AC impedance spectroscopy

Abstract:

In gas turbine, thermal barrier coating system (TBCs) is usually used to increase the operation temperature and protect the turbine components like blades and vanes. A TBC system consists typically of a superalloy substrate, a metallic bond coat, the thermally grown oxide and a heat-insulating ceramic top coat. The thermally grown oxide(TGO) grows at bond coat / top coat interface is a dominate factor leading to the failure of TBCs, thus determining the thickness of TGO non-destructively is essential for monitoring the performance of TBCs and predicting the lifetime of TBCs in service. Our work here uses impedance spectroscopy to analysis the TBCs samples with 0.1mm YSZ top coat after oxidation at 1000℃. The SEM and EDS result shows that a continuous alumina layer formed after 100h oxidation and the thickness increases with increasing oxidation time, meanwhile, the chemical composition changes from alumina to mixed oxides. The oxidation kinetics demonstrates that the oxidation procedure is controlled by diffusion. By equivalent circuit model analysis, we obtain the direct proportional relationship between TGO thickness and capacitance, so the TGO thickness can be measured.