JPH08283071A - Silicon nitride sintered body and its production - Google Patents

Abstract

PURPOSE: To produce a silicon nitride sintered body excellent in strength of a cast body and strength at high temp. by preparing a starting material slurry for casting from specified components at the time of producing a silicon nitride sintered body. CONSTITUTION: When a cast body made of a ceramic material cast in a prescribed shape is fired to obtain a silicon nitride sintered body, the cast body is formed by casting a slurry prepd. by kneading silicon nitride powder, AlN and oxide of a rare earth element as sintering aids and fibers as a shape retaining agent with an org. solvent not practically contg. water. The fibers are preferably silicon nitride fibers having Si-N bonds and/or Si-C bonds.

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 having high strength at high temperature and a method for producing the same.

[0002]

2. Description of the Related Art Since silicon nitride sintered bodies are superior in strength at high temperatures as compared with other ceramics, they are being developed as parts for automobile engines, rotors for gas turbines and the like.

On the other hand, since a silicon nitride-based sintered body is difficult to sinter, a dense sintering body is obtained by using a sintering aid such as Y ₂ O ₃ , Al ₂ O ₃ or MgO. . However,
If a sintering aid in the form of an oxide such as Y ₂ O ₃ , Al ₂ O ₃ or MgO is used, a glass phase with a low melting point remains at the grain boundaries of the silicon nitride sintered body and the high temperature strength tends to decrease. is there.

As a method for improving the strength at high temperature, the cooling rate after firing is slowed down so that XSi is added to the grain boundary phase of silicon nitride.
A method of precipitating a crystal such as O ₂ N (X is a group IIIa element) (Japanese Patent Laid-Open No. 62-100066), and an oxide component constituting a grain boundary phase is solid-dissolved in Si ₃ N ₄ to form a crystal lattice as a sialon. Incorporation method (Japanese Patent Laid-Open No. 4-46062), rare earth element oxide and HfO ₂ or the like IV as a firing aid
A method using a group a oxide (Japanese Patent Laid-Open No. 4-280871) has been proposed. However, high temperatures, especially 13
The strength and fracture toughness at 00 ° C or higher were not satisfactory.

Therefore, as a result of earnest research, the present inventor added AlN (aluminum nitride) and a rare earth element oxide as an auxiliary agent for sintering a silicon nitride-based material.
It was found that a silicon nitride-based sintered body excellent in both high temperature strength and fracture toughness can be manufactured.

On the other hand, in the case of a silicon nitride-based sintered body, an unsintered compact (green) is heated and sintered. As a method for molding this compact, press molding, extrusion molding, cast molding ( Slip cast) has been used conventionally.

Molded products obtained by press molding and extrusion molding are limited to simple shapes, and cannot be applied to complicated shapes such as turbine rotors.
In addition, cast molding is usually to prepare a slurry using water as a solvent, and to inject the slurry into a mold having excellent water absorption such as a gypsum mold to absorb the solvent into the mold to produce a molded body,
A molded product having a complicated shape can be obtained.

[0008]

As described above, cast molding is suitable for obtaining a molded body having a complicated shape, but when AlN is contained as a sintering aid,
Specific problems occur. That, AlN is highly reactive with water, the NH ₃ generated by reacting with water, pH of the slurry by the NH ₃ becomes abnormally high, not a slurry. Therefore, it is conceivable to prepare a slurry using an organic solvent. However, when an organic solvent is used, since the vapor pressure is generally higher than that of water, cracking may occur during drying, resulting in a high-density molded body. I can't. Therefore, it is not possible to manufacture a large-sized and complex shaped body.

[0009]

In order to solve the above problems, the present invention provides a silicon nitride powder and AlN as a sintering aid.
(Aluminum nitride) and a rare earth element oxide, and a silicon nitride fiber having a Si—N bond and / or a Si—C bond as a shape maintaining material are mixed with an organic solvent to prepare a slurry, and the slurry is absorbed by water. A molded body having a predetermined shape was produced by pouring the molded body into a mold having a good property, and the molded body was fired to obtain a silicon nitride-based sintered body.

Either α-type or β-type can be used as Si ₃ N ₄ , and the production method thereof is a direct nitriding method of Si, a reduction / nitridation method of silica, a thermal decomposition method of silicon diimide, and SiH ₄ + NH. There is a gas phase reaction method of ₃ + N ₂ and Si ₃ N ₄
The average particle size of the powder is about 3 to 0.01 μm.

As the rare earth element oxide, Ce
₂ O ₃ , Pr ₂ O ₃ , Nd ₂ O ₃ , Dy ₂ O ₃ , Ho ₂ O ₃ , Er
_{At least one of 2} O ₃ , Tm ₂ O ₃ , Yb ₂ O ₃ , Lu ₂ O ₃ and Y ₂ O ₃ .

As the silicon nitride fiber,
For example, the composition ratio is 50 to 70% by weight of Si, 10 to 4
Amorphous or crystalline fibers consisting of 0% by weight N, 20% by weight or less C and 5% by weight or less O are used. Specifically, polysilazane is a raw material of a silicon nitride fiber having a Si—N bond, and polycarbosilazane is a raw material of a silicon nitride fiber having a Si—C bond.

In order to obtain a silicon nitride fiber by ceramicizing a silicon-containing organic polymer such as polysilazane or polycarbosilazane, the silicon-containing organic silicon polymer is melt-spun, and N ₂ , NH ₃ , H ₂ , 800 in a vacuum, an inert atmosphere, or a combination of these
Heat treatment at ℃ or above. In particular, in the case of a polymer having a Si-C bond, heat treatment in ammonia gives a Si ₃ N ₄ -free fiber containing no C.

Polysilazane is cyclosilazane (R ₂ SiN
It can be synthesized from R) ₃ and chlorosilane (RnSiCl ₄ -n). However, R is H or an alkyl group, and n =
Is an integer of 1, 2, 3

First, hexamethylcyclosilazane (Me ₂ SiNH) ₃ is used as cyclosilazane, and trichloromethylsilane is mixed as chlorosilane. The mixing ratio of hexamethylcyclosilazane and trichloromethylsilane is 1: 1 to 1: 5, and preferably about 1: 3 in terms of molar ratio. With such a blending ratio, melt spinning is possible and polysilazane with a high ceramic yield can be obtained.

The above mixture of hexamethylcyclosilazane and trichloromethylsilane is heated to reflux at 190 to 195 ° C. As a result, hexamethylcyclosilazane is ring-opened and a chlorosilazane oligomer is produced. This process takes about 12 hours.

Furthermore, ammonia gas is blown into this solution to perform ammonolysis. The amount of ammonia gas blown in is about 70 liters / hour, and it is preferable to perform it for 3 to 4 hours. By this ammonolysis, the chlorosilazane oligomer becomes an aminosilazane oligomer.

Next, the aminosilazane oligomer is subjected to a deammonification step while being heated to about 250 to 400 ° C. in an atmosphere of nitrogen gas or the like to prepare a polysilazane exhibiting thermoplasticity. The softening point of polysilazane can be adjusted by heating conditions, but it is 50 for melt spinning.
It is preferably about 200 ° C.

The polysilazane thus obtained is melted and spun at a temperature above the softening point. At this time, the winding speed is preferably about 20 to 400 m / min, which makes the diameter 5 to 5 m.
A 30 μm fiber is obtained. By setting this to 800 ° C. or higher in the atmosphere as described above, the diameter is 1 to 20 μm.
m ceramic fibers are obtained.

The preferred diameter of the ceramic fiber is 3 to
10 μm, preferred length is 10-500 μm, especially 1
The optimum value is 00 to 300 μm. If the average diameter and the fiber length are too large, the dispersibility may decrease, and the baked product may have defects or the density may decrease. On the contrary, if the average diameter and the fiber length are too small, the reinforcing effect cannot be sufficiently exhibited.

As the organic solvent, alcohols such as ethanol, hydrocarbons such as toluene, esters such as ethyl acetate, and ketones such as acetone can be used.

Further, as a ratio of components constituting the molded body, the ratio of AlN is 0.5% by weight to 10% by weight, the ratio of rare earth element oxide is 2% by weight to 20% by weight, and the ratio of fibers is 0.1% by weight. It is preferable that the content of the organic solvent is 01 to 15% by weight, and the ratio of the organic solvent is 35 to 150 parts by weight with respect to all the powder components. If the proportion of AlN added is less than 0.5% by weight, a dense fired body cannot be obtained, and if it exceeds 10% by weight, the strength is lowered even if the Al element is added in the form of a nitride. If the addition ratio of the rare earth element oxide is less than 2% by weight, a dense fired body cannot be obtained, or even if it becomes dense to some extent, sufficient strength cannot be obtained. Too much, a dense fired body cannot be obtained, or sufficient strength cannot be obtained even if it becomes dense to some extent. Further, if the addition ratio of the fiber is less than 0.01% by weight, a sufficient reinforcing effect cannot be obtained, and if it exceeds 15% by weight, the dispersibility of the fiber is deteriorated and the portion where a large amount of the fiber exists is sintered. When it becomes a defect, the strength decreases.

[0023]

By using both AlN and a rare earth element oxide as a sintering aid, a silicon nitride sintered body having excellent strength and toughness at high temperature can be obtained. Also, by mixing the silicon nitride fiber in the slurry when manufacturing the molded body before becoming a sintered body by cast molding, shrinkage of the molded body during drying is reduced, and the mechanical properties of the molded body are reduced. It is possible to improve strength and strain resistance.

In the firing step, the silicon nitride fiber melts and fluidizes to lose its fiber shape, and is incorporated into the matrix to obtain a silicon nitride sintered body having a uniform structure.

[0025]

EXAMPLES Examples of the present invention and comparative examples will be described below.

(Example) Si ₃ N ₄ , AlN, Yb ₂ O ₃ , Al
Each powder of ₂ O ₃ was mixed with water or ethanol as a solvent at a ratio shown in (Table 1) below for 16 hours to form a slurry. Then, silicon nitride fibers were added to this slurry at a ratio shown in (Table 1), mixed for 2 hours, cast-molded in a plaster mold, and after 3 hours, demolded to obtain 10 × 10 × 60 m.
m rod-shaped molded body and a turbo rotor were obtained. After that, the compact was put into a BN crucible filled with Si ₃ N ₄ and a powdery mixture of BN of 1: 1. The BN crucible was further put into a carbon crucible, and the maximum temperature was 1850 ° C. and the maximum pressure was 1000 kg / cm ² . HIPed in ₂ atmospheres.

(Comparative Example) As shown in (Table 1), a molded product having the same shape as that of (Example) was obtained without adding AlN, without adding a silicon nitride fiber, or using water as a solvent. I did it. After this, HIP treatment was performed under the same conditions as in (Example).

The strength of the molded body (green) obtained in the above (Example) and (Comparative example) and 1 after firing the molded body
The strength at 400 ° C. was measured. The results are shown in (Table 1). The strength of the compact was measured 24 hours after release from the rod-shaped compact by three-point bending with a span of 50 mm. The strength of the sintered compact was measured from a turbo rotor with a test piece of 3 × 3 × 40 mm. Cut out, JI
Three-point bending according to SR1601 was performed at 1400 ° C.

[0029]

[Table 1]

From Table 1 it can be seen that the silicon nitride sintered body according to the present invention is excellent in both the strength of the molded body and the strength at high temperature (1400 ° C.). On the other hand, in the case of using water as a solvent and not adding the silicon nitride-based fiber, it is not possible to prepare a molded product without making it into a slurry, and even if the silicon nitride-based fiber is added, Al
When N is not added, the strength at high temperature (1400 ° C) is poor,
Further, even when both AlN and Y ₂ O ₃ were added, satisfactory results could not be obtained both in the strength of the molded body and the high temperature strength when the silicon nitride fiber was not added.

[0031]

According to the present invention as described above,
In preparing the slurry for casting, fibers such as silicon nitride having Si-N bonds and / or Si-C bonds are uniformly mixed in the slurry, so that the organic substance substantially free of water is used. Even if a molded body is produced using a solvent, the strength of the molded body can be maintained high. Further, since water is not contained in the slurry, the pH of the slurry does not become abnormally high even if AlN is used as a firing aid, and it is possible to produce a large-sized molded article having a complicated shape. By using AlN and a rare earth element oxide as the agent, a silicon nitride-based sintered body having excellent strength at high temperature can be obtained.

[Procedure amendment]

[Submission date] May 31, 1995

[Procedure Amendment 1]

[Document name to be amended] Statement

[Correction target item name] 0026

[Correction method] Change

[Correction content]

(Example) Powders of Si ₃ N ₄ , AlN and Y ₂ O ₃ were mixed with water or ethanol as a solvent at a ratio shown in the following (Table 1) for 16 hours to form a slurry. Then, silicon nitride fibers were added to this slurry at a ratio shown in (Table 1), mixed for 2 hours, and cast into a gypsum mold,
After 3 hours, the mold was released to obtain a 10 × 10 × 60 mm rod-shaped compact and a turbo rotor. Then, the compact was put into a BN crucible filled with Si ₃ N ₄ and a BN powder having a ratio of 1: 1.
Put the N crucible in the carbon crucible further and the maximum temperature is 1850.
HI in a N ₂ atmosphere with a maximum pressure of 1000 kg / cm ²
P treated.

[Procedure Amendment 2]

[Document name to be amended] Statement

[Correction target item name] 0028

[Correction method] Change

[Correction content]

The strength of the molded body (green) obtained in the above (Example) and (Comparative example) and 1 after firing the molded body
The strength at 400 ° C. was measured. The results are shown in (Table 1). The strength of the compact was measured 24 hours after release from the rod-shaped compact by three-point bending with a span of 50 mm, and the strength of the sintered compact was measured with a test piece of 3 × 4 × 40 mm from a turbo rotor. Cut out, JI
Three-point bending according to SR1601 was performed at 1400 ° C.

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical display location C04B 35/64 302Z (72) Inventor Yuji Miki 1-4-1 Chuo, Wako-shi, Saitama Stock Company Honda Technical Research Institute

Claims (7)

[Claims] 1. A silicon nitride-based sintered body obtained by firing a molded body made of a ceramic material molded into a predetermined shape, the molded body comprising silicon nitride-based powder and AlN (sintering aid). Aluminum nitride) and a rare earth element oxide, and a fiber formed as a shape-maintaining material are kneaded in an organic solvent substantially free of water to form a slurry, which is formed by a casting method. Sintered body. 2. The silicon nitride-based sintered body according to claim 1, wherein the fiber has a Si—N bond and / or a Si—N bond.
A silicon nitride-based sintered body, which is a silicon nitride-based fiber having a C bond. 3. The silicon nitride-based sintered body according to claim 1 or 2, wherein the rare earth element oxide is Ce _{2
O 3, Pr 2 O 3,} Nd 2 O 3, Dy 2 O 3, Ho 2 O 3, Er 2 O 3,
A silicon nitride-based sintered body characterized by being at least one of Tm ₂ O ₃ , Yb ₂ O ₃ , Lu ₂ O ₃ and Y ₂ O ₃ . 4. The silicon nitride-based sintered body according to claim 1, wherein the proportion of AlN in the compact formed by the casting method is 0.5% by weight or more and 10% by weight.
Below, the proportion of the rare earth element oxide is 2% by weight or more and 20% by weight or less, the proportion of the fiber is 0.01% by weight or more and 15% by weight or less, and the proportion of the organic solvent is 35 parts by weight or more and 150% by weight based on all the powder components. A silicon nitride-based sintered body characterized by being no more than 10 parts. 5. The silicon nitride-based sintered body according to claim 1, wherein the silicon nitride-based fiber is S.
i-N bond and / or melt-spinning an organosilicon polymer having a Si-C bond, which in _{N 2, NH 3, H 2} , a vacuum or inert atmosphere or in an atmosphere in which these combinations 8
A silicon nitride-based sintered body characterized by being heat-treated at a temperature of 00 ° C. or higher. 6. A slurry is prepared by uniformly dispersing silicon nitride-based powder, AlN (aluminum nitride) and a rare earth element oxide, and fibers as a shape-maintaining material in an organic solvent. A method for producing a silicon nitride-based sintered body, characterized in that a cast molded body is manufactured in step 1, and then the molded body is subjected to HIP treatment. 7. The method for producing a silicon nitride based sintered body according to claim 6, wherein the fiber is a silicon nitride based fiber having Si—N bond and / or Si—C bond. A method for manufacturing a silicon-based sintered body.