Contents lists available at ScienceDirect Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat Effect of processing and interface on the durability of single and bilayer 7YSZ / gadolinium zirconate EB-PVD thermal barrier coatings Uwe Schulz a,* , Andrzej Nowotnik b , Stefan Kunkel c , Georg Reiter c a German Aerospace Center (DLR), Cologne, 51147, Germany b Rzeszow University of Technology, 35-959, Rzeszow, Poland c ALD Vacuum Technologies GmbH, 63457, Hanau, Germany ARTICLE INFO Keywords: Thermal barrier coatings EB-PVD Gadolinium zirconate Bilayer ABSTRACT Gadolinium Zirconate (GZO) is known for its low thermal conductivity, high thermal stability and a favorable fast reaction with CMAS deposits that makes such a thermal barrier coating (TBC) more stable and less infiltrated in dusty turbine environments. In the present study, Electron Beam Physical Vapor deposited (EB-PVD) GZO/ 7YSZ bilayers were coated on superalloy bars having a NiCoCrAlY bond coat. Two different coaters that were either pilot production/lab scale size suitable for coating development (coater ESPRI) or small scale production/ development size (SMART Coater) suitable for both development and production were used. The bilayers were compared to single layers of 7YSZ or GZO, respectively. Beside lifetime investigations of the various TBC systems special emphasis was put on the interface architecture between 7YSZ and GZO. The coatings where the transition between the two layers was done rapidly achieved the longest lifetime of all bilayers, regardless of the EB-PVD coater used. Nearly all GZO top coats showed a longer lifetime than the standard 7YSZ systems in furnace cycling testing at 1100°C. 1. Introduction Thermal barrier coatings (TBCs) having compositions other than the commonly used yttria-stabilized zirconia (7–8 wt% Y 2 O 3 –ZrO 2 in- cluding up to around 2 wt% HfO 2 as natural impurity - 7YSZ) have attracted much interest over the last decade. Among them, gadolinium zirconate Gd 2 Zr 2 O 7 (GZO) is one of the most prominent examples in- troduced into aero-engines several years ago. Numerous studies have been published on GZO TBCs manufactured by a variety of techniques such as air plasma spraying, suspension plasma spraying, and electron- beam physical vapor deposition (EB-PVD) [1–8]. The advantages of GZO comprise an improved behavior under deposits such as CMAS or volcanic ash by drastically lowering the infiltration depth and the stiffened layer thickness, a lower thermal conductivity, and improved phase stability. Drawbacks of this material include higher raw material cost, a reduced fracture toughness that leads to a lower erosion re- sistance and a different cracking behavior, a lower coefficient of thermal expansion, and a possible incompatibility with alumina as main constituent of the thermally grown oxide (TGO) on top of the bond coat [2,3,9–12]. Alumina in the TGO and the pyrochlore and/or cubic phase of GZO are not in equilibrium and tend to form GdAlO 3 after prolonged high temperature exposure [10,13]. While on NiCoCrAlY [6] and CoNiCrAlY bond coats [7,8] such phase formation was observed during high temperature annealing and testing of GZO single EB-PVD layers, no reaction was found on a Hf-doped NiCoCrAlY that consist of alumina and hafnia particles [5]. To overcome the limitations of single GZO layers, bilayer TBCs consisting of a first 7YSZ base layer and a thicker GZO top layer have been introduced. This has been successfully demonstrated for both thermal sprayed coatings [3,4,14,15] and EB-PVD coatings [6–8,16–18]. Most of those studies revealed a longer lifetime of the bilayers compared to single layer GZO, single layer 7YSZ, or both. In contrast to those findings, in some of our previous studies we could achieve a longer lifetime of single layer GZO coatings if applied prop- erly on a NiCoCrAlY bond coat, albeit of some reactions with the TGO [5,6,19]. While phase formation in the top layer, reactions with CMAS or volcanic ash, TGO formation, failure behavior under isothermal, furnace cycle testing (FCT), and temperature gradient conditions have been widely evaluated for 7YSZ/GZO bilayers, the interface archi- tecture between the two layers has not been studied in detail. There- fore, the current study concentrates on the influence of processing conditions and the interface design of the two ceramic layers on mi- crostructure and lifetime of the TBCs. The coatings were manufactured in two different EB-PVD coaters to elaborate the impact of different https://doi.org/10.1016/j.surfcoat.2019.125107 Received 22 August 2019; Received in revised form 18 October 2019; Accepted 23 October 2019 * Corresponding author. E-mail address: Uwe.Schulz@dlr.de (U. Schulz). Surface & Coatings Technology 381 (2020) 125107 Available online 02 November 2019 0257-8972/ © 2019 Elsevier B.V. All rights reserved. T