Note: Descriptions are shown in the official language in which they were submitted.
21521~ .
METHOD OH FABRIC.~TING HOT PRESSED
SILICON ~IITRIDE BILLETS
BACKGROUND OF THE INVENTION
. .
AND PR I OR A RT S TATE ~IENT
The present invention is directed to a method of
making a more economical and distortion free silicon
nitride product suitable for use as 2 cutting tool by hot
pressing with significantly reduced need for subsequent
shaping.
Hot pressing of ceramic starting materials has
been kno~n for some time (see Refractory .laterials, by
Allen M. Alper, Vol. 5, III, "High Temperature Oxides,'l
1970, pages 184-189, for an explanation of the typical hot
pressing process and equipment). The hot pressing sequence
15 usually involves placing a loose particulate powder mixture
or semidense pressed block of the powder mixture into a
pressing assembly and heating the assembly while applying
pressure to the mass sufficient to densify and fuse the
particles to a desired degree. Typically, the pressing
20 assembly is a cylinder die closed by end plungers or
pistons, one or both of such end plungers or pistons being
forceably moved by platens of a p!eSs to apply pressure to
- the mass within the assembly. The cylinder and end
plungers are close fitting and are typically constructed of
25 graphite. A refractory insulation shell is wrapped about
the cylindrical die assembly and heat is applied thereto by
induction coils or by resistance heating. Hot pressing
is typically carried out in the temperature range of
1500-1800C, the pressures employed usually are in the
30 range of 2000-7000 psi, and the time period usually com-
prises 5-180 minutes. The resulting density for silicon
nitride so hot pressed is usually in the range of 3.0-3.35
~m/cm3.
If the above conventional hot pressing sequence
35 were to be employed for the simultaneous pressing of a
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plurality of stacked, cold compacted, flat plates
(particularly larse diameter plates on the order of
a 6 inch diameter), several problems would be encountered.
First, a temperature gradient is created across the lateral
width of the plates which results in a corresponding
viscosity gradient. Such viscosity gradient causes the
pressing assembly to apply a nonuniform pressure dis-
tribution across and through the mass of the plates.
Secondly, there exists a drag force (a friction force
between the pressing assembly walls an~ the plate
sides) which also contributes to a nonuniform pressure
distrib~tion across the lateral width of the plates. ~nese
two factors together cause material transport under the hot
pressing conditions which in turn results in "dishing" or
severe distortion of the flat plates in their fully
densified conditionO
Such problems as above indicated are amplified
when economy is sought by the simultaneous production of a
greater number of products, such as four or more plates
(hereafter called billets) not separated by rigid spacers.
A billet is defined herein to mean a mass of material of
suitable thickness fro~ which several useful cutting tools
can be directly shaped by merely subdividing the billet by
cutting through the thickness thereof. Such billets are
either formed in a solid, single layer with a thickness to
width ratio of 1:3 to 1:40, or they may be formed in a slab
which is segregated by scoring to define a multiple number
of cutting tools within a single slab, the segregated
pieces being hinged together by the unscored membrane.
In only one instance has the prior art attempted
to simultaneously hot press a plurality of silicon nitride
components. In British patent 1,405,171, a number of cold
compacted preforms are placed in a single layer within a
pressing assembly, each preform having a thickness
generally equal to its width. Each preform is separated
from all others by a release agent. No greater than two
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3~ 21528
layers are used. The problems overcome by this invention
would not be ex?erienced in the application of this 2ritish
patent. Side wall drag ~ould be insuEficient to promote
distortion since there is little dif~erence in ,~ovement
5 between layers; the preforms do not contact the die wall
and the thickness to width ratio is only 1:1. Material
transport cannot take place as a result of pressure and
thermal gradients because there is little relative movement
between layers, little or no side wall drag, and the
10 thickness to width ratio is only 1:1. The disclosure thus
fails to appreciate the need for a unique stacking sequence
that would eliminate dishing in the hot pressing of
multiples of billets having a thickness to width ratio of
1:3-1:40.
SUMMAP~Y OF THE I~V~NTION
The invention is a method of making a plurality of
dimensionally accurate hot pressed ceramic bodies.
The method comprises: (a) preparing a plurality of
ceramic billets having a thickness to width ratio in the
20 range of 1:3 to 1:40 and a density of 2.0-2.7 gm/cm3; (b)
uniquely stacking the billets into a pressure assembly
having walls to support said billets normal to the
direction of pressure; and (c) hot pressing said stacked
billet groups under pressure and temperature to densify
25 each of said billets to at least 95% of theoretical density
with a compression ratio of 1.2:1 to 2:1. The stacking is
in groups of progressively decreasing number so that for a
billet group residing in a zone of compression that will
experience the least movement along the pressing direction,
30 the stacked number of billets is greatest within such
group. For a billet group residing in a zone of
compression that will experience the most movement alon~
pressure direction, the stacked number of billets within
such group is the lowest, each group being separated ,rom
35 adjacent groups by an inert rigid spacer.
4 12215Z8
It is preferable if (a) 'he hot pressing is
carried out with uniaxial pressure and the number of
billets in eacn of said groups proceeds from a maximum
number of 5 to 3 to 2 to 1, the latter group has the lowest
5 number and experiences the most relative movement; or (b)
the hot pressing is ap~lied biaxially and the sequence oE
groups contains the following numbers in order: 1, 2, 3,
10, 3, 2, 1, with the group having the single number
experiencing the highest movement during compression and
the group with the number 10 experiencing the lowest
relative movement during compression.
Advantageously, the billets are prepared by cold
compacting a dry milled mixtuee of silicon powder, Y2O3,
and A12O3, and then heating the mixture in a nitrogen
atmosphere for a period of time and at a temperature to
agglomerate the mass to a density of about 2.3 gm/cm3 and
a chemical content of ~ and ~ phase Si3N4, silicon yttrium
oxynitrides, and some amorphous silicate. The cold com-
paction is carried out with a pressure of 1500-2000 psi.
The spacers are preferably fine grained, high
strength graphite with a thickness (.25-1.00") appropriate
for withstanding the hot pressing forces and fully trans-
mitting such forces comparable to a die wall. The thick-
ness of the spacers is determined by the rule that in the
zone of greatest axial movement for the body, the spacer
associated therewith is the thickest because the forces
encountered will be the greatest.
SUMMARY OF THE DRAWINGS
Figure 1 is a schematic illustration in central
sectional view of a pressing assembly showing the unique
sequence of stacking in conformity with this invention for
uniaxial pressure;
F~gure 2 is a view similar to that of Figure 1
used in the environment of biaxial pressing; and
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Figure 3 is a schematic illustration of a pressins
assembly after densification using a stacking sequence
which shows the disadvantages of using a stackin~ sequence
not in conformity with this invention.
DETAILED DESCRIPTION
A preferred method for makins silicon nitride
comprising objects according to this invention is as
follows:
1. Compacting
A compact is formed from a mixture of powdered
silicon and reactive oxygen carrying powder agents.
Reactive powder oxygen carrying agents are defined herein
to mean powder ingredients that are effective to form
oxynitrides and appropriate silicates when reacted with the
silicon under a heated nitrogen atmosphere. The powder
agents can be advantageously selected from the group con-
sisting of Y2O3, A12O3, SiO2, tlgO, CeO2~ ZrO2~ HfO2~ andrare earths. Use of selected quantities of Y2O3 and A12O3
will result in the formation of a silicon yttrium oxy-
nitride phase which (a) will uniformly be disbursed, and(b) displace the detrimental glassy silicate phase normally
formed except for a controlled and limited amount of the
latter. For purposes of the preferred method, a uniform
powder mixture is prepared with 2000 grams of silicon
powder (86.6 weight percent of the mixture), 278 grams of
Y2O3 (12 weight percent of the mixture and 13.9 weight
percent of silicon powder), and 32 grams of A12O3 (1.4
weight percent of the mixture and 1.6 weight percent of the
silicon).
Silicon is selected to have 98% or greater purity
and a starting average particle size of 8-9.2 microns.
The major trace metal contaminants experienced ~ith such
impurity include iron, aluminum, Ca and Mn. Nonmetallic
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contaminants incl~de carbon and 2- The mixture is COIT,-
minuted and blended by being charged into an inert milling
jar along with grinding media in the form of cylinders,
~illed ~or 4~ hours, at ~4 rpm, then the mixture is
separated from the media. '~he resulting milled mixture
will have at least 90~ sized to an average particle size
of less tnan 23 micronsJ the oxygen level after milling in
air will be increased to 1.6 weight percent of the silicon,
and an oxide coating will be present on the silicon in an
amount of 3.0 weight percent. The ratio of the Y2O3/SiO2
is controlled to be in the range of 1-7 and preferably
about 4.
A measured quantity of the milled mixture is
loaded into a cold pressed die arrangement and pressed at
ambient conditions by use of 1400-1500 psi to form a com-
pact of a size about 6 inches in diameter and 1/2 inch in
thickness, having a green density of 1.4 gm/cm3.
2. Heating to Nitride
The compact is heated in a nitriding atmosphere to
20 produce a silicon nitride comprising body (billet) having
at least one dispersed silicon yttrium oxynitride phase,
and up to .5~ by weight free silicon and unreacted oxygen
carrying agents. The body will have a size greater than
the object to be subsequently formed and a density less
25 than such object.
To carry out the heating the compact is placed in
an enclosed furnace, preferably evacuated to a pressure of
less than 1 micron, and heated at a fast rate to 1200F
(649C). The furnace is then filled with a gaseous mixture
30 consisting of 72~ by weight nitrogen, 3~ hydrogen, and 25%
helium, at a pressure of about 2.7 psig. The total 2 and
H2O content in such gaseous mixture is less than 4 ppm.
The temperature of the furnace is then increased to a
nitriding temperature of 2000-2500F (1093-1427C) at a
_ 7 _ ~ ~1528
slower rate. Fresh nitro3en is continuously sup~lied to
the furnace to replace the nitro~en consumed in forming
Si3~4 and silicon yttrium oxynitrides.
The nitrided body (billet~ will preferably con-
5 sist of silicon nitride (at least 6G% of which is in thealpha form), silicon yttrium oxynitride in the YlSiO2N
phase, and up to .5~ of either unreacted silicon or yttria.
This body is an intermediate product or commodity that has
particular utility as a starting material for the hot
10 pressing technique to follow. The billet has a thickness
in the range of .3-.1.0 inches and a width or diameter of
3-12 inches; the thickness/width ratio of 1:3 to 1:40.
3. Hot Pressing
The billets are stacked within a pressing
15 assembly, the assembly having walls to support the series
of aligned billets normal to the direction of pressure.
As shown in Figure 1, the upper piston is used to apply
uniaxial pressure to the stacked series of billets.
The pressing assembly has gra~hite walls 40-41-
20 42-43. The walls and nitrided body are both coated with a
slurry of boron nitride and dried. The graphite walls are
additionally covered by graphite foil and/or molybdenum
foil.
The sequence of stacking of the billets is criti-
25 cal to this invention. The biliets are stacked in groupsin progressively decreasing numbers so that for a billet
group residing in a zone of compression that will experi-
ence the least movement along the pressure direction, the
stacked number of billets is highest. For the billet group
30 residing in a zone of compression that will experience the
most movement along said pressure dicection, the stacked
number of oillets is lowest. Each billet group is
separated from adjacent billet groups by an inert rigid
spacer comprised of either graphite, substantially full
35 density boron nitride, or other pressure transferring
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1221~i28
material. Preferabiy the graphite spacers are covered with
boron nitride which in turn is coated with graphite foil.
The spacers maintain geometry, undergo no material
transport, and are not affected by thermal pressure
5 sradients. The thickness of the spacers is determined by
the rule that in the zone of greatest axial movement for
the body, the spacer associated therewith is the thickest
because of the greater forces encountered.
In particular, the stacking sequence for Figure 1
10 shows that there are five billets, 10-11-12-13-14, placed
in contiguous relationship with one another in the zone of
movement 15 which is at the lowest portion oE the stack,
the latter experiencing the least amount of relative axial
movement in the direction of compression 16.
The next billet group comprises three billets,
18-19-20, and will reside in an intermediate zone of move-
ment 17 and therefore has a reduced number. The billet
group containing the least number has billet 21 and is in
the zone of highest movement 22.
Other combinations of billet grouping can be en-
visioned as long as the basic criteria of using decreasing
numbers of billets in the billet groupings when progressing
in a direction from the smallest amount of billet movernent
(billet 14) to the largest zone of billet movement (billet
25 21) is utilized.
In the environment of biaxial pressure, as shown
in Figure 2, the stacking sequence is as indicated. In the
zone of least movement 23, there are 10 billets (2x5); in
the zone of most movement 24 there is one billet. The
30 stacking sequence is 1-2-3-10-3-2-1.
If the stacking sequence of this invention was not
employed, the following phenomena would have resulted. As
shown in Figure 3, a pressure distribution is present which
is brought about in part from side wall drag; that is, the
35 edges of the billets 29-34 will contac- the graphite side
walls and impart a resistance to compression and promote a
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drag. The pressure gradient facilitates material transport
from th2 ou.side diameter to the center of billets 30-31-
32-33 and from the center to the outside diameter on
billets 34, 35 and 36. These effects, u?on cooling, yield
- 5 "dished" hot pressed Si3~14 billets that require substantial
amounts of material to be removed (diamond grindins~ to
produce a flat product.
It is important that the compression ratio for hot
pressing be in the range of 1.2:1 to 2:1. The pressure is
10 preferably ap21ied in steps, about 150 psi at room tempera-
ture before heat-up. Pressure is then increased to 500 psi
at 1800F (982C), pressure is next increased to 2500 psi
as the temperature is increased to 2500F (1371C), finally
the temperature is increased to 3000F (1649C) and pres-
15 sure to 3700 psi. The latter conditions are maintaineduntil full density is achieved. This usually requires
2.0 hours at the ultimate pressing temperature. The object
is then cooled at any rate to room temperature.
The resulting object will consist essentially of
20 beta phase silicon nitride, silicon yttrium oxynitrides
(predominantly YlSiO2N) enveloped by amorphous silicate
phase having a 'hickness of 4-10 angstroms and haviny no
microporosity. The object preferably possesses a hardness
of 89.0-91.0 on the 45-N scale, a density of 3.30-3.33
25 gm/cm3, a fracture strength of greater than 85,0000 psi at
1200C in a four-point bend test, and an oxidation
resistance that prevents weight pickup by the object after
450 hours in air at 1000C.
