JP2691295B2 - Silicon nitride sintered body - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial application] The present invention relates to a silicon nitride material, and more particularly to a sintered body excellent in oxidation resistance and strength in a medium temperature range.

(Prior art)

Sintered bodies made of silicon nitride are mainly composed of covalent bonds, and have excellent properties in strength, hardness, and thermal chemical stability. Therefore, application of engineer ceramics for heat engines is progressing. Has been.

In recent years, it has been considered that the operating temperature of the heat engine becomes 1400 ° C. or higher as the efficiency of the heat engine increases, and a material usable under this condition is desired.

Therefore, various studies have heretofore been made in terms of the composition of the sintered body or the sintering method in order to obtain a sintered body having excellent high temperature characteristics.

In terms of composition, in particular, rare earth element oxides such as Y ₂ O ₃ as well as alkaline earth element oxides such as Al ₂ O ₃ and MgO and CaO are used as sintering aids, and compositions containing these are subjected to atmospheric pressure. It is known to obtain a high density sintered body by firing, hot press firing, gas pressure firing, or hot isostatic firing.

However, in the above system, the room temperature strength has sufficient strength, but the high temperature characteristics are still insufficient.

It has been pointed out that the melting point of the grain boundary phase existing between the silicon nitride crystals of the sintered body is lowered as a major cause of the deterioration of the high temperature characteristics. Therefore, among the sintering aids that form the grain boundary phase, so-called silicon nitride (Si ₃ N ₄ ) -rare earth oxide (RE ₂ O ₃ ) that does not include low-melting point oxides such as Al ₂ O ₃ , MgO, and CaO. A system consisting of) -silicon oxide (SiO ₂ ) is attracting attention as a system excellent in high temperature characteristics, and improvements such as crystallization of grain boundaries in this system are being made.

[Problems to be solved by the invention]

However, although the silicon nitride sintered body composed of Si ₃ N ₄ —RE ₂ O ₃ —SiO ₂ system is excellent in oxidation resistance and strength in the high temperature range of 1200 ° C or higher, the temperature range of 800 to 1200 ° C is high. Has the drawback that its oxidation resistance deteriorates and its strength decreases.

One of the reasons for this is that the presence of a grain boundary phase containing a large amount of RE ₂ O ₃ as a high melting point compound makes it difficult for an oxide film to form on the surface of the sintered body in the middle temperature range. It is thought that this is because cracks occur.

Therefore, one measure is to lower the melting point of the grain boundary phase by adding Al ₂ O ₃ and SiO ₂ components, but if a large amount of Al ₂ O ₃ and SiO ₂ is added as the overall composition, the sintered body itself The high temperature strength decreases, making it unsuitable as a high temperature material.

[Object of the invention]

The present invention provides a silicon nitride sintered body made of Si ₃ N ₄ —RE ₂ O ₃ —SiO ₂ system, which has improved oxidation resistance and strength, especially in the middle temperature range around 1000 ° C. The purpose is to

[Means for solving the problem]

The inventors of the present invention have conducted extensive research on the above object, and as a result, as rare earth element oxides (RE ₂ O ₃ ), Sc ₂ O ₃ , Er ₂ O ₃ , Yb ₂ O ₃ , Ho ₂
O ₃ and Dy ₂ O Si ₃ having a specific composition containing at least one ₃
By depositing or volatilizing the rare earth elements present on the surface of the N ₄ —RE ₂ O ₃ —SiO ₂ -based sintered body and converting only the surface into a composition in which SiO ₂ is present in excess, the medium temperature range It has been found that the oxidation resistance and strength can be improved.

The silicon nitride sintered body of the present invention has a total composition of 85 to 99 mol% silicon nitride (Si ₃ N ₄ ), particularly 88 to 95 mol%, and Sc as a rare earth element oxide (RE ₂ O ₃ ). 0.5-5 mol% of at least one selected from the group of Yb, Er, Ho and Dy, especially 2
4 mol% and excess oxygen (converted to SiO ₂ ) 10 mol% or less. Here, the excess oxygen is obtained by subtracting the oxygen mixed as the rare earth element oxide from the total oxygen amount of the sintered body and converting the remaining oxygen amount into SiO ₂ .

A feature of the present invention is that excess oxygen / rare earth element oxide (molar ratio) on the surface portion of the sintered body having the above composition (hereinafter, simply referred to as SiO ₂
/ RE ₂ O ₃ ) is greater than the internal ratio, which lowers the melting point of only the grain boundary phase on the surface of the sintered body, making it easier to form an oxide film in the middle temperature range. In addition, the composition is less likely to cause volume expansion and the like, and the oxidation resistance and strength in the medium temperature range can be improved.

In the present invention, the composition of the sintered body is limited to the above range when silicon oxide is less than 85 mol% or rare earth element oxide is more than 5 mol% and excess oxygen is 10 mol%.
If the content exceeds 99 mol%, on the contrary, if the silicon nitride content exceeds 99 mol%, or if the rare earth element oxide content is less than 0.5 mol%, it is difficult to obtain a dense sintered body.

Incidentally, according to the present invention, SiO ₂ / RE ₂ O ₃ of the surface portion of the sintered body
The ratio is 2-10 due to its effect, and the internal SiO ₂ / RE ₂ O ₃ ratio is 0.5-7.
And the difference is preferably 1 or more.

Also has an internal SiO ₂ / RE ₂ O ₃ ratio greater than SiO ₂ / RE ₂ O ₃ ratio, the thickness is particularly the difference is 1 or more surface portions 100μm
It is desirable that:

In order to obtain such a sintered body, first, a molded body containing silicon nitride, a rare earth element and excess oxygen (SiO ₂ ) is fired under normal pressure, hot press firing, atmospheric pressure firing or hot isostatic pressing. To obtain a sintered body obtained by firing in a nitrogen-containing atmosphere at a temperature of 1600 to 2000 ° C. and having a composition within the above-mentioned composition range.

The SiO ₂ / RE ₂ O ₃ ratio of the inside and outside of the sintered body obtained by the above-mentioned ordinary method is substantially the same. According to the present invention, after polishing the surface of the sintered body as desired, if desired, at a temperature of 1400 to 1600 ° C. and in an atmosphere of CO gas or Si.
2 in a high-purity nitrogen atmosphere containing no O gas, etc.
When subjected to 20 hours of about heat treatment, the rare earth element content of the sintered body surface portion is reduced by the rare earth elements of the sintered body surface portion to aggregate precipitation or volatilization, SiO ₂ / RE ₂ of substantially surface portion
The O ₃ molar ratio becomes larger than the internal SiO ₂ / RE ₂ O ₃ molar ratio. Such a phenomenon is caused by Sc, Yb, Er, Ho and D among rare earth elements.
This is considered to be a property peculiar to y. The reason for this is that these elements have higher diffusivity at the grain boundaries as compared to rare earth elements such as Y, and the degree of freedom at the grain boundaries softened during heat treatment is high, so compositionally It is considered that an excessive amount is deposited on the surface portion.

Note that the atmosphere during the heat treatment preferably contains substantially no oxygen or oxide gas, and the presence of these oxygens inhibits the movement and precipitation of the rare earth elements.
At the time of heat treatment, it is desirable to place the sintered body in a container made of SiC, BN, or the like.

 Hereinafter, the present invention will be described with reference to the following examples.

(Example) A molded body containing powders of silicon nitride, a rare earth element, and silicon oxide was baked at 1850 ° C. for 3 hours in an atmosphere of N ₂ 9.3 atm,
Further, hot isostatic pressing was performed for 1 hour in an argon atmosphere at 1800 ° C. and 2000 atm to obtain sintered bodies having different compositions shown in Table 1.

The obtained sintered body was placed in a silicon carbide container 1400.
Heat treatment was performed at a temperature of ~ 1600 ° C in a nitrogen atmosphere of 1 atm.

The obtained sintered body is kept in an oxidizing atmosphere at 1000 ° C for 100 hours to increase the weight of the oxidized product, and JIS R1601 at 1000 ° C.
The 4-point bending strength based on the above was measured.

Further, a SiO ₂ / RE ₂ O ₃ ratio of the surface portion and the interior of the SiO ₂ / RE ₂ O ₃ ratio was determined by IPC analysis.

As is clear from the results in Table 1, the sintered body No. 2 before heat treatment had almost the same SiO ₂ / RE ₂ O ₃ ratio both inside and outside.
Although the oxidation resistance at ℃ is poor, in contrast to this, the heat-treated No. 1 sample showed the precipitation of Er ₂ O _{3 on} the surface, and the SiO ₂ / RE ₂ O ₃ ratio was higher on the surface. Is large, and the oxidation resistance and strength are greatly improved. All the samples of the present invention achieved an increase in oxidation weight at 1000 ° C. of 0.1 mg / cm ² or less and a strength of 85 Kg / mm ² .

〔The invention's effect〕

As described above in detail, by reducing the rare earth elements on the surface of the sintered body and making only the surface of the sintered body an excessive composition of SiO ₂ , the oxidation resistance and strength in the medium temperature range can be obtained without deteriorating the high temperature characteristics. Both can be improved, and the application to turbo rotors, turbines, etc. can be further advanced.

Claims (1)

(57) [Claims] 1. 85 to 99 mol% of silicon nitride, 0.5 to 5 mol% of at least one rare earth element oxide (RE ₂ O ₃ ) selected from the group of Sc, Yb, Er, Ho and Dy, and excess oxygen. (SiO ₂ conversion) 10
A silicon nitride sintered body which is a sintered body having a ratio of less than or equal to mol%, wherein an excess oxygen / rare earth element oxide (molar ratio) on a surface portion of the sintered body is higher than an internal ratio.