JP6858374B2 - Manufacturing method of high-strength silver sintered body - Google Patents

Description

The present invention relates to a method for producing a high-strength silver sintered body having high room temperature strength.

In recent years, in order to reduce the environmental load and improve recyclability, the reduction of alloying elements has been regarded as important in metal materials, and even when the strength is increased, it is possible to improve the characteristics without significantly changing the chemical composition from the conventional one. It has been demanded. For example, in silver-indium alloys used as dental materials, indium is also a rare metal, and its use is desired to be reduced.

ここで、ｄは粒径、σ_Ｙは降伏応力、ｋは定数、σ_０は粒界をもたない単結晶の場合の降伏応力である。It is well known that the mechanical properties are improved by refining the crystal grains. There is the following hole petch relationship between the strength at room temperature and the particle size, and the smaller the particle size, the higher the room temperature strength.
[Equation (1)]

Here, d is the particle size, σ _Y is the yield stress, k is a constant, and σ ₀ is the yield stress in the case of a single crystal having no grain boundary.

As a conventional sintered body, there is one disclosed in Patent Documents 1-3. Patent Document 1 is a stainless steel sintered body produced by molding and sintering a mixed powder obtained by mixing a powder mainly composed of a ferrite phase and a powder mainly composed of an austenite phase in a predetermined ratio. The particle size is large because it is manufactured by the water spray method. Therefore, the Vickers hardness Hv of the sintered body is about 230, which is low strength as a steel sintered body.

Patent Document 2 is produced by injection molding a composition consisting of a raw material powder obtained by adding Ni powder having an average particle size of 10 μm or less to a ferrite alloy powder and sintering the molded product after debinding. Although it is an austenite-based stainless steel sintered body, the room temperature strength is low because the particle size of the raw material powder is as large as over 1 μm.

Patent Document 3 discloses that silver clay obtained by adding a binder or the like to silver powder containing copper or the like and kneading it is molded and then fired to produce a silver sintered body. There is. Although such a silver sintered body is suitable for jewelry and arts and crafts, there remains a concern about its safety as a medical member because it contains a large number of components.

Japanese Unexamined Patent Publication No. 7-109540 Japanese Unexamined Patent Publication No. 2000-129309 Japanese Patent No. 3274960

In view of the above problems of the prior art, the present invention has been made to provide a method for producing a high-strength silver sintered body having high room temperature strength without the addition of alloying elements or binders.

The method for producing a high-strength silver sintered body of the present invention, which has been made to solve the above problems, is an ultrafine powder having an average particle size of 10 to 300 nm by an arc plasma forced evaporation method in which arc discharge is performed on silver in a hydrogen gas-containing atmosphere. The first step of producing silver powder and
The ultrafine silver powder is heated to 200 to 500 ° C. by discharge plasma while being pressurized to produce a sintered body having an average crystal grain size of 40 to 500 nm after sintering and a Vickers hardness of 50 or more at room temperature. It is characterized by consisting of a second step.

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Method of producing a high strength silver sintered body according to the present invention, silver powder as a sintering material, so prepared by arc plasma forced evaporation of arcing silver in a hydrogen gas-containing atmosphere, an average particle diameter of 10 to An ultrafine silver powder having a diameter of 300 nm can be produced.

Further, in the method for producing a high-strength silver sintered body according to the present invention, the ultrafine silver powder is heated to 200 to 500 ° C. by discharge plasma while being pressurized, so that the average crystal grain size after sintering is 40 to 500 nm. A sintered body having a Vickers hardness of 50 or more at room temperature can be produced. Therefore, a high-strength silver sintered body can be produced without using alloying elements such as indium, zinc, and tin, or a binder.

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It is a schematic block diagram of the ultrafine silver powder production apparatus for carrying out the arc plasma forced evaporation method. It is a schematic block diagram of the discharge plasma sintering apparatus. It is a graph which shows the cooling water temperature dependence of the average particle diameter of the ultrafine silver powder. It is a graph which shows the sintering holding time dependence of the Z-axis displacement amount of a sintered body. It is a graph which shows the sintering temperature dependence of the crystal grain size of a sintered body. 6 is an SEM image showing crystal grains at each sintering holding time of a 327 ° C. sintered body. It is a graph which shows the Dependence of the Vickers hardness of a sintered body for the sintering holding time. It is a correlation diagram which shows the relationship between Vickers hardness and particle size. It is a figure which shows the tensile test result of each temperature sintered body with a sintering holding time of 1 minute. It is a figure which shows the tensile test result of the 327 ° C. sintered body at each sintering holding time.

The method for producing the high-strength silver sintered body of the present invention will be described in detail below.

The method for producing ultrafine silver powder for a high-strength silver sintered body of the present invention is a method for producing an ultrafine silver powder for a high-strength silver sintered body, in which, in the first step, an arc plasma forced evaporation method in which arc discharge is performed on silver in a hydrogen gas-containing atmosphere causes an average particle size of more than 10 to 300 nm. Manufacture fine silver powder.

FIG. 1 is a schematic configuration diagram of an ultrafine silver powder manufacturing apparatus for carrying out the arc plasma forced evaporation method. In the figure, a water-cooled copper hearth 2 and a plasma electrode 3 which are anodes are arranged in the nanoparticle generation chamber 1, and both are connected via a power source 4. An exhaust rotary pump 5 and an Ar cylinder and a hydrogen cylinder (not shown) are connected to the nanoparticle generation chamber 1. Further, a nanoparticle collection chamber 6 is connected to the nanoparticle generation chamber 1, and the nanoparticle generation chamber 1 and the nano are via a filter 7 and a diaphragm pump 8 provided above the nanoparticle collection chamber 6. The particle collection chamber 6 is connected to form a gas circulation path. The entire device is cooled by cooling water.

When silver is placed on a water-cooled copper hearth 2 and an arc discharge is performed in an atmosphere containing hydrogen gas, the silver is heated to a high temperature, and hydrogen ions are dissolved in the silver and recombined and released. Ions are released to generate nanoparticles. At this time, 40 to 100 A can be used as the discharge current value. The output is small below 40 A, and the ultrafine silver powder, which is nanoparticles sucked by the diaphragm pump 8 capable of achieving the target up to 100 A, is captured by the filter 7 and recovered. As the silver, commercially available silver, high-purity silver and the like can be used. Further, as the hydrogen gas-containing atmosphere, for example, a mixed gas of 50% argon gas and 50% hydrogen gas can be used.

An ultrafine silver powder having an average particle size of 10 to 300 nm is produced by an arc plasma forced evaporation method. This is because the particle size does not have to be less than 10 nm, and fine crystals can be obtained even after sintering at 10 nm or more. Further, the reason why the nm is set to 300 nm or less is that if it exceeds this, it becomes difficult to obtain fine crystal grains after sintering, which causes a decrease in room temperature strength.

In the method for producing a high-strength silver sintered body of the present invention, in the second step, the ultrafine silver powder obtained as described above is heated to 200 to 500 ° C. by discharge plasma while being pressurized, and after sintering. A sintered body having an average crystal grain size of 40 to 500 nm and a Vickers hardness of 50 or more at room temperature is produced.

That is, the ultrafine silver powder is sintered using the discharge plasma sintering apparatus shown in FIG. The device includes a cylindrical graphite die in a vacuum chamber. The ultrafine silver powder is charged into the inside of the die, and the ultrafine silver powder is sintered by heating by passing a pulse current while pressurizing from above and below with a graphite punch. In the discharge plasma sintering apparatus, since it is possible to rapidly raise the temperature of the ultrafine silver powder by self-heating only on the particle surface, there is an advantage that the grain growth of the powder can be suppressed and sintering can be performed.

The sintering temperature is 200 ° C to 500 ° C. This is because if the temperature is lower than 200 ° C, it becomes difficult to obtain a sintered body having weak sintering strength and high adhesion, while if the temperature exceeds 500 ° C, the crystal grains become coarse and the hardness is lowered. ..

The average crystal grain size after sintering is 40 nm to 500 nm. This is because if it is less than 40 nm, a high-density sintered body cannot be obtained, and if it is more than 500 nm, the room temperature strength is significantly reduced. Further, if the Vickers hardness of the silver sintered body is less than 50, the object cannot be achieved, so the Vickers hardness is set to 50 or more.

Examples of the present invention will be described below.

An ultrafine silver powder was produced by an arc plasma forced evaporation method at an arc discharge current of 40 to 100 A and a cooling water temperature of 15 ° C. About 300 spherical particles were selected from the SEM image, and their diameters were measured with calipers. As a result, the average particle size was 14.75 nm at an arc discharge current of 40 A, 12.06 nm at 50 A, 89.1 nm at 80 A, and 144.7 nm at 100 A, and the one at 80 A was the smallest. In addition, the standard deviation of 80 A was the smallest at 31.25, and the particle groups were concentrated in 41 to 80 nm.

FIG. 3 shows the relationship between the cooling water temperature and the average particle size. The one produced at a cooling water temperature of 10 ° C. had the smallest average particle size of 78.5 nm.

The ultrafine silver powder produced as described above was sintered using the discharge plasma sintering apparatus shown in FIG. FIG. 4 shows the compression amount of the object to be sintered at a compressive stress of 60 MPa, that is, the Z-axis displacement amount depending on the sintering temperature. At 127 ° C., sintering is completed in the middle of densification. At 227 ° C., 327 ° C., 427 ° C., and 527 ° C., densification was completed, and the apparently recorded displacement reached about 1 mm through the change in the expansion direction.

FIG. 5 shows the dependence of the crystal grain size of the sintered body on the sintering time. The size of the crystal grains is the minimum crystal grain size of 243.7 nm at a sintering temperature of 227 ° C. and a sintering time of 1 minute, and the maximum crystal grain size of 8.5 μm at a sintering temperature of 527 ° C. and a sintering time of 15 minutes. became. The average crystal grain size of industrial silver, which is the starting material, was 41.6 μm, and the size of the crystal grains could be increased 1/17 times. FIG. 6 shows an SEM image at each holding time of the 327 ° C. sintered material for reference.

FIG. 7 shows the Vickers hardness of the sintered body at each temperature, and FIG. 8 shows the relationship between the Vickers hardness and the crystal grain size. The maximum hardness was 120.7 MPa at a sintering temperature of 227 ° C. and a holding time of 5 minutes, and the minimum hardness was 67.15 MPa at a sintering temperature of 127 ° C. and a holding time of 1 minute. The maximum hardness was about 3.7 times higher than the hardness of the starting material, 32.37 MPa. As a whole, as the sintering time increased, the hardness decreased, and the Vickers hardness tended to stagnate at about 70 to 80 MPa regardless of the size of the crystal grain size.

FIG. 9 shows the results of a room temperature tensile test of each temperature sintered material at a sintering time of 1 minute. The sintered body at 327 ° C. or lower broke with a strain of about 1%. In the 427 ° C. sintered body, the tensile strength reached 249 MPa and the breaking elongation reached 16%. In the 527 ° C. sintered body, the tensile strength was 227 MPa and the breaking elongation was 19%.

FIG. 10 shows the results of a room temperature tensile test of each holding time sintered body at 327 ° C. An improvement in ductility was observed as the sintering time increased, and the elongation was 9.3% at a sintering time of 30 minutes. The tensile strength was 332 MPa, which was the highest when the sintering time was 5 minutes, and 290 MPa, which was the lowest when the sintering time was 30 minutes.

The tensile strength of 332 MPa is about 4.1 times that of the tensile strength of aluminum 1050 A of 65 to 95 MPa. As described above, the high-strength silver sintered material according to the present invention has great industrial value as a high-strength and high-ductility metal material containing no alloying elements, especially in the field of dental materials and the like.

1 Nanoparticle generation chamber 2 Water-cooled copper hearth 3 Plasma electrode 4 Power supply 5 Rotary pump 6 Nanoparticle collection chamber 7 Filter 8 Diaphragm pump

Claims (1)

The first step of producing ultrafine silver powder having an average particle size of 10 to 300 nm by the arc plasma forced evaporation method in which silver is arc-discharged in a hydrogen gas-containing atmosphere.
The ultrafine silver powder is heated to 200 to 500 ° C. by discharge plasma while being pressurized to produce a sintered body having an average crystal grain size of 40 to 500 nm after sintering and a Vickers hardness of 50 or more at room temperature. A method for producing a high-strength silver sintered body, which comprises a second step.

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