Enabling Ultra-Stable Bearing Performance: Design of a Self-Lubricating PI Composite Retainer

Abstract

To address challenges such as temperature rise, operational instability, and premature failure in rolling bearings caused by retainer friction, this study designed and developed a high-performance polyimide (PI)-based composite self-lubricating retainer to enable “ultra-stable” bearing operation. Both solid and oil-porous self-lubricating retainers were fabricated through material composition and structural design. Systematic tests under controlled load and speed conditions were conducted to compare their temperature rise behavior and wear morphology. The results demonstrated that the temperature rise in the YSU-PI1 bearing with a solid retainer decreased by approximately 57% compared to a conventional bearing. The YSU-PA2 bearing with an oil-porous retainer exhibited a further improvement in thermal performance. Notably, under high-speed conditions, the equilibrium temperature of the YSU-PA2 bearing was lower than that under low-speed conditions, confirming a centrifugal-force-driven self-regulating oil-supply mechanism. Wear surface analysis revealed that the porous structure promoted the formation of a continuous and uniform transfer film, effectively mitigating wear and pitting. This study successfully integrates “material–structure–function” innovation. The oil-porous PI-based composite retainer transforms centrifugal force—typically considered detrimental—into a beneficial lubrication mechanism, effectively suppressing temperature rise and enabling “ultra-stable operation”. These findings provide crucial theoretical and technical support for developing bearings for high-end equipment.

1. Introduction

As critical “joints” in modern industrial equipment, the performance of rolling bearings directly influences the precision, efficiency, reliability, and service life of major technical equipment [1,2]. The field of tribology provides the fundamental principles governing these performance aspects, as extensively covered in practical guides and recent bibliometric analyses of the research landscape [3]. With advancing demands in aerospace, precision machine tools, and high-speed motor spindles—requiring higher speeds, heavier loads, greater precision, and operating under extreme conditions—the performance requirements for bearings have become increasingly stringent [4,5,6]. Under high-speed, starved-lubrication or variable operating conditions, traditional metal or non-metal retainers often lead to excessive temperature rise and lubrication failure due to frictional heat accumulation. This can result in catastrophic failures such as scuffing, burning, and even seizure, representing a bottleneck in the development of high-end bearing technology [7,8,9].

To address these challenges, researchers have explored multiple approaches: optimizing oil–air lubrication parameters and developing new nano-additive lubricants to improve interfacial tribological behavior [10,11,12]; applying wear-resistant and friction-reducing coatings such as diamond-like carbon (DLC) and iron-based alloys on bearing rings and rolling elements [13,14]; using silicon nitride (Si₃N₄) ceramic rolling elements to reduce centrifugal forces and minimize friction and wear [15]; and implementing advanced sensors and algorithms for real-time monitoring of bearing health [16]. Among these, the retainer—which connects and spaces the rolling elements—plays a vital role in mediating bearing performance. Its multifunctional nature makes it a critical, yet often overlooked, component. Self-lubricating retainer technology aims to form a continuous transfer film at the friction interface through inherent lubricity or smart structural design, thereby reducing friction and suppressing temperature rise [17,18]. Researchers have investigated polymer retainers made of polyetheretherketone (PEEK), polytetrafluoroethylene (PTFE), and their composites, taking advantage of their self-lubricating properties, low density, and wear resistance [19,20].

Polyimide (PI)-based composites are considered ideal candidates for manufacturing high-performance self-lubricating retainers due to their exceptional thermal stability (maintaining performance −269 °C to 400 °C), high mechanical strength, inherent lubricity, and good fatigue resistance [21,22]. Recent studies have explored PI-based composites. N. Daniel et al. [23] investigated the wear mechanisms of carbon-fiber-reinforced PI-based composites under dry sliding conditions, confirming their ability to form effective transfer films. Similarly, Hu et al. [24] demonstrated improved tribological performance in bearings using nano-MoS₂-filled PI-based composites. Concurrently, research on other polymer systems, such as porous PEEK retainer materials, has also advanced. Zhang et al. systematically investigated the mechanical and high-temperature tribological properties of porous PEEK cage materials fabricated via fused deposition modeling, highlighting the challenge of balancing porosity with mechanical strength [25]. Collectively, these works lay a solid foundation for the application of PI in bearings.

Despite significant progress, two critical issues remain inadequately addressed in current research and practice. First, most studies focus primarily on optimizing solid PI-based composites, paying insufficient attention to the integrated “structure–function” design of the retainer itself. Under long-term or varying conditions, the lubricating film may degrade, leading to performance loss [26,27,28]. Second, evaluations of bearing “operational stability” often rely solely on absolute temperature rise values, overlooking the “dynamic stability” of the temperature rise process. Significant temperature fluctuations directly reflect poor lubrication and operational instability, contributing to vibration, noise, and contact fatigue [29,30,31]. Therefore, achieving “ultra-stable operation”—characterized by low temperature rise with minimal fluctuation—represents a more challenging and meaning goal.

Given these considerations, merely modifying PI materials is insufficient to meet the stability demands of next-generation high-end bearings. Crucially, the lubrication mechanism of porous PI-based retainers and their effects on dynamic temperature rise and wear behavior remain poorly understood. To bridge this gap, we designed and prepared high-performance PI-based self-lubricating retainers with synergistic structural and material optimization. Using PI as the matrix, compounded with PTFE as a solid lubricant and nano-Al₂O₃ for enhanced wear resistance and dimensional stability, we manufactured both solid and oil-porous retainers. This work aims to demonstrate the system-level thermal regulation capability of porous retainers and introduce “ultra-stable operation” as a vital performance indicator. Under strictly controlled test conditions, we systematically compared temperature rise curves and wear morphology, thereby elucidating the mechanism of sustained lubrication and temperature stability enabled by the porous oil-containing structure. This study clarifies the fundamental principle behind achieving “ultra-stable operation” and provides an important experimental and theoretical basis for advancing new self-lubrication technologies.

2. Materials and Methods

2.1. Raw Materials

Polytetrafluoroethylene (PTFE) powder (white), average particle size of 500 μm, density 2.16 g/cm³, tensile strength 22–30 MPa, was supplied by Dongguan Xingwang Plastic Raw Materials Co., Ltd. (Dongguan, China). Polyimide (PI) molding powder (yellow, grade SK-0130), particle size of 300 mesh, heat deflection temperature above 230 °C, density ~1.4 g/cm³, tensile strength greater than 100 MPa, was provided by Changzhou Shine Special Polymer Material Co., Ltd. (Changzhou, China); α-Al₂O₃ nanoparticles (30 nm) were supplied by Shanghai Hanlang New Material Technology Co., Ltd. (Shanghai, China). Pore-forming agent: azodicarbonamide (AC), molecular formula C₂H₄O₂N₄.

This specific composite system was strategically designed to meet the extreme demands of high-speed, self-lubricating bearing retainers. The PI matrix was chosen for its exceptional thermal stability, high mechanical strength, and inherent good tribological properties, providing a robust foundation [32]. PTFE was incorporated as a solid lubricant due to its extremely low friction coefficient, facilitating the formation of a beneficial transfer film. Nano-Al₂O₃ particles were added to counteract the potential softening effect of PTFE and to improve the composite’s overall wear resistance, hardness, and dimensional stability through dispersion strengthening. This finesse builds upon our previous findings in material science [33,34] and aims to achieve an optimal balance between lubrication efficiency and mechanical robustness for practical bearing applications.

2.2. Main Experimental Equipment

A ZRY-120 vacuum hot-press sintering furnace was supplied by Jinzhou Hangxing Vacuum Equipment Co., Ltd. (Jinzhou, China). OLS3100 laser confocal scanning microscope (LCSM, Olympus Corporation, Tokyo, Japan). A self-developed rolling bearing fatigue testing machine.

2.3. Bearing Retainer Preparation

PI matrix, fillers, and pore-forming agent (where applicable) were mixed in predetermined proportions. The mixture was placed into a custom-made mold and sintered using the vacuum hot-press sintering system. The time–temperature profile and pore structure characterization followed previously reported methods [33]. After cooling, the sintered composite was machined to match the dimensions of the original bearing retainer for subsequent testing.

2.4. Bearing Properties Tests

Bearing tests were conducted on a specialized testing machine, detailed in reference [35]. Prior to each friction test, all bearings underwent a standardized conditioning procedure to ensure consistent initial lubrication. The bearings were fully immersed in a bath of L-AN46 mechanical oil (GB 443-89, kinematic viscosity: 46 mm²/s at 40 °C) heated and maintained at 100 °C for 2 h. Subsequently, the bearings were removed and meticulously wiped with lint-free cloths to remove excess oil from external surfaces, leaving lubrication primarily within the bearing assembly.

The testing system enables real-time monitoring of temperature variations during operation. An axial load was applied to the test bearing through a lever mechanism. The temperature at the outer ring was continuously recorded. After testing, samples were extracted from the bearings for raceway surface morphology analysis using laser confocal microscopy.

3. Results and Discussion

3.1. Comparison of Solid Retainer Bearings

Figure 1 compares temperature rise curves of a conventional 7206C angular contact ball bearing and the YSU-PI1 bearing equipped with a solid retainer (PI + 20 wt.% PTFE). Tests were conducted at a constant speed of 1376 r/min. The conventional bearing was tested for 3 h under 700 N, while YSU-PI1 bearing was tested under 700 N and 1500 N for 3 h and 5 h, respectively. The maximum Hertzian contact pressure was estimated using the classical formula for point contact [1]:

$$P_{\max }={\displaystyle \frac{1}{\pi \mu \nu }}{\left({\displaystyle \frac{3Q}{2}}{\left({\displaystyle \frac{E^{\prime }}{{\displaystyle \sum \rho }}}\right)}^2\right)}^{1/3}$$

where $\mu$ and $\nu$ are Hertz contact coefficients, which are functions of curvature $\cos (\tau )=F(\rho )$. The estimated maximum contact pressure on the outer ring is approximately 890 MPa under the 700 N axial load and 1100 MPa under the 1500 N axial load.

As shown in Figure 1, under the same axial load of 700 N, the YSU-PI1 bearing showed significantly lower temperature rise and less fluctuation compared to the conventional bearing. This improvement is attributed to the excellent self-lubricating properties of PTFE, which reduces friction at the interface. In contrast, the conventional bearing showed a rapid temperature increase with considerable curve oscillation, indicating unstable lubrication. Under the higher load of 1500 N, the YSU-PI1 bearing displayed a faster initial temperature rise rate due to increased frictional heat generation. Nevertheless, even during extended testing (300 min), its temperature rise remained below 45 °C, demonstrating outstanding long-term thermal stability.

Figure 2 presents optical micrographs of the worn surfaces on the outer ring raceway of the YSU-PI1 bearing under different load conditions. Under 700 N (Figure 2a), the worn surface exhibited fine scratches and numerous micro-pits, suggesting the absence of a significant continuous transfer film. In contrast, under 1500 N (Figure 2b), severe abrasion and broader wear tracks were observed, accompanied by noticeable surface burning. This distinct behavior is attributed to the increased Hertzian contact pressure under the elevated load. At a constant rotational speed, the higher pressure likely induced transient localized temperature spikes, which hindered the formation and stability of transfer films, thereby preventing the development of a continuous self-lubricating layer.

Figure 3 illustrates the temperature rise data for the YSU-PA1 bearing, equipped with a self-lubricating retainer made of PI + 20 wt.% PTFE + 5 wt.% Al₂O₃ composite. The test was conducted under 700 N and rotational speed 2412 r/min for 7.5 h. As displayed in the curve, the bearing temperature increased gradually but the rate of temperature rise diminished over time. The fitted curve indicates that thermal equilibrium was attained after approximately 6.5 h. Notably, the maximum temperature rise did not exceed 50 °C under these conditions, highlighting the exceptional thermal stability of the YSU-PA1 material.

Figure 4 shows an optical micrograph of the worn surface on the outer ring raceway of the YSU-PA1 bearing. A continuous and uniform transfer film with a cotton-like morphology has formed on the raceway surface. This lubricating film effectively reduces friction during operation and protects the raceway from premature pitting failure. The improved performance compared to YSU-PI1 is attributed to two main factors. Firstly, the PTFE filler facilitates the formation of a self-lubricating layer due to its exceptionally low friction coefficient [36,37]. Secondly, the incorporated α-Al₂O₃ nanoparticles act as rigid reinforcements. More importantly, at the microscopic contact points under boundary lubrication conditions where sliding dominates, these particles can function “micro-bearings”, altering the local friction mode by introducing a rolling component, which effectively reduces shear stress in the micro-contact zone [38,39]. This synergistic mechanism results in a substantial reduction in frictional heat, thereby ensuring stable and reliable operation.

3.2. Performance of Bearings with Porous Oil-Containing Retainers

To evaluate the superior performance of the newly developed porous oil-containing retainer (YSU-PI2, PI+PTFE), its temperature rise characteristics were compared against the solid retainer (YSU-PI1) made of the identical base material. This controlled comparison was conducted under identical conditions (700 N, 1376 r/min, 3 h) to highlight the intrinsic advantage of the porous structure itself. As illustrated in Figure 5, although the YSU-PI2 bearing exhibited a marginally higher temperature rise, its temperature quickly stabilized with minimal fluctuation, plateauing around 25.5 °C. In contrast, the temperature of the YSU-PI1 bearing with the solid retainer continued to climb, ultimately surpassing that of YSU-PI2.

This result clearly demonstrates the superior long-term thermal stability of the porous retainer. While existing research, such as the porous PEEK retainer material [25], effectively characterizes material-level tribological properties, our findings provide complementary, system-level insights by demonstrating the translation of this principle into stable thermal performance at the full-bearing level under continuous operation. The achievement of “ultra-stable operation” with a consistent temperature plateau underscores the value of system-integrated performance evaluation for assessing retainers in conditions closer to actual application.

Figure 6 shows the micrograph of the wear morphology on the outer ring raceway of the YSU-PI2 bearing. Compared to the YSU-PI1 bearing with a solid retainer (Figure 2a), the raceway surface of the YSU-PI2 bearing equipped with a porous oil-containing self-lubricating retainer is considerably smoother, with markedly reduced wear defects and noticeably finer, narrower wear tracks. This improvement is attributed to the continuous and stable oil supply from the porous structure, which effectively maintains lubricating film integrity and reduces direct metal-to-metal contact. The superior surface condition correlates with the enhanced thermal stability shown in Figure 5, underscoring the effectiveness of the porous oil-containing retainer design.

Figure 7 presents the temperature variation curves of the YSU-PA2 bearing—equipped with a self-lubricating porous oil-containing retainer (PI+PTFE+Al₂O₃)—under a constant load of 700 N at different rotational speeds. The results demonstrate that the introduction of micro-pores significantly enhances the thermal performance. Contrary to the typical trend observed in conventional lubrication, the bearing operated at a lower equilibrium temperature under the higher rotational speed. This counterintuitive phenomenon is attributed to an enhanced lubrication mechanism enabled by the porous retainer at elevated speeds. The increased centrifugal force actively promotes more effective oil release from the porous structure, thereby improving lubricant availability at the critical contact interfaces. Furthermore, the formation of a self-lubricating transfer film is facilitated under these high-speed conditions, synergistically reduced friction and enhancing heat dissipation. These findings highlight the critical role of the structurally self-regulated lubrication in achieving superior thermal stability under varying operational conditions.

Figure 8 displays optical micrographs of the worn surfaces on the outer ring raceway of the YSU-PA2 bearing under different rotational speed conditions. At lower speed (Figure 8a), the raceway surface exhibited a relatively rough morphology with limited micro-defects and deeper, and wider scratches. In contrast, under high-speed conditions (Figure 8b), the worn surface appeared noticeably smoother, with significantly finer and narrower scratches. These observations indicate that the self-lubricating performance of the porous oil-containing retainer is more effectively activated under high-speed conditions, consistent with the improved thermal performance. The combination of improved wear resistance and reduced friction under high-speed operation demonstrates the functional advantage of the porous retainer design in demanding applications.

In summary, the bearing equipped with an oil-porous self-lubricating retainer demonstrates superior performance compared to one with a solid retainer, achieving the desired “ultra-stable” operational behavior. This performance stems from the unique structural characteristics of the porous retainer. The interconnected oil-filled micro-pores function simultaneously as an oil reservoir and a capillary oil-locking system. The numerous micropores ensure a sufficient lubricant supply throughout operation, while the capillary action within the pores retains the oil, preventing leakage and making this design particularly suitable for high-cleanliness applications.

Under high-speed conditions, the centrifugal force acts as a “pump”, actively delivering the oil stored in the pores toward the peripheral contact zones (rolling elements and raceways). This dynamic supply overcomes the slow migration rate governed solely by capillary action, ensuring rapid replenishment of oil to critical friction interfaces. An additional advantage under high-speed operation is the hydrodynamic effect. When oil is carried into the converging gap between rolling elements and raceways, the relative motion and fluid viscosity generate significant pressure, forming a hydrodynamic lubricating film that effectively separates the surfaces. The higher the rotational speed, the stronger this effect, resulting in a thicker and more stable oil film. This phenomenon elucidates the self-regulating lubrication mechanism of the porous retainer, which actively adjusts oil supply in response to operational conditions, transforming a typically detrimental force (centrifugal force) into a beneficial pumping action.

4. Conclusions

This study systematically investigated the system-level performance of PI-based self-lubricating retainers in functional bearings, with a focus on their temperature rise characteristics and wear morphology. The results demonstrate that the oil-filled porous retainer achieves “ultra-stable operation” by effectively controlling temperature rise through a synergistic mechanism combining “material self-lubrication” and “structural active oil supply”. This work provides a critical advancement from material-level studies to system-level validation, offering theoretical and practical foundations for developing high-performance bearings. The main findings are summarized as follows:

(1) Synergistic Innovation is Key: Integrating the intrinsic self-lubricating properties of PI-based composites with a rationally designed porous oil-storage structure is paramount to the enhanced performance. This approach achieved significant reductions in operating temperature and ultimately enabled the “ultra-stable operation” of the bearing.

(2) A Self-regulating Lubrication Mechanism was Unveiled: This study reports the counterintuitive phenomenon of a lower equilibrium temperature at higher rotational speed. It conclusively demonstrates that the porous retainer converts centrifugal force—typically considered detrimental—into a beneficial pumping action, enabling active regulation of the oil supply rate according to rotational speed. This is the core mechanism behind the performance breakthrough.

(3) Porous Design Excels under Demanding Conditions: While solid PI-based retainers perform adequately under light loads and short duration, the porous design exhibits superior performance in high-speed and long-duration operations. Its continuous lubrication capacity effectively prevents performance degradation, making it essential for applications requiring long life and high reliability, such as in aerospace and precision machine tool spindles.

These findings provide new insights into the design of self-regulating lubrication systems and underscore the significant potential of structural–functional integration in advancing high-performance bearing technology.