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Lessons Learned from Atomic Oxygen
Interaction with Spacecraft Materials in Low Earth Orbit
There have been five Materials International Space Station Experiment
(MISSE) passive experiment carriers (PECs) (MISSE 1-5) to date that
have been launched, exposed in space on the exterior of International
Space Station (ISS) and than returned to Earth for analysis. An
additional four MISSE PECs (MISSE 6A, 7A, and 7B) are in various stages
of completion. The PECs are two-sided suitcase to size sample
carriers that are intended to provide information on the effects of the
low Earth orbital environment on a wide variety of materials and
components. As a result of post retrieval ananlyses of the
retrieved MISSE 2 experiments, and numerous prior space experiments,
there have been valuable lessons learned and needs identified that are
worthy of being documented so that planning, design, and analysis of
future space environment experiments can benefit from the experience in
order to maximize the knowleged gained. Some of the lessons
learned involve the techniques, concepts, and issues associated with
measuring atomic oxygen erosion yields. These are presented along
with several issues to be considered when desigining experiments, such
as the uncertainty in mission duration, scattering and contamination
effects on results, and the accuracy of measuring atomic oxygen
erosion.
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Use of Atomic Oxygen for Increased Water
Contact Angles of
Various Polymers for Biomedical Applications
In the low Earth
orbit (LEO) space environment, spacecraft surfaces can be altered
during atomic
oxygen exposure through oxidation and erosion. There can be
terrestrial
benefits of such interactions, such as the modification of hydrophobic
or
hydrophilic properties of polymers due to chemical modification and
texturing. Such modification of the surface may be useful for
biomedical
applications. For example, atomic oxygen texturing may increase the
hydrophilicity of polymers, such as Aclar, thus allowing increased
adhesion and
spreading of cells on textured Petri dishes. The purpose of this
study was
to determine the effect of atomic oxygen exposure on the hydrophilicity
of nine
different polymers. To determine whether hydrophilicity remains static
after
atomic oxygen exposure or changes with exposure, the contact angles
between the
polymer and a water droplet placed on the polymer’s surface were
measured. The
polymers were exposed to atomic oxygen in a radio frequency (RF) plasma
asher.
Atomic oxygen plasma treatment was found to significantly alter the
hydrophilicity
of non-fluorinated polymers. Significant decreases in the water contact
angle
occurred with atomic oxygen exposure. Fluorinated polymers were
found to
be less sensitive to changes in hydrophilicity for equivalent atomic
oxygen
exposures, and two of the fluorinated polymers became more hydrophobic.
The
majority of change in water contact angle of the non-fluorinated
polymers was
found to occur with very low fluence exposures, indicating potential
cell
culturing benefit with short treatment time.
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Comparison of the Atomic Oxygen
Erosion Depth and Cone Height of Various Materials at Hyperthermal
Energy
Atomic oxygen readily reacts with most spacecraft polymer
materials exposed to the low Earth orbital (LEO) environment. If
the atomic oxygen arrival comes from a fixed angle of impact, the
resulting erosion will foster the development of a change in surface
morphology as material thickness decreases. Hydrocarbon and
halopolymer materials, as well as graphite, are easily oxidized and
textured by directed atomic oxygen in LEO at energies of
~4.5eV. What has been curious is that the ratio of cone height to
erosion depth is quite different for different materials. The
formation of cones under fixed direction atomic oxygen attack may
contribute to a reduction in material tensile strength in excess of
that which would occur if the cone height to erosion depth ratio was
very low because of greater opportunities for crack initiation. In
an effort to understand how material composition affects the ratio of
cone height to erosion depth, an experimental investigation was
conducted on 18 different materials exposed to a hyperthermal energy
directed atomic oxygen source (~70eV). The materials were first
salt-sprayed to provide microscopic local areas that would be protected
from atomic oxygen. This allowed erosion depth measurements to be made
by scanning microscopy inspection. The polymers were then exposed
to atomic oxygen produced by an end Hall ion source that was operated
on pure oxygen. Samples were exposed to an atomic oxygen effective
fluence of 1.0x1020 atoms/cm2 based on Kapton H polyimide
erosion. The average erosion depth and average cone height were
determined using field emission scanning electron microscopy
(FESEM). The experimental ratio of average cone height to erosion
depth is compared to polymer composition and other properties.
MISSE PEACE Polymers Atomic
Oxygen Erosion Results
Forty-one different polymers, collectively called the PEACE
(Polymer Erosion and Contamination Experiment) Polymers, have been
exposed to the low Earth orbit environment on the exterior of the
International Space Station (ISS) for nearly four years as part of
Materials International Space Station Experiment 1 & 2 (MISSE 1
& 2). The objective of the MISSE PEACE Polymers experiment is
to accurately determine the atomic oxygen erosion yield of a very wide
variety of polymeric materials. The polymers range from those
commonly used for spacecraft applications, such as TeflonÒ FEP, to more
recently developed polymers, such as high temperature polyimide PMR
(polymerization of monomer reactants). Additional polymers were
included to explore erosion yield dependence upon chemical
composition. The MISSE PEACE Polymers experiment was flown in
MISSE Passive Experiment Carrier 2 (PEC 2), Tray 1, on the exterior of
the ISS Quest Airlock and was exposed to atomic oxygen along with solar
and charged particle radiation. MISSE PEC 1 & 2 were
successfully retrieved during a space walk on July 30, 2005 during
Discovery’s STS-114 Return to Flight mission. Details on the
specific polymers flown, flight sample fabrication, and pre-flight and
post-flight characterization techniques will be discussed along with a
summary of the atomic oxygen erosion yield results. The long
duration low Earth orbit erosion yield data obtained from this
experiment will be compared with predictive model results for the same
polymers. The PEACE Polymers erosion yield data is unique and
provides extremely valuable information for spacecraft design purposes.
MISSE Scattered Atomic Oxygen
Characterization Experiment
An experiment designed to measure the atomic oxygen (AO) erosion
profile of scattered AO was exposed to low Earth orbital (LEO) AO for
almost 4 years as part of the Materials International Space Station
Experiment 1 & 2 (MISSE 1 & 2). The experiment was flown
in MISSE Passive Experiment Carrier
2 (PEC 2), Tray 1, attached to the exterior of the International Space
Station
(ISS) Quest Airlock. The experiment consisted of an aperture disk
lid of
Kapton H polyimide coated on the space exposed surface with a thin AO
durable
silicon dioxide film. The aperture lid had a small hole in its
center to
allow AO to enter into a chamber and impact a base disk of
aluminum. The
AO that scattered from the aluminum base could react with the under
side
of the aperture lid which was coated sporadically with microscopic
sodium
chloride particles. Scattered AO erosion can occur to materials
within a
spacecraft that are protected from direct AO attack but because of
apertures in the spacecraft the AO can attack the interior materials
after scattering. The erosion of the underside of the Kapton lid
was sufficient to be able to
use profilometry to measure the height of the buttes that remained
after washing
off the salt particles. The erosion pattern indicated that peak
flux of
scattered AO occurred at and angle of approximately 45o
degrees
from the incoming normal incidence on the aluminum base unlike the
erosion
pattern predicted for scattering based on Monte Carlo computational
predictions
for AO scattering from Kapton H polyimide. The effective erosion
yield for
the scattered AO was found to be a factor of 0.214 of that for direct
impingement
on Kapton H polyimide.
Comparison of Atomic Oxygen Erosion
Yields of Materials at Various Energies and Impact Angles
The atomic oxygen erosion yields of various materials, measured
in volume of material oxidized per incident atomic oxygen atom, are
compared
to the commonly accepted standard of Kapton H polyimide. The ratios of
the
erosion yield of Kapton H to the erosion yield of various materials are
not consistent at different atomic oxygen energies. Although it is most
convenient to use isotropic thermal energy RF plasma ashers to assess
atomic
oxygen durability, the results can be misleading because the relative
erosion
rates at thermal energies are not necessarily the same as low Earth
orbital
(LEO) energies of ~4.5 eV. An experimental investigation of the
relative
atomic oxygen erosion yields of a wide variety of polymers and carbon
was
conducted using isotropic thermal energy (~0.1 eV) and hyperthermal
energy
(~70 eV) atomic oxygen using an RF plasma asher and an end Hall ion
source.
For hyperthermal energies, the atomic oxygen erosion yields relative to
normal incident Kapton H were compared for sweeping atomic oxygen
arrival
with that of normal incidence arrival. The results of isotropic thermal
energy, normal incident, and sweeping incident atomic oxygen are also
compared
with measured or projected LEO values.
Preliminary Analysis of Polymer Film
Thermal Control and Gossamer Materials Experiments on Materials
International Space Station Experiment (MISSE 1 and MISSE 2)
A total of 31 samples were included in the National Aeronautics and
Space Administration (NASA) Glenn Research Center (GRC) Polymer Film
Thermal Control (PFTC) and Gossamer Materials experiments, which were
exposed to the low Earth orbit environment for nearly four years on the
exterior of
the International Space Station (ISS) as part of the Materials
International Space Station Experiment (MISSE 1 and MISSE 2).
MISSE is a materials flight experiment sponsored by the Air Force
Research Lab/Materials Lab and NASA. This paper describes
objectives, materials, and characterizations for the MISSE 1 and MISSE
2 GRC PFTC and Gossamer Materials samples. Samples included films
of polyimides, fluorinated polyimides, and TeflonÒ fluorinated ethylene
propylene (FEP) with and without second-surface metalizing layers
and/or surface
coatings. Also included were films of polyphenylene
benzobisoxazole (PBO)
and a polyarylene ether benzimidazole (TOR-LMTM). Polymer film
samples were
examined post-flight for changes in mechanical and optical properties.
The
environment in which the samples were located was characterized through
analysis
of sapphire contamination witness samples and samples dedicated to
atomic
oxygen (AO) erosion measurements. Results of the preliminary
analyses of
the PFTC and Gossamer Materials experiments are discussed.
Effects of the Space Environment on
Polymer Film Materials Exposed on the Materials International Space
Station Experiment (MISSE 1 and MISSE 2)
A total of 28 polymer film samples were included in the National
Aeronautics and Space Administration (NASA) Glenn Research Center (GRC)
Polymer Film Thermal Control (PFTC) and Gossamer Materials Experiments,
which were exposed to the low Earth orbit environment for nearly 4
years on the exterior of
the International Space Station (ISS) as part of the Materials
International Space Station Experiment (MISSE 1 and MISSE 2).
MISSE is a materials flight experiment sponsored by the Air Force
Research Lab/Materials Lab and NASA. This paper will describe
objectives, materials, and characterizations for the MISSE 1 and MISSE
2 GRC PFTC and Gossamer Materials samples. Samples included films
of polyimides, fluorinated polyimides, and TeflonÒ fluorinated ethylene
propylene (FEP) with and without second-surface metalizing layers
and/or surface coatings. Also included were films of
polyphenylene benzobisoxazole (PBO) and a polyarylene ether
benzimidazole (TOR-LMTM). Polymer film samples were examined
post-flight for changes in mechanical and optical properties and
for atomic oxygen (AO) erosion. Results of the preliminary
analyses of the PFTC and Gossamer Materials Experiments are discussed.
MISSE PEACE Polymers Atomic Oxygen
Erosion
Results
Forty-one different polymer samples, collectively called the Polymer
Erosion
and Contamination Experiment (PEACE) Polymers, have been exposed to the
low
Earth orbit (LEO) environment on the exterior of the International
Space
Station (ISS) for nearly 4 years as part of Materials International
Space
Station Experiment 2 (MISSE 2). The objective of the PEACE
Polymers experiment
was to determine the atomic oxygen erosion yield of a wide variety of
polymeric
materials after long term exposure to the space environment. The
polymers
range from those commonly used for spacecraft applications, such as
Teflon
FEP, to more recently developed polymers, such as high temperature
polyimide
PMR (polymerization of monomer reactants). Additional polymers
were included
to explore erosion yield dependence upon chemical composition.
The MISSE
PEACE Polymers experiment was flown in MISSE Passive Experiment Carrier
2
(PEC 2), tray 1, on the exterior of the ISS Quest Airlock and was
exposed
to atomic oxygen along with solar and charged particle radiation.
MISSE
2 was successfully retrieved during a space walk on July 30, 2005,
during
Discovery’s STS-114 Return to Flight mission. Details on the
specific polymers
flown, flight sample fabrication, pre-flight and post-flight
characterization
techniques, and atomic oxygen fluence calculations are discussed along
with
a summary of the atomic oxygen erosion yield results. The MISSE 2
PEACE
Polymers experiment is unique because it has the widest variety of
polymers
flown in LEO for a long duration and provides extremely valuable
erosion
yield data for spacecraft design purposes.
Ground-to-Space Effective Atomic Oxygen
Fluence
Correlation for DC 93-500 Silicone
The objective of this research was to calibrate the ground-to-space
effective
atomic oxygen fluence for DC 93-500 silicone in a thermal energy
electron
cyclotron resonance (ECR) oxygen plasma facility. A technique has been
developed
at NASA Glenn Research Center to determine the equivalent amount of
atomic
oxygen exposure in an ECR ground-test facility to produce the same
degree
of atomic oxygen damage as in space. The approach used was to compare
changes
in the surface hardness of ground test (ECR)-exposed DC 93-500 silicone
with
DC 93-500 exposed to low Earth orbit (LEO) atomic oxygen as part of a
shuttle
flight experiment. The ground-to-space effective atomic oxygen fluence
correlation
was determined based on the fluence in the ECR source that produced the
same
hardness for the fluence in space. A nanomechanical measurement system
operated
in conjunction with an atomic force microscope (AFM) was used to
determine
the surface hardness of the silicones. Hardness vs contact depth
measurements
were obtained for five ECR-exposed DC 93-500 samples (ECR exposed for
18
to 40 h, corresponding to Kapton effective fluences of 4.2×1020 to
9.4×1020
atoms/cm2, respectively) and for space-exposed DC 93-500 from the
Evaluation
of Oxygen Interactions with Materials III (EOIM III) shuttle flight
experiment,
exposed to LEO atomic oxygen (2.3×1020atoms/cm2). Pristine controls for
the
ECR tests and for the EOIM III flight sample were also evaluated. A
ground-to-space
correlation value was determined based on correlation values for four
contact
depths (150, 200, 250, and 300 nm), which represent the near-surface
depth
data. The results indicate that the Kapton effective atomic oxygen
fluence
in the ECR facility needs to be 2.64 times higher than in LEO to
replicate
equivalent exposure damage in the ground test silicone as occurred in
the
space exposed silicone.
Determination of
Ground-Laboratory
to In-Space Effective Atomic Oxygen Fluence for DC 93-500 Silicone
The objective of this research was to calibrate the ground-to-space
effective
atomic oxygen fluence for DC 93-500 silicone in a thermal energy
electron
cyclotron resonance (ECR) oxygen plasma facility. Silicones, a
commonly
used spacecraft material, do not chemically erode with atomic oxygen
attack
like other organic materials. Silicones react with atomic oxygen
and form
an oxidized hardened silicate surface layer. Therefore, the effective
atomic
oxygen fluence in a ground test facility should not be determined based
on
mass loss measurements, as they are with organic polymers, such as
Kapton,
a polyimide. A technique has been developed at the Glenn Research
Center
to determine the equivalent amount of atomic oxygen exposure in an ECR
ground
test facility to produce the same degree of atomic oxygen damage as in
space.
The approach used was to compare changes in the surface hardness of
ground
test (ECR) exposed DC 93-500 silicone with DC 93-500 exposed to low
Earth
orbit (LEO) atomic oxygen as part of a shuttle flight experiment.
The ground
to in-space effective atomic oxygen fluence correlation was deter¬mined
based
on the fluence in the ECR source that produced the same hardness for
the
fluence in-space. Because microhardness measurements need to be
obtained
on the very surface layer of a rubber substrate (with primarily elastic
deformation)
traditional techniques for microhardness that apply large forces and
indenta¬tions
based on plastic deforma¬tion, could not be used. Therefore, a
nanomechanical
measurement system operated in conjunction with an atomic force
microscope
(AFM) was used to determine the surface hardness of the
silicones. The nanomechanical
system can provide ultra light load indentations and can continuously
measure
force and displacement as an indent is made. Hardness versus
contact depth
measurements were obtained for five ECR exposed DC 93-500 samples (ECR
exposed
for 18 hrs to 40 hrs, corresponding to Kapton effective fluences of 4.2
x
1020 to 9.4 x 1020 atoms/cm2, respectively) and for a space exposed DC
93-500
from the Evaluation of Oxygen Interactions with Materials III (EOIM
III)
shuttle flight experiment, exposed to LEO atomic oxygen for 2.3 x 1020
atoms/cm2.
Pristine controls for the ECR tests and for the EOIM III flight sample
were
also evaluated. A ground-to-space correlation value was
determined based
on correlation values for four contact depths (150, 200, 250 & 300
nm),
which represent the near surface depth data. The results indicate
that the
Kapton effective atomic oxygen fluence in the ECR facility needs to be
2.64
times higher than in LEO to replicate equivalent exposure damage in the
ground
test silicone as occurred in the space exposed silicone.
Fast Three-Dimensional Modeling of
Atomic
Oxygen Undercutting of Protected Polymers
A method is presented to model atomic oxygen
erosion
of protected polymers in low Earth orbit. Undercutting of
protected
polymers by atomic oxygen can occur due to the presence of scratch,
crack
or pin-window defects in the protective coatings. As a means of
providing
a better understanding of undercutting processes, a fast method of
modeling
atomic-oxygen undercutting of protected polymers has been
developed.
Current simulation methods often rely on computationally expensive
ray-tracing
procedures to track the surface-to-surface movement of individual
“atoms”.
To reduce the burden of time consuming calculations, the method
introduced
in this paper replaces computationally demanding individual particle
simulations by substituting a model that utilizes both a geometric
configuration-factor technique, which collectively governs the diffuse
transport of atoms between surfaces, and an efficient algorithm, which
rapidly computes the cumulative effects stemming from the series of
atomic oxygen collisions at the surfaces of an undercut cavity.
This new method facilitates the systematic study of three-dimensional
undercutting by allowing rapid simulations to be made over a wide range
of erosion parameters.
Atomic Oxygen Effects on
Spacecraft
Materials
Low Earth orbital (LEO) atomic oxygen cannot only erode the external
surfaces of polymers on spacecraft, but can cause degradation of
surfaces internal to components on the spacecraft where openings to the
space environment exist. Although atomic oxygen attack on internal or
interior surfaces may not have direct exposure to the LEO atomic oxygen
flux,
scattered impingement can have can have serious degradation effects
where
sensitive interior surfaces are present. The effects of atomic oxygen
erosion of polymers interior to an aperture on a spacecraft is
simulated
using Monte Carlo computational techniques. A 2-dimensional model is
used to provide quantitative indications of the attenuation of atomic
oxygen flux as a function of distance into a parallel walled cavity.
The
degree of erosion relative is compared between the various interior
locations and the external surface of an LEO spacecraft.
Space Flight Experiments to Measure
Polymer
Erosion and Contamination on Spacecraft
Atomic oxygen erosion and silicone contamination are serious issues
that could damage or destroy spacecraft components
after orbiting for an extended period of time, such as on a space
station or satellite. An experiment, the Polymer Erosion And
Contamination
Experiment (PEACE) will be conducted to study the effects of atomic
oxygen (AO) erosion and silicone contamination, and it will provide
information and contribute to a solution for these problems. PEACE will
fly 43 different polymer materials that will be analyzed for AO erosion
effects through two techniques: mass loss measurement and recession
depth measurement. Pinhole cameras will provide information about the
arrival direction of AO, and silicone contamination pinhole cameras
will identify the source of silicone contamination on a spacecraft. All
experimental
hardware will be passively exposed to AO for up to two weeks in the
actual
space environment when it flies in the bay of a space shuttle. A second
set of the PEACE Polymers is being exposed to the space environment for
erosion yield determination as part of a second experiment, Materials
International Space Station Experiment (MISSE). MISSE is a
collaboration
between several federal agencies and aerospace companies. During a
space
walk on August 16, 2001, MISSE was attached to the outside of the
International
Space Station (ISS) during an extravehicular activity (EVA), where it
began its exposure to AO for approximately 1 1/2 years. The PEACE
polymers,
therefore, will be analyzed after both short-term and long-term AO
exposures
for a more complete study of AO effects.
Techniques for Measuring Low Earth Orbital
Atomic Oxygen Erosion of Polymers
Polymers such as polyimide Kapton® and Teflon® FEP (fluorinated
ethylene propylene) are commonly used spacecraft materials due to their
desirable properties such as flexibility, low density, and in the case
of FEP, a low solar absorptance and high thermal emittance. Polymers on
the exterior of spacecraft in the low Earth orbit (LEO) environment are
exposed to energetic atomic oxygen. Atomic oxygen reaction with
polymers causes erosion, which is a threat to spacecraft durability. It
is therefore important to understand the atomic oxygen erosion yield
(E, the volume loss per incident oxygen atom) of polymers being
considered in spacecraft design. The most common technique for
determining E is through mass loss measurements. For limited duration
exposure experiments, such as shuttle experiments, where the atomic
oxygen fluence is often so low that mass loss measurements can not
produce acceptable uncertainties, recession
measurements based on atomic force microscopy analyses can be used.
Equally
necessary to knowing the mass loss or recession depth for determining
the erosion yield of polymers is the knowledge of the atomic oxygen
fluence
that the polymers were exposed to in space. This paper discusses the
procedures
and relevant issues for mass loss and recession depth measurements for
passive atomic oxygen erosion yield characterization of polymers, along
with techniques for active atomic oxygen fluence and erosion
characterization. One active atomic oxygen erosion technique discussed
is a new technique
based on optical measurements. Details including the use of both
semi-transparent and opaque polymers for active erosion measurement are
reviewed.
Direct C-C Bond Breaking in the Reaction
of
O(3P) with Flouropolymers in Low Earth Orbit (and) Degradation of
Fluoropolymers by O(3P) in Low Earth Orbit
Spacecraft flying in low Earth orbit (LEO) are exposed to
a harsh environment which includes frequent bombardment by fast atomic
oxygen (AO) and ultraviolet (UV) radiation. As a result, many
spacecraft surface materials are severely eroded. In the case of
fluoropolymers, a controversy exists whether AO or UV or AO/UV synergy
is responsible for the degradation. In this study, with the use of ab
initio calculations, we address the question whether the most abundant
species in LEO, viz., atomic oxygen in its ground state, O(3P), alone
can cause the degradation in fuoropolymer materials. The smallest
fluorocarbons CNF2N+2 (N = 2, 3, 5) serve as models of fluoropolymers.
Since electronegativity of fluorine seems to preclude F-abstraction by
O(3P), we concentrate on direct O(3P) attacks on carbon-carbon bonds.
For the case of fluoroethane (N = 2), we explore the triplet potential
energy surface of the following reaction:
O(3P) + CF3 – CF3 –> •O – CF3 + •CF3. Analogous reactions, where
O(3P)
attacks on a central carbon atom, are studied for the higher
fluorocarbons.
Results obtained using the Hartree-Fock method and density functional
theory
are reported. We conclude that O(3P) species in LEO possesses enough
translational energy to degrade fluorocarbon materials.
Issues and Consequences of Atomic Oxygen
Undercutting of Protected Polymers in Low Earth Orbit
Hydrocarbon based polymers that are exposed to atomic oxygen in low
Earth orbit are slowly oxidized which results in recession of their
surface. Atomic oxygen protective coatings have been developed which
are both durable to atomic oxygen and effective in protecting
underlying polymers. However, scratches, pin window defects, polymer
surface roughness and protective coating layer configuration can result
in erosion and potential failure of protected thin polymer films even
though the coatings are themselves atomic oxygen durable. This paper
will present issues that cause protective coatings to become
ineffective in some cases yet effective in others dues to the details
of their specific application. Observed in-space examples of failed and
successfully protected materials using identical protective thin films
will be discussed and analyzed. Proposed approaches to prevent the
failures that have been observed will also be presented.
Fast Three-Dimensional Method of Modeling
Atomic Oxygen Undercutting of Protected Polymers
A method is presented to model atomic oxygen erosion of
protected polymers in low Earth orbit (LEO). Undercutting of protected
polymers by atomic oxygen occurs in LEO due to the presence of scratch,
crack
or pin-window defects in the protective coatings. As a means of
providing a better understanding of undercutting processes, a fast
method of modeling atomic-oxygen undercutting of protected polymers has
been developed. Current simulation methods often rely on
computationally expensive ray-tracing procedures to track the
surface-to-surface movement of individual “atoms”. The method
introduced in this paper replaces slow individual particle approaches
by substituting a model that utilizes both a geometric
configuration-factor technique, which governs the diffuse transport of
atoms between surfaces, and an efficient telescoping series algorithm,
which rapidly integrates the cumulative effects stemming from the
numerous atomic oxygen events
occurring at the surfaces of an undercut cavity. This new method
facilitates
the systematic study of three-dimensional undercutting by allowing
rapid
simulations to be made over a wide range of erosion parameters.
Vacuum Ultraviolet Radiation and Atomic
Oxygen Durability Evaluation of HST Bi-Stem Boom Thermal Shield
Materials
Bellows-type thermal shields were proposed for use on the
Hubble Space Telescope (HST) solar array bi-stem booms to reduce the
thermal gradient-induced jitter during orbital thermal cycling.
Candidate thermal shield materials included aluminized FEP Teflon with
and without protective coatings for durability to atomic oxygen (AO)
and combined AO and ultraviolet (UV) radiation. NASA Lewis (now Glenn)
Research Center
performed vacuum ultraviolet (VUV) radiation and AO durability testing
of candidate materials as part of an overall program coordinated by
NASA Goddard Space Flight Center (GSFC) to evaluate the on-orbit
durability
of these thermal shield materials.
Coating adhesion problems were observed for samples having AO-
and the combined AO/UV-protective coatings which were attributed to
exposure to rapid thermal cycling used to simulate thermal cycling on
orbit.
Such adhesion problems led to production of coating flakes from the
material which could pose a significant risk to HST optics if the
coated materials were used for the bi-stem boom thermal shields. No
serious degradation
was observed for the uncoated aluminized Teflon as evaluated by optical
microscopy, although atomic force microscopy (AFM) revealed that an
embrittled surface layer would build up on the uncoated Teflon surface
due to ultraviolet radiation exposure. This embrittled layer was not
completely
removed by AO erosion. Despite the formation of this embrittled layer,
no cracks or particle flakes were produced for the uncoated material
upon
exposure to VUV and AO.
Uncoated aluminized FEP Teflon was determined to be the most
appropriate thermal shield material and was used on the replacement
solar arrays installed during the December 1993 First HST Servicing
Mission.
Atomic Oxygen Protection of Materials in
Low Earth Orbit
Spacecraft polymeric materials as well as polymer-matrix carbon-fiber
composites can be significantly eroded as a result of exposure to
atomic oxygen in low Earth orbit (LEO). Several new materials now
exist, as well as modifications to conventionally used materials that
provide much more resistance to atomic oxygen attack than conventional
hydrocarbon polymers. Protective coatings have also been developed
which are resistant to atomic oxygen attack and provide protection of
underlying materials. However, in actual spacecraft applications, the
configuration, choice of materials, surface characteristics and
functional
requirements of quasidurable materials or protective coatings can have
great impact on the resulting performance and durability. Atomic oxygen
degradation phenomena occurring on past and existing spacecraft will be
presented. Issues and considerations involved in providing atomic
oxygen
protection for materials used on spacecraft in low Earth orbit will be
addressed. Analysis of in-space results to determine the causes of
successes
and failure of atomic oxygen protective coatings is presented.
A Sensitive Technique Using Atomic Force
Microscopy to Measure the Low Earth Orbit Atomic Oxygen Erosion of
Polymers
Polymers such as polyimide Kapton and Teflon FEP (fluorinated ethylene
propylene) are commonly used spacecraft materials due to their
desirable properties such as flexibility, low density, and in the case
of FEP low solar absorptance and high thermal emittance. Polymers on
the exterior of spacecraft in the low Earth orbit (LEO) environment are
exposed to energetic atomic oxygen. Atomic oxygen erosion of polymers
occurs in LEO and is a threat to spacecraft durability. For example,
depths
of more than 0.0127 cm thickness of Kapton and Mylar were eroded away
after 5.8 years in LEO on the Long Duration Exposure Facility (LDEF).
It
is therefore important to understand the atomic oxygen erosion yield
(E,
the volume loss per incident oxygen atom) of polymers being considered
in spacecraft design. Because long-term space exposure data is rare and
very costly, short-term exposures such as on the shuttle are often
relied
upon for atomic oxygen erosion determination. The most common technique
for determining E is through mass loss measurements. For limited
duration
exposure experiments, such as shuttle experiments, the atomic oxygen
fluence
is often so small that mass loss measurements can not produce
acceptable uncertainties. Therefore, a recession measurement technique
has been developed using selective protection of polymer samples,
combined with post-flight atomic force microscopy (AFM) analysis, to
obtain accurate erosion yields of polymers exposed to low atomic oxygen
fluences. This paper discusses the procedures used for this recession
depth technique along with relevant characterization issues. In
particular, a polymer is salt-sprayed prior
to flight, then the salt is washed off post-flight and AFM is used to
determine the erosion depth from the protected plateau. A small sample
was salt-sprayed for AFM erosion depth analysis and flown as part of
the Limited Duration
Candidate Exposure (LDCE-4,-5) shuttle flight experiment on STS-51.
This
sample was used to study issues such as use of contact versus
non-contact
mode imaging for determining recession depth measurements. Error
analyses
were conducted and the percent probable error in the erosion yield when
obtained
by the mass loss and recession depth techniques has been compared. The
recession depth technique is planned to be used to determine the
erosion yield of
42 different polymers in the shuttle flight experiment PEACE (Polymer
Erosion And Contamination Experiment) planned to fly in 2002 or 2003.
MISSE PEACE Polymers: an International
Space Station Environmental Exposure Experiment
Forty-one different polymers are being exposed to the low
Earth orbit (LEO) environment on the exterior of the International
Space Station (ISS) for one year as part of MISSE (Materials
International
Space Station Experiment). MISSE is a materials flight experiment
sponsored by the Air Force Research Lab/Materials Lab and the National
Aeronautics and Space Administration (NASA). A second set of the same
polymers is planned to be flown as part of PEACE (Polymer Erosion And
Contamination Experiment), a short duration shuttle flight experiment,
and therefore these forty-one polymers on ISS are collectively called
the MISSE PEACE Polymers. The objective of the MISSE PEACE Polymers
experiment
is to accurately determine the atomic oxygen (AO) erosion yield of a
wide
variety of polymeric materials. The polymers range from those commonly
used for spacecraft applications, such as Teflon® FEP, to more recently
developed polymers, such as high temperature polyimide PMR
(polymerization
of monomer reactants). Additional polymers were included to explore
erosion
yield dependence upon chemical composition. Details on the specific
polymers being flown, flight sample fabrication, and pre-flight
characterization techniques will be discussed. The MISSE PEACE Polymers
experiment was
placed on the exterior of ISS during a spacewalk on August 16, 2001 and
is planned to be retrieved in the fall of 2002. The erosion yield data
obtained from this experiment will be compared with data from the
future
short duration experiment PEACE and with predicted results from models
developed by a Canadian group that predicts the AO erosion yield of
organic
materials based on their chemical structure. Having flight data, and
comparing
flight data with the predictive model results, will be valuable for
spacecraft
design purposes.
Zr-Containing 4,4’-ODA/PMDA Polyimide
Composites
The objective of this research is to improve the atomic oxygen
resistance of KaptonTM, a polyimide (PI) made from pyromellitic acid
dianhydride (PMDA) and 4,4’-oxydianiline (ODA), while retaining or
enhancing the desirable properties of the pure polymer. Toward this
end, zirconium-containing complexes and polymers were used to make
composites and blends. Tetra(acetylacetonato)zirconium(IV), Zr(acac)4,
which is commercially available, was identified as the
best zirconium-containing complex for enhancing the atomic oxygen
resistance of polyimide composites of the 10 complexes screened. Films
prepared from the commercially-available polyamic acid (PAA) of
PMDA-ODA (DuPont) have good uniformity, flexibility, and tensile
strength. A 24-layer
10% (mol) Zr(acac)4/PI composite film showed significant improvement
(ca. 20 fold) of atomic oxygen resistance over the pure polyimide.
However, 10% (mol) Zr(acac)4 represents an upper concentration limit,
above which films undergo cracking upon thermal imidization. In order
to increase
the Zr complex concentration in PMDA-ODA PI films, while retaining good
film properties, [Zr(adsp)2-PMDA]n coordination polymer
[bis(4-amino-N,N’-disalicylidene-1,2-phenylenediamino)zirconium(IV)-pyromellitic
dianhydride] and [Zr(adsp)2-PMDA-ODA-PMDA]n terpolymer were synthesized
and blended with commercial PAA, respectively. Several techniques were
used to characterize the films made from the polymer containing
Zr(acac)4. Plasma studies of films having 2% (mol) incremental
concentrations of Zr in the Kapton up to 10% (mol) show that the
overall rate of erosion is reduced about 75 percent.
Low Temperature Testing of a Radiation
Hardened CMOS 8-Bit Flash Analog-to-Digital (A/D) Converter
Power processing electronic systems, data acquiring probes, and signal
conditioning circuits are required to operate reliably under harsh
environments in many of NASA’s missions. The environment of the space
mission as well as the operational requirements of some of the
electronic systems, such as infrared-based satellite of telescopic
observation
stations where cryogenics are involved, dictate the utilization of
electronics
that can operate efficiently and reliably at low temperatures. In this
work, radiation-hard CMOS 8-bit flash A/D converters were characterized
in terms of voltage conversion and offset in the temperature range of
+25 to –190 °C. Static and dynamic supply currents, ladder resistance,
and gain and offset errors were also obtained in the temperature range
of +125 to –190 °C. The effect of thermal cycling on these properties
for a total of ten cycles between +80 and -150 °C was also determined.
The experimental procedure along with the data obtained are reported
and
discussed in the paper.
Modeling of Transmittance Degradation
Caused by Optical Surface Contamination by Atomic Oxygen Reaction With
Adsorbed Silicones
A numerical procedure is presented to calculate transmittance
degradation caused by contaminant films on spacecraft surfaces produced
though the interaction of orbital atomic oxygen (AO) with volatile
silicones and hydrocarbons from spacecraft component. In the model,
contaminant accretion is dependent on the adsorption of species,
depletion
reactions due to gas-surface collisions, desorption, and surface
reactions
between AO and silicone producing SiOx (where x is near 2). A detailed
description of the procedure used to calculate the constituents of
the contaminant layer is presented, including the equations that govern
the evolution of fractional coverage by specie type. As an illustrative
example of film growth, calculation results using a prototype code that
calculates the evolution of surface coverage by specie type is
presented
and discussed. An example of the transmittance degradation caused by
surface
interaction of AO with deposited contaminant is presented for the case
of exponentially decaying contaminant flux. These examples are
performed
using hypothetical values for the process parameters.
Can Hydrocarbon Chains be Disrupted by
Fast
O(3P) Atoms?
O(3P) is a highly reactive species which may cause damage
to materials on contact. In low Earth orbit (LEO) high energy
collisions (~4.5 eV) of O(3P) with spacecraft materials can lead to
extensive degradation. In this study we use ab initio molecular orbital
calculations to investigate the possibility of chain breaking in
polyethylene caused by a single O(3P) attack under LEO conditions,
since the occurrence of such reactions could greatly accelerate the
erosion. The smallest alkanes (N=2,3,5,7) serve as models of
polyethylene. For the case of ethane (N=2), we explore the
triplet potential energy surface of the following reaction: O(3P)
+ CH3
~ CH3 –> •O ~ CH3 + •CH3. Analogous reactions, where O(3P)
attacks a
central carbon atom, are studied for the higher alkanes. Results
obtained using the Hartree-Fock method, density functional theory, and,
in the simplest case, i.e., ethane, second-order MØller-Plesset
perturbation theory, Gaussian theoretical models (G1, G2, and G2MP2),
and complete basis set (CBS-QB3) approach are reported. We conclude
that conditions in LEO are conductive to chain breaking in polyethylene
caused by a single O(3P) attack.
Prediction and Measurement of the Atomic
Oxygen Erosion Yield of Polymers in Low Earth Orbital Flight
Recently developed models of the erosion of polymeric materials by AO
in low Earth orbit (LEO) have been used for predictive evaluation of
the erosion resistance in LEO for a representative, comprehensive set
of polymers. The established correlations of erosion yield values with
the chemical composition and structure of hydrocarbon polymers,
and with their flammability have been used for predictive evaluation of
the behavior of those materials in LEO. Among these materials, a
variety
of aromatic and aliphatic hydrocarbon polymers, including homopolymers,
copolymers, and terpolymers, have been considered. Predictive estimates
have also been given for linear-chain fluoro- and
fluoro-chloropolymers.
With different degrees of fluorination, using a recently modified
version
of the predictive model, and the results were in good agreement with
the
flight data, that exist to date. Altogether, predictive evaluations
have
been performed for more than 40 polymers, including a few recommended
materials
for which the lower and higher extremes in erosion yield in LEO can be
expected,
based on their chemical composition and structure.
For almost half of the selected materials, there is no date
from neither space nor ground-based experimental testing. For the rest,
the data was collected mostly from the Long Duration Exposure Facility
(LDEF) and several other flight experiments. The predicted erosion
yield
values ReLEO were found to be, mostly, in good agreement with the
flight
data for materials, already tested in LEO. A reasonable agreement
between
the two mentioned above predictive correlations, i.e., the one related
to the chemical composition and structure of the materials, and the one
related to the material’s flammability was found for ReLEO (pred) for
the
majority of untested materials.
A low Earth orbital space experiment entitled “Polymers
Erosion
and Contamination Experiment,” has been designed and is planned to
allow
measurement of the atomic oxygen erosion yield of a set of 40 different
polymeric materials, whose erosion yields were predicted as described
above. This will allow direct comparison between predicted and measured
in-space
atomic oxygen erosion yield. The experiment is a Get-Away-Special (GAS
can)
experiment to be conducted in the Shuttle bay that allows atomic oxygen
to impinge on two sets of the 40 types of polymers. One set of polymer
samples
will be analyzed later, using weigh loss to measure atomic oxygen
erosion
yields, and the other set will be evaluated using erosion depth to
measure
atomic oxygen erosion yield. Erosion depth will be measured by means of
salt
or mica flake particles on the polymer surfaces to act on local
protective
coatings, which will result in the production of step height changes
that
are measured by atomic force microscopy. Using this latter technique,
erosion
yield measurements with uncertainties of ~3% can be achieved for
typical
polymers with atomic oxygen fluences of ~5 x 1019 atoms/cm2.
The Dependence of Atomic Oxygen
Undercutting of Protected Kapton® H Upon Defect Size
Understanding the behavior of polymeric materials when exposed to the
low-Earth-orbit (LEO) environment is important in predicting
performance characteristics such as in-space durability. Atomic oxygen
(AO) present in LEO is known to be the principle agent in causing
undercutting erosion of SiOx protected polyimide Kapton® H film, which
serves as a mechanically stable blanket material in solar arrays. The
rate of undercutting is dependent on the rate of arrival,
directionality and energy of the AO with respect to the film surface.
The erosion rate also depends on the distribution of the size of
defects existing in the protective coating. This paper
presents results of experimental ground testing using low energy,
isotropic
AO flux together with numerical modeling to determine the dependence of
undercutting erosion upon defect size.
International Test Program for
Synergistic
Atomic Oxygen and VUV Exposure of Spacecraft Materials
Spacecraft in low Earth orbit (LEO) are subject to degradation in
thermal and optical performance of components and materials through
interaction with atomic oxygen and vacuum ultraviolet radiation which
are predominant in LEO. Due to the importance of LEO durability and
performance to manufacturers and users, an international test program
for assessing the durability of spacecraft materials and components was
initiated. Initial tests consisted of exposure of samples representing
a variety of thermal control paints and multiplayer insulation
materials that have been used in space. Materials donated from various
international sources were tested alongside a material whose
performance is well known such as Teflon FEP or Kapton H for
multiplayer insulation, or Z-93-P for white thermal control paints. The
optical, thermal or mass loss data generated during the test was then
provided to the participating material supplier. Data was not published
unless the participant donating the material consented to
the publication. This paper presents a description of the types of test
and facilities that have been used for the test program as well as some
examples of data that have been generated. The test program is intended
to give spacecraft builders and users a better understanding of
degradation
processes and effects to enable improved prediction of spacecraft
performance.
Consequences of Atomic Oxygen Interaction
with Silicone and Silicone Contamination on Surfaces in Low Earth Orbit
The exposure of silicones to atomic oxygen in low Earth orbit causes
oxidation of the surface, resulting in conversion of silicone to
silica. This chemical conversion increases the elastic modulus to
the surface and initiates the development of a tensile strain.
Ultimately, with sufficient exposure, tensile strain leads to cracking
of the surface enabling the underlying unexposed silicone to be
converted to silica
resulting in additional depth and extent of cracking. The use of
silicone
coatings for the protection of materials from atomic oxygen attack is
limited because of the eventual exposure of underlying unprotected
polymeric
material due to deep tensile stress cracking of the oxidized silicone.
The use of moderate to high volatility silicones in low Earth orbit has
resulted in a silicone contamination arrival at surfaces which are
simultaneously being bombarded with atomic oxygen, thus leading to
conversion of the
silicone contaminant to silica. As a result of these processes, a
gradual
accumulation of contamination occurs leading to deposits, which at
times
have been up to several microns thick (as in the case of a Mir solar
array
after 10 years in space). The contamination species typically consist
of silicon, oxygen, and carbon, which in the synergistic environment of
atomic oxygen and UV radiation leads to increased solar absorptance and
reduced solar transmittance. A comparison of the results of atomic
oxygen interaction with silicones and silicone contamination will be
presented
based on the LDEF, EOIM-III, Offeq-3 spacecraft and Mir solar array
in-space
results. The design of a contamination pin-hole camera space
experiment,
which uses atomic oxygen to produce an image of the sources of silicone
contamination, will also be presented.
A Space Experiment to Measure the Atomic
Oxygen Erosion of Polymers and Demonstrate a Technique to Identify
Sources of Silicone Contamination
A low Earth orbital space experiment entitled, "Polymers Erosion and
Contamination Experiment" (PEACE) has been designed as a Get-Away
Special (GAS Can) experiment to be accommodated as a Shuttle in-bay
environmental exposure experiment. The first objective is to measure
the atomic
oxygen erosion yields of ~40 different polymeric materials by mass
loss and erosion measurements using atomic force microscopy. The second
objective is to evaluate the capability of identifying sources of
silicone
contamination through the use of a pin-hole contamination camera, which
utilizes environmental atomic oxygen to produce a contaminant source
image on an optical substrate.
Investigation of Atomic Oxygen Erosion of
Polyimide Kapton H Exposed to a Plasma Asher Environment
Experimental results are presented on the erosion characteristics of
the polyimide Kapton H, which serves as a blanket material in solar
arrays. This polymer has a number of characteristics that make it a
suitable choice for both terrestrial and space applications. In this
paper attention is focused on the durability of protected Kapton hen
exposed to atomic oxygen (AO) in a plasma asher. A strip of 0.025-mm
thick
Kapton film, coated on both sides with SiO2, was studied
during
a 1360-hour exposure. The erosion, located at defect sites in the
protective
coating and measured optically, is described in terms of volume loss
as a function of AO fluence. Three simple geometric profiles are used
to generate a useful array of cavity shapes to model erosion evolution.
These models connect the volume erosion rate to the observed lateral
expansion
of the developing cavities via their diameters, measured adjacent to
the upper and lower protective film, and fitted by least-squares
regression
to simple power law functions of fluence. The rationale for the choice
of models is discussed. It was found later that growth in cavity size
evolves
less than linearly with fluence.
Atomic Oxygen Durability of Graphite
Epoxy
Composite Silver Mirrors for Space Power Applications
Two light-weight graphite epoxy composite mirrors, each having a silver
reflective layer and a silicon dioxide protection layer, were exposed
to two levels of atomic oxygen fluence in a ground-based plasma asher
facility for the purpose of evaluating their atomic oxygen durability.
Total reflectivity and specular reflectivity were monitored during
the course of atomic oxygen exposure. Optical microscope photographs
were also taken during the course of exposure to document the process
of atomic oxygen undercutting at pin window defect sites. Although
there was evidence of atomic oxygen undercutting at pin window defects
sites, functional performance of the mirrors remained fair over the
course of atomic oxygen exposure.
Monte Carlo Computational Modeling of the
Energy Dependence of Atomic Oxygen Undercutting of Protected Polymers
A Monte Carlo computational model has been developed which simulates
atomic oxygen attack of protected polymers at defect sites in the
protective coatings. The parameters defining how atomic oxygen
interacts with polymers and protective coatings as well as the
scattering process which occur have been optimized to replicate
experimental results observed from protected polyimide Kapton on the
Long Duration Exposure Facility (LDEF) mission. Computational
prediction of atomic oxygen undercutting at defect sites in protective
coatings for various arrival energies was investigated. The atomic
oxygen undercutting energy dependence predictions enable one to predict
mass loss that would occur in low Earth orbit, based on lower energy
ground laboratory atomic oxygen beam systems. Results of computational
model prediction of undercut cavity size as a function of energy
and defect size will be presented to provide insight into expected
in-space
mass loss of protected polymers with protective coating defects based
on
lower energy ground laboratory testing.
Atomic Oxygen Durability of Second
Surface
Silver Microsheet Glass Concentrators
Second surface silver microsheet glass concentrators are being
developed for potential use in future solar dynamic space power
systems. Traditional concentrators are aluminum honeycomb sandwich
composites with either aluminum or graphite epoxy face sheets, where a
reflective aluminum layer is deposited onto an organic leveling layer
on the face sheet. To protect the underlying layers, a SiO2
layer is applied on top of the aluminum reflective layer. These
concentrators may be
vulnerable to atomic oxygen degradation due to possible atomic oxygen
attack of the organic layers at defect sites in the protective and
reflective
coatings. A second surface microsheet glass concentrator would be
inherently
more atomic oxygen durable than these first surface concentrators. In
addition, a second surface microsheet glass concentrator design
provides
a smooth optical surface and allows for silver to be used as a
reflective
layer, which would improve the reflectivity of the concentrator and the
performance of the system. A potential threat to the performance of
second
surface microsheet glass concentrators is atomic oxygen attack of the
underlying
silver at seams and edges or at micrometeoroid and debris (MMD) impact
sites.
Second surface silver microsheet glass concentrator samples were
fabricated
and tested for atomic oxygen durability. The samples were initially
exposed to an atomic oxygen environment in a plasma asher. Samples were
evaluated for potential degradation at fabrication seams, simulated MMD
impact sites, and edges. Optical microscopy was used to evaluate atomic
oxygen degradation. Reflectance was obtained for an impacted sample
prior to and after atomic oxygen exposure. After an initial atomic
oxygen exposure to an effective fluence of » 1 x 1021
atoms/cm², oxidation of that silver at defect sites and edges was
observed. Exposure to an additional » 1 x 1021 atoms/cm²
caused no observed increase in oxidation. Oxidation at an
impact site caused negligible changes in reflectance. In all cases
oxidation
was found to be confined to the vicinity of the seams, impact sites,
edges, or defect sites. Asher to in-space atomic oxygen correlation
issues
will be addressed.
Atomic Oxygen Undercutting of Long
Duration Exposure facility Aluminized Kapton Multilayer Insulation
Atomic oxygen undercutting is a potential threat to vulnerable
spacecraft materials which have atomic oxygen protective coatings. Such
undercutting is due to the atomic oxygen attack of oxidized materials
at microscopic defects in the protective coatings. These defects occur
during fabrication and handling, or from micrometeoroid and debris
bombardment in space. An aluminized-polyimide Kapton multi-layer
insulation sample that was located on the leading edge of the Long
Duration Exposure Facility has been used to study low Earth orbit
atomic oxygen undercutting. Cracks in the aluminized coating located
around vent holes provided excellent defect sites for the evaluation of
atomic oxygen undercutting. The
experimentally observed undercut profiles were compared to predictions
from Monte Carlo models for normal incident space ram atomic oxygen
attack. The shape of the undercut profile was found to vary with crack
width, which is proportional to the number of oxygen atoms entering the
crack. The resulting profiles of atomic oxygen undercutting which
occurred
on the aluminized-Kapton sample indicated wide undercut cavities in
spite of the fixed ram orientation. Potential causes of the observed
undercutting are presented. Implications of the undercutting profiles
relevant to Space Station Freedom are also discussed.
Determination of Atomic Oxygen Fluence
Using Spectrophotometric Analysis of Infrared Transparent Witness
Coupons for Long Duration Exposure Tests
Atomic oxygen degradation is one of the several major threats to the
durability of spaceborn systems in low Earth orbit. Ground-based
simulations are conducted to learn how to minimize the adverse effects
of atomic oxygen exposure. Assessing the fluence of atomic oxygen
chambers such as the plasma asher over long periods of time is
necessary for accurate determination of atomic oxygen exposure.
Currently, an atomic oxygen susceptible organic material such as Kapton
is placed next to samples as a witness coupon and its mass loss is
monitored and used to determine the effective atomic oxygen fluence.
However, degradation of the Kapton witness coupons occurs so rapidly in
plasma ashers that for any long term test many witness coupons must be
used sequentially in order to keep track of the fluence. This
necessitates opening vacuum to substitute fresh coupons. A passive
dosimetry technique was sought to monitor atomic oxygen exposure over
longer periods without the need to open the plasma asher to the
atmosphere.
This paper investigates the use of spectrophotometric analysis of
durable
IR transparent witness coupons to measure atomic oxygen exposure for
longer duration testing. The method considered would be conductive to
making in situ measurements of atomic oxygen fluence.
Space Station Freedom Solar Array Blanket
Coverlay Atomic Oxygen Durability Testing Results
The power system for the Space Station Freedom used a flexible solar
array for photovolatic power generation. Support for the solar cells
and current carriers on the flexible array is provided by the solar
array blanket. The main structural member of the array blanket is the
coverlay (laminate), which is composed of Kapton, fiberglass scrim
cloth
and silicone adhesive. The anti-solar facing side of the laminate is
protected
from the atomic oxygen environment with a thin film coating of silicone
dioxide. Coated Kapton and laminate samples were exposed to simulated
atomic
oxygen environments (plasma asher and directed beam) to determine
whether
the coated Kapton is durable and the degree to which the coating is
damaged
by the lamination process. Test results indicated that the mass loss
relative to unprotected Kapton (relative reactivity) for the laminate
was roughly a factor of 10 higher than for the coated Kapton possibly
due in part to an increase in the number of scratches in the coating.
This increase is
probably due to handling during the lamination process. These results
were not dependant on whether the exposure was performed in the plasma
asher or the directed beam. Although atomic oxygen at thermal energies
can produce results which are pessimistic indicators of in space
durability,
the data indicates that if surface scratching of the coating is limited
and the coated Kapton is adherent to the underlying silicone, the
laminate
should survive for its desired lifetime of 15 years.
Atomic Oxygen Durability Evaluation of
Protected Polymers Using Thermal Energy Plasma Systems
The durability evaluation of protected polymers intended for use in low
Earth orbit (LEO) has necessitated the use of large-area, high-fluence,
atomic oxygen exposure systems. Two thermal energy atomic oxygen
exposure systems which are frequently used for such evaluations are
radio frequency (RF) plasma ashers and electron cyclotron resonance
plasma sources. Plasma source testing practices such as sample
preparation, effective fluence prediction, atomic oxygen flux
determination, erosion measurement, operational considerations, and
erosion yield measurements are presented. Issues which influence the
prediction of in-space durability based on ground laboratory thermal
energy plasma system testing are also addressed.
Atomic Oxygen Durability Testing of an
International Space Station Solar Array Validation Coupon
An International Space Station solar array validation coupon was
exposed in a directed atomic oxygen beam for space environment
durability testing. At the NASA Glenn Research Center. Exposure to
atomic oxygen and intermittent tensioning of the solar array were
conducted to verify the solar array's durability to low Earth orbital
atomic oxygen and to the docking threat of plume loading both of which
are anticipated over
its expected mission life of fifteen years. The validation coupon was
mounted on a specially designed rotisserie. The rotisserie mounting
enabled the
solar and anti-solar facing side of the array to be exposed to directed
atomic oxygen in a sweeping arrival process replicating space exposure.
The rotisserie mounting also enabled tensioning, in order to examine
the
durability of the array and its hinge to simulated plume loads. Flash
testing
to verify electrical performance of the solar array was performed with
a
solar simulator before and after the exposure to atomic oxygen and
tensile
loading. Results of the flash testing indicated little or no
degradation in the solar array's performance. Photographs were also
taken of the array before and after the durability testing and are
included along with along with comparisons and discussions in this
report. The amount of atomic oxygen damage appeared minor with the
exception of a very few isolated defects. There was also no indications
that the simulated plume loadings had weakened or damaged the array,
even though there was some erosion of Kapton due to atomic oxygen
attack. Based on the results of this testing, it is apparent that the
International Space Station's solar array should survive the low Earth
orbital atomic oxygen environment and docking threats which are
anticipated over its expected mission life.
Atomic Oxygen Interaction at Defect Sites
in Protective Coatings on Polymers Flown on LDEF
Although the Long Duration Exposure Facility (LDEF) had exposed
materials with a fixed orientation relative to the ambient
low-Earth-orbital environment, arrival of atomic oxygen is angularly
distributed as
a result of the atomic oxygen's high temperature Maxwellian velocity
distribution and the LDEF's orbital inclination. Thus, atomic oxygen
entering defects in protective coatings on polymeric surfaces can cause
wider undercut cavities than the size of the defect in the protective
coating. Because only a small fraction of atomic oxygen reacts upon
first impact with most polymeric materials, secondary reactions with
lower
energy thermally accommodated atomic oxygen can occur. The secondary
reactions of scattered and/or thermally accommodated atomic oxygen also
contribute to widening the undercut cavity beneath the protective
coating
defect. As the undercut cavity enlarges, exposing more polymer, the
probability of atomic oxygen reacting with underlying polymeric
material
increases because of multiple opportunities for reaction. Thus, the
effective
atomic oxygen erosion yield for atoms entering defects above that of
the
unprotected material. Based on the results of analytical modeling and
computational modeling, aluminized Kapton multilayer insulation exposed
to atomic oxygen on row 9 lost the entire externally exposed player of
polyimide Kapton, yet based on the results of this investigation, the
bottom surface aluminum film must have remained in place, but crazed.
Atomic oxygen undercutting at defect sites in protective coatings on
graphite epoxy composites indicates that between 40 to 100 percent of
the atomic oxygen thermally accommodates upon impact, and that the
reaction
probability of thermally accommodated atomic oxygen may range from
1.1x10-6 to 2.1x10-3, depending upon the degree
of thermal accommodation upon each impact.
Atomic Oxygen Interactions with Protected
Organic Materials on the Long Duration Exposure Facility
The Long Duration Exposure Facility (LDEF) has provided an excellent
opportunity to understand the nature of directed atomic oxygen
interactions with protected polymers and composites. Although there
were relatively few samples of materials with protective coatings on
their external surfaces on LDEF which were exposed to a high atomic
oxygen
fluence, analysis of such samples has enabled an examination of the
shape
of atomic oxygen undercut cavities at defect sites in the protective
coatings.
Samples of front-surface aluminized (Kapton®) polyimide were
inspected by scanning electron microscopy to identify and measure crack
defects in the aluminum protective coatings. After chemical removal of
the aluminum coating, measurements were also made of the width of the
oxidized
undercut cavities below the crack defects. The LDEF flight undercut
cavity
geometries were then compared to the Monte Carlo computational model
undercut
cavity predictions. The comparison of the LDEF results and
computational modeling indicates agreement in specific undercut cavity
geometries for atomic oxygen reaction probabilities dependant upon the
0.68 to 3.0 power if the energy. However, no single energy dependency
was adequate to replicate flight results over a variety of aluminum
crack widths.
Atomic Oxygen Erosion Phenomena
The surface textures resulting from directed atomic oxygen interaction
with materials which produce fully volatile oxidation products are
similar to those produced by more energetic physical sputter texturing.
A Monte Carlo computational model has been developed which simulated
both low Earth orbital energetic atomic oxygen attack as well as
isotropic thermal energy plasma atomic oxygen interactions with
materials with volatile
oxides. The surface roughening predicted by the model agrees with
experimental
observations, indicating that surface texture develops under the
simplest
interaction assumptions and grows in a less than linear manner with
atomic
oxygen fluence. The more paraxial the atomic oxygen arrival is, the
greater
the surface roughness for the same atomic oxygen fluence. The detailed
nature
of the scattering interactions appears to play a negligible role in the
development of surface roughness.
Monte Carlo Computational Modeling for
Simulation of Atomic Oxygen Interactions with Composites at Defect
Sites in Protective Coatings
Spacecraft orbiting the earth at altitudes below 500 kilometers are
exposed to the remnants of the earth's upper atmosphere. This
low Earth orbital (LEO) environment consists predominantly of atomic
oxygen caused by photo-dissociation of O2 by ultraviolet
radiation from the sun. Organic matrix carbon fiber composite materials
exposed to this environment are oxidized at a rate which would limit
the durability of many spacecraft components. As a result, atomic
oxygen
protective coatings consisting of metals and metal oxides are being
used
to protect materials from oxidation degradation in LEO. The use of
Monte
Carlo computational modeling to simulate the effects of atomic oxygen
undercutting oxidation of composite materials both in the ground
laboratory
and in space can greatly assist in improving the ability to project
in-space
durability testing. This modeling was used to test coating materials
for
performance in LEO.
Plasma and Beam Facility Atomic Oxygen
Erosion of a Transition Metal Complex
Glassy residues of the complex bis (N, N1-disalicylidene-1,
2-phenylenediamino) zirconium (IV), Zr (dsp)2, on glass
slides were exposed to atomic oxygen in a plasma asher or an atomic
beam facility for various amounts of time in order to study the erosion
process, determine the rate of erosion, and learn the chemical identity
of the residue. The exposed films were characterized by weight loss,
optical
photography, profilometry, diffuse reflectance and total transmittance
spectroscopy, scanning electron microscopy (SEM) with wavelength
dispersive x-ray spectrometry (WDS), x-ray diffraction , and x-ray
photoelectron spectroscopy (XPS). Results indicate that these films
erode much more slowly polyimide (Kapton®) film under identical
conditions, that the erosion is very non-uniform, and that the
zirconium dioxide is the predominant product after extended exposure.
This complex is currently being evaluated as a polymer additive.
Leveling Coatings for Reducing Atomic
Oxygen Defect Density in Graphite Fiber-Epoxy Composites
Pinholes or other defect sites in a protective oxide coating provide
pathways for atomic oxygen in low Earth orbit to reach underlying
material. One concept for enhancing the lifetime of materials in low
Earth orbit is to apply a leveling coating to the material prior to the
material prior to applying any reflective and protective coatings.
Using a surface-tension-leveling coating concept, a low-viscosity epoxy
was applied to the surface of several composite coupons. A protective
layer of 1000 Å of SiO2 was deposited on top of the leveling
coating, and the coupons were exposed to an atomic oxygen environment
in a plasma asher. Pinhole populations per unit area were estimated by
counting the
number of undercut sites observed by scanning electron microscopy.
Defect
density values of 180,000 defects/cm² were reduced to about 1000
defects/cm²
as a result of applied leveling coating. These improvements occur at a
mass penalty of about 2.5 mg/cm².
Prediction of In-Space Durability of
Protected Polymers Based on Ground Laboratory Thermal Energy Atomic
Oxygen
The probability of atomic oxygen reacting with polymeric materials is
orders of magnitude lower at thermal energies (<0.1 eV) than at
orbital impact energies (4.5 eV). As a result, absolute atomic oxygen
fluxes at thermal energies must be orders of magnitude higher than
orbital energy fluxes, to produce the same effective fluxes (or same
oxidation rates) for polymers. These differences can cause highly
pessimistic durability predictions for protected polymers, and polymers
which develop protective metal oxide surfaces as a result of oxidation
if one does not make suitable calibrations. A comparison was conducted
of undercut cavities below defect sites in protected polyimide Kapton
samples flown on the Long Duration Exposure Facility (LDEF) with
similar samples exposed in thermal energy oxygen plasma. The results of
this comparison were used to quantify predicted material loss in space
based on material loss in ground laboratory thermal energy plasma
testing. A microindent hardness comparison of surface oxidation of a
silicone flown on the Environmental Oxygen Interaction with Materials
III (EOIM-III) experiment with samples exposed in thermal energy
plasmas was similarly used to calibrate the rate of oxidation of
silicone in space relative to samples in thermal energy plasmas exposed
to polyimide Kapton effective fluences.
Monte Carlo Computational Techniques for
Prediction of Atomic Oxygen Erosion of Materials
Materials on the surface of spacecraft in low Earth orbit
(LEO) are exposed to the remnants of the Earth's upper atmosphere.
Energetic solar photons cause photodissociation of O2 to
produce highly reactive atomic oxygen. As spacecraft orbit through
the atomic oxygen, impact energies of 4.5± 1 eV result with an arrival
flux sufficient to cause polymeric materials to be oxidized at rates
high enough durability concerns. To increase materials durability
adequate
to meet spacecraft mission lifetime requirements, atomic oxygen
protective
coatings have been applied over polymers. Such coatings typically
consist
of metal oxide thin films. The durability of such protected polymers
used for solar array blankets and thermal control is limited as a
result
of microscopic defects in the protective films.
Recovery of a Charred Painting Using
Atomic Oxygen Treatment
A non-contact method is described which uses atomic oxygen to remove
soot and char from the surface of a painting. The atomic oxygen was
generated by the dissociation of oxygen in low-pressure air using radio
frequency energy. The treatment, which is an oxidation process, allows
control of the amount of material to be removed. The effectiveness of
char removal from half of a fire-damaged oil painting was studied using
reflected light measurements from selected areas of the painting and by
visual and photographic observation. The atomic oxygen was able to
effectively remove char and soot from the treated half of the painting.
The remaining loosely bound pigment was lightly sprayed with a mist to
replace the binder and then varnish was reapplied. Caution should be
used
when treating an untested paint medium using atomic oxygen. A
representative edge or corner should be tested first in order to
determine if the process would be safe for the pigments present. As
more testing occurs, a greater knowledge base will be developed as to
what types of paints and varnishes can or cannot be treated using this
technique. With proper precautions, atomic oxygen treatment does appear
to be a technique with great potential for allowing charred, previously
unrestorable art to be salvaged.
Atomic Oxygen Treatment as a Method of
Recovering Smoke Damaged Paintings
Smoke damage, as a result of fire, can be difficult to remove from some
types of painting media without causing swelling, leaching, or pigment
movement or removal. A non-contact technique has been developed which
can remove soot from the surface of a painting by use of a gently
flowing gas containing atomic oxygen. The atomic oxygen chemically
reacts with the soot on the surface creating gasses such as carbon
monoxide
and carbon dioxide which can be removed through the use of an exhaust
system. The reaction is limited to the surface so that the process can
be timed to stop when the paint layer is reached. Atomic oxygen is a
primary
component of the low Earth orbital environment, but it can be generated
on Earth through various methods. This paper will discuss the results
of
atomic oxygen treatment of soot exposed acrylic gesso, ink on paper,
and
a varnished oil painting. Reflectance measurements were used to
characterize
the surfaces before and after treatment.
An Atmospheric Atomic Oxygen Source for
Cleaning Smoke Damaged Art Objects
Soot and other carbonaceous combustion products deposited
on the surfaces of porous ceramic, stone, ivory, and paper can be
difficult to remove and can have potentially unsatisfactory results
using wet chemical and/or abrasive cleaning techniques. An atomic
oxygen source which
operates in air at atmospheric pressure, using a mixture of oxygen and
helium, has been developed to produce an atomic oxygen beam which is
highly effective in oxidizing soot deposit on surfaces by burning
candles
made of paraffin, oil, or rendered animal fat. Atomic oxygen source
operating conditions and the results of cleaning soot from paper,
gesso,
ivory, limestone, and water color-painted limestone are presented.
A Comparison of Atomic Oxygen Erosion
Yields of Carbon and Selected Polymers Exposed in Ground Based
Facilities
and in Low Earth Orbit
A comparison of the relative erosion yields (volume of material removed
per oxygen atom arriving) for FEP Teflon, polyethylene, and pyrolytic
graphite with respect to Kapton HN was performed in an atomic oxygen
directed beam system, in a plasma asher, and in space on the
EOIM-III (Evaluation of Oxygen Interaction with Materials-III) flight
experiment. This comparison was performed to determine the sensitivity
of material reaction to atomic oxygen flux, atomic oxygen fluence, and
vacuum ultraviolet radiation for enabling accurate estimates of
durability
in ground based facilities. The relative erosion yield of pyrolytic
graphite
was found not to be sensitive to these factors that for FEP was
sensitive
slightly to fluence and possibly ions, and that for polyethylene was
found
to be partially VUV and flux sensitive but more sensitive to an unknown
factor. Results indicate that the ability to use these facilities for
material relative durability prediction is great as long as the
sensitivity
of particular materials to conditions such as VUV, and atomic oxygen
flux
and fluence are taken into account. When testing materials of a
particular
group such as Teflon, it may be best to use a witness sample made of a
similar material that has some available space data on it. This would
enable one to predict an equivalent exposure in the ground based
facility.
Evaluation of Space Power Materials Flown
on the Passive Optical Sample Assembly
Evaluating the performance of materials on the exterior of spacecraft
id of continuing interest, particularly in anticipation of
those applications that will require a long duration in low Earth
orbit.
The Passive Optical Sample Assembly (POSA) experiment flown on the
exterior of Mir as a risk mitigation experiment for the International
Space Station was designed to better understand the interaction of
materials with the low Earth orbit environment and to better understand
the potential contamination threats that may be present in the vicinity
of spacecraft. Deterioration in the optical performance of candidate
space power materials due to the low Earth orbit environment, the
contamination environment, or both, must be evaluated in order to
propose measures to mitigate such deterioration. The thirty-two samples
of space power materials studied here include solar array blanket
materials such as polyimide Kapton H and SiOx coated polyimide Kapton
H, front surface aluminized sapphire, solar dynamic concentrator
materials such as silver on spin coated polyimide and aluminum on spin
coated polyimide, CV1144 silicone, and the thermal control paint
Z-93-P. The physical and optical properties that were evaluated prior
to and after the POSA flight include mass, total, diffuse, and specular
reflectance, solar absorptance, and infrared emittance. Additional post
flight evaluation included scanning electron microscopy to observe
surface features caused by the low Earth orbit environment and the
contamination environment, and variable angle spectroscopic
ellipsometry to identify contaminant
type and thickness. This paper summarizes the results of pre- and
post-flight
measurements, identifies the mechanisms responsible for optical
properties
deterioration, and suggests improvements for the durability of
materials
in future missions.
Environmental Durability Issues for Solar
Power Systems in Low Earth Orbit
Space solar power systems for use in low Earth orbit (LEO) environment
experience a variety of harsh environmental conditions. Materials used
for solar power generation in LEO need to be durable to
environmental threats such as atomic oxygen, ultraviolet (UV)
radiation, thermal cycling, and micrometeoroid and debris impact.
Another threat to LEO solar power performance is due to contamination
from other spacecraft components. This paper gives an overview of these
LEO environmental issues as they relate to space solar power system
materials. Issues addressed include atomic oxygen erosion of organic
materials, atomic oxygen undercutting of protective coatings, UV
darkening of ceramics, UV embrittlement of Teflon, effects of thermal
cycling on organic composites, and contamination due to silicone and
organic materials. Specific examples of samples from the Long Duration
Exposure Facility (LDEF) and materials returned from the first
servicing mission of the Hubble Space Telescope (HST) are presented.
Issues concerning ground laboratory facilities which simulate the LEO
environment are discussed along with ground-to-space correlation
issues.
Atomic Oxygen/Vacuum Ultraviolet
Radiation
Exposure of Z-93 and Z-93-P Coatings
Laboratory testing was conducted in order to assess the long-term
atomic oxygen and vacuum ultraviolet radiation durability of the
thermal control coating Z-93-P to be used on the International Space
Station radiator surfaces. This testing provided atomic oxygen
equivalent to approximately four years and vacuum ultraviolet radiation
equivalent
to approximately twenty-five years on Space Station radiator surfaces.
Solar absorptance data were obtained in vacuo at various exposure
increments. Facility limitations resulted in the inability to provide
the appropriate atomic oxygen to vacuum ultraviolet radiation ratio
that would be experienced by Space Station radiator surfaces, and
unexpected sputtering of components in the vacuum chamber caused a
contaminant layer to be deposited on the
samples. However, some conclusions can be made from the data. First,
Z-93-P
samples performed comparably to the Z-93 control sample assuring that
the
successful flight history of the original Z-93 formulation can be
applied
to the reformulated Z-93-P coating. Second, solar absorptance increases
of no more than 0.1 were calculated for the combined atomic oxygen and
vacuum
ultraviolet radiation exposure environment used in this test.
A Technique for Synergistic Atomic Oxygen
and Vacuum Ultraviolet Radiation Durability Evaluation of Materials for
use in LEO
Material erosion data collected during flight experiments
such as the Environmental Oxygen Interaction with Materials (EOIM)-III
and the Long Duration Exposure Facility (LDEF) have raised questions
as to the sensitivity of materials erosion to levels of atomic oxygen
exposure and vacuum ultraviolet (VUV) radiation. The erosion
sensitivity
of some materials such as FEP Teflon used as a thermal control material
on satellites in low Earth orbit (LEO), is particularly important but
difficult to determine. This is in large part due to the inability to
hold all but one exposure parameter constant during a flight
experiment.
This is also difficult to perform in a ground based facility, because
often
the variation of the level of atomic oxygen or VUV radiation also
results
in a change in the level of the other parameter. A facility has been
developed
which allows each parameter to be changed almost independently and
offer
broad area exposure. The resulting samples can be made large enough for
mechanical testing. The facility uses an electron cyclotron resonance
plasma
source to provide the atomic oxygen. A series of glass plates is used
to
focus the atomic oxygen while filtering the VUV radiation from the
plasma
source. After filtering, atomic oxygen effective flux levels can still
be measured which are as high as 7x1015 atoms/cm²-sec which
is adequate for accelerated testing. VUV radiation levels after
filtering can be as low as 0.3 suns. Additional VUV suns can be added
with the use of deuterium lamps which allow the VUV level to be changed
while keeping
the flux of atomic oxygen constant. This paper discusses the facility,
and
the results from exposure of Kapton and FEP at pre-determined atomic
oxygen
flux and VUV sun levels.
Modify Surfaces with Ions and Arcs
NASA originally conducted research in the field of electron bombardment
because the technology involves generation of high-velocity ions, which
have the potential to produce much higher propellant exhaust velocities
for spacecraft than chemical propulsion. As a consequence, considerable
data were collected about the effects of ion beams on a
wide range of materials. Based on this information, researchers
designed
specialized surface modification techniques such as ion beam sputter
texturing, etching, and simultaneous deposition and etching.
Arc-texturing
technology was developed as a result of research on
high-thermal-emittance
radiators. In this process, an electric arc is formed between a carbon
or silicone-carbide electrode and a moving metal surface, resulting in
durable, microscopically rough surfaces that emit heat more efficiently
than coated materials. Atomic-oxygen texturing is a by-product of
studies
about the effects of atomic oxygen on the surfaces of spacecraft. The
purpose of the original research was to find coatings that could
withstand atomic-oxygen attack, but it evolved into deliberate
bombardment of
polymeric materials to increase thermal emittance or reduce coefficient
of friction.
Optical Property Enhancement and
Durability Evaluation of Heat and Receiver Aperture Shield Materials
Solar Dynamic (SD) power systems have been investigated by the National
Aeronautics and Space Administration (NASA) for electrical power
generation in space. As part of the International Space Station (ISS)
program, NASA Glenn Research Center (GRC) teamed with the Russian Space
Agency (RSA) to build a SD system to be flown on the Russian Space
Station MIR. Under the US/Russian SD Flight Demonstration (SDFD)
program, GRC worked with AlliedSignal Aerospace, the heat receiver
contractor, on
the development, characterization, and durability testing of materials
to obtain appropriate optical and thermal properties for the SDFD heat
receiver aperture shield. The aperture shield is composed of refractory
metal multi-foil insulation (MFI) attached to an aperture back plate.
Because of anticipated off-pointing periods, the aperture shield was
designed to withstand the extreme temperatures that 80 W/cm² would
produce. To minimize the temperature that the aperture shield will
reach during off-pointing, it was desired for the aperture shield
exterior layer to have a solar absorptance (a s) to thermal
emittance (e ) ratio as small as possible. In addition, a very low
specular reflectance (r s < 0.1) was also necessary,
because reflected concentrated sunlight could cause overheating of the
concentrator which is undesirable. Testing was conducted at GRC
to evaluate pristine and optical property enhanced molybdenum and
tungsten foils and screen covered foils. Molybdenum and tungsten foils
were grit-blasted using silicon carbide or alumina grit under various
grit-blasting conditions for optical property enhancement. Black
rhenium coated tungsten foil
was also evaluated. Tungsten, black rhenium-coated tungsten, and
grit-blasted tungsten screens of various mesh sizes were placed over
the pristine and grit-blasted foils for optical property
characterization. Grit-blasting was found to be effective in decreasing
the specular reflectance and absorptance/emittance ratio of the
refractory foils. The placement of a screen further enhanced these
optical properties, with a grit-blasted screen over a grit-blasted foil
producing the best results. Based on the optical property enhancement
results, samples were tested for atomic oxygen (AO) and vacuum heat
treatment (VHT) durability. Grit-blasted (Al2O3
grit) 2 mil tungsten foil was chosen for the exterior layer of the SDFD
heat receiver shield. A 0.007 in. diameter, 20x20 mesh tungsten screen
was chosen to
cover the tungsten foil. Based on these test results, a heat receiver
aperture shield test unit has been built by Aerospace Design and
Development (A.D.D.) with the screen covered grit-blast tungsten foil
exterior layers. The aperture shield was tested in GRC's Solar Dynamic
Ground Test Demonstration (SDGTD) system to verify the thermal and
structural durability of the outer foil layers during an off-pointing
period.
Advances in Optical Property Measurements
of Spacecraft Materials
This report describes some of the instruments and experimental
approaches available for measuring optical properties of thermal
control materials. It also describes the instruments' uses in
laboratory studies of the effects of combined contaminants and the
space environment on these materials, and in the qualification of
hardware for spacecraft. In recent years, several instruments for
measurement of solar absorptance (a ) and infrared emittance (e ) have
been introduced. These instruments offer improved speed, accuracy, and
data-handling, all of which substantially improve the study of
contaminated thermal control materials. A transient method for directly
measuring material e is also described, and the results are compared
with other instruments. In addition, our understanding of oxygen
exposure effects on the ( of materials following contamination or
exposure to simulated space conditions shows that oxygen exposure
before measuring of e should be avoided.
Indium Tin Oxide-Magnesium Fluoride
Co-Deposited Films for Spacecraft Applications
Highly transparent coatings with a maximum sheet resistivity between
108 and 109 ohms/square are desired to prevent charging of solar arrays
for low Earth polar orbit and geosynchronous orbit missions. Indium tin
oxide (ITO) and magnesium fluoride (MgF2) were ion beam
sputter co-deposited onto fused silica substrates and were evaluated
for transmittance, sheet resistivity and the effects of simulated space
environments including atomic oxygen (AO) and vacuum ultraviolet (VUV)
radiation. Optical properties and sheet resistivity as a function of MgF2
content in the films will be presented. Films containing 8.4 wt.% MgF2
were found to be highly transparent and provided sheet resistivity in
the required range. These films maintained a high transmittance upon
exposure to AO and to VUV radiation, although exposure to AO in the
presence of charged species and intense electromagnetic radiation cause
significant degradation in film transmittance. Sheet resistivity of the
as-fabricated films increased with time in ambient conditions. Vacuum
heat treatment following
film deposition caused a reduction in sheet resistivity. However,
following
heat resistivity values remained stable during storage in ambient
conditions.
Performance and Properties of
Atomic
Oxygen Protective Coatings for Polymeric Materials
Polymeric materials intended for use on spacecraft surfaces in
low Earth orbit need protective coatings to prevent oxidation resulting
from reaction with environmental atomic oxygen. The effectiveness
of
atomic oxygen protective coatings relies upon the inherent atomic
oxygen
durability of the coating itself, and the number and area of scratch
and pin window defects in the protective coating. Highly
effective coatings
result in protected polymer oxidation mass losses which are a very
small
fraction of that of unprotected materials. Such coatings are
required
for high atomic oxygen fluence missions such as Space Station Freedom.
Typically, SiOx (where 1.9 < X < 2.0) coatings have been shown to
be effective atomic oxygen protection. This paper will present
the results
of various protective and/or electrically conductive coatings,
including
germanium, SiOx, and indium tin oxide, which have been exposed to
atomic
oxygen in RF plasma ashers and compares the results with
state-of-the-art
SiOx coatings. Resulting protected polymeric material mass loss,
electrical
conductivity, and optical properties dependence upon atomic oxygen
exposure
are presented.
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