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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.
![[horizontal rule]](file:///W:/epbranch/images/rule.gif)
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.
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 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 texture-height to
erosion-depth
ratio was very low because of greater opportunities for crack
initiation.
In an effort to try to understand how material composition affects the
cone
height to erosion depth, an experimental investigation was conducted on
18
different materials exposed to a hyperthermal energy directed atomic
oxygen
source (~90eV). The materials were first salt-sprayed to provide
microscopic
local areas that would be protected from atomic oxygen to allow 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
which
was operated on pure atomic oxygen. Samples were exposed to an
atomic oxygen
fluence of 1.0E+20 atoms/cm2. The average erosion depth and
average cone
height was determined using field emission scanning electron microscopy
(FESEM).
The experimental ratio of average cone height to erosion depth will be
compared
to polymer composition and other properties.
Effects of Vacuum Ultraviolet
Radiation on Dow Corning (DC) 93-500 Silicone
Vacuum ultraviolet radiation is among the space
environment elements that can be hazardous to DC93-500 silicone film,
which has been
proposed for use on spacecraft exterior surfaces. Investigations
have been
conducted to examine vacuum ultraviolet effects on DC93-500 film.
Laboratory
exposure tests were used to determine the effectiveness of various
wavelength
ranges in causing optical and mechanical degradation and to determine
intensity-dependence of optical and mechanical properties
degradation. Results indicated that wavelengths between 185 nm
and 200 nm were significantly more effective
in causing degradation than wavelengths between 140 nm and 185
nm. These
findings were consistent with results of vacuum ultraviolet
ellipsometric
optical measurements which provided data on depth of penetration in
DC93-500
as a function of wavelength. Wavelengths between 185 and 200 nm
penetrate
to depths between 1 m and 3 m in DC93-500, depths where bulk
degradation
is likely, whereas the penetration of shorter wavelengths is much more
shallow
and more likely to result only in surface degradation. Results of
exposures
of DC93-500 film samples to vacuum ultraviolet of intensities between
1.5
and 5.5 times the sun’s intensity indicated no intensity-dependence of
optical
and mechanical property degradation.
Issues and Advancements in
Space
Durable Multi-Functional Thermal Control Coatings
Passive spacecraft thermal control coatings are required to possess
properties of low solar absorptance, high thermal emittance, and
stability to survive the space environment for the mission
duration. The white paint coatings Z-93, YB-71 and S13G/LO,
originally developed in the 1960s, have been successfully used for
satellite thermal control and have served as standards for spacecraft
white thermal control paints. Since their original development,
these coatings have gone through re-formulations as original raw
materials became unavailable; however, their replacement products
continue to serve as standards for spaceflight thermal control.
Unique conditions of space exploration and space science missions have
required that additional functionalities be incorporated into
spacecraft thermal control coatings. Coating development efforts
have addressed needs for long-life stability, surface conductivity, and
the ability to
clean coating surfaces. Advancements in development of
lightweight composite
structures for spacecraft have led to the need for thermal control
coatings
that are adherent and compatible with these composite substrates,
whereas
the original formulations of white paints were developed for
application
to aluminum substrates. The pursuit of nuclear reactor powered
spacecraft
for future missions requires coating/substrate systems which are not
only
compatible with harsh space radiation environmental exposures, but must
also
perform at higher temperatures than have been previously
required. Future
missions to the lunar and Martian surfaces will additionally require
thermal
control coatings for which dust accumulation can be mitigated.
Although
advancements continue in the area of thermal control materials
technologies,
thermal control coatings are not currently commercially available to
meet
all of these advanced requirements. This paper presents some of
the unique
challenges for thermal control material systems for future space
missions
and some current approaches to meeting these challenges.
Effects of Vacuum Ultraviolet
Radiation of Various Wavelength Ranges on Teflon FEP Film, June 2004
This paper describes testing to investigate the
effects of vacuum ultraviolet (VUV) radiation on Teflon® fluorinated
ethylene
propylene (FEP) film, examining differences in mechanical properties
degradation for samples of 50.8 m thickness exposed to VUV of various
lower cut-off wavelengths. Samples were illuminated in a
high vacuum facility by deuterium lamps, which provide radiation in the
115-400 nm wavelength range, but
with the highest intensity being below 200 nm. Windows of
fused silica,
crystalline quartz, and magnesium fluoride provided lower cut-off
wavelengths
of 155, 140, and 115 nm, respectively. Lamp intensity was
measured in
the 115-200 nm wavelength range throughout the sample exposures. The
determined
intensities were used to estimate intensity and incident energy of
various
wavelength ranges, between 115 and 400 nm. Samples were analyzed
for tensile
strength and elongation at failure. The effects of radiation
exposures
of different wavelength ranges were compared and discussed in terms of
the
expected depth to which radiation of various wavelengths is deposited
into
FEP.
Fast Three-Dimensional Modeling of Atomic
Oxygen Undercutting of Protected Polymers, May-June 2004
Snyder, Aaron and Banks, Bruce, ““Fast Three-Dimensional Modeling
of Atomic Oxygen Undercutting of Protected Polymers”, Journal of
Spacecraft and Rockets, Vol. 41, Number 3, pp. 340-344, May-June 2004
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.
Ground-Laboratory to In-Space
Effective Atomic Oxygen Fluence Determination for DC 93-500 Silicon
The objective of this research was to determine
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 away 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 cannot be determined
based on mass loss measurements, as they are with
most polymers such as Kapton. A new 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 amount of
atomic
oxygen damage 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
was determined 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 indentations based on
plastic deformation, could not be used. Therefore, a Hysitron
Inc. TriboScope Nanomechanical Test System operated in conjunction with
a Park Scientific AutoProbe atomic force microscope (AFM) was used to
determine the surface hardness of the silicones. The Hysitron
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 2.64X higher than in LEO to replicate
equivalent
exposure damage in the ground test silicone as occurred in the space
exposed
silicone.
Vacuum Ultraviolet Radiation Effects on
Dow Corning (DC) 93-500 Silicone Film
A space-qualified silicone polymer (Dow Corning
DC93-500) has been used as a spacecraft solar cell adhesive and has
been more recently proposed for use in a Fresnel lens solar
concentrator for space power applications. Potential future
applications of DC93-500 for exterior spacecraft surfaces require an
understanding of its overall space environment durability. Vacuum
ultraviolet (VUV) radiation is among the space environment elements
that
can be hazardous to the properties of DC93-500. This paper
describes investigations into the effects of VUV radiation on DC93-500
silicone film.
Vacuum ultraviolet ellipsometric optical measurements were made on
DC93-500 silicone to determine the depth of absorption of vacuum
ultraviolet light as a function of wavelength. These data
indicate the depth within which
VUV radiation can cause material degradation. Laboratory VUV
exposures were
used to examine effects of various VUV exposure wavelength ranges and
various
VUV exposure intensities to determine whether there exist wavelength or
intensity dependencies of degradation. In one set of experiments,
transmittance degradation of DC93-500 was examined as a function of
exposure to narrow wavelength
bands (~ 20 nm bandwidth) of VUV in the 140 to 200 nm wavelength
range.
In another set of experiments, broad spectrum VUV exposures (greater
than
115 nm) were used to examine effects of VUV intensity on rates of
optical
and mechanical properties degradation. Correlations between
observed degradation
and the measured depth of VUV penetration will be discussed.
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.
Hubble Space
Telescope Degradation
Data for Ground-based Durability Projection of EPTFE on ISS
Ground-based environmental durability tests
have indicated
that exposing materials in accelerated tests to environmental model
predicted
spacecraft mission exposures of known degradation sources does not
simulate
the extent of damage that occurs in the space environment. One
approach
to overcoming the difficulties in simulating the space environment
using
ground-based testing is to calibrate the facility using data from
actual space
exposed materials to determine exposure levels required to replicate
degraded
properties observed in space. This paper describes a
ground-to-space correlation
method that uses a multiple step process to determine the durability of
expanded-polytetrafluoroethylene
(ePTFE) for International Space Station (ISS) applications based on
ground-based
x-ray irradiation and heating exposure that simulates bulk
embrittlement
as occurs in fluorinated ethylene propylene (FEP) thermal insulation
covering
the Hubble Space Telescope (HST). This method was designed to
damage the
back surface of equivalent thickness ePTFE to the same amount of
scission
damage as occurred in HST FEP (based on elongation data) and then
correct
for differences in ground test ionizing radiation versus space
radiation
effects, temperature variations, space ionizing radiation environment
variations
(spacecraft altitude, inclination and duration), and thickness
variations.
The analysis indicates that after a 10 year mission, the ISS ePTFE will
have
an extremely embrittled front surface, with surface cracks induced
under
any given strain, and a very ductile back surface. This study
also found
that a thermal induced strain of 0.1 will develop in the ePTFE, and
under
this strain condition, microscopic cracks will start developing very
early
in the mission at the exposed surface and develop to a depth of ≈ 300
μm
after 10 years.
The Effect of Heating on
the
Degradation of Ground Laboratory and Space Irradiated Teflon FEP
The outer most layer of the multilayer insulation (MLI) blankets on the
Hubble Space Telescope (HST) is back surface aluminized Teflon¨ FEP
(fluorinated ethylene propylene). As seen by data collected after
each of the three servicing missions and as observed during the second
servicing mission (SM2), the FEP has become embrittled in the space
environment, leading to degradation of the mechanical properties and
severe on-orbit cracking of the FEP. During SM2, a sample of
aluminized-FEP was retrieved from HST that had cracked and curled,
exposing its aluminum backside to space. Because of the difference in
optical properties between FEP and aluminum, this insulation piece
reached 200 ¡C on-orbit, which is significantly higher than the nominal
MLI temperature extreme of 50 ¡C. This piece was more brittle than
other retrieved material from the first and third servicing missions
(SM1 and SM3A, respectively). Due to this observation and the fact that
Teflon thermal shields on the solar array bi-stems were heated on-orbit
to 130 ¡C, experiments have been conducted to determine the effect of
heating on the degradation of FEP that has been irradiated in a ground
laboratory facility or in space on HST. Teflon FEP samples were x-ray
irradiated in a high vacuum facility in order to simulate the damage
caused by radiation in the space environment. Samples of pristine FEP,
x-ray irradiated FEP and FEP retrieved from the HST during SM3A were
heat treated from 50 to
200 ¡C at 25¡ intervals in a high vacuum facility and then tensile
tested.
In addition, samples were tested in a density gradient column to
determine
the effect of the radiation and heating on the density of FEP. Results
indicate that although heating does not degrade the tensile properties
of non-irradiated Teflon, there is a significant dependence of the
percent elongation at
failure of irradiated Teflon as a function of heating temperature.
Irradiated
Teflon was found to undergo increasing degradation in the elongation at
failure as temperature was increased from room temperature to 200 ¡C.
Rate
of degradation changes, which were consistent with the glass I
transition
temperatures for FEP, appeared to be present in both tensile and
density
data. The results indicate the significance of the on-orbit temperature
of Teflon FEP with respect to its degradation in the low Earth orbital
space environment.
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.
![[horizontal rule]](../images/rule.gif)
Radiation Durability of Candidate Polymer
Films for the Next Generation Space Telescope Sunshield
The Next Generation Space Telescope (NGST), anticipated to
be launched in 2009 for a 10-year mission, will make observations in
the infrared portion of the spectrum to examine the origins and
evolution of our universe. Because it must operate at cold temperatures
in order to make these sensitive measurements, it will use a large,
lightweight, deployable sunshield, comprised of several polymer film
layers, to block heat and stray light. This paper describes laboratory
radiation durability testing of candidate NGST sunshield polymer film
materials. Samples of fluorinated polyimides CP1 and CP2; and a
polyarylene ether benzimidazole, TOR-LMTM, were exposed to 40 keV
electron and 40 keV proton radiation followed by exposure to vacuum
ultraviolet (VUV) radiation in the 115-200 nm wavelength range. Samples
of these materials were also exposed to VUV without prior electron and
proton exposure. Samples of polyimides Kapton® HN, Kapton® E, and
Upilex-S were exposed to electrons and protons, only, due to limited
available exposure area in the VUV facility. Exposed samples were
evaluated for changes in solar absorptance and thermal emittance and
mechanical properties of ultimate tensile strength and elongation at
failure. Data obtained are compared with previously published data for
radiation durability
testing of these polymer film materials.
Simulated Space Vacuum Ultraviolet (VUV)
Exposure Testing for Polymer Films
Vacuum ultraviolet (VUV) radiation of wavelengths between 115 and 200
nm produced by the sun in the space environment can cause degradation
to polymer films producing changes in optical, mechanical, and chemical
properties. These effects are particularly important for thin polymer
films being considered for ultra-lightweight space structures, because,
for most polymers, VUV radiation is absorbed in a thin surface layer.
NASA Glenn Research Center has developed facilities and methods for
long-term ground testing of polymer films to evaluate space
environmental VUV radiation effects. VUV exposure can also be used as
part of sequential simulated space environmental exposures to determine
combined damaging effects. This paper will describe the effects of VUV
on polymer films and the necessity for ground testing. Testing
practices used at Glenn Research Center for VUV exposure testing will
be described including characterization of the VUV radiation source
used, calibration procedures traceable to the National Institute of
Standards and Technology (NIST), and testing techniques for VUV
exposure of polymer surfaces.
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.
Effect of Air and Vacuum Storage on the
Tensile Properties of X-ray Exposed Aluminized-FEP
Metallized Teflon® FEP (fluorinated ethylene propylene), a common
spacecraft thermal control material, from the exterior layer of the
Hubble Space Telescope (HST) has become embrittled and suffers from
extensive cracking. Teflon samples retrieved during Hubble servicing
missions and from the Long Duration Exposure Facility (LDEF) indicate
that there may be continued degradation in tensile properties over
time. An investigation has been conducted to evaluate the effect of air
and vacuum storage on
the mechanical properties of x-ray exposed FEP. Aluminized-FEP (Al-FEP)
tensile samples were irradiated with 15.3 kV Cu x-rays and stored in
air
or under vacuum for various time periods. Tensile data indicate that
samples
stored in air display larger decreases in tensile properties than for
samples
stored under vacuum. Air-stored samples developed a hazy appearance,
which
corresponded to a roughening of the aluminized surface. Optical
property
changes were also characterized. These findings indicate that air
exposure
plays a role in the degradation of irradiated FEP, therefore proper
sample
handling and storage is necessary with materials retrieved from space.
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.
Effects of Vacuum Ultraviolet Radiation on
Thin Polyimide Films
This paper describes the vacuum ultraviolet (VUV) radiation durability
screening testing of thin (12.7 to 25.4 µm) polyimide films proposed
for use on the Next Generation Space Telescope (NGST) sunshield.
Materials included in this screening test were Kapton®E, Kapton®HN,
Upilex®S,
CP1, CP1 with vapor deposited aluminum (VDA) on its back surface, and
CP2 with a VDA coating on its back surface. Samples were exposed to
approximately 1000 equivalent sun hours (ESH) of VUV radiation and
examined for changes in solar absorptance, thermal emittance, ultimate
tensile strength,
and elongation-to-failure. Changes in solar absorptance were observed
for some materials, and additionally, significant changes in spectral
reflectance were observed in the ultraviolet to visible wavelength
region
for all polyimide materials tested. Changes in ultimate tensile
strength
and elongation at failure were within the experimental uncertainty for
all samples. Longer exposures are needed to verify the observed trends
and to develop performance predictions for these materials on the NGST
sunshield.
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.
Steady Sate Vacuum Ultraviolet Exposure
Facility With Automated Calibration Capability
NASA Glenn Research Center at Lewis Field designed and developed a
steady state vacuum ultraviolet automated (SSVUVa) facility with
in situ VUV intensity calibration capability. The automated feature
enables a constant accelerated VUV radiation exposure over long periods
of testing without breaking vacuum. This test facility is designed to
simultaneously accommodate four isolated radiation exposure tests
within
the SSVUVa vacuum chamber. Computer-control of the facility for long
term continuous operation also provides control and recording of
thermocouple
temperatures, periodic recording of VUV lamp intensity, and monitoring
of vacuum facility status. This paper discusses the design and
capabilities
of the SSVUVa facility.
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.
Low Earth Orbit Durability of Protected
Silicone for Refractive Photovoltaic Concentrator Arrays
Photovoltaic power systems with novel refractive silicone solar
concentrators are being developed for use in low Earth orbit (LEO).
Because of the vulnerability of silicones to atomic oxygen and
ultraviolet radiation, these lenses are coated with a multi-layer metal
oxide protective coating. The objective of this work was to evaluate
the effects of atomic oxygen and thermal exposures on multi-layer
coated silicone. Samples were exposed to high-fluence ground-laboratory
and low-fluence in-space atomic oxygen. Ground testing resulted in
decreases in both total and specular transmittance, while in-space
exposure resulted in only small decreases in specular transmittance. A
contamination film, attributed to
exposed silicone at coating crack sites, was found to cause
transmittance decreases during ground testing. Propagation of coating
cracks was found to be the result of sample heating during exposure.
The potential for silicone
exposure, with the resulting degradation of optical properties from
silicone
contamination, indicates that his multi-layer coated silicone is not
durable
for LEO space applications where thermal exposures will cause coating
crack
development and propagation.
Combined Contamination and Space
Environmental Effects on Solar Cells and Thermal Control Surfaces
For spacecraft in low Earth orbit (LEO), contamination can
occur from thruster fuel, sputter contamination products, and from
products of silicone degradation. This paper describes laboratory
testing
in which solar cell materials and thermal control surfaces were exposed
to simulated spacecraft environmental effects including contamination,
atomic oxygen, ultraviolet radiation and thermal cycling. The objective
of these experiments was to determine how the interaction of the
natural
LEO environmental effects with contaminated spacecraft surfaces impacts
the performance of these materials. Optical properties of samples were
measured and solar cell performance data was obtained. In general,
exposure
to contamination by thruster fuel resulted in degradation of solar
absorptance
for fused silica and various thermal control surfaces and degradation
of
solar cell performance. Fused silica samples which were subsequently
exposed
to an atomic oxygen/vacuum ultraviolet radiation environment showed
reversal
of this degradation. These results imply that solar cells and thermal
control
surfaces which are susceptible to thruster fuel contamination and which
also receive atomic oxygen exposure may not undergo significant
performance
degradation. Materials which were exposed to only vacuum ultraviolet
radiation
subsequent to contamination showed, slight additional degradation in
solar absorptance.
The Effects of Simulated Low Earth Orbit
Environments on Spacecraft Thermal Control Coatings
Candidate Space Station Freedom radiator coatings including Z-93,
YB-71, anodized aluminum, and SiOx-coated silvered Teflon
have been characterized for optical properties degradation upon
exposure to environments containing atomic oxygen, vacuum ultraviolet
(VUV)
radiation and/or silicone contamination. YB-71 coatings showed a
blue-gray
discoloration, which has not been observed in space, upon exposure in
atomic oxygen facilities which also provide exaggerated VUV radiation.
This is evidence that damage mechanisms occur in these ground
laboratory
facilities which are different from those which occur in space.
Radiator
coatings exposed to an electron cyclotron resonance (ECR) atomic oxygen
source in the presence of silicone-containing samples showed severe
darkening form the intense VUV radiation provided by the ECR and from
silicone contamination. Samples exposed to atomic oxygen from the ECR
source and to VUV lamps, simultaneously, with in situ reflectance
measurement,
showed that significantly greater degradation occurred when samples
received line-of-site ECR beam exposure than when samples were exposed
to atomic oxygen scattered off of quartz surfaces without line-of-site
view of the ECR beam. For white paints, exposure to air following
atomic oxygen/VUV exposure reversed the darkening due to VUV damage.
This illustrates the importance of in situ reflectance measurement.
Evaluation of Low Earth Orbit
Environmental Effects
on International Space Station Thermal Control Materials
Samples of International Space Station (ISS) thermal control coatings
were exposed to simulate low Earth orbit (LEO) environmental conditions
to determine effects on optical properties. In one test,
samples of the white paint coating Z-93P were coated with outgassed
products
from Tefzel® (ethylene tetrafluoroethylene copolymer) power
cable insulation as may occur on ISS. These samples were then exposed,
along with an uncontaminated Z-93P witness sample, to vacuum
ultraviolet
(VUV) radiation to determine solar absorptance degradation. The Z-93P
samples coated with Tefzel® outgassing products experienced
greater increases in solar absorptance than witness samples not coated
with Tefzel® outgassing products. In another test, samples
of second surface silvered Teflon® FEP (fluorinated ethylene
propylene), SiOx (where x(2)-coated silvered Teflon®
FEP, and Z-93P witness samples were exposed to the combined
environments
of atomic oxygen and VUV radiation to determine optical properties
changes
due to these simulated ISS environmental effects. This test verified
the
durability of these materials in the absence of contaminants.
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.
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.
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.
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 intermittant 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 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.
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.
Simulated Solar Flare X-Ray and Thermal
Cycling Durability Evaluation of Hubble Space Telescope Thermal Control
Candidate Replacement Materials
During the Hubble Space Telescope (HST) second servicing mission (SM2),
astronauts noticed that the multi-layer insulation (MLI) covering the
telescope was damaged. Large pieces of the outer layer of MLI
(aluminized Teflon® fluorinated ethylene propylene (Al-FEP))
were cracked in several locations around the telescope. A piece of
curled up Al-FEP was retrieved by the astronauts and was found to be
severely embrittled, as witnessed by ground testing. The national
Aeronautics and Space Administration (NASA) Goddard Space Flight Center
(GSFC) organized a HST MLI Failure Review Board (FRB) to determine the
damage mechanism of the Al-FEP in the HST environment, and to recommend
a replacement thermal control outer layer to be installed on HST during
the subsequent servicing missions. Candidate thermal control
replacement materials were chosen by the FRB and tested for
environmental durability under various exposures and durations by GSFC
and NASA Glenn Research Center (GRC). This paper describes durability
testing at GRC of candidate materials which were exposed to charged
particle radiation, simulated solar flare x-ray radiation, and thermal
cycling under load. Samples were evaluated for changes in solar
absorptance and tear resistance. Descriptions of environmental
exposures and durability evaluations of these materials are presented.
Synchrotron VUV and Soft X-Ray Radiation
Effects on Aluminized Teflon® FEP
Surfaces of the aluminized Teflon® FEP multi-layer thermal
insulation on the Hubble Space Telescope were found to be cracked and
curling in some areas at the time of the second servicing mission in
February 1997, 6.8 years after HST was deployed in low Earth orbit
(LEO). As a part of a test program to assess environmental conditions
which would produce embrittlement sufficient to cause cracking of Teflon®
on HST, samples of Teflon® FEP with a backside layer of
vapor deposited aluminum were exposed to vacuum ultraviolet (VUV) and
soft x-ray radiation of various energies using facilities at the
National Synchrotron
Light Source, Brookhaven National Laboratory. Samples were analyzed for
ultimate tensile strength and elongation. Results will be compared to
those of aluminized Teflon® FEP retrieved from HST after 3.6
years and 6.8 years on orbit and will be referenced to estimated HST
mission
doses of VUV and soft x-ray radiation.
A Comparison of Space and Ground Based
Facility Environmental Effects on FEP Teflon®
Fluorinated Ethylene Propylene (FEP) Teflon® is
widely used as a thermal control material for spacecraft, however, it
is susceptible to erosion, cracking, and subsequent mechanical failure
in low Earth orbit. One of the difficulties in determining whether FEP
Teflon® will survive during a mission is the wide disparity
of erosion rates observed for this material in space and in ground
based
facilities. Each environment contains different levels of atomic
oxygen,
ions, and vacuum ultraviolet (VUV) radiation in addition to parameters
such as the energy of the arriving species and temperature. These
variations
make it difficult to determine what is causing the observed differences
in erosion rates. This paper attempts to narrow down which factors
affect
the erosion rate of FEP Teflon® through attempting to
change only one environmental constituent at a time. This was attempted
through the use of a single simulation facility (plasma asher)
environment
with a variety of Faraday cages and VUV transparent windows. Isolating
one factor inside of a radio frequency (RF) plasma proved to be very
difficult.
Two observations could be made. First, it appears that the erosion
yield
of FEP Teflon® with respect to that of polyimide Kapton is
not greatly affected by the presence or lack of VUV radiation present
in
the RF plasma and the relative erosion yield for the FEP Teflon®
may decrease with increasing fluence. Second, shielding from charged
particles appears to lower the relative erosion yield of the FEP to
approximately
that observed in space, however, it is difficult to determine for sure
whether ions, electrons, or some other components are causing the
enhanced erosion.
Evaluation and Selection of Replacement
Thermal Control Materials for the Hubble Space Telescope
The mechanical and optical properties of the metalized Teflon®
FEP thermal control materials on the Hubble Space Telescope (HST) have
degraded over the nearly 7 years the telescope has been in orbit. Given
the damage to the outer layer of the multi-layer insulation (MLI) that
was apparent during the second servicing mission (SM2), the decision
was made to replace the outer layer during subsequent servicing
missions. A Failure Review Board (FRB) was established to investigate
the damage to
the MLI and identify a replacement material. The replacement material
had
to meet the stringent thermal requirements of the spacecraft and
maintain structural integrity for at least 10 years. Ten candidate
materials were selected
and exposed to ten-year HST-equivalent doses of simulated orbital
environments.
Samples of the candidates were exposed sequentially to low and
high-energy
electrons and protons, atomic oxygen, x-ray radiation, ultraviolet
radiation,
and thermal cycling. Following the exposures, the mechanical integrity
and
optical properties of the candidates were investigated using Optical
Microscopy,
Scanning Electron Microscopy (SEM), and a Laboratory Portable
Spectroreflectometer
(LPSR). Based on the results of these simulations and analyses, the FBR
selected
a replacement material and two alternates that showed the highest
likelihood
of providing the requisite thermal properties and surviving for 10
years
in orbit.
Ground Based Testing of Replacement
Thermal Control Materials for the Hubble Space Telescope
The mechanical and optical properties of the metallized Teflon
FEP thermal control materials on the Hubble Space Telescope (HST) have
degraded over the nearly seven years the telescope has been in orbit.
Given
the damage to the outer layer of the multi-layer insulation (MLI)
blanket that was apparent during the second servicing mission (SM2),
the decision was made to replace the outer layer during subsequent
servicing missions. A Failure Review Board was established to
investigate the damage to the
MLI and identify a replacement material. The replacement material had
to
meet the stringent thermal requirements of the spacecraft and maintain
mechanical integrity for at least ten years. Ten candidate materials
were selected and exposed to ten-year HST-equivalent doses of simulated
orbital environments. Samples of the candidates were exposed
sequentially to low- and high-energy electrons and protons, atomic
oxygen, x-ray radiation, ultraviolet radiation, and thermal cycling.
Following the exposures, the mechanical integrity
and optical properties of the candidates were investigated using
optical
microscopy, scanning electron microscopy (SEM), a laboratory portable
spectroreflectometer (LPSR) and a Lambda 9 spectroreflectometer. Based
on the results of these simulations and analyses, the Failure Review
Board selected a replacement material and two alternatives that showed
the highest likelihood of providing the requisite thermal properties
and surviving for ten years in orbit.
Effects of Heating on Teflon® FEP Thermal
Control Material from the Hubble Space Telescope
Metallized Teflon® FEP (fluorinated ethylene propylene)
thermal control material on the Hubble Space Telescope (HST) is
degrading in the space environment. Teflon® FEP thermal
control blankets (space-facing FEP) retrieved during the first service
mission (SM1) were found to be embrittled on solar facing surfaces and
contained microscopic cracks. During the second servicing mission (SM2)
astronauts noticed
that the FEP outer layer of the multi-layer insulation (MLI) covering
the telescope was cracked in many locations around the telescope. Large
cracks were observed on the light shield, forward shell, and equipment
bays. A tightly curled piece of cracked FEP from the light shield was
retrieved during SM2 and was severely embrittled, as witnessed by
ground
testing. A Failure Review Board (FRB) was organized to determine the
mechanism
causing the MLI degradation. Density, x-ray crystallinity, and solid
state
nuclear magnetic resonance (NMR) analyses of FEP retrieved during SM1
were
inconsistent with results of FEP retrieved during SM2. Because the
retrieved
SM2 material curled while in space, it experienced a higher temperature
extreme during thermal cycling, estimated at 200° C, than the SM1
material,
estimated at 50° C. An investigation on the effects of heating pristine
and FEP exposed on HST was therefore conducted. Samples of pristine,
SM1,
and SM2 FEP were heated to 200° C and evaluated for changes in density
and
morphology. Elevated temperature exposure was found to have a major
impact
on the density of the retrieved materials. Characterization of polymer
morphology
of as-received and heated FEP samples by NMR provided results that were
consistent with the density results. These findings have provided
insight
to the damage mechanisms of FEP in the space environment.
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.
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.
Effect of X-Rays on the Mechanical
Properties of Aluminized FEP Teflon
Pieces of the multilayer insulation (MLI) that is integral
to the thermal control of the Hubble Space Telescope (HST) have been
returned by two servicing missions after 3.6 and 6.8 years in orbit.
They reveal that the outer layer, which is made from 5 mil (0.13 mm)
thick aluminized fluorinated ethylenepropylene (FEP) Teflon®,
has become severely embrittled. Although possible agents of
embrittlement
include electromagnetic radiation across the entire solar spectrum,
trapped particle radiation, atomic oxygen, and thermal cycling,
intensive
investigations have not yielded unambiguous causes. Previous studies
utilizing monoenergenic photons in the 69-1900 eV range did not cause
significant embrittlement, even at much higher doses than were
experienced by the
HST MLI. Neither did x-rays in the 3 to 10 keV range generated in a
modified electron beam evaporator. An antidotal aluminized FEP sample
that was
exposed to an intensive dose from unfiltered Mo x-ray radiation from a
rotating anode generator, however, did show the requisite brittlement.
Thus, a study was undertaken to determine the effects of x-ray exposure
on the embrittlement of aluminized FEP in hopes that it might elucidate
the HST MLI degradation mechanism. Tensile specimens of aluminized 5
mil
thick FEP were exposed to a constant fluence of unfiltered x-ray
radiation
from a Mo target whose maximum energy ranged from 20-60 kV. Other
samples
were annealed, thermally cycled (100x) between 77-333 K, or cycled and
irradiated. Tensile tests and density measurements were then performed
on the samples which had been irradiated had the drastically reduced
elongation-to-break, characteristic of the HST samples. Thermal cycling
may accelerate the embrittlement, but the effect was near the scatter
in the measurements. Annealing and thermal cycling had no apparent
effect. Only the samples which had been irradiated and annealed showed
significant density increases, likely implicating polymer chain
scission and annealing.
Ground Laboratory Soft X-Ray Durability
Evaluation of Aluminized Teflon® FEP Thermal Control Insulation
Metallized Teflon® fluorinated ethylene propylene (FEP)
thermal control insulation is mechanically degraded if exposed to a
sufficient fluence of soft x-ray radiation. Soft x-ray photons
(4 to 8 Å in wavelength or 1.55 to 3.2 keV) emitted during solar flares
have been proposed as a cause of mechanical properties degradation of
aluminized Teflon® FEP thermal control insulation on the
Hubble
Space Telescope (HST). Such degradation can be characterized by a
reduction
in elongation-to-failure of the Teflon® FEP. Ground
laboratory soft x-ray exposure tests of aluminized Teflon®
FEP were
conducted to assess the degree of elongation degradation, which would
occur as a result of exposure to soft x-rays in the range of 3 to 10
keV. Test results indicate that soft x-ray exposure in the 3 to 10 keV
range,
at mission fluence levels, does not alone cause the observed reduction
in
elongation of flight retrieved samples. The soft x-ray exposure
facility
design, mechanical properties degradation results, and implications
will
be presented.
Performance and Durability of High
Emittance Heat Receiver Surfaces for Solar Dynamic Power Systems
Haynes 188, a cobalt-based super-alloy, will be used to make thermal
energy storage (TES) containment canisters for a 2 kW solar
dynamic ground test demonstrator (SDGTD). Haynes 188 containment
canisters
with a high thermal emittance (e ) are desired for radiating heat away
from local hot spots, improving the heat distribution, which will in
turn
improve canister service life. In addition to needing a high emittance,
the surface needs to be durable in an elevated temperature, high vacuum
(» 830° C, <10-7 torr) environment for an extended time period.
Thirty-five
Haynes 188 samples were exposed to 14 different types of surface
modification
techniques for emittance and vacuum heat treatment (VHT) durability
enhancement.
Optical properties were obtained for the modified surfaces. Emittance
enhanced
samples were exposed to VHT for up to 2692 hours at 827° C and <10-6
torr with integral thermal cycling. Optical properties were taken
intermittently
during exposure, and after final VHT exposure. The various surface
modification treatments increased the emittance of pristine Haynes 188
from 0.11 to 0.86. Seven different surface modification techniques were
found to provide surfaces which met the SDGTD receiver VHT durability
requirement (e ³ 0.70 after
1000 hours). Of the 7 surface treatments, 2 were found to display
excellent
VHT durability: alumina-titania (AlTi) coatings (e = 0.85 after 2695
VHT
hours) and zirconia-titania-yttria coatings (e = 0.86 after 2024.3 VHT
hours).
The AlTi coating was chosen for the e enhancement surface modification
technique for the SDGTD receiver. Details of the alumina-titania
coating and other
Haynes 188 emittance surface modification techniques are discussed.
Technology
from this program will lead to successful demonstration of solar
dynamic
power for space applications, and has potential for applications in
other
systems requiring high emittance.
Optical Property Enhancement and
Durablility 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.
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.
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