Material for ceramic ball, ceramic ball, and method for producing the same

Abstract

To provide a material for a ceramic ball having high sphericity, capable of shortening the processing time of a ceramic material, especially in surface plate processing, during polishing.SOLUTION: A material for a ceramic ball according to an embodiment has a spherical part and a band-shaped part formed in a band. When the roundness observed in a height direction of the band-shaped part is defined as C, the material for a ceramic ball has the roundness C falling within a range of more than 0% and 2.5% or less. When this material for a ceramic ball is polished, the durability of the grindstone can be improved. Further, the ceramic may be applied as one or more selected from alumina, silicon nitride, boron nitride, and zirconia.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The embodiments described below relate to a ceramic ball material, a ceramic ball, and a method for manufacturing the same. [Background technology]

[0002] Various ceramic materials possess properties such as high hardness, electrical insulation, and wear resistance. Fine ceramics, which have particularly high purity and uniform particle size, exhibit properties that make them suitable for a variety of applications, including capacitors, actuator materials, and refractory materials. Among these, bearing balls are products that take advantage of their wear resistance and electrical insulation properties, and materials such as aluminum oxide, silicon nitride, zirconium oxide, silicon carbide, and sialon are used. For example, Japanese Patent Laid-Open No. 6-48813 (Patent Document 1) and Japanese Patent No. 2764589 (Patent Document 2) disclose bearing balls made of silicon nitride materials, and Japanese Patent Laid-Open No. 60-18620 (Patent Document 3) discloses bearing balls made of zirconium oxide materials. Furthermore, the aforementioned ceramic balls may be polished to suit their intended use, such as bearing applications, using a surface plate processing technique such as that described in Japanese Patent No. 5334040 (Patent Document 5).

[0003] The process for manufacturing these bearing ball materials involves sintering a green compact. Furthermore, press molding using a mold is used as the molding method. Press molding generally involves inserting powder between an upper mold 1 and a lower mold 2 and applying pressure, as shown in FIG. 1. To protect the molds during press molding, a gap must be provided between the tip 3 of the upper mold 1 and the tip 4 of the lower mold 2. For this reason, the green compacts produced by press molding have a spherical portion and a strip-shaped portion. For example, Japanese Patent No. 4761613 (Patent Document 4) discloses a bearing ball material having a spherical portion and a strip-shaped portion. Figure 2 shows the ceramic ball material described in Patent Document 4. In Figure 2, 7 denotes the ceramic ball material, 6 denotes the spherical portion, and 5 denotes the strip-shaped portion. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 6-48813 [Patent Document 2] Patent No. 2764589 [Patent Document 3] Japanese Patent Application Publication No. 18620 / 1983 [Patent Document 4] Patent No. 4761613 [Patent Document 5] Patent No. 5334040 Summary of the Invention [Problem to be solved by the invention]

[0005] Ceramic balls are produced by polishing a ceramic ball material having a spherical portion 6 and a strip portion 5. Polished ceramic balls are particularly suitable for use as bearing balls. A ceramic ball material 7 having a spherical portion 6 and a strip portion 5 is sometimes called a bare ball. For example, bearing balls are mirror-finished to a surface roughness Ra of 0.1 μm or less. A surface plate is used for the mirror-finishing process.

[0006] Ceramic materials have excellent wear resistance, but because they are brittle and have high hardness, they require more processing steps than metals. In particular, the initial process of increasing sphericity to the required value often takes up the majority of the processing time due to the large processing costs. The higher the sphericity (roundness) before processing, the longer this processing time will be, so it is necessary to minimize the sphericity of the ceramic ball material before processing.

[0007] The present invention is intended to solve these problems and provides a ceramic ball material with high sphericity that can shorten the processing time of ceramic materials during polishing, particularly during surface plate processing. [Means for solving the problem]

[0008] The ceramic ball material according to the present embodiment has a spherical portion and a band-shaped portion formed in a band shape. When the circularity of the band-shaped portion is C when observed from the height direction, the circularity C is in the range of more than 0% to 2.5%. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a diagram showing an example of die press molding. [Figure 2] FIG. 1 is a diagram showing an example of a conventional ceramic ball material. [Figure 3] FIG. 2 is a diagram schematically illustrating a measurement direction (first measurement direction) in the material for ceramic balls according to the embodiment. [Figure 4] FIG. 2 is a diagram schematically illustrating an image obtained by observing the ceramic ball material according to the embodiment from a first measurement direction. [Figure 5] FIG. 2 is a diagram schematically illustrating a measurement direction (second measurement direction) in the material for ceramic balls according to the embodiment. [Figure 6] FIG. 3 is a schematic view showing an image obtained by observing the ceramic ball material according to the embodiment from a second measurement direction. [Figure 7] 1 is a diagram illustrating an example of the width of a band-shaped portion of a ceramic ball material according to an embodiment. FIG. [Figure 8] 1 is a diagram illustrating an example of the height of a band-shaped portion of a ceramic ball material according to an embodiment. FIG. [Figure 9] FIG. 7 is a diagram schematically illustrating the positional relationship between the image shown in FIG. 4 and the image shown in FIG. 6. [Figure 10] FIG. 2 is a diagram showing an example of the radius obtained by measuring the roundness C of a ceramic ball material according to an embodiment. [Figure 11] FIG. 2 is a diagram showing an example of the radius obtained by measuring the roundness C′ of a ceramic ball material according to an embodiment. [Figure 12] FIG. 1 is a diagram showing an example of rubber mold isostatic pressing. DETAILED DESCRIPTION OF THE INVENTION

[0010] A ceramic ball material according to an embodiment has a spherical portion and a band-shaped portion. The ceramic ball material has a circularity C of more than 0% to 2.5% when observed from the height direction of the band-shaped portion, where C is the circularity. The height direction is the direction perpendicular to the width direction of the band-shaped portion. It is also preferable that the band-shaped portion be formed along the circumference, because if the band-shaped portion is present only partially along the circumference, stress is likely to be concentrated locally during, for example, surface plate processing, which may cause cracks or chips in the ball material. More preferably, the circularity C is a value satisfying the range of 0.01% to 2%. When the circularity C is 2.5% or less, particularly 2% or less, the initial contact area with the surface plate becomes uniform during machining of the ceramic ball material, thereby reducing the amount of machining required. Therefore, considering processability, the smaller the circularity C, the better. On the other hand, to achieve a circularity C of less than 0.01%, particularly 0%, the band-shaped portion must be extremely small, and the circularity of the spherical portion must also be small. If the band-shaped portion itself is made too small in order to reduce the circularity of the compact having the band-shaped portion, there is a risk that the granulated powder will not be sufficiently filled at the boundary between the upper and lower dies. This may result in a deterioration in yield. Therefore, considering yield, a value of 0.01% or more is more preferable. Furthermore, it is even more preferable that the circularity C be a value of 0.01% to 1.5% of the average radius rA of the ceramic ball material.

[0011] FIG. 3 shows a ceramic ball blank according to an embodiment. 7 denotes a ceramic ball blank, 6 denotes a spherical portion, and 5 denotes a band-shaped portion. The ceramic ball blank 7 includes the spherical portion 6 and the band-shaped portion 5. In the ceramic ball blank 7 shown in FIG. 3, a first measurement direction 8, which is horizontal to the equatorial plane formed by the band-shaped portion 5, i.e., the measurement direction "when viewed from the height direction along the height of the band-shaped portion 5," is indicated by an arrow in the schematic diagram. The location of the band-shaped portion 5 preferably includes the equatorial plane of the ceramic ball blank 7 so that the upper spherical portion 61 and the lower spherical portion 62 of the spherical portion 6 are approximately symmetrical with respect to the equatorial plane of the band-shaped portion 5 formed around the circumference. Furthermore, the shape of the band-shaped portion 5 is preferably approximately symmetrical with respect to the equatorial plane formed by the band-shaped portion 5. The equatorial plane formed by the band-shaped portion 5 is a plane passing through the center of the width of the band-shaped portion 5. An image observed from the first measurement direction 8 is shown in FIG. 4. 4, the belt-like portion 5 and the spherical portion 6 are simultaneously observed in the measurement from the above measurement direction. When the belt-like portion 5 is present in a location including the equatorial plane of the ceramic ball material 7 as shown in FIG. 4, the location of the belt-like portion 5 observed from the height direction of the belt-like portion 5 may have the largest radius.

[0012] FIG. 5 shows a ceramic ball material according to an embodiment. 7 denotes the ceramic ball material, 6 denotes the spherical portion, and 5 denotes the band-shaped portion. In FIG. 5, a schematic diagram of a second measurement direction 9, which is perpendicular to the equatorial plane of the band-shaped portion 5, i.e., a direction perpendicular to the height 11 (shown in FIG. 8) of the band-shaped portion 5, is shown. FIG. 6 shows an example of an image observed from this second measurement direction 9.

[0013] Similarly to FIG. 4 , FIG. 2 shows the ceramic ball blank 7 observed from the first measurement direction 8. As shown in FIG. 2 , one or more diameters of the band-shaped portion 5 of the ceramic ball blank 7 are designated R1, and one or more diameters of the spherical portion 6 are designated R2. In the ceramic ball blank 7 shown in FIG. 2 , the diameter R1 is defined as a line connecting a point on the outer circumferential surface of one band-shaped portion 5 to a point on the outer circumferential surface of the opposing band-shaped portion 5 where the length is the longest. In the ceramic ball blank 7 shown in FIG. 2 , the diameter R2 is defined as a line connecting a point on the outer circumferential surface of the spherical portion 6 to a point on the outer circumferential surface of the spherical portion 6 where the length is the longest. Thus, the ceramic ball blank 7 shown in FIG. 2 is divided into 12 equal-spaced sections by 12 diameters, and each diameter is divided by 2 to obtain a radius. The resulting radii are designated r1 to r12 according to the division locations. The average of the radii r1 to r12 is defined as an average radius rA. This number of divisions may be changed if a sufficient number of samples can be obtained when evaluating the roundness described below. However, it is desirable to measure at least 12 divisions isotropically to increase the reliability of the data. In addition, in Figures 2 and 4, the roundness may be calculated from a single measurement direction 8, but it may also be calculated by measuring the radii described below from multiple measurement directions 8 and calculating the maximum, minimum, and average values.

[0014] Here, the radii r1 to r12 are measured using a non-contact image dimension measuring instrument, such as the IM-7000 manufactured by KEYENCE Corporation or an instrument with equivalent performance.

[0015] Furthermore, the measurement magnification is not particularly limited, but should be such that the ceramic ball material 7 occupies an area of ​​at least 1 / 10 of one field of view and the entire ceramic ball material 7 is within one field of view. If the ceramic ball material 7 occupies less than 1 / 10 of one field of view, it is not preferable because it is susceptible to errors and the accuracy may decrease. On the other hand, if the entire ceramic ball material 7 does not fit within one field of view, it is not preferable because the center may become unclear and there may be some areas where the diameter cannot be determined.

[0016] The roundness (roundness C (%)) of the ceramic ball blank 7 when observed from the first measurement direction 8 is defined as the difference in radius between the inscribed circle and the circumscribed circle of the object divided by the average value of the radii. The radius of the inscribed circle is the minimum radius (r min ) On the other hand, the circumscribed circle is the circle having the maximum radius (r max Therefore, the roundness C (%) of the ceramic ball material 7 is calculated by the following formula (1). Roundness C(%) =(r max -r min ) / rA×100 …(1)

[0017] For example, when the ceramic ball blank 7 observed from the first measurement direction 8 is isotropically divided into 12 parts, the difference between the radii of the circumscribed circle and the inscribed circle is the maximum value (r max ) and minimum value (r min ) is the difference between the circumscribed circle radius and the maximum value of r1 to r12 (r max ) The radius of the inscribed circle is the smallest value of r1 to r12 (r min ) The average radius (rA) is the average value of r1 to r12. As can be seen from formula (1), the smaller the value of circularity C, the closer it is to a perfect circle. An example of the radii obtained by the measurement at this time is shown in FIG. 10, where radii r1 to r12 obtained by the measurement when measuring the roundness of the raw material for ceramic balls are indicated by radius 14.

[0018] The ceramic ball material 7 has a spherical portion 6 and a band-like portion 5. The band-like portion 5 is formed in a band shape around the circumference of the spherical portion 6. The band-like portion 5 may have a partial recess. The spherical portion 6 may be spherical. Therefore, the shape of the spherical portion 6 may be a perfect circle or an ellipse. The band-like portion 5 is provided on the circumference of the spherical portion 6.

[0019] The circularity C calculated by the above formula (1) is preferably a value exceeding 0% and not exceeding 2.5%. More preferably, the circularity C is a value that is not less than 0.01% and not exceeding 2%. The value of the circularity C is the maximum value (r max ) and minimum value (r min ) is calculated, and the radius difference is divided by the average radius (rA) and multiplied by 100 to convert to a percentage (%). When the circularity C is small enough to satisfy the requirement of 2.5% or less, particularly 2% or less, the initial contact area with the surface plate becomes uniform during machining of the ceramic ball material 7, thereby reducing the amount of machining required. Therefore, from the viewpoint of processability, the smaller the circularity C, the better. On the other hand, to achieve a circularity C of less than 0.01%, particularly 0%, the band-shaped portion 5 must be extremely small, and the circularity of the spherical portion 6 must also be small. If the band-shaped portion 5 itself is made too small in order to reduce the circularity of the compact that forms the basis of the ceramic ball material 7 having the band-shaped portion 5, there is a risk that the granulated powder will not be sufficiently filled at the boundary between the upper mold 1 and the lower mold 2 (shown in FIG. 1) that mold the compact. This may result in a decrease in yield. Therefore, from the viewpoint of yield, it is more preferable that the circularity C be 0.01% or more.

[0020] Furthermore, it is more preferable that the roundness C calculated by the above formula (1) is a value of 0.01% or more and 1.5% or less. The roundness C is the maximum value (r max) and minimum value (r min ) is calculated, and the difference in radius is divided by the average radius (rA) and multiplied by 100 to convert the result into a percentage (%). By keeping the roundness C within this range, when further polishing is performed using a surface plate, it is possible to increase the effectiveness of uniforming the initial contact area with the surface plate. Furthermore, it is possible to reduce the incidence of defects such as chips, cracks, and color unevenness on the surface of the ceramic ball material 7 on the bearing surface.

[0021] Furthermore, it is preferable that the circularity (circularity C') of the spherical portion 6 (excluding the belt-shaped portion) when observed from the first measurement direction 8 is a value of 0% to 1.5%. The circularity C' of the spherical portion 6 is the maximum value (r' max ) and minimum value (r´ min ) is calculated, and the difference in radius calculated is divided by the average radius value (r'A), multiplied by 100, and converted to a percentage (%). Furthermore, for the measurement point of only the spherical portion 6 excluding the measurement value of this band-like portion 5, the value from the first measurement is not used, but the area of ​​only the spherical portion 6 is divided into 12 and measured separately from Figure 10. At this time, the average value of the radii excluding the measurement value of the band-like portion 5 is taken as the average radius rA'. At this time, a roundness C' of the spherical portion 6 of 0 indicates that the difference is a value below the resolution of the device. An example of the radii obtained by the measurement at this time is shown in FIG. 11, where radii r'1 to r'12 obtained by measuring only the spherical surface portion 6 are indicated by a radius 15.

[0022] Furthermore, it is more preferable that the roundness C' of the spherical portion 6 is a value between 0% and 1.3%. However, the average value of the radii excluding the measured values ​​of the band-shaped portion 5 is defined as the average diameter rA'. The roundness C' of the spherical portion 6 is calculated by the following formula (2). Roundness C´(%) =(r´ max -r´ min ) / rA´×100 …(2)

[0023] The measurement direction perpendicular to the height direction of the band-shaped portion is defined as the second measurement direction. This second measurement direction is shown as 9 in Figure 5. For Figure 6, which is a schematic diagram projected from the second measurement direction 9, the diameters are obtained at 12 locations as in Figure 4, and the radius is calculated by following the same procedure. The maximum radius (r´´ max ), minimum value (r´´ min The roundness C" (%) is calculated by deriving the diameter (rA") and the average value (rA"). In this case, the average diameter of the ceramic ball blank 7 is defined as rA" and the roundness C" is defined as C". The roundness C" of the ceramic ball blank 7 is calculated by the following formula (3). Roundness C´´(%) =(r´´ max -r´´ min ) / rA´´×100 …(3)

[0024] The circularity C" is preferably 0% or more and 1.5% or less. The circularity C" is preferably 0.01% or more and 1.0% or less. The circularity C" is defined as the maximum value (r") of r"1 to r"12 (12 radii not shown). max ) and minimum value (r´´ min ) and then divide the difference in radius by the average radius (r´´A) and multiply the result by 100 to convert it into a percentage (%). If the roundness C´ is within this range, the initial contact area with the surface plate becomes uniform when machining the ceramic ball material 7, which reduces the amount of machining required. Furthermore, if the roundness C´ is not within this range, the roundness of other measurement surfaces will also increase, which may increase the amount of machining required.

[0025] 3 is the measurement surface 12 when measured from the first measurement direction 8. The measurement surface 12 when measured from the first measurement direction 8 is sometimes called the plane 12. 5 is measurement surface 13 when measured from second measurement direction 9. Measurement surface 13 when measured from second measurement direction 9 is sometimes called plane 13. As shown in FIG. 9, plane 12 and plane 13 intersect approximately perpendicularly.

[0026] The ceramic ball material 7 preferably contains at least 85 mass% of one or more of aluminum oxide, silicon nitride, boron nitride, silicon carbide, zirconium oxide, and sialon. The ceramic ball material 7 is made of a ceramic sintered body. The content of at least 85 mass% of one or more of aluminum oxide, silicon nitride, boron nitride, and zirconium oxide refers to the content in the ceramic sintered body. In other words, the ceramic sintered body may contain up to 15 mass% of substances other than those listed above.

[0027] Materials used for bearing balls include aluminum oxide sintered compacts, silicon nitride sintered compacts, boron nitride sintered compacts, zirconium oxide sintered compacts, silicon carbide sintered compacts, sialon sintered compacts, argil (a mixture of aluminum oxide and zirconium oxide, with a total content of 85% by mass or more), and sialon sintered compacts. Therefore, the content of only one material, such as aluminum oxide sintered compacts, silicon nitride sintered compacts, boron nitride sintered compacts, zirconium oxide sintered compacts, and silicon carbide sintered compacts, may account for 85% by mass or more. Alternatively, the content of two or more materials, such as argil sintered compacts and sialon sintered compacts, may account for 85% by mass or more. Among these, bearing ball materials made of silicon nitride sintered compacts are particularly excellent in terms of wear resistance. Therefore, sintered compacts containing 85% by mass or more of silicon nitride are particularly preferred.

[0028] Aluminum oxide sintered body and zirconium oxide sintered body have a Vickers hardness of 1200 to 1700, but a toughness value of 3 MPa m 1 / 2 More than 6MPa m 1 / 2 It is about the following.

[0029] On the other hand, silicon nitride sintered body has a Vickers hardness of 1400 to 1800, and a toughness value of 5 MPa m 1 / 2 More than 10MPa m 1 / 2 It is about the following. In other words, silicon nitride sintered bodies have excellent toughness and Vickers hardness among ceramics. Because of these properties, silicon nitride sintered bodies are particularly excellent in wear resistance. Sintered silicon nitride has a structure mainly composed of β-type silicon nitride crystal grains. β-type silicon nitride crystal grains have an elongated shape. Because these elongated crystal grains are intricately intertwined, sintered silicon nitride has a high toughness value. Silicon nitride sintered bodies have high mechanical strength. This can lead to very poor polishing efficiency. However, as mentioned above, by improving (i.e., reducing) the roundness, the amount of cutting can be reduced. This reduction in cutting allowance improves the polishing efficiency even for ceramic ball materials 7 made of high-strength ceramic sintered bodies such as silicon nitride sintered bodies.

[0030] Next, a method for manufacturing the ceramic ball material 7 will be described. The method for manufacturing the ceramic ball material 7 according to the embodiment is not particularly limited as long as it satisfies the above-mentioned configuration. However, the following manufacturing method can be mentioned as a method for efficiently manufacturing the ceramic ball material 7. The manufacturing method will be described using a silicon nitride sintered body as an example.

[0031] First, the raw material silicon nitride is mixed with appropriate amounts of sintering aids, additives, solvents, binders, etc., and then crushed and granulated using a spray dryer. This process produces granulated powder from the raw material powder. Furthermore, when the total of the silicon nitride powder and sintering aid powder is 100% by mass, it is preferable that the silicon nitride powder account for 85% by mass or more. The additive is a plasticizer.

[0032] The solvent is water, an organic solvent, or a mixture thereof. Examples of organic solvents include alcohols, ketones, benzene, and compounds containing ethers. Examples of alcohols include methanol, ethanol, propanol, butanol, hexanol, heptanol, octanol, and phenol. Examples of ketones include acetone and diethyl ketone. Examples of ethers include diethyl ether and dimethyl ether. The organic solvent preferably has a linear carbon chain length of 25 or less. A linear chain length greater than 25 makes the organic solvent less volatile, potentially making it difficult to control during the degreasing process. The binder is an organic substance. The amount of binder added is 3% to 20% by mass, where the total amount of silicon nitride powder and sintering aid powder is 100% by mass. By adjusting the binder amount and the mold shape (changing the spherical portion into an aspherical shape), the roundness of the surface including the spherical portion 6 and the strip portion 5 can be reduced in the process described below.

[0033] Next, the granulated powder is press-molded. Examples of press molding include a molding method using an upper die 1 and a lower die 2. The spherical shape inside the upper die 1 and the lower die 2 determines the spherical portion 6 of the ceramic ball blank 7. During this process, the sphere formed by uniaxial pressing experiences variations in molding density within the sphere depending on the direction perpendicular to the press, the vertical direction, and the proximity of the upper die 1 and the lower die 2. Therefore, especially when die press molding is combined with isostatic pressing (e.g., CIP processing), the circularity during die molding differs from that after isostatic pressing. Therefore, to improve circularity in subsequent processes, the three-dimensional shape of the spherical portion formed by the die is made aspherical, deviating from a perfect sphere. This enables the molding of a ceramic ball blank 7 with improved circularity, making it possible to keep the circularity C calculated from the above formula (1) within a range of more than 0% to 2.5% of the average diameter rA.

[0034] The compact obtained by press molding has a spherical portion corresponding to the spherical portion 6 and a strip portion corresponding to the strip portion 5. The resulting strip of the molded body may be subjected to a removal process. Any tool can be used in this removal process as long as it can remove the strip. Examples of tools used to remove the strip include files such as sandpaper, emery paper, and abrasive paper.

[0035] The strip-shaped portion is preferably removed after press molding and before HIP (hot isostatic pressing). Considering the ease of removal of the strip-shaped portion, it is preferably removed before sintering, more preferably before degreasing, and even more preferably before cold isostatic pressing (CIP). It is preferable not to apply too much pressure during this strip removal step.

[0036] Excessive pressure can cause chipping and cracking, so the applied pressure must be controlled. Furthermore, even after the band-like portion removal process, the stripe-like traces of the band-like portion remain, so it can be determined that the product once had a band-like portion. Therefore, even when the band-like portion removal process is performed, the product is still defined as having a band-like portion.

[0037] It is also preferable to perform isostatic pressing on the compact after press molding (including those that have undergone a strip-shaped portion removal step, etc.). By performing isostatic pressing, it is possible to uniformly compress the granulated powder in the compact. This makes it possible to reduce the amount of granulated powder that remains crushed in the compact. By reducing the amount of granulated powder that remains crushed, it is possible to control the shrinkage rate during the sintering step.

[0038] As an example of isostatic molding, we will explain the isostatic molding method using a rubber mold. An example of a disk-shaped rubber mold is shown in Figure 12. In the figure, 16 and 17 are disk-shaped rubber molds, and 18 is a space.

[0039] The disc-shaped rubber dies 16, 17 have hemispherical holes on both sides that are approximately 1% to 35% larger than the diameter of the molded body. The molded body 20 is placed in the holes and the disc-shaped rubber dies 16, 17 are then stacked on top of each other to seal the space 18 between the molded body 20 and the disc-shaped rubber dies 16, 17. A hydrostatic pressure higher than the pressure during molding is applied to the disc-shaped rubber dies 16, 17. This allows for uniform compression of the molded body 20. This process reduces residual crushed granulated powder. It is also preferable to position the band-shaped portion of the molded body 20 perpendicular to the cylindrical plane of the disc-shaped rubber dies 16, 17. It is also preferable to use disc-shaped rubber dies 16, 17 with a Shore hardness Hs of 30 to 50. By keeping the hardness of the disc-shaped rubber dies 16, 17 within this range, the deformability is ensured, allowing for uniform contact between the surface of the molded body 20 and the disc-shaped rubber dies 16, 17. Furthermore, the durability of the disc-shaped rubber molds 16 and 17 is also good.

[0040] Since pressure is applied isotropically to the green body obtained in this process, it is possible to obtain a shape similar to the sintered ceramic ball material 7. The roundness C at this time may be measured and the curved surfaces formed by the upper die 1 and the lower die 2 during press molding may be adjusted.

[0041] Next, a degreasing step is carried out to degrease the compact after press molding (including those that have undergone the strip-shaped portion removal step and isostatic pressing step). 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 evaporate the organic components. The degreasing step may be carried out in a nitrogen atmosphere or in the air. A degreased body can be obtained by the degreasing step.

[0042] Next, the degreased body is sintered in a sintering process. The sintering temperature is preferably between 1700°C and 2000°C. The sintering process is preferably performed in a nitrogen atmosphere. The sintering pressure is preferably between atmospheric pressure and 300 MPa. The atmospheric pressure is 0.10133 MPa (=1 atm). The sintered body obtained in the sintering process may be subjected to HIP (hot isostatic pressing). This process allows for the production of a ceramic ball material 7 with minimal density variation. The ceramic ball material 7 thus obtained is a sintered ceramic body with a theoretical density of 98% or more. The spherical portion 6 and the band-shaped portion 5, i.e., the roundness C, can also be adjusted by polishing the finished ceramic ball material 7. However, this method requires an additional polishing step and is therefore not desirable. The above-described manufacturing method is preferable.

[0043] Ceramic balls can be manufactured by polishing the ceramic ball material 7. A typical example of polishing a sphere is surface plate processing.

[0044] For example, the polishing process involves inserting the ceramic ball material 7 between two parallel plates. The polishing plates then move to polish the ceramic ball material 7 into a perfect sphere. Alternatively, fixed diamond abrasive grains may be used to improve the precision of the polished surface. When the ceramic balls obtained by polishing the ceramic ball material 7 are used for bearing applications, they are referred to as bearing balls.

[0045] The surface roughness of bearing balls is specified in ASTM F2094. Bearing balls are graded according to ASTM F2094 depending on the application. They are polished to an arithmetic surface roughness Ra that conforms to that grade. Higher grades can also be mirror-finished to an arithmetic surface roughness Ra of 0.01 μm or less.

[0046] The ceramic ball material 7 according to the embodiment has a characteristic that the roundness C of the curved surface formed by the spherical portion 6 and the band-shaped portion 5, measured in the first measurement direction 8, is in the range of more than 0% to 2.5%. This allows for surface contact with a grinding stone such as a polishing table. This prevents damage to the ceramic ball material 7 during the polishing process. It also improves the durability of the polishing table during polishing. Therefore, providing a ceramic ball as a bearing ball material with a small amount of cutting allowance is expected to reduce the time and cost required for polishing, the amount of polishing material, and the amount of polishing required.

[0047] Furthermore, as mentioned above, (small) ceramic balls with improved sphericity are particularly suitable for bearing applications that require polishing. Bearing applications may use races, such as inner and outer rings, made of other materials, such as stainless steel, or the outer and inner rings may also be made of ceramics. Therefore, the entire wear-resistant component used in the bearing may be made of ceramic, or the bearing may consist solely of ceramic bearing balls. Furthermore, when used in all-ceramic bearing applications, the races, such as the inner and outer rings, do not need to be made primarily of the same material as the bearing balls, but they can be made primarily of the same material. They may also be used in bearings that do not use grease.

[0048] (Example) (Examples 1 to 8, Comparative Examples 1 to 3) Sintering aids, additives, solvents, binders, etc. were added to the raw ceramic powder, which was then mixed and crushed, and granulated using a spray dryer. Examples 1 to 3 and 6 to 8 are silicon nitride sintered bodies, Example 4 is an aluminum oxide sintered body, and Example 5 is a zirconium oxide sintered body. The silicon nitride sintered body contains 85 mass% or more of silicon nitride. The aluminum oxide sintered body contains 85 mass% or more of aluminum oxide. The zirconium oxide sintered body contains 85 mass% or more of zirconium oxide. When the total of the main component and sintering aids was 100 mass parts, the amount of binder added was 3 mass parts or more and 20 mass parts or less.

[0049] Next, the granulated powder was subjected to press molding. Press molding was performed using upper and lower dies. After die molding, isostatic pressing was performed. For isostatic pressing, a disc-shaped rubber mold with a Shore hardness Hs of 30 to 50 was used (shown in Figure 12). For isostatic pressing, disc-shaped rubber molds 16 and 17 were provided on both sides with hemispherical holes that were 1% to 35% larger than the diameter L1 of compact 20. Furthermore, the band-shaped portion of compact 20 (corresponding to band-shaped portion 5 of ceramic ball material 7) was positioned perpendicular to the cylindrical direction of the rubber mold. In this state, a hydrostatic pressure higher than the pressure during molding was applied during the isostatic pressing process.

[0050] Next, a sintering process was carried out at 1700°C to 1900°C in a nitrogen atmosphere at atmospheric pressure, followed by HIP treatment at 1600°C to 1900°C in a nitrogen atmosphere at a pressure of 150 MPa to 300 MPa.

[0051] The ceramic ball materials according to the examples were prepared by this process. In the comparative examples, the binder was added in an amount of 3 parts by mass when the total amount of the main component and sintering aid was 100 parts by mass. Furthermore, isostatic pressing was not performed after the molding process.

[0052] The shapes of the ceramic ball materials according to the examples and comparative examples were measured. Each sample had a band-shaped portion with a width of 0.5 mm to 3 mm and a height of 0.05 mm to 0.2 mm. The width of the band-shaped portion is shown schematically as 10 in FIG. 7, and the height of the band-shaped portion is shown schematically as 11 in FIG. 8. The measurement methods for each were as described above. The results are shown in Table 1. [Table 1]

[0053] Example 1 is a ceramic ball material for ceramic balls that will be 1 / 4 inch (6.35 mm) in size after grinding. Example 2 and Comparative Example 1 are ceramic ball materials for 5 / 16 inch (7.9375 mm) ceramic balls. Examples 3 and 7 are ceramic ball materials for 1 inch (25.4 mm) ceramic balls. Examples 4, 8 and Comparative Example 2 are ceramic ball materials for 5 / 8 inch (15.875 mm) ceramic balls. Example 5 and Comparative Example 3 are ceramic ball materials for 3 / 4 inch (19.05 mm) ceramic balls. Example 6 is a ceramic ball material for 1-3 / 8 (1 1 / 8) inch (34.925 mm) ceramic balls. All of these materials can be used as bearing balls.

[0054] In Comparative Examples 1, 2, and 3, the circularity C calculated from the above formula (1) exceeds 2.5%, and the circularity C' calculated from the above formula (2) exceeds 1.5%. In Comparative Example 3, the circularity C'' calculated from the above formula (3) does not exceed 1.5%, but exceeds 1.0%.

[0055] The polishing efficiency was evaluated using ceramic ball materials of the Examples and Comparative Examples. Each ceramic ball material was processed using a grinding wheel with a grit size of #180. One batch consisted of a number of pieces according to the size of the ceramic ball material, and the number of batches the grinding wheel could withstand was determined. The polishing process was performed so that the surface roughness Ra of the ceramic balls was 0.01 μm or less. The rate of defects, such as chipping of the ceramic ball material during the polishing process (referred to as "material chipping defect rate" in Table 2), was also determined. The average diameter variation of 10 randomly selected ceramic balls after polishing was also determined. The diameter variation was defined as the difference between the minimum and maximum diameters measured around the entire spherical surface. The results are shown in Table 2. [Table 2]

[0056] As can be seen from Table 2, when ceramic balls were obtained by grinding the ceramic ball materials according to the examples, the durability of the grindstone was improved. Furthermore, the rate of defective ceramic ball materials was reduced. Furthermore, deviation from the intended diameter after grinding was also reduced. Therefore, it can be seen that the ceramic ball materials according to the embodiments have good grinding efficiency.

[0057] 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. [Explanation of symbols]

[0058] 1...Upper mold 2...Lower mold 3...Tip of the upper mold 4...Tip of the lower mold 5...Strip 6...Spherical part 61...Upper spherical part 62...Lower spherical part 7...Ceramic ball material 8...First measurement direction 9...Second measurement direction 10...Width of the strip 11...Height of the belt 12...Measurement surface when measured from the first measurement direction 13...Measurement surface when measured from the second measurement direction 14...Radius obtained by measuring the roundness of the ceramic ball material 15...Radius obtained by measuring only the spherical portion 16...Disc-shaped rubber mold 17...Disc-shaped rubber mold 18...Space portion of the inner surface of the rubber mold 20...Molded body

Claims

1. A spherical portion; a strip-shaped portion; A ceramic ball material having When the circularity when observed from the height direction of the band-shaped portion is defined as C, the circularity C is in the range of more than 0% to 1.3% or less, The ceramic ball material is characterized in that, when the circularity C'' is defined as the circularity when observed from a direction perpendicular to the height direction of the band-shaped portion, the circularity C'' is in the range of more than 0% to 1.5%.

2. 2. The ceramic ball material according to claim 1, wherein the circularity C' of the spherical portion when observed from the height direction of the band-shaped portion is in the range of more than 0% to 1.5%.

3. 2. The ceramic ball material according to claim 1, wherein the spherical portion has a circularity C' of more than 0% and not more than 1.3% when observed from the height direction of the band-shaped portion.

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