JPS5935870B2 - Silicon nitride object manufacturing method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The use of isostatic pressing shows many advantages in producing objects with silicon nitride with high densities (greater than 90% of the theoretical density) by sintering the powders together.

That is, the manufactured object has approximately the same strength in all directions because the pressure is applied on all sides, whereas this is not the case with other manufacturing methods.

Moreover, objects of complex shapes can be produced directly by pressing without subsequent machining with tools, e.g. by grinding, i.e. with virtually no machining, since silicon nitride has a very high hardness. It's very important for what you do.

Another important characteristic of isostatic squeezing is that the use of squeezing tools is avoided, thus also avoiding the very important material problems associated with the use of tools caused by the required high pressures and temperatures, at least 20 MPa and 1600° C., respectively. It can be avoided.

Prior to isostatic pressing and associated sintering of the silicon nitride powder, the powder is preformed into a manageable powder object by pressing the powder placed in a sealed capsule of flexible material, such as a plastic capsule. is appropriate.

The compaction is at room temperature or another temperature significantly lower than the temperature during compaction associated with sintering, and at a pressure of at least 100 MPa;
It can advantageously be carried out without the use of temporary binders.

The product can then be machined into the desired shape.

For the preforming, inter alia, also the customary techniques for producing ceramic products can be used.

The silicon nitride powder is then usually mixed with a temporary binder, such as methyl cellulose, cellulose nitrate, an acrylic binder, a wax or a mixture of waxes, before shaping.

The binder is driven off by heating after preforming so that the preformed powder body is substantially free of binder.

Since the preformed powder body is subjected to isostatic pressing at the sintering temperature, it can be depressurized before pressing and used in conjunction with it during pressing in order to obtain the desired dense sintered product. The pressure medium, usually a gas, must be contained in a casing that can prevent the pressure medium, usually a gas, from penetrating into the powder body.

Of course, the casing must also have a sufficiently high strength or viscosity during compression to prevent penetration into the pores of the powder body.

If a preformed capsule of glass is used as a casing, this must be of a high melting point type in order to not run off or penetrate into the powder body at high sintering temperatures, but if the glass softens the preform It is not possible to prevent powder objects from collecting in cavities and other recesses.

This often results in damage to the protruding parts of the sintered body during cooling due to the difference in thermal expansion coefficients of silicon nitride and glass.

Rather, a preformed powder object is immersed in a suspension of high-melting glass particles, or in some other way the object is surrounded by a layer of such glass particles, and the particles are then tightly packed around the powder object. A casing can be formed instantly by heating it in vacuum at a temperature that will form a casing.

The last-mentioned method makes it possible to apply a casing that is thin and adapts to the shape of the powder body, thus also avoiding the accumulation of glass on the sintered body and the associated disadvantages.

However, since the glass must be of a high melting type in order not to be washed away or penetrate into the powder body during the sintering of the silicon nitride, this method has a significant disadvantage associated with the fact that a dense casing can only be obtained at high temperatures. It has its drawbacks.

The fact that dense casings are obtained only at high temperatures
This means that it is impossible to avoid the dissociation of the silicon nitride, which undergoes nitrogen elimination, which results in a deterioration of the quality of the sintered object.

The present invention solves the problem of obtaining a dense casing around a preformed powder body of silicon nitride without causing harmful glass build-up on the sintered body and preventing harmful silicon nitride dissociation. be.

The invention allows the production of complex parts of silicon nitride with homogeneous composition and homogeneous properties.

The present invention applies an inner porous layer of a first material to the preformed powder object, and an outer porous layer of a second material applied outside this, the inner porous layer cooperating as much as possible with the second material. can be converted into a layer impermeable to the pressure medium at a temperature below the sintering temperature for silicon nitride, and the outer porous layer is impermeable to the pressure medium at a temperature below the temperature for the inner porous layer. It is necessary to first degas the preformed object and form a layer impermeable to the pressure medium with the outer porous layer, but with the inner porous layer. Heating is applied to a temperature that keeps the porous layers porous, and then heating is applied to the temperature necessary to form a layer impermeable to the pressure medium in the inner porous layer, while The preformed object is degassed before isostatic pressing, characterized by maintaining a pressure on the outside of the layer that is greater than the gas pressure inside and then performing isostatic pressing of the preformed product. The invention relates to a method for producing objects from silicon nitride by isostatic pressing of preformed objects of silicon nitride powder with a pressure medium at the temperatures necessary to sinter the silicon nitride powder.

The second substance in the inner porous layer is 5iO296,7
Vycor glass containing 2.9% by weight of B203 and 0.4% by weight of Al203, as well as quartz glass and a substance that forms a gas-permeable glass layer during heating, such as a mixture of particles of 5i02 and B2O3. High melting glass powder can be used.

The primary material in the inner porous layer is a powder of a high-melting metallic material capable of forming a metallic layer impermeable to pressure media, such as molybdenum, tungsten and other refractory metals. You can also do that.

The second substance in the outer porous layer is 5i0280.3
Pyrex (Pyre
x) Glass, further 5iO2 58% by weight, B2039% by weight, Al20320% by weight, Ca0 5% by weight and M
Aluminum silicate, 5i0 containing g0 8% by weight
aluminum silicate containing 260% by weight, 20% by weight of Al203, 15% by weight of Ca0 and 5% by weight of Mg0, and a substance that forms a gas-impermeable glass layer during heating;
Low melting glass powders can be used, such as mixtures of particles of SiO2, B2O3, Al2O3 and alkali and alkaline earth metal oxides.

The two porous layers, each suitably having a thickness in the range from 0.05 to 1 mm, are prepared inter alia by immersing the preformed powder body in a suspension of granulated material, or by thermal spraying or spraying. Can be applied by other thermal sprays.

The particles may suitably have a particle size in the range 0, 1 to 100 microns.

Degassing is suitably initiated and allowed to continue in the chamber for a time determined by the dimensions of the preformed powder object.

Under continued reduced pressure, the temperature is increased so that the outer porous layer is converted into a layer impermeable to the pressure medium.

If this is done, pressure can be applied to the encapsulated powder pair with a gas pressure medium to prevent dissociation of the silicon nitride during continued temperature rise.

If the layer consists of glass or glass-forming material during the continuation of the temperature increase, the glass in the outer layer reacts with the material in the inner porous layer, forming at the same time a glass of increasingly higher melting point, and at the same time increasing the temperature of the layer. impermeable to the pressure medium and finally in the innermost part of the inner porous layer before the glass in the outer layer can flow away. A glass layer is formed.

This last-formed glass layer forms a dense casing around the powder body when isostatic pressing of the preformed product is carried out at sintering temperatures.

If a metallic material is used in the inner porous layer and a glass or glass-forming material is used in the outer porous layer, the glass layer formed from the outer porous layer will be impermeable by conversion of at least the inner metal layer. It acts as an impermeable layer until it becomes a sexual layer.

The temperature at which the outer porous layer is converted into an impermeable layer is 60
The temperature at which the inner porous layer is converted into an impermeable layer is suitably within the range of 0 to 1100°C, and the temperature at which the inner porous layer is converted to an impermeable layer is 1.
It is within the range of 300 to 1600°C.

However, when squeezing the inner layer under isostatic pressure, which can be done after the outer layer has become airtight, the temperature 10
00 to 1300 degrees Celsius may also cause problems.

For such squeezing, size 20 to 300 MP
A pressure of about a is required.

The powder body is sintered at a temperature of at least 1600°C, preferably at 1
It is carried out at 600 to 1900°C.

The pressure during sintering of the preformed silicon nitride body depends on whether sintering promoting additives such as magnesium oxide have been added to the silicon nitride.

If such additives are not used, the pressure is at least 100 MPa, preferably between 200 and 300 MPa.
should be.

Although lower pressures can be used when using sintering accelerating additives, less (20 MPa is suitable).

The invention will now be explained in greater detail by way of embodiments with reference to the accompanying drawings, in which FIG. 1 schematically shows a preformed powder body of silicon nitride with two porous layers. and FIG. 2 schematically depicts a processing cycle for producing a sintered body of a preformed powder body.

It has a powder particle size of 7μ or less, and contains about 0 magnesium oxide.
．． A silicon nitride powder containing 1% by weight is placed in a capsule of plastic, e.g. It is placed in the apparatus shown in FIGS. 1 and 2 of Japanese Patent Application No. 50-133928.

The powder is subjected to compression at 600 MPa for 5 minutes.

After completion of the compression, the capsules are removed and the preformed powder bodies thus produced are machined into the desired shape.

The preformed powder body 1 is now initially composed of 296.7% by weight of Si0, 2032.9% by weight of B2030.4 and 2030.4% by weight of B20.
After immersion in an aqueous suspension of glass powder consisting of 0.3% by weight of 5i0280.3% by weight and then drying this layer
312.2% by weight, Al20328% by weight, Na204
．． The inner porous layer 2, as is evident from FIG. and an outer porous layer 3.

The preformed powder body treated in this way can then be degassed to degas the powder body and can be supplied with gas to generate the pressure necessary for isostatic squeezing. It is placed in a high pressure furnace equipped with a conduit and equipped with a heating device.

Such a high pressure furnace is disclosed, for example, in the above-mentioned Japanese Patent Application No. 50-1339.
It is stated in the specification of No. 28.

As shown in FIG. 2, the preformed powder body is first degassed in a high pressure oven at room temperature for approximately 2 hours.

While the vacuum continues, the temperature is increased to about 900°C.

The temperature is raised very gradually so that the pressure does not exceed 0.1 Torr at any time during this time.

The temperature is kept constant for about 2 hours at about 900° C., thus providing final degassing and the glass powder in the outer porous layer sintering together into a gas-impermeable layer.

The temperature is reduced to 700° C. in order to reduce the viscosity of the glass in the outer layer and thus reduce the risk of glass melt penetrating into the inner porous layer.

Argon or helium is then supplied to a pressure level giving a pressure of 200 to 300 MPa at the final sintering temperature.

The temperature is then increased to 1700-1800° C., the appropriate sintering temperature for silicon nitride.

The pressure increases at the same time. This temperature increase is achieved slowly enough that the molten glass in the outer layer has time to react with the glass powder in the inner layer, simultaneously forming an increasingly high-melting glass, and the glass powder in the inner layer. The deepest layers have time to sinter into an impermeable layer before the glass in the outer layers can be washed away.

1700 to 1800°C and 200 to 300M
Suitable times for sintering at Pa are at least 2 hours without sintering-promoting additives and at least 0.5 hours with such additives.

After the cycle is completed, the furnace is allowed to cool to the appropriate unloading temperature and the sintered body is blown to clean the glass.

400 before or after application of the porous layer if binders such as methylcellulose, cellulose nitrate, acrylic binders, waxes or mixtures of waxes with different melting points are used in the production of the preformed object as exemplified above. or 700°
It is appropriate to remove it by heating to
Furthermore, processing can be carried out as described for the preformed powder without binder.

The invention is particularly suitable for use in continuous manufacturing.

In this case (a) removal of the binder, (b) degassing in vacuum and dense sintering of the outer layer, (C) additional heating in gas at a pressure greater than the pressure inside the layer, and (d) final The various processing steps, such as heating and hot isostatic pressing, can be carried out in high temperature conditions in different types of furnace equipment, moving between them.

As examples of objects which are very well suited for the use of the invention, mention may be made, inter alia, of blades for gas turbines and single turbine rotors.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram showing a preformed powder object of silicon nitride with two porous layers, 1 being a preformed powder object;
2 is an inner porous layer, and 3 is an outer porous layer. FIG. 2 shows temporal changes in pressure and temperature during the treatment according to the present invention, where the vertical axis is temperature and pressure, and the horizontal axis is time.

Claims (1)

[Claims] 1. Preparation of a silicon nitride object by subjecting a preform of silicon nitride powder to a degassing treatment and then subjecting the preform to isostatic pressure using a pressure medium at the sintering temperature of silicon nitride. In the manufacturing method, a first layer comprising a high-melting glass, a high-melting glass-forming substance or a high-melting metallic substance capable of being converted into a layer impermeable to the pressure medium at a temperature below the sintering temperature of silicon nitride is provided on the preform. an inner porous layer of material, and an outer porous layer of a second material consisting of a low melting glass or a low melting glass-forming material capable of being converted into a layer impermeable to a pressure medium at a temperature lower than the temperature in said inner porous layer. After the application, the preform is first subjected to a degassing treatment, and then the inner porous layer is porous at a temperature necessary for forming an impermeable layer to the pressure medium of the outer porous layer. and then maintaining the gas pressure outside these layers greater than the gas pressure inside the layers to the temperature required for the formation of an impermeable layer to the pressure medium of the inner porous layer. A method for producing a silicon nitride object, which comprises heating the preform while applying pressure to the preform. 2 Each particle size of the first and second substances used to form the inner porous layer and the outer porous layer is 0.1 to 10.
The method according to claim 1, wherein the particle size is 0 microns. 3 Each thickness of the inner porous layer and the outer porous layer is 0.
605-1 mrIL. 4. The heating temperature of the outer porous layer is 600 to 1100°C.
The method according to claim 1. 5. The heating temperature of the inner porous layer is 1300 to 1600.
The method according to claim 1, wherein the temperature is .degree. 6. Heating the preformed body to a temperature of 1000 to 1300° C. under simultaneous isostatic squeezing in order to form a layer of the inner porous layer that is impermeable to the pressure medium. The method described in any one of paragraphs.