Self-Lubricating Properties of Polyether-Ether-Ketone Composites Filled with CNTs@RC2540 Nano-Capsules

: Polyether-ether-ketone (PEEK) exhibits great potential in being a replacement for metal components across various applications relying on the mechanical and tribological properties. However, there is still much to be done to improve its properties. The main motivation of this paper is to improve the tribological and mechanical properties of PEEK simultaneously for more severe working environment. Therefore, dialkyl pentasulﬁde (RC2540) was proposed to ﬁll into the cavity of CNTs to prepare nano-capsules, which were then ﬁlled into PEEK to prepare PEEK/nano-capsules composites. The existence of nano-capsules in PEEK was analyzed, and the friction and wear properties exhibited by PEEK composites against GCr15 steel were examined using pin-disk friction pairs, and the self-lubricating mechanism of PEEK composites in friction was revealed. Findings of this study indicated that when the mass fraction of nano-capsules was less than 5%, the ﬁlling of nano-capsules could improve the tensile strength of PEEK and reduced the friction coefﬁcient and speciﬁc wear rate of PEEK by ﬁlling nano-capsules. During the friction process, RC2540 in the nano-capsules can be released as PEEK wears so that a self-lubricating layer can be formed for reducing PEEK composites’ friction and wear.


Introduction
Polyether-ether-ketone (PEEK) is a commonly used polymer material with a low coefficient of friction and good thermal stability.It is increasingly applied in aerospace, automobile manufacturing, electronic and electrical, and healthcare fields, finding itself in an irreplaceable role in many fields.When it comes to mechanical application, PEEK is often used to manufacture bearings, insulation, seals and other components [1][2][3].When PEEK is used as the material of pure water components such as pumps and valves, severe wear often occurs when it rubs against steel under water lubrication, especially at higher speeds and loads.This is due to the high coefficient of linear expansion and poor creep resistance of pure PEEK.Therefore, graphite, MoS 2 , carbon fiber and glass fiber are often applied to modify it to meet the requirements of use.Literature reviews [4][5][6][7].on the effect of SiO 2 , ZrO 2 , Si 3 N 4 , SiC and other nanoparticles on PEEK composites' friction and wear properties (tribological properties) have shown that the coefficient of friction (COF) of the material decreases with the increase in nanoparticles content.The composites showed the lowest COFs when the content of SiO 2 , ZrO 2 and Si 3 N 4 was about 15%.As the content of nanoparticles increased, the wear front decreased and then rose.The COF decreased with the increase in loadings, which indicated that the COF could be more obviously impacted by nanoparticles under higher loadings.M. Zalaznik and Moskalewicz [8,9] investigated the influence of filling content of MoS 2 on PEEK materials' mechanical and tribological properties.The results indicate that PEEK composites showed the optimal tribological properties when the MoS 2 content was 5%, with a 25% and 20% reduction in the COF and vacuum filtering, the obtained filter cake underwent drying treatment at 85 • C and 10 h of ball milling and acidified CNTs were obtained at last.
The liquid-phase wet chemical method was used to prepare nano-capsules [19,20].10 g of RC2540 was dissolved in 400 mL of acetone, added with 20 g of acidified CNTs.After the complete mixing, the mixture was packed into a spherical flask, followed by being vacuumized to −0.1 MPa pressure.Under this condition, the mixture was ultrasonic stirred for 2 h at 65 • C.Then, the mixture was filtered under vacuum condition and washed in acetone to remove the unfilled RC2540.Finally, the filter cake underwent 12 h of drying treatment in an oven at 75 • C, and ball milling was applied to obtain CNTs@RC2540 nano-capsules. Figure 1 displays the preparation process of CNTs@RC2540 nano-capsules.
Lubricants 2023, 11, x FOR PEER REVIEW 3 of 17 obtained filter cake underwent drying treatment at 85 °C and 10 h of ball milling and acidified CNTs were obtained at last.The liquid-phase wet chemical method was used to prepare nano-capsules [19,20].10 g of RC2540 was dissolved in 400 mL of acetone, added with 20 g of acidified CNTs.After the complete mixing, the mixture was packed into a spherical flask, followed by being vacuumized to −0.1 MPa pressure.Under this condition, the mixture was ultrasonic stirred for 2 h at 65 °C.Then, the mixture was filtered under vacuum condition and washed in acetone to remove the unfilled RC2540.Finally, the filter cake underwent 12 h of drying treatment in an oven at 75 °C, and ball milling was applied to obtain CNTs@RC2540 nanocapsules.Figure 1 displays the preparation process of CNTs@RC2540 nano-capsules.A Nicolet 6700 infrared spectrometer was adopted to detected the Fourier transform infrared (FTIR) spectrometry of RC2540, acidified CNTs and the nano-capsules.A FEI TECNAI G20 transmission electron microscope (TEM) detected specimen' microstructure at an acceleration voltage of 200 KV [21,22].Thermogravimetric analysis of CNTs and nano-capsules was conducted using an STA449 thermal analyzer with a temperature range of 20-800 °C and a rise rate of 20 °C/min), and the atmosphere was N2 with a flow rate of 40 mL/min [23].

Preparation of PEEK Composites
150XF PEEK powder was purchased from VICTREX Plc.Acidified CNTs and CNTs@RC2540 nano-capsules were added into PEEK powder at levels ranging from 1% to 9%, respectively.After thorough mechanical mixing, the mixture powder was ballmilled for 8 h and then the mixture was poured into an "Eight-Shaped" cavity mold and a "Ring-Shaped" cavity mold, respectively.Cold-pressing technology was adopted to pressure the powder into a Ф 40 mm × 10 mm "Eight-Shaped" block specimen and Ф 30 mm × 12 mm ring specimen under the molding pressure of 30 MPa, respectively.After pressing, the specimens were placed in a programmable oven for curing, as shown in Figure 2a.The specimens were cured under the following conditions [24]: curing temperature: 343 °C; and temperature rise rate: 10 °C/h.After natural cooling, the specimens were finally prepared.The eight-shaped block specimens were used for a mechanical property test, and the ring specimens were used for friction test after being cut into pin specimen.
Actone dissolved RC2540 ̶ 0.05 Mpa Vacuum Ultrasonic 24 h, 65 ºC A Nicolet 6700 infrared spectrometer was adopted to detected the Fourier transform infrared (FTIR) spectrometry of RC2540, acidified CNTs and the nano-capsules.A FEI TECNAI G20 transmission electron microscope (TEM) detected specimen' microstructure at an acceleration voltage of 200 KV [21,22].Thermogravimetric analysis of CNTs and nano-capsules was conducted using an STA449 thermal analyzer with a temperature range of 20-800 • C and a rise rate of 20 • C/min, and the atmosphere was N 2 with a flow rate of 40 mL/min [23].

Preparation of PEEK Composites
150XF PEEK powder was purchased from VICTREX Plc.Acidified CNTs and CNTs@RC2540 nano-capsules were added into PEEK powder at levels ranging from 1% to 9%, respectively.After thorough mechanical mixing, the mixture powder was ball-milled for 8 h and then the mixture was poured into an "Eight-Shaped" cavity mold and a "Ring-Shaped" cavity mold, respectively.Cold-pressing technology was adopted to pressure the powder into a Φ 40 mm × 10 mm "Eight-Shaped" block specimen and Φ 30 mm × 12 mm ring specimen under the molding pressure of 30 MPa, respectively.After pressing, the specimens were placed in a programmable oven for curing, as shown in Figure 2a.The specimens were cured under the following conditions [24]: curing temperature: 343 • C; and temperature rise rate: 10 • C/h.After natural cooling, the specimens were finally prepared.The eight-shaped block specimens were used for a mechanical property test, and the ring specimens were used for friction test after being cut into pin specimen.

Mechanical and Friction Properties Tests
The tensile strength of the eight-shaped block was tested using an RG 4100 universal material testing machine (Figure 2b) according to ISO standards.The tensile rate was 1 mm/min, and the deformation curve breaking strength of PEEK and its composites were automatically recorded by the testing machine [25,26].The specimen hardness was tested by an HR-150A Rockwell hardness tester with Ф12.7 mm diameter of indenter with a main load of 588 N and a pre-load of 98 N in the hardness test (HRR).The hardness values were measured at four locations (three points at each location) and then averaged.
According to the ASTM standards, we conducted friction tests on the MMW-1 friction and wear testing machine (Figure 2c), and Table 1 lists the basic experimental conditions.The COF is calculated as: where T is friction torque, P is test loading, r is radius of the contact point trajectory and is about 11.25 mm.The COF is taken as the average value in the test period.The mass of the pin specimen before and after tests was weighed.In the calculation formula of the specific wear rate: where ΔV is the wear volume, and d is the sliding distance and can be calculated by test time, rotation radius r and rotation speed.The experimental protocol is as follows: (a) The influence of filler content on the PEEK composites' tribological properties was tested at a test load of 50 N and a sliding speed of 0.5 m/s.At this stage, the filler content varied from 1 wt.% to 10 wt.%.(b) The influence of sliding speed on the tribological properties exhibited by 5 wt.% nanoparticles-filling PEEK was tested at a test load of 50 N.In this test, the sliding speed was changed from 0.1 m/s to 0.9 m/s.(c) The influence of test load on the tribological properties exhibited by 5 wt.% nanoparticles-filling PEEK was investigated at a sliding speed of 0.5 m/s.In this test, the load was changed from 20 N to 80 N.

Mechanical and Friction Properties Tests
The tensile strength of the eight-shaped block was tested using an RG 4100 universal material testing machine (Figure 2b) according to ISO standards.The tensile rate was 1 mm/min, and the deformation curve breaking strength of PEEK and its composites were automatically recorded by the testing machine [25,26].The specimen hardness was tested by an HR-150A Rockwell hardness tester with Φ 12.7 mm diameter of indenter with a main load of 588 N and a pre-load of 98 N in the hardness test (HRR).The hardness values were measured at four locations (three points at each location) and then averaged.
According to the ASTM standards, we conducted friction tests on the MMW-1 friction and wear testing machine (Figure 2c), and Table 1 lists the basic experimental conditions.The COF is calculated as: where T is friction torque, P is test loading, r is radius of the contact point trajectory and is about 11.25 mm.The COF is taken as the average value in the test period.The mass of the pin specimen before and after tests was weighed.In the calculation formula of the specific wear rate: where ∆V is the wear volume, and d is the sliding distance and can be calculated by test time, rotation radius r and rotation speed.The experimental protocol is as follows: (a) The influence of filler content on the PEEK composites' tribological properties was tested at a test load of 50 N and a sliding speed of 0.5 m/s.At this stage, the filler content varied from 1 wt.% to 10 wt.%.(b) The influence of sliding speed on the tribological properties exhibited by 5 wt.% nanoparticles-filling PEEK was tested at a test load of 50 N.In this test, the sliding speed was changed from 0.1 m/s to 0.9 m/s.(c) The influence of test load on the tribological properties exhibited by 5 wt.% nanoparticlesfilling PEEK was investigated at a sliding speed of 0.5 m/s.In this test, the load was changed from 20 N to 80 N.

Surface Analysis
The FTIR spectra of PEEK and nano-capsules were measured before and after curing using a Nicolet 6700 infrared spectrometer [27].The microstructure of PEEK composites was observed using a SIRION-100 FESEM.The worn disks were removed under a test load of 50 N and a sliding speed of 0.5 m/s and cleaned with acetone under ultrasound for 30 min, which corresponds to a filler content of 5% in the PEEK composite.An AXIS Ultra DLD X-ray photoelectron spectroscope (XPS) helped to analyze the binding energy of the main elements on the worn surface.The electron pass energy was 80 eV, the ray source was monochromatic AlKα, and a binding energy of 284.8 eV for adventitious carbon C 1s was taken as the internal standard.SEM also served for observing the worn surface microstructure of PEEK composites.

Characterization of CNTs@RC2540 Nano-Capsules
Figure 3 displays the results of thermogravimetric (TG) and differential scanning calorimetry (DSC) of RC2540, ordinary CNTs and nano-capsules.As indicated in Figure 3a, the TG curve of normal CNTs changes gently under low temperature, while the TG curve of nano-capsule has a significant weight loss process at about 295 • C, which is the result of RC2540 escaping from the nano-capsules under the condition of heat.This also demonstrates the presence of RC2540 in the CNTs.Equation (3) explains the calculation of the filling rate (η) of RC2540 in CNTs [28][29][30].
where H f denotes the phase-transition latent heat of RC2540-filled CNTs, and J/g and H p refers to that of RC2540 at the same quality.As for the DSC curve of the nano-capsules in Figure 3b, the calculated result of the phase transition latent heat of RC2540 is 29.5 J/g (A1: Area enclosed by the DSC curve and baseline), while that of RC2540 under the same conditions is 121.3J/g (A2: Area enclosed by the DSC curve and baseline).Hence, we confirmed the filling rate of RC2540 in CNTs as 24.3%.

Surface Analysis
The FTIR spectra of PEEK and nano-capsules were measured before and after curing using a Nicolet 6700 infrared spectrometer [27].The microstructure of PEEK composites was observed using a SIRION-100 FESEM.The worn disks were removed under a test load of 50 N and a sliding speed of 0.5 m/s and cleaned with acetone under ultrasound for 30 min, which corresponds to a filler content of 5% in the PEEK composite.An AXIS Ultra DLD X-ray photoelectron spectroscope (XPS) helped to analyze the binding energy of the main elements on the worn surface.The electron pass energy was 80 eV, the ray source was monochromatic AlKα, and a binding energy of 284.8 eV for adventitious carbon C1s was taken as the internal standard.SEM also served for observing the worn surface microstructure of PEEK composites.

Characterization of CNTs@RC2540 Nano-Capsules
Figure 3 displays the results of thermogravimetric (TG) and differential scanning calorimetry (DSC) of RC2540, ordinary CNTs and nano-capsules.As indicated in Figure 3a, the TG curve of normal CNTs changes gently under low temperature, while the TG curve of nano-capsule has a significant weight loss process at about 295 °C, which is the result of RC2540 escaping from the nano-capsules under the condition of heat.This also demonstrates the presence of RC2540 in the CNTs.Equation (3) explains the calculation of the filling rate (η) of RC2540 in CNTs [28][29][30].
where Hf denotes the phase-transition latent heat of RC2540-filled CNTs, and J/g and Hp refers to that of RC2540 at the same quality.As for the DSC curve of the nano-capsules in Figure 3b, the calculated result of the phase transition latent heat of RC2540 is 29.5 J/g (A1: Area enclosed by the DSC curve and baseline), while that of RC2540 under the same conditions is 121.3J/g (A2: Area enclosed by the DSC curve and baseline).Hence, we confirmed the filling rate of RC2540 in CNTs as 24.3%.Figure 4 shows the FTIR spectra of acidified CNTs, RC2540 and CNTs@RC2540 nanocapsules.It can be seen from Figure 4a that the new absorption peaks of CNTs around 3432 cm −1 and 1822 cm −1 wavelengths after acidification treatment are a result of hydroxyl and carboxyl groups [31], revealing the binding of acidified CNTs to oxygen-containing groups, that is good for improving the bonding strength of CNTs and PEEK during curing.Furthermore, the peak near 1643 cm −1 is a result of the plane absorption peak of the carbon ring structure [32,33].That is to say, CNTs preserve tubular structures, which is the prerequisite for the CNTs to be filled with RC2540.It can be found from Figure 4a that the FTIR spectrum of nano-capsules does not present any new characteristic peaks, and the characteristic peaks are partial superimpositions of these of the acidified CNTs and RC2540, indicating that in the process of filling CNTs with RC2540, physical combination mainly occurs between them, but no chemical reaction.By observing the microstructure of carbon tubes with TEM, the state of RC2540 in CNTs tubes can be visually determined.Figure 4b shows a TEM of the nano-capsules.Several sections of the tubes were clearly infiltrated with RC2540, which visually demonstrates the presence of RC2540 in the CNTs tubes.
Figure 4 shows the FTIR spectra of acidified CNTs, RC2540 and CNTs@RC2540 nano-capsules.It can be seen from Figure 4a that the new absorption peaks of CNTs around 3432 cm −1 and 1822 cm −1 wavelengths after acidification treatment are a result of hydroxyl and carboxyl groups [31], revealing the binding of acidified CNTs to oxygen-containing groups, that is good for improving the bonding strength of CNTs and PEEK during curing.Furthermore, the peak near 1643 cm −1 is a result of the plane absorption peak of the carbon ring structure [32,33].That is to say, CNTs preserve tubular structures, which is the prerequisite for the CNTs to be filled with RC2540.It can be found from Figure 4a that the FTIR spectrum of nano-capsules does not present any new characteristic peaks, and the characteristic peaks are partial superimpositions of these of the acidified CNTs and RC2540, indicating that in the process of filling CNTs with RC2540, physical combination mainly occurs between them, but no chemical reaction.By observing the microstructure of carbon tubes with TEM, the state of RC2540 in CNTs tubes can be visually determined.Figure 4b shows a TEM of the nano-capsules.Several sections of the tubes were clearly infiltrated with RC2540, which visually demonstrates the presence of RC2540 in the CNTs tubes.

Mechanical Property and Interior Structure of PEEK Composites
Figure 5 indicates how the mass fractions of CNTs and nano-capsules affect the PEEK composites' tensile strength and hardness.In Figure 5a, with the increase in the filling amount, the tensile strength presents an increase-to-decrease trend.Because CNTs show higher surface activity and many unsaturated bonds and reactive groups are formed [34].The reinforced bond of PEEK can be strengthened by physical or chemical crosslinking of polymer chain and CNTs [35][36][37].Further addition may lead to problems with the aggregation of CNTs, thereby reducing this cross-linking effect as well as reducing the PEEK composite strength.CNTs have an optimum content of about 5%, and the strength of PEEK can be increased by about 35% at this point, which is better than the method of filling CNTs in most of the literature reports above.At a filling level of approximately 8%, the strength of the PEEK composite is comparable to that of pure PEEK.The filling of nano-capsules more remarkably impacts PEEK strength and hardness because the nanocapsules have a smaller aspect ratio and are more closely bound to PEEK. Figure 5b suggests that the PEEK composite hardness decreases as the filling amount elevates.In practice, the appropriate filling amount can be selected according to the requirements of different self-lubricating parts [38].

Mechanical Property and Interior Structure of PEEK Composites
Figure 5 indicates how the mass fractions of CNTs and nano-capsules affect the PEEK composites' tensile strength and hardness.In Figure 5a, with the increase in the filling amount, the tensile strength presents an increase-to-decrease trend.Because CNTs show higher surface activity and many unsaturated bonds and reactive groups are formed [34].The reinforced bond of PEEK can be strengthened by physical or chemical crosslinking of polymer chain and CNTs [35][36][37].Further addition may lead to problems with the aggregation of CNTs, thereby reducing this cross-linking effect as well as reducing the PEEK composite strength.CNTs have an optimum content of about 5%, and the strength of PEEK can be increased by about 35% at this point, which is better than the method of filling CNTs in most of the literature reports above.At a filling level of approximately 8%, the strength of the PEEK composite is comparable to that of pure PEEK.The filling of nano-capsules more remarkably impacts PEEK strength and hardness because the nanocapsules have a smaller aspect ratio and are more closely bound to PEEK. Figure 5b suggests that the PEEK composite hardness decreases as the filling amount elevates.In practice, the appropriate filling amount can be selected according to the requirements of different self-lubricating parts [38].
Due to the complexity of the PEEK curing reaction, CNTs@RC2540 nano-capsules can possibly be destroyed in the curing process, so the stability of nano-capsules in the curing process and its compatibility with PEEK shall be studied.According to the TG curves of CNTs and nano-capsules in Figure 3, the nano-capsules show no significant weight loss until 300 • C, making them strong enough to deal with the curing temperature of PEEK. Figure 6a displays the FTIR spectra of PEEK, nano-capsules and the PEFE composites powder.The peaks of 1112 cm −1 , 1635 cm −1 , 2924 cm −1 and 3424 cm −1 in the nano-capsules spectrum, respectively, represent the absorption peaks of carbon nanotube skeleton vibration, C-H stretching vibration, C-H deformation vibration and O-H bond stretching vibration [39].Among them, the first three peaks clearly appeared in the FTIR spectrum of the cured PEEK.That is to say, the CNTs' skeleton structure was not destroyed in the curing process, which can well protect the RC2540 within it.Figure 6b shows the SEM images of PEEK composites tissue.Nano-capsules are distributed in the PEEK organization in the form of lump-like particles [40].Each particle is an oil-containing nano-capsules that is destroyed during friction, while RC2540 is uniformly released for lubrication in the process of friction.Due to the complexity of the PEEK curing reaction, CNTs@RC2540 nano-capsules can possibly be destroyed in the curing process, so the stability of nano-capsules in the curing process and its compatibility with PEEK shall be studied.According to the TG curves of CNTs and nano-capsules in Figure 3, the nano-capsules show no significant weight loss until 300 °C, making them strong enough to deal with the curing temperature of PEEK. Figure 6a displays the FTIR spectra of PEEK, nano-capsules and the PEFE composites powder.The peaks of 1112 cm −1 , 1635 cm −1 , 2924 cm −1 and 3424 cm −1 in the nano-capsules spectrum, respectively, represent the absorption peaks of carbon nanotube skeleton vibration, C-H stretching vibration, C-H deformation vibration and O-H bond stretching vibration [39].Among them, the first three peaks clearly appeared in the FTIR spectrum of the cured PEEK.That is to say, the CNTs' skeleton structure was not destroyed in the curing process, which can well protect the RC2540 within it.Figure 6b shows the SEM images of PEEK composites tissue.Nano-capsules are distributed in the PEEK organization in the form of lump-like particles [40].Each particle is an oil-containing nano-capsules that is destroyed during friction, while RC2540 is uniformly released for lubrication in the process of friction.

Effect of Filling Content on PEEK Composites' Tribological Properties
Figure 7 shows the influence of the filling content of CNTs or nano-capsules on the COF and WR of PEEK at a load of 50 N and a sliding speed of 0.5 m/s.In Figure 7a, the  Due to the complexity of the PEEK curing reaction, CNTs@RC2540 nano-capsules can possibly be destroyed in the curing process, so the stability of nano-capsules in the curing process and its compatibility with PEEK shall be studied.According to the TG curves of CNTs and nano-capsules in Figure 3, the nano-capsules show no significant weight loss until 300 °C, making them strong enough to deal with the curing temperature of PEEK. Figure 6a displays the FTIR spectra of PEEK, nano-capsules and the PEFE composites powder.The peaks of 1112 cm −1 , 1635 cm −1 , 2924 cm −1 and 3424 cm −1 in the nano-capsules spectrum, respectively, represent the absorption peaks of carbon nanotube skeleton vibration, C-H stretching vibration, C-H deformation vibration and O-H bond stretching vibration [39].Among them, the first three peaks clearly appeared in the FTIR spectrum of the cured PEEK.That is to say, the CNTs' skeleton structure was not destroyed in the curing process, which can well protect the RC2540 within it.Figure 6b shows the SEM images of PEEK composites tissue.Nano-capsules are distributed in the PEEK organization in the form of lump-like particles [40].Each particle is an oil-containing nano-capsules that is destroyed during friction, while RC2540 is uniformly released for lubrication in the process of friction.

Effect of Filling Content on PEEK Composites' Tribological Properties
Figure 7 shows the influence of the filling content of CNTs or nano-capsules on the COF and WR of PEEK at a load of 50 N and a sliding speed of 0.5 m/s.In Figure 7a, the

Effect of Filling Content on PEEK Composites' Tribological Properties
Figure 7 shows the influence of the filling content of CNTs or nano-capsules on the COF and WR of PEEK at a load of 50 N and a sliding speed of 0.5 m/s.In Figure 7a, the lower content of CNTs leads to a certain diminution of the COF of PEEK, which is due to the molecular structure of the CNTs being destroyed during friction, and the adsorption and sedimentation of the CNT' molecular fragments may produce certain an anti-friction effect [41].Then, the CNTs content exceeded 4% and continued to rise, the anti-friction effect of PEEK/CNTs became less significant, as excess CNTs might act as a deterrent.When filling with nano-capsules, the COF of PEEK/nano-capsule composite decreased with the increase in the content.It can be found that during friction, RC2540 was released from the nano-capsule inside PEEK, and RC2540 was adsorbed on the friction interface, which formed a more adequate lubrication and thus served to reduce the friction [42,43].In Figure 7b, the WR is highest for pure PEEK.A transfer film can be formed at the friction interface of PEEK, that is not very strongly bonded to the metal.Filling CNTs or nanocapsules reduces the WR of PEEK, with minimum WRs of 2.7 × 10 −6 mm 3 /N•m and Lubricants 2023, 11, 511 8 of 17 1.9 × 10 −6 mm 3 N•m at 15% or 10% filling, respectively.The value of this wear rate and the friction coefficient values mentioned above are superior to all relevant literature reports above.The adsorption and deposition of CNTs molecular fragment produced anti-friction effect on the friction interface, but also prevented a PEEK transfer film from being formed to some extent, reducing the WR of composites.The increasing amount of filler affects the hardness of composites [44], resulting in increased WR.The PEEK/nano-capsule composite had a lower WR than the PEEK composites under various contents.The WR result also shows that RC2540 is released during friction, and that the lubrication layer composed of RC2540 is more resistant to the adhesion and transfer of PEEK and therefore more effective against wear.
effect of PEEK/CNTs became less significant, as excess CNTs might act as a deterrent.When filling with nano-capsules, the COF of PEEK/nano-capsule composite decreased with the increase in the content.It can be found that during friction, RC2540 was released from the nano-capsule inside PEEK, and RC2540 was adsorbed on the friction interface, which formed a more adequate lubrication and thus served to reduce the friction [42,43].In Figure 7b, the WR is highest for pure PEEK.A transfer film can be formed at the friction interface of PEEK, that is not very strongly bonded to the metal.Filling CNTs or nanocapsules reduces the WR of PEEK, with minimum WRs of 2.7 × 10 −6 mm 3 /N•m and 1.9 × 10 −6 mm 3 N•m at 15% or 10% filling, respectively.The value of this wear rate and the friction coefficient values mentioned above are superior to all relevant literature reports above.The adsorption and deposition of CNTs molecular fragment produced anti-friction effect on the friction interface, but also prevented a PEEK transfer film from being formed to some extent, reducing the WR of composites.The increasing amount of filler affects the hardness of composites [44], resulting in increased WR.The PEEK/nano-capsule composite had a lower WR than the PEEK composites under various contents.The WR result also shows that RC2540 is released during friction, and that the lubrication layer composed of RC2540 is more resistant to the adhesion and transfer of PEEK and therefore more effective against wear.

Effect of Sliding Speed on PEEK Composites' Tribological Properties
Figure 8 displays the effect of sliding speed on the COF and WR of pure PEEK, PEEK/CNT composite and PEEK/nano-capsule composite materials under a 50 N test load at a 5% filling of CNTs or nano-capsules.The COFs of the three materials have a decreasing tendency with the increase in sliding speed, which is mainly related to the temperature change, deformation, roughness and vibration between contact surfaces caused by the increasing speed [45,46].In Figure 8a, PEEK/CNT composite has a smaller COF than pure PEEK under various velocities, with the PEEK/nano-capsule composite having the lowest coefficient of friction.The WR of the three materials increases with the increase in the speed in Figure 8b, and the PEEK/nano-capsule composite has the lowest WR.The high speed leads to an increase in temperature between the friction interfaces, which is more favorable to the release of RC2540 in the PEEK/nano-capsule composite material.The formation of a more adequate lubrication layer, as a result, leads to a significant downward trend in the COF of PEEK/nano-capsule composites.The linear velocity of high-speed machine parts when velocity can be above 40 m/s [47].Under such conditions, the sliding

Effect of Sliding Speed on PEEK Composites' Tribological Properties
Figure 8 displays the effect of sliding speed on the COF and WR of pure PEEK, PEEK/CNT composite PEEK/nano-capsule composite materials under a 50 N test load at a 5% filling of CNTs or nano-capsules.The COFs of the three materials have a decreasing tendency with the increase in sliding speed, which is mainly related to the temperature change, deformation, roughness and vibration between contact surfaces caused by the increasing speed [45,46].In Figure 8a, PEEK/CNT composite has a smaller COF than pure PEEK under various velocities, with the PEEK/nano-capsule composite having the lowest coefficient of friction.The WR of the three materials increases with the increase in the speed in Figure 8b, and the PEEK/nano-capsule composite has the lowest WR.The high speed leads to an increase in temperature between the friction interfaces, which is more favorable to the release of RC2540 in the PEEK/nano-capsule composite material.The formation of a more adequate lubrication layer, as a result, leads to a significant downward trend in the COF of PEEK/nano-capsule composites.The linear velocity of high-speed machine parts when velocity can be above 40 m/s [47].Under such conditions, the sliding between friction interfaces should be more conducive for the cleavage of nano-capsules, which promote RC2540 to be released and the self-lubrication layer to be formed.between friction interfaces should be more conducive for the cleavage of nano-capsules, which promote RC2540 to be released and the self-lubrication layer to be formed.

Effect of Test Load on Tribological Properties of PEEK Composites
Figure 9 shows the variation of the COF and WR of the three materials with the test load when the test sliding speed was 0.5 m/s.The coefficient of friction of the pure PEEK or PEEK/CNT composites gradually increases in Figure 9a for increasing loads that lead to larger frictional contact areas and heat generation [48].However, the COF of the PEEK/nano-capsule composite did not change much or even decreased as a more sufficient lubrication layer was formed with the releasing of more RC2540.The PEEK/CNT composite had a smaller COF than ordinary PEEK under various loads.In Figure 9b, the WR of the three kind materials elevated with the TL, and the WR of the nano-capsule composite is the smallest.This may be due to the fact that at higher loads, more lubricating layers will be formed as the RC2540 is released.

SEM Analysis of the Worn Surface on Steel Disc
SEM morphology regarding the worn surface on steel disc and EDS energy spectrum of selected areas under the friction of the pure PEEK, PEEK/CNT composite and PEEK/nano-capsule composites are indicated in Figure 10.In Figure 10(a1-c1), the steel disc wear surface has deeper furrows in the presence of pure PEEK, better surface quality

Effect of Test Load on Tribological Properties of PEEK Composites
Figure 9 shows the variation of the COF and WR of the three materials with the test load when the test sliding speed was 0.5 m/s.The coefficient of friction of the pure PEEK or PEEK/CNT composites gradually increases in Figure 9a for increasing loads that lead to larger frictional contact areas and heat generation [48].However, the COF of the PEEK/nano-capsule composite did not change much or even decreased as a more sufficient lubrication layer was formed with the releasing of more RC2540.The PEEK/CNT composite had a smaller COF than ordinary PEEK under various loads.In Figure 9b, the WR of the three kind materials elevated with the TL, and the WR of the nano-capsule composite is the smallest.This may be due to the fact that at higher loads, more lubricating layers will be formed as the RC2540 is released.
Lubricants 2023, 11, x FOR PEER REVIEW 9 of 17 between friction interfaces should be more conducive for the cleavage of nano-capsules, which promote RC2540 to be released and the self-lubrication layer to be formed.

Effect of Test Load on Tribological Properties of PEEK Composites
Figure 9 shows the variation of the COF and WR of the three materials with the test load when the test sliding speed was 0.5 m/s.The coefficient of friction of the pure PEEK or PEEK/CNT composites gradually increases in Figure 9a for increasing loads that lead to larger frictional contact areas and heat generation [48].However, the COF of the PEEK/nano-capsule composite did not change much or even decreased as a more sufficient lubrication layer was formed with the releasing of more RC2540.The PEEK/CNT composite had a smaller COF than ordinary PEEK under various loads.In Figure 9b, the WR of the three kind materials elevated with the TL, and the WR of the nano-capsule composite is the smallest.This may be due to the fact that at higher loads, more lubricating layers will be formed as the RC2540 is released.

SEM Analysis of the Worn Surface on Steel Disc
SEM morphology regarding the worn surface on steel disc and EDS energy spectrum of selected areas under the friction of the pure PEEK, PEEK/CNT composite and PEEK/nano-capsule composites are indicated in Figure 10.In Figure 10(a1-c1), the steel disc wear surface has deeper furrows in the presence of pure PEEK, better surface quality

SEM Analysis of the Worn Surface on Steel Disc
SEM morphology regarding the worn surface on steel disc and EDS energy spectrum of selected areas under the friction of the pure PEEK, PEEK/CNT composite and PEEK/nanocapsule composites are indicated in Figure 10.In Figure 10(a1-c1), the steel disc wear surface has deeper furrows in the presence of pure PEEK, better surface quality in the presence of the PEEK/CNT composite and the smoothest wear surface in the presence of the PEEK/nano-capsules.In addition, the worn surface under the action of pure PEEK has more adhesions that are transfer films formed by PEEK.The adhesion of the CNT/PEEK composite or PEEK/nano-capsule composite is relatively low.The involvement of CNTs in the friction process is capable of reducing the PEEK adhesion and improving the surface quality of worn surface, while the release of RC2540 from the nano-capsules further reduces the adhesion of PEEK, and a lubricating layer may have some polishing effect, which may also improve the surface quality [49].Figure 10(a2-c2) suggest that the worn surface under PEEK friction has a high elemental content of C and Fe, which is mainly a result of the transfer of PEEK materials to the metal surface.When CNTs or nano-capsules are involved in the friction process, the content of C increases and that of Fe decreases.This suggests that CNTs are decomposed and deposited on the metal surface for creating a lubrication layer during the friction process, and RC2540 can be released to the friction surface during the action of nano-capsules.The releasing and adsorption of RC2540 further increases the content of C and forms a more adequate lubrication layer, resulting in the reduced adhesion of PEEK.As shown in Figure 10(c2), the release of RC2540 is well demonstrated by the significant increase in elemental S content on the wear surface in the presence of the PEEK/nano-capsule composite.
Lubricants 2023, 11, x FOR PEER REVIEW 10 of 17 in the presence of the PEEK/CNT composite and the smoothest wear surface in the presence of the PEEK/nano-capsules.In addition, the worn surface under the action of pure PEEK has more adhesions that are transfer films formed by PEEK.The adhesion of the CNT/PEEK composite or PEEK/nano-capsule composite is relatively low.The involvement of CNTs in the friction process is capable of reducing the PEEK adhesion and improving the surface quality of worn surface, while the release of RC2540 from the nanocapsules further reduces the adhesion of PEEK, and a lubricating layer may have some polishing effect, which may also improve the surface quality [49].

XPS Analysis of Worn Surface on Steel Disc
To further confirm the composition of the self-lubricating layer formed by CNTs@RC2540 nano-capsules, XPS energy spectrum analysis was conducted on the steel disc worn surface (Figure 11).XPSPEAK 41 software is applied to separate peaks of C, O and S. The obtained fitted peaks together with their possible corresponding functional groups are marked in the figure.Table 2 presents the relative atomic concentrations of elements on the worn surface.

XPS Analysis of Worn Surface on Steel Disc
To further confirm the composition of the self-lubricating layer formed by CNTs@RC2540 nano-capsules, XPS energy spectrum analysis was conducted on the steel disc worn surface (Figure 11).XPSPEAK 41 software is applied to separate peaks of C, O and S. The obtained fitted peaks together with their possible corresponding functional groups are marked in the figure.Table 2 presents the relative atomic concentrations of elements on the worn surface.

XPS Analysis of Worn Surface on Steel Disc
To further confirm the composition of the self-lubricating layer formed by CNTs@RC2540 nano-capsules, XPS energy spectrum analysis was conducted on the steel disc worn surface (Figure 11).XPSPEAK 41 software is applied to separate peaks of C, O and S. The obtained fitted peaks together with their possible corresponding functional groups are marked in the figure.Table 2 presents the relative atomic concentrations of elements on the worn surface.The C and O spectra of the GCr15 steel surface in Figure 11(a1,a2) have a single peak shape, which mainly marks the oxygen in polluting carbon and iron oxides.The steel disc wear surface under pure PEEK friction has two new peaks in the C and O spectra compared to the GCr15 steel surface, which may be a result of the adhesive segments of PEEK.In Table 2, the adhesion of PEEK fragments is demonstrated by the increased atomic concentrations of C and the decreased atomic concentrations of O on the wear surface under friction with pure PEEK as compared to the surface of GCr15 steel.Such adhesion mainly depends on Van der Waals' force, hydrogen bond force and electrostatic force, which cause the PEEK fragments to bond chemically to the iron atoms, consequently reducing the atomic concentrations of O [50][51][52].
The peak shape for the C element of the worn surface rubbed by PEEK/CNT composite is similar to that of pure PEEK, and the fitting peak is primarily resulted from the carbon types in C-C, C-O and C=O in Figure 11(c1,c2) [53][54][55][56].In Table 2, the highest atomic concentrations of C can be observed when the PEEK/CNT composites were rubbed, which suggests that the friction products of CNTs are adsorbed and deposited in the friction process.The PEEK/CNT-composite-forming lubrication layer is different from that formed by pure PEEK.In the friction process, the molecular structure of CNTs is destroyed, with some of the carbonized fragments deposited on or absorbed into the worn surface for weakening the friction.The C and O spectra of the GCr15 steel surface in Figure 11(a1,a2) have a single peak shape, which mainly marks the oxygen in polluting carbon and iron oxides.The steel disc wear surface under pure PEEK friction has two new peaks in the C and O spectra compared to the GCr15 steel surface, which may be a result of the adhesive segments of PEEK.In Table 2, the adhesion of PEEK fragments is demonstrated by the increased atomic concentrations of C and the decreased atomic concentrations of O on the wear surface under friction with pure PEEK as compared to the surface of GCr15 steel.Such adhesion mainly depends on Van der Waals' force, hydrogen bond force and electrostatic force, which cause the PEEK fragments to bond chemically to the iron atoms, consequently reducing the atomic concentrations of O [50][51][52].
The peak shape for the C element of the worn surface rubbed by PEEK/CNT composite is similar to that of pure PEEK, and the fitting peak is primarily resulted from the carbon types in C-C, C-O and C=O in Figure 11(c1,c2) [53][54][55][56].In Table 2, the highest atomic concentrations of C can be observed when the PEEK/CNT composites were rubbed, which suggests that the friction products of CNTs are adsorbed and deposited in the friction process.The PEEK/CNT-composite-forming lubrication layer is different from that formed by pure PEEK.In the friction process, the molecular structure of CNTs is destroyed, with some of the carbonized fragments deposited on or absorbed into the worn surface for weakening the friction.
In Figure 11(d3), the worn surface under action of the PEEK/nano-capsule composite has a distinct peak shape of S element, which appears at 161.2 eV and 168.3 eV, respectively.The two peeks may be attributed to the sulfur species in Fe-S and S-O, respectively [57-59], which is good evidence for the release of RC2540 lubricant during friction.In Figure 11(d1,d2), the peak shape of C on the worn surface under friction of the PEEK/nano-capsule composite is similar to that of the PEEK/CNT composite.The fitting peak belonging to C-O has a stronger strength, and the peak shape of O element is more complex.These results indicates that CNTs in nano-capsules affect the friction, while the releasing of RC2540 in the nano-capsules impacts the formation of the lubricating layer of CNTs at the same time.Table 2 shows that the PEEK/nano-capsule composite has the lowest atomic concentration of C and the highest atomic concentration of O, while there is an intermediate concentration of Fe among the three kinds of worn surfaces.During frictions, RC2540 was released from the composite.The S element in the RC2540 molecule is highly reactive and readily binds to iron atoms through chemical adsorption, resulting in increased atomic concentrations of O and Fe elements and decreased atomic concentrations of C elements.

SEM Analysis of Worn Surface on Composites
Figure 12 gives the SEM analysis results of the worn surface of PEEK, the PEEK/CNT composite, and the PEEK/nano-capsule composite at a test load of 50 N, a sliding speed of 0.5 m/s and a filler content of 5%.With adhesion on the surface of friction pair and cyclic alternating shear stress on PEEK surface, the pure PEEK material wears mainly as a secondary surface fatigue spalling wear [60].In addition, the surface material of PEEK can easily undergo thermoplastic deformation and flow under the action of friction temperature and stress, as shown in Figure 12a, as a result, the COF and the WR of pure PEEK materials are higher [61].
Lubricants 2023, 11, x FOR PEER REVIEW 13 of 17 In Figure 11(d3), the worn surface under action of the PEEK/nano-capsule composite has a distinct peak shape of S element, which appears at 161.2 eV and 168.3 eV, respectively.The two peeks may be attributed to the sulfur species in Fe-S and S-O, respectively [57-59], which is good evidence for the release of RC2540 lubricant during friction.In Figure 11(d1,d2), the peak shape of C on the worn surface under friction of the PEEK/nanocapsule composite is similar to that of the PEEK/CNT composite.The fitting peak belonging to C-O has a stronger strength, and the peak shape of O element is more complex.These results indicates that CNTs in nano-capsules affect the friction, while the releasing of RC2540 in the nano-capsules impacts the formation of the lubricating layer of CNTs at the same time.Table 2 shows that the PEEK/nano-capsule composite has the lowest atomic concentration of C and the highest atomic concentration of O, while there is an intermediate concentration of Fe among the three kinds of worn surfaces.During frictions, RC2540 was released from the composite.The S element in the RC2540 molecule is highly reactive and readily binds to iron atoms through chemical adsorption, resulting in increased atomic concentrations of O and Fe elements and decreased atomic concentrations of C elements.

SEM Analysis of Worn Surface on Composites
Figure 12 gives the SEM analysis results of the worn surface of PEEK, the PEEK/CNT composite, and the PEEK/nano-capsule composite at a test load of 50 N, a sliding speed of 0.5 m/s and a filler content of 5%.With adhesion on the surface of friction pair and cyclic alternating shear stress on PEEK surface, the pure PEEK material wears mainly as a secondary surface fatigue spalling wear [60].In addition, the surface material of PEEK can easily undergo thermoplastic deformation and flow under the action of friction temperature and stress, as shown in Figure 12a, as a result, the COF and the WR of pure PEEK materials are higher [61].In Figure 12b, the PEEK/CNT composites' worn surface shows no thermoplastic deformation and flow onto the material surface caused by friction temperature and stress, mainly as a result of the improved flexural strength, modulus and hardness of the PEEK composite as well as the load-bearing capacity of the CNTs.On the other hand, CNTs are directly involved in friction and also act as a lubricant.CNTs partially replace the direct contact between the matrix PEEK and GCr15 steel, reducing the real contact area between the resin and GCr15 steel, and effectively lowers the adhesion between the resin and the steel surface.The lubricating layer formed by CNTs also reduces the abrasive wear effect during the friction process and inhibits PEEK composites' thermoplastic deformation and wear, so that the wear resistance can be improved.However, the lubrication layer formed by CNTs only has a limited lubrication effect on the wearing surface, as the CNT.sIn Figure 12b, the PEEK/CNT composites' worn surface shows no thermoplastic deformation and flow onto the material surface caused by friction temperature and stress, mainly as a result of the improved flexural strength, modulus and hardness of the PEEK composite as well as the load-bearing capacity of the CNTs.On the other hand, CNTs are directly involved in friction and also act as a lubricant.CNTs partially replace the direct contact between the matrix PEEK and GCr15 steel, reducing the real contact area between the resin and GCr15 steel, and effectively lowers the adhesion between the resin and the steel surface.The lubricating layer formed by CNTs also reduces the abrasive wear effect during the friction process and inhibits PEEK composites' thermoplastic deformation and wear, so that the wear resistance can be improved.However, the lubrication layer formed by CNTs only has a limited lubrication effect on the wearing surface, as the CNT.s molecules do not have effective lubricating groups.The poor anti-friction and anti-wear effect of PEEK/CNTs resulted in the deep furrows on the worn surface.
The wear surface of the PEEK/nano-capsule composite also shows no significant plastic deformation or flow, as shown in Figure 12c, and the surface is much smoother.In the friction process, the CNTs of the nano-capsules shell can first act as a friction reducer, and then the RC2540 in the nano-capsules can also be released to form a more sufficient lubricating layer, providing better friction and wear reduction, so the PEEK/nano-capsule composite exhibits the best tribological properties.Figure 13 gives the friction contact area model when applying PEEK/nano-capsules.The friction process is accompanied by the release of RC2540 from the nano-capsules, acting together with the friction products of CNTs for the formation of a more sufficient self-lubrication layer.RC2540 can also strengthen chemisorption, and the self-lubricating layer formed by it possesses three functions: First, the COF between PEEK and steel is reduced.Second, the WR is lowered.Third, the worn surface quality of PEEK composites and the steel disc are improved.
Lubricants 2023, 11, x FOR PEER REVIEW 14 of 17 molecules do not have effective lubricating groups.The poor anti-friction and anti-wear effect of PEEK/CNTs resulted in the deep furrows on the worn surface.The wear surface of the PEEK/nano-capsule composite also shows no significant plastic deformation or flow, as shown in Figure 12c, and the surface is much smoother.In the friction process, the CNTs of the nano-capsules shell can first act as a friction reducer, and then the RC2540 in the nano-capsules can also be released to form a more sufficient lubricating layer, providing better friction and wear reduction, so the PEEK/nano-capsule composite exhibits the best tribological properties.Figure 13 gives the friction contact area model when applying PEEK/nano-capsules.The friction process is accompanied by the release of RC2540 from the nano-capsules, acting together with the friction products of CNTs for the formation of a more sufficient self-lubrication layer.RC2540 can also strengthen chemisorption, and the self-lubricating layer formed by it possesses three functions: First, the COF between PEEK and steel is reduced.Second, the WR is lowered.Third, the worn surface quality of PEEK composites and the steel disc are improved.

Conclusions
Compared to the previous methods of adding CNTs for reinforcement in previous publications, the method of adding CNTs@RC2540 composites in this article can better increase the mechanical strength of PEEK materials and make it have better self-lubricating properties, which have broader application potential.The conclusions of this article are as follows: (a) This study demonstrated the successful preparation of CNTs@RC2540 nano-capsules whose filling rate reached approximately 25% TG, and DSC analysis proved the thermal stability of the composites.The nano-capsules completely resist the curing temperature of PEEK with their intact structure during curing.PEEK tissue exists in the form of cluster particles, which forms a good protection for RC2540.(b) After filling with nano-capsules, the tensile strength and hardness of PEEK are changed, but the actual mechanical property conditions of the self-lubricating parts can still be met within a certain filling range.(c) The filling of nano-capsules can significantly reduce the COF and WR of PEEK.
Within a certain range, the higher the filling amount, the more significant the improvement in the COF.With the increase in experimental speed and loading, the COF of PEEK/nano-capsule composites also decreases, while the WR increases.(d) PEEK composite material filled with CNTs@RC2540 nano-capsules can release RC2540 during friction, which is adsorbed on the worn surface and acts with CNTs

Conclusions
Compared to the previous methods of adding CNTs for reinforcement in previous publications, the method of adding CNTs@RC2540 composites in this article can better increase the mechanical strength of PEEK materials and make it have better self-lubricating properties, which have broader application potential.The conclusions of this article are as follows: (a) This study demonstrated the successful preparation of CNTs@RC2540 nano-capsules whose filling rate reached approximately 25% TG, and DSC analysis proved the thermal stability of the composites.The nano-capsules completely resist the curing temperature of PEEK with their intact structure during curing.PEEK tissue exists in the form of cluster particles, which forms a good protection for RC2540.(b) After filling with nano-capsules, the tensile strength and hardness of PEEK are changed, but the actual mechanical property conditions of the self-lubricating parts can still be met within a certain filling range.(c) The filling of nano-capsules can significantly reduce the COF and WR of PEEK.Within a certain range, the higher the filling amount, the more significant the improvement in the COF.With the increase in experimental speed and loading, the COF of PEEK/nanocapsule composites also decreases, while the WR increases.(d) PEEK composite material filled with CNTs@RC2540 nano-capsules can release RC2540 during friction, which is adsorbed on the worn surface and acts with CNTs for forming a self-lubrication layer.The surface layer of the contact area from the combined action of the two components acts as a friction reduction, anti-wear and the surface quality improvement.

Figure 1 .
Figure 1.Schematic illustration for the preparation process of CNTs@RC2540 nano-capsules.

Figure 1 .
Figure 1.Schematic illustration for the preparation process of CNTs@RC2540 nano-capsules.

Figure 2 .
Figure 2. The experimental process of this article: preparation of samples (a), tensile test (b) and tribological test (c).

Figure 2 .
Figure 2. The experimental process of this article: preparation of samples (a), tensile test (b) and tribological test (c).

Figure 5 .
Figure 5.Effect of filler content on tensile strength (a) and hardness (b) of PEEK composites.

Figure 6 .
Figure 6.Characterization of structure: FTIR spectra of PEEK and composite (a), SEM image of composites (b).

Figure 5 .
Figure 5.Effect of filler content on tensile strength (a) and hardness (b) of PEEK composites.

Figure 5 .
Figure 5.Effect of filler content on tensile strength (a) and hardness (b) of PEEK composites.

Figure 6 .
Figure 6.Characterization of structure: FTIR spectra of PEEK and composite (a), SEM image of composites (b).

Figure 6 .
Figure 6.Characterization of structure: FTIR spectra of PEEK and composite (a), SEM image of composites (b).

Figure 7 .
Figure 7. Effect of content on friction coefficient (a) and wear rate (b) of PEEK composites.

Figure 7 .
Figure 7. Effect of content on friction coefficient (a) and wear rate (b) of PEEK composites.

Figure 8 .
Figure 8. Influence of sliding speed on friction coefficient (a) and wear rate (b) of PEEK composites.

Figure 9 .
Figure 9. Influence of test load on PEEK composites' tribological properties of friction coefficient (a) and wear rate (b).

1 • m − 1 Figure 8 .
Figure 8. Influence of sliding speed on friction coefficient (a) and wear rate (b) of PEEK composites.

Figure 8 .
Figure 8. Influence of sliding speed on friction coefficient (a) and wear rate (b) of PEEK composites.

Figure 9 .
Figure 9. Influence of test load on PEEK composites' tribological properties of friction coefficient (a) and wear rate (b).

Figure 9 .
Figure 9. Influence of test load on PEEK composites' tribological properties of friction coefficient (a) and wear rate (b).

Figure 13 .
Figure 13.The model of self-lubrication layer under the friction of PEEK/nano-capsule composites.

Figure 13 .
Figure 13.The model of self-lubrication layer under the friction of PEEK/nano-capsule composites.

Table 1 .
Friction and wear test conditions.

Table 2 .
Relative atomic content of main elements on GCr15 steel and worn surface under the friction of different materials.

Table 2 .
Relative atomic content of main elements on GCr15 steel and worn surface under the friction of different materials.
, funding acquisition, conceptualization, data curation.L.Y.: Conceptualization, data curation.J.G. and S.H.: Resources, formal analysis, Conceptualization, data curation.All authors have read and agreed to the published version of the manuscript.Funding was provided by National Natural Science Foundation of China (51805345 and 52275468).