Material for ceramic balls, processing apparatus for ceramic molded bodies, and processing method for ceramic molded bodies

Abstract

To provide a ceramic ball material that offers excellent polishing efficiency and durability. [Solution] The ceramic ball material according to the embodiment is characterized by having a sphericity of 2% or less and an arithmetic mean roughness Ra of 0.2 μm or more and 2 μm or less. Furthermore, it is preferable that the maximum cross-sectional height Rt of the ceramic ball material is 4 μm or more and 20 μm or less. Furthermore, it is preferable that when the surface roughness Ra in the circumferential direction of the band-shaped mark is Ra1 and the surface roughness Ra in the circumferential direction perpendicular to the band-shaped mark is Ra2, the ratio Ra1 / Ra2 of the ceramic ball material is 0.2 or more and 2 or less.

Description

Technical Field

[0001] The embodiments described below relate to a material for ceramic balls, a processing apparatus for a ceramic molded body, and a processing method for a ceramic molded body.

Background Art

[0002] Ceramic balls are used in various bearings. The ceramic balls used in bearings are called bearing balls. Examples of bearing balls include ceramic sintered bodies such as silicon nitride sintered bodies, aluminum oxide sintered bodies, zirconium oxide sintered bodies, and aldyl sintered bodies. Aldyl is a mixture of aluminum oxide and zirconium oxide. The grade of the surface roughness and material properties of bearing balls is defined in ASTM (American Society for Testing and Materials)_F2094. A grade corresponding to the application field of the bearing is required. A true spherical ceramic ball is required for bearing balls. For example, in ASTM_F2094, the surface roughness of bearing balls is set to Ra0.013 μm or less.

[0003] To obtain a true spherical ceramic ball, the material for the ceramic ball is polished. The material for the ceramic ball refers to a ceramic sintered body before being polished into a ceramic ball. The material for the ceramic ball is sometimes also called a raw ball. For example, Patent No. 4761613 (Patent Document 1) and Patent No. 3853197 (Patent Document 2) disclose a material for a ceramic ball having a belt-shaped portion. Since the molded body of the material for the ceramic ball is manufactured by die pressing, a belt-shaped portion is always formed. In Patent Document 1 and Patent Document 2, the polishing efficiency of the material for the ceramic ball is improved by devising the shape such as the belt-shaped portion. Although the polishing efficiency is improved in the materials for the ceramic balls of Patent Document 1 and Patent Document 2, no further improvement is seen because the belt-shaped portion exists.

[0004] On the other hand, Japanese Patent Publication No. 2000-185976 (Patent Document 3) discloses a method for producing ceramic ball material by rolling granulation. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Patent No. 4761613 [Patent Document 2] Patent No. 3853197 [Patent Document 3] Japanese Patent Publication No. 2000-185976 [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] As described in Patent Document 3, the ceramic ball material obtained by sintering a ceramic molded body obtained by rolling granulation does not have a strip-like portion formed on it. Because the ceramic ball material does not have a strip-like portion, the polishing efficiency of the ceramic ball material is improved. However, rolling granulation has a weak press pressure, so the molded body is not dense enough. When a molded body that is not dense enough is sintered, the density of the sintered ceramic ball material also decreases.

[0007] Patent document 3 states that this method is effective for bearing balls with a diameter of 4.5 mm or less. Bearing balls with a diameter of 4.5 mm or less are used in hard disk drives (HDDs), etc. The load on the bearing balls in HDD motors is small. In fields with heavy loads, bearing balls produced by rolling granulation have lacked durability.

[0008] The embodiment provides a material for ceramic balls that has good polishing efficiency and excellent durability. [Means for solving the problem]

[0009] The ceramic ball material according to the embodiment is characterized by having a sphericity of 2% or less and an arithmetic mean roughness Ra of 0.2 μm or more and 2 μm or less. The sphericity of the ceramic ball material can be determined as a single roundness ((maximum diameter - minimum diameter) / average diameter) based on a single projection image when projected onto a plane from one direction, or as the average value of multiple roundnesses based on multiple projection images when projected onto a plane from each of multiple directions. For example, the roundness of a projection image can be determined from eight equally spaced diameters centered on the centroid of the projection image. In other words, the ceramic ball material according to the embodiment is characterized by having a roundness of 2% or less, determined from eight equally spaced diameters centered on the centroid of a projection image when projected onto a plane from any direction, and an arithmetic mean roughness Ra of 0.2 μm or more and 2 μm or less. [Brief explanation of the drawing]

[0010] [Figure 1] An external view showing an example of a material for ceramic balls with a strip-like portion. [Figure 2] An external view showing an example of a ceramic ball material in which the strip-shaped portion has been sufficiently removed, according to the embodiment. [Figure 3] A diagram illustrating a roughly circular projection image projected onto a plane, and eight equally spaced diameters centered around the centroid of the projection image. [Figure 4] A diagram showing an example of a processing apparatus for ceramic molded bodies for manufacturing ceramic ball material according to the embodiment. [Figure 5] A conceptual diagram illustrating an example of the displacement amount of the outer surface of a ceramic molded body used to manufacture a material for ceramic balls according to the embodiment. [Figure 6] An external view showing an example of a spherical ceramic molded body for manufacturing a material for ceramic balls according to the embodiment. [Modes for carrying out the invention]

[0011] The ceramic ball material according to this embodiment is characterized by having a sphericity of 2% or less and an arithmetic mean roughness Ra of 0.2 μm or more and 2 μm or less. The ceramic ball material according to this embodiment refers to the ceramic sintered body before it is polished to form a ceramic ball. The ceramic ball material is sometimes also called a primary ball.

[0012] Figure 1 shows an example of a conventional ceramic ball material having a strip-shaped portion. In Figure 1, reference numeral 1A denotes a ceramic ball material obtained by sintering a ceramic molded body having a strip-shaped portion, reference numeral 2A denotes the spherical portion of the ceramic ball material 1A, and reference numeral 3A denotes the strip-shaped portion of the ceramic ball material 1A. Figure 2 also shows an example of a ceramic ball material according to an embodiment. In Figure 2, reference numeral 1 denotes a ceramic ball material obtained by sintering a ceramic molded body 13 (shown in Figure 6) from which the strip-shaped portion has been sufficiently removed, according to the embodiment.

[0013] Conventional ceramic ball material 1A has a spherical portion 2A and a strip-shaped portion 3A formed around the circumference of the spherical portion 2A, as shown in Figure 1. In contrast, the ceramic ball material 1 according to this embodiment has a spherical portion 2, as shown in Figure 2, but does not have the strip-shaped portion 3A as shown in Figure 1.

[0014] The ceramic ball material 1 has a sphericity of 2% or less. For example, the roundness of ceramic ball material 1, which can be determined from eight equally spaced diameters centered on the centroid of a single projected image when projected onto a plane from one direction, is 2% or less. Figure 3 shows an example of a roughly circular projected image 1E projected onto a plane and eight equally spaced diameters r1 to r8 centered on the centroid of the projected image. In Figure 3, the symbol 1E is the projected image of ceramic ball material 1, and the symbols r1 to r8 are the eight diameters.

[0015] Shape measurement using the projection image 1E shall be performed using an optical three-dimensional shape measurement device. The three-dimensional shape measurement device shall use the VR-5000 manufactured by KEYENCE and shall be performed using the analysis software of the device. The measurement device may be any device having equivalent functions.

[0016] First, for the ceramic ball material 1, eight diameters r1 to r8 evenly spaced are measured in the projection image 1E when projected onto a plane from an arbitrary direction. In FIG. 3, the diameters are measured at the points where the circumference of the projection image 1E is equally divided into 16 parts. The diameters r1 to r8 are each taken as the measured diameters. The maximum value of the measured diameters r1 to r8 is rmax, the minimum value is rmin, and the average value is taken as the average diameter. The circularity of the projection image 1E of the ceramic ball material 1 is obtained from the following formula. Circularity (%) = (|rmax - rmin| / average diameter) × 100

[0017] Thus, it can be seen that the smaller the circularity value, the closer it is to a perfect circle. Also, "high circularity" and "good circularity" indicate that the numerical value of the circularity is small. Conversely, "low circularity" and "bad circularity" indicate that the numerical value of the circularity is large. The sphericity of the ceramic ball material 1 is obtained as one circularity based on one projection image 1E when projected onto a plane from one direction, or as the average value of a plurality of circularities based on a plurality of projection images 1E when projected onto a plane from each of a plurality of directions.

[0018] Further, the material 1 for ceramic balls is characterized in that the sphericity (that is, one roundness obtained from one projection image 1E or the average value of a plurality of roundnesses obtained from a plurality of projection images 1E) is 2% or less. That the sphericity of the material 1 for ceramic balls is 2% or less means that the material 1 for ceramic balls has a shape approximated to a true sphere (the projection image is a perfect circle). Since the projection image 1E of the material 1 for ceramic balls has a shape approximated to a perfect circle, it does not substantially have an elliptical shape. Also, the material 1 for ceramic balls does not substantially have a band-shaped portion (band-shaped portion 3A shown in FIG. 1). By setting the sphericity of the material 1 for ceramic balls to 2% or less, the polishing cost can be reduced. Therefore, the sphericity of the material 1 for ceramic balls is preferably 2% or less, more preferably in the range of 0% or more and 1.5% or less.

[0019] Also, the material 1 for ceramic balls has an arithmetic mean roughness Ra of 0.2 μm or more and 2 μm or less. The arithmetic mean roughness Ra is measured in accordance with JIS-B-0601 (2013). Also, the measurement of the surface roughness is performed with a measurement length of 0.8 mm or more. JIS-B-0601 corresponds to ISO4287. The surface roughness measuring machine shall use SURFCOM2000 manufactured by Tokyo Seimitsu Co., Ltd. and shall be performed using the evaluation analysis software of the device. The measuring device may have the same functions as this.

[0020] The ceramic ball material 1 has an arithmetic mean roughness Ra of 0.2 μm to 2 μm, regardless of where it is measured on the surface. To reduce the arithmetic mean roughness Ra to less than 0.2 μm, a polishing process is required for ceramic ball material 1. Furthermore, if the arithmetic mean roughness Ra exceeds 2 μm, the surface irregularities are large, which may cause damage during polishing. Therefore, the arithmetic surface roughness Ra of ceramic ball material 1 is preferably within the range of 0.2 μm to 2 μm, and more preferably between 0.5 μm and 1.0 μm. As mentioned above, ASTM F2094 specifies the arithmetic surface roughness Ra according to the grade of the bearing ball. The largest Ra is 0.013 μm. Ceramic ball material 1 has an arithmetic surface roughness Ra of 0.2 to 2 μm. Therefore, it can be distinguished from polished ceramic balls that can be used as bearing balls.

[0021] Furthermore, the surface of the ceramic ball material 1 is preferably the sintered surface, the sintered surface that has been honed, or the sintered surface that has been cleaned. The sintered surface refers to the surface of the ceramic sintered body after the sintering process. Honing and cleaning have the effect of removing dirt from the surface of the sintered surface. For this reason, honing and cleaning are distinguished from polishing.

[0022] As will be described later, removing the strip-shaped portion 11 (shown in Figure 4) of the ceramic molded body 9 before sintering is effective in controlling the sphericity of the ceramic ball material 1. By controlling the removal of the strip-shaped portion 11 of the ceramic molded body 9, the arithmetic surface roughness Ra of the sintered surface (including the sintered surface that has undergone honing or cleaning treatment) can be controlled. For example, in rolling granulation as in Patent Document 3, no press pressure is applied, so the arithmetic surface roughness Ra becomes large.

[0023] Furthermore, the surface of the ceramic ball material 1 preferably has a maximum cross-sectional height Rt of 4 μm or more and 20 μm or less. The maximum cross-sectional height Rt shall be measured in the same manner as the arithmetic mean roughness Ra described above. To make the maximum cross-sectional height Rt of the ceramic ball material 1 less than 4 μm, polishing of the ceramic ball material 1 is necessary. Also, if the maximum cross-sectional height Rt is larger than 20 μm, the burden on the ceramic ball material 1 during polishing for manufacturing ceramic balls (e.g., bearing balls) may increase. For this reason, the maximum cross-sectional height Rt is preferably within the range of 4 μm or more and 20 μm or less, and more preferably within the range of 5 μm or more and 17 μm or less.

[0024] Furthermore, when Ra1 is the arithmetic surface roughness Ra in the circumferential direction of the band-shaped mark and Ra2 is the surface roughness Ra of the circumference perpendicular to the band-shaped mark, it is preferable that Ra1 / Ra2 is between 0.2 and 2. It is also preferable that the band-shaped mark is formed by removing the band-shaped portion 11 (shown in Figure 4) from the ceramic molded body 9 and then sintering it. The band-shaped mark can be identified because it appears as a streak after the band-shaped portion has been removed. In addition, the band-shaped mark can usually be confirmed visually. If the band-shaped mark is difficult to identify, the arithmetic surface roughness Ra of the circumference in an arbitrary circumferential direction and its perpendicular direction may be measured to determine Ra1 / Ra2.

[0025] As mentioned above, the arithmetic surface roughness Ra of material 1 for ceramic balls is between 0.2 μm and 2 μm. Furthermore, the fact that Ra1 / Ra2 is between 0.2 and 2 indicates that the arithmetic surface roughness Ra is non-directional. This suppresses variations in the amount of material removed during polishing. As a result, the amount of material to be polished can be reduced.

[0026] Furthermore, when Rt1 is the maximum cross-sectional height Rt in the circumferential direction of the band-shaped mark on the ceramic ball material 1, and Rt2 is the maximum cross-sectional height Rt of the circumference perpendicular to the band-shaped mark, it is preferable that Rt1 / Rt2 is between 0.2 and 2. If the band-shaped mark is difficult to distinguish, Rt1 / Rt2 may be determined by measuring the maximum cross-sectional height Rt of the circumference in any circumferential direction and the direction perpendicular to it. The fact that Rt1 / Rt2 is between 0.2 and 2 indicates that Rt has no directionality. This makes it possible to suppress variations in the amount of material removed during the polishing process of the ceramic ball material 1. As a result, the polishing allowance can be reduced. Furthermore, the arithmetic surface roughness Ra, maximum cross-sectional height Rt, Ra1 / Ra2, and Rt1 / Rt2 can be used in combination. A synergistic effect can also be obtained by satisfying all of these conditions.

[0027] Furthermore, it is preferable that the ceramic ball material 1 is one selected from silicon nitride sintered body, aluminum oxide sintered body, zirconium oxide sintered body, and argyl sintered body. It is even more preferable that the ceramic ball material 1 is a silicon nitride-based sintered body.

[0028] Silicon nitride sintered bodies have a three-point bending strength of 850 MPa or more or a fracture toughness of 6 MPa·m. 1 / 2 The above is preferable. Three-point bending strength is measured in accordance with JIS-R-1601 (2008). Fracture toughness is measured in accordance with the IF method of JIS-R-1607 (2015) and calculated using Niihara's formula. JIS-R-1601 corresponds to ISO 14704, and JIS-R-1607 corresponds to ISO 15732.

[0029] The aluminum oxide sintered body has a three-point bending strength of 450-550 MPa and a fracture toughness of 4 MPa·m. 1 / 2 It is to that extent. Furthermore, zirconium oxide sintered bodies and algil sintered bodies have a three-point bending strength of 800-1000 MPa and a fracture toughness of 6 MPa·m. 1 / 2 There are varying degrees.

[0030] Silicon nitride sintered bodies are primarily composed of silicon nitride crystal particles with an aspect ratio of 1.5 or higher, and even 2 or higher. The wear resistance of silicon nitride sintered bodies is improved by the complex intertwining of elongated silicon nitride crystal particles. Zirconium oxide sintered bodies are primarily composed of zirconium oxide crystal particles with an aspect ratio of less than 1.5. Argile sintered bodies are also primarily composed of argile crystal particles with an aspect ratio of less than 1.5. Therefore, silicon nitride sintered bodies offer superior wear resistance.

[0031] On the other hand, since silicon nitride sintered bodies are mainly composed of elongated silicon nitride crystal particles, they tend to have large arithmetic surface roughness Ra and maximum cross-sectional height Rt. As mentioned above, by controlling the arithmetic surface roughness Ra and maximum cross-sectional height Rt, it is possible to provide a ceramic ball material 1 with excellent wear resistance while reducing the amount of polishing required.

[0032] Furthermore, bearing balls can be obtained by polishing ceramic ball material 1. A bearing ball is an example of a polished ceramic ball. The arithmetic surface roughness Ra of the polished bearing ball is 0.1 μm or less. Also, ASTM_F2094 sets the arithmetic surface roughness Ra to 0.013 μm or less. The arithmetic surface roughness Ra decreases as the grade of the bearing ball increases. In order to make bearing balls from ceramic ball material 1, polishing is necessary to reduce the arithmetic surface roughness Ra.

[0033] Furthermore, the bearing balls are preferably 5 mm or larger in diameter. To improve the wear resistance of the bearing balls, it is preferable to prepare a dense ceramic molded body 9. Using a die press is effective in obtaining a dense ceramic molded body 9. The diameter of the bearing balls is not particularly limited, but 70 mm or less is preferred. The diameter of the bearing balls may exceed 70 mm, but this may increase manufacturing costs. For this reason, the diameter of the bearing balls is preferably within the range of 5 mm to 70 mm, and more preferably between 8 mm and 55 mm.

[0034] The ceramic balls manufactured from ceramic ball material 1 are exemplified for use as bearing balls. They can also be used as media in mixing processes. Therefore, ceramic balls manufactured from ceramic ball material 1 represent both bearing balls and media.

[0035] Next, a method for manufacturing the ceramic ball material 1 will be described. As long as the ceramic ball material 1 has the above configuration, the method for manufacturing it is not particularly limited, but the method for obtaining it with good yield is as follows. The ceramic ball material 1 is manufactured by sintering a spherical ceramic molded body after the strip-shaped portion has been sufficiently removed from the ceramic molded body using a processing device 100 described later. The processing device 100 is characterized by comprising a molded body fixing unit that clamps and fixes the spherical portion of a ceramic molded body having a spherical portion and a strip-shaped portion formed around its circumference so that the spherical portion can rotate, and a processing unit that presses against the strip-shaped portion to remove the strip-shaped portion.

[0036] Figure 4 shows an example of a processing apparatus for a ceramic molded body. Figure 5 shows an example of a method for measuring the displacement of the outer surface of a ceramic molded body. In the figures, reference numeral 4 denotes the rotating shaft end (upper rotating shaft end) which is part of the molded body fixing part, reference numeral 5 denotes the rotating shaft end (lower rotating shaft end) which is another part of the molded body fixing part, reference numeral 6 denotes the processing jig which serves as the processing part, reference numeral 7 denotes the processing jig fixing part, reference numeral 8 denotes the displacement measurement part, reference numeral 9 denotes the ceramic molded body having a strip-shaped part, reference numeral 10 denotes the spherical part, reference numeral 11 denotes the strip-shaped part, reference numeral 12 denotes the control unit, reference numeral 100 denotes the processing apparatus, reference numeral R denotes the center curve of the outer circumference of the strip-shaped part 11, reference numeral V denotes the straight line connecting the rotating shaft ends 4 and 5 (rotation axis), and reference numeral H denotes the displacement.

[0037] The following describes a method for manufacturing ceramic ball material 1 using a silicon nitride sintered body (including a method for processing the ceramic molded body 9) as an example. First, a ceramic molded body 9 having a strip-shaped portion 11 as shown in Figure 4 is prepared. A raw material powder slurry, which is a mixture of silicon nitride powder, sintering aid powder, and binder, is pressed into a mold. A mold with a spherical inner surface is used for the mold press. By pressing the mold using an upper mold and a lower mold with spherical inner surfaces, a ceramic molded body 9 having a spherical portion 10 and a strip-shaped portion 11 formed around the circumference can be manufactured.

[0038] The ceramic molded body 9 set in the processing apparatus 100 may be a molded body before CIP treatment or a molded body after CIP treatment. It is preferable that the ceramic molded body 9 set in the processing apparatus 100 is a molded body before CIP treatment. This is because a molded body that has undergone CIP treatment becomes somewhat harder due to the CIP treatment, which may increase the load on the processing apparatus 100 when removing the strip-shaped portion 11. process A molded body that has undergone this process is called a CIP body.

[0039] It should be noted that the ceramic molded body 9 set in the processing apparatus 100 is not limited to being a molded body before CIP treatment. The ceramic molded body 9 set in the processing apparatus 100 may be a molded body that has undergone at least one of the CIP treatment process, a degreasing process, and a calcination process. A molded body that has undergone the degreasing process is called a degreasing body. A molded body that has undergone the calcination process is called a calcined body. The calcination process is a heat treatment at a lower temperature than the sintering process described later. It should be noted that even if the ceramic molded body 9 set in the processing apparatus 100 is a degreasing body or a calcined body, it will become somewhat harder, similar to the case of a CIP body. Therefore, the load on the process of removing the strip-shaped portion 11 may increase slightly. The following describes the case where the ceramic molded body 9 set in the processing apparatus 100 is a molded body before CIP treatment.

[0040] Next, the strip-shaped portion 11 of the ceramic molded body 9 before CIP treatment is removed to obtain a spherical ceramic molded body 13 (shown in Figure 6) (removal step). The removal step of the strip-shaped portion 11 is performed using the processing apparatus 100 shown in Figure 4.

[0041] The molded body fixing section of the processing apparatus 100 has rotating shaft ends 4 and 5. For convenience, rotating shaft end 4 may be called the upper rotating shaft end 4, and rotating shaft end 5 may be called the lower rotating shaft end 5. The tips of the upper rotating shaft end 4 and the lower rotating shaft end 5 have fixing members for fixing the ceramic molded body 9. The ceramic molded body 9 is fixed between the upper rotating shaft end 4 and the lower rotating shaft end 5. At this time, it is preferable to arrange the strip-shaped portion 11 so that it is horizontal, as shown in Figure 4. By arranging the strip-shaped portion 11 horizontally, the process of removing the strip-shaped portion 11 becomes easier.

[0042] Next, the fixed ceramic molded body 9 is rotated. After fixing the ceramic molded body 9 between the upper rotation shaft end 4 and the lower rotation shaft end 5, the rotation shaft ends 4 and 5 themselves are rotated around the rotation shaft V. The processing jig 6 is pressed against the strip-shaped portion 11 of the ceramic molded body 9, which is rotating together with the rotation shaft ends 4 and 5. The strip-shaped portion 11 is removed by pressing the processing jig 6 against the strip-shaped portion 11 of the rotating ceramic molded body 9.

[0043] Furthermore, the plane (horizontal plane) containing the central curve R on the outer circumference of the strip-shaped portion 11 of the fixed ceramic molded body 9 is set to be perpendicular to the rotation axis V. The displacement measurement unit 8 then sequentially measures the amount of displacement of the outer surface of the strip-shaped portion 11, which is rotated around the rotation axis V, on the plane containing the central curve R. The displacement measurement unit 8 is a device that detects the amount of physical change of an object using various sensor elements and calculates the amount of displacement of the object (distance from the sensor to the object) by converting the amount of change into distance. The displacement measurement unit 8 converts the amount of displacement of the outer circumference of the strip-shaped portion 11 into digital data and transmits it to the control unit 12.

[0044] Preferably, the control unit 12 has a function to measure the runout value of the outer surface of the strip-shaped portion 11 based on the displacement amount of the outer surface of the strip-shaped portion 11 measured by the displacement amount measuring unit 8 (runout value measurement function), and a function to adjust the amount of processing of the strip-shaped portion 11, that is, the force with which the processing jig 6 pushes the strip-shaped portion 11, according to the runout value (processing amount adjustment function). For example, the control unit 12 is composed of a computer equipped with a processor, and the runout value measurement function and the processing amount adjustment function are realized by the processor executing a computer program. In addition to the displacement amount of the outer surface of the strip-shaped portion 11, the runout value of the outer surface of the strip-shaped portion 11 may also take into account the deviation of the rotation axis V from the center of the ceramic molded body 9.

[0045] By setting the surface containing the central curve R of the outer circumference of the strip-shaped portion 11 of the fixed ceramic molded body 9 to be perpendicular to the rotation axis V, the rotation direction of the strip-shaped portion 11 and the circumferential direction of the strip-shaped portion 11 become the same. This allows the processing jig 6 to be applied perpendicularly to the rotating strip-shaped portion 11. The rotation speed of the ceramic molded body 9 is within the range of 30 rpm to 250 rpm. If the rotation speed is less than 30 rpm, the processing time may be prolonged. Also, if the rotation speed exceeds 250 rpm, the ceramic molded body 9 may break. For this reason, the rotation speed is preferably within the range of 30 rpm to 250 rpm, and more preferably within the range of 50 rpm to 200 rpm.

[0046] The displacement measurement unit 8 measures the amount of material removed from the strip-shaped portion 11 of the rotating ceramic molded body 9 as the displacement. The displacement measurement unit 8 is preferably a laser displacement meter using laser light (dashed arrow shown in Figure 4). A laser displacement meter allows for the measurement of the displacement of the outer surface of the strip-shaped portion 11 without contacting the rotating ceramic molded body 9.

[0047] Furthermore, the displacement measurement unit 8 has the function of measuring the runout value of the outer surface of the strip-shaped portion 11 based on the displacement of the outer surface of the strip-shaped portion 11. Figure 5 shows a conceptual diagram of how the strip-shaped portion 11 measures the displacement of its outer surface. In Figure 5, the ceramic molded body 9 is rotated counterclockwise, but the direction of rotation is arbitrary.

[0048] When the strip-shaped portion 11 of the ceramic molded body 9 is removed using the processing jig 6, the distance between the surface of the ceramic molded body 9 on the displacement measurement section 8 side and the displacement measurement section 8 changes by the amount removed. This change in distance is the displacement amount H. The difference between the maximum and minimum values ​​of the displacement amount H is called the runout value. In other words, the runout value can be calculated using the following formula. Break value = Maximum value of displacement H - Minimum value of displacement H

[0049] Preferably, the control unit 12 has a function to adjust the amount of processing on the strip-shaped portion 11 according to this runout value. The function to adjust the amount of processing on the strip-shaped portion 11 involves moving the processing jig 6 in the depth direction of the strip-shaped portion 11 (the radial direction of the ceramic molded body 9) to change the amount that the processing jig 6 contacts the strip-shaped portion 11, i.e., the applied force. Furthermore, if the control unit 12 feeds back the displacement amount H obtained by the displacement amount measuring unit 8 and sets the processing jig 6 to move automatically, the mass production efficiency of the spherical ceramic molded body 13 is improved. In addition, the runout value is measured while the ceramic molded body 9 is rotated and the strip-shaped portion 11 is removed, until the sphericity of the ceramic molded body 9 is 2% or less. This makes it possible to control the sphericity of the ceramic ball material 1 after sintering to 2% or less.

[0050] Furthermore, after removing the strip-shaped portion 11, it is also effective to change the fixing point of the ceramic molded body 9 by 90° and measure the runout value of the spherical portion 10 in the same manner as the measurement of the runout value of the strip-shaped portion 11 described above. It is also effective to confirm the sphericity of the ceramic molded body 9 by fixing the strip-shaped portion 11 in contact with the rotational shaft ends 4 and 5 and measuring the runout value of the spherical portion 10.

[0051] Furthermore, it is preferable that the part of the processing jig 6 that contacts the strip-shaped portion 11 of the ceramic molded body 9 (hereinafter referred to as the "contact portion") is made of abrasive paper. It is also preferable that the contact portion of the processing jig 6 is mainly composed of tungsten carbide (WC). Abrasive paper is sometimes called sandpaper or abrasive paper. Abrasive paper is coated with an abrasive material. By changing the particle size of the abrasive material, the amount removed and the surface roughness can be controlled. Abrasive paper of the same grit number may be used, or it may be changed to a different grit number midway through the process. The grit number of the abrasive paper is preferably in the range of P100 to P600. The higher the number of the grit number of the abrasive paper, the finer the grit. If the grit is less than P100, the surface of the strip-shaped portion 11 of the ceramic molded body 9 may become rough. If the surface of the strip-shaped portion 11 becomes rough, the surface of the ceramic ball material (sintered body) may become rough. Also, if the grit exceeds P600, the processing efficiency may decrease. Therefore, the grit size of the abrasive paper should preferably be within the range of P100 to P600, and even more preferably P150 to P400. Furthermore, JIS-R-6252(2022) and JIS-R-6010(2000) are referenced for the abrasive paper. These JIS standards correspond to ISO3366(1999), ISO21948(2001), ISO21950(2001), ISO6344-1~3(1998), etc.

[0052] Furthermore, the abrasive paper may be attached to the base material of the processing jig 6, or it may be fixed around its perimeter. When fixing the perimeter of the abrasive paper, it is preferable to apply tension to the extent that the abrasive paper forms an arc (including a bow shape). Methods for fixing the perimeter of the abrasive paper include stretching it straight or stretching it in an arc. As described above, when removing the strip-shaped portion 11 while rotating the ceramic molded body 9 having the strip-shaped portion 11, the contact with the strip-shaped portion 11 can be softened by arranging the abrasive paper in an arc shape. This allows the strip-shaped portion 11 to be removed uniformly. In addition, various methods can be applied to fix the perimeter, such as up and down, left and right, or all around.

[0053] Furthermore, materials with WC as the main component are those containing 50% or more by mass of WC (tungsten carbide). These are sometimes called cemented carbide. Examples of cemented carbide include those containing WC along with one or more elements selected from Fe (iron), Ni (nickel), and Co (cobalt). Materials with WC as the main component are hard and therefore suitable for processes that remove strip-shaped portions. In addition, by flattening the tip of the contact portion of the processing jig 6 containing WC, the amount removed and the surface roughness can be controlled.

[0054] The above illustrates a method for removing the strip-shaped portion 11 from a ceramic molded body 9 while rotating the molded body 9 having the strip-shaped portion 11. However, this method is not limited to this one; another effective method involves fixing the ceramic molded body 9 and moving the contact portion of the processing jig 6 around the strip-shaped portion 11. Furthermore, for stripping hard materials such as CIP bodies, degreased bodies, or calcined bodies, there is also a method of rolling the ceramic molded body 9 on a rotating polishing plate. Additionally, there is a method of stripping the ceramic molded body 9 by barrel polishing.

[0055] As described above, the processing apparatus according to the embodiment can be used to remove the strip portion 11 from the ceramic molded body 9, which has a spherical portion 10 and a strip portion 11 formed around its circumference. This makes it possible to obtain a spherical ceramic molded body 13 without the strip portion 11. Furthermore, the molded body scraps generated by stripping can be reused as raw materials. Since the processing is performed on the ceramic molded body 9, the molded body scraps can be reused. If necessary, the spherical ceramic molded body 13 may be subjected to CIP treatment.

[0056] Next, the spherical ceramic molded body 13 is degreased. The degreasing process involves heating in a non-oxidizing atmosphere at a temperature of 500°C to 800°C for 1 to 4 hours to remove most of the organic binder that was added beforehand. Examples of non-oxidizing atmospheres include nitrogen gas atmospheres and argon gas atmospheres. If necessary, the process is carried out in an oxidizing atmosphere such as air to control the amount of organic matter remaining in the degreased body.

[0057] Next, the degreased body is sintered. The sintering process is preferably carried out at a temperature between 1650°C and 2000°C. A nitrogen gas atmosphere or a reducing atmosphere containing nitrogen gas is preferred as the non-oxidizing atmosphere. Furthermore, the pressure inside the firing furnace is preferably a pressurized atmosphere. If sintering is performed at a low temperature of less than 1650°C, the grain growth of silicon nitride crystals is insufficient, making it difficult to obtain a dense sintered body. On the other hand, if sintering is performed at a temperature higher than 2000°C, there is a risk of decomposition into Si and N2 if the atmospheric pressure inside the furnace is low, so it is preferable to control the sintering temperature within the above range. Furthermore, the sintering time is preferably within the range of 3 hours to 12 hours.

[0058] Furthermore, a pre-heating process may be performed before the sintering process, if necessary. The pre-heating process involves heat treatment at a lower temperature than the sintering process, which will be described later. The temperature of the pre-heating process is higher than that of the degreasing process but lower than that of the sintering process.

[0059] Furthermore, it is preferable to perform HIP (Hot Isostatic Pressing) treatment after the above sintering process. HIP treatment involves treating the sintered body. The process of sintering the degreased body is called the first sintering process, and the process of treating the sintered body with HIP is called the second sintering process. HIP treatment is preferably performed at a temperature between 1600°C and 1900°C and a pressure between 80 MPa and 200 MPa. HIP treatment can reduce the pores within the sintered body, thereby obtaining a dense sintered body. If the pressure is less than 80 MPa, the effect of applying pressure is insufficient. Conversely, if the pressure is higher than 200 MPa, the load on the manufacturing equipment may increase.

[0060] The ceramic ball material 1 is completed upon completion of the sintering process. Furthermore, the sintered ceramic ball material 1 may be subjected to honing as needed. Honing is a process of removing foreign matter such as dust by spraying abrasive particles onto the surface of the sintered body.

[0061] The resulting ceramic ball material 1 has a sphericity of 2% or less and a surface roughness Ra of 0.2 μm to 2 μm. Furthermore, by polishing ceramic ball material 1, it can be made into a bearing ball. The surface roughness Ra of the bearing ball shall be 0.1 μm or less. In addition, it shall be polished to a surface roughness corresponding to the grade specified in ASTM F2094.

[0062] (Examples) (Examples 1-6, Comparative Examples 1-6) First, a ceramic molded body 9 having a spherical portion 10 and a strip-shaped portion 11 formed around its circumference was prepared. The ceramic molded bodies 9 in Examples 1 to 5 are for obtaining a silicon nitride sintered body. The ceramic molded body 9 in Example 6 is for obtaining an aluminum oxide sintered body.

[0063] To manufacture the ceramic ball material 1 according to Examples 1 to 6, a stripping process was performed on the ceramic molded body 9 using the processing apparatus 100 shown in Figure 4. The processing jig 6 of the processing apparatus 100 was made of abrasive paper. The abrasive paper used had a grit size in the range of P100 to P600. The spherical portion 10 of the ceramic molded body 9 was fixed to the upper and lower ends of the rotating shaft 4 and 5, and the strip portion 11 was positioned so that it was horizontal. A laser displacement meter was used as the displacement measurement unit 8. The processing jig 6 was set to move automatically according to the runout value obtained from the displacement amount H determined by the laser displacement meter. The same process was followed for manufacturing the ceramic ball material according to Comparative Example 1.

[0064] Examples 1-6 and Comparative Example 1 were obtained by stripping ceramic molded bodies 9, which were acquired by die pressing, using a processing device 100. Comparative Examples 2-4 and 6 were not stripped. Comparative Example 5 was produced by rolling granulation, resulting in a ceramic molded body without strips. Comparative Examples 1-5 used aluminum oxide molded bodies. Comparative Example 6 also used an aluminum oxide molded body.

[0065] Each ceramic molded body was subjected to a degreasing process, a sintering process, and a HIP process to produce a ceramic ball material consisting of a silicon nitride sintered body or an aluminum oxide sintered body. The degreasing body for producing the ceramic ball material 1 in Examples 1 to 6 is obtained by degreasing a spherical ceramic molded body 13. The same applies to the degreasing body for producing the ceramic ball material in Comparative Example 1. The degreasing body for producing the ceramic ball material in Comparative Examples 2 to 4 and 6 is obtained by degreasing a ceramic molded body 9 having a strip-shaped portion 11. The degreasing body for producing the ceramic ball material in Comparative Example 5 is obtained by degreasing a ceramic molded body without a strip-shaped portion 11.

[0066] The sphericity, arithmetic surface roughness Ra, maximum cross-sectional height Rt, Ra1 / Ra2, and Rt1 / Rt2 of the ceramic ball material obtained after the HIP process were measured. Ra1 and Rt1 are values ​​in the circumferential direction of the band-shaped indentation. Ra2 and Rt2 are values ​​perpendicular to the band-shaped indentation. The measured results are shown in Table 1.

[0067] [Table 1]

[0068] The diameter of approximately 10 mm in Example 1 and Comparative Examples 2 and 5 is suitable for producing ceramic balls with a diameter of 3 / 8 inch. The diameter of approximately 31 mm in Examples 2 to 3 and 6 and Comparative Examples 1, 3 and 6 is suitable for producing ceramic balls with a diameter of 1-3 / 16 inches. The diameter of approximately 47 mm in Examples 4 to 5 and Comparative Example 4 is suitable for producing ceramic balls with a diameter of 1-7 / 8 inches. In Comparative Example 1, although stripping was performed using the processing apparatus 100 in the same manner as in Examples 1 to 6, the sphericity of the spherical ceramic molded body 9 before obtaining the ceramic ball material was inherently poor.

[0069] Polishing efficiency and yield were investigated using ceramic ball materials related to the examples and comparative examples shown in Table 1. Polishing efficiency was measured by the time required to perform finish polishing to a surface roughness of Ra 0.005 μm. Ra 0.005 μm is equivalent to grade 5 of ASTM F2094 for bearing balls.

[0070] When the finishing polishing time for Comparative Examples 2-4 and Comparative Example 6 was set to 100, the ratio of the finishing polishing time for the Example was calculated. A ratio of 70% or less was classified as "improvement," and a ratio between 70% and 100% was classified as "maintenance." Specifically, for bearing balls with a diameter of 3 / 8 inch manufactured from the ceramic ball material of Example 1 and Comparative Examples 2 and 5, the polishing time of Comparative Example 2 was set to 100. For bearing balls with a diameter of 1-3 / 16 inch manufactured from the ceramic ball material of Examples 2-3 and 6 and Comparative Examples 1, 3, and 6, the polishing time of Comparative Example 3 was set to 100. Furthermore, for bearing balls with a diameter of 1-7 / 8 inch manufactured from the ceramic ball material of Examples 4-5 and Comparative Example 4, the polishing time of Comparative Example 4 was set to 100.

[0071] Regarding yield, we investigated the rate of breakage of the ceramic ball material during the above-mentioned finish polishing process. 1000 balls were polished using the finish polishing process, and the percentage of broken balls was counted. Balls with a breakage rate of 0% to 0.5% were classified as "improved," those with a breakage rate between 0.5% and 3% were classified as "maintained," and those with a breakage rate above 3% were classified as "poor." The results are shown in Table 2.

[0072] [Table 2]

[0073] As can be seen from Table 2, the ceramic ball material used in the examples showed improved polishing efficiency and yield compared to the comparative examples. In contrast, materials with a small amount of stripping, such as Comparative Example 1, showed only a small improvement in polishing efficiency and yield. Furthermore, materials formed by tumbling granulation, such as Comparative Example 5, had insufficient strength, resulting in a lower yield. This indicates that tumbling granulation is not suitable for finishing processes that require a surface roughness Ra of approximately 0.005 μm. It was also found to be effective for ceramic sintered bodies other than silicon nitride sintered bodies, as shown in Example 6.

[0074] Although several embodiments of the present invention have been illustrated above, these embodiments are presented as examples only and are not intended to limit the scope of the invention. These novel embodiments can be implemented in various other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. Modifications of these embodiments are included in the scope and spirit of the invention, as well as in the claims of the invention and its equivalents. Furthermore, the embodiments described above can be implemented in combination with each other. [Explanation of symbols]

[0075] 1…Material for ceramic balls 1A...Material for ceramic balls having a strip-shaped portion 2,2A…Spherical part 3A...band-shaped area 4…End of the rotating shaft (upper end of the rotating shaft) 5…End of the rotating shaft (lower end of the rotating shaft) 6. Machining jigs 7…Processing jig fixing part 8…Displacement measurement unit 9. Ceramic molded body having a strip-shaped portion 10...Spherical part 11…band-shaped area 12…Control Unit 13…Ceramic molded body 100...Processing equipment

Claims

1. The sphericity is 2% or less. Regardless of where on the surface it is measured, the arithmetic mean roughness Ra is between 0.2 μm and 2 μm. A ceramic ball material characterized by having a maximum cross-sectional height Rt of 4 μm or more and 20 μm or less.

2. The ceramic ball material according to claim 1, characterized in that the sphericity is 1.5% or less.

3. The material for ceramic balls according to claim 1 or 2, characterized in that the maximum cross-sectional height Rt is 5 μm or more and 17 μm or less.

4. The ceramic ball material according to claim 1 or 2, characterized in that the ceramic ball material is one selected from silicon nitride sintered body, aluminum oxide sintered body, zirconium oxide sintered body, and argyl sintered body.

5. The ceramic ball material according to claim 3, characterized in that the material for the ceramic ball is one selected from silicon nitride sintered body, aluminum oxide sintered body, zirconium oxide sintered body, and argyl sintered body.

6. A bearing ball characterized by being obtained by polishing the ceramic ball material described in claim 1.

7. A bearing ball characterized by being obtained by polishing the ceramic ball material described in claim 5.

8. The bearing ball according to claim 6, wherein the diameter is 5 mm or more and 70 mm or less.

9. The bearing ball according to claim 7, wherein the diameter is 5 mm or more and 70 mm or less.

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