NASA / TMm1998-207918 / REV1

A Comparison of Space and Ground Based

Facility Environmental Effects

for FEP Teflon

Sharon K. Rutledge and Bruce A. Banks

Lewis Research Center, Cleveland, Ohio

Michael Kitral

Cleveland State University, Cleveland, Ohio

Prepared for the

Fourth International Space Conference on Protection of Materials and

Structures from the LEO Space Environment

sponsored by the University of Toronto Institute for Aerospace Studies

Toronto, Canada, April 23-24, 1998

National Aeronautics and

Space Administration

Lewis Research Center

July 1998
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Aeronautics and Space Administration.

NASA Center for Aerospace Information

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A COMPARISONOFSPACEANDGROUNDBASEDFACILITY

ENVIRONMENTALEFFECTSFORFEPTEFLON

SHARON K. RUTLEDGE AND BRUCE A. BANKS

NASA Lewis Research Center

21000 Brookpark Rd.

Cleveland, Ohio 44135; U.S.A.

MICHAEL KITRAL

Cleveland State University

Euclid Ave. at East 24th Street

Cleveland, Ohio 44115; U.S.A.

1.0 Abstract

Fluorinated Ethylene Propylene (FEP) Teflon is widely used as a thermal control

material for spacecraft, however, it is susceptible to erosion, cracking, and subsequent

mechanical failure in low Earth orbit. One of the difficulties in determining whether

FEP Teflon will survive during a mission is the wide disparity of erosion rates observed

for this material in space and in ground based facilities. Each environment contains

different levels of atomic oxygen, ions, and vacuum ultraviolet (VUV) radiation in

addition to parameters such as the energy of the arriving species and temperature. These

variations make it difficult to determine what is causing the observed differences in

erosion rates. This paper attempts to narrow down which factors affect the erosion rate

of FEP Teflon through attempting to change only one environmental constituent at a

time. This was attempted through the use of a single simulation facility (plasma asher)

environment with a variety of Faraday cages and VUV transparent windows. Isolating

one factor inside of a radio frequency (RF) plasma proved to be very difficult. Two

observations could be made. First, it appears that the erosion yield of FEP Teflon with

respect to that of polyimide Kapton is not greatly affected by the presence or lack of

VUV radiation present in the RF plasma and the relative erosion yield for the FEP

Teflon may decrease with increasing fluence. Second, shielding from charged particles

appears to lower the relative erosion yield of the FEP to approximately that observed in

space, however it is difficult to determine for sure whether ions, electrons, or some

other components are causing the enhanced erosion.

NASA/TM--1998-207918 1
2.0 Introduction

Materials qualification for atomic oxygen durability in low Earth orbit has been

performed in both ground based facilities and in space. The ground based facilities

have been developed out of the necessity to study long term atomic oxygen effects to

levels greater than can be achieved with a typical mission on the Space Shuttle. They

also have the advantage of quick turnaround, lower cost per exposure, and ease of

experiment tailorability.

Some questions naturally occur about the applicability of ground based test data

for use in predicting how a material will perform in low Earth orbit (LEO). These

questions arise due to the differences in the species, energy or charged state of the

arriving atoms and the intensity and wavelength of VUV radiation present. Other

parameters also may play a role, such as temperature. There is currently no known

facility, whether thermal plasma or directed beam, which will exactly duplicate the

conditions in low Earth orbit [1-7]. Such facilities, however, are needed to screen

materials for use in LEO. So it is important to understand why differences in erosion

rates occur so that the test equipment can be modified to produce results closer to those

in space, or to allow calibration factors to be developed.

Of all the materials, FEP Teflon seems to provide the widest disparity of data in

space and ground based systems. Typically FEP Teflon Exposed in RF plasmas has an

erosion yield relative to polyimide Kapton which is an order of magnitude greater than

in LEO. This paper attempts to determine what is present in one type of simulation

facility (RF plasma), that affects the erosion of FEP Teflon, by isolating selected

components in the environment that may have an effect on the erosion rate. VUV

radiation and charged species were the components selected to be the focus of this study

due to the wide belief among the LEO testing community that these are the important

factors.

3.0 Experimental Procedure

3.1. MATERIALS

Sample coupons, 2.54 cm diameter, were punched from 0.005 cm thick sheets of

polyimide Kapton I-IN (DuPont) and fluorinated ethylene propylene (FEP Teflon)

(DuPont). All of the samples were fully dehydrated for 48 hours in a vacuum of 8-13 Pa

(60-100 mTorr) prior to weighing and subsequent exposure in the atomic oxygen

plasma. Samples were quickly weighed after dehydration and upon removal from the

vacuum chamber after exposure to atomic oxygen in order to minimize errors in mass

due to water absorption [8].

3.2 PLASMA ASHER

An SPI Plasma Prep II asher operated on air was used as the environment simulator for

these tests. The asher operates by using RF (13.56 Mhz) to create a discharge between

two electrodes which surround a glass reaction chamber. A thermal plasma is produced

NASA/TM--1998-207918 2
whichisatanenergyof<<0.1eV.Theplasmacontainsatomicandexcitedstatespecies

aswellasmolecules, ions,electrons, andVUVradiation.Theamountsandintensityof

thelatterfivehavenotbeendetermined. Theeffectivearrivalofatomicoxygenis

estimatedbydeterminingthereactionrateofpolyimideKapton,whosespace

degradationrateiswellknown.Thenitrogenintheplasmahasbeenshowninearlier

testsnottoreactwiththematerialsthatarebeingtestedinthisstudy[8].Typical

vacuumchamber pessureduringoperationwas16-27Pa(120-200mTorr).

Temperaturemeasuredinthepastinsidetheplasmachamberwas65°C[9].

3.3FARADAYCAGESANDVUVWINDOWS

AluminumFaradaycageswereconstructedfromsinglesheetsof0.0127cmaluminum

foilsothatboxeswithattachedlidswereformed.Openingsunderthelidsonthesides

producedopenre-entrant boxesandpreventedline-of-sight exposureofthesamplesto

theexternalirplasma. Several modificationsweremadetotheboxestoallowvarious

plasmacomponents tobeexcludedorallowedintotheboxes.

Figure1showsthevariousboxdesignsusedfortesting.Figure1Aisaclosed

Faradaycage.ThisdesignissupposedtoblockVUVradiationandchargedspecies

fromthesamplesinsidewhileallowingatomicoxygentoscatterintotheboxthrough

theopeningsunderthelidonthesideofthebox.Thisboxdesignwasmodifiedin

Figure1Btoincludeanopeninginthelidsurroundedbyaluminumfoiltabstohold

variouswindowsandfiltersthatwere2.54cmdiameter. Thewindoworfilterafter

insertionwascoveredwithathinaluminumfoilsheetwithacircularholecutinthe

centerthatwasbondedtothesurfaceofthewindowandboxwithacrylicadhesive. This

preventedplasmafromleakingintotheboxaroundthewindow.

A1 Foil Holder

_---At ?oil Box with _in A1

A_:,_ached Lid Foil Pocket

@

Figure 1A. Closed Faraday cage

NASA/TM--1998-207918 3
_A1 !oil Holder

_Window or Fil_.er _in A1

Foil Pocket

®

Figure lB. Faraday cage with window or mesh filter

A magnesium fluoride window was used to allow VUV radiation of wavelengths

greater than approximately 120 nm, and visible light to impinge onto one sample placed

inside of one end of the box. The lransmission versus wavelength for the window is

shown in Figure 2. Another sample could be placed at the other end of the interior of the

box so that it would see all of the same conditions but not be exposed to the VUV

radiation entering through the window. A gold mesh filter was also used to allow all

wavelengths to enter the box but block most of the charged particles from entering. A

single mesh screen spot welded to an aluminum foil ring (0.0127 cm thick) was found to

actually accelerate charged species into the box, so a double screen was made from two

sheets of gold mesh spot welded to either side of an aluminum foil ring (0.0127 cm

thick). The gold mesh was manufactured by Buckbee Mears to have 80% transparency

at roughly 39 lines per centimeter of 0.00267 cm diameter gold wire.

In order to keep samples from sliding out from under the window during exposure

due to vibration, a 0.0127 cm thick aluminum foil rectangle that just fit into the interior

of the box was cut, as shown in Figure 1, to have tabs that would fold around the

samples to hold them in place. Each sample was wrapped with approximately 0.002 cm

thick aluminum foil with a 1.905 cm diameter hole punched in the front surface to allow

the atomic oxygen to reach the sample of FEP Teflon or polyimide Kapton HN.

3.4 DATA MEASUREMENT AND ANALYSIS

Changes in mass were recorded for FEP Teflon and polyimide Kapton HN for each test

configuration. The effective fluence (number of atoms of atomic oxygen arriving per

square centimeter over the test duration) was determined by measuring the mass loss per

unit area of polyimide Kapton HN and using Equation 1.

NASA/TM--1998-207918 4
_D

_D

100 -1-

90 4

80 q

70 4

604

50 4

40 4

30 -_

20 -_

10 4

0 I

100 150 200

I I I

250 300 350 400

Wavelength (nm)

Figure 2. Transmission of MgF2 Window

M/A

Fp*E

O)

Where:

F = Fluence (atoms/cm 2)

M= Mass loss of polyimide Kapton (g)

A = Area ofpolyimide Kapton exposed to atomic oxygen (cm 2)

p = Density ofpolyimide Kapton ( 1.42 g/cm _)

E = Erosion Yieldforpolyimide Kapton in LEO (3xlO -24 cm3/atom)

The effective erosion yield for FEP Teflon, which is the volume of FEP Teflon removed

for each effective atom that arrives, was determined by using Equation 2. The erosion

yield was then divided by the known erosion yield for polyimide Kapton in low Earth

orbit (3xl0 24 cm3/atom) to obtain the measurement relative to a known standard.

Erosion yields relative to polyimide Kapton were then plotted to look for potential

trends in the data.

M/A

E FEP --

p*F

(2)

NASA/TM-- 1998 -207918 5
Where:

ErEe = Erosion Yield for FEP Teflon (cm3/atom)

M = Mass loss of FEP Teflon (g)

A = Area of FEP Teflon exposed to atomic oxygen (cm 2)

p = Density of FEP Teflon (2.15 g/cm 3)

F = Fluence (atoms/cm 2) (based on polyimide Kapton)

4.0 Results and Discussion

4.1 EXPOSURE DIRECTLY IN THE PLASMA

FEP Teflon exposed in the plasma should see the total environment present including

charged species and VUV radiation. Several tests were conducted with FEP Teflon and

polyimide Kapton samples exposed to the plasma while sitting on top of glass

microscope slides that were approximately 7.62 cm x 2.54 cm in area. The average

erosion yield with respect to Kapton measured over approximately the same fluence

exposure was 1.9 + 0.14. Samples of FEP Teflon and polyimide Kapton were also

placed on top of some of the Faraday cages during testing to record what the effective

atomic oxygen fluence was outside of the Faraday cage. It was noticed that during these

tests, the erosion yield with respect to Kapton in the plasma but on top of the aluminum

surface was lower than that observed on glass slides (between 1 and 1.2). This was

unexpected, because the plasma environment was believed to not be affected by the

sample holders. Another test was conducted, but this time the FEP Teflon and

polyimide Kapton were placed in aluminum foil covers and held down to an aluminum

sheet (identical sample holders used in the Faraday cages and described earlier) with

aluminum foil tabs. This placed the samples in good electrical contact with the

aluminum and also allowed them to sit in the same location in the asher as the samples

on glass slides had been during previous testing. The erosion yield for FEP Teflon with

respect to Kapton was even lower for this test (0.374 ± 0.036). A comparison bar chart

with error bars is shown in Figure 3.

It can be seen that the differences in the erosion yields are much greater than the

measurement error. Flux levels varied during each test, but the overall fluence was

approximately 2x102° atoms/cm 2on the average. Table I contains the data for all of the

tests performed including the effective atomic oxygen flux. There does not appear to be

any relationship between flux and relative erosion yield. It is possible that the surface

conductivity may affect the arrival of charged species at the sample surface or

that VUV radiation intensities may be altered near the sample. In order to try to narrow

down the reason for these differences, testing was conducted inside Faraday cages

where some of these environments could potentially be reduced or isolated.

NASA/TM--1998-207918 6
4, --

Z 3.5-

[-

_ 2.5.

E-

_ 1.5

Z

_ 1

N

11

0.5

I

On Glass Slides On Single Mesh Cage

m m

I )

On I_uble Mesh Cage In Aluminum Holder

Figure 3. Comparison of relative erosion yields for FEP Teflon Exposed In Plasma

Table I. Data From Plasma Asher Exposures

Description

1118 In plasma

on glass

1118 In plasma

on glass

1120 In plasma

on g]ass

1120 In plasma

on glass

406 In plasma

on glass

406 In plasma

on glass

409 In plasma

on glass

409 In plasma

on glass

402 In plasma

on aluminum

holder

Mamrial

Kapton

FEP

Kapton

FEP

Time (In's)

7.98

7.98

9.18

9.18

Effective Atomic Oxygen

Flux _atoms/cm2-sec)

7.43E15 ± 7.37e14

7.43E15 ± 7.37e14

6.5E15 ± 6.485E14

6.5E15 ± 6.485E14

Erosion Yield With

Respect to Kapton

1.0

2.0065 ± 0.28

1.0

1.968 ± 0.278

Kapton 1.0 4.67 5.11E15 ± 1.13E14

FEP 4.67 5.11E15 ± 1.13E14 1.6898 ± 0.050

Kapton 5 5.14E15 ± 5.13E14 1.0

FEP 5 5.14E15 ± 5.13E14 1.93 ± 0.271

Kapton 5.633 1.406E16 ± 5.037E14 1.0

402 In plasma FEP 5.633 1.406E16 ± 5.037E14 0.3742 ± 0.03575

on aluminum

holder

NASA/TM--1998-207918 7
Description

415 In plasma

on top of double

mesh cai[e

415 In plasma

on top of double

mesh cage

410 In plasma

on top of single

mesh cage

410 In plasma

on top of single

mesh ca_e

410 In single

mesh windowed

ca[e

410 In single

mesh windowed

ca_e

410 In single

mesh windowed

ca_e

415 In double

mesh windowed

case

415 In double

mesh windowed

cage

415 In double

mesh windowed

case

130 Closed

Cage with One

M£F2 Window

130 Closed

Cage with One

MBF2 Window

130 Closed

Cage with one

MI_F2 Window

331 Closed

Cage with

MgF2 Window

331 Closed

Cage with

MsF2 Window

401 Closed

Cage with

M[[F2 Window

401 Closed

Cage with

M_F2 Window

Ma_ri_

Kapton

FEP

Kapton

Time (hrs)

42.1

42.1

5.05

Effective Atomic Oxygen

Flux (atoms/em2-sec)

4.366E15 ± 8.66E13

4.366E15 ± 8.66E13

4.86E15 ± 1.07E14

FEP 5.05 4.86E15 ± 1.07E14

Kapton 5.05 4.72E15 ± 1.856E14

Erosion Yield With

Respect to Kapton

1.0

1.015 ± 0.0284

1.0

1.223 ± 0.037

1.0

FEP (Under 5.05 4.72E15 ± 1.856E14 3.53 ± 0.1857

Mesh)

FEP (Shielded) 5.05 4.72E15 ± 1.856E14 0.0204 ± 0.0167

Kapton 42.1 4.666E15 ± 1.64E14 1.0

FEP (Under 42.1 4.666E15 ± 1.64E14 0.117 ± 0.00596

Mesh)

FEP (Shielded) 42.1 4.666E15 ± 1.64E14 0.0321 ± 0.0027

Kapton 24.3 4.07E15 ± 4.03E14 1.0

(shidded)

FEP (under 24.3 4.07E15 ± 4.03E14 0.103 ± 0.0143

window)

FEP (shidded) 24.3 4.07E15 ± 4.03E14 0.091 ± 0.0127

Kapton (under 24.017 8.75E15 ± 3.08E14 0.9432 ± 0.047

window)

Kapton 24.017 8.75E15 ± 3.08E14 1.0

Kapton 24.8 1.48E16 ± 5.19E14 1.0

FEP (under 24.8 1.48E16 ± 5.19E14 0.0325 ± 0.0018

window)

NASA/TM-- 1998 -207918 8
Description

401Closed

Cagewith

MgF2Window

406Closed

Cagewith

MgF2Window

406Closed

Cage with

MgF2 Window

406 Closed

Cage with

MgF2 Window

522 Closed

Faraday cage

522 Closed

Faraday ca_e

530 Closed

Faraday cage

530 Closed

Faraday ca6e

609 Closed

Faraday ca_e

609 Closed

Faraday ca_e

1231A Closed

cage with MgF2

window

Mamri_

FEP (Shielded)

Time (hrs)

24.8

Effective Atomic Oxygen

Flux (atoms/cm2-sec)

1.48E16 ± 5.19E14

Kapton 48.967 1.283E16 ± 4.5E14 1.0

Erosion Yield With

Respect to Kapton

0.0217 ± 0.0011

FEP (Under 48.967 1.283E16 + 4.5E14 0.0327 ± 0.00163

window)

FEP (Shielded) 48.967 1.283E16 ± 4.5E14 0.0085 ± 0.00118

Kapton 47.88 4.77E15 ± 4.71E14 1.0

FEP 47.88 4.77E15 ± 4.71E14 0.0374 ± 0.0062

Kapton 5.4 6.85E15 ± 6.8E14 1.0

FEP

Kapton

6.85E15 ± 6.8E14

6.64E15 -t. 6.86E14

5.4

5.08

0.195 ± 0.0422

1.0

FEP 5.08 6.64E15 ± 6.86E14 0.15 ± 0.028

Knpton (under 5 2.55E15 ± 2.56E14 1.0

window)

1231A Closed Kapton 5 2.55E15 ± 2.56E14 0.982 ± 0.139

cage with MgF2 (shielded)

window

1231B Closed Kapton (under 27.033 3.09E15 ± 3.05E14 1.0

cage with MgF2 window)

window

1231B Closed Kapton 27.033 3.09E15 ± 3.05E14 0.814 ± 0.114

cage with MgF2 (shielded)

window

4.2 EXPOSURE INSIDE A CLOSED FARADAY CAGE

The closed Faraday cage was used in an attempt to prevent VUV radiation from

reaching the surface of the samples by allowing no line-of-sight viewing of the samples

to the plasma. It was also designed to prevent charged species from entering by allowing

the box to float to plasma potential. Several tests at a variety of fluence levels were

conducted with FEP Teflon inside the Faraday cage. Results varied but the erosion

yield for FEP Teflon relative to Kapton was less than 0.2 in all cases as shown in

Figure 4. It was noticed that the relative erosion yield seemed to decrease with

increasing fluence. The data plotted as a function of effective fluence is shown in Figure

5. The relative erosion yield does appear to take on an almost exponential decay

NASA/TM--1998-207918 9
0.25

0.2

O

EEt_

e. 0.15

_ 0.1

,-1

_ 0.05

_a

m I I t I I

522 530 609 130 401 406

SAMPLE NUMBER

Figure 4. Comparison of relative erosion yields for FEP Teflon inside a closed

Faraday cage

with increasing fluence. It is not known why this would occur in an isotropic plasma.

More testing and data are needed to verify this trend.

Another puzzling feature noticed during these tests is that the effective flux inside

and outside of the Faraday cage is approximately the same. One would expect the flux

inside the Faraday cage to be greatly reduced because atoms would need to scatter in

through slots underneath the lid in order to enter the box. However, this does not appear

to be the case. It is possible that the RF field is exciting a plasma inside the box, but no

glow was observed inside during testing. The greatly reduced relative erosion yield for

FEP Teflon also does not support this theory. The relative erosion yield measured inside

the Faraday cage is near that observed in space on the Long Duration Exposure Facility

(LDEF) (0.11 _+0.002) [10].

4.3 EXPOSURE INSIDE FARADAY CAGE WITH MAGNESIUM FLUORIDE

WINDOW

Because it is difficult to draw conclusions from samples experiencing different fields

and fluxes, a Faraday cage was developed which could expose two samples inside the

same Faraday cage environment, but allow only one of them to have line of sight to the

plasma. This was accomplished by putting a magnesium fluoride window in the lid

towards one end of the Faraday cage box. Initially, polyimide Kapton HN erosion was

NASA/TM--1998-207918 10
0.25 .... " :

0.2

@

[..

au 0.15

,-1

Z

._ 0.05

0

0

I I I

5E+20 1E+21 1.5E+21

FLUENCE (_onH/on^2)

I I

2E+21 2.5E+21

Figure 5. FEP Teflon relative erosion yield as a function of effective atomic oxygen

fluence inside a closed Faraday cage

compared in order to verify that the window seal did not let additional plasma into the

box. The results shown in Figure 6 verify that the polyimide Kapton HN is not affected

by VUV radiation and that the seal functions as expected, because the erosion rates both

under the window and shielded from the plasma are nearly identical and within

experimental error.

The relative erosion yield of FEP Teflon was tested in a similar manner, but a

sample of polyimide Kapton was placed between the two FEP Teflon samples so that

the exposure flux could be measured. Figure 7 contains the data for the FEP Teflon. The

effect of VUV radiation transmitted by magnesium fluoride appears to be very low.

There may be a slight increase in relative erosion yield with VUV radiation, but not

enough to account for the nearly order of magnitude difference observed when exposed

directly in the plasma.

4.4 EXPOSURE INSIDE FARADAY CAGE WITH GOLD MESH SCREEN FILTER

In order to determine if shorter wavelength VUV radiation plays a role, it was necessary

to block charged species from entering the box but allow all wavelengths of radiation to

enter. A single gold mesh screen was installed in the Faraday cage replacing the

magnesium fluoride window used in the previous test. FEP Teflon and polyimide

Kapton HN were tested in the same manner as previously. During exposure,

NASA/TM--1998-207918 11
1.5

0.5  ̧

[] Undo" MgF2 W_dow

[] s_k_d

331 1231A 12311)

SAMPLE NUMBF_

Figure 6. Comparison of relative erosion yields for Kapton HN exposed in a

Faraday cage with and without VUV radiation

a directed plasma beam was observed that was approximately 1 cm in length extending

perpendicular to the mesh, centered on it and extending down through it. The erosion

yield of the FEP relative to Kapton in the shielded area of the box was roughly the same

as in the closed Faraday cage test. The relative erosion yield for the sample under the

mesh, however, was 3.53 ± 0.186. This is much higher than what was observed when

the sample was sitting directly in the plasma. It appears that the mesh screen

may have been acting like an electrostatic accelerator grid increasing the flux of

charged species into the box.

Because it was still not possible to determine if other wavelengths of VUV

radiation played a role, a double mesh was developed and installed in place of the single

mesh making sure that it was electrically connected to the box to provide an

equipotential gap to reduce the flux of charged species entering. The relative erosion

yield was then observed to drop for the sample underneath the double mesh in the

Faraday cage to approximately 0.12 ± 0.006. This value is near that observed inside the

closed Faraday cage and is similar to that observed in space.

NASA/TM-- 1998-207918 12
Z

O

<

"* 0.75

O

[..,

(..,

_ 0.5

Z 0.25

[] Under" MgF2 Window

m s_J_'

_ _ r

130 401 406

SAMPLE NUMBER

Figure 7. FEP Teflon exposed inside a Faraday cage with and without VUV

radiation

4.5 TESTING SUMMARY

Figure 8 contains a comparison chart of samples tested in the plasma asher under

various conditions as compared to the value measured in space. It appears that placing

FEP Teflon inside a Faraday cage will yield results very close to those observed in low

Earth orbit. It does not appear to matter whether VUV radiation can enter the Faraday

cage, only if charged species can enter. The greatly reduced erosion yield for FEP

Teflon samples sitting on a conductive plate but directly inside the plasma also indicates

that charged species may play a role. It is very difficult to isolate the effects of charged

species inside of an RF field. In order to draw more firm conclusions about

what is causing the differences in erosion yields, tests will need to be conducted in a

facility where the sample region is not inside the RF field such as in a directed beam

facility.

NASA/TM-- 1998-207918 13
3.5

O 3

2.5

m 2

1.5

.1

Z !

_ 0.5

I )

In Plasma In Plasma In Cage In Cage In C_e In CIo_d In Spac_

on Glass In Under Under Unde¢ Cage (LDEF)

Alummmn Single Double MgF2

Holder Mesh Mesh Window

Figure 8. Comparison of FEP Teflon relative erosion yields

5.0 CONCLUSIONS

These tests have shown that the accelerated erosion can be reduced by exposing the

samples on a conductive plate or inside a Faraday cage whether or not the samples are

exposed to the VUV radiation from the plasma. This may provide a means for testing of

samples such as FEP Teflon in plasma environments which more closely mimics the

erosion observed in LEO. More work needs to be done to determine if electrons, ions,

or some effect of the RF field is causing the accelerated erosion of FEP Teflon in

plasma asher facilities.

6.0 REFERENCES

1. Stidham, C.R.;et al. "Low Earth Orbital Atomic Oxygen Environmental Simulation Facility for Space

Materials Evaluation", NASA TM- 106128, 1993.

2. Johnson, C.L.; et al. "The Vanderbilt University Neuta-al O-Beam Facility", 18 th International SAMPE

Technical Conference Proceedings, Oct. 7-9, 1986.

3. Arnold, G.S.; Peplinski, D.R.; "A Facility for Investigating Interactions of Energetic Atomic Oxygen

with Solids", Aerospace Report No. ATR-94(8540)-3, 1985.

4. Brim, D.E.; et al. "Production of Pulsed Atomic Oxygen Beams Via Laser Vaporization Methods",

18 th International SAMPE Technical Conference Proceedings, Oct. 7-9, 1986.

5. Rempt, R.D.; "Production of a Beam of Ground State Oxygen Atoms of Selectable Energy", 18 th

International SAMPE Technical Conference Proceedings, Oct. 7-9, 1986.

NASA/TM-- 1998-207918 14
6. Sjolander, G.W. and Bareiss, L.E.; "Martin Marietta Atomic Oxygen Beam Facility", 18m International

SAMPE Technical Conference Proceedings, Oct. 7-9, 1986.

7. Caledonia, G.E.; et al. "A High Flux Source of Energetic Oxygen Atoms for Material Degradation

Studies", AIAA 85-7015.

8. Rufledge, S.K.; et al. "An Evaluation of Candidate Atomic Oxygen Resistant Materials for Space

Applications in LEO", NASA TM-100122, 1986.

9. Rutledge, S.K,, Banks, B.A. and Cales, M.; "A Comparison of Atomic Oxygen Erosion Yields of

Carbon and Selected Polymers Exposed in Ground Based Facilities and in Low Earth Orbit", AIAA-94-

2628.

l 0. Banks, B.A.; "The Use of Fluoropolymers in Space Applications", Modern Fluoropolymers, Scheirs, J.

Ed.; John Wiley & Sons, New York, (1997), 103-111.

NASA/TM-- 1998-207918 15
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OMB No. 0704-0188

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1. AGENCY USE ONLY (Leave blank) 2. REPORT DATE 3. REPORTTYPE AND DATES COVERED

July 1998 Technical Memorandum

4. TITLE AND SUBTITLE 5. FUNDING NUMBERS

A Comparison of Space and Ground Based Facility Environmental Effects for

FEP Teflon

6. AUTHOR(S)

Sharon K. Rutledge, Bruce A. Banks, and Michael Kitral

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESSEES)

National Aeronautics and Space Administration

Lewis Research Center

Cleveland, Ohio 44135-3191

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration

Washington, DC 20546-0001

WU-632-1A-IE-00

8. PERFORMING ORGANIZATION

REPORT NUMBER

E-11194

10. SPONSORING/MONITORING

AGENCY REPORT NUMBER

NASA TM-- 1998-207918/REV 1

11. SUPPLEMENTARY NOTES

Prepared for the Fourth International Space Conference on Protection of Materials and Structures from the LEO Space

Environment sponsored by the University of Toronto Institute for Aerospace Studies, Toronto, Canada, April 23-24, 1998.

Sharon K. Rutledge and Bruce A. Banks, NASA Lewis Research Center; and Michael Kitral, Cleveland State University,

Cleveland, Ohio 44115-2403. Responsible person, Sharon K. Rutledge, organization code 5480, (216) 433-2219.

12a. DISTRIBUTION/AVAILABILITY STATEMENT

Unclassified - Unlimited

Subject Category: 27 Distribution: Nonstandard

This publication is available from the NASA Center for AeroSpace Information, (301) 621--0390.

12b. DISTRIBUTION CODE

13. ABSTRACT (Meximum2OOworde)

Fluorinated Ethylene Propylene (FEP) Teflon is widely used as a thermal control material for spacecraft, however, it is susceptible to erosion, cracking, and subsequent mechanical failure in low Earth orbit. One of the difficulties in determining whether

FEP Teflon will survive during a mission is the wide disparity of erosion rates observed for this material in space and in ground

based facilities. Each environment contains different levels of atomic oxygen, ions, and vacuum ultraviolet (VUV) radiation in

addition to parameters such as the energy of the arriving species and temperature. These variations make it difficult to determine

what is causing the observed differences in erosion rates. This paper attempts to narrow down which factors affect the erosion

rate of FEP Teflon through attempting to change only one environmental constituent at a time. This was attempted through the

use of a single simulation facility (plasma asher) environment with a variety of Faraday cages and VUV transparent windows.

Isolating one factor inside of a radio frequency (RF) plasma proved to be very difficult. Two observations could be made. First,

it appears that the erosion yield of FEP Teflon with respect to that of polyimide Kapton is not greatly affected by the presence

or lack of VUV radiation present in the RF plasma and the relative erosion yield for the FEP Teflon may decrease with increasing fluence. Second, shielding from charged particles appears to lower the relative erosion yield of the FEP to approximately

that observed in space, however it is difficult to determine for sure whether ions, electrons, or some other components are

causing the enhanced erosion.

14. SUBJECT TERMS

FEP; Teflon; Atomic oxygen; Low Earth Orbit; Plasma asher

17. SECURITY CLASSIFICATION

OF REPORT

Unclassified

NSN 7540-01-2fl0-5500

18. SECURITY CLASSIFICATION

OF THIS PAGE

Unclassified

19. SECURITY CLASSIRCATION

OF ABSTRACT

Unclassified

15. NUMBER OF PAGES

21

16. PRICE CODE

A03

20. LIMrrATION OF ABSTRACT

Standard Form 298 (Rev. 2-89)

Prescribed by ANSI Std, Z39-18

29B-102