Zirconia sintered body

Abstract

To provide a zirconia sintered body that can secure a high level of fracture toughness and can be easily manufactured even by atmospheric pressure sintering. [Solution] The Y2O3 content in the total composition of the sintered body is set to 1.6 to 2.9 mol%, the area ratio of the first region consisting of zirconia crystal particles with an area-average particle size of 0.3 to 0.6 μm and a particle size of less than 0.1 to 0.4 μm is set to 20 to 80% or less, the area ratio of the second region consisting of zirconia crystal particles with a particle size of 0.4 to 1.6 μm or less is set to 20 to 80% or less, the average second region Y2O3 concentration C2 of the zirconia crystal particles forming the second region is set to 1.9 to 3.8 mol% or less, the average first region Y2O3 concentration C1 of the zirconia crystal particles forming the first region is set to 1.4 to 2.3 mol% or less, and the difference in Y2O3 concentration is set to ΔC ≡ C2 - C1, the value of ΔC / C2 is set to 0.1 to 0.5 or less, and the fracture toughness value by the IF method is set to 10.5 MPa·m 0.5 That's all.

Description

[Technical Field]

[0001] This invention relates to a zirconia sintered body, and more particularly to a zirconia sintered body in which a good fracture toughness value is ensured. [Background technology]

[0002] As shown in Fig. 9 of Non-Patent Document 1 (referred to as Figure 27 in this specification), pure ZrO2 (zirconium oxide) has polymorphisms from high temperature to high temperature, including a CaF2-type cubic phase (C phase), a tetragonal phase (T phase), and a monoclinic phase (M phase). Adding 2-5 mol% of stabilizers such as Y2O3 (yttrium oxide) or CaO (calcium oxide) results in partially stabilized zirconia (PSZ) containing the metastable T phase at room temperature, and adding 8 mol% or more results in the C phase, i.e., fully stabilized zirconia (FSZ). In particular, PSZ has been shown to exhibit excellent mechanical properties due to a martensitic transformation from the high-temperature T phase to the low-temperature M phase. When cracks propagate within the sintered body of PSZ, the stress causes a transformation from the T phase to the M phase, and at that time, the volume expands by about 4%. This expansion applies compressive stress to the crack tip, suppressing crack propagation (hereinafter, this mechanism of strengthening of zirconia sintered bodies will be referred to as "transformation-induced high toughness"). PSZ using Y2O3 as a stabilizer is chemically stable and possesses high strength and toughness, making it widely used as a mechanical structural material such as engine materials, cutting tools, dies, seals, and bearings, as well as a biomaterial such as dental bone material.

[0003] The zirconia sintered body disclosed in Patent Document 1 employs a 5mol%Y2O3 composition, which provides greater stability to the C phase, and is manufactured using the HIP method, which minimizes residual air bubbles. While it is disclosed that yttria-containing zirconia powder was used as the raw material, the specific manufacturing method is not disclosed. The flexural strength of the sintered body is approximately 800M to 1100MPa, but the fracture toughness is 3.5 to 4.0MPa·m despite the use of the HIP method. 0.5 The values ​​remain at a low level. Furthermore, it has been disclosed that the average grain size of the sintered body, measured by the line intercept method, is approximately 0.49 to 0.95 μm. The raw material powder used has an average grain size of 0.028 to 0.030 μm and a specific surface area of ​​15 to 16 m². 2 The particles used are extremely fine, measured in grams.

[0004] Patent Document 2 discloses a sintered body of zirconia with a composition of 1.6-2 mol% Y2O3, manufactured using the HIP method. While it is disclosed that commercially available yttria-containing zirconia powder containing Y2O3 was used as the raw material powder, the specific manufacturing method is not disclosed. Due to the adoption of a composition with a higher T-phase content, which is a factor in the transformation and increased toughness, the sintered body has a bending strength of 1470 M-2140 MPa and a fracture toughness of 6.0-10.3 MPa·m. 0.5 It exhibits high strength and toughness. The average grain size of the sintered body, measured by the planimetric method, is 0.28 to 0.55 μm. The raw material powder used has a bimodal distribution with peaks at 0.14 μm and 0.34 to 0.35 μm, respectively, and a median diameter of 0.15 to 0.18 μm, with a specific surface area of ​​15.1 to 17.9 m². 2 It is / g.

[0005] Patent Document 3 discloses a sintered body produced using an atmospheric pressure sintering method, primarily for zirconia with a 3mol%Y2O3 composition. Regarding the raw material powder, a method is disclosed in which a coprecipitate obtained by mixing zirconium salt, a stabilizer source, and alkali is calcined and pulverized. While the resulting sintered body exhibits a bending strength of 980-1280 MPa, the fracture toughness value is not disclosed. Furthermore, the average grain size of the sintered body, measured by the planimetric method, is 0.30-0.34 μm. The raw material powder used has an average grain size of 0.4-0.7 μm and a specific surface area of ​​11-15 m². 2 The value is / g. The raw material powder used had an average particle size of 0.4-0.7 μm and a specific surface area of ​​11-15 m². 2 It is / g.

[0006] Patent Document 4 discloses a sintered body manufactured using an atmospheric pressure sintering method, primarily for zirconia with a 4mol% Y2O3 composition. While it states that "zirconia powder is produced by crushing zirconia," it does not disclose the specific method for adding stabilizing components such as yttria. The flexural strength of the sintered body is 10¹⁶ to 1220 MPa, which is comparable to the sintered body described in Patent Document 3, which has a lower Y2O3 content. However, the fracture toughness is 4.0 to 4.5 MPa·m. 0.5 The degree is low. Although the specific surface area value of the powder used is not disclosed, according to the description in 0043, the particle size distribution of the zirconia crystal particles of the raw material powder has at least two peaks, and in the particle size distribution, the first peak is preferably located at 0.05 μm to 0.11 μm (small particle size powder), and the second peak is preferably located at 0.1 μm to 0.7 μm (large particle size powder). As a specific example, a mixture of equal amounts of small particle size powder with a peak of 0.08 μm and large particle size powder with a peak of 0.4 μm is used. This raw material is considered to be a concept similar to the bimodal distribution raw material powder disclosed in Patent Document 2.

[0007] In Patent Document 4, analysis of the crystal grain size distribution was performed using SEM observation images of the sintered body. In the circular-equivalent particle size distribution of each crystal particle observed on the image, the cross-sectional area ratio of zirconia particles in the class less than 0.4 μm is 4% or more and 35% or less, and the cross-sectional area ratio of zirconia particles in the class of 0.4 μm or more and less than 0.76 μm is 24% or more and 57% or less. Furthermore, it is disclosed that the cross-sectional area ratio of zirconia particles in the class of 0.76 μm or more is preferably 16% or more and 62% or less. Although the value of the average crystal grain size is not disclosed, it is estimated that the value would be in the range of 0.06 μm to 0.085 μm from the area ratio of the crystal particles in each class. The summary of the disclosure content of each of the above patent documents is shown in Table 1.

[0008]

Table 1

Prior Art Documents

Patent Documents

[0009]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Non-Patent Documents

[0010]

Non-Patent Document 1

Non-Patent Document 2

Non-Patent Document 3

[0011] To improve the mechanical properties of zirconia sintered bodies, it is considered effective to reduce the Y2O3 content to increase the T-phase ratio, which contributes to transformation toughening, and to refine the grain size of the sintered body. However, as is clear from comparing the disclosures in Patent Documents 1 to 3 as shown in Table 1, in zirconia sintered bodies, it is not possible to secure a high fracture toughness value by reducing the Y2O3 content, especially 10 MPa·m. 0.5 Achieving fracture toughness values ​​exceeding this level has been difficult. For example, paragraph 0071 of Patent Document 4 states that "the distribution balance or area balance of grain size is considered to contribute to improving bending strength, fracture toughness, and phase transition suppression," but judging from the disclosed characteristic values, it is difficult to say that high fracture toughness values ​​have been achieved. Furthermore, as shown in Figure 30, there is almost no correlation between the fracture toughness value and the area ratio of crystal grains smaller than 0.4 μm, indicating that grain refinement does not necessarily contribute to improving fracture toughness.

[0012] Furthermore, since the HIP method requires large-scale equipment, it is desirable to use a simpler atmospheric pressure sintering method for dental materials, etc. However, as is clear from the disclosures in Patent Documents 3 and 4, the mechanical properties of zirconia sintered bodies produced by atmospheric pressure sintering have been significantly inferior to those produced by the HIP method. Therefore, there is a need for a zirconia sintered body that can be produced by atmospheric pressure sintering while having particularly high fracture toughness.

[0013] Furthermore, even though bending strength and fracture toughness are the same mechanical properties, they are in a trade-off relationship, as mentioned in paragraph 0010 of Patent Document 4, and it is difficult to increase both bending strength and fracture toughness. For example, when the bending strength and fracture toughness values ​​disclosed in Patent Documents 1, 2, and 3 are plotted in two dimensions, the result is as shown in Figure 31, and it is difficult to say that there is a clear correlation between these two properties. The data points enclosed by the dashed lines in the figure are the values ​​for the sintered bodies of Patent Documents 1 and 2 manufactured using the HIP method, and for these, there does seem to be a negative correlation between bending strength and fracture toughness. However, the remaining data points for the sintered bodies manufactured using the atmospheric pressure sintering method deviate significantly from the data points for the sintered bodies manufactured using the HIP method, indicating that bending strength and fracture toughness cannot be understood through a unambiguous correlation trend.

[0014] According to Non-Patent Literature 4, the fracture of brittle materials occurs when stress concentrates, causing cracks to form, which then rapidly grow and lead to fracture. This rapid crack growth is called unstable fracture, while fracture achieved by controlling crack propagation in response to slowly applied stress, such as in a bending strength test, is called stable fracture. As mentioned above, when a crack propagates within a zirconia sintered body, the stress concentrated at the crack tip causes the surrounding T phase to undergo martensitic transformation into the M phase. The compressive stress resulting from the volume expansion caused by this transformation prevents further crack propagation. The phenomenon where the crack propagates despite this compressive stress is called unstable fracture, and the fracture toughness value represents the critical stress intensity factor at which unstable fracture begins. On the other hand, once the mode shifts to unstable fracture, the sintered body cannot control crack propagation and fractures; therefore, it is thought that most of the phase transformation during bending strength tests occurs during stable fracture.

[0015] Thus, the way in which phase transformations are involved in the test values ​​differs fundamentally between fracture toughness tests and bending strength tests. It should also be noted that, for example, in cases where the results of the fracture toughness test are not disclosed, as in Patent Document 3, it is fundamentally impossible to estimate the fracture toughness value from the disclosed bending strength test value.

[0016] The object of the present invention is to provide a zirconia sintered body that has particularly high fracture toughness and can be easily manufactured even by atmospheric pressure sintering, by improving the crystal structure, taking into account the distribution of the Y2O3 component as a stabilizer. [Means for solving the problem]

[0017] The zirconia sintered body of the embodiment for solving the above problems is a zirconia sintered body with a relative density of 99% or more, wherein the content of Y2O3 in the total composition of the sintered body is 1.6 mol% or more and 2.9 mol% or less, and the balance is composed of ZrO2 and inevitable impurities. In the electron microscope observation image of the sintered structure of the zirconia sintered body, the area average particle size of the zirconia crystal particles is 0.3 μm or more and 0.6 μm or less, and the area ratio of zirconia crystal particles with a particle size of less than 0.1 μm and zirconia crystal particles with a particle size exceeding 1.6 μm is less than 1% each. The area ratio of the first region composed of zirconia crystal particles with a particle size of 0.1 μm or more and less than 0.4 μm is 20% or more and 80% or less, and the area ratio of the second region composed of zirconia crystal particles with a particle size of 0.4 μm or more and 1.6 μm or less is 20% or more and 80% or less. The average second region Y2O3 concentration C2 of the zirconia crystal particles forming the second region is 1.9 mol% or more and 3.8 mol% or less, and the average first region Y2O3 concentration C1 of the zirconia crystal particles forming the first region is 1.4 mol% or more and 2.3 mol% or less. And, taking the difference in Y2O3 concentration between the second region Y2O3 concentration C2 and the first region Y2O3 concentration C1 as ΔC≡C2 - C1, the value of ΔC / C2 is 0.1 or more and 0.5 or less, and the fracture toughness value measured by the IF method in accordance with JIS Z 2244-1(2024) is 10.5 MPa·m 0.5 and characterized by the above.

Effect of the Invention

[0018] The zirconia sintered body of the present invention has a Y2O3 content of 1.6 mol% to 2.9 mol% in the total composition of the sintered body, with the remainder being ZrO2 and unavoidable impurities, and a relative density of 99% or more. In electron microscope images of the sintered structure of the zirconia sintered body, the area-average particle size of the zirconia crystal particles is 0.3 μm to 0.6 μm, the area ratio of zirconia crystal particles with a particle size of less than 0.1 μm and zirconia crystal particles with a particle size of more than 1.6 μm is both less than 1%, and the area ratio of the first region consisting of zirconia crystal particles with a particle size of 0.1 μm to less than 0.4 μm is... The material has a ratio of 20% to 80%, an area ratio of 20% to 80% for the second region consisting of zirconia crystal particles with a particle size of 0.4 μm to 1.6 μm, an average second region Y2O3 concentration C2 of the zirconia crystal particles forming the second region of 1.9 mol% to 3.8 mol%, an average first region Y2O3 concentration C1 of the zirconia crystal particles forming the first region of 1.4 mol% to 2.3 mol%, and a difference in Y2O3 concentration between the second region Y2O3 concentration C2 and the first region Y2O3 concentration C1, where ΔC≡C2-C1, is used, and the value of ΔC / C2 is 0.1 to 0.5, and the fracture toughness value measured by the IF method in accordance with JIS Z 2244-1(2024) is 10.5 MPa·m 0.5 Therefore, by improving the crystal structure, taking into account the distribution of the Y2O3 component as a stabilizer, we were able to obtain a zirconia sintered body that has particularly high fracture toughness and can be easily manufactured even by atmospheric pressure sintering. [Brief explanation of the drawing]

[0019] [Figure 1] This histogram shows the particle size distribution (volume-based) of the sintering raw material powder used to manufacture the zirconia sintered body test sample No. 4 (Example) in Table 3, along with its statistical characteristics, as measured by a laser scattering particle size analyzer. [Figure 2] This histogram shows the particle size distribution (volume-based) of the sintering raw material powder used to manufacture the zirconia sintered body test sample No. 8 (Example) in Table 3, along with its statistical characteristics, as measured by a laser scattering particle size analyzer. [Figure 3]This histogram shows the particle size distribution (volume-based) of the sintering raw material powder used to manufacture the zirconia sintered body test sample No. 14 (Comparative Example) in Table 3, along with its statistical characteristics, as measured by a laser scattering particle size analyzer. [Figure 4] Table 3, item 4, shows a scanning electron microscope image (magnification 20,000x) of the zirconia sintered body (example sample). [Figure 5] Table 3, item 8, shows a scanning electron microscope image (magnification 20,000x) of the zirconia sintered body (comparative example). [Figure 6] Table 3, item 14, shows a scanning electron microscope image (magnification 20,000x) of the zirconia sintered body (comparative example). [Figure 7] This is a histogram showing the microstructure and grain size distribution (area-based) of the zirconia sintered body of item 4 (Example) in Table 3, analyzed using the observation image in Figure 2. [Figure 8] This is a histogram showing the microstructure and grain size distribution (area-based) of the zirconia sintered body of item 8 (Example) in Table 3, analyzed using the observation image in Figure 3. [Figure 9] This is a histogram showing the microstructure and grain size distribution (area-based) of the zirconia sintered body of item 14 (comparative example) in Table 3, analyzed using the observation image in Figure 4. [Figure 10] Figure 4 shows typical examples of setting the Y2O3 concentration analysis circle for crystal grains in the first and second regions, as observed in the image. [Figure 11] This figure shows the correspondence between the scanning electron microscope observation image (magnification 20,000x) of the zirconia sintered body of Table 3 No. 1 (comparative example) and the EPMA radiation analysis results, which measured the Y2O3 concentration along analysis lines crossing multiple particles in the image. [Figure 12] This figure shows the scanning electron microscope (magnification 5000x) observation image of the zirconia sintered body of Table 3 No. 1 (Comparative Example), and the corresponding mapping image of the effective Y concentration region identified based on the EPMA surface analysis results on the image. [Figure 13]This figure shows the correspondence between a scanning electron microscope observation image (magnification 20,000x) of the zirconia sintered body of Table 3 No. 4 (Example), and the EPMA radiation analysis results, which measured the Y2O3 concentration along analysis lines crossing multiple particles in the image. [Figure 14] This figure shows the scanning electron microscope observation image (magnification 5000x) of the zirconia sintered body of Table 3 No. 4 (Example), and the corresponding mapping image of the effective Y concentration region identified based on the EPMA surface analysis results on the image. [Figure 15] This figure shows the correspondence between a scanning electron microscope image (magnification 20,000x) of the zirconia sintered body of Table 3 No. 8 (Example), and the EPMA radiation analysis results, which measured the Y2O3 concentration along analysis lines crossing multiple particles in the image. [Figure 16] This figure shows the scanning electron microscope observation image (magnification 5000x) of the zirconia sintered body of Table 3 No. 8 (Example), and its correspondence with the mapping image of the effective Y concentration region identified based on the EPMA surface analysis results on the image. [Figure 17] This figure shows the correspondence between the scanning electron microscope observation image (magnification 20,000x) of the zirconia sintered body of Table 3 No. 13 (comparative example) and the EPMA radiation analysis results, which measured the Y2O3 concentration along analysis lines crossing multiple particles in the image. [Figure 18] This figure shows the scanning electron microscope observation image (magnification 5000x) of the zirconia sintered body of Table 3 No. 13 (Comparative Example), and its correspondence with the mapping image of the effective Y concentration region identified based on the EPMA surface analysis results on the image. [Figure 19] This figure shows the scanning electron microscope observation image (magnification 5000x) of the zirconia sintered body of Table 3 No. 14 (comparative example), and the corresponding mapping image of the effective Y concentration region identified based on the EPMA surface analysis results on the image. [Figure 20] This diagram schematically shows the estimated diffusion modes of Zr and Y when small crystal particles with closely spaced particle sizes coalesce during the sintering process of a zirconia sintered body. [Figure 21]This is a schematic diagram illustrating the initial process of crystal structure formation in the zirconia sintered body of the present invention. [Figure 22] Figure 21 is an explanatory diagram illustrating how the Y2O3 powder particle component diffuses into the ZrO2 powder particles, causing concentration fluctuations according to their size during the process shown. [Figure 23] This figure schematically shows the estimated diffusion modes from small crystal particles of various sizes toward larger crystal particles in contact with them during the sintering process of the zirconia sintered body of the present invention. [Figure 24] This diagram schematically shows the estimated diffusion mode of the Y component when small crystal particles are absorbed by large crystal particles. [Figure 25] This diagram illustrates how fluctuations in the concentration of the Y2O3 component due to diffusion become less likely in ZrO2 powder particles in which the Y2O3 component of the reference raw material is uniformly dissolved. [Figure 26] This diagram illustrates the sintering process of a conventional zirconia sintered body using a reference raw material powder. [Figure 27] This is the ZrO2-Y2O3 binary phase diagram disclosed in Fig. 9 of Non-Patent Document 1. [Figure 28] This graph plots the measured fracture toughness of each sintered body against the equivalent cubic area fraction S(C) value obtained for each numbered zirconia sintered body in Table 3. [Figure 29] This graph plots the measured fracture toughness of each sintered body against the effective Y concentration area ratio obtained for each zirconia sintered body number in Table 3. [Figure 30] This graph plots the fracture toughness values ​​of the zirconia sintered body disclosed in Patent Document 4 against the area ratio of crystal grains smaller than 0.4 μm. [Figure 31] This is a two-dimensional plot of the bending strength and fracture toughness values ​​of zirconia sintered bodies disclosed in Patent Documents 1, 2, and 3. [Modes for carrying out the invention]

[0020] The zirconia sintered body of this embodiment has a Y2O3 content (core composition) of 1.6 mol% to 2.9 mol% in the total composition of the sintered body, with the remainder consisting of ZrO2 and unavoidable impurities. Furthermore, the relative density of the zirconia sintered body is ensured to be 99% or higher to obtain sufficient mechanical properties. Preferably, the relative density is 99.5% or higher.

[0021] The fracture toughness value used in the zirconia sintered body of the embodiment is the value measured by the IF (Indentation Fracture) method. According to Non-Patent Literature 3 and Non-Patent Literature 4, it has been pointed out that the fracture toughness value measured and calculated by the IF method tends to appear lower than the fracture toughness value measured by other methods due to the accuracy of reading the crack tip. On the other hand, the IF method has the advantage that it can be measured without problems even when it is difficult to take a test piece out of the sintered product. Also, the fact that the measured value of fracture toughness appears lower can be considered as providing a safety margin in ensuring a higher fracture toughness value of the material. The aforementioned IF method is a method that uses a Vickers hardness tester compliant with JIS Z 2244-1 (2024) (corresponding standard ISO 6507-1:2023) to measure the length of the indentation and crack generated by pressing a Vickers indenter into the test piece for a period of 15 seconds. From this measurement, the fracture toughness value "K IC The result is calculated as follows. Fracture toughness value K IC = 0.018 × (E / H) 0.5 ×(P / c 1.5 ) K IC Fracture toughness value (MPa·m) 0.5 ) E: Young's modulus (GPa), where 205 GPa was used as the Young's modulus. H: Vickers hardness (HV) = 0.1891P / (2a) 2 c: Half the average crack length (m) a: Half the average length of the indentation (m) P: Vickers indenter press load (N) (1kgf = 9.80665N)

[0022] In the embodiment, the zirconia sintered body has an area-average particle size of zirconia crystal particles of 0.3 μm or more and 0.6 μm or less in electron microscope images of the sintered body structure. If the area-average particle size is less than 0.3 μm, it becomes difficult to ensure the density of the sintered body, leading to a decrease in the hardness and flexural strength of the sintered body. Furthermore, the fracture toughness value may also be insufficient. Also, if the area-average particle size exceeds 0.6 μm, the fracture toughness value of the sintered body will be insufficient. Preferably, the area-average particle size of the zirconia crystal particles is 0.35 μm or more and 0.58 μm or less, and more preferably 0.4 μm or more and 0.55 μm or less. In addition, the area ratio of zirconia crystal particles with a particle size of less than 0.1 μm and zirconia crystal particles with a particle size of more than 2 μm should both be less than 1%. If the area ratio of zirconia crystal particles with a particle size of more than 2 μm is 1% or more, the fracture toughness value of the sintered body will be insufficient. Furthermore, it is difficult to achieve a relative density of 99% or more in the sintered body while maintaining a zirconia crystal particle content of 1% or more (less than 1 μm) through atmospheric pressure sintering.

[0023] Furthermore, the area ratio of the first region, consisting of zirconia crystal particles with a particle size of 0.1 μm or more and less than 0.4 μm, shall be 20% or more and 80% or less, while the area ratio of the second region, consisting of zirconia crystal particles with a particle size of 0.4 μm or more and less than 1.6 μm, shall be 20% or more and 80% or less.The average Y2O3 concentration of the zirconia crystal particles forming the second region shall be 1.9 mol% or more and 3.8 mol% or less, while the average Y2O3 concentration of the zirconia crystal particles forming the first region shall be 1.4 mol% or more and 2.3 mol% or less, and the difference in Y2O3 concentration between the second region Y2O3 concentration and the first region Y2O3 concentration shall be ΔC ≡ C2 - C1, with the value of ΔC / C2 being 0.1 or more and 0.5 or less.

[0024] If the area ratio of zirconia crystal particles in the first region and the area ratio of zirconia crystal particles in the second region fall outside the range of the present invention, it becomes impossible to secure a sufficient fracture toughness value for the zirconia sintered body. Furthermore, for the average Y2O3 concentration C1 in the first region and Y2O3 concentration C2 in the second region, if the value of ΔC / C2 when ΔC ≡ C2 - C1 falls below 0.1, the fracture toughness value of the zirconia sintered body will not reach 10.5 MPa·m. 0.5It becomes impossible to secure the above. Furthermore, it is difficult to obtain a sintered body with a ΔC / C2 value exceeding 0.5, at least by normal atmospheric pressure sintering.

[0025] If the core composition of Y2O3 falls below 1.6 mol%, it becomes difficult to ensure a ΔC / C2 value of 0.1 or higher, and the fracture toughness value becomes 10.5 MPa·m. 0.5 It becomes impossible to secure the above. On the other hand, if the core composition of Y2O3 becomes too high, even if the ΔC / C2 value is secured to 0.1 or higher, it may not be possible to secure a sufficient fracture toughness value, and in the embodiment, it is 10.5 MPa·m. 0.5 To reliably obtain the above fracture toughness values, the upper limit is set at 2.9 mol%. The core composition of Y2O3 should preferably be 1.7 mol% or more, and more preferably 1.8 mol% or more. Furthermore, the core composition of Y2O3 should preferably be 2.7 mol% or less, and more preferably 2.5 mol% or less.

[0026] While there are no particular restrictions on the upper limit of the fracture toughness value of the zirconia sintered body in the embodiment, it can be increased to, for example, 15-22 MPa·m by adjusting various conditions of atmospheric pressure sintering. 0.5 It is also possible to raise it both forward and backward.

[0027] In the zirconia sintered body of the embodiment, the first region consisting of zirconia crystal particles with a particle size of 0.1 μm or more and less than 0.4 μm is a region mainly composed of fine crystal particles, and it has generally been thought that increasing the number of crystal particles in this region is advantageous in improving the mechanical strength of the material. However, when considering the viewpoint of improving fracture toughness, as is clear from the disclosure in Patent Document 4 (see Figure 30), simply increasing the number of crystal particles in this region will not lead to a significant improvement in fracture toughness. The reason for this is that if the proportion of the T phase, which contributes to suppressing the rapid propagation of cracks, i.e., unstable fracture, is small, even a region with refined crystal particles will not lead to an improvement in fracture toughness.

[0028] Here, the raw material powder for the zirconia sintered body disclosed in Patent Document 3 is derived from a manufacturing method in which a coprecipitate obtained by mixing a zirconium salt, a stabilizer source, and alkali is calcined and pulverized. This manufacturing method is a common method widely used in the production of raw material powder for commercially available partially stabilized zirconia, and the powder prepared has a uniform solid solution of Y2O3 component in each zirconia powder particle (although Patent Documents 1, 2, and 4 do not disclose the specific manufacturing method of the raw material powder, they contain descriptions such as "yttria-containing zirconia powder," so it is thought that powder manufactured by a similar method is used). Therefore, it is thought that fluctuations in the concentration of the Y2O3 component are unlikely to occur in the crystalline structure of the zirconia sintered body manufactured using such raw material powder, and it is thought that there is almost no difference in the distribution of T-phase domains and C-phase domains in both the first region consisting of zirconia crystal particles with a particle size of 0.1 μm or more and less than 0.4 μm, and the second region consisting of zirconia crystal particles with a particle size of 0.4 μm or more and less than 1.6 μm.

[0029] Even though zirconia sintered bodies are toughened, their fracture mode is essentially brittle fracture, and the cracks associated with fracture propagate through the crystal grains. Furthermore, considering that the toughening mechanism of zirconia sintered bodies involves suppression of unstable fracture due to martensitic transformation of the T-phase domain, it is difficult to significantly improve fracture toughness even by controlling the ratio of the first and second regions when there is no difference in the distribution of the T-phase and C-phase.

[0030] Therefore, the zirconia sintered body of this embodiment is characterized by setting the average Y2O3 concentration of the entire sintered body (hereinafter also referred to as the "core composition") to a relatively low range of 1.6 mol% to 2.9 mol%, while setting the average Y2O3 concentration of the second region, which is mainly composed of large crystal grains, higher than the average Y2O3 concentration of the first region, which is mainly composed of small crystal grains. With this configuration, the fracture toughness value of the sintered body is 10.5 MPa·m 0.5 It becomes possible to improve to the above high level. From a histological perspective, the mechanism is presumed to be as follows.

[0031] In the composition range of 1.6 mol% to 2.9 mol%, the equilibrium ratio of the C phase (equilibrium Y2O3 concentration around 6.3 mol%) and the T phase (equilibrium Y2O3 concentration around 1.5 mol%) near the sintering temperature (e.g., 1475°C) is considered to be around 1:10 when the central composition is 2.0 mol% (composition point O). For the T phase domains formed in the high-temperature range to contribute to improved fracture toughness, they must maintain a metastable T phase structure down to near room temperature. However, according to the phase diagram in Figure 27, the T→M transformation point ML rises sharply as the Y2O3 concentration in the T phase decreases. It is thought that most of the T phase in the zirconia sintered body obtained in the above central composition range will have already transformed into the M phase by the time it cools to room temperature, and therefore a significant improvement in fracture toughness cannot be expected.

[0032] On the other hand, in the situation described above where the Y2O3 concentration in the first region with fine grains decreases and the Y2O3 concentration in the second region with coarse grains increases, if we consider the phase diagram shown in Figure 27, for example around 800°C, the Y2O3 concentration in the first region shifts from the central composition towards point A, which indicates the solid solubility limit on the T phase side, and similarly, the Y2O3 concentration in the second region shifts towards point B, which indicates the solid solubility limit on the C phase side. In terms of the equilibrium phase diagram, if the shifted compositions fall within the composition range indicated by the line segment AB, then the Y2O3 concentrations of the T phase and C phase in both the first and second regions will be the equilibrium concentrations indicated by points A and B.

[0033] However, as mentioned in Non-Patent Documents 5 and 6, it is known that in ZrO2 ceramics with added Y2O3, the separation of the T phase from the C phase is accompanied by spinodal decomposition-like concentration fluctuations. In a phase separation mechanism accompanied by concentration fluctuations, a modulated structure is more likely to be created in which the Y2O3 concentration changes continuously between the C phase domain with a high Y2O3 concentration and the T phase domain with a low Y2O3 concentration. Furthermore, it is thought that even in composition regions where the Y2O3 concentration is higher than the T phase solid solution line ST (solid solution separation line) on the T phase side in Figure 27, a larger amount of the T phase with supersaturated Y2O3 solid solution is more likely to remain. In Figure 27, the line indicating the T→M transformation point ML is shown to decrease monotonically as the Y2O3 concentration increases, moving from the T phase single-phase region into a supersaturated region beyond the Y2O3 solid solution line ST. Furthermore, it is believed that the transformation toughness of the zirconia sintered body is primarily due to the T phase (hereinafter referred to as the "supersaturated T phase"), which has a composition in the supersaturated region and remains metastable without transforming into the M phase at room temperature.

[0034] In the zirconia sintered body of this embodiment, the lower limit of the central composition is set to 1.6 mol%, and the upper limit is also relatively low at 2.9 mol%. In the first region, since the Y2O3 concentration in the first region is even lower than the central composition described above, the proportion of the T phase, which has a high transformation point to the M phase, increases, and the contribution to improving fracture toughness is reduced. However, in the second region, where the Y2O3 concentration is higher than the Y2O3 central composition, the proportion of the supersaturated T phase, which has a lower transformation point to the M phase, increases compared to the first region, and it is expected that this will contribute to improving fracture toughness. The supersaturated T phase is thought to be more likely to form mainly in the region where the Y2O3 concentration in the second region gradually decreases, near the boundary between the second region, which is mainly composed of large crystal grains, and the first region, which is mainly composed of small crystal grains (for example, the peripheral region of large crystal grains within the second region, as described later). Therefore, by forming the aforementioned second region within the sintered body in an appropriate area ratio, a supersaturated T phase with a large crack propagation inhibition effect based on stress-induced martensitic transformation is uniformly dispersed within the sintered body, resulting in a fracture toughness value of 10.5 MPa·m. 0.5 This achieves a significantly higher level of toughness through transformation.

[0035] Next, the zirconia sintered body of the embodiment can be manufactured by atmospheric pressure sintering using the following specific zirconia raw material powder. That is, the zirconia raw material powder is (1) Zirconia raw material powder obtained by mixing and grinding ZrO2 powder that does not contain Y2O3 component (for example, pure ZrO2 powder) and pure Y2O3 powder that constitutes a stabilizing component using a ball mill as starting materials. (2) In the particle size distribution measured by volume using a laser scattering particle size analyzer, • Arithmetic mean particle size is 0.10 μm or more and 0.15 μm or less. • The arithmetic standard deviation is between 0.03 μm and 0.05 μm, Particles with a diameter greater than 0.8 μm and particles with a diameter less than 0.03 μm were not measured. Furthermore, the measured values ​​of particles with a particle size greater than 0.4 μm and particles with a particle size of 0.04 μm or less are both 0.5% or less. The following two conditions must be met (such zirconia raw material powder will be referred to as "recommended zirconia raw material powder" below).

[0036] If raw material powder deviating from condition (2) is used, the average first-region Y2O3 concentration C1 of the zirconia crystal particles forming the first region will be lower than the average second-region Y2O3 concentration C2 of the zirconia crystal particles forming the second region (specifically, with ΔC ≡ C2 - C1, the value of ΔC / C2 will be between 0.1 and 0.5), making it difficult to obtain a characteristic concentration distribution morphology. Furthermore, considering the formation of secondary particles in the powder, the specific surface area value of the raw material powder measured by the BET method should be set to 13 m² to more reliably exclude particles smaller than 0.04 μm at the primary particle level. 2 It is preferable to keep it below / g. Furthermore, the specific surface area value of the raw material powder is related to converging the arithmetic mean particle size to 0.10 μm or more and 0.15 μm or less, which is 8m 2 It is best to set it to / g or higher.

[0037] According to the above condition (1), the Y2O3 component and the ZrO2 component in the zirconia raw material powder will be separated at the particle level. Therefore, the step of forming a state in which the Y2O3 component is solid-dissolved in the ZrO2 substrate must depend on diffusion during sintering. Since it is difficult to obtain a ZrO2 substrate in which the Y2O3 component is uniformly solid-dissolved when a zirconia sintered body is manufactured using such zirconia raw material powder, conventional zirconia sintered bodies have been manufactured using zirconia raw material powder (hereinafter referred to as "reference zirconia raw material powder") that has been prepared by a method in which the Y2O3 component is solid-dissolved in the ZrO2 substrate during the chemical synthesis stage of the powder raw material, such as the coprecipitation method disclosed in Patent Document 3.

[0038] However, as a result of detailed investigations by the present inventors, it was found that, in order to obtain a sintered body in which a difference in Y2O3 concentration is generated between two regions with different average crystal particle sizes, it is advantageous to use zirconia raw material powder in which the Y2O3 component and the ZrO2 component are separated, as described in (1) above. However, in order to obtain a sintered body with good fracture toughness, the zirconia raw material powder must be finely ground by a ball mill until the particle size distribution is sufficiently sharpened at the arithmetic mean particle size, so as to satisfy the conditions in (2).

[0039] In this case, the ZrO2 powder particles, which serve as the starting material for the recommended zirconia raw material powder, are significantly harder than the Y2O3 powder particles. Therefore, in ball mill grinding, not only the media (e.g., zirconia spheres) introduced into the pot along with the raw material, but also the ZrO2 powder particles introduced as raw material, function as fine media for the Y2O3 powder particles. At that time, the Y2O3 powder particles receive high-energy grinding impact forces from the larger grinding media through contact with the fine ZrO2 powder particles, making it easier to obtain a state in which the Y2O3 powder particles are uniformly mixed and distributed in the final raw material powder. It is considered that the Y2O3 powder particles in the recommended zirconia raw material powder obtained in this way are ground to an average particle size smaller than that of the ZrO2 powder particles.

[0040] The conditions in (2) above are necessary to obtain a state in which the Y2O3 powder particles are sufficiently and uniformly mixed and distributed among the ZrO2 powder particles. In order to finely grind the starting material to this level, it is effective to set the grinding time in the ball mill to a correspondingly long time. However, according to the inventors' research, it has been found that using a mixture of multiple spherical media of different dimensions as input media is effective in shortening the grinding time.

[0041] Figure 1 shows the particle size distribution measurement results for an example of a recommended zirconia raw material powder. This raw material powder has a Y2O3 content of 2.0 mol%, and the particle size distribution measured by volume using a laser scattering particle size analyzer (equipment used: HORIBA LA-960) shows the following: ·Arithmetic mean particle size: 0.128μm ·Arithmetic standard deviation: 0.038μm The measured values ​​for particles larger than 0.4 μm and particles smaller than 0.04 μm are zero (therefore, the measured values ​​for particles larger than 0.8 μm and particles smaller than 0.03 μm are also zero). More specifically, the maximum value of the particle size in the measured class is 0.339 μm, and the class exceeding 0.35 μm, which is the large-diameter peak of the bimodal powder in Patent Document 2, is less than 1% (zero in Figure 1). The powder was obtained by placing a total of 1 kg of commercially available pure ZrO2 powder and pure Y2O3 powder, 7.7 kg of 2 mm diameter zirconia media and 2.3 kg of 3 mm diameter zirconia media (consisting of a partially stabilized zirconia sintered body with a Y2O3 content of 2.75 mol%), totaling 10 kg, and 1.7 kg of pure water into a pot with an inner diameter of approximately 245 mm and grinding at a rotation speed of 78 rpm for 20 hours.

[0042] Furthermore, as another example of a recommended zirconia raw material powder, Figure 2 shows the particle size distribution measurement results of a raw material powder with a Y2O3 content of 2.9 mol%. In the particle size distribution measured by volume using a laser scattering particle size analyzer (equipment used: HORIBA LA-960), ·Arithmetic mean particle size: 0.125μm Arithmetic standard deviation: 0.036 μm The measured values ​​for particles larger than 0.4 μm and particles smaller than 0.04 μm are zero (therefore, the measured values ​​for particles larger than 0.8 μm and particles smaller than 0.03 μm are also zero). More specifically, the maximum particle size measured was 0.296 μm, and less than 1% (zero in Figure 1) of the particles exceeded 0.35 μm, which is the large-diameter peak for bimodal powder in Patent Document 2. The grinding conditions for this powder are the same as those for the powder in Figure 1.

[0043] On the other hand, Figure 3 shows the particle size distribution measurement results for an example of a reference zirconia raw material powder. This raw material powder has a Y2O3 content of 3.0 mol%, and the particle size distribution measured by volume using a laser scattering particle size analyzer shows that ·Arithmetic mean particle size: 0.404μm ·Arithmetic standard deviation: 0.666μm The measured values ​​for particles smaller than 0.04 μm were zero, but the measured values ​​for particles larger than 0.4 μm were 19.31%. The powder was obtained by adding yttrium chloride to hydrated zirconia sol obtained by hydrolyzing an aqueous solution of zirconium oxychloride, then drying and calcining it, and then adding 15 kg of similar zirconia media with a diameter of 3 mm and 1.7 kg of pure water to 1 kg of Y2O3-containing calcined zirconia powder, which was then placed in a pot with an inner diameter of approximately 245 mm and ground at a rotation speed of 78 rpm for 10 hours.

[0044] Table 2 shows the application volume frequency for each particle size class value of each raw material powder in Figure 1 (recommended zirconia raw material powder) and Figure 3 (reference zirconia raw material powder), along with the total surface area per gram of powder and the surface area ratio for each class, calculated by approximating the crystal particles as spheres. The left column of Table 2 shows the raw material powder in Figure 1, and the right column shows the raw material powder in Figure 3.

[0045] [Table 2]

[0046] Furthermore, the zirconia powder particles contained in the recommended zirconia raw material powder may contain less than 1.5 mol% Y2O3 as a solid solution component. Zirconia powder particles within this composition range are less prone to transformation toughening due to the low amount of Y2O3 solid solution, and are therefore less hard and more susceptible to fracture crack propagation compared to tough zirconia media made of partially stabilized zirconia sintered bodies (with a Y2O3 solid solution content of, for example, 2.0 mol% to 5.0 mol%). Consequently, fine pulverization due to impact force from the zirconia media is more likely to occur, reducing the average diameter of the powder particles through relatively short pulverization times, and also reducing the likelihood of coarse powder particles remaining.

[0047] Figure 4 is a scanning electron microscope image (magnification 20,000x) of the surface of a zirconia sintered body (Table 3: No. 4 (Y2O3 content: 2.0 mol%)) produced by atmospheric pressure sintering at 1475°C for 2 hours using the recommended zirconia raw material powder shown in Figure 1. It can be seen that the structure consists of a mixture of relatively fine crystalline grains of 0.4 μm or less (first region) and slightly coarser crystalline grains of 0.4 μm or more (second region). Figure 5 is a scanning electron microscope image (magnification 20,000x) of the surface of a zirconia sintered body (Table 3: No. 8 (Y2O3 content: 2.9 mol%)) produced by atmospheric pressure sintering at 1475°C for 2 hours using the recommended zirconia raw material powder shown in Figure 2. Although the crystalline grains of 0.4 μm or more (second region) have grown somewhat compared to the sintered body in Figure 4, it can be seen that the relatively fine structure is still maintained.

[0048] On the other hand, Figure 6 is a scanning electron microscope image (magnification 20,000x) of a zirconia sintered body (Table 3: No. 14, corresponding to the comparative example of the present invention) produced by atmospheric pressure sintering at 1500°C for 2 hours using the reference zirconia raw material powder of Figure 3. Despite employing almost the same core composition as the raw material powder of Figure 2, it can be seen that it exhibits a coarse structure, completely different from the sintered bodies of Figures 2 and 6, with a significantly increased proportion of crystalline particles larger than 0.4 μm.

[0049] In this specification, the grain size distribution on electron microscope images is quantified using the following method. First, 300 to 400 crystal particles are randomly selected from the image. In this specification, as shown in Figure 4, multiple sampling lines SL are set on the image, and the crystal particles that each sampling line crosses are selected as the target for extraction. At this time, the setting interval is adjusted so that no single crystal particle is passed through by multiple sampling lines. Then, for each crystal particle that passes through each sampling line, parallel lines tangent to the outline of the crystal particle are drawn on the image in the vertical and horizontal directions, and when the intervals between the parallel lines are D1 and D2, the value Dm = (D1 + D2) / 2 is calculated, and the diameter of the circle having the same area as Dm is determined as the particle size of each extracted crystal particle.

[0050] Figure 7 is a histogram showing the microstructure grain size distribution (area-based) for the zirconia sintered body number 4 in Table 3, analyzed using the observation image in Figure 4 and the method described above. Figure 8 is a histogram showing the microstructure grain size distribution (area-based) for the zirconia sintered body number 12 in Table 3, analyzed using the observation image in Figure 5 and the method described above. Figure 9 is a histogram showing the microstructure grain size distribution (area-based) for the zirconia sintered body number 14 in Table 3, similarly analyzed using the observation image in Figure 6. The histograms of the zirconia sintered bodies produced using the recommended zirconia raw material powder (Figures 7 and 8) show that the area ratio of crystal particles in the first region with a crystal particle diameter of 0.4 μm or less falls within the range of 20% to 80%, whereas the histogram of the comparative example zirconia sintered body (Figure 9) shows that the area ratio of crystal particles in the first region is significantly lower at 9.7%.

[0051] Furthermore, since all sintered bodies exhibited relatively smooth surface conditions, no polishing was performed, and the sintered surfaces were directly observed under magnification. For confirmation, the grain size distribution of a microstructure obtained by mirror polishing the surface of a sintered body and then thermal etching it at a temperature 50°C lower than the sintering temperature for 1 hour was also measured using the same method. Within the scope of the study described herein, it should be noted that the calculation results of the area ratios of the first and second regions were in agreement with the results obtained using observation images of the unpolished sintered body surface within ±5%.

[0052] Furthermore, the average Y2O3 concentration in the first and second regions can be analyzed using a field emission electron probe microanalyzer (EPMA) in the following manner. Specifically, as shown in Figure 10, in the first region where crystal particles smaller than 0.4 μm are densely packed in the observation field of the electron microscope (magnification 20,000x), the spot diameter is adjusted within the range of 0.2 to 0.8 μm so that multiple crystal particles smaller than 0.4 μm are contained within the spot of the irradiated electron beam, as indicated by the dashed white circle AC1, and the concentrations of Zr, Y, and O are analyzed. The same measurement is then performed five times while changing the spot position in the field of view, and the Y2O3 concentration is calculated from the average value of these results.

[0053] On the other hand, for crystal grains in the second region exceeding 0.4 μm, as shown by the dashed white circle AC2 in Figure 10, the spot diameter was adjusted within the range of 0.3 to 0.8 μm so that the spot of the irradiated electron beam sufficiently covered the region including the center inside the crystal grain, and the concentrations of Zr, Y, and O were analyzed. The same measurement was then performed for 10 crystal grains while changing the spot position in the field of view, and the Y2O3 concentration was calculated from the average value of these results. For the zirconia sintered body shown in Figure 6 (crystal grain size distribution: Figure 9), where the area ratio of crystal grains smaller than 0.4 μm is small, 10 crystal grains smaller than 0.4 μm were individually extracted, and the spot diameter was adjusted so that the spot of the irradiated electron beam was contained within each crystal grain for analysis.

[0054] Figures 7 to 9 show the analysis results of the average Y2O3 concentration in the first and second regions, along with the measurement results of the fracture toughness values. For the zirconia sintered body produced using the raw material powder in Figure 1 (recommended zirconia raw material powder) (histogram in Figure 7), the average first region Y2O3 concentration C1 in the first region, where the crystal particle size is 0.4 μm or less, is 1.86 mol%, which is lower than the central composition of 2.00 mol%. In contrast, the average second region Y2O3 concentration C2 in the second region, where the crystal particle size is greater than 0.4 μm, is 2.65 mol%, which is higher than the central composition. If we assume that the difference in Y2O3 concentrations in both the second and first regions (second region Y2O3 concentration C2 and first region Y2O3 concentration C1) is ΔC ≡ C2 - C1, then the value of ΔC / C2 reaches 0.30, which is sufficiently larger than the lower limit of 0.1 for the present invention. Furthermore, the fracture toughness value of the zirconia sintered body is 15.8 MPa·m. 0.5 And it is extremely high.

[0055] Similarly, for the zirconia sintered body produced using the raw material powder shown in Figure 2 (histogram in Figure 8), the average Y2O3 concentration C1 in the first region is 2.27 mol%, which is lower than the central composition of 2.9 mol%. On the other hand, the average Y2O3 concentration C2 in the second region is 3.87 mol%, which is higher than the central composition. Furthermore, the ΔC / C2 value is 0.39, and the fracture toughness value of the zirconia sintered body is 10.8 MPa·m. 0.5 And it's expensive.

[0056] On the other hand, the zirconia sintered body produced using the raw material powder shown in Figure 3 (reference zirconia raw material powder) (histogram in Figure 9) has a low area ratio in the first region (2.9%: outside the scope of the present invention). Furthermore, although the average first region Y2O3 concentration C1 in the first region is 2.83 mol%, which is lower than the central composition of 3.0 mol%, the average second region Y2O3 concentration in the second region, which accounts for 97% of the total and has a crystal grain size of over 0.4 μm, is 3.01 mol%, which is almost the same as the central composition. As a result, the value of ΔC / C2 is 0.06, which is less than the lower limit of 0.1 in the present invention, and the fracture toughness value of the zirconia sintered body is 5.9 MPa·m 0.5 And it remains at a low value.

[0057] Furthermore, more detailed analysis using EPMA revealed that in the crystalline grains constituting the structure of the zirconia sintered body of the embodiment (particularly the crystalline grains in the second region), the Y2O3 concentration in the center is significantly higher than that in the outer periphery. In crystalline grains with such a concentration structure, the proportion of the C phase increases in the central region with high Y2O3 concentration, and a region is formed where the Y2O3 concentration gradually decreases toward the periphery of the crystalline grain. The supersaturated T phase, which is mainly responsible for the transformation toughness of the zirconia sintered body, is thought to be more likely to form in the part of this gradually decreasing Y2O3 concentration region at the periphery of the crystalline grain where the Y2O3 concentration falls within a specific range.

[0058] As a result of detailed investigations by the inventors, it was found that a zirconia sintered body with particularly good fracture toughness can be obtained when the area ratio of the effective Y2O3 concentration region, in which the Y2O3 concentration is within a specific range of 2.0 mol% to 2.4 mol%, is 17% or more. It is presumed that the area ratio of this effective Y2O3 concentration region reflects the area ratio of the supersaturated T phase formation, and it is thought that the formation morphology and absolute amount change depending on the central composition of Y2O3 set in the zirconia sintered body (see Figure 29 below).

[0059] Figure 11 shows a scanning electron microscope image (magnification 20,000x) of the surface of a zirconia sintered body (Table 3: No. 1, described later) manufactured by atmospheric pressure sintering at 1425°C for 2 hours using a raw material powder with a central Y2O3 composition of 1.5 mol% (below the lower limit of the embodiment) as the recommended zirconia raw material powder, and the EPMA line analysis results, in which the Y2O3 concentration was measured along an analysis line crossing the large particles belonging to the second region in the field of view. The region where the Y2O3 concentration is 1.5 mol% belongs to the T-phase (M) single-phase region, which is close to the solid solubility limit on the T(M) phase side in the phase diagram of Figure 27, and the formation of the C phase is hardly expected.

[0060] The EPMA line analysis results shown in Figure 11 below show that the Y2O3 concentration, regardless of whether it is a large or small particle, exhibits a profile with small concentration fluctuations near the central composition. In the graph, the two horizontal dashed lines indicate the Y2O3 concentration range (2.0 mol% to 2.4 mol%) corresponding to the effective Y2O3 concentration region mentioned above. However, the line analysis profile shows that most of the data lies below the lower limit of the effective Y2O3 concentration range, and the area ratio of the effective Y2O3 concentration region is also small. Figure 12 shows the surface of the same zirconia sintered body as in Figure 11, observed with a scanning electron microscope at 5000x magnification, and correlated with a mapping image of the effective Y concentration region identified based on the EPMA surface analysis results on the image (the area shown in black in the image is the effective Y concentration region; the same applies hereafter). The area ratio of the effective Y concentration region in this zirconia sintered body is small at 6.5%, and the fracture toughness value is 5.5 MPa·m 0.5 This falls outside the scope of the present invention. Furthermore, when attempting to sinter a powder of the same composition at 1450°C, numerous cracks occurred in the sintered body, making evaluation impossible.

[0061] Figure 13 shows a scanning electron microscope image (magnification 20,000x) of the surface of a zirconia sintered body (Table 3: No. 4, described later) manufactured by atmospheric pressure sintering at 1475°C for 2 hours using a recommended zirconia raw material powder with a central Y2O3 composition of 2.0 mol% (within the range of the embodiment), and the results of EPMA radiation analysis measuring the Y2O3 concentration along an analysis line crossing large particles belonging to the second region in the same field of view. The region where the Y2O3 concentration is 2.0 mol% occupies a position close to the solid solubility limit on the T(M) phase side within the T(M) + C phase multiphase region in the phase diagram of Figure 27, and is a region where the T(M) phase is predominant but the formation of a certain amount of C phase is expected. The sintered body structure can be seen to have a structure in which zirconia crystal particles belonging to the second region with a particle size of 0.4 μm or more are dispersed in a background region consisting of zirconia crystal particles belonging to the first region with a particle size of less than 0.4 μm.

[0062] As shown in the EPMA line analysis results in Figure 13, a concentration profile is obtained in which the Y2O3 concentration changes continuously from the maximum point formed within the large particle toward the periphery. Looking at it in more detail, it can be seen that the Y2O3 concentration increases significantly above the central composition at the location of the large particle belonging to the second region, while in the background region, which is mainly composed of particles from the first region, a profile is observed in which a gentle concentration fluctuation occurs near the lower limit of the effective Y concentration interval. The effective Y2O3 concentration interval is a relatively long section of the profile interval in which the Y2O3 concentration decreases with a significant gradient from the center to the periphery of the large particle, and in addition, there are parts of the background region that fall into the effective Y2O3 concentration interval. Figure 14 shows the same zirconia sintered body surface as in Figure 13, observed with a scanning electron microscope at a magnification of 5000x, and the mapping image of the effective Y concentration interval identified based on the EPMA surface analysis results on the image. This zirconia sintered body exhibits a large area ratio of 24.3% for the effective Y concentration region, and its fracture toughness value is 15.8 MPa·m². 0.5 This is extremely large. In this sintered body structure, the effective Y2O3 concentration region is thought to be dispersed in a network-like manner along the boundary between the large particles and the background region. Corresponding to such an effective Y2O3 concentration region, it is thought that the supersaturated T phase, which can inhibit the propagation of fracture cracks, is also dispersed in a network-like manner, which is more advantageous for improving fracture toughness.

[0063] Figure 15 shows a scanning electron microscope image (magnification 20,000x) of the surface of a zirconia sintered body (Table 3: number 8, described later) manufactured by atmospheric pressure sintering at 1475°C for 2 hours using a similar raw material powder with a central Y2O3 composition of 2.9 mol% (within the range of the embodiment), and the results of EPMA line analysis measuring the Y2O3 concentration along an analysis line crossing large particles belonging to the second region in the same field of view. The region where the Y2O3 concentration is 2.9 mol% is a region within the T phase (M) + C phase mixed phase region of the phase diagram in Figure 27, slightly away from the solid solubility limit on the T (M) phase side, where the T phase (M) remains the main component, but the formation of more C phase is expected.

[0064] The EPMA line analysis profile shown in Figure 15 (bottom) reveals that the Y2O3 concentration at the location of large particles belonging to the second region increases significantly compared to the central composition, similar to Figure 13. However, almost the entire gradient section of Y2O3 concentration from the center to the periphery of the large particles, and a considerable portion of the background region mainly composed of particles in the first region, fall outside the upper limit of the effective Y concentration range. Figure 16 shows the same zirconia sintered body surface as in Figure 15, observed with a scanning electron microscope at 5000x magnification, and correlated with a mapping image of the effective Y concentration region identified based on the EPMA surface analysis results on the image. The area ratio of the effective Y concentration region in this zirconia sintered body is 18.9%, which is lower than in Figure 14, and the fracture toughness value is also 10.5 MPa·m. 0.5 While this is good, it is lower than that of the zirconia sintered bodies shown in Figures 13 and 14.

[0065] Figure 17 shows a scanning electron microscope image (magnification 20,000x) of the surface of a zirconia sintered body (Table 3: No. 13, described later) manufactured by atmospheric pressure sintering at 1500°C for 2 hours using a similar raw material powder with a central Y2O3 composition of 5.0 mol% (outside the scope of the embodiment), and the results of EPMA radiation analysis measuring the Y2O3 concentration along an analysis line crossing large particles belonging to the second region in the same field of view. The region where the Y2O3 concentration is 5.0 mol% is the region in the phase diagram of Figure 27 where the amount of C phase formation is expected to be greater than that of the T(M) phase within the T(M) phase multiphase region.

[0066] As is clear from the observed images, the proportion of coarse particles with a particle size of 1 μm or more, belonging to the second region, is increasing. Furthermore, the EPMA radiation analysis profile shows a continuously wave-like change in Y2O3 concentration, with a maximum point within the coarse particles and a minimum point within the background region formed by the surrounding small particle group. It can be seen that almost the entire profile, including the minimum point, is outside the upper limit of the effective Y concentration range. Figure 18 shows the same zirconia sintered body surface as in Figure 17, observed with a scanning electron microscope at a magnification of 5000x, and correlated with a mapping image of the effective Y concentration region identified based on the EPMA surface analysis results on the image. The area ratio of the effective Y concentration region in this zirconia sintered body is almost zero, and the fracture toughness value is 4.6 MPa·m 0.5 And it's low.

[0067] Figure 19 shows a scanning electron microscope image (magnification 5000x) of the surface of a zirconia sintered body (Table 3: No. 14, described later) manufactured using the aforementioned reference zirconia raw material powder by atmospheric pressure sintering at 1500°C for 2 hours in air, and its correspondence with a mapping image of the effective Y concentration region identified based on the EPMA surface analysis results on the image. The area ratio of the effective Y concentration region of this zirconia sintered body is 8.28%, and the fracture toughness value is 5.9 MPa·m 0.5 And it's low.

[0068] Next, while the region where the supersaturated T phase is formed in close proximity to the C phase tends to appear in the larger crystal grains of the second region, where the average Y2O3 concentration is higher than the central composition, it can also appear in the first region, where the average Y2O3 concentration is lower than the central composition, if the Y2O3 concentration is locally high, although the amount formed will be less than in the second region. Importantly, in the zirconia sintered body of the embodiment in which the Y2O3 content in the total composition of the sintered body (central composition of Y2O3) is set relatively low, between 1.6 mol% and 2.9 mol%, the high-concentration Y2O3 region where the C phase is likely to appear and the region where the supersaturated T phase may appear are closely related, and there is a unique relationship between the amount of C phase formation inferred from the phase diagram, the amount of supersaturated T phase formation, and consequently the fracture toughness value of the sintered body.

[0069] Specifically, let S1 be the area ratio of the first region in the sintered body structure, C1 be the average Y2O3 concentration of the first region, S2 be the area ratio of the second region, and C2 be the average Y2O3 concentration of the second region. (C) The equivalent cubic area fraction S is calculated using the formula: =S1 × {(7.4-C1) / 6} × 100 + S2 × {(7.4-C2) / 6} × 100 (C) When the percentage (%) is between 5% and 25%, particularly high fracture toughness values ​​are achieved (details are explained in the "Experimental Examples" section).

[0070] The following describes the presumed mechanism by which a characteristic structure is obtained that results in a difference in the average Y2O3 concentration between the first and second regions (Y2O3 concentration in the first region, Y2O3 concentration in the second region) by using the recommended zirconia raw material powder having the central composition of the embodiment. First, in this raw material powder, there are almost no particles with a particle size exceeding 0.4 μm, and almost all powder particles have a sharp dispersion peak converging in the particle size range of 0.06 to 0.3 μm. When such raw material powder is molded and sintering is started, in the initial stage of sintering, as shown in Figure 20, there are almost no large crystalline particles exceeding 0.4 μm, and small crystalline particles SG are the main component. At this stage, the main mechanism is bonding and coalescing of adjacent small crystalline particles SG, and it is thought that the state of slow, uniform growth continues for a while without generating any protruding large particles.

[0071] However, as shown on the left side of Figure 21, it is thought that the Y2O3 powder particles Y in the raw material powder are pulverized to an average particle size considerably smaller than that of the ZrO2 powder particles BG and SG. In the initial stages of sintering, it is likely that the reaction in which the finely pulverized Y2O3 powder particles Y diffuse into the ZrO2 powder particles BG and SG will proceed easily.

[0072] As shown with reference to Figure 23, according to known solid-state sintering neck formation theory, the larger the dimensional difference between the small crystal grains SG and large crystal grains BG that are in contact with each other, the smaller the radius of curvature ρ of the neck N, which is the joint between them, and the lower the vapor pressure. This results in an excess vacancy concentration increase ΔD in the neck N compared to the bulk. The vacancy concentration gradient in the neck N is expressed as ΔD / ρ, and this acts as a driving force, causing vacancies to diffuse into the interior of the crystal grains, while in exchange, constituent atoms of the crystal grains flow into the voids V between the crystal grains. This causes the voids V to contract, and solid-state sintering proceeds.

[0073] According to the neck formation theory described above, in Figure 21, the atomic diffusion velocity of Y2O3 powder particles Y toward the large crystal particles BG is considered to be high, while the atomic diffusion velocity toward the small crystal particles SG is considered to be low. Consequently, as shown in Figures 21 and 22, it is presumed that in the early stages of sintering, when the growth of ZrO2 powder particles BG and SG is not progressing significantly, a contrast structure is formed in which the Y2O3 concentration is slightly higher in the large crystal particles BG and slightly lower in the small crystal particles SG.

[0074] Next, as solid-phase sintering progresses further, as shown in Figure 23, the excess vacancy concentration increment ΔD is thought to concentrate on the side of the small crystal particles SG with a smaller surface curvature radius. Therefore, atomic diffusion proceeds in a direction that fills the voids V and causes the small crystal particles SG to contract, growing the large crystal particles BG. At this time, the smaller crystal particles SG(3) have a higher atomic diffusion velocity toward the large crystal particles BG and are absorbed and disappear. Since both the small crystal particles SG(3) and the large crystal particles BG are mainly composed of ZrO2, and there is not as much of a Y2O3 concentration difference as between the ZrO2 powder particles and Y2O3 powder particles contained in the starting material, even if a certain number of small crystal particles SG(3) are absorbed by the large crystal particles BG, it does not significantly increase the Y2O3 concentration of the large crystal particles BG.

[0075] On the other hand, smaller crystalline particles SG(1) or SG(2), which are larger than the smaller crystalline particles SG(3), have a relatively small atomic diffusion velocity toward the larger crystalline particles BG, and most of them remain without being absorbed by the larger crystalline particles BG. In this case, although there is no literature to support the atomic diffusion coefficient from the smaller crystalline particles SG(1) and SG(2) toward the larger crystalline particles BG, the inventors speculate that the diffusion coefficient of Y is larger than that of Zr. According to this speculation, it is thought that the larger the particle size of the smaller crystalline particles SG(1), the more likely Y diffusion toward the larger crystalline particles BG will occur.

[0076] If we assume that the Y2O3 concentration of large crystal particles BG, which have grown using small crystal particles as seeds, is increased by the mechanism hypothesized by the inventors, then the large crystal particles would be in contact with small crystal particles that have a lower Y concentration than the large crystal particles. In other words, the Y atom concentration gradient would be formed in the direction from the large crystal particles to the small crystal particles, that is, in the opposite direction to the hypothesized Y atom diffusion direction. However, judging from the EPMA analysis results shown in the illustration above, there is no doubt that Y is diffusing from the small crystal particles to the large crystal particles, overcoming the Y atom concentration gradient.

[0077] According to the equilibrium phase diagram in Figure 27, the ZrO2-Y2O3 binary system composition tends to separate into a T(M) phase and a C phase with different free energies on the ZrO2 side. According to Fig. 10 of Non-Patent Literature 1, which is the source of this diagram, in the Y2O3 concentration range assumed in the embodiment, the T phase has a lower free energy ΔGv than the C phase, and the difference between the two decreases as the Y2O3 concentration increases. Furthermore, when the size of the crystal grains is taken into consideration, the total energy of the crystal can be expressed as the sum of the above-mentioned phase free energy term and the interfacial tension energy term. As the crystal grains become finer, the contribution of the interfacial tension energy term to the total energy increases. At this time, a portion of the C phase decomposes into the T(M) phase with a lower phase free energy, which lowers the total energy of the crystal and makes it thermodynamically stable. Therefore, it can be considered that small crystal grains release the Y component toward the large crystal grains, accompanied by the decomposition of the T phase from the C phase.

[0078] As the outflow of Y component from small crystal particles SG to large crystal particles BG progresses, the Y2O3 concentration in large crystal particles BG increases, while the Y2O3 concentration in small crystal particles SG decreases. As mentioned above, coupled with the fact that differences in Y component concentration already exist between small crystal particles SG and large crystal particles BG in the initial stages of sintering, the Y2O3 concentration decreases more rapidly in the small crystal particles SG that are left behind later. As a result, as shown in Figures 11, 13, and 15, the Y2O3 concentration in the large crystal particles is thought to be lower in the outer regions that are added later in time.

[0079] Returning to Figure 23, as the void V decreases, the shrinkage of the sintered body slows down, and the vacancy concentration, which is the driving force for atomic diffusion, also decreases. Therefore, the small crystal particles SG remaining at that point are less likely to coalesce and absorb with the large crystal particles BG. In this way, it is thought that the formation of a structure with a large Y2O3 concentration difference between the small crystal particles SG and the large crystal particles BG is largely completed. Furthermore, the lower the Y2O3 concentration of the small crystal particles SG, the higher the T(M) phase ratio becomes, and the greater the phase free energy difference with the large crystal particles BG, which have a high C phase ratio. Therefore, coalescing with the large crystal particles BG is considered to be less likely to occur.

[0080] Next, the reason why a difference in Y2O3 concentration depending on the crystal grain size is less likely to occur when using reference zirconia raw material powder can be estimated as follows. First, as shown in Figure 24, in the reference zirconia raw material powder, the Y component is solid-solved at roughly the same concentration from the beginning between the large crystal particles BG and the small crystal particles SG at the raw material stage. Therefore, even in the initial stage of sintering when this powder is molded and sintered, no difference in Y2O3 concentration occurs between the large and small crystal particles. When such raw material powder is used, a large number of large crystal particles that are present in the raw material powder from the beginning are already included in the initial stage of sintering, and their size is considerably larger than when the recommended zirconia raw material powder is used. In particular, when using bimodal powder that has two peaks at the 0.3-0.5 μm position and at the 0.15 μm position or less, as in Patent Documents 2 and 4, the proportion of large crystal particles exceeding 0.4 μm is especially high.

[0081] Furthermore, the Y2O3 concentration of the large crystal particles BG is close to the composition at the time of formulation, and the proportion of particles with an irregularly increased Y2O3 concentration is considered to be small. In this case, as shown in Figure 25, in the initial sintering stage where many voids remain, the number ratio of minute powder particles FG and small crystal particles SG with a small degree of growth to the particles in contact with such large crystal particles BG is much higher than in the case of Figure 13, and most of them are absorbed and disappear into the large crystal particles BG. As already explained, in such a case, the Y2O3 concentration of the large crystal particles BG is unlikely to increase.

[0082] Therefore, until a critical size is reached in which absorption into the large crystal particles BG becomes less likely, the process of surrounding small crystal particles SG being absorbed and disappearing into one of the many large crystal particles BG continues for a long time. Consequently, until the large crystal particles BG grow to a considerable size, there is little inflow of Y from the small crystal particles SG, and the Y2O3 concentration difference between the large crystal particles BG and the small crystal particles SG remains relatively small. When using a bimodal distribution raw material powder such as that described in Patent Documents 2 or 4, sintering is completed with a relatively large number of small crystal particles SG remaining between the large crystal particles BG, where the Y2O3 concentration has not decreased significantly (see the left side of Figure 26). Furthermore, when using a raw material powder with a relatively large average particle size and a broad particle size distribution with one peak, the growth of the large crystal particles BG progresses further until it has almost completely consumed the small crystal particles SG, and it is thought to reach an equiaxed structure with a large average crystal grain size (see the right side of Figure 26). Furthermore, it has been confirmed that in zirconia sintered bodies exhibiting an equiaxed crystal structure like the latter, Y segregates near the grain boundaries where large crystal particles are adjacent to each other (Non-Patent Literature 5). It should be noted that this Y2O3 concentration distribution shows a trend completely opposite to that of the embodiment shown in Figure 9, where the Y concentration is lower in the outer periphery of the large crystal particle BG and in the surrounding small crystal particles.

[0083] In the zirconia sintered body of the embodiment, a portion of the remaining ZrO2 may be substituted with Al2O3 (aluminum oxide) in an amount of 0.2 mol% or less (preferably 0.15 mol% or less). Alternatively, it may be substituted with TiO2 (titanium oxide) in an amount of 0.1 mol% or less (preferably 0.075 mol% or less). The inclusion of Al2O3 to TiO2 within such compositional ranges does not particularly hinder the realization of the characteristic structure and component distribution morphology of the zirconia sintered body of the present invention described in detail above, and consequently the resulting fracture toughness and total light transmittance.

[0084] Furthermore, in the embodiments, "unavoidable impurities" refer to components (so-called contaminations) that can be included in a range that does not impair the characteristics of the zirconia sintered body of the embodiment. This does not necessarily mean only residual components that cannot be actively removed in the raw material powder or manufacturing process of the zirconia sintered body, but the total of these components does not exceed 0.5 mol%. For example, in the zirconia sintered body of the embodiment, Y2O3 is used as a stabilizer, and other stabilizers such as CaO, MgO (magnesium oxide), and CeO2 (cerium oxide) may be included as unavoidable impurities in this concept. In addition, SiO2 (silicon dioxide) may be included as an unavoidable impurity, and it is desirable that its content in the total composition of the sintered body be 0.1% by mass or less.

[0085] The Vickers hardness Hv of the zirconia sintered body of the embodiment configured as described above should be 1100 or higher, preferably 1150 or higher. Furthermore, assuming the use of atmospheric pressure sintering, it is desirable that the bending strength be at least 600 MPa when a three-point bending strength test is performed. The three-point bending strength test is measured in accordance with standards such as JIS R 1601 (2008) (ISO 14704:2000).

[0086] The method for manufacturing the zirconia sintered body of the embodiment has already been partially described. To further explain, first, pure zirconia powder (average particle size: for example, 20 μm or less) and pure Y2O3 powder (and oxide powders that make up other components), which are stabilizer powders, are blended to achieve the desired composition. Zirconia media with a diameter of 1.5 mm to 9 mm (preferably 1.5 mm to 5 mm) and a solvent (for example, pure water) are blended and placed in a pot of a grinding device such as a ball mill, and the contents are ground at a predetermined rotational speed that causes an avalanche phenomenon. After grinding, an appropriate amount of water-soluble binder is added. The ground material is dried in a spray dryer or the like to prepare a composition for molding.

[0087] The composition may be formed into a green molded body by press molding (green processing), and if necessary, the green molded body may be subjected to secondary processing such as cutting, and then immediately subjected to main sintering. Alternatively, a calcined body may be created first, and then the calcined body may be subjected to secondary processing such as cutting before main sintering. The sintering temperature when creating the calcined body should be, for example, 800°C to 1200°C, preferably 900°C to 1000°C. The main sintering of the green molded body should be, for example, 1400°C to 1500°C, preferably 1420°C to 1490°C. Both calcination and main sintering can be carried out in an atmospheric atmosphere at normal pressure.

[0088] When the green molded body is immediately subjected to the main sintering process, the sintering time required to obtain a sufficiently dense sintered body is, for example, 70 to 180 minutes after degreasing the water-soluble binder required during molding at 300°C to 500°C. On the other hand, if the calcined body is manufactured using raw material powder with a particle size distribution that satisfies the aforementioned conditions, the main sintering time required to densify the calcined body can be significantly reduced. When using such a calcined body, the main sintering time is 60 minutes or less, preferably 30 minutes or less, and more preferably 20 minutes or less.

[0089] Furthermore, the method for manufacturing the zirconia sintered body in the embodiment can be modified as appropriate, provided that it does not particularly hinder the securing of the characteristic structure, component distribution, and consequently the resulting fracture toughness of the zirconia sintered body in the embodiment.

[0090] Furthermore, the zirconia sintered body of the embodiment can be used in a variety of applications, such as dental materials like prosthetic materials, optical fiber connectors like ferrules and sleeves, various tools (e.g., grinding balls, grinding tools), various parts (e.g., screws, bolts and nuts, bearing balls), various sensors, electronic components, and decorative items (e.g., watch bands). When the zirconia sintered body is used as a dental material, it can be used for copings, frameworks, crowns, crown bridges, abutments, implants, implant screws, implant fixtures, implant bridges, implant bars, brackets, denture bases, inlays, onlays, onlays, orthodontic wires, laminate veneers, etc.

[0091] (Example of experiment) The following describes further detailed experimental examples to support the effects of the zirconia sintered body of the embodiment. First, to obtain zirconia sintered bodies with various central compositions shown in Table 3, a total of 1 kg of commercially available pure zirconia powder mixed with pure Y2O3 powder in various proportions, 10 kg of zirconia media (7.7 kg of 2 mm diameter media and 2.3 kg of 3 mm diameter media) and 1.7 kg of pure water were placed in a pot with an inner diameter of approximately 245 mm and ground at a rotation speed of 78 rpm for 20 hours. After adding an appropriate amount of water-soluble binder, the slurry was dried with a spray dryer to obtain raw material powders adjusted to various values. Furthermore, raw material powders were similarly prepared by substituting a portion of the ZrO2 with 0.1 mol% Al2O3 (numbers 9 and 10) and 0.05 mol% TiO2 (numbers 11 and 12) for compositions numbered 1 to 9 in Table 3, where the Y2O3 concentration was 1.9 and 2.0 mol%.

[0092] For comparison, a reference zirconia raw material powder with a Y2O3 concentration of 3.0 mol% was prepared by adding yttrium chloride to a hydrated zirconia sol obtained by hydrolyzing an aqueous solution of zirconium oxychloride, then drying and calcining. 1 kg of Y2O3-containing calcined zirconia powder was then mixed with 15 kg of 3 mm diameter zirconia media and 1.7 kg of pure water in a pot with an inner diameter of approximately 245 mm. The mixture was ground at a rotation speed of 78 rpm for 10 hours, then an appropriate amount of water-soluble binder was added, and the slurry was dried with a spray dryer (Table 3: No. 14 below). A portion of the slurry before the addition of the water-soluble binder was not dried and was subjected to measurement of the powder particle size distribution using a laser scattering particle size analyzer. Except for powder number 14, all powders yielded a particle size distribution nearly identical to that shown in Figure 1, confirming that the arithmetic mean particle size fell within the range of 0.10 μm to 0.15 μm, and the arithmetic standard deviation fell within the range of 0.03 μm to 0.05 μm.

[0093] Next, the raw material powder was molded into compacts with a hydraulic press to create N=3 compacts, which were then sintered at a temperature of 1425-1500°C in atmospheric pressure to produce zirconia sintered bodies. The dimensions of the sintered bodies were 40mm x 30mm x 3mm in the shape of plates. The density of the sintered bodies was measured by the Archimedes method, and by comparing it with the calculated true density for each composition, it was confirmed that all were densified to over 99.5%.

[0094] Next, one of the three sintered bodies was cut into a first specimen measuring 30 mm × 30 mm × 3 mm and a second specimen measuring 30 mm × 10 mm × 3 mm. The sintered surface forming one of the main surfaces of the second specimen was observed using a Schottky electron microscope (HITACHI: SU5000) in low vacuum mode at magnifications of 5000x, 10000x, and 20000x to obtain microstructure images.

[0095] Using the obtained microstructure images, the grain size distribution was measured and a histogram was created using the method previously described. The area ratios of the first region, consisting of grains between 0.1 μm and 0.4 μm, and the second region, consisting of grains between 0.4 μm and 1.6 μm, were calculated as ratios to the total area of ​​the sampled grains for particle size measurement. Furthermore, using the measured grain size distribution data, the area-average grain size of the sampled grains (a value obtained by weighting the particle diameter of each grain by its circular particle area on the image, summing the results, and dividing by the total area of ​​the sampled grains) was calculated. For the second specimen, EPMA (equipment used: JXA-8530F (JEOL Ltd.)) was used to measure the average Y2O3 concentration C1 in the first region and the average Y2O3 concentration C2 in the second region using the method described above, and the value of ΔC / C2 was calculated. EPMA surface analysis was also performed, and the area ratio of the effective Y2O3 region was calculated based on the results.

[0096] Next, both sides of the first test specimen were ground to a thickness of 0.5 mm, and then both sides were mirror-polished using diamond abrasive grains. Using this test specimen, the total light transmittance was measured using a commercially available UV-Vis spectrophotometer (JASCO: V-780) with the CIE standard light D65 in the visible light wavelength range of 380 nm to 780 nm. In addition, the Vickers hardness and fracture toughness values ​​were measured using a commercially available Vickers hardness tester on one of the mirror-polished surfaces of the first test specimen. For each sample, Vickers hardness and fracture toughness values ​​were measured at 12 points at different positions, and the maximum and minimum measured values ​​were excluded to obtain the average value.

[0097] Furthermore, the remaining two sintered bodies were processed into 3mm x 4mm x 40mm bending test specimens, and bending strength tests were performed by three-point bending. Ten bending strength measurements were taken for each specimen, and the maximum and minimum measurements were excluded to obtain the average value. The results are shown in Table 3. In Table 3, numbers marked with "*" indicate that the product is outside the scope of the present invention.

[0098] [Table 3]

[0099] For test specimens numbered 2 to 12, where the central composition, the area ratio of the first region, and the area ratio of the second region are within the range specified in the claim, and the value of ΔC / C2 is 0.1 or greater, the fracture toughness value is 10.5 MPa·m 0.5 It can be seen that the values ​​are as high as above. For core composition in the range of 1.9 mol% to 2.5 mol%, the fracture toughness value is 13.5 MPa·m. 0.5 As can be seen, particularly good values ​​were obtained. Furthermore, test samples 2 to 12 also showed relatively good total light transmittance values ​​of 37% or higher. In addition, the evaluation results for test samples 9 to 12, which had Al2O3 and TiO2 added, were all almost the same as those for the corresponding Y2O3 compositions (numbers 3 and 4) without Al2O3 and TiO2 added, indicating that there were no adverse effects from the addition of Al2O3 and TiO2.

[0100] On the other hand, in test specimen No. 1, whose central composition is less than the lower limit of the claimed range (1.6 mol%), the area ratio of the first region exceeds the upper limit of the claimed range (80%), the area ratio of the second region falls below the lower limit (20%), and the area-average particle size is less than the lower limit of the claimed range (0.3 μm). Furthermore, the ΔC / C2 value is significantly below the lower limit of the claimed range (0.1), and the fracture toughness value is 5.5 MPa·m 0.5 This is low and outside the scope of the claims (the present invention). On the other hand, in test specimen No. 13, in which the central composition exceeds the upper limit of the claims (2.9 mol%), the area ratio of the first region is less than the lower limit of the claims (20%), and the area ratio of the second region exceeds the upper limit (80%), and the area-average particle size also exceeds the upper limit of the claims (0.6 μm). Although the ΔC / C2 value is within the specified range of the claims, the fracture toughness value is 4.6 MPa·m 0.5 This is low and falls outside the scope of the claims (of this invention).

[0101] Furthermore, in test specimen No. 14, manufactured using reference zirconia raw material powder, the area ratio of the first region was less than the lower limit of the claimed range (20%), and the area ratio of the second region exceeded the upper limit (80%). The area-average particle size also exceeded the upper limit of the present invention (0.6 μm). The ΔC / C2 value was below the lower limit of the claimed range (0.1), and the fracture toughness value was 5.9 MPa·m 0.5 This is low and falls outside the scope of the claims (of this invention).

[0102] Based on the above results, let S1 be the area ratio of the first region in the sintered body structure, and C1 be the average Y2O3 concentration of the first region. S (C) (%)=S1×{(7.4-C1) / 6}×100+S2×{(7.4-C2) / 6}×100 The converted cubic area fraction S is calculated using the method described above. (C) The values ​​for each are shown in Table 3. In the above equation, the denominator value "6" represents the distance between point A (Y2O3 concentration: approximately 1.4 mol%), which represents the solid solubility limit of Y2O3 to ZrO2 at 800°C where atomic diffusion slows down to some extent, and point B (Y2O3 concentration: approximately 7.4 mol%), which represents the solid solubility limit of ZrO2 to Y2O3, in the equilibrium phase diagram of Figure 27. In Figure 27, when the given composition point is O, the abundance ratio of the C phase in the phase equilibrium state can be expressed by AO / AB. The first term on the right side of the above equation gives the area fraction of the C phase in the first region, and the second term gives the area fraction of the C phase in the second region. S is defined as the sum of these two. (C) This refers to the expected cubic area ratio in the entire zirconia sintered body when there is a Y2O3 concentration difference between the first and second regions, and this is referred to as the "converted cubic area ratio" in this specification. These values ​​do not immediately represent the amount of C phase actually present in the zirconia sintered body, however S (C) From a phase equilibrium perspective, it can be expected that a larger value of will result in a larger area ratio of tetragonal crystals present within the sintered zirconia.

[0103] In the central composition region of Y2O3 adopted by the claims (the present invention), as described above, it is considered that a supersaturated T phase is likely to form in close proximity to the C phase formation region. Therefore, the fracture toughness values ​​(K) in Table 3. IC ) is the tetragonal area fraction S converted above. (C) The results plotted against the value of S are shown in Figure 28. (C) When the percentage (%) is between 5% and 25%, the fracture toughness value measured by the IF method is 12 MPa·m 0.5 It can be seen that particularly high values ​​have been achieved. This is due to the converted cubic crystal area ratio S (C) It is presumed that if the value is too large or too small, the formation of the supersaturated T phase mentioned above will be insufficient. Also, the fracture toughness value (K) in Table 3. IC Figure 29 shows the results of plotting the effective Y2O3 concentration region against the area fraction of the effective Y2O3 concentration region. It can be seen that the larger the area fraction of the effective Y2O3 concentration region, which is presumed to reflect the amount of supersaturated T phase formation, the more monotonically the fracture toughness value measured by the IF method (a method that measures the length of the indentation and crack generated by pressing a Vickers indenter into a test specimen for a period of 15 seconds using a Vickers hardness tester compliant with JIS Z 2244-1 (2024) (corresponding standard ISO 6507-1:2023)) increases.

Claims

1. Y in the overall composition of the sintered body ２ O ３ The content of is 1.6 mol% to 2.9 mol%, and the remainder is ZrO ２ A zirconia sintered body having a relative density of 99% or more, consisting of unavoidable impurities, In the electron microscope image of the sintered structure of the zirconia sintered body, The area-average particle size of the zirconia crystal particles is 0.3 μm or more and 0.6 μm or less. The area ratio of zirconia crystal particles with a particle size of less than 0.1 μm and zirconia crystal particles with a particle size of more than 1.6 μm is both less than 1%. The area ratio of the first region, which consists of zirconia crystal particles with a particle size of 0.1 μm or more and less than 0.4 μm, is 20% or more and 80% or less. The area ratio of the second region, which consists of zirconia crystal particles with a particle size of 0.4 μm or more and 1.6 μm or less, is 20% or more and 80% or less. The average second region Y of the zirconia crystal grains that make up the second region ２ O ３ The concentration C2 is 1.9 mol% or more and 3.8 mol% or less. The average first region Y of the zirconia crystal particles forming the first region ２ O ３ The concentration C1 is 1.4 mol% or more and 2.3 mol% or less, and the second region Y ２ O ３ The concentration C2 and the first region Y ２ O ３ The Y of the concentration C1 ２ O ３ Taking the difference in concentration as ΔC≡C2−C1, the value of ΔC / C2 is 0.1 or more and 0.5 or less, The fracture toughness value measured by the IF method in accordance with JIS Z 2244-1 (2024) was 10.5 MPa·m. ０．５ A zirconia sintered body characterized by the above.

2. In the zirconia crystal grains forming the second region in the sintered body structure, the outer Y region is more prominent than the central Y region. ２ O ３ A zirconia sintered body according to claim 1 with a low concentration.

3. The sintered structure has a background region consisting of zirconia crystal particles belonging to the first region, with zirconia crystal particles belonging to the second region dispersed within it, and along the analysis line that crosses the background region and the plurality of large particles, Y ２ O ３ When the concentration is measured, a continuous Y-shaped pattern is observed from the maximum point formed within each large particle toward the surrounding area. ２ O ３ A zirconia sintered body according to claim 2, which exhibits a concentration profile in which the concentration changes.

4. In the aforementioned sintered body structure, Y ２ O ３ Effective Y with a concentration of 2.0 mol% or more and 2.4 mol% or less ２ O ３ The zirconia sintered body according to claim 1, wherein the area ratio of the concentration region is 17% or more.

5. In the sintered body structure, the effective Y ２ O ３ The zirconia sintered body according to claim 4, wherein the concentration region is dispersed in a network-like manner along the boundary between the large particles and the background region.

6. S1 is the area ratio of the first region in the sintered body structure, and Y is the average of the first region. ２ O ３ C1 is the concentration, S2 is the area ratio of the second region, and Y is the average of the second region. ２ O ３ Let the concentration be C2. S （Ｃ） =S1×{(7.4-C1) / 6}×100+S2×{(7.4-C2) / 6}×100 The calculation is performed by Converted cubic area ratio S （Ｃ） A zirconia sintered body according to claim 1, wherein the percentage is 5% or more and 25% or less.

7. The remaining portion ZrO ２ A portion of it is Al, which has a content of 0.2 mol% or less in the total composition of the sintered body. ２ O ３ A zirconia sintered body according to claim 1, which is substituted with [the specified substance].

8. The remaining portion ZrO ２ A portion of it is TiO, which is present in a concentration of 0.1 mol% or less in the total composition of the sintered body. ２ A zirconia sintered body according to claim 1, which is substituted with [the specified substance].

9. The zirconia sintered body according to claim 1, wherein the Vickers hardness Hv of the sintered body structure of the zirconia sintered body is 1100 or more.

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