Al₂O₃/PTFE Composites for Marine Self-Lubricating Bearings: Modulation Mechanism of Alumina Particle Size on Material Mechanical Properties and Tribological Behavior

Abstract

Polytetrafluoroethylene (PTFE) is one of the alternative materials suitable for seawater-lubricated bearings, favored for its excellent corrosion resistance and good self-lubricating properties. As marine equipment develops towards higher load, higher reliability, and longer service life, more stringent requirements are imposed on the wear resistance of bearing materials. However, traditional PTFE materials struggle to meet the performance requirements for long-term stable operation in modern marine environments. To improve the wear resistance of PTFE, this study used alumina (Al₂O₃) particles with three different particle sizes (50 nm, 3 μm, and 80 μm) as fillers and prepared Al₂O₃/PTFE composites via the cold pressing and sintering process. Tribological performance tests were conducted using a ball-on-disk reciprocating friction and wear tester, with Cr12 steel balls as counterparts, under an artificial seawater lubrication environment, applying a normal load of 10 N for 40 min. The microstructure and wear scar morphology were characterized by scanning electron microscopy (SEM), and mechanical properties were measured using a Shore hardness tester. A systematic study was carried out on the microstructure, mechanical properties, friction coefficient, wear rate, and limiting PV value of the composites. The results show that the particle size of Al₂O₃ particles significantly affects the mechanical properties, friction coefficient, wear rate, and limiting PV value of the composites. The 50 nm Al₂O₃/PTFE formed a uniformly spread friction film and transfer film during the friction process, which has better friction and wear reduction performance and load bearing capacity. The 80 μm Al₂O₃ group exhibited poor friction properties despite higher hardness. The nanoscale Al₂O₃ filler was superior in improving the wear resistance, stabilizing the coefficient of friction, and prolonging the service life of the material, and demonstrated good seawater lubrication bearing suitability. This study provides theoretical support and an experimental basis for the design optimization and engineering application of PTFE-based composites in harsh marine environments.

1. Introduction

With the promotion of the green shipping concept and increasingly strict environmental protection regulations, self-lubricating bearings can directly use seawater as a lubricating medium to eliminate the risk of oil pollution, and have gradually gained wide application in the ship propulsion system. However, the bearing materials serving in the seawater medium need to cope with a high load, high speed, and complex corrosive environment at the same time, and traditional metal bearings are prone to corrosion and cavitation erosion in water, which makes it difficult to operate stably for a long period of time. Therefore, the development of polymer matrix composites with excellent mechanical and tribological properties has become a key method to improve the performance of ship bearings [1,2,3,4,5]. Polytetrafluoroethylene (PTFE) exhibits a very low coefficient of friction and excellent self-lubricating properties, which effectively reduce frictional resistance in water lubrication and other working conditions [6,7,8]. However, the low mechanical strength of its body, which is prone to wear during high load or long-time operation, limits its application under severe service conditions [9,10,11,12,13]. For this reason, a large number of scholars have attempted to add fillers to PTFE to improve its wear resistance and load-carrying capacity. Feng, W et al. [14] explored the feasibility of adding PTFE particles to NBR-bearing materials to form self-lubricating composites under water lubrication conditions by means of ring block tests. The results show that PTFE particles form a lubrication film on the surface under water lubrication conditions, which reduces friction and wear and improves the load-carrying capacity and lubricity, providing a reference for the application of self-lubricating bearing materials. Dong, SK et al. [15] investigated the effects of SiC content and particle size on the tribological properties of PTFE water-lubricated composites. Namely, 3~5 wt% SiC significantly enhanced the wear resistance, of which 5 wt% SiC reduced the wear by 99.44%; increasing SiC particle size led to a decrease in the friction and wear performance of the composites, and the mechanism of the wear resistance shifted from adhesion to abrasion; the composite containing 40 nm SiC reduced the wear by 97.03% and had the best wear resistance. The composites with 40 nm SiC have 97.03% wear reduction and the best wear resistance. This study provides an experimental basis for the development of high-performance PTFE water-lubricated bearings. Wang, JZ et al. [16] compared the tribological performance of PTFE and GCr15 steel in air, distilled water, seawater, and 3.5 wt% NaCl solution. The results show that the friction process of PTFE in seawater is stable, the coefficient of friction is slightly lower than that of distilled water, significantly lower than that of air and NaCl solution, and the wear rate is between distilled water and NaCl solution; the aqueous medium triggers indirect corrosive wear on the counter-abrasive surfaces through corrosion; the green rust generated in the salt solution has a lubricating effect; and Mg²⁺ and Ca²⁺ in seawater can be used in the counter-abrasive surfaces. In seawater, Mg(OH)₂ and CaCO₃ are present on the counter-abrasive surfaces, which attenuate the corrosion and enhance the lubrication of green rust, thus reducing the frictional wear of PTFE. Khan, MJ et al. [17] investigated pin-on-disk friction and wear tests of 25 wt% glass fiber reinforced PTFE against AISI 420 stainless steel in air, distilled water, and seawater environments. The results show that the composites have the lowest coefficient of friction (0.028) and the lowest specific wear rate (5.85 × 10⁻⁶ mm³/N-m) in the seawater environment; the lubrication film formed on the surface under the seawater is the key to the excellent performance, and the indirect corrosive wear also affects the roughness of the counter-abrasive surface. Wang, X et al. [18] studied modified graphene by amino functionalization, characterized by FT-IR, Raman, and XRD, and morphology observed by TEM. The modified graphene reinforced PTFE composites were subjected to face contact friction tests under dry sliding and water lubrication conditions, and the wear surfaces were analyzed by SEM-EDS. The results show that amino-functionalized graphene promotes the formation of a friction film and significantly improves the anti-friction and wear resistance of the composites. The study elucidates the wear mechanism and provides a reference for the design of water-lubricated industrial materials for new energy vehicles.

Alumina (Al₂O₃) is a commonly used reinforcing filler due to its high hardness, chemical stability, and good dispersibility [19,20]. First, the particle size of Al₂O₃ directly affects its dispersion behavior. Smaller particles (e.g., 50 nm) possess a high specific surface area, leading to stronger interfacial bonding, but they tend to agglomerate, resulting in uneven distribution. In contrast, larger particles (e.g., 80 μm) have a lower risk of agglomeration, but their interfacial bonding strength is often insufficient. In terms of tribological properties, interfacial bonding strength determines the efficiency of stress transfer between particles and the matrix. Strong interfacial bonding allows small particles to absorb frictional stress more effectively, promoting the formation of transfer films. Large particles generally enhance load-bearing capacity through their own rigidity. Therefore, the regulation of tribological behavior by particle size essentially depends on the synergistic effect of dispersibility and interfacial bonding. The agglomeration mechanism is likely to hinder stress transfer and affect performance stability [21]. At present, systematic comparative studies on the effects of different particle sizes of Al₂O₃ on the performance of PTFE composites are still insufficient, especially in the context of seawater water lubrication bearing applications, the regulatory mechanisms between different particle sizes of Al₂O₃ on the microstructure of the material, friction behavior, and load-bearing capacity [22]. Therefore, in this study, three types of Al₂O₃/PTFE composites were prepared using Al₂O₃ particles with particle sizes of 50 nm, 3 μm, and 80 μm as variables, and systematically investigated in terms of microstructure, mechanical properties, friction and wear behaviors, and load-carrying capacity.

This paper reveals the enhancement mechanism of Al₂O₃ particles of different sizes in a PTFE matrix, clarifies the influence of particle size changes on the tribological behavior and ultimate PV value of the material, and selects the optimal combination of filler particle sizes suitable for water-lubricated bearings in ships. The results of the study can provide a theoretical basis and experimental support for the engineering application of PTFE composites in complex water lubrication conditions, which has both significant academic value and practical engineering applications.

2. Tests

2.1. Materials

α-Al₂O₃ powder was purchased from Almatis, Frankfurt am Main, Germany, and PTFE powder was purchased from Chemours, Wilmington, DE, USA. To simulate the marine environment, the seawater was prepared by using self-made 3.5% sodium chloride (NaCl). A three-dimensional powder blending machine (model XGJ-5) was purchased from Yanling Drying Equipment, Changzhou, China.

2.2. Preparation of Al₂O₃/PTFE Composites

The preparation method of Al₂O₃/PTFE composites with different particle sizes is as follows: Al₂O₃ powders with three different particle sizes (50 nm, 3 μm, and 80 μm) were cleaned in an ultrasonic cleaner with absolute ethanol for 30 min to remove surface impurities, and then placed in a vacuum drying oven at 80 °C for 2 h to dry. Al₂O₃ particles of 50 nm, 3 μm, and 80 μm, accounting for 3% by mass, were weighed, respectively, to ensure the consistency of the mass of the three types of Al₂O₃ particles. PTFE and Al₂O₃ were mixed in a volume ratio and then loaded into a three-dimensional powder blending machine (model XGJ-5). This was stirred at a speed of 45 revolutions per minute for 2 h to ensure the complete mixing of the two powders. The mixed powder was loaded into a stainless steel mold and cold pressed by a hydraulic press (YA32-315 type, from TMA Machine, Shanghai, China) at 10 MPa pressure, holding pressure for 10 min to obtain a Φ50 mm × 5 mm blank. Finally, the billet was placed in a muffle furnace (SX2-12-16, from Shanghai Yiheng Scientific Instruments Co., Ltd., Shanghai, China) for sintering, which was ramped up to 380 °C at 10 °C/min, held for 3 h, and then taken out after cooling to 100 °C with the furnace. Different particle size Al₂O₃/PTFE groups were prepared synchronously to compare the properties (Figure 1).

2.3. Friction Wear Test

Figure 2a is CFT-1 reciprocating friction and wear tester (Jinan Hengxu Testing Machine Technology Co., Ltd., Jinan, China) was used to evaluate the tribological properties of Al₂O₃/PTFE composites. The average roughness (Ra) of the specimen surfaces was approximately 0.15 ± 0.02 μm. Before the tests, the specimens were cleaned with anhydrous ethanol in an ultrasonic bath for 30 min, and then thoroughly dried 0.6 mm diameter Cr12 steel balls (hardness 62 HRC, surface roughness Ra = 0.05 μm) were used for the grinding pair and were fixed in the loading arm of the tester. Under the lubrication condition of seawater at room temperature, the normal load was set at 10 N, the reciprocating frequency was 5 Hz, the stroke was 5 mm, and the friction coefficient was continuously rubbed for 40 min. The friction coefficient was recorded in real-time by the built-in strain transducer, with a sampling frequency of 10 Hz. All experiments were repeated five times to ensure data reproducibility.

At the end of the experiment, the Al₂O₃/PTFE Composites and PTFE specimen wear areas were ultrasonically cleaned for 10 min using anhydrous ethanol. The cross-sectional morphology at the wear track was measured using a three-dimensional optical profilometer (Contour GT-K, Bruker, Billerica, MA, USA). To ensure consistency and reproducibility, wear contour measurements were taken at five fixed locations evenly distributed along the centerline of the wear track-one at the center and two symmetrically positioned on each side. These measurements were used for the calculation of wear rates, and the wear volume (V/m³) was calculated by integrating the wear depth and width. The wear rate (W/m³(N·mm)⁻¹) was calculated according to the following equation:

$$W=V/F\cdot L$$

where W is the wear rate, m³/(N·m); V is the wear volume; F is the normal load, N; L is the total sliding distance, mm.

Figure 2b is the schematic diagram of the MMU-5G end face wear testing machine. the MMU-5G end face friction and wear tester (Jinan Hengxu Testing Machine Technology Co., Ltd.) was used to determine the ultimate PV value of Al₂O₃/PTFE composites. The average roughness (Ra) of the specimen surfaces was approximately 0.15 ± 0.02 μm. The specimens were cleaned with anhydrous ethanol in an ultrasonic bath for 30 min, then thoroughly dried, and formed an end-face friction pair with a rotary 42CrMo alloy ring (hardness 48-52 HRC after quenching, Ra = 0.1 μm). The tester is a unidirectional movement. The experiments were carried out under seawater lubrication at 25 °C, with an initial contact pressure of 10 MPa and a constant rotational velocity V of 0.2 m/s, followed by an incremental pressure increase of 2 MPa at 10 min intervals until the torque monitoring system triggered a threshold value of 10 N·m (indicating that Seizure failure had occurred), and a record was made of the critical pressure at this point, P. The data were collected in real time by a strain-gauge transducer (sampling frequency of 50 Hz). This ultimate PV value enables a quantitative comparison of the differences in load-bearing performance among various material systems under extreme operating conditions.

2.4. Characterization and Testing Methods

A variety of instruments were used to test and characterize the material properties systematically. The mass difference in the specimens before and after abrasion was used to calculate the wear rate for the end-friction test. The mass of the specimens before and after abrasion was weighed by the ESJ30-5A electronic analytical balance produced by Shanghai Measurement Precision Instrumentation Co., Ltd., Shanghai, China. with a weighing accuracy of 0.01 mg. To minimize the testing error, each group of specimens was weighed five times, and the average value was recorded. The hardness of the material was tested by the ASTM D2240 standard [23] using a D-type Shore hardness tester. For the friction performance test, the coefficient of friction was measured by the CFT-1 reciprocating friction and wear tester under ball-face contact conditions. The limiting PV value test was performed by an MMU-5G end face friction and wear tester, which was obtained by step-by-step loading. The microstructure and wear morphology of the specimen surface were observed and analyzed using a Sigma-300 scanning electron microscope (SEM, Carl Zeiss AG, Oberkochen, Germany) and an energy spectrometer (EDS) to further identify the composition of the wear surface and products.

3. Results and Discussion

3.1. Powder Morphology and Specimen Micromorphology

Figure 3 presents the microstructures of Al₂O₃ particles with three distinct sizes (50 nm, 3 μm, and 80 μm) and their corresponding Al₂O₃/PTFE composites. Figure 3a–c depicts the SEM morphologies of pristine Al₂O₃ particles: all three sizes exhibit quasi-spherical or near-spherical shapes, demonstrating favorable sphericity. Specifically, the 50 nm Al₂O₃ particles display well-defined boundaries with negligible agglomeration, indicative of excellent monodispersity. The 3 μm Al₂O₃ particles show a broad size distribution ranging from 1 μm to 3 μm, accompanied by irregular spatial arrangement. In contrast, the 80 μm Al₂O₃ particles feature relatively uniform particle sizes with minimal size variation, presenting a more homogeneous morphological characteristic. Figure 3d–f show the micro-morphologies and the distribution states of Al elements of the Al₂O₃/PTFE composites with three different sizes. The composite microstructures reveal that Al₂O₃ particles are uniformly embedded within the PTFE matrix. The EDS mapping results further confirm the homogeneous distribution of Al elements across the composite surfaces, as no local agglomeration is detectable. This superior dispersion homogeneity implies that Al₂O₃ particles of all three sizes establish effective interfacial interactions with the PTFE matrix. Such a favorable dispersion state is conducive to subsequent investigations of mechanical and tribological properties, as it minimizes the adverse effects of particle clustering on composite performance.

3.2. Shore Hardness

Hardness is an important indicator of the mechanical properties of a material [24], and Figure 4 shows a comparison of the Shore hardness of pure PTFE and three different particle sizes of Al₂O₃-filled PTFE composites. The results show that the hardness of all composites is significantly higher than that of pure PTFE, indicating that the introduction of Al₂O₃ particles significantly enhances the deformation resistance of the material regardless of the filler particle size. The 80 μm Al₂O₃/PTFE composites exhibit the highest hardness values, followed by the 3 μm and 50 nm groups, in that order. The main reason is that the Al₂O₃ particles with a larger particle size of 80 μm are more likely to form a stable spatial support structure during cold compression molding and Sintering, which can bear a larger local load and form a “rigid pillar” effect under the state of stress, thus effectively inhibiting the elastic deformation of the PTFE matrix, and significantly increasing the overall hardness of the composites [25]. The overall hardness of the composite material is significantly improved [25]. The 3 μm particle size Al₂O₃ has both dispersive and supportive effects and also has a significant improvement in hardness [26]. Moreover, 50 nm nanoscale Al₂O₃ is very prone to agglomeration, which in turn restricts its reinforcing efficacy, and ultimately manifests itself in the macro-hardness enhancement lower than that of the micron-sized filler group. Pure PTFE exhibits a lower hardness level due to its high molecular chain flexibility and lack of rigid support, which itself is difficult to resist the stress concentration generated during the indentation process. Therefore, different particle size Al₂O₃ has obvious differences in mechanical properties, especially in surface hardness enhancement.

3.3. Friction Wear Properties

The coefficient of friction is one of the important parameters that characterize the wear performance of materials, directly reflecting the size of sliding resistance between the contact interfaces and affecting the energy consumption, wear rate, and service life of materials. Figure 5 and Figure 6 show the friction coefficient versus time curves of Al₂O₃/PTFE composites with different particle sizes under reciprocating friction conditions. As seen from the results, the average coefficient of friction of the pure PTFE material is 0.156, with minimal fluctuations. The coefficient of friction of the 50 nm Al₂O₃/PTFE composites is approximately 0.13, which is the lowest among the four groups, exhibiting the smallest fluctuations and excellent friction stability. The friction coefficient of 3 μm Al₂O₃/PTFE was about 0.168, which was higher than that of the nano group. The highest friction coefficient of 80 μm Al₂O₃/PTFE composites was 0.193, which fluctuated obviously at the beginning, and then gradually stabilized. The results indicate that the nano-Al₂O₃ particles are beneficial in enhancing the friction-reducing and self-lubricating properties of PTFE materials. This is due to the low-shear-strength nanoparticle agglomerates, reducing the overall shear force required to slide the PTFE composite, resulting in a lower and more stable coefficient of friction [27].

Wear rate reflects the wear resistance of a material under specific load and motion conditions, and is an important tribological property parameter for assessing its durability and service life. Figure 7 shows the wear rates of pure PTFE and three different particle sizes of Al₂O₃-filled PTFE composites, and it can be seen that the introduction of Al₂O₃ significantly reduces the wear rate of the material, in which 50 nm Al₂O₃/PTFE exhibits the best wear resistance, with a wear rate of only 5.8 × 10⁻¹⁴ m³/(N·m), a significant reduction of about 69% compared to 18.7 × 10⁻¹⁴ m³/(N·m) for pure PTFE. Although the wear resistance of 3 μm and 80 μm particle sizes of Al₂O₃ particles was better than that of pure PTFE, their wear rates were 8.4 and 9.6 × 10⁻¹⁴ m³/(N·m), corresponding to reductions of approximately 54% and 48%, respectively, which were significantly higher than those of the nanoparticle size group. This is because the large particle size of micron-sized Al₂O₃ will cause damage to the transfer film, thereby reducing the wear resistance of the composite material. Therefore, the 50 nm Al₂O₃/PTFE material has the best wear resistance. Nanoscale Al₂O₃, due to its extremely small size, large specific surface area, and high surface activity, is able to promote the directional arrangement of polymer molecules at the interface during friction, which helps to form an effective lubricating and protective layer on metal-to-metal surfaces, reduce direct contact and shear damage, and significantly inhibit the material volume loss [28]. On the other hand, pure PTFE itself has soft molecular chains, limited pressure resistance, and is prone to plastic flow and shear shedding under frictional loads, lacks an effective interfacial stabilization mechanism, and therefore wears the most severely [29].

3.4. Wear Mark Analysis

Figure 8 shows the morphology of the abrasion marks and their three-dimensional profiles of Al₂O₃/PTFE composites with different particle sizes after reciprocating friction tests, revealing the differences in the surface structural features and wear mechanisms formed by different particle sizes of the Al₂O₃-filled PTFE composites in the friction process. Figure 8a shows that the distribution of Al₂O₃ on the surface of nano-Al₂O₃/PTFE composites is basically consistent with the sliding direction, which forms a friction film, effectively isolating the direct contact and shear damage to the friction parts, reducing the friction and wear, and the corresponding three-dimensional morphology of the abrasion marks is shallow and flat, which further verifies its excellent friction and wear reduction performance. Figure 8b shows obvious scratches, shallow cracks, and flaking blocks, and the three-dimensional morphology can be observed, showing that the depth of the abrasion marks is slightly larger than that of group Figure 8a, and the surface undulation is intensified, which indicates that its abrasion resistance and stability are relatively weak. Figure 8c has the roughest abrasion mark morphology, with large deep cracks on the surface, and more obvious wear phenomenon on the surface layer of the material, indicating that large particles are more likely to form interfacial discontinuity and stress concentration during friction, which leads to problems such as interfacial debonding, microcracks extension, and abrasive damage, etc. The depth of abrasion grooves in the three-dimensional contour map is the most significant, and the surface concavity and convexity are drastic, which indicates that it has poorer wear-resistant properties. Therefore, the nanoscale Al₂O₃ filler is easier to form a dense and stable friction film in the PTFE matrix to show excellent friction reduction and wear resistance properties during the friction process. As the particle size increases, it fails to form an effective friction film, and the surface damage increases, leading to a significant increase in the degree of wear. These morphological characteristics are consistent with the trends of the friction coefficient and wear rate data obtained previously, further confirming the correspondence between the friction film and the friction reduction and wear resistance properties.

The EDS distribution of Al elements in Figure 8 shows the following: in the 50 nm group, the Al elements are continuously and uniformly enriched along the sliding direction, which indicates that the nanoscale fillers can be stably aggregated and self-assembled to form a protective film covering the surface of the specimen during the friction process; in the 3 μm group, the distribution of Al is fragmented, mostly aggregated on the edges of the shallow cracks and flakes, and the medium-sized particles are easy to debond at the interface and difficult for forming a film continuously; and there is almost no coherent Al enrichment band in the 80 μm group. In the 3 μm group, the Al distribution is scattered, mostly at the edges of shallow cracks and flaking blocks, and the medium-sized particles are easy to debond at the interface and difficult for sustaining the film formation. The Al distribution characteristics at these three scales are highly consistent with the trends of the friction coefficient and wear resistance.

3.5. Transfer Film Analysis

Figure 9 presents the wear-scar morphologies and elemental distribution characteristics formed on the Cr12 steel balls’ surfaces. As shown in Figure 9(a1–d1), in the wear scar areas of the dual components of pure PTFE, 3 μm Al₂O₃/PTFE, and 80 μm Al₂O₃/PTFE, obvious particle accumulation was observed. Moreover, tear and peeling marks could be clearly seen in the boundary areas. The 50 nm Al₂O₃/PTFE material significantly reduced the amount of wear debris produced by the dual components over the entire range of wear marks, with regular edges to the abrasion zones and a more intact surface, indicating that the amount of material detachment during the friction process is minimal. From the morphology of Figure 9(a2–d2), it can be seen that the pure PTFE and micron-sized Al₂O₃-filled sample’s counterpart exhibit more tensile grooves and microscopic cracks on the surface of the dyadic parts, with poorer continuity of the material surface, indicating stronger shear wear and tear damage characteristics. In contrast, the surface of the 50 nm Al₂O₃/PTFE’s counterpart showed continuous and fine parallel abrasion marks with shallow and regular grooves, which indicated that the interfacial shear process was more moderate, no large-scale pulling cracks or localized damages were observed, and a relatively dense transfer film structure was formed at the friction interface [30]. As can be seen from the distribution of Al elements and the elemental content table, the 50 nm Al₂O₃/PTFE’s counterpart exhibited a dense and uniform distribution of aluminum elements on the surface of the CR12 steel ball counterface, with an aluminum content of 7.8 wt%, while the oxygen content is 12.4 wt%, which indicates that the material is capable of forming an Al₂O₃-based transfer film on the surface of the metal dyad during the friction process. In addition, the sample has the highest F element content of 6.5 wt%, indicating that the polymer component is also involved in the construction of the transfer film, and the form of interfacial contact tends to be more like polymer–polymer contact, which reduces the interfacial frictional stress and slows down the wear process on the metal surface [31]. The lowest F element on the surface of the pure PTFE material pair is only 3.1%, indicating that the interface lacks effective transfer film protection, and is always in the direct friction state between the polymer and the metal. The 80 μm Al₂O₃/PTFE’s counterpart has a certain amount of Al element (1.4 wt%), but its distribution is sparse, because the large particles of Al₂O₃ are embedded in the process of friction and detachment, making it more likely to cause mechanical plowing of the metal surface, which cannot effectively slow down the wear process. The mechanical plowing during the friction process caused by 80 μm Al₂O₃ will damage the integrity of the transfer film. Different particle sizes of Al₂O₃ fillers have significant effects on the ability of PTFE composites to form transfer films at the friction interface. The nano-sized Al₂O₃, due to its extremely small size and huge specific surface area, has strong interfacial bonding with PTFE. Therefore, during the friction process, it is more likely to interact with the substrate, thereby forming a uniform and continuous inorganic-organic composite transfer film. This significantly improves the friction performance of the material and the protective performance of the coupling component.

The friction coefficient curves of the materials under different loading conditions are shown in Figure 10, which shows that with the gradual increase in the friction load (from 4 MPa to 22 MPa), the friction factor shows a decreasing trend of fluctuation in phases, and finally terminates due to the failure of the surface wear of the materials. The 50 nm Al₂O₃/PTFE composites have the lowest friction factor throughout the whole test process and have not failed even at the maximum load of 22 MPa, showing remarkable stability and load-carrying capacity. In contrast, the pure PTFE material exhibited obvious failure characteristics at 18 MPa, whereas the 3 μm and 80 μm Al₂O₃-filled PTFE samples displayed a sudden change in the friction factor at 20 MPa, indicating that they had entered the severe wear stage. For pure PTFE materials, due to their limited intrinsic strength and pressure resistance, when the friction load continues to increase to the critical value, it is difficult to maintain the surface transfer film structure stably, the lubrication performance rapidly decays, and the friction factor fluctuates drastically, which ultimately leads to interfacial spalling failure [32]. The 50 nm Al₂O₃ filler with smaller particle size and higher specific surface area is easier to be distributed in the interface during the friction process and participate in the construction of the transfer film, which effectively inhibits the expansion of the surface microcracks, and the friction factor is still low and smooth in the high load section without destructive fluctuations [25]. With the load increment, the initial friction factor of the four groups of samples have a certain degree of decline, indicating that in the lower load stage, the surface of each composite material is in the process of friction, the rough peaks come into contact with each other to form the initial shear zone, accompanied by the gradual establishment of the transfer film, the interface gradually tends to be stabilized, and frictional resistance is reduced accordingly. Thereafter, with the further increase in load, the real contact area between the rough peaks increased significantly, and interfacial debonding and spalling started to occur in pure PTFE and micrometer-sized Al₂O₃/PTFE, resulting in a rapid increase in the friction factor until failure. On the other hand, the 50 nm Al₂O₃/PTFE exhibited excellent compressive stability and wear resistance throughout the test interval, with an ultimate PV value of more than 4.4 MPa·m/s, which is significantly higher than that of other comparative materials, indicating that it is more suitable for high-load engineering scenarios. Therefore, the effects of different particle sizes of Al₂O₃ on the frictional properties of PTFE composites are significant, in which 50 nm Al₂O₃ significantly enhances the load-bearing stability and tribological properties of the material under high loading conditions due to its excellent dispersion and interfacial synergy.

4. Conclusions

In this paper, three kinds of PTFE matrix composites reinforced by Al₂O₃ particles with different particle sizes (50 nm, 3 μm, and 80 μm) were prepared by the cold pressing-sintering process to investigate the differences in mechanical properties, friction behaviors, and load-bearing capacity under a seawater lubrication environment, and the main findings are as follows:

(1) The hardness of all Al₂O₃/PTFE composites is significantly higher than that of pure PTFE, indicating that the introduction of Al₂O₃ particles can effectively enhance the deformation resistance of the material. Among them, the 80 μm Al₂O₃/PTFE composite exhibits the highest hardness, followed by the 3 μm and 50 nm groups. This is mainly because 80 μm particles tend to form a stable spatial support structure and exert a “rigid pillar” effect during the molding process, thereby inhibiting the elastic deformation of the matrix. The 3 μm particles combine both dispersion and support functions. In contrast, the 50 nm particles are prone to agglomeration, which limits their reinforcing effect, resulting in a lower hardness improvement compared to the micrometer-sized filler groups. Pure PTFE has a lower hardness due to the lack of rigid support and the high flexibility of its polymer chains.

(2) Al₂O₃ particle size effectively enhances the friction behavior and wear mechanism of the composites. In the reciprocal friction test, the friction factor and wear rate of all Al₂O₃-containing samples were lower than those of the pure PTFE group, and the samples in the 50 nm group exhibited the lowest friction factor (0.13) and the smallest wear rate (5.8 × 10⁻¹⁴ m³/N·m). Comparative analysis shows that as the particle size increases, it becomes more difficult to form a uniform and dense transfer film at the friction interface, the interfacial shear damage increases, and the wear resistance is poor. Therefore, 50 nm Al₂O₃ filler has the most significant effect in inhibiting abrasive wear and microscopic shear damage.

(3) Nano-Al₂O₃ enhances the ultimate PV value and load resistance through interfacial synergy. In the seawater-lubricated end-face friction test, the ultimate PV value of 50 nm Al₂O₃/PTFE composites exceeded 4.4 MPa·m/s, which was superior to that of pure PTFE and micron filler groups. This improvement is mainly attributed to the formation of a dense and uniform transfer film enriched with fluorine and Al₂O₃ particles, which effectively protects the counterface. Meanwhile, the nanoparticles fill interfacial depressions, homogenize contact stresses, and suppress microscopic adhesion damage, thereby enhancing the load-bearing performance of the composites.

Nanoscale Al₂O₃ fillers have significant advantages in enhancing the wear resistance, friction stability, and ultimate load-carrying capacity of PTFE-based composites. The particle size regulation mechanism revealed in this paper provides important theoretical support and an experimental basis for the engineering design and performance optimization of PTFE-based composites under seawater lubrication conditions, which has good application prospects and promotion value.