JPH0510295B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application _Field ] The present invention relates to a sintering aid for silicon nitride _and a method for producing a silicon nitride sintered body using the sintering aid.
This invention relates to a method for producing a dense and high-strength silicon nitride sintered body using a mixture of La group oxides as a sintering aid. Silicon nitride sintered bodies have excellent heat resistance, corrosion resistance, wear resistance, and thermal shock resistance, and are therefore used in structural materials such as bearings, nozzles, bearings, and turbine members. [Prior Art] Since it is difficult to sinter silicon nitride by itself, MgO, Al ₂ O ₃ , Y ₂ O ₃ or the like is generally added as a sintering aid. However, when sintering using the pressureless sintering method, it is difficult to obtain a dense sintered body with a porosity of 2% or less unless a large amount of sintering aid is added; Although it is dense, it has the disadvantage that the inherent characteristics of silicon nitride, such as high strength, are lost. In addition, as a sintering method that compensates for the above drawbacks, there is a method of sintering while applying pressure such as hot press (HP) (Japanese Patent Publication No. 52-3415).
It has industrial disadvantages such as difficulty in obtaining sintered bodies with complicated shapes. [Problems to be Solved by the Invention] The present inventor has investigated various methods for solving the above-mentioned drawbacks and producing a dense and high-strength silicon nitride sintered body by an atmospheric pressure sintering method. As a result, they discovered that cobalt aluminate spinel (CoAl ₂ O ₄ ) and a La group oxide can be used together as a sintering aid, thereby completing the present invention. [Means for Solving the Problems] That is, the present invention has the following gist. 1 Cobalt aluminate spinel (CoAl ₂ O ₄ ) and
A sintering aid for silicon nitride containing a La group oxide. 2 80% by weight or more of silicon nitride powder, 0.5 to 15% by weight of cobalt aluminate spinel (CoAl ₂ O ₄ ), and La
A method for producing a dense silicon nitride sintered body, which comprises molding a mixed powder containing 0.5 to 18% by weight of group oxides and then firing it in a non-oxidizing atmosphere. The present invention will be explained in more detail below. Cobalt aluminate spinel (CoAl ₂ O ₄ ), which is a sintering aid component, contributes to the densification and high strength of silicon nitride sintered bodies, and cobalt oxide (CoO) and aluminum oxide (Al ₂ O 4 ) ₂ ) is a compound with a spill structure obtained through a high temperature reaction with Even if a mixture of cobalt oxide and aluminum oxide is used instead of this spinel, the effects of densification and high strength as in the present invention cannot be obtained. The other sintering aid component, the La group oxide, is 1
A species or two or more species may be used. Any type of La group oxide may be used, but particularly preferred are cerium oxide, praseodymium oxide, samarium oxide, and gadolinium oxide. Even if the above-mentioned cobalt aluminate spinel and La group oxide are used alone, the effects of densification and high strength cannot be obtained, so it is necessary to use both together. Their ratio is 80% silicon nitride powder as the sum of both.
It is 20% by weight or less compared to 20% by weight or more. If the proportion of the sintering aid is greater than this, the inherent properties of silicon nitride will be significantly impaired. The preferred blending ratio is
Silicon nitride powder 80% by weight or more, cobalt aluminate spinenel 0.5 to 15% by weight, particularly preferably 1 to 7
% by weight, 0.5 to 18% by weight of La group oxide, particularly preferably 2 to 8% by weight. As the silicon nitride powder, one containing 50% by weight or more of α phase is suitable,
If the α phase is less than 50% by weight, α will be generated during the sintering process.
The ratio of long columnar crystals from the phase to the β phase in the sintered body decreases, which is disadvantageous for increasing the strength. There are no particular limitations on the method of mixing the silicon nitride powder and the sintering aid. As the molding method, a conventional molding method such as slurry casting, injection molding, mold press molding, extrusion molding, etc. is appropriately selected depending on the desired shape. Furthermore, in order to suppress the decomposition of silicon nitride, the sintering atmosphere is preferably a non-oxidizing atmosphere, particularly preferably a nitrogen gas atmosphere, but mixed with another gas that acts as a non-oxidizing atmosphere. There is nothing wrong with that either. The sintering method is not limited to pressureless sintering; hot pressing (HP), hot isostatic pressing (HIP), etc. may be used depending on the shape and desired physical properties of the sintered body. can. [Example] Hereinafter, the present invention will be described in more detail by giving examples and comparative examples. Example: 90% α phase with an average particle size of 0.73 μm (measured with a particle size analyzer [trade name: Microtrack, manufactured by N&L Co., Ltd.])
Si ₃ N ₄ powder 90% by weight, CoAl ₂ O ₄ with average particle size 1.0 μm
(Commercial product) CeO ₂ with 5% by weight and average particle size of 0.8μm
(Commercial product) Add 1,1,
Add 1-trichloroethane, wet mix in a ball mill for 4 hours, and after drying, mold 6×10×60 with a molding pressure of 100Kg/ ^cm2 .
After molding into a mm shape, CIP with a molding pressure of 2000/cm ²
Molded. For comparison, CoO (commercially available) instead of CoAl ₂ O ₄
A molded body was obtained by molding under the same conditions as above except that a mixture of and Al ₂ O ₃ (commercially available) was used. These molded bodies are set in a carbon crucible,
A sintered body was produced by firing at 1650° C. for 12 hours in an N ₂ gas atmosphere. Porosity of these sintered bodies and room temperature 3
When measuring the point bending strength (according to JIS R 1601), the present example had a porosity of 1.2% and a room temperature bending strength of 90 Kg/ ^mm2 , whereas the comparative example had a rate 3.9%, normal temperature 3-point bending strength 75Kg/mm ²
It was hot. The porosity was determined by dividing the bulk specific gravity measured by the Archimedes method (according to JIS2205) by the theoretical density based on the raw material composition, subtracting it from 1, and multiplying by 100. Next, Si ₃ N ₄ powder with 90% α phase, CoAl ₂ O ₄ powder and La group oxides such as CeO ₂ , Pr ₆ O ₁₁ , Sm ₂ O ₃ ,
Using Gd ₂ O ₃ powder (all commercially available) as a raw material, molded bodies were made under the same conditions as above for 18 types of raw material powder, including comparative examples, and after firing, the porosity and three-point bending strength at room temperature were measured. . The results are shown in the table.

〔Effect of the invention〕

According to the present invention, a dense and high-strength silicon nitride sintered body can be manufactured.

Claims (1)

[Claims] 1. Cobalt aluminate spinel (CoAl ₂ O ₄ ) and
A sintering aid for silicon nitride containing a La group oxide. 2. After molding a mixed powder containing 80% by weight or more of silicon nitride powder, 0.5 to 15% by weight of cobalt aluminate spinel (CoAl ₂ O ₄ ), and 0.5 to 18% by weight of La group oxide, a non-oxidizing powder is formed. A method for producing a dense and high-strength silicon nitride sintered body, which is characterized by firing in an atmosphere.