3.1 Surface morphology
Figure 1 shows the surface morphologies of NCSTCFs developed at a fixed PF of 10 Hz 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 films derived at 50% and 70% DCs are exhibited in Fig. 2. The magnification of the surface morphology of those composite films shown in Fig. 2 was comparatively higher than that of Fig. 1. During the electrodeposition process, it was discovered that NCSTCFs obtained at 50% DC had a finer and harder surface morphology than those derived at 70%.
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Different pulse frequencies with a constant DC of 50% resulted in distinct surface morphologies for NCSTCFs (Fig. 3). At 10 Hz 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 60 Hz. Figure 4 illustrated superficial characteristics for SiC and TiN NPs embedded within NCSTCFs. Typically, SiC and TiN NPs agglomerated and produced particle clouds, and they were implanted being second phases within Ni-Co grains. This result was similar to the findings of Liu et al. [18].
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3.2 SiC deposition content
Figure 5 and Fig. 6 depict, respectively, DC/PF influence upon SiC and TiN NPs levels embedded in NCSTCFs. It was found from Fig. 5 that NCSTCFs formed at 10% DC had the smallest volume among all Ni-Co/SiC + TiN composite films, under fixed condition of 10 Hz PF. SiC and TiN NPs incorporated within films were measured to reveal peak SiC and TiN NP level (11.6 v/v% and 11.7 v/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 NCSTCFs. 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 films [19]. 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 film [20].
Furthermore, under identical conditions (50% DC), as shown in Fig. 6, NCSTCFs developed through 60 Hz PF had minimal SiC and TiN contents with a volume percentage of 5.6 v/v% and 5.4 v/v% respectively. The film formed through 10 Hz PF, on the other hand, possessed peak SiC and TiN levels (10.2 v/v% and 10.5 v/v%). The experimental findings demonstrated that SiC and TiN contents within NCSTCFs exhibited a decrease as the PF was increased. Selecting an optimal PF, such as a frequency of 10 Hz, potentially drives significant over-potential across PCE depositing process, leading to enhanced energy production for the adsorption of SiC and TiN NPs onto the electrode [21].
Figure 7 presents a three-dimensional surface diagram depicting the effects of DC and PF on the levels of SiC and TiN NPs embedded in NCSTCFs. The SiC and TiN contents in the NCSTCFs increased with an increase in DC (from 10–50%) and a decrease in PF (from 60 Hz 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.
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3.3 XRD characteristic analysis
Figures 8 and 9 show the XRD patterns of NCSTCFs 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 fraction of SiC and TiN NPs within films was relatively low, resulting in the weak SiC and TiN peaks within XRD pattern. Furthermore, XRD patterns exhibited the formation of NiCo with other solid mixtures containing two phases. This can be attributed through Co/Ni salt presence within plating mixture.
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It is noteworthy that the strength of the characteristic diffraction peaks in composite films 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 NCSTCFs. Results showed that elevating DC or decreasing PF decreased grain dimensions. When the DC and PF were kept at 50% and 10 Hz, the average NiCo size in Ni-Co/SiC + TiN nano-composite coatings was approximately ~ 85.2 nm. Therefore, finer-grained NCSTCFs have been developed.
Table 2
Average NiCo sizes in NCSTCFs fabricated at varied DCs and PFs.
Plating parameters | Average NiCo size (nm) |
Pulse frequency (Hz) | Duty cycle (%) |
10 | 10 | 124.2 |
30 | 113.5 |
50 | 85.1 |
70 | 92.4 |
10 | 50 | 85.3 |
30 | 102.0 |
60 | 118.9 |
Insert Table 2 here
NCSTCFs 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 10–50%) together with reduced PF (from 60 Hz to 10 Hz), grain size for NCSTCFs decreased, influencing SiC and TiN NPs-deposition within composite films. 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 films. This, in turn, has an impact on the grain size of the films during the PCE process.
3.4 Microhardness test
Figures 10 and 11 demonstrate the microhardness values of Ni-Co/SiC + TiN composite films under different pulse frequencies (PFs) and duty cycles (DCs). Figure 11 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 films ranged from 510 to 670 kg/mm2. An increase in DC (from 10–50%) and a decrease in PF (from 60 Hz to 10 Hz) resulted in an increase of microhardness value. The composite films prepared with a PF of 10 Hz and a DC of 50% exhibited the peak microhardness value of 667.4 kg/mm2. In comparison, the composite films prepared with a PF of 60 Hz and a DC of 10% yielded a minimal value of 514.1 kg/mm2.
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NCSTCFs microhardness depends on hardness from metal substrate while amount of each of the ceramic NPs within matrix, 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. Fine grain strengthening and dispersion strengthening are the two main types of strengthening mechanisms that are determined by the size and content of NPs [22]. 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 NCSTCFs has a significant factor for improving the microhardness of coatings [23]. The deposited NPs are uniformly dispersed within films, and the strength of Ni-Co/SiC + TiN composite films is improved. Besides, SiC and TiN NPs have high microhardness (SiC: ~2840 kg/mm2, TiN:~3200 kg/mm2), which further enhances the hardness of the coating.
3.5 EIS analysis
Figures 13 and 14 illustrate respectively Nyquist/Bode curves for NCSTCFs attained through differing PF/DC variables. 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 (4915.7-4927.2 Ω·cm2), indicating an excellent corrosion resistance. This study's findings are consistent with those of Xia and colleagues [24].
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Corrosion behavior by NCSTCFs 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 films 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.
Table 3
Charge-transfer resistances collected from EIS plots on Ni-Co/SiC + TiN composite films.
Plating parameters | Charge transfer resistance (Ω·cm2) |
Pulse frequency (Hz) | Duty cycle (%) |
10 | 10 | 2901.4 |
30 | 3654.3 |
50 | 4915.7 |
70 | 4297.6 |
10 | 50 | 4927.2 |
30 | 3416.9 |
60 | 2479.7 |
Strength, hardness, and structural stability for NCSTCFs were considerably improved through deposition of SiC and TiN NPs, which may further improve the films' corrosion properties [25]. 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.