Ceramic sintered compact, bearing ball, and cutting tool

Abstract

This ceramic sintered compact contains: zirconia crystal particles containing yttria; and alumina crystal particles. The ratio ({Y / (Zr+Y) × 100} between elemental yttrium (Y) and elemental zirconium (Zr) contained in the zirconia crystal particles is 1.5-20 at%.

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

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

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

[0007] (1) According to one aspect of the present invention, there is provided a ceramic sintered body, which includes zirconia crystal grains containing yttria and alumina crystal grains, and the ratio of zirconium (Zr) to yttrium (Y) contained in the zirconia crystal grains ({Y / (Zr+Y)}×100) is 1.5 at % or more and 20 at % or less.

[0008] According to this configuration, the ratio of zirconium to yttrium contained in the zirconia crystal grains ({Y / (Zr+Y)}×100) is 1.5 at% or more and 20 at% or less. The inclusion of yttrium in this concentration range makes the zirconia crystal grains relatively stable, and the alumina crystal grains restrict phase transformation. This prevents a decrease in the strength of the ceramic sintered body in high-temperature environments.

[0009] (2) 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. This makes it easier for zirconia crystal grains to be bound by 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.

[0010] (3) 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.

[0011] (4) 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 zirconia crystal grains in which the ratio of zirconium element (Zr) to yttrium element (Y) ({Y / (Zr+Y)}×100) is 1.5 at% or more and 20 at% or less. This makes it possible to suppress a decrease in the strength of the bearing ball in a high-temperature environment.

[0012] (5) 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 zirconia crystal grains in which the ratio of zirconium element (Zr) to yttrium element (Y) ({Y / (Zr+Y)}×100) is 1.5 at% or more and 20 at% or less. This makes it possible to suppress a decrease in the strength of the cutting tool in high-temperature environments.

[0013] 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.

[0014] 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 schematic diagram of a cutting tool according to a second embodiment; Fig. 5 is a schematic diagram illustrating a state in which the cutting tool according to the second embodiment is used;

[0015] <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.

[0016] The bearing ball 1 is formed of a ceramic sintered body. The ceramic sintered body forming the bearing ball 1 of this embodiment is an alumina-based sintered body, containing zirconia (ZrO) crystal grains containing yttria (YO) and alumina (AlO) crystal grains. In the ceramic sintered body of the bearing ball 1, the ratio of zirconium (Zr) to yttrium (Y) contained in the zirconia crystal grains, {Y / (Zr+Y)}×100, is 1.5 at% to 20 at%. This makes the bearing ball 1 of this embodiment less susceptible to strength degradation even when used in high-temperature environments, and therefore less susceptible to breakage even when used in high-temperature environments. The ratio of zirconium to yttrium contained in the zirconia crystal grains of the ceramic sintered body is measured using a scanning transmission electron microscope and an energy dispersive X-ray analyzer. Details of a method for detecting zirconium and yttrium contained in the zirconia crystal grains will be described later.

[0017] The ceramic sintered body forming the bearing ball 1 contains 60% by weight or more and 93% by weight or less of alumina and 7% by weight or more and 40% by weight or less of zirconia. The ceramic sintered body of the bearing ball 1 of this embodiment contains 60% by weight of alumina and 40% by weight of zirconia. The ceramic sintered body of the bearing ball 1 of this embodiment also contains a small amount of yttrium (Y) as an additive.

[0018] 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, 39.7% by weight of the zirconia crystal particles are tetragonal, and 0.3% by weight of the zirconia crystal particles are cubic. In the zirconia crystal particles, both the tetragonal and cubic crystals are crystalline phases that can exist stably, which improves the strength of the bearing ball 10.

[0019] 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.

[0020] 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 grind and mix the raw material powder, thereby producing 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 6.5. 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.

[0021] In the manufacturing method of the bearing ball 1 of this embodiment, the pH of the mixed slurry is adjusted to 6.5 to prevent a decrease in the strength of the ceramic sintered body in a high-temperature environment. When the pH of the mixed slurry reaches 6.5, the surfaces of the alumina powder and the 3YSZ powder are positively charged, and the zirconia powder is negatively charged. In this charged state, the zirconia powder is attracted to the alumina powder and the 3YSZ powder, but the alumina powder and the 3YSZ powder repel each other. As a result, the zirconia powder is appropriately dispersed while in contact with the alumina powder and the 3YSZ powder, and the yttrium contained in the 3YSZ powder diffuses into the zirconia crystal particles, thereby suppressing excessive segregation of the yttrium. Furthermore, the alumina powder is attracted to the zirconia powder and restrains it, thereby suppressing the segregation of the yttrium and suppressing the phase transformation of the zirconia crystal particles in the ceramic sintered body. 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 10.

[0022] A binder, such as an acrylic resin, is added to the mixed slurry, and the mixture is mixed for another 30 minutes. The mixed slurry is then dried by spray drying to produce a mixed powder. The mixed powder is then filled into a dedicated mold and subjected to powder press molding using a uniaxial press (pressing pressure: 100 MPa) to produce a molded body. The produced molded body is degreased by heat treatment in air at a maximum temperature of 800°C. The degreased molded body is then subjected to air firing (firing temperature: 1500°C, firing time: 2 hours) and then hot isostatic pressing (HIP, treatment temperature: 1500°C, treatment pressure: 50 MPa, argon atmosphere, treatment time: 2 hours), to produce a base sphere that will serve as the base for the bearing ball 1. The surface of the produced base sphere is polished to produce the bearing ball 1.

[0023] 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 difference in the ratio of zirconium and yttrium contained in the zirconia crystal grains and the numerical values ​​related to the strength of the ceramic sintered body was evaluated.

[0024] FIG. 2 is a diagram illustrating the results of a first evaluation test of the ceramic sintered body. Six types of samples were prepared using different manufacturing methods in this evaluation test. Each of the six types of samples was manufactured using a method similar to the manufacturing method for the ceramic sintered body of the bearing ball 1 of this embodiment. Samples 1 to 5 were manufactured by weighing the raw material powders so that the weight percentages of alumina and zirconia were the values ​​shown as "Al2O3" and "ZrO2" in the "Component (wt %)" section of FIG. 2, and adjusting the pH of the mixed slurry to 6.5. Sample 6 was manufactured by adjusting the weight percentages of alumina and zirconia to the same values ​​as Sample 3, but adjusting the pH of the mixed slurry to 10.

[0025] Figure 2 shows the "density" and "composition" for each of Samples 1 to 6 of the ceramic sintered bodies produced by the above-mentioned manufacturing method. "Density" refers to the density (unit: g / cm) of each sample. 3 The "density" indicates a value measured according to the procedure described in JIS R1634 using a ceramic sintered body in which the surface was polished to remove 0.25 mm or more from the sintered surface.

[0026] 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.

[0027] 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 6 were evaluated for each of the five items shown in Fig. 3. Here, the evaluation methods for each will be explained.

[0028] 3 indicates the ratio of zirconium element to yttrium element contained in the zirconia crystal particles. Specifically, in the zirconia crystal particles contained in each sample, the atomic percentage AF of zirconium element contained in the zirconia crystal particles is Zr and the atomic percentage AF of the yttrium element Y The atomic percentage AF of the yttrium element relative to the total YThe ratio of yttrium to yttrium (hereinafter simply referred to as "yttrium ratio") is shown. In measuring the value of "yttrium ratio", the surface of the produced ceramic sintered body was polished to remove 0.25 mm or more from the baked surface, and then ion milled to obtain a thin slice of the sample. A scanning transmission electron microscope (STEM, JEM-F200 manufactured by JEOL Ltd.) and an energy dispersive X-ray analyzer (EDS / EDX, Silicon Drift Detector / Pathfinder manufactured by Thermo Fisher Scientific Ltd.) were used. Specifically, elemental analysis was performed on any five locations on the surface of the thin slice of the sample, targeting a square area with one side measuring 8 μm, and the atomic percentage AF of zirconium element contained in the zirconia crystal particles was determined. Zr and the atomic percentage AF of the yttrium element Y The "yttrium ratio" shown in FIG. 3 is the atomic percentage AF of zirconium element obtained by the measurement. Zr and the atomic percentage AF of the yttrium element Y The yttrium ratio was calculated using the formula (1). Note that FIG. 3 shows the "minimum value" and the "maximum value" of the measurement results at any five points where elemental analysis was performed. "Yttrium ratio" = AF Y / (AF Zr +AF Y ) × 100 ... (1)

[0029] 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 (2) using the value "before autoclaving," Sf, and the value "after autoclaving," Sa, obtained by the measurement. "Rate of change" = (1 - Sa / Sf) × 100 (2)

[0030] In the numerical values ​​for "yttrium ratio" shown in Figure 3, both the minimum and maximum values ​​for Samples 1 to 5 are 1.5 at% or more and 20 at% or less. 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 six types of 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 a rate of change in bending strength of 6% or less before and after heating in an autoclave, confirming that strength reduction in a high-temperature environment was suppressed.

[0031] It was confirmed that Sample 6 had a minimum "yttrium ratio" of 1 at% and a maximum of 25 at% compared to each of Samples 1 to 5. The pH of the mixed slurry during production of Sample 6 was adjusted to 10, and it is believed that the raw material powders repel each other in the mixed slurry. Therefore, the number of contact points between the 3YSZ powder and the ZrO2 powder decreased, making it difficult for the yttrium element to move, leading to segregation of the yttrium element and an increase in unstable zirconia crystal particles. This is thought to have resulted in a significant decrease in the bending strength after autoclaving compared to Samples 1 to 5.

[0032] According to the bearing ball 1 of this embodiment described above, the ceramic sintered body forming the bearing ball 1 has a ratio of zirconium to yttrium contained in the zirconia crystal grains ({Y / (Zr+Y)}×100) of 1.5 at% to 20 at%. This relatively increases the stability of the zirconia crystal grains, and, because the zirconia crystal grains are constrained by the alumina crystal grains, phase transformation is less likely to occur. This makes it possible to suppress a decrease in the strength of the bearing ball 1 in high-temperature environments.

[0033] 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 makes it easier for the zirconia crystal grains to be bound 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.

[0034] 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.

[0035] Second Embodiment FIG. 4 is a schematic diagram of a cutting tool according to a second embodiment. Cutting tools 2a to 2g shown in FIG. 4 are formed of ceramic sintered bodies. The ceramic sintered bodies forming cutting tools 2a to 2g according to this embodiment are alumina-based sintered bodies containing zirconia crystal grains containing yttria and alumina crystal grains. In the ceramic sintered bodies of cutting tools 2a to 2g, the ratio of zirconium (Zr) to yttrium (Y) contained in the zirconia crystal grains ({Y / (Zr+Y)}×100) is 1.5 at% or more and 20 at% or less. As a result, cutting tools 2a to 2g according to this embodiment are less likely to lose strength even when used in a high-temperature environment, and are therefore less likely to break even when used in a high-temperature environment. As shown in FIG. 4, cutting tools 2a to 2g according to this embodiment can be used in various shapes.

[0036] Fig. 5 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. 5.

[0037] According to the cutting tool 2 of this embodiment described above, the ceramic sintered body forming the cutting tool 2 has a ratio of zirconium to yttrium contained in the zirconia crystal grains ({Y / (Zr+Y)}×100) of 1.5 at% to 20 at%. This relatively increases the stability of the zirconia crystal grains, and the zirconia crystal grains are constrained by the alumina crystal grains, making phase transformation less likely to occur. This therefore makes it possible to suppress a decrease in the strength of the cutting tool 2 in high-temperature environments.

[0038] <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.

[0039] [Modification 1] 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.

[0040] [Modification 2] 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.

[0041] 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.

[0042] <Application Example 1> A ceramic sintered body, comprising zirconia crystal particles containing yttria and alumina crystal particles, wherein the ratio of zirconium (Zr) to yttrium (Y) contained in the zirconia crystal particles ({Y / (Zr+Y)}×100) is 1.5 at% to 20 at%. <Application Example 2> The ceramic sintered body according to Application Example 1, comprising 60 wt% to 93 wt% alumina and 7 wt% to 40 wt% zirconia. <Application Example 3> The ceramic sintered body according to Application Example 1 or Application Example 2, wherein the zirconia crystal particles have a crystal phase that is at least one of a tetragonal system and a cubic system. <Application Example 4> A bearing ball, formed from the ceramic sintered body according to any one of Application Examples 1 to 3. <Application Example 5> A cutting tool, characterized in that it is formed from the ceramic sintered body according to any one of Application Examples 1 to 3.

[0043] 1...Bearing ball 2...Cutting tool

Claims

1. A ceramic sintered body comprising zirconia crystal grains containing yttria and alumina crystal grains, wherein the ratio of zirconium (Zr) to yttrium (Y) contained in the zirconia crystal grains ({Y / (Zr+Y)} x 100) is 1.5 at% or more and 20 at% or less.

2. A ceramic sintered body according to claim 1, 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.

3. 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.

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

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

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