Method for manufacturing sintered body

Abstract

To provide a method for manufacturing a sintered body which simplifies a manufacturing process, can reduce a manufacturing cost, and has a dense layer having no hole in a surface layer part.SOLUTION: A method for manufacturing a sintered body includes the steps of: molding raw material powder, and molding a powder molding 20; raising the temperature of the powder molding 20 from a normal temperature to a predetermined temperature at a temperature raising rate of 20°C / min or more and blocking a hole in only the surface layer part, performing primary sintering to form a dense layer 21 for airtightly sealing the inside, and obtaining a primary sintered body SB1; and subjecting the primary sintered body SB1 to HIP treatment, and thereby performing secondary sintering to sinter the primary sintered body SB1, and obtaining a secondary sintered body SB2.SELECTED DRAWING: Figure 3

Description

Technical Field

[0001] The present invention relates to a method for manufacturing a sintered body in which a powder compact made of metal powder or the like is sintered by a three-dimensional lamination molding method to obtain a three-dimensional object.

Background Art

[0002] Conventionally, a layer manufacturing method is known in which a step of laminating a raw material powder for layer manufacturing (metal powder, alloy powder, or ceramic powder) on a powder bed and a step of binding the laminated layer of raw material powder into a predetermined shape are alternately repeated to finally obtain a three-dimensional object. As this layer manufacturing method, there is a selective sintering method (Powder Bed Fusion method, that is, PBF method) in which a step of directly sintering by irradiating a raw material powder with a laser beam or an electron beam is repeated to bond the sintered portions to obtain a target three-dimensional object. On the other hand, there is a binder jet method in which a binder is printed on the raw material powder to be laminated to form a powder compact in which the raw material powder and the binder are bonded, and the powder compact is sintered to obtain a three-dimensional object. In recent years, this binder jet method has been particularly developed and put into practical use because the apparatus, raw materials, and process costs are low and it can be efficiently implemented (see Patent Documents 1, 2, etc.).

[0003] When sintering a powder compact obtained by the binder jet method, that is, a laminate of a plurality of powder layers, solid diffusion of metal powder and bonding of the powder occur at a high temperature, so that the powder compact is sintered to a high density. Usually, the density of a powder compact by the binder jet method is about 50% (4 g / cm 3 ) or so in relative density, but the relative density is increased to 97% or more by sintering. If such a sintering density is not ensured, it is difficult to obtain industrial mechanical strength. However, for example, when sintering a relatively large part having a weight of 1 kg to 80 kg to a high density, sintering conditions at a high temperature for a long time are required as compared with MIM (metal injection molding method) or the like for small parts of 100 g or less.

[0004] During sintering, the larger the volume of the powder compact to be sintered, the greater the sintering energy required for densification. As a result, the crystal grains of the sintered body grow larger. A sintered body having enlarged crystal grains has most of its major mechanical properties, such as impact value, strength, elongation, drawing, yield point, etc., decreased as can also be seen in Hall-Petch's law. Therefore, for sintered bodies of large parts using the binder jet method, refinement of the sintered structure is required.

[0005] The energy cost invested in sintering large parts runs counter to the current trend of sustainable technologies. On the other hand, the demand for manufacturing large parts by the binder jet method is increasing, and in that case, a manufacturing technology that enables densification with less energy cost is required.

[0006] In the conventional powder metallurgy method, parts having a weight of about 100 g have been said to be the largest. However, with the binder jet method, it has become possible to manufacture powder compacts of large parts having a weight of about 30 kg. However, in the subsequent sintering, since the volume of the powder compact to be sintered is larger than before, the shrinkage behavior accompanying sintering also becomes larger, and for example, there is a large difference in the amount of shrinkage between the surface installed on the setter (floor plate) and the free surface. Therefore, in order to make the shrinkage amount as uniform as possible, for example, a buffer material of the same quality as the sintered powder material is interposed between the work to be sintered and the ceramic setter, and while absorbing the shrinkage amount at the bottom of the work, measures are taken to reduce the difference in the shrinkage amount between the upper and lower parts of the work.

[0007] In view of the above circumstances, the present inventor has proposed a method for manufacturing a sintered body that can appropriately manufacture sintered parts larger than before by reducing the deformation accompanied by shrinkage during sintering of the laminate caused by high-temperature sintering, while achieving densification, suppressing the growth of crystal grains, and reducing energy costs (Patent Document 3).

[0008] The method for manufacturing a sintered body disclosed in Patent Document 3 includes a step of forming a powder compact containing a first powder for sintering by a binder jet method, a step of forming a coating layer containing a second powder for sintering having an average particle size smaller than that of the first powder on the surface of the powder compact, a step of performing primary sintering to sinter the coating layer with the second powder by heating the powder compact having the coating layer on the surface layer to obtain a primary sintered body, and a step of performing secondary sintering to sinter the first powder by subjecting the primary sintered body to HIP treatment at a temperature lower than the temperature during the primary sintering to obtain a secondary sintered body.

Prior Art Documents

Patent Documents

[0009]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0010] An object of the present invention is to further develop the above method for manufacturing a sintered body to simplify the manufacturing process, reduce the manufacturing cost, and provide a method for manufacturing a sintered body having a dense surface layer portion without pores.

Means for Solving the Problems

[0011] In the technique described in Patent Document 3, a slurry obtained by mixing a resin solution with a second powder is applied to the surface of a compact of the first powder, dried, and sintered to form a high-density outer shell made of the second powder on the surface of the compact. As a result of earnestly studying whether such a series of steps can be simplified, the present inventor came to the idea of forming a high-density sintered portion in the surface layer portion of the compact of the first powder. For this purpose, the inventor reached the inference that the surface layer portion could be selectively sintered by applying a large amount of heat to the compact of the first powder in a short time.

[0012] Therefore, as a result of sintering the powder compact at various heating rates, the present inventor found that when the temperature was raised from room temperature to a predetermined temperature at a heating rate of 20°C / min or more and held for a short time, the pores in the surface layer portion were almost closed, but the temperature inside did not rise as much as the powder compact was larger, and it remained almost in the form of powder due to the short time. And to obtain a primary sintered body in which the surface layer portion had a relative density of approximately 94% or more and the communication between the pores almost disappeared, it was found that when this primary sintered body was subjected to HIP treatment, a secondary sintered body with a relative density close to 100% could be obtained.

[0013] The present invention has been made based on the above findings, and includes a step of molding raw material powder to form a powder compact, a step of performing primary sintering to obtain a primary sintered body by raising the temperature of the powder compact from room temperature to a predetermined temperature at a heating rate of 20°C / min or more to close the pores only in the surface layer portion and form a dense layer that hermetically seals the inside, and a step of performing secondary sintering to sinter the primary sintered body to obtain a secondary sintered body by subjecting the primary sintered body to HIP treatment.

[0014] In the present invention, in order to selectively sinter only the surface layer to obtain a dense layer, the temperature is raised from room temperature to a predetermined temperature at a heating rate of 20 °C / min or more in the first sintering. If the heating rate is less than 20 °C / min, sintering of the powder compact due to the internal powder proceeds easily together with the surface layer. When the internal powder sinters, the pores remain large and the powders stick together, so the pores may not be filled by the subsequent HIP treatment. The heating rate is preferably 25 °C / min or more, more preferably 40 °C / min or more, and even more preferably 60 °C / min or more. The heating rate may be 90 °C / min or more.

[0015] The heating temperature and the heating holding time are appropriately selected according to the type of the raw material powder. When the raw material powder is an iron-based metal, the heating holding temperature in the first sintering is preferably 1350 to 1450 °C. In this temperature range, neck growth of the powder in the surface layer proceeds rapidly, the structure contracts, and the pores are blocked. Also, the heating holding time is preferably 3 to 15 minutes. By sintering under such conditions, a large amount of heat can be applied to the powder compact in a short time.

[0016] When the raw material powder is an iron-based metal, if the heating holding temperature in the first sintering is lower than 1350 °C and the heating holding time is less than 3 minutes, sintering in the surface layer becomes insufficient and it becomes difficult to obtain a dense layer having a desired relative density. On the other hand, if the heating holding time exceeds 15 minutes, the dense layer dissolves and sintering of the powder compact inside the dense layer tends to proceed. The adverse effects are as described above. The heating holding temperature in the first sintering is limited to 1450 °C, which is lower than the melting point of iron. The heating holding temperature in the first sintering is more preferably 1400 to 1430 °C, and the heating holding time is more preferably 3 to 10 minutes.

[0017] The thickness of the dense layer rapidly sintered by the first sintering is desirably 100 μm or more. When the thickness of the dense layer is less than 100 μm, it becomes difficult to obtain the effect of sealing the inner molded portion of the first sintered body with the dense layer and hermetically sealing it. In addition, there is a risk that the dense layer may be deformed or destroyed by the pressure of the HIP treatment during the second sintering by the HIP treatment. On the other hand, when the thickness of the coating layer exceeds 800 μm, the processing cost of surface polishing performed after the completion of the sintered body tends to increase, and the productivity decreases. Therefore, the thickness of the surface layer portion is desirably 100 to 800 μm, more desirably 500 to 700 μm.

[0018] As a method for molding the raw material powder, the binder jet method can be preferably adopted, but it is not limited thereto, and any molding method can be used. For example, a binder can be mixed with the raw material powder to make it clay-like, and it can be molded into a desired shape by handwork or using a press or a cutting machine.

[0019] Examples of the raw material powder include metal powders, alloy powders, and among them, steel powders. The raw material powder will be selected according to the target product made of the sintered body. For example, if the product is a mold, an alloy powder of alloy steel for hot work molds typified by SKD61 can be mentioned. Also, if the product is a cast steel product, high-tensile carbon steel cast iron typified by SCC material can be mentioned. In addition to these, at least one of the metal powders and alloy powders that are generally used in the above-mentioned MIM, such as stainless steel and Inconel (registered trademark), carbonyl iron powder, carbonyl nickel powder, nickel-based alloy powder, cobalt-based alloy powder, copper and copper alloy powder, aluminum and aluminum alloy powder, titanium and titanium alloy powder, magnesium and magnesium alloy powder, ceramic powder, etc. can be mentioned, but it is not limited to these. When the binder jet method is used as the molding method, water atomized powder can be preferably used.

[0020] Next, the primary sintered body that has been sealed (canned state) by the dense layer at the above high density is subjected to secondary sintering by HIP treatment to obtain a secondary sintered body. The heating temperature in the secondary sintering is desirably 900 to 1100 °C. This heating temperature is lower than that of normal metal sintering, and since the inner part of the high-density dense layer receives high pressure from the dense layer, compression solidification occurs effectively. For this reason, recrystallization in the interior of the secondary sintered body remains to a minimum, and densification close to 100% is achieved.

[0021] When the heating temperature during the secondary sintering is lower than 900 °C, it becomes difficult for the powder compact to sinter, and it becomes difficult to obtain a secondary sintered body having a desired relative density. On the other hand, when the heating temperature during the secondary sintering exceeds 1100 °C, grain growth due to powder coarsening may occur. Also, there is a risk of causing an increase in energy costs. Therefore, the heating temperature during the secondary sintering is desirably 900 to 1100 °C, and more desirably 950 to 1000 °C.

[0022] In the present invention, even if the weight of the sintered body is relatively large, for example, about 30 kg, the densification by HIP treatment is always almost constant. Also, due to the compression solidification by HIP treatment in a relatively low temperature range, the occurrence of distortion, deformation, etc. caused by different shrinkage amounts at a plurality of sites, as in normal high-temperature sintering, is suppressed. In addition, it has become possible to make a sintered body with a deformed shape, which was difficult in the above canning treatment, have the same quality as the canning treatment.

Effects of the Invention

[0023] According to the present invention, by using one type of raw material powder and closing only the pores in the surface layer portion of the powder compact by high-speed sintering to form a dense layer, the dense layer acts the same as the can of canning, and HIP treatment in the next step is possible. Thus, in the present invention, the manufacturing process is simplified and the manufacturing cost can be reduced. In addition, the sintered body manufactured in this way has a dense dense layer without pores, and thus is excellent in strength and hardness.

Brief Description of the Drawings

[0024]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Mode for Carrying Out the Invention

[0025] Hereinafter, embodiments of the present invention will be described with reference to the drawings. The method for manufacturing a sintered body according to the embodiment includes a step of forming a powder compact made of raw material powder for sintering by a binder jet method, a step of heating the powder compact from room temperature to a predetermined temperature at a heating rate of 20°C / min or more to close pores only in the surface layer portion to form a dense layer, and performing a first sintering to obtain a first sintered body, and a step of performing a second sintering to sinter the first sintered body by HIP treatment to obtain a second sintered body.

[0026] First, an outline of a method for obtaining a sintered body using the binder jet method will be described. FIG. 1 schematically shows a process of three-dimensionally laminating and shaping an object by a binder jet method, and sintering the shaped powder compact of the target shape to obtain a sintered body.

[0027] The method for manufacturing the sintered body shown in Fig. 1 first, as shown in Fig. 1(A), on a horizontally set powder bed 11 having a predetermined area, raw material powder P made of iron-based metal powder for sintering (for example, steel powder, including alloy powder composed of multiple types of metals) is supplied while being allowed to fall naturally from a hopper 12 and spread to form a raw material powder layer PL of a single layer with a predetermined thickness. As shown in Fig. 2, the raw material powder layer PL is leveled by a roller 13 that moves in conjunction with the hopper 12 so as to have a flat and uniform thickness. The thickness of the single-layer raw material powder layer PL is, for example, about 40 to 50 μm, but is appropriately set within a range of generally 100 μm or less.

[0028] Next, as shown in Fig. 1(B), a liquid binder B (for example, BA005: manufactured by ExOne US, etc.) is selectively ejected from an inkjet dispenser 14 onto the laminated raw material powder layer PL. The raw material powder P in the portion that receives the ejection of the binder B is bonded and cured by the binder B. The inkjet dispenser 14 is computer-controlled based on three-dimensional data corresponding to the shape of the target three-dimensional sintered body and is driven over the raw material powder layer PL.

[0029] Next, raw material powder P is again supplied from the hopper 12 onto the first raw material powder layer PL selectively bonded with the binder B and leveled with the roller 13 to laminate the second-layer raw material powder layer PL. Then, the binder B is selectively ejected from the inkjet dispenser 14 onto the second-layer raw material powder layer PL to bond the raw material powder with the binder.

[0030] In this way, the process of laminating the raw material powder P on the raw material powder layer PL on which the bonding portion by the binder B is selectively formed to form the next raw material powder layer PL, and then selectively ejecting the binder B onto the raw material powder layer PL is repeated a number of times to form a powder molded body G, which is a combination of the binder B and the raw material powder P, inside the multi-layer raw material powder layers PL (shown in Fig. 1(C)). In order to form an integral three-dimensional powder molded body, the raw material powder layers PL that are adjacent and stacked vertically at least partially overlap in the supply portion of the binder B and bond to each other, thereby forming a vertically continuous powder molded body G.

[0031] Next, as shown in Fig. 1(D), the powder compact G is taken out from inside the raw material powder layer PL. To take out the powder compact G from inside the raw material powder layer PL, the laminated raw material powders P that surround the powder compact G and on which no binder is printed and that are not bonded can be removed by a method such as suction using a suction nozzle, for example. The method for removing the raw material powder P not bonded by the binder B is not limited to this, and an appropriate method is selected. Next, the taken-out powder compact G is sintered under predetermined sintering conditions to obtain a sintered body.

[0032] The above is an outline of the method for manufacturing a three-dimensional shaped sintered body using the binder jet method. In the embodiment of the present invention, up to the step of obtaining the powder compact G among the above steps is performed. Subsequently, the details of the embodiment will be described.

[0033] Figs. 3(A) to (C) schematically show the steps of the method for manufacturing a sintered body according to the embodiment. The method for manufacturing a sintered body according to the embodiment first forms a powder compact 20 by the above-described binder jet method as shown in Fig. 3(A). The average particle size of the raw material powder is, for example, 3 to 50 μm.

[0034] Next, the powder compact 20 is loaded into a high-speed sintering furnace, and the temperature increase is started to perform primary sintering. As the high-speed sintering furnace, for example, an indirectly heated type using molybdenum as a heating element is used. The rate of temperature increase is 25°C / min, and when the temperature reaches 1430 to 1450°C, it is held for 10 minutes. During this holding period, the surface layer portion of the powder compact 20 is rapidly sintered, pores are closed, and a dense layer 21 is formed. The thickness of the dense layer 21 is 50 to 800 μm. During this holding time, the inner low-density layer 10A inside the dense layer 21 is not sintered or is in an intermediate state of sintering. As shown in Fig. 3(B), when the powder compact 20 is taken out from the high-speed sintering furnace and cooled after the holding time has elapsed, a primary sintered body SB1 having a dense layer 21 is obtained.

[0035] The first sintered body SB1 has a high-density dense layer 21. Since the dense layer 21 is rapidly sintered, its relative density is 94% or more, which is a high density. Generally, in powder metallurgy, when the relative density exceeds 94%, the internal pores become closed pores and HIP treatment becomes possible. In the first sintered body SB1, the dense layer 21 forms an outer shell, and the outside and the inner low-density layer 10A outside the dense layer 21 are hermetically isolated by the dense layer 21. That is, the surface of the porous inner low-density layer 10A is sealed by the dense layer 21.

[0036] Figure 4 is a micrograph showing a partial cross-section of the first sintered body SB1 in which the dense layer 21 is sintered. As shown in Figure 4, the first sintered body SB1 has a structure in which a porous inner low-density layer 10A is covered by a dense dense layer 21. In Figure 4, the dense layer 21 is a structure in which pores indicated by black dots are scattered in a white base tissue. Thus, the dense layer 21 has few pores and a relative density of 94% or more. Therefore, there is almost no communication between the pores, and the inner low-density layer 10A is hermetically sealed.

[0037] Next, as shown in Figure 3, the first sintered body SB1 is subjected to a second sintering in which HIP treatment is performed at a relatively low temperature. The HIP treatment during the second sintering is performed, for example, at about 1150 °C × 100 MPa × 3 hours, thereby obtaining a second sintered body SB2. In this embodiment, this second sintered body SB2 is obtained as the target sintered body. Note that the HIP treatment in the second sintering is not limited to the above conditions. For example, the sintering temperature is about 900 to 1100 °C, the applied pressure is about 80 to 120 MPa, and it is appropriately carried out over time.

[0038] The heating temperature (sintering temperature) during the second sintering is lower than about 1300°C to 1400°C of normal HIP treatment. Therefore, the energy cost is reduced, and thermal deformation is less likely to occur. In the sintering by HIP treatment, the internal low-density layer 10A is also sintered together with the dense layer 21 and becomes the internal layer 10B after HIP. The internal layer 10B after HIP can be sintered at a temperature lower than the normal sintering temperature by compression solidification while receiving pressure evenly from the dense layer 21. As a result, in the second sintered body SB2, the recrystallization of the powder in the internal low-density layer 10A is minimized, the coarsening of crystal grains is suppressed, and a high density close to 100% relative density is achieved.

[0039] During this second sintering, since the surface of the internal low-density layer 10A is hermetically sealed by the dense layer 21 that serves as the outer shell, the high pressure by HIP treatment is applied as an external pressure to the entire internal low-density layer 10A without penetrating into the inside of the internal low-density layer 10A and acts on the internal low-density layer 10A. For this reason, the internal low-density layer 10A is compressed at the same pressure as the external pressure. And since the dense layer 21 covers the entire surface, the high pressure during HIP treatment acts evenly on the entire area of the internal low-density layer 10A through the dense layer 21.

[0040] As a result, as shown in Fig. 3(C), the internal low-density layer 10A is densified to become the internal layer 10B after HIP, and the second sintered body SB2 is obtained. In the second sintered body SB2, the shrinkage amount is likely to be uniform at any part. As a result, the occurrence of distortion, deformation, etc. caused by different shrinkage amounts at a plurality of parts is suppressed. Also, it becomes possible to perform HIP treatment without interposing a buffer material between the powder compact 20 and the ceramic setter on which the powder compact 20 is placed as in the conventional case. Also, it becomes possible to manufacture a sintered body with an irregular shape, which was difficult in the conventional canning treatment, to have the same quality as the canning treatment.

[0041] FIG. 5 is a micrograph showing a partial cross-section of the secondary sintered body SB2. In FIG. 5, the portion where the grain boundaries are clear is the internal layer 10B after HIP sintering of the internal low-density layer 10A. There are slight pores in the internal layer 10B after HIP, and the relative density is 98% or more. Also, the entire circumference of the internal layer 10B after HIP is covered with the dense layer 21B after HIP. The dense layer 21B after HIP is obtained by subjecting the dense layer 21 to HIP treatment and compressing it, and the pores have disappeared. Also, the crystal grains of the dense layer 21B after HIP have grown larger due to the first sintering. There are no pores in the dense layer 21B after HIP, and the relative density is 100%. Therefore, the dense layer 21B after HIP has higher strength and hardness than the internal layer 10B after HIP, and the secondary sintered body SB2 is promising as, for example, a press molding die.

[0042] According to the method for manufacturing a sintered body according to the above embodiment, since the surface layer portion of the powder compact 20 is rapidly sintered by rapid sintering using one type of raw material powder to form the dense layer 21, the dense layer 21 functions in the same way as a can for canning, and HIP treatment in the next step is possible. Thus, in the above embodiment, the manufacturing process is simplified, and the manufacturing cost can be reduced. Also, the secondary sintered body SB2 manufactured in this way has a high density with a relative density almost close to 100% and is provided with a dense HIP-treated dense layer 21B without pores, and thus is excellent in strength and hardness. Therefore, the secondary sintered body SB2 is promising as a press molding die.

[0043] The present invention is not limited to the above embodiment, and various modifications are possible as follows. i) As the molding method of the powder compact 20, in addition to the binder jet method, any three-dimensional layer molding method such as the fused deposition molding (FDM) method or the selective laser sintering (SLS) method can be adopted.

[0044] ii) As the raw material powder P, a metal powder produced by the water atomization method is preferable. Since the water atomized powder has an irregular shape, it has a large angle of repose and poor fluidity. When the raw material powder P made of a metal powder for sintering is supplied and spread while being naturally dropped from the hopper 12 onto the powder bed 11 with open sides as in the above embodiment, the metal powder hardly spreads unnecessarily in all directions and forms a single-layer raw material powder layer PL with a predetermined thickness. On the other hand, when using a metal powder close to a spherical shape, when leveled with the roller 13, since the fluidity is good, there is a portion where the metal powder spreads too much in all directions, and there is a risk that the thickness becomes uneven. Also, since a metal powder close to a spherical shape has a small angle of repose, as shown in Fig. 1(C), in order to form multiple layers, the skirt of the raw material powder becomes considerably wider, and accordingly, the powder bed 11 also needs to be enlarged.

Industrial Applicability

[0045] The present invention can be used in the industrial field of relatively large sintered bodies such as press molding dies.

Explanation of Reference Numerals

[0046] 10A... Inner low-density layer, 10B... Inner layer after HIP, 11... Powder bed, 12... Hopper, Roller 13, 14... Inkjet dispenser, 20... Powder compact, 21... Dense layer, 21B... Dense layer after HIP, B... Binder, SB1... First sintered body, SB2... Second sintered body.

Claims

1. A step of forming a raw material powder into a powder compact; A step of obtaining a first sintered body by performing a first sintering in which the powder compact is heated from room temperature to a predetermined temperature at a heating rate of 20°C / min or more to close pores only in the surface layer portion and form a dense layer that hermetically seals the inside; A step of obtaining a second sintered body by performing a second sintering in which the first sintered body is sintered by HIP treatment, the method for manufacturing a sintered body comprising these steps.

2. The method for manufacturing a sintered body according to Claim 1, wherein the HIP treatment is performed at a heating temperature of 900 to 1100°C and a pressure of 80 MPa or more.

3. The method for manufacturing a sintered body according to Claim 1 or 2, wherein the thickness of the dense layer is 100 µm or more.

4. The method for manufacturing a sintered body according to Claim 1 or 2, wherein the powder compact is formed by a binder jet method.

5. The raw material powder is at least one of an iron-based alloy powder, a carbon steel cast iron powder, a stainless steel powder, an Inconel (registered trademark) powder, a carbonyl iron powder, a carbonyl nickel powder, a nickel-based alloy powder, a cobalt-based alloy powder, a copper and copper alloy powder, an aluminum and aluminum alloy powder, a titanium and titanium alloy powder, a magnesium and magnesium alloy powder, and a ceramic powder, the method for manufacturing a sintered body according to Claim 1 or 2.

6. A sintered body in which the entire circumference of a sintered structure having grain boundaries and pores is covered with a HIP post-dense layer composed of grains that are larger than those of the sintered structure without pores.

7. The sintered body according to Claim 6, wherein the thickness of the dense layer is 100 µm or more.

8. The sintered body according to Claim 6 or 7, wherein the relative density of the sintered structure is 98% or more and less than 100%, and the relative density of the dense layer is 100%.

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