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Article

Effect of Grain Size and Incidence Angle on Erosive Wear of Polyurea Coating

by
Justyna Sokolska
* and
Piotr Sokolski
Department of Fundamentals of Machine Design and Mechatronic Systems, Faculty of Mechanical Engineering, Wroclaw University of Science and Technology, ul. I. Lukasiewicza 5, 50-371 Wroclaw, Poland
*
Author to whom correspondence should be addressed.
Appl. Sci. 2025, 15(13), 7568; https://doi.org/10.3390/app15137568 (registering DOI)
Submission received: 23 May 2025 / Revised: 1 July 2025 / Accepted: 3 July 2025 / Published: 5 July 2025
(This article belongs to the Special Issue Feature Papers in 'Surface Sciences and Technology' Section, 3rd Edition)
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Abstract

This study investigated the erosive wear of a polyurea coating with a hardness of 95 ShA and a thickness of 3 mm applied to a 3 mm thick plate made of S235 steel. The process of erosive wear was carried out using a stream of compressed air containing abrasive grains of aluminum oxide (Al2O3). The erosive wear was studied using different incidence angles (45°, 60° and 90°) and erosive grain sizes. Thus, the effects of the incidence angle and erosive grain size on the erosive wear of the polyurea coating were analyzed. Erosive wear was determined as linear wear: the depth of the wear trace was measured using an optical profilometer. This study showed a non-linear correlation between erosive wear, incidence angle and erosive particle size. In addition, a qualitative study of the surface of the coating after a wear test was carried out using a scanning electron microscope, which made it possible to describe the mechanisms of erosive wear of the 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, plowing 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]. 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°, while it reaches a minimum in the range of 15 ÷ 30° [27]. When silica sand was used as the erosive material against thermoplastic polymers, erosion was the highest at 30° and the lowest at 90°.
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-based coating occurred at an incidence angle of 15°. An increase in the incidence angle resulted in a linear decrease in the wear rate. Such a relation was observed up to 90°, 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° (incidence angle of erosive particles). In another study, particle incidence angles were analyzed as a factor influencing polyurea coatings’ erosive wear. At a 45° angle, wear was approximately three times greater than at 90° [39]. Similar conclusions were given in [40], where 30 ÷ 45° 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°, 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°, 60° and 90°) and grain size (53 µm, 75 µm, 106 µm, 125 µm, 150 µm, 180 µm, 212 µm and 250 µm). 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. Additionally, wear mechanisms for different erosive conditions were distinguished.

2. Materials and Methods

2.1. Materials

Erosive wear tests were conducted on a two-component, solvent-free polyurea coating obtained by mixing resin and hardener at a volume ratio of 100:100 (weight ratio of 100:112). The 3 mm thick coating was applied to a 3 mm thick S235 steel sheet via air spraying. Selected parameters of the coating are presented in Table 1. The samples for erosive wear tests were in the form of plates with dimensions of 30 mm × 10 mm × 6 mm and were cut from a coated sheet via water jet cutting to avoid heating the coating (Figure 1).
In this study, brown fused aluminum oxide was used as erosive grains. Its basic properties are shown in Table 2. Aluminum oxide with five different grain sizes, defined according to the FEPA standard as F70, F90, F120, F150 and F220, was selected for this study (Table 3). In addition, microscopic images of the grains were taken to show their shape (Figure 2).

2.2. 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 m3/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 erosive grain (the angle at which the sample is inclined) used in the tests were 90°, 60° and 45°. The mass of the delivered aluminum oxide for each test was constant for all grain sizes used, at 100 ± 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 °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 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.
Using statistical methods is important for analyzing the repeatability of the measurements and the stability of the test conditions. It made it possible not only to compare the tested samples more precisely, but also to formulate correct conclusions.
Microscopic examination of the samples before and after the wear process was carried out using a Phenom 5G scanning electron microscope (ThermoFisher Scientific, Waltham, MA, USA). Microscopic images were acquired under low-vacuum conditions with an accelerating voltage of 10 kV, at 1000× magnification. Profilometric studies were carried out using a Leica DCM8 optical profilometer (Leica Microsystems, Wetzlar, Germany). A profilometric analysis was carried out in confocal mode at 5× magnification, with the sample surface illuminated using a green LED. Analysis of the images obtained from the Leica DCM8 profilometer was carried out using the custom software package Leica Map. A scanning electron microscope is commonly used for detailed studies of the wear resistance of coatings [4,32,47].

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 μm. 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 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 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° and 60° angles, the graphs show a similar pattern, and the differences between the grain sizes are clearly visible. At a 90° 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.
In Figure 8 the effect of the incidence angle on the surface roughness after the wear process is presented. The relationship is decreasing and non-linear. As the incidence angle decreases, the rate of increase in roughness intensifies. The higher surface roughness at smaller incidence angles occurs due to the different nature of the interaction of the aluminum oxide particle on the coating surface. At high angles, the particles cause the surface to crack, while at low angles, plowing occurs. The bruises formed by the abrasive activity of the particles results in changes in roughness to a greater extent than the cracks formed by the erosive action of the particles.
Figure 9, Figure 10 and Figure 11 show a comparison of the coating surface before and after erosion at 45°, 60° and 90°, respectively, using F120 grains as a representative example. The wear marks at all grain sizes have a similar appearance, varying in size and quantity. Microscopic images reveal differences in the surface appearance depending on the incidence angle of erosive particles. At a 90° incidence angle, numerous cracks can be seen on the surface of the sample—the impacting particles cause fatigue-like cracking, i.e., cracks are not caused by a single impact but by the repeated, intense impacts of particles on the same areas. As the process continues, the material begins to chip at the cracks, leading to material loss. As the incidence angle decreases, fewer microcracks can be observed; instead, there is a greater proportion of material deformation in the form of bruises. The formed uplifts are peeled off by the impact of successive grains. The wear product formed by the impact of an erosive particle is smaller than the wear product formed by the abrasive action of the particle. This is caused by the fact that the erosive action of the particle, which leads to cracking and eventually spalling, occurs on a significantly smaller surface area than the abrasive action of the particle, which leads to the formation of uplift, which is peeled off at a later stage, hence the relationship discussed earlier showing greater wear at lower incidence angles. This occurs because the particles impacting at low angles first impact and then rub against the surface of the sample, similar to abrasion, causing cracks followed by deformation of the material. At an incidence angle of 60°, an in-between form (45° and 90°) of wear marks is visible. The images also show fragments of aluminum oxide (bright spots) embedded in the surface of the coating, further affecting its condition. At a 45° incidence angle, similar wear marks to those formed after abrasive wear were observed (Figure 12).
For an incidence angle of 45°, a similarity of wear marks to those formed after abrasive wear was observed (Figure 12).

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°), 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.

Author Contributions

Conceptualization, J.S.; methodology, J.S.; 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, J.S. 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.

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Figure 1. Shape and dimensions of the erosive wear test specimen.
Figure 1. Shape and dimensions of the erosive wear test specimen.
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Figure 2. Microscopic images of the aluminum oxide used in this study: (a) F70, (b) F90, (c) F120, (d) F150, (e) F220 [27].
Figure 2. Microscopic images of the aluminum oxide used in this study: (a) F70, (b) F90, (c) F120, (d) F150, (e) F220 [27].
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Figure 3. Experimental test rig: (a) Photo. (b) Diagram: 1—nozzle; 2—erosive stream; 3—sample.
Figure 3. Experimental test rig: (a) Photo. (b) Diagram: 1—nozzle; 2—erosive stream; 3—sample.
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Figure 4. Examples of wear trace profiles for different incidence angles of the F120 erosive particles: (a) 45°, (b) 60°, (c) 90°.
Figure 4. Examples of wear trace profiles for different incidence angles of the F120 erosive particles: (a) 45°, (b) 60°, (c) 90°.
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Figure 5. Examples of three-dimensional wear trace images for different incidence angles of the F120 erosive particles: (a) 45°, (b) 60°, (c) 90°.
Figure 5. Examples of three-dimensional wear trace images for different incidence angles of the F120 erosive particles: (a) 45°, (b) 60°, (c) 90°.
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Figure 6. Effect of incidence angle of erosive particles on erosive wear of polyurea coating for different erosive grain sizes.
Figure 6. Effect of incidence angle of erosive particles on erosive wear of polyurea coating for different erosive grain sizes.
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Figure 7. Effect of erosive grain size on erosive wear of polyurea coating for different incidence angles.
Figure 7. Effect of erosive grain size on erosive wear of polyurea coating for different incidence angles.
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Figure 8. Effect of incidence angle on the surface roughness of the polyurea coating after the erosive wear process: (a) Ra parameter, (b) Sa parameter.
Figure 8. Effect of incidence angle on the surface roughness of the polyurea coating after the erosive wear process: (a) Ra parameter, (b) Sa parameter.
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Figure 9. Microscopic images of surfaces of polyurea coatings after erosive wear tests at incidence angle of 45°, with different erosive grain sizes: (a) before test, (b) F70, (c) F90, (d) F120, (e) F150, (f) F220.
Figure 9. Microscopic images of surfaces of polyurea coatings after erosive wear tests at incidence angle of 45°, with different erosive grain sizes: (a) before test, (b) F70, (c) F90, (d) F120, (e) F150, (f) F220.
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Figure 10. Microscopic images of surfaces of polyurea coatings after erosive wear tests at incidence angle of 60°, with different erosive grain sizes: (a) before test, (b) F70, (c) F90, (d) F120, (e) F150, (f) F220.
Figure 10. Microscopic images of surfaces of polyurea coatings after erosive wear tests at incidence angle of 60°, with different erosive grain sizes: (a) before test, (b) F70, (c) F90, (d) F120, (e) F150, (f) F220.
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Figure 11. Microscopic images of surfaces of polyurea coatings after erosive wear tests at incidence angle of 90°, with different erosive grain sizes: (a) before test, (b) F70, (c) F90, (d) F120, (e) F150, (f) F220.
Figure 11. Microscopic images of surfaces of polyurea coatings after erosive wear tests at incidence angle of 90°, with different erosive grain sizes: (a) before test, (b) F70, (c) F90, (d) F120, (e) F150, (f) F220.
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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°, (b) abrasive wear.
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°, (b) abrasive wear.
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Table 1. Selected properties of the polyurea coating.
Table 1. Selected properties of the polyurea coating.
Shore HardnessDensity [g/cm3]Tensile Strength [MPa]Elongation at Break [%]
95 ShA1.1021.0425
Table 2. Selected properties of the aluminum oxide used in this study [43].
Table 2. Selected properties of the aluminum oxide used in this study [43].
Properties
Al2O3 content94.5 ÷ 97%
AdmixturesTiO2–2.5 ÷ 3.2%
SiO2–0.5 ÷ 0.8%
Fe2O3–0.4%
CaO + MgO–0.4 ÷ 0.8%
Specific density3.9 ± 0.05 g/cm3
Bulk density1.52–1.87 g/cm3
(depending on the granulation)
Grain shapeSharp-edged
Hardness9 on the Mohs scale
Table 3. The size of the aluminum oxide grains used according to the FEPA standard.
Table 3. The size of the aluminum oxide grains used according to the FEPA standard.
MarkingGrain Size [µm]
FromTo
F70212250
F90150180
F120106125
F15075106
F2205375
Table 4. Summary of erosive wear test results for polyurea coating (We—linear erosive wear; σ—standard deviation; c—confidence).
Table 4. Summary of erosive wear test results for polyurea coating (We—linear erosive wear; σ—standard deviation; c—confidence).
Incidence Angle Erosive Wear [μm]
F70F90F120F150F220
W ¯ e σc W ¯ e σc W ¯ e σc W ¯ e σc W ¯ e σc
45°12911141287915291213415191122024
60°4546638106213165311143034
90°221512155627111316671634
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Sokolska, J.; Sokolski, P. Effect of Grain Size and Incidence Angle on Erosive Wear of Polyurea Coating. Appl. Sci. 2025, 15, 7568. https://doi.org/10.3390/app15137568

AMA Style

Sokolska J, Sokolski P. Effect of Grain Size and Incidence Angle on Erosive Wear of Polyurea Coating. Applied Sciences. 2025; 15(13):7568. https://doi.org/10.3390/app15137568

Chicago/Turabian Style

Sokolska, Justyna, and Piotr Sokolski. 2025. "Effect of Grain Size and Incidence Angle on Erosive Wear of Polyurea Coating" Applied Sciences 15, no. 13: 7568. https://doi.org/10.3390/app15137568

APA Style

Sokolska, J., & Sokolski, P. (2025). Effect of Grain Size and Incidence Angle on Erosive Wear of Polyurea Coating. Applied Sciences, 15(13), 7568. https://doi.org/10.3390/app15137568

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