JPH0280371A - Silicon nitride-based composition - Google Patents

Abstract

PURPOSE:To obtain a silicon nitride-based compsn. forming a silicon nitride sintered body whose strength lowers hardly at high temp. by blending silicon nitride with spinel and aluminum nitride. CONSTITUTION:This silicon nitride-based compsn. consists of 0.5-5wt.% spinel (MgAl2O4), 0.5-5% aluminum nitride(AlN) and the balance silicon nitride (Si3N4). The average particle size of the spinel used is preferably regulated to <=1.0mum so as to enhance the sinterability of the silicon nitride. In the case of >1.0mum average particle size, the density of a formed sintered body and the strength at room temp. and high temp. lower. unpreferably. It is desirable that the aluminum nitride is used in the form of fine particles and the pref. average particle size is <=1mum. The silicon nitride is preferably used in the form of fine powder so as to enhance the sinterability and the pref. average particle size is <=2.0mum.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a silicon nitride composition for structural members applied to parts of gasoline engines, gas turbine engines, etc., for example.

[Prior Art 1] Silicon nitride sintered bodies have high strength at high temperatures, so their application to parts used under harsh conditions, such as parts for internal combustion engines, is being considered. These applications generally require complex shapes. That is, there is a need for a silicon nitride'R sintered body that is easy to form and has high strength at high temperatures. In order to achieve this high strength and ease of formability, compositions have been proposed in which additives, sintering aids, etc. are added to silicon nitride. For example, in Japanese Patent Publication No. 63-23153,
A method is disclosed in which 0.5 to 5% by weight of a spinel compound and 0.5 to 5% by weight of titanium carbide are added to silicon nitride as sintering aids for sintering.

It is claimed that this method of sintering a silicon nitride composition allows a silicon nitride sintered body with high strength at high temperatures to be obtained.

Also, in Japanese Patent Application Laid-Open No. 178473/1983, the average grain t!
A composition in which at least one weight of a sintering aid of aluminum oxide, yttrium oxide, and magnesium oxide of 0.1 μm or less F is added to silicon nitride in a total amount of 2.0%,
There is disclosed a method for producing a sintered body made of silicon nitride, which reduces oxidation weight gain during the sintering process and improves strength at high temperatures.

[Problems to be Solved by the Invention] A sintered body made of a composition in which spinel is mixed with silicon nitride as a sintering aid as described above tends to form an amorphous portion, and the silicon nitride sintered body is difficult to form at high temperatures. tends to reduce the strength of Further, when titanium carbide is added together with a sintering aid, titanium carbide is not dissolved in the grain boundary portion of the sintered body but exists as a fine powder. Therefore, when using titanium carbide, there is a problem in that it must be well dispersed in silicon nitride together with a sintering aid. In addition, a total of 2 types of sintering aids such as alumina, magnesia, and ittria are added.
．． The method of limiting the amount to 0% or less requires a small amount of sintering aid to be uniformly dispersed in the silicon nitride.

The technical object of the present invention is to provide a silicon nitride composition that forms a silicon nitride sintered body with little loss of strength at high temperatures.

[Means for Solving the Problems 1 The silicon nitride composition of the present invention has spinel (MgAl2
O4) 0.5-5% and aluminum nitride (A+N
>0.5 to 5% by weight, with the remainder being silicon nitride.

The amount of spinel (Ir14 to Mg) added is 0.5 to 5% by weight. If the amount added is less than 0.5% by weight, the spinel will not be effective as a sintering aid, and the sinterability of the silica nitride solution will not improve. Additionally, if the amount added exceeds 5% by weight, an excessive liquid phase is generated during the sintering process, which improves the quality of the bamboo, but it is not preferable because the strength of the nitride-based sintered body at high temperatures decreases. 1. The average particle diameter of spinel (MOAl t Oa) is 1
In order to improve the sinterability of silicon nitride, it is preferable that the thickness is 0 μm or more. If the average particle diameter exceeds 1.0 μm, it is not preferable because the density of the obtained sintered body and the strength at room temperature and high temperature decrease.

The amount of aluminum nitride (△IN> added is 0°5.~5
The weight is 1r1%. If the amount added is less than 0.5 rfu%, the addition has no effect, and if it exceeds 511%, no improvement in the strength of the silicon nitride sintered body is observed, and on the contrary, it causes a decrease in high temperature strength. This is undesirable.

This aluminum nitride is dissolved into silicon nitride during sintering and reduces the suspension of amorphous parts at grain boundaries, so it is thought that slippage of grain boundaries at high temperatures is prevented and strength is improved (for example, solid solution as Sialon 5I3N-Al2O2). This means that titanium carbide, which does not dissolve in solid form during sintering,
This is a current situation that cannot be recognized.

The silicon nitride composition of the present invention reduces the number of amorphous portions at the grain boundaries of the sintered body, thereby preventing the strength from decreasing at high temperatures. In order to exhibit this effect, aluminum nitride is desired to be fine particles, and preferably has an average particle size of 1 μm or less.

In addition, since aluminum nitride has components similar to spinel and silicon nitride, it can improve sinterability and become a solid solution, reducing the hardness of the sintered body.

The silicon nitride used here is
Preferably, fine powder is used. Therefore, it is preferable that silicon nitride has an average particle diameter of 2.0 μm or less.

Further, in order to improve the sinterability of silicon nitride, spinel and aluminum nitride are preferably fine powders, and preferably have an average particle size of 1.0 μm or less.

Silicon nitride is a high-strength, high-hardness material, but in the present invention, in order to fully exhibit its characteristics, it is preferable to use high-purity powder. In particular, it is preferable to use a material with a high α-phase content.

The silicon nitride composition of the present invention can be easily produced not by a specific sintering method but by a conventional molding method and a sintering method in a non-oxidizing atmosphere.

That is, each component powder is mixed, filled into a mold of a predetermined shape by a conventional method, and pressed to form a green compact. Then,
This green compact is sintered under normal pressure or under pressure in a non-oxidizing atmosphere. In order to obtain a particularly dense sintered body, it is preferable to sinter under pressure. For example, hot press method or H
The fP method is used. and the temperature conditions (1500~
(1800° C.) and pressure conditions (100 to 9 f/cmZ or higher) can be selected from various ranges.

[Operations and Effects of the Invention] The silicon nitride composition of the present invention has spinel at an angle of 0°5 to 5°.
It is formed by blending 0.5 to 5% aluminum nitride with silicon nitride and sintering the mixture.

The silicon nitride composition obtained by blending spinel and aluminum nitride in specific amounts in this manner has improved strength in a high-temperature atmosphere. The reason is not fully understood, but when aluminum nitride is added, the additive dissolves into solid solution, and the amorphous parts in the grain boundaries of the silicon nitride sintered body crystallize. It is thought that by reducing the amount, slipping of the silicon nitride sintered body at grain boundaries at high temperatures can be prevented and strength at high temperatures can be maintained.

It is also presumed that the density of the obtained silicon nitride sintered body increases, contributing to the improvement in strength.

[Example] Hereinafter, this will be explained in detail using examples.

This silicon nitride composition consists of silicon nitride and spinel (M
(JA I t 04) and aluminum nitride (Al
N) is pressure-molded into a molded body, and this molded body is sintered to yield a sintered body.

Silicon nitride powder with an average particle size of 0.7 μm, spinel (MgAl t 04 ) powder with an average particle size of 0.2 μm, and aluminum nitride (AlN) powder with an average particle size of 0.3 μm.
) powder in the proportions shown in Table 1, and an organic solvent (
ethanol) and mixed in a ball mill for 48 hours. After that, the organic solvent was distilled off (the mixed powder was
After drying at 10° C. for about 12 hours, it was molded in a predetermined mold, and a pressure of 3000 kQ/crn was applied using a hydrostatic molding method to form a molded product.

Next, this molded body was heated to 1,700 to 1
A silicon nitride sintered body was obtained by sintering at 800°C.

It was confirmed using a TEM (transmission electron microscope) that almost no aluminum nitride existed as fine particles in the grain boundaries of the silicon nitride sintered body. Further, as characteristics of this silicon nitride sintered body, the density of the sintered body and the three-point bending strength at room temperature and 1000° C. were measured. The results are shown in Table 1.

Note that the three-point bending strength at 1000° C. was measured based on the JIS standard using a test piece having the same shape as that at room temperature in a nitrogen gas atmosphere.

No. 2 with specified amounts of spinel and aluminum nitride added
-6 and 8 showed a sintered density of 95.0% or more (approximately % of the theoretical value). On the other hand, N001 with less spinel addition has a low sintered density of 90.0%. However, more N007 and N009 than the range between additions of the present invention
The sintered density is as high as 97.8% and 97.7%, and the density is high due to the large number of spinel suspensions. Furthermore, the bending strength at room temperature also showed high strength.

However, the bending strength at 1000°C is reduced to about 50% of the strength at room temperature, and the strength at high temperatures is not maintained.

Further, in No. 1, which contains less than the predetermined amount of spinel, the bending strength at room temperature and at 1000° C. is extremely low. However, if the sintering aid is within the predetermined range of the present invention,
In No. 008, there is little difference in bending strength between room temperature and 1000° C., indicating that the strength at high temperatures is improved. In particular, No. 5 to 6 have sintered density and bending strength of room temperature and 10
It is an excellent silicon nitride sintered body with little difference from 00°C.

Note that the sintering aid N098 has a sintered density of 98
．． It is the highest at 0%, but the bending strength is 1000℃ compared to room temperature.
The case of 74% is close to the limit value for strength reduction.

Therefore, this silicon nitride composition shows that a sintered body with high density and little decrease in strength even at high temperatures can be obtained.

Patent applicant Toyoku Automatic Width Co., Ltd. Agent
Patent attorney Hiroshi Okawa

Claims (1)

[Claims] (1) A silicon nitride composition consisting of 0.5 to 5% by weight of spinel (MgAl_2O_4), 0.5 to 5% by weight of aluminum nitride (AlN), and the remainder silicon nitride (Si_3N_4).