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Solar Effects on Tensile and Optical
Properties
of Hubble Space Telescope Silver-Teflon Insulation
A section of the retrieved Hubble Space Telescope (HST) solar array
drive
arm (SADA) multilayer insulation (MLI), which experienced 8.25 years of
space
exposure, was analyzed for environmental durability of the top layer of
silver-Teflon
fluorinated ethylene propylene (Ag-FEP). Because the SADA MLI had
solar
and anti-solar facing surfaces and was exposed to the space environment
for
a long duration, it provided a unique opportunity to study solar
effects
on the environmental degradation of Ag-FEP, a commonly used spacecraft
thermal
control material. Data obtained included tensile properties,
solar absorptance,
surface morphology and chemistry. The solar facing surface was
found to
be extremely embrittled and contained numerous through-thickness
cracks.
Tensile testing indicated that the solar facing surface lost 60% of its
mechanical
strength and 90% of its elasticity while the anti-solar facing surface
had
ductility similar to pristine FEP. The solar absorptance of both
the solar
facing surface (0.155 0.032) and the anti-solar facing surface (0.208
0.012) were found to be greater than pristine Ag-FEP (0.074).
Solar facing
and anti-solar facing surfaces were microscopically textured, and
locations
of isolated contamination were present on the anti-solar surface
resulting
in increased localized texturing. Yet, the overall texture was
significantly
more pronounced on the solar facing surface indicating a synergistic
effect
of combined solar exposure and increased heating with atomic oxygen
erosion.
The results indicate a very strong dependence of degradation,
particularly
embrittlement, upon solar exposure with orbital thermal cycling having
a
significant effect.
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Effects of
Vacuum Ultraviolet Radiation of Various Wavelength Ranges on Teflon FEP
Film
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.
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 ˇC on-orbit, which is
significantly
higher than the nominal MLI temperature extreme of 50 ˇC. 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 ˇC, 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
ˇC at 25ˇ 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 ˇC. 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.
Mechanical Properties of Teflon® FEP
Retrieved From the Hubble Space Telescope
Teflon® FEP (fluorinated ethylene propylene) surfaces on the Hubble
Space Telescope (HST) have experienced significant degradation in
mechanical properties during nearly ten years of exposure in the low
Earth orbit (LEO) environment. This paper describes results of
mechanical properties testing of Teflon® FEP materials exposed on HST
for 9.7 years between launch and the third servicing mission (SM3A) and
for 2.8 years between the second servicing
mission (SM2) and SM3A. Results of tensile testing, bend testing and
microscopic
examination of crack morphology are described. Effects of
post-retrieval
heating and air vs. vacuum storage on the mechanical properties of FEP
surfaces
are described as they significantly affect interpretation of results
regarding
the durability of FEP on HST. This paper provides comparisons of the
properties
of FEP surfaces retrieved during SM3A to previously reported results
for
FEP materials retrieved during the first servicing mission (SM1) and
SM2.
Environmental exposure conditions for the HST exposed materials are
also
described.
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Hubble Space Telescope Third Servicing
Mission Retrieved Metallized Teflon FEP Analysis
Following the third servicing mission (SM3A in December ’99) to
the Hubble Space Telescope, analysis was performed on the two returned
panels of multilayer insulation (MLI) as well as two patches. The MLI
panels had been in space since the telescope was launched in April ’90
(9.7 years), while the patches were installed during the second
servicing
mission in February ’97 (2.8 years). This paper provides an overview of
the tests performed on the returned metallized Teflon FEP along with a
summary of results. Testing including determination of mechanical and
optical
properties, crystallinity and fractography. Because of the amount of
material
retrieved and the nominal environmental exposures of the retrieved
materials,
these analyses resulted in a fairly complete understanding of the
degradation
process affecting the materials on the telescope. Test results from
SM3A
materials showed significantly better mechanical strength than second
servicing
mission (SM2) samples.
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.
Thermal Contributions to the Degradation
of Teflon® FEP on the Hubble Space Telescope
Metallized Teflon® fluorinated ethylene propylene (FEP) thermal
control material on the Hubble Space Telescope (HST) is degrading in
the
space environment. Teflon® FEP insulation was retrieved during
servicing
missions, which occurred in 1993, 1997 and 1999. During the second
servicing
mission (SM2), the 5 mil aluminized-FEP (Al-FEP) outer layer of
multilayer
insulation (MLI) covering the telescope was found to be cracked in many
locations around the telescope. Teflon® FEP retrieved during SM2 was
more
embrittled than FEP retrieved 2.8 years later from a different
location,
during the third servicing mission (SM3A). Studies have been conducted
to understand the degradation of FEP on HST, and the difference in the
degree
of degradation of FEP from each of the servicing missions. The
retrieved
SM2 material experienced a higher temperature extreme during thermal
cycling
(200°C) than first servicing mission (SM1) and SM3A materials (upper
temperature
of 50°C), therefore an investigation on the effects of heating FEP was
also
conducted. Samples of pristine FEP and SM1, SM2, and SM3A retrieved FEP
were heated to 200°C and evaluated for changes in properties. Heating
at
130°C was also conducted because FEP bi-stem thermal shields are
expected
to cycle to a maximum temperature of 130°C on-orbit. Tensile, density,
x-ray
diffraction (XRD) crystallinity and differential scanning calorimetry
(DSC)
data were evaluated. It was found that heating pristine FEP caused an
increase
in the density and practically no change in tensile properties.
However,
when as-retrieved space samples were heated, the density increased and
tensile
properties decreased. Upon heating, all samples experienced an increase
in crystallinity, with larger increases in the space exposed FEP. These
results indicate that irradiation of FEP in space causes chain
scission,
resulting in embrittlement, and that excessive heating allows increased
mobility of space-environment-induced scissioned chains. Thermal
exposure
was therefore found to have a major impact on the extent of
embrittlement
of FEP on HST.
Quantitative Study of Degradation of PTFE
and FEP Films by Soft X-Rays
The deterioration of mechanical properties experienced by the thermal
control blankets on the Hubble Space Telescope may be due, at least in
part, to the soft x-ray component of the space environment. We have
performed a quantitative study of the degradation of the mechanical
properties of both Teflon PTFE and FEP films exposed to soft x-rays in
high vacuum.
A quantitative x-ray intensity spectrum of the radiation at the
specimen
surface (in photons/cm2-sec-eV) was calculated with the NIST
Desk Top Spectrum Analyzer and experimentally verified in-situ with a
silicon
photodiode radiation detector. Most of the exposures were performed
with
a tube potential of 15.3 kV and with the radiation filtered through a
Be
plate to eliminate the softer x-rays that would be absorbed by the
front
surface of the specimen. Knowledge of the intensity spectrum allowed a
calculation
of the distribution of energy deposition within the specimen film. This
distribution was reasonably uniform throughout the film for these
conditions.
The quantitative measure of radiation dose most closely correlated with
the degradation of mechanical properties was found to be the energy
absorbed
per unit area – in kJ/m2 – throughout the film’s thickness
(areal
dose). The strength of the films was assessed by a hydrostatic burst
test
devised in our laboratory .It was found that the strength of the 75 µm
thick
PTFE films fell by 25% at an areal dose of 10 kJ/m2. This
corresponded
to an interior dose 58.3 kGy. The strength continued to decrease with
exposure
until it reached about 10% of its initial value at areal doses of 350
kJ/m2
In contrast, the 50 µm FEP films were much more robust than the PTFE
films,
retaining much of their original strength up to areal doses of 50
kJ/m2.
Environmental Exposure Conditions for
Teflon® FEP on the Hubble Space Telescope
The outer layer of Teflon® fluorinated ethylene propylene
(FEP) multi-layer insulation (MLI) on the Hubble Space Telescope (HST)
was observed to be significantly cracked at the time of the Second HST
Servicing Mission (SM2), 6.8 years after HST was launched into low
Earth orbit (LEO). Comparatively minor embrittlement and cracking were
also observed in FEP materials retrieved from solar-facing surfaces on
HST at the time of the First Servicing Mission (3.6 years exposure).
After SM2, a Failure Review Board was convened to address the problem
of degradation of MLI on HST.
In order for this board to determine possible degradation mechanisms,
it
was necessary to consider all environmental constituents to which the
FEP
MLI surfaces were exposed. Based on measurements and various models,
environmental exposure conditions for FEP surfaces on HST were
estimated including; number and temperature ranges of thermal cycles;
equivalent sun hours; fluence
and absorbed radiation dose of x-rays, trapped protons, and plasma
electrons and protons; and atomic oxygen (AO) fluence. This paper
presents the environmental exposure conditions for FEP on the Hubble
Space Telescope, briefly describing the possible roles of the
environmental factors in the observed FEP embrittlement and providing
references to the published works which describe in detail testing and
analysis related to FEP degradation on HST.
Insights into the Damage Mechanism of
Teflon® FEP from the Hubble Space Telescope
Metallized Teflon® FEP (fluorinated ethylene propylene) thermal
control material on the Hubble Space Telescope (HST) has been found to
be degrading in the space environment. Teflon® FEP thermal control
blankets
(space-facing FEP) retrieved during the first servicing 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 FEP and FEP retrieved from the 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 by
NMR
provided results that were consistent with the density results.
Differential
scanning calorimetry (DSC) was conducted on pristine, SM1 and SM2 FEP.
DSC
results provided evidence of chain scission and increased crystallinity
in
the space exposed FEP, which supported the density and NMR results.
Samples exposed to simulated solar flare x-rays, thermal cycling and
long-term thermal exposure provided information on environmental
contributions to degradation. These findings have provided insight into
the damage mechanisms of FEP in the space environment.
Environmental Exposure Conditions for
Teflon® FEP on the Hubble Space Telescope
The outer layer of Teflon® fluorinated ethylene propylene (FEP)
multi-layer insulation (MLI) on the Hubble Space Telescope (HST) was
observed to be significantly cracked at the time of the Second HST
Servicing
Mission (SM2), 6.8 years after HST was launched into low Earth orbit
(LEO).
Comparatively minor embrittlement and cracking were also observed in
FEP
materials retrieved from solar-facing surfaces on HST at the time of
the
First Servicing Mission (3.6 years exposure). After SM2, a Failure
Review
Board was convened to address the problem of degradation of MLI on HST.
In order for this board to determine possible degradation mechanisms,
it
was necessary to consider all environmental constituents to which the
FEP
MLI surfaces were exposed. Based on measurements and various models,
environmental
exposure conditions for FEP surfaces on HST were estimated including;
number
and temperature ranges of thermal cycles; equivalent sun hours; fluence
and absorbed radiation dose of x-rays, trapped protons, and plasma
electrons
and protons; and atomic oxygen (AO) fluence. This paper presents the
environmental
exposure conditions for FEP on the Hubble Space Telescope, briefly
describing
the possible roles of the environmental factors in the observed FEP
embrittlement
and providing references to the published works which describe in
detail
testing and analysis related to FEP degradation on HST.
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.
Analysis of Retrieved Hubble Space
Telescope Thermal Control Materials
The mechanical and optical properties of the thermal control materials
on the Hubble Space Telescope (HST) have degraded over the nearly 7
years the telescope has been in orbit. Astronaut observations and
photographs from the second servicing mission (SM2) revealed large
cracks in the metallized Teflon® FEP, the outer layer of the
mulit-layer insulation
(MLI), in many locations around the telescope. Also, the emissivity of
the bonded metallized Teflon® FEP radiator surfaces of the
telescope has increased over time. Samples of the top layer of the MLI
and radiator material were retrieved during SM2, and a thorough
investigation
into the degradation following in order to determine the primary cause
of damage. Mapping of the cracks on HST and the ground testing showed
that
thermal cycling with deep-layer damage from electron and proton
radiation
are necessary to cause the observes embrittlement. Further, strong
evidence
was found indicating that chain scission (reduced molecule weight) is
the
dominant form of damage to the metallized Teflon® FEP.
Evaluation and Selection 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 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.
Mechanical Properties Degradation of
Teflon® FEP Returned form the Hubble Space Telescope
After 6.8 years in orbit, degradation has been observed in the
mechanical properties of second-surface metallized Teflon®
FEP (fluorinated ethylene propylene) used on the Hubble Space telescope
(HST) on the outer surface of the multi-layer insulation (MLI) blankets
and on radiator
surfaces. Cracking of FEP surfaces on HST was first observed upon close
examination of samples with high solar exposure retrieved during the
first servicing mission (SM1) conducted 3.6 years after HST was put
into
orbit. Astronaut observations and photographs from the second servicing
mission (SM2), conducted after 6.8 years on orbit, revealed severe
cracks
in the FEP surfaces of the MLI on many locations around the telescope.
This paper describes results of mechanical properties testing of FEP
surfaces
exposed for 3.6 and 6.8 years to the space environment on HST. These
tests
include bend testing, tensile testing, and surface micro-hardness
testing.
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.
Investigation of Teflon FEP
Embrittlement
on Spacecraft in Low Earth Orbit
Teflon FEP (fluorinated ethylene-propylene) is commonly used on
exterior spacecraft surfaces in the low Earth orbit (LEO) environment
for thermal control. Silverized or aluminized FEP is used for the outer
layer of thermal control blankets because of its low solar absorptance
and high thermal emittance. FEP is also preferred over other spacecraft
polymers because of its relatively high resistance to atomic oxygen
erosion.
Because of its low atomic oxygen erosion yield, FEP has not been
protected
in the space environment. Recent, long term space exposures such as on
the Long Duration Exposure Facility (LDEF, 5.8 years in space), and the
Hubble Space Telescope (HST, after 3.6 years in space) have provided
evidence
of LEO environmental degradation because of long durations and the
different
conditions (such as differences in altitude) of the exposures. Samples
of
FEP from LDEF and from HST (retrieved during its first servicing
mission)
have been evaluated for solar induced embrittlement and for synergistic
effects of solar degradation and atomic oxygen. Micro-indenter results
indicate
that the surface hardness increased as the ratio of atomic oxygen
fluence
to solar fluence decreased for the LDEF samples, but the solar
exposures
were higher. Cracks induced during bend testing were significantly
deeper
for the HST samples with the higher solar exposure than for LDEF
samples with similar oxygen fluence to solar fluence ratios. If solar
fluences are compared, the LDEF samples appear as damaged as the HST
samples, except
that HST had deeper induced cracks. The results illustrate difficulties
in comparing LEO exposed materials from different missions. Because the
HST FEP appears more damaged than the LDEF FEP based on the depth of
embrittlement,
other causes for FEP embrittlement in addition to the atomic oxygen and
ultraviolet (UV) radiation, such as thermal effects and the possible
role
of soft x-ray radiation, need to be considered. FEP that was exposed to
soft x-rays in a ground test facility, showed embrittlement similar to
that
witnessed in LEO, which indicates that the observed differences between
LDEF
and HST FEP might be attributed to the different soft x-ray fluences
during
these two missions.
Degradation of FEP Thermal Control
Materials Returned from the Hubble Space Telescope
After an initial 3.6 years of space flight, the Hubble Space Telescope
(HST) was serviced through a joint effort with the National Aeronautics
and Space Administration (NASA) and the European Space Agency (ESA).
Multi-layer insulation (MLI) was retrieved from the electronics boxes
of the two magnetic sensing systems (MSS), also called the
magnetometers, and from the returned solar array (SA-I) drive arm
assembly. The top layer of each MLI assembly is fluorinated ethylene
propylene (FEP, a type of Teflon). Dramatic changes in material
properties were observed when comparing areas of high solar fluence to
areas of low solar fluence. Cross sectional analysis shows atomic
oxygen (AO) erosion values of up to 25.4m m (1 mil). Greater
occurrences of through-thickness cracking and surface microscopy were
observed in areas of high solar exposure. Atomic force microscopy (AFM)
showed increases in surface microhardness measurements with increasing
solar exposure. Decreases in FEP tensile strength and elongation were
measured when compared to non-flight material. Erosion yield and
tensile results are compared with FEP data from the Long Duration
Exposure Facility (LDEF). AO erosion yield data, solar fluence values,
contamination, micrometeoroid or debris (MMD) impact sites, and optical
properties are presented.
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.
Hubble Space Telescope Metalized Teflon®
FEP Thermal Control Materials: On-Orbit Degradation and Post-Retrieval
Analysis
During the Hubble Space Telescope (HST) second servicing mission (SM2),
degradation of unsupported Teflon® FEP (fluorinated ethylene
propylene), used as the outer layer of the multi-layer insulation (MLI)
blankets, was evident as large cracks on the telescope light shield. A
sample
of the degraded outer layer was retrieved during the mission and
returned
to Earth for ground testing and evaluation. The results of the Teflon®
FEP sample evaluation and additional testing of pristine Teflon®
FEP led the investigative team to theorize that the HST damage was
caused by thermal cycling with deep-layer damage from electron and
proton radiation which allowed the propagation of cracks along stress
concentrations, and
that the damage increased with the combined total dose of electrons,
protons,
ultraviolet and x-ray radiation along with thermal cycling. This paper
discusses
the testing and evaluation of the retrieved Teflon® FEP.
Effects of Radiation and Thermal Cycling
on Teflon® FEP
Surfaces of the aluminized Teflon® FEP (fluorinated
ethylene propylene) multilayer thermal insulation on the Hubble Space
Telescope (HST) were found to be cracked and curled in some areas at
the time of
the second servicing mission (SM2) in February 1997, 6.8 years after
HST
was deployed into low Earth orbit (LEO). In an effort to understand
what
elements of the space environment might cause such damage, pristine
second-surface aluminized Teflon® FEP was tested for
durability to various
types of radiation, to thermal cycling and to radiation followed by
thermal
cycling. Types of radiation included synchrotron vacuum ultraviolet and
soft x-ray radiation, electrons and protons. Thermal cycling was
conducted
in various temperature ranges to simulate HST orbital conditions for
Teflon® FEP. Results of tensile testing of the exposed
specimens showed that exposure to high fluences of radiation caused
degradation in tensile properties
of FEP. However, exposure to radiation alone in exposures comparable to
those experienced by HST did not produce reduction in ultimate tensile
strength and elongation of Teflon® similar to that observed
for HST-retrieved aluminized Teflon®. Synergism of radiation
exposure and thermal cycling was evident in the results of three
experiments: thermal cycling following electron and proton irradiation,
thermal cycling following x-ray exposure, and additional thermal
cycling of a sample retrieved from HST. However, irradiation and
thermal cycling with comparable HST SM2 exposure conditions did not
produce the degradation observed in the FEP material
retrieved during HST SM2.
Thermal Cycling-Caused Degradation of
Hubble Space Telescope Aluminized FEP Thermal Insulation
The Hubble Space Telescope (HST) was launched in April of 1990 and was
visited during service missions in December of 1993 and February of
1997. During the latter servicing mission, astronauts observed that the
top layer of multi-layer insulation, which consisted of second surface
aluminized FEP Teflon®, has occasional tears in its 0.127 mm
thick outer layer. A sample was retrieved which had torn and rolled up
under
its own stress such that the aluminized layer was on the exposed
surface.
The sample was found to have an increase in solar absorptance and has
multiple
cracks in the aluminization in a mud-tile configuration. Tests
conducted
in a ground laboratory high-rate thermal cycling system indicate that a
signification portion of the observed increase in solar absorptance may
have been caused by cracks in the fatigued aluminum as a result of
approximately
40.000 thermal cycles it received in space.
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
Ĺ 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. |
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