Note: Descriptions are shown in the official language in which they were submitted.
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.~ET~D OF PREPARING POr~DER INGREDIENTS
FOR S~BSEQUENT CONSOLIDATION
BACKGROUND OF THE INVENTION
In the manufacture of ceramic parts, powder
5 ingredients are first admixed and dry milled to uniformly
mix the ingredients and to create the proper proportions if
attrition or wear from the milling media is desired as part
of the ingredients. The milled powder mixture can then be
subjected to cold compaction, gas chemical treat~ent, and
10 hot pressing to produce the desired product. If the
milling is not complete, that is, the powder ingredients
are not reduced to a desired average or mean particle size
and desired homogeniety, then incomplete or unreacted
ingredients may result from subsequent gas chemical
15 treatment, resulting in a hot pressed product with internal
flaws. In the case of the manufacture cf a silicon nitride
cutting tool material, these flaws may originate by the -
oxidation of free silicon and impurities, such as iron
oxide, or unreacted additives to form silicide particles,
20 such as iron silicide. The silicide forms a solute which
dissolves silicon nitride during hot pressing and, when
cooled, recrystallization results in large beta Si3N4
particles adjacent the silicide along with porosity. These
are soft spots in the ceramic which pull out or degrade
25 during use of the ceramic, particularly as a cutting tool.
~ hat is needed is a method of milling the starting
powder ingredients to insure that the particles of the
milled mixture are not above a critical average or mean
particle size, such as 35 microns, thus preventing packing
30 of the mixture along the sides of the milling chamber and
thus insuring a constant milling action. Such method
should not incorporate materials to achieve such object
that will provide for undesirable side reactions should
optimally reduce the milling time, should improve the green
35 strength of the milled mixture when compacted, and should
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minimize, if not totally eliminate, contamination that
results in flaws in the final ceramic product. Heretofore,
prior ar' methods have incorporated milling aids in a high
amount of about 5~ by weight, such as stearic acid, Zn
5 stearate, oleic acid and carbowax, each of which have a
high carbon content which can cause contamination.
SUMMARY OF THE INVENTION
The invention i5 a method of making a compacted
body useful in the fabrication of an improved, fully
10 densified, silicon nitride ceramic product which has par-
ticular utility as a cutting tool. The method comprises
mixing a supply of silicon powder and a supply of oxygen
carrying agents with an effective but small amount of a dry
lubricant of the type having a low surface energy value
15 less than 15 dynefcm, and further characterized by a glass
transition temperature of -54 to -82C and a molecular
weight of 5000. The admixture is then milled for a
sufficient period of time to insure that the average
particle size of the mixture is no greater than 35 microns.
20 Finally, the milled mixture is formed into a compact with
pressure sufficient to provide a green strength of at least
42% of full theoretical.
It is preferable that the dry lubricant be
selected from the group consisting of polybutylacrylate,
25 poly 2-ethylhexylacrylate and polyisobutyacrylate. The dry
lubricant may be further characterized by its ability to
render lubrication even though added in an extremely small
amount, thus avoiding carbon contamination. The lubricant
is effective to avoid packing of the powder mixture along
30 the sides of the milling chamber during the milling opera-
tion, thereby permitting said milling to reduce the
particles to the desired mean size of no greater than 35
microns within a milling period equal to or less than 48
hours. It is advantageous to add the dry lubricant in an
3samount of .001-.006% by weight of the mixture or about .001
grams per 100 grams of mixture.
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It is preferable if the starting materials have
defined purities. For example, silicon powder may have a
purity of 97~ or greater, and the oxygen carrying powder
agents may have a purity such as 99.99% for Y2O3 and 99.5%
5 for A12O3. All of the starting powder materials can have
an average particle size greater than 270 mesh (10
microns), and operably the starting average particle size
can be as high as 10-60 microns.
Optimally, the starting powder ingredients may
10 comprise silicon powder, Y2O3, and A12O3, the powder
ingredients containing trace contaminants of up to 1.0% Fe,
.3% Al, 1.0~ Mn, and 0.1% Ca, and an 2 content of no
greater than .5%.
DETAILED DESCRIPTION
A preferred method for making a compacted body
useful in the fabrication of an improved, fully densified,
silicon nitride ceramic product according to this invention
is as follows.
1. Mixing
An admixture of powder inyredients comprising
silicon powder, reactive oxygen carrying powder agents, and
a dry milling lubricant is prepared. The reactive oxygen
carrying powder agents is defined herein to mean powder
ingredients that are effective to form second phase
25 crystallites, particularly oxynitrides, when reacted with
the silicon powder under a heated nitrogen atmosphere. The
oxygen carrying agents can be advantageously selected from
the group consisting of SiO2, Y2O3, CeO2, ZrO2, HfO2, and
other rare earths. Use of these agents will improve
30 physical characteristics and formation of a second phase
crystallite which will be uniformly disbursed and which
substantially displaces a detrimental glassy silicate phase
normally formed except for a controlled and limited amount
of the oxygen carrying agent.
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For purposes of the preferred method, a uniform
powder mixture i5 typically prepared with 2000 grams of
silicon (~6.6 weight percent of mixture), 278 grams of Y2O3
(12 weight percent of mixture and 13.9~ of silicon), and 32
grams A12O3 (1.4 weight percent of mixture and 1.6% of
silicon). The usable range for the oxygen carrying agent
is .4-2.3 molar percent of the mixture and 0.4-2.4 molar
percent of silicon. Y2O3 is normally used in the range of
3-19% by weight of the silicon and 3-16% by weight of the
mixture. A glass forming oxide, such as A12O3, is used in
the range of .4-5% by weight of the silicon, .4-4% by
weight of the mixture. SiO2 is present usually as an oxide
on the silicon powder and increased to 1-3% by weight of
the silicon by milling. The oxide that is added to be
reactive with SiO2 and Y2O3, such as A12O3, can be selected
from the group consisting of `L~lgO, CeO, BO, A12O3, Fe2O3,
CaO2, Cr2O3, ZrO2, BeO, and rare earth oxides.
Silicon is preferably selected to have 98% or
greater purity and a starting average particle size of
about 10-20 microns with random particles in the 100-540
micron range. The major trace metal contaminants
experienced with such purity include: iron up to 1.0%,
aluminum up to .5~, and manganese up to .09%. Nonmetallic
contaminants include: carbon up to .05%, and oxygen less
25 than .5%. The average powder size of the Y2O3 powder is
about .0438 microns (438A) with random particles as large
as 40 microns. The average particle size of the A12O3
powder is .3-.5 microns with random particles as large as
50 microns.
To the above ingredients is added a dry milling
lubricant which must have a low surface energy value less
than dyne/cm, and is further characterized by a molecular
weight of 5000. The dry milling lubricant is preferably
selected from the group consisting of polybutylacrylate,
35 poly 2-ethylhexylacrylate, and polyisobutlyl. The dry
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milling lubricant is added in an effective small amount to
insure that the powder ~ass will not collect and pack along
the sides of the milling chambee during the milling
operation, thereby permitting the milling to reduce all
5 particles to the desired size of no greater than 40 microns
within a milling period equal to or less than 48 hours. An
efective small amount is in the rar.ge of .001-.006% by
weight of the mixture; this amount is insufficient to cause
a problem by carbon contamination.
10 2. Milling
The mixture is then comminuted and blended by
being charged into an inert milling jar along with grinding
media in the form of Burundum cylinders (85% A12O3 and 15%
SiO2, which adds A12O3 to the mixture by attrition), and is
15 milled for 48 hours at 64 rpm. Thereafter the mixture is
separated from the media by use of a ~10 mesh screen. The
resulting milled mixture must have no particle of a size
greater than 35 microns. If any of the particles in the
milled mixture are greater than 35 microns, the deleterious
20 effects indicated earlier would ensue. Such efects
include (a) insufficient nitriding of all particles leaving
free silicon which leads to flaws in the final product and
thus decreases strength, (b) local concentrations of Y2O3,
A12O3, or silicon may appear causing strength variations in
25 the final product.
The oxygen level after milling in air will be
increased to about 1.6 weight percent of the silicon, and
be present as an oxide coating on the silicon in an amount
of about 3 weight percent. The oxide coating should never
30 be stripped off for milling purposes. The ratio of
Y2O3/SiO2 is controlled to be in the range of 1.1-6.4 and
preferably about 4.
3. Cold Compaction
A measured quantity of the milled mixture is then
35 loaded into a cold pressed die arrangement and pressed at
ambient conditions by use of 1400-1500 psi to form a
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compact of a size about 6" x .6", and a density of about
1.4 grams/c~3. The pressure of compaction should be
sufficient to provide a green strength of at least 1.4
gm/c~3 (a2%~ in the compact.
4. Heating to Nitride
The compact is then heated in a nitriding
atmosphere, without the use of pressure normally associated
with hot pressing, to produce a silicon nitride comprising
body consisting of silicon nitride, at least one dispersed
second phase crystallite (silicon oxynitride), .2-1~
silicate (by weight of the body), and up to .5~ by weight
of free silicon and unreacted oxygen carrying agents (here
Y2O3 and A12O3). The body will have a size greater than
and a density less than the object to be formed.
5. Hot Pressing
The nitrided body is then hot pressed to produce a
silicon nitride comprising object of required dimension and
density. A- pressing fixture having graphite walls is
typically used to carry out the hot pressing. The walls
and nitrided body are both coated with a slurry of boron
nitride and dried.
The resulting product will have a strength
~modulus of rupture) in a 4-point bend test of 110-120 hsi,
as compared to 80-100 hsi for material prepared according
to the prior art. The coelesced particles in the product
will have a Weilbull Slope for variation of strength data
which is 10-18 as opposed to 7-12 for the prior art. This
shows greater uniformity. The hardness will be increased
to 92 (45-N scale) as opposed to 90 for the prior art.
