Effect of Current Density on Magnetic and Hardness Properties of Ni-Cu Alloy Coated on Al via Electrodeposition

Nickel (Ni)-rich single-phase nickel-copper (Ni-Cu) alloy coatings were produced on aluminum (Al) substrates by electrodeposition in stabilized citrate baths. Electrodeposition experiments were performed at four different current densities. Increasing the current density resulted in the metal deposition rate increasing faster than the hydrogen evolution rate; thus, the cathodic current efficiency increased. The crystal systems of the Ni-Cu alloys were face center cubic (fcc), with the (111) plane as the preferred crystal plane. Scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDS) measurements showed that the Ni content


INTRODUCTION
Researchers widely study nickel (Ni) and copper (Cu) alloys as engineering materials due to their unique mechanical, magnetic, and anti-corrosion properties (1,2).Ni-Cu alloys are known as monel in the industry and are typically comprised of 70 wt% Ni and 30 wt% Cu (3).These alloys have outstanding capabilities in acidic and alkaline environments (4).Ni-Cu alloys are singlephase alloys throughout their composition on the phase diagram, and these alloys formed because Ni and Cu are fully soluble in their solid and liquid states (5).Ni and Cu both have a face center cubic (fcc) crystal structure, and they have almost similar electronegativity and atomic radii (6,7).Given that conventional casting as a manufacturing method for monel results in substantial production costs and that Ni-Cu alloy coated on aluminum (Al) has potential as a replacement for monel as a bulk material (8), Al-based metals have received considerable attention (9,10).They are lightweight and demonstrate high resistance to wear and corrosion and a high strength-stiffness combination (11).
Several techniques have been proposed to successfully modify the surface morphology and chemical composition, including sol-gel, chemical etching, chemical vapor deposition, thermal embossing, and electrodeposition (12,13).The electrodeposition technique is a cost-effective, scalable, and easy-tocontrol process for coating Ni-Cu alloy (14).Specific methods have also been developed to determine the structure, morphology, and phase composition of the coated Ni-Cu alloys (15,16).Goranova et al. (17) investigated how changing the concentration of Ni ions and the current density affected the structure and composition of Ni-Cu alloys formed by electrodeposition in alkaline citrate baths.Higher concentrations of Ni ions in the bath led to notably smoother deposits and enhanced current efficiency.However, producing a uniform Ni-Cu coating can be challenging due to the difference in reduction potential between Ni and Cu.The reduction potential of Ni atoms is -0.25 V vs. SHE, and that of Cu atoms is +0.34 V vs. SHE (18).As a result, controlling the concentrations of Ni and Cu is vital.Complexing agents must be added to narrow the potential difference between Ni and Cu.The most frequently used complexing agent is citrate due to its low toxicity, cost-effectiveness, and buffering characteristics (19).
The electrodeposition process affects the physical properties of the resultant Ni-Cu alloy, as does the current density.A high current density causes the crystal plane to be oriented in the (111) plane, the lattice size to be smaller, and the atomic distance to be less (20).The grain size becomes smaller when the current density is high, and the morphological shape resembles that of a cauliflower (21).As a result of a high current density, the coating will be thicker, and the composition of the Ni weight fraction will also be higher.The amount of Ni deposited on the substrate and the thickness of the coating both have an impact on the product's magnetic properties (22).In addition, a smaller grain size results in an increase in hardness (23).Karunakaran et al. (24) reported a hardness of 153 HV when the current density was 40 mA/cm 2 , and Karunakaran and Pugazh (25) Vadivu reported a magnetic saturation value of 0.0004 emu/g at 40 mA/cm 2 .Nevertheless, the researchers did not examine the impact of the coating electrodeposition factors, structure, and morphology on the magnetic and hardness properties.
The aims of this research were 1) to produce a Nirich Ni-Cu alloy coating on Al via electrodeposition and 2) to investigate the link between magnetic and hardness properties and the coating's microstructure and surface morphology.We varied the current density, and the process was conducted at room temperature.We examined the influence of various process variables on the cathodic current efficiency, structure, morphology, composition, grain size, and thickness of the produced coatings.Finally, the magnetic properties and hardness of the coatings were investigated.

Characterization
The deposition rate was calculated using the previously reported method (26).The following formula, Equation 1, was used to calculate the efficiency of the cathodic current (27): Wm and Wf were calculated using Faraday's law, as shown in Equations 2 and 3.
The crystal structure of the Ni-Cu coating was determined using X-ray diffraction (XRD-PANanalytical Aeris Instrument Suit) (Cu-Kα radiation, λ = 0.15418 nm).XRD data were collected from 20° to 80° with a step size of 0.020°.The Materials Analysis Using Diffraction (MAUD) program was used to determine the crystal parameters of the sample after Rietveld refinement.The preferential crystallite orientation was determined from the texture coefficient , as shown in Equation 4 (28): Based on XRD results, the lattice strain  was calculated using Equation ( 5 coating thickness.Measurement of magnetic properties was conducted using a vibrating sample magnetometer (VSM, Oxford 1.2H).The hardness of the Ni-Cu coatings was measured using a MicroMct® 5100 Series Microindentation Hardness Tester.The ATM E384 standard was used for the tests, which were performed with a load of 100 g for 10 s at five places.

1. Cathodic Current Efficiency and Deposition Rate
Figure 1 depicts the relationship between the average CCE and deposition rate in the citrate electrolyte bath.The CCE was found to be high, with a value of 82-89%.The basic concept of current efficiency can be understood as the fraction of total current used for metal plating (30).Apart from metal deposition, hydrogen evolution is the only other necessary process that must occur on the substrate surface.Ni and Cu precipitation are both antagonistic to the hydrogen evolution reaction.In this study, the metal deposition rate increased faster than the hydrogen evolution rate when the current density increased from 15 to 40 mA/cm 2 ; hence, the CCE increased.The highest CCE (89.96%) was associated with the Ni-Cu 40 sample, and the lowest CCE (82.55%) was associated with the Ni-Cu 15 sample.Basori et al. and Syamsuir et al. (31) found that the deposition rate and current efficiency are considered linear.(32,33).As the Ni content of the alloy coating increased, the diffraction angle also increased.

2. Structural Properties
Crystal size calculation using MAUD resolved refinement was used to determine the size of the crystallites in the Ni-Cu alloys, and the results (Table 2) show that the crystallite size of the Ni-Cu coating ranged from approximately 24 to 50 nm.The crystallite size of a pure Ni layer is 60 nm, meaning that the Ni-Cu alloys had smaller crystallite sizes than a pure Ni layer.This result is also similar to that obtained by Li et al. (34).In contrast to the typical watt-Ni coating, we found that the presence of a sodium citrate complexing agent resulted in a finer crystallite size.This is consistent with the findings of Sarac and Baykul (35), who observed that Cu atoms affect grain refinement in Ni-Cu alloys.Cu atoms can restrain the surface diffusion of Ni atoms during the deposition process and inhibit the growth of crystallites.
The evolution of the crystallographic orientation of the Ni-Cu coatings produced with varying current densities is shown in detail in Figure 3.It can be observed that a strong (111) fiber texture appeared in all the samples, while the (002) texture gradually decreased as the current density increased.
The µ values of different crystal planes are also used to evaluate the degree of crystallographic orientation (34).Moreover, the Ni-Cu coating electrodeposited at the current density of 40 mA/cm 2 was found to have a strong (111) texture.
The texture coefficient for every preference was calculated using Equation 4to ascertain the preferred crystal orientation direction of each Ni-Cu alloy obtained at the various current densities, and the results are shown in Table 3 (32).
It seems that the texture coefficient was also dependent on the peak current, and the preferred orientation was the (111) plane.Li et al. (34) found that the higher the current density, the more dominant the (111) plane.The findings suggest that the (111) crystallographic orientation was preferable for all the Ni-Cu coatings electrodeposited at the tested current densities.
The lattice strain of the prepared coatings was determined using Equation 5, and Figure 4 illustrates the changes in the crystal size and lattice strain of the Ni-Cu coatings based on the current density applied in the plating bath.
The crystal size increased and the lattice strain decreased as the current density increased (36).This result aligns with that obtained by Devi et al. (33), who showed that the higher the current density, the more the

Surface Morphology
EDS was used to determine the elemental composition of the Ni-Cu coatings, and the results are shown in Figure 5 and Table 4. Cu and Ni were the only elements present in the deposits.The alloy composition was influenced by the current density: as the current density increased, the Cu content decreased.This phenomenon can be caused by the [Ni 2+ ]/[Cu 2+ ] ratio in the bath, which changes the composition of Ni and Cu.Goranova et al. discovered that as the Cu content of deposits decreased, so did the CCE (37).This phenomenon occurs because of the orderly deposition of Ni and Cu (38).In regular deposition, increasing the current density leads to an increased proportion of less noble metals in the deposited material (39).In our scenario, Ni is the less noble metal.As a result, when the current density was higher, the deposits were richer in Ni.
Another notable feature was that as the current density increased, the peak shifted to the right (i.e., to higher 2θ values).The change in the alloy composition may have also caused this peak shift.The Ni concentration increased as the current density increased (see Table 4).Because Ni and Cu combine to produce a single-phase alloy, the diffraction peak shifted toward that of pure Ni as the Ni percentage increased.This finding is similar to the observations of Goranova et al. (17), who found that the fcc reflection for Ni-rich Ni-Cu alloy deposits shifted to the right as the Ni concentration increased.Indeed, it is logical to expect the Ni-Cu alloy peak to shift as the Ni content increase.The surface morphological structure and the cross section of the coated samples were observed using SEM. Figure 6 shows SEM micrographs of the four samples' surface morphological structures.The deposits developed a fine-grained and compact spherical shape when lower deposition current densities were applied (Figure 5(a)).Deo et al. ( 27) and Goranova et al. ( 37) also observed this morphology at low current densities.The shape changed to a coarser cauliflower form when higher current densities were applied (Figure 6(d)).A diffusionlimited deposition mechanism in which a multigeneration spherical diffusion layer creates a cauliflower shape is likely to produce this type of morphology (40).As the current density increased, the cauliflower-like protrusions became more spaced and separated, creating gaps.The Ni-Cu 40 sample, produced with the highest current density, was found to have the largest gaps between the cauliflower-like bulges.
The increased nucleation rate can explain the observed decrease in grain size with increasing current density (41,42).Ni-Cu ion flow to the cathode is faster at higher current densities.Further investigation is  2b).The reduction in dspacing is ascribed to residual stress induced at a higher deposition rate (43).The statistical results of the grain size distribution presented in Figure 6 indicate that the grain size ranged from 4.63 to 1.94 µm.The decrease in grain size with the increase in current density is evident in the data shown in Table 5.
Figure 7 depicts the relationship between the deposited alloy composition and the applied current density.EDS was used to determine the composition.
Figure 8 (a-d) displays SEM cross-section images of the produced Ni-Cu coatings.The absence of cracks between the substrate and coating demonstrates that appropriate adhesion occurred between the two entities (44).The thickness of the electrodeposited Ni-Cu coating was also measured for each sample (27), and the   8; the thickness increased as the current density increased (19).Hence, a higher current density results in more mass and a thicker coating.The findings presented in Figure 1 show that as the current density increased, so too did the CCE.
According to Faraday's law, when the deposition time remains constant for all samples, coatings formed at lower current densities will be thinner than those produced at higher current densities.A thinner covering may lead to severe interference from the Al substrate.The cathodic current density also affects the coating, as Deo et al. (27) discovered that increasing the current density improves the thickness of the film due to an increase in the CCE.

4. Magnetic Properties
Figure 9 depicts the magnetic characteristics and fluctuations in the current density magnetization measured with a VSM at room As the Ni content of the Ni-Cu alloy coatings increased, so did the saturation magnetization (see Table 6).Wang et al. (43) reported that saturation magnetization depends on the Ni content of Ni-Cu alloy coatings.In addition, Awasthi (22) reported that magnetization is enhanced by increasing the coating thickness.A possible reason for this enhanced magnetization is the magnetic disorder caused by the coating.It has been shown that the trend in saturation magnetization enhancement is associated with the coating level (47).Demidenko et al. (48) found that monel has paramagnetic properties at room temperature, while the Ni-Cu alloys in this study had ferromagnetic properties.Here, we have improved upon the results of previous studies in which phosphorus (P) and tungsten (W) were added (25).In this study, the Ni-Cu 40 sample exhibited the strongest magnetic properties.

5. Hardness
Figure 10 depicts the dependence of the microhardness of the Ni-Cu coatings on the current density in the plating bath.From the data presented in Figure 10, it is clear that the coating of the Ni-Cu 40 sample had the highest microhardness value (404 HV).In general, the microhardness increased with the current density and was attributed to the grain size and thickness of the coatings (42).Pingale et al. (19) found that hardness increased with the thickness of the coating.The  Moreover, the hardness reported in previous studies that resulted from electrodeposition of Ni-Cu on Al in the presence of P was lower than that recorded in the present study (24).This is due to the smaller grain size that resulted from applying a different current density.In addition, the increase in microhardness reported here is related to the role that Ni atoms play in grain refinement (31)

CONCLUSION
In this study, Ni-Cu alloys were electrodeposited onto Al substrates using citrate baths.The effects of adding a citrate solution on the properties of the deposited coatings were studied, and the results illustrate that single-phase Ni-Cu alloy layers were produced on the Al surface at all current densities.The CCE increased as the current density increased.The coatings formed at lower current densities showed a more compact and spherical morphology, while those formed at higher current densities showed a less uniform structure with a cauliflower-like morphology.Both the surface morphology and composition of the coating showed a strong dependence on the current density.The Ni-Cu alloy coating deposited at a low current density had a layer thickness of 32 μm, while the coating deposited at a high current density had a thickness of 50 μm.The saturation magnetization of the coating increased with the Ni content in the Ni-Cu alloy and with the coating thickness.The hardness increased with the coating thickness, grain size, and Ni composition in the solution.The hardness of the produced Ni-Cu alloy coatings was found to be greater than that of monel.

TABLE 1 .
(Thermofisher Quanta 650 EDAX EDS Analyzer) with 1000× magnification was used to analyze the surface morphology of the Ni-Cu coatings.EDS was used to determine the chemical composition of the coatings, and the statistical distribution of grain sizes was calculated using ImageJ software.The cross sections of the coated samples were also examined to assess how the current density and CCE affected the Bath composition and deposition parameters

Figure 1 .
Figure 1.The cathodic current efficiency and deposition rate at a current density of 15, 20, 30, and 40 mA/cm 2

Figure 4 .
Figure 4.The lattice strain and crystallize size of the Ni-Cu alloys coated on Al at various current densities

Figure 5 .
Figure 5. Eds graph of the Ni-Cu 15

Figure 6 .
Surface SEM images of the deposited Ni-Cu alloy coatings and plots showing the statistical distribution of the grain size needed to determine the exact relationship between the current density and the grain size of the coating.In the Ni-Cu 15, Ni-Cu 20, Ni-Cu 30, and Ni-Cu 40 samples, the peak that corresponded to the (111) plane shifted toward the right (see Figure 20 4.38 ± 0.365 Ni-Cu 30 2.28 ± 0.068 Ni-Cu 40 1.94 ± 0.032 following results were recorded: Ni-Cu 15 = 32 μm, Ni-Cu 20 = 42 μm, Ni-Cu 30 = 49 μm, and Ni-Cu 40 = 50 μm.The effect of the current density on the thickness of the Ni-Cu coating is depicted in Figure

Figure 7 .Figure 8 .Figure 9 .
Figure 7.The dependence of the deposited alloy's composition (shown as wt% of the single electrolytes) on current density

Figure 10 .
Figure 10.The microhardness of the electrodeposited Ni-Cu coatings of the samples produced with different current densities Ramkumar et al. (50)  reported a monel hardness value of 165 HV, which is lower than the peak hardness value recorded in the current study.

Figure 11 .
Figure 11.The relationship found between wt% Ni and microhardness, based on the data obtained from the four experimental samples

TABLE 2 .
Parameters of the Ni-Cu alloys after Rietveld refinement using MAUD

TABLE 3 .
Texture coefficient analysis of Ni-Cu deposits

TABLE 4 .
The chemical composition of Ni-Cu coatings on the Al substrate prepared at different current densities

TABLE 5 .
Average grain size found in each sample

TABLE 6 .
The results of the magnetic analysis of the Ni-Cu/Al samples