MoS2 coatings for simultaneously improved

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General Research

POSS grafted PAI/MoS2 coatings for simultaneously improved tribological properties and atomic oxygen resistance Chuanyong Yu, Pengfei Ju, Hongqi Wan, Lei Chen, Hongxuan Li, Huidi Zhou, and Jianmin Chen Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b02439 • Publication Date (Web): 16 Aug 2019 Downloaded from pubs.acs.org on August 20, 2019

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Industrial & Engineering Chemistry Research

POSS grafted PAI/MoS2 coatings for simultaneously improved tribological properties and atomic oxygen resistance Chuanyong Yua, b, Pengfei Juc, Hongqi Wana, Lei Chen a, b*, Hongxuan Lia, b*, Huidi Zhoua, b, and Jianmin Chena, b aState

Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics,

Chinese Academy of Sciences, Lanzhou 730000, P. R. China bCenter

of Materials Science and Optoelectronics Engineering, University of Chinese

Academy of Sciences, Beijing 100049, P. R. China cShanghai

Aerospace Equipment Manufacture, Shanghai 200245, P. R. China

Corresponding author： Lei Chen，[email protected]； Hongxuan Li，[email protected]

Abstract: Polyamide−imide (PAI) is extensively used onboard spacecraft mainly as surface protective material owing to its inherent excellent mechanical strength, chemical inertness, irradiation resistance，high-temperature stability, and high wear resistance. Nevertheless, the PAI in the low earth obit (LEO) is severely eroded by the presence of atomic oxygen (AO). To solve this problem, in this work, the octa- and mono-amino polyhedral oligomeric silsesquioxane (OMPOSS) were incorporated into the PAI matrix by covalent grafting, and the lubricating coatings were prepared using molybdenum disulfide (MoS2) as the solid lubricant. Additionally, the effects of AO exposure on pure PAI/MoS2 and POSS reinforced PAI/MoS2 coatings were investigated. More importantly, the results indicated that the coatings exhibited outstanding tribological properties and anti-atomic oxygen performance owning to the addition of octa-amino POSS (OPOSS) monomers into the main chain and mono-amino POSS (MPOSS) as the end-capping reagent of the polymer. The cooperation of OPOSS and

ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

MPOSS significantly improve the AO resistance result from the formation of a connected and networked silicon dioxide passivating layer on the surface. As a result, the PAI based lubricating coatings modified by OMPOSS possessing excellent tribological properties and anti-atomic oxygen performance would be a novel spacesuitable material in LEO. Keywords: polyhedral oligomeric silsesquioxane; low earth orbit; atomic oxygen resistance; wear resistance 1. Introduction Because of their remarkable performances, such as lightweight, chemical stability, high mechanical strength and good radiation resistance, polymers are extensively used in low Earth orbit (LEO) to meet with the requirements for lubrication, protection and other issues [1-3]. Especially, because of its excellent tribological properties of molybdenum disulfide (MoS2) in the vacuum environment [4-5], the lubricating coatings used polyamide-imide (PAI) as the binder and molybdenum disulfide as the solid lubricants have made a great contribution over the past decades [6]. However, the organic lubricating coatings applied in the space environment are often easily eroded and broken by the various terrible conditions, such as ionizing radiation, atomic oxygen (AO), ultraviolet (UV), high vacuum and plasma [7-9]. Among them, AO as a strong oxidant is the most fatal to the safety and reliability of spacecraft [10]. In LEO, longtime irradiation under AO exposure will lead to the physical, chemical, and structural changes of polymer materials used on satellite surface via a lot of complicated processes [11, 12]. As a result, traditional organic lubricating coatings cannot meet the demand of long wear life and terrible service environment while the lifetime of spacecraft increases from 3-5 years to 10 years [13-15]. Therefore, it is of great significance to develop novel high-performance lubricating coatings with excellent AO resistance and tribological properties. Polyhedral oligomeric silsesquioxane (denoted as POSS) is a new type of nanoscale organic/inorganic hybrid materials that emerged in recent years [16-18]. POSS contains a silicon/oxygen inorganic hollow cage-like framework (RSiO1.5). Here R is the external hydrocarbon which comprises inert or active groups; the inert group


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can improve the compatibility with polymers, and the active group can react with polymers [19-20]. Therefore, the active groups in POSS solved the weaknesses of the inorganic nanoparticles as fillers by physical blending, such as poor compatibility, sedimentary, and agglomeration [21, 22]. Also, the POSS monomer can be incorporated as a side group on the polymer or incorporated into the main chain of the polymer, and its incorporation modes can be easily controlled by adjusting the numbers of the active group in POSS [23, 24]. Namely, the octa-functional POSS can be incorporated into the main chain of polymer matrix as a coss-linking agent which concentrated in the inner of the polymer system [23]; and the monofunctional POSS in the molecular chain of polymer is incorporated as a blocking agent or pendant group and concentrated on the surface of the polymer system [24]. Previous researches demonstrate that POSSs are advanced reinforcing agents for organic–inorganic composite materials because POSS has an inorganic cage-like structure composed of Si—O—Si bond, which exhibits better oxidation resistance than the C—C or C—H bond in the polymer [2529]. More important, the Si—O—Si bond can form a SiO2 ceramic layer on the surface of the copolymers upon exposure to AO, thereby further hindering the oxidation and erosion of the inner organic materials [4, 8]. To date, some researchers have attempted to improve the AO resistance of POSSbased composites by incorporating POSS with different numbers of functional groups [30-32]. Wang et al. [33] found that TBS-PBO-POSS copolymer composite, obtained by the incorporation of POSS monomer with cage-like structure into the main chain of the copolymer, exhibits outstanding anti-atomic oxygen performance, which is because the passivating silica layer formed on the surface of the polymers can drastically reduce the AO-induced erosion. Qian et al. [8] reported that the blends of trisilanolphenyl (TSP) POSS and PI with different weight percentages of Si7O9POSS cage exhibit increased AO resistance with increasing POSS cage loading upon exposure to a hyperthermal AO beam, which is due to the formation of passivating SiOx layers on the surfaces of POSScontaining polymers during AO exposure. Moreover, the effects of AO on various polymeric monolithic materials modified by POSS have been widely investigated by laboratory simulation experiments in recent years. However, the studies on POSS-



containing composites as lubricating coatings under AO irradiation are rarely reported. These lubricating coatings are very thin (about 20 ± 5 μm) and do not possess their own bearing capacity, and they contain only a small amount of POSS as compared with POSS-containing monolithic materials. In addition, the formation of SiO2 passivation layer is based on the degradation of the organic components on the surface while the organic composite coatings are applied [3, 17, 27, 34, 35], and it remains unknown whether POSS can improve the tribological properties of the lubricating coatings in the space environment. Therefore, it is imperative to study the AO resistance and tribological properties of the POSS-containing lubricating coatings under atomic oxygen irradiation. In this work, octa- and mono-amino functionalized POSS with different numbers of amino groups are mixed with PAI matrix to afford advanced multifunctional coatings via copolymerization. The AO resistance of the PAI-matrix coatings is investigated in detail, and the effects of AO irradiation on their tribological properties are also discussed. This study, hopefully, could help to provide a novel and efficient method for fabricating advanced lubricating coatings with excellent anti-oxidative performance and mechanical properties in LEO. 2. Experimental details 2.1 Materials The POSS monomer with eight 3-amino propyl groups (OPOSS) was synthesized through dehydration condensation reaction [32], and the POSS monomer with single 3amino propyl groups (MPOSS) was obtained from Beijing HWRK Chemical Company (Beijing, China). The POSS cage can be incorporated into the main chain of polymer or as a side- or end-groups in the polymer (Figure 1) [4, 8]. Ethanol, hydrochloric acid, and N, N-dimethyl formamide (DMF) was commercially obtained from the East Instrument Chemical Glass Company Limited (Shanghai, China). All reactants are analytically pure. The MoS2 powder (ground with a planetary ball mill; average size: 3-8 μm) and PAI resin were provided by Shanghai Research Institute of Synthetic Resins (Shanghai, China).


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Fig. 1. Schematic diagram of a) PAI precursor, b) POSS in the main chain of PAI, and c) POSS as the side- or end-group of PAI molecule. 2.2 Preparation of POSS/PAI/MoS2 coatings NH2−POSS with different numbers of amino group were added into PAI resins to afford POSS/PAI hybrid composites via copolymerization. Namely, the OPOSS and OMPOSS were added into PAI resin respectively to afford OPOSS/PAI and OMPOSS/PAI composites through the route reported elsewhere [25]. In addition, the detailed composition of the materials is list in Table 1. The POSS/PAI hybrid nano-composites and MoS2 solid lubricant with a certain proportion were mechanically mixed into a slurry. Prior spraying of the coating, the steel substrate surfaces were sand blast and ultrasonically treated with acetone for 15 min to improve the bond strength of the coatings. Then the modified POSS/PAI/MoS2 composite slurry was sprayed onto the steel substrates with 0.2 MPa gas. The as-



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sprayed coatings were incubated in an air electric drying oven at 150 ℃ for 1 h and cured at 280 ℃ for 1 h to achieve the lubricating coatings with a thickness of 25 ± 5 μm (measured with a MINITEST 1100 microprocessor coating thickness gauge). Table 1 Weight compositions of the pristine and POSS-reinforced coatings Samples

Compositions (g)

PAI/MoS2 OPOSS/PAI/ MoS2 OMPOSS/PAI/MoS

POSS content (wt.%)

PAI

MoS2

OPOSS

MPOSS

5 4.5 4.5

5 5 5

0 0.5 0.25

0 0 0.25

0 5 5

2

2.3 AO exposure experiment The AO exposure experiments were conducted under vacuum (＜1.5 × 10−2 Pa) with an AO ground simulation device (Yanfei Electrical Technology Company Limited, Hefei, China). The oxygen plasma produced by a microwave power source is transformed into a beam under an electromagnetic field [12, 13]. The AO strikes the coating at an average energy of 5 eV and an AO flux of 1.23 × 1016 AO/cm2·s for an irradiation time of 35 h. The effective AO flux is calculated by Eq. (1). ∆M

(1)

AO flux = ρ × A × t × AORC

Where ΔM is the mass loss of Kapton after AO irradiation, ρ is the density of Kapton, A is the exposed surface area, t is the exposure time, and AORC is the AO reactivity coefficient of Kapton with an AO reactivity of 3.0 × 10-24 cm3/atom [33]. The specimens of the composite coatings are irradiated in AO space environment, which is approximately equivalent to the exposure of a satellite surface to the LEO environment for 100 days. 2.4 Structure characterizations of the coatings The chemical constituents and functional groups of the coatings were obtained with a Nexus 870 Fourier-transform infrared (FT-IR) spectrometer within a spectral range of 500−4000 cm-1 and a HORIBA Jobin LabRAM HR evolution micro-Raman spectrometer within a scanning range of 100−1100 cm-1. The chemical composition of the composite coatings before and after AO irradiation was determined by X-ray


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photoelectron spectroscopy (XPS; ESCALAB 250Xi, USA). The morphologies of the coating surfaces and worn surfaces were observed with a sca nning electron microscope (SEM, JSM-5600LV, Japan) equipped with an energy dispersive spectrometer (EDS). 2.5 Friction and wear behavior The room temperature friction and wear behaviors of POSS/PAI/MoS2 hybrid coatings before and after AO irradiation were comparatively investigated with a ballon-disc tribometer (CSM, Switzerland). The counterpart steel balls (HRC 58-61, Ra = 0.02 μm, E = 208×103 MPa) with a diameter of 6 mm were driven to slide against the coatings at a speed of 10 cm/s, an applied load of 10 N, an amplitude of 5 mm and friction distance of 100m and 500m. The wear volume was measured with a MicroXAM-3D non-contact surface profiler (AEP, USA), and the wear rate was calculated to evaluate the anti-wear performance of the composite coatings before and after AO irradiation. The lifetime of all the coatings before and after AO exposure were tested until worn out in the same conditions. Three repeat experiments were conducted to confirm the reported results. 3. Results and discussion 3.1 Change in macroscopic appearance

Fig. 2. Appearance changes of the coating surfaces before (a, c, and e) and after (b, d and f) AO irradiation.



Fig. 3. Thickness a) and mass loss b) of coatings before and after AO exposure. The changes in the macroscopic images of the coating surfaces before and after AO exposure (observed with a camera) are shown in Fig. 2. The original coating is swarthy; and it becomes gray after AO irradiation, due to the surface damage upon AO irradiation. Namely, under the AO irradiation, the PAI molecules on the coating surface are eroded, fractured, and decomposed, thereby exposing the inner MoS2 fillers on the coating surface in association with the change in the color of the coating surface. The mass and thickness of the coatings before and after AO irradiation are presented in Fig. 3. The thickness of the coating is obviously reduced after AO exposure. Particularly, the thickness of the PAI/MoS2 coating is reduced by about 9 ± 1.5 μm; and that of the coatings with OPOSS and OMPOSS is reduced by 6.5 μm and 5.0 μm, respectively. In the meantime, the mass of the coatings is significantly reduced after AO exposure, and the mass loss of the coatings decreases with the addition of OPOSS and OMPOSS (the mass loss of the pristine coating as well as the coatings incorporated with OPOSS and OMPOSS is 1.504, 1.315 and 1.16 mg/cm2). During AO exposure experiments, the atomic oxygen reacts with the PAI resin on the surface to generate small molecules such as COx and NOx of volatile products [36-38]. As a result, the organic components on the coating surface are eroded and degraded significantly, in association with the decreases in the thickness and mass of the coatings [36].


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Fig. 4. Mass loss of PAI/MoS2 coatings upon AO irradiation. The mass loss per unit surface area versus AO irradiation time is illustrated in Fig. 4 to reveal the anti-atomic oxygen performance of POSS in detail. It can be seen that the mass loss of all the coatings increases quickly during the initial irradiation of up to 10 h, due to the destruction and degradation of the PAI resin on the coating surface. When the AO irradiation time is over 10 h, however, the incorporation of different functionalized POSS leads to changes in the variation trend of the mass loss of PAImatrix coatings. Particularly, at a given irradiation time, the coatings incorporated with functionalized POSS exhibit less mass loss than those without functionalized POSS, and their mass loss dramatically decreases with increasing irradiation time. This implies that the mass loss of the composite coatings mainly occurs at the beginning of AO exposure, due to the degradation of PAI on the coating surface and the removal of the organic functional groups from POSS. At an extended AO irradiation time, the mass loss of the composite coatings tends to be slowed down, due to the exertion of the MoS2 and SiOx protective layer on the coating surfaces.



Fig. 5. Low magnification SEM micrographs of coating surfaces before (a, b, c) and after (d, e, f) AO exposure.

Fig. 6. Sectional profiles of coating surfaces before (a-c) and after (d-f) AO exposure. (a, d) PAI/MoS2; (b, e) OPOSS/PAI/MoS2; (c, f) (O+M) POSS/PAI/MoS2.


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Fig. 7 High magnification SEM images and EDS spectras of the composite coatings before (a-c) and after (d-f) AO irradiation, (a, d) PAI/MoS2, (b, e) OPOSS/PAI/MoS2, and (c, f) (O+M) POSS/PAI/MoS2 The low magnification SEM images of coatings before and after AO exposure are shown in Fig. 5. The surfaces of the composite coatings are relatively smooth and continuous before AO exposure (Figure 5a, b and c); and the surface roughness of PAI/MoS2, OPOSS/PAI/MoS2, and (O+M) POSS/PAI/MoS2 coatings is relatively small (Ra = 1.09, 1.39 and 1.93 μm; see Figure 6a, b and c). The surfaces of the composite coatings incorporated with different functionalized POSS are rougher than that of pristine PAI/MoS2 coating, which results from the surface migration effect and enrichment of POSS on coating surfaces [25]. When the PAI-matrix composite coatings are exposed to AO irradiation, however, their surfaces become dramatically rough and contain numerous holes (Figure 5d, e and f as well as Figure 6d, e and f; Ra = 3.49, 2.87 and 2.25 μm). The roughness of all the coatings increased significantly after AO exposure compared with the unexposed coatings, which can be attributed to that AO irradiation results in serious erosion and degradation of the coating surfaces. Also, it is



worth noting that the roughness decreased with the addition of OPOSS and OMPOSS after AO irradiation for the reason that the POSS improved the crosslink density and compactness of the coatings and protected the organic materials on the surface from erosion and degradation. Figure 7 shows the high magnification SEM images of the PAI-matrix composite coatings before and after AO irradiation. It can be seen that all the coatings before AO exposure were relatively smooth and consecutive, additionally, some MoS2 sheets were distributed outside or inside of the PAI matrix. The PAI/MoS2 coating after AO irradiation has an irregular porous structure, and the surface of MoS2 is relatively smooth (Fig. 7a). After the AO irradiation of the OPOSS/PAI/MoS2 composite coating, the surface of MoS2 is covered by some silicon dioxide (Fig. 7b). Besides, the AO irradiation of the (O+M) POSS/PAI/MoS2 composite coating gives rise to more apparent network structure composed of silicon dioxide on the exposed surface (Fig. 7c). In addition, comparing with the unexposed coatings, a regular change can be found that the all the carbon contents of these exposed coatings decreased distinctly, which indicates that the PAI on the coating surface weas oxidized and degraded severely. The PAI on the coating surface was degraded and oxidized seriously, and the surface was covered with the elements Si, O, S and Mo after AO exposure. Therefore, the network structure on the surface of (O+M) POSS/PAI/MoS2 composite coating could be result from the oxidation of POSS molecules affording SiO2 and SiOx remnants. The networked remnants could prevent the underlying organic materials from AO irradiation even though they are only locally continuous, which is corresponding to the results of thickness and mass loss of the coatings. As a result, the OMPOSS/PAI/MoS2 coating is highly resistant to AO attack, due to the formation of SiOx networked layer on the surface. 3.2 Changes in surface chemical composition


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Fig. 8. FTIR spectra of a) PAI/MoS2, b) OPOSS/PAI/MoS2, and c) (O+M) POSS/PAI/MoS2 composite coatings before and after AO irradiation. Figure 8 shows the FTIR-ATR spectra of the coatings before and after AO exposure. The AO irradiation leads to obvious changes in the intensity of the characteristic absorbance peaks (Fig. 8a). Namely, the absorbance peaks at 1641 cm-1 (C = O) and 1530 cm-1 (C=C) are dramatically weakened after AO irradiation, which indicates that some PAI chains on the coating surfaces are oxidized and degraded thereafter. Besides, the intensity of the absorbance peaks at 1238 cm-1 (C -O-C) and 3265 cm-1 (-NH) is decreased after AO exposure, and the intensity of C=O absorbance at 715 cm-1 is increased after that. The intensity of the C-H peaks increases very slightly according to the new scale. The results may be attributed to some C-H bonds in POSS were protected by the oxidized MoS2 and SiO2 under AO irradiation. These FTIR-ATR data confirm that some complex chemical reactions and degradation of PAI molecules occur after the AO irradiation of PAI/MoS2 coating [40]. However, as shown in Fig. 8b and c, the composite coatings incorporated with OPOSS and OMPOSS retain nearly the same FTIR-ATR absorbance features before and after AO irradiation, possibly



because the oxidation protective layers formed on their surfaces exhibit excellent stability under AO exposure.

Fig. 9. Raman spectra of PAI-matrix composite coatings before (a-c) and after (d-f) AO irradiation, (a, d) refer to PAI/MoS2, (b, e) refer to OPOSS/PAI/MoS2, (c, f) refer to (O+M) POSS/PAI/MoS2. The Raman spectra of PAI-matrix composite coatings before and after AO irradiation are shown in Fig. 9. The composite coatings before AO irradiation show two strong absorbance bands at 384 cm-1 and 410 cm-1 (Fig. 9a, b and c), and these two absorbance bands correspond to the Raman spectrum of MoS2[41,42]. After AO exposure, the intensity of MoS2 absorbance band is reduced obviously in association with the appearance of two weak absorbance peaks of MoO3 at 820 cm-1 and 996 cm-1 (Fig. 9d, e and f), which is due to the oxidation of MoS2 upon AO irradiation. In addition, the intensity of MoS2 band after AO exposure increases with the addition of octa- and mono-amino POSS, and the intensity of MoO3 band decreases therewith. This means that the oxidation degree of MoS2 decreases significantly after the incorporation of octa- and mono-amino POSS into the PAI-matrix composite coatings, which should be closely related to the enrichment of SiOx on the coating surfaces (see the surface morphology of the coatings shown in Fig. 7).


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Fig. 10 XPS spectra of the composite coatings before and after AO irradiation. a) PAI/MoS2, b) OPOSS/PAI/MoS2, c) (O+M) POSS/PAI/MoS2

Table 2 Surface composition (at.%) of the composite coatings before and after AO exposure: a) PAI/MoS2, b) OPOSS/PAI/MoS2, c) (O+M) POSS/PAI/MoS2. Coatings

a a-AO b b-AO c c-AO

Surface composition (at.%)

C/N

C

N

O

S

Mo

74.92 27.97 68.77 17.96 67.9 16.06

6.00 27.56 6.33 13.54 5.88 11.12

18.03 22.16 19.44 35.3 22.33 40.03

0.73 12.19 0.84 10.57 0.70 11.88

0.33 10.12 0.18 7.00 0.10 3.01


O/Si

Si

4.43 15.64 5.09 17.91

12.48 1.01 10.86 1.33 11.54 1.44

4.39 2.26 4.38 2.23


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Fig. 11. High-resolution Si 2p (a, b) and Mo 3d (c, d and e) XPS spectras of the coatings after AO irradiation. c) PAI/ MoS2, (a, d) OPOSS/PAI/MoS2, (b, e) (O+M) POSS/PAI/MoS2. Table 3 The intensity ratio of silicon and molybdenum valence after AO irradiation. ISiO1.5 /ISiO2

IMoO3/IMoS2

PAI/MoS2

－

0.907

OPOSS/PAI/MoS2

0.05

0.492

(O+M) POSS/PAI/MoS2

0.16

0.202

The changes in the surface chemical compositions of the PAI-matrix composite coatings before and after AO irradiation were further investigated by XPS. The composite coatings show XPS peaks of C1s, O1s, N1s, S2p, Mo3d and Si2p; and in particular, the intensity of their Si 2p and O 1s peaks tends to rise after AO exposure (Fig. 10a, b and c). The atomic concentrations of the composite coatings before and after AO irradiation, derived from the XPS survey scans, are shown in Table 2. AO with strong oxidizing ability can react with the organic components on the coating surfaces to produce some volatile gases escaping therefrom [8, 17, 33]. As a result, the concentration of carbon on the surfaces of PAI/ MoS2, OPOSS/PAI/MoS2, and (O+M) POSS/PAI/MoS2 composite coatings decreases from 74.92%, 68.77% and 66.9% to 27.97%, 17.96% and 16.06% after AO irradiation (Table 1). Besides, the concentrations


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of oxygen and silicon of the composite coatings increase after AO irradiation, due to the formation of nonvolatile silicon dioxide and other oxides during AO exposure process [33, 37]. Therefore, the relative atomic concentrations of O and Si of the PAImatrix composite coatings increase significantly while the atomic concentrations of C and N decrease dramatically after AO exposure. In addition, the Si/O ratios of the composite coatings after AO irradiation are about 1: 2.36, which indica tes that a silicon-oxygen layer, corresponding to the network structure, emerges on the coating surfaces after AO exposure [8]. Figure 11 shows the high-resolution Si 2p and Mo 3d XPS spectra of the composite coatings after AO exposure. The Si 2p peaks of the AO-exposed coatings shift to higher binding energies as compared with those of the unexposed counterparts (Fig. 11a and b), which indicates that Si2O3 is oxidized into SiO2 after AO irradiation [24, 43]. Besides, the Mo 3d XPS peaks of the PAI-matrix composite coatings before and after AO irradiation are assigned to MoS2 and MoO3 (Fig. 11c, d and e), respectively, which indicates that AO irradiation causes the oxidation of MoS2 generating MoO3 [4, 11, 44]. On the one hand, the organic components on the coating surfaces are preferentially destroyed by AO to generate volatile gases such as CO and CO2 that tend to escape therefrom. On the other hand, MoS2 and POSS are exposed on the coating surfaces and oxidized into MoO3 and SiO2 upon AO irradiation, thereby affording an oxidation passivation layer to protect the inner organic materials from further erosion and degradation under AO irradiation. The de-convoluted Si 2p- and Mo 3d-XPS spectra of the PAI-matrix composite coatings after AO exposure are centered at 102.2, 103.3, 229.4, 232.2, 232.9 and 236.1 eV, respectively (Fir. 11); and these de-convoluted Gaussian peaks are assigned to SiO1.5, SiO2, MoS2 and MoO3. The peak intensity ratios of SiO1.5/SiO2 and MoO3/MoS2 are listed in Table 3. The SiO1.5/SiO2 ratio of the PAI-matrix composite coatings increases with the addition of mono-POSS; and the coatings with OPOSS and OMPOSS have lower MoO3/MoS2 ratios than pristine PAI/MoS2 coating. This means that the oxidation degree of MoS2 decreases significantly after the incorporation of OPOSS and OMPOSS. The peak intensity ratios of SiO1.5/SiO2 and MoO3/MoS2 give



further evidences that SiO2 passivating layer is formed on the coating surfaces under AO irradiation. Therefore, at the initial exposition stage, the organic components on the surface were oxidized and degraded, resulting in the formation of volatile products that escaped from the coating, such as COx and NOx [14]. After an extended AO irradiation, the SiO and Si-C bonds on the topmost surface were oxidized to form a networked passivating layer. The networked structure observed in Figure 7 may be the remnants of POSS-POSS lamellae that oxidized to SiO2. In addition, the oxidation of POSS in the polymer might result in a SiO2 layer that is not entirely continuous and therefore not entirely passivating, such that it would allow some penetration of incident O atoms to the underlying pristine material and lead to a steadily decreasing but finite erosion rate of the material. The combination of the Raman spectra and XPS data confirms that POSS contributes to the formation of SiO2 networked layer and the significant reduce in the oxidation degree of MoS2, which is in agreement with corresponding high magnification SEM images shown in Fig. 7. 3.3 Friction and wear behavior

Fig. 12. Friction coefficient curves (a-c), mean friction coefficient (d) and wear rate (e) of the coatings, a) PAI/MoS2, b) OPOSS/PAI/MoS2, c) OMPOSS/PAI/MoS2


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AO irradiation could bring out serious oxidation and degradation of organic material, thereby influencing the friction and wear behavior of the coatings. The influence of POSS on the friction and wear behavior of the PAI-matrix composite coatings are illustrated in Fig. 12. The friction coefficient curves are shown in Fig. 12(ac), all the curves after AO exposure are lower than that before AO irradiation, which may be attributed to the surface oxidation and degradation. In the friction process, the friction coefficient is reduced due to the rapid formation of a transfer film composed of MoS2 on the counterpart ball at the initial stage. The friction coefficients of PAI/MoS2 coating, OPOSS/PAI/MoS2 coating and OMPOSS/PAI/MoS2 coating are 0.088, 0.084 and 0.081, respectively (Fig. 12d). After AO irradiation, the friction coefficient of the three kinds of PAI-matrix coatings increases, possibly due to the formation of hard oxidation layer in association with the change in the interface interaction of the frictional pair. In the meantime, the wear rates of all pristine and modified PAI/MoS2 coatings, whether before or after AO exposure, decrease with the incorporation of POSS (Fig. 12e), and the coating incorporated with OMPOSS exhibits the lowest wear rate. The result indicates that both the two types of POSS contribute to significantly improving the wear resistance of the PAI-matrix composite coatings. The decrease of wear rate before AO exposure can be attributed to the addition of OPOSS and MPOSS into PAI matrix as a coss-linking agent and blocking agent respectively. The addition of POSS significantly contributes to the decrease of total surface energy and consequently to the increase of the hardness, mechanical properties and wear resistance according to our previous works [25].



Fig. 13. The friction coefficient curves (a-c) and the average lifetime (d) of the three coatings, a) PAI/MoS2, b) OPOSS/PAI/MoS2, c) OMPOSS/PAI/MoS2 Fig. 13 shows the friction coefficient curves and average lifetime of the three coatings before and after AO irradiation. It can be seen that the average lifetime of the PAI-matrix composite coatings before and after AO exposure increases with the incorporation of OPOSS and OMPOSS. The decreases of lifetime vary with the addition of OPOSS and OMPOSS according to the friction coefficient curves in Fig. 13a-c, and the average lifetime (per μm) were calculated. Therefore, their average lifetime is decreased after AO irradiation: the lifetime of PAI/MoS2, OPOSS/PAI/MoS2 and (O+M) POSS/PAI/MoS2 coatings is decreased by 22.7%, 12.5% and 7.4% after AO irradiation. This indicates that the cooperation of octa- and mono-functional POSS can effectively improve the AO resistance as well as wear resistance of the PAI-matrix composite coatings [45, 46].


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Fig. 14. SEM morphologies and EDS spectra of wear tracks of PAI-matrix composite coatings before (a, b and c) and after (d, e and f) AO irradiation: (a, d) PAI/ MoS2, (b, e) OPOSS/PAI/MoS2, (c, f) (O+M) POSS/PAI/MoS2. The SEM morphologies and EDS spectra of the worn coating surfaces before and after AO exposure are shown in Fig. 14. As seen in Fig. 14a, b and c, the PAI-matrix composite coatings before AO irradiation produce broad and deep wear tracks, which indicates that they undergo serious plastic deformation during the friction process. Particularly, the wear track of the OMPOSS/PAI/MoS2 coating is relatively small and smooth in association with few signs of lamellar peeling as compared with that of pristine PAI/MoS2 coating, which well corresponds to relevant lifetime data. After AO irradiation, the wear tracks become shallow and narrow and contain some pits and slight furrows (Fig. 14d, e and f), due to the formation of hard SiO2 nanoparticles. In addition, the largest amount of furrows appear on the worn surface of the AO-exposed OMPOSS/PAI/MoS2 coating, which corresponds to the oxidation of octal- and monoamino POSS generating SiO2 networked passivation layer upon AO irradiation. In addition, the distribution of Mo element showed different results in Fig. 14. Before AO irradiation, the Mo element dispersed uniformly on the coating surface and wear tracks in the friction process; however, the content of Mo element in the wear tracks decreased significantly after AO exposure. The result may be attributed to the oxidation and degradation of PAI, which causing the MoS2 to be exposed to the surface.



In the friction process, the direct contact with MoS2 results in the rapid formation of a transfer film composed of MoS2 on the counterpart ball. In summary, the incorporated OMPOSS helps to enhance the cross-linking of the PAI-matrix composite coatings and transform their plastic deformation and fatigue wear before AO irradiation to mild abrasive wear under AO attack [25, 45]. The octa- and mono-amino POSS jointly contribute to improving the AO resistance and wear resistance of the PAI/MoS2 coatings in LEO.

Fig. 15 Wear schematic diagram of the structural evolution of the pristine a) and OMPOSS modified b) coatings. The structures changes of the pristine and OMPOSS modified coatings before and after AO exposure during the friction are shown in Fig.15. It can be seen that, the incorporation of OMPOSS enhance the cross-linking and hardness of the PAI-matrix composite coatings before AO irradiation, and jointly to improve the wear resistance of the PAI/MoS2 coating. After AO exposure, most MoS2 sheets in the pristine coating were oxidized and became MoO3 and worn out quickly (Fig.15a) [7, 30]. On the contrary, the POSS-containing coatings were oxidized and a SiO2 passicating layer formed on the surface, which protected the underlying PAI and MoS2 from further oxidation and degradation. Compared with the OPOSS reinforced coating, the content of Si on the surface is relatively high which result from the surface migration and enrichment of MPOSS on the coating surface. Thus only the MoS2 on the most


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superficial surface were oxidized, which corresponding to the results of Raman and XPS [31]. In addition, the synergistic effect of SiO2 and MoS2 on the surface improved the wear resistance significantly due to the low surface energy and the effect of supporting, thus making the OMPOSS modified coating to have a long lifetime in the space environment. 4

Conclusions POSS/PAI/MoS2 composite coatings with different numbers of amino groups are

prepared by copolymerization, and the effects of POSS on the AO resistance and wear resistance of the PAI/MoS2 coatings before and after AO exposure are investigated. As evidenced by Raman spectroscopy and XPS chemical analysis, AO attack leads to the oxidation of POSS on the surfaces of OMPOSS-containing coatings and generates a networked SiO2 passivating layer thereon. The passivating SiO2 layer can prevent the underlying organic materials from further erosion and degradation, which could account for the low mass loss of POSS-containing PAI-matrix composite coatings under AO exposure. Particularly, the octa- and mono-amino POSS jointly function to enhance the cross-linking of the PAI-matrix composite coatings. As a result, the OMPOSS/PAI/MoS2 coatings generally exhibit longer lifetime as well as better AO resistance and wear resistance than PAI/MoS2 coating, due to the formation of networked SiO2 protective layer under AO irradiation. The present approach, hopefully, could help to provide an efficient and significant route for developing high-performance lubricating coatings with excellent AO resistance and wear resistance as potential protective materials of spacecraft in low earth orbit. Acknowledgements The funding from the National Natural Science Foundation of China (U1637204) is gratefully acknowledged. References [1] De Groh, K. K., Banks, B. A., Mccarthy, C. E., Rucker, R. N., Roberts, L. M., & Berger, L. A. MISSE 2 PEACE polymers atomic oxygen erosion experiment on the international space station. High Performance Polymers, 2008, 20(4-5), 388-



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Table of Contents Graphic:


MoS2 coatings for simultaneously improved

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