JPH0159996B2 - - Google Patents

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to hot isostatic pressurization (hereinafter referred to as HIP) using glass as a sealing material for pressure medium gas.
The present invention relates to a method for manufacturing a silicon nitride sintered body by a method, and particularly to a method for manufacturing a clean high-density silicon nitride sintered body without being contaminated by a sealing material.

[Conventional technology] High-temperature gas turbines,
The development of high-temperature operating equipment such as diesel engines and MHD generators has been actively conducted in recent years. The development of these devices is all about the development of high-temperature structural materials, and the development of these high-temperature structural materials is attracting attention. However, the development of new materials is desired. On the other hand, there is a need to conserve heat-resistant metal materials, which are scarce resources, and a direction is being taken to use ceramics made from relatively abundant materials such as Si, Al, O, and N as high-temperature structural materials. be.

The development of such high-temperature structural materials is also recognized as being of great importance for use as high-hardness members (for example, tools) and corrosion-resistant materials, and is attracting great interest.

Among these ceramic high-temperature structural materials, silicon nitride (Si ₃ N ₄ ) is particularly attracting attention as a material that has sufficient strength at high temperatures, is chemically stable, and is resistant to thermal shock.

In this case, since silicon nitride-based materials are difficult to sinter, the techniques for obtaining a high-density sintered body are as follows:
Widely used methods include atmospheric pressure sintering, which facilitates sintering by adding sintering aids such as Al ₂ O ₃ , MgO, and Y ₂ O ₃ , and methods that apply pressure during sintering. . Especially for the latter, 1000-3000
Can apply pressure as high as Kg/ ^cm2 isotropically
The HIP method is expected to be an excellent method from the viewpoint of densification and homogeneity.

When using the HIP method, there are broadly two methods. One is to add a certain amount of sintering aid,
In this method, sintering is performed to the extent that the pores are closed using a pressureless sintering method, etc., and then the HIP method is used to crush the pores and increase the density. This is a method of compressing and sintering by blocking the surface of the molded body from pressure gas and applying only pressure while preventing the pressure gas from penetrating into the pores. The latter has characteristics such as being able to obtain a high-density sintered body without adding a sintering aid, but it requires a high temperature of 1700°C or higher during HIP treatment.
There are many technological development factors, such as the extremely difficult selection of materials to seal the surface of the molded product. As sealing materials, high-melting point metals such as molybdenum, tantalum, and tungsten, and glass-based materials are used in the laboratory, but from the perspective of industrial use, glass-based materials are advantageous because they have fewer problems in terms of price. It is believed that.

Methods of using glass-based materials as sealants include vacuum-sealing the molded body in a glass container (glass capsule method), and placing the molded body in low-softening point glass powder and then placing it under reduced pressure as it is. A method in which the temperature is raised by heating to melt the glass, and the molded body is compressed through the molten glass (Glass bath method: JP-A-Sho
52-58714, Japanese Unexamined Patent Publication No. 54-89405, etc.). For example, JP-A-54-89405 describes an example of application to silicon nitride, and it is said that glass containing B ₂ O ₃ is preferable as the glass material. Examples of such glasses include 80.3% by weight SiO ₂ , 12.2% by weight
wt% _B2O3 , 2.8wt% _{Al2O3 ,} 4.0wt% _{Na2O ,}
Glass (Pyrex: registered trademark) consisting of 0.4% by weight K ₂ O and 0.3% by weight CaO; 58% by weight SiO ₂ ,
Glass consisting of 9% by weight B ₂ O ₃ , 20% by weight Al ₂ O ₃ , 5% by weight CaO and 8% by weight MgO; 96.7% by weight
Suitable examples include glass (Vycor: registered trademark) consisting of SiO ₂ , 2.9% by weight B ₂ O ₃ , and 0.4% by weight Al ₂ O ₃ .

[Problems to be Solved by the Invention] The present inventors have been conducting research on methods of using low softening point glasses such as those described above, and have identified the problems when using these low softening point glasses. I found out.

This problem is that glass components other than SiO ₂ such as Na ₂ O and MgO may
It volatilizes and contaminates the internal silicon nitride, and tends to form a glass phase with a low softening point at grain boundaries, which is an important obstacle to the production of sintered bodies with high high-temperature strength. This problem was observed both with and without sintering aids. In this case, it was considered to apply the CIP method to form a barrier layer (e.g., BN layer) on the surface of the molded product before HIP treatment using the method shown in JP-A-57-106575, but this method was not conducive to contamination due to volatile components. It was found that it was not very effective in preventing. Of course, when a BN layer is not applied, the reaction due to direct contact is significant, and when Y ₂ O ₃ and Al ₂ O ₃ are added as sintering aids, the thickness of the reaction layer can reach 1 mm or more. I am discovering that.

In addition, various problems have been found in the implementation of HIP processing, such as the use of glass containing such volatile components contaminates the furnace materials inside the HIP equipment and impairs their durability.

[Problems to be Solved by the Invention] The present invention has been made to eliminate the drawbacks of the prior art as described above. In order to eliminate the above-mentioned drawbacks, the present inventors conducted experiments using quartz glass as a sealing glass material, which contains not only volatile components but also almost no other components. As a result, quartz glass has the property of being easily crystallized at high temperatures of 1200°C or higher, so even if a temporary seal is formed due to softening at high temperatures, it is necessary to hold it further at the high temperatures as described above. As the glass crystallizes, by the time HIP pressure starts, all of the glass has already crystallized, and this crystallized glass will not only be unable to perform its sealing function properly but will also be damaged by the impact of the pressure. I have experienced many cases of damage.
As a result of various studies on ways to avoid this problem, the present invention was arrived at.

[Means for Solving the Problems] The main point of the present invention is that the following steps are included in the entire HIP process.

A step of covering the surface of the porous molded body with a barrier material that is difficult to sinter and has low reactivity with silicon nitride, and placing the molded body embedded in silicon dioxide powder in a heat-resistant container. , and a step of surrounding the silicon dioxide powder with a flexible carbon material and arranging a barrier material between the flexible carbon material and the inner wall of the heat-resistant container, and applying a hot isostatic pressure device to the entire heat-resistant container. The process of vacuum degassing and nitriding gas replacement is performed by heating and increasing the temperature while keeping the inside of the hot isostatic pressurizing device in a nitriding gas atmosphere, and after reaching a temperature equal to or higher than the melting point of cristobalite, pressurized gas is A step of supplying cristobalite, a step of holding it under a predetermined pressure at a temperature above the melting point of cristobalite.

[Function] In the present invention, the surface of the molded body is first covered with a barrier material (for example, a BN layer) that is difficult to sinter and has low reactivity with silicon nitride, and then the molded body is covered with a barrier material that is difficult to sinter and has low reactivity with silicon nitride. It is embedded in silicon powder and placed in a heat-resistant container, which is then installed in a HIP device.
When performing HIP treatment, after performing vacuum deaeration and then nitrogen gas replacement, preferably 100Kg/
The melting point of ^cristobalite , a high-temperature crystalline phase of silicon dioxide (approximately 1725
After the silicon dioxide is completely melted by raising the temperature to above (°C), a pressure medium gas is injected and pressurized to perform HIP treatment. The holding temperature during the HIP treatment is preferably a temperature equal to or higher than the melting point of cristobalite.

By the way, the pressure of the nitriding gas at the time when the silicon dioxide (silica glass, etc.) is completely melted, that is, when the sealing of the compact is completed, is the pressure [atmospheric pressure] equal to or higher than the following formula with respect to the temperature [〓] at that time. It is preferable to do this, and thereby the decomposition of silicon nitride can be suppressed as much as possible.

P _N2 = exp {(100.2−213400/T+100)/3.974} The reason why we stated above that the nitrogen gas atmosphere is preferably 100Kg/ ^cm2 or less is because if too high pressure is trapped inside the seal layer, it will cause damage after HIP treatment. This is because the seal layer may rupture due to internal pressure during the depressurization process.

Silicon nitride molded bodies can be molded using existing molding methods, such as the CIP method, injection molding, or slip casting, or those molded using these methods can be further calcined to improve the strength of the molded body. It is possible to use the following materials.

In addition, in the present invention, since the HIP treatment holding temperature is quite high, products containing 15% by weight or more of a sintering aid may result in coarse grains, and products containing a small amount of sintering aid may result in coarse grains. It is preferable to select As the sintering aid, oxides such as Y ₂ O ₃ , Sc ₂ O ₃ , and Al ₂ O ₃ as well as nitrides such as AlN and TiN can be used, but these are, of course, non-limiting examples. Needless to say, it is also applicable to those containing no sintering aid.

When carrying out the present invention under conditions such that silicon nitride and silicon dioxide come into direct contact, silicon oxynitride (Si ₂ N ₂ O) is generated at the contact interface.
This Si ₂ N ₂ O may have unfavorable effects depending on the application, and in such cases,
Between the molded body and silicon dioxide as a reaction prevention layer,
It is necessary to interpose a barrier layer that is difficult to sinter and does not easily react with silicon nitride. As the barrier material, BN (boron nitride) can be mentioned without limitation.
The method for forming the BN layer is also not limited, but for example, the pressing method described in JP-A-57-106578 can be used. By forming this barrier layer, HIP
Another effect is that the silicon dioxide sintered body after the treatment can be separated very easily.

The temperature lowering step after the HIP treatment may be performed by natural cooling, but in this case, if the glass is quartz glass, its thermal expansion coefficient (7 × 10 ^-7 /℃) and that of silicon nitride (3 ~3.4×10 ^-6 /°C) may cause cracks in the sintered body. As a method for preventing this, in addition to forming the barrier layer described above, a method of crystallizing silicon dioxide by further slowing down the temperature drop rate can also be adopted.
In this case, if there is no barrier layer, in addition to the above-mentioned Si ₂ N ₂ O, a silica glass layer of about 100 μm or thinner often remains at the interface between silicon nitride and silicon dioxide. This quartz glass layer can be removed by sandblasting or the like, but as mentioned above, this may be undesirable depending on the application, so it is recommended to form a barrier layer from the beginning.

The silicon dioxide to be used preferably has a purity of 99% or more for the purpose of the present invention, and transparent quartz glass, opaque quartz glass, crystal, cristobalite, etc. can be used, and as for the form, of course, powder, Platy or cylindrical quartz may be added to a portion of the powder to compensate for the low bulk of the powder.

[Example] Example 1 Silicon nitride powder (grade
LC-12) was molded by the CIP method at a pressure of 2 tons/cm ² and then processed into a size of 100φ×80H (mm). The density of this molded article was 1.86 g/cm ³ . On the surface of this molded body
A BN layer with a thickness of 8 mm was applied by the CIP method. Next, as shown in Fig. 1, the BN coated molded body 1 (2 is
BN layer) was embedded in quartz glass powder 3 and placed in graphite crucible 4. Note that 5 is BN powder and serves to prevent the molten quartz glass 3 from sticking to the crucible 4. Further, 6 is graph oil, which plays a role of preventing mixing of the BN powder 5 and quartz glass. Place the entire body in a HIP device, vacuum degas it, and perform a nitrogen gas replacement operation to reduce the volume to approximately 5Kg/cm ²
of nitrogen gas. After applying heating power to the heater of the HIP equipment and raising the temperature to 1800℃ and holding it for 1.5 hours, argon gas was injected as a pressure medium and the temperature was raised further, finally reaching 2000℃.
The pressure was set at 1500 Kg/cm ² and held for 2 hours. After lowering the temperature and reducing the pressure, the graphite container was removed from the HIP device, and the silicon nitride sintered body covered with transparent glass-like quartz was recovered. The density of the sintered body obtained by breaking the quartz glass and BN was approximately 99%.

Comparative Example 1 The same silicon nitride powder as in Example 1 was treated in the same manner as in Example 1 except that the initial holding temperature was 1700°C. After the HIP treatment, the quartz glass taken out from the graphite crucible was blackish and crack-like. When this glass and BN were removed and the silicon nitride sintered body was taken out, there were 5 large cracks.
~6 pieces were formed, and the density was just over 60%, hardly densified, and powder that appeared to be metal Si was attached to the surface.

In order to investigate the cause of seal failure during this process, we separately held the sample at 100Kg/cm ² after holding it at 1700℃.
Tests were conducted up to the point where the pressure was applied to 1700
After being held at ℃, the quartz glass was taken out after lowering the temperature and reducing the pressure.The quartz glass was completely crystallized in white and had been severely cracked by external pressure.

Example 2 A silicon nitride powder containing 5% by weight of yttrium oxide and 2% by weight of aluminum oxide was molded by the CIP method and then primarily fired at 1750° C. for 30 minutes in a nitrogen atmosphere. After applying a BN layer to a thickness of about 3 mm on the surface of this molded body by the CIP method, it was placed in a graphite crucible 4 as shown in FIG. This graphite container was placed in a HIP device and treated in the same manner as in Example 1. However, both the initial holding temperature and final holding temperature
The temperature was 1750℃. The quartz glass in the graphite container taken out is transparent, and the density of the sintered body inside is 3.2 g/cm ³
It was almost true density. Moreover, the surface of the sintered body was almost black as a whole, and no color unevenness was observed.

Comparative Example 2 The same test as in Example 2 was conducted except that the embedded glass was replaced with Pyrex glass. The density of the obtained sintered body was almost the same as that of Example 2, but the surface was white and mottled, and a reaction appeared to have occurred due to the penetration of the glass component. When this sintered body was cut and the thickness of the reaction layer was measured, the thick part was approximately
It was found that the diameter was 1.5 mm and that it could not be used as a sintered body.

[Effects of the Invention] Since the present invention is configured as described above, a high-density sintered body can be obtained by using a glass-based material as a seal for a pressure medium, and at the same time, it is possible to obtain a high-density sintered body by using a glass-based material as a seal for a pressure medium. It is now possible to avoid contamination and reactions caused by components, and to supply products with high purity and excellent surface quality.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view showing the state of preparation for HIP treatment.

Claims (1)

[Claims] 1. A method of manufacturing a high-density silicon nitride sintered body by sealing the surface of a porous molded body mainly composed of silicon nitride with a glass-based material and performing hot isostatic pressing treatment. A step of covering the surface of the porous molded body with a barrier material that is difficult to sinter and has low reactivity with silicon nitride: placing the molded body embedded in silicon dioxide powder in a heat-resistant container; and a step of surrounding the silicon dioxide powder with a flexible carbon material and arranging a barrier material between the flexible carbon material and the inner wall of the heat-resistant container, and placing the entire heat-resistant container in a hot isostatic pressurizing device. A process of vacuum degassing and nitriding gas replacement.The inside of the hot isostatic pressurizing device was heated and heated in a nitrogen gas atmosphere to reach a temperature higher than the melting point of cristobalite and completely melt the silicon dioxide powder. A method for producing a high-density silicon nitride sintered body, the method comprising: supplying a pressurized gas; and maintaining a predetermined pressure at a temperature equal to or higher than the melting point of cristobalite. 2 When the nitrogen gas partial pressure when forming a nitrogen gas atmosphere is the temperature at the start of pressure gas supply as T [°C], the following formula P _N2 = exp {(100.2−213400/T+100)/3.974} A method for manufacturing a high-density silicon nitride sintered body according to claim 1, wherein the value is