Experimental study on friction-assisted electroforming of Ni-Co alloys

Owing to the complexity of the electroforming mechanism, the structure of electroformed deposits is affected by many process parameters, and defects such as pinholes and nodules in electroformed deposits must be solved. In this study, the mechanism by which hard particles inhibit the formation of the above defects in the electroforming process of Ni-Co alloys was analyzed. Through the friction-assisted electroforming Ni-Co alloy tests, the effects of friction and current density on the hardness, texture and microstructure of the Ni-Co electroformed deposits were studied. The results show that the microhardness of the Ni-Co coatings obtained is between 651 HV and 669 HV with the increase of the cathode speed from 8 rpm to 512 rpm. When the cathode speed is 16rpm, the grain sizes of Ni-Co coatings increase with the increase of cathode current density from 2 A dm−2 to 8 A dm−2, and the microhardness of the coatings is between 672 HV and 590 HV.


Introduction
Electroforming technology is a special processing technology that uses the principle of electrodeposition to form parts. Nickel can easily prepare different nickel-based composites with other metal or non-metallic materials by electroforming to meet different physical and chemical performance requirements, which has aroused the interest of many researchers [1][2][3][4][5][6]. Electroformed Ni-Co alloy is mainly used to replace electroformed nickel in precision molds to improve the wear resistance and high-temperature resistance of the molds [7][8][9][10]. In addition, Ni-Co alloy deposits have good mechanical, chemical, physical and electrocatalytic properties and are widely used in electronics, computers, automobiles, energy storage devices, aerospace and other fields [11][12][13]. However, restricted by the basic theory of electroforming, the preparation of electroformed deposits with required properties often depends on trial and error methods [14][15][16][17]. Therefore, the current electroforming research on single metal, alloy and composite materials is carried out together. Additionally, the traditional electroforming process has problems such as pinholes, pockmarks and nodules. The electroforming process can overcome these barriers [18][19][20][21]. At present, the common solutions are using pulse power supply, using additives and adjusting anode contour, but these methods cannot completely solve the problems [22,23]. For example, when a pulse power supply is used, good results can be obtained only when the electroforming deposits are thin; however, additives will be consumed in the electroforming process, which is difficult to determine, and the stability of the electroforming solution is poor. In the long-term electroforming process, the quality of the electroforming layer is unstable, and the material structure is uneven.
In this paper, friction-assisted electroforming technology is adopted without using a pulse power supply and adding any additives to the electroforming solution. The microstructure of electroformed Ni-Co deposits was controlled by changing the current density and cathode speed.
In the process of friction-assisted electroforming, due to the friction between the hard particles and the cathode surface, the hard particles hinder the adsorption of hydrogen bubbles in the solution on the cathode surface and drive away the hydrogen bubbles adsorbed on the cathode surface, thereby preventing the hydrogen Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. bubbles from being adsorbed on the electroforming deposits or mixed in the electroforming deposits, as shown in figure 1.
The blocking effect will lead to an increase in the cathodic hydrogen evolution overpotential, which will reduce the hydrogen evolution reaction rate and hydrogen evolution amount so that electroformed deposits without pinholes, pits and other defects can be prepared. Additionally, the movement of hard particles will interfere with the electroforming solution, thereby improving the mass transfer process. In addition, the hard particles near the cathode may also interfere with the mass transfer process in the diffusion layer, driving the solution to timely supplement the metal ions consumed in the diffusion layer so that the ion concentration in the diffusion layer tends to be uniform and the thickness of the diffusion layer decreases. Therefore, compared with traditional electroforming technology, friction-assisted electroforming can adopt a higher current density, thereby improving the electroforming speed. Figure 2 illustrates the schematic diagram of the experimental apparatus. The system consists of a power supply, cathode unit, anode basket, heater with temperature control equipment and electrolyte circulating system. The electrolyte contained Ni(NH 2 SO 3 ) 2 ·6H 2 O 400 g l −1 , Co(NH 2 SO 3 ) 2 40 g l −1 , H 3 BO 3 30 g l −1 , NiCl 2 15 g l −1 , and T = 45°C. A nickel plate and a cobalt plate were used as soluble anodes, and their dimensions were 10 mm wide, 15 mm high and 4 mm thick. The cathode adopts a stainless steel cylindrical mandrel with a deposition area of Ø12 mm × 15 mm. Ceramic beads with a diameter of Ø1-2 mm were chosen as the hard particles. All solutions were prepared with fresh distilled water.

Experiment
According to the above analysis, compared with traditional electroforming, friction-assisted electroforming can adopt a higher cathode current density. In this experiment, the motor drives the small end of the cathode to rotate, and the hard particles were placed in the anode basket, which could rub freely with the cathode surface. A DC regulated power supply was adopted, and the cathode currents were 2 A dm −2 , 4 A dm −2 , 6 A dm −2 , 8 A dm −2 . It was found that when the rotating speed of the cathode is lower than 8 rpm, the friction effect is too weak and has little impact on the electrodeposition process. When it was higher than 512 rpm, the difference in electroformed deposits decreased. Therefore, the microstructure of the Ni-Co alloy electroformed deposits with the rotating speed of the cathode in the range of 8 rpm to 512 rpm was analyzed. In order to compare the effect of friction-assisted electroforming, the Ni-Co deposit was also electroformed without friction-assisted when the  cathode current density is 4 A dm −2 at rotating speeds of 16 rpm. The thickness of the electroformed deposits was 30 μm for microhardness detection, SEM, and material composition analysis when the electroforming time was controlled. With a digital intelligent microhardness tester, model HXS-1000A, the microhardness of the electroformed deposits was measured. Each sample was measured at six different locations under conditions of 0.49 N load and 10 s holding time. A scanning electron microscope, model JSM-6510, was used to look into the morphology of electroformed deposits. When the electroforming time was controlled, the thickness of the electroformed deposits was 10 μm for x-ray analysis. A photoelectron spectrometer model K-Alpha+ and an x-ray diffractometer model Ultima IV were used to analyze the sediment's composition.

Results and discussion
3.1. Microhardness Figure 3(a) shows the variation trend of microhardness of the Ni-Co alloy electroformed deposits with the rotating speed of the cathode. When the rotating speed of the cathode is 8 rpm~32 rpm, the microhardness of the electroformed Ni-Co alloy increases slowly; when the rotating speed of the cathode increases to 128 rpm, the microhardness increases to 665 HV. Compared with the Ni-Co alloy microhardness of approximately 550 HV without friction-assisted, the microhardness of the Ni-Co alloy electroformed deposits prepared in this test is significantly improved because the hard particles change the microstructure of the deposits and refine the grains during the electroforming process. When the current density is constant, the increase in the rotating speed of the cathode will enhance the friction so that the hardness of the electroformed deposits will be increased correspondingly. Figure 3(b) shows that when the rotating speed of the cathode is 16 rpm, the microhardness decreases from 672 HV to 590 HV with increasing cathode current density. The results show that the microhardness of Ni-Co alloy electroformed deposits prepared by friction-assisted electroforming is higher than that of traditional electroformed layers [24,25]. Figure 4 shows the SEM images of the Ni-Co electroformed deposits when the cathode current density is 4 A/dm 2 at rotating speeds of 8 rpm, 32 rpm, 128 rpm and 512 rpm. When the rotating speed of the cathode is 8 rpm, as shown in figure 4(a), the grains of the Ni-Co alloy are relatively coarse, when the rotating speed is 32 rpm, as shown in figure 4(b), the grains become fine. When the rotating speed is 128 rpm or 512 rpm, there is little difference in the grain sizes of the electroformed Ni-Co deposits, as shown in figures 4(c) and (d). Figure 5 shows the SEM image of the Ni-Co electroformed deposits prepared when the rotating speed of the cathode is 16 rpm at current densities of 2 A dm −2 , 4 A dm −2 , 6 A dm −2 and 8 A dm −2 . Comparing figures 5(a)-(c), we can see that when the cathode current density increases from 2 A dm −2 to 6 A dm −2 , the grain size of the Ni-Co alloy changes little and does not increase significantly. However, when the current density continues to increase to 8 A dm −2 , as shown in figure 5(d), the grains are obviously rough, and the surface of the electroformed deposits is relatively rough.

SEM
The above results show that when the cathode current density is constant, the friction between the hard particles and the cathode surface will refine the grains of Ni-Co electroformed deposits, and the grain refinement degree will increase at the rotating speed of the cathode increases (friction intensity). However, when the rotating speed of the cathode is increased above 128 rpm, the grain size does not change significantly. When the rotating speed of the cathode is 16 rpm, and the cathode current density is within 6 A dm −2 , the grain size difference is small, and the electrodeposition process is well controlled. However, when the cathode current density is increased to more than 8 A dm −2 , the grain size is obviously coarse.
It can be seen that the friction between the hard particles and the cathode surface gradually decreases with the increasing cathode current density, and with the increase of cathode current density, the effect of friction between hard particles and the cathode surface on grain refinement of electroformed deposits decreases gradually. In addition, the SEM images show that the grain size of the same Ni-Co electroformed deposits is the same at different positions, which indicates that when the rotating speed of the cathode reaches a certain value, Ni-Co alloy electroformed deposits with good material uniformity can be prepared.

Effect of the rotating speed of the cathode on XRD
The XRD results show that the electroformed deposits shown in figure 6(a) are the face-center cubic structure of α-Co(Ni). With the increase in the rotating speed of the cathode, a new surface (101) appears, indicating that a new phase appears in the electroformed deposit, that is, the close-packed hexagonal structure of ε-Co(Ni), as shown in the figure 6(b). When the cathode speed continues to increase to 128 rpm, a new (010) surface appears, as shown in figure 6(c). As the rotating speed of the cathode further increased, as shown in figure 6(d), the closepacked hexagonal structure of the ε-Co(Ni) crystalline phase increased significantly.
The XRD results show that the friction between hard particles and the cathode surface will affect the crystal structure of electroformed deposits at a certain cathode current density and increases with increasing friction strength. The electroformed deposits are transformed from a face-centered cubic structure of α-Co(Ni) to a two-phase structure consisting of a face-centered cubic structure of α-Co(Ni) and a close-packed hexagonal structure of ε-Co(Ni). Figure 7(a) shows the variation trend of the Co content of the Ni-Co alloy electroformed deposits with the rotating speed of the cathode. When the rotating speed of the cathode was 8 rpm ∼ 128 rpm, the Co content increased from 51.2% to 65.1%, and when it increased to 512 rpm, the Co content decreased to 64.7%. The above results show that when the rotating speed of the cathode is 128 rpm, the Co content in the Ni-Co alloy is the highest. Figure 7(b) shows that when the rotating speed of the cathode is 16 rpm, the Co content decreases from 64.3% to 46.1% with increasing cathode current density. The results show that with increasing current density, the content of Co in the Ni-Co alloy decreases gradually. This is consistent with the conclusions of other studies [26].

Conclusion
The Ni-Co deposits were prepared by friction-assisted electroforming, and their microstructure was analyzed. The experimental results show that the friction of particles has an obvious effect on grain refinement. When the rotating speed of the cathode is accelerated, the grain refinement is enhanced, and Ni-Co alloy electroforming  deposits with microhardness between 590 HV and 670 HV are prepared. With the increase in the cathode current density, the friction effect of hard particles on the cathode surface decreases, the microhardness of Ni-Co alloy electroformed deposits decreases gradually, and the grain size increases. It is found that the friction of hard particles on the cathode surface will affect the grain growth and its relative growth rate, resulting in a change in the surface diffraction density and orientation index. With the increase in the rotating speed of the cathode, the Ni-Co alloy electroformed deposits change from a single-phase structure to a two-phase structure.

Acknowledgments
This work was financially supported by the Young Teachers' Scientific Research Fund Project of Shazhou Institute of Technology, China (Grant No. SGJJ2021A01).

Data availability statement
All data that support the findings of this study are included within the article (and any supplementary files).

Conflicts of interest
The authors declare no conflict of interest.