Graphite surfaces can be hardened and protected from erosion by hydrogen at high temperatures by refractory metal carbide coatings, which are usually prepared by chemical vapor deposition (CVD) or chemical vapor reaction (CVR) methods. These techniques rely on heating the substrate to a temperature where a volatile metal halide decomposes and reacts with either a hydrocarbon gas or with carbon from the substrate. For CVR techniques, deposition temperatures must be in excess of 2000 C in order to achieve favorable deposition kinetics. In an effort to lower the bulk substrate deposition temperature, the use of laser interactions with both the substrate and the metal halide deposition gas has been employed. Initial testing involved the use of a CO2 laser to heat the surface of a graphite substrate and a KrF excimer laser to accomplish a photodecomposition of TaCl5 gas near the substrate. The results of preliminary experiments using these techniques are described

Barietta, R. E.

Branch, D.

Vanier, P. E.

Veligdan, James

English

NASA Technical Reports Server

N94-3047'5
DEPOSITION OF TANTALUM CARBIDE COATINGS
ON GRAPHITE BY LASER INTERACTIONS
Dr. James Vcligdan,
D. Branch, P.E.. Vanier and R.E. Barletta
Engineering Research and Applications Division
Department of Nuclear Energy
Brookhaven National Laboratory
Upton, NY 11973
.2
/ /
ABSTRACT
Graphite surfaces can be hardened and protected from erosion by hydrogen at high temperatures by refractory
metal carbide coatings, which are usually prepared by chemical vapor deposition (CVD) or chemical vapor
reaction (CVR) methods. These techniques rely on heating the substrate to a temperature where a volatile metal
halide decomposes and reacts with either a hydrocarbon gas or with carbon from the substrate. For CVR
techniques, deposition temperatures must be in excess of 2000 ° C in order to achieve favorable deposition
kinetics. In an effort to lower the bulk substrate deposition temperature, the use of laser interactions with both
the substrate and the metal halide deposition gas has been employed. Initial testing involved the use of a CO2
laser to heat the surface of a graphite substrate and a KrF excimer laser to accomplish a photodecomposition of
TaCI 5 gas near the substrate. The results of preliminary experiments using these techniques are described.
INTRODUCTION
The use of refractory carbide coatings for protecting materials from corrosive gases at high temperatures or to
provide surface toughness is well established. Recendy, we have become involved with thc use of these materials
for the protection of graphites and carbon-carbon composites from attack by hot hydrogen at high temperatures
[1]. Typically, these coatings have been applied to the carbon substrate using either conventional chemical vapor
deposition (CVD) or chemical vapor reaction (CVR) techniques. The latter process has proven quite effective in
producing dense, adherent and protective coatings. In this process a metal halide vapor reacts with carbon in the
substrate to produce the carbide. For a refractory carbide such as tantalum, the reaction is
5
TaCI5 + C(substrate) --->TaC + _ C12. (i)
This reaction typically occurs at temperatures in the range of 1700 to 2600 K, depending on the carbide with
coating thicknesses on the order of 10 to 100 micrometers [2]. Since these coatings are produced at high
temperatures, the grain size is quite large.
In an effort to reduce the overall substrate temperature as well as to provide a means to perform "spot repairs"
for coatings, work has been initiated to develop an alternative coating method which could achieve the same grain
morphology and coating stoichiometry as the CVR process. Thermochemical data [3,4] indicate that high surface
temperatures are necessary for the carbide formation to proceed. Since the region of carbon diffusion is small,
however, bulk heating of the substrate is not required. Thus, the use of surface heating of the carbon with an
infrared laser was chosen. Further, experiments on the photo-assisted formation of Tat from TaCi 5 indicate the
efficacy of UV photons in the photo-decomposition of the starting metal
halide [5,6]. Given these considerations, a coating process in which the metal halide is decomposed by ultraviolet
radiation while the surface is heated using an infrared laser could result in coatings which are effectively the same
as high temperature CVR coatings while being made at lower overall substrate coating temperatures. In order to
test this hypothesis, initial coating studies have been performed using the TaC/graphite system.
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EXPERIMENTAL
The basic experimental setup consisted of a gas handling system for the TaCI 5 vapor and a deposition cell (see
Figure 1). The TaCI 5 vapor was produced by heating solid TaCI 5 in a reservoir. The vapor pressure in the
reservoir was calculated using the following relationship between 363 and 490 K [7]
4729
Log{P(Torr)} = 12.323- T (2)
The gas from the reservoir was throttled through a heated needle valve and ducted to the deposition cell where it
was directed onto the graphite sample through a nozzle. The graphite sample was attached to a heated stage whose
temperature could be varied between room temperature and 700 K. Light from a carbon dioxide laser was focused
on the graphite sample through a KBr window. Light from an excimer laser was admitted through a quartz
window at 90 ° to the CO2 beam. The excimer laser used was a KrF laser operating at 248 nm. This line was
chosen since it is coincident with the absorption maximum of the TaCI5 [6]. The graphite was locally heated
with a 3.5 W, cw-CO2 laser focused to - 1 mm in diameter.
Figure 1. Schematic of deposition cell. a. Silica window; b. KBr window; c. TaCI 5 inlet nozzle; d. Graphite
substrate; e. Heated sample pedestal; f. Excimer laser beam; g. CO2 laser beam.
Experiments were performed by heating the deposition tube to a temperature some 10 to 20 K higher than the
reservoir temperature. The sample in these tests consisted of a small piece of graphite (0.16 cm) embedded in a
refractory disk and attached to the pedestal. The substrate pedestal was also heated at this time to the test
temperature. Gas flow was then initiated by opening the reservoir needle valve and irradiation (infrared alone or
infrared plus UV) was begun. Throughout the course of the experiment, system pressure, reservoir and substrate
temperatures and excimer laser power were monitored. Upon completion of the test the gas flow was stopped and
the apparatus was cooled to room temperature. The graphite sample was then examined using scanning electron
microscopy (SEM) and energy dispersive x-ray analysis (EDX).
An initial experiment was also performed with a graphite disk in place of the heated platform. In this test, only
infrared radiation was used. Since the graphite remained at room temperature, TaCI 5 condensation on the disk in
areas not heated by the laser was expected. In other respects, the experimental procedure was identical to that
described above.
308
RESULTS AND DISCUSSION
Infrar_ Irradiation
The initial experiment on the graphite disk resulted in a limited thermal decomposition of the pentachloride in
the region heated by the laser. During the course of CO2 irradiation (about 5 minutes), the temperature of the
area heated by the laser appeared to decrease dramatically. This was evidenced by a change in color of the graphite
at the laser focus from bright white to dull orange to dark during the test. These changes indicate that the
temperature went from a point where decomposition and carbide formation is thermochemically favorable to a
temperature where only condensation of the chloride is expected. EDX examination revealed that the region
irradiated by the CO2 laser was in fact Ta rich and CI deficient with respect to other areas on the disk. The
presence of TaC could not be confirmed in this test.
Irradiation of graphite on the heated platform proved much more successful. With the platform at 573 K only
minimal chloride was deposited. This was presumably due to thermal decomposition of the pentachloride at the
surface resulting in traces of a lower vapor pressure chloride condensing during the course of the experiment.
Direct heating of the pentachloride molecule itself by the 10.6 lam radiation can be eliminated from a
consideration of the vibrational spectroscopy of the molecule [8]. EDX analysis was used to subtract the
contribution of this chloride and the graphite background and, when this was done, only an x-ray spectrum of Ta
remained (Figure 2). Thus, with CO2 radiation alone, we observed complete thermal decomposition of the
pentachloride can be accomplished leaving only Ta on the surface. The substrate surface temperature in this case
was insufficient to cause a reaction of the Ta with the graphite substrate.
C
o
u
n
t
s
C
Ta
C1
0.0 1.0 2.0 3.0 4.0 5.0
Energy (keV)
Figure 2. EDX spectrum of deposit on graphite disk after irradiation with infrared photons alone. The background
contributions to this spectrum have been subtracted using the spectrum of an unirradiated graphite specimen under
identical conditions. The deposition time in this experiment was 30 s.
Infrared plus Excimer Irradiation
Under experimental conditions identical to that of the infrared irradiation of a heated graphite substrate described
above, the coincident irradiation of the gas phase pentachloride stream with excimer light at 248 nm resulted in
the production of a coating which contained both Ta and C. Figure 3 shows an EDX spectrum of this coating
after background subtraction. In this experiment, the UV flux density was on the order of 2 W/cm 2 as a result of
irradiation with an unfocused excimer light. The pulse energy was - 145 mJ pulse and the pulse duration was on
the order of 20 ns. The results of this experiment when compared with those of infrared irradiation alone indicate
that UV absorption by the TaCI5 vapor is instrumental in allowing the carbide formation to proceed. The TaC
film formed by this process is quite fine grained (on the order of a few micrometers). Figure 4 shows an SEM
photograph of the film after 10 minutes of deposition. Although film coverage appeared to be uniform across the
graphite surface, holes in the coating are evident at high magnification. Detailed characterization and testing of
this coating has not been performed at this time. It should be noted that the grain size of these initial coatings is
much smaller than that seen in the TaC films produced by the process described in Reference 2.
309
The photon flux density in these experiments was approximately three orders of magnitude higher than that
used in the photo-CVD production of Tat [6]. Indeed, the laser power used is more consistent with multiphoton
dissociation processes [9]. In those experiments, photodissociation of a variety of metal halides and carbonyls as
well as other organometallic compounds was found to proceed in a step-wise fashion, ultimately producing
electronically excited metal atoms. For photons at 248 nm, it can be readily calculated from thermoehemical data
[3] that a minimum of 2 photons would be necessary to completely dissociate TaCI5. Such a process is expected
to result in photon emission from the Ta(I). We are currently in the process of investigating the photophysics of
this system.
C
o
u
n
t
s
C
Ta
CI
0.0 1.0 2.0 3.0 4.0 5.0
Energy (keV)
Figure 3. EDX spectrum of deposit on graphite disk after irradiation with both infrared and UV photons. The
background contributions to this spectrum have been subtracted using the spectrum of an unirradiated graphite
specimen under identical conditions. The deposition time in this experiment was 30 s.
Figure 4. SEM photograph of TaC coating on graphite after 10 minute deposition (2000 X).
310
CONCLUSIONS
The results of these initial experiments indicate that it is possible to produce refractory metal carbides at
very low substrate temperatures (< 700 K) using a two laser process. The photochemical process produces
carbides via a CVR-like reaction in which the carbon source is the substrate itself. This carbide formation
reaction has only been observed thermally at temperatures almost 2000 K higher. The UV photons in this carbide
reaction give rise to dissociation of the halide reactant while infrared photons cause heating of the immediate
substrate surface. We are currently in the process of investigating the details of the process (photochemistry,
photophysics, deposition rate, coating characteristics and hydrogen permeability, etc.) and expect to extend our
investigations of this multiple laser coating process to other coating systems.
ACKNOWLEDGEMENTS
The authors would like to thank R. E. Davis for his helpful discussions of the thermochemistry of the
system as well as J. Adams for his assistance with the SEM and EDX work. This work was carried out under the
auspices of the U. S. Department of Energy under
Contract No. DE-AC02-76CH00016.
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and Propulsion, 1993.
2. Union Carbide Advanced Ceramics Technical Information Bulletin, Metal Carbide Coatings on Graphite ;
NbC. TaC. TiC and Zr_, no date.
3. O. Kubashewski and C. B. Alcock, Metallurgical Thermochemistry, 5th ed., (Pergamon Press, Oxford,
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4. H. Schafer and F. Kahlenberg, Z. Anorg. Allgem. Chemie, 305, 178-89 (1960).
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Deposition of tantalum carbide coatings on graphite by laser interactions

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