Microstructural characterization of thermal barrier coating on Inconel 617 after high temperature oxidation

Association of Metallurgical Engineers of Serbia AMES Scientific paper UDC: 667.613:621.224.3 MICROSTRUCTURAL CHARACTERIZATION OF THERMAL BARRIER COATING ON INCONEL 617 AFTER HIGH TEMPERATURE OXIDATION ∗ Mohammadreza Daroonparvar1 , Mehdi MazarAtabaki1,2, Muhammad Azizi Mat Yajid1, Mohammad Sakhawathussain1, Mehdi Asgharifar2, Noordin Mohd.Yusof1 1 Department of Materials, Manufacturing and Industrial Engineering, Faculty of Mechanical Engineering, Universiti Teknologi Malaysia, 81310 Johor Bahru, Johor, Malaysia 2 Research Center for Advanced Manufacturing (RCAM), Department of Mechanical Engineering, Southern Methodist University, 3101 Dyer Street, Dallas, TX 75205, USA. Received 31.07.2012 Accepted 09.11.2012 Abstract A turbine blade was protected against high temperature corrosion and oxidation by thermal barrier coatings (TBCs)using atmospheric plasma spraying technique (APS) on a Ni-based superalloy (Inconel 617). The coatings (NiCr6AlY/ YSZ and NiCr10AlY/YSZ) consist of laminar structure with substantial interconnected porosity transferred oxygen from Yittria stabilized Zirconia (YSZ) layer toward the bond coat (NiCrAlY). Hence, a thermally grown oxide layer (TGO) was formed on the metallic bond coat and internal oxidation of the bond coat occurred during oxidation. The TBC systems were oxidized in a normal electrically heated furnace at 1150 °C for 18, 22, 26, 32 and 40h.Microstructural characterization of coatings demonstrated that the growth of the TGO layer on the nickel alloy with 6wt. % Al is more rapid than TGO with 10wt. % Al. In addition, many micro-cracks were observed at the interface of NiCr6AlY/YSZ. X-ray diffraction analysis (XRD) showed the existence of detrimental oxides such as NiCr2O4, NiCrO3 and NiCrO4 in the bond coat containing 6wt. % Al, accompanied by rapid volume expansion causing the destruction of TBC. In contrast, in the bond coat with 10wt. % Al, NiO, Al2O3and Cr2O3 oxides were formed while very low volume expansion occurred. The oxygen could not penetrate into the TGO layer of bond coat with 10 wt. % Al during high temperature oxidation and the detrimental oxides were not ∗ Corresponding author: Mohammadreza Daroonparvar : 96 Metall. Mater. Eng. Vol 19 (2) 2013 p. 95-106 extensively formed within the bond coat as more oxygen was needed. The YSZ with higher Al content showed higher oxidation resistance. Keywords: Oxidation; Atmospheric plasma spray; NiCrAlY; TGO and YSZ. Introduction Thermal barrier coatings (TBCs) have been extensively applied on nickel based superalloys to improve the efficiency and durability of hot section components of gas turbines. A typical TBC system consists of a thermally insulating ceramic top-coat (yttria stabilized Zirconia (YSZ) layer) and an oxidation-resistant metallic bond coat on the nickel-based superalloy substrates. (Ni, Co) CrAlY is usually utilized as a bond coat to provide a suitable thermal expansion match between the topcoat and the substrate and to protect the substrate from rapid oxidation and hot corrosion [1]. Applying MCrAlY bond coating at high temperature causes the transfer of oxygen from the topcoat to the bond coat; so an oxide scale can be formed on the bond coat designated as thermally grown oxide (TGO). Although this scale protects the substrate against further oxidation, the growth of TGO during thermal cycling can lead to failure of the yttria stabilized Zirconia (YSZ) layer [2, 3]. Two mechanisms have been proposed for transferring oxygen from the plasma sprayed Zirconia coatings. Ionic diffusion from the crystalline structure of ZrO2 and gas penetration through some porosity and micro cracks are the main mechanisms [4]. The thickness of TGO layer can be increased during the oxidation process. This can produce stress at the interface of the bond coat and the ceramic layer. This stress is higher than the strain tolerance of MCrAlY bond coating, causing delamination of the coating at the interface of bond coat and YSZ layer [5]. There are some studies about the bond coatings applying different percentages of aluminum [6,7]. It was found that a bond coating with low aluminum content cannot be utilized as a protective coating at high temperature, while the coatings prepared with a higher percentage of Al showed better oxidation behavior. This favorable oxidation performance can be mainly attributed to the microstructure of coating [6]. It can be said that a homogeneous adhesive TGO layer on the bond coat is the main cause of this behavior. In addition, the TGO layer protects the coating from rapid oxidation and avoids the formation of detrimental oxides [7-10]. However, the ever-increasing requirement of a higher operating temperature for better efficiency of the turbine blades gives a strong support to investigate new compositions. It is generally considered that the YSZ is transparent to oxygen due to lattice diffusion and the multitude of connected cracks. Therefore, it would be considered that the YSZ does not hinder oxygen attacking the bond coat. The aim of this study is to reduce the thickness of TGO layer and decrease the detrimental oxides using NiCr(x)AlY layer. Two TBC systems (NiCr6AlY/YSZ and NiCr10AlY/YSZ) were oxidized in a normal electrical furnace at 1150°C for 22 and 40h.This study is also focused on the development of a new coating combination while plasma spray technique is applied as the coating process. Microstructural changes in TBC surfaces were examined using scanning electron microscopy (SEM), X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM) equipped with energy dispersive X-Ray analysis (EDS). Daroonparvar at al. - Microstructural characterization of thermal barrier coating ... 97 Experimental procedure Materials A nickel-based superalloy (Inconel 617) disc of 30×5mm, grit blasted with alumina particles was used as substrate. Two powders were used as bond coatings and had the composition of Ni-22 (wt. %) Cr-10 (wt. %) Al-1 (wt. %) Y (with an average grain size of 57 µm) and Ni-25 (wt. %) Cr-6 (wt. %) Al-0.4 (wt. %) Y (with an average grain size of 55 µm). For the thermal barrier coating ZrO2-8%Y2O3 (with an average grain size of 33 µm)was utilized. Coating preparation NiCr6AlY/YSZ and NiCr10AlY/YSZ coatings were generated applying atmospheric plasma spray (APS) method. Tables 1 and 2 show the characteristics of the coatings and the parameters of atmospheric plasma spraying process, respectively. Table 1. Details of two different YSZ thermal barrier coating compositions on Inconel 617. Layer thickness (µm) TBC NiCrAlY Yittria stabilized Zirconia NiCr10AlY/YSZ 110 260 NiCr6AlY/YSZ 120 250 Table 2. Parameters of air plasma spraying. Parameter Current Voltage Primary gas, Ar Secondary gas, H2 Powder feed rate Spray distance Unit A V 1/min 1/min g/min cm NiCr(x)AlY 450 50 85 15 15 15 YSZ 550 70 38 17 35 7.5 High temperature oxidation test and microstructural characterization The coatings were insert (...truncated)