**1. Introduction**

Thermal barrier coatings (TBCs) are insulating overlayers deposited on superalloy substrates, which are usually employed in high-temperature components of gas turbines. TBCs reduce the surface temperature of the metallic component substrate, improving the thermal durability and increasing the fuel e fficiency in gas turbines [1–6]. The TBC is usually comprised of four di fferent layers: (1) a ceramic top coat, (2) a metallic bond coat, (3) a Ni- and/or Co-based superalloy substrate, and (4) a thin thermally grown oxide (TGO) layer. The TGO layer acts as a protective layer to retard the thermal and oxidation di ffusion. However, the TGO layer may increase the internal stress in TBC systems, which causes potential cracking at the interface between the bond and top coats, eventually leading to spallation or delamination of the top coat [7–10]. The bond coat in a TBC system is to ensure the structural integrity and to protect the superalloy substrate from oxidation. Moreover, the metallic

bond coat can reduce the coe fficient of thermal expansion (CTE) mismatch between the superalloy substrate and the ceramic top coat, and enhance adhesion with the top coat [11–15]. The bond coat can be deposited by a variety of thermal spraying processes, such as vacuum plasma spray, high-velocity oxygen fuel (HVOF), and air-plasma spray (APS). Although MCrAlY (M = Ni and/or Co) feedstock has been used for a bond coat for several decades, the failure of the TBC system is often resulted from the thermomechanical mismatch between the bond and top coats. The durability and stability of TBC systems can be improved by reducing the CTE mismatch between the top and bond coats, decreasing the excessive TGO layers, and eliminating the resultant residual stresses. For example, TGO layer growth may be modified through powder oxidation which forms a duplex oxide scale with an outer layer and an inner Al2O3 layer composed of NiAl2O4, Cr2O3, and other spinel structures [16].

In the present study, a new combined experimental and modeling study of the e ffect of bond coat species on the microstructure evolution of electron beam-physical vapor deposition processed (EB-PVD) yttria stabilized zirconia (YSZ) TBCs is conducted. Three types of thermal exposure tests, i.e., flame thermal fatigue (FTF), cyclic furnace thermal fatigue (CFTF), and thermal shock (TS), are employed in order to understand the TBCs' thermomechanical properties in thermal cyclic environments. A finite element (FE) model is developed to simulate the distribution of stresses in di fferent bond coats and thermal exposure environments. The relationship between coating failure behavior and the bond coat is investigated, based on the microstructure evaluation in the thermal cyclic tests.

## **2. Experimental Procedure**

## *2.1. Coating Materials and Sample Preparation*

In this study, Ni-based superalloy (GTD–111, with the nominal composition of Ni–14Cr–9.5Co– 4.9Ti–3.8W–3Al–2.8Ta–1.5Mo–0.1C–0.03Zr, in wt.%) is used as the substrate. The diameter and thickness of the test specimen are 25.4 and 5 mm, respectively. The surface of the substrate is blasted using an alumina powder, cleaned before coating processes, and then the coatings are deposited within 2 h. AMDRY 962 (Nominal composition of Ni–22Cr–10Al–1.0Y in wt.% and particle size of 56–106 μm; Sulzer Metco Holding AG, Winterthur, Switzerland) and AMDRY 9951 (Nominal composition of Co–32Ni–21Cr–8Al–0.5Y in wt.% and particle size of 5–37 μm; Sulzer Metco Holding AG) are used as the feedstock powders to fabricated the bond coats by APS and HVOF process, respectively. The top coat is formed by the EB-PVD process on the bond coats using 204C-NS (particle size of 45–140 μm, Oerlikon Metco AG, Pfä ffikon, Switzerland). The thicknesses of the bond and top coats are designed as 300 ± 20 and 600 ± 50 μm, respectively. In the spray process of TBCs, the parameters recommended by the Chrome-Alloying Co. Ltd (London, UK) are employed.
