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Preparation of tungsten coatings on graphite by electro-deposition via Na2WO4–WO3 molten salt system
Fusion Engineering and Design 89 (2014) 2529–2533
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Preparation of tungsten coatings on graphite by electro-deposition via Na2 WO4 –WO3 molten salt system Ning-bo Sun a , Ying-chun Zhang a,∗ , Fan Jiang a , Shao-ting Lang a , Min Xia a,b a b
School of Materials Science and Engineering, University of Science and Technology Beijing, 30 Xueyuan Road, Haidian District, Beijing 100083, PR China Institute of Powder Metallurgy and Advanced Ceramics, Southwest Jiaotong University, 111, 1st Section, Northern 2nd Ring Road, Chengdu, PR China
h i g h l i g h t s • • • •
Tungsten coatings on graphite were firstly obtained by electro-deposition method via Na2 WO4 –WO3 molten salt system. Uniform and dense tungsten coatings could be easily prepared in each face of the sample, especially the complex components. The obtained tungsten coatings are with high purity, ultra-low oxygen content (about 0.022 wt%). Modulate pulse parameters can get tungsten coatings with different thickness and hardness.
a r t i c l e
i n f o
Article history: Received 28 October 2013 Received in revised form 7 March 2014 Accepted 30 May 2014 Available online 30 June 2014 Keywords: Tungsten coating Graphite Electro-deposition Melts Pulse current density
a b s t r a c t Tungsten coating on graphite substrate is one of the most promising candidate materials as the ITER plasma facing components. In this paper, tungsten coatings on graphite substrates were fabricated by electro-deposition from Na2 WO4 –WO3 molten salt system at 1173 K in atmosphere. Tungsten coatings with no impurities were successfully deposited on graphite substrates under various pulsed current densities in an hour. By increasing the current density from 60 mA cm−2 to 120 mA cm−2 an increase of the average size of tungsten grains, the thickness and the hardness of tungsten coatings occurs. The average size of tungsten grains can reach 7.13 m, the thickness of tungsten coating was in the range of 28.8–51 m, and the hardness of coating was higher than 400 HV. No cracks or voids were observed between tungsten coating and graphite substrate. The oxygen content of tungsten coating is about 0.022 wt%. © 2014 Elsevier B.V. All rights reserved.
1. Introduction It has been discovered that tungsten has a good compatibility with fusion plasma [1] due to its high melting point, good thermal properties and low erosion rate. However, it is counteracted by a number of critical issues, such as poor processing, poor welding performance, high cost and heavy weight. Carbon materials, like graphite and C/C fiber composite (CFC) have been used for plasma facing materials (PFMs), because of their high thermal shock resistance, light weight and high strength. However, one critical issue is high erosion rates of graphite or CFC at elevated temperature. A solution to overcome this issue is tungsten coating of carbon materials like graphite or CFC. This solution has been successfully applied in ASDEX Upgrade [2,3] for many years and recently in the ITER-like Wall at JET [4]. Nowadays the techniques such as
∗ Corresponding author. Tel.: +86 01062334951; fax: +86 01062334951. E-mail address: [email protected] (Y.-c. Zhang). http://dx.doi.org/10.1016/j.fusengdes.2014.05.027 0920-3796/© 2014 Elsevier B.V. All rights reserved.
plasma spray (PS), physical vapor deposition (PVD) and chemical vapor deposition (CVD), combined magnetron sputtering and ion implantation (CMSII) techniques have been applied in tungsten coating of graphite as PFMs [5–9]. However, there are still some disadvantages, for example, detrimental phases is prone to being introduced; the cost is high; the procedures are complicate, etc. Electro-deposition is a viable and promising method for preparing tungsten coatings due to the simple operation and low cost. Furthermore, uniform and dense tungsten coatings could be easily prepared on each face of the sample. Recently, Liu et al. obtained pure tungsten coatings on Al2 O3 –Cu substrates from the Na2 WO4 –WO3 melt [10,11]. It is a great prospect of application, because this molten salt system is not sensitive to oxygen or water, it also has the advantages of simple operation, low cost, low equipment requirement, etc. This stimulated us to study the possibility of fabrication tungsten coatings on graphite. By using this method, tungsten coatings on graphite substrates with high purity, low oxygen content are expected to be prepared.
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Fig. 1. XRD patterns of tungsten coatings obtained at current density of (a) 60 mA cm−2 , (b) 80 mA cm−2 , (c) 100 mA cm−2 , and (d) 120 mA cm−2 .
Fig. 3. Average grain size of tungsten coatings obtained at current density of (a) 60 mA cm−2 , (b) 80 mA cm−2 , (c) 100 mA cm−2 , and (d) 120 mA cm−2 .
In this paper, tungsten coatings on graphite substrates were electro-deposited from the Na2 WO4 –WO3 melt at 1173 K. In order to obtain well defined structures for tungsten coatings, different pulsed current densities were investigated. The study of pulsed current density on tungsten nucleation and its performance were conducted.
of the tungsten coating were analyzed by Energy Dispersive Xray Fluorescence (EDXRF, EDX-720) and X-ray diffraction (XRD, Rigaku Industrial Co., Ltd., D/MAX-RB) with a scanning electron microscope (SEM, JSM 6480LV). Micro-hardness of the coatings was performed by a MH-6 micro-hardness instrument with a loading force of 50 g and loading time of 15 s. The adhesion was detected by nano scratch tests. The oxygen content was measured by the Nitrogen/Oxygen Analyzer (TC600, LECO, USA).
2. Experimental procedure Graphite (IG-430) substrate of 15 mm × 10 mm × 5 mm was coated with tungsten by electro-deposition, using anhydrous Na2 WO4 and WO3 (Na2 WO4 :WO3 = 3:1, by mole ratio) molten salts. The working electrodes (graphite substrates) were polished by mechanical and chemical methods to obtain high quality surface. And counter electrode was a tungsten plate (15 mm × 10 mm × 5 mm). Molten salts were mixed into the eutectic composition in an alumina crucible. Then the crucible was put in an electric furnace and was heated at a rate of 5 ◦ C/min. Electrodeposition was performed at 1173 K using a pulse power supply (HPMCC-5) holding for one hour, the duty cycle and period were 0.8 and 2 ms, respectively. After the deposition, the sample was fetched out and cooled in the air. Subsequently, the samples were ultrasonic-cleaned in a 10 M NaOH solution and deionized water to remove adherent salts. The composition, structure, thickness
3. Results and discussion The coatings were electro-deposited in the Na2 WO4 –WO3 melt at 1173 K for 1 h with different pulse current densities (60 mA cm−2 , 80 mA cm−2 , 100 mA cm−2 and 120 mA cm−2 ). The duty cycle was 0.8 for a period of 2 ms. Then the W coatings were investigated in terms of composition, morphology, oxygen content, adhesion and hardness. 3.1. Composition The intrinsic structure of tungsten coating was analyzed by XRD. Fig. 1(a)–(d) is the XRD patterns refer to current density of 60 mA cm−2 , 80 mA cm−2 , 100 mA cm−2 , and 120 mA cm−2 , respectively. As can be seen, the crystal planes of (1 1 0), (2 0 0), (2 1 1)
Fig. 2. The surface SEM images of electro-deposition tungsten coatings obtained at duty cycle of 0.8 and current density of (a) 60 mA cm−2 , (b) 80 mA cm−2 , (c) 100 mA cm−2 , and (d) 120 mA cm−2 .
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Fig. 4. Cross-sectional morphology and line scanning map of tungsten coatings obtained at current density of (a) 60 mA cm−2 , (b) 80 mA cm−2 , (c) 100 mA cm−2 , and (d) 120 mA cm−2 .
and (2 2 0) of all samples were corresponded to the diffraction 2 peaks at 40◦ , 58◦ , 73◦ and 87◦ , respectively, which matches well with the body centered cubic (BCC) structured standard tungsten (PDF-04-0806), and no other phases was detected. Table 1 shows the EDXRF results and 100.0000% tungsten coatings were achieved under various current densities. As described in Section 2, all samples were placed in the Na2 WO4 –WO3 molten salt, and no other impurities or detrimental phases were introduced. And the whole molten salt system was isolated from the air, which implied the
absence of oxide process. Moreover, during the experimentation, electrochemical processes in the molten salt are as follows [12]: The anode :
W − 6e = W6+
In the molten salt : The cathode :
WO4 2− + WO3 ↔ W2 O7 2−
4 W2 O7 2− + 6e = W0 + 7WO4 2−
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Table 1 The EDXRF result of tungsten coating with different current densities (comment: 8◦ /min). Analyte
Result
60 mA cm−2 W coatings 80 mA cm−2 W coatings 100 mA cm−2 W coatings 120 mA cm−2 W coatings
100.0000% 100.0000% 100.0000% 100.0000%
There are no impurities formed. All these features of electrodeposition are attributed to the tungsten coatings with high purity and ultra-low oxygen content. The consistency of XRD and EDXRF results of different samples prepared in different current densities reveals that current density has no influence on the formation of pure tungsten coatings, and pure tungsten coating can be obtained by the method. 3.2. Morphology The typical microstructure of different current densities was shown in Fig. 2. It illustrates that all coatings are uniform and fairly compact, and no voids or cracks were observed, which indicates that pure tungsten coatings can be obtained under different current densities. Similar microstructures for tungsten coatings deposited on copper were reported by Liu et al. [10]. The average sizes of tungsten grains were found to increase slightly when the current density was increased. Fig. 3(a) 7.13 m, (b) 8.76 m, (c) 11.17 m, (d) 15.82 m shows the average grain sizes of tungsten and the variation trend. It is inferred that the formation of tungsten coatings is an electrochemical process. Since the nucleation rate is lower than the growth rate of nuclei owing to the increase of current density, the nucleation rate of tungsten grains is considered to be restrained. The cross-sectional micrographs and line scanning maps of the coatings are showed in Fig. 4, which indicated that the bonding between tungsten coating and graphite substrate is compact with no observable cracks, voids or desquamations. The line analysis was used to further confirm that the tungsten coatings were pure. Moreover, after one hour electro-deposition, tungsten coatings with the thickness in the range of 28.8–51 m can be prepared, and Fig. 5 shows that the coating thickness increased with the increasing of current density. The results show that higher current density accelerates the nucleation and growth rate of grain sizes. It can be explained by the theory (grain growth and overlap of new grain’s nucleation) proposed by M.Y. ABYANEH [13]. Na2 WO4 and WO3 were in the molten state, when the external current was applied to the electrode, tungsten ions first deposited on the cathode of the substrate for the reason that the equilibrium potential of the tungsten ions in the molten salt is higher than that of the cathode. With the increasing of current density, cathode over potential increases,
Fig. 5. Thickness and hardness of tungsten coatings obtained at current density of (a) 60 mA cm−2 , (b) 80 mA cm−2 , (c) 100 mA cm−2 , and (d) 120 mA cm−2 .
therefore, tungsten ions on the cathode increases. For the duration, when it was increased, more tungsten ions gathered. This is good for the forming of new crystal nuclei. On the other hand, it would be helpful to accelerate the growth of tungsten crystalline grains. So increasing the current density can get thicker tungsten coatings.
3.3. Oxygen content For nuclear fusion PFMs, tungsten coatings with low-oxygen content are highly desired. It can improve longevity of PFMs, as the fewer oxygen content, the fewer chances of preferential broken [14]. In this research, the oxygen contents in the coatings which were measured by a Nitrogen/Oxygen Analyzer were about 0.022 wt%, 0.023 wt%, 0.025 wt% and 0.027 wt%, corresponding to the current densities 60 mA cm−2 , 80 mA cm−2 , 100 mA cm−2 and 120 mA cm−2 , respectively. The values are much lower than the oxygen content of tungsten coating prepared by Atmospheric Plasma Spraying (APS) method (0.48 wt%) [15], and Vacuum Plasma Spraying (VPS) method (0.35 wt%) [16]. It is hard for the oxygen atoms to go into tungsten coatings during the electro-deposition process. There are two explanations for the phenomenon. Firstly, during the process of the experiment, the samples were totally immersed into molten salt in an anaerobic condition at 1173 K. Additionally, the reaction was an electrochemical process, in which ion exchange (W6+ + 6e = W0 ) is the reaction [17]. In conclusion, electro-deposition from the Na2 WO4 –WO3 melt at 1173 K is a simple and feasible method to prepare tungsten coatings. Besides, the 100.0000% tungsten coating as described in Section 3.1 is not incompatible with the oxygen content (<0.030 wt%), which should be attributed to the fact that Energy Dispersive X-ray Fluorescence instrument exhibits lower precision than Nitrogen/Oxygen Analyzer as well as the oxygen content was too low to be detected by EDXRF.
3.4. Hardness and adhesion The study of current density, which was measured by Vickers micro-hardness instrument on the tungsten coatings’ hardness, is shown in Fig. 5. They were measured by Vickers micro-hardness instrument. The results indicate that the micro-hardness of the coatings show an upward tendency along with the current density. Overall, current density is found to increase the micro-hardness, which is consistent with the thickness of the coatings. These coatings’ hardness could be greater than 400 HV due to the lower porosity and fewer defects. These prove that increasing current density may be benefit to the increase of coating thickness and hardness. However, it has not been found the same result in other reports that a small grain size could result in higher hardness [18]. It is proposed that the main reason is the major influence of graphite substrate on thinner tungsten coating in the present investigation. The adhesion of the W coating on the graphite substrate was measured by scratch-test method. During the test, a progressive load was drawn across the surface of the coating. The coating will start to fail at a certain critical load which is used to quantify the adhesive properties of different coating-substrate combinations. In this study, the critical loads are all close to 60 N; it is much higher than the adhesion of tungsten coating (21 N) that prepared by double-glow plasma method [19]. The trend of adhesion is given in Fig. 6. The illustration showed that even the current density increased, the adhesions of the coatings rarely changed. The reason is that the electrocrystallization theories are the same even with different current densities, during the process of the electrodeposition.
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and the National Natural Science Foundation of China (Nos. 51172019 and 51372017). References
Fig. 6. Adhesion of tungsten coatings obtained at current density of (a) 60 mA cm−2 , (b) 80 mA cm−2 , (c) 100 mA cm−2 , and (d) 120 mA cm−2 .
4. Conclusions Uniform and pure tungsten coatings on graphite substrate with low oxygen content, high adhesion strength and high hardness have been successfully electro-deposited from the Na2 WO4 –WO3 molten salt. It was revealed that current density has great influence on the nucleation, growth and the properties of tungsten coatings. As the current density increases, the grain size (7.13–15.82 m) of tungsten increased, so do the thickness (28.8–51 m) and the hardness (405.85–445.08 HV). The bonding between tungsten coating and graphite substrate is compact. The oxygen content is lower than 0.03 wt%. Overall, electro-deposited from the Na2 WO4 –WO3 melt at 1173 K is a simple and feasible method to prepare tungsten coatings. Acknowledgments The study was supported by International Thermonuclear Experimental Reactor (ITER) Project of China (No. 2014GB123000)
[1] H. Bolt, V. Barabash, W. Krauss, W. Krauss, J. Linke, R. Neu, et al., ASDEX Upgrade Team, J. Nucl. Mater. 329–333 (2004) 66–73, Part A. [2] R. Neu, R. Dux, A. Kallenbach, T. Eich, A. Herrmann, C. Maggi, et al., The ASDEX Upgrade Team, Operation with the new tungsten divertor: ASDEX Upgrade en route to an all tungsten device, in: 31st EPS Conference on Plasma Phys, London, UK, 2004. [3] P.T. Lang, A. Kallenbach, J. Bucalossi, G.D. Conway, A. Degeling, R. Dux, et al., Integrated exhaust scenarios with actively controlled ELMs, in: 20th IAEA Fusion Energy Conference, Vilamoura, Portugal, 2004. [4] C. Ruset, E. Grigore, I. Munteanu, H. Maier, H. Greuner, C. Hopf, et al., JET-EFDA Contributors, Fusion Eng. Des. 84 (2009) 1662–1665. [5] C. Ruset, E. Grigore, H. Maier, R. Neu, X. Li, H. Dong, et al., JET EFDA Contributors, Phys. Scr. T128 (2007) 171–174. [6] X. Liu, F. Zhang, S. Tao, Y.Z. Cao, Z.G. Xu, Y. Liu, et al., J. Nucl. Mater. 363–365 (2007) 1299–1303. [7] R. Neu, H. Maier, E. Gauthier, H. Greuner, T. Hirai, Ch. Hopf, et al., Phys. Scr. T128 (2007) 150–156. [8] G.S. Cho, K.H. Choe, Surf. Coat. Technol. 209 (2012) 131–136. [9] K. Tokunaga, N. Yoshida, N. Noda, Y. Kubota, S. Inagaki, R. Sakamoto, et al., J. Nucl. Mater. 266-269 (1999) 1224–1229. [10] Y.H. Liu, Y.C. Zhang, Q.Z. Liu, X.L. Li, F. Jiang, Int. J. Refract. Metals Hard Mater. 35 (2012) 241–245. [11] Y.H. Liu, Y.C. Zhang, Q.Z. Liu, X.L. Li, F. Jiang, Fusion Eng. Des. 87 (2012) 1861–1865. [12] B.A.N. Nauka, Electrocrystallization of Metals from Fused Salts, Nauka, Moscow, Russia, 1976. [13] M.Y. Abyaneh, Electrochim. Acta 27 (1982) 1329–1334. [14] A. Oliva, R. Kelly, G. Falcone, Surf. Sci. 166 (1986) 403–418. [15] J.J. Huang, X.J. Li, J. Chen, Y. Liu, B. Qi, S.S. Jiang, et al., J. Nucl. Mater. 432 (2013) 16–19. [16] F.L. Chong, J.L. Chen, J.G. Li, D.Y. Hu, X.B. Zheng, J. Nucl. Mater. 375 (2008) 213–217. [17] K. Nitta, T. Nohira, R. Hagiwara, M. Majima, S. Inazawa, Electrochim. Acta 55 (2010) 1278–1281. [18] K. Nitta, T. Nohira, R. Hagiwara, M. Majima, S. Inazawa, J. Appl. Electrochem. 40 (2010) 1443–1448. [19] Y. Zhang, Z.F. Chen, L.B. Wang, B. Yan, C.L.D. Fang, Fusion Eng. Des. 84 (2009) 15–18.
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Preparation of tungsten coatings on graphite by electro-deposition via Na2 WO4 –WO3 molten salt system Ning-bo Sun a , Ying-chun Zhang a,∗ , Fan Jiang a , Shao-ting Lang a , Min Xia a,b a b
School of Materials Science and Engineering, University of Science and Technology Beijing, 30 Xueyuan Road, Haidian District, Beijing 100083, PR China Institute of Powder Metallurgy and Advanced Ceramics, Southwest Jiaotong University, 111, 1st Section, Northern 2nd Ring Road, Chengdu, PR China
h i g h l i g h t s • • • •
Tungsten coatings on graphite were firstly obtained by electro-deposition method via Na2 WO4 –WO3 molten salt system. Uniform and dense tungsten coatings could be easily prepared in each face of the sample, especially the complex components. The obtained tungsten coatings are with high purity, ultra-low oxygen content (about 0.022 wt%). Modulate pulse parameters can get tungsten coatings with different thickness and hardness.
a r t i c l e
i n f o
Article history: Received 28 October 2013 Received in revised form 7 March 2014 Accepted 30 May 2014 Available online 30 June 2014 Keywords: Tungsten coating Graphite Electro-deposition Melts Pulse current density
a b s t r a c t Tungsten coating on graphite substrate is one of the most promising candidate materials as the ITER plasma facing components. In this paper, tungsten coatings on graphite substrates were fabricated by electro-deposition from Na2 WO4 –WO3 molten salt system at 1173 K in atmosphere. Tungsten coatings with no impurities were successfully deposited on graphite substrates under various pulsed current densities in an hour. By increasing the current density from 60 mA cm−2 to 120 mA cm−2 an increase of the average size of tungsten grains, the thickness and the hardness of tungsten coatings occurs. The average size of tungsten grains can reach 7.13 m, the thickness of tungsten coating was in the range of 28.8–51 m, and the hardness of coating was higher than 400 HV. No cracks or voids were observed between tungsten coating and graphite substrate. The oxygen content of tungsten coating is about 0.022 wt%. © 2014 Elsevier B.V. All rights reserved.
1. Introduction It has been discovered that tungsten has a good compatibility with fusion plasma [1] due to its high melting point, good thermal properties and low erosion rate. However, it is counteracted by a number of critical issues, such as poor processing, poor welding performance, high cost and heavy weight. Carbon materials, like graphite and C/C fiber composite (CFC) have been used for plasma facing materials (PFMs), because of their high thermal shock resistance, light weight and high strength. However, one critical issue is high erosion rates of graphite or CFC at elevated temperature. A solution to overcome this issue is tungsten coating of carbon materials like graphite or CFC. This solution has been successfully applied in ASDEX Upgrade [2,3] for many years and recently in the ITER-like Wall at JET [4]. Nowadays the techniques such as
∗ Corresponding author. Tel.: +86 01062334951; fax: +86 01062334951. E-mail address: [email protected] (Y.-c. Zhang). http://dx.doi.org/10.1016/j.fusengdes.2014.05.027 0920-3796/© 2014 Elsevier B.V. All rights reserved.
plasma spray (PS), physical vapor deposition (PVD) and chemical vapor deposition (CVD), combined magnetron sputtering and ion implantation (CMSII) techniques have been applied in tungsten coating of graphite as PFMs [5–9]. However, there are still some disadvantages, for example, detrimental phases is prone to being introduced; the cost is high; the procedures are complicate, etc. Electro-deposition is a viable and promising method for preparing tungsten coatings due to the simple operation and low cost. Furthermore, uniform and dense tungsten coatings could be easily prepared on each face of the sample. Recently, Liu et al. obtained pure tungsten coatings on Al2 O3 –Cu substrates from the Na2 WO4 –WO3 melt [10,11]. It is a great prospect of application, because this molten salt system is not sensitive to oxygen or water, it also has the advantages of simple operation, low cost, low equipment requirement, etc. This stimulated us to study the possibility of fabrication tungsten coatings on graphite. By using this method, tungsten coatings on graphite substrates with high purity, low oxygen content are expected to be prepared.
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Fig. 1. XRD patterns of tungsten coatings obtained at current density of (a) 60 mA cm−2 , (b) 80 mA cm−2 , (c) 100 mA cm−2 , and (d) 120 mA cm−2 .
Fig. 3. Average grain size of tungsten coatings obtained at current density of (a) 60 mA cm−2 , (b) 80 mA cm−2 , (c) 100 mA cm−2 , and (d) 120 mA cm−2 .
In this paper, tungsten coatings on graphite substrates were electro-deposited from the Na2 WO4 –WO3 melt at 1173 K. In order to obtain well defined structures for tungsten coatings, different pulsed current densities were investigated. The study of pulsed current density on tungsten nucleation and its performance were conducted.
of the tungsten coating were analyzed by Energy Dispersive Xray Fluorescence (EDXRF, EDX-720) and X-ray diffraction (XRD, Rigaku Industrial Co., Ltd., D/MAX-RB) with a scanning electron microscope (SEM, JSM 6480LV). Micro-hardness of the coatings was performed by a MH-6 micro-hardness instrument with a loading force of 50 g and loading time of 15 s. The adhesion was detected by nano scratch tests. The oxygen content was measured by the Nitrogen/Oxygen Analyzer (TC600, LECO, USA).
2. Experimental procedure Graphite (IG-430) substrate of 15 mm × 10 mm × 5 mm was coated with tungsten by electro-deposition, using anhydrous Na2 WO4 and WO3 (Na2 WO4 :WO3 = 3:1, by mole ratio) molten salts. The working electrodes (graphite substrates) were polished by mechanical and chemical methods to obtain high quality surface. And counter electrode was a tungsten plate (15 mm × 10 mm × 5 mm). Molten salts were mixed into the eutectic composition in an alumina crucible. Then the crucible was put in an electric furnace and was heated at a rate of 5 ◦ C/min. Electrodeposition was performed at 1173 K using a pulse power supply (HPMCC-5) holding for one hour, the duty cycle and period were 0.8 and 2 ms, respectively. After the deposition, the sample was fetched out and cooled in the air. Subsequently, the samples were ultrasonic-cleaned in a 10 M NaOH solution and deionized water to remove adherent salts. The composition, structure, thickness
3. Results and discussion The coatings were electro-deposited in the Na2 WO4 –WO3 melt at 1173 K for 1 h with different pulse current densities (60 mA cm−2 , 80 mA cm−2 , 100 mA cm−2 and 120 mA cm−2 ). The duty cycle was 0.8 for a period of 2 ms. Then the W coatings were investigated in terms of composition, morphology, oxygen content, adhesion and hardness. 3.1. Composition The intrinsic structure of tungsten coating was analyzed by XRD. Fig. 1(a)–(d) is the XRD patterns refer to current density of 60 mA cm−2 , 80 mA cm−2 , 100 mA cm−2 , and 120 mA cm−2 , respectively. As can be seen, the crystal planes of (1 1 0), (2 0 0), (2 1 1)
Fig. 2. The surface SEM images of electro-deposition tungsten coatings obtained at duty cycle of 0.8 and current density of (a) 60 mA cm−2 , (b) 80 mA cm−2 , (c) 100 mA cm−2 , and (d) 120 mA cm−2 .
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Fig. 4. Cross-sectional morphology and line scanning map of tungsten coatings obtained at current density of (a) 60 mA cm−2 , (b) 80 mA cm−2 , (c) 100 mA cm−2 , and (d) 120 mA cm−2 .
and (2 2 0) of all samples were corresponded to the diffraction 2 peaks at 40◦ , 58◦ , 73◦ and 87◦ , respectively, which matches well with the body centered cubic (BCC) structured standard tungsten (PDF-04-0806), and no other phases was detected. Table 1 shows the EDXRF results and 100.0000% tungsten coatings were achieved under various current densities. As described in Section 2, all samples were placed in the Na2 WO4 –WO3 molten salt, and no other impurities or detrimental phases were introduced. And the whole molten salt system was isolated from the air, which implied the
absence of oxide process. Moreover, during the experimentation, electrochemical processes in the molten salt are as follows [12]: The anode :
W − 6e = W6+
In the molten salt : The cathode :
WO4 2− + WO3 ↔ W2 O7 2−
4 W2 O7 2− + 6e = W0 + 7WO4 2−
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Table 1 The EDXRF result of tungsten coating with different current densities (comment: 8◦ /min). Analyte
Result
60 mA cm−2 W coatings 80 mA cm−2 W coatings 100 mA cm−2 W coatings 120 mA cm−2 W coatings
100.0000% 100.0000% 100.0000% 100.0000%
There are no impurities formed. All these features of electrodeposition are attributed to the tungsten coatings with high purity and ultra-low oxygen content. The consistency of XRD and EDXRF results of different samples prepared in different current densities reveals that current density has no influence on the formation of pure tungsten coatings, and pure tungsten coating can be obtained by the method. 3.2. Morphology The typical microstructure of different current densities was shown in Fig. 2. It illustrates that all coatings are uniform and fairly compact, and no voids or cracks were observed, which indicates that pure tungsten coatings can be obtained under different current densities. Similar microstructures for tungsten coatings deposited on copper were reported by Liu et al. [10]. The average sizes of tungsten grains were found to increase slightly when the current density was increased. Fig. 3(a) 7.13 m, (b) 8.76 m, (c) 11.17 m, (d) 15.82 m shows the average grain sizes of tungsten and the variation trend. It is inferred that the formation of tungsten coatings is an electrochemical process. Since the nucleation rate is lower than the growth rate of nuclei owing to the increase of current density, the nucleation rate of tungsten grains is considered to be restrained. The cross-sectional micrographs and line scanning maps of the coatings are showed in Fig. 4, which indicated that the bonding between tungsten coating and graphite substrate is compact with no observable cracks, voids or desquamations. The line analysis was used to further confirm that the tungsten coatings were pure. Moreover, after one hour electro-deposition, tungsten coatings with the thickness in the range of 28.8–51 m can be prepared, and Fig. 5 shows that the coating thickness increased with the increasing of current density. The results show that higher current density accelerates the nucleation and growth rate of grain sizes. It can be explained by the theory (grain growth and overlap of new grain’s nucleation) proposed by M.Y. ABYANEH [13]. Na2 WO4 and WO3 were in the molten state, when the external current was applied to the electrode, tungsten ions first deposited on the cathode of the substrate for the reason that the equilibrium potential of the tungsten ions in the molten salt is higher than that of the cathode. With the increasing of current density, cathode over potential increases,
Fig. 5. Thickness and hardness of tungsten coatings obtained at current density of (a) 60 mA cm−2 , (b) 80 mA cm−2 , (c) 100 mA cm−2 , and (d) 120 mA cm−2 .
therefore, tungsten ions on the cathode increases. For the duration, when it was increased, more tungsten ions gathered. This is good for the forming of new crystal nuclei. On the other hand, it would be helpful to accelerate the growth of tungsten crystalline grains. So increasing the current density can get thicker tungsten coatings.
3.3. Oxygen content For nuclear fusion PFMs, tungsten coatings with low-oxygen content are highly desired. It can improve longevity of PFMs, as the fewer oxygen content, the fewer chances of preferential broken [14]. In this research, the oxygen contents in the coatings which were measured by a Nitrogen/Oxygen Analyzer were about 0.022 wt%, 0.023 wt%, 0.025 wt% and 0.027 wt%, corresponding to the current densities 60 mA cm−2 , 80 mA cm−2 , 100 mA cm−2 and 120 mA cm−2 , respectively. The values are much lower than the oxygen content of tungsten coating prepared by Atmospheric Plasma Spraying (APS) method (0.48 wt%) [15], and Vacuum Plasma Spraying (VPS) method (0.35 wt%) [16]. It is hard for the oxygen atoms to go into tungsten coatings during the electro-deposition process. There are two explanations for the phenomenon. Firstly, during the process of the experiment, the samples were totally immersed into molten salt in an anaerobic condition at 1173 K. Additionally, the reaction was an electrochemical process, in which ion exchange (W6+ + 6e = W0 ) is the reaction [17]. In conclusion, electro-deposition from the Na2 WO4 –WO3 melt at 1173 K is a simple and feasible method to prepare tungsten coatings. Besides, the 100.0000% tungsten coating as described in Section 3.1 is not incompatible with the oxygen content (<0.030 wt%), which should be attributed to the fact that Energy Dispersive X-ray Fluorescence instrument exhibits lower precision than Nitrogen/Oxygen Analyzer as well as the oxygen content was too low to be detected by EDXRF.
3.4. Hardness and adhesion The study of current density, which was measured by Vickers micro-hardness instrument on the tungsten coatings’ hardness, is shown in Fig. 5. They were measured by Vickers micro-hardness instrument. The results indicate that the micro-hardness of the coatings show an upward tendency along with the current density. Overall, current density is found to increase the micro-hardness, which is consistent with the thickness of the coatings. These coatings’ hardness could be greater than 400 HV due to the lower porosity and fewer defects. These prove that increasing current density may be benefit to the increase of coating thickness and hardness. However, it has not been found the same result in other reports that a small grain size could result in higher hardness [18]. It is proposed that the main reason is the major influence of graphite substrate on thinner tungsten coating in the present investigation. The adhesion of the W coating on the graphite substrate was measured by scratch-test method. During the test, a progressive load was drawn across the surface of the coating. The coating will start to fail at a certain critical load which is used to quantify the adhesive properties of different coating-substrate combinations. In this study, the critical loads are all close to 60 N; it is much higher than the adhesion of tungsten coating (21 N) that prepared by double-glow plasma method [19]. The trend of adhesion is given in Fig. 6. The illustration showed that even the current density increased, the adhesions of the coatings rarely changed. The reason is that the electrocrystallization theories are the same even with different current densities, during the process of the electrodeposition.
N.-b. Sun et al. / Fusion Engineering and Design 89 (2014) 2529–2533
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and the National Natural Science Foundation of China (Nos. 51172019 and 51372017). References
Fig. 6. Adhesion of tungsten coatings obtained at current density of (a) 60 mA cm−2 , (b) 80 mA cm−2 , (c) 100 mA cm−2 , and (d) 120 mA cm−2 .
4. Conclusions Uniform and pure tungsten coatings on graphite substrate with low oxygen content, high adhesion strength and high hardness have been successfully electro-deposited from the Na2 WO4 –WO3 molten salt. It was revealed that current density has great influence on the nucleation, growth and the properties of tungsten coatings. As the current density increases, the grain size (7.13–15.82 m) of tungsten increased, so do the thickness (28.8–51 m) and the hardness (405.85–445.08 HV). The bonding between tungsten coating and graphite substrate is compact. The oxygen content is lower than 0.03 wt%. Overall, electro-deposited from the Na2 WO4 –WO3 melt at 1173 K is a simple and feasible method to prepare tungsten coatings. Acknowledgments The study was supported by International Thermonuclear Experimental Reactor (ITER) Project of China (No. 2014GB123000)
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