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Laminar iridium coating produced by pulse current electrodeposition from chloride molten salt
Applied Surface Science 282 (2013) 820–825
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Laminar iridium coating produced by pulse current electrodeposition from chloride molten salt Li’an Zhu ∗ , Shuxin Bai, Hong Zhang, Yicong Ye Department of Materials Science and Engineering, College of Aerospace Science and Engineering, National University of Defence Technology, Changsha 410073, PR China
a r t i c l e
i n f o
Article history: Received 29 November 2012 Received in revised form 14 April 2013 Accepted 13 June 2013 Available online 20 June 2013 Keywords: Iridium coating Molten salt Pulse current electrodeposition Laminar structure
a b s t r a c t Due to the unique physical and chemical properties, Iridium (Ir) is one of the most promising oxidationresistant coatings for refractory materials above 1800 ◦ C in aerospace field. However, the Ir coatings prepared by traditional methods are composed of columnar grains throughout the coating thickness. The columnar structure of the coating is considered to do harm to its oxidation resistance. The laminar Ir coating is expected to have a better high-temperature oxidation resistance than the columnar Ir coating does. The pulse current electrodeposition, with three independent parameters: average current density (Jm ), duty cycle (R) and pulse frequency (f), is considered to be a promising method to fabricate layered Ir coating. In this study, laminar Ir coatings were prepared by pulse current electrodeposition in chloride molten salt. The morphology, roughness and texture of the coatings were determined by scanning electron microscope (SEM), profilometer and X-ray diffraction (XRD), respectively. The results showed that the laminar Ir coatings were composed of a nucleation layer with columnar structure and a growth layer with laminar structure. The top surfaces of the laminar Ir coatings consisted of cauliflower-like aggregates containing many fine grains, which were separated by deep grooves. The laminar Ir coating produced at the deposition condition of 20 mA/cm2 (Jm ), 10% (R) and 6 Hz (f) was quite smooth (Ra 1.01 ± 0.09 m) with extremely high degree of preferred orientation of 1 1 1, and its laminar structure was well developed with clear boundaries and uniform thickness of sub-layers. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Iridium (Ir) has extremely low oxygen permeability and vapor pressure at high temperatures with the melting point of 2440 ◦ C, high chemical compatibility, and does not react with carbon below the eutectic temperature of 2280 ◦ C [1–4]. It has drawn considerable attention recently as a promising anti-oxidation coating for the refractory materials at high temperatures [5–7]. The most successful application of Ir as a coating is the Ultramet’s flagship product iridium/rhenium (Ir/Re) combustion chamber [8], which allows hours of operation up to 2200 ◦ C using earth-storable propellants [9,10]. Among kinds of preparation methods [11–19], electrodeposition in molten salt is an ideal one for fabricating uniform Ir coating with high quality and productivity on the substrates with complex shapes [20–23]. However, Ir coatings produced by general electrodeposition process in molten salt, i.e., direct current electrodeposition, is composed of columnar grains throughout the coating thickness [20,24]. Hamilton et al. [25] found that the
∗ Corresponding author. Tel.: +86 0731 84573180; fax: +86 0731 84576578. E-mail address: mr [email protected] (L. Zhu). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.06.064
diffusion rate of Re atoms along grain boundaries of Ir coating is much higher than that through the Ir grains. Unluckily, the boundaries of columnar grains are parallel to the growth direction of the grains, which is the shortest diffusion path for oxygen inward and substrate material outward. Therefore, the oxidation resistance of the Ir coating composed of columnar grains is deteriorated by this kind of microstructure. Maury and Senocq [14] indicated that the high-temperature oxidation resistance of layered Ir coating was much higher than that of Ir coating with columnar structure. The Ir coating with laminar structure breaks the continuity of the boundary structure of columnar grains, and thus increases the diffusion distance of oxygen and underlying material, which improves the oxidation resistance of the Ir coating. Yang et al. [26] prepared the multilayered Ir coating on the quartz using metal-organic chemical vapor deposition by multiple depositions at various deposition temperatures. It was found that the defects within the Ir coating during the deposition processes could be sealed perfectly by the subsequent depositions. Compared with the traditional direct current electrodeposition, the pulse current electrodeposition process contains three independent parameters: average current density (Jm ), duty cycle (R) and pulse frequency (f). Therefore, the Ir coating with laminar structure is supposed to be prepared using one-step electrodeposition
L. Zhu et al. / Applied Surface Science 282 (2013) 820–825
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Table 1 Electrodeposition parameters of Ir coating.a Sample b
DC PC-1c PC-2c PC-3c a b c
Fig. 1. Schematic diagram of the apparatus for electrodeposition of Ir coating from molten salt.
method by adjusting parameters of the pulse current. In the present research, the multilayered Ir coatings were prepared by pulse current electrodeposition in chloride molten salt. We investigated the microstructure, roughness and orientation of the layered Ir coatings and made a preliminary analysis of the formation mechanism of the laminar structure, simultaneously compared with that of the Ir coating with columnar structure obtained by direct current electrodeposition. 2. Experimental 2.1. Preparation of Ir coating A schematic diagram of the apparatus used in electrodeposition of Ir coating from molten salt is shown in Fig. 1. The electrodeposition of Ir coating was performed in a quartz chamber, sealed by a rubber plug on which electrodes and a thermal couple were fixed. The electrodes and thermal couple could be moved up and down without gas leakage. The molten salt electrolyte was stored in a graphite crucible and heated by a vertical resistance furnace. The atmosphere above the molten salt was maintained by putting the high-purity argon (purity: 99.999%) into the quartz chamber continuously. The tail gas was scrubbed by passing through the saturated NaOH solution. A graphite planchet ( 21 mm × 3 mm) coated with Re produced by chemical vapor deposition (CVD) was used as the substrate (so-called Re/C). Prior to the CVD process, the graphite planchet was polished using metallographic abrasive paper, cleaned in acetone by ultrasonic cleaning and then dried in an oven for 20 min. The Re coating was prepared by thermal decomposition of ReCl5 produced by in situ chlorination of Re powder (purity: 99.999%, ZHU ZHOU KETE INDUSTRIES CO., LTD.). The detailed description of the home-made CVD device was reported
Jm /(mA/cm2 )
R/(%)
f/(Hz)
20 20 20 20
– 20 10 10
– 4 4 6
Deposition time was 0.5 h for all samples. DC = Direct current. PC = Pulse current.
in the previous investigation [27]. The CVD conditions were as follows: chlorination temperature 730 ◦ C; deposition temperature 1130 ◦ C; flow rate of chlorine (purity: 99.999%) 90 mL/min; flow rate of argon (purity: 99.999%) 500 mL/min; total pressure 20 kPa and deposition time 1 h. The CVD Re coating, with a flat topped tower-like surface morphology, was composed of columnar grains throughout the coating thickness. The detailed description of the Re coating, including morphology, orientation and roughness, was reported in another work [28]. The as-received Re/C planchet and two graphite plates (50 mm × 40 mm × 5 mm) were used as the cathode and anode, respectively (Fig. 1). The cathode sample, connected to a carbon fiber, was overall immersed into the molten salt, while each of the anode plates, tied to an iron chrome alloy wire, was partly dipped in the electrolyte with the designed dipping area of about 25 cm2 . The salts were dried in a vacuum drying chamber at 150 ◦ C for 5 h before being mixed. Ir coating was electrodeposited on the Re/C substrate in a ternary eutectic molten salt of NaCl-KCl-CsCl (29.8, 29.8 and 40.4 mol.%, respectively) containing 1.9 mol.% Ir ion at 580 ◦ C under the argon atmosphere. The Ir ions were introduced by adding iridium trichloride (purity: 99.99%, SHAANXI KAIDA CHEMICAL ENGINEERING CO., LTD.) directly into the ternary molten salt solvent at room temperature. The detailed parameters of the experiment are listed in Table 1. 2.2. Characterization The surface and fracture morphologies of Ir coatings were observed by Hitachi S-4800 scanning electron microscope (SEM). The phase identifications and growth directions of the Ir coatings were conducted by X-ray diffraction (XRD, Rigaku D/Max 2550VB+ ) using Ni-filtered Cu K␣ radiation at a scanning rate of 5 ◦ /min and scanning from 15 ◦ to 85 ◦ of 2. The surface roughness measurement of the coatings was conducted using a Form Talysurf PGI 1240 type profilometer (Taylor Hobson Ltd.) with stylus radius of 2 m. The result of surface roughness Ra was the mean value of 9 measurements performed along 3 random directions on the coating surface. 3. Results and discussion The macroscopic photographs of the Ir coatings obtained at different electrodeposition conditions are shown in Fig. 2. The Ir coatings of sample DC and PC-1 were grayish-white and compact with a few of macroscopic nodules. The Ir coating of sample PC-2 was silvery-white, compact and smooth with only a few of microscopic nodules. The Ir coating of sample PC-3 was silvery white, compact and quite smooth without any defects. The surface and cross sectional morphologies of the Ir coatings obtained at different deposition conditions are shown in Fig. 3. The Ir coating produced by direct current electrodeposition consisted of typical columnar grains with a tapered top. The coating was composed of two sub-layers, i.e., a dense nucleation layer of about 3 m comprising columnar grains of lower slenderness ratio, and
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L. Zhu et al. / Applied Surface Science 282 (2013) 820–825
Fig. 2. Macroscopic photographs of the Ir coatings obtained at different deposition conditions: (a) DC, (b) PC-1, (c) PC-2 and (d) PC-3.
a growth layer comprising columnar grains of higher slenderness ratio. Some micropores were found at the interface between the two sub-layers, which might arise during the growth transition process of the two sub-layers. The microstructures of the Ir coatings prepared by pulse current electrodeposition were quite different from that of the Ir coating produced by direct current electrodeposition (Fig. 3(c)–(h)). The coatings produced by pulse current electrodeposition (sample PC-1, PC-2 and PC-3) were all composed of cauliflower-like aggregates containing many fine grains, which were separated by deep grooves (Fig. 3(c), (e) and (g)). A significant change of the cross sectional morphologies of the Ir coatings by pulse current electrodeposition was observed in Fig. 3(d), (f) and (h). The coating of sample PC-1 was composed of both laminar and columnar grains as shown in Fig. 3(d). The nucleation layer, with thickness of about 3 m, comprising columnar grains of low slenderness ratio was observed adjacent to the substrate. The growth layer was composed of both a laminar structure layer and a columnar structure layer. The laminar structure layer comprised two sub-layers with the thicknesses of 1.5 m and 0.5 m, respectively. The columnar structure layer was about 5 m thick and grew on the laminar structure layer. The result showed that the microstructure of Ir coating changed partly from columnar structure to laminar structure under this pulse current electrodeposition condition. The Ir coating of
sample PC-2 was composed of nucleation layer and growth layer as well. The nucleation layer was about 2.5 m thick and composed of small columnar grains (Fig. 3(f)), while the growth layer consisted of laminar grains entirely with the thickness of about 7.5 m. The thicknesses of the sub-layers within the growth layer were non-uniform and the boundaries between sub-layers were fuzzy. It could be found in Fig. 3(h) that the laminar structure of the Ir coating of sample PC-3 was well developed, with clear boundaries and uniform thicknesses of sub-layers. Further more, it was observed that none of the micropores existed at the interfaces of the nucleation layers and growth layers of the coatings prepared by pulse current electrodeposition (Fig. 3(d), (f) and (h)), which could be attributed to the re-nucleation process under high peak current density (Jp ) of pulse current electrodeposition process. As is known, in conventional direct current electrodeposition, there is only one parameter, namely current density. In pulse current electrodeposition, by contrast, we have three independent variables, which are on-time (ton ), off-time (toff ) and peak current density (Jp ). In practice, we prefer to use three other parameters, viz., average current density (Jm ), duty cycle (R) and pulse frequency (f). The relations between them are given by:
Jm = Jp R
(1)
L. Zhu et al. / Applied Surface Science 282 (2013) 820–825
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Fig. 3. SEM micrographs of the surface and fracture morphologies of the Ir coatings obtained at different deposition conditions: (a) and (b) DC, (c) and (d) PC-1, (e) and (f) PC-2, (g) and (h) PC-3.
R = ton / ton + toff
f = 1/ ton + toff
(2) (3)
Therefore, at the same Jm , the Jp of pulse current electrodeposition could be quite high corresponding to the low R. In our research, the Jp of the three samples (PC-1, PC-2 and PC-3) produced by pulse current electrodeposition was not less than 100 mA/cm2 , which was much higher than that used by sample DC, although they had the same Jm . The increased current density enhances the electric field strength of the electric double layer near the cathode surface, and thus increases the adion population and overpotential [29]. As a result, both the driving force and nucleation rate are improved. The columnar growth trend of grains gets weakened, while the laminar growth trend gets strengthened. It could be found that the laminar growth trend was enhanced with increasing Jp as shown
in Fig. 3(b), (d) and (f). However, the nucleation process did not occur within every pulse period. It seemed that the formation of nuclei occurred every hundreds of pulse cycles according to the thickness of the sub-layers (Fig. 3(f) and (h)). The reason for that was not clear for now. But it could be confirmed that the electrocrystallization and growth mechanism of Ir coating could be affected by pulse current parameters, and furthermore the laminar structure of Ir coating could be adjusted by adopting varied pulse current parameter combinations. It was observed in Fig. 2 and Table 2 that the surface roughness of the Ir coatings decreased as the decreasing R (from sample PC-1 to PC-2) and increasing f (from sample PC-2 to PC-3). The decrease of R decreases the ratio of ton /toff , which means that in a pulse cycle, the consumption time (ton ) of metal ions is shortened, while the replenishment time (toff ) is lengthened. The concentration polarization is suppressed and thus the surface roughness of the coating decreases. The increase of
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L. Zhu et al. / Applied Surface Science 282 (2013) 820–825
Table 2 Surface roughness of the Ir coatings obtained at different deposition conditions. Sample
DC
PC-1
PC-2
PC-3
Surface roughness (Ra )/m
1.64 ± 0.16
1.55 ± 0.16
1.21 ± 0.12
1.01 ± 0.09
f further reduces the ton , and thus the concentration of metal ions near the electrode surface could always keep a quite high level. As a result, the electrodeposition process is controlled by electrochemical reaction kinetics all the while, and the surface roughness of the coating will maintain a low level. It was noticed in Fig. 3 that there was a 2–3 m thick nucleation layer comprising small columnar grains for almost all samples in spite of the electrodeposition conditions. It may be ascribed to the influence of the substrate on the coating growth at the initial stage of the electrodeposition, since it has been shown, e.g. for other metal electrodeposits, that the influence of the substrate appears as far as a few microns distance from the substrate [30]. The XRD patterns of the Ir coatings obtained at different deposition conditions are shown in Fig. 4. The strong Ir peaks, with (1 1 1), (2 0 0), (2 2 0) and (3 1 1) reflections were observed in all the XRD patterns. It indicated that all the Ir coatings had a highly textured, polycrystalline structure. According to the standard X-ray powder diffraction pattern of Ir (Pattern: 87-0715), the preferred growth direction of the Ir coatings can be determined using the texture coefficient (TC(h k l) ), which can be calculated by the following formula [31]:
TC(h
k l)
I(h k l) /I(h k l)
= 1/N
I(h k l) /I(h k l)
(4)
where TC(h k l) is the texture coefficient of the (h k l) plane, I(h k l) is the measured intensity of (h k l) plane, I (h k l) is the corresponding recorded intensity in JCPDS data file, and N is the number of preferred growth directions. The texture coefficients of the Ir coatings at different deposition conditions are listed in Table 3. The results in Table 3 indicated that the preferred growth directions of the Ir coatings at different deposition conditions were all 1 1 1. The degree of the preferred orientation for sample PC-3 was extremely high compared with the other samples. As is known, the lower the crystal plane index is, the higher the atomic planar density is. Thus the diffusion-barrier property against oxygen of the low-index plane is better than that of high-index plane. The
Table 3 Texture coefficients of the Ir coatings obtained at different deposition conditions. Sample
TC(1 1 1)
TC(2 0 0)
TC(2 2 0)
TC(3 1 1)
DC PC-1 PC-2 PC-3
1.6986 1.4355 1.7045 3.6734
0.5312 0.7987 0.7128 0.1002
0.8695 0.9912 0.8847 0.1224
0.9006 0.7746 0.6979 0.1040
higher the degree of 1 1 1 preferred orientation is, the higher the ratio of (1 1 1) crystal planes in the Ir coating is. Therefore, the Ir coating of sample PC-3 was expected to have a better oxygen permeability resistance compared with other samples due to both its highly preferred orientation of 1 1 1 and well-developed laminar structure. 4. Conclusions (1) The laminar Ir coatings were produced on Re/C substrates by pulse current electrodeposition from chloride molten salt under inert atmosphere. (2) The laminar Ir coatings were composed of a nucleation layer with columnar structure and a growth layer with laminar structure. None of the micropores existed at the interfaces of the nucleation layers and growth layers. The top surfaces of the laminar Ir coatings consisted of cauliflower-like aggregates containing many fine grains, which were separated by deep grooves. (3) Under the pulse current electrodeposition condition of low R (10%) and high f (6 Hz), the obtained Ir coating was quite smooth (Ra 1.01 ± 0.09 m) and its laminar structure was well developed with clear boundaries and uniform thicknesses of sub-layers. (4) The laminar Ir coating produced at the deposition condition of 20 mA/cm2 (Jm ), 10% (R) and 6 Hz (f) had an extremely high degree of the preferred orientation of 1 1 1. Therefore, it was expected to have a better oxygen permeability resistance compared with traditional Ir coatings with columnar structure due to both its highly preferred orientation of 1 1 1 and welldeveloped laminar structure. References
Fig. 4. XRD patterns of the Ir coatings obtained at different deposition conditions: (a) DC; (b) PC-1; (c) PC-2; (d) PC-3.
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Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Laminar iridium coating produced by pulse current electrodeposition from chloride molten salt Li’an Zhu ∗ , Shuxin Bai, Hong Zhang, Yicong Ye Department of Materials Science and Engineering, College of Aerospace Science and Engineering, National University of Defence Technology, Changsha 410073, PR China
a r t i c l e
i n f o
Article history: Received 29 November 2012 Received in revised form 14 April 2013 Accepted 13 June 2013 Available online 20 June 2013 Keywords: Iridium coating Molten salt Pulse current electrodeposition Laminar structure
a b s t r a c t Due to the unique physical and chemical properties, Iridium (Ir) is one of the most promising oxidationresistant coatings for refractory materials above 1800 ◦ C in aerospace field. However, the Ir coatings prepared by traditional methods are composed of columnar grains throughout the coating thickness. The columnar structure of the coating is considered to do harm to its oxidation resistance. The laminar Ir coating is expected to have a better high-temperature oxidation resistance than the columnar Ir coating does. The pulse current electrodeposition, with three independent parameters: average current density (Jm ), duty cycle (R) and pulse frequency (f), is considered to be a promising method to fabricate layered Ir coating. In this study, laminar Ir coatings were prepared by pulse current electrodeposition in chloride molten salt. The morphology, roughness and texture of the coatings were determined by scanning electron microscope (SEM), profilometer and X-ray diffraction (XRD), respectively. The results showed that the laminar Ir coatings were composed of a nucleation layer with columnar structure and a growth layer with laminar structure. The top surfaces of the laminar Ir coatings consisted of cauliflower-like aggregates containing many fine grains, which were separated by deep grooves. The laminar Ir coating produced at the deposition condition of 20 mA/cm2 (Jm ), 10% (R) and 6 Hz (f) was quite smooth (Ra 1.01 ± 0.09 m) with extremely high degree of preferred orientation of 1 1 1, and its laminar structure was well developed with clear boundaries and uniform thickness of sub-layers. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Iridium (Ir) has extremely low oxygen permeability and vapor pressure at high temperatures with the melting point of 2440 ◦ C, high chemical compatibility, and does not react with carbon below the eutectic temperature of 2280 ◦ C [1–4]. It has drawn considerable attention recently as a promising anti-oxidation coating for the refractory materials at high temperatures [5–7]. The most successful application of Ir as a coating is the Ultramet’s flagship product iridium/rhenium (Ir/Re) combustion chamber [8], which allows hours of operation up to 2200 ◦ C using earth-storable propellants [9,10]. Among kinds of preparation methods [11–19], electrodeposition in molten salt is an ideal one for fabricating uniform Ir coating with high quality and productivity on the substrates with complex shapes [20–23]. However, Ir coatings produced by general electrodeposition process in molten salt, i.e., direct current electrodeposition, is composed of columnar grains throughout the coating thickness [20,24]. Hamilton et al. [25] found that the
∗ Corresponding author. Tel.: +86 0731 84573180; fax: +86 0731 84576578. E-mail address: mr [email protected] (L. Zhu). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.06.064
diffusion rate of Re atoms along grain boundaries of Ir coating is much higher than that through the Ir grains. Unluckily, the boundaries of columnar grains are parallel to the growth direction of the grains, which is the shortest diffusion path for oxygen inward and substrate material outward. Therefore, the oxidation resistance of the Ir coating composed of columnar grains is deteriorated by this kind of microstructure. Maury and Senocq [14] indicated that the high-temperature oxidation resistance of layered Ir coating was much higher than that of Ir coating with columnar structure. The Ir coating with laminar structure breaks the continuity of the boundary structure of columnar grains, and thus increases the diffusion distance of oxygen and underlying material, which improves the oxidation resistance of the Ir coating. Yang et al. [26] prepared the multilayered Ir coating on the quartz using metal-organic chemical vapor deposition by multiple depositions at various deposition temperatures. It was found that the defects within the Ir coating during the deposition processes could be sealed perfectly by the subsequent depositions. Compared with the traditional direct current electrodeposition, the pulse current electrodeposition process contains three independent parameters: average current density (Jm ), duty cycle (R) and pulse frequency (f). Therefore, the Ir coating with laminar structure is supposed to be prepared using one-step electrodeposition
L. Zhu et al. / Applied Surface Science 282 (2013) 820–825
821
Table 1 Electrodeposition parameters of Ir coating.a Sample b
DC PC-1c PC-2c PC-3c a b c
Fig. 1. Schematic diagram of the apparatus for electrodeposition of Ir coating from molten salt.
method by adjusting parameters of the pulse current. In the present research, the multilayered Ir coatings were prepared by pulse current electrodeposition in chloride molten salt. We investigated the microstructure, roughness and orientation of the layered Ir coatings and made a preliminary analysis of the formation mechanism of the laminar structure, simultaneously compared with that of the Ir coating with columnar structure obtained by direct current electrodeposition. 2. Experimental 2.1. Preparation of Ir coating A schematic diagram of the apparatus used in electrodeposition of Ir coating from molten salt is shown in Fig. 1. The electrodeposition of Ir coating was performed in a quartz chamber, sealed by a rubber plug on which electrodes and a thermal couple were fixed. The electrodes and thermal couple could be moved up and down without gas leakage. The molten salt electrolyte was stored in a graphite crucible and heated by a vertical resistance furnace. The atmosphere above the molten salt was maintained by putting the high-purity argon (purity: 99.999%) into the quartz chamber continuously. The tail gas was scrubbed by passing through the saturated NaOH solution. A graphite planchet ( 21 mm × 3 mm) coated with Re produced by chemical vapor deposition (CVD) was used as the substrate (so-called Re/C). Prior to the CVD process, the graphite planchet was polished using metallographic abrasive paper, cleaned in acetone by ultrasonic cleaning and then dried in an oven for 20 min. The Re coating was prepared by thermal decomposition of ReCl5 produced by in situ chlorination of Re powder (purity: 99.999%, ZHU ZHOU KETE INDUSTRIES CO., LTD.). The detailed description of the home-made CVD device was reported
Jm /(mA/cm2 )
R/(%)
f/(Hz)
20 20 20 20
– 20 10 10
– 4 4 6
Deposition time was 0.5 h for all samples. DC = Direct current. PC = Pulse current.
in the previous investigation [27]. The CVD conditions were as follows: chlorination temperature 730 ◦ C; deposition temperature 1130 ◦ C; flow rate of chlorine (purity: 99.999%) 90 mL/min; flow rate of argon (purity: 99.999%) 500 mL/min; total pressure 20 kPa and deposition time 1 h. The CVD Re coating, with a flat topped tower-like surface morphology, was composed of columnar grains throughout the coating thickness. The detailed description of the Re coating, including morphology, orientation and roughness, was reported in another work [28]. The as-received Re/C planchet and two graphite plates (50 mm × 40 mm × 5 mm) were used as the cathode and anode, respectively (Fig. 1). The cathode sample, connected to a carbon fiber, was overall immersed into the molten salt, while each of the anode plates, tied to an iron chrome alloy wire, was partly dipped in the electrolyte with the designed dipping area of about 25 cm2 . The salts were dried in a vacuum drying chamber at 150 ◦ C for 5 h before being mixed. Ir coating was electrodeposited on the Re/C substrate in a ternary eutectic molten salt of NaCl-KCl-CsCl (29.8, 29.8 and 40.4 mol.%, respectively) containing 1.9 mol.% Ir ion at 580 ◦ C under the argon atmosphere. The Ir ions were introduced by adding iridium trichloride (purity: 99.99%, SHAANXI KAIDA CHEMICAL ENGINEERING CO., LTD.) directly into the ternary molten salt solvent at room temperature. The detailed parameters of the experiment are listed in Table 1. 2.2. Characterization The surface and fracture morphologies of Ir coatings were observed by Hitachi S-4800 scanning electron microscope (SEM). The phase identifications and growth directions of the Ir coatings were conducted by X-ray diffraction (XRD, Rigaku D/Max 2550VB+ ) using Ni-filtered Cu K␣ radiation at a scanning rate of 5 ◦ /min and scanning from 15 ◦ to 85 ◦ of 2. The surface roughness measurement of the coatings was conducted using a Form Talysurf PGI 1240 type profilometer (Taylor Hobson Ltd.) with stylus radius of 2 m. The result of surface roughness Ra was the mean value of 9 measurements performed along 3 random directions on the coating surface. 3. Results and discussion The macroscopic photographs of the Ir coatings obtained at different electrodeposition conditions are shown in Fig. 2. The Ir coatings of sample DC and PC-1 were grayish-white and compact with a few of macroscopic nodules. The Ir coating of sample PC-2 was silvery-white, compact and smooth with only a few of microscopic nodules. The Ir coating of sample PC-3 was silvery white, compact and quite smooth without any defects. The surface and cross sectional morphologies of the Ir coatings obtained at different deposition conditions are shown in Fig. 3. The Ir coating produced by direct current electrodeposition consisted of typical columnar grains with a tapered top. The coating was composed of two sub-layers, i.e., a dense nucleation layer of about 3 m comprising columnar grains of lower slenderness ratio, and
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Fig. 2. Macroscopic photographs of the Ir coatings obtained at different deposition conditions: (a) DC, (b) PC-1, (c) PC-2 and (d) PC-3.
a growth layer comprising columnar grains of higher slenderness ratio. Some micropores were found at the interface between the two sub-layers, which might arise during the growth transition process of the two sub-layers. The microstructures of the Ir coatings prepared by pulse current electrodeposition were quite different from that of the Ir coating produced by direct current electrodeposition (Fig. 3(c)–(h)). The coatings produced by pulse current electrodeposition (sample PC-1, PC-2 and PC-3) were all composed of cauliflower-like aggregates containing many fine grains, which were separated by deep grooves (Fig. 3(c), (e) and (g)). A significant change of the cross sectional morphologies of the Ir coatings by pulse current electrodeposition was observed in Fig. 3(d), (f) and (h). The coating of sample PC-1 was composed of both laminar and columnar grains as shown in Fig. 3(d). The nucleation layer, with thickness of about 3 m, comprising columnar grains of low slenderness ratio was observed adjacent to the substrate. The growth layer was composed of both a laminar structure layer and a columnar structure layer. The laminar structure layer comprised two sub-layers with the thicknesses of 1.5 m and 0.5 m, respectively. The columnar structure layer was about 5 m thick and grew on the laminar structure layer. The result showed that the microstructure of Ir coating changed partly from columnar structure to laminar structure under this pulse current electrodeposition condition. The Ir coating of
sample PC-2 was composed of nucleation layer and growth layer as well. The nucleation layer was about 2.5 m thick and composed of small columnar grains (Fig. 3(f)), while the growth layer consisted of laminar grains entirely with the thickness of about 7.5 m. The thicknesses of the sub-layers within the growth layer were non-uniform and the boundaries between sub-layers were fuzzy. It could be found in Fig. 3(h) that the laminar structure of the Ir coating of sample PC-3 was well developed, with clear boundaries and uniform thicknesses of sub-layers. Further more, it was observed that none of the micropores existed at the interfaces of the nucleation layers and growth layers of the coatings prepared by pulse current electrodeposition (Fig. 3(d), (f) and (h)), which could be attributed to the re-nucleation process under high peak current density (Jp ) of pulse current electrodeposition process. As is known, in conventional direct current electrodeposition, there is only one parameter, namely current density. In pulse current electrodeposition, by contrast, we have three independent variables, which are on-time (ton ), off-time (toff ) and peak current density (Jp ). In practice, we prefer to use three other parameters, viz., average current density (Jm ), duty cycle (R) and pulse frequency (f). The relations between them are given by:
Jm = Jp R
(1)
L. Zhu et al. / Applied Surface Science 282 (2013) 820–825
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Fig. 3. SEM micrographs of the surface and fracture morphologies of the Ir coatings obtained at different deposition conditions: (a) and (b) DC, (c) and (d) PC-1, (e) and (f) PC-2, (g) and (h) PC-3.
R = ton / ton + toff
f = 1/ ton + toff
(2) (3)
Therefore, at the same Jm , the Jp of pulse current electrodeposition could be quite high corresponding to the low R. In our research, the Jp of the three samples (PC-1, PC-2 and PC-3) produced by pulse current electrodeposition was not less than 100 mA/cm2 , which was much higher than that used by sample DC, although they had the same Jm . The increased current density enhances the electric field strength of the electric double layer near the cathode surface, and thus increases the adion population and overpotential [29]. As a result, both the driving force and nucleation rate are improved. The columnar growth trend of grains gets weakened, while the laminar growth trend gets strengthened. It could be found that the laminar growth trend was enhanced with increasing Jp as shown
in Fig. 3(b), (d) and (f). However, the nucleation process did not occur within every pulse period. It seemed that the formation of nuclei occurred every hundreds of pulse cycles according to the thickness of the sub-layers (Fig. 3(f) and (h)). The reason for that was not clear for now. But it could be confirmed that the electrocrystallization and growth mechanism of Ir coating could be affected by pulse current parameters, and furthermore the laminar structure of Ir coating could be adjusted by adopting varied pulse current parameter combinations. It was observed in Fig. 2 and Table 2 that the surface roughness of the Ir coatings decreased as the decreasing R (from sample PC-1 to PC-2) and increasing f (from sample PC-2 to PC-3). The decrease of R decreases the ratio of ton /toff , which means that in a pulse cycle, the consumption time (ton ) of metal ions is shortened, while the replenishment time (toff ) is lengthened. The concentration polarization is suppressed and thus the surface roughness of the coating decreases. The increase of
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Table 2 Surface roughness of the Ir coatings obtained at different deposition conditions. Sample
DC
PC-1
PC-2
PC-3
Surface roughness (Ra )/m
1.64 ± 0.16
1.55 ± 0.16
1.21 ± 0.12
1.01 ± 0.09
f further reduces the ton , and thus the concentration of metal ions near the electrode surface could always keep a quite high level. As a result, the electrodeposition process is controlled by electrochemical reaction kinetics all the while, and the surface roughness of the coating will maintain a low level. It was noticed in Fig. 3 that there was a 2–3 m thick nucleation layer comprising small columnar grains for almost all samples in spite of the electrodeposition conditions. It may be ascribed to the influence of the substrate on the coating growth at the initial stage of the electrodeposition, since it has been shown, e.g. for other metal electrodeposits, that the influence of the substrate appears as far as a few microns distance from the substrate [30]. The XRD patterns of the Ir coatings obtained at different deposition conditions are shown in Fig. 4. The strong Ir peaks, with (1 1 1), (2 0 0), (2 2 0) and (3 1 1) reflections were observed in all the XRD patterns. It indicated that all the Ir coatings had a highly textured, polycrystalline structure. According to the standard X-ray powder diffraction pattern of Ir (Pattern: 87-0715), the preferred growth direction of the Ir coatings can be determined using the texture coefficient (TC(h k l) ), which can be calculated by the following formula [31]:
TC(h
k l)
I(h k l) /I(h k l)
= 1/N
I(h k l) /I(h k l)
(4)
where TC(h k l) is the texture coefficient of the (h k l) plane, I(h k l) is the measured intensity of (h k l) plane, I (h k l) is the corresponding recorded intensity in JCPDS data file, and N is the number of preferred growth directions. The texture coefficients of the Ir coatings at different deposition conditions are listed in Table 3. The results in Table 3 indicated that the preferred growth directions of the Ir coatings at different deposition conditions were all 1 1 1. The degree of the preferred orientation for sample PC-3 was extremely high compared with the other samples. As is known, the lower the crystal plane index is, the higher the atomic planar density is. Thus the diffusion-barrier property against oxygen of the low-index plane is better than that of high-index plane. The
Table 3 Texture coefficients of the Ir coatings obtained at different deposition conditions. Sample
TC(1 1 1)
TC(2 0 0)
TC(2 2 0)
TC(3 1 1)
DC PC-1 PC-2 PC-3
1.6986 1.4355 1.7045 3.6734
0.5312 0.7987 0.7128 0.1002
0.8695 0.9912 0.8847 0.1224
0.9006 0.7746 0.6979 0.1040
higher the degree of 1 1 1 preferred orientation is, the higher the ratio of (1 1 1) crystal planes in the Ir coating is. Therefore, the Ir coating of sample PC-3 was expected to have a better oxygen permeability resistance compared with other samples due to both its highly preferred orientation of 1 1 1 and well-developed laminar structure. 4. Conclusions (1) The laminar Ir coatings were produced on Re/C substrates by pulse current electrodeposition from chloride molten salt under inert atmosphere. (2) The laminar Ir coatings were composed of a nucleation layer with columnar structure and a growth layer with laminar structure. None of the micropores existed at the interfaces of the nucleation layers and growth layers. The top surfaces of the laminar Ir coatings consisted of cauliflower-like aggregates containing many fine grains, which were separated by deep grooves. (3) Under the pulse current electrodeposition condition of low R (10%) and high f (6 Hz), the obtained Ir coating was quite smooth (Ra 1.01 ± 0.09 m) and its laminar structure was well developed with clear boundaries and uniform thicknesses of sub-layers. (4) The laminar Ir coating produced at the deposition condition of 20 mA/cm2 (Jm ), 10% (R) and 6 Hz (f) had an extremely high degree of the preferred orientation of 1 1 1. Therefore, it was expected to have a better oxygen permeability resistance compared with traditional Ir coatings with columnar structure due to both its highly preferred orientation of 1 1 1 and welldeveloped laminar structure. References
Fig. 4. XRD patterns of the Ir coatings obtained at different deposition conditions: (a) DC; (b) PC-1; (c) PC-2; (d) PC-3.
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