JPH0116790B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a silicon carbide sintered body, particularly a silicon carbide sintered body whose surface layer is composed of silicon carbide and silicon nitride.

For silicon carbide sintered bodies, silicon carbide and carbon are mixed with an organic binder, molded, calcined, machined as necessary, and then molten or gaseous metal silicon is added to the sintered body. The carbon in the calcined body is made into silicon carbide by contacting the
A method of forming a silicon sintered body is known (see US Pat. Nos. 2,938,807 and 3,495,939). This method requires a hot pressing method to produce this type of silicon carbide sintered body, or a method in which fine particulate silicon carbide of 1 μm or less is heated at 1,900 to 2,300°C, the reaction temperature is lower than that of metal silicon. The melting point of 1400℃ to 1700℃ is sufficient, and the sintered body obtained by this method has a dimensional change of 1 to 2% or less during shaping and sintering, and there is no distortion during sintering. It has the advantage that a homogeneous sintered body can be obtained.

However, since this method involves contacting and reacting metallic silicon with a calcined silicon carbide body, in addition to remaining unreacted metallic silicon in the target silicon carbide sintered body, this method Metallic silicon adheres to the surface of the sintered body, and this adhesion is also caused by surface blowing due to volume expansion when the metallic silicon remaining in the sintered body solidifies. Metallic silicon must be removed. Removal of this metal silicon is generally carried out by mechanical methods such as sandblasting, but this method leads to high costs when applied to mass production of small items, and this also has the disadvantage of Removal of this metallic silicon has the disadvantage that it may impair the dimensional accuracy of the metal.
Attempts have also been made to treat the sintered body with a chemical that dissolves only silicon metal without attacking silicon carbide, such as a mixed solution of fluoric acid and nitric acid, or a warm solution of caustic soda at a temperature of 50°C or higher. This has the serious disadvantage that the surface of the sintered body changes color to yellow-green and its hardness drastically decreases.
Another disadvantage was that this yellow-green surface layer had to be machined by sandblasting, barreling, or the like.

Note that if this silicon carbide sintered body is used as a crucible for melting high-purity metal, the metallic silicon will melt and contaminate the target object, so it should be kept in the air beforehand. It has also been proposed to convert silicon metal into SiO ₂ through high-temperature treatment, but this SiO ₂ is reduced to metal silicon in a reducing atmosphere, and under high-temperature vacuum, this SiO ₂ vaporizes. It pollutes the atmosphere, and when this crucible treatment melts ferrite oxide, SiO ₂
This oxidation treatment is not always considered to be useful, since it dissolves into the melt.

The present invention relates to a method for manufacturing a silicon carbide sintered body that solves these disadvantages, and is a method for manufacturing a silicon carbide sintered body consisting of silicon carbide and metal silicon by reducing the amount of silicon metal present in the surface layer of the silicon carbide sintered body to 900% It is characterized by reacting with nitrogen gas at a temperature of 0.degree. C. or higher to form silicon nitride.

To explain this, the present inventors have conducted various studies on the modification of reactive sintered silicon carbide consisting of silicon carbide and metal silicon, and found that the metal silicon carbide present on the surface of this silicon carbide sintered body If silicon nitride is obtained by nitriding, the heat resistance will increase from 1400℃, the melting point of metal silicon, to 1900℃, the decomposition temperature of silicon nitride.
℃, so even if this crucible is used as a metal melting crucible, the target metal will not be contaminated by the decomposition of silicon nitride. We discovered that silicon nitride is neither reduced nor evaporated during processing, and that silicon nitride is formed only on the surface layer, with the inner layer remaining unchanged as a SiC-Si sintered body. Therefore, the present invention was completed after confirming that this can be widely used as a non-oxide ceramic for various purposes.

The reactive sintered silicon carbide sintered body used as the starting material in the method of the present invention may be any conventionally known one,
This is done by mixing silicon carbide and carbon powder with a binder such as methylcellulose, phenolic resin, silicone resin, or tarpitz, molding this into a desired shape using a rubber press or extrusion molding, and then using nitrogen, argon, It is heated to 500-1000℃ in an inert gas such as neon to form a calcined body, and then this is heated in an inert gas or vacuum.
It can be obtained by heating to 1400°C or higher and contacting it with molten or gaseous metal silicon. In this case, most of the metallic silicon reacts with carbon to become silicon carbide, and the remainder remains inside the calcined silicon carbide body, but some of it remains when cooled as described above. It will blow out to the outer layer due to volumetric expansion.

The method of the present invention is for nitriding the metallic silicon present in the outer layer of this silicon carbide sintered body,
This is done by heating the silicon carbide sintered body obtained by the above method in a nitrogen gas atmosphere at a temperature of 900°C or higher, preferably 1250°C.
This can be done by heating the temperature between ℃ and 1450℃.
Since this reaction does not extend to the inner layer of the silicon carbide sintered body due to its dense structure, only the metallic silicon in the surface layer is nitrided.
Before carrying out this nitriding reaction, of the metallic silicon adhering to the surface layer of this silicon carbide sintered body, those that may impair the shape or dimensional stability of the molded body are removed in advance. This may be done by mechanical means such as sandblasting, or by treatment with a hydrofluoric acid-nitric acid mixture that dissolves silicon metal, hot caustic soda water, or the like. However, when treated with this hydrofluoric acid-nitric acid mixture or caustic soda mixture, the surface becomes yellowish-green and the hardness decreases as described above, so in this case, the calcined silicon carbide body before reacting with metal silicon is It is preferable to treat the material in advance with a solution containing a synthetic resin such as a phenolic resin, various oils and fats, paraffin, or polycarbosilane, thereby avoiding such disadvantages. Note that when carrying out this nitriding reaction, it is optional to carry out this in the presence of known fluorides, iron powder, etc. as a nitriding catalyst, and furthermore to mix 0.01 to 20% hydrogen into this nitrogen gas. According to this, the nitriding can proceed more effectively.

The silicon carbide sintered body obtained by the method of the present invention has a surface layer composed of silicon carbide and silicon nitride, and since metallic silicon is not present in this surface layer, this is made up of silicon nitride. 1900 which is the decomposition temperature
There are no accidents due to its decomposition at temperatures below 0.degree. C., and it is chemically stable and unaffected by a mixture of hydrofluoric acid and nitric acid, so it has a wide range of uses as a ceramic agent.

Next, examples of the present invention will be given.

Example 1 200 g of commercially available silicon carbide powder with an average particle size of 9.5 μm,
120g of artificial graphite with an average particle size of 1.0μm and silicone resin KR-260 (manufactured by Shin-Etsu Chemical Co., Ltd., trade name)
160 g was added to 200 ml of toluene, mixed in a ball mill, and the toluene was removed to obtain a powder mixture of 100 mesh or less.

Next, use a hydrostatic press to create a diameter of 50 mm.
Formed into a cylindrical body with a length of 70 mm and placed in a nitrogen gas atmosphere.
After calcining at 800℃, the outer diameter is 45mm and the inner diameter is lathe-processed.
It is formed into a crucible shape of 35mm long, 60mm long, and 5-6mm thick at the bottom, reacted with metal silicon in a vacuum furnace at 1600℃, and the surface is sandblasted to produce a silicon carbide-silicon sintered body. I created a crucible.

Next, this crucible was placed in a 4:1 mixed gas atmosphere furnace of nitrogen gas and hydrogen gas maintained at 1100°C, and the temperature inside the furnace was raised to 1400°C at a rate of 50°C/hour.
After keeping it at this temperature for 4 hours and then cooling it to room temperature, the composition of its surface was examined by X-ray analysis, and it was found that there was no metallic silicon, and the surface was composed of silicon carbide and silicon nitride. It was confirmed that there is.

In addition, when 30g of metal aluminum was placed in this crucible and heated to 850℃ in a vacuum furnace to melt it,
The metallic silicon present in this aluminum is
The metal silicon content was 10 ppm, but when using a crucible without the above nitriding treatment, the metal silicon content is 200~
It was 400ppm.

Example 2 A ring with an outer diameter of 76 mm, an inner diameter of 65 mm, and a length of 6 mm was made from the same raw materials as in Example 1 using a mold press, and this was heated to 850°C in a nitrogen gas atmosphere to form a calcined body. After that, 50% of polycarbosilane was added to this.
% toluene solution and then heated to 1600℃.
This was reacted with metallic silicon in a vacuum furnace to form a ring made of silicon carbide-silicon sintered body.

Next, this was immersed in a 30% caustic soda solution heated to 70°C for 8 hours to remove the silicon present on its outer surface, and then it was heated to 1300°C and soaked in nitrogen gas and hydrogen gas for 8 hours. The rings were placed in a mixed gas atmosphere furnace (No. 1), held for 8 hours while being evacuated and filled with gas every hour, and then allowed to cool to room temperature. When the surface was subjected to X-ray analysis, no metallic silicon was detected at all.

Please note that this product is mixed with 3% hydrofluoric acid-nitric acid (1:
When immersed in the aqueous solution of 1), there was no change in weight at all, but the weight of the material without nitriding continued to decrease, and 85% of the metallic silicon weight was lost in 200 hours.
has dissolved.

Claims (1)

[Claims] 1. Metallic silicon in the surface layer of a silicon carbide sintered body consisting of silicon carbide and metallic silicon is reacted with nitrogen gas at a temperature of 900°C or higher to form silicon nitride. A method for producing a silicon carbide sintered body, characterized by: 2. The silicon carbide sintered body according to claim 1, wherein the silicon carbide sintered body is etched with a chemical that dissolves silicon metal, and then reacted with nitrogen gas at a temperature of 900°C or higher. manufacturing method.