Six different types of carbon and carbon-boron nitride composites were exposed to low Earth orbit aboard Space Shuttle flight STS-46. The samples received a nominal atomic oxygen fluence of 2.2 x 10(exp 20) atoms/sq cm in 42 hours of exposure. Pyrolytic graphite and highly oriented pyrolytic graphite showed significant degradation, and the measured erosion yield was within a factor of two of published values. The erosion yield of pyrolytic boron nitride was found to be 2.6 x 10(exp 26) cu cm/atom in plasma asher exposure, over 42 times lower than that of pyrolytic graphite. This low erosion yield makes graphite plus boron nitride mixtures quite resistant to low Earth orbit exposure. Evidence suggests that the graphitic component was preferentially etched, leaving the surface boron nitride rich. Degradation resistance increases with boron nitride composition. Carbon fiber/carbon composites degraded in low Earth orbit, and the carbon pitch binder was found to etch more easily than the graphite fibers which have much higher degradation resistance

Devries, M. J.

Hale, Jeffrey S.

Lake, Max

Moore, Arthur W.

Spady, Blaine R.

Synowicki, R. A.

Woollam, John A.

English

NASA Technical Reports Server

N95- 27636
LEO DEGRADATION OF GRAPHITE AND CARBON-BASED COMPOSITES ABOARD
SPACE SHUTrLE FLIGHT STS-46"
Blaine R. Spady
R.A. Synowicki
Jeffrey S. Hale
M.J. DeVries
John A. Woollam
Center for Microelectronic and Optical Materials Research, and Department of Electrical
Engineering, University of Nebraska-Lincoln
Lincoln, NE 68588-0511
Phone: 402/472-1978, Fax: 402/472-7987
Arthur W. Moore
Union Carbide Corporation, Parma Technical Center
Parma, Ohio
Max Lake
Applied Sciences, Incorporated
Cedarville, Ohio
SUMMARY
Six different types of carbon and carbon-boron nitride composites were exposed to low
Earth orbit aboard Space Shuttle flight STS-46. The samples received a nominal atomic oxygen
fluence of 2.2X102° atoms/cm 2 in 42 hours of exposure. Pyrolytic graphite and highly oriented
pyrolytic graphite showed significant degradation, and the measured erosion yield was within a
factor of two of published values. The erosion yield of pyrolytic boron nitride was found to be
2.6X 10 -26 cm3/atom in plasma asher exposure, over 42 times lower than that of pyrolytic graphite.
This low erosion yield makes graphite plus boron nitride mixtures quite resistant to low earth
orbit exposure. Evidence suggests that the graphitic component was preferentially etched, leaving
the surface boron nitride rich. Degradation resistance increases with boron nitride composition.
Carbon fiber/carbon composites degraded in low Earth orbit, and the carbon pitch binder was
found to etch more easily than the graphite fibers which have much higher degradation resistance.
*Research Supported by NASA-Lewis Research Center Grant NAG-3-95
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https://ntrs.nasa.gov/search.jsp?R=19950021215 2020-06-16T07:54:55+00:00Z
INTRODUCTION
High temperature radiators are necessary for rejection of waste heat generated aboard
orbiting spacecraft (refs. 1,2). Radiator materials must be able to operate at temperatures up to
900 K. Other materials requirements for space radiators are high thermal conductivity and surface
emittance. Furthermore, conductivity and emittance must not change with exposure to the
corrosive low Earth orbit (LEO) space environment.
Atomic oxygen (AO) is the primary reactive species present in LEO (ref. 3). Oxygen atoms
strike spacecraft surfaces with an average kinetic energy of 4.5 eV due primarily to the orbital
speed of the spacecraft. The AO attack is directional along the spacecraft's velocity vector since
the spacecraft is moving into the oxygen atoms. Ultraviolet light from the sun is also present and
synergistic degradation effects are suspected.
Ion beams have been extensively used to modify the surface morphology of many haaterials,
resulting in a "carpet-like" surface texturing (ref. 4). Surface modification of this type can
significantly alter the surface emittance of a waste heat radiator.
Bulk basal plane pyrolytic graphite, as well as vapor grown carbon fibers, shows the highest
thermal conductivity of all materials (ref. 5). Also, graphite can withstand temperatures up to
2800 K and has found extensive use in high temperature applications. These properties make
graphite an excellent choice for radiator applications. However, it has been shown in both
laboratory simulations and actual LEO exposure that graphite is readily etched by AO (ref. 6).
This paper considers the AO degradation resistance of six different types of graphite,
graphite plus boron nitride mixtures, and carbon fiber/graphite composites. Their usefulness as
space radiator materials has been evaluated through exposure to LEO aboard Space Shuttle flight
STS-46 in July 1992.
EXPERIMENT
Six different types of materials were prepared in bulk form. Two different types of graphite
were chosen, namely pyrolytic graphite (PG) and highly oriented pyrolytic graphite (HOPG) (ref.
5). Two samples consisted of carbon and boron nitride mixtures (C-BN). These two C-BN
samples consisted of one hot-pressed and one as-deposited containing 40% and 60% pyrolytic
graphite, respectively (ref. 7). It will be shown that the presence of boron nitride in the mixture
serves to greatly decrease erosion. Boron nitride is not as good a thermal conductor as graphite,
so the thermal conductivity of C-BN materials is less than that of pure graphite, but should still be
quite acceptable for space radiator applications.
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The remaining two samples were composites of vapor grown carbon fibers embedded in a
graphite (pitch) matrix. These carbon/carbon fiber (C/C) composites were made by either liquid
or gas infiltration of the graphite precursor into the fiber weave.
Sample characterization consisted of accurate measurements of mass and dimensions. Mass
was measured using an ultramicrobalance with a resolution of 10 -7 g and the physical dimensions
were measured using a vernier caliper with a resolution of 25 _tm. Dimension and mass
measurements allow for measurement of erosion yield (ref. 6).
Scanning electron microscopy (SEM) and atomic force microscopy (AFM) were employed to
measure topographic changes and quantify surface roughness. The AFM is a contact method
which gives extremely fine scale measurements of surface roughness over small areas, on the
order of 100 lam or less.
The bulk samples were cut into circular geometries to fit into the provided sample holder. In
order to isolate the effects of the AO, the back sides of the samples were used fis control
surfaces.
All samples were part of the limited duration space environment candidate materials
exposure-3 payload, aboard STS-46. The samples were exposed to the space environment
throughout the entire shuttle flight. However, the samples received direct ram exposure to AO
for only 42 hours during the 8-day mission and the oxygen fluence received during this exposure
was nominally 2.2X102° atoms/cm 2. Oxygen fluence was determined by both atmospheric
modeling using the MSIS-86 thermospheric model (ref. 8) and by a mass spectrometer flown
aboard STS-46 on the EOIM-3 payload. Kapton ® mass loss measurements (ref. 9) showed the
oxygen fluence to be 2.3X 1020 atoms/cm 2.
RESULTS AND DISCUSSION
The PG, HOPG, and C/C composites all showed visible degradation. This is shown in
Fig 1. The C-BN samples showed no visible degradation except for a slight "darkening" of the
60% C-BN sample.
SEM results for the PG, HOPG, and C/C composite made by gas infiltration showed
uniform degradation. The C/C composite made by liquid infiltration showed preferential
degradation of the graphite pitch surrounding the carbon fibers as shown in Fig 2. The 40% C-
BN sample showed no changes in the SEM photos.
The hot-pressed 40% C-BN sample is comprised of microscopic domains of pure pyrolytic
boron nitride and boronated PG; the size of the domains was found to slightly exceed 0.1 I.tm.
Evidence on the as-deposited 60% C-BN was inconclusive of whether or not the material is a
single-phase mixture of C, B, and N or a two-phase mixture of pyrolytic boron nitride domains
and boronated PG domains (ref. 7). Fig 3 shows an unexposed and exposed AFM picture of the
surface of the 60% C-BN sample. The figure appears to show the preferential etching of either
991
PG or pyrolytic boron nitride domains. A preliminary study of pyrolytic boron nitride using
oxygen plasma ashers showed the erosion yield to be 2.6X 10 .26 cm3/atom. This is over 42 times
lower than the published 1.1X 10.24 cm3/atom erosion yield of PG (ref. 6). This suggests that the
PG is being preferentially etched, leaving pyrolytic boron nitride domains. This behavior was
observed in both the as-deposited and hot-pressed materials. Table I shows the surface roughness
data for the 40% and 60% C-BN samples taken for three scan sizes with the AFM. The 1 l.tm
scan has the greatest percentage change between the unexposed and exposed samples; namely,
59% for the 40% C-BN, and 238% for the 60% C-BN samples. The percentage change for the
100 lam scan was only 29% for the 40% C-BN and 36% for the 60% C-BN. This indicates the
surface roughened on a sub micron scale, for which the small scans were more sensitive. From
this data it can be seen that the 40% C-BN sample was more stable in LEO than the 60% C-BN
sample; this is to be expected since pure pyrolytic boron nitride has a lower erosion yield than PG.
Table I. RMS Surface Roughness of 40% and 60% C-BN
SAMPLE AFM
l_m scan
9.88 nm40% C-BN
unexposed
40% C-BN
exposed
60% C-BN
unexposed
60% C-BN
exposed
15.713 nm
9.343 nm
31.586 nm
AFM
10lam scan
56.714 nm
AFM
100_tm scan
108.500 nm
70.460 nm 139.600 nm
15.619 nm 133.137 nm
37.377 nm 181.667 nm
The densities (p) of the samples were determined from the physical dimensions and mass.
The mass loss (AM) of each sample was the pre-flight mass minus the post-flight mass. The At
fluence (F) was nominally 2.2X102° atoms/cm 2. The erosion yield was calculated as:
AM
E =
oAF
where A was the exposed area of the sample. The mass loss, density, and erosion yield for all
samples are shown in Table II. The densities and erosion yields of the PG and HOPG samples
were within a factor of two of published data (ref. 6). The C/C composites and C-BN samples
were new materials in space so there were no published data for comparison. The erosion yields
for the C-BN samples were an order of magnitude lower than either the PG or the HOPG. The
gas infiltrated C/C composite erroneously lost mass due to handling, making the data unusable.
The degradation observed on pure graphite samples and the pitch in C/C composites
indicates that these materials are not stable in LEO and will require protective coatings if
deployed. The degradation resistance of C-BN mixtures is very encouraging. These materials
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show promise as radiator materials without the need for protective overcoats due to the presence
of boron nitride. The sample containing 40% carbon outperformed that containing 60% carbon.
Much more work could be done studying the effectiveness of bulk mixtures of carbon and boron
nitride, as well as graphite fiber composites containing boron nitride in the binding matrix. Fiber
composites of this type would be advantageous since the thermal conductivity would be
dominated by the graphite fibers, and atomic oxygen resistance would be enhanced by the
presence of boron nitride.
Table H. Measured Density, Mass Loss, and Calculated Erosion Yield for all Samples
Material
PG
HOPG
Density
(g/cm 3)
2.17I
2.096
Mass Loss (mg)
.62
.77
Erosion Yield (cm3/atom)
.538x10 -24
.692x10 -24
.071xlff 2440%C-BN 2.134 .08
60%C-BN 1.782 .21 .222x10 24
C-C Liquid lnfilvation 1.403 .22 1.093x10 -24
C-C Gas Infiltration 1.517 11.5398 ******
CONCLUSION
Six different carbon based materials were exposed to 42 hours of LEO aboard Space
Shuttle flight STS-46. The PG, HOPG, and C/C composites were degraded by AO exposure,
making them less desirable for space radiator applications. Measured erosion yields agreed well
with published results. Samples of C-BN showed a strong resistance to atomic oxygen erosion.
This resistance increased as the percentage of carbon decreased since the erosion yield of
pyrolytic boron nitride is lower than that of PG by a factor of 42. In these C-BN mixtures, atomic
oxygen preferentially etches the graphitic component of the material at the surface, leaving the
surface boron nitride rich. C/C composites degraded easily when exposed to LEO. The graphite
binder eroded similarly to bulk pyrolytic graphite; however, graphite fibers in the C/C composites
were found to erode much more slowly than the surrounding pitch binder.
REFERENCES
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*
°
Mirtich, Michael J., Kussmaul, Michael T., Enhanced Thermal Emittance of Space
Radiators by Ion-Discharge Chamber Texturing, NASA TM-100137, March 1987.
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994
Unexposed Exposed
PG
HOPG
Gas Infiltrated
C/C Composite
Liquid Infiltrated
C/C Composite
Figure 1. Visible degradation of samples after LEO exposure.
995
Unexposed
Exposed
Figure 2. SEM images showing preferential etching of graphite binder in the liquid infiltrated C/C
composite.
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Unexposed
Exposed
Figure 3. AFM data showing preferential etching of graphite domains in 60% C-BN.
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LEO degradation of graphite and carbon-based composites aboard Space Shuttle Flight STS-46

LEO degradation of graphite and carbon-based composites aboard Space Shuttle Flight STS-46

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