Note: Descriptions are shown in the official language in which they were submitted.
1067524
The present inventiOn is directed to the formation
of silicon carbide articles.
In recent years there has been much interest in
fabrication of articles of complex shape from silicon carbide~
One particular area of interest has been the forming of
complex shaped articles for gas turbine engine application
from silicon carbide as this material is capable of withstand-
ing temperatures substantially higher than the temperatures
which can be withstood by present day super alloys used in
gas turbine engines. When such turbine engines are operated
at higher temperatures, such as the temperatures which can be
used with silicon carbide components in a gas turbine engine,
they become much more efficient by giving a greater amount
of power for the same fuel consumption.
- In attempting to form silicon carhide articles of
-~ complex shape, injection molding processes have been developed.
In general, these molding processes are carried out by mixing
silicon carbide particles, and optionally graphite particles,
with a predetermined amount of a thermosetting binder. The
article is formed in an injection molding operation, removed
from the mold and subjected to heat in the absence of oxygen
to reduce the thermosetting binder to carbon. The article
is silicided to transform the carbon and any graphite present
to silicon carbide thereby to produce a finished article of
silicon carbide. A process for producing such an article is
disclosed in Canadian Patent No. 1,026,939 assigned to
Ford Motor Company of Canada, Limited.
We have found that the prior art processes of
siliciding an injection molded article containing silicon
carbide and a thermosetting binder had some drawbac~s. In
particular, the prior art processes were slow, generally did
not produce an article which was fully silicided, and were
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1~675Z4
difficult to perform on a body having any substantial thick-
ness within a reasonable period of time. We have uncovered
several reasons why the prior art siliciding processes had
such difficulty.
One principal difficulty is that almost all commer-
cially available silicon carbide powder has some silicon
dioxide contained therein. This silicon dioxide is not wetted
by molten silicon metal thus making a siliciding operation on
an article containing this material difficult.
Another reason that the prior art processes had
some difficulty in achieving a fully dense silicon carbide
article is that the prior art processes did not try to adjust
the total amount of carbon in the article after both its
formation and heating to break down the thermosetting binder
into carbon. In other words, the prior art did not recognize
that if more carbon was present in the article after pyroli-
zing thereof than pore volume available for growth of new
silicon carbide, when silicon reacted with the carbon to form
silicon carbide, the result would be that the reaction would
close off the pores and carbon remaining in the interior of
the article would not be reached by the silicon. In such a
case the surface of the article would be formed of substan-
tially pure silicon carbide and the interior volume would be
a mixture of the original silicon carbide particles and un-
, reacted carbon.
As another problem, the article manufactured in aninjection molding process generally will have a s]ightly
higher ~oncentration of thermosetting binder at its sur~ace.
When the article is heated to pyrolyze the binder, a slightly
; 30 greater carbon concentration therefore develops at its
`~ surface. This extra amount of carbon can cause a closing off
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10675Z4
of the article's pore structure during a siliciding operation.
, Still another difficulty found in the urior art pro-
cesses is that the prior art processes did not control the
onset of the siliciding operation by application of a gaseous
pressure of nitrogen to coincide with a point at which the
article to be silicided was both at a proper siliciding tem-
perature and was properly cleaned. The article is properly
cleaned when the excess carbon is removed from the surface,
all of the silicon dioxide is removed therefrom and its pore
structure adjusted so that the pore volume o~ the article is
sufficient to permit penetration of the article with a reac-
table form of silicon and a reaction of that silicon with all
available carbon and graphite, if present, in the article.
In accordance with the teachings of this invention
a silicon carbide article is formed by carrying out the
~ following process. A flow molding mixture is formed by mixing
-- together 60 to 80 percent, preferably 65 to 75 percent,
by weight of silicon carbide particles having an average
particle size in a range from about 40 microns to less than
about one micron; and 40 to 20 percent, preferably 35 to 25
percent, by weight of a thermosetting binder which produces a
1Owable liquid phase when melted and which produces carbon
upon nonoxitive pyrolysis. Up to 7 percent by weight graphite
` particles having an average particle size in a range from
about 10 to 0.1 microns may be substituted for a portion of
the silicon carbide particles and thermosetting binder. A
mold release agent may also be employed in the mixture. ~
The mixture is then molded into an article by a -
molding technique which operates on-the basis that the thermo-
setting binder forms a continuous matrix about the silicon
carbide particles and graphite particles, if present, supported -
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~0675Z4
thereby, whereby the mixture is moldable as if it contained
only the thermosetting binder. The molded article is pyro-
lyzed in the absence of oxygen, whereby the thermosetting
binder undergoes a volumetric reduction in breaking do~7n to
form a low density vitreous carbon phase. Sucn action also
develops a generally interconnected pore structure throughout
the article.
The surface of the article is treated to remove any
excess carbon thereon. This action assures that the pore
structure of the article is open to the surface of the article.
The porous article is heated in the absence of
oxygen to a siliciding temperature above the melting tempera-
ture of silicon. The article is maintained, once it is
heated to a temperature of at least 2800~F, for a period of
time in a gaseous environment consisting principally of
nitrogen with from 0 to 10 percent, preferably 3-7 percent, --
hydrogen contained therein. The hydrogen, if present, is
`; active to remove carbon from the article to ensure that the
article has sufficient free volume to accommodate conversion
of the remaining carbon to silicon carbide. In this dual gas
treatment, the nitrogen is active to transform any oxides of
silicon present in the silicon carbide to transform it into
silicon nitride. This dual gas treatment accomplishes two
major functions, one of cleaning up the article by removing
- oxides of silicon and the other of insuring that sufficient
free volume is available within the article so that the
carbon may be transformed into silicon carbide in a manner
which does not prematurely seal off the pore structure of
the article.
-`' 30 The dual nitrogen-hydrogen atmosphere is withdrawn
from associated with the article prior to a siliciding oper-
ation. The article preferably-then is brought to its
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~0675Z4
siliciding temperature with some nitrogen gas surrounding the
same. This nitrogen gas is replaced by a vacuum lower than
the vapor pressure of silicon at the siliciding temperature.
Following withdrawal o~ the nitrogen atmosphere, the silicon
nitride formed in the article by the action of the nitrogen on
the oxides of silicon and on clean silicon surfaces gives up
the nitrogen reacted therewith and is transformed to silicon
metal. The now clean article is silicided at an elevated
temperature by permitting penetration of the article through
its pore structure with a reactable form of silicon. This
silicon reacts with the available carbon to form silicon
carbide.
The siliciding of the article can be carried out by
introducing silicon metal into the chamber containing the
article when a nitrogen containing environment surrounds the
article. This nitrogen containing environment may be either
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some nitrogen-hydrogen gases or pure nitrogen by itself. The
article is brought in the presence of the nitrogen containing
environment to its siliciding temperature in a range from the
melting temperature of silicon to 3300F. The nitrogen con-
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taining environment is withdrawn when the siliciding temper-
ature is reached thereby leaving behind silicon metal in a
form which penetrates the pore structure of the article and
rapidly reacts with the available carbon and graphite, if
present, of the article. While the nitrogen environment
is present in the chamber holding the article, the nitrogen
reacts with the liquid silicon metal to form a silicon nitride
,~, skin thereon which stops any penetration of the porous body
by the silicon in any ~orm.
' 30 As stated above, ~raphite particles may be mixed
with the molding mixture in order to provide another source
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1067524
of reactable carbon in the molded article. If graphite is
present in the articler some of the thermosetting binder
during the nonoxitive pyrolysis operation will shrink about
individual particles of graphite. When the article is sub-
sequently subjected to the hydrogen-nitrogen treatment, the
hydrogen eliminates a portion of the graphite thus making
pores in the vitreous carbon formed about the graphite -
particles.
The method of this invention will be covered in the
discussions set forth below. The particular materials set
forth in the discussions are not intended to limit the scope
of this invention. Any thermosetting polymeric material which
originally contains aromatic components or produces such
aromatic components upon pyrolysis and is in a flowable liquid
phase at temperatures of a plastic molding operation is
suitable for use in this method of forming silicon carbide
articles by injection molding.
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~0675Z4
The method of the invention is initiated by mixing
together 60 to 80 percent, preferably 65-75 percent, by weight
of silicon carbide particles. These silicon carbide particles
should have an average particle size in a range from about
40 microns down to less than about 1 micron. Such silicon
carbide material is commercially available and is generally
alpha silicon carbide. The higher amounts of silicon carbide
particles in a mixture can be obtained when the larger
particle sizes are used. As the average particle size is
reduced towards the lower particle size limit, the amount of
silicon carbide which can be loaded into a mix and still be
~ totally surrounded by a liquified thermosetting material with ~-
; the particles not contacting one another is reduced. This
comes about, of course, because for the same given weight
; of material the smaller particles have a larger surface area
to be wet than do the larger particles.
` The silicon carbide particles are mixed with
40 to 20 percent, preferably 35 to 25 percent, by weight of
a thermosetting binder which produces a flowable liquid
phase when melted and which produces carbon upon nonoxitive
pyrolysis. The carbon is produced when the thermosetting
material breaks down upon pyrolysis to form aromatic components.
These aromatic components subsequently form a vitreous, low
` density carbon phase. Some thermosetting materials which
are satisfactory for use in the method of this invention are
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j the following: phenol furfural, phenol formaldehyde,
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1 polybenzimindazole, phenolicnaphthalenediol terpolymer,
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polyphenylenes, polyvinyl chloride, polyvinylidiene chloride -
and polyphenol polymer. At room temperatures these materials
are generally in a solid state, but when they are heated to
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10675Z4
a temperature of an injection molding operation, they melt
and produce a liquid phase.
If desired, graphite particles may be added to the
molding mix. If these particles are added, they are added
up to 7 percent by weight of the mix. If graphite is used,
it mainly displaces the silicon carbide particles but it
also displaces a small amount of the thermosetting binder.
For molding mix quantities, when 0 to 7 pe~cent by weight of
graphite is used, the broad limits are 60 to 75 percent by
weight of silicon carbide particles and 40 to 23 percent
by weight of the thermosetting material. Also, the graphite
particles should have an average particle size in a range
from about 10 to about 0.1 microns. Graphite particles
having an average particle size of around 0.5 microns are
preferred.
No matter what combination of materials are used
in forming the molding mixture, the essential feature is
A
that sufficient thermosetting material is present with the
particles that when the thermosetting binder is liquified,
' 20 it forms a continuous phase about the particles which are
suspended therein. This allows the molding mixture to be
injection molded by flow molding techniques.
After the molding mixture has been formed, the
mixture is heated to a temperature at which the thermosetting
binder is in a liquid phase. The liquid phase must
completely surround the individual silicon carbide particles
and any graphite particles present. The thermosetting
material must form a continuous phase about these particles
so that the molding mixture ifi a flowable mass which can
be injection molded.
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1067524
The mixture is injection molded by forcing it under
pressure in an injec'~ion molding machine into a mold. Such
an injection molding tec~mique operates on the basis that
the flowable thermosetting binder forms a continuous phase
about the particles contained therein. Since the mixture
is a flowable mass, the mold into which it is injection molded
can be of a complex shape. For example, the mold may define
the shape of a rotor or a stator of a gas turbine engine.
After this molding operation, the thermosetting
binder is stiffened by permitting the material sufficient
time in the mold that the thermosetting material crosslinks.
This stiffening lends strength to the molded article so that
it may be removed from the mold without damage thereto.
The molded article is then subjected to a pyrolyzing
operation in the absence of oxygen. Under these conditions,
the thermosetting binder in the molded article undergoes a
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volumetric reduction in breaking down to form a vitreous
carbon phase. A portion of the thermosetting material is
driven off as volatile matter but a portion of it remains
~20 behind by forming aromatic components which subsequently
form carbon. The carbon formed is a vitreous carbon phase
which serves to bond the silicon carbide particles and any
graphite particles present together. With respect to the
smaller graphite particles, the vitreous carbon may surround
portions thereof.
, The pyrolyzing action develops a generally inter-
connecting pore structure throughout the article as a result
of the volumetric reduction of the thermosetting material. -
i The pyrolyzing operation may be carried out at a final
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10675Z4
temperature in any manner which accomplishes the intended
function. The heating and cooling rates should be such
that no substantial stresses are set up in the article
whichmight cause damage thereto. A typical pyrolyzing
operation is o~e which is carried out by heating the
article from room temperature to 700F at a rate of about
50F per hour, from 700 F to 1200 F at a rate of about
25F per hour, and from 1200F to a final temperature in a
range of from 1850 to 2500F at a rate of 50F per hour.
The article may be cooled back to room temperature at a
` rate of about 150F per hour.
`3~ After the pyrolyzing operation, the article generally
; has a slightly greater concentration of carbon at its surface
than in its central portions. This extra carbon may cause
trouble in a siliciding operation by sealing up pores of the
article when it reacts with the silicon. This difficulty is
avoided by a cleansing operation which eliminates some of the
excess surface carbon. For example, the article is heated in
nitrogen to a treatment temperature in the range of 700F
to 850F. When the treatment temperature is reached, oxygen
' i5 introduced into the treatment chamber and the article is
exposed to the oxygen for a period of time sufficient to
remove excess surface carbon and insure that the pore
'i structure of the article is open to the article's surface.
For example, oxygen in a concentration of 10 to 30 percent
by volume may be used for treating the article up to five
minutes.
After the surface treatment operation, the article
may be cooled to room temperature, if desired and then
subsequently reheated so that it may be silicided in a
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1067S24
separate operation. On the other hand, the article may be
brought from the surface treatment operation ~irectly to
a temperature for the siliciding operation. Generally, the
article is cooled to room temperature and reheated as it is
silicided in a different furnace than that in which it is
pyrolyzed.
After surface treatment, the article is heated in
the absence of oxygen to a siliciding temperature which is
genera~ly a temperature above the melting point of silicon -
but less than 3300F. The article may be heated at any
heating rate which does not cause substantial thermal
stresses to be set up therein thus possibly leading to
damage to the article. For example, a heating rate of about
200F per minute is satisfactory. A lower siliciding temper- -
ature, for example just above thé melting point of silicon,
~ is used for small articles. A higher siliciding temper-
- ature is used for articles of larger cross section. In
either case, a siliciding of the article is obtained in a
relatively short period of time. For example, an article
having a thickness of a quarter of an inch can be silicided
in a period of less than one minute whereas an article having
a thickness of 1 inch can be silicided in one minute.
As one of the important steps of the process of
this invention, while the article is being brought to its
siliciding temperature, the article is maintained for a
period of time at a temperature of at least 2800F in a
gaseous environment consisting principally of nitrogen with
from 0 to 10 percent, prèferably 3 to 7 percent, hydrogen
by volume. This dual atmosphere of nitrogen and hydrogen
is maintained about the article at a pressure less than about
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1067524
one-quarter atmosphere. The temperature of treatment may be
any temperature above 2800F. Below 2800F the treatment's
effectiveness is reduced substantially. The period of
time for which the article is held in the dual gas treatment
is determined by the amount of carbon material which must
be removed from the article. For example, treatment times
may be from 15 minutes to 2 hours.
This dual gas treatment is an effective way of
cleaning up the article prior to the siliciding operation.
The cleaning up comes about in that the hydrogen is active
in removing some of the carbon and some of the graphite
if any is present in the article. This action develops
sufficient free volume in the article to accommodate
conversion of the article's remaining carbon and any graphite
to silicon carbide. One may determine the amount of time
that the article should be subjected to the dual gas treatment
by knowing 1) the amount of graphite in the article, 2) the
amount of carbon in the article produced by decomposition
of the thermosetting material, and 3) the pressure of the
gas and amount of hydrogen present to react with the carbon.
If the article is rich in silicon carbide particles and has
a relatively low amount of carbon produced by decomposition
of the thermosetting material, the time of dual gas treatment
; is reduced. Hydrogen also aids in cleaning up oxides of
'! silicon in the silicon carbide powder.
The nitrogen of the dual gas treatment reacts
with any oxides of silicon present in the silicon carbide
particles. Oxides of silicon, such as silicon dioxide, are
found in almost all commercially sold silicon carbide powders.
The nitrogen reacts with the oxides of silicon to transform
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10675Z4
them into silicon nitride. Silicon nitride is generally
unstable at these temperatures but in the presence of
nitrogen it does not break down. However, when a vacuum
is drawn on the system, the silicon nitride breaks down
to give off the nitrogen and the silicon remains behind as
pure silicon.
In a situation where graphite particles are used
in the mixture, the thermosetting binder which was
thermally decomposed may form about some of these graphite
particles. The dual gas treatment eliminates a portion of
the graphite particles thus placing a pore in any of the
vitreous carbon which may have been formed thereabout.
Thus, additional pores can be developed in the dual gas
treatment if graphite particles have been used in the
initial mixture.
As a result of the dual gas treatment, the article
is now cleansed of silicon oxides as well as having a very
well developed pore structure therein. The pore structure
is sufficient to accommodate the conversion of the remaining
t 20 carbon and any graphite to silicon carbide. The silicon
for accomplishing this transformation is permitted entry
into and penetration of the article because of its open
and adequate pore structure.
The nitrogen-hydrogen environment can remain while
the article is brought to its siliciding temperature or it can
be replaced by a substantially pure nitrogen environment which
. .
surrounds the article. This environment around the article
being brought to its siliciding temperature is maintained
at a pressure less than one-quarter atmosphere. If a lower
siliciding temperature is to be used then the temperature
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~067524
of the dual gas treatment, the article may actually be cooled
off to be brought to its siliciding temperature. However, if
a higher siliciding temperature is to be used, the article
is heated to the siliciding temperature at a rate which does
not cause any thermal distortions therein. For example, if
the article has been treated at 2850F for the dual gas
treatment and the siliciding temperature is to be 3150F, the
article might be heated at a rate of 200F per minute to
the higher temperature.
In order to carry out the siliciding operation,
it is necessary to make available in the vicinity of the
; article pure silicon metal. This metal may be introduced
into the chamber through appropriate apparatus already
known in the art at a time when the nitrogen containing
environment is present and the article is being brought to
its siliciding temperature. On the other hand, if desired,
the silicon may be made available to the article after the
article has achieved its siliciding temperature. It is
preferred, however, to introduce the silicon adjacent to
the article at the time that the article is being heated
in a nitrogen containing environment to its siliciding
temperature.
The temperature of the chamber in which the article
is resting is above the melting point of the silicon metal
which is introduced into the chamber as a solid metal. If a
nitrogen atmosphere is present, the silicon will first melt
j and as it does so, a thin skin of silicon nitride will form
thereover by the action of the nitrogen on the silicon.
~- The siIicon is disabled in this manner from reacting with
the carbon and graphite in the article because of the
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1067524
formation of this skin. We have found that it is important
to keep the silicon from reacting with the article until the
article is at its siliciding temperature.
Once the article has been brought to its siliciding
temperature, the nitrogen containing environment surrounding
the same is replaced with a vacuum lower than the vapor
pressure of the liquid silicon at the siliciding temperature.
By drawing a lower vacuum, some of the silicon volatilizes -
into the atmosphere surrounding the article and thereby
enters the pore structure of the article. It is apparent
that the higher the temperature, the higher the vapor
pressure of the silicon metal and the less vacuum needed on
the system. For example, higher vacuums are needed to accom-
plish the volatilization of the silicon metal at temperatures
closer to the melting point of the silicon. The drawing
of the vacuum initially operates on the unstable silicon
. :
o~ nitride skin to break it down and leave pure silicon
behind. Likewise the drawing of the vacuum also strips
;1 the nitrogen from the silicon nitride formed when the
nitrogen reacted with oxides of silicon in the silicon
carbide. This also leaves behind pure silicon.
When the skin of silicon nitride has been removed
I from the molten silicon metal, the silicon in a reactable
- state penetrates the article through its pore structure.
This silicon reacts with the carbon developed as a result
of decomposition of the thermosetting material and it also
reacts with any graphite that is present in the article
to transform the same into silicon carbide. The controlling
of the onset of siliciding as well as the siliciding of
a body which has been cleansed in the dual gas treatment
;! allows the process to proceed at a very rapid rate. For
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1067S24
example, as described above, the siliciding of a body of
inch thickness would take less than one minute whereas the
siliciding of a l inch thick section would take about one
minute. However, the silicon treatment may remain in
effect for a period of time up to 30 minutes or more to
insure that all carbon in the article is converted to silicon
carbide. The completed silicon carbide article is generally
of almost theoretical density of 3.21 g/cc.
The finished article is recovered by cooling the
article to room temperature at a rate which does not cause
any thermal fracturing of the material. For example the
' furnace may be turned off and allowed to cool back to room
: temperature with the article therein.
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