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Research of in situ modified PS-PVD thermal barrier coating against CMAS (CaO–MgO–Al2O3–SiO2) corrosion
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Research of in situ modified PS-PVD thermal barrier coating against CMAS (CaO–MgO–Al2O3–SiO2) corrosion Jinbing Songa,b, Xiaofeng Zhanga,c,n, Chunming Denga,b, Changguang Denga,b, Min Liua,b, Kesong Zhoua,b, Xin Tonga,b a
National Engineering Laboratory for Modern Materials Surface Engineering Technology, Guangzhou 510650, China b Guangzhou Research Institute of Non-ferrous Metals, Guangzhou 510650, China c School of Materials Science and Engineering, South China University of Technology, Guangzhou 510640, China Received 4 August 2015; received in revised form 10 September 2015; accepted 20 October 2015
Abstract Feather-like columnar structured 7YSZ thermal barrier coating (TBC) was prepared by plasma spray-physical vapor deposition (PS-PVD). An aluminum (Al) film was deposited on the TBC surface by magnetron sputtering. The Al-deposited TBC was vacuum heat-treated at 665 1C, 808 1C and 900 1C for 1 h separately. Then an α-Al2O3 layer was observed on the top of 7YSZ coating, which resulted from in situ reaction of Al and ZrO2 at high temperature under vacuum condition. The microstructure evolution and phase composition of the TBC were characterized by FE-SEM and XRD respectively. The CMAS (CaO–MgO–Al2O3–SiO2) corrosion property of as-sprayed and treated Al-deposited TBC was compared, which was conducted at 1200 1C for 24 h. The results show that the Al-deposited TBC after vacuum heat treatment have better CMAS corrosion resistance than the as-sprayed TBC. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: Thermal barrier coating; PS-PVD; Aluminum; In situ synthesis; CMAS corrosion
1. Introduction Thermal barrier coating (TBC) is essential for hotcomponents in gas turbine engine and it has been widely used as an effective measure to improve the high temperature resistance of hot components in the aircraft and land based engine [1,2]. To increase the operation temperature of the hot components, two technologies are widely used to prepare ceramic TBC system, namely electron beam-physical vapor deposition (EB-PVD) and atmospheric plasma spraying (APS), which have been applied for over 30 years in aircraft engine manufacture [3]. Since the 1980s, a new plasma spray technology, low pressure plasma spray (LPPS) has been n Corresponding author at: School of Materials Science and Engineering, South China University of Technology, Guangzhou 510640, China. Tel.: þ 86 20 37238263; fax: þ86 20 37238531. E-mail address: [email protected] (X. Zhang).
introduced in preparation of bond coating in TBC system to reduce the oxidation of deposition process [4]. Recently, an advanced technology based on LPPS was presented called plasma spray-physical vapor deposition (PS-PVD) [5–7]. As a new process in TBC, PS-PVD is intended to combine the advantages of plasma spray and physical vapor deposition to deposit columnar structured coating. The columnar TBC not only has high deposition rate and cost efficiency, but also has a feather-like structure [8,9]. Feather-like columnar structured 7YSZ (ZrO2–7 wt% Y2O3) coating has lower thermal conductivity around 0.2 W/mK than APS (0.8–1.7 W/mK) and EB-PVD coating (1.5–2.0 W/mK), due to its higher porosity (about 60%) compared with APS (15–25%) and EB-PVD coating (10–20%) [10,11]. In high engine operation temperature, the TBC surface will be covered with a molten CMAS glass deposit which penetrates into the porous TBC, resulting in the loss of strain tolerance and premature failure [12]. Higher porosity for PS-PVD TBC means more amount of
http://dx.doi.org/10.1016/j.ceramint.2015.10.106 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: J. Song, et al., Research of in situ modified PS-PVD thermal barrier coating against CMAS (CaO–MgO–Al2O3–SiO2) corrosion, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.10.106
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CMAS penetrating into ceramic coating. Therefore, CMAS attack is a critical problem for PS-PVD TBC that applied in next-generation gas-turbine engines [13]. Corrosion protection of TBC caused by CMAS has attracted increasing attention in the last years. But the CMAS corrosion of 7YSZ TBC prepared by PS-PVD has rarely been investigated, as opposed to conventional APS and EB-PVD TBC. For these conventional TBCs, various approaches had been proposed as follows: doping of more resistant stabilizers in the ceramic coating, such as Al, Ti and so on [14,15]; laser glazing and re-melting of the ceramic coating surface providing a dense layer to prevent molten CMAS penetration into the porous ceramic coating [16,17]; fabrication of a dense alumina overlay on the surface of the ceramic coating to hinder CMAS corrosion of 7YSZ coating [18,19]. Regretfully, above methods were just applied in conventional APS and EB-PVD TBC instead of PS-PVD TBC. Thus, based on the advantages and disadvantages of proposed methods, a new approach was presented in this paper. An α-Al2O3 layer on top of the PSPVD 7YSZ coating was in situ synthesized through the reaction of Al and ZrO2 after vacuum heat treatment, where the Al was deposited on the surface of 7YSZ coating by magnetron sputtering. This method not only decreases the porosity of the top ceramic coating but also increases the CMAS corrosion resistance. 2. Experimental 2.1. Preparation of TBC The TBCs including bond and ceramic coating were prepared by a Sulzer Metco (now Oerlikon Metco) PS-PVD Multicoating System, as seen in Fig. 1(a). The working pressure, current and power of the system can reach 100 Pa, 3000 A and 180 kW respectively. The materials used for bond and ceramic coating were commercial NiCoCrAlYTa powders (997, Oerlikon-Metco) and ZrO2–7 wt% Y2O3 (7YSZ) powders (6700, Oerlikon-Metco) respectively. The nickel-based superalloy K4169 was used as substrate material. Prior to bond coating, the substrates were degreased and cleaned with petrol and ethanol, followed by grit blasting with alumina under
0.2 MPa. Subsequently, the samples with bond coating were prepared by PS-PVD using a F4 gun (Oerlikon-Metco). The 7YSZ ceramic coatings were then fabricated by a high power O3CP gun (Oerlikon-Metco), whose operation image is shown in Fig. 1(b). Before deposition of the ceramic coating, the bond coating was polished (roughness o 2 μm). The morphologies of the NiCoCrAlYTa and 7YSZ powders are shown in Fig. 2. The deposition parameters of the bond and ceramic coating are presented in Table 1. The prepared TBCs were deposited with Al film on the surface by direct current circular magnetron sputtering (J-1250, Jingzhou Industrial Coating, China). The thermal–physical properties of free-standing Al-deposited 7YSZ coating were analyzed by thermogravimetricdifferential scanning calorimeter analyzer (TG-DSC, STA449C, Netsch) as described in [20,21]. Thus vacuum heat treatment of the Al-deposited TBC samples was carried out at 665, 808 and 900 1C for 1 h separately in a closed furnace ( 2 10 3 Pa). 2.2. CMAS corrosion test The CMAS corrosion tests were carried out on the columnar 7YSZ TBCs prepared by PS-PVD. Vermiculite powders provided from Henan province of China were used as the CMAS material. Vermiculite is a hydrous silicate mineral and its main composition is presented in Table 2, obtained by Xray fluorescence (XRF). Besides, the thermophysical properties including glass transition and melting temperature were attained by TG-DSC with a heating rate of 10 1C/min, as shown in Fig. 3. The glass transition and melting temperature were 1043 and 1190 1C respectively. The vermiculite powders were sprinkled on the surface of 7YSZ coating (0.1 g/mm2). Then, the TBC samples were placed in a furnace kept at 1200 1C for 24 h followed by air cooling. Cross-sectional microstructure and phase transformation of both as-sprayed TBC and treated TBC were characterized by field emissionscanning electron microscope (FE-SEM, Nova-Nono430, FEI, Holland) and X-ray diffraction (XRD, D8-Advance, Bruker) with a step of 0.021 (Cu-Kα, incident angle 31, a 2θ range of 10–901) respectively. Besides, composition diffusion of CMAS in 7YSZ coating after CMAS corrosion test was
Fig. 1. (a) PS-PVD facility at the Guangzhou Research Institute of Non-ferrous Metals, and (b) PS-PVD plasma jet. Please cite this article as: J. Song, et al., Research of in situ modified PS-PVD thermal barrier coating against CMAS (CaO–MgO–Al2O3–SiO2) corrosion, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.10.106
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Fig. 2. Morphologies of (a) NiCoCrAlYTa powders and (b) 7YSZ powders.
Table 1 Parameters of 7YSZ thermal barrier coating by PS-PVD. Materials
Powder size (μm)
NiCoCrAlYTa 15–45 7YSZ 5–22
Power (kW)
Ar (slpm) N2 (slpm) Feed rate (g/ min)
25 127
40 35
8 60
2 15 29
Table 2 Compositions of Vermiculite obtained by XRF. Oxides
CaO
MgO
Al2O3
SiO2
Rest (Fe2O3, Na2O, K2O, etc.)
Weight (wt%)
2
21
17
42
18
Stand-off distance (mm)
Pre-heating temperature (1C)
Chamber pressure (mbar)
340 950
300 850
70 1.5
the 7YSZ coating are presented in Fig. 4(a) and (b) correspondingly. Magnified image of marked area in Fig. 4(a) shows that different size of gap exists between 7YSZ columns and the columns have lots of micropores. In order to in situ synthesize an α-Al2O3 layer on top of the 7YSZ TBC, an Al film was deposited on the coating surface by magnetron sputtering. Fig. 4(c) shows the cross-sectional image of the Aldeposited 7YSZ coating, where magnified image of marked area reveals the interface of Al film and 7YSZ coating. Fig. 4 (d) indicates the surface of the Al film deposited on the columnar structured 7YSZ coating. Based on observation of Fig. 4(c) and (d), it can be inferred that the fine Al grains grown along the original 7YSZ crystal orientation during the deposition of Al film. And the cauliflower structure of TBC has not changed obviously. 3.2. In situ synthesis of α-Al2O3 layer
Fig. 3. TG-DSC of Vermiculite with a heating rate of 10 1C/min.
studied by energy dispersive spectroscopy (EDS, Oxford INCAx-sight 6427).
An Al–ZrO2 system has existed on the top of TBC after deposition of Al film, which will lead to a formation of Al– ZrO2 interface. Based on the thermodynamic data reported in [22-24], the possible reaction in the Al–ZrO2 system and their corresponding G0T can be described as follows: 4Al þ 3ZrO2 2α Al2 O3 þ 3½Zr ð1Þ
3. Results and discussion
ΔG0T ¼ 176; 603 þ 54:63T
3.1. Deposition of Al film on columnar TBC
3Al þ ½Zr—Al3 Zr
Columnar 7YSZ TBCs were prepared by PS-PVD. Feathery-structured cross-section and cauliflower surface of
ΔG0T ¼ 254; 330 þ 41:4T
ð2Þ
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Fig. 4. Microstructure of 7YSZ coating prepared by PS-PVD: (a) as-sprayed cross-section, (b) as-sprayed surface, (c) cross-section after deposition of Al film, and (d) surface after deposition of Al film.
From reaction (1), when ΔG0T ¼ 0, that is 176,603 þ 54.63T¼ 0. So T ¼ 3232 K. In fact, all the temperatures during vacuum heat treatment in the present investigation are less than 3232 K. Hence, ΔG0T o 0 , this indicates that reaction (1) occurred spontaneously. Similarly, reaction (2) can also happen spontaneously. Based on the thermodynamic analysis above, the total reaction in the Al–ZrO2 system can be described by the following reaction: 13Al þ 3ZrO2 ——2α Al2 O3 þ 3Al3 Zr
ð3Þ
According to above analysis, the α-Al2O3 layer can be obtained by the reaction of Al and ZrO2 [20]. Cross-section and surface of the Al-deposited TBC after vacuum heat treatment are shown in Fig. 5(a) and (b) respectively. A dense overlay was observed on the as-sprayed cauliflower surface. Compared with the magnified image of Fig. 4(a), under the dense layer, the microstructure of 7YSZ coating after Almodification has no distinct change for the column still keeps porous structure. Fig. 6(a) shows the cross-sectional element distribution of the Al-deposited columnar 7YSZ coating after vacuum heat treatment indicating the presence of Zr, Al and O. During this treatment, the Al film was melted and infiltrated into the porous ceramic coating when it was heated above the melting
point of 665 1C. So the opening pores and cracks of the top coating would be filled with molten Al resulting in a decrease of ceramic coating porosity. There was an infiltration area in the top ceramic coating with a depth of 40 mm revealed in Fig. 6(b). In the infiltration area, the Al will react with ZrO2 forming α-Al2O3. The phase composition of surface before and after vacuum heat treatment was analyzed. And the phase of the 7YSZ feedstock powder was also tested. Fig. 7(a) shows that the 7YSZ powder is made of M-ZrO2, Tʹ-ZrO2 and C-ZrO2 phase (where M¼ monoclinic, T’ ¼ metastable tetragonal, C ¼ cubic phase). The as-sprayed TBC prepared by PS-PVD was mainly composed of C-ZrO2 and Tʹ-ZrO2 phase, as indicated in Fig. 7 (b). The M-ZrO2 phase had disappeared since the 7YSZ coating was deposited by gas phase transformed from solid powders. Phases of α-Al2O3 and Al3Zr were observed in the Al-deposited TBC after vacuum heat treatment (seen in Fig. 7 (c)) resulting from the in situ synthesis of Al and ZrO2 [15,16]. The formation of α-Al2O3 layer on top of the porous 7YSZ coating is expected to improve the CMAS corrosion resistance. Besides, the simultaneous formed Al3Zr phase has no negative effect on the TBC performances in terms of high melting temperature, as well as physical and chemical stability, etc. [25,26].
Please cite this article as: J. Song, et al., Research of in situ modified PS-PVD thermal barrier coating against CMAS (CaO–MgO–Al2O3–SiO2) corrosion, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.10.106
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Fig. 5. Microstructure of the Al-deposited 7YSZ coating after vacuum heat treatment: (a) cross-section and (b) surface.
Fig. 7. XRD patterns of (a) 7YSZ powder, (b) as-sprayed TBC and (c) Aldeposited TBC after vacuum heat treatment.
Fig. 6. Cross-sectional element distribution of Al-deposited columnar 7YSZ after vacuum heat treatment tested by EDS.
3.3. Comparison of CMAS corrosion resistance The CMAS corrosion resistance of as-sprayed and treated Al-deposited TBC was compared, which was carried out through isothermal heating at 1200 1C for 24 h. Taking vermiculite as the CMAS material, Si was detected in both types of TBCs by EDS. The results indicate that the penetrating amount of CMAS in the as-sprayed 7YSZ coating (Fig. 8 (a)) is far more than that in the treated Al-deposited 7YSZ coating (Fig. 8(b)). Fig. 9(a) and (b) are magnified images of the marked area after polishing shown in Fig. 8, which corresponding to the assprayed and treated Al-deposited TBC respectively. During
CMAS corrosion test, the 7YSZ grain of as-sprayed coating would dissolve in the molten CMAS and separate from the oriented structure into small independent blocks [27]. As opposed to the as-sprayed 7YSZ coating, the top of the treated Al-deposited coating has no obvious change due to an α-Al2O3 overlay which was in situ synthesized on the top of porous 7YSZ coating. The α-Al2O3 overlay has a good CMAS corrosion resistance and acts as a CMAS diffusion barrier. 4. Conclusions In this paper, columnar 7YSZ TBCs were prepared by plasma spray-physical vapor deposition (PS-PVD). In order to improve the CMAS corrosion resistance of the TBC, an αAl2O3 layer on the top of the TBC was in situ synthesized by the reaction of Al and ZrO2 after a vacuum heat treatment at 665, 808 and 900 1C for 1 h separately, where the Al was deposited on the TBC surface by magnetron sputtering. A dense overlay was observed on the surface of TBC after vacuum heat treatment. Under the dense overlay, the coating
Please cite this article as: J. Song, et al., Research of in situ modified PS-PVD thermal barrier coating against CMAS (CaO–MgO–Al2O3–SiO2) corrosion, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.10.106
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Si element EDS line
Si element
EDS line
Fig. 8. Cross-sectional image of PS-PVD TBC after CMAS corrosion testing at 1200 1C for 24 h: (a) as-sprayed TBC and (b) Al-deposited TBC after vacuum heat treatment.
α-Al2O 3overlay
Fig. 9. Cross-section of TBC after CMAS corrosion, (a) as-sprayed TBC, (b) Al-deposited TBC after vacuum heat treatment.
still keeps porous structure and it has no distinct change compared with the as-sprayed TBC. Using silicate mineral vermiculite as the CMAS materials, the CMAS corrosion resistance of the as-sprayed and treated Al-deposited TBCs
was compared through isothermal heating at 1200 1C for 24 h. The results show that the Al-deposited TBC after vacuum heat treatment has better CMAS corrosion resistance than the assprayed TBC.
Please cite this article as: J. Song, et al., Research of in situ modified PS-PVD thermal barrier coating against CMAS (CaO–MgO–Al2O3–SiO2) corrosion, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.10.106
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Acknowledgments This work was supported by the National Basic Research Program of China (“973” program, No. 2012CB625100) and the Guangzhou Science and Technology Program key projects (201510010095). References [1] G.D. Girolamo, C. Blasi, A. Brentari, M. Schioppa, Microstructural, mechanical and thermal characteristics of zirconia-based thermal barrier coatings deposited by plasma spraying, Ceram. Int. 41 (2015) 11776–11785. [2] I. Gurrappa, A.S. Rao, Thermal barrier coatings for enhanced efficiency of gas turbine engines, Surf. Coat. Technol. 201 (2006) 3016-2029. [3] W.R. Chen, L.R. Zhao, Review—volcanic ash and its influence on aircraft engine components, Procedia Eng. 99 (2015) 795–803. [4] R. Vassen, A. Stuke, D. Stover, Recent developments in the field of thermal barrier coatings, J. Therm. Spray Technol. 18 (2009) 181–186. [5] A. Hospach, G. Mauer, R. Vaben, D. Stover, Columnar-structured thermal barrier coatings (TBCs) by thin film low-pressure plasma spraying (LPPS-TF), J. Therm. Spray Technol. 20 (2011) 116–120. [6] K.V. Niessen, M. Gindrat, Plasma spray-PVD: a new thermal spray process to deposit out of the vapor phase, J. Therm. Spray Technol. 20 (2011) 736–743. [7] L.H. Gao, H.B. Guo, L.L. Wei, C.Y. Li, S.K. Gong, H.B. Xu, Microstructure and mechanical properties of yttria stabilized zirconia coatings prepared by plasma spray physical vapor deposition, Ceram. Int. 41 (2015) 8305–8311. [8] G. Mauer, A. Hospach, R. Vaßen, Process development and coating characteristics of plasma spray-PVD, Surf. Coat. Technol. 220 (2013) 219–224. [9] L.H. Gao, H.B. Guo, L.L. Wei, C.Y. Li, H.B. Xu, Microstructure, thermal conductivity and thermal cycling behavior of thermal barrier coatings prepared by plasma spray physical vapor deposition, Surf. Coat. Technol. 276 (2015) 424–460. [10] M. Kambara, A. Shinozawa, K. Aoshika, K. Eguchi, T. Yoshida, Development of porous YSZ coatings with modified thermal and optical properties by plasma spray physical vapor deposition, J. Solid Mech. Mater. Eng. 4 (2010) 95–106. [11] N.P. Padture, M. Gell, E.H. Jordan, Thermal barrier coatings for gasturbine engine applications, Science 296 (2002) 280–284. [12] S. Krämer, J. Yang, C.G. Levi1, C.A. Johnson, Thermochemical interaction of thermal barrier coatings with molten CaO–MgO–Al2O3– SiO2 (CMAS) deposits, J. Am. Ceram. Soc. 89 (2006) 3167–3175. [13] C. Mercera, S. Faulhabera, A.G. Evansa, R. Daroliab, A delamination mechanism for thermal barrier coatings subject to calcium–magnesium– alumino-silicate (CMAS) infiltration, Acta Mater. 53 (2005) 1029–1039.
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CERAMICS INTERNATIONAL
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Research of in situ modified PS-PVD thermal barrier coating against CMAS (CaO–MgO–Al2O3–SiO2) corrosion Jinbing Songa,b, Xiaofeng Zhanga,c,n, Chunming Denga,b, Changguang Denga,b, Min Liua,b, Kesong Zhoua,b, Xin Tonga,b a
National Engineering Laboratory for Modern Materials Surface Engineering Technology, Guangzhou 510650, China b Guangzhou Research Institute of Non-ferrous Metals, Guangzhou 510650, China c School of Materials Science and Engineering, South China University of Technology, Guangzhou 510640, China Received 4 August 2015; received in revised form 10 September 2015; accepted 20 October 2015
Abstract Feather-like columnar structured 7YSZ thermal barrier coating (TBC) was prepared by plasma spray-physical vapor deposition (PS-PVD). An aluminum (Al) film was deposited on the TBC surface by magnetron sputtering. The Al-deposited TBC was vacuum heat-treated at 665 1C, 808 1C and 900 1C for 1 h separately. Then an α-Al2O3 layer was observed on the top of 7YSZ coating, which resulted from in situ reaction of Al and ZrO2 at high temperature under vacuum condition. The microstructure evolution and phase composition of the TBC were characterized by FE-SEM and XRD respectively. The CMAS (CaO–MgO–Al2O3–SiO2) corrosion property of as-sprayed and treated Al-deposited TBC was compared, which was conducted at 1200 1C for 24 h. The results show that the Al-deposited TBC after vacuum heat treatment have better CMAS corrosion resistance than the as-sprayed TBC. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: Thermal barrier coating; PS-PVD; Aluminum; In situ synthesis; CMAS corrosion
1. Introduction Thermal barrier coating (TBC) is essential for hotcomponents in gas turbine engine and it has been widely used as an effective measure to improve the high temperature resistance of hot components in the aircraft and land based engine [1,2]. To increase the operation temperature of the hot components, two technologies are widely used to prepare ceramic TBC system, namely electron beam-physical vapor deposition (EB-PVD) and atmospheric plasma spraying (APS), which have been applied for over 30 years in aircraft engine manufacture [3]. Since the 1980s, a new plasma spray technology, low pressure plasma spray (LPPS) has been n Corresponding author at: School of Materials Science and Engineering, South China University of Technology, Guangzhou 510640, China. Tel.: þ 86 20 37238263; fax: þ86 20 37238531. E-mail address: [email protected] (X. Zhang).
introduced in preparation of bond coating in TBC system to reduce the oxidation of deposition process [4]. Recently, an advanced technology based on LPPS was presented called plasma spray-physical vapor deposition (PS-PVD) [5–7]. As a new process in TBC, PS-PVD is intended to combine the advantages of plasma spray and physical vapor deposition to deposit columnar structured coating. The columnar TBC not only has high deposition rate and cost efficiency, but also has a feather-like structure [8,9]. Feather-like columnar structured 7YSZ (ZrO2–7 wt% Y2O3) coating has lower thermal conductivity around 0.2 W/mK than APS (0.8–1.7 W/mK) and EB-PVD coating (1.5–2.0 W/mK), due to its higher porosity (about 60%) compared with APS (15–25%) and EB-PVD coating (10–20%) [10,11]. In high engine operation temperature, the TBC surface will be covered with a molten CMAS glass deposit which penetrates into the porous TBC, resulting in the loss of strain tolerance and premature failure [12]. Higher porosity for PS-PVD TBC means more amount of
http://dx.doi.org/10.1016/j.ceramint.2015.10.106 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: J. Song, et al., Research of in situ modified PS-PVD thermal barrier coating against CMAS (CaO–MgO–Al2O3–SiO2) corrosion, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.10.106
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CMAS penetrating into ceramic coating. Therefore, CMAS attack is a critical problem for PS-PVD TBC that applied in next-generation gas-turbine engines [13]. Corrosion protection of TBC caused by CMAS has attracted increasing attention in the last years. But the CMAS corrosion of 7YSZ TBC prepared by PS-PVD has rarely been investigated, as opposed to conventional APS and EB-PVD TBC. For these conventional TBCs, various approaches had been proposed as follows: doping of more resistant stabilizers in the ceramic coating, such as Al, Ti and so on [14,15]; laser glazing and re-melting of the ceramic coating surface providing a dense layer to prevent molten CMAS penetration into the porous ceramic coating [16,17]; fabrication of a dense alumina overlay on the surface of the ceramic coating to hinder CMAS corrosion of 7YSZ coating [18,19]. Regretfully, above methods were just applied in conventional APS and EB-PVD TBC instead of PS-PVD TBC. Thus, based on the advantages and disadvantages of proposed methods, a new approach was presented in this paper. An α-Al2O3 layer on top of the PSPVD 7YSZ coating was in situ synthesized through the reaction of Al and ZrO2 after vacuum heat treatment, where the Al was deposited on the surface of 7YSZ coating by magnetron sputtering. This method not only decreases the porosity of the top ceramic coating but also increases the CMAS corrosion resistance. 2. Experimental 2.1. Preparation of TBC The TBCs including bond and ceramic coating were prepared by a Sulzer Metco (now Oerlikon Metco) PS-PVD Multicoating System, as seen in Fig. 1(a). The working pressure, current and power of the system can reach 100 Pa, 3000 A and 180 kW respectively. The materials used for bond and ceramic coating were commercial NiCoCrAlYTa powders (997, Oerlikon-Metco) and ZrO2–7 wt% Y2O3 (7YSZ) powders (6700, Oerlikon-Metco) respectively. The nickel-based superalloy K4169 was used as substrate material. Prior to bond coating, the substrates were degreased and cleaned with petrol and ethanol, followed by grit blasting with alumina under
0.2 MPa. Subsequently, the samples with bond coating were prepared by PS-PVD using a F4 gun (Oerlikon-Metco). The 7YSZ ceramic coatings were then fabricated by a high power O3CP gun (Oerlikon-Metco), whose operation image is shown in Fig. 1(b). Before deposition of the ceramic coating, the bond coating was polished (roughness o 2 μm). The morphologies of the NiCoCrAlYTa and 7YSZ powders are shown in Fig. 2. The deposition parameters of the bond and ceramic coating are presented in Table 1. The prepared TBCs were deposited with Al film on the surface by direct current circular magnetron sputtering (J-1250, Jingzhou Industrial Coating, China). The thermal–physical properties of free-standing Al-deposited 7YSZ coating were analyzed by thermogravimetricdifferential scanning calorimeter analyzer (TG-DSC, STA449C, Netsch) as described in [20,21]. Thus vacuum heat treatment of the Al-deposited TBC samples was carried out at 665, 808 and 900 1C for 1 h separately in a closed furnace ( 2 10 3 Pa). 2.2. CMAS corrosion test The CMAS corrosion tests were carried out on the columnar 7YSZ TBCs prepared by PS-PVD. Vermiculite powders provided from Henan province of China were used as the CMAS material. Vermiculite is a hydrous silicate mineral and its main composition is presented in Table 2, obtained by Xray fluorescence (XRF). Besides, the thermophysical properties including glass transition and melting temperature were attained by TG-DSC with a heating rate of 10 1C/min, as shown in Fig. 3. The glass transition and melting temperature were 1043 and 1190 1C respectively. The vermiculite powders were sprinkled on the surface of 7YSZ coating (0.1 g/mm2). Then, the TBC samples were placed in a furnace kept at 1200 1C for 24 h followed by air cooling. Cross-sectional microstructure and phase transformation of both as-sprayed TBC and treated TBC were characterized by field emissionscanning electron microscope (FE-SEM, Nova-Nono430, FEI, Holland) and X-ray diffraction (XRD, D8-Advance, Bruker) with a step of 0.021 (Cu-Kα, incident angle 31, a 2θ range of 10–901) respectively. Besides, composition diffusion of CMAS in 7YSZ coating after CMAS corrosion test was
Fig. 1. (a) PS-PVD facility at the Guangzhou Research Institute of Non-ferrous Metals, and (b) PS-PVD plasma jet. Please cite this article as: J. Song, et al., Research of in situ modified PS-PVD thermal barrier coating against CMAS (CaO–MgO–Al2O3–SiO2) corrosion, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.10.106
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Fig. 2. Morphologies of (a) NiCoCrAlYTa powders and (b) 7YSZ powders.
Table 1 Parameters of 7YSZ thermal barrier coating by PS-PVD. Materials
Powder size (μm)
NiCoCrAlYTa 15–45 7YSZ 5–22
Power (kW)
Ar (slpm) N2 (slpm) Feed rate (g/ min)
25 127
40 35
8 60
2 15 29
Table 2 Compositions of Vermiculite obtained by XRF. Oxides
CaO
MgO
Al2O3
SiO2
Rest (Fe2O3, Na2O, K2O, etc.)
Weight (wt%)
2
21
17
42
18
Stand-off distance (mm)
Pre-heating temperature (1C)
Chamber pressure (mbar)
340 950
300 850
70 1.5
the 7YSZ coating are presented in Fig. 4(a) and (b) correspondingly. Magnified image of marked area in Fig. 4(a) shows that different size of gap exists between 7YSZ columns and the columns have lots of micropores. In order to in situ synthesize an α-Al2O3 layer on top of the 7YSZ TBC, an Al film was deposited on the coating surface by magnetron sputtering. Fig. 4(c) shows the cross-sectional image of the Aldeposited 7YSZ coating, where magnified image of marked area reveals the interface of Al film and 7YSZ coating. Fig. 4 (d) indicates the surface of the Al film deposited on the columnar structured 7YSZ coating. Based on observation of Fig. 4(c) and (d), it can be inferred that the fine Al grains grown along the original 7YSZ crystal orientation during the deposition of Al film. And the cauliflower structure of TBC has not changed obviously. 3.2. In situ synthesis of α-Al2O3 layer
Fig. 3. TG-DSC of Vermiculite with a heating rate of 10 1C/min.
studied by energy dispersive spectroscopy (EDS, Oxford INCAx-sight 6427).
An Al–ZrO2 system has existed on the top of TBC after deposition of Al film, which will lead to a formation of Al– ZrO2 interface. Based on the thermodynamic data reported in [22-24], the possible reaction in the Al–ZrO2 system and their corresponding G0T can be described as follows: 4Al þ 3ZrO2 2α Al2 O3 þ 3½Zr ð1Þ
3. Results and discussion
ΔG0T ¼ 176; 603 þ 54:63T
3.1. Deposition of Al film on columnar TBC
3Al þ ½Zr—Al3 Zr
Columnar 7YSZ TBCs were prepared by PS-PVD. Feathery-structured cross-section and cauliflower surface of
ΔG0T ¼ 254; 330 þ 41:4T
ð2Þ
Please cite this article as: J. Song, et al., Research of in situ modified PS-PVD thermal barrier coating against CMAS (CaO–MgO–Al2O3–SiO2) corrosion, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.10.106
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Fig. 4. Microstructure of 7YSZ coating prepared by PS-PVD: (a) as-sprayed cross-section, (b) as-sprayed surface, (c) cross-section after deposition of Al film, and (d) surface after deposition of Al film.
From reaction (1), when ΔG0T ¼ 0, that is 176,603 þ 54.63T¼ 0. So T ¼ 3232 K. In fact, all the temperatures during vacuum heat treatment in the present investigation are less than 3232 K. Hence, ΔG0T o 0 , this indicates that reaction (1) occurred spontaneously. Similarly, reaction (2) can also happen spontaneously. Based on the thermodynamic analysis above, the total reaction in the Al–ZrO2 system can be described by the following reaction: 13Al þ 3ZrO2 ——2α Al2 O3 þ 3Al3 Zr
ð3Þ
According to above analysis, the α-Al2O3 layer can be obtained by the reaction of Al and ZrO2 [20]. Cross-section and surface of the Al-deposited TBC after vacuum heat treatment are shown in Fig. 5(a) and (b) respectively. A dense overlay was observed on the as-sprayed cauliflower surface. Compared with the magnified image of Fig. 4(a), under the dense layer, the microstructure of 7YSZ coating after Almodification has no distinct change for the column still keeps porous structure. Fig. 6(a) shows the cross-sectional element distribution of the Al-deposited columnar 7YSZ coating after vacuum heat treatment indicating the presence of Zr, Al and O. During this treatment, the Al film was melted and infiltrated into the porous ceramic coating when it was heated above the melting
point of 665 1C. So the opening pores and cracks of the top coating would be filled with molten Al resulting in a decrease of ceramic coating porosity. There was an infiltration area in the top ceramic coating with a depth of 40 mm revealed in Fig. 6(b). In the infiltration area, the Al will react with ZrO2 forming α-Al2O3. The phase composition of surface before and after vacuum heat treatment was analyzed. And the phase of the 7YSZ feedstock powder was also tested. Fig. 7(a) shows that the 7YSZ powder is made of M-ZrO2, Tʹ-ZrO2 and C-ZrO2 phase (where M¼ monoclinic, T’ ¼ metastable tetragonal, C ¼ cubic phase). The as-sprayed TBC prepared by PS-PVD was mainly composed of C-ZrO2 and Tʹ-ZrO2 phase, as indicated in Fig. 7 (b). The M-ZrO2 phase had disappeared since the 7YSZ coating was deposited by gas phase transformed from solid powders. Phases of α-Al2O3 and Al3Zr were observed in the Al-deposited TBC after vacuum heat treatment (seen in Fig. 7 (c)) resulting from the in situ synthesis of Al and ZrO2 [15,16]. The formation of α-Al2O3 layer on top of the porous 7YSZ coating is expected to improve the CMAS corrosion resistance. Besides, the simultaneous formed Al3Zr phase has no negative effect on the TBC performances in terms of high melting temperature, as well as physical and chemical stability, etc. [25,26].
Please cite this article as: J. Song, et al., Research of in situ modified PS-PVD thermal barrier coating against CMAS (CaO–MgO–Al2O3–SiO2) corrosion, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.10.106
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Fig. 5. Microstructure of the Al-deposited 7YSZ coating after vacuum heat treatment: (a) cross-section and (b) surface.
Fig. 7. XRD patterns of (a) 7YSZ powder, (b) as-sprayed TBC and (c) Aldeposited TBC after vacuum heat treatment.
Fig. 6. Cross-sectional element distribution of Al-deposited columnar 7YSZ after vacuum heat treatment tested by EDS.
3.3. Comparison of CMAS corrosion resistance The CMAS corrosion resistance of as-sprayed and treated Al-deposited TBC was compared, which was carried out through isothermal heating at 1200 1C for 24 h. Taking vermiculite as the CMAS material, Si was detected in both types of TBCs by EDS. The results indicate that the penetrating amount of CMAS in the as-sprayed 7YSZ coating (Fig. 8 (a)) is far more than that in the treated Al-deposited 7YSZ coating (Fig. 8(b)). Fig. 9(a) and (b) are magnified images of the marked area after polishing shown in Fig. 8, which corresponding to the assprayed and treated Al-deposited TBC respectively. During
CMAS corrosion test, the 7YSZ grain of as-sprayed coating would dissolve in the molten CMAS and separate from the oriented structure into small independent blocks [27]. As opposed to the as-sprayed 7YSZ coating, the top of the treated Al-deposited coating has no obvious change due to an α-Al2O3 overlay which was in situ synthesized on the top of porous 7YSZ coating. The α-Al2O3 overlay has a good CMAS corrosion resistance and acts as a CMAS diffusion barrier. 4. Conclusions In this paper, columnar 7YSZ TBCs were prepared by plasma spray-physical vapor deposition (PS-PVD). In order to improve the CMAS corrosion resistance of the TBC, an αAl2O3 layer on the top of the TBC was in situ synthesized by the reaction of Al and ZrO2 after a vacuum heat treatment at 665, 808 and 900 1C for 1 h separately, where the Al was deposited on the TBC surface by magnetron sputtering. A dense overlay was observed on the surface of TBC after vacuum heat treatment. Under the dense overlay, the coating
Please cite this article as: J. Song, et al., Research of in situ modified PS-PVD thermal barrier coating against CMAS (CaO–MgO–Al2O3–SiO2) corrosion, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.10.106
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Si element EDS line
Si element
EDS line
Fig. 8. Cross-sectional image of PS-PVD TBC after CMAS corrosion testing at 1200 1C for 24 h: (a) as-sprayed TBC and (b) Al-deposited TBC after vacuum heat treatment.
α-Al2O 3overlay
Fig. 9. Cross-section of TBC after CMAS corrosion, (a) as-sprayed TBC, (b) Al-deposited TBC after vacuum heat treatment.
still keeps porous structure and it has no distinct change compared with the as-sprayed TBC. Using silicate mineral vermiculite as the CMAS materials, the CMAS corrosion resistance of the as-sprayed and treated Al-deposited TBCs
was compared through isothermal heating at 1200 1C for 24 h. The results show that the Al-deposited TBC after vacuum heat treatment has better CMAS corrosion resistance than the assprayed TBC.
Please cite this article as: J. Song, et al., Research of in situ modified PS-PVD thermal barrier coating against CMAS (CaO–MgO–Al2O3–SiO2) corrosion, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.10.106
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Please cite this article as: J. Song, et al., Research of in situ modified PS-PVD thermal barrier coating against CMAS (CaO–MgO–Al2O3–SiO2) corrosion, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.10.106