Ceramic sintered body, bearing ball, and cutting tool

Abstract

This ceramic sintered body contains a plurality of alumina crystal grains and zirconia crystal grains. A crystal grain boundary that is formed between alumina crystal grains adjacent to each other among the plurality of alumina crystal grains includes each of a rare earth element and a group 4 element.

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 technology for suppressing a decrease in strength of a ceramic sintered body in a high-temperature environment.

[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 including a plurality of alumina crystal grains and zirconia crystal grains, wherein grain boundaries formed between adjacent alumina crystal grains include a rare earth element and a Group 4 element.

[0008] According to this configuration, the grain boundaries formed between adjacent alumina crystal grains in the ceramic sintered body contain both a rare earth element and a Group 4 element. This improves the stability of the grain boundaries at high temperatures, saturates the element concentrations in the grain boundaries, and reduces the segregation of the rare earth element in the zirconia crystal grains. This prevents a decrease in the strength of the ceramic sintered body at high temperatures.

[0009] (2) In the ceramic sintered body of the above embodiment, the grain boundaries may contain at least one of zirconium and hafnium as a Group 4 element. According to this configuration, at least one of zirconium and hafnium is present as a Group 4 element in the grain boundaries formed between adjacent alumina crystal grains. This improves the bonding strength between the alumina crystal grains, thereby improving the strength of the ceramic sintered body in high-temperature environments.

[0010] (3) In the ceramic sintered body of the above embodiment, the grain boundaries may contain yttrium as a rare earth element. According to this configuration, yttrium is present as a rare earth element in the grain boundaries formed between adjacent alumina crystal grains. This relatively increases the stability of the zirconia crystal grains, thereby further improving the strength of the ceramic sintered body under high-temperature environments.

[0011] (4) In the ceramic sintered body of the above embodiment, the ceramic sintered body may contain 60% by weight to 93% by weight of alumina and 7% by weight to 40% by weight of zirconia. According to this configuration, the ceramic sintered body contains 60% by weight to 93% by weight of alumina and 7% by weight to 40% by weight of zirconia. As a result, the zirconia crystal grains are constrained by the alumina crystal grains, and the toughness of the ceramic sintered body as a structure is improved. Therefore, the decrease in strength of the ceramic sintered body in a high-temperature environment can be further suppressed.

[0012] (5) 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.

[0013] (6) 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 containing a rare earth element and a Group 4 element at the crystal grain boundaries formed between adjacent alumina crystal grains. This makes it possible to suppress a decrease in the strength of the bearing ball in a high-temperature environment.

[0014] (7) According to yet another aspect of the present invention, there is provided a cutting tool. The 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 containing a rare earth element and a Group 4 element at the grain boundaries formed between adjacent alumina crystal grains. This makes it possible to suppress a decrease in the strength of the cutting tool in a high-temperature environment.

[0015] 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.

[0016] FIG. 1 is a schematic diagram of a bearing according to a first embodiment; FIG. 2 is a diagram illustrating the results of a first evaluation test of a ceramic sintered body; FIG. 3 is a diagram illustrating the results of a second evaluation test of a ceramic sintered body; FIG. 4 is a first diagram illustrating a method for detecting elements present in a grain boundary; FIG. 5 is a second diagram illustrating a method for detecting elements present in a grain boundary; FIG. 6 is a third diagram illustrating a method for detecting elements present in a grain boundary; FIG. 7 is a schematic diagram of a cutting tool according to a second embodiment; FIG. 8 is a schematic diagram illustrating a state in which the cutting tool according to the second embodiment is used.

[0017] <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.

[0018] The bearing ball 1 is formed from a ceramic sintered body. The ceramic sintered body forming the bearing ball 1 of this embodiment is an alumina-based sintered body containing a plurality of alumina (Al2O3) crystal grains and zirconia (ZrO2) crystal grains. In the ceramic sintered body of the bearing ball 1, the crystal grain boundaries formed between adjacent alumina crystal grains contain both a rare earth element and a Group 4 element. This improves the stability of the crystal grain boundaries at high temperatures, saturates the element concentrations in the crystal grain boundaries, and makes it difficult for the rare earth element to segregate in the zirconia crystal grains. This therefore makes it possible to suppress a decrease in the strength of the ceramic sintered body in high-temperature environments.

[0019] In the ceramic sintered body forming the bearing ball 1, the grain boundaries formed between adjacent alumina crystal grains contain at least one of zirconium (Zr) and hafnium (Hf) as a Group 4 element. In this embodiment, the grain boundaries contain both zirconium and hafnium. This improves the bonding strength between the alumina crystal grains, thereby improving the strength of the ceramic sintered body in high-temperature environments. Furthermore, in the ceramic sintered body forming the bearing ball 1, the grain boundaries formed between adjacent alumina crystal grains contain yttrium (Y) as a rare earth element. This relatively increases the stability of the zirconia crystal grains. Therefore, the strength of the ceramic sintered body in high-temperature environments can be further improved. Elements contained in the grain boundaries formed between adjacent alumina crystal grains in the ceramic sintered body are detected using a scanning transmission electron microscope and an energy dispersive X-ray analyzer. Details of the method for detecting elements contained in the grain boundaries will be described later.

[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 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, 39.5% by weight of the zirconia crystal particles are tetragonal, and 0.5% by weight of the zirconia crystal particles are cubic. In the zirconia crystal particles, both the tetragonal and cubic crystals are crystal phases that can exist stably, which improves the strength of the bearing ball 10.

[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 72 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 8. For example, if the pH of the mixed slurry is 9, the pH of the mixed slurry is corrected to the acid side by adding nitric acid 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 8 to suppress a decrease in the strength of the ceramic sintered body in a high-temperature environment. When the pH of the mixed slurry reaches 8, the surface of the alumina powder is positively charged, and the surfaces of the zirconia powder and the 3YSZ powder are both negatively charged. In this charged state, the alumina powder and the zirconia powder are attracted to each other, while the zirconia powder and the 3YSZ powder are repelled by each other. This allows the zirconia powder to be appropriately dispersed while in contact with the alumina powder and the 3YSZ powder. The yttrium and zirconium elements contained in the 3YSZ powder migrate to the zirconia crystal particles through the grain boundaries formed between the alumina crystal particles, improving the stability of the grain boundaries between the alumina crystal particles and saturating the element concentrations in the grain boundaries, thereby making it difficult for the rare earth elements to segregate in the zirconia crystal particles. Due to this phenomenon, the decrease in strength of the ceramic sintered body in a high temperature environment can be suppressed more effectively than in a ceramic sintered body produced using a mixed slurry with a pH of 9.

[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 compact. The produced compact is degreased by heat treatment in air at a maximum temperature of 800°C. The degreased compact is then subjected to two air sintering stages. Specifically, the first air sintering stage is performed at a sintering temperature of 1500°C for one hour. The second air sintering stage, which follows the first air sintering stage, is performed at a sintering temperature of 1300°C for one hour. By performing such air sintering, the yttrium and zirconium elements can migrate to the grain boundaries without grain growth in the compact. After the two-stage atmospheric firing, hot isostatic pressing (HIP, treatment temperature 1500°C, treatment pressure 50 MPa, argon atmosphere, treatment time: 2 hours) is performed 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, ceramic sintered body samples were produced using different production methods, and the relationship between the types of elements present in the grain boundaries of alumina crystal grains and the numerical values ​​related to the strength of the ceramic sintered body was evaluated.

[0027] FIG. 2 is a diagram illustrating the results of a first evaluation test of the ceramic sintered body. In this evaluation test, seven types of samples were prepared using different manufacturing methods. Each of the seven types of samples was manufactured using a method similar to the manufacturing method of the ceramic sintered body of the bearing ball 1 of this embodiment. Samples 1 to 5 were manufactured by using raw material powders weighed so that the weight percentages of alumina and zirconia were the values ​​shown as "Al2O3" and "ZrO2" in the "Component (wt %)" section of FIG. 2, adjusting the pH of the mixed slurry to 8, and conducting a first-stage air firing (firing temperature: 1500°C, firing time: 1 hour) in the same manner as the bearing ball 1 of this embodiment, followed by a second-stage air firing at a firing temperature of 1300°C for 1 hour ("Maintain" in the "Maintain 1300°C" section of FIG. 2). Sample 6 was produced by adjusting the weight percentages of alumina and zirconia to the same as Sample 3, adjusting the pH of the mixed slurry to 9, and conducting a first-stage air firing (firing temperature: 1500°C, firing time: 1 hour) in the same manner as Samples 1 to 5, followed by a second-stage air firing at 1300°C for 1 hour (shown as "Maintained" in "Maintained at 1300°C" in Figure 2). Sample 7 was produced by adjusting the weight percentages of alumina and zirconia to the same as Sample 3, adjusting the pH of the mixed slurry to 8, and conducting a first-stage air firing (firing temperature: 1500°C, firing time: 1 hour) in the same manner as Samples 1 to 5, followed by cooling (shown as "Not Maintained" in "Maintained at 1300°C" in Figure 2).

[0028] Figure 2 shows the "density" and "composition" for each of Samples 1 to 7 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 whose 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 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 2, 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. 3 is a diagram illustrating the results of the second evaluation test of the ceramic sintered body. In this evaluation test, Samples 1 to 7 were evaluated for each of the four items shown in Fig. 3. Here, the evaluation methods for each will be described.

[0031] 3 indicates the types of elements confirmed at the grain boundaries formed between alumina crystal grains (hereinafter simply referred to as "types of elements at the grain boundaries"). In this evaluation test, the types of elements at the grain boundaries were identified using a scanning transmission electron microscope (STEM, JEM-F200 manufactured by JEOL Ltd.) and an energy dispersive X-ray analyzer (EDS / EDX, Silicon Drift Detector manufactured by Thermo Fisher Scientific / Pathfinder manufactured by Thermo Fisher Scientific).

[0032] For the analysis of the elements at the grain boundaries, the surface of the produced ceramic sintered body was polished to remove at least 0.25 mm from the baked surface, and then the resulting sample was subjected to ion milling to create a thin slice. Linear analysis was performed on the surface of the thin slice at any five locations on the grain boundaries between adjacent alumina crystal grains along the direction perpendicular to the grain boundaries.

[0033] FIG. 4 is a first diagram illustrating a method for detecting elements present at grain boundaries. FIG. 4 shows a cross-sectional STEM image of a sample captured by a scanning transmission electron microscope. In the analysis of the types of elements at grain boundaries, an analysis line A1 perpendicular to the grain boundary B1 formed between adjacent alumina crystal grains P1 and P2 was defined in the STEM image shown in FIG. 4 , and the intensities of L lines of elements such as yttrium, zirconium, and hafnium were measured along the analysis line A1. The length of the analysis line A1 was 82 nm, and the midpoint (42 nm from the end of the analysis line L1) was defined as the position of the grain boundary B1. The measurement interval for the L line intensity along the analysis line A1 was 1 nm.

[0034] FIG. 5 is a second diagram illustrating a method for detecting elements present at grain boundaries. FIG. 6 is a third diagram illustrating a method for detecting elements present at grain boundaries. FIG. 5 shows an example of the change in intensity of the L line of yttrium element along the analysis line described in FIG. 4 . FIG. 6 shows an example of the change in intensity of the L line of zirconium element along the analysis line described in FIG. 4 . In FIGS. 5 and 6 , the horizontal axis represents the distance from one end of the analysis line described in FIG. 4 , and the vertical axis represents the L line intensity of the element to be detected. In the analysis of the type of element at the grain boundaries, the average intensity Ea of all measurement points was calculated for each of the elements to be detected. Next, the maximum intensity Eb was calculated within a range of 41 nm ± 5 nm (range Rg shown in FIGS. 5 and 6 ), which included the position of the grain boundary (see FIGS. 5 and 6 ). Finally, the magnitude Re of the maximum value Eb relative to the average value Ea was calculated. If the magnitude Re was greater than 2.5, it was determined that the element to be detected was present at the grain boundary. The elements shown in "Types of elements present at grain boundaries" in Figure 3 are listed as elements detected by this detection method. In Samples 6 and 7, no elements were detected at the grain boundaries (below the detection limit), so the "Types of elements present at grain boundaries" is listed as "none."

[0035] The "bending strength" shown in Figure 3 indicates the bending strength of the sample before heating in an autoclave as "before autoclaving," the bending strength of the sample after heating in an autoclave as "after autoclaving," and the change in bending strength before and after heating as the "percent change (%)" of the sample. To measure the bending strength, the surface of the prepared ceramic sintered body was first polished to remove at least 0.25 mm from the baked surface, and then processed into a 3 mm x 4 mm x 50 mm size sample for "bending strength" measurement. According to 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 single point in the center between the supports. The maximum bending stress at break was recorded as the "before autoclaving" value Sf. The sample for "bending strength" measurement was then placed in a dedicated autoclave container with water, sealed, and stored in a constant temperature oven at 150 °C for two weeks. After two weeks of storage, the sample for measuring "bending strength" was taken out of the autoclave and subjected to a three-point bending strength test in accordance with JIS R1601. The maximum bending stress at which the sample broke was taken as the value "after autoclaving," Sa. The "rate of change" shown in FIG. 3 was calculated by formula (1) using the value "before autoclaving," Sf, and the value "after autoclaving," Sa, obtained by the measurement. "Rate of change" = (1 - Sa / Sf) × 100 (1)

[0036] In the analysis results for "types of elements present at grain boundaries" shown in Figure 3, elements were confirmed at the grain boundaries in all of Samples 1 to 5. As shown in Figure 3, Samples 1 to 5 were confirmed to be ceramic sintered bodies with relatively high bending strength and small "rate of change" among the seven samples evaluated in this evaluation test. Specifically, Samples 1 to 5 had bending strengths of 1000 MPa or more before and after heating in an autoclave. Furthermore, Samples 1 to 5 had bending strengths that changed by 5% or less before and after heating in an autoclave, confirming that strength reduction in high-temperature environments was suppressed. Note that hafnium (Hf) was detected at the grain boundaries in Sample 5. This is because hafnium is inevitably contained in zirconia powder and 3YSZ powder, which are the raw materials for zirconia. Sample 5 had the highest zirconia content among Samples 1 to 5, resulting in a high amount of hafnium present at the grain boundaries, exceeding the detection limit of the energy dispersive X-ray analyzer, which is believed to have confirmed its presence.

[0037] As mentioned above, no elements present at the grain boundaries were confirmed in Sample 6. In Sample 6, the pH of the mixed slurry during production was adjusted to 9, which reduced the surface charge of the alumina powder, causing the alumina powder to aggregate more than in Samples 1 to 5, which had a mixed slurry pH of 8, resulting in a longer distance between the zirconia powder particles in the mixed slurry. This is thought to be why elements did not diffuse sufficiently at the grain boundaries formed within the ceramic sintered body, resulting in an increase in the amount of relatively unstable m-ZrO2 (see Figure 2), and therefore the strength after autoclaving was significantly lower than that of Samples 1 to 5.

[0038] In Sample 7, like Sample 6, no elements were found to be present at the grain boundaries. This is thought to be because Sample 7 was sintered in air at a sintering temperature of 1500°C for one hour and then cooled, which made it difficult for elements to diffuse sufficiently at the grain boundaries formed within the ceramic sintered body. For this reason, like Sample 6, Sample 7's strength after autoclaving was thought to be significantly lower than that of Samples 1 to 5.

[0039] According to the bearing ball 1 of this embodiment described above, the ceramic sintered body forming the bearing ball 1 contains a plurality of alumina crystal grains and zirconia crystal grains, and the grain boundaries formed between adjacent alumina crystal grains contain both a rare earth element and a Group 4 element. This improves the stability of the grain boundaries at high temperatures, saturates the element concentrations in the grain boundaries, and makes it difficult for the rare earth element to segregate in the zirconia crystal grains. This therefore makes it possible to suppress a decrease in the strength of the ceramic sintered body in high-temperature environments.

[0040] Furthermore, according to the bearing ball 1 of this embodiment, both zirconium and hafnium are present as Group 4 elements at the grain boundaries formed between adjacent alumina crystal grains in the ceramic sintered body that forms the bearing ball 1. This improves the bonding strength between the alumina crystal grains, thereby improving the strength of the ceramic sintered body in high-temperature environments.

[0041] Furthermore, according to the bearing ball 1 of this embodiment, the ceramic sintered body that forms the bearing ball 1 contains yttrium as a rare earth element at the grain boundaries formed between adjacent alumina crystal grains. The yttrium is incorporated into the zirconia crystal grains via the grain boundaries. This relatively increases the stability of the zirconia crystal grains, further improving the strength of the ceramic sintered body in high-temperature environments.

[0042] Furthermore, according to the bearing ball 1 of this embodiment, the ceramic sintered body forming the bearing ball 1 contains 60% by weight to 93% by weight of alumina and 7% by weight to 40% by weight of zirconia. This allows the zirconia crystal grains to be constrained by the alumina crystal grains, and improves the toughness of the ceramic sintered body as a structure. Therefore, it is possible to further suppress a decrease in the strength of the ceramic sintered body in a high-temperature environment.

[0043] Furthermore, in the bearing ball 1 of this embodiment, the zirconia crystal grains have at least one of the tetragonal and cubic crystal phases, which are relatively stable, thereby further improving the strength of the ceramic sintered body.

[0044] Second Embodiment FIG. 7 is a schematic diagram of a cutting tool according to a second embodiment. The cutting tools 2a to 2g shown in FIG. 7 are formed of a ceramic sintered body. The ceramic sintered body forming the cutting tools 2a to 2g of this embodiment is an alumina-based sintered body containing a plurality of alumina crystal grains and zirconia crystal grains. In the ceramic sintered body of the cutting tools 2a to 2g, the grain boundaries formed between adjacent alumina crystal grains contain a rare earth element and a Group 4 element. This configuration makes the cutting tools 2a to 2g of this embodiment less susceptible to loss of strength even when used in a high-temperature environment, and therefore less susceptible to breakage even when used in a high-temperature environment. As shown in FIG. 7, the cutting tools 2a to 2g of this embodiment can be used in various shapes.

[0045] Fig. 8 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. 8.

[0046] According to the cutting tool 2 of this embodiment described above, the ceramic sintered body forming the cutting tool 2 includes a plurality of alumina crystal grains and zirconia crystal grains, and the grain boundaries formed between adjacent alumina crystal grains include a rare earth element and a Group 4 element. This improves the stability of the grain boundaries at high temperatures, saturates the element concentrations in the grain boundaries, and reduces the likelihood of segregation of the rare earth element in the zirconia crystal grains. This therefore makes it possible to suppress a decrease in the strength of the ceramic sintered body in high-temperature environments.

[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, in the ceramic sintered body, the grain boundaries formed between adjacent alumina crystal grains contain both zirconium and hafnium as Group 4 elements. However, the grain boundaries may contain at least one of zirconium and hafnium. Furthermore, the grain boundaries may not contain either zirconium or hafnium, as long as they contain a Group 4 element. By containing at least one of zirconium and hafnium, the bonding strength between the alumina crystal grains can be improved.

[0049] [Modification 2] In the above-described embodiment, the grain boundaries formed between adjacent alumina crystal grains in the ceramic sintered body contain yttrium as a rare earth element. However, the grain boundaries do not necessarily contain yttrium, as long as they contain a rare earth element. By containing yttrium, the stability of the zirconia crystal grains can be improved.

[0050] [Modification 3] 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.

[0051] [Modification 4] 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.

[0052] 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.

[0053] <Application Example 1> A ceramic sintered body including a plurality of alumina crystal grains and zirconia crystal grains, wherein a grain boundary formed between adjacent alumina crystal grains contains a rare earth element and a Group 4 element. <Application Example 2> The ceramic sintered body according to Application Example 1, wherein the grain boundary contains at least one of zirconium and hafnium as the Group 4 element. <Application Example 3> The ceramic sintered body according to Application Example 1 or Application Example 2, wherein the grain boundary contains yttrium as the rare earth element. <Application Example 4> The ceramic sintered body according to any one of Application Examples 1 to 3, wherein the ceramic sintered body contains 60% by weight to 93% by weight of alumina and 7% by weight to 40% by weight of zirconia. <Application Example 5> The ceramic sintered body according to any one of Application Examples 1 to 4, wherein the crystalline phase of the zirconia crystal particles is at least one of a tetragonal system and a cubic system. <Application Example 6> A bearing ball, wherein the bearing ball is formed from the ceramic sintered body according to any one of Application Examples 1 to 5. <Application Example 7> A cutting tool, wherein the cutting tool is formed from the ceramic sintered body according to any one of Application Examples 1 to 5.

[0054] 1... Bearing ball 2... Cutting tool P1, P2... Alumina crystal grains B1... Crystal grain boundary

Claims

1. A ceramic sintered body comprising a plurality of alumina crystal grains and zirconia crystal grains, wherein the grain boundaries formed between adjacent alumina crystal grains contain a rare earth element and a Group 4 element, respectively.

2. A ceramic sintered body according to claim 1, characterized in that the grain boundaries contain at least one element selected from the group 4 elements of zirconium and hafnium.

3. A ceramic sintered body according to claim 1, characterized in that the grain boundaries contain yttrium as a rare earth element.

4. 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.

5. 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.

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

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

Images