Microstructure and mechanical properties of disperse Ni–Co alloys electrodeposited on Cu substrates

https://doi.org/10.1016/j.matchemphys.2009.11.029Get rights and content

Abstract

The nickel and cobalt disperse alloy deposits of five different compositions were obtained by electrodeposition on Cu substrates in the galvanostatic regime from an ammonium sulfate-chloride solution. The effect of cathodic current density and Ni2+/Co2+ ions concentration ratio in the electrolyte on the composition, microstructure, morphology and mechanical properties of Ni–Co alloys were investigated. Conditions for formation of nanostrustured disperse deposits and surface roughness was determined by 3D SEM reconstruction of the specimen surface. It was established that formation of the disperse deposits with highly developed structures is favored from bath with equal Ni2+/Co2+ ions concentration ratio in the electrolyte. Cathodic polarization diagrams determined for all investigated alloys have shown shift of the cathodic potential for alloy deposition to the more negative values with increasing Ni2+/Co2+ ions concentration ratio in the electrolyte. An increase of in the cobalt content in the alloy was observed with decreasing the current density and increasing of the Co2+ ions concentration ratio in the bath. X-ray analyses of nanocrystalline Ni–Co deposits show formation of a single phase face-centered cubic (FCC), a single phase hexagonal-close packed (HCP) and mixture of FCC solid solutions and HCP phase depending on the current density applied and electrolyte composition. The increase of HCP phase content in the nanocrystalline deposits appears as a result of both, the increase in Co2+ ions concentration in the bath and decrease of deposition current density. The mechanical properties of nanocrystalline deposits have shown increase of the hardness with increasing Ni content in the alloy. The cross-section of the samples electrodeposited on Cu substrates from electrolytes with equal ion metal concentration at lower current density values revealed the beginning of a dendrite structure formation.

Introduction

The recent interest in the electrodeposition of iron-group metals (Ni, Co and Fe) and their alloys is due to their unique magnetic and thermophysical properties [1], [2]. Electrodeposition is a process which is capable for depositing nanocrystalline metals and alloys [3] onto recessed and non-uniform surfaces, and therefore has found a role in microelectromechanical systems (MEMS) [2], [4], [5], [6], [7]. The hardness and strength of the electrolytic deposits are better than alloys prepared by conventional metallurgical processes [2]. One of the most important aspects of electrodeposition process is possibility for production of nanocrystalline materials which exhibit unique properties compared to the microcrystalline counterparts [3].

A metal powder represents a loose deposit which can spontaneously fall off from the electrode or can be removed by tapping or by other similar way [8], [9], [10]. The electrodeposition of the powders from the solutions, established by the work of Calusaru [11] possesses significant advantages over other methods for synthesis of nanocrystalline materials [3]. This method usually yields products of requested chemical composition and high purity, which can be well pressed and sintered [12], [13], [14]. The effect of operating conditions, electrolyte composition, temperature and pH on the Ni–Co composition and properties of obtained deposits are widely investigated [15], [16], [17], [18]. On the other hand, electrodeposition at very high current density and high overpotential is only investigated for the case of the obtaining of powders [19], [20], [21].

However, electrochemical synthesis of disperse deposits on the substrate is insufficiently investigated [8]. Investigations of such Cu deposits formed at high current densities characterized by open and very porous structures with extremely high surface areas were initiated recently [22]. It has been stated that the open and porous structures of copper deposits obtained at high current densities were ideally suited for use as electrodes in electrochemical devices such as fuel cells, batteries and chemical sensors [22], while the extremely high surface area is relevant for evaluating some electrochemical reactions [23]. Surface microstructure plays a crucial role in application such as a magnetic storage, printing devices and all other similar technological procedures, but correlation between surface morphology and roughness structure with deposition parameters is still open topic [24]. Three-dimensional (3D) characterization seems more adapted for applied research. Therefore, the aim of this study is investigation of the influence of deposition current density on microstructure of disperse Ni–Co deposits and correlation between electrolysis conditions for formation of such rough deposits on the substrates with morphology, surface roughness and mechanical properties.

Section snippets

Experimental

The deposits were obtained in an open glass electrochemical cell with a volume of 1 dm3, thermostatically controlled at a temperature of 298 K. A Ti-plate covered with RuO2/TiO2 (10 cm2 geometric area), placed close and parallel to the Cu plate, was used as an anode (DSA). The solutions were made from analytical grade chemicals and triple distilled water. The alloys were electrodeposited from mixed ammonium sulfate-chloride solutions of different Ni2+/Co2+ ions concentration ratios of 0.25, 0.5,

Current efficiency

Fig. 1 shows the polarization curves for different compositions of the electrolyte performed on a Cu cathode. The shape and position of the polarization curves strongly depend on the electrolyte composition. A decrease in the content of Co2+ as well as an increase in the content of Ni2+ shifts the position of the corresponding polarization curves towards negative values of the potentials corresponding to the potential of the Ni/Ni2+ deposition of pure nickel.

Metal and alloy deposition of the

Conclusions

The microstructure as well as the morphology of the Ni–Co alloy deposits electrochemically obtained from an ammonium sulfate-chloride solution depends on the deposition current density and the bath composition. With a decrease in the deposition current density the volume fraction of the HCP phase in the deposits increases accompanied by the crystal grain growth. The increase of HCP phase content in the nanocrystalline deposits appears as a result of both an increase in the Co2+ ions

Acknowledgments

This research was supported by the research project “Bulk Nanostructured Materials” within the research focus “Material Science” of the University of Vienna. The investigation was partially supported by the Ministry of Science and Environmental Protection of Serbia, under Project 142025 and within the K plus programme at CEST by the Austrian Research Promotion Agency (FFG) and the government of Lower Austria. L.R is grateful for the support by the I.K. “Experimental Materials Science –

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