Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
BACKGROUND OF THE INVENTION
Field of the Invention:
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This invention relates generally to hot-pressing
of ceramic articles and more specifically to the production
of gas turbine blades from hot isostatically pressed silicon
nitride or silicon carbide.
Description o~ the_Prior Art:
Current high density refractory ceramic gas turbine
blades and rotors are manufactured by diamond machining
blocks of unidirectional hot-pressed silicon nitride or
silicon carbide.
Silicon nitride and silicon carblde are the leading
ceramic material candidates for high temperature applications
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because of their relatively unique mechanical and thermal
properties whlch allow them to resist moderate to severe
thermal shock conditions. Although they are nonoxides, both
form a coherent, protective surface layer of SiO2 during
thelr lnitial stage of oxldation which permits their use
in high temperature oxidizing environments for extended
periods.
Silicon nitride is prepared by reacting silicon
powder with nitrogen. The sllicon is pre-pressed into a
10 desired shape before reacting, and the finished product is -
reaction-sinter-ed silicon nitride. Densities of 70 to 85
percent o~ theoretical may be achieved by the reaction sin-
tering process. The strength of silicon nitride was improved
when a technique was developed to densify the material by
hot-pressing, as described by G. G. Deeley, J. M. Herbert
and N. C. Moore in an article entitled "Dense Silicon Nitride"
in Powder Metallurgy, No. 8, pages 145-151 (1961). The method
consisted of mixing the silicon nitride powder with an addi-
tive, and hot-pressing the mixture at a temperature of 1800
to 1850C with 3000 p.5.i. applied pressure. Different
additives were tried, but magnesium compounds such as MgO
or Mg3N2 were ~ound to be best for promoting densification
and high strength.
A powder-vehicle hot-pressing technique to hot-
press complex shapes was developed to densify engineering
shapes as described by F. F. Lange and G. R. Terwilliger in
an article entltled "The Hot-Vehicle Pressing Technique" in
Ceram c Bulletin9 No. 52, pages 563-565, (1973). The process
involved preshaping the ob~ect using a conventional ceramic
forming technlque such as isostatic pressing~ slip casting,
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in~ection molding and the like. The ob~ect then was embedded
in a powder, termed the powder-vehicle, which was contained
within a cylindrically shaped hot-pressing die, having end
plungers. At the temperature required to densify the pre-
formed ob~ect, a load ~s applied to the powder-vehicle
through the end plungers. The powder-vehicle conveys the
pressure to the ob~ect which thereupon densifies. Two ob-
~ects had to be met using the powder-vehicle technique,
whlch are: (a~ the powder-vehicle should not react with the
ob~ect to be densi~ied, and (b) the ob~ect should be easily
removed from the powder-vehicle after hot-pressing. At
temperatures above 1400C, both boron nitride and graphite
powders can easily be removed as long as no reaction occurs.
Other ceramic powders are useful as powder-vehicles at lower
temperatures.
The transfer of pressure to the complex shape is
important to determine the proper hot-pressing schedules for
complete densification. The pressure transfer is related
to the behavior of the powder-vehicle under an applied
stress at high temperatures.
Loose powders have characteristics Or a fluid since
they are unable to support shear stresses, whereas pressed
powders resemble a solid. The powder-vehicle, con~ined with
a die and being subjected to an applied axial pressure,
unfortunately exhibits each o~ these characteristics during
the hot-pressing period. During the initial loading, a
limlted-amount of fluid-like flow occurs. Subsequently, some
densi~ication o~ the powder-vehicle occurs and it then must
be treated as a solid which precludes an isostatic pressure
transfer to the ob~ect to be densified.
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In the complex object, most of the densification
occurs ln the direction Or the applied axial force. The
amount of densification in the direction perpendicular to
the applied force depends on the obJect dimensions in these
two directions.
An ob~ect of the present invention i6 to completely
eliminate any unidirectional densiflcation. Further dis-
advantages of the prior art include the required use of a
complex-shaped encapsulating container which houses the
porous pre-formed ob~ect to be densi~ied, which must be used
to maintain a pressure differential when hot-gases are used
to promote densification; and the container materials must
be carefully selected to prevent reaction wlth the materials
to be densified. An article by E. S. Hodge entitled "Elevated-
Temperature Compaction of Metals and Ceramics by Gas Pressures"
in Powder M tallurg~, No. 7, pages 168-201, (1964), further
illustrates the problem. Glass containers which are often
used for metals, may readily react with ceramics such as
silicon nitride at elevated temperatures. The glass contain-
ers are also limited to maximum temperatures of about 15$0C.
Another ob~ect o~ the present invention is to over-
come the above cited dl~advantages o~ the prior art.
SUMMARY OF THE INVEN~ION
This invention describes a process for manufacturing
complex shapes of high density silicon nitride and silicon
carbide. ~he process comprises embedding a low density com-
plex pre-formed ob~ect in a inert powder-vehicle. The entire
arrangement being sealed in a container which is then evac-
uated of air. The container is comprised of a collapsible
refractory material. The container is then disposed in a
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pressure chamber, sealed therein, and sub~ected to a hot
pressurized gas. The pressure exerted in this way is ex-
tended uniformly to all the surfaces of the complex shaped
pre-formed ob~ect, causing a uniform densification ~herein.
BRIEF DESCRIPTION OF THE DRAWINGS
For a better understanding of the nature and ob~ects
o~ the invention~ re~erence may be had to the following de-
tailed descriptions, taken in con~unction wlth the accompany-
ing drawings in which:
Figure 1 is an elevational view, partly in section
of an ob~ect disposed in a hot isostatic gas powder-vehicle
densification arrangement constructed according to the prin-
ciple~ of this invention; and,
Figure 2 is a graph showing the relationship be-
tween flexural strength and temperature of silicon nitride.
DESCRIPTION OF THE PREFERRED EMBODIMENT
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~ he process for a uniform densification of a com-
plex ob~ect 10 3 iS shown in Figure 1. The complex-shaped
ob~e¢t 10, for this example, a gas turbine blade, is embedded
in an inert powder 12. The complex-shaped ob~ect 10 is made
~rom a pre-~ormed mix of silicon nitride, Si3N4, having a
1 to 5 weight percent magnesium oxLde, MgO, disposed therein,
to aid densification. The complex-shaped ob~ect 10 may also
be made primarily from silicon carbide, SiC. Alumina, A1203,
or a boron3 B, additive would be used to aid densification,
in that case. The inert powder 12 may be comprised of boron
nitride BN, or graphite.
The complex-shaped ob~ect 10 and inert powder 12,
are dispo~ed in a suitable sealable non-reactive container
14. The container 14 may be made from any refractory sheet-
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thickness metal such as molybdenum, tungsten or stainless
steel. The contalner 14 must be able to withstand hot-gas
temperatures of approximately 1600 to 1800C. The con- -
tainer 14 must also be deformable under a minimum pressure
of 5000 p.s.i. Pressures may range as hlgh as 60,000 p.s.l.
The proGess deæcribed herein permits the container 14 to be
of simple cylindrical shape, rather than being of any com-
plex geometry which requires machining to match the complex
shape to be denslfied which was typical of the prior art.
The simple shape of the container 14 is poss1ble because the
pressure exertion upon the hot surfaces of the pre-formed
ob~ect 10 is due to the inert powder 12 belng uniformly pres-
surized and the inert powder 12 belng the vehicle of pressure
transmission. Several pre-formed ob~ects may be placed in
one container, 14. Since the container 14 does not come into
contact with the pre-~ormed ob~ect 10 to be densified, a
better selection of material ~or making the container 14 is
permitted.
The container 14 is heated to about 120C to drive
off moisture, sealed by welding or the like, and then eva-
cuated of ~lr or additional molsture throu~h an evacuation
member 15, to less than 1 mm pressure. The container 14 is
then placed in a heated pressure chamber 16, and the chamber
16 sealed. A hot pressurized gas at a temperature of 1600
to 1800C is supplied through supply means 18, from a hot
pressurized gas source, not shown, to the pressure chamber
16. Heating colls 20 may be used to keep the chamber 16 at
the required temperature.
By application of the hot pressurized gas, uniform
mechanical properties may be achieved in the complex-shaped
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ob~ect 10 that would be unattainable in the prior art, which
comprised axial or unidirectional hot-pressing of silicon
nltride~ or silicon carbide billets. Thi conventional axial
hot-pressing results in anisotropic mechanical properties in
sllicon nitride, and weak flexural strength, as lndicated by
t'A" in ~igure 2. High temperature flexural strength produced
by the hot isostatic gas powder-vehicle technique is indicated
by the curve "B" in Figure 2. The high strength produced is
important in demanding structural applications such as rotor
blades in high temperature gas turbines. The use of an inert
power-vehicle 12 permits a wide selection of material ~rom
whlch to manuracture a container 14 which would otherwise
come into contact and possibly react with the pre-~ormed
ob~ect 10.
Although the invention has been described with a
certain degree of particularity, it is understood that the
olaims are interpretative only and not in a limiting sense.
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