Interface failure behavior of yttria stabilized zirconia (YSZ), La2Zr2O7, Gd2Zr2O7, YSZ/La2Zr2O7 and YSZ/Gd2Zr2O7 thermal barrier coatings (TBCs) in thermal cyclic exposure
Introduction
Thermal barrier coatings (TBCs) have been widely used on components that run at high service temperatures such as gas turbines and aircraft engines [1,2]. The main intended use of TBCs is to enable high temperature components to operate with higher efficiencies through preventing heat transfer to substrate materials [3,4]. MCrAlY-type coatings have been widely applied to protect nickel based super alloy substrates of gas turbines from oxidation and hot corrosion. This type of coatings is generally preferred for TBC systems. In MCrAlY-type coatings, M refers to either nickel or cobalt, or both metals [2,5]. Chromium and aluminum are added in the composition of metallic bond coat feedstock to increase oxidation and hot corrosion resistance. Minor amounts of reactive elements such as yttrium (Y) are also added to enhance the toughness of the formed oxide and to improve adhesion [[6], [7], [8]].
Recently, MCrAlY type coatings have been deposited on substrate surfaces using thermal spray coating processes such as high velocity oxy fuel (HVOF) and cold gas dynamic spray (CGDS) [9]. Coatings deposited with HVOF and CGDS techniques have been preferred over the conventional thermal spray methods such as atmospheric plasma spray (APS), detonation gun (D-gun) and arc spray due to their significantly dense and porosity-free structures [[10], [11], [12]]. CGDS is still being investigated, and arc spray cannot be used due to severe oxidation of metallic phases [13]. Another effective approach is low pressure plasma spray which is more advantageous than APS, HVOF and CGDS. APS and HVOF can be used for those with short lifetime due to low cost [14]. Except for dense structure with low porosity, diffusion treatment is critical to improve the oxidation resistance and adhesion strength as well [[14], [15], [16], [17], [18]].
Oxidation that arises between the bond coat and top coat at high temperatures significantly impairs the coating structure. During the oxidation of MCrAlY-type metallic bond coats, a thermally grown oxide layer (TGO) forms on the coating surface. This TGO layer mainly consists of alpha-alumina (α-Al2O3) and trace amounts of different oxides [10,[19], [20], [21]].
The design of TBC systems involves deposition of ceramic top coating layers consisting of Zr and rare earth elements with low porosity on the MCrAlY metallic bond coat to provide thermal insulation [10,[22], [23], [24]]. Such ceramic coatings with significantly high melting points are generally deposited with two deposition techniques. The first method is APS as the most commonly known, cost-efficient and applicable method. The latter is the electron beam physical vapor deposition (EB-PVD) technique, which is a more expensive and more complex process. Coatings deposited with APS technique feature higher amounts of oxide (in metallic materials) and porosity content as the process inherently takes place under atmospheric conditions, hence the name. In addition, the high kinetic energy and high processing temperatures result in deposition of laminar shaped particles. As an innovative technique with an ever-increasing application field, EB-PVD process involves deposition of less porous coatings [25,26]. Moreover, in this technique, the deposition process results in a columnar coating microstructure. As a consequence of the expansion tolerance provided by EB-PVD process, strains that arise during thermal cyclic oxidation of TBCs are tolerated to a greater extent [[27], [28], [29]].
In the hot sections of gas turbine engines, the porosity of fabricated ceramic coatings, sintering speed and the mechanical and thermo-mechanical characteristics of the applied coatings are the critical parameters for thermal barrier coating systems. The failure mechanisms of thermal barrier coatings can be categorized as external and internal failures. The internal mechanism involves the sintering of the top ceramic coating, distortion, and bending of the thermally grown oxide layer, the porosity of coating layer and residual stresses [30,31]. Mechanical stresses induced by the growth of TGO layer and the particular effect of thermal coefficient mismatch during cooling are the dominating factors in occurrence of failure [[31], [32], [33]]. In addition, formation of undesired phases in TGO may arise as a result of Al depletion in the bond coat. The rapid growth of the occurring phases in addition to their brittle structure results in an expedited damage formation in the further stages of oxidation [[34], [35], [36]].
Zirconia stabilized with 6–8% yttria (YSZ) has proven to be the most successful and the most widely used TBC material so far [[37], [38], [39], [40], [41], [42]]. However, as its main drawback, YSZ undergoes phase transformation from semi tetragonal to tetragonal and cubic and, from tetragonal phase to monoclinic during cooling, and it is exposed to sintering effect under long-term usage at high temperatures. Such phase transformations induce spalling and delamination of TBC material. YSZ can be exposed to distinct phase change period at high temperatures in addition to sintering effect. There is not an exact temperature for phase transformation, as it is a function of lifetime [[43], [44], [45], [46], [47]]. Increased thrust-to-weight ratios of new-generation engines necessitate higher gas temperatures. The intense interest on rare earth elements in recent decades has led to the notion that these are likely the ideal ceramic materials for future TBC applications [[48], [49], [50], [51]]. Among these materials, La2Zr2O7 and Gd2Zr2O7 stand out as alternatives to YSZ with their pyrochlore structures, high phase stability and low thermal conductivity [52,53]. New generation TBC systems lanthanum-zirconium-cerium oxide (LaZrCeO) has been the focus of recent studies. It has been observed that the thermal cycling behavior of these types of coatings produced by EB-PVD methods has a longer service life than those produced by APS method. LaZrCeO has a low thermal conductivity, high melting point and high phase stability. However, the low thermal durability of LaZrCeO can lead to high residual stresses during thermal cycles between the bond coating and the top coating. In order to overcome this disadvantage, double-layer ceramic coatings are being used. The use of YSZ coating as an intermediate layer in dual-layer TBC systems is a factor that increases chemical and thermal stability [[54], [55], [56], [57]]. In order to increase the thermal expansion coefficient of La2Zr2O7 coatings, CeO2 addition was provided a higher thermal expansion coefficient and lower thermal conductivity to La2Zr2O7 [58].
In this research, CoNiCrAlY metallic bond coat was deposited on Inconel 718 substrate material with CGDS method. Afterwards, YSZ, La2Zr2O7, Gd2Zr2O7, YSZ/La2Zr2O7 and YSZ/Gd2Zr2O7 rare earth zirconates were deposited as the ceramic top coat using EB-PVD technique. YSZ, La2Zr2O7, Gd2Zr2O7, YSZ/La2Zr2O7 and YSZ/Gd2Zr2O7 TBC systems were subjected to thermal cyclic tests at 1150 °C to determine their service lives, and the resulting microstructural changes were evaluated via characterization of the tested samples with SEM and mapping analyses.
Section snippets
Production and characterization of TBCs
TBCs were deposited on nickel-based Inconel 718 superalloy substrates. The substrate material used was cut into dimensions of 25 mm diameter and 5 mm thickness using a wire erosion method. Substrate surfaces were subjected to sandblasting with Al2O3 abrasive particles prior to the deposition of bond coats with a coating thickness of 100 μm. This process was performed as a means to increase the mechanical bonding of the bond coat and to remove undesired structures on the bond coat surface. Grit
Cross-sectional microstructure morphology
The microstructures before the thermal cycling of the TBC systems, which have YSZ, Gd2Zr2O7, La2Zr2O7, YSZ/Gd2Zr2O7 and YSZ/La2Zr2O7 top coats and CoNiCrAlY metallic bond coatings, were analyzed by taking cross-sectional microstructure and top surface SEM images. The upper surface and cross-sectional microstructure view of the YSZ sample is given in Fig. 1. In the cross-sectional microstructure image given in Fig. 1a, the top coating deposited by EB-PVD is clearly shown. In this image with
Conclusion
In this study, the thermal cycling resistance of Gd2Zr2O7 and La2Zr2O7 as the state of the art ceramic top coating materials alternative to YSZ were compared. Obtained results were mentioned as following;
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The TBC systems that involved a Gd2Zr2O7/YSZ and La2Zr2O7/YSZ coating, exhibited significantly higher strength than the traditional YSZ and other TBC systems. As a result of the performed cycling tests the best performance was exhibited by the YSZ/Gd2Zr2O7 coating system whereas La2Zr2O7 TBC
Declaration of competing interest
The authors declare no conflict of interest.
Acknowledgments
This investigation was financially supported by The Scientific and Technological Research Council of Turkey (TUBITAK, 113R049).
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