Impact of thermo-oxidative aging on the dry tribological performance and wear mechanisms of UHMWPE/ZrO₂ friction pairs Open Access

Ultra-high molecular weight polyethylene (UHMWPE), lauded for its superior wear resistance, low friction coefficient, and robust impact resistance, is widely adopted in both industrial and medical arenas, notably in artificial joint replacements.^1–5 ZrO₂, as a high-performance ceramic, exhibits commendable strength, hardness, self-lubrication, remarkable thermal stability, and biocompatibility, underpinning its diverse applications.^6–8 The synergistic integration of UHMWPE with ZrO₂ produces enhanced capabilities at frictional interfaces, particularly pronounced within medical contexts, highlighting substantial clinical promise.^9,10 Therein lies a critical need to investigate the underlying interactions between UHMWPE and ZrO₂, as such understanding is pivotal to improving interface longevity and efficacy, with profound implications for the optimization of functional performance in practical deployments.

Thermo-oxidative aging is widely acknowledged as a critical cause of polymer material property degradation.^11–15 Oral et al.¹⁶ have observed that even minimal oxidation (an oxidation index of 0.1) can impair the mechanical properties of UHMWPE, negatively influencing its clinical performance. Chang et al.¹⁷ found that accelerated aging unfavorably affects the mechanical and tribological properties of UHMWPE with water lubrication. Chen et al.¹⁸ determined, through tribological testing, that the friction coefficient of UHMWPE diminishes post-thermo-oxidative aging. Tsinas et al.¹⁹ demonstrated that thermal aging, especially above the α-relaxation temperature for polyethylene, leads to oxidation processes, chain scission, and the generation of unsaturated entities and polymerization radicals. Baena and Peng²⁰ report that increased aging temperatures intensify plastic deformation and adhesion in UHMWPE, correspondingly accelerating the wear rate. Ketsamee et al.²¹ suggest that MgO nanoparticles can mitigate the progression of thermal-oxidative aging to a degree. Furthermore, Cheng et al.²² discovered significant phase separation in UHMWPE after aging at elevated temperatures (150 °C), leading to subpar tribological properties.

Despite the extensive body of research on UHMWPE and its applications in artificial joints, the specific behaviors of UHMWPE/ZrO₂ interfaces under thermo-oxidative aging conditions remain underexplored.^23,24 This is particularly true concerning wear debris generation, a critical issue in orthopedic implants due to its potential to induce inflammatory responses and compromise implant longevity.^25–28 Our study uniquely focuses on elucidating the tribological behavior and aging processes of UHMWPE/ZrO₂ pairs post-aging, aiming to provide a comprehensive understanding of how thermo-oxidative aging impacts wear debris formation. This approach not only advances our knowledge in the field of material science, particularly in the context of orthopedic applications but also offers valuable insights for the development of more durable and reliable implant materials.^29,30

In conclusion, this study aims to (1) investigate the effects of thermo-oxidative aging on the friction and wear behaviors of UHMWPE/ZrO₂ friction pairs, with a particular focus on wear debris generation, and (2) understand the underlying mechanisms of wear and aging processes in these materials under varied thermo-oxidative conditions. Through this research, we aspire to contribute to the development of more durable and reliable materials for orthopedic implants, addressing a critical gap in the current literature and paving the way for future investigations in this field.

In this study, UHMWPE and ZrO₂ were used as the materials for the friction pair. The molecular weight of the UHMWPE is ∼5 × 10⁶ g/mol, which was processed from the knee joint spacers provided by Beijing Huakang Tianyi Biotechnology Co., Ltd. Before testing, the UHMWPE samples were cut and machined into specimens of 15 × 10 × 5 mm³, and then polished to a mirror-like finish (Ra ≈ 0.02 µm) to ensure the consistency of the contact surface properties of all samples. The ZrO₂ used was yttria-stabilized zirconia, which possesses excellent mechanical properties and biocompatibility. The ZrO₂ spheres utilized in the experiments had a diameter of 6 mm and were processed through sintering and precision grinding to achieve a dense, uniform structure with fine surface accuracy, thus providing enhanced toughness and wear resistance. The specific performance parameters of the ZrO₂ ceramic balls are shown in Table I.

The chosen temperatures of 40 and 70 °C, set below the crystalline transition temperature of UHMWPE (110–140 °C), were meticulously selected to avoid melting, degradation, or pronounced oxidation of the material. These conditions are intended to simulate accelerated aging phenomena that polymers might experience during actual use, thereby offering a realistic approximation of in-service conditions. The durations of 100, 200, and 500 h were determined based on insights from previous studies and our preliminary experiments, aiming to provide a comprehensive assessment of the effects of thermo-oxidative aging over time. This methodological approach enables us to observe significant changes in the material’s tribological performance, offering valuable insights into the longevity and reliability of UHMWPE/ZrO₂ pairs in clinical applications.

In the thermo-oxidative aging process, experiments were conducted in an oxygen-rich environment, maintaining an oxygen concentration of ∼21%, which is equivalent to the concentration in ambient air. This approach was chosen to simulate exposure conditions that closely resemble natural aging processes, ensuring the relevance and applicability of our findings to real-world scenarios. Samples were aged in a thermo-oxidative chamber with circulating heated air, which ensured uniform exposure throughout their surfaces. Post-aging, the surfaces of both UHMWPE samples and zirconia balls were thoroughly cleaned using alcohol and ultrasonic treatment, guaranteeing surface cleanliness and experiment repeatability. Detailed assessments of aging effects were conducted via tribological performance testing and analyses of surface morphology and chemical composition.

The wear tests were conducted using a ball-on-flat contact model under dry conditions, utilizing a CFT-I multifunctional material surface performance tester developed by Lanzhou Zhongke Kaihua Technology Development Co., Ltd. The schematic diagram of the testing machine is shown in Fig. 1. The sliding speed was carefully adjusted to mirror the typical articulation speed observed in artificial joints. Specifically, a reciprocation frequency of 1 Hz was set, corresponding to a sliding speed of 10 cm/s, based on the stroke length utilized in our tests. The total sliding distance was determined by the number of cycles, resulting in distances of ∼50 m for 500 cycles, 200 m for 2000 cycles, 500 m for 5000 cycles, and 1000 m for 10 000 cycles. To closely replicate the load conditions of artificial joints, the stress level was maintained at ∼50 MPa, which corresponds to a vertical load of about 5N.

Post-aging friction coefficients between UHMWPE and ZrO₂ pairs were measured and interpreted through the use of Origin software. Focusing on the UHMWPE, which primarily sustains wear in the friction pair, post-test characterization included macroscopic and microscopic morphology examinations using a stereoscopic microscope (SM; SXZ7, Olympus), and a field emission scanning electron microscope (FESEM; Apreo 2C, Thermo Fisher Scientific). Additionally, wear debris was inspected using FESEM. Chemical analysis of the wear scar areas was performed using an energy-dispersive x-ray spectroscope (EDS, Ultim Max65, Oxford). Fourier-transform infrared spectroscopy (FT-IR; Nicolet iS50, Thermo Fisher Scientific) was employed for characterizing the samples subjected to various aging treatments.

Figures 2 and 3 present the morphological alterations observed in UHMWPE samples subjected to varying thermo-oxidative aging conditions. Macroscopically, as depicted in Fig. 2, the aged UHMWPE samples maintained their original color and shape, indicating negligible outward change at both 40 and 70 °C. Microscopically, Fig. 3 shows that UHMWPE samples, both unaged and aged at 40 °C, preserved their fibrous textures with observable microcracks interspersed between fibers. In contrast, aging at 70 °C resulted in a noticeable loss of the fibrous definition, with the inter-fiber gaps appearing filled, yielding a comparatively smoother surface. This smoothing effect suggests a measure of surface material flow at 70 °C, which, despite being below the UHMWPE’s melting point, signifies that the elevated temperature combined with oxygen exposure exerts a transformative influence on the polymer’s microstructure over extended periods. The implications of these changes on the material’s tribological behavior warrant further investigation to determine potential impacts on its performance.

Figure 4 illustrates the evolution of the friction coefficient of UHMWPE samples under three distinct conditions: unaged, aged at 40 °C for 500 h, and aged at 70 °C for 500 h, plotted over the number of sliding cycles. The data indicate that the unaged samples exhibit a friction coefficient of intermediate value, ∼0.135, and it stabilizes at this level. For samples aged at 40 °C, the friction coefficient is observed to be the highest, reaching a stable value of about 0.165. Conversely, samples aged at 70 °C exhibit the lowest friction coefficient, with a stable value around 0.118. These findings suggest a non-linear relationship between the aging temperature and the friction coefficient of UHMWPE, wherein the friction coefficient first increases with rising aging temperatures up to 40 °C and subsequently decreases at 70 °C.

Additionally, the friction coefficients of the unaged and 40 °C aged samples incrementally ascend during the initial 4000 cycles until they plateau. In contrast, the 70 °C aged samples’ friction coefficient maintains a consistent level following an early stabilization period of ∼300 cycles. This behavior implies that thermo-oxidative aging influences the frictional stability of UHMWPE, with higher aging temperatures facilitating faster attainment of equilibrium. These observations underscore the significant influence of thermo-oxidative aging on the tribological properties of UHMWPE.

Figure 5 illustrates the wear mark characteristics of UHMWPE samples after 10 000 cycles of testing under three distinct aging conditions. These conditions include unaged samples, samples aged at 40 °C for 500 h, and samples aged at 70 °C for 500 h, as depicted in images from the supplementary material. The unaged samples maintain shallow wear marks, indicating their superior wear resistance and structural integrity. Conversely, samples aged at 40 °C for 500 h exhibit more pronounced wear, including deeper tracks with visible furrows and micro-cutting marks. This suggests compromised wear resistance. Specimens aged at 70 °C for 500 h show markedly non-uniform and complex wear marks, featuring pronounced plowing grooves and spalled pits. This indicates a substantial decline in the material’s wear resistance, potentially due to thermal softening or microstructural changes. These findings underscore that thermo-oxidative aging significantly reduces the wear resistance of UHMWPE, as evidenced by the increasingly severe and irregular wear mark formations at higher aging temperatures.

Figure 6 displays scanning electron microscopy (SEM) images of wear marks on unaged UHMWPE samples subjected to various test cycles (500, 5000, and 10 000). The figure shows a minimal increase in wear mark damage with the increasing cycle count. Notably, even after 10 000 cycles, the wear marks on the surfaces exhibit only slight wear, maintaining considerable uniformity and smoothness with no evident furrows or spalling. The predominant feature observed on the UHMWPE wear marks is a polishing effect rather than substantial wear damage. Consequently, the unaged UHMWPE demonstrates excellent tribological properties, maintaining an impressive surface condition despite the high number of cycles.

Figure 7 shows SEM images illustrating the morphologies of aged UHMWPE wear marks after 500 h at 40 °C under varying test cycles. Initially, after 500 cycles, the wear mark surface remains relatively flat and exhibits uniformity, indicating limited wear. However, increasing the test cycles to 5000 results in the development of discernible scratches on the wear mark surface, oriented in the direction of reciprocating frictional motion. This indicates an increase in abrasive wear. Further increasing the test cycle to 10 000, the surface roughness escalates along with the scratches, revealing a more intricate and abrasive texture, indicative of significant fatigue wear. These observations suggest that thermo-oxidative aging at 40 °C for 500 h notably degrades the wear resistance of UHMWPE. This degradation likely stems from thermo-oxidative aging that weakens surface integrity, making the material more susceptible to abrasion. Additionally, an increased coefficient of friction may enhance frictional heat generation, further exacerbating wear.

Figure 8 shows the microscopic morphology of the wear marks on UHMWPE samples after undergoing thermal aging treatment at 70 °C for 500 h, observed after 500, 5000, and 10 000 test cycles. From the image, it can be seen that the surface of the samples is in very poor condition, with a lot of spalling damage both inside and outside the wear marks. This spalling damage is caused by thermo-oxidative aging, not friction. This indicates that during the 70 °C, 500 h aging process, the surface of the UHMWPE samples underwent severe degradation and molecular chain breakage, leading to a significant decrease in the material’s strength and toughness. Interestingly, as the number of cycles increased from 500 to 5000, the wear mark area became relatively smooth, suggesting that the friction behavior to some extent enhanced the flow of the UHMWPE surface material, forming a more uniform protective film at the friction interface. The formation of this protective film might be due to the heat generated during the friction process, which increased the viscosity of the UHMWPE material, making it flow more easily. Additionally, the friction pair generated during the friction process might also have played a filling role, filling the gaps on the surface of the wear marks. As the number of cycles further increased to 10 000, the spalling pits on the inner surface of the wear marks almost disappeared, but a large number of black “inclusions” appeared on the surface. These “inclusions” might be wear debris or other impurities from the friction pair embedded into the protective film on the UHMWPE surface during the friction process. It can be seen that after 70 °C and 500 h of thermo-oxidative aging under high cycle friction conditions, the surface of UHMWPE shows significant wear and forms a protective film. This protective film can slow down the rate of wear to some extent, but it may also lead to impurities appearing on the surface of the wear marks.

Figure 9 illustrates the impact of varying aging durations on the friction coefficient of UHMWPE samples at two aging temperatures: 40 and 70 °C. Under the 40 °C condition, the friction coefficient generally exceeds that of unaged samples, especially notable after 100 and 200 h of aging, where it shows significant fluctuation. Beyond 500 h of aging, while the friction coefficient remains higher than in unaged samples, its fluctuation reduces, showing a tendency to stabilize after 4000 cycles. In contrast, the 70 °C aging treatment results in a gradual decrease in the friction coefficient over time. The coefficients after 100 and 200 h of aging are initially higher than those of unaged samples, with a marked increase in fluctuation after 200 h. However, after 500 h, the friction coefficient drops below that of unaged samples, exhibiting minimal fluctuation and enhanced stability.

Figure 10 shows SEM images of wear marks on UHMWPE samples after 10 000 test cycles, aged at 40 and 70 °C for varying durations. In images (a) to (c), the samples aged at 40 °C display relatively smooth surfaces with minor wear damage, suggesting minimal aging impact. The smoother interior of the wear marks compared to the exterior implies a polishing effect from friction behavior. With increased aging duration, notably at 500 h, a slight increase in roughness is observed inside the wear marks, yet severe wear damage remains absent.

Conversely, in images (d) to (f), the samples aged at 70 °C exhibit progressively worse surface damage with increased aging time, particularly after 200 and 500 h, characterized by visible spalling and pitting. The wear characteristics evolve with aging duration: at 100 h, scratches dominate, indicating abrasive wear; at 200 h, the rough texture inside the wear marks suggests a shift to fatigue or adhesive wear; and at 500 h, numerous black impurities aligned with the friction direction appear, possibly indicating melting damage or advanced abrasive wear.

As depicted in Fig. 11, the wear debris of UHMWPE samples after 10 000 cycles of test under different aging conditions exhibits distinct morphological characteristics. In image (a), the wear debris from the unaged sample primarily consists of sharp and elongated particles, mostly ranging in size from 1 to 2 µm. In image (b), after aging at 40 °C for 500 h, the wear debris is mainly composed of flake-like peeled-off materials, larger in size, reaching up to 6.5 µm. The magnified image indicates signs of impending layer-like peeling in the material. In image (c), after aging at 70 °C for 500 h, the wear debris predominantly consists of block-like, peeled-off materials, ∼2.5 µm in size. Black “inclusions” are visible on the surface of the wear marks.

Figure 12 presents the EDS analysis results at points A and B in the wear area. This analysis shows that the black “inclusions” have a higher O content, increasing from 1.95% in other wear areas to 10.31%. This observation stands in contrast to that in Fig. 8, where, under the 70 °C aging condition, the wear mark surfaces of samples subjected to low cycle numbers did not exhibit significant black “inclusions.” It is hypothesized that these “inclusions” are likely formed by wear debris being repeatedly rolled over and embedded into the friction surface. This suggests a progression in the wear process, where prolonged exposure to friction and aging conditions leads to the incorporation of wear debris into the surface, thereby altering its composition and appearance.

As depicted in Fig. 13, the infrared spectroscopy analysis of the UHMWPE sample surface shows typical absorption peaks characteristic of UHMWPE. This includes two methylene (–CH2–) asymmetric and symmetric stretching vibration peaks at 2914 and 2847 cm⁻¹, a double peak of long-chain –CH2– bending vibration at 1472 and 1462 cm⁻¹, and a double peak of –CH2– in-plane rocking vibration at 730 and 718 cm⁻¹. After thermo-oxidative aging, new absorption peaks emerge at 1735, 1645, and 1170 cm⁻¹ on the sample surface. The peak at 1735 cm⁻¹ corresponds to the stretching vibration of carbonyl C=O groups formed due to oxidation, indicating an increase in carbonyl content. The peak at 1645 cm⁻¹ may represent the stretching vibration of C=C double bonds, suggesting possible polymer chain breakage on the surface, typically caused by free radical reactions under high temperatures and oxygen. The peak at 1170 cm⁻¹ is associated with the stretching vibration of the C–O–C functional group, indicating increased formation of C–O–C bonds, particularly ether bonds, during thermal oxidation aging. These three peaks intensify with increasing aging temperature, suggesting that thermal oxidation aging accelerates the oxidation of the sample surface. The surface of the aged samples tends to form C–O and C=O bonds, a consequence of the negative inductive effect of oxygen atoms. This chemical alteration, compounded by the temperature rise due to friction, diminishes their mechanical strength and increases brittleness. As a result, microscopic cracks develop more readily, and the sample surfaces become prone to easy spalling.

In light of our observations on wear debris characteristics and the chemical alterations indicated by infrared spectroscopy, it becomes pertinent to discuss the underlying mechanisms contributing to the observed wear resistance variations post-aging. Specifically, the role of cross-link density merits attention in this context. As aging progresses, particularly under conditions of 70 °C, which intensify the wear damage, the increase in cross-link density within UHMWPE plays a pivotal role. This structural change, induced by thermal-oxidative aging, enhances the material’s ability to resist wear by improving its mechanical strength and reducing its susceptibility to abrasive and adhesive wear mechanisms. The formation of C–O and C=O bonds, as evidenced by the infrared spectroscopy analysis, points to an increased cross-link density that, while contributing to a certain degree of brittleness, also aids in forming a more robust tribological layer on the UHMWPE surface. This layer acts as a protective barrier, mitigating the material’s wear rate by preventing direct contact and reducing the severity of mechanical interactions with the ZrO₂ counterpart.

Furthermore, the altered wear debris morphology post-aging—transitioning from sharp and elongated particles to more flake-like and block-like structures—indicates a change in the wear mechanism, potentially facilitated by the enhanced cross-link density. These observations underscore the complex interplay between thermal-oxidative aging, cross-link density, and wear behavior, illustrating how aging not only affects the chemical composition but also significantly influences the mechanical and tribological properties of UHMWPE.

In summary, UHMWPE exhibits significant variations in dry friction damage characteristics and wear mechanisms under different aging conditions. Higher thermal oxidation aging temperatures more adversely affect UHMWPE’s wear performance. Unaged UHMWPE demonstrates excellent wear resistance, characterized mainly by a polishing effect during dry friction, with minimal wear damage even at high cycle numbers. This is attributed to its long molecular chains, orderly structure, high crystallinity, mechanical strength, and low friction coefficient. At 40 °C, UHMWPE wear is characterized by slight scratches and surface roughening inside the wear marks while maintaining some degree of friction polishing effect. This indicates that under these conditions, UHMWPE’s wear mechanism may involve abrasive wear and fatigue wear as cycle numbers increase. Although 40 °C thermal oxidation aging somewhat deteriorates UHMWPE’s wear resistance, the overall damage is relatively mild. However, at 70 °C, UHMWPE exhibits significantly poor wear characteristics. Post-aging, the sample surface shows extensive spalling and pitting, severely compromising the material’s surface integrity. Oxidation wear becomes more evident during the wear process, and as aging time increases, it significantly exacerbates wear damage. This highlights the profound impact that temperature has on the wear resistance of UHMWPE under aged conditions.

This study investigated the impact of thermal-oxidative aging on the dry tribology performance of UHMWPE/ZrO₂ friction pairs. The main conclusions are as follows:

1. Unaged UHMWPE exhibits excellent wear resistance and is characterized by a low friction coefficient and minimal wear damage. The wear debris is predominantly sharp and elongated, and the dry friction process mainly results in a polishing effect.

2. With thermal-oxidative aging at 40 °C, there is a noticeable decrease in UHMWPE’s wear resistance. This decrease is evidenced by a significant increase in the friction coefficient and visible changes on the wear surface, including scratching and roughening. The wear debris also transforms into predominantly flake-like peelings, indicating a shift in the wear mechanism from abrasive wear to fatigue wear.

3. At 70 °C thermal-oxidative aging, UHMWPE samples show extensive spalling and pitting on the surface, along with a significant decrease in wear resistance. The wear process at this temperature predominantly exhibits fatigue wear and oxidative wear.

4. The study confirms that higher thermal-oxidative aging temperatures have a more detrimental impact on UHMWPE’s tribology performance. This is primarily due to the formation of oxidation products on the surface, such as carbonyl C=O groups, C=C double bonds, and C–O–C functional groups, which increase surface polarity and brittleness. In practical applications, it is advisable to minimize the exposure of UHMWPE materials to high-temperature and oxygen-rich environments and to implement strategies to delay their thermal-oxidative aging.

The authors are grateful for the support of the National Natural Science Foundation of China (Grant No. 52105132). We express our sincere gratitude to Beijing Huakang Tianyi Biotechnology Co., Ltd. for their generous assistance in providing the raw materials essential for this study.

The authors have no conflicts to disclose.

Xinyue Zhang: Conceptualization (equal); Data curation (equal); Formal analysis (equal); Investigation (equal); Writing – original draft (equal). Deqiang Tan: Funding acquisition (equal); Supervision (equal); Methodology (equal); Writing – review & editing (equal). Qi Tang: Formal analysis (equal); Methodology (equal). Bin Hou: Conceptualization (equal); Data curation (equal); Formal analysis (equal); Investigation (equal); Methodology (equal); Resources (equal); Writing–review & editing (lead). Jialiang Tian: Conceptualization (equal); Investigation (equal); Methodology (equal); Resources (equal). Min Wei: Investigation (equal); Methodology (equal); Resources (equal); Software (equal).

The data that support the findings of this study are available from the corresponding author upon reasonable request.