JPH07115936B2 - Method for manufacturing silicon nitride sintered body - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention relates to a silicon nitride sintered body having excellent heat resistance, and more specifically, a silicon nitride sintered body containing α · Si ₃ N _{4 and} having excellent high-temperature strength characteristics. The present invention relates to a method for manufacturing a bound body.

(Prior Art) Since silicon nitride is superior in heat resistance to metals,
Suitable for high temperature structural members. Further, when producing this silicon nitride sintered body, since silicon nitride itself has a large covalent bond and is also a hardly sinterable substance, various oxides such as Al ₂ O ₃ and basic earth oxides are used. It is known that a dense sintered body can be obtained by adding singly or in combination of several kinds (Japanese Patent Publication Sho
48-7486, Japanese Patent Publication No. 49-21091). In addition, by using silicon nitride powder with a high proportion of α ・ Si ₃ N ₄ as much as possible, the influence of surrounding sintering aids during high temperature sintering,
Silicon nitride undergoes a transition from α ・ Si ₃ N ₄ to β ・ Si ₃ N ₄ , and this transition causes densification and formation of columnar crystals, resulting in a high-strength sintered body entangled with columnar crystals. It is considered.
However, when an oxide is used as a sintering aid, the oxide forms a grain boundary phase after contributing to the densification of silicon nitride and formation of columnar crystals, and the grain boundary phase softens at high temperature, resulting in high temperature. However, there is a drawback that the mechanical properties at the point of time are deteriorated.

(Problems to be Solved by the Invention) As a result of various investigations by the present inventor, the inventors achieved various objectives as a result of various studies aimed at obtaining a silicon nitride sintered body excellent in high-temperature strength and suitable for an engine member of a gas turbine or a diesel engine. In order to achieve this, the inventors have found that it is sufficient to control the crystal structure of silicon nitride to densify the sintered body and to contain α.Si ₃ N ₄ and have completed the present invention.

(Means for Solving Problems) That is, according to the present invention, the specific surface area is 5 to 12 m ² / g, α · Si ₃ N ₄
Of 90% by weight or more and the dissolved oxygen content of 1.5% by weight or less, 100 parts by weight of silicon nitride powder, 0.1 to 5 parts by weight of MgO, and A
One or more selected from l ₂ O ₃ , SiO ₂ and Group IIIa element oxides
After molding 0.1 to 10 parts by weight of mixed powder, sintering at a temperature of 1,450 to 1,800 ° C under normal pressure in a non-oxidizing atmosphere for 30 minutes by weight or less of α ・ Si ₃ N ₄ (not including 0). And a method for manufacturing a silicon nitride sintered body.

Hereinafter, the present invention will be described in more detail.

First, a silicon nitride sintered body (hereinafter referred to as a sintered body) manufactured by the method for manufacturing a silicon nitride sintered body of the present invention will be described. Conventionally, all α ・ Si ₃ N ₄ was converted to β ・ Si ₃ by sintering.
We obtained a high-strength sintered body that was densified and entangled with columnar crystals after it was transformed to N ₄ because this sintered body has a grain boundary phase that deteriorates mechanical properties at high temperatures in its grain boundaries. In addition, in order to improve the high temperature strength properties, it was necessary to further reduce the grain boundary phase, prevent the formation of compounds having a low melting point, change the grain boundary phase from the glass phase to the crystalline phase, and so on.

The sintered body obtained by the manufacturing method of the present invention is α · Si ₃ N
It is a sintered body that is characterized by suppressing the transition from ₄ to β · Si ₃ N ₄ as much as possible to make it dense, and has no entangled crystal structure of columnar crystals, but has excellent high-temperature strength characteristics. This will be explained with reference to the drawings. FIG. 1 shows a crystal structure of a sintered body obtained by the manufacturing method of the present invention, and FIG. 2 shows a crystal structure of a sintered body obtained by a conventional sintering method. Double SEM
(Scanning electron microscope) Photographs showing large crystal grains and many columnar crystals in FIG. 2, whereas grain growth and columnar crystal development are not so noticeable in FIG. .

The method for analyzing α · Si ₃ N _{4 in} the silicon nitride sintered body is the same as the method for analyzing the ratio of α · Si ₃ N _{4 in} the silicon nitride powder. That is, (102) plane of α · Si ₃ N ₄ which is measured by X-ray diffraction, (210) plane and the β · Si ₃ N ₄ (10
It is obtained from the ratio of the heights of the diffraction peaks of the 1) and (210) planes. An example of the X-ray diffraction diagram is shown in FIG. 3 (Example 1) and FIG. 4 (Comparative Example 1).

In the present invention, it is necessary that α.Si ₃ N ₄ is contained in an amount of 30% by weight or less (0 is not included), and if it exceeds 30% by weight, the compactness of the sintered body is extremely lowered, High temperature strength decreases. When the content of α · Si ₃ N ₄ is 30% by weight or less, there is almost no difference in high temperature strength characteristics due to the difference in the content.

Further, the relative density (absolute value with respect to the theoretical density) of the sintered body obtained by the production method of the present invention is 95% or more, and for example, the hot strength at 1200 ° C. required for an engine member such as a gas turbine or diesel is 800 MPa. The above is obtained. Thus, the reason why the sintered body produced by the present invention is excellent in high temperature strength is not clear, but in addition to the fact that the sintering aid component is evenly distributed throughout the sintered body, a small amount of oxygen It is believed that this is because there is no remarkable grain boundary phase as in the past because it dissolves in α · Si ₃ N ₄ .

The sintered body as described above is made of a carefully selected silicon nitride powder as a raw material, and MgO, Al ₂ O ₃ , SiO and II
It can be produced by using in combination with at least one selected from Group Ia element oxides and sintering at an appropriate temperature and time under a normal pressure non-oxidizing atmosphere. Hereinafter, the manufacturing method of the present invention will be described.

The transition from α ・ Si ₃ N ₄ to β ・ Si ₃ N ₄ is
Since · Si ₃ N ₄ liquid phase and β · Si ₃ N ₄ particles around the particles proceeds if present, in order to minimize suppress metastasis to β · Si ₃ N ₄ from α · Si ₃ N ₄ is As the raw material silicon nitride powder,
It is necessary that the specific surface area is 5 to 12 m ² / g, the ratio of α · Si ₃ N ₄ is 90% by weight or more, and the solid solution oxygen content is 1.5% by weight or less.

When the specific surface area of the powder exceeds 12 m ² / g, the contact between the silicon nitride molecules and the surrounding liquid phase increases, and α ・ Si ₃ N ₄ to β ・ Si ₃ N ₄
It is not preferable because the transition to On the other hand, if the specific surface area is less than 5 m ² / g, the sinterability will be poor.

In addition, solid solution oxygen contained in silicon nitride powder is 1.5% by weight.
Above this, the transition from α · Si ₃ N ₄ to β · Si ₃ N ₄ progresses, making it impossible to manufacture a sintered body with excellent high-temperature strength. Here, the solid solution oxygen is not essentially oxygen that has a Si ₂ ON ₂ structure or a Si oxide structure that is unavoidably present on the surface of the Si ₃ N ₄ particles due to its presence, and is only a Si ₃ N ₄ particle. It refers to oxygen dissolved inside, and is, for example, oxygen measured as follows. That is, 50 ml of hydrofluoric acid aqueous solution (HF content: 5% by weight) and 1 g of Si ₃ N ₄ powder were placed in a 100 ml beaker and vacuum filtered with stirring at room temperature for 20 minutes (rotation speed of magnetic stirrer: 300 to 600 rpm). Then, the obtained solid is dried in a dryer at a temperature of 100 to 150 ° C. for 2 hours, and then the amount of oxygen is quantified by an analyzer, for example, LECO O / N simultaneous analyzer “TC-136 type”. The measured value of the amount of dissolved oxygen does not change significantly within the condition range of the rotation speed of the magnetic stirrer and the drying temperature.

If the proportion of α ・ Si ₃ N _{4 in} the silicon nitride powder is less than 90% by weight, the transition from α ・ Si ₃ N ₄ to β ・ Si ₃ N ₄ is promoted,
It becomes difficult for α ・ Si ₃ N ₄ to remain in the sintered body. Preferred α ・ Si ₃
The proportion of N ₄ is 95% by weight or more.

Next, the sintering aid will be described. In the present invention, Mg
1 selected from O and Al ₂ O ₃ , SiO ₂ and Group IIIa element oxides
Use together with seeds or more.

The amount of MgO added is 100 parts by weight of the silicon nitride powder.
It is 0.1 to 5 parts by weight. If it is less than 0.1 part by weight, the densification will be insufficient, and if it exceeds 5 parts by weight, the high-temperature strength properties will deteriorate. When MgO is added, the X-ray diffraction peak of the MgO crystal disappears from the sintered body at a sintering temperature near 1,500 ° C. This phenomenon is considered to be due to the fact that the MgO crystal partially became a liquid phase from around 1,500 ° C, which contributes to the densification of the sintered body, that is, low temperature sintering becomes possible, and α ・ Si ₃ N ₄ to β ・ Si ₃
It is considered that the transition to N ₄ was suppressed and the densification proceeded.

If a sintering aid other than MgO is used, densification of the sintered body will occur, but sintering will require higher temperature and a longer time than in the case of using MgO, and α ・ Si ₃ N ₄ will be added to the sintered body. It becomes difficult to make it remain.

However, since the densification is not sufficient when the above MgO alone is added, 0.1 to 10 parts by weight of one or more selected from Al ₂ O ₃ , SiO ₂ and Group IIIa element oxides is added in the present invention. If it is less than 0.1 part by weight, the effect of improving the densification is small, and if it exceeds 10 parts by weight, the high temperature strength properties are deteriorated. Examples of group IIIa elements include Y, Ce, Nd, and the like. MgO and Al ₂ O ₃
Alternatively, it is within the scope of the present invention to add both compounds such as MgAl ₂ O ₄ and Mg ₂ SiO ₄ instead of separately adding SiO ₂ . It is preferable to add 5 to 15 parts by weight of MgO and one or more selected from Al ₂ O ₃ , SiO ₂ and Group IIIa element oxides as a combination of the two.

Then, a mixture of the above-mentioned silicon nitride powder and a sintering aid is molded at a temperature of 1,450 to 1,800 ° C. in a non-oxidizing atmosphere, and α · Si ₃ N ₄ is 30 wt% or less (0 is not included. ) Sinter for the remaining time. If the sintering temperature is less than 1,450 ° C, the densification is insufficient, and if it exceeds 1,800 ° C, the densification progresses, but the silicon nitride powder is transformed into β ・ Si ₃ N ₄ and the high temperature strength decreases. . Even between 1450 ° C and 1800 ° C, it densifies due to long-term sintering on the low temperature side, and α ・ Si ₃ N ₄
However, α ・ Si ₃ N ₄ does not remain on the high temperature side unless the time is short. Therefore, when raising the sintering temperature, it is necessary to shorten the sintering time. The sintering method employed in the present invention is pressureless sintering, which has an advantage that a silicon nitride sintered body having a complicated shape or a large size can be manufactured.

(Example) Next, the present invention will be described more specifically with reference to Examples and Comparative Examples.

Examples 1-5, Comparative Examples 1-8 100 parts by weight of raw material silicon nitride powder shown in Table 1, 3.3 parts by weight of MgO, 2.2 parts by weight of Al ₂ O ₃ and 5.6 parts by weight of Y ₂ O ₃ are 1,1 parts by weight. 1,1-Trichloroethane was added, wet-mixed in a ball mill for 4 hours, dried, and then molded with a molding pressure of 100 kg / cm ² into a shape of 6 × 10 × 60 mm, and then CIP molded with a molding pressure of 2,000 kg / cm ² . . This compact was set in a carbon crucible and sintered under the conditions shown in Table 2. Sintering is pressureless sintering in a nitrogen atmosphere.

Regarding the sintered body obtained as described above, JIS R2205
Based on the relative density and X-ray diffraction pattern in accordance with, the α ・ Si ₃ N ₄ (102) plane, (210) plane, β ・ Si ₃ N ₄ (101) plane, (21
0) plane diffraction peak intensity ratio [α (102) + α (210) / α (102) + α (210) + β (101) + β (210)] α ratio and temperature of 1,200 according to JIS R1601
Three-point bending strength at ° C was measured.

The results are shown in Table 2.

It can be seen from Table 2 that the silicon nitride powders A, B and C according to the present invention, which were sintered so that α · Si ₃ N ₄ remained at 30% or less, had a relative density of 95% or more, Also, it is understood that the powder A has the high temperature strength of 800 MPa or more, and the powder A gives the best results.

SEM photographs (magnification: 5000 times) of the sintered bodies of Example 1 and Comparative Example 1 are shown in FIGS. 1 and 2, and X-ray diffraction is shown in FIGS. 3 and 4.

Examples 6 to 12 Comparative Examples 9 to 17 Using silicon nitride powder A, sintered bodies were produced under the conditions shown in Table 3. From Table 3, it can be seen that the examples of the present invention are superior in high temperature strength to the comparative examples. In addition, Example 7,
8 and Comparative Example 15 are HIP sintering.

(Effect of the Invention) According to the present invention, a silicon nitride sintered body having a hot strength at 1,200 ° C. of 800 MPa or more can be obtained.

[Brief description of drawings]

FIG. 1 is an SEM (scanning electron microscope) photograph showing the crystal structure of a silicon nitride sintered body obtained in Example 1 and FIG. 2 is in Comparative Example 1. 3 and 4 are X-ray diffraction diagrams of the silicon nitride sintered bodies obtained in Example 1 and Comparative Example 1, respectively.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI technical display location 102 C 102 D

Claims (2)

[Claims] 1. A specific surface area of 5 to 12 m ² / g and a proportion of α · Si ₃ N ₄ of 9
100 parts by weight of silicon nitride powder having a solid solution oxygen content of 0% by weight or more and 1.5% by weight or less, MgO 0.1 to 5 parts by weight, Al ₂ O ₃ , SiO ₂
And 0.1 to 10 parts by weight of mixed powder of at least one selected from Group IIIa element oxides, and then molding under a non-oxidizing atmosphere under normal pressure.
Α ・ Si ₃ N ₄ is 30 wt% or less (0
The method for producing a silicon nitride sintered body is characterized by performing sintering for the remaining time. 2. The method for producing a silicon nitride sintered body according to claim 1, wherein the proportion of α.Si ₃ N _{4 in} the silicon nitride powder is 95% by weight or more.