JPH05117032A - Production of silicon nitride sintered compact - Google Patents

Abstract

PURPOSE:To provide the silicon nitride sintered compact small in the mechanical strength drop at an elevated temperature. CONSTITUTION:A mixture of (A) 85-95wt.% of silicon nitride powder and (B) 5-15wt.% of sintering auxiliary powder comprising spinel (MgAl2O4) powder and zircon (ZrSiO4) powder is made into a form which is then sintered in a non-oxidative atmosphere at 1500-1700 deg.C to obtain the objective sintered compact. Based on the fact that a sintered compact made from the above zircon powder alone hardly varies in its mechanical strength in the temperature range from the room temperature to 1400 deg.C, the objective sintered compact is considered as being resistant to its mechanical strength drop even in the presence of the above-mentioned sintered compact as grain boundary phase after sintering.

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 which can be used as a heat resistant structural material for gas turbine parts, diesel engine parts and the like.

[0002]

2. Description of the Related Art In a silicon nitride sintered body having excellent heat resistance, silicon nitride itself is a covalent bond crystal, and it is difficult to sinter with silicon nitride powder alone. It is manufactured by adding a co-agent. Further, since the above-mentioned sintering aid contributes to the promotion of sinterability and also greatly affects the strength of the sintered body, various kinds and combinations of sintering aids have been conventionally studied.

For example, in Japanese Patent Publication No. 52-3647 and Japanese Patent Publication No. 57-77072, a compact is formed from a mixed powder of silicon nitride powder to which spinel (MgAl ₂ O ₄ ) as a sintering aid is added. There is disclosed a method for manufacturing a silicon nitride sintered body which is formed and sintered in a non-oxidizing atmosphere. Further, Japanese Patent Application Laid-Open No. 59-184771 discloses a manufacturing method in which yttria (Y ₂ O ₃ ) is added as a sintering aid in addition to spinel to further increase the strength.

[0004]

However, since the sintered body obtained by the above-mentioned conventional manufacturing method using yttria in addition to spinel as a sintering aid has low strength at high temperature, It was difficult to use for a long time. It is considered that this is because the yttria used as a sintering aid is an oxide, and therefore the strength of the yttria itself at high temperature is reduced.

The present invention has been made in view of the above circumstances, and it is an object of the present invention to provide a silicon nitride sintered body which has a relatively small decrease in strength at high temperatures and can be suitably used as a structural component even at high temperatures. To aim.

[0006]

A method for manufacturing a silicon nitride sintered body according to the present invention comprises: 85 to 95% by weight of silicon nitride powder, spinel (MgAl ₂ O ₄ ) powder and zircon (ZrSiO).
₄ ) A mixing step of mixing and mixing 5 to 15% by weight of a sintering aid powder made of powder to form a molded body, and sintering the molded body at 1500 to 1700 ° C. in a non-oxidizing atmosphere and firing. It is characterized by comprising a sintering step of forming a unit.

The molding step is a step of molding a compact from a mixed powder obtained by adding spinel powder and zircon powder as a sintering aid to silicon nitride powder. The average particle size of the silicon nitride powder is preferably 0.5 μm or less in order to improve the sinterability and obtain a dense sintered body. The mixing amount of the sintering aid powder is 5 to 15% by weight. If the amount of the sintering aid powder blended is less than 5% by weight, the sinterability will not be improved and a dense sintered body cannot be obtained. If the mixing amount of the sintering aid powder is more than 85% by weight, a dense sintered body can be obtained, but the amorphous phase (glass phase) has many grain boundaries in the sintered body, and The strength is significantly reduced. It is considered that this is because the sintering aid component existing as an amorphous phase at the grain boundary in the sintered body is exposed to high temperature to form a liquid phase again.

The mixing ratio of the spinel powder as the sintering aid powder and the zircon powder is preferably 1/3 to 3/1. When the amount of the spinel powder mixed is small with respect to the zircon powder and the ratio is less than 1/3, the sintering temperature becomes too high (higher than 1700 ° C.) and the grain size becomes coarse, which causes strength reduction, which is preferable. Absent. Further, if the amount of the spinel powder mixed with the zircon powder is large and the ratio thereof is larger than 3/1, the sinterability is improved but the high temperature strength is lowered, which is not preferable.

Regarding the particle size of the sintering aid powder, it is preferable that the average particle size is 0.5 μm or less in order to improve the sinterability and obtain a dense sintered body. The sintering step is a step of sintering the molded body obtained in the molding step at 1500 to 1700 ° C. in a non-oxidizing atmosphere to obtain a sintered body. As the non-oxidizing atmosphere, for example, nitrogen gas which does not oxidize nitrogen of silicon nitride and silicon can be adopted.

If the sintering temperature is lower than 1500 ° C., a sufficiently densified sintered body cannot be obtained. The sintering temperature is 170
If the temperature is higher than 0 ° C, the structure will not be refined due to abnormal grain growth of silicon nitride particles, and the strength will decrease. In the sintering step, a sufficiently dense sintered body can be obtained in nitrogen under normal pressure. For example, it should be performed under pressure in a nitrogen atmosphere of about 1000 atm after the surface layer of the sintered body has been densified. Is preferable, and the strength of the sintered body can be further enhanced, which is preferable.

[0011]

According to the method for producing a silicon nitride sintered body of the present invention, silicon nitride powder is mixed with a predetermined amount of spinel and zircon as a sintering aid, and the mixture is molded in a non-oxidizing atmosphere.
It is sintered at 00 to 1700 ° C. The zircon powder used as a sintering aid in the present invention has the following features. That is, it has been confirmed that the strength of a sintered body composed only of zircon powder hardly changes from room temperature to 1400 ° C. Therefore, it is considered that zircon, which is added together with spinel as a sintering aid, is resistant to strength reduction even if it exists as a grain boundary phase after sintering.

Therefore, the silicon nitride sintered body produced by the method of the present invention has a high room temperature strength and a relatively small decrease in strength at high temperatures.

[0013]

EXAMPLES The present invention will be specifically described below with reference to examples. Si
₃ N ₄ powder (average particle size: 0.5 μ or less, α-formation ratio: 92-
97%), MgAl ₂ O ₄ powder (average particle size: 0.3 μm, purity: 99.9%) as a sintering aid, and ZrSi.
O ₄ powder (average particle size: 0.5 μm or less, purity: 97% or more) was added in the mixing ratio shown in Table 1, and uniformly mixed in ethanol with a ball mill made of Si ₃ N ₄ and then dried. A mixed powder was obtained.

The above-mentioned mixed powder is pressure-molded at a pressure of 200 kgf / cm ² , the molded body is packed in a thin rubber, vacuum-enclosed, and then CIP (Cold Isostatic Pres).
In the process (s), a pressure of 3000 kgf / cm ² was applied to obtain a molded product having a predetermined shape (6 mm × 5 mm × 45 mm). The above-mentioned molded body was heated at the maximum heating temperature shown in Table 1 for 4 hours,
Sintering was performed in a furnace in a nitrogen atmosphere to obtain a sintered body. In addition,
The heating rate at this time was 1 ° C./min.

For each of these sintered bodies, room temperature 4-point bending strength (JIS R 1601) and high temperature (1000 ° C.) 4-point bending strength (JIS R 1604) were measured for each 20 pieces, and the average strength was determined. The results are shown in Table 1. The density in Table 1 means the relative density (% T) of the sintered body obtained by dividing the bulk density obtained by the n-butanol substitution method by the theoretical density.
D).

[0016]

[Table 1] As is clear from the results shown in Table 1, all of the sintered bodies according to the present examples have a high room temperature strength of 1000 MPa or more, and a high temperature strength of 1000 ° C. of 80%.
It was possible to secure 0 MPa or more. Further, in the present example, the effect of sufficiently improving the strength was obtained even by pressureless sintering. this is,
It is considered that zircon contributes to the improvement of sinterability. The sintered bodies according to Examples 6 to 8 which were sintered under pressure can be further strengthened and have a high temperature strength of 1.
000 MPa or more could be secured. On the other hand, the sintered body of Comparative Example 1 in which the total amount of the sintering aid added was too small did not sinter densely. In addition, the strength of the sintered body according to Comparative Example 2 in which the total amount of the sintering aid added was too large decreased. Further, the sintered body according to Comparative Example 3 having a too low sintering temperature did not sinter densely. The strength of the sintered body according to Comparative Example 4, which had an excessively high sintering temperature, decreased. ZrO ₂ instead of zircon powder
The sintered body according to Comparative Example 5 in which the powder and the SiO ₂ powder were added alone did not sinter densely. Further, the sintered body according to Comparative Example 6 in which ZrO ₂ powder was added instead of zircon powder and the sintered body according to Comparative Example 7 in which SiO ₂ powder was added instead of zircon powder did not sinter densely.

[0017]

As described above in detail, the method for producing a silicon nitride sintered body of the present invention can produce a silicon nitride sintered body having a high room temperature strength and a relatively small decrease in strength at a high temperature. Thus, it becomes possible to provide a silicon nitride sintered body that can be suitably used as a structural component even at high temperatures.

Further, according to the manufacturing method of the present invention, the high strength of the sintered body can be sufficiently obtained even by the normal pressure sintering, and the productivity is improved.

Claims (1)

[Claims] 1. 85-95% by weight of silicon nitride powder, spinel (MgAl ₂ O ₄ ) powder and zircon (ZrSiO).
₄ ) A molding step of mixing and mixing 5 to 15% by weight of a sintering aid powder consisting of powder to form a molded body, and sintering the molded body at 1500 to 1700 ° C. in a non-oxidizing atmosphere and firing. A method of manufacturing a silicon nitride sintered body, which comprises a sintering step of forming a unit.