U.S. patent number RE30,286 [Application Number 06/021,878] was granted by the patent office on 1980-05-27 for method of producing high density silicon carbide product.
This patent grant is currently assigned to The Carborundum Company. Invention is credited to John A. Coppola, Richard H. Smoak.
United States Patent |
RE30,286 |
Coppola , et al. |
May 27, 1980 |
Method of producing high density silicon carbide product
Abstract
Disclosure is made of a high-density, high-strength silicon
carbide ceramic material that is produced using a silicon carbide
powder containing boron or boron-containing compound as a
densification additive by the utilization of boron in the sintering
atmosphere.
Inventors: |
Coppola; John A. (Lewiston,
NY), Smoak; Richard H. (Lewiston, NY) |
Assignee: |
The Carborundum Company
(Niagara Falls, NY)
|
Family
ID: |
26695205 |
Appl.
No.: |
06/021,878 |
Filed: |
March 19, 1979 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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Reissue of: |
743448 |
Nov 22, 1976 |
04080415 |
Mar 21, 1978 |
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Current U.S.
Class: |
264/674; 264/332;
264/676; 501/92 |
Current CPC
Class: |
C04B
35/565 (20130101); C04B 35/575 (20130101) |
Current International
Class: |
C04B
35/575 (20060101); C04B 35/565 (20060101); C04B
035/56 () |
Field of
Search: |
;264/65,332 ;106/44 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Alliegro et al., "Pressures Sintered Silicon Carbide", J. Am. Cer.
Soc., vol. 39, pp. 386-389..
|
Primary Examiner: Parrish; John A.
Attorney, Agent or Firm: Dougherty; David E. Green; Raymond
W. Studley; Donald C.
Claims
What is claimed is:
1. A method of sintering silicon carbide powders containing boron
or boron-containing compounds as densification aids to produce a
high density silicon carbide ceramic material which comprises the
step of sintering such powders in an atmosphere containing boron,
wherein the partial pressure of boron in the sintering atmosphere
is equal to or greater than the equilibrium vapor pressure of the
boron in the silicon carbide powder during sintering of said
powder.[...]..Iadd., said partial pressure of boron in the
sintering atmosphere being provided by boron from a
boron-containing slurry or solution applied to the interior of the
sintering chamber prior to sintering, or by the addition of a
boron-containing gas into the chamber during sintering
.Iaddend..
2. The method of claim 1 wherein the silicon carbide containing
powders include boron in an amount between about 0.1 and about 5.0
percent by weight.
3. The method of claim 1 wherein the atmosphere containing boron
includes an inert gas.
4. The method of claim 3 wherein the inert gas is nitrogen.
5. The method of claim 3 wherein the inert gas is argon.
6. The method of claim 3 wherein the inert gas is helium.
7. The method of claim 1 wherein the boron in the atmosphere is
introduced as boron.[.chloride.]. .Iadd.trichloride..Iaddend.
8. The method of claim 1 wherein the boron in the atmosphere is
introduced as boron carbide.
Description
BACKGROUND OF THE INVENTION
Silicon carbide, a crystalline compound of metallic silicon and
nonmetallic 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 powders has been
developed. Methods include reaction bonding, chemical vapor
deposition, hot pressing and pressureless sintering (initially
forming the article and subsequently sintering). Examples of these
methods are described in U.S. Pat. Nos. 3,853,566; 3,852,099;
3,954,483; and 3,960,577. The high density silicon carbide bodies
so produced are excellent engineering materials and find utility in
fabrication of components 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 invention
relates methods of producing silicon carbide articles that have
high-density and high-strength characteristics.
In order to obtain high-density and high-strength silicon carbide
ceramic materials, various additives have been utilized. For
example, a method of hot pressing silicon carbide to densities in
order of 98% of theoretical by addition of aluminum and iron as
densification aids is disclosed by Alliegro, et al., J. .[.Cram..].
.Iadd.Ceram. .Iaddend.Soc., Vol. 39, No. 11, Nov., 1956, pages 386
to 389. They found that a dense silicon carbide could be produced
from a powder mixture containing 1% by weight of aluminum. Their
product had a modulus of rupture of 54,000 psi at room temperature
and 70,000 psi at 1371.degree. C. More recent advance is the use of
boron as a densification 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
boron-containing silicon carbide powders may be found in U.S. Pat.
Nos. 3,852,099; 3,954,483; and 3,968,194.
SUMMARY OF THE INVENTION
It has been found that high densification can be obtained when the
sintering operation of silicon carbide containing powders which
include boron as a densification aid are carried out in the
presence of a boron consisting atmosphere. It is postulated that,
by performing the sintering operation in an atmosphere containing
boron, the amount of boron which would be normally removed from the
powder compact is reduced and that the sintered ceramic product has
a more consistent composition and is less porous than sintered
products produced when boron is only used as an additive. Boron may
be fed into the sintering operation in the form of a gas such as
boron trichloride in mixture with inert gases usually used, namely
nitrogen, argon, or helium. Boron may also be added to the furnace
atmosphere by inclusion into the sintering chamber of compounds of
boron which have a significant vapor pressure at the sintering
temperature. Such compounds may suitably be introduced into the
sintering chamber by forming a solution or slurry of the boron
compound and applying the solution or slurry to the interior of the
chamber. Suitably, acetone is used as the carrier, but other
carriers such as water or other available solvents may be employed,
their only purpose is to enable good distribution of the boron
material on the walls of the sintering chamber. Alternatively,
boron may be added to the furnace atmosphere by the use in the
sintering chamber of a boron compound, per se, or by the use of
furnace components, parts and the like, which contain a significant
amount of boron.
DETAILED DESCRIPTION
The silicon carbide powders which may be utilized to produce
high-density, high-strength silicon carbide ceramic material which
may be used in the present invention are those found in the prior
art. For example, those described in U.S. Pat. Nos. 3,852,099;
3,954,483; and 3,968,194. The present invention relates to the use
of a boron-containing atmosphere during the sintering operation.
The use of boron in the sintering atmosphere yields marked
improvement when the partial pressure of boron in the atmosphere is
equal to or greater than the equilibrium vapor pressure of the
boron contained in the silicon carbide powder compact.
The silicon carbide powders containing boron or boron-containing
compounds as densification aids generally contain boron in amounts
between about 0.2 and about 3.0 percent by weight. The final
sintered material usually contains about the same percentage of
boron. It has been found that sintering in a boron containing
atmosphere does not appear to substantially change the amount of
boron in the final product. The boron atmosphere appears to inhibit
the escape of boron from the powder compact during the sintering
operation without adding any significant amount of boron to the
product. Thus, in pressureless sintering, a silicon carbide powder
having from about 0.1 to about 2.0 percent by weight excess carbon
and containing from about 0.1 to about 5.0 percent by weight of
boron added as boron carbide is pressed into a powder compact and
sintered at 2100.degree. C. in a furnace in an inert atmosphere
such as argon or helium which is free of boron. The bulk density of
sintered compacts formed by this method is typically less than 2.9
gm/cm.sup.3 (90.3% of theoretical density). However, if a similar
powder compact is sintered in the same manner in an inert
atmosphere such as argon or helium in which the partial pressure of
boron is about 10.sup.-7 atmospheres or greater, the resultant bulk
density of the sintered compact is typically greater than 2.98
gm/cm.sup.3 (92.8% of theoretical density).
EXAMPLE 1
CONTROL EXPERIMENT
A submicron silicon carbide powder having the characteristics
listed below was used to demonstrate this invention.
Oxygen--0.3 wt %
Free Carbon--2.0
Aluminum--0.002
Iron--0.01
Specific Surface Area--12 m.sup.2 /gm
This powder, 99.5 parts, was mixed with 0.7 parts of boron carbide,
100 parts deionized water and 3 parts polyvinyl alcohol. The
mixture was rolled in a plastic jar for five hours using tungsten
carbide balls to promote mixing. The resultant mixture was poured
into a glass tray and the moisture removed by drying in a vacuum
oven. The dried powder cake was screened through a 60 mesh screen
and pressed at 12,000 psi into pellets, 11/8 inches diameter and
weighing approximately 10 gm each. These pellets were inserted into
a graphite crucible, the ends of the crucible were closed, and the
crucible plus pellets were pushed at approximately 1/2 inch per
minute through a 6 .Iadd.inch .Iaddend.diameter graphite resistance
heating element tube furnace. The hot zone of this tube furnace was
maintained at 2,150.degree. C. and the residence time of the powder
compacts in this hot zone was approximately 25 minutes. The
sintered powder compacts which contained approximately 0.5 percent
by weight boron before being run through the furnace were found to
contain approximately 0.05 percent by weight boron after the
sintering operation was complete. The bulk density of these powder
compacts averaged 2.57 gm/cm.sup.3 (80.1% of theoretical).
EXAMPLE 2
BORON IN THE FURNACE COMPONENTS
A graphite crucible, similar to the one described in Example 1 was
painted with a slurry of boron carbide and acetone to form a liquid
vehicle in an amount sufficient to provide 0.7% by weight boron
carbide based on the weight of the graphite crucible. A second set
of powder compacts of the composition of Example 1 was prepared by
the method described in Example 1 and was placed into the prepared
crucible containing a thin coating of boron carbide. The bulk
density of these compacts, measured after undergoing a sintering
operation described in Example 1, was determined to average 3.08
gm/cm.sup.3 or 96% of theoretical density. The boron content of the
sintered compacts was determined to be 0.5% by weight.
EXAMPLE 3
HOT PRESSING
The powder described in Example 1, 99.5 parts, was admixed with 1.2
parts boron nitride (approximately 0.43% boron by weight) in slurry
form using acetone as the liquid vehicle. The resultant mixture was
then dried and granulated by passing it through a 60 mesh
screen.
A hot pressing mold and plungers made of graphite and containing no
boron were used in this experiment. The granulated powder was
placed into this graphite mold, the plungers inserted and a
pressure of approximately 100 psi was applied. The mold was heated
within an induction coil to 2000.degree. C. over a period of two
hours; pressure was applied when the temperature reached
1650.degree. C. and after a hold period of 30 minutes at
2000.degree. C. the power to the furnace was shut off, the pressure
being released when the temperature reached 1750.degree. C. After
cooling in the mold, the hot pressed silicon carbide article was
removed and found to have a bulk density of 2.95 gm/cm.sup.3 (91.9%
of theoretical).
A second set of hot pressing mold and plungers, similar to that
described above, was coated with a slurry of boron carbide in
acetone to an extent that boron carbide in an amount equal to
approximately 0.7% by weight of the weight of the plungers and mold
set was applied. Approximately 100 gm of the granulated mix
described in Example 1, but with the addition of 1.2 parts of boron
nitride was placed into this mold and .[.pluggers.]. .Iadd.plungers
.Iaddend.set. After performing the hot pressing procedure similar
to that described above, the hot pressed silicon carbide article
was removed from the mold and found to have a bulk density of 3.18
gm/cm.sup.3 (99.1% of theoretical).
* * * * *