COMBINED EFFECT OF ATOMIC OXYGEN AND VACUUM ULTRAVIOLET FROM DEUTERIUM LAMP ON THE EROSION OF FLUORINATED POLYMER

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1 COMBINED EFFECT OF ATOMIC OXYGEN AND VACUUM ULTRAVIOLET FROM DEUTERIUM LAMP ON THE EROSION OF FLUORINATED POLYMER Kumiko Yokota (1), Kazuhiro Kishida (2), Akio Okamoto (3), Junichiro Ishizawa (4), Masahito Tagawa (5) (1) Graduate School of Engineering, Kobe University, Rokko-dai 1-1, Nada, Kobe, Japan, , (2) Graduate School of Engineering, Kobe University, Rokko-dai 1-1, Nada, Kobe, Japan, , (3) Research Institute of Osaka Prefecture, Ayumino 2-7-1, Izumi, Osaka, Japan, , (4) Japan Aerospace Exploration Agency, Sengen 2-1-1, Tsukuba, Japan, (5) Graduate School of Engineering, Kobe University, Rokko-dai 1-1, Nada, Kobe, Japan, , ABSTRACT In order to confirm the absence of synergistic effect, mass loss of a fluorinated polymer was measured under simultaneous exposure of hyperthermal atomic oxygen beam and deuterium lamp. It was confirmed that the fluorinated polymer does not show significant synergistic effect on mass-loss by atomic oxygen and vacuum ultraviolet (VUV) from a deuterium and excimer lamps; it was an additive effect. It was confirmed that the origin of the accelerated erosion of fluorinated polymers in atomic oxygen ground testing is not a VUV from the oxygen plasma. Contribution of ion component was also evaluated using a retarding grid. It was found that the contribution of ions in the beam cannot be ignored for fluorinated polymers if the oxygen ions are involved in the beam. 1. INTORODUCTION Synergistic effect is one of the key issues in space environmental effect of materials. Among many environmental factors in space, synergistic effect of atomic oxygen and ultraviolet is most important from a practical point of view; strong ultraviolet (UV) exposure always affects the results of atomic oxygen (AO) tests in ground-based facilities. Because of the difficulty for eliminating UV from a ground-based AO testing, effect of UV from oxygen plasma is always included in erosion property by AO evaluated in a ground-based facility. As a result, some UV-sensitive materials could indicate an erosion rate greater than that by AO. To evaluate the effect of AO without UV on material erosion accurately, the experimental/analytical methods to eliminate the effect of UV in ground based AO testing are required. In this process, degree of synergistic effect, if present, has to be evaluated. One of the most suitable materials to be tested is believed to be UV-sensitive material such as fluorinated polymers. It has been reported that fluorinated ethylene polymer (FEP) Teflon eroded much faster in ground-based facilities than in space [1, 2]. This phenomenon is believed due to ultraviolet, which is a byproduct from oxygen plasma, and many studies regarding synergistic effect of AO and vacuum ultraviolet (VUV) have been conducted [3-6]. However, in our previous results using quartz crystal microbalance (QCM) and high-speed chopper wheel, we have confirmed that UV from oxygen plasma does not affect significant synergistic effect; i.e., it is almost an additive effect [7]. This experiment was performed with 172 nm monochromatic VUV source. If this is true for other UV wavelength, ground-based AO tests for fluorinated materials become rather simple. Namely, UV effect can be simply subtracted from AO+UV results which can be obtained by conventional AO testing. In this study, we have evaluated the effect of combined effect of AO beam and UV from deuterium lamp, which is commonly used as a UV source in many space environmental effect studies. The main purpose of this study is to confirm the presence or absence of synergistic effect of AO and deuterium lamp on fluorinated polymer. Also the effect of charged particles included in the beam on the erosion of fluorinated polymers was evaluated. 2. APPARATUS AND PROCEDURES The laser-detonation atomic oxygen beam source used in this study has been developed at Kobe University in order to study gas/surface reactions of hyperthermal atomic oxygen with solid materials. This

2 type of the source was known as a PSI-type source [8]. Figure 1 shows a schematic diagram of the facility. This type of beam source uses pulsed supersonic valve (PSV) and carbon dioxide laser (10.6 µm, 5-7 J/pulse), and delivers intense atomic oxygen beam pulses with velocity of approximately 8 km/s. In order to block the UV from oxygen plasma in the nozzle, a high-speed chopper system was installed. Figure 2 shows the block diagram of the chopper system. A stainless steel disk was driven by an AC hysteresis synchronous motor at 150 Hz. A photodiode detects the position of two slits on the disk. A 300 Hz TTL signal, thus generated, is deduced to 1/300 and 1Hz system clock was created. The PSV and laser operation was based on this system clock. Velocity of the atomic oxygen pulse was adjusted by the laser delay, however, blocking of ultraviolet was Sample on the Rotatable QCM QMS & Scintillator Excimer or D 2 lamp 172nm VUV Chopper Au mirror Nozzle PSV Laser Figure 1. The laser detonation atomic oxygen beam source used in this study. Excimer and deuterium lamps are equipped as an external source of VUV. made by adjusting the system delay that controls position of the slit at the moment of laser flash. The hyperthermal beam was characterized by a time-of-flight (TOF) distribution measured by the quadrupole mass spectrometer (QMS) and scintillation detector which are installed in the beamline. Translational energies of the species in the beam were calculated using TOF distributions with the flight length of 238 cm. The UV signal also be detected by the scintillation detector, such that the elimination of UV from oxygen plasma was identified by the TOF spectra measured. A 30 W deuterium lamp with MgF 2 window (Hamamatsu L-8998, C-9598) was used as an external VUV source. The axes of atomic oxygen and deuterium lamp are crossed 90 degrees, and the sample can be rotated with the axis perpendicular to both AO and VUV (See Figure 3). Thus, the intensities of AO and VUV per unit area can be adjusted by rotating the sample. The mass change of the polymeric materials (fluorinated polymer and polyimide) was detected from the frequency shift of QCM on which the specimens were coated. Samples used in this study are two types of polymers; polyimide and fluorinated polymer. A polyimide amide acid was spin-coated on a quartz crystal at 12,000 rpm for 30 s, and curing treatments at 150 C for 1 hr. and 300 C for 1 hr. were carried out in order to form the spin-coated polyimide structure with a thickness of approximately 0.1 µm. The spincoated polyimide film, thus formed, and Kapton-H film has the same repeating unit of polymer (PMDA- ODA polyimide). In contrast, fluorinated polymer was prepared by the plasma-assisted physical vapor deposition technique developed at Technology Sample Scintillation detector Skimmer PSV Laser QMS Amp PD Laser delay 1/300 Chopper Wheel (150 Hz) Shaper System delay PSV t MCS, PC Figure 2 The block diagram of the high-speed chopper system used in this study. Operation of whole system was based on the 300 Hz signal generated by the chopper wheel. Figure 3 Photograph of the deuterium lamp installed in the atomic oxygen beam source. The axis of VUV light is crossed 90 degrees from that of atomic oxygen.

3 500 T08z15AA Intensity (counts) T08401AO Intensity (counts) Flight time (µs) Figure 5 The TOF spectrum of AO beam sliced by the chopper wheel. System Delay (SD) was settled to 2.92 ms to block the VUV from oxygen plasma Flight time (µs) Figure 4 Photograph of the laser-sustained plasma (exposure time: 1/30 s) and the TOF distribution of m/q=16 without the chopper wheel. Average beam energy: 4.4 ev Research Institute of Osaka Prefecture [9]. All exposure experiments were carried out with sample temperature at 38 C. 3. RESULTS AND DISCUSSION Figure 4 represents the photograph of the lasersustained plasma and typical TOF distribution of mass/charge ratio (m/q) =16 without chopper wheel. Luminescence from the excited oxygen atoms/molecules is clearly observed in the photograph. The sharp spike at time-zero in the TOF spectrum in Figure 4 corresponds to the photon signal from the laser-sustained plasma shown in the upper panel. In this TOF spectrum, the peak near µs comes from the hyperthermal AO component in the beam. The TOF spectra measured with the chopper wheel are shown in Figure 5. The system delay was settled to 2.92 ms to block UV from the oxygen plasma. It has been clearly indicated that the only AO signal is obvious in Figure 5; UV signal at time zero was lost. The slit of the wheel is selected to be large enough to pass the entire AO beam. The effect of external VUV exposure on the AOinduced erosion was examined. (The experimental setup is shown in Figure 3.) The 172 nm excimer light or 30 W deuterium lamp with MgF 2 window irradiated the fluorinated polymer located at the downstream of the chopper wheel. Again, the exposure axis was 90 degrees from the AO beam axis such that relative intensity of AO and VUV can be adjusted by rotating the QCM. Figures 6 shows the experimental results for mass-loss of fluorinated polymer by 172 nm excimer and deuterium lamp, respectively. The abscissa shows the incident angle of AO beam (θ) such that 90 - θ corresponds to that of VUV light. The gray and white circles represent the data for the individual exposures of AO or VUV. These two data plots follow cosine distribution on the incident angle of AO and VUV respectively (dashed lines). These erosion properties agree with those measured for polyimide in our previous study [10]. Sum of the best-fit cosine curves of the individual exposures (dashed lines) are shown in the solid line in Figure 6. On the other hand, the black circles in Figure 6 represent the erosion data measured under the simultaneous exposure condition of AO + VUV. It is clearly indicated that the data points shown in the black circles follow the solid line, which is the sum of the erosion rates under the individual exposures of AO and VUV. This is true for both lamps. This experimental result is evidence that the fluorinated polymer does not show synergistic effect of AO and VUV, i.e., the erosion rate of fluorinated polymer in the simultaneous exposure condition is expressed simply by the sum of those in the individual exposures under the VUV exposures by excimer and deuterium lamps. On the other hand, polyimide does not react with VUV from deuterium lamp (See Figure 7) as expected by the previous results using excimer lamp [11]. Note that the synergistic effect of AO and VUV

4 Erosion Rate (Hz/s) AO only Excimer only AO + Excimer Erosion Rate (Hz/s) FEP+D2 AO + D2 D2 only AO only Incident Angle of AO (deg.) Incident angle of AO (deg.) Figure 6 The relationship between the incident angle of AO and the frequency shift of fluorinated polymer under individual and simultaneous exposures of AO and VUV. (left panel) Excimer lamp and (right panel) deuterium lamp. White circle: VUV alone, grey circle: AO alone, and black circle: AO+VUV condition from deuterium lamp was not obvious in Figure 7. This is probably due to the difference in intensity of VUV. The results of this study clearly indicate there exists no synergistic effect of AO and VUV on the accelerated erosion of the fluorinated polymer. In Figure 6, the erosion rate of the fluorinated polymer in the AO alone exposure condition (0.006 Hz/s) is calculated to be 1.95 x cm 3 /atom with considering the sample area and densities (0.40 cm 2 and 2.2 g/cm 3 for fluorinated polymer). This value is still much greater than that reported in the flight experiment [12]. This experimental data suggests that there exist other origins of accelerated erosion Erosion Rate (Hz/s) Polyimide Incident angle (deg.) Figure 7 The relationship between the incident angle of AO and the frequency shift of polyimide under individual and simultaneous exposures of AO and VUV from deuterium lamp. White circle: VUV alone, grey circle: AO alone, and black circle: AO+VUV condition phenomena of fluorinated polymer in the laserdetonation AO beam source beside UV from the oxygen plasma. 4. POSSIBLE ORIGINS The origin of the accelerated erosion of fluorinated polymers in AO ground testing should be included in AO beam itself; i.e., it was not affected by VUV. The remaining possible origins are either (1) internal states of oxygen atom, (2) wide distributed impact energy of AO, (3) ion component and/or (4) collision-induced desorption. Among these possible origins, internal states of oxygen atom generated by the laser detonation source were analyzed by Minton and coworkers [13]. They clearly indicated that the oxygen atoms included in the beam are mainly O( 3 P), which is a ground-state. This is consistent with actual LEO space environment. The contribution of high-energy component of AO beam is discussed in the companion paper [14]. The contribution of ion component could simply be evaluated by a retarding grid located in front of the chopper wheel. Such experiment was performed in this study. Figure 8 represents the relationship between applied potential and erosion rate of polyimide and fluorinated polymer. It was found that the mass-loss rate of fluorinated polymer film decreased when negative voltage was applied to the grid. In contrast, that of polyimide was independent on the grid potential. Note that the sample potential was kept at ground potential in these experiments. The ion charge at the sample position was measured to be 1.84 x 10-7 C/pulse without grid potential, which is 0.38 % of the AO flux measured by the Ag-QCM if

5 Reaction efficiency (x10-24 cm 3 /atom) : Fluorinated Polyimide polymer : Polyimide Fluorinated polymer Figure 8 Reaction efficiency of polyimide and fluorinated polymer as a function of the grid voltage. considered to be single charged ions. 0.38% of ion content was decreased to % with the grid potential of 350 V. From the results indicated in Figure 8, it was concluded that negative ion (may be O - or electrons) could affect the erosion of fluorinated polymer, but not for polyimide. Thus, the contribution of negative ions or electrons in the beam cannot be ignored for fluorinated polymers if they are involved in the beam. 5. CONCLUSIONS In order to confirm the absence of synergistic effect, mass loss of a fluorinated polymer was measured under simultaneous exposure of hyperthermal atomic oxygen beam and deuterium lamp. It was confirmed that the fluorinated polymer does not show significant synergistic effects on mass loss by atomic oxygen and VUV both from excimer and deuterium lamps. They showed additive effect. Since erosion rate of fluorinated polymer is greater than polyimide, the origins of the accelerated erosion of fluorinated polymers were included in the atomic oxygen beam itself. Contribution of ions was evaluated in this study. It was found that the mass-loss rate of fluorinated polymer film was affected by negative ions or electrons in the beam. ACKNOWLEDGMENTS Grid voltage (V) The authors would like to thank Prof. T. K. Minton of Montana State University for his help for providing the high-speed chopper device technology. A part of this work was supported by the Grant-in-Aids for Scientific Research from JSPS. REFERENCES [1] Weihs B., van Eesbeek M., Secondary VUV erosion effect on polymers in the ATOX atomic oxygen exposure facility, Proceedings of 6 th International Symposium on Materials in a Space Environment, ESA SP-368, 1994, pp [2] van Eesbeek M., Levadou F., Skurat V. E., Dorofeev Y. I., Vasilets V. N., Babashev E. A., Degradation of Teflon FEP due to VUV and atomic oxygen exposure, Proceedings of 6 th International Symposium on Materials in a Space Environment, ESA SP-368, 1994, pp [3] Grossman E., Noter Y., Lifshitz Y., Oxygen and VUV irradiation of polymers: atomic force microscopy (AFM) and complementary studies, Proceedings of the 7th International Symposium on Materials in a Space Environment, ESA SP- 399, Toulouse, France (1997) pp [4] Zhang J., Lindholm N. F., Brunsvold A. L., Upadhyaya H. D., Monton T. K., Tagawa M., Erosion of FEP Teflon and PMMA by VUV radiation and hyperthermal O or Ar atoms, ACS Appl. Mater. Interfaces, Vol.1, No.3 (2009) pp [5] Skurat V. E., Samsonov P. V., Nikiforov A. P., Vacuum ultraviolet radiation in sources of hyperthermal atomic oxygen. Photodestruction of polytetrafluoroethylene (PTFE) and Teflon FEO for indication of radiation, High Performance Polymers, Vol.16, No.2 (2006) pp [6] Intrater R., Lempert G., Gouzman I., Grossman E., Hoffman A., The effect of polyethylenes structure on its interaction with a simulated LEO environment, Proceedings of the 9th international Symposium on Materials in a Space Environment, ESA SP-540, Noordwijk, The Netherlands (2003) pp [7] Tagawa M., Abe S., Kishida K., Yokota K., Okamoto A., Synergistic effect of EUV from the laser-sustained oxygen plasma in the groundbased atomic oxygen simulation of fluorinated polymers, Proceedings of the 9th International Conference on the Protection of Materials and Structures from the LEO Space Environment, 2008, Toronto, Canada pp

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