CA1063325A - Preparing complex shapes of ultra-high density silicon nitride and silicon carbide by a hot isostatic gas powder vehicle - Google Patents
Preparing complex shapes of ultra-high density silicon nitride and silicon carbide by a hot isostatic gas powder vehicleInfo
- Publication number
- CA1063325A CA1063325A CA224,830A CA224830A CA1063325A CA 1063325 A CA1063325 A CA 1063325A CA 224830 A CA224830 A CA 224830A CA 1063325 A CA1063325 A CA 1063325A
- Authority
- CA
- Canada
- Prior art keywords
- vehicle
- high density
- complex shapes
- container
- silicon nitride
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
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Abstract
PREPARING COMPLEX SHAPES OF ULTRA-HIGH DENSITY
SILICON NITRIDE AND SILICON CARBIDE BY A HOT
ISOSTATIC GAS POWDER VEHICLE
ABSTRACT OF THE DISCLOSURE
A method is presented for the production of complex shapes of ultra-high density silicon nitride and silicon carbide materials. This method consists of embedding low density complex pre-formed objects in an inert powder vehicle.
The inert powder vehicle is then placed in a deformable container. The deformable container is sealed and evacuated, whereupon the container is isostatically compressed in a pressure vessel by a hot gas. The pressure exerted in this manner is uniform on all surfaces of the complex pre-formed objects due the compression of the container and transmission therethrough of the pressure by the powder vehicle. This technique is especially useful in the production of oxidation resistant high temperature strength gas turbine blades.
SILICON NITRIDE AND SILICON CARBIDE BY A HOT
ISOSTATIC GAS POWDER VEHICLE
ABSTRACT OF THE DISCLOSURE
A method is presented for the production of complex shapes of ultra-high density silicon nitride and silicon carbide materials. This method consists of embedding low density complex pre-formed objects in an inert powder vehicle.
The inert powder vehicle is then placed in a deformable container. The deformable container is sealed and evacuated, whereupon the container is isostatically compressed in a pressure vessel by a hot gas. The pressure exerted in this manner is uniform on all surfaces of the complex pre-formed objects due the compression of the container and transmission therethrough of the pressure by the powder vehicle. This technique is especially useful in the production of oxidation resistant high temperature strength gas turbine blades.
Description
BACKGROUND OF THE INVENTION
Field of the Invention:
~ .
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 ; ;r , ~ .
33;~:S
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,
Field of the Invention:
~ .
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 ; ;r , ~ .
33;~:S
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,
-2-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.
- ~ .. . . , - . . . .. " . , . . . .. ,:
~ii3325i ~
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 _ _ :
.. . .
.. .. ' , ' ' '.',;`' . ~,' ' `":
~633;2~
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
-- ~.
~ 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-~ 633;~5 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 .
~ 633Z5 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.
.. . . . .
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.
- ~ .. . . , - . . . .. " . , . . . .. ,:
~ii3325i ~
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 _ _ :
.. . .
.. .. ' , ' ' '.',;`' . ~,' ' `":
~633;2~
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
-- ~.
~ 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-~ 633;~5 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 .
~ 633Z5 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.
.. . . . .
Claims (5)
1. A method of making complex shapes of a high density ceramic material comprising the steps of:
embedding a low-density, pre-formed, complex shaped object in an inert powder-vehicle;
placing said inert powder-vehicle containing said low density, preformed, complex shaped object into a deformable sheet metal container so that said low density preformed, complex shaped object does not contact said deformable sheet metal container;
sealing said deformable sheet metal container;
evacuating any gases from said sealed deformable sheet metal container;
disposing said deformable sheet metal container in a pressure vessel;
pressurizing said pressure vessel with a hot gas to isostatically collapse said deformable sheet metal container about said powder-vehicle, said inert powder vehicle isostatically compressing said low density, pre-formed complex shaped object.
embedding a low-density, pre-formed, complex shaped object in an inert powder-vehicle;
placing said inert powder-vehicle containing said low density, preformed, complex shaped object into a deformable sheet metal container so that said low density preformed, complex shaped object does not contact said deformable sheet metal container;
sealing said deformable sheet metal container;
evacuating any gases from said sealed deformable sheet metal container;
disposing said deformable sheet metal container in a pressure vessel;
pressurizing said pressure vessel with a hot gas to isostatically collapse said deformable sheet metal container about said powder-vehicle, said inert powder vehicle isostatically compressing said low density, pre-formed complex shaped object.
2. A method of making complex shapes of high density ceramic material as recited in claim 1, wherein said hot gas has a pressure range of 5,000 p.s.i. to 60,000 p.s.i.
3. A method of making complex shapes of high density ceramic material as recited in claim 1, wherein said inert powder-vehicle is comprised of particles selected from the group consisting of boron nitride and graphite.
4. A method of making complex shapes of high density ceramic material as recited in claim 2, wherein said deformable container is manufactured from a refractory metal taken from the group comprising molybdenum, tungsten and stainless steel.
5. A method of making complex shapes of a high density ceramic material as recited in claim 1, wherein said ceramic material is selected from a group consisting of silicon carbide and silicon nitride.
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US46369674A | 1974-04-24 | 1974-04-24 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1063325A true CA1063325A (en) | 1979-10-02 |
Family
ID=23840996
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA224,830A Expired CA1063325A (en) | 1974-04-24 | 1975-04-17 | Preparing complex shapes of ultra-high density silicon nitride and silicon carbide by a hot isostatic gas powder vehicle |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1063325A (en) |
-
1975
- 1975-04-17 CA CA224,830A patent/CA1063325A/en not_active Expired
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