GB2050926A - Method of manufacturing articles of ceramic or metallic material - Google Patents

Abstract

For manufacturing an article of a ceramic or metallic material by sintering and simultaneous isostatic pressing of a powder of the material, the powder is enclosed in an embedding material of glass which is made impermeable to gas before the isostatic pressing is carried out with a gaseous pressure medium. The powder is introduced into the mould cavity of a mould 10 made of glass powder, the mould cavity having a shape corresponding to that of the article to be manufactured, whereafter the mould is covered with glass 14 which, together with the mould, forms an embedding material for the powder. The powder and the embedding material are then located in a vessel 15, and the embedding material is transformed into a melt having a surface limited by the walls of the vessel, below which surface the powder in the mould cavity is located, before the pressure necessary for the isostatic pressing is applied to the melt with the gaseous pressure medium. Alternatively the tube through which the mould cavity is filled may be covered with a plate of glass rather than using the covering of glass 14. <IMAGE>

Description

SPECIFICATION Method of manufacturing articles of ceramic or metallic material Technical field This invention relates to a method of manufacturing an article of ceramic or metallic material, by sintering powder of the material while it is being isostatically pressed.

When employing this method, it is necessary, in order to achieve a sintered product having the desired high density, to seal the powder in a casing which, during the pressing, prevents the pressure medium used, normally a gas, from penetrating into the powder. The casing as well as its contents are normally liberated from undesirable gases during a process stage prior to the sealing of the casing.

According to one known method, the casing employed is a preformed capsule of homogeneous glass having a shape corresponding to the shape of the article to be produced but having larger dimensions. Particularly if the article to be produced has a complicated shape, such a capsule becomes very expensive because of the forming tools and other equipment required for its manufacture.

The present invention aims to provide a simple and inexpensive method of manufacturing an article of ceramic or metallic material, which does not require the employment of complicated and expensive equipment.

Disclosure of the invention According to the invention a method of manufacturing an article of ceramic or metallic material by sintering and simultaneously isostatically pressing a powder of the ceramic or metallic material with a gaseous pressure medium, comprises the steps of introducing said powder into the mould cavity of a mould of glass powder, said mould cavity having a shape corresponding to the shape of the article that is to be manufactured, covering said mould with glass which together with the mould forms an embedding material for said powder, locating said powder and the surrounding embedding material in a vessel resistant to the temperature at which the sintering is to be carried out, heating said vessel and its contents to raise the temperature of said powder to the sintering temperature thereof, with conse quent transformation of said embedding material into a gas-impermeable melt having a surface limited by the walls of said vessel, below which surface the powder in said mould cavity is located, and applying to said melt the pressure required for the isostatic pressing employing said gaseous pressure medium.

The term "melt" used in this specification means a gas-impermeable mass which at least for the main part is in the molten phase. It is thus not necessary that all the constituents of said embedding material should have melted in their entirety for an adequate gas-impermeable mass to have been formed.

The mould of glass powder, into which the powder of ceramic or metallic material is introduced, may be manufactured by surrounding a pattern of the article to be manufactured with the glass powder and by binding the glass particles to each other to form a coherent unit, before the pattern is removed.The pattern is made of a material which is capable of being removed from the mould without the latter being destroyed or damaged, for example of wax or other low-melting material such as an alloy composed of 60 per cent of Bi and 40 per cent of Sn or Woods metal (50 per cent of Bi, 25 per cent of Pb, 14 per cent of Sn and 11 per cent of Cd) which, of course, is removed by allowing the material to run out after melting, of a plastics material, such as polystyrene, or other material which can be broken down by heating and caused to depart from the mould in gaseous form, or of a material which may be dissolved out with a solvent, for example a metal capable of being dissolved with acid. The glass particles may be bonded to each other in various ways.For example, an inorganic binder, for example B2O3, or SiO2 formed in the mould from ethyl silicate, may be used, or an organic binder, for example starch or a resinous binder, which may be burnt away, may be used. It is also possible to effect a bonding of the glass particles to each other by sedimentation or cold compaction without the use of a binder.

The glass is preferably of a low-melting type. As examples of suitable glasses may be mentioned the glass known undertheTrade Mark "Pyrex" and containing (by weight) 80.3 per cent of SiO2, 12.2 per cent of B203, 2.8 per cent of Al2O3, 4.0 per cent of Na2O, 0.4 per cent of K2O and 0.3 per cent of CaO, an aluminium silicate glass containing (by weight) 58 per cent of SiO2, 9 per cent of B2O3, 20 per cent of Al2O3, 5 per cent of CaO and 8 per cent of MgO, as well as mixtures of particles of substances, for example SiO2, 13203, Al203 as well as alkali metal and alkaline earth metal oxides which form glass upon heating.The term "glass" used in this specification includes glass-forming materials, for example the mixtures just mentioned.

Before being introduced into the mould cavity, the powder of ceramic or metallic material is given a suitable consistency, if required, by the addition of an additive, for example a solvent (for example, methanol, ethanol or water), a binder (for example, polyvinyl alcohol), a plasticizer (for example, ethylene glycol), an anti-flocculating agent (for example, benzene sulphonic acid) and/or a wetting agent (for example, ethyl pentyl glycol). The introduction of the material into the mould cavity may be done by slip casting or with the aid of a vacuum pressure, centrifugal force or ultrasonic vibration.

The mould may be provided with suction means, which facilitate introduction of the material by slip casting. The porous nature of the mould and its ability to suck up liquid facilitates a high degree of filling of the mould cavity with the powder of ceramic or metallic material, especially in the case of slip casting. At the inner surface of the mould, the pores of the mould wall preferably have a smaller size (diameter) than the size of the powder grains of the ceramic or metallic material.

The vessel in which the powder and the surrounding embedding material are located is preferably made of graphite, but other materials, for example boron nitride or molybdenum, may be used instead.

The embedding material may be made gasimpermeable while maintaining a vacuum around it.

Alternatively, the embedding material may be made gas-impermeable while it is maintained in contact with a gas in which a pressure is maintained which is at least as great as the pressure of the gas present in the powder in the mould cavity. In this way, the risk of gas leaving the powder in the mould cavity damaging the mould is avoided.

The preferred pressure medium for use in the method of the invention is an inert gas, for example argon, helium or nitrogen gas.

Ceramic articles which may be made by the method of the invention include articles made from nitrides, for example silicon nitride, metal oxides, for example aluminium oxide and carbides, for example silicon carbide. Metallic articles which may be made by the method of the invention include articles made from steel, from iron-based alloys, for example 3 per cent Cr-Mo steel containing 0.33 percent of C, 0.30 percent of Si, 0.40 percent of Mn, 0.01 percent of P, 0.01 per cent of S, 2.8 per cent of Cr, and 0.6 per cent of Mo, the balance being Fe or 12 percent Cr-Mo-V Nb steel containing 0.18 per cent of C, 0.25 percent of Si, 0.60 percentof Mn, 0.01 percent of P, 0.01 per cent of S, 11.5 percent of Cr, 0.5 percent of Ni, 0.5 percentof Mo, 0.30 percent of V and 0.25 percent of Nb, the balance being Fe, or an alloy containing 1.27 percentofC, 0.3 percent of Si, 0.3 percentof Mn, 6.4 per cent of W, 5.0 per cent of Mo, 3.1 per cent of V and 4.2 per cent of Cr, the balance being Fe, or from nickel-based alloys, for example an alloy containing 0.03 per cent of C, 15 per cent of Cr, 17 per cent of Co, 5 per cent of Mo, 3.5 per cent of Ti, 4.4 per cent of Al and 0.03 percent of B, the balance being Ni, or an alloy containing 0.06 per cent of C, 12 per cent of Cr, 17 percent of Co, 3 per cent of Mo, 0.06 per cent of Zr, 4.7 percent of Ti, 5.3 percent of Al, 0.01 per cent of Band 1.0 per cent of V, the balance being Ni, all these percentages being by weight.

Briefdescription of drawings The invention will now be described, by way of example, with reference to the accompanying drawings, in which Figures 1 to 3 are sketches illustrating three different stages during the manufacture of an article by the method in accordance with the invention, and figure 4 is a partly sectioned side view of one embodiment of a high pressure furnace for performing the isostatic pressing and sintering steps of the method in accordance with the invention.

Description ofpreferred embodiments As a first example of the method of the invention, there will be described the manufacture of a bladed turbine disc from silicon nitride powder.

Awax pattern is first manufactured, the shape of which corresponds to the shape of the turbine disc but which has larger dimensions to compensate for the reduction in dimensions which occurs during manufacture of the disc. At the centre of the wax pattern there is attached a tube of boron silicate glass. The wax pattern and the part of the glass tube lying closest to the pattern are dipped repeatedly into a suspension of gruel-like consistency containing 65 parts by weight of boron silicate glass having a grain size less than 1 micron in 35 parts by weight of ethyl silicate until a coating layer having a thickness of from 0.3 to 0.5 mm is obtained. The pore size in the layer is from 0.2 to 0.5 micron.The pattern is then further built on by dipping it into a suspension of boron silicate glass particles in ethyl silicate and containing 47 parts by weight of the glass particles having a grain size of 45 microns, 24 parts by weight of the glass particles having a grain size of 177 microns and 29 parts by weight of ethyl silicate, until a layer of a suitable thickness, about 3 to 5 mm, is obtained around the wax pattern, thereafter, the pattern with its coating is dried.

The coated pattern is then positioned in a vessel, the bottom of which is provided with a hole corresponding to the inner diameter of the glass tube and a shoulder corresponding to the outer diameter of the glass tube. The coated pattern is positioned in the vessel with the glass tube extending downwardly and with its lower end resting against the shoulder in the bottom of the vessel. Thereafter, an aqueous suspension of boron silicate particles having a grain size of less than 44 microns is slip cast or sedimented into the vessel until a mould with a thickness of about 20 mm is obtained. When the mould has been allowed to dry in the vessel, the vessel with the mould is placed in an autoclave to which steam at a temperature of about 1 500C is supplied to melt out the wax.The mould is then heated to a temperature of from 5500 to 600"C in a furnace, to remove all residues of wax and to increase the strength of the mould. The mould thus obtained is hard and porous with a good adhesion between the glass particles. In Figure 1 the mould designated by the numeral 10, is shown with the glass tube 12 extending downwardly. The mould cavity formed when the wax has run out is designated bythe numeral 11.

A suspension of silicon nitride powder in methanol is then prepared, the silicon nitride having a grain size less than 5 microns and containing about 0.5 percent by weight of free silicon and about 5 per cent by weight of yttrium oxide. The proportions of the silicon nitride powder and the methanol are chosen so that the suspension has a gruel-like consistency. The suspension is also provided with additives consisting of polyvinyl alcohol, polyethylene glycol and benzene sulphonic acid, the weights of these additives amounting to 2,4 and 1 per cent, respectively, of the weight of the suspension. The suspension is then degassed to remove blisters and then poured into the mould cavity 11, as shown in Figure 2, in which the suspension is designated by the numeral 13. From this Figure it will be seen that the tube 12 is used to introduce the suspension into the mould cavity. It also acts as a riser for the air displaced from the mould cavity.

Because the mould 10 is porous, the main part of the methanol and the additives is sucked up by the mould from the suspension and the mould cavity becomes filled up with silicon nitride powder. Final ly, when the levei of the suspension 13 in the tube 12 stops sinking, indicating that the mould cavity 11 has been filled, a gaseous pressure of about 2 megapascals (MPa) is applied to the material in the mould cavity. This improves the filling of the mould cavity and reduces the shrinkage during the subsequent drying step. The filled mould is then dried for a period of from 4 to 5 days at room temperature and atmospheric pressure.

The mould with its contents is then heated to around 500"C in vacuum, to remove from the mould and the mould cavity the substances added to the silicon nitride in the suspension. As shown in Figure 3, the mould is then placed in a vessel 15 of graphite, which is provided internally with a release layer of boron nitride, this vessel being resistant to the sintering temperature for the silicon nitride. The mould is covered with a powder 14 of boron silicate glass. Alternatively, the powder 14 may be replaced, for example, with a plate of glass covering the opening of the glass tube 12, whether this still has its original length or has been cut down to the level of the upper surface of the mould. In either case, the silicon nitride powder has been provided with an embedding material of glass.All the boron silicate glass referred to in this example is of a low-melting type and contains by weight, 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 per cent of CaO.

One or more of the vessels 15 are then placed in the high-pressure furnace shown in Figure 4, although for the sake of clarity only one vessel 15 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 highpressure chamber 42. The press stand is of the type consisting 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 hold together the strip sheath axially and constitute suspension devices by which the high-pressure chamber is secured to the column 49. The chamber 42 has a lower end closure 53 which protrudes into the tube 50 of the high-pressure cylinder. The end closure 53 has a groove into which a sealing ring 54 is fitted, a channel 55 for degassing the products to be pressed and for the supply of a gaseous pressure medium, suitably argon, helium or nitrogen gas, and a channel 56 for electric supply cables leading to heating elements 57 for heating the furnace. The elements 57 are supported by a cylinder 58 resting on an insulating bottom 59, which protrudes into an insulating shell 60.The upper end closure of the furnace comprises an annular portion 61 with a sealing ring 62 which seals against the tube 50. The shell 60 is suspended from the portion 61 and is connected thereto in a gas-tight manner. The upper end closure also comprises a lid 63 for closing the opening in the portion 61,which is usually permanently mounted in the high-pressure cylinder. The lid 63 is provided with a sealing ring 64 which seals against the inner surface of the portion 61 and with an insulating lid 65 which, when the high-pressure chamber is closed, protrudes into the cylinder 60 and constitutes part of the insulating shell which surrounds the actual furnace space 66. The lid 63 is secured to a bracket 67 carried by an operating rod 68 which is rotatable and vertically movable.The yokes 26 and 27 take up the forces acting on the end closure 53 and the lid 63 when pressure is applied in the furnace chamber.

When the vessel 15 with its contents has been placed in the furnace space 66, the powder in the cavity 11 and the surrounding glass 10 and 14 are degassed at room temperature for approximately two hours. Then, while continuing the degassing, the temperature is raised to approximately 1 1 50"C.

The increase in temperatures is made so slowly that the pressure does not exceed 0.1 torr at any time. At approximately 1 1 50"C the temperature is maintained constant for approximately one hour, the final degassing then taking place and the glass forming a low viscosity melt which completely surrounds the silicon nitride powder. Thereafter, argon, helium or nitrogen gas is supplied at the same temperature and at a pressure which will give a pressure of from 200 to 300 MPa at the final sintering temperature.

The temperature is then raised to a suitable sintering temperature for the silicon nitride, i.e. a temperature of from 1700" to 1800"C, during a period of one hour, the pressure rising simultaneously. A suitable time for sintering under the conditions stated is at least two hours. When the sintering operation has been completed, the furnace is allowed to cool to a suitable discharging temperature. The vessel 15 then contains a blank cake, in which the sintered powder body is visible through the solidified and clear glass.

The powder body is completely embedded in the glass and has thus been entirely located below the surface of the glass melt during the pressing.

Because it is possible to apply the necessary high pressure when the glass has low viscosity and because the glass, in its solidified form, has the same coefficient of thermal expansion as the silicon nitride, flawless turbine discs may be manufactured with good reproducibility. The cake is easily released from the vessel 15 because of the presence of the release agent. The glass may then be removed from the turbine disc by blasting or pickling.

In an alternative embodiment of the above described procedure, after the vessel 15 and its contents have been degassed at room temperature for approximately two hours, the furnace space 66 is filled with nitrogen gas at atmospheric pressure and the temperature of the furnace is raised to 11 500C while successively supplying nitrogen gas to a pressure of 0.1 MPa. When the temperature has reached 1150"C, the glass powder forms a low viscosity melt which entirely surrounds the silicon nitride powder body. Thereafter, argon, helium or nitrogen gas is supplied at the same temperature, and the pressing and the sintering are carried out under the previously described conditions.

The process described above for the manufacture of a turbine disc of silicon nitride can be employed in modified form forthe manufacture of a cutting tool from a powder of an iron-based alloy having the following composition by weight: 1.27 percent of C, 0.3 percent of Si, 0.3 per cent of Mn, 6.4 per cent of W, 5.0 per cent of Mo, 3.1 per cent of V and 4.2 per cent of Cr, the balance being Fe. The steel powder has a grain size below 177 microns. Prior to the application of the above-mentioned layer of boron silicate glass having a thickness of from 0.3 to 0.5 mm, the wax pattern is in this case coated with a high-melting type glass, for example the glass known under the Trade Mark "Vycor" containing, by weight, 96.7 percent of SiO2, 2.9 per cent of B203 and 0.4percent of A1203 and having a grain size of less than 1 micron.Apart from this, all the glass materials used are the same boron silicate glasses used in the previous example. The high-melting glass powder is sintered to the glass powder in the rest of the mould during the sintering thereof and prevents the penetration of glass into the steel powder body during the isostatic pressing, which is carried out at a temperature of around 1 00 C and a pressure of around 100 MPafora period oftwo hours. To prevent deformation of the cutting tool produced, caused by different coefficients of thermal expansion of the glass and steel, the main part of the embedding glass material around the tool is removed while the glass still has such a temperature that it is molten.

The pressures and the temperatures employed during the isostatic pressing and sintering of ceramic or metallic materials are, of course, dependent on the type of this material: Normally, the pressure should amount to at least 100 MPa, preferably to at least 150 MPa. In the case of silicon nitride, sintering is suitably carried out at a temperature of from 1600 to 1900"C and the pressure should amount to at least 100 MPa if no sintering-promoting additive such as magnesium oxide or yttrium oxide is used and to at least 20 MPa if such additive is used.As regards materials other than the silicon nitride and highspeed steel specifically exen:plified above, it may be mentioned that the temperåtsre for aluminum oxide should be at least 1 2000C, prcsferably from 1300 to 1500 C, for iron-based alloys least 1000"C, prefer ably from 1100 to 1 200 C and for nickel-based alloys at least 1 050 C, preferably from 1100 to 1 250 C.

For the manufacture of articles of ceramic or metallic materials with a very high sintering temper ature, it is possible to use in the mould, instead of the low-melting type glass powder, a powder of glass of a high-melting type such as the previously mentioned "Vycor", quartz glass or mixtures of particles of, for example, SiO2 and B203 which form a gas-tight glass layer when heated.

Claims (7)

1. f. A method of manufacturing an article of a ceramic or metallic material by sintering and simul taneously isostatically pressing a powder of the ceramic or metallic material with a gaseous pressure medium, said method comprising the steps of introducing said powder into the mould cavity of a mould of glass powder, said mould cavity having a shape corresponding to the shape of the article that is to be manufactured, covering said mould with glass which together with the mould forms an embedding material for said powder, locating said powder and the surrounding embedding material in a vessel resistant to the temperature at which the sintering is to be carried out, heating said vessel and its contents to raise the temperature of said powder to the sintering temperature thereof, with conse quenttransformation of said embedding material into a gas-impermeable melt, as hereinbefore defined, having a surface limited by the walls of said vessel, below which surface the powder in said mould cavity is located, and applying to said melt the pressure required for the isostatic pressing employing said gaseous pressure medium.
2. 2. A method according to claim 1, in which said embedding material is made gas-impermeable while maintaining said powder and said embedding material under a vacuum.
3. 3. A method according to claim 1, in which said embedding material is made gas-impermeable while said powder and said embedding material are maintained in contact with a gas in which a pressure is maintained which is at least as great as the pressure, at the temperature in question, of the gas which is present in said powder.
4. 4. A method according to any of the preceding claims, in which said powder is silicon nitride powder.
5. 5. A method according to any of claims 1 to 3, in which said powder is steel powder, an iron-based alloy powder or a nickel-based alloy powder.
6. 6. A method of manufacturing an article of a ceramic or metallic material substantially as hereinbefore described with reference to the accompanying drawings.
7. 7. A ceramic or metallic article when made by the method claimed in any of the preceding claims.