Carbon-fiber reinforced SiC (C/SiC) composites are proposed for leading edge applications of hypersonic vehicles due to the superior strength of carbon fibers at high temperatures (greater than 1500 C). However, the vulnerability of the carbon fibers in C/SiC to oxidation over a wide range of temperatures remains a problem. Previous oxidation studies of C/SiC have mainly been conducted in air or oxygen, so that the oxidation behavior of C/SiC at reduced oxygen partial pressures of the hypersonic flight regime are less well understood. In this study, both carbon fibers and C/SiC composites were oxidized over a wide range of temperatures and oxygen partial pressures to facilitate the understanding and modeling of C/SiC oxidation kinetics for hypersonic flight conditions

Opila, Elizabeth J.

Serra, Jessica

NASA Technical Reports Server

OXIDATION OF C/SiC COMPOSITES AT REDUCED 
OXYGEN PARTIAL PRESSURES 
Elizabeth J. Opila*, Jessica Serra^ 
NASA Glenn Research Center, Cleveland, OH 44135 
^The Pennsylvania State University, University Park, PA, 
16802 
 
Carbon-fiber reinforced SiC (C/SiC) composites are 
proposed for leading edge applications of hypersonic 
vehicles due to the superior strength of carbon fibers at 
high temperatures (>1500°C).  However, the vulnerability 
of the carbon fibers in C/SiC to oxidation over a wide 
range of temperatures remains a problem.  Previous 
oxidation studies of C/SiC have mainly been conducted in 
air or oxygen1-3, so that the oxidation behavior of C/SiC at 
reduced oxygen partial pressures of the hypersonic flight 
regime are less well understood.  In this study, both 
carbon fibers and C/SiC composites were oxidized over a 
wide range of temperatures and oxygen partial pressures 
to facilitate the understanding and modeling of C/SiC 
oxidation kinetics for hypersonic flight conditions. 
 
T-300 carbon fiber and C/SiC coupons were oxidized at 
temperatures of 816°, 1149°, 1343°, and 1538°C (1500°, 
2100°, 2450°, and 2800°F) and in gas mixtures containing 
50% O2/Ar, 5% O2/Ar, 0.5% O2/Ar, and 0.1% O2/Ar at 
one atmosphere total pressure.  The T-300 fibers were 
held in slotted alumina crucibles for the oxidation testing.  
Oxidation kinetics were monitored using 
thermogravimetric analysis.  Post test characterization of 
selected coupons included optical microscopy, x-ray 
diffraction, scanning electron microscopy, and energy 
dispersive spectroscopy. 
   
Results of the T-300 carbon fiber oxidation are shown in 
Figure 1.  The oxidation rates reported are instantaneous 
rates when 50% of the fibers are consumed.  It can be seen 
from  Figure  1 that the oxidation kinetics are only weakly 
dependent on temperature, but strongly dependent on 
oxygen partial pressure.  The weak temperature 
dependence and oxygen pressure dependence shown in 
Figure 2 are both consistent with oxidation limited by 
gaseous transport. 
4 6 8 10 12 14
10,000/T (K)-1
-6
-5
-4
-3
Lo
g 
(o
xi
da
tio
n 
ra
te
)
Lamouroux, O2
Halbig, O2
Serra/Opila 50% O2
Serra/Opila 5% O2
Serra/Opila 0.5% O2
Serra/Opila 0.1% O2
50060070080010001600
Temperature (°C)
12001400
Figure 1.  Oxidation rates of T-300 carbon fibers as a 
function of temperature and oxygen partial pressure. 
 
The results in Figure 1 include oxidation rates for T-300 
fibers in 100% oxygen that are found in the literature1,4.  
The results of Halbig et al.4 were obtained in the same 
laboratory and show consistent pressure dependence with 
the new results as shown in Figure 2. 
 
Kinetic results for C/SiC coupon oxidation are shown for 
all temperatures in 50% O2/Ar in Figure 3.  Weight loss is 
typically observed indicating that oxidation of carbon 
fibers rather than oxidation of SiC dominates the kinetics 
even at the highest oxygen partial pressure studied.  
Weight loss is largest at 816°C where cracks in the SiC 
coating and matrix are open allowing ingress of oxygen 
and complete consumption of the C fibers.  As the 
temperature is increased thermal expansion and oxidation 
of the SiC both contribute to crack closure.  The variation 
in the observed weight change is explained by variation in 
the number and size of cracks in the SiC seal coat as well 
as varying extents of crack closure.  At lower oxygen 
partial pressures and higher temperatures crack closure is 
attributed exclusively to thermal expansion rather than 
formation of silica in the cracks. 
-3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0
Log (PO2)
-6
-5
-4
-3
Lo
g 
(o
xi
da
tio
n 
ra
te
)
816°C,   slope=0.84±0.12
1149°C, slope=0.79±0.18
1343°C, slope=0.88±0.08
1538°C, slope=0.93±0.09
0.05 10.50.005
PO2 (atm)
Halbig data
0.001
 
Figure 2.  Oxygen partial pressure dependence of T-300 
fiber oxidation rates obtained at all temperatures. 
0 5 10 15 20 25 30
Time (h)
-80
-60
-40
-20
0
Sp
ec
ifi
c 
w
ei
gh
t c
ha
ng
e 
(m
g/
cm
2 )
816°C
1149°C
1343°C
1538°C
Figure 3.  Oxidation weight loss of C/SiC composites in 
0.4 cm/sec flowing 50% O2/Ar as a function of 
temperature. 
 
At the highest temperature (1538°C) and lowest oxygen 
partial pressure (0.1% O2/Ar) the C/SiC coupon was 
rapidly consumed due to active oxidation of the SiC and 
oxidation of the C fibers as they were exposed. 
 
Acknowledgment 
This work was funded by the NASA Aeronautics 
Research Mission Directorate/Fundamental Aeronautics 
Program in Hypersonics. 
 
References 
1 F. Lamouroux, X. Bourrat, and R. Naslain, 
“Structure/Oxidation Behavior Relationship in the 
Carbonaceous Constituents of 2D-C/PyC/SiC 
Composites,” Carbon 31 [8] 1273-1288 (1993). 
 
2F. Lamouroux, G. Camus, J. Thebault, “Kinetics and 
Mechanism of 2D Woven C/SiC Composites:  I, 
Experimental Approach,” J. Am. Ceram. Soc. 77 [8]2049-
57 (1994). 
 
3F. Lamouroux, R. Naslain, J.-M. Jouin, “Kinetics and 
Mechanism of 2D Woven C/SiC Composites:  II, 
Theoretical Approach,” J. Am. Ceram. Soc. 77 [8]2058-
68 (1994). 
 
4M.C. Halbig, D.N. Brewer, and A.J. Eckel, “Degradation 
of Continuous Fiber Ceramic Matrix Composites Under 
Constant Load Conditions,” NASA TM-2000-209681. 
https://ntrs.nasa.gov/search.jsp?R=20100040695 2019-08-30T13:34:16+00:00Z
National Aeronautics and Space Administration
www.nasa.gov 1
Oxidation of C/SiC Composites at Reduced Oxygen 
Partial Pressures
Elizabeth Opila
NASA Glenn Research Center
Cleveland, OH
Jessica Serra
The Pennsylvania State University
University Park, PA
216th Meeting of the Electrochemical Society Meeting
Vienna Austria
October 8, 2009
National Aeronautics and Space Administration
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Introduction
• C/SiC is proposed for leading edges, hot structure 
control surfaces, and acreage thermal protection 
systems for hypersonic vehicles
• Re-entry and hypersonic cruise environments: 
high temperatures, low oxygen partial pressures
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Introduction:  C/SiC oxidation
• Oxidation of C/SiC and carbon fibers studied in air or 
oxygen
– Ismail, Carbon 29 [6] 777-792 (1991).
– Lamouroux et al., Carbon 31 [8] 1273-1288 (1993).
– Lamouroux, et al., J. Am. Ceram. Soc. 77 [8] 2049-57 (1994).
– Halbig et al., NASA TM-2000-209681.
• Bulk carbon oxidation studied at reduced oxygen 
partial pressures
– Walker et al., Adv. Catal., Vol. XI, 133-221 (1959).
– Gulbransen et al., J. Electrochem. Soc. 110 [6] 476-483 (1963).
• Three reactions to consider
C + 1/2 O2(g) = CO(g)
SiC + 3/2 O2(g) = SiO2 + CO(g)   passive oxidation
SiC + O2(g) = SiO(g) + CO(g)      active oxidation
National Aeronautics and Space Administration
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Objectives
• Quantify the oxidation rates of the constituent 
carbon fibers and C/SiC composites at high 
temperatures and low oxygen partial pressures
• Understand the dominant oxidation mechanisms
• Provide input to a finite element model for 
predicting the life expectancy of C/SiC in 
hypersonic vehicle applications
National Aeronautics and Space Administration
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Experimental materials
• T-300 carbon fibers 
– 1k tow
– heat treated to 1538°C (2800°F)
– 0.5 g batches of material
– fibers bundled in slotted alumina crucible
• C/SiC composite coupons
– GE Power Systems
– 1538°C heat treated T-300 fibers
– CVI SiC matrix
– 2.5 cm x 1.3 cm x 0.25 cm coupons
– 0.25 cm dia hole
– CVD SiC seal coat
0.1 cm
1 cm
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Experimental technique
Thermogravimetric Analysis (TGA)
• Temperatures studied
– 816°C (1500°F)
– 1149°C (2100°F)
– 1343°C (2450°F)
– 1538°C (2800°F)
• Total pressure 1 atm
• Oxygen partial pressures
– 50% O2 / argon
– 5% O2 / argon
– 0.5% O2 / argon
– 0.1% O2 / argon
• Flow rate:  100 ccm, 0.4 cm/sec
• Exposure times
– C fibers oxidized to completion
– C/SiC coupons 25 or 100h
• Post-exposure microscopy, SEM/EDS, XRD
National Aeronautics and Space Administration
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T-300 carbon fiber oxidation
National Aeronautics and Space Administration
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0 20 40 60 80 100
Time (h)
0
20
40
60
80
100
%
 in
iti
al
 w
ei
gh
t
816°C
1149°C
1343°C
1538°C
0 2 4 6 8 10 12
Time (h)
0
20
40
60
80
100
%
 in
iti
al
 w
ei
gh
t
816°C
1149°C
1343°C
1538°C
8
T-300 fiber oxidation TGA results
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
Time (h)
0
20
40
60
80
100
%
 in
iti
al
 w
ei
gh
t
816°C
1149°C
1343°C
1538°C
• Results fairly repeatable
• Reported rates - instantaneous rate at 
50% consumed
• Oxidation rates vary by about a factor of 
two over temperature range of interest
• Oxidation rates vary by orders of 
magnitude over pressure range of interest
50% O2 / argon
0.5% O2 / argon
5% O2 / argon
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Temperature dependence for T-300 fiber oxidation
Low temperature (<700°C)  surface reaction control
High temperature (>700°C):  gas phase diffusion control
C
 +
 O
2
C
O2
CO gas 
boundary 
layer
4 6 8 10 12 14
10,000/T (K)-1
-6
-5
-4
-3
Lo
g 
(o
xi
da
tio
n 
ra
te
)
Lamouroux, O2
Halbig, O2
Serra/Opila 50% O2
Serra/Opila 5% O2
Serra/Opila 0.5% O2
Serra/Opila 0.1% O2
50060070080010001600
Temperature (°C)
12001400
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Temperature dependence for T-300 fiber oxidation
Kinetic theory of gases predicts 
T3/2 dependence of gas phase 
interdiffusion, D
2.90 2.95 3.00 3.05 3.10 3.15 3.20
Log (Temperature, °C)
-6
-5
-4
Lo
g(
ox
id
at
io
n 
ra
te
)
50% O2: slope=1.04±1.05
5% O2: slope=0.96±0.73
0.5% O2: slope=1.17±0.73
816 1149 1343 1538
Temperature, °C
predicted slope = 1.0
0.1% O2: slope=0.98±3.75
L
DSck oxox
ρ3/12/1Re=
1Tkox ∝
1−∝ DSc
32Dkox ∝
Gas boundary layer limited 
oxidation rate, kox
Only considers temperature 
dependence of gas phase 
diffusion
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Pressure dependence for T-300 fiber oxidation
Oxidation limited by gas diffusion 
through a laminar boundary layer:
2/1
3/12/1Re
total
oxox
ox P
P
L
DSck ∝= ρ
816°C
-3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0
Log (PO2)
-6
-5
-4
-3
Lo
g 
(o
xi
da
tio
n 
ra
te
)
816°C,   slope=0.84±0.12
1149°C, slope=0.79±0.18
1343°C, slope=0.88±0.08
1538°C, slope=0.93±0.09
0.05 10.50.005
PO2 (atm)
Halbig data
0.001
0 50 100 150 200 250 300 350
Time (h)
0
20
40
60
80
100
%
 in
iti
al
 w
ei
gh
t
0.5 atm O2
0.05 atm O2
0.005 atm O2
0.001 O2
Theory predicts Pn, n=1
Experiment:  n=0.79 to 0.93
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• T-300 fiber oxidation is limited by gas phase 
diffusion through the gas boundary layer
• Actual hypersonic environment will have 
reduced total pressure
Summary of T-300 fiber oxidation
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C/SiC coupon oxidation
National Aeronautics and Space Administration
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0 20 40 60 80 100
Time (h)
-3
-2
-1
0
Sp
ec
ifi
c 
w
ei
gh
t c
ha
ng
e 
(m
g/
cm
2 )
816°C
1538°C
1343°C
1149°C
0 20 40 60 80 100
Time (h)
-30
-20
-10
0
Sp
ec
ifi
c 
w
ei
gh
t c
ha
ng
e 
(m
g/
cm
2 )
816°C
1149°C
1343°C
1538°C
14
C/SiC coupon oxidation TGA results
0 5 10 15 20 25 30
Time (h)
-80
-60
-40
-20
0
Sp
ec
ifi
c 
w
ei
gh
t c
ha
ng
e 
(m
g/
cm
2 )
816°C
1149°C
1343°C
1538°C
• Weight loss observed in almost all 
conditions: oxidation of carbon fibers 
through cracks in SiC seal coat
• Results show scatter due to varied crack 
structure in seal coat
• Weight loss rates are greatest at lowest 
temperatures, seal coat cracks open
• Weight loss rates are strongly dependent 
on oxygen partial pressure
50% O2 / argon
0.5% O2 / argon
5% O2 / argon
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C/SiC coupon oxidation: carbon oxidation
C/SiC composite, 816°C, 0.5 atm O2 / argon, 100ccm, 25h
C/SiC composite, 1538°C, 0.5 atm O2 / argon, 100ccm, 25h
Complete C fiber oxidation at 816°C
Minimal C fiber oxidation at 1538°C
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C/SiC coupon oxidation: SiC oxidation
816°C 1149°C 1343°C 1538°C 816°C 1149°C 1343°C 1538°C
816°C 1149°C 1343°C 1538°C816°C 1149°C 1343°C 1538°C
0.5 atm O2 0.05 atm O2
0.005 atm O2 0.001 atm O2
Colors indicate thin film of SiO2 has formed on surface of SiC
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C/SiC coupon oxidation: SiC oxidation
PO2, atm
Temperature, °C
Time, h
816* 1149* 1343^ 1538^
0.5 0.02 0.43 0.74 1.17 25
0.05 0.01 0.25 0.47 0.74 100
0.005 0.00 0.06 0.15 0.24 100
0.001 0.00 0.01 0.07 n/a 100
Predicted oxide thickness (µm) formed 
on SiC at specified oxidation conditions
*Ramberg et al., J. Am. Ceram. Soc. 79 [11] 2897-2911 (1996).
^Ogbuji et al., J. Electrochem. Soc. 142 [3] 925-930 (1995).
tBx =BtAxx =+2
Low temperature, short times, thin 
scales:  linear parabolic kinetics
High temperature, long times, thick 
scales:  parabolic kinetics
X=oxide thickness, B=parabolic rate constant, B/A=linear rate constant
2O
PB∝
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Transport of oxidant through cracks in SiC 
coating controls fiber oxidation rate
• Typical crack width on order of 1 µm (green) 
• Significant number of wider cracks (red)
• Average spacing between cracks 73 µm
0.1 cm
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Crack closure mechanisms as temperature is increased
Oxidation: growth of SiO2Thermal Expansion of SiC
Dominant mechanism of crack closure
Neither mechanism can completely close cracks >∼ 1 µm
PO2, atm
Temperature, °C
Time, h
816* 1149* 1343^ 1538^
0.5 0.02 0.43 0.74 1.17 25
0.05 0.01 0.25 0.47 0.74 100
0.005 0.00 0.06 0.15 0.23 100
0.001 0.00 0.01 0.07 n/a 100
T, °C % ∆L/L
∆L (µm) for 73 µm 
segment
816 0.38 0.28
1149 0.57 0.41
1343 0.69 0.50
1538 0.81 0.59
Silica thickness, µm
PO2, atm
Temperature, °C
816 1149 1343 1538
0.5 TE ≈ ox ox
0.05 TE TE ≈ ox
0.005 TE TE TE TE
0.001 TE TE TE TE
%∆L/L from Touloukian
C/SiC
SiC SiC
C/SiC
i i SiC SiC
SiO2
National Aeronautics and Space Administration
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0 20 40 60 80
time, h
-0.6
-0.5
-0.4
-0.3
-0.2
-0.1
0.0
0.1
w
ei
gh
t c
ha
ng
e,
 g
20
Active oxidation of C/SiC
SiC + O2 = SiO(g) + CO(g)
1538°C (2800°F), 1000 ppm O2/Ar, exposure time: 72.3h
Sample fell from hanger when hole breached, weight loss 0.468g
sample fell from hanger
after exposure
before exposure
flow direction
after exposure
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Summary of C/SiC oxidation 
• C/SiC oxidation is dominated by carbon 
fiber oxidation through cracks in the 
coating
• Oxidation rates of C and SiC in actual 
hypersonic environments will differ due 
to the presence of atomic oxygen
• Active oxidation of SiC at low oxygen 
partial pressures and high temperatures 
is a major concern for C/SiC in 
hypersonic environments
National Aeronautics and Space Administration
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Ongoing work
• Active oxidation of SiC under study 
– Determine conditions under which it occurs
– Develop predictive capability for rate of attack
• C/SiC protective coating development task in place  
– Task objectives
• 200h lifetime
• T≥1650°C
• leading edge environment
– Coating needs
• Seal cracks over all temperature ranges and under mechanical 
loading conditions
• Mitigate SiC active oxidation
22
National Aeronautics and Space Administration
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Acknowledgments
This work was funded by the NASA Aeronautics Research Mission 
Directorate, Fundamental Aeronautics Program on Hypersonics
Thanks to
Bob Mattingly (NASA-GRC) for XRD 
Matt Rowan (U. Toledo, Ohio)
Meredith Boyd (Rochester U., New York)


Oxidation of C/SiC Composites at Reduced Oxygen Partial Pressures

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