Candidate atomic oxygen protective coatings for long-term low Earth orbit (LEO) spacecraft were evaluated using the Los Alamos National Laboratory O-atom exposure facility. The coatings studied include Teflon, Al2O3, SiO2, and SWS-V-10, a silicon material. Preliminary results indicate that sputtered PTFE Teflon (0.1 micrometers) has a fluence lifetime of 10 to the 19th power O-atoms/cm (2), and sputtered silicon dioxide (0.1 micrometers), aluminum oxide (0.1 micrometers), and SWS-V-10, a silicone, (4 micrometers) have fluence lifetimes of 10 to the 20th power to 10 to the 21st power O-atoms/cm (2). There are large variations in fluence lifetime data for these coatings

Cross, J. B.

Lan, E. H.

Smith, Charles A.

English

NASA Technical Reports Server

N8 9 - 1259 2
ATOMIC OXYGEN EFFECTS ON CANDIDATE COATINGS FOR
LONG-TERM SPACECRAFT IN LOW EARTH ORBIT
E. H. Lan and C. A. Smith
McDonnell Douglas Astronautics Company
J. B. Cross
Los Alamos National Laboratory
ABSTRACT
Candidate atomic oxygen protective coatings for long-term low Earth orbit
(LEO) spacecraft have been evaluated using the Los Alamos National Laboratory
O-atom exposure facility. The coatings studied include Teflon, A1203, SiO2,
and SWS-V-10, a silicone material. Preliminary results indicate that sput-
tered PTFE Teflon (0.i _Lm) has a fluence lifetime of 1019 O-atoms/cm 2, and
sputtered silicon dioxide (0.I _m), aluminum oxide (0.i _m), and SWS-V-10, a
silicone, (4 _m) have fluence lifetimes of 1020 to 1021 O-atoms/cm 2. There
are large variations in fluence lifetime data for these coatings. Further
investigations are under way to determine the failure mechanisms.
INTRODUCTION
The low Earth orbit (LEO) environment is known to react with many space-
craft materials, especially organic polymers. I'2 Samples retrieved from Space
Shuttle flights after being exposed to LEO had mass loss as well as surface
morphology changes. The primary constituent of the LEO environment at Space
Shuttle altitudes, atomic oxygen, is suspected to be the principal cause of
the erosion of materials because of its high oxidative ability, flux
(1015 atoms/cm2-sec) and collision energy (5 eV) for a spacecraft traveling
at 8 km/sec.
Spacecraft for long duration missions such as the Space Station must
utilize materials that can withstand the environment without undergoing sig-
nificant changes in material properties. Reactive materials must be coated
with a protective coating if they are to achieve longevity in LEO. A variety
of protective coatings, such as ion-beam sputter-deposited thin films, 3 sili-
cones and Teflon 4 have been proposed and developed for extended use in LEO.
Some of these coatings have been tested on Space Shuttle flight experiments,
but Space Shuttle flight data provide only limited information on the feasi-
bility of long-term use of these materials in LEO. Space Shuttle flight
experiments provide relatively small fluence exposures (1020 O-atoms/cm 2) in
comparison to the exposure a spacecraft such as Space Station will experience
during 20-30 years in LEO (1022 - 1023 O-atoms/cm2) .1 It is, therefore, nec-
essary to evaluate materials using ground-based simulation facilities which
can perform accelerated testing at flux levels up to 1017 O-atoms/cm2-sec.
110
https://ntrs.nasa.gov/search.jsp?R=19890003221 2020-03-20T05:50:24+00:00Z
INSTRUMENTATION
The Los Alamos National Laboratory O-atom source 5'6 uses a CW plasma pro-
duced by focusing a high power CO 2 laser beam in a hydrodynamic expansion
nozzle that has an oxygen/rare gas mixture flowing into it. The resulting
temperature of the plasma is about 15,000 ° K. Using argon or neon as the rare
gas, the percentage dissociation of molecular oxygen into atomic oxygen is
about 85%. The facility is capable of producing atomic oxygen fluxes of
up to 1017 O-atoms/cm2-sec, energies of 1-5 eV, and total fluences of
1022 O-atoms/cm2. 6
T_CHNIOUE TO EVALUATE COATINGS
A technique to evaluate candidate coatings for atomic oxygen resistance has
been developed, which uses silver oxidation as an atomic oxygen detector. 7
Silver oxidizes to silver oxide in the presence of atomic oxygen, and the
electrical resistance of the oxide is dramatically higher than that of pure
silver. Coatings of known thicknesses are deposited on top of a thin (250 A)
silver film and as atomic oxygen penetrates or diffuses through the coatings,
the measured electrical resistance increases. The technique has been demon-
strated to be highly sensitive to atomic oxygen and can also provide quanti-
tative measurements of atomic oxygen flux. 6 A schematic of the sensor is
shown in figure i.
Samples are mounted on a sample manipulator and accurately placed perpen-
dicular to the O-atom beam axis. Electronic instrumentation was arranged so
that continuous in-situ resistance measurements can be obtained during atomic
oxygen exposure. Figure 2 shows the resistance-measuring circuit for the
actinometers. A constant current is supplied to the silver films and resis-
tance data is obtained through voltage measurements. A high impedance
voltage-to-frequency converter feeds a multichannel analyzer, which stores the
voltage measurements as a function of exposure time. The data is then fed to
a computer for data analysis.
SAMPLE PREPARATION
The actinometers were coated with PTFE Teflon, A1203, Si02, and silicone
SWS-V-10 materials. The PTFE Teflon was sputtered using a Teflon target at
the NASA-Lewis Research Center. The thicknesses of the PTFE Teflon coatings
were 800 A and 1250 A, as determined by performing profilometry on separate
witness samples. The AI203 and SiO 2 coatings were sputtered using AI203 and
SiO 2 targets at the MDAC-HB facility. The AI203 and SiO 2 coatings were 950 A,
as determined by performing ellipsometry on witness silicon wafer samples. We
estimate the measured thicknesses of all the sputtered coatings to be within
±10%. The SWS-V-10 silicone coating was brushed on at NASA-Marshall Space
Flight Center. The thickness of the brushed coating is estimated to be between
4 and I0 _m. For data analysis of these samples, we will assume that the
coatings had a thickness of 4 _m.
111
Q-ATOM BEAM FLUX CALIBRATION
The time history of oxidation of 250 A thick bare silver films when exposed
to the O-atom beam can be used to calculate the oxidation rate in monolayers/
second, which can then be converted to a beam flux in atoms/cm2-sec. The
oxidation rate and beam flux were calculated using the assumptions that: i)
the sticking probability of O-atoms on bare polycrystalline silver is unity
over the entire range of oxide formation, 2) the nearest neighbor distance
(NND) in polycrystalline silver is 2.9 A, the same as that for the silver face
centered cubic structure, and 3) the 02 molecules in the beam (about 15%) do
not react with silver to form silver oxide. The flux is then calculated from:
Oxidation rate (monolayers/sec) = Thickness of Aq film (_)
Time of oxidation (sec) x NND (_/monolayer)
Beam flux (atoms/cm2-sec) = Oxidation rate x No. of Ag atoms/cm2-monolayer,
where No. of Ag atoms/cm2-monolayer = 1.2 x 1015 , as determined from the NND.
In these experiments, when bare 250 A thick silver films were placed 15 cm
from the nozzle, the estimated atomic oxygen flux was 1 x 1017 atoms/cm2-sec.
With the exception of Teflon, all of the coatings were exposed to an O-atom
beam (using an O2/Ar mixture) having a kinetic energy of 1.5 eV (Teflon 3 eV)
and a flux of 1 x 1017 atoms/cm2-sec (Teflon 6 x 1016 atoms/cm2-sec) .
RESULTS
An example plot of the electrical resistance/conductance of silver under-
lying a protective coating as a function of O-atom fluence is provided in
figure 3. The shapes of the resistance/conductance vs. fluence graphs,
however, do vary from coating to coating as well as from sample to sample of
the same coating. Please see Table I for a summary of the results obtained
for the coatings studied.
PTFE Teflon:
ESCA analysis of the sputtered PTFE Teflon coatings before exposure showed
that their atomic concentration was 45% C and 55% F. Two PTFE Teflon-coated
actinometers (800 A and 1250 _) were exposed to an O-atom beam (using an O2/Ne
mixture) having a 3 eV kinetic energy and 6 x 1016 atoms/cm2-sec flux. Visual
examination of the beam spot on the actinometers after O-atom exposure
indicated that the Teflon was eroded away. Bulk Teflon has been tested in the
LANL O-atom beam and profilometry measurements after exposure indicate there
is erosion of Teflon through reaction with atomic oxygen. The PTFE Teflon
tested at 3 eV kinetic energy gave a reaction efficiency of 0.22 x 10 -24
cm3/atom for the 800 _ sample and 0.82 x 10 -24 cm3/atom for the 1250 _ sample.
Previous exposure of a 800 A thick PTFE Teflon-coated actinometer to a flux of
9 x 1015 atoms/cm2-sec at 1.5 eV kinetic energy (using an O2/Ar mixture) gave
a reaction efficiency of 0.13 x 10 -24 cm3/atom. The fluxes used in the Teflon
exposures were reduced because of reduced gas flows in the nozzle.
112
A1203:
Auger analysis of the A1203 coating before exposure showed an atomic con-
centration of 33% AI, 65% O, and <2% C. SEM photographs of the coating up to
a magnification of 20,000X did not reveal any obvious porosity. Two AI203-
coated actinometers, both of 950 A, were exposed to an O-atom beam (using an
O2/Ar mixture) having a 1.5 eV kinetic energy and 1.0 x 1017 atoms/cm2-sec
flux. Complete oxidation of the underlying silver occurred after a fluence of
1.0 x 1020 atoms/cm 2 for one sample, whereas complete oxidation of the under-
lying silver occurred after a fluence of 8.3 x 1020 atoms/cm 2 for the other
sample. Analyses of the AI203 coating after exposure are under way.
SiO2:
Auger analysis of the SiO 2 coating before exposure showed an atomic concen-
tration of 35% Si, 58% O, 5% N and 2% C. SEM photographs of the sputtered
coating up to a magnification of 20,000X did not reveal any obvious porosity.
Three SiO2-coated (950 A) actinometers were exposed under the same conditions
as the AI203 samples. Complete oxidation of the underlying silver occurred
after fluences of 1.2 xil020 atoms/cm 2, 7.3 x 1020 atoms/cm 2, and ii.0 x 1020
atoms/cm 2. Visual inspection of the SiO2-coated actinometers after the test
runs indicated that the coating was still intact, and erosion of the coating
as observed in the exposed PTFE Teflon samples did not occur.
ESCA and Auger analyses were performed on the SiO 2 coated silver strip
after O-atom exposure and only a small percentage of silver (about 2%) was
present on the sample surface. Auger elemental depth profile of a SiO2-coated
actinometer after exposure to a ii.0 x 1020 O-atom/cm 2 fluence is provided in
figure 4. The profile confirms that the SiO 2 coating was not eroded away.
There is a layer of SiO 2 on the surface, and silver does not appear until part
of the coating is sputtered away. The profile does show, however, silver over
a broad depth range, which may be due to the high surface roughness of the
alumina substrate (>I _m) and/or diffusion of silver into the coating.
SWS-V-10 Silicone:
The three SWS-V-10 silicone-coated actinometers (4 _m thickness) tested
under the same conditions as the AI203 and SiO 2 samples showed complete
oxidation of the underlying silver after fluences of 1.8 x 1020 atoms/cm 2,
4.9 x 1020 atoms/cm 2, and 6.0 x 1020 atoms/cm 2. Visual inspection of the
silicone samples after exposure indicated that the coating was also intact,
although the underlying silver had oxidized. Further analysis of these
samples will be performed by NASA/MSFC.
DISCUSSION
The oxygen atom source provides, in addition to atomic oxygen, a flux of
visible and infrared photons. With the exception of the PTFE Teflon-coated
samples, which did not experience a temperature rise during the course of the
113
test runs, all of the other samples, aluminum oxide, silicon dioxide, and
SWS-V-10 silicone, did experience sample heating. A thermocouple placed on
the side of the sample substrate, but not in the beam spot, indicated that the
substrates reached temperatures of up to i00 ° C by the end of the exposures.
Readings from another thermocouple that was placed directly in the beam spot
indicated that the beam spot temperature was another 20 ° C higher. The rate
of temperature rise was not controlled.
Measurements of the vacuum UV (VUV) intensities 6 produced by the plasma
have been taken using a vacuum monochrometer with a CsTe sQlar blind and
bialkali photomultiplier tubes. The 1.5-eV (02/Ar) beam produced VUV intensi-
ties about 3 times higher than that produced by solar radiation, while the
3-eV (O2/Ne) beam produced VUV intensities about equal to solar VUV. The
inorganic coatings (SiO 2 and AI203) are not likely to be affected by the VUV
radiation, but organic coatings such as Teflon and silicones may degrade to
the point where atomic oxygen has an enhanced reactivity with the photoproduct
backbone of the polymer.
Auger analysis (figure 4) and visual inspection after exposure indicated
that the SiO 2 coating was intact, although the underlying silver had oxidized.
Diffusion of O-atoms into inorganic thin film coatings, therefore, may be a
significant process that warrants additional attention. Since Auger analysis
after exposure also revealed a broad distribution of silver in the thickness
of the coated area, diffusion of silver into the coatings may have also
occurred. Temperature effects could be significant in both of these
processes.
The test results of the sputter-coated actinometers reveal a large scatter
in the data (see table I), even though the coatings were coated during the
same run and should be of the same thickness. Some of the parameters that
would contribute to scatter in the data include temperature, coating con-
tinuity, coating impurities, and substrate surface roughness. Since the
surface of the actinometers used in these experiments had roughness on the
order of microns and the sputtered coatings were only about 0.i _m in thick-
ness, it is unknown whether continuous uniform coatings were protecting the
actinometer surfaces. The quali£y of the coatings in terms of chemical com-
position and codeposited impurities is another factor to be considered in
assessing their ability to protect reactive substrates. For future spacecraft
parts, if thin film coatings are to be used, the ability to apply the coatings
consistently on a large-scale basis and to certify the coating quality will be
essential to ensure the substrate protection for the life of the spacecraft.
Of the coatings we tested to date (0.i Jlm of sputtered Teflon, A1203, and
SiO 2 and 4 _m of silicone), none of them have provided protection for the full
atomic oxygen fluence of 1022 to 1023 atoms/cm 2 expected for long-term space-
craft such as Space Station. Our results indicate that the coatings of the
above-specified thicknesses may provide protection for O-atom fluences of 1020
to 1021 atoms/cm 2. It is unclear at this point whether increasing the thick-
ness of the coatings will proportionately increase the total O-atom fluence
that these coatings can withstand. Future work will emphasize the fundamental
mechanisms responsible for the interaction of these materials with O-atoms.
114
Of particular interest are thin film inorganic coatings and the relationship
of diffusion-limited kinetics to the atomic oxygen kinetic energy.
ACKNOWLEDGEMENTS
The authors wish to thank Dr. Lubert Leger, James Visentine, and
Dr. Steve Koontz (NASA/Johnson Space Center) for many useful discussions
and support for the development of the O-atom beam facility, as well as
Sharon Rutledge and Bruce Banks (NASA/Lewis Research Center) and Roger Linton
(NASA/Marshall Space Flight Center) for many useful discussions and their
coating of the samples. We would also like to thank Frank Archuleta (Los
Alamos National Laboratory) and Robert Atkinson (summer student at Los Alamos
from Montana State University) for their valuable technical assistance in this
program.
115
1. Leger, L.; Visentine, J.; and Santos-Mason, B.: "Selected Materials
Issues Associated with Space Station," 18th International SAMPE
Technical Conference, October 1986.
2 o Whitaker, A. F.; Little, S. A.; Harwell, R. J.; Griner, D. B.; DeHaye,
R.F.; and Fromhold, A. T., Jr.: "Orbital Atomic Oxygen Effects on
Thermal Control and Optical Materials," STS-8 Results, AIAA-85-0146,
AIAA 23rd Aerospace Sciences Meeting, January 1985.
° Banks, B. A.; Mirtich, M. J.; Rutledge, S. K.; Swec, D. M.; and Nahra,
H. K.: "Ion Beam Sputtered Deposited Thin Film Polymer Coatings for
Protection of Spacecraft in Low Earth Orbit", AIAA 23rd Aerospace
Sciences Meeting, January 1985.
4o Leger, L. J.; Visentine, J. T.; and Schliesing, J. A.: "A Consideration
of Atomic Oxygen Interactions with Space Station," AIAA-85-0476, AIAA
23rd Aerospace Sciences Meeting, January 1985.
5° Cross, J. B.; Spangler, L. H.; Hoffbauer, M. A.; and Archuleta, F. A.:
"High Intensity 5 eV CW Laser Sustained O-Atom Exposure Facility for
Material Degradation Studies," 18th International SAMPE Technical
Conference, October 1986.
6, Cross, J. B.; and Blais, N. C.: "High Energy/Intensity Atomic Oxygen Beam
Source for Low Earth Orbit Materials Degradation Studies," 16th
International Symposium on Rarefied Gas Dynamics, July 1988.
7 o Cross, J. B.; Lan, E. H.; and Smith, C. A.: "A Technique to Evaluate
Coatings for Atomic Oxygen Resistance," 33rd International SAMPE
Technical Conference, March 1988.
116
TABLE I. - SUMMARY OF COATINGS RESULTS
Coating Thickness Fluence Lifetime (Atoms/cm 2)
PTFE Teflon 800 A 6.0 x 1019
AI203
SiO 2
SWS-V-10
8O0 A
1250
950 A
950 A
950 A
95o A
950 i
4 _m
4 _m
4 _m
3.5 x 1019
1.5 x 1019
(Reaction Efficiency of 0.13 x 10 -24
cm3/atom)
(Reaction Efficiency of 0.22 x 10 -24
cm3/atom)
(Reaction Efficiency of 0.82 x 10 -24
cm3/atom)
1.0 x 1020
8.3 x 1020
1.2 x 1020
7.3 x 1020
ii.0 x 1020
1.8 x 1020
4.9 x 1020
6.0 x 1020
117
/
Bare _ I
CM
Figure i. The O-atom actinometer pattern used in which
constant current is supplied to each strip
and voltage is then measured using the same
contacts (see also figure 2). The actinometers
were parepared by first imprinting gold leads
onto an alumina substrate. Silver was then
vacuum evaporatedoonto the substrate to a
thickness of 250 A.
118
30 V DC
+
500K Ohms
....i_i,_/_,_i_i_!-
Voltage to Frequency
Converter
Multichannel
Scalar
Figure 2. Electrical Circuit for Actinometer Resistance Measurements.
119
10 10 0
10 5
10 4
10"I
10"2
0 2 4 6 8
Fluence x 1 E20 atoms/sq, cm
101
10 0
10-5
-m.- Resistance
4- Conductance
Figure 3. Plot of electrical resistance/conductance of silver
underlying a protective coating (950 A SiO2) shows the
variation of log(electrical resistance) and log(electrical
conductance) as a function of O-atom fluence. The shapes
of the silver electrical resistance/conductance vs.
fluence graphs do vary from coating to coating and from
sample to sample of the same coating.
120
!o
_d
6
4
Si
0
Ag
AI
0
0 500 1000 1500 2000
Approximate Depth (Angstroms)
Figure 4. Auger elemental depth profile of Si, O, Ag, and A1 in a
Si02-coated silver actinometer after an O-atom exposure
fluence of ii.0 x 1020 atoms/cm 2 shows that the SiO 2
coating was not eroded away. The actinometer has an
alumina substrate, which accounts for the presence of AI.
The depth is only an approximation, because it is based on
an estimated sputtering rate.
121
Session IV
DYNAMIC TESTING
PRI_C_DINO PAO8 BLANK NOTFIL_ED


Atomic oxygen effects on candidate coatings for long-term spacecraft in low earth orbit

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