Journal of Materials Science & Technology  2019 , 35 (8): 1797-1802 https://doi.org/10.1016/j.jmst.2019.03.019

Orginal Article

A new insight into promotion action of Co2+ in Ni-diamond composite electrodeposition

Haifei Zhoua*, Nan Dub, Jingdong Guoc, Shuan Liud

a Zhejiang Electric Power Corporation Research Institute, Hangzhou 310014, China
b Nanchang Hangkong University, Nanchang 330063, China
c Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China
d Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, China

Corresponding authors:   *Corresponding author.E-mail address: zhouhf@alum.imr.ac.cn (H. Zhou).

Received: 2018-09-29

Revised:  2018-11-9

Accepted:  2018-11-29

Online:  2019-08-05

Copyright:  2019 Editorial board of Journal of Materials Science & Technology Copyright reserved, Editorial board of Journal of Materials Science & Technology

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Abstract

A new insight into the promotion action of Co2+ on both particle and metal deposition in Ni-diamond composite electrodeposition system was analyzed according to electrochemical measurements. The results showed that the addition of Co2+ made particles content in deposits increased remarkably. The change of particles content in deposits was related inversely to the change of cathodic zero potential with the increase of the concentration of cobalt sulfate. Zero charge potential of cathode was shifted to much more negative region. The negative shift of the zero potential, combining with positive shift of the zeta potential, increased the electrostatic force between the particle-adsorbed metallic cations and the cathode. It not only benefits to the transportation of particles in solution towards cathode, but also shortens their residence time on cathodic surface. Meanwhile, entry of particles is also promoted. For metals deposition, reduction resistance of metallic cations rises greatly and deposition current at cathodic potentiodynamic polarization decreases after cobalt sulfate has been added into electrolyte. These factors are favorable for increasing particles content in deposits. In addition, physical model of diamond particles deposition state before and after the addition of Co2+ has been discussed.

Keywords: Cobalt sulfate ; Composite electrodeposition ; Ni-diamond ; Zero charge potential ; Particle content in deposits

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Haifei Zhou, Nan Du, Jingdong Guo, Shuan Liu. A new insight into promotion action of Co2+ in Ni-diamond composite electrodeposition[J]. Journal of Materials Science & Technology, 2019, 35(8): 1797-1802 https://doi.org/10.1016/j.jmst.2019.03.019

1. Introduction

Composite electrodeposition has been fascinated in the last decades because the properties could be adapted by controlling the fraction of incorporated particles in coating according to design [[1], [2], [3]]. Composite coatings are also endowed with new functions in order to satisfy different demands. For example, many researches have focused on the excellent tribological properties [4] and corrosion resistance [5] of metal-diamond composite coatings. Recent research has proved that thermal conductivity of nickel-diamond coating can be much higher than that of pure nickel coating [6].

It is well known that the fraction of incorporate particle in composite coating plays an essential role in improving properties of coating [[1], [2], [3],[7], [8], [9], [10], [11], [12]]. Many methods, such as surfactant, magnetic field and double-pulse technique and so on, were studied in order to favor particle entries into deposits [10,[8], [9], [10], [11], [12]]. Addition of Co2+ cations in Watt's bath has been suggested to enhance particle incorporation in composite coatings several decades ago [4,11,[13], [14], [15], [16], [17]]. It was explained that stronger adsorption of Co2+ on particle surface than that of Ni2+ would change zeta potential of particles, which increased positive charge of the surface and enhanced the electrostatic attractive forces between the charged particles and cathode surface [4,11,17]. However, Wu et al. [14,16] indicated that the zeta potentials kept more positive with increasing concentration ratio of [Co2+] to ([Ni2+]+[Co2+]), i.e., [Co2+]/([Ni2+]+[Co2+]), in Ni-Co-Al2O3 composite system. Consequently, the content of particles in deposits increased with the increasing concentration of Co2+ and held up to 0.4 ratio of [Co2+]/([Ni2+]+[Co2+]) with further marginal increment at higher Co2+ concentrations, indicating that the change of particles content in deposits was not just determined by the change of zeta potential.

It is obvious that cathodic potential also has a significant effect on the electrostatic attraction between the particles and the cathode surface. A sharp positive shift in the cathode potential can not only result in a decrease of electrostatic attraction between positively charged particles and the cathode surface, but also affect the motions of particles in solution and their immobilization on cathode surface. However, there has been no report about the influences from the addition of cobalt sulfate on the change of cathodic potential.

In this work, the promotion action of cobalt sulfate in Ni-diamond composite electrodeposition system was verified by various technical experiments. Deposition state of diamond particles was discussed based on the changes of the cathodic zero potential and the zeta potential data that are caused by the addition of cobalt sulfate. The effect of the concentration of cobalt sulfate on the metal deposition was discussed according to Faraday resistance and cathodic potentiodynamic polarization. Physical model of diamond particles deposition state before and after the addition of Co2+ was established. And so the promotion actions of Co2+ in Ni-diamond composite electrodeposition were studied from a new perspective.

2. Experimental

2.1. Deposition of Ni-diamond coatings on carbon steel sheet

Ni-diamond coatings were electrodeposited on carbon steel sheets (10 × 10 × 0.2 mm) with experimental conditions as present in Table 1. Bath composition for composite coatings was similar to our previous work [18]. Prior to coating, the steel sheets were polished with emery paper up to #2000 grade and then washed in a degreasing bath containing 30 g/L Na2CO3, 30 g/L Na3PO4 and 40 g/L NaOH at 70 °C for 3 min. Finally, the sheets were activated in dilute sulfuric acid at room temperature for 30 s. Diamond particles (30 μm in diameter) were used in this study, which was etched with H2SO4 10 vol.% for 4 h before electrodeposition.

Table 1   Bath composition and operating conditions.

ComponentBath
NiSO4·12H2O300 g/L
CoSO4·7H2O0-8 g/L
NiCl2·6H2O20 g/L
H3BO335 g/L
Diamond Particle150 g/L (30 μm)
Stirring Rate440 rpm
Current Density5 A/dm2
Temperature30 °C

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During electrodeposition, suspension of the diamond particles were mechanically stirred at a speed of 440 rpm, and the temperature was maintained by a water jacket at 30 °C. The carbon steel specimen and a pure Ni plate were used as cathode and anode, respectively. Deposition was carried out for 2 h and then the coatings were cleaned using deionized water.

Analysis of the compositions of the coatings was carried out by dissolving them into. The deposits were stripped and its weight was measured by a BS210S electronic balance (Sartorius, Germany). The coatings were etched in 1:1 (v/v) HNO3 solution and then the weight of diamond particle was weighed. Then content of diamond particle in deposits was obtained. The surface morphology of the deposits was observed using a fields emission scanning electron microscope (FESEM, super 35). Film substrate interfacial adhesion was measured by scratch testing (SCM Revetest, Switzerland). Samples were displaced at a constant speed of 3 mm/min with maximum scratched coating length of 5 mm.

2.2. Electrochemical measurements

Electrochemical measurements were performed through a microcomputer-controlled potentiostat/galvanostat Autolab with PGSTAT30 equipment on a three-electrode cell, where the carbon steel specimen with an exposed area of 1 cm2 was used as working electrode, a saturated calomel electrode (SCE) as reference electrode, and a platinum plate as counter electrode. The solution for electrochemical tests was similar to the above deposition bath. Prior to testing, working electrode was immersed in the bath for 30 min in order to make the system stable.

Potentiodynamic polarization experiments were performed at a low sweep rate of 1 mV/s. Electrochemical impedance spectroscopy (EIS) was measured with a 10 mV disturbance and a measuring frequency range from 10 KHz to 1 Hz. Since the electrolyte would obviously change if the final frequency was too low, the final frequency of 1 Hz was chosen. During the measurement, the electrolyte was kept at 30 °C under stirring.

3. Results and discussion

3.1. Cobalt sulfate vs. Weight percentage of diamond particles in composite

Fig. 1 displays the relationship between the content of diamond particles in the deposits and the concentration of cobalt sulfate in the electrolyte. It is observed that the weight percentage of diamond particles in the deposits first increased from 4.6 wt.% to 8.2 wt.% and then reduced to 7.8 wt.% as the concentration of CoSO4 increased from 0 g/L to 1.25 g/L and further to 2.5 g/L. Finally it tended to remain a steady value of 6.4 wt.% when the concentration of CoSO4 was increased to 5 g/L. These results indicated that the addition of cobalt sulfate could remarkably increase the weight percentage of diamond particles incorporated into the coating. It's showed that CoSO4 with a concentration of 1 g/L has the most significant promotion action on the content of diamond particles in the deposit from our experiments. Bakhit et al. [13] obtained similar rules in their Ni-Co-SiC composite system. In their work, it is reported that the content of SiC particles in the coating firstly increased from 2.1 vol.% to 8.1 vol.% when the concentration of cobalt in the electrolyte increased from 0 g/L to 50 g/L, and then decreased slightly as the concentration of cobalt increased up to 70 g/L.

Fig. 1.   Relationship between concentration of cobalt sulfate and mass fraction of Ni-diamond composite electrodeposition (5 A/dm2, 440 r/min, 30 μm, 30 °C).

Fig. 2(a-d) displays the FESEM images of Ni-diamond films without CoSO4 (A,B,C) and with the addition of 5 g/L CoSO4 (D). The deposits uniformly covered the substrate surface, and no micro-cracks or pinholes were observed in any of the plantings. The diamond particles were evenly distributed on the surface. Scratch test results showed that adhesion strength of the coating on substrates was 15.55 N. The excellent interface between the coating and the matrix ensured the good interface cohesion ability of the coating, as shown in Fig. 2(c).

Fig. 2.   Surface and cross-section images of Ni-diamond films without CoSO4 (a,b,c) and with the addition of 5 g/L CoSO4 (d).

3.2. Cobalt sulfate vs. co-deposition of diamond particle

Electrophoretic force between cathode surface and charged particles has direct influences on the co-deposition process of particles [4,11,13,14]. It is thought that ions can be adsorbed onto surface of particles in the solution, leading to the change of the electrophoretic force. Therefore, both cathode potential and zeta potential of the particles are critical factors for electrophoretic forces.

The cathode potential can be reflected by the change of cathodic zero potential. The differential capacitance curves of Ni-diamond composite electrodeposit system under different concentrations of cobalt sulfate were studied, and the results were shown in Fig. 3. Values of the corresponding zero potential are illustrated in Table 2. The corresponding zero potentials were -844 mV, -948 mV, -931 mV and -861 mV when the concentration of CoSO4 was 0 g/L, 1 g/L 3 g/L and 5 g/L, respectively, showing a “V”-shaped trend. It can be seen that the zero-charge potential were more negative with the presence of CoSO4 and the most negative shift occured when the concentration of CoSO4 was 1 g/L.

Fig. 3.   Differential capacitance curves at different concentration of cobalt sulfate.

Table 2   Zero potential at different concentration of cobalt sulfate.

Concentration of CoSO4·7H2O (g/L)0135
Zero potential(V)-0.844-0.948-0.931-0.861

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As for the zeta potential of diamond particles, our previous study has detected that it is positive in the Ni-diamond system [18]. It means that cations can be adsorbed on the surface of diamond particles in watts bath. It has been reported that the zeta potential is increasing along with the concentration of Co2+ in Ni/nano-diamond [4], Ni/SiC [11,13] and Ni/Al2O3 [14] composition systems because of stronger adsorption capacity of Co2+ cations onto the surfaces of particle than that of Ni2+ cations. The zeta potential tends to no longer change when the adsorption reaches saturation with increasing concentration of Co2+.

Fig. 4 shows a plot of cathodic zero potential and the content of diamond in deposits as a function of the concentration of cobalt sulfate in bath. Interestingly, the change in particle content in the deposits was found to be inversely proportional to the change in zero potential. When the concentration CoSO4 was about 1 g/L, the diamond content reached its maximum value, while the zero potential reached its minimum value. According to the corresponding trend of the two curves, there must be an internal logic between zero charge potential and diamond content in deposits.

Fig. 4.   Zero charge potential of cathode and diamond particle content in deposits VS concentration of cobalt sulfate in bath.

Zeta potential tends to be steady with increasing concentration of Co2+ [11,14,17]. The electrophoretic force is subject to the change of cathode potential. It is reasonable that the negative shift of cathodic zero potential promotes electrophoresis force of diamond particles adsorbing metallic cations towards cathode. The promotion actions lie in three aspects. Firstly, the electrophoretic force can be ignored for particles in whole solution according to the fact that electrophoretic velocity (about 10-5 cm/s) of particles in the bath is much slower than its movement velocity (about 1 cm/s), which means that the promotional function of negative shift of zero potential is weak for these particles. Secondly, intensity of electric field is very high for particles arriving at diffusion double electric layer between the cathode and the electrolyte in view of the fact that the potential difference of diffusion double electric layer occurs in micron-sized distance. Electrophoretic force of particles can't be neglected at this case. The promotion function of negative shift of the zero potential is prominent in this situation. Finally, the diamond particles covering temporarily on the surface of the cathode will be successfully captured into the coating only if scouring action can not make them out of the electrode surface. There must be a residence time before these particles are rooted in matrix of metal. It is obvious that negative shift of the zero potential shortens the residence time of the particles. At the same time, the particles have more intense resistance of scouring action. The entry of diamond particles is promoted by the addition of cobalt sulfate. Of course, the greater the negative shift of the zero potential, the more significant the promotion effect on co-deposition of diamond particles.

In addition, negative shift of the cathodic zero potential must also accelerate the transportation of Ni2+ and Co2+ towards the surface of the cathode. However, the effect of accelerated transportation on diamond content in deposits should be negligible in view of the fact that reduction process of Ni2+ and Co2+ is not controlled by ion diffusion.

3.3. Cobalt sulfate vs. metal deposition

The change of metal deposition owing to the addition of CoSO4 also has big influences on particle content in deposits. Electrochemical impedance spectroscope (EIS) was performed to detect the effects of the concentration of CoSO4 on metal deposition. Fig. 5 shows the Nyquist diagrams measured in the bath at -880 mV with different concentrations of CoSO4: 0 g/L, 1 g/L, 3 g/L and 5 g/L. It is consisted of only one capacitance loop for all of the scan frequency regions. The equivalent circuit of Rs-Rr∥Cd is used to fit the EIS data (Fig. 6) where Rs is solution resistance between the reference electrode and the working electrode, Rr is Faraday resistance which corresponds to charge transfer and mass diffusion during oxidation-reduction reaction, Cd is the double layer capacity in parallel which corresponds to interfacial charge. In most cases, the circuit allows to obtain an excellent agreement between experimental and simulated impedance plots [19].

Fig. 5.   Nyquist plots for Ni-diamond system at different concentration of cobalt sulfate.

Fig. 6.   Electrical equivalent circuit used for simulating the Impedance spectroscopy.

The simulated values of Faraday resistance (Rr) with different concentrations of CoSO4 are shown in Table 3. As the concentration of CoSO4 increased from 0 g/L to 1 g/L, 3 g/L to 5 g/L, respectively, Rr values increased from 17.28 Ω to 71.2 Ω, 178.5 ohm-257 ohm, respectively. Particularly, Rr value at a concentration of CoSO4 of 1 g/L is four times larger than that without any CoSO4. It can be observed that the addition of CoSO4 significantly increases the reduction resistance of metal ions. Undoubtly, this is a favorable factor for increasing the content of diamond particles in deposits.

Table 3   Faraday resistance at different concentration of cobalt sulfate.

Concentration of CoSO4·7H2O (g/L)0135
Faraday resistance Rr(ohm)17.2871.2178.5257

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The increase in reduction resistance may not be equal to a lower rate of reduction reaction. Fig. 7 compares the cathodic polarization curves for the Ni-diamond dispersion electrolyte under different concentrations of cobalt sulfate with the same remaining parameters. It is proved that the reduction of metal ions started in the vicinity of -880 mV, which is the reason why a potential of -880 mV has been selected in EIS. The addition of cobalt sulfate in the bath depresses current density of cathode under the same cathode potential. It is in accordance with the effect resulted from increasing Faraday resistance (Rr), shown in EIS. It means that the addition of CoSO4 slowed down reduction process of Ni2+ and Co2+. It is also one of the keys to promote particle content in deposits by cobalt sulfate. Moreover, the cathodic current density at a concentration of CoSO4 of 1 g/L is the lowest within the range of cathode potential from -880 mV to -1050 mV. That is the reason for lower reduction rate of metallic cations in spite of less Rr.

Fig. 7.   Cathodic polarization curves for Ni-diamond system at different concentration of cobalt sulfate.

3.4. Cathodic zero potential vs. EIS and cathodic polarization

The addition of Co2+ increases Faraday resistance Rr in the vicinity of -880 mV according to the analysis of EIS. It also decreased the cathodic current density according to cathodic polarization curves, indicating that neutralization velocity of cathodic electrons slows down. Zero potential showed negative shifts with the addition of Co2+. On the other hand, the adsorption of Co2+ on the surface of the cathode should have made the zero potential positively shift. However, its adsorption form of Co(OH) restrains the positive shift of zero potential [14]. Based on the fact that the cathodic current density at a concentration of 1 g/L CoSO4 is the lowest in the long potential range, it can be suggested that the neutralization velocity of electrons is the slowest at this situation. It may be the reason that negative shift degree of zero potential is the largest a concentration of 1 g/L CoSO4.

Fig. 8 shows the schematic diagrams of diamond particles deposition state without (A→B) or with (C→D) addition of Co2+. Negative shift of zero potential because of addition of Co2+ makes more electrons distribute on the surface of cathode. Positive shift of zeta potential means that more cations be adsorbed onto the surface of particles. Both the increase of Faraday resistance and the decrease of cathodic current density lead to thinner coatings in the same period of time. All of the above factors are of great benefit to increasing the weight percentage of particles in composite.

Fig. 8.   Schematic diagrams of diamond particles deposition state without (A→B) or with (C→D) addition of Co2+.

4. Conclusions

In summary, the addition of cobalt sulfate in Watt's bath remarkably increased the content of diamond particles in deposits for Ni-diamond system. It also shifts cathodic zero potential to more negative regions. It is observed that the change of the content of particle in deposits is inversely related to the change of the cathodic zero potential. The negative shift of the zero potential, combining with the positive shift of the zeta potential, greatly raises the electrostatic attractive force between charged particles and the surface of the cathode and then promotes the entry process of diamond particles into coating. For deposition of metals, the addition of CoSO4 can not only increase Faraday resistance coming from charge transfer and mass diffusion during oxidation-reduction reaction but also reduce the reduction rate of metallic cations. All these effects are benefit to increase the content of particles in deposits.

The cathodic zero potential at a concentration of CoSO4 of 1 g/L is negatively shifted to -948 mV compared with that of -844 mV without any CoSO4. The degree of negative shift is more than those with concentration of CoSO4 of 3 g/L or 5 g/L. In addition, metallic deposition velocity is also the lowest when the concentration of CoSO4 was 1 g/L. These are the reasons that why CoSO4 with a concentration of 1 g/L has more significant effects on the co-deposition of particles than that of 3 g/L and 5 g/L.

Acknowledgement

This study was supported by the State Grid Scientific and Technological Research Program of China (Grant No. 5211DS17001X).

The authors have declared that no competing interests exist.


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