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LEGAL NOTICE
This report was prepared as an account of
Government sponsored work. Neither the
United States, nor the Commission, nor any
person acting on behalf of the Commission:
A. Makes any warranty or representa-
tion, expressed or implied, with respect to
the accuracy, completeness, or usefulness
of the information contained in this report,
or that the use of any information, appa-
ratus, method, or process disclosed in this
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B. Assumes any liabilities with respect
to the use of, or for damages resulting from
the use of any information, apparatus,
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As used in the above, “person acting on
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that such employee or contractor of the
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to, any information pursuant to his employ-
ment or contract with the Commission, or
his employment with such contractor.
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1961
8 / 19
DATE
ISSUANCE
MICROCARD
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OKN2 P 84
To be published in the Proceedings of the 18th AEC Metallographic Group Meeting of
Atomics International, Conogo Park, California, June 22-24, 1964.
CONF-607-4
JUL 1 5 1964LECTRON MI
OGLECTRON METALLOGRAPHY OF PYROLYTIC CARBON COATINGS
ON FUEL PARTICLES*
Facsimile Price S klad
C.K.H. DuBose and J. O. Stiegler
C.K.H. Du
Microfilm Price S _, SC
Oak Ridge National Laboratory
Available from the
Oak Ridge, Tennessee
Office of Technical Services
Department of Commerce
Washington 25, D. C.
SUMMARY

VASTER
A replica electron microscope study of as-polished and cathodically etched sur-
fuces of Fyrolytic carbon coatings on fuel particles has been made in an attempt to char-
acterize coatings that showed as much as 30% difference in bulk density. High and low
density coatings could be characterized by the ir polished surface textures; however, these
features were not indicative of the true structure as seer. by direct electron transmission.
Microvoids detected by the transmission study of cleavage flakes exist on too fine a scale
to be observed either optically or by electron microscope examination of replicas of the
polished surfaces. Other features such as the effect of cathodic and chemical etching,
coating delamination, and duplex coating interfaces have also been examined.
1. INTRODUCTION
Pyrolytic carbon deposits produced by the decomposition of gaseous hydrocarbons
on heated substrates currently are being considered as coatings for fuel particles for nu-
clear reactors. By varying the deposition conditions, it is possible to alter the physical
and mechanical properties of the carbon over very wide ranges. For example, deposits
ranging in density between about 1.4 and 2.2 gm/cmº can be formed by changing the
*Research sponsored by the U.S. Atomic Energy Commission under contract with
the Union Carbide Corporation.
deposition temperature or rate. Furthermore, by altering the deposition conditions during
formation of a batch of coatings, it is possible to prepare multilayer coatings having a
combination of desirable properties (e.g., a low density, spongy inner layer to absorb
fission recoils and a high density outer layer to provide strength and fission product reten-
tion)...
It was the purpose of this siudy to characterize on a microscale deposit formed under
varied conditions in order to correlate physical and mechanical properties with structure.
Transmission electron microscopy and electron diffraction were used to determine the true
nature of the various pyrolytic carbons studied. Replica studies of polished and etched
surfaces of the same materials were also carried out to determine distinguishing features
of the different deposits.
II. NATURE OF THE TRUE STRUCTURE
Pyrolytic carbon exists in the unique class of crystalline solids having two dimensional
crystal lattices. X-ray' and electron? diffraction studies have shown that the deposits
consist of monolayer crystallites on the order of 100 Å in diameter having the hexagonal
Bernal graphite structure that are twisted and rotated randomly with respect to one another.
The C axis parameter of 3.43 Å and greater indicates that the stacking is not totally ran-
dom, but nearly so.
The optical microstructures of the deposits fall into two broad classes depending on
the deposition conditions: 1) a laminar structure typified by the coating shown in Fig. 1(a),
and 2) a columnar structure characterized in Fig. 1(b). Although deposits belonging to
either class appear quite similar to one another in bright field illumination, their physical
- 3
•
.
and mechanical properties may vary over a wide range. It is felt that the microstructural
differences occur on too fine a scale to be detected by optical microscopy.
•
Transmission electron micrographs of cleavage flakes extracted from both high and
low-density laminar and columnar deposits are shown in Figs. 2 and 3. Contrast variations
in these micrographs arise from differences in thickness. That is, the degree of darkening
is proportional to the mass of material traversed by the electron beam. The small white
areas, therefore, correspond to thinner regions in the sample or, in this case, microvoids.
It is readily apparent that as the bulk density decreases, the pore density increases. The
lowest-density materials appear quite spongy; they evidently consist of blocks of carbon
and void, both on the order of 100 Å in diameter.
III. RELATED SURFACE MICROSTRUCTURES
Obtaining suitably thin cleavage flakes from particle coatings is a difficult and
time consuming procedure. An electron microscope study was made of replicas taken from
polished and etched surfaces in order to determine whether or not details of the structure
could be inferred from such observations, which are considerably easier to make than
transmission observations.
..
A. Polished Surfaces
The resulting microstructures of the high and low densities of the laminar and co-
lumnar material after standard metallographic polishing are shown in Fig. 4. It is obvious
that the replica studies of the asapolished surfaces do not suggest the same structure of the
pyrolytic carbon as seen in transmission; they do, however, reveal that the polishing char-
acteristics of these materials vary with the density. . The voids that exist in these coatings
apparently are too small to be distinguished above the general surface roughness caused
by their characteristic polishing behavior. There are, however, significant differences
in the structure between the high density and low density material observed by the repli-
cation technique. The low density material always exhibits a smoother surface than does
the high density material, see Fig. 4 (a) and (c), and shows polishing scratches. This leads
one to the assumption that the low density material is less brittle and smears more easily
during polishing than does the high density material. The high density material is more
brittle and possibly fractures on a microscale during the polishing operation, and thus
exhibits the coarser texture as seen in Fig. 4(b) and (d).
The microstructures of the various degrees of density are reproducible for a partic-
ular range of density, i.e., a low density coating always gives a rather smooth texture
upon replication for electron microscopy. The high density material, on the other hand,
gives a much coarser texture. The intermediate ranges of densities lead to intermediate
ranges of polished surface textures.
Slight differences exist in the as-polished surface textures between the laminar
and columnar materials, as may be seen in Fig. 4. In the low density material the dif-
ference in texture is rather subtle, whereas, in the high density materials considerably
greater differences are evident. The exact nature of the swirled texture of the high den-
sity columnar material is not known; it is, however, typical of high density columnar pyro-
lytic carbon.
In many cases surface texture differences were noted within the same coating, in-
dicating varying densities within the same coating. The specimen shown in Fig. 5(a) and
(b), for example, has a much coarser texture near the outer edge than near the core, I see
also Fig. 6(a) and (b) l. This may arise from changes in the deposition variables or from
the increase in surface area of the coating as deposition progresses. Cleavage flakes for
- 5-
transmission examination of selected areas such as this would be impossible to obtain.
With this background informotion, one can infer a great deal about a cooting
from looking at a replica of its polished surface in the electron microscope. It is posa
sible, for example, to observe quickly and easily how the density of a coating varies
with thickness or if densities on different particles are appreciably different. In addi-
tion, rough estimates of the actual coating density can be made.
B.
Cathodic Etching
Particles representative of high density (2.01 gm/cm) and a low density (1.71
gm/cm) laminar coatings were mounted in an electrical conductive mount for cathodic
etching. To ensure identical etching conditions, they were located side by side in the
same mount. Prior to etching, a replica was made of the polished surfaces for examina-
tion in the electron microscope. In the as-polished state, the high density material ex-
hibited a coarser texture than did the low density material. Near the outer edge of the
high density particles, a much coarser texture was noted than was found near the core,
see Fig. 5 (a) and (b). The reverse of this was found in the low density particles, see
Fig. 6 (a) and (b). These variations in texture could be related to density changes during
deposition of the coatings. After cathodic etching for 15 min at 3500 v with a current of
35 ma, numerous pits were found in the area of the less coarse structure. The high density
coatings showed many more pits than did the low density material. The pits were located
near the core area in the high density material (Fig. 5(c) and (d) I, whereas they were near
the outer surface of the low density coatings (Fig. 6 (c) and (d) l.
Based on the transmission work of the coatings, the pits appear to bear no relation-
ship to true structure. They may be sites of "micro-inclusions that have been greatly
..
.
enlarged by the nature of cathodic etching.
C. Chemical Etching
In an attempt to reveal the pore structure obscured by flowed material due to pol-
ishing, a chemical etchant also was employed on high and low density laminar pyrolytic
carbon coatings. A standard graphite etchant of hot potassium dichromate-phosphoric
acid was used. The structure, however, was virtually unchanged from the as-polished
condition. Considerable experimentation would be necessary to develop a meaningful
chemical etchont for pyrolytic carbon.
D. Macrostructures
1.
Delaminations
Delaminations in the laminar coatings were very common, and appeared to be
related to some possible changes in the system during the coating process. The size of .
the cracks varied considerably in width and depth. Large cracks, as shown in Fig. 7(a)
are clearly visible optically, however, small fissures, as shown in Fig. 7(b) at 12,500 X,
may not be seen in the optical microscope.
2. Duplex Interfaces
.
The interface between the laminar and columnar structures of duplex coatings was
very smooth, for the most part, in the particles that were examined. However, for some
applications, small voids at the interface of the duplex coatings is beneficial. Such is the
case in frying to reduce crack propogation due to irradiation effects.
A duplex coating consisting of a columnar coating on top of a laminar coating is
-7.
.
shown in Fig. 8(a). Only slight differences in densities between coatings comprise the
duplex coating represented in Fig. 8(b). Good interface bonding is represented in these
micrographs; however, many particles examined did exhibit cracks at the duplex interface.
- 8 -
.
REFERENCES
1. 0, J. Guentert, J. Chem. Phys., 37, 884 (1962).
2. J. O. Stiegler, C.K.H. DuBose and J. L. Cook, ORNL-TM-863 (in press).
3. J. Biscoe and B. E. Warren, J. Appl. Phys., 13, 364 (1942).
LIST OF FIGURES
Fig. 1. Polarized Light Microstructures of Pyrolytic Carbon Coatings. (c) Lominar
structure. (b) Columnar structure.
Fig. 2 Transmission Electron Micrographs of Low and High Laminar Coatings. (a) Low
density (p= 1.41 gm/cm3). Light spots are microvoids. 90,000 X. (b) High
density (p = 2.08 gm/cm3). 105,000 X.
Fig. 3 Transmission Electron Micrographs of Low and High Density Columnar Coatings.
(a) Low density (p = 1.39 gm/cm3). 102,000 X. (b) High density (p = 2.08
gm/cm3). 75,000 X.
Fig. 4 Electron Micrographs of Replicas Taken of As-Polished Surfaces of Various
Coatings. (a) Laminar, low density (p=1.41 gm/cm3). (b) Laminar, high
density (p = 2.08 gr/cm3). (c) Columnar, low density (p= 1.39 gm/cm3).
(d) Columnar, high density (p = 2,08 gm/cm3).
Fig. 5 Replicas Taken of High Density (p = 2.01 gm/cm3) Laminar Coating. (a) As-
polished, coarse texture near outer edge, and (b) near core. (c) After cathodic
etching, smooth surface near edge, and (d) pits near core area. 25,000 X.
Fig. 6
Replicas Taken of Low Density (p = 1.7 gm/cm) Laminar Coating. (a) As-
polished, smooth surface near outer edge, and (b) coarser texture near core.
(c) After cathodic etching, pits near outer edge, and (d) smooth surface near
core. 25,000 X.
Electron Micrographs of Replicas Taken of Delaminations in Lominar Coatings.
(a) Large cracks, as polished. 6,250 X. (b) Micro cracks, cathodic etch.
Fig. 7
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Coating of Varying Densities. Arrows indicate interface.
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END