JPH0791044B2 - Method for producing easily sinterable silicon nitride powder - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention relates to a silicon nitride powder used as a high-temperature structural material for a gas turbine member, a nozzle, a bearing, etc., particularly, it is easily sinterable and exhibits high-temperature strength. The present invention relates to a method for producing silicon nitride powder.

[Conventional technology]

Conventionally, as methods for producing silicon nitride powder, (1) direct metal nitriding method, (2) silica reduction nitriding method, and (3) silicon halide method are known. Probably because the powders produced by these methods have different production histories, even if the amount of metal impurities, the amount of oxygen, the particle size, and the specific surface area are the same, the sinterability of the powder and the sintered body after sintering are There are many differences in characteristics such as bending strength.

Generally, the powder produced by the method (1) is easily sinterable but has low high-temperature bending strength, and the powder of the method (2) is hardly sinterable but has high high-temperature bending strength, The powder of the method (3) is said to exhibit intermediate performance.

In particular, since the silicon nitride powder produced by the method (1) receives strain due to pulverization on the surface of the powder as compared with the other two methods, it is possible to release some strain energy by releasing the strain energy. Research has been done on imparting characteristics to powders. One example is JP-A-53-88011.

In this prior art, silicon nitride powder is heat-treated at a temperature of 1,400 to 1,900 ° C. to remove oxygen contained in silicon nitride into Si.
The amount of the grain boundary phase in the sintered body can be reduced by removing it as O and not dissolving a part of oxygen on the surface of the silicon nitride powder as a solid solution and improving the wettability with the sintering aid. It is a technology, and as a result, high temperature and high strength are developed.

However, since this method utilizes mass transfer on the surface of the silicon nitride powder as the starting material, the starting material is naturally limited, that is, it is required to be a fine powder or a material with high oxygen content. There is a problem. Therefore,
There is a limit to the modification of powder, and one example of the results is the use of only hot press, which leaves a problem in that the powder characteristics could not be completely grasped.

[Problems to be Solved by the Invention]

In view of the above problems in JP-A-53-88011, the present inventors have made various investigations on the method of improving powder and the binding of improved powder, and as a result, stick to the starting material. It was found that if powder of silicon oxide is added to and mixed with silicon nitride powder without heat treatment and heat-treated at a temperature of 1,500 to 1,800 ° C. under a non-oxidizing atmosphere, the powder can be applied to atmospheric pressure sintering. Was completed.

[Means for Solving the Problems]

That is, the present invention is a method for producing an easily sinterable silicon nitride powder characterized by adding and mixing silicon oxide fine powder to silicon nitride powder, and heat-treating at a temperature of 1,500 to 1,800 ° C. under a non-oxidizing atmosphere. is there.

Hereinafter, the present invention will be described in more detail.In the present invention, the reason for adding the silicon oxide fine powder is that the surface modification of the silicon nitride powder is performed only by the fine powder when oxygen in the silicon nitride powder is biased to the fine powder, This is because coarse particles that are considered to have a small amount of oxygen, for example, powder having a particle size of about 5 to 10 μm may be left behind from the reforming. That is, the present invention intends to solve this problem by positively adding fine silicon oxide powder and reacting with the surface of the silicon nitride powder.

The amount of silicon oxide added is determined in consideration of the following purposes and oxygen of the raw material silicon nitride powder, but is usually within 6% by weight. That is, one of the purposes is to improve the powder having easy sinterability, and the other is to reduce oxygen and rearrange fine powders. The former is intended to improve the wettability of the powder surface by adding the fine powder of silicon oxide within 2% by weight and performing heat treatment at a relatively low temperature, for example, at a temperature up to about 1600 ° C. On the other hand, the latter is the Japanese Patent Application No. 63-2773.
Aiming at the technical contents of No. 60 specification, it is a heat treatment at a high temperature, for example, about 1750 ° C., and as much silicon oxide fine powder as possible is added, and the reaction is positively carried out to perform deoxidation and low oxygen content. In addition to the formation of fine particles, the fine particles are rearranged to reduce the fine particles. The heat treatment time also affects the temperature, but about 3 to 15 hours is sufficient.

As described above, the particle size of the silicon oxide is preferably smaller because it is intended to positively react with the surface of the silicon nitride powder. Specifically, it is desirable that the specific surface area of the raw material silicon nitride powder be larger, and it is convenient that the fine powder is 10 m ² / g or more. Specific examples of such fine particles of silicon oxide include aerogel, white carbon and the like.

The heat treatment atmosphere is preferably a non-oxidizing atmosphere, for example, an atmosphere of nitrogen, argon, hydrogen, ammonia, or the like, for the purpose of reducing oxygen. The optimum heat treatment temperature is 1,500-1,800 ° C. 1,500 ° C
If it is less than the range, the reaction between the silicon oxide fine powder and the silicon nitride powder, that is, the mass transfer does not sufficiently occur, and the powder cannot be modified. On the other hand, when the temperature exceeds 1,800 ° C, sublimation transition and partial decomposition of silicon nitride itself occur.

As described above in detail, the present invention has the disadvantages of silicon nitride powder, particularly silicon nitride powder obtained by the direct nitriding method. The silicon nitride powder is modified to easily sinter and develop high-temperature strength. The basic idea of the present invention is based on the reactions shown in the following formulas (1) and (2).

α-Si ₃ N ₄ + SiO ₂ → 2Si ₂ ON ₂ (1) 2Si ₂ ON ₂ → β-Si ₃ N ₄ + SiO ↑ (2) [Example] Hereinafter, more specific examples and comparative examples will be given. The present invention will be described.

Examples 1 to 11 and Comparative Examples 1 to 4 100 parts by weight of a commercially available silicon nitride powder is used, and S having a non-surface area of 50 m ² / g is added.
iO ₂ was added and mixed at a ratio shown in Table 1. This mixed powder was put into a carbon crucible and heated under the heat treatment conditions shown in Table 1 to produce a silicon nitride powder. The characteristics of the obtained silicon nitride powder are shown in Table 1.

Next, Y ₂ O ₃ (average particle diameter of 1.3 μm) and Al ₂ O ₃ (average particle diameter of 1.
4 μm), MgO (average particle size 1.2 μm) and MgO ・ Al ₂ O
_As shown in Table 2, ₃ (average particle size 1.2 μm) was internally blended in various proportions, and 1,1,1-trichloroethane was added to give 4
After wet mixing with a ball mill for 2 hours, dry and mold with a molding pressure of 100 kg / cm ² into a 6 × 10 × 60 mm shape, then 2700 kg / cm ²
CIP molding was performed at the molding pressure of. These compacts were set in a carbon crucible and fired under N ₂ gas atmosphere under the conditions shown in Table 2 to obtain sintered compacts. After grinding the obtained sintered body, the relative density and bending strength at room temperature (δ _RT ) and 1200 ° C (δ ₁₂₀₀ ) were measured. The results are shown in Table 3.

The measured values shown in Tables 1 and 3 were obtained by the following method.

(1) Oxygen (% by weight): by LECO TC-136 type O / N simultaneous analyzer.

(2) Specific surface area (m ² / g): According to Cantersorb Jr BET 1-point method manufactured by Yuasa Ionics.

(3) α fraction (%): by X-ray diffraction of Geiger Frax RAD-II B type manufactured by Rigaku Denki KK

(4) Relative density (%): By Archimedes method.

(5) Bending strength (MPa): Autograph AG manufactured by Shimadzu
-By type 2000A.

〔The invention's effect〕

The silicon nitride powder produced by the present invention is easily sinterable in spite of low oxygen content, and high temperature bending strength at 1200 ° C. can be 800 MPa or more. This is a result of controlling the powder properties related to the generation and growth of β-columnar crystals in the sintered body.

Claims (1)

[Claims] 1. A method for producing an easily sinterable silicon nitride powder, which comprises adding and mixing fine silicon oxide powder to silicon nitride powder and heat-treating at a temperature of 1,500 to 1,800 ° C. in a non-oxidizing atmosphere.