Research on Polymer Wear under Water Conditions: A Review

Abstract

Polymeric materials are widely used in aerospace, biomedical, marine, and agricultural applications due to their viscoelasticity and corrosion resistance. Polymeric materials fail due to wear during their service life, so studying their wear behavior is essential to control and predict their service life. This paper summarizes the progress of water lubrication research as well as experimental studies on the wear of polymeric materials under aqueous conditions. The effects of lubrication conditions, material formulation ratios, load, sliding speed, impact angle, abrasive particles, and temperature factors on the wear behavior of commonly used polymeric materials ideal for water lubrication (NBR, SBR, NR, EP, polyethylene, and their composites, etc.) are summarized in terms of the three most frequently occurring forms of wear, namely, two-body wet sliding wear, two-body erosive wear, and three-body wet abrasive particle wear. The results show that the mechanical properties, such as hardness, can be effectively changed by altering the formulation ratios of the materials, and the hardness and hydrophilicity of the formulations can further affect the wear and lubrication. In general, the coefficient of friction and the wear rate decrease with the increase in hardness, and the increase in temperature leads to the localized lubrication failure and the aging of the materials, which in turn leads to the intensification of wear. Among the working condition factors, load and sliding speeds are the most important factors affecting the wear, and the wear rate increases with the increase in the load and sliding speed; in contrast, the three-body wet abrasive wear is more obviously affected by the load. In the study of the impact angle effect, the overall trend of the erosion wear rate with the increase in the angle shows the first rise and then fall, the maximum value is mostly concentrated in the 45–60° between. Usually, the increase in the abrasive particle size can make the wear rate increase. Overall, the three-body wet abrasive wear of the rubber material wear rate shows first an increase and then a decrease. The research in this paper provides theoretical support and reference ideas for the tribological study of polymer materials in the water environment and puts forward the outlook for future water lubrication and material improvement of the research directions and applications.

1. Introduction

Polymer materials have the advantages of good viscoelasticity, corrosion resistance, fatigue resistance, easy molding, etc., and at the same time, due to their economic advantages, they make the application value higher and propose a wider range of applications. They can be used in aerospace [1], marine [2], exploration [3], industrial machinery [4], and other fields. Polymer materials can be prepared according to specific working conditions and are indispensable engineering materials for the defense industry and national economy.

With the use of polymeric materials, friction can cause wear between components, leading to the failure and shortened service life of mechanical parts. Component failure consists of two main aspects, wear damage and lubrication failure, the end result of which will also lead to wear. Reducing the wear between two sliding surfaces has been the focus of research. Distinguished from materials such as ceramics [5,6,7] and metals [8,9,10,11], polymer materials exhibit different wear patterns due to their ductility and viscoelasticity [12,13,14].

The inherent properties of the material itself are internal factors that directly affect the wear results. Many friction experts and scholars have conducted a lot of research [15,16,17,18,19,20], and some of them aim to improve the formulation ratios of polymer composites to enhance their specialized mechanical properties. The level of the material’s hardness directly affects the actual contact area between the wear partners, and is therefore often used to study the relationship with wear behavior. The development of modified polymers and polymer matrix composites has also put forward higher requirements for tribological studies of polymers [21,22,23].

Another important factor affecting wear performance is the lubrication conditions. The common lubrication methods are water lubrication [24,25], oil lubrication [26,27,28], and grease lubrication [5]. As environmental pollution continues to increase, the green industry has become a global hot topic. The application of water, with its clean and environmentally friendly characteristics, is bound to become more and more widespread. Therefore, the study of the wear performance under water lubrication conditions has very important engineering applications and environmental significance.

In order to predict the service life of polymeric materials under abrasive service conditions, a systematic study of the mechanism and evolution of their wear behavior is required, and experimental research is the main method [29]. Among them, two-body sliding wear, two-body erosive wear, and three-body abrasive wear are the three common forms of wear under aqueous conditions. Load and sliding speeds are necessary for wear to occur, grain size as well as orientation affect the load and stress on the material in contact, and grain orientation is influenced by the velocity and impact angle. Therefore, load, sliding speed, impact angle, and abrasive grains have a great influence on abrasive wear. In the practical application in the working environment, usually the forms of wear, at the same time or alternately, using comparative analysis have rarely been studied. The comprehensive study of the different types of wear behavior of polymer materials is of profound significance when revealing the wear law and mechanism of water-lubricated components in engineering applications and reducing the loss of wear energy and material consumption.

This review summarizes the effects of operating conditions, such as lubrication conditions, material formulation ratio, load, sliding speed, abrasive particles, and temperature, on the tribological behavior of polymer materials from the abovementioned three types of wear. This study provides ideas for the modification of polymer wear-resistant materials under the working condition of a water environment, and provides a theoretical basis for the selection and life prediction of ideal water lubrication materials. It also provides a feasible research basis for related simulation and modeling studies, such as finite element analysis [27] and molecular dynamics [30,31,32,33,34]. This study aims to keep the readers informed of the research progress of various types of wear behavior and to provide a reference for future research. The general structure of this paper is shown in Figure 1.

2. Lubrication Conditions

2.1. Water Lubrication

Under various operating conditions, friction inevitably leads to the wear and tear of mechanical components, resulting in failure or a reduced service life. Hence, selecting appropriate lubrication conditions is crucial to minimizing these effects and ensuring the reliable operation of machinery. This paper delves into studies related to water lubrication, investigating how water-based lubrication can be effectively employed to mitigate friction and extend the lifespan of mechanical parts. Numerous scholars have undertaken extensive testing to examine the impact of water lubrication on the wear behavior of polymer materials. For example, Rajdeep et al. conducted a study on the frictional behavior of bamboo powder-reinforced polymer matrix composites against stainless-steel discs, using a pin-on-disc friction machine under wet sliding conditions. Their research revealed that, following water lubrication, as the sliding distance increased from 200 m to 2000 m, the composites with 2.5% and 5% bamboo filler exhibited the lowest specific wear rate (SWR) values. Conversely, composites with a higher bamboo filler content, specifically 10% and 12.5%, showed increased SWR values. This suggests that, while moderate bamboo filler content can enhance the wear resistance under water-lubricated conditions, an excessive filler content may lead to higher wear rates [35]. Yang and his team employed the SSB-100 Block and Ring Friction Tester to evaluate the wear performance of nitrile butadiene rubber (NBR) materials used in marine water-lubricated stern tube bearings. Their study revealed that, in water, the coefficient of friction is inversely related to the sliding speed, regardless of the sediment type or particle size. Additionally, as the applied load increased from 100 N to 300 N, the larger load effectively prevented particles from entering the friction pair, thereby influencing the wear characteristics [36]. Liu et al. investigated the microbubble phenomenon in the contact region between soft tribological interfaces under sliding conditions [37]. Lv et al. studied the tribological behavior of nitrile rubber (NBR) under varying CO₂ gas flow rates using impregnation and abrasion tests. Their results demonstrated that CO₂ gas bubbles infiltrating the water caused significant damage to the rubber’s cross-linking network, leading to defects, such as cracks, holes, and lamellar bumps, on the rubber surface. The presence of CO₂ bubbles in the water lubrication film was found to reduce the steady-state friction coefficient of the rubber. Despite the different CO₂ gas flow rates, the steady-state friction coefficient in water remained consistently around 0.1 [38]. Gao et al. investigated the tribological performance of epoxy (EP) composites under water lubrication conditions. Their findings showed that the coefficient of friction for the epoxy material decreased over time. Specifically, at a sliding speed of 0.2 m/s, the coefficient of friction reduced from 0.24 to 0.2. At a higher sliding speed of 1.8 m/s, the coefficient of friction decreased more significantly, from 0.18 to 0.08. These results indicate that the presence of water lubrication improves the frictional performance of epoxy composites, with a greater reduction in friction observed at higher sliding speeds [39]. Wang et al. investigated the micromotion-induced friction and wear characteristics of ultra-high-molecular-weight polyethylene (UHMWPE) with varying surface roughness under aqueous lubrication conditions, using SRV-4 micromotion friction and wear testing apparatus. This study revealed that, under the water lubrication condition, an increase in the surface roughness of the UHMWPE from 0.07 μm to 0.50 μm resulted in a higher specific wear rate, which rose from 6 × 10⁻⁶ mm³/Nm to 8 × 10⁻⁶ mm³/Nm, respectively. These findings suggest that the greater surface roughness of UHMWPE under water lubrication conditions leads to increased wear [24].

2.2. Lubrication Improvement

Reducing the friction loss between two sliding surfaces has become a cutting-edge topic in the field of tribology [40,41]. Over the years, many scholars have focused on studying lubrication mechanisms (Figure 2) to reduce the coefficient of friction and minimize material losses under water-related lubrication. Zhang et al. summarized a decade of research on liquid superlubricating materials, categorizing them into five groups: water- and acid-based solutions, hydrated materials, ionic liquids (ILs), two-dimensional (2D) materials as lubricant additives, and oil-based lubricants. They demonstrated how hydrodynamics and hydration contribute to achieving liquid super lubrication [42]. Han et al. demonstrated in their research that macroscopic superlubricity can be achieved using hydrated ions (cations and anions) between Si₃N₄ (silicon nitride) balls and sapphire disks, even under high contact pressure [6,43,44,45,46]. These findings indicate that hydration, derived from the hydration layer structure, along with the tribochemical boundary layer, which is easy to shear, both contribute to providing an excellent boundary lubrication performance [47,48]. Due to water’s poor film-forming ability, Luo et al. investigated the lubricating effect of self-emulsifying esters (SEEs) and other components on the surface of a titanium alloy (TB6) using a high-frequency reciprocating linear vibration machine. The results showed that SEEs can significantly enhance the film-forming ability of aqueous solutions, and the combination of SEEs and phosphate ester in an aqueous solution can provide effective lubrication for the titanium alloy [49]. Changing the ionic potential can also help reduce the coefficient of friction. Gao et al. proposed a method to achieve a very low friction state by applying an external potential. A particular anionic surfactant solution, when subjected to a distinctively applied negative potential, was found to induce an exceedingly low friction condition on the surface of gold. This discovery revealed a unique interaction phenomenon between the surfactant solution and the gold surface under the influence of the applied potential. As the duration of the applied negative potential increased, the friction decreased from 8.3 to 3.5 × 10⁻² N, representing a reduction of 99.6%. The final coefficient of friction could be lowered to 3 × 10⁻⁴ [50]. Wang et al. synthesized P (MPC-co-SBMA) copolymerized hydrogels using two amphiphilic ionic polymers: 2-methacryloyloxyethyl phosphorylcholine (MPC) and sulfobetaine methacrylate (SBMA). When the copolymerized hydrogel hemisphere was combined with sulfide water, a superlubricated state with a friction coefficient of about 0.002 was achieved. These findings offer new insights into the mechanism of superlubrication in amphiphilic hydrogels based on the hydration effect and suggest potential applications for artificial joint cartilage [51]. Advances in lubrication research are placing greater demands on biotechnology research focused on polymeric materials [52,53,54].

3. Wear Test Studies

Wear research involves a multidisciplinary approach, utilizing a range of techniques including theoretical analysis, simulation, experimental testing, and characterization analysis. In this chapter, within the context of the experimental investigation, the influences of six distinct factors on the tribological characteristics of polymeric materials are comprehensively outlined. This study covers three principal wear mechanisms: two-body wet sliding wear, two-body erosive wear, and three-body wet abrasive wear. Figure 3a illustrates the distribution of the main factors influencing the three wear mechanisms discussed in this review. Meanwhile, Figure 3b provides a summary of the number of literature sources addressing these wear forms.

3.1. Two-Body Wet Sliding Wear

In two-body wet sliding wear (Figure 4), the wear behavior of polymeric materials is primarily influenced by factors such as the polymer material [55], the load applied to the grinding pairs [56,57,58], the sliding speed between the grinding pairs [59,60,61], and the temperature at which the grinding pairs are exposed [26,62]. In a water environment, the wear behavior differs from that under other lubrication conditions, and some scholars have explored the underlying mechanisms of this behavior.

3.1.1. Materials

Polymeric materials are prone to severe wear in engineering applications. Typically, their suitability for practical use is determined by their mechanical load-carrying capacity and specific wear rate. Both of these properties can be enhanced by incorporating hard fillers, nanoparticles, and fibers.

Some specialists focus on modifying polymer materials by adjusting their formulation to meet the specific mechanical and tribological properties required for engineering applications. Wu and colleagues conducted a comprehensive examination of the thermal stability and structural characteristics of reinforced SSBR (Solution Styrene Butadiene Rubber) composites. They employed various techniques, including Thermogravimetric Analysis (TGA), Differential Scanning Calorimetry (DSC), X-ray Diffraction (XRD), X-ray Photoelectron Spectroscopy (XPS), Field-Emission Scanning Electron Microscopy (FESEM), and Transmission Electron Microscopy (TEM). Additionally, they investigated the tribological behavior of butadiene rubber composites against marble in a wet environment through a series of sliding contact experiments. The results revealed that the average coefficients of the friction and wear rate for SSBR filled with micron silica particles were 0.265 and 1.34, respectively. For SSBR composites filled with nanoscale silica, the average friction coefficient and wear rate were 0.229 and 0.232, respectively. Meanwhile, the SSBR composites filled with carbon black (N660) exhibited a mean frictional coefficient and wear rate of 0.188 and 0.105, respectively, under wet operating conditions. Notably, the inclusion of N660 effectively reduced both the friction coefficient and abrasion levels experienced by the marble blocks, demonstrating a significant reduction [21]. Song et al. investigated the tribological properties of three NBR (nitrile butadiene rubber) samples with acrylonitrile mass percentages of 18%, 26%, and 41%, and corresponding hardness values of 71, 73, and 74 (Shore A). Unidirectional sliding wear tests were conducted in humid conditions to assess their properties. The results showed that, as the acrylonitrile content in the NBR samples increased from 18% to 41%, the coefficient of friction initially increased from 0.113 to 0.126 and then decreased to 0.116. Additionally, the abrasion decreased from 10.5% to 5.4%. This indicates that the abrasion resistance of NBR improves with increasing the acrylonitrile content [63]. Paveena and her team explored the influence of varying proportions of bagasse ash silica (BASi), ranging from 0 to 15 phr (parts per hundred rubber) in increments of 5 phr, on the tribological properties of styrene–butadiene rubber (SBR) under wet conditions. The hardness (Shore A) values of the BASi/SBR composites with different BASi contents was measured as 57.25, 57.75, 59.25, and 60.75 ± 0.50. The results show that, as the BASi addition increases from 0 to 15 phr, the hardness of the material also increases from 57.25 to 60.75. However, the specific wear rate of the SBR/BASi composites increased from 11.82 ± 2.35 to 13.69 ± 0.19, indicating a decrease in the abrasion resistance with a higher BASi content [64].

In order to ensure that resin materials exhibit high-quality mechanical properties and wear resistance in various applications, some researchers have investigated the impact of varying compositions on the tribological characteristics of these materials. By analyzing how changes in material composition affect properties like friction, wear resistance, and durability, these studies aim to optimize the performance of resin materials in real-world conditions. Gao and his research team conducted an investigation to evaluate the influence of reinforced fillers, specifically carbon fiber, graphite, and silica nanoparticles, on the friction and wear properties of epoxy resin (EP) under water lubrication conditions. The outcomes of this investigation, presented in Figure 5A, reveal a significant decrease in the specific wear rate from 10.5 × 10⁻⁷ mm³/Nm to 0.7 × 10⁻⁷ mm³/Nm with the inclusion of carbon fibers in the EP. Furthermore, the addition of silica nanoparticles to EP composites already reinforced with carbon fibers and graphite led to a further reduction in both friction and wear. The scanning electron microscopy (SEM) results supporting these findings are shown in Figure 5B [39]. Yang et al. prepared and investigated a series of epoxy resin (EP) composites filled with layered nickel silicate (NiPS) nanofluids, with varying filler percentages of 1%, 3%, 5%, and 7%. This study focused on examining the tribological properties of these composites under wet sliding conditions, providing an in-depth analysis of how different NiPS filler concentrations influence the friction and wear behavior of the EP composites. The results show that the material hardness increased from 87.6 HD with 0% filler to 90.0 HD with 7% filler. The wear rate exhibited an inverse parabolic trend as the NiPS filler content increased, reaching a minimum of 0.82 × 10⁻⁶ mm³/Nm with 3% filler. This indicates that the abrasion resistance improved as the filler content increased up to 3%, after which it began to decrease with higher filler percentages [65]. Bicer et al. conducted a study comparing the two-body wear characteristics of hybrid, micro-hybrid, and nano-hybrid direct and indirect composite resins. An examination was conducted using a pin-on-disk wear apparatus to analyze the influence of water on tribological behavior, specifically focusing on the two-body wear rate, of six distinct composite materials. The results showed that the average wear rate of the nanocomposite resin was 73 × 10⁻⁴ mm³/Nm, while that of the micro-composite resin was 143 × 10⁻⁴ mm³/Nm. Additionally, the abrasive wear of the nanocomposite resin was significantly lower than that of the micro-composite resin, indicating superior wear resistance of the nanocomposite material [66].

3.1.2. Load

When a polymer composite material slides against a grinding vice, the load exerted on the grinding vice fluctuates, leading to variations in the wear patterns of the material.

Wu et al. conducted wear tests on aircraft tires to investigate the effects of different loads on the friction and wear performance of tires in the presence of water. The results showed that the friction coefficient of the tire decreased from 0.7 to 0.6 when the load was increased from 100 N to 300 N. This indicates that higher loads can reduce the frictional performance of tires in wet conditions, potentially affecting their grip and safety during operation [67]. Utilizing the orthogonal design approach, Hu and colleagues investigated the microstructural alterations in the worn surface of styrene-butadiene rubber (SBR) conveyor belts and the evolution of their wear mechanisms under alternating seawater dry and wet conditions (W-SDWs). Their findings revealed a linear increase in the friction coefficient of the SBR conveyor belts, which positively correlated with the applied load. Additionally, the dominant wear mechanism transitioned from primarily fatigue wear to a composite damage mechanism characterized by the coexistence of adhesive wear and fatigue wear. This shift in wear mechanisms underscores the complex interactions between load, environmental conditions, and material behavior in such challenging settings [68]. The rubber-plate bearing, operating with water lubrication, plays a crucial role in the propulsion shaft system of underwater vehicles, ensuring an efficient and reliable performance. Its ability to function effectively in water-lubricated environments is essential for maintaining the smooth operation of the propulsion system in challenging underwater conditions. Yang et al. investigated the effects of different loads (0.1, 0.2, 0.3, and 0.4 MPa) on the wear and friction vibration suppression mechanism of rubber slats. The results showed that the friction moment of the rubber slat increased significantly, from 40 N·m to 140 N·m, as the load was increased from 0.1 MPa to 0.4 MPa. This increase in the friction moment with higher loads highlights the impact of load on the wear characteristics and vibration suppression capability of rubber slats [69]. Budi et al. investigated the effect of different applied loads on the wear and vibration suppression mechanism of silica-reinforced styrene-butadiene rubber (SBR) during the sliding friction of a steel-blade indenter in wet-contact conditions. Using a needle-and-disk tribometer, they observed the frictional wear patterns that emerged during sliding friction. The experimental results showed that the total coefficient of friction decreased from 1.75 μ to 1.5 μ as the applied load increased from 1 N to 2 N. Additionally, the spacing between wear patterns increased with the increase in load, indicating a load-dependent alteration in the wear characteristics [70]. Styrene–butadiene rubber (SBR), as the main component of tire tread, is widely used in the rubber industry to manufacture automobile treads. Wu et al. conducted a study to elucidate the tribological characteristics and underlying wear mechanisms of styrene-butadiene rubber (SBR) in a wet environment. They used a modified MM-200 ring-on-block friction and wear testing apparatus to perform sliding wear experiments between SBR specimens and marble blocks. The results revealed a significant trend: the friction coefficient underwent a sharp decline, falling within the range from 0.1 μ to 0.2 μ, as the applied load was increased from 60 N to 90 N. Concurrently, the wear rate initially increased from 0.266 to 0.521, followed by a rapid decrease to 0.222. These findings, as illustrated in Figure 6, highlight the complex relationship between the load, friction, and wear rate of SBR in wet conditions [71]. Hua et al. investigated the effect of loading history on the friction and wear behavior of a PESU-based nanocomposite/steel friction system. The results showed that the wear of the nanocomposites increased significantly when the loading conditions were higher than the critical loading limit of 2 Mpa. The reason for this is that the sulfur dioxide (SO₂) decomposed by PESU undergoes a redox reaction with iron, leading to significant changes in the steel surface [72].

3.1.3. Sliding Speed

When studying the effect of the sliding velocity on polymer composites, many scholars have employed friction and wear testers to explore the relationship between various parameters, such as the friction coefficient, wear rate, wear volume, and sliding velocity.

Yang et al. investigated the trend of friction torque and the coefficient of friction of water-lubricated rubber-plate bearings under varying speeds. The selected speeds for the study were 100, 200, 300, 400, and 500 r/min. The results of the wear test revealed that both the frictional torque and the coefficient of friction of the rubber plate decreased by 15.5% as the speed was increased from 100 to 500 r/min under the same conditions. This decrease indicates that higher rotational speeds can lead to reduced frictional forces in water-lubricated rubber bearings, which may contribute to improved performances in certain applications [69]. Wang et al. conducted an experimental study to investigate the friction and wear characteristics of water-lubricated rubber- plate bearings using a high-speed ring-block friction and wear tester. This study focused on analyzing the effect of varying sliding velocities on the coefficient of friction. A thorough experimental investigation was performed, and the SEM image of the tested surface is shown in Figure 7A. The results indicate that, under water lubrication conditions, the coefficient of friction remains relatively stable with the increasing speed. However, as the speed increases, the associated rise in frictional heat can lead to the onset of dry friction. Additionally, the wear rate was observed to increase linearly with the increase in speed [73]. In their study of resin-based materials, Michael et al. examined the effect of different sliding speeds on the coefficient of friction between PEEK (polyether ether ketone) and human tooth enamel under wet conditions. They employed a rotational block-on-ring test setup, with boundary lubrication using deionized water, to measure the sliding friction and the resulting material degradation. The results demonstrate that, during the initial stage, the coefficient of friction decreases from 0.43 μ to 0.27 μ as the rotational speed increases from 0.1 m/s to 1 m/s, respectively, indicating a significant reduction in friction with increased speed. This trend, as illustrated in Figure 7B, highlights the role of speed and lubrication in mitigating friction between these materials [53].

3.1.4. Temperature

Temperature plays a crucial role in the aging and failure of polymer materials. To address this, many experts conduct comparative tests that control temperature variables to determine the optimal working temperature, with the goal of extending the service life of these materials.

Water-lubricated rubber bearings are among the most suitable bearings widely used in submersible pump drainage systems. To better understand their performance, a cylindrical dynamic friction model was developed based on the LuGre friction model, taking into account the effects of wet aging and thermal aging on rubber seals.

As shown in Figure 8a, the friction analysis at different temperatures was conducted with a compression ratio of 80%. The friction curve was then fitted using a steady-state friction model. Figure 8b illustrates the Coulomb friction coefficients and static friction coefficients at varying temperatures and aging times. The results indicate that the static coefficient of friction decreases from 0.935 to 0.855 as the aging temperature increases from 20 °C to 80 °C, respectively. This decrease highlights the impact of thermal aging on reducing friction, which is critical for optimizing the longevity and performance of rubber bearings in such applications [74]. Wang et al. tested the friction and wear characteristics of water-lubricated rubber/nickel-coated pairs under water-lubricated conditions using a high-speed ring-block friction and wear tester. This study revealed that, under high-speed conditions, the heat generated from friction causes an increase in temperature, leading to the evaporation of water. This reduction in lubrication results in a linear increase in the wear rate as the temperature rises, emphasizing the critical role of temperature control in maintaining the integrity of water-lubricated systems [73].

Rajdeep and his team conducted a tribological analysis of bamboo filler-reinforced polymer composites, assessing their performance under wet and heated sliding conditions using a pin-on-disk tribometer. Their findings revealed an interesting trend: as the temperature increased from 40 °C to 80 °C, the specific wear initially rose from 7.5 × 10⁻⁵ mm³/Nm to 12 × 10⁻⁵ mm³/Nm, respectively, but then it decreased significantly to 2.5 × 10⁻⁵ mm³/Nm. This suggests a complex interaction between the temperature and wear behavior in the composites, where certain conditions may lead to reduced wear, despite the higher temperatures. [35].

Table 1. Summary of experimental parameters in the two-body wet sliding wear condition.

Type	Load (N)		Ref.	Other Parameters
				Sliding Speed (m/s)	Temperature (°C)	Materials
Rubber	0–50	10	[64]	0.1	Room Temperature	NR, SBR, NR/SBR
		1.0, 1.5, 2.0	[70]	0.05	Room Temperature	SBR
		50	[74]	0–0.1	20, 40, 60, 80	Rubber
		10, 50, 100	[68]	0.00083	25 ± 3	SBR
	50–100	50, 60, 70, 80, 90, 100	[21]	20 0 (r/min)	15–25	SSBR
		60, 70, 80, 90	[71]	200 (r/min)	15–25	SBR
	>100	200	[63]	1.4	Room Temperature	NBR
		100, 200, 300	[67]	0.06	Room Temperature	Rubber
		300	[73]	900 (r/min)	20	Rubber
Plastic/Resin/Fiber	0–50	50	[54]	0.0026	5, 37, 55	PMMA, PEEK
		15	[66]	0.6	25 ± 3	Resins
		20–40	[35]	200–2000 (m)	Room Temperature	Bamboo Filler
	50–100	100	[53]	0.1, 0.5, 1	18.8 ± 0.63	PEEK
		100	[39]	0.1, 0.2, 0.6, 1.0, 1.8, 3.0	Room Temperature	EP
	>100	120	[65]	100, 125, 150, 175, 200 (r/min)	Room Temperature	EP

offers a comprehensive review of previous studies, summarizing the specific parameters of load, sliding velocity, and temperature used in various polymer material experiments. This table is designed to provide readers with a clearer understanding of the experimental conditions and outcomes across different studies, facilitating comparisons and insights into how these factors influence the performance of polymer materials exposed to different conditions.

3.2. Two-Body Erosion and Wear

Two-body erosive wear (as shown in Figure 9) is a common wear mode characterized by the loss of surface material due to the relative motion that occurs when a solid surface contacts a flowing liquid containing abrasive particles. Unlike sliding wear, erosive wear is primarily influenced by factors such as the impact angle [75,76,77], impact velocity [78], particle size [79,80,81], and the composite formulation ratio [62,82,83]. In contrast to dry sand erosion, erosion under fluid-solid coupling conditions in aqueous environments is more complex. This complexity arises from interactions between the liquid, solid particles, and the material surface. As humans’ exploration and utilization of the ocean continue to expand, marine tribology has become an increasingly significant field. In order to better prevent the failure of materials affected by erosion wear in the ocean, scholars are actively studying detection schemes [84,85].

3.2.1. Materials

The mechanical properties of polymers can be effectively enhanced to minimize erosive wear by adjusting the formulation ratios and incorporating modified fillers. These modifications help to improve the material’s resistance to wear, making it more durable under conditions where erosive wear is a concern.

Nie et al. investigated the effect of seawater on the tribological properties of NBR with varying acrylonitrile contents (18%, 26%, and 41%) through static dissolution and erosion tests. The hardness of the materials was measured at 74.8, 75.1, and 75.7. This study revealed that, as the acrylonitrile content increased, the wear rate significantly decreased from 33.97 to 12.07, indicating improved wear resistance. This suggests that a higher acrylonitrile content enhances the material’s durability in erosive environments, particularly in seawater conditions [86]. Chen Y et al. developed composites with varying ratios of carbon fibers (CFs), polyaniline (PANI), and epoxy (EP) coatings to test their corrosion resistance. The results indicate that the volume loss of the modified composite PANI4@CF5/EP decreases from 240 mm³ to 120 mm³ compared to pure EP. Similarly, the volume loss of CF5/EP decreased from 240 mm³ to 140 mm³. These findings demonstrate that the addition of carbon fibers significantly improves the wear resistance, enhancing the durability of the composites in water [87]. Xu and colleagues conducted an in-depth investigation into the erosive degradation behavior of hydrophobic silica-reinforced biomimetic epoxy resin composites, focusing on samples with different proportions of reinforcing agents. This study revealed a complex trend: as the concentration of hydrophobic silica increased from 0% to 6%, the material’s rigidity (measured as HV0.1 hardness) initially increased from 20.5 to 26.8, respectively, before slightly decreasing to 24.2. Concurrently, the erosion velocity of the epoxy resin matrix decreased from 1.86% to 1.5%, but then slightly increased to 1.53%. This study identified 4% hydrophobic silica content as the optimal concentration, where the erosion rate dropped to its lowest point of 1.494%, indicating the most effective protection against erosive degradation. This concentration also corresponded to the highest hardness level, highlighting its superior performance in terms of abrasion resistance [88].

Hernandez and colleagues conducted a comprehensive study to evaluate the impact of epoxy resin matrix composites reinforced with glass fibers (GFRPs), carbon fibers (CFRPs), and hybrid fiber combinations (HFRPs) on the physical properties and tribological performance under abrasive wear conditions. The Shore D hardness values of the composites after reinforcement were 88.66 for the GFRP, 88.43 for the CFRP, and 88.22 for the HFRP. This study revealed significant findings: over time, the mass loss of the GFRP, CFRP, and HFRP composites were 5.1 × 10⁻³ g, 3.0 × 10⁻³ g, and 1.7 × 10⁻³ g, respectively. These results indicate improved abrasion resistance, with carbon fiber-reinforced composites (CFRPs) showing greater effectiveness in enhancing wear resistance compared to glass fibers (GFRPs). The hybrid fiber combination (HFRP) exhibited the best overall performance in terms of reducing mass loss under abrasive conditions [14]. Jiang and colleagues conducted a study on the impact of different alumina (Al₂O₃) concentrations—0 wt%, 2 wt%, 4 wt%, 6 wt%, and 8 wt%—on the erosive wear resistance of high-density polyethylene (HDPE) composites reinforced with bamboo fibers. The hardness (HD) values of the composites were measured as 61.1, 64.8, 66, 67.9, and 67. The spinning water-jet technique was used to evaluate erosive wear. The results, as depicted in Figure 10a, indicate that the composite with a 6 wt% Al₂O₃ concentration (hardness: 67.9) exhibits the best wear resistance. Additionally, Figure 10b shows the SEM images of the five different composites after impact wear testing, highlighting the surface characteristics and wear mechanisms at various concentrations. This study demonstrates that an optimal Al₂O₃ content (6 wt%) significantly enhances the wear resistance of bamboo fiber-reinforced HDPE composites [89]. Talib et al. studied the impact of different nanoclay ratios (1%, 3%, and 5%) on the wear performance of basalt fiber-reinforced polymer (BFRP) composites under slurry tank wear conditions, as illustrated in Figure 10c. The hardness values (HRRs) of the composites were recorded as 121.58 ± 0.70, 122.15 ± 0.44, and 121.63 ± 0.34. Scanning Electron Microscopy (SEM) was used to examine the wear surface features of these nanoclay-modified BFRP composites, with the SEM images presented in Figure 10d. This study’s findings reveal that the incorporation of nanoclay significantly improves the abrasion resistance of the BFRP composites. The wear rate of the EP/BF composites was 3.4432 × 10⁻² mm³/g, while the wear rate of the EP/BF/1NC composite specimens decreased to 1.6797 × 10⁻² mm³/g. However, as the nanoclay content increased beyond 1%, the wear rate also increased, indicating a reduction in abrasion resistance at higher nanoclay concentrations. This suggests that, while a small amount of nanoclay enhances the wear resistance, an excessive nanoclay content may negatively affect the composite’s durability [22]. Prabina et al. prepared epoxy composites reinforced with polyester fibers and varying contents of blast furnace slag (0%, 5%, 10%, and 15% by weight) to investigate their abrasion behavior in mud. The hardness values of the composites were measured as 23.9, 28.7, 33.3, and 35.2. This study showed that, as the content of blast furnace slag increased, the wear rate of the composites significantly decreased from 39 mm³/Nm to 20 mm³/Nm. However, despite the reduction in the wear rate, the overall abrasion resistance of the composites diminished. This indicates that, while the addition of blast furnace slag improves the hardness and reduces the wear rate, it may also lead to a decrease in the composite’s ability to resist abrasion effectively under certain conditions [90].

3.2.2. Abrasive Grain

In the erosion wear experiment, the size of abrasive particles directly affected the effect of surface erosion. Ojala et al. conducted tests on natural rubber and polyurethane lining materials, using a wide range of particle sizes, ranging from coarse 8/10 mm granite particles to finer 0.1/0.6 mm quartz particles. A 3D profilometer image of the elastomer wear surface is provided in Figure 11A. This study found that, in general, elastomers exhibited a better wear performance, with a reduction in the wear loss of 40–95% when edge wear was suppressed. This indicates that both large and fine elastomers are effective in minimizing wear when abrasive particles do not cause significant edge damage. However, as the size of the abrasive particles increased, edge wear became more pronounced, leading to greater material loss at the edges. Additionally, with larger particles, the phenomenon of particle nesting decreased, meaning that larger particles were less likely to embed in the elastomer surface, potentially leading to more extensive wear in those regions. These findings suggest that controlling the size of abrasive particles and minimizing edge wear are key strategies for enhancing the durability of elastomer linings in abrasive environments [91]. Suihkonen et al. explored the erosive wear characteristics of glass fiber-reinforced vinyl composites (FRPs) in slurries containing different abrasive grain sizes. These tests were conducted in high-temperature water and acidic environments to simulate challenging operating conditions. This study found that, when the abrasive material was changed from fine quartz (50–200 μm) to coarse quartz (100–600 μm), there was a significant increase in the polymeric material’s volume loss on the pressure-bearing surface, which rose from 0.7% to 1.2% by weight. Conversely, on the suction side of the material, the volume loss decreased from 1.75% to 1.5% by weight. This data are depicted in Figure 11B. These findings suggest that larger abrasive particles contribute to greater erosion on the pressure-bearing surface, likely due to their increased impact force and the resulting material displacement. However, the reduction in volume loss on the suction side with coarser particles indicates a complex interaction between the abrasive size and the wear mechanisms, where finer particles may cause more significant material removal from certain areas due to factors like particle nesting or different erosion dynamics [92].

Some research experts have utilized the univariate method to examine the surface erosion patterns resulting from changes in the abrasive grain concentration through comparative tests. Padmaraj et al. specifically studied the effect of slurry concentration on the erosion behavior of carbon/epoxy quasi-isotropic laminates. In their experiments, they found that, at a slurry concentration of 70%, the maximum mass loss recorded was 0.321 g. This result highlights the substantial impact that slurry concentration can have on the erosion of composite materials, with higher concentrations leading to more significant material loss [93]. Pankaj and his team conducted a study on the erosive wear characteristics of hybrid polymer composites reinforced with glass and steel fibers, examining their performance at varying slurry concentrations. Their findings revealed that, as the slurry concentration increased within the range of 160 to 265 g/min, the composites experienced more severe damage. This indicates a direct correlation between higher slurry concentrations and the extent of wear and degradation in these hybrid composites, underscoring the importance of optimizing slurry conditions to minimize erosion in such materials. This study provides valuable insights into the durability of hybrid polymer composites in erosive environments, particularly in applications where exposure to high slurry concentrations is inevitable [94].

3.2.3. Impact Angle

The impact angle is an important basis for studying the behavior of erosive wear. The measurement of the erosion rate as a function of the impact angle reveals distinct response behaviors among various polymers. This relationship between the impact angle and erosion rate helps in characterizing the wear resistance of various polymers and provides insights into their performance under different operational conditions.

Many experts have conducted erosive wear experiments on different types of polymers under conditions involving abrasive particles and water impact. Wichain and colleagues specifically investigated how varying erosion angles affect the corrosion resistance of neoprene rubber. Their results indicated that the erosion rate of neoprene (CR) was the highest at an impact angle of 15°. Interestingly, at a very low impact angle of 5°, the erosion rate initially increased, but then sharply decreased. For impact angles between 20° and 60°, the influence of the impact angle on erosion was much less significant, showing a relatively stable erosion rate. These findings, illustrated in Figure 12A, suggest that neoprene rubber is particularly susceptible to erosion at specific low angles, with the erosion rate diminishing or stabilizing as the impact angle increases beyond this range [95]. Yang et al. investigated the erosive wear behavior and mechanisms of chlorinated rubber coatings using a rotating-disc scouring test in a seawater environment containing sea sand. Their study examined the effects of different impact angles—22.5°, 45°, 67.5°, and 0°—on the wear of the coatings. The results revealed that the maximum erosive wear of the chlorinated rubber coating occurred at an impact angle of 67.5°, while minimum wear was observed at 22.5°. This study also identified distinct damage mechanisms depending on the impact angle. At low angles, such as 22.5°, the coating predominantly experienced damage in the form of cutting and ploughing, where the abrasive particles scraped and gouged the surface. In contrast, at higher angles, like 67.5°, the damage was primarily due to deformation and chiseling, where the particles caused more substantial material displacement and cracking [96].

Pankaj et al. fabricated hybrid polymer composites reinforced with glass and steel fibers, incorporating varying weight percentages, using the vacuum-assisted resin transfer molding (VARTM) method. Their study focused on investigating the steady-state slurry scour properties of these composites, specifically analyzing the erosion behavior at different impact angles within a slurry scour environment. The results revealed that the peak erosion rate occurred at impact angles ranging from 45° to 75°, while keeping all other variables constant. This indicates that the composites are most vulnerable to erosion within this specific range of impact angles, where the combination of abrasive particles and impact force leads to maximum material removal. This study underscores the importance of understanding how the impact angle influences erosion in hybrid polymer composites, which is crucial for optimizing their performance and durability in environments where they are exposed to slurry and similar abrasive conditions [94]. Prakash et al. investigated the erosion and abrasive wear properties of porous nano-activated carbon epoxy composites. The erosion wear rate was examined under different impact angles, with the most significant change observed in composites containing 2% activated carbon. The results indicated that all the composites exhibited semi-ductile erosive wear behavior, with the highest wear rate occurring at a 45° impact angle, as shown in Figure 12B [97]. Panchal et al. investigated the erosive wear properties of epoxy composites reinforced with uncooked and cooked eggshell fillers. This study evaluated the erosive wear behavior in mineral water conditions at various impact angles, including 30°, 45°, 60°, and 90°, to determine their specific effects. The results showed that the highest erosion rate occurred at a 60° impact angle [98].

Two-body erosive wear is not limited to jet erosion, but also includes agitated erosive wear. Ojala et al. conducted wear tests on natural rubber (NR) and polyurethane (PU) lining materials using a high-speed slurry tank at impact angles of 45° and 90°. The results showed that, under the same conditions, NR exhibited greater volume loss at a 45° impact angle compared to 90°, while PU showed the opposite wear behavior [91]. Padmaraj et al. investigated the effect of the rotational angle on the erosive properties of carbon/epoxy quasi-isotropic laminates exposed to seaweed particles. As shown in Figure 12C, erosive wear experiments were conducted at rotational angles of 15°, 30°, and 45°. The results reveal that peak erosion occurs at a 15° rotational angle, with the scouring process displaying characteristics of semi-ductile scouring behavior. A comparison of the erosive wear experiments is illustrated in Figure 12D [93].

3.2.4. Impact Speed

Impact velocity and impact angle are indeed critical factors for determining the behavior of erosive wear. In two-body erosive wear, the impact velocity of abrasive particles, such as mortar, directly influences the load applied to the material surface. As the impact velocity changes, it affects not only the erosion rate, but also the overall wear magnitude and related wear mechanisms. Higher impact velocities typically lead to the increased kinetic energy of the particles, resulting in greater material removal and a higher erosion rate. This can intensify the wear process, causing more severe damage to the material surface.

The researchers conducted comparative experiments to observe the erosive wear behavior and its evolutionary pattern. Li et al. specifically investigated the wear behavior of contaminants, such as sediment particles in water, on the rubber surface of water-lubricated rubber bearings with spiral grooves at various rotational speeds (1000, 3000, and 5000 r/min). They utilized Finne’s basic mathematical model of wear to analyze the results. This study found that, at a rotational speed of 1000 r/min, the wear on the bearing surface was more significant when the fluid was close to a laminar flow state. In this condition, the movement of sediment particles was relatively uniform, leading to consistent abrasion on the surface. However, at a higher rotational speed of 5000 r/min, the fluid transitioned to a turbulent flow state, where independent vortex phenomena appeared in the infeed grooves. These vortices caused sediment particles to aggregate within the vortex, which resulted in intensified wear erosion on the surfaces where these vortices formed [99]. Kopchenkov investigated the wear behavior of the bearing surfaces of water-lubricated rubber bearings under various impact velocities, specifically focusing on the impact wear behavior of solid particles on rubber surfaces in aqueous environments. This study’s findings indicated that, at low impact velocities (up to 15 m/s), the solid particles generated tensile stresses on the rubber surface, leading to the formation of subsurface cracks. These cracks, under the influence of liquid microjets, resulted in the removal of rubber micro-fragments from the entire contact area. As the impact velocity increased (exceeding 23 m/s), the rubber material began to experience fatigue, which led to the development of more extensive cracks in the subsurface layer. These cracks eventually propagated to the central contact zone, where the stress concentration was the highest, extending toward the main crack on the surface [100].

Panchal et al. studied the erosive wear properties of epoxy composites reinforced with raw and cooked eggshell fillers at three different impact velocities (86, 101, and 119 m/s) using an erosion test rig in a mineral water environment. The findings demonstrated a clear relationship between the impact velocity and wear behavior. As the impact velocity increased from 86 m/s to 119 m/s, the abrasion resistance of the composites decreased, resulting in a corresponding increase in the erosion rate. Specifically, the wear rate rose from 0.25 to 0.75 with the increase in impact velocity. This indicates that higher impact velocities lead to more severe material removal, reducing the effectiveness of the eggshell fillers in reinforcing the epoxy matrix against erosive forces. This study highlights the importance of considering the impact velocity when designing composite materials for erosive environments, as higher velocities can significantly diminish their wear resistance, particularly in materials reinforced with natural fillers, like eggshells [98]. Padmaraj et al. investigated the effect on the erosion behavior of carbon/epoxy quasi-isotropic laminates by using sea sand slurry as the erosive materials in the slurry basin erosion test rig. The results showed that the maximum mass loss of the composite was 0.315 g at an erosion rate of 1500 r/min [93]. Marlin and Chahine performed cavitation spraying experiments on polyurethane coating samples using cavitation jet erosion test equipment to study the relationship between erosion velocity and the erosion of polyurethane coatings. The results demonstrated that, as the velocity increased from 0.01 in/s to 0.42 in/s, the erosion effect on the polyurethane coatings became progressively more severe. The coatings thinned out and eventually disappeared under higher velocities. This indicates that the intensity of cavitation erosion is closely tied to the velocity, with higher speeds leading to more rapid and significant material degradation. Additionally, Figure 13b, illustrates the relationship between failure pressure and translation speed, further emphasizing how these factors interact to influence the durability of polyurethane coatings under cavitation conditions [101].

Pankaj et al. conducted experiments to characterize the slurry erosion rate of glass/steel fiber-reinforced polymer composites in the context of water environments. They varied the impact velocity from 10 m/s to 40 m/s to study how the target material’s surface eroded due to changes in the slurry injection velocity. The results revealed a linear correlation between the erosion rate and the impact velocity. As the impact velocity increased, the rate of fiber breakage also rose, driven by the sustained impact of abrasive particles traveling at varying speeds. This relationship, illustrated in Figure 13a, highlights how higher velocities exacerbate the erosion of these composite materials, leading to more significant surface degradation [94]. Shanmugam and his team evaluated the influence of the erosion rate on the erosion-related parameters of novel coir fiber and clay-reinforced polyester composites. They used response surface methodology (RSM) combined with analysis of variance (ANOVA) to model and analyze the central composite design. The results demonstrate that the erosion rate of these composites changes linearly as the erosion velocity increases from 70 m/s to 130 m/s. This linear relationship indicates that, as the velocity of the erosive particles increases, the rate at which the composites erode also increases proportionally [102].

Table 2. Summary of experimental parameters in the two-body erosive wear experiment.

Type	Impact Speed (m/s)		Ref.	Other Parameters
				Impact Angle (°)	Abrasive Grain (μm)	Materials
Rubber	0–10	3	[96]	0°, 22.5°, 45°, 67.5°	Quicksand (Hainan, CHN)	CR
	10–50	14	[91]	45, 90	100–600, 8000–10,000	Rubber Polyurethane
		22.8	[86]	45	380–830	NBR
		15	[100]	45	4000	Rubber
		30	[95]	10, 15, 20, 30, 45, 60, 90	392	CR
Plastic/Resin/Fiber	0–10	3.13	[87]	90	150	EP
		4.8, 7.2	[92]	45	50–600	FRP
		4.7	[14]	90	400	EP/Aramid Fiber
		3.4	[89]	0–90	149	HDPE/Bamboo Fiber
	10–50	10–50	[94]	30–90	160–265 (g/min)	EP
	>50	86, 101, 119	[98]	30, 45, 60, 90	200 ± 50	EP
		101, 119, 148	[97]	30, 45, 60, 90	1.467 ± 0.02 (g/min)	EP
		70, 100, 130	[102]	30, 60, 90	3–5 (g/min)	Modified Coconut Sheath and Red Mud-Reinforced Hybrid Composites/EP
	300 r/min		[22]	90	200–630	BFRP
	500/1000/1500 r/min		[93]	15, 30, 45	230 ± 35.061	C/EP

offers a comprehensive review of various studies, detailing the specific parameters, such as the impact speed, impact angle, and abrasive grain size, used in different polymer material experiments. This table serves to provide the reader with a clearer understanding of how these parameters were manipulated in previous research, providing a valuable context for comparing and interpreting the results of different studies on polymer material erosion.

3.3. Three-Body Wet Abrasive Wear

The study of three-body abrasive wear (as shown in Figure 14) considers the combined effects of the abraded part, the abrasive grain, and the counter-abrasive subassembly simultaneously. This approach allows for a more comprehensive understanding of wear mechanisms under varying operating conditions. Experts and scholars have conducted extensive research on this topic, focusing on different aspects, such as the material composition [13,103,104], the characteristics of abrasive grains [105,106,107], the applied load [108,109,110], and the velocity [111,112,113] of the counter-abrasive pair. This paper reviews the effects of abrasive particles, load, and velocity on the abrasive wear behavior of various polymeric materials in aqueous environments. By synthesizing the findings from these studies, this paper aims to provide a deeper understanding of how these factors influence wear, which is critical for improving the durability and performance of polymeric materials in practical applications.

3.3.1. Materials

When external hard particles intrude between the friction surfaces, three-body wear is formed on top of the two-body sliding wear, making the wear form more complex [114]. The nature of the polymer material plays a critical role in its wear behavior. This wear resistance is not only determined by the formulation ratios and properties of the fillers used, but is also significantly influenced by the surface roughness of the polymer. The material composition, including the type and proportion of fillers, affects the polymer’s hardness, strength, and resistance to abrasion.

The influence of polymeric materials is a significant factor in three-body abrasive wear, as demonstrated by the study conducted by Song et al. They investigated the abrasive wear behavior of nitrile butadiene rubber (NBR) with varying acrylonitrile mass fractions (18%, 26%, and 41%) after immersion in deionized water, using a ring–block micro-controlled testing machine. The results indicated that increasing the acrylonitrile content in the Buna-N rubber led to an increase in hardness (Shore A) from 62 to 73. Additionally, the friction coefficient of the dissolved material decreased from 0.787 to 0.504 when subjected to spherical abrasive particles. This reduction in the friction coefficient was accompanied by a decrease in the wear quality change (Qwc), from 17.6% to 9.7%, demonstrating enhanced abrasion resistance. This study, visualized in the SEM diagram shown in Figure 15A, highlights how modifying the polymer composition can effectively improve its wear performance in abrasive environments [115]. Yousry et al. investigated the effect of fiber volume fraction on the wear properties of man-made fiber-reinforced polymer composites. Utilizing a high-velocity clay-pot abrasion apparatus, tribological examinations were conducted under three-body abrasive conditions to assess the wear characteristics. The results showed that increasing the fiber length from 1.5 mm to 3 mm improved the wear resistance of man-made fiber-reinforced polymer materials. The application of chemical modification techniques to fibers enhances the durability against wear in fiber-reinforced polymeric composites, thereby improving their overall wear resistance [116]. Wang et al. conducted a study on the micromotional wear behavior of ultra-high-molecular-weight polyethylene (UHMWPE) with varying surface roughness levels of 0.07 μm, 0.35 μm, and 0.50 μm. They used an Optimal SRV-IV vibratory friction and wear tester under water lubrication conditions to evaluate how surface roughness affects wear properties. This study revealed a notable trend: as the surface roughness increased, the coefficient of friction initially decreased from 0.2 to 0.17, but then slightly increased to 0.19 as the roughness continued to rise. This indicates that, while smoother surfaces reduce friction initially, further increases in roughness may lead to greater friction. Additionally, the specific wear rate rose from 6 × 10⁻⁶ mm³/N·m to 8 × 10⁻⁶ mm³/N·m with the increasing surface roughness, suggesting that rougher surfaces result in greater material loss. Figure 15B illustrates the microscopic wear state of UHMWPE with different levels of surface roughness, showcasing the variations in the wear patterns. Figure 15C presents an SEM image, providing a detailed view of the wear surfaces and the effects of surface roughness on the material’s wear behavior [24].

3.3.2. Abrasive Grain

The impact of abrasive particles on three-body wear is significant and is one of the primary factors contributing to direct polymer composite wear and grooving. These particles act as a third body between the two sliding surfaces, intensifying the wear process by causing more severe material removal and creating grooves on the surface of the polymer composite. This interaction often leads to the accelerated degradation of the material, highlighting the crucial role of abrasive particles in determining the wear performance and longevity of polymer composites in various applications.

In studying the effect of abrasive grain size on the three-body abrasive wear mechanism, Shen and colleagues conducted a comprehensive investigation into the influence of abrasive particle dimensions on the tribological performance of an O-ring rubber-seal pairing, specifically under aqueous lubrication conditions, utilizing a versatile tribometer for friction and wear assessments. The experimental setup is shown in Figure 16. The results show the following characteristics: when the particle size is larger than the critical value of about 75 μm, a few particles can be embedded in the friction side of the contact area on both sides of the vice, with plowing action on the surface of the grinding vice to produce a deeper furrow; when the particle size is between the two critical values, there is contact on both sides of the vice “particle embedded belt”, showing a lower coefficient of friction and a slowing down of rubber wear; and when the particle size is smaller than the critical value of 12.5 μm, the particles can pass through the friction interface more freely, accelerating the rubber erosion and wear, and the rubber wear surface shows “ridge–furrow–ridge” alternating characteristics [117]. Furthermore, the research group devised a novel abrasive wear evaluation method tailored for acrylonitrile butadiene rubber (NBR) cylinders, which were subjected to reciprocating sliding tests within a 316L stainless-steel plate friction clamp. This approach enabled assessments across varying particle densities. The results showed that, when the particle size was too large (i.e., 70–250 mesh), the wear volume of the metal counterpart became more significant as the particle concentration increased, and the inhibition of the NBR wear loss by particle embedding became more significant as the particle concentration increased. With the decrease in the particle size (i.e., 500 mesh), the abrasive flow of the small particles erodes the rubber surface and increases the wear volume of the NBR [118]. Nitrile rubber (NBR) is widely used in water-lubricated stern tube bearings in ships. However, sediments in the water can significantly impact the friction and wear performance of these bearings. Yang et al. investigated the tribological performance of water-lubricated rubber bearings under different sediment grain sizes and sand types. The results indicate that, as the grain size increases, the wear volume of the NBR is the highest at 75–90 μm. The coefficient of friction initially increased and then decreased, reaching its maximum value at 48–60 μm [36]. Yuan et al. conducted sliding wear tests on rubber-tail shaft tube bearings using sand with different grain sizes (4.5 μm, 23 μm, 48 μm, and 75 μm) in water at a consistent mass concentration of 1.1%. The results showed that the coefficient of friction, wear volume, and wear surface morphology of the specimens increased with the grain size. Sand with a grain size of 4.5 μm had the least impact on the wear behavior of the friction pair [119]. Sun et al. employed orthogonal experimental methods to study the wear behavior of ultra-high-molecular-weight polyethylene (UHMWPE) materials in aqueous environments. The results showed that irregular wrinkling wear primarily occurred with grain sizes below 5000 mesh. For medium-sized particles, specifically 2000 mesh and 1000 mesh, the material experienced wear predominantly through plowing, resulting in the formation of furrows along the sliding direction on the surface. In environments with larger particles, exceeding 600 mesh, the rolling of these particles led to the production of fibrous chips on the abraded surface [120]. Song et al. conducted a study to examine the impact of different abrasive grain types on the wear behavior of acrylonitrile/nitrile butadiene rubber (NBR) after it had undergone swelling in an aqueous environment. Their results indicated that, under the same particle size, the wear caused by angular abrasive grains was significantly greater than that caused by circular abrasive grains. Specifically, the wear due to angular particles was more sensitive to the effects of swelling, with the maximum wear increment after swelling being 1.56-times higher than that caused by circular abrasive particles. This study further revealed that wear mechanisms differ, based on the shape of the abrasive grains. Circular abrasive particles primarily roll against the rubber surface, creating circular, crater-like indentations. In contrast, angular abrasive grains tend to slide and embed themselves into the rubber surface, leading to the formation of angular embedded pits [115].

The magnitude and distribution of the contact pressure play a critical role in determining the extent of abrasive particle intrusion, which directly affects the wear behavior of materials. In response to this, some researchers conducted experiments aimed at reducing the impact of abrasive wear. Sui et al. designed an innovative seal with a unique contact pressure profile to address this issue. They tested this new seal design against a traditional o-ring design using a rotary seal tester. The results confirmed the improved performance of the new seal, as evidenced by the lower friction torque, reduced surface temperature, and less material loss during testing. Surface wear traces further indicated that the proposed design was more effective than the o-ring seal in preventing the intrusion of abrasive particles. This suggests that optimizing the contact pressure distribution can significantly enhance the durability of seals in abrasive environments, offering better protection against wear and extending the service life of mechanical components [121]. In efforts to prevent hard debris from penetrating elastomeric dynamic seals, Farfan-Cabrera et al. proposed a suitable test method for assessing abrasive wear. They employed a TE66 microscale wear tester under slurry conditions to evaluate the abrasive wear rate of three-body elastomers. Their results indicated that, when comparing the wear data obtained from an optical profilometer with the calculated wear results, the discrepancies were minimal. This suggests that the proposed model demonstrated significant accuracy when predicting abrasive wear [122]. The intrusion behavior of particles in drilling fluid under vibration conditions can easily lead to seal failure. Zhou et al. designed a test setup to simulate the working condition of FKM o-rings under vibration. The intrusion behavior of particles was also observed in the field. The results show that the two behaviors of seal boundary movement and particle displacement together lead to particle intrusion into the seal interface; both behaviors are caused by vibration, and their significance is affected by the frequency of vibration [123].

3.3.3. Load

In three-body abrasive wear, the nature of the wear is closely related to the applied load. When lower loads are applied, abrasive grains are less likely to penetrate the surfaces and tend to roll between them, leading to a type of wear known as rolling wear. This wear mechanism is typically less severe, as the particles primarily cause surface deformation without significant material removal. However, under higher loads, the abrasive grains are more likely to become embedded in the wear surface. This causes them to dig into the material, creating deep grooves, a process known as groove wear. Groove wear is generally more destructive than rolling wear, as it involves the removal of material along the paths of the embedded particles, leading to more significant wear and surface damage.

Many researchers have conducted comparative tests to examine the effect of load on the wear behavior of polymeric materials. S.K. et al. systematically investigated the wear of stainless steel in contact with three different rubbers—thermoplastic polyurethane (TPU), nitrile butadiene rubber (NBR), and liquid silicone rubber (LSR)—in a muddy environment, which involved water containing sand particles. To achieve this, they developed a new experimental setup. The wear measurements revealed that the coefficient of friction for all three rubbers decreased as the applied load increased. Notably, LSR exhibited the most significant reduction in the coefficient of friction, decreasing from 0.5 to 0.22 as the load was increased from 5 N to 15 N, respectively. This behavior suggests that, under higher loads, the interaction between the rubber materials and the abrasive particles changes, potentially due to increased contact pressure causing the rubber to conform more closely to the stainless-steel surface, thereby reducing friction [124]. Dong et al. studied the wear behavior of a nitrile butadiene rubber (NBR) pin and a 1Cr18Ni9Ti stainless-steel disc friction pair under sand-water lubrication conditions. They conducted sliding wear tests using the CBZ-1 friction testing machine, comparing the effects of sand and pure water as lubricants. The results, illustrated in Figure 12a, show that the wear volume of the friction pair increases with the applied load. Similarly, the coefficient of friction also rose as the load increased. This study identified an optimal load range for this friction pair, which was found to be between 0.1 and 0.5 MPa. Within this range, the wear and friction were more controlled, suggesting that operating within these load parameters could enhance the longevity and performance of the friction pair under abrasive conditions [125]. Nitrile butadiene rubber (NBR) is extensively used in marine environments, particularly as the preferred material for water-lubricated tail shaft tube bearings in ships, due to its durability and versatility. Yuan et al. conducted a study to investigate the wear performance of these rubber tail shaft tube bearings under conditions involving both sand and water, with a focus on the impact of applied load. This study employed a CBZ-1 friction tester to perform sliding wear tests on a combination of NBR and ZCuSn₁₀Zn₂ cast copper. The results revealed that the applied load had a significant effect on the wear performance of this material pair. Specifically, when the load was increased from 0.7 MPa to 1.1 MPa, the mass loss of the NBR/ZCuSn₁₀Zn₂ cast copper bearings increased from 0.1 g to 0.35 g, respectively. This indicates that higher loads lead to more severe wear, which is crucial for understanding how to optimize the performance and longevity of these bearings in abrasive marine conditions [119].

Vishwas et al. conducted a study on the three-body abrasive wear behavior of jute/natural rubber, flexible green composites under both dry and lubricated conditions. They used a three-body wear tester to perform wear tests at various loads of 10, 20, 30, and 40 N. The results, illustrated in Figure 17b, demonstrate a significant reduction in mass loss under lubricated wear conditions compared to dry conditions. Specifically, the mass loss was reduced by 44.44%, 15.87%, and 29.96% at different loads, highlighting the effectiveness of lubrication in minimizing wear. This finding emphasizes the importance of lubrication in extending the life of jute/natural rubber composites, particularly in applications where abrasive conditions are prevalent [126]. Yousry et al. utilized a high-speed slush pot wear tester to examine the effect of load on the wear performance of man-made fiber-reinforced polymers under slurry conditions. Their findings revealed that the wear rate of these reinforced polymer materials increased proportionally with the applied load. This indicates a direct relationship between the load and the rate of wear, suggesting that higher loads result in greater material degradation under abrasive slurry conditions [116].

3.3.4. Sliding Speed

Sliding speed plays a critical role in the friction performance of polymer composites. Many researchers employ friction and wear testing machines to investigate the factors influencing sliding speed and its effects on wear behavior. These studies are instrumental in understanding how sliding speed impacts the friction process, providing insights into optimizing conditions to minimize friction loss. By controlling the sliding speed, it is possible to enhance the durability and efficiency of polymer composites in various applications, making this an essential aspect of materials engineering and design.

Dong et al. investigated the wear behavior of an NBR pin and stainless-steel disk friction pair under sand–water lubrication conditions using a CBZ-1 friction tester. The results, illustrated in Figure 18a, demonstrate that the sliding speed has a significant influence on the tribological properties of the friction pair. Specifically, as the sliding speed increases, the wear volume also increases. Additionally, the coefficient of friction initially decreases with the increasing speed, but then rises again. The optimal sliding speed range for maintaining favorable tribological properties was found to be between 0.33 and 1.1 m/s, balancing both the wear and friction performance [125]. Yuan et al. explored the wear performance of rubber tailstock tube bearings in water mixed with sand, using a CBZ-1 friction tester to conduct sliding wear tests with a sand concentration of 1.1%. The results revealed that, as the sliding speed increased, it became easier to form a water film between the NBR and ZCuSn₁₀Zn₂ cast copper. This formation of the water film played a protective role, reducing wear. Specifically, when the sliding velocity increased from 0.11 m/s to 1.1 m/s, the volume loss of the material decreased from 0.4 g to 0.2 g, respectively. This indicates that higher sliding speeds can enhance the lubrication effect and reduce wear in such environments [119]. Sun et al. investigated the influence of rotational speed on the abrasive wear performance of ultra-high-molecular-weight polyethylene (UHMWPE) under water lubrication conditions. Their study analyzed the wear mechanisms by considering both the coefficient of friction and wear characteristics as crucial factors. The results demonstrate that the wear rate of the UHMWPE reaches its minimum value, approximately 8.49 × 10⁻⁶ mm³/Nm, at a rotational speed of 360 r/min (Figure 18b). Below the threshold rotational velocity of 360 r/min, the wear mechanisms were primarily dominated by three-body abrasive wear and adhesive wear. However, once this critical rotational speed was exceeded, the predominant wear mechanism shifted to the plowing effect caused by particles on the material surface. This resulted in plow furrows with widths as large as 40 μm, leading to a more severe wear pattern [127]. Li et al. studied the frictional wear mechanism of polyamide 66 (PA66) using a ring–block wear tester under lubrication conditions involving a suspension of 99.9% brine and 0.1% sediment. The findings indicate that the tribological performance of PA66 is relatively favorable at a line speed of 0.51 m/s. Under these conditions, the material exhibited stable frictional behavior, suggesting that this speed is suitable for reducing wear and maintaining a good performance in the presence of brine and sediment [128].

Table 3. Summary of experimental parameters in the three-body abrasive wear test.

Type	Load (N)		Ref.	Other Parameters
				Sliding Speed (m/s)	Abrasive Grain (μm)	Materials
Rubber	0–50	10	[117]	0.06	5.6–164	NBR
		10	[118]	0.06	2.8–212	NBR
		1.5	[122]	0.11 ± 0.01	4	Silicone Rubber
		5–15	[124]	2900–4500 (r/min)	133	TPU, NBR, LSR
		10, 20, 30, 40	[126]	200 (r/min)	AFS60	Jute/Natural Rubber, Flexible Green Composite
		7.85, 23.55, 39.25, 54.95, 70.65	[125]	0.11, 0.33, 0.55, 0.77, 1.1, 2.2	5–60	NBR
	50–100	100	[115]	0.93	420–850	NBR
		54.53, 70.11, 85.69	[119]	0.11, 0.55, 1.1	4.5, 23, 48, 75	NBR
	>100	100–300	[36]	0.05–0.91	0–200	NBR
Plastic/Resin/Fiber	0–50	10	[24]	0.005	120	UHMWPE
		30	[120]	300 (r/min)	5–100	UHMWPE
		5	[116]	0.026–0.115	500, 714, 1430	Man-made Fibers
		30	[127]	240, 300, 360, 420, 480 (r/min)	25	UHMWPE
	>100	225, 250, 275, 300	[128]	0.12, 0.51, 0.90, 1.29	0.1 wt%	PA66

provides a review of the studies, the specific parameters of load, sliding velocity, and abrasive grain in the various types of polymer material experiments, to provide the reader with a better understanding of previous studies.

4. Discussion

4.1. Material

Currently, the research on the wear of polymer materials in water lubrication environments primarily focuses on materials such as NBR, SBR, NR, EP, polyethylene, and their composites, which are considered ideal for water lubrication. Efforts to improve the wear resistance of these polymers and their composites have centered on wear modification through changes in formulations and ratios. By identifying the most suitable material formulations for specific working conditions, researchers aim to enhance the durability and performance of these materials under water-lubricated conditions.

In the study of rubber formulations, the content of acrylonitrile added to NBR was varied at 18%, 26%, and 41%, resulting in material hardness levels of 71, 73, and 74, respectively. As the hardness increased, the coefficient of friction during the two-body wet sliding test wear remained relatively stable, around 0.11 to 0.12, while the change in wear mass decreased from 10.5% to 5.4% [63]. In the two-body abrasive wear test, the erosion rate significantly decreased from 15.81 cm³/kg to 6.53 cm³/kg [86]. Similarly, in three-body wet abrasive wear, the coefficient of friction decreased from 0.663 to 0.456, and the wear mass change (Qwc) dropped from 11.7% to 8.8% [115]. These findings suggest that, as the acrylonitrile content increases, the material’s hardness also increases, leading to a reduction in the wear rate by approximately 25% to 50% across different types of wear. However, due to the hydrophilic nature of acrylonitrile, the increased swelling behavior may decrease the material’s hardness and wear resistance over time. This balance between the hardness, wear rate, and the material’s swelling behavior is critical to optimizing the formulation for specific applications.

In a study examining the effect of fillers on the wear performance of SSBR (solution-styrene-butadiene rubber) under two-body wet sliding conditions, the addition of different fillers—micrometer silica, nanoscale silica, and carbon black—resulted in notable changes in both the coefficient of friction and wear rate [21]. The average coefficient of friction was reduced as follows: 0.265 with micrometer silica, 0.229 with nanoscale silica, and 0.188 with carbon black. Correspondingly, the wear rate also decreased significantly: 1.34% with micrometer silica, 0.232% with nanoscale silica, and 0.105% with carbon black. These results indicate that the incorporation of nanoscale silica significantly reduce the coefficient of friction and wear rate of the SSBR. However, the SSBR formulation with carbon black exhibited the best wear resistance under wet sliding conditions, suggesting that carbon black is a more effective filler for enhancing wear resistance in such environments.

Silicon dioxide (silica) has been extensively used as a modified filler in studies focused on enhancing the wear resistance of resin materials. For instance, adding 6% silica to epoxy resins resulted in a hardness increase of approximately 17% and a reduction in the wear rate by about 21% under erosive wear conditions [88]. In another study, under two-body wet sliding conditions, incorporating silica into epoxy resin containing carbon fiber and graphite led to a significant decrease in the average coefficient of friction by around 47% and a reduction in the wear rate by about 33.1% [39]. Additionally, the inclusion of nickel foliated silicate (NiPS) up to 7% in epoxy resin resulted in a 3% increase in hardness, a 14% reduction in the coefficient of friction, and a 9% reduction in wear rate [65]. These findings suggest that the addition of silicon dioxide can effectively enhance the wear resistance of resin materials, making them more durable in abrasive and erosive environments.

Some scholars have explored the impact of carbon combination fillers on the tribological properties of polymers. Specifically, the incorporation of bagasse ash silica (BASi) into natural rubber (NR), styrene–butadiene rubber (SBR), and NR/SBR blends led to varying changes in material hardness: an increase of about 19% for NR, 6% for SBR, and 14% for the NR/SBR blend. Under two-body wet sliding conditions, the specific wear rate of these materials showed differing trends. For NR, the specific wear rate was reduced by approximately 33%, indicating improved wear resistance. However, for SBR and the NR/SBR blend, the specific wear rate increased by about 16% and 42%, respectively, suggesting a decline in the wear performance in these cases [64]. In the modification of epoxy resin, studies have shown that incorporating specific fillers can significantly enhance the material’s properties. For instance, basalt fiber-reinforced polymer (BFRP) composites filled with nanoclay exhibited an increase in material hardness by 1.7% to 2.2%, along with a reduction in the specific wear rate (Ks) by 51.12% under two-body erosive wear conditions [22]. Similarly, the addition of blast furnace slag to the polyester fiber epoxy resin composites resulted in substantial hardness increases of 20% to 47%, and a reduction in the specific wear rate by approximately 50% under erosive wear conditions [90]. These results suggest that carbon combination fillers can effectively improve the hardness of polymer materials. However, the impact on the tribological properties varies depending on the material. While resin materials like epoxy resin show enhanced wear resistance, rubber materials, such as SBR, experience a decrease in wear resistance when modified with certain carbon combination fillers, indicating that this approach may not be suitable for improving the wear resistance of SBR. Therefore, the selection of carbon-combination modified rubber materials for use under water lubrication conditions requires further research and verification to determine the most effective formulations for specific applications.

Fiber materials are frequently employed as modifiers in resin systems to enhance their mechanical and tribological properties, which can make it better used in different occasions [129]. The incorporation of carbon and glass fibers into epoxy resin has been found to significantly alter its performance under two-body wet sliding conditions. Specifically, the addition of carbon fibers reduces the coefficient of friction by approximately 26%, while the inclusion of glass fibers increases it by about 71%. Despite this contrast in frictional behavior, both types of fibers greatly improve the wear resistance, with carbon fibers reducing the specific wear rate by around 96% and glass fibers by approximately 88% [39]. Moreover, when carbon, glass, and hybrid fibers are added to epoxy resin, the resulting composites exhibit different levels of wear resistance under erosive wear conditions. For instance, the hardness values of the epoxy composites with carbon, glass, and hybrid fibers were 89.13, 88.47, and 88.32, respectively. These composites experienced mass losses of 2.8%, 2.0%, and 1.2%, indicating that hybrid fibers may provide superior protection against erosive wear [14]. Additionally, the introduction of 5.0 wt% carbon fibers into epoxy resin resulted in a volumetric loss reduction of about 42% during erosive wear, further demonstrating the effectiveness of carbon fibers in enhancing the wear resistance of epoxy-based materials [87].

Plant fibers have garnered significant attention from researchers due to their abundant availability, biodegradability, and cost-effectiveness. Various studies have explored their potential as fillers to enhance the tribological properties of polymers and resins. For example, adding 80 wt% bamboo fiber to polyethylene resulted in a hardness of about 61 and a volume loss of 43 mm³ at an erosion distance of 0.5 cm and an erosion time of 180 s [89]. In epoxy resins, 2.5 wt% bamboo filler was found to significantly improve the wear resistance under wet sliding conditions, reducing the friction coefficient by approximately 66% and the specific wear rate by about 67% under a 40 N load compared to the material without bamboo fiber. However, it is important to note that increasing the bamboo fiber content to 12.5 wt% actually reduced the wear resistance, indicating that there is an optimal filler concentration for achieving the best performance [35]. Similarly, when sorghum straw (SS) was added to polyvinyl chloride (PVC) composites after alkalization treatment with a 4.5% sodium hydroxide solution, the hardness increased by about 16%. After soaking for 12 days, the friction coefficient was reduced by about 16%, and the specific wear rate decreased by about 35% [130]. Other natural fibers, such as coconut fiber [131,132,133], hazelnut husk [134,135], and rice husk [136,137], have also been studied for their potential to improve the tribological properties of resins and plastics [138]. However, most of the existing research has focused on dry friction or oil lubrication conditions, with relatively few studies addressing the effects of water lubrication. This gap in the research presents an opportunity for scholars to explore the tribological behavior of plant fiber-reinforced materials under water lubrication conditions, which could lead to new insights and applications in environments where water is a primary lubricant.

The relationship between hardness, hydrophilicity, and wear resistance in rubber and resin materials is complex and influenced by multiple factors, especially under water lubrication conditions. Generally, for both rubber [21,63,115] and resin materials [65,88,89,130], wear rates tend to decrease as hardness increases. However, exceptions arise when hydrophilic fillers, like bagasse ash silica (BASi) [64], are added to hydrophobic SBR, or when hydrophobic fillers, such as carbon and glass fibers, are incorporated into hydrophobic epoxy resin [14]. In these cases, the expected trend of wear reduction with increasing hardness may reverse. In water-lubricated environments, both the hardness and hydrophilicity of materials significantly affect their lubrication performance. The limited data reviewed in this study suggest that the coefficient of friction generally decreases as the material hardness increases [21,63,65,115,130]. In addition, the hardness affects the wear surface morphology and thus the state of the lubrication film, which deserves a more systematic study. In rubber materials, for instance, increasing the acrylonitrile content in hydrophilic NBR enhances lubrication conditions, leading to a reduced coefficient of friction [63,115]. In contrast, in hydrophobic SBR, incorporating hydrophobic carbon black results in better wear resistance and a lower coefficient of friction compared to adding hydrophilic silica [21]. For resin materials, which are typically hydrophobic, adding hydrophilic fillers tends to reduce the coefficient of friction [65,89,130]. On the other hand, when hydrophobic fillers, like carbon fibers, are added, the friction coefficient decreases, whereas the addition of glass fibers tends to increase it [39]. This indicates that the tribological performance is not solely dependent on hardness, but also on the interaction between the material’s inherent properties (such as hydrophobicity or hydrophilicity) and the type of fillers used. Overall, the selection of wear-resistant polymer materials under water lubrication conditions requires a comprehensive understanding of the material’s base properties, the nature of the fillers, lubricant characteristics, the abrasive particles involved, surface properties, wear mechanisms, and environmental factors, such as temperature. Systematic studies addressing these variables are essential for developing optimized materials with a superior tribological performance in specific conditions.

Temperature plays a significant role in both the aging process and the lubrication conditions of materials. For instance, in the case of rubber seals, the Coulomb friction coefficient decreases by approximately 19% when the temperature is raised from 20 °C to 80 °C after four days of aging [74]. On the other hand, the specific wear rate of an epoxy composite filled with 12.5% bamboo increases by about 92% as the temperature rises from 40 °C to 80 °C [35]. High-speed friction exacerbates these effects as the frictional heat, combined with the material’s low thermal diffusivity, leads to the softening of the material [139]. Additionally, as the temperature increases, the evaporation of the water film can cause lubrication failure, resulting in a change in the wear mode [73]. Given that temperature is a critical factor influencing wear, it represents an important avenue for further research, particularly in the context of water lubrication. This highlights the need for comprehensive studies that explore the interplay between temperature and tribological performance under various operating conditions to optimize material selection and design for specific applications.

Nowadays, many scholars have studied new polymer composites and showed better performances. For example, aromatic thermosetting co-polyester (ATSP)-reinforced PTFE composites [140], graphitized iron carbon nitride (Fe₃O₄@g-C₃N₄)/ethylene methacrylate (EMA) composites [141], polypropylene wear-resistant composites filled with polypropylenepotassium titanate (PPT) [142], graphene (GNS), carbon nanotubes (CNTs), and fullerene (C60)-filled composites of fluoroelastomers (FKM) [143], among others. These polymer composites are expected to address the current limitations in studying polymer wear in water and have wider applications in water lubrication conditions.

4.2. Work-Related Condition

4.2.1. Load

The research on the conditions affecting wear behavior under water lubrication conditions mainly focuses on the factors such as the load/impact velocity, sliding velocity, impact angle, and abrasive properties. These factors collectively impact the wear rate and the type of wear that occurs, making them essential considerations when studying the tribological performance under water-lubricated conditions. Understanding these variables helps in designing materials and systems that can better withstand wear and maintain their integrity in various operational environments. Among them, as the necessary condition of wear, load is the first factor affecting wear, and the research on it is more common. In the two-body wet sliding wear condition, when the load range is 1–2 N, the friction coefficient of SBR decreases by about 7% for every 0.5 N increase in the load, and the length of the wear pattern spacing is in the range of 15–17 μm on average [70]. When the load range is 10–100 N, the friction coefficient of the SBR increases by about 14% and the mass loss increment is about 1.2 mg for every 50 N increase [68]. When the applied load is in the range of 60–90 N, the wear rate of the SBR reaches the maximum value of 0.521 × 10⁻³% when the load is 80 N, which is about twice that of 60 N [71]. For aircraft tire materials, when the applied load is in the range of 100–300 N, the friction coefficient decreases by about 5% for every 100 N increase [67]. For two-body wet sliding wear, when the load range is 1–2 N, the wear rate changes little with the increase in the load. In the range of 10–80 N, the wear rate increases with the increase in the load. The friction coefficient decreases with the increase in the load range of 1–2 N, and the influence of lubrication on the load is more sensitive at a low load. In the range of 1–100 N, the friction coefficient increases with the increase in the load. In the range of 100–300 N, the friction coefficient decreases with the increase in the load due to the change in lubrication conditions and the easier removal of polymer materials under a high load.

In the three-body wet wear test, LSR materials exhibit better wear resistance when loads of 5 N to 15 N were applied to polyurethane (TPU), liquid silicone rubber (LSR), and nitrile butadiene rubber (NBR) [124]. For NBR/stainless-steel composites, when the load is applied at a range of 0.1–0.9 MPa, with the increase in the load, the wear volume growth rate is slower in the range of 0.1–0.5 MPa, and when it is increased to 0.7 MPa, the growth rate is faster, and the friction coefficient increases about 2.5 times when the load reaches 0.9 MPa. The wear volume increases by about four times [125]. Under the sliding speed of 0.55 m/s and an abrasive particle size of 23 μm, when the load borne by the NBR/casting copper alloy composite material increases from 0.7 to 1.1 MPa, the mass loss increases by about 2.5 times [119]. When the fiber material was used as the filling material, we found that, for the jute/natural rubber composite, as the load increased from 10 N to 30 N, the incremental volume loss of the material was significantly affected by the load with each increase of 10 N, about 110%, and the volume loss increased by about 20% when the load increased from 30 N to 40 N [126]. When the load applied to the resin/glass fiber composite increases from 4.45 N to 22.24 N, the wear rate increases by about 4.15 times [116]. The analysis shows that, under the three-body wear mode, the wear rate increases with the increase in the load. In the range of 0.1–1.1 MPa, the unit MPa increment of the wear rate is about 1.25 times of that of a load less than 0.7 MPa. In the range of 10–30 N, with the increase in the load, the wear rate increases by about 1–2 times/10 N. When the load increases in the range of 30–40 N, the increment of the wear rate decreases. Considering that it may be affected by the number of abrasive particles that can enter, the specific reasons are worthy of further study.

For erosion wear, the impact velocity affects the load acting on the surface of the material. When conducting erosion wear tests on eggshell particulate epoxy composites, the impact velocity increased from 86 m/s to 101 m/s and then to 119 m/s, and the wear rate increased by roughly the same amount, about 0.25 mg [98]. When carbon/epoxy composites were applied from 500 r/min to 1500 r/min, the mass loss increased by about five times [93]. When the impact velocity is in the range of 10–40 m/s, the average wear rate of the glass/steel fiber-reinforced epoxy resin composite increases by about 45% with every increase of 10 m/s, and the slope of the impact velocity/wear rate curve gradually increases with the increase in speed [94]. When the impact velocity of coconut sheath and red mud-reinforced hybrid composites increases from 70 to 130 m/s, the wear rate increases by about 22%, and the impact on the wear rate gradually decreases with the increase in speed [102]. In the form of erosion wear, with the increase in speed, the increase in the force acting on the material surface leads to the increase in the wear rate. In the range of 10–40 m/s, the increase in the wear rate is about 45%/10 m/s; in the range of 70–130 m/s, the increase in the wear rate is about 3.6%/10 m/s; and the increase in the wear rate decreases with the increase in speed. The study of erosion wear should be combined with the impact velocity, impact angle, and the influence of abrasive particles.

4.2.2. Sliding Speed

The wear of materials occurs in relative motion, and the sliding velocity affects the tangential force exerted on the material surface, while the effect on frictional heat is further reflected in the effect on lubrication conditions and material properties, which is an important influence in the study of polymer material wear. In two-body wet sliding wear, when the sliding velocity increases from 0.1 m/s to 1 m/s, the friction coefficient decreases by about 0.06–0.1 for every 0.5 m/s increase in the sliding velocity applied to the PEEK, and the specific wear rate rises and then decreases, with an overall increase of about 87%, and reaches a maximum value of 0.9 × 10⁻⁶ mm³/Nm at 0.5 m/s [53]. For NBR/aluminum alloy-bearing materials, both the coefficient of friction and friction moment decreased by about 50–67% when the sliding speed was increased from 100 r/min to 500 r/min, and the slopes of the sliding speed/coefficient of friction (friction moment) curves decreased gradually with the increase in the speed [69]. In the three-body wet abrasive wear test on the NBR/stainless-steel-bearing material applied to the sliding velocity from 0.11 m/s to 2.2 m/s, the wear volume rises by a factor of about seven, and the increment in the wear volume from 1.1 to 2.2 m/s is twice as large as that from 0.11 to 1.1 m/s [125]. At a load of 0.9 MPa with an abrasive grain size of 23 μm, when sliding velocities acting on NBR/cast copper alloy-bearing materials ranged from 0.11 to 1.1 m/s, the coefficient of friction increased within 0–0.1 and the wear mass loss decreased by about 0.05–0.1 g for each increase of 0.55 m/s [119]. For UHMWPE, when sliding speed was increased from 240 r/min to 480 r/min, the overall mass loss was elevated by about 1.25 times, with the most pronounced effect between 360–420 r/min, where the mass loss was elevated by nearly a factor of one [127]. For the PA66-bearing material, the friction coefficient exhibits a slight fluctuation when the sliding speed is increased from 0.12 to 1.29 m/s, and the wear mass friction changes in the range of 2.6–0.9 mg, with a small increase occurring at 0.9 m/s, and an overall decrease of about 10% [128]. For two-body wet sliding wear, the friction coefficient decreases, and the wear rate increases as the sliding velocity increases; three-body wet abrasive wear studies show different trends in wear rates with increasing sliding velocities. In comparison, three-body wet abrasive wear is more significantly affected by load.

4.2.3. Impact Angle

For erosive wear, the impact angle affects the normal and tangential forces acting on the material surface by the abrasive particles and their sliding velocities on the material surface. For neoprene rubber using multiple angles for erosion, 15° has the highest impact, with a rate of erosion up to about 1.65 (g/tone) [95]. For chlorinated rubber coatings, the rate of erosion increases and then decreases as the impact angle increases from 22.5° to 90°, with a maximum of 0.71 × 10⁻⁴ g/(cm²-h) at 67.5° [96]. For glass/steel fiber-reinforced epoxy composite erosion, the erosion rate first increased and then decreased when the erosion angle was increased from 30° to 90°, with the maximum erosion wear rate of 34 mg/kg at 45–60° [94]. When the porous nano-activated carbon composites containing 1–3% porous activated carbon (AC800) were eroded at different velocities, the erosion rate increased and then decreased when the erosion angle was increased from 30° to 90°, with the maximum erosion wear rate at 45°, where the maximum erosion wear rate could reach about 34 mg/kg [97]. When eggshell particulate epoxy composites were eroded at the temperature range of 30–90°, the erosion rate increased and then decreased, reaching a maximum of about 0.2–0.35 mg/g at 60–70° [98]. When erosive wear was carried out on polyurethane and natural rubber using abrasive grains of 8–10 mm particle sizes, the erosion rate was nearly one-third higher at 45° than at 90° for both materials [91]. In the abovementioned multi-angle erosive wear studies, most of the maximum values of the erosive wear rate were concentrated in the range of 45–60°, and the overall trend of the erosive wear rate with increasing angles showed an increase and then a decrease.

4.2.4. Abrasive Grain

Characteristic parameters, such as the particle size of abrasive particles, can affect the size, velocity, angle, and area of the load acting on the material surface. In the study of two-body erosive wear, glass fiber-reinforced vinyl ester composites (FRPs) were tested with two abrasive grain sizes in the ranges of 0.05–0.2 mm and 0.1–0.6 mm in the tank erosive wear study, and the large grain size had a greater impact on the material compared to the small grain size, and the mass loss increased by about 78% [92]. Using two abrasive grain sizes in the ranges of 0.1–0.6 mm and 8–10 mm, respectively, in polyurethane and natural rubber erosion wear tests, at the erosion angle of 45°, the impact of the wear rate on the large-grain-size abrasive grains is much higher than the small-grain-size abrasive grains. The polyurethane abrasion rate increased by about 480 cm³/m²h; the wear rate of natural rubber increased by about 260 cm³/m²h [91]. In the three-body wet abrasive wear test, abrasive particle sizes in the range of 5.6–164 μm and below 16 μm in NBR seals had a greater impact on the wear surface and average mass wear rate at 0.6 (g/m³) [117]. Wear experiments were conducted on NBR seals with abrasive grain sizes in the range of 2.8–212 μm; the average mass wear rate was roughly 6 × 10^–6 g/m when the grain size was below 25 μm, and the average mass wear rate was about 16 × 10^–6 g/m when the grain size was larger than 25 μm [118]. Four types of silica abrasive grains, 0–25 μm; 48–60 μm; 75–90 μm and 150–200 μm; three types of Yangtze River sediment particles, 48–60 μm, 75–90 μm, and 120–150 μm, were used for the wear testing of NBR-bearing materials. The 75–90 μm and 120–150 μm sand grains were used for the experiments, and the wear volume increased and then decreased with the grain size, with the 48–90 μm grains having the greatest effect [36]. Under the experimental conditions of a load of 0.7 MPa, 1.1 m/s, for the NBR/cast copper alloy, the particle size increased from 4.5 μm to 75 μm, and the wear mass loss was elevated about 1.67 times [119]. The wear tests on UHMWPE seals using abrasive particles with sizes ranging from 5 μm to 96 μm showed that the wear of the UHMWPE reached a maximum of about 2.5 × 10⁻³ g when the particle size reached 96 μm [120]. The study of erosive wear indicates that the wear rate increases with the increase in the abrasive grain size. The wear rate of the resin material increases with the increase in the abrasive grain size in the three-body wear test, and the wear rate of the rubber material increases with the increase in abrasive grain size. The overall wear rate shows the law of increasing and then decreasing, and the maximum wear rate shows the abrasive grain size is affected by the factors of the load, sliding speed, and hardness of the material, which needs to be specifically analyzed according to the different working conditions.

In addition to the abovementioned three common forms of wear, in water lubrication conditions, with the increase in the sliding speed, frictional heat intensification leads to the evaporation of the water film. Lubrication is insufficient to make the material localized dry friction situation [73]. Therefore, some scholars have conducted a comparative study on the friction of dry and wet lubrication environments [63,144]. There is an appearance of cracks on the surface of the abraded material, and the cracks expand to produce fatigue wear under the action of cyclic loads, which is also a form of wear that is easy to produce in water lubrication conditions [140]. Dry friction and fatigue wear can be studied as an extension of wear in aqueous environments.

5. Conclusions

This paper addresses the study of the wear of polymeric materials under water-lubricated conditions and presents a comprehensive summary of their experimental studies under three major forms of wear. By organizing and analyzing the effects of several important factors on the three forms of wear, the main conclusions are obtained to facilitate the reader or researcher to quickly grasp the characteristics of the wear behavior of polymer materials in the friction and wear process. The aim is to provide a research basis for scholars and engineers to solve the tribological problems of polymer materials under specific working conditions.

From the perspective of lubrication, the influence of water lubrication conditions on the wear of polymer materials is emphasized, the wear mechanism of polymer materials under water lubrication conditions is summarized, and the effects of hardness and material hydrophilicity on wear and lubrication are specifically analyzed, which can provide guidance for the research on material modifications under water lubrication conditions. By discussing water-related lubrication research, the lubrication improvement research for ultra-low friction and wear is outlined, which provides relevant research scholars with a research basis from which they can draw ideas.

From the material point of view, the more ideal water lubrication polymer materials are pointed out by collating the relevant literature on rubber and resin materials, and the laws and limitations of the influence of formulation ratios on the mechanical and tribological properties of the materials are categorized and summarized, so as to provide a research methodology and theoretical basis for the selection and design of polymer components and the improvement of formulations of polymer materials for abrasion resistance under different abrasion conditions in the aqueous environment. Fillers and composites that may be used in the future for material improvement under water lubrication conditions are proposed for the improvement and application field expansion of polymers and their composites.

From the tribological system perspective, the parameters of the three wear forms, two-body wet sliding wear, two-body erosive wear, and three-body abrasive wear, are summarized in order to facilitate the relevant researchers to find references to the literature. The importance of five factors, namely, the load, sliding speed, impact angle, abrasive particles, and temperature, is proposed. By analyzing the experimental studies and conclusions of a large number of studies, the factors and influences involved in different forms of wear are sorted out, and the influences of the five factors on the different forms of wear are discussed, which can help researchers and scholars to seek an effective way to control wear and to improve the wear-resistant properties of materials. It can be helpful for related-research scholars to seek effective ways to control wear and improve the wear resistance of materials.

The study of the tribological behavior of materials is an interdisciplinary subject with multidisciplinary significance. The improvement of polymers and their composites has become a hot research topic. Recent investigations have revealed that superslip, marine tribology, and biotribology research are the main trends of the future. There is still room for improvement in the basic and applied research on the wear of polymers under water-based lubrication conditions. By summarizing the related research conducted in recent years, it can provide readers with references in future research.