Current density dependent microstructure and texture evolution and related effects on properties of electrodeposited Ni-Al coating
Graphical abstract
Introduction
In last two decades, composite coatings prepared by electrodeposition technique are at the center of attraction due to their widespread technological applications with superior mechanical and corrosion resistant properties, like high hardness, better wear resistant along with excellent anti-corrosion and oxidation resistant properties than pure metal coatings [[1], [2], [3]]. Reported second phase reinforced Ni-based composite coatings comprise mainly rare earth oxides (REO) (CeO2, La2O3 and Y2O3) [[3], [4], [5]], hard ceramic particles (TiN, Si3N4, SiC, Al2O3 and TiO2) [[6], [7], [8], [9], [10], [11], [12], [13], [14], [15]], metal particles (Cr, Ti, W and Al) [[16], [17], [18], [19], [20], [21], [22]], and intermetallics (NbAl3) [23,24].
Electro-codeposition is one of the most convenient techniques to synthesize such composite coatings owing to numerous benefits such as easy operation, cost-effective, precise control, and competence to coat complex geometrical components. To get the required properties, one can manipulate the deposition parameters such as pulse parameter, current density, stirring rate, types of second phase particles and concentrations, temperature of plating bath and pH of the solution [6,12,16,20]. Among these plating parameters, applied current density plays an important character by altering the electric field between cathode, electrodeposited ions and second phase dispersed particles. Consequently, the amount of second phase particles codeposition in the coating is significantly varied [6,12,25].
Ni-Al coatings developed by electro-codeposition have been studied by various researchers due to their exceptional corrosion resistance and high temperature oxidation resistance [21,22,[26], [27], [28], [29], [30], [31], [32]]. Uniform and higher amount of Al codeposition is the promising way to increase the corrosion resistant of the synthesized coatings. Composite coatings like Ni-Al and Ni/Cu-Al having 29–35 vol.% of Al particles into the coating matrix are synthesized by sediment codeposition (SCD) technique utilizing the codeposition performance of Al (particles) similar to inert particles and this could easily be elucidated by Guglielmi's model [20]. The size factor of second phase Al powder also affected the microstructure of the Al reinforced coatings [33]. Some published literature report that Ni-Al coating has better corrosion resistant compared to pure Ni coating because of the formation of aluminum oxide containing passive film on the Ni-Al coating surface [26,27]. Moreover, Al incorporation in the Ni-Al coating matrix prevents the excess outward oxygen diffusion by forming a uniform, adherent, dense and protective oxide film on the surface of the Ni-Al composite coating even at high temperatures [[29], [30], [31]]. Recently, Cai et al. reported about electrodeposition of Ni/Co-Al coatings with 2.9 and 5.5 wt% of Al concentrations and its corrosion behavior [21]. They have described that Al concentration in the coating matrix significantly improves the corrosion resistance and also observed that with codeposition of Co and Al, the typical (200) fiber texture of pure Ni coating changes to random texture of Ni/Co-Al coating. However, they have not studied any current density dependent microstructure and texture evolution and related mechanical and electrical properties. Similarly, another recent paper published on pulse electrodeposition of Ni-Al coating which recorded different Al concentrations at a constant current density of 430 mA/cm2 [22]. They have categorically reported the effect of specific crystallographic texture with respect to different values of Al nanoparticle loadings of on tensile strength of the developed coatings. Some of the literature has also reported the formation of NiAl and Ni3Al intermetallic alloys obtained from annealed Ni-Al coatings, which can be used as high temperature structural material in aerospace as well as power industry applications [[28], [29], [30]]. Polycrystalline Ni and Ni-based alloy/composite films with thickness of few micrometers are widely being used as electrical interconnects in various integrated circuits [34]. Mechanical (wear resistance and hardness) and electrical conductivity behavior of Ni-based films are significantly affected by the structure of the coating surfaces, grain geometry, and type of grain boundaries, surface impurities as well as intragranular defects. Therefore to improve the service life of Ni coatings in above applications, development of wear-resistant coating as well as maintaining the electrical conductivity is also important.
After reviewing sufficient amount of literature, the authors of the current work observed that, though a fair amount of work based on Ni-Al electrodeposited coatings has been studied, most of the studies are focused on synthesis, oxidation resistance and anti-corrosion of the developed coatings. In addition, two recent reports on Ni/Co-Al and Ni-Al electrodeposits depict about effect of Al concentration on texture formation correlating to corrosion and tensile behavior of the synthesized coatings, respectively. Therefore, the authors of the present work believe that deposition parameters (specifically, the applied current density) dependent microstructure and texture variation and its correlation with mechanical and electrical behavior of the Ni-Al coating can be an area of study to strengthen the understanding of existing literature.
The present work discusses the synthesis of electrodeposited 10 g/l Al reinforced Ni coatings at different applied current densities (1, 5, and 8A/dm2). Microstructure and texture evolutions with respect to applied current densities and correlation of them to the mechanical and electrical properties of the developed coatings have also been studied.
Section snippets
Ni and Ni-Al deposition
Pure Ni coating and Ni-Al composite coatings were electrodeposited on copper substrates from standard Watt's bath and Al nanopowder dispersed Watt's bath. The details of electrolyte compositions and plating parameters are shown in Table 1. Specific dimensions (20 mm × 10 mm × 2 mm) of Cu strips were used as a cathode (substrate), whereas, Ni plate was used as an anode. Prior to the electrodeposition process, the copper substrates were mirror polished with the help of different grades of emery
Particle size of Al powder
Transmission electron microscopy (TEM) image of procured Al powder with particle size distribution (inset) is presented in Fig. 1. The micrograph shows that Al particles of near circular shapes are distributed in different sizes. The size distribution graph (inset) confirms the particles diameters in the range of 6–48 nm. However, the average particle size of the Al nanoparticles is calculated as 14.5 nm.
Phase and elemental analysis
XRD pattern of Ni-Al coating developed with 1A/dm2 is presented in Fig. 2(a). The result
Conclusions
The current study which investigates electrodeposition of pure Ni and Ni-Al coatings at different current densities concludes the followings:
Incorporation of Al nanoparticle in the coating matrix was decreased with increase in current density and significantly affects the grain size distribution. The grain size distribution of Ni-Al coatings is found to be finer to uneven and equiaxed with progressive increase in applied current density. Not only grain size distribution, but also average
Acknowledgements
The author (H. S. Maharana) acknowledges the financial support from the Science and Engineering Research Board, India (N-pdf scheme) with grant No:PDF/2016/003346. XRD-Texture laboratory at Department of Metallurgical and Materials Engineering, NIT Rourkela supported by DST-FIST, India (Grant No: SR/FST/ETI-344-/2013C and G) is also greatly acknowledged.
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