# Effect of Grain Size and Incidence Angle on Erosive Wear of Polyurea Coating
## 3 Results and Discussion  
Table 4 summarizes the results of the erosive wear tests on the polyurea coating for different erosive grain sizes and incidence angles of erosive particles. Erosive wear was determined as linear wear, defined as the depth of the wear trace, obtained using a Leica DCM8 optical profilometer with an accuracy of up to 1 \(\upmu\)m. Figure 4 shows examples of wear trace profiles at different incidence angles of erosive particles. Based on these, the linear erosive wear of the tested polyurea coating was determined. In Figure 5, three-dimensional profilometric images of wear traces are shown. At lower incidence angles, the wear traces were observed to be separated into two zones: a central zone, where the particles hit the surface for the first time with maximum impact  
\begin{table}
\begin{tabular}{c c c c c c c c c c c c c c c} \hline \hline \multirow{3}{*}{ \begin{tabular}{c} **Incidence** \\ **Angle** \\ \end{tabular}  } & \multicolumn{8}{c}{**F70**} & \multicolumn{8}{c}{**F90**} & \multicolumn{8}{c}{**F120**} & \multicolumn{8}{c}{**F150**} & \multicolumn{8}{c}{**F220**} \\ \cline{2-13}  & \(\overline{W}_{e}\) & \(\sigma\) & \(c\) & \(\overline{W}_{e}\) & \(\sigma\) & \(c\) & \(\overline{W}_{e}\) & \(\sigma\) & \(c\) & \(\overline{W}_{e}\) & \(\sigma\) & \(c\) & \(\overline{W}_{e}\) & \(\sigma\) & \(c\) \\ \hline
45\({}^{\circ}\) & **129** & 11 & 14 & **128**energy, causing the most wear, and an outer zone, where there is a secondary impact of the erosive particles, causing much less, but still noticeable, wear. Figure 6 presents a graph showing the effect of the incidence angle of erosive particles on the erosive wear of the polyurea coating. It can be observed that as the incidence angle of erosive particles decreases, the erosive wear increases. However, this is not a linear  
Figure 4: Examples of wear trace profiles for different incidence angles of the F120 erosive particles: (a) \(45^{\circ}\), (b) \(60^{\circ}\), (c) \(90^{\circ}\). Figure 5: Examples of three-dimensional wear trace images for different incidence angles of the F120 erosive particles: (a) \(45^{\circ}\), (b) \(60^{\circ}\), (c) \(90^{\circ}\). correlation. At high incidence angles, wear occurs due to the repeated impact of particles on the same areas, which leads to cracking and then the detachment of material fragments. However, at lower angles, the erosive activity is combined with the abrasive activity of the particles. The particles not only impact the surface but also abrade it, causing material loss. The abrasive action of the particles occurs through plowing or microcutting the material, while their erosive activity is fatigue-like. Figure 7 shows a notable correlation between erosive grain size and erosive wear. At 45\({}^{\circ}\) and 60\({}^{\circ}\) angles, the graphs show a similar pattern, and the differences between the grain sizes are clearly visible. At a 90\({}^{\circ}\) angle, the differences are noticeably smaller. At all tested incidence angles, the greatest erosive wear was observed for grains of medium size (F120). This is because there are more small grains than large grains in the same volume (or mass). A single small grain causes less wear than a single large grain--larger particles act on a larger surface area and with greater kinetic energy. However, collectively, smaller grains cause higher wear. This is because F120 grain has the most unfavorable ratio of grain size to number of grains in a given volume in terms of wear. A single F120 grain is of such a size as to cause more wear than F150 and F220 grains. On the other hand, there are significantly more F120 grains in the same volume than F90 and F70 grains, resulting in the greatest sum wear. Figure 6: Effect of incidence angle of erosive particles on erosive wear of polyurethane coating for different erosive grain sizes. Figure 7: Effect of erosive grain size on erosive wear of polyurethane coating for different incidence angles. In Figure 8 the effect of the incidence angle on the surface roughness after the wear process is presented.