Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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BACKGR~UND OF THE INVENTION
Field of the Invention
The invention relates to processes for fabricating
refractory articles. More specifically, the invention concerns
high temperature molding of fully dense silicon nitride.
Prior Art Statement
The following patents are representative of the
most relevant prior art known to the Applicant at the time
of filing of the application:
UNITED STATES_PATENTS
3,4Ç8,992 September 23, 1969 Lubatti et al
3,549,402 December 22, 1970 Whitney et al
3,819,786 June 25, 1974 May
3,830,652 August 20, 1974 Gazza
3,835,211 September 10, 1974 Coe et al
- 3,839,540 October 1, 1974 Arrol
While the essence of the present invention resides
in a novel hot-pressing method of forming silicon nitride
products, the other commonly used forming methods, sintering
and reaction bonding, are also relevant. The relevance of
the latter two forming methods is in the fact that they may
be used to preform product that may subsequently be subjected
to the process of the present invention, particularly when
` the configuration of the final product is relatively simple.
Lubatti et al discloses a typical sintering method
for forming silicon nitride consisting of mixing finely
particulate silicon nitride with up to 25% of a binder which
may be boric acid, boron anhydride, or boron phosphate;
' compressing the mixture in a mold at room temperature under
;, 30 from 0.5 to 5 tons per square centimeter; and sintering the ~-
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compact in a nitrogenous atmosphere at a temperature of from
850 to 1250C to produce essenti~lly ~-silicon nitride.
A reaction bonding or reaction sinterlng process is
described by May involving a mixture of ~00 mesh silicon
powder and up to 35~ of a temporary organic binder system e.g.
butyl methacrylate and trichloroethylene. Some of the
trichloroethylene solvent i5 removed to produce a mix that is
stiff at room temperature but flexible and doughy at 50C; the
preformed article is molded at ambient temperature and then ~
10 heat treated up to 130C to remove the remaining solvent. The ~ ~ -
preform is then heated to 1000C at a rate of 50C per hour
with a continuous flow of nitrogen through the furnace,
resulting in decomposition and removal of the temporary
organic binder. The preform which is now made up essentially ;~
.j of silicon is further heat treated in a static pressure of
- nitrogen at 1 psi to an ultimate temperature of 1450C; the l -
final product is reaction bonded ~-silicon nitride with a bulk
density of 2.24 g/cc. '-
Both Gazza and Whitney et al teach the fabrication of ~ ~
`, 20 silicon nitride products by the hot-pressing of silicon nitride. ;
~! The principal difference between the two references is the
compound each adds to the powdered silicon nitride prior to
hot-pressing. Gazza adds 1.0 to 3.5% by weight of a yttrium
compound, while Whitney et al add a lithium compound. In both
references the silicon nitride-additive mixture is placed in
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, a graphite mold; Gazza presses between 1750 and 1800C at a
- pressure of 6000 to 7000 psi while Whitney et al presses at
-~i 1395 to 1900C and a pressure as low as 2000 psi.
The process taught by Arrol involves the concept of
forming an initial compact by a first step of reaction bonding
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followed by a hot-pressin~ step. This dual step approach may
also be employed within the scope of the present invention.
Arrol takes 3 micron silicon powder, blends in a magnesium
oxide fluxing ayent and a temporary organic binder system.
The mixture is heat treated to remove the vehicle for the
organic binder. The resulting molding powder is then compacted
to the desired shape but with dimensions slightly greater than
those desired in the final product. The silicon preform is
then nitrided by heating at a temperature not to exceed 1300C ;~
in an atmosphere of 90~ nitrogen, forming a porous ~-silicon
nitride body with a density of 1.7 to 2.7 g/cc. The ~-silicon
nitride shape is then placed in a graphite mold and hot-pressed
at a pressure of 4000 psi and a temperature of 1750C, to the ;;
articles desired final dimensions and density. In another
embodiment, a modified nitriding step is employed wherein the
nitriding takes place first at 1250C whereafter the temperature
is increased to 1400C and then to 1550C to convert the silicon
to predominantly ~-phase silicon nitride. The porous low density
article is then coated with a release agent and placed in a
graphite mold, and finally hot-pressed at 4,000 psi at a
temperature of 1750C to give a finished product with a density
- between 2.2 and 2.7 g/cc.
The most relevant reference is the patent to Coe et al -~
which is directed specifically at a method of hot-pressing
silicon nitride and the conversion of the silicon nitride
from the ~ to the ~ form. Coe et al take ~-silicon nitride
powder, into which 2~ magnesium oxide has been blended, place
it in a graphite mold set-up of desired internal configuration
and hot-press the mixture to full density. The pressing
conditions for full densification are a temperature of from
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1500-1850C and a pressure of from 1000-5000 psi. Once full
j- densification is indicated, the temperature and pressure are
maintained for at least another 10 minutes to convert the
~-silicon nitride to ~-silicon nitride. Alternatively, the
silicon nitride powder may be hot-pressed to full density and
heated further to fully convert the ~-silicon nitride to the
' ~ form without pressure.
SUMMARY OF THE INVENTION
Thus, in accordance with the present teachings, a
method is provided of forming fully dense ~-silicon nitride
products which comprises the steps of placing silicon nitride
which contains a substantial amount of ~-silicon nitride in a
mold, converting the a-silicon nitride to ~-silicon nitride by
heating and pressing the powder at a temperature suf~iciently
high to cause conversion of the ~-silicon nitride to ~-silicon
nitride but at a temperature and at a pressure low enough to
avoid full densification of the silicon nitride, holding the
temperature and pressure conditions until essentially all of
the ~-silicon nitride has been converted to ~-silicon nitride
and fully densifying the ~-silicon nitride by increasing the
temperature and/or pressure~
The invention process is a hot-pressing method
directed specifically at producing fully dense articles of
silicon nitride, in which the silicon nitride is converted
from a-silicon nitride to ~-silicon nitride during the hot-
pressing process. A molding powder which is predominantly
~-silicon nitride (and which may include any of the known
; sintering aids or fluxing agents) is placed in an appropriately
designed graphite hot-pressing mold. The mold and its contents
are slowly heated to 1500C at a pressure below that which
would bring about full densification of the silicon nitride
e.g. atmospheric pressure to 1500 psi, but preferably about
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500 psi. The temperature and low pressure conditions are
maintained for a time period sufficient to allow essentially
complete conversion of the a-silicon nitride to the ~ form.
At this point the temperature is increased to as high as
1850C and the pressure increased to at least 2000 psi, thus
bringing about full densification of the ~-silicon nitride.
The principal objective is to heat the a-sllicon
nitride, convert it to the ~ form without bringing about full
densification, and then finally fully densifying. This can
be accomplished with a variety of temperature-pressure
conditions. The ~ to ~ conversion step can be carrled out
without
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significant densification using, up to a maximum pressure of
1500 psi at 1400C, up to 1850C at about atmospheric pressure,
or any combination of temperature and pressure therebetween.
Similarly, the step of densifying the ~-silicon nitride can be
carried out at from 1500-1850C at a pressure of at least
2000 psi.
; According to the Coe et al hot-pressing method, full
~ densification must take place first, followed by the a to ~
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phase conversion caused to take place under pressure. The
major problem attendant the Coe et al process is that during
the pressurized ~ to ~ convarsion phase of the process, the
- costly graphite mold band fre~uently ruptures, probably due ;
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to the change in volume of the silicon nitride which takes
place as a result of the ~ to ~ phase change.
` DESCRIPTION OF THE PREFERRED EMBODIMENTS ` ` -:
While it is not a requirement for the sucessful ;
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practice of the invention, it is preferred that the particle ~~"
size of the powders employed, have an average particle size of
about 3 microns or finer. As is well known in the art, the
highest strength final product is produced when 3 micron or
` finer powders are used. The starting silicon nitride powder
should be at least 50% alpha.
The processing parameters which constitute the best
mode of practicing the invention are as follows: for the -~
to conversion step heat at from 1400-1750C at a pressure
of from 0 - 1000 psi for 30 to 180 minutes; for full ~;~
densification, increase tha temperature to 1600 - 1850C,
increase the pressure to at least 2000 psi and maintain these
conditions for 10 - 60 minutes. These parameters of course ;~
are variable to a substantial degree in both directions as a ~ ~
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function of the presence of other materials such as sintering
aids, the thickness and configuration of the piece being
processed, the amount of a versus ~ silicon nitride in the
; original powder, and so on. The following is an example of
the best mode of practicing the invention.
EXAMPLE
- . ,
A mixture of 4000 grams of a-silicon nitride and 120
grams of magnesium carbonate were placed in a tungsten carbide
lined milling jar 1/3 full of 1/2" tungsten carbide balls. To
this was added 9000 milliliters of isopropanol. The mixture
was milled for 16 hours after which it was screened through
a 500 mesh screen and dried at 90C. The resulting cake was
broken up by tumbling in a plastic jar containing rubber balls,
for 2 hours.
- A 2254 gram quantity of the mixture was placed in a
boron nitride coated graphite mold set~up having a cavity
therein, measuring 6-1/4 x 6-1/4. The mold was fully assembled
` and the contents thereof hot-pressed according to the
following schedule to obtain a piece approximately 6-1/4 x -
`;; 20 6-1/4 x 1
-- (a) Temperature increased from room temperature to ~ ;
1500C over a period of 100 minutes while
` maintaining a pressure of 500 psi;
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-;i (b) Temperature and pressure maintained at 1500C
and 500 psi for 90 minutes;
(c) Pressure increased 250 psi every 10 minutes
until pressure reached 3000 psi while
maintaining temperature at 1500C;
(d) Pressure held at 3000 psi while temperature
raised from 1500C to 1740C over a period ;
of 40 minutes;
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(e) Temperature held at 1740C and pressure at :
- 3000 psi for about 25 minutes;
: (f) Heat and pressure were terminated ant the
mold and its contents allowed to cool.
The resulting piece measured approximately 6-1/4 x
6-1/4 x 1" had a density of 3~22 g/cm3, a cross-bending strength
of 130,050 psi under 4 point loading, and was essentially all
~-silicon nitride. ~ ~:
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