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Comparison of oxidation behavior of YSZ and Gd2Zr2O7 thermal barrier coatings (TBCs)
SCT-21935; No of Pages 10 Surface & Coatings Technology xxx (2016) xxx–xxx
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Comparison of oxidation behavior of YSZ and Gd2Zr2O7 thermal barrier coatings (TBCs) Kadir Mert Doleker ⁎, Abdullah Cahit Karaoglanli Metallurgical and Materials Engineering Department, Bartin University, 74100 Bartin, Turkey
a r t i c l e
i n f o
Article history: Received 19 July 2016 Revised 19 December 2016 Accepted in revised form 20 December 2016 Available online xxxx Keywords: Thermal barrier coating (TBC) Cold gas dynamic spray (CGDS) Oxidation Rare-earth oxides YSZ Gd2Zr2O7
a b s t r a c t Yttria Stabilized Zirconia (YSZ) is widely used as a traditional ceramic top coat material in gas turbine engine components. Nowadays, rare earth zirconates, as alternative materials to YSZ, are enhanced for use as top coat layer of TBC owing to their better thermal isolation properties. In the present study, metallic CoNiCrAlY bond coat powder was sprayed on Inconel 718 superalloy substrate by cold gas dynamic spray (CGDS) technique. After the deposition of bond coats, YSZ and Gd2Zr2O7 top coats were produced using EB-PVD process. TBCs were exposed to isothermal oxidation tests at 1100 °C for 8, 24, 50 and 100 h. Oxidation and growth behaviors of thermally grown oxide (TGO) layer were observed. TBC samples were investigated using scanning electron microscope (SEM), EDS elemental mapping and X-ray diffractometer (XRD) analysis before and after the oxidation tests. Oxidation performances of two different TBC systems were compared to each other according to the analysis results. © 2016 Elsevier B.V. All rights reserved.
1. Introduction In gas turbine engines, thermal barrier coatings (TBCs) are generally deposited onto superalloys to decrease turbine inlet temperature and to obtain higher efficiency and endurance of hot section components [1–3]. Generally, a TBC system consists of two different layers as bond coat (MCrAlY; M = Co, Ni or Co/Ni) and ceramic top coat. Bond coat reduces the thermal expansion mismatch and provides better adhesion between the top coat and the substrate, increasing the oxidation and corrosion resistance, whereas top coating structure reduces the thermal conductivity and protects the metallic parts against the adverse effects of high temperatures [4,5]. Several processes are used for production of bond coat such as atmospheric plasma spray (APS), high velocity oxy fuel (HVOF), low pressure plasma spray (LPPS) and cold gas dynamic spray (CGDS). However, only a few deposition processes such as APS and Electron Beam Physical Vapor Deposition (EB-PVD) are available for the top coats with high melting point [6–8]. CGDS system enables deposition on substrate material in lower temperatures when compared to other thermal spray process. In this technique, powder is deposited on the substrate by means of the supersonic gas jet reaching to a critical velocity. Upon impact, powders become plastically deformed [9]. Another important advantage of this process is that powders do not oxidize due to deposition with low temperature. Depletion of Al-rich phases significantly decrease during the deposition
⁎ Corresponding author. E-mail address: [email protected] (K.M. Doleker).
[10–12]. Thus, CGDS process provides an advantage for better oxidation resistance. In EB-PVD system, process occurs through deposition of evaporated ingot on heated substrate from molten pool with the aid of high vapor pressure in vacuum atmosphere. Intra-columnar fine porosities cause to fall thermal conductivity in EB-PVD while obtained columnar structure provides high strain tolerance and adhesion to bond coat for TBCs [13–15]. TBCs are exposed to harsh environments such as oxidation, corrosion, CMAS (CaO, MgO, Al2O3 and SiO2) attack or erosion under service conditions. From this point of view, TBC components should be improved and alternative materials should be developed especially for the top coat of TBC systems [16–18]. As top coat material, Yttria Stabilized Zirconia (YSZ) is conventionally preferred in TBC systems because of its low thermal conductivity and high coefficient of thermal expansion (CTE) which is close to metals [19–21]. However, above 1200 °C, YSZ is exposed to phase transformation from tetragonal phase to tetragonal and monoclinic phase with an increase in the sintering effect [22,23]. The main characteristic of a top coat is its ability to reduce the heat transfer to the substrate. This is directly related with the thermal conductivity of the top coat. Lattice defects result in phonon scattering in materials. Thus, thermal conductivity can decrease depending on increased lattice defects. Similarly, fine grain structures, point defects, oxygen vacancies, porosities and inclusions contribute to the decrease in the thermal conductivity [24]. Melting point of rare-earth zirconates (Re2Zr2O7) such as Gd2Zr2O7, Sm2Zr2O7 or La2Zr2O7 are above 2000 °C. In addition, phase transformation temperatures of rare-earth zirconates are higher than YSZ. Their CTE is close to that of YSZ. However, toughness values of Re2Zr2O7 are lower than YSZ [25–27]. Ion doping, heavier than Y3 +,
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Table 1 Deposition parameters of top coats and bond coat. Bond coat
Spray pressure (bar)
Gas temperature (°C)
Working gas (slpm)
Spray distance (mm)
Gun speed (mm/s)
CoNiCrAlY
30
600
Helium(1000)
15
20
Top coat
Voltage (kV)
Temperature (°C)
Vacuum (Torr)
Rotation speed (rpm)
Condensation ratio (μm/min)
YSZ
20 kV
800 ± 20 °C
25
4,5
Gd2Zr2O7
20 kV
800 ± 20 °C
5 × 10−5– 1 × 10−4 5 × 10−5– 1 × 10−4
25
3,7
decreases the thermal conductivity because of increasing atomic weight. Substitution of Zr4+ with rare earth zirconates causes formation of oxygen vacancy. One oxygen vacancy similar to YSZ and great number of displaced oxygen atoms provide lower thermal conductivity [28]. There are two main oxygen transfer mechanisms in oxidation. The first one is the ionic diffusion which is related to the top coat structure, and the second one is the penetration of gas through porosities and micro-cracks. Oxygen diffuses to interface and sub-interface through ionic conductivity of top coating at high temperatures and also, intercolumnar voids and porosities simplify oxygen penetration [29,30]. In this study, Gd2Zr2O7 and YSZ were deposited on CoNiCrAlY coated Inconel 718 substrates using EB-PVD technique. Then, the produced TBCs were subjected to isothermal oxidation tests with different periods at 1100 °C in furnace to make a comparison between their oxidation performances.
min, respectively). The spray parameters of the bond coats and the deposition parameters of the YSZ and Gd2Zr2O7 top coats, produced by EB-PVD (UE-202, Kiev, Ukraine) technique, are given in Table 1. Porosity contents of TBCs were calculated using ImageJ software program by taking the percentage of the porous area in overall area of the analyzed coating cross-section. Mean porosity values were calculated using 10 different SEM backscattered electron detector (BSD) images of each coating at 1,5kx before the oxidation test. Surface roughness values of the bond coats and the top coats were evaluated using SJ310, Mitutoyo instrument. Roughness measurements were conducted with 0.8 mm cut off length, 4 mm measurement distance and 0.5 mm/s indenter speed, 4 mN measuring force, 5 μm stylus radius, 90° conical taper angle. 5 measurements were taken from the top surface of each coating to calculate average roughness values.
2.2. Oxidation and characterization 2. Materials and methods 2.1. Components of TBC manufacturing process and properties Inconel 718 disk samples having 25 mm diameter and 5 mm height were used as the substrate material. Before the deposition of the bond coat, samples were grit blasted using Al2O3 powder with 40 ± 5 μm particle size under 2.5 bar pressure at 75° angle with horizontal axis. CoNiCrAlY (Co - 32Ni - 21Cr - 8Al - 0.5Y (wt.%)) powders (−38 + 5.5 μm particle size, Amdry 9951-Sulzer Metco) were sprayed on the substrates using CGDS technique. The ceramic ingots were used for deposition of top coats. YSZ (ZrO2–8wt.%Y2O3) ceramic ingot has 68.5 mm diameter and 40–50 mm height while Gd2Zr2O7 (Gd2O3– 30 vol.% ZrO2) ceramic ingot has 68.5 mm diameter and 40–60 mm height. The main difference in deposition of YSZ and Gd2Zr2O7 ingots was observed in their condensation ratio (4,5 μm/min and 3,7 μm/
Vacuum heat treatment with 10−5 Torr pressure and 2 h at 1080 °C was applied to TBCs before the oxidation tests. The main purpose of this process is to obtain thin Al2O3 layer and β-NiAl phases rich in Al phases. Isothermal oxidation test in air atmosphere was carried out on TBC samples using Protherm high temperature furnace (PLF 130/12, Turkey) under 1100 °C for 8, 24, 50 and 100 h periods. The heating rate was 8 °C/min and the cooling rate was 5 °C/min. Isothermally oxidized TBC samples were examined using scanning electron microscopy (SEM) and elemental mapping spectroscopy. X-ray diffractometer (XRD) Cu Kα radiation was applied to identify the presence of the phase transformation of oxidized TBC samples using the data obtained from the top coat surface. The TGO thickness measurements included the measurement of mixed oxides and Al2O3. TGO thicknesses of all oxidized TBC samples were measured at 3kx magnification using Image Pro Plus 6 software program. 15 measurements were taken from each of 10
Fig. 1. As-Sprayed CoNiCrAlY bond coat and XRD spectra.
Please cite this article as: K.M. Doleker, A.C. Karaoglanli, Surf. Coat. Technol. (2016), http://dx.doi.org/10.1016/j.surfcoat.2016.12.078
K.M. Doleker, A.C. Karaoglanli / Surface & Coatings Technology xxx (2016) xxx–xxx Table 2 Porosity rate and surface roughness values of substrate and bond coat.
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Table 3 Porosity and surface roughness values of top coat.
Coatings
Ra (μm)
Porosity rate (%)
Coatings
Ra (μm)
Porosity (%)
Inconel-718 CoNiCrAlY bond coat
5,38 7,20
– 0,65 ± 0,2
YSZ top coat Gd2Zr2O7 top coat
7,92 7,49
1,6 ± 0,3 1,9 ± 0,3
different SEM images during TGO thickness measurements (150 measurements in total), and the mean TGO thickness was evaluated for each image. Finally, the average of the mean values of all images was taken to evaluate the overall TGO thickness average.
3. Results and discussion 3.1. Cross-sectional morphology 3.1.1. As-sprayed bond coat structure As-sprayed bond coat structure and XRD spectrum are given in Fig. 1. The thickness of the bond coat layer is approximately 100 μm. Bond coats of the TBCs were homogenously coated. In the microstructure analysis of the bond coat, porosity content is at a quite low level (0,65 ± 0,2) % compared to other studies [11,31]. The areas close to the surface include porosities in the bond coat due to the characteristics of the CGDS technique [10,32,33]. CoNiCrAlY surface morphology exhibited small-sized undeformed particles which result in a rough bond coat surface. This can be ascribed to the facilitated deposition of initial particles by the impact of subsequent particles whereas the final particles do not have such impacting support [10,32,33]. Furthermore, oxide structure is not visible in the bond coat according to the SEM examination due to the use of inert He gases and the kinetics of using CGDS technique during spraying. According to the XRD results of assprayed bond coat, there is only one phase which consists of γphase rich in Co and Ni. Severe plastic deformation causes grain refinements. Thus, β-NiAl phases can be dissolved into the γ-matrix [31]. Porosity and surface roughness values of the substrate and bond coating are given in Table 2. Porosity [1,11] and surface roughness [1,34–37] values of bond coat affect the oxidation behavior TBC system.
3.1.2. As-deposited TBC structure The cross-sectional microstructure of YSZ and Gd2Zr2O7 TBC system is illustrated in Fig. 2. Thickness of the top coat layer is approximately 200 μm. Both top coatings showed a columnar structure as the common characteristic of EB-PVD coatings. Columnar growing and vertical separation of evaporated ingots provide low porosity in EB-PVD coatings. Since APS coating is deposited on the substrate in the form of splats, it has high porosity content. During the EB-PVD deposition, the used parameters such as rotation speed, chamber pressure, substrate temperature or condensation ratio affect the top coat porosity. There are visible inter-columnar voids in the top coat structure. This can also be attributed to the surface roughness of the bond coat [38–40]. Because, growing morphology of columns is affected from bond coat roughness. Growing columns can intersect with each other. As a result, porosities and vertical separation between columns occur. Porosity and surface roughness values of the top coats are given in Table 3. TBCs were exposed to vacuum annealing to get a thin α-Al2O3 layer at the interface, to enable general stabilization of the coating, to facilitate the formation of β-NiAl phases and to provide better adhesion of the ceramic layer to the metal bond coat [41–43]. After deposition of TBCs, TGO layer is quite visible as a result of vacuum annealing according to as-sprayed SEM examination and mapping microstructure (Figs. 3 and 4). In addition, homogeneous βNiAl and γ-phases dispersion in the bond coat was observed in Figs. 3 and 4. Colors of Al and O2 elements overlap in Figs. 3 and 4. Also, Cr colors are not seen in these areas. This is an advantage of CGDS process due to the high spray velocity and the presence of inert gas atmosphere during coating process. Normally, presence of β-NiAl phases was not detected in SEM. Because, severe plastically deformed particles provide grain refinements which may induce dissolution of β-NiAl phases in γ-matrix. These phases can be observed under high magnifications [31] before the vacuum annealing but they can be detected after vacuum annealing which gives sufficient energy for their formation.
Fig. 2. As-deposited TBC cross-sectional SEM microstructure; a) YSZ TBC and b) Gd2Zr2O7 TBC.
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Fig. 3. Elemental mapping microstructure belonging to as-deposited YSZ TBC at 3kx magnification.
3.2. The oxidation behavior of TBCs Cross-sectional SEM microstructures of YSZ and Gd2Zr2O7 TBCs for each different period are given in Figs. 5 and 6 after the oxidation
tests. Thin darker oxide scales represent continuous Al2O3 layer whereas grey areas can show spinel or mixed oxide formations. They can be understood from elemental mapping of 100 h oxidized samples (Figs. 7 and 8). Dark colors in SEM image coincide with Al and O2 colors
Fig. 4. Elemental mapping microstructure belonging to as-deposited Gd2Zr2O7 TBC at 3kx magnification.
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Fig. 5. Cross-sectional SEM microstructures of YSZ TBC after oxidation tests at 1100 °C; a) 8 h, b) 24 h, c) 50 h and d) 100 h.
in elemental mapping microstructures. Grey colors overlap with colors of Co, Ni, Cr and O2 elements in mapping images. In the first stage of oxidation, there is no visible mixed oxide formation while after 50 h oxidation period, mixed oxide formation can be clearly observed according to SEM microstructure. Formation of TGO at the interface of the top coat and the bond coat causes residual stress. Substantial increase in stress during TGO growth induces spallation of top coat from bond coat [44,45]. In TBC microstructures, micro-crack formations occurred after 50 and 100 h oxidation period, because of thermal stresses. However, no spallation of coatings was observed during the oxidation experiments. TGO layers for both coating structures prevent its uniformity. Elemental mapping microstructures of YSZ and Gd2Zr2O7 TBCs after 100 h oxidation period are given in Figs. 7 and 8. According to the mapping results, TGO layer mainly consists of Al2O3 and mixed oxides such as spinels, Cr, Ni and Co oxides for YSZ and Gd2Zr2O7 TBC. Al2O3 formation is desirable for TBC because of lower oxygen permeability compared to mixed oxides. However, depleted NiAl phases cause diffusion of other elements to interface. Thus, TGO layer rapidly grows resulting in stress formation at the interface. There is a little difference in porosity values of top coats according to Table 3. It is absolutely effective in oxidation rate. However, the use of top coatings with different crystal structures is a dominant factor in the oxidation rate. In terms of oxidation mechanism, Gd2Zr2O7 have higher resistance against oxygen penetration due to having stable Frenkel pairs in its structure
when compared with YSZ [46,47]. This can be attributed to the fact that, Gd2Zr2O7 includes an oxygen vacancy at 48f position and oxygen anion at interstitial 8b position in its crystal structure. These two oxygen anions provide stable Frenkel pair defects, that require higher activation energy for oxygen transfer, thus a better oxidation resistance is exhibited by Gd2Zr2O7 when compared to YSZ [46,48–50]. Mahade et al. [46] compared the oxidation behavior of YSZ and YSZ/Gd2Zr2O7 in terms of TGO thickness. The results show that double layer system with Gd2Zr2O7 has more resistance against oxidation. The reason of better resistance is ascribed to the presence of stable Frenkel pair defects in Gd2Zr2O7. Its oxygen permeability is lower than YSZ. During oxidation, initially Y is oxidized since it has the highest affinity. Afterwards it forms Yttrium Aluminum Garnet (YAG) phases after its reaction with the TGO layer formed by Al2O3. Its effect was discussed in the literature [51]. It was explained in the previous studies that GdAlO3 phases occur at the interface as a result of the reaction between Al2O3 and Gd2Zr2O7 under high temperature. It was also stated that TBC's lifetime is reduced due to the porosity formation induced by GD's diffusion into the interface [52–54]. In the present study it was hard to make an accurate distinction with the SEM and mapping images taken with higher magnification after the 100 h oxidation test (Fig. 9.). Also, Gd, Al and O2 coexist in only few regions and YAG phases are comparatively more detectable as seen in Fig. 9. TBC/TGO interface was also found to involve mixed oxides.
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Fig. 6. Cross-sectional SEM microstructures of Gd2Zr2O7 TBC after oxidation tests at 1100 °C; a) 8 h, b) 24 h, c) 50 h and d) 100 h.
Crystallographic transformation may affect the oxidation behavior at the interface of the bond coat and the top coat. Because, this transformation may cause volumetric change, thus leading to formation of higher stresses at the interface in addition to the stresses of newly formed oxides due to oxidation. All these effects increase the bond coat(BC)/top coat (TC) mismatch. In addition, after phase transformation, potential crack formations can lead to oxygen ingress to the BC/TC interface. The X-ray diffraction patterns of as-sprayed and 100 h oxidized TBCs are given in Fig. 10. YSZ has tetragonal phase (t) while Gd2Zr2O7 has cubic phase (c) during all oxidation periods. Metastable tetragonal (t) phase does not transform into monoclinic or cubic phase until 1200 °C [55]. In contrast to YSZ, Gd2Zr2O7 preserves its phase stability up to 1550 °C [56]. As a result, XRD peaks show that there is no phase transformation during oxidation for each TBCs. TGO layer thickness for YSZ-TBC is higher than that of Gd2Zr2O7-TBC as shown in Fig. 11. After 50 h oxidation period, the growth rate of the TGO layer decreased compared to 8 and 24 h oxidation. However, mixed oxide formation for YSZ-TBC is lower compared to Gd2Zr2O7TBC. Also, the crack formation is observed more clearly in Gd2Zr2O7TBC as a result of its elastic modulus. Low elastic modulus and toughness can cause early crack formation which provides simpler oxygen penetration to structure [30]. In fact, SEM examination and TGO thickness show that Gd2Zr2O7 has better oxidation resistance but crack formation can lead to more dangerous oxidation related results. Thus, mixed oxide formation and internal
oxidation content can be seen at higher rates in Gd2Zr2O7-TBC which exhibits higher rates of crack formation as seen in Fig. 12. Another important factor for crack formation is thermal expansion coefficient of the coatings. YSZ has a slightly higher thermal expansion coefficient compared to Gd2Zr2O7 [27,57,58]. Depending on increased oxidation time, thermal expansion mismatch increases even more and ceramic layer is exposed to higher stresses. In the first stages of 8 and 24 h oxidation periods, Gd2Zr2O7-TBCs exhibited lower TGO growth as mixed oxide formation was not visible. This is attributed to the fact that the stress level has not yet reached a level to form cracks in the coating. However, as the stress level increases with increasing time, crack formations will result in expedited oxidation, hence formations of mixed oxides. Despite all these disadvantages, TGO layer thickness in Gd2Zr2O7-TBC was observed at slightly lower rates according to TGO thickness measurements. Better strain tolerance gained by EBPVD positively contributed to the oxidation performance of Gd2Zr2O7TBC. Vaßen et al. [27] used APS coating technique to deposit Gd2Zr2O7 and YSZ on NiCoCrAlY bond coated Inconel 738 to compare their thermal cycling performance with YSZ. Results showed that YSZ-TBC exhibited longer lifetime due to its higher fracture toughness and CTE. To overcome thermal expansion mismatch, double ceramic layered TBCs (Gd2Zr2O7/YSZ) were used. Similar to this study, Bobzin et al. [59] performed oxidation test on Gd2Zr2O7 and YSZ TBCs at 1300 °C with 25 h. YSZ TBC prevented its uniformity whereas Gd2Zr2O7 spalled in spite of usage of EB-PVD technique. However, double ceramic layered TBCs
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Fig. 7. Elemental mapping microstructure belonging to YSZ TBC after 100 h oxidation.
(Gd2Zr2O7/YSZ) exhibited better stability. In contrast to these studies, Munawar et al. [53] compared thermal cycling performance of YSZ and Gd2Zr2O7 TBCs produced by EB-PVD technique but their results did not support each other due to different top coat deposition techniques. Gd2Zr2O7 TBC exhibited better performance through EB-PVD
process providing higher lateral compliance. In the literature, some studies focused on the use of double ceramic layer to overcome the thermal expansion mismatch between the bond and top coat for single layered Gd2Zr2O7 TBCs and to minimize the low fracture toughness of Gd2Zr2O7 [27,46,59–61].
Fig. 8. Elemental mapping microstructure belonging to Gd2Zr2O7 TBC after 100 h oxidation.
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Fig. 9. Elemental distribution images of Gd2Zr2O7 TBC interface after 100 h oxidation.
4. Conclusion In this study, homogenously produced CoNiCrAlY bond coats with low porosity content, deposited with CGDS technique, were combined with EB-PVD technique which provides higher expansion tolerance; and oxidation behavior of TBC systems including Gd2Zr2O7 as a new top coating material and YSZ as the conventional top coating material, was investigated. In the first stages of oxidation, the growth behavior of TGO layer occurred at an increased level compared to other stages of growth behavior of TGO. TGO thickness increased with increasing
time and formations of mixed oxides were observed. At the end of the oxidation tests, no spallation was observed at the coating interfaces of both TBC systems having YSZ and Gd2Zr2O7 as top coat material, and the TGO layer maintained its integrity. In terms of the growth behavior of TGO layer, better results were obtained from the TBC system with Gd2Zr2O7 top coat. Its lower coefficient of thermal conductivity, lower oxygen permeability, relatively high coefficient of thermal expansion and structural stability at higher temperatures render the Gd2Zr2O7 a good alternative top coating material for the TBCs. On the other hand its lower fracture toughness and relatively lower coefficient of thermal
Fig. 10. XRD spectra of as-deposited and 100 h oxidized a) YSZ and b) Gd2Zr2O7 TBC.
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Fig. 11. TGO growth of YSZ and Gd2Zr2O7 TBC.
Fig. 12. SEM cross-sectional microstructure of 100 h oxidized YSZ and Gd2Zr2O7 TBC.
expansion compared to YSZ, result in early formation of cracks during the thermal stresses occurring at sustained high temperatures. In this regard, the forthcoming studies will focus on the modifications for improved Gd2Zr2O7 toughness, new material dopants and/or double or gradient layered coatings. Acknowledgements This investigation was financially supported by The Scientific and Technological Research Council of Turkey (TUBITAK, 113R049). References [1] A.C. Karaoglanli, K.M. Doleker, B. Demirel, A. Turk, R. Varol, Effect of shot peening on the oxidation behavior of thermal barrier coatings, Appl. Surf. Sci. 354 (2015) 314–322. [2] J.R. Davis, Handbook of thermal spray technology, ASM International, 2004. [3] R.A. Miller, Thermal barrier coatings for aircraft engines: history and directions, J. Therm. Spray Technol. 6 (1997) 35–42. [4] W. Gao, Developments in High Temperature Corrosion and Protection of Materials, first ed. Woodhead Publishing, CRC Press, New York, 2008. [5] S. Ghosh, Advanced Ceramic Processing, in: A. Mohamed (Ed.), Thermal Barrier Ceramic Coatings-a Review, InTech, Croatia 2016, pp. 111–138.
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Comparison of oxidation behavior of YSZ and Gd2Zr2O7 thermal barrier coatings (TBCs) Kadir Mert Doleker ⁎, Abdullah Cahit Karaoglanli Metallurgical and Materials Engineering Department, Bartin University, 74100 Bartin, Turkey
a r t i c l e
i n f o
Article history: Received 19 July 2016 Revised 19 December 2016 Accepted in revised form 20 December 2016 Available online xxxx Keywords: Thermal barrier coating (TBC) Cold gas dynamic spray (CGDS) Oxidation Rare-earth oxides YSZ Gd2Zr2O7
a b s t r a c t Yttria Stabilized Zirconia (YSZ) is widely used as a traditional ceramic top coat material in gas turbine engine components. Nowadays, rare earth zirconates, as alternative materials to YSZ, are enhanced for use as top coat layer of TBC owing to their better thermal isolation properties. In the present study, metallic CoNiCrAlY bond coat powder was sprayed on Inconel 718 superalloy substrate by cold gas dynamic spray (CGDS) technique. After the deposition of bond coats, YSZ and Gd2Zr2O7 top coats were produced using EB-PVD process. TBCs were exposed to isothermal oxidation tests at 1100 °C for 8, 24, 50 and 100 h. Oxidation and growth behaviors of thermally grown oxide (TGO) layer were observed. TBC samples were investigated using scanning electron microscope (SEM), EDS elemental mapping and X-ray diffractometer (XRD) analysis before and after the oxidation tests. Oxidation performances of two different TBC systems were compared to each other according to the analysis results. © 2016 Elsevier B.V. All rights reserved.
1. Introduction In gas turbine engines, thermal barrier coatings (TBCs) are generally deposited onto superalloys to decrease turbine inlet temperature and to obtain higher efficiency and endurance of hot section components [1–3]. Generally, a TBC system consists of two different layers as bond coat (MCrAlY; M = Co, Ni or Co/Ni) and ceramic top coat. Bond coat reduces the thermal expansion mismatch and provides better adhesion between the top coat and the substrate, increasing the oxidation and corrosion resistance, whereas top coating structure reduces the thermal conductivity and protects the metallic parts against the adverse effects of high temperatures [4,5]. Several processes are used for production of bond coat such as atmospheric plasma spray (APS), high velocity oxy fuel (HVOF), low pressure plasma spray (LPPS) and cold gas dynamic spray (CGDS). However, only a few deposition processes such as APS and Electron Beam Physical Vapor Deposition (EB-PVD) are available for the top coats with high melting point [6–8]. CGDS system enables deposition on substrate material in lower temperatures when compared to other thermal spray process. In this technique, powder is deposited on the substrate by means of the supersonic gas jet reaching to a critical velocity. Upon impact, powders become plastically deformed [9]. Another important advantage of this process is that powders do not oxidize due to deposition with low temperature. Depletion of Al-rich phases significantly decrease during the deposition
⁎ Corresponding author. E-mail address: [email protected] (K.M. Doleker).
[10–12]. Thus, CGDS process provides an advantage for better oxidation resistance. In EB-PVD system, process occurs through deposition of evaporated ingot on heated substrate from molten pool with the aid of high vapor pressure in vacuum atmosphere. Intra-columnar fine porosities cause to fall thermal conductivity in EB-PVD while obtained columnar structure provides high strain tolerance and adhesion to bond coat for TBCs [13–15]. TBCs are exposed to harsh environments such as oxidation, corrosion, CMAS (CaO, MgO, Al2O3 and SiO2) attack or erosion under service conditions. From this point of view, TBC components should be improved and alternative materials should be developed especially for the top coat of TBC systems [16–18]. As top coat material, Yttria Stabilized Zirconia (YSZ) is conventionally preferred in TBC systems because of its low thermal conductivity and high coefficient of thermal expansion (CTE) which is close to metals [19–21]. However, above 1200 °C, YSZ is exposed to phase transformation from tetragonal phase to tetragonal and monoclinic phase with an increase in the sintering effect [22,23]. The main characteristic of a top coat is its ability to reduce the heat transfer to the substrate. This is directly related with the thermal conductivity of the top coat. Lattice defects result in phonon scattering in materials. Thus, thermal conductivity can decrease depending on increased lattice defects. Similarly, fine grain structures, point defects, oxygen vacancies, porosities and inclusions contribute to the decrease in the thermal conductivity [24]. Melting point of rare-earth zirconates (Re2Zr2O7) such as Gd2Zr2O7, Sm2Zr2O7 or La2Zr2O7 are above 2000 °C. In addition, phase transformation temperatures of rare-earth zirconates are higher than YSZ. Their CTE is close to that of YSZ. However, toughness values of Re2Zr2O7 are lower than YSZ [25–27]. Ion doping, heavier than Y3 +,
http://dx.doi.org/10.1016/j.surfcoat.2016.12.078 0257-8972/© 2016 Elsevier B.V. All rights reserved.
Please cite this article as: K.M. Doleker, A.C. Karaoglanli, Surf. Coat. Technol. (2016), http://dx.doi.org/10.1016/j.surfcoat.2016.12.078
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Table 1 Deposition parameters of top coats and bond coat. Bond coat
Spray pressure (bar)
Gas temperature (°C)
Working gas (slpm)
Spray distance (mm)
Gun speed (mm/s)
CoNiCrAlY
30
600
Helium(1000)
15
20
Top coat
Voltage (kV)
Temperature (°C)
Vacuum (Torr)
Rotation speed (rpm)
Condensation ratio (μm/min)
YSZ
20 kV
800 ± 20 °C
25
4,5
Gd2Zr2O7
20 kV
800 ± 20 °C
5 × 10−5– 1 × 10−4 5 × 10−5– 1 × 10−4
25
3,7
decreases the thermal conductivity because of increasing atomic weight. Substitution of Zr4+ with rare earth zirconates causes formation of oxygen vacancy. One oxygen vacancy similar to YSZ and great number of displaced oxygen atoms provide lower thermal conductivity [28]. There are two main oxygen transfer mechanisms in oxidation. The first one is the ionic diffusion which is related to the top coat structure, and the second one is the penetration of gas through porosities and micro-cracks. Oxygen diffuses to interface and sub-interface through ionic conductivity of top coating at high temperatures and also, intercolumnar voids and porosities simplify oxygen penetration [29,30]. In this study, Gd2Zr2O7 and YSZ were deposited on CoNiCrAlY coated Inconel 718 substrates using EB-PVD technique. Then, the produced TBCs were subjected to isothermal oxidation tests with different periods at 1100 °C in furnace to make a comparison between their oxidation performances.
min, respectively). The spray parameters of the bond coats and the deposition parameters of the YSZ and Gd2Zr2O7 top coats, produced by EB-PVD (UE-202, Kiev, Ukraine) technique, are given in Table 1. Porosity contents of TBCs were calculated using ImageJ software program by taking the percentage of the porous area in overall area of the analyzed coating cross-section. Mean porosity values were calculated using 10 different SEM backscattered electron detector (BSD) images of each coating at 1,5kx before the oxidation test. Surface roughness values of the bond coats and the top coats were evaluated using SJ310, Mitutoyo instrument. Roughness measurements were conducted with 0.8 mm cut off length, 4 mm measurement distance and 0.5 mm/s indenter speed, 4 mN measuring force, 5 μm stylus radius, 90° conical taper angle. 5 measurements were taken from the top surface of each coating to calculate average roughness values.
2.2. Oxidation and characterization 2. Materials and methods 2.1. Components of TBC manufacturing process and properties Inconel 718 disk samples having 25 mm diameter and 5 mm height were used as the substrate material. Before the deposition of the bond coat, samples were grit blasted using Al2O3 powder with 40 ± 5 μm particle size under 2.5 bar pressure at 75° angle with horizontal axis. CoNiCrAlY (Co - 32Ni - 21Cr - 8Al - 0.5Y (wt.%)) powders (−38 + 5.5 μm particle size, Amdry 9951-Sulzer Metco) were sprayed on the substrates using CGDS technique. The ceramic ingots were used for deposition of top coats. YSZ (ZrO2–8wt.%Y2O3) ceramic ingot has 68.5 mm diameter and 40–50 mm height while Gd2Zr2O7 (Gd2O3– 30 vol.% ZrO2) ceramic ingot has 68.5 mm diameter and 40–60 mm height. The main difference in deposition of YSZ and Gd2Zr2O7 ingots was observed in their condensation ratio (4,5 μm/min and 3,7 μm/
Vacuum heat treatment with 10−5 Torr pressure and 2 h at 1080 °C was applied to TBCs before the oxidation tests. The main purpose of this process is to obtain thin Al2O3 layer and β-NiAl phases rich in Al phases. Isothermal oxidation test in air atmosphere was carried out on TBC samples using Protherm high temperature furnace (PLF 130/12, Turkey) under 1100 °C for 8, 24, 50 and 100 h periods. The heating rate was 8 °C/min and the cooling rate was 5 °C/min. Isothermally oxidized TBC samples were examined using scanning electron microscopy (SEM) and elemental mapping spectroscopy. X-ray diffractometer (XRD) Cu Kα radiation was applied to identify the presence of the phase transformation of oxidized TBC samples using the data obtained from the top coat surface. The TGO thickness measurements included the measurement of mixed oxides and Al2O3. TGO thicknesses of all oxidized TBC samples were measured at 3kx magnification using Image Pro Plus 6 software program. 15 measurements were taken from each of 10
Fig. 1. As-Sprayed CoNiCrAlY bond coat and XRD spectra.
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K.M. Doleker, A.C. Karaoglanli / Surface & Coatings Technology xxx (2016) xxx–xxx Table 2 Porosity rate and surface roughness values of substrate and bond coat.
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Table 3 Porosity and surface roughness values of top coat.
Coatings
Ra (μm)
Porosity rate (%)
Coatings
Ra (μm)
Porosity (%)
Inconel-718 CoNiCrAlY bond coat
5,38 7,20
– 0,65 ± 0,2
YSZ top coat Gd2Zr2O7 top coat
7,92 7,49
1,6 ± 0,3 1,9 ± 0,3
different SEM images during TGO thickness measurements (150 measurements in total), and the mean TGO thickness was evaluated for each image. Finally, the average of the mean values of all images was taken to evaluate the overall TGO thickness average.
3. Results and discussion 3.1. Cross-sectional morphology 3.1.1. As-sprayed bond coat structure As-sprayed bond coat structure and XRD spectrum are given in Fig. 1. The thickness of the bond coat layer is approximately 100 μm. Bond coats of the TBCs were homogenously coated. In the microstructure analysis of the bond coat, porosity content is at a quite low level (0,65 ± 0,2) % compared to other studies [11,31]. The areas close to the surface include porosities in the bond coat due to the characteristics of the CGDS technique [10,32,33]. CoNiCrAlY surface morphology exhibited small-sized undeformed particles which result in a rough bond coat surface. This can be ascribed to the facilitated deposition of initial particles by the impact of subsequent particles whereas the final particles do not have such impacting support [10,32,33]. Furthermore, oxide structure is not visible in the bond coat according to the SEM examination due to the use of inert He gases and the kinetics of using CGDS technique during spraying. According to the XRD results of assprayed bond coat, there is only one phase which consists of γphase rich in Co and Ni. Severe plastic deformation causes grain refinements. Thus, β-NiAl phases can be dissolved into the γ-matrix [31]. Porosity and surface roughness values of the substrate and bond coating are given in Table 2. Porosity [1,11] and surface roughness [1,34–37] values of bond coat affect the oxidation behavior TBC system.
3.1.2. As-deposited TBC structure The cross-sectional microstructure of YSZ and Gd2Zr2O7 TBC system is illustrated in Fig. 2. Thickness of the top coat layer is approximately 200 μm. Both top coatings showed a columnar structure as the common characteristic of EB-PVD coatings. Columnar growing and vertical separation of evaporated ingots provide low porosity in EB-PVD coatings. Since APS coating is deposited on the substrate in the form of splats, it has high porosity content. During the EB-PVD deposition, the used parameters such as rotation speed, chamber pressure, substrate temperature or condensation ratio affect the top coat porosity. There are visible inter-columnar voids in the top coat structure. This can also be attributed to the surface roughness of the bond coat [38–40]. Because, growing morphology of columns is affected from bond coat roughness. Growing columns can intersect with each other. As a result, porosities and vertical separation between columns occur. Porosity and surface roughness values of the top coats are given in Table 3. TBCs were exposed to vacuum annealing to get a thin α-Al2O3 layer at the interface, to enable general stabilization of the coating, to facilitate the formation of β-NiAl phases and to provide better adhesion of the ceramic layer to the metal bond coat [41–43]. After deposition of TBCs, TGO layer is quite visible as a result of vacuum annealing according to as-sprayed SEM examination and mapping microstructure (Figs. 3 and 4). In addition, homogeneous βNiAl and γ-phases dispersion in the bond coat was observed in Figs. 3 and 4. Colors of Al and O2 elements overlap in Figs. 3 and 4. Also, Cr colors are not seen in these areas. This is an advantage of CGDS process due to the high spray velocity and the presence of inert gas atmosphere during coating process. Normally, presence of β-NiAl phases was not detected in SEM. Because, severe plastically deformed particles provide grain refinements which may induce dissolution of β-NiAl phases in γ-matrix. These phases can be observed under high magnifications [31] before the vacuum annealing but they can be detected after vacuum annealing which gives sufficient energy for their formation.
Fig. 2. As-deposited TBC cross-sectional SEM microstructure; a) YSZ TBC and b) Gd2Zr2O7 TBC.
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Fig. 3. Elemental mapping microstructure belonging to as-deposited YSZ TBC at 3kx magnification.
3.2. The oxidation behavior of TBCs Cross-sectional SEM microstructures of YSZ and Gd2Zr2O7 TBCs for each different period are given in Figs. 5 and 6 after the oxidation
tests. Thin darker oxide scales represent continuous Al2O3 layer whereas grey areas can show spinel or mixed oxide formations. They can be understood from elemental mapping of 100 h oxidized samples (Figs. 7 and 8). Dark colors in SEM image coincide with Al and O2 colors
Fig. 4. Elemental mapping microstructure belonging to as-deposited Gd2Zr2O7 TBC at 3kx magnification.
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K.M. Doleker, A.C. Karaoglanli / Surface & Coatings Technology xxx (2016) xxx–xxx
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Fig. 5. Cross-sectional SEM microstructures of YSZ TBC after oxidation tests at 1100 °C; a) 8 h, b) 24 h, c) 50 h and d) 100 h.
in elemental mapping microstructures. Grey colors overlap with colors of Co, Ni, Cr and O2 elements in mapping images. In the first stage of oxidation, there is no visible mixed oxide formation while after 50 h oxidation period, mixed oxide formation can be clearly observed according to SEM microstructure. Formation of TGO at the interface of the top coat and the bond coat causes residual stress. Substantial increase in stress during TGO growth induces spallation of top coat from bond coat [44,45]. In TBC microstructures, micro-crack formations occurred after 50 and 100 h oxidation period, because of thermal stresses. However, no spallation of coatings was observed during the oxidation experiments. TGO layers for both coating structures prevent its uniformity. Elemental mapping microstructures of YSZ and Gd2Zr2O7 TBCs after 100 h oxidation period are given in Figs. 7 and 8. According to the mapping results, TGO layer mainly consists of Al2O3 and mixed oxides such as spinels, Cr, Ni and Co oxides for YSZ and Gd2Zr2O7 TBC. Al2O3 formation is desirable for TBC because of lower oxygen permeability compared to mixed oxides. However, depleted NiAl phases cause diffusion of other elements to interface. Thus, TGO layer rapidly grows resulting in stress formation at the interface. There is a little difference in porosity values of top coats according to Table 3. It is absolutely effective in oxidation rate. However, the use of top coatings with different crystal structures is a dominant factor in the oxidation rate. In terms of oxidation mechanism, Gd2Zr2O7 have higher resistance against oxygen penetration due to having stable Frenkel pairs in its structure
when compared with YSZ [46,47]. This can be attributed to the fact that, Gd2Zr2O7 includes an oxygen vacancy at 48f position and oxygen anion at interstitial 8b position in its crystal structure. These two oxygen anions provide stable Frenkel pair defects, that require higher activation energy for oxygen transfer, thus a better oxidation resistance is exhibited by Gd2Zr2O7 when compared to YSZ [46,48–50]. Mahade et al. [46] compared the oxidation behavior of YSZ and YSZ/Gd2Zr2O7 in terms of TGO thickness. The results show that double layer system with Gd2Zr2O7 has more resistance against oxidation. The reason of better resistance is ascribed to the presence of stable Frenkel pair defects in Gd2Zr2O7. Its oxygen permeability is lower than YSZ. During oxidation, initially Y is oxidized since it has the highest affinity. Afterwards it forms Yttrium Aluminum Garnet (YAG) phases after its reaction with the TGO layer formed by Al2O3. Its effect was discussed in the literature [51]. It was explained in the previous studies that GdAlO3 phases occur at the interface as a result of the reaction between Al2O3 and Gd2Zr2O7 under high temperature. It was also stated that TBC's lifetime is reduced due to the porosity formation induced by GD's diffusion into the interface [52–54]. In the present study it was hard to make an accurate distinction with the SEM and mapping images taken with higher magnification after the 100 h oxidation test (Fig. 9.). Also, Gd, Al and O2 coexist in only few regions and YAG phases are comparatively more detectable as seen in Fig. 9. TBC/TGO interface was also found to involve mixed oxides.
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Fig. 6. Cross-sectional SEM microstructures of Gd2Zr2O7 TBC after oxidation tests at 1100 °C; a) 8 h, b) 24 h, c) 50 h and d) 100 h.
Crystallographic transformation may affect the oxidation behavior at the interface of the bond coat and the top coat. Because, this transformation may cause volumetric change, thus leading to formation of higher stresses at the interface in addition to the stresses of newly formed oxides due to oxidation. All these effects increase the bond coat(BC)/top coat (TC) mismatch. In addition, after phase transformation, potential crack formations can lead to oxygen ingress to the BC/TC interface. The X-ray diffraction patterns of as-sprayed and 100 h oxidized TBCs are given in Fig. 10. YSZ has tetragonal phase (t) while Gd2Zr2O7 has cubic phase (c) during all oxidation periods. Metastable tetragonal (t) phase does not transform into monoclinic or cubic phase until 1200 °C [55]. In contrast to YSZ, Gd2Zr2O7 preserves its phase stability up to 1550 °C [56]. As a result, XRD peaks show that there is no phase transformation during oxidation for each TBCs. TGO layer thickness for YSZ-TBC is higher than that of Gd2Zr2O7-TBC as shown in Fig. 11. After 50 h oxidation period, the growth rate of the TGO layer decreased compared to 8 and 24 h oxidation. However, mixed oxide formation for YSZ-TBC is lower compared to Gd2Zr2O7TBC. Also, the crack formation is observed more clearly in Gd2Zr2O7TBC as a result of its elastic modulus. Low elastic modulus and toughness can cause early crack formation which provides simpler oxygen penetration to structure [30]. In fact, SEM examination and TGO thickness show that Gd2Zr2O7 has better oxidation resistance but crack formation can lead to more dangerous oxidation related results. Thus, mixed oxide formation and internal
oxidation content can be seen at higher rates in Gd2Zr2O7-TBC which exhibits higher rates of crack formation as seen in Fig. 12. Another important factor for crack formation is thermal expansion coefficient of the coatings. YSZ has a slightly higher thermal expansion coefficient compared to Gd2Zr2O7 [27,57,58]. Depending on increased oxidation time, thermal expansion mismatch increases even more and ceramic layer is exposed to higher stresses. In the first stages of 8 and 24 h oxidation periods, Gd2Zr2O7-TBCs exhibited lower TGO growth as mixed oxide formation was not visible. This is attributed to the fact that the stress level has not yet reached a level to form cracks in the coating. However, as the stress level increases with increasing time, crack formations will result in expedited oxidation, hence formations of mixed oxides. Despite all these disadvantages, TGO layer thickness in Gd2Zr2O7-TBC was observed at slightly lower rates according to TGO thickness measurements. Better strain tolerance gained by EBPVD positively contributed to the oxidation performance of Gd2Zr2O7TBC. Vaßen et al. [27] used APS coating technique to deposit Gd2Zr2O7 and YSZ on NiCoCrAlY bond coated Inconel 738 to compare their thermal cycling performance with YSZ. Results showed that YSZ-TBC exhibited longer lifetime due to its higher fracture toughness and CTE. To overcome thermal expansion mismatch, double ceramic layered TBCs (Gd2Zr2O7/YSZ) were used. Similar to this study, Bobzin et al. [59] performed oxidation test on Gd2Zr2O7 and YSZ TBCs at 1300 °C with 25 h. YSZ TBC prevented its uniformity whereas Gd2Zr2O7 spalled in spite of usage of EB-PVD technique. However, double ceramic layered TBCs
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Fig. 7. Elemental mapping microstructure belonging to YSZ TBC after 100 h oxidation.
(Gd2Zr2O7/YSZ) exhibited better stability. In contrast to these studies, Munawar et al. [53] compared thermal cycling performance of YSZ and Gd2Zr2O7 TBCs produced by EB-PVD technique but their results did not support each other due to different top coat deposition techniques. Gd2Zr2O7 TBC exhibited better performance through EB-PVD
process providing higher lateral compliance. In the literature, some studies focused on the use of double ceramic layer to overcome the thermal expansion mismatch between the bond and top coat for single layered Gd2Zr2O7 TBCs and to minimize the low fracture toughness of Gd2Zr2O7 [27,46,59–61].
Fig. 8. Elemental mapping microstructure belonging to Gd2Zr2O7 TBC after 100 h oxidation.
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Fig. 9. Elemental distribution images of Gd2Zr2O7 TBC interface after 100 h oxidation.
4. Conclusion In this study, homogenously produced CoNiCrAlY bond coats with low porosity content, deposited with CGDS technique, were combined with EB-PVD technique which provides higher expansion tolerance; and oxidation behavior of TBC systems including Gd2Zr2O7 as a new top coating material and YSZ as the conventional top coating material, was investigated. In the first stages of oxidation, the growth behavior of TGO layer occurred at an increased level compared to other stages of growth behavior of TGO. TGO thickness increased with increasing
time and formations of mixed oxides were observed. At the end of the oxidation tests, no spallation was observed at the coating interfaces of both TBC systems having YSZ and Gd2Zr2O7 as top coat material, and the TGO layer maintained its integrity. In terms of the growth behavior of TGO layer, better results were obtained from the TBC system with Gd2Zr2O7 top coat. Its lower coefficient of thermal conductivity, lower oxygen permeability, relatively high coefficient of thermal expansion and structural stability at higher temperatures render the Gd2Zr2O7 a good alternative top coating material for the TBCs. On the other hand its lower fracture toughness and relatively lower coefficient of thermal
Fig. 10. XRD spectra of as-deposited and 100 h oxidized a) YSZ and b) Gd2Zr2O7 TBC.
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Fig. 11. TGO growth of YSZ and Gd2Zr2O7 TBC.
Fig. 12. SEM cross-sectional microstructure of 100 h oxidized YSZ and Gd2Zr2O7 TBC.
expansion compared to YSZ, result in early formation of cracks during the thermal stresses occurring at sustained high temperatures. In this regard, the forthcoming studies will focus on the modifications for improved Gd2Zr2O7 toughness, new material dopants and/or double or gradient layered coatings. Acknowledgements This investigation was financially supported by The Scientific and Technological Research Council of Turkey (TUBITAK, 113R049). References [1] A.C. Karaoglanli, K.M. Doleker, B. Demirel, A. Turk, R. Varol, Effect of shot peening on the oxidation behavior of thermal barrier coatings, Appl. Surf. Sci. 354 (2015) 314–322. [2] J.R. Davis, Handbook of thermal spray technology, ASM International, 2004. [3] R.A. Miller, Thermal barrier coatings for aircraft engines: history and directions, J. Therm. Spray Technol. 6 (1997) 35–42. [4] W. Gao, Developments in High Temperature Corrosion and Protection of Materials, first ed. Woodhead Publishing, CRC Press, New York, 2008. [5] S. Ghosh, Advanced Ceramic Processing, in: A. Mohamed (Ed.), Thermal Barrier Ceramic Coatings-a Review, InTech, Croatia 2016, pp. 111–138.
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