JPH0556308B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a silicon nitride sintered body, and particularly to a silicon nitride sintered body with improved thermal shock resistance (resistance to thermal shock stress).

[Conventional technology]

Silicon nitride (Si ₃ N ₄ ) sintered bodies have attracted attention as ceramics for automobile engine parts and other high-temperature structures, and research and development is progressing and they are being put into practical use (for example, ``ceramics for highly reliable engine parts''). , Mekei New Material, pre-launch 1
No., May 20, 1985, pp. 39-46 [Nikkei McGraw-Hill]).

Silicon nitride sintered bodies are generally produced by pressure-molding α-silicon nitride powder with a suitable sintering aid such as alumina, ittria, or magnesia.
It is manufactured by molding using methods such as injection molding, extrusion molding, and cast molding, and then firing the molded product in a non-oxidizing atmosphere (typically, a nitrogen atmosphere). Control of the grain boundary phase of silicon nitride crystals is a factor in improving the properties of sintered bodies, such as high-temperature strength, oxidation resistance, and thermal shock resistance. Therefore,
The applicant is Japanese Patent Application No. 61-125906 (June 2, 1986)
In order to increase high-temperature _strength , _{XSiO 2} N, _{7N2 ,}
and X ₁₀ (SiO ₄ ) ₆ N ₂ [In these formulas, X represents a Group 3A element (including lanthanide series elements) in the long period periodic table. We have proposed a silicon nitride sintered body characterized by precipitating one or more crystals. In addition, the patent application 1986-
No. 141449 (filed on June 8, 1986), Y ₅ N (SiO ₄ ) ₃ , Y ₁₀ N ₂ which are hexagonal crystals are present at the grain boundaries of the sintered body.
(SiO ₄ ) ₆ and Y ₂₀ N ₄ (SiO ₄ ) ₁₂ and YAG (5Al ₂ O ₃ .3Y ₂ O ₃ ) crystals are precipitated. suggested the body.

[Problem that the invention seeks to solve]

The conditions under which silicon nitride sintered bodies are used may lead to the occurrence of thermal shock stress, and this is particularly the case when silicon nitride sintered bodies are used in automobile engine parts. For this purpose, it is necessary to reduce thermal shock stress and increase thermal shock resistance (thermal shock resistance). Therefore,
An object of the present invention is to provide a silicon nitride sintered body with improved thermal shock resistance.

[Means for solving problems]

In order to solve the above problems, the present invention uses at least Li ₂ O ₃ , Al ₂ O ₃ and SiO ₂ as sintering aids.
This is a silicon nitride sintered body obtained by sintering a silicon nitride raw material using a sintered body, in which low-expansion or heat-shrinkable Li ₂ O−Al ₂ O ₃ −SiO ₂ crystals are precipitated at the grain boundaries of the sintered body. A thermal shock resistant silicon nitride sintered body is provided.

[Effect]

The Li ₂ O−Al ₂ O ₃ −SiO ₂ crystals precipitated at the grain boundaries of the sintered body are Li ₂ O・Al ₂ O ₃・8SiO ₂ petalite, Li ₂ O・Al, which exhibit negative thermal expansion. ₂ O ₃・4SiO ₂ spodumene, Li ₂ O・Al ₂ O ₃・2SiO ₂ eucryptite, Li ₂ O・Al ₂ O ₃・
It contains at least one type of crystalline phase of 6SiO ₂ lithium orthoclase, and exhibits heat shrinkage or lower expansion than conventional grain boundary phases. Note that the equilibrium diagram for the Li ₂ O ₃ -Al ₂ O ₃ -SiO ₂ system shown in Figure 1 is known, and the
P: petalite, R: lithium orthoclase, S: spodiume, and E: eucryptite, which are on the line connecting Li ₂ O・2Al ₂ O ₃ and SiO ₂ , and in their vicinity exhibit negative thermal expansion (thermal contraction). show.

It is preferable that the grain boundary phase is heat-shrinkable and combined with silicon nitride crystals exhibiting positive thermal expansion to form a non-expandable silicon nitride sintered body.

To improve thermal shock resistance, use the following formula for thermal shock stress (σ): σ=AEαΔΤ/1−ν Where: A: Constant E: Young's modulus α: Coefficient of linear expansion ΔΤ: Temperature difference ν: Poisson The following can be considered. That is, in order to reduce the thermal shock stress (σ), (1) the Young's modulus is reduced, (2) the coefficient of linear expansion is reduced, and (3) the Poisson's ratio is reduced. Furthermore, the higher the thermal conductivity, the smaller the thermal shock stress, and the higher the strength of the object, the more effective it is against thermal shock stress. In the present invention, the grain boundary phase is made to have thermal shrinkage or low expansion property as described above in order to reduce thermal shock stress and improve thermal shock resistance by reducing the coefficient of thermal expansion.

The larger the amount of sintering aids (Li ₂ O ₃ , Al ₂ O ₃ and SiO ₂ ) added according to the present invention, the more effective it is in terms of thermal shrinkage or low expansion, but due to the relationship with strength, It is preferably 5 to 20 wt% of the total.

The above-mentioned Li ₂ O ₃ , Al ₂ O ₃ and SiO ₂ are used as sintering aids.
In addition, Y ₂ O ₃ , MgO, MgAl ₂ O ₄ , CeO ₂ , SrO, etc. can be used, and Y ₂ O ₃ is preferred because it contributes to improving the strength.

〔Example〕

Hereinafter, the present invention will be explained in detail using embodiment examples and comparative examples of the present invention.

Experiment The raw material composition of sample A was Si ₃ N ₄ 92wt%,
Li ₂ O 1wt%, Al ₂ O ₃ 3wt% and SiO ₂ 4wt%,
The raw material composition of sample B is Si ₃ N ₄ 90wt%, Li ₂ O 1wt%.
%, _{Al2O3 3wt} %, _SiO2 3wt% and _{Y2O3 3wt} %
And the raw material composition of sample C of comparative example is
Si ₃ N ₄ 90wt%, Al ₂ O ₃ 5wt% and Y ₂ O ₃ 5wt%. Next, a cylindrical test piece with a diameter of 30 mm and a height of 20 mm was manufactured by press molding. After further applying 2 tons of isostatic pressure using a rubber press, the product was heated to a sintering temperature of 1750° C. in a pressurized nitrogen (N ₂ ) atmosphere (9 atm) to obtain a silicon nitride sintered body. next,
The sintered body was reheated at 1200°C (preferably 800 to 1400°C) to crystallize the grain boundary phase.

As a result of identifying the crystal phase of the silicon nitride sintered body thus obtained by X-ray diffraction, sample A
In sample B, Li ₂ O・Al ₂ O ₃・2SiO ₂
Li ₂ O・Al ₂ O ₃・2SiO ₂ , Al ₅ Y ₃ O ₁₂ , Y ₂₀ N ₄ Si ₁₂ O ₄₈
And in sample C, Al ₂ Y ₃ O ₁₂ ,
It was confirmed that Y ₂₀ N ₄ Si ₁₂ O ₄₈ was precipitated at grain boundaries.

Then, in order to examine the thermal shock resistance of the obtained silicon nitride sintered body, a rapid cooling test was conducted in which the body was heated to a temperature of 300 to 600°C in an electric furnace, and immediately after being taken out, it was put into water. As a result, sample A was destroyed by rapid cooling from 450°C, sample B was destroyed by rapid cooling from 500°C, and sample C was destroyed by rapid cooling from 400°C. From this, in Samples A and B according to the present invention, the _Li2O - _{Al2O3 -} SiO crystal phase is precipitated at the grain boundaries, so that the thermal shock resistance is improved compared to the conventional one.

In addition, in order to investigate the high temperature strength of the obtained silicon nitride sintered body, we conducted a three-point bending fracture test at 800℃ and 1000℃.
and at room temperature. As shown in Figure 2, the three-point bending strength of sample A (black circle ●) is smaller than samples B and C, sample C (white circle ○) is the highest, and sample B (× mark) was between samples A and C.
It can be seen that high-temperature strength is improved when Y ₂ O ₅ (Ittria) is used as a sintering aid. Samples A and B of the present invention have lower high-temperature strength than the comparative sample C, but have higher thermal shock resistance, so they are suitable for applications that involve thermal shock due to rapid heating and cooling. In particular, sample B has higher high-temperature strength than sample C. It is desirable because it is close to C and has the best thermal shock resistance.

〔Effect of the invention〕

According to the present invention, Li ₂ O−Al ₂ O ₃ precipitated at the grain boundaries of silicon nitride crystal grains in a silicon nitride sintered body
By including heat-shrinkable (negative thermal expansion coefficient) crystals in the -SiO ₂ -based crystals, the grain boundary phase becomes low-expandable or heat-shrinkable, and the thermal shock resistance of the sintered body can be improved.

[Brief explanation of drawings]

Figure 1 is an equilibrium phase diagram of the Li ₂ O-Al ₂ O ₃ -SiO ₂ system, and Figure 2 is a graph showing the relationship between the three-point bending strength of a silicon nitride sintered body and the test temperature. .

Claims (1)

[Claims] 1 A silicon nitride sintered body obtained by sintering a silicon nitride raw material using at least Li ₂ O ₃ , Al ₂ O ₃ and SiO ₂ as sintering aids, with low expansion or heat shrinkage at the grain boundaries of the sintered body. A thermal shock-resistant silicon nitride sintered body characterized by precipitating Li ₂ O−Al ₂ O ₃ −SiO ₂ crystals.