## 2 Experimental Research
### Main Experiment and Analysis of Obtained Results  
The main experiment was carried out on three different types of material: carbon steel (S235), galvanized steel (S235 JR + Z) and aluminum (AlMg3). As in the preliminary test, the size of the test samples was 150 \(\times\) 100 mm. The research aim was to define the optimal parameters (voltage and strength of electric current) for the application of powder coating in a satisfactory thickness. The coded values of the experiment factors are presented in Table 3. Design-Expert Software 22.0.8 (Stat-Ease, Inc. 1300 Godward Street Northeast, Suite 6400 Minneapolis, MN 55413) was used for experiment planning and statistical data processing. All experiment runs and the results obtained from the measurement of the coating thickness are summarized in Table 4. The presented results refer to the mean value of three repeated measurements for each test sample. \begin{table}
\begin{tabular}{c c} \hline \hline
**Test Sample** & **Thickness of the Applied Coating/\(\upmu\)m** \\ \hline PL1 & 26 \\ PL2 & 14 \\ PL3 & 62 \\ PL4 & 69 \\ UC1 & 23 \\ UC2 & 21 \\ UC3 & 43 \\ UC4 & 78 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Results of the preliminary experiment. Figure 3: Test sample made out of carbon steel. Figure 4 shows the measurement of the thickness of the applied layer (SaluTron ComBi D1000, SaluTron Messtechnik GmbH, Dr.-Gottfried-Cremer-Allee 30/7, D-50226 Frechen) on the test sample. The measurement of the thickness of the applied layer was carried out by measuring the thickness of the applied layer at fifteen different locations on each test sample. Of all the measured values, the mean value of all measurements on each sample was recorded as the reference value for the planned experiment output (output variable). \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Test Sample** & **Experiment Run** & **Current/\(\mu\)A** & **Voltage/kV** & **Material** & **Coating Thickness/\(\mu\)m** \\ \hline  
\hline \hline \end{tabular}
\end{table} 
Table 3: Coded values for the experiment plan. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Coded Values** & **Factor** & **Factor** & **Factor 3—Material** \\
**Categorical** & **Current/\(\mu\)A** & **2—Voltage/kV** & **(Categorical)** \\ \hline –1.414 & 20 & 20 & & \\ –1 & 31.7157 & 31.7157 & \multirow{2}{*}{\
\begin{tabular}{} \end{tabular} } & \multirow{2}{*}{
\begin{tabular}{} \end{tabular} } \\
0 & 60 & 60 & 60 \\
1 & 88.2843 & 88.2843 & & \\
1.414 & 100 & 100 & & \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Measured coating thickness. Within this experiment, results were obtained by calculating the arithmetic mean of three repeated measurements. When performing the experiment, the principle of randomization was applied, so the samples were processed in a random order, as presented in Table 4. According to the results, the application of the coating was carried out first on test sample 8 (experiment run 1), while sample 23 (experiment run 33) was the last one to be coated. Analysis of the experimental research results (Table 4) showed that the minimum response value (coating thickness) was 23.5 \(\upmu\)m, while the maximum was 77.86 \(\upmu\)m. The arithmetic mean of the response in this case was 51.85 \(\upmu\)m.  
Referring to the data obtained for the thickness of the powder-coated layer (Table 4), testing of the linear model, the linear model with second-order interaction 2 FI, the quadratic model and the cubic model confirmed that the quadratic model was the best, as presented in Table 5. The model was tested in relation to the \(p\)-value (mean square deviation), \(p\)-value for deviation from the model and coefficients of determination (coefficient of determination, adjusted coefficient of determination and predicted coefficient of determination). The \(p\)-value (probability) for the F variable for the model is less than 0.05, indicating that at least one of the 11 regression variables has a non-zero regression coefficient, i.e., the contribution to the model is significant.