JPS61264103A - Production of high melting point metallic sintered body - Google Patents

Abstract

PURPOSE:To suppress the reaction of a coating material and molding during hot pressurizing and to produce a sintered body having high purity by coating preliminarily titanium nitride powder to the molding in the stage of hot pressurizing the molding consisting of high melting point metallic powder in a vacuum atmosphere, etc. CONSTITUTION:The titanium nitride is coated on the powder molding formed of the high melting point metallic powder such as molybdenum or tungsten in the stage of hot pressurizing such powder molding in the vacuum atmosphere or inert atmosphere. The titanium nitride powder adjusted to 1-10mu average grain size is suspended into a volatile org. solvent such as acetone at about 2:8-3:7 by volume and such suspension is coated on the molding so that the coating layer has about 0.5-2.5mm thickness. The reaction between the coating material and the molding is thereby suppressed and the easy removal of the reaction layer is made possible. The product having the high purity is thus obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a method for producing a sintered body of a high melting point metal such as tungsten or molybdenum.

As a method of obtaining a sintered body of a high-melting point metal such as tungsten, a powder compact made of these metal powders is hot heated in a vacuum atmosphere or an inert gas (N2 or Ar, etc.) atmosphere. A method of pressure sintering has been put into practical use.

Compared to conventional sintering methods, the hot pressing method allows a sintered body close to the theoretical density to be obtained at a relatively low temperature, but high melting point metals such as tungsten easily oxidize in hot conditions and become brittle when oxidized. Therefore, it is necessary to prevent surface oxidation of the powder compact.

For example, in the hot press method, when a powder compact is placed directly in a mold and hot pressurized, the powder compact and carbon forming the mold etc. react during the hot press.
Brittle carbides are formed on the surface of the sintered body. In addition, when hot pressing a powder compact through a powder pressure transmission medium in the mold for the purpose of applying uniform pressure to the powder compact, the reaction between the powder compact and the carbon forming the mold etc. Although this can be prevented, oxides are formed on the surface of the sintered body due to the air remaining in the powder pressure transmission medium. In order to prevent the formation of such carbides, oxides, etc., conventionally, the surface of a powder compact is coated with a paste-like substance of boron nitride, and this powder compact is placed in a hot pressing mold. Alternatively, a method has been proposed in which nitride, which is the powder pressure transmission medium in the mold, is buried in a bed of boron powder and hot pressurized (Japanese Patent Laid-Open No. 5
3-62709).

1. In the method of coating the boron nitride described above, the powder compact and the boron nitride react during hot pressing, and a very dense and hard coating is formed on the surface of the sintered compact. There is a problem in that a compound is formed, and it is difficult to remove this reaction layer by cutting or grinding after hot pressing.

As a result of intensive studies to solve the above problems, the inventor of the present invention found that titanium nitride, which is more thermally stable than boron nitride and has less reactivity with the powder compact, should be used as a coating material for the powder compact. By minimizing the reaction between the coating material and the powder compact during hot pressurization, titanium nitride powder serves as an excellent mold release material when removing the sintered compact from a carbon mold etc. after hot pressurization. I discovered that it can be done.

The present invention was made based on such knowledge, and an object of the present invention is to minimize the reaction between the powder compact and the coating material during hot pressing, and to prevent sintering with carbon, oxygen, nitrogen, etc. Prevent body pollution.

An object of the present invention is to provide a method for producing a high-melting point metal sintered body that yields a product of high purity.

The present invention for determining the temperature is achieved by hot pressing a powder compact made of high melting point metal powder in a vacuum atmosphere or an inert atmosphere.
This is a method for producing a high melting point metal sintered body, which comprises applying titanium nitride powder to the powder compact.

The high melting point metals in the present invention are tungsten, molybdenum, tantalum, niobium, rhenium, and alloys mainly composed of these.

The powder compact in the present invention is a powder compact formed by molding high-melting point metal powder into a predetermined shape by pressing such as a mold press or a rubber press, or a powder compact formed in a hydrogen atmosphere to increase the strength of this compact. A material pre-sintered at a medium to relatively low temperature may be used, and furthermore, a powder compact or a pre-sintered material may be used that has been temporarily assembled by cutting or the like.

As a method for hot pressing the powder compact, for example, a hot pressing method or a hot static pressing method is used.

The powder pressure transmission medium used in the present invention is a medium that transmits pressure to the powder compact, and includes, for example, powdered boron nitride, powdered titanium nitride newly discovered by the applicant, and the like. Specifically, for example, upper and lower punches and a mold are filled with this powder medium, and the powder compact to be compressed is embedded in the powder medium and compressed. Therefore, since pressure is applied to the powder compact through this powder medium, the pressure on the powder compact becomes more uniform than when pressure is applied directly by upper and lower punches.

A method for applying titanium nitride powder to the surface of a powder compact will be described below. First, titanium nitride powder is suspended in a volatile organic solvent to form a suspension, and then this suspension is applied to the entire surface of the powder compact with a brush, or the powder compact is placed in the suspension. Soak one or more times. After coating, the solvent is evaporated in the atmosphere to obtain a uniform coating layer of titanium nitride powder. The thickness of the coating layer is, for example, 0.5 to 2.5 m. The average particle size of the titanium nitride powder used in the present invention is preferably 1 to 10 μm. Also.

The volatile organic solvents used here include, for example, acetone, methanol, ethanol,
Propatool and isopropatool are preferred. moreover,
The mixing ratio of titanium nitride powder and solvent in the suspension is as follows:
A capacity ratio of 2:8 to 3 is preferred.

In addition, as another example of the above-mentioned method, coating may be performed using a paste-like material of titanium nitride powder, but in this case, the paste etc. may be used to prevent contamination of the sintered body by carbon caused by the paste. It is preferable to have as little as possible.

Below, a hot press method will be described as an embodiment of the invention.

The high melting point metal powder is molded into a powder compact of a predetermined shape by applying a molding pressure of, for example, 500 to 3000 kg/cd using a press such as a mold press or a rubber press. If necessary, the molded body may be pre-sintered at a relatively low temperature in a hydrogen atmosphere in order to further increase the molding strength of the molded body.

Next, as a preliminary step to hot pressing, titanium nitride powder is applied to the surface of the powder compact, and the powder compact is placed in upper and lower carbon punches and a mold, or is filled into the mold. embedded in a powder pressure transmission medium.

Next, a resistance heating element is heated to 1800 to 20
While heating at a temperature of 00°C, pressurize with a press pressure of 300 to 500 kg/aI using carbon upper and lower punches, and perform hot pressing in an inert gas (Nt or Ar, etc.) or vacuum atmosphere. A sintered body having a predetermined shape and close to theoretical density is obtained.

Further, as another embodiment, a case of hot static pressing method will be described.

Applying titanium nitride powder to the surface of a powder compact formed in the same manner as in the above embodiment, embedding this powder compact in a powder pressure transmission medium in a closed metal container (capsule),
After deaerating and sealing, place it in a sintering furnace and heat it to a temperature of 1300~
Sintering is performed at 2000° C. with an inert gas at a cuff pressure of 00 to 2000 kg/d to obtain a sintered body having a predetermined shape and close to theoretical density.

Chimpanzee (Example 1) Tungsten powder with a particle size of 1 μm was used as the raw material powder, and it was pressed into a mold at a rate of 1700 kg/aJ.
After applying the molding pressure to form a powder compact into a predetermined shape,
Further, it was pre-sintered at 1700° C. in a hydrogen atmosphere to form a powder compact having a density ratio of about 90%.

Next, titanium nitride powder was applied to the surface of the powder compact, and the powder compact was placed in upper and lower carbon punches and a mold.

Next, in a nitrogen atmosphere, heating temperature was 1800°C and pressing pressure was 5.
Hot pressing was carried out under the conditions of 00 kg/at and 120 minutes of press time. After hot pressing, the sintered body could be easily taken out from the carbon mold. Further, the reaction layer between the coating material and the powder compact on the surface of the sintered body could be removed by light cutting or grinding, and no reactions such as diffusion and penetration into the interior of the sintered body were observed.

Note that the thickness of the reaction layer was about 10 μm.

The tungsten powder sintered body thus obtained was a homogeneous sintered body with no surface oxidation or internal oxidation, no contamination by carbon or nitrogen, and a theoretical density ratio of about 95%. Ta.

(Comparative Example 1) Boron nitride powder was applied to a powder compact obtained in the same manner as in Example 1, and hot pressing was performed under the same hot pressing conditions and pressing time as in Example 1. The thickness of the reaction layer on the surface of the sintered body was about 50 μm. It was difficult to remove this reaction layer by cutting or grinding.

(Example 2) As raw material powders, tungsten powder and molybdenum powder with a particle size of 1 μm were mixed at a weight ratio of 9=1, and then about 2000 kg of this was mixed with a die press.
After applying a molding pressure of /aJ to form a powder compact of a predetermined shape, the powder was further pre-sintered at 1700° C. in a hydrogen atmosphere to form a powder compact with a density ratio of about 93%.

Next, titanium nitride powder was applied to the surface of the powder compact, and the powder compact was embedded in titanium nitride powder, which was a powder pressure transmission medium filled in upper and lower carbon punches and the inside of the mold.

Next, hot pressing was performed in a nitrogen atmosphere at a heating temperature of 1800° C., a pressing pressure of 500 kg/cd, and a pressing time of 120 minutes. After hot pressing, the reaction layer on the surface of the sintered body could be easily cut and ground, and no reaction such as diffusion into the interior of the sintered body was observed.

Note that the thickness of the reaction layer was about 10 μm. The obtained sintered body showed no contamination and was homogeneous with a theoretical density ratio of about 97%.

(Comparative Example 2) Boron nitride powder was applied to a powder compact obtained in the same manner as in Example 2, and hot pressing was performed under the same hot pressing conditions and pressing time as in Example 2. The thickness of the reaction layer on the surface of the sintered body was about 50 μm. This reaction layer could not be easily removed by cutting or grinding.

According to the present invention, since the surface of the powder compact is covered with a coating material of titanium nitride powder during hot pressing,
When there is no reaction between the hot pressing mold and the powder compact, and there is no contamination of the sintered body with carbon, oxygen, or nitrogen, and when the sintered body is taken out from the hot pressing mold at the end of hot pressing. The titanium nitride powder acts as a mold release agent, resulting in good mold release properties.

Further, the thickness of the reaction layer between the coating material and the powder compact is about 115 or less when boron nitride is used as the coating material,
Moreover, since the reaction layer itself is brittle, this reaction layer can be easily removed by cutting or grinding after hot pressing, which is a special effect.

The sintered body thus obtained has extremely high practical value as a high purity sintered body of a high melting point metal.

Claims (1)

[Claims] A high melting point metal sintered body characterized in that titanium nitride powder is applied to the powder compact formed from the high melting point metal powder when the powder compact is hot pressed in a vacuum atmosphere or an inert atmosphere. Production method.