JP2684275B2 - 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 having a high bending strength and a high toughness, and particularly to engine parts,
The present invention relates to a silicon nitride sintered body having a large bending strength and a high toughness, which is suitable for use in rubbing parts and the like.

[0002]

2. Description of the Related Art Silicon nitride (Si ₃ N ₄ ), which is one of the ceramics high-temperature structural materials, has attracted attention as a material that has sufficient strength at high temperatures, is chemically stable, and is resistant to thermal shock. However, the bending strength and toughness of the silicon nitride sintered body obtained by the conventional manufacturing method were not sufficient for use as an actual ceramic part.

Therefore, various studies have been conducted for the purpose of improving the bending strength, toughness, etc. of the silicon nitride sintered body.
For example, JP-A-3-8771 discloses a silicon nitride sintered body containing columnar crystals having a specific diameter and aspect ratio. However, since the silicon nitride powder used has an average particle size of more than 0.5 μm, the sinterability is not sufficient, and sufficient toughness for use as an actual ceramic part has not been obtained.

[0004]

[Patent Document 1] Japanese Patent Application Laid-Open No. 3-8772
As a means for improving bending strength and toughness, a silicon nitride powder having an average particle size of 0.1 to 0.5 μm and an average particle size of 0.5 to 0.
A method for producing a silicon nitride sintered body in which a ratio of the numbers of large particles and small particles is specified by mixing 7 μm of silicon nitride powder and a sintering aid is disclosed.

However, in this method, since a relatively large silicon nitride powder having an average particle size of 0.5 to 0.7 μm is used as a raw material, the sinterability is poor, and therefore a large amount of sintering aid is used during sintering. It is compounded. As a result, the sintering aid often forms a low-melting glass phase (grain boundary phase) at the grain boundaries of silicon nitride, often impairing bending strength and toughness.

Therefore, an object of the present invention is to solve the above-mentioned problems and to provide a silicon nitride sintered body which is excellent in bending strength and toughness.

[0007]

As a result of earnest research in view of the above objects, the inventors of the present invention have found that if silicon nitride powders having a small average particle size and different specific surface areas are mixed, the crystals at the time of sintering are crystallized. It was discovered that there is a difference in the growth of the above, and the bending strength and toughness of the obtained silicon nitride sintered body are improved, and the present invention was completed.

That is, the silicon nitride sintered body of the present invention is
A silicon nitride powder having a specific surface area of 12 to 8 m ² / g and an average particle diameter of 0.40 to 0.45 μm, a silicon nitride powder having a specific surface area larger than that of the silicon nitride powder and a small average particle diameter, and a sintering aid are given in predetermined weights. A silicon nitride sintered body obtained by mixing the raw material powders at a ratio of 1900 ° C. or higher and sintering the raw material powder at a temperature of 1900 ° C. or higher.
Β-acicular crystals with a diameter of 1 μm or less and an aspect ratio of 3 to 5
.Beta.-crystal has a surrounding structure.

Hereinafter, the present invention will be described in detail. As a raw material silicon nitride powder for obtaining the silicon nitride sintered body of the present invention, a specific surface area of silicon nitride powder having a specific surface area (BET) of 12 to 8 m ² / g (hereinafter referred to as powder A) and powder A A large silicon nitride powder (hereinafter referred to as powder B) is used.

The average particle size of powder A is 0.40 to 0.45 μm.
And the average particle size of the powder B is smaller than the average particle size of the powder A. Since a powder having a small average particle diameter is used as a raw material in this way, a sintered body having a sufficient bending strength and toughness with a relatively small amount of the sintering additive added can be obtained as described later. Although the specific surface area of the powder and the average particle diameter do not necessarily have a quantitative correlation, generally, the larger the specific surface area of the powder, the smaller the average particle diameter tends to be.

The mixing ratio (weight ratio) of powder A and powder B is 7
It is preferably 5:25 to 50:50.

Examples of the sintering aid added to the above silicon nitride powder include Y ₂ O ₃ , Al ₂ O ₃ and MgO.
It is preferable to use Y ₂ O ₃ and Al ₂ O ₃ .

When both Y ₂ O ₃ and Al ₂ O ₃ are used as sintering aids, the amount of Y ₂ O ₃ added is 1.5 to 3.5% by weight, and the amount of Al ₂ O ₃ added is 0.1 to 1.0% by weight. %, The balance is preferably substantially silicon nitride.

Next, a method for producing a silicon nitride sintered body from the above raw materials will be described.

First, the powder A and the powder B are blended in a predetermined ratio, a predetermined amount of a sintering aid and a solvent are added, wet mixed with a ball mill or the like, dried and molded into a predetermined shape. Molding methods include die press or cold isostatic press (C
A known method using IP) or the like can be used.

Next, the obtained compact is sintered under an inert gas, for example, nitrogen gas. Sintering temperature is 1900 ℃ or higher. If the sintering temperature is lower than 1900 ° C, sufficient bending strength and toughness cannot be obtained.

The silicon nitride sintered body of the present invention obtained as described above has a diameter of 3 to 10 determined by SEM observation.
β-acicular crystals with an aspect ratio of 10 to 20 at μm (hereinafter simply referred to as β-
Needle-like crystals) with a diameter of 1 μm or less and an aspect ratio of 3 to
And β-crystal of No. 5 (hereinafter simply referred to as β-crystal), and has a structure structure in which elongated β-acicular crystals are surrounded by fine β-crystals. As described above, the combination of needle-like crystals with a large aspect ratio and a large diameter and fine crystals with a small aspect ratio is one of the major features of the present invention.

[0018]

It is not always clear that the silicon nitride sintered material of the present invention has large bending strength and high toughness, but by mixing silicon nitride powders having different specific surface areas and sintering at 1900 ° C. or higher, Differences occur in the growth direction and shape development of each crystal during sintering, the growth of elongated β-needle crystals is promoted,
It is considered that the silicon nitride sintered body having the above-mentioned texture structure can be obtained. Further, since the silicon nitride powders each having a specific surface area within a specific range used in the present invention have a relatively small particle size, the addition amount of the sintering aid can be reduced.
Thereby, the bending strength and toughness of the silicon nitride sintered body obtained by sintering are further improved.

[0019]

EXAMPLES Examples of the present invention will be described below, but the present invention is not limited to these examples.

Examples 1 to 8 and Comparative Examples 1 to 10 Silicon nitride powder A (manufactured by Ube Industries, Ltd., specific surface area (BE
T): 10 m ² / g) and silicon nitride powder B (Ube Industries, Ltd.)
And specific surface area (BET): 15 m ² / g) so that the compounding ratio (weight ratio) shown in Table 1 is obtained, and further Y ₂ O ₃ is added to 2.
Add 5 wt% and Al ₂ O ₃ to 1.0 wt% (
However, silicon nitride powder A, silicon nitride powder B, Y ₂ O ₃ and Al
The total amount of ₂ O ₃ and 100% by weight) and ethanol were added and ball mill wet mixing was performed for 16 hours. In addition,
SEM photographs of the silicon nitride powders A and B were taken, and the average particle size was measured to be about 0.4 μm and 0.4 μm respectively.
It was below.

Each of the obtained mixtures was dried by a rotary evaporator and molded into a size of 55 × 35 × 6 mm by a die press.

As shown in Table 1, each of the above moldings was
A test piece was obtained by sintering at 50 ° C., 1900 ° C. or 1800 ° C. under a nitrogen gas atmosphere of 9 atm for 4 hours.

Regarding the obtained test piece, JIS R
According to 1601, the three-point bending strength and fracture toughness were measured.
Table 1 shows the results. The relationship between the mixing ratio of the powder A and the powder B and the bending strength and fracture toughness of the sintered body is shown in FIGS. 1 to 3 for each sintering temperature. Fig. 1 shows the case where the sintering temperature is 1950 ℃.
Shows the case of 1900 ° C, and Fig. 3 shows the case of 1800 ° C.

Further, Examples 3, 5 and Comparative Examples 1, 3, 4,
The test pieces obtained in 6 and 9 were photographed with a scanning electron microscope (SEM) at a magnification of 500 to 2000 times, and the diameter and aspect ratio of β-acicular crystals and β-crystals were measured from the photographs. The length of some of the sintered bodies was also measured. Table 2 shows the results.

Table 1Raw material mixture ratio Sintered body characteristics (Powder B / Sintering temperature Bending strength ρ Fracture toughness K_{I c}  Example No. Powder A) (℃) (kgf / mm ²⁾  (MPa m ^1/2 ) Example 1 10/90 1950 105 8.0 Example 2 10/90 1900 110 6.6 Example 3 25/75 1950 120 9.8 Example 4 25/75 1900 122 8.1 Example 5 50/50 1950 120 9.8 Example 6 50 / 50 1900 123 8.4 Example 7 75/25 1950 100 7.9 Example 8 75/25 1900 107 6.2 Comparative Example 1 100/0 1950 76 5.2 Comparative Example 2 100/0 1900 79 4.2 Comparative Example 3 100/0 1800 67 3.9 Comparative Example 4 0/100 1950 60 6.0 Comparative Example 5 0/100 1900 71 4.2 Comparative Example 6 0/100 1800 63 4.4 Comparative Example 7 10/90 1800 69 4.4 Comparative Example 8 25/75 1800 76 4.1 Comparative Example 9 50/50 1800 78 3.7 Comparative example 10 75/25 1800 75 3.7

Table 2 β-Needles β-Crystal Diameter Aspect Ratio Diameter Diameter Aspect Ratio Length Example No. (μm) (μm) (μm) Example 3 3 to 10 10 to 20 <1 3 to 5 Example 5 3 to 10 10 to 20 <1 3 to 5 Comparative Example 1 5 to 10 3 to 5 <1 <3 Comparative Example 3 (β− No needle crystals) <1 <3 2 to 3 Comparative example 4 5 to 10 3 to 5 <1 <3 Comparative example 6 (no β-needle crystals) <1 <3 2 to 3 Comparative example 9 (β-needle (No crystals) <1 <3 2-3

As is clear from Table 1, the silicon nitride sintered bodies of Examples 1 to 8 of the present invention are superior to Comparative Examples 1 to 10 in both bending strength and fracture toughness. .

As is apparent from Table 2, the silicon nitride sintered bodies of Examples 3 and 5 obtained by the method of the present invention have the crystal structure defined by the present invention.

Further, as is clear from FIGS. 1 to 3,
It can be seen that the bending strength and fracture toughness are good at a sintering temperature of 1900 ° C or higher.

[0030]

As described above, the silicon nitride sintered body of the present invention has high strength and high toughness. Further, according to the present invention, the addition amount of the sintering aid can be reduced, so that the obtained sintered body has high bending strength and high toughness.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between the compounding ratio of silicon nitride raw material powder and the bending strength and fracture toughness of a silicon nitride sintered body sintered at 1950 ° C.

FIG. 2 is a graph showing the relationship between the compounding ratio of silicon nitride powder raw materials and the bending strength and fracture toughness of a silicon nitride sintered body sintered at 1900 ° C.

FIG. 3 is a graph showing the relationship between the compounding ratio of the silicon nitride powder raw material and the bending strength and fracture toughness of the silicon nitride sintered body sintered at 1800 ° C.

Claims (3)

(57) [Claims] 1. A specific surface area of 12 to 8 m ² / g and an average particle size of 0.
40 to 0.45 μm silicon nitride powder, a silicon nitride powder having a larger specific surface area and a smaller average particle diameter than the silicon nitride powder, and a sintering aid are mixed in a predetermined weight ratio, and the raw material powder is molded,
A silicon nitride-based sintered body obtained by sintering at 1900 ° C. or higher, in which β-acicular crystals having a diameter of 3 to 10 μm and an aspect ratio of 10 to 20 determined by SEM observation have an aspect ratio of 1 μm or less and an aspect ratio of 10 to 20 μm. A silicon nitride-based sintered body having a texture structure surrounded by 3 to 5 β-crystals. 2. The silicon nitride sintered body according to claim 1, wherein the sintering aid is Y ₂ O ₃ and Al ₂ O ₃ . 3. The silicon nitride sintered body according to claim 2, wherein the Y ₂ is based on the total amount of the silicon nitride powder.
O ₃ is the 1.5 to 3.5 wt%, and the Al ₂ O ₃ is 0.
A silicon nitride-based sintered body characterized by being 1 to 1.0% by weight.