Highly stable tribological performance and hydrophobicity of porous polyimide material filled with lubricants in a simulated space environment

Abstract

Space exploitation and development need high-performance materials for spacecraft so as to maintain the long service life and reliability of mechanical equipment. The purpose of the present study was to exploit a new material with durable life, stable friction coefficient and low wear rate in harsh space environments. Two kinds of solid–liquid synergetic lubricating composites have been prepared using perfluoropolyethers (PFPE) or chlorinated phenyl and methyl terminated silicone oil (CPSO) filled in porous polyimide (PPI). The tribological performance and hydrophobicity of the oil-filled PPI were evaluated by contact angle analyses and a ball-on-disk tribometer before and after proton irradiation in a simulated space environment. After proton irradiation, two composites can maintain stable hydrophobic performance. More importantly, the friction coefficients of CPSO/PPI and PFPE/PPI increased slightly from 0.07 and 0.05 to 0.1 and 0.14, respectively. The wear rates of CPSO/PPI and PFPE/PPI also increased slightly from 5.13 × 10⁻⁵ mm³ N⁻¹ m⁻¹ and 4.23 × 10⁻⁵ mm³ N⁻¹ m⁻¹ to 5.75 × 10⁻⁵ mm³ N⁻¹ m⁻¹ and 6.19 × 10⁻⁵ mm³ N⁻¹ m⁻¹, respectively. The CPSO/PPI composite showed the smallest change in hydrophobicity, friction coefficient and wear rate before and after proton irradiation. The mechanism of highly stable hydrophobicity and tribological performance was mainly based on a continuous self-healing surface; the stored oil in the pores of PPI can creep to the surface of the material to repair the damage induced by proton irradiation, which ensured that the material had stable and durable hydrophobicity and tribological properties in a proton irradiation environment.

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Introduction

The durability of a material in rigorous environments is important for the service life and reliability of mechanical equipment, especially for the assemblies applied to space exploitation and development.^1–5 Spacecraft carrying out their missions are affected by many complicated and changeable environmental factors, including thermal cycles ranging in hundreds of degrees centigrade, ultra-high vacuum, high-energy cosmic radiation particles, etc. Among them, protons, electrons, atomic oxygen and ultraviolet rays could severely erode the materials used in the spacecraft and then induce equipment failure, especially for proton radiation, which causes the worst damage to polymer materials.^6–11 Therefore, developing durable materials that can be applied to the extremely rigorous space environment is still a highly challenging task.

In recent years, considerable efforts have been dedicated to develop new materials for space applications, and mostly to studies on the single stable performance by mixing materials, composite films, etc. The incorporation of carbon nanotubes into a polyimide matrix resulted in enhanced thermo-optical properties in the space environment.⁵ Polyhedral oligomeric silsesquioxane incorporated into a Kapton-like polyimide showed favorable radiation resistance.^12,13 A Pb/PbS composite film could keep a stable low friction coefficient in vacuum conditions for a long time.¹⁴ Perfluropolyether-filled anodic aluminum oxide showed highly durable hydrophobicity in a simulated space environment.¹⁵ However, these materials could not completely meet the property requirements of spacecraft-specific materials.

A desired goal is that the materials have durable life, stable friction coefficient, low wear rate and durable hydrophobicity in harsh space environments.^16–20 In this context, the development of a simple and efficient strategy by solid–liquid synergetic lubrication, which would achieve this goal, would be a highly desirable and attractive alternative. Liquid lubricants coated on the surface of materials would be degraded in a space irradiation environment,^21–23 so are not necessarily the best choice. Herein, we designed a kind of material with a porous structure that can be used as a reservoir to store liquid lubricants. When the outermost oil molecules of the porous material are degraded by space irradiations, stored lubricants in the porous gaps of the material can creep and spread all over the surface of the material to maintain the self-healing lubrication, which provides a new strategy to construct a new type of material for the applications of space science.

Polyimide (PI), a type of important self-lubricating material, is widely applied in space science because PI exhibits superior friction and wear characteristics under vacuum.^24–27 In our previous work, the PI material had initially high friction coefficient and surface carbonization under a proton irradiation environment, which could affect the stability of moving parts.²⁸ A type of porous PI film with an ordered surface was also prepared, but the PI film could not be applied as a lubricating material because of poor mechanical strength.²⁹ On the basis of this work, a new PI composite material was designed by filling the liquid lubricants into the porous PI material to address the durable lubrication problems in irradiative environments. To the best of our knowledge, there has been no attempt via filling porous PI with lubricants to study the tribological behavior and hydrophobicity in an extremely rigorous space environment, thereby making it an important research topic.

In this paper, the porous polyimide (PPI) block material was prepared by cold press and sintering technology using polyimide powder, and then the PPI block was immersed in the liquid lubricants to obtain the oil-containing PPI material. The perfluoropolyethers (PFPE) and chlorinated-phenyl and methyl terminated silicone oil (CPSO) were selected as liquid lubricants owing to their excellent characteristics including very low volatility, high thermal resistance, non-flammability and low surface energy.^30,31 Then the PFPE and CPSO-filled PPI materials were subjected to a proton irradiation test in a simulated space environment because proton irradiation seriously affects the tribological behavior of the PI materials.^21,32 The hydrophobicity and tribological properties of the oil-filled PPI materials were evaluated by contact angle (CA) analyses and a ball-on-disk tribometer. Multiple characterization techniques were employed to investigate changes in the surface structures using attenuated total reflectance infrared spectroscopy (ATR-FTIR), micro-Raman spectroscopy and scanning electron microscopy (SEM).

Experimental

Materials

The PI powders used with a particle size of 400 mesh which have a density of 1.35 g cm⁻³ and a glass transition temperature of 250 °C, were purchased from Shanghai Research Institute of Synthetic Resins. The PPI material was prepared by cold press and sintering technology, during which the PI powders were cold pressed in a mold under a pressure of 30 MPa for 30 min and then sintered in an oven at 300 °C for 240 min to form a 20 × 12 × 15 mm³ block which was finally cut into 20 × 12 × 2 mm³ blocks for the irradiation and wear tests. The SEM picture of the cross section of the PPI sample is shown in Fig. 1. The prepared PPI samples were firstly dried under vacuum at 120 °C for 2 h and then quickly immersed in the selected lubricating oil for 24 h under vacuum at 120 °C to fully infiltrate into the inner pore, followed by wiping the oil on the surface with a cotton cloth. The liquid lubricants used were chlorinated phenyl and methyl terminated silicone oil (CPSO) and perfluoropolyethers (PFPEs, Fomblin M30), and the corresponding structural formulae are shown in Fig. 2. CPSO was supplied by Lanzhou Institute of Chemical Physics. PFPE was obtained from Solvay solexis. Inc. and used as received.

Fig. 1 SEM picture of a cross section of the PPI sample.

Fig. 2 The structural formulae of the CPSO (a) and PFPE (b).

Test methods

The oil-filled PPI was then put into a chamber of a space environment simulation facility at Lanzhou Institute of Chemical Physics to conduct proton (Pr) irradiation at 10⁻³ Pa (as shown in Fig. 3). 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 exposure time of Pr irradiation was controlled as 5 min which was selected due to Pr irradiation possessing higher energy for corroding organic compound molecules.^15,21

Fig. 3 A schematic diagram of the proton irradiation equipment.

The friction and wear behavior of the bare PPI and oil-filled PPIs before and after Pr irradiation against a GCr15 steel ball were tested on a ball-on-disk tribometer at a vacuum level of 3 × 10⁻⁴ Pa. The GCr15 steel ball with a standard 3 mm diameter has the chemical composition (in wt%) of 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 an Fe balance. The hardness and elastic modulus are 6.9 GPa and 208 GPa, respectively. The steel ball slid on the sample disk that rotated at a speed of 0.126 m s⁻¹ under the load of 5 N for 1800 s with a rotational diameter of 10 mm. Fig. 4 shows the calculation for the wear rate, where b and d, respectively, denote the width and the diameter of the wear track (10 mm), r refers to the radius of the steel ball, V is the wear volume loss of the PPI block (mm³), K (mm³ N⁻¹ m⁻¹) corresponds to the wear rate value, P is the applied load (N) and L is the sliding distance (m). In order to minimize the error, three specimens were tested under each condition to attain the average wear rate of samples.

Fig. 4 Formula for calculation of the wear rate.

Characterization

Contact angles (CA) were measured using a DSA-100 optical contact-angle meter (Kruss, Germany) at room temperature (20 °C) by injecting 5 μL double distilled water on the sample’s surface. Images were captured with a digital camera (Sony, Japan) and the average CA values were obtained by testing the same sample at five different positions. The infrared spectroscopic measurements were carried out on a Nexus 870 FTIR spectrometer (Nicolet, USA) using the attenuated total reflection (ATR) technique with a germanium crystal. A JEM-5600LV scanning electron microscope (SEM, JEOL, Japan) and an optical microscope were used to observe surface micrographs of the samples. The ingredients of the worn surfaces on the steel ball were analyzed by micro-Raman spectroscopy (LabRam HR800, Japan) at an excitation wavelength of 633 nm and by energy dispersive X-ray spectroscopy (EDS).

Results and discussion

Hydrophobicity

The CA of water was measured to investigate the hydrophobic performance of bare PPI and oil-filled PPI before and after Pr irradiation, and the results are presented in Fig. 5. The bare PPI, without any liquid lubricant, had a water CA of 90°. After impregnation with CPSO and PFPE liquid lubricating oils, the CA increased from 90° to 113° and 109°, respectively. After Pr irradiation for 5 min, it could be easily seen that the CA of the bare PPI, CPSO/PPI and PFPE/PPI increased to 112°, 116° and 120°, respectively. The above results indicated that filling PPI with the oils can enhance its hydrophobic properties. In addition, these new composites can better maintain stable hydrophobic performance after Pr irradiation compared with the bare PPI, especially for the CPSO/PPI sample.

Fig. 5 Contact angles of the bare PPI and oil-filled PPI before and after Pr irradiation.

Tribological performance

Friction coefficient and wear rate are the main indexes to evaluate tribological properties. The changes in friction and wear behaviors of PPI and oil-filled PPI against GCrl5 steel balls before and after Pr irradiation were comparatively investigated using a ball-on-disk tribometer. Fig. 6a displays the friction coefficient variations of the bare PPI, CPSO/PPI and PFPE/PPI before and after Pr irradiation. Before Pr irradiation, it can be seen that the friction coefficient of the bare PPI sample is the highest with an obvious fluctuation in the range of 0.26–0.29. In contrast, the friction coefficients of CPSO/PPI and PFPE/PPI remained respectively very stable at about 0.07 and 0.05, which is almost 1/4 and 1/6 of the friction coefficient of the bare PPI. After Pr irradiation, the friction coefficient curve of the bare PPI sample exhibited a drastic fluctuation, in which the initial friction coefficient increased to 0.34 and the steady friction coefficient decreased to 0.10. Whereas, the friction coefficients of CPSO/PPI and PFPE/PPI increased slightly from to 0.07 and 0.05 to 0.10 and 0.14, respectively, and the curve was still stable. The bar charts in Fig. 6b present the wear rates of PPI, CPSO/PPI and PFPE/PPI before and after Pr irradiation. The wear rates of all samples have different degrees of increment after Pr irradiation. For the bare PPI, the wear rate after Pr irradiation is 18.36 × 10⁻⁵ mm³ N⁻¹ m⁻¹, which is about 2.3 times that before Pr irradiation (8.06 × 10⁻⁵ mm³ N⁻¹ m⁻¹). In the case of CPSO/PPI, the wear rate after Pr irradiation is 5.75 × 10⁻⁵ mm³ N⁻¹ m⁻¹, which is about 1.1 times that before Pr irradiation (5.13 × 10⁻⁵ mm³ N⁻¹ m⁻¹). As for PFPE/PPI, the wear rate after Pr irradiation is 6.19 × 10⁻⁵ mm³ N⁻¹ m⁻¹, which is about 1.5 times that before Pr irradiation (4.23 × 10⁻⁵ mm³ N⁻¹ m⁻¹). Therefore, filling oil into PPI is effective in decreasing the friction coefficient and wear rate to achieve super low-friction and wear resistant materials. Meanwhile, the friction and wear behavior of the bare PPI is very sensitive to Pr irradiation, but the oil-filled PPI showed wonderful stability under a Pr irradiation environment. Especially for CPSO/PPI that exhibited the smallest change in friction and wear behavior before and after Pr irradiation.

Fig. 6 The friction coefficient variation (a) and the wear rate (b) of the bare PPI and oil-filled PPI before and after Pr irradiation.

Microstructural analysis

To investigate the reason for the durable and stable hydrophobicity and tribological properties of the oil-filled PPI under a Pr irradiation environment, the chemical structures, surface morphologies, and worn surfaces of the oil-filled PPI and steel ball were characterized by SEM, ATR-FTIR, Raman and EDS.

The surface morphologies of the PPI and oil-filled PPI were observed by SEM before and after Pr irradiation, and the results are shown in Fig. 7. Compared to the bare PPI, the surfaces of CPSO/PPI and PFPE/PPI are relatively smooth, indicating that both CPSO and PFPE completely covered the rough PPI surface. After Pr irradiation, the surface of the bare PPI was seriously eroded and became much rougher, but the CPSO/PPI and PFPE/PPI still kept relatively smooth surfaces.

Fig. 7 SEM surface morphologies of PPI and oil-filled PPI before and after Pr irradiations.

The changes in the chemical structures of the bare PPI and oil-filled PPI surface before and after Pr irradiation were investigated by FTIR-ATR, and the result is given in Fig. 8. It is clear that the bare PPI has the characteristic peaks at 1717 cm⁻¹ (C=O), 1498 cm⁻¹ (C=C), 1372 cm⁻¹ (C–N–C) and 1240 cm⁻¹ (C–O–C). When filled with CPSO or PFPE oil, except for the characteristic peaks of PPI, the characteristic peaks of the CPSO at 1255 cm⁻¹ (–CH₃), 1065 cm⁻¹ (Si–O–Si) and 795 cm⁻¹ (–Si–C–) and the characteristic peaks of PFEP at 1090 cm⁻¹ (C–O–C), 1194 cm⁻¹ (CF) and 1230 cm⁻¹ (CF₂) can be seen.^15,33 The chemical degradation of material induced by irradiation can be conveniently investigated by following the intensity changes of various FTIR peaks.³⁴ In addition, many research results showed that Pr irradiation could induce a chain scission reaction occurring in organic polymer materials and then the intensity of the characteristic peaks may reduce obviously.^33,35–37 In this test, after Pr irradiation, the intensity of the characteristic peaks for PPI in both bare PPI and oil-filled PPI decreased greatly. In contrast, the intensity of the characteristic peaks for both CPSO and PFPE oil reduced very slightly, which is not consistent with evidence in the reference that the Pr irradiation can seriously erode the oil molecules. The reason for this inconsistency is that the stored oil molecules in the porous structure of PPI tend to spread out from the pore structure to the top of the surface of PPI due to the low surface energy.³⁸

Fig. 8 FTIR-ATR spectra of PPI and oil-filled PPI before and after Pr irradiation.

The worn surfaces of the steel ball were analyzed in detail by SEM, Raman and EDS, and the results are presented in Fig. 9. For the bare PPI, there was an accumulated continuous transfer layer presented on the steel ball surface before and after Pr irradiation (Fig. 9a). As shown in Fig. 9b, the Raman spectrum of the transfer layer of the bare PPI before irradiation did not have specific peaks, which indicated that the composition of the transfer layer was considered as polymer-like carbon. On the other hand, the spectrum of the transfer layer of the bare PPI after irradiation displayed two broad peaks at 1357 cm⁻¹ and 1580 cm⁻¹ that correspond to the D and G bands of disordered and ordered graphite, which indicated that a graphite-like structure was formed in the transfer layer. In contrast, there was no continuous transfer layer on the steel ball surface of the oil-filled PPI before and after Pr irradiation (Fig. 9a). In addition, no characteristic peaks in the Raman spectra were observed most likely because the changes that occurred on the steel ball were out of the detection limit of the applied Raman system. Moreover, EDS was used to analyze the elements on the steel ball. As shown in Fig. 9c, the elements Si and F were detected on the steel ball surfaces of the oil-filled PPI before and after Pr irradiation, indicating that the steel ball surface of the oil-filled PPI contained lubricating oil which can prevent the solid-to-solid contact and then reduce the friction coefficient and wear rate.

Fig. 9 (a) SEM images of the steel ball worn surfaces for PPI and oil-filled PPI before and after Pr irradiation, (b) the corresponding Raman spectra and (c) the corresponding EDS spectra.

According to the above analysis of the results, it is easy to deduce the mechanism of the stability and durability of the hydrophobicity and tribological performance of the oil-filled PPI under Pr irradiation, and the schematic is illustrated in Fig. 10. After the Pr irradiation in a simulated space environment, the bare PPI has a significant change in the hydrophobic properties and tribological performance because Pr irradiation erodes the surface structure. Though the surface of the oil-filled PPI also suffered some damage, the stored oil molecules in the porous structure of PPI tend to creep on the surface of PPI, especially for the CPSO oil, due to the lower surface energy. Therefore, the ability to withstand Pr irradiation damage was mainly based on the continuous self-healing of the surface as a result of the stored oil in the porous structure of PPI, and the lubricant oils could spread out from its micropores onto the contact surface to form a homogeneous and plain oil film which enhanced the hydrophobicity and reduced its friction coefficient and wear rate. Meanwhile, a balance may be maintained between the irradiation damage and self-healing of the oil-filled PPI, which ensured that the material had stable and durable hydrophobicity and tribological properties in the Pr irradiation environment.

Fig. 10 Schematic illustration explaining the possible self-healing mechanism of oil-filled PPI under Pr irradiation.

Conclusions

In sum, two kinds of PPI composite materials were designed and fabricated by filling PFPE or CPSO oil into the PPI pores. Oil-filled PPI showed highly stable and durable hydrophobicity and tribological performance compared with the bare PPI, especially for the CPSO/PPI composite which had the smallest changes in hydrophobicity, friction coefficient and wear rate before and after Pr irradiation. Combined with the characterizations of SEM, ATR-FTIR, Raman and EDS, Pr irradiation could induce surface damage of all the test samples. The graphite-like substance was formed on the bare PPI surface, but the CPSO/PPI and PFPE/PPI still kept relatively smooth surfaces. These excellent performances of the oil-filled PPI in a simulated space environment were mainly attributed to the stored oil in the pores of the PPI which tends to spread all over the PPI surface to maintain the self-healing hydrophobicity and lubrication.

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 Chinese Academy of Sciences (CXJJ-14-M43). And the authors would like to thank Mr Jiazheng Zhao for his assistance in SEM observation.

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