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The effect of tungsten addition on metallurgical state and solute content in nanocrystalline electrodeposited nickel
Accepted Manuscript The effect of tungsten addition on metallurgical state and solute content in nanocrystalline electrodeposited nickel N. Shakibi Nia, J. Creus, X. Feaugas, C. Savall PII: DOI: Reference:
S0925-8388(14)01025-1 http://dx.doi.org/10.1016/j.jallcom.2014.04.192 JALCOM 31157
To appear in: Received Date: Revised Date: Accepted Date:
5 March 2014 24 April 2014 25 April 2014
Please cite this article as: N. Shakibi Nia, J. Creus, X. Feaugas, C. Savall, The effect of tungsten addition on metallurgical state and solute content in nanocrystalline electrodeposited nickel, (2014), doi: http://dx.doi.org/ 10.1016/j.jallcom.2014.04.192
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
The effect of tungsten addition on metallurgical state and solute content in nanocrystalline electrodeposited nickel N. Shakibi Nia*, J. Creus, X. Feaugas, C. Savall Laboratoire des Sciences de l'Ingénieur pour l'Environnement, CNRS UMR 7356, Université de La Rochelle, Av. Michel Crépeau, F-17042 La Rochelle, France Abstract It is usually difficult to control the incorporation of foreign species in electrodeposited coatings originating from the solvent or the chemical species used for the electrodeposition bath. However, the presence of these impurities can modify their physicochemical properties. In the present study, complementary analytical techniques were used to evaluate the chemical contamination in nickel and nickel-tungsten alloys, electrodeposited from additive free baths. In order to better understand the relationship between impurity content and grain size refinement, the concentration of light elements (H, O and N) was systematically quantified by hot extraction analysis. Also, the distribution of contaminants was evaluated by SIMS analysis. We have shown that in nanocrystalline electrodeposited nickel the grain size refinement and the impurity contents are strongly related. However, in Ni-W alloys the evolution of the contamination is more complex, with a maximum amount for W contents around 10 at.%. 1. Introduction Nanocrystalline materials have been the subject of many studies due to their specific physicochemical properties, generally correlated to the grain size refinement. However, as the grain size decreases, other parameters as crystallographic orientation, solute content, internal stresses, grain boundary or misorientation and density of defects are also modified. Several studies reported that even small amount of impurities can have a dramatic effect on the properties of nanocrystalline and polycrystalline metals (ex. strength and ductility or corrosion) [1-8], which shows the importance of evaluating these elements. Electrodeposition of metals and alloys are usually accompanied by the incorporation of foreign species originating from the solvent (often water, leading to hydrogen co-deposition) or from the chemicals species used for the bath (metallic salts, complexing agents, additives …). Metallic impurities can be analyzed using elementary analysis techniques such as X-Ray Fluorescence, GDOES (Glow Discharge Optical Emission Spectroscopy) or GDMS (Glow Discharge Mass Spectrometry). However, the detection and quantification of light elements, as H, C, N, and O, is quite difficult explaining why these elements are generally investigated in bulk materials rather than in electrodeposited metals. Few studies have investigated the non-metallic contamination in electrodeposited nickel and nickel alloys using hot extraction method [9-11]. The nitrogen content in nanocrystalline electrodeposited nickel was influenced by the bath composition and increased up to 620 wt. ppm in coatings elaborated from a bath containing tartrate (complexing agent) and Na-saccharin (inhibitor) [11]. Chassaing et al. [9] have shown that the amounts of H and O are more important in electrodeposited Ni-Mo alloys than in pure nickel deposited from the same bath.
* Corresponding author Email address: [email protected] Tel: +335 46 45 82 68
1
A significant impact of the bath deposition temperature was reported, which resulted in maximum amounts of around 3800 wt. ppm of oxygen at 60-80°C and about 400 wt. ppm of hydrogen at 60°C. In electrodeposited Ni-P alloys the amount of absorbed hydrogen increased with the phosphorus content in alloys [10]. More recently, SIMS analysis (Secondary Ion Mass Spectrometry) was used to evaluate the influence of deposition parameters on chemical contamination in electrodeposited nickel coatings obtained from an additive-free sulphamate bath [12, 13]. According to the obtained results, an important increase of contamination by light elements is observed with the decrease in deposition current density and with grain size refinement. However, a better understanding of the relationship between metallurgical state and solute content can be achieved through a broader grain size range. Several studies have reported that the grain size range in electrodeposited nickel can be expanded at the nanometer scale by addition of tungsten alloying element. Also, it has been suggested that light elements incorporation during the electrodeposition process may affect the mechanical properties of these alloys, but the amounts of these impurities have not been yet quantified [14-20]. In this paper, the relation between grain size refinement and impurity content in nanocrystalline electrodeposited Ni-W alloys is being discussed. For this purpose, the hot extraction method is used to quantify the amounts of light elements in Ni-W alloys, for the first time to our knowledge. 2. Experimental procedure Nickel deposits were obtained by direct current (DC) and pulsed current (PC) on annealed nickel substrates (99.7 %) in a classical three electrode cell, using a VSP potentiostat from Biologic. An additive free sulphamate bath was used in order to minimize the incorporation of contaminants [12]. Ni-W alloys were also electrodeposited on polycrystalline nickel substrates (99 %) by direct current (DC) and reverse pulsed current (RPC). The bath proposed by Yamasaki et al. [16] was used without additives at 65°C for most of the deposits (Table 1) and for some other the concentration of nickel sulfate and sodium tungstate was modified in order to vary the W content. Nickel substrates were mechanically polished with 1µ m diamond paste before electrodeposition and the anode was a platinum mesh. For each condition the cathodic efficiency (η) was estimated by weighting the specimen before and after deposition and the deposition time (or the number of pulses) was adjusted to obtain a thickness of 50 µm. The applied cathodic current density in DC method was 5 and 50 mA/cm². The conditions in RPC method were chosen according to the study of Detor et al. [21] for which a constant forward current density of 100 mA/cm² was applied during 20 ms and the reverse current density was varied between 50 and 200 mA/cm² during 3 ms. A multi-scale approach was used to characterize the microstructure in nickel and Ni-W alloys deposits. X-ray analysis was performed with Cu-Kα radiation (λ=0.15405 nm) in θ-2θ mode using a Bruker AXS D8-Advanced operating at 40 kV and 40 mA. Spectra were obtained between 40° and 100° and then the instrumental broadening, Kα2 ray and background were removed. In order to estimate the grain size, Scherrer formula was applied on (111) peak. TEM micrographs were obtained using Transmission Electron Microscope (TEM JEOL JEM 2011) operating at 200 kV. Composition of deposits was analyzed using a Bruker M4 Tornado X-ray micro-fluorescence (µ-XRF) operating with a rhodium filament at 35 kV and 300 µA and Secondary Ion Mass Spectrometry (SIMS) with IMS 4FE6 Apparatus from CAMECA and two ionic sources Cs+ (at 14.5 keV) and O2+ (at 5.5 keV) to obtain the best sensitivity. 2
The concentration profiles were obtained after 5 to 10 µm pulverization in order to eliminate the surface contamination effects. The distribution of the elements can also be revealed through surface and cross section mapping using this technique. Hot extraction analyses were carried out using EMGA-621W oxygen / nitrogen / hydrogen analyzer to determine the light elements contamination in Ni and Ni-W deposits. 3. Results and discussion The hydrogen and oxygen contamination was first evaluated by hot extraction method for the bulk nickel and for two nickel deposits elaborated by direct current at 5 mA/cm² (C5) and 50 mA/cm² (C50) with different grain sizes and crystallographic textures (Table 2), elaborated from an additive free sulphamate bath. The obtained results reveal an increase in oxygen and hydrogen contamination when the grain size is reduced. As presented in Fig. 1, the oxygen contamination in the deposits depends on the period between their elaboration and the hot extraction analysis. In order to limit the oxidation of samples and to optimize the reproducibility of these measurements, the samples are systematically stored after electrodeposition for 12 hours under vacuum (70 mbar), and they are then analyzed. SIMS analysis was used to evaluate the light elements distribution for the same coatings. According to the concentration profiles and mappings, the impurities were homogeneously distributed laterally and through the thickness of the coatings. Several profiles were obtained for each sample and the calibration was realized with bulk nickel of known composition [13]. However, this semi-quantitative analysis is limited for the study of alloys, because reference samples with the same composition are required to avoid an important matrix effect. For comparison, Fig. 2 presents the results obtained by hot extraction and SIMS analysis for bulk nickel and two Ni based coatings elaborated from the sulphamate bath. It is observed that the amounts of H and O incorporated in nickel deposits present the same trend using these two techniques, despite the fact that hot extraction is a non-localized method. This result validates the developed sample preparation procedure for hot extraction analysis. It was observed that in electrodeposited nickel the grain size refinement is strongly influenced by the incorporation of impurities, especially the light element species. However, the restricted grain size range strongly limits the discussion on the relation between grain size refinement and light elements contamination. Therefore, as the addition of tungsten permits to reach grain sizes down to nanometer scale without the addition of additives in the deposition bath, Ni-W alloys were elaborated and investigated. The hot extraction analysis was systematically used to evaluate the amounts of light elements in nanocrystalline Ni-W alloys electrodeposited at 50 mA/cm² by direct current at 25°C and 65°C. Fig. 3 shows that the bath temperature has an important influence on the light elements incorporation in these deposits. The influence of the bath deposition temperature between 40 and 160°C was discussed in the study of Chassaing et al. on electrodeposited Ni-Mo alloys, in which a maximum amount of contamination for oxygen was found in deposits elaborated at 60-80°C [9]. This result highlights the importance of optimizing the elaboration conditions to limit the incorporation of impurities in deposits. For the following Ni-W deposits the bath temperature was set to 65°C, as, in our case, this temperature led to the lowest contamination levels. In order to enhance the reproducibility, the influence of different storage conditions on the hydrogen concentration evolution was then studied. For this purpose, a 15 at.% Ni-W deposit, (elaborated by DC method) was stored in various conditions before hot extraction measurements. As observed in Fig. 4, the 3
amounts of measured hydrogen immediately after electrodeposition is 3 times higher than that measured after 12 hours to 7 days of storage under vacuum (70 mbar). However, the hydrogen content is negligible as the deposit is heated at 90°C for 1 hour after electrodeposition. The presence of hydrogen in deposits after elaboration can be attributed to the co-deposition of hydrogen during the plating process [14]. As it was suggested, the release of diffusible hydrogen may influence the mechanical stresses in Ni-W deposits [15, 18, 20] so in order to ensure the results reliability, samples were systematically stored under vacuum (70 mbar) for 12 hours and just before hot extraction analysis were taken out of the vacuum storage. The influence of the tungsten addition from 1.47 to 18 at.% on the grain size evaluated by Scherrer formula on (111) peak, is summarized in Table 3. In accordance with published results, W incorporation leads to a decrease of the grain size [16, 17, 20, 21]. The X-ray diffraction patterns obtained for deposits with three different W contents are presented in Fig. 5. The comparison with the peak intensities of the hkl reflections for a randomly oriented nickel sample (JCPDS: 00-004-0850) shown that the crystallographic texture is modified from non-textured to <111> for alloys containing higher than 10 at.% of W. TEM observations (Fig. 6) were also realized for some of these alloys, in order to verify the reliability of determined grain sizes by Scherrer formula. The results show that the correlation between XRD and TEM techniques is only valid for alloys containing more than 10 at.% of W. It is suggested that the formation of twins in low alloyed deposits (Fig. 6) affects the grain size estimation deduced from XRD analyses [5, 22] and leads to an underestimation of the grain size by this method which actually corresponds to the size of twins. Hydrogen, oxygen and nitrogen amounts measured by hot extraction analysis present a marked evolution with W content and grain size refinement (Table 3). The distribution of hydrogen and oxygen in these alloys was evaluated by SIMS analysis in profile mode. The obtained profiles for two alloys containing 10 and 18 at.% of W are compared in Fig. 7. It is observed that the hydrogen and oxygen profiles are stable and they present higher signals for alloy containing 10 at.% of W. In agreement with SIMS profiles, the hot extraction results show that the H, O and N concentrations first increase with W incorporation and a maximum amount occurs for W contents between 7 and 14 at.% of W and grain sizes between 10 and 30 nm (Fig. 8). A decrease is then observed when high W alloy contents with finer grain sizes are reached. It is also observed that the crystallographic texture is modified from non-textured to <111> in less contaminated deposits at a similar transition in term of W content. The addition of W in nickel deposits results in finer grain sizes and thus higher volume fraction of grain boundaries. According to the equilibrium phase diagram of W [23], solubility limit for W in Ni matrix at room temperature is around 12 at.% [24]. By using atom-probe tomography, Detor et al. [25] shown that W atoms have a weak tendency to segregate at grain boundaries. Tang et al. [3] recently discussed, with experimental evidence, the influence of low levels of impurities on microstructural stability in nanocrystalline Al films obtained by magnetron sputtering. In their study, segregation of oxygen at grain boundaries was observed for average oxygen contents between 0.18 and 1.76 at.% and grain sizes about 20 nm. Generally, in electrodeposited coatings the impurity content is given in weight ppm. However, if the values are expressed in atomic percent the most contaminated Ni-W alloy presents 1.1 at.% of hydrogen and 0.24 at.% of oxygen. As the solubility of H and O in nickel at room temperature is very low (for example, H/Ni ~ 3 × 10-5) [26, 27], the incorporation of these elements can contribute to the grain refinement, particularly for low W concentrations (below the W solubility limit). When the W solubility limit is reached, W segregation on grain boundaries could affect the incorporation of oxygen and hydrogen on grain
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boundaries, which probably limits the introduction of these species in the material and modifies the texture. As shown in figure 9, the grain size in Ni-W deposits evolves with an increase in reverse pulsed current density from 0 to 200 mA/cm², applied during 3 ms. The obtained results are in good agreement with those presented by Detor et al. [21], for which this effect is explained by a preferential stripping of W atoms during the anodic pulse, leading to a lower incorporation of W and higher grain sizes. Moreover, Natter et al. [28] shown that in electrodeposited nickel the oxygen co-deposition is controlled by the current density of the anodic pulse. The present results show that in electrodeposited Ni-W the reverse pulsed current density influences both W content and light elements contamination. The origin of the light elements in Ni-W deposited alloys is due to the adsorption of different compounds during the electrocrystallization process. Several studies have shown that the chemical composition of these alloys depends strongly on the electrodeposition bath, and so the incorporation of associated species, in relation with the elaboration parameters [29-32]. In our case, the high oxygen contents may be related to the presence of tungstate. Also, the complexing agents as ammonia and citrate may influence the hydrogen and oxygen content and in a lesser proportion the nitrogen content. In order to complete these results, further analyses are in progress to evaluate the carbon contamination in deposits, due to the presence of the citrate complexing agent in the electrodeposition bath. 4. Conclusions The light elements contamination in electrodeposited Ni and Ni-W alloys was determined systematically, for the first time to our knowledge, by hot extraction measurements. The obtained results were then compared with those evaluated by SIMS analysis. We have shown that the contamination in nickel deposits increases drastically with grain size refinement and crystallographic texture evolution. As the grain size range at the nanometer scale was restricted in electrodeposited nickel, Ni-W alloys were elaborated from an additive free bath in order to better understand the relation between grain size refinement and light elements contamination. According to the hydrogen and oxygen profiles obtained from SIMS analysis, these elements were uniformly distributed in Ni-W deposited alloys and the hot extraction measurements showed a maximum contamination for H, O and N around 10 at.% of W. We suggest that below the solubility limit of W in Ni (~12 at.%) the incorporation of these elements can contribute to the grain refinement, however, beyond this limit the W addition seems to be the major parameter affecting the grain refinement and the texture modification. The presence of light elements is influenced by the bath composition and elaboration parameters. As we have shown, in alloys deposited by reverse pulsed current the anodic pulsed current density controls both W and light elements contents. Further analyses are in progress to evaluate the carbon and sulfur contents in these alloys. Acknowledgments The authors would like to thank the Poitou-Charentes region for their financial support, Dr. T. Hungria (INSA Toulouse, France) for SIMS analysis, Mrs C. Rébéré and Mr. Cyril Berziou (LaSIE, Université de La Rochelle, France) for their technical support and advice on the hot extraction measurements and Dr. E. Conforto (Université de La Rochelle, France) for TEM facilities.
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References [1] S.X. McFadden, A.K. Mukherjee, Materials Science and Engineering: A, 395 (2005) 265-268. [2] G.D. Hibbard, K.T. Aust, U. Erb, Materials Science and Engineering: A, 433 (2006) 195-202. [3] F. Tang, D.S. Gianola, M.P. Moody, K.J. Hemker, J.M. Cairney, Acta Materialia, 60 (2012) 10381047. [4] I. Matsui, T. Uesugi, Y. Takigawa, K. Higashi, Acta Materialia, 61 (2013) 3360-3369. [5] I. Matsui, Y. Takigawa, T. Uesugi, K. Higashi, Materials Letters, 99 (2013) 65-67. [6] S. Armyanov, G. Sotirova-Chakarova, Journal of The Electrochemical Society, 139 (1992) 3454-3457. [7] P. Marcus, J. Oudar, I. Olefjord, Materials Science and Engineering, 42 (1980) 191-197. [8] G. Palumbo, K.T. Aust, Acta Metallurgica Et Materialia, 38 (1990) 2343-2352. [9] E. Chassaing, K. Vu Quang, M.E. Baumgärtner, M.J. Funk, C.J. Raub, Surface and Coatings Technology, 53 (1992) 257-265. [10] J.P. Bonino, S. Bruet-Hotellaz, C. Bories, P. Pouderoux, A. Rousset, Journal of Applied Electrochemistry, 27 (1997) 1193-1197. [11] H. Natter, M. Schmelzer, R. Hempelmann, Journal of Materials Research, 13 (1998) 1186-1197. [12] A. Godon, J. Creus, X. Feaugas, E. Conforto, L. Pichon, C. Armand, C. Savall, Materials Characterization, 62 (2011) 164-173. [13] C. Savall, A. Godon, J. Creus, X. Feaugas, Surface and Coatings Technology, 206 (2012) 4394-4402. [14] D.R. Gabe, Journal of Applied Electrochemistry, 27 (1997) 908-915. [15] T. Yamasaki, Scripta Materialia, 44 (2001) 1497-1502. [16] T. Yamasaki, P. Schlossmacher, K. Ehrlich, Y. Ogino, Nanostructured Materials, 10 (1998) 375-388. [17] C.A. Schuh, T.G. Nieh, H. Iwasaki, Acta Materialia, 51 (2003) 431-443. [18] I. Mizushima, P.T. Tang, H.N. Hansen, M.A.J. Somers, Electrochimica Acta, 51 (2006) 6128-6134. [19] R. Juškenas, I. Valsiunas, V. Pakštas, R. Giraitis, Electrochimica Acta, 54 (2009) 2616-2620. [20] T.D. Ziebell, C.A. Schuh, Journal of Materials Research, 27 (2012) 1271-1284. [21] A.J. Detor, C.A. Schuh, Acta Materialia, 55 (2007) 371-379. [22] B.Y.C. Wu, P.J. Ferreira, C.A. Schuh, Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science, 36 (2005) 1927-1936. [23] ASM Handbook : Alloy Phase Diagrams, vol.3, American Society for Metals, Metals Park, OH,1992. [24] A. Gabriel, H.L. Lukas, C.H. Allibert, I. Ansara, Zeitschrift fuer Metallkunde/Materials Research and Advanced Techniques, 76 (1985) 589-595. [25] A.J. Detor, M.K. Miller, C.A. Schuh, Philosophical Magazine, 86 (2006) 4459-4475. [26] M.L. Wayman, G.C. Weatherly, Bulletin of Alloy Phase Diagrams, 10 (1989) 569-580. [27] J.-W. Park, C. Altstetter, MTA, 18 (1987) 43-50. [28] H. Natter, R. Hempelmann, Electrochimica Acta, 49 (2003) 51-61. [29] H. Cesiulis, A. Baltutiene, M. Donten, M.L. Donten, Z. Stojek, Journal of Solid State Electrochemistry, 6 (2002) 237-244. [30] O. Younes, E. Gileadi, Journal of The Electrochemical Society, 149 (2002) C100-C111. [31] M.D. Obradović, R.M. Stevanović, A.R. Despić, Journal of Electroanalytical Chemistry, 552 (2003) 185-196. [32] N. Eliaz, T.M. Sridhar, E. Gileadi, Electrochimica Acta, 50 (2005) 2893-2904.
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Table 1. Bath composition for Ni-W electrodeposition [16]
Chemical species Sodium citrate dihydrate Ammonium Chloride Sodium tungstate Nickel sulfate Sodium bromide
Formula Na3C6H5O7.2H2O NH4Cl Na2WO4.2H2O NiSO4.6H2O NaBr
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Concentration (mol/L) 0.5 0.5 0.14 0.06 0.15
Table 2. Light elements evaluation in bulk nickel and nickel deposits with different grain sizes and crystallographic textures Specimen
Grain size
Texture
Bulk nickel (Ni-D)
168 (µm)
NT
Nickel coating (C50)
750 (nm)
200
Nickel coating (C5)
112 (nm)
220
18
Technique
H (wt. ppm)
O (wt. ppm)
Hot Extraction Inert Gas Fusion Hot Extraction SIMS Hot Extraction SIMS
2 0.8 9.8 1 41 70
11 9.6 16 25 150 400
Table 3. Summary of microstructural and chemical properties of Ni-W alloys electrodeposited at 65°C. Forward current density of 100 mA/cm² was applied during 20ms. Reverse current densities were varied between 0 and 200 mA/cm² and applied during 3ms. Ni-15at.%W was elaborated by direct current at 50 mA/cm².
W (at. %)
1.47 1.5 3 5 5.6 7 10 14 15 18
Reverse j (mA/cm²) 200 150 100 50 100 50 50 0 dc 25
Grain size dXRD (nm) 224 158 83.5 38 42 28 13.5 9 8 5
Grain size dTEM (nm) 760 236 16.8 9 5.4
Crystallographic Texture NT NT NT NT NT NT 111 111 111 111
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H (wt. ppm) 37 29 33.5 64.5 114 152 164 91.5 83 51
O (wt. ppm) 319 319 336 216.5 487 426.5 580 117.5 381 382.5
N (wt. ppm) 9.5 9.5 49 45 90 103 171.5 34.5 151.5 27
Highlights • • •
• •
The light elements contamination in electrodeposited nickel and Ni-W alloys is influenced by the elaboration conditions. In electrodeposited nickel the light elements contamination increases drastically with grain size refinement and crystallographic texture evolution. SIMS analysis showed that the hydrogen and oxygen were uniformly distributed in Ni-W deposited alloys and a maximum contamination for H, O and N around 10 at.% of W was found by hot extraction method. The incorporation of light elements below the solubility limit of W in Ni (~12 at.%) may contribute to the grain refinement. The W addition beyond the solubility limit seems to be the major parameter affecting the grain refinement and the texture modification.
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S0925-8388(14)01025-1 http://dx.doi.org/10.1016/j.jallcom.2014.04.192 JALCOM 31157
To appear in: Received Date: Revised Date: Accepted Date:
5 March 2014 24 April 2014 25 April 2014
Please cite this article as: N. Shakibi Nia, J. Creus, X. Feaugas, C. Savall, The effect of tungsten addition on metallurgical state and solute content in nanocrystalline electrodeposited nickel, (2014), doi: http://dx.doi.org/ 10.1016/j.jallcom.2014.04.192
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
The effect of tungsten addition on metallurgical state and solute content in nanocrystalline electrodeposited nickel N. Shakibi Nia*, J. Creus, X. Feaugas, C. Savall Laboratoire des Sciences de l'Ingénieur pour l'Environnement, CNRS UMR 7356, Université de La Rochelle, Av. Michel Crépeau, F-17042 La Rochelle, France Abstract It is usually difficult to control the incorporation of foreign species in electrodeposited coatings originating from the solvent or the chemical species used for the electrodeposition bath. However, the presence of these impurities can modify their physicochemical properties. In the present study, complementary analytical techniques were used to evaluate the chemical contamination in nickel and nickel-tungsten alloys, electrodeposited from additive free baths. In order to better understand the relationship between impurity content and grain size refinement, the concentration of light elements (H, O and N) was systematically quantified by hot extraction analysis. Also, the distribution of contaminants was evaluated by SIMS analysis. We have shown that in nanocrystalline electrodeposited nickel the grain size refinement and the impurity contents are strongly related. However, in Ni-W alloys the evolution of the contamination is more complex, with a maximum amount for W contents around 10 at.%. 1. Introduction Nanocrystalline materials have been the subject of many studies due to their specific physicochemical properties, generally correlated to the grain size refinement. However, as the grain size decreases, other parameters as crystallographic orientation, solute content, internal stresses, grain boundary or misorientation and density of defects are also modified. Several studies reported that even small amount of impurities can have a dramatic effect on the properties of nanocrystalline and polycrystalline metals (ex. strength and ductility or corrosion) [1-8], which shows the importance of evaluating these elements. Electrodeposition of metals and alloys are usually accompanied by the incorporation of foreign species originating from the solvent (often water, leading to hydrogen co-deposition) or from the chemicals species used for the bath (metallic salts, complexing agents, additives …). Metallic impurities can be analyzed using elementary analysis techniques such as X-Ray Fluorescence, GDOES (Glow Discharge Optical Emission Spectroscopy) or GDMS (Glow Discharge Mass Spectrometry). However, the detection and quantification of light elements, as H, C, N, and O, is quite difficult explaining why these elements are generally investigated in bulk materials rather than in electrodeposited metals. Few studies have investigated the non-metallic contamination in electrodeposited nickel and nickel alloys using hot extraction method [9-11]. The nitrogen content in nanocrystalline electrodeposited nickel was influenced by the bath composition and increased up to 620 wt. ppm in coatings elaborated from a bath containing tartrate (complexing agent) and Na-saccharin (inhibitor) [11]. Chassaing et al. [9] have shown that the amounts of H and O are more important in electrodeposited Ni-Mo alloys than in pure nickel deposited from the same bath.
* Corresponding author Email address: [email protected] Tel: +335 46 45 82 68
1
A significant impact of the bath deposition temperature was reported, which resulted in maximum amounts of around 3800 wt. ppm of oxygen at 60-80°C and about 400 wt. ppm of hydrogen at 60°C. In electrodeposited Ni-P alloys the amount of absorbed hydrogen increased with the phosphorus content in alloys [10]. More recently, SIMS analysis (Secondary Ion Mass Spectrometry) was used to evaluate the influence of deposition parameters on chemical contamination in electrodeposited nickel coatings obtained from an additive-free sulphamate bath [12, 13]. According to the obtained results, an important increase of contamination by light elements is observed with the decrease in deposition current density and with grain size refinement. However, a better understanding of the relationship between metallurgical state and solute content can be achieved through a broader grain size range. Several studies have reported that the grain size range in electrodeposited nickel can be expanded at the nanometer scale by addition of tungsten alloying element. Also, it has been suggested that light elements incorporation during the electrodeposition process may affect the mechanical properties of these alloys, but the amounts of these impurities have not been yet quantified [14-20]. In this paper, the relation between grain size refinement and impurity content in nanocrystalline electrodeposited Ni-W alloys is being discussed. For this purpose, the hot extraction method is used to quantify the amounts of light elements in Ni-W alloys, for the first time to our knowledge. 2. Experimental procedure Nickel deposits were obtained by direct current (DC) and pulsed current (PC) on annealed nickel substrates (99.7 %) in a classical three electrode cell, using a VSP potentiostat from Biologic. An additive free sulphamate bath was used in order to minimize the incorporation of contaminants [12]. Ni-W alloys were also electrodeposited on polycrystalline nickel substrates (99 %) by direct current (DC) and reverse pulsed current (RPC). The bath proposed by Yamasaki et al. [16] was used without additives at 65°C for most of the deposits (Table 1) and for some other the concentration of nickel sulfate and sodium tungstate was modified in order to vary the W content. Nickel substrates were mechanically polished with 1µ m diamond paste before electrodeposition and the anode was a platinum mesh. For each condition the cathodic efficiency (η) was estimated by weighting the specimen before and after deposition and the deposition time (or the number of pulses) was adjusted to obtain a thickness of 50 µm. The applied cathodic current density in DC method was 5 and 50 mA/cm². The conditions in RPC method were chosen according to the study of Detor et al. [21] for which a constant forward current density of 100 mA/cm² was applied during 20 ms and the reverse current density was varied between 50 and 200 mA/cm² during 3 ms. A multi-scale approach was used to characterize the microstructure in nickel and Ni-W alloys deposits. X-ray analysis was performed with Cu-Kα radiation (λ=0.15405 nm) in θ-2θ mode using a Bruker AXS D8-Advanced operating at 40 kV and 40 mA. Spectra were obtained between 40° and 100° and then the instrumental broadening, Kα2 ray and background were removed. In order to estimate the grain size, Scherrer formula was applied on (111) peak. TEM micrographs were obtained using Transmission Electron Microscope (TEM JEOL JEM 2011) operating at 200 kV. Composition of deposits was analyzed using a Bruker M4 Tornado X-ray micro-fluorescence (µ-XRF) operating with a rhodium filament at 35 kV and 300 µA and Secondary Ion Mass Spectrometry (SIMS) with IMS 4FE6 Apparatus from CAMECA and two ionic sources Cs+ (at 14.5 keV) and O2+ (at 5.5 keV) to obtain the best sensitivity. 2
The concentration profiles were obtained after 5 to 10 µm pulverization in order to eliminate the surface contamination effects. The distribution of the elements can also be revealed through surface and cross section mapping using this technique. Hot extraction analyses were carried out using EMGA-621W oxygen / nitrogen / hydrogen analyzer to determine the light elements contamination in Ni and Ni-W deposits. 3. Results and discussion The hydrogen and oxygen contamination was first evaluated by hot extraction method for the bulk nickel and for two nickel deposits elaborated by direct current at 5 mA/cm² (C5) and 50 mA/cm² (C50) with different grain sizes and crystallographic textures (Table 2), elaborated from an additive free sulphamate bath. The obtained results reveal an increase in oxygen and hydrogen contamination when the grain size is reduced. As presented in Fig. 1, the oxygen contamination in the deposits depends on the period between their elaboration and the hot extraction analysis. In order to limit the oxidation of samples and to optimize the reproducibility of these measurements, the samples are systematically stored after electrodeposition for 12 hours under vacuum (70 mbar), and they are then analyzed. SIMS analysis was used to evaluate the light elements distribution for the same coatings. According to the concentration profiles and mappings, the impurities were homogeneously distributed laterally and through the thickness of the coatings. Several profiles were obtained for each sample and the calibration was realized with bulk nickel of known composition [13]. However, this semi-quantitative analysis is limited for the study of alloys, because reference samples with the same composition are required to avoid an important matrix effect. For comparison, Fig. 2 presents the results obtained by hot extraction and SIMS analysis for bulk nickel and two Ni based coatings elaborated from the sulphamate bath. It is observed that the amounts of H and O incorporated in nickel deposits present the same trend using these two techniques, despite the fact that hot extraction is a non-localized method. This result validates the developed sample preparation procedure for hot extraction analysis. It was observed that in electrodeposited nickel the grain size refinement is strongly influenced by the incorporation of impurities, especially the light element species. However, the restricted grain size range strongly limits the discussion on the relation between grain size refinement and light elements contamination. Therefore, as the addition of tungsten permits to reach grain sizes down to nanometer scale without the addition of additives in the deposition bath, Ni-W alloys were elaborated and investigated. The hot extraction analysis was systematically used to evaluate the amounts of light elements in nanocrystalline Ni-W alloys electrodeposited at 50 mA/cm² by direct current at 25°C and 65°C. Fig. 3 shows that the bath temperature has an important influence on the light elements incorporation in these deposits. The influence of the bath deposition temperature between 40 and 160°C was discussed in the study of Chassaing et al. on electrodeposited Ni-Mo alloys, in which a maximum amount of contamination for oxygen was found in deposits elaborated at 60-80°C [9]. This result highlights the importance of optimizing the elaboration conditions to limit the incorporation of impurities in deposits. For the following Ni-W deposits the bath temperature was set to 65°C, as, in our case, this temperature led to the lowest contamination levels. In order to enhance the reproducibility, the influence of different storage conditions on the hydrogen concentration evolution was then studied. For this purpose, a 15 at.% Ni-W deposit, (elaborated by DC method) was stored in various conditions before hot extraction measurements. As observed in Fig. 4, the 3
amounts of measured hydrogen immediately after electrodeposition is 3 times higher than that measured after 12 hours to 7 days of storage under vacuum (70 mbar). However, the hydrogen content is negligible as the deposit is heated at 90°C for 1 hour after electrodeposition. The presence of hydrogen in deposits after elaboration can be attributed to the co-deposition of hydrogen during the plating process [14]. As it was suggested, the release of diffusible hydrogen may influence the mechanical stresses in Ni-W deposits [15, 18, 20] so in order to ensure the results reliability, samples were systematically stored under vacuum (70 mbar) for 12 hours and just before hot extraction analysis were taken out of the vacuum storage. The influence of the tungsten addition from 1.47 to 18 at.% on the grain size evaluated by Scherrer formula on (111) peak, is summarized in Table 3. In accordance with published results, W incorporation leads to a decrease of the grain size [16, 17, 20, 21]. The X-ray diffraction patterns obtained for deposits with three different W contents are presented in Fig. 5. The comparison with the peak intensities of the hkl reflections for a randomly oriented nickel sample (JCPDS: 00-004-0850) shown that the crystallographic texture is modified from non-textured to <111> for alloys containing higher than 10 at.% of W. TEM observations (Fig. 6) were also realized for some of these alloys, in order to verify the reliability of determined grain sizes by Scherrer formula. The results show that the correlation between XRD and TEM techniques is only valid for alloys containing more than 10 at.% of W. It is suggested that the formation of twins in low alloyed deposits (Fig. 6) affects the grain size estimation deduced from XRD analyses [5, 22] and leads to an underestimation of the grain size by this method which actually corresponds to the size of twins. Hydrogen, oxygen and nitrogen amounts measured by hot extraction analysis present a marked evolution with W content and grain size refinement (Table 3). The distribution of hydrogen and oxygen in these alloys was evaluated by SIMS analysis in profile mode. The obtained profiles for two alloys containing 10 and 18 at.% of W are compared in Fig. 7. It is observed that the hydrogen and oxygen profiles are stable and they present higher signals for alloy containing 10 at.% of W. In agreement with SIMS profiles, the hot extraction results show that the H, O and N concentrations first increase with W incorporation and a maximum amount occurs for W contents between 7 and 14 at.% of W and grain sizes between 10 and 30 nm (Fig. 8). A decrease is then observed when high W alloy contents with finer grain sizes are reached. It is also observed that the crystallographic texture is modified from non-textured to <111> in less contaminated deposits at a similar transition in term of W content. The addition of W in nickel deposits results in finer grain sizes and thus higher volume fraction of grain boundaries. According to the equilibrium phase diagram of W [23], solubility limit for W in Ni matrix at room temperature is around 12 at.% [24]. By using atom-probe tomography, Detor et al. [25] shown that W atoms have a weak tendency to segregate at grain boundaries. Tang et al. [3] recently discussed, with experimental evidence, the influence of low levels of impurities on microstructural stability in nanocrystalline Al films obtained by magnetron sputtering. In their study, segregation of oxygen at grain boundaries was observed for average oxygen contents between 0.18 and 1.76 at.% and grain sizes about 20 nm. Generally, in electrodeposited coatings the impurity content is given in weight ppm. However, if the values are expressed in atomic percent the most contaminated Ni-W alloy presents 1.1 at.% of hydrogen and 0.24 at.% of oxygen. As the solubility of H and O in nickel at room temperature is very low (for example, H/Ni ~ 3 × 10-5) [26, 27], the incorporation of these elements can contribute to the grain refinement, particularly for low W concentrations (below the W solubility limit). When the W solubility limit is reached, W segregation on grain boundaries could affect the incorporation of oxygen and hydrogen on grain
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boundaries, which probably limits the introduction of these species in the material and modifies the texture. As shown in figure 9, the grain size in Ni-W deposits evolves with an increase in reverse pulsed current density from 0 to 200 mA/cm², applied during 3 ms. The obtained results are in good agreement with those presented by Detor et al. [21], for which this effect is explained by a preferential stripping of W atoms during the anodic pulse, leading to a lower incorporation of W and higher grain sizes. Moreover, Natter et al. [28] shown that in electrodeposited nickel the oxygen co-deposition is controlled by the current density of the anodic pulse. The present results show that in electrodeposited Ni-W the reverse pulsed current density influences both W content and light elements contamination. The origin of the light elements in Ni-W deposited alloys is due to the adsorption of different compounds during the electrocrystallization process. Several studies have shown that the chemical composition of these alloys depends strongly on the electrodeposition bath, and so the incorporation of associated species, in relation with the elaboration parameters [29-32]. In our case, the high oxygen contents may be related to the presence of tungstate. Also, the complexing agents as ammonia and citrate may influence the hydrogen and oxygen content and in a lesser proportion the nitrogen content. In order to complete these results, further analyses are in progress to evaluate the carbon contamination in deposits, due to the presence of the citrate complexing agent in the electrodeposition bath. 4. Conclusions The light elements contamination in electrodeposited Ni and Ni-W alloys was determined systematically, for the first time to our knowledge, by hot extraction measurements. The obtained results were then compared with those evaluated by SIMS analysis. We have shown that the contamination in nickel deposits increases drastically with grain size refinement and crystallographic texture evolution. As the grain size range at the nanometer scale was restricted in electrodeposited nickel, Ni-W alloys were elaborated from an additive free bath in order to better understand the relation between grain size refinement and light elements contamination. According to the hydrogen and oxygen profiles obtained from SIMS analysis, these elements were uniformly distributed in Ni-W deposited alloys and the hot extraction measurements showed a maximum contamination for H, O and N around 10 at.% of W. We suggest that below the solubility limit of W in Ni (~12 at.%) the incorporation of these elements can contribute to the grain refinement, however, beyond this limit the W addition seems to be the major parameter affecting the grain refinement and the texture modification. The presence of light elements is influenced by the bath composition and elaboration parameters. As we have shown, in alloys deposited by reverse pulsed current the anodic pulsed current density controls both W and light elements contents. Further analyses are in progress to evaluate the carbon and sulfur contents in these alloys. Acknowledgments The authors would like to thank the Poitou-Charentes region for their financial support, Dr. T. Hungria (INSA Toulouse, France) for SIMS analysis, Mrs C. Rébéré and Mr. Cyril Berziou (LaSIE, Université de La Rochelle, France) for their technical support and advice on the hot extraction measurements and Dr. E. Conforto (Université de La Rochelle, France) for TEM facilities.
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References [1] S.X. McFadden, A.K. Mukherjee, Materials Science and Engineering: A, 395 (2005) 265-268. [2] G.D. Hibbard, K.T. Aust, U. Erb, Materials Science and Engineering: A, 433 (2006) 195-202. [3] F. Tang, D.S. Gianola, M.P. Moody, K.J. Hemker, J.M. Cairney, Acta Materialia, 60 (2012) 10381047. [4] I. Matsui, T. Uesugi, Y. Takigawa, K. Higashi, Acta Materialia, 61 (2013) 3360-3369. [5] I. Matsui, Y. Takigawa, T. Uesugi, K. Higashi, Materials Letters, 99 (2013) 65-67. [6] S. Armyanov, G. Sotirova-Chakarova, Journal of The Electrochemical Society, 139 (1992) 3454-3457. [7] P. Marcus, J. Oudar, I. Olefjord, Materials Science and Engineering, 42 (1980) 191-197. [8] G. Palumbo, K.T. Aust, Acta Metallurgica Et Materialia, 38 (1990) 2343-2352. [9] E. Chassaing, K. Vu Quang, M.E. Baumgärtner, M.J. Funk, C.J. Raub, Surface and Coatings Technology, 53 (1992) 257-265. [10] J.P. Bonino, S. Bruet-Hotellaz, C. Bories, P. Pouderoux, A. Rousset, Journal of Applied Electrochemistry, 27 (1997) 1193-1197. [11] H. Natter, M. Schmelzer, R. Hempelmann, Journal of Materials Research, 13 (1998) 1186-1197. [12] A. Godon, J. Creus, X. Feaugas, E. Conforto, L. Pichon, C. Armand, C. Savall, Materials Characterization, 62 (2011) 164-173. [13] C. Savall, A. Godon, J. Creus, X. Feaugas, Surface and Coatings Technology, 206 (2012) 4394-4402. [14] D.R. Gabe, Journal of Applied Electrochemistry, 27 (1997) 908-915. [15] T. Yamasaki, Scripta Materialia, 44 (2001) 1497-1502. [16] T. Yamasaki, P. Schlossmacher, K. Ehrlich, Y. Ogino, Nanostructured Materials, 10 (1998) 375-388. [17] C.A. Schuh, T.G. Nieh, H. Iwasaki, Acta Materialia, 51 (2003) 431-443. [18] I. Mizushima, P.T. Tang, H.N. Hansen, M.A.J. Somers, Electrochimica Acta, 51 (2006) 6128-6134. [19] R. Juškenas, I. Valsiunas, V. Pakštas, R. Giraitis, Electrochimica Acta, 54 (2009) 2616-2620. [20] T.D. Ziebell, C.A. Schuh, Journal of Materials Research, 27 (2012) 1271-1284. [21] A.J. Detor, C.A. Schuh, Acta Materialia, 55 (2007) 371-379. [22] B.Y.C. Wu, P.J. Ferreira, C.A. Schuh, Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science, 36 (2005) 1927-1936. [23] ASM Handbook : Alloy Phase Diagrams, vol.3, American Society for Metals, Metals Park, OH,1992. [24] A. Gabriel, H.L. Lukas, C.H. Allibert, I. Ansara, Zeitschrift fuer Metallkunde/Materials Research and Advanced Techniques, 76 (1985) 589-595. [25] A.J. Detor, M.K. Miller, C.A. Schuh, Philosophical Magazine, 86 (2006) 4459-4475. [26] M.L. Wayman, G.C. Weatherly, Bulletin of Alloy Phase Diagrams, 10 (1989) 569-580. [27] J.-W. Park, C. Altstetter, MTA, 18 (1987) 43-50. [28] H. Natter, R. Hempelmann, Electrochimica Acta, 49 (2003) 51-61. [29] H. Cesiulis, A. Baltutiene, M. Donten, M.L. Donten, Z. Stojek, Journal of Solid State Electrochemistry, 6 (2002) 237-244. [30] O. Younes, E. Gileadi, Journal of The Electrochemical Society, 149 (2002) C100-C111. [31] M.D. Obradović, R.M. Stevanović, A.R. Despić, Journal of Electroanalytical Chemistry, 552 (2003) 185-196. [32] N. Eliaz, T.M. Sridhar, E. Gileadi, Electrochimica Acta, 50 (2005) 2893-2904.
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Table 1. Bath composition for Ni-W electrodeposition [16]
Chemical species Sodium citrate dihydrate Ammonium Chloride Sodium tungstate Nickel sulfate Sodium bromide
Formula Na3C6H5O7.2H2O NH4Cl Na2WO4.2H2O NiSO4.6H2O NaBr
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Concentration (mol/L) 0.5 0.5 0.14 0.06 0.15
Table 2. Light elements evaluation in bulk nickel and nickel deposits with different grain sizes and crystallographic textures Specimen
Grain size
Texture
Bulk nickel (Ni-D)
168 (µm)
NT
Nickel coating (C50)
750 (nm)
200
Nickel coating (C5)
112 (nm)
220
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Technique
H (wt. ppm)
O (wt. ppm)
Hot Extraction Inert Gas Fusion Hot Extraction SIMS Hot Extraction SIMS
2 0.8 9.8 1 41 70
11 9.6 16 25 150 400
Table 3. Summary of microstructural and chemical properties of Ni-W alloys electrodeposited at 65°C. Forward current density of 100 mA/cm² was applied during 20ms. Reverse current densities were varied between 0 and 200 mA/cm² and applied during 3ms. Ni-15at.%W was elaborated by direct current at 50 mA/cm².
W (at. %)
1.47 1.5 3 5 5.6 7 10 14 15 18
Reverse j (mA/cm²) 200 150 100 50 100 50 50 0 dc 25
Grain size dXRD (nm) 224 158 83.5 38 42 28 13.5 9 8 5
Grain size dTEM (nm) 760 236 16.8 9 5.4
Crystallographic Texture NT NT NT NT NT NT 111 111 111 111
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H (wt. ppm) 37 29 33.5 64.5 114 152 164 91.5 83 51
O (wt. ppm) 319 319 336 216.5 487 426.5 580 117.5 381 382.5
N (wt. ppm) 9.5 9.5 49 45 90 103 171.5 34.5 151.5 27
Highlights • • •
• •
The light elements contamination in electrodeposited nickel and Ni-W alloys is influenced by the elaboration conditions. In electrodeposited nickel the light elements contamination increases drastically with grain size refinement and crystallographic texture evolution. SIMS analysis showed that the hydrogen and oxygen were uniformly distributed in Ni-W deposited alloys and a maximum contamination for H, O and N around 10 at.% of W was found by hot extraction method. The incorporation of light elements below the solubility limit of W in Ni (~12 at.%) may contribute to the grain refinement. The W addition beyond the solubility limit seems to be the major parameter affecting the grain refinement and the texture modification.
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