PEEK

Article pubs.acs.org/JPCC

Structure and Function of Transfer Film Formed from PTFE/PEEK Polymer Blend Tasuku Onodera,*,† Jun Nunoshige,† Kenji Kawasaki,† Koshi Adachi,‡ Kazue Kurihara,§ and Momoji Kubo∥ †

Center for Technology InnovationMaterials, Research & Development Group, Hitachi, Ltd., 7-1-1 Omika, Hitachi 319-1292, Japan ‡ Department of Nanomechanics, Graduate School of Engineering, Tohoku University, 6-6-1 Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan § Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan ∥ Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan ABSTRACT: Improving the tribological performance of polytetrafluoroethylene (PTFE) resin is important for industrial use of PTFE. An effective way to do this is to blend pure PTFE with another resin material to improve the quality of the transfer film, i.e., adhesion to a counter metallic surface. It is well-known that PTFE/polyetheretherketone (PEEK) polymer blends show significantly less wear than pure PTFE and PEEK. However, the structure and function of the transfer film formed from the blend, which is the key process for reducing friction and wear, have not yet been understood well. Accordingly, the tribological properties and the structure of transfer film of PTFE/PEEK polymer blends were investigated. The blends were prepared by compression and calcination of mixed powders, and a conventional pin-on-disk friction test was performed using pure aluminum as a counter material. Both the friction coefficient and wear rate of the polymer blends were lower than those of pure PTFE and PEEK, respectively. This is consistent with the results of an earlier study [Burris and Sawyer, Wear 2006, 261, 410−418]. The present study especially focused on the microstructure and function of the transfer film formed from the PTFE/PEEK polymer blend. The microstructure of the transfer film was analyzed by X-ray photoelectron spectroscopy (XPS) with argon etching. The XPS depth profile of the carbon 1s and fluorine 1s photoelectrons in the transfer film revealed the existence of a unique microstructure. The film was probably heterogeneous: the PTFE content was on the topmost surface of the film, and the PEEK content was mainly inside the film. To gain a better understanding of this transfer film formation, a density functional theory simulation was performed, and the results indicated that PEEK is more likely to adsorb onto an aluminum surface than PTFE. The PEEK contents in the polymer blend apparently migrated to the counter aluminum surface prior to PTFE addition. Subsequent investigation of the tribological function of the transfer film by molecular dynamics simulation showed that the PEEK prevents detachment of the film and that the PTFE reduces friction by interlayer sliding. These findings clarify the tribological advantage of PTFE/PEEK polymer blends in terms of elucidating the structure and function of the transfer film. material because this lubricious film, despite its nanometricscale thickness,5 inhibits direct contact between the PTFE and metallic surface. Improving the mechanical properties of PTFE by several hard materials such as carbon particle is, of course, important for engineering use of PTFE. However, consideration should also be given to the tribochemical reaction of PTFE. Krick et al. reported that composites of PTFE and α-alumina nanoparticles produce wear rates which are extremely less than that of pure

1. INTRODUCTION Polytetrafluoroethylene (PTFE)-based composite resin is widely used as a bearing material for metallic parts in machines as it reduces friction and wear, eliminating the need to use a liquid-phase lubricant. To improve the wear resistance property of pure PTFE, several types of filler particles are typically added (carbon,1 glass fiber,2,3 molybdenum disulfide,3 aluminum oxide,4 and so on). PTFE and its composites form a transfer film on the counter metal surface during the friction process. Since the transfer film is mainly composed of PTFE, the original PTFE-based material contacts the thin PTFE film on the counter metal surface. Formation of this transfer film is important for reducing friction and wear of the PTFE-based © XXXX American Chemical Society

Received: March 26, 2017 Revised: June 6, 2017

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DOI: 10.1021/acs.jpcc.7b02860 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

structure and function of transfer films formed from polymer blends are still unclear. In the study reported here, we clarified the tribochemistry of polymer blends by analyzing the structure of the transfer film using X-ray photoelectron spectroscopy (XPS) with argon ion etching. Density functional theory (DFT) and molecular dynamics (MD) simulations were performed to investigate the detailed mechanism for the formation of transfer films from polymer blends. We studied PTFE/PEEK polymer blends because using this type of blend results in excellent tribological properties.12 The resultant tribological performance and structure of the transfer film were compared with those for pure PTFE.

PTFE.4,6 They concluded that the mechanism for this wear reduction is not a mechanical effect because the wear rate of PTFE/alumina composites is dependent on the humidity of the environment. This tendency suggested a tribochemical mechanism is responsible for the wear behavior of PTFE and its composites. Further investigation has been done by Pitenis et al.7 Several surface analyses showed that PTFE chains break due to the mechanical stresses at the wear surface and produce carboxylic acid end groups in humid air condition. These end groups can absorb onto the exposed metal on a counter surface and help to form the transfer film. Our previous studies using quantum chemistry simulation methods also showed that the formation of carboxylic acid end groups causes strong adherence to a metal counter surface and is effective to form transfer film.8,9 In order to further investigate low wear mechanism for PTFE composite, a microscopy measurement was strenuously done by Ye et al. for observing evolution of transfer film development in detail.10 A time evolution of morphology of the transfer film was discussed. In the running-in period the transfer film was removed each cycle, while nanoscale and oxidized fragments of PTFE (carboxylic acid end groups may be included) were transferred to the counterface in the low wear transition period. These fragments initiated the transfer film evolution by collecting trace material from the bulk of PTFE composite, growing into small islands. This small island then merged with a neighboring one to be an obvious transfer film. From their study, it can be suggested that the nature of the transfer film is critical to wear resistance of PTFE-based material and their tribochemistry is the key process to improving the quality of the transfer film. Thus, PTFE composites should be designed from the tribochemical point of view for considering the quality of the transfer film. As described above, it is well-known that PTFE composite is dependent on the humidity of the environment.4,6 Friction under a low-humidity condition significantly increases the amount of wear of PTFE because its chemical structure changes less; viz., there is less formation of carboxylic acid end groups. In an actual situation of engineering use of PTFE composites, they are often used in an extreme low-humidity condition such as sealing materials (piston ring) for a reciprocating dry gas compressor.11 In a low-humidity condition, formation of the transfer film is hardly activated due to less water contents which are effective to form carboxylic acid end groups. One way to improving the quality of transfer film formation even in a low-humidity condition is to construct a polymer blend using another engineering plastic because several polymer materials are endowed with polar groups which more easily adhere to counter metal surface than PTFE and thus help to form transfer film. Several such blends have been reported, including ones with polyetheretherketone (PEEK),12,13 polyimide (PI),14 and polyphenylenesulfide (PPS).15 It is well-known that these polymers drastically increase the antiwear performance of PTFE. As an example,12 PTFE/PEEK polymer blends showed a substantially lower specific wear rate (10−9 mm3/(N·m) order of magnitude at minimum) when slid along steel than both pure PTFE and PEEK. Although polymer blends thus effectively improve the antiwear performance of PTFE, the effect of polymer blending on the tribochemical effects (transfer film formation on counter metal surface) has yet to be reported so far. Moreover, the

2. METHODS 2.1. Material Preparation. Several bulk materials of PTFE/PEEK polymer blend were prepared using PTFE molding powder from Asahi Glass (Fluon G163) and PEEK molding powder from Daicel-Evonik (VESTAKEEP 2000 UFP20). Examination of scanning electron microscope (SEM) images (Figure 1) showed that the average particle

Figure 1. SEM images of (a) PTFE and (b) PEEK powders.

size of both materials was ∼20 μm (specified sizes were 25 and 20 μm, respectively). The two powders were blended using a mixer, which also broke apart the agglomerations of particles. Four sample types were prepared with different PEEK concentrations: 0, 20, 50, and 100 wt %. After mixing, the samples were compressively molded in a stainless steel cylinder using a laboratory press machine at room temperature and at a pressure over 60 MPa, which was maintained for 1 min. The compressed samples were removed from the machine, placed in an electric furnace, and sintered. The temperature inside the furnace was increased to around the melting temperature of PTFE (362 °C) at a rate of ∼50 °C/h. The maximum temperature of 375 °C was then maintained for 6 h. The furnace was then cooled down to room temperature at the same rate. Figure 2 shows the surface microstructure of the sample with a 20 wt % PEEK concentration. Note that the surface was finished using #2000 emery paper. A SEM image and energydispersive X-ray spectroscopy (EDX) mapping images for B


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Figure 2. SEM and EDX images of PTFE/PEEK polymer blend with 20 wt % PEEK concentration. For EDX, carbon and fluorine mapping images are shown.

carbon and fluorine are shown. Two domains are evident in the SEM image: a light gray part with a fibrous structure and a dark gray part with a flat surface. These two domains are clearly separated by ∼100 μm order of magnitude. Reference to the EDX images enabled identification of the fibrous and flat surfaces as PTFE (with fluorine) and PEEK domains (without fluorine), respectively. Therefore, both the PTFE and PEEK domains probably directly contacted the counter material when a contact pressure is applied. The durometer hardness (Shore D scale) and density for the four sample types prepared are shown in Figure 3. Measure-

2.2. Friction Test. The tribological performance of the samples was investigated by conducting a friction test using a pin-on-disk tribometer (Bruker UMT-3) with a rotary drive. A cylindrical pin made by one of the sample types was slid along a metallic disk, as schematically shown in Figure 4. Aluminum

Figure 4. Schematic illustration of pin-on-disk friction test setup.

(99% purity) was chosen as the metal counter material because it is used as a conventional material for frictional parts in many machines. In fact, several reciprocating gas compressors equip cylinders made of cast aluminum which is rubbed with PTFE composites as a piston ring. The diameter and length of the cylindrical pin were 3 and 20 mm, respectively. The pin was inserted into a designated holder with a 3 mm hole. Almost 2− 3 mm length of pin material was out of the holder. The diameter and thickness of the disk were 30 and 4 mm, respectively. The arithmetic average roughness (Ra) of the disk surface was 0.05 μm, and its micro-Vickers hardness was 40 HV, as measured by using a micro-Vickers hardness tester (Shimadzu HMV-G) with 1 N loading. Prior to the friction test, the resin sample was polished on the same pin-on-disk tribometer so as to make a condition of conformal contact. An emery paper (#1000) was fixed on dummy steel disk (diameter of 30 mm and thickness of 4 mm), and a pin sample was contacted with a normal load of 5 N. Then the disk was rotated at 100 rpm for 5 min duration. After this polishing, the emery paper used was replaced by that of #1500 and the same polishing process was applied again. In the friction test, a normal load of 10 N was applied, and the disk was rotated at 100 rpm. The contact pressure was 1.41 MPa. The equivalent sliding speed was 78.5 mm/s, and it was constant during the friction test (not elevated). The test was conducted for 180 min. Although a final goal of our investigation was to improving an antiwear performance of

Figure 3. Durometer hardness (Shore D scale) and density of PTFE/ PEEK polymer blends against PEEK concentration.

ments by durometer were done five times for each sample, and the density was measured for cylindrical pins in a friction test (see section 2.2). In the density measurement, the material volume was determined by carefully measuring the diameter and length of each sample within 10−2 mm order of magnitude. The average values for hardness and density are plotted in the figure with error bars. The hardness linearly increased with the PEEK concentration. The increasing number of PEEK domains on the surface of the polymer blend affected its mechanical response. There was also a linear relationship between the density and PEEK concentration, which is reasonable given that the density of pure PEEK is lower than that of pure PTFE. C


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The Journal of Physical Chemistry C the materials in a low-humidity condition, the test was done in ambient air (room temperature 20−25 °C; relative humidity 50−60%) as a first step of our investigation on the tribochemistry of polymer blends. Results of the friction test in a low-humidity condition will be reported in a future report. The sliding scar on the aluminum disk was analyzed using a XPS instrument (Ulvac-Phi PHI5600), in which a focused monochromatic Al Kα X-ray (14 keV) beam was scanned over an 800-μm-diameter spot. The photoelectron takeoff angle relative to the sample normal was 45°. 2.3. Simulation. As stated above, DFT and MD simulations were performed to investigate the formation of transfer films formed from polymer blends. In the DFT simulation, the DMol3 program implemented in Accelrys Materials Studio Version 4.3 was used. Double numerical basis sets with polarization and generalized gradient approximation in terms of Perdew−Burke−Ernzerhof (PBE) exchange-correlation functionals16,17 were used. The calculation models are described in section 3.2. Since directly observing the dynamic behavior of the sliding interface between resin and metal can provide important insights into behavior of the transfer film,8,9 MD simulation was performed using the LAMMPS18 code developed at the Sandia National Laboratory. The consistent valence force field (CVFF)19 was used to express the atomic and molecular interactions. The system temperature was controlled by scaling the velocity of atoms. The velocity Verlet algorithm20 was used to solve the equation of motion. Accelrys Materials Studio Visualizer was used to visualize the simulation models.

Figure 6. Wear rate of PTFE/PEEK polymer blends.

and the specific wear rate depended on the PEEK concentration, but the relationships were not linear. The 20 wt % sample had an average friction coefficient of 0.18, slightly lower than those of the 0 and 50 wt % samples (0.21 and 0.20, respectively). The 100 wt % sample had the highest friction coefficient (0.29). The range of friction coefficient obtained in the test with 0 wt % sample is in good agreement with that found in several papers that also discussed the friction between PTFE and metal.4,6,10 The 20 and 50 wt % samples had specific wear rates of 5.4 × 10−6 and 6.6 × 10−6 mm3/(N·m), respectively, which are close to the lowest specific wear rate. The 0 wt % sample had the highest specific wear rate (5.2 × 10−4 mm3/(N·m)). The 100 wt % sample had a specific wear rate of 1.6 × 10−5 mm3/(N·m), which was approximately three times larger than that of the 20 wt % sample. The differences in material wear were observed using an optical microscope. As shown by the image in Figure 7a, the sliding of the 0 wt % sample produced a substantial amount of fibrous debris around the scar. In contrast, the sliding of the 20 and 50 wt % samples produced less debris (Figures 7b and 7c). The 100 wt % sample apparently produced several types of debris (Figure 7d). Also observed in Figure 7 is that the scar was covered by transfer film. The 0 wt % sample created a stripe-like scar (Figure 7a). The dark and light parts in the stripe are attributed to differences in the thickness of the transfer film. In contrast, the 20 and 50 wt % samples produced a scar made with a uniform surface (Figures 7b and 7c). A transfer film with a constant thickness may have covered the surface. This formation of a uniform transfer film would result in the PTFE/PEEK polymer blends having better tribological properties than pure PTFE. The 100 wt % sample produced a completely different scar (Figure 7d). It had a discontinuous morphology, and scaly matter clung to the surface. Since this sample had a larger friction coefficient than the others and a high frequency noise was emitted during the friction test, stick− slip had apparently occurred, resulting in the formation of discontinuous transfer film. These friction test results demonstrate that the frictionreduction and antiwear performances of PTFE are improved by blending it with PEEK. This is consistent with an earlier study by Burris et al.12 although their friction test conditions (linear reciprocation, sliding speed of 50.8 mm/s, normal load of 250 N, and AISI 304 stainless steel as a counter material) were substantially different from our test conditions (rotation, sliding

3. RESULTS AND DISCUSSION 3.1. Tribological Performance of PTFE/PEEK Polymer Blends. The first step in this investigation was to evaluate the friction and wear performances of the PTFE/PEEK polymer blend samples by conducting a friction test using the pin-ondisk tribometer. The test was conducted three times for each sample to determine the reproducibility of the test results. Figures 5 and 6 show the steady friction coefficient and specific wear rate, respectively. The average values are plotted with error bars. Wear volume was calculated from the downward displacement of the cylindrical pin, which was continuously monitored by a displacement sensor (micrometer accuracy) equipped with UMT-3 tribometer, multiplied by bottom area of a cylindrical pin (3 mm diameter). Both the friction coefficient

Figure 5. Steady friction coefficient of PTFE/PEEK polymer blends. D


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Figure 7. Optical microscope images of scar on aluminum surface produced by sliding of pin composed of different PTFE/PEEK polymer blends with PEEK concentrations of (a) 0, (b) 20, (c) 50, and (d) 100 wt %.

speed of 78.5 mm/s, normal load of 10 N, and pure aluminum as a counter material). 3.2. Structure of Transfer Film. The friction test results suggest that the tribological performance of PTFE/PEEK polymer blends is affected by transfer film formation. The validity was investigated by examining the chemical nature of the transfer film for the 20 wt % sample, which had the lowest friction coefficient and wear rate. The atomic concentration in the depth direction of the transfer film on aluminum disk was analyzed using XPS with argon ion etching technique (etching rate of 9 nm/min for silica surface). XPS analysis was performed for the 0 and 20 wt % samples. Figure 8a shows the XPS depth profiles of Al 2p, O 1s, C 1s, and F 1s photoelectrons for the 0 wt % sample. The etching time as a horizontal axis corresponds to the depth position. Both fluorine and carbon existed on the topmost surface of the scar, meaning that PTFE transfer film formed during the friction process. Their atomic concentrations sharply decreased as the etching time was increased whereas those of the aluminum and oxygen from the original aluminum surface increased. This atomic distribution suggests that the PTFE transfer film was quite thin (at least 50 nm as calculated by the etching rate of silica). The depth profile was completely different for the 20 wt % sample (Figure 8b). Although both fluorine and carbon existed on the topmost surface of the scar, only the fluorine concentration sharply decreased as the etching time was increased. The carbon concentration gradually decreased in a deeper direction. Other elements (aluminum and oxygen) were also mixed in sublayer. The fluorine behavior indicates that the PTFE is mainly on the topmost surface of the transfer film. The carbon behavior indicates that PEEK from the polymer blend is mainly inside the transfer film, but aluminum and oxygen are also mixed. From the etching time when the atomic concentrations of Al 2p and O 1s are almost saturated, the thickness of the transfer film was determined to be over 300 nm (calculated on the basis of the silica etching rate). These results

Figure 8. XPS depth profile of Al 2p, O 1s, C 1s, and F 1s photoelectrons for samples with (a) 0 and (b) 20 wt % PEEK.

show that a gradient structure of the transfer film was constructed from the PTFE/PEEK polymer blends: PTFE E


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Figure 9. DFT-optimized models of (a) fluorocarbon, (b) diphenyl ether, and (c) benzophenone molecules adsorbed on γ-alumina (100) surface. Size of each surface area is a = 1.117 nm and b = 0.841 nm.

3.3. Function of Transfer Film. Use of XPS analysis and DFT calculation enabled identification of the unique microstructure of the gradient structure of transfer film formed from the PTFE/PEEK polymer blend. The tribological function of the transfer film was further investigated by MD simulation using the LAMMPS code to directly visualize the dynamic behavior during the friction process of the transfer film. The MD simulation model (Figure 10) consisted of three layers:

was on the topmost surface, and PEEK was mainly inside the film. A more detailed experiment such as cross-section analysis by SEM-EDX is required for fully determining the structure of transfer film. We now plan to conduct further analysis and report in our next paper. To clarify why the gradient structure of transfer film was formed in the polymer blend case, the adsorption energies of PTFE and PEEK on an aluminum surface were calculated by using a DFT method. The calculation model consisted of a small fragment molecule of PTFE or PEEK adsorbed on a γalumina (100) surface, which was used as a representative oxidized surface of aluminum in previous studies.21,22 The C5F12 fluorocarbon molecule was chosen as a fragment of PTFE. One fluorine atom in a C5F12 molecule was attached to an aluminum atom on a γ-alumina (100) surface. Similarly, diphenyl ether ((C6H5)2O) and benzophenone ((C6H5)2CO) molecules were used for the case of PEEK. Oxygen atoms (a polar part) in both molecules were placed above an aluminum atom on the γ-alumina (100) surface. The model geometry was optimized using the DMol3 program, and the electronic energy was obtained. The adsorption energy, EA, was calculated by the following equation: EA = ES/M − (ES + EM)

(1)

where ES is the energy of the γ-alumina (100) surface, EM is the energy of the fragment molecule of each polymer, and ES/M is the energy of the molecule adsorbed on the γ-alumina (100) surface. A larger negative value means stronger adsorption of the fragment molecule. The DFT-optimized structure of the adsorbing molecule is shown in Figure 9. Weaker adsorption of the fluorocarbon molecule was observed because the interatomic distance between the fluorine and aluminum molecules was greater than that for the other two molecules. Both the diphenyl ether and benzophenone molecules apparently adsorbed firmly due to strong interaction between their oxygen and aluminum atoms on the γ-alumina (100) surface. The resultant adsorption energies were −49.6, −248.1, and −284.1 kJ/mol for the fluorocarbon, diphenyl ether, and benzophenone, respectively. Adsorption of the PEEK fragment molecule (diphenyl ether and benzophenone) was more than five times stronger than that of the PTFE fragment molecule. These DFT calculation results indicate that the PEEK contents in PTFE/PEEK polymer blend preferentially adsorb onto a counter aluminum surface prior to PTFE adsorption. We think that this is one reason why gradient structure of transfer film formed from the PTFE/PEEK polymer blend.

Figure 10. MD simulation model consisting of crystalline PTFE, crystalline PEEK, and γ-alumina layers. PEEK layer directly contacted γ-alumina surface; PTFE layer is below PEEK layer.

crystalline PTFE, crystalline PEEK, and γ-alumina mimicking an aluminum surface. The PEEK layer directly contacted the γalumina surface on the basis of the results of the XPS analysis described in the previous section. The PTFE layer was below the PEEK layer, so it did not contact the γ-alumina surface. A periodic boundary condition was applied, and the system temperature was maintained at 300 K in the MD simulation. To simulate the friction condition, vertical pressure was applied to the topmost surface of the γ-alumina layer while it was slid at a certain horizontal velocity. A contact pressure of 500 MPa was applied, and the sliding was done parallel to each polymer chain at a certain velocity of 20 m/s to observe the sliding effects with a reasonable computation time. The bottom layer of the PTFE F


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The Journal of Physical Chemistry C was completely fixed. The MD simulation was performed for over 5 × 106 steps with an integration time of 0.1 fs. A snapshot of this friction simulation in the final MD step is shown in Figure 11. In this figure, the periodic boundary is not

were lower than those of pure PTFE and PEEK, respectively. The microstructure of the transfer film was investigated by XPS analysis with argon etching technique. Depth profiling of several element concentrations showed that the transfer film was probably gradient structure: PTFE was on the topmost surface of the film, and PEEK was mainly inside the film. Formation of this gradient structure was due to the difference in molecular adsorption onto the counter aluminum surface. DFT simulation for model compounds clearly showed that PEEK adsorption was energetically more likely to occur than that of PTFE. That is, the PEEK contents in the polymer blend preferentially transferred to the counter aluminum surface prior to PTFE addition. Investigation of the tribological function of the transfer film by using MD simulation showed that the PEEK prevents detachment of the transfer film and that the PTFE reduces friction. An advantage of using PTFE/PEEK polymer blends is thus to prevent detachment of the transfer film as well as lowering friction. This study has elucidated the mechanism of the excellent tribological performance of the PTFE/PEEK polymer blend by clarifying the microstructure and function of the transfer film. Use of PTFE/PEEK polymer blends would be a way to improving the quality of transfer film formation even in low-humidity condition. Results of friction tests for PTFE/PEEK polymer blends in a low-humidity condition will be reported in our next paper.

Figure 11. Dynamic behavior of PEEK and PTFE layers under friction condition. Periodic boundary is not considered to clarify understanding of atom motions.

considered to provide a clear understanding of atomic motion during the friction simulation. It is clearly evident in the figure that the overall PEEK layer tracked with the sliding γ-alumina. This happened because the PEEK polymer chain tightly adhered to the γ-alumina surface, as described in the previous section. Sliding inside the PEEK layer was not observed, possibly due to a strong molecular interaction between the PEEK polymer chains. Therefore, the PEEK in the gradient structure of PTFE/PEEK transfer film apparently functions to adhere to the film firmly, preventing the film from detaching from the counter surface. On the other hand, the top few layers of the PTFE polymer chain track with the γ-alumina and PEEK sliding motion, thereby producing interlayer sliding inside the PTFE layer. This sliding between the PTFE layers should be the source of the low friction, so the PTFE layer in the gradient structure of transfer film apparently functions to reduce friction. Interlayer sliding in a solid PTFE phase was also observed in another MD simulation using a model consisting of γ-alumina and PTFE without a PEEK layer (data not shown here). This MD simulation reported here revealed the tribological function of the gradient structure of transfer film formed from PTFE/PEEK polymer blend. This function prevents detachment of the transfer film by the PEEK contents inside the film and reduces friction by PTFE on the topmost surface of the film. In conclusion, the mechanism responsible for the excellent tribological performance of the PTFE/PEEK polymer blend samples observed in the friction test has been elucidated from the insights gained into the microstructure and function of the transfer film.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +81-294-525111. Fax: +81-294-52-7622. ORCID

Tasuku Onodera: 0000-0001-9126-5044 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by “Tohoku Innovative Materials Technology Initiatives for Reconstruction (TIMT)” funded by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) and Reconstruction Agency, Japan.

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REFERENCES

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4. CONCLUSION The tribochemistry of PTFE/PEEK polymer blends was investigated by using a conventional friction test coupled with XPS analysis and molecular-scale simulation, with the focus on the microstructure and function of the transfer film. The polymer blend was prepared by compression and calcination of mixed powders. In the friction test, cylindrical pins made of various polymer blends were slid along a pure aluminum disk. The friction coefficients and wear rates of the polymer blends G


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