Chemical vapor deposition of pyrolytic carbon on SiC fibers

Article abstract of DOI:10.1134/S002016850603006X

We carried out thermodynamic analysis of reactions underlying the preparation of pyrolytic carbon from a number of carbon-containing compounds and experimentally determined the optimal temperature range for the deposition of thin (1-3 μm) pyrolytic carbon layers onto continuous silicon carbide fibers. The results indicate that, among the compounds studied, the highest deposition rate of pyrolytic carbon is ensured by the pyrolysis of toluene and n-heptane. The slowest deposition rate is observed in methane pyrolysis. Rate data are used to determine the apparent activation energies for the pyrolysis of C ₇H₈, C₇H₁₆, (CH₃) ₂CO, CCl₄, and CH₄. Pleiades Publishing, Inc., 2006.

Full text of DOI:10.1134/S002016850603006X

ISSN 0020-1685, Inorganic Materials, 2006, Vol. 42, No. 3, pp. 246–249. © Pleiades Publishing, Inc., 2006.
Original Russian Text © P.M. Silenko, A.N. Shlapak, V.P. Afanas’ev, 2006, published in Neorganicheskie Materialy, 2006, Vol. 42, No. 3, pp. 288–291.
Chemical Vapor Deposition
of Pyrolytic Carbon on SiC Fibers
P. M. Silenko*, A. N. Shlapak*, and V. P. Afanas’ev**
* Frantsevich Institute of Materials Science Problems, National Academy of Sciences of Ukraine,
ul. Krzhizhanovskogo 3, Kiev, 03142 Ukraine
** Institute of Applied Problems of Physics and Biophysics, National Academy of Sciences of Ukraine,
Sluzhebnaya ul. 3, Kiev, 03142 Ukraine
e-mail: ps_2005@pochta.ru
Received July 27, 2005
Abstract—We carried out thermodynamic analysis of reactions underlying the preparation of pyrolytic carbon
from a number of carbon-containing compounds and experimentally determined the optimal temperature range
for the deposition of thin (1–3 µm) pyrolytic carbon layers onto continuous silicon carbide fibers. The results
indicate that, among the compounds studied, the highest deposition rate of pyrolytic carbon is ensured by the
pyrolysis of toluene and n-heptane. The slowest deposition rate is observed in methane pyrolysis. Rate data are
used to determine the apparent activation energies for the pyrolysis of C7H8, C7H16, (CH3)2CO, CCl4, and CH4.
DOI: 10.1134/S002016850603006X
INTRODUCTION
In particular, Nutt and Wawner [8] reported that depo-
sition of a pyrolytic carbon layer increased the strength
of silicon carbide fibers by a factor of 2.
Chemical vapor deposition (CVD) offers a number
of advantages over other widespread processes for pro-
ducing coatings (thermal evaporation, cathode sputter-
ing, electron-beam evaporation, laser ablation, and oth-
ers), in particular, simplified apparatus and high depo-
sition rates. The key features of CVD were described in
detail in many works (see, e.g., [1–3]). CVD is used to
deposit both metals and nonmetals, in particular, car-
bides, nitrides, silicides, and pyrolytic carbon.
In depositing antidiffusion barrier layers, including
those of pyrolytic carbon, one must taken into account
that long-term anneals at temperatures above 1470 K
and even short-term anneals above 1570–1670 K mark-
edly reduce the strength of silicon carbide fibers. This
is particularly true of W-SiC fibers [9, 10]. In connec-
tion with this, barrier layers must be deposited at lower
temperatures.
Continuous silicon carbide fibers grown by CVD are
used as reinforcing agents in metal- and ceramic-matrix
composites. SiC-fiber-reinforced composites can with-
stand temperatures as high as 1273 K and above. A seri-
ous impediment to many potential applications of SiC-
fiber-reinforced metal-matrix composites is the chemi-
cal interaction between the fibers and matrix at elevated
The objective of this work was to carry out thermo-
dynamic analysis of reactions underlying the prepara-
tion of pyrolytic carbon from a number of carbon-con-
taining compounds (toluene, methane, carbon tetra-
chloride, acetone, and n-heptane) and to determine the
optimal temperature range for the deposition of thin (1–
temperatures. One effective way to prevent (or reduce) 3 µm) pyrolytic carbon layers onto continuous silicon
fiber–matrix interaction is to protect reinforcing fibers carbide fibers.
with antidiffusion barrier layers from materials nonre-
active with both the fibers and matrix. Commonly
employed materials for such barrier layers include
refractory metal carbides and nitrides and pyrolytic car-
bon [4–7].
THEORY
Thermodynamic analysis allows one to establish
conditions favorable for gas-phase reactions. To this
end, we evaluated the Gibbs energy of reactions of the
type
Pyrolytic carbon barrier layers are substantially eas-
ier to produce than coatings from refractory metal car-
bides and nitrides. At the same time, pyrolytic carbon
ensures sufficiently effective protection of silicon car-
bide fibers in metallic matrices. Moreover, pyrolytic
carbon layers are capable of eliminating surface defects
CxHy
xC + yH
(1)
of fibers, thereby substantially enhancing their strength. in the temperature range 773–1773 K.
246

CHEMICAL VAPOR DEPOSITION OF PYROLYTIC CARBON ON SiC FIBERS
247
The Gibbs energy was calculated using well-known
equations of chemical thermodynamics [11]:
∆G, kJ/mol
–1000
–800
–600
–400
–200
∆G(T) = ∆H(T) – T∆S(T),
(2)
(3)
5
T
∆H(T) = ∆H0(298 K) + ∆CpdT,
∫
298
1
4
3
2
T
∆S(T) = ∆S0(298 K) + (∆Cp/T)dT,
(4)
(5)
0
500
1000
1500
2000
∫
298
T, K
Fig. 1. Gibbs energy as a function of temperature for pyrol-
ysis of (1) C H , (2) CH , (3) CCl , (4) (CH ) CO, and
∆G(T) = ∆H0(298 K) – T∆S0(298 K)
7
8
4
4
3 2
(5) C H
.
T
T
7
16
–
(∆Cp/T)dT + ∆CpdT.
∫
∫
Hydrocarbon + argon
298
298
5
3
4
In calculations, we used Ulich’s second approximation:
∆Cp = const.
2
2
1
After integration, Eq. (5) takes the form
6
7
∆G(T) = ∆H0(298 K)
– T∆S0(298 K) – ∆Cp(298 K)TM0,
Fig. 2. Experimental setup for pyrolytic carbon deposition:
(1) SiC fiber feed spool, (2) mercury contacts, (3) inlet tube,
(4) deposition chamber, (5) outlet tube, (6) C/SiC fiber
takeup spool, (7) power supply.
(6)
where
298.2
------------
T
time of the fiber in the reactor was 20, 40, 60, 80, or
100 s.
T
M0 = ln------------ – 1 +
298.2
.
(7)
The carbon resulting from the pyrolysis of the car-
bon-containing compound deposited on the immobile
silicon carbide fiber. The thickness and quality of pyro-
lytic carbon layers grown using different temperature–
time combinations were determined with an MIM-8
optical microscope.
The input thermodynamic data (enthalpies, entropies,
and heat capacities) for the carbon-containing com-
pounds of interest were taken from Karapet’yants [12].
The calculated Gibbs energies in the range 773–
1773 K are presented in Fig. 1. As seen, n-heptane and
toluene are best suited (have the lowest pyrolysis tem-
peratures) and methane and carbon tetrachloride are
least suited to carbon CVD.
RESULTS AND DISCUSSION
Our results indicate that the optimal temperature
range for pyrolytic carbon deposition from the carbon-
containing compounds in question is 1473–1573 K.
The coatings grown at lower temperatures were too thin
(<0.1 µm). Higher temperature deposition reduced the
strength of the silicon carbide fibers, preventing them
from being used as reinforcing agents of composites.
EXPERIMENTAL
To establish the optimal temperature range for pyro-
lytic carbon deposition, we used the experimental setup
schematized in Fig. 2.
Pyrolytic carbon was deposited by the following
procedure: After saturation with carbon-containing
The thicknesses of pyrolytic carbon layers grown at
vapor in a glass bubbler thermostated at 290 K, the 1473 and 1573 K and different deposition times are
argon carrier gas was introduced into the deposition listed in Table 1. As seen, the highest deposition rate of
chamber (4). The silicon carbide fiber in the chamber pyrolytic carbon is ensured by the pyrolysis of toluene
was heated by an electric current flowing through mer- and n-heptane, and the slowest deposition rate is
cury contacts (2). The fiber temperature was measured observed for the pyrolysis of methane and carbon tetra-
by an OMP-054 optical micropyrometer. The residence chloride. The pyrolytic carbon deposits grown from tol-
INORGANIC MATERIALS Vol. 42 No. 3 2006

248
SILENKO et al.
The temperature effect on the pyrolysis rate can be
represented by the Arrhenius equation [13]
K = Ae–E /(RT)
,
(8)
a
where K is the reaction rate constant; A is the preexpo-
nential factor, proportional to the number of molecular
collisions; and Ea is the apparent activation energy.
Activation energy is a key parameter for elucidating
the reaction mechanism: only Ea allows one to ascertain
whether a particular reaction is plausible.
Knowing the reaction rate constant K at several tem-
peratures, one can evaluate the apparent activation
energy by the equation [14]
10 µm
Fig. 3. Cross-sectional fracture surface of a silicon carbide
4.58log(KT /KT )
2
1
fiber coated with pyrolytic carbon.
------------------------------------------
Ea =
.
(9)
1
1
----- – -----
T1 T2
2.0
1
1.8
To calculate a reaction rate constant, one must know
the form of the rate equation. It is, therefore, preferable
to determine the apparent activation energy from the
temperature dependence of the rate of some process
(e.g., deposition rate), rather than from that of the rate
constant of the process. This approach to determining
Ea as a function of temperature was described by Eman-
uel’ and Knore [15] and Emyashev [16] for static
experimental conditions.
1.6
2
1.4
1.2
1.0
0.8
0.6
0.4
0.2
20
40
60
80
100
According to Emanuel’ and Knore [15], to evaluate
the apparent activation energy it is sufficient to deter-
mine the time it takes for the deposit to rich a predeter-
mined thickness at different temperatures,
Time, s
Fig. 4. Thickness as a function of deposition time for pyro-
lytic carbon layers grown from acetone at (1) 1573 and
(2) 1473 K.
1/τT
1/τT
4.575T2T1
-------------------------
T2 – T1
2
-----------
Ea =
log
,
(10)
1
uene and n-heptane had smooth surfaces and consisted
of fine grains. Figure 3 shows a fracture surface of a sil-
icon carbide fiber coated with pyrolytic carbon at
1573 K using toluene.
where τT is the deposition time at temperature T2, and
2
τT is the deposition time at temperature T1.
1
Table 1. Thicknesses of pyrolytic carbon layers grown from carbon-containing compounds at 1473 and 1573 K and different
deposition times
∆d, µm
Compound
1473 K
60 s
1573 K
60 s
20 s
40 s
80 s
100 s
20 s
40 s
80 s
100 s
C7H8
1.18
0.22
0.28
0.40
0.42
2.20
0.28
0.41
0.68
1.41
2.40
0.31
0.45
0.98
1.84
3.42
0.35
0.60
1.26
2.48
4.10
0.41
0.75
1.50
3.45
1.22
0.39
0.50
0.65
0.80
2.24
0.41
0.60
1.10
1.65
3.02
0.42
0.75
1.25
2.40
3.75
0.45
0.90
1.65
3.00
5.00
0.48
1.10
1.95
3.75
CH4
CCl4
(CH3)2CO
C7H16
INORGANIC MATERIALS Vol. 42 No. 3 2006

CHEMICAL VAPOR DEPOSITION OF PYROLYTIC CARBON ON SiC FIBERS
249
lated under the title Osazhdenie iz gazovoi fazy, Mos-
cow: Atomizdat, 1970.
Table 2. Apparent activation energies of pyrolysis reactions
Reaction
7C + 8H2
7C + 4H2
3C + 2H2 + H2O
Ea, kJ/mol
2. Krivoruchko, V.M., Poluchenie tugoplavkikh soedinenii
iz gazovoi fazy (Preparation of Refractory Compounds
by Vapor-Phase Processes), Moscow: Atomizdat, 1976.
C7H16
C7H8
36
72
3. Samsonov, G.V. and Epik, G.V., Zashchitnye pokrytiya
(Protective Coatings), Moscow: Metallurgiya, 1973.
(CH3)2CO
CCl4
78
4. Ning, X.J., Pirouz, P., Lagerlof, K., and DiCarlo, J., The
Structure of Carbon in Chemically Vapor Deposited SiC
Monofilaments, J. Mater. Res., 1990, vol. 5, no. 12,
pp. 2865–2876.
C + 2Cl2
C + 2H2
171
398
CH4
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gle Fiber Test Technique for Titanium MMC’s, Proc.
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don, 1990, pp. 55–64.
In this work, we calculated the apparent activation
energy by Eq. (10). The values of τT and τT were
2
1
6. Choy, K.L., Effect of Surface Modification on the Inter-
facial Chemical Stability and Strength of Continuous
SiC Fibers after Exposure to Molten Aluminum, Scr.
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determined from the layer thicknesses measured as a
function of deposition time at 1473 and 1573 K. Fig-
ure 4 displays such data for acetone.
The apparent activation energies evaluated in this
manner for all of the carbon-containing compounds
studied here are listed in Table 2. The highest values of
Ea were obtained for the pyrolysis of methane and car-
bon tetrachloride, and the lowest values were obtained
for the pyrolysis of n-heptane and toluene, which corre-
lates with the rate data for these compounds.
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Microstructure, J. Mater. Sci., 1985, vol. 20,
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CONCLUSIONS
We carried out thermodynamic analysis of reactions
underlying the preparation of pyrolytic carbon from
carbon-containing compounds in a broad temperature
range, 500–2000 K. The results indicate that n-heptane
and toluene are best suited (have the lowest pyrolysis
temperatures) and methane and carbon tetrachloride are
least suited to carbon CVD.
Kinetic studies of pyrolytic carbon deposition on
silicon carbide fibers using carbon-containing com-
pounds demonstrate that the highest deposition rate of
pyrolytic carbon is ensured by the pyrolysis of toluene.
The slowest deposition rate is observed for methane
pyrolysis.
11. Karapetyants, M.Kh., Khimicheskaya termodinamika
(Chemical Thermodynamics), Moscow: Khimiya, 1975.
12. Karapet’yants, M.Kh. and Karapet’yants, M.L., Osnov-
nye termodinamicheskie konstanty neorganicheskikh i
organicheskikh veshchestv (Fundamental Thermody-
namic Constants of Inorganic and Organic Substances),
Moscow: Khimiya, 1968.
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rastvorakh (Principles of Chemical Kinetics in Gases
and Solutions), Moscow: Mosk. Gos. Univ., 1971.
14. Kiperman, S.L., Osnovy khimicheskoi kinetiki v getero-
gennom katalize (Principles of Chemical Kinetics in
Heterogeneous Catalysis), Moscow: Khimiya, 1979.
15. Emanuel’, N.M. and Knore, D.G., Kurs khimicheskoi
kinetiki (A Course in Chemical Kinetics), Moscow:
Vysshaya Shkola, 1969.
The apparent activation energies for the pyrolysis of
C7H8, C7H16, (CH3)2CO, CCl4, and CH4 are deter-
mined.
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INORGANIC MATERIALS Vol. 42 No. 3 2006