Bearing balls and method for producing bearing balls

Abstract

The present invention addresses the problem of providing bearing balls which have high toughness even in high-temperature environments and are less apt to break. The bearing balls of the present invention are each a ceramic sphere comprising zirconia as a main component and containing CaO in an amount of 4.0-5.0 mol% and have an average diameter of 1-100 mm, a relative density of 99% or greater, and a proportion of monoclinic crystals to all the crystal structures of 1 vol% or less.

Description

Bearing ball and method of manufacturing the same

[0001] The present invention relates to a bearing ball and a method for manufacturing the same.

[0002] Ceramic spherical bodies containing zirconia as a main component are used as bearing balls for use in bearings for semiconductor manufacturing equipment, industrial machine motors, on-vehicle motors, wind power generators, etc. Examples of bearing balls using ceramic spherical bodies include zirconia-containing ceramic balls (see, for example, Patent Document 1) in which a zirconia-containing ceramic material containing 10% by volume or more of a zirconia-based ceramic phase mainly composed of zirconium oxide is used as a bearing rolling element between an inner ring and an outer ring, and in which the maximum void size observed in the surface layer region is 3 μm or less; ２ -Y ２ O ３ The zirconia-based sintered body is composed of a zirconia-based sintered body (see, for example, Patent Document 2), and contains stabilizers CaO and Y. ２ O ３ , Er ２ O ３ , or Yb ２ O ３ and zirconia bearing balls (see, for example, Patent Document 2) that contain the above-mentioned material and have specified properties such as crushing load value, media diameter, fracture toughness, and bending strength.

[0003] Japanese Patent Application Publication No. 2001-294483 International Publication No. 2023 / 210268

[0004] Bearing balls used in industrial machinery motors, automotive motors, wind power generation, and other applications are generally large, with diameters ranging from 1 to 100 mm. Large-diameter bearing balls with diameters of 1 mm or more have a greater weight and higher impact energy than small and medium-diameter bearing balls, making them more susceptible to cracking and requiring high toughness. Furthermore, because bearing balls are continuously subjected to impact energy, they are prone to high temperatures, and therefore require durability that prevents cracking even in high-temperature environments.

[0005] SUMMARY OF THE INVENTION An object of the present invention is to provide a bearing ball that has high toughness even in a high-temperature environment and is less likely to crack.

[0006] In order to solve the above problems, the present invention and its preferred embodiments mainly have the following configurations: <1> A bearing ball made of ceramic spheres containing zirconia as a main component and 4.0 mol % to 5.0 mol % of CaO, having an average diameter of 1 mm to 100 mm, a relative density of 99% or more, and a monoclinic phase ratio of 1 vol % or less to the total crystalline structure. <2> The bearing ball according to <1>, in which the monoclinic phase ratio of the total crystalline structure is 1 vol % or less after heat treatment at a temperature of 200°C for 1,000 hours. <3> The bearing ball has a fracture toughness of 10 MPa m after heat treatment at a temperature of 200°C for 1,000 hours, as measured by the IF method (JIS R1607:2015). １／２ <4> The bearing ball according to any one of <1> to <3>, wherein the number of voids with a maximum length of 5 μm or more within an area of ​​200 μm × 200 μm of the cross section of the sphere is less than one. <5> The bearing ball according to any one of <1> to <4>, wherein the coefficient of variation Y of the crushing strength satisfies the following formula (2): Y≦0.20 (2) <6> The method for producing a bearing ball according to any one of <1> to <5>, comprising, in this order: a molding step of molding zirconia powder containing zirconia as a main component and 4.0 mol% to 5.0 mol% of CaO into a spherical shape to obtain a green body; a CIP step of cold isostatically pressing the green body; a sintering step of sintering the green body after the CIP step to obtain a sintered body; and a HIP step of hot isostatically pressing the sintered body. <7> The method for producing a bearing ball according to <6>, wherein the molding step is performed by die press molding at a pressure of 25 MPa or more.

[0007] The bearing ball of the present invention has high toughness even in high temperature environments and is less likely to crack.

[0008] 1 is a graph showing the relationship between average diameter X and crushing strength W in Examples 1 and 9 to 11.

[0009] The bearing ball of the present invention is made of a ceramic spherical body containing zirconia as the main component and 4.0 mol % to 5.0 mol % of calcia. In this specification, "containing zirconia as the main component" means containing 90 mol % or more of zirconia. In addition, in this specification, "ceramic spherical body" refers to a body after sintering, and "green body" refers to a body before sintering.

[0010] The bearing ball of the present invention preferably contains 94 mol % or more of zirconia from the viewpoint of further improving toughness in a high-temperature environment.

[0011] Calcia acts as a stabilizer. Known stabilizers for ceramic spheres include yttria, ceria, alumina, and magnesia, with yttria being widely used. In the present invention, by selecting calcia as a stabilizer, the rate of localized zirconia transformation from tetragonal to monoclinic crystals due to the application of external stress is increased compared to when yttria is used as a stabilizer. This suppresses stress propagation due to volume expansion associated with the transformation, thereby further improving toughness in high-temperature environments. If the calcia content is less than 4.0 mol%, the proportion of monoclinic crystals increases after sintering, reducing toughness in high-temperature environments and making the material more susceptible to cracking. On the other hand, if the calcia content exceeds 5.0 mol%, the zirconia content decreases relatively, reducing toughness in high-temperature environments due to zirconia, making the material more susceptible to cracking. The calcia content is preferably 4.6 mol% or less.

[0012] In the present invention, the ceramic spherical bodies may contain other stabilizers in addition to calcia. However, since yttria contains yttrium, which is a rare earth, it is preferable that the ceramic spherical bodies are substantially free of yttria from the viewpoints of cost reduction and environmental load reduction, and the yttria content is preferably 0.1 mol% or less.

[0013] The ceramic spheres may further contain alumina, which can lower the sintering temperature. If alumina is contained, the alumina content is preferably 0.2 mol% or more, more preferably 0.4 mol% or more. On the other hand, the alumina content is preferably 1.0 mol% or less, more preferably 0.8 mol% or less.

[0014] The content (mol %) of each component in the ceramic spheres can be determined as follows. First, the ceramic spheres are crushed using a universal testing machine, and approximately 0.3 g of crushed pieces are placed in a platinum crucible and melted with potassium hydrogen sulfate. This is then dissolved in dilute nitric acid to obtain a constant solution, and each metal element is quantified using ICP atomic emission spectrometry. The determined amount of metal element is converted to its oxide, and then further divided by the molecular weight of each component to convert it to mol %.

[0015] The average diameter of the bearing balls of the present invention is 1 mm or more and 100 mm or less. Such large-diameter ceramic spheres have a larger weight and higher impact energy than small- to medium-diameter ceramic spheres, which has traditionally been problematic in that they are prone to cracking, particularly in high-temperature environments. However, the present invention has high toughness and is less likely to crack, even in high-temperature environments, making it particularly effective for large-diameter bearing balls.

[0016] Here, the average diameter of the bearing balls can be measured using a vernier caliper. For 10 randomly selected bearing balls, the lengths in three orthogonal axial directions are measured using the vernier caliper, the average of the measured values ​​is taken as the diameter of each bearing ball, and the number average of the measured values ​​is taken as the average diameter.

[0017] The bearing ball of the present invention has a relative density of 99% or more. The relative density is an index of the degree of sintering, expressed as a percentage of the sintered density relative to the theoretical sintered density. If the relative density is less than 99%, the particles that form the bearing ball are not sintered together sufficiently, resulting in insufficient toughness in high-temperature environments and making the ball more susceptible to cracking. A relative density closer to 100% is more preferable.

[0018] Here, the relative density of the bearing ball can be calculated by the following formula (3): Relative density (%) = (sintered density / theoretical sintered density) × 100 (3) The sintered density is measured by the Archimedes method specified in JIS Z8807:2012. The theoretical sintered density is calculated by the following formula (4): Theoretical sintered density [g / cm ３ ]=6.17[g / cm ３ ]-0.04[g / cm ３ mol%] × CaO molar concentration [mol%] (4) For example, if the CaO content is 4.0 mol%, the theoretical sintered density is 6.01 g / cm ３ , 5.0 mol% is 5.97 g / cm ３ is.

[0019] A method for achieving a relative density of 99% or more includes, for example, a method in which the heating temperature and pressure in the HIP process are set within the preferred ranges described below in the manufacturing method of bearing balls described below.

[0020] The proportion of monoclinic crystals in the entire crystal structure of the bearing ball of the present invention is 1% by volume or less. If the monoclinic crystals exceed 1% by volume, deterioration is likely to occur starting from the monoclinic crystals, resulting in insufficient toughness in high-temperature environments and making the ball more susceptible to cracking in high-temperature environments. The smaller the proportion of monoclinic crystals, the better.

[0021] Here, the proportion of monoclinic crystals can be measured by X-ray diffraction. A thin plate-like test piece having a thickness of 1 mm is cut out from the bearing ball and attached to a sample holder of an X-ray diffractometer (for example, MiniFlex 600 manufactured by Rigaku Corporation). The diffraction intensity of each crystalline phase is measured by wide-angle X-ray diffraction (microscopic X-ray diffraction), and the proportion of monoclinic crystals relative to the entire crystalline structure is calculated. However, if the raw materials and manufacturing method of the bearing ball are known, the proportion of monoclinic crystals relative to the entire crystalline structure may be calculated using a thin plate-like test piece having a thickness of 1 mm produced using the same raw materials and manufacturing method.

[0022] Examples of methods for making the proportion of monoclinic crystals 1% by volume or less include a method in which the calcia concentration is set within the above-mentioned range, and a method in which the thermal history (heating temperature, heating time) in the sintering step and the HIP step is set within the preferred ranges described below.

[0023] In the bearing ball of the present invention, it is preferable that the number of voids (number of internal voids) with a maximum length of 5 μm or more within an area of ​​200 μm × 200 μm of the cross section of the sphere is less than one, which further improves toughness in high-temperature environments and makes it possible to further suppress cracking.

[0024] The number of internal voids in a bearing ball can be measured using the following method. A bearing ball is ground using a grinding machine to a size of 40 to 60% of its pre-grinding diameter, and then finished by polishing it for 10 minutes or more using a diamond slurry with a particle size of 6 μm to obtain a cross section. The obtained cross section is observed using a digital microscope at a magnification of 10 to 200 times within a field of view of 200 μm x 200 μm, the maximum length of the voids is measured, and the number of voids with a maximum length of 5 μm or more is counted.

[0025] An example of a method for reducing the number of internal voids to less than 1 is a method of carrying out a HIP process, which will be described later, in the method for producing a bearing ball, which will be described later. As will be described later, the ceramic raw material powder preferably used in the present invention has a larger than usual specific surface area to enable low-temperature sintering at the 1,200°C level, and therefore the raw material powder (granules) are less likely to be crushed during press molding, which will be described later, and gaps between the powder particles are likely to remain as voids even after sintering, making densification by a HIP process particularly useful.

[0026] The fracture toughness value of the bearing ball of the present invention obtained by the IF method of JIS R1607:2015 is 10 MPa m １／２ The fracture toughness value is preferably 15 MPa m or more, and cracking in a high-temperature environment can be further suppressed. １／２ The above is more preferable.

[0027] Here, the fracture toughness value of a bearing ball can be measured by the following method. After using a grinding machine to grind a bearing ball to a size of 40 to 60% of the diameter before grinding, a diamond slurry with a particle size of 6 μm is used to finish polish it for 10 minutes or more to obtain a cross section. Five locations are randomly selected from the obtained cross section, and a diamond indenter is pressed into the surface using a microhardness tester according to the IF method of JIS R1607:2015 to measure the crack length and indentation length generated, and the fracture toughness value is calculated using the following formula, and the number average value is calculated. K = 0.018 × (E / H) ０．５ × (P / c １．５ ) K: Fracture toughness value (unit: MPa m １／２ E: Young's modulus (unit: GPa): 230 GPa measured by another evaluation method H: Vickers hardness (unit: HV) = 0.1891 × P / (2a) ２ c: Half the average crack length (unit: m) a: Half the average indentation length (unit: m) P: Indenter pressing load (unit: N): 98N.

[0028] Fracture toughness value of 10 MPa m １／２ As a method for achieving the above, for example, a method in which the calcia content and relative density are set within the above ranges can be mentioned.

[0029] In the bearing ball of the present invention, it is preferable that the average diameter X (mm) and the crushing strength W (kN) satisfy the following formula (1). By doing so, the crushing strength in a high-temperature environment can be improved. ２ +0.92X≦W×1.2 (1) The coefficient 1.2 multiplied by W on the right side of the equation represents a manufacturing margin that includes variations in dimensional accuracy, press pressure accuracy, and the like during the manufacturing process.

[0030] The crushing strength W of the bearing balls can be determined by clamping 20 randomly selected bearing balls between cylindrical zirconia jigs having a diameter of 20 mm, applying a compressive load at a rate of 0.5 mm / min using an electronic universal testing machine, measuring the load value at which the balls break, and calculating the number average value.

[0031] Since the crushing strength is proportional to the area of ​​the bearing ball, i.e., the square of the diameter of the bearing ball, the inventors derived formula (1) by obtaining an approximation curve using the square of the average diameter from the actual measurement values ​​of the crushing strength of bearing balls with an average diameter of 1 to 10 mm.

[0032] It is also preferable that the coefficient of variation Y of the crushing strength satisfies the following formula (2), which allows for improved crushing strength in high-temperature environments. In particular, bearing balls obtained by a manufacturing method including a molding step of press-molding zirconia powder into a spherical shape tend to have a lower degree of densification in the portions corresponding to the gaps in the mold (band portions) than in other portions when the pressing pressure is low, and therefore tend to have anisotropic crushing strength. However, by satisfying the following formula (2), the bearing balls are less likely to be crushed regardless of the direction of the load on them: Y≦0.20 (2).

[0033] Examples of methods for satisfying the above formulas (1) and (2) include a method of minimizing the thermal history in the sintering step and HIP step described below to a range necessary for bonding primary particles and eliminating internal voids, and a method of setting the pressure used in molding into a sphere by a mold molding method within a preferred range described below.

[0034] The bearing ball of the present invention preferably has a monoclinic crystal structure with a ratio of 1% by volume or less to the total crystal structure after heat treatment at 200°C for 1,000 hours. Furthermore, the fracture toughness value obtained by the IF method of JIS R1607:2015 after heat treatment at 200°C for 1,000 hours is 10 MPa m １／２ Heat treatment under these conditions simulates the thermal history of a bearing ball used in a bearing under high temperature conditions, and when the proportion of monoclinic crystals under these conditions is 1% by volume or less, deterioration originating from the monoclinic crystals is suppressed, toughness under high temperature environments is further improved, and cracking under high temperature environments can be further suppressed. In addition, the fracture toughness value under these conditions is 10 MPa m １／２ When the fracture toughness value is 15 MPa m or more, the toughness in a high-temperature environment is higher, and cracking in a high-temperature environment can be further suppressed. １／２The above is more preferable.

[0035] Here, the proportion of monoclinic crystals in the total crystal structure and the fracture toughness value after heat treatment at a temperature of 200°C for 1,000 hours can be measured by heat treating the bearing ball at a temperature of 200°C for 1,000 hours and then using the same method as for measuring the proportion of monoclinic crystals and the fracture toughness value described above.

[0036] After heat treatment at 200°C for 1,000 hours, the proportion of monoclinic crystals is 1% by volume or less, and the fracture toughness is 10 MPa m １／２ As a method for achieving the above, for example, a method of setting the calcia concentration in the above-mentioned range can be mentioned. In addition, it is preferable to keep the thermal history in the sintering step and HIP step described later to a small value within a range necessary for bonding primary particles and eliminating internal voids, and a method of setting the maximum temperature and heating time in each step within the preferred ranges described later can be mentioned.

[0037] The bearing ball of the present invention can be obtained, for example, by forming a zirconia powder containing zirconia as the main component and 4.0 mol% to 5.0 mol% of CaO into a spherical shape and then sintering it. The process preferably includes, in this order: a molding step of forming a zirconia powder (hereinafter sometimes referred to as "raw material powder") containing zirconia as the main component and 4.0 mol% to 5.0 mol% of CaO into a spherical shape to obtain a molded body; a cold isostatic pressing (CIP) step of cold isostatically pressing the molded body; a sintering step of sintering the molded body after the CIP step to obtain a sintered body; and a hot isostatic pressing (HIP) step of hot isostatically pressing the sintered body. A preliminary polishing step of polishing the molded body may also be included, which can remove burrs and other impurities from the mold during molding. In this case, a semi-baking step of heating the molded body may be performed before the preliminary polishing step, thereby improving the strength of the spherical body in the preliminary polishing step. Furthermore, if the molded body contains a binder resin such as a binding agent, a degreasing step of heating and removing the binder resin may be performed before the sintering step. Furthermore, a polishing step may be performed after sintering to smooth the surface.

[0038] The raw material powder may further contain 0.2 to 1.0 mol % of alumina.

[0039] The raw material powder may also contain an organic component. Examples of the organic component include a binder resin and a mold release agent. The binder resin has the function of binding the raw material powder together, thereby maintaining the strength of the molded body. The mold release agent has the function of making it easier to release the molded body from the molding die. The content of the organic component in the raw material powder is preferably 1 to 5 wt %, more preferably 3 to 5 wt %. The content of the binder resin is preferably 3 to 5 wt %.

[0040] First, in the molding process, raw material powder is molded into a spherical shape. From the viewpoint of dimensional accuracy, it is preferable to mold the spherical shape using mold press molding. More specifically, the raw material powder is filled into a hemispherical mold and then pressed from above and below using another hemispherical mold of the same shape and size. From the viewpoint of densification, the pressing pressure is preferably 10 MPa or more. Furthermore, the higher the pressing pressure, the stronger the band portion of the spherical body corresponding to the gap in the mold becomes, improving the crushing strength when a load is applied to the bearing ball. This allows the average diameter X and the crushing strength W to satisfy the relationship of formula (1). Furthermore, the anisotropy of the crushing strength in the load direction is suppressed, allowing the coefficient of variation Y of the crushing strength to satisfy formula (2). From the viewpoint of satisfying the above-mentioned formulas (1) and (2), the pressing pressure is more preferably 25 MPa or more, and even more preferably 30 MPa or more. On the other hand, from the viewpoint of releasability when removing the spherical body after mold molding, the pressing pressure is preferably 50 MPa or less.

[0041] In the compacting step, it is preferable to compact with a dimensional accuracy of about ±5%. The density of the compacted body is generally 1.2 g / cm of the raw material powder. ３ From about 2.5 g / cm ３ It is densified to some extent.

[0042] Next, in the CIP process, the molded body is preferably vacuum-packed and subjected to a CIP device for pressure application. The pressure in the CIP process is preferably about 50 to 200 MPa, and the pressure application time is preferably about 1 to 10 minutes. The density of the molded body is generally 2.8 g / cm3 by the CIP process. ３ It is densified to some extent.

[0043] Next, in the debinding step, it is preferable to heat and remove the binder resin inside the molded body. In the debinding step, it is preferable to raise the temperature to a maximum temperature of about 400 to 600° C. The temperature rise rate is preferably about 5 to 15° C. / hour from the viewpoint of suppressing cracking, and it is preferable to gradually remove the binder resin inside the molded body.

[0044] Next, in the semi-baking step, the molded body is preferably heated. In the semi-baking step, from the viewpoint of appropriately hardening the molded body, it is preferable to raise the temperature to a maximum temperature of about 950 to 1,050°C. From the viewpoint of suppressing the occurrence of cracks, the temperature rise rate is preferably about 5 to 15°C / hour. The heating time after reaching the maximum temperature is preferably 1 hour or more. The semi-baking step may be carried out consecutively to the degreasing step.

[0045] Next, in a preliminary polishing step, it is preferable to remove burrs and the like that are generated by the mold during molding. Examples of the preliminary polishing method include barrel polishing using water. If necessary, an abrasive such as alumina particles may be used.

[0046] Next, in the sintering process, the compact that has undergone the molding process and, if necessary, the CIP process, debinding process, semi-firing process, and preliminary polishing process is placed in a sagger or the like and fired in a firing furnace, thereby bonding the raw material powder and obtaining spherical ceramics. Note that, if the semi-firing and preliminary polishing processes are not performed, the sintering process may be performed consecutively to the debinding process. The maximum temperature reached in the firing process is preferably 1,150 to 1,300°C, and the sintering time is preferably 0.5 to 4 hours. By setting the maximum temperature to 1,150°C or higher, densification can be promoted and density can be improved. The maximum temperature is more preferably 1,200°C or higher. On the other hand, the maximum temperature is preferably 1,250°C or lower. This suppresses thermal diffusion of calcia, uniforming the calcia concentration distribution, making it possible to easily reduce the occurrence of monoclinic crystals in the bearing balls to 1% by volume or less, and also to easily reduce the proportion of monoclinic crystals to 1% by volume or less after the bearing balls are heat-treated at 200°C for 1,000 hours.

[0047] Next, in the HIP process, high temperature and isotropic pressure are applied simultaneously, thereby further densifying the spherical bodies. The HIP process removes defects such as voids and cracks remaining inside without changing the shape, further improving the relative density of the bearing ball and its toughness under high-temperature conditions, and further suppressing cracking. Note that if the HIP process is performed before the sintering process, the gas acting as the pressure medium will penetrate into the pores inside the compact, which has a low relative density, and will not act as a force to compress the compact. Therefore, in the manufacturing method of the bearing ball of the present invention, it is preferable to perform the HIP process after the sintering process.

[0048] Furthermore, the maximum temperature reached in the HIP process is preferably 1,150 to 1,300°C. By setting the maximum temperature reached in the HIP process to 1,150°C or higher, the diffusion of ceramic powder such as zirconia during the HIP treatment can be sufficiently promoted, the toughness under high temperature conditions can be further improved, and cracking under high temperature conditions can be further suppressed. On the other hand, by setting the maximum temperature reached in the HIP process to 1,300°C or lower, more preferably 1,250°C or lower, the generation of monoclinic crystals due to the thermal diffusion of calcia can be suppressed.

[0049] The heating time in the HIP step is preferably 0.5 to 4 hours.

[0050] The pressure in the HIP process is preferably 100 MPa or more. In order to achieve a high pressure state, the HIP process is preferably performed in an Ar gas atmosphere.

[0051] For example, if the heating time after reaching the maximum temperature in both the sintering step and the HIP step is 2 hours, it is preferable to keep the maximum temperature in both the sintering step and the HIP step at 1,200°C or higher and 1,250°C or lower. By setting the maximum temperature in both the sintering step and the HIP step to 1,200°C or higher, bonding of primary particles is promoted, the relative density and toughness in high-temperature environments are further improved, and cracking in high-temperature environments can be further suppressed. On the other hand, by setting the maximum temperature in both the sintering step and the HIP step to 1,250°C or lower, calcia is uniformly thermally diffused, zirconia crystals are maintained in a tetragonal system, and transformation to monoclinic crystals in high-temperature environments is suppressed, thereby further suppressing cracking in high-temperature environments. Similarly, if the heating time to the maximum temperature in both the sintering step and the HIP step is 1 hour, the maximum temperature in both the sintering step and the HIP step is preferably 1,225°C or higher and 1,275°C or lower, and if the heating time to the maximum temperature in both the sintering step and the HIP step is 0.5 hour, the maximum temperature in both the sintering step and the HIP step is preferably 1,250°C or higher and 1,300°C or lower.

[0052] Next, in the polishing step, the surface of the resulting ceramic spherical bodies is polished to reduce the surface roughness and friction during ball rotation. Examples of surface polishing equipment include wet polishing equipment such as a barrel polishing machine and a ball mill.

[0053] After the polishing step, a cleaning step for removing abrasive residue and a drying step may be further carried out.

[0054] The present invention will be specifically described below based on examples, but the present invention should not be construed as being limited to these examples. Evaluation methods in each example and comparative example are as follows.

[0055] (Composition of Raw Material Powder) The content of each component in the ceramic spheres coincides with the content of the inorganic components in the raw material powder, so the composition of the inorganic components in the raw material powder was determined.

[0056] Approximately 0.3 g of the raw material powder used in each example and comparative example was placed in a platinum crucible and melted with potassium hydrogen sulfate. This was dissolved in dilute nitric acid to a constant solution, and each metal element was quantified using ICP atomic emission spectroscopy. The amount of the metal element thus determined was converted to its oxide, and then divided by the molecular weight of each component to convert it to mol%.

[0057] (Specific Surface Area) The specific surface area of ​​the raw material powders used in each of the Examples and Comparative Examples was measured using a specific surface area measuring device (FlowSorbIII2305, manufactured by Shimadzu Corporation) according to the constant volume method of JIS R1626:1996 "Method for measuring the specific surface area of ​​fine ceramic powders by the gas adsorption BET method."

[0058] (Average diameter) Ten bearing balls were randomly selected from the bearing balls obtained in each Example and Comparative Example, and their lengths in three orthogonal axial directions were measured using a vernier caliper, and the average value was taken as the diameter. The number average value of the diameters of the ten balls was calculated and taken as the average diameter.

[0059] (Relative Density) The sintered density of the bearing balls obtained in each Example and Comparative Example was measured by the Archimedes method specified in JIS Z8807:2012. The theoretical sintered density in each Example and Comparative Example was calculated using the following formula (4). Theoretical sintered density [g / cm ３ ]=6.17[g / cm ３ ]-0.04[g / cm ３ mol %]×CaO molar concentration [mol %] (4) The relative density was calculated from the obtained sintered density and the theoretical sintered density using the following formula (3): relative density (%)=(sintered density / theoretical sintered density)×100 (3).

[0060] (Monoclinic Crystal Ratio) Using the raw material powder used in each Example and Comparative Example, thin plate-like pellets with a thickness of 1 mm were produced by the same production method as in each Example and Comparative Example. The obtained thin plate-like pellets were attached to a sample holder of an X-ray diffractometer (MiniFlex 600, manufactured by Rigaku Corporation), and the diffraction intensity of each crystalline phase was measured by wide-angle X-ray diffraction (micro-area X-ray diffraction) under the following measurement conditions: X-ray source: CuK ray (using a multilayer mirror); Output: 50 kV, 22 mA; Slit system: 100 μmφ pinhole; Measurement range: 2θ = 23° to 33°, 70° to 77°; Accumulation time: 3600 seconds / frame.

[0061] From the measurement results, the monoclinic ratio of zirconia was calculated using the following formula: Monoclinic ratio (%) = [{I ｍ (111) + I ｍ (1-1-1)} / {I ｍ (111) + I ｍ (1-1-1) + I ｔ＋ｃ (111)} × 100 where I indicates the diffraction intensity. The subscripts m, t, and c indicate monoclinic, tetragonal, and cubic crystals, respectively. The parentheses for the diffraction intensity indicate the index of each crystal.

[0062] (Number of Internal Voids) The bearing balls obtained in each Example and Comparative Example were ground using a grinding machine to a size of 40 to 60% of their pre-grinding diameter, and then further polished for 10 minutes or more using a diamond slurry with a particle size of 6 μm to obtain a cross section. The obtained cross section was observed using a digital microscope at a magnification of 10 to 200 times within a field of view of 200 μm × 200 μm, the maximum length of the voids was measured, and the number of voids with a maximum length of 5 μm or more was counted to obtain the number of internal voids.

[0063] (Fracture toughness value) The bearing balls obtained in each example and comparative example were ground using a grinding machine to a size of 40 to 60% of the diameter before grinding, and then further polished for 10 minutes or more using a diamond slurry with a particle size of 6 μm to obtain a cross section. Five points randomly selected from the obtained cross section were measured according to the IF method of JIS R1607:2015 using a microhardness tester (HM2000XYp manufactured by Fisher Instruments Co., Ltd.) to press a diamond indenter into the surface, and the crack length and indentation length generated were measured, and the fracture toughness value was calculated using the following formula. K = 0.018 × (E / H) ０．５ × (P / c １．５ ) K: Fracture toughness value (unit: MPa m １／２ E: Young's modulus (unit: GPa): 230 GPa measured by another evaluation method H: Vickers hardness (unit: HV) = 0.1891 × P / (2a) ２ c: Half the average crack length (unit: m) a: Half the average indentation length (unit: m) P: Indenter pressing load (unit: N): 98N.

[0064] (Crushing Strength) Twenty bearing balls randomly selected from the bearing balls obtained in each Example and Comparative Example were clamped in a cylindrical jig made of zirconia having a diameter of 20 mm, and a compressive load was applied at a rate of 0.5 mm / min using an electronic universal testing machine (CATY-2000YD, manufactured by Yonekura Seisakusho Co., Ltd.), the load value at which they broke was measured, and the number average value was calculated to obtain the crushing strength W. In addition, the coefficient of variation Y (standard deviation / average value) was determined as an index of uniformity of the crushing strength.

[0065] (Monoclinic Crystal Ratio After Heat Treatment) Using the raw material powder used in each Example and Comparative Example, thin plate-like pellets having a thickness of 1 mm were produced by the same production method as in each Example and Comparative Example. The obtained thin plate-like pellets were subjected to heat treatment at 200°C for 1,000 hours using a constant temperature dryer (EO-300V) manufactured by AS ONE Corporation, and then the monoclinic crystal ratio after heat treatment was measured in the same manner as the above-mentioned method for evaluating the monoclinic crystal ratio.

[0066] (Fracture toughness value after heat treatment) The bearing balls obtained in each of the examples and comparative examples were heat treated at 200°C for 1,000 hours using a constant temperature dryer (EO-300V, manufactured by AS ONE Corporation), and then the fracture toughness value after the heat treatment was measured in the same manner as described above for the fracture toughness value. This fracture toughness value after heat treatment was used as an index of toughness in a high-temperature environment.

[0067] (Collapse Strength After Heat Treatment) The bearing balls obtained in each of the Examples and Comparative Examples were heat treated at 200°C for 1,000 hours using a constant temperature dryer (EO-300V, manufactured by AS ONE Corporation), and then the collapse strength after heat treatment was measured in the same manner as in the method described above for the collapse strength.

[0068] (Number of cracks after heat treatment) The bearing balls obtained in each of the examples and comparative examples were heat treated at 200°C for 1,000 hours using a constant temperature dryer (EO-300V, manufactured by AS ONE Corporation), and then a diamond indenter was pressed into the surface 10 times under a load of 294 N. The surface was then observed using a digital microscope and the number of cracks in the indenter mark was counted. This number of cracks was used as an index of the susceptibility to cracking in a high-temperature environment.

[0069] (Surface Peeling) Ten bearing balls were randomly selected from the bearing balls obtained in each Example and Comparative Example, and were immersed in a fluorescent flaw detection liquid (OD-2800III manufactured by Marktec Co., Ltd.) for one minute, water was replaced, the balls were washed with running water, and the balls were air-dried. Thereafter, the surfaces were visually observed under black light irradiation, and the glowing areas were counted as surface peeling areas, and the number of such areas was counted and the average value was taken as the number of surface peeling areas.

[0070] [Example 1] (Raw material powder) A powder having a specific surface area of ​​22 m2 measured by the BET method ２ / g, and the molar ratio of oxides in the resulting ceramic molded body was calcia / zirconia / alumina = 4.2 / 95.2 / 0.6. "HSY-0480" manufactured by Daiichi Kigenso Kagaku Kogyo Co., Ltd., which contains about 3 wt% of an acrylic resin as a binder resin, was used as the raw material powder.

[0071] (Molding step) The raw material powder was molded into spheres with an average diameter of about 14 mm by die press molding, with a pressure (pressing pressure) applied per sphere being 30 MPa, to obtain molded bodies.

[0072] (CIP Step) The obtained compacts were individually vacuum-packed and pressed at a pressure of 100 MPa for 5 minutes using a CIP device to densify the compacts.

[0073] (Degreasing Step) After the CIP step, the molded body was heated in an electric furnace at a temperature rising rate of 10° C. / hour to a final temperature of 500° C., and then kept at a constant temperature for 2 hours to remove the binder resin inside the molded body.

[0074] (Semi-baking step) Following the degreasing step, the temperature was raised in an electric furnace at a rate of 10°C / hour to a final temperature of 1,000°C, and the temperature was maintained constant for 2 hours to obtain a semi-baked body.

[0075] (Preliminary Polishing Step) 60 kg of the obtained semi-fired body and water (approximately half the capacity of the polishing machine) were filled into a barrel polishing machine (Yamashina Seiki Co., Ltd., tiltable barrel polishing machine 120HD), and co-polishing was carried out for 14 hours at a rotation speed of 100 rpm until band-like burrs that had occurred at the interface between the upper and lower molds during molding could no longer be seen with the naked eye.

[0076] (Sintering step) Thereafter, the temperature was increased in an electric furnace at a rate of 70°C / hour to a maximum temperature (sintering temperature) of 1,225°C, and the temperature was kept constant for 2 hours to obtain a dense ceramic sintered body.

[0077] (HIP Process) The ceramic sintered body was further subjected to HIP treatment in an Ar atmosphere under conditions of 117 MPa and a maximum temperature (HIP temperature) of 1,200° C., where the temperature was kept constant for 2 hours.

[0078] (Polishing process) The ceramic sintered body obtained by the above process, water, 0.5 wt% of a surfactant (Cerna D-305, manufactured by Chukyo Yushi Co., Ltd.), and 3 wt% of silicon carbide particles (GC-8000 (F), average diameter 1 μm, end-treated, manufactured by Tipton Co., Ltd.) as an abrasive were loaded into a barrel polishing machine (Yamashina Seiki Co., Ltd., tiltable barrel polishing machine 120HD), and polished for 4 hours at a rotation speed of 100 rpm. Here, the amount of surfactant and abrasive added was expressed as wt% of the total weight of the polishing slurry consisting of water, surfactant, and abrasive. After completion, the polishing slurry was replaced with new one, and the same polishing process was carried out, and this was repeated for a total of 24 hours. Thereafter, the polishing slurry in the machine was replaced with water, and the bearing ball was again polished for 2 hours at a rotation speed of 100 rpm to remove abrasive residue, followed by washing and drying to obtain a bearing ball.

[0079] Examples 2 and 3 Bearing balls were produced in the same manner as in Example 1, except that the sintering temperature in the sintering step was changed as shown in Table 1.

[0080] Example 4 Bearing balls were produced in the same manner as in Example 1, except that the calcia content in the raw material powder was changed as shown in Table 1.

[0081] Examples 5 to 8 Bearing balls were produced in the same manner as in Example 1, except that the pressing pressure in the molding step was changed as shown in Table 1.

[0082] Examples 9 to 11 Bearing balls were produced in the same manner as in Example 1, except that the die used for die press molding in the molding step was changed so that the average diameter would be as shown in Table 2.

[0083] Example 12 Bearing balls were produced in the same manner as in Example 1, except that the sintering temperature in the sintering step was changed as shown in Table 1.

[0084] Comparative Example 1 A bearing ball was produced in the same manner as in Example 1, except that the HIP process was not carried out.

[0085] Comparative Examples 2 and 3 Bearing balls were produced in the same manner as in Example 1, except that the sintering temperature in the sintering step and / or the HIP temperature in the HIP step were changed as shown in Table 1.

[0086] Comparative Examples 4 and 5 Bearing balls were produced in the same manner as in Example 1, except that the calcia content in the raw material powder was changed as shown in Table 2.

[0087] Comparative Example 6 The raw material powder was changed to "HSY-3FSD-J" manufactured by Daiichi Kigenso Kagaku Kogyo Co., Ltd., which had a molar ratio of yttria / zirconia / alumina = 3.2 / 96.5 / 0.3 in terms of oxides in the resulting ceramic molded body and contained approximately 3% by weight of acrylic resin as the binder resin. In addition, due to the decrease in the specific surface area of ​​the raw material powder, the sintering temperature and HIP temperature were changed to 1400°C and 1390°C, respectively. Otherwise, bearing balls were produced in the same manner as in Example 1.

[0088] Comparative Example 7 Bearing balls were produced in the same manner as in Comparative Example 6, except that the yttria content in the raw material powder was changed as shown in Table 1.

[0089] The main configurations and evaluation results of each example and comparative example are shown in Tables 1 to 3. In the tables, the sintering temperature and HIP temperature represent the maximum temperatures reached in the sintering and HIP processes, respectively, and the sintering time and HIP time represent the holding times at the maximum temperatures reached in the sintering and HIP processes, respectively.

[0090] The relationship between the average diameter X and the crushing strength W in Examples 1 and 9 to 11 is shown in Figure 1. The main conditions and evaluation results for each Example and Comparative Example are shown in Tables 1 to 3.

[0091]

[0092]

[0093]

Claims

1. A bearing ball consisting of a ceramic sphere containing zirconia as the main component and 4.0 mol% to 5.0 mol% of CaO, with an average diameter of 1 mm to 100 mm, a relative density of 99% or more, and a monoclinic crystal ratio of 1% by volume or less to the total crystal structure.

2. The bearing ball according to claim 1, wherein the proportion of monoclinic crystals in the total crystal structure is 1% by volume or less after heat treatment at 200°C for 1,000 hours.

3. After heat treatment at 200°C for 1,000 hours, the fracture toughness value obtained by the IF method (JIS R1607:2015) is 10 MPa m １／２ 3. The bearing ball according to claim 1 or 2.

4. A bearing ball according to claim 1 or 2, wherein the number of voids having a maximum length of 5 μm or more within an area of ​​200 μm×200 μm of the cross section of the sphere is less than one.

5. The bearing ball according to claim 1 or 2, wherein the coefficient of variation Y of the crushing strength satisfies the following formula (2): Y≦0.20 (2) 6. A method for manufacturing a bearing ball as described in claim 1 or 2, comprising, in this order, a molding step of forming zirconia powder, the main component of which is zirconia and containing 4.0 mol% to 5.0 mol% of CaO, into a spherical shape to obtain a green body; a CIP step of cold isostatically pressing the green body; a sintering step of sintering the green body after the CIP step to obtain a sintered body; and a HIP step of hot isostatically pressing the sintered body.

7. The method for manufacturing a bearing ball according to claim 6, wherein the molding step is performed by die press molding at a pressure of 25 MPa or more.

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