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 oxides of rare earth elements or alkaline earth elements. In the cross-section of the ceramic sintered body, if A is the average particle size of the alumina crystal particles contained in a rectangular region with sides of 10 μm, B is the average particle size of the zirconia crystal particles contained in the region, N is the number of zirconia crystal particles contained in the region, Lc is the average perimeter of the zirconia crystal particles contained in the region, and Lb is the average interface length, which is the sum of the lengths of the parts where the zirconia crystal particles contained in the region touch each other divided by N, then the following equations (1) to (2) are satisfied. B / A < 0.8 ···(1) Lb / Lc < 0.35 ···(2)

Description

Technical Field

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

Background Art

[0002] Conventionally, a ceramic sintered body based on alumina has 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 was still room for improvement in the technology for improving the strength of the ceramic sintered body.

[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 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 an oxide of a rare earth element or an alkaline earth element, and in the cross-section of the ceramic sintered body, if A is the average particle size of the alumina crystal particles contained in a rectangular region with sides of 10 μm, B is the average particle size of the zirconia crystal particles contained in the region, N is the number of zirconia crystal particles contained in the region, Lc is the average perimeter of the zirconia crystal particles contained in the region, and Lb is the average interface length which is the sum of the lengths of the parts in contact with each other by N, then the following equations (1) to (2) are satisfied. B / A < 0.8 ···(1) Lb / Lc < 0.35 ···(2)

[0008] According to this configuration, a ceramic sintered body comprising alumina crystal particles and zirconia crystal particles containing oxides of rare earth elements or alkaline earth elements satisfies equations (1) and (2). In a ceramic sintered body satisfying equation (1), the zirconia crystal particles are stabilized because they are more easily constrained by the alumina crystal particles. In a ceramic sintered body satisfying equation (2), the zirconia crystal particles are stabilized because the rare earth elements or alkaline earth elements, which are stabilizing elements of the zirconia crystal particles, are less likely to concentrate at the interfaces between the zirconia crystal particles. As a result of this stabilization of the zirconia crystal particles, phase transformation of the zirconia crystal particles in high-temperature environments can be suppressed. Therefore, the strength of the ceramic sintered body can be improved.

[0009] (2) In the ceramic sintered body of the above form, the average interface length of the multiple zirconia crystal particles may be less than 95 μm. With this configuration, the interfaces between zirconia crystal particles, where yttrium contained in the zirconia crystal particles tends to concentrate, become even smaller. As a result, the zirconia crystal particles are further stabilized, and the phase transformation of the zirconia crystal particles under high-temperature conditions can be further suppressed. Therefore, the strength of the ceramic sintered body can be further improved.

[0010] (3) 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, a 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.

[0011] (4) 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. According to this configuration, the cutting tool is formed from a ceramic sintered body comprising zirconia crystal grains that are stable even in high-temperature environments. This improves the strength of the cutting tool and thus extends the product life of the cutting tool.

[0012] (5) 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. According to this configuration, the bearing ball is formed from a ceramic sintered body comprising zirconia crystal grains that are stable even in high-temperature environments. This improves the strength of the bearing ball and thus extends the product life of the bearing ball.

[0013] 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]

[0014] [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 schematic cross-sectional view of a ceramic sintered body. [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]

[0015] <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 oxides of rare earth elements or alkaline earth elements. 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.

[0016] Figure 3 is a schematic cross-sectional view of the ceramic sintered body forming the cutting tool 1. The schematic cross-sectional view of the ceramic sintered body shown in Figure 3 is a schematic representation of a portion of an image taken using a scanning electron microscope (SEM). In Figure 3, the areas indicated by hatching with diagonal lines are alumina crystal particles CPa, and the areas indicated by hatching with dots 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 zirconia crystal particles in the ceramic sintered body forming the cutting tool 1 of this embodiment contain yttria (Y2O3), which is an oxide of a rare earth element. In this case, yttrium (Y) acts as a stabilizing element for the zirconia crystal particles. Furthermore, the oxides contained in the zirconia crystal particles are not limited to yttrium; they may also be oxides of other rare earth elements, or oxides of alkaline earth elements such as magnesium oxide (MgO) or calcium oxide (CaO).

[0017] In the ceramic sintered body forming the cutting tool 1 of the present embodiment, the relationship between the particle size of the alumina crystal particles CPa and the particle size of the zirconia crystal particles CPb, and the relationship between the crystal particles around the zirconia crystal particles CPb have the following characteristics. In the cross-section of the ceramic sintered body, the average particle size of the alumina crystal particles contained in a rectangular region with a side of 10 μm is denoted as A, the average particle size of the zirconia crystal particles contained in the region is denoted as B, the number of zirconia crystal particles contained in the region is denoted as N, the average circumferential length of the zirconia crystal particles contained in the region is denoted as Lc, and the average interface length Lb is the value obtained by dividing the total length of the portions where the zirconia crystal particles contained in the region contact each other by N. Then, the following formulas (1) to (2) are satisfied. Note that each of the average particle sizes A and B, the number N, the average circumferential length Lc, and the average interface length Lb was measured by using a thermal etching method in which the cross-section of the ceramic sintered body to be measured was mirror-polished and then heat-treated. Specifically, SEM images at 10,000 times magnification of arbitrary multiple locations were acquired for the cross-section of the ceramic sintered body treated by the thermal etching method, and the measurement was performed using image analysis software WinROOF (manufactured by Mitani Corporation) for the acquired SEM images. The average particle sizes A of the zirconia crystal particles CPb and B of the alumina crystal particles CPa respectively correspond to the equivalent circle diameters of the respective crystal particles in the above-mentioned image analysis software. B / A < 0.8 ···(1) Lb / Lc < 0.35 ···(2)

[0018] Using the cross-sectional schematic diagram shown in FIG. 3, the average perimeter length Lc of the zirconia crystal particles CPb will be specifically described. The perimeter length Lc1 of the zirconia crystal particle CPb located approximately at the center of FIG. 3 is the length of the outer contour line of the zirconia crystal particle CPb, which is the length of the solid line Sc in FIG. 3. Therefore, when a plurality of zirconia crystal particles CPb are included in a rectangular region with a side length of 10 μm, the average perimeter length Lc of the zirconia crystal particles CPb is the value obtained by dividing the sum of the respective perimeter lengths Lc1 of the plurality of zirconia crystal particles CPb by the number of zirconia crystal particles CPb included in the rectangular region with a side length of 10 μm. Note that the zirconia crystal particles CPb that are not entirely included in the rectangular region with a side length of 10 μm are excluded from the calculation of the average perimeter length Lc.

[0019] Using the cross-sectional schematic diagram shown in FIG. 3, the average interface length Lb of the zirconia crystal particles CPb will be specifically described. The interface length Lb1 of the zirconia crystal particle CPb located approximately at the center of FIG. 3 is the length of the portion of the outer contour line of the zirconia crystal particle CPb that contacts other zirconia crystal particles CPb. Specifically, among the outer contour line Sc of the zirconia crystal particle CPb shown in FIG. 3, the sum of the length of the outer contour line Sc1 between the boundary point P1 and the boundary point P2 and the length of the outer contour line Sc2 between the boundary point P3 and the boundary point P4 is the interface length Lb1 of the zirconia crystal particle CPb. Therefore, when a plurality of zirconia crystal particles CPb are included in a rectangular region with a side length of µm, the average interface length Lb of the zirconia crystal particles CPb is the value obtained by dividing the sum of the respective interface lengths Lb1 of the plurality of zirconia crystal particles CPb by the number of zirconia crystal particles CPb included in the rectangular region with a side length of 10 μm. Note that the zirconia crystal particles CPb that are not entirely included in the rectangular region with a side length of 10 μm are excluded from the calculation of the average interface length Lb.

[0020] Equation (1) shows that the average particle size of zirconia crystal particles CPb is a certain degree smaller than the average particle size of alumina crystal particles CPa. When the ceramic sintered body forming the cutting tool 1 satisfies equation (1), the zirconia crystal particles CPb are more easily constrained by the alumina crystal particles CPa, thus stabilizing the zirconia crystal particles CPb. Equation (2) shows that the length of the contact area between zirconia crystal particles CPb around a single zirconia crystal particle CPb is relatively short. When the ceramic sintered body forming the cutting tool 1 satisfies equation (2), the yttrium contained in one zirconia crystal particle CPb is less likely to move to other zirconia crystal particles CPb, thus reducing its concentration at the interface between zirconia crystal particles CPb. As a result, yttrium is more likely to remain in each of the multiple zirconia crystal particles CPb, thus stabilizing the zirconia crystal particles CPb. Therefore, since the phase transformation of zirconia crystal particles is suppressed even in high-temperature environments, the strength of the ceramic sintered body can be improved. The ceramic sintered body forming the cutting tool 1 of this embodiment has a B / A ratio of 0.59 and a Lb / Lc ratio of 0.30.

[0021] In the ceramic sintered body forming the cutting tool 1 of this embodiment, the average interface length Lb of the multiple zirconia crystal particles CPb in a rectangular region of the cross-section of the ceramic sintered body with sides of 10 μm is less than 95 μm. When the average interface length Lb of the zirconia crystal particles CPb is less than 95 μm, the interfaces between the zirconia crystal particles CPb, where yttrium contained in the zirconia crystal particles CPb tends to concentrate, become even smaller. As a result, the yttrium contained in the zirconia crystal particles CPb becomes less likely to migrate, and the zirconia crystal particles are further stabilized. Therefore, the phase transformation of the zirconia crystal particles CPb under high-temperature environments can be further suppressed, and the strength of the ceramic sintered body can be further improved. In the ceramic sintered body forming the cutting tool 1 of this embodiment, the average interface length Lb of the zirconia crystal particles CPb is 71 μm.

[0022] Next, the manufacturing method of cutting tool 1 will be described. In the manufacturing method of cutting tool 1, alumina powder (average particle size: 0.9 μm) and yttria-stabilized zirconia powder (hereinafter referred to as "3YSZ powder," average particle size: 0.7 μm) are used as raw material powders. First, the alumina powder and 3YSZ powder are weighed out so that the content of alumina in the ceramic sintered body of cutting tool 1 is 70% by mass and zirconia is 30% by mass.

[0023] The weighed raw material powder is placed in a pot along with alumina balls, pure water is added as a solvent, and ball milling is performed at a rotation speed of 60 revolutions per minute for 72 hours to pulverize and mix the raw material powder, thereby producing a mixed slurry. A binder, such as an acrylic resin, is added to the mixed slurry and mixed for a further 30 minutes, after which the mixed slurry is dried by spray drying to produce a mixed powder.

[0024] In the manufacturing method of the cutting tool 1 of this embodiment, the mixed slurry is prepared by placing the weighed raw material powder into an alumina pod. This improves the grinding efficiency of the alumina powder and 3YSZ powder compared to using a resin pod, allowing the alumina powder and 3YSZ powder to be made into even finer powders. A polycarboxylic acid-based dispersant is used as the dispersant. In particular, ammonium polycarboxylic acid salts as dispersants are suitable for hydrophilic oxides, so they can reduce the amount of water in the mixed slurry while appropriately dispersing the alumina powder and 3YSZ powder. The amount of polycarboxylic acid-based dispersant added is preferably about 0.2 wt% to 0.5 wt% in terms of solid content. Adding a small amount of dispersant makes it easier to disperse the 3YSZ powder.

[0025] 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 prepare 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. In the manufacturing method of the cutting tool 1 of this embodiment, the mixing ratio of 10 mm diameter spheres to 5 mm diameter spheres is 8:1 by mass. In ball milling of the weighed raw material powder, the 3YSZ powder is first introduced into the pod to perform preliminary crushing of the 3YSZ powder. This allows for efficient crushing, disintegration, and dispersion of the 3YSZ powder. In the manufacturing method of the cutting tool 1 of this embodiment, the appropriate time for ball milling targeting only the 3YSZ powder out of the 72 hours of ball milling is approximately 20% to 40%.

[0026] 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.

[0027] Next, we will describe the evaluation test of ceramic sintered bodies. In this evaluation test, samples of ceramic sintered bodies were prepared using different manufacturing methods, and the relationship between the differences in crystal particle size and the relationship between the crystal particles surrounding the zirconia crystal particles and the change in the strength of the ceramic sintered bodies due to high-temperature storage was evaluated.

[0028] 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 similar to the manufacturing method of the ceramic sintered body of 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.

[0029] In the preparation of the mixed slurry for Samples 1 to 8, the weighed raw materials were placed into pods made of the material shown in "Pot Material" in Figure 4, and the amount of dispersant shown in "Dispersant (wt%)" in Figure 4 was used. "Pre-grinding" in Figure 4 indicates whether or not pre-grinding of the 3YSZ powder was performed by ball milling of the raw material powder in Samples 1 to 8. Sample 6 differs from Samples 1 to 5 in "Pot Material". Sample 7 differs from Samples 1 to 5 in "Dispersant (wt%)". Sample 8 differs from Samples 1 to 5 in "Pre-grinding".

[0030] 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.

[0031] Figure 5 is the second figure illustrating the results of the evaluation test of the ceramic sintered body. Figure 5 shows the "composition (wt%)" indicating the type and amount of crystalline phase for each of Samples 1 to 8. Specifically, Figure 5 shows the mass ratio (unit: wt%) of alumina crystalline particles to zirconia crystalline particles contained in each sample, and the mass percentage (unit: wt%) of the crystalline phase in the zirconia crystalline particles relative to the total sample. For the measurement of "composition," 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 was removed was further mirror-polished to prepare a surface for "composition" measurement. The crystalline phase of the crystalline particles contained in the sample was identified on the prepared surface for "composition" measurement using X-ray diffraction (XRD). The quantification 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 using the WPPF method, the ICDD card numbers for "Al2O3," "ZrO2," "t-ZrO2," "c-ZrO2," and "m-ZrO2" shown in "Components (wt%)" in Figure 5, which were referenced, are as follows. 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

[0032] As shown in Figure 5, in each of Samples 1 to 5, all or most of the zirconia crystal grains are tetragonal zirconia (t-ZrO2). On the other hand, in each of Samples 6 to 8, in addition to tetragonal zirconia, approximately 25% each of cubic zirconia (c-ZrO2) and monoclinic zirconia (m-ZrO2) is found.

[0033] Figure 6 is the third figure illustrating the results of the evaluation test of the ceramic sintered body. For each of Samples 1 to 8, Figure 6 shows the "average particle size (μm)" indicating the average particle size of the crystal grains, the "perimeter length (μm)" showing the relationship between the particle size of the zirconia crystal grains and the surrounding crystal grains, and the "bending strength (MPa)" showing the change in bending strength due to high-temperature storage.

[0034] Figure 6 shows the "Average Particle Size (μm)" for each of the eight samples, from sample 1 to sample 8, representing the average particle size of the alumina crystal particles ("Al2O3") and the average particle size of the zirconia crystal particles ("ZrO2"). The average particle sizes of the alumina crystal particles and zirconia crystal particles for each of the eight samples were measured using the same method as the measurement method for the average particle size B of the alumina crystal particles CPa and the average particle size A of the zirconia crystal particles CPb in the ceramic sintered body forming the cutting tool 1 of this embodiment. Figure 6 shows the "Particle Size Ratio (-)" representing the ratio of the average particle size of the zirconia crystal particles to the average particle size of the alumina crystal particles. Of the eight samples, samples 1 to 5, sample 7, and sample 8 all have a "Particle Size Ratio (-)" of 0.8 or less.

[0035] In Figure 6, the "Perimeter Length (μm)" shows the average perimeter length of the zirconia crystal grains as "Lc" and the average interface length as "Lb" for each of the eight samples (1 to 8). The average perimeter length Lc and average interface length Lb for each of the eight samples were measured using the same method as the measurement method for the average perimeter length Lc and average interface length Lb of the zirconia crystal grains CPb in the ceramic sintered body forming the cutting tool 1 of this embodiment. The "Perimeter Ratio (-)" in Figure 6 shows the ratio of the average interface length Lb to the average perimeter length Lc. Of the eight samples (1 to 8), each of the eight samples (1 to 6) has a "Perimeter Ratio (-)" of 0.35 or less.

[0036] Figure 6 shows the "Bending Strength (MPa)" for each of Samples 1 to 8, before and after high-temperature storage in an autoclave. For the measurement of "Bending Strength (MPa)," samples were prepared by polishing the surface of the samples to remove at least 0.5 mm from the hardened surface, and then processing them into 3 mm x 4 mm x 50 mm size samples for bending strength measurement. First, in accordance with JIS R1601, a three-point bending strength test was performed by placing the sample before high-temperature storage on two support points 30 mm apart, and then applying a load to the center point between the support points. The maximum bending stress at which the sample broke was defined as the bending strength of the sample before high-temperature storage ("before autoclave"). Next, the samples with the measured bending strength before high-temperature storage were sealed in an autoclave with water and stored in a constant temperature oven at 150°C for two weeks. Subsequently, the samples removed from the autoclave and stored at high temperature were subjected to a three-point bending strength test, similar to the samples before high-temperature storage. The maximum bending stress at which the sample broke was defined as the bending strength of the sample after high-temperature storage ("after autoclaving"). The "Change Rate (-)" in Figure 6 shows the ratio of the bending strength of the sample after high-temperature storage to the bending strength of the sample before high-temperature storage.

[0037] Of the eight samples shown in Figure 6, samples 1 through 5 all showed bending strengths of 1000 MPa or higher before and after high-temperature storage, with a maximum "change rate (-)" of 7% or less. On the other hand, samples 6 through 8 showed bending strengths below 1000 MPa after high-temperature storage, with a "change rate (-)" exceeding 40% in all cases. In other words, it was confirmed that samples 1 through 5, each with a "particle size ratio (-)" of 0.8 or less and a "perimeter ratio (-)" of 0.35 or less, suppressed the decrease in bending strength even in high-temperature environments.

[0038] Samples 1 to 5, shown in Figure 6, have an average interface length Lb of zirconia crystal particles smaller than 95 μm. This further reduces the interface between zirconia crystal particles, where yttrium, a stabilizing element contained in zirconia crystal particles, tends to concentrate. As a result, the yttrium contained in the zirconia crystal particles CPb is less likely to migrate. Therefore, the zirconia crystal particles are further stabilized, and the strength of the ceramic sintered body can be further improved.

[0039] In ceramic sintered bodies containing alumina and zirconia crystal particles, the zirconia crystal particles may undergo a phase transformation from tetragonal to monoclinic under high-temperature conditions of 100°C to 300°C. This phase transformation of zirconia crystal particles from tetragonal to monoclinic occurs because excessive segregation of yttrium contained in the 3YSZ powder during firing in the manufacturing process of the ceramic sintered body leads to the formation of unstable zirconia crystal particles. The phase transformation from tetragonal to monoclinic in zirconia crystal particles results in a volume expansion of approximately 4%, leading to defects such as cracks and delamination, and reducing the strength of the ceramic sintered body. Cutting tools become hot during use, which may cause defects such as cracks and delamination during use.

[0040] As described above, the ceramic sintered body forming the cutting tool 1 of this embodiment has a relationship between the particle size of alumina crystal particles CPa and the particle size of zirconia crystal particles CPb, and a relationship between the particle size of zirconia crystal particles CPb and the surrounding crystal particles, where B / A < 0.8 and Lb / Lc < 0.35. In other words, the ceramic sintered body forming the cutting tool 1 has a particle size ratio (-) of 0.8 or less and a perimeter ratio (-) of 0.35 or less. As a result, the ceramic sintered body forming the cutting tool 1, like samples 1 to 5 shown in Figure 6, has its bending strength suppressed even in high-temperature environments, thus improving the strength of the ceramic sintered body.

[0041] As described above, the cutting tool 1 of this embodiment comprises alumina crystal particles CPa and zirconia crystal particles CPb containing yttria, and satisfies equations (1) and (2). In a ceramic sintered body satisfying equation (1), the zirconia crystal particles CPb are more easily constrained by the alumina crystal particles CPa, thus stabilizing the zirconia crystal particles CPb. In a ceramic sintered body satisfying equation (2), the yttrium contained in the zirconia crystal particles CPb is less likely to concentrate at the interface between the zirconia crystal particles CPb, thus stabilizing the zirconia crystal particles CPb. In this way, the zirconia crystal particles CPb are stabilized, and phase transformation of the zirconia crystal particles CPb in high-temperature environments can be suppressed. Therefore, the strength of the ceramic sintered body can be improved.

[0042] Furthermore, according to the cutting tool 1 of this embodiment, the average interface length of the multiple zirconia crystal particles CPb in the ceramic sintered body forming the cutting tool 1 is less than 95 μm. As a result, the interfaces between the zirconia crystal particles CPb, where yttrium contained in the zirconia crystal particles CPb tends to concentrate, become even smaller, thus further stabilizing the zirconia crystal particles. Therefore, the phase transformation of the zirconia crystal particles CPb under high-temperature environments can be further suppressed, and the strength of the ceramic sintered body can be further improved.

[0043] 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 and 7% to 30% by mass of zirconia crystal particles. This makes it possible to achieve both chemical stability and toughness as a structure formed from ceramics, and further suppress the shedding of zirconia crystal particles CPb. Therefore, the strength of the ceramic sintered body can be further improved.

[0044] Furthermore, the cutting tool 1 of this embodiment is formed from a ceramic sintered body containing zirconia crystal particles CPb, which are stable even in high-temperature environments. This improves the strength of the cutting tool, thereby extending the product life of the cutting tool 1.

[0045] <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.

[0046] 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 oxides of rare earth elements or alkaline earth elements. In the cross-section of the ceramic sintered body forming the bearing ball 2 of this embodiment, if A is the average particle size of the alumina crystal particles contained in a rectangular region with sides of 10 μm, B is the average particle size of the zirconia crystal particles contained in the region, N is the number of zirconia crystal particles contained in the region, Lc is the average perimeter of the zirconia crystal particles contained in the region, and Lb is the average interface length which is the sum of the lengths of the parts where the zirconia crystal particles contained in the region touch each other divided by N, then the following equations (1) to (2) are satisfied. B / A < 0.8 ···(1) Lb / Lc < 0.35 ···(2) In the ceramic sintered body forming the bearing ball 2, the average interface length Lb of the multiple zirconia crystal grains CPb in a rectangular region of the cross-section of the ceramic sintered body with one side of 10 μm is less than 95 μm.

[0047] As described above, the bearing ball 2 comprises alumina crystal particles CPa and zirconia crystal particles CPb containing yttria, satisfying equations (1) and (2). In this way, the zirconia crystal particles CPb are stabilized, and the phase transformation of the zirconia crystal particles CPb under high-temperature conditions can be suppressed. Therefore, the strength of the ceramic sintered body can be improved.

[0048] Furthermore, the bearing ball 2 of this embodiment is formed from a ceramic sintered body containing zirconia crystal particles CPb, which are stable even in high-temperature environments. This improves the strength of the cutting tool, thereby extending the product life of the bearing ball 2.

[0049] <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.

[0050] [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.

[0051] [Differentiation 2] In the above-described embodiment, the average circumference of the zirconia crystal particles was assumed to be less than 95 μm. However, the average circumference of the zirconia crystal particles may be 95 μm or more, but the shorter the circumference, the smaller the interface between the zirconia crystal particles becomes, making the zirconia crystal particles easier to stabilize.

[0052] [Difference 3] 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.

[0053] 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.

[0054] <Application Example 1> A ceramic sintered body, Alumina crystal particles and A zirconia crystal particle containing an oxide of a rare earth element or an alkaline earth element, In the cross-section of the ceramic sintered body, Let A be the average particle size of alumina crystal particles contained in a rectangular region with sides of 10 μm. Let B be the average particle size of the zirconia crystal particles contained in the aforementioned region. Let N be the number of zirconia crystal particles contained in the region. Let Lc be the average perimeter length of the zirconia crystal grains contained in the aforementioned region. If Lb is the average interface length, which is the sum of the lengths of the parts where the zirconia crystal particles in the aforementioned region are in contact with each other, divided by N, then The following equations (1) and (2) are satisfied, Ceramic sintered body. B / A < 0.8 ···(1) Lb / Lc < 0.35 ···(2) <Application Example 2> A ceramic sintered body as described in Application Example 1, The average interface length of multiple zirconia crystal particles is less than 95 μm. Ceramic sintered body. <Application Example 3> A ceramic sintered body as described in Application Example 1 or Application Example 2, 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 4> A cutting tool, The material is characterized by being formed from a ceramic sintered body described in any one of the three application examples, from Application Example 1 to Application Example 3. cutting tools. <Application Example 5> It is a bearing ball, The material is characterized by being formed from a ceramic sintered body described in any one of the three application examples, from Application Example 1 to Application Example 3. Bearing balls. [Explanation of symbols]

[0055] 1...Cutting tools 2…Bearing balls CPa... Alumina crystal grains CPb... Zirconia crystal grains

Claims

1. A ceramic sintered body, Alumina crystal particles and A zirconia crystal particle containing an oxide of a rare earth element or an alkaline earth element, In the cross-section of the ceramic sintered body, Let A be the average particle size of alumina crystal particles contained in a rectangular region with sides of 10 μm. Let B be the average particle size of the zirconia crystal particles contained in the aforementioned region. Let N be the number of zirconia crystal particles contained in the region. Let Lc be the average perimeter length of the zirconia crystal particles contained in the aforementioned region. If Lb is the average interface length, which is the sum of the lengths of the parts where the zirconia crystal particles in the region are in contact with each other, divided by N, The following equations (1) to (2) are satisfied, Ceramic sintered body. B / A<0.8...(1) Lb / Lc<0.35...(2)

2. A ceramic sintered body according to claim 1, The average interface length of multiple zirconia crystal particles is less than 95 μm. Ceramic sintered body.

3. A ceramic sintered body according to claim 1 or claim 2, The alumina crystal particles comprise 60% 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.

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

5. A cutting tool, The ceramic sintered body described in claim 3 is formed from a ceramic sintered body, cutting tools.

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

7. It is a bearing ball, The ceramic sintered body described in claim 3 is formed from a ceramic sintered body, Bearing balls.

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