JPS61186265A - Silicon nitride sintered body and manufacture - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a silicon nitride sintered body having excellent high-temperature strength and a method for manufacturing the same.

[Prior art]

Silicon nitride ceramic is a material that has excellent properties such as heat resistance, thermal shock resistance, abrasion resistance, and chemical stability, and is expected to be applied to various structural parts.

Silicon nitride (Si3N) has strong covalent bonds and is difficult to sinter, so in order to obtain a dense sintered body, an auxiliary agent must be added to promote sintering. Until now, much research has been done on the sintering aids, such as magnesia (MgO) and alumina (A7!2).
o3), spinel (MgAl2O3), rare earth oxides (Y2O2, La20., etc.),
It is known to be extremely effective in sintering silicon nitride, and has been widely praised.

When these oxide-based auxiliary agents are added, sintering is promoted by the amorphous phase (glass phase) of the oxide that is generated at the grain boundaries of silicon nitride, resulting in sufficient sintering and dense formation. A hardened sintered body can be obtained.

[Problem to be solved]

By using an oxide sintering aid as mentioned above,
A dense sintered body can be obtained, but although this sintered body exhibits excellent properties at room temperature, it cannot be used in a high temperature range (approximately 8
(00° C. or higher), its initial characteristics deteriorate significantly.

For example, its bending strength is approximately 14 kg at room temperature.
When f is around 712, at 1200℃,
Approximately 30 kg f/mu” and 40 at room temperature
% level.

This decrease in high-temperature strength is caused by the grain boundary phase (amorphous phase) within the sintered body.
This is due to the softening of the grain boundaries, and in order to improve the high-temperature strength, it is necessary to increase the heat resistance of the grain boundaries.

The heat resistance of grain boundaries can be improved by crystallizing the grain boundary phase, but oxide sintering aids generally become amorphous during the cooling process after sintering. A conceivable method for crystallizing this amorphous phase is to perform heat treatment again after sintering to crystallize it. However, according to experiments conducted by the present inventors, although this method can prevent a decrease in high-temperature strength, it has the disadvantage that the strength at room temperature decreases by about 20 to 30%.

Expanding the use of silicon nitride sintered bodies as various structural parts
In order to achieve diversification, it is necessary to improve the properties in the high temperature range, especially at 800° C. or higher, without impairing the properties at room temperature.

[Technical means and effects]

In the present invention, by devising the composition of the sintering aid, a mixed phase of an amorphous phase and a crystalline phase is formed as the grain boundary phase of the silicon nitride sintered body, without impairing the room temperature characteristics.
It has improved characteristics in high temperature range.

In the silicon nitride sintered body of the present invention, the grain boundary phase is an oxide-based amorphous phase and a Si-W-based, Si'-Co-based, or Si-C
It is characterized in that it consists of a mixed phase with one or more o-W crystal phases.

The sintered body of the present invention having such a grain boundary phase is sufficiently densified, has a high relative density, and has excellent room temperature characteristics, and at the same time, as shown in the examples below, it can be heated to 800°C.
It exhibits high-temperature properties superior to conventional materials, with little loss of strength in temperature ranges exceeding

According to the method of the present invention, the silicon nitride sintered body having the grain boundaries is produced by adding tungsten carbide and cohal to silicon nitride powder together with an oxide compound as a sintering aid.
) as a starting material.

The grain boundary crystal phase produced by adding tungsten carbide and cobalt powder to silicon nitride powder is Si-
It is broadly classified into W-based, Si-Co-based, and Si-W-Co-based.
Examples of the i-Co type include SiCo2, and examples of the Si-W-Co type include CoWSi. However, the crystal phase that is formed varies depending on the sintering temperature, and at relatively low sintering temperatures, it is mainly Si3W5 and Si3W5.
Crystal phases such as Co□ are easily generated, and as the sintering temperature increases, Si3W, SiWCo, etc. are generated. However, regardless of the crystal phase, the effect of improving high-temperature properties is the same, and it is possible to prevent a significant decrease in strength in the high-temperature range.

The amount of tungsten carbide powder and cobalt powder in the starting material powder mixture is approximately proportional to the proportion of the crystal phase in the grain boundaries of the resulting sintered body.

The amount of tungsten carbide blended is preferably in the range of 0.5 to 10% by weight. This is because if it is less than 0.5% by weight, the proportion of crystalline phase at grain boundaries will be small and the effect of improving high-temperature properties will be insufficient, and if it exceeds 10% by weight, the particles will remain even after sintering. It remains unreacted as a defect in the sintered body,
This is because it causes damage to mechanical properties. As the tungsten carbide, either 3WC or W2C, or a mixture of both may be used as desired, and when both are used, the total amount thereof may be 0.5 to 10% by weight.

The appropriate amount of cobalt powder is 0.1 to 5% by weight. If the amount is less than 0.1% by weight, the added effect will be insufficient, and if it exceeds 5% by weight, it will remain as an unreacted substance and cause destruction of the sintered product, as in the case of tungsten carbide. Because it will be.

In particular, within the above range, the ratio to the amount of tungsten carbide (cobalt/tungsten carbide) is 1/2 to 115.
It has been found that a greater effect can be obtained when blended so that the following is achieved.

The oxide compound is blended with the tungsten carbide and cobalt powder to promote sintering. This oxide compound is not particularly limited, and includes, for example, magnesia (MgO), alumina (A Ilz 03 ),
Zirconia (ZrO2), spinel (MgA7!zo
One or more oxides selected from rare earth oxides such as a), yttria (yzo, ), lanthania (T-a203), etc. can be used as appropriate. The blending amount (total amount when two or more types are used together) is suitably in the range of 1 to 20% by weight. This is 1
If it is less than 20% by weight, the sintering promotion effect will be insufficient and densification will be insufficient, while if it exceeds 20% by weight, the high temperature strength will decrease due to an increase in the amorphous phase at the grain boundaries.

There are no particular restrictions on other manufacturing conditions in the method for manufacturing a sintered body of the present invention, and kneading and preparation of the starting material powder mixture, molding and sintering of the compact, etc. can be carried out using general methods and under normal conditions. good. For example, the starting material is a mixture of a silicon nitride powder bed and a sintering aid in the ratio specified above, and if necessary, an appropriate amount of a suitable binder (a diluted solution of methyl cellulose, acrylic acid methyl ester, etc.) as a forming aid. (e.g. 1 to 10% by weight) and sufficiently kneaded, or the kneaded material is mixed with an appropriate particle size (e.g. 3% by weight) using a spooled dryer method or the like.
0 to 100 μm).

Further, the forming and sintering of the starting material is performed by a pressureless sintering method, a pressure sintering method, or the like. In the case of the pressureless sintering method, the starting material powder mixture is pressure-molded into the desired shape by an appropriate molding method (rubber press, -shaft molding, etc.), and the molded body is placed in a nitrogen gas atmosphere (for example, 1 to 10 atm). ) and maintained at an appropriate temperature (usually about 1650 to 1900'C) for a predetermined period of time. In addition, according to the pressure sintering method, the starting material powder mixture or its compact is loaded into, for example, a graphite mold, and a predetermined pressure (for example, 150 to 500 k) is applied.
gf7c+J) at an appropriate temperature (approximately 1650~18
00'c) for a predetermined period of time. As the pressure sintering method, geothermal isostatic sintering method or the like can also be applied.

〔Example〕

Example 1 Silicon nitride powder bed (gelatinization rate of 90% or more) 90% by weight, sintering aids: spinel powder 4% by weight, zirconia powder 4%
wt%, tungsten carbide (WC) powder 1.5 wt%
, and a mixed powder consisting of 0.5% by weight of cobalt powder (
Each powder has an average particle size of 1 μm or less) in a ball mill for 24 hours.
After wet kneading for an hour, it is dried in a spooled dryer.
Granules (particle size 70 to 100 μm) are obtained. This was formed using a -shaft metal press (pressure force 200 kg f/cJ), and then rubber press molding method (press force 2000 kg f/cJ).
/ cJ) to form a disk-shaped compact of 60 mmφ x 10 mm', and then under nitrogen atmosphere (1 atm) at
The temperature was maintained at 00° C. for 2 hours to complete the sintering.

Example 2 Powder mixture consisting of 88% by weight of silicon nitride powder bed (gelatinization rate of 90% or more), 4% by weight of spinel powder, 4% by weight of zirconia, 3% by weight of tungsten carbide (WC) powder, and 1% by weight of cobalt powder. Kneading, molding and sintering were carried out under the same conditions as in Example 1.

[Comparative example]

Using a powder mixture consisting of 5% by weight of spinel and 5% by weight of zirconia, kneading, molding and sintering were performed under the same conditions as in Example 1.・ Comparison flame ↓ Using a powder mixture consisting of 90% by weight of silicon nitride powder (gelatinization rate of 90% or more), 4% by weight of spinel, 4% by weight of zirconia, and 2% by weight of tungsten carbide (WC), The same kneading, shaping and sintering were carried out.

The bending strength at room temperature and high temperature of the sintered bodies obtained in the above Examples and Comparative Examples (by three-point bending method) was
Shown in the table.

z As shown in Table 1, the sintered body obtained by the conventional method (Comparative Example 1) showed a decrease in bending strength at 800°C,
At 1000℃, it decreases to about 40% of normal temperature, and at 1200℃
It has declined even more significantly. On the other hand, in the sintered body of the example of the present invention, there was no decrease in strength at 800'C,
Even at 1000'C, it has a strength that is 70% or more of that at room temperature, and even at 1200'C, it maintains a strength that is 50% or more of that at room temperature. In Comparative Example 2, in which tungsten carbide was used as part of the sintering aid, the high temperature strength was slightly improved compared to Comparative Example 1;
The strength decreases significantly in the above cases, and is not at all comparable to the examples of the present invention.

The X-ray diffraction results of the sintered body obtained in Example 1 are shown in FIG. In addition to β-3i, N4, and 3W in the silicon nitride powder world,
A mixture of crystal phases such as s S i3, S it W, and S 1WCo is observed.

〔Effect of the invention〕

According to the present invention, a silicon nitride sintered body having grain boundaries consisting of a mixed phase of an amorphous phase and a crystalline phase can be obtained, unlike a conventional silicon nitride sintered body whose grain boundaries are composed only of an amorphous phase. It will be done.

The silicon nitride sintered body obtained by the present invention not only has good properties at room temperature, but also has high grain boundary heat resistance, so there is little decrease in strength even in a high temperature range, and it has excellent high temperature properties. Therefore, it is suitable for high-temperature mechanical parts, such as engine parts and furnace material parts, and can guarantee durability and stability superior to conventional silicon nitride sintered bodies.

[Brief explanation of drawings]

FIG. 1 is an X-ray diffraction diagram of a silicon nitride sintered body obtained according to the present invention.

Claims (4)

[Claims] (1) The silicon nitride grain boundary phase is separated from the oxide-based amorphous phase by Si
- A silicon nitride sintered body comprising a mixed phase with one or more of W-based, Si-Co-based, or Si-W-Co-based crystal phases. (2) Si-W crystal phase is Si_2W and/or S
i_3W_5, and the Si-Co crystal phase is CoSi_
2, and the silicon nitride sintered body according to item (1) above, wherein the Si-W-Co crystal phase is CoWSi. (3) An oxide-based amorphous material characterized by using as a starting material a powder mixture of silicon nitride powder, tungsten carbide powder, and cobalt powder as well as an oxide-based compound as a sintering aid. phase, Si-W system, S
A method for manufacturing a silicon nitride biscuit having a grain boundary phase consisting of a mixed phase with one or more of i-Co or Si-W-Co crystal phases. (4) The blending amount of the sintering aid in the starting material powder mixture is 5 to 10% by weight for tungsten carbide powder, 0.1 to 5% by weight for cobalt powder, and 1 to 1% by weight for oxide compound powder. The method for producing a silicon nitride sintered body according to item (3) above, wherein the content is 20% by weight.