# Effect of Grain Size and Incidence Angle on Erosive Wear of Polyurea Coating
## 2 Materials and Methods
### Methods  
The erosive wear of the polyurea coating was induced using a pressurized jet of erosive particles directed at its surface. The tests were carried out on the test stand presented in Figure 3. The test specimen was placed in a holder that allowed for a change in the angle of the specimen. An erosive stream, a mixture of compressed air and aluminum oxide, was directed at the surface of the sample. The stream caused the particles to hit the surface of the test material, which induced erosive wear. The design of the test rig also allowed for changing the nozzle's distance from the surface of the sample. The entire system was enclosed in a sealed chamber to limit the leakage of aluminum oxide particles into the environment. The tests were carried out at a constant air pressure (0.4 MPa on the compressor), constant air flow (10 m\({}^{3}\)/h) and constant distance of the nozzle from the sample surface (10 mm). With these parameters, the velocity of both the erosive grains and the air contained in the stream was assumed to be the same (about 141 m/s). The incidence angles of the  
Figure 1: Shape and dimensions of the erosive wear test specimen. \begin{table}
\begin{tabular}{c c c} \hline \hline  & \multicolumn{2}{c}{**Properties**} \\ \hline \multicolumn{2}{c}{Al\({}_{2}\)O\({}_{3}\) content} & 94.5 \(\div\) 97\% \\ \hline \multirow{4}{*}{Admixtures} & TiO\({}_{2}\)–2.5 \(\div\) 3.2\% \\  & SiO\({}_{2}\)–0.5 \(\div\) 0.8\% \\  & Fe\({}_{2}\)O\({}_{3}\)–0.4\% \\  & CaO + MgO–0.4 \(\div\) 0.8\% \\ \hline \multicolumn{2}{c}{Specific density} & 3.9 \(\pm\) 0.05 g/cm\({}^{3}\) \\ \hline \multirow{2}{*}{Bulk density} & 1.52–1.87 g/cm\({}^{3}\) \\  & (depending on the granulation) \\ \hline \multicolumn{2}{c}{Grain shape} & Sharp-edged \\ \hline \multicolumn{2}{c}{Hardness} & 9 on the Mohs scale \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Selected properties of the aluminum oxide used in this study [43]. erosive grain (the angle at which the sample is inclined) used in the tests were 90\({}^{\circ}\), 60\({}^{\circ}\) and 45\({}^{\circ}\). The mass of the delivered aluminum oxide for each test was constant for all grain sizes used, at 100 \(\pm\) 5 g. The duration of each trial was 180 s. For each set of variable parameters, 5 trials were carried out, followed by statistical analysis of the obtained results. The tests were conducted under ambient conditions, at 22-23 \({}^{\circ}\)C with 18-22% humidity. A cumulative influence of ambient temperature and humidity was observed in polymer composites [44], and the protective performance of polyurea coating is heavily affected by these factors [45]. What is more, at higher temperatures, an increase in a polyurea  
Figure 3: Experimental test rig: (**a**) Photo. (**b**) Diagram: 1—nozzle; 2—erosive stream; 3—sample. Figure 2: Microscopic images of the aluminum oxide used in this study: (**a**) F70, (**b**) F90, (**c**) F120, (**d**) F150, (**e**) F220 [27]. coating debonding risk was observed [26,46]. Therefore, the ambient conditions during testing should be related to the real working conditions of the tested coating. To ensure the reliability and validity of the wear test results, statistical tools such as standard deviation and confidence intervals were used. Standard deviation is a statistical measure that determines how much single data points differ from the mean. The lower the standard deviation, the more the data is clustered around the mean value. It helps assess the reliability and consistency of the results. Low standard deviation suggests that the results are consistent. The confidence interval is the range of values in which the true value of a parameter is most likely to be found in the population. The confidence interval is defined with a certain level of confidence (e.g., 95%). The narrower the range, the more accurate the result. A confidence interval allows for indicating the range of uncertainty associated with the estimate of the mean. It provides more information than the mean itself; it shows the precision of the estimate.