JP2710865B2 - Manufacturing method of 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 method for producing a silicon nitride sintered body having excellent strength at room temperature and high temperature and excellent creep characteristics at high temperature, which is used as a component for a heat engine structure such as a gas turbine.

[0002]

2. Description of the Related Art Conventionally, silicon nitride-based sintered bodies have attracted attention as materials having excellent strength, hardness and thermochemical stability at high temperatures, and have been applied as engineering ceramics, particularly as structural materials for heat engines. I have.

In general, since silicon nitride itself is difficult to sinter, it is necessary to add a rare earth element oxide such as Y ₂ O ₃ and an Al ₂ O ₃ as a sintering aid. .

[0004] Also, when used as a structural material for heat engines such as turbo rotors and gas turbine rotors, silicon nitride based sintered bodies have high flexural strength at high temperatures, excellent oxidation resistance, and low room temperature. It is required that the deterioration of strength from low to high temperature is small.

[0005] Such a silicon nitride-based sintered body is usually formed by molding a mixture obtained by adding a sintering aid as described above to silicon nitride powder into a desired shape, and then forming a nitrogen-containing non-oxidizing atmosphere. Among them, densification is achieved by a method such as normal pressure firing, hot press firing, nitrogen gas pressure firing, hot isostatic pressure firing, and the like. Of these, nitrogen gas pressurized firing and hot isostatic firing are being widely used as firing methods suitable for obtaining high-strength sintered bodies of complicated shapes such as heat engine parts.

[0006] In addition, silicon nitride, which contains a large amount of α-type silicon nitride as a raw material powder to be used in view of its sinterability, has a high sinterability and a sintered body having a high strength can be obtained.
% Or more is generally used.

[0007]

Problems to be Solved by the Invention Silicon nitride-based sintered bodies have hitherto been examined with respect to sintering additives to be added and sintering conditions, but most of them are at room temperature and at high temperatures. It has been studied for the purpose of improving strength and oxidation resistance. However, it is known that a silicon nitride-based sintered body itself has a property (creep property) that the sintered body exhibits plastic deformation when held for a long time in a state where a load is applied at a high temperature. For heat engine construction,
A material having small plastic deformation in such creep characteristics is required.

However, the creep characteristics have not been studied much and there is no concrete solution at present. For example, when sintering is performed using a raw material containing a large amount of α-type silicon nitride as in the past, silicon nitride particles grow as the transition from α-type to β-type silicon nitride occurs during the sintering process, and the particle size increases. There is a problem in that particles are generated and become non-uniform in structure, whereby the creep characteristics are easily deteriorated.

[0009]

[Means for Solving the Problems] The present inventors have studied the creep characteristics and found that Yb ₂ O ₃ and Er ₂ O ₃ were used as sintering aids, and β-type silicon nitride was used. By adding and mixing to a relatively large amount of silicon nitride powder and firing it in a high-pressure nitrogen gas atmosphere, a sintered body with excellent strength and oxidation resistance at high temperatures and excellent creep resistance can be obtained. This led to the present invention.

That is, according to the method for producing a silicon nitride sintered body of the present invention, Yb ₂ O ₃ and / or Er ₂ O ₃ are added to silicon nitride powder containing β-type silicon nitride of 30% or more and mixed. After molding into a shape, nitrogen gas pressure 30 ~ 100at
m in a non-oxidizing atmosphere at 1800 to 2100 ° C.
It is characterized by hot isostatic firing at 1500 to 2000 ° C. at 00 to 2000 atm.

[0011] Silicon nitride is roughly classified into two types, α-type and β-type, based on its crystal structure.
In view of the fact that the creep characteristics at high temperatures change depending on the ratio in the raw materials used, a major feature is that a raw material containing 30% or more, particularly 40 to 70%, of β-type silicon nitride is used as the silicon nitride raw material powder.

The reason that the amount of β-type silicon nitride in the silicon nitride raw material is set in the above range is that if the amount is less than 30%, the creep resistance characteristics, particularly the creep characteristics at 1400 ° C., are greatly deteriorated. . It is known that silicon nitride powder usually contains oxygen. This oxygen contributes to sinterability, and the amount is 0.8 to 1.5% by weight. Is desirable, and the average particle size is 0.3 to
Those having a diameter of about 0.6 μm are preferably used.

[0013] Further, it is added and mixed with the silicon nitride powder.
Y as a sintering aid_TwoO_ThreeAnd other periodic tables
Al in addition to Group 3a element oxides_TwoO_ThreeKnow various things
According to the present invention, among these, Yb_Two
O_ThreeAnd / or Er_TwoO _ThreeAdd as an essential component
You. These sintering aids are used in an amount of 0.5 to
It is desirable to add at a ratio of 3.5 mol%.

As for the composition, silicon oxide can be added to the above-mentioned system of silicon nitride powder and Yb ₂ O ₃ and / or Er ₂ O ₃ powder. This silicon oxide, including amounts SiO ₂ by converting the oxygen content of the silicon nitride raw material powder, it is desirable to add such a ratio of less than 10 mol%, particularly the Yb ₂ O and the amount of SiO _{2 3} , the molar ratio (SiO ₂ / RE ₂ O) to the oxide of a group 3a element of the periodic table containing Er ₂ O ₃ (RE ₂ O ₃ )
₃ ) is preferably 1 to 2.

These raw material powders are weighed and mixed at the above-mentioned predetermined ratio, and then mixed to a predetermined shape by a known molding means, for example, a method such as press molding, injection molding, extrusion molding, casting molding, or cold isostatic pressing. After forming into a shape, it is fired.

Next, the compact is subjected to a nitrogen gas pressure of 30 to 30.
The firing is performed under a high pressure atmosphere of 100 atm, particularly 30 to 70 atm. The reason why the nitrogen gas pressure in this firing is limited to the above range is that if the pressure is 30 atm or less, the high temperature strength does not improve, and if it exceeds 100 atm, the sample becomes rough. In addition, the firing temperature is caused by the fact that the raw material powder of the present invention contains a large amount of β-type silicon nitride, and it is difficult to densify at a temperature lower than 1800 ° C. On the other hand, if the firing temperature is higher than 2100 ° C., the silicon nitride itself is decomposed, and the surface of the sintered body is roughened and the strength is deteriorated. Therefore, the sintering temperature is 1800 to 2100 ° C., particularly 1900
Set to ~ 2000 ° C.

According to this firing, the ratio of the theoretical density to 95%
According to the present invention, the sintered body obtained by the above-mentioned sintering can be further subjected to a nitrogen gas pressure of 500 to 2000 atm under a high-pressure atmosphere.
By performing hot isostatic firing at a temperature of 500 to 1900 ° C., higher densification can be achieved, and strength and creep resistance at high temperatures can be improved.

Further, in some cases, the sintered body obtained by the above method may be appropriately treated in a non-oxidizing atmosphere at 1400 to 1800 ° C. to improve crystallization of grain boundaries and improve high-temperature characteristics. it can.

[0019]

The creep characteristics are greatly affected by the homogeneity of the structure of the sintered body. In particular, if large particles are non-uniformly present in the structure due to grain growth, the creep characteristics are greatly deteriorated.

According to the present invention, by using a silicon nitride raw material powder containing a large amount of β-type silicon nitride as the raw material powder, the grain growth due to α-β transition is reduced in the firing process, and the silicon nitride particles are uniformly formed. Since a uniform structure is formed, high-temperature creep characteristics can be greatly improved.

In addition, Yb ₂ O contained as a sintering aid
_3, Er ₂ O ₃ is mostly present in the grain boundaries in the sintered body, these oxides are commonly used Y
_Since the viscosity of the grain boundary at a high temperature can be increased as compared with ₂ O ₃ , the effect of suppressing the slip of the silicon nitride particles can be obtained, and the high temperature creep characteristics can be also improved.

Further, the nitrogen gas pressure during firing is 30 atm.
The reason described above is because N ₂ dissolves in the silicon nitride grain boundary phase and increases the viscosity of the grain boundary phase at high temperatures, thereby leading to improvement in high temperature creep characteristics and high temperature strength.

[0023]

EXAMPLES As silicon nitride raw material powders, raw materials having different contents of α-type and β-type silicon nitride raw materials were prepared. All of these raw materials have an oxygen content of 1.0 to 1.2% by weight,
The average particle size is 0.4 to 0.6 μm.

The silicon nitride raw material has an average particle size of 0.
Powders of 5 to 0.6 μm of Yb ₂ O ₃ and Er ₂ O ₃ , and for comparison, Y ₂ O ₃ powder were weighed and mixed in proportions shown in Tables 1 and 2.

After appropriately adding a binder to this mixed powder,
Press molding was performed under a pressure of 1 ton / cm ² . Next, this molded body was fired under the firing conditions shown in Tables 1 and 2 to obtain various sintered bodies.

In Tables 1 and 2, samples No. 11, 11, 1
5, 16 and 21 were further subjected to hot isostatic firing under the conditions shown in Table 3.

[0027]

[Table 1]

[0028]

[Table 2]

[0029]

[Table 3]

For each of the obtained sintered bodies, JISR16
Based on No. 01, the four-point bending strength at 1400 ° C. and the increase in oxidized weight after being kept in an oxidizing atmosphere at 1400 ° C. for 24 hours were measured, and the maximum load that could be held at 1400 ° C. for 1 hour was determined. The results are shown in Tables 4 and 5.

[0031]

[Table 4]

[0032]

[Table 5]

According to the results shown in Tables 1 to 5, β-
Sample No. 1 containing less than 30% of Si ₃ N ₄ exhibited room temperature strength and high temperature strength to some extent, but had low creep characteristics in terms of characteristics. Also,
In samples No. 3 and 3 having a firing pressure lower than 30 atm and samples No. 4 having a firing temperature higher than 2100 ° C., the surface of the sintered body had surface roughness which was considered to be due to decomposition of silicon nitride. When the firing temperature is 1750 ° C., sintering cannot be performed, and the pressure of the sample No. 14 is as high as 200 atm.
However, the surface of the sintered body was rough.

In Sample No. 20 using Y ₂ O ₃ as a sintering aid, even when a raw material containing β-Si ₃ N ₄ was used, stains were observed on the surface of the sintered body, and the characteristic Was also low.

On the other hand, the sample of the present invention fired at a predetermined high temperature and high pressure using a raw material containing high β-Si ₃ N ₄
All had excellent properties in room temperature strength, high temperature strength and high temperature creep properties. Furthermore, the properties could be improved by hot isostatic firing of the sintered body.

[0036]

As described in detail above, according to the method for producing a silicon nitride-based sintered body of the present invention, a specific sintering aid is added to silicon nitride powder containing high β-type silicon nitride. By baking under high pressure, a sintered body having excellent high-temperature strength and high-temperature oxidation resistance and excellent high-temperature creep resistance can be obtained. As a result, excellent durability is exhibited even under operating conditions in which the operating temperature of the heat engine material for a gas turbine or the like exceeds 1400 ° C., and practical use can be further promoted.

Claims (2)

(57) [Claims] 1. A method in which Yb ₂ O ₃ and / or Er ₂ O ₃ is added to silicon nitride powder containing 30% or more of β-type silicon nitride and mixed to form a predetermined shape.
A method for producing a silicon nitride-based sintered body, characterized in that firing is performed in a non-oxidizing atmosphere at 1800 to 2100 ° C. at 00 atm. 2. A silicon nitride powder containing 30% or more of β-type silicon nitride is mixed with an oxide of an element belonging to Group 3a of the periodic table, and molded into a predetermined shape.
m in a non-oxidizing atmosphere at 1800 to 2100 ° C., and then 1500 to 20 at 500 to 2000 atm.
A method for producing a silicon nitride-based sintered body, characterized by performing hot isostatic firing at 00 ° C.