Spark plasma sintering method

Abstract

To provide a discharge plasma sintering apparatus and a discharge plasma sintering method which can more effectively remove impurities between particles of a powder material and impurities stuck to surfaces of the particles.SOLUTION: A discharge plasma sintering apparatus comprises: a first punch and a second punch which are inserted into an internal space of a die; electrodes for energizing the first and second punches; and a movement mechanism for moving at least either the first punch or the die so as to enable the insertion of the first punch into the internal space and the removal thereof from the internal space. A punch holding part for detachably holding the first punch between the first punch and the electrode for energizing the first punch. The first punch has a corresponding part to be held which can be held by the punch holding part.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a spark plasma sintering technique in which a powder material is sintered by passing an electric current through it while being pressurized, and more specifically, to a spark plasma sintering apparatus and a spark plasma sintering method that make it possible to effectively remove gas present between particles of the powder material and impurities attached to the particles. [Background technology]

[0002] Spark plasma sintering (also called electric current sintering or pulse current pressure sintering) is used as a sintering method for metals, ceramics, etc. Spark plasma sintering is a method of sintering a material by applying pressure to a powdered material while applying pulse current heating. Compared to hot press sintering and hot isostatic pressure sintering, which have been used as manufacturing methods for sintered bodies, spark plasma sintering has the following advantages: it can sinter at a low temperature in a short time, it has good moldability and can sinter materials that could not be molded by conventional sintering methods, it can produce dense sintered bodies with controlled grain growth, and it consumes less power than hot press sintering. These advantages have attracted attention in recent years.

[0003] A typical configuration of a conventional spark plasma sintering apparatus is as shown in the schematic diagram of FIG. 9. The spark plasma sintering apparatus 5 includes a cylindrical die 52 having an internal space in which the powder material M is filled, and two punches 53, 54 inserted into the internal space from the upper and lower ends of the die 52. The die 52 and the two punches 53, 54 are usually formed using graphite. The die 52 and the two punches 53, 54 are arranged in a vacuum chamber 50 so as to be sandwiched between electrodes 57, 58 for applying a pulse current to the powder material M via spacers 55, 56. The lower punch 54 is movable in the axial direction of the cylindrical die 52 by a press mechanism 70, thereby allowing the powder material M filled in the internal space of the die 52 to be pressurized. When performing spark plasma sintering, the powder material M is placed in the internal space of the die 52, the punches 53 and 54 are inserted into the internal space until they come into contact with the powder material, and the combination of the die 52 and the punches 53 and 54 is placed in the vacuum chamber 50. Next, the on-off valve 62 is opened, the air in the vacuum chamber 50 is evacuated by the vacuum pump 61, and the press mechanism 70 is operated to pressurize the powder material M with the punches 53 and 54 while applying a pulse current to the powder material M with the electrodes 57 and 58. As a result, a sintered body is produced from the powder material M.

[0004] It is desirable to use powder material with as high purity as possible for filling the die in order to stabilize the physical properties of the sintered body. However, there are spaces between the particles of the filled powder material, and these spaces contain gases such as air and foreign matter. In addition, it is inevitable that gases such as oxygen and nitrogen contained in the air, moisture, oxide films, and foreign matter such as sebum that has adhered during handling will adhere to the surfaces of each particle of the powder material. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2006-131921 A [Patent Document 2] Japanese Patent Application Publication No. 9-324228 [Patent Document 3] JP 2003-49207 A Summary of the Invention [Problem to be solved by the invention]

[0006] Patent Document 1 proposes an apparatus that is provided with a glove box-type pre-processing chamber connected to the sintering process chamber and controlled to a specific atmosphere in order to remove gases and foreign matter present in the spaces between particles and gases and foreign matter attached to the particle surfaces (hereinafter referred to as "impurities"). In this apparatus, the weighing of raw material powder, filling into a sintering mold (die), punch insertion into the sintering mold, and chamber depressurization are carried out in the pre-processing chamber with an inert gas atmosphere, and these are then transferred to the sintering process chamber without exposure to the outside air, which is said to enable the production of sintered bodies that are free from contamination during the preparation of raw material powder. However, this technology requires large-scale and complicated equipment and operations, and the impact on the environment due to the constant use of inert gas is problematic.

[0007] Technologies for removing impurities between particles and on the particle surfaces are also disclosed in Patent Document 2 and Patent Document 3. Patent Document 2 discloses a method of filling metal powder between a graphite die and upper and lower punches that also serve as electrodes, arranging this on a sintering stage in a container, and reducing the pressure inside the container to remove gases and the like adsorbed to the metal powder. Patent Document 3 also discloses a method of removing material degraders such as water and hydroxides from the aluminum alloy powder by reducing the air inside the container to a vacuum atmosphere while heating the container containing the aluminum alloy powder.

[0008] However, as disclosed in Patent Document 2, sufficient impurity removal cannot be achieved by simply removing gases and the like adsorbed on metal powder by reducing pressure. In addition, the technology disclosed in Patent Document 3 requires reducing pressure while heating a metal container filled with raw material powder, compressing the raw material powder inside to a predetermined bulk density to obtain a sintered body, and then removing the sintered body from the container. Therefore, there are many steps and the process is complicated.

[0009] In a spark plasma sintering apparatus, the die and punch are usually fitted with high precision so that there is no escape gap for the powder material, and the fitting gap between the inner wall of the die and the punch is extremely small. In addition, when the punch comes into contact with the powder material filled in the die, it is considered that the powder material may enter the boundary between the die and the punch. Therefore, simply evacuating the die with the punch inserted does not easily remove impurities between the particles of the powder material or impurities attached to the particle surfaces. If the powder material is sintered without removing the impurities, the impurities may be absorbed in the sintered body, causing a decrease in purity. In addition, voids that are maintained under high pressure may be trapped inside the sintered body, becoming the starting point of cracks, etc., and reducing the mechanical strength of the sintered body.

[0010] An object of the present invention is to provide a spark plasma sintering apparatus and a spark plasma sintering method that enable impurities between particles of a powder material and impurities attached to the particle surfaces to be removed more effectively than conventional methods. [Means for solving the problem]

[0011] The inventors of the present invention discovered that impurities between particles of powder material and impurities attached to the particle surfaces can be more effectively removed by evacuating the chamber with the space filled with powder material open and removing impurities by heating with the space filled with powder material open, or by removing impurities by heating multiple times with the space filled with powder material open, and thus completed the present invention.

[0012] In a first aspect, the present invention provides a spark plasma sintering apparatus for sintering a powder material filled in an internal space of a cylindrical die by applying pressure to the powder material with a punch in a vacuum atmosphere while applying current to the powder material. The spark plasma sintering apparatus includes a first punch and a second punch inserted into the internal space of the die, an electrode for applying current to the first and second punches, and a moving mechanism for moving at least one of the first punch and the die so that the first punch can be inserted into the internal space and removed from the internal space. A punch holding part for detachably holding the first punch is disposed between the first punch and the electrode for applying current to the first punch. The first punch has a corresponding held part that can be held by the punch holding part. In one embodiment, a spacer may be disposed between the first punch and the electrode for applying current to the first punch, and the punch holding part may be provided on the spacer.

[0013] In one embodiment, the held portion of the first punch and the punch holding portion are preferably configured to allow the first punch to move. By allowing the first punch to move, when the first punch is inserted into the internal space in a vacuum atmosphere by the operation of the moving mechanism, the first punch can be automatically aligned with the internal space. To achieve this, in one embodiment, the held portion of the first punch preferably has a flange, and the punch holding portion preferably has a hook that engages with the flange.

[0014] In one embodiment, the die preferably has a guide portion for aligning the first punch with the internal space. The guide portion is preferably a tapered portion formed toward the back of the internal space at an entrance portion of the die on the side where the first punch is inserted.

[0015] In a second aspect, the present invention provides a spark plasma sintering method for sintering a powder material filled in an internal space of a cylindrical die by applying current to the powder material while applying pressure with a punch in a vacuum atmosphere. The spark plasma sintering method includes a vacuum step of creating a vacuum atmosphere around the die filled with the powder material, a heating step of heating the powder material, a holding step of holding the heated powder material without inserting the punch into the internal space, and a sintering step of inserting the punch into the internal space and applying current to the powder material while applying pressure to the powder material with the punch. In the vacuum step, it is more preferable to create a vacuum atmosphere around the die filled with the powder material without inserting the punch into the internal space of the die.

[0016] In one embodiment, the heating step preferably includes inserting a punch into the internal space, heating the powder material by passing an electrical current through the punch, and then removing the punch from the internal space. Effect of the Invention

[0017] According to the present invention, since the punch can be inserted into the die and removed from the die in a vacuum atmosphere without the operator touching the die or punch, it is possible to continuously perform non-thermal impurity removal and heated impurity removal in a series of steps, or to perform the heated impurity removal step multiple times. Therefore, the apparatus and method according to the present invention can effectively remove impurities, thereby reducing the variation in purity, and can produce sintered bodies of stable quality that are densified by evacuating the powder space. [Brief description of the drawings]

[0018] [Figure 1] 1 is a diagram showing a configuration of a main part of a spark plasma sintering apparatus according to an embodiment of the present invention. [Diagram 2] FIG. 2 is a diagram showing an upper punch used in a spark plasma sintering apparatus, where (a) is a vertical cross-sectional view and (b) is a bottom view. [Diagram 3]1A and 1B are diagrams showing an upper spacer used in a spark plasma sintering apparatus, in which (a) is a vertical cross-sectional view and (b) is a bottom view. [Figure 4] FIG. 2 is a vertical cross-sectional view showing a die used in a spark plasma sintering apparatus. [Diagram 5] FIG. 13 illustrates how the upper punch is automatically aligned when it is inserted into the inner space of the die. [Figure 6] FIG. 1 is a flow diagram showing steps of a spark plasma sintering method according to an embodiment of the present invention. [Figure 7] This is an EDS elemental mapping image of a sintered sample after impurities were removed by heating. [Figure 8] This is an EDS elemental mapping image of a sintered sample that was not subjected to thermal impurity removal. [Figure 9] FIG. 1 is a diagram showing a configuration of a main part of a conventional spark plasma sintering apparatus. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] Hereinafter, the embodiments of the present invention will be described in detail with reference to the drawings. The present invention is not limited to the following embodiments, and can be appropriately modified and implemented within the scope of the object of the present invention.

[0020] [Configuration of spark plasma sintering equipment] FIG. 1 is a schematic diagram showing the configuration of the main part of a spark plasma sintering apparatus 1 (hereinafter, simply referred to as the apparatus 1) according to one embodiment of the present invention. The main part shown in FIG. 1 is housed inside a housing (not shown) of the apparatus 1. The apparatus 1 includes a cylindrical die 12 having an internal space 11, and an upper punch 13 and a lower punch 14 inserted into the internal space 11 of the die 12. The upper punch 13 is inserted into the internal space 11 from above the die 12, and the lower punch 14 is inserted into the internal space 11 from below the die 12. The shapes of the parts of the upper punch 13 and the lower punch 14 inserted into the internal space 11 are shaped to match the internal space 11. The internal space 11 is filled with a powder material M, and the powder material M becomes a sintered body by passing an electric current through it while being pressurized by the upper punch 13 and the lower punch 14.

[0021] The die 12, the upper punch 13, and the lower punch 14 are preferably made of a conductive material. Specifically, they are preferably made of graphite, steel material, silicon carbide, tungsten carbide, or the like, and more preferably made of graphite. The size and shape of the die 12 and the inner space 11, as well as the upper punch 13 and the lower punch 14, are not particularly limited and can be appropriately designed according to the size and shape of the target sintered body and the scale of the equipment. In addition, the die 12, the upper punch 13, and the lower punch 14 are formed so that the fitting gap between the inner wall of the die 12 and the upper punch 13 and the lower punch 14 is extremely small so that no escape gap for the powder material M is generated (high pressure is not achieved if an escape gap exists).

[0022] An upper electrode 17 is connected to the upper punch 13 via an upper spacer 15, and a lower electrode 18 is connected to the lower punch 14 via a lower spacer 16. The upper spacer 15 and the lower spacer 16 are also made of a conductive material similar to that of the die 12, the upper punch 13, and the lower punch 14, and are preferably made of graphite. The upper electrode 17 and the lower electrode 18 are connected to a power source (not shown) that generates DC pulse power or single-phase full-wave power. The power generated by the power source is applied to the die 12 (or to the powder material M itself if the powder material M is a conductive material) via the upper electrode 17 and the lower electrode 18, the upper spacer 15 and the lower spacer 16, and the upper punch 13 and the lower punch 14.

[0023] The die 12, the upper punch 13, the lower punch 14, the upper spacer 15, the lower spacer 16, and parts of the upper electrode 17 and the lower electrode 18 are disposed inside the vacuum chamber 10. The inside of the vacuum chamber 10 can be made into a vacuum atmosphere by opening an opening / closing valve 22 and evacuating air with a vacuum pump 21.

[0024] The upper electrode 17 is fixed at a position inside the device 1, and the lower electrode 18 is movable in the vertical direction by a press mechanism (moving mechanism) 30 (not shown). When the press mechanism 30 moves the lower electrode 18 upward, the lower punch 14 moves upward inside the internal space 11, and high pressure can be applied to the powder material M filled in the internal space 11. Note that the vacuum chamber 10 is configured so that the lower electrode 18 can be moved up and down while the inside is maintained in a vacuum. In this embodiment, the lower electrode 18 is configured to be moved by the press mechanism 30, but this is not limited thereto, and the upper electrode 17 may be configured to be movable, or both the upper electrode 17 and the lower electrode 18 may be configured to be movable.

[0025] (Upper spacer and upper punch) Fig. 2 shows an upper punch 13 used in the device 1, where (a) is a vertical cross-sectional view and (b) is a bottom view. Also, Fig. 3 shows an upper spacer 15 used in the device 1, where (a) is a vertical cross-sectional view and (b) is a bottom view.

[0026] The upper punch 13 includes a cylindrical main body 131 and a held portion for being held by the upper spacer 15. In this embodiment, the held portion is a circular flange 132 provided so as to protrude from the side surface of the main body 131, as shown in FIG. 2. Meanwhile, the upper spacer 15 includes a cylindrical main body 151 and a punch holding portion for detachably holding the upper punch 13. In this embodiment, the punch holding portion is hooks 152, 153 provided so as to protrude downward (i.e., toward the upper punch 13) from the lower surface of the main body 151. The hooks 152, 153 have protruding portions 152a, 153a formed so as to face each other, as shown in FIG.

[0027] In this embodiment, as shown in FIG. 1, the upper punch 13 is placed so that the lower surface of the flange 132 contacts the upper surface of the protruding parts 152a, 153a of the upper spacer 15. Therefore, the upper punch 13 can freely move laterally even when engaged with the upper spacer 15. With this structure, the press mechanism 30 moves the lower electrode 18 up and down, so that the upper punch 13 can be inserted into the inner space 11 of the die 12 in a vacuum atmosphere, and the upper punch 13 can be removed from the inner space 11 of the die 12. Furthermore, when the press mechanism 30 moves the lower electrode 18 upward to insert the upper punch 13 into the inner space 11 of the die 12 in a vacuum atmosphere, even if the central axis of the upper punch 13 does not coincide with the central axis of the inner space 11, the upper punch 13 moves laterally and is automatically aligned with the inner space 11. This point will be explained again with reference to FIG. 5 in the description of the die 12 described later.

[0028] In another embodiment, a configuration may be adopted in which the held portion provided on the upper punch 13 is a male screw, and the punch holding portion provided in the center part of the upper spacer 15 is a female screw, and the male screw is screwed into the female screw, so that the upper punch 13 can be detachably attached to the upper spacer 15. With this structure, the press mechanism 30 moves the lower electrode 18 up and down, so that the upper punch 13 can be inserted into the internal space 11 of the die 12 in a vacuum atmosphere, or the upper punch 13 can be removed from the internal space 11 of the die 12. In yet another embodiment, the upper punch 13 can be detachably attached to the upper spacer 15 using, for example, a pin that penetrates the hooks 152 and 153 of the upper spacer 15 and reaches the upper punch 13. In yet another embodiment, the upper punch 13 can be detachably attached to the upper spacer 15 by, for example, attaching a wire to an appropriate position, such as the internal space of the upper spacer 15, and hanging the upper punch 13 from the wire.

[0029] In yet another embodiment, when the upper spacer 15 is not used, that is, when the upper punch 13 is in direct contact with the upper electrode 17, a punch holding portion (hooks 152, 153 in the case of FIG. 1) may be provided on the lower surface of the upper electrode 17. For example, a hook portion as shown in FIG. 3 can be provided so as to protrude downward from the lower surface of the upper electrode 17.

[0030] (Thailand) FIG. 4 is a vertical cross-sectional view showing the die 12 used in the device 1. The die 12 preferably has a guide portion at the entrance portion 111 where the upper punch 13 is inserted to assist in aligning the upper punch 13 with the internal space 11. In this embodiment, as shown in FIG. 1 and FIG. 4, the die 12 preferably has a shape in which the corner facing the internal space 11 is cut out over the entire circumferential direction at the entrance portion 111 on the side where the upper punch 13 is inserted. That is, the guide portion of the die 12 in this embodiment is a tapered portion 122 provided at the corner facing the internal space 11 so that the diameter of the internal space 11 gradually narrows from the entrance portion 111 toward the back of the internal space 11. By providing the tapered portion 122 at the entrance portion 111 of the die 12 in this way, it becomes easier to align the upper punch 13 with the internal space 11 when inserting the upper punch 13 in a facing atmosphere. The angle (angle θ in FIG. 4) between two points when tapered portion 122 is lowered into internal space 11 along tapered portion 122 at opposing radial positions of die 12 is preferably, but not limited to, 45° to 80°. If this angle is too small, the tolerance for misalignment between die 12 and upper punch 13 becomes small, and if this angle is too large, upper punch 13 becomes less likely to slide on tapered portion 122, so neither is preferable.

[0031] The alignment of the die 12 and the upper punch 13 will be described with reference to FIG. 5. FIG. 5(a) shows the state in which the lower punch 14 is inserted and the die 12 filled with the powder material M is placed inside the vacuum chamber 10 (more specifically, on the lower spacer 16). The upper punch 13 is attached to the upper spacer 15. Thereafter, as will be described later in the description of the discharge plasma sintering method, when a heating step is performed with the inside of the vacuum chamber 10 in a vacuum atmosphere, the lower electrode 18 is raised by the press mechanism 30. As shown in FIG. 5(b), when the die 12 moves in the direction of the arrow A, if the central axis of the upper punch 13 and the central axis of the internal space 11 are misaligned, a part of the lower end of the upper punch 13 comes into contact with a part of the die 12. At this time, since the upper punch 13 is held by the upper spacer 15 so as to be able to move laterally, and the die 12 is provided with the tapered portion 122, as the die 12 moves further in the direction of arrow A, the upper punch 13 slides along the inclination of the tapered portion 122 with the flange 132 resting on the protruding portions 152a, 153a of the upper spacer 15, and moves in the direction of arrow B, as shown in Fig. 5(c). In this way, the upper punch 13 is automatically aligned with the internal space 11 when it is inserted into the internal space 11 in a vacuum atmosphere by the operation of the press mechanism 30.

[0032] (heater) In another embodiment of the present invention, a heater 42 may be provided as a heating unit for heating the powder material M from outside the die 12. The heater 42 has, for example, a ring shape arranged so as to surround the outer periphery of the die 12, and is held by a heater holder 41. The heater 42 can be moved from above inside the vacuum chamber 10 to the side of the die 12 by the operation of a vertical drive unit 43, so that there is no problem with the operability when the die 12 is put in and taken out of the vacuum chamber 10.

[0033] [Spark plasma sintering method] 6 is a flow chart 600 showing steps of a spark plasma sintering method according to an embodiment of the present invention. The flow chart 600 shows a step of installing the die 12 and the upper punch 13, a step of removing impurities, and a step of sintering and cooling the powder material M. Before the flow chart 600, a pre-process and a post-process are performed.

[0034] (pre-process) First, an operator inserts the lower punch 14 into the inner space 11 of the die 12 outside the vacuum chamber 10, and fills the inner space 11 formed by the die 12 and the lower punch 14 with a required amount of powder material M. In this embodiment, the die 12 is cylindrical, and the lower punch 14 is columnar, but this is not limited thereto, and a die and punch having a shape that matches the desired shape of the sintered body to be produced may be prepared. The powder material M is not limited as long as it can form a sintered body by spark plasma sintering, and for example, metals, alloys, and ceramics such as oxides, carbides, and nitrides may be appropriately used.

[0035] (Installation process) Next, the die 12 filled with the powder material M and with the lower punch 14 inserted therein is placed at a predetermined position on the lower spacer 16 in the vacuum chamber 10 (s601). A thermocouple (not shown) for temperature control is attached to the die 12. The upper punch 13 is attached to the upper spacer 15 as shown in Figures 1 and 5 (s602). At this time, the upper punch 13 is moved laterally and inserted between the hooks 152, 153 so that the flange 132 rests on the protruding portions 152a, 153a of the hooks 152, 153. Finally, the door of the vacuum chamber 10 is closed.

[0036] (Impurity removal process) The step of removing impurities preferably includes a non-heating impurity removal step of removing gas present between particles of the powder material M without heating in a vacuum atmosphere, and a heating impurity removal step of removing gas and foreign matter attached to the particles of the powder material M while applying heat to the powder material M in a vacuum atmosphere. First, the door of the vacuum chamber 10 is closed, and then the on-off valve 22 is opened and the air in the vacuum chamber 10 is exhausted by the vacuum pump 21 (s603). At this time, the upper punch 13 is not inserted into the internal space 11 of the die 12. By exhausting the air in this manner without inserting the upper punch 13 into the internal space 11 of the die 12, the gas present between particles of the powder material M can be removed (s604: non-heating impurity removal). The degree of vacuum can be appropriately selected depending on the type of powder material M, the target performance and characteristics of the sintered body to be produced, and the like, but is typically 10 Pa to 1×10 -3 Pa is preferred.

[0037] Next, the press mechanism 30 is operated, and the lower electrode 18 is moved upward while the inside of the vacuum chamber 10 is kept in a vacuum, thereby inserting the upper punch 13 into the inner space 11 (s605). At this time, the alignment between the upper punch 13 and the inner space 11 is performed as described above with reference to FIG. 5. When the pressure applied to the powder material M reaches a predetermined pressure, electricity is applied by the upper electrode 17 and the lower electrode 18, and the powder material M is heated until it reaches a predetermined temperature (s607). The pressure applied to the powder material M and the predetermined temperature can be appropriately selected according to the type of powder material M, the target performance and characteristics of the sintered body to be produced, and the like. The pressure is preferably 0.1 kN or more, and can be used up to the maximum pressure of the device 1 as necessary. In addition, the temperature can be any temperature from a temperature higher than room temperature to a sintering temperature as necessary. As an example, the pressure can be 0.1 kN and the temperature can be 150°C.

[0038] After heating to a predetermined temperature, the press mechanism 30 is operated to move the lower electrode 18 downward, thereby removing the upper punch 13 from the inner space 11. The heated powder material M is held in this state in a vacuum atmosphere for a predetermined time (s608). The holding time can be appropriately selected according to the type of powder material M and the target performance and characteristics of the sintered body to be produced, and can be 1 minute to 99 minutes as necessary. As an example, the holding time can be 3 minutes. By holding the heated powder material M in a vacuum atmosphere while the upper punch 13 is not inserted into the die 12, gas and impurities attached to the surface of the particles of the powder material M are removed (s609: thermal impurity removal). As described above, by performing both the non-thermal impurity removal step and the thermal impurity removal step, a sintered body with high purity and high mechanical strength can be obtained. The thermal impurity removal may be performed multiple times as necessary, and in that case, the steps of s605 to s609 are repeated multiple times. In the device 1 according to the present invention, the upper punch 13 can be inserted into and removed from the die 12 in a vacuum atmosphere, so that the impurities can be removed by heating multiple times.

[0039] In another embodiment, the impurity removal step may be only the thermal impurity removal. In this case, after the upper punch 13 is attached to the upper spacer 15 in s602, the press mechanism 30 is operated to insert the upper punch 13 into the die 12, and simultaneously or before or after that, the air in the vacuum chamber 10 is exhausted (s606). Thereafter, the upper electrode 17 and the lower electrode 18 are energized to heat the powder material M until the temperature of the powder material M reaches a predetermined temperature (s607). Furthermore, the upper punch 13 is removed from the inner space 11, and the heated powder material M is held in this state in a vacuum atmosphere for a predetermined time (s608), thereby performing the thermal impurity removal (s609). In this case, since the non-thermal impurity removal is not performed, it is particularly preferable to perform the thermal impurity removal multiple times by repeating the steps of s605 to s609 multiple times.

[0040] In still another embodiment, the powder material M can be heated by heat from outside the die 12 using the heater 42. In this case, after the non-heating impurity removal step is completed, the heater holder 41 is moved so that the heater 42 is positioned to the side of the die 12, and the powder material M is heated without the upper punch 13 being inserted into the internal space 11, and the powder material M is held in a vacuum atmosphere for a predetermined time while the heating is continued or stopped.

[0041] (Sintering and cooling process) After the heated powder material M is held in a vacuum atmosphere for a predetermined time, the press mechanism 30 is operated to move the lower electrode 18 again while maintaining the vacuum inside the vacuum chamber 10, thereby inserting the upper punch 13 into the inner space 11 again (s610). The press mechanism 30 is operated until the pressure applied to the powder material M reaches a predetermined value. After the predetermined pressure is reached, electricity is passed through the upper electrode 17 and the lower electrode 18 at a predetermined heating rate and sintering temperature for a predetermined sintering time, and sintering is performed (s611). The pressure, heating rate, sintering temperature, and sintering time during sintering may be appropriately selected depending on the type of powder material M, the target performance and characteristics of the sintered body to be produced, and the like. For example, the pressure can be 7 kN, the heating rate 240°C / min, the sintering temperature 500°C, and the sintering time 2 minutes. When sintering is complete, the current is stopped, and the die 12 is left to stand until its temperature drops to the set temperature, after which it is rapidly cooled by blowing cooling gas onto the die 12 as necessary (s612).

[0042] (post-process) After the cooling of the die 12 is completed, the post-processing is performed. Specifically, it is as follows. First, the vacuum state of the vacuum chamber 10 is released. Next, the press mechanism 30 is operated to move the lower electrode 18 downward, and the die 12, the upper punch 13, and the lower punch 14 are removed from the vacuum chamber 10. At this time, since the flange 132 of the upper punch 13 is placed on the protruding parts 152a, 152b of the upper spacer 15, the upper punch 13 is moved laterally and removed from between the hooks 152, 153. If necessary, a jig is used to remove the upper punch 13 and the lower punch 14 from the die 12, and the sintered body is removed. If necessary, burrs are removed to complete the sintered body. EXAMPLES

[0043] Example 1 The device according to the present invention can insert and remove the upper punch into and from the die in a vacuum atmosphere. Therefore, it is possible to perform the thermal impurity removal multiple times. Here, the impurity removal effect when the thermal impurity removal was performed multiple times was confirmed. Three types of sintered samples were prepared using a punch with a diameter of 15 mm and a corresponding die, and the impurity removal effect was compared. The powder materials, weights of sintered bodies, sintering process conditions, and thermal impurity removal conditions of the three types of samples are as shown in Tables 1 to 3. In the tables, "heated Degas" stands for "heated impurity removal". In samples No. 8 and No. 9, stearic acid powder was assumed to be an impurity and mixed into the powder raw material. The number of times the heated Degas was repeated was two.

[0044] [Table 1]

[0045] [Table 2]

[0046] [Table 3] The experimental results are shown in Table 4. Also, Fig. 7 and Fig. 8 are element mapping images obtained by cutting and polishing the cut surfaces of the sintered bodies of Samples No. 8 and No. 9 and measuring them with an EDS (energy dispersive X-ray analyzer).

[0047] [Table 4]

[0048] Comparing sample No. 8 and sample No. 9, the sintered weight of sample No. 8 was lighter, and the inside of the vacuum chamber turned white overall after sintering of No. 8. This is thought to be because the stearic acid was vaporized by the thermal impurity removal, and adhered to the cold inner walls of the vacuum chamber and to the viewport glass of the vacuum chamber. Also, the EDS observation results show that there is less C (carbon) remaining in the cross section of the sample in No. 8 (Figure 7) than in No. 9 (Figure 8). From these findings, it is thought that impurities in the powder material can be removed before sintering by thermal impurity removal, making it possible to increase purity. [Explanation of symbols]

[0049] 1. Spark plasma sintering equipment 10 Vacuum Chamber 11 Inner Space 12 Die 121 Main unit 122 Tapered section 13 Upper punch 131 Main unit 132 Brim 14 Lower punch 15 Upper spacer 151 Main unit 152, 153 Hook 152a, 153a protruding parts 16 Lower spacer 17 Upper electrode 18 Lower electrode 20 Exhaust system 21 Vacuum Pump 22 Opening and closing valve 30 Press mechanism 40 Heating mechanism 41 Heater holder 42 Heater 43 Vertical drive unit M powder material

Claims

1. A spark plasma sintering method for sintering a powder material filled in an inner space of a cylindrical die by applying electric current while pressing the powder material with a punch in a vacuum atmosphere, comprising: a vacuum process for creating a vacuum atmosphere around the die filled with the powder material; a heating step of heating the powder material in a vacuum atmosphere; a holding step of holding the heated powder material in a vacuum atmosphere with the punch not inserted into the internal space; a sintering step of inserting the punch into the internal space after the holding step in a vacuum atmosphere, and applying a current to the powder material while pressing the powder material with the punch; A spark plasma sintering method comprising:

2. The spark plasma sintering method according to claim 1 , wherein the heating step includes inserting the punch into the internal space, heating the powder material by passing current through the punch, and removing the punch from the internal space.

3. The vacuum process includes creating a vacuum atmosphere around the die filled with the powder material while the punch is not inserted into the internal space of the die. The spark plasma sintering method according to claim 1 or 2.

4. The spark plasma sintering method according to claim 1 , wherein the heating step comprises heating the powder material by externally heating the die.

Images