GB2048952A

GB2048952A - Isostatic Hot Pressing Metal or Ceramic

Info

Publication number: GB2048952A
Application number: GB7943542A
Authority: GB
Original assignee: ASEA AB
Current assignee: ABB Norden Holding AB
Priority date: 1978-12-19
Filing date: 1979-12-18
Publication date: 1980-12-17
Also published as: GB2048952B; DE2950158A1; IT1119639B; CA1148772A; AU531109B2; DE2950158C2; AU5384779A; FR2444523A1; IT7969434A0; FR2444523B1

Abstract

A ceramic or metallic article is manufactured by isostatically hot pressing a preformed powder body which is embedded in glass or glass- forming material, for example in a mass of glass particles, in a vessel which is resistant to the temperature at which the sintering of the metallic or ceramic material is to be carried out. The embedding material is then transformed into a melt having a surface limited by the walls of the vessel, below which surface the preformed body is located, and the pressure necessary for the isostatic pressing of the preformed body is then applied on the melt by a gaseous pressure medium. Materials which may be isostatically pressed are Si3N4, silicon aluminium oxynitride, SiC, Fe and Ni based alloys.

Description

SPECIFICATION Method of Manufacturing an Article of Metallic or Ceramic Material In the manufacture of articles of metallic or ceramic material by sintering powder of the material while it is subjected to isostatic pressure, the powder is suitably preformed into a manageable powder body. This can be done by loose sintering, i.e. by sintering a powder, filled into a forming cavity, under vacuum or in a protective gas atmosphere, so that a coherent body is formed but no significant densification takes place. It can also be done by subjecting the powder to isostatic compaction, for example with the powder arranged in a sealed capsule of a yielding material, such as a plastics capsule.The compaction can be carried out with advantage without the use of a binder at room temperature or any other temperature which is considerably lower than the temperature during the subsequent compression in connection with the sintering. The compacted product can thereafter be given its desired shape by machining. The powder body may also be preformed by a conventional technique used in the manufacture of ceramic goods. Thus, the powder may be mixed before the preforming with a temporary binder, for example methyl cellulose, cellulose nitrate, an acrylate binder, a wax or a mixture of waxes. After the preforming the binder is driven off by heating so that the performed powder body in all essentials becomes free from binder.

When the preformed powder body is subjected to the isostatic pressing at the sintering temperature, it must, in order to give a desired dense, sintered product, be enclosed in a sealed casing which, during the pressing, is able to prevent the pressure medium then used, normally a gas, from penetrating into the powder body. The casing, like its contents, is liberated from undesirable gases during a process stage prior to its sealing. Various ways of forming the casing are known. According to one known method, a preformed capsule of glass is used as the casing.

According to another known method, the casing is manufactured on the spot by dipping the preformed powder body into a suspension of glass particles, or surrounding it in some other manner with a layer of glass particles, and then heating it under vacuum at such a temperature that the particles form a tight casing around it. As far as silicon nitride is concerned, it is also known to use a porous layer of glass of a low-melting type outside a porous layer of glass of a highmelting type. In this known case, the outer porous layer is transformed into a layer impermeable to the pressure medium while the powder body is degassed. When a tight layer has been formed, pressure is applied to the enclosed powder body employing argon or helium to counteract dissociation of the silicon nitride when the temperature is continually raised.During the continued temperature increase, the glass in the outer layer reacts with the material in the inner porous layer while forming an increasingly highmelting glass and while maintaining a layer impenetrable to the pressure medium, and finally a glass layer which is impenetrable to the pressure medium is formed from the innermost part of the inner porous layer before the glass in the outer layer has time to run off. This last formed glass layer forms a casing around the powder body when the isostatic pressing thereof is carried out at the sintering temperature.

In certain cases it has been found that there are problems in achieving a desirably high degree of reproducibility in the manufacture of articles of powder material when using the known methods described above, especially when it is a question of articles having a complicated shape such as articles having sharp corners or edges or having thin-walled portions, for example turbine discs with blades. If a preformed capsule of glass is used, there is a risk that the glass, when softening, will accumulate in pockets and, because of a relatively high viscosity, may cause damage there in thin-walled portions of the preformed powder body when high pressure is applied for the isostatic pressing of the powder body.If the casing is manufactured on the spot by surrounding the powder body with layers of glass particles, there is a risk, which is also present when using a preformed body of glass, that the powder body in certain places, especially at sharp corners or edges, is not covered by any casing material when the pressing is to be carried out, because the glass is not retained there.

The present invention aims to provide a method of manufacturing articles of powder material of high density with greater reproducibility than with the previously mentioned methods.

According to the invention a method of manufacturing an article of metallic or ceramic material by isostarically pressing a preformed body of powder of the metallic or ceramic material with a gaseous pressure medium, the preformed body being embedded in an embedding material consisting of glass or of a material forming glass upon heating, which embedding material is transformed into a casing impermeable to said pressure medium before the isostatic pressing is carried out while sintering the preformed body, is characterised in that the preformed body and the embedding material are placed in a vessel which is resistant to the temperature at which the sintering of the metallic or ceramic material is carried out, in that the embedding material is transformed into a melt, as hereinafter defined, having a surface limited by the walls of the vessel, with the preformed body located below the surface of the melt, and in that the pressure required for the isostatic pressing is applied on the melt with the gaseous pressure medium.

In this specification the word "melt" means a gas-impermeable mass which, at least partly and preferably at least for the main part, consists of molten phase. It is thus not necessary that all the constituents of the embedding material have melted in their entirety in order for a functioning gas-impermeable mass, here included in the concept melt, to have formed. The melt has an at least substantially horizontal surface.

It is essential that the melt is subjected to pressure by a gaseous pressure medium, for example in a heatable high-pressure chamber, and not by- a piston in a mould cavity, in which the melt is enclosed while making contact with the walls of the mould cavity. In the latter case it is not possible, or at any rate extremely difficult, to avoid damage to weak portions of the powder body, because it is not possible to maintain a sufficiently low viscosity of the glass melt, since in that case it tends to penetrate out between the piston and the mould cavity.

The invention is of extremely great importance in connection with isostatic pressing of articles of silicon nitride or of materials comprising silicon nitride as the main constituent. The invention will therefore first be described with respect to its application for the manufacture of articles of silicon nitride.

As the pressure medium for carrying out the present invention, there are preferred inert gases, for example argon and helium, and nitrogen gas.

The pressure during the sintering of a preformed silicon nitride body is dependent on whether a sintering-promoting additive, for example magnesium oxide or yttrium oxide, has been added to the silicon nitride. If no such additive is used, the pressure should amount to at least 100 MPa, preferably from 200 to 300 MPa. When using a sintering-promoting additive, a lower pressure may be used, however suitably at least 20 MPa. The sintering of the preformed body is suitably carried out at a temperature of from 16000 to 1 9000C, preferably from 17000 to 1 8000C.

Graphite is preferred as the material of the vessel which is resistant to the sintering temperature, but other materials such as nitride or molybdenum may be used.

Advantageously, the glass, or material forming glass on heating, in which the preformed body is embedded, is in the form of particles. The preformed silicon nitride body is then embedded in the particles in the temperature resistant vessel and the particles are transformed into a melt in the vessel. It is also possible to use larger pieces of the glass or of the glass-forming material, such as preformed pieces which at least substantially follow the shape of the preformed body. For example, it is possible to use a preformed piece on the lower side of the preformed body and another preformed piece on the upper side of the body, the edges of the pieces then suitably making contact with each other.Depending on the shape of the article, it is also possible in principle to use a capsule of glass or glassforming material made in one piece and having an opening enabling the preformed body to be inserted into the capsule. When the embedding material is made from one piece or a few pieces of glass or glass-forming material, it is possible to transform it into a melt either before or after it has been placed in the vessel. In the first-mentioned case this can be done with advantage in a separate process in a furnace which is suited for this purpose and in the latter case it may be done with advantage during transformation of the embedding material into a. melt in the highpressure furnace in which the isostatic pressing is carried out.

Advantageously, the embedding material is made gas-impermeable while maintaining a vacuum around it. In this case, in order to avoid a dissociation of the silicon nitride, a glass or a glass-forming material should'be used which is converted into a melt at a relatively low temperature. Thus, if the embedding material consists of particles of glass or glass-forming material which are transformed into a melt in the temperature resistant vessel while maintaining a vacuum across it, a glass or glass-forming material should be used which results in a melt with a low viscosity, suitably at most 106 poises at a temperature of about 11 50 C, so that a high pressure may be applied at this temperature without the risk of damage to the preformed body arising.

The embedding material may also with advantage be transformed into a melt while keeping it in contact with a gas which at least for the main part consists of nitrogen gas and which is maintained at a pressure which is at least as great as the pressure in the nitrogen gas in the pores of the preformed silicon nitride at the temperature in question. When using this method, and particularly when the embedding material consists of particles of glass or a glass-forming material which are transformed into a melt in the temperature resistant vessel in the presence of nitrogen gas under pressure, a glass or glassforming material may be used which gives a melt with low viscosity, suitably at most 106 poises, at considerably higher temperatures than the glass materials which may be used in the vacuum method.The high pressure required for the isostatic pressing is then applied, as in the preceding case, when the melt has acquired a low viscosity, which, depending on the type of glass, may be done in a temperature range of from around 11500 to around 17000C.

The density of the glass or glass-forming material which is used for silicon nitride should be at the most 2.4 g/cm3 in order to avoid the risk that the powder body will rise to such an extent from the melt that parts of the body will not be covered by the melt.

If a limited superficial penetration of the glass or glass-forming material into the pores of the powder body is permissible it is possible to use a pluraiity of different glasses or glass-forming materials which provide melts of such low viscosity, possibly while applying nitrogen gas pressure during the formation of the melt, that the preformed body of silicon nitride is not damaged when the high pressure required for the isostatic pressing is applied. Among other things, different types of lead silicate glass and aluminium silicate glass may be used, as well as quartz and mixtures of different glass-forming oxides. In certain cases it may then be necessary to remove the surface layer on the pressed silicon nitride body, for example by blasting.

However, it has been found possible to avoid a penetration of the glass or glass-forming melt into the pre-formed silicon nitride body by using a glass containing B203 around the silicon -nitride body and a sufficiently small grain size of the silicon nitride, preferably a grain size of less than 5 microns. A plausable explanation as to why a boron-containing glass does not penetrate into the silicon nitride body is that a boron nitrogen compound, probably boron nitride, is formed at the boundary surface between the glass and the silicon nitride before the glass forms a lowviscous melt and that this boron nitrogen compound counteracts the penetration of the glass into the pores of the powder body. The content of B203 in the glass may advantageously amount to from 2 per cent to 70 per cent by weight.As examples of applicable boroncontaining glasses may be mentioned a "Pyrex" (Trade Mark) glass containing, by weight, 80.3 per cent of SiO2, 12.2 per cent of B203, 2.8 per cent of Awl2 03, 4.0 per cent of Na2O, 0.4 per cent of K2O and 0.3 per cent of CaO, a glass containing 58 per cent SiO2, 9 per cent of B203, 20 per cent of Awl203, 5 per cent of CaO and 8 per cent of MgO, a "Vycor" (Trade Mark) glass containing 96.7 per cent of SiO2, 2.9 per cent of B203 and 0.4 per cent of Awl203, and a glass containing 38 percent of SiO2, 60 per cent-of B203 and 2 per cent of Al203.

It is also possible to use mixtures of particles of substances, for example SiO2, Al203, B203 as well as alkali metal oxides and alkaline earth metal oxides, which form glass when heated.

After the pressing and the sintering, the finished article of silicon nitride is embedded in the glass. According to an advantageous embodiment of the invention, a glass is used which has approximately the same coefficient of thermal expansion as silicon nitride within a considerable part of the range between the solidification temperature of the glass and room temperature, preferably a coefficient of thermal expansion of from 3.0 to 3.8 xl 0-8 per C within the temperature range of from 5000 to 200C.

This prevents damage to the article caused by cracks or rupture during the cooling. A suitable glass is the previously mentioned "Pyrex" glass which has a coefficient of thermal expansion of 3.2x10-6 per C between 5000 and 200C. For silicon nitride the corresponding value is 3.2x 10-6 per OC. It is possible, although it complicates the manufacture of the article to a considerable extent, to contrive the exposure of the finished article when using a glass the coefficient of thermal expansion of which is considerably different from that of the silicon nitride. An example of such a glass is the previously mentioned "Vycor" glass.Such a glass can be removed substantially completely from the silicon nitride article, for example by reducing the outer pressure below the dissociation pressure of the silicon nitride, for example at 16000C, the glass then lifting from the silicon nitride body and the article being allowed to cool without being damaged by the glass. In certain cases it may be suitable to remove the glass by increasing the temperature above the temperature used during the sintering, so that the glass acquires such a low viscosity as to leave only a thin film which will not damage the article and which, if necessary, may be removed by blasting.

The method of the invention may be used, in the same way as described above for silicon nitride for manufacturing articles of materials containing silicon nitride as the main constituent, for example silicon aluminium oxynitride and-the said oxynitride in which aluminium has been at least partly replaced with yttrium, as well as other silicon metal oxynitrides and further mixtures of silicon nitride and silicon metal oxynitride.

Examples of other materials for which the method of the present invention may be used are iron and nickel-based alloys, for example an ironbased alloy containing, by weight, 0.33 per cent of C, 0.30 per cent of Si, 0.40 per cent of Mn, 0.01 per cent of P, 0.01 per cent of S, 2.8 per cent of Cr, 0.6 per cent of Mo, the remainder being Fe (3 per cent Cr-Mo steel), an iron-based alloy containing, by weight, 0.1 8 per cent of C, 0.25 per cent of Si, 0.60 per cent of Mn, 0.01 per cent of P, 0.01 per cent of S, 1 1.5 per cent of Cr, 0.5 per cent of Ni, 0.5 per cent of Mo, 0.30 per cent of V, 0.25 per cent of Nb, the remainder being Fe (12 per cent Cr-Mo-V-Nb-steel), a nickel-based alloy containing, by weight, 0.03 per cent of C, percent of Cr, 17 per cent of Co,5 per cent of Mo, 3.5 per cent of Ti, 4.4 percent of Al, 0.03 per cent of B, the remainder being Ni, or a nickel-based alloy containing, by weight, 0.06 per cent of C, 12 per cent of Cr, 17 per cent of Co, 3 per cent of Mo, 0.06 per cent of Zr, 4.7 per cent of Ti, 5.3 per cent of 0.014 per cent of B, per cent of V, the remainder being Ni, certain metal oxides, for example AI203, and certain carbides, for example silicon carbide.

In addition to the gases previously mentioned, hydrogen gas is suitable as the pressure medium when pressing metallic materials, particularly if the sealing of the embedding material takes place while supplying gas. The pressure and the temperature during the sintering of the preformed body are, of course, dependent on the properties of the metallic or ceramic material. Normally, the pressure should amount to at least 50 MPa, preferably to at least 100 MPa. If the material consists of an iron-based alloy the temperature should be at least 100000, preferably from 11000 to 12000C, and if the material consists of a nickel-based alloy the temperature should be at least 105000, preferably from 11000 to 12500C.

If the material is aluminium oxide the temperature should be at least 12000C, preferably from 13000 to 1 5000C, and if the material is silicon carbide the temperature should be at least 1 7000C and preferably from 1 8000 to 20000C.

For materials with particularly high sintering temperatures, such as silicon carbide, quartz glass may suitably be used in the embedding material.

In order to prevent glass from penetrating into pores of powder bodies of metallic or ceramic material, it is suitable to surround the powder body with a blocking layer, for example a layer of finely-divided boron nitride or of finely-divided glass having a higher melting temperature than the glass in the embedding material.

The invention will now be illustrated by the following non-limitative Examples, (in which the percentages of compositions are expressed by weight), which Examples are described with reference to the accompanying schematic drawings, in which: Figure 1 is a plan of a body preformed from silicon nitride powder in the form of a turbine wheel for a gas turbine engine.

Figure 2 is a sectional side view of the body of Figure 1, Figure 3 is a sectional view showing the body of Figures 1 and 2 placed in a temperatureresistant vessel and embedded in a mass of glass particles, and Figure 4 is a partly sectioned side view of a high-pressure furnace in which isostatic pressing and sintering of the preformed powder body are carried out.

Example 1 Silicon nitride powder having a powder grain size of less than 5 microns and containing about 0.5 per cent of free silicon and about 0.1 per cent of magnesium oxide is placed in a capsule of plastic, for example plasticised polyvinyl chloride, or of rubber, having approximately the same shape as the preformed powder body to be manufactured, whereafter the capsule is sealed and placed in a press device, for example the device shown in Figures 1 and 2 of British Patent Specification No. 1,522,705. The powder is subjected to a compaction at a pressure of 600 MPa for a period of 5 minutes. When this compaction has been effected the capsule is removed and the preformed powder body thus manufactured is machined to the desired shape.

The powder body has a density of 60 per cent of the theoretical density.

The preformed powder body 10, which is shown in Figures 1 and 2, consists of a turbine wheel having a hub 1 a web 12, a rim 13 and blades 1 4.

As shown in Figure 3, the powder body 10 is placed in an open-topped vessel 1 5 which is resistant to the sintering temperature to be used, the powder body then being embedded in glass powder 1 6. The vessel in the exemplified case consists of graphite and is provided internally with a release layer 1 7 of boron nitride. The glass powder consists of particles of a glass containing 80.3 per cent of SiO2, 12.2 per cent of B203, 2.8 per cent of Al203, 4.0 per cent of Na2O, 0.4 per cent of K2O and 0.3 percent of CaO.

One or more of the vessels 1 5, each containing a powder body 10 embedded in glass powder 16, are then placed in a high-pressure furnace according to Figure 4, but for sale of clarity, only one vessel 1 5 is shown in this Figure. In Figure 4, the numeral 22 designates a press stand which is supported by wheels 23 and is displaceable on rails 24 on the floor 25 between the position shown in the Figure and a position in which the stand surrounds a high-pressure chamber 42. The press stand is of the type which consists of an upper yoke 26, a lower yoke 27 and a pair of spacers 28, which are held together by a prestressed strip sheath 29.The high-pressure chamber 42 is supported by a column 49 and comprises a high-pressure cylinder which is built up of an inner tube 50, a surrounding prestressed strip sheath 51 and end rings 52 which axially secure the strip sheath and constitute suspension means by which the high-pressure chamber is attached to the column 49. The chamber 42 has a lower end closure 53 projecting into the tube 50 of the high-pressure cylinder. The end closure 53 is provided with a groove, in which a sealing ring 54 is inserted, a channel 55 for degassing the products to be pressed and for supplying pressure medium, suitably argon, helium or nitrogen gas, and a channel 56 for cables for feeding heating elements 57 for the heating of the furnace. The elements 57 are supported by a cylinder 58 which rests on an insulating bottom 59, which projects into an insulating mantle 60.The chamber 42 also has an upper end closure comprising an annular portion 61 with a sealing ring 62 which seals against the tube 50. The mantle 60 is suspended from, and connected in a gas-tight manner to, the portion 61. The upper end closure also comprises a lid 63 for sealing the opening of the portion 61, which is usually permanently applied in the high-pressure cylinder. The lid 63 is provided with a sealing ring 64 sealing against the inner surface of the portion 61, and with an insulating cover 65 which, when the highpressure chamber is closed, projects into the cylinder 60 and constitutes part of the insulating shield which surrounds the furnace space proper 66. The lid 63 is attached to a bracket 67, which is supported by a raisable, lowerable and turnable operating rod 68.When the stand 22 surrounds the chamber 42, the yokes 26 and 27 absorb the compressive forces acting on the end closures 53 and the lid 63 when pressure is applied to the furnace space.

When the vessel 1 5 with its contents has been placed in the furnace space 66, the powder body 10 with the surrounding glass powder 1 6 is degassed at room temperature for approximately 2 hours. While continuing the degassing, the temperature is then raised to approximateiy 11 500C. The temperature increase is carried out so slowly that the pressure does not exceed 0.01 torr during any part of the time. At approximately 11 500C the temperature is maintained constant for about 1 hour, whereafter the final degassing takes place and the glass powder forms a low viscosity melt which completely surrounds the powder body 10 and has a horizontal surface.

Thereafter, argon or helium is supplied at the same temperature to a pressure level which provides a pressure of from 200 to 300 MPa at the final sintering temperature. The temperature is then raised to a value of from 1 700" to 1800 C, that is, to a suitable sintering temperature for the silicon nitride, for a period of 1 hour, and at the same time the pressure rises. A suitable time for sintering under the conditions mentioned is at least 2 hours. At the end of the sintering operation, the furnace is allowed to cool to a suitable discharging temperature. The vessel 1 5 then contains a blank cake, in which the powder body is visible through the solidified and clear glass.The powder body is completely embedded in the glass and was therefore located below the surface of the melt in its entirety during the pressing. Because of the high pressure required for the pressing was applied when the glass had low viscosity and because the glass in solidified form has the same coefficient of thermal expansion as the silicon nitride, flawless articles can be produced with a good reproducibility. The cake is easily released from the vessel because of the presence of the release layer 17. The glass can then be removed from the article by blasting.

The density of the finished article exceeds 99.5 per cent of the theoretical density.

Example 2 The equipment used in this Example was the same as in Example 1, but the glass powder 16 used contained 96.7 per cent of SiO2, 2.9 per cent of B203 and 0.4 per cent of A/203. With this glass a sufficiently low-viscous melt may be achieved only at a temperature of 1 6000C. To counteract the dissociation of silicon nitride that occurs at this temperature, the glass mass 16 is transformed into a melt while maintaining a pressure of nitrogen gas in the furnace space 66.

When the vessel with the powder body 10 and surrounding glass powder has been degassed in the furnace space 66 at room temperature for approximately 2 hours, the furnace space is filled with nitrogen gas at atmospheric pressure and the temperature of the furnace is raised to 1600 C while successively supplying nitrogen gas to a pressure of 0.1 MPa. When the temperature has reached 1 6000C, the glass powder forms a low viscosity melt which completely surrounds the powder body 1 0.

Thereafter argon or helium is supplied at the same temperature and the pressing and the sintering are carried out under the conditions described in Example 1. Since the glass in this case has a coefficient of thermal expansion which differs considerably from that of the silicon nitride, only a limited cooling of the furnace may be allowed before the vessel 1 5 with its contents is removed.

The pressed object is then heated to a temperature of about 1 8000C so that the glass runs off the finished article and only leaves a thin film on the article. After cooling to room temperature, the film is suitably removed by blasting.

Example 3 In this Example a preformed body was prepared in a divisible form or mould having a cavity shaped as a turbine disc and composed of an aluminium-silicate based material, for example of the same type as is normally used in cores for the investment casting of turbine blades having cooling channels. This form is filled with spherical powder of an iron-based alloy containing 0.1 8 per cent of C, 1 1.5 per cent of Cr, 0.25 per cent of Si, 0.5 per cent of Mo, 0.60 per cent of Mn, 4.4 per cent of Al, 0.01 per cent of P, 0.01 per cent of S, 0.5 per cent of Ni, 0.30 per cent of V and 0.25 per cent of Nb, the remainder being Fe and having a grain size of less than 250 microns. The powder is vibrated together by striking lightly on the form and the powder is sintered under vacuum at a temperature of about 1 2000C for about 2 hours.

After cooling, the reusable form is divided and a porous turbine disc having essentially the same dimensions as the forming cavity is removed. The porous turbine disc is then provided with a blocking layer by being coated with a powder of a high-melting glass having a grain size of less than 1 micron, to a thickness of about 0.3 mm. The glass consists of 96.7 per cent of SiO2, 2.9 per cent of B203 and 0.4 per cent of AI2O3.

The turbine disc is completely embedded in glass particles in a graphite crucible, the glass consisting of 80.3 per cent of SiO2, 1 2.2 per cent of B203, 2.8 per cent of Awl203, 4.0 per cent of Na2O, 0.4 per cent of K20 and 0.3 per cent of CaO. The graphite crucible with its contents is then placed in a high-pressure furnace of the kind described in Example 1.

The preformed powder body embedded in the glass particles is first degassed in the highpressure furnace for approximately 2 hours at room temperature, after which the furnace is heated to 9500C. When the temperature has reached 9500C and the embedding glass has sintered densely, the pressure is raised by pumping in argon gas. The temperature is also raised to 1200"C, the embedding glass then forming a melt with a horizontal surface. At this temperature a pressure of 100 MPa is maintained for 2 hours, the powder body then being completely densified. After completion of the pressing operation, the furnace is allowed to cool to a suitable discharging termperature. The vessel then contains a blank cake in which the powder body is visible through the solidified and clear glass. The powder body is completely embedded in the glass and has thus been located below the surface of the melt in its entirety during the pressing. The glass may be removed in the manner described previously.

Example 4 The divisible form or mould used in Example 3 was filled with the same spherical, iron-based alloy powder as in Example 3 and the powder was sintered under vacuum, under the same conditions as in Example 3 to provide a porous turbine disc. The porous turbine disc is then provided with a blocking layer by being coated with a fine-grained powder having a grain size of less than 1 micron either of the same highmelting glass as in Example 3 or of boron nitride, to a thickness of about 0.3 mm.

The turbine disc is completely embedded in glass particles of the same kind as in Example 3 in a graphite crucible, and the crucible with its contents is then placed in a high-pressure furnace of the kind described in Example 1.

The preformed powder body embedded in the glass particles is first degassed in the highpressure furnace for approximately 2 hours at room temperature, after which the furance is heated to 7500C. The pressure is then increased at a rate of about 100 mbar/min by supplying hydrogen gas while the temperature is raised at a rate of about 5 C/min.

When the temperature has reached 9500C and the embedding glass has sintered densely, the pressure is raised further by pumping in argon gas. The temperature is also raised to 12000 C, the glass then forming a melt. At this. temperature the pressure is maintained at 100 MPa for 2 hours, the powder body then being completely densified. The pressure and the temperature are then reduced and the crucible may be removed from the hot furnace. The sintered powder body is then obtained from a blank cake of glass, as described in Example 1.

Claims

1. A method of manufacturing an article of metallic or ceramic material by isostatically pressing a preformed body of a powder of the metallic or ceramic material with a gaseous pressure medium, the preformed body being embedded in an embedding material consisting of glass or of a material forming glass upon heating,.

which embedding material is transformed into a casing impermeable to said pressure medium before the isostatic pressing is carried out while sintering the preformed body, characterised in that the preformed body and the embedding material are placed in a vessel which is resistant to the temperature at which the sintering of the metallic or ceramic material is carried out, in that the embedding material is transformed into a melt, as hereinbefore defined, having a surface limited by the walls of the vessel with the preformed body located below the surface of the melt, and in that the pressure required for the isostatic pressing is applied on the melt with the gaseous pressure medium.

2. A method according to claim 1, in which the glass or glass-forming material is in the form of particles or pieces.

3. A method according to claim 1 or 2, in which the embedding material is transformed into said melt while the powder body and the embedding material are held under a vacuum.

4. A method according to claim 1 or 2, in which the embedding material is transformed into said melt while the powder body and the embedding material are held in contact with a gas, in which a pressure is maintained which is at least as great as the pressure of the gas which is present in the pores of the powder body at the temperature in question.

5. A method according to any of the preceding claims, in which the melt has a viscosity of at the most 1 OB poises when the pressure required for the isostatic pressing is applied on the melt.

6. A method according to any of the preceding claims, in which the powder material consists of silicon nitride or a material containing silicon nitride as the main constituent.

7. A method according to claim 6, in which glass containing B203 is employed as the embedding material.

8. A method according to claim 7, in which the B203 content of the glass is from 2 to 70 per cent by weight.

9. A method according to any of claims 6 to 8, in which the glass has a coefficient of thermal expansion of from 3.0 to 3.8 xl 0-6 per OC within the temperature range of from 20 to 5000C.

10. A method according to any of claims 1 to 5, in which the powder material consists of a metallic material and that a blocking layer is arranged around the preformed body, which layer prevents glass from the embedding material from penetrating into the pores of the preformed body.

11. A method of manufacturing an article of metallic or ceramic material substantially as described in any of the foregoing Examples.

12. An article of metallic or ceramic material when made by the method claimed in any of the preceding claims.