JP3116701B2 - Method for producing 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 widely used as a ceramic product.

[0002]

2. Description of the Related Art Conventionally, in order to improve the strength of a silicon nitride sintered body, yttria (Y ₂ O ₃ ), scandium oxide, hafnium oxide or spinel (MgAl
_A sintering aid such as ₂ O ₄ ) is used. For example, it is known that columnar crystals are generated in the structure of a sintered body by using yttria as a sintering aid, and high strength is obtained.

It is also effective to use a combination of yttria and spinel. For example, Japanese Patent Application Laid-Open No. 5-32460 discloses that yttria and spinel are added as sintering aids in a total amount of 5 to 15% by weight, and yttria to spinel is used. Is described to be used in a weight ratio of 0.3 to 10.
By using yttria and spinel in this manner, sintering at a low temperature of 1400 to 1650 ° C. becomes possible.

[0004]

SUMMARY OF THE INVENTION However, Japanese Patent Application Laid-Open No.
When yttria and spinel are used in combination as a sintering aid and sintered at a low temperature as described in JP-A-32460, the resulting silicon nitride sintered body has improved room temperature strength, but its structure is not uniform. Notwithstanding, coarse particles were also observed, and it was also clarified that the degree of growth of columnar crystals varied.
It is fully conceivable that the unevenness of the structure adversely affects the strength.

Further, there is a problem that rare earth oxides such as yttria are expensive, and the obtained silicon nitride sintered body is also expensive. The present invention has been made in view of such circumstances, and has pioneered a sintering aid that promotes the growth of columnar crystals and makes the structure of a sintered body fine and uniform, thereby achieving high-strength silicon nitride sintering. It is intended to be a body.

[0006]

[MEANS FOR SOLVING THE PROBLEMS]
The method for producing a silicon nitride sintered body is
After mixing with auxiliaries and forming a molded body of a predetermined shape, molding
The body in a nitrogen atmosphere at a temperature of 1400-1650 ° C
A method for producing a silicon nitride sintered body to be sintered, comprising a sintering aid
Yttria (Y_TwoO_Three) And titania (TiO_Two)
And spinel (MgAl_TwoO_Four) With a total addition of 5 to 15
% By weight, and the weight ratio (Y _TwoO_Three+ Ti
O_Two) / MgAl_TwoO_Four0.3 to 4, weight ratio Y_TwoO_Three
/ TiO_TwoIs set to 0.2 or more.

According to a second aspect of the present invention, there is provided a method for manufacturing a silicon nitride sintered body according to the present invention, comprising mixing a silicon nitride powder and a sintering aid to form a molded body having a predetermined shape. A method for producing a silicon nitride sintered body which is sintered in a temperature range of 1400 to 1650 ° C. in zirconia (ZrO ₂ ), titania (TiO ₂ ) and spinel (M
with Gal ₂ O ₄₎ such that the total addition amount is 5-15 wt%, the weight ratio _{(ZrO 2 + TiO 2) /} MgAl 2 O
₄ is 0.3 to 4, and the weight ratio ZrO ₂ / TiO ₂ is 0.2 or more.

[0008]

In the method for producing a silicon nitride sintered body of the present invention, a sintering aid is a three-component system of yttria, titania and spinel,
Alternatively, a ternary system of zirconia, titania, and spinel is used, and the mixing ratio of each oxide is set as described in claim 1 or claim 2. The structure of the compact is further refined, and the growth of columnar crystals is promoted. Therefore, the strength of the sintered body is improved.

Further, by using zirconia instead of yttria, the sintering aid becomes inexpensive, and as a result, the silicon nitride sintered body becomes inexpensive. 5-15 total sintering additive
The reason for setting the content to 5% by weight is that if the content is less than 5% by weight, sintering in a low temperature range becomes difficult and the strength of the sintered body is reduced.
This is because if it is added in excess of weight%, a dense sintered body is obtained, but the strength is reduced.

The weight ratio (Y ₂ O ₃ + TiO ₂ ) / Mg
The reason for setting the Al ₂ O ₄ to 0.3 to 4 is that if the ratio is smaller than 0.3, columnar crystals do not grow and the strength is reduced. This is because it becomes difficult to densify. The reason why the weight ratio Y ₂ O ₃ / TiO ₂ is 0.2 or more is that if it is smaller than 0.2, densification is hindered. Although the upper limit is not specified,
The strength tends to gradually decrease with a peak at a weight ratio of Y ₂ O ₃ / TiO ₂ = 1, and asymptotically approaches the strength of a sintered body sintered using a yttria / spinel binary sintering aid. I do.

The weight ratio (ZrO ₂ + TiO ₂ ) / Mg
The reason for setting the Al ₂ O ₄ to 0.3 to 4 is that if this ratio is smaller than 0.3, growth of columnar crystals is not observed and the strength is reduced. This is because it becomes difficult to densify. The reason why the weight ratio ZrO ₂ / TiO ₂ is 0.2 or more is that if the weight ratio is smaller than 0.2, densification is hindered. Although the upper limit is not specified,
The strength tends to gradually decrease with the peak of the weight ratio ZrO ₂ / TiO ₂ = 1, and approaches the strength of the sintered body sintered using the sintering agent of the two-component system of zirconia / spinel.

[0012]

The present invention will be specifically described below with reference to examples. (Examples 1 to 25, Comparative Examples 1 to 7) First, silicon nitride powder obtained by a pyrolysis method of silicon diimide (having an average particle diameter of 0. 1).
Two types of silicon nitride (Si ₃ N ₄ ) powders, 2 μm, α conversion 97%) and silicon nitride powder (average particle size 0.9 μm, α conversion 92%) obtained by direct nitriding of metal silicon Was prepared.

Separately, yttria (Y ₂ O ₃ ) powder (average particle size 0.3 μm, purity 99.9%), zirconia (ZrO ₂ ) powder (average particle size 0.3 μm, purity 99.9)
%), Titania (TiO ₂ ) powder (average particle size 0.2 μm)
m, purity 99.9%) and spinel (MgAl ₂ O ₄ )
Powder (average particle size 0.3 μm, purity 99.9%) was prepared.

These powders were charged into a silicon nitride ball mill at the ratios shown in Table 1 and mixed. The silicon nitride powder formed by the direct nitridation method was used only in Examples 10 to 12, the Example 16, and the Comparative Example 15, and the silicon nitride powder formed by the thermal decomposition method of silicon diimide was used in all other cases. Was. The silicon nitride used in Example 16 and Comparative Example 15 was of a refractory grade.

Next, the obtained mixed powder was mixed with 19.6 MPa.
, Pressed into thin rubber, filled in vacuum, and then subjected to CIP treatment at a pressure of 294 MPa to form a molded body. The compact was sintered in a nitrogen gas atmosphere under the conditions shown in Table 1. The temperature was raised at a rate of 1 ° C./min in a nitrogen gas atmosphere at a pressure of 0.1 Pa until the maximum temperature was reached.
Pressurization was performed at a pressure increase rate of 6 Pa. The holding time at the highest temperature is 4 hours.

With respect to each of the obtained sintered bodies, the relative density and ten-point bending strength at room temperature defined by JIS-R-1601 were measured for ten pieces each, and the results as average values are shown in Table 1. The relative density is a value obtained by dividing the bulk density obtained by the n-butanol substitution method by the theoretical density. (Comparative Examples 8 to 20) The same yttria powder, zirconia powder and spinel powder as described above were prepared, and as a mixed sintering aid of a two-component system of yttria-spinel or zirconia-spinel, silicon diimide was thermally decomposed. It was mixed with the formed silicon nitride powder at a ratio shown in Table 2.

The mixed powder was molded in the same manner as described above, heated at the same temperature as above, and sintered under the conditions shown in Table 1.
Then, the relative density and the four-point bending strength at room temperature were measured in the same manner as above, and the results are shown in Table 2.

[0018]

[Table 1]

[0019]

[Table 2]

(Evaluation) Examples 1 to 15 and Comparative Examples 8 to 20
It is clear that the room temperature strength is improved by using the three-component sintering aid to which titania is added, as compared with the two-component system. In addition, the degree of the variation in the strength was a Weibull coefficient, and the sintered body of the example showed a value 1 to 5 larger than that of the comparative example, and it was also confirmed that the variation of the strength was smaller in the example. The high strength of the sintered body of the example is considered to be due to the fine structure of the sintered body and the remarkable growth of columnar crystals.

The internal structure photographs (SEM) of the sintered bodies of Example 16 and Comparative Example 15 are shown in FIGS. From this, it is clear that the structure of the sintered body of Example 16 is finer and more uniform than that of Comparative Example 15. In the examples, the weight ratio (Y ₂ O ₃ + TiO ₂ ) / MgAl ₂ O ₄ and the weight ratio (ZrO ₂ + TiO ₂ ) / MgAl ₂ O ₄ are 1.5, and those sintered at 1550 ° C. are the best. It shows strength and is the most preferable condition. And in this composition,
Even at a sintering temperature as low as 1400 ° C., a strength exceeding 900 MPa is exhibited.

In Comparative Example 1 in which the total amount of the sintering aids is less than 5% by weight, the sintering at 1550.degree. In Comparative Example 2 in which the content is more than 15% by weight, the sintered body is dense but high strength cannot be obtained. Weight ratio Y
_In Comparative Example 3 in which ₂ O ₃ / TiO ₂ is smaller than 0.2, it is difficult to densify and the strength is low.

Weight ratio (Y ₂ O ₃ + TiO ₂ ) / MgAl
_In Comparative Example _{4 in} which ₂ O ₄ is smaller than 0.3, a dense sintered body is obtained, but the strength is low. This is because columnar growth is not remarkable. In Comparative Example 5 in which the weight ratio is greater than 4, the strength is low because the material is not densified. Furthermore, in Comparative Example 6 in which the sintering temperature is lower than 1400 ° C., the strength is low because densification is difficult, and in Comparative Example 7 in which the sintering is performed at a temperature exceeding 1650 ° C., although densification is achieved,
High strength cannot be obtained because the structure of the sintered body becomes large.

[0024]

According to the method for manufacturing a silicon nitride sintered body of the present invention, a high-strength silicon nitride sintered body can be stably and easily manufactured even by low-temperature sintering. If zirconia is used instead of yttria, an inexpensive silicon nitride sintered body can be manufactured while maintaining high strength while the raw material is inexpensive.

[Brief description of the drawings]

FIG. 1 is a substitute drawing of a micrograph showing a crystal structure of a sintered body obtained in one example of the present invention.

FIG. 2 is a substitute drawing of a micrograph showing a crystal structure of a sintered body obtained in a comparative example.

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int. Cl. ⁷ , DB name) C04B 35/584

Claims (2)

(57) [Claims] 1. A silicon nitride sintering method comprising: mixing a silicon nitride powder and a sintering aid to form a molded body having a predetermined shape; and sintering the molded body in a nitrogen atmosphere at a temperature of 1400 to 1650 ° C. A method for producing a sintered body, wherein yttria (Y ₂ O ₃ ), titania (TiO ₂ ) and spinel (MgAl ₂ O ₄ ) are added as the sintering aid in a total amount of 5 to 15% by weight. Weight ratio (Y ₂ O
_{3 + TiO 2) / MgAl 2} O 4 of 0.3 to 4, weight ratio Y ₂ O ₃ / method for producing a silicon nitride sintered body, characterized in that TiO ₂ was 0.2 or more. 2. A silicon nitride sintering method comprising mixing a silicon nitride powder and a sintering aid to form a molded body having a predetermined shape, and then sintering the molded body in a nitrogen atmosphere at a temperature of 1400 to 1650 ° C. A method for producing a sintered body, wherein zirconia (ZrO ₂ ), titania (TiO ₂ ) and spinel (MgAl ₂ O ₄ ) are used as the sintering aid in a total addition amount of 5 to 15% by weight, Weight ratio (ZrO
₂ + TiO ₂ ) / MgAl ₂ O ₄ : 0.3 to 4, and a weight ratio ZrO ₂ / TiO ₂ : 0.2 or more.