# Effect of Grain Size and Incidence Angle on Erosive Wear of Polyurea Coating
## 4 Conclusions  
The erosive wear tests conducted on polyurea coatings using different incidence angles and erosive grain sizes led to the following conclusions:  
* The incidence angle of the particles has a significant effect on the erosive wear of the polyurea coating. This was observed both quantitatively (the amount of erosive wear) and qualitatively (the nature of the particle interaction). As the incidence angle decreases, the erosive wear increases, and the nature of the interaction between the particles and the coating surface changes. At high angles, this interaction leads to the cracking and chipping off of fragments of the material. At lower angles, there is increasing deformation, resulting in the formation of bruises and the eventual removal of the deformed material by subsequent particles.
* The size of erosive grains has an impact on erosive wear. This is because of the variations in the number of particles of different sizes within a certain volume or mass, and their different kinetic energies and contact areas.
* The incidence angle of the particles also affects the roughness of the coating surface. The higher the incidence angle, the lower the surface roughness. This can be linked to the greater penetration of the particles, which cause deep bruises when eroded at a low angle. No clear pattern was observed regarding the effect of grain size on surface roughness. Based on these results, it is necessary to continue to study a wider range of incidence angles (e.g., up to 15\({}^{\circ}\)), the effects of the velocity of erosive particles and the distance between the erosive stream and the surface. In addition, it would be worthwhile to analyze the results in comparison to a metallic material, such as steel. The results of the tribological studies conducted on the erosive wear of polyurea coating under the impact of particles of different sizes and angles can be widely applied in various fields of materials engineering and industry. The results can help select appropriate parameters for applying protective coatings in erosion-prone environments, such as in mining, petrochemical, energy, construction and marine industry installations. Understanding the effects of incidence angle and particle size improves the design or modification of coating materials. The results of this research allow for the adaptation of coatings to specific operating conditions. The results indicate under which conditions the coating is most effective. The results can also contribute to the development or improvement of test methods for the resistance of materials to erosion, including protective coatings.  
Figure 12: Examples of microscopic images of Surface of polyurea coating after tests with F120 aluminum oxide grains: (**a**) erosive wear at angle of 45\({}^{\circ}\), (**b**) abrasive wear.  
**Author Contributions:** Conceptualization, [PERSON]; methodology, [PERSON]; validation, J.S. and P.S.; formal analysis J.S.; investigation, J.S. and P.S.; resources, J.S.; data curation, J.S.; writing--original draft preparation, J.S. and P.S.; writing--review and editing, J.S. and P.S.; visualization, J.S. and P.S.; supervision, [PERSON] All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research received no external funding.  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** The data presented in this study are available upon request from the corresponding author. The data are not publicly available due to privacy.  
**Conflicts of Interest:** The authors declare no conflicts of interest.