Note: Descriptions are shown in the official language in which they were submitted.
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BERYLLIUM CONTAINING SILICON CARBIDE POWDER COMPOSITION
BACKGP~OUND OF TLIE INVENTION
Silicon carbide, a crystalline compound of silicon and
non-metallic carbon, has long been known for its hardness,
its strength, and its excellent resistance to oxidation and
corrosion. Silicon carbide has a low coefficient of expansion ,
good heat transfer properties, and maintains high strength at
elevated temperatures. In recent years, the art of producing
high density silicon carbide bodies from silicon carbide pow-
ders has been developed. Methods include reaction bonding,
chemical vapor deposition, hot pressing, and pressureless sin-
tering (initially forming the article and subsequently sinter-
ing). Examples of these methods are described in U. S. Patent s
Nos. 3,853,566; 3,852,099, 3,954,483; and 3,960,577. The hig~
density silicon carbide bodies so produced are excellent engir _
eering materials and find utility in the fabrication of compo-
nents for turbines, heat exchange units, pumps, and other
equipment or tools that are exposed to severe wear and/or
operation under high temperature conditions. The present in-
vention relates both to silicon carbide powder mixtures that
are adapted to use in the various methods of producing a high-
density silicon carbide body by hot pressing or sintering and
to the ceramic articles produced therefrom.
In order to obtain high density and high strength silicor
carbide ceramic materials, various additives have been utiliz~ d.
For example, a method of hot pressing silicon carbide to den-
sities in order of 98 percent of theoretical by addition of
aluminum and iron as densification aids is disclosed by
11
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.
~Alliegro, et al, J. Ceram. Soc., Vol. 39, ~lo. 11, Nov., 1965,
pages 386 to 389. They found that a dense silicon carbide
¦could be produced from a powder rnixture containing 1 percent
¦by weight of aluminum. Their product had a modulus of rup-
¦ture of 54,000 psi at room temperature and 70,000 psi at
~1371C. More recent advance is the use of boron as a densi-
ication additive, usually in the range of between about 0.3
and about 3.0 percent by weight of the powder. The boron
additive may be in the form of elemental boron or in the form
of boron-containing compounds, for example, boron carbide.
Examples of silicon carbide powders containing boron may be
found in U. S. Patents Nos. 3,852,099; 3,954,483; and
3,968,194.
:.
I SUM~5ARY OF THE INVENTION
It has now been found that high densification may be ob-
tained when the sintering of silicon carbide-containing pow-
ders which include beryllium as a densification aid is carried
out in the presence of a beryllium-containing atmosphere. By
performing the sintering operation in an atmosphere containing
; 20 beryllium, the amount of beryllium which would be normally
removed from the powder compact is reduced, and the sintered
ceramic product has a more consistent composition and is less
porous than sintered products produced when beryllium is
simply used as an additive in the powder. Beryllium may be
added to the furnace atmosphere by inclusion into the sinterin~
chamber of compounds of beryllium which produce a significant
vapor pressure in the sintering temperature range. Such com-
pounds mdy sui ably be introduced into the sintering chamber
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by forming a solution or slurry of the beryllium compound and
applying the solution or slurry to the interior of the cham-
ber. Suitably, acetone is used as the carrier, but other
carriers, such as water or other available liquids, may be
~employed, their only purpose being to enable good distribution
of the beryllium material on the walls of the sintering cham-
ber. A beryllium atmosphere may suitably be provided by a
cover mix, a powder composition containing a beryllium source,
for example, a mixture of silicon carbide and beryllium car-
bide. When using a cover mix, the article to be sintered is
placed within the cover mix and the article in the mix exposed
to sintering conditions. Alternatively, beryllium may be
added to the furnace atmosphere by the use in the sintering
chamber of a beryllium compound, per se, or by the use of
furnace components, containers, crucibles and the like which
contain a significant amount of beryllium. Crucibles utilized
repeatedly in the production of sintered silicon carbide arti-
cles by the present process may build up a concentration of
beryllium. The beryllium content of such crucibles may be
monitored by standard analytical techniques, e.g., emission
spectroscopy, to determine the amount of beryllium in the
crucible and if additional beryllium is required to produce
the beryllium atmosphere of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
The starting silicon carbide powder, containing from about
0.5 to about 5.0 percent by weight excess carbon, is admixed
with finely-divided beryllium or a beryllium-containing com-
pound. Preferably, the particle size of both components is
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Illess than 5 microns and, more preferably, less than 2 microns.
¦IExceptionally good distribution is obtained when the compon-
ents are less than 1.0 microns. In order to obtain densifi-
~¦cation, the beryllium or beryllium-containing additive should
¦be utilized in an amount whereby between about 0.03 and about
~1.5 percent by weight of the powder is beryllium. The use of
less than about 0.03 percent by weight has not been found to
substantially increase the density of the sintered product.
l The addition of more than about 1.5 percent by weight of
beryllium may be detrimental to densification.
A bulk density of at least 75 percent of theoretical is
required for most applications, and bulk densities of at
least 85 percent of theoretical are more often required.
Sintered products having densities of 85 percent of theoreti-
cal may be obtained by the process of the present invention.
The beryllium additive of the present invention may be
utilized alone or may be mixed with other densification aids,
the most usual being boron in the form of elemental boron or
boron-containing compounds. Boron in amounts between about
0.10 and about 1.5 percent by weight are useful; however, den-
sities of over 90 percent of theoretical are obtainable when
the boron additive is included in amounts of from about 0.1 to
about 0.3 percent by weight. In general, such mixtures, when
ready for sintering, contain from about 0.03 to about 1.5
percent by weight of beryllium and a total of between about
0.03 and about 3.0 percent by weight of densification aids.
The silicon carbide source material is preferably a sub-
micron powder having a surface area greater than 8.0 m2/gm
and containing from about 0.5 to about 5.0 percent by weight
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of excess carbon. Generally, powder compositions having
surface areas between about 5 and about 20 m2/gm are found
eminently useful. The excess carbon may be introduced, for
¦ example, during the production process, by the subsequent
¦ addition of carbon or a carbonaceous material, or as a
¦ binder prior to sintering.
The beryllium or beryllium-containing additive starting
materials found useful are generally less than 50 microns in
particle size and, preferably, less than 10 microns in
particle size. A particle size of less than 5 microns is
eminently useful for ease of even distribution of the beryl-
lium or beryllium-containing additive with the silicon carbide
powder to obtain a homogeneous mixture useful in sintering.
Other additives may be utilized but are not necessary for the
promotion of densification during the sintering process.
Preferably, the sintering operation is carried out in
an inert atmosphere; gases, such as argon or helium, being
inert to silicon carbide at the sintering temperature range
are aptly suited to use. A reducing atmosphere may also be
utilized.
The present invention utilizes a beryllium-containing
atmosphere during the sintering operation. The use of beryl-
lium in the sintering atmosphere yields marked improvement
when the partial pressure of beryllium in the atmosphere
during sintering is equal to or greater than the equilibrium
vapor pressure of the beryllium contained in the silicon car-
bide powder compact. When the partial pressure of berylllum
in the sintering atmosphere is the same or greater than that
of beryllium in the article to be sintered, there will be no
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loss of volatile beryllium during the sintering operation.
The now residual beryllium in the article acts as an aid in
densification. At sintering temperatures, the partial pres- ¦
~sure of beryllium in the atmosphere is usually at least 10 4
atmosphere and, more preferably, at least lO 3 atmosphere.
The silicon carbide powders containing beryllium or berylL
lium-containing compounds as densification aids generally con-
tain beryllium in amounts between about 0.03 and about 1.5
percent by weight and, more preferably, from about 0.04 to
about 1.25 percent by weight. The final sintered material
usually contains about the same percentage of beryllium. It
has been found that sintering in a beryllium-containing atmos-
phere does not appear to substantially change the amount of
beryllium in the final product. The beryllium atmosphere
functions to inhibit the escape of beryllium from the powder
compact during the sintering operation without adding any
significant amount of beryllium to the product.
Thus, in pressureless sintering, a silicon carbide powder ,
containing from about 0.5 to about 5.0 percent by weight of
excess carbon, is mixed to form a homogeneous mixture with
beryllium or a beryllium-containing additive so that a total
of between about 0.03 and about 1.5 percent by weight of
beryllium is present. The homogeneous mixture is then shaped
into a green product. Suitable additives to increase flow
and binding of the particles may be incorporated into the
starting mixture. The green product is subsequently sinteredj
in an inert or in a reducing atmosphere in which the partial
pressure of beryllium is equal to or greater than the equili-
brium vapor pressure of the beryllium contained in the silicor
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jlcarbide powder compact at a temperature of between about 1950
~and about 2300C. for a time sufficient to obtain a silicon
~carbide product having a density greater than 75 percent of
theoretical. More particularly, a silicon carbide powder
having a surface area of approximately 11 m2/gm and containing
about 2.0 percent by weight excess carbon may be admixed with
between about 0.04 and about 1.25 percent by weight of beryl-
lium, suitably added as Be2C, or in elemental form. The
resultant mixture is then pressed to a density of about 1.76
gm/cm3. Binders may be used to increase the flowability of
the powder or to increase the green strength of the pressed
product. The pressed compacted powder is then sintered, pre-
ferably in an inert atmosphere, in which the partial pressure
of beryllium during sintering is about 10 4 atmospheres or
greater. The sintering operation is generally carried out at
a temperature of about 2100C. for a period of about 30 min-
utes. After cooling, the sintered product typically has a
density of greater than 85 percent of theoretical.
The invention will now be illustrated by more specific
examples which further illustrate various aspects of the inven- .
tion but are not intended to limit it. Where not otherwise
specified in this specification and claims, temperatures are
given in degrees Celsius, and all parts and percentages are
by weight.
l EXAMPLE 1
i CONTROL
A silicon carbide powder having the following specifica-
tions was uti ized as a starting material. The silicon car-
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bide powder had a surface area greater than 8.0 m2/gm and the
~following analysis in percent by weight:
Oxygen less than 0.8
Iron less than 0.2
Aluminum less than 0.4
Nickel less than 0.1
Titanium less than 0.1
Tungsten less than 0.5
Free Silicon less than 0.4
Silicon Carbide greater than 97.5
A composition comprised of 95% of the silicon carbide
powder characterized above was mixed in acetone with 5% of
a phenolic resin known as Resin No. 8121, a product of Varcum
Chemical Company. The slurry was comprised of about one part
by weight mixture to about one part by weight acetone. The
slurry was mixed for about 30 minutes and the acetone then
allowed to evaporate. The resulting powder mixture was
pressed into 1/2 inch diameter pellets weighing about 1-1/2
grams each. The pellets or powder compacts typically had a
density of about 1.76 gm/cm3.
Pellets prepared by the above procedure were placed in a
graphite crucible, the crucible covered and pushed through a
graphite resistance heating element tube furnace having a hot
zone temperature of 2080C. using an argon atmosphere. The
bulk density of the pellets after passing through the tube
furnace was 1.83 gm/cm3, about 57% of theoretical.
EXAMPLES 2 AND 3
BERYLLIUM ADD I T ION
Six mixes having the powder composition of Example 1 were
1,
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prepared, except that varying amounts of beryllium in the form
¦of beryllium carbide, having a particle size of less than 10
microns, were added to each mix. The composition of the mixes
is shown in Table I. Four pellets 1/ inch in diameter and
weighing about 1-1/2 grams each were pressed from each mix,
using the technique of ~xample 1. These pellets were divided
into two sets, A and B, with two pellets per mix in each set.
The pellets from Set ~ were fired in a graphite resistanc~
heating element tube furnace in accord with the procedure
utilized in Example 1. Pellets from Set B were fired in a
manner similar to Set A, with the exception that a cover mix
was used to surround the pellets in the crucible. The cover
mix was in the form of a powder having the composition 97.5%
silicon carbide, 2.0P6 carbon and 0.5% beryllium in the form of
beryllium carbide. The purpose of the cover mix was to in-
crease the amount of beryllium in the atmosphere around the
pellets.
The bulk density of the pellets was determined both befor~
and after sintering and is shown in Table I. Thus, the
pellets in Set A, fired in crucibles having no beryllium-con-
taining atmosphere, ranged in fired density from 68.5 to 75.4%
of theoretical. ~he pellets in Set B, fired in crucibles
having a beryllium-containing atmosphere, ranged in fired
ensity from 68.8 to 93.5% of theoretical.
EXAMPLE 4
ADDITIVE MIXTURES
A silicon carbide powder having a composition similar to
that in Example 1 was prepared and divided into batches. Var-
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ious amounts of finely-divided boron carbide and beryllium
~carbide were separately added to each batch to obtain the pow-
~der compositions recited in Table II. The various batches
were then mixed wi-th a carbon source and pressed into pellets
as in Example 1.
Two pellets from each batch were placed into a crucible
of the same composition, covered with a crucible lid and
placed within a graphite boat 4 inches in diameter and 19
inches in length. The pellets were sintered by pushing the
¦ boat containing the pellets through a graphite resistance
element tube furnace operated under an argon atmosphere. The
pellets had a residence time at 2150C. of 30 minutes. The
results are shown in Table II. Thus, following this proce-
dure, in Mix ~8, a silicon carbide starting material contain-
~ing 0.10% boron, 0.10% beryllium, 2.0% carbon and 97.80%
¦silicon carbide was pelleted to a cured density of 1.72 gm/cm3 ,
or 53.6% of theoretical. After firing, the density was found
to be 2.98 gm/cm3, or 92,8~ of the theoretical density of
¦silicon carbide.
A control sample containing only 0.5% boron and no beryl-
lium was prepared as above described. The control sample was
sintered in a manner similar to that described above, except
that beryllium and boron were absent from the sintering atmos-
phere. After firing, the control sample was found to have a
¦bulk density f 79.0~ oi theoreticel.
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