GROUND SIMULATION TESTS OF SIMULTANEOUS IRRADIATION FROM THREE BEAM SOURCES ON MATERIALS

AT THE COMBINED SPACE EFFECTS TEST FACILITY

Eiji MIYAZAKI(1), Hiroyuki SHIMAMURA(1) and Yugo KIMOTO(1)

(1) Space Materials Section, Electronic Devices and Materials Group, Aerospace Research and Development Directorate, Japan Aerospace Exploration Agency (JAXA), 2-1-1 Sengen, Tsukuba, Ibaraki 305-8505, Japan.

*Phone: +81-29-868-2321, E-mail: miyazaki.eiji@jaxa.jp

ABSTRACT Atomic Oxygen (AO), Ultraviolet rays, Radiation, etc., are known as factors that damage materials used on spacecraft in orbit. Quantitative understanding of the degradation of materials used in flight hardware is required when the materials are exposed to the space environment. In the present report, we performed a simultaneous irradiation with AO, Electron Beam (EB), and Vacuum Ultraviolet rays (VUV) on Kapton® H and silver-coated FEP at the Combined Space Effects Test Facility, to simulate a LEO environment around a 400 km altitude for 3.5 months. As a result, the simultaneous irradiation did not have any additional effect on Kapton® H, i.e., it can be concluded that AO attack was the dominant factor for Kapton® H even in simultaneous three-beam irradiation. For silver-coated FEP, it revealed that an increase in surface roughness by AO erosion and surface smoothing by VUV occurred concurrently, resulting in a unique surface shape and a smaller solar absorptance change after simultaneous irradiation. The appearance change and the mass loss of the silver-coated FEP that occurred by three-beam simultaneous irradiation can be understood by adding the single beam effects. However, the decrease of normal infrared emittance did not change by simultaneous irradiation which was expected to decrease due to the thickness loss caused by AO attack. 1. INTRODUCTION

In low earth orbit (LEO), Atomic Oxygen (AO) is one of the severe environmental factors degrading the outboard materials of spacecraft1,2. Originating from the Earth’s atmosphere, oxygen molecules are dissociated into neutral atoms by ultraviolet rays (UV). The resultant AO remains at LEO altitude. Spacecraft fly in LEO at approximately 8 km/s, colliding with AO at a high relative velocity that imparts the equivalent of approx. 5 eV of translational energy, thereby causing erosion called AO Attack. In addition to AO, the effects of UV itself, radiation, etc., on materials are also known as degradation factors. Quantitative understanding of the degradation of materials when the materials are exposed to the space environment is important. Though all environmental

factors should be imposed simultaneously on objects in ground simulation tests to simulate a real space environment, a sequential combination of single-beam irradiations is usually applied to simulate the space environment. Sequential irradiation techniques have been studied and are considered established procedures. So far, the simultaneous irradiation with two beams, i.e. AO / UV, and AO / Electron Beam (EB), have been investigated in terms of effects of simultaneous irradiation on the materials3-6. 3 4 5 6 However, the results obtained from simultaneous three-beam irradiation, e.g. AO / UV / EB, were not reported.

In the present report, we performed a simultaneous irradiation with AO, EB, and Vacuum Ultraviolet rays (VUV) on materials being used commonly on spacecraft, e.g., polyimide film, silverized FEP, etc. In order to study this, we used one of our facilities called the “Combined Space Effects Test Facility,” which is able to radiate the three beams into a single vacuum chamber with a high vacuum of 10-5 Pa. Single beam irradiation with each source was also done for comparison. Macroscopic observations, calculations of erosion yield, thermo-optical properties, and microscopic observations of the surface were done for the evaluations of the samples’ property changes that occurred due to irradiations. 2. EXPERIMENT OVERVIEW 2.1 Materials

The polyimide films (25-µm thickness, Kapton® H; DuPont) and the silver-coated (one side) FEP films (25-µm thickness; Sheldahl) were used as test samples. The 25-mm-dia. samples were cut from a sheet by using a punch to fit the irradiation facility’s sample holder. For Ag-coated FEP, the FEP surface side was exposed. 2.2 Irradiation Tests 2.2.1 Facility The simultaneous irradiation test of three kinds of beams, AO, VUV, and EB, and the single irradiation tests of each beam were conducted by using the “Combined Space Effects Test Facility”4 at Tsukuba Space Center, Japan Aerospace Exploration Agency. Equipped with AO, VUV, and EB sources, the Combined Space Effects Test

Facility can expose samples to these beams simultaneously under a high vacuum. The schematic view of the facility is presented in Fig. 1. The AO generation in the facility is based on a laser detonation phenomenon7. The facility is equipped with deuterium lamps and a filament electron gun as VUV and EB sources, respectively. The linear motion feedthrough installed in the irradiation chamber is located over the sample holder. The linear motion feedthrough can move parallel to the sample holder’s plane. A diamond UV ray monitor or a quartz crystal microbalance can be attached on the linear motion feedthrough. 2.2.2 Irradiation tests

The conditions and fluence of each beam were as follows:

The AO velocity was controlled at approximately 8.0 km/s to simulate the LEO environment around a 400 km altitude; the translational energy at that velocity is 5 eV. In usual procedures of AO irradiation tests, the fluence of AO is measured by using a monitoring material, Kapton® H, which is installed on the sample holder as well as on the samples to be evaluated. The fluence is calculable by measuring the mass loss, the irradiated area, the density, and the reaction yield of Kapton® H by using Eq (1). monmonmonholdermonmon EAmF ρ_Δ= (1)

where Fmon: AO fluence at the monitor, atoms/cm2; Δmmon_holder: mass loss of the AO fluence monitor, Kapton® H, placed on the sample holder, g; Amon: exposure area of the AO fluence monitor, Kapton® H,

3.14 cm2; ρmon: density of the AO fluence monitor, Kapton® H, 1.42 g/cm3; and Emon: erosion yield of the AO fluence monitor, Kapton® H, 3 × 10-24 cm3/atom. The AO monitor is expected to be affected by the EB or VUV in simultaneous irradiation, resulting in an error in the AO fluence measurement. In our previous study, the EB did not affect the erosion yield of polyimide5. However, it was reported that the erosion yield of Kapton® can be increased by simultaneous irradiation with VUV and AO6. In order to avoid such effect on the monitor, a location was selected on the linear motion arm (see Fig. 1) where AO reaches but VUV does not reach, and the AO fluence monitor was placed at that location. Before the simultaneous irradiation test, the ratio of AO fluence on the sample holder to that at the monitoring position was measured4. For the present condition, the ratio was calculated to be 2.65, i.e., the AO fluence can be described as Eq. (2).

monmonmonarmmonsample EAmF ρ_65.2 Δ= (2)

where Fsample: AO fluence on the sample holder, atoms/cm2; and Δmmon_arm: mass loss of the AO fluence monitor, Kapton® H, placed on the linear motion arm, g.

The intensity of the VUV beam at the source of the Combined Space Effects Test Facility can be controlled with a variable lamp current. For the irradiation test, the intensity of the VUV beam is measured by a Diamond UV sensor (DCU-105HS; Iwasaki Electric Co., Ltd.) at the linear motion arm in the beginning and the end of a test to interpolate the intensity during the irradiation test. We calculate the integral intensity of 120 - 200 nm of wavelength at the sample holder based on the measured value at the Diamond UV sensor with a sensitivity spectrum and a Deuterium lamp emission spectrum.

The EB source can be controlled in the accelerating voltage and the source current. In the present experiment, we selected the source condition as: 200 kV for the accelerating voltage, and 1 mA for the source current. The total irradiation dose was controlled by the irradiation time based on a dose rate measured before the test with a Thermo Luminescence Dosimeter (TLD) in the condition of 200 kV and 1 mA.

Table 1 provides the AO and VUV fluence and the EB total dose in the simultaneous irradiation test and each single irradiation test, along with irradiation time and vacuum in each irradiation test. The fluence levels were equivalent to those in the LEO environment around a 400 km altitude for 3.5 months, according to the analysis performed by using the Space Environment and Effect System (SEES: JAXA’s database system for providing data and models related to space environments and the effects of space environments). The beam conditions in each single irradiation test were equivalent to those in the simultaneous irradiation test mentioned above.

Fig. 1 Schematic illustration of the Combined Space Effects Test Facility

AOEB VUV

Filamentelectrongun

Sample

AO source(Laser detonation)

Sample holder

Linear motion arm

UV sensor

Deuteriumlamps

Irradiationchamber

AO monitor

Table 1 AO and VUV fluence, EB total dose, irradiation time, and vacuum in each irradiation test.

Irradiation method Simultaneous Single

AO+VUV+EB AO VUV EB AO fluence, 1020 atoms/cm2 5.65 5.43 N/A N/A VUV fluence, 104 mJ/cm2 * 1.69 N/A 1.63 N/A

EB total dose, Gy 216 N/A N/A 216 Irradiation time, h 25 25 25 25

Vacuum, Pa 1 × 10-3 - 1 × 10-2

1 × 10-3 - 1 × 10-2

1 × 10-5 - 1 × 10-4

1 × 10-5 - 1 × 10-4

N/A: not applicable

* Integral intensity at 120–200 nm

2.3 Evaluation 2.3.1 Appearance Evaluation

The samples were photographed by a digital still camera to investigate whether any changes had occurred in their appearance due to the irradiation tests or not. 2.3.2 Mass and Erosion Yield Measurements A microbalance (XP6; Mettler Toledo International Inc.) was used for the measurement of the mass of samples before and after the irradiation tests. For the samples, water absorption into a sample is apparent during weighing. For this reason, it must be compensated by extrapolation of the mass increase curve plotted during weighing of the sample to calculate the dry mass. The weighing procedure is as follows: the sample is first stored in a desiccator for at least 24 h; it is brought onto the microbalance under controlled ambient conditions, 23±2 °C and 50±5% relative humidity; the mass is plotted for 180 s after removal of the sample from the desiccator. That measurement is then fitted to a quadratic curve to obtain the estimated dry mass from the intercept of the fitted curve. For simultaneously irradiated and AO-irradiated samples, the erosion yield was calculated from the mass change, using Eq. (3): FAmE SSSS ρΔ= (3)

where Es: erosion yield of the samples, cm3/atom; Δms: mass loss of the samples, g; As: exposure area of the samples, cm2; and ρs: density of the samples, Kapton® H = 1.42 g/cm2, Ag-coated FEP = 2.15 g/cm2. 2.3.3 Thermo-Optical Property Evaluation The thermo-optical properties, solar absorptance (αS) and normal infrared emittance (εN), were measured by

means of a spectrophotometer (U-4100; Hitachi High-Technologies Corp.) and a portable reflectometer (TESA 2000; AZ Technology Corp.) before and after the irradiation tests. 2.3.4 Irradiated Surface Observations

The irradiated surfaces of the samples were observed by means of scanning electron microscopy (SEM) (JSM-6360NS; JEOL). The samples were deposited with Pt in preparation for SEM observations. The samples were tilted by approximately 20 degrees during observations to facilitate viewing of the surface topography. 3. RESULTS 3.1 Appearance Change Figure 2 shows the appearance of Kapton® H samples. After the simultaneous irradiation, the center of the sample, i.e. the irradiated area, changed to a white diffusing surface (Fig. 2(b)). The change seen in the AO-irradiated sample (Fig. 2(c)) was similar to that in the simultaneously irradiated sample. The samples of VUV irradiation and EB irradiation showed no significant change (Fig. 2(d), (e)). Figure 3 shows the appearance of silver-coated FEP samples. Note that the pictures of simultaneously irradiated and EB-irradiated samples were taken with a cover glass on because the samples were curled up strongly (Fig. 3(b),(e)). On the simultaneously irradiated and EB-irradiated samples, wrinkling was found around the irradiated area, which could have resulted from shrinkage of the irradiated area. It was found that the area under simultaneous and EB irradiation became brittle, which resulted in tear. The clearness of the area under simultaneous irradiation seemed to have changed slightly. The AO-irradiated sample changed to a white diffusing surface (Fig. 3 (c)). The VUV-irradiated sample did not change significantly (Fig. 3 (d)).

3.2 Mass Change and Erosion Yield Figure 4 shows the results of average mass loss in irradiation tests. The initial mass is approx. 18.0 mg for Kapton® H, and approx. 28.5 mg for silver-coated FEP, respectively. According to Fig. 4 (a), which shows the results of Kapton® H, a 2.5 - 2.6 mg/cm2 loss was observed after simultaneous and AO irradiations; no significant mass loss was observed for EB and VUV irradiations. The erosion yield for Kapton® H calculated from the measured mass loss was approx. 3.0 × 10-24 cm3/atom.

According to Fig. 4 (b), which shows the results of silver-coated FEP, a 1.6 - 1.8 mg/cm2 loss was observed after simultaneous and AO irradiations; no significant mass loss was observed for EB and VUV irradiations, which seemed to have a similar tendency to Kapton® H. The erosion yield for silver-coated FEP was calculated to be approx. 1.5 × 10-24 cm3/atom. 3.3 Thermo-Optical Property Changes Figure 5 shows the measured results of solar absorptance, αS. The initial values are approx. 0.26 for

Fig. 4 Average mass loss for (a) Kapton® H and (b) silver-coated FEP.

(a)

(b)

(a) (b) (c)

(d) (e) Fig. 3 Appearance of silver-coated FEP of (a)

control, and after (b) three-beam simultaneous irradiation, (c) AO irradiation, (d) VUV irradiation, and (e) EB irradiation.

10mm

(a) (b) (c)

(d) (e) Fig. 2 Appearance of Kapton® H of (a) control, and

after (b) three-beam simultaneous irradiation, (c) AO irradiation, (d) VUV irradiation, and (e) EB irradiation.

10mm

Kapton® H, and approx. 0.02 for silver-coated FEP, respectively. According to Fig. 5 (a), which shows the results of Kapton® H, a ~0.02 gain was observed after simultaneous and AO irradiations; no significant change was observed for EB and VUV irradiations. According to Fig. 5 (b), which shows the results of silver-coated FEP, a ~0.04 gain was observed after AO irradiations; no significant change was observed for simultaneous, EB and VUV irradiations. Figure 6 shows the measured results of normal infrared emittance, εN. The initial values are approx. 0.52 for Kapton® H, and approx. 0.60 for silver-coated FEP, respectively. According to Fig. 6 (a), which shows the results of Kapton® H, a 0.16 - 0.18 decrease was observed after simultaneous and AO irradiations; no significant change was observed for EB and VUV irradiations. According to Fig. 6 (b), which shows the results of Silver-coated FEP, a ~0.07 decrease was observed after AO irradiations; no significant change was

observed for simultaneous, EB and VUV irradiations 3.4 Irradiated Surface Morphology Figure 7 shows the SEM images of Kapton® H samples. After simultaneous and AO irradiations, the surface obtained a needle-like shape which is commonly seen on AO-eroded surfaces (Fig. 7(b), (c)). The samples of VUV irradiation and EB irradiation showed no significant change (Fig. 7(d), (e)). Figure 8 shows the SEM images of silver-coated FEP samples. After simultaneous irradiation, the surface became slightly rough (Fig. 8(b)). It is quite different from the surfaces seen in other results. The surface of the AO-irradiated sample showed a needle-like shape which was similar to that of AO-eroded Kapton® H. The samples of VUV irradiation and EB irradiation showed no significant change (Fig. 8(d), (e)).

Fig. 6 Average normal infrared emittance (εN) change for (a) Kapton® H and (b) silver-coated FEP.

(a)

(b)

Fig. 5 Average solar absorptance (αS) change for (a) Kapton® H and (b) silver-coated FEP.

(a)

(b)

4. DISCUSSION The results of Kapton® H are summarized in Table 2. From Table 2, the results obtained from simultaneous irradiation and AO irradiation in which significant changes were observed were almost the same. Thus, simultaneous irradiation did not have any special effect on Kapton® H, i.e. it can be concluded that AO attack was the dominant factor for Kapton® H even in simultaneous three-beam irradiation. In Table 3, the results obtained from the irradiation tests on silver-coated FEP are summarized. the microscopic shape of the simultaneously irradiated sample was different from the others, which is a unique result of the simultaneous irradiation. As shown in Fig. 8,

the roughness of the surface increased due to AO irradiation (Fig. 8(c)). A smoothing effect of VUV on the FEP surface in a smaller scale was reported; this effect was observed by means of an Atomic Force Microscope4. However, no significant change can be found on the VUV-irradiated sample surface as shown in Fig. 8(d). The unique surface observed on the simultaneously irradiated sample may have resulted from an increase in roughness due to AO erosion, occurring simultaneously with smoothing by VUV. Such phenomenon was also found in our previous investigation where a two-beam simultaneous irradiation experiment was performed with AO and VUV4. Though the simultaneously irradiated silver-coated FEP sample underwent mass loss, which was evidence of erosion by AO (Fig. 4(b)), the appearance of the simultaneously irradiated sample did not show the white diffusing surface that was seen in the AO-irradiated sample (Fig. 3(b),(c)). This can be explained by the same mechanism mentioned above: an AO-attacked rough surface would be smoothed by VUV, resulting in a reduced roughness. Curl up, wrinkling, and embrittlement found in the simultaneously irradiated sample (Fig. 3(b)) were similar to those found in the EB irradiated sample (Fig. 3 (e)). FEP is known as one of the materials that can be affected by EB and result in decomposition and embrittlement8. From the experimental results and other reports, it could be concluded that the appearance change of simultaneously irradiated silver-coated FEP was caused by EB. A similar result to the present experiment has been reported before: silver-coated FEP retrieved from the Hubble Space Telescope showed shrinkage on the exposed surface and curl-up9. The mass loss of the simultaneously irradiated silver-coated FEP was almost the same as that of the AO-irradiated sample (Fig. 4(b)). It is known that silver-coated FEP can be affected by both AO and VUV, resulting in mass loss3. When AO and VUV are irradiated simultaneously, the mass loss is the sum of the mass loss caused by each irradiation3. For the present experiment, the mass loss would be the sum; however, it could not be observed because the mass loss by VUV irradiation was too small. Thus, the mass loss of the simultaneously irradiated sample was almost the same as that of the AO-irradiated sample. In the αS measurement for silver-coated FEP, no significant change was observed for EB and VUV irradiations; an increase was observed for AO irradiation, which resulted from the AO-attacked rough surface (Fig. 5(b), Fig. 8(c)). For simultaneous irradiation, the surface roughness is less than that of the AO-irradiated sample (Fig. 8(b)), resulting in no significant change in αS after simultaneous irradiation. On the other hand, an εN decrease was observed only for the AO-irradiated sample

(a) (b) (c)

(d) (e) Fig. 8 SEM images of silver-coated FEP of (a)

control, and after (b) three-beam simultaneous irradiation, (c) AO irradiation, (d) VUV irradiation, and (e) EB irradiation.

5μm

(a) (b) (c)

(d) (e) Fig. 7 SEM images of Kapton® H of (a) control, and

after (b) three-beam simultaneous irradiation, (c) AO irradiation, (d) VUV irradiation, and (e) EB irradiation.

5μm

Table 2 Evaluation results of Kapton H after each irradiation test.

Irradiation method Simultaneous Single

AO+VUV+EB AO VUV EB

Appearance Diffusing surface Y Y N N

Curled N N N N Wrinkled N N N N

Mass loss, mg/cm2 ca. 2.5 ca. 2.6 N N Erosion yield, 10-24 cm3/atom ca. 3.2 ca. 3.4 N/A N/A

αS change ca. +0.03 ca. +0.02 N N εN change ca. -0.16 ca. -0.18 N N

Surface morphology needle-like needle-like N N Embrittlement N N N N

Y: occurred N: almost no change or did not occur N/A: not applicable

Table 3 Evaluation results of silver-coated FEP after each irradiation test.

Irradiation method Simultaneous Single

AO+VUV+EB AO VUV EB

Appearance Diffusing surface N Y N N

Curled Y N N Y Wrinkled Y N N Y

Mass loss, mg/cm2 ca. 1.6 ca. 1.8 N N Erosion yield, 10-24 cm3/atom ca. 1.4 ca. 1.5 N/A N/A

αS change N ca. +0.04 N N εN change N ca. -0.08 N N

Surface morphology slightly-rough needle-like N N Embrittlement Y N N Y

Y: occurred N: almost no change or did not occur N/A: not applicable

(Fig. 6(b)). It might have been caused by the thickness decrease by AO erosion. However, the result of the simultaneously irradiated sample did not show any decrease, though the thickness should decrease. It would be expected that the εN of the simultaneously irradiated sample might have the same tendency as that of the AO-irradiated sample. It is necessary to carry out additional investigation, such as qualitative analysis. 5. CONCLUSIONS We performed a simultaneous irradiation with AO, EB, and VUV on Kapton® H and silver-coated FEP at the Combined Space Effects Test Facility, to simulate a LEO environment around a 400 km altitude for 3.5 months. For Kapton® H, the results obtained from simultaneous irradiation and AO irradiation were almost the same. This revealed that the simultaneous irradiation did not have any additional effect on Kapton® H, i.e. it can be concluded that AO attack was the dominant factor

for Kapton® H even in simultaneous three-beam irradiation. For silver-coated FEP, the roughness of the simultaneously irradiated surface increased slightly, but it was less than that of the AO-irradiated sample. It could be caused by the smoothing effect of VUV on the FEP surface. The solar absorptance change can also be explained by the AO and VUV concurrence effects. The appearance change and mass loss that occurred by three-beam simultaneous irradiation can be understood by summing each single beam effect. It was reported that wrinkling and embrittlement of FEP could be caused by the EB. The mass loss of the simultaneously irradiated sample was almost the same as that of the AO-irradiated sample; thus the thickness loss was also the same. Though it should result in a decrease of εN, the experimental result did not agree with the expected one. It is necessary carry out additional investigation, especially a qualitative analysis, to find out the reason.

ACKNOWLEDGMENTS The authors gratefully acknowledge the support of Ms. Kaori Umeda, Mr. Kensuke Takahashi, Mr. Yoshinobu Miyagawa, Advanced Engineering Services Co., Ltd., and Mr. Susumu Baba, JAXA, in conducting the experiments described in this paper. REFERENCES 1 F. W. Crossman, “Spacecraft Material Applications –

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