Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
109;~793
The subject matter of the present invention is an improved method
for manufacturing, at relatively low cost, reaction bonded silicon carbide
bodies which are of superior quality particularly in that they are substan-
tially non-porous.
Thus the present invention provides a method for manufacturing a
reaction bonded silicon carbide body comprising: heating, in a vacuum or
an inert atmosphere, to at least the melting temperature of elemental
silicon, a porous compact consisting essentially of a uniform mixture of
silicon carbide grain and finely divided elemental carbon while said compact ~ -
is in contact with a mixture of finely divided elemental silicon and a small
amount of finely divided uniformly distributed elemental carbon, whereby
upon said heating a portion of the elemental silicon in the silicon-carbon
mixture reacts with the carbon in said silicon-carbon mixture to form a
friable, porous matrix of silicon carbide which guides the flow of molten
elemental silicon from the mixture into said porous compact wherein at least
some of the molten silicon reacts with the carbon in said compact to form
additional silicon carbide thereby to provide a reaction bonded silicon
carbide body.
The present invention also provides a method for manufacturing a
reaction bonded silicon carbide body comprising: ~1) forming an assembly of
two compacts in contact with each other, one of said compacts consisting
essentially of a uniform mixture of from about 70% to 95% by weight silicon
carbide grain and the remainder finely divided elemental carbon and the
other of said compacts consisting essentially of a uniform mixture of from
about 87% to 97% by weight finely divided elemental silicon and the remainder
finely divided elemental carbon; (2) heating s.aid assembly in a vacuum or an
inert atmosphere to the melting temperature of silicon whereby a portion of
said elemental silicon reacts with the carbon in said second-mentioned compact
to form a friable, porous, silicon carbide matrix and whereby the remainder
of said silicon permeates said first-mentioned compact wherein at least a
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105~;~793
portion of said silicon reacts with the carbon in said first-mentioned compact
such that said first-mentioned compact is converted to a reaction bonded
silicon carbide body; and (3) thereafter removing said friable, porous,
silicon carbide matrix from said reaction bonded carbide body.
The present invention further provides a method for manufacturing a
reaction bonded silicon carbide body comprising: (1) forming an assembly of
two compacts in contact with each other, one of said compacts consisting
essentially of a uniform mixture containing from about 75 to 95 parts by weight
silicon carbide grain, from about 5 to 25 parts by weight finely divided
elemental carbon and from about 5 to 15 parts by weight organic binter and the
other of said compacts consisting essentially of a uniform mixture containing
from about 90 to 97 parts by weight finely divided elemental silicon, from
about 3 to 10 parts by weight finely tivided elemental carbon and from about
3 to 10 parts by weight organic binder; (2) heating said assembly to a tem-
perature sufficient to decompose the organic binder in said compacts; (3)
heating said assembly in a vacuum or an inert atmosphere to the melting tem- -
perature of said silicon whereby a portion of said silicon reacts with the
carbon in said second-mentioned compact to form a friable, porous, silicon
carbide matrix and whereby the remainder of said silicon permeates said first-
mentioned compact wherein at least a portion of said silicon reacts with the
carbon in said first-mentioned compact such that said first-mentioned compact
is converted to a reaction bonded silicon carbide body; and (4) thereafter
re ving said friable, porous, carbide matrix from said reaction bonded
silicon carbide body.
The present invention additionally provides a method for manufac-
turing a body containing at least about 60~ by weight silicon carbide and
the remainder substantially all elsmental silicon, said method comprising
heating, in a vacuum or an inert at sphere, to at least the melting temper-
ature of elemental silicon for from one-half to six hours, a compact having
3Q a porosity such that from about 20% to 50% by volume thereof consists of
voids, and consisting essentially of a uniform mixture of from about 70% to
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1092793
95% by weight silicon carbide grain and from about 5% to 30% by weight finely
divided elemental carbon, said heating being performed while said compact is
in contact with a uniform mixture of from about 87% to 97% by weight finely
divided elemental silicon and the remainder finedly divided carbon, the
amount of elemental silicon being greater than that stoichiometrically
required to react with the total of the carbon in said silicon-carbon mixture
and in said compact.
The invention will be described largely with reference to the
manufacture of silicon carbide seal rings though it will be understood that
the method can be used for the manufacture of bodies for numerous other uses.
It is already known that silicon carbide seal rings used, for
example, in sealing the rotary shafts of slurry pumps and the like, are
advantageous because of the hardness, and hence great wear resistance, the
high thermal conductivity, and the high thermal shock resistance of the
silicon carbide. To optimize these properties it is desirable that such
seal rings be of high density, and hence low porosity. Of course, it is
also desirable that they be manufactured at low cost so that the end user
does not have to pay an excessive premium for using such seal rings as
compared with seal rings of other materials.
It is known that silicon carbide seal rings can be manufactured by
first making a ring-shaped compact of silicon carbide and carbon and then
permeating this compact with silicon, either in vapor or liquid form, such
that the silicon reacts with the carbon in the compact to form additional
silicon carbide in situ, the amount of silicon used being that stoichio-
metrically necessary to react with all of the carbon in the compact. Such
bodies are referred to as being of reaction bonded silicon carbide. However,
it is difficult to attain a substantially non-porous structure using such
techniques. Further, the subsequent cutting and machining operations to
àccomplish the required dimensions and smooth surface finish for the individ-
ual seal rings are difficult, and hence expensive, because of the extreme
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109;~793
hardness of silicon carbide.
; In particular, the present invention provides a method for manufac-
turing reaction bonded silicon carbide seal rings which are substantially
non-porous and yet which can be made at relatively low cost by reason of the
relatively easy machinability of the seal ring surfaces to smooth finish
subsequent to silicon impregnation of the silicon carbide-carbon compact and
also by reason of the elimination of stringent composition control and other
control measures heretofore necessary.
Briefly, and with reference to the manufacture of seal rings, the
method of the present invention in its preferred form comprises forming an
assembly of a porous silicon carbide-carbon ring-shaped compact in axial
face-to-axial face intimate contact with a ring-shaped compact predominantly
of finely divided elemental silicon but also containing finely divided carbon
uniformly distributed therethrough, such assembly thereafter being heated to
the melting temperature of the silicon. The amount of elemental silicon in
the last-mentioned compact is greater than required to react with all the
carbon in both compacts. As the silicon reaches its melting temperature a
portion thereof reacts with the carbon with which it is mixed thereby to
form silicon carbide which provides a highly porous, friable matrix. This
highly porous matrix then functions to guide the flow of the molten silicon
into the porous silicon carbide-carbon compact whereupon a portion of the
molten silicon reacts with the carbon in that compact to form additional
silicon carbide, a remaining portion of the molten silicon filling any voids
between the silicon carbide grains thereby to provide a substantially
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109;~7~3
P-397
non-porous body. After cooling,the friable silicon carbide and
any excess of elemental silicon on the surface of the non-porous
body can be easily removed, as by a machining or o~her abrading
operation, this because of the friable nature of the porous
silicon carbide matrix and the relative softness of the elemental
silicon. Because the amount of silicon used is greater than
that stoichiometrically required to react with all the carbon,
there is no requirement for close control of the amount of sili-
con in the silicon-carbon compact, any excess being easily
removable from the manufactured non-porous body for the reason
aforesaid. By the same token, there is no requirement for
stringent control of the porosity of the silicon carbide-carbon
compact or of the precise amount of carbon therein since the
use of excess elemental silicon in the silicon-carbon compact
assures adequate quality control and non-porosity of the manu-
factured bodies. That is, minor variations, between the indi-
vidual finished seal rings, in the amount of elemental silicon
they contain has no significant effect on the quality and
excellent performance characteristics of the seal rings. This,
aiong with the ease of final machining operations to provide
the precise size and surface finish desired, enables relatively
low cost manufacture.
Other features, particulars and advantages of the
invention will appear more clearly from the following more de-
tailed description of the invention.
MATERIALS --
, . .
The silicon carbide grain used as the starting material
can be alpha silicon carbide of conventional commercial grade
readily availa~le in the market and with a grain size of anywhere
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P-397
from 200 to 1200 grit. It is preferred that the grain size be
variated. Typical for use in the practice of the invention is
1000 grit size alpha silicon carbide, the particle size distri-
bution being such that approximately 50% of the grains are of a
S size ranging upwardly from 10 microns to as high as 25 microns
and with the remaining S0~ ranging relatively uniformly down
from 4 microns to as low as .S microns--and hence, the average
particle size of the grain being approximately 10 microns. As
has been indicated, larger or smaller grain size silicon carbide
can be used if desired.
The carbon for each of the compacts can be either
amorphous carbon or graphite. The particle size of the carbon
should preferably be submicron, a particle size distribution of
from about .01 to 1 microns, and with an average particle size
of .1 microns, being excellent.
The silicon used in the silicon-carbon compact can be
of commercial grade, preferably with a particle size within the
range of about lOa to 325 mesh, 200 mesh being typical. The
particle size distribution in 200 mesh silicon powder is approx-
imately 10 to lS0 microns, the average particle size being about75 microns.
In the formulation of the mixtures for making the
compacts it is desirable to include an organic binder to insure
that the compacts have good green strength and, hence, can be
handled without danger of crumbling or breaking. Of course,
substantially before the silicon reaches its melting temperature
in the practice of the method of the invention, the organic
binder decomposes to elemental carbon thereby providing a small
amount of carbon in addition to that added as such to the mix-
tures from which the compacts are formed. Any of a wide variety
, ~,
10~;~793P-397
of organic binders can be used, for example, the acrylic resins,
polyvinyl butyral, cellulose acetate, methyl cellulose or poly-
ethylene glycol. For simple, low cost manufacture it is desir-
able tnat the organic binder be water soluble, as will be clear
from the further description of the method which follows.
FORMULATION AND PREPARATION OF TE~E COMPACTS
The mixture used for the silicon carbide-carbon com-
pact should preferably contain from about 75 to 95 parts by
weight silicon carbide, from about 5 to 25 parts by weight ele-
mental carbon ~i.e., either amorphous carbon or graphite) andfrom about S to 15 parts by weight organic binder. With such
formulations, by the time the silicon becomes molten in the
; practice of the method ~prior to which time the organic binder
~-~ will have decomposed to carbon), the relative amounts of silicon
. ~ . .
carbide and carbon in the compact will be approximately from
70% to 95% by weight silicon carbide and from 5% to 30% by
weight carbon. The upper end of the silicon carbide range and
the lower end of the carbon range remain approximately the same
as in the initial` batch since,where only 5 parts by weight
binder are use and where only a minor percentage of the binder
molecule is carbon, the amount of carbon contributed by decompo~
sition of the binder is insignificant. A typical batch formula-
tion for forming the silicon carbide-carbon compact is: 80
parts by weight silicon carbide (1000 grit), 20 parts by weight -
carbon ~lack, 10 parts by weight polyethylene glycol and 100
parts by weight water. In forming the mixture it is preferable
that the water, polyethylene glycol and carbon first be mixed
to form a slurry, and with the silicon carbide then being added
to this slurry and mixed therewith. After such mixing the water
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1()9;~7~3
P-397
is evaporated and the resulting material consists of silicon
carbide grains coated with a mixture of the binder and the car-
bon. A measured amount of this coated loose granular silicon
carbide material can then be pressed in matched metal dies to
form the silicon carbide-carbon compact. Of course, the shape
of the dies is selected to impart the desired shape to the com-
pact, ring-shaped dies being used to form the compacts for
making seal rings. Depending upon the particular binder selected,
heat may or may not be necessary in the pressing operation to
attain a compact having good green strength. Where polyethylene
glycol is used as the binder, no heat is required.
In the pressing operation to form the silicon carbide-
carbon compact, the amount of pressure applied should preferably
be such as to result in the compact having a bulk density of
from about 1.4 to 2.5 grams per cubic centimeter, 1.8 grams per
cubic centimeter being typical. Where the bulk density of the
compact is within the aforesaid range, the porosity of the com-
pact is such that from about 20g to 50% by volume of the compact
consists of voids between the coated grains of silicon carbide.
These voids are uniformly distributed and communicate with each
other. Depending upon the precise mixture used and the porosity
desired, pressing pressure of from 3000 to 20,000 psi can be
used.
The mixture for the silicon-carbon compact should
preferably contain from about 90 to 97 parts by weight silicon,
~rom about 3 to 10 parts by weight elemental carbon and from
about 3 to 10 parts by weight binder. The carbon can be either
amorphous carbon or graphite and the binder can be the same as
that used in the silicon carbide-carbon compact. Just as dis-
cussed above with reference to the silicon carbide-carbon compact,
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109;~793
P-397
so also in the silicon-carbon compact, by the time the silicon
reaches its melting temperature the organic binder will have
decomposed to provide a small amount of carbon in addition to
that added as such. With mixture formulations within the afore-
said ranges, the relative amounts of silicon and carbon in thecompact at the conclusion of the decomposition of the binder is
approximately 87% to 97% by weight silicon and 3% to 13% by
weight carbon. A typical mixture for making the silicon-carbon
compact is: 94 parts by weight silicon (200 mesh), 6 parts by
we;ght carbon black or graphite with an average particle size
of about .1 micron, 5 parts by weight polyethylene glycol and
100 parts by weight water. The mixture is formed and then dried
the same as described above with reference to the silicon car-
bide-carbon compact, and the resulting coated silicon powder can
then be formed in matched metal dies into a compact of the shape
~- ~ desired. The pressure used for the pressing operation can be
.j` :
~' from 3000 to 20,000 pounds per square inch. The silicon-carbon
compact can be pressed to high density, and hence without voids,
-
though the presence of voids does no harm.
In the practîce of the preferred embodiments the amount
of silicon used in the silicon-carbon compact should be at least
equal to, and preferably at least slightly in excess of that re-
quired to react with all the carbon in both compacts to form
silicon carbide, plus the amount required to fill all the voids
remaining between the silicon carbide after all the carbon in
the silicon carbide-carbon compact has reacted to form silicon
carbide. Hence, the greater the amount of carbon in the two
compacts and the greater the porosity of the silicon carbide-
carbon compact, the greater is the amount of silicon which
should be used in the silicon-carbon compact.
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109~7~3
P-397
PROCESSING OF THE COMPACTS
. TO FORM THE FINAL PRODUCT
An assembly is formed comprising a silicon carbide-
carbon compact in surface-to-surface abutment with a silicon-
S carbon compact. Por the manufacture of a seal ring, such
- assembly consists of a ring-shaped silicon carbide-carbon com-
pact with a ring-shaped silicon-carbon compact laid on top therof
in axial alignment therewith. The silicon-carbon ring should
preferably be, though need not necessarily be, of the same
` 10 internal and external diameter as the silicon carbide-carbon
- ring. The thickness of the silicon carbide-carbon ring is at
least approximately, but not less than, that desired for the seal
ring being made, and the thickness of the silicon-carbon ring is
~: determined by the amount of silicon u~ed in accordance with the
factors discussed above.
1~ Instead of forming the two compacts separately and
then stacking one on top of the other to form the assembly, the
two compacts can be formed as one unitary compact having a layer
of silicon carbide-carbon and a layer of silicon-carbon. That
is, the mixture ùsed for the silicon carbide-carbon compact can
be pressed to the desired shape in a set of matched metal dies
and then, with the male die removed and with the silicon carbide-
carbon compact remaining in the female die, the mixture for the
silicon-carbon compact can be placed on top of the silicon car-
2S bide-carbon compact and the silicon-carbon mixture then pressed
into a layer adherent to the silicon carbide-carbon bottom layer
after which the resulting composite compact is removed from the
die. This technique is advantageous for high production opera-
- tions in that it eliminates the need subsequently to stack one
compact on top of the other and it assures tha~ the two compacts
....... .. . .
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p_397 109~7~3
which form the assembly are assembled and remain assembled pre-
cisely as desired.
The assem~ly of the compacts is then heated to a tem-
perature sufficient to decompose the binder. This heating step
can be performed in air though it is better that it be in a
non-oxidizing atmosphere such as a mixture of nitrogen and hydro-
gen, 85% ~y volume nitrogen and 15% by volume hydrogen being
excellent. During the decomposition, at least most of the carbon
atoms in the binder will remain as carbon in the compact and the
other ingredients in the binder, for example hydrogen and oxygen,
, leave the compact in gaseous or vapor form. In general t the
temperature used for this heating operation to decompose the
binder can be from about 300 to 450 C, the precise temperature -
and time for this operation depending upon the particular binder
;~ 15 used. Where the binder is polyethylene glycol, heating to a
~`~ temperature of 375 C for one hour in a nitrogen-hydrogen atmo-
sphere is quite satisfactory.
After the binder has been decomposed as aforesaid, the
assembly is heated to a temperature at least equal to the melting
temperature of the silicon in an inert atmosphere or in a vacuum,
preferably the latter. Of course, the higher the vacuum the
better; a vacuum of from 1 x 10 2 to 1 x 10 1 mm Hg provides
excellent results. If it is desired to use an inert atmosphere,
it is preferable to use a higher temperature than required with
the use of a vacuum in order to obtain optimum, relatively rapid
infiltration of the molten silicon through the silicon carbide-
carbon compact. Suitable inert atmospheres are argon, helium,
and hydrogen~ The latter, though it may function as a reducing
atmosphere with respect to any oxide contaminant as might be
present, is inert with respect to the essential ingredients.
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109;~753
P-397
The preferred temperature and time used, particularly where the
heating is in a vacuum,are from 1450 to 1650 C for from one-
half to six hours, the precise temperature and time being
dependent on the thickness of the body being made and the por-
osity of the silicon carbide-carbon compact. In general, the
greater the thickness and the lesser the porosity, the higher
the temperature and the greater the time used for this operation.
Typically, for a body having a thickness of one-half inch made
from a silicon carbide-carbon compact having a bulk density of
about 1~8 grams per cubic centimeter, heating in a vacuum to a
temperature of 1500 C for two hours is satisfactory. If an
inert atmosphere is used, temperatures as high as 2000 C may
be desirable to attain optimum infiltration within a short
period.
During this heating operation, by or at the time the
silicon becomes molten, a portion thereof reacts with the carbon
in the silicon-carbon compact to form silicon carbide. This
silicon carb~de forms a highly porous, friable matrix which ~ -
functions to contain and guide the flow of the remaining silicon,
now in molten form, into the porous silicon carbide-carbon com-
pact. That is, the friable porous matrix prevents or inhibits
flow of the molten silicon over the edge and down the sides of
the silicon carbide-carbon compact. As the molten silicon
infiltrates the silicon carbide-carbon compact, a portion there-
of reacts with the carbon in that compact to form additionalsilicon carbide and, after all of the carbon has-reacted to form
silicon carbide, any remaining pores are filled with the ele-
mental silicon. Hence, the resulting body is of high strength,
unitary, reaction bonded silicon carbide with the pores thereof
filled with silicon to provide a substantially non-porous struc-
ture.
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109;~793
P-397
After the infiltration is complete and the resulting
body removed from the heating chamber and cooled, the friable
silicon carbide matrix, along with any excess silicon on the
surface of the body, can be easily removed by a simple abrading
or machining operation. Where seal rings are being manufactured,
this operation is then followed by a polishing operation to pro-
vide the desired smooth surface finish for the seal ring, pre-
, ferably a surface finish of less than 60 microinches (root mean
square~.
Hence, by means of the invention, silicon carbide-
silicon seal rings and other bodies can be made to substantial
~:~ non-porosity and at a relatively low manufacturing cost.
It is desirable during the heating operation that the
assembly of compacts be oriented vertically with the silicon
i; 15 carbide-carbon compact on the bottom and the silicon-carbon
compact on top since with this orientation the molten silicon
~. .
flows into and permeates through the silicon carbide-carbon com-
pact both by reason of capillary action and by reason of gravity.
However, since the molten silicon can flow into the silicon car-
bide-carbon compact solely by way of capillary action, the afore-
said orientation of the assembly is not essential. Further,
,. . .
whereas there is generally no reason to use an assembly of more
~; than two compacts, it is possible to do so if desired--an
~ example being a silicon carbide-carbon compact sandwiched between.~,.
~ 25 two silicon-carbon compacts.
,...... . .
In all of the embodiments of the invention described
^,~ above, the mixture of elemental silicon and carbon is used in
,.....
the form of a compact thereof, made as by the technique described
wherein a binder is included in the mixture and the mixture is
'- 30 pressed to form the compact. This use of the silicon-carbon
,
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109;~793
P-397
mixture in the form of a compact thereof is preferred; however,
it is not essential for the practice of the invention in its
broader scope. That is, it is within the purview of the inven-
tion to use the silicon-carbon mixture for the practice of the
5 invention in a loose powder form and hence without need for a
binder therein. In so practicing the invention the finely
divided silicon and carbon are mixed to form the desired uniform
mixture thereof in non-compacted loose form and then the desired
quantity of this loose particulate mass can be poured onto and
10 around the silicon carbide-carbon compact. Where the silicon
carbide-carbon compact is a ring, it is desirable that most of
the loose silicon-carbon mixture be placed in the center and
- around the faces of the ring, and hence in intimate surface-to-
surface contact with the ring. From then on the method is the
15 same as has been described, the silicon carbide-carbon compact
being heated, while in such contact with the loose mass, to the
melting temperature of the silicon. The particle sizes and
~ percentages of the powdered elemental silicon and carbon, and
: the form of carbon, used in che loose mixture can be as describ-
2Q ed above with reference to the embodiments wherein the mixture
! is used in the form of a compact thereof.
As has been discussed above, the method of the present
-~ invention serves to particular advantage for the manufacture of
s substantially non-porous bodies wherein any interstitial voids
25 between the carbide grains are filled with elemental silicon and
~ indeed, excellent bodies can be manufactured containing as
`~ little as about 60% by weight silicon carbide and the remainder
elemental silicon. However, it will be understood that the
invention can be used to form bodies wherein there is little or
30 no free silicon, this being accomplished by using only that
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P-397
amount of silicon as is stoichiometrically required to react
with the carbon.
Hence, it will be understood that while the invention
has been described particularly with reference to preferred
embodiments thereof, various changes may be made all within the
full and intended scope of the claims which follow.
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