JP2742622B2 - Silicon nitride sintered body and method for producing the same - Google Patents

Description

Description: FIELD OF THE INVENTION The present invention relates to a silicon nitride sintered body which is excellent in flexural strength and oxidation resistance at high temperatures and is particularly used for heat engines such as gas turbines and the production thereof. About the method.

(Prior Art) Conventionally, a silicon nitride sintered body has a high strength at a high temperature,
Engineering ceramics as a material having excellent hardness and thermal and chemical stability are being applied, particularly for heat engines. When applied to these uses, the sintered body is required to have excellent mechanical properties from room temperature to a high temperature of 1400 ° C., but recently, improvement of the properties at 1500 ° C. is also desired.

Generally, as a method for producing a silicon nitride-based sintered body, a sintering aid is added due to its difficulty in sintering itself, and a sintering method such as a hot press method, a normal pressure sintering method, or a gas pressure sintering method is used. By baking in a non-oxidizing atmosphere.

On the other hand, from the viewpoint of composition, rare earth oxides, alumina, magnesia, etc. are most commonly used as sintering aids. These oxides do not contain Si ₃ N ₄ —RE ₂ O ₃ (RE: rare earth elements) —SiO ₂
The composition of the original system is being studied. Particularly, in this system, in order to improve the strength of the grain boundary phase in the sintered body, a high melting point crystal phase such as melilite, apatite, YA
Attempts have been made to precipitate M, wollastonite and the like.

(Problems to be Solved by the Invention) However, when melilite is precipitated in the grain boundary phase, the high-temperature strength in an inert atmosphere is excellent, but the stability in an oxidizing atmosphere which is a practical condition is poor, and the strength changes due to a volume change due to oxidation. Is deteriorated. Also, apatite,
In the present condition, wollastonite or YAM precipitates have a slightly improved stability in an oxidizing atmosphere as compared with those in which melilite precipitates, but their oxidation resistance at 1500 ° C. is greatly reduced, so that they can hardly be used.

(Object of the Invention) An object of the present invention is to provide a silicon nitride-based sintered body having excellent high-temperature strength and excellent oxidation resistance at 1500 ° C. and a method for producing the same.

(Means for solving the problem) The present inventors have solved the above problem by using Si ₃ N ₄ -RE ₂ O ₃ (RE:
As a result of repeated studies on a simple ternary system of (rare earth element) -SiO ₂ , sintering containing a large amount of SiO ₂ component and silicon oxynitride (Si ₂ N ₂ O) at a specific ratio in the grain boundary phase The body was found to have excellent high temperature properties and oxidation resistance at 1500 ° C.

That is, the silicon nitride-based sintered body of the present invention is composed of 0.5 to 2 mol% of rare earth element oxide, 10 to 25 mol% of excess oxygen (in terms of SiO ₂ ), the balance being β-type silicon nitride, and / A sintered body having a molar ratio represented by a rare earth element oxide in the range of 5 to 25, wherein a crystal phase composed of silicon oxynitride (Si ₂ N ₂ O) is formed at a grain boundary of the sintered body. The peak intensity of the (111) plane of β-type silicon nitride (β-Si ₃ N ₄ ) in the X-ray diffraction curve is represented by h ₁ , and the silicon oxynitride (Si ₂ N
When the peak intensity of the (111) plane of the ₂ O) was h _2, the peak intensity ratio h ₂ / h ₁ is from 1.5 to 1.96, and room-temperature strength is more than 800 MPa, 1400 ° C. strength having the above characteristics 500MPa It is characterized by the following.

In addition, as a manufacturing method, rare earth element oxide is 0.5 to 2 mol%, excess oxygen (SiO ₂ conversion) is 10 to 25 mol%, and the remainder is made of silicon nitride, and is represented by excess oxygen / rare earth element oxide. The molded body having a molar ratio of 5 to 25 is heated to 1800 to 2000 ° C.
It is fired in a nitrogen gas atmosphere containing SiO, and the peak intensity of the (111) plane of β-type silicon nitride (β-Si ₃ N ₄ ) in the X-ray diffraction curve is h ₁ , and the silicon oxynitride (Si ₂ N ₂ when the peak intensity of the (111) plane of the O) was h _2, the peak intensity ratio h ₂ /
h ₁ is characterized in that to produce a sintered body of from 1.5 to 1.96.

 Hereinafter, the present invention will be described in detail.

The sintered body of the present invention, the composition of which is rare earth element oxide 0.5
2 to 2 mol%, especially 1 to 1.5 mol%, excess oxygen (in terms of SiO ₂ ) 10 to 25 mol%, particularly 10 to 15 mol%, the balance being β-type silicon nitride, and excess oxygen / rare earth element oxidation The molar ratio represented by the product is in the range of 5 to 25, particularly 7 to 15. The excess oxygen is the amount of oxygen obtained by subtracting oxygen mixed in a stoichiometric amount as a rare earth element oxide from the total amount of oxygen contained in the entire system of the sintered body, and specifically, the amount of impurities in the silicon nitride raw material. It is composed of oxygen or oxygen in added SiO ₂ , and in the present invention, each of them indicates the amount in terms of SiO ₂ .

In the present invention, the composition of the sintered body is limited to the above-described ratio, which is an important factor for obtaining excellent properties. If the content of the rare-earth element oxide is less than 0.5 mol%, the sintering method described later will be omitted. It is difficult to densify the aggregate,
If the amount exceeds mol%, the oxidation resistance at 1500 ° C. is deteriorated.
Also, if the excess oxygen is less than 10 mol% or exceeds 25 mol%, it becomes difficult to densify. Further, if the molar ratio of excess oxygen / rare earth element oxide is less than 5, the oxidation resistance at high temperatures tends to deteriorate, and if it exceeds 25, glass with a low melting point tends to be formed, and the high-temperature characteristics deteriorate.

Further, the sintered body of the present invention is structurally composed of silicon nitride crystal grains and a grain boundary phase, and silicon oxynitride crystals represented by Si ₂ N ₂ O precipitate in the grain boundary phase, and the amount thereof is reduced. In the X-ray diffraction curve of the sintered body, the peak intensity of the (111) plane of β-type silicon nitride is set to h ₁ , and the peak intensity of (11) of silicon oxynitride is set to (11).
1) when the peak intensity of the face was h _2, the peak intensity ratio h ₂ / h ₁ is
1.5 to 1.96, particularly 1.6 to 1.96. Thereby, high-temperature strength and oxidation resistance at a high temperature of 1500 ° C. can be improved. The reason why the peak intensity ratio represented by h ₁ / h ₂ is limited to the above range is that if the peak intensity ratio is lower than 1.5, the oxidation resistance at 1500 ° C. is deteriorated. On the other hand, if the peak intensity ratio is larger than 1.96, the absolute amount of silicon nitride decreases, so that the intended intensity characteristics cannot be obtained.

According to the method for producing a sintered body of the present invention, silicon nitride, a rare earth element oxide and silicon oxide are used as raw materials, and are prepared in the above-described case. At this time, it is assumed that the impurity oxygen in the silicon nitride powder is present as silicon oxide in the silicon oxide, and the amount of the oxygen converted into SiO ₂ is also included. The silicon nitride used preferably 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 amount of impurity oxygen is suitably 0.8 to 1.5% by weight. On the other hand, a rare earth element oxide or silicon oxide has a BET specific surface area of preferably ^{1 to} 10 m ² / g.

After the prepared powder is sufficiently mixed, a binder is appropriately added, and the mixture is granulated and then molded. The molding is performed into a desired shape by a known method, for example, press molding, extrusion molding, injection molding, casting molding, or the like. The molded body thus obtained is baked after removing the binder.

The firing is performed in a non-oxidizing atmosphere at 1800 to 2000 ° C, particularly 1800 to 1900 ° C. According to the present invention, since the composition of the compact contains a large amount of low-melting-point silicon oxide, it is baked while containing SiO in the atmosphere to suppress fluctuations in the composition. In order to suppress the decomposition of silicon nitride due to high-temperature firing, it is also important to introduce a nitrogen gas at a temperature higher than the decomposition equilibrium pressure of silicon nitride at the firing temperature. SiO in the atmosphere can be generated by arranging SiO ₂ powder or the like together with the compact in a firing furnace.

During this firing process, rare earth element oxides
A liquid phase composed of silicon nitride and silicon oxide is generated, and sintering proceeds and densification occurs. For example, the grain boundary phase is crystallized in a cooling process after firing for about 4 to 10 hours. According to the present invention, a desired amount of silicon oxynitride is precipitated in the grain boundary phase by slow cooling at about 300 ° C./Hr or less, since the grain boundary tends to be amorphous if the cooling rate at that time is high. be able to. On the other hand, α-type silicon nitride undergoes phase transformation into β-type silicon nitride to form needle-like crystals, and finally a fibrous structure having an average aspect ratio of 3 or more is formed, thereby increasing the strength and toughness of the sintered body. Is achieved.

As the rare earth element oxide described above, Y ₂ O ₃ is the most common, but heavy rare earth element oxides such as Yb ₂ O ₃ , Er ₂ O ₃ , Ho ₂ O ₃ , and Dy ₂ O ₃ are used. It is desirable because the stability of the characteristics of the sintered body and the high characteristics can be obtained.

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

(Example) Silicon nitride powder (BET specific surface area 5 m ² / g,
Using an oxide ratio of 95% and an impurity oxygen amount of 1.0% by weight) and various rare earth element oxides or silicon oxide powders, they were mixed and mixed so as to have the composition shown in Table 1, and then press-molded at 1 t / cm ² .

The obtained compact was fired under the firing conditions shown in Table 1 in a 50 aatm nitrogen gas atmosphere in which a SiO ₂ powder was placed in a furnace.

Next, the obtained sintered body was subjected to the Archimedian method for the theoretical density ratio, the 4-point bending strength at room temperature and 1400 ° C according to JISR1601, and the oxidation resistance test at 1500 ° C × 24 hours. Was measured for the increase in oxidation weight.

Further, from the X-ray diffraction curve of the sintered body,
The peak intensity h ₁ of the (111) plane existing near θ = 39 ° and S
The peak intensity h ₂ of the (111) plane existing near 2θ = 26.6 ° of i ₂ N ₂ O was measured, and the peak intensity ratio h ₂ / h ₁ was calculated.

 The results are shown in Table 1.

In Table 1, the X-ray diffraction curve of Sample No. 1 is shown in FIG.

According to Table 1, Samples 18 and 19 in which the amount of excess oxygen exceeds 25 mol% or the amount of rare earth element oxide is small and the ratio of excess oxygen to rare earth element oxide exceeds 25 are both samples.
The formation of crystals of Si ₂ N ₂ O was often observed, but the sinterability was poor and the high temperature strength was greatly deteriorated. In Sample 20, in which the ratio of excess oxygen to the rare earth oxide was lower than 5, the generation of crystals of Si ₂ N ₂ O was small, and the oxidation resistance at 1500 ° C. was deteriorated. Furthermore, Sample 21 with a low firing temperature and Sample 23 with a fast cooling rate showed little generation of Si ₂ N ₂ O crystals, and deteriorated at 1500 ° C. oxidation resistance.

Samples 24 and 2 in which apatite and YAM were precipitated in the grain boundary phase
In the case of 5, the oxidation resistance at 1500 ° C was insufficient.

On the other hand, in all of the samples of the present invention, generation of crystals of Si ₂ N ₂ O was observed, and room temperature strength of 800 MPa
Above, 1400 ° C strength 500MPa or more, oxidation weight increase at 1500 ° C is 0.
It exhibited excellent properties of 15 or less.

(Effects of the Invention) As described in detail above, the present invention relates to a Si containing a large amount of SiO _2
3 N ₄ -RE ₂ O _3: by precipitating the silicon oxynitride in a specific ratio in the grain boundary in the (RE rare earth element) simple ternary -SiO _2, the oxidation resistance and high-temperature strength at 1500 ° C. An excellent sintered body can be obtained.

This makes it possible to further expand the application of the silicon nitride-based sintered body as a heat engine component, and to provide a material that can sufficiently cope with a particularly high operating temperature of the heat engine.

[Brief description of the drawings]

FIG. 1 is a chart of an X-ray diffraction curve of the silicon nitride sintered body of the present invention.

Claims (2)

(57) [Claims] 1. A rare earth element oxide of 0.5 to 2 mol%, excess oxygen (SiO ₂ equivalent) of 10 to 25 mol%, and a balance of β-type silicon nitride, which is represented by excess oxygen / rare earth element oxide. A sintered body having a molar ratio in the range of 5 to 25, wherein a crystal phase composed of silicon oxynitride (Si ₂ N ₂ O) exists at the grain boundary of the sintered body, The peak intensity of the (111) plane of the β-type silicon nitride (β-Si ₃ N ₄ ) was h ₁ , and the peak intensity of the (111) plane of the silicon oxynitride (Si ₂ N ₂ O) was h ₂ . when the peak intensity ratio h ₂ / h ₁ is 1.5 to 1.
96, and room temperature strength of 800 MPa or more, 1400 ° C strength of 50
A silicon nitride based sintered body having characteristics of 0 MPa or more. 2. A rare earth element oxide of 0.5 to 2 mol%, an excess of oxygen (in terms of SiO ₂ ) of 10 to 25 mol%, a balance of silicon nitride, and a molar ratio represented by excess oxygen / rare earth element oxide The compact having a ratio of 5 to 25 is fired in a nitrogen gas atmosphere containing SiO at 1800 to 2000 ° C., and the (111) plane of β-type silicon nitride (β-Si ₃ N ₄ ) in the X-ray diffraction curve is obtained. The peak intensity is h ₁ ,
When the peak intensity of the (111) plane of silicon oxynitride (Si ₂ N ₂ O) is h ₂ , the peak intensity ratio h ₂ / h ₁ is 1.5 to ₁ .
A method for producing a silicon nitride-based sintered body, comprising producing 96 sintered bodies.