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METHOD OF MAKING HIGH STRENGTH HOT PRESSED Sweeney
BACKGROUND OF THE INVENTION
AND PRIOR ART STATEMENT
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This invention is directed to the art ox making
silicon nitride from silicon powder as a starting material;
the powder and selected densiication aids are subjected to
a reactive nit riding gas to form a mixed phase (alpha and
beta) silicon nitride body, the body being then hot
pressed. The prior art has consistently measured the mean
particle size of powder ingredients (including silicon
powder and oxygen carrying agents such as Moo) for making
a hot pressed silicon nitride body (see US. patents
3,839,541; 3,839,540; and 3,591,337). This allows a
truncated particle size distribution permitting some, but
15 still significant, particles larger than the mean size and,
of course, several particles smaller than the mean. such
distribution, based on a mean measurement along with a
known comminution mode, has been useful because it
correlated well with packing efficiency for making a cold
compacted preform with good density. Unfortunately, the
presence of several large particles tolerated by a mean
particle size measurement, permits an increased number of
flaws to form in the reliant silicon nitride product.
The average eyeball characteristic strength (modulus of
rupture) is about 100 ski for a silicon nitride product
formed with only mean particle size control.
Even though mean particle size control, a per the
prior art, failed to control flaw formation in the final
product, the broad particle size distribution did facile-
late greater packing efficiency when consolidating the powders under pressure. It would be useful if a method
were available which not only eliminated much of the
present flaws in a silicon nitride hot pressed product
which tend to hold the Wobbly characteristic strength
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level to no greater than 100 ski, but also would not reduce
the packing efficiency of the powder ingredients if a
special compression ratio was desired.
SYRIA OF THE INVENTION
The invention is a method of making an improved
hot pressed silicon nitride comprising object by (a)
forming a powder mixture of silicon and reactive oxygen
carrying agents, the mixture being dry, wet or jet milled
to an absolute particle size distribution having sub-
10 staunchly no particle greater than 16 microns, (b)
compacting the mixture to form a preform having density
of about 1.0 gm/cm3, (c) heating the preform in a nit riding
atmosphere without the use of pressure, normally associated
with hot pressing, to substantially fully react said powder
15 ingredients with said atmosphere to form a body consisting
substantially of silicon nitride, and (d) hot pressing the
nitride preform to produce silicon nitride comprising
object of desired dimension and density.
. Preferably the reactive oxygen carrying agents
20 are selected from the group consisting of YO-YO, Moo, Sue,
Zr2 and AYE. The amount of the oxygen carrying agents
permitted in the mixture is preferably limited to percent
by weight of the mixture): 3.2-16~ YO-YO and .4-4% AYE.
Optimally, the maximum absolute particle size of the oxygen
25 carrying agents should be substantially less than the
maximum silicon particle size
The mixture can additionally contain a silicon
nitride composite in an amount of 2 US% by weight of said
mixture, said silicon nitride composite increasing the
30 particle packing efficiency, increasing green density to
about 1.4 gm/cm3, and decreasing the hot pressing
compression ratio.
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The silicon nitride object resulting from the
practice of this method will possess improved physical
properties, specifically four point flexor] strengths in
excess of 150 ski at room temperature and hardness (45-N)
of 92. The room temperature flexural four point bend
strength was determined in an In~tron~machine (Model 1122)
using a self aligning steel fixture with an outer span of
19.05 mm (0.750") and an inner span of 9.525 mm (0.375")
respectively. The test bars were 31.75 mm (1.25") long by
6.35 mm (0.250") wide by 3.175 mm (0.125") thick.
SUMMARY OF THE DRAWINGS
Figure 1 is a photograph~~l000 X-magnification) of
a silicon nitride material produced in accordance with
prior technology, and without the teachings of this
15 invention, it a particle size distribution curve
containing particles greater than lo microns. The
photograph illustrates a high concentration of dejects,
here iron solaced, in the fully hot pressed product, which
is a strength determining flaw in the final product;
Figure 2 is a photograph of a diamond ground
surface of the material of Figure 1 showing in greater
clarity the large grain beta silicon nitride particles
associated with the presence of iron solaced, a portion
having been pulled out as a result of the go inning
25 operation.
DETAILED DESCRIPTION
To eliminate flaws that might occur in the final
product, as illustrated in Figures 1 and 2, the particles
that cannot nitride must be eliminated A considerable
30 amount of such flaws are associated with the presence of
free silicon particles after the nit riding step has been
completed. These free silicon particles are available to
form silicides with the impurities that are present in the
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starting powder ingredients. When iron solaced is
subjected to high temperature, such as in the final
nit riding stage and during hot pressing, it forms a
liquid into which the alpha silicon nitride dissolves.
5 Cooling or solidification of these iron rich solutions
causes a growth of very large sized beta silicon nitride
particles surrounded by considerable porosity and
consistently associated with the iron solaced solidified
adjacent thereto (see Figure 1 at A. Large beta Sweeney
10 grains or defects can also be produced by other silicon
metal impurities, such as chromium, manganese, etc., in
like fashion. These localized areas of solaced,
accompanied by large grain beta silicon nitride and
porosity (see Figure 2 at B), form a weak area or zone
15 within the final hot pressed product which limits the
overall Wobbly characteristic strength level (a modulus of
rupture) of the silicon nitride material to consistently
about 100 ski or less, and with relatively low associated
values (Wobbly modulus) of 8 or less. The presence of
20 free silicon is a direct result of inadequate nit riding;
nit riding is a diffusion process dependent upon the
distance the gas phase must penetrate to obtain chemical
reaction. Oxygen is known to reduce nitrogen diffusion and
therefore interfere with efficient nit riding of silicon.
25 Oxygen carrying agents used in reaction bonded silicon
nitride technology impede this nit riding process. Silicon
particles that could be nitride efficiently in the absence
of oxygen carrying agents are found to no longer nitride
efficiently in the presence of oxygen carrying agents using
30 state of the art nit riding technology. when there are
large sized particles in the mixture, the diffusion
distance is not sufficient to entirely penetrate such
particles and incomplete nit riding results, allowing such
free silicon to remain in isolated pockets within the
35 nitride product. It is difficult, and in most cases
undesirable, to remove impurities within the starting
ingredients which may combine with such free silicon.
For example, up to 1.0~ iron, either as metallic or
combined, is present in silicon powder purchased
commercially on the open market. The silicon also contains
silicon dioxide as an oxide film on each of the grains
which impedes the nit riding process and is difficult to
strip or reduce. The iron tends to break up the Sue film
on the silicon, facilitating nit riding. Iron in the form
of Foe is sometimes added to facilitate the breakup of
the Sue layers. Moreover, the expense of attempting to
purify the starting ingredients beyond that which is
commercially available is highly exorbitant and is not a
desirable alternative solution.
A preferred method for eliminating such flaws in
the final product of a hot pressed silicon nitride body
according to the invention herein is as follows.
Forming Mixture
1. A mixture of powdered silicon and reactive
oxygen carrying powder agents is prepared. The reactive
oxygen powder carrying agents is defined herein to mean
powder ingredients that are effective to form second phase
crystallizes, particularly oxynitrides, when reacted with
the silicon under a heated nitrogen atmosphere. The powder
agents can be advantageously selected from the group
consisting of YO-YO, AYE, Moo, Sue, ZrO2, HfO2, and other
rare earths. Use of these agents will improve physical
characteristics and formation of a second phase crystallite
which will be uniformly dispersed and substantially
displace the detrimental glassy phase normally formed,
except for a controlled and limited amount of such glassy
phase. For purposes of the preferred method, YO-YO and
AYE are used as the reactive oxygen carrying agents
along with Sue, which is normally inherent as an oxide
:~276;;~3
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film on the silicon powder. YOKE it normally required to
be present in an amount of about 3-16~ by weight ox the
mixture and AYE it normally required to be prevent in an
amount of about .4-4%. For purpose of the preferred
method, a uniform powder mixture is prepared with 2000
grams of silicon (86.6% by weight of the mixture), 278
grams of YO-YO (12% by weight of the mixture), and 32 grams
of AYE (1.4% by weight of the mixture for purposes of
this invention).
The silicon starting powder that it usually
commercially available is 98~ pure or greater. The average
starting particle size of such silicon powder is usually in
the range of 10-20 microns, which permits particles of 100,
150, and up to 540 microns to be present as well as smaller
15 particles lower than 16 microns. The major trace metal
contaminants normally experienced with such commercially
available silicon powder include: iron in an amount of
loot aluminum up to .5%, manganese up to .09%, and calcium
up to .09%. Nonmetallic contaminants normally include:
20 carbon up to .05%, and oxygen less than .5%. The yttria is
usually available on a commercial market with a purity of
99.99% and average crystal size of about .0438 microns (438
angstroms) with particles as large as 40 microns. The
alumina usually has a purity of 99.5~ and is usually added
25 separately, but can be added by attrition during the
milling operation. The alumina will have an average
particle size of about .3-.5 microns with some particles
as large as 50 microns.
Milling to Strict Particle Size Control
2. The mixture is then commented and blended by
being charged into a dry milling apparatus or by being
subjected to jet milling. The dry milling normally is
carried out by use of a ball milling device, where balls or
other equivalent milling media are introduced to the
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mixture and the apparatus rotated so as to cause the
particles to grind against the milling media over an
extended period of time. Dry milling has to be coupled
with classification to ensure no particles are larger than
5 16 microns. Roy wet milling normally is carried out by use
off wet milling device, where balls or other equivalent
milling media and a liquid are introduced to the mixture
and the apparatus rotated so as to cause the particles to
be reduced to the desired size. Jet milling is a method by
10 which the particles are caused to impinge upon each other
under jets of gas (air or inert) so as to cause
comminution.
For purposes of this preferred method, the mixture
is charged into an inert milling jar along with grinding
15 media in the form of Burundum cylinders (85% AYE, 11%
Sue, and 4% other ingredients) for a period of 48 hours at
64 rum. The mixture is then separated from the media by
use of a #10 mesh screen. The dry milled mixture is air
classified and/or sieved to insure that no particle in the
20 mixture is greater than 16 microns and optimally no
particle should be greater than 3-4.5 microns. The milling
is carried out for a sufficient period of time to insure
that no particle in the mixture is greater than 9 microns
and optimally no particle should be greater than 3-4. 5
25 microns. In order to determine whether the mixture is of
slush limited particle size, x-ray sedimentation,
centriEugation, sieving, and laser light scattering tests
may be employed
O By reducing the absolute maximum particle size,
30 the particle size distribution becomes such that packing
efficiency is detrimentally affected. Without the
intermingling of somewhat large and small particles, the
packing of the entire powder mixture becomes limited. The
poor packing does not detrimentally affect the chemical
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reaction. The thicker preforms that result prom a poor
particle size distribution results in very long hot
pressing strokes.
If a small compression ratio is desired, the poor
packing efficiency can be remedied by adding 2-25% of
silicon nitride composite powder. This silicon nitride
composite will have a particle size distribution which when
combined with the silicon mixture will give the desired
packing efficiency. The silicon nitride composite is made
according to the teachings of this invention (using a
maximum particle size of 16 microns or less) for forming a
mixture, compacting, and heating to nitride. The nitride
body is then dry milled to the desired particle size
distribution. The silicon nitride composite powder must be
the same chemistry as the resulting nitride body without
the addition of composite. This injures a completely
homogeneous body with the same chemistry throughout. This
homogeneous approach (adding silicon nitride composite of
the same chemistry as the resulting nitride body)
20 eliminates areas of local high silicon nitride concern-
traction without second phase chemistry (low density areas)
that is obtained by the addition of silicon nitride to a
silicon powder mixture. The larger sized particles of the
silicon nitride composite are useful and present no bad
25 effects; no free silicon results from the use of the
silicon nitride composite.
Com~actiny
3. A measured quantity of the milled mixture is
then loaded into a cold pressed die arrangement and pressed
30 at ambient conditions by use of typically 1400-1500 psi to
form a compact of a size of 6 x .6" and a density of about
1.0 gm/cm3. With the addition of 2-25% of the silicon
nitride composite, the density of the compact becomes at
least 1.4 gm/cm3.
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Heath to Nitride
4. The compact is heated in a nit riding atoms
phone without the use of pressure normally associated with
hot pressing to produce a silicon nitride comprising body
consisting of mixed phase (alpha or beta) silicon nitride,
at least one dispersed second phase crystallite (silicon
oxynitride), .2-1~ silicate (by weight of the body), and
unmeasurable mounts of free silicon and unrequited oxygen
carrying agents (here YO-YO and AYE). The body will have
a size greater than and a density lest than the object to
be formed. As the result of the use of a critical particle
size no greater than 16 microns, conventional nit riding
techniques can be used to nitride silicon in the presence
of oxygen carrying agents as used in reaction bonded
silicon nitride technology (see nit riding techniques in
Mantels US. patent No. 4,356,136.
Because of the use of an absolute maximum particle
size of 16 microns, the nitridPd body will preferably
consist of silicon nitride which has at least 60~ alpha
phase silicon nitride, 3-15~ silicon oxynitride in the
YlSiO2N phase, and the remainder silicate glass (which may
be theorized to be aluminosilicate.
The resulting thickness of the nit idled bodies
will be approximately the same thickness size as that of
25 the compact going into the nicker idling step.
5. The nitride body is then hot pressed to
produce a silicon nitride comprising object of Required
dimension and density. A pressing fixture having graphite
walls is used to carry out the hot pressing. The walls and
nitride body are both coated with a slurry of boron
nitride and dried. The pressing fixture with the nitride
body therein is placed in the hot pressing furnace. The
i . ., .,
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heating and pressing is carried out preferably in
increments up to the final hot pressing temperature of
3000F and pressure of 3700 psi, the latter conditions
being maintained until at least 99~ or desirably 99.5~ of
5 theoretical full density is achieved. This usually
requires .25-3.0 hours at the hot pressing temperature.
The object is then cooled at any rate, even quenched, to
room temperature. The resulting hot pressed object will
have a strength level (modulus of rupture) in the range of
10 150-190 ski and a density in the range of 3.28-3.32 gm/cm3.
The hardness is also increased as a result of the practice
of this invention from a typical hardness level (45-N
scale) of 90 to 92. The system using the silicon nitride
additive to the mixture, will experience less side wall
15 drag (between sides of the preforms and the die walls)
which will result in less distortion in the final product.
The nitride preforms formed from a mixture containing a
silicon nitride composite will exhibit a smaller height
dimension for the preforms and thus reduce the hot pressing
20 stroke.
