Note: Descriptions are shown in the official language in which they were submitted.
RD-7361
1 334677
m e chemical and physical properties of silicon
carbide make it an excellent material for high temperature
structural applications. However, silicon carbide is produced
in the form of particles or powder from which dense bodies
must be formed and it is the formation and properties of these
dense bodies that have presented problems.
Hot pressing of silicon carbide powder has been
used to produce small dense bodies under closely controlled
conditions. However, hot-pressing methods require high
pressures and temperatures necessitating expensive energy
consuming machinery equipped with graphite dies. Also,
hot-pressing yields pressed bodies in the form of billets
of simple geometric shape only which require time-consuming
machining to produce a complex shaped part.
Silicon carbide powder alone is not sinterable.
However, in my Canadian application Serial No. 208,705, filed
September 9, 1974, there is disclosed a method of producing
a ~ -silicon carbide sintered body by forming a mixture of
submicron powder composed of ~-silicon carbide, a boron
additive and free carbon into a green body and sintering it
at a temperature of about 1900-2100 C.
me onset of exaggerated growth of large tabular
~-silicon carbide crystals on densification of ~-silicon
carbide powders doped with boron is a limitation to obtaining
the uniform fine-grained microstructures necessary to withstand
fracture, especially at temperatures of the order of about
2000C. m is phenomenon is related to the transformation of
~-silicon carbide into the thermo-dynamically more stable
o~Sic phase at temperatures of about 2000C and higher.
While several means to suppress this kind of
grain growth on hot pressing have been devised, none of them
is currently applicable to sintering. m us, for instance,
_ 1 334677 RD-7361
when the hot-pressing temperature is decreased and compensated
for by increased pressing pressure, conditions may be found
where exaggerated grain growth does not occur. Also various
additions such as aluminum, silicon nitride, aluminum nitride,
and boron nitride to ~-SiC powder have been proven effective
in controlling the growth of tabular ~ crystals on hot-pressing:
these means, however, cannot be used for sintering as they
interfere with the densification process and prevent obtaining
high densities.
The present process relates to improved grain growth
control on sintering of silicon carbide by transforming a
substantial mass of the sintering ~-SiC powder to the a form,
i.e. into the thermodynamically more stable form, by seeding
with an addition of ~-SiC. The ~-SiC nuclei thus provided
induce a rapid ~ to ~ transformation during sintering. m e
growing ~-SiC grains impinge on each other early in their
development, cease to grow and result in a sintered product
with a substantially uniform, relatively fine-grained micro-
structure wherein at least 70% by weight of the silicon carbide
present is composed of ~-SiC in the form of platelets or elon-
gated grains which may range in the long dimension from about
5 to 150 microns, and preferably from about 5 to 25 microns.
m e present invention provides a number of advantages.
One advantage is that since the present process provides grain
growth control, sintering can be carried out through a wide
temperature range which is particularly economical and
practical since it eliminates the need for critical temperature
controls. The second advantage is that the presentsintered
produce has a shape and mechanical properties which do not
change significantly through a wide temperature range, i.e.
temperatures ranging from substantially below 0C to temperatures
higher than 2300C.
1 334677 R~7361
-
m ose skilled in the art will gain a further and
better understanding of the present invention from the
detailed description set forth below, considered in conjunction
with th~ figures accompanying and forming a part of the
specification, in which:
FIGURE 1 is a photomicrograph (magnified 500 X) of
an etched specimen produced according to the present invention
but without the addition of ~-SiC, sintered at 2080C
illustrating the uniform microstructure of ~-SiC.
FIGURE 2 is a photomicrograph (magnified 500X)
of an etched specimen of the same initial composition as
FIGURE 1 sintered at 2150C showing a feather-like morphology
of large grains of ~-SiC polytypes in a matrix of ~ -SiC.
FIGURE 3 is a photomicrograph (magnified 500 X)
of an etched specimen prepared in accordance with the present
invention sintered at 2175C showing a substantially uniform
microstructure of ~-SiC.
Briefly stated, the process of the present invention
comprises providing a substantially homogeneous particulate
dispersion or mixture, wherein the particles are submicron
in size, of ~-silicon carbide powder, ~-silicon carbide
seeding powder, boron additive and a carbonaceous additive
which is free carbon or a carbonaceous organic material which
is heat-decomposible to produce free carbon, shaping the
mixture into a green body, and sintering the green body at a
temperature ranging from about 1950C to 2300 C in an
atmosphere in which the green body and resulting sintered body
having a density of at least 80% of the~theoretical density
for silicon carbide and a significantly uniform microstructure
wherein at least 70% by weight of the silicon carbide is ~-SiC.
In the present invention single phase ~ -silicon
carbide powder is used having an average particle size ranging
RD-7361
1 334677
,
up to about 0.45 micron, and generally from about 0.05 micron
to 0.4 micron. A~ a practical matter and for best results
the ~-SiC powder preferably ranges in size from an average
particle size of about 0.1 micron to 0.2 micron. ~-silicon
carbide powder of this size can be prepared by a number of
techniques as, for example, by direct synthesis from the elements,
by reduction of silica, or by pyrolysis of compounds containing
silicon and caxbon. A number of processes which involve the
pyrolysis of silicon compounds and organic compounds to produce
silicon and carbon are particularly advantageous since they
can be controlled to produce ~-silicon carbide of desired
submicron particle size composed mainly of isolated crystallites.
Hot plasma techniques are especially preferred for producing
the powders useful in the present invention. The final product
generally requires leaching, especially with acid to remove
any silicon which may be present, to produce a sinterable
phase pure ~-silicon carbide powder.
In the present process, to achieve the desired
grain growth control, the particle size of the a-SiC seed
powder should be at least about twice as large as the average
particle size of the ~-SiC. Also, the ~-SiC seed powder is
always submicron in size, and generally has a particle size
ranging from about 0.1 micron to about 0.6 micron. All
polytype compositions of the ~-SiC are operable in the present
invention.
The present fine-size ~-SiC can be prepared by a
number of techniques. For example, abrasive grade silicon
carbide, which is always totally ~-SiC, can be milled and
the milled powder admixed with a liquid,such as water to
separate fractions of large and fine-sized particles by
se~i -~tation. Specifically, the large-sized particles are
allowed to settle and the liquid in which the desired
1 334677 R~7361
finer-sized particles float is decanted and evaporated to
yield the fine-sized, submicron particle fraction.
The ~-SiC powder is used in amounts ranging from
about 0.05% by weight to 5% by weight based on the ~-SiC. The
larger the amount of ~-SiC used, the lower is the density of
the sintered product. Amounts of a-SiC ranging from 1% by
weight to 3% by weight based on the ~ -SiC produce the finest
and most uniform microstructures. Amounts of ~-SiC powder
larger than 5% by weight are likely to produce a sintered pro-
duct having a density lower than 80%, and amounts of ~-SiC
smaller than 0.05% by weight do not provide sufficient nuclei
for grain growth control.
The d-sic powder is admixed with ~-SiC powder alone,
or with ~-SiC powder containing the boron additive and/or
carbonaceous additive to produce a homogeneous dispersion.
Specifically, the 0~SiC should be dispersed through ~ -SiC
powder substantially uniformly in order to produce a sintered
product with the desired uniform microstructure.
The~-SiC powder can be admixed with the ~-SiC
powder by a number of techniques such as, for example, ball
milling or jet milling, to attain the necessary uniform
distribution and produce a substantially homogeneous dispersion.
One technique for introducing ~-SiC powder into
~-SiC powder utilizes milling balls formed of silicon
carbide containing C~-SiC in significant amount, i.e. at least
10% by weight. In this technique ~ -SiC powder is milled with
the SiC balls which introduce d-SiC seeds by wear due to
milling. Milling is preferably carried out in a liquid
dispersion. The a~ount of -~-SiC introduced is controlled
by controlling milling time. Introduction of the proper amount
of ~ -SiC is determinable empirically. For example, in
accordance with the present process, the resulting powder
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1 334677
can be sintered and the product sectioned and ~x~mined
metallographically. The proper amount of ~-SiC has been
introduced in accordance with the present invention when the
product has a significantly uniform microstructure, contains
~-SiC in an amount of at least 70% by weight of the total
amount of SiC and has a density of at least 80% of the
theoretical density for SiC.
m e boron additive in the powder mixture from which
the green body is shaped is in the form of elemental boron or
boron carbide. In order to obtain significant densification
- ~ during SinterinQ, the amount of boron additive is critical
and is e~uivalent to about 0.3% to about 3.0% by weight of
elemental boron based on the total amount of silicon carbide.
The particular amount of boron additive used is determinable
empirically and depends largely on the degree of dispersion
achieved in the mixture since the more thoroughly it is dispersed
the more uniform is the density of the sintered product.
However, amounts of elemental boron below 0.3% by weight do
not result in the necessary degree of densification whereas
amounts of elemental boron greater than 3.0% by weight produce
no significant additional densification and may deteriorate
the oxidation resistance of the product. During sintering,
the boron additive enters into solid solution with the
silicon carbide. In addition, generally when amounts of the
additive in excess of that e~uivalent to about 1% by weight
of elemental boron are used, a boron carbide phase also
precipitates.
The carbonaceous additive is used in an amount
equivalent to 0.1% by weight to 1.0% by ~eight of free carbon
based on the total amount of silicon carbide. Specifically,
the carbonaceous additive is particulate free carbon of
submicron size such as, for example, acetylene black, or a
..
RD-7361
1 334677
carbonaceous organic material which is heat-decomposible to
produce particulate free carbon of submicron size in the
re~uired amount. In addition, the carbonaceous organic material
is a solid or liquid at room temperature and completely
decomposes at a temperature in the range of about 50C to
1000C to yield free carbon and gaseous products of decomposi-
tion. Also, the carbonaceous organic material is one which has
no significant deteriorating effect on the silicon carbide,
boron additive or the resulting sintered product.
To produce the present sintered product having a
density of at least 80%, the oxygen content of the silicon
carbide powder should be less than 1% by weight of the
total amount of 9 ilicon carbide used, and preferably, less
than about 0.4% by weight. m is oxygen content is determinable
by standard techniques and generally, it is present largely in
the form of silica.
The function of free carbon in the present process,
is to reduce silica which always is present in silicon carbide
powders in small amounts or which forms on heating from oxygen
absorbed on the powder surfaces. The free carbon reacts
during heating with silica according to the reactions:
SiO2 + 3C ~ SiC + 2C0. Silica, when present in the SiC
powders in appreciable amounts, halts densification of silicon
carbide completely so that little or no shrinkage, i.e.
densification, is obtained.
The free carbon also acts as a getter for free
silicon if present in the powders or if it is formed by the
following reaction during heating up to the sintering
temperature: SiO2 + 2SiC ~ 3Si + 2C0. The presence of
silicon, just as the silica, tends to halt or retard
densification of SiC.
The specific amount of submicron free carbon
1 334677 RD-7361
required in the present process depends largely upon the
oxygen content in the starting SiC powder and ranges from
about 0.1% to 1.0% by weight of the total amount of silicon
carbide used. Specifically, green bodies of the present
invention which contain about 1% by weight of free carbon
that do not sinter also will not sinter with amounts of free
carbon significantly in excess of 1% by weight to a density
of at least 80%. Also, amounts of free carbon significantly
in excess of 1% by weight function much like permanent pores
in the sintered product limiting its ultimate achievable
density and strength.
Free carbon in the form of a submicron powder
can be admixed with the silicon carbide powder by a ~u~ber
of conventional techniques such as, for example, jet milling
or ball milling in a liquid dispersion.
In carrying out the present process, the carbo-
naceous organic material can be introduced by a number of
techniques and heat-decomposed before or after the green body
is formed. If the carbonaceous organic material is a solid,
it is preferably admixed in the form of a solution with the
silicon carbide powder and boron additive to substantially
coat the particles. The wet mixture can then be treated to
remove the solvent, and the resulting dry mixture can be heated
to decompose the carbonaceous organic material producing free
carbon in situ before the mixture is formed into a green body.
If dèsired, the wet mixture can be formed into a green body and
the solvent removed therefrom. In this way, a substantially
uniform coating of the organic material on the silicon carbide
powder is obtained which on decomposition produces a uniform
distribution of free carbon. The green body is then heated
to decompose the carbonaceous organic material to produce free
carbon in situ and diffuse away gaseous products of decomposition
before sintering initiates. The solvent can be removed by a
RD-7361
1 334677
number of techniques such as by evaporation or by freeze drying,
i.e. subliming off the solvent in vacuum from the frozen
dispersion. Likewise, if the carbonaceous organic material
is a liquid, it can be admixed with the silicon carbide powder
and boron additive, and the wet mixture heated to decompose
the organic material and form free carbon, or the wet mixture
can be formed into a green body which is then heated to
decompose the organic material to form free carbon in situ
and diffuse away gaseous products of decomposition. The heat-
decomposition of the carbonaceous organic material should be
carried out in an atmosphere in which the components being
heated are substantially inert or which has no significant
deteriorating effect on the components being heated such as
argon or a vacuum. Preferably, the carbonaceous organic
material in the green body is heat-decomposed in the sintering
furnace as the temperature is being raised to sintering
temperature.
High molecular weight aromatic compounds are the
preferred carbonaceous organic materials for making the carbon
addition since they ordinarily give on pyrolysis the required
yield of particulate free carbon of submicron size. Examples
of such aromatic compounds are a phenol-formaldehyde condensate-
novolak which is soluble in acetone or higher alcohols, such
as butyl alcohol, as well as many of the related condensation
products, such as resorcinol-formaldehyde, aniline-formaldehyde,
and cresolformaldehyde. Another satisfactory group of compounds
are derivatives of polynuclear aromatic hydrocarbons contained
in coal tar, such as dibenzanthracene and chrysene. A
preferred group of carbonaceous additivès- are polymers of
aromatic hydrocarbons such as polyphenylene or polymethylphenylene
which a~e soluble in aromatic hydrocarbons and yield on heat-
decomposition up to 90% of free carbon.
.~
_ 9 _
1 334677 R~7361
-
Another approach to improved carbon distribution
on a submicron particle size level is the application of jet
milling. The silicon carbide powder is soaked with a solution
of, for instance, a novolak resin in acetone,dried in air and
heated up to 500C to 800C in nitrogen to pyrolyze the resin.
The actual amount of carbon introduced by this process is
determined as weight gain after the pyrolysis or by analysis
of free carbon. The powder with the added carbon is then ~et
milled which greatly improves the distribution of carbon and
eliminates major carbon grains in the sintered product.
A number of techniques can be used to shape the powder
mixture into a green body. For example, the powder mixture can
be extruded, injection molded, die-pressed isostatically
pressed or slip cast to produce the green body of desired
shape. Any lubricants, binders or similar materials used in
shaping the powder mixture should have no significant
deteriorating effect on the green body or the resulting sintered
body. Such materials are preferably of the type which evaporate
on heating at relatively low temperatures, preferably below
200C, leaving no significant residue. The green body,
preferably, should have a density of at least 45% of the
theoretical density for silicon carbide to promote densification
during sintering and achieve attainment of the desired density
of at least 80%.
Sintering of the green body is carried out in an
atmosphere in which it is substantially inert, i.e. an
atmosphere which has no significant deteriorating effect on
its properties such as, for example, argon, helium or a
vacuum. The sintering atmosphere can range from a substantial
vacuum to atmospheric pressure.
Sintering is carried out at a temperature ranging
from about 1950 C to about 2300C. The particular sintering
-- 10 --
1 3 3 4 6 7 7 RD-7361
- temperature is determinable empirically and depends largely
on particle size, density of the green body, and final density
desired in the sintered product with higher final densities
requiring higher sintering temperatures. Also, lower sintering
temperatures would be used with sintering atmospheres below
atmospheric pressure. Specifically, the smaller the size of
the particles in the green body and the higher its density,
the lower is the required sintering temperature. Sintering
temperatures lower than 1950 do not produce the present
sintered bodies with a density of at least 80%. Temperature
higher than 2300C can be used since the present process
provides sufficient grain growth control but the use of
temperatures significantly higher than 2300C provide no
particular advantage and bring about evaporation of silicon
carbide.
The sintered body of the present invention has a
density ranging from ~0% to about 95% of the theoretical
density for silicon carbide. m e product is composed of
silicon carbide, boron or boron and boron carbide, and free
elemental carbon. The composition of the silicon carbide-
in the present product ranges from o~sic alone to a composition
composed of 70% by weight a-SiC and 30% by weight ~-SiC.
The ~-SiC is present in the form of a substantially uniform
microstructure in the form of elongated grains or platelets
which, in the long dimension, may range from about 5 microns
to about 150 microns with an average length ranging from about
10 microns to 30 microns, and preferably have a grain size of
from about 5 microns to 25 microns in the long dimension with
an average length of about 10 microns. ~he ~-SiC is present in
the form of fine grains ranging from about 1 micron to about
10 microns with an average grain size of about 3 microns. me
boron is present in an amount ranging from 0.3% by weight to 3%
RD-7361
1 334677
by weight based on the total amount of silicon carbide. The
boron is in solid solution with the ~-and ~-silicon carbides
and also may be present as a boron carbide phase in a very
fine-sized precipitated form detectable by X-ray analysis. m e
boron or boron and boron carbide are substantially uniformly
distributed throughout the sintered body. m e sintered body
also contains from 0.1% to 1% by weight of free carbon based
on the total amount of silicon carbide. The free carbon is
in the form of particles, substantially submicron in size,
which are substantially uniformly distributed throughout the
sintered body.
Since the present sintered produce has a substantially
stable microstructure, it retains its room temperature shape
and mechanical properties at high temperatures. Specifically,
the sintered product undergoes no significant change in density
or mechanical properties after substantial exposure in air to
temperatures ranging up to about 1700C and after substantial
exposure in an atmosphere in which it is substantially inert
such as argon to temperatures above 1700C ranging up to about
2300C. Such properties make it particularly useful for high
temperature structural applications such as gas turbine blades.
Although, at temperatures of 2000C or higher, ~-SiC in the
present sintered product will transform to ~-SiC, the newly
form ~-SiC grains cannot grow significantly because they
impinge on and are blocked by the substantial number of d-sic
grain already present substantially uniformly throughout
the product. As a result, any additional transformation of
~-SiC has no significant effect on shape or mechanical
properties of the product.
The present invention makes it possible to
fabricate complex shaped polycrystalline silicon carbide
ceramic articles directly which heretofore could not be
RD-7361
1 334677
- manufactured or which were produced by expensive and tedious
machining because of the hardness of the material. The present
sintered product requires no machining and it can be made in
the form of a useful complex shaped article, such as a gas
turbine airfoil, an impervious crucible, a thin walled tube,
a long rod, a spherical body, or a hollow shaped article such
as a gas turbine blade. Specifically, the dimensions of the
present sintered product differ from those of its green body
by the extent of shrinkage, i.e. densification, which occurs
during sintering. Also, the surface characteristics of the
sintered body depend on those of the green body from which
it is formed, i.e. it has a substantially smooth surface if
the green body from which it is formed has a smooth surface.
m e invention is further illustrated by the following
Examples which, unless otherwise noted, were carried out as
followed:
All sintering and firing was carried out in a
carbon-element resistor furnace by bringing the furnace up to
sintering or firing temperature in about one hour, holding at
sintering or firing temperature for 20 minutes, shutting the
furnace off and furnace-cooling to room temperature.
~-SiC powder used had an average particle size
of 0.17 micron.
~-SiC powder used had an average particle size
of 0.32 micron.
The powder dispersion was pressed into a green body
in the form of a cylinder, 1.5 cm x 1.5 cm, which had a density
of 55% of the theoretical density for silicon carbide.
% Density given herein is fractional % of the
theoretical density for silicon carbide.
Sintered and fired products were subjected to
metallographic analyses and X-ray analyses.
RD-7361
1 334677
EX~MPLE 1
A carbon-rich silicon carbide powder prepared by a
pyrolytic process was used. Specifically, it was a powder
disper~ion, submicron in size, consisting essentially of cubic
~ -silicon carbide with free carbon uniformly and intimately
dispersed therein in an amount of 0.35% by weight of the ~ -SiC,
The ~-SiC contained 0.17% by weight 2' had an average particle
size of 0.17 micron and a surface area of 9.2 m2/g. This
powder dispersion was ball milled with particles of amorphous
elemental boron of submicron size to produce a uniform powder
dispersion containing 0.4% by weight boron baced on the ~ SiC,
A portion of the resulting powder dispersion was pressed into
a cylinder which was sintered in argon at a temperature of
2020 C, The resulting sintered product was Px~mined and the
results are shown as Example 1 in Table I. Specifically, the
sintered product had a uniform microstructure of ~-SiC such
as that shown in FIGURE 1.
EXAMPLE 2
m e sintered product of Example 1 was fired at a
temperature of 2080C.
Examination of the resulting product showed that
raising the temperature from 2020C to 2080C resulted in a 4%
transformation into (~-sic, which phase appeared in the
form of large plates, twenty times larger than the average
grain size of the ~-SiC matrix.
Ex~MæLE 3
The product of Example 2 was fired at a temperature
of 2150 C.
Examination of the resulting product showed that
this further increase in temperature resulted in a high degree
of conversionto ~-SiC accompanied by catastrophic exaggerated
grain growth of huge ~-grains of feather-like morphology as
- 14 -
RD-7361
1 334677
shown in FIGURE 2. This microstructure development is
manifested by a substantial decrease in strength as shown by
additional specimens which were prepared in the form of bars
4 mm x 4 mm x 40 mm according to Example 2 and which had at
room temperature in three point bending a modulus of rupture,
i.e. strength, of 80,000 psi, and which after firing at a
temperature of 2150C showed a modulus of rupture of 40,000 psi.
EXAMPLE 4
This example illustrates the present invention
utilizing ~-SiC.
The procedure used in this example was the same as
set forth in Example 1 except as shown in Table I.
To prepare the submicron a-SiC, abrasive grade
silicon carbide, 325 mesh grit, was milled in an aqueous
dispersion in a steel jar with steel balls for 50 hours. m e
product was then repeatedly leached with concentrated HCl and
washed with distilled water until all iron cont~m~nation due
to ball wear was removed, filtered and dried. The resulting
powder was dispersed in water to obtain a 2% dispersion which
was stabilized by the addition of 1 cc sodium silicate solution
per 500 g. of SiC. m e liquid was left standing for all particles
of about one micron or larger to settle. The dispersion was
siphoned off and the submicron SiC was recovered from the
dispersion by an addition of nitric acid to obtain pH3, filtered
and dried. The resulting powder contained particles ranging
up to one micron.
m e resulting submicron powder was characterized
and found to consist essentially of ~-SiC with 0.2% by weight
2 and 0.2% by weight of free carbon. Th-e d-sic particles
had a surface area of 5.5 m2/g, an average particle size of
0.32 micron and X-ray analysis showed it to be composed of
~-SiC polytypes 6H, 15R (4H,3C).
- 15 -
1 3 3 4 6 7 7 RD-7361
A portion of the powder dispersion prepared in
Example 1, which consisted essentially of ~-SiC, and based
on the ~ -SiC, 0.35% by weight free carbon and 0.4% by weight
boron, was used in this example. To this dispersion there
was added the a-SiC having an average size of 0.32 micron in
an amount of 0.1% by weight based on the amount of ~-SiC.
The resulting mixture was balled milled in benzene in a plastic
jar with cemented balls. After 5 hours of milling, the benzene
was removed by evaporation and the resulting powder was pressed
into a green cylinder which was sintered at 2080C. The
resulting sintered product was examined and found to have a
uniform microstructure. The results as shown in Table I
illustrate that the addition of only 0.1% of ~-SiC to the
starting powder brought about a high degree of conversion to
~-SiC on sintering at 2080C which ~-SiC phase crystallized
in the form of a uniform network of platelet-like grains.
EXAMPLE 5
The sintered product of Example 4 was fired at
2180C.
F.x~m; nation of the resulting product showed that
although additional ~-SiC phase formed after firing at 2180C,
the platelet-like grains of the product of Example 4 grew
relatively very little, i.e. to 48 microns, and that the
microstructure retained its uniformity.
EXAMPLE 6
The product of Example 5 was fired at 2250C.
Examination of the resulting product showed that
although still additional ~-SiC phase formed after firing
at 2250C, the average d-sic grain size;grew very little,
i.e. to 67 microns, which illustrates the stability of the
microstructure of the present sintered product to temperature
fluctuation.
- 16 -
RD-7361
1 334677
EXAMPLES 7-9
In these examples, which also illustrate the present
invention, the procedure and materials used were the same
as set forth in Example 3 except as shown in Table I.
The green body of Example 9 consisted essentially
of ~-SiC, 5% by weight of ~-SiC based on the ~-SiC, and
based on the total amount of SiC about 0.35% by weight of free
carbon and about 0.38% by weight of boron.
RD-7 361
1 334677
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-- 18 --
RD-7361
1 334677
-
It can be seen from Examples 4 to 9 in Table I,
which illustrate the present invention, that the addition of
d-SiC results in a lower terminal density of the sintered
product which cannot be further increased significantly by
increasing the sintering temperature. Table I illustrates
that the terminal density obtained on sintering is determined
by the amount of C~-sic used for seeding as demonstrated by
Examples 4, 7, 8 and 9 where increases in the amount of -SiC
resulted in decreases in terminal densities.
Also, the sintered or fired products of Examples
4 to 9 showed particles of free carbon, substantially submicron
in size, and present in an amount of less than about 0.5% by
weight of the total amount of silicon carbide distributed
substantially uniformly throughout each product. Also, analysis
showed the boron to be in solid solution with the silicon
carbide substantially uniformly throughout the product. In
addition, the sintered cylinders had a smooth surface since
the green bodies from which they were formed had a smooth
surface.
EXAMPLE 10
The ~-silicon powder used in this example was the
same as that used in Example 1 except that it contained 0.05%
by weight of free carbon. This powder was mixed with, submicron
in size particles and based on the amount of ~-SiC, 0.3% by
weight of acetylene black and 0.4% by weight of amorphous boron.
me mixture was ball milled in a solution of 1 g. of polyethylene-
glycol per 100 cc of benzene. 200 cc of the solution was used
per 100 g. of the powder mi~ture. After 5 hours milling with
cemented carbide balls the slurry was spray-dried.
A portion of the resulting ball milled powder was
pressed into cylinders which were sintered in argon at 2130C.
The sintered cylinders were subjected to X-ray diffraction
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analysis and found to consist on the average of 80% by weight
~ -SiC and 20% by weight ~-SiC. The microstructure was
characterized by large feather-like ~-SiC crystals in a fine-
grained ~-SiC matrix and the density was 95.5% of theoretical.
These SiC sintered cylinders were used as grinding
balls with the remaining portion of the ball milled ~ -SiC
powder mixture to introduce ~-SiC into the powder mixture by
wear of the cylinders. After 8 hours, the resulting a-SiC
contAi n; ng powder was pressed into pellets which were sintered
under the same conditions at 2130 C in argon. m e sintered
pellets had a density of 93.5%, a phase composition which was
100% ~-SiC and a substantially uniform microstructure composed
of a network of platy ~-SiC crystals.
These sintered pellets were then fired at 2175 C
in argon. The resulting pellets showed the same microstructure
as obtained at 2130C. The ~-SiC crystals were about 40 microns
long and are shown in FIGURE 3. This illustrates that the
exaggerated grain growth observed in the sintered pellets
processed with cemented carbide balls containing no ~-SiC
could be eliminated entirely by the present invention.
,. . ~ .,
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