# Preparation of Co-Ni Alloy Coating with Stable Composition by Jet-Electrodeposition
## 1 Introduction  
Electrodeposited cobalt-nickel (Co-Ni) alloy coatings usually have superior properties that are better than both pure Co and Ni metals [1]. The reason for this is its microstructure, which can be varied as a function of composition. The Co-Ni co-deposition is an anomalous process, in which the deposition of Co is preferred, while that of Ni is inhibited [2]. Therefore, the Co/Ni atom ratio in the coating is generally higher than the Co\({}^{2+}\)/Ni\({}^{2+}\) cation ratio in the plating solution. Ni-Co co-deposition is also a non-equilibrium one, and is controlled by the mass transfer of Co\({}^{2+}\) in the solution [3]. When Co content in the coating changes, the phase of coating's structure may change from HCP to FCC or HCP + FCC [4], therefore showing dramatic variation in its properties. Coatings with HCP structure have better wear resistance than those with FCC structure, while coatings with FCC structure possess better corrosion resistance than those with HCP structure [5; 6; 7]. Therefore, Ni-Co coatings have found a wide range of applications, including in microelectromechanical system (MEMS) due to their good soft magnetic properties [8; 9], or for surface protection due to their good resistance to corrosion, temperature and/or abrasion [10; 11].  
Electrodeposition is commonly undertaken in a plating bath, and it has been found that many parameters, such as bath composition [4; 12; 13], pH [14], current density [15; 16], and others, will significantly affect the deposition process. [PERSON] et al. [17] studied the influence of plating solution on the electroplating with grooves, and the results showed that the coating surface was uniform, fine and dense when the plating solution was stirred. This is because the stirring of the plating solution in the bath plating affects the mass transfer of the cation. When the current density is fixed, the Co content in the coating is associated with the ratio of Co\({}^{2+}\)/Ni\({}^{2+}\) concentration in the plating solution. Similarly, electrodeposition can be carried out in jet plating format. [PERSON] et al. [18] studied the effect of plating solution flow rate on the composition and properties of jet electrodeposited Co-Ni alloy coatings. The results showed that the flow rate increased the Co content in the coatings, while reducing the thickness of the diffusion layer with refined grain and increasing the microhardness of the coating. The increase of the solution flow rate in the jet electrodeposition process is equivalent to the increase of the stirring speed in the plating bath.  
The plating solution flow rate is also one of the key parameters in jet electrodeposition for preparing particle composite coatings. This affects the mass transfer of the particles and the entry of the particles into the composite coating [19]. When jet electrodepositing the Co-Ni alloy matrix and particle composite coating, the plating solution flow rate may affect both the Co content and the particle content in the coating. In this case, it is difficult to tell the property effect from the alloy matrix between the composited particles. Therefore, it is necessary to obtain high flow-independent quality of Co-Ni coating by jet electrodeposition. Unfortunately, there is still a lack of research on this. In this paper, we conducted a systematic investigation of the effect of jet flow rate on the microstructure and mechanical properties of jet electrodeposited coatings. Six different flow rates and four kinds of solutions were tested.