Thermal, Hardness, and Tribological Assessment of PEEK/CoCr Composites

Thermal, Hardness, and Tribological Assessment of PEEK/CoCr Composites | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Thermal, Hardness, and Tribological Assessment of PEEK/CoCr Composites Bakytzhan Sariyev, Andas Amrin, Aiat Mergenbay, H Jeevan Rao, and 3 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-6885923/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 06 Aug, 2025 Read the published version in Scientific Reports → Version 1 posted 10 You are reading this latest preprint version Abstract Poly(ether-ether-ketone) (PEEK) is a high-performance thermoplastic with excellent mechanical strength, thermal stability, and chemical resistance, making it attractive for applications like biomedical implants and prostheses. However, neat PEEK suffers from a high friction coefficient and pronounced wear in sliding contacts. In this work, composites of PEEK with Cobalt–Chromium (CoCr) alloy powder were fabricated by centrifugal powder compaction and vacuum sintering. Four composite compositions, with weight percentages of 10%, 20%, 30%, and 40% of CoCr, were produced. Comprehensive characterization was conducted, including particle size distribution analysis, scanning electron microscopy with energy dispersive X-ray spectroscopy, differential scanning calorimetry, thermogravimetric analysis, microhardness testing, and ball-on-disk tribological testing against a steel counterface. The PEEK/CoCr composites showed uniform dispersion of CoCr particles in the PEEK matrix. Thermal analysis indicated that the addition of CoCr did not significantly alter PEEK’s melting temperature or thermal stability with residual weights corresponding closely to the filler fractions. The microhardness of the composites increased with CoCr content, with the 40% CoCr composite showing a 35% increase in hardness compared to the neat PEEK. In ball-on-disk tests, all PEEK/CoCr composites exhibited lower wear rates than neat PEEK, owing to the hard CoCr particles reinforcing the polymer. The coefficient of friction was in the typical range for PEEK sliding on steel for all composites, as the metallic filler does not provide lubrication; however, the improved hardness and load-bearing capacity of the composites led to reduced wear depth and volume loss. Overall, the PEEK/CoCr composites demonstrate enhanced hardness and wear resistance while retaining PEEK’s favorable thermal properties, suggesting their potential for applications requiring better tribological performance than unfilled PEEK. Physical sciences/Engineering/Biomedical engineering Physical sciences/Engineering/Mechanical engineering PEEK Cobalt–Chromium alloy Powder metallurgy Tribological performance Microhardness Thermal stability Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 1. Introduction Polymer-metal composites represent a significant advancement in materials science, offering unique combinations of properties that cannot be achieved by either constituent alone. Among high-performance thermoplastics, PEEK has garnered substantial attention due to its exceptional thermal stability, mechanical strength, and chemical resistance, which make it suitable for high-demand applications such as aerospace, automotive, and biomedical devices 1 – 3 . Particularly in orthopedics and dentistry, PEEK has gained popularity due to its favorable biocompatibility, radiolucency, and sterilization resistance 2 , 4 – 6 . Despite these advantages, neat PEEK demonstrates poor tribological behavior under dry sliding conditions, including a high coefficient of friction (COF) and substantial wear, limiting its service life in load-bearing or articulating components 7 – 10 . To overcome these limitations, researchers have developed various PEEK-based composites by incorporating functional fillers aimed at improving frictional and wear behavior. These can be broadly categorized into solid lubricants, ceramic reinforcements, fiber reinforcements, and metallic fillers 11 – 17 . Among solid lubricants, polytetrafluoroethylene (PTFE), graphite, and molybdenum disulfide (MoS₂) have been widely used to reduce COF by forming transfer films during sliding 9 , 15 , 18 . For example, Burris et al. 19 showed that PEEK/PTFE composites achieved ultralow wear rates (2×10⁻⁹ mm³/N·m) and a low COF (0.12). Similarly, carbon fiber/PTFE/graphite hybrid composites have achieved COF values below 0.2 against Co-Cr alloy surfaces 20 . Ceramic fillers such as titanium dioxide (TiO₂), alumina (Al₂O₃), and silica (SiO₂) significantly improve hardness and wear resistance due to their intrinsic rigidity and load-bearing capabilities 21 , 22 . Titanium-based ceramic composites have been effectively used in biomedical crowns and load-bearing implants 23 . Fiber reinforcements, especially carbon and glass fibers, provide substantial improvements in strength and stiffness, although often at the cost of increased brittleness and anisotropy 24 – 26 . Metallic fillers present a relatively underexplored but promising route to enhance both the mechanical and thermal properties of PEEK. Metals such as silicon, titanium, and aluminum offer improved dimensional stability, thermal conductivity, and surface hardness 21 , 27 . Oladele et al. 28 provided a comprehensive review of polymer composites, emphasizing the potential of metallic fillers to improve mechanical strength and durability for structural applications. Goyal et al. 29 demonstrated that the incorporation of aluminum nitride particles into PEEK significantly enhanced its thermomechanical properties, including glass transition temperature and modulus. Ochoa-Putman et al. 30 emphasized the critical role of interfacial adhesion in determining the mechanical performance of metal–polymer composites, highlighting how chemical treatment of metal surfaces improves compatibility with polymer matrices. Furthermore, Dobrzańska-Danikiewicz et al. 31 explored processing techniques for metal–polymer composites and noted that conventional methods often result in non-uniform dispersion of metal fillers, adversely affecting structural integrity. More specifically, Thiruchitrambalam et al. 32 , reviewed the current state and future prospects of metal-reinforced PEEK composites, identifying processing challenges and potential applications. However, as observed by Siraj et al. 7 , systematic investigations into the tribological properties of metal-reinforced PEEK composites remain limited. Notably, CoCr alloys are established biomaterials due to their wear resistance, corrosion resistance, and proven biocompatibility, making them ideal candidates for reinforcing polymers intended for medical applications 33 – 35 . Li et al. 36 characterized the microstructure of biocompatible CoCr alloys, highlighting their potential for medical implants. Wu et al. 37 investigated the properties of CoCr dental alloys fabricated via selective laser melting, demonstrating superior mechanical performance compared to conventional casting methods. In the context of composite materials, Senra et al. 38 explored the potential of CoCr-reinforced polymers for biomedical applications, reporting enhanced bioactivity and mechanical properties. The superior wear resistance of CoCr alloys, as documented by Yan et al. 39 suggests their potential as reinforcement materials for improving the tribological performance of polymers. Although a few hybrid composite studies have combined CoCr with solid lubricants like hexagonal boron nitride to balance hardness and lubricity, the independent contribution of CoCr to PEEK's tribological and thermal behavior remains largely unexplored 40 – 42 . Ma et al. 43 demonstrated the potential of hard fillers to improve microhardness by 19% using calcium silicate, but there are no studies that have systematically examined PEEK/CoCr binary systems. Moreover, understanding the role of metal content in influencing thermal stability and crystallinity is critical for expanding the applicability of these composites to more demanding environments. In terms of characterization, Doumneg et al. 8 employed thermal analysis techniques to study the crystallization behavior of PEEK composites, while Puhan et al. 44 conducted comprehensive tribological assessments of PEEK-based materials under various wear modes. Beckford et al. 15 investigated the effect of ceramic particles on the tribological characteristics of PEEK, providing valuable insights into wear mechanisms. Rui et al. 45 demonstrated that powder metallurgy techniques, including compaction and sintering, enable the incorporation of high-volume hydroxyapatite fillers into polymer without degrading the polymer matrix, achieving uniform dispersion and enhanced bioactivity. Traditional processing methods like melt compounding, compression, and injection molding often lead to issues such as particle agglomeration and weak interfacial bonding. 32 Advanced processing techniques have been developed to address the challenges associated with polymer-metal composites. Tsukamoto demonstrated that centrifugal casting could effectively distribute carbon nanotubes in metallic matrices, suggesting its potential for polymer-metal systems 46 . Similarly, Lu et al. 47 optimized processing parameters for CF/PEEK composites, highlighting the importance of controlling the thermal history to achieve the desired properties. Although centrifugal powder compaction is known to improve filler dispersion, its use with PEEK/CoCr composites has not been well studied. This work applies the method to evaluate its effectiveness in enhancing structure and properties for these systems 48 . This study aims to investigate the microstructural, thermal, mechanical, and tribological characteristics of PEEK/CoCr composites fabricated by powder centrifugal compaction and vacuum sintering. Four compositions, PC(10), PC(20), PC(30), and PC(40), containing 10%, 20%, 30%, and 40% of CoCr by weight, respectively, were synthesized and characterized using scanning electron microscopy with energy-dispersive spectroscopy, differential scanning calorimetry, thermogravimetric analysis, microhardness testing, and ball-on-disk tribometry. By focusing on a binary PEEK/CoCr system, this research fills a critical gap in the literature and evaluates the potential of metal-filled PEEK composites for advanced engineering and biomedical applications where enhanced wear resistance and hardness are required without compromising thermal stability. 2. Experimental 2.1. Materials Polyetheretherketone (PEEK) powder (Victrex 450PF, UK) and cobalt–chromium (CoCr) alloy powder (MSE Supplies, USA) were employed as the base materials in this study. The particle size distribution (PSD) of both powders was measured by laser diffraction using a Mastersizer 3000 with an Aero S dry powder disperser operating at 1 bar (Malvern Panalytical, UK). The median particle diameter of the PEEK powder was determined to be approximately 38 µm. For the CoCr powder, the PSD analysis yielded D 10 = 40.5 µm, D 50 = 62.4 µm, and D 90 = 86.1 µm, indicating a relatively broad particle size distribution. These percentile values correspond to the particle diameters below which 10%, 50%, and 90% of the total volume of particles are found, respectively. The span S, a parameter commonly used to characterize distribution width, was calculated using the formula: $$\:S=\frac{{D}_{90}-{D}_{10}}{{D}_{50}}$$ 1 The morphology of the CoCr powder, observed using a scanning electron microscope (SEM), is shown in Fig. 1 . 2.2. Composite preparation PEEK and CoCr powders were measured and premixed in proportions to yield 10%, 20%, 30%, and 40% by weight of CoCr. The weight and corresponding volume fractions of PEEK and CoCr are presented in Table 1 . The prepared powder mixtures were inserted into the aluminum dies and loaded into the centrifugal machine [Allegra X-14 Beckman Coulter Inc. Brea, USA]. The powder mixture in the die, tilted along the horizontal axis, was subjected to a centrifugal force of 3000 G for 180 minutes. This method promotes uniform blending of the dense metal particles with the lighter polymer powder while simultaneously compacting the mixture. After mixing, the compacted powder blends were molded and sintered. Sintering was carried out in a vacuum furnace to avoid oxidation of the metal. The specimens were slowly heated to 360°C, slightly above the melting temperature of PEEK 343°C, and held for a duration for 30 min, to ensure complete melting of PEEK matrix and encapsulation the metal particles. Then the specimens cooled down slowly to minimize residual stresses. The sintered composites were removed and cut into test samples for further characterization. Table 1 Weight and volume fraction of PEEK/CoCr compositions for the different tested samples. Sample CoCr weight content (%) PEEK weight content (%) CoCr volume fraction (%) PEEK volume fraction (%) PEEK 0 100 0 100 PC(20) 10 90 1.7 98.3 PC(20) 20 80 3.8 96.2 PC(30) 30 70 6.3 93.7 PC(40) 40 60 9.5 90.5 2.3. Characterization methods The fabricated PEEK/CoCr composites were sectioned for microstructural analysis using a wet abrasive cut-off machine (Brillant 220, QATM, Mammelzen, Germany). The cross-sectional surfaces were subsequently polished using a series of silicon carbide abrasive papers with grit sizes of 800, 1200, and 2500, followed by fine polishing with alumina suspensions of 1 µm, 0.3 µm, and 0.05 µm particle sizes. Microstructural characterization was performed using a scanning electron microscope (JSM-IT200, JEOL, Japan) operated at an accelerating voltage of 15 kV in secondary electron mode. Prior to imaging, a thin conductive gold layer (~ 5 nm) was sputter-coated onto the samples to facilitate high-resolution SEM and Energy Dispersive X-ray Spectroscopy (EDS) analysis. Thermogravimetric analysis (TGA) was conducted using STA 449 thermal analyzer (Netzsch, Germany) to determine the thermal decomposition behavior of both neat PEEK and its CoCr-filled composites. The measurements were carried out under a nitrogen atmosphere with a constant heating rate of 10°C/min, ranging from 30°C to 1000°C, followed by a 90-minute isothermal hold at the maximum temperature. Differential scanning calorimetry (DSC) was performed using a 300 Caliris Supreme analyzer (Netzsch, Germany) to evaluate the melting behavior and degree of crystallinity of the materials. Approximately 10 mg of each sample was heated from 30°C to 500°C at a constant rate of 10°C/min under a nitrogen atmosphere. Based on the difference in melting enthalpy of the crystalline region and amorphous region, the crystallinity X c can be calculated according to the following Eq. 4 9 : $$\:Xc=\frac{\varDelta\:Hm}{\varDelta\:H^\circ\:}\times\:100\%$$ 2 where ΔHm​ is the melting enthalpy of PEEK or PEEK/CoCr composites, and ΔH⁰ is the enthalpy of 100% crystalline PEEK (ΔH⁰=130 J/g). Vickers microhardness testing was conducted on the polished cross-sections using a standard microhardness tester. A load of 10 N was applied with a dwell time of 10 seconds for each indentation. Ten measurements were taken for each sample at randomly selected regions, carefully avoiding direct indentation on large metal particles. The Vickers hardness (HV) was calculated based on the diagonal lengths of the indentations, and average values with standard deviations were reported. All tribological tests were conducted in a laboratory environment under dry sliding conditions. The ambient temperature and relative humidity were maintained at approximately 26°C and 12%, respectively. A ball-on-disk tribometer (Anton Paar, Austria) was used to evaluate the friction and wear behaviour in a reciprocating sliding configuration. The counterface was a steel ball with a diameter of 6 mm, sliding against a flat sample surface. The test setup had a radius of 10 mm and a reciprocating angle of 10°, generating linear back-and-forth motion. The sliding frequency was set at 2.00 Hz, corresponding to a maximum linear speed of 0.99 cm/s. A constant normal load of 10N was applied during testing, and the sliding motion was carried out for 5000 cycles. The apparent coefficient of friction was calculated as the ratio of the measured frictional force to the applied normal load, allowing for the evaluation of frictional behavior under controlled and repeatable conditions. 3. Results and Discussion 3.1. Composite Morphology and Interfacial Characteristics The microstructural evolution of PEEK/CoCr composites with varying metallic content was systematically examined using SEM. Figure 2 displays representative SEM images of PC(10), PC(20), PC(30) and PC(40) composites. It is noteworthy that during sectioning and polishing, partial detachment of CoCr particles occurred, leading to the formation of hollows on the polished surfaces. The incorporation of CoCr particles leads to substantial changes in microstructural features across the composition range. In PC(20) (Fig. 2 b), spherical metallic particles ranging from approximately 5 to 50 µm are uniformly embedded in the PEEK matrix, with minimal signs of agglomeration. The particle–matrix interfaces appear clearly defined, suggesting interfacial diffusion or bonding during the sintering process. With increasing CoCr content in PC(30) (Fig. 2 c), a notable increase in particle number density is observed, accompanied by a reduction in interparticle spacing. Nevertheless, the distribution remains relatively uniform, indicating that centrifugal compaction effectively mitigates particle clustering even at elevated filler contents. In PC(40) (Fig. 2 d), the highest particle concentration is achieved, and instances of particle–particle contact become more frequent. This may indicate the onset of a percolation threshold, wherein the metallic phase begins to form quasi-continuous networks within the polymer matrix. Higher magnification SEM analysis (Fig. 1 ) reveals significant morphological details of the CoCr particles. Many particles exhibit internal porosity and complex surface textures, likely originating from the powder metallurgy process used for their fabrication. Some particles display crater-like surface indentations and protuberances, which could contribute to mechanical interlocking with the surrounding PEEK matrix, thereby enhancing interfacial bonding and mechanical performance. The preservation of particle sphericity throughout all composite formulations implies that the processing conditions of powder compaction were sufficiently controlled to prevent deformation or fragmentation of the metallic phase. Such morphological uniformity is essential for ensuring consistent mechanical and thermal behavior of the composite system. Furthermore, the lack of visible interfacial gaps suggests good wetting and adhesion between the polymer matrix and the CoCr reinforcement, which is critical for efficient stress transfer in load-bearing applications. Further high-resolution SEM observations (Fig. 3 ) provide insight into the interface between CoCr particles and the PEEK matrix. CoCr particle surfaces exhibit varied microscale roughness, which may facilitate mechanical interlocking with the thermoplastic matrix. Detailed inspection reveals localized zones of intimate contact between the matrix and CoCr particles. These regions represent potential sites of enhanced interfacial bonding. Notably, the CoCr particles exhibit excellent shape transcription within the PEEK matrix, indicating no deformation or smearing during processing and suggesting strong mechanical interlocking at the interface. Interfacial adhesion, internal porosity, and particle morphology are key microstructural features expected to strongly influence the mechanical response of the composites. 3.2. DSC analysis The curves and results of DSC analysis for PEEK and PEEK/CoCr composites are presented in Fig. 4 and Table 2 . These results indicate that both PEEK and its composites exhibit a melting range rather than a sharp melting point, which is typical of semi-crystalline polymers. The variation in crystal size and perfection leads to melting over a range of temperatures, with PEEK typically exhibiting melting between 290–360°C. The melting point of neat PEEK was determined to be approximately 343°C, a value that remains unchanged with the incorporation of CoCr. The melting behavior of PEEK/CoCr composites shows that the inclusion of metallic reinforcement does not significantly shift the melting point but does impact the enthalpy of fusion and, thus, the degree of crystallinity. For instance, the melting enthalpy (ΔHₘ) for neat PEEK was measured at 53 J/g, which corresponds to a crystallinity of 41%. In contrast, PEEK composites with 10%, 20%, 30%, and 40% CoCr exhibited ΔHₘ values of 48.3 J/g, 48.5 J/g, 49.2 J/g, and 49.3 J/g, resulting in crystallinity levels of 37%, 37%, 38%, and 38%, respectively. These values demonstrate a general trend of decreasing crystallinity with increasing CoCr content. This can be attributed to the reduced mass fraction of PEEK in the composite, which inherently limits the total amount of crystallizable material, as well as the physical presence of CoCr particles that hinder polymer chain mobility and alignment during crystallization. Although crystallinity decreased with filler content, the melting and crystallization peaks remained clearly visible, suggesting that the filler does not completely suppress crystal formation. Table 2 Crystallinity of PEEK and PEEK/CoCr composites as determined by DSC analysis. Sample Melting range (°C) Tm (°C) ΔH m ​ (J/g) X c (%) PEEK 320–360 343 53 41 PС(10) 316–355 343 48.3 37 PС(20) 320–360 343 48.5 37 PС(30) 320–360 343 49.2 38 PС(40) 316–355 343 49.3 38 The DSC data also supports the understanding that crystallization in PEEK and its composites is determined by multiple factors. In pure PEEK, crystallization is governed by intrinsic polymer behavior, largely dependent on temperature and cooling rate. In composites, however, crystallization is influenced both by the polymer matrix and the embedded particles. Unlike fiber-reinforced systems, where carbon fibers provide nucleation sites, metallic particles like CoCr may interfere with nucleation depending on their distribution and interaction with the matrix 49 . In summary, DSC analysis reveals that CoCr content significantly influences the thermal behavior and crystallinity of PEEK-based composites. While the melting point remains stable, crystallinity slightly decreases with the filler content, reinforcing the notion that the structural organization of PEEK is sensitive to the presence and distribution of reinforcement within the matrix. 3.3. TGA analysis TGA was performed under a nitrogen atmosphere to investigate the thermal stability and resin content of PEEK and PEEK/CoCr composites. The TGA curves are presented in Fig. 5 , and the corresponding residual mass data are summarized in Table 3 . All samples of PEEK and PEEK/CoCr composites exhibited a characteristic weight loss starting at approximately 520°C, in agreement with typical decomposition behavior of PEEK-based materials 50 . A rapid mass loss was observed in the range of 550–620°C, indicating the primary decomposition phase, which is attributed to the cleavage of aromatic ether and ketone bonds in the PEEK backbone. As the temperature increased, this bond dissociation was accelerated by radical-induced chain scission, leading to the evolution of volatile decomposition products such as CO, CO₂, and low molecular weight phenolics 49 . Beyond 620°C, the decomposition rate gradually decreased and stabilized after approximately 750°C, marking the transition to a residual carbonaceous phase and thermally stable inorganic components. Notably, PEEK exhibited the highest total weight loss, retaining approximately 54% of its original mass at 800°C. In contrast, the PEEK/CoCr composites exhibited a progressive increase in residual mass with higher CoCr content. At 800°C, the PC(10), PC(20), PC(30), and PC(40) samples retained 63%, 67%, 73%, and 76% of their original mass, respectively. This trend confirms the presence and high thermal stability of the CoCr filler, which remains undegraded within the evaluated temperature range. The thermal degradation process can be divided into four stages. From 25–500°C, the material remains stable with minimal mass loss. Between 500–640°C, rapid decomposition occurs due to volatile product release. From 640–700°C, the degradation rate slows as stable residues form. Above 700°C, the process stabilizes, with no further significant mass change due to the presence of thermally inert residues like carbonized matrix and CoCr. Table 3 Residual mass (%) of neat PEEK and PEEK/CoCr composites determined by TGA. Sample Onset temperature (°C) Mass at 600°C (%) Mass at 800°C (%) PEEK 570 62 54 PC(10) 570 69 63 PC(20) 570 73 67 PC(30) 570 77 73 PC(40) 570 82 76 3.4. Microhardness Investigation Figure 6 presents the microhardness values of neat PEEK and PEEK/CoCr composites as a function of cobalt–chromium filler content. The hardness increases from 26.5 HV for unfilled PEEK to 28.3 HV, 31.5 HV, 32.0 HV, and 37 HV for PC(10), PC(20), PC(30), and PC(40), respectively. This represents a maximum hardness enhancement of approximately 37% in PC(40) compared to the pristine matrix. The incorporation of 10% filler in PC(10) resulted in an initial 6.8% improvement, followed by a more pronounced increase of 19% and 21% for PC(20) and PC(30) respectively. Further increasing the CoCr content in PC(40) led to a significant 40% improvement over neat PEEK, highlighting the continued reinforcement effect at higher loadings. The overall increase of 40% in PC(40) aligns well with previously reported results for PEEK composites filled with 40wt% ceramic reinforcements (Al₂O₃), which demonstrate comparable improvements in microhardness 51 . The observed increase in microhardness of the PEEK/CoCr composites may be primarily attributed to the higher intrinsic hardness of the CoCr alloy (approximately 450 HV) compared to that of the PEEK matrix (24 HV). The incorporation of hard metallic particles within the softer thermoplastic matrix effectively constrains local plastic deformation under indentation. In addition, the relatively uniform dispersion of CoCr particles and the decrease in interparticle distance with increasing filler loading contribute to enhanced indentation resistance. As the metallic content rises, the spatial proximity of CoCr particles within the matrix increases, forming a more interconnected reinforcing network that impedes localized deformation. To support the experimental findings, the microhardness of the composites was also predicted using a modified Halpin–Tsai model, which is commonly used in micromechanics to estimate the effective mechanical properties of composite materials 52 . In this adaptation, the modulus terms are substituted with hardness values, as follows 51 : $$\:{H}_{c}={H}_{m}\left[\frac{1+\xi\:\eta\:{V}_{f}}{1-\eta\:{V}_{f}}\right]$$ 3 where H c ​ is the predicted composite hardness, H m and H f are the matrix and filler hardnesses, respectively, V f is the filler volume fraction, η = [( H f / H m − 1)/( H f / H m + \(\:\xi\:\) )] and \(\:\xi\:\) = 1 (spherical particles). A comparison between the experimentally measured and Halpin–Tsai-predicted hardness values is presented in Fig. 6 . While the model accurately reflects the increasing trend in hardness with filler loading, it underestimates the absolute values by a margin that grows with increasing CoCr content. The differences between experimental and predicted values ranged from 3.6 HV for PC(10) to 8.5 HV for PC(40), suggesting that the Halpin–Tsai model, although useful as a first approximation, does not fully capture the complexity of the reinforcement mechanisms involved. These deviations can be attributed to factors not accounted for in the analytical model, such as enhanced interfacial adhesion, localized micromechanical interlocking, crystallinity changes in the PEEK matrix, and possible particle–particle interaction. Moreover, as the filler content approaches the percolation threshold, the formation of quasi-continuous metallic reinforcement networks may further amplify the resistance to deformation. 3.5. Tribological Assessment 3.5.1. Friction Figure 7 presents the evolution of friction coefficients for PEEK composites with varying filler contents over 5000 sliding cycles. All materials exhibited characteristic tribological behavior with an initial running-in period followed by steady-state friction regimes. Neat PEEK demonstrated a relatively stable friction coefficient starting at approximately 0.177 and gradually decreasing to 0.17 by test completion. This behavior aligns with previous studies on PEEK materials, which typically exhibit friction coefficients of 0.18–0.2 against steel counterfaces 13 , 18 , 53 . The observed reduction in friction coefficient during extended cycling suggests the development of plastic deformation, a phenomenon well-described for PEEK-based materials 8 . The PEEK/CoCr composites demonstrated distinct frictional responses depending on filler concentration. PC(10) exhibited the highest steady-state coefficient of friction (µ ≈ 0.183) along with a gradually increasing trend over time. This behavior suggests that the filler content is insufficient to provide effective load-bearing support, resulting in limited improvement in tribological performance. In contrast, PC(20) achieved a lower and more stable COF (µ ≈ 0.182), indicating a more favorable filler-to-matrix ratio that enhances frictional stability under dry sliding conditions. Higher filler concentrations in PC(30) and PC(40) yielded progressively lower friction coefficients µ ≈ 0.18 and µ ≈ 0.175, respectively. This mild decrease can be attributed to the increased surface hardness reducing the real contact area and adhesive junctions at the interface, because the CoCr particles themselves do not provide any solid lubrication, the overall friction levels remain moderate. Notably, the PC(40) sample showed more frequent friction fluctuations during sliding, indicating slight instabilities in the tribolayer at the highest filler loading. This is due to the micro-scale inhomogeneities and occasional protruding metal particles causing intermittent metal–metal contacts, a phenomenon also reported in other highly filled PEEK composites where excessive filler leads to uneven contact conditions​ 13 . In summary, adding CoCr up to 40% produces no drastic change in COF; all samples maintain friction in the range expected for PEEK–steel sliding, unlike composites with solid lubricant fillers that show dramatic friction reductions. 3.5.2. Wear Performance Analysis Figure 8 presents SEM micrographs of the worn surfaces of neat PEEK and its composites after dry sliding against stainless steel. The worn surface of neat PEEK, shown in Fig. 8 (a, b), displays a characteristic combination of abrasive and adhesive wear mechanisms. Pronounced ploughing grooves and wedge-like formations indicate the prevalence of adhesive interactions, where localized material transfer and plastic deformation dominate. The central region of the wear track exhibits distinct wedge-shaped features, resulting from the reattachment of loosened wear debris back onto the polymer surface. This re-deposition is attributed to the cyclic loading and unloading during reciprocating motion at low sliding speeds, which promotes the entrapment and compaction of flaky debris within the tribological contact. Additionally, interaction with the hard asperities of the stainless steel counterface induces micro-cutting, leading to the removal of material in the form of fine flakes or wear particles. PC(10) exhibited severely damaged surface regions and relatively larger metallic wear debris compared to PC(20) after the sliding test. Unlike neat PEEK, the composite surfaces showed no evident signs of adhesive wear under dry sliding conditions. This improvement is attributed to the presence of CoCr filler particles, which act as protective reinforcements, shielding the polymer matrix from direct interaction with the hard asperities of the stainless steel counterface and thereby reducing the wear rate. Minimal material removal was observed on the wear tracks of PC(30) and PC(40), with only slight localized deposits appearing near the trailing edges. Figure 9 further supports these findings by presenting the mass loss data for all tested compositions. Neat PEEK exhibited a wear mass loss of 19.7 µg/N·m, significantly higher than its reinforced counterparts. The addition of CoCr particles led to a progressive decrease in both wear depth and mass loss, with mass loss values for PC(10), PC(20), PC(30), and PC(40) measured at approximately 11.0, 7.0, 3.3, and 3.1 µg/N·m, respectively. This represents an overall reduction in wear mass of 84% for PC(40) compared to unfilled PEEK. However, a diminishing effect was observed at the highest filler concentration: the wear reduction from PC(30) to PC(40) was minimal. This plateau suggests the existence of threshold filler content beyond which additional reinforcement offers a limited tribological advantage. Similar behavior has been reported in other PEEK composite systems. For instance, Yan et al. showed that beyond 0.7 wt% GO or 20 wt% MoS₂, additional filler did not significantly reduce wear rates and, in some cases, introduced instability due to poor dispersion or matrix weakening 18 . The trend of decreasing wear with increasing filler content follows the predictions of Archard’s wear law, which correlates wear volume inversely with material hardness under constant load and sliding conditions 54 . This relationship is corroborated by the progressive increase in microhardness observed with higher CoCr content further confirming the dominance of mechanical reinforcement in governing wear performance. This pronounced improvement in wear resistance can be primarily attributed to the mechanical reinforcement effect of the CoCr particles. Acting as hard, load-bearing elements within the polymer matrix, these particles reduce the effective contact area subjected to plastic deformation, thereby limiting micro-ploughing, abrasive cutting, and adhesive tearing of the matrix during sliding. As a result, the composite surfaces exhibit shallower and smoother wear traces compared to the deep grooves and plastic smearing observed in neat PEEK. Overall, even a relatively low filler content in PC(10) provides a notable enhancement in wear resistance, while higher CoCr concentrations in PC(30) and PC(40) yield up to an order of magnitude reduction in wear rate without a significant increase in friction. These findings highlight the effectiveness of metallic fillers like CoCr in extending the service life of PEEK components subjected to dry sliding contact, especially in load-bearing applications where abrasive and adhesive wear mechanisms dominate. The simultaneous consideration of friction and wear data provides valuable insights into the operating tribological mechanisms. Unlike solid lubricant fillers (such as PTFE or graphite) that can reduce both friction and wear through the formation of low-shear-strength transfer films, the fillers employed in this study primarily enhance wear resistance while maintaining moderate friction coefficients 19 , 55 . This behavior suggests a wear mechanism dominated by mechanical reinforcement rather than lubrication. The hard filler particles effectively distribute contact stresses, reducing plastic deformation and material removal of the softer polymer matrix. As filler content increases, the composite surface presents a higher proportion of wear-resistant particles to the counterface, resulting in shallower wear tracks and reduced material loss. The modest reduction in friction coefficient observed with higher filler contents (PC(30) and PC(40)) may be attributed to changes in real contact area due to increased surface hardness rather than a true lubrication effect. This explains why friction reduction is less pronounced than wear improvement—a characteristic distinction between reinforcing fillers and lubricating fillers. The tribological performance improvements achieved in this study, particularly the 84% wear reduction for PC(40), represent a significant enhancement over unfilled materials. Future investigations could explore hybrid composites incorporating both reinforcing and lubricating fillers to optimize both wear resistance and friction characteristics. Overall, the materials developed in this study, specifically PC(30) and PC(40), offer valuable combinations of moderate friction (µ ≈ 0.18 − 0.175) and excellent wear resistance. These characteristics make them suitable candidates for numerous tribological applications where extended component lifetime under moderate load conditions is prioritized over extremely low friction. 4. Conclusions In this study, PEEK/CoCr composites were successfully fabricated using centrifugal powder compaction and vacuum sintering, and their microstructure, thermal properties, microhardness, and tribological behavior were systematically investigated. The key conclusions are as follows: The centrifugal powder compaction technique effectively dispersed CoCr particles within the PEEK matrix, enabling uniform distribution and intimate matrix-particle contact without significant agglomeration or void formation. The addition of CoCr filler up to 40 wt% increased the microhardness of PEEK by approximately 40%, with PC(40) reaching 37 HV compared to 26.5 HV for neat PEEK. This enhancement is primarily attributed to the intrinsic hardness of CoCr and improved particle–matrix interlocking. Thermal analysis confirmed that the inclusion of CoCr particles did not significantly alter the melting temperature of PEEK (343°C) but resulted in a slight reduction in crystallinity, indicating minimal interference with the polymer’s thermal stability. Tribological testing under dry sliding conditions demonstrated a substantial reduction in wear rate for PEEK/CoCr composites, with up to 84% wear volume loss reduction in PC(40) compared to neat PEEK. The coefficient of friction remained stable across all compositions, showing no lubricating effect from the metallic filler. SEM analysis of worn surfaces revealed a transition from adhesive and abrasive wear mechanisms in neat PEEK to predominantly abrasive wear with reduced plastic deformation in CoCr-filled composites, confirming the reinforcing effect of CoCr particles in wear resistance. Future studies could investigate the fatigue behavior and long-term stability of these composites and explore hybrid filler systems to simultaneously optimize wear resistance and friction behavior. Additionally, validation in application-relevant environments would further support the use of PEEK/CoCr composites in high-performance engineering and biomedical applications. Declarations Acknowledgement This research work was funded by the Committee of Science of the Ministry of Education and Science of the Republic of Kazakhstan via grant №AP23488642 “Compact Compliant Resilient Wind turbine architecture (COREWind)” project and grant №AP22686136. Author contributions Conceptualization, BS; methodology, BS; synthetic materials and performance characterization, BS, AM, AA and AK; investigation, BS and AA; resources, AM and AA; data curation, BS and AA; writing of the original draft preparation, BS, AK and AM; writing of review and editing, BS BG and HJR; visualization, BS, HJR; supervision, BG and CS; project administration, BG and CS; funding acquisition, AA and CS; All authors have read and agreed to the published version of the manuscript. Data availability All data generated or analyzed during this study are included in this published article. Competing interests The authors declare no competing interests. Additional information Correspondence and requests for materials should be addressed to BS and CS. References Kurtz, S. M. PEEK Biomaterials Handbook (William Andrew, 2019). Kurtz, S. M. & Devine, J. N. PEEK biomaterials in trauma, orthopedic, and spinal implants. 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2","display":"","copyAsset":false,"role":"figure","size":1455948,"visible":true,"origin":"","legend":"\u003cp\u003eSEM micrographs of PC(10) (a), PC(20) (b), PC(30) (c), and PC(40) (d) specimens\u003cem\u003e.\u003c/em\u003e\u003c/p\u003e","description":"","filename":"Figure2.png","url":"https://assets-eu.researchsquare.com/files/rs-6885923/v1/1e63c5ed0a17a8cd9e048c67.png"},{"id":85078669,"identity":"0fb15d92-66f3-4f4e-9cd7-a194767e1fa1","added_by":"auto","created_at":"2025-06-20 17:08:31","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":1735654,"visible":true,"origin":"","legend":"\u003cp\u003eDetailed SEM micrographs of metal-polymer contact\u003cem\u003e.\u003c/em\u003e\u003c/p\u003e","description":"","filename":"Figure3.png","url":"https://assets-eu.researchsquare.com/files/rs-6885923/v1/1891235099e55e0671350522.png"},{"id":85078656,"identity":"11193385-8c40-4ae5-8ed6-40c61266c898","added_by":"auto","created_at":"2025-06-20 17:08:31","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":149309,"visible":true,"origin":"","legend":"\u003cp\u003eDSC graph of PEEK, PC(10), PC(20), PC(30) and PC(40) composites\u003cem\u003e.\u003c/em\u003e\u003c/p\u003e","description":"","filename":"Figure4.png","url":"https://assets-eu.researchsquare.com/files/rs-6885923/v1/4c4949a07488785f8b6a1871.png"},{"id":85078697,"identity":"9c4f17e0-cfb5-4ddd-860c-2ecc3654415e","added_by":"auto","created_at":"2025-06-20 17:08:33","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":824936,"visible":true,"origin":"","legend":"\u003cp\u003eTGA curve of PEEK, PC(10), PC(20), PC(30) and PC(40) composites\u003cem\u003e.\u003c/em\u003e\u003c/p\u003e","description":"","filename":"Figure5.png","url":"https://assets-eu.researchsquare.com/files/rs-6885923/v1/a443eb873812ff67e7c53145.png"},{"id":85078653,"identity":"6030a4d3-6958-4da5-81a8-3464a816a45f","added_by":"auto","created_at":"2025-06-20 17:08:31","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":461425,"visible":true,"origin":"","legend":"\u003cp\u003eExperimental and predicted (from Eq. 2) microhardness values of neat PEEK, PC(10), PC(20), PC(30) and PC(40) composites.\u003c/p\u003e","description":"","filename":"Figure6.png","url":"https://assets-eu.researchsquare.com/files/rs-6885923/v1/f096036b8400e449bb9ca73f.png"},{"id":85079035,"identity":"6568e246-017c-4bd9-a8e1-df6552cf9de0","added_by":"auto","created_at":"2025-06-20 17:16:32","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":667351,"visible":true,"origin":"","legend":"\u003cp\u003eFriction coefficient of neat PEEK, PC(10), PC(20), PC(30) and PC(40) composites.\u003c/p\u003e","description":"","filename":"Figure7.png","url":"https://assets-eu.researchsquare.com/files/rs-6885923/v1/a57e66bd5dbebec88aa8ea44.png"},{"id":85078674,"identity":"33ff3807-b6bb-4be6-8298-4ecee4f005a7","added_by":"auto","created_at":"2025-06-20 17:08:32","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":3605346,"visible":true,"origin":"","legend":"\u003cp\u003eSEM micrographs of worn surfaces: (a,b) neat PEEK, (c) PC(10), (d) PC(20), (e) PC(30), and (f) PC(40) composites.\u003c/p\u003e","description":"","filename":"Figure8.png","url":"https://assets-eu.researchsquare.com/files/rs-6885923/v1/c6a031e209b19a818bbb450f.png"},{"id":85078654,"identity":"214c9073-aef0-467b-833a-39bbcf232c1d","added_by":"auto","created_at":"2025-06-20 17:08:31","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":840672,"visible":true,"origin":"","legend":"\u003cp\u003eWear behavior of neat PEEK, PC(10), PC(20), PC(30) and PC(40) composites.\u003c/p\u003e","description":"","filename":"Figure9.png","url":"https://assets-eu.researchsquare.com/files/rs-6885923/v1/e6961452a3079c6047b233dd.png"},{"id":88814212,"identity":"59fa45a8-42dc-4ed7-8cc4-0a8e11ae6ff9","added_by":"auto","created_at":"2025-08-11 16:08:32","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":10868849,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-6885923/v1/70409cb1-ebb4-44f3-9a45-7d083248334d.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Thermal, Hardness, and Tribological Assessment of PEEK/CoCr Composites","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003ePolymer-metal composites represent a significant advancement in materials science, offering unique combinations of properties that cannot be achieved by either constituent alone. Among high-performance thermoplastics, PEEK has garnered substantial attention due to its exceptional thermal stability, mechanical strength, and chemical resistance, which make it suitable for high-demand applications such as aerospace, automotive, and biomedical devices\u003csup\u003e\u003cspan additionalcitationids=\"CR2\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u003c/sup\u003e. Particularly in orthopedics and dentistry, PEEK has gained popularity due to its favorable biocompatibility, radiolucency, and sterilization resistance\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e,\u003cspan additionalcitationids=\"CR5\" citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u003c/sup\u003e. Despite these advantages, neat PEEK demonstrates poor tribological behavior under dry sliding conditions, including a high coefficient of friction (COF) and substantial wear, limiting its service life in load-bearing or articulating components\u003csup\u003e\u003cspan additionalcitationids=\"CR8 CR9\" citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eTo overcome these limitations, researchers have developed various PEEK-based composites by incorporating functional fillers aimed at improving frictional and wear behavior. These can be broadly categorized into solid lubricants, ceramic reinforcements, fiber reinforcements, and metallic fillers\u003csup\u003e\u003cspan additionalcitationids=\"CR12 CR13 CR14 CR15 CR16\" citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u003c/sup\u003e. Among solid lubricants, polytetrafluoroethylene (PTFE), graphite, and molybdenum disulfide (MoS₂) have been widely used to reduce COF by forming transfer films during sliding\u003csup\u003e\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e,\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e,\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003e. For example, Burris et al. \u003csup\u003e\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u003c/sup\u003e showed that PEEK/PTFE composites achieved ultralow wear rates (2\u0026times;10⁻⁹ mm\u0026sup3;/N\u0026middot;m) and a low COF (0.12). Similarly, carbon fiber/PTFE/graphite hybrid composites have achieved COF values below 0.2 against Co-Cr alloy surfaces\u003csup\u003e\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eCeramic fillers such as titanium dioxide (TiO₂), alumina (Al₂O₃), and silica (SiO₂) significantly improve hardness and wear resistance due to their intrinsic rigidity and load-bearing capabilities\u003csup\u003e\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e,\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u003c/sup\u003e. Titanium-based ceramic composites have been effectively used in biomedical crowns and load-bearing implants\u003csup\u003e\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u003c/sup\u003e. Fiber reinforcements, especially carbon and glass fibers, provide substantial improvements in strength and stiffness, although often at the cost of increased brittleness and anisotropy\u003csup\u003e\u003cspan additionalcitationids=\"CR25\" citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eMetallic fillers present a relatively underexplored but promising route to enhance both the mechanical and thermal properties of PEEK. Metals such as silicon, titanium, and aluminum offer improved dimensional stability, thermal conductivity, and surface hardness\u003csup\u003e\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e,\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e\u003c/sup\u003e. Oladele et al.\u003csup\u003e\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e\u003c/sup\u003e provided a comprehensive review of polymer composites, emphasizing the potential of metallic fillers to improve mechanical strength and durability for structural applications. Goyal et al.\u003csup\u003e\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u003c/sup\u003e demonstrated that the incorporation of aluminum nitride particles into PEEK significantly enhanced its thermomechanical properties, including glass transition temperature and modulus. Ochoa-Putman et al.\u003csup\u003e\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e\u003c/sup\u003e emphasized the critical role of interfacial adhesion in determining the mechanical performance of metal\u0026ndash;polymer composites, highlighting how chemical treatment of metal surfaces improves compatibility with polymer matrices. Furthermore, Dobrzańska-Danikiewicz et al.\u003csup\u003e\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e\u003c/sup\u003e explored processing techniques for metal\u0026ndash;polymer composites and noted that conventional methods often result in non-uniform dispersion of metal fillers, adversely affecting structural integrity. More specifically, Thiruchitrambalam et al.\u003csup\u003e\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e\u003c/sup\u003e, reviewed the current state and future prospects of metal-reinforced PEEK composites, identifying processing challenges and potential applications. However, as observed by Siraj et al. \u003csup\u003e\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u003c/sup\u003e, systematic investigations into the tribological properties of metal-reinforced PEEK composites remain limited. Notably, CoCr alloys are established biomaterials due to their wear resistance, corrosion resistance, and proven biocompatibility, making them ideal candidates for reinforcing polymers intended for medical applications\u003csup\u003e\u003cspan additionalcitationids=\"CR34\" citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e\u003c/sup\u003e. Li et al.\u003csup\u003e\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e\u003c/sup\u003e characterized the microstructure of biocompatible CoCr alloys, highlighting their potential for medical implants. Wu et al.\u003csup\u003e\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e\u003c/sup\u003e investigated the properties of CoCr dental alloys fabricated via selective laser melting, demonstrating superior mechanical performance compared to conventional casting methods. In the context of composite materials, Senra et al.\u003csup\u003e\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e\u003c/sup\u003e explored the potential of CoCr-reinforced polymers for biomedical applications, reporting enhanced bioactivity and mechanical properties. The superior wear resistance of CoCr alloys, as documented by Yan et al.\u003csup\u003e\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e\u003c/sup\u003e suggests their potential as reinforcement materials for improving the tribological performance of polymers.\u003c/p\u003e \u003cp\u003eAlthough a few hybrid composite studies have combined CoCr with solid lubricants like hexagonal boron nitride to balance hardness and lubricity, the independent contribution of CoCr to PEEK's tribological and thermal behavior remains largely unexplored\u003csup\u003e\u003cspan additionalcitationids=\"CR41\" citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e\u003c/sup\u003e. Ma et al. \u003csup\u003e\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e\u003c/sup\u003e demonstrated the potential of hard fillers to improve microhardness by 19% using calcium silicate, but there are no studies that have systematically examined PEEK/CoCr binary systems. Moreover, understanding the role of metal content in influencing thermal stability and crystallinity is critical for expanding the applicability of these composites to more demanding environments.\u003c/p\u003e \u003cp\u003eIn terms of characterization, Doumneg et al.\u003csup\u003e\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u003c/sup\u003e employed thermal analysis techniques to study the crystallization behavior of PEEK composites, while Puhan et al.\u003csup\u003e\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e\u003c/sup\u003e conducted comprehensive tribological assessments of PEEK-based materials under various wear modes. Beckford et al.\u003csup\u003e\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u003c/sup\u003e investigated the effect of ceramic particles on the tribological characteristics of PEEK, providing valuable insights into wear mechanisms. Rui et al.\u003csup\u003e\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e\u003c/sup\u003e demonstrated that powder metallurgy techniques, including compaction and sintering, enable the incorporation of high-volume hydroxyapatite fillers into polymer without degrading the polymer matrix, achieving uniform dispersion and enhanced bioactivity.\u003c/p\u003e \u003cp\u003eTraditional processing methods like melt compounding, compression, and injection molding often lead to issues such as particle agglomeration and weak interfacial bonding.\u003csup\u003e\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e\u003c/sup\u003e Advanced processing techniques have been developed to address the challenges associated with polymer-metal composites. Tsukamoto demonstrated that centrifugal casting could effectively distribute carbon nanotubes in metallic matrices, suggesting its potential for polymer-metal systems\u003csup\u003e\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e\u003c/sup\u003e. Similarly, Lu et al. \u003csup\u003e\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e\u003c/sup\u003e optimized processing parameters for CF/PEEK composites, highlighting the importance of controlling the thermal history to achieve the desired properties. Although centrifugal powder compaction is known to improve filler dispersion, its use with PEEK/CoCr composites has not been well studied. This work applies the method to evaluate its effectiveness in enhancing structure and properties for these systems\u003csup\u003e\u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e48\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eThis study aims to investigate the microstructural, thermal, mechanical, and tribological characteristics of PEEK/CoCr composites fabricated by powder centrifugal compaction and vacuum sintering. Four compositions, PC(10), PC(20), PC(30), and PC(40), containing 10%, 20%, 30%, and 40% of CoCr by weight, respectively, were synthesized and characterized using scanning electron microscopy with energy-dispersive spectroscopy, differential scanning calorimetry, thermogravimetric analysis, microhardness testing, and ball-on-disk tribometry. By focusing on a binary PEEK/CoCr system, this research fills a critical gap in the literature and evaluates the potential of metal-filled PEEK composites for advanced engineering and biomedical applications where enhanced wear resistance and hardness are required without compromising thermal stability.\u003c/p\u003e"},{"header":"2. Experimental","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1. Materials\u003c/h2\u003e \u003cp\u003ePolyetheretherketone (PEEK) powder (Victrex 450PF, UK) and cobalt\u0026ndash;chromium (CoCr) alloy powder (MSE Supplies, USA) were employed as the base materials in this study. The particle size distribution (PSD) of both powders was measured by laser diffraction using a Mastersizer 3000 with an Aero S dry powder disperser operating at 1 bar (Malvern Panalytical, UK). The median particle diameter of the PEEK powder was determined to be approximately 38 \u0026micro;m. For the CoCr powder, the PSD analysis yielded D\u003csub\u003e10\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;40.5 \u0026micro;m, D\u003csub\u003e50\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;62.4 \u0026micro;m, and D\u003csub\u003e90\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;86.1 \u0026micro;m, indicating a relatively broad particle size distribution. These percentile values correspond to the particle diameters below which 10%, 50%, and 90% of the total volume of particles are found, respectively. The span S, a parameter commonly used to characterize distribution width, was calculated using the formula:\u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e\n$$\\:S=\\frac{{D}_{90}-{D}_{10}}{{D}_{50}}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e1\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eThe morphology of the CoCr powder, observed using a scanning electron microscope (SEM), is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2. Composite preparation\u003c/h2\u003e \u003cp\u003ePEEK and CoCr powders were measured and premixed in proportions to yield 10%, 20%, 30%, and 40% by weight of CoCr. The weight and corresponding volume fractions of PEEK and CoCr are presented in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. The prepared powder mixtures were inserted into the aluminum dies and loaded into the centrifugal machine [Allegra X-14 Beckman Coulter Inc. Brea, USA]. The powder mixture in the die, tilted along the horizontal axis, was subjected to a centrifugal force of 3000 G for 180 minutes. This method promotes uniform blending of the dense metal particles with the lighter polymer powder while simultaneously compacting the mixture. After mixing, the compacted powder blends were molded and sintered. Sintering was carried out in a vacuum furnace to avoid oxidation of the metal. The specimens were slowly heated to 360\u0026deg;C, slightly above the melting temperature of PEEK 343\u0026deg;C, and held for a duration for 30 min, to ensure complete melting of PEEK matrix and encapsulation the metal particles. Then the specimens cooled down slowly to minimize residual stresses. The sintered composites were removed and cut into test samples for further characterization.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eWeight and volume fraction of PEEK/CoCr compositions for the different tested samples.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"5\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSample\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eCoCr weight\u003c/p\u003e \u003cp\u003econtent (%)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003ePEEK weight\u003c/p\u003e \u003cp\u003econtent (%)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eCoCr volume\u003c/p\u003e \u003cp\u003efraction (%)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003ePEEK volume\u003c/p\u003e \u003cp\u003efraction (%)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePEEK\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e100\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e100\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePC(20)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e90\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e1.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e98.3\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePC(20)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e20\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e80\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e3.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e96.2\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePC(30)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e30\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e70\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e6.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e93.7\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePC(40)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e40\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e60\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e9.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e90.5\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e2.3. Characterization methods\u003c/h2\u003e \u003cp\u003eThe fabricated PEEK/CoCr composites were sectioned for microstructural analysis using a wet abrasive cut-off machine (Brillant 220, QATM, Mammelzen, Germany). The cross-sectional surfaces were subsequently polished using a series of silicon carbide abrasive papers with grit sizes of 800, 1200, and 2500, followed by fine polishing with alumina suspensions of 1 \u0026micro;m, 0.3 \u0026micro;m, and 0.05 \u0026micro;m particle sizes. Microstructural characterization was performed using a scanning electron microscope (JSM-IT200, JEOL, Japan) operated at an accelerating voltage of 15 kV in secondary electron mode. Prior to imaging, a thin conductive gold layer (~\u0026thinsp;5 nm) was sputter-coated onto the samples to facilitate high-resolution SEM and Energy Dispersive X-ray Spectroscopy (EDS) analysis.\u003c/p\u003e \u003cp\u003eThermogravimetric analysis (TGA) was conducted using STA 449 thermal analyzer (Netzsch, Germany) to determine the thermal decomposition behavior of both neat PEEK and its CoCr-filled composites. The measurements were carried out under a nitrogen atmosphere with a constant heating rate of 10\u0026deg;C/min, ranging from 30\u0026deg;C to 1000\u0026deg;C, followed by a 90-minute isothermal hold at the maximum temperature.\u003c/p\u003e \u003cp\u003eDifferential scanning calorimetry (DSC) was performed using a 300 Caliris Supreme analyzer (Netzsch, Germany) to evaluate the melting behavior and degree of crystallinity of the materials. Approximately 10 mg of each sample was heated from 30\u0026deg;C to 500\u0026deg;C at a constant rate of 10\u0026deg;C/min under a nitrogen atmosphere.\u003c/p\u003e \u003cp\u003eBased on the difference in melting enthalpy of the crystalline region and amorphous region, the crystallinity \u003cem\u003eX\u003c/em\u003e\u003csub\u003e\u003cem\u003ec\u003c/em\u003e\u003c/sub\u003e can be calculated according to the following Eq.\u0026nbsp;4\u003csup\u003e9\u003c/sup\u003e:\u003cdiv id=\"Equ2\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ2\" name=\"EquationSource\"\u003e\n$$\\:Xc=\\frac{\\varDelta\\:Hm}{\\varDelta\\:H^\\circ\\:}\\times\\:100\\%$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e2\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003ewhere ΔHm​ is the melting enthalpy of PEEK or PEEK/CoCr composites, and ΔH⁰ is the enthalpy of 100% crystalline PEEK (ΔH⁰=130 J/g).\u003c/p\u003e \u003cp\u003eVickers microhardness testing was conducted on the polished cross-sections using a standard microhardness tester. A load of 10 N was applied with a dwell time of 10 seconds for each indentation. Ten measurements were taken for each sample at randomly selected regions, carefully avoiding direct indentation on large metal particles. The Vickers hardness (HV) was calculated based on the diagonal lengths of the indentations, and average values with standard deviations were reported.\u003c/p\u003e \u003cp\u003eAll tribological tests were conducted in a laboratory environment under dry sliding conditions. The ambient temperature and relative humidity were maintained at approximately 26\u0026deg;C and 12%, respectively. A ball-on-disk tribometer (Anton Paar, Austria) was used to evaluate the friction and wear behaviour in a reciprocating sliding configuration. The counterface was a steel ball with a diameter of 6 mm, sliding against a flat sample surface. The test setup had a radius of 10 mm and a reciprocating angle of 10\u0026deg;, generating linear back-and-forth motion. The sliding frequency was set at 2.00 Hz, corresponding to a maximum linear speed of 0.99 cm/s. A constant normal load of 10N was applied during testing, and the sliding motion was carried out for 5000 cycles. The apparent coefficient of friction was calculated as the ratio of the measured frictional force to the applied normal load, allowing for the evaluation of frictional behavior under controlled and repeatable conditions.\u003c/p\u003e \u003c/div\u003e"},{"header":"3. Results and Discussion","content":"\u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003e3.1. Composite Morphology and Interfacial Characteristics\u003c/h2\u003e \u003cp\u003eThe microstructural evolution of PEEK/CoCr composites with varying metallic content was systematically examined using SEM. Figure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e displays representative SEM images of PC(10), PC(20), PC(30) and PC(40) composites. It is noteworthy that during sectioning and polishing, partial detachment of CoCr particles occurred, leading to the formation of hollows on the polished surfaces.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe incorporation of CoCr particles leads to substantial changes in microstructural features across the composition range. In PC(20) (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb), spherical metallic particles ranging from approximately 5 to 50 \u0026micro;m are uniformly embedded in the PEEK matrix, with minimal signs of agglomeration. The particle\u0026ndash;matrix interfaces appear clearly defined, suggesting interfacial diffusion or bonding during the sintering process.\u003c/p\u003e \u003cp\u003eWith increasing CoCr content in PC(30) (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec), a notable increase in particle number density is observed, accompanied by a reduction in interparticle spacing. Nevertheless, the distribution remains relatively uniform, indicating that centrifugal compaction effectively mitigates particle clustering even at elevated filler contents. In PC(40) (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ed), the highest particle concentration is achieved, and instances of particle\u0026ndash;particle contact become more frequent. This may indicate the onset of a percolation threshold, wherein the metallic phase begins to form quasi-continuous networks within the polymer matrix.\u003c/p\u003e \u003cp\u003eHigher magnification SEM analysis (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e) reveals significant morphological details of the CoCr particles. Many particles exhibit internal porosity and complex surface textures, likely originating from the powder metallurgy process used for their fabrication. Some particles display crater-like surface indentations and protuberances, which could contribute to mechanical interlocking with the surrounding PEEK matrix, thereby enhancing interfacial bonding and mechanical performance. The preservation of particle sphericity throughout all composite formulations implies that the processing conditions of powder compaction were sufficiently controlled to prevent deformation or fragmentation of the metallic phase. Such morphological uniformity is essential for ensuring consistent mechanical and thermal behavior of the composite system. Furthermore, the lack of visible interfacial gaps suggests good wetting and adhesion between the polymer matrix and the CoCr reinforcement, which is critical for efficient stress transfer in load-bearing applications.\u003c/p\u003e \u003cp\u003eFurther high-resolution SEM observations (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e) provide insight into the interface between CoCr particles and the PEEK matrix. CoCr particle surfaces exhibit varied microscale roughness, which may facilitate mechanical interlocking with the thermoplastic matrix.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eDetailed inspection reveals localized zones of intimate contact between the matrix and CoCr particles. These regions represent potential sites of enhanced interfacial bonding. Notably, the CoCr particles exhibit excellent shape transcription within the PEEK matrix, indicating no deformation or smearing during processing and suggesting strong mechanical interlocking at the interface. Interfacial adhesion, internal porosity, and particle morphology are key microstructural features expected to strongly influence the mechanical response of the composites.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003e3.2. DSC analysis\u003c/h2\u003e \u003cp\u003eThe curves and results of DSC analysis for PEEK and PEEK/CoCr composites are presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e and Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. These results indicate that both PEEK and its composites exhibit a melting range rather than a sharp melting point, which is typical of semi-crystalline polymers. The variation in crystal size and perfection leads to melting over a range of temperatures, with PEEK typically exhibiting melting between 290\u0026ndash;360\u0026deg;C. The melting point of neat PEEK was determined to be approximately 343\u0026deg;C, a value that remains unchanged with the incorporation of CoCr.\u003c/p\u003e \u003cp\u003eThe melting behavior of PEEK/CoCr composites shows that the inclusion of metallic reinforcement does not significantly shift the melting point but does impact the enthalpy of fusion and, thus, the degree of crystallinity. For instance, the melting enthalpy (ΔHₘ) for neat PEEK was measured at 53 J/g, which corresponds to a crystallinity of 41%. In contrast, PEEK composites with 10%, 20%, 30%, and 40% CoCr exhibited ΔHₘ values of 48.3 J/g, 48.5 J/g, 49.2 J/g, and 49.3 J/g, resulting in crystallinity levels of 37%, 37%, 38%, and 38%, respectively.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThese values demonstrate a general trend of decreasing crystallinity with increasing CoCr content. This can be attributed to the reduced mass fraction of PEEK in the composite, which inherently limits the total amount of crystallizable material, as well as the physical presence of CoCr particles that hinder polymer chain mobility and alignment during crystallization. Although crystallinity decreased with filler content, the melting and crystallization peaks remained clearly visible, suggesting that the filler does not completely suppress crystal formation.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eCrystallinity of PEEK and PEEK/CoCr composites as determined by DSC analysis.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"5\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSample\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eMelting range (\u0026deg;C)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eTm (\u0026deg;C)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eΔH\u003csub\u003em\u003c/sub\u003e​ (J/g)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eX\u003csub\u003ec\u003c/sub\u003e (%)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePEEK\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e320\u0026ndash;360\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e343\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e53\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e41\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePС(10)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e316\u0026ndash;355\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e343\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e48.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e37\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePС(20)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e320\u0026ndash;360\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e343\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e48.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e37\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePС(30)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e320\u0026ndash;360\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e343\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e49.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e38\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePС(40)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e316\u0026ndash;355\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e343\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e49.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e38\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eThe DSC data also supports the understanding that crystallization in PEEK and its composites is determined by multiple factors. In pure PEEK, crystallization is governed by intrinsic polymer behavior, largely dependent on temperature and cooling rate. In composites, however, crystallization is influenced both by the polymer matrix and the embedded particles. Unlike fiber-reinforced systems, where carbon fibers provide nucleation sites, metallic particles like CoCr may interfere with nucleation depending on their distribution and interaction with the matrix\u003csup\u003e\u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e49\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eIn summary, DSC analysis reveals that CoCr content significantly influences the thermal behavior and crystallinity of PEEK-based composites. While the melting point remains stable, crystallinity slightly decreases with the filler content, reinforcing the notion that the structural organization of PEEK is sensitive to the presence and distribution of reinforcement within the matrix.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec9\" class=\"Section2\"\u003e \u003ch2\u003e3.3. TGA analysis\u003c/h2\u003e \u003cp\u003eTGA was performed under a nitrogen atmosphere to investigate the thermal stability and resin content of PEEK and PEEK/CoCr composites. The TGA curves are presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e, and the corresponding residual mass data are summarized in Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e.\u003c/p\u003e \u003cp\u003eAll samples of PEEK and PEEK/CoCr composites exhibited a characteristic weight loss starting at approximately 520\u0026deg;C, in agreement with typical decomposition behavior of PEEK-based materials\u003csup\u003e\u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e50\u003c/span\u003e\u003c/sup\u003e. A rapid mass loss was observed in the range of 550\u0026ndash;620\u0026deg;C, indicating the primary decomposition phase, which is attributed to the cleavage of aromatic ether and ketone bonds in the PEEK backbone. As the temperature increased, this bond dissociation was accelerated by radical-induced chain scission, leading to the evolution of volatile decomposition products such as CO, CO₂, and low molecular weight phenolics\u003csup\u003e\u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e49\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eBeyond 620\u0026deg;C, the decomposition rate gradually decreased and stabilized after approximately 750\u0026deg;C, marking the transition to a residual carbonaceous phase and thermally stable inorganic components.\u003c/p\u003e \u003cp\u003eNotably, PEEK exhibited the highest total weight loss, retaining approximately 54% of its original mass at 800\u0026deg;C. In contrast, the PEEK/CoCr composites exhibited a progressive increase in residual mass with higher CoCr content. At 800\u0026deg;C, the PC(10), PC(20), PC(30), and PC(40) samples retained 63%, 67%, 73%, and 76% of their original mass, respectively. This trend confirms the presence and high thermal stability of the CoCr filler, which remains undegraded within the evaluated temperature range.\u003c/p\u003e \u003cp\u003eThe thermal degradation process can be divided into four stages. From 25\u0026ndash;500\u0026deg;C, the material remains stable with minimal mass loss. Between 500\u0026ndash;640\u0026deg;C, rapid decomposition occurs due to volatile product release. From 640\u0026ndash;700\u0026deg;C, the degradation rate slows as stable residues form. Above 700\u0026deg;C, the process stabilizes, with no further significant mass change due to the presence of thermally inert residues like carbonized matrix and CoCr.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab3\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 3\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eResidual mass (%) of neat PEEK and PEEK/CoCr composites determined by TGA.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"4\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSample\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eOnset\u003c/p\u003e \u003cp\u003etemperature (\u0026deg;C)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eMass at 600\u0026deg;C (%)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eMass at 800\u0026deg;C (%)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePEEK\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e570\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e62\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e54\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePC(10)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e570\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e69\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e63\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePC(20)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e570\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e73\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e67\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePC(30)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e570\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e77\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e73\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePC(40)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e570\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e82\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e76\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec10\" class=\"Section2\"\u003e \u003ch2\u003e3.4. Microhardness Investigation\u003c/h2\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e presents the microhardness values of neat PEEK and PEEK/CoCr composites as a function of cobalt\u0026ndash;chromium filler content. The hardness increases from 26.5 HV for unfilled PEEK to 28.3 HV, 31.5 HV, 32.0 HV, and 37 HV for PC(10), PC(20), PC(30), and PC(40), respectively. This represents a maximum hardness enhancement of approximately 37% in PC(40) compared to the pristine matrix.\u003c/p\u003e \u003cp\u003eThe incorporation of 10% filler in PC(10) resulted in an initial 6.8% improvement, followed by a more pronounced increase of 19% and 21% for PC(20) and PC(30) respectively. Further increasing the CoCr content in PC(40) led to a significant 40% improvement over neat PEEK, highlighting the continued reinforcement effect at higher loadings.\u003c/p\u003e \u003cp\u003eThe overall increase of 40% in PC(40) aligns well with previously reported results for PEEK composites filled with 40wt% ceramic reinforcements (Al₂O₃), which demonstrate comparable improvements in microhardness\u003csup\u003e\u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e51\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eThe observed increase in microhardness of the PEEK/CoCr composites may be primarily attributed to the higher intrinsic hardness of the CoCr alloy (approximately 450 HV) compared to that of the PEEK matrix (24 HV). The incorporation of hard metallic particles within the softer thermoplastic matrix effectively constrains local plastic deformation under indentation. In addition, the relatively uniform dispersion of CoCr particles and the decrease in interparticle distance with increasing filler loading contribute to enhanced indentation resistance. As the metallic content rises, the spatial proximity of CoCr particles within the matrix increases, forming a more interconnected reinforcing network that impedes localized deformation.\u003c/p\u003e \u003cp\u003eTo support the experimental findings, the microhardness of the composites was also predicted using a modified Halpin\u0026ndash;Tsai model, which is commonly used in micromechanics to estimate the effective mechanical properties of composite materials\u003csup\u003e\u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e52\u003c/span\u003e\u003c/sup\u003e. In this adaptation, the modulus terms are substituted with hardness values, as follows \u003csup\u003e\u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e51\u003c/span\u003e\u003c/sup\u003e:\u003cdiv id=\"Equ3\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ3\" name=\"EquationSource\"\u003e\n$$\\:{H}_{c}={H}_{m}\\left[\\frac{1+\\xi\\:\\eta\\:{V}_{f}}{1-\\eta\\:{V}_{f}}\\right]$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e3\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003ewhere \u003cem\u003eH\u003c/em\u003e\u003csub\u003e\u003cem\u003ec\u003c/em\u003e\u003c/sub\u003e​ is the predicted composite hardness, \u003cem\u003eH\u003c/em\u003e\u003csub\u003e\u003cem\u003em\u003c/em\u003e\u003c/sub\u003e and \u003cem\u003eH\u003c/em\u003e\u003csub\u003e\u003cem\u003ef\u003c/em\u003e\u003c/sub\u003e are the matrix and filler hardnesses, respectively, \u003cem\u003eV\u003c/em\u003e\u003csub\u003e\u003cem\u003ef\u003c/em\u003e\u003c/sub\u003e is the filler volume fraction, \u003cem\u003eη\u003c/em\u003e = [(\u003cem\u003eH\u003c/em\u003e\u003csub\u003ef\u003c/sub\u003e/\u003cem\u003eH\u003c/em\u003e\u003csub\u003em\u003c/sub\u003e \u0026minus;\u0026thinsp;1)/(\u003cem\u003eH\u003c/em\u003e\u003csub\u003ef\u003c/sub\u003e/\u003cem\u003eH\u003c/em\u003e\u003csub\u003em\u003c/sub\u003e + \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\xi\\:\\)\u003c/span\u003e\u003c/span\u003e)] and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\xi\\:\\)\u003c/span\u003e\u003c/span\u003e = 1 (spherical particles).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eA comparison between the experimentally measured and Halpin\u0026ndash;Tsai-predicted hardness values is presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e. While the model accurately reflects the increasing trend in hardness with filler loading, it underestimates the absolute values by a margin that grows with increasing CoCr content. The differences between experimental and predicted values ranged from 3.6 HV for PC(10) to 8.5 HV for PC(40), suggesting that the Halpin\u0026ndash;Tsai model, although useful as a first approximation, does not fully capture the complexity of the reinforcement mechanisms involved. These deviations can be attributed to factors not accounted for in the analytical model, such as enhanced interfacial adhesion, localized micromechanical interlocking, crystallinity changes in the PEEK matrix, and possible particle\u0026ndash;particle interaction. Moreover, as the filler content approaches the percolation threshold, the formation of quasi-continuous metallic reinforcement networks may further amplify the resistance to deformation.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003e3.5. Tribological Assessment\u003c/h2\u003e \u003cdiv id=\"Sec12\" class=\"Section3\"\u003e \u003ch2\u003e3.5.1. Friction\u003c/h2\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e presents the evolution of friction coefficients for PEEK composites with varying filler contents over 5000 sliding cycles. All materials exhibited characteristic tribological behavior with an initial running-in period followed by steady-state friction regimes.\u003c/p\u003e \u003cp\u003eNeat PEEK demonstrated a relatively stable friction coefficient starting at approximately 0.177 and gradually decreasing to 0.17 by test completion. This behavior aligns with previous studies on PEEK materials, which typically exhibit friction coefficients of 0.18\u0026ndash;0.2 against steel counterfaces\u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e,\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e,\u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e53\u003c/span\u003e\u003c/sup\u003e. The observed reduction in friction coefficient during extended cycling suggests the development of plastic deformation, a phenomenon well-described for PEEK-based materials\u003csup\u003e\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eThe PEEK/CoCr composites demonstrated distinct frictional responses depending on filler concentration. PC(10) exhibited the highest steady-state coefficient of friction (\u0026micro;\u0026thinsp;\u0026asymp;\u0026thinsp;0.183) along with a gradually increasing trend over time. This behavior suggests that the filler content is insufficient to provide effective load-bearing support, resulting in limited improvement in tribological performance. In contrast, PC(20) achieved a lower and more stable COF (\u0026micro;\u0026thinsp;\u0026asymp;\u0026thinsp;0.182), indicating a more favorable filler-to-matrix ratio that enhances frictional stability under dry sliding conditions.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eHigher filler concentrations in PC(30) and PC(40) yielded progressively lower friction coefficients \u0026micro;\u0026thinsp;\u0026asymp;\u0026thinsp;0.18 and \u0026micro;\u0026thinsp;\u0026asymp;\u0026thinsp;0.175, respectively. This mild decrease can be attributed to the increased surface hardness reducing the real contact area and adhesive junctions at the interface, because the CoCr particles themselves do not provide any solid lubrication, the overall friction levels remain moderate. Notably, the PC(40) sample showed more frequent friction fluctuations during sliding, indicating slight instabilities in the tribolayer at the highest filler loading. This is due to the micro-scale inhomogeneities and occasional protruding metal particles causing intermittent metal\u0026ndash;metal contacts, a phenomenon also reported in other highly filled PEEK composites where excessive filler leads to uneven contact conditions​\u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003e. In summary, adding CoCr up to 40% produces no drastic change in COF; all samples maintain friction in the range expected for PEEK\u0026ndash;steel sliding, unlike composites with solid lubricant fillers that show dramatic friction reductions.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec13\" class=\"Section3\"\u003e \u003ch2\u003e3.5.2. Wear Performance Analysis\u003c/h2\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e presents SEM micrographs of the worn surfaces of neat PEEK and its composites after dry sliding against stainless steel. The worn surface of neat PEEK, shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e(a, b), displays a characteristic combination of abrasive and adhesive wear mechanisms. Pronounced ploughing grooves and wedge-like formations indicate the prevalence of adhesive interactions, where localized material transfer and plastic deformation dominate. The central region of the wear track exhibits distinct wedge-shaped features, resulting from the reattachment of loosened wear debris back onto the polymer surface. This re-deposition is attributed to the cyclic loading and unloading during reciprocating motion at low sliding speeds, which promotes the entrapment and compaction of flaky debris within the tribological contact. Additionally, interaction with the hard asperities of the stainless steel counterface induces micro-cutting, leading to the removal of material in the form of fine flakes or wear particles.\u003c/p\u003e \u003cp\u003ePC(10) exhibited severely damaged surface regions and relatively larger metallic wear debris compared to PC(20) after the sliding test. Unlike neat PEEK, the composite surfaces showed no evident signs of adhesive wear under dry sliding conditions. This improvement is attributed to the presence of CoCr filler particles, which act as protective reinforcements, shielding the polymer matrix from direct interaction with the hard asperities of the stainless steel counterface and thereby reducing the wear rate. Minimal material removal was observed on the wear tracks of PC(30) and PC(40), with only slight localized deposits appearing near the trailing edges.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e further supports these findings by presenting the mass loss data for all tested compositions. Neat PEEK exhibited a wear mass loss of 19.7 \u0026micro;g/N\u0026middot;m, significantly higher than its reinforced counterparts. The addition of CoCr particles led to a progressive decrease in both wear depth and mass loss, with mass loss values for PC(10), PC(20), PC(30), and PC(40) measured at approximately 11.0, 7.0, 3.3, and 3.1 \u0026micro;g/N\u0026middot;m, respectively. This represents an overall reduction in wear mass of 84% for PC(40) compared to unfilled PEEK. However, a diminishing effect was observed at the highest filler concentration: the wear reduction from PC(30) to PC(40) was minimal. This plateau suggests the existence of threshold filler content beyond which additional reinforcement offers a limited tribological advantage. Similar behavior has been reported in other PEEK composite systems. For instance, Yan et al. showed that beyond 0.7 wt% GO or 20 wt% MoS₂, additional filler did not significantly reduce wear rates and, in some cases, introduced instability due to poor dispersion or matrix weakening\u003csup\u003e\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003e. The trend of decreasing wear with increasing filler content follows the predictions of Archard\u0026rsquo;s wear law, which correlates wear volume inversely with material hardness under constant load and sliding conditions\u003csup\u003e\u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e54\u003c/span\u003e\u003c/sup\u003e. This relationship is corroborated by the progressive increase in microhardness observed with higher CoCr content further confirming the dominance of mechanical reinforcement in governing wear performance.\u003c/p\u003e \u003cp\u003eThis pronounced improvement in wear resistance can be primarily attributed to the mechanical reinforcement effect of the CoCr particles. Acting as hard, load-bearing elements within the polymer matrix, these particles reduce the effective contact area subjected to plastic deformation, thereby limiting micro-ploughing, abrasive cutting, and adhesive tearing of the matrix during sliding. As a result, the composite surfaces exhibit shallower and smoother wear traces compared to the deep grooves and plastic smearing observed in neat PEEK.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eOverall, even a relatively low filler content in PC(10) provides a notable enhancement in wear resistance, while higher CoCr concentrations in PC(30) and PC(40) yield up to an order of magnitude reduction in wear rate without a significant increase in friction. These findings highlight the effectiveness of metallic fillers like CoCr in extending the service life of PEEK components subjected to dry sliding contact, especially in load-bearing applications where abrasive and adhesive wear mechanisms dominate.\u003c/p\u003e \u003cp\u003eThe simultaneous consideration of friction and wear data provides valuable insights into the operating tribological mechanisms. Unlike solid lubricant fillers (such as PTFE or graphite) that can reduce both friction and wear through the formation of low-shear-strength transfer films, the fillers employed in this study primarily enhance wear resistance while maintaining moderate friction coefficients\u003csup\u003e\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e,\u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e55\u003c/span\u003e\u003c/sup\u003e. This behavior suggests a wear mechanism dominated by mechanical reinforcement rather than lubrication. The hard filler particles effectively distribute contact stresses, reducing plastic deformation and material removal of the softer polymer matrix. As filler content increases, the composite surface presents a higher proportion of wear-resistant particles to the counterface, resulting in shallower wear tracks and reduced material loss.\u003c/p\u003e \u003cp\u003eThe modest reduction in friction coefficient observed with higher filler contents (PC(30) and PC(40)) may be attributed to changes in real contact area due to increased surface hardness rather than a true lubrication effect. This explains why friction reduction is less pronounced than wear improvement\u0026mdash;a characteristic distinction between reinforcing fillers and lubricating fillers.\u003c/p\u003e \u003cp\u003eThe tribological performance improvements achieved in this study, particularly the 84% wear reduction for PC(40), represent a significant enhancement over unfilled materials. Future investigations could explore hybrid composites incorporating both reinforcing and lubricating fillers to optimize both wear resistance and friction characteristics.\u003c/p\u003e \u003cp\u003eOverall, the materials developed in this study, specifically PC(30) and PC(40), offer valuable combinations of moderate friction (\u0026micro;\u0026thinsp;\u0026asymp;\u0026thinsp;0.18\u0026thinsp;\u0026minus;\u0026thinsp;0.175) and excellent wear resistance. These characteristics make them suitable candidates for numerous tribological applications where extended component lifetime under moderate load conditions is prioritized over extremely low friction.\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e"},{"header":"4. Conclusions","content":"\u003cp\u003eIn this study, PEEK/CoCr composites were successfully fabricated using centrifugal powder compaction and vacuum sintering, and their microstructure, thermal properties, microhardness, and tribological behavior were systematically investigated. The key conclusions are as follows:\u003c/p\u003e \u003cp\u003e \u003col\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003eThe centrifugal powder compaction technique effectively dispersed CoCr particles within the PEEK matrix, enabling uniform distribution and intimate matrix-particle contact without significant agglomeration or void formation.\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003eThe addition of CoCr filler up to 40 wt% increased the microhardness of PEEK by approximately 40%, with PC(40) reaching 37 HV compared to 26.5 HV for neat PEEK. This enhancement is primarily attributed to the intrinsic hardness of CoCr and improved particle\u0026ndash;matrix interlocking.\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003eThermal analysis confirmed that the inclusion of CoCr particles did not significantly alter the melting temperature of PEEK (343\u0026deg;C) but resulted in a slight reduction in crystallinity, indicating minimal interference with the polymer\u0026rsquo;s thermal stability.\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003eTribological testing under dry sliding conditions demonstrated a substantial reduction in wear rate for PEEK/CoCr composites, with up to 84% wear volume loss reduction in PC(40) compared to neat PEEK. The coefficient of friction remained stable across all compositions, showing no lubricating effect from the metallic filler.\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003eSEM analysis of worn surfaces revealed a transition from adhesive and abrasive wear mechanisms in neat PEEK to predominantly abrasive wear with reduced plastic deformation in CoCr-filled composites, confirming the reinforcing effect of CoCr particles in wear resistance.\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003c/ol\u003e \u003c/p\u003e \u003cp\u003eFuture studies could investigate the fatigue behavior and long-term stability of these composites and explore hybrid filler systems to simultaneously optimize wear resistance and friction behavior. Additionally, validation in application-relevant environments would further support the use of PEEK/CoCr composites in high-performance engineering and biomedical applications.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgement\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis research work was funded by the Committee of Science of the Ministry of Education and Science of the Republic of Kazakhstan via grant №AP23488642 \u0026ldquo;Compact Compliant Resilient Wind turbine architecture (COREWind)\u0026rdquo; project and grant №AP22686136.\u003c/p\u003e\n\u003cp\u003eAuthor contributions\u003cstrong\u003e\u003cbr /\u003e\u003c/strong\u003eConceptualization, BS; methodology, BS; synthetic materials and performance characterization, BS, AM, AA and AK; investigation, BS and AA; resources, AM and AA; data curation, BS and AA; writing of the original draft preparation, BS, AK and AM; writing of review and editing, BS BG and HJR; visualization, BS, HJR; supervision, BG and CS; project administration, BG and CS; funding acquisition, AA and CS; All authors have read and agreed to the published version of the manuscript.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData availability\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAll data generated or analyzed during this study are included in this published article.\u003c/p\u003e\n\u003cp\u003eCompeting interests\u003cstrong\u003e\u003cbr /\u003e\u003c/strong\u003eThe authors declare no competing interests.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAdditional information\u003cbr /\u003e Correspondence \u003c/strong\u003eand requests for materials should be addressed to BS and CS.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eKurtz, S. 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Tribological behavior of PEEK components with compositionally graded PEEK / PTFE surfaces. \u003cb\u003e262\u003c/b\u003e, 220\u0026ndash;224 (2007). \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.wear.2006.03.048\u003c/span\u003e\u003cspan address=\"10.1016/j.wear.2006.03.048\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"scientific-reports","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"scirep","sideBox":"Learn more about [Scientific Reports](http://www.nature.com/srep/)","snPcode":"","submissionUrl":"","title":"Scientific Reports","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Scientific Reports","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"PEEK, Cobalt–Chromium alloy, Powder metallurgy, Tribological performance, Microhardness, Thermal stability","lastPublishedDoi":"10.21203/rs.3.rs-6885923/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-6885923/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003ePoly(ether-ether-ketone) (PEEK) is a high-performance thermoplastic with excellent mechanical strength, thermal stability, and chemical resistance, making it attractive for applications like biomedical implants and prostheses. However, neat PEEK suffers from a high friction coefficient and pronounced wear in sliding contacts. In this work, composites of PEEK with Cobalt\u0026ndash;Chromium (CoCr) alloy powder were fabricated by centrifugal powder compaction and vacuum sintering. Four composite compositions, with weight percentages of 10%, 20%, 30%, and 40% of CoCr, were produced. Comprehensive characterization was conducted, including particle size distribution analysis, scanning electron microscopy with energy dispersive X-ray spectroscopy, differential scanning calorimetry, thermogravimetric analysis, microhardness testing, and ball-on-disk tribological testing against a steel counterface. The PEEK/CoCr composites showed uniform dispersion of CoCr particles in the PEEK matrix. Thermal analysis indicated that the addition of CoCr did not significantly alter PEEK\u0026rsquo;s melting temperature or thermal stability with residual weights corresponding closely to the filler fractions. The microhardness of the composites increased with CoCr content, with the 40% CoCr composite showing a 35% increase in hardness compared to the neat PEEK. In ball-on-disk tests, all PEEK/CoCr composites exhibited lower wear rates than neat PEEK, owing to the hard CoCr particles reinforcing the polymer. The coefficient of friction was in the typical range for PEEK sliding on steel for all composites, as the metallic filler does not provide lubrication; however, the improved hardness and load-bearing capacity of the composites led to reduced wear depth and volume loss. Overall, the PEEK/CoCr composites demonstrate enhanced hardness and wear resistance while retaining PEEK\u0026rsquo;s favorable thermal properties, suggesting their potential for applications requiring better tribological performance than unfilled PEEK.\u003c/p\u003e","manuscriptTitle":"Thermal, Hardness, and Tribological Assessment of PEEK/CoCr Composites","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-06-20 17:08:12","doi":"10.21203/rs.3.rs-6885923/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2025-07-15T09:45:00+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-07-14T11:30:48+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-06-28T04:35:38+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"201187589083779761248482539757964663969","date":"2025-06-23T05:39:24+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"270751689681154835998465783020381508799","date":"2025-06-18T07:41:34+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-06-18T03:15:51+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-06-18T03:14:21+00:00","index":"","fulltext":""},{"type":"editorInvited","content":"","date":"2025-06-18T03:11:05+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-06-16T10:57:42+00:00","index":"","fulltext":""},{"type":"submitted","content":"Scientific Reports","date":"2025-06-13T07:40:21+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"scientific-reports","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"scirep","sideBox":"Learn more about [Scientific Reports](http://www.nature.com/srep/)","snPcode":"","submissionUrl":"","title":"Scientific Reports","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Scientific Reports","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"e02ad92d-c6ec-4c3a-95d0-42e7a9f40da5","owner":[],"postedDate":"June 20th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[{"id":50224054,"name":"Physical sciences/Engineering/Biomedical engineering"},{"id":50224055,"name":"Physical sciences/Engineering/Mechanical engineering"}],"tags":[],"updatedAt":"2025-08-11T16:03:56+00:00","versionOfRecord":{"articleIdentity":"rs-6885923","link":"https://doi.org/10.1038/s41598-025-14776-5","journal":{"identity":"scientific-reports","isVorOnly":false,"title":"Scientific Reports"},"publishedOn":"2025-08-06 15:58:03","publishedOnDateReadable":"August 6th, 2025"},"versionCreatedAt":"2025-06-20 17:08:12","video":"","vorDoi":"10.1038/s41598-025-14776-5","vorDoiUrl":"https://doi.org/10.1038/s41598-025-14776-5","workflowStages":[]},"version":"v1","identity":"rs-6885923","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-6885923","identity":"rs-6885923","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}