Determining oxide growth in thermal barrier coatings (TBCs) non-destructively using impedance spectroscopy

Abstract

Thermal barrier coatings (TBCs) have been applied to gas turbine blades and vanes for providing thermal protection and corrosion resistance. One of the major concerns for application of TBCs is their reliability and durability. It has been established that the oxidation of bond coat underlying yttria stablized zirconia (YSZ) is a dominant factor in controlling the failure of TBCs [1, 2]. The oxidation resistance of bond coat is attributed to the formation of a protective oxide layer at the surface of bond coat. The growth of the oxide layer increases the stress at the bond coat/YSZ interface and leads to the spallation of TBCs [3]. The thickness of the oxide layer is therefore a useful indicator of life expectancy of TBCs. For this reason the non-destructive detection of oxidation layer growth is of great importance in the application of TBCs. Impedance is a cheap and quick non-destructive testing method. It is particularly efficient in characterizing multi-layer materials when different layers have very different electrical properties [4, 5]. In this investigation, we have successfully used impedance spectroscopy in determining the alumina layer growth in TBCs. The TBCs used in this investigation consist of the top coat YSZ (8 wt% yttria stabilized zirconia) and the bond coat MCrAlY (with 38.5wt%Co-32wt%Ni21wt%Cr-8wt%Al-0.5wt%Y). The substrate is Haynes 230 alloy. The TBC samples were cut into 10 mm by 10 mm squares. The samples were oxidized at 1100 ◦C for 400–1500 h. Impedance measurements were made using a Solartron SI 1255 HF Frequency Response Analyser coupled with a 1296 Dielectric Interface (Solartron, UK.) with applied voltage of 1 volt at frequencies from 106 Hz to 10−3 Hz, with 5 data reading per decade. Electrodes were applied by painting a thin layer of silver ink on the top coat and subsequent firing at 400 ◦C for 15 min. Then impedance measurements were made at 350 ◦C. Microstructure of the TBC samples was examined using SEM coupled with energy dispersive X-ray spectroscopy (EDS). Fig. 1 shows the Nyquist plot obtained from impedance measurements of TBCs. For all the oxidized samples, the Nyquist plot is composed of two semicircles: one is a small semicircle at high frequency (HF), another is a large semicircle at low frequency (LF). The small HF semicircle can be identified clearly in an enlarged diagram of Fig. 1. By comparison with the impedance spectrum of the as-received TBC without