Effect of Pulse Electrodeposition Mode on Microstructures and Properties of Ni-TiN Composite Coatings

Abstract

Mild steel is a kind of material commonly used in the chemical industry and to manufacture machinery, farm tools, and other parts in order to improve the surface performance of the parts and prolong their service life. The Ni-TiN composite coating was fabricated through ultrasonic electroplating using a Ni-based sulfamic acid bath with added nano-TiN particles. The effects of three distinct current modes, direct current (DC), positive pulse current (PC), and positive–negative pulse current (PNPC), on various aspects of the coating, including surface morphology, TiN content and distribution, interface bonding strength, microhardness, friction and wear properties, as well as corrosion resistance, were investigated. The findings demonstrated that, in comparison to DC electroplating, PC electrodeposited Ni-TiN composite coatings yielded finer grains, smoother surfaces, reduced surface cracks, increased interface bonding strength, and enhanced corrosion resistance. Furthermore, PNPC electrodeposited Ni-TiN composite coatings showed higher interface bonding strength than those of PC electrodeposited Ni-TiN coatings and had the densest structure, leading to the best corrosion resistance. Pulse current electroplating enhanced the incorporation of nano-TiN particles, with a higher deposition rate observed in positive–negative pulse current electroplating compared to positive pulse current electroplating. Furthermore, the PNPC electrodeposited coating displayed improved microhardness and demonstrated the best friction and wear properties, while the DC electroplated coating displayed the least favorable performance in these aspects.

1. Introduction

Recently, researchers globally have been captivated by the remarkable performance of composite coatings, prompting extensive investigations [1]. Electroplating, a method proven effective for the deposition of coatings, involves co-depositing particles from the plating solution onto metal or alloy surfaces [2,3,4]. This process forms composite coatings that enhance wear resistance, provide self-lubrication, resist corrosion, offer decorative features, enable electrical contact, and more.

Nickel-based coatings are among the earliest and most widely used coatings in research [5,6]. Combining nickel metal with diverse nanoparticles enables the fabrication of composite coatings capable of functioning in diverse ways, including wear resistance and friction reduction, as well as high-temperature oxidation resistance. These nickel-based composite coatings have found widespread applications in the aerospace, automotive, and chemical industries and other fields. Currently, methods to improve the performance of composite coatings mainly include (1) modification of process conditions (i.e., duty cycle, peak current density, pH, stirring intensity, temperature) to enhance the overall performance of composite coatings; (2) utilization of organic additives (i.e., saccharin, coumarin, sodium dodecyl sulfate) to enhance the brightness and other properties of composite coatings; and (3) co-deposition of nano-ceramic particles like TiN and SiC to improve the wear resistance and corrosion resistance of composite coatings.

TiN, a metallic bond compound, demonstrates exceptional electrical conductivity among nano-ceramic materials, coupled with a high melting point, low brittleness, robust interfacial bonding strength, and outstanding chemical stability [7]. The addition of nano-TiN to nickel-based coatings contributes to heightened strength, improved wear resistance, and enhanced overall properties of the composite coatings. Ultrasonic pulse electroplating is widely employed for preparing Ni-TiN composite coatings due to its ability to uniformly disperse nanoparticles in the deposited coating [8,9]. Consequently, Ni-TiN composite coatings significantly enhance the overall performance of machinery, chemical metal parts, farm tools, and other components. Thus, the depositing of such coatings and their properties maintain significant importance for industrial and agricultural parts.

However, limited research has delved into the impact of different current modes on the performance of ultrasonic pulse electroplated Ni-TiN composite coatings. Hence, this study employed three different current modes, namely direct current (DC), positive pulse, and positive–negative pulse, to fabricate Ni-TiN composite coatings on mild steel substrates. The effects of the current modes on the microstructure and mechanical properties of the coatings were also investigated.

2. Experiment

2.1. Sample Preparation

A nickel sulfamate bath with sodium nano-sized TiN was used in the experiment, and its composition was as follows: Ni(SO₃NH₂)₂·4H₂O (300 g·L⁻¹), NiC1₂·6H₂O (30 g·L⁻¹), H₃BO₃ (30 g·L⁻¹), sodium dodecyl sulfate (0.2 g·L⁻¹), and TiN (8 g·L⁻¹). TiN powder with a particle size of 20 nm was purchased from Beijing Deke Co., Ltd. (Beijing, China). All other reagents used in this study were analytically pure and procured from Tianjin Kerui Reagent Co., Ltd. (Tianjin, China). They were utilized without further purification. The anode material comprised an electrolytic nickel plate, with a purity exceeding 99.99%, and had dimensions of 60 mm × 40 mm × 3 mm. In addition, the nickel plate was bought from Tianjin Heda Nonferrous Metals Co., Ltd. (Tianjin, China). The cathode material consisted of mild steel with dimensions of 40 mm × 30 mm × 3 mm, providing a working area of 0.09 dm². The mild steel was purchased from the Daqing Gangcai Co., Ltd. (Daqing, China), and the chemical composition and mechanical properties were listed as follows: 0.20% C, 0.6% Mn, 0.3% Si, 0.04% S, 0.045% P, 400 N/mm² tensile strength, and 225 N/mm² yield strength. The mild steel sample was first polished sequentially with 200#, 400#, 800#, 1000#, and 1200# sandpapers. It was then degreased chemically in a NaOH solution (2 mol/L) for 10 min at 25 °C, activated in an HCl solution (1 mol/L) for 15 s, and then washed with deionized water before undergoing the electrodeposition process.

A KQ-1000-type ultrasonic generator as used to disperse and stir the plating solution. The ultrasonic power was 300 W. The power supply adopted a KDMZ12 intelligent pulse power supply. The current operation modes were DC, positive pulse current (PC), and positive–negative pulse current (PNPC), respectively, and the electrodeposition time was 30 min. The DC current density was 6 A·dm⁻², pulse current turn-on time t_on = 250 μs, turn-off time t_off = 750 μs, duty cycle γ = 1/4, current density 6 A·dm⁻², forward working time t_F = 5 ms, reverse working time t_R = 1 ms, positive pulse, and positive–negative pulse frequencies, f, were 1000 Hz and 66.7 Hz, respectively. The plating solution temperature was controlled at ~55 °C by using an HS-600-type heating solution unit, and the pH value of the bath was 4.5 (See Figure 1).

2.2. Test Methods

The microhardness of the coating surface was measured with an HXD-1000 microhardness tester. The load was 0.98 N, the loading time was 4 s, and the average value was measured at 5 points. The sample was cut with a SODICK-AD360Ls machine tool, and the coating surface and cross-section morphology were observed with a JES-550OLV scanning electron microscope (SEM). The uniformity of SiC particle incorporation of the coating was measured by using an HT7800 transmission electron microscope (TEM). The CSM-type high-load scratch meter was used to measure the binding force of the coating. Single-pass scratch was adopted, the initial load was 10 N, the final load was 50 N, the loading mode was linear, the scratch speed was 5 mm·min⁻¹, the scratch length was 5 mm, and the radius of the indenter tip was 200 μm. The friction and wear test was conducted using the MW-1A friction and wear testing machine. The test involved dry friction with a test force of 50 N, a speed of 300 r·min⁻¹, and a duration of 20 min. The weight of the sample before and after friction and wear was measured using an FA2004B electronic balance with an accuracy of 0.1 mg. The post-wear morphology of the coating was analyzed using a scanning electron microscope.

The corrosion resistance of the coating was tested by the weight loss method (Standard’s Number: GB/T 21621-2008; Test method for corrosion to metals). The corrosive medium comprised a 5 wt.% NaCl solution, and the soaking times were 48 h, 72 h, 96 h, 144 h, and 192 h. The corrosion rate was calculated as follows:

$$v={\displaystyle \frac{m_0-m}{A·t}}$$

(1)

where v is the corrosion rate, mg·mm⁻²·h⁻¹; m₀ and m, respectively, represent the mass of the sample before and after immersion tests, mg; A is the surface area of the sample, mm²; t is the soaking time, h.

3. Results and Discussion

3.1. SEM Morphology

Figure 2 displays the surface morphologies of the electrodeposited Ni-TiN coatings under different current modes. The coating deposited through direct current electroplating shows a large grain size and a rough surface. In contrast, the coating obtained via positive pulse electroplating has a smaller grain size compared to the direct current electroplating, resulting in a relatively smoother surface. Employing Image Tool software 3.4 for image analysis, it can be determined that the average grain size of the coating deposited through positive–negative pulse electroplating is the smallest, approximately 1 μm. Generally, the formation of new nuclei tends to coincide with their growth. Fine-grained coatings can be achieved only when the rate of new nuclei formation exceeds the rate of their growth. During pulse electroplating, the peak current density is significantly higher compared to direct current electroplating, causing a greater overpotential. This elevated overpotential enhances nucleation probability, where the rate of nuclei formation surpasses their growth rate [10,11]. Consequently, this phenomenon results in finer grains within the coating and diminishes the impact of concentration polarization on the electrocrystallization process. The outcome is a coating with a dense and smooth surface.

Figure 3 presents the cross-section morphologies of electrodeposited Ni-TiN coatings under different current modes. The TEM images of the coatings under varying current modes are displayed in the upper right corners of Figure 3. The coating deposited through direct current electroplating shows poor thickness uniformity, along with inclusions at the interface between the coating and the substrate. In contrast, the coating obtained through positive pulse current electroplating shows improved uniformity in thickness, accompanied by a reduction in the number of inclusions at the interface. The coating achieved through positive–negative pulse current electroplating displays even greater uniformity in thickness, resulting in a smooth and flat interface between the coating and the substrate. It is believed that during direct current electroplating, concentration polarization leads to uneven thickness and inclusions in the coating [12]. In positive pulse current electroplating, when the pulse current density transitions from 0.6 A·dm⁻² to 0 A·dm⁻², the ongoing growth of grains halts, and the growth centers of the grains are shielded. This forces the generation of new nuclei during the subsequent pulse cycle, ensuring a consistent growth rate of the coating along the thickness direction and yielding a uniform thickness. During positive–negative pulse current electroplating, in addition to the features of positive pulse electroplating, the negative pulse current can eliminate protrusions and inclusion defects formed during positive pulse current electroplating [13], thereby further enhancing the uniformity of coating thickness and achieving a smoother interface.

Figure 4 shows the TiN distribution on the Ni-TiN coating surface under different current modes. The coating produced by direct current electroplating shows a relatively low concentration of TiN particles, with uneven distribution and a certain degree of agglomeration. In the coating formed through positive pulse current electroplating, there is an increase in the number of TiN particles, leading to improved distribution and reduced agglomeration [14]. Furthermore, in the coating resulting from positive–negative pulse current electroplating, the TiN particle count is relatively high, and the nano-TiN particles display a uniform distribution within the coating with a significant reduction in agglomeration. In comparison to direct current electroplating, positive pulse current electroplating hinders the formation of a diffusion layer at the cathode surface due to the increased ion concentration, allowing for more efficient deposition of the nano-TiN particles in the coating. During positive–negative pulse current electroplating, the negative current action dissolves micro-protrusions within the coating. This enhances the ion concentration at the cathode surface, suppressing the formation of the diffusion layer. The improved cathode current efficiency accelerates the co-deposition rate of nano-TiN particles, leading to an increased number of nano-TiN particles in the coating.

3.2. Interface Bonding Force

Figure 5 reveals the SEM micrographs of the scratches on the surfaces of the Ni-TiN coating deposited at different current modes. Under a 25 N load, significant cracks emerge in the scratches of the coating deposited through direct current electroplating. With a 40 N load, small cracks appear in the scratches of the coating from positive pulse current electroplating. Remarkably, under a 50 N load, no cracking occurs in the scratches of the coating from positive–negative pulse current electroplating, and the scratches remain uniform.

According to the B-W formula:

$$F={\displaystyle \frac{KAH}{\sqrt{R^2-A^2}}}$$

(2)

$$A=\sqrt{{\displaystyle \frac{L_C}{\pi H}}}$$

(3)

where L_c is the critical load, N; K is the empirical constant with the value being equal to 1; H is the matrix hardness, HV; R is the radius of indenter, mm; F is the binding force, MPa.

The interface binding force of positive–negative pulse current electrodeposited coatings is the highest (1047.1 MPa), and that of the PC and DC electrodeposited coatings is 851.9 and 380.7 MPa, respectively.

During direct current electroplating, the continuous conducting state of the current prevents the recovery of nickel ion concentrations near the cathode to their normal levels. This condition easily triggers hydrogen evolution reactions, leading to an elevated pH in the cathode region. This pH increase results in the formation of alkaline salts or hydroxides, which become trapped in the deposited layer, ultimately reducing the adhesion strength of the coating. In positive pulse current electroplating, the off-time of the pulse allows for a partial recovery in the concentration of nickel ions near the cathode. This recovery suppresses the occurrence of hydrogen evolution reactions and enhances the adhesion strength of the coating [15,16,17]. In positive–negative pulse current electroplating, the negative pulse dissolves the slight micro-protrusions formed during the positive pulse, thereby increasing the concentration of nickel ions and nano-TiN near the cathode. This further suppresses hydrogen evolution reactions and improves the adhesion strength of the coating. Therefore, the adhesion strength of the coating under positive–negative pulse action is higher than that under direct current and positive–positive pulse action.

3.3. Microhardness

Microhardness values of Ni-TiN coatings electrodeposited with different current modes are displayed in

Table 1. Microhardnesess of Ni-TiN coatings electrodeposited at different current modes.

Current Mode	Test Microhardness (HV)					Mean Microhardness (HV)
DC	313.5	276.9	318.3	324.1	354.2	317.4
PC	377.0	342.5	402.6	377.4	405.9	381.1
PNPC	479.3	533.9	486.4	470.0	482.6	490.4

. The positive–negative pulse current electroplating coating shows the highest microhardness, boasting an average value of 490.4 HV. In comparison, the average microhardness values for the positive pulse current and direct current electroplating coatings are 381.1 HV and 317.4 HV, respectively. Examination of Figure 2 leads to the conclusion that the positive–negative pulse current electroplating coating features the finest grain size and the most compact structure. Conversely, the direct current electroplating coating gives the coarsest grain size, while the positive pulse current electroplating coating falls in between the two [18]. Therefore, the positive–negative pulse current electroplating coating has the highest microhardness, while the direct current electroplating coating has the lowest.

3.4. Friction and Wear Property

Figure 6 presents the wear amounts of the Ni-TiN coatings deposited using different current modes. The wear amounts of Ni-TiN coatings prepared by direct current, positive pulse current, and positive–negative pulse current electroplating were measured to be 9.2 mg, 3.1 mg, and 1.7 mg, respectively. Under identical friction and wear conditions, the coating from direct current electroplating has the highest wear amount, suggesting poorer wear resistance. Conversely, the coating from positive–negative pulse current electroplating demonstrates the lowest wear amount, indicating excellent wear resistance performance.

Figure 7 illustrates the friction coefficient curves of the Ni-TiN coatings deposited using different current modes. The coating from positive–negative pulse current electroplating shows the smallest friction factor (average value: ~0.38), followed by positive pulse current electroplating, while direct current electroplating has the highest friction coefficient (average value: ~0.71). The above analysis leads to the conclusion that as the degree of grain refinement increases, the coating becomes denser, contains more nano-TiN particles, displays higher hardness, and consequently, displays increased resistance to plastic deformation with a lower friction factor [19,20,21].

Figure 8 reveals that the direct current electroplating coating shows considerable surface wear, evident in substantial grinding blocks, step-like wear marks accompanied by pitting, and extensive exfoliation. Compared to the direct current electroplating coating, the pitting and exfoliation on the surface of the positive pulse current electroplating coating are significantly reduced, and the wear marks become sparse, resulting in a significant improvement in wear resistance. The wear surface of the positive–negative pulse current electroplating coating shows no prominent grinding blocks, and the wear marks appear narrow and shallow, indicating excellent wear resistance performance. In summary, the shift in the current application mode (from direct current to positive pulse current to positive–negative pulse current) results in enhanced grain refinement, density, and hardness of the coating [22]. Additionally, there is an increase in the number of nano-TiN particles within the coating, and they are more uniformly distributed. Throughout the friction and wear process, this phenomenon effectively impedes the movement of dislocations within the coating and the sliding of grain boundaries, offering both protection and lubrication for the coating. Consequently, the wear resistance performance of the coating steadily improves.

3.5. Corrosion Behavior

Figure 9 shows the average corrosion rates of the mild steel samples and Ni-TiN coatings deposited under different current modes. The average corrosion rates of the mild steel samples and the coatings obtained under different current application modes are presented. It can be observed that the average corrosion rate of the mild steel sample is 7.1 × 10⁻² mg·mm⁻²·h⁻¹, while those of the direct current, positive pulse current, and positive–negative pulse current electroplating samples are 4.9 × 10⁻² mg·mm⁻²·h⁻¹, 3.7 × 10⁻² mg·mm⁻²·h⁻¹, and 2.9 × 10⁻² mg·mm⁻²·h⁻¹, respectively. In summary, the positive–negative pulse current electroplating sample displays the lowest average corrosion rate, suggesting outstanding corrosion resistance. Furthermore, the mild steel sample attains its peak corrosion rate after 72 h of immersion, followed by a continuous decline. In the case of the direct current electroplating sample, the maximum corrosion rate is reached after 96 h of immersion, also displaying a subsequent decrease. Meanwhile, the corrosion rates of the positive pulse current and positive–negative pulse current electroplating samples remain relatively stable [23,24].

Figure 10 displays the corrosion morphologies of the mild steel samples and the coatings produced under various current modes following immersion in a 5% NaCl solution for 144 h. It can be observed that severe corrosion occurs on the surface of the mild steel sample, with significant unevenness, along with the presence of large amounts of corrosion products [25,26]. Visible corrosion cracks can also be observed. Obvious corrosion pits are present in the coating obtained under direct current electroplating conditions. The coatings obtained under positive pulse current and positive–negative pulse current conditions are relatively flat with fewer corrosion pits. In particular, the coating obtained under positive–negative pulse current conditions has a finer and denser surface structure.

Coatings obtained through different electrodeposition methods (DC, PC, and PNPC) show different levels of corrosion resistance. In direct current electrodeposition, the coating’s grain structure becomes coarser due to a shortage of metal ions on the electrode surface. With pulse current, the cathodic polarization effect is strengthened, leading to a higher nucleation rate and slower growth of crystal grains, resulting in a finer structure. When subjected to a positive–negative pulse current, in addition to maintaining the original positive pulse, the reverse pulse current significantly boosts the concentration of metal ions on the cathode surface. This process leads to a finer grain size and a more compact structure in the coating. The increased density of the structure enhances the corrosion resistance of the coating. This research provides technical support for the application of Ni-TiN coating in machinery, the chemical industry, agriculture, aviation, and other fields.

4. Conclusions

In conclusion, pulse electrodeposited coatings, when compared to DC electrodeposition coatings, exhibit finer grains, smoother surfaces, and fewer cracks, resulting in higher bonding strength. Specifically, positive–negative pulse electrodeposition coatings demonstrate exceptional performance with a bonding strength of 1047.1 MPa and a microhardness of 490.4 HV, surpassing that of positive pulse and DC coatings. Furthermore, the coating from positive–negative pulse current electroplating shows the smallest friction factor (average value: ~0.38), followed by positive pulse current electroplating, while direct current electroplating has the highest friction coefficient (average value: ~0.71). In addition, the average corrosion rate of the mild steel sample is 7.1 × 10⁻² mg·mm⁻²·h⁻¹, while those of the direct current, positive pulse current, and positive-negative pulse current electroplating samples are 4.9 × 10⁻² mg·mm⁻²·h⁻¹, 3.7 × 10⁻² mg·mm⁻²·h⁻¹, and 2.9 × 10⁻² mg·mm⁻²·h⁻¹, respectively. Notably, positive–negative pulse electrodeposited coatings showcase superior wear resistance, excellent tribological properties, and remarkable corrosion resistance, making them a standout choice in terms of overall performance.