Evaluation of ZrO 2 –24MgO ceramic coating by eddy current method A. Nusair Khan a , S.H. Khan b, * , Farhad Ali c , M.A. Iqbal b a Metallurgy Division, GPO Box No. 502, Rawalpindi, Pakistan b Department of Physics, University of the Punjab, Quaid-e-Azam Campus, Lahore, Pakistan c 537- D/6, Allahabad, WSTG III, Rawalpindi, Pakistan article info Article history: Received 29 January 2008 Received in revised form 8 July 2008 Accepted 15 July 2008 Available online 29 August 2008 PACS: 81.70.Ex 82.60.Fa 81.70.Pg Keywords: Ceramic coatings High temperature ceramics Air plasma spraying Eddy current evaluation Microstructure Metallurgical phase abstract A novel study was carried out using eddy current technique for non-destructive evaluation of Thermal Barrier Coatings (TBC). For this purpose, zirconium oxide, stabilized with magnesium oxide as a top-coat, and Ni-5Al as a bond-coat were air plasma sprayed onto a nickel base alloy substrates. Microstructure and phase changes were observed during thermal treatment. Formation of nickel-oxide was noticed dur- ing the experimentation. The values of eddy current and the phase-angle changes, were correlated with the micro-structural and metallurgical phase changes, observed at the interface of bond-coat and top- coat. Ó 2008 Elsevier B.V. All rights reserved. 1. Introduction The use of protective coating for industrial applications is grow- ing day by day. Thermal Barrier Coatings (TBCs) are widely used in gas turbines for propulsion and power generation [1–4]. The TBCs can be considered as three layer materials systems, consisting of (1) a super alloy substrate, (2) an oxidation-resistant metallic bond-coat, and (3) the ceramic top coating deposited either by plasma spraying or electron beam physical vapor deposition. The zirconia’s top-coat has excellent thermal shock resistance, low thermal conductivity and a relatively high coefficient of thermal expansion (CTE). The bond-coats provide a rough surface for mechanical bonding of the ceramic top-coat, protect the underly- ing alloy substrate against high temperature oxidation corrosion, and minimize the effect of CTE mismatch between the substrate and ceramic top-coat materials. Pure zirconia transforms from tetragonal to monoclinic phase and thus volumetric changes are associated with this transforma- tion when temperature changes. This transformation is not desir- able in the application of coating. Therefore, it can be stabilized with either CaO (5 wt%), MgO (15–24 wt%) [4] or Y 2 O 3 (6– 12 wt%) [5,6]. The fully or partially stabilized zirconia is usually deposited by plasma spraying method on the metal surface. These coatings are bad heat conductors and have good thermal shock resistant properties. Among them ZrO 2 –MgO are relatively cheaper than ZrO 2 –Y 2 O 3 ‘that is’ why it is utilized in those regions where the temperature intensity is relatively low e.g. in the exhaust of the jet engines. Further, an MgO stabilized system can be used for the development of intermediate coating in a three part graded coating system with magnesium zirconate as a top-coat. Since the TBCs are used on the hot sections of an engine there- fore, it is very difficult and not economical to carryout destructive tests. Eddy current technique being easy and non-destructive in nature can be utilized as an effective method to evaluate the life of TBC. Eddy current testing is an inspection method applied to conduc- tive materials. Numerous advantages like high sensitivity, rapid scanning, contact less inspection, and versatility contribute to its widespread utilization. This technique can be utilized for the eval- uation of metallurgical variations [7,8], in addition to defect detec- tion [9]. This method works on the principle of Faraday’s law. It measures change in coil impedance in the same exciter coil or induced voltage in a separate coil. The excitation frequency is 0927-0256/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.commatsci.2008.07.021 * Corresponding author. Tel.: +92 051 9285017; fax: +92 051 9285018. E-mail address: shoaibhkhan@yahoo.com (S.H. Khan). Computational Materials Science 44 (2009) 1007–1012 Contents lists available at ScienceDirect Computational Materials Science journal homepage: www.elsevier.com/locate/commatsci