Crab Leg-Inspired Poly(p‑phenylene benzobisoxazole) Hybrid Fibers for Boosting the Tribological Performance of UHMWPE Composites

Abstract

The performance of ultra-high-molecular-weight polyethylene (UHMWPE) is frequently limited by issues such as low strength and wear resistance. Inspired by the hierarchical structure of crab legs, this study develops a hybrid fiber filler designed to synergistically enhance the tribological and mechanical properties of UHMWPE composites. The filler (PBO@PDA@ZnO) architecture utilizes poly­(p-phenylene benzobisoxazole) (PBO) fibers as a rigid structural skeleton, employs polydopamine (PDA) as a functional intermediate layer, and incorporates uniformly loaded zinc oxide (ZnO) nanoparticles on the surface to create a “rigid skeleton-functional particle” system. The results indicate that the PBO@PDA@ZnO hybrid filler significantly optimizes both mechanical and tribological performance. Specifically, the UHMWPE composite reinforced with 1 wt % filler exhibits the best overall performance, with the elongation at break increasing from 650% to 850% while simultaneously improving tensile strength. Furthermore, under both low and high load conditions, the friction coefficient of the UHMWPE composite is all reduced by over 60%, and the wear rate is decreased by an order of magnitude, with the lowest value reaching 8.46 × 10^–7 mm³/N·m. This superior performance is attributed to the synergistic effects of fiber reinforcement, stable transfer film formation, and the “micro-bearing” rolling effect of the ZnO nanoparticles. This study provides a biomimetic strategy for the fabrication of high-performance self-lubricating UHMWPE composites.

Introduction

Ultra-high-molecular-weight polyethylene (UHMWPE) is widely used in critical fields, such as artificial joints and high-performance bearings, owing to its exceptional impact toughness, low friction coefficient, and outstanding wear resistance. − However, neat UHMWPE exhibits inherent limitations in its mechanical load-bearing capacity and tribological stability when subjected to severe operating conditions, particularly high contact stresses and elevated temperatures. Its relatively low hardness and elastic modulus, along with a high susceptibility to plastic deformation and creep, restrict its deployment in more demanding industrial environments. ,

To overcome these performance limitations of UHMWPE, various strategies such as molecular structure modification, filler reinforcement, and surface modification have been extensively explored. − Among these, the incorporation of fiber reinforcements has emerged as a highly effective approach. It can not only significantly enhance the mechanical strength and Young’s modulus of composites through load transfer effects , but also contribute to the transfer tribofilm formation on the counterpart, thereby improving tribological properties. , Recently, the incorporation of multiple fillers together with fibers into polymeric matrices frequently induces a synergistic effect, thereby enhancing the overall performance characteristics. In these hybrid fillers, the presence of fiber can provide structural reinforcement and improve load transfer efficiency, while hard particles (e.g., SiO₂, and TiO₂ ) will help resist abrasive wear by bearing localized stresses. Furthermore, thermally conductive fillers (e.g., graphite, and boron nitride) can increase heat dissipation at the sliding interface, reducing the fatigue wear.

Among various reinforcement materials, poly­(p-phenylene benzobisoxazole) (PBO) fiber, an organic fiber with high mechanical strength, high thermal stability, and low density, has received widespread attention. Currently, it has been applied to enhance the properties of various polymers, including epoxy resin, polyimide, − phenolic resin, polyester, etc. However, the smooth and inert surface of PBO leads to poor interfacial interaction with the polymer matrix, severely restricting the strength, friction, and other properties of its composites. , As a result, reports on its enhancement of the friction performance of UHMWPE are still scarce. Therefore, there is an urgent need to develop surface modification methods to improve its surface roughness and reactivity, such as acid–base treatment, plasma treatment, irradiation, chemical grafting, etc.

In nature, sophisticated structural designs are widely observed in natural animals, offering valuable insights into the development of engineered materials and the achievement of exceptional multifunctional properties. For example, the crab leg is found to be a biocomposite of calcium carbonate nanoparticles filled in spaces between chitin–protein fibers, which are embedded in a hardened protein matrix. This elegant hierarchical structure solves the engineering challenges of supporting a body, resisting predation, and enabling efficient movement in a complicated environment. Inspired by crab legs, we realized that PBO–nanoparticle hybrid fibers could be achieved if efficient molecular interactions are introduced. We therefore propose that an adhesive interlayer would provide multiple strong interactions between PBO fibers and inorganic nanoparticles, which is appealing and yet challenging.

In this work, the crab-leg-inspired PBO@PDA@ZnO hybrid fiber was designed and prepared to mimic the hierarchical structure. By incorporating this filler as a reinforcement phase into the UHMWPE matrix, it is anticipated that the PBO fibers will provide core mechanical enhancement, the ZnO nanoparticles will contribute to the rolling friction reduction effect, and the PDA interlayer will ensure synergistic interaction between them. This strategy seeks to simultaneously improve the mechanical and tribological characteristics of UHMWPE composites, providing new ideas and an experimental foundation for the development of high-performance self-lubricating materials that are suitable for extreme operational environments.

Experimental Section

Materials

The materials used in this experiment include PBO chopped fibers with a length of 6 mm (obtained from Toyobo Co., Ltd., Osaka, Japan), dopamine hydrochloride (Shanghai Macklin Biochemical Technology Co., Ltd.), Tris–HCl buffer solution (Shanghai Sunbio Biotechnology Co., Ltd.), sodium hydroxide (Tianjin Yongda Chemical Reagent Co., Ltd.), zinc acetate dihydrate (Shanghai Aladdin Biochemical Technology Co., Ltd.), and UHMWPE powder with a viscosity-average molecular weight of 2 × 10⁶ g/mol (purchased from Shanghai Lianle Chemical Technology Co., Ltd., Shanghai, China). The selection of UHMWPE with a molecular weight of 2 × 10⁶ g/mol was primarily based on considerations to balance processing performance and mechanical properties.

Synthesis of PBO@PDA Fiber

The PDA-decorated PBO fiber (PBO@PDA) was synthesized following our previous literature. Scheme shows the synthesis and preparation of PBO@PDA@ZnO fiber and UHMWPE composites. The synthesis of PDA is a mild and facile oxidation process, so the reaction was conducted under ambient temperature and pressure in air, using a 1 L open beaker as the container to ensure sufficient contact area between the reaction solution and air. Two grams of PBO chopped fibers were dispersed into 500 mL of Tris–HCl buffer solution (pH = 8.5, 0.1 mol/mL) and ultrasonicated for 5 min. Then, 1 g of dopamine hydrochloride was added, and the mixture was stirred vigorously for 24 h. During this process, a PDA layer formed on the surface of the PBO fibers, and the color of the fibers changed from golden yellow to dark brown. The resulting mixture was filtered and washed three times with deionized water, followed by vacuum drying at 45 °C. The obtained product was PBO@PDA. This procedure was repeated multiple times to obtain a sufficient quantity of PBO@PDA. Scheme 1 illustrates the synthesis reaction of PBO@PDA, as well as the synthesis and preparation routes of other substances.

1. Synthesis and Preparation of PBO@PDA@ZnO Fiber and UHMWPE Composites.

Synthesis of PBO@PDA@ZnO Hybrid Fiber

PBO@PDA@ZnO was synthesized via a hydrothermal reaction. A 100 mL hydrothermal autoclave was used as the reactor. 60 mL of deionized water, 155 μL of a 0.02 g/mL sodium hydroxide solution, and 0.5 g of PBO@PDA were mixed. After shaking to mix thoroughly, 0.3 g of zinc acetate dihydrate was added, and the mixture was shaken again to ensure homogeneity. The reaction was then carried out at 110 °C for 12 h to allow for the nucleation and growth of ZnO on the active sites of PBO@PDA, yielding the crude product of ZnO nanoparticle-coated PBO@PDA. The product was washed three times with deionized water and dried. Finally, it was annealed in air at 350 °C for 2 h to obtain PBO@PDA@ZnO. The color of the fibers changed from dark brown to brownish-orange, intermediate between those of PBO and PBO@PDA. This procedure was repeated multiple times to obtain a sufficient quantity of PBO@PDA@ZnO.

Preparation of UHMWPE/PBO@PDA@ZnO Composites

The composites were prepared via melt blending and hot pressing. PBO@PDA@ZnO and UHMWPE powder were mixed according to predetermined proportions, with the fiber content relative to the total composite mass being 0%, 0.5%, 1%, 2%, 4%, and 6%, respectively. The mixtures with different ratios were then melt-mixed using an internal mixer (MetaStation 8E, Brabender, Germany) at 200 °C for 8 min. Then, the blends were shaped by hot pressing under 220 °C and 10 MPa pressure for 10 min by a plate vulcanizer (POTOP, Guangzhou Putong, China). Finally, they were cut into various forms for testing. Furthermore, to comparatively illustrate the role of ZnO, a UHMWPE/PBO@PDA composite containing 1 wt % PBO@PDA was also prepared using the same method. The formulations and sample names of various composite materials are shown in Table S1.

Characterization

The structure and properties of the materials were characterized by scanning electron microscopy (SEM, Thermo Fisher, Verios 5 UC), energy-dispersive X-ray spectrometry (EDS, coupled with SEM), X-ray diffraction (XRD, Bruker, D8 ADVANCE Davinci), X-ray photoelectron spectroscopy (XPS, Escalab Xi+, Thermo Fisher), thermal gravimetric analysis (TGA, TA, TGA 5500), differential scanning calorimetry (DSC, TA, DSC 250), tensile test (Sasev, CMT5504), friction test (Rtec, MFT 2000A), and white light interferometry (Bruker, Contour GT-X).

Before the friction test, the composites were first cut into a 25 mm diameter disc, bonded to a stainless-steel base in order to avoid errors introduced by deformation or vibration of the soft UHMWPE material, and then polished by the same procedure to achieve the same initial surface conditions. Finally, the composites were tested under loads of 5, 15, and 30 N, respectively. Because of their universality as a standard grinding material, 10 mm diameter GCr15 bearing steel balls were used as the grinding side. The single-travel distance was 5 mm, the frequency of reciprocating travel (10 mm) was 5 Hz, and the continuous test lasted for 1 h. These are typical testing conditions for evaluating the tribological behavior of polymer composites under reciprocating sliding.

The wear rate is calculated using formula : where V is the wear volume (mm³), measured by a white light interferometer; F is the normal load (N); and L is the total sliding distance (m).

$$W=\frac{V}{F\times L}$$ 1

Results and Discussion

Design and Synthesis of PBO@PDA@ZnO Fiber

The core of this design is to construct a “robust skeleton–functional particle” system. The presence of using PBO fibers serve as the load-bearing skeleton, mimicking the main structure of crab legs. Simultaneously, the incorporation of zinc oxide (ZnO) nanoparticles, which have high hardness and controllable morphology, is employed as the functional units. , Here, PBO fibers reinforce the matrix, bear the load, and suppress plastic deformation, thereby reducing friction and wear. While ZnO nanoparticles act as “micro-bearings” at the friction interface, they can convert sliding friction into rolling friction, thereby significantly reducing the friction coefficient, a process known as the “micro-bearing” rolling effect. ,

To realize this design, the key is to firmly and uniformly load ZnO nanoparticles onto the chemically inert surface of PBO fibers. Polydopamine (PDA), as a biomimetic polymer with strong adhesion, provides an ideal solution to this interface engineering challenge. , First, PDA is coated on the surface of PBO fibers to improve the interfacial adhesion between the fibers and the polymer matrix. , Then, the abundant functional groups on the surface serve as ideal nucleation sites to guide the growth of ZnO nanoparticles. − Thus, the specific implementation pathway was established, culminating in the design of PBO@PDA@ZnO hybrid fillers.

Structure of PBO@PDA@ZnO Fiber

Figure a shows the SEM image of pristine PBO fiber. It is clear that pure PBO has a very smooth surface. After surface functionalization, the surface of PBO@PDA@ZnO fiber (Figure b) becomes noticeably rough, covered with a layer of uniform tiny nanoparticles. This significant morphological difference indicates the successful loading of the PDA coating and ZnO nanoparticles. Correspondingly, the EDS analysis in Figure S1 reveals distinct signals corresponding to nitrogen (N) and zinc (Zn) on the surface of the modified PBO fibers. Since Zn is not an intrinsic component of pristine PBO, its presence indicates successful incorporation of ZnO nanoparticles after modification. Quantitative elemental analysis further confirms the introduction of these exogenous elements, thereby providing compositional evidence that supports the morphological observations from SEM. Information regarding the material’s crystalline structure is presented through the XRD pattern (Figure c). Pure PBO fiber displays its characteristic crystalline diffraction peaks at 16.0°, 25.6°, and 27.2°, which correspond to (200), (010), and (210) planes, respectively. Upon introducing an amorphous PDA layer, the intensity of these peaks weakens, but the peak shape remains unchanged. Most crucially, a series of new diffraction peaks emerge in the XRD pattern of the PBO@PDA@ZnO hybrid. The positions of all these new peaks perfectly match the standard pattern for hexagonal wurtzite ZnO. The characteristic peaks are observed at approximately 31.8°, 34.4°, 36.3°, 47.5°, and 56.6°, which correspond to the (100), (002), (101), (102), and (110) crystal planes of ZnO, respectively. The presence of these diffraction peaks confirm the successful formation of well-crystallized ZnO on the fiber surface via the hydrothermal and annealing processes. To further elucidate the surface chemical composition and bonding states of the hybrid fibers, XPS analysis was performed. The survey spectrum (Figure d) confirms the presence of C, N, O, and Zn elements on the surface. High-resolution spectra were subsequently deconvoluted to extract detailed chemical state information. The N 1s spectrum (Figure e) shows that nitrogen primarily exists in amine/imine forms characteristic of PDA, and a subtle shift in its binding energy compared to pure PDA suggests that some N atoms have likely coordinated with Zn ions. Furthermore, the high-resolution Zn 2p spectrum (Figure f) displays two distinct peaks at binding energies of 1022.7 and 1045.8 eV, corresponding to Zn 2p_3/2 and Zn 2p_1/2 spin–orbit components, respectively, with a spin–orbit splitting of 23.1 eV. These values are in good agreement with those reported results of ZnO nanoparticles. , These above results indicate the successful preparation of a hierarchical structured hybrid fiber with PBO as the substrate, PDA as the adhesive interfacial layer, and crystalline ZnO as the functional surface.

1.

SEM images of (a) pure PBO fiber and (b) PBO@PDA@ZnO fiber. (c) XRD patterns, (d) XPS full scan spectrum, (e) N 1s high resolution spectrum, and (f) Zn 2p high resolution spectrum of PBO@PDA@ZnO fiber.

Morphology of PBO@PDA@ZnO/UHMWPE Composites

The cryo-fractured surface morphologies of pure UHMWPE and its composites were characterized using SEM, complemented by EDS for elemental mapping (Figure ). The cross sections of all the samples (Figure a–d) exhibit characteristic ductile tearing ridges, indicative of significant plastic deformation and effective energy absorption during fracture. This confirms an efficient stress transfer and dissipation within the composites. With the increase of fiber content, a correspondingly higher density of dispersed fibers is observed. Notably, there is no evident macroscopic interfacial debonding or significant void formation between the embedded fibers and the UHMWPE matrix, implying a robust interfacial adhesion. , The EDS analysis result reveals that pure UHMWPE (UHMWPE-0) shows no signals for O, N, or Zn, while UHMWPE-PDA-1 can detect O and N elements but no Zn. In contrast, UHMWPE-1 and UHMWPE-6 reveal distinct, overlapping signals for N, O, and Zn localized exactly along the fiber pathways. This elemental distribution perfectly matches our theoretical design, confirming the co-location of the PBO skeleton and PDA layer (the sources of N and O) with the ZnO nanoparticles (the source of Zn). ,

2.

Morphology images and EDS mapping images of the cryo-fracture surface of UHMWPE and its composites: (a) UHMWPE-0, (b) UHMWPE-1, (c) UHMWPE-6, and (d) UHMWPE-PDA-1.

Thermal Stability of UHMWPE/PBO@PDA@ZnO Composites

The thermal stability of pure UHMWPE and its composites is evaluated by TGA and DTG curves, as shown in Figure S3. All of the samples exhibit a similar single decomposition stage, with the rapid decomposition occurring between 450 and 520 °C. Compared to pure UHMWPE, the degradation curves for the composites containing PBO@PDA@ZnO fillers shift toward higher temperature regions and have higher char residual mass. These results suggests that the hybrid fillers effectively enhance the thermal stability of the UHMWPE composites. The quantitative data in Table S2 further elucidated this trend. As the PBO@PDA@ZnO loading increases from 0 wt % to 6 wt %, the initial decomposition temperature (T ₁₀) steadily rises from 421.51 °C to 438.99 °C, while the temperature at the maximum decomposition rate (T _p) remained relatively stable, fluctuating slightly between 469.6 °C and 463.4 °C. Simultaneously, we examined the char residues at 700 °C, where the primary thermal decomposition of UHMWPE and its composites has been completed, allowing for a clear comparison of the residual mass contributed by the filler. The results showed that a 6 wt % PBO@PDA@ZnO loading significantly increased the char residue from 0.89% to 1.71%. These above results confirm that the addition of the PBO@PDA@ZnO hybrid not only delays the initial decomposition of the polymer but also promotes the formation of a protective carbon layer. This thermal stability enhancement primarily stems from the synergistic effects of the PBO@PDA@ZnO hybrid filler. The PDA coating improves interfacial adhesion between PBO fibers and the UHMWPE matrix, which restricts the thermal mobility of the polymer chains. On the other hand, the uniformly dispersed ZnO nanoparticles and PBO fibers form a physical barrier network within the composite, inhibiting the volatilization of pyrolysis products and heat penetration. Additionally, the ZnO component may act as a Lewis acid catalyst during pyrolysis, promoting dehydrogenation and cross-linking reactions within the polymer matrix, thereby facilitating the formation of a more stable and dense protective carbon layer and contributing to the increased char residue. Naturally, higher filler content amplify these barrier and catalytic effects, resulting in dose-dependent improvement of the composite’s thermal stability.

Crystallization of UHMWPE/PBO@PDA@ZnO Composites

The XRD patterns of pure UHMWPE and its composites are shown in Figure . All the samples exhibit two dominant diffraction peaks at approximately 21.5° and 23.8°, corresponding to the (110) and (200) crystal planes of its orthorhombic crystal system, respectively, indicating that the introduction of the hybrid filler does not disrupt the bulk crystalline structure of the polymer matrix. However, a subtle structural evolution occurs near 16.7°. No distinct diffraction signal is observed at this position in pure UHMWPE. Subsequently, a new diffraction peak gradually emerges at this location, with its intensity continuously enhancing with increasing filler content as the filler content of PBO@PDA@ZnO increases. This signal corresponds precisely to the (200) crystal plane characteristic of the rigid PBO fibers. Its progressive appearance not only confirms the successful incorporation the hybrid filler within the UHMWPE matrix but also provides direct structural evidence that the inherent crystalline framework of the PBO skeleton is completely preserved during the composite fabrication process.

3.

(a) XRD patterns of UHMWPE composites, and crystallization and melting behavior of UHMWPE composites: (b) cooling and (c) second heating.

The DSC curves of pure UHMWPE and its composites are shown in Figure a, and the integrated thermal data is summarized in Table . During the cooling process (Figure b), the crystallization peak temperature (T _c) exhibits a distinct non-monotonic trend. With the addition of 1 wt % PBO@PDA@ZnO, the T _c of UHMWPE-1 shifts noticeably from 113.6 °C to 115.7 °C as compared to pure UHMWPE. This initial increase is a classic indicator that the well-dispersed PBO@PDA@ZnO fibers act as highly effective heterogeneous nucleating agents, providing abundant active sites that encourage the polymer chains to crystallize. However, with the further increase of PBO@PDA@ZnO content, the T _c drops back down to 114.6 °C and 113.8 °C for UHMWPE-4 and UHMWPE-6, respectively. These results suggest that the crowded physical network at high filler content severely restricts the mobility of the polymer chains, making it more difficult for them to fold and arrange into a crystalline lattice. The crystallization enthalpy data in Table reveals that the UHMWPE-1 composite shows the highest crystallization enthalpy value of 80.7 J/g, while the UHMWPE-4 composite shows the lowest value of 77.0 J/g. This trend indicates the transition from a nucleation-promoting regime to a mobility-restricting regime as the filler content rises. In the second heating process (Figure c), the melting peak temperature (T _m) decreases slightly from 133.4 °C in the pure polymer to approximately 131.1 °C for UHMWPE-1 and UHMWPE-2, before climbing back to 132.9 °C at the 6 wt % loading. Notably, the melting peak for UHMWPE-6 becomes significantly broadened. It is ascribed to a wider variation in lamellar thickness and reduced crystal perfection, which aligns perfectly with the hypothesis of a crowded, restricted interfacial environment. The melting enthalpy values in Table show a similar trend. The calculated crystallinity mirrors this non-monotonic variation, starting at 20.7% for UHMWPE-0, peaking at 21.6% for UHMWPE-1, dipping to 21.2% for UHMWPE-4, and ultimately reaching a maximum of 22.5% for UHMWPE-6. Ultimately, the DSC curves highlight the dual nature of the PBO@PDA@ZnO hybrid filler for the UHMWPE composites. At low content (≤1 wt %), it can serve as an efficient nucleating agent that enhances crystallization. At elevated loadings, physical crowding and steric hindrance restrict chain mobility, altering both crystal growth and melting behavior.

1. Crystallization and Melting Behavior of UHMWPE Composites .

samples	crystallization enthalpy (J/g)	melting enthalpy (J/g)	crystallinity (%)	crystallization temperature (°C)	melting point (°C)
UHMWPE-0	78.9	60.8	20.7	113.6	133.4
UHMWPE-0.5	79.2	60.8	20.8	113.7	133.6
UHMWPE-1	80.7	62.7	21.6	115.7	131.1
UHMWPE-2	80.0	61.4	21.4	115.6	131.0
UHMWPE-4	77.0	59.7	21.2	114.6	131.8
UHMWPE-6	79.4	61.9	22.5	113.8	132.9

^aNote: The theoretical melting enthalpy (ΔH _m°) of UHMWPE used in the calculation of crystallinity is 293 J/g.

Mechanical Properties of UHMWPE/PBO@PDA@ZnO Composites

Figure displays the tensile properties of pure UHMWPE and its composites. As shown in Figure a, pure UHMWPE exhibits high ductility with an elongation at break of approximately 650%. It is noted that the incorporation of small content of PBO@PDA@ZnO synergistically enhances both ductility and strength. Specifically, the elongation at break of UHMWPE-1 achieves a maximum value of roughly 850% at a 1 wt % loading. However, the ductility of the UHMWPE/PBO@PDA@ZnO composites is highly sensitive to filler concentration. With the further increase of content to 2 wt %, the elongation at break of UHMWPE-2 falls below that of pure UHMWPE. In addition, at higher loadings (≥4 wt %), the elongation at break drops drastically to below 30%, marking a clear transition to brittle fracture behavior. Conversely, the tensile strength of the composites is consistently enhanced by the addition of PBO@PDA@ZnO (Figure b). As the filler content increases from 0 to 2 wt %, the tensile strength steadily increases from 23.3 to 24.6 MPa. A more substantial reinforcement is observed at 4 wt %, where the strength peaks at 33.2 MPa before slightly declining at 6 wt % (while still remaining well above 30 MPa). These results indicate that the addition of PBO@PDA@ZnO can effectively reinforce the UHMWPE matrix, but an excessively high content can lead to a deterioration in ductility. This is because at low contents, the rigid fibers can effectively transfer and disperse applied stress, which simultaneously improves both strength and toughness. − While at high content, the excessive rigidity of the filler network and the tendency for fibers to agglomerate create localized stress concentration points. This ultimately leads to premature brittle fracture and the observed sharp decline in elongation at break. ,

4.

(a) Stress versus strain curves and (b) tensile properties of UHMWPE composites.

Furthermore, UHMWPE-PDA-1 exhibits higher tensile strength but significantly lower elongation at break compared to UHMWPE-1 with the same 1 wt % filler content. This is attributed to the absence of ZnO on the surface. PDA is renowned for promoting robust interfacial adhesion. Without ZnO covering the PDA layer surface, the completely exposed PDA layer creates excessively strong bonds between the rigid PBO fibers and the UHMWPE matrix. , This tight coupling can maximize load transfer, pushing the tensile strength to an impressive value of 30.9 MPa. However, it also makes it very difficult for the polymer chains to naturally change their shape when they are under tension. This restriction causes stress to build up sharply at the fiber–matrix interface, which causes brittle fracture to happen too soon at an elongation of less than 30%. In contrast, the uniform deposition of ZnO nanoparticles effectively regulates the interfacial adhesion. The nanoparticles partially mask the highly adhesive PDA layer, creating a tempered moderate interface. This tailored bonding restricts the matrix just enough to provide significant structural reinforcement yet remains flexible enough to permit localized polymer chain mobility and ductile yielding.

Tribological Properties of UHMWPE/PBO@PDA@ZnO Composites

Figure presents the tribological performance of the UHMWPE composites under different loads (5, 15, and 30 N). As shown in Figure a,d–f, the average friction coefficients of all samples overall show a downward trend as the load increases. The COF values distinctly cluster into two regimes: a higher friction regime (0.15–0.25) and a low friction regime (0.05–0.075). Under the same load, as the content of PBO@PDA@ZnO increases, the average friction coefficient first increases, followed by a dramatic reduction at a 1 wt % loading, after which the COF rises again. Among them, UHMWPE-0.5 under a load of 5 N has the highest average friction coefficient of 0.236, whereas UHMWPE-1 under a load of 30 N achieves the lowest average friction coefficient of 0.057. Across all tested loads, the sample UHMWPE-1 consistently demonstrates superior lubricity, showing COF reductions of 70%, 67%, and 64% under 5, 15, and 30 N loads, respectively, compared to UHMWPE-0. Interestingly, while the general trend dictates a lower COF at higher loads, UHMWPE-2 deviates from this behavior. The friction coefficient of UHMWPE-2 first jumps from 0.065 to 0.193 at 15 N and then slightly decreases to 0.176 at 30 N. This anomalous spike at 15 N for the 2 wt % composite may originate from an unstable intermediate regime in filler distribution. At this higher concentration, the fillers might not form a consistent and robust transfer film under moderate load (15 N), leading to irregular contact and increased friction. As the load further increases to 30 N, the higher contact pressure could partially improve the film stability, resulting in a slight COF reduction compared to that at 15 N. Additionally, at an equivalent 1 wt % filler loading, the composite containing ZnO nanoparticles (UHMWPE-1) exhibits better lubrication performance than the composite without ZnO loading (UHMWPE-PDA-1).

5.

(a) Average coefficient of friction (COF), (b) wear volume, and (c) friction roughness of composites under different loads, as well as different types and contents of fillers. Variations of the friction coefficient of composites with different filler types and contents under (d) 5 N, (e) 15 N, and (f) 30 N loads. Height curve of the scratch cross section (the cross-section is perpendicular to the grinding direction) of composites with different filler types and contents under (g) 5 N, (h) 15 N, and (i) 30 N loads.

These results indicate that the incorporation of PBO@PDA@ZnO as a lubricating filler can effectively enhance the tribological properties of UHMWPE composites. The general decrease in the friction coefficient with increasing load can be attributed to the higher contact pressure facilitating a more uniform dispersion of the filler at the sliding interface, which promotes the formation of a robust transfer film. This can reduce the direct surface contact and lower the friction coefficient. The abrupt transition in the COF between the 0.15–0.25 and 0.05–0.075 regimes suggests a fundamental shift in the dominant wear mechanism, likely transitioning from sliding friction to rolling friction. The anomalous behavior of UHMWPE-2 highlights that applied load, filler type, and concentration are highly interdependent. For instance, at a 5 N load, UHMWPE-2 appears to strongly favor a rolling friction mechanism, resulting in a distinctly low friction coefficient. However, at 15 N, the instability in the transfer film due to the higher filler loading (2 wt %) might temporarily disrupt this mechanism, causing a reversion to a higher-friction sliding regime. A similar load-dependent mechanistic shift explains the friction behavior observed in UHMWPE-PDA-1.

Figure b,g–i shows the wear rates of the composites and the corresponding cross-sectional profiles of the wear surfaces. In the same formula, the wear rates and wear scratch depth of the samples overall show an increasing trend as the load increases. Under a constant load, the wear rate of UHMWPE/PBO@PDA@ZnO composites initially decreases and then increases as a function of filler content. Among them, pure UHMWPE (UHMWPE-0) exhibits the lowest wear resistance across all tests and has the maximum wear rate of 1.50 × 10^–5 mm³/N·m under a 15 N load, while UHMWPE-1 maintains low wear rates and minimal track depths regardless of the applied load. Specifically, the wear rates of UHMWPE-1 are 8.46 × 10^–7, 1.11 × 10^–6, and 1.23 × 10^–6 mm³/N·m, respectively, under 5, 15, and 30 N loads. In addition, UHMWPE-PDA-1 without ZnO loading displays a unique wear progression. The wear rate of UHMWPE-PDA-1 initially increases and then decreases as the load escalates from 5 to 30 N.

As shown in Table , to clarify the positioning of this study within the field of similar research, a selection of friction performance studies on composite materials based on UHMWPE was conducted for comparison. The results demonstrate that this work exhibits a leading advantage in both the COF and wear rate.

2. Tribological Properties of Composites Based on UHMWPE .

filler	content of filler (wt %)	counterpart	friction type	load (N)	COF	wear rate (mm3/N·m)
PBO@PDA@ZnO (this work)	1	GCr15 steel ball	reciprocating dry sliding	30	0.057	1.23 × 10–6
HNTs@TiO2	3	GCr15 steel ball	reciprocating water lubrication	40	0.073	6.67 × 10–6
CZrP	5	GCr15 steel ball	rotary dry sliding	100	0.029	5.3 × 10–5
ATP–PEI–CF	15	GCr15 steel ball	reciprocating dry sliding	10	0.26	3.2 × 10–6
MoS2–CF	20	GCr15 steel ball	rotary dry sliding	10	0.15	3.8 × 10–5
BN–Gt	70	steel ring	rotary dry sliding	200	0.17	4.48 × 10–5

^aNote: The samples selected in the table represent those with the best overall performance in the study, and the COF and wear rate presented values were obtained under the conditions specified in the table.

Figures and S4 provide 2D and 3D topographical images of the wear tracks of UHMWPE composites under different loads, respectively. With the exception of the UHMWPE-PDA-1 formulation, the width and depth of the wear tracks of UHMWPE composites consistently increase with the applied load. It is primarily driven by elevated contact stresses, which exacerbate interfacial plastic deformation, microcutting, and fatigue delamination at the interface. It is noted that UHMWPE-0 exhibits severe wear characteristics under all loads, highlighting its inherently poor wear resistance. With the incorporation of PBO@PDA@ZnO, all of the UHMWPE composites have an improvement in wear resistance to some extent. Among these tests, UHMWPE-1 consistently displays the narrowest and shallowest wear tracks, confirming that this formulation offers an optimal structural stability and wear resistance.

6.

2D modeling images of wear scratches on UHMWPE composites under different loads.

To further quantify interfacial damage, Figure c presents the surface roughness of the wear scratches. At a constant applied load, the roughness overall shows a trend of first decreasing and then increasing as the filler content increases. The initial reduction in roughness is attributed to optimal filler loadings, facilitating a uniform, smooth transfer film and restricting localized matrix deformation. However, excessive filler content will lead to fiber agglomeration and severe interfacial stress concentrations, which disrupt surface integrity and cause a sharp rebound in the wear track roughness. For the reinforced composites, the wear track roughness generally scales with the applied load. Elevated contact pressures intensify localized plastic deformation, abrasive plowing effects, and filler–matrix debonding, thereby generating more severe topographical variations and surface damage.

Figure presents the SEM images of the wear surface morphologies of different composite materials after the friction tests. For the given filler formulations, the severity of surface damage is visibly exacerbated at the higher applied load (30 N) compared to the lower load (5 N). Extensive plastic deformation, material tearing, and deep pits can be observed on the worn surface of UHMWPE-0 (Figure a,e), which indicate severe adhesion and plastic flow failure during the friction process. In contrast, the wear surface of optimally loaded UHMWPE-1 is relatively smooth, primarily showing fine, parallel ploughing-like scratches, indicative of abrasive wear, with no large-scale plastic deformation zones. High-magnification SEM images confirm the uniform distribution of ZnO nanoparticles across the wear surface. However, incorporating an excessive filler yields detrimental effects. The wear surfaces of UHMWPE-6 (Figure c,g) are substantially more damaged than those of UHMWPE-1. Densely distributed ZnO particles protruding from the matrix are visible on its surface, acting as third-body abrasives that severely exacerbate abrasive plowing and microcutting. Furthermore, no ZnO nanoparticle features are observed in the SEM images of UHMWPE-PDA-1. UHMWPE-PDA-1 exhibits more pronounced wear at 5 N than its ZnO-containing counterpart (UHMWPE-1), although their wear morphologies converge somewhat under a 30 N load. This suggests that the ZnO nanoparticles play a critical role in enhancing the antiwear properties of the PBO fibers, particularly under lower loads. These morphological changes are consistent with the wear rate data (Figure b), revealing a synergistic reinforcement mechanism provided by hybrid fillers.

7.

SEM images of wear scratches of UHMWPE composites under different loads: (a–d) 5 N and (e–h) 30 N.

The high-modulus PBO fibers act as a robust skeleton that bears and disperses the contact load, suppressing large-scale adhesion and plastic deformation of the UHMWPE matrix and shifting the dominant wear mechanism from adhesive wear to abrasive wear. At the optimal content (1 wt %), the uniformly distributed ZnO nanoparticles on the fiber surfaces act as solid lubricants at the sliding interface. They provide a dynamic “microbearing” effect that facilitates rolling friction, thereby smoothing the interfacial contact and drastically reducing shear forces. Conversely, at a high filler content, severe filler agglomeration compromises the structural integrity of the composite. These agglomerates induce localized stress concentrations that lead to microspalling, deep abrasive grooving, and a consequent deterioration in overall tribological performance.

To support the proposed wear mechanisms, the morphologies of the transfer films deposited on the GCr15 bearing steel ball counterfaces were examined via SEM (Figure ). When sliding against UHMWPE-0, the resulting transfer film appears discontinuous, flaky, and loosely adhered to the steel substrate (Figure a,e). Such poorly consolidated transfer film is highly susceptible to cyclic detachment and regeneration during the sliding process. This continuous cycle leads to interfacial instability, pronounced fluctuations in the friction coefficient, and accelerated wear of the polymer matrix. In contrast, the grinding ball against UHMWPE-1 exhibits a highly uniform, dense, and continuous transfer film that firmly adheres to the counterface (Figure b,f). This robust, high-quality tribofilm effectively prevents direct asperity contact between the UHMWPE matrix and the steel ball, smoothing the sliding interface and significantly enhancing the wear resistance. Furthermore, the morphology of the transfer film is highly sensitive to filler content. At excessive filler loadings (UHMWPE-6), the tribofilm becomes thick, irregular, and dominated by the accumulation of large, fragmented wear debris and agglomerates, as shown in Figure c,g. This phenomenon confirms that excessive rigid fibers disrupt the continuous film-forming process, acting instead as third-body abrasives that destabilize the interface. Similarly, the transfer film generated by UHMWPE-PDA-1 (Figure d,h) exhibits blocky debris and inferior continuity compared to UHMWPE-1. This contrast reinforces the conclusion that the ZnO nanoparticles on the fiber surfaces play a catalytic role in promoting the deposition of a smooth, stable, and lubricious transfer film under varying contact pressures. In additional, the analysis of EDS mapping (Figure S5) also confirmed the previously proposed “micro-bearing” mechanism, demonstrating that these nanoparticles are actively retained at the sliding interface to function as solid lubricants and alleviate interfacial shear stress.

8.

SEM images of wear scratch on friction pairs with different composites and under different loads: (a–d) 5 N, (e–h) 30 N.

In summary, by integration of the analysis of both the composite wear morphologies and the counterface transfer films, a comprehensive synergistic wear mechanism for the UHMWPE/PBO@PDA@ZnO composites can be clarified, as shown in Figure . In the optimal formulation (UHMWPE-1), the rigid hybrid fillers can effectively bear the applied normal load, heavily restricting the macroscopic plastic deformation of the polymer matrix. Concurrently, these fillers actively participate in constructing and maintaining a robust, continuous tribofilm at the sliding interface enriched with lubricating ZnO phases. This synergistic action fundamentally shifts the dominant wear mechanism from severe adhesive tearing to mild abrasive plowing and is continuously buffered by the protective transfer film. This transition drastically reduces the friction coefficient and enhances the overall wear resistance. Furthermore, the elevated wear observed in the ZnO-free UHMWPE-PDA-1 composite at a low load directly corroborates the critical function of the ZnO nanoparticles. As depicted in Figure , these ZnO nanoparticles act as solid-state lubricants, providing a dynamic “microbearing” rolling effect that is essential for optimizing interfacial lubrication.

9.

Synergistic friction-reducing and anti-wear mechanism of UHMWPE/PBO@PDA@ZnO composites.

Conclusions

In this work, a type of hierarchically structured PBO@PDA@ZnO hybrid fiber was prepared by a biomimetic strategy for fabricating high-performance UHMWPE composites. By mimicking the “rigid skeleton-functional particle” architecture of crab legs, the hybrid filler achieves synergistic improvements in the mechanical and tribological properties of the UHMWPE composites. The results demonstrated that at the optimal loading (1 wt %), the composite (UHMWPE-1) showed the best comprehensive performance. Specifically, UHMWPE-1 exhibited a slight increase in tensile strength with maintaining high ductility (an elongation at break increases to 850%). In addition, whether subjected to loads of 5 N, 15 N, and 30 N, the modified composite material (UHMWPE-1) exhibits a COF reduction of over 60% compared to pure UHMWPE, and the wear rate achieved a minimum value of 8.46 × 10^–7 mm³/N·m. The enhancement in tribology is mainly attributed to the PBO fibers effectively bearing the contact load, coupled with the ZnO nanoparticles converting sliding friction into rolling friction via a “micro-bearing” effect and stabilizing the interfacial transfer film. Notably, excessive filler loadings (e.g., 6 wt %)P will lead to leads agglomeration, reverse these benefits by triggering nanoparticle agglomeration, which compromises the composite’s toughness and wear resistance. This work provides new design insights for engineering advanced self-lubricating UHMWPE composites for demanding industrial applications, including bearing bushings for heavy machinery, seals operating in abrasive environments, and components in offshore engineering and mining equipment that must withstand high wear and impact.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/polymscitech.6c00046.

• EDS mapping images of PBO@PDA@ZnO (Figure S1); TGA curves and C ls high-resolution spectra of PBO@PDA@ZnO (Figure S2); thermal stability curves of UHMWPE composites (Figure S3); 3D modeling images of wear scars on UHMWPE composites (Figure S4); EDS mapping images of the wear scar on friction pair with UHMWPE-1 (Figure S5); and tables summarizing the composite formulations (Table S1) and thermal stability data (Table S2) (PDF)

The authors declare no competing financial interest.