JP2801455B2 - Silicon nitride sintered body - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a silicon nitride sintered body excellent in bending strength and oxidation resistance at a high temperature suitable for a heat engine such as a gas turbine or a turbo rotor, and a method for producing the same.

[0002]

2. Description of the Related Art Conventionally, silicon nitride sintered bodies have been attracting attention as engineering ceramics, particularly as materials for heat engines, because of their excellent strength, hardness and thermochemical stability at high temperatures. Specific heat engines include components for turbo rotors and gas turbines, and when applied to these, have excellent mechanical properties in the range of room temperature to about 1200 ° C. for sintered bodies, and at high temperatures of 1400 ° C. for some components. Is required.

[0003] In response to such requirements, from the viewpoint of composition,
In addition to rare earth element oxides such as Y ₂ O ₃ , Al ₂ O ₃ , Mg
Oxides such as O are most commonly used as sintering aids. However, considering the high temperature characteristics of the sintered body,
_{When 2} O ₃ or MgO is contained, a low-melting substance is generated at the grain boundary of the sintered body, so that high-temperature strength and high-temperature oxidation resistance decrease. From these viewpoints, Si ₃ N ₄ —RE ₂ O ₃ (rare earth element oxide) —Si substantially free of the above oxides
The composition of a simple ternary system of O ₂ has also been studied.
i ₃ N ₄ -RE ₂ O ₃ apatite consisting -SiO _2, etc., attempts have been made to improve the high temperature properties by precipitating a crystal phase such as YAM.

[0004]

However, the problems to be solved by the invention
₃ N ₄ -RE ₂ O ₃ (rare earth oxide) in a simple ternary -SiO _2, sintered bodies precipitated various crystal phase in the grain boundaries is improved in strength at high temperatures of room temperature and 1400 ° C. or acid Although the effect of improving the oxidizing property is recognized to some extent, it has the disadvantage that the characteristics in the middle temperature range (700 to 1000 ° C.), particularly the oxidation resistance, are significantly deteriorated.

Accordingly, an object of the present invention is to provide a silicon nitride sintered body having improved oxidation resistance in a medium temperature range while maintaining excellent high temperature characteristics.

[0006]

[Means for Solving the Problems] The present inventors have studied the above problems, and as a result, have added chromium to the above simple ternary system, which is present as chromium silicide. It has been found that the above-mentioned object is achieved by doing so.

That is, the silicon nitride sintered body of the present invention comprises 70 to 98 mol% of silicon nitride and 1.0 to 1 of rare earth element oxide.
0 mol%, excess oxygen (SiO ₂ conversion amount) 25 mol% or less, and a chromium compound in an amount of 0.01 to 5 mol% in terms of oxide. Excess oxygen (SiO ₂ conversion amount) / rare earth element oxide Is greater than 0.7 and less than or equal to 3, and the chromium is present as silicide.

Hereinafter, the present invention will be described in detail. The sintered body of the present invention has a composition of 70 to 99 mol% of silicon nitride, 1.0 to 10 mol% of rare earth element oxide, and excess oxygen (SiO ₂
(Equivalent amount) of 25 mol% or less, and 0.01 to 5 mol% of chromium compound in terms of oxide. Here, the excess oxygen is an oxygen amount obtained by removing oxygen mixed in a stoichiometric composition as a rare earth oxide (RE ₂ O ₃ ) from the total oxygen amount contained per unit volume of the sintered body. More specifically, it is composed of impurity oxygen in a silicon nitride raw material or oxygen added as SiO ₂ , and each of them indicates a SiO ₂ equivalent amount.

The reason why the composition of the sintered body is limited to the above range is that the room temperature strength and the high temperature strength are deteriorated even if any of silicon nitride, rare earth element oxide and excess oxygen deviates from the above range. In particular, when the amount of the chromium compound is 0.
If the amount is less than 01 mol%, the effect of improving the oxidation resistance in a medium temperature range of the sintered body is not obtained. If the amount exceeds 5 mol%, a glass having a low melting point is easily generated, and the high-temperature characteristics are deteriorated.

According to the sintered body of the present invention, it is important that the molar ratio expressed by excess oxygen (amount in terms of SiO ₂ ) / oxide of the rare earth element is larger than 0.7 and equal to or smaller than 3. It is. This is because when the molar ratio is 0.7 or less, it is difficult to densify the resin, and when it exceeds 3, the high-temperature strength is deteriorated.

Next, as a method for producing the silicon nitride sintered body of the present invention, a silicon nitride powder, a rare earth element oxide powder, a chromium compound, and optionally a silicon oxide powder are used as raw material powders, and Weigh and mix as needed. At this time, the silicon nitride powder desirably has a BET specific surface area of 3 to ²⁰ m ² / g and a pregelatinization ratio of 95% or more in order to promote sinterability. The silicon nitride powder generally contains an impurity oxygen content of about 0.8 to 1.5% by weight, but the total oxygen content can be arbitrarily adjusted by adding silicon oxide.
Note that the chromium compound is an oxide, a nitride, a carbide, a silicide, a boride, or the like.

A binder is appropriately added to the above mixed powder, granulated, and then molded. A well-known method can be employed for molding, and specifically, press molding, extrusion molding, casting molding, injection molding or the like can be employed.

The compact obtained in this way is calcined after removing the binder. The firing is performed in a non-oxidizing atmosphere at 1450 to 2000 ° C., depending on the firing means. As the firing means, a nitrogen gas pressure firing method, a hot isostatic pressure firing method, or the like is preferable. At this time, in order to convert the chromium compound into a chromium silicide, 1450-1850
What is necessary is just to hold once at about ° C.

Further, the above-mentioned calcination may crystallize grain boundaries. Specifically, apatite (RE ₅ Si ₃ O ₁₂
N, RE ₁₀ Si ₇ O ₂₃ N ₄ , RE ₄ Si ₄ O ₉ N ₄ ),
YAM (RE ₄ Si ₂ O ₇ N ₂ ), wollastonite (R
Desirably, ESiO ₂ N) or the like is precipitated, and heat treatment may be performed in a non-oxidizing atmosphere at 1200 to 1600 ° C. to promote crystallization.

[0015] Thus, the sintered body of the present invention is mainly composed of β-silicon nitride crystals, chromium silicide is dispersed in the grain boundaries, and Si ₃ N ₄ is further dispersed in the grain boundaries. -R
E ₂ O ₃ -SiO ₂ system crystals having the structure precipitated.
At this time, the chromium silicide controls the particle diameter of the chromium compound and is preferably distributed in a size of 0.1 to 10 μm. These particle sizes can be controlled by the particle size of the raw material of the chromium compound.

According to the present invention, an oxide which is present at the grain boundary and easily forms a glass phase to enhance the crystallinity of the grain boundary in the sintered body, specifically, Al ₂ O ₃ , MgO, CaO , Fe ₂ O
It is desirable that the oxide such as ₃ is 0.05% by weight or less in the total amount of the sintered body. In addition, as the rare earth element oxide,
Y ₂ is O ₃ is _{generally, Yb 2 O 3, Er 2} O 3,
It is desirable to use heavy rare earth element oxides such as Ho ₂ O ₃ and Dy ₂ O _{3 in} that the occurrence of color unevenness and the like can be prevented and a sintered body having stable characteristics can be obtained.

[0017]

According to the present invention, Si ₃ N ₄ —RE ₂ O ₃ —S
It is important to add a chromium compound to the composition of iO ₂ . Conventionally, it is known that a ternary composition of Si ₃ N ₄ —RE ₂ O ₃ —SiO ₂ has excellent high-temperature properties exceeding 1200 ° C., but has poor oxidation resistance at 700 to 1000 ° C. This is considered to be because, in this temperature range, no protective film is formed on the surface of the sintered body, so that the grain boundary phase is destructively oxidized. In response to this problem, the present inventors
The problem was solved by adding a chromium compound. That is, the added chromium compound is converted into a silicide in the sintered body and remains at the grain boundary. Then, in an oxidizing atmosphere in a medium temperature range, the chromium silicide forms an amorphous protective film on the surface of the sintered body and prevents subsequent oxidation. Further, since the chromium silicide does not react with the grain boundaries even in a high temperature range, it does not deteriorate the high temperature characteristics.

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

[0019]

【Example】

Example 1 As a raw material powder, silicon nitride powder (BET specific surface area 5 m ²
/ G, pregelatinization rate 95%, impurity oxygen amount 1.0% by weight),
Various rare earth oxides having an average particle size of 1 μm and an average particle size of 1
Using various chromium compounds or SiO ₂ powders having a composition shown in Table 1 and mixing them,
Press molding was performed at t / cm ² .

The obtained compact was heated at 1600 ° C. under a nitrogen gas atmosphere of 50 atm in which SiO ₂ powder was placed in a furnace.
After holding for a time, it was further fired at 1900 ° C. for 1 hour.

Each sintered body was subjected to a four-point bending strength at room temperature and 1400 ° C. and an oxidation resistance test at 900 ° C. for 24 hours in accordance with JISR1601, and the increase in oxidized weight after the test was measured. The measurement results are shown in Table 1.

[0022]

[Table 1]

According to Table 1, Samples Nos. 13 and 14 to which no chromium compound was added and Sample No. 16 which had a small amount of chromium compound added had a high strength at 1400 ° C. Large weight increase. Sample Nos. 15 and 1 in which the amount of the chromium compound added exceeded 5 mol%.
In No. 7, the strength at 1400 ° C. was deteriorated. Furthermore, in Samples Nos. 18 to 23, in which the amount of rare earth elements in terms of oxide and the molar ratio of SiO ₂ / RE ₂ O ₃ are out of the range of the present invention, problems such as insufficient high-temperature strength and densification are not obtained. was there.

On the other hand, each of the sintered bodies of the present invention has a strength at 1400 ° C. of 600 MPa or more and 900 MPa or more.
It exhibited excellent properties with an increase in oxidation weight at 0.3 ° C. of 0.3 mg / cm ² or less.

In the table, when the X-ray diffraction measurement was performed on the sufficiently compacted sintered body, it was found that all the chromium compounds were present as silicides having an average particle size of 1 to 5 μm. It was confirmed. In the table, apatite was precipitated for Samples Nos. 1 to 17 and YAM phase was precipitated for Sample No. 20 as other crystal phases. No crystal phase was detected in sample No. 18.

[0026]

As described in detail above, according to the silicon nitride sintered body of the present invention, a chromium compound is added to a simple ternary system of Si ₃ N ₄ —RE ₂ O ₃ (rare earth element oxide) —SiO ₂ . By adding, it is possible to improve oxidation resistance in a medium temperature range while maintaining high-temperature strength at a high temperature of 1400 ° C.

Therefore, the application of the present invention to various high-temperature structural materials including heat engine parts such as turbo rotor and gas turbine parts can be further expanded.

Claims (1)

(57) [Claims] (1) 70 to 98 mol% of silicon nitride, 1.0 to 10 mol% of a rare earth element in terms of oxide, and excess oxygen
It contains chromium (Cr) in an amount of 0.01 to 5 mol% in terms of oxide in terms of iO ₂ equivalent of 25 mol% or less, and the excess oxygen in terms of SiO _{2 in} relation to oxide equivalent of rare earth element in terms of oxide. A silicon nitride-based sintered body characterized in that the molar ratio is greater than 0.7 and is 3 or less, and the chromium is dispersed as silicide.