The high-temperature oxidation behavior of reaction-bonded porous silicon carbide ceramics in dry oxygen

Article abstract of DOI:10.1111/j.1551-2916.2010.03708.x

The oxidation behavior of reaction-bonded porous silicon carbide (RPSC) ceramics in dry oxygen between 1100° and 1500°C was investigated based on four specimens with different porosities. RPSC ceramics exhibited a rapid mass increase in the initial stage of oxidation but a slow mass increase in the following oxidation, which was considerably different from the oxidation behavior of dense SiC. The oxidation kinetics for RPSC can be better represented by an asymptotic law rather than the parabolic law for dense SiC. We suppose that, although oxidation occurred in the entire pore channels at the beginning, the pores were rapidly blocked by the oxide as their growth rate near the pore mouth was very fast due to sufficient oxygen. As the result, the oxidation of the pore interior was stopped in the absence of further oxygen supply.

Full text of DOI:10.1111/j.1551-2916.2010.03708.x

J. Am. Ceram. Soc., 93 [7] 2062–2067 (2010)
DOI: 10.1111/j.1551-2916.2010.03708.x
r 2010 The American Ceramic Society
ournal
J
The High-Temperature Oxidation Behavior of Reaction-Bonded Porous
Silicon Carbide Ceramics in Dry Oxygen
Chuanwei Zheng, Zhenming Yang, and Jinsong Zhangw
Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China
The oxidation behavior of reaction-bonded porous silicon car-
bide (RPSC) ceramics in dry oxygen between 11001 and 15001C
was investigated based on four specimens with different poros-
ities. RPSC ceramics exhibited a rapid mass increase in the ini-
tial stage of oxidation but a slow mass increase in the following
oxidation, which was considerably different from the oxidation
behavior of dense SiC. The oxidation kinetics for RPSC can be
better represented by an asymptotic law rather than the para-
bolic law for dense SiC. We suppose that, although oxidation
occurred in the entire pore channels at the beginning, the pores
were rapidly blocked by the oxide as their growth rate near the
pore mouth was very fast due to sufficient oxygen. As the result,
the oxidation of the pore interior was stopped in the absence of
further oxygen supply.
oxidation of RBSN and SiCN was governed by the radius of the
pore channels allowing the oxygen to penetrate the specimen. It
is noticed that the RPSC contains mainly micrometer-scale open
pores other than nanoscale pores like in RBSN and SiCN. It was
interesting to know whether the oxidation mechanism of RBSN
and SiCN can still be applied to the case of porous SiC. More-
over, the influence of porosities on the oxidation behaviors of
porous ceramics has not been systematically studied. In this
manuscript, we report a detailed study on the oxidation behav-
iors of RPSC with four different open porosities in dry oxygen
and in the temperature range of 11001–15001C.
II. Experimental Procedure
Porous SiC materials were prepared by the reaction bonding
method. In brief, aminophenolic resin, ethanol, a-SiC powder
(
about 6.5 mm in size), and Si powder (about 5 mm in size) were
mixed together and then pressed at 2001C for 30 min under
0 MPa. The preforms were pyrolyzed at 8001C for 30 min,
I. Introduction
Porous silicon carbide (SiC) ceramics have attracted significant
attention for a number of industrial applications such as cata-
lytic supports, hot-gas and molten metal filters, membrane sup-
ports, gas-burner media, and lightweight structural parts for
2
and then sintered at 18001C for 2 h in an argon atmosphere.
The resulting compacts were heated in vacuum at 17001C to
volatilize the residual silicon. Samples with a dimension of 40
mm ꢀ 10 mm ꢀ 4 mm were mechanically polished with 1 mm
diamond paste, followed by degreasing in acetone and ultra-
sonic cleaning in deionized water. RPSC materials with different
porosities can be fabricated by controlling the distribution of
silicon particles, pressure, and hot-pressing temperatures. For
reference, dense SiC samples were also prepared by the reaction
bonding method but without silicon volatilization.
1
–4
elevated temperature applications.
There are typically four
methodological categories developed for manufacturing porous
SiC ceramics: replication, reaction bonding, partial sintering,
and expandable microspheres. Among them, the reaction bond-
5
ing technique is a promising method, which involves oxidation
and carbother-
6
–8
bonding techniques, silicizing techniques,
9,10
1
1
mal reduction techniques. The reaction bonding process is
characterized by a low processing temperature and a short sinte-
ring time, and is suitable for the preparation of large-size and
Four RPSC materials with a porosity of 12.770.5, 17.670.2,
2
1.370.5, and 28.070.4 vol% were studied in detail, and la-
beled as A, B, C, and D, respectively, as shown in Table I. The
bulk density (d ) was determined by dividing the weights of the
1
2
complex-shaped components. In comparison with the porous
SiC prepared by other methods, reaction-bonded porous SiC
B
(RPSC) retains its strength up to very high temperatures and has
excellent thermal stability.
The chemical inertness of SiC results from the formation of a
samples by their volumes. The solid density (d , also called skel-
s
etal density) was measured by the Archimedean method with
distilled water as the immersion medium. The open porosity was
obtained from the two densities using the following equation:
protective SiO
2
layer. Intensive investigations have been focused
on the oxidation behavior of SiC materials in the form of pow-
It is gener-
1
3,14
15,16
17,18
ders,
sintered ceramics,
and CVD films.
e ¼ 1 ꢁ dB=ds
(1)
ally accepted that the oxidation behavior of SiC is controlled by
the diffusion of oxygen molecules through the growing oxide
scale as characterized by a parabolic rate model over a wide
temperature range, e.g. 11001–15001C. A detailed review has
The pore volumes and volume distribution of RPSC speci-
mens were determined with the aid of mercury porosimetry
(MIP, Win9400, Micromeritics Instrument Corp., Norcross, GA).
The dimension of the specimens was 10 mm ꢀ 4 mm ꢀ 2 mm.
The diameter at 50% of the intruded mercury volume was
treated as the median pore diameter. The total pore area and
total intrusion volume of all RPSC materials determined by
mercury porosimetry are given in Table I. The open porosity
derived from the total intrusion volume is consistent with that
obtained by Eq. (1). The presence of impurities in the initial sam-
ples was analyzed semiquantitatively by X-ray photoelectron spec-
troscopy (XPS, ESCALAB250, Thermo VG, Waltham, MA)
after sputtering (for 60 s duration) with a 2 kV beam of argon
ions. The results are given in Table I. No metallic silicon can be
detected by X-ray diffractometry (XRD), indicating that the
amount of residual silicon, if there is any, is below the detection
sensitivity of the X-ray diffractometer used.
1
9
been published by Presser and Nickel. Porous SiC obviously
differs from dense SiC in terms of porosity and high internal
surface area, which may result in a much different oxidation
behavior. However, till date, a detailed study on the oxidation
behavior of porous SiC is still lacking in literatures. Similar
3 4
studies have been carried out on reaction-bonded Si N (RBSN)
2
¨
by Porz and Thummler, and SiC nitride (SiCN) by Raj et al.
0
21
In both works, the authors found that the extent of internal
N. Jacobson—contributing editor
Manuscript No. 27000. Received October 22, 2009; approved February 3, 2010.
Author to whom correspondence should be addressed. e-mail: jshzhang@imr.ac.cn
w
2
062

July 2010
High-Temperature Oxidation Behavior
2063
Table I. Properties of the Selected RPSC Materials
order to allow for free oxygen flowing. The weights of the sam-
ples (ranging from 3.4000 to 4.4000 g) were measured using an
electronic balance with the sensitivity of 0.1 mg. At chosen time
intervals, the sample was extracted from the furnace, cooled,
weighted, and then reinserted into the furnace. In order to min-
imize the contamination, the alumina tube and crucible were
aged under flowing oxygen for about 500 h at 15001C before the
oxidation experiments.
The morphology and composition of the samples were ex-
amined by scanning electron microscopy (SEM, S3400, Hitachi,
Tokyo, Japan) and energy-dispersive spectroscopy (Inca, Oxford,
Abingdon, U.K.). The crystalline phases in the samples were
identified by XRD (D/max-2500PC, Rigaku, Tokyo, Japan).
Material
Properties
A
B
C
D
Bulk density,
d (g/cm )
2.79
2.65
2.53
2.32
3
B
Solid density,
d (g/cm )
Open porosity
3.20
12.7
409
3.22
17.6
620
3.21
21.3
761
3.22
28.0
1018
0.57
0.123
28.2
3
S
w
(
vol%)
Median pore
diameter (nm)
Total pore area
3.16
0.053
14.5
1.83
1.42
0.086
21.5
2
(
m /g)
III. Results and Discussion
Total intrusion
volume (mL/g)
Open porosity
0.075
19.3
(1) Oxidation Kinetics
The oxidation profiles of four specimens at different tempera-
tures are plotted in Fig. 1 as a function of oxidation time. As can
be seen, almost all profiles, except that for specimen D at
z
(
vol%)
Impurities (wt%)
Al
Na
Ca
K
1.27
0.85
0.50
0.85
1
4001C, are typically with a high mass increase in the initial
stage but a slow mass increase in the following. This is signifi-
cantly different from the oxidation behavior of dense SiC, which
typically follows the linear parabolic oxidation kinetics. In
fact, the RPSC oxidation results are better represented by an
asymptotic treatment of the oxidation kinetics. Porz and
Thummler have used an asymptotic equation to fit the oxida-
¨
tion of porous Si N , in which the pores were self-blocking, and
the rate of oxygen transport was determined by the number of
pores still remaining open. In this work, the asymptotic equation
can also be adopted to represent the oxidation results of RPSC,
as shown by the following equation:
2
2
w
z
Calculated using Eq. (1). Calculated from the total intrusion volume.
20
3
4
The oxidation apparatus used in this work was an alumina
tube furnace with MoSi windings and alumina insulation. Ox-
ygen (UHP, 99.995%) at atmospheric pressure and a flow rate
of 100 standard cm /min (SCCM), dried by passing through
silica gel and dispersed phosphorus pentaoxide desiccant towers,
was used for all experiments. The oxidation temperatures were
2
3
ꢁ1
Dm Dm1
À
Á
1
1001–15001C with an accuracy of 721C. The samples were
1 ꢁ eꢁkat
¼
(2)
placed in an alumina crucible and separated by SiC foam in
m
m
Fig. 1. Oxidation kinetics of reaction-bonded porous silicon carbide in flowing oxygen at different temperatures. The solid lines are the fitting results
using the asymptotic equation.

2
064
Journal of the American Ceramic Society—Zheng et al.
Vol. 93, No. 7
where Dm
of the oxidation curve is reached and k
coefficient. Referring to Eq. (2), the mass gain Dm/m can never
exceed Dm /m. Although there is still a very low oxidation rate
N
/m is the final mass gain when the horizontal region
a
is the asymptotic rate
N
in the horizontal region because of the further oxidation at the
external surface, this can be neglected compared with the high
initial oxidation rate.
As seen from the solid lines in Fig. 1, the experimental data
can be well fitted by the asymptotic Eq. (2). The fitting results
for the four samples are summarized in Table II. The parameter
t99, representing the time by which 99% of the final mass gain
was reached, was calculated using Eq. (2) and listed in Table II.
We can see that the higher the oxidation temperature, the
shorter the time t . In other words, the mass gain at higher
9
9
temperature was lower than that at the lower temperature be-
cause of the limited time for internal oxidation for the former.
From Table II, we can also see that the Dm
are extremely sensitive to the amount and type of open porosity,
namely the higher porosity, the higher Dm /m and t99 values.
N
/m and t99 values
N
Fig. 2. Arrhenius plot of the asymptotic rate coefficients.
We believe that the higher porosity allows more oxygen to pen-
etrate and produce more oxide. However, this silica growth re-
sults in intrinsic compressive stress in the pore channels. With
enough stress being accumulated, the SiC matrix deforms and
breaks, and henceforth breakaway oxidation occurs. This can be
seen from Fig. 1, at 13001 and 14001C, material D has a much
higher mass gain than the fitting curves because of the break-
away oxidation.
of this residual silicon results in micrometer- and nanometer-
sized pores, which are connected to the holes created during the
pyrolysis process and form open pore channels.
SEM images of material B before and after oxidation at
1
2001C are shown in Fig. 3. Before oxidation, RPSC is mainly
composed of SiC matrix and a mixture of micrometer- and
nanometer-sized pores and associated open pore channels. Most
of the pores were open and exposed to the environment, as they
were created by silicon volatilization. The micrometer-sized
pores were as large as 5 mm, which might be due to the mi-
crometer-sized silicon particles used. There was a large amount
of voids created after the sintering and volatilizing process. Fig-
ure 3 also shows the surface second electron image and the
cross-section backscattered electron image of material B after
oxidation at 12001C for 20 (Fig. 3(c)) and 100 h (Figs. 3(d)–(f)).
The surface becomes coarse and the diameters of the pores are
narrowed after oxidation (see Figs. 3(c) and (d)). The cross-sec-
tion image (Figs. 3(e) and (f)) shows that not only is the surface
covered with an oxide layer but also the pores have been filled
The asymptotic rate coefficient k
pendent, as shown in Fig. 2, which plots the usual Arrhenius
plot of ln k vs. 1/T (T in Kelvin). Because of the rapid change in
a
is strongly temperature de-
a
the kinetics at 15001C, there were no data points recorded for
the early stage of the oxidation of material A. This affects the
calculation of ka and gives a very low value. The apparent
a
activation energy (E ) calculated from the Arrhenius plot is
proportional to the open porosity, which means that the rate-
controlling step of the oxidation process is correlated to the
porosity.
(
2) Microstructure Development
Pore-free bulk SiC materials cannot be prepared by the reaction
bonding method because of the escape of organic gaseous spe-
cies during the solid-state pyrolysis of polymers, which inevita-
bly leaves a certain amount of open and closed pores. Residual
silicon is also inevitable in reaction-bonded SiC. Volatilization
with SiO . This means that large amounts of oxide have formed
2
in the interior of the specimen. Oxidation of RPSC materials
occurs at both the external and internal surfaces. If there is
enough oxygen in the pores to support the oxidation, all of them
2
would be completely filled with SiO after a certain time. How-
ever, this is obviously not the case for our results. As seen in
Figs. 3(e) and (f), there are three kinds of pores in the RPSC
specimens: pores fully filled with SiO (label A), pores with some
Table II. Results of Asymptotic Treatment of the
Oxidation Plots
2
2 2
annular SiO (label B), and pores without SiO (label C). This
indicates that the oxidation in some pore channels has ceased,
which will be discussed in later sections.
Oxidation
temperature
Dm1
ꢁ1
)
Material
(1C)
k
a
(h
t99 (h)
m
SEM images of material D before and after oxidation at
13001C are shown in Fig. 4. Compared with material B, material
D has a higher porosity (Fig. 4(a)), which allows more oxygen to
penetrate and produces more oxide. As mentioned above, the
growth of silica results in intrinsic compressive stress in the pore
channels. With enough stress being accumulated, the SiC matrix
deforms and breaks. This leads to breakaway oxidation, as
shown in Fig. 4(b).
A
1100
0.040670.0003 0.07870.003 58.78
0.035370.0003 0.32670.020 14.11
0.031870.0001 1.16370.022 3.96
0.029270.0001 2.18570.072 2.11
0.020670.0001 3.31670.385 1.39
0.072670.0007 0.04070.002 116.38
0.062370.0006 0.14770.009 31.37
0.054370.0005 0.44670.033 10.34
0.049070.0002 1.08370.050 4.25
0.043170.0001 2.38170.101 1.93
0.098070.0013 0.03570.002 131.95
0.094470.0011 0.12070.008 38.33
0.081270.0009 0.35770.030 12.89
0.077470.0006 0.80170.056 5.75
0.057270.0002 1.87270.067 2.46
0.150970.0026 0.02270.001 213.60
0.140770.0021 0.07170.006 64.53
0.134470.0044 0.17070.030 27.05
0.121170.0116 0.40870.178 11.28
1
1
1
1
200
300
400
500
B
1100
1
1
1
1
200
300
400
500
(3) Porosity Analysis
The porosities of four RPSC specimens before and after oxida-
tion at 12001C for different times, which were measured by
mercury porosimetry, are shown in Fig. 5. Before oxidation, the
porosities of the four materials with pore diameters larger than
C
D
1100
1
1
1
1
200
300
400
500
2
mm were about 3 vol%. The pore diameter distribution cen-
tered at around 0.41 mm for material A, 0.62 mm for material B,
.76 mm for material C, and 1.02 mm for material D. After
1100
0
1
1
1
200
300
400
oxidation, the pore channels were narrowed and the amounts of
the open pores diminished with the increase of oxidation time
for all four samples. For example, the pore diameter distribution

July 2010
High-Temperature Oxidation Behavior
2065
Fig. 3. Scanning electron microscopy images of surface ((a), (c), (d): SE) and cross-section ((b), (f): SE; (e): BSE) for material B before (a, b) and
after oxidation at 12001C for 20 h (c) and 100 h (d, e, f). (f) Microstructures of the cross section in the middle of the sample. SE, second electron;
BSE, backscattered electron.
of material C centered at around 0.63, 0.25, and 0.04 mm after
oxidation at 12001C for 4, 20, and 40 h, respectively. Moreover,
the four materials with different initial porosities show different
pore distribution profiles after a certain oxidation time. For ex-
ample, after oxidation for 20 h, 40% of the initial open pores are
still open and can be filled with mercury for material D. This
value is 35% for material C and 31% for material B. However,
for material A, only a few of the initially open pores are still
open at this reaction stage. Most of them have been filled with
silica. This means that the RPSC material having larger porosity
Fig. 4. Surface images of material D before (a) and after oxidized at 13001C for 100 h. (b) A breakaway microstructure in the sample.

2
066
Journal of the American Ceramic Society—Zheng et al.
Vol. 93, No. 7
Fig. 5. Smoothed dV/d log D pore volume versus pore diameter plots for four materials before and after oxidation measured by mercury porosimetry.
23,24
gases away from the surface.
needs longer time to completely fill the pores. This is consistent
with the calculated t99 by asymptotic Eq. (2) for the materials
with different starting porosities, as given in Table II.
It should be pointed out that the size of the pores obtained
from the mercury porosimetry is not in agreement with that ob-
tained from SEM observations. As illustrated in Fig. 6, the
RPSC samples have micrometer-sized pores (or particle pores),
nanopores, and associated open pore channels (or called ‘‘neck
pores,’’ the ‘‘neck’’ part of the open pore channels that are con-
nected to particle pores). From the mercury porosimetry, the
intrusion curves give the volume that is accessible through open-
ings to the surface greater than a certain diameter; therefore, the
pore diameter distribution from the intrusion curves is not the
distribution of all the pores in the entire samples, but of the open
pore channels the mercury intruded. The median pore diameter
is characterized to be the neck pores that are connected to in-
ternal pores.
During internal oxidation, the transport process of oxygen is
similar to the intrusion process of mercury. The extent of inter-
nal oxidation of these RPSC materials is governed by the size
and amount of these neck pore channels. When all the channels
were completely filled with silica, the internal oxidation termi-
nated and the horizontal region of the oxidation curve was
reached.
Most investigations suggest
that the rate-controlling step of dense SiC oxidation would be
one of (2), (3), or (4). Generally, linear kinetics of silica growth
would indicate interface control mechanisms, and a parabolic
time law corresponds with in- or outward diffusion-controlled
2
4
processes. However, unlike the case for dense SiC materials,
the transport of molecular oxygen in the gas phase would be
very important in porous SiC. The typical oxidation behavior of
RPSC materials can be described as follows.
The pore channels in RPSC can be represented by a bundle of
parallel cylindrical pore channels of different diameters. In one
pore channel, the diffusion process of oxygen into the pore goes
with the chemical reaction at the wall of the channel. As illus-
trated in Fig. 7, at the outer surface of the RPSC sample, there
was enough oxygen to support the formation and growth of a
silica layer. However, in the pore channels, the diffusion of
oxygen was not fast enough to provide sufficient oxygen for
the oxidation reaction with the channel wall. As the result, the
oxygen concentration inside the pores was reduced gradually
with the oxidation being continued. A concentration profile c(x)
20
along the pore channel has been built and shown in Fig. 7(a).
The silica layer at the channel wall was thickened in the region of
the pore mouth, which reduced the open diameter of the pore
channels. After a certain time, the channels were completely
clogged near the mouth (Fig. 7(c)). When all the pore channels
were choked, the internal oxidation terminated. The three
kinds of pores labeled as A, B, and C in the cross-section im-
age (Figs. 3(e) and (f)) were considered to correspond to the
different positions in one channel (the secants marked A, B, and
C in Fig. 7(c)). It is believed that a higher temperature may cause
a faster growth of the silica layer near the pore mouth
(Fig. 7(b)), which then leads to a quicker blocking of the pores
(
4) Mechanism of the RPSC Oxidation
There are five steps in the oxidation procedure: (1) diffusion of
oxygen in the atmosphere to the sample, (2) inward diffusion of
oxygen through the silica layer, (3) oxidation reaction at the
2
SiC/SiO interface, (4) outward diffusion of the gaseous product
through the silica layer, and (5) outward diffusion of product
Fig. 6. Schematic illustration of a pore structure of RBSC. (a) Cross section, and (b) three kinds of pores.

July 2010
High-Temperature Oxidation Behavior
2067
tion of oxygen by reaction with SiC to form silica. The oxide
growth rate near the pore mouth was faster than the rate of ox-
ygen supply to the interior of the pore, and thus it clogged the
pore and stopped the oxidation. The oxidation of RPSC was
found to be extremely sensitive to the amount and type of open
porosity. Material with small pores showed good oxidation re-
sistance. A pre-oxidation at high temperatures may limit oxygen
penetration and prevent the oxidation of RPSC.
Acknowledgments
The authors thank Dr. Wensheng Zhang and Mr. Jiayuan Ye (Research In-
stitute of Cements & New Building Materials, China Building Materials Academy)
for porosity measurement. C. W. Zheng thanks Dr. Chunhai Jiang for helpful
discussions.
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0
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16
J. A. Costello and R. E. Tressler, ‘‘Oxidation Kinetics of Silicon Carbide
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(1986).
17
T. Narushima, T. Goto, and T. Hirai, ‘‘High Temperature Passive Oxidation
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1989).
(
18
C. E. Ramberg, G. Cruciani, K. E. Spear, and R. E. Tressler, ‘‘Passive Ox-
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19
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20
¨
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21
IV. Conclusion
R. Raj, L. An, S. Shah, R. Riedel, C. Fasel, and H. J. Kleebe, ‘‘Oxidation
Kinetics of an Amorphous Silicon Carbonitride Ceramic,’’ J. Am. Ceram. Soc., 84
8] 1803–10 (2001).
The oxidation of RPSC with porosities of 12.7, 17.6, 21.3, and
28.0 vol% has been analyzed in detail. The oxidation behavior
of RPSC exhibited a typical pattern with a high initial oxidation
rate, which then falls off rapidly toward longer oxidation time.
In the channels of RPSC, two simultaneous processes were ac-
tive: the diffusion of oxygen into the channels and the consump-
[
22
B. E. Deal and A. S. Grove, ‘‘General Relationship for the Thermal Oxidation
of Silicon,’’ J. Appl. Phys., 36 [12] 3770–8 (1965).
N. S. Jacobson, ‘‘Corrosion of Silicon-Based Ceramics in Combustion Envi-
23
ronments,’’ J. Am. Ceram. Soc., 76 [1] 3–28 (1993).
K. L. Luthra, ‘‘Some New Perspectives on Oxidation of Silicon Carbide and
24
Silicon Nitride,’’ J. Am. Ceram. Soc., 74 [5] 1095–103 (1991).
&