JPH06668B2 - Method for producing homogeneous sintered body - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing a homogeneous sintered body made of silicon nitride which is homogeneous and has high strength and the like.

[Prior Art] Since a silicon nitride (Si ₃ N ₄ ) sintered body is excellent in heat resistance, thermal shock resistance, corrosion resistance, and high-temperature strength, it is a high-temperature structural material for a gas turbine engine, a diesel engine, or a wear-resistant material. It is used for etc. This silicon nitride sintered body is liable to cause a large difference in sintered body characteristics between the inside and the surface of the sintered body due to decomposition of silicon nitride generated during firing and volatilization of oxygen and a sintering aid. In order to suppress the inhomogeneity of the, the firing is performed in a pod.

Conventionally, as a box for firing such a silicon nitride, a box made of carbon or a box made of carbon coated with chemical vapor deposition silicon carbide (Japanese Patent Laid-Open No. 6-58242).
No. 0-210575), a box comprising an inner case of silicon nitride and an outer case of a refractory (Japanese Patent Publication No. 49-40123) and the like.

[Problems to be Solved by the Invention] However, in a carbon box, evaporation on the surface of nitrogen silicon is large, SiC is easily generated on the surface, and further, volatilization of oxygen and a sintering additive is suppressed. Absent.

Further, in a carbon box covered with chemical vapor deposition silicon carbide, SiC generation due to evaporation on the surface of silicon nitride is suppressed, but volatilization of oxygen and a sintering aid is not suppressed, and further, between carbon and silicon carbide layers. Easy to peel off, and the durability of the box is small.

On the other hand, in the case of the box made of the inner case of silicon nitride and the outer case of the refractory, evaporation of the silicon nitride surface suppresses generation of SiC and volatilization of oxygen, but does not suppress volatilization of the sintering aid.

As described above, the conventional boxes have a problem that the decomposition of silicon nitride and the volatilization of oxygen and the sintering aid that occur during firing cannot be suppressed at the same time, and the silicon nitride firing obtained using these boxes As for the bound body, the surface layer is deteriorated and discolored, so that the one having sufficient characteristics cannot be obtained. In addition, especially as a sintering aid Y
_Since a silicon nitride sintered body using a high melting point additive such as ₂ O ₃ or Yb ₂ O ₃ is manufactured by high temperature firing, there is a problem that decomposition of silicon nitride and volatilization of oxygen and a sintering aid are more likely to occur. There is.

[Means for Solving the Problems] Accordingly, the present invention is made of silicon nitride which has solved the problems in the above-mentioned conventional methods and has sufficient strength and the like without causing alteration or discoloration of the surface layer of the sintered body. It is an object to provide a method for producing a homogeneous sintered body.

According to the present invention, an object of the present invention is to provide a body to be fired composed of silicon nitride powder and a sintering aid, a jar containing silicon nitride composed of the same components as the body to be fired and a sintering aid, or According to a method for producing a homogeneous sintered body, which is characterized in that a film made of silicon nitride composed of the same components as that of the object to be fired and a sintering aid is placed in a box having an inner surface and fired. To be achieved.

[Operation] In the method for producing a homogeneous sintered body according to the present invention, the silicon nitride molded body is installed in the box in which the box made of the same component as the body to be fired or the film made of the same component as the body to be fired is formed on the inner surface. Then, the body to be fired is fired under predetermined conditions. The box or the coating film formed on the inner surface of the box may be the same component as that of the body to be fired, and the content of each component is not limited.

That is, in the case of the present invention, since the main body of the box or the inner surface of the box has the same composition as that of the body to be fired, decomposition of Si ₃ N ₄ does not easily occur, and oxygen (SiO) and additives volatilize from the box material. Therefore, the partial pressure of oxygen and additives in the box becomes high, the volatilization of oxygen and additives from the body to be fired is suppressed, and a homogeneous and high-strength silicon nitride sintered body can be obtained.

There is no particular limitation on the kind of the material for the case in the case of forming a coating film of the same component as that of the body to be fired on the inner surface of the case, but carbon is preferable. Carbon products are inexpensive and do not undergo high temperature deformation.

Further, as a method of forming a coating film on the inner surface of the box, by applying a substance consisting of the same component as the object to be baked into the box in which powder of the same component as the object to be baked is applied on the inner surface, the inner surface of the box material is baked. A dense coating that firmly adheres can be formed. If the thickness of the coating is usually 0.1 mm or more, the diffusion of the ingredients of the box material into the box can be suppressed.

Next, the firing is carried out under almost normal pressure, usually in the range of 0 to 0.5 kg / cm ² as a gauge pressure.

The firing temperature condition is usually in the range of 1500 to 1800 ° C. If the firing temperature is lower than 1500 ° C., the silicon nitride is difficult to be densified, while if it exceeds 1800 ° C., decomposition and evaporation of the silicon nitride are likely to occur, and it becomes difficult to obtain the one having sufficient characteristics.

In the present invention, it may be carried out under a pressure of up to 10 kg / cm ² with a gauge pressure, and in that case, the firing temperature is 1500 to
2000 ° C is preferred.

The box used in the present invention preferably has a wall thickness of 5 to 30 mm. If the wall thickness is less than 5 mm, the box will be easily deformed by the load, and if it exceeds 30 mm, the thermal conductivity will be poor, and the volume will not be large, and the weight will be heavy and the workability will be poor. There is.

The body to be fired used in the present invention is a molded body composed of silicon nitride powder and a sintering aid.

[Examples] Hereinafter, the present invention will be described in more detail based on examples, but the present invention is not limited to these examples.

Example 1 92% by weight of α-type silicon nitride powder having an average particle diameter of 0.7 μm
(Hereinafter, all% represent% by weight.), Yttrium oxide (Y ₂ O ₃ ) 3%, ytterbium oxide (Yb)
₂ O ₃ ) 5% was added to 100 parts by weight of the mixed powder,
150 parts by weight of water was added, and mixed and pulverized by a pulverizer for 7 hours.

To this pulverized product, 1 part by weight of polyvinyl alcohol was added and spray-dried. 2500 kg after molding the obtained dried product into a 35 mm (φ) × 20 mm disc using a mold
Cold isostatic pressing was performed at a pressure of / cm ² to prepare a molded body (body to be fired).

This molded body is used as a box for firing and has an outer diameter of 100 mm and an inner diameter of 8
mm, height 80 mm cylinder, outer diameter 100 mm, thickness 10 mm bottom plate and lid, installed in each box No. 1-5 shown in Table 1, at 1750 ° C in a nitrogen atmosphere at atmospheric pressure. 1.
Firing was performed for 5 hours.

In addition, the No. 2 box shown in Table 1 is an object to be fired by putting a substance having the same component as the object to be fired into a carbon box whose inner surface is coated with powder having the same components as the object to be fired It was prepared by forming a coating film of the same component as that on the inner surface of the carbon box.

Regarding the obtained sintered body, the amount of oxygen (Sio ₂ ), Y ₂ O ₃ and Yb ₂ O was measured for the surface portion and the inner portion.
The content of ₃ (sintering aid) and the bulk density were measured, and the ratio was determined. The results are shown in Table 1.

As can be seen from Table 1, when fired according to the method of the present invention, the amount of oxygen, the amount of additives, and the bulk density of the surface portion and the inner portion of the sintered body are almost the same, In this case, the amount of additive and the bulk density are extremely small. That is, when fired according to the method of the present invention, a homogeneous sintered body can be obtained with less volatilization of oxygen, additives and the like from the surface of silicon nitride.

[Effects of the Invention] As described above, according to the method for producing a homogeneous sintered body of the present invention, the silicon nitride compact is installed in the box body or in the box whose inner surface is made of the same component as the material to be fired. By firing, it is possible to easily obtain a homogeneous sintered body made of silicon nitride having excellent characteristics. Further, the method of the present invention is a complex shape product such as a vane or a turbocharger rotor,
It is extremely useful as a sintering method for shaped articles that are used as they are without being processed.

Claims (1)

[Claims] 1. A body to be fired composed of silicon nitride powder and a sintering aid is a box made of silicon nitride and a sintering aid composed of the same components as the body to be fired, or the same as the body to be fired. A method for producing a homogenous sintered body, which comprises placing a coating film comprising silicon nitride composed of the components and a sintering aid in a box formed on the inner surface and firing.