# Effect of Grain Size and Incidence Angle on Erosive Wear of Polyurea Coating
## 1 Introduction  
Polymers and polymer composites have become popular materials for structural applications due to their advantageous performance parameters, including their strength-to-density ratio, which is higher than that of standard metal alloys [1]. As protective layers, they also have great chemical resistance, hydrophobicity, corrosion and ageing resistance and can be easily manufactured as coatings with high adhesion. Additionally, these coatings can be thick and cure quickly, ensuring impact protection, improved shear-resistance capacity, ductility and dissipated energy [2, 3, 4, 5, 6, 7, 8]. Self-healing abilities and hyper-elasticity are also observed in polymer coatings subjected to wear [9, 10]. Therefore, polymers are increasingly used to protect elements exposed to abrasive or erosive wear, with the potential for ultra-high performance [11]. Among the best materials applied as coatings are polyurethane, polyurea and polyurethane-urea [12, 13]. Polyurea coatings are considered to be highly effective for erosive wear protection [14]. In some cases, non-polymeric coatings are also used [15]. Tribological wear of polymers is a very complex process and can occur through various mechanisms. Abrasive wear occurs when the surface layer of a material is subjected to the influence of abrasive elements such as hard loose particles or wear products. These particles penetrate the surface of the material and cause wear in the form of microcutting, following or microscratching. These are the three basic processes of abrasive wear [16, 17, 18]. Erosive wear, on the other hand, is the process of the surface layer wear due to the action of solid particles, liquids or gases with high kinetic energy (mechanical erosion) or electric current (electrical erosion) on its surface. Mechanical erosion can thus be divided into gas erosion, cavitation erosion and abrasion erosion [19; 20; 21]. The adhesion between polymer coating and metallic substrate depends, most of all, on coating technology, polymer chemical modification and the condition of the surface layer of the substrate [22; 23]. Additionally, hydrogen bonding and phase segregation are the main factors influencing the mechanical parameters of polyurea [24]. To ensure reliable coating, the substrate surfaces must be flat, dry, durable, strong, free from loose and brittle particles and contaminants like greases or oils, and to some degree, rough [25]. Relational bonding between coating and substrate must be ensured to protect a given surface from wear [26]. An increase in coating hardness reduces the level of protection because of lowered damping properties. However, this phenomenon is reversed at low impact angles [27]. In another study, it was observed that erosion resistance is higher for lower hardness of polyurea coating [28]. Polyurea and polyurethane are widely applied to protect parts subjected to erosive wear; polyurea coatings are recommended when a higher tensile strength and a shorter curing time are required [9]. Furthermore, the bonding process is faster during the preparation of polyurea coatings [29]. The thickness of a polyurea layer is also significant, as thin coatings may not provide sufficient protection, while coatings that are too thick may unnecessarily increase the weight and structural complexity of the protected part [4]. The structure of polyurea can be modified to obtain even better properties. When the base polyurea's properties are insufficient, structure modification can be performed [30]. One of the research projects dedicated to studying the improvement of polyurea coatings is presented in [2]. The addition of basalt fiber increased the mechanical and thermomechanical properties of these coatings. The reinforcement with basalt rigid fibers caused a strengthening effect and changes in the structure of polyurea. A separate study found that a higher number of carbon fabric layers results in a greater impact. The main benefits of such a modification were increased coating resistance to mechanical indentation, stiffness and flexural load [31].