Note: Descriptions are shown in the official language in which they were submitted.
KN 50~5
~Q9~
Method of manufacturin~ an ob~ect of~silicon _ tride
In the manufaoture of ob~ects of silicon nitride wlth hi~h density (more than
go10 of the theoretical density) by sintering together powder, the use Or iso-
~tatlc pressing offers many advantages. ~hus, the manufactured obJect8 will
have approximately the same streneth in all direotions beoause of the all-
sided pressure, which is not the case with other methods of manufacture.
Furthermore, object~ of complicated ~hapes can be manufactured directly by
the pressin~ and without, or substantially without, a subsequent machinin~
by means of tools, for example by grinding, which is exceedingly important
beoause of the faot that silioon nitride has a very Breat hardness. One
further important property of isostatic pressin~ is that the use of press
tools is avoided and thus also thevery oonsiderable material problems connec-
ted thexewith, whioh problems Rre oaused by the high pressures and temperatures,at lea~t 20 MPa and 1600C, respeoti~ely, which are required.
Prior to the isostatic pres~in~ and the sintering together of the ~ilicon ni-
tride powder in connection therewith, it is suitable that the powder is pre-
formed into a manageable powder body by sub~ecting the powder to a compaction,
for example arranged in a sealed capsule of gieldi~g matarial, such as a plas-
tic capsule. The compaction can be performed with advantage without the use
of a tempora~y binder at a pressure of at least 100 MPa at room temperature
or another temperature which iB considerably lower than the temperature during
the compression in oonnection ~ith the sintering. The procluct can thereafter
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be given its desired shape by means o~ machining. For the preforming it is
also possible to employ, among other thin~s, conventional techniques for ma-
nuacturin~ ceramic g~ods. The silicon nitride powder is then usually mixed
before the forming with a temporary binder, for example methyl oellulose,
cellulose nitrate, an a¢rylic binder, a wax or a misture of waxes. After
the preforming the binder is driven off by heating 80 that the preformed
powder body becomes substantially free from binder.
Since the preformed powder body is subjected to the iso~ta~ic pressing at
the æinterinB temperature, it must, in order to giva a desired, dense sinte-
red product, be enclosed in a casing capable of being evacuated before the
pressing and which, during pressing, is able to prevent the pressure medium
used in this ¢onnection, normally a gas, from penetrating into the powder
body. The casing must of course also have a sufficiently high ~trength or
viscosity during the pressing in order not to penetrate lnto the
pores of the powder body. If a preformed capsule of glass is used as the
casing, which then has to be of a high-melting type in order not to run
away or penetrate into the powder body at the high sintering temperature,
it can not be prevented that the gla~s, when softening, i8 collected in
pockets And other reoe~es o~ the preformed powder body, ThiB often
. . . ................... , . . .. . .. . _ _ .. , . __ _ ..
leadc to fraoture at pro~ectin~ portions Or the sintered ob~ect duxing it~
cooling because Or dirrerences in the ooef~icient of thexmal expansion of
silicon nitride and glass. Instead, the oa~ing can be allowed to form on
the spot by dipping the preformed powder body in a su~pension of parti¢les
of high-melting glass or in some other way surroundipg the body with a layer
of partioles of such glass, and then heating the powder body under vacuum
at such a temperature that the particles form a dense casing ~round it~
The last-mentioned method permits the application of a casi~g which can be
made thin and which conforms to the shape of the powder body, thus avoiding
accun~ations of glass on the sintered ob~ect ag well as the disadvantages
associated therewith. However, the method has one serious drawback connec-
ted with the fact that a dense casing is only achieved at high temperatures
since the glass has to be of high-melting type in order not to run away or
penetrate into the powder body during the sintering of the silicon nitride.
The fnct that a dense casing is a¢hieved only at high temperatures means that
dis~ociation of the silioon nitride under departure of nitrogen cannot be
avoided, which results in a deteriorated quality of the object as sintered.
The present invention will solve the problem of achieving a dense casing
around a preformed powder body of silicon nitride, which does not give rise
to harmful accumulations of glaas on the sintered body and ~hioh prevent~
a harmful dissociation of the silicon nitride. Accordi~g to the in~ention,
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this makes possible the manufacture of complicated parts of
silicon nitride with a homogeneous composition and with homo-
geneous properties.
Therefore, according to the present invention
there is provided a method of manufacturing an object of silicon
nitride by isostatic pressing of a preformed body of silicon
nitride powder with a pressure medium at a temperature required
for sintering of the silicon nitride, -the preformed body being
subjected to a degassing prior to the isostatic pressing, charac-
terised in tha-t on the preformed powder body there a:re applied
an inner porous layer of a first material and outside this an
outer porous layer oE a second material, the inner porous layer
being transformable into a layer impermeable to the pressure
medium at a temperature below the sintering temperature for the
silicon nitride, and the outer porous layer being transformable
into a layer impermeable to the pressure medium at a temperature
which is lower than that for the inner porous layer, whereafter
the preformed body is first subjected to a degassing and to a
heating to a temperature required for forming a layer, imper-
meable to the pressure medium, of the outer porous layer bu-t
which maintains the inner porous layer porous, and then to a
heating to a temperature required for forming a layer, impermeable
to the pressure medium, of the inner porous layer while main-
taining a pressure outside said layers which is greater than the
gas pressure inside these layers, and that the isostatic pressing
of the preformed product is then carried out.
As said first material in the inner porous layer
there may be used a powder of a high-melting glass such as Vycor
glass containing 96,7 per cent by weight SiO2, 2.9 pe:r cent by
weight B2O3 and 0.~ per cent by weight A12O3, further quartæ
glass and mixtures of par-ticles of substances, for example SiO2
and B2O3, which, during hea-ting, form a gas-impermeable glass
layer. It is a~so possible to use as said Eirst material in the
inller porous layer a powder oE a high-meltincJ metallic material
capable of forming a metallic layer impermeable to the pressure
medium, such as molybdenum, tungsten and other refractory metals.
As said second material in the outer porous layer
there may be used a powder of a low-melting glass such as Pyrex
glass containing 80.3 per cent by weight SiO2, 12.2 per cent by
weight B203, 2.8 per cent by weight A1203, 4.0 per cent by weight
Na20, 0.4 per cen-~ by weight K20 and 0.3 per cent by weight CaO,
further an aluminium silicate containing 58 per cent by weight
SiO2, 9 per cent by weight B203, 20 per cent by weight A1203,5
per cent by weight CaO and 8 per cent by weight MgO, an aluminium
silicate containing 60 per cent by weight SiO2, 20 per cent by
weight ~120~, 15 per cent by weight CaO and
5 per cent by ~eight MgO9 and mixtures of particles o ~ub~tances, for
example SiO2, B203~ A120~ and alkali and easth alkali metal oxlde~ ~hich,
during heatiDg, for~ a ~as-imper~eable ~laas layer.
; i
The two porous layers, each o~ which Euitably has a thickness ~ithi~ the
ran~e oP 0.05 to 1 mm, may, among other thl~a, be applied by dipping the
preformed powder body in su~pen~ions of the p~rtioulate ~aterials or b~
M ame ~prayi~g or other thermsl spraying. ~he partioles may suitably ha~e
a grain size within the range of 0.1 to 100 microns.
~he dega~sin~ i~ suitably ~tarted and is allowed to ¢o~tinu0 at room tempe-
rature for a period which 1~ dependent on the 8iZ9 0~ the pre~ormed powaer
body. Under continusd evacuation, the temperature is rai~ed BO that the
outer porous layer iB tran8ferrea into a layer whioh iB impermeable for the
pressure medium. When this is done, prasBure o~n be applied with a gaseous
pressure medium on the en~losed powder body to counteraot dissociation Or
the silicon nitride d~ring continued temperature increase. During the con-
tinued increase in temperature, provided the layers oo~ist Or a 61ass or
a glass-forming material, ~he glass in the outer layer reacts wlth the mat-
erial in the inner porous layer while forming an increa~in~ly hi~h-melting
glass and while m~int~lnlng a layer impermeable for the pressure medium, and
finally a glass layer, impermeable ror the pressure, i8 for~ed of the i~ner-
most part of the inner pcrous lsyer before the glsss i~ the outer layer is
a~le tv run a~ay. This last formed glass layer ~orms ~ den~e oasin& around
the powder body, when the $sostatic pressiDg of the preformed product i~
carried out at the sintering temperature. When using a metallio material
in the inner, porous layer and a FIass or a glass-forming mat~rial i~ the
outer porous layer, the glass layer formed ~rom the outer poroua layer aots
as impermeable layer at least until the inner metallic layer has been trans-
formed to an i~permeable layer.
The temperature at which the outer porous laycr is caused to be tr~nsformed
into ~n impermeable layer lios ~uitably within the range of 600 to 1100C,
and the temperature at which $he inner porous layer is caused to be tra~s-
formed into an ~mpermeable layer lieB wlthin the ra~g~ 1300 to 1600G. If
the inner layer iB compressed under isos~atic pressure, whioh can be achieved
after the outer layer ha~ become ga8-tight9 there ~ay, howe~er, also be a
question Or ~cmporatures Or 1000 to 1300C. For ~uch a compres~io~ there iB
required a pre~sure o~ the order Or magnitude of 20 to 300 ~Pa. The sintering
of the powder body i~ carried out at at least 1600C, prefe:rabl~ at 1600 to
1 900C ~
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The pressure during the sintering of the pre-
formed silicon nitride body is dependent on whether a sintering-
promoting additive, such as magnesium oxide, has been added to the
silicon nitride or not. If no such additive is used, the pressure
should amo~mt to at least 100 Ml~a, preferably to 200 to 300 MPa.
When using a sin-tering-promo-ting addi-tive, a lower pressure can be
used, howover suitably at leas-t 20MPa.
The invention will be explained in greater
detail by describing an embodiment with reference to the accompa-
nying drawing, in which Figure 1 schematically shows a preformed
powder body of silicon nitride provided with two porous layers,
and Figure 2 schema-tically a treatment cycle for manufacturing
a sintered object of the preformed powder body.
Silicon nitride powder with a powder grain size
below 7 microns and containing around 0.1 per cent by weight of
magnesium oxide is placed in a capsule of plastic, for example a
softened polyvinyl chloride, or of rubber, having approximately the
same shape as the preformed powder body to be manufactured, where-
after the capsule is sealed and placed in a press device, for
example the device shown in the British patent 1,522,705 to ASEA
Aktiebolag. The powder is subjected to a compaction at 600 MPa for
a time of five minutes. After completed compaction the capsule
is removed, and the preformed powder body thus manufac-tured is
machined into the desired shape.
The preformed powder body 1 is then provided,
as is clear from Figure 1, with an inner porous layer 2 and an
outer porous layer 3 by being dipped first in a water suspension
of powder of a glass consisting of 96.7 per cent by weight SiO2,
2.9 per cent by weight B203 and 0.4 per cent by weight A1203 and
then, after drying this layer, in a water suspension of a powder
of a glass consisting of 80.3 per cent by weight SiO2, 12.2 per
cent by weight B203, 2.8 per cent by weight A1203, 4.0 per cent by
wei~ht Na20, 0.4 per cent by weight K20 and 0.3 per cent by weight
CaO, followed by a renewed drying.
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' The preformed powder body thus trea-ted is
thereafter placed ln a hi~Jh-pressure furnace which is provided
with a conduit through which gas can be discharged for degassing
of the powder body and through which gas can be supplied for
generating the required pressure for the isostatic pressing and
which is provided with heating devices. Such a high pressure
furnace is described, for example, in the preveously mentioned
British patent No. 1,522,705.-
07
As illustrated in Figure 2, the preformed powder body i~ first degassed inthe high pressure furnace at room temperature for approximately 2 hours.
During continued evacuation the temperature i8 raised to about 900C. The
t~mperature is raised BO slowly that the pressure doe~ not exceed 0.1 torr
during any part of the time. At about 900C the temperature is maintained
con~tant for about 2 hours, the final dega~sing thus being performed and the
glass powder in the outer porous layer sintering together into a gas-imper-
meable layer To reduce the viscosity on the glass in the outer layer and
thus reduce the risk of penetration Or glass melt into the inner porous
_,, _ ., , . _ , . ...
layer, the temperature is reduced to 700& . Thereafter argon or helium is
supplied to a pres~ure level which provides a pras~ure of 200 to 300 MPa at
the final sintering temperature. ~he temperatur0 is then raised to 1700 to
1800C, that iB~ to a suitable sintering temperature for the silicon nitride.
The pressure then rise6 simNltaneously. ~hi~ temperature increase i~
achieved sufficiently slowly for the molten glass in the outer layer to ha~e
time to react with the glass powder in the inner layer while forming an in-
creasingly high-melting glass, and for the innermost layer of the glass pow-
der in the inner layer to have time to sinter into a gas-~mpermeable lsyer
before the glass in the outer layer is able to run off~ A suitable time for
~intering at 1700 to 1800C and 200 to 300 MPa is at least 2 hours, if no
sintering-promobing additive iB u~ed and at least 0.5 houra if such additive
is used. After a completed cycle, the furnaoe is allowed to cool to a
suitable deoharging temperature and the sintered ob~ect i9 blasted clean of
glass.
If a binder, such as the previously exemplified methyl aellulose, cellulose
nitrate, an acrylic binder, a wax or a mixture of ~axes with different melt-
ing points, has been used in the manufaotu~e of the preformed powder body,the
binder i~ remo~ed before or after the application of the porous layers, suiba-
bly by heating the powder body to 400 to 700C in vaauum. Thereafter degass-
in~ and further treatment oan be done, as de~cribed for a preformed powder
body without a binder.
The pxesent in~ention is particularly suitable for use in series manufacture.
In this case the various stage~ o~ treatment,such as a) removal of the binder,
b) degassing in vacuum and dense sintering of the outer layer, o) ~urther
heating of the parts in ga8 with pressure excesding the pre6sure inside the
layers, and d) final heating and hot i~ostatic pr*6sing, can be per-
formed in different kinds of furnace equipment with transfer between them
in hot condition.
As examples of ob~ect~ for which the present invention is extremely well
suited for use may be mentioned, among other things, va~es and mono-
lithic turbine rotors_for g~s`turbines.
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It is to be noted that 0.1 MPa (MPa being used as pres-
sure unit in the present application) is the same as 1 bar,
which means that 1 MPa is the same as 10 bars and approximately
the same as 10 atm. and approximately the same as 10 kp/cm2,
