Ceramic setter, method for producing material for ceramic balls, and method for producing ceramic balls

Abstract

The ceramic setter according to an embodiment holds the ceramic molded body in place during the execution of degreasing and / or sintering on a ball-shaped ceramic molded body. This ceramic setter has, in a first bottom surface that is one bottom surface of two bottom surfaces of a cylinder or prism, a first bottom surface groove that has a shape in which a surface parallel to the bottom surface decreases along the depth. A material of the ceramic setter is preferably a material in common with the material for ceramic balls. The material for ceramic balls, which is obtained after the ceramic molded body has been sintered using the ceramic setter, preferably comprises any one of a silicon nitride sintered body, an aluminum oxide sintered body, and a zirconium oxide sintered body.

Description

Ceramic setter, method for manufacturing ceramic ball material, and method for manufacturing ceramic ball

[0001] The embodiments described below relate to a ceramic setter, a method for manufacturing a ceramic ball material, and a method for manufacturing a ceramic ball.

[0002] Various ceramic materials possess properties such as high hardness, insulation, and wear resistance. Fine ceramics, in particular, which have high purity and uniform particle size, exhibit properties that make them useful in a variety of fields, including capacitors, actuator materials, and refractory materials. Ceramic materials are used in balls, which are products that take advantage of their wear resistance and insulation. Ceramic balls, which are balls made of ceramic, are used in bearings, jigs, tools, gauges, solenoid valves, check valves, and various valves. Among these, ceramic bearing balls, which are ceramic balls used for bearing applications, are made from materials such as aluminum oxide, silicon nitride, and zirconium oxide (Patent Documents 1 to 3). For example, Patent Documents 1 and 2 disclose bearing balls made from silicon nitride, and Patent Document 3 discloses bearing balls made from zirconium oxide.

[0003] In a process for producing ceramic ball materials, which are ball-shaped ceramic sintered bodies (hereinafter simply referred to as "sintered bodies") for producing ceramic balls, including these ceramic bearing balls, a method of sintering a ball-shaped ceramic green body (hereinafter simply referred to as "green body") is used. Furthermore, press molding using a mold is used as a molding method. Press molding generally involves inserting powder between an upper punch and a lower punch and applying pressure to obtain a green body. Because a pressed body is formed by uniaxial loads from above and below, a CIP body, which is a homogeneous green body, is obtained by applying pressure from all directions using cold isostatic pressing (CIP) (Patent Document 4). According to Patent Document 4, a gap must be provided between the tips of the upper and lower punches during press molding to protect the mold. Therefore, the green body has a spherical portion and a band-like portion due to the gap, and therefore the ceramic ball material also has a spherical portion and a band-like portion.

[0004] Also, a method for molding a ceramic ball material by rolling granulation without using a press die has been disclosed (Patent Document 5). According to Patent Document 5, a ceramic ball material without a band-shaped portion is molded by rolling granulation.

[0005] Japanese Patent Application Laid-Open No. 6-48813 Japanese Patent No. 2764589 Japanese Patent Application Laid-Open No. 60-18620 International Publication No. 2023 / 003040 Japanese Patent Application Laid-Open No. 2001-146479

[0006] As the market expands, larger ceramic bearing balls are being used in larger bearings. The process for manufacturing ceramic balls, including ceramic bearing balls, begins with mixing ceramic powder with an organic binder to obtain a green compact through powder molding. Next, the green compact is degreased and sintered to obtain a sintered compact. This sintered compact is called a ceramic ball material. Next, the ceramic ball material is polished to produce ceramic balls. Of the ceramic balls, those manufactured for bearing use are ceramic bearing balls.

[0007] However, as the size of the compacts increases, the compact's own weight can cause deformation and breakage during the heat treatment process (especially the debinding and sintering processes). Because the surface of the compact is spherical, the compact contacts the flat setter, which serves as the base for sintering, at a point rather than a surface. Even if the compact maintains its shape before debinding due to the organic binder, debinding removes the organic components from the compact, making it brittle and prone to deformation and other defects. For example, the surface of the compact that contacts the setter becomes flat, resulting in poor deformation of the ceramic ball material after sintering. Furthermore, the compact is subjected to a single point of force, which can easily react with the setter and burn, resulting in poor appearance (dentation). Therefore, it was necessary to increase the contact area between the compact and the setter during the debinding and sintering processes to distribute the compact's weight. Furthermore, if the setter is flat, the spherical compact tends to roll, resulting in damage to the compact due to collisions or contact.

[0008] Furthermore, when ceramic ball materials obtained by sintering molded bodies with deformation or dent defects are processed into ceramic balls, there is the problem that a long polishing process is required to make them spherical.Furthermore, if defects remain that cannot be removed in the polishing process, there is also the problem that only ceramic balls with low reliability can be obtained.

[0009] The present invention solves these problems by providing a ceramic setter that can reduce the occurrence of defects (e.g., deformation defects and dent defects) in the ceramic ball material after sintering by avoiding the concentration of the weight of the molded body at one point when at least one of degreasing and sintering the molded body.

[0010] In one embodiment, a ceramic setter on which a ball-shaped ceramic body is placed when at least one of degreasing and sintering the body is performed has a first bottom surface groove on the first bottom surface, which is one of the two bottom surfaces of a cylinder or rectangular prism, in which the surface parallel to the bottom surface becomes smaller along the depth.

[0011] FIG. 1 is a diagram showing an example of a molded body to be placed on a ceramic setter according to an embodiment. FIG. 2 is a diagram showing an example of die press molding for producing a molded body to be placed on a ceramic setter according to an embodiment. FIG. 3 is a diagram showing an example of a molded body to be placed on a ceramic setter according to an embodiment. FIG. 4 is a cross-sectional view showing an example of a ceramic setter in use according to an embodiment. FIG. 5 is a diagram showing a first example of a ceramic setter according to an embodiment. FIG. 6 is a diagram showing a second example of a ceramic setter according to an embodiment. FIG. 7 is a cross-sectional view showing an example of a ceramic setter in use according to a comparative example. FIG. 8 is a diagram showing a third example of a ceramic setter according to an embodiment. FIG. 9 is a diagram showing a fourth example of a ceramic setter according to an embodiment. FIG. 10 is a diagram showing a fifth example of a ceramic setter according to an embodiment. Embodiment

[0012] Hereinafter, embodiments of a ceramic setter, a method for manufacturing a ceramic ball material, and a method for manufacturing a ceramic ball will be described in detail with reference to the drawings.

[0013] In one embodiment, a ceramic setter on which a ball-shaped ceramic body is placed when at least one of degreasing and sintering the body is performed has a first bottom surface groove on the first bottom surface, which is one of the two bottom surfaces of a cylinder or rectangular prism, in which the surface parallel to the bottom surface becomes smaller along the depth.

[0014] FIG. 1 shows a compact. The compact broadly includes a press-molded body after press molding, a CIP body after CIP treatment, and a degreased body after degreasing treatment. In FIG. 1, reference numeral 1 denotes the compact, reference numeral 2 denotes a spherical portion, and reference numeral 3 denotes a band-like portion. FIG. 2 also shows a press die for molding the press-molded body as the compact 1. In FIG. 2, reference numeral 4 denotes a press die, and reference numeral 5 denotes a punch portion. FIG. 3 shows a compact 1 having only the spherical portion 2. The compact 1 shown in FIG. 3 is produced by, for example, rolling granulation. Note that the compact 1 shown in FIG. 3 is not limited to being produced by rolling granulation, and may be a compact 1 shown in FIG. 1 in which the band-like portion 3 has been polished off.

[0015] 4 is a cross-sectional view of an example in which a CIP body, which is a molded body 1, is placed on a ceramic setter according to an embodiment. In FIG. 4, reference numeral 1 denotes the molded body, reference numeral 6 denotes a ceramic setter having a generally cylindrical shape, and reference numeral 7 denotes a non-penetrating first bottom groove (e.g., a conical upper surface groove) formed on the first bottom surface (upper surface), which is one of the two bottom surfaces (first and second bottom surfaces) of the generally cylindrical ceramic setter 6. The molded body 1 may have a band-shaped portion 3 as shown in FIG. 1, or may not have a band-shaped portion 3 as shown in FIG. 3.

[0016] The upper surface grooves 7 of the ceramic setter 6 are provided on the upper surface, which is one of the bottom surfaces of a cylinder or a rectangular pillar, and have a shape in which the surface parallel to the bottom surface becomes smaller along the depth. For example, the upper surface grooves 7 have a conical shape, a truncated conical shape, a pyramidal shape, or a truncated pyramidal shape. In the following drawings, unless otherwise specified, the non-penetrating upper surface grooves 7 will be described using the case where they have a conical shape (shown in FIGS. 4 to 6 ) as an example, but are not limited to this case.

[0017] Furthermore, the slope of the upper surface groove 7 is not limited to a straight line, but may be curved (for example, arcuate). When the upper surface groove 7 has a conical or truncated conical shape, the green body 1 contacts the ceramic setter 6 and the slope of the upper surface groove 7 at a ring-shaped surface, so that the weight of the green body 1 is dispersed rather than concentrated at a single point, thereby suppressing deformation of the green body 1 due to its own weight. Furthermore, the green body 1 is supported by the contact surface rather than at a point. Therefore, by using the ceramic setter 6 having the shape shown in FIG. 4, it is possible to prevent the green body 1 from rolling after being placed on it.

[0018] FIG. 5 shows a first example of a ceramic setter 6, which is a ceramic setter 6 having a generally cylindrical shape. FIG. 5(A) is a top view of the generally cylindrical ceramic setter 6, and a conical upper surface groove 7 is provided on the upper surface, which is one of the bottom surfaces of the cylinder. FIG. 5(B) is a front view of the generally cylindrical ceramic setter 6, FIG. 5(C) is a bottom view of the generally cylindrical ceramic setter 6, and FIG. 5(D) is a VD-VD cross-sectional view of FIG. 5(A). The upper surface groove 7 of the ceramic setter 6 shown in FIG. 5 is provided on the upper surface of a cylinder with a bottom diameter D, and has a conical shape with a bottom diameter D including a vertex (deepest part) G. The molded body 1 is placed so as to contact the slope of this upper surface groove 7.

[0019] FIG. 6 shows a second example of a ceramic setter 6, which is a ceramic setter 6 having a generally prismatic shape (e.g., a generally rectangular prism shape). FIG. 6(A) is a top view of the generally prismatic ceramic setter 6, and a conical upper surface groove 7 is provided on the upper surface, which is one of the bottom surfaces of the cylinder. FIG. 6(B) is a front view of the generally prismatic ceramic setter 6, FIG. 6(C) is a bottom view of the generally prismatic ceramic setter 6, and FIG. 6(D) is a VID-VID cross-sectional view of FIG. 6(A). The upper surface groove 7 of the ceramic setter 6 shown in FIG. 6 is provided on the upper surface of a prism with a bottom side length D, and has a conical shape with a bottom diameter D including a vertex G. The molded body 1 is placed so as to contact the slope of this upper surface groove 7.

[0020] In contrast, Figure 7 is a cross-sectional view of an example of a comparative example in which a compact 1 is placed on a ceramic setter. Reference numeral 8 denotes a flat ceramic setter. The compact 1 contacts the ceramic setter 8 at one point on the spherical surface 2. Therefore, the load of the compact 1 is applied to the contact point, and the direction of the load is perpendicular to the flat plate. Therefore, if the compact 1 is intended for a small, light ceramic ball material, there is no problem. However, if the compact 1 is intended for a large, heavy ceramic ball material, the spherical shape may be deformed by its own weight. Furthermore, there is a possibility that the compact 1 may react with the ceramic setter 8, causing a portion of the compact 1 to peel off, resulting in a defect in the compact 1.

[0021] Moreover, the ceramic setter 6 according to the embodiment is preferably made of the same material as the ceramic ball material.

[0022] When a pressed compact or CIP compact, which is the compact 1, is degreased in a degreasing process to obtain a degreased body, only the organic binder of the compact 1 is removed, and there is almost no change in the outer diameter between the pressed compact (or CIP compact) and the degreased body. In contrast, when a sintered body is obtained by sintering the degreased body in a sintering process, the sintered ceramic ball material shrinks by approximately 15 to 25% from the compact 1. This is because adjacent raw material particles in the ceramic compact 1 gradually bond together, reducing the gaps between the particles. In this way, ceramics become more dense as they are sintered. Densified ceramics shrink as the temperature drops. If a setter made of a material with a different thermal expansion coefficient from the ceramic material is used, the difference in thermal expansion will cause force to be applied at the contact point between the compact 1 and the setter, resulting in deformation of the compact 1.

[0023] For this reason, it is preferable that the setter have the same thermal expansion coefficient as the ceramic ball material. A "same material" refers to a material of the same type as the material to be sintered. Furthermore, "same material" means that the material has the same composition or that 85% or more of the main raw material is the same. For example, if the main raw material of the ceramic ball material is silicon nitride, the ceramic setter 6 has the same composition as the ceramic ball material, or is made of silicon nitride containing 85% or more of silicon nitride by mass and a different type and ratio of auxiliary components. Since the thermal expansion coefficient of ceramics is the same or close depending on the main raw material, if the ceramic setter 6 is made of the same material as the ceramic ball material, deformation due to differences in thermal expansion can be prevented.

[0024] Furthermore, even if the ceramic setter 6 has the same composition as the ceramic ball material, it is also effective to over-sinter the ceramic setter 6 by increasing the sintering temperature or lengthening the sintering time when manufacturing the ceramic setter 6 compared to the sintering conditions for the ceramic ball material. This is because the over-sintered ceramic setter 6 can be used for a long life under the sintering conditions for the normal ceramic ball material.

[0025] Furthermore, when the ceramic setter 6 is a co-material of the ceramic ball material, it is also effective to reduce the amount of sintering aid added to the ceramic setter 6 compact compared to the amount of sintering aid added to the ceramic ball material compact 1. By reducing the amount of sintering aid added to the ceramic setter 6 compact, it is possible to suppress the formation of a glass phase and to suppress reaction with the ceramic ball material. Furthermore, when sintering the ceramic setter 6 compact, it is possible to sinter at a higher temperature than when sintering the ceramic ball material compact 1 in order to suppress the formation of a glass phase, which allows for a longer service life as described above.

[0026] Furthermore, when the thermal expansion coefficient (e.g., linear expansion coefficient) of the ceramic ball material is αb and the linear expansion coefficient of the ceramic setter 6 is αs, it is preferable that the linear expansion coefficient ratio (αs / αb), which is the ratio of the linear expansion coefficient of the ceramic ball material to the linear expansion coefficient of the ceramic setter 6, satisfies the following formula (1): 0.9<αs / αb<1.1 (1)

[0027] If the linear expansion coefficient ratio is 0.9 or less, the shrinkage of the ceramic setter 6 is too small compared to the shrinkage of the ceramic ball material, so that stress opposing the shrinkage is applied at the contact points of the ceramic ball material. On the other hand, if the linear expansion coefficient ratio is 1.1 or more, the shrinkage of the ceramic setter 6 is too large compared to the shrinkage of the ceramic ball material, so that stress opposing the shrinkage is applied at the contact points of the ceramic ball material. These stresses lead to deformation of the green body 1 during sintering, and ultimately to deformation of the ceramic ball material.

[0028] It is more preferable that the linear expansion coefficient ratio satisfies the following formula (2), and it is even more preferable that the linear expansion coefficient ratio satisfies the following formula (3): 0.93<αs / αb<1.07 (2) 0.96<αs / αb<1.03 (3) These linear expansion coefficient ratios are defined by JIS R1618:2002.

[0029] Furthermore, it is preferable that the ceramic setter 6 according to the embodiment has a through hole formed in the bottom surface of the upper surface groove 7, which extends toward the second bottom surface (lower surface), which is the other of the two bottom surfaces of the ceramic setter 6.

[0030] FIG. 8 shows a third example of a ceramic setter 6, which has a generally cylindrical shape. Reference numeral 9 denotes a through-hole. FIG. 8(A) is a top view of the generally cylindrical ceramic setter 6, with an upper surface groove 7 provided on the upper surface. FIG. 8(B) is a front view of the generally cylindrical ceramic setter 6, FIG. 8(C) is a bottom view of the generally cylindrical ceramic setter 6, and FIG. 8(D) is a IIXD-IIXD cross-sectional view of FIG. 8(A). In FIG. 8, a through-hole 9 extending toward the lower surface of the ceramic setter 6 is formed in the bottom surface of the upper surface groove 7 of the ceramic setter 6.

[0031] Gases are generated from the compact 1 during debinding and sintering. These gases are generated by the components that are the bases of the gas components contained in the CIP compact or debound compact. The components that are the bases of the gas components include various substances that gasify when heated, such as the organic binder added during molding, gas components in the pores present between the granulated powder or particles of the compact, and even moisture in the air absorbed by the CIP compact or debound compact.

[0032] As shown in Figure 4, during debinding and sintering, the compact 1 contacts the ceramic setter 6 at a ring-shaped surface. There is a space below the contact surface, and gas generated from the bottom of the compact 1 during debinding and sintering is discharged into this lower space and rises through gaps formed at the contact surface between the compact 1 and the ceramic setter 6, before being released to the outside. In this case, shortening the heating time improves cost performance. Furthermore, if through-holes 9 are provided in the ceramic setter 6, gas can also be released from the underside of the ceramic setter 6, allowing for smooth gas release.

[0033] In the ceramic setter 6 of FIG. 8 , the through-hole 9 imposes a lower limit on the diameter of the compact 1. That is, if a compact 1 with a small diameter is placed on the ceramic setter 6, no contact can be made with the slope of the upper surface groove 7. Furthermore, even if contact with the slope is made in the degreased state, it may not be possible to make contact with the slope in the sintered state after shrinking. Therefore, the through-hole 9 must have a diameter Db that allows it to contact the spherical surface of the sintered ceramic ball material. Furthermore, as shown in FIGS. 9 and 10 (described later), the through-hole 9 may have a height. The height of the through-hole 9 prevents the pole of the spherical surface 2 of the compact 1 having the strip-shaped portion 3 from contacting the slope of the ceramic setter 6.

[0034] Furthermore, the ceramic setter 6 according to the embodiment may have a bottom groove formed on its bottom surface, extending from the bottom of the through hole 9 toward the side of the ceramic setter 6 in order to connect the side of the ceramic setter 6 to the through hole 9 .

[0035] FIG. 9 shows a fourth example of a ceramic setter 6, which has a generally cylindrical shape. FIG. 9(A) is a top view of the generally cylindrical ceramic setter 6, in which an upper surface groove 7 is provided on the upper surface. FIG. 9(B) is a front view of the generally cylindrical ceramic setter 6, in which a lower surface groove 10 is provided extending from the lower side of the through-hole 9 toward the side of the ceramic setter 6. FIG. 9(C) is a bottom view of the generally cylindrical ceramic setter 6, in which the lower surface grooves 10 are formed in four locations across the lower surface of the ceramic setter 6, and are connected to the through-hole 9. In other words, the upper surface grooves 7, the through-hole 9, and the lower surface grooves 10 are connected in that order by cavities. FIG. 9(D) is a cross-sectional view taken along XID-XID of FIG. 9(A).

[0036] The lower surface grooves 10 shown in FIG. 9 are grooves formed to more smoothly discharge gases discharged from the through holes 9 shown in FIG. 8 . Therefore, one to three lower surface grooves 10 may be provided, or five or more may be provided. The shape of the lower surface grooves 10 may be a semi-cylindrical shape as shown in FIG. 9(B) or a rectangular prism shape. The depth of the lower surface grooves 10 is preferably equal to or less than the height of the through holes 9. That is, as shown in FIG. 9(D), when the depth of the lower surface grooves 10 is Hm and the height of the through holes 9 is Ha, Hm≦Ha is satisfied. If the depth Hm of the lower surface grooves 10 is greater than the height Ha of the through holes 9, when the compact 1 is placed on the ceramic setter 6, there will be portions that do not contact the ceramic setter 6, preventing a uniform load from being applied, which may result in deformation of the compact 1 and, ultimately, the ceramic ball material.

[0037] Furthermore, the ceramic setter 6 according to the embodiment may have a flat portion formed on the outer periphery of the opening on the top surface.

[0038] Figure 10 shows a fifth example of the ceramic setter 6, which is a ceramic setter 6 having a roughly cylindrical shape. Figure 10(A) is a top view of the roughly cylindrical ceramic setter 6, and a flat portion 11 is formed on the outer periphery of the opening on the top surface. Figure 10(B) is a front view of the roughly cylindrical ceramic setter 6, and Figure 10(C) is a bottom view of the roughly cylindrical ceramic setter 6. Figure 10(D) is a cross-sectional view of the XD-XD plane of Figure 10(A).

[0039] In Figure 10(D), if the diameter or diagonal length of the bottom surface of the ceramic setter 6 is D and the diameter of the opening of the upper surface groove 7 is Da, in Figures 5, 6, 8, and 9, the diameter D of the ceramic setter 6 and the diameter Da of the opening of the upper surface groove 7 are approximately the same (D = Da). The ceramic setter 6 can accommodate various sizes of molded body 1. As mentioned above, the lower limit of the diameter of the molded body 1 that can be accommodated is determined by the diameter of the through hole 9. In contrast, there is no upper limit to the diameter of the molded body 1 in design, and it is possible to mount the molded body 1 no matter how large its diameter. However, as the molded body 1 becomes larger, the load of the molded body 1 on the ceramic setter 6 increases, making the upper surface edge of the ceramic setter 6 more likely to deform. For this reason, it is preferable to form a flat portion 11 on the ceramic setter 6 to prevent deformation of the ceramic setter 6.

[0040] As described above, the ceramic setter 6 is ideal for degreasing and sintering the compact 1. In this case, the compact 1, i.e., the ceramic ball material, is made of one of silicon nitride sintered body, aluminum oxide sintered body, and zirconium oxide sintered body.

[0041] In this case, the molded body 1, i.e., the ceramic ball material, preferably contains one or more of silicon nitride, aluminum oxide, and zirconium oxide as the main component (50 mass% or more). Furthermore, the molded body 1, i.e., the ceramic ball material, more preferably contains 85 mass% or more of one or more of silicon nitride, aluminum oxide, and zirconium oxide. The ball-shaped sintered body is used as a ceramic ball, for example, a ceramic bearing ball. The above-mentioned materials are used for ceramic bearing balls. In particular, silicon nitride sintered bodies have excellent wear resistance and are effective as ceramic bearing balls.

[0042] Furthermore, the fact that the molded body 1, i.e., the ceramic ball material, contains 85 mass% or more of one or more of silicon nitride, aluminum oxide, and zirconium oxide means that in addition to these main components, it may also contain 15 mass% or less of a sintering aid.

[0043] For example, the aluminum oxide sintered body or the zirconium oxide sintered body has a Vickers hardness of about 1200 or more and 1700 or less. On the other hand, the toughness value is 3 MPa m １／２ 6MPa・m or more １／２ In contrast, silicon nitride sintered bodies have a high Vickers hardness of 1400 or more and 1800 or less. Also, the toughness value is 5 MPa m １／２ 10MPa・m or more １／２ The toughness is high, at about the same level as above. Silicon nitride sintered bodies have both high toughness and Vickers hardness, which gives them excellent wear resistance. This is because silicon nitride sintered bodies have a structure that is mainly composed of β-type silicon nitride crystal grains. β-type silicon nitride crystal grains have a long, thin shape, and these long, thin crystal grains are intricately intertwined, which is why they achieve high toughness.

[0044] The outer diameter of the ceramic ball material is 15 mm or more. As described above, when the ceramic ball material is a silicon nitride sintered body, an aluminum oxide sintered body, or a zirconium oxide sintered body, the weight of a compact, degreased body, or sintered body with an outer diameter of less than 15 mm is light, and deformation is unlikely to occur even when placed on a flat ceramic setter 8 (shown in FIG. 7). Therefore, the outer diameter of the ceramic ball material is more preferably 20 mm or more, and even more preferably 25 mm or more.

[0045] The ceramic ball material obtained as described above is suitable for ceramic balls, such as ceramic bearing balls, which can be obtained by polishing. In other words, polishing is required to turn a ball-shaped sintered body into a ceramic bearing ball. Furthermore, a spherical shape obtained by polishing the ceramic ball material is called a ceramic bearing ball.

[0046] Furthermore, the ceramic ball obtained by polishing the ceramic ball material obtained as described above has an arithmetic surface roughness Ra of 0.01 μm or less.

[0047] Ceramic balls can be manufactured by polishing ceramic ball materials. A typical polishing method for obtaining ceramic balls is surface plate processing. For example, the ceramic ball material is inserted between parallel surface plates. The movement of the polishing platen allows the ceramic ball material to be processed into a perfect sphere. The surface roughness of ceramic balls used in bearing balls is specified in ASTM F2094. Depending on the application, ceramic balls used in ceramic bearing balls are graded according to ASTM F2094, ISO 26602, or JIS R1669. They are polished to an arithmetic surface roughness Ra according to that grade. As the grade increases, a mirror finish is applied to an arithmetic surface roughness Ra of 0.01 μm or less.

[0048] Next, the ceramic setter 6, the ceramic ball material, and the method for manufacturing the ceramic balls will be described. The ceramic setter 6, the ceramic ball material, and the method for manufacturing the ceramic balls may be configured as described above, but a method for reducing the defective rate of the ceramic ball material, that is, for improving the yield, will be described below.

[0049] A method for manufacturing the ceramic setter 6 will be described. A method for manufacturing the ceramic setter 6 will be described when silicon nitride is used for the molded body 1, i.e., the material for ceramic balls. When the material for ceramic balls contains at least one of aluminum oxide and zirconium oxide as the main component (50 mass % or more), the term "silicon nitride" is substituted and the respective manufacturing conditions are applied.

[0050] First, appropriate amounts of sintering aid powder, additives, solvent, binder, etc. are added to the raw material silicon nitride powder, mixed, crushed, and granulated using a spray dryer. This process produces a granulated powder from the raw material powder. Furthermore, when the total of the silicon nitride powder and sintering aid powder is 100 mass%, the silicon nitride powder is preferably 85 mass% or more. The additive is a plasticizer, etc. The solvent is water or an organic solvent. Examples of organic solvents include alcohol, ketone, and benzene. The binder is an organic substance. The amount of binder added is preferably 3 mass parts to 20 mass parts, when the total of the silicon nitride powder and sintering aid powder is 100 mass parts. Adjusting the amount of binder can adjust the shape retention and density uniformity of the compact during uniaxial pressing and CIP. Furthermore, granulation allows for uniform mixing of the silicon nitride powder and sintering aid powder.

[0051] Next, the granulated powder is subjected to a CIP process. A rubber mold is used for the CIP process. For example, when forming a cylindrical ceramic setter, the granulated powder is filled into a rubber mold with a cylindrical interior. The rubber mold filled with the granulated powder is then subjected to the CIP process. The CIP process can be performed using either a WET-CIP device or a DRY-CIP device. When isotropic pressure is applied to the granulated powder during the CIP process, the granulated powder is crushed, thereby suppressing density variations. A cylindrical molded body, that is, a CIP body, can be obtained through the CIP process.

[0052] Next, the CIP body is subjected to green machining. Green machining can be performed using a general-purpose lathe, NC lathe, machining center, or the like, and cutting can be performed using high-speed steel or carbide cutting tools. By cutting, compacts having the shapes shown in Figures 5, 6, and 8 to 10 are obtained.

[0053] Next, a degreasing step is performed to degrease the compact. The degreasing step is a step in which the compact is heated to a temperature equal to or higher than the decomposition temperature of organic components such as binders to remove the organic components. The degreasing step may be performed in a nitrogen atmosphere or an air atmosphere. A degreased body can be obtained by the degreasing step.

[0054] Next, a sintering process is carried out to sinter the degreased body. The sintering process is preferably carried out at a temperature of 1700°C or higher and 2000°C or lower. The sintering process is preferably carried out in a nitrogen atmosphere. The sintering pressure is preferably in the range of atmospheric pressure or higher and 300 MPa or lower. The atmospheric pressure is 0.10133 MPa (=1 atm). By carrying out the sintering process, a silicon nitride ceramic setter 6 can be obtained.

[0055] In addition to the manufacturing method of cutting and sintering a green material as described above, it is also possible to form it into the shape of the ceramic setter 6 by post-processing. It is also possible to form it into the shape of the ceramic setter 6 using a 3D printer. However, since the former method is difficult to process, such as grinding, because the silicon nitride sintered body is hard, and the latter method is less suitable for mass production, it is preferable to form it by green processing.

[0056] Next, a method for manufacturing a ceramic ball material will be described. First, a method for preparing a molded body 1 for obtaining a ceramic ball material will be described using silicon nitride. When the ceramic ball material contains at least one of aluminum oxide and zirconium oxide as the main component (50 mass% or more), the term "silicon nitride" is substituted, and the respective manufacturing conditions apply. Furthermore, in the examples of the present invention, uniaxial pressure molding is used as a method for obtaining a molded body, but the molding method is not limited to this. For example, a rolling granulation method may also be used as a molding method. Another molding method is to directly mold a sphere using CIP.

[0057] First, appropriate amounts of sintering aid powder, additives, solvent, binder, etc. are added to the raw material silicon nitride powder, mixed, crushed, and granulated using a spray dryer. This process produces a granulated powder from the raw material powder. Furthermore, when the total of the silicon nitride powder and sintering aid powder is 100 mass%, the silicon nitride powder is preferably 85 mass% or more. The additive is a plasticizer, etc. The solvent is water or an organic solvent. Examples of organic solvents include alcohol, ketone, and benzene. The binder is an organic substance. The amount of binder added is preferably 3 mass parts to 20 mass parts, when the total of the silicon nitride powder and sintering aid powder is 100 mass parts. Adjusting the amount of binder can adjust the shape retention and density uniformity of the compact during uniaxial pressing and CIP. Furthermore, granulation allows for uniform mixing of the silicon nitride powder and sintering aid powder.

[0058] Next, uniaxial pressure molding is performed using the granulated powder. Examples of uniaxial pressure molding include a die molding method using an upper punch and a lower punch as shown in Figure 2. The shape of the press-molded body, which is the compact 1, can be adjusted depending on the shape of the die. A spherical press-molded body can be obtained by making the insides of the upper punch and the lower punch hemispherical. The compact 1 obtained by uniaxial pressure molding has a spherical shape having a spherical portion 2 and a band-like portion 3 as shown in Figure 1.

[0059] Next, a step of CIP processing is performed on the press-molded body, which is the molded body 1. A rubber mold is used for the CIP processing. The molded body 1 is filled into the hole in the rubber mold. Because the molded body is formed using granulated powder, when isostatic pressure is applied to the molded body 1 by the CIP processing, the granulated powder is crushed, making it possible to suppress density variations. By using granulated powder to form the molded body 1, the silicon nitride powder and sintering aid powder are uniformly dispersed, and density variations can be suppressed.

[0060] The pressure of the CIP molding is preferably higher than the pressing pressure of the uniaxial press molding. The pressure of the CIP treatment is preferably in the range of 30 MPa to 300 MPa. When the pressure is in this range, the density variation of the CIP body, which is the compact 1 after the CIP treatment, can be reduced.

[0061] Next, the CIP body is placed on a ceramic setter 6 and subjected to a degreasing process in which the CIP body is degreased. The degreasing process involves heating the CIP body at a temperature equal to or higher than the decomposition temperature of organic components such as binders to remove the organic components. In the degreasing process, the compact 1 is placed on the ceramic setter 6. At this time, in the case of the compact 1 having a band-shaped portion 3 formed thereon, the spherical portion 2 is placed in contact with the slope of the upper surface groove 7 of the ceramic setter 6. The degreasing process may be performed in a nitrogen atmosphere or an air atmosphere. A degreased body can be obtained by the degreasing process.

[0062] Next, a sintering process is performed to sinter the degreased body. The degreased body placed on the ceramic setter 6 is placed in a sintering furnace and heated to perform the sintering process. The sintering process is preferably performed at a temperature of 1700°C to 2000°C. The sintering process is preferably performed in a nitrogen atmosphere. The sintering pressure is preferably within a range of atmospheric pressure to 300 MPa. The atmospheric pressure is 0.10133 MPa (=1 atm). After the sintering process, the degreased body is removed from the ceramic setter to obtain a ceramic ball material (sintered body). The sintered body obtained by the sintering process may also be subjected to HIP (hot isostatic pressing). This process allows the production of a ceramic ball material. The ceramic ball material is a sintered ceramic body with a theoretical density of 98% or more.

[0063] Ceramic balls can be manufactured by polishing ceramic ball materials. A typical polishing method for obtaining ceramic balls is surface plate processing. For example, the ceramic ball material is inserted between parallel surface plates. The movement of the polishing platen can be used to process the ceramic ball material into a perfect sphere. The surface roughness of bearing balls is specified in ASTM F2094. Depending on the application, bearing balls are graded according to ASTM F2094, ISO 26602, or JIS R1669. They are polished to an arithmetic surface roughness Ra according to that grade. Higher grades can be mirror-finished to an arithmetic surface roughness Ra of 0.01 μm or less.

[0064] (Examples 1 to 5, Comparative Examples 1 to 5) The molded body 1, i.e., ceramic powder used as the raw material for the ceramic ball material, was mixed with sintering aids, additives, solvents, binders, etc., crushed, and granulated using a spray dryer. As shown in Table 1, the molded body 1 sintered using the ceramic setter 6 in Examples 1 to 3 and the molded body 1 sintered using the ceramic setter 8 in Comparative Examples 1 to 3 were silicon nitride molded bodies containing 85% by mass or more of silicon nitride. The molded body 1 sintered using the ceramic setter 6 in Example 4 and the molded body 1 sintered using the ceramic setter 8 in Comparative Example 4 were aluminum oxide (alumina) molded bodies containing 85% by mass or more of aluminum oxide. The molded body 1 sintered using the ceramic setter 6 in Example 5 and the molded body 1 sintered using the ceramic setter 8 in Comparative Example 5 were zirconium oxide (zirconia) molded bodies containing 85% by mass or more of zirconium oxide. In the compact 1, when the total amount of the main component and the sintering aid was 100 parts by mass, the amount of the binder added was 3 to 20 parts by mass. The materials of the ceramic setter 6 of the example and the ceramic setter 8 of the comparative example were the same as the material for the ceramic balls, respectively.

[0065] Next, a ceramic setter 6 was fabricated. In the examples, a ceramic setter 6 (shown in FIG. 5 or FIG. 10) was used, in which a conical upper surface groove 7 was formed on the upper surface of a roughly cylindrical shape. In the comparative examples, a flat ceramic setter 8 (shown in FIG. 7) was used. The conical shape of the upper surface groove 7 of the ceramic setter 6 used in the examples was obtained by placing granulated powder in a cylindrical rubber mold and performing CIP molding. The obtained CIP body was then green-processed to obtain a green processed body. After degreasing the green processed body, the silicon nitride was sintered at 1820°C, and the aluminum oxide and zirconium oxide were sintered at 1520°C, to obtain the ceramic setters 6 and 8.

[0066] Two types of ceramic setters 6 were fabricated: small and large. The silicon nitride setter 6 and the aluminum oxide and zirconium oxide setter 6 were fabricated in large sizes. The small ceramic setter 6 had the shape shown in Figure 5. Specifically, it was cylindrical with outer dimensions of 30 mm diameter x 9 mm height, and had a conical upper surface groove 7 with a depth of 6 mm on the upper surface. The large ceramic setter 6 had the shape shown in Figure 10. Specifically, it was cylindrical with outer dimensions of 50 mm diameter x 10 mm height, and had a conical upper surface groove 7 extending from the upper surface at a diameter of 40 mm, and a flat portion 11 with a width of 5 mm on the upper surface. Furthermore, a through hole 9 with a diameter of 20 mm x height 5 mm was formed on the lower surface, directly connecting to the upper surface groove 7. Furthermore, four lower surface grooves 10 with a semicircular cross section and a radius of 3 mm were formed on the lower surface. One hundred ceramic setters 6 with the shapes shown in Table 1 were fabricated for each example.

[0067] In contrast, the flat-plate-shaped ceramic setters 8 used in the comparative examples were prepared by press-molding granulated powder into a flat plate shape to obtain press-molded bodies. The obtained press-molded bodies were degreased and then sintered under the above-mentioned sintering conditions to obtain flat-plate-shaped ceramic setters 8 measuring 100 mm x 100 mm x 10 mm. 100 ceramic setters 8 having the shapes shown in Table 1 were produced for each comparative example.

[0068] Next, a green body 1 was produced to obtain a ceramic ball material. The green body 1 was produced by press molding using the above-mentioned granulated powder. The press molding was performed by die molding using upper and lower punches in a press die 4 shown in FIG. 2. The press molding using upper and lower punches was uniaxial pressure molding. The die was used to produce spherical green bodies. In this way, 100 green bodies 1 having the diameters shown in Table 1 were produced for each example, and 100 for each comparative example.

[0069] The CIP bodies, which are molded bodies obtained by placing the molded body 1 in a rubber mold and performing CIP processing, were placed on ceramic setters 6, 8. 100 molded bodies 1 were placed on each of 100 small ceramic setters 6 according to Example 1 so that the spherical portion 2 was in contact with the slope of the upper surface groove 7 of the ceramic setter 6. Similarly, in Examples 2 to 5, 100 molded bodies 1 were placed on each of 100 large ceramic setters 6 according to each Example so that the spherical portion 2 was in contact with the slope of the upper surface groove 7 of the ceramic setter 6.

[0070] In contrast, in Comparative Examples 1 to 5, 100 compacts 1 were placed on 100 flat ceramic setters 8 according to each Comparative Example. In Table 1, the small ceramic setter 6 (shown in FIG. 5) is indicated as "small conical groove," and the large ceramic setter 6 (shown in FIG. 10) is indicated as "large conical groove."

[0071] The CIP bodies placed on the ceramic setters 6 and 8 were degreased. Then, while the degreased bodies were still placed on the ceramic setters 6 and 8, the silicon nitride bodies were sintered at 1800°C, and the aluminum oxide and zirconium oxide bodies were sintered at 1500°C. The spherical surface of the sintered bodies was measured, and if any one of appearance defects (e.g., dent defects), deformation defects, or peeling defects was found by visual inspection of the surface, the body was deemed to be "defective" and the "defect rate" was calculated.

[0072] Deformation defects mean that, when the minor axis length of the sintered body is L1 and the major axis length is L0, the ratio of the minor axis length L1 to the major axis length L0, L1 / L0, satisfies the following formula (4): L1 / L0≦0.99 (4) For example, in each example (and the same for comparative examples), 100 CIP bodies of the same size before degreasing are prepared, and deformation defects can be inspected using a perforated sorting plate or roller sorter that drops sintered bodies with depressions due to deformation. Note that a sintered body without a band-like portion can be obtained by sintering a compact 1 without a band-like portion 3, or by sintering a compact 1 with a band-like portion 3 and then polishing and removing the band-like portion.

[0073] Peeling defects refer to peeling with a diameter of approximately 0.5 mm or more. The defect rates are shown in Table 1. The defect rates for each example and comparative example are the ratio of defective ceramic ball materials to 100 ceramic ball materials (ball-shaped sintered bodies).

[0074]

[0075] In the examples, the defect rate was less than 3%, which was within a desirable range. This was because the ceramic setter 6 used dispersed the stress on the compact 1. In contrast, in the comparative examples, the defect rate was 3% or more, which was poor. This was because the use of a flat ceramic setter 8 caused stress to concentrate at one point on the spherical surface 2, resulting in deformation of the compact 1 due to its own weight and the application of pressure, which caused a reaction between the compact 1 and the ceramic setter 8.

[0076] Next, after excluding the defective ceramic ball materials, the ceramic ball materials made of silicon nitride were subjected to HIP treatment at a temperature of 1700° C. to 1900° C. in a nitrogen atmosphere under a pressure of 50 MPa to 200 MPa, and the ceramic ball materials made of aluminum oxide and zirconium oxide were subjected to HIP treatment at a temperature of 1300° C. to 1500° C. in an argon atmosphere under a pressure of 50 MPa to 200 MPa.

[0077] The ceramic setter 6 according to Example 1 corresponds to a ceramic ball material for ceramic balls that will be 17 / 32 inches (13.49 mm) after polishing. The ceramic setters 6 according to Examples 2, 4, and 5 correspond to a ceramic ball material for 1-inch (25.4 mm) ceramic balls. The ceramic setter 6 according to Example 3 corresponds to a ceramic ball material for 1¾ inch (44.45 mm) ceramic balls. All of the ceramic ball materials according to the Examples can be used as ceramic balls, for example, bearing balls.

[0078] In the examples, the defective rate of the ceramic ball material was reduced by using a ceramic setter 6 having upper surface grooves 7 for both degreasing and sintering the compact 1 through the heat treatment process. On the other hand, even when a ceramic setter 6 having upper surface grooves 7 was used for only degreasing the compact 1 or when a ceramic setter 6 having upper surface grooves 7 was used for only sintering the compact 1, the defective rate was lower than when a ceramic setter 8 was used (especially when the diameter of the compact 1 was large). In addition, since the ceramic setters 6 shown in Figures 6, 8, and 9 also have upper surface grooves 7, it is believed that the same results as those of the ceramic setters 6 shown in Figures 5 and 10 can be obtained.

[0079] According to the embodiment described above, when the molded body 1 is subjected to at least one of debinding and sintering, the weight of the molded body 1 is prevented from concentrating on one point on the ceramic setter, thereby maintaining a high yield of the material for ceramic balls.

[0080] Although several embodiments of the present invention have been described above, these embodiments are presented by way of example only and are not intended to limit the scope of the invention. These novel embodiments can be embodied in various other forms, and various omissions, substitutions, modifications, etc. can be made without departing from the spirit of the invention. Modifications of these embodiments are included within the scope and spirit of the invention, as well as within the scope of the invention and its equivalents as set forth in the claims. Furthermore, the above-described embodiments can be implemented in combination with each other.

Claims

1. A ceramic setter for placing a ball-shaped ceramic molded body when performing at least one of degreasing and sintering on the said ceramic molded body, A first base surface, which is one of the two base surfaces of a cylinder or prism, has a first base groove whose surface parallel to the base surface becomes smaller with increasing depth. The first bottom groove is a non-penetrating groove having a frustoconical shape. A ceramic setter characterized in that a through hole is formed in the bottom surface of the first bottom groove, extending toward the second bottom surface, which is the other of the two bottom surfaces.

2. The ceramic setter according to claim 1, characterized in that the material is the same material as the ceramic ball material obtained by sintering the ceramic molded body.

3. The ceramic setter according to claim 1 or 2, characterized in that a second bottom groove is formed on the second bottom surface, which is the other of the two bottom surfaces, extending from the through hole toward the side surface of the ceramic setter.

4. The ceramic setter according to claim 1 or 2, characterized in that a flat portion is formed on the outer circumference of the opening of the first bottom surface.

5. A degreasing step of degreasing the ceramic molded body using the ceramic setter described in claim 1 or claim 2, A sintering step is performed to obtain a material for ceramic balls by sintering the degreased body obtained in the degreasing step, It has, A method for manufacturing a ceramic ball material, characterized in that the ceramic ball material is one of the following: a silicon nitride sintered body, an aluminum oxide sintered body, or a zirconium oxide sintered body.

6. The method for manufacturing a ceramic ball material according to claim 5, characterized in that the outer diameter of the ceramic ball material is 15 mm or more.

7. A method for manufacturing ceramic balls, characterized by comprising a polishing step to obtain ceramic balls by polishing the ceramic ball material obtained by the manufacturing method described in claim 5.

8. A method for manufacturing ceramic balls, characterized by comprising a polishing step to obtain ceramic balls by polishing the ceramic ball material obtained by the manufacturing method described in claim 6.

9. The method for producing ceramic balls according to claim 7, characterized in that the arithmetic surface roughness Ra of the ceramic ball is 0.01 μm or less.

10. The method for producing ceramic balls according to claim 8, characterized in that the arithmetic surface roughness Ra of the ceramic ball is 0.01 μm or less.