JPS5891072A - Manufacture of silicon nitride sintered body - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a high-density silicon nitride sintered body having excellent mechanical strength and high oxidation resistance.

Since silicon nitride sintered bodies have excellent properties such as mechanical strength, heat resistance, and corrosion resistance, they are used as high-temperature structural materials such as gas turbine parts. However, since silicon has a high degree of covalent bonding, it has poor sinterability and it is difficult to obtain a sintered body with high density and high strength. Conventional methods for producing silicon oxide include a reactive sintering method in which metallic silicon is nitrided and sintered, a hot press method in which a sintering aid is added to silicon nitride powder and sintered, and an atmospheric pressure sintering method.

Although the reactive sintering method allows sintering of complex shapes, the density is low, resulting in low strength and oxidation resistance. The pot press method still has the best strength and oxidation resistance, but it can only sinter into simple shapes. The pressureless sintering method yields high-strength products, but it requires the most sintering aids, and the reaction sintering method suffers from slight deterioration in strength under high temperatures, and its oxidation resistance is inferior to that of the reactive sintering method. is low. However, if the amount of the sintering aid can be reduced in the pressureless sintering method and sufficient densification can be achieved, it should be possible to create a high-temperature structural material with high strength and good oxidation resistance. Therefore, in order to improve the oxidation resistance of the silicon nitride sintered body in accordance with this objective, the present inventors conducted various studies on the sintering method using the pressureless sintering method. As a result, the mixture of silicon nitride powder and sintering aid was molded, and the resulting molded body was heated in an atmosphere of nitrogen gas or a mixed gas of nitrogen and non-oxidizing gas at a temperature of 100-1130°C. It was discovered that high heat resistance and high temperature strength can be obtained by sintering at a temperature of /1r00-2.200℃ in an atmosphere of nitrogen gas or a mixed gas of nitrogen and non-oxidizing gas after heat treatment for more than an hour. Completed the invention.

The present invention will be explained in detail below. Generally, sintering of silicon nitride involves adding a sintering aid to silicon nitride powder to form a liquid phase, and acicular crystals of β-8i s N4 precipitate from the liquid phase, resulting in high strength υ. It is believed that a silicon nitride sintered body can be obtained.

Furthermore, it is difficult to obtain a dense and high-strength sintered body using the pressureless sintering method, which requires a large amount of sintering aid, which significantly reduces mechanical strength and oxidation resistance at high temperatures. There was a problem. In particular, in order to improve the oxidation resistance of the silicon nitride sintered body, it is important to further reduce the amount of sintering aid and to obtain a dense sintered body. Therefore, in the present invention, a mixture of emptied silicon powder and a sintering aid is molded, and the resulting molded body is heated to 100°C in an atmosphere of nitrogen gas or a mixed gas of nitrogen and a non-oxidizing gas. at a temperature of 3
After heat treatment for more than an hour, sintering is performed in an atmosphere of nitrogen gas or a mixed gas of nitrogen and non-oxidizing gas at a temperature of 100-2200℃, which is more effective than the conventional pressureless sintering method. A dense, high-strength, and highly oxidation-resistant sintered body was obtained with a small amount of sintering aid.

Although it is not clear why a dense, high-strength, and highly oxidation-resistant sintered body can be obtained through such a firing process, it is presumed that it is due to the following reasons. That is, the sintering process of silicon nitride varies depending on the type and function of the sintering aid, but in general, when an oxide sintering aid such as magnesium oxide is used, the following can be said.

First, the densification of silicon nitride is greatly influenced by its sintering temperature. In silicon nitride, at temperatures above 1100°C, densification due to particle rearrangement begins to occur, albeit slightly.
At temperatures above 300°C, the rate of densification is quite fast. However, if the amount of sintering aid is reduced, this densification rate will become slower, and it will take more time for densification even at the same temperature. Compared to such a densification process, the temperature at which the α-to-β transition occurs without significantly affecting the bending strength of silicon nitride is quite different, and the α-β transition is
It starts to occur at 100°C, and the rate of transition becomes faster when the temperature exceeds 1100°C. In particular, if the amount of sintering aid is small, the densification rate will be quite slow, α-β transition will occur, and once β-8i a N4 needle crystals have grown, densification will no longer progress as much. being sidelined. In other words, since there is a difference between the temperature at which densification progresses rapidly and the temperature at which needle crystals grow due to α-β transition, /100
℃～/! Pretreatment at a temperature of 10°C for 3 hours or more promotes densification of the kana, and furthermore, β is added to the sintered body.
-8i3N4 is sufficiently nucleated to grow uniformly, and then further densified and uniform β-8i a N4 acicular crystals are grown at a temperature exceeding /AOOt. It is thought that a dense and high-strength silicon sintered body was obtained using less sintering aid than the sintering method. Further, the effect of such pretreatment is influenced by the size and shape of the molded article, and the larger the article, the greater the effect of pretreatment than when no pretreatment is applied. Note that the temperature of this pretreatment is desirably 7300-/60[theta]C in consideration of the energy, time, etc. required for densification.

Next, the present invention will be explained in more detail with reference to examples, but the present invention is not limited to the following examples unless it exceeds the gist thereof.

Example / Silicon nitride containing 50% or more of the α type with an average particle size of 7 μm and MgO with an average particle size of 7 μm as a sintering aid were mixed and pulverized in a ball mill in the proportions shown in the first coating. 200
It was molded at a pressure of 0 Ky/d and fired under the conditions shown in the same table, and the density, bending strength at room temperature, and oxidation weight gain of the obtained sintered body were measured, and the results shown in Table 1 were obtained. . The bending strength was measured using JIS B≠IQ method using a test piece with a width of ≠X and a length of 2jm (span, 20B). The increase in perilla was measured.

Table 1 Note) A//R, A/, and 2R are comparative products.

The others are products of the present invention.

As is clear from the table, ILi/~sio heat-treated according to the present invention exhibits superior bulk density, bending strength, and especially superior oxidation resistance compared to comparative products A//R and /2R, and is suitable for automobiles. This material provides excellent heat resistance, wear resistance, and structural parts for gas turbines, engine parts, cutting tools, and other general high-temperature applications.

Example λ In Example 1, it was cooled after heat treatment and then reheated.
Similar results were obtained even when the main firing was carried out after the heat treatment.

Although MgO was used as the sintering aid in Example 2.2, the present invention is not limited to this, and any known glaze sintering aid can be used.

Claims (1)

[Claims] A molded body obtained by molding a mixture of silicon nitride powder and a sintering aid is heat-treated at a temperature of /100 to /j10°C for 3 hours or more in an atmosphere of nitrogen gas or a mixed gas of nitrogen and a non-oxidizing gas. After that, #00-. 2. A method for producing a silicon nitride sintered body, characterized by sintering at a temperature of 200°C.