Research Progress on Polyether Ether Ketone (PEEK) Composite Coatings: A Review

Abstract

As modern industrialization accelerates, traditional metallic materials face challenges in meeting critical surface protection requirements. Constrained by their physicochemical properties, these materials exhibit significant performance degradation. This leads to frequent peeling of surface coatings on critical components. Polyetheretherketone (PEEK) is a high-performance semi-crystalline thermoplastic used in advanced engineering applications. Its composite coating systems have emerged as a promising alternative to metallic coatings. This paper systematically reviews the recent advances in coating preparation techniques for PEEK composites. The current status of the use of mainstream preparation methods such as thermal spray technology, rapid prototyping and electrophoretic deposition is highlighted. The strengths and weaknesses of each method are also compared. Critical parameters including substrate roughness, temperature, and substrate elasticity are systematically examined. The effects of these variables are evaluated with respect to critical performance indicators, including porosity levels and interfacial bonding strength of PEEK composite coatings. A comparative investigation was carried out on different reinforcement materials. Their interfacial interactions with the matrix are examined in detail at the microscopic level. The impact of these modification strategies on coating performance was comprehensively evaluated.

1. Introduction

Polyetheretherketone (PEEK) is a high-performance, semi-crystalline engineering plastic within the polyaryletherketone family. PEEK exhibits thermal transitions with a glass transition temperature (Tg) of approximately 143 °C and a melting temperature (Tm) near 334 °C. The material exhibits excellent thermal stability and outstanding wear resistance. PEEK is inherently self-lubricating and can be used in air at temperatures up to 250 °C. Figure 1 shows the chemical structure of PEEK. It has a long service life and significantly outperforms conventional thermoplastics. PEEK exhibits good impact strength, with a strain at break of 15% under tensile loading. Due to its semi-crystalline structure, it maintains relatively high mechanical strength at elevated temperatures, making PEEK a suitable reinforcement for coatings [1,2]. Mechanical interlocking and chemical interactions occur between PEEK and the matrix. These mechanisms, such as hydrogen bonding and possible covalent bonding, significantly enhance interfacial adhesion and wear resistance. PEEK is commonly used as a structural material for monolithic components and bearing housings due to its superior mechanical properties and low friction characteristics. The elastic modulus of pure PEEK coatings can reach up to 4 GPa [3]. The base hardness of the PEEK coating is approximately 24 HV [4]. Furthermore, PEEK-based composite coatings exhibit outstanding tribological performance, with reduced friction coefficients and enhanced interfacial adhesion at contact interfaces [5]. Such properties have led to a wide range of applications of polyether ether ketone as special functional materials in medical, chemical and mechanical fields [6,7,8,9].

Pure PEEK materials face several challenges in practical engineering applications. For example, wear is likely to occur during prolonged operation due to its high intrinsic friction coefficient, and the limited surface hardness reduces impact and scratch resistance. The interfacial bonding strength and fatigue resistance of PEEK require further enhancement. Notably, its stability is insufficient in coupled multi-physical field environments [10,11,12]. To address these limitations, researchers have proposed functional composite coatings on PEEK surfaces [13,14,15]. By incorporating reinforcing phases, the coating shows significant improvements in hardness, wear resistance, and interfacial adhesion, as well as enhanced corrosion resistance and environmental stability. Compared to pure PEEK coatings, composite coatings achieve a coordinated improvement in both mechanical and functional performance [16,17]. They offer the dual benefits of lightweight and multifunctionality, demonstrating significant practical value and potential for engineering applications [18,19,20,21]. PEEK-based coating systems are widely used for surface modification of mechanical components in industrial applications. These improvements have significantly increased both production efficiency and economic benefits [22,23,24,25]. These successful industrial cases also provide strong support and demonstration for the broader promotion and application of PEEK composite coatings [26,27,28,29].

This paper overviews recent studies on PEEK materials and summarizes recent advances in the preparation of PEEK composite coatings. This paper provides an overview of the theoretical development and practical applications of PEEK-based wear-resistant composite coatings. A wide range of innovative methods and techniques is discussed. In addition, the optimization strategies for coating performance under various processing conditions are analyzed. Furthermore, based on the current research landscape, this paper also provides an outlook on the future development of PEEK composite coating materials. The article suggests possible challenges and opportunities for future research. Figure 2 presents the overall structure of this review.

2. Preparation of PEEK Composite Coatings

2.1. Preparation of PEEK Coatings by Thermal Spray Technology

Thermal spraying technology is the primary process for preparing PEEK composite coatings, including methods such as flame spraying, high-velocity air fuel (HVAF) spraying, high-velocity oxy fuel (HVOF) spraying, and plasma spraying. These techniques are widely used to enhance the surface properties of engineering components. Flame spraying is considered the primary method for preparing PEEK coatings due to its relatively low processing temperature. Prior to spraying, the substrate was preheated to 400 °C. During the spraying process, the disk was fixed in front of the spray gun, which scanned at a linear speed of 200 mm/s, while the substrate temperature was maintained between 380 °C and 420 °C. Following spraying, the PEEK coating was rapidly quenched in cold water. However, studies have shown that flame spraying results in a powder buildup on the substrate. The high porosity of these coatings leads to inferior mechanical properties, Figure 3 illustrates the surface morphology of the as-sprayed PEEK coating [30]. This preparation method does not provide optimal material performance [31,32,33]. Figure 4 presents a schematic of the coating fabrication process by flame spraying. Temperature has the most significant impact on the degradation of PEEK material. Since the processing temperature of flame spraying is the lowest, the degradation of PEEK is minimized [9,16]. The process parameters of flame spraying are shown in

Table 1. Flame spraying parameters.

Flame Spraying Parameters
Acetylene flow rate (L/min)	6
O2 flow rate (L/min)	16
Flux rate of compressed air (L/min)	230
Flux rate of carrier gas (N2, L/min)	35
Spraying distance (mm)	180
Powder feed rate (g/min)	12

. HVAF and HVOF processes produce particle velocities up to 30 times higher than those in flame spraying, resulting in greater kinetic energy. As a consequence, an intensified viscoplastic response is generated at the interfacial region. This is accompanied by enhanced microscale interfacial engagement between the deposited layer and the substrate. These two factors are the primary mechanisms underlying the improved adhesion and cohesive strength of coatings produced by HVAF or HVOF processes. In contrast, coatings fabricated by conventional flame spraying generally exhibit weaker interfacial bonding.

In a comparative study of HVAF and HVOF technologies, Guo et al. demonstrated that the nature of the process gas plays a pivotal role in governing the coating’s densification and adhesion behavior. Unlike HVAF, HVOF employs oxygen as a combustion gas, resulting in more pronounced oxidation [34]. Krishal Patel’s experimental analysis revealed that shorter spraying distances during HVAF processing led to more severe degradation of the PEEK material. Nozzle size, however, has only a minor influence on PEEK degradation [9]. Plasma spraying, with flame temperatures up to 15,000 K, inevitably induces degradation of PEEK.

Enhancing coating properties requires optimizing the post-treatment of PEEK composite coatings formed by thermal spraying. Post-treatment parameters significantly affect crystallization behavior as well as the structural integrity and surface functionality of the coatings. Zhang et al. conducted a comprehensive analysis of the effects of different annealing treatments on the crystallinity of flame-sprayed coatings. The formation of a thick crystalline layer during isothermal treatment is the key factor in enhancing coating hardness. The crystal stacking structure can be effectively regulated by optimizing annealing parameters, such as elevated temperature and prolonged holding duration. Such control helps minimize surface degradation and sliding-induced energy dissipation in the coating [35]. In laser remelting, Soveja A et al. developed a synergistic process combining diode and CO₂ laser. Experiments shown that this combined process significantly reduces the coating porosity compared to a single laser treatment [36,37,38,39]. Zhang et al. investigated the microscopic mechanisms of various laser remelting processes from the perspective of phase transition kinetics. They found that both Nd:YAG and CO₂ lasers enable rapid cooling during remelting, refining grain size and produces a densified surface structure [40,41,42]. Zhang et al. conducted XRD tests on the crystallinity of coatings treated by different post-treatment methods, as shown in Figure 5. The as-sprayed coating and CO₂ laser remelted coating exhibit a semi-crystalline structure, whereas the Nd:YAG laser remelted coating displays an amorphous structure due to the high cooling rate. This is primarily because CO₂ laser treatment has a lower scanning speed, allowing more laser energy to be applied to the substrate surface, which helps reduce the cooling rate, thus maintaining a semi-crystalline structure. The higher cooling rate, as in the case of Nd:YAG laser treatment, leads to the amorphization of the coating [30]. In the process of preparing PEEK coatings through plasma spraying, Dupuis A et al. enhanced the bond strength of these coatings considerably by extending their exposure time. The enhancement is ascribed to the introduction of chemically active polar moieties on the surface of the coating. This resulted in improved interfacial wettability and mechanical interlocking [43].

Current research on PEEK coatings is mainly focused on process-performance, with limited understanding of the coupled mechanisms between process, material degradation, and performance evolution. Long-term service performance validation under comparative multi-process conditions is lacking. For example, the extremely high flame temperature in plasma spraying is considered to increase the degradation risk of PEEK [44,45,46,47]. Traditional perspectives have focused primarily on the link between ‘temperature and degradation’, overlooking the potential for degradation suppression through process parameter optimization. This has resulted in somewhat one-sided conclusions. The phase transition, molecular chain scission, and interfacial bonding mechanisms of PEEK during thermal spraying have not been systematically elucidated. Post-treatment research on annealing has primarily focused on its effect on crystallinity, while the roles of porosity evolution and interfacial characteristics have been less studied. Performance evaluation has mainly concentrated on short-term performance, with limited long-term stability validation under complex service conditions such as friction, corrosion, and thermal cycling. These limitations have hindered the translation of laboratory results into engineering applications.

2.2. Additive Manufacturing Technology for PEEK Coating Preparation

Recent advances in additive manufacturing have sparked significant interest in the study of PEEK composite coatings prepared using this method. Among various additive manufacturing techniques, fused deposition molding (FDM) has emerged as the leading technology for preparing PEEK composite coatings, due to its simplicity and cost-effective equipment requirements. Figure 6 illustrates a schematic diagram of the coating prepared by additive manufacturing. The arrows in Figure 6 indicate the lateral movement of the nozzle and the vertical movement of the build platform during the FDM process. FDM process parameters are key determinants of the coating’s final performance, making their optimization a central focus of current research. Temperature critically governs the crystallization behavior of PEEK. Increasing the nozzle temperature to 400 °C significantly improves its tensile strength (He Y et al.). This observation confirms a positive correlation between processing temperature and mechanical performance. However, beyond this critical temperature, thermal degradation occurs with a sudden decrease in crystallinity. This leads to a significant deterioration of the mechanical performance of the PEEK coating [48]. Zhao et al. experimentally demonstrated that the combined effect of grid angle and filler spacing plays a crucial role in regulating extension force distribution in the molten state. When the angle between grid alignment and extrusion direction reaches certain values, the coating’s interfacial bonding strength is significantly reduced. Conversely, an optimized spacing of the filler can result in a notable enhancement in mechanical properties [49,50,51,52,53]. Figure 7 shows the friction coefficient and wear rate of PEEK before and after annealing, as well as their relationship with the relative crystallinity of PEEK, particularly in the case of different filament orientations relative to sliding. It can be seen that the relationship between the crystallinity of PEEK and the friction coefficient and wear rate is not a simple linear one. Specifically, the increase in crystallinity may lead to different friction and wear behaviors depending on the filament orientation relative to the sliding direction. The annealing process not only affects the crystallinity of PEEK but also influences its friction and wear performance by altering its microstructure, especially for different filament orientations. Huang et al. introduced laser-assisted preheating technology innovatively. Reducing the temperature gradient at the interlayer interface effectively suppresses transverse cracks caused by residual stresses. This substantially reduces interlayer porosity and improves interlayer bonding quality [54]. As shown in Figure 8, it can be observed that heat treatment and laser-assisted preheating have a significant impact on the molecular structure and interfacial crystallinity of PEEK materials. This process not only facilitates the disentanglement and re-entanglement of molecules but also enhances the crystallinity of the interfacial layer, thereby improving the interfacial bonding strength and the overall mechanical performance of the material. Annealing post-treatment has been shown to effectively optimize the metastable crystalline structure formed during rapid cooling in the FDM process. During annealing, the reorganization of the crystalline structure is incomplete. Interdiffusion of molecular chains promotes the formation of a homogeneous crystalline phase. This improves the interfacial adhesion strength [55,56,57,58,59,60,61].

Compared to conventional manufacturing methods, the FDM-based additive manufacturing process can create high-performance PEEK coatings on complex curved substrates through precise layer-by-layer stacking. In addition, the process is solvent-free and has a low heat impact. These features ensure its biosafety. As it does not introduce cytotoxic substances, the FDM process represents a significant advancement in the development of biocompatible PEEK-based coatings for biomedical applications. In precision coating technology, Liu et al. developed a directional deposition device that enables uniform preparation of ultra-thin PEEK coatings through precise temperature control and an array of micro-nozzles. This technology significantly enhances processing efficiency [62]. Oladapo’s innovative used FDM molding to co-extrude hydroxyapatite with PEEK, thereby enhancing the bone bonding strength of the composite coating through in situ crystallization [63]. The microtextured surface produced by FDM molding significantly promotes adult cell growth [64,65,66,67,68]. The optimized parameters for preparing PEEK coatings using FDM technology are: nozzle temperature 440 ± 5 °C, molding platform temperature (110 °C at the center and 85 °C at the edges), printing speed 40 mm/s, and layer thickness 0.2 mm. By optimizing the combination of FDM parameters, customized PEEK structures with excellent mechanical properties can be prepared. This underscores the considerable potential of rapid prototyping for fabricating complex structures and biomedical applications.

Although significant progress has been made in preparing PEEK composite coatings using FDM technology, several limitations remain in current research. While temperature optimization in process parameters has been widely emphasized, the precise mechanisms by which high temperature affects the coating’s microstructure and molecular chain properties are not yet fully understood. The relationship between crystallinity and mechanical properties, as well as their balance, requires further exploration. Initial studies have examined the impact of grid angle and filler spacing on interfacial bonding strength. However, a systematic comparison of the performance differences between various geometric grid structures, such as hexagonal, triangular, and others, is still lacking. The relationship between grid complexity and coating performance remains unclear. There are conflicting conclusions regarding the effect of nozzle temperature on mechanical properties. While some studies suggest that increasing temperature improves performance, others indicate that excessively high temperatures may cause degradation and reduce performance. This suggests a nonlinear relationship between temperature and performance, necessitating further exploration of the underlying microstructural mechanisms. Research on the fabrication of PEEK composite coatings using FDM technology is rapidly advancing. However, in-depth investigation is still needed to understand the integrated effects of process parameter optimization, microstructural mechanisms, and surface properties.

2.3. Electrophoretic Deposition (EPD) Technology for PEEK Coating Preparation

Electrophoretic deposition (EPD), a simple and efficient coating technique, offers significant advantages in forming uniform and adherent PEEK composite layers on various substrates. Figure 9 presents a schematic illustration of the PEEK-based composite layer formed via the EPD process. Numerous studies have successfully fabricated PEEK coatings on a variety of metallic substrates. Ilaria and Ur Rehman successfully achieved uniform deposition of PEEK-based films onto stainless steel surfaces by optimizing key parameters in the EPD process. The study revealed that electric field characteristics can be enhanced by increasing field strength, using dilute suspensions, and employing cylindrical electrodes. These measures inhibit the settling of PEEK particles during deposition, significantly improving coating quality [7,69]. Various PEEK composite coatings have been successfully developed using the EPD process. Fiołek et al. incorporated graphite nanoparticles into PEEK, enabling uniform deposition of graphite-PEEK composite coatings on metallic substrates. An analysis of the adsorption behavior of graphite particles on PEEK particle interfaces revealed key interaction patterns. Furthermore, the observation of anodic deposition phenomena indicated that the co-deposition mechanism is predominantly driven by electrostatic forces. The study also identified 70 V and 40 S as the optimal process parameters [70,71]. As shown in Figure 10, the coatings were deposited using the EPD method, which involves transporting coating particles to the cathode surface and utilizing the electric field to deposit the particles onto the substrate. Seuss S et al. successfully fabricated PEEK-TiO₂ composite coatings on metallic substrates using the EPD process at 30 V for 1 min with an ethanol-based suspension system [72]. In the biomedical field, Bastan et al. developed a PEEK/hydroxyapatite (HA) system. A biologically active composite coating was created via electrophoresis by co-depositing positively charged HA microparticles and PEEK particles onto the cathode [73]. Rehman M A U et al. have prepared a PEEK-bioactive glass (BG) composite coating with excellent osseointegration properties by tuning the suspension system and using the EPD process. The composite coating provides an innovative solution for surface modification of orthopedic implant materials. This study expands new directions for the application of biomedical materials [74].

The mechanical hardness exhibited by the deposited layer is strongly influenced by the crystallinity of PEEK. This crystallinity can be modulated through adjustments in EPD parameters and post-deposition annealing and cooling protocols [1]. Kruk A et al. investigated the synergistic mechanism of EPD combined with heat treatment. After obtaining a uniform and dense pre-coating via deposition optimization, the material was subjected to heat treatment at 355 °C for 30 min. This post-heat treatment significantly enhances the interfacial adhesion strength between the deposited layer and the Ti-6Al-4V substrate. At the same time, it promotes the densification of the deposits. The researchers designed a slow cooling process to control the microstructure and successfully prepared a semi-crystalline coating. Experimental comparisons show that semi-crystalline coatings have significantly better scratch durability than conventional amorphous coatings. Additionally, the interface of the semi-crystalline coating exhibits no signs of peeling damage. This provides an important theoretical basis for the design and preparation of high-performance coatings on metal surfaces [70,75].

The researchers developed a coating system applicable to various metallic substrates, including biomedical metals such as austenitic steels and Ti-based alloys. This was achieved by fine-tuning several key process parameters. These parameters included electric field strength, suspension concentration, and electrode arrangement. The resulting system exhibited a uniform and dense PEEK composite coating.

Table 2. Comparison of different PEEK coating fabrication techniques.

Preparation Method	Principle of the Process	Post-Processing Requirements	Quality of the Coating	Equipment and Cost	References
Flame spraying technology	Gas flame heating of powder, low-speed particle deposition	Annealing/laser remelting required to improve structure	High porosity, poor density, Weak interfacial bonding	Simple equipment, low cost	[32,43]
HVAF Thermal Spray Technology	High-velocity air-fuel combustion, high particle kinetic energy	Optional annealing to further improve density	Significant densification, low porosity, Strong bonding, good toughness	Complex equipment, moderate cost	[9,34]
HVOF Thermal Spray Technology	High-velocity oxy-fuel combustion, high particle kinetic energy	Optional annealing to reduce oxidation effects	Significant densification, but prone to oxidation, Strong bonding, but may be affected by oxidation	Complex equipment, moderate cost
Plasma spraying technology	Plasma arc heating of powder, high-temperature deposition	Required heat treatment to reduce porosity	High density, Improved bonding, but with a risk of thermal cracking	Expensive equipment, requires plasma source	[32,35]
Additive Manufacturing Processes	Heating and melting PEEK filament, layer-by-layer stacking	Annealing/laser-assisted optimization of crystallinity and interlayer bonding	Layered structure, significant porosity and interlayer cracking, Interlayer bonding strength depends on temperature and cooling rate	Simple equipment, low cost	[48,63]
Electrophoretic deposition process	Electric field-driven deposition of charged particles on the electrode	Heat treatment can improve crystallinity and hardness	High coating uniformity, good particle dispersion, Excellent interfacial bonding, further enhanced after annealing	Simple equipment, higher cost	[7,49,69,73,74]

provides a summary of the strengths and limitations of the commonly used PEEK coating preparation methods. During the development of functionalized coatings, uniform nanoparticle dispersion was achieved through the electrostatic co-deposition mechanism. The research further elucidates the intrinsic relationship between the mechanical performance of the coatings and the degree of structural ordering within the polymer matrix. The semi-crystalline coating on Ti-6Al-4V significantly improved Vickers hardness and scratch resistance by precisely adjusting the EPD process parameters. Meanwhile, the coating achieved simultaneous optimization of interfacial bond strength and structural densification. The PEEK composite coatings prepared using the EPD process exhibited excellent bioactivity, providing a new way for the surface modification of biomedical implants.

While electrophoretic deposition (EPD) technology shows great potential for preparing PEEK composite coatings, several discrepancies and gaps remain in current research. A consensus on optimal deposition parameters has yet to be reached. Experiments show that uniform coatings can be obtained under both 70 V/40 s and 30 V/60 s conditions, indicating a significant variation in the optimal voltage and time parameters. This difference may be attributed to the experimental setup and material characteristics. Regarding post-treatment, annealing has been widely shown to enhance hardness and wear resistance, but its underlying mechanisms remain debated. Some studies attribute performance improvement mainly to increased crystallinity, while others highlight the role of enhanced interfacial bonding strength. Most research on EPD-based PEEK composite coatings lacks systematic modeling and cross-scale validation of the electric field distribution and deposition layer microstructure formation, limiting the repeatability and applicability of findings under different experimental conditions.

3. Factors Affecting the Performance of PEEK Composite Coatings

3.1. Influence of Substrate Roughness and Temperature on Coating Properties

By studying the interfacial behavior of PEEK coatings from multiple perspectives, scholars have revealed the key influences of substrate surface properties and temperature effects on coating performance. In terms of the interfacial bonding mechanism, the team led by Otto M et al. made an important observation. They found that substrate roughness has no significant effect on the coating’s hardness and elastic modulus. This conclusion remains valid provided that the indentation depth does not exceed 10% of the total coating thickness. The results were determined through a series of indentation and scratch tests [5]. The micro-rough structure of the substrate surface significantly enhances the mechanical interlocking effect by increasing the actual contact area of the interface and forming effective physical anchoring points, thus directly determining the scratch hardness performance of the coating. This finding corroborates the study of Wu J et al., who used a low-power plasma spraying technique to confirm that etched and heat-treated aluminum substrates significantly enhanced the contact density of PEEK particles with the substrate [76]. Henriques B’s team further validated the science of micromechanical interlocking as the dominant mechanism of interfacial adhesion through laser structuring [75]. A comprehensive investigation conducted by Sändker H et al. demonstrated that the Sa value of the substrate’s surface roughness could be accurately regulated within the range of 0.60 ± 0.10 µm to 3.80 ± 0.30 µm through adjustments to the parameters of the laser pulses. The experimental data showed that the PEEK coatings presented the best adhesion properties when the Sa value reached 2.3 ± 0.5 µm [77,78]. G. Zhang et al. demonstrated that the frictional thermal effects responsible for the stick-slip behavior markedly intensify interfacial shear stress and accelerate surface damage [79,80,81]. Figure 11 Schematic representation of the transfer film formation mechanism at the polymer-metal contact interface during sliding illustrates the mechanistic schematic of transfer film formation at the polymer-metal sliding interface. This discovery provides meaningful support for the findings of Nunez. The study revealed that once the interfacial temperature exceeds PEEK’s glass transition threshold, significant changes in its crystallization behavior are triggered. This transition leads to a rise in interfacial frictional resistance, while simultaneously reducing the material loss per unit sliding distance. This anomaly stems from the double effect of crystallization: the increase in material stiffness enhances wear resistance but simultaneously exacerbates the accumulation of fatigue damage, with typical brittle spalling features observed at the wear track [6]. These findings provide an important theoretical basis for the adaptive design of PEEK coatings for working conditions.

The interfacial properties and service behavior of PEEK coatings are synergistically regulated by substrate surface roughness and temperature effects. The substrate’s micro-roughness structure enhances the bonding strength of the coating through a mechanical interlocking effect. The best adhesion performance is achieved when the surface roughness Sa value is optimized to 2.3 ± 0.5 µm. Fluctuations in interfacial temperature trigger phase transformation processes, such as material crystallization. These transformations result in a nonlinear evolution of frictional behavior and wear rate over time.

3.2. Effect of Coating Thickness and Lubrication on Coating Performance

Coating technology serves as a crucial method for improving the surface characteristics of materials; however, its true effectiveness is frequently limited by the interplay between coating thickness and lubrication conditions. It has been shown that differences in coating thickness can change the stress distribution pattern. This may lead to a shift in the dominant failure mechanism, either at the interface or on the surface. In addition, variations in coating thickness have a pronounced influence on the tribological performance, impacting both frictional response and wear resistance. This phenomenon primarily arises from interactions between the coating and lubricant medium. Wang et al. conducted a finite element analysis that demonstrated the substantial influence of coating thickness on mechanical properties. Thinner PEEK coatings exhibited elevated stress concentrations within the substrate material and along the interfacial region. Conversely, for coatings of greater thickness, the highest stresses were directed toward the upper surface of the coating. Differences in the elastic modulus of substrate materials can alter the contact state. A highly elastic substrate tends to undergo greater deformation under load. This deformation increases the actual contact area. As a result, higher friction, more pronounced plastic deformation, and increased wear are induced [82,83]. Zhang et al. reported that under boundary and mixed lubrication regimes, trace amounts of diesel fuel significantly increase both the coefficient of friction (COF) and specific wear rate of unfilled PEEK. In contrast, when lubricated with engine oil, PEEK-based composites exhibit enhanced wear performance, surpassing that of St50-2 steel. This improved performance is mainly attributed to the formation of an effective friction film, which plays a critical role in lubrication [84].

Coating thickness and lubrication conditions, as the core parameters of coating technology, jointly determine the interfacial mechanical response and tribological properties of materials. Thin coatings tend to induce stress concentration at the substrate-coating interface. In contrast, thick coatings shift the stress distribution toward the surface of the coating. Furthermore, the lubrication medium critically influences the material’s wear characteristics. This modulation occurs through the formation of a friction film. These findings reveal a dynamic correlation among matrix elasticity, contact stress distribution, and tribological properties. This correlation is observed under the synergistic influence of the coating thickness gradient and the lubrication state. The results provide essential theoretical support for the design and optimization of coating systems in engineering applications.

4. Current Status of Research on Friction-Modulated PEEK Composite Coatings

4.1. Frictional Wear Mechanism of PEEK-PTFE Composite Coatings

Polytetrafluoroethylene (PTFE) is a polymer synthesized from tetrafluoroethylene monomers. Due to its unique chemical structure, PTFE exhibits excellent chemical inertness, a low friction coefficient, and high-temperature resistance. Its molecular chains, composed of highly polar C-F bonds, form a dense fluorocarbon shell. PTFE’s flexible molecular chains and low surface energy give it outstanding self-lubricating and non-stick properties. These attributes make PTFE widely used in engineering applications such as corrosion-resistant linings, seals, bearings, self-lubricating coatings, and high-frequency electrical insulating materials. However, its relatively low mechanical strength and wear resistance limit its performance in high-load and durability applications. This has led researchers to explore combining PTFE with high-performance polymers like PEEK to optimize tribological properties and structural stability.

PEEK-PTFE composite coatings combine the high-temperature stability of PEEK with the self-lubricating properties of PTFE. This combination forms a high-performance material system. The system combines excellent chemical resistance with a low coefficient of friction. In this system, PTFE is enriched on the surface of the composite coating to form a lubricating layer. PEEK is mainly distributed within the coating to maintain mechanical stability. This spatial distribution inhibits transfer film detachment and reduces the friction coefficient. Onodera T et al. further reveals the phase distribution law of the coating based on the microstructural characterization of PEEK-PTFE composites. The low friction coefficient results from the formation of a solid PTFE transfer film. Surface-enriched PTFE effectively reduces friction through interfacial slip. On the other hand, PEEK distributed in the interior maintains the structural stability of the transfer film through a mechanical anchoring effect [85]. Figure 12 presents a schematic representation illustrating the formation mechanism of tribological films derived from PTFE/PEEK polymer composite systems. Vail J.R. et al. demonstrated through tribological testing that the incorporation of PTFE significantly reduces composite material degradation and sliding resistance [86,87]. Sun W et al. found that PEEK-PTFE heterostructured surfaces have excellent friction properties. This property is mainly achieved by the interpenetration of the two phases and the degradation polarization process of PTFE [88,89]. Li S et al. investigated the molecular-level C-C bond cleavage in PTFE under friction conditions. They found that the fluorinated radicals generated upon fracture can react with environmental molecules to form high-performance friction films. This synergistic effect between the friction partners is a key mechanism for achieving superlubrication [90,91,92,93,94].

The aforementioned research presents a comprehensive examination of the structural features of PEEK-PTFE composite coatings using multidimensional characterization techniques. The results reveal a complex correlation mechanism between the group distribution ratio, interfacial properties, phase distribution pattern and friction reduction behavior.

The temperature factor has a significant effect on the friction and wear behavior of PEEK-PTFE composites. The tensile strength and hardness of the composite material gradually decrease with increasing temperature, particularly as the temperature approaches the coating’s glass transition temperature. Studies have also demonstrated that functionalizing PEEK-PTFE composite coatings by incorporating various additives can endow them with diverse functionalities. Luo, H et al. found that the introduction of micro and nano-sized SiO₂ particles into PEEK-PTFE composite coatings could confer superhydrophobicity to the coatings. PTFE reduce the surface energy of the coating, while SiO₂ particles further enhance its superhydrophobicity by constructing micro-and nano-multi-level rough structures [95]. As shown in Figure 13, the effect of component content on the friction coefficient and wear rate of the composite coatings was studied. The research by Tasuku Onodera investigated the influence of PEEK content variation on the friction coefficient and wear rate in PEEK-PTFE composite coatings. The study found that the friction coefficient ranged from 0.18 to 0.29, showing a trend of initially decreasing and then increasing. The wear rate exhibited a similar trend to that of the friction coefficient. However, the experimental data in the study were limited, and adding more intermediate concentration data within the range of 10% to 60% would help better reveal the behavior of the transition zone and precisely determine the optimal wear resistance and friction coefficient range [85]. This finding was also validated by Jayashree Bijwe’s research, which showed that the friction performance gradually improved as the PTFE content increased from 0 to 30% [11]. Shuren Qu’s study, however, indicated that as the PTFE content increased from 0 to 20%, the friction coefficient decreased [96]. This differs from previous findings, possibly due to the different loads applied under varying experimental conditions.

The PEEK-PTFE system combines the excellent properties of both materials, with PEEK providing structural stability at high temperatures and PTFE offering good self-lubricating properties. In this composite coating, surface-enriched PTFE forms the lubricating layer, while the internal PEEK acts as a mechanical support. This synergistic structure significantly improves the stability and overall tribological properties of the transfer film. Optimizing the wear balance of the coating for various operating conditions can be achieved by modifying the PTFE concentration. The friction-reducing mechanism of the coating primarily originates from the development of a lubricating layer composed of PTFE residues at the contact interface. In addition, the reconstruction of the frictional film, induced by the scission of carbon-carbon bonds within the polymer matrix, contributes to the dynamic evolution of the tribological interface. The synergistic interactions between the two phases at the interface further enhance the overall friction-reducing capability. The problems of temperature sensitivity and mechanical property decay are particularly significant near the glass transition temperature. Further functionalization and modification studies showed that the introduction of micro-and nanoscale SiO₂ particles can build multistage rough structures. This structure gives the coating excellent superhydrophobic properties. Furthermore, the incorporation of the two-dimensional material Ti₃C₂T_X reinforces interfacial adhesion within the composite system. It also acts as a facilitator for tribolayer formation, resulting in a significant improvement in both hardness and wear resistance of the composites. The coating significantly extends the service life of components by suppressing adhesive wear and reducing interface temperature rise. It can perform anti-friction and sealing functions in demanding operating conditions, such as in automotive air conditioning compressor piston rings and dry bearings.

Despite significant progress in PEEK-PTFE composite coatings research, discrepancies remain across different studies. Regarding PTFE content optimization, Bijwe J et al. suggest that 7.5% PTFE content provides the best wear resistance, while other studies report excellent performance in low-amplitude vibration wear with 30% PTFE. These discrepancies are mainly due to inconsistencies in tribological testing conditions, highlighting the lack of standardization in experimental protocols. Some studies show that increasing temperature reduces coating strength and hardness, thereby weakening wear resistance. However, adding reinforcement phases like SiO₂ and Ti₃C₂T_X improves tribological properties as temperature rises. This suggests that temperature effects and reinforcement phase introduction are often coupled, making it difficult to isolate their individual contributions. Additionally, the lack of comprehensive multi-scale validation for interfacial mechanisms results in unclear hierarchy and consistency among proposed mechanisms.

4.2. Frictional Wear Mechanism of PEEK-SiC Composite Coatings

Silicon carbide (SiC) is a covalent-bonded ceramic known for its exceptional hardness, wear resistance, and thermal stability. Its crystal structure consists mainly of strong Si-C covalent bonds, providing excellent mechanical strength and chemical inertness, and enabling stable performance in high-temperature, corrosive, and high-load environments. SiC combines low density with high elastic modulus, offering excellent specific strength and stiffness, along with high thermal conductivity and thermal shock resistance. These properties make it an ideal material for friction pair applications.

In recent years, researchers have begun introducing SiC nanoparticles into PEEK-based composite coatings. This study aims to address the inherent deficiencies of pure PEEK coatings, particularly their suboptimal structural robustness and surface durability under frictional conditions. This strategy has become an important technical approach to breaking through the material performance bottleneck. Zhang G et al. discovered that the presence of SiC particles facilitated the densification process of coatings while preparing PEEK coatings through microwave sintering. Simultaneously, the incorporation of SiC particles can enhance the tribological characteristics of the coating by improving heat transfer efficiency. However, the uneven distribution of SiC particles can lead to localized overheating, with triggers PEEK degradation [97]. A systematic study by Kadiyala et al. showed that the energy storage modulus and glass transition temperature of composite coatings increased significantly with increasing SiC content. The coating exhibited notable enhancements in scratch resistance and interfacial adhesion. The stiffness and thermal stability of the coating show an increasing trend. The improved performance of nanoparticles is attributed to their larger specific surface area and smaller particle spacing. The enhanced stiffness and thermal stability are primarily attributed to the higher filler content [98]. The frictional characteristics of PEEK-SiC composite coatings are altered when the dominant mechanism shifts to interfacial molecular bond shearing. In the process, the coating gradually develops a dynamically balanced surface contact area. The change in the friction mechanism is primarily due to the combined impact of the SiC particles and the surrounding matrix. On the one hand, SiC particles effectively inhibit the slippage and reorientation of polymer chains by reducing the true contact area and increasing matrix hardness. On the other hand, they also mitigate the plowing effect under high load conditions. The introduction of SiC particles results in a minor increase in the coefficient of friction, which can be attributed to enhanced shear resistance. Figure 14 presents a schematic representation of the interfacial reinforcement mechanism induced by SiC particles in PEEK coatings. The wear surface primarily exhibits characteristics of mild abrasive wear due to predominant abrasive action [32,99,100]. Figure 15 shows the variations in friction coefficient and wear rate of PEEK–SiC composite coatings under different loads and component contents. In the study of thermal conductivity, Liu S’s used the non-equilibrium molecular dynamics (NEMD) method for comparative analysis. Three composite systems, PEEK/SiO₂, PEEK/graphene and PEEK/SiC-were studied. The findings indicate that the heat transfer efficiency at the interface of PEEK/SiO₂ and PEEK/graphene composites is notably superior to that of the PEEK/SiC system. In terms of surface treatments, Caglar I’s have shown that silica coatings and sandblasting can significantly improve the shear bond strength of PEEK. This combination exhibits optimal bonding properties across a wide range of treatments. It was also found that the modulation of the surface morphology had a significantly higher effect on bonding performance than the conventional roughness parameter [101,102]. The PEEK-SiC composite coating has significant application potential in areas such as electronic packaging under extreme conditions and marine lubrication bearings. It enhances heat dissipation and packaging reliability in high-temperature, high-humidity, and corrosive environments. In marine bearings, its exceptional wear and corrosion resistance extends equipment service life in harsh marine conditions.

In the study by G. Zhang, the variation in the friction coefficient and wear rate of PEEK and PEEK-SiC composite coatings at different sliding speeds was presented. Under the conditions of a 1 N load and a sliding speed of 0.5 m/s, the friction coefficient of the PEEK coating exhibited significant fluctuations, indicating substantial frictional variations during the sliding process. In contrast, the PEEK-SiC coating displayed a more stable friction coefficient, suggesting that the addition of SiC improved the friction performance of the coating. The friction coefficient of the PEEK coating showed larger variations at higher loads, while the PEEK-SiC coating demonstrated better load-bearing capacity and friction stability, especially at high sliding speeds, where the friction coefficient change was smaller. This indicates that the PEEK-SiC coating performs better than the pure PEEK coating under high loads and high sliding speeds. The wear rate of the PEEK coating increased with the sliding speed, particularly at higher loads, where the wear rate significantly increased. In contrast, the wear rate of the PEEK-SiC coating showed little variation under different loads and maintained a relatively low wear rate at high sliding speeds, proving the wear resistance of the SiC-enhanced material. The study by Ajay Kumar Kadiyala focused more on the relationship between friction coefficient, load, and sliding distance, providing additional details on coating failure, such as the correlation between AE signals and coating failure. While both studies presented wear rate data under different loads, they did not provide results from long-term friction tests, which limits the conclusions to short-term tests and lacks an evaluation of the coating’s long-term wear resistance.

The incorporation of SiC nanoparticles effectively addresses the limitations of pure PEEK coatings, including insufficient mechanical strength and poor wear resistance. At present, numerous studies have focused on optimizing preparation processes, regulating tribological properties, analyzing heat conduction mechanisms, and exploring applications. Microwave sintering technology leverages the heat conduction capability of SiC particles to significantly enhance coating densification. However, the uniform dispersion of particles remains a critical factor limiting the consistency and stability of coating performance. Although SiC particles enhanced the thermal conductivity of the PEEK matrix, their interfacial thermal resistance is still higher than that of SiO₂ and graphene reinforced systems. This finding provides a theoretical basis for developing multifaceted composite filler systems. Optimally dispersed SiC nanoparticles can significantly enhance the wear resistance and thermal stability of PEEK coatings. However, their tendency to agglomerate may cause localized stress concentration and heat accumulation, leading to matrix softening or interfacial failure, highlighting the importance of dispersion control for consistent performance. The reinforcement mechanisms are multifaceted: the physical mechanism involves mechanical interlocking and barrier effects, the thermal perspective focuses on heat conduction and noise reduction, and the chemical mechanism concerns interfacial bonding. In thermo-mechanical coupled environments, SiC-reinforced coatings exhibit significant performance discrepancies: some studies report good stability under high temperature and high load conditions, while others observe increased brittleness near the glass transition temperature, underscoring the need for standardized experimental parameters.

4.3. Tribological Mechanisms of Other PEEK Composite Coatings

The structural performance and wear resistance of PEEK composite coatings can be effectively tailored through the incorporation of various reinforcing agents. These include nanoparticles, two-dimensional materials, inorganic fillers, and functional additives, all of which collectively contribute to enhanced functionality and durability. Additionally, these materials and additives enhance the functional characteristics of the coating. The PEEK101 coating developed by Weisbach et al., which contains 10% perfluoroalkanes and 1% graphite, provides markedly higher yield strength relative to other coating types. The coating exhibits better shape stability and very low wear. These properties have led to successful applications in key components of hydrostatic lifting systems [103]. Tharajak J et al. fabricated composite coatings based on PEEK reinforced with hexagonal boron nitride (h-BN) using a flame spraying technique. The roughness of unmelted PEEK and the coatings was notably influenced by the h-BN content. The incorporation of h-BN particulates, regardless of their size or morphology, resulted in a noticeable enhancement in microhardness. Differences in the dimensional characteristics of h-BN particles were found to significantly influence the degree of crystallinity within the composite structure. Coarse particles in the micrometer range (0.5–1.5 μm) hinder crystallization by disrupting polymer chain alignment, while finer particles at the submicron scale (0.1 μm) promote nucleation within the PEEK composite matrix [104,105]. The findings indicate that the incorporation of TaN nanoparticles enable effective tailoring of the microstructure and functional performance of PEEK coatings. The properties of these hard particles enhance the substrate and elevate the coating’s hardness [106,107]. The results in Figure 16 confirm the positive effect of graphite fillers on the tribological performance of PEEK. In particular, under constant load, the graphite/PEEK coating exhibited a lower friction coefficient, and the friction tests did not induce significant coating deformation. This provides important evidence supporting the application of graphite-reinforced PEEK materials in friction-related field [70]. As shown in Figure 17, the PEEK/BG coating, owing to the reinforcing effect of BG, exhibits superior wear resistance and a lower friction coefficient compared with the pure PEEK coating, making it suitable for applications in medical devices that demand high wear resistance. The variation in friction coefficient further demonstrates the long-term stability of the PEEK/BG coating, indicating its suitability for use under sustained loading conditions [69].

The structural strength and frictional behavior of the coating can be significantly influenced by material modifications. Incorporating nanoparticles, two-dimensional materials, and inorganic fillers into the PEEK matrix allows for precise tuning of these performance attributes. Furthermore, these composites exhibit a strong capability to enhance the overall performance of the coating. Specifically, improvements are observed in multifunctional attributes such as thermal stability, chemical resistance, and surface durability. The combined action of perfluoroalkyl alkanes and graphite led to a marked enhancement in the yield strength and dimensional stability of the coatings. They achieve extremely low wear rates, a characteristic well-suited to precision mechanical components. The introduction of nanofillers can affect the microstructure of the material by reducing the spacing of the PEEK lattice planes through spatial confinement effects. In combination with heat treatment processes, the crystallinity of PEEK composite coatings can be further optimized. This optimization significantly enhances the functional integrity and performance reliability of the coating system. A suitable concentration of uniformly distributed nanoparticles can markedly improve the coating’s resistance to surface damage induced by mechanical abrasion.

In nanoparticle-reinforced systems, nanoparticles can fill defects, improve density, and enhance thermal conductivity. However, uneven distribution may cause stress concentration and high-temperature embrittlement. Additionally, the effects of particle size (nano vs. micron) on interfacial stability can be opposing, suggesting that a systematic control strategy for particle content, size, and dispersion is still lacking. Two-dimensional materials, such as graphene and its derivatives, can significantly reduce friction and improve wear resistance when uniformly dispersed. However, their optimal content and reinforcement effect are highly dependent on dispersion quality. High content or aggregation may degrade mechanical properties instead of enhancing them. Furthermore, variations in load, friction pair, and environmental conditions across studies exacerbate the inconsistency of results. Inorganic fillers, such as Si₃N₄, oxides, and mica, exhibit a dual nature: they improve flame retardancy and wear resistance but can reduce mechanical properties if added excessively or with poor interfacial bonding. This highlights the lack of theoretical guidance for synergistic optimization of these fillers’ multi-performance.

Table 3. Tribological properties of various PEEK composites.

Type of Composite	Optimal Component	Coating Function	Friction Mechanism	References
PEEK-PTFE	7.5% PTFE	Enhanced impact strength of coatings (abrasive wear)	formation of a low-friction transfer film	[11,96,108]
	20%–30% PTFE	Increased wear resistance (low amplitude oscillatory wear)
	Incorporation of 8% micronized SiO2 particles	Superhydrophobicity of the coating
	Addition of Ti3C2TX material	Improvement of coating hardness
PEEK-SiC	-	Enhances coating compactness and improves its hardness, and retards the rate of increase in the coefficient of friction. (Shearing of interfacial molecular bonds)	Enhancement of interfacial shear resistance, suppression of polymer chain mobility, and mitigation of the plowing effect	[32,97,99]
PEEK101	89% PEEK, 10% perfluoroalkanes, 1% graphite	Increased yield strength, better shape stability, very low wear rate	constructing a synergistic lubrication network	[103]
PEEK-TaN	-	Increases coating thickness, improves coating hardness and scratch resistance, and reduces shear and furrow forces.	Enhancement of interfacial bonding strength and suppression of crack propagation	[107]
PEEK-h-BN	Addition of hexagonal boron nitride	Increased coating hardness	Solid lubrication effect, hardness enhancement, and crystallinity variation induced by particle size regulation	[104,105]

and

Table 4. The performance and application scenarios of PEEK composite coatings.

Type of Composite	Friction Coefficient	Wear Rates (mm3/(N·M))	Typical Application Scenarios	Experimental Conditions	Years	References
Pure PEEK	0.29	1.6 × 10−5	-	-	-	-
PEEK-PTFE	0.18–0.29	5.4 × 10−6	Car air conditioning compressor piston ring, oil-free bearing sleeve	Pin-on-disk dry friction test	2017	[85]
	0.015–0.44	1 × 10−7		Low amplitude oscillating wear	2005	[11]
	0.373–0.22	-		Pin-on-disk dry friction test	2021	[96]
PEEK-SiC	0.28–0.3	1 × 10−5–2.5 × 10−5	Extreme environment electronic packaging, seawater lubricated bearings	Under a 1 N load	2006	[99]
		2.5 × 10−5–7.5 × 10−5		Under a 5 N load
		2.5 × 10−5–1.25 × 10−4		Under a 9 N load
PEEK-BG	0.21–0.36	-	Medical devices	Electrophoretic deposition fabrication	2020	[69]
PEEK-graphite	0.09–0.11	-	Medical devices	Electrophoretic deposition fabrication	2020	[70,109]

present the tribological properties of various PEEK composites, as well as the performance and application scenarios of PEEK composite coatings.

5. Conclusions and Future Research Directions

Performance requirements for materials in harsh industrial applications continue to escalate. This trend has driven growing research interest in PEEK composite coatings. These coatings exhibit remarkable mechanical strength, exceptional thermal stability, and high wear resistance. These superior properties establish them as a critical alternative to conventional coating systems. This paper reviews the significant advancements in the preparation techniques, performance enhancement, and mechanisms of friction and wear related to PEEK composite coatings, summarizing the findings as follows:

Flame spraying combined with laser remelting reduces coating porosity, while annealing optimizes the crystalline structure. HVAF technology significantly improves coating bond strength due to high particle velocities (30 times higher than flame spraying). However, controlled spray distances are required to minimize PEEK degradation. The optimal FDM parameters for fabricating PEEK coatings include a nozzle temperature of 440 °C and a deposition rate of 40 mm/s. In combination with post-processing (annealing at 300 °C for 4 h), high-density coatings for orthopedic implants were successfully prepared. The EPD technique enables uniform coating deposition on substrates with complex geometries. This is achieved through precise control of electric field intensity and colloidal suspension concentration.

In PEEK-PTFE composite coatings, PTFE significantly reduces the coefficient of friction by forming a low friction transfer film. However, excessive addition of PTFE can weaken the mechanical strength. During frictional processes, fluorine- and oxygen-containing radicals derived from PTFE molecular chain scission interact synergistically with tribologically induced species. This synergistic interaction promotes the development of high-performance tribofilms with enhanced protective characteristics. In PEEK-SiC composite coatings, the dispersed SiC particles restrict polymer chain mobility, thereby enhancing coating hardness through reinforcement effects. However, particle agglomeration may induce localized thermal degradation due to uneven heat distribution during frictional contact. Uneven distribution of SiC particles can easily lead to local overheating and degradation. Under frictional loading, the coating surface attains a dynamic equilibrium state characterized by competing wear and self-repair mechanisms. The dominant friction mechanism subsequently transitions to interfacial molecular bond shearing, which is governed by the relative strength of adhesive interactions. The load-bearing effect of the hard particles significantly reduces the plowing effect. Within PEEK-carbon fiber composite coatings, the incorporated fibers mitigate substrate shear deformation via efficient load distribution, functioning as mechanical reinforcement. The fiber alignment (parallel/perpendicular relative to sliding direction) dictates transfer film characteristics and abrasive wear resistance by inducing anisotropic stress distribution.

Substrate surface roughness significantly influences the adhesion performance of PEEK composite coatings. Optimal adhesion is achieved at a surface roughness of Sa = 2.3 ± 0.5 μm, where mechanical interlocking effects are maximized. Increasing temperature promotes PEEK crystallization, enhancing hardness and wear resistance. However, surpassing the glass transition temperature of 143 °C results in stick-slip behavior and a notable rise in the friction coefficient. Thin coatings exhibit concentrated stress at the coating-substrate interface, whereas thick coatings demonstrate more uniform stress distribution across the surface. The elastic matrix properties result in elevated wear rates owing to expanded real contact area during sliding.

PEEK composite coatings demonstrate substantial performance advantages in demanding applications. However, critical technological challenges must be resolved to fully exploit their potential. The following outlines the key future challenges and research directions:

The coupling mechanisms between process parameters, material degradation, and performance evolution are still not fully understood, especially in terms of long-term service performance validation under multi-process conditions. Furthermore, the mechanisms by which temperature affects the microstructure and molecular chain properties of the coatings remain unclear, and there are significant differences between studies in optimizing PTFE content, temperature effects, and filler dispersion, leading to a lack of consistency. Although some studies have proposed optimizing coating performance by introducing functional fillers and enhancing interfacial bonding strength, existing research still lacks in-depth exploration of multi-factor coupling effects. Therefore, future research should focus on the optimization of process parameters, control of filler dispersion, and systematic modeling of interfacial mechanisms to achieve long-term stability and high performance of PEEK coatings under extreme operating conditions.

Preparation process cost and efficiency balance. Thermal spraying processes, including HVAF and plasma spraying, involve substantial equipment and operational costs. Flame spraying necessitates additional laser remelting treatment, thereby elevating overall process complexity. Although additive manufacturing (FDM) can customize complex structures, the printing speed (40 mm/s) and post-processing time (4 h annealing) limit the production efficiency. To break through this bottleneck, future research focuses on developing multi-process synergistic techniques to reduce costs.

Long-term stability and reliability of coatings. The long-term performance stability of PEEK composite coatings is one of the core challenges for their application in extreme conditions. In high temperature or cyclic loading environments, changes in the crystallinity of PEEK may lead to degradation of coating hardness and attenuation of mechanical properties. In biomedical applications, the degradation behavior of PEEK coatings in body fluid environments and the long-term effects of osteointegration need further verification. At present, efforts to improve the environmental adaptability of coatings are centered on surface modification techniques, such as sulfonation and titanium deposition, to enhance resistance to environmental corrosion.

Optimization of interfacial bonding and reinforcing phase dispersion. The effectiveness of PEEK composite coatings is strongly influenced by the uniformity of reinforcement dispersion and the strength of interfacial bonding. However, existing technologies still face a key bottleneck: uneven dispersion of nanofillers. Nanoparticles such as SiC and graphene (GNP) are prone to agglomeration, resulting in local overheating, coating degradation, and limited interface heat transfer efficiency. The mechanism of fiber orientation influence is ambiguous, and CF/PEEK composites are prone to performance degradation due to fiber-matrix interface debonding under cyclic loading. Through the optimization of the dispersion process, the heat transfer mechanism of the filler-matrix interface is studied to guide ultrasonic dispersion or in situ synthesis process. Interface modifiers have been engineered to optimize interfacial adhesion between fibers and matrix. Chemical grafting technology is employed to functionalize carbon fiber surfaces with reactive moieties, thereby strengthening interfacial chemical interactions with the PEEK matrix.

With their customizable mechanical, tribological, and biological properties, PEEK composite coatings have shown irreplaceable advantages in the field of high-end manufacturing. In the future, it will be necessary to break through the existing performance bottleneck through multi-scale interface design, process innovation, and functional integration. These breakthroughs will promote the large-scale application of PEEK composite coatings in aerospace precision parts, smart medical implants, new energy equipment, and other fields.