epoxy composite interface with a silane coupling agent

    Improvement of atomic oxygen erosion resistance of carbon fiber and carbon fiber/epoxy composite interface with a silane coupling agent Nan Zheng, Jinmei He, Dan Zhao, Yudong Huang, Jiefeng Gao, Yiu-Wing Mai PII: DOI: Reference:

S0264-1275(16)30895-4 doi: 10.1016/j.matdes.2016.07.004 JMADE 2008

To appear in: Received date: Revised date: Accepted date:

18 May 2016 29 June 2016 3 July 2016

Please cite this article as: Nan Zheng, Jinmei He, Dan Zhao, Yudong Huang, Jiefeng Gao, Yiu-Wing Mai, Improvement of atomic oxygen erosion resistance of carbon fiber and carbon fiber/epoxy composite interface with a silane coupling agent, (2016), doi: 10.1016/j.matdes.2016.07.004

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ACCEPTED MANUSCRIPT Improvement of Atomic Oxygen Erosion Resistance of Carbon Fiber and Carbon Fiber/Epoxy Composite Interface with a Silane Coupling Agent

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Nan Zheng1,2, Jinmei He1*, Dan Zhao1, Yudong Huang1*, Jiefeng Gao2,3, Yiu-Wing Mai2

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1MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage, State Key Laboratory of Urban Water Resource and Environment, School of Chemical Engineering and Technology, Harbin Institute of Technology, Harbin, 150001, China 2

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Centre for Advanced Materials Technology (CAMT) School of Aerospace, Mechanical and Mechatronic Engineering J07 The University of Sydney, Sydney, NSW 2006, Australia 3

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The College of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou,

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Abstract

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It is critical for carbon fiber/epoxy (CF/EP) composites to have a high resistance to atomic oxygen (AO) erosion when they are utilized in a low earth orbit (LEO) environment. Herein, we proposed a simple method whereby a silane coupling agent (SCA) was applied onto the carbon fiber surface to fabricate CF/EP composites with both improved interfacial shear strength (IFSS) and AO erosion resistance. The SCA was first hydrolyzed, then reacted with the hydroxyl groups on the pretreated CF surface, and finally formed a continuous uniform layer of siloxane oligomers. Atomic force microscopy images exhibited relatively smooth surfaces for SCA-treated CFs after AO erosion, when compared to the rough surface for bare CFs. It was found that the IFSS and AO erosion resistance were improved for SCA-coated CFs and CF/EP composite interface since a silica (SiO2) layer was formed upon exposure to AO as confirmed by XPS results.

Keywords: Atomic Oxygen; Interface; Carbon Fiber/Epoxy; Silane Coupling Agent 1. Introduction Owing to the low density, good electrical conductivity, high specific strength and modulus , carbon fiber/epoxy (CF/EP) composites are widely used as spacecraft materials. However,

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Corresponding author. Tel: +86 451 86414806; Fax: +86 451 86221048 E-mail: [email protected] -1-

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when a spacecraft operates in a low earth orbit (LEO) at altitudes between 200 and 700 km, many environmental factors, such as atomic oxygen (AO), high-energy ultraviolet (UV), vacuum ultra violet (VUV), thermal cycling, micro-meteoroid and space debris, can severely impact the material and affect its structural reliability and lifetime [3,4]. Among these, AO with high chemical reactivity is regarded as the most dominant and harsh species [5]. Although the density of AO is not high in the LEO, the high orbital speed of the spacecraft can still lead to high AO fluxes (1014-1015 atoms/cm2s) and high collision energies (ca 5 eV) [6]. When AO attacks composite materials, the interactions between them are quite complex. For example, AO (a) can scatter over the composites without chemical reaction, (b) reacts with composite surface forming volatile oxides like carbon oxide and nitrogen oxide, and (c) penetrates into and damages the composites [7]. Particularly, for CF/EP composites, AO can interact with both the carbon fibre and epoxy matrix. The reaction with CF generally leads to the formation of volatile oxides on the surface resulting in surface recession. Degradation of the epoxy matrix under AO attack also causes significant mass loss and irreversible reductions of physical and chemical properties [8,9] due to polymer bond breakages and thence molecule fragmentations leading to the erosion of the epoxy matrix. Hence, CF/EP composites are vulnerable to AO exposure, which affects their performance and may even cause material failure. So, it is critical to fabricate CF/EP composites with high resistance to AO erosion. To this end, many approaches have been developed, such as surface protective coating (e.g., SiO2, Al2O3) [10-12], surface modification (e.g., ion implantation) [13,14], and incorporation of nanofiller (e.g., CNTs, SiO2, graphene) in epoxy matrix[15-17]. While these methods can, to some degree, protect the composites from AO erosion, the interface between CF and epoxy is rarely investigated even though a fiber-matrix interface with good bonding is required for effective load transfer from matrix to fiber [18] to achieve the ultimate mechanical performance of the composite. When CF/EP composites are used in LEO under thermal cycling, cracks occur inevitably at the interface due to the differential thermal expansion between CF and epoxy matrix. Once AO concentrates inside these cracks, the ensuing interface failure may even tear off the CF/EP composite leaving the CF fully exposed and severely corroded [19]. To address this problem, it is necessary to achieve improved interfacial adhesion and enhanced AO erosion resistance for the interface of CF/EP composites and the CFs. In our previous work, Wei et al. [20] applied gold (Au) coating on CF surface to protect the interface of CF/EP composites and it was found that the untreated composite interface was severely eroded by AO and its interface shear strength (IFSS) decreased dramatically with the greatest loss of 13.0 MPa/h, while the interface treated by Au coating could retain 95% of the IFSS value after 8 h exposure. However, the Au-coated CF sacrificed the interfacial adhesion between CF and epoxy due to the incompatibility between Au and epoxy. He et al. [21] further introduced a thiol self-assembly film on the Au-coated CF surface to improve the interfacial resistance against AO erosion. Although this method solved the problem of a weak interfacial adhesion and further increased the AO erosion resistance with the highest IFSS retention of 96.3% compared to an Au-coated interface, Au-coating is prohibitively expensive. By contrast, silane coupling agent (SCA) treatment is a practical economic method widely used

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to enhance interfacial adhesion in fiber reinforced composites, where SCA can react with fiber and epoxy based on their bifunctional groups thereby forming chemical bridges between themselves [22, 23]. However, its influence on AO erosion resistance is, to our best knowledge, not studied. In fact, SCA can be oxidized to form silica (SiO2) that is inert to AO and thus acts as a barrier to CFs in the composites. Here, in this paper, we proposed a SCA treatment method to improve the interfacial interaction and AO erosion resistance for the interface of CF/EP composites. Carbon fiber was first modified with three types of SCA having different end groups. Then, the effects of AO attack on the mechanical properties of SCA-treated carbon fibers and the interface of CF/EP composites were studied under a ground simulation AO facility. The interfacial shear strength (IFSS) was chosen to evaluate the interfacial property and the AO resistance of the composite interface. It was found that after SCA-treatment, the IFSS was much increased and showed very good retention after AO exposure.

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2. Experimental Work 2.1. Materials

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T300 carbon fibers were provided by Toray Company, Japan. The matrix was epoxy resin E-51 and hardener H-256. Three types of silane coupling agents, viz., methyltrimethoxysilane (CH3Si(OCH3)3, MS, 96%), γ-aminopropyltriethoxysilane (H2N(CH2)3Si(OC2H5)3, APS, 97%) and γ-glycidoxypropyltrimethoxysilane (CH2OCHCH2O(CH2)3Si(OCH3)3, GPS, 99%), were purchased from Aldrich Chemical Co. These SCAs with different end groups possess different reactivity with the epoxy matrix. Other chemical reagents including nitric acid (HNO3), lithium aluminum hydride (LiAlH4), tetrahydrofuran (THF), acetone and ethanol (96%) were obtained from Sinopharm Group Co., Ltd. 2.2. Preparation of SCA modified carbon fibers

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Surface sizing and contaminants on carbon fibers were removed using acetone at 70 oC for 48 h. Then, CFs were pretreated to obtain hydroxylated fibers by three steps: (a) oxidizing in HNO3 (6M, 100 ml) for 1 h at 80 oC, (b) reducing in LiAlH4-THF saturated solution for 1 h, and (c) rinsing with deionized water and drying at 100 oC for 10 min. The SCA solution was prepared by dissolving three types of SCAs (MS, APS and GPS) (1 ml) in 100 ml ethanol at room temperature by vigorous stirring. The as-prepared hydroxylated CFs were separately immersed into each of three SCA-ethanol solution and were then stirred at ambient conditions for 8 h. Finally, the SCA modified CFs were obtained after rinsing with deionized water and drying at 80 oC, and they were designated as MS-CF, APS-CF and GPS-CF, respectively. 2.3. Preparation of SCA modified CF/EP composites Microbond samples for IFSS tests were prepared by dripping epoxy resin droplets onto a SCA modified CF monofilament with an embedded length of 60-80 µm, followed by curing first at 80 oC for 1 h, then at 120 oC for 2 h, and finally at 150 oC for 3 h. For convenience of description, samples from these three types of composites are named: MS-CF/EP, APS-CF/EP and GPS-CF/EP. For comparison, bare CF/EP samples were also prepared as controls. -3-

ACCEPTED MANUSCRIPT 2.4 Experiments on AO exposure

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The AO exposure experiments were conducted on a ground-based AO effects simulation facility at Beihang University, Beijing. The working principle of this facility is as follows. First, the oxygen influx into a vacuum chamber is controlled to reach a certain working pressure; second, the cathode filament is heated electrically. As the temperature and discharging voltage between filament and vacuum chamber walls increase, electrons are emitted from the surface of the cathode filament and reach a sufficiently high-energy level. Third, the oxygen plasma is formed through the collision ionizations and dissociation of oxygen molecules by the electrons. The main components of the plasma are O2, O2+, O, O+ and e-, which impinge on the samples during the experiment. Among these, atomic oxygen O is the dominant component owing to its strongest characteristic spectral band. More details on the configuration and characteristics of the facility can be found elsewhere [24]. In our experiments, unmodified and SCA modified CF mono-filaments and CF/epoxy microbond samples were first attached to a concave iron holder (7x2 mm2), and then vertically placed in another circular holder with a diameter of 160 mm in the vacuum chamber for AO exposure. The temperature and pressure in the system were 60-70 oC and 1.4×10-1 Pa, respectively. Kapton, a commonly applied polymer for spacecraft, can be severely eroded by AO and its erosion yield is a constant [25]. Thus, it was chosen as the standard material to calculate the AO flux through its mass loss. AO exposure time for various samples was chosen to 1 h, 2 h, 4 h and 8 h, respectively, and the AO flux was calculated to be 8.90×1015 atoms/cm2s within 8 h. 2.5. Materials characterization and IFSS measurements

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Fourier transform infrared (FTIR) spectra with wavelengths 500 to 4000 cm-1 were obtained from a Bruker Vertex 80v spectrometer. X-ray photoelectron spectroscopy (XPS) spectra were conducted on a Thermo ESCALAB 250 photoelectron energy spectrometer. The

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sample surface morphology was examined by scanning electron microscopy (SEM, Zeiss ULTRA Plus) and a Solver P47 AFM (NT-MDT, Russia). The monofilament tensile strength was measured on an Instron 5500R testing machine using ASTMD 3379-75 at room temperature with a gauge length of 20 mm and strain rate of 10 mm/min. At least 50 specimens were tested for each material type, and finally Weibull statistical method was used to analyse the obtained results. Interfacial shear strengths (IFSS) of carbon fibre/epoxy composites before and after AO exposure were recorded on an interfacial microbond evaluation instrument made by Tohei Sangyo Corporation, Japan at a crosshead displacement rate of 0.5 µm/s. The IFSS values were calculated by: IFSS=Fmax/πdl

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where Fmax is the maximum pullout load, d average diameter of carbon fiber, and l embedded length of single fiber in epoxy. At least 40 measurements for fibers which were successfully pulled out but not broken were obtained for each type of composites. 3. Results and Discussion 3.1. Characteristics of pretreated carbon fiber surface -4-

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original oxidated reduced

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The surface element contents before and after HNO3 oxidation and subsequent LiAlH4 reduction were measured by XPS. Fig. 1 shows the XPS spectra of the original, oxidized, and reduced CF surfaces. Compared with the original CF, the oxygen content in the oxidized CF is significantly increased, while the carbon content is dramatically decreased, leading to an increase of the O:C ratio from 0.227 to 0.425. This result reveals that nitric acid treatment effectively oxidizes CF through the unsaturated carbon and defect atoms and produces a large number of oxygen-containing groups. After further LiAlH4 reduction, a moderate decrease of the O:C ratio from 0.425 to 0.296 occurs. To accurately obtain the concentration of the oxygen-containing functional groups on CF surfaces, the XPS C1s curves were fitted and the fractions given in Table 1. Compared to the original CF, all the contents of various oxygencontaining groups in the oxidized CF, e.g., hydroxyl, carbonyl and carboxyl, increase to some degree, indicating a conversion from the -C-H group to oxygen-containing groups after the oxidation treatment. With subsequent reduction reaction, fractions of carbonyl and carboxyl groups are clearly decreased, while that of hydroxyl group is increased from 21.84% to 25.78%. That is, during LiAlH4 reduction, the original carboxyl and carbonyl groups have been partially converted to the hydroxyl group that is essential for subsequent silane coupling agent reaction on CFs.

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Intensity (a.u.)

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Fig. 1 XPS spectra of pretreated CF surfaces Table 1 Contents of correlative functional groups on pretreated CF surface Contents of correlative functional groups Samples

C-C BE (eV)

C-OH %

BE (eV)

C=O %

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BE (eV)

COOH %

BE(eV)

%

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286.1

12.43

287.3

1.34

288.5

6.04

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284.6

66.21

286.3

21.84

287.3

5.16

288.6

6.78

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284.6

68.05

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25.78

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Table 2 Surface energy of pretreated CF surface

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Surface energy ( mJ/m2)

Ethylene

water

glycol

Original CF

76 .08

45.59

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35.59

Reduced CF

69.59

34.59

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8.12

36.09

30.12

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30.40

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The surface energy of the original CF is relatively low due to the non-polar inert fiber surface, but it is increased from 36.09 to 40.55 mJ/m2 after the oxidization process as shown in Table 2. This improvement could be attributed to the plentiful hydroxyl, carboxyl and other polar groups introduced during the oxidation process, which is consistent with the enhanced O:C ratio in the XPS analysis. After the LiAlH4 reduction, most of -COOH, -CO- and -COOfunctional groups on the oxidized CF surface are transformed to the -C-OH group as shown in Table 1. However, the surface energy varies little (Table 2). Hence, it can be concluded that the surface energy of CF does not have a direct correlation with a specific oxygen-containing group. The hydroxyl group, however, benefits the wetting of SCA on CF surface, facilitating subsequent SCA self-assembly reaction on the CF. 3.2. Characteristics of SCA modified carbon fiber surface The surface morphologies of untreated CF and SCA-treated CFs (MS, APS and GPS) are shown in Fig. 2. Clearly, the untreated CF possesses a diameter of ~7.5 µm and many intrinsic grooves along the axial direction of the fiber (Fig. 1a). However, the grooves become invisible after SCA-treatment due to the formation of a thin layer of siloxane oligomers on the CF surface (Fig. 1(b, c and d)) activated by the reaction of the hydrolyzed SCA and hydroxyl groups on CF [26,27]. Furthermore, there are no differences in the surface morphologies for the three types of SCA-treated CFs because of the same reaction mechanism on the fibers.

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Fig. 2 SEM images of (a) untreated CF; (b) MS-CF; (c) APS-CF and (d) GPS-CF.

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Fig. 3 shows the FTIR spectra of unmodified CF, MS-CF, APS-CF and GPS-CF. It is noted that there are no obvious characteristic peaks for the unmodified CF. However, the characteristic peaks at ~3448 and 1634 cm-1 in all the SCA-treated samples are attributed to the stretching vibration modes of the hydroxyl group generated from the redox pre-treatment process. Besides, as for MS-CF, the peaks appearing at 2925 and 2848 cm-1 belong to the anti-symmetric and symmetric stretching vibration peak of -CH3. For APS-CF, the peak near 3440 cm-1 may be due to the overlapping peak of the stretching vibration mode of the hydroxyl and the amino groups. For the GPS-CF, the weak peaks at 1261 and 874 cm-1 are attributed to the stretching and bending vibration absorption peaks of the epoxy ring. More importantly, the new absorption peaks reflecting the -Si-O-C and -Si-O-Si stretching bands are found at around 1107 and 1021 cm-1 in the spectra of all the SCA-modified CFs. The characteristic peaks of -Si-O-C confirm the dehydration and condensation reactions between SCA and CF have occurred, and the -Si-O-Si peaks indicate formation of the polysiloxane chain by condensation reactions between silane molecules [28]. From the above SEM and FTIR analyses, it could be concluded that there was a thin film of silane derivative with different end groups on the CF surface. Specifically, -Si(OCH3)3 or -Si(OC2H5)3 groups in SCA were first hydrolyzed to produce -Si(OH)3 groups, and subsequently -Si(OH)3 groups combined with the hydroxylated CF surface by dehydration and condensation to form Si-O-C bond. Meantime, silane molecules aggregated by van der Waals interaction and the condensation reactions which occurred to form a thin film of polysiloxane on the CF surface.

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Fig. 3 FTIR spectra of SCA-treated CF samples

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3.3. Characteristics of SCA-treated carbon fibers before and after AO erosion

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The AFM images and roughness values (Ra) of untreated CF and SCA-treated CF surfaces before and after 2 h and 8 h AO erosion, respectively, are presented in Fig. 4 and Table 3. The untreated CF surface was very smooth with a Ra of 120.6 nm before exposure to AO, and some grooves along the fiber length can be clearly seen. After the SCA-treatment, a uniform siloxane oligomer film is formed on the three types of CFs with slight increases of Ra to 137.2, 127.4 and 129.3 nm, respectively. After exposure to AO, the untreated CF surface becomes extremely rough with a larger Ra of 199.5 nm (i.e., 65% increase), displaying a “corduroy-like” structure after only 2 h. But a relatively intact surface with slight erosion is found on the SCA-treated CFs whose surface roughness is increased to 149.4, 139.8 and 140.2 nm after 8 h for MS-CF, APS-CF and GPS-CF, respectively, representing 8.9, 9.7 and 8.4% increases only. These results indicate that CFs are well protected by SCA-treatment during AO exposure. The compositions of the CF surfaces before and after AO erosion were measured by X-ray photoelectron spectroscopy (XPS) and the results are given in Table 4. It can be seen that there are significant changes for the surface element content after AO exposure. For untreated CF, the contents of C and N decrease slightly but O increases moderately, indicating when the highly active AO impinges the CF surface with a certain collision energy, the C-C bond reacts with AO and forms some volatile fragments such as short chain oxidation products (CO, CO2, H2O, NO2, etc.) that leave the surface, yielding reductions in C and N.

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Fig. 4 Atomic force microscopy images of untreated CF and SCA-treated CF surfaces before and after atomic oxygen exposure.

Besides C, N and O elements, Si can also be found on the three types of SCA-treated CFs, which further confirm the existence of siloxane oligomers on CF surfaces. After AO exposure for 8 h, the contents of C and N on all three treated CFs are much reduced owing to the formation of volatile oxides. However, the contents of O are much increased but those for Si are only slightly increased. The O:Si atomic ratios are increased by 1.51 for MS-CF, 2.81 for APS-CF, and 2.58 for GPS-CF, which means Si is oxidized during AO exposure, probably forming a layer of SiO2.

Table 3 Surface roughness of four types of CF surfaces before and after AO exposure Ra (nm)

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After AO exposure

Change (%)

Untreated CF

120.6

199.5 (2 h)

65.42

MS-CF

137.2

149.4 (8 h)

8.89

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127.4

139.8 (8 h)

9.73

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129.3

140.2 (8 h)

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Table 4 Composition and contents of untreated and SCA-treated CF surfaces before and after AO exposure

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C 78.28

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74.01

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Si

16.97

4.75

22.97

3.02

75.01

19.37

1.48

4.14

62.51

31.70

0.67

5.12

72.65

19.26

4.51

3.58

APS-CF (Post-AO)

64.06

29.41

2.94

3.59

GPS-CF (Pre-AO)

77.89

18.27

1.84

2.00

68.50

27.86

1.26

2.38

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Composition and contents of elements (%)

MS-CF (Pre- AO)

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3.4. AO erosion resistance of SCA-treated CFs and CF/EP composite interface The tensile strength (TS) results of untreated CF, MS-CF, APS-CF and GPS-CF are shown in Fig. 5. The bare CF has a tensile strength of 3.44 GPa, and the tensile strengths of the treated CFs by MS, APS and GPS are 3.21, 3.10 and 3.16 GPa, respectively, representing on average an 8% reduction. This marginal negative effect is due to chemical etching during the redox pre-treatment. Fig. 5 also shows the microbond interfacial shear strength (IFSS) results for the four types of CF/EP composites. IFSS of the control CF/EP is 58.15 MPa, and this increases to 72.09, 76.99 and 79.09 MPa for MS-CF/EP, APS-CF/EP and GPS-CF/EP, respectively, which means corresponding improvements of 24%, 32% and 36% compared to the control composite. These IFSS results are expected due to the increasingly compatible interface: (a) in MS-CF, the alkyl group increases compatibility with epoxy; (b) in APS-CF, occurrence of crosslinking and curing reaction between amino groups in APS and epoxy further increase the interfacial bond strength; and (c) in GPS-CF, compatibility is the largest facilitated by the interaction between epoxy end groups in GPS-treated CFs and epoxy matrix.

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Fig. 5 Effect of SCA-treatment on TS of CF and IFSS of CF/EP composites.

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Fig. 6 shows untreated CFs are heavily eroded after only 2 h exposure to AO with a TS retention of 34% compared to ~90% retention of MS-CF, APS-CF and GPS-CF. Even after 8 h AO exposure, TS retentions of SCA -treated CFs are 73.21, 78.4 and 77.2%, respectively, which agree with the surface roughness values shown in Fig. 4 and Table 3. These results clearly confirm that SCA treatment on CFs has provided excellent protection against AO attack.

Fig. 6 Tensile strength retention ratio of CFs after AO exposure Tto reveal the mechanisms responsible for the AO erosion resistance of SCA-treated CFS, the samples have been analyzed by XPS, and best fits of the Si2p curves before and after AO attack are given in Fig. 7. Without AO exposure, most of the silicon atoms are present at a relatively low binding energy (~102 eV). In MS-CF, the Si2p spectrum can be de-convoluted into double peaks at 102.8 and 102.0 eV in Fig. 8(a), attributed to –SiO2(OH) and –SiO(OH)2, which indicate the formation of polysilanoxyl chains on CFs. In APS-CF, the Si2p spectrum - 11 -

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also displays a double-peak (Fig. 8(c)): the lower peak at 101.68eV is due to –Si(OH)3, which illustrates the formation of hydrogen bonds on CF surface; and the higher peak at 102.43 eV is assigned to –SiO2(OH) structure, which indicates the condensation reaction taken place on the treated CFs. However, only a single peak exists at 102.10 eV assigned to –SiO(OH)2 for GPS-CF (see Fig.8(e)), indicating a relatively uniform condensation degree of SCA molecules [29,30] .

Fig. 7 Deconvoluted XPS Si2p spectrum from SCA-treated CFs

After AO erosion, a new peak at 103.6 eV can be found in the de-convoluted Si2p peaks of all SCA-treated CFs as shown in Fig. 8 (b, d and f), which is associated with the characteristic peak of Si-O-Si. This indicates that the siloxane formed at the composite interface has reacted with AO, and the original oxidation state of Si, such as –SiO2(OH) and –SiO(OH)2, have been dissociated and reconfigured into the new Si-O-Si band during AO erosion, corresponding to the generation of silica (SiO2). Since SiO2 with high dissociation energy does not react with AO, it protects the underneath CF from AO erosion [31,32]. Fig. 8 displays the IFSS retention of all four types of CF/EP composites for different AO exposure time. For untreated control CF/EP, IFSS decreases sharply with a retention of 67.7% after 2 h AO exposure. The severe AO attack at the composite interface may have introduced many interfacial detects and weakened the bond strength, leading to very much reduced IFSS.

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By comparison, for MS-CF, there is only a small loss of 8.5% in IFSS after even 8 h AO exposure. IFSS retentions for APS-CF/EP and GPS-CF/EP specimens are about 93% and 95%, respectively, after 8 h exposed to AO. These results also indicate that SCA-treated CFs having proper functional groups not only increase the interfacial adhesion between CF and epoxy but also effectively improve the interfacial resistance to AO erosion. Put simply, in SCA-treated CFs, the fiber surface is coated by a layer of uniform siloxane oligomer, which, when exposed to AO, forms a SiO2 passivation layer avoiding further degradation at the composite interface. We note that the IFSS retentions of all the SCF-CF/EP samples (~95%) after 8 h AO exposure are similar to our previous studies [20, 21] in which CFs were treated with an expensive coating.

Fig. 8 Interfacial shear strength retention ratio of CF/EP composites after AO exposure.

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4. Conclusion

Three types of silane coupling agents (SCA) with different end groups were selected to treat carbon fibers (CFs). A thin siloxane oligomer layer was produced on all the treated CF surfaces through a dehydration and condensation reaction. The treated CF surfaces had good compatibility with the epoxy matrix and thus increased the interfacial adhesion between fibre and matrix of the CF/EP composites. When CF/epoxy composites were utilized in low earth orbit experiments, untreated CFs could be severely eroded by AO and the tensile strength retention was only ~34% after only 2 h exposure. However, for MS-CF, APS-CF and GPS-CF, the tensile strength retentions were 73.2%, 78.4% and 77.2%, respectively, after 8 h AO exposure. Furthermore, the single fiber microbond tests showed that the IFSS of all SCA-treated CF/EP composites maintained almost their initial values after 8 h AO erosion compared to the control CF/EP composite with 67.7% retention after 2 h AO erosion. XPS results showed that a passive inorganic SiO2 layer was formed on the SCA-treated CF surface due to the conversion of the siloxane oligomer under AO exposure. Hence, once AO penetrated into the composite interface, the SiO2 layer could effectively protect the interface from AO erosion. Thus, SCA-treatment on CFs not only - 13 -

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This work was financially supported by Grant No.51003019 from the National Natural Science Foundation of China. NZ was also supported by the Chinese Scholarship Council. References

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reduction followed by silsesquioxane coating treatment on interfacial mechanical properties of carbon fibre/polyarylacetylene composites, Composites Part A 38 (2007) 936-944. [31] A. Brunsvold, T. Minton, I. Gouzman, et al. An investigation of the resistance of polyhedral oligomeric silsesquioxane polyimide to atomic-oxygen attack, High Perform. Polym. 16 (2004) 303-318 [32] R.I. Gonzalez, S.H. Phillips, G.B. Hoflund, In situ oxygen-atom erosion study of polyhedral oligomeric silsesquioxane-siloxane copolymer. J. Spacecr. Rockets. 37(4) (2000) 463-467.

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graphical abstract

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Highlights

carbon fiber/epoxy composites was proposed.

Three types of silane coupling agents with different ending groups were used to

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A facile method to improve atomic oxygen erosion resistance for the interface of

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investigate their effect on the AO erosion resistance.

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Both the interfacial adhesion and AO erosion resistance ability of CF/EP

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composites had an obvious improvement through the generation of SiO2 on the

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interface of composites.

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