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Al2O3 composite tritium permeation barrier coating on surface of 316L stainless steel
Surface & Coatings Technology 383 (2020) 125282
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Preparation and properties of FeAl/Al2O3 composite tritium permeation barrier coating on surface of 316L stainless steel ⁎
Jun Huanga, Hao Xiea, Lai–Ma Luoa,c,d, , Xiang Zana,c, Dong–Guang Liub, Yu–Cheng Wua,c,d,
T ⁎
a
School of Materials Science and Engineering, Hefei University of Technology, Hefei 230009, China Industry & Equipment Technology, Hefei University of Technology, Hefei 230009, China c National-Local Joint Engineering Research Centre of Nonferrous Metals and Processing Technology, Hefei 230009, China d Research Centre for Powder Metallurgy Engineering and Technology of Anhui Province, Hefei 230009, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Tritium permeation barrier FeAl/Al2O3 composite coating Pack cementation Bonding strength Hydrogen resistance performance
In fusion reactors, preparing barrier coatings on the surface of steel structures is one of the most effective ways to reduce the penetration of hydrogen and its isotopes. An FeAl/Al2O3 composite tritium permeation barrier is a good choice for the current tritium permeation prevention technology because it has low enthalpy permeability, small thermal mismatch, good compatibility with lithium lead, and metallurgical bonding. In this study, FeAl/ Al2O3 composite tritium permeation barrier coating was successfully prepared on the surface of 316L stainless steel by embedding method combined with in-situ thermal oxidation. By using test methods, such as TEM, SEM, EDS, and XRD, we found that the FeAl alloy transition layer was successfully formed on the surface of the aluminized matrix, thereby effectively relieving the thermal mismatch between the Al2O3 coating and the substrate. The results showed that the thickness of the aluminum-rich layer and the FeAl transition layer on the surface of the substrate were approximately 32 μm and 8 μm, respectively. In addition, α-Al2O3 layer formed on the surface of the aluminum-rich layer after oxidation was approximately 4 μm thick. According to the scratch test, the bond strength of the coating showed that the adhesion of the FeAl/Al2O3 composite coating was 52.4 N. The hydrogen permeation test results showed that the FeAl/Al2O3 composite coating remarkably improved the hydrogen barrier properties of the 316L stainless steel matrix.
1. Introduction 316L stainless steels are widely used in fields of hydrogen storage devices, vacuum solar energy receivers and fusion reactors etc. [1]. However, in a reactor environment, hydrogen and its isotopes are the main fuels for fusion reaction, among which, tritium has certain radioactivity and activity, strong dispersal ability for structural materials, which can easily cause leakage, fuel loss, material embrittlement, and radioactive contamination [2–4]. Preparing a tritium permeation barrier on the surface of steel structural materials is one of the most effective ways to reduce the penetration of hydrogen and its isotopes [5,6]. At present, ceramic coating is the first choice for tritium permeation barrier coating because it can reduce hydrogen isotope penetration under high temperature and neutron irradiation [7,8]. Oxide ceramic coating (Al2O3, Cr2O3, Er2O3, Y2O3, SiO2, and ZrO2) [9–15], titaniumbased ceramic coating (TiN, TiC, and TiN/TiC) [16], silicide coating (SiC, Si3N4, and SiC/Si3N4) [17–19], aluminide coating (FeAl/Al2O3
⁎
and AlN) [20–24], and their composites [25–29] have excellent properties such as low tritium permeability, high strength, and high temperature resistance. Among these properties, Al2O3 coatings is a typical representative of protective coatings [30], tritium permeation resistance factor of the Al2O3 coating is much higher than that of other materials. Furthermore, resistivity is high and thermodynamic stability is good. The coating is compatible with lithium lead and has excellent comprehensive performance. However, the oxide ceramic coating generally has a thermal mismatch with the matrix, and the coefficient of thermal expansion between the metal matrix and oxide ceramic coating is hugely different, thereby causing the coating to peel off easily [31,32]. The current mainstream solution to this problem is to form a gradient layer of transition between the substrate and the coating. The FeAl alloy transition layer in the FeAl/Al2O3 coating system can alleviate the thermal mismatch between the Al2O3 coating and the matrix [20,33]. In a fusion reactor, the tritium permeation barrier coating is usually prepared on the surface or inner wall of a large complex-shaped
Corresponding authors at: School of Materials Science and Engineering, Hefei University of Technology, Hefei 230009, China. E-mail addresses: [email protected] (L. Luo), [email protected] (Y. Wu).
https://doi.org/10.1016/j.surfcoat.2019.125282 Received 7 September 2019; Received in revised form 16 December 2019; Accepted 17 December 2019 Available online 19 December 2019 0257-8972/ © 2019 Elsevier B.V. All rights reserved.
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structural vessel or pipe [6,34]. Therefore, selecting a suitable barrier coating preparation process according to the production conditions is necessary. A simple embedding process can be performed to coat a workpiece with a complex shape, and the coating is thick and uniform with a strong adhesive force [35–37]. First, the aluminum and iron atoms in the steel matrix were mutually diffused on the steel surface to form a transition layer of FeAl alloy, and then combined with in-situ thermal oxidation to selectively oxidize the surface of the transition layer to form an Al2O3 film [38]. Al2O3 can have various phase structures (α, δ, γ, and θ) in which the steady-state α-Al2O3 is granular and the formed oxide film is dense, thereby effectively preventing tritium permeation and ensuring excellent capacity of the tritium barrier [39]. Reduced Activation Ferrite Martensite (RAFM) is expected to be used in future fusion reactors. 316 L stainless steel is also an important nuclear fusion structural material [40]. In this study, 316L stainless steel is used as the substitute of RAFM steel. The FeAl/Al2O3 tritium permeation barrier coating was prepared by in-situ thermal oxidation on the surface of 316L stainless steel. Effects of different preparation temperatures on the morphology of the embedded layer were studied. Moreover, a systematic analysis of the embedded layer and coating was conducted, and the phase structure and bonding strength and hydrogen barrier properties of the coating were tested.
Fig. 1. Electrochemical hydrogen permeation barrier test experimental schematic.
2.2. Characterization Tritium is a tightly controlled radioactive military resource, and hydrogen is usually used instead of testing [25]. As shown in Fig. 1, the electrochemical hydrogen permeation experiment adopts the Devanathan and Stachurski double electrolyzer model [41]. 0.2 mol/L NaOH was used as electrolyte in both cathode chamber and anode chamber, and the sample was sandwiched between the cathode chamber and anode chamber. At the beginning of the experiment, the voltage of the anode chamber was controlled at 0.115 V, and after the background current dropped to a stable value, the cathode chamber was applied with 10 mA/cm2 constant current of hydrogen charging. The hydrogen reduced in the cathode chamber penetrates the sample into the anode chamber, and the hydrogen entering the anode chamber is oxidized, and the corresponding current data is recorded through the connected computer. The current density increases to the steady state value with the increase of time. The current density decreases when the hydrogen charge constant current in the cathode chamber is closed. The difference between the steady state value and the initial point of hydrogen charge reflects the hydrogen permeation barrier property of the sample. An SU8020 field emission scanning electron microscope (SEM) combined with an EDS accessory was used to detect the microstructure and composition distribution of the sample before and after oxidation. An X'Pert PRO MPD X-ray diffractometer (XRD) was used to analyze the phase composition of the sample before and after oxidation. The crosssection morphology, structure, and interface of the seepage layer were analyzed by a JEM-2100F field emission transmission electron microscope (TEM). The bonding strength of the coating was analyzed by a WS-2005-type coating adhesion automatic scratch tester. The hydrogen barrier performance of the coating was analyzed by a self-made electrochemical hydrogen permeation device.
2. Experimental materials and methods 2.1. FeAl/Al2O3 coating preparation In this study, the base material used in this experiment was 316L stainless steel, and its chemical composition is shown in Table 1 [8]. The sample size was Φ20 mm × 1 mm. The sample was polished with 120#, 320#, 600#, and 800# metallographic sandpaper step by step, and then ultrasonically cleaned and dried. The ratio of the embedding penetrant in terms of mass percentage was 30% Al powder, 5% NaF powder, and 65% Al2O3 powder. The Al2O3 powder could prevent adhesion in the embedding penetrant, and the powder purity was not less than 99.0%. After being configured, the powder was placed in a planetary ball mill to be uniformly mixed and refined. The speed of the ball mill was 300 rpm, the ball-to-material ratio was 10:1, and the milling time was 8 h. The experiment includes aluminizing and oxidation. First, the sample was embedded in an ark with infiltrant, and the ark was sealed and placed in a high-temperature tubular furnace that had been pumped to a vacuum. Argon gas was introduced into the furnace to atmospheric pressure, and then the furnace temperature was raised to a certain level with a heating rate of 5°/min. Three temperatures were selected for this experiment: 750, 850, and 950 °C. After being kept in the furnace for 5 h, the sample was cooled to room temperature under argon atmosphere. The in-situ thermal oxidation experiment was conducted on the samples. The aluminized sample was placed in an ark in a high-temperature tubular furnace. After the tube furnace was evacuated to a vacuum, argon gas was introduced into the furnace to atmospheric pressure, and the furnace temperature was raised to 850 °C with a heating rate of 5°/min. Then, 20 vol% of O2 and 80 vol% of N2 were introduced into the furnace, and the mixture was kept in this atmosphere for 2 h. After heat preservation, the furnace was cooled to room temperature under the protection of an argon atmosphere. The sample was ultrasonically cleaned and dried to remove loose particles on the surface.
3. Experimental results and discussion 3.1. Organization and structure of embedded Al layer Fig. 2 shows the surface morphology and corresponding EDS spectra of three types of embedded aluminized samples at different preparation temperatures. Fig. 2(a) shows the sample prepared at 750 °C × 5 h, Fig. 2(b) shows the sample at 850 °C × 5 h, and Fig. 2(c) shows the sample at 950 °C × 5 h. As the temperature increased, the Al content on the surface of the layer also increased. Many experiments show that with the increase of temperature, the aluminum content on the surface of aluminum layer also increased, and more and more holes appeared on the surface. The surface of samples prepared at 750 °C × 5 h and 850 °C × 5 h were relatively flat with no cracks. The surface of samples prepared at 950 °C × 5 h had more holes, which may be caused by higher temperature. Pits, cracks and holes will increase the surface roughness of the sample, which is not conducive to the preparation of
Table 1 Chemical composition of the 316L stainless steel [8]. Element
Ni
Cr
Mo
Mn
Fe
Total
Wt%
10.30
17.21
2.33
1.23
68.93
100
2
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Fig. 2. SEM morphology of 316L stainless steel surface and energy spectrum of permeable Al layer at different temperatures: (a) 750 °C × 5 h, (b) 850 °C × 5 h, and (c) 950 °C × 5 h.
the subsequent oxide films [42]. Therefore, the sample prepared at 850 °C × 5 h with high Al content and better surface of permeable layer was used for the subsequent oxidation experiment and characterization. A physical diagram of the aluminized sample is shown in Fig. 3. Fig. 4 presents the surface XRD pattern of an embedded aluminized sample at three different preparation temperatures. As shown in the figure, the main phase component of the sample layer at the three preparation temperatures was Al5FeNi, indicating that the aluminum had diffused from the exterior to the interior of the layer. Furthermore, the inward diffusion of aluminum was expected for high-activity embedding aluminizing, which was similar to the literature [43]. Fig. 5 shows the cross-sectional morphology of the embedded aluminized sample prepared at 850 °C × 5 h and the corresponding element line scan, the cross-section of the infiltrated layer had no obvious
defects. Fig. 5(a) shows a cross-sectional shape of the infiltrated layer consisting of ① a rich Al region, ② a diffusion transition region, and ③ a 316L stainless steel matrix along the white arrow from top to bottom. The Al-rich region was approximately 32 μm thick, and the diffusion region was approximately 8 μm thick. Metallurgical bonding is formed between aluminizing layer and transition layer and between aluminizing layer and 316 L stainless steel substrate [37]. Fig. 5(b) shows an enlarged view of the cross-sectional shape of the infiltrated layer, which corresponds to the position of the white rectangular frame in Fig. 5(a). Fig. 5(c) presents an elemental line scan diagram of the cross-section of the infiltrated layer. The line scan direction is consistent with the direction of the white arrow in Fig. 5(a). According to the EDS results, Al element diffused more in region ① and Fe element was less, corresponding to ① a rich Al region in Fig. 5(a). In region ②, the diffusion 3
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Fig. 3. Real object diagram of aluminized layer prepared on surface of 316L stainless steel at 850 °C for 5 h.
Fig. 4. Surface XRD of 316L stainless steel at different temperatures: 750 °C × 5 h, 850 °C × 5 h, and 950 °C × 5 h.
degree and content of Al decreased, while Fe gradually increased, corresponding to ② a diffusion transition region in Fig. 5(a). In the region ③, Al element could not spread to this position, Al element content decreased to 0, and Fe content increased to a stable level, corresponding to ③ a 316L stainless steel matrix in Fig. 5(a). Therefore, the cross section seepage layer presents a three-layer structure as shown in Fig. 5(a). Fig. 6 presents a cross-sectional TEM image of the embedded aluminized sample prepared at 850 °C × 5 h. Fig. 6(a) shows a crosssectional TEM image of the low-permeability layer; regions ①, ②, and ③ correspond to the ① Al-rich region, ② diffusion transition region, and ③ 316L stainless steel matrix in Fig. 5, respectively. Fig. 6(b) provides an electron diffraction diagram of the diffusion transition region in the cross-section of the infiltrated layer, and the diffraction point is on the Al5Fe2 surface of (112), (021), and (020). This result indicates that the main phase component in the diffusion transition region was Al5Fe2, which was consistent with the literature [44]. Fig. 6(c) shows a highresolution image of the Al-rich region, the diffusion transition region, and their interfaces at the cross-section of the layer. As shown in Fig. 6(c), the diffusion transition region was mainly Al5Fe2 phase (400) surface, which had a high Al diffusivity, and Al deposited on the surface of the coating diffused rapidly on the Al5Fe2 layer and reacted with Fe in the matrix at the coating/substrate interface. Therefore, the Al5Fe2 transition layer was formed by the inward diffusion of Al [45].
Fig. 5. 316L stainless steel surface (850 °C × 5 h) cross-section morphology SEM and element line scanning diagram: (a) cross-section topography of infiltrated layer, (b) section topography magnification of infiltrated layer, and (c) cross-section element line scanning diagram of infiltrated layer.
3.2. Organization and structure of 316 L-FeAl/Al2O3 composite coating Fig. 7 shows the surface morphology, cross-sectional morphology, and EDS spectrum of an embedded aluminized sample prepared at 850 °C × 5 h after in-situ thermal oxidation at 850 °C × 2 h. As shown by the surface topography of the coating in Fig. 7(a), the coating was uniform and dense with no obvious crack holes, and the surface of the coating was granular α-Al2O3 [46]. Fig. 7(b) presents the cross-sectional morphology of the coating and the EDS point scan of the Al2O3 layer. A α-Al2O3 layer with thickness of approximately 4 μm was formed on the surface of the 316L stainless steel. The coating was uniformly prepared on the surface of the 316L stainless steel and bonded well to the matrix. Fig. 8 shows an XRD pattern on the surface of the coating. After oxidation at 850 °C × 2 h, the surface of the sample consisted of a FeAl diffusion layer and an α-Al2O3 layer, which was consistent with the literature [47]. The composite coating
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Fig. 8. XRD diagram of 316L-FeAl/Al2O3 composite coating.
Fig. 9. Bonding strength of 316L-FeAl/Al2O3 composite coating.
Fig. 6. 316L stainless steel surface (850 °C × 5 h) cross-section TEM diagram: (a) low osmotic layer cross-section TEM diagram; (b) infiltrated layer section Al-rich zone, transition zone, and high-resolution image; and (c) infiltrated layer section transition zone electron diffraction pattern at interface.
Fig. 7. 316L-FeAl/Al2O3 composite coating surface morphology, cross-section morphology, and EDS energy spectrum. 5
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large abrupt peaks appeared on the acoustic signal curve, indicating that the peeling degree of the coating was increasing. Finally, the acoustic signal curve reached the peak value and tended to be stable, indicating that the coating had completely fallen off at this time. Fig. 10 provides a topographical view of the coating surface. Fig. 10(a) shows that the depth and width of the scratch gradually increased from the starting point of the scratch along its direction. When the load was 52.4 N, the coating began to peel off, and when the scratch end point was reached, the coating peeled off completely. Fig. 10(b) presents an enlarged view of the peeling-off region shown in Fig. 10(a). The scratched edge coating had a small number of cracks with no significant damage. 3.4. Hydrogen resistance performance of 316L-FeAl/Al2O3 composite coating Fig. 11(a) shows a hydrogen permeation curve of the 316L stainless steel matrix and the 316L-FeAl/Al2O3 composite coating. Fig. 11(b) presents a hydrogen permeation curve of the 316L-FeAl/Al2O3 composite coating. The steady-state hydrogen permeation current density of the 316L stainless steel matrix was 1.37 × 10−6 A/cm2, and the difference between it and the current density at the beginning of hydrogen charging was 4.64 × 10−7 A/cm2. The steady-state hydrogen permeation current density of the 316L-FeAl/Al2O3 composite coating was 8.41 × 10−7 A/cm2. The difference between it and the current density at the beginning of hydrogen charging was 0.22 × 10−7 A/cm2. The ratio of the steady-state hydrogen permeation current density of the matrix to the composite coating was 1.6:1, the current density difference ratio was 21.1:1. Hydrogen permeation resistance of the coating is closely related with the microstructure of the coating [25]. The 316LFeAl/Al2O3 composite coating significantly improved the hydrogen barrier properties of the 316L stainless steel matrix.
Fig. 10. 316L-FeAl/Al2O3 composite coating scratch SEM diagram: (a) topography of coating surface scratch, and (b) magnification of area where coating begins to peel off.
consisting of the FeAl diffusion layer had a self-repairing mechanism, and α-Al2O3 had excellent tritium barrier properties [38,39].
4. Conclusion 3.3. Bonding strength and scratch morphology of 316 L-FeAl/Al2O3 composite coating
1) The FeAl/Al2O3 composite tritium barrier coating was successfully prepared on the surface of 316L stainless steel through embedded aluminizing combined with in-situ thermal oxidation. The effects of three different temperatures on coating preparation were explored. The prepared coating was uniform and dense without obvious defects, and had a thickness of approximately 4 μm. The surface of the sample consisted of a FeAl diffusion layer with a self-repairing mechanism and an α-Al2O3 layer with excellent tritium barrier properties. 2) The bonding strength of the prepared 316 L-FeAl/Al2O3 composite
The bonding strength of the coating to the substrate is an important performance indicator (such as titanium coatings) [48], which can be expected in Al2O3 coatings. As shown in Fig. 9, the binding force of the 316 L-FeAl/Al2O3 composite coating was determined by the relation between emission intensity and load of acoustic signal for the scratch test. The Fig. 9 showed that when the load reached 52.4 N, the acoustic signal curve exhibited a sudden peak, indicating that the coating began to peel off under this load. With the continuous increase of load, several
Fig. 11. Hydrogen permeation curves: (a) 316L stainless steel matrix, and (b) 316L-FeAl/Al2O3 composite coating. 6
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coating was tested by the scratch method, which was approximately 52.4 N between the coating and the matrix. The microscopic morphology showed a small number of cracks on the edge of the coating, but no obvious damage was observed. 3) Hydrogen permeation test of the prepared 316 L-FeAl/Al2O3 composite coating was performed by using a self-made electrochemical hydrogen permeation device. The results showed that the ratio of the steady-state hydrogen permeation current density of the matrix to the 316 L-FeAl/Al2O3 composite coating was 1.6:1, the current density difference ratio was 21.1:1, and the 316L-FeAl/Al2O3 composite coating significantly improved the hydrogen barrier properties of the 316L stainless steel matrix.
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Author contribution statement Jun Huang and Hao Xie conducted the experiment. Jun Huang, Lai–Ma Luo, and Hao Xie drafted the manuscript. Lai–Ma Luo and Yu–Cheng Wu supervised the experiments. Xiang Zan, Dong–Guang Liu, Lai–Ma Luo, Jun Huang, and Yu–Cheng Wu were involved in the data analysis and discussions. Declaration of competing interest The authors declare no competing financial interests. Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant No. 2017YFE0300304), the Fundamental Research Funds for the Central Universities (Grant No. PA2018GDQT0010 and PA2019GDPK0043), the National Natural Science Foundation of China (Grant No. 21671145), the State Key Laboratory of Powder Metallurgy, the Foundation of Laboratory of Nonferrous Metal Material and Processing Engineering of Anhui Province (Grant No. 15CZS08031), and the 111 Project (Grant No. B18018). References [1] D. He, S. Li, X. Liu, et al., Preparation of Cr2O3 film by MOCVD as hydrogen permeation barrier[J], Fusion Engineering and Design 89 (1) (2014) 35–39. [2] X. Wang, G. Ran, H. Wang, et al., Current progress of tritium fuel cycle technology for CFETR[J], J. Fusion Energ. 38 (2019) 125–137. [3] H. Ushida, K. Katayama, H. Matsuura, et al., Tritium permeation behavior through pyrolytic carbon in tritium production using high-temperature gas-cooled reactor for fusion reactors[J], Nuclear Materials and Energy 9 (2016) 524–528. [4] M. Tamura, T. Eguchi, Nanostructured thin films for hydrogen-permeation barrier [J], J. Vac. Sci. Technol. A 33 (4) (2015) 041503. [5] T. Chikada, M. Shimada, R.J. Pawelko, et al., Tritium permeation experiments using reduced activation ferritic/martensitic steel tube and erbium oxide coating[J], Fusion Engineering and Design 89 (7–8) (2014) 1402–1405. [6] G. Zhang, Y. Lu, X. Wang, Hydrogen interactions with intrinsic point defects in hydrogen permeation barrier of α-Al2O3: a first-principles study[J], Phys. Chem. Chem. Phys. 16 (33) (2014) 17523–17530. [7] V. Nemanic, Hydrogen permeation barriers: basic requirements, materials selection, deposition methods, and quality evaluation[J], Nuclear Materials and Energy 19 (2019) 451–457. [8] Q. Yu, L. Hao, S. Li, et al., Microstructure and deuterium permeation resistance of the oxide scale prepared by initial oxidation method on vacuum solar receiver[J], Solid State Ionics 231 (2013) 5–10. [9] M. Zhang, B. Xu, G. Ling, Preparation and characterization of α-Al2O3 film by low temperature thermal oxidation of Al8Cr5 coating[J], Appl. Surf. Sci. 331 (2015) 1–7. [10] L. Wang, J.J. Yang, Y.J. Feng, et al., Preparation and characterization of Al2O3 coating by MOD method on CLF-1 RAFM steel[J], J. Nucl. Mater. 487 (2017) 280–287. [11] F. Jun, C. Meiyan, T. Honghui, et al., Preparation of alumina coatings as tritium permeation barrier by a composite treatment of low temperature plasma[J], Rare Metal Materials & Engineering 46 (10) (2017) 2837–2841. [12] T. Chikada, S. Naitoh, A. Suzuki, et al., Deuterium permeation through erbium oxide coatings on RAFM steels by a dip-coating technique[J], J. Nucl. Mater. 442 (1–3) (2013) 533–537. [13] Q. Li, J. Wang, Q.-Y. Xiang, et al., Thickness impacts on permeation reduction factor of Er2O3 hydrogen isotopes permeation barriers prepared by magnetron sputtering
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coating by plasma assisted tempering of aluminized P91 steels[J], J. Nucl. Mater. 464 (2015) 73–79. [47] Q. Zhan, H.G. Yang, W.W. Zhao, et al., Characterization of the alumina film with cerium doped on the iron-aluminide diffusion coating[J], J. Nucl. Mater. 442 (1–3) (2013) S603–S606. [48] N.M. Lin, L.L. Zhao, Q.L. Liu, et al., Preparation of titanizing coating on AISI 316 stainless steel by pack cementation to mitigate surface damage: estimations of corrosion resistance and tribological behavior[J], J. Phys. Chem. Solids 129 (2019) 387–400.
oxidation behavior of aluminized AISI 316 stainless steel[J], Mater. Manuf. Process. 31 (1) (2015) 1–8. [44] X.X. Yang Feilong, Z. Guikai, et al., Effect of steel substrates on the formation and deuterium permeation resistance of aluminide coatings[J], Coatings 9 (2) (2019) 95. [45] Y.Q. Wang, Y. Zhang, D.A. Wilson, Formation of aluminide coatings on ferritic–martensitic steels by a low-temperature pack cementation process[J], Surface & Coatings Technology 204 (16) (2010) 2737–2744. [46] N.I. Jamnapara, S. Mukherjee, A.S. Khanna, Phase transformation of alumina
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Preparation and properties of FeAl/Al2O3 composite tritium permeation barrier coating on surface of 316L stainless steel ⁎
Jun Huanga, Hao Xiea, Lai–Ma Luoa,c,d, , Xiang Zana,c, Dong–Guang Liub, Yu–Cheng Wua,c,d,
T ⁎
a
School of Materials Science and Engineering, Hefei University of Technology, Hefei 230009, China Industry & Equipment Technology, Hefei University of Technology, Hefei 230009, China c National-Local Joint Engineering Research Centre of Nonferrous Metals and Processing Technology, Hefei 230009, China d Research Centre for Powder Metallurgy Engineering and Technology of Anhui Province, Hefei 230009, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Tritium permeation barrier FeAl/Al2O3 composite coating Pack cementation Bonding strength Hydrogen resistance performance
In fusion reactors, preparing barrier coatings on the surface of steel structures is one of the most effective ways to reduce the penetration of hydrogen and its isotopes. An FeAl/Al2O3 composite tritium permeation barrier is a good choice for the current tritium permeation prevention technology because it has low enthalpy permeability, small thermal mismatch, good compatibility with lithium lead, and metallurgical bonding. In this study, FeAl/ Al2O3 composite tritium permeation barrier coating was successfully prepared on the surface of 316L stainless steel by embedding method combined with in-situ thermal oxidation. By using test methods, such as TEM, SEM, EDS, and XRD, we found that the FeAl alloy transition layer was successfully formed on the surface of the aluminized matrix, thereby effectively relieving the thermal mismatch between the Al2O3 coating and the substrate. The results showed that the thickness of the aluminum-rich layer and the FeAl transition layer on the surface of the substrate were approximately 32 μm and 8 μm, respectively. In addition, α-Al2O3 layer formed on the surface of the aluminum-rich layer after oxidation was approximately 4 μm thick. According to the scratch test, the bond strength of the coating showed that the adhesion of the FeAl/Al2O3 composite coating was 52.4 N. The hydrogen permeation test results showed that the FeAl/Al2O3 composite coating remarkably improved the hydrogen barrier properties of the 316L stainless steel matrix.
1. Introduction 316L stainless steels are widely used in fields of hydrogen storage devices, vacuum solar energy receivers and fusion reactors etc. [1]. However, in a reactor environment, hydrogen and its isotopes are the main fuels for fusion reaction, among which, tritium has certain radioactivity and activity, strong dispersal ability for structural materials, which can easily cause leakage, fuel loss, material embrittlement, and radioactive contamination [2–4]. Preparing a tritium permeation barrier on the surface of steel structural materials is one of the most effective ways to reduce the penetration of hydrogen and its isotopes [5,6]. At present, ceramic coating is the first choice for tritium permeation barrier coating because it can reduce hydrogen isotope penetration under high temperature and neutron irradiation [7,8]. Oxide ceramic coating (Al2O3, Cr2O3, Er2O3, Y2O3, SiO2, and ZrO2) [9–15], titaniumbased ceramic coating (TiN, TiC, and TiN/TiC) [16], silicide coating (SiC, Si3N4, and SiC/Si3N4) [17–19], aluminide coating (FeAl/Al2O3
⁎
and AlN) [20–24], and their composites [25–29] have excellent properties such as low tritium permeability, high strength, and high temperature resistance. Among these properties, Al2O3 coatings is a typical representative of protective coatings [30], tritium permeation resistance factor of the Al2O3 coating is much higher than that of other materials. Furthermore, resistivity is high and thermodynamic stability is good. The coating is compatible with lithium lead and has excellent comprehensive performance. However, the oxide ceramic coating generally has a thermal mismatch with the matrix, and the coefficient of thermal expansion between the metal matrix and oxide ceramic coating is hugely different, thereby causing the coating to peel off easily [31,32]. The current mainstream solution to this problem is to form a gradient layer of transition between the substrate and the coating. The FeAl alloy transition layer in the FeAl/Al2O3 coating system can alleviate the thermal mismatch between the Al2O3 coating and the matrix [20,33]. In a fusion reactor, the tritium permeation barrier coating is usually prepared on the surface or inner wall of a large complex-shaped
Corresponding authors at: School of Materials Science and Engineering, Hefei University of Technology, Hefei 230009, China. E-mail addresses: [email protected] (L. Luo), [email protected] (Y. Wu).
https://doi.org/10.1016/j.surfcoat.2019.125282 Received 7 September 2019; Received in revised form 16 December 2019; Accepted 17 December 2019 Available online 19 December 2019 0257-8972/ © 2019 Elsevier B.V. All rights reserved.
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structural vessel or pipe [6,34]. Therefore, selecting a suitable barrier coating preparation process according to the production conditions is necessary. A simple embedding process can be performed to coat a workpiece with a complex shape, and the coating is thick and uniform with a strong adhesive force [35–37]. First, the aluminum and iron atoms in the steel matrix were mutually diffused on the steel surface to form a transition layer of FeAl alloy, and then combined with in-situ thermal oxidation to selectively oxidize the surface of the transition layer to form an Al2O3 film [38]. Al2O3 can have various phase structures (α, δ, γ, and θ) in which the steady-state α-Al2O3 is granular and the formed oxide film is dense, thereby effectively preventing tritium permeation and ensuring excellent capacity of the tritium barrier [39]. Reduced Activation Ferrite Martensite (RAFM) is expected to be used in future fusion reactors. 316 L stainless steel is also an important nuclear fusion structural material [40]. In this study, 316L stainless steel is used as the substitute of RAFM steel. The FeAl/Al2O3 tritium permeation barrier coating was prepared by in-situ thermal oxidation on the surface of 316L stainless steel. Effects of different preparation temperatures on the morphology of the embedded layer were studied. Moreover, a systematic analysis of the embedded layer and coating was conducted, and the phase structure and bonding strength and hydrogen barrier properties of the coating were tested.
Fig. 1. Electrochemical hydrogen permeation barrier test experimental schematic.
2.2. Characterization Tritium is a tightly controlled radioactive military resource, and hydrogen is usually used instead of testing [25]. As shown in Fig. 1, the electrochemical hydrogen permeation experiment adopts the Devanathan and Stachurski double electrolyzer model [41]. 0.2 mol/L NaOH was used as electrolyte in both cathode chamber and anode chamber, and the sample was sandwiched between the cathode chamber and anode chamber. At the beginning of the experiment, the voltage of the anode chamber was controlled at 0.115 V, and after the background current dropped to a stable value, the cathode chamber was applied with 10 mA/cm2 constant current of hydrogen charging. The hydrogen reduced in the cathode chamber penetrates the sample into the anode chamber, and the hydrogen entering the anode chamber is oxidized, and the corresponding current data is recorded through the connected computer. The current density increases to the steady state value with the increase of time. The current density decreases when the hydrogen charge constant current in the cathode chamber is closed. The difference between the steady state value and the initial point of hydrogen charge reflects the hydrogen permeation barrier property of the sample. An SU8020 field emission scanning electron microscope (SEM) combined with an EDS accessory was used to detect the microstructure and composition distribution of the sample before and after oxidation. An X'Pert PRO MPD X-ray diffractometer (XRD) was used to analyze the phase composition of the sample before and after oxidation. The crosssection morphology, structure, and interface of the seepage layer were analyzed by a JEM-2100F field emission transmission electron microscope (TEM). The bonding strength of the coating was analyzed by a WS-2005-type coating adhesion automatic scratch tester. The hydrogen barrier performance of the coating was analyzed by a self-made electrochemical hydrogen permeation device.
2. Experimental materials and methods 2.1. FeAl/Al2O3 coating preparation In this study, the base material used in this experiment was 316L stainless steel, and its chemical composition is shown in Table 1 [8]. The sample size was Φ20 mm × 1 mm. The sample was polished with 120#, 320#, 600#, and 800# metallographic sandpaper step by step, and then ultrasonically cleaned and dried. The ratio of the embedding penetrant in terms of mass percentage was 30% Al powder, 5% NaF powder, and 65% Al2O3 powder. The Al2O3 powder could prevent adhesion in the embedding penetrant, and the powder purity was not less than 99.0%. After being configured, the powder was placed in a planetary ball mill to be uniformly mixed and refined. The speed of the ball mill was 300 rpm, the ball-to-material ratio was 10:1, and the milling time was 8 h. The experiment includes aluminizing and oxidation. First, the sample was embedded in an ark with infiltrant, and the ark was sealed and placed in a high-temperature tubular furnace that had been pumped to a vacuum. Argon gas was introduced into the furnace to atmospheric pressure, and then the furnace temperature was raised to a certain level with a heating rate of 5°/min. Three temperatures were selected for this experiment: 750, 850, and 950 °C. After being kept in the furnace for 5 h, the sample was cooled to room temperature under argon atmosphere. The in-situ thermal oxidation experiment was conducted on the samples. The aluminized sample was placed in an ark in a high-temperature tubular furnace. After the tube furnace was evacuated to a vacuum, argon gas was introduced into the furnace to atmospheric pressure, and the furnace temperature was raised to 850 °C with a heating rate of 5°/min. Then, 20 vol% of O2 and 80 vol% of N2 were introduced into the furnace, and the mixture was kept in this atmosphere for 2 h. After heat preservation, the furnace was cooled to room temperature under the protection of an argon atmosphere. The sample was ultrasonically cleaned and dried to remove loose particles on the surface.
3. Experimental results and discussion 3.1. Organization and structure of embedded Al layer Fig. 2 shows the surface morphology and corresponding EDS spectra of three types of embedded aluminized samples at different preparation temperatures. Fig. 2(a) shows the sample prepared at 750 °C × 5 h, Fig. 2(b) shows the sample at 850 °C × 5 h, and Fig. 2(c) shows the sample at 950 °C × 5 h. As the temperature increased, the Al content on the surface of the layer also increased. Many experiments show that with the increase of temperature, the aluminum content on the surface of aluminum layer also increased, and more and more holes appeared on the surface. The surface of samples prepared at 750 °C × 5 h and 850 °C × 5 h were relatively flat with no cracks. The surface of samples prepared at 950 °C × 5 h had more holes, which may be caused by higher temperature. Pits, cracks and holes will increase the surface roughness of the sample, which is not conducive to the preparation of
Table 1 Chemical composition of the 316L stainless steel [8]. Element
Ni
Cr
Mo
Mn
Fe
Total
Wt%
10.30
17.21
2.33
1.23
68.93
100
2
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Fig. 2. SEM morphology of 316L stainless steel surface and energy spectrum of permeable Al layer at different temperatures: (a) 750 °C × 5 h, (b) 850 °C × 5 h, and (c) 950 °C × 5 h.
the subsequent oxide films [42]. Therefore, the sample prepared at 850 °C × 5 h with high Al content and better surface of permeable layer was used for the subsequent oxidation experiment and characterization. A physical diagram of the aluminized sample is shown in Fig. 3. Fig. 4 presents the surface XRD pattern of an embedded aluminized sample at three different preparation temperatures. As shown in the figure, the main phase component of the sample layer at the three preparation temperatures was Al5FeNi, indicating that the aluminum had diffused from the exterior to the interior of the layer. Furthermore, the inward diffusion of aluminum was expected for high-activity embedding aluminizing, which was similar to the literature [43]. Fig. 5 shows the cross-sectional morphology of the embedded aluminized sample prepared at 850 °C × 5 h and the corresponding element line scan, the cross-section of the infiltrated layer had no obvious
defects. Fig. 5(a) shows a cross-sectional shape of the infiltrated layer consisting of ① a rich Al region, ② a diffusion transition region, and ③ a 316L stainless steel matrix along the white arrow from top to bottom. The Al-rich region was approximately 32 μm thick, and the diffusion region was approximately 8 μm thick. Metallurgical bonding is formed between aluminizing layer and transition layer and between aluminizing layer and 316 L stainless steel substrate [37]. Fig. 5(b) shows an enlarged view of the cross-sectional shape of the infiltrated layer, which corresponds to the position of the white rectangular frame in Fig. 5(a). Fig. 5(c) presents an elemental line scan diagram of the cross-section of the infiltrated layer. The line scan direction is consistent with the direction of the white arrow in Fig. 5(a). According to the EDS results, Al element diffused more in region ① and Fe element was less, corresponding to ① a rich Al region in Fig. 5(a). In region ②, the diffusion 3
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Fig. 3. Real object diagram of aluminized layer prepared on surface of 316L stainless steel at 850 °C for 5 h.
Fig. 4. Surface XRD of 316L stainless steel at different temperatures: 750 °C × 5 h, 850 °C × 5 h, and 950 °C × 5 h.
degree and content of Al decreased, while Fe gradually increased, corresponding to ② a diffusion transition region in Fig. 5(a). In the region ③, Al element could not spread to this position, Al element content decreased to 0, and Fe content increased to a stable level, corresponding to ③ a 316L stainless steel matrix in Fig. 5(a). Therefore, the cross section seepage layer presents a three-layer structure as shown in Fig. 5(a). Fig. 6 presents a cross-sectional TEM image of the embedded aluminized sample prepared at 850 °C × 5 h. Fig. 6(a) shows a crosssectional TEM image of the low-permeability layer; regions ①, ②, and ③ correspond to the ① Al-rich region, ② diffusion transition region, and ③ 316L stainless steel matrix in Fig. 5, respectively. Fig. 6(b) provides an electron diffraction diagram of the diffusion transition region in the cross-section of the infiltrated layer, and the diffraction point is on the Al5Fe2 surface of (112), (021), and (020). This result indicates that the main phase component in the diffusion transition region was Al5Fe2, which was consistent with the literature [44]. Fig. 6(c) shows a highresolution image of the Al-rich region, the diffusion transition region, and their interfaces at the cross-section of the layer. As shown in Fig. 6(c), the diffusion transition region was mainly Al5Fe2 phase (400) surface, which had a high Al diffusivity, and Al deposited on the surface of the coating diffused rapidly on the Al5Fe2 layer and reacted with Fe in the matrix at the coating/substrate interface. Therefore, the Al5Fe2 transition layer was formed by the inward diffusion of Al [45].
Fig. 5. 316L stainless steel surface (850 °C × 5 h) cross-section morphology SEM and element line scanning diagram: (a) cross-section topography of infiltrated layer, (b) section topography magnification of infiltrated layer, and (c) cross-section element line scanning diagram of infiltrated layer.
3.2. Organization and structure of 316 L-FeAl/Al2O3 composite coating Fig. 7 shows the surface morphology, cross-sectional morphology, and EDS spectrum of an embedded aluminized sample prepared at 850 °C × 5 h after in-situ thermal oxidation at 850 °C × 2 h. As shown by the surface topography of the coating in Fig. 7(a), the coating was uniform and dense with no obvious crack holes, and the surface of the coating was granular α-Al2O3 [46]. Fig. 7(b) presents the cross-sectional morphology of the coating and the EDS point scan of the Al2O3 layer. A α-Al2O3 layer with thickness of approximately 4 μm was formed on the surface of the 316L stainless steel. The coating was uniformly prepared on the surface of the 316L stainless steel and bonded well to the matrix. Fig. 8 shows an XRD pattern on the surface of the coating. After oxidation at 850 °C × 2 h, the surface of the sample consisted of a FeAl diffusion layer and an α-Al2O3 layer, which was consistent with the literature [47]. The composite coating
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Fig. 8. XRD diagram of 316L-FeAl/Al2O3 composite coating.
Fig. 9. Bonding strength of 316L-FeAl/Al2O3 composite coating.
Fig. 6. 316L stainless steel surface (850 °C × 5 h) cross-section TEM diagram: (a) low osmotic layer cross-section TEM diagram; (b) infiltrated layer section Al-rich zone, transition zone, and high-resolution image; and (c) infiltrated layer section transition zone electron diffraction pattern at interface.
Fig. 7. 316L-FeAl/Al2O3 composite coating surface morphology, cross-section morphology, and EDS energy spectrum. 5
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large abrupt peaks appeared on the acoustic signal curve, indicating that the peeling degree of the coating was increasing. Finally, the acoustic signal curve reached the peak value and tended to be stable, indicating that the coating had completely fallen off at this time. Fig. 10 provides a topographical view of the coating surface. Fig. 10(a) shows that the depth and width of the scratch gradually increased from the starting point of the scratch along its direction. When the load was 52.4 N, the coating began to peel off, and when the scratch end point was reached, the coating peeled off completely. Fig. 10(b) presents an enlarged view of the peeling-off region shown in Fig. 10(a). The scratched edge coating had a small number of cracks with no significant damage. 3.4. Hydrogen resistance performance of 316L-FeAl/Al2O3 composite coating Fig. 11(a) shows a hydrogen permeation curve of the 316L stainless steel matrix and the 316L-FeAl/Al2O3 composite coating. Fig. 11(b) presents a hydrogen permeation curve of the 316L-FeAl/Al2O3 composite coating. The steady-state hydrogen permeation current density of the 316L stainless steel matrix was 1.37 × 10−6 A/cm2, and the difference between it and the current density at the beginning of hydrogen charging was 4.64 × 10−7 A/cm2. The steady-state hydrogen permeation current density of the 316L-FeAl/Al2O3 composite coating was 8.41 × 10−7 A/cm2. The difference between it and the current density at the beginning of hydrogen charging was 0.22 × 10−7 A/cm2. The ratio of the steady-state hydrogen permeation current density of the matrix to the composite coating was 1.6:1, the current density difference ratio was 21.1:1. Hydrogen permeation resistance of the coating is closely related with the microstructure of the coating [25]. The 316LFeAl/Al2O3 composite coating significantly improved the hydrogen barrier properties of the 316L stainless steel matrix.
Fig. 10. 316L-FeAl/Al2O3 composite coating scratch SEM diagram: (a) topography of coating surface scratch, and (b) magnification of area where coating begins to peel off.
consisting of the FeAl diffusion layer had a self-repairing mechanism, and α-Al2O3 had excellent tritium barrier properties [38,39].
4. Conclusion 3.3. Bonding strength and scratch morphology of 316 L-FeAl/Al2O3 composite coating
1) The FeAl/Al2O3 composite tritium barrier coating was successfully prepared on the surface of 316L stainless steel through embedded aluminizing combined with in-situ thermal oxidation. The effects of three different temperatures on coating preparation were explored. The prepared coating was uniform and dense without obvious defects, and had a thickness of approximately 4 μm. The surface of the sample consisted of a FeAl diffusion layer with a self-repairing mechanism and an α-Al2O3 layer with excellent tritium barrier properties. 2) The bonding strength of the prepared 316 L-FeAl/Al2O3 composite
The bonding strength of the coating to the substrate is an important performance indicator (such as titanium coatings) [48], which can be expected in Al2O3 coatings. As shown in Fig. 9, the binding force of the 316 L-FeAl/Al2O3 composite coating was determined by the relation between emission intensity and load of acoustic signal for the scratch test. The Fig. 9 showed that when the load reached 52.4 N, the acoustic signal curve exhibited a sudden peak, indicating that the coating began to peel off under this load. With the continuous increase of load, several
Fig. 11. Hydrogen permeation curves: (a) 316L stainless steel matrix, and (b) 316L-FeAl/Al2O3 composite coating. 6
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coating was tested by the scratch method, which was approximately 52.4 N between the coating and the matrix. The microscopic morphology showed a small number of cracks on the edge of the coating, but no obvious damage was observed. 3) Hydrogen permeation test of the prepared 316 L-FeAl/Al2O3 composite coating was performed by using a self-made electrochemical hydrogen permeation device. The results showed that the ratio of the steady-state hydrogen permeation current density of the matrix to the 316 L-FeAl/Al2O3 composite coating was 1.6:1, the current density difference ratio was 21.1:1, and the 316L-FeAl/Al2O3 composite coating significantly improved the hydrogen barrier properties of the 316L stainless steel matrix.
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Author contribution statement Jun Huang and Hao Xie conducted the experiment. Jun Huang, Lai–Ma Luo, and Hao Xie drafted the manuscript. Lai–Ma Luo and Yu–Cheng Wu supervised the experiments. Xiang Zan, Dong–Guang Liu, Lai–Ma Luo, Jun Huang, and Yu–Cheng Wu were involved in the data analysis and discussions. Declaration of competing interest The authors declare no competing financial interests. Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant No. 2017YFE0300304), the Fundamental Research Funds for the Central Universities (Grant No. PA2018GDQT0010 and PA2019GDPK0043), the National Natural Science Foundation of China (Grant No. 21671145), the State Key Laboratory of Powder Metallurgy, the Foundation of Laboratory of Nonferrous Metal Material and Processing Engineering of Anhui Province (Grant No. 15CZS08031), and the 111 Project (Grant No. B18018). References [1] D. He, S. Li, X. Liu, et al., Preparation of Cr2O3 film by MOCVD as hydrogen permeation barrier[J], Fusion Engineering and Design 89 (1) (2014) 35–39. [2] X. Wang, G. Ran, H. Wang, et al., Current progress of tritium fuel cycle technology for CFETR[J], J. Fusion Energ. 38 (2019) 125–137. [3] H. Ushida, K. Katayama, H. Matsuura, et al., Tritium permeation behavior through pyrolytic carbon in tritium production using high-temperature gas-cooled reactor for fusion reactors[J], Nuclear Materials and Energy 9 (2016) 524–528. [4] M. Tamura, T. Eguchi, Nanostructured thin films for hydrogen-permeation barrier [J], J. Vac. Sci. Technol. A 33 (4) (2015) 041503. [5] T. Chikada, M. Shimada, R.J. Pawelko, et al., Tritium permeation experiments using reduced activation ferritic/martensitic steel tube and erbium oxide coating[J], Fusion Engineering and Design 89 (7–8) (2014) 1402–1405. [6] G. Zhang, Y. Lu, X. Wang, Hydrogen interactions with intrinsic point defects in hydrogen permeation barrier of α-Al2O3: a first-principles study[J], Phys. Chem. Chem. Phys. 16 (33) (2014) 17523–17530. [7] V. Nemanic, Hydrogen permeation barriers: basic requirements, materials selection, deposition methods, and quality evaluation[J], Nuclear Materials and Energy 19 (2019) 451–457. [8] Q. Yu, L. Hao, S. Li, et al., Microstructure and deuterium permeation resistance of the oxide scale prepared by initial oxidation method on vacuum solar receiver[J], Solid State Ionics 231 (2013) 5–10. [9] M. Zhang, B. Xu, G. Ling, Preparation and characterization of α-Al2O3 film by low temperature thermal oxidation of Al8Cr5 coating[J], Appl. Surf. Sci. 331 (2015) 1–7. [10] L. Wang, J.J. Yang, Y.J. Feng, et al., Preparation and characterization of Al2O3 coating by MOD method on CLF-1 RAFM steel[J], J. Nucl. Mater. 487 (2017) 280–287. [11] F. Jun, C. Meiyan, T. Honghui, et al., Preparation of alumina coatings as tritium permeation barrier by a composite treatment of low temperature plasma[J], Rare Metal Materials & Engineering 46 (10) (2017) 2837–2841. [12] T. Chikada, S. Naitoh, A. Suzuki, et al., Deuterium permeation through erbium oxide coatings on RAFM steels by a dip-coating technique[J], J. Nucl. Mater. 442 (1–3) (2013) 533–537. [13] Q. Li, J. Wang, Q.-Y. Xiang, et al., Thickness impacts on permeation reduction factor of Er2O3 hydrogen isotopes permeation barriers prepared by magnetron sputtering
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