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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)
The Development of Surface Roughness and
Implications for Cellular Attachment in Biomedical Applications
The application of a microscopic surface texture produced by ion beam
sputter texturing to the surfaces of polymer implants has been shown to
result
in significant increases in cellular attachment compared to smooth
surface
implants in animal studies. A collaborative program between NASA Glenn
Research
Center and the Cleveland Clinic Foundation has been established to
evaluate
the potential for improving osteoblast attachment to surfaces that have
been
microscopically roughened by atomic oxygen texturing. The range of
surface
textures that is feasible depends upon both the texturing process and
the
duration of treatment. To determine whether surface texture saturates
or
continues to increase with treatment duration, an effort was conducted
to
examine the development of surface textures produced by various
physical
and chemical erosion processes. Both experimental tests and
computational modeling were performed to explore the growth of surface
texture with treatment time. Surface texturing by means of abrasive
grit blasting of glass, stainless steel and polymethylmethacrylate
surfaces was examined to measure the growth in roughness with grit
blasting duration by surface profilometry measurements. Laboratory
tests and computational modeling was also conducted to examine the
development of texture on Aclar® (chlorotrifluoroethylene) and Kapton®
polyimide, respectively. For the atomic oxygen texturing tests of
Aclar®, atomic force microscopy was used to measure the development of
texture with atomic oxygen fluence. The results of all the testing and
computational
modeling support the premise that development of surface roughness
obeys
Poisson statistics. The results indicate that surface roughness does
not
saturate but increases as the square root of the treatment time.
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.
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.
Modify Surfaces with Ions and Arcs
NASA originally conducted research in the field of electron bombardment
because the technology involves generation of high-velocity ions, which
have the potential to produce much higher propellant exhaust velocities
for spacecraft than chemical propulsion. As a consequence, considerable
data were collected about the effects of ion beams on a wide range of
materials. Based on this information, researchers designed specialized
surface modification techniques such as ion beam sputter texturing,
etching, and simultaneous deposition
and etching. Arc-texturing technology was developed as a result of
research
on high-thermal-emittance radiators. In this process, an electric arc
is
formed between a carbon or silicone-carbide electrode and a moving
metal
surface, resulting in durable, microscopically rough surfaces that emit
heat
more efficiently than coated materials. Atomic-oxygen texturing is a
by-product of studies about the effects of atomic oxygen on the
surfaces of spacecraft. The purpose of the original research was to
find coatings that could withstand atomic-oxygen attack, but it evolved
into deliberate bombardment of polymeric materials to increase thermal
emittance or reduce coefficient of friction.
Electric Arc and Electrochemical Surface
Texturing Technologies
Surface texturing of conductive materials can readily be accomplished
by means of a moving electric arc which produces a plasma from the
environmental gases as well as form the vaporized substrate and arc
electrode materials. As the arc is forced to move across the substrate
surface, a condensate
from the plasma redeposits an extremely rough surface which is
intimately
mixed and attached to the substrate material. The arc textured surfaces
produce greatly enhanced thermal emittance and hold potential for use
as high temperature radiator surfaces in space, as well as in systems
which use radiative heat dissipation such as computer assisted
tomography (CAT) scan systems. Electrochemical texturing of titanium
alloys can be accomplished by using sodium chloride solutions along
with ultrasonic agitation to produce a random distribution of craters
on the surface. The crater size and density can be controlled to
produce surface craters appropriately sized for direct bone in-growth
of
orthopedic implants. Electric arc texturing and electrochemical
texturing techniques, surface properties, and potential applications
will be presented.
Optical Property Enhancement and
Durability Evaluation
of Heat and Receiver Aperture Shield Materials
Solar Dynamic (SD) power systems have been investigated by the National
Aeronautics and Space Administration (NASA) for electrical power
generation in space. As part of the International Space Station (ISS)
program, NASA Glenn Research Center (GRC) teamed with the Russian Space
Agency (RSA) to build a SD system to be flown on the Russian Space
Station MIR. Under the US/Russian SD Flight Demonstration (SDFD)
program, GRC worked with AlliedSignal Aerospace, the heat receiver
contractor, on the development, characterization, and durability
testing of materials to obtain appropriate optical and thermal
properties for the SDFD heat receiver aperture shield. The aperture
shield is composed of refractory metal multi-foil insulation (MFI)
attached to an
aperture back plate. Because of anticipated off-pointing periods, the
aperture
shield was designed to withstand the extreme temperatures that 80 W/cm²
would
produce. To minimize the temperature that the aperture shield will
reach
during off-pointing, it was desired for the aperture shield exterior
layer
to have a solar absorptance (a s) to thermal emittance (e )
ratio
as small as possible. In addition, a very low specular reflectance (r s
< 0.1) was also necessary, because reflected concentrated sunlight
could cause overheating of the concentrator which is undesirable.
Testing was
conducted at GRC to evaluate pristine and optical property enhanced
molybdenum
and tungsten foils and screen covered foils. Molybdenum and tungsten
foils
were grit-blasted using silicon carbide or alumina grit under various
grit-blasting conditions for optical property enhancement. Black
rhenium coated tungsten foil was also evaluated. Tungsten, black
rhenium-coated tungsten, and grit-blasted tungsten screens of various
mesh sizes were placed over the pristine and
grit-blasted foils for optical property characterization. Grit-blasting
was
found to be effective in decreasing the specular reflectance and
absorptance/emittance ratio of the refractory foils. The placement of a
screen further enhanced these optical properties, with a grit-blasted
screen over a grit-blasted
foil producing the best results. Based on the optical property
enhancement
results, samples were tested for atomic oxygen (AO) and vacuum heat
treatment
(VHT) durability. Grit-blasted (Al2O3 grit) 2 mil
tungsten foil was chosen for the exterior layer of the SDFD heat
receiver shield.
A 0.007 in. diameter, 20x20 mesh tungsten screen was chosen to cover
the
tungsten foil. Based on these test results, a heat receiver aperture
shield
test unit has been built by Aerospace Design and Development (A.D.D.)
with
the screen covered grit-blast tungsten foil exterior layers. The
aperture shield was tested in GRC's Solar Dynamic Ground Test
Demonstration (SDGTD) system to verify the thermal and structural
durability of the outer foil
layers during an off-pointing period.
Effects of Ambient High Temperature
Exposure on Alumina-Titania High Emittance Surfaces for Solar Dynamic
Systems
Solar dynamic (SD) space power systems require durable, high emittance
surfaces on a number of critical components, such as heat receiver
interior surfaces and parasitic load radiator (PLR) elements. To
enhance surface
characteristics, an alumina-titania coating has been applied to 500
heat
receiver thermal energy containment canisters and the PLR of NASA Glenn
Research
Center's (GRC) 2kW SD ground test demonstrator (GTD). The
alumina-titania
coating was chosen because it had been found to maintain its high
emittance
under vacuum at high temperatures for an extended period. However,
preflight
verification of SD system components, such as the PLR, require
operation
at ambient pressure and high temperatures. Therefore, the purpose of
this
research was to evaluate the durability of he alumina-titania coating
at
high temperature in air. Fifteen of sixteen alumina-titania coated
Incoloy
samples were exposed to high temperatures for various durations (2 to
32
hours). Samples were characterized prior to, and after, heat treatment
for
reflectance, solar absorptance, room temperature emittance, and
emittance
at 1200° F. Samples were also examined to detect physical defects and
to
determine surface chemistry using optical microscopy, scanning electron
microscopy,
operated with an energy dispersive spectroscopy (EDS) system, and x-ray
photoelectron
spectroscopy (XPS). Visual examination of the heat-treated samples
showed
a whitening of samples exposed to temperatures of 1000° F and above.
Correspondingly,
the optical properties of these samples degraded. A sample exposed to
1500°
F for 24 hours had whitened and the thermal emittance at 1200° F had
decreased
from the non-heat treated value of 0.94 to 0.62. The coating on this
sample
had become embrittled, with spalling off the substrate noticeable at
several
locations. Based on this research it is recommended that preflight
testing
of SD components with alumina-titania coatings be restricted to
temperatures
no greater than 600° F in air to avoid optical degradation. Moreover,
components
with the alumina-titania coating are likely to experience optical
property
degradation with direct atomic oxygen exposure in space. |
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