Mechanical properties and wear and corrosion resistance of electrodeposited Ni–Co/SiC nanocomposite coating
Introduction
Ceramic or metal matrix nanocomposites containing dispersed second-phase particulates usually have various special properties such as dispersion hardening, self-lubricity, high temperature inertness, good wear and corrosion resistance, and chemical and biological compatibility [1], [2], [3], [4], [5]. This accounts for the increased application of Ni-based nanocomposites in automobile industry. In order to meet the requirement for developing novel ceramic- and metal-based nanocomposites, many preparation techniques have been investigated. As a technique conducted at a normal pressure and ambient temperature and of low cost and high deposition rate, electrodeposition is considered to be one of the most important techniques for producing nanocomposites and nanocrystals [6], [7], [8].
With a view to the correlation among the structures and properties of composites, it could be practicable to endow electrodeposited nickel–cobalt alloy coatings with special properties different from that of the plated alloy coating, by incorporation of second-phase ceramic nano-particulates, because the inorganic nano-particulates such as SiC, Al2O3, and ZrO2 possess good chemical stability, high microhardness, and good wear resistance and corrosion resistance at elevated temperature [9], [10], [11]. Therefore, it would be rational to anticipate that electrodeposted Ni–Co/ceramic nano-particulates composite coatings have higher strength and hardness, specific magnetic properties, better chemical stability, and better wear and corrosion resistance at elevated temperature than the Ni–Co alloy coating [12], [13], [14], [15], [16], since the composite coatings combine the advantages of both the electrodeposited Ni–Co coating and the ceramic nano-particulates. In such a way the application fields of the Ni–Co alloy-based electrodeposited coatings as a kind of high temperature wear-resistant and anticorrosive coating would be greatly broadened to the severe environments containing water or corrosive compounds which may cause severe wear and oxide scaling at elevated temperatures. Thus it is unsurprising that the electrodeposited composite coatings consisting of alloy matrixes and dispersed nano-particulates, for example, Ni–Fe–nano-Si3N4 [17], Co–Ni–nano-Al2O3 [18], Zn–Ni-nano-SiC coatings [19], and Ni–Co–nano-Si3N4 [20], have been recently extensively focused on. Ni–Co alloy has been widely used as the recording head materials for computer hard drive industries. In the case of micro-electrical mechanical system (MEMS), the magnetic layer thickness can vary from a few nanometers to a few millimeter, depending on the applications. The magnetic thin films must also have good adhesion, low-stress, and good corrosion resistance, and be thermally stable with excellent magnetic properties. The well-dispersed nano-sized SiC particles in a Ni–Co matrix can not only enhance the mechanical properties, but also would be a necessity for the use as the composite materials in microdevices. Unfortunately, most of the availed reports in this respect are focusing on the SiC powders in a size of micron, and so far no work has been reported on the preparation and performance investigation of nano-sized SiC reinforced Ni–Co composite coatings.
Thus Ni–Co/SiC nanocomposite coatings were prepared by the electrodeposition in a nickel–cobalt plating bath containing SiC nano-particulates. The microstructure and surface morphology of the composite coatings were investigated. The effects of the incorporated SiC on the mechanical properties and wear and corrosion resistance of the nanocomposite coatings were analyzed.
Section snippets
Experimental
The plating bath is composed of 200 g L−1 NiSO4, 40 g L−1 NiCl2, 40 g L−1 CoSO4, 50 g L−1 Na3C6H5O7, 30 g L−1 H3BO3, 1–20 g L−1 SiC, and 5 g L−1 saccharin. Analytical reagents and distilled water were used to prepare the plating solution. Prior to plating, the SiC nano-particulates of a mean diameter 50 nm (Kaier, Hefei, China) were dispersed in the electrolyte in the presence of saccharin. The electroplating tests were performed on a model 273A potentiostat/galvanostat device (EG&G Princeton Applied
TEM observation of SiC nano-particulates
Fig. 1 shows the TEM image of the SiC nano-particulates (the specific surface area is reported to be 90 m2 g−1 by the producer). It is seen that they appear as microspheres of a diameter about 50 nm. The β-SiC nano-particulates have a density of 0.05 g cm−3 and a face centered cubic structure.
Polarization behavior of Ni–Co/SiC electrolyte
Fig. 2 shows the cathodic polarization curves of the Ni–Co/SiC electrolytes containing different concentrations of SiC nano-particulates. It is seen that the addition of SiC nano-particulates to the electrolyte
Conclusions
It is feasible to prepare Ni–Co/SiC nanocomposite coating by properly incorporating the nano-particulates to be co-deposited in the Ni–Co plating bath. The cathodic polarization potential of the Ni–Co/SiC electrolyte increases with increasing SiC concentration in the plating bath, but the co-deposited SiC nano-particulates do not significantly affect the electrodepostion process of the Ni–Co alloy coating. It is recommended to prepare the Ni–Co/SiC nano-particulates composite coating of the
Acknowledgements
The work was supported by the National Natural Science Foundation of China (Grant No. 50405040), the Ministry of Science and Technology of China (Grant No. 2002AA302609), and the Innovative Group Foundation from NSFC (Grant No. 50421502). The authors thank Prof. J.Z. Zhao and Mr. D.K. Song for their help in the SEM, EDS, and XRD measurements.
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