Vacuum 177 (2020) 109401 Available online 27 April 2020 0042-207X/© 2020 Elsevier Ltd. All rights reserved. Performance of single YSZ, Gd2Zr2O7 and double-layered YSZ/Gd2Zr2O7 thermal barrier coatings in isothermal oxidation test conditions Kadir Mert Doleker a , Abdullah Cahit Karaoglanli b, * , Yasin Ozgurluk b , Akira Kobayashi c, d a Metallurgical and Materials Engineering Department, Ondokuz Mayis University, 55139, Samsun, Turkey b Metallurgical and Materials Engineering Department, Bartin University, 74100, Bartin, Turkey c Faculty of Science, Chulalongkorn University, Bangkok, 10330, Thailand d Graduate School of Engineering, University of Tokyo, Bunkyo, Tokyo, 113-8656, Japan A R T I C L E INFO Keywords: Double-layered thermal barrier coating Cold gas dynamic spray (CGDS) Isothermal oxidation Thermally grown oxide (TGO) Gadolinium zirconate (Gd 2 Zr 2 O 7 ) ABSTRACT Oxidation is an inevitable failure mechanism under the operating temperature in gas turbines. To avoid negative effects of oxidation, ceramic-based materials having low thermal conductivity and high stability should be used to hot section components. In accordance with this purpose, thermal barrier coatings (TBCs) are used in order to increase the lifetime of gas turbine engine components that have not reached to desired levels yet. Yttria sta- bilized zirconia (YSZ) has been used as a conventional top coat material in TBCs. Increased the turbine inlet temperatures (TIT) promote to researchers to try higher stable material such as rare earth zirconates. In this study, CoNiCrAlY metallic powders were sprayed using a new emerging technique as called cold gas dynamic spray (CGDS) on Inconel 718 substrates. Single layer YSZ, Gd 2 Zr 2 O 7 and double-layered YSZ/Gd 2 Zr 2 O 7 were deposited by electron beam physical vapor deposition (EB-PVD) technique as top coat materials. In high tem- perature furnace, both TBC samples were isothermally oxidized at 1000 � C under different time periods. TBCs were examined as microstructural before and after oxidation tests. Thermally grown oxide (TGO) layer forming at the interface during oxidation were investigated and compared for each TBC systems. Oxidation and TGO growth behaviors were discussed. 1. Introduction Thermal barrier coatings (TBCs) assist to prolong the lifetime of the hot section parts such as gas turbine blades, vanes and combustion chambers [1]. A TBC system consists of a Ni-based superalloy substrate, MCrAlY (M ¼ Co, Ni, or Co/Ni) as a metallic bond coat for oxidation protection and YSZ (yttria-stabilized zirconia) top coat for thermal insulation [2]. Substrate materials must have high strength, fatigue and creep resistance in TBC systems. The bond coat provides adhesion be- tween the substrate and top coat as well as oxidation and corrosion resistance. Top coat thermally insulates the substrate and bond coat against hot gas fows. Top coat materials and production techniques have great importance for long service lifetime [3]. Generally, 7-8% stabilized ZrO2 (YSZ) was used due to its superior mechanical and thermal properties [4]. However, there are some limitations related to sintering and phase transformation temperature of YSZ TBC which is stable up to 1200 � C. Since YSZ is subjected to phase separation through diffusion after passing the temperature of about 1200 � C, it becomes a monoclinic phase from tetragonal during cooling. This transformation may cause volumetric changes, which may lead to decomposition in the coating [5,6]. Therefore, alternative materials instead of YSZ are very attractive for many TBC researchers. Rare earth zirconates (Re2Zr2O7) like La2Zr2O7, Nd2Zr2O7, Gd2Zr2O7 are popular material groups for top coat materials due to their high phase transformation and sintering temperature as well as their low thermal conductivities compared to YSZ [7–10]. Their limitations compared to YSZ are in terms of lower thermal expansion coeffcient (TEC), lower fracture toughness and compatibility with TGO layer [11]. To overcome these disadvantages, double layered TBCs are more suitable compared to single layer rare earth zirconates or YSZ [12]. In high temperature applications of the gas turbines, some failure mechanisms such as oxidation, corrosion, erosion or calcia-magnesia-alumina-silicates (CMAS) attack can be exposed. Gd2Zr2O7 has higher resistance against CMAS attack and hot corrosion [13]. Furthermore, its oxygen penetration is lower than YSZ due to its Frenkel pair defects required higher activation energy [14]. During service conditions, high temperature oxidation causes formation of * Corresponding author. E-mail address: cahitkaraoglanli@gmail.com (A.C. Karaoglanli). Contents lists available at ScienceDirect Vacuum journal homepage: http://www.elsevier.com/locate/vacuum https://doi.org/10.1016/j.vacuum.2020.109401 Received 10 February 2020; Received in revised form 13 April 2020; Accepted 15 April 2020