Surface morphology
Figure1 shows the surface morphologies of NCSTCCs developed at a fixed PF of 10Hz with varying DCs. The films were found to have cauliflower-like globular morphological structures. When the DC was increased to 50%, the cauliflower-like structural morphology changed to nodular morphology, in which the cauliflower-like grains became small and fine. Furthermore, the surface morphological characteristics of the composite coatings derived at 50% and 70% DCs are exhibited in Fig.2. The magnification of the surface morphology of those composite coatings shown in Fig.2 was comparatively higher than that of Fig.1. During the electrodeposition process, it was discovered that NCSTCCs obtained at 50% DC had a finer and harder surface morphology than those derived at 70%.
NCSTCCs surface morphologies obtained with various DCs at 10Hz of fixed PF: (a) 10%, (b) 30% (c) 50%, and (d) 70%.
Amplified SEM pictures of NCSTCCs obtained at different duty cycles: (a) 50% and (b) 70%.
Different pulse frequencies with a constant DC of 50% resulted in distinct surface morphologies for NCSTCCs (Fig.3). At 10Hz PF, the structure property of the film was primarily nodular in shape, with essentially minimal presence of large grains. The structure of the film changed to cauliflower-like as the PF gradually increased. The amount and size of large grains in the developed film increased as the PF was increased towards 60Hz. Figure4 shows a cross-section of the composite film and shows the surface characteristics of the SiC and TiN NPs embedded in the ncstcf. It can be seen from the figure that SiC and TiN particles exist not only on the surface of the film, but also in the film, and the dispersion is high. The thickness of the film is 37.2μm, in which the SiC and TiN nanoparticles are aggregated to form particle clouds and implanted into the Ni–Co grains as the second phase. This result is similar to that of Liu et al.17.
NCSTCCs surface morphologies obtained at 50% DC and different PFs: (a) 10Hz, (b) 30Hz, and (c) 60Hz.
Superficial characteristics for SiC and TiN NPs embedded within NCSTCCs obtained at 50% DC and 10Hz PF.
SiC deposition content
Figures5 and 6 depict, respectively, DC/PF influence upon SiC and TiN NPs levels embedded in NCSTCCs. It was found from Fig.5 that NCSTCCs formed at 10% DC had the smallest volume among all Ni–Co/SiC + TiN composite coatings, under fixed condition of 10Hz PF. SiC and TiN NPs incorporated within films were measured to reveal peak SiC and TiN NP level (11.6v/v% and 11.7v/v%) once DC raised towards 50%. Thus far, it has been determined that higher DC conditions (i.e., 50% DC) allow for greater deposition of SiC and TiN NPs in NCSTCCs. According to the adsorption mechanism proposed by Guglielmi, the adsorption process consists of two successive stages, leading to the complete co-deposition of SiC and TiN NPs within films18. SiC and TiN NPs moving around the cathode were first relatively loosely adsorbed onto the electrode substrate, while metal ions completely covered the cathode surface, so that SiC and TiN NPs were deposited within the films, thereby increasing its deposition content within the films. However, the contents of SiC and TiN NPs decreased slightly when DC reached 70%. Because a high duty cycle resulted in an excessive current density, which caused the coating surface to be scorched. And part of the coating was stripped from the film, resulting in the reduction of SiC and TiN NPs contents in the film19.
DC influence upon SiC and TiN NPs levels embedded in NCSTCCs obtained at 10Hz PF.
PF influence upon SiC and TiN NPs levels embedded in NCSTCCs obtained at 50% DC.
Furthermore, under identical conditions (50% DC), as shown in Fig.6, NCSTCCs developed through 60 Hz PF had minimal SiC and TiN contents with a volume percentage of 5.6v/v% and 5.4v/v% respectively. The film formed through 10Hz PF, on the other hand, possessed peak SiC and TiN levels (10.2v/v% and 10.5v/v%). The variance in SiC and TiN content between films deposited at 10Hz and 60Hz PF, despite maintaining a constant 50% DC, can be primarily attributed to the effects of pulse frequency on the deposition kinetics and dynamics of the particles within the plating bath. At a lower pulse frequency of 10Hz, the longer pulse duration allows for more time during each cycle for the nanoparticles to migrate and adhere to the substrate. This increased contact time enhances the likelihood of SiC and TiN nanoparticles being captured and embedded in the forming film. Conversely, at a higher frequency of 60Hz, although the deposition cycles are more frequent, each pulse is significantly shorter. This reduction in pulse duration limits the time available for nanoparticle migration and adsorption per cycle, leading to a lower overall content of SiC and TiN nanoparticles in the deposited film. Furthermore, higher frequencies can induce more turbulent fluid dynamics in the electrolyte, potentially leading to less stable deposition conditions and reduced efficiency in nanoparticle incorporation. The experimental findings demonstrated that SiC and TiN contents within NCSTCCs exhibited a decrease as the PF was increased. Selecting an optimal PF, such as a frequency of 10Hz, potentially drives significant over-potential across PCE depositing process, leading to enhanced energy production for the adsorption of SiC and TiN NPs onto the electrode20.
Figure7 presents a three-dimensional surface diagram depicting the effects of DC and PF on the levels of SiC and TiN NPs embedded in NCSTCCs. The SiC and TiN contents in the NCSTCCs increased with an increase in DC (from 10 to 50%) and a decrease in PF (from 60 to 10 Hz). The Ni–Co/SiC + TiN composite film deposited at 50% DC and 10 Hz PF displayed the highest SiC and TiN contents among all the films. These findings were consistent with those of Figs.5 and 6.
DC and PF influence upon SiC and TiN NPs levels embedded in NCSTCCs.
The increase in the embedded amount of SiC and TiN with pulse frequency (PF) can be attributed to the effect of shorter current pulses. These shorter pulses promote rapid nucleation, resulting in increased adsorption of SiC and TiN nanoparticles onto the metal surface. Furthermore, higher PF prevents nanoparticle agglomeration, enhances bath agitation, and leads to finer grain structures, which offer more surface area for nanoparticle adhesion. Consequently, the higher mobility of the nanoparticles and the frequent current pulses facilitate their incorporation into the growing composite layer, resulting in a greater amount of embedded SiC and TiN as PF increases.
XRD characteristic analysis
Figures8 and 9 show the XRD patterns of NCSTCCs derived through varied DC and PF variables. Weaker characteristic lines were observed when the diffraction angle of 2θ varied between 20°–40° and 50°–70°. The reason for this phenomenon was primarily associated with the fact that the density of Ni and Co was nearly three-fold that of SiC and TiN, while the fraction of SiC and TiN NPs within films was relatively low, resulting in the weak SiC (JCPDS No. 29-1129) and TiN (JCPDS No. 38-1420) peaks within the XRD patterns. Furthermore, XRD patterns exhibited the formation of NiCo with other solid mixtures containing two phases, which can be attributed to the presence of Co/Ni salts within the plating mixture. The formation of the NiCo alloy phase was confirmed by the characteristic diffraction peaks at 2θ values corresponding to the JCPDS card numbers [e.g., NiCo (JCPDS No. 45-1027)].
XRD patterns of NCSTCCs derived through varied DC: (a) 10% and (b) 50%.
XRD patterns of NCSTCCs derived through varied PF: (a) 10Hz and (b) 30Hz.
It is noteworthy that the strength of the characteristic diffraction peaks in composite coatings decreased with increasing PF together with reduced DC, though appearance by films’ XRD patterns did not change. Additionally, Table 2 shows the XRD spectroscopy measurements of individual grains within developed NCSTCCs. Results showed that elevating DC or decreasing PF decreased grain dimensions. When the DC and PF were kept at 50% and 10Hz, the average NiCo size in Ni–Co/SiC + TiN nanocomposite coatings was approximately ~ 85.2nm. Therefore, finer-grained NCSTCCs have been developed.
NCSTCCs deposited by the PCE technique are predominantly hexagonal close-packed structures (h c p) (Figs.8 and 9), because the Ni/Co-based solid mixture within XRD spectrum reflects primarily from the (100) and (110) planes. With increasing DC (from 10 to 50%) together with reduced PF (from 60 to 10Hz), grain size for NCSTCCs decreased, influencing SiC and TiN NPs-deposition within composite coatings. This phenomenon can be attributed to the nucleation-promoting properties of SiC and TiN NPs. The presence of SiC and TiN NPs within films led to reduced grain dimensions together with accelerating nucleation process. Additionally, the growth of grains was effectively inhibited, leading to further refinement of the grains. As previously stated, modifying DC/PF variables can result in the increase of SiC and TiN concentrations within Ni–Co/SiC + TiN composite coatings. This, in turn, has an impact on the grain size of the films during the PCE process.
Microhardness test
Figures10 and 11 demonstrate the microhardness values of Ni–Co/SiC + TiN composite coatings under different pulse frequencies (PFs) and duty cycles (DCs). Figure12 shows a three-dimensional surface diagram illustrating the effects of DC and PF on the microhardness values of the films. The microhardness values of the composite coatings ranged from 510 to 670kg/mm2. An increase in DC (from 10 to 50%) and a decrease in PF (from 60 to 10Hz) resulted in an increase of microhardness value. The composite coatings prepared with a PF of 10Hz and a DC of 50% exhibited the peak microhardness value of 667.4kg/mm2. In comparison, the composite coatings prepared with a PF of 60Hz and a DC of 10% yielded a minimal value of 514.1kg/mm2.
DC influence upon microhardness of NCSTCCs obtained at 10Hz PF.
PF influence upon microhardness of NCSTCCs obtained at 50% DC.
DC and PF influence upon microhardness of NCSTCCs.
In the co-electrodeposition process of Ni–Co/SiC + TiN composite coatings, the distribution of nanoparticles can be influenced by several factors. Non-uniform distribution is often observed due to variations in local current densities, which affect the electrochemical environment at different locations on the substrate. This variation can lead to uneven deposition rates of nanoparticles, which are further compounded by factors such as particle agglomeration and the hydrodynamic conditions within the plating bath. During our analysis, SEM images revealed areas of dense nanoparticle clusters alongside regions with fewer particles, indicating a degree of heterogeneity in particle distribution. This heterogeneity can contribute to the variability in microhardness measurements across different sample areas. Furthermore, the measured microhardness values, ranging from 510 to 670kg/mm2, exhibited standard deviations that reflect the material's heterogeneous nature. Such variations underline the complex interplay between nanoparticle distribution and the composite's mechanical properties. Addressing these challenges requires careful optimization of the electrodeposition parameters and possibly the incorporation of mechanical or magnetic agitation to enhance particle dispersion.
NCSTCCs microhardness primarily depends on the amount of embedded ceramic nanoparticles (NPs) within the matrix and the fine grain structure of the composite, both of which contribute significantly to the overall hardness of the material. The microhardness of the resulting composite film is defined by the hard NPs deposited when a specific metal substrate is utilized within experiment. In this context, the contribution of the hardness of the steel substrate is minimal due to the significant thickness (~ 70μm) of the coating. Instead, factors such as surface morphology and crystallographic orientation have a more substantial effect on microhardness. Fine grain strengthening and dispersion strengthening are the two main types of strengthening mechanisms that are determined by the size and content of NPs21. On the one hand, a large number of fine particles are dispersed within material strengthened by dispersion strengthening. Such fine particles obstruct dislocation movements, while matrix bears such load. On the other hand, it is demonstrated that dispersion strengthening of NPs in NCSTCCs has a significant factor for improving the microhardness of coatings22. The deposited NPs are uniformly dispersed within films, and the strength of Ni–Co/SiC + TiN composite coatings is improved. The cited hardness values for SiC (approximately 2840kg/mm2) and TiN (approximately 3200kg/mm2) are based on well-established literature and standard material properties, which illustrate the inherent hardness characteristics of these materials. These values further enhance the understanding of how such hard particles contribute to the overall hardness of the composite coating.
EIS analysis
Figures13 and 14 illustrate respectively Nyquist/Bode curves for NCSTCCs attained through differing PF/DC variables. Figure15 shows the equivalent circuit diagram in the EIS test. The high bright spots on these curves were extended across the entire frequency range using a single half-circle shape. Furthermore, as the electrode or plating bath charge was transferred, the resistance experienced a change, impacting the size of the half-circle; raising DC together with reducing PF resulted in a larger half-circle. When the DC and PF were 50% and 10 Hz, the Ni–Co/SiC + TiN composite film had the maximum charge transfer resistance (4915.7–4927.2Ω·cm2), indicating an excellent corrosion resistance. This study’s findings are consistent with those of Xia and colleagues23.
(a) Nyquist and (b) Bode curves of NCSTCCs obtained at various DC values (PF: 10Hz).
(a) Nyquist and (b) Bode curves of NCSTCCs obtained at various PF values (DC: 50%).
The equivalent circuit diagram in the EIS test.
Corrosion behavior by NCSTCCs produced under varying DC and PF electrodeposition settings was investigated using an analogous circuit. This equivalent circuit model contains charge transfer resistor and a mixture resistor linked in parallel, together with a double-charge layer capacitor used to assess electrochemical parameters. Table 3 illustrates influence from DC/PF variables upon charge transfer resistance, which is inversely related to corrosion rate. Obviously, since DC was raised, the resistance steadily increased, implying that the corrosion rate of manufactured composite coatings decreased faster while corrosion resistance was excellent. The corrosion rate slowed down when the PF was steadily decreased, further confirming the greater resistance of composite coatings to corrosion.
Strength, hardness, and structural stability for NCSTCCs were considerably improved through deposition of SiC and TiN NPs, which may further improve the films’ corrosion properties24. In addition, presence of numerous SiC and TiN NPs upon cathode surface of films reduces functional regions concerning such reduction reaction occurring upon cathode surface, thereby decreasing the anode dissolution rate.
Potentiodynamic polarization test analysis
To further investigate the corrosion resistance of the Ni–Co/SiC + TiN composite coatings (NCSTCCs), a potentiodynamic polarization (PDP) test was conducted. The polarization curves are presented in Figs.16 and 17. The test was carried out using an electrochemical workstation in a seawater-like solution (NaCl 25g/L, MgSO4 3g/L, MgCl2 2g/L, CaCl2 1g/L) with a platinum electrode as the counter electrode and a saturated calomel electrode (SCE) as the reference electrode.
The polarization curves at a duty cycle (DC) of 50% and a pulse frequency (PF) of 10Hz.
The polarization curves at a duty cycle (DC) of 10% and a pulse frequency (PF) of 60Hz.
The obtained polarization curves were analyzed to derive corrosion potential (Ecorr) and corrosion current density (Icorr). The corrosion current density was used to calculate the corrosion rate of the films, providing a quantitative measure of corrosion resistance, which was calculated by using Eq.(1). The results indicate that the NCSTCCs prepared at a duty cycle (DC) of 50% and a pulse frequency (PF) of 10Hz exhibited the lowest corrosion current density (1 × 10−6A/cm2) and the most positive corrosion potential (− 0.45V vs. SCE), indicating the highest corrosion resistance (3.68mm/year). In contrast, the films produced at 10% DC and 60Hz PF showed a higher corrosion current density (1 × 10−5A/cm2) and more negative corrosion potential (− 0.55V vs. SCE), reflecting a reduced corrosion resistance (18.49mm/year).
$$CR=\frac{K\times {I}_{coorr}\times Eq.Weight}{\rho \times A}$$
(1)
where K denotes the constant that depends on the unit system, where \({I}_{coorr}\) refers to corrosion current density, where \(Eq.Weight\) represents the equivalent weight of the material being tested, where \(\rho \) expresses the density of the material and A represents the area of the exposed surface.
These findings are consistent with the results from the Electrochemical Impedance Spectroscopy (EIS) tests, confirming that the corrosion resistance improves with increasing DC and decreasing PF. The presence of SiC and TiN nanoparticles in the NCSTCCs contributes to this enhancement, as they provide barrier effects, refine the grain size, and improve the passivation behavior.
