# Study on Preparation of Aluminum Ash Coating Based on Plasma Spray
## 3 Results and Discussion
### Effects of Spray Process on Coating Properties  
#### 3.2.1 Orthogonal Experiment  
As a new type of coating, no relevant spray process parameters are available for reference. The spraying process has a great influence on the coating performance, while the spraying process contains multiple parameters and each parameter affects each other. An orthogonal experiment is an experimental method used to deal with multi-factor and multi-level design. The main tool of orthogonal experiment design is an orthogonal table. The tester can find the corresponding orthogonal table according to the number of factors and their levels. The tester performs experiments based on the orthogonal experimental table and obtains an optimal solution by analyzing the measurement results. The orthogonal experimental table is shown in Table 4. The spray current, spray voltage, powder flow rate, and main air flow are selected as the research variables. Other spraying process parameters such as spray distance (100 mm) and spray angle (60\({}^{\circ}\)) remain unchanged. The microhardness, adhesive strength, porosity, and abrasion rate of the coating were taken as survey indicators.  
The influence degree of each index on the coating performance was analyzed, the multi-index comprehensive balance method was used to select the optimal parameters, and the comprehensive weighted score was used to evaluate the performance of the coating. First, assign weights to each indicator according to variable importance, then calculate the weighted indicators for each experiment, and turn it into a single indicator problem. Take a comprehensive score of 100 points, including microhardness, porosity, abrasion rate, and adhesive strength of 25 points each. The coating porosity and abrasion rate are negatively correlated with the coating quality, so they are negative. The microhardness, adhesive strength, porosity, and abrasion rate listed in Table 6 were measured by experimental calculations. In the lower half of Table 6, \(k_{1}\), \(k_{2}\), and \(k_{3}\) are the average values of the first, second, and third levels of the respective factors. R is the extreme difference (i.e., the difference between the maximum and minimum values in \(k_{1}\), \(k_{2}\), and \(k_{3}\)), which reflects the influence degree of the listed factors on the investigation index of the sample, that is, the larger the R, the greater the influence of the listed factors on the investigation index.  
According to the comprehensive scoring method, the primary and secondary factors affecting the spray performance of the UAA are spray current \(>\) spray voltage \(>\) powder flow rate \(>\) main air flow. When the spray current is 600 A, the spray voltage is 55 V, the powder flow rate is 24 g/min, and the main air flow is 33 lspm, the obtained coating performance is superior.  
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \multirow{2}{*}{**No.**} & \multirow{2}{*}{**A**} & \multirow{2}{*}{**B**} & \multirow{2}{*}{**C**} & \multirow{2}{*}{**D**} & \multicolumn{2}{c}{**Microhardness/**} & \multirow{2}{*}{**Porosity/\%**} & \multirow{2}{*}{**Abrasion Rate**} & \multirow{2}{*}{**Adhesive**} & \multirow{2}{*}{**Comprehensive**} \\  & & & & & & & **HV** & & **10\({}^{-3}\) g/min** & & \\ \hline
1 & 1 & 1 & 1 & 1 & 1 & 93.44 & 0.15 & 32.4 & 8 & \(-\)66.15 \\
2 & 1 & 2 & 2 & 2 & 370.92 & 0.18 & 11.2 & 5 & \(-\)45.2 \\
3 & 1 & 3 & 3 & 3 & 37.18 & 0.17 & 20.02 & 5 & \(-\)51.53 \\
4 & 2 & 1 & 2 & 3 & 382.96 &