## 3 Results and Discussions  
### Effects of Electrodeposition Parameters on Morphology and Composition of the Ni/SiCw Composite  
Figure 1 shows the surface morphology of various current density for the Ni/SiC_w_ composite coatings, which was prepared at a stirring speed of 300 rpm, a temperature of 50 \({}^{\circ}\)C, and a current density from 2 mA/cm\({}^{2}\) to 40 mA/cm\({}^{2}\). As shown in Figure 1, obviously, the microstructure and content of SiC_w_ in coatings were appreciably affected by the current density. When the current density was lower than 18 mA/cm\({}^{2}\), the glossy surface of the nickel matrix with uniformly distributed SiC whiskers could be seen, and the content of SiC_w_ increased with the increase of current density (Figure 1a-d). However, when the current density was higher than 25 mA/cm\({}^{2}\), the whiskers content reduced, and the surface morphology of coatings became nodular and inhomogeneous (Figure 1e-f). These phenomena made clear that the electrodeposits prepared at low current density tend to form smooth and luminous surface morphology with fine grains. The influences of current density on composite coatings could be explained by the following aspects. Firstly, as the cathode current density increases, the Coulomb force between the whisker-adsorbed metal ions and the cathode increases, which can increase the deposition rate of the matrix metal and save time for SiCw to be embedded in Ni matrix. That is, the content of SiCw in the nickel matrix increases at per unit time. Secondly, as the cathode current density increases, the cathode overpotential and the electric field force increases correspondingly, which facilitates the electrostatic attraction of the cathode to the solid particles adsorbing the positive ions. At higher overvoltage, the transportation of metal ions to cathode becomes an important factor in electrodeposition, and it is well known that \begin{table}
\begin{tabular}{c c c c} \hline \hline \multicolumn{2}{c}{**Electrolyte Composition**} & \multicolumn{2}{c}{**Electrodeposition Parameters**} \\ \hline Ni(NH\({}_{2}\)SO\({}_{3}\))\({}_{2}\)-4H\({}_{2}\)O & 300 g/L & Current density & 2\(\sim\)40 mA/cm\({}^{2}\) \\ NiCl\({}_{2}\)6H\({}_{2}\)O & 40 g/L & Temperature & 20\(\sim\)60 \({}^{\circ}\)C \\ H\({}_{3}\)BO\({}_{3}\) & 30 g/L & Magnetic stirring & 0\(\sim\)500 rpm \\ SiC whisker & 0.8 g/L & pH & 4.1 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Operating parameters for Ni/SiC_w_ composite electroplating. Figure 1: SEM images of Ni/SiC_w_ composite coating fabricated from the electrolyte at stirring speed of 200 rpm, and temperatures of 35 \({}^{\circ}\)C, and current density of (**a**) 2 mA/cm\({}^{2}\), (**b**) 6 mA/cm\({}^{2}\), (**c**) 10 mA/cm\({}^{2}\), (**d**) 18 mA/cm\({}^{2}\), (**e**) 25 mA/cm\({}^{2}\), (**f**) 40 mA/cm\({}^{2}\). the addition of inert particles could enhance this transportation [23]. Compared with lower current density, the rate of whisker that transfer to cathode surface and embed into the composite coatings increases remarkably when the cathode current density is too high. Thirdly, part of the surface of the cathode is covered by whiskers, due to its embedding behavior, which results in the increase of the cathode overpotential, the enhancement of hydrogen adsorption capacity and the formation of alkaline salts near the cathode [4]. These by-products that adsorbed on the cathode surface or embedded into the coating also prevent the whisker from co-depositing with nickel metal and lead to a decrease of the SiCw content in electrodeposits. Besides, at a higher overpotential, the dendrites or nodules were formed on the coating's surface, due to the discharge of metal ions near the cathode, which cause the discharge at the swelling of deposits and it is consistent with previous literature reported by other researchers [2]. During the electrodeposition process, the deposited crystal and grew behavior are closely related to the electrolyte temperature, which plays an essential role in controlling its texture and grain size.