# Preparation of Co-Ni Alloy Coating with Stable Composition by Jet-Electrodeposition
## 3 Results and Discussion
### Microhardness and Wear Volume of the Coatings  
Figure 10 shows the variation of the microhardness of the coatings obtained with different concentration ratios of Co\({}^{2+}\):Ni\({}^{2+}\) and solution flow rates. It can be seen that the microhardness of the coatings generally increases with the increasing flow rate. The microhardness for M = 2:1 is the highest (about 420 @ (HV)) for all flow rates, and the microhardness for M = 1:3 is the lowest for each flow rate. There is only a small change for M = 2:1 or 1:1, but a large change for M = 1:3. The microhardness of the coating is mainly related to the content of the elements and the grain size in the coating [19]. In Co-Ni alloy coatings, the presence of Ni element plays an important role in solid solution strengthening. The increase of the plating solution flow rate decreases the content of Ni in the coating, which leads to the reduction of the solid solution strengthening. On the other hand, the decrease of the Ni content makes the grain size of the coatings smaller, leading to the enhancement of the dominating fine grain strengthening. This dominated grain strengthening mechanism of the coating can explain the higher microhardness for M = 2:1 or 1:1. As the grain size for M = 1:2 or 1:3 decreases significantly with the increasing flow rate, there is a relatively large change in the microhardness of the coating.  
Figure 10 shows the wear volume of coatings prepared with different mass concentration ratios of Co\({}^{2+}\):Ni\({}^{2+}\) and solution flow rates. It can be seen that when M = 1:2 or 1:3, the wear volume of the coating remains largely unchanged. When M is 1:2, the wear volume of the coating decreases slightly. When M is 1:3, the wear volume of the coatings is high, and the wear volume of the coating decreases rapidly. Therefore, the wear volume of the coating is affected by the element content, microhardness, and surface microstructure of the coatings. When M is 2:1 or 1:1 in the plating solution, the increase of the flow rate of the plating solution does not substantially change the element content of the coating, and the decrease in the crystal size of the coatings and the reduction in the surface roughness are not obvious, so that the wear volume of the coating changes little. When M is 1:2 or 1:3 in the plating solution, the increase of the flow rate of the plating solution increases the Co content in the coatings, and the grain of the coatings are obviously refined, the surface roughness is reduced, and the microhardness is increased, so the wear volume of the coating is reduced and wear resistance is improved.  
Figure 11 shows the wear volume of coatings prepared with different mass concentration ratios of Co\({}^{2+}\):Ni\({}^{2+}\) and solution flow rates. It can be seen that the wear volumes are relatively small for all coatings, except for M = 1:3. The coating with M = 2:1 has the highest wear resistance among all coatings. The wear resistance decreases gradually with the increasing Co\({}^{2+}\):Ni\({}^{2+}\) ratio. There is also a decreasing trend in the wear volume with the increasing flow rate for all coatings. In general, the trend of wear volume variation is inversely correlated with the microhardness  
Figure 10: Microhardness of the coatings.  
It can be seen that the wear volume of the coating is affected by the element content, microhardness, and surface microstructure of the coatings.  
When M is 2:1 in the plating solution, the coating has a high Co content and fine grain for all flow rates, so the wear resistance is high and there is no substantial variation among the coatings. For M = 1:3 in the plating solution, there is a large variation in grain size, surface roughness, and microhardness as the flow rate changes, and similar for the wear resistance of this coating. It can be seen that the flow rate plays a significant role in the performance of Co-Ni coatings by jet electrodeposition, and that high-quality flow-independent Co-Ni coatings can be obtained when Co\({}^{2+}\)/Ni\({}^{2+}\) ratio in the solution is 2:1.