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CoCrAlYTaSi thermal sprayed barrier coating on GH202 superalloy
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Thermal shock behavior of a 8YSZ/CoCrAlYTaSi thermal sprayed barrier coating on GH202 superalloy Jiangdong Cao∗, Keke Gao, Xue yu Cao, Bocheng Jiang Mechanical and Electrical Department, Nantong Shipping College, Nantong, Jiangsu, 226010, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Thermal shock 8YSZ CoCrAlYTaSi Thermal barrier coating Plasma spraying
The thermal shock behavior of a thermal barrier coating (TBC) prepared by plasma spraying at 1100 °C was investigated. The TBC consisted of a double layer structure of 8YSZ/CoCrAlYTaSi. The morphology, microstructure, phases and the elemental distribution of the TBCs were characterized using scanning electron microscopy (SEM), transmission electron microscope (TEM), scanning transmission electron microscope (STEM), Xray diffraction (XRD) and electron probe micro-analysis (EPMA). The characterization results showed that the film consisted primarily of metastable tetragonal phases (t′), and a large number of micro-cracks were present in the 8YSZ crystals. Following eighty-six thermal shock cycles of the specimens a large areal spallation was observed on the 8YSZ coating. The decreased concentration of yttrium at the coating interfaces weakened the inhibition of crystal growth and the phase transition of the Al2O3. The growth of TGO (Thermal growth oxide) and the diffusion into the 8YSZ coating produced deformation and stress in the ceramic coating. Tantalum appeared to absorb the oxygen that diffused into the coatings and delayed the growth of TGO in the interface between the CoCrAlYTaSi and substrate, which was beneficial to prolonging the life of the TBC.
1. Introduction Turbine blades are one of the most important components of a jet engine, they serve at extremely high temperatures that can produce many corrosion issues [1,2]. As the thrust to weight ratios increase, the working temperature of the engine can exceeded the limit of the turbine blade superalloys and metal coatings. In order to decrease the working temperature of the alloys, thermal barrier coatings (TBCs) are applied to the turbine blades. These coatings provide excellent thermal insulation [3], and resistance to high temperature oxidation and corrosion [4,5], which protect the alloy. This approach has been internationally recognized as a most effective method for defeating these issues. Over the past several decades, research on thermal barrier coatings has been conducted to better understand the issues of structures, materials and preparation processes. Most thermal barrier coatings consist of a double-layer structure, where the outer layer is a ceramic that contains Y2O3 (wt.6%–8%) stabilized zirconia (YSZ) [6,7]. This composition is widely used in the ceramic layer due to its low thermal conductivity and relatively high thermal expansion coefficient [8]. The inner layer is a bonding layer, where the main component is MCrAlY (M: Ni, Co or Fe) [9], such as NiCrAlY [10], CoCrAlY [11], or CoNiCrAlY [12] et al. which decreases the overall gradient of the thermal ∗
expansion coefficient of this layer. Usually, the YSZ is sintered at high temperature for a long period, and the strain mismatch between the two layers causes internal stress in the YSZ after extreme cold and heat cycles [13,14]. This has been found to produce spallation in the outer layer of the coating. In order to improve the stability of YSZ coatings at high temperature, several different oxides (CeO2、La2O3、HfO2 et al.) are mixed in the ZrO2 powder to restrain the crystal transformation of ZrO2 [15–17]. Some researchers have experimented with fluorite structured compounds (A2B2O7) to replace YSZ, such as Yb2Si2O7 [18], La2Zr2O7 [19], Gd2Zr2O7 [20] et al. Compared to the YSZ ceramic, fluorite structured compounds have a lower Young's modulus, lower thermal conductivity and better high-temperature phase stability [21,22]. Based on the results of these studies thermal barrier coatings with a multilayer structure or gradient structure [23] have been developed to reduce the mismatch in the thermal expansion coefficient of the coatings. In our earlier work, we investigated the high temperature oxidation behavior of CoCrAlYTaSi coatings [24], and the results demonstrated the excellent resistance of this material to high temperature oxidation. In this current work the thermal barrier coating (8YSZ/CoCrAlYTaSi) was designed and prepared by plasma spraying, and thermal shock behavior of the TBC at 1100 °C were investigated. The microstructures, phase structure and morphology of the TBC were characterized by
Corresponding author. Tel.: 86 0513 85965515. E-mail address: [email protected] (J. Cao).
https://doi.org/10.1016/j.ceramint.2019.11.247 Received 3 October 2019; Received in revised form 22 November 2019; Accepted 27 November 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Jiangdong Cao, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.11.247
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strengthening phases (γ′) with the diameter of 20–50 nm are dispersed in the alloy in Fig. 1(a). According to the electron diffraction in Fig. 1(b), the alloy is a Ni–Cr solid solution and the γ′ phase is mainly Ni3(Al, Ti). A small amount of crystal defects occur, such as twins, dislocations in Fig. 1(c) and (d), and some blocky carbides exist at the grain boundaries. However, this superalloy does not meet the requirements of increasing thrust-weight ratios, so coating the GH202 alloy with a good thermal barrier coatings is an effective method to improve the alloy's performance.
Table 1 Chemical compositions of Ni-based superalloy GH202 (wt. %). Material
Content
Chemical compositions Al
Ti
Cr
Mo
W
Ni
1.38
2.02
20.39
4.08
5.62
all
STEM (Scanning transmission electron microscopy), XRD (X-ray diffraction), EPMA (Electron probe micro analysis), and SEM (Scanning electron microscopy). The experimental TBC exhibited excellent performance in extreme hot and cold environments, which should provide a good reference for the development of improved thermal barrier coating.
2.1.2. Coating materials The thermal barrier coating used in this work had a double layer structure, with the outer layer composed of 8YSZ, and the inner layer was CoCrAlYTaSi. The morphologies of 8YSZ and CoCrAlYTaSi powders are shown as Fig. 2(a) and (b). To improve the fluidity of the powders during the plasma spraying, the 8YSZ and CoCrAlYTaSi powder particles were prepared as spheroids or ellipsoids, the diameter of which was from 30 μm to 80 μm. The energy spectrum analyses of the powders are shown as Fig. 2(c) and (d).
2. Experimental procedure 2.1. Materials
2.2. Preparation process of the TBCs
2.1.1. Substrate material A Ni-based superalloy GH202 is widely used as a key component in high thrust liquid rocket engines, in components such as turbine blades, turbine rotors, bellows, gas pipelines etc., because it exhibits excellent resistance to high temperature oxidation below 1000 °C [25]. The chemical composition of the alloy used as the substrate in this reported work is shown in Table 1. The microstructure of the superalloy was characterized by TEM in Fig. 1. A large number of the globular
The 8YSZ/CoCrAlYTaSi coatings were prepared by plasma spraying, and the detailed experimental procedures was as follows: (1) Ni-based superalloy foils GH202 were cut into the squares with dimensions of 40 × 40 × 5 mm3 using a WEDM (Wire Electrical Discharge Machining). Then the specimens were polished using 180
Fig. 1. The microstructure of the Ni-based superalloy GH202: (a) γ′ phase, (b) the electron diffraction pattern, (c) Twins, (d) dislocations and Carbides. 2
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Fig. 2. The SEM images of the powders: (a) 8YSZ, (b) CoCrAlYTaSi; the energy spectrum analysis: (c) 8YSZ, (d) CoCrAlYTaSi.
mesh SiC sandpaper to remove pits and tool marks. (2) These metal specimens were immersed into acetone for 24h, cleaned in alcohol using an ultrasonic cleaner, and washed with deionized water. (3) The specimens were clamped on a special fixture and sandblasted with 1.18 mm using 4 bar compressed air. (4) The surface of the specimen was heated to 300 °C and the CoCrAlYTaSi and 8YSZ powders were sprayed onto the heated metal using a plasma arc gun under argon gas, as shown in Fig. 3. The process parameters of plasma spraying for the TBCs are shown in Table 2.
Table 2 Process parameters of plasma spraying for the TBCs. Coatings
Bonding layer Ceramic layer
Process parameters of plasma spraying Power (KW)
Main air (L/min)
Auxiliary gas (L/min)
Carrier gas (L/min)
Spraying distance (mm)
30 40
60 60
25 25
7 7
100 80
2.3. Thermal shock test Based on the HB 7269-96 Chinese quality inspection standard for TBCs the coated specimens were placed in a high temperature furnace
Fig. 3. The sketch of a plasma arc gun. 3
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The cross-sectional elemental distribution of the thermal barrier coating was characterized using EPMA and the results are shown in Fig. 6. From this image it appeared that the concentration of zirconium was high near the interface of the YSZ/CoCrAlYTaSi, coatings and the several pores were present in the ceramic coating. Based on the distribution of oxygen and aluminum, it appeared that there was an abundance of banded Al2O3 produced at the bottom and top of the CoCrAlYTaSi coating. From the elemental distribution map for yttrium, it can be seen that the yttrium was distributed primarily at the interface of CoCrAlYTaSi/8YSZ and CoCrAlYTaSi/substrate. Tantalum in the bonding layer was evenly distributed except for a small amount of large particles. The amount of silicon in the coatings did not appear to be significant in the elemental mapping due to its low concentration in the components. 3.3. Surface morphology of the TBCs after the thermal shock
The morphologies and microstructures of the powders and TBCs were examined using a scanning electron microscope (SEM) with an energy dispersive spectrometer (EDS), transmission electron microscope (TEM) and scanning transmission electron microscopy (STEM). The phases of the TBCs were characterized by X-ray diffraction (XRD). The distribution of the elements in the TBCs before and after the thermal shock test was determined by electron probe micro-analysis (EPMA).
Fig. 7 shows the surface morphology of the 8YSZ/CoCrAlYTaSi after many thermal shock cycles at 1100 °C. After successive, rapid heating and cooling cycles, the 8YSZ coating expanded and contracted drastically, which produced cracks in the 8YSZ coating due to the intrinsic brittleness of this ceramic. After 20 cycles several long and narrow cracks appeared to have been generated on the surface of the TBCs as shown in Fig. 7(a). After 40 cycles, a few of these cracks widened as shown in Fig. 7(b). With the thermal shocks proceeding the cracks of the 8YSZ coating gradually propagated to become continuous, which caused spallation of the 8YSZ coating. A few small area of spallation appeared on the 8YSZ coating after 60 cycles as shown in Fig. 7(c), but the compromised coating still protected the bonding coating and the substrate. After 86 cycles spallation of the 8YSZ coating occurred over a wider area in Fig. 7(d). Due to the spallation the cracks after 60 cycles and 80 cycles are less than those with 20 and 40 cycles. The results showed that the 8YSZ/CoCrAlYTaSi exhibited excellent resistance to thermal shock at 1100 °C.
3. Results and discussion
3.4. Cross-sectional microstructure of the TBCs after thermal shock cycles
3.1. Phase analysis of the TBCs
Fig. 8 shows the cross-sectional microstructure of the TBCs after the thermal shock testing at 1100 °C. It can be easily seen from the data in Fig. 8 (a) that the 8YSZ was intact after the 20 cycles, but some tiny pores and narrow cracks that extended along the direction perpendicular to the interface were present. After 40 cycles, the pores in the 8YSZ coating became larger as shown in Fig. 8 (b), and the cracks propagated from the surface to the interface. Some mixed oxide (Al2O3, SiO2, CoO, Cr2O3) filled in the narrow cracks in the CoCrAlYTaSi coating due to the internal diffusion of the oxygen. After 60 cycles the many vertical cracks closed to the interface were formed as shown in Fig. 8 (c), and several large pores appeared near the coatings interface, which decreased the bonding strength between the 8YSZ coating and the CoCrAlYTaSi coatings. After 86 cycles, the 8YSZ coating was fractured and spalled as shown in Fig. 8 (d). Meanwhile, oxygen propagated to the substrate through the CoCrAlYTaSi coating, and caused the internal oxidation and cracks of the superalloy.
Fig. 4. XRD pattern of the TBCs after the plasma spraying.
at 1100 °C for 10 min, and then quickly removed and placed into the water at 20 °C ± 5 °C. This operation was repeated continuously until large areas of spallation appeared on the ceramic layer. 2.4. Microstructural characterization of TBCs
Fig. 4 shows the phases of the ceramic layer surface of the TBCs. From this image it can be seen that only the metastable tetragonal phase (t′) was generated in the ZrO2 ceramic layer after the plasma spraying. The content of Y2O3 in the coating stabilized the phase, and a transformation of the phase (t′) to monoclinic phase (m′) during the rapid cooling of the specimen from high temperature was not found. Splitting of the diffraction peaks occurred at near 35° and 60°, which resulted from the stretching of the C axis of the t′ phase. 3.2. Morphology and microstructures of the TBCs Fig. 5 shows the morphology and microstructures of the TBCs, where it can be seen that the ceramic 8 YS, the bonding layer CoCrAlYTaSi and the substrate all bonded well, and the each layer of the coating had a thickness of approximately 200 μm as shown in Fig. 5 (a). The molten 8YSZ particles were sprayed onto the CoCrAlYTaSi coating at a high speed and they condensed to form the ZrO2 ceramic layer. In Fig. 5 (b) it can be seen that the polyhedral crystals were intact, but a few micro-cracks were present. There were several micro-pores at the interface between the ceramic layer and the bonding layer. Fig. 5 (c) shows the CoCrAlYTaSi coating with a multi-layer structure, and a few micro-pores and bands of Al2O3 were present at the interface between the layers. It can be clearly seen that there was a small amount of Ta present in the form of white large particles. From the magnification of this area a large number of tiny Ta particles appeared to be evenly distributed in the coating, which appeared as the white spots in Fig. 5 (d).
3.5. Evolution of thermally grown oxides Fig. 9 shows the evolution of the thermally grown oxides at the interface of 8YSZ/CoCrAlYTaSi coatings after the thermal shock experiments at 1100 °C. It can be seen from the data represented in Fig. 9(a) that a few microcracks near the interface had been generated in the 8YSZ coating, which provided channels for oxygen diffusion to the substrate. After 40 cycles a thin and continuous oxide layer at the interface was gradually formed as shown in Fig. 9(b). As the cycling continued the TGO layer became thicker and more dense, and some larger pores had been formed near the interface as shown in Fig. 9(c). After 86 cycles the enlarged pores connected with each other and 4
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Fig. 5. STEM images of the TBCs: (a) the macrostructure, (b) the microstructure of 8YSZ, (c) the microstructure of CoCrAlYTaSi, (d) the magnification of the figure c.
Fig. 6. The across-sectional elements distribution of the thermal barrier coating.
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Fig. 7. The surface morphologies of the TBCs after the thermal shock cycles: (a) 20 cycles, (b) 40 cycles, (c) 60 cycles, (d) 86 cycles.
Fig. 8. The cross-sectional microstructures of the TBCs after the thermal shock cycles: (a) 20 cycles, (b) 40 cycles, (c) 60 cycles, (d) 86 cycles. 6
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Fig. 9. SEM images of thermally grown oxides at the interface after the thermal shock cycles: (a) 20 cycles, (b) 40 cycles, (c) 60 cycles, (d) 86 cycles.
plasma spraying and yttrium was found at the upper and lower interfaces of the CoCrAlYTaSi coating as shown in Fig. 11(a). The inhibition effect of yttrium on the growth and phase transition of the Al2O3 improved the adhesion of the coating to the substrate [29]. The brittleness of ZrO2 ceramic produced cracks on the surface of the TBCs at the initial stages of thermal shock. These narrow cracks extended from the surface to the interface between 8YSZ and CoCrAlYTaSi. Aluminum in the bonding layer reacted with oxygen to form a discontinuous amount of Al2O3 at the interface. In addition, the yttrium moved towards the middle of the CoCrAlYTaSi coating as shown in Fig. 11(b). At mid-stage of the thermal shock treatments the cracks gradually propagated to the interface due to the 8YSZ deformation that was caused by the significant growth of the TGO [30], and a few pores filled with the mixed oxide became larger. Over time, the yttrium content at the interface decreased, which may have produced a decrease in the bond strength of the ZrO2 coating [31] as shown in Fig. 11(c). At the last stage of the cycling, the TGO layer with a few gaps grew larger, which increased the extension of the cracks. When the cracks at the coating surface connected with the cracks at the bottom as shown in Fig. 11(d), spallation of the ceramic coating occurred. Meanwhile, the spallation of the 8YSZ coating in some areas accelerated the oxidation of the CoCrAlYTaSi coating and promoted Cr2O3 to react with CoO and produced CoCr2O4 [24]. It was caused that Cr content in the bond-coat was decreased significantly after 86 cycles. Almost all of the yttium migrated to the middle of the coating, which did not inhibit the growth and phase transition of the Al2O3. In addition, tantalum added to the bond coating absorbed most of oxygen diffusing into the coatings [24], which slowed the growth of the TGO in the interface between the CoCrAlYTaSi and substrate, and it reduced the deformation of the TBCs and prolonged their service life. The Si ions encountered the oxygen diffusing from the ceramic layer to form the molten SiO2 at the high temperature, which filled the cracks in the CoCrAlYTaSi coating due to the good liquidity
formed a huge gap between the ceramic layer and the bonding coating were generated in Fig. 9(d), which caused the spallation of the 8YSZ coating. 3.6. Elemental distribution of TBCs during thermal shock Fig. 10 shows the distribution of the cross-sectional elements in TBCs during the thermal shock cycling at 1100 °C. Based on the mapping of zirconium, the amount and size of the pores in the 8YSZ coating gradually increased, and the cracks nearly penetrated the entire 8YSZ coating. From the elemental distribution data of the oxygen and aluminum in the specimen it can be seen that there was a significant amount of Al2O3 formed in the CoCrAlYTaSi coating, and the diffusion of Al2O3 into the 8YSZ coating was significant during the cycling. In addition, the micro-cracks in the CoCrAlYTaSi coating were filled with Al2O3 due to the higher reactivity of aluminum to oxygen compared to cobalt and nickel. The distribution of yttrium in the CoCrAlYTaSi coating exhibited a tendency to move from the interface to the middle of the coating. Tantalum was diffusely distributed in the CoCrAlYTaSi coating, and the tantalum oxides gradually grew at high temperature, because of the high reactivity of Ta with oxygen. 3.7. Discussion the mechanism of the resistance to thermal shock The molten 8YSZ particles were sprayed on the surface of the CoCrAlYTaSi coating at a high rate, and the ZrO2 ceramic coating was formed after it cooled and crystallized. The crystals in the coating were near polyhedron in shape, and a large number of micro-cracks occurred among the crystals, which provided space for thermal expansion and contraction [26,27], which partially lowered the thermal stress [28]. These micro-cracks also provided channels for the diffusion of oxygen. A small amount of Al2O3 was present at the coating interface after the 7
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Fig. 10. The EPMA image of the elements distribution at the cross-sectional of TBCs during the thermal shock at 1100 °C.
polyhedrons, and a large number of micro-cracks were present among the crystals. The phases of the coatings are mainly metastable tetragonal phases (t′), and the transformation of the phase (t′) to monoclinic phase (m′) was not found. (2) The spallation of the 8YSZ coating in a wide area occurred after 86 thermal shock cycles at 1100 °C. It was caused by the growth of the TGO layer and the movement of yttrium toward the middle of the coating from the interfaces (8YSZ/CoCrAlYTaSi, CoCrAlYTaSi/ GH202).
[32]. It is beneficial to improve the service life of the coating. 4. Conclusions The thermal shock behavior of a thermal barrier coating prepared by plasma spray at 1100 °C was investigated, which produced the following conclusions: (1) The crystals of the 8YSZ coating prepared by plasma spraying were 8
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Fig. 11. The schematic of the thermal shock behavior of the TBCs at 1100 °C.
(3) Oxygen diffusing into the coatings was absorbed by tantalum, which delayed the growth of TGO in the interface between the CoCrAlYTaSi and substrate, and it reduced the deformation caused by oxide overgrowth and prolonged their service life.
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Declaration of competing interest We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled, “Thermal shock behavior of a 8YSZ/CoCrAlYTaSi thermal sprayed barrier coating on GH202 superalloy”. Acknowledgment This work is supported by Natural Science Foundation of Jiangsu Province (BK20191204), Natural Science Foundation of Jiangsu Province for universities and colleges (19KJB430031), Nantong science and technology project (GY12018032) and Oinglan Project of Jiangsu Province. References [1] C.M. Sim, H.S. Oh, T.J. Kim, et al., Detecting internal hot corrosion of in-service turbine blades using neutron tomography with Gd tagging, J. Nondestruct. Eval. 33 (2014) 493–503. [2] P. Mechnich, W. Braue, J. Smialek, Solid-state CMAS corrosion of an EB-PVD YSZ coated turbine blade: Zr\r, 4+\r, partitioning and phase evolution, J. Am. Ceram. Soc. 98 (2015) 296–302. [3] X.X. Ling, Y.Z. Wang, X. Wang, et al., Numerical study of effect of pore microstructure of layered thermal barrier coatings on thermal insulation performance, Chin. J. Nonferrous Metals 25 (2015) 408–414. [4] K.M. Doleker, H. Ahlatci, A.C. Karaoglanli, Investigation of isothermal oxidation behavior of thermal barrier coatings (TBCs) consisting of YSZ and multilayered YSZ/Gd2Zr2O7 ceramic layers, Oxid. Metals 88 (2017) 1–11. [5] M.G. Gok, G. Goller, Microstructural characterization of GZ/CYSZ thermal barrier coatings after thermal shock and CMAS+hot corrosion test, J. Eur. Ceram. Soc. 37 (2017) 2501–2508.
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Thermal shock behavior of a 8YSZ/CoCrAlYTaSi thermal sprayed barrier coating on GH202 superalloy Jiangdong Cao∗, Keke Gao, Xue yu Cao, Bocheng Jiang Mechanical and Electrical Department, Nantong Shipping College, Nantong, Jiangsu, 226010, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Thermal shock 8YSZ CoCrAlYTaSi Thermal barrier coating Plasma spraying
The thermal shock behavior of a thermal barrier coating (TBC) prepared by plasma spraying at 1100 °C was investigated. The TBC consisted of a double layer structure of 8YSZ/CoCrAlYTaSi. The morphology, microstructure, phases and the elemental distribution of the TBCs were characterized using scanning electron microscopy (SEM), transmission electron microscope (TEM), scanning transmission electron microscope (STEM), Xray diffraction (XRD) and electron probe micro-analysis (EPMA). The characterization results showed that the film consisted primarily of metastable tetragonal phases (t′), and a large number of micro-cracks were present in the 8YSZ crystals. Following eighty-six thermal shock cycles of the specimens a large areal spallation was observed on the 8YSZ coating. The decreased concentration of yttrium at the coating interfaces weakened the inhibition of crystal growth and the phase transition of the Al2O3. The growth of TGO (Thermal growth oxide) and the diffusion into the 8YSZ coating produced deformation and stress in the ceramic coating. Tantalum appeared to absorb the oxygen that diffused into the coatings and delayed the growth of TGO in the interface between the CoCrAlYTaSi and substrate, which was beneficial to prolonging the life of the TBC.
1. Introduction Turbine blades are one of the most important components of a jet engine, they serve at extremely high temperatures that can produce many corrosion issues [1,2]. As the thrust to weight ratios increase, the working temperature of the engine can exceeded the limit of the turbine blade superalloys and metal coatings. In order to decrease the working temperature of the alloys, thermal barrier coatings (TBCs) are applied to the turbine blades. These coatings provide excellent thermal insulation [3], and resistance to high temperature oxidation and corrosion [4,5], which protect the alloy. This approach has been internationally recognized as a most effective method for defeating these issues. Over the past several decades, research on thermal barrier coatings has been conducted to better understand the issues of structures, materials and preparation processes. Most thermal barrier coatings consist of a double-layer structure, where the outer layer is a ceramic that contains Y2O3 (wt.6%–8%) stabilized zirconia (YSZ) [6,7]. This composition is widely used in the ceramic layer due to its low thermal conductivity and relatively high thermal expansion coefficient [8]. The inner layer is a bonding layer, where the main component is MCrAlY (M: Ni, Co or Fe) [9], such as NiCrAlY [10], CoCrAlY [11], or CoNiCrAlY [12] et al. which decreases the overall gradient of the thermal ∗
expansion coefficient of this layer. Usually, the YSZ is sintered at high temperature for a long period, and the strain mismatch between the two layers causes internal stress in the YSZ after extreme cold and heat cycles [13,14]. This has been found to produce spallation in the outer layer of the coating. In order to improve the stability of YSZ coatings at high temperature, several different oxides (CeO2、La2O3、HfO2 et al.) are mixed in the ZrO2 powder to restrain the crystal transformation of ZrO2 [15–17]. Some researchers have experimented with fluorite structured compounds (A2B2O7) to replace YSZ, such as Yb2Si2O7 [18], La2Zr2O7 [19], Gd2Zr2O7 [20] et al. Compared to the YSZ ceramic, fluorite structured compounds have a lower Young's modulus, lower thermal conductivity and better high-temperature phase stability [21,22]. Based on the results of these studies thermal barrier coatings with a multilayer structure or gradient structure [23] have been developed to reduce the mismatch in the thermal expansion coefficient of the coatings. In our earlier work, we investigated the high temperature oxidation behavior of CoCrAlYTaSi coatings [24], and the results demonstrated the excellent resistance of this material to high temperature oxidation. In this current work the thermal barrier coating (8YSZ/CoCrAlYTaSi) was designed and prepared by plasma spraying, and thermal shock behavior of the TBC at 1100 °C were investigated. The microstructures, phase structure and morphology of the TBC were characterized by
Corresponding author. Tel.: 86 0513 85965515. E-mail address: [email protected] (J. Cao).
https://doi.org/10.1016/j.ceramint.2019.11.247 Received 3 October 2019; Received in revised form 22 November 2019; Accepted 27 November 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Jiangdong Cao, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.11.247
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strengthening phases (γ′) with the diameter of 20–50 nm are dispersed in the alloy in Fig. 1(a). According to the electron diffraction in Fig. 1(b), the alloy is a Ni–Cr solid solution and the γ′ phase is mainly Ni3(Al, Ti). A small amount of crystal defects occur, such as twins, dislocations in Fig. 1(c) and (d), and some blocky carbides exist at the grain boundaries. However, this superalloy does not meet the requirements of increasing thrust-weight ratios, so coating the GH202 alloy with a good thermal barrier coatings is an effective method to improve the alloy's performance.
Table 1 Chemical compositions of Ni-based superalloy GH202 (wt. %). Material
Content
Chemical compositions Al
Ti
Cr
Mo
W
Ni
1.38
2.02
20.39
4.08
5.62
all
STEM (Scanning transmission electron microscopy), XRD (X-ray diffraction), EPMA (Electron probe micro analysis), and SEM (Scanning electron microscopy). The experimental TBC exhibited excellent performance in extreme hot and cold environments, which should provide a good reference for the development of improved thermal barrier coating.
2.1.2. Coating materials The thermal barrier coating used in this work had a double layer structure, with the outer layer composed of 8YSZ, and the inner layer was CoCrAlYTaSi. The morphologies of 8YSZ and CoCrAlYTaSi powders are shown as Fig. 2(a) and (b). To improve the fluidity of the powders during the plasma spraying, the 8YSZ and CoCrAlYTaSi powder particles were prepared as spheroids or ellipsoids, the diameter of which was from 30 μm to 80 μm. The energy spectrum analyses of the powders are shown as Fig. 2(c) and (d).
2. Experimental procedure 2.1. Materials
2.2. Preparation process of the TBCs
2.1.1. Substrate material A Ni-based superalloy GH202 is widely used as a key component in high thrust liquid rocket engines, in components such as turbine blades, turbine rotors, bellows, gas pipelines etc., because it exhibits excellent resistance to high temperature oxidation below 1000 °C [25]. The chemical composition of the alloy used as the substrate in this reported work is shown in Table 1. The microstructure of the superalloy was characterized by TEM in Fig. 1. A large number of the globular
The 8YSZ/CoCrAlYTaSi coatings were prepared by plasma spraying, and the detailed experimental procedures was as follows: (1) Ni-based superalloy foils GH202 were cut into the squares with dimensions of 40 × 40 × 5 mm3 using a WEDM (Wire Electrical Discharge Machining). Then the specimens were polished using 180
Fig. 1. The microstructure of the Ni-based superalloy GH202: (a) γ′ phase, (b) the electron diffraction pattern, (c) Twins, (d) dislocations and Carbides. 2
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Fig. 2. The SEM images of the powders: (a) 8YSZ, (b) CoCrAlYTaSi; the energy spectrum analysis: (c) 8YSZ, (d) CoCrAlYTaSi.
mesh SiC sandpaper to remove pits and tool marks. (2) These metal specimens were immersed into acetone for 24h, cleaned in alcohol using an ultrasonic cleaner, and washed with deionized water. (3) The specimens were clamped on a special fixture and sandblasted with 1.18 mm using 4 bar compressed air. (4) The surface of the specimen was heated to 300 °C and the CoCrAlYTaSi and 8YSZ powders were sprayed onto the heated metal using a plasma arc gun under argon gas, as shown in Fig. 3. The process parameters of plasma spraying for the TBCs are shown in Table 2.
Table 2 Process parameters of plasma spraying for the TBCs. Coatings
Bonding layer Ceramic layer
Process parameters of plasma spraying Power (KW)
Main air (L/min)
Auxiliary gas (L/min)
Carrier gas (L/min)
Spraying distance (mm)
30 40
60 60
25 25
7 7
100 80
2.3. Thermal shock test Based on the HB 7269-96 Chinese quality inspection standard for TBCs the coated specimens were placed in a high temperature furnace
Fig. 3. The sketch of a plasma arc gun. 3
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The cross-sectional elemental distribution of the thermal barrier coating was characterized using EPMA and the results are shown in Fig. 6. From this image it appeared that the concentration of zirconium was high near the interface of the YSZ/CoCrAlYTaSi, coatings and the several pores were present in the ceramic coating. Based on the distribution of oxygen and aluminum, it appeared that there was an abundance of banded Al2O3 produced at the bottom and top of the CoCrAlYTaSi coating. From the elemental distribution map for yttrium, it can be seen that the yttrium was distributed primarily at the interface of CoCrAlYTaSi/8YSZ and CoCrAlYTaSi/substrate. Tantalum in the bonding layer was evenly distributed except for a small amount of large particles. The amount of silicon in the coatings did not appear to be significant in the elemental mapping due to its low concentration in the components. 3.3. Surface morphology of the TBCs after the thermal shock
The morphologies and microstructures of the powders and TBCs were examined using a scanning electron microscope (SEM) with an energy dispersive spectrometer (EDS), transmission electron microscope (TEM) and scanning transmission electron microscopy (STEM). The phases of the TBCs were characterized by X-ray diffraction (XRD). The distribution of the elements in the TBCs before and after the thermal shock test was determined by electron probe micro-analysis (EPMA).
Fig. 7 shows the surface morphology of the 8YSZ/CoCrAlYTaSi after many thermal shock cycles at 1100 °C. After successive, rapid heating and cooling cycles, the 8YSZ coating expanded and contracted drastically, which produced cracks in the 8YSZ coating due to the intrinsic brittleness of this ceramic. After 20 cycles several long and narrow cracks appeared to have been generated on the surface of the TBCs as shown in Fig. 7(a). After 40 cycles, a few of these cracks widened as shown in Fig. 7(b). With the thermal shocks proceeding the cracks of the 8YSZ coating gradually propagated to become continuous, which caused spallation of the 8YSZ coating. A few small area of spallation appeared on the 8YSZ coating after 60 cycles as shown in Fig. 7(c), but the compromised coating still protected the bonding coating and the substrate. After 86 cycles spallation of the 8YSZ coating occurred over a wider area in Fig. 7(d). Due to the spallation the cracks after 60 cycles and 80 cycles are less than those with 20 and 40 cycles. The results showed that the 8YSZ/CoCrAlYTaSi exhibited excellent resistance to thermal shock at 1100 °C.
3. Results and discussion
3.4. Cross-sectional microstructure of the TBCs after thermal shock cycles
3.1. Phase analysis of the TBCs
Fig. 8 shows the cross-sectional microstructure of the TBCs after the thermal shock testing at 1100 °C. It can be easily seen from the data in Fig. 8 (a) that the 8YSZ was intact after the 20 cycles, but some tiny pores and narrow cracks that extended along the direction perpendicular to the interface were present. After 40 cycles, the pores in the 8YSZ coating became larger as shown in Fig. 8 (b), and the cracks propagated from the surface to the interface. Some mixed oxide (Al2O3, SiO2, CoO, Cr2O3) filled in the narrow cracks in the CoCrAlYTaSi coating due to the internal diffusion of the oxygen. After 60 cycles the many vertical cracks closed to the interface were formed as shown in Fig. 8 (c), and several large pores appeared near the coatings interface, which decreased the bonding strength between the 8YSZ coating and the CoCrAlYTaSi coatings. After 86 cycles, the 8YSZ coating was fractured and spalled as shown in Fig. 8 (d). Meanwhile, oxygen propagated to the substrate through the CoCrAlYTaSi coating, and caused the internal oxidation and cracks of the superalloy.
Fig. 4. XRD pattern of the TBCs after the plasma spraying.
at 1100 °C for 10 min, and then quickly removed and placed into the water at 20 °C ± 5 °C. This operation was repeated continuously until large areas of spallation appeared on the ceramic layer. 2.4. Microstructural characterization of TBCs
Fig. 4 shows the phases of the ceramic layer surface of the TBCs. From this image it can be seen that only the metastable tetragonal phase (t′) was generated in the ZrO2 ceramic layer after the plasma spraying. The content of Y2O3 in the coating stabilized the phase, and a transformation of the phase (t′) to monoclinic phase (m′) during the rapid cooling of the specimen from high temperature was not found. Splitting of the diffraction peaks occurred at near 35° and 60°, which resulted from the stretching of the C axis of the t′ phase. 3.2. Morphology and microstructures of the TBCs Fig. 5 shows the morphology and microstructures of the TBCs, where it can be seen that the ceramic 8 YS, the bonding layer CoCrAlYTaSi and the substrate all bonded well, and the each layer of the coating had a thickness of approximately 200 μm as shown in Fig. 5 (a). The molten 8YSZ particles were sprayed onto the CoCrAlYTaSi coating at a high speed and they condensed to form the ZrO2 ceramic layer. In Fig. 5 (b) it can be seen that the polyhedral crystals were intact, but a few micro-cracks were present. There were several micro-pores at the interface between the ceramic layer and the bonding layer. Fig. 5 (c) shows the CoCrAlYTaSi coating with a multi-layer structure, and a few micro-pores and bands of Al2O3 were present at the interface between the layers. It can be clearly seen that there was a small amount of Ta present in the form of white large particles. From the magnification of this area a large number of tiny Ta particles appeared to be evenly distributed in the coating, which appeared as the white spots in Fig. 5 (d).
3.5. Evolution of thermally grown oxides Fig. 9 shows the evolution of the thermally grown oxides at the interface of 8YSZ/CoCrAlYTaSi coatings after the thermal shock experiments at 1100 °C. It can be seen from the data represented in Fig. 9(a) that a few microcracks near the interface had been generated in the 8YSZ coating, which provided channels for oxygen diffusion to the substrate. After 40 cycles a thin and continuous oxide layer at the interface was gradually formed as shown in Fig. 9(b). As the cycling continued the TGO layer became thicker and more dense, and some larger pores had been formed near the interface as shown in Fig. 9(c). After 86 cycles the enlarged pores connected with each other and 4
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Fig. 5. STEM images of the TBCs: (a) the macrostructure, (b) the microstructure of 8YSZ, (c) the microstructure of CoCrAlYTaSi, (d) the magnification of the figure c.
Fig. 6. The across-sectional elements distribution of the thermal barrier coating.
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Fig. 7. The surface morphologies of the TBCs after the thermal shock cycles: (a) 20 cycles, (b) 40 cycles, (c) 60 cycles, (d) 86 cycles.
Fig. 8. The cross-sectional microstructures of the TBCs after the thermal shock cycles: (a) 20 cycles, (b) 40 cycles, (c) 60 cycles, (d) 86 cycles. 6
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Fig. 9. SEM images of thermally grown oxides at the interface after the thermal shock cycles: (a) 20 cycles, (b) 40 cycles, (c) 60 cycles, (d) 86 cycles.
plasma spraying and yttrium was found at the upper and lower interfaces of the CoCrAlYTaSi coating as shown in Fig. 11(a). The inhibition effect of yttrium on the growth and phase transition of the Al2O3 improved the adhesion of the coating to the substrate [29]. The brittleness of ZrO2 ceramic produced cracks on the surface of the TBCs at the initial stages of thermal shock. These narrow cracks extended from the surface to the interface between 8YSZ and CoCrAlYTaSi. Aluminum in the bonding layer reacted with oxygen to form a discontinuous amount of Al2O3 at the interface. In addition, the yttrium moved towards the middle of the CoCrAlYTaSi coating as shown in Fig. 11(b). At mid-stage of the thermal shock treatments the cracks gradually propagated to the interface due to the 8YSZ deformation that was caused by the significant growth of the TGO [30], and a few pores filled with the mixed oxide became larger. Over time, the yttrium content at the interface decreased, which may have produced a decrease in the bond strength of the ZrO2 coating [31] as shown in Fig. 11(c). At the last stage of the cycling, the TGO layer with a few gaps grew larger, which increased the extension of the cracks. When the cracks at the coating surface connected with the cracks at the bottom as shown in Fig. 11(d), spallation of the ceramic coating occurred. Meanwhile, the spallation of the 8YSZ coating in some areas accelerated the oxidation of the CoCrAlYTaSi coating and promoted Cr2O3 to react with CoO and produced CoCr2O4 [24]. It was caused that Cr content in the bond-coat was decreased significantly after 86 cycles. Almost all of the yttium migrated to the middle of the coating, which did not inhibit the growth and phase transition of the Al2O3. In addition, tantalum added to the bond coating absorbed most of oxygen diffusing into the coatings [24], which slowed the growth of the TGO in the interface between the CoCrAlYTaSi and substrate, and it reduced the deformation of the TBCs and prolonged their service life. The Si ions encountered the oxygen diffusing from the ceramic layer to form the molten SiO2 at the high temperature, which filled the cracks in the CoCrAlYTaSi coating due to the good liquidity
formed a huge gap between the ceramic layer and the bonding coating were generated in Fig. 9(d), which caused the spallation of the 8YSZ coating. 3.6. Elemental distribution of TBCs during thermal shock Fig. 10 shows the distribution of the cross-sectional elements in TBCs during the thermal shock cycling at 1100 °C. Based on the mapping of zirconium, the amount and size of the pores in the 8YSZ coating gradually increased, and the cracks nearly penetrated the entire 8YSZ coating. From the elemental distribution data of the oxygen and aluminum in the specimen it can be seen that there was a significant amount of Al2O3 formed in the CoCrAlYTaSi coating, and the diffusion of Al2O3 into the 8YSZ coating was significant during the cycling. In addition, the micro-cracks in the CoCrAlYTaSi coating were filled with Al2O3 due to the higher reactivity of aluminum to oxygen compared to cobalt and nickel. The distribution of yttrium in the CoCrAlYTaSi coating exhibited a tendency to move from the interface to the middle of the coating. Tantalum was diffusely distributed in the CoCrAlYTaSi coating, and the tantalum oxides gradually grew at high temperature, because of the high reactivity of Ta with oxygen. 3.7. Discussion the mechanism of the resistance to thermal shock The molten 8YSZ particles were sprayed on the surface of the CoCrAlYTaSi coating at a high rate, and the ZrO2 ceramic coating was formed after it cooled and crystallized. The crystals in the coating were near polyhedron in shape, and a large number of micro-cracks occurred among the crystals, which provided space for thermal expansion and contraction [26,27], which partially lowered the thermal stress [28]. These micro-cracks also provided channels for the diffusion of oxygen. A small amount of Al2O3 was present at the coating interface after the 7
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Fig. 10. The EPMA image of the elements distribution at the cross-sectional of TBCs during the thermal shock at 1100 °C.
polyhedrons, and a large number of micro-cracks were present among the crystals. The phases of the coatings are mainly metastable tetragonal phases (t′), and the transformation of the phase (t′) to monoclinic phase (m′) was not found. (2) The spallation of the 8YSZ coating in a wide area occurred after 86 thermal shock cycles at 1100 °C. It was caused by the growth of the TGO layer and the movement of yttrium toward the middle of the coating from the interfaces (8YSZ/CoCrAlYTaSi, CoCrAlYTaSi/ GH202).
[32]. It is beneficial to improve the service life of the coating. 4. Conclusions The thermal shock behavior of a thermal barrier coating prepared by plasma spray at 1100 °C was investigated, which produced the following conclusions: (1) The crystals of the 8YSZ coating prepared by plasma spraying were 8
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Fig. 11. The schematic of the thermal shock behavior of the TBCs at 1100 °C.
(3) Oxygen diffusing into the coatings was absorbed by tantalum, which delayed the growth of TGO in the interface between the CoCrAlYTaSi and substrate, and it reduced the deformation caused by oxide overgrowth and prolonged their service life.
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Declaration of competing interest We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled, “Thermal shock behavior of a 8YSZ/CoCrAlYTaSi thermal sprayed barrier coating on GH202 superalloy”. Acknowledgment This work is supported by Natural Science Foundation of Jiangsu Province (BK20191204), Natural Science Foundation of Jiangsu Province for universities and colleges (19KJB430031), Nantong science and technology project (GY12018032) and Oinglan Project of Jiangsu Province. References [1] C.M. Sim, H.S. Oh, T.J. Kim, et al., Detecting internal hot corrosion of in-service turbine blades using neutron tomography with Gd tagging, J. Nondestruct. Eval. 33 (2014) 493–503. [2] P. Mechnich, W. Braue, J. Smialek, Solid-state CMAS corrosion of an EB-PVD YSZ coated turbine blade: Zr\r, 4+\r, partitioning and phase evolution, J. Am. Ceram. Soc. 98 (2015) 296–302. [3] X.X. Ling, Y.Z. Wang, X. Wang, et al., Numerical study of effect of pore microstructure of layered thermal barrier coatings on thermal insulation performance, Chin. J. Nonferrous Metals 25 (2015) 408–414. [4] K.M. Doleker, H. Ahlatci, A.C. Karaoglanli, Investigation of isothermal oxidation behavior of thermal barrier coatings (TBCs) consisting of YSZ and multilayered YSZ/Gd2Zr2O7 ceramic layers, Oxid. Metals 88 (2017) 1–11. [5] M.G. Gok, G. Goller, Microstructural characterization of GZ/CYSZ thermal barrier coatings after thermal shock and CMAS+hot corrosion test, J. Eur. Ceram. Soc. 37 (2017) 2501–2508.
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