Jet Electrodeposition of Ni-SiO 2 Nanocomposite Coatings with Online Friction and Its Performance

To improve the surface quality and performance of Ni-SiO 2 nanocomposite coatings, jet electrodeposition of SiO 2 nanoparticles (diameter 20–30 nm) with Watt’s nickel solution was used to process on graphite substrate accompanied by ceramic stick rolling and friction (R&F) online. The crystal structure and microstructure of coatings were analyzed by using X-ray diffraction and ﬁeld emission scanning electron microscope, respectively. The corrosion resistance of the samples was evaluated by potentiodynamic polarization (Tafel). By studying the inﬂuences of different concentrations of SiO 2 in the solution and the effect of R&F on the coatings, we found that adding a certain amount of nano-SiO 2 in the solution can signiﬁcantly increase the microhardness of coatings (from 550 HV to 627 HV or a 14% increase). R&F further caused the microhardness to reach 711 HV, which is a 20% increase relative to pure Ni coatings. Moreover, R&F can help to remove cellular bulge on the surface of nanocomposite coatings, thereby improving surface quality. The corrosion resistance of Ni-SiO 2 nanocomposite coatings is better than that of pure Ni coatings.

In recent years, nanocomposite coating processed by electrodeposition has received widespread attention because it performs better than pure metal coating. [1][2] In the process of electrodeposition, nanoparticles deposited in the coating as the second phase, thereby improving the performance of the coating. 3-13 P. Baghery 14 obtained Ni-TiO 2 nanocomposite coating by using electrodeposition, which greatly improved the wear resistance and corrosion resistance of the coating. Ping Yu 15 improved the high-temperature oxidation resistance of Ni coatings by adding nano-SiO 2 to the solution. Yi Wang 16 obtained a Ni-W-SiO 2 coating by direct electrodeposition, which increased the microhardness by nearly 60 HV but worsened the surface quality.
Ni-based coating is widely used as a protective layer for various mechanical instruments because of its excellent anti-corrosion property, wear resistance, high hardness, and good appearance. 1,6,8 Our team has conducted related research on this coating for years. [17][18][19][20] Nano-SiO 2 has a spherical microstructure, which can improve the strength and chemical resistance of materials. However, when nano-SiO 2 are deposited with nickel, the surface will form cellular bulges, 14 which influence the surface quality. Few studies focus on improving the surface quality of Ni-SiO 2 coating. To improve the hardness of Ni-based coating and solve the problem of cellular bulge, this paper proposes the use of jet electrodeposition to process nanocomposite coatings accompanied by ceramic stick rolling and friction (R&F) online. R&F prevents reunited particles from depositing into the coatings and removes surface bulge, thereby improving the quality of the coatings.

Experimental
Experimental device.-The experimental device is shown in Figure 1. The workpiece is fixed in a plating bath with a special fixture as the cathode. Plating solution is pumped into the anode chamber from the plating bath and then injected onto the surface of the workpiece. A nickel stick is inserted through the anode chamber and connects to the anode of the power supply. The anode chamber and jet nozzle constitute the jet nozzle system, which is fixed on the XZ platform. During the electrodeposition process, the workpiece remains stationary while the X platform drives the nozzle system in a horizontal reciprocating motion.
R&F device.-To ensure that the ceramic stick is glued to the workpiece for R&F and considering that it should float as the thickness z E-mail: ldshen@nuaa.edu.cn changes, the device was designed with a spring, as shown in Fig. 2. The device is mainly composed of four parts, namely, the shell, the ceramic stick, the spring, and the spring seat. The shell of the device was manufactured by using 3D laser printing technology.
The device's related parameters are shown in Table I. Ten springs were used, and pressure F was calculated.
(n: spring number; k: modulus of elasticity; x: spring deformation; m: ceramic weight) The shape of the nozzle exit was approximate to a 1 mm × 18 mm narrow rectangular mouth. The flow control was 200 L/h. The distance between the nozzle and the cathode was 1 mm. The flow velocity was calculated as follows:  Table II. The diameter of nano-SiO 2 was 20-30 nm (as shown in Figure 3). Two experiments (with or without R&F) were conducted with different SiO 2 concentrations.
Pretreatment.-To conveniently strip the coating for the test, a 30 mm × 15 mm square graphite substrate was selected. The actual area of deposition was 20 mm × 10 mm. Before the experiment, the graphite substrate was polished sequentially with #1-#6 sandpaper to remove the surface texture and then placed in an alcohol solution to remove grease through ultrasonic cleaning. Deionized water was used in the ultrasonic generator to remove residual alcohol and other substances.
Nano-SiO 2 powder was added to the electrolyte, placed in an ultrasonic generator, and oscillated for 60 minutes. A glass rod was used for stirring to scatter the agglomerate powder as much as possible. Water bath was used to heat the electrolyte at a temperature of approximately 50 ± 1 • C, and the DC power supply output was set at approximately 0.18 A. The distance between the nozzle and the surface was approximately 1 mm while the voltage fluctuated at 28 V. The calculation result of current density was 100 A/dm 2 .
Characterization of Ni-SiO 2 composite coating.-The instruments and parameters for coating characterization are shown in Table III.

Results and Discussion
Surface morphology.-The photos taken by field emission scanning electron microscope are shown in Figure 4. The surface morphology of pure nickel coating (Figure 4a) is smooth and has no obvious surface defects. However, after adding nano-SiO 2 particles to the solution (Figure 4b), the surface formed cellular bulges (The bulges is not SiO 2 but Ni-coating with some nano-SiO 2 embeded) with diameters that ranged from 20-100 μm. With the increase of SiO 2 content, the size of the cellular bulge increased. When the concentration of SiO 2 reached 10 g/L (Figure 4d), a large number of small cellular bulges increased in size. The morphology also changed, becoming higher and larger than that shown in Figure 4b (4 g/L). This change is due to the increased particle density of SiO 2 ; these particles dispersed in the solution, thereby increasing the number of cellular bulges with SiO 2 as nucleation. However, cellular bulges were not removed on time. Thus, nickel deposited on the bulges, thereby causing a point discharge effect. Thus, the cellular bulges grew bigger and became uncontrolled, thereby affecting the surface quality of the coatings. After introducing the method of mechanical friction, cellular bulges were smoothed by the effect of R&F in a timely manner, leaving behind very small scratch marks, as shown in Figure 5a. Moreover, the surface morphology of coatings greatly improved. But when the content of SiO 2 improved, the effect of R&F declined (Figure 5b). The principle of R&F is shown in Figure 6. In the electrodeposition process, the effect of mechanical R&F could smooth the rapidly growing crystal on time, thereby hindering the point discharge. As a result, the     Compositions of different coatings were analyzed by means of EDS analysis and the results are shown in Figure 7, in which the SiO 2 content corresponding to Si content in the coating. As shown in Figure 7, SiO 2 content incresed slightly with the increase of SiO 2 nano-particles in electroplating bath. But the coating with R&F has slightly lower SiO 2 content which caused by the friction that brought away some particles during electrodeposition.
Coating structure.- Figure 8 shows the X-ray diffraction patterns of pure Ni coating (a) and Ni-SiO 2 composite coating (b, 6 g/L; c, 10 g/L) without friction. A comparison with the Ni standard map shows that the structure is face-centered cubic structure. Crystal surfaces (111) and (200) of the pure nickel coating grows uniformly, whereas the Ni-SiO 2 composite coating has an obvious preferred orientation in the (111) surface.
Phase analysis indicates that no SiO 2 diffraction peak existed in the patterns, but the 'b' and 'c' diffraction peak shifted to the left contrast to the 'a'. This shift implies that the crystal lattice constant became larger; some particles whose diameters were larger than that of the nickel atoms were mixed into the coating, thereby resulting in crystal lattice distortion. Thus, a possible reason for the absence of a SiO 2 diffraction peak is that the SiO 2 content in the coating is too low (The highest content of SiO 2 is only 0.83%) to be detected.
The diffraction peak of 'b' is obviously wider than that of 'a', while that of 'c' is slightly narrower. Calculation indicates that the grain size of 'c' is the biggest (12.33 nm), 'a' is the second (11.53 nm), and 'b' is the minimal (10.93 nm). This finding is consistent with the idea that adding nanoparticles can refine the grains, but the added amount of SiO 2 must be appropriate. Figure 9 shows the XRD patterns of coatings with R&F. Compared to the coatings without R&F, preferred orientation in the (111) surface weakened, and the diffraction peaks shifted to the left slightly. Calculation indicates that the grain size of 'a ' and 'b ' is 11.86 nm and 12.00 nm, which is a little bigger than 'a' and 'b'.  Microhardness.- Figure 10 shows the effect of different SiO 2 concentrations and the influence of R&F on the microhardness of the coating when other parameters are the same. Within a certain concentration range, the increase of the microhardness of the coating was approximately linear with the increasing of SiO 2 concentration. The microhardness of the coating with R&F was obviously superior to that without R&F. When the concentration of SiO 2 reached 6 g/L, the microhardness peaked (627 HV), exhibiting a 14% increase over that of pure nickel coating. As the concentration increased, the microhardness decreased gradually. With R&F, the microhardness peaked (711 HV) when the concentration of SiO 2 was 8 g/L, thereby indicating a 29.3% increase. Then, as the concentration increased, the microhardness decreased continuously.
The microhardness increased for the following reasons: Nanoparticles were dispersed in the plating solution, which provided a large number of nucleation areas for the deposition process. Thus, the coating grain number increased. During the process of grain growth, nanoparticles embedded into the Ni-coating, thereby resulting in lattice distortion, which induced micro-distortion strengthening in the coating. Also, nano-SiO2 as the 2 nd phase co-deposit with Ni matrix which acted as strong obstacles to dislocation movement, thereby strengthening dispersion. However, the microhardness declined when   the SiO 2 concentration exceeded a certain value. The high density of dispersed SiO 2 affected the discharge process of nickel ions, and the reunion rate increased greatly as concentration increased. The reunited particles in the coating obviously affected the coating performances.
Microhardness peaks at different concentrations with or without R&F, thereby proving that R&F can improve the surface quality, remove surface defects in a timely manner, and eliminate some negative effects due to high SiO 2 concentration, thus improving the coating performance. However, when the concentration of SiO 2 is too high, the effect of R&F cannot improve the coating performance very well, thereby reducing the hardness.
Electrochemical corrosion behavior.-The potentiodynamic polarization curves of pure Ni and Ni-SiO 2 nanocomposite coatings in 3.5 wt% NaCl solution are depicted in Figure 11. The corrosion potential (E corr ) and corrosion current (i corr ) are obtained from the intersection of cathodic and anodic Tafel curve tangents and listed in Table IV. (d ,e with R&F; others without). Table IV shows that the corrosion resistance of nanocomposite coating is better than that of pure nickel coating and is influenced by the concentration of SiO 2 . The E corr and i corr of the pure Ni coating are −0.37 V and 3.58 μA/cm 2 , respectively. For the Ni-SiO 2 nanocomposite coatings, however, the E corr values are more positive, and the i corr values are lower than those of pure Ni coating, thereby indicating that the Ni-SiO 2 nanocomposite coatings have higher corrosion resistance than the pure Ni coating in 3.5 wt% NaCl solution. With the increase of the concentration of SiO 2 , the corrosion resistance improved. However, the performance dropped when the concentration reached 8-10 g/L, similar to the result of microhardness. The nanoparticle can refine the grain. However, excess nanoparticles causes many nanoparticles  to reunite, thereby resulting in cellular bulges and surface defects. As a result, the corrosion resistance drops. A comparison between 'd' and 'd ' shows that at the concentration of 6 g/L, the coating with R&F has more negative E corr but lower i corr . To better understand the corrosion behavior, the electrochemical impedance spectra (EIS) in 3.5 wt% NaCl solution were obtained and shown as Nyquist plots in Fig. 12. The capacitive loops of the coating with R&F has a larger radius than that without R&F, which means R&F can improve the corrosion resistance. Compare 'e' and 'e ', it shows that at the concentration of 8 g/L, the coating with R&F has more positive E corr and lower i corr , which also means coating with R&F has better corrosion resistance. The corrosion of materials originates from the surface, especially micro-holes, micro-cracks, and other defects. R&F can make the coating compact and uniform, thereby improving the corrosion resistance.

Conclusions
1. Adding a proper amount of nano-SiO 2 in the plating solution can refine the grains and improve the microhardness of the coating significantly from 550 HV to 627 HV. However, when the content of nano-SiO 2 is too high, microhardness drops.