JPS5851911B2 - Method for manufacturing fiber-reinforced silicon nitride sintered body - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a fiber-reinforced silicon nitride sintered body, and more particularly to a method for manufacturing a silicon nitride sintered body compositely reinforced with fibrous silicon carbide as a reinforcing material.

Silicon nitride and silicon carbide have extremely excellent heat resistance and corrosion resistance, and are increasingly being used as components for high-temperature gas turbines and structural materials for diesel engines in place of conventional heat-resistant metals.

However, silicon nitride and improved silicon are difficult to sinter by themselves, and conventionally various oxides and nitrides are
% fJO was added to facilitate sintering.

For example, magnesium oxide, yttrium oxide, aluminum oxide, etc. are used as sintering aids for silicon nitride.
By adding ~20% and performing pressure sintering (hot pressing), a sintered body having a density close to the theoretical density can be obtained.

However, the glass phase caused by these sintering aids softens at high temperatures, and the strength of the sintered body is significantly lower than the room temperature value.

In order to prevent this decrease in strength at high temperatures, attempts have been made to crystallize the glass at the grain boundaries caused by the sintering aid, and to reduce the amount of the sintering aid added as much as possible. No solution has been reached.

On the other hand, apart from the study of sintering aids, there is also the idea of a fiber reinforcement method that increases the high-temperature strength of the sintered body by using composites, especially high-strength fibers.

It goes without saying that the fibers used for this purpose have sufficient strength, but it is also desirable that the fibers do not break during pressure sintering and that their strength does not decrease even at high temperatures.

Potential candidates for such fiber reinforcing agents include carbon fibers and ceramic fibers such as silicon carbide, silicon nitride, boron nitride, and alumina.

In particular, fibrous crystals of silicon carbide, commonly called whiskers, have attracted attention because they are close to single crystals and have extremely high strength and excellent heat resistance.

Furthermore, in recent years, silicon carbide long fibers obtained from organosilicon polymers are also considered to be effective as composite reinforcing materials.

However, these silicon nitride composite sintered bodies made of silicon carbide whiskers or silicon carbide long fibers are
Since the bulk density of the fiber itself is low, it is not possible to form a high-density sintered body when it is subjected to composite treatment, which has the disadvantage that sufficient strength cannot be obtained.

Therefore, the present invention was conceived in view of the current situation, and provides a composite sintered body which has been completely sintered, that is, the relative density of the theoretical value has been densified to 99% or more. It has the advantage of being able to obtain a sintered body that maintains excellent strength even at high temperatures of 1000° C. or higher.

That is, the present invention provides silicon nitride powder and its 10-4
Fiber-reinforced nitriding characterized by dispersing 0% by weight of fibrous silicon carbide in a binder to form a paste, forming this into a plate shape, drying it, stacking it, and sintering it under 7711 pressure. This is a method for manufacturing a silicon sintered body.

The present invention will be explained in detail below.

In the present invention, first, silicon nitride powder and 10 to 40% by weight of the fibrous silicon carbide are dispersed in a binder and thoroughly mixed to produce a paste.

Here, fibrous silicon carbide refers to silicon carbide whiskers,
Alternatively, it is silicon carbide long fibers obtained from organosilicon polymers, and if the amount mixed with silicon nitride is less than 10% by weight, sufficient composite reinforcing effect cannot be obtained, so it is not preferable, and if the amount mixed with silicon nitride is less than 10% by weight, it is not preferable. If it exceeds this value, the bulk density of the plate-shaped molded product will be significantly reduced, making it difficult to obtain a dense sintered body.

Any binder may be used as long as it has a viscosity that allows molding of a paste made of silicon nitride and fibrous silicon carbide, and can be easily removed by drying and degreasing. For densification, it is preferable to use polyvinyl alcohol, acrylic resin, cellulose, a solution of sodium alginate in water, alcohol or other organic solvent as a binder.

The amount of binder used varies depending on the type of binder, but
Normally, the amount is enough to form the paste into a plate shape, that is, 1% for the total amount of silicon nitride and fibrous silicon carbide
~20% by weight.

This paste is then shaped into a plate and dried.

Examples of the molding method include normal pressure filtration or pressure filtration using paper or cloth, a doctor blade method from a paste, or an injection molding method.

Although the thickness of this molded plate is not particularly limited, it is usually a thin plate of 0.5 to 3 gm, preferably 1 to 2 mm in order to facilitate the pressurization and sintering described later.

The obtained molded plate is usually dried under heat or reduced pressure, and the binder is completely removed by heating at a temperature of 600° C. or less.

After drying, the molded plates are stacked and sintered under pressure to obtain a sintered body.

The pressurization and sintering method is the so-called hot press method, in which the molded plate is first densified under a hydrostatic pressure of 3 to 10 tons, and then densified under a nitriding gas atmosphere at a pressure of 1,600 to 1
The sintered body obtained by the normal pressure sintering method, which is sintered at 800°C, or the hot press method or the normal pressure sintering method, is further sintered at 1,500 to 2,000 atmospheres in an inert gas at 1,600 to 1,850°C. A method such as hot isostatic pressing is used.

As mentioned above, in conventional silicon nitride composite sintered bodies, even if a mixture of silicon nitride powder and reinforcing fibers was pressurized and sintered, a high-density sintered body could not be obtained. This was because the bulk density was extremely low, and pores remained inside if only the pressure was applied during the sintering operation.

However, according to the pressing and firing method via plate forming of the present invention, at the stage of forming the plate from the paste mixture, the relative value to the theoretical density is 20 to 4, depending on the forming method.
A density of about 0% can be obtained.

For this reason, pores can be almost completely removed through the sintering process.
High density is easily achieved.

In addition, when obtaining a relatively dense plate-shaped molded product, as mentioned above, a binder such as cellulose, polyvinyl alcohol, or acrylic resin is used to promote densification and at the same time increase the strength of the molded product when dry. It is preferable to increase it appropriately.

By adding such a binder, the molded product that has undergone various densification operations such as filtration and injection molding has a relatively high density even after drying, and has a certain degree of strength, making it convenient for subsequent handling operations. .

However, when molding a plate by injection molding, in which a considerable number of binders are added to give fluidity, the plate-shaped molded product is pretreated with a so-called degreasing process to vaporize the binder. , it is necessary to remove it.

If this degreasing treatment is insufficient, carbon content may remain in the sintered body, resulting in a decrease in strength.

Furthermore, in the present invention, when the paste is made to flow in one direction when forming the plate-shaped article, the orientation of the reinforcing fibers can be promoted along this flow.

Therefore, in the sintered body obtained according to the present invention, the fibrous silicon carbide is primarily or secondarily oriented, so it is more effective as a reinforcing agent than a sintered body in which the fibrous silicon carbide is oriented in a tertiary or random manner. It can be extremely effective,
The result is a sintered body with significantly high strength.

As described above, according to the present invention, a plate-like object is first formed from a paste of silicon nitride powder and fibrous silicon carbide, and after the formed objects are laminated, they are pressurized and sintered. The pores remaining in the product are significantly reduced after sintering, resulting in a high-density sintered product.

Further, if the paste is made to flow in a fixed direction to form a plate-like object, the random orientation of the fibrous silicon carbide is reduced, and therefore the strength of the sintered body is further increased.

That is, the present invention can be said to be an epoch-making invention that solves all the drawbacks of the conventional silicon nitride-fibrous silicon carbide sintered body.

The present invention will be explained below using examples.

Example 1 Silicon nitride powder (9 containing 5% by weight magnesium oxide)
5% α-type silicon nitride) with silicon carbide whiskers as reinforcing fibers at 10, 20, 30° and 40% by weight, respectively.
In addition, this was irradiated with ultrasonic waves in water to untangle the fibers, and at the same time, the fibers were thoroughly mixed.

Next, about 5% by weight of polyvinyl alcohol was added as a binder to make a paste, and then it was made into a paste with a thickness of 5% by vacuum filtration.
A thin plate with a diameter of less than m was molded.

Cut this thin plate into the size of a graphite mold for hot press,
After drying, a predetermined number of layers were laminated, heated and degreased in a graphite mold, and then hot pressed at 1800° C. and 4ook/cr/1 for 60 minutes.

After cooling, the densities of the four types of sintered bodies (samples A to D) thus obtained were measured.

The results are shown in Table 1 below.

For comparison, the density of a sintered body obtained by hot pressing directly from a powder state without going through thin plate forming is also shown.

As is clear from Table 1, the sintered body of the present invention has a higher density than that obtained by hot pressing in a powder state.

Example 2 A sintered body containing 30% by weight of silicon carbide whiskers was obtained in the same manner as in Example 1.

However, as raw material nitride**silicon, α phase is 90% and 9
Powders containing 5% (A powder and B powder) were used.

The room temperature of the obtained sintered i body was 1100°C and 1300°C.
The bending strength at ℃ is shown in Table 2 below.

Further, for comparison, the bending strength of the sintered body without reinforcing whiskers was similarly measured.

From this Table 2, it can be seen that the bending strength of the sintered body of Example 2 at room temperature is lower than that of the sintered body with no whisker added for both A and B powders, but at high temperatures of 1100°C or higher, fiber reinforcement It is clear that the sintered body exhibits higher bending strength.

This proves that the manufacturing method of the present invention is effective in improving the high-temperature strength of silicon nitride sintered bodies.

Example 3 B powder silicon nitride of Example 2 (containing 5% by weight of magnesium oxide) with 30% by weight of silicon carbide whiskers added was dispersed in water, and about 5% by weight of sodium alginate was added as a binder. In addition, a thin plate was formed by a doctor blade method, cut into a predetermined size, dried, stacked, and hot pressed at 1750°C and 300°C/cIl for 90 minutes, and after cooling, the bending strength of this sintered body was measured.

The results are shown in Table 3.

A sintered body directly hot-pressed from a powder state without using the doctor blade method was also measured in the same manner for comparison.

The sintered body of Example 3, in which the orientation of the reinforcing fibers was promoted by the doctor blade method, exhibited higher bending strength at any temperature than that hot-pressed from non-oriented powder.

This clearly shows the effect of reinforcing fiber orientation.

Claims (1)

[Claims] 1 Disperse silicon nitride powder and 10 to 40% by weight of the fibrous silicon carbide in a binder to form a paste,
A method for producing a fiber-reinforced silicon nitride sintered body, which is characterized by forming this into a plate shape, drying it, stacking it, pressurizing it, and sintering it.