Electrodeposited Composite Coatings Based on Ni Matrix Filled with Solid Lubricants: Impact of Processing Parameters on Tribological Properties and Scratch Resistance

Abstract

Electrodeposited composite coatings are widely studied for their potential to improve surface properties such as wear resistance and friction reduction. This study investigates the effect of electrodeposition parameters on the structure, morphology, and tribological performance of three coatings: pure nickel (Ni), Ni–graphite (Ni-G), and Ni–MoS₂ (Ni-MoS₂). Three deposition conditions were selected based on a review of key electrochemical parameters commonly used in the literature. The coatings were analyzed in terms of morphological characteristics, friction and wear resistance. The findings reveal that higher current densities led to increased friction and wear in Ni coatings, while lower pH values promoted finer crystallite sizes and improved tribological behavior. Ni-G coatings exhibited larger cluster formations with reduced friction and wear, especially at low pH, whereas Ni-MoS₂ coatings developed a stable cauliflower-like morphology at pH 2, but showed reduced adhesion and structural integrity at higher pH levels. Scratch resistance tests performed under optimal deposition conditions showed that Ni-G coatings provided the highest resistance to mechanical damage, while Ni-MoS₂ coatings were more susceptible to microcracking and adhesion failure. These results underscore the importance of optimizing deposition parameters to tailor the microstructure and functional properties of composite coatings for enhanced tribological and mechanical performance.

1. Introduction

The pursuit of advanced materials with exceptional tribological properties is paramount for improving the durability and efficiency of components in aerospace, automotive, and industrial applications, such as engine components and industrial tooling. Electrodeposited nickel (Ni)-based composite coatings, enhanced with solid lubricants like graphite and molybdenum disulfide (MoS₂), are prized for their ability to minimize friction and wear, thereby promoting energy efficiency and environmental sustainability [1,2]. As a cost-effective and precise deposition technique, electrodeposition has matured significantly, with nickel’s hardness, corrosion resistance, and versatility making it an ideal matrix for such coatings. However, despite these advancements, critical challenges persist in tailoring these coatings to meet the rigorous demands of high-performance applications, revealing gaps that necessitate further investigation.

The morphology, tribological performance, and mechanical integrity of Ni-based composite coatings are profoundly influenced by deposition parameters such as pH, current density, stirring rate, temperature, and particle concentration. Prior studies have shown that pulse current density refines the microstructure in Ni–SiC coatings [3], while a pH of 5 optimizes particle embedding in similar systems [4]. Current density plays a crucial role in the codeposition process, with higher current densities often leading to alterations in particle concentration within the coating [4,5]. For instance, Mahrana et al. demonstrated that higher current densities result in increased aluminum particle concentrations during the electrodeposition of Ni–Al, thereby influencing grain size [6], whereas Adabi et al. found that increasing current density decreases Al particle content in composite coatings while enhancing adhesion strength [7]. Temperature also significantly affects deposition rates, with studies identifying 50 °C as an optimal temperature for balancing energy efficiency and coating quality in many cases [8,9]. Recent work on Ni–graphene nanoplatelet (Ni-GnP) coatings further illustrates that low stirring rates produce rougher coatings with low friction coefficients (~0.15) due to exposed graphene particles, whereas high stirring rates yield smoother but less lubricious surfaces [10]. In Ni–MoS₂ coatings, temperature and MoS₂ particle concentration significantly influence the weight percentage of MoS₂ within the coatings, with friction coefficients exhibiting a notable reduction as particle incorporation increases and particle size decreases [11,12,13,14,15]. Similarly, for Ni–graphite coatings, numerous studies have investigated the impact of graphite concentration on tribological behavior, showing that higher concentrations often reduce friction and wear, while also examining the effects of pH, current density, and bath temperature on coating thickness [16,17,18]. Despite these findings, the synergistic interactions between pH and current density, and their combined effects on coating structure and performance, remain poorly understood, restricting the ability to design coatings with optimized tribological properties tailored to specific lubricants like graphite and MoS₂.

Beyond deposition parameters, the comparative performance of graphite and MoS₂ under identical, optimized electrodeposition conditions is underexplored, leaving a critical gap in knowledge regarding the application specific coatings. Several operating conditions have been demonstrated to be effective in generating high-quality nickel coatings containing graphite or MoS₂, leading to favorable tribological properties. However, determining the most suitable conditions for achieving reduced friction coefficients and wear remains unclear, necessitating a comparative analysis that correlates coating morphologies with tribological characteristics.

To date, scratch resistance—a key indicator of durability in abrasive environments—remains largely unexplored for lubricant-enhanced Ni coatings, despite its importance for ensuring mechanical integrity under high-contact conditions. Beyond friction and wear resistance, scratch resistance is a critical factor influencing the mechanical durability of composite coatings, particularly in high-contact and abrasive environments such as automotive and aerospace applications. Research has shown that incorporating particles like nano-alumina or nano-NiTi can enhance scratch resistance, with improvements attributed to factors such as microstructure compaction, surfactant effects, and variations in scratch speed, loading rate, and stylus type [19,20,21,22,23]. For example, electrophoretic deposition (EPD) studies on graphene nanoplatelet (GNP) coatings on copper reveal that optimal scratch resistance is achieved at intermediate deposition conditions (30 V, 2 min), where the coating’s adhesion is maximized, though higher voltages or longer times lead to brittle failure due to hydrogen embrittlement [24]. Additionally, while these EPD studies report significant wear resistance improvements (~88.67% over uncoated substrates) alongside enhanced scratch resistance, the mechanical durability of electrodeposited Ni–graphite and Ni–MoS₂ coatings under similar abrasive conditions remains poorly understood, highlighting a critical research gap [24]. These unresolved challenges represent significant barriers to achieving coatings that simultaneously deliver low friction, high wear resistance, and robust mechanical durability—properties essential for demanding applications. This study addresses these gaps by systematically examining the effects of pH and current density on the structure, morphology, and tribological properties of Ni, Ni–graphite, and Ni–MoS₂ coatings electrodeposited at 50 °C. Through cyclic voltammetry and potentiostatic experiments, we identify optimal deposition conditions that minimize friction and wear, correlating these outcomes with coating structure. By further evaluating scratch resistance, this work provides novel insights into the durability of lubricant-enhanced coatings, advancing their suitability for high-performance applications.

2. Materials and Methods

In this study, the substrate, with dimensions of 30 × 20 × 15 mm³, was machined from an S235 mild steel bar (provided by PROSID-SUD, Sfax, Tunisia). The research involved the preparation of three types of coatings: pure nickel coating, referred to as Ni throughout the manuscript; a nickel graphite composite, denoted as Ni-G; and a nickel MoS₂ composite, denoted as Ni-MoS₂.

These coatings were obtained by electrodeposition from a Watts nickel bath, consisting of various components. The primary source of nickel was nickel sulfate NiSO₄.6H₂O, acquired from SUVCHEM, with a purity of 98%. To prevent anode passivity and improve the bath conductivity, nickel chloride NiCl₂.6H₂O (97% purity, supplied by Loba Chemie, Mumbai, India) was incorporated. Moreover, boric acid H₃BO₃ (99% purity, supplied by SUVCHEM, Mumbai, India) was used as the pH buffer. Prior to each electroplating process, the pH was adjusted using sulfuric acid (H₂SO₄) or sodium hydroxide (NaOH) to a predetermined value.

To obtain the Ni composites coating, 1 g/L of G (with an average size of 7–11 µm) or MoS₂ (with an average below 2 µm) particles were introduced in the Watts nickel bath along with 20 mg of Sodium Dodecyl Sulfate (SDS), which served as a surfactant. No surfactant was used in the pure Ni solution because it serves as a baseline for comparison with the composite coatings in our study. Since the Ni solution contains no particles, a surfactant was unnecessary, enabling us to isolate the effects of pH and current density on the Ni matrix’s intrinsic properties, such as microstructure and mechanical behavior. The temperature was fixed as 50 °C as required. The baths were magnetically stirred for 2 h to ensure homogenization. Prior to each electrodeposition, the electrolyte solutions were sonicated for 30 min using a Lab Companion Ultrasonic Cleaner UC-10 (Jeio Tech, Korea), operating at a frequency of 40 kHz and a tank capacity of 2 L, to achieve uniform dispersion and prevent particle agglomeration.

Table 1. Components quantities in the electrolytic bath for elaborated coatings.

	Components Quantities
Coatings	NiSO4.6H2O (g/L)	NiCl2.6H2O (g/L)	H3BO3 (g/L)	Graphite (g/L)	MoS2 (g/L)	SDS (mg/L)
Ni	270	50	35	-	-	-
Ni-G	270	50	35	1	-	20
Ni-MoS2	270	50	35	-	1	20

provides a summary of the quantities of the components used in the electrolytes.

Two pH levels (2.0 and 3.8) were selected based on reports in the literature demonstrating their effectiveness in promoting uniform and adherent coatings [9,25,26]. Following this selection, electrochemical studies involving cyclic voltammetry and potentiostatic experiments were conducted to determine suitable current density values. These experiments were carried out using a Solartron 1287 apparatus (Solartron Analytical, Farnborough, United Kingdom) with a three-electrode cell setup: a saturated calomel electrode (SCE) as the reference, a glassy carbon disk in a Teflon holder as the working electrode, and a carbon bar as the counter electrode. Glassy carbon was chosen for its resistance to dissolution and ease of polishing. The electrochemical data obtained under the selected pH conditions enabled the identification of three suitable operating conditions for electrodeposition. Coatings were subsequently fabricated on steel substrates using galvanostatic methods. Prior to initiating the electroplating process, a thorough pretreatment of the substrate’s active surface was conducted to ensure the optimal adhesion and uniformity of the coating. This pretreatment included successive mechanical polishing using abrasive papers (grit P180 followed by P600), followed by rinsing with distilled water. The samples were then degreased in an alkaline solution containing 51 g/L NaOH and 49 g/L Na₂CO₃ at 80 °C for 2 min, and rinsed again. An anodic pickling step was subsequently performed in a 20% sulfuric acid solution under a current density of 48 mA/dm² for 1 min to remove surface oxides and activate the surface. A final rinse with distilled water completed the preparation. A flowchart summarizing this pretreatment sequence is presented in Figure 1.

This study employed four characterization techniques. First, X-ray diffraction (XRD) is used to assess the structure and phase compositions of the composite coatings. X-ray diffraction (XRD) analyses were performed on a Siemens D5000 diffractometer (Siemens/ Bruker SAS, Karlsruhe, Germany) using filtered CuKα (λ = 0.15406 nm) radiation for 2θ angles between 30°and 40°. The diffractometer was operated at 40 kV, at room temperature, with a scanning rate of 10°/min Second, a scanning electron microscope (SEM) combined with an Energy-dispersive X-ray Spectroscopy (EDS) technique (PhilipsXL30 ESEM, Philips Electron Optics, Eindhoven, Netherlands, 20KV) was employed to scrutinize the morphologies of the deposits and evaluate the particle percentage within the coatings. Tribological tests were conducted using a reciprocating tribometer developed in our laboratory (Figure 2) to evaluate the friction coefficient and wear volume of Ni, Ni–G, and Ni–MoS₂ coatings. A normal load of 6 N, a tangential motion amplitude of 7.5 mm, a reciprocating frequency of 1 Hz, and a total of 2000 cycles were selected based on the ASTM G133 standard for linearly reciprocating ball-on-flat sliding wear, which is widely used for evaluating thin-film coatings [27]. These parameters fall within the standard’s recommended ranges (e.g., loads of 5–25 N, stroke lengths of 5–25 mm, frequencies of 1–5 Hz, and 1000–10,000 cycles) and were chosen to simulate the cyclic sliding conditions encountered in applications such as sliding seals in pumps, linear actuators in industrial automation, and automotive bearing surfaces, where low friction and wear resistance are critical. The moderate load and low frequency minimize thermal effects and substrate deformation, while the selected stroke length and cycle count ensure the establishment of steady-state friction and measurable wear volumes. This test configuration provides a reliable basis for comparing coating performance under conditions representative of real-world tribological systems.

Wear tests were performed using a ball-on-plan contact sliding test at room temperature (25 °C) and 55.5% relative humidity. Each test was repeated at least three times to ensure result reliability. The counter ball, with a diameter of 15 mm and a surface roughness of Ra 0.02, was made of high chromium steel (100Cr6). Before each friction test, both the substrate and the ball were thoroughly rinsed with ethanol. The setup facilitated the application of a constant normal load, enabling contact between the steel ball and the coating surface. Subsequently, the coated specimen experienced tangential cyclic motion. A load cell positioned between the coated specimen and its holder permitted the measurement of the tangential force with data continuously recorded using a data acquisition system. At the end of the test, the wear volume was evaluated via wear profile tracing using a tactile profilometer (SJ-210 Hand-held Roughness Tester).

Scratch tests were conducted using a CSM Instruments scratch tester to evaluate the mechanical integrity of the coatings. The device was equipped with a Rockwell HRC indenter featuring a 120° cone angle and a 100 µm tip radius. During the test, a progressively increasing normal load, ranging from 0.3 to 25 N, was applied at a loading rate of 10 N/min over a 5 mm track length. To assess failure mechanisms, the scratch tester was integrated with an optical microscope, allowing the identification of the critical loads corresponding to adhesive and/or cohesive failures. To ensure the reliability of the results, three tests were performed for each coating.

3. Results

3.1. Determination of Current Densities

Cyclic voltammetry was conducted to determine the optimal operating conditions for obtaining nickel and nickel composite coatings. This technique makes it possible to identify the electrochemical reactions occurring at the interfaces and pinpoints the potential for nickel and nickel composite deposits. In this study, the temperature was set at 50 °C, based on prior research optimization, and two pH levels (2 and 3.8) were chosen in accordance with the literature [9,28]. The voltammetric curves were acquired using a sweep rate of 4 mV/s within the potential range of 0 to −1.2 V during the forward sweep, and from −1.2 to 1.2 V in the back scan.

Figure 3 and Figure 4 show three voltammograms illustrating the response of a glassy carbon substrate at pH = 3.8 and pH = 2, respectively, immersed in three distinct electrolytes: pure Ni Watts electrolyte, or Watts baths loaded either with 1 g/L of MoS₂ or graphite particles (Table 1).

The voltammograms suggest that the primary mechanism of the cathodic reaction is charge transfer. Moreover, they reveal three distinct anodic peaks, suggesting the oxidation of various nickel-based compounds. These peaks are probably associated with the oxidation of nickel, nickel hydroxide (Ni(OH)₂), and chlorine (Cl₂), as elaborated by Mimani et al. [28].

The voltammograms reveal that both types of particles enhance and accelerate the reduction of Ni²⁺ ions, leading to higher anodic and cathodic currents. For example, the presence of MoS₂ particles in the nickel bath induces a quantity of electricity approximately 50% greater, as detailed in

Table 2. Anodic and cathodic current quantities and Watts bath efficiency from the composite Watts bath loaded with 1 g/L of MoS₂ and from the Watts bath composite loaded with 1 g/L of graphite at pH = 2 or 3.8 and T = 50 °C.

	pH 2			pH 3.8
Coatings	Anodic Current Quantity (Qa)	Cathodic Current Quantity (Qc)	Efficiency (%)	Anodic Current Quantity (Qa)	Cathodic Current Quantity (Qc)	Efficiency (%)
Ni	7.3	−7.8	93	7.63	−8.17	93
Ni-G	10.1	−11	91	5.23	−6,15	85
Ni-MoS2	15.7	−18	87	6.08	−7.47	81

.

The small sulfide particle dimensions (below 2 µm) in contrast to graphite (7–11 µm) favor their suspension in solution. Despite the potentially lower conductivity of MoS₂-containing solutions, the ease of transporting MoS₂ particles to the cathode surface is facilitated by their lighter nature.

The introduction of particles resulted in a decrease in the cathodic efficiency of the deposits. This decline was notably more pronounced when MoS₂ particles were added, suggesting that at a concentration of 1 g/L, MoS₂ particles were more effectively integrated into the nickel matrix.

A potentiostatic study at −1.2 V, chosen as the potential reduction for the three types of coatings, was conducted to determine the current density of deposition. Figure 5 illustrates the impact of pH variations on the potentiostatic curves conducted at −1.2 V on a steel electrode immersed in pure nickel electrolytes at T = 50 °C, with pH levels differing from 3.8 to 2. Despite the variation in the pH, these curves exhibited similar patterns. Initially, there was a rapid decline in the current density owing to the charging and discharging of the double layer [29]. Subsequently, the current density was stabilized at a constant value, denoted as i₀. The shift in pH from 3.8 to 2 led to an increase in the stabilization current i₀, transitioning from 25 mA/cm² to 48 mA/cm².

The incorporation of graphite or MoS₂ particles into the Watts bath at pH = 3.8 resulted in a notable increase in the current density (i₀) from 25 mA/cm² for the pure nickel coating to approximately 66 mA/cm² for the composite coatings, as shown in Figure 6. This finding is consistent with the cyclic voltammetry observations. Specifically, the introduction of a surfactant (SDS) to the particles enhanced their adsorption on the cathode surface by amplifying their positive charges. Consequently, these particles with their semiconductor properties function as miniature cathodes, contributing to an overall increase in the current.

The next part of the study presents a comparative analysis of the structural, morphological, and tribological characteristics of Ni and Ni composite coatings obtained under three operating conditions (C1, C2, and C3), as summarized in

Table 3. Selected electrodeposition conditions for coating elaboration.

	pH	Current Density (mA/cm2)
C1	3.8	25
C2	3.8	66
C3	2	48

. The pH values were selected based on a literature survey, while the current densities were derived from the previous potentiostatic curves.

3.2. Structural Characterizations

Figure 7 illustrates the XRD patterns of the pure Ni coatings under three distinct operating conditions, revealing a consistent face-centered cubic (FCC) structure. Notably, a pronounced intensity peak emerges at (200), which is particularly evident in the pure nickel coating obtained under C1 conditions, denoting a slip plane with a diminished deformation capacity [30]. An analysis of the average crystallite size of pure nickel was performed using the Debye Scherrer equation,

D = 0.9λ/(FWHM cos θ),

(1)

where D is the average crystallite size, λ is the X-ray wavelength, FWHM is the full-width half maximum at the 2θ peak (measured in radians), and θ is the Bragg angle of the peak.

The broadening of the full width at half maximum (FWHM) signifies a refinement in the crystallite size. This refinement progressed from 70 nm under C2 conditions to 65 nm under C1 conditions, before further diminishing to 60 nm under C3 conditions. Conversely, the XRD patterns of the composite coatings under C3 conditions exhibit a distinct preferential peak (111), indicative of three slip planes, facilitating easier deformation (Figure 8). This peak, which is prominent in the randomly oriented nickel XRD pattern, characterizes the composite coatings as being more randomly grained. Moreover, for FCC metals such as nickel, the (111) peak boasts the lowest surface energy among the various textures, thereby favoring surface energy minimization [31,32]. Analysis of the nickel crystallite size using the Debye–Scherrer equation revealed a decrease to 40 nm for Ni-G and 20 nm for Ni-MoS₂ composite coatings. This reduction is directly linked to the smaller size of MoS₂ particles. Notably, the smaller particles played an important role in reducing the size of the Ni crystals, consequently leading to a substantial enhancement in the hardness of the composite coatings. This enhancement was primarily driven by the strengthening mechanisms of grain refinement and dispersion owing to the reduced particle size [33].

3.3. Morphological Characterizations

An examination of the morphologies of the pure nickel coatings formed under diverse operating conditions (Figure 9) revealed that the pyramidal morphology of Ni was evident across all three types of coatings. Notably, increasing the current density at a constant pH led to a larger pyramidal morphology and the appearance of significant bubbles, which were likely attributed to hydrogen release. This enlarged pyramidal morphology appears to be linked to the larger crystallite size of nickel, consistent with the findings of Rasmussen et al., suggesting that the augmentation of current density is linked to an increase in grain size [34]. In fact, elevating the current density appears to correlate with a rise in residual stress within the deposited material [34].

In contrast, a reduction in pH (C3) resulted in a finer pyramidal morphology, likely attributed to the more refined crystallite size of nickel. The pH of the electrodeposition bath significantly influenced the crystallite size, possibly owing to differences in the nucleation modes. Competition between growth and nucleation is a well-known factor that determines the granularity of the deposit. At a lower pH value, a decrease in the intensity and a broadening of the peak was observed, suggesting that the crystallinity and crystallite size in the deposits decreased with a reduction in the pH of the solution owing to high polarization [35].

Through electrochemical measurements, Boubatra et al. demonstrated that a higher pH reduces the activation and diffusion polarization, thus reducing the total overpotential (η) during film electrodeposition. Hence, a higher pH augments the crystallite size of the deposit by decreasing the nucleation rate [36]. The higher the nucleation rate during deposition, the finer the crystal grains of the deposits are [36].

Figure 10 shows the morphologies, carbon EDS mappings and percentages of C obtained under the three operating conditions. The Ni-G coating obtained under the C1 condition presents small clusters, accredited by the addition of graphite particles to the nickel matrix. The size of these clusters increased with a higher current density (C2) or lower pH (C3).

Figure 10d–f present the EDS mappings and carbon quantities within these Ni-G layers. The figures illustrate an approximately homogeneous distribution of the graphite particles. In addition, the carbon content in the coating obtained under the C3 operating conditions was considerably higher than those under the other operating conditions.

The Ni-MoS₂ morphology exhibited stability across distinct operating conditions, displaying porous deposits with a cauliflower structure composed of very small crystallites (Figure 11a–c). These cauliflower formations became more numerous and smaller with an increase in the current density (C2) or decrease in the bath pH (C3). In Figure 11d–f, the EDS map of Ni-MoS₂ coatings reveals the molybdenum (Mo) distribution, demonstrating that an elevated current density corresponds to an increased Mo content. Nonetheless, the particle distribution remained uniform under the three operating conditions. Remarkably, only coatings synthesized at pH = 2 displayed cohesion; at pH = 3.8, even a minor impact such as a nail stroke leads to considerable detachment, which is likely ascribable to the internal stress introduced during electrodeposition, which agrees well with the findings of Güler et al. [9]. Indeed, the latter has formerly confirmed that internal stress reaches a minimum at pH = 2, a current density of 48 mA/cm², and a temperature of 50 °C during Ni-MoS₂ electrodeposition from a Watts bath [9].

3.4. Tribological Characterizations

Table 4. Wear volumes and friction coefficients of pure Ni, Ni-G and Ni-MoS₂ coatings under different operating conditions (C1, C2 and C3).

	Ni			Ni-G			Ni-MoS2
	C1	C2	C3	C1	C2	C3	C1	C2	C3
Wear Volumes (mm3)	0.171 ± 0.017	0.213 ± 0.060	0.158 ± 0.007	0.16 ± 0.02	0.210 ± 0.003	0.120 ± 0.007	Not Available	Not Available	0.187 ± 0.012
Friction Coefficients	0.94 ± 0.02	0.970 ± 0.031	0.840 ± 0.013	0.77 ± 0.02	0.790 ± 0.014	0.710 ± 0.021	Not Available	Not Available	0.78 ± 0.04

presents a summary of the stabilized friction coefficients and wear volumes after 2000 sliding cycles for various coatings elaborated under three distinct operating conditions with an average thickness of 50 µm. Notably, the pure Ni coatings demonstrated improved tribological properties under C3 conditions, attributed to a finer grain size that contributed to a harder coating with elevated grain boundaries, thereby enhancing wear resistance. This enhancement is evidenced by an increase in both the friction coefficient and wear volume with a higher current density, as larger grain sizes induced a shift in the deformation mechanism from intergranular slippage to dislocation slip. This phenomenon is supported by Mishra et al. [37], who reported that decreasing the grain size of nanocrystalline nickel leads to a decrease in the friction coefficient and enhanced wear resistance, with a five-fold increase in hardness associated with a two-fold increase in wear resistance.

For Ni–graphite coatings, the suppression of graphite particle clusters leads to the formation of a transfer film primarily composed of graphite, mitigating abrasive wear across all coatings. Larger cluster sizes result in the quicker formation of this transfer film, leading to a reduced friction coefficient and wear volume. However, coatings under C3 conditions exhibited enhanced tribological features compared to those under C1 and C2 conditions.

Nevertheless, for Ni-MoS₂ coatings, because only coatings synthesized under C3 conditions displayed cohesion, tribological characterization was exclusively carried out on the coatings obtained under C3 operating conditions. Although a reduction in the friction coefficient was observed, the coatings still demonstrated limited tribological performance, as evidenced by the high wear volume.

Overall, the most effective operational parameters for achieving coatings with favorable tribological properties are associated with the C3 conditions. These conditions, characterized by a current density of 48 mA/cm², pH level of 2, and temperature of 50 °C, proved to be optimal for all three types of coatings, Ni, Ni-G, and Ni-MoS₂.

The comparison of friction coefficient values for the three types of coatings deposited under optimal C3 operating conditions clearly demonstrates that incorporating solid lubricant particles such as MoS₂ or graphite into nickel matrices effectively reduces friction. Among the coatings, Ni–G exhibits the lowest friction coefficient (0.710 ± 0.021), attributed to the formation of a tribofilm predominantly composed of graphite particles. In contrast, the Ni–MoS₂ coating showed a higher friction coefficient (0.78 ± 0.04), likely due to the partial oxidation of MoS₂ into MoO₃, which disrupted its layered structure and diminished its lubricating ability [38,39]. These findings are consistent with those from previous work by Sangeetha et al., who reported friction coefficients of 0.5–0.7 for Ni–W/BN coatings under dry sliding conditions. While these values are lower than those observed for pure Ni coatings (0.840 ± 0.013), they align well with the friction behavior of both Ni–G and Ni–MoS₂ coatings, suggesting that BN particles, like graphite and MoS₂, contribute to friction reduction [40].

Regarding wear performance, the Ni–G coating showed a significant reduction in wear volume (0.120 ± 0.007 mm³) compared to pure Ni (0.158 ± 0.007 mm³). This improvement is attributed to the formation of a soft debris layer composed of graphite particles, which covered the wear track, reduced direct metal-to-metal contact, and minimized frictional energy dissipation. These results align with findings by García-Lecina et al., who demonstrated enhanced wear resistance through the incorporation of SiC particles into nickel matrices [41]. Conversely, the Ni–MoS₂ coating exhibited a larger wear volume (0.187 ± 0.012 mm³), indicating that under the tested conditions, MoS₂ addition did not sufficiently improve wear resistance, likely due to oxidative degradation and mechanical instability.

Figure 12 presents SEM micrographs of the wear tracks (a–c) and optical micrographs of the corresponding counterbody worn surfaces (d–f) for Ni, Ni–G, and Ni–MoS₂ coatings after 2000 sliding cycles under optimal C3 conditions. A detailed analysis of the wear morphology reveals distinct mechanisms for each coating, with significant differences in severity and evolution throughout the test’s duration. The pure Ni coating (Figure 12a) displayed a pronounced wear track characterized by deep plastic deformation zones, extensive adhesive spallation covering (average size ~20 µm, based on SEM scale), and severe delamination. The corresponding counterbody surface (Figure 12d) exhibited prominent abrasive grooves, indicating a combination of adhesive wear, abrasive wear, and surface fatigue. Similar wear behavior has been reported by Farokhzadeh et al. for titanium alloys under high-load conditions, although they noted even greater material loss than was observed here [42].

Upon incorporation of graphite particles (Ni–G coating, Figure 12b), the wear track morphology became significantly smoother, with fewer and smaller adhesive craters (average size ~10 µm, approximately 50% smaller than in pure Ni), indicating improved surface protection. The corresponding counterbody surface (Figure 12e) still showed residual abrasive grooves, suggesting that although adhesive wear was markedly reduced, mild abrasive wear and third-body abrasion due to nickel debris persisted. The formation of a graphite-rich tribofilm, covering approximately 40% of the wear track, resulted from the progressive release of graphite particles during sliding. This tribofilm played a crucial role in mitigating adhesive wear. However, its incomplete continuity over the 2000 cycles allowed abrasive debris to remain active, leading to persistent mild third-body wear. Similar behavior has been observed for Ni–CNT coatings, where Srinivasan et al. reported that a CNT-rich tribofilm effectively reduced adhesive wear under comparable conditions [43].

In contrast, the Ni–MoS₂ coating (Figure 12c) presented a significantly wider and deeper wear track (~40% larger than Ni), marked by severe plowing (plow marks ~100 µm wide), the presence of adhesive craters, numerous microcracks, and evidence of oxidative wear associated with the transformation of MoS₂ into MoO₃. The corresponding counterbody surface (Figure 12f) revealed a heavily damaged area approximately 1 mm in diameter, confirming the dominance of severe adhesive, abrasive, and oxidative wear mechanisms. These effects were exacerbated by local frictional heating, which promoted MoS₂ oxidation. The emergence of microcracks further indicated the occurrence of fatigue wear under cyclic loading, contributing to extensive material removal during the sliding cycles. Initially, adhesive wear dominated due to weak cohesion between sliding surfaces, followed by oxidative wear as MoS₂ degraded, generating abrasive MoO₃ particles that intensified material loss. As the test progressed, fatigue-induced microcracking led to further surface degradation. Similar wear evolution, though without significant oxidative effects, has been documented by Farokhzadeh et al. in titanium alloys subjected to severe sliding conditions [42].

Collectively, these findings highlight that pure Ni coatings predominantly experience adhesive, abrasive, and fatigue wear mechanisms, whereas the addition of graphite in Ni–G coatings effectively reduces adhesive wear, but allows mild abrasive and third-body wear to persist. In contrast, the Ni–MoS₂ coatings are subjected to a complex combination of severe adhesive, abrasive, oxidative, and fatigue wear. Comparisons with previous literature reveal both alignments and contrasts: the protective tribofilm formation observed in Ni–G coatings closely mirrors the behavior reported for Ni–CNT composites [43], while the relatively high wear volume recorded for Ni–MoS₂ coatings contrasts with the improved performance seen in WS₂/Ni systems, highlighting the oxidative instability of MoS₂ under the tested conditions [44]. These observations emphasize the critical importance of optimizing the stability and dispersion of solid lubricants within the nickel matrix to ensure continuous tribofilm formation and to limit oxidative degradation, as successfully demonstrated in coatings reinforced with WS₂ and CNT particles [43,44].

3.5. Scratch Resistance at Optimal Conditions

To emphasize the impact of particle incorporation on the adhesion and mechanical integrity of Ni-based coatings, progressive load scratch tests were performed under C3 conditions (48 mA/cm², pH 2, 50 °C).

Figure 13 illustrates the evolution of penetration depth and friction coefficient as a function of progressive normal load for Ni, Ni-MoS₂ and Ni-G coatings. When examining penetration depth under increasing normal load, the Ni coating exhibited the most uniform and stable response, whereas composite coatings, particularly those enriched with MoS₂ particles, showed increasing irregularities. This behavior resulted from localized plastic deformation and ridge formation in front of the indenter, which obstructed its movement and introduced fluctuations in penetration depth. The Ni-MoS₂ coating, characterized by a rougher surface texture, exhibited the most pronounced ridges and enhanced plastic flow, highlighting its reduced resistance to indentation. Additionally, composite coatings demonstrated greater penetration depths compared to pure Ni, primarily due to the non-homogeneous distribution of solid lubricant particles, which generated stress concentration zones and facilitated localized deformation. The MoS₂-rich coating, in particular, underwent rapid plastic deformation, further diminishing its ability to resist indentation. A similar pattern was observed in the friction coefficient, which increased with both the incorporation of particles and the progressive rise in normal load during the scratch test. The correlation between friction and penetration depth confirms that ridge formation and plastic flow significantly influenced the coatings’ tribological response. In progressive scratch mode, MoS₂-containing coatings experienced significant plastic deformation, forming ridges ahead of the indenter that altered the contact area and intensified resistance to movement. This interaction manifested as undulations in the friction coefficient curve, reflecting the unstable nature of the interface. More pronounced ridges resulted in greater resistance, leading to higher and more fluctuating friction values.

The critical load (LC) is a key parameter for assessing the adhesion strength of a coating to a substrate during a scratch test. As the scratch progresses under an increasing load, multiple coating failure events may occur. The critical load (LC), defined as the normal load at failure, is essential for characterizing the coating–substrate adhesion. As the Rockwell indenter moves across the surface, coating failure is typically detected through acoustic emissions, optical microscopy, changes in penetration depth, or variations in tangential frictional force between the tip and the sample. Friction coefficient and penetration depth variations were disregarded as failure indicators due to their irregularity and inconsistent correlation with coating detachment. Therefore, the critical load was determined using optical microscopy and acoustic emission. The failure process occurred in three stages: LC1 (crack initiation), where microcracks first appear; LC2 (crack propagation), where cracks extend along the scratch path; and LC3 (complete coating failure), where the coating fully detaches from the substrate. Figure 14 and Figure 15 provide a detailed analysis of these critical loads and their impact on the mechanical integrity of Ni, Ni-MoS₂, and Ni-G coatings. Figure 14, which presents optical micrographs of the scratch tracks, highlights the evolution of damage across these stages. The Ni coating demonstrates strong adhesion to the substrate, as indicated by a smooth scratch track with small subsurface microcracks at LC1, minor localized cracking at LC2, and no LC3 failure. In contrast, the Ni-MoS₂ coating exhibits early plastic deformation and weak interfacial bonding, characterized by surface roughness and ridge formation at LC1, significant crack propagation at LC2, and complete delamination at LC3, confirming poor adhesion properties. Meanwhile, the Ni-G coating exhibits minimal damage at LC1, limited crack propagation at LC2, and no LC3 failure, suggesting that graphite reinforcement enhances stress distribution and mechanical stability.

Figure 15 quantifies these observations, demonstrating that the Ni-MoS₂ coating consistently exhibited the lowest critical loads, with LC1 ~10 N, LC2 ~15 N, and LC3 ~20 N. These values highlight its poor mechanical resistance and weak adhesion. In contrast, the Ni-G coating significantly outperformed both pure Ni and Ni-MoS₂ coatings, displaying higher critical loads of LC1~15 N and LC2~20 N, and no observed LC3 failure, thereby confirming its superior toughness and resistance to crack propagation. The pure Ni coating, while showing moderate mechanical stability with LC1 ~12 N and LC2 ~18 N, lacked the reinforcement benefits provided by graphite, which enhanced crack resistance and load-bearing capacity.

These results are consistent with findings reported in previous studies. Gajewska-Midziałek et al. observed that the incorporation of boron particles in Ni-B/B composite coatings improved mechanical properties; however, they also noted that particle agglomeration led to local stress intensities, significantly reducing scratch resistance by accelerating crack initiation. A similar phenomenon is evident in the present work for Ni-MoS₂, where the non-uniform particle dispersion undermines coating integrity [45].

Moreover, the behavior of the Ni-G coating aligns closely with the findings of Li et al., who demonstrated that the incorporation and controlled annealing of Ti particles in Ni-P coatings greatly enhanced scratch resistance. Mechanisms such as crack deflection, crack bridging, and crack shielding were identified as critical factors in improving mechanical durability. Their work emphasized that uniform particle distribution and strong matrix–particle bonding are essential to achieving higher critical loads—principles that are clearly reflected in the enhanced performance of the Ni-G composite [46].

Overall, these findings establish a strong correlation between tribological performance and scratch resistance. While MoS₂ is effective in reducing friction, it significantly compromises mechanical durability, leading to accelerated plastic deformation, early crack initiation, and complete delamination. Conversely, the incorporation of graphite enhances both the tribological and mechanical stability of the Ni coating by effectively distributing stress and inhibiting crack propagation. As a result, Ni-G emerges as the most robust and reliable coating under the tested conditions.

4. Conclusions

This study advanced the optimization of electrodeposited Ni-based composite coatings for superior tribological and mechanical performance, meeting the demand for tailored coatings in challenging industrial applications. By employing cyclic voltammetry and potentiostatic experiments, three operating conditions were established (C1—pH 3.8, 25 mA/cm²; C2—pH 3.8, 66 mA/cm²; C3—pH 2, 48 mA/cm²) at 50 °C. Pure Ni, Ni-G, and Ni-MoS₂ coatings were systematically deposited and evaluated under these conditions, revealing the synergistic effects of deposition parameters, enabling a detailed performance comparison and the assessment of durability in abrasive environments.

The study’s key contributions are as follows:

• Optimal deposition conditions enabled the formation of coatings with refined microstructures and enhanced performance. Ni-G coatings exhibited a fine crystallite size (≈40 nm) and superior tribological behavior, with a reduced friction coefficient (0.710 ± 0.021) and lower wear volume compared to pure Ni and Ni-MoS₂ coatings;
• Lubricant-dependent tribological behavior—Under C3, Ni–graphite coatings outperformed both pure Ni and Ni-MoS₂, reducing friction and wear through the formation of a lubricating graphite-rich transfer film. Conversely, Ni-MoS₂ coatings exhibited higher wear due to debris detachment and MoS₂ oxidation to MoO₃, which compromised its lubricating effect;
• Enhanced mechanical durability—Scratch testing under C3 conditions highlighted the Ni-G coatings’ superior adhesion and crack resistance, addressing mechanical durability challenges in abrasive environments. In contrast, Ni-MoS₂ coatings showed stress-induced detachment and premature failure at higher pH levels, while pure Ni displayed moderate stability without the reinforcement benefits of graphite.

Overall, this work establishes a strong link between deposition parameters, microstructural evolution, and functional properties, offering a robust framework for designing advanced nickel-based composite coatings. The Ni-G system, in particular, emerges as a highly promising candidate for aerospace, automotive, and industrial tooling applications where both low friction and mechanical durability are critical.

Future research will focus on evaluating the long-term durability of these coatings under cyclic and dynamic loading conditions, as well as improving Ni-MoS₂ cohesion at higher pH levels through advanced surfactant strategies or hybrid reinforcements. In addition, planned cross-sectional SEM and profilometric analyses will provide deeper insights into subsurface crack propagation and failure mechanisms, complementing the current surface-level assessments.