Ceramic sintered bodies, cutting tools, and bearing balls

Abstract

This invention provides a technology to improve the strength of ceramic sintered bodies. [Solution] The ceramic sintered body comprises alumina crystal particles and zirconia crystal particles containing tetragonal zirconia. In the cross-section of the ceramic sintered body, if N is the number of zirconia crystal particles contained in a rectangular region with sides of 10 μm, if a is the number of zirconia crystal particles surrounded only by zirconia crystal particles, if b is the number of zirconia crystal particles surrounded only by alumina crystal particles, and if c is the number of zirconia crystal particles surrounded by both zirconia crystal particles and alumina crystal particles, then the following equations (1) to (4) are satisfied. 110 ≤ N ···(1) 0 ≤ a / N ≤ 0.03 ···(2) 0.23 ≤ b / N ≤ 0.50 ···(3) 0.50 ≤ c / N ≤ 0.76 ···(4)

Description

Technical Field

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

Background Art

[0002] Conventionally, alumina-based ceramic sintered bodies have been known (for example, Patent Document 1).

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

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

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

Means for Solving the Problems

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

[0007] (1) According to one embodiment of the present invention, a ceramic sintered body is provided. This ceramic sintered body comprises alumina crystal particles and zirconia crystal particles containing tetragonal zirconia. In the cross-section of the ceramic sintered body, if N is the number of zirconia crystal particles contained in a rectangular region with sides of 10 μm, if a is the number of zirconia crystal particles surrounded only by zirconia crystal particles, if b is the number of zirconia crystal particles surrounded only by alumina crystal particles, and if c is the number of zirconia crystal particles surrounded by both zirconia crystal particles and alumina crystal particles, then the following equations (1) to (4) are satisfied. 110 ≤ N ···(1) 0 ≤ a / N ≤ 0.03 ···(2) 0.23 ≤ b / N ≤ 0.50 ···(3) 0.50 ≤ c / N ≤ 0.76 ···(4)

[0008] According to this configuration, a ceramic sintered body comprising alumina crystal particles and zirconia crystal particles containing tetragonal zirconia satisfies equations (1) to (4). In a ceramic sintered body satisfying equation (2), relatively brittle zirconia particles are less likely to form aggregates during the manufacturing of the ceramic sintered body. In a ceramic sintered body satisfying equation (3), the growth of zirconia crystal particles is suppressed by the pinning effect of the alumina particles during the manufacturing of the ceramic sintered body, making it less likely for the zirconia crystal particles to undergo phase transformation, and thus reducing the influence of tensile stress caused by the phase transformation of the zirconia crystal particles. In a ceramic sintered body satisfying equation (4), the zirconia crystal particles are less likely to undergo phase transformation, thus reducing the influence of tensile stress caused by the phase transformation of the zirconia crystal particles. In this way, even when a load is applied to the ceramic sintered body, the zirconia crystal particles are less likely to undergo phase transformation, thus suppressing the shedding of zirconia crystal particles. Therefore, the strength of the ceramic sintered body can be improved.

[0009] (2) In the ceramic sintered body of the above form, the ceramic sintered body may contain 70% to 93% by mass of alumina crystal particles and 7% to 30% by mass of zirconia crystal particles. With this configuration, the ceramic sintered body containing 70% to 93% by mass of alumina crystal particles and 7% to 30% by mass of zirconia crystal particles can achieve both chemical stability and toughness as a structure formed from ceramics. This further suppresses the shedding of zirconia crystal particles, thereby further improving the strength of the ceramic sintered body.

[0010] (3) According to another embodiment of the present invention, a cutting tool is provided. This cutting tool is formed from the above-described ceramic sintered body. With this configuration, the cutting tool is formed from a ceramic sintered body in which the influence of tensile stress generated by the phase transformation of zirconia crystal grains is relatively small. This makes it possible to improve the strength of the cutting tool and thus extend the product life of the cutting tool.

[0011] (4) According to yet another embodiment of the present invention, a bearing ball is provided. This bearing ball is formed from the above-described ceramic sintered body. With this configuration, the bearing ball is formed from a ceramic sintered body in which the tensile stress generated by the phase transformation of zirconia crystal grains is relatively small. This makes it possible to improve strength and thus extend the product life of the bearing ball.

[0012] Furthermore, the present invention can be realized in various forms, including a structure formed from a ceramic sintered body, an apparatus equipped with 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. [Brief explanation of the drawing]

[0013] [Figure 1] This is a schematic diagram of a cutting tool according to the first embodiment. [Figure 2] This is a schematic diagram showing the usage state of the cutting tool according to the first embodiment. [Figure 3] This is a cross-sectional photograph of the ceramic sintered body forming the cutting tool of the first embodiment. [Figure 4] Figure 1 illustrates the results of the evaluation test of the ceramic sintered body. [Figure 5] This is the second figure illustrating the results of the evaluation test of the ceramic sintered body. [Figure 6] This is the third figure illustrating the results of the evaluation test of the ceramic sintered body. [Figure 7] This is a schematic diagram of a ball bearing according to the second embodiment. [Modes for carrying out the invention]

[0014] <First Embodiment> Figure 1 is a schematic diagram of the cutting tool 1 of the first embodiment. Figure 2 is a schematic diagram showing the cutting tool 1 of this embodiment in use. The cutting tool 1 shown in Figure 1 is formed from a ceramic sintered body. The ceramic sintered body forming the cutting tool 1 of this embodiment comprises alumina crystal particles and zirconia crystal particles containing tetragonal zirconia. The cutting tool 1 of this embodiment can be used in various shapes, as shown in Figure 1. Figure 2 shows the cutting tool 1 attached to the shank 11. The cutting tool 1 of this embodiment can be used, for example, in the state shown in Figure 2.

[0015] Figure 3 is a cross-sectional image of the ceramic sintered body forming the cutting tool 1. The cross-sectional image of the ceramic sintered body shown in Figure 3 is an image (magnification: 2000x) taken using a scanning electron microscope (SEM). In Figure 3, the relatively dark areas are alumina crystal particles CPa, and the relatively white areas are zirconia crystal particles CPb. The ceramic sintered body forming the cutting tool 1 of this embodiment contains 70% to 93% by mass of alumina crystal particles CPa and 7% to 30% by mass of zirconia crystal particles CPb. The ceramic sintered body forming the cutting tool 1 of this embodiment contains tetragonal zirconia as zirconia crystal particles CPb.

[0016] The ceramic sintered body forming the cutting tool 1 of this embodiment has the following characteristics regarding the positional relationship between alumina crystal particles CPa and zirconia crystal particles CPb in the cross-section of the ceramic sintered body as shown in Figure 3. If N is the number of zirconia crystal particles CPb contained in a rectangular region with sides of 10 μm in the cross-section of the ceramic sintered body, a is the number of zirconia crystal particles CPb surrounded only by zirconia crystal particles CPb, b is the number of zirconia crystal particles CPb surrounded only by alumina crystal particles CPa, and c is the number of zirconia crystal particles CPb surrounded by both zirconia crystal particles CPb and alumina crystal particles CPa, then the following equations (1) to (4) are satisfied. Note that the numbers N, a, b, and c were measured by using a thermal etching method, in which the cross-section of the ceramic sintered body to be measured is mirror-polished and then heat-treated. Specifically, SEM images at 10,000x magnification are acquired for any multiple locations on the cross-section of a ceramic sintered body treated by thermal etching. For each acquired SEM image, the type of crystal grains surrounding the zirconia crystal grain CPb is identified and measured within any rectangular region with sides of 10 μm. The location for capturing the SEM images to measure the number N, a, b, c is preferably the region near the center of gravity of the cutting tool 1. 110 ≤ N ···(1) 0 ≤ a / N ≤ 0.03 ···(2) 0.23 ≤ b / N ≤ 0.50 ···(3) 0.50 ≤ c / N ≤ 0.76 ···(4)

[0017] By satisfying formula (2), the ceramic sintered body makes it difficult for relatively fragile zirconia particles to form aggregates during the production of the ceramic sintered body described later. By satisfying formula (3), the ceramic sintered body suppresses the growth of zirconia crystal particles CPb due to the pinning effect of alumina particles during the production of the ceramic sintered body, so that the influence of the tensile stress caused by the phase transformation of the zirconia crystal particles CPb can be reduced. By satisfying formula (4), the ceramic sintered body can reduce the influence of the tensile stress caused by the phase transformation of the zirconia crystal particles CPb. Thus, even when a load is applied to the ceramic sintered body, it becomes difficult for the zirconia crystal particles CPb to undergo a phase transformation, so that the grain detachment of the zirconia crystal particles CPb can be suppressed. Therefore, the strength of the ceramic sintered body can be improved.

[0018] Next, a method for manufacturing the cutting tool 1 will be described. In the method for manufacturing the cutting tool 1, as raw material powders, alumina powder (average particle size: 0.6 μm) and yttria-stabilized zirconia powder (hereinafter referred to as "3YSZ powder", average particle size: 0.3 μm) are used. First, the contents in the ceramic sintered body of the cutting tool 1 are weighed such that alumina is 70% by mass and zirconia is 30% by mass for each of the alumina powder and the 3YSZ powder.

[0019] The weighed raw material powders are put into a resin pot together with alumina balls, pure water is added as a solvent, and ball milling is performed at a rotation speed of 60 rotations per minute for 72 hours to pulverize and mix the raw material powders, thereby producing a mixed slurry. A binder, for example, an acrylic resin, is added to the mixed slurry, and after further mixing for 30 minutes, the mixed slurry is dried by spray drying to produce a mixed powder.

[0020] In the manufacturing method of the cutting tool 1 of this embodiment, a polycarboxylic acid-based dispersant is used to prepare the mixed slurry. In particular, ammonium polycarboxylic acid salts as dispersants are suitable for hydrophilic oxides, and can appropriately disperse the alumina powder and 3YSZ powder while reducing the water content in the mixed slurry. The amount of polycarboxylic acid-based dispersant added is preferably about 0.5 wt% to 0.8 wt% in terms of solid content. Adding a larger amount of dispersant makes it easier to disperse the 3YSZ powder.

[0021] In the manufacturing method of the cutting tool 1 of this embodiment, a mixture of alumina spheres with a diameter of 10 mm and spheres with a diameter of 5 mm is used to produce the mixed slurry. This allows for further crushing of the relatively fine 3YSZ powder, thereby dispersing the 3YSZ powder and suppressing the formation of aggregates. The mixing ratio of the 10 mm diameter spheres to the 5 mm diameter spheres used in the manufacturing method of the cutting tool 1 of this embodiment is 8:1 by mass.

[0022] Next, using the prepared mixed powder, a preliminary mold is created by filling a special mold with the mixed powder and then performing powder press molding (press pressure: 30 MPa) using a uniaxial press. The prepared preliminary mold is then bagged in a special sheet and subjected to cold isostatic pressing (press pressure: 150 MPa) to produce the final mold. The produced final mold is degreased by heat treatment in air at a maximum temperature of 800°C. The degreased final mold is then fired in air (fire temperature: 1450°C, firing time: 2 hours) and then subjected to hot isostatic pressing (HIP, processing temperature: 1400°C, processing pressure: 150 MPa, argon atmosphere, processing time: 2 hours) to become the base material for cutting tool 1. Finally, the cutting tool 1 is produced by polishing the surface of the base material.

[0023] Next, we will describe the evaluation test of ceramic sintered bodies. In this evaluation test, samples of ceramic sintered bodies made using different manufacturing methods were prepared, and the relationship between the differences in the positional relationship between zirconia crystal particles and alumina crystal particles in the cross-section and numerical values ​​related to the strength of the ceramic sintered bodies was evaluated.

[0024] Figure 4 is the first diagram illustrating the results of the evaluation test of the ceramic sintered body. In this evaluation test, eight different samples were prepared using different manufacturing methods. Each of the eight samples was manufactured using a method consistent with the manufacturing method of the ceramic sintered body that forms the cutting tool 1 of this embodiment. Samples 1 to 8 were prepared by weighing the raw material powders so that the respective mass percentages of alumina and zirconia were the values ​​shown for "Al2O3" and "ZrO2" in the "Mass Ratio (wt%)" section of Figure 4.

[0025] In preparing the mixed slurry for Samples 1 to 8, the amount of dispersant shown in "Dispersant (wt%)" in Figure 4 was used, and spheres with the diameter shown in "Sphere Diameter (mm)" in Figure 4 were used. Sample 6 differs from Samples 1 to 5 in "Dispersant (wt%)". Sample 7 differs from Samples 1 to 5 in "Sphere Diameter (mm)". Sample 8 differs from Samples 1 to 5 in both "Dispersant (wt%)" and "Sphere Diameter (mm)".

[0026] Figure 4 shows the density (in units: g / cm³) of each of the ceramic sintered bodies, Samples 1 to 8, produced by the manufacturing method described above. 3 The value shown is "density". The density of each sample was measured according to the procedure described in JIS R1634, using a ceramic sintered body whose surface was polished with a 220-grit abrasive to remove at least 0.5 mm of the hardened surface.

[0027] Figure 4, "Cross-sectional Analysis Results," shows the characteristics of the positional relationship between zirconia crystal particles and alumina crystal particles in samples 1 to 8, similar to the characteristics of the positional relationship between zirconia crystal particles CPb and alumina crystal particles CPa in the cross-section of the ceramic sintered body forming the cutting tool 1 of this embodiment. Specifically, in the cross-section of each sample, the number of zirconia crystal particles contained in a specific rectangular region with sides of 10 μm is shown as "N (particles)," the number of zirconia crystal particles surrounded only by other zirconia crystal particles is shown as "a (particles)," the number of zirconia crystal particles surrounded only by alumina crystal particles is shown as "b (particles)," and the number of zirconia crystal particles surrounded by both zirconia crystal particles and alumina crystal particles is shown as "c (particles)." Each of "N (pieces)", "a (pieces)", "b (pieces)", and "c (pieces)" was measured using the same method as the method for measuring the number of pieces N, a, b, and c in the cross-section of the ceramic sintered body forming the cutting tool 1 of this embodiment.

[0028] Figure 4, "Cross-sectional analysis results," shows the values ​​calculated for each of Samples 1 to 8 using "N (particles)," "a (particles)," "b (particles)," and "c (particles)." "A (%)" represents the proportion of zirconia crystal particles surrounded only by other zirconia crystal particles within a specific region (= a / N × 100). "A (%)" represents the proportion of zirconia crystal particles surrounded only by alumina crystal particles within a specific region (= b / N × 100). "C (%)" represents the proportion of zirconia crystal particles surrounded by both zirconia crystal particles and alumina crystal particles within a specific region (= c / N × 100). Of Samples 1 to 8 shown in Figure 4, each of Samples 1 to 7 has an "A (%)" of 3% or less. Of samples 1 through 8, each of samples 1 through 5 has a "B (%)" ratio between 23% and 50%. Of samples 1 through 8, each of samples 1 through 5 has a "C (%)" ratio between 50% and 76%.

[0029] Figure 5 is the second figure illustrating the results of the evaluation test of the ceramic sintered body. Figure 6 is the third figure illustrating the results of the evaluation test of the ceramic sintered body. Figures 5 and 6 show, for each of Samples 1 to 8, the "composition (wt%)" indicating the type and amount of crystalline phase, the "flexural strength (MPa)" indicating the magnitude of the flexural strength, and the "Vickers test" showing the results of the Vickers test. Figure 5 shows the results for each of Samples 1 to 8 after polishing with a grinding wheel with a grit of 1000. Figure 6 shows the results for each of Samples 1 to 8 after polishing with a grinding wheel with a grit of 220.

[0030] The "Components (wt%)" in Figures 5 and 6 show the mass ratio (unit: wt%) of alumina crystal particles to zirconia crystal particles contained in each sample, and the mass percentage (unit: wt%) of the crystalline phase in the zirconia crystal particles relative to the total sample. For the measurement of "Components," the surface of the fabricated ceramic sintered body was polished to remove 0.5 mm or more from the sintered surface, and then the surface from which the sintered surface had been removed was further mirror-polished to prepare a surface for "Components" measurement. On the prepared surface for "Components" measurement, the crystalline phase of the crystal particles contained in the sample was identified using X-ray diffraction (XRD). The quantitative analysis of the identified crystalline phase was performed using the WPPF method with the X-ray diffraction pattern obtained by X-ray diffraction. In the quantitative analysis by the WPPF method, the ICDD card numbers for "Al2O3," "ZrO2," "t-ZrO2," "c-ZrO2," and "m-ZrO2" shown in "Components (wt%)" in Figures 5 and 6, which were referenced, are as follows. Furthermore, the ratio of alumina to total zirconia in each sample was the same as the mass ratio of the raw material powders weighed during sample preparation. α-Al2O3:01-089-7715 t-ZrO2:01-081-154 c-ZrO2:01-083-9463 m-ZrO2:00-037-1484

[0031] In the measurement of "bending strength (MPa)" in Figures 5 and 6, first, the surface of the fabricated ceramic sintered body was polished to remove at least 0.5 mm from the fired surface, and then processed into a size of 3 mm x 4 mm x 50 mm to prepare as a sample for "bending strength" measurement. In accordance with JIS R1601, the sample for "bending strength" measurement was placed on two support points arranged 30 mm apart, and then a three-point bending strength test was performed by applying a load to the center point between the support points, and the maximum bending stress at which it broke was defined as the "bending strength".

[0032] The "Vickers test" shown in Figures 5 and 6 was evaluated based on the presence or absence of delamination when a predetermined load was applied using a Vickers indenter. Specifically, in accordance with JIS R1610, a Vickers indenter was driven into the polished surface of each sample polished with an abrasive wheel under a load condition of 10 kgf, and the presence or absence of delamination near the indentation was observed. For each sample, the test was performed at three arbitrary points, and if there was no delamination at any of them, it was judged as "○", and if there was delamination at even one point, it was judged as "×".

[0033] Samples 1 to 8 shown in Figure 5 were polished relatively precisely with a 1000-grit grinding wheel and were not subjected to a very large load. In other words, Samples 1 to 8 shown in Figure 5 can be said to be ceramic sintered bodies in a condition close to that of a new, unused product. All of Samples 1 to 8 shown in Figure 5 had a "flexural strength (MPa)" of 1000 MPa or more and passed the "Vickers test" with a "✓".

[0034] Samples 1 to 8 shown in Figure 6 were polished relatively roughly with a 220-grit grinding wheel. In other words, Samples 1 to 8 shown in Figure 6 were subjected to a relatively large load on the ceramic sintered body, and can be said to be ceramic sintered bodies in a state close to that of being used. Of Samples 1 to 8 shown in Figure 6, Samples 1 to 5 all had a "flexural strength (MPa)" of 1000 MPa or more, and the "Vickers test" was "✓". On the other hand, in Samples 6 to 8, the "flexural strength (MPa)" was significantly lower than the values ​​shown in Figure 5, and the "Vickers test" was "✗".

[0035] A ceramic sintered body containing alumina crystal grains and zirconia crystal grains containing tetragonal zirconia may undergo a phase transformation from tetragonal to monoclinic when subjected to relatively large loads during use. This phase transformation from tetragonal to monoclinic in zirconia crystal grains results in a volume expansion of approximately 4%, which can cause detachment of zirconia crystal grains and a decrease in strength. Comparing Figures 5 and 6, it can be seen that some of the zirconia crystal grains in samples 6 to 8, polished with a 220-grit grinding wheel, have undergone a phase transformation from tetragonal (t-ZrO2) to monoclinic (m-ZrO2). Consequently, each of samples 6 to 8, polished with a 220-grit grinding wheel, shows a decrease in "flexural strength (MPa)" and a "fail" rating in the "Vickers test". On the other hand, in each of the samples 1 to 5 shown in Figure 4, where "A(%)" is 3% or less, "B(%)" is 23% to 50%, and "C(%)" is 50% to 76%, the majority of the zirconia crystal grains in samples 1 to 5 remain in a tetragonal (t-ZrO2) state even after being polished with a 220-grit grinding wheel (see Figure 6). In other words, in samples 1 to 5, the phase transformation from tetragonal to monoclinic in the zirconia crystal grains is less likely to occur. As a result, the volume expansion of the zirconia crystal grains is suppressed, and thus the shedding of zirconia crystal grains is suppressed. Therefore, the strength of the ceramic sintered body is maintained even under load from use.

[0036] As described above, the cutting tool 1 of this embodiment comprises alumina crystal particles CPa and zirconia crystal particles CPb containing tetragonal zirconia, and satisfies equations (1) to (4). In a ceramic sintered body satisfying equation (2), relatively brittle 3YSZ powder is less likely to form aggregates during the manufacturing of the ceramic sintered body. In a ceramic sintered body satisfying equation (3), the growth of zirconia crystal particles CPb is suppressed by the pinning effect of the alumina powder during the manufacturing of the ceramic sintered body, making it less likely for zirconia crystal particles CPb to undergo phase transformation, and thus reducing the influence of tensile stress caused by the phase transformation of zirconia crystal particles CPb. In a ceramic sintered body satisfying equation (4), since zirconia crystal particles CPb are less likely to undergo phase transformation, the influence of tensile stress caused by the phase transformation of zirconia crystal particles CPb can be reduced. Thus, even when a load is applied to the ceramic sintered body, the zirconia crystal particles CPb are less likely to undergo phase transformation, thereby suppressing the shedding of zirconia crystal particles CPb. Consequently, the strength of the ceramic sintered body can be improved.

[0037] Furthermore, according to the cutting tool 1 of this embodiment, the ceramic sintered body forming the cutting tool 1 contains 70% to 93% by mass of alumina crystal particles CPa and 7% to 30% by mass of zirconia crystal particles CPb. This allows for both chemical stability and toughness as a structure formed from ceramics, thereby further suppressing the shedding of zirconia crystal particles CPb. Consequently, the strength of the ceramic sintered body can be further improved.

[0038] Furthermore, according to the cutting tool 1 of this embodiment, the cutting tool 1 is formed from a ceramic sintered body in which the influence of tensile stress generated by the phase transformation of zirconia crystal grains CPb is relatively small. As a result, the strength of the cutting tool 1 can be improved, and thus the product life of the cutting tool 1 can be extended.

[0039] <Second Embodiment> Figure 7 is a schematic diagram of a bearing 20 according to the first embodiment. The bearing 20 comprises an inner ring 21, an outer ring 22, and a plurality of bearing balls 2. The bearing balls 2 are held by a cage (not shown) and sandwiched between the inner ring 21 and the outer ring 22. The bearing balls 2 stably hold the rotation axis of the inner ring 21, to which a rotating shaft (not shown) is fixed, with respect to the outer ring 22, which is fixed to a machine or the like. Note that the method of use of the bearing 20 is not limited to this.

[0040] The bearing ball 2 is formed from a ceramic sintered body. The ceramic sintered body forming the bearing ball 2 of this embodiment comprises alumina crystal particles and zirconia crystal particles containing tetragonal zirconia. In the cross-section of the ceramic sintered body forming the bearing ball 2 of this embodiment, if N is the number of zirconia crystal particles contained in a rectangular region with sides of 10 μm, a is the number of zirconia crystal particles surrounded only by zirconia crystal particles, b is the number of zirconia crystal particles surrounded only by alumina crystal particles, and c is the number of zirconia crystal particles surrounded by both zirconia crystal particles and alumina crystal particles, then the following equations (1) to (4) are satisfied. 110 ≤ N ···(1) 0 ≤ a / N ≤ 0.03 ···(2) 0.23 ≤ b / N ≤ 0.50 ···(3) 0.50 ≤ c / N ≤ 0.76 ···(4)

[0041] As described above, the bearing ball 2 is formed by a ceramic sintered body comprising alumina crystal particles CPa and zirconia crystal particles CPb containing tetragonal zirconia, satisfying equations (1) to (4). In this way, even when a load is applied to the ceramic sintered body, the zirconia crystal particles CPb are less likely to undergo phase transformation, thus suppressing the shedding of zirconia crystal particles CPb. Therefore, the strength of the ceramic sintered body can be improved.

[0042] Furthermore, according to the bearing ball 2 of this embodiment, the bearing ball 2 is formed from a ceramic sintered body in which the influence of tensile stress generated by the phase transformation of zirconia crystal grains CPb is relatively small. As a result, the strength of the bearing ball 2 can be improved, and thus the product life of the bearing ball 2 can be extended.

[0043] <Modified form of this embodiment> The present invention is not limited to the embodiments described above, and can be implemented in various forms without departing from its spirit, for example, the following modifications are also possible.

[0044] [Example 1] In the above-described embodiment, the ceramic sintered body forms either a cutting tool 1 or a bearing ball 2. However, the components formed by the ceramic sintered body are not limited to these. It is applicable in any technical field where improving the strength of the ceramic sintered body used can extend the product life.

[0045] [Differentiation 2] In the above-described embodiment, the ceramic sintered body contained 70% to 93% by mass of alumina crystal particles and 7% to 30% by mass of zirconia crystal particles. The respective mass percentages of alumina and zirconia in the ceramic sintered body are not limited to these.

[0046] The embodiments of this specification have been described above based on the embodiments and modifications described above. The embodiments described above are for the purpose of facilitating understanding of this specification and do not limit it. This specification may be modified and improved without departing from its spirit and the scope of the claims, and equivalents thereof are included in this specification. Furthermore, any technical features that are not described as essential in this specification may be deleted as appropriate.

[0047] <Application Example 1> A ceramic sintered body, Alumina crystal particles and It comprises zirconia crystal particles containing tetragonal zirconia, In the cross-section of the ceramic sintered body, Let N be the number of zirconia crystal particles contained in a rectangular region with sides of 10 μm. Among the zirconia crystal particles included in the aforementioned region, Let a be the number of zirconia crystal particles surrounded only by other zirconia crystal particles. Let b be the number of zirconia crystal particles surrounded only by alumina crystal particles. If c is the number of zirconia crystal particles surrounded by zirconia crystal particles and alumina crystal particles, A ceramic sintered body characterized by satisfying the following equations (1) to (4). 110 ≤ N ···(1) 0 ≤ a / N ≤ 0.03 ···(2) 0.23 ≤ b / N ≤ 0.50 ···(3) 0.50 ≤ c / N ≤ 0.76 ···(4) <Application Example 2> A ceramic sintered body as described in Application Example 1, The alumina crystal particles comprise 70% by mass or more and 93% by mass or less, The following features include containing zirconia crystal particles in an amount of 7% by mass or more and 30% by mass or less: Ceramic sintered body. <Application Example 3> A cutting tool, The ceramic sintered body described in Application Example 1 or Application Example 2 is formed from a ceramic sintered body, cutting tools. <Application Example 4> It is a bearing ball, The ceramic sintered body described in Application Example 1 or Application Example 2 is formed from a ceramic sintered body, Bearing balls. [Explanation of symbols]

[0048] 1...Cutting tools 2…Bearing balls CPa... Alumina crystal grains CPb... Zirconia crystal grains

Claims

1. A ceramic sintered body, Alumina crystal particles and It comprises zirconia crystal particles containing tetragonal zirconia, In the cross-section of the ceramic sintered body, Let N be the number of zirconia crystal particles contained in a rectangular region with sides of 10 μm. Among the zirconia crystal particles included in the aforementioned region, Let a be the number of zirconia crystal particles surrounded only by other zirconia crystal particles. Let b be the number of zirconia crystal particles surrounded only by alumina crystal particles. If c is the number of zirconia crystal particles surrounded by zirconia crystal particles and alumina crystal particles, A ceramic sintered body characterized by satisfying the following equations (1) to (4). 110 ≤ N ... (1) 0 ≤ a / N ≤ 0.03 ... (2) 0.23 ≤ b / N ≤ 0.50 ... (3) 0.50 ≤ c / N ≤ 0.76 ... (4)

2. A ceramic sintered body according to claim 1, The alumina crystal particles comprise 70% by mass or more and 93% by mass or less, The following features include containing 7% by mass or more and 30% by mass or less of the zirconia crystal particles, Ceramic sintered body.

3. A cutting tool, A ceramic sintered body formed according to claim 1 or claim 2, cutting tools.

4. It is a bearing ball, A ceramic sintered body formed according to claim 1 or claim 2, Bearing balls.

Images