Fabrication of silicon carbide-mullite composite by melt infiltration

Article abstract of DOI:10.1111/j.1151-2916.2003.tb03274.x

The melt infiltration method was used to fabricate a SiC-mullite composite at high temperature. Mullite was successfully obtained from a SiO₂ and Al₂O₃ powder mixture by melting above 1830°C in a BN crucible with a lid. When infiltrated into a porous SiC preform, the mullite significantly reacted with SiC to form gaseous SiO and CO, even at the lowest investigated temperature of 1830°C, consuming SiO₂ and leaving Al₂O₃ and silicon phases in the sample. The relevant reactions were studied in detail. A closed system was adopted to suppress the reaction, and a dense composite was successfully obtained.

Full text of DOI:10.1111/j.1151-2916.2003.tb03274.x

J. Am. Ceram. Soc., 86 [1] 39–42 (2003)
journal
Fabrication of Silicon Carbide–Mullite Composite by Melt Infiltration
Jintao Tian and Kazuhisa Shobu
National Institute of Advanced Industrial Science and Technology (AIST), Kyushu Center, Tosu, Saga 841-0052, Japan
The melt infiltration method was used to fabricate a SiC–
mullite composite at high temperature. Mullite was success-
fully obtained from a SiO2 and Al2O3 powder mixture by
melting above 1830°C in a BN crucible with a lid. When
infiltrated into a porous SiC preform, the mullite significantly
reacted with SiC to form gaseous SiO and CO, even at the
lowest investigated temperature of 1830°C, consuming SiO2
and leaving Al2O3 and silicon phases in the sample. The
relevant reactions were studied in detail. A closed system was
adopted to suppress the reaction, and a dense composite was
successfully obtained.
ratio of 69:31 in ethyl alcohol using a ball mill for 1 h and then
dried at 75°C in an oven (hereafter referred to as Al2O3 ϩ SiO2
powder). Formation of mullite and infiltration into the porous SiC
preform were conducted in an induction furnace above the mullite
melting point of 1830°C under argon at atmospheric pressure. A
typical sample with dimensions of 13 mm ϫ 10 mm ϫ 6 mm was
placed in a boron nitride (BN) crucible with a loose-fitting BN lid
and manually heated to 1000°C, then automatically to a tempera-
ture Ͼ1830°C in 5 min to form molten mullite. The formed molten
mullite, as expected, infiltrated into the porous SiC preform via
capillary force. After the SiC was infiltrated by mullite, the furnace
was cooled to 1000°C in 5 min, followed by natural cooling to
room temperature, which led to crystallization of mullite and
fabrication of a dense SiC–mullite composite. Crystalline phases
of the obtained composite were identified using X-ray diffractom-
etry (XRD). The microstructure of the composite was observed
using optical microscopy.
I. Introduction
ILICON CARBIDE (SiC) has been recognized as
a high-
S
performance material for structural applications because of
its unique combination of properties, such as good mechanical
properties maintained to high temperature, high wear and
oxidation resistance, and high thermal conductivity.1,2 Mullite
(3Al2O3⅐2SiO2) is an important ceramic material and is used for a
wide range of purposes, including structural, optical, and electrical
applications.3 Mullite has a close thermal expansion match and
good chemical compatibility with SiC. Thus, the combination of
SiC and mullite to form SiC–mullite composites is very attractive.
There have been several studies on the subject reported in the
literature.4–6 Pressureless sintering or hot pressing commonly has
been adopted as the fabrication method. Melt infiltration is an
alternative to these conventional methods, with the notable advan-
tage of fabrication of essentially fully dense composites of com-
plex shapes with good geometric and dimensional fidelity.7 An-
other advantage of melt infiltration is that there is little grain-
boundary phase in the composite. Furthermore, the method is cost
effective; therefore, the fabrication of SiC–mullite composite using
the melt infiltration process is attractive. Until now, investigations
related to the subject have not been reported in the literature.
In the present study, the fabrication of SiC–mullite composite
using melt infiltration was explored. The processing conditions for
the fabrication were studied in detail, including the formation of
mullite and the reactions with SiC during infiltration.
III. Results and Discussion
(1) Formation of Mullite from Al2O3 ؉ SiO2 Powder
The formation of mullite from Al2O3 ϩ SiO2 powder was
studied at temperatures above the melting point of mullite
(ϳ1830°C). The experimental details and the results are listed in
Table I. Mullite could be synthesized by melting Al2O3 ϩ SiO2
powder in a BN crucible with a lid in a temperature range from
1830° to 1900°C. At 1950°C, however, a hold Ͼ5 min led to a
definite decrease in the SiO2 content, leaving Al2O3 phase in the
sample. The lid was found open after the experiment at 1950°C,
which indicated extensive gas evolution inside the crucible,
whereas the lid stayed closed at lower temperatures. This was
explained by the decomposition and evaporation of mullite at high
temperature. By assuming molten mullite as an ideal solution of
the constituents, where the activities for SiO2 and Al2O3 were
simply assumed to be concentrations, and using the thermody-
namic data taken from the JANAF Thermochemical Table,8 we
calculated the equilibrium partial pressures of the gaseous species
over molten mullite at high temperature, as shown in Fig. 1.
Molten mullite has the main gaseous species SiO, O2, and O, and
the total vapor pressures at 1900° and 1950°C are ϳ11 and ϳ21
Pa, respectively. The loose-fitting lid had a weight of 1.84 g and
could be open when the gas pressure inside the crucible was higher
than that outside the crucible by ϳ10 Pa. The calculated vapor
pressure of ϳ21 Pa reasonably explained the opening of the lid at
II. Experimental Procedure
High-purity commercial SiC powder (99% pure, average parti-
cle size of ϳ5 ␮m, Showa-Denko, Tokyo, Japan) was molded in
a die and kept at 700°C for 1 h in air to remove residual carbon
impurities. The relative density of the obtained SiC preform was
ϳ50%. High-purity commercial Al2O3 powder (99.5% pure,
average particle size of ϳ0.6 ␮m, Sumitomo Chemicals, Tokyo,
Japan) and SiO2 powder (99.5% pure, average particle size of ϳ5
␮m, Mitsuwa–Kagaku, Tokyo, Japan) were mixed in a weight
Table I. Mullite Formation Using
Al2O3 ؉ SiO2 Powder as the Raw Material
Temperature (°C)
Time (min)
XRD results
1830
1850
1875
1900
1925
1950
1950
1950
10
10
10
10
10
5
Mullite
Mullite
Mullite
J. Smialek—contributing editor
Mullite
Mullite
Mullite
10
20
Al2O3, mullite
Al2O3
Manuscript No. 187365. Received October 29, 2001; approved October 10, 2002.
Supported by the Japan Society for the Promotion of Science (JSPS).
39

40
Journal of the American Ceramic Society—Tian and Shobu
Vol. 86, No. 1
melting point of mullite. The infiltration was conducted at tem-
peratures from 1830° to 1950°C in a BN crucible with a lid.
Unfortunately, extensive reactions were observed in all the exper-
iments between SiC and mullite, as shown in Table II, and the
phases SiC, Al2O3, mullite, and silicon were identified using XRD
analysis of the obtained sample. The mullite phase was detected
only as a minor phase at the lowest temperatures of 1830° and
1850°C. Many bubbles were observed in the molten infiltrant, and
the lid was found open after the experiment, which indicated
extensive gas evolution. An additional infiltration experiment was
also conducted using mullite powder, which was formerly pre-
pared in the experiment described in Section III(1). The results
were almost identical.
Many studies have been conducted theoretically and experimen-
tally on the possible reactions between SiO2 and SiC.9–11 These
studies identified some typical reactions, as follows:
SiC͑s͒ ϩ 2SiO2͑l͒ 3 3SiO͑g͒ ϩ CO͑g͒
2SiC͑s͒ ϩ SiO2͑l͒ 3 3Si͑l͒ ϩ 2CO͑g͒
Si͑l͒ ϩ SiO2͑l͒ 3 2SiO͑g͒
(1)
(2)
(3)
Fig. 1. Equilibrium vapor pressures of gas species on molten mullite.
Figure 3 shows the equilibrium total vapor pressures for these
reactions as a function of temperature. SiC reacts with SiO2 via
reaction (1) or (2) to form various products: SiO(g) and CO(g), or
Si(l) and CO(g). Reaction (3) also leads to the consumption of
SiO2. To determine the reaction by which SiC and SiO2 actually
reacted in the present case, experiments were designed and
performed as shown in Table III. In experiment A, the powder
mixture of SiC and SiO2 with a mole ratio of 1:2 almost
completely disappeared (ϳ99 wt% of the powder), whereas, in
experiments B and C, the mixtures with mole ratios of 2:1 and 2:3
lost weights by only ϳ55 and ϳ91 wt%, respectively. Thus,
reaction (1) appears to have occurred in experiment A, with almost
full loss of the sample. However, no single reaction above is
responsible for the results of experiments B and C.
An almost complete thermodynamic description of the Si-O-C
system is available in the literature,12 but it is complicated and not
easy to understand. We present here a slightly simpler description.
All the possible reactions must actually occur, so that reactions
(1)–(3) must be at equilibrium. As shown in Fig. 3, where the
equilibrium of each single reaction is assumed, the vapor pressure
of reaction (1) is higher than that of reaction (3). This implies that
the reverse reaction of reaction (3) is possible, so that the activity
of Si(l) becomes 1. Three phases—SiC(s), SiO2(l), and Si(l)—can
coexist with the gas phase, but the gas-phase composition and
pressure are fixed by the temperature (the phase rule). In the
present case, we have only two species, SiC and SiO2, initially
under external atmospheric pressure (101 kPa). With this obser-
vation and by equilibrating reactions (1) to (3), we obtain the
equilibrium vapor pressure of the system, as shown in Fig. 4,
which is a function of temperature only. There are two extremes
for the reaction of SiC and SiO2: one is a carbon-saturated
extreme, where the carbon activity is 1; and the other is a
Fig. 2. Equilibrium vapor pressures of gas species on molten mullite with
carbon presence.
1950°C. After the lid was open, continuous SiO2 consumption was
attributed to the reaction with carbon from the heating element,
which left the Al2O3 phase in the sample. This result could be
explained by gaseous CO with much higher equilibrium partial
pressure over molten mullite in a carbonaceous environment, as
shown in Fig. 2.
(2) Reaction between SiC and Mullite
Although it has been well documented that SiO2 reacts with SiC
at high temperatures, we have attempted infiltration of mullite into
the SiC preform, hoping that the lower activity of SiO2 in mullite
might result in a successful infiltration, at least just above the
Table II. SiC Preform Infiltrated with Al2O3 ؉ SiO2 and Mullite Powders
Raw material
Preform
Temperature (°C)
Time (min)
XRD results
Al2O3 ϩ SiO2 powder
SiC
SiC
SiC
SiC
SiC
SiC
1830
1850
1875
1900
1900
1950
10
10
10
10
15
10
SiC, Al2O3, mullite, silicon
SiC, Al2O3, silicon, mullite
SiC, Al2O3, silicon
SiC, Al2O3, silicon
SiC, silicon, Al2O3
SiC, silicon, Al2O3
Mullite powder
SiC
SiC
SiC
SiC
SiC
1830
1850
1875
1900
1950
10
10
10
10
5
SiC, Al2O3, mullite, silicon
SiC, Al2O3, silicon, mullite
SiC, silicon, Al2O3
SiC, silicon, Al2O3
SiC, silicon, Al2O3

January 2003
Fabrication of Silicon Carbide–Mullite Composite by Melt Infiltration
41
Fig. 3. Equilibrium total vapor pressures of several reaction schemes
between SiC and SiO2.
Fig. 4. Equilibrium total pressure and gas composition of SiC–SiO2
system (the activity of silicon is specified to be 1 for the calculations).
silicon-saturated extreme, where the silicon activity is 1.13,14 In the
case of calculations for Fig. 4, the activity of silicon is specified to
be 1, because possible carbon impurity from raw materials has
been removed and silicon has been observed in our experiments.
The gas composition is important and also is depicted in Fig. 4 in
the SiO:CO ratio of partial pressure (PSiO:PCO). The total pressure
is Ͼ101 kPa at ϳ1810°C, so that, above this temperature, an
extensive reaction proceeds under an external pressure of 101 kPa.
(Actually, the involved reactions are all endothermic, and the
temperature is the invariant point where the three condensed
phases coexist at the gas-phase pressure of 101 kPa, so that the
reaction is almost completed at the temperature.) Therefore, the
gas-phase composition is approximately 2SiO(g) ϩ CO(g) at
temperature, as shown in Fig. 4. The near equilibrium reaction,
therefore, must proceed without violating the ratio and equilibrium
of the overall system. Therefore, the reaction is approximated by
reaction occurred at ϳ1840°C, only a little higher than that for
pure SiO2, which was well within experimental uncertainty.
Because of this marginal stability of mullite with SiC, a minor
mullite phase should have been observed in the experiments at
1830° and 1850°C.
(3) Infiltration of Mullite into the SiC Preform
To suppress the reaction described above, a load was applied to
the BN lid to force the system to remained closed. The gas pressure
inside was obtained from Fig. 4, and a sufficiently higher load than
this was applied. The experimental details and the results are
shown in Table IV. As shown in Table IV, mullite could be
successfully infiltrated into a porous SiC preform under such
conditions, and only small bubbles were identified in the melted
infiltrant. The XRD analysis of the obtained composite indicated
no phase presence other than SiC and mullite. The calculation
indicated that only 0.36 wt% of the total SiO2 was consumed
because of the reactions with SiC to form gaseous SiO and CO.
The amount of formed silicon was too little to be detected using
XRD.
If mullite melted nonhomogeneously to form solid Al2O3
precipitates and liquid aluminosilicate, as the phase diagram
indicates,15 the aluminosilicate melt must have infiltrated into the
SiC preform, resulting in a SiC–aluminosilicate composite. The
present result appeared to imply that mullite melted homoge-
neously under the present conditions. Therefore, the melt was
presumably metastable.
2SiC͑s͒ ϩ 3SiO2͑l͒ 3 Si͑l͒ ϩ 4SiO͑g͒ ϩ 2CO͑g͒
(4)
which is the only reaction scheme that meets the necessary
conditions. When a higher external pressure is applied, the net
reaction proceeds at higher temperature, according to a different
reaction scheme (with lower PSiO:PCO).
Reaction (4) was used to reanalyze the present experimental
results. For experiment A, excess SiO2 was supplied compared
with that of reaction (4), which then reacted with the produced
silicon to form gaseous SiO by reaction (3), leading to the total
reaction (1). For experiments B and C, the weight losses according
to reaction (4) were calculated to be 56 and 89 wt%, respectively,
which were in close agreement with the experimental results of 55
and 91 wt%, respectively. These results also indicated that the
reactions in our experiments occurred as a near-equilibrium
process.
As was expected from the low viscosity of the molten mullite
(Ͻ0.2 Pa⅐s)15 and its good wettability, almost full infiltration was
achieved within 10 min for the present preform dimensions (13
mm ϫ 10 mm ϫ 6 mm). Figure 5 shows the microstructure of the
composite obtained by infiltration at 1900°C. The absence of fine
SiC grains indicated that dissolution–reprecipitation of SiC
through molten mullite occurred to form a coarser SiC network.
This indicated that sintering shrinkage occurred after infiltration
Using the assumed activity of SiO2 in mullite to be that of the
concentration as before, a similar calculation was conducted for
the reaction between SiC and mullite. The result indicated that a
Table III. Experiments A, B, and C and Their Results†
Experiment A
Experiment B
Experiment C
Mole ratio of SiC:SiO2
1:2
2:1
2:3
Weight of sample before reaction (g)
Weight of sample after reaction (g)
Weight lost of sample (g)
1.00
0.01
0.99
0.88
0.40
0.48
1.30
0.12
1.18
Percent of weight lost of sample
Observed
Calculated from reactions (3) and (4)
Phase composition of sample
99
55
56
91
89
Silicon
100
Silicon
Silicon, SiC
†Raw materials were mixtures of SiC and SiO2 powders and the reaction condition was a 10 min hold at 1900°C.

42
Journal of the American Ceramic Society—Tian and Shobu
Vol. 86, No. 1
Table IV. Mullite Formation and SiC Infiltration Using
Al2O3 ؉ SiO2 Powder as the Raw Materials in a
Closed System
evaporation, which left Al2O3 in the sample. When molten mullite
was infiltrated into the SiC preform, an extensive reaction between
SiC and SiO2 was observed, and only a small amount of mullite
was found in the infiltrated sample at the lowest temperatures
investigated. The relevant reaction scheme was discussed in detail
and a forced closed system was adopted to suppress the reaction.
A dense SiC–mullite composite was then successfully obtained.
Preform
Loading (kg)
Temperature (°C)
Time (min)
XRD results
8
8
1900
1900
10
10
Mullite
SiC, mullite
SiC
Acknowledgments
The authors thank the National Institute of Advanced Industrial Science and
Technology (AIST), Kyushu Center, for the performance of the experiments. The
authors are also grateful to Dr. N. S. Jacobson for his suggestion on the thermody-
namic calculations and analyses of the paper.
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