Part I: Ni-Co alloy foils electrodeposited using ionic liquids

1 Introduction

Nickel (Ni)-cobalt (Co) alloys exhibit desirable magnetic, mechanical, chemical, physical, and thermophysical characteristics, including wear resistance, high thermal conductivity, high hardness, and superior corrosion resistance. Owing to their electrocatalytic properties, Ni-Co alloys (Al Radadi and Ibrahim, 2020; El-Feky et al., 2013; Lupi et al., 2017; Chai and Jiang, 2019) are widely used in several industrial applications, such as electronics, computers, automotive devices, and energy storage devices, especially under environments that require the devices to be heat and corrosion resistant (Al Radadi and Ibrahim, 2020; Al Raddadi, 2014). Electrodeposition is a promising technique to obtain Ni-Co alloys, as the associated cost is low, device maintenance is convenient, a uniform coating thickness can be obtained, and complex geometries can be handled with a low toxicity (Al Radadi and Ibrahim, 2020). Several researchers have investigated the characteristics of Ni-Co alloy coatings obtained from different baths (Al Radadi and Ibrahim, 2020; Vijayakumar et al., 2013; Tian et al., 2011) including or not including additives (El-Feky et al., 2013; Vijayakumar et al., 2013; Nemţoi et al., 2009). However, the use of an acid sulfate bath is becoming increasingly popular as it is relatively inexpensive, safe, and nonpolluting (Loto, 2012) and can help obtain smooth and fine surfaces (El-Feky et al., 2013). With the rapid development in science and technology, material property requirements have become more stringent. Recent techniques to prepare composite coatings involve the use of additives to realize specific applications of the materials. Additives are widely used in electrodeposition baths as they influence the growth, morphology and structure of the resulting deposits. In general, additives are added to plating solvents to improve the properties of the electrodeposit, such as the grain size, brightness, internal stress, hardness and resistance to corrosion (El-Feky et al., 2013; Jamil et al., 2017; Alesary et al., 2019). The additives are often incorporated in the deposit (Omar, 2012), and they inhibit the crystal growth toward other crystallographic axes (Hamilakis et al., 2015; Hashemi et al., 2017) as they are adsorbed on the plated surface. However, Ni, Co or Ni-Co alloys electrodeposited from baths including additives have been noted to exhibit a granular coating with extensive microcracking on the surface (Al Raddadi, 2014; Ji et al., 2016), low hardness and yield strength (Zamani et al., 2016), low corrosion resistance (Cârâc and Ispas, 2012), and low cathodic current efficiency (Lupi et al., 2017; Al Raddadi, 2014) under a high temperature and current density. Consequently, certain environmentally unfriendly materials such as Cd²⁺ compounds (Mohanty et al., 2002); thiourea (Zhu et al., 2012), or additives with a high flammability, high volatility (Zhu et al., 2012), easy decomposition (Ibrahim and Omar, 2013), and low thermal stability (Qibo and Yixin, 2017) have been used. In this regard, ionic liquids (ILs) represent a promising alternative chemical group for many existing materials to be used in electrodeposition due to their excellent properties (Yao et al., 2019; Ibrahim et al., 2016; Anicai et al., 2011). Ionic liquids (ILs) are molten salts with melting points below 100 °C and they consist solely of cations and anions. The charge on these ions is delocalized or shielded by side groups, which is one reason why the melting points of ionic liquids are remarkably low (Barbato, 2009). ILs are usually large electrochemical windows: The most important advantage of ILs is their large electrochemical windows (>5V) which gives access to elements that cannot be electrodeposited from aqueous or organic solutions such as for example, Al, Mg, Ti and Ta, at moderate temperatures. The effect of cation and anion: Generally, the cation controls the physical properties of the salt such as its structure and morphology and the anion affects the stability and chemical reactivity. Moreover, the anion affects coordination geometry around the metal ion which effects reduction potential, reduction current and nucleation. Both the anion and cation affect the conductivity and viscosity. Properties of ILs, such as dissolving ability from metal salts, polarity, viscosity and density can be tuned by an appropriate choice of the anion and the cation. Overall, ionic liquids are inevitably advanced, technological solvents that can be designed to fit a particular application. Conductivity: ILs have reasonably good ionic conductivities, between 10⁻³ and 10⁻² Ω⁻¹ cm⁻¹, compared with those of organic solvents/electrolyte systems. These conductivities make ionic liquids interesting solvents for low-temperature electrodeposition studies. Low vapor pressures: most ILs have at or near room temperature vapor pressures of 10⁻¹¹−10⁻¹⁰ mbar allowing experiments to be done at temperatures up to 400 °C. The very small vapor pressure of ILs also allows their use in vacuum experiments even at elevated temperatures. Thermal stability: ILs can be used at wider temperature ranges and still have great thermal control of electrochemical processes, unlike when using conventional molecular solvent/electrolyte systems. Less toxic: from the environmental perspective ILs are environmentally cleaner than other media. When used in electroplating processes, strongly aqueous electrolytes create quantities of metal-laden, corrosive effluent solution, whereas in ILs electrolytes the metals will precipitate and be readily separated and recycled. Good solvents for both organics and inorganics: metal ion electrodeposition potentials are much closer together in ILs compared with water, enabling easier preparation of alloys and the possibility of a much wider range of possible electroplated alloys, which are difficult or impossible in water (Yao et al., 2019; Ibrahim et al., 2016; Anicai et al., 2011). Many researchers have used ILs to prepare an electroplating bath (Alesary et al., 2019; Costovici et al., 2016; Ismail, 2017). However, relatively few studies have investigated the role of ILs as additives in the electrodeposition processes (Allahyarzadeh et al., 2011; Qibo and Yixin, 2017). This means that study the effect of ILs as additives during electrodeposition process is insufficient. Therefore, in the current work, the effect of two novel synthesized ILs, namely, 1-methyl-3-(2-oxo-2-((2,4,5 trifluorophenyl) amino) ethyle)-1H-imidazol-3-ium iodide ([MOFIM]I) and 1-(4-fluorobenzyl)-3-(4-phenoxybutyl)imidazol-3-ium bromide ([FPIM]Br) on the Ni-Co alloy coatings generated from a sulfate bath was studied for the first time as new ionic liquid addition agents in Ni-Co alloy electrodeposition and compared. The structures, morphologies, and compositions of the Ni-Co alloy coatings were determined using a scanning electron microscope (SEM) combined with the energy dispersive X-ray spectroscopy (EDS), EDS mapping, X-ray diffraction (XRD) and atomic force microscopy (AFM) techniques.

2 Experimental details

2.2

2.2 Additive preparation

The imidazolium iodide incorporating an aromatic amide, namely, 1-methyl-3-(2-oxo-2-((2,4,5-trifluorophenyl) amino)ethyl)-1H-imidazol-3-ium iodide (MOFIM]I), M.W = 397.15 g/mol, was synthesized through simple quaternization of methylimidazole with fluorinated aromatic acetamide bromide in acetonitrile (Rezki et al., 2020; Omar et al., 2020), as shown in Fig. 1a. The imidazolium bromide incorporating an aromatic amide, namely, 1-(4-fluorobenzyl)-3-(4-phenoxybutyl)imidazol-3-ium bromide ([FPIM]Br), as shown in Fig. 1b, M.W = 405.31 g/mol, was synthesized using ultrasonic irradiation, as reported in Omar et al. (2020). Both studied ILs soluble in warm aqueous solution (50–60 °C).

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Fig. 1

The molecular structure of the ionic liquids namely (a) 1-methyl-3-(2-oxo-2-((2,4,5 trifluorophenyl)amino)ethyle)-1H-imidazol-3-ium iodide ([MOFIM]I), (b) 1-(4-fluorobenzyl)-3-(4-phenoxybutyl)imidazol-3-ium bromide ([FPIM]Br).

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3 Results and discussion

3.2

3.2 Cathodic current efficiency (CCE%)

The impact of the bath composition and plating operation conditions on the CCE was analyzed, and the results are present in Fig. 2 and Table 2. In all the experiments, the CCE% was slightly lower than 100% owing to the simultaneous H₂ evolution associated with the coating deposition process, as reported in (Al Radadi and Ibrahim, 2020). The presence of [MOFIM]I and [FPIM]Br considerably increased the CCE%, which exhibited maximum values of 99.8%, 97.91% and 97.76% in baths 2, 5 and 8, respectively, when using 1 × 10⁻⁵ M [MOFIM]I, as shown in Fig. 2b. Fig. 2c shows that the improvement in the CCE% was smaller when using [FPIM]Br (97.07%, 96.26% and 98.76% for baths 3, 6 and 9, respectively). The efficiency of the alloy codeposition improved in the presence of ILs due to the increase and decrease in the efficiency of the Co deposition and hydrogen evolution, respectively. According to the results presented in Table 2 and Fig. 2, the optimal bath composition for the Ni-Co alloy deposition, corresponding to the highest CCE% values, included 70 g/L and 30 g/L of Ni²⁺ and Co²⁺ concentrations, respectively, with the IL concentration as 1 × 10⁻⁵ M. The CCE% pertaining to the bath including [MOFIM]I was higher than that for the bath including [FPIM]Br owing to the stronger adsorption of the former molecule than the latter molecule on the cathode surface, which inhibited the hydrogen evolution. The Ni-Co alloy could be obtained with high current efficiencies (more than 95%) when using either [MOFIM]I or [FPIM]Br. When performing Ni-Co and codeposition from a sulfate bath using 1 × 10⁻⁵ M [MOFIM]I, the current efficiency value was nearly 100% compared to the CCE% values of 90% and 84.8% in the presence of sodium citrate (Lupi et al., 2017), glycine (Al Radadi and Ibrahim, 2020) respectively and CCE% values of Cu-Zn alloy about 68% in presence of polyethylene glycol (Ibrahim et al., 2019); Table 3.

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Fig. 2

Effect of [Co^2+] / [Ni²⁺] ratios in the bath on the CCE% of Ni-Co alloys deposits (a) Ni-Co1, Ni-Co4 and Ni-Co7 alloys (free IL), (b) Ni-Co2, Ni-Co5 and Ni-Co8 alloys (with 1 × 10⁻⁵ M [MOFIM]I), (c) Ni-Co3, Ni-Co6 and Ni-Co9 alloys (with 1 × 10⁻⁵ M [FPIM]Br), i = 20 mA cm⁻², t = 10 min., T = 25 °C.

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Table 3 Comparison of obtained results of Ni-Co2 and Ni-Co3 alloys coating in presence of 1 × 10^-5 M [MOFIM]I and [FPIM]Br with other best bath composition in presence of additives.

Additives	CCE%	Ra	Microhardness (kg f mm−2)	Reference No.
[MOFIM]I	99.8	28.14 nm	852.5	Current work
[FPIM]Br	97.07	51.07 nm	775.0	Current work
Sodium citrate	90%	–	–	Lupi et al. (2017)
Glycine	84.8%	–	–	Al Radadi and Ibrahim (2020)
Polyethylene Glycol	68%	–	–	Kumar et al. (2019)
Fe2+		–	510.0	Ji et al. (2016)
Fe2+ + H3PO3		–	690	Ji et al. (2016)
Fe		–	220	Deng et al. (2018)
Co		–	20	Deng et al. (2018)
Gd2O3 nanoparticles		–	590	Franklin (1987)
Ascorbic acid		0.389 µm	–	Budi et al. (2017)
Glycine		812.89 nm	–	Al Raddadi (2014)
Saccharin		266.96 nm	–	Tian et al. (2011)
Saccharin			280	Liu et al. (2019)

3.3

3.3 Physical characterizations

Figs. 3–5 show SEM images of the Ni-Co alloys obtained from the nine baths. As shown in Fig. 3a and a', the surface morphology of the Ni-Co1 alloy obtained from a silent electrolyte (without ILs additive) in bath 1 was granulated and exhibited a lustrous structure despite the presence of many small (grain size 278 nm) and large circular grains (the maximum grain size 589 nm). The quality of the deposit was unsatisfactory due to the co-evolution of H₂ (Bahramian et al., 2018). The surface morphology of the alloy was enhanced significantly after 1 × 10⁻⁵ M [FPIM]Br was added, and the alloy became more compact and homogeneous; however, the presence of [MOFIM]I more effectively enhanced the Ni-Co2 alloy coating, as observed in Fig. 3b and b'. In particular, extremely few large grains were present in the Ni-Co3 alloy surface obtained using [FPIM]Br (the maximum grain size 225 nm), as shown in Fig. 3c and c'. In general, the [MOFIM]I and [FPIM]Br cations attach to the substrate and form layers through adsorption with a strong interaction (El-Feky et al., 2013). The ionic liquids undergo nonblocking adsorption in the active sites of the crystal planes of the substrates and on the emerging surfaces of the crystallizing phases (El-Feky et al., 2013) of the Ni-Co alloy. In other words, the ILs adsorb on the energetically favorable surfaces (low energy sites) on the substrate and the first stages of the Ni-Co alloy, which considerably inhibits the H₂ evolution, leading to pitting and cracking on the deposit surface. Consequently, only high energy sites are available for the metal deposition (Hashemi et al., 2017), and the ILs decelerate the surface diffusion of the adsorbed ion surface. This phenomenon impedes the grain growth, leading to a higher nucleation rate and the presence of finer grains, free cracks and more compact deposits. The deposits are compact as the deposits obtained in the presence of saccharin (Qibo and Yixin, 2017) and cysteine (El-Feky et al., 2013). The additives primarily occupy the active sites, which are the edges of a growth step, and the metal ions must form a new layer on the less active areas (El-Feky et al., 2013).

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Fig. 3

SEM images, high magnification SEM of (a,a') Ni-Co1 alloy (free IL), (b,b') Ni-Co2, alloy (with 1 × 10⁻⁵ M [MOFIM]I), (c,c') Ni-Co3 alloy (with 1 × 10⁻⁵ M [FPIM]Br).

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Fig. 4

SEM images, high magnification SEM of (a,a') Ni-Co4 alloy (free IL), (b,b') Ni-Co5, alloy (with 1 × 10⁻⁵ M [MOFIM]I), (c,c') Ni-Co6 alloy (with 1 × 10⁻⁵ M [FPIM]Br).

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Fig. 5

SEM images, high magnification SEM of (a,a') Ni-Co7 alloy (free IL), (b,b') Ni-Co8, alloy (with 1 × 10⁻⁵ M [MOFIM]I), (c,c') Ni-Co9 alloy (with 1 × 10⁻⁵ M [FPIM]Br).

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To further understand the role of the two studied ILs in enhancing the morphology of the Ni-Co alloy, as shown in Fig. 1, the chemical composition of the two compounds was considered. [MOFIM]I and [FPIM]Br cations have a strongly cationic configuration, and thus, they readily adsorb on the cathode surface under an electric field. Specifically, —C⚌N—, which is the functional group, and the electronegative nitrogen of imidazolium in both [MOFIM]I and [FPIM]Br cations enable the spontaneous adsorption of these ILs on the metal surface due to the specific interaction between these functional groups and the metal surface (Qibo and Yixin, 2017). Furthermore, the oxygen atoms in the amide and ether functional groups in the [MOFIM]I and [FPIM]Br structures, respectively, tend to donate electrons, which contributes to the strong adsorption ability of these ILs on the metal surface (Kumar et al., 2019). Moreover, both [MOFIM]I and [FPIM]Br molecules have double bonds in the aromatic rings in their structures. As the number of double bond increases, the number of π electrons increases, thereby facilitating the adsorption on the cathode surface (Deng et al., 2018). However, owing to the presence of an amide functional group in the [MOFIM]I structure, which contains highly electronegative nitrogen and oxygen atoms, this IL is a more effective additive compared to the [FPIM]Br.

When the Co²⁺ concentration in the bath increased to 50 g/L, the grain structure of Ni-Co4 changed from a circular to a starry shape with a maximum grain size of 691.7 nm, as shown in Fig. 4a and a'. The micrographs of the Ni-Co5 alloy indicate that after 1 × 10⁻⁵ M [MOFIM]I was added in bath 5, the starry shape changed to an elongated structure with needlelike crystals, which is not desirable for deposits, and the maximum grain size increased to 2.1 µm as shown in Fig. 4b and b'. Moreover, bath 6 including 1 × 10⁻⁵ M [FPIM]Br resulted in Ni-Co6, which exhibited undesirable sharp needlelike crystals with considerable elongation, distributed in completely random directions. Moreover, the maximum grain size increased to 2.8 µm, as shown in Fig. 4c and c'. A similar effect was noted when the ionic liquid 1-butyl-3-methylimidazolium hydrosulphate [BMIM]HSO₄ was used as an additive in the zinc electrodeposition process involving an acidic sulfate electrolyte (Qibo and Yixin, 2017).

With a further increase in the Co percentage, the Ni-Co7 alloy was deposited from an IL free bath (bath 7), and this alloy exhibited a granular, nodular, and spherical structure with cauliflower like grains, as shown in Fig. 5a and a'. A mixture of small and large grains was obtained, with the maximum grain size being 1.63 µm. When 1 × 10⁻⁵ M [MOFIM]I was added, in bath 8, the granular spherical grains of the alloy retained their shape, as shown in Fig. 5b and b'; however, the maximum grain size reduced to 1.03 µm. The presence of [MOFIM]I did not considerably influence the shape of the Ni-Co8 alloy obtained from bath 8. However, when 1 × 10⁻⁵ M [FPIM]Br was added in bath 9, as shown in Fig. 5c and c', the surface of the Ni-Co9 alloy became rough and exhibited fibril-like maximum grains sized 2.06 µm, which is not a preferable deposit quality. Increasing the Co content in the alloy composition led to larger grain structures. This phenomenon was especially notable in the cases with Ni50%-Co50% and Ni30%-Co70% baths, as the H₂ evolution increased owing to the higher overpotential, which is not desirable in the case of deposits. Granular Ni-Co alloys were also obtained from the silent bath (without ILs additive) due to the increase in the Co content in the Ni-Co alloys (El-Feky et al., 2013; Franklin, 1987). Moreover, a small valley was observed in Ni-Co7, Ni-Co8, Ni-Co9 alloys, which was similar to that observed in the of Ni-Co alloy morphology when the Co content in the alloys was more than 71% (El-Feky et al., 2013).

The elemental EDS mapping images shown in S2 indicate that the Ni and Co were uniformly distributed on the Cu substrate, even at a high magnification at which the crystallites could be clearly observed. This result suggests that Ni and Co are found within individual crystallite domains and confirms the formation of the Ni-Co solid solution, as shown in S2. a–c. The images also indicate the successful incorporation of the [MOFIM]I (S2. a'–c') and [FPIM]Br ILs (S2. a''–c'') into the Ni-Co film. The ILs exhibited a homogeneous distribution of the Ni and Co elements. A similar result has been reported for Ni-Co alloys deposited in the presence of saccharin (Budi et al., 2017), samarium oxide (Liu et al., 2019) and hydroxides (Chai and Jiang, 2019).

4 Analysis of the phase composition

Fig. 6 shows the XRD diagrams of the Cu substrate as well as the nine Ni-Co alloys electrodeposited from nine different bath compositions. The XRD patterns agree with the face center cubic (fcc) Cu substrate (JCPDS 00-004-0836) and Co-Ni (JCPDS 01-074-5694). The fcc structures of the Ni-Co alloys were not influenced by the change in the Ni²⁺ and Co²⁺ concentrations or the presence of [MOFIM]I and [FPIM]Br in the electrolytes, as shown in Fig. 6a–c. The only differences were in terms of the XRD peak intensity and width of the patterns. The XRD patterns of the Ni-Co1, Ni-Co2 and Ni-Co3 alloys exhibited a large amount of (2 2 0), preferred growth orientation, and diminutive amounts of (1 1 1) and (2 0 0), as shown in Fig. 6a. Fig. 6b. shows that the XRD patterns of the Ni-Co7, Ni-Co8 and Ni-Co9 alloys were similar. Specifically, most of the alloy crystallites were oriented parallel to the (2 2 0) plane. The intensity of the Ni-Co (2 0 0) and (1 1 1) peaks changed slightly when [MOFIM]I and [FPIM]Br were introduced in the system. However, the XRD patterns of the Ni-Co4, Ni-Co5 and Ni-Co6 alloys exhibited diminutive amounts of fcc (2 2 0), (2 0 0) and (1 1 1). The introduction of [MOFIM]I and [FPIM]Br led to a significant and slight increase in the intensity of all the XRD pattern peaks of the Ni-Co5 and Ni-Co6 alloys, respectively, as shown in Fig. 6c. The average crystallite size of the fcc (2 2 0) of the Ni-Co2 film electrodeposited from bath 2 (including 1 × 10⁻⁵ M [MOFIM]I), calculated using the Scherrer equation, was 12.97 nm, which was significantly smaller than the corresponding values (15.54 and 17.63 nm, respectively) for the Ni-Co3 deposited using bath 3 with 1 × 10⁻⁵ M [FPIM]Br and Ni-Co1 deposited from IL free bath 1. This result indicates that [MOFIM]I could more effectively improve the surface morphology of the Ni-Co alloy deposits. The considerable decrease in the crystallite size in the presence of both the considered ILs could be due to an increase in the nucleation rate, which likely decreased the crystal growth rate during the deposition process. In general, electrodeposition occurs through the nucleation of the crystallites on the substrate surface followed by their growth along with the formation of new nuclei (Budi et al., 2017). In the system containing [MOFIM]I and [FPIM]Br (baths 2 and 3, respectively), the cations of [MOFIM]I and [FPIM]Br could be absorbed on the active sites of the surface substrate, on which the metallic nuclei form and grow. This condition generated a physical barrier and impeded the kinetic transport at the electrolyte–electrode interface, thereby increasing the overpotential in the system, which favored the nucleation rate, as reported in (Budi et al., 2017), and suppressed the grain growth. This suppression could explain the small crystallite size in the Ni-Co2 and Ni-Co3 alloys. However, both the ILs did not lead to a considerable enhancement in the Ni-Co8 and Ni-Co9 alloys, although [MOFIM]I was more effective than [FPIM]Br. The average crystallite sizes of the fcc (2 2 0) of the Ni-Co8 and Ni-Co9 films electrodeposited from baths containing [MOFIM]I and [FPIM]Br were 20.48 nm and 24.3 nm, respectively, which are slightly smaller than the corresponding value (25.45 nm) for Ni-Co7 deposited from free ILs.

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Fig. 6

XRD patterns of (a)Ni-Co1, Ni-Co2, Ni-Co3 alloys, (b)Ni-Co7, Ni-Co8, Ni-Co9 alloys, (c) Ni-Co4, Ni-Co5, Ni-Co6 alloys without and with 1 × 10⁻⁵M [MOFIM]I and [FPIM]Br.

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5 Surface topography and roughness

The surface topography and roughness of the Cu substrate and Ni70%-Co30% bath coatings without and with different concentrations of [MOFIM]I and [FPIM]Br were investigated using the AFM technique. Fig. 7a and a' show the topography of the Cu substrate as 2D and 3D nuclei imaging in a flat surface with low average roughness (R_a, deviation in height) and the total roughness (R_t, vertical distance between the deepest valley and highest peak (Emran and Al-Refai, 2017), respectively. The Ni-Co1 alloy deposited from the silent electrolyte (without ILs additive) in bath 1 exhibited a high roughness, as indicated by the large R_a (111.9 nm) and R_t (140.6 nm) values, as shown in Fig. 7b and b', respectively. This variation likely occurred due to the presence of randomly distributed and irregularly sized grain during co-electrodeposition due to the high polarization and co-evolution of H₂, as discussed in the SEM analysis. The lowest roughness (R_a of 28 nm and R_t of 35.5 nm) was obtained when [MOFIM]I was added in the system, as shown in Fig. 7c and c', respectively. Nevertheless, the coating quality was satisfactory in the case of [FPIM]Br (R_a of 51. 1 nm and R_t of 65.7 nm), as shown in Fig. 7d and d', respectively, although the efficiency in this case was lower than that with [MOFIM]I. This finding is in agreement with the more regular, compact and uniform alloy grain growth observed in the SEM images pertaining to the solution containing [MOFIM]I than that containing [FPIM]Br. Thus, lower concentrations (approximately 5 × 10⁻⁷ M) of both the considered ILs should be avoided as they lead to undesirably high roughness surfaces on the Ni-Co alloy deposits (R_a 105 nm, R_t 134.2 nm in the presence of [MOFIM]I; R_a 87.5 nm and R_t 111.6 nm in the presence of [FPIM]Br), as shown in Fig. 7e and e' and Fig. 7f and f', respectively. This phenomenon likely occurs owing to an increase in the activation overpotential of both the H₂ evolution and metal ions reduction, as reported in (Jamil et al., 2017). Moreover, this result is in agreement with the findings of the visual observation performed in the SEM analysis. A similar behavior of saccharin as the additive (Manhabosco and Müller, 2008) and glycine as the complexing agent (Al Raddadi, 2014) has been obtained during Co and Ni electrodeposition, Table 3.

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Fig. 7

AFM 3D, 2D images, of (a, a') Cu substrate, Ni-Co alloy deposits in (b, b') free ILs, with (c, c') 1 × 10⁻⁵ M [MOFIM]I, (d, d') 1 × 10⁻⁵ M [FPIM]Br, (e, e') 5 × 10⁻⁷ M [MOFIM]I, (f, f') 5 × 10⁻⁷ M [FPIM]Br.

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Fig. 7

AFM 3D, 2D images, of (a, a') Cu substrate, Ni-Co alloy deposits in (b, b') free ILs, with (c, c') 1 × 10⁻⁵ M [MOFIM]I, (d, d') 1 × 10⁻⁵ M [FPIM]Br, (e, e') 5 × 10⁻⁷ M [MOFIM]I, (f, f') 5 × 10⁻⁷ M [FPIM]Br.

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6 Microhardness

The microhardness of the Ni-Co alloys obtained in the presence of [MOFIM]I and [FPIM]Br from baths 2 and 3, respectively, and from the silent electrolyte (without ILs additive) in the case of bath 1 was examined. The microhardness of the Ni-Co2 and Ni-Co3 alloys deposited in the presence of 1 × 10⁻⁵ M [MOFIM]I and [FPIM]Br was approximately 852.5 kg f mm⁻² and 775 kg f mm⁻², respectively, compared to the microhardness of 541.75 kg f mm⁻² of the Ni-Co1 alloy deposited from the silent bath. The hardness of the Ni-Co2 and Ni-Co3 alloys deposited in the presence of the two considered ILs was significantly increased compared to that of the Ni-Co1 deposit obtained in the absence of the ILs. Furthermore, [MOFIM]I was more efficient in increasing the hardness of the Ni-Co2 alloy film compared to [FPIM]Br. This phenomenon occurred because nanocoatings with smoother, more compact, denser and finer grains were obtained when using the [MOFIM]I and [FPIM]Br systems, as indicated by the SEM and XRD measurements. The hardening effect by the added agent can be attributed to the grain refinement effect. It has been reported that the mechanical properties of crystalline metals and alloys depend on the grain size (El-Feky et al., 2013). Consequently, the strength and hardness of a metallic material significantly increases when the grain size decreases. It is well-established that as the grain size of metal films increases, the surface roughness increases and metal hardness typically decreases (Pissolati and Majuste, 2018). Moreover, the microhardness of the Ni-Co alloys in the Ni70%-Co30% bath was affected by the crystal structure of the alloys, and the Ni-Co alloy with the fcc structure corresponded to a notably high hardness (El-Feky et al., 2013). It has been reported that Ni-Co alloy coatings exhibit a higher hardness with a higher Ni content in the alloys, owing to the formation of the solid solution of the Ni-Co alloys (El-Feky et al., 2013). Moreover, the microhardness values obtained in this study are considerably larger than those reported for Ni and Co alloys deposited under similar conditions (Ji et al., 2016; Oliveira et al., 2017; Sivasakthi and Sangaranarayanan, 2019); Table 3.

7 Conclusion

The surface morphology of Ni-Co alloy coatings was successfully enhanced by using 1-methyl-3-(2-oxo-2-((2,4,5trifluorophenyl)amino)ethyle)-1H-imidazol-3-ium iodide [MOFIM]I and 1-(4-fluorobenzyl)-3-(4-phenoxybutyl)imidazol-3-ium bromide [FPIM]Br ionic liquids as additives in an acidic sulfate bath under three different [Ni²⁺] and [Co²⁺] concentrations. The addition of 1 × 10⁻⁵ M [MOFIM]I and [FPIM]Br in a bath with a composition of Ni70%-Co30% led to optimal Ni-Co alloy coatings, which exhibited characteristics such as finer grains, and a more coherent and compact deposit, as verified by SEM and AFM measurements. However, larger grains and more oriented crystals were obtained when using [MOFIM]I than when using [FPIM]Br. Both the considered ILs led to favorable results as they inhibited the Ni²⁺ and Co²⁺ reduction owing to the adsorption of the IL cations on the energetically favorable surfaces on the substrate and the first stages of the Ni-Co alloy deposits. Owing to this adsorption, the ILs hindered the H₂ evolution, controlled the grain growth and increased the nucleation rate. The XRD spectra indicated that the addition of the considered ILs did not change the crystal structure of the co-electrodeposited Ni-Co alloys. In all the considered Ni-Co alloy coatings corresponding to baths 1–9, the Co content (the less noble metal) in the alloy was higher than that in the bath, thereby indicating that the codeposition of Co and Ni corresponded to anomalous plating. The optimal bath conditions that led to the highest CCE% values for the co-electrodeposition of the Ni-Co alloys involved a current of 20 mA cm⁻², deposit potential of 6.5 V, pH of 4.5, temperature of 20 °C and deposit time of 10 min. The CCE% values attained maximum values of 99.8% and 97.07% in the presence of [MOFIM]I and [FPIM]Br, respectively, under the composition of Ni 70% and Co 30%.