# Effect of Grain Size and Incidence Angle on Erosive Wear of Polyurea Coating
## 1 Introduction
In another research paper, the addition of fiberglass to polyurea was analyzed, and this was found to increase strength and dissipated energy [6]. Different studies have demonstrated self-healing capabilities by incorporating carbon nanotubes [32]. Fiber reinforcements improve the impact and tensile resistance of polyurea [11]. Additionally, nanoparticles are commonly introduced into the polymer matrix to increase wear and erosion resistance [33]. In some materials, such as concrete, water erosion can be caused by three factors: First, two-dimensional stress occurs from the impact of erosive materials. Then, increased pressure enables water to penetrate the cracks. Finally, further flow results in shear stress in the surface layer [34; 35]. Overall, polyurea shows above-average resistance to water erosion and abrasion [36; 37]. The intensity of erosion is influenced by several factors, such as the type, shape, size and speed of erosive particles [1; 34; 35; 38]; the angle of impact [1; 34; 35]; temperature and humidity [27]; and the direction of the impact [27]. In the case of thermoplastics, it was discovered that erosion intensity increases with an incidence angle up to 75\({}^{\circ}\), while it reaches a minimum in the range of 15 \(\div\) 30\({}^{\circ}\)[27]. When silica sand was used as the erosive material against thermoplastic polymers, erosion was the highest at 30\({}^{\circ}\) and the lowest at 90\({}^{\circ}\). Compared to polyurethane coatings, polyurea-based coatings showed the lowest erosive wear from sand [13]. A study found that the maximum wear of a polyurea-basedcoating occurred at an incidence angle of 15\({}^{\circ}\). An increase in the incidence angle resulted in a linear decrease in the wear rate. Such a relation was observed up to 90\({}^{\circ}\), the highest value evaluated in this study. The temperature changes did not influence that relation [14]. Among the parameters with the highest influence on erosive wear resistance, this paper analyzes the impact angle and erosive particle size. Based on the literature review, the erosive grain size significantly influences the wear of coatings because it influences the kinetic energy transferred into the coating. Different grain sizes result in different particles at a given time [35]. Additionally, in the same study, the highest wear rate for polyurethane coatings was observed at 90\({}^{\circ}\) (incidence angle of erosive particles). In another study, particle incidence angles were analyzed as a factor influencing polyurea coatings' erosive wear. At a 45\({}^{\circ}\) angle, wear was approximately three times greater than at 90\({}^{\circ}\)[39]. Similar conclusions were given in [40], where 30 \(\div\) 45\({}^{\circ}\) was identified as the range that resulted in the most intense erosive wear on polymer coatings. At low impact angles, solid erosive particles cause erosion through abrasion via microcutting and plowing. The same process is expected for liquid at lower impingement angles [27]. For incidence angles above 45\({}^{\circ}\), plastic deformation becomes the main wear mechanism [35; 40]. In another study, it was found that an increase in strain rate in polyurea coating improves its mechanical properties like elastic and tangent modulus and stiffness as well [41]. What is more, among challenging working conditions, in case of a dry-wet circulation, polyurea coatings have excellent resistance [42]. In the present paper, the erosive wear of polyurea coatings was examined. The influence of two parameters on that type of wear was evaluated: incidence angle (45\({}^{\circ}\), 60\({}^{\circ}\) and 90\({}^{\circ}\)) and grain size (53 \(\upmu\)m, 75 \(\upmu\)m, 106 \(\upmu\)m, 125 \(\upmu\)m, 150 \(\upmu\)m, 180 \(\upmu\)m, 212 \(\upmu\)m and 250 \(\upmu\)m). To study the impact of those factors on coating erosion, profilometric wear imaging, microscopic images and surface roughness determination were utilized. The correlation between erosive particle size, incidence angle and the wear volume was assessed.