Synthesis and analysis of Ni-Co-TiN composite coating via ultrasonic vibration-assisted jet electrodeposition

2.1. Preparation

The experimental plating solution system comprised a combination of main salts, conductive salts, functional additives, and a buffer. NiSO₄·6H₂O (240 g L^-1) and CoSO₄·7H₂O (30 g L^-1) served as the primary sources of Ni²⁺ and Co²⁺ ions. NiCl₂·6H₂O (30 g L^-1) functioned as an anode activator, not only improving conductivity through Cl⁻ ions but also facilitating the sustained dissolution of the nickel anode to prevent passivation. Furthermore, C₇H₄NO₃SNa (4 g L^-1) was incorporated as a brightener, which adsorbed onto the cathode surface to regulate grain growth and improve coating smoothness. C₁₂H₂₅SO₄Na (0.1 g L^-1) served as an anionic surfactant, with its hydrophobic group interacting with hydrogen bubbles to effectively lower the liquid-solid interfacial tension, minimizing pinhole defects in the coating. H₃BO₃ (25 g L^-1) functioned as a pH buffer, maintaining the plating solution within a stable pH range of 4 to 5 [15]. All reagents were analytically pure, while the TiN nanoparticles had an average particle size of 50 nm, a purity of more than 99%. The DLS-Zeta potential of TiN nanoparticles in the plating solution was measured using a Malvern particle size meter, and the result was +25.23 mV, indicating that the plating solution was in a steady state.

A 304 stainless steel substrate with dimensions of 50 mm×25 mm×2 mm was used in the experiments. It was mainly composed of 10 wt% Ni, 8 wt% Cr, 2 wt% Mn, 68 wt% Fe, and a small amount of other elements. Before electrodeposition, the surface of the specimen was progressively sanded using 400#, 800#, 1200#, 1600#, and 2000# water-abrasive sandpaper. Following this, the specimen was immersed in a 10 wt% NaOH solution for 15 min to perform alkaline washing and degreasing. The surface was then activated through acid washing with a 15 wt% HCl solution for 30 s. After activation, the specimen was thoroughly rinsed with deionized water to eliminate any remaining dilute hydrochloric acid from the surface. A pure nickel rod (Φ3 mm×60 mm) and the pretreated 304 stainless steel were employed as the anode and cathode, respectively [16]. To make TiN particles more dispersed, the plating solution was subjected to ultrasonic vibration at 20 kHz, 200 W for 30 min before deposition. Ni-Co-TiN composite coatings were then fabricated using the UVAJE method under varying TiN additions (0-6 g L^-1). For comparison, similar coatings were also produced using the TJE technique. The detailed preparation parameters have been presented in Table 1.

2.2. Characterization

The surface morphology and chemical composition of the coatings were examined using a FEI-Prisma-E SEM equipped with an EDS detector and a backscattered electron (BSE) detector. The surface roughness was analyzed using a WI-5000 white light interferometer. Five different regions were tested per sample, and the arithmetic mean roughness (R_a) and root mean square roughness (R_q) were calculated. An image-based method was employed to determine the porosity: SEM surface images were analyzed using ImageJ software. Five non-overlapping regions were selected, and the porosity was statistically determined via threshold segmentation. XRD analysis was performed with an ADVANCE X-ray diffractometer utilizing a Cu Kα radiation source (λ=1.5406 Å) at 5°/min to investigate the crystal structure. Crystallite sizes were calculated using Scherrer’s formula (1) [17].

Where D denotes the crystallite size (nm), K is 0.89, λ is the wavelength (0.15406 nm), θ represents the Bragg angle and β is the full width at half maxima of the diffraction peak (rad).

The microhardness was determined using an HVS-1000 microhardness tester under a 200 g load with a 15 s loading time. Five distinct regions were selected from each sample, with three points tested in each region, and the average value was taken. The scratch test to assess adhesion strength was performed with an RST 100 scratch tester using a 40 N load, a 3 mm scratch length, a 2 mm static load length, a reciprocating frequency equal to 1, and a static pressure duration equaling 30 s. Three scratches were tested per sample, and the average value was adopted for analysis. The compression performance of the coatings was tested using a domestic CMT-4254 universal tensile testing machine. The test specimens were prepared into strip shapes with dimensions of 3 cm×3 mm, then fixed in the clamps, and a gauge length of 20 mm was marked. During the test, a constant tensile rate of 0.5 mm min^-1 was set. Three tests were performed per sample, and the average value was adopted for analysis. GCr15 bearing steel balls (diameter: 6 mm) were used as the counter body. The surface hardness of the steel balls was verified with an HVS-1000 Vickers hardness tester: five test points were selected at different positions of each ball, with a 200 g load applied for 15 s. The measured hardness was 60±1 HRC, conforming to the standard hardness range of GCr15 (58-62 HRC). Friction tests were conducted in reciprocating mode using an MFT-5000 friction and wear tester, under the conditions of 10 N load, 30 mm s^-1 reciprocating speed, and 20 min test duration. Each sample was tested five times. After testing, the wear loss was measured, and the wear scars were scanned by a VK-X2003D laser confocal profilometer to determine their length. The wear rate (W) was calculated using the following formulas (2,3) [18]:

Where l denotes the wear scar length (mm), A represents the average cross-sectional area (mm²), V is the wear volume (mm³), F is the applied load (N), and S is the sliding distance (m).

The corrosion properties were analyzed using a CHI660E electrochemical workstation in a three-electrode configuration, where the coating sample served as the working electrode, a saturated calomel electrode as the reference, and a platinum electrode as the auxiliary. Potentiodynamic polarization tests were conducted in a NaCl solution (3.5 wt%), with a scan range of ±500 mV relative to the open-circuit potential and 1 mV/s scan rate. Electrochemical impedance spectroscopy (EIS) was performed over a frequency range between 10^-2 and 10⁵ Hz to evaluate the interfacial charge transfer behavior of the coatings. The long-term corrosion performance was assessed through a neutral salt spray test using a NaCl solution (5 wt%). Similarly, the aforementioned corrosion resistance tests also employed the average of three trials.

Additionally, at an ultrasonic power of 100 W, the variations of ultrasonic parameters during deposition were measured using a WT3000 high-precision power meter, an SP-100 sound pressure distribution analyzer, and a thermocouple thermometer (repeated 3 times), followed by performance testing. The results indicated that significant differences in coating properties were observed when the ultrasonic parameters varied excessively, and the coating properties were highly sensitive to the ultrasonic parameters. For instance, at a TiN concentration of 4 g L^-1, when the power fluctuation decreased from ±10 W to ±5 W, the microhardness relative standard deviation (RSD) of the coating dropped from 12.5% to 3.1%, and the RSD of corrosion current density decreased from 18.2% to 9.8%. Therefore, to improve experimental reproducibility, the actual ultrasonic power at the workpiece was controlled within 80±5 W, the sound field exhibited a cylindrical focused distribution, and the maximum temperature rise was ≤5°C.

3.1. Surface morphology

A comparison of different electrodeposition techniques and TiN additions revealed a strong correlation between the deposition process and particle dispersion. In the absence of TiN (0 g L^-1), the Ni-Co coating produced by TJE displayed a honeycomb-like microbump surface structure (Figure 1a), whereas the coating synthesized via UVAJE exhibited a much smoother surface with the microbumps effectively eliminated (Figure 1b).

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Figure 1.

SEM images of the different Ni-Co-TiN coatings: (a) 0 g/L, (b) 0 g/L, (c) 2 g/L, (d) 2 g/L, (e) 4 g/L, (f) 4 g/L (g) 6 g/L by TJE; (h) 6 g/L by UVAJE.

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The surface of the Ni-Co-TiN (2 g L^-1) coating fabricated via TJE displayed pronounced nodular or cauliflower-like features (Figure 1c). The coating synthesized via UVAJE displayed a relatively smooth surface with only minor microbumps (Figure 1d). Increasing the TiN addition to 4 g L^-1 resulted in a smoother surface with significantly reduced nodule size (Figure 1e), attributed to the increased availability of nucleation sites provided by well-dispersed TiN nanoparticles, which also helped fill surface voids and refine the structure. The Ni-Co-TiN coatings by UVAJE had a smooth and compact surface with almost no honeycomb protrusions (Figure 1f). However, when the TiN addition was further increased to 6 g L^-1, particle agglomeration occurred, leading to surface roughening and the reappearance of prominent nodular formations (Figure 1g). However, the UVAJE process significantly mitigated this issue, ensuring a uniform surface with no visible nodules (Figure 1h). At the same TiN addition, the surface quality of Ni-Co-TiN coatings fabricated by UVAJE was particularly improved to those produced by TJE. This could be attributed to the cavitation effect, which reduced particle aggregation, and the ultrasonic agitation, which promoted even distribution of particles across the coating surface. Li et al. [19] evaluated the effect of different stirring methods on the preparation of Ni-Co/ZrO₂ coatings by electrodeposition, reporting that ultrasonic stirring could disperse the ZrO₂ nanoparticles more uniformly than mechanical stirring, resulting in a flat and dense coating.

Further analysis showed that the dispersion of TiN particles in the electrolyte played a crucial role in the coating growth mechanism. In the traditional TJE process, the non-uniform distribution of TiN particles, combined with the tip-discharge effect, led to the excessive growth of metal ions in areas where particles agglomerated, resulting in coarse nodules. Liu et al. [20] demonstrated that nanoparticles undergo overgrowth in particle agglomeration regions. However, UVAJE utilized the microturbulence and shock waves generated by ultrasonic cavitation to enhance particle uniformity and refine the grain structure, leading to a significant improvement in the surface smoothness of the coating. It was worth noting that the surface quality of coatings without TiN was improved to those with TiN, regardless of the preparation method. This suggested that while the UVAJE process improved particle dispersion, it did not fully eliminate the inherent physical differences between TiN and the metal substrate [21], which was supported by the data listed in Table S1. At a TiN concentration of 4 g L^-1, the R_a, R_q, and porosity of coatings prepared by TJE and UVAJE were determined to be 545.3±10 nm, 621.34±12 nm, 2.5±0.3% and 392.6±8.1 nm, 432.4±9.2 nm, 1.8 ± 0.3%, respectively. These values were lower than those of other coatings with TiN but higher than those of TiN-free samples without TiN (TJE: 465.3±10.3 nm, 492.3±10.1 nm, 1.5±0.3%; UVAJE: 274.8±6.3 nm, 332.5±7.7 nm, 1.4±0.3%). Furthermore, under the same TiN addition, the R_a, R_q, and porosity of UVAJE-prepared coatings were significantly lower than those of TJE-prepared counterparts.

3.2. Energy-dispersive X-ray spectroscopy (EDS)

The component characteristics of the Ni-Co-TiN (4 g L^-1) composite coating produced by UVAJE were confirmed through EDS analysis (Figure 2), which revealed a uniform distribution of Ni, Ti, Co, O, and N elements across the surface. O mainly came from the oxidation products of Ni and Co and adsorbed oxygen, and its distribution was related to the morphology of Ni and Co, so the uniform distribution of O could indirectly support the uniform distribution of Ni and Co. The co-localization of Ti and N elements indicated the successful embedding of TiN particles into the coating. The BSE image (Figure S1) showed a strong and uniform bond between the coating and the substrate, with no visible defects. EDS analysis of the cross-section (Figure S2) further confirmed the even distribution of TiN particles throughout the coatings, indicating the formation of a stable composite structure with the metal matrix. The thicknesses of different coatings have been shown in Figure S3. When the TiN addition ranged from 0 to 6 g L^-1, the coating thickness was 18.2±1.2-20.4±2 μm under the TJE process and 22.5±1.5-26.5±2.2 μm under the UVAJE process. This difference was attributed to the critical regulatory effect of nanoparticle codeposition on deposition rate and coating thickness: TiN particles in the TJE process were prone to agglomeration, which impeded ion mass transfer and interfered with codeposition efficiency, resulting in a gentle deposition rate and thinner coatings. In contrast, the ultrasonic cavitation effect of UVAJE could inhibit particle agglomeration, strengthen mass transfer, and accelerate codeposition kinetics, leading to a significantly enhanced deposition rate and thicker coatings [22,23].

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Figure 2.

EDS images of Ni-Co-TiN coating (4 g L^-1) by UVAJE: (a) Overall; (b) Ni; (c) Co; (d) N; (e) Ti; (f) O.

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EDS quantitative analysis results have been presented in Table 2. At the same TiN addition level, the Ni/Co atomic ratio of TJE and UVAJE coatings showed a negligible difference. As TiN addition increased from 0 to 6 g L^-1, the Ni/Co ratio of both coatings remained stable within 3.76-3.91. These results indicate that ultrasonic vibration exerted a negligible effect on Ni-Co codeposition kinetics without altering the reduction competition between Ni²⁺ and Co²⁺. The incorporation of TiN nanoparticles also did not disturb the codeposition equilibrium, showing no significant regulatory effect on the Ni/Co ratio.

As listed in Table S2, when the TiN addition was 0-4 g L^-1, the TiN incorporation content in both coatings increased linearly (reaching 2.15±0.02 wt% for TJE and 2.32±0.01 wt% for UVAJE), which was attributed to the inhibition of particle agglomeration and improved bath stability by ultrasonic cavitation. When the TiN addition increased to 6 g L^-1, the growth of incorporation content slowed down (only increasing to 2.28±0.03 wt% for TJE and 2.45±0.02 wt% for UVAJE), as excessive particles tended to undergo secondary agglomeration, making mass transfer to the cathode difficult and prone to detachment. Notably, the coating by UVAJE still maintained a higher TiN incorporation content, further validating that ultrasound can refine agglomerates, enhance mass transfer efficiency, and improve the effective TiN incorporation rate.

3.3. X-ray diffraction (XRD)

The XRD patterns of composite coatings prepared with varying TiN additions and different processes have been presented in Figure 3. All coatings exhibited the characteristic face-centered cubic (FCC) crystal structure. The main diffraction peaks of the coatings corresponded to the (111), (200), and (311) crystal planes, with the (111) plane showing the highest intensity, consistent with the densest plane arrangement in the FCC structure. However, diffraction peaks at the (220) and (222) crystal planes exhibited weaker intensities [24]. With the increase in TiN particle content, the diffraction intensity of the (111) crystal plane exhibited a trend of first decreasing and then increasing. At low TiN content, the particles interfered with the continuous growth of (111) grains in the Ni-Co matrix, disrupting their orientational consistency and leading to a reduced diffraction intensity of this plane. At excessively high TiN content, the (111) plane of TiN underwent epitaxial growth with that of the Ni-Co matrix due to lattice matching; meanwhile, excessive particles restricted the competitive growth of grains with other orientations. Under this combined effect, the diffraction intensity of the (111) plane recovered, which was consistent with the findings of Chakraborti et al. [25]. In the UVAJE system, the ultrasonic cavitation effect facilitated uniform grain nucleation and suppressed the growth of abnormal orientations, which could explain the weaker diffraction peaks for the non-dense-row facets such as (220) and (222). No distinct Co diffraction peaks were observed in the XRD patterns, which could be attributed to the favorable radius matching between Co atoms (atomic radius 1.67 Å) and Ni atoms (1.62 Å) during the Ni-Co co-deposition process, allowing Co to fully integrate into the Ni lattice, thus forming a solid solution with a single-phase structure.

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Figure 3.

XRD patterns of the differentre Ni-Co-TiN coatings: (a) 0 g/L, (b) 0 g/L (c) 2 g/L, (d) 2 g/L, (e) 4 g/L, (f) 4 g/L (g) 6 g/L by TJE; (h) 6 g/L by UVAJE.

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The average crystallite size data calculated from the XRD diffraction peaks have been shown in Figure 4. Although partial error bars overlapped, all p values were < 0.05, indicating significant differences among the obtained data. Under identical conditions, crystallite size decreased and then increased as TiN addition increased. At 0 g L^-1 TiN, the largest crystallite sizes were observed, 14.5±1.2 nm for coating prepared by TJE and 12.8±1.4 nm for that by UVAJE. The smallest crystallites, 12.1±1.4 nm for TJE and 11.3±1.3 nm for UVAJE, were recorded at 4 g L^-1 TiN addition. This trend was primarily attributed to the heterogeneous nucleation effect induced by TiN nanoparticles, which, at optimal additions, offered abundant nucleation sites, accelerated metal ion crystallization, and suppressed grain growth. Excessive TiN particle content caused nanoparticle agglomeration, resulting in a high particle size up. At the same TiN addition, the mean particle size of the coatings developed via UVAJE was particularly smaller compared to those prepared by TJE. This difference was attributed to the cavitation phenomenon induced by ultrasonic vibrations. The microjets generated by the collapse of cavitation bubbles effectively disrupted nanoclusters, while the acoustic flow enhanced the mass transfer of the electrolyte, therefore preventing abnormal grain growth. Yu et al. [26] explored the effect of ultrasonic power on the preparation of Fe-Ni-Co thin films by ultrasonic electrodeposition, and the results revealed that appropriate ultrasonic power could significantly refine the coating grains.

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Figure 4.

The average crystallite size of the different Ni-Co-TiN coatings.

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3.4. Microhardness

The microhardness of the composite coatings prepared using different methods at various TiN additions is shown in Figure 5. Although partial error bars overlapped, all p values were < 0.05, indicating significant differences among the obtained data. For both deposition methods, the microhardness of the coatings initially increased and then decreased with increasing TiN addition. The highest microhardness values were observed at 4 g L^-1 TiN, with values of 573.5±18.8 HV and 612.4±20.2 HV, respectively. The lowest microhardness was recorded at 0 g L^-1 TiN, with values of 453.8±19.2 HV and 510.8±20.6 HV, respectively. This trend was attributed to the presence of TiN nanoparticles, which acted as heterogeneous nucleation sites, promoting a higher nucleation rate. The nanoparticles, dispersed throughout the interlayer and pores, facilitated grain refinement and consequently enhanced the microhardness of the coatings. At this addition, the TiN nanoparticles were strongly anchored to the substrate surface, effectively restricting dislocation motion between grains and contributing to a dispersion-strengthening effect. However, when the TiN addition increased to 6 g L^-1, particle agglomeration became more pronounced, leading to reduced uniformity and a diminished dispersion strengthening effect, which in turn caused a decline in the coating’s microhardness. Li et al. [27] prepared Ni-TiN coatings with different TiN additions using jet electrodeposition, revealing that the hardness of the fabricated coatings exhibited a tendency to increase and then decrease with the increase of TiN addition.

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Figure 5.

Microhardness of the different Ni-Co-TiN coatings.

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Further analysis showed that at the same TiN addition, coatings fabricated using UVAJE exhibited higher microhardness compared to those produced by TJE. This improvement was primarily attributed to the influence of ultrasonics introduced during the UVAJE process, which improved the microstructure and enhanced the mechanical properties. The cavitation effect generated by ultrasonics in the plating solution produced high temperatures and pressures, which helped break up the growing grains, leading to the formation of uniformly distributed fine crystals. This process increased the number of nuclei and nucleation points, refining the crystallite size and subsequently improving the microhardness. Furthermore, ultrasonic vibrations promoted the even distribution and rapid diffusion of ions within the solution, reducing defects such as pores and cracks, which further contributed to the enhancement of the microhardness.

The maximum indentation depth and the ratio of indentation depth to coating thickness of different coatings have been shown in Figure S4. For the TJE process, with TiN addition ranging from 0 to 6 g L^-1, the maximum indentation depth was 1.2±0.1-1.8±0.2 μm, accounting for 5.8±0.5%-8.9±0.6% of the coating thickness. For the UVAJE process, under the same TiN addition range (0-6 g L^-1), the maximum indentation depth was 1.1±0.2-1.6±0.3 μm, with a corresponding ratio of 4.9±0.2%-6.0±0.3%. These data indicated that the indentation depth did not exceed 10% of the coating thickness in all cases, suggesting no significant interference from the substrate on the microhardness test results and validating the reliability of the microhardness data [28,29].

3.5. Adhesion

The adhesion of coatings prepared using different deposition methods and varying TiN additions has been shown in Figure 6. For both methods, the adhesion strength exhibited an initial increase followed by a decrease as the TiN addition rose. The coatings synthesized via UVAJE and TJE showed the lowest adhesion at 0 g L^-1 TiN, with values of 20.4±2.1 N and 15.8±1.6 N, respectively, and the highest adhesion at 4 g L^-1 TiN, with values of 35.4±2.0 N and 26.2±1.8 N. This trend was attributed to TiN acting as a heterogeneous nucleation site that refined the Ni-Co matrix grains, reduced pores and microcracks, and filled the gaps in the Ni-Co matrix. This resulted in a more homogeneous composite structure, increasing the contact area between the coating and the substrate, thus strengthening adhesion. However, at a higher TiN addition (6 g L^-1), the increased interaction between nanoparticles led to poorer dispersion in the plating solution, causing particle agglomeration. This negatively impacted the uniformity and densification of the coating, ultimately reducing adhesion.

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Figure 6.

Adhesion of the different Ni-Co-TiN coatings.

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The coatings prepared with UVAJE exhibited significantly higher adhesion compared to those prepared with TJE at the same TiN addition. This could primarily be attributed to the ultrasonic effect during UVAJE. The cavitation effect induced by ultrasonic agitation of the liquid layer on the coating surface, refining the particles and increasing the bonding area between the coating and the substrate, improves adhesion. Ren et al. [30] synthesized Ni-B/WC@MoS₂ coatings on 45# steel prepared by ultrasonic-assisted jet electrodeposition, and found that the ultrasonic power significantly promoted the deposition of WC@MoS₂ particles in the coatings, which resulted in a better surface quality of the prepared coatings and stronger performance. Similarly, the high-frequency ultrasonic vibrations effectively removed H₂ bubbles produced during deposition on the coating surface, lowering H₂ embrittlement and internal stress. This further improved the adhesion between the coating and the substrate. The surface morphologies of the various coatings after the scratch test are shown in Figure S5. The scratching was performed from left to right, and the results revealed evident differences in deformation behavior. Specifically, the coating by TJE displayed considerable plastic deformation at the end of the scratch path, whereas the coating by UVAJE showed much less deformation in the same area. This difference could be attributed to the introduction of ultrasonics. Ultrasonic promoted the uniform dispersion of TiN particles in the coating, and the uniformly dispersed TiN particles acted as a rigid reinforcing phase, hindering the dislocation slip in the Ni-Co matrix through the pinning effect and improving the deformation resistance of the coating.

3.6. Compression performance

The stress-strain curves of different Ni-Co-TiN coatings (Figure 7) exhibited a four-stage characteristic: initial elastic, yield, plastic deformation (work hardening), and fracture stages. At low external forces, atoms in the Ni-Co matrix only underwent reversible elastic displacement; TiN, with its high elastic modulus, further enhanced the coatings’ elastic recovery. Stress was proportional to strain in this stage, and no dislocation movement was initiated—corresponding to the initial elastic stage. When the external force exceeded the elastic limit, the yield stage was triggered. The Ni-Co phase in the coatings had a nanocrystalline structure with a high grain boundary fraction, so dislocation movement was hindered by these boundaries; meanwhile, TiN pinned dislocations, slowing stress growth while strain continued to increase. In the plastic deformation stage, TiN continuously blocked dislocations in a dispersed manner and simultaneously inhibited grain boundary sliding of Ni-Co nanocrystals. These two strengthening mechanisms acted synergistically, causing stress to rise rapidly with strain and inducing a significant work hardening effect. When the strain reached a critical value, the fracture stage commenced. As a brittle ceramic phase, TiN was prone to microcrack formation; these microcracks propagated rapidly along Ni-Co/TiN interfaces or nanocrystal grain boundaries, exceeding the plastic bearing capacity of the Ni-Co matrix. Consequently, stress dropped sharply after reaching the peak, and the coatings ultimately fractured.

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Figure 7.

Strain-stress curves of the different Ni-Co-TiN coatings.

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As listed in Table 3, under the same preparation process, the yield strength and ultimate strength of the coatings first increased and then decreased as TiN addition increased, peaking at 4 g L^-1, indicating optimal compressive performance at this addition level. For TiN addition of 0-4 g L^-1, TiN nanoparticles were uniformly dispersed in the Ni-Co matrix: dispersion strengthening effectively hindered dislocation movement and grain boundary sliding under compressive load, while refining hard phase particle size to significantly improve strength. Moreover, uniform particle dispersion did not notably impair matrix plasticity, enabling the coatings to retain a certain deformation capacity before fracture. However, when TiN addition reached 6 g L^-1, particles tended to agglomerate; these agglomerates formed stress concentration sites. During compression, cracks were prone to initiating and propagating along agglomerate-matrix interfaces, leading to a sharp decline in compressive strength and a fracture mode inclined to brittle cleavage. Furthermore, with fixed TiN addition, coatings prepared by UVAJE exhibited higher yield and ultimate strength. This improvement was attributed to the synergistic effect of multiple ultrasonic physical effects. Ultrasonic cavitation broke TiN agglomerates, and mechanical stirring promoted uniform dispersion of TiN particles in the Ni-Co matrix. Simultaneously, ultrasonic waves refined Ni-Co grains, reduced coating internal stress, and lowered porosity, preventing early failure caused by internal stress concentration or pore collapse during compression. In addition, the ultrasonic jet effect improved plating solution mass transfer, increased coating density, and further enhanced resistance to compressive deformation [31].

3.7. Wear resistance

The wear loss of coatings prepared using various deposition processes has been shown in Figure 8. Compared to the Ni-Co coating, the Ni-Co-TiN composite coatings exhibited reduced wear loss, indicating higher wear resistance. At a TiN addition equal to 4 g L^-1, the coatings synthesized by TJE and UVAJE showed the lowest wear losses of 0.29±0.02 and 0.18±0.01 mg, respectively. However, when the TiN addition was increased to 6 g L^-1, the wear loss for both TJE and UVAJE coatings slightly increased, with values of 0.33±0.02 and 0.23±0.01 mg, respectively. The wear rate of the different Ni-Co-TiN coatings is listed in Table S3. Similarly, the lowest wear rate of the coatings produced by both methods was observed at 4 g L^-1 TiN addition. This could be attributed to the reinforcing effect of TiN particles and the refinement of the coating’s microstructure. When TiN particles were evenly distributed within the Ni-Co matrix, they served as a rigid reinforcing phase, effectively countering the abrasive-induced cutting and plowing forces. Throughout the wear process, these particles impeded dislocation movement and delayed plastic deformation, therefore minimizing material loss. The presence of TiN also contributed to grain refinement through both heterogeneous nucleation and pinning grain boundaries, resulting in a denser microstructure that enhanced both the strength and toughness of the coating. However, excessive TiN particles were prone to agglomeration, which not only resulted in uneven dispersion in the Ni-Co matrix but also acts as stress concentration points. The agglomerates exhibited weak interfacial adhesion with the matrix and were liable to detach during friction, generating abrasive particles. Furthermore, agglomeration hindered the increase in actual TiN incorporation content, limiting further improvements in coating hardness and density and thereby compromising the coating’s shear resistance and wear resistance. Chen et al. [32] prepared Ni-Mo-TiN coatings on the surface of metal parts by pulse electrodeposition to improve their wear resistance. The results showed that the addition of TiN reduced the internal stress of the Ni-Mo-TiN coatings and mitigated the tendency of crack formation during coating preparation and friction testing. At a constant TiN addition, coatings synthesized through UVAJE demonstrated significantly reduced wear loss compared to those obtained via TJE, emphasizing the beneficial impact of ultrasonic in the electrodeposition process. This improvement in durability was closely associated with the refined surface structure achieved under ultrasonic influence. The cavitation induced by ultrasonic produced a more compact and uniform coating surface than that generated by conventional methods. As a result, the improved structural integrity contributed to improved wear resistance, showcasing the effectiveness of ultrasonic assistance in elevating coating quality.

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Figure 8.

Wear loss of the different Ni-Co-TiN coatings.

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The friction coefficient curves of the various coatings have been shown in Figure 9. The coefficient of friction of the coating with TiN was lower than that of the coating without TiN, and reached the lowest at the TiN addition of 4 g L^-1. This suggested that TiN’s self-lubricating properties contributed to maintaining a stable boundary lubrication state on the surface, which effectively reduced friction during the wear process. The coefficient of friction of the coatings synthesized by UVAJE was lower than that of the coatings generated by TJE at the same TiN addition level. This could be attributed to the ultrasonic, which enhanced the TiN particle dispersion within the coating. The improved dispersion helped reduce surface roughness, enhanced the flatness of the coating (at 4 g L^-1 TiN, R_a=392.6±8.1 nm), and improved its microhardness, thus contributing to better wear resistance and a lower friction coefficient. Surface morphology analysis following the friction wear tests (Figure S6) revealed evident differences between the Ni-Co alloy coating and the Ni-Co-TiN composite coatings. The wear surface of the Ni-Co coating exhibited considerable superficial flaking and plastic deformation, along with extensive crusting and crack propagation, indicating substantial material loss during the high-stress friction process. However, the wear surfaces of the Ni-Co-TiN composite coatings displayed shallower furrows and fewer microcracks, particularly in the sample prepared by UVAJE (Figure S6d). The wear surface of the coating by UVAJE features “shallow plowing grooves with smooth edges” and no obvious spallation, which are fully consistent with the 3D profilometer results: narrow wear scars (80±5 μm), shallow wear scars (0.8±0.1 μm). This confirmed that the ultrasonic effect significantly reduced the wear degree by optimizing particle dispersion and coating compactness.

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Figure 9.

Friction coefficient curves for the different Ni-Co-TiN coatings.

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3.8. Corrosion resistance

The Tafel curves of coatings prepared using different deposition processes in 3.5 wt% NaCl solution have been shown in Figure 10(a). The Tafel curves of the composite coatings with TiN exhibited a rightward shift compared to the coatings without TiN, indicating a significant improvement in corrosion resistance for the Ni-Co-TiN composite coatings compared to the Ni-Co alloy coating. The electrochemical parameters for the different coatings have been summarized in Table 4. When the deposition method was TJE, the corrosion current density (I_corr) of the composite coating with TiN was considerably lower compared to that of the coating without TiN. The corrosion resistance further increased as the TiN addition increased, reaching its peak at 4 g L^-1, where the I_corr declined from 10.31±0.42 μA cm^-2 for the Ni-Co alloy coating to 3.72±0.23 μA cm^-2 for the Ni-Co-TiN composite coating. However, some particle aggregation occurred as the TiN addition reached 6 g L^-1, resulting in increased surface defects on the coatings. This led to an increase in the I_corr, although it remained lower than that of the Ni-Co alloy coating. Zhou et al. [33] synthesized a Ni-Cu-P-TiN coating using chemical plating with a I_corr of 7.92 μA cm^-2, which was significantly lower than that of 13.60 μA cm^-2 for the coating without TiN. A comparison of the coatings prepared by different processes revealed that the I_corr values of the coatings synthesized via UVAJE were significantly lower compared to those prepared by TJE, indicating higher corrosion resistance. This improvement was mainly attributed to the cavitation and vibration effects of the ultrasonic, which promoted more homogeneous TiN distribution and refined the grain structure of the coatings. The Nyquist plots of the different coatings revealed that the Ni-Co-TiN composite coating exhibited a significantly larger impedance arc radius compared to the Ni-Co alloy coating (Figure 10b). The impedance arc radius of the coating synthesized via UVAJE was greater compared to that of the coating developed through TJE, which provided further evidence that the incorporation of TiN hard particles and the use of ultrasonic technology considerably improved the corrosion resistance. The I_corr of different Ni-Co-based coatings have been listed in Table S4. Compared with the Ni-Co-WC coating prepared by Chen et al. [18] (I_corr=4.10 μA cm^-2, the Ni-Co-SiC coating fabricated by Das et al. [34] (I_corr=19.11 μA cm^-2), and the Ni-Co-Al₂O₃ coating synthesized by Zhang et al. [35] (I_corr=4.37 μA cm^-2), the Ni-Co-TiN coating prepared via UVAJE with a TiN addition of 4 g L^-1 in this work exhibited the lowest I_corr, indicating its superior corrosion resistance.

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Figure 10.

The electrochemical curves of the Ni-Co-TiN coatings: (a) Tafel curves; (b) Nyguist plots; (c) Equivalent circuit.

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To assess the long-term corrosion resistance, the coatings were exposed to a 96 h salt spray test, and the corrosion loss was calculated, as shown in Figure S7. The Ni-Co-TiN composite coatings demonstrated lower corrosion loss compared to the Ni-Co alloy coatings with the same deposition method. At a TiN addition equal to 4 g L^-1, the coatings synthesized via UVAJE and TJE exhibited minimal corrosion losses of 1.4±0.1 and 2.7±0.2 mg, respectively. Further, the coatings fabricated via UVAJE consistently showed lower corrosion loss compared to those made using the TJE method for the same TiN addition. The SEM analysis after the salt spray test (Figure S8) revealed distinct differences between the Ni-Co-TiN coatings prepared by the two processes. The coating fabricated by UVAJE (Figure S8b) showed remarkable resistance to corrosion, with its surface largely intact and free from significant damage. The coating prepared by TJE (Figure S8a) displayed considerable localized corrosion, featuring clear defects like pits and cracks on the surface.

Figure 10(c) illustrates the equivalent circuit of the coating (95% confidence interval), where R_s represents the solution resistance of the corrosive solution, R_ct denotes the charge transfer resistance, and CPE indicates the double-layer capacitance between the coating and the electrolyte. The R_s is predominantly determined by electrolyte conductivity, exhibiting a weak correlation with the coating microstructure. In contrast, the R_ct and constant CPE are tightly coupled with structural evolution. At a TiN addition of 4 g L^-1, ultrasonic cavitation in UVAJE refined TiN particles and inhibited agglomeration, reducing the coating porosity from 2.5±0.3% (TJE) to 1.8±0.3%. This blocked Cl^- penetration paths and reduced the exposed corrosion interface area, leading to an increase in R_ct from 1250±50 Ω·cm² (TJE) to 2800±80 Ω cm^-2 (UVAJE). When TiN addition was 6 g L^-1, particle agglomeration in both processes induced increased porosity, accompanied by a subsequent decrease in R_ct (950±45 Ω·cm² for TJE and 1620±60 Ω cm^-2 for UVAJE). Meanwhile, coatings by TJE exhibited concentrated porosity and localized pitting corrosion, with a CPE value of 5.8±0.4×10^-5 F·cm^-2 at 0 g L^-1 TiN. In contrast, coatings by UVAJE were dense and uniformly structured, resulting in a more homogeneous corrosion interface and a reduced CPE of 1.8±0.1×10^-5 F cm^-2 at 4 g L^-1 TiN (at this time, R_a=392.6±8.1 nm).

Further analysis revealed the impact of surface morphology on corrosion resistance. Metal corrosion initiates at coating surface defects. Tip discharge affected in TJE form honeycomb-like protrusions and cracks, reducing surface uniformity and accelerating Cl^- penetration. In contrast, cavitation microjets in UVAJE eliminated protrusions and formed a continuous dense layer, extending Cl^- diffusion paths by 3-5 times. Additionally, TiN particles further filled microcracks and pores in the matrix, forming a physical barrier to block Cl^- penetration. They also promoted the formation of inert passive films (e.g., TiO₂) at heterogeneous interfaces, preventing direct contact between the substrate and corrosive media.