Effects of individual and sequential irradiation with atomic oxygen and protons on the surface structure and tribological performance of polyetheretherketone in a simulated space environment

Abstract

The changes in the surface structure and the tribological performance of polyetheretherketone (PEEK) induced by individual and sequential irradiations with atomic oxygen (AO) and protons (Prs) were investigated in a space environment simulation facility. The experimental results showed that Pr irradiation induced the surface carbonization of PEEK which induced the greatest degree of decrease in the surface roughness from 29.61 nm to 16.15 nm, surface energy from 49.16 mJ m⁻² to 46.96 mJ m⁻², friction coefficient from 0.28 to 0.08 and wear rate from 10.28 × 10⁻⁵ mm³ N⁻¹ m⁻¹ to 5.45 × 10⁻⁵ mm³ N⁻¹ m⁻¹. The AO irradiation induced the surface oxidation of PEEK, and then increased the surface roughness from 29.61 nm to 58.77 nm, surface energy from 49.16 mJ m⁻² to 73.75 mJ m⁻², friction coefficient from 0.28 to 0.35 and wear rate from 10.28 × 10⁻⁵ mm³ N⁻¹ m⁻¹ to 18.22 × 10⁻⁵ mm³ N⁻¹ m⁻¹. The surface structural variations and tribological performance of PEEK induced by the sequential Pr–AO and AO–Pr irradiations were respectively similar to the results of the individual AO and Pr irradiations, and the final form of irradiation has a bigger effect on the changes in the surface structure and tribological performance during the sequential irradiation tests. The erosion stacking effect of the sequential irradiations was observed, and the AO–Pr irradiations caused the biggest changes in infrared spectra and the surface composition of C and O elements in X-ray photoelectron spectroscopy. The Pr–AO irradiations gave the biggest increment in the surface energy from 49.16 mJ m⁻² to 74.03 mJ m⁻² and wear rate from 10.28 × 10⁻⁵ mm³ N⁻¹ m⁻¹ to 24.07 × 10⁻⁵ mm³ N⁻¹ m⁻¹.

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Introduction

It is known that polymer materials are widely applied in friction materials as critical moving assemblies of satellites and spacecraft in space systems, owing to their good mechanical performance, low weight, high wear resistance, easy manufacturing processes, resistance to irradiation, self-lubrication properties and chemical inertness.^1,2 Polyetheretherketone (PEEK) as one of the high performance engineering thermoplastic polymers has attracted increasing interest due to its attractive physical and chemical properties, such as excellent mechanical properties, good chemical resistance and high long-term working temperatures, and is considered to be one of the most prospective polymers for biomedical applications, the automotive industry, electronics and spacecraft design among various polymers.^3–7

The cosmic space has many rigorous environmental factors including high vacuum, thermal cycles, ultraviolet rays, atomic oxygen, proton irradiation, electron irradiation and so on, which could severely affect the service life and reliability of mechanical equipment.^8–10 Therefore the materials used in satellites or spacecraft that can defend against a rigorous space environment are in higher demand. However, another important research subject on the damage to polymer materials in a cosmic space environment has also induced broad interest.

In our previous work, the changes in the surface structure and tribological performance of polytetrafluoroethylene, polyimide, phenolphthalein poly(ether sulfone) and their composites enforced with various fibers or nano-oxides have been investigated under a single form of irradiation with protons (Prs), electrons, atomic oxygen (AO) or ultraviolet rays in a simulated space environment.^11–15 With the design improvements of materials, a kind of porous polyimide material with a highly stable tribological performance and hydrophobicity in a simulated space environment has been reported.¹⁶ The experimental results showed that Pr or AO irradiation induced more effects on the surface performance of these polymers compared to other forms of single irradiation, especially on the tribological performance, and the order of irradiations have a big effect on the tribological performance and surface properties of the polymer materials. PEEK is an important space material and the changes in the surface structure and tribological performance in a simulated space environment are particularly important to design airspace parts. Moreover, the AO and Pr irradiations as the most destructive irradiations may result in a different impact on the properties of PEEK. To the best of our knowledge, there has been no attempt to investigate the effect of sequential irradiation with AO and Prs on polymer materials, which would make a very valuable research contribution. More importantly, the study of sequential irradiation on polymer materials is very meaningful for both basic research and practical applications. On the one hand, the irradiation damage mechanism of polymer materials can be further revealed using sequential irradiation. On the other hand, it contributes to improving the service reliability and lifetime when designing a spacecraft.

In this paper, the effect of individual and sequential irradiations with Prs and AO on the surface properties and tribological performance of PEEK were investigated in the ground simulation facility. The changes in the surface structure before and after irradiations were detected using attenuated total reflectance infrared spectroscopy (ATR-FTIR), X-ray photoelectron spectroscopy (XPS), contact angle measurements and X-ray diffraction (XRD). The changes in the tribological performance of PEEK before and after irradiations were investigated using a ball-on-disc tribometer that was used for the tribological test because of its low cost, small space and the ease of handling little samples. The morphologies of the wear track were observed using scanning electron microscopy (SEM).

Experimental

Materials

The PEEK powder (450P, molecular structure shown in Fig. 1) used in this study was supplied by Victrex (Lancashire, UK). The density of PEEK is 1.32 g cm⁻³, and its glass transition temperature, the melting point, and the decomposition temperature are 143, 334 and 590 °C, respectively. PEEK powders were pressed in a mold, heated to 375 °C, and held at 20 MPa for 120 min to form a 50 × 60 × 8 mm³ block. The thermoformed PEEK was then cut into 18 × 18 × 2 mm³ blocks for the irradiation and wear tests. Every sample surface was polished carefully to the roughness R_a ≤ 0.2 μm. All the samples were cleaned with ultrasonication in acetone before the irradiation test.

Fig. 1 The chemical repeating unit of PEEK.

Irradiation test

The experiments of AO and Pr irradiations were performed in a space simulation facility at the Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences. The basic principle schematic illustration of the irradiations is referred to in our previous papers.^17,18 As for the AO irradiation, a microwave power source with an electron convolute resonance technique was used to excite O₂ to produce oxygen plasma which would become a beam and be accelerated towards a molybdenum plate with a negatively charged electric field. When colliding with the plate the accelerated positive oxygen ions are neutralized by the negative charges and rebounded to form a neutral AO beam with a mean kinetic energy of about 5 eV that is similar to the energy of the AO impinging on the surface of a spacecraft in a space environment.^19,20 The flux of the AO beam was determined to be 6.0 × 10¹⁵ atoms per cm² per s using the standard method of Kapton mass loss.^19,21 The Pr irradiation was carried out at an accelerative voltage of 25 kV and the flux of protons was determined to be about 6.25 × 10¹⁵ ions per cm² per s. The tests of AO and Pr irradiations were performed individually and sequentially. The individual irradiation time of AO and Pr irradiations was controlled for about 180 min and 5 min, respectively. For Pr irradiation, 5 min was selected due to the Pr possessing higher energy for corroding polymer molecules compared with AO.²² The experimental procedure for the sequential irradiation is that the sample was first irradiated with Prs for 5 min and then with AO for 180 min (or AO for 180 min and then Pr irradiation for 5 min).

Surface characterization of PEEK

The infrared spectroscopic measurements of the PEEK samples before and after irradiations were performed using a Nexus 870 FTIR spectrometer (Nicolet, America) using an attenuated total reflection accessory (ATR) technique with a germanium crystal. The surface chemical composition before and after irradiations was analyzed using an ESCALAB 250Xi X-ray photoelectron spectroscopy instrument (ThermoFisher, America). All spectra were acquired using an Al-Kα X-ray source (1391 eV) with a binding energy range of 0–1400 eV. All binding energies were referenced to the C 1s hydrocarbon peak at 284.6 eV. Contact angle measurements were performed using the static sessile drop method using a DSA-100 optical contact-angle meter (Kruss Company Ltd, Germany) at room temperature (25 °C). The average contact angle values were obtained by measuring the same sample at five different positions with 5 μL of double distilled water or diiodomethane. Images were captured with a Sony Digital Camera (Sony Ltd, Japan). The total surface energy and its polar and dispersive components were calculated using the method of Owens and Wendt.²³ The surface morphologies and the width of the wear track were observed using a JEM-5600LV scanning electron microscope (JEOL, Japan). The average roughness (R_a) of samples was measured using a MicroXAM 3D non-contact surface mapping profiler (ADE Corporation, America). The three-dimensional (3D) images and the root-mean-square roughness (RMS) values of the samples before and after irradiations have been acquired using a scanning probe microscope integrated in a Hysitron Triboindenter TI-950 system (Hysitron, America).

Friction and wear test

The friction and wear behaviors of PEEK before and after Pr irradiation against a GCr15 steel ball were tested using a ball-on-disk tribometer in a vacuum level of 3 × 10⁻⁴ Pa. The GCr15 steel ball has a standard 3 mm diameter with the chemical composition (in wt%): Mn (0.20–0.40), Si (0.15–0.35), Cr (1.30–1.65), C (0.75–0.85), P (≤0.026), S (≤0.020) and Fe to balance. The steel ball slid on the sample disk that rotated at a speed of 0.126 m s⁻¹ under the load of 0.5 N for 1800 s with a rotational diameter of 12 mm. The corresponding starting stress level is about 100 MPa using Hertz-contact formulae. The calculation for the wear rate is shown in Fig. 2, where b and d refer to the width and the diameter of the wear track (12 mm), respectively, r denotes the radius of the counterpart steel ball, V corresponds to the wear volume loss (mm³), K (mm³ N⁻¹ m⁻¹) is the wear rate value, L is the sliding distance (m) and P is the applied load (N). In order to minimize the error, three specimens were tested under each condition to get the average wear rate of the samples.

Fig. 2 Calculation formulae of the wear rate.

Results and discussion

Surface morphologies

The surface morphologies of PEEK samples before and after individual and sequential irradiations with AO and Prs were studied using SEM, and the results are given in Fig. 3. The surface morphologies of the untreated and Pr irradiated PEEKs were relatively flat in Fig. 3a and b. However, the surface morphologies of AO, Pr–AO and AO–Pr irradiated PEEK had significantly changed and exhibited a ‘blanket-like’ structure shown in Fig. 3c–e. In order to get a precise analysis of the surface roughness, the 3D images and RMS before and after individual and sequential irradiations with AO and Prs are presented in Fig. 4. The surface roughness of the PEEK obviously decreased from 29.61 nm to 16.15 nm after Pr irradiation, while the surface roughness of the PEEK significantly increased from 29.61 nm to 58.77 nm after AO irradiation, which could be ascribed to the numerous larger short cones formed on the surface. In comparison with the analytical results, the changes in the surface morphologies of PEEK induced by the AO and Pr irradiations were opposite as Pr irradiation made the PEEK surface become smooth, whereas AO irradiation made the PEEK surface become coarser. Thus, the surface roughness of PEEK changed little in the sequential AO–Pr and Pr–AO irradiated experiments compared to that of untreated PEEK. However, the short cones became smaller and denser probably due to the combined effect of the AO and Pr irradiations and the different erosion mechanism.

Fig. 3 SEM images of PEEK specimens before and after irradiations: (a) untreated PEEK, (b) Pr, (c) AO, (d) Pr–AO, (e) AO–Pr.

Fig. 4 3D images (5 μm × 5 μm) and RMS of PEEK specimens before and after irradiations: (a) untreated PEEK, (b) Pr, (c) AO, (d) Pr–AO, (e) AO–Pr.

XRD analysis

PEEK is a semi-crystalline thermoplastic polymer. The effect of irradiations on the crystallinity variation of PEEK was investigated using an XRD technique, and the results are shown in Fig. 5. The four distinct peaks were observed at about 18.81°, 20.81°, 22.80° and 28.80° and can be assigned to the (110), (111), (200) and (211) planes of crystallized PEEK, respectively, which indicated that PEEK mainly exhibited an orthorhombic crystalline form.^24,25 In addition, the sharp and diffuse patterns of PEEK were characteristic of semi-crystalline polymers. All the diffraction peak positions did not shift after individual and sequential irradiations with AO and Prs, signifying that the lattice parameters did not change, which is due to the fact that the Pr and AO irradiations only lead to the degradation of the outmost surface of the polymer material.^17,26 The microscopic changes on the PEEK surface were not observed using XRD, which shows that the irradiations could not affect the performance of the main body materials.

Fig. 5 The XRD profiles of the PEEK specimens before and after irradiations.

ATR-FTIR spectra

The changes in the chemical structure of the PEEK surface induced by individual and sequential irradiations with AO and Prs were studied using ATR-FTIR, and the results are presented in Fig. 6. The characteristic peaks of the original PEEK were at 1651 cm⁻¹ due to the C=O stretching vibration, 1598, 1490 and 1413 cm⁻¹ due to the aromatic skeletal vibration, 1306 cm⁻¹ due to the bending motion of C–C(=O)–C, 1280 and 1187 cm⁻¹ due to the asymmetric stretching of C–O–C, 1157 and 1103 cm⁻¹ due to a number of aromatic hydrogen in-plane deformation bands, 927 cm⁻¹ due to the diphenyl ketone band, and 860 and 841 cm⁻¹ due to the out of plane bending modes of the aromatic hydrogens, which was consistent with the previous reports of this material.^27,28 After individual and sequential irradiations with AO and Prs, the intensity of these characteristic peaks for the PEEK samples decreased to a different degree which indicated that the irradiations induced a different level of breakage of the molecular chains of the PEEK samples, and a complex chemical reaction may take place during the irradiation process. During all the irradiated conditions, AO–Pr irradiation caused the signal peaks of PEEK to nearly disappear, which indicated that performing AO irradiation first and then Pr irradiation could give rise to the worst erosion on the PEEK surface, and this case also confirmed that the stacking effect could happen with multiple forms of irradiation.

Fig. 6 The ATR-FTIR spectra of the PEEK specimens before and after irradiations.

Changes in the surface chemical composition

In order to reveal the possible chemical reaction occurring on the PEEK surface during various irradiation processes, the changes in the surface chemical composition induced by individual and sequential irradiations with AO and Prs were studied using XPS and the corresponding results are summarized in Table 1. The untreated PEEK has a composition of C 77.64% and O 21.89%. The surface composition has experienced obvious changes after different irradiations in this test. It is found that the surface concentrations of the C element were respectively increased to 80.56% and 80.64% whereas the concentrations of the O element were respectively decreased to 19.43% and 19.36% after individual Pr irradiation and sequential AO–Pr irradiations. However, the surface concentrations of the C element were respectively decreased to 66.47% and 67.44% whereas the concentrations of the O element were respectively increased to 33.53% and 31.96% after individual AO irradiation and sequential Pr–AO irradiations. These results indicated that both the individual Pr irradiation and sequential AO–Pr irradiations could lead to the surface carbonization of PEEK, while the individual AO irradiation and sequential Pr–AO irradiations could result in the surface oxidation of PEEK. As shown by the chemical structure of PEEK in Fig. 1, there are three different carbon environments: (1) carbon atoms in the aromatic rings (284.72 eV), (2) carbon atoms in the C–O groups (286.40 eV), and (3) carbon atoms in the carbonyl groups (288.94 eV). Fig. 7 depicts the high resolution (HR) C 1s XPS spectra of PEEK before and after the irradiations. The relative content of C around 284.72 eV increased during the Pr and AO–Pr irradiation processes, which may be due to the chain scission occurring during the irradiations to form the amorphous carbon species in the surface of the PEEK and the irradiation region was also changed into black. The relative content of C in the C–O groups obviously increased during the AO and Pr–AO irradiation processes, which indicated that the molecular chains of PEEK were destroyed by the oxidation reaction to produce some oxygen-containing functional groups. Moreover, a tendency is that the latter irradiation would occupy a dominant effect during the sequential irradiations. The results of the two forms of irradiations showed that sequential irradiations induced more of an influence than individual irradiation, from the changes in the elements C and O, and the AO–Pr irradiation process caused the worst erosion, which is consistent with the results of ATR-FTIR.

Table 1 The surface composition of the PEEK specimens before and after irradiations using XPS

Specimens	Surface composition (at%)
	C	O
Untreated PEEK	77.64	21.89
Pr irradiated	80.56	19.43
AO irradiated	66.47	33.53
Pr–AO irradiated	67.44	31.96
AO–Pr irradiated	80.64	19.36

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Fig. 7 The HR C 1s spectra of the PEEK specimens before and after irradiations.

Changes in the surface energy

The surface energy has different effects on the sliding friction behavior of the polymer.^29,30 Some researchers have shown that ion irradiation could affect the surface energy of the polymer.^31,32 In order to evaluate the changes in the surface energy of the PEEK samples, contact angle tests were carried out using a static sessile drop method. The changes in the contact angles of PEEK in water and diiodomethane are shown in Fig. 8a before and after individual and sequential irradiations with Prs and AO. Meanwhile, the changes in surface energy were determined by the evaluation of contact angles according to the method of Owens and Wendt, and the calculated values of the total surface energy (γ_tot) and its polar (γ_polar) and dispersive components (γ_disp) are presented in Fig. 8b. It can be seen that untreated PEEK has a surface energy of 49.16 mJ m⁻² which is very close to the dispersive component because of the negligible polar component. After the individual Pr irradiation and sequential AO–Pr irradiations, the total surface energy decreased slightly to 46.96 and 48.20 mJ m⁻² which may be attributed essentially to the formation of a carbonized layer on the PEEK surface. After the individual AO irradiation and sequential Pr–AO irradiations, the total surface energy obviously increased to 73.75 and 74.03 mJ m⁻² which was mainly attributed to the great increment of the polar component due to the formation of oxygen-containing functional groups. The individual and sequential irradiations induced different changes in the contact angles and surface energies, and the changing trends were in accordance with the results of the latter irradiation. These results indicated that the changes in surface energy were related to the changes in the surface composition.

Fig. 8 The changes in the contact angles (a) and the total, polar and dispersive surface energies (b) of the PEEK specimens before and after irradiations.

Friction and wear properties

The friction coefficients and wear rates of a material are the key parameters for the evaluation of tribological performance. The influence of the irradiation environment on the friction and wear properties of the PEEK samples was investigated using a ball-on-disk tribometer. The friction coefficient variations of PEEK before and after the irradiations are displayed in Fig. 9a. The friction coefficient of the untreated PEEK sample is stable at around 0.28. After individual Pr irradiation, the friction coefficient obviously decreased to 0.08, which is about 3.5 times lower than that of the untreated PEEK. After individual AO irradiation, the friction coefficient increased to 0.35, that is around 1.25 times higher than that of the untreated PEEK. It is worth noting that the friction coefficient of PEEK after the sequential AO–Pr irradiations also significantly decreased to 0.08 and after the sequential Pr–AO irradiations, the coefficient increased to 0.31. The bar charts in Fig. 9b display the wear rates of PEEK before and after irradiations. The wear rate of the untreated PEEK was 10.28 × 10⁻⁵ mm³ N⁻¹ m⁻¹. After the individual Pr irradiation, the wear rate obviously decreased to 5.45 × 10⁻⁵ mm³ N⁻¹ m⁻¹, which is about 1.89 times lower than that of the untreated PEEK. After the individual AO irradiation, the wear rate increased to 18.22 × 10⁻⁵ mm³ N⁻¹ m⁻¹, that is about 1.77 times higher than that of the untreated PEEK. Similarly, the wear rate of PEEK after the sequential AO–Pr irradiations decreased to 6.89 × 10⁻⁵ mm³ N⁻¹ m⁻¹ and that after the sequential Pr–AO irradiations increased to 24.07 × 10⁻⁵ mm³ N⁻¹ m⁻¹. All the above results indicated that both the individual Pr irradiation and sequential AO–Pr irradiations could obviously decrease the friction coefficient and wear rate of PEEK. Both the individual AO irradiation and the sequential Pr–AO irradiations could induce an increase in the friction coefficient and wear rate of PEEK, and the Pr–AO irradiations have the biggest impact on the wear rate of PEEK, which also confirmed that the latter irradiation would play leading roles regarding the friction and wear properties of the material. Moreover, the changes in the tribological properties induced by the individual and sequential irradiations with AO and Prs were consistent with the surface energy and not the surface roughness.

Fig. 9 The friction coefficients (a) and wear rates (b) of the PEEK specimens before and after irradiations.

The damage to the PEEK blocks after the friction test before and after irradiations were further investigated using SEM, and the results are shown in Fig. 10. The low magnification images (Fig. 10a–e) mainly present the wear track whereas the high magnification images (Fig. 10f–j) focus on the worn surface. The worn surface of the untreated PEEK was relatively smooth and mainly presented plastic deformation (Fig. 10a and f). The individual Pr irradiated PEEK had a relatively small wear track width with obvious grooves (Fig. 10b and g), which is because the carbon-enriched wear debris during the friction process could introduce three-body abrasive wear arising from their high hardness and small size, and three-body abrasion could reduce the wear rate and friction coefficient. The individual AO irradiated PEEK displayed a relatively larger wear track width with a more serious plastic deformation (Fig. 10c and h), indicating that the AO irradiation aggravated the friction and wear, which displayed the higher friction coefficient and wear rate by AO irradiation.

Fig. 10 SEM micrographs of the typical wear scars seen under low (a–e) and high (f–j) magnification: (a and f) PEEK, (b and g) Pr, (e and h) AO, (d and i) Pr–AO, (e and j) AO–Pr.

The sequential Pr–AO irradiated PEEK showed a similar wear morphology to that of the AO irradiated PEEK except for the difference of the more severe plastic deformation (Fig. 10d and i), which indicated that the sequential Pr–AO irradiations aggravated the friction and wear compared to the individual AO irradiation. Compared with the individual Pr irradiated sample, the sequential AO–Pr irradiated PEEK exhibited a similar width of the wear track, but displayed quite a different aspect by the production of massive flaky debris instead of the grooves (Fig. 10e and j), which indicated that the sequential AO–Pr irradiation aggravated the friction and wear compared to the individual Pr irradiation.

Conclusions

In this article, the effects of individual and sequential irradiations with AO and Prs on the surface structure and tribological performance of PEEK were evaluated using various characterization techniques. The individual Pr irradiation decreased the surface roughness of PEEK, but the individual AO irradiation greatly increased the surface roughness of PEEK. The sequential AO–Pr or Pr–AO irradiations had little effect on the surface roughness of PEEK. The results of XRD and ATR-FTIR indicated that individual and sequential irradiations with AO and Prs only led to the degradation of the outmost surface of the PEEK material. The individual Pr and sequential AO–Pr irradiations resulted in the carbonization of the PEEK surface, which decreased the surface energy, friction coefficient and wear rate. Compared with the individual Pr irradiation, the sequential AO–Pr irradiations displayed the higher surface energy, friction coefficient and wear rate. The individual AO and sequential Pr–AO irradiations led to surface oxidation, which caused an increment of the surface energy, friction coefficient and wear rate. Compared with the individual AO irradiation, the sequential Pr–AO irradiations presented the higher surface energy, friction coefficient and wear rate. The effect of the sequential irradiation with Pr and AO on the surface structure and performance of PEEK was determined by the final form of irradiation.

Acknowledgements

The authors would like to acknowledge the financial support of the National Basic Research Program of China (973 Program, Grant No. 2015CB057502) and the National Defense Innovation Fund of the Chinese Academy of Sciences (CXJJ-14-M43).

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