Kapton polyimide oxidizes at significant rates (4.3x10(-24) gram/incident oxygen atom) when exposed in low Earth orbit to the ram atomic oxygen flux. Ion beam sputter deposited thin films of Al2O3 and SiO2 as well as a codeposited mixture of predominantly SiO2 with a small amount of polytetrafluoroethylene were evaluated and found to be effective in protecting Kapton from oxidation in both laboratory plasma ashing tests as well as in space on board shuttle flight STS-8. A protective film of  or = 96 percent SiO2 and  or = 4 percent polytetrafluoroethylene was found to be very flexible compared to the pure metal oxide coatings and resulted in mass loss rates that were 0.2 percent of that of the unprotected Kapton. The optical properties of Kapton for wavelengths investigated between 0.33 and 2.2 microns were not significantly altered by the presence of the coatings or changed by exposure of the coated Kapton to the low Earth orbital ram environment

Banks, B. A.

Mirtich, M. J.

Rutledge, S. K.

Swec, D. M.

English

NASA Technical Reports Server

NASA Technical Memorandum 83706
N84_680_
Sputtered Coatings for Protection
of Spacecraft Polymers
Bruce A. Banks, Michael J. Mirtich, Sharon K. Rutledge,
and Diane M. Swec
Lewis Research Center
Cleveland, Ohio
Prepared for the
Eleventh International Conference on Metallurgical Coatings
sponsored by the American Vacuum Society
San Diego, California, April 9-13, 1984
N/ A
https://ntrs.nasa.gov/search.jsp?R=19840018735 2020-03-20T21:59:11+00:00Z

SPUTTERED COATINGS FOR PROTECTION OF SPACECRAFT POLYMERS
Bruce A. Banks, Michael J. Mirtlch, Sharon K. Rutledge and Diane M. Swec
National Aeronautics and Space Administration
Lewis Research Center
Cleveland, Ohio 44135
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SUMMARY
Kapton® polylmlde oxidizes at significant rates (4.3x10 -24 gram/Incldent
oxygen atom) when exposed in low earth orbit to the ram atomic oxygen flux.
Ion beam sputter deposited thin films of Al203 and SiO 2 as well as a
codeposlted mixture of predominantly SiO 2 with a small amount of polytetra-
fluoroethylene were evaluated and found to be effective in protecting Kapton®
from oxidation in both laboratory plasma ashlng tests as well as in space on
board Shuttle flight STS-8. A protective film of _96 percent SiO 2 and _4
percent polytetrafluoroethylene was found to be very flexible compared to the
pure metal-oxlde coatings and resulted in mass loss rates that were 0.2 percent
of that of the unprotected Kapton®. The optical prQpertles of Kapton® for
wavelengths investigated between 0.33 and 2.2 pm were not significantly altered
by the presence of the coatings or changed by exposure of the coated Kapton®
to the low earth orbital ram environment.
INTRODUCTION
Early Shuttle flights have demonstrated that materials such as polylmlde
(Kapton®), carbon coatings, and some paints undergo welght loss and changes
in optical properties when exposed in low earth orbit. ! The postulated
mechanism for these material changes is oxidation by atomic oxygen atoms which
energetically (~5 eV) impact into the exposed spacecraft surfaces because of
the spacecrafts orbital ram velocity relative to the geostatlonary Iow-earth-
orbital environment which is predominantly atomic oxygen at altitudes between
180 km (97 n ml) and 650 km (351 n mi). 2 Materials which form volatile
oxides when exposed to ram atomic oxygen typically become mat in appearance due
to microscopic surface fibrils or cone llke structures which are a result of
oxidation by the directed atomic oxygen flux. 3 The observed rates of oxlda-
tlon of Kapton® and Mylar® are sufficiently high as to cause concern for
their survival as solar cell blankets or thermal blankets when used in low
earth orbital applications of any significant duratlon. 4 Thus considerable
interest exists in identifying oxidation resistant substitute polymeric mate-
rials or protective coatings suitable to prevent oxidation on existing polymers
such as Kapton® and Mylar®.
This paper presents an approach which utilizes thin films of predominantly
metal oxide coatings as a means of preventing oxidation of polymers. Thus the
protection of the polymer surface from oxidation is afforded by nonvolatile
metal oxides which are already in their highest oxidation state. Because metal
oxides such as SiO 2 and Al203 are brittle a molecularly mixed film of
predominantly metal oxide with a little fluoropolymer was thought to be an
attractive approach to gain both oxidation protection from the oxide component
and perhaps added flexibility as a result of the polymeric constituent.
APPARATUS AND PROCEDURE
Coating Deposition and Selection
Two types of thin film coatings were investigated for oxidation protection
for Kapton® polylmlde. These were pure metal oxides and a molecular mixture
of metal oxide and fluoropolymer. The thin films were deposited by ion beam
sputter deposition after 2 minutes of ion beam sputter cleaning the Kapton®
substrates as shown in figure I. An 8 cm diameter ion source was used to prod-
uce a lO00 eV, 65 mA argon ion beam for sputter cleaning and deposltlon. 5 A
ion beam current density of up to N2.8 mA/cm _ resulted in the vicinity of
the sputter target and Kapton® substrates which were located approximately
20 cm downstream of the ion source. The ion source was operated with a hot
wire neutralizer in a vacuum facility 4.5 m long by 1.5 m in diameter which
maintained pressures of 3xlO -5 torr during ion source operation. Smooth
fused silica slides were also mounted with the Kapton® substrates to allow
documentation of film thickness. Deposited film thlckneses were measured by
means of a surface profiling instrument (Alpha-Step Profiler®, lencor Instru-
ments) by tracing the surface with a stylus passing from a virgin portion of
the fused silica surface (which was protected by means of a polylmlde tape) to
the deposited surface. Sputter targets were 15.24 cm diameter disks of SiO 2
or Al203 for deposition of single constituent metal oxide films, and SiO 2
with polytetrafluoroethylene (Teflon®) bars or tubes ranging from 19.1 to
0.44 mm wide stretched across the diameter of the SiO 2 target for the
codeposlted films.
Because solar array and thermal blankets of Kapton® may require such
oxidation protective coatings, the flexibility of the coated blanket is a
pertinent consideration. The deposited thin film coatings on 0.127 mm Kapton®
were subjected to compressive and tensile stresses by wrapping the coated
Kaptonm (with the coated surface concave then convex) around various size
mandrels ranging in radii from 10.16 to 0.008 cm. The coated surfaces were
then microscopically examined for crazing and/or spalling.
The oxidation protection capability of various thickness and composition
coatings were evaluated with an RF plasma asher (SPI Plasma Prep II®) to
produce atomic oxygen bombardment of samples. Both coated and uncoated Kapton®
samples were subjected to low pressure RF air discharge for approxi- mately 15
hours during which typically 18 percent of the unprotected Kapton's® O.12l mm
total thickness would be removed by oxidation. Samples were protected by
glass slides such that only one surface (the coated surface for the protected
samples) was exposed to the RF plasma. Samples were evaluated to compare
weight loss after allowing sufficient time for reabsorptlon of moisture.
Thin film coating composition and thickness selected for space flight
testing were based on the results of the flexibility tests and plasma asher
tests.
Space Experiment
Circular 2.54 cm diameter 0.127 mm thick unprotected and three thin film
protected Kapton® samples were mounted in aluminum trays whlch allowed space
exposure of a 2.06 cm diameter central portion of each sample. The unprotected
sample of Kapton® was a control for the coated samples as well as others not
described in this paper and had an aluminum film deposited on its unexposed
surface. The control sample aluminum film as well as the protective coatings
were all deposited on the smoothest side of the Kapton® substrates. The
samples as mounted in their flight tray are shown figure 2. This tray was
located within the shuttle bay of STS-8 as shown in figure 3. This allowed
normally incident ram atomic oxygen flux to impinge upon the samples.
Samples were docummented by optical microscopic photography, scanning
electron microscopy (Amray® 1400), energy dispersive spectroscopy (Kevex®
EDS system), mass change, optical reflectance, absorptance and transmittance
(by means of a Gier-Dunckle® integratlng sphere in conjunction with a
tungsten strip lamp and monochrometer) to allow an evaluation of the charac-
teristics and effectiveness of the thin film protective coatings.
RESULTS AND DISCUSSION
Coating Selection for Space Flight Test
Thin film coatings selected for space flight testing were 700A of AI203,
650A of SiO 2, and 650A of _96 percent SiO 2 with <4 percent (by volume) poly-
tetrafluoroethylene (PTFE). The codeposltlon target for the _96 percent SiO 2
<4 percent PTFE film utilized a 0.44 mm PTFE tube stretched across the SiO 2
target as the source of the <4 percent PTFE in the codeposlted film. Film
thlckneses evaluated ranged from 400A to lO00A. The minimum radius of curva-
ture that each selected coating could survive on 0.127 mm thick Kapton®
without crazing or spalllng was found to be 6.35 mm for ?OOA of Al203,
3.18 mm for 650A of Si02, and near zero for 650A of _96 percent SiO 2 _4
percent PTFE. The codeposlted film showed no evidence of failure even with a
180 ° fold back of the Kaptonm on itself. Codeposlted films with a signif-
icant fractional fluoropolymer content such as 65 percent SiO 2 35 percent
PTFE were found to be ineffective in protecting the Kapton in the plasma
ashing tests.
Space Flight Test Results
The samples were exposed to the ram atomic oxygen environment of 222 km
(120 n ml) during Shuttle Flight 8 in three separate exposure periods on
September 3, 4, and 5, 1983 for a total of 41.17 hours. This was accompllshed
by orbiting the earth with the shuttle bay doors open to allow the sample trays
to ram with normal incidence into the environmental atmosphere.
The unprotected Kapton® sample developed a mat surface (or increase in
diffuse reflectance) as a result of the space exposure whereas no significant
change beyond the experimental measurement error was found to occur in the
Al203, Si02, or _96 percent Si02, _4 percent PTFE samples. Figure 4
an 5 compare the changes In optical properties of unprotected and _96 percent
SiO 2, _4 percent PTFE protected Kapton®. The transmittance is not shown in
figure 4 because its value is zero for the wavelength region shown due to the
aluminum coating on the unexposed surface. The optical properties of reflect-
ance, absorptance, and transmittance for Al203 and SiO 2 are not presented
in this paper because they were indistinguishable from each other and the >96
percent SiO 2 _4 percent PTFE sample. From Figs. 4 and 5 one can conclude
that the thin film coatings of A1203, S102, and >96 percent SiO 2 _4
percent PTFE do not significantly (over the wavelength region of 0.33 - 2.2 um)
alter the optical properties of uncoated Kapton® and upon exposure to the low
earth orbital environment also did not change as does the unprotected Kapton®.
Table I compares the mass losses' of the protected and unprotected Kapton®
samples that resulted from the exposure to the low earth orbital environment.
The unprotected Kapton® was found to lose an equivalent thickness of
2.6xi0 -4 mm/hr (IxlO -5 In./hr) while being exposed to the ram atomic oxygen
flux. Thls rate was found to be conslstant with several other investigators
also onboard STS-8. The rate of mass loss of the Al203 coated sample would
indicate that slx times the mass of the protective coating was removed. How-
ever energy dispersive spectroscopy substantiated the presence of the coating.
Even though the mass loss of the Al203 protected Kapton® was only II
percent of that of the unprotected, there Is a significant probability that a
mlsslng Kapton shard (lost during sample mounting or removal) contributed to
this mass loss. Thus the Al203 coatings actual performance Is probably
more similar to the SiO 2 coating which lost only O.l percent of the mass of
unprotected Kapton®. One can also see that the more flexible >96 percent
SIO 2 _4 percent PTFE coating was also very effective In protection of the
polylmlde with only 0.2 percent of the unprotected loss rate. Energy disper-
sive X-ray spectrometry (EDS) also confirmed the presence of both the SiO 2
and _96 percent SiO 2 _4 percent PTFE coatings. Scanning electron microscopy
also gave strong support that the atomic oxygen which slgnlflcantly textured
the unprotected Kapton® did not alter any of the three coated Kapton®
surfaces. Figure 5 compares scanning electron microscope photographs of the
unprotected and _96 percent SIO 2 _4 percent PTFE protected Kapton® samples
before and after space flight exposure. As can be seen significant surface
texturing of the unprotected Kapton® occurs. The textured surface obviously
Is the predominant cause of the mat appearance of the post flight unprotected
Kapton®. All of the three protective coatings yielded the same results upon
scanning electron microscope inspection. That Is, the thln film coatings
showed now significant alteratlbn in Surface morphology upon deposition on the
Kapton® or change as a result of exposure In low earth orbit.
CONCLUSIONS
Ion beam sputter deposited thin film of 700A of AI203, 650A of SI02,
and _96 percent SiO 2 _4 percent PTFE were found to be effective in prevent
ing oxidation of Kapton® exposed to ram atomic oxygen in low earth orbit.
The codeposlted film of _96 percent SiO 2 _4 percent PTFE was found to be more
flexible than the either of the pure metal-oxide films. Kapton® coated wlth
the codeposlted fllm and exposed for 41.17 hr to the orbital ram environment at
222 km (120 n ml) resulted In mass loss rates of 0.2 percent of that of the un-
protected Kaptonm. The optical properties of reflectance, absorptance, and
transmittance over the wavelengths of 0.33 to 2.2 _m were not slgnlflcantly
altered by the presence of the films, and furthermore, dld not change upon ex-
posure of the thln-film-coated Kaptonm to the low earth orbital ram
environment.
ACKNOWLEDGMENTS
The authors gratefully acknowledge the efforts of Dale Ferguson, Bob
Roman,Ralph 3acko, and David Bates who significantly contributed to thls
investigation.
l °
2.
3.
4.
o
REFERENCES
L. J. Leger, Oxygen Atom Reaction with Shuttle Materials at Orbital
Altitudes, NASA Tech. Memo. TM-58246, 1982.
NOAA, NASA, and USAF, U.S. Standard Atmosphere, 1976, NASA Tech. Memo. Tm
X-74335, 1976.
D. C. Ferguson, Laboratory Degradation of Kapton in a Low Energy Oxygen
Ion Beam, NASA Tech. Memo. TM-83530, 1984.
A. F. Whltaker, LEO Atomic Effects on Spacecraft Materials, Paper
presented at the AIAA Shuttle Environment and Operations Meeting,
Washington D.C., Oct. 31 to Nov. 2, 1983, AIAA-83-2632-CP.
B. A. Banks and S. K. Rutledge, Ion Beam Sputter Deposited Dlamondllke
Films, NASA Tech. Memo. TM-82873, 1982.
TABLE I. - MASS LOSS OF PROTECTED AND UNPROTECTED KAPTON®
SAMPLES TO LOW EARIH ORBITAL ENVIRONMENT
Protective coating
on Kaptone
None (Unprotected)
Al203
SiO 2
96% SiO 2 _ 4% PTFE
Thickness of
protective
coating, A
0
700
650
650
Mass
loss, #g
5020 + 9.9
k
567 + 5.2
5.9 + 5.2
I0.3 + 5.2
Mass loss per
incident oxygen
atom*, g/atom
4.3x10-24
4.8xI0- 25
5.0x10-27
8.8xi0-27
*Based on an estimated atomic oxygen fluence of 3.5xi020
atoms/cm 2
TABLE I. - MASS LOSS OF PROTECTED AND UNPROTECTED KAPTON®
SAMPLES TO LOW EARTH ORBITAL ENVIRONMENT
Protective coating
on Kapton®
None (Unprotected)
A1203
SiO 2
96% SiO 2 _ 4_ PTFE
Thickness of
protective
coating, A
0
700
650
650
Mass
loss, _g
5020 + 9.9
56? + 5.2
5.9+5.2
I0.3+5.2
Mass loss per
incident oxygen
atom*, g/atom
4.3xi0-24
4.8xi0-25
5.0xi0-27
8.8xi0-27
*Based on an estimated atomic oxygen fluence of 3.5xi020
atoms/cm 2.
ION
SOURCE
I I,,----_Ar -,,,,,.c'--"---'% , ----.
_-"_._ il
l-T--;'7_
NEUTRALIZER-/I I
SiO2 OR
AI203 TARGET-...,
Ar +
ION
BEAM \
/X_../ L OPTIONALFLUOROPOLYMERTARGET
FOR CO-DEPOSITION/ II"_"_Kapton®
,' II SUBSTRATE
Kapton® SUBSTRATEIN /z II IN DEPOSITION
SPUTTERCLEANINGPOSITION-/ c_ POSITION
"l J"
i\
Figure 1. - Ion beam sputter cleaning and deposition configuration.
Figure 2. - Shuttle experi ment sample tray.
Figure 3. - Shuttle experimentin the bayof STS-8.
ZtJ
t*,.-
....I
O
I=.-
Z
t_
,.=I
Z
O
ram,
1.0
.8
.6
.4
.2
0
.6
/
.4 .6 .8 1.0 1.2
I I I I I
1.4 1.6 1.8 2.0 2.2
(a)Total reflectance.
.4
.2
oY
1,0 r_-
.8--
.4 --
.2
0
---! I
.4 .6 .8
(b)Diffuse reflectance.
EXPOSEDSAMPLE
UNEXPOSEDSAMPLE
I I I I I I I I I
.4 .6 .8 1.0 1.2 1.4 1.6 1.8 2.0
WAVELENGTH,_ , pm
(c) Absorptance.
Figure 4. - Optical propertiesof unprotected Kapton®(0.12/mm thick with an
aluminum film on the exposedsurface) for samplesunexposedand exposedto
low earth orbital environment.
I
2.2
i,i
t.J
i,i
L
0
ILl
Z
t_
0
W
Z
Z
1.0
.8
.6
.4
.2
1.0--
o4-
I I I I I -I
(a) Total reflectance.
f
(b) Absorptance.
I " EXPOSEDAND COATEDSAMPLE
NJ I I I I I I I I I
.4 .6 .8 1.0 1.2 1.4 1.6 1.8 2.0 2.2
WAVELENGTH, _,, pm
(c) Transmittance.
Figure 5. - Optical properties of >--96%SiO2 -<4%PTFEcoated Kapton®samples unex-
posed and exposed to low earth orbital environment compared with uncoated and
unexposed Kapton®.
(a) UnprotectedKapton®beforespaceexposure. (c) _>96%SiO2 _<4% PTFEcoatedKapton®beforespaceexposure.
(b) UnprotectedKapton®afterspaceexposure. (d)_>96%SiO2 _<4%PTFEcoatedKapton®afterspaceexposure.
Figure 6. - Scanning electron microscope photographs of unprotectedand >_96%SiO2 <_4%PTFEcoatedKapton_beforeand after spaceflight exposure.
1. Report No,
NASA TM- 83706
4. Title and Subtitle
2. Government Accession No.
Sputtered Coatings for Protection of Spacecraft
Pol yme rs
7 Author(s)
Bruce A. Banks, Michael O. Mirtich, Sharon K. Rutledge
and Diane M. Swec
9. Performing Organization Name and Address
National Aeronautics and Space Administration
Lewis Research Center
Cleveland, Ohio 44135
12. Sponsoring Agency Name and Address
National Aeronautics and Space Administration
Washington, D.C. 20546
3. Recipient's Catalog No.
5. Report Date
6. Performing Organization Code
506- 55- 72
8. Performing Organization Report No
E-2092
10. Work Unit No,
11. Contract or Grant No.
13. Type of Report and Period Covered
Technical Memorandum
14. Sponsoring Agency Code
15. Supplementary Notes
Prepared for the Eleventh International Conference on Metallurgical Coatings
sponsored by the American Vacuum Society, San Diego, California, April 9-13, 1984.
16. Abstract
Kapton ® polyimide oxidizes at significant rates (4.3x10 -24 gram/incident
oxygen atom) when exposed in low earth orbit to the ram atomic oxygen flux. Ion
beam sputter deposited thin films of Al203 and SiO 2 as well as a codepos-
ited mixture of predominantly SiO 2 with a small amount Qf polytetrafluoro-
ethylene were evaluated and found to be effective in protecting Kapton ® from
oxidation in both laboratory plasma ashing tests as well as in space on board
Shuttle flight STS-8. A protective film of >96 percent SiO 2 and <4 percenL
polytetrafluoroethylene was found to be very-flexible compared to-the pure metal--
oxide coatings and resulted in mass loss rates that were 0.2 percent of that of
the unprotected Kapton ®. The optical properties of Kapton ® for wavelengths
investigated between 0.33 and 2.2 um were not significantly altered by the pre-
sence of the coatings or changed by exposure of the coated Kapton ® to the low
earth orbital ram environment.
17. Key Words (Suggested by Author(s))
Sputtered
Atomic oxygen
Coati ngs
Oxi da ti on
Polyimi de
19. Security Classif. (of this report)
Unclassified
20. Security Classif. (of this page)
Unclassified
18. Distribution Statement
Unclassified - unlimited
STAR Category 27
21. No. of pages
22. Price"
*For sale by the National Technical Information Service, Springheld, Virginia 22161


Sputtered coatings for protection of spacecraft polymers

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