JPH0361627B2 - - Google Patents

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention relates to a method for producing a silicon nitride sintered body with excellent high-temperature strength, and more specifically to a method for producing a silicon nitride sintered body having excellent high-temperature strength. The present invention relates to a method for manufacturing a silicon nitride sintered body with an improved surface structure by isostatic pressing (abbreviated as "HIP").

(Prior Art) In recent years, the HIP method has been seen as a promising method for sintering ceramic materials to make them more dense and using a small amount of sintering aid, and various processing techniques using this method have been developed. Particularly when obtaining a complex-shaped sintered body from shaped powder, the powder, formed body, or pre-sintered body is generally placed in a container made of glass (for example, Corning's product name "Vycor" or "Pyrex" glass, etc.). This is prefired under an inert gas pressure near the softening point of the glass container, the periphery of the compressed object is covered with glass, and this glass coating layer is heated in a high pressure and high temperature gas as a pressure medium. There is a method (glass capsule HIP method) in which the desired sintered body is obtained by densifying the powder, formed body, or pre-fired body and removing the glassy material around the obtained sintered body using a dissolving liquid or sandblasting. It's mainstream.

Among this glass capsule HIP method, especially
The method disclosed in Japanese Patent No. 52-9369 involves coating a preformed silicon nitride powder with two layers of high-melting point glass and low-melting point glass, degassing it before applying equal pressure, and first coating the low-melting point glass with two layers of high-melting point glass and low-melting point glass. The glass is heated to around its melting point, then heated to around the melting point of high-melting point glass to encapsulate the glass, and then the preform is sintered while applying equal pressure (this method is referred to as the "first method"). .

Furthermore, the method disclosed in JP-A-54-144412 covers a preformed silicon nitride powder with a gas-permeable frame made of high-melting point glass and/or low-melting point glass, and then processes this frame. It is transformed into an impermeable frame in a pressurized gas and then sintered under isostatic pressure (this method is referred to as the "second method").

(Problems to be Solved by the Invention) However, in the methods disclosed in the above two types of publications, the first method performs heating after degassing, so there is no gas movement due to gas convection.
All heating of the heated object must be done by thermal radiation. Therefore, some parts of the sample are not irradiated with thermal radiation, which creates a difference in heat distribution compared to other parts of the sample that are irradiated with thermal radiation. The presence of glass layers makes complete encapsulation extremely difficult.
Moreover, in some cases, the inconvenience may even occur that the inner skin partially dissolves before the encapsulation of the outer skin is completed. In the second method, heating is performed in pressurized gas (nitrogen gas), so heat transfer is performed by both gas convection and thermal radiation, and therefore it is difficult to notice differences in heat distribution in each part of the sample. There is an advantage. However, since the sample is encapsulated with pressurized gas inside the container, when the sample is pressurized at equilibrium, if the pressure outside the sample increases, the internal gas pressure increases accordingly, resulting in HIP.
This has the disadvantage of reducing the compacting effect of the sample.

SUMMARY OF THE INVENTION An object of the present invention is to provide a method for producing silicon nitride, which eliminates the above-mentioned disadvantages, enables good encapsulation of a preform, and enables efficient balanced pressing.

(Means and Effects for Solving the Problems) The present invention provides a method of converting a preform formed from silicon nitride powder into a porous material that is permeable to a gaseous pressure medium and that is gas-impermeable by heating. and sintering the preform under isostatic pressure, the gas permeable material is heated in a gaseous pressure medium to change the gas permeability. This method is characterized in that the coating layer is degassed immediately before changing into a gas-impermeable coating layer, and the temperature is further increased to change the coating layer into a gas-impermeable coating layer. According to the invention, before the gas-permeable coating layer is transformed into a gas-impermeable coating layer,
Since the preformed body having the coating layer can be heated uniformly and degassed immediately before changing to the gas-impermeable coating layer, the powder compact itself or the generated gas can be released immediately after degassing. This is prevented by the formation of a gas-impermeable coating layer, and after changing to a gas-impermeable coating layer, there is almost no gas pressure in the coating layer, and the gas pressure around the preform is lower than in conventional known methods. Deposit densely. The temperature at which the gas is degassed just before the coating layer changes from gas permeable to gas impermeable is, for example, in the case of one glass coating layer, the temperature is 95% of the melting temperature of the glass; In this case, it is preferable to set the temperature to 95% of the melting temperature of the outer layer glass. The purity of the Si ₃ N ₄ powder raw material used here is preferably at least 99% or higher. The reason for this is that impurities in silicon nitride evaporate during high-temperature firing, causing pores, and reacting with additives to create a low-melting-point liquid phase, which significantly deteriorates the high-temperature properties of the sintered body. .

As the gaseous pressure medium, it is preferable to use an inert gas, such as argon, helium, or nitrogen gas, in order to prevent decomposition and oxidation of silicon nitride during firing.

In a preferred embodiment of the invention, the material constituting the coating layer is preferably made of high melting point glass. As high melting point glass, SiO ₂ 96.7% by weight,
Preference is given to a mixture of Vycor glass, quartz glass, SiO _{2 and} B ₂ O ₃ containing _2.9 % by weight of B ₂ O ₃ and 0.4% by weight of Al 2 O 3 .

Furthermore, in another embodiment of the present invention, the coating layer is composed of two layers, an inner layer and an outer layer, the inner coating layer is made of high melting point glass, the outer coating layer is made of low melting point glass, and the inner coating layer is replaced by the outer coating layer. It is preferred to change to the gas-impermeable layer after changing to the gas-impermeable layer. By providing two coating layers in this manner, it is possible to more effectively prevent the gas from leaving the powder compact as described above, and it is possible to perform dense encapsulation. Application of these coating layers can be carried out, for example, by dipping into a suspension of silicon nitride powder,
Performed by spray painting or other hot atomization. In addition to _SiO2 , for example, low melting point glasses include
Pyrex glass containing small amounts of B ₂ O ₃ , Al ₂ O ₃ , Na ₂ O, K ₂ O and CaO, as well as SiO ₂ and
Aluminum silicate containing Al ₂ O ₃ and also small amounts of B ₂ O ₃ , CaO and MgO can be used.

Furthermore, in an embodiment of the present invention, the pressure of the gaseous pressure medium is in the range of 0.3 to 10 atmospheres.
Gas convection does not occur when the gas pressure is below 0.3 atmospheres,
This is because an incomplete coating layer is formed due to non-uniform heat radiation as described above, and at temperatures above 10 atmospheres, the high temperature exhaust gas flow is generated during deaeration just before the coating layer changes from gas permeable to gas impermeable. is rapidly excreted,
This is because there is a risk of damaging the degassing device, for example, the seals at the joints of the gas exhaust pipes.

(Example) Examples of the manufacturing method of the present invention will be described in detail.

Example 1 100% by weight of silicon nitride (Si ₃ N ₄ ) powder with a purity of 99.9% and an average particle size of about 1 μm, 1% by weight of SrO,
Add 4% by weight of MgO and 5% by weight of _CeO2 and mix;
Sample 1 having a cylindrical shape with a bottom as shown in cross section in FIG. 1 was manufactured by injection molding.

Next, 96.7% by weight of SiO ₂ was added to this bottomed cylindrical sample 1.
Applying Vycor glass powder containing 2.9% by weight of B ² O ₃ and 0.4% by weight of Al ₂ O ₃ by spraying,
A porous glass coating layer 2 having a thickness of 1.0 mm was formed (see FIG. 2).

This glass-covered molded body is loaded into a HIP device. This HIP device is a pressure vessel that has a heating device inside, and a conduit for discharging internal gas and a conduit for supplying inert gas are connected to this device. The inside of this HIP apparatus is degassed at room temperature, then filled with nitrogen gas, and the temperature is continuously raised to 1520°C under an atmosphere in the range of 0.3 to 10 atm.

Once the temperature reaches 1520℃, the inside of the device is degassed to create a near-vacuum state, and then heated continuously from 1520℃ to 1600℃ for 50 minutes using heat radiation from the heating device.
Increase in °C/hr. After a while after the temperature reached 1600°C, a gas-impermeable glass coating layer 2' was formed (see Figure 3).

After that, fill the HIP device with nitrogen gas, argon gas, or helium gas to reduce the pressure to 100
Make the pressure 200 atm. Next, set the temperature inside the device to 1800-2000.
The temperature was raised to ℃ to sinter silicon nitride. After the HIP process was completed, the sintered body was taken out from the HIP apparatus and cooled, and the glass adhering to the sintered body was removed by sandblasting or the like.

For comparison, we used the same material and changed the glass coating layer from gas-permeable to gas-impermeable in vacuum, as described in conventional known literature, and when the glass coating layer was changed from gas-permeable to gas-impermeable in vacuum, and Two types of sintered body samples were prepared, one in which the coating layer was changed from gas permeable to gas impermeable (these are referred to as "Sample 1" and "Sample 2").

From these three types of sintered compact samples, JIS R1601−
When a square bar specified in 1981 was cut out and tested for four-point bending strength, sample 1 had a strength of 92Kgf/ ^mm2 , sample 2 had a strength of 95Kgf/ ^mm2 , and the sample according to the present invention had a strength of 124Kgf/mm2.
It was Kgf/ ^mm2 .

Example 2 A preformed body having the same components as in Example 1 shown in FIG _.
wt%, _{B2O3 12.2wt} %, _{Al2O3 2.8wt} %,
Pyrex glass containing 4.0% by weight of Na ₂ O, 0.4% by weight of K ₂ O and 0.3% by weight of CaO was applied, resulting in two layers with thicknesses of 0.1-0.2 mm and 0.5-0.8 mm, respectively. Porous glass coating layers 2 and 3 were formed (see FIG. 4).

This glass coating layer molded product is loaded into the HIP apparatus described above. The inside of this HIP device is degassed at room temperature, and then filled with nitrogen gas to a gas pressure of 0.3~
The temperature is raised continuously to 950℃ in an atmosphere of 10 atm.

When the temperature reaches 950℃, it is degassed to create a vacuum state, and then heated to 1000℃ by heat radiation from the heating device.
Raise the temperature to ℃. At this time, the coating layer 3 contains almost no gas inside the coating layer and is gas-impermeable (see FIG. 5). The temperature is then raised continuously from 1000°C to 1450°C under near-vacuum conditions. Shortly after the temperature reached 1450°, a gas-impermeable glass coating layer 2' was formed.

Thereafter, in the same manner as in Example 1, the molded body was filled with inert gas and subjected to HIP sintering at a gas pressure of 100 to 200 atm and a temperature of 1900°.
After the HIP process was completed, the glass adhering to the sintered body was removed.

For comparison, sintered bodies were produced using the two conventional methods as in Example 1, and the four-point bending strength specified in JIS R1601 was measured for the samples according to the present invention and the samples according to the two conventional methods. However, the four-point bending strength of the sample obtained using the conventional method was 90Kgf/ ^mm2 and 93
Kgf/mm ² while that of the sample according to the invention was 4
The point bending strength was 120Kgf/ ^mm2 .

(Effects of the Invention) As is clear from the above, the method for producing silicon nitride of the present invention uniformly heats the coating layer applied to the preform to make the entire coating layer uniformly gas-impermeable. Moreover, since there is almost no gas inside the gas-impermeable coating layer, the external pressure can compact the compact during the HIP process, which improves the mechanical strength of the produced sintered compact compared to conventional about
It can be increased to 1.3 times.

[Brief explanation of drawings]

Fig. 1 is a sectional view showing the sample shape in an embodiment of the present invention, Figs. 2 and 3 are explanatory diagrams showing steps in an embodiment of the invention, and Figs. 4 and 5 are steps in another embodiment of the invention. FIG. 1... Sample, 2, 2'... High melting point glass coating layer, 3,
3'...Low melting point glass coating layer.

Claims (1)

[Scope of Claims] 1. A preform formed from silicon nitride powder is coated with at least one layer of a porous material that is permeable to a gaseous pressure medium and that changes into a gas-impermeable material upon heating. coating and sintering the preform under isostatic pressure, the gas-permeable material is heated in a gaseous pressure medium so that the gas-permeable coating layer becomes gas-impermeable. A method for producing a silicon nitride sintered body, which comprises degassing the material immediately before changing into a coating layer, and further increasing the temperature to convert the material into a gas-impermeable coating layer. 2. The method for producing a silicon nitride sintered body according to claim 1, wherein the material constituting the coating layer is made of high melting point glass. 3. The coating layer is composed of two layers, an inner layer and an outer layer, the inner coating layer is made of high melting point glass, the outer coating layer is made of low melting point glass, and the outer coating layer is replaced by a gas impermeable layer. 2. The method for producing a silicon nitride sintered body according to claim 1, wherein the layer is later changed to a gas-impermeable layer. 4. The method for producing a silicon nitride sintered body according to any one of claims 1 to 3, characterized in that the pressure of the gaseous pressure medium is in the range of 0.3 to 10 atmospheres.