Influence of CuSO₄ concentration on microstructures and properties of electroless deposited Ni/Cu-P coatings

Abstract

Ni/Cu–P composite coatings were fabricated on Q235 steel substrates with dimensions of 20 mm×15 mm×2 mm by introducing different CuSO₄ concentrations (0–0.30 g/L) into the Ni–P plating bath, denoted as SCU-0, SCU-0.1, SCU-0.15, SCU-0.2, and SCU-0.3, respectively. The effects of Cu addition on the structural, mechanical, magnetic, and corrosion properties were systematically investigated. The SCU-0.15 coating, deposited with 0.15 g/L CuSO₄, exhibited the most compact microstructure with a Cu content of 1.28 wt% and a thickness of 68.7 μm. TEM analysis confirmed that amorphous phosphorus was uniformly distributed in the Ni/Cu solid-solution matrix, forming nanosized grains (~ 34.6 nm) that improved structural uniformity. Compared with the undoped Ni–P coating (447.6 HV), the SCU-0.15 coating showed a markedly enhanced hardness of 713.5 HV and the lowest wear loss. The saturation magnetization decreased from 443 to 284 emu/cc as the CuSO₄ concentration increased from 0.1 to 0.3 g/L. Electrochemical testing in 15 wt% HNO₃ revealed that the SCU-0.15 coating possessed the most positive corrosion potential and the lowest corrosion current density (30.79 µA/cm²). Its charge-transfer resistance reached 4678.4 Ω·cm², and the largest Nyquist arc radius further confirmed superior corrosion protection. Overall, moderate Cu incorporation refined grains, strengthened the Ni–P matrix, and significantly improved the hardness, wear resistance, and corrosion durability of the coatings, demonstrating an effective route to optimize Ni-P coating performance.

Introduction

Electroless deposition (ED) is widely used in the machinery, agriculture, chemical, and aviation industries due to its simple operation, low cost, and high processing efficiency^1–3. In particular, nickel-based coatings have attracted sustained interest because they can significantly enhance surface hardness, wear resistance, and corrosion resistance, thereby extending the service life of mechanical components operating under harsh environments such as friction, corrosion, and high load conditions. As a result, nickel-based coatings are extensively applied in agricultural machinery parts, chemical reactor components, petroleum equipment, molds, and electronic connectors. Meng et al.⁴ prepared titanium-nickel-based coatings by adding titanium to nickel-based alloys and investigated the effect of titanium content on the corrosion resistance of nickel-based alloy coatings. The results showed that Ti would change the selection preference of different crystal planes and microstructure morphologies in the matrix phase, and the nickel-based alloy coating containing 1 wt% titanium had the densest microstructure and the most uniform distribution of precipitated phases. Dong et al.⁵. deposited nickel-based alloys with different boron contents on AISI 1045 carbon steel by laser cladding and investigated the influence of boron on the microstructure of the nickel-based alloy coatings through characterization techniques. The results showed that the microstructure of the nickel-based alloy coatings mainly consisted of γ-Ni dendrites, interdendritic eutectics, CrB-type chromium borides and dispersed NbC particles.

Among various nickel-based coatings, Ni–P coatings fabricated by ED are widely used for surface modification of both metallic and non-metallic substrates owing to their excellent adhesion, uniform microstructure, and tunable properties. The unique amorphous or amorphous–nanocrystalline structure of Ni–P coatings endows them with superior corrosion resistance and relatively low friction coefficients compared with crystalline nickel coatings. For example, it was reported by Rabizadeh et al.⁶ that Ni-P coatings could be deposited via ED and that the incorporation of SiO₂ nanoparticles enhanced the corrosion resistance of the coatings. Successful fabrication of Ni-P-Cu coatings was achieved in a study by Bhattacharyya et al.⁷, in which their corrosion and thermal performance were also thoroughly investigated. Ma et al.⁸ analyzed the influence of phosphorous acid concentration on the characteristics of Ni-P-PTFE coatings and found that these coatings exhibited low friction coefficients against steel. Despite these advantages, pure Ni–P coatings still suffer from inherent limitations, including relatively low hardness and insufficient wear resistance under severe service conditions, which restrict their broader industrial applications^9,10. Alloying nickel with a third element has therefore become an effective strategy to overcome these drawbacks. By introducing elements such as Cu, W, Co, and Mo, the microstructure, phase constitution, and defect density of Ni–P coatings can be tailored, leading to synergistic improvements in mechanical, tribological, and corrosion properties^11–14.

Among these alloying elements, copper is of particular interest due to its dual role during the electroless deposition process. Generally, Cu ions can act as stabilizers by suppressing excessive nickel deposition and improving coating uniformity, while simultaneously serving as catalytic centers that promote Ni reduction under appropriate conditions¹⁵. However, an excessive Cu concentration may disrupt the deposition balance, resulting in coarser microstructures and degraded coating performance. Therefore, precise control of CuSO₄ concentration is crucial for optimizing the structure and properties of Ni/Cu–P coatings. Zarebidaki et al.¹⁶ investigated the corrosion resistance and wear resistance of electroless copper coatings with different copper contents. The research results indicated that the microhardness value of the coating decreased with the increase of copper content. In the corrosion resistance test, the corrosion current density of the coating increased with the increase of copper content in the coating. Ashassi-Sorkhabi et al.¹⁷ successfully deposited Ni-Cu-P alloy on 302b steel plates and investigated the effects of pH value, temperature and bath composition on the hardness and composition of the deposits. The research results showed that the average deposition rate of the alloy was 9 mg cm^− 2·h^− 1, and the optimal pH and temperature were 8.5 and 80 °C, respectively. Marchewka et al.¹⁸ conducted electrochemical deposition of copper (II) ions from copper sulfate solution on the surface of stainless steel using carbon nanotubes. The variation in the concentration of copper (II) ions in the solution and the deposition time led to the formation of various crystal morphologies. The study found that the size of copper (I) oxide crystals varied depending on the concentration of the copper sulfate solution.

Although previous studies have demonstrated that Cu incorporation can influence the mechanical and corrosion behavior of Ni–P coatings, most reports remain phenomenological and lack a clear correlation between microstructural evolution and property optimization. In particular, the role of Cu in regulating the amorphous–nanocrystalline transition and its subsequent impact on multifunctional properties has not yet been fully clarified. Unlike previous reports, this work establishes a direct link between structural refinement (Scherrer-calculated crystallite size and TEM evidence) and property optimization, providing a clear mechanistic explanation for the Cu-modulated amorphous–nanocrystalline transition. The effects of CuSO₄ concentration on the structure, hardness, surface roughness, corrosion and wear resistance, and magnetic characteristics of the Ni/Cu-P coatings were systematically investigated. This study offers technical support for enhancing the hardness, corrosion, and wear resistance of the Ni-P coatings and presents a new approach for their application in agricultural, industrial, and petroleum fields.

Experiment

Materials and plating condition

The Q235 steel substrates, with dimensions equal to 20 mm × 15 mm × 2 mm, were polished sequentially using 200#, 600#, 800#, and 1200# abrasive papers. After polishing, the substrates underwent degreasing, etching, sensitization, and activation treatments. The degreasing step removed residual grease and organic contaminants from the steel surface. The subsequent etching treatment slightly roughened the surface and dissolved the passive oxide layer, thereby enhancing mechanical interlocking between the coating and substrate. During sensitization, Sn²⁺ ions were adsorbed onto the substrate surface to provide catalytic sites, and the following activation step introduced Pd²⁺ ions, which were reduced to Pd⁰ nuclei serving as active centers for the initiation of Ni–P deposition. These pretreatments ensured uniform nucleation, improved adhesion, and promoted the formation of continuous, defect-free coatings.

The solution compositions and process parameters for each step are detailed in Table 1. Each chemical served a distinct role: NaOH, Na₂CO₃, and Na₃PO₄ acted as alkaline cleaners; HF and H₂SO₄ promoted oxide removal and micro-etching; SnCl₂ and PdCl₂ were used sequentially for sensitization and activation to create catalytic Pd sites; and NaH₂PO₂·H₂O served as the reducing agent in the electroless bath, while CuSO₄ controlled Cu incorporation for microstructural modification. To utilize the stabilizing effect of CuSO₄ for achieving a fine and compact coating with enhanced wear and corrosion resistance, CuSO₄ was added to the electroless Ni–P plating solution at concentration values ranging from 0.1 to 0.3 g/L. All reagents used in the experiment were of analytical grade. After each treatment step, the obtained specimens were thoroughly cleaned with deionized water and dried before ED. The resulting coatings were designated as SCU-0, SCU-0.1, SCU-0.15, SCU-0.2, and SCU-0.3, corresponding to CuSO₄ concentrations of 0, 0.1, 0.15, 0.2, and 0.3 g/L, respectively.

Table 1.

Solution compositions and process parameters for fabricating Ni/Cu-P coatings.

Operation	Chemical	Concentration	Function	Conditions
Degreasing	NaOH	60 g/L	grease removal	Temperature: 80℃, Time: 10 min, pH = 8.5
	Na2CO3	50 g/L	emulsification
	Na3PO4	40 g/L	dispersion of oils
Etching	HF	80 mL/L	Oxide removal, surface micro-etching	Temperature: 25℃, Time: 5 min pH = 3.5
	H2SO4	80 mL/L	Acidic activator, surface roughening
Sensitization	PdCl2	0.4 g/L	Adsorption of Sn²⁺ for catalytic activation	Temperature: 25℃, Time: 5 min pH = 4.0
	38% HCl	10 mL/L	Stabilization of Sn²⁺ ions
Activation	SnCl2	30 g/L	Formation of Pd⁰ catalytic nuclei	Temperature: 25℃, Time: 10 s pH = 4.8
	35% HCl	40 mL/L	Dissolution and stabilization of Pd²⁺
Electroless deposition	NiSO4·6H2O	30 g/L	Ni source for coating deposition	Temperature: 80℃, Depositing time: 100 min pH = 6.0
	Na3C6H5O7·2H2O	10 g/L	Complexing agent, bath stabilization
	NaC2H3O2·3H2O	15 g/L	Buffer agent, pH control
	C3H6O3	30 g/L	pH adjustment and buffering
	NaH2PO2·H2O	25 g/L	P source and reducing agent
	CH4N2S	0.01 g/L	suppression of bath decomposition
	CuSO4	0, 0.1, 0.15, 0.2, 0.3 g/L	Cu source; microstructure modifier

Characterization

A JSM-3400 scanning electron microscope (SEM) was employed to examine the cross-sectional and surface morphologies of the obtained coatings. The elemental compositions of the coatings were analyzed using an INC250 energy-dispersive spectrometry (EDS) system. For transmission electron microscopy (TEM) analysis, thin foils of the coatings were prepared by mechanically polishing the cross-sectional samples to a thickness of approximately 30–50 μm, followed by ion milling (Gatan PIPS, 5 kV Ar⁺ beam) until electron transparency was achieved. In some cases, site-specific lamellae were also extracted using focused ion beam (FIB, FEI Helios Nanolab 600i) to observe the interface and nanocrystalline regions. The microstructure of the coatings was then examined using a JEM-2100 F transmission electron microscopy (TEM) system operated at 200 kV. The phase structures of the coatings were studied via an AL-Y3500 X-ray diffractometer (XRD) using Cu Kα radiation, a tube current equal to 300 mA, an accelerating voltage equaling 50 kV, a scan rate equal to 2°/min, a scan range extending from 20°–90°, and a scanning step size equal to 0.02°. The average crystallite sizes of the coatings were calculated from the full width at half maximum (FWHM) of the main diffraction peaks using the Scherrer equation (Eq. 1):

1

where D is the crystallite size (nm), K is the shape factor (0.9), λ is the wavelength of Cu Kα radiation (0.15406 nm), β is the FWHM of the diffraction peak (in radians), and θ is the Bragg diffraction angle. The obtained values represent the average coherent domain size of the nanocrystalline phases within the amorphous Ni–P matrix. The coating hardness was determined via a VMH-002 V microhardness tester under a load equal to 1 N and a holding time equaling 15 s. For each specimen, five measurements were performed at randomly selected locations on the coating surface to minimize local heterogeneity, and the average value was taken as the coating hardness. Surface roughness (Ra) was evaluated with a TR300 surface roughness tester, and coating brightness was assessed using a YG47 gloss meter at an incident angle of 45°.

Wear experiments were carried out utilizing an MM-200 abrasion testing machine. In this configuration, the coated specimen (block, 20 mm×15 mm×2 mm) was fixed horizontally against the rotating GCr15 quenched steel ring (Φ30 mm, hardness ≈ 60 HRC), serving as the counterpart material. The sample was positioned in contact with the outer circumferential surface of the ring and pressed under a normal load of 20 N. The ring rotated at 200 rpm, corresponding to a linear sliding speed of approximately 0.47 m/s (≈ 28.3 m/min). Given the 30 min test duration, the total sliding distance was about 850 m. The specimen was weighed every 5 min using an electronic balance, and the mass loss before and after each interval was recorded as the wear amount. Although intermittent weighing could slightly interrupt the wear process, this approach was adopted to monitor the progressive wear behavior and ensure accurate cumulative mass-loss measurement. The specimens were allowed to cool to ambient temperature before each weighing to minimize thermal expansion effects. It is acknowledged that such interruptions might partially influence the stabilization of the tribolayer compared with a continuous test; however, the short cooling intervals (less than 1 min) and the consistent experimental conditions ensured the reproducibility and comparability of the results. This procedure follows standard block-on-ring test practice and ensures consistent contact geometry throughout the experiment. The total wear test duration was 30 min. Moreover, the magnetic properties of the SCU-0, SCU-0.1, SCU-0.15, SCU-0.2, and SCU-0.3 coatings were evaluated using a vibrating sample magnetometer (VSM, Lakeshore-7304).

The polarization curves and electrochemical impedance spectra of the coatings in a 15 wt% HNO₃ solution were measured using a CS350 electrochemical testing system. Nitric acid (HNO₃) was selected as the corrosive medium because it is a strong oxidizing acid capable of promoting the formation and breakdown of passive films on Ni–P-based coatings. A conventional 3-electrode setup was employed, with a Pt electrode (10 mm × 10 mm) as the auxiliary electrode, the coated sample (7 mm × 7 mm test area) as the working electrode, and a saturated calomel electrode (SCE) as the reference electrode. The prepared specimen was then immersed within the test solution for 15 min before testing to stabilize the open-circuit potential. The polarization curve was obtained over a voltage range of ± 250 mV relative to the open-circuit potential, with a scan rate equal to 50 mV/min. Electrochemical impedance spectroscopy (EIS) was performed at the stabilized open-circuit potential over a frequency range of 100 kHz to 10 mHz, using a sinusoidal AC perturbation of 10 mV (rms) and acquiring 10 points per frequency decade. All measurements were conducted at 25 ± 1 °C. Each spectrum was collected at least in duplicate to confirm reproducibility. EIS data were fitted with an equivalent circuit of the form Rs, Rct∥CPE using ZView (Scribner), and the goodness-of-fit was evaluated by the χ² value and the randomness of residuals.

Results and discussion

Surface and cross-sectional morphologies

The surface morphologies of the obtained coatings obtained at various CuSO₄ concentration values (0–0.3 g/L) are shown in Fig. 1. All coatings displayed the typical nodular morphology characteristic of Ni-P coatings, appearing dense and continuous. As illustrated in Fig. 1a, the surface of the Ni-P coating without CuSO₄ displayed non-uniform nodular clusters with various sizes, resulting in a relatively coarse morphology. However, the SCU-0.15 coating exhibited a finer and more compact morphology compared to the SCU-0 and SCU-0.1 coatings. Furthermore, with the increase in CuSO₄ concentration, the nodules in the SCU-0.15 coating became smaller and more uniform in size, as displayed in Fig. 1c. The improvement in coating morphology was primarily attributed to the standard electrode potential of Cu²⁺, which allowed it to preferentially precipitate and adsorb onto the substrate surface. The precipitated Cu acted as nucleation sites for subsequent Ni deposition, leading to finer nodules in the coating¹⁹. Cu²⁺ inhibited the rapid precipitation of Ni, promoting a more uniform and compact coating morphology²⁰. When the CuSO₄ concentration reached 0.2 g/L and 0.3 g/L, distinct pyramid-like morphology appeared on the surfaces of the SCU-0.2 and SCU-0.3 coatings, respectively, and their density increased with rising CuSO₄ concentration (Fig. 1d and e). This morphological evolution can be attributed to the change in local deposition kinetics and crystallographic orientation induced by excessive Cu incorporation. At higher Cu²⁺ levels, the catalytic reduction of Ni²⁺ accelerates preferentially along the (111) and (200) crystal planes, resulting in anisotropic growth and the formation of faceted, pyramid-like surface features. Meanwhile, the increased Cu content reduces the overall P incorporation, lowering the amorphous fraction and enhancing the degree of crystallinity. This transition from an amorphous to a partially crystalline morphology favors three-dimensional nucleation and the outward growth of pyramidal grains. Moreover, localized Cu enrichment on the coating surface promotes heterogeneous nucleation clusters that act as growth centers for these pyramid-like features. These changes in surface morphology indicate that the addition of a small amount of CuSO₄ to the electroless Ni-P plating solution can enhance the surface quality of the coating. In this study, the surface quality was primarily evaluated based on the average nodule size and surface compactness observed in SEM images. A smaller and more uniformly distributed nodule size reflects a higher nucleation density and lower surface roughness, which are desirable for achieving compact coatings with fewer micro-defects and improved corrosion resistance. In this context, CuSO₄ acted as a stabilizer during the deposition process of the Ni-P coatings. In this context, CuSO₄ acted as a stabilizer during the deposition process of the Ni–P coatings by moderating the reduction rate of Ni²⁺ ions and promoting uniform growth of fine nodules. However, as the CuSO₄ concentration exceeded 0.2 g/L, the nodule size increased and pyramid-like features appeared, indicating a transition from uniform fine-grained growth to coarse crystalline growth. Larger nodules generally lead to higher surface roughness and more exposed grain boundaries, which may serve as preferential sites for corrosion initiation and wear. This morphological evolution suggests that while moderate Cu²⁺ addition improves surface smoothness and coating uniformity, excessive Cu²⁺ promotes anisotropic grain growth and surface coarsening, thereby reducing the overall surface quality²¹.

Fig. 1.

Surface morphologies of (a) SCU-0, (b) SCU-0.1, (c) SCU-0.15, (d) SCU-0.2, and (e) SCU-0.3 coatings.

The EDS spectra of the different coatings are presented in Fig. 2, and their corresponding compositions are given in Table 2. As anticipated, no Cu was detected in the SCU-0 coating. In the SCU-0.1 coating, the Cu content was also very low (0.31 wt%), resulting in a weak diffraction peak. This may be attributed to the low Cu²⁺ concentration in the SCU-0.1 bath and the preferential precipitation of Cu²⁺ ions, most of which were consumed during the initial stage of the ED process²². However, distinct diffraction peaks corresponding to Cu appeared in the SCU-0.15, SCU-0.2, and SCU-0.3 coatings when the CuSO₄ concentrations reached 0.15, 0.2, and 0.3 g/L. The Cu contents in these coatings were 1.28, 3.82, and 7.43 wt%, respectively. The contents of P and Ni in all coatings decreased with an increase in the concentration of CuSO₄. The P content in each coating remained above 10 wt%, indicating the formation of high-phosphorus coatings.

Fig. 2.

EDS spectra of different coatings.

Table 2.

Element compositions in different coatings (wt%).

Sample	Ni	Cu	P
SCU−0	87.18	0	12.82
SCU−0.1	86.15	0.31	13.54
SCU−0.15	85.34	1.28	13.38
SCU−0.2	84.31	3.82	12.88
SCU−0.3	81.25	7.43	11.32

The cross-sectional morphologies of the SCU-0, SCU-0.15, and SCU-0.3 coatings are presented in Fig. 3. The SCU-0 coating had a coarse and porous morphology, with a thickness of approximately 67.9 μm. However, the SCU-0.15 and SCU-0.3 coatings exhibited compact and refined cross-sectional morphologies, with coating thicknesses of approximately 68.7 μm and 68.2 μm, respectively. In the electroless Ni–P system, copper ions (Cu²⁺) possess a more positive standard reduction potential (Cu²⁺/Cu, + 0.34 V vs. SHE) than nickel ions (Ni²⁺/Ni, − 0.25 V vs. SHE). As a result, Cu²⁺ is reduced more readily by the hypophosphite reducing agent, leading to the initial codeposition of Cu atoms on the substrate surface. These Cu nuclei serve as catalytic sites that accelerate the subsequent autocatalytic reduction of Ni²⁺ and enhance the overall nucleation density²³.

Fig. 3.

Cross-sectional morphologies of (a) SCU-0, (b) SCU-0.15, and (c) SCU-0.3 coatings.

TEM structure

The TEM microstructures of the SCU-0, SCU-0.15, and SCU-0.3 coatings are displayed in Fig. 4. The SCU-0 coating had a coarse microstructure, with an average Ni grain diameter of approximately 89.5 nm. When an appropriate amount of CuSO₄ (0.15 g/L) was added to the plating bath, the coating structure became finer and denser, with the average diameter of the Ni/Cu solid solution measuring approximately 34.6 nm. However, the coating displayed a coarser structure as the CuSO₄ concentration was raised to 0.3 g/L, and the average size of the Ni/Cu solid solution increased to approximately 63.9 nm.

Fig. 4.

TEM images of (a) SCU-0, (b) SCU-0.15, and (c) SCU-0.3 coatings.

Only Ni grains were observed in the Ni-P coating, as no Cu²⁺ ions were added to the plating solution. However, Ni/Cu solid solutions were present in the SCU-0.15 and SCU-0.3 coatings due to the introduction of CuSO₄ into the plating bath. Since the concentration value of the Cu ions within the solution was relatively low, the XRD profiles of the prepared Ni/Cu-P coatings primarily showed nickel grain characteristics, as illustrated in the upper right corners of Fig. 4b and c.

Phase structure

The XRD spectra of the SCU-0, SCU-0.1, SCU-0.15, SCU-0.2, and SCU-0.3 coatings are provided in Fig. 5. The SCU-0, SCU-0.1, and SCU-0.15 coatings displayed only a broad, bun-shaped nickel diffraction peak at 2θ = 45°, which is characteristic of the short-range order of amorphous Ni–P alloys. However, TEM analysis (Sect.  3.4) revealed the presence of weak diffraction rings and discrete spots, suggesting that the coatings were not fully amorphous but rather composed of an amorphous matrix containing uniformly dispersed nanocrystallites. Therefore, the Ni–P–Cu coatings should be regarded as amorphous–nanocrystalline composite structures, where nanosized Ni or Ni/Cu solid-solution grains are embedded within an amorphous Ni–P matrix. The addition of a small amount of CuSO₄ did not alter the amorphous structure of the coating, which contributed to its excellent wear and corrosion resistance^24,25. When the CuSO₄ concentration increased to 0.2 and 0.3 g/L, additional diffraction peaks appeared at 2θ ≈ 43.4° and 50.3°, which can be indexed to the (111) and (200) planes of a Cu₀.₈₁Ni₀.₁₉ solid solution (PDF#04-0836). This indicates the formation of a significant amount of Ni–Cu solid solution in the SCU-0.2 and SCU-0.3 coatings²⁶, confirming a gradual structural transition from a predominantly amorphous matrix to a mixed amorphous–nanocrystalline state with increasing Cu incorporation. The average crystallite size, microstrain, and relative texture coefficient (RTC) were further evaluated from the broadening and intensity distribution of the main (111) diffraction peak, and the results are summarized in Table 3. The crystallite size decreased from 24.1 nm (SCU-0) to 10.3 nm (SCU-0.15), accompanied by a pronounced increase in microstrain, reflecting intensified lattice distortion induced by Cu incorporation and inhibited grain growth. At higher CuSO₄ concentrations (SCU-0.2 and SCU-0.3), the microstrain slightly decreased, which can be attributed to partial stress relaxation associated with the formation of Ni–Cu solid solutions and improved crystallinity.

Fig. 5.

XRD spectra of different coatings.

Table 3.

Crystallite size of Ni–P–Cu coatings calculated from scherrer equation.

Sample	Main diffraction peak (2θ, °)	FWHM (°)	Crystallite size D (nm)	microstrain ε (×10⁻³)	RTC_Ni(111) (%)
SCU−0	45.0 Ni(111)	0.35	24.1	6.8	90.91
SCU−0.1	45.0 Ni(111)	0.48	17.6	9.3	97.89
SCU−0.15	45.0 Ni(111)	0.82	10.3	15.9	85.00
SCU−0.2	43.4 Cu₀.₈₁Ni₀.₁₉ (111)	0.76	11.1	14.7	86.63
SCU−0.3	43.4 Cu₀.₈₁Ni₀.₁₉ (111)	0.61	13.8	11.8	79.42

Furthermore, the RTC_Ni(111) values reveal a strong preferred orientation along the (111) direction for all coatings. The RTC initially increased with the addition of a small amount of CuSO₄ and then gradually decreased at higher Cu contents, indicating that excessive Cu incorporation weakened the (111) texture dominance due to competitive growth of Ni–Cu solid-solution crystallites^27,28.

Hardness

Figure 6 illustrates the relationship between coating hardness and CuSO₄ concentration. The coating hardness increased with rising CuSO₄ concentration. At 0 g/L CuSO₄, the hardness of the SCU-0 coating was approximately 447.6 HV. The hardness of the SCU-0.15 coating increased significantly to 713.5 HV when the concentration reached 0.15 g/L. This enhancement correlates well with the XRD and TEM observations, which confirmed that the Cu-containing coatings exhibit an amorphous–nanocrystalline composite structure. The presence of fine Ni/Cu nanocrystallites embedded within an amorphous Ni–P matrix results in grain-boundary strengthening and constraint of plastic deformation, following the Hall–Petch relationship. The refined grain size and the high density of Ni/Cu interfaces increase dislocation barriers, thereby enhancing hardness²⁹.

Fig. 6.

Effect of CuSO₄ concentration on hardness of coatings.

This led to a denser and more uniform coating structure, therefore enhancing the coating’s hardness. Solid-solution strengthening from Cu incorporation—caused by atomic-size and lattice-strain mismatch between Ni and Cu atoms—combined with reduced P content and higher crystallinity (as evidenced by sharper XRD peaks) contributed to this further improvement. As the CuSO₄ concentration further increased to 0.3 g/L, Ni and Cu were co-deposited to form a Ni/Cu solid solution, which effectively inhibited coating deformation and further increased hardness. Thus, the SCU-0.3 coating demonstrated the highest hardness of 728.9 HV among all samples. Overall, the progressive increase in hardness with Cu addition can be attributed to the synergistic effects of grain refinement, amorphous–nanocrystalline structure formation, and solid-solution strengthening, as consistently supported by the XRD and TEM results.

Surface roughness and brightness

The impact of the CuSO₄ concentration value on the surface roughness and brightness of the coatings is illustrated in Fig. 7. The Ra values of SCU-0, SCU-0.1, SCU-0.15, SCU-0.2, and SCU-0.3 coatings first decreased and then increased as the CuSO₄ concentration value was enhanced from 0 to 0.3 g/L. The SCU-0.15 coating revealed the lowest surface roughness of 0.78 μm at a CuSO₄ concentration value equal to 0.15 g/L, whereas the SCU-0 coating had the highest surface roughness of 1.23 μm among all samples. Moreover, the brightness of the coatings initially increased and then followed a decreasing trend with a rise in CuSO₄ concentration. The SCU-0.15 coating, deposited at 0.15 g/L CuSO₄, indicated the highest brightness of 196.8 GU, indicating excellent surface quality. The addition of an appropriate concentration of CuSO₄ to the plating solution resulted in a fine and uniform coating structure, enhancing the surface brightness³⁰. However, at CuSO₄ concentrations of 0.2 to 0.3 g/L, numerous Ni-Cu solid solutions formed on the coating surfaces, leading to the development of pyramid-like structures on SCU-0.2 and SCU-0.3 coatings, which resulted in decreased brightness.

Fig. 7.

Effect of CuSO₄ concentration on roughness and brightness of coatings.

Wear resistance

The wear loss curves of SCU-0, SCU-0.1, SCU-0.15, SCU-0.2, and SCU-0.3 coatings are shown in Fig. 8. The wear losses of all coatings increased rapidly during the initial wear period from 0 to 15 min, which can be attributed to the time required to smooth the rough surfaces at the beginning of the test. From 15 to 30 min, the wear losses of all coatings continued to increase gradually with wear time. As the wear test progressed, the rough surface of the coating became smoother, resulting in full contact between the coating surface and the friction pair. Therefore, the wear resistance of the coating slowly increased³¹. Moreover, the SCU-0.15 coating indicated the lowest wear loss among all coatings, while the SCU-0 coating showed the highest wear loss during the wear test. This trend is in excellent agreement with the hardness results (Sect.  3.4), confirming that higher hardness and denser microstructures effectively improve wear resistance. The enhanced hardness of the SCU-0.15 coating, resulting from grain refinement, solid-solution strengthening, and the amorphous–nanocrystalline composite structure, provides greater resistance to plastic deformation and microcutting by abrasive asperities. Consequently, the dominant wear mechanism for this coating is mild abrasive wear, characterized by shallow grooves and limited material removal.

Fig. 8.

Wear loss curves of different coatings.

In contrast, the SCU-0 coating, which has a coarse and porous morphology and lower hardness, experiences severe abrasive and adhesive wear. The weak interfacial bonding and low hardness facilitate the detachment of wear debris, which adheres to the counterface and leads to secondary abrasion. For the SCU-0.3 coating, despite the formation of a Ni–Cu solid solution that improves intrinsic hardness, the emergence of pyramid-like surface features increases local stress concentration and promotes microfracture under sliding contact, resulting in slightly higher wear loss compared to SCU-0.15. This difference is attributed to the microstructure of the coatings, as a finer and more compact structure significantly enhances wear resistance³².

The worn surface morphologies of various coatings after the wear test are shown in Fig. 9. No pyramid-like structures were observed on the surfaces of the Ni-P and Ni/Cu-P coatings, as the rough surfaces were smoothed during the wear experiment. The SCU-0 coating (Fig. 9a) revealed deep and wide furrows, resulting from plastic deformation during wear testing. Material buildup occurred along the edges of the wear tracks³³. However, the depth and width of the furrows decreased as the concentration of CuSO₄ concentration was raised up to 0.15 g/L (Fig. 9c), indicating the excellent wear resistance of the SCU-0.15 coating. As previously mentioned, the microstructure played a pivotal role in the wear resistance of the coating. The SCU-0.15 coating revealed a fine and compact microstructure, which enhanced its wear resistance, resulting in minimal wear loss and shallower furrows. For the SCU-0.3 coating (Fig. 9e), several small bumps were observed on the worn surface. These protrusions correspond to regions enriched in the Ni/Cu solid-solution phase that formed uniformly within the Ni–P matrix during deposition. In this coating, Ni and Cu atoms co-deposited to form nanoscale solid-solution domains embedded in the amorphous matrix rather than isolated particles. During sliding wear, the relatively softer Ni–P matrix experienced preferential material removal, while the harder Ni/Cu solid-solution domains resisted abrasion and remained slightly protruded above the worn surface^34,35.

Fig. 9.

Worn SEM images of (a) SCU-0, (b) SCU-0.1, (c) SCU-0.15, (d) SCU-0.2, and (e) SCU-0.3 coatings.

Magnetic property

The coercivity (H_c) and saturation magnetization (M_s) profiles of the Ni-P and Ni/Cu-P coatings are presented in Fig. 10. All coatings showed low H_c values, indicating soft magnetic behavior. Among them, the SCU-0 coating displayed the highest M_s value of 802 emu/cc. The M_s value of the coating dropped from 443 to 284 emu/cc as the CuSO₄ concentration was raised from 0 to 0.3 g/L. This trend can be attributed to the fact that copper is a non-magnetic element with negligible M_s, whereas nickel is ferromagnetic with a high M_s³⁶. Therefore, increasing Cu content in the coating resulted in a gradual reduction in its overall M_s.

Fig. 10.

Magnetic hysteresis curves of Ni-P and Ni/Cu-P coatings.

To explore the impact of CuSO₄ concentration on the magnetic properties of the coatings further, their Cu contents were analyzed using the Lakeshore-7304 vibrating sample magnetometer. The influence of CuSO₄ concentration on the M_s of the coatings is presented in Fig. 11. It was observed that the M_s value decreased from approximately 440 emu/cc to 285 emu/cc as the CuSO₄ concentration value was enhanced from 0.1 to 0.3 g/L, which aligns with the magnetization measurement results.

Fig. 11.

Effect of CuSO₄ concentration on saturation magnetization of coatings.

From an application perspective, the combination of low coercivity and tunable saturation magnetization makes the Ni/Cu–P coatings promising candidates for soft magnetic functional coatings used in electromagnetic and electromechanical components. For instance, coatings with relatively high Ms and low Hc, such as SCU-0 and SCU-0.1, are suitable for applications requiring efficient magnetic flux conduction, including magnetic shielding layers, inductive components, and electromagnetic actuator surfaces. In contrast, coatings with reduced Ms achieved at higher CuSO₄ concentrations may be advantageous for applications where excessive magnetic interaction must be avoided, such as precision mechanical components, sensor housings, and tribological parts operating in magnetic-field-sensitive environments. Moreover, the ability to tailor magnetic properties through CuSO₄ concentration adjustment enables the integration of wear-resistant and corrosion-resistant Ni/Cu–P coatings into multifunctional components that simultaneously require controlled magnetic responses, which is particularly relevant for micro-electromechanical systems (MEMS), electrical connectors, and protective coatings for electronic devices.

Corrosion resistance

The electrochemical polarization curve of the coatings in 15 wt% HNO₃ solution are shown in Fig. 12, while their corresponding corrosion current density (J_corr), corrosion potential (E_corr), polarization resistance (R_p), and charge transfer resistance (R_ct) values are provided in Table 3. All electrochemical data were normalized to the exposed surface area of the working electrode (7 mm×7 mm, i.e., 0.49 cm²). The corrosion potential and polarization resistance of the coatings initially increased and then followed a decreasing trend as the CuSO₄ concentration was increased, while the corrosion current density showed the opposite trend, first decreasing and then rising. The SCU-0.15 coating displayed the highest corrosion potential (E_corr = − 0.002 V) and polarization resistance (R_p = 1190.3 Ω·cm²) among all samples. Moreover, the corrosion current density of the SCU-0.15 coating was the lowest at 30.79 µA/cm², indicating its higher corrosion resistance. This outstanding performance is attributed to the coating’s dense and homogeneous amorphous structure formed at a CuSO₄ concentration of 0.15 g/L, which lacks grain boundaries, enhancing its corrosion resistance^37,38. Similarly, the corrosion potentials and polarization resistances of the SCU-0.2 and SCU-0.3 coatings were lower, while their corrosion current densities were higher than those of the SCU-0 coating, indicating poorer corrosion resistance. This decline is attributed to the co-deposition of Ni and Cu at CuSO₄ concentrations of 0.2 and 0.3 g/L, which transformed the coating structure from amorphous to a mixed crystalline phase. Therefore, various corrosive microcells formed at the grain boundaries of the microcrystalline structure, accelerating the corrosion process³⁹.

Fig. 12.

Polarization curves and electrochemical impedance spectra of different coatings in 15 wt% HNO₃ solution.

Although the incorporation of Cu modified the microstructure and surface morphology of the Ni–P coatings, the corrosion current densities (Jcorr) among all samples were relatively close, varying only between 30.8 and 34.2 µA·cm⁻² (Table 4). This indicates that the addition of Cu had only a limited influence on the overall corrosion rate under the strong oxidizing conditions of 15 wt% HNO₃. The slight improvement observed for the SCU-0.15 coating is mainly attributed to its smoother surface and denser amorphous–nanocrystalline structure, which reduce the number of active corrosion sites. The anodic and cathodic Tafel slopes (β_a and β_c) exhibited only small variations, suggesting that the fundamental electrochemical reactions—metal dissolution and hydrogen evolution—were not significantly altered by Cu incorporation. Therefore, while moderate Cu addition improved the coating compactness and slightly increased R_ct, the intrinsic corrosion kinetics of the Ni–P matrix remained largely unchanged.

Table 4.

Electrochemical parameters of different coatings.

Sample	Ecorr (V)	Jcorr (µA/cm2)	βa (V·dec⁻¹)	βc (V·dec⁻¹)
SCU−0	−0.153	33.02	0.124	0.167
SCU−0.1	−0.086	31.61	0.118	0.162
SCU−0.15	−0.002	30.79	0.115	0.158
SCU−0.2	−0.237	34.05	0.126	0.172
SCU−0.3	−0.291	34.18	0.129	0.175

Figure 13a presents the Bode plots of the Ni–Cu–P coatings in 15 wt% HNO₃ solution, showing the variation of impedance modulus (|Z|) as a function of frequency. All samples display a single time constant, consistent with the semicircular Nyquist plots, confirming that the corrosion process is dominated by a single charge-transfer mechanism at the coating/electrolyte interface. The corresponding Bode-phase plots further confirm the presence of a single time constant for all coatings. As shown in Fig. 13b, each sample exhibits only one broad phase-angle maximum over the entire frequency range, with no additional peaks or shoulders observed. This characteristic single peak indicates that the electrochemical response is governed by one dominant electrochemical process, validating the use of a single-time-constant equivalent circuit for EIS fitting. The phase-angle maximum is located in the intermediate-frequency region, which is typically associated with charge-transfer reactions coupled with interfacial capacitance at the coating/electrolyte interface. At low frequencies (below 10 Hz), the impedance modulus represents the total polarization resistance of the system and is closely related to the coating’s protective ability. The SCU-0.15 coating exhibits the highest |Z| value (≈ 10²․¹ Ω cm²) at low frequency, indicating the greatest resistance to charge transfer and the slowest corrosion rate. In contrast, the SCU-0.3 coating shows the lowest |Z| (≈ 10¹․¹ Ω cm²), suggesting a more permeable surface and inferior corrosion protection.

Fig. 13.

Bode plots of the Ni–Cu–P coatings in 15 wt% HNO₃ solution, (a) impedance modulus |Z| versus frequency and (b) phase angle versus frequency.

In the mid-frequency region (10–10³ Hz), the impedance gradually decreases with increasing frequency, corresponding to the relaxation of the interfacial capacitance. The parallel shift of the |Z| curves with Cu concentration reveals that Cu incorporation affects the compactness and dielectric properties of the coatings. Moderate Cu addition (0.15 g L⁻¹) produces a dense amorphous–nanocrystalline structure that suppresses electrolyte penetration, whereas excessive Cu (≥ 0.2 g L⁻¹) results in rougher, pyramid-like surfaces and a more porous morphology, reducing the barrier effect. At high frequencies (> 10³ Hz), all samples approach similar |Z| values, reflecting the solution resistance (Rₛ) and confirming that differences in corrosion behavior arise mainly from interfacial rather than bulk-electrolyte effects.

Figure 14 shows the electrochemical impedance spectra of the different coatings in 15 wt% HNO₃ solution. In addition, the specific values of the equivalent circuit fitting parameters are shown in Table 5. All spectra showed similar semicircular shapes, indicating that the corrosion process involves a single time constant. The SCU-0.15 coating indicated the largest arc radius in the impedance spectrum, corresponding to the highest charge transfer resistance of 4678.4 Ω·cm². However, the SCU-0.3 coating showed the smallest arc radius, with a charge transfer resistance of 4251.5 Ω·cm². The results also confirmed that the SCU-0.15 coating exhibited the best corrosion resistance, while the SCU-0.3 coating showed the poorest corrosion resistance⁴⁰.

Fig. 14.

Electrochemical impedance spectra of different coatings in 15 wt% HNO₃ solution.

Table 5.

Equivalent circuit fitting parameters of coatings in 15 wt% HNO₃ solution.

Sample	R s (Ω·cm²)	R ct (Ω·cm2)	CPE-T (S·sⁿ·cm⁻²)	CPE-n	χ² (×10⁻³)
SCU−0	6.24	4381.8	3.52 × 10⁻⁴	0.87	1.9
SCU−0.1	5.87	4531.9	3.28 × 10⁻⁴	0.89	1.6
SCU−0.15	5.65	4678.4	3.05 × 10⁻⁴	0.91	1.5
SCU−0.2	6.43	4316.7	3.76 × 10⁻⁴	0.86	2.0
SCU−0.3	6.71	4251.5	3.94 × 10⁻⁴	0.84	2.2

It should be noted that surface morphology and roughness play a crucial role in determining the electrochemical response of the coatings. In particular, the CPE-n values obtained from EIS fitting are closely related to surface roughness and interfacial heterogeneity. As shown in Table 5, the SCU-0.15 coating exhibits the highest CPE-n value (0.91), indicating behavior closer to an ideal capacitive response. This can be attributed to its relatively smooth and compact surface, as confirmed by SEM observations and the lowest Ra value, which reflects a more homogeneous electrode–electrolyte interface and reduced dispersion of relaxation times. In contrast, the SCU-0.3 coating displays the lowest CPE-n value (0.84), corresponding to a more pronounced deviation from ideal capacitive behavior. This decrease in CPE-n is associated with its higher surface roughness and the presence of pyramid-like protrusions and microdefects, which increase surface heterogeneity and create a non-uniform current distribution at the interface. The increased Ra enlarges the effective electrochemical surface area and introduces localized variations in charge transfer kinetics, thereby lowering both the CPE-n value and the charge transfer resistance.

Conclusions

1. The SCU-0.15 coating exhibited a finer and more compact structure compared to the SCU-0 and SCU-0.1 coatings. Furthermore, with the increase in CuSO₄ concentration, the nodules in the SCU-0.15 coating became smaller and more uniform in size. Furthermore, distinct diffraction peaks corresponding to Cu appeared in the SCU-0.15, SCU-0.2, and SCU-0.3 coatings when the CuSO₄ concentrations reached 0.15, 0.2, and 0.3 g/L. The Cu contents in these coatings were 1.28, 3.82, and 7.43 wt%, respectively.

2. The SCU-0.15 and SCU-0.3 coatings exhibited compact and refined cross-sectional morphologies, with coating thicknesses of approximately 68.7 μm and 68.2 μm, respectively. When an appropriate amount of CuSO₄ (0.15 g/L) was added to the plating bath, the coating structure became finer and denser, with the average diameter of the Ni/Cu solid solution measuring approximately 34.6 nm. However, the coating displayed a coarser structure as the CuSO₄ concentration was raised to 0.3 g/L, and the average size of the Ni/Cu solid solution increased to approximately 63.9 nm.

3. The SCU-0, SCU-0.1, and SCU-0.15 coatings displayed only a broad, bun-shaped nickel diffraction peak at 2θ = 45°, indicating that these three coatings possessed amorphous Ni-P structures. When the CuSO₄ concentration reached 0.2 and 0.3 g/L, characteristic diffraction peaks of the Cu element appeared at 2θ = 43.4° and 50.3°, corresponding to the planar directions (111) and (200) of Cu₀.₈₁Ni₀.₁₉, respectively, indicating the formation of numerous Ni/Cu solid solutions in the SCU-0.2 and SCU-0.3 coatings.

4. The SCU-0.15 coating exhibited the lowest wear loss among all samples, whereas the SCU-0 coating showed the highest wear loss. With increasing CuSO₄ concentration, the saturation magnetization decreased from 443 to 284 emu/cc due to the dilution of the ferromagnetic Ni phase by non-magnetic Cu. Electrochemical measurements demonstrated that the SCU-0.15 coating possessed the best corrosion resistance, as evidenced by the highest charge transfer resistance (R_ct = 4678.4 Ω·cm²) obtained from EIS analysis. Tafel polarization results further confirmed this trend, with the SCU-0.15 coating exhibiting the highest corrosion potential (E_corr = − 0.002 V), the highest polarization resistance (R_p = 1190.3 Ω·cm²), and the lowest corrosion current density (J_corr = 30.79 µA·cm⁻²) among all coatings. The anodic and cathodic Tafel slopes showed only minor variations for different samples, indicating that Cu incorporation mainly influenced the coating microstructure and surface compactness rather than fundamentally altering the corrosion reaction kinetics.

Author contributions

Qiang Li, Huaxing Li and Qiang Zhang wrote the main manuscript text, Zhenghai Han and Jun Hu prepared Figs. 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12 and 13. All authors reviewed the manuscript.

Data availability

Data are contained within the article.

Declarations

Competing interests

The authors declare no competing interests.