Ceramic sintered compact, bearing ball, and cutting tool

Abstract

To provide a technique for suppressing strength degradation of a ceramic sintered compact under a high-temperature environment.SOLUTION: 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%, inclusive.SELECTED DRAWING: Figure 3

Description

[Technical Field]

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

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

[0003] [Patent Document 1] Japanese Patent Application Publication No. 2016-132577 Summary of the Invention [Problem to be solved by the invention]

[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. [Means for solving the problem]

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

[0007] (1) According to one aspect of the present invention, there is provided a ceramic sintered body comprising 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)}×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 the ceramic sintered body from losing strength in high-temperature environments.

[0009] (2) In the ceramic sintered body of the above embodiment, the ceramic sintered body may contain 60% to 93% by weight of alumina and 7% to 40% by weight of zirconia. According to this configuration, the ceramic sintered body contains 60% to 93% by weight of alumina and 7% 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 grains may have a crystalline phase of at least one of tetragonal and cubic. According to this configuration, the crystalline phase of the zirconia crystal grains 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-mentioned 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. 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 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 a high-temperature environment.

[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. [Brief explanation of the drawings]

[0014] [Figure 1] FIG. 2 is a schematic diagram of a bearing according to the first embodiment. [Figure 2] FIG. 10 is a diagram illustrating the results of a first evaluation test of the ceramic sintered body. [Figure 3] FIG. 10 is a diagram illustrating the results of a second evaluation test of the ceramic sintered body. [Figure 4] FIG. 4 is a schematic diagram of a cutting tool according to a second embodiment. [Figure 5] FIG. 6 is a schematic view showing a state in which the cutting tool of the second embodiment is used. DETAILED DESCRIPTION OF THE INVENTION

[0015] First Embodiment 1 is a schematic diagram of a bearing according to a 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 (ZrO2) crystal grains containing yttria (YO3) and alumina (Al2O3) 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% or more and 20 at% or less. This makes the bearing ball 1 of this embodiment less susceptible to deterioration in strength even when used in a high-temperature environment, and therefore less susceptible to breakage even when used in a high-temperature environment. 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. A method for detecting the zirconium and yttrium contained in the zirconia crystal grains will be described in detail below.

[0017] 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. 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 compact 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 compact 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 crystal 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 compact 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 zirconia powder to 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 at 60 revolutions per minute for 72 hours to pulverize and mix the raw material powder, thereby producing a mixed slurry. In the method for manufacturing 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, nitric acid or the like is added to correct the pH of the mixed slurry to be more acidic.

[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 after mixing for another 30 minutes, the mixed slurry is dried by spray drying to produce a mixed powder. Next, the produced mixed powder is used to fill a dedicated mold, and then a green body is produced by powder press molding using a uniaxial press (pressing pressure: 100 MPa). The produced green body is degreased by heat treatment in air at a maximum temperature of 800°C. The degreased green body is then air-fired (fired at 1500°C, for 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 become 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. In this evaluation test, six types of samples were prepared using different manufacturing methods. 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 out 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 setting the weight percentages of alumina and zirconia to the same values ​​as in Sample 3, while adjusting the pH of the mixed slurry to 10.

[0025] FIG. 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 the value measured according to the procedure described in JIS R1634, using the surface of the produced ceramic sintered body, which was polished to remove 0.25 mm or more from the sintered surface.

[0026] The "composition" column indicates the weight ratio (unit: wt%) of alumina crystal particles and zirconia crystal particles contained in each sample, as well as the weight ratio (unit: wt%) of the crystalline phase in the zirconia crystal particles relative to the entire sample. For the "composition" measurements, 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 "composition" measurement surface. The crystalline phases of the crystalline particles contained in the samples were identified using X-ray diffraction (XRD) on the prepared "composition" measurement surface. The quantification of the identified crystalline phases was performed using the WPPF method, which uses the X-ray diffraction patterns obtained by X-ray diffraction. The ICDD card numbers for "Al2O3," "t-ZrO2," "c-ZrO2," and "m-ZrO2" in the "composition" column of Figure 2, which were referenced in the quantitative analysis using the WPPF method, are as follows: The ratio of alumina to the total amount of zirconia in each sample was the same as the weight ratio of the raw material powder weighed when the sample was produced. α-Al2O3:01-089-7715 t-ZrO2:01-081-154 c-ZrO2:01-083-9463 m-ZrO2:00-037-1484

[0027] Fig. 3 is a diagram illustrating the results of a 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 respective evaluation methods will be explained.

[0028] The "yttrium ratio" shown in Figure 3 indicates the ratio of zirconium element to yttrium element contained in the zirconia crystal particles. Specifically, the atomic percentage AF of zirconium element contained in the zirconia crystal particles in each sample is Zr and the atomic percentage of yttrium element AF Y Atomic percentage AF of yttrium element relative to the total of YThe ratio of yttrium to yttrium (hereinafter simply referred to as "yttrium ratio") is shown. To measure the "yttrium ratio," the surface of the prepared ceramic sintered body was polished to remove at least 0.25 mm from the sintered surface, and then ion milled to create a thin slice. The sample was then analyzed using a scanning transmission electron microscope (STEM, JEOL Ltd. JEM-F200) and an energy dispersive X-ray analyzer (EDS / EDX, Thermo Fisher Scientific Ltd. Silicon Drift Detector / Thermo Fisher Scientific Ltd. Pathfinder). Specifically, elemental analysis was performed on five randomly selected points on the surface of the thin sliced ​​sample, covering a square area with sides of 8 μm, to determine the atomic percentage AF of zirconium contained in the zirconia crystal grains. Zr and the atomic percentage of yttrium element AF Y The "yttrium ratio" shown in Figure 3 is the atomic percentage AF of zirconium element obtained by the measurement. Zr and the atomic percentage of yttrium element AF Y The values ​​were calculated using formula (1) using the above. Note that Figure 3 shows the "minimum value" and "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 is the bending strength of the sample before heating in an autoclave ("before autoclaving"), the bending strength of the sample after heating in an autoclave ("after autoclaving"), and the change in bending strength before and after heating is shown as the "percent change (%)." To measure the bending strength, the surface of the ceramic sintered body was first polished to remove at least 0.25 mm from the baked surface. This was then cut into a 3 mm x 4 mm x 50 mm sample. According to JIS R1601, the sample for "bending strength" measurement was placed on two supports spaced 30 mm apart, and a load was applied to the center point between the supports. A three-point bending strength test was performed. The maximum bending stress at breakage 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 samples for measuring "bending strength" were removed from 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 "after autoclaving" value Sa. The "rate of change" shown in Figure 3 was calculated using the "before autoclaving" value Sf and the "after autoclaving" value Sa obtained by the measurement, according to formula (2). "Rate of change" = (1 - Sa / Sf) × 100 (2)

[0030] In the "yttrium ratio" values ​​shown in Figure 3, Samples 1 to 5 have both minimum and maximum values ​​of 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] Compared to each of Samples 1 to 5, Sample 6 was confirmed to have a minimum "yttrium ratio" of 1 at% and a maximum of 25 at%. For Sample 6, the pH of the mixed slurry during production was adjusted to 10, which is thought to be due to repulsion between the raw material powders in the mixed slurry. This reduced the number of contact points between the 3YSZ powder and the ZrO2 powder, 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 significantly lower bending strength after autoclaving than 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 the zirconia crystal grains are restrained 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 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% to 93% by weight of alumina and 7% 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. This further reduces the decrease in strength of the ceramic sintered body in high-temperature environments.

[0034] Furthermore, in the bearing ball 1 of this embodiment, the zirconia crystal grains have at least one of the relatively stable tetragonal and cubic crystal phases, which further improves 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 the cutting tools 2a to 2g of this embodiment are alumina-based sintered bodies, containing zirconia crystal grains containing yttria and alumina crystal grains. In the ceramic sintered bodies of the 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. Thus, the cutting tools 2a to 2g of 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, the cutting tools 2a to 2g of 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. Fig. 5 shows the cutting tool 2 attached to the 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] <Modification of this embodiment> The present invention is not limited to the above-described embodiment, and can be embodied in various forms without departing from the spirit of the invention. For example, the following modifications are also possible.

[0039] [Variation 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 weight percentages of alumina and zirconia in the ceramic sintered body are not limited to these.

[0040] [Variation 2] In the above-described embodiment, the crystalline phase of the zirconia crystal particles is mostly tetragonal, with the remainder being cubic. However, 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 it 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, The zirconia crystal particles contain yttria, and the alumina crystal particles contain yttria. a ratio ({Y / (Zr+Y)}×100) of zirconium (Zr) to yttrium (Y) contained in the zirconia crystal particles is 1.5 at % or more and 20 at % or less. Sintered ceramics. <Application example 2> The ceramic sintered body according to Application Example 1, The ceramic sintered body is characterized in that it 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. Sintered ceramics. <Application example 3> The ceramic sintered body according to Application Example 1 or Application Example 2, The crystalline phase of the zirconia crystalline particles is at least one of a tetragonal phase and a cubic phase. Sintered ceramics. <Application Example 4> A bearing ball, A ceramic sintered body according to any one of Application Examples 1 to 3, Bearing ball. <Application example 5> A cutting tool, A ceramic sintered body according to any one of Application Examples 1 to 3, cutting tools. [Explanation of symbols]

[0043] 1...Bearing ball 2…Cutting tools

Claims

1. A ceramic sintered body, The zirconia crystal particles contain yttria, and the alumina crystal particles contain yttria. a ratio ({Y / (Zr+Y)}×100) of zirconium (Zr) to yttrium (Y) contained in the zirconia crystal particles is 1.5 at % or more and 20 at % or less; Sintered ceramics.

2. The ceramic sintered body according to claim 1, The ceramic sintered body is characterized in that it 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. Sintered ceramics.

3. The ceramic sintered body according to claim 1 or 2, The crystalline phase of the zirconia crystalline particles is at least one of a tetragonal phase and a cubic phase. Sintered ceramics.

4. A bearing ball, A ceramic sintered body according to claim 1 or 2, Bearing ball.

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

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