JP3127225B2 - Low temperature sintering aid for silicon nitride and sintering method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a sintering aid and a sintering method for microwave sintering of silicon nitride ceramics. The present invention relates to a sintering aid for microwave sintering of silicon nitride ceramics having an action of promoting the growth of silicon columnar crystals, and a method of sintering silicon nitride ceramics at a low temperature using the sintering aid. INDUSTRIAL APPLICABILITY The present invention is useful as a method for producing a silicon nitride ceramics by microwave sintering which is a member for a mechanical structure requiring strength and toughness such as a heat engine such as a gasoline engine or a diesel engine or a drawing die. The present invention provides a simple and economical method for increasing the toughness of silicon nitride ceramics, which is simpler and more economical than conventional gas pressure sintering and seed crystal addition methods, which are methods for increasing the toughness of silicon nitride ceramics. Things.

[0002]

2. Description of the Related Art Engineering ceramics are used as high-temperature structural materials and sliding members from the viewpoint of heat resistance and corrosion resistance. In order to spread engineering ceramics more widely and as a general material for mechanical structures, it is necessary to reduce manufacturing costs and improve characteristics by controlling microstructure. Conventionally, sintering of toughened silicon nitride ceramics is performed by adding a sintering aid composed of a rare earth oxide such as aluminum oxide and yttrium oxide to silicon nitride powder, and heating the mixture to 1850 ° C. or more under a nitrogen atmosphere of 10 atm or more. Sintered at temperatures.

In general, in order to increase the fracture toughness of silicon nitride ceramics, a high nitrogen gas pressure of 100 atm or more is applied to suppress the decomposition of silicon nitride while sintering at a high temperature of 1900 ° C. or more. A gas pressure sintering method for forming a structure composed of developed silicon nitride crystals and a seed crystal addition method for sintering by adding a silicon nitride single crystal of 1 to 2 μm to fine silicon nitride powder are known.

For example, Am. Ceram. Soc. Bull., 65 [9] 13
In 11-1315 (1986), alumina-rare earth oxide was added as a sintering aid to α-Si ₃ N ₄
By sintering at 2000 ° C. in nitrogen at 0 atm, a silicon nitride sintered body having a fracture toughness of about 9 MPa · m ^1/2 can be obtained. J. Am. Ceram. Soc., 76 [7] 1892-1894 ( 1993), by adding Y ₂ O ₃ —Nd ₂ O ₃ to a β-Si ₃ N ₄ raw material as a sintering aid and performing calcination at 2000 ° C. for 2 to 8 hours in nitrogen at 100 atm. Fracture toughness 8.5-1
A silicon nitride sintered body of 0.3 MPa · m ^1/2 is obtained.
In J. Am. Ceram. Soc., 77 [7] 1857-1862 (1994), single-crystal β-silicon nitride columnar particles whose morphology is controlled in advance are added as seed crystals to a silicon nitride raw material, and anisotropically added. An attempt was made to control the shape and size of the β-silicon nitride columnar particles that grew, resulting in relatively high strength and high toughness of 900 to 1000 MPa in strength and 8.2 to 8.6 MPa · m ^1/2 in fracture toughness. We have succeeded in obtaining a silicon nitride sintered body.

Further, for example, a β-type silicon nitride seed crystal having an average aspect ratio of the major axis and the minor axis of 1.5 to 10 and an average diameter of the minor axis of 0.5 μm to 10 μm is added to silicon nitride powder. 0.1% by weight or more and 20% by weight or less,
To this, a mixed powder is prepared by adding a sintering aid of an oxide or a nitride, and after molding, a method for producing a silicon nitride sintered body which is sintered at a temperature of 1600 ° C. to 2100 ° C. No. 34670), and the like.

[0006] However, in the gas pressure sintering method, not only must a complicated and expensive gas pressure sintering furnace be used to perform sintering under a high nitrogen gas pressure, but also the destruction of the manufactured silicon nitride ceramics. Although the toughness is improved, there is a disadvantage that the strength is reduced. Although the seed crystal addition method can be manufactured using a normal atmosphere sintering furnace that sinters under a nitrogen gas pressure of about 10 atm, it must be manufactured by adding a silicon nitride single crystal synthesized by a special method. However, none of these methods is a simple method for increasing the fracture toughness of silicon nitride ceramics.

[0007] The sintering by microwaves, compared with the conventional sintering technology of the external heating method, generates heat by microwaves and is directly heated. Instead, it is possible to perform selective heating using the difference in the microwave absorption characteristics of the raw materials constituting the molded body, and it is expected to be a sintering technology capable of controlling the microstructure.

[0008]

In such a situation, various sintering aids that generate a liquid phase at a low temperature have been studied in order to lower the sintering temperature and reduce the production equipment and production cost. However, since the growth of silicon nitride columnar crystals does not proceed sufficiently due to the decrease in sintering temperature, the manufactured sintered body has high strength, but the one with high fracture toughness characteristic of silicon nitride ceramics is manufactured. could not.

The present inventors have proposed silicon nitride ceramics as a typical engineering ceramics.
In the course of intensive research on microstructure control and mechanical property improvement by microwave heating, the microwave heating method has shown that β
-It has been found that silicon nitride columnar crystals are well developed, and a sintered body having high toughness can be obtained more easily than the synthesis of existing high toughness silicon nitride ceramics such as a gas pressure sintering method or a seed crystal addition method. Under these circumstances, based on the inventors' discovery that the growth of crystal planes, which are diffusion-controlled by microwaves, is promoted, the inventors have further developed low-temperature firing suitable for microwave heating. By searching for binders and mixing magnesia in a specific ratio with ordinary sintering aids for silicon nitride ceramics, the sintering temperature is reduced, and silicon nitride ceramics with both high strength and high fracture toughness can be obtained. This led to the completion of the present invention. That is, as a sintering aid, addition of magnesium oxide, which lowers the generation temperature of a liquid phase formed from aluminum oxide and yttrium oxide, which are general sintering aids for silicon nitride, and silicon nitride long columnar crystals by microwave By promoting the growth, the sintering temperature can be reduced by 10 times compared with the conventional method.
A method for sintering silicon nitride ceramics having high strength and high fracture toughness despite being 0 to 350 ° C. was established. An object of the present invention is to provide a sintering aid and a sintering method which lower the sintering temperature of silicon nitride ceramics and promote the growth of silicon nitride columnar crystals.

[0010]

The present invention for solving the above problems comprises the following technical means. (1) Microwave firing in which the weight ratio of aluminum oxide to yttrium oxide is in the range of 2: 3 to 1: 4 and the weight ratio of aluminum oxide to magnesium oxide is in the range of 1: 1 to 4: 1. Silicon nitride ceramic sintering aid. (2) The silicon nitride ceramic sintering aid for microwave sintering according to the above (1), wherein aluminum oxide, yttrium oxide, and magnesium oxide are mixed at an optimal ratio of 2: 5: 1 by weight. (3) A method of producing silicon nitride ceramics having high strength and high fracture toughness by growing silicon nitride columnar crystals at a low sintering temperature, wherein the sintering aid of the above (1) is added to silicon nitride powder. 3 to 15% by weight, and a molded article molded by an appropriate method was placed in a nitrogen atmosphere at normal pressure from 1500 to 17%.
A method for producing silicon nitride ceramics, comprising sintering using microwaves at a temperature of 50 ° C.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in more detail. In producing a silicon nitride sintered body according to the present invention, first, a predetermined amount of a sintering aid is added to silicon nitride raw material powder. As the silicon nitride raw material, any of α-type, β-type, and amorphous crystalline systems may be used, but it is preferable to use fine powder having an average particle size of 0.5 μm or less. Al ₂ O ₃ , Y ₂ O ₃ , and MgO are used as sintering aids.

The combination of these sintering aids is such that the weight ratio of aluminum oxide to yttrium oxide is in the range from 2: 3 to 1: 4 and the weight ratio of aluminum oxide to magnesium oxide is from 1: 1. Mixing in the range of 4: 1, preferably Al ₂ O ₃ , Y ₂ O ₃ , M
It is desirable to mix gO in a weight ratio of 2: 5: 1 at an optimum ratio.
%, And preferably, 3 to 15 sintering aids are added to the silicon nitride powder in order to grow silicon nitride into columnar anisotropic grains during sintering.
It is desirable to add wt%.

In mixing these raw materials, an ordinary machine used for mixing or kneading powder can be used. In this case, either of a wet type or a dry type may be used, but the mixing is desirably performed in a wet type. In the wet mixing, a solvent such as water, methanol, ethanol, and toluene is used, and it is preferable to use an organic solvent to suppress oxidation of silicon nitride. When an organic solvent is used, mixing can be performed effectively by using a dispersant such as sorbitan monooleate.

Next, the mixed slurry obtained as described above is mixed with an appropriate amount of an organic binder, and then formed into a formed product by sheet forming by a doctor blade method or the like or by extrusion. Further, a molded product is produced from a mixed powder obtained by drying the mixed slurry by a die molding method, an isostatic pressing method, or the like. Next, if necessary, the molded body is subjected to a normal firing method,
After performing calcination at a temperature of about 0 to 800 ° C. and removing the molding binder by heating, heat treatment is performed in a microwave sintering furnace,
A sintered body is produced.

In the sintering method using microwave heating, the compact formed by adding a sintering aid is subjected to a 3
A method of sintering at a temperature of 1500 ° C. to 1750 ° C. for 0 minutes to 2 hours is exemplified as a preferable method.

In the present invention, the sintering is performed only by microwave heating at a temperature of 1500 ° C. to 1750 ° C. under a normal pressure nitrogen atmosphere, compared to the normal sintering of silicon nitride at about 10 atm. It is possible.

Sintering of silicon nitride ceramics proceeds through a process of dissolution-precipitation in a liquid phase generated by the reaction of a sintering aid. At this time, by forming a structure in which the β-silicon nitride developed in a columnar shape and the fine matrix β-silicon nitride are entangled with each other, the material has excellent strength and toughness.

In the silicon nitride ceramics sintered by the method of the present invention, β-silicon nitride columnar crystals are selectively grown, and as a result, β-silicon nitride columnar crystals form fibers entangled with each other. In addition, the fracture toughness of silicon nitride ceramics can be increased without lowering the strength. The method of the present invention is a simple, economically superior method for increasing the toughness of silicon nitride ceramics, compared to conventional gas pressure sintering and seeding methods, which are methods for increasing the toughness of silicon nitride ceramics. is there. According to the method of the present invention, lowering the sintering temperature not only reduces the energy cost required for sintering, but also requires high-pressure nitrogen gas atmosphere which is indispensable to prevent decomposition of silicon nitride in high-temperature sintering. Without
Since it can be sintered in a nitrogen gas atmosphere at normal pressure, high-strength and high-toughness silicon nitride ceramics can be manufactured without using equipment such as a pressure sintering furnace.

[0019]

Next, the present invention will be specifically described based on examples, but the present invention is not limited to the examples. Example 1 α-silicon nitride powder and 5 wt% yttrium oxide and 2
A molded body obtained by adding aluminum oxide of 1 wt% and magnesium oxide of 1 wt% under a nitrogen atmosphere of 1 atm.
It was sintered at 1600 ° C. for 2 hours by microwaves of GHz. It has a fracture toughness of 8 MPa · m ^1/2 and a strength of 82
It was a silicon nitride sintered body of 0 MPa.

Example 2 A compact produced in the same manner as in Example 1 was sintered at 1700 ° C. for 2 hours by a microwave of 28 GHz under a nitrogen atmosphere of 1 atm. It has a fracture toughness of 9 MPa · m
It was a silicon nitride sintered body having a strength of ^1/2 and a strength of 920 MPa.

REFERENCE EXAMPLE 1 A compact produced in the same manner as in Example 1 was sintered at 1600 ° C. for 2 hours in a resistance heating atmosphere furnace under a nitrogen atmosphere at 1 atm. In this product, pores remained and a dense silicon nitride sintered body was not obtained.

REFERENCE EXAMPLE 2 In the same manner as in Reference Example 1, 175
Sintering was performed at 0 ° C. in a resistance heating atmosphere furnace for 2 hours. This was a silicon nitride sintered body having a fracture toughness of 5 MPa · m ^1/2 and a strength of 850 MPa.

REFERENCE EXAMPLE 3 In the same manner as in Reference Example 1, under a nitrogen atmosphere of 10 atm.
Sintering was performed at 50 ° C. in a resistance heating atmosphere furnace for 2 hours. It has a fracture toughness of 7 MPa · m ^1/2 and a strength of 700 MPa.
Was obtained.

[0024]

According to the present invention, not only the energy cost required for sintering is reduced by lowering the sintering temperature, but also a high-pressure nitrogen gas atmosphere which is indispensable for preventing decomposition of silicon nitride in high-temperature sintering. Therefore, high-strength and high-toughness silicon nitride ceramics can be manufactured without using equipment such as a pressure sintering furnace since sintering can be performed in a nitrogen gas atmosphere at normal pressure without the need for sintering. Furthermore, since the grain boundary phase is selectively heated, the silicon nitride crystal grows even at a temperature lower than the decomposition temperature of silicon nitride at normal pressure, so that a high nitrogen gas pressure is applied to prevent the decomposition of silicon nitride. In addition, a silicon nitride ceramics structure developed in a columnar shape can be formed without adding a silicon nitride single crystal serving as a nucleus for crystal growth.

[Brief description of the drawings]

FIG. 1 shows a sintered structure of silicon nitride subjected to microwave sintering by the method shown in Example 2 of one embodiment of the present invention.

FIG. 2 shows a change in fracture toughness due to microwave sintering.

────────────────────────────────────────────────── ─── Continuing from the front page (72) Inventor Hiroshi Watari 1-5-5 Shiroyama, Komaki-shi, Aichi Prefecture 1 San Heights F Building 306 (72) Inventor Manuel E Brito 3-2-2 Kita Chikusa, Chikusa-ku, Nagoya-shi, Aichi Prefecture No. 4 Chikusa Higashi House 17-302 (72) Inventor Masayuki Hirota 1-31 Hiratecho, Kita-ku, Nagoya City, Aichi Prefecture 1-1 River Palace 101 (72) Inventor Maria Baresiros Kita Chikusa, Chikusa-ku, Nagoya City, Aichi Prefecture Examiner Hitoshi Misaki, No. 17-302, 4,000 kinds of east housing at 3-chome-2 (56) Reference JP-A-10-226577 (JP, A) JP-A-10-226576 (JP, A) JP-A-9- 30867 (JP, A) (58) Field surveyed (Int. Cl. ⁷ , DB name) C04B 35/584 C04B 35/64

Claims (3)

(57) [Claims] 1. The weight ratio of aluminum oxide to yttrium oxide is in the range of 2: 3 to 1: 4, and the weight ratio of aluminum oxide to magnesium oxide is 1: 1 to 4:
Silicon nitride ceramics sintering aid for microwave sintering mixed in range 1. 2. An aluminum oxide, a yttrium oxide,
A silicon nitride ceramics sintering aid for microwave sintering in which magnesium oxide is mixed at a weight ratio of 2: 5: 1 to be optimal. 3. A method for producing silicon nitride ceramics having a high strength and high fracture toughness by growing silicon nitride columnar crystals at a low sintering temperature, wherein the sintering aid according to claim 1 is a silicon nitride powder. A method for producing silicon nitride ceramics, comprising: sintering a molded body formed by an appropriate method by adding 3 to 15% by weight to a mixture at a temperature of 1500 to 1750 ° C. using a microwave in a normal pressure nitrogen atmosphere. .