JPS5848507B2 - Manufacturing method of silicon nitride molded body - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a molded article mainly containing silicon nitride.

Silicon nitride has many advantages such as high heat resistance, low coefficient of thermal expansion, high strength, and therefore excellent thermal shock resistance, high hardness and excellent wear resistance, and not being wetted by molten metal and excellent corrosion resistance. It is one of the new materials that has been in the spotlight in recent years due to its unique characteristics.

However, since silicon nitride has poor sinterability, the reality is that a rod-shaped body with excellent properties is not easily available.

As a method for manufacturing a silicon nitride molded body, a hot pressing method, a reaction sintering method (or a nitriding sintering method), and the like are known.

When using the hot press method, the silicon nitride molded product obtained is dense and has the advantage of having properties close to the original characteristics of silicon nitride, but in terms of shape, only extremely simple shapes such as cylinders and prisms can be obtained. When trying to obtain a complex shaped body, mechanical processing such as diamond grinding is required from these large blocks, which has the disadvantage of being expensive.

In the reaction sintering method, the shape of the shaped body is given in advance from metallic silicon or a mixed powder of metallic silicon and silicon nitride by methods such as pressure shaping, extrusion, injection shaping, and casting shaping. A silicon nitride molded body is obtained by heating in a nitrogen atmosphere.

Therefore, this method has the advantage that it is relatively easy to obtain a large amount of shaped bodies with quite complex shapes, but there is almost no dimensional shrinkage during nitriding sintering.
For this reason, the apparent density of the compact obtained by the reaction sintering method is
There is a drawback that it is limited by the apparent density of the green compact and it is difficult to obtain a dense silicon nitride molded body.

Currently, the silicon nitride shaped body obtained by the reaction sintering method is 2.0
~2.4971 apparent density, 900~1400kg/
It is common to have a bending strength of crit, which limits its uses.

An object of the present invention is to provide a manufacturing method that improves the properties of a metal silicon molded body while taking advantage of the economic advantages of the reactive sintering method.

In other words, by using the reaction sintering method, the bending strength is 2000 kg/
It is an object of the present invention to provide a method for manufacturing a silicon nitride shaped body having a silicon nitride shape of Crlt or higher.

In the reaction sintering method, in order to obtain a compact with a high apparent density, it is necessary to increase the apparent density of the green compact as much as possible.

However, when the apparent density of the compact increases, it becomes difficult for nitrogen gas to penetrate into the compact during sintering, and the nitriding reaction does not proceed easily, requiring a longer sintering schedule.

Furthermore, an increase in the apparent density of the green compact impairs the dissipation of the heat generated during the nitriding reaction, and a portion of the metal silicon tends to melt, resulting in a compact that lacks uniformity. There were drawbacks.

As a result of various studies in order to solve these drawbacks, the inventor of the present invention used a powder mainly composed of fine metallic silicon with an average particle size of 5μ or less, and added an appropriate dispersion medium to it to make it into lumps. By drying and pulverizing the powder into a powder of 150μ or less, pressurizing it and nitriding it, it has a higher apparent density than conventional products and a bending strength of 2000K.
It is an object of the present invention to provide a method for producing a silicon nitride-based hollow body that yields a silicon nitride hollow body having a silicon nitride density of 1 or more.

The present invention involves adding and kneading a dispersion medium of 36 to 45 volumes φ to a powder mainly composed of metallic silicon with an average particle size of 5μ or less to form a lump, and drying and pulverizing the lump to obtain a particle size The method is characterized in that it is made into a powder with secondary particles of 150 μm or less, and then the green compact obtained by pressurizing the powder is heated in a nitrogen atmosphere.

The present invention will be further explained in detail below.

The particle size of the metallic silicon used as the raw material of the present invention is determined so that the nitriding reaction can proceed rapidly and to obtain a uniform shaped body.
It is desirable to be as detailed as possible.

Furthermore, when fine metallic silicon is used as a raw material, its high reactivity allows the sintering time to be shortened, which has a great economical effect.

The metallic silicon used in the present invention has an average particle size of 5μ or less, and if the average particle size is less than 5μ, the nitriding reaction does not proceed easily during nitriding sintering, and unreacted metallic silicon remains and is uniform. It is difficult to obtain a rod-shaped body with high flexural strength, and as a result, one with high flexural strength cannot be obtained. Although these drawbacks can be naturally improved by sintering the green compact for a long time, it is not economical.

In the present invention, iron or iron fluoride is used as a catalyst when fine metallic silicon cannot be used.

By adding a catalyst, the nitriding reaction can proceed easily and the rod-shaped body can be manufactured economically.

The average particle size of the catalyst is preferably 5 μm or less, similar to metal silicon.

In addition, for the purpose of changing the microstructure of silicon nitride,
Yttrium oxide or the like may be added.

In the present invention, a binder is added in order to increase the strength of the secondary particles and make compacting under high pressure easy, and also to increase the green strength of the powder compact and improve workability.

This binder must be removed from the surface of metal silicon during the nitriding reaction, and generally an organic binder such as polyethylene glycol or polyvinyl alcohol is added to the raw material after being dissolved in 1 to 3 times a dispersion medium. be done.

A dispersion medium is a substance that acts as a shield at room temperature, but it should not be used as a material that is poisonous, highly toxic, easily ignitable, expensive, or that easily reacts with metal silicon. It's not appropriate.

Specific examples of the dispersion medium are preferably those that can completely dissolve the binder; for example, when using methyl cellulose as the binder, organic solvents such as toluene and acetone are used; when using hollyhinyl alcohol as the binder, water is used. and so on.

When water is used as a dispersion medium, it may react with metal silicon and oxidize its surface to form a film, but water is one of the superior dispersion media in terms of workability and economy. .

In this case, the selection is economically determined by considering the effect on the physical properties of the molded product obtained through the entire process.

In the present invention, if the amount of the dispersion medium is not 36 volumes or more relative to the raw material powder, it is difficult to obtain a certain shape with substantially high density and sufficient uniformity, and especially to improve the bending strength. It is not possible.

This is presumed to be because the binder is difficult to disperse sufficiently uniformly, and therefore the contact between the primary particles is not sufficient, leaving relatively large pores between the primary particles.

In this sense, sufficient dispersion of the binder and close contact of the primary particles are expected at a volume of 36 φ or more, with or without pressurizing the powder.

However, if the volume of the dispersion medium exceeds 45, the dispersion medium will be in a substantially nullary state, and more heat than necessary will be used to volatilize the dispersion medium, which is economically disadvantageous.

In other words, in the case of 36 to 45 volume rice cakes, by appropriately applying pressure to the wet powder, it is possible to achieve good quality and economical results, and to ensure that the binder is sufficiently uniformly dispersed and the primary particles are in close contact with each other. This makes it possible to produce secondary particles such as powder.

The method of crushing is not essential to the present invention; it is possible to use a hammer minor, bo! Although conventional methods such as remilling are used, a pulverization method that makes the shape of the secondary particles obtained by pulverization as round as possible is suitable for the present invention.

Regarding secondary particle powders, it varies depending on the particle size distribution, but generally those with large particle sizes leave large pores in the green compact, and these pores are not easily erased even after nitriding and sintering. ζ tends to remain in the molded bodies, often resulting in molded bodies with a non-uniform structure and insufficient properties.

In addition, if too fine secondary particles are used, the compression ratio will be large during compaction, and it will be difficult to increase the density of the resulting green compact, resulting in insufficient properties, especially high strength. It becomes difficult to obtain a molded body.

The present invention is based on the discovery that a molded article with high density and high strength can be obtained by using secondary particles of 150 μm or less.

The silicon nitride shaped body produced by the method of the present invention is 2.5 to 27 g/
Since it has an apparent density of 2000K and a bending strength of 9/door or more, it is closer to the original properties of silicon nitride than conventional products.

Since the product of the present invention has such high bending strength, its range of use can be expanded to fields such as sliding members such as mechanical seals and refractory structural parts that require high-temperature strength.

The present invention will be further explained in detail below with reference to Examples and Comparative Examples.

Examples and Comparative Examples The raw materials shown in the table were placed in a 500 g polyethylene container and dry mixed for 24 hours.

Next, 8 weights of a binder (polyvinyl alcohol was used when the dispersion medium was water, and carboxymethyl cellulose was used when the dispersion medium was toluene) was dissolved in advance in the dispersion medium. After adding the required amount, the mixture was kneaded using a mortar mixer.

In addition, when the dispersion medium amount is 30 and 40 volumes, it is in the state of "wet powder", and these are 500 ky in the mold die.
The mixture was pressurized with a pressure of 1/2 to form a lump, and then dried at 105° C. for 24 hours.

Further, when the dispersion medium is 50 volumes diagonally, the dispersion medium is in a very soft soil state and is substantially close to a nullary state.

In this case, after mixing, the mixture was transferred to an enamel container and dried at 105° C. for 48 hours.

The thus-obtained lumpy sample was ground in a mortar and sieved to form a powder of secondary particles of 150 μm or less.

In addition, in Experiment A7, the thickness was set to 297μ or less for comparison.

These secondary powder particles were processed at 2100k using a mold die.
By pressure molding at a pressure of g/ffl, 10 mm x 8
A green compact was obtained with dimensions of m7IL×80r/L7rt.

This green compact was heated at 400°C for 48 hours in a nitrogen atmosphere to volatilize the organic matter, and then heated to 1250°C.
Nitriding sintering was carried out by holding at C for 20 hours and further holding at 1450°C for 10 hours.

The results of measuring the physical properties of these molded bodies are shown in the table.

Further, Experiment/I68 was conducted in the same manner as in Example 1 except that the dispersion medium was not used in the method of Example 1.

The results are also shown in the table.

As is clear from the table, all of Examples Experiments/161 to 4 have a bending strength of 2o00 kg/1 or more, and Comparative Examples Experiments/165 to 8 have an apparent density that is different from that of Examples. There are some that do not, but the bending strength is 2000kg/crj
．． None of the above was observed, indicating that the present invention is superior.

Claims (1)

[Claims] 1. Add and knead 36 to 45 volumes of dispersion medium to a powder mainly composed of metallic silicon with an average particle size of 5μ or less to form a lump, dry and crush the lump to form a powder with a particle size of 150μ or less. 1. A method for producing a silicon nitride-based molded body, which comprises making a powder into secondary particles, then press-molding the powder, and heating the green compact obtained in a nitrogen atmosphere.