Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
52536
The present invention relates to a sintered silicon
carbide. More particularly, the present invention relates
to a dense sintered silicon carbide having high strength
obtained By pressureless sintering such as normal sinter-
ing.
Silicon carb~de is well known as a useful ceramic
source having high hardness, excellent wear resistance, a
low thermal expansion coefficient, a high decomposition
temperature, h~gh oxidation resistance and chemical resis-
tance, and a relatively high electrical conductivity. A
dense sintered silicon carbide has these characteristics
and also has high strength even at high temperature, high
heat shock resistance, and is considered to be an effec-
tive source for high temperature structural products andto be useful for various uses, such as in gas turbines.
Silicon carbide has relatively high covalent bond whereby
it is difficult to sinter silicon carbide by itself. In
order to obtain a dense sintered product, it is necessary
to incorporate a certain sintering additive. In the hot
press process, boron, B4C, aluminum, AQN or AQ2O3 has been
used as a sintering additive. In, for example, U.S. Patent
No. 3,836,673, a sintered silicon carbide having a strength
of 104 psi (70 kg/mm2) is obtained by the hot press process
with the addition of 0.5 to 5 wt.% of aluminum. However,
the use of metallic aluminum and the hot press process have
the disadvantages given hereafter.
~'
..
,. - -
:- - .
.
.
2536
In the pressureless sinte~ing, it is known to incor-
porate aluminum and carbon. In, or example, U.S. Patent
No. 4,23Q,479, a yroduct having a high flexural strength
is obtained by pressureless sintering of a mixture of
silicon carbide, 0.2 to 2 wt.% of an aluminum component
and 0.1 to 2.0 wt.% of a carbon component. In this pro-
cess, aluminum is mainly used and carbon is added for accel-
eration of the sintering. As the carbon source, a resin
is used and hardening of the resin causes problems in the
process. Moreover, the conventional pressureless sintered
product has not been satisfactory with respect to its
character~stics and process.
The present invention provides a sintered silicon
carbide having superior characteristics with respect to
conventional products, produced by a pressureless sintering
process avoiding a hot press process.
According to the present invention there is provided
a sintered silicon carbide ceramic having high strength
and a flexural strength of at least 25 kg/mm2 at room temp-
erature and 1400C which is obtained by pressureless sin-
tering of a mixture of 0.5 to 35 wt.% of aluminum oxide
and a remainder portion essentially of silicon carbide.
- 3
2536
It is known to use aluminum oxide as a sintering addi-
tive for silicon carbide. However~ the characteristics and
the production of the convent~onal products are quite dif-
ferent from those of the present invention. One type of
conventional product is refractory ~ire brick as disclosed
in U.S. Patent No. 3,759,725 and British Patent No. 1,460,635
and is obtained by molding a mixture of coarse aggregate
of silicon carbide and aluminum oxide and then sintering
the molded product at about lZ00 to 1500C. The sintered
product comprises a thick layer of aluminum oxide or sili-
con oxide disposed around silicon carbide grains.
It has also been proposed in J. Am. Ceram. Soc. 39(11)
386-389, 1956 by Alliegro et al to use aluminum oxide as
a sintering additive in the hot press process to produce a
dense sintered silicon carbide ceramic.
It was initially considered that the effect of aluminum
oxide on silicon carbide in the hot press process is similar
to that in the pressureless sintering process. It was also
considered that the sintered product obtained by the hot
press process has higher density and strength than the
sintered product obtained by conventional pressureless
sintering when the same molded product made of a mixture
having the same formulation is sintered at the same temp-
erature by the hot press process and the pressureless process
because of the pressure effect. However, it has now been
found that a sintered product having higher strength is
obtained by the pressureless sintering process compared
with the hot press process.
L~
3~52536
As shown in the examples, the strength at high temperature
is different to a great extent. It is considered that the
sintering mechanism ~n the pressureless sintering process
is different from that in the hot press process. The
microstructure of the resulting sintered product is also
different.
In the hot press process, pressure is applied in the
liquid phase comprising aluminum oxide as a main component
to easily form a dense structure. However, the sintered
product has a microstructure having equiaxed grains (block-
like) with aluminum oxide disposed around the silicon car-
bide grain. At high temperature, aluminum oxide disposed
around the silicon carbide grains softens causing a serious
decrease in strength. However, in the pressureless sin-
tering process, the sintering mechanism is unknown and it
is considered that the desired grain growth of silicon
carbide results from the presence of sufficient liquid
phase comprising aluminum oxide as a main component during
' the sintering and the decomposition and evaporation of the
components comprising aluminum oxide as a main component
result and aluminum oxide contributed to the dense structure
is separated from the molded product to form a strong
microstructure intertwining grown prismatic or plate-like
grains.
According to the observation through an electron micro-
scope, aluminum oxide grains are often found among silicon
car~ide grains in the sintered product but a second phase
of aluminum oxide is not found at most of the grain boun-
dary between the silicon carbide grains. In some of the
sintered products, aluminum oxide is not found.
~Z536
A desired structureof the sintered product obtained in
the present invention is of the illtertwined ~- and ~-SiC
grains compristng fine prismati~c or plate-like B-SiC grains
as a main component. In most sintered products obtained
in the present invention, ~-SiC is 80-10 vol% and ~-SiC
is 20-90 vol%. It is found that such structure is obtained
by pressureless sintering of a fine ~-SiC powder having a
specific surface area of at least 10 m2/g as a source of
the strong structure. When ~-SiC is used as a source, it
usually provides the structure of block-like SiC grains as
a main component.
The sintered product having high strength at high
temperature obtained by the hot press process using aluminum
oxide as a sintering additive is disclosed in Example 34
of U.S. Patent No. 3,520,656. The structure of the sintered
product is that of the hot press process. The strength
of the sintered product is not satisfactory at temperatures
higher than 1200aC and the disadvantages of the hot press
process are not overcome.
It is disclosed in U.S. Patent No. 3,998,646 to obtain
a sintered silicon carbide having high strength. It is
not clear, whether the process is a hot press process or
a pressureless sintering process, and the necessity of an
addition of aluminum oxide is not disclosed. A sintered
silicon carbide ceramic having the characteristic structure
is not disclosed.
This shows that the sintered silicon carbide ceramic
of the present invention is different from the conventional
silicon carbide-aluminum oxide refractory, the silicon
carbide ceramics obtained by the hot press process using
aluminum oxide additive or the silicon carbide ceramic ob-
tained by the conventional pressureless sintering process.
D
36
The sources and the production of the present invention
will be illust~ated in detail,
The silicon carbide CSic~ source can be both of ~-form
and ~-form though ~-form is preferable as described above.
Th~ pur~ty IS most preferably at least 98% and is more pre-
ferably at least 95~ though it can be 90 to 95% for prac-
tical use. The particle size of the fine grains is usually
shown by a specific surface area rather than an average
partic e diameter.
In order to obtain a sintered product having a flexural
strength of at least 25 kg/mm2, especially at least 30 kg/mm2,
at room temperature and l~OO~C and a density of at least
3.0 as at least 90% of theoretical density, it is necessary
for the source to have a specific surface area of at least
5 m2/g. In order to obtain a sintered product having a
flexural strength of at least 35 kg/mm at room temperature
and 1400C, it is preferable for the source to have a
specific surface area of at least 10 m2/g in the case of
6 to 35 wt.% of aluminum oxide and at least 15 m2/g in the
case of 0.5 to ~ wt.% of aluminum oxide and the flexural
strength of at least 40 kg/mm .
The aluminum oxide (AQ2O3) source is preferably corrun-
dum ~-AQ2O3, but can be another crystalline material, such
as the y-form. Aluminum sources, such as aluminum hydroxide
and aluminum sulfate, which can be converted into aluminum
oxide by heating in a non-oxidative atmosphere can also be
used. Thus aluminum oxide also includes a precursor which
easily forms aluminum oxide. Aluminum oxide preferably
has a purity of at least 98%, a low sodium content and an
average particle diameter of up to l~m, preferably up to
0.2~m.
~2536
In the process of the present invention a mixture of
the aluminum oxide source `~Q.5 to 35 wt.% as AQ2O3) and
silicon carbide is used, It is possible to incorporate a
small amount of anotfier aluminum source, such as aluminum
nitride (AQN), aluminum carbide (AQ4C3), aluminum diboride
(A~B2), aluminum phosphide (A~P), aluminum silicon carbide
(AQ4SiC4) and aluminum (AQ).
The amount of the aluminum oxide source relative to
the total of aluminum oxide and silicon carbide is in the
range of 0.5 to 35 wt.% as A~2O3. When it is less than
0.5 wt.~, the dense structure is not formed in the sinter-
ing and a dense sintered product having a density of at
least 90% of the theoretical density cannot be obtained.
When it is more than 35 wt.%, the dense structure is formed
but the strength is too low even though it is sintered at
a lower temperature of up to 1900C. When it is more than
35 wt.% and the molded product is sintered at 1900 to
2300C, the decomposition is severe to form a porous
structure and undesired aluminum oxide remains in the
sintered product. The content of the aluminum oxide source
is preferably in the range of 2 to 20 wt.% as AQ2O3.
When the content of the aluminum oxide source is in a
range of 6 to 35 wt.%, and silicon carbide having a speci-
fic surface area of at least 10 m2/g and a purity of at
least 95%, preferably 98~, is used, it is easily possible
to obtain a sintered product having a density of at least
90% of the theoretical density and a flexural strength of
at least 35 kg~mm2 at room temperature and 1400C.
-- 8 --
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~2~36
When the content of aluminum oxide is in the range of 0.5to 5 wt~ and silicon carhide having a specific sur~ace
area of at le~St 15 m2~g and a pur~ty of at least ~8~ is
used, it is easily possible to obtain a sintered product
having a density of at least ~0% of the theoretical density
and a flexural strength of at least 40 kg/mm2 at room temp-
erature and 1400C.
In the present invention, it is preferable to prepare
a mixture of aluminum oxide source and the substantial resi-
dual portion of silicon carbide. It is one of the advan-
tages of the process of the present invention to be able
to use such sources. It is possible to contain impurities
of the silicon carbide source and a small amount of other
components incorporated in a pulverizing step. As des-
cribed below, certain components, such as silicon oxide,
can he incorporated at a relatively large amount. This
is one of the advantages of the process of the present
invention.
All molding processes for molding ceramics can be em-
ployed in the molding step. A press molding process, a
slip casting process, an injection molding process and an
extrusion molding process can be employed. The sintering
is carried out in a non-oxidative atmosphere under pres-
sureless conditions at 1900 to 2300C. The non-oxldative
atmosphere is suitably selected from an atmosphere of nitro-
gen, argon, helium, carbon monoxide and hydrogen. It is
especially preferable to have an atmosphere of argon and
helium. As mentioned below, the treatment in the atmos-
phere containing the aluminum component is especially pre-
ferable. As the method of the formation of the non-oxida-
tive atmosphere, it is preferable to form the atmosphere
containing certain carbon or silicon components as well
as the aluminum component and to - -
_ g _
3~L5Z536
place the molded product made of the aluminum oxide source
and silicon car~ide in s~ch an atmosphere The temperature
fo~ sinterin~ i`s pre~erafily in the ran~e of 1950 to 2100C.
When the sintering temperature is lower than 1900C, the
density is not satisfactory and a desired dense sintered
product cannot be obtained, whereas when it is hi~her than
2300QC, the molded product is decomposed yielding a porous
product. The sintering time is in the range of 1 to ~8
hours, preferably 2 to 24 hours. When the sintering time
is too short, the dense structure is not formed, or a sat-
isfactory strength is not obtained even though the dense
structure is formed. When the sintering time is too lon~,
the decomposition is too great and a porous product is
disadvantageously formed.
The desired process for producing the sintered product
in the present invention will no~ be illustrated. The
aluminum oxide source as AQ203 is incorporated as a sinter-
ing additive. In the conventional process, the aluminum
oxide source is rapidly decomposed or evaporated to be re-
moved before contributing to the densifying of the molded
product. Therefore, a satisfactory density is not attained
and a dense sintered product is not obtained.
In order to overcome these problems, various tests
have been made. As a result, it is found to preferably
sinter the molded silicon carbide containing the aluminum
oxide source in an atmosphere including aluminum or aluminum
component. That is, a dense sintered product is easily
o~tained by sintering the molded product in the atmosphere
including one or more of aluminum and aluminum compounds.
In accordance with such process, an amount of aluminum
oxide removed
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B
i2536
be~ore completing the densifying of the molded product is
reduced to obtain a dense ~intered product having a stable
formulation and struc-ture. However, silicon carbide itself
begins to decompose at the sintering temperature for the
molded silicon carbide. That is, silicon carbide is not
meltea at the atmospheric pressure and begins to sublimate
at a temperature higher than 2000C and is decomposed into
carbon and silicon rich vapor at higher temperature. The
sintering temperature for producing the dense sintered
product in the process of the present invention is in the
range of l900 to 2300C. In the high temperature zone,
sublimation and decomposition of silicon carbide are caused
to generate a silicon vapor and disilicon carbide Si2C.
When the molded silicon carbide is sintered in the atmos-
phere containing the Si vapor and Si2C, the sublimationand decomposition of silicon carbide in the molded product
can be reduced. But, the decomposition of silicon carbide
is not simple in a practical operation. Thus, it results
in certain mutual reactions of the aluminum oxide source
as the sintering additive, and a silica layer of the surface
of silicon carbide grains and other impurities and a small
amount of oxygen in the atmosphere. In order to prevent
a decomposition of the molded product during the sintering
and to obtain a dense sintered product, it is preferable
to maintain a partial pressure of the gas in the atmosphere
over the equilibrium vapor pressure of the gas generated
by the dec~mposition of the molded product.
When the molded silicon carbide containing the aluminum
oxide source is sintered, it is difficult to confirm the
type of reactions and gases generated. In various tests,
it is found to be preferable to sinter
-- 11 ~
~52536
the molded silicon carbide containing aluminum oxide in the atmosphere
including aluminum and silicon and/or carbon components to obtain a
dense sintered product having uniform formulation and structure.
In the decomposition of the molded product in the sintering
it is considered to mainly result in the following reaction:
SiC + AQ203 ~ AQ20 + SiO + CO
When the partial pressure of the gases of AQ2O, SiO and
CO in the atmosphere during the sintering is more than the equilibrium
vapor pressure of the gases generated bythe decoIrpositlon of themolded
product, the decomposition of the molded product is reduced to obtain
a sintered product having higher density.
The process will be further illustrated.
The atmosphere including the ~luminum component or the
aluminum and silicon and/or carbon components is provided by feeding
the gas of these components in to the sintering furnace. The aluminum
component gas can be fed as AQ, AQCQ3, AQ2C or AQO etc.; the
silicon component gas can be fed as Si, SiCQ4, SiH4 or SiO etc. and
the carbon component gas can be fed as a hydrocarbon or CO etc.
Usually, the atmosphere is prepared by mixing the gas of these compo-
nents with a non-oxidative gas, such as nitrogen, ar~n and helium.
In another method, it is effective to place a powder or a molded
product or a sintered product for generating the gas of these compo-
nents at the sintering temperature around the molded silicon carbide.
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5316
Thus for example;
~ 1) The atmosphere is ~ormed by aluminum powder, one
or more aluminum compound powders or a molded product of
the powder which is placed around the molded silicon carbide;
(2) The atmosphere is formed by one or more of alu-
minum powders, aluminum compound powders and one or more of
silicon powders, one or more silicon compound powders, car-
bon powder, and carbon compound powders or an unsintered
molded product of the powder which is placed around the
molded silicon carbide; or
(3) The atmosphere is formed by a sintered product of
silicon carbide containing aluminum and or an aluminum
compound which is placed around the molded silicon carbide.
As the method of placing the powder around the molded
silicon carbide, the molded product is buried in the powder
or is placed in a casing having a carbon or silicon car-
bide powder coating on the inner wall. The method of
burying the molded product in the powder is preferable be-
cause the decomposition of the molded product is greatly
reduced. However, this method is not suitable for a large
molded product or a molded product having a complicated
shape. The method of coating the powder on the inner wall
of the casing is suitable for molded products having various
shapes. The surface condition of the sintered product is
superior and the dense sintered product having the charac-
teristics similar to the burying method can be obtained.
The coating method is attained by coating a slurry
of the powder and an organic medium, such as alcohol and
acetone, or water on the casing. It is possible to incor-
porate a binder, such as polyvinyl alcohol, in the slurry.
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2536
In the method of burying into the powder or coating
with the powder, the above-mentioned powders can be used.
It is preferable to use a mixture o~ aluminum or an alum-
inum co~pound and silicon carbide powder and/or carbon
powder. As the aluminum powder or the aluminum compound
powder, it is preferable to use aluminum oxide powder
though it is possible for example t~ use aluminum hydroxide,
aluminum nitride or aluminum car~ide. In the powder coat-
ing method, it is possible to use an aromatic polymer which
leaves high carbon residue, such as phenol resins, and
polymethyl phenylene instead of carbon powder.
When the silicon carbide powder and/or carbon powder
is mixed with aluminum powder or the aluminum compound
powder, an amount of the aluminum component is in the range
of 2 to 40 wt.% as AQ. When it is less than 2 wt.%, the
prevention of the decomposition of the molded product is
not sufficiently high and a desired dense sintered product
is not obtained. When it is more than 40 wt.%, the decom-
position velocity of the powder is too high and the weightloss is, disadvantageously, too great even though it has
high density.
When the molded product is buried in aluminum powder
or aluminum oxide powder, the immersion of the liquid phase
of the aluminum component is disadvantageously caused.
It is preferable to use a sintered product instead
of the powder or the unsintered molded product. It is also,
preferable to use the sintered silicon carbide containing
aluminum or the
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~2536
aluminu~ compound. In this case~ it is preferable to place
the sintered product having a similar formulation axound
the molded product for sintering though it is possible to
place a sintered product having different formulation.
When a sintered product is used, the surface area of
the sintered product is small and the decomposition velocity
is low which has the effect of maintaining the atmosphere
in a desired condition for a long time in comparison with
the use of the powder or the unsintered molded product.
This is suitable for sintering over a long period of time.
The eharacteristic feature and advantage of the present
invention are further illustrated.
1) A dense sintered silicon carbide ceramic having
high strength can be easily obtained by the pressureless
sintering process.
For example, when a sintering additive of AQ, A~N, B
or B4C is used, the hot press process is needed and a pro-
duct having a complicated shape or a large size cannot be
produced. Aluminum is easily oxidized so as to be diffi-
cult for use and reacts to cause foaming or contact with
water, and a fine aluminum powder is dangerously explosive.
However, B, B4C and AQN are expensive and fine powder there-
of is not easily available, and these materials are not
easily ground.
2) A sintered product having higher strength than
the conventional product can ~e obtained.
For example, a sintering additive, such as B+C, B4C+C,
AR+C and AQN+C, can be used for the pressureless sintering.
- 15 -
~,~
~2536
However, an aromatic pol~mer, such as polyphenylmethylene
and phenol resins, is usually used as the C source. The
handling is not easy and the C source should be uniformly
mixed re~uiring a long operating time. The strength of
the product in the case of s-C additive, is relatively
low, such as 40 to 50 kg~mm2, at room temperature. In the
case of such sintering additive, the silicon carbide source
is quite fine, such as a specific surface area of at least
15 m /g. It is necessary to use the sources containing
less SiO2 though the sources having a high SiO2 content
can be used in the present invention.
3) Aluminum oxide is stable and is not reactive with
water. A step of contacting with water can be employed.
Wet mixing, grinding and a slip casting using water can be
employed and the atmosphere is not limited.
4~ Even though the purity of the silicon carbide
source is low (even lower than 95%) or the grain size is
relatively large teven a specific surface area of less than
lOm /g), such factors do not greatly affect the sintering
process and characteristics of the product. It is unneces-
sary to remove the silicon oxide layer from the surface
of the silicon carbide powder. It is possible to add
silicon oxide.
The present invention is remarkably advantageous in
an industrial scale.
The present invention will be further illustrated, by
the following Examples and References.
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2536
EXAMP~ES 1 to 6 and RE:FERENCES 1 to 3:
In Examples 1 to 6, ~- o:r c~- silicon carbide powder and
aluminum oxide powder (corundum) having a purity higher than
95% and an average particle diameter less than l,um shown in Table 1
were thoroughly mixed with ethanol and each mixture was molded by
hydraulic isostatic pressure molding under a pressure of 2000 kg/cm2
to form a molded product having a size of 20 x 40 x 15 mm. Each
molded product was held in a carbon casing having a cover slightly :
larger than the volume of the molded product. The carbon casing
was placed in an argon gas atmosphere and the molded product was
sintered under the conditions shown in Table 1.
In References 1 to 3, each mixture was treated by a hot
press in a carbon mold having an inner diameter of 30 mm under a
pressure of 200 kg/cm2. The densities and flexural strengths of the
sintered products are shown in Table 1.
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B
.. ~ ..
. . . . .;. . . .
. ~ . .. ~. . ..
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S36
Table i
SiC Exp.l Exp~ 2 _~E~ ~ Exp. S Exp. S :~
Ma~in crystalline ~ ~ ~ ~ ~ a
Purity (%) 99< 99< 99< 99< 99< 95<
Specific surface 13 . 4 13 . 4 13 . 413 . 4 18.1 7 . 0
A Q 2 3 ~ ~
Content ( %) 25 15 3 2 3 13
Sinterin g
Temperature~C) 2000 2000 2000 2000 2000 1950
Time (hr. ) 5 5 5 5 5 5
Density (g/cm3) 3. 06 3.11 3.13 3.14 3.18 3.13
Flexural strength
(kg/mm2)
Room temperature 76 . 7 87 . 9 61. 6 45 . 6 65 . 1 56 . 2
1400C 44.7 53. 0 34.3 32.0 46. 1 38. 7 :
- 18 -
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~5;~536
Table 1 (cont'à)
¦ Ref. 1 Ref. 2 Ref. 3
sic
Main crystalline ~ ~ ~
Purity (%) 99< 99< 99<
Speci~lc surface 7 4 13. 4 7. 4
area (m2/g) .
A~?~
Content (96) 13
Sintering
Temperature (C) 1950 2100 2200
Time (hr. ) 1
Density (g/cm3) 3.19 3, 20 3.19
Flexural strength
(kg/mm2)
Room temperature 43 . 0 54. 9 77. 0
1400C 15. 8 19. 3 43. 2
- 19 -
~ . ~
:~:
S3~i
The samples were respectiYely cut and the structures
of the cut sur~aces of the samples were obseryed under a
microscope~ The results are a~ ~ollows.
Samples l, 2~ ~ and 5:
A microstructure uni~ormly intertwining prismatic or
plate-like SiC grains having a long axis of about 2-5~ which
comprises fine SiC grains.
Sample 4:
The same as Samples 1, except containing, large grown
grains and having slightly non-uniform grain sizes.
Sample 6:
r
A microstructure closely bonding SiC grains in a block-
like manner having a diametex of l to 5~.
EXAMPLES 7 to 16:
~ ilicon carbide powder having a purity of 99% and a
specific surface area of at least 10 m2/g (commercially
available) was used. Each sintering additive shown in Table
2 was mixed in the amount shown in Table 2. Each mixture
was charged in a plastic pot and thoroughly mixed with
plastic balls in the presence of acetone and the mixture
was dried and pressed under a pressure of 300 kg/cm to
30 prepare tne molded product having a size of 20 x 20 x 40 mm.
Each molded product was sintered in an electric resistance
furnace under the conditions shown in Table 2 at 2000C
for 3 hours.
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~52S36
Table 2
Exp. 7 Exp. 8 I Exp. 9Exp. 10Exp . 11
Sintering addi~ive
Kind AQ2O3 AQ2O3 AQ2O3 AQ2O3 AQ2O3
Content (wt . %)4 4 4 4 4
Atmosphere
Method Bury Coat*3 Coat Coat Coat
Kind AQ2O3 20 AQ2 3 AQ2O3 30AQ2O3 40 AQ2O3 30
C 80 100 SiC 70SiC 60 resin30
Content (wt. %) SiC 40
Density (g/cm3) 3.16 3. 06 3 14 3. 09 3 .13
Flexural stren gth .
(kg/mm2)
Room temperature 70 . 2 58. 3 69. 4 52 . 6 64 . 7
1400C 41. 5 38. 1 40 . 3 41. 5 39. 7
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31~
Table 2 (c~nt'd)
Exp. 12 Exp. 13 Exp. 14 Exp. 15 Exp. 16
_
Sintering additive
Kind 2 3 AQ2O3 AQ2O3 AQ2O3 ~Q23
Content (wt. %) 4 6 9 12 1
Atmosphere *~
l 0 Method Coat pMrodd . B ury B ury B ury
Kind AQ2O3 50 AQ2O3 20 AQ2O3 10 AQ2O3 10 AQ2O3 lQ
Content (wt . ~ ) SiO2 50 SiC 80 SiC 90 iC 90 SiC 90
_
Density (g/cm3) 3.04 3.14 3.12 3.11 2.92
Flexural strength
(kg/mm2)
Room temperature 50.5 75.4 72.8 fi9.3 27.1
1400C 35.8 45.1 42.9 ~7.3 25.3
Note: *1 ~-SiC was used as the silicon carbide source
(the other sources are ,B-SiC)
*2 Burying method: A molded product was buried in a
- mixed powder.
(the kind and content are shown in the
Table)
*3 Coating method: A slurry of the mixed powder and
~ ethanol was coated on an inner wall of
a carbon casing and was dried.
The molded product was held therein.
me coated thickness was akout 0.5 rr,m.
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,
.. .. . ~ .
, -
~152536
*4 Molded product: A molded product was placed in anunsintered molded product casing made
of the mixed powder.
The structures of the cut surfaces of the sintered products
are as follows:
Samples 7 to 9 and 11 to 15:
The same as Sample 1.
Sample 10:
The same as Sample 6.
Sample 16:
The same as Sample 4.
~5 ~:
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