JPS589785B2 - Manufacturing method of silicon carbide sintered body - Google Patents

Description

[Detailed description of the invention] The present invention relates to an improvement in the manufacturing method of silicon carbide sintered bodies.

Silicon carbide sintered bodies have excellent heat resistance and a low coefficient of thermal expansion, and high-density sintered bodies have high strength at high temperatures, so they are attracting attention as structural materials for high temperatures.

Conventionally, as a manufacturing method for silicon carbide sintered bodies, (1) 0.5 to 5% by weight of aluminum or an aluminum compound is added to silicon carbide powder of 10 microns or less and sintered under pressure at 1950°C or higher in an inert atmosphere. Method of tying (Japanese Unexamined Patent Publication No. 52-6
716), (2), β-type silicon carbide powder, 0.3
A boron additive corresponding to ~3.0 wt% boron and 0.
A method is known in which a carbon additive equivalent to 1 to 1.0% by weight of carbon is added and sintered at 1950 to 2300° C. in an inert atmosphere (Japanese Unexamined Patent Publication No. 1983-6716).

As a result of research on both types of sintering accelerators, the present inventors found that (1) when using an aluminum-based sintering accelerator alone to sinter silicon carbide, the amount of Al added is 0.5 It requires at least 1% by weight, preferably 1% by weight.

When such a large amount of Al is added, the high-temperature strength of the resulting silicon carbide sintered body decreases.

However, if a boron-based sintering accelerator is added at the same time, the amount of Al added can be significantly reduced due to the synergistic effect of the two, and the total amount added can be reduced to a small amount.

(2) Al imparts toughness to the resulting sintered body, but the high temperature strength is lower than the room temperature strength.

On the other hand, B has somewhat increased brittleness, but the high temperature strength is equal to or higher than the room temperature strength.

Therefore, it has been found that by setting a specific ratio of Al and B, a silicon carbide sintered body can be obtained that has high strength even at room temperature and does not lose its strength even at high temperatures.

The present invention was completed based on this knowledge.

The gist of the present invention is that the total amount of silicon carbide powder in the sintered body is 0.15 to 0.60% by weight, and the atomic ratio of B/Al is 0.
Boron-based and aluminum-based sintering accelerators in amounts in the range of 15≦B/Al<2, oxygen accompanying the raw materials and additives as carbon monoxide, and boron, aluminum, and silicon produced by the reaction as carbide. The carbon necessary for fixation as well as a carbon-based additive that provides 0.1-0.5% by weight of the silicon carbide are mixed and heated under an inert atmosphere to 18
A method for producing a sintered silicon carbide body characterized by sintering at a temperature of 50 to 2200°C.

Raw material silicon carbide has many polymorphs, but the present invention does not require a specific polymorph, and all can be used.

The particle size of this powder is preferably 5 microns or less, particularly 2 microns or less.

This is because, if the sintering conditions are the same, the smaller the particle size and the larger the specific surface area, the faster the density can be obtained.

Further, the content of impurity metal elements is preferably 0.5% by weight or less, particularly preferably 0.4% by weight or less.

Examples of boron-based sintering accelerators include boron, boron carbide (
Typical composition B4C), silicon boride (SiB4 or Si
B6), and examples of aluminum sintering accelerators include:
For example, aluminum, alumina aluminum hydroxide,
Aluminum carbide is mentioned.

The amount of these sintering accelerators added is such that the total amount of boron and aluminum in the sintered body is 0.15 to 0.60% by weight, preferably 0.15 to 0.40% by weight, and the atomic ratio of B and Al (
B/Al) is 0.15≦B/Al<2.

If the silicon carbide powder contains boron or aluminum as a solid solution, or if these are present as impurities, the amount added is the amount added to the above amount.

The total amount of boron and aluminum in the sintered body is 0.60
If it exceeds % by weight, the high-temperature melt generated around 1500 to 1600°C tends to remain up to the sintering temperature, which promotes abnormal grain growth and deteriorates the mechanical strength of the sintered body.

When the total amount is less than 0.15% by weight, sintering becomes poor.

Moreover, when the atomic ratio of B/Al becomes less than 0.15, B
A synergistic sintering effect of Al and Al cannot be expected, and 2 or more, for example 2 to 4
When this happens, it becomes difficult to densify by sintering due to grain growth.
The resulting silicon carbide sintered body has lower strength at high temperatures (for example, 1300° C.) than at room temperature.

In the present invention, when boron-based and aluminum-based sintering accelerators are added at the same time in the above ratio,
A high temperature melt is formed near °C.

This is a special phenomenon that occurs when both sintering accelerators are added at the same time, and is a phenomenon that cannot be observed when boron or aluminum is added alone.

The formation of such a high-temperature melt will harm sintering if it exists in large amounts, but if it is added in the amount specified in the present invention, it will result in uniform dispersion of the auxiliary agent at elevated temperatures during sintering. It is thought that this contributes to the sintering process, making it possible to perform sintering with a smaller amount of sintering accelerator than in the conventional method.

The carbon-based additive added in the present invention generally has the following three functions.

(1) Of the oxygen content accompanying the silicon carbide powder due to surface oxidation and the oxygen content accompanying the additives, the oxygen content that cannot be dissipated in the form of water vapor or the like at high temperatures is removed from the system mainly as carbon monoxide.

(2) Free silicon that previously existed in silicon carbide, the above (
Free silicon produced in the process of 1), silicon monoxide vapor, and silicon such as free silicon that may be produced in the solid solution process of additives, as well as boron and aluminum are combined and fixed as carbides.

(3) Some of what remained during sintering remains at the grain boundaries,
Suppress grain boundary movement or grain growth.

Examples of carbon-based additives include graphite or fine carbon powder.

However, in order to uniformly disperse carbon, it is necessary to use sugars, condensates of furfuryl alcohol and formaldehyde, or condensates of phenol and formaldehyde, etc., which are soluble in water or organic solvents, and heated at 400 to 1000°C.
It is preferable to add the organic compound in the form of an organic compound that disperses and generates free carbon when heated in a non-oxidizing atmosphere.

The amount of addition is determined by removing the oxygen content accompanying the silicon carbide powder and other additives as carbon monoxide, fixing boron, aluminum, and silicon produced by the reaction as carbide, and the carbon necessary to fix silicon carbide. 0.1-1.0% by weight
, preferably 0.1 to 0.55% by weight of carbon.

Addition of silicon carbide in an amount exceeding 1.0% by weight may cause more carbon than necessary to remain in the sintered body, which may deteriorate the mechanical properties of the sintered body.
This is not preferable because it deteriorates oxidation resistance.

Moreover, when it is less than 0.1% by weight, smooth progress of sintering is inhibited.

These silicon carbide and additives are manufactured using jet mills whose main passages are lined with silicon carbide sintered bodies, ball mills whose interior is lined with silicon carbide sintered bodies, and silicon carbide sintered balls to prevent contamination with impurities. Mix and grind using a grinding mixer or a mixer lined with rubber or resin.

When using an organic compound as described above as a carbon-based additive, mix an aqueous solution or an organic solvent solution of the organic compound with the pulverized mixture obtained as described above to homogeneously disperse it, and then dry. You may.

In this case, the resulting mixture was heated under a non-oxidizing atmosphere for 4 hours.
After heating to 00 to 1000°C to decompose the organic compound and generate free carbon, sintering may be performed.

Alternatively, the organic compound may be decomposed using an inert atmosphere such as argon or helium at atmospheric pressure or below atmospheric pressure during sintering, and then sintering may be performed.

In this case, in addition to the organic compound that forms free carbon, if necessary, an organic binder such as polyvinyl butyral, polyvinyl acrylate, etc. (along with a plasticizer if necessary) is used to facilitate cold molding.

According to the method of the present invention, since both Al and B sintering accelerators are added, the atomic ratio of B/Al is set to 0.15≦B/Al<
In particular, silicon carbide raw material powder is difficult to obtain with high purity, and usually contains aluminum as an impurity.

When using such a silicon carbide raw material, if a single compound such as AlB2 is used, the atomic ratio of B/Al cannot be adjusted, but according to the present invention, it is possible to adjust the atomic ratio even when such a raw material is used. is easy.

In addition, the atomic ratio of B/Al is set to a specific ratio (0.15≦B/Al
<2), by using a small amount of sintering accelerator, it is possible to obtain a silicon carbide sintered body that has high strength even at room temperature and does not lose its strength even at high temperatures.

In addition, by adding an appropriate amount of carbon-based additives, the oxygen content in the sintered body can be removed, thereby preventing the sintering accelerator from forming a foreign phase within the sintered body, resulting in high strength even at high temperatures.
This has the effect that a silicon carbide sintered body having physical properties with excellent oxidation resistance can be obtained.

Example 1 Two types of commercially available industrial silicon carbide products, green and black (both 200 mesh or less), were ground repeatedly in a jet mill with a powder flow path made of steel, while being classified with an air flow, and then sieved with water. A fine powder with a particle size of 1.5 microns or less was obtained.

This fine powder was oxidized in an oxygen stream at 600°C and treated with hydrochloric acid and nitric-fluoric acid.

The analysis results were as follows.

Boron-based and aluminum-based sintering accelerators and carbon-based additives are added to these fine powders under the conditions shown below.
Calcining and sintering were performed under the conditions shown below.

The properties of the sintered body are as follows. In addition, when the above-mentioned sintered body of the present invention was examined using an electron beam irradiation X-ray microanalyzer, no segregation of aluminum and boron was observed in the sintered body.

Note that since the raw material black already contained 0.22% by weight of aluminum at the raw material stage, no aluminum-based additives were added to this material.

Example 2 High-purity amorphous silica fine powder and carbon black were mixed at a molar ratio of 1:4 using an agate ball mill, and this was heated at 1900° C. for 30 minutes under an argon atmosphere at atmospheric pressure. Subsequently, the temperature was maintained at 2300° C. for 10 hours and cooled, and then free carbon was oxidized and removed.

Next, in the same manner as in Example 1, the mixture was crushed and purified using a jet mill to obtain a fine silicon carbide powder.

Its composition and analysis results were as follows.

A boron- and aluminum-based sintering accelerator and a carbon-based additive were added to this powder under the conditions shown below, and calcination and sintering were performed under the conditions shown below.

The properties of the sintered body were as follows.

When the sintered body was examined using an electron beam irradiation X-ray microanalyzer, no segregation of boron or aluminum was observed in the sintered body.

Example 3 Coarse particles of 1 micron or more were removed from the black industrial silicon carbide fine powder shown in Example by a sedimentation method, and fine particles were obtained from the suspension by centrifugation, which was then treated with nitric hydrofluoric acid. A fine powder with a specific surface area of 8 m2/g was obtained.

The oxygen content of this fine powder was 0.24% by weight.

Other analysis results were almost the same as in Example 1.

To this fine powder, add 0.15% by weight of boron fine powder and 2.5% by weight of boron fine powder.
% by weight of a condensate of furfuryl alcohol and formaldehyde, 3% by weight of polyvinyl butyral (binder)
and 40 parts by weight of acetone (dispersion medium) were added, mixed in a polyethylene mixing ball mill, and dried to obtain a powder for cold-open molding.

This powder was rubber pressed to obtain a cylindrical molded body (approximately 10φ×10) containing 48% by weight in terms of silicon carbide.

This molded body was placed in a graphite container and heated to 400°C in an argon stream.
After slowly raising the temperature to 50° C./min to scatter the binder, the temperature was raised to 2050° C. at 200° C./min and held at this temperature for 30 minutes.

A sintered body with a porosity of about 12% was obtained.

During this sintering process, sintering shrinkage of about 18% was exhibited, but the shrinkage was uniform and no cracks were observed.

The bending strength was about 30 kg/mm2 at room temperature.

The distribution of boron, aluminum, and iron within the sintered body was uniform, and no segregation to grain boundaries was observed using an X-ray microanalyzer.

Claims (1)

[Claims] 1 Silicon carbide powder contains a total amount of 0.15 to 0.15 in the sintered body.
At 0.60% by weight, the atomic ratio of B/Al is 0.15≦B/
Both boron-based and aluminum-based sintering accelerators in an amount in the range of Al<2, and the oxygen accompanying the raw materials and additives are converted into carbon monoxide, which is necessary to fix boron, aluminum, and silicon produced by the reaction as carbide. carbon and a carbon-based additive that provides 0.1 to 0.5% carbon by weight of silicon carbide, and 1850 to 220
A method for producing a silicon carbide sintered body, characterized by sintering at 0°C.