Erosion yield of epoxy–silica nanocomposites at the lower earth orbit environment of the International Space Station

Abstract

The erosive effect of ultraviolet radiation and atomic oxygen at the lower earth orbit space environment of the International Space Station on Epon 862 based epoxy composites with 12 and 100 nm silica particles was investigated. Although exposure to ultraviolet radiation had a small effect on surface erosion, restricted to 2 µm of the top surface, concurrent exposure to ultraviolet radiation and atomic oxygen resulted in significant erosion. Atomic oxygen erosion of nanocomposites with 1–5 wt% silica particles resulted in a carpet-like residual surface layer whose thickness and morphology were dependent on the size and concentration of the embedded silica particles. The eroded surface of the control epoxy had high surface roughness in the form of 10–40 µm long conical protrusions. With the addition of silica particles, the residual surface layer became fibrous and rich in silica particles, and its density increased with the weight fraction and size of the silica particles. The large and uneven erosion depth of samples exposed to atomic oxygen and ultraviolet radiation resulted in a surface damage layer with average thickness between 5 and 100 µm with significantly reduced mechanical properties compared to the surface of the as-fabricated nanocomposites. The erosion yield of the control epoxy due to atomic oxygen was 4.36 × 10⁻²⁴ cm³/atom and the addition of silica nanoparticles reduced it significantly to 1.78 × 10⁻²⁵ cm³/atom. In particular, silica nanoparticles of diameter 12 and 100 nm and weight fraction 5% reduced the erosion yield of the control epoxy by 90% and 96%, respectively.

Keywords

Radiation effects polymer nanocomposites mechanical properties surface damage

Introduction

Epoxies are suitable structural materials for space applications due to their high strength-to-weight ratio, flexibility, low electrical and thermal conductivities, and relatively low coefficient of thermal expansion. In particular, polymer films are used in solar sails, photovoltaics¹ and as adhesives² and thermal and electrical insulators in space vehicles and satellites. However, the space environment in lower earth orbit (LEO) consists of ultraviolet (UV) radiation in the range 100–400 nm,³ atomic oxygen (AO),⁴ highly accelerated ions, and space debris,^5,6 which cause degradation, corrosion, and surface damage of polymeric materials.⁷ The UV radiation emitted by the sun in the LEO is composed of vacuum UV (100–200 nm) and near field UV (200–400 nm) that accounts for 8% of the total solar constant (1.36 kW/m²).^3,8 UV radiation can cause either cross-linking or polymer chain scission⁹ with immediate consequences on mechanical properties.^10,11 The AO in LEO environment, predominantly in the altitude range 180–650 km,¹² readily oxidizes the majority of metals and polymers. In LEO, O₂ disassociates into AO which does not easily recombine because of the high available energy and the very long mean free paths (∼10⁸m).¹³ Although the concentration of AO in LEO is small, the flux of AO impinging a spacecraft’s outer surface is significant (10¹⁴–10¹⁵ atoms/cm²) due to the high orbital velocity (7–8 km/s) of the spacecraft.¹⁴ Thus, AO impacting polymeric materials over extended periods of time will cause surface erosion and mass loss.

In the past decade, there has been a concerted effort to investigate the effect of long exposure of various materials to the harsh LEO environment through the Materials International Space Station Experiment (MISSE) project.¹⁵ Samples were mounted in a suitcase-like passive experiment container (PEC) which was mounted externally on the International Space Station (ISS). These experiments included NASA’s ongoing Polymer Erosion and Contamination Experiment to investigate the effect of space environment on polymeric materials for space applications.^16,17 Under this project, the erosion yield of various polymers due to extended exposure to AO in LEO was measured and presented in several reports by de Groh et al.¹⁶ and Banks et al.¹⁸ AO erosion changes the surface morphology by producing standing surface cones¹⁸ and reducing the thickness of polymeric materials.

Due to difficulty in conducting experiments in true space environment, ground-based facilities were developed to simulate space conditions.^10,19,20 In an experimental study that simulated space conditions in a ground-based facility, Zhao et al.¹⁹ investigated the individual and synergistic effects of UV and AO on polytetrafluroethylene (Teflon). UV alone caused negligible mass loss. However, surfaces exposed to UV radiation became dark due to loss of fluorine with increased carbon content. Upon subsequent exposure to AO, the carbon accumulated on the surface oxidized to form volatile compounds and the surface regained its original color. The authors concluded that AO has greater effect on erosion than UV radiation. Similarly, Grossman and Gouzman¹⁰ investigated the effect of UV and AO on polymers as a function of the fluorine-to-carbon (F/C) ratio. They observed a negligible effect of UV on mass loss for polymers with F/C < 1 when compared to the combined effect of AO and UV. The mass loss upon simultaneous exposure to AO and UV was directly proportional to F/C ratio and ∼40 times higher compared to UV exposure alone for polymers with F/C ratio of zero. The spectral mismatch between UV radiation generated at ground-based facilities and in the LEO results in different degradation mechanisms and trends.¹³ Fluoropolymers in particular are very sensitive to UV radiation: it has been reported that erosion of fluorinated ethylene propylene (FEP) is much higher in ground-based facilities than in space environment (see works^10,16,21 and included references), which has been attributed to irradiance peaks at wavelengths that are easily absorbed by FEP. Moreover, the lack of an effective method to block the additional UV radiation generated as a byproduct during oxygen disassociation in ground-based experiments leads to erosion yields that differ from those measured in space environment.

Usually, polymers deployed in space are protected from AO by a 100 nm layer of metal, metal oxide or silicon dioxide.^22–25 Although this method is effective in protecting the underlying polymer layer, microcracks in the protective layer can cause continued erosion of the polymer due to AO. Therefore, polymers with dispersed inorganic particles, such as SiO₂ and Al₂O₃, have been proposed for enhanced resistance to AO due to the shadowing effect of the fillers onto the underlying polymer.²⁶ Kiefer et al.²⁶ observed that the addition of bis(triphenyltin) oxide (BTO) to polyetherimide, commercially known as Ultem, and to polypyromellitimide, with the same chemical formulation as Kapton, lowered the AO erosion yield.²⁵ The addition of BTO in the range 8–16% to both polymers reduced the erosion yield by 38–66%.

An alternative approach to decelerate the eroding effect of AO is the dispersion of ceramic nanoparticles in a polymer matrix, which will shield the underlying polymer. This approach does not make the polymer impervious to AO but it eliminates the effects of microcracking and delamination due to thermal mismatch between a protective ceramic coating and the underlying polymer. The viability of this approach has not been tested in an actual space environment. This study investigated the effect of LEO environment on the surface degradation and mechanical properties of Epon 862 based composites filled with different sizes and concentrations of silica nanoparticles. The specimens were exposed to: (a) UV radiation and (b) UV radiation and AO at the ISS during the MISSE-6 experiment. Upon return to earth, the method of instrumented nanoindentation was employed to measure the reduced elastic modulus of the specimen surface, which was compared to the reduced modulus of the original nanocomposites based on our earlier study.²⁷ The surface degradation and erosion yield were calculated as a function of size and concentration of the embedded silica nanoparticles, pointing to significant improvements in the erosion resistance of the original epoxy.

Materials and experimental methods

The polymer matrix in this study was Epon 862 (diglycidylbisphenol-F) resin cross-linked with curing agent W (diethyltoluenediamine). Two types of nanoparticles were employed: (a) 12 nm silica particles that were incorporated at weight fractions 1, 3, and 5 wt% and (b) 100 nm silica particles that were agglomerates of 14 nm individual silica particles at weight fractions 0.6, 3, and 5 wt%. For simplicity, a specimen with 1% weight fraction of 12 nm silica nanoparticles is referred to as (12 nm, 1 wt%). The preparation of uniformly dispersed 100 nm aggregated silica particles was conducted by dispersing 14 nm spherical silica nanoparticles (fumed silica powder, Aldrich) in Epon 862 by high-shear ultrasonication and curing with curing agent W. The precise size of the 100 nm particle agglomerates, as measured from atomic force microscopic images, was 94 ± 13 nm.²⁷ The 14 nm silica nanoparticles in the 100 nm agglomerates were held together by weak van der Waals’ forces. A scanning electron microscopic (SEM) image of the morphology of the silica agglomerates is shown in Figure 1. Preparation of 12 nm individually dispersed silica particles was done by dispersion (MEK-ST suspension, Nissan Chemicals) in Epon 862 via ultrasonication, followed by curing with W. Finally, dog-bone specimens with gage sections 0.6 × 1.2 × 8 mm³ were machined from composite plates, avoiding any mechanical damage to the originally smooth specimen surface that had root mean square roughness of about 1 nm.²⁷

Figure 1.

SEM image showing morphology of 100 nm aggregated silica particles. SEM: scanning electron microscope.

The composite specimens, listed in Table 1, were firmly mounted onto two aluminum plates for placement in the two PECs shown in Figure 2. As a part of MISSE-6, the PECs were transported to the ISS by the STS-123 Endeavour mission on 11 March 2008. The PECs were installed on the exterior of the ISS on 22 March 2008 such that one PEC faced the ram side (where AO ‘rammed’ onto the specimens) and the other the wake side. The specimens facing the ram side of the ISS experienced UV radiation and high flux of AO (UV + AO) and those facing the wake side were exposed to UV radiation only. The composite specimens were exposed to space conditions from the date of installation until 1 September 2009 for an approximate duration of 12,936 h. The PECs were returned to earth on 11 September 2009 by the STS-128 Discovery mission.

Figure 2.

Specimens placed inside PECs. The circles mark the two aluminum plates with the composite specimens. The photograph was provided by the NASA Langley Research Center. PEC: passive experiment container.

Table 1.

Specimens included in each PEC

Specimen	Description	Number of specimens
Epoxy	Control epoxy	2
(12 nm, 1 wt%)	12 nm particle, 1 wt%	2
(12 nm, 3 wt%)	12 nm particle, 3 wt%	2
(12 nm, 5 wt%)	12 nm particle, 5 wt%	2
(100 nm, 3 wt%)	100 nm particle, 3 wt%	1
(100 nm, 5 wt%)	100 nm particle, 5 wt%	1

PEC: passive experiment container.

Due to the very limited number of available specimens for each kind, the nanoindentation technique was employed to measure the reduced modulus, E_r and compare it to that of the original materials. The reduced modulus is defined as E/(1 − ν²), where E is the Young’s modulus and ν the Poisson’s ratio of a material. The E_r for each specimen was obtained by a commercial nanoindenter using a Berkovich diamond tip. A minimum of 36 indents with 4 µm indentation depth were performed on each specimen at a loading rate of 25 nm/s. A standard epoxy whose properties were accurately measured in uniaxial tension was used as the calibration material to calculate the tip area function.²⁸ Due to the viscoelastic nature of the polymeric samples, a trapezoidal shaped loading function with constant loading and unloading rates was used in displacement control. Figure 3(a) shows typical loading and unloading profiles of indentations. The force vs indentation depth curve at the peak load is zoomed in Figure 3(b) to show the short holding segment of the trapezoidal shape function (5 s) compared to the loading and unloading segments (160 s). The applied force was reduced during the holding segment in Figure 3(b) due to stress relaxation of the polymer. The dashed line in Figure 3(a) is the initial slope of the unloading curve used to calculate E_r. From the load vs indentation depth curves and the tip area function, the reduced modulus of the composites was calculated following the methodology by Oliver and Pharr.²⁹

Figure 3.

(a) Nanoindentation loading and unloading curves for a composite specimen and (b) close view of the holding segment encircled in (a).

Results and discussion

The aluminum plates containing the epoxy specimens exposed to UV and UV + AO are shown in Figure 4. The dog-bone shaped epoxy composite specimens exposed to UV and UV + AO were darker compared to the original materials. However, the control epoxy specimens exposed to UV + AO were lighter in color compared to the composite specimens. The top surface of each specimen was examined by an SEM to quantify any changes in the surface morphology due to exposure to space environment. As shown in Figure 5(a) to (d), a porous carpet-like residue was formed on the entire surface of all specimens exposed to UV radiation. This residue layer was a result of polymer chain scission and had uniform thickness of about 2 µm for all specimens. Figure 5(c) shows a cross-sectional view of a (100 nm, 5 wt%) specimen with the residue layer being clearly discernible at the top surface. A detailed view of the residue layer on the top surface of the same specimen caused by exposure only to UV radiation is shown in Figure 5(d).

Figure 4.

Specimens exposed to: (a) UV radiation and (b) UV + AO. Each plate is 5 × 5 cm². The photographs were obtained at the NASA Langley Research Center upon the return of the samples to earth. UV: ultraviolet and AO: atomic oxygen.

Figure 5.

Specimens exposed to UV radiation: (a) top view of carpet layer on control epoxy; (b) top view of carpet layer on (100 nm, 5 wt%); (c) cross-sectional view of carpet layer on (100 nm, 5 wt%); and (d) top view of carpet layer on (100 nm, 5 wt%) specimen. UV: ultraviolet.

The top surface of the composite specimens exposed to UV + AO also had a thin carpet layer, Figure 6(d), (f) and (h), similar to the one shown in Figure 5(a) to (d), overlying a much thicker residual surface layer where all surface damage was accumulated. The cross-sectional and top views of the residual layers for the control epoxy, (12 nm, 1 wt%), (12 nm, 5 wt%), and (100 nm, 5 wt%) specimens are shown in Figure 6. The thickness of the residual layer was measured from specimen cross sections and it was found to decrease dramatically with the size and the weight fraction of the embedded silica nanoparticles. Interestingly, no substantial carpet-like layer was found on the control epoxy. Instead, conical protrusions were observed on the entire surface of the control epoxy exposed to UV + AO, as shown in Figure 6(a) and (b) and also reported before in Banks et al.¹⁸ The texture of the residual surface layer was different across different composites. In composites with 12 nm silica particles, the residual layer was composed of fine and wavy fibrous structures, as shown in Figure 6(c). A close side view of the residual layer at the top surface of (12 nm, 1 wt%), (12 nm, 3 wt%), (12 nm, 5 wt%), and (100 nm, 5 wt%) samples is shown in Figure 7. For larger particle weight fractions, the fibrous structures were more closely spaced, as shown in Figure 7(a) and (b). The residual layer found on the composites with 100 nm silica particles, Figure 6(g) and (h), was composed of very fine and dense particles, which provided protection to the underlying material and reduced the erosion depth compared to composites with 12 nm particles. Although the thickness of the residual layer varied with location and sample type, in general, it was always thicker than that observed for specimens exposed only to UV radiation. The fibrous structures formed on (12 nm, 1 wt%), (12 nm, 3 wt%), and (12 nm, 5 wt%) specimens were composed of fine silica particles held together by the epoxy trapped between them, as shown in Figure 7(c) to (e). A close view of the particles that formed the carpet and the fibrous surface layers on a (100 nm, 5 wt%) specimen is shown is Figure 7(f). The image is taken at one of the cracks shown in Figure 6(g), and hence, the particles are spaced far apart.

Figure 6.

Cross-sectional view of the residual layer due to UV + AO exposure of: (a) control epoxy; (c) (12 nm, 1 wt%); (e) (12 nm, 5 wt%); and (g) (100 nm, 5 wt%) specimens. Oblique view of the residual layer on: (b) control epoxy; (d) (12 nm, 1 wt%); (f) (12 nm, 5 wt%); and (h) (100 nm, 5 wt%) specimens. UV: ultraviolet and AO: atomic oxygen.

Figure 7.

Close-up side view of the fibrous structure of the residue layer due to UV + AO exposure of: (a) (12 nm, 1 wt%); (b) (12 nm, 5 wt%); (c–d) (12 nm, 3 wt%); (e) (12 nm, 5 wt%); and (f) (100 nm, 5 wt%) specimens. Figure 7 (c) to (f) were taken near the cracks pointed out in Figure 6. UV: ultraviolet and AO: atomic oxygen.

Because of the large thickness of the specimens used in this experiment (the dog-bone specimens were milled down from large and thick plates to ∼500 µm in thickness), the erosion thickness and eroded material mass were calculated by comparing the cross section of the exposed specimen surface with parts of the cross section covered by metal washers used for specimen mounting. The specimens were firmly mounted onto aluminum base plates using screws, as shown in Figure 8(a), before they were placed in the PECs. The washer between the mounting screw and the sample surface, as shown in Figure 8(a), protected the underlying material from UV + AO corrosion, thereby causing a step, as shown in Figure 8(b). The cross section of each specimen at this surface step was imaged using an SEM and the average thickness of the material removed from the top surface exposed to UV + AO was calculated.

Figure 8.

(a) Specimens firmly mounted to an aluminum plate with washers masking part of the specimen surface and (b) SEM image of control epoxy specimen showing the step created due to (AO + UV) erosion in the region surrounding a metal washer. SEM: scanning electron microscope; AO: atomic oxygen; and UV: ultraviolet.

The residual damage layer on the top surface of specimens exposed only to UV radiation was at the same level as the material under the washer. On the contrary, the control epoxy specimens exposed to UV + AO exhibited the largest step of ∼86 µm, clearly indicating the vulnerability of the control epoxy to AO attack. In comparison, the composite specimen (100 nm, 5 wt%) proved to be quite resistant to AO attack. It should be noted that, even though material erosion was uniform across the specimen width, there were regions where the erosion crevices were significantly deeper, which appear, for example, as deep cracks in Figure 6(c), (e) and (g). According to Dressler,³⁰ the erosion yield R_e (cm³ of material removed by each incident atom of AO, i.e. cm³/atom) is calculated as

R_{e} = \frac{V_{er}}{F}

(1)

where V_er is the volume of the polymer eroded and F the total number of incident O atoms. The total number of O atoms (fluence) is obtained from a Kapton calibration sample (also called witness sample), which is commonly used as structural material in space applications. Its erosion yield due to AO has been extensively evaluated in the past decade^31–33 and Kapton is currently used as the reference material to calculate the erosion yield of other polymeric materials exposed to the same conditions. Kapton standards were installed inside the PECs of the MISSE-6 mission and the average AO fluence in UV + AO exposed PEC was measured as 1.97 × 10²¹ atoms/cm².³⁴ The results of the eroded material thickness and the erosion yield are given in Table 2 and are plotted in Figure 9. In the presence of UV + AO, the control epoxy was eroded much faster than the silica nanoparticle composites: it is notable, that the addition of 100 nm particles at 5 wt% retarded the erosion of the polymer matrix by almost 25 times (or 96%).

Figure 9.

Erosion yield of composite specimens exposed to UV + AO. The error bars are upper bounds for erosion yield calculated from the average depth of crevices (cracks) shown in Figure 6(c), (e) and (g). UV: ultraviolet and AO: atomic oxygen.

Table 2.

Erosion thicknesses and yield of polymer nanocomposites exposed to UV + AO

Specimen	Original thickness (µm)	Average eroded thickness (µm)^a	Average erosion yield (10⁻²⁴ cm³/atom)
Control epoxy	534	86 + 23	4.36 + 1.16
(12 nm, 1 wt%)	516	24 + 12.88	1.26 + 0.65
(12 nm, 3 wt%)	573	17 + 5	0.863 + 0.25
(12 nm, 5 wt%)	605	9 + 3.48	0.457 + 0.17
(100 nm, 3 wt%)	651	6 + 2.65	0.35 + 0.13
(100 nm, 5 wt%)	617	3.5 + 2.87	0.178 + 0.15

The data in this column represent the average eroded thickness plus the depth of the cracks (refer to Figure 6(c), (e), and (g)) taken as the upper variance bound. The data in this column are used to compute the average erosion yield.

UV: ultraviolet and AO: atomic oxygen.

Due to the small number of available specimens, the mechanical behavior was evaluated by instrumented nanoindentation. The reduced modulus of the reference specimens was consistent with that calculated using the Young’s modulus reported by Chen et al.²⁷ The reduced modulus of the specimens exposed to UV radiation was very similar to that of the as-fabricated samples. However, the specimens exposed to UV + AO showed large variations in the reduced modulus within each specimen. Also, the reduced modulus of the control epoxy was approximately 48% lower than the original material and only 14% lower than that of the (100 nm, 5 wt%) composite. This variability in the measured moduli of the composites exposed to UV + AO was due to high surface roughness which renders the nanoindentation measurements representative only for the very top specimen surface. For example, the force vs nanoindentation depth plots for the control epoxy exposed to UV and UV + AO are shown in Figure 10. Due to the surface roughness and the residual layer at the top surface, the sample contact resistance during indentation was not significant until the indenter tip reached the non-degraded material under the residue. When the control epoxy was exposed to UV + AO, this indentation depth was in the range 30–50 µm, indicating that there is a significant layer of surface damage after material has been eroded away.

Figure 10.

Force vs indentation depth plots of control epoxy exposed to UV and UV + AO. The initial loading curve for specimens exposed to UV + AO had a very slow ascending slope due to the presence of 10–40 µm long conical protrusions on the eroded surface. UV: ultraviolet and AO: atomic oxygen.

A summary of the mechanical properties of the original composites and those exposed to UV and (UV + AO) is given in Table 3. Unfortunately, the available data do not allow us to determine whether there was a synergistic effect of UV and AO that may have resulted in erosion yields higher than those caused by each process alone. Literature reports are not fully conclusive on the synergistic effect of UV and AO, although it appears that for several polymers, such as polymethylmethacrylate (PMMA), polyethylene (PE), polyimide, and Teflon FEP, no particular synergy is discernible.^14,35

Table 3.

Reduced modulus of the surface of nanocomposites subjected to different exposures

Specimen	Ground conditions (GPa)	UV exposure (GPa)	UV and AO exposure (GPa)
Control epoxy	3.7 ± 0.169	3.65 ± 0.06	1.98 ± 0.17
(12 nm, 1 wt%)	3.85 ± 0.027	3.93 ± 0.18	2.41 ± 0.23
(12 nm, 3 wt%)	3.90 ± 0.09	3.92 ± 0.08	3.01 ± 0.3
(12 nm, 5 wt%)	3.92 ± 0.14	4.01 ± 0.06	3.23 ± 0.37
(100 nm, 3 wt%)	3.98 ± 0.16	4.04 ± 0.06	3.47 ± 0.19
(100 nm, 5 wt%)	4.17 ± 0.12	4.21 ± 0.06	3.59 ± 0.12

UV: ultraviolet and AO: atomic oxygen.

UV radiation is absorbed only in the top 1–3 µm of the specimen surface; hence, the polymer specimens subjected to UV radiation underwent surface degradation that created a ∼ 2 µm thick carpet layer at the top surface.¹⁹ The degraded polymer retarded further degradation of the underlying polymer due to UV radiation. When the same materials were exposed to UV radiation and high flux of AO, significantly additional erosion took place due to the reaction with AO of the layer that was initially degraded by UV radiation, forming volatile products and, thus, erosion of the carpet layer (see also Zhao et al.¹⁹ and included references). No carpet layer was found at the top surface of control epoxy exposed to UV + AO. On the contrary, a carpet layer was present in all composite specimens, whose thickness varied with the size and the weight fraction of the silica particles. Because of the shadowing effect of the silica nanoparticles to UV radiation and ram AO, the structure of the residual layer was dependent on the particle size and weight fraction. The residual layer was composed of fine fibers in the case of 12 nm silica particle composites, whose density was directly proportional to the particle weight fraction. Figure 6(c) and (e) show cross-sectional views of the fibrous structure formed on composites with 12 nm silica particles with weight fractions 1 and 5 wt%, respectively. The composite with 12 nm silica particles and weight fraction 1 wt% had a thicker residual layer with less-dense threads compared to the composite with 5 wt% silica. In comparison, the composites with 100 nm particles formed a thin carpet layer of silica particles, as shown in Figure 6(g) and (h) due to the larger size of the particles. As the density of silica particles increased in the residual surface layer, it became harder for AO to penetrate and erode the underlying epoxy. Thus, the depth of erosion was also dependent on the size and concentration of the silica particles.

The erosion yield for various polymers due to UV + AO at LEO is compared in Figure 11 to the results of this study. The erosion data for PMMA, epoxy, polystyrene, PE, polyimide (Kapton H), polyacrylonitrile, polyvinylfluoride, and FEP (Teflon FEP) were adopted from de Groh et al.¹⁶ The erosion yield of the control epoxy tested in this study compares very well with the epoxy data reported in de Groh et al.¹⁶ The addition of silica nanospheres of diameter 12 and 100 nm and weight fraction 5% lowered the erosion yield of the epoxy by 90% and 96%, respectively. The erosion yield due to AO depends on many factors such as chemical structure, type of bonds (single, double, and triple), density of the material, presence of fluorine atoms in the polymer chains, and the nature of the residue.³⁶ The inorganic silica particles produced a non-volatile residual layer that shielded the underlying polymer from further significant erosion. From the comparison plot in Figure 11, it can be concluded that the (100 nm, 5 wt%) nanocomposite is a very good alternative to the majority of polymers that have been investigated in the past for space applications.

Figure 11.

Comparison of erosion yields of various polymers reported in de Groh et al.¹⁶ with those calculated for the composite materials tested in this study. PMMA: polymethylmethacrylate; PS: polystyrene; PE: polyethylene; PAN: polyacrylonitrile; PPD-T: polyvinylfluoride; and FEP: fluorinated ethylene propylene.

Conclusions

The effect UV radiation and AO on the degradation of Epon 862 based composites with 12 and 100 nm silica particles was investigated as a function of particle size and weight fraction at the true LEO conditions of the ISS. UV-induced radiation degradation was restricted to 2 µm of the exposed surfaces, irrespective of particle size and weight fraction. Moreover, the reduced modulus of the composites exposed to UV radiation was very comparable to that of as-fabricated nanocomposites. On the contrary, the effect of UV + AO was severe on the control epoxy, and the inorganic silica nanoparticles were found to reduce the erosion yield of the epoxy by shadowing the polymer underneath the particles from ram AO attack. The UV + AO erosion yield of the control epoxy was 4.36 × 10⁻²⁴ cm³/atom and was found to decrease dramatically with the weight fraction and the size of the embedded silica nanoparticles. The addition of silica nanoparticles of either 12 or 100 nm in size at 5 wt% lowered the erosion yield of the epoxy by 90% and 96%, respectively. Thus, (100 nm, 5 wt%) silica nanocomposites are a very good alternative to most of the polymeric materials explored in the past for LEO space applications.

Footnotes

Funding

This research received the support of the Air Force Office of Scientific Research through grants FA9550-06-1-0140 and DURIP FA9550-06-1-0428 with Dr BL Lee as the program manager.

Acknowledgments

The authors gratefully acknowledge the support of the NASA Langley Research Center for the opportunity to participate in the Materials International Space Station Experiment 6 at the International Space Station.

Conflicts of interest

None declared.

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