GB1564851A

GB1564851A - Method of manufacturing an object of silicon nitride

Info

Publication number: GB1564851A
Application number: GB3490/77A
Authority: GB
Original assignee: ASEA AB
Current assignee: ABB Norden Holding AB
Priority date: 1976-01-29
Filing date: 1977-01-28
Publication date: 1980-04-16
Also published as: SU651687A3; AU507155B2; SE7600919L; JPS5935870B2; JPS5293699A; SE396371B; AU2157477A; DE2702073C2; DE2702073A1; ZA77476B; IT1192228B; FR2339582B1; CA1091907A; FR2339582A1

Description

(54) METHOD OF MANUFACTURING AN OBJECT OF SILICON NITRIDE (71) We, ASEA AKTIEBOLAG, a Swedish Company of Västeras, Sweden, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: In the manufacture of objects of silicon nitride with high density (more than 90% of the theoretical density) by sintering together powder, the use of isostatic pressing offers many advantages. Thus, the manufactured objects will have approximately the same strength in all directions because of the allsided pressure, which is not the case with other methods of manufacture.Furthermore, objects of complicated shapes can be manufactured directly by the pressing and without, or substantially without, a subsequent machining by means of tools, for example by grinding, which is exceedingly important because silicon nitride is of very great hardness. One further important property of isostatic pressing is that the use of press tools is avoided and thus also the very considerable material problems connected therewith, which problems are caused by the high pressures and temperatures, at least 20 MPa and 1600"C, respectively, which are required. (A pressure of 1 MPa=10 atmo spheres.) Prior to the isostatic pressing and sintering of the silicon nitride powder, it is suitable that the powder is preformed into a manageable powder body by subjecting the powder to compaction, for example arranged in a sealed capsule of yielding material, such as a plastic capsule. The compaction can be performed with advantage without the use of a temporary binder at a pressure of at least 100 MPa either at room temperature or at a temperature which is considerably lower than the temperature employed during the high pressure sintering process. The product can thereafter be given its desired shape by means of machining. The preforming may also be carried out employing, among other things, conventional techniques for manufacturing ceramic goods.The silicon nitride powder is then usually mixed before the preforming operation with a temporary binder, for example methyl cellulose, cellulose nitrate, an acrylic binder, a wax or a mixture of waxes. After the preforming operation the binder is driven off by heating so that the preformed powder body becomes substantially free from binder.

Since the preformed powder body is subjected to the isostatic pressing operation at the sintering temperature, it must in order to give a desired, dense sintered product, be enclosed in a casing capable of being evacuated before the pressing operation and which, during pressing, is able to prevent the pressure medium used (which is normally a gas) from penetrating into the powder body. The casing must, of course, also have a sufficiently high strength or viscosity during the pressing operation in order not to penetrate into the pores of the powder body. If a preformed capsule of glass is used as the casing, which then has to be of a high-melting type in order not to run away or penetrate into the powder body at the high sintering temperature, the glass, when softening, collects in pockets and other recesses of the preformed powder body.This often leads to fracture at projecting portions of the sintered object during its cooling because of differences in the coefficient of thermal expansion of silicon nitride and glass. Instead the casing can be allowed to form on the spot by dipping the preformed powder body in a suspension of particles of high-melting glass or in some other way surrounding the body with a layer of particles of such glass, and then heating the powder body under vacuum at such a temperature that the particles form a dense casing around it. The last-mentioned method permits the application of a casing which can be made thin and which conforms to the shape of the powder body, thus avoiding accumulations of glass on the sintered object as well as the disadvantages associated therewith.However, the method has one serious drawback connected with the fact that a dense casing is only achieved at high temperatures since the glass has to be of high-melting type in order not to run away or penetrate into the powder body during the sintering of the silicon nitride. The fact that a dense casing is achieved only at high temperatures means that dissociation of the silicon nitride with evolution of nitrogen cannot be avoided, which impairs the quality of the sintered object.

The present invention aims to solve the problem of achieving a dense casing around a preformed powder body of silicon nitride, which does not give rise to harmful accumulations of glass on the sintered body and which prevents a harmful dissociation of the silicon nitride. This makes possible the manufacture of complicated parts of silicon nitride having a homogeneous composition and homogeneous properties.

According to the invention a method of manufacturing an object of silicon nitride by the isostatic pressing of a preformed body of silicon nitride powder with a pressure medium at a temperature required for sintering of the powder, comprises applying on the preformed powder body an inner porous layer of a first material and outside this an outer porous layer of a second material, the inner porous layer, possibly in cooperation with the second material, being transformable into a layer, impermeable to the pressure medium, at a temperature below the sintering temperature of the silicon nitride powder, and the outer porous layer being transformable into a layer, impermeable to the pressure medium, at a temperature which is lower than that for the inner porous layer, whereafter the preformed body is first subjected to degassing and to heating to a temperature which is required for forming a layer, impermeable to the pressure medium, of the outer porous layer but which maintains the inner porous layer porous, and then to heating to a temperature which is required for forming a layer, impermeable to the pressure medium, of the inner porous layer while maintaining a pressure outside said layers which is greater than the gas pressure inside these layers, and that the isostatic pressing of the preformed product then being carried out.

As said first material for the inner porous layer, there may be used a powder of a highmelting glass, for example "Vycor" (RTM) glass containing 96.7 per cent by weight of SiO2, 2.9 per cent by weight of B2O3, and 0.4 per cent by weight of Awl203, or quartz glass or mixtures of particles of substances, for example SiO2 and B2O,, which, during heating, form a gas-impermeable glass layer. It is also possible to use as said first material, for the inner porous layer, a powder of a highmelting metallic material capable of forming a metallic layer impermeable to the pressure medium, for example molybdenum, tungsten and other refractory metals.

As said second material, for the outer porous layer, there may be used a powder of a lowmelting glass for example "Pyrex" (RTM) glass containing 80.3 per cent by weight of SiO2, 12.2 per cent by weight of BoO35 2.8 per cent by weight of Awl4033 4.0 per cent by weight of Na2O, 0.4 per cent by weight of K,O and 0.3 per cent by weight of CaO, or an aluminium silicate containing 58 per cent by weight of SiO, 9 per cent by weight of B203, 20 per cent by weight of Aloe3) 5 per cent by weight of CaO and 8 per cent by weight of MgO, or an aluminium silicate containing 60 per cent by weight of SiO2, 20 per cent by weight of Al3O,, 15 per cent by weight of CaO and 5 per cent by weight of MgO, or mixtures of particles of substances, for example SiO2, B203) Awl203 and alkali metal oxides and alkaline earth metal oxides which, during heating, form a gas-impermeable glass layer.

The two porous layers, each of which suitably has a thickness within the range of from 0.05 to 1 mm, may, among other things, be applied by dipping the preformed powder body in suspensions of the particulate materials or by flame spraying or other thermal spraying. The particles suitably have a grain size within the range of from 0.1 to 100 microns.

The gas sing is suitably started, and allowed to continue, at room temperature for a period which is dependent on the size of the preformed powder body. Under continued evacuation, the temperature is raised so that the outer porous layer is transferred into a layer which is impermeable to the pressure medium. When this is done, pressure can be applied with a gaseous pressure medium on the enclosed powder body to counteract dissociation of the silicon nitride during continued temperature increase.During the continued increase in temperature, provided the layers consist of a glass or a glass-forming material, the glass of the outer layer reacts with the material of the inner porous layer while forming an increasingly high-melting glass and while maintaining a layer impermeable to the pressure medium, and finally a glass layer, impermeable to the pressure medium, is formed of the innermost part of the inner porous layer before the glass in the outer layer is able to run away. This last formed glass layer forms a dense casing around the powder body, when the isostatic pressing of the preformed product is carried out at the sintering temperature. When using a metallic material in the inner porous layer and a glass or a glass-forming material in the outer porous layer, the glass layer formed from the outer porous layer acts as impermeable layer at least until the inner metallic layer has been transformed to an impermeable layer.

The temperature at which the outer porous layer is caused to be transformed into an impermeable layer suitably lies within the range of from 600" to 1100 C, and the temperature at which the inner porous layer is caused to be transformed into an impermeable laver suitablv lies within the range of from 1300 to 1600"C. If the inner layer is compressed under isostatic pressure, which can be achieved after the outer layer has become gas-tight, there may, however, also be a question of temperatures of from 1000" to 1300"C.

For such a compression there is required a pressure of the order of magnitude of from 20 to 300 MPa. The sintering of the powder body is carried out at a temperature of at least 1600"C, preferably at a temperature of from 16000 to 1900"C.

The pressure during the sintering of the preformed silicon nitride body is dependent on whether a sintering-promoting additive, such as magnesium oxide, has been added to the silicon nitride or not. If no such additive is used, the pressure should amount to at least 100 MPa, preferably from 200 to 300 MPa.

When using a sintering-promoting additive, a lower pressure can be used, suitably at least 20 MPa.

The invention will now be described, by way of example, with reference to the accompanying drawing, in which Figure 1 is a schematic view of a preformed powder body of silicon nitride provided with two porous layers, and Figure 2 is a diagram illustrating a treatment cycle for manufacturing a sintered object by the method according to the invention.

Silicon nitride powder having a powder grain size of less than 7 microns and containing approximately 0.1 per cent by weight of magnesium oxide is placed in a capsule of plastics material, for example a softened 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 our British Patent Specification No.

1,522,705 (Application No. 44575/75). The powder is subjected to compaction at a pressure of 600 MPa for a time of five minutes.

After completed compaction the capsule is removed, and the preformed powder body thus manufactured is machined to the desired shape.

As shown in Figunre 1, the preformed powder body 1 is then provided with an inner porous layer 2 and an outer porous layer 3 by being dipped first in an aqueous suspension of powder of a glass consisting of 96.7 per cent by weight of SiO2, 2.9 per cent by weight of B2O, and 0.4 per cent by weight of Awl203 and then, after drying this layer, in an aqueous suspension of a powder of a glass consisting of 80.3 per cent by weight of SiO22, 12.2 per cent by weight of B2O3, 2.8 per cent by weight of At203, 4.0 per cent by weight of Na2O, OA per cent by weight of K20 and 0.3 per cent by weight of CaO, followed by a renewed drying.

The preformed powder body thus treated is thereafter placed in a high-pressure furnace which is provided with a conduit through which gas can be discharged for degassing of the powder body and through which gas can be supplied for generating the required pressure for the isostatic pressing and which is provided with heating devices. Such a high pressure furnace is described, for example, in the previously mentioned British Patent Specification No. 1,522,705 (Application No.

44575/75).

As illustrated in Figure 2, the preformed powder body is first degassed in the high pressure furnace at room temperature for approximately 2 hours. While evacuation continues the temperature is then raised to about 900"C, the temperature being raised so slowly that the pressure does not exceed 0.1 torr during any part of the time. At about 900"C the temperature is maintained constant for about 2 hours, the final degassing thus being performed and the glass powder in the outer porous layer sintering together into a gasimpermeable layer.To reduce the viscosity of the glass in the outer layer and thus reduce the risk of penetration of glass melt into the inner porous layer, the temperature is reduced to 700"C. Thereafter argon or helium is supplied 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 1700 to 1800"C, that is, to a suitable sintering temperature for the silicon nitride, the pressure rising simultaneously.

This temperature increase is achieved sufficiently slowly for the molten glass in the outer layer to have time to react with the glass powder in the inner layer to form an increasingly high-melting glass, and for the innermost layer of the glass powder in the inner layer to have time to sinter into a gas-impermeable layer before the glass in the outer layer is able to run off. A suitable time for sintering at 1700 to 1800"C and 200 to 300 MPa is at least 2 hours, if no sintering-promoting additive is used and at least 0.5 hours of such additive is used. After a completed cycle, the furnace is allowed to cool to a suitable discharging temperature and the sintered object is blasted clean of glass.

If a binder, such as the previously exemplifier methyl cellulose, cellulose nitrate, an acrylic binder, a wax or a mixture of waxes with different melting points, has been used in the manufacture of the preformed powder body, the binder is removed before or after the application of the porous layers, suitably by heating the powder body to 400" to 700"C in vacuum. Thereafter degassing and further treatment can be done, as described for a preformed powder body without a binder.

The method of the present invention is particularly suitable for being carried on as a series manufacturing process. In this case the various stages of treatment, such as a) removal of the binder, b) degassing in vacuum and dense sintering of the outer layer, c) further heating of the parts in gas at a pressure exceeding the pressure inside the layers, and d) final heating and hot isostatic pressing, can be performed in different kinds of furnace equipment with transfer from one equipment to another taking place with the powder in hot condition.

The method of the present invention is extremely well suited for the manufacture of, inter alia vanes and monolithic turbine rotors for gas turbines.

WHAT WE CLAIM IS: 1. A method of manufacturing an object of silicon nitride by the isostatic pressing of a preformed body of silicon nitride powder with a pressure medium at a temperature required for sintering of the powder said method comprising applying on the preformed powder body an inner porous layer of a first material and outside this an outer porous layer of a second material, the inner porous layer, possibly in cooperation with the second material, being transformable into a layer, impermeable to the pressure medium, at a temperature below the sintering temperature of the silicon nitride powder and the outer porous layer being transformable into a layer, impermeable to the pressure medium, at a temperature which is lower than that for the inner porous layer, whereafter the preformed body is first subjected to gas sing and to heating to a temperature which is required for forming a layer, impermeable to the pressure medium, of the outer porous layer but which maintains the inner porous layer porous, and then to heating to a temperature which is required for forming a layer, impermeable to the pressure medium, of the inner porous layer while maintaining a pressure outside said layers which is greater than the gas pressure inside these layers, the isostatic pressing of the preformed product then being carried out.

2. A method according to claim 1, in which a high-melting glass or a high-melting glassforming material is employed as said first material.

3. A method according to claim 1 in which a high-melting metallic material is employed as said first material.

4. A method according to any of claims 1 to 3, in which a low-melting glass or a lowmelting glass-forming material is employed as said second material.

5. A method according to any of claims 1 to 4, in which the preformed body is heated to a temperature of from 600" to 1100 C to render a layer of the outer porous layer impermeable to the pressure medium.

6. A method according to any of claims 1 to 5 in which the preformed body is heated to a temperature of from 1300"C to 1600"C to render a layer of the inner porous layer impermeable to the pressure medium.

7. A method according to any of claims 1 to 5, in which the preformed body is heated to a temperature of 1000" to 1300"C with simultaneous isostatic compaction to render a layer of the inner porous layer impermeable to the pressure medium.

8. A method of manufacturing an object of silicon nitride substantially as herein described with reference to the accompanying drawing.

9. An object of silicon nitride when made by the method claimed in any of the pre

Claims

ceding claims.