Role of Ceria Nanoparticles on the Electrodeposited ...

ECS Electrochemistry Letters, 3 (9) D33-D35 (2014)

2162-8726/2014/3(9)/D33/3/$31.00 © The Electrochemical Society

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Role of Ceria Nanoparticles on the Electrodeposited Zinc Coating’s Growth: Interest of a TEM-Scale Investigation L. Exbrayat,a,b,z E. Calvié,b T. Douillard,b G. Marcos,c C. Savall,a C. Berziou,a J. Creus,a and P. Steyerb a LaSIE, Universit´ e de la Rochelle, F-17042 La Rochelle, France b MATEIS, INSA de Lyon, F-69621 Villeurbanne Cedex, France c Institut Jean Lamour, Ecole des Mines de Nancy, Parc de Saurupt,

CNRS UMR 7198, F-54042 Nancy Cedex, France

The study deals with a Zn-ceria nanocomposite coating synthesized by direct current electrodeposition. The aim of the study was to determine how the presence of the ceramic particles may affect the coating’s growth. Distribution of the ceramic phase into the metallic matrix is deduced from a deep TEM characterization. It was observed that the formation of nanocomposite coating was strongly governed by a preferential lateral growth of zinc. Nanoparticles were mainly located on the top-surface and in a lesser extent into the film’s thickness. Optimization of the incorporated nanoparticules’ rate is then discussed on the basis of hosting sites, such as grain boundaries. © 2014 The Electrochemical Society. [DOI: 10.1149/2.0011410eel] All rights reserved. Manuscript submitted May 28, 2014; revised manuscript received July 10, 2014. Published July 24, 2014.

Metal Matrix Composite (MMC) is one of the most promising way to improve the durability of sacrificial coatings deposited onto steels.1 Some nickel- and cobalt-based coatings were thoroughly investigated. Recently, an increasing interest toward zinc-based composites is raised.2,3 Indeed, pure zinc coatings suffer from poor mechanical properties and the incorporation of a second hard phase during the electrodeposition process (e.g. ceramic nanoparticles) would normally permit to enhance them. In order to better understand the relationships involved between functional properties and particles, the localisation of this phase needs to be deeply characterized. In most of the papers, a simple comparative methodology is adopted based on the consequences of the presence or not, of particles on some functional properties: corrosion,4 mechanical properties.5 Besides, concerning more specifically zinc based composite coatings, authors did not focus their investigation on the embedded particles into the film’s thickness, probably due to the difficulty to prepare cross sections from a so soft metal as zinc. Adopting such an approach, the role of particles’ characteristics (nature, morphology, distribution. . . ) is then neglected.6,7 Our objective was to deeply characterize embedded ceria nanoparticles, in order to study their influence on the coating growth. A model susceptible to explain and optimize their incorporation in the electrodeposited zinc matrix is then proposed. Materials and Methods To make easier the interpretation, a simple coating was considered: a 8 μm-thick pure zinc matrix containing 2%wt of 35 nm-sized CeO2 nanoparticles. Coatings were deposited at a 30 mA.cm−2 direct current density from an alkaline ammonia bath containing nanoparticles in suspension. More details are given in reference.8 The coating’s surface was examined by Scanning Electron Microscopy (SEM, FEI Quanta 200 ESEM FEG). The cross section of the coating was also observed in the Transmission Electron Microscope-scale (TEM) using a JEOL 2010F TEM. The High Angle Annular Dark Field (HAADF) mode was preferred for its high sensitivity to chemical contrasts. Zinc thin foils were obtained by ultramicrotomy (Leica EM UC7). For this method, film was deposited on copper to preserve the diamond knife.9 Morphologies and properties of coatings electrodeposited onto copper substrate have been controlled to ensure that the nature of the substrate does not influence their characteristics. Results SEM surface observations of pure zinc and Zn-CeO2 nanocomposite coatings are presented on Fig. 1. The typical microstructure of zinc-based electrodeposited coatings is observed,10 with packets z

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of micrometers-sized hexagonal platelets randomly orientated, and stacked on each other (Fig. 1a). It is worth mentioning that such microstructure is unchanged for the composite coating (Fig. 1b). Ceria nanoparticles are present as agglomerates of some tens to some hundreds nanometers (Fig. 1c). They are preferentially adsorbed on the edges of the hexagonal plates, confirming Kondo’s results on Zn-SiO2 coatings.11 The presence of ceria nanoparticles is confirmed on the coating surface. However, durability of the film, regarding especially their mechanical properties, is mainly influenced by the nanoparticles introduced into the film thickness, which has also to be characterized. Conventional cross-section methods involving mechanical preparations are not recommended owing to the zinc intrinsic ductility. Therefore ultramicrotomy sectioning was used to obtain thin foils, and their observation allowed us to determine the particle nature and distribution into the coating. The chemical nature of particles was clearly identified in the film through X-ray images showing that ceria nanoparticles are clearly entrapped in the zinc thickness. Particles were associated in some tens nm-sized agglomerates (Fig. 2) and segregated into small cavities mostly located along grain boundaries (Fig. 3a). Such porosities are not inherent to the second phase as they were also observed for the pure zinc coating, devoid from particles (Fig. 3b). These narrow cavities could be due to the numerous defects resulting from the disordered zinc microstructure. They can play the role of hosting sites where nanoparticles could be entrapped during the zinc electrodeposition process.

Discussion: Toward a Control of the Nanoparticle’s Rate Distribution of CeO2 was characterized at the microscopic scale. Particles are present mainly in top-surface, but also inside the film itself. The specific microstructure of electrolytic zinc coatings is on the basis of a growth model susceptible to explain the ceramic dispersion inside the zinc matrix (Fig. 4). At the early stage of deposition, nuclei are formed randomly throughout the cathode surface (Fig. 4a). Due to the high tendency of zinc to form platelets, a lateral growth is favored, and particles are preferentially adsorbed to the edges of platelets (Fig. 4b). Most of the particles are rejected toward the surface, the others being trapped in small cavities into the film thickness. These porosities are mainly located in the intergranular space, which results from an imperfect contact between adjacent growing grains (Fig. 4c). Particles may then be found in different hosting sites: either through large aggregates in isolated holes (Figs. 4d, 4e) or along the grain boundaries adopting a string-like structure (Figs. 4d, 4f). Similarities between the morphology of the pure zinc and the Zn/CeO2 nanocomposite coatings, imply that nanoparticles do not really affect the specific zinc growth (Fig. 1). In both cases, we observe on the one hand the disordered structure (pile-up of large hexagonal

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ECS Electrochemistry Letters, 3 (9) D33-D35 (2014)

Figure 1. Surface SEM image of pure zinc (a) and zinc-ceria nanocomposite coatings (b and c).

Figure 2. TEM cross section: electronic image (b) and X-Ray images (a, c) of ceria nanoparticles incorporated in the zinc matrix.

Figure 3. TEM cross section images of a zinc-ceria nanocomposite coating (a) and of a pure zinc coating synthesized with the same parameters (b).

plates), and on the other hand, the presence of small cavities localized in grain boundaries between the stacks of platelets. This could explain why ceria content measured in the coating is quite low. A solution to improve the ceria incorporation would then be to increase the amount of hosting sites inside the film thickness. This goal can be reached modifying the zinc crystals’ growth, hindering the development of the lateral growth. Such a growth modification may concern chemistry of the bath on the one hand (alloying effect with nickel for instance12 ), or the deposition process on the other hand (pulsed plate current13 ). These advanced alternatives should lead to new coatings more enriched in particles, characterized by improved functional properties. Conclusions Using a TEM-scale investigation, we proposed a model of growth for an electrodeposited zinc-CeO2 nanocomposite coating susceptible to explain the heterogeneous distribution of nanoparticles. It was shown that nanoparticles did not affect the typical morphology of zinc matrix, with formation of packets of micrometers-sized hexagonal disordered platelets. Due to this specific platelet shape, growth

Figure 4. Different steps for the growth of electrodeposited zinc nanocomposite coatings (a) nucleation b) lateral growth, c) grain formation, d) particles entrapment and comparison with TEM images (e, f) explaining how particles may be introduced in the film.


ECS Electrochemistry Letters, 3 (9) D33-D35 (2014) is highly favored in the lateral direction. Therefore, nanoparticles are mainly rejected on top-surface, or, in a lesser extent, introduced into the coating along the grain boundaries. The solution to increase the particles’ incorporation rate would be to drastically refine the microstructure. Acknowledgment We acknowledge the French Agence National pour la Recherche for supporting this research under the Chameleon Project 2010 BLAN 939. We would like also to thank gratefully Helmut Gnaegi, from the Diatome company, for the delivery of ultramicrotomic sectioning. Moreover, authors acknowledge the CLYM (Center Lyonnais de Microscopie, http://www.clym.fr) for the access to the JEOL 2010 F microscope. References 1. C. T. J. Low, R. G. A. Wills, and F. C. Walsh,“Electrodeposition of composite coatings containing nanoparticles in a metal deposit,” Surface and Coatings Technology 201, 371 (2006). 2. J. Fustes and A. Gomes, “Silva Pereira MI. Electrodeposition of Zn–TiO2 nanocomposite films–effect of bath composition,” J Solid State Electrochem 12, 1435 (2008).

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3. X. Xia, I. Zhitomirsky, and J. R. McDermid, “Electrodeposition of zinc and composite zinc–yttria stabilized zirconia coatings,” Journal of Materials Processing Technology 209, 2632 (2009). 4. M. Azizi, W. Schneider, and W. Plieth, “Electrolytic co-deposition of silicate and mica particles with zinc,” J Solid State Electrochem. 9, 429 (2005). 5. S. Ranganatha, T. V. Venkatesha, K. Vathsala, and M. K. P. kumar, “Electrochemical studies on Zn/nano-CeO2 electrodeposited composite coatings,” Surface and Coatings Technology 208, 64 (2012). 6. N. Guglielmi, “Kinetics of the Deposition of Inert Particles from Electrolytic Baths,” Journal of The Electrochemical Society 119, 1009 (1972). 7. J. P. Celis, J. R. Roos, and C. Buelens, “A Mathematical Model for the Electrolytic Codeposition of Particles with a Metallic Matrix,” Journal of The Electrochemical Society 134, 1402 (1987). 8. L. Exbrayat, P. Steyer, C. Rébéré, C. Berziou, C. Savall, and P. Ayrault, et al, “Electrodeposition of zinc–ceria nanocomposite coatings in alkaline bath,” J Solid State Electrochem. 18, 223 (2014). 9. S. Shimada, H. Tanaka, M. Higuchi, T. Yamaguchi, S. Honda, and K. Obata, “ThermoChemical Wear Mechanism of Diamond Tool in Machining of Ferrous Metals,” CIRP Annals - Manufacturing Technology 53, 57 (2004). 10. C. A. Loto, “Electrodeposition of Zinc from acid based solutions: A review and experimental study,” Asian J Applied Sci. 5, 314 (2012). 11. K. Kondo, A. Ohgishi, and Z. Tanaka, “Electrodeposition of Zinc-SiO2 Composite,” Journal of The Electrochemical Society 147, 2611 (2000). 12. R. Rizwan, M. Mehmood, M. Imran, J. Ahmad, M. Aslam, and J. I. Akhter, “Deposition of nanocrystalline zinc-nickel alloys by DC plating in additive free chloride bath,” Materials transactions 48, 1558 (2007). 13. K. Saber, C. Koch, and P. Fedkiw, “Pulse current electrodeposition of nanocrystalline zinc,” Materials Science and Engineering: A 341, 174 (2003).
