Ceramic sintered compact, bearing ball, and cutting tool

Abstract

This ceramic sintered compact contains: alumina crystal particles having pores; and zirconia crystal particles. The total surface area of the pores in the alumina crystal particles is at least 95% of the total surface area of pores contained in a cross section of the ceramic sintered compact.

Description

Sintered ceramics, bearing balls, and cutting tools

[0001] The present invention relates to a ceramic sintered body, a bearing ball, and a cutting tool.

[0002] BACKGROUND ART Alumina-based ceramic sintered bodies have been known for some time (for example, see Patent Document 1).

[0003] Japanese Patent Application Laid-Open No. 2016-132577

[0004] However, even with the prior art such as that of Patent Document 1, there is still room for improvement in the technique for improving the strength of a ceramic sintered body.

[0005] The present invention has been made to solve the above-mentioned problems, and an object of the present invention is to provide a technique for improving the strength of a ceramic sintered body.

[0006] The present invention has been made to solve at least part of the above-mentioned problems, and can be realized in the following aspects.

[0007] (1) According to one aspect of the present invention, there is provided a ceramic sintered body comprising alumina crystal grains having pores and zirconia crystal grains, wherein the total area of ​​the pores in the alumina crystal grains in a cross section of the ceramic sintered body is 95% or more of the total area of ​​the pores in the cross section.

[0008] According to this configuration, in a ceramic sintered body containing alumina crystal grains having pores and zirconia crystal grains, the total area of ​​the pores in the alumina crystal grains in the cross section accounts for 95% or more of the total area of ​​the pores in the cross section. In other words, a relatively large number of pores are present inside the alumina crystal grains. As a result, when an external force is applied to the ceramic sintered body, crack propagation is less likely to progress than in cases where pores are present at the grain boundaries. Therefore, the strength of the ceramic sintered body can be improved.

[0009] (2) In the ceramic sintered body of the above embodiment, the equivalent circle diameter of the pores included in the cross section may be 0.3 μm or less. According to this configuration, the equivalent circle diameter of the pores included in the cross section of the ceramic sintered body is 0.3 μm or less. This makes it difficult for stress concentration to occur when an external force is applied to the ceramic sintered body, thereby further improving the strength of the ceramic sintered body.

[0010] (3) In the ceramic sintered body of the above embodiment, the ceramic sintered body may contain 60% by weight or more and 93% by weight or less of alumina and 7% by weight or more and 40% by weight or less of zirconia. According to this configuration, the ceramic sintered body contains 60% by weight or more and 93% by weight or less of alumina and 7% by weight or more and 40% by weight or less of zirconia. This can further improve the strength of the ceramic sintered body.

[0011] (4) In the ceramic sintered body of the above embodiment, the zirconia crystal particles may have a crystalline phase of at least one of tetragonal and cubic. According to this configuration, the crystalline phase of the zirconia crystal particles is at least one of tetragonal and cubic, which exist relatively stably. This can further improve the strength of the ceramic sintered body.

[0012] (5) According to another aspect of the present invention, there is provided a bearing ball. This bearing ball is formed from the above-described ceramic sintered body. According to this configuration, the bearing ball is formed from a ceramic sintered body in which, in a cross section, the total area of ​​pores possessed by alumina crystal grains accounts for 95% or more of the total area of ​​pores contained in the cross section. This can improve the strength of the bearing ball.

[0013] (6) According to yet another aspect of the present invention, there is provided a cutting tool. This cutting tool is formed from the above-described ceramic sintered body. According to this configuration, the cutting tool is formed from a ceramic sintered body in which, in a cross section, the total area of ​​pores possessed by alumina crystal grains accounts for 95% or more of the total area of ​​pores contained in the cross section. This makes it possible to improve the strength of the cutting tool.

[0014] The present invention can be realized in various forms, such as a structure formed from a ceramic sintered body, an apparatus including a structure formed from a ceramic sintered body, a method for manufacturing a ceramic sintered body and a structure formed from a ceramic sintered body, and a computer program that causes a manufacturing apparatus to manufacture a ceramic sintered body and a structure formed from a ceramic sintered body.

[0015] FIG. 1 is a schematic diagram of a bearing according to a first embodiment; FIG. 2 is a cross-sectional photograph of a ceramic sintered body forming a bearing ball; FIG. 3 is a diagram illustrating the results of a first evaluation test of the ceramic sintered body; FIG. 4 is a diagram illustrating the results of a second evaluation test of the ceramic sintered body; FIG. 5 is a schematic diagram of a cutting tool according to a second embodiment; and FIG. 6 is a schematic diagram illustrating a state in which the cutting tool according to the second embodiment is used.

[0016] <First embodiment> Fig. 1 is a schematic diagram of a bearing according to the first embodiment. The bearing 10 includes an inner ring 11, an outer ring 12, and a plurality of bearing balls 1. The bearing balls 1 are held by a cage (not shown) and are sandwiched between the inner ring 11 and the outer ring 12. The bearing balls 1 stably hold the rotation axis of the inner ring 11, to which a rotating shaft (not shown) is fixed, relative to the outer ring 12, which is fixed to a machine or the like, for example. However, the method of use of the bearing 10 is not limited to this.

[0017] The bearing ball 1 is formed from a ceramic sintered body. The ceramic sintered body that forms the bearing ball 1 of this embodiment is an alumina-based sintered body that includes alumina (Al2O3) crystal grains with pores and zirconia (ZrO2) crystal grains. In the ceramic sintered body of the bearing ball 1, the total area of ​​the pores in the alumina crystal grains in the cross section of the ceramic sintered body is 95%, which is 95% or more of the total area of ​​the pores contained in the cross section. This gives the bearing 10 of this embodiment relatively high durability.

[0018] Fig. 2 is a cross-sectional photograph of the ceramic sintered body that forms the bearing ball 1. The cross-sectional photograph of the ceramic sintered body shown in Fig. 2 is an image taken using a transmission electron microscope (TEM, JEM-F200 manufactured by JEOL Ltd.) of a ceramic sintered body that was made into a thin slice by ion milling the bearing ball 1. In Fig. 2, the relatively dark parts are alumina crystal particles B1, and the relatively white parts are zirconia crystal particles W1. The even darker parts contained in the alumina crystal particles B1 shown in Fig. 2 are pores Pr. As can be seen from the TEM image in Fig. 2, in the ceramic sintered body that forms the bearing ball 1, most of the pores Pr are present within the alumina crystal particles B1, and almost no pores Pr are present at the grain boundaries between the crystal particles.

[0019] In the ceramic sintered compact that forms bearing ball 1, the pores contained in the cross section have an equivalent circle diameter of 0.11 μm, which is 0.3 μm or less. As a result, even if an external force acts on bearing ball 1, stress is less likely to concentrate in the pores contained in the ceramic sintered compact, thereby improving the strength of bearing ball 1.

[0020] The ceramic sintered body forming the bearing ball 1 contains 60% by weight or more and 93% by weight or less of alumina and 7% by weight or more and 40% by weight or less of zirconia. The ceramic sintered body of the bearing ball 1 of this embodiment contains 60% by weight of alumina and 40% by weight of zirconia. The ceramic sintered body of the bearing ball 1 of this embodiment also contains a small amount of yttrium (Y) as an additive.

[0021] In the ceramic sintered body forming the bearing ball 1, the crystalline phase of the zirconia crystal particles is at least one of tetragonal and cubic. Of the 40% by weight of zirconia crystal particles contained in the ceramic sintered body of the bearing ball 1 of this embodiment, 39.0% by weight of the zirconia crystal particles are tetragonal, and 1.0% by weight of the zirconia crystal particles are cubic. In the zirconia crystal particles, both the tetragonal and cubic crystals are crystalline phases that can exist stably, which improves the strength of the bearing ball 1.

[0022] Next, a method for manufacturing the bearing ball 1 will be described. In the method for manufacturing the bearing ball 1, alumina powder (average particle size: 0.6 μm), zirconia powder (average particle size: 0.5 μm), and yttria-stabilized zirconia powder (hereinafter referred to as "3YSZ powder", average particle size: 0.5 μm) are used as raw material powders. First, the alumina powder, zirconia powder, and yttria-stabilized zirconia powder are weighed out so that the content of the ceramic sintered body of the bearing ball 1 is 60% by weight of alumina, 40% by weight of zirconia, and a small amount of yttrium. The ratio of the zirconia powder to the 3YSZ powder is mixed to be 0.1.

[0023] The weighed raw material powder is placed in a resin pot together with alumina balls, water is added as a solvent, and ball milling is performed for 60 hours at a rotation speed of 60 revolutions per minute to pulverize and mix the raw material powder, thereby preparing a mixed slurry. In the manufacturing method of the bearing ball 1 of this embodiment, the pH of the mixed slurry is measured one hour after the start of ball milling, and the pH of the mixed slurry is adjusted to 9. For example, if the pH of the mixed slurry is 8, the pH of the mixed slurry is corrected to the alkaline side by adding a sodium hydroxide solution, ammonia water, or the like.

[0024] In the manufacturing method of the bearing ball 1 of this embodiment, the pH of the mixed slurry is adjusted to 9 to improve the strength of the ceramic sintered body. When the pH of the mixed slurry is 9, the alumina powder has no surface charge, while the zirconia powder and the 3YSZ powder each have a negative surface charge. In this charged state, the alumina powder particles do not repel or attract each other, but the zirconia powder and the 3YSZ powder repel each other. This makes it difficult for the ceramic sintered body to have a homogeneous crystalline structure. Furthermore, since the alumina powder in the mixed slurry moderately aggregates, pores are easily enclosed within the alumina crystal particles, and the proportion of pores within the alumina crystal particles is greater than the proportion of pores formed at the grain boundaries. Furthermore, the pinning effect of the moderate dispersion of the zirconia powder and the 3YSZ powder in the mixed slurry makes it difficult for relatively large alumina crystal particles to form, which tends to result in small equivalent circle diameters of the pores. Due to this phenomenon, the strength of the ceramic sintered body is improved compared to ceramic sintered bodies produced using a mixed slurry with a pH of 10 or 11. In addition, in the mixed slurry with a pH of 9, by mixing zirconia powder and 3YSZ powder, the balance between the zirconia powder, which has a stronger surface charge, and the 3YSZ powder, which has a slightly weaker charge, makes it easier to further improve the strength of the ceramic sintered body.

[0025] A binder, such as an acrylic resin, is added to the mixed slurry, and the mixture is mixed for another 30 minutes. The mixed slurry is then dried by spray drying to produce a mixed powder. The mixed powder is then filled into a dedicated mold and subjected to powder press molding using a uniaxial press (pressing pressure: 100 MPa) to produce a molded body. The produced molded body is degreased by heat treatment in air at a maximum temperature of 800°C. The degreased molded body is then air-fired (fired at 1500°C for 2 hours) and then subjected to hot isostatic pressing (HIP, treatment temperature: 1500°C, treatment pressure: 50 MPa, argon atmosphere, treatment time: 2 hours), to produce a base sphere that will become the bearing ball 1. The surface of the produced base sphere is polished to produce the bearing ball 1.

[0026] Next, we will explain the evaluation test of the ceramic sintered body. In this evaluation test, samples of ceramic sintered bodies were produced using different production methods, and the relationship between the differences in pore characteristics in the cross section and the numerical values ​​related to the strength of the ceramic sintered body was evaluated.

[0027] FIG. 3 is a diagram illustrating the results of a first evaluation test of the ceramic sintered body. For this evaluation test, eight types of samples were prepared using different manufacturing methods. Each of the eight types of samples was manufactured using a method similar to the manufacturing method for the ceramic sintered body of the bearing ball 1 of this embodiment. Samples 1 to 5 were manufactured by weighing raw material powders so that the weight percentages of alumina and zirconia were the values ​​shown as "Al2O3" and "ZrO2" in the "Component (wt %)" section of FIG. 3, and adjusting the pH of the mixed slurry to 9. Sample 6 was manufactured by setting the weight percentages of alumina and zirconia to the same values ​​as Sample 3, while adjusting the pH of the mixed slurry to 10. Sample 7 was manufactured by setting the weight percentages of alumina and zirconia to the same values ​​as Sample 3, while adjusting the pH of the mixed slurry to 11. Sample 8 was produced by using only 3YSZ powder as the raw material powder of zirconia without using any zirconia powder, and adjusting the pH of the mixed slurry to 9.

[0028] 3 shows the "density" and "composition" for each of Samples 1 to 8 of the ceramic sintered bodies produced by the above-mentioned manufacturing method. "Density" refers to the density (unit: g / cm) of each sample. 3 The "density" indicates a value measured according to the procedure described in JIS R1634 using a ceramic sintered body in which the surface was polished to remove 0.25 mm or more from the sintered surface.

[0029] The "components" column indicates the weight ratio (unit: wt%) of alumina crystal grains and zirconia crystal grains contained in each sample, and the weight ratio (unit: wt%) of the crystalline phase in the zirconia crystal grains relative to the entire sample. For the "components" measurement, the surfaces of the prepared ceramic sintered bodies were polished to remove at least 0.25 mm of the sintered surface. The surface from which the sintered surface was removed was then mirror-polished to prepare the "components" measurement surface. The crystalline phases of the crystalline grains contained in the samples were identified using X-ray diffraction (XRD) on the prepared "components" measurement surface. The quantification of the identified crystalline phases was performed using the WPPF method using the X-ray diffraction patterns obtained by X-ray diffraction. The ICDD card numbers for "Al2O3," "t-ZrO2," "c-ZrO2," and "m-ZrO2" in the "components" column of Figure 3, which were referenced in the quantitative analysis using the WPPF method, are as follows: The ratio of alumina to total zirconia in each sample was the same as the weight ratio of the raw material powder weighed out when the sample was prepared.

[0030] Fig. 4 is a diagram illustrating the results of the second evaluation test of the ceramic sintered body. In this evaluation test, evaluation was performed on each of Samples 1 to 8 for the five items shown in Fig. 4. Here, the evaluation methods for each will be described.

[0031] For the "pores" shown in Figure 4, the "intragranular porosity (%)," which indicates the ratio of the total area of ​​pores in alumina crystal grains to the total area of ​​pores in the cross section of the ceramic sintered body, and the "circle equivalent diameter (μm)," which is a standardized value of pore size, were measured. To measure the numerical values ​​related to "pores," samples were prepared by polishing the surface of the prepared ceramic sintered body to remove 0.25 mm or more from the baked surface and then ion milling the resulting thin slices. Specifically, five randomly selected areas on the surface of the thin sliced ​​sample, each 8 μm square, were imaged using a transmission electron microscope (TEM, JEM-F200 manufactured by JEOL Ltd.), and TEM images were obtained. For each of the five areas where TEM images were obtained, the circle equivalent diameter of the pores was calculated by image analysis, and the average value was used as the "circle equivalent diameter." Image analysis was performed using image analysis software such as WinROOF (registered trademark). In addition, the area value of the pores was determined in each of the five ranges where the TEM images were obtained, and the total area of ​​the pores inside the alumina crystal particles and the total area of ​​the pores outside the alumina crystal particles were calculated to determine the "intragranular porosity."

[0032] In the measurement of "hardness" shown in Figure 4, the surface of the produced ceramic sintered body was first polished to remove 0.25 mm or more from the fired surface, and the surface from which the fired surface had been removed was further mirror-polished to prepare a sample for "hardness" measurement. In accordance with JIS R1610, a diamond Vickers indenter was pressed into the sample for "hardness" measurement with a constant load (10 kg), and the Vickers hardness (unit: HV10) determined from the shape of the indentation was taken as the "hardness".

[0033] In the measurement of "fracture toughness" shown in Figure 4, the surface of the produced ceramic sintered body was first polished to remove 0.25 mm or more from the fired surface, and the surface from which the fired surface had been removed was further mirror-polished to prepare a sample for "fracture toughness" measurement. In accordance with JIS R1607, a diamond Vickers indenter was pressed into the sample for "fracture toughness" measurement with a constant load (10 kg), and the fracture toughness (unit: MPa m 1 / 2 ) was defined as the "fracture toughness."

[0034] In the measurement of "bending strength" shown in Figure 4, the surface of the produced ceramic sintered body was first polished to remove 0.25 mm or more from the fired surface, and then processed into a size of 3 mm x 4 mm x 50 mm to prepare a sample for "bending strength" measurement. In accordance with JIS R1601, the sample for "bending strength" measurement was placed on two supports spaced 30 mm apart, and a three-point bending strength test was performed in which a load was applied to a point in the center between the supports, and the maximum bending stress at which the sample broke was taken as the "bending strength."

[0035] In the "pore" values ​​shown in Figure 4, Samples 1 to 5 have an "intragranular porosity" of 95% or more and an "equivalent circle diameter" of 0.3 μm or less. As shown in Figure 4, Samples 1 to 5 were confirmed to be ceramic sintered bodies with high "bending strength" among the eight types of samples evaluated in this evaluation test. Specifically, Samples 1 to 5 were confirmed to have a maximum bending stress of 1000 MPa or more, which is the standard value for "bending strength" in this evaluation test.

[0036] It was confirmed that Sample 6 had a smaller "intragranular porosity" and a smaller "bending strength" than each of Samples 1 to 5. This is thought to be because the pH of the mixed slurry used in Sample 6 was adjusted to 10 during production, and the raw material powder particles in the mixed slurry repelled each other, forming a relatively homogeneous crystalline structure. When a homogeneous crystalline structure is formed, the intragranular porosity decreases, and bending strength tends to decrease.

[0037] Sample 7 has a smaller "intragranular porosity" and a larger "equivalent circle diameter" than each of Samples 1 to 5. Since the pH of the mixed slurry during production of Sample 7 was adjusted to 11, it is thought that the degree of repulsion between the raw material powder particles in the mixed slurry was greater than that of Sample 6, resulting in a lower intragranular porosity than Sample 6 and a larger equivalent circle diameter than Sample 6. This resulted in Sample 7 having the lowest bending strength among the eight types of samples evaluated in this evaluation test.

[0038] Although Sample 8 has an "intragranular porosity" of 95% or more, its "equivalent circle diameter" is larger than those of Samples 1 to 5. Sample 8 was produced using 3YSZ powder as the raw zirconia powder. When the pH of the mixed slurry during production was 9, the 3YSZ powder had a weaker surface charge than the zirconia powder, and therefore the degree of dispersion of the raw powder was relatively reduced. This is thought to have made it easier for relatively coarse alumina crystal particles to be formed, which increased the equivalent circle diameter of the pores contained in the alumina crystal particles.

[0039] The "hardness" shown in FIG. 4 is smaller for Samples 6 to 8 than for Sample 3. This is thought to be because the equivalent circle diameters of Samples 6 to 8 are larger than that of Sample 3. Furthermore, the "fracture toughness" shown in FIG. 4 is larger for Samples 6 to 8 than for Sample 3. This is thought to be because the intragranular porosity of Samples 6 to 8 is larger than that of Sample 3, making it easier to suppress crack propagation.

[0040] According to the bearing ball 1 of this embodiment described above, the ceramic sintered body forming the bearing ball 1 contains alumina crystal grains and zirconia crystal grains having pores, and in a cross section, the total area of ​​the pores in the alumina crystal grains accounts for 95% or more of the total area of ​​the pores contained in the cross section. In other words, a relatively large number of pores are present inside the alumina crystal grains. As a result, when an external force is applied to the ceramic sintered body, cracks are less likely to propagate than when pores are present at the grain boundaries. This makes it possible to improve the strength of the bearing ball 1.

[0041] Furthermore, according to the bearing ball 1 of this embodiment, the circle-equivalent diameter of the pores contained in the cross section of the ceramic sintered body is 0.08 μm. This makes it difficult for stress concentration to occur when an external force is applied to the ceramic sintered body, thereby further improving the strength of the bearing ball 1.

[0042] Furthermore, according to the bearing ball 1 of this embodiment, the ceramic sintered body contains 60% by weight of alumina and 40% by weight of zirconia, which further improves the strength of the bearing ball 1.

[0043] Furthermore, in the bearing ball 1 of this embodiment, the crystalline phase of the zirconia crystal grains is at least one of tetragonal and cubic crystals, which exist in a relatively stable state, thereby further improving the strength of the bearing ball 1.

[0044] Second Embodiment FIG. 5 is a schematic diagram of a cutting tool according to a second embodiment. Cutting tools 2a to 2g shown in FIG. 5 are formed of ceramic sintered bodies. The ceramic sintered bodies forming the cutting tools 2a to 2g of this embodiment are alumina-based sintered bodies containing alumina crystal grains having pores and zirconia crystal grains. In the ceramic sintered bodies forming the cutting tools 2a to 2g, the total area of ​​the pores of the alumina crystal grains in the cross section of the ceramic sintered body is 95% or more of the total area of ​​the pores included in the cross section. This improves the durability of the cutting tools 2a to 2g of this embodiment. As shown in FIG. 5, the cutting tools 2a to 2g of this embodiment can be used in various shapes.

[0045] Fig. 6 is a schematic diagram showing a state in which the cutting tool of the second embodiment is used, in which the cutting tool 2 is attached to a shank 21. The cutting tool 2 of this embodiment is used, for example, in the state shown in Fig. 6.

[0046] According to the cutting tool 2 of this embodiment described above, the ceramic sintered body forming the cutting tool 2 contains alumina crystal grains and zirconia crystal grains having pores, and in a cross section, the total area of ​​the pores in the alumina crystal grains accounts for 95% or more of the total area of ​​the pores in the cross section. As a result, when an external force is applied to the ceramic sintered body, crack propagation is less likely to progress than in a case where pores exist at the grain boundaries. Therefore, the strength of the cutting tool 2 can be improved.

[0047] <Modifications of this embodiment> The present invention is not limited to the above-described embodiment, and can be implemented in various forms without departing from the spirit of the present invention. For example, the following modifications are also possible.

[0048] [Variation 1] In the above-described embodiment, the equivalent circle diameter of the pores included in the cross section is 0.3 μm or less. The equivalent circle diameter of the pores included in the cross section may be greater than 0.3 μm. By making the equivalent circle diameter of the pores 0.3 μm or less, stress concentration is less likely to occur when an external force acts on the ceramic sintered body, and the strength of the ceramic sintered body can be further improved.

[0049] [Modification 2] In the above-described embodiment, the ceramic sintered body contains 60% by weight or more and 93% by weight or less of alumina and 7% by weight or more and 40% by weight or less of zirconia. The respective weight percentages of alumina and zirconia in the ceramic sintered body are not limited to these.

[0050] In the above-described embodiment, the crystalline phase of the zirconia crystal particles is mostly tetragonal, with the remainder being cubic. It is not necessary for the crystalline phase to be both tetragonal and cubic, but it is sufficient for the crystalline phase to be at least one of tetragonal and cubic, and the crystalline phase may be only tetragonal or only cubic.

[0051] This aspect has been described above based on embodiments and modifications. However, the above-described embodiments are intended to facilitate understanding of this aspect and are not intended to limit this aspect. This aspect may be modified or improved without departing from the spirit and scope of the claims, and equivalents thereof are included in this aspect. Furthermore, if a technical feature is not described as essential in this specification, it may be deleted as appropriate.

[0052] <Application Example 1> A ceramic sintered body comprising alumina crystal grains having pores and zirconia crystal grains, wherein the total area of ​​the pores in the alumina crystal grains in a cross section of the ceramic sintered body is 95% or more of the total area of ​​the pores in the cross section. <Application Example 2> The ceramic sintered body according to Application Example 1, wherein the pores in the cross section have an equivalent circle diameter of 0.3 μm or less. <Application Example 3> The ceramic sintered body according to Application Example 1 or Application Example 2, wherein the ceramic sintered body comprises 60% to 93% by weight of alumina and 7% to 40% by weight of zirconia. <Application Example 4> The ceramic sintered body according to any one of Application Examples 1 to 3, wherein the zirconia crystal grains have a crystal phase that is at least one of a tetragonal system and a cubic system. <Application Example 5> A bearing ball, characterized in that it is formed from the ceramic sintered body according to any one of Application Examples 1 to 4. <Application Example 6> A cutting tool, characterized in that it is formed from the ceramic sintered body according to any one of Application Examples 1 to 4.

[0053] 1... Bearing ball 2... Cutting tool B1... Alumina crystal particle Pr... Pore W1... Zirconia crystal particle

Claims

1. A ceramic sintered body comprising alumina crystal grains having pores and zirconia crystal grains, wherein in a cross section of the ceramic sintered body, the total area of ​​the pores in the alumina crystal grains is 95% or more of the total area of ​​the pores contained in the cross section.

2. A ceramic sintered body according to claim 1, characterized in that the equivalent circle diameter of pores contained in the cross section is 0.3 μm or less.

3. A ceramic sintered body according to claim 1 or 2, characterized in that the ceramic sintered body contains 60% by weight or more and 93% by weight or less of alumina and 7% by weight or more and 40% by weight or less of zirconia.

4. A ceramic sintered body according to claim 1 or 2, characterized in that the crystalline phase of the zirconia crystal grains is at least one of a tetragonal system and a cubic system.

5. A bearing ball, characterized in that it is formed from the ceramic sintered body according to claim 1 or 2.

6. A cutting tool, characterized in that it is formed from the ceramic sintered body according to claim 1 or 2.

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