JP7697196B2 - Zirconia sintered body and its manufacturing method - Google Patents

Description

This disclosure relates to a zirconia sintered body and a method for producing the same.

Ceramics are widely used in industrial components due to their excellent heat resistance, wear resistance, and corrosion resistance. In particular, the applications of transparent ceramics are expanding due to their high aesthetic quality and texture. For example, applications of transparent ceramics are being considered for use in electronic device components such as mobile phones, watch components, and jewelry. As applications expand in this way, there is a demand for ceramic components that are made of transparent ceramics and ceramics that have a different color tone, as components that not only have higher aesthetic quality but also have higher designability.

However, ceramics are a highly tough material and are difficult to process into complex shapes. For this reason, ceramic components with complex shapes have traditionally been made by joining pieces of ceramic together.

For example, a ceramic joint has been reported in which a colored zirconia sintered body and a transparent zirconia sintered body are joined by shrinking the colored zirconia sintered body and physically fixing the transparent zirconia sintered body (Patent Document 1).

JP 2013-14471 A

The ceramic joint described in Patent Document 1 is a joint produced by utilizing the difference in thermal contraction during heat treatment, and the sintered bodies are joined together by physical force. The strength of this joint is low, and the applications for which it can be used are limited.

The objective of the present disclosure is to provide at least one of a zirconia sintered body and a method for producing the same that can be used in a wider range of applications than conventional ceramic joints that contain transparent zirconia.

In light of the above issues, the researchers conducted research and discovered that a specific zirconia sintered body can solve the above issues.

That is, the present invention is as defined in the claims, and the gist of the present disclosure is as follows.
[1] A zirconia sintered body having a transparent zirconia portion and an opaque zirconia portion, the zirconia sintered body having a biaxial bending strength of 300 MPa or more, the opaque zirconia portion being made of a medium-light zirconia sintered body.
[2] The zirconia sintered body according to the above [1], in which the transparent zirconia portion and the opaque zirconia portion are on the same plane.
[3] The zirconia sintered body according to the above [1] or [2], wherein the transparent zirconia portion has an in-line transmittance of 50% or more.
[4] The zirconia sintered body according to any one of the above [1] to [3], wherein the in-line transmittance of the opaque zirconia portion is less than 5%.
[5] The zirconia sintered body according to any one of the above [1] to [4], wherein the L ^* in the L ^* a ^* b ^* color system of the opaque zirconia sintered body is more than 30 and is 60 or less.
[6] The zirconia sintered body according to any one of the above [1] to [5], wherein the transparent zirconia portion contains zirconia containing a stabilizer and titania.
[7] The zirconia sintered body according to the above [6], wherein the stabilizer is at least one selected from the group consisting of yttria, calcia and magnesia.
[8] The zirconia sintered body according to the above [6] or [7], wherein the stabilizer for the transparent zirconia portion is yttria, and the yttria content is 6 mol% or more and 12 mol% or less.
[9] The zirconia sintered body according to any one of the above [6] to [8], wherein the stabilizer for the opaque zirconia portion is yttria, and the yttria content is 2 mol% or more and less than 6 mol%.
[10] The zirconia sintered body according to any one of the above [1] to [9], wherein the opaque zirconia portion contains a coloring element.
[11] The zirconia sintered body according to the above [10], wherein the coloring element is at least one selected from the group consisting of transition metal elements, alkali metal elements, alkaline earth metal elements, aluminum, silicon, boron, phosphorus, germanium, and rare earth elements.
[12] The zirconia sintered body according to any one of [1] to [11] above, characterized in that the biaxial bending strength is 350 MPa or more.
[13] A method for producing a zirconia sintered body according to any one of the above [1] to [12], characterized in that it comprises a sintering step of sintering a secondary molded body formed by stacking a primary molded body made of either one of a raw material powder of a transparent zirconia part or a raw material powder of an opaque zirconia part and a molded body made of the other raw material powder.
[14] The method for producing a zirconia sintered body according to the above [13], wherein the raw material powder of the transparent zirconia part is a mixed powder containing a stabilizer-containing zirconia source and a titania source.
[15] The method for producing a zirconia sintered body according to the above [13] or [14], wherein the sintering includes at least a HIP treatment.
[16] The method according to any one of the above [13] to [15], comprising pressureless sintering at 1300°C to 1400°C, followed by HIP treatment at 1450°C to 1550°C.

The zirconia sintered body disclosed herein can provide at least one of a zirconia sintered body and a method for producing the same that can be used in a wider range of applications than conventional ceramic joints that contain transparent zirconia.

FIG. 2 is a schematic diagram showing the measurement of biaxial bending strength. FIG. 2 is a schematic diagram showing a cross section of a secondary molded body. FIG. 2 is a schematic diagram showing the appearance (front and cross section) of the zirconia sintered body of Example 1.

The zirconia sintered body of the present disclosure will be described below with reference to an example embodiment.

This embodiment is a zirconia sintered body having a transparent zirconia portion and an opaque zirconia portion, characterized in that the biaxial bending strength is 300 MPa or more, and the opaque zirconia portion is made of a medium-light zirconia sintered body.

The zirconia sintered body of this embodiment has a transparent zirconia part and an opaque zirconia part. This tends to improve the aesthetics and design. In another embodiment, the zirconia sintered body of this embodiment is a multi-color zirconia sintered body, that is, a zirconia sintered body having two or more zirconia sintered bodies with different color tones. In yet another embodiment, the zirconia sintered body of this embodiment is a sintered body having a zirconia sintered body that transmits light and a zirconia sintered body that does not transmit light, and further, a zirconia sintered body consisting of a zirconia sintered body that can be visually recognized as transparent and a zirconia sintered body that can be visually recognized as opaque.

The zirconia sintered body of this embodiment preferably has a structure in which a transparent zirconia part and an opaque zirconia part are sintered, and more preferably has a structure in which the transparent zirconia part and the opaque zirconia part are sintered in a state in which they form an interface. It is even more preferable that the interface does not have gaps. Note that "having no gaps" means that an interface between the transparent zirconia part and the opaque zirconia part is formed to such an extent that the strength of the zirconia sintered body of this embodiment is expressed, and the zirconia sintered body of this embodiment may have fine defects that do not affect its strength. By having a structure in which the transparent zirconia part and the opaque zirconia part are joined without the intervention of a third component such as a binder, the zirconia sintered body of this embodiment becomes a sintered body consisting of an integrated sintered structure, and the source of fracture is reduced. In addition, by having a structure in which the transparent zirconia part and the opaque zirconia part are sintered in a state in which they form an interface, the mechanical strength is likely to be higher.

Furthermore, by having a structure in which the transparent zirconia part and the opaque zirconia part are joined without the third component, the zirconia sintered body of this embodiment includes a grain structure having a crystal grain structure in which the crystal grains of the transparent zirconia part and the crystal grains of the opaque zirconia part are sintered. Therefore, it is different from a zirconia sintered body that does not have such a grain structure and a zirconia joined body in which two or more zirconia sintered bodies are simply fitted together. In other words, the zirconia sintered body of this embodiment is a joined body in which the transparent zirconia part and the opaque zirconia part are joined by sintering, and is different from a joined body in which the transparent zirconia part and the opaque zirconia part are joined only by physical force.

Generally, transparent zirconia sintered bodies have a lower strength than opaque zirconia sintered bodies. In contrast, the zirconia sintered body of the present embodiment has a structure in which the transparent zirconia portion is sintered with the opaque zirconia portion, and therefore the strength of the transparent zirconia portion itself tends to be higher than when the transparent zirconia sintered body is used alone.

The shape of the zirconia sintered body of this embodiment is not particularly limited, but it is preferable that at least the transparent zirconia portion and the opaque zirconia portion are on the same plane. In this embodiment, "on the same plane" means on the same plane or the same curved surface, and it is more preferable that the transparent zirconia portion and the opaque zirconia portion are on the same visible surface. A structure in which the transparent zirconia portion and the opaque zirconia portion are arranged on the same plane tends to provide high designability.

The shape of the zirconia sintered body of this embodiment can be, for example, a sphere, an approximately sphere, a disk, a cylinder, an elliptical cylinder, a plate, a cube, a rectangular parallelepiped, a polygonal shape, an approximately polygonal shape, or any other shape according to the application. Furthermore, the shape may include a structure in which either the transparent zirconia part or the opaque zirconia part is arranged to surround the other, such as the shape shown in FIG. 1 of Patent Document 1, and it is preferable that the opaque zirconia part has a structure surrounding the transparent zirconia part.

The zirconia sintered body of this embodiment can also be obtained by sintering a molded body in which the precursor of the transparent zirconia part and the precursor of the opaque zirconia part are in contact with each other without any gaps. Therefore, the zirconia sintered body of this embodiment has a high degree of freedom in shape, and it is possible to obtain a zirconia sintered body having a complex shape. For example, the zirconia sintered body of this embodiment may include a structure in which either the transparent zirconia part or the opaque zirconia part has an uneven shape, and the other is laminated so that the uneven shapes are combined.

The ratio of the transparent zirconia portion to the opaque zirconia portion in the zirconia sintered body of this embodiment can be selected arbitrarily depending on the desired aesthetics and shape, and for example, the volume ratio of the transparent zirconia portion: the opaque zirconia portion can be 1:99 to 99:1.

The zirconia sintered body of this embodiment may have a structure in which either the transparent zirconia part or the opaque zirconia part forms a pattern on the surface of the other. Here, the "pattern" refers to a line, figure, or combination thereof formed on either the transparent zirconia part or the opaque zirconia part of the other zirconia part in a visible part of the zirconia sintered body of this embodiment, such as the surface. Examples of the line can be solid lines, dashed lines, wavy lines, numbers, letters, symbols, etc., and examples of the figure can be polygons such as triangles, squares, and pentagons, and geometric shapes such as circles and ellipses. For example, the pattern can be formed in an area of 1 cm ² or less, or even an area of 1 mm ² or less, or even an area of 0.5 mm ² or less, or even an area of 0.05 mm ² or less, or even an area of 0.005 mm ² or less. Further examples include diagrams consisting of lines with a thickness of about 150 μm, diagrams or figures with intervals of about 150 μm, and figures with a diameter of 1 mm or less, or even 0.5 mm or less.

Since the zirconia sintered body of this embodiment has particularly excellent aesthetics, it is preferable that it does not have color bleeding. Color bleeding is thought to occur when a certain amount or more of a coloring element in either the transparent zirconia part or the opaque zirconia part diffuses into the other zirconia part. Here, the "coloring element" refers to an element that produces a coloring effect in zirconia, and its existence state is not limited to ions, oxides, complex oxides, etc. In this embodiment, "color bleeding" refers to a state in which a coloring element in either the transparent zirconia part or the opaque zirconia part is contained in the other, and is observed visually or with an optical microscope mainly at the interface between the transparent zirconia part and the opaque zirconia part and the region near the interface (hereinafter also referred to as the "transition region").

In order to obtain a zirconia sintered body with superior aesthetics, it is preferable that the content of coloring elements in the transparent zirconia region within 20 μm from the interface is 0.5 mass% or less, and more preferably 0.3 mass% or less. The content of coloring elements in the transition region can be measured by composition analysis using EPMA, etc.

The zirconia sintered body of the present embodiment has a density (hereinafter also referred to as "measured density") calculated from the ratio (g/cm ³ ) of the volume measured by the Archimedes method to the mass measured by mass measurement, which can be exemplified as 5.8 g/cm ³ or more and 6.10 g/cm ³ or less, and further 5.9 g/cm ³ or more and 6.0 g/cm ³ or less.

The relative density of the zirconia sintered body of this embodiment is preferably 99.5% or more, more preferably 99.7% or more, and even more preferably 99.9% or more.

In this embodiment, the relative density of the zirconia sintered body can be calculated from the following formula.
Relative density (%) = measured density of zirconia sintered body (g/cm ³ ) / apparent true density of zirconia sintered body (g/cm ³ ) × 100

The measured density (sintered body density) of the zirconia sintered body is a density determined by Archimedes' method, and the apparent true density of the zirconia sintered body is a density calculated from the apparent true density and volume ratio of each of the transparent zirconia portion and the opaque zirconia portion according to the following formula.
M = (Ma・X+Mb・Y)/(X+Y)

In the above formula, M is the apparent true density (g/cm ³ ) of the zirconia sintered body, Ma is the apparent true density (g/cm ³ ) of the transparent zirconia part, Mb is the apparent true density (g/cm ³ ) of the opaque zirconia part, X is the volume ratio of the transparent zirconia part to the volume of the zirconia sintered body, and Y is the volume ratio of the opaque zirconia part to the volume of the zirconia sintered body. Ma and Mb are the densities of the HIP-treated bodies of the respective sintered bodies measured by Archimedes' method. The HIP-treated bodies are sintered bodies having a density equivalent to a relative density of 100%, and can be produced by HIPing a primary sintered body having a relative density of 97% or more but less than 100% at 150 MPa and 1500° C. for 1 hour using argon as a pressure medium.

The zirconia sintered body of this embodiment has a biaxial bending strength of 300 MPa or more. If the biaxial bending strength is less than 300 MPa, it is easily broken and the applications in which it can be used are limited. Since the zirconia sintered body of this embodiment can be applied to components that require higher strength, it is preferable that the biaxial bending strength is 350 MPa or more, and further 400 MPa or more, 450 MPa or more, 500 MPa or more, or 600 MPa or more. In addition, the biaxial bending strength of this embodiment is 2000 MPa or less, further 1000 MPa or less, 900 MPa or less, or 800 MPa or less.

The biaxial bending strength in this embodiment can be measured using a method conforming to the biaxial bending strength measurement method specified in ISO/DIS6872.

In the measurement of biaxial bending strength, multiple supports are used to place the measurement sample. The supports are placed so that the interface surrounding the transparent zirconia part or the opaque zirconia part of the measurement sample fits within a circle (support circle) drawn by connecting the supports. The biaxial bending strength can be measured by applying a load with a size smaller than the interface to the center of gravity of the support diameter for the measurement sample placed so that the interface fits within the support circle. For example, when a zirconia sintered body having a shape in which a disc-shaped transparent zirconia part with a diameter of 5 mm is surrounded by an opaque zirconia part is used as the measurement sample, three or more supports (e.g., three to five ceramic balls) can be placed so that the diameter of the support circle is more than 5 mm. For the measurement, a load can be applied to the center of gravity (center) of the support circle using an indenter with a diameter of less than 5 mm.

Figure 1 is a schematic diagram showing the measurement of biaxial bending strength, and shows a zirconia sintered body placed on a support. 100a shows a view of the zirconia sintered body when measuring the biaxial bending strength from the bottom (i.e., the surface where the support and the zirconia sintered body contact), and 100b shows a cross section of the zirconia sintered body. The supports (110a to 110c) are arranged so that the transparent zirconia part (101) fits inside the support circle. The zirconia sintered body is arranged so that the opaque zirconia part (102) is placed on each support. The biaxial bending strength can be measured by applying a load (120) to the center of gravity of the support circle.

In this embodiment, the "transparent zirconia part" is made of a clear zirconia sintered body, and is made of a zirconia sintered body having transparency. Furthermore, the transparent zirconia part is preferably made of a zirconia sintered body that can be visually recognized as having transparency, more preferably made of a colorless zirconia sintered body, and even more preferably made of a zirconia sintered body having a high transmittance of incident light, particularly a high linear transmittance.

The transparent zirconia portion preferably has a linear transmittance of 50% or more. If the linear transmittance is 50% or more, it is easily recognized as transparent. The linear transmittance is more preferably 60% or more, and even more preferably 70% or more. This provides the transparent zirconia portion with aesthetics that are particularly suitable for applications such as watch covers and display components for electronic devices. The linear transmittance of the transparent zirconia portion is 75% or less.

In this embodiment, the "linear transmittance" refers to the linear transmittance when the sample is 1 mm thick and the light source is a D65 light source, and is a transmittance having the following relationship:
Ti = Tt - Td
Tt: Total light transmittance (%)
Td: Diffuse transmittance (%)
Ti: Linear transmittance (%)

The D65 light source is one of the standards for light sources that can be used as a substitute for the standard light source defined by the International Commission on Illumination (Commission internationale l'eclairage; CIE). This light source is equivalent to natural daylight. Therefore, when the zirconia sintered body of this embodiment has a sample thickness of 1 mm, it can be confirmed that the zirconia sintered body of this embodiment has a transparent zirconia portion by confirming that the linear transmittance is 50% or more, or even 60% or more, or even 70% or more.

The transparent zirconia portion is preferably a zirconia sintered body containing zirconia having a cubic fluorite structure, more preferably a zirconia sintered body having zirconia having a cubic fluorite structure as the main phase, and even more preferably a zirconia sintered body made of zirconia having a cubic fluorite structure.

The zirconia sintered body contained in the transparent zirconia part has an average crystal grain size of 5 μm or more, 10 μm or more, or 15 μm or more, and 200 μm or less, 100 μm or less, 50 μm or less, or 30 μm or less, and preferably 5 μm or more and 200 μm or less. In this embodiment, the average crystal grain size is the average diameter of the zirconia crystal grains of the zirconia sintered body, which can be obtained by the intercept method from an observation diagram obtained by a scanning electron microscope (SEM). The surface of the sintered body is observed at a magnification of 15,000 times, and 200 or more, preferably 250 ± 30, zirconia crystal grains are extracted from the obtained SEM observation diagram, and the crystal grain size is measured by the intercept method (k = 1.78), and the average can be obtained. The average crystal grain size can be measured, for example, by using a scanning electron microscope (apparatus name: JSM-IT100, manufactured by JEOL Ltd.) to analyze an SEM observation image obtained under the following measurement conditions by an intercept method using commercially available analysis software (product name: Intouch Scope), and determining the average value of the obtained particle diameters.
Acceleration voltage: 10 kV
Measurement magnification: 400 to 10,000 times

The transparent zirconia portion may have any composition as long as it is a sintered body exhibiting transparency. The transparent zirconia portion may be made of zirconia containing a stabilizer and titania (TiO ₂ ).

The stabilizer may be at least one selected from the group consisting of yttria (Y ₂ O ₃ ), calcia (CaO) and magnesia (MgO), and is preferably yttria.

The content of the stabilizer in the transparent zirconia portion is preferably an amount that stabilizes the zirconia in a cubic fluorite structure. For example, when the stabilizer is yttria, the yttria content is 6 mol% to 12 mol%, preferably 7 mol% to 12 mol%, more preferably 8 mol% to 11 mol%, and even more preferably 8 mol% to 10 mol%.

In this embodiment, the content of the stabilizer is the ratio (mol %) of the stabilizer to the total of the stabilizer and zirconia (ZrO ₂ ) calculated as oxide, and is a value calculated from {[stabilizer (mol)]/[zirconia (mol)+stabilizer (mol)]}×100.

Since the transparency of the transparent zirconia portion tends to be high, the titania content of the transparent zirconia portion is 3 mol% or more and 20 mol% or less, preferably 5 mol% or more and 15 mol% or less, and more preferably 8 mol% or more and 12 mol% or less.

In this embodiment, the titania content is the ratio (mol %) of titania (TiO ₂ ) to the total of zirconia, stabilizer, and titania, and is a value calculated from {[titania (mol)]/[zirconia (mol) + stabilizer (mol) + titania (mol)]}×100.

The transparent zirconia portion may contain, in addition to inevitable impurities such as hafnia (HfO ₂ ), coloring elements, etc., as long as the transparency is not impaired. For example, the transparent zirconia portion may contain 0% by mass or more and 0.1% by mass or less of aluminum, calculated as Al ₂ O ₃ .

The "opaque zirconia portion" is made of an opaque zirconia sintered body, and is made of a zirconia sintered body that does not have transparency. The opaque zirconia portion is made of a medium-light zirconia sintered body. It is preferable that the opaque zirconia portion is made of a zirconia sintered body that has a low transmittance of incident light.

The opaque zirconia portion preferably has a linear transmittance of less than 5%, more preferably less than 4%, even more preferably less than 3%, and even more preferably less than 2%. A zirconia sintered body with a linear transmittance of less than 5% is easily recognized as opaque. When all incident light is reflected or when all transmitted light is diffusely transmitted, the linear transmittance of the zirconia sintered body is 0%. Therefore, the linear transmittance of the opaque zirconia portion is 0% or more. Therefore, the zirconia sintered body of this embodiment may be confirmed to have an opaque zirconia portion by including an area with a linear transmittance of less than 50%, or even less than 10%, or even less than 5%, or even less than 4%, or even less than 2% when the sample thickness is 1 mm.

The opaque zirconia portion is preferably a zirconia sintered body containing zirconia having a tetragonal fluorite structure, more preferably a zirconia sintered body having zirconia having a tetragonal fluorite structure as the main phase, and even more preferably a zirconia sintered body made of zirconia having a tetragonal fluorite structure.

The average crystal grain size of the zirconia contained in the opaque zirconia portion is preferably 0.1 μm or more and 50 μm or less.

The opaque zirconia portion is a zirconia sintered body that does not have transparency, and its composition is arbitrary as long as it is an opaque zirconia sintered body. A preferred composition of the opaque zirconia portion is zirconia containing a stabilizer and titania, and it is preferable that the opaque zirconia portion contains a coloring element, and the remainder is zirconia containing a stabilizer and titania.

The stabilizer contained in the opaque zirconia portion is at least one selected from the group consisting of yttria, calcia, and magnesia, and is preferably yttria.

The content of the stabilizer is preferably an amount that stabilizes zirconia in a tetragonal fluorite structure. For example, when the stabilizer is yttria, the yttria content of the opaque zirconia portion is 2 mol% to 6 mol%, preferably 2 mol% to 4 mol%, and more preferably 2.5 mol% to 3.5 mol%.

Since the mechanical strength tends to be high, the titania content of the opaque zirconia portion is 1 mol% or more and 7 mol% or less, preferably 1.5 mol% or more and 6 mol% or less.

In the zirconia sintered body of this embodiment, the transparent zirconia portion and the opaque zirconia portion preferably contain zirconia containing a stabilizer and titania.

When the transparent zirconia portion and the opaque zirconia portion each contain titania, it is preferable that the opaque zirconia portion has a lower titania content than the transparent zirconia portion, and further, it is preferable that the difference in titania content between the transparent zirconia portion and the opaque zirconia portion is 2 mol% or more and 10 mol% or less, and more preferably 3 mol% or more and 7 mol% or less.

The opaque zirconia portion is made of medium-light zirconia sintered body. The opaque zirconia sintered body is a zirconia sintered body that does not have transparency, and the medium-light zirconia sintered body is a zirconia sintered body that has a color tone between light and dark tones.

The medium-light color zirconia sintered body is preferably a zirconia sintered body having a lightness L ^* (hereinafter also simply referred to as "L ^* ") in the L ^* a ^* b ^* color system of more than 30 and not more than 60. A zirconia sintered body having an L ^* of 40 or more or 45 or more and not more than 60, 55 or not more than 50 is more preferable, and an L* of 48 or more and not more than 60 is even more preferable.

The specific color tone of the opaque zirconia portion is arbitrary, and may be any of orange, yellow, red, purple, green, yellow-green, blue-green, blue, gray, and light gray. On the other hand, it is preferable that the color tone of the opaque zirconia portion is other than black.

As a more profound and aesthetically pleasing color tone, blue tones can be mentioned, as a general-purpose color tone, gray tones can be mentioned, and as a decorative color tone, any color tone selected from the group consisting of red, yellow, and orange.

The medium-light color tone is preferably any color tone selected from the group consisting of orange to yellow, red to purple, green to yellow-green, blue-green to blue, and gray to light gray.

When the color tone exhibited by the opaque zirconia portion is expressed in the L ^* a ^* b ^* color system, it can have, for example, the following L ^* and chroma a ^* b ^* (hereinafter, chroma a ^* b ^* will also be referred to as "a ^* " and "b ^* ", respectively).

L ^* : greater than 30, 40 or more, 45 or more, or 48 or more, and
60 or less, 55 or less, or 50 or less a ^* : -100 or more, -50 or more, or 0 or more, and
100 or less, 50 or less, or 30 or less, and
b ^* : -100 or more, -50 or more, or 0 or more, and
The color tone of the opaque zirconia portion is the above-mentioned L ^* a ^* b ^* , and does not include a color tone in which, when expressed in the L ^* a ^* b ^* color system, L ^* is 10 or more, a ^* is -2 or more and 2 or less, and b ^* is -2 or more and 5 or less. The color tone represented by L ^* a ^* b ^* in this embodiment is preferably a color tone in a state sintered in an oxidizing atmosphere after sintering in a reducing atmosphere.

On the other hand, the ranges of L ^* a ^* b ^* for each color tone are exemplified as follows.

Orange to yellow shades:
L ^* is more than 30 and not more than 60, 40 or more and not more than 60, or 45 or more and not more than 60,
a ^* is -5 or more and 100 or less, preferably -5 or more and 30 or less, or 10 or more and 40 or less, and
b ^* is 5 or more and 100 or less, preferably 5 or more and 30 or less, or 15 or more and 80 or less. Red to purple color tone:
L ^* is more than 30 and not more than 60, preferably 45 or more and not more than 60;
a ^* is 5 or more and 100 or less, preferably 5 or more and 30 or less, and
b ^* is -100 or more and 5 or less, preferably -30 or more and 5 or less. Green to yellow-green color tone:
L ^* is more than 30 and not more than 60, preferably 45 or more and not more than 60;
a ^* is -100 or more and -5 or less, preferably -30 or more and -5 or less, and
b ^* is -5 or more and 100 or less, preferably -5 or more and 30 or less. Blue-green to blue color tone:
L ^* is more than 30 and not more than 60, preferably 45 or more and not more than 60;
a ^* is -100 or more and 5 or less, preferably -30 or more and 5 or less, and
b ^* is -100 or more and -5 or less, preferably -30 or more and -5 or less. Gray to light gray color tone:
L ^* is more than 30 and not more than 60, preferably 45 or more and not more than 60;
a ^* is greater than -5 and less than 5, preferably greater than -3 and less than 2; and
b ^* is more than -5 and less than 5, preferably -2 to 4, or -3 to 3. The color tone of the L ^* a ^* b ^* color system is obtained by measuring a sintered body having a surface roughness (Ra) of 0.02 nm or less by a method conforming to JIS Z 8722. The color tone can be measured using a general color difference meter (e.g., Spectrophotometer SD 3000, manufactured by Nippon Denshoku Industries Co., Ltd.). The color tone can be measured using a white plate as the background (so-called white back measurement) under the following conditions.
Light source: D65 light source Viewing angle: 10°
Measurement method: SCE

In order to impart such a color tone, it is preferable that the opaque zirconia portion contains an element that produces a coloring effect in the zirconia (hereinafter also referred to as a "coloring element"; a coloring element that produces a red color is referred to as a "red element", and each coloring element that produces a different color tone is also referred to as an "~ color element"). The color tone of the opaque zirconia portion can be freely changed by changing the type and combination of coloring elements, as well as the content. Examples of coloring elements contained in the opaque zirconia portion include at least one selected from the group consisting of transition metal elements, alkali metal elements, alkaline earth metal elements, aluminum (Al), silicon (Si), boron (B), phosphorus (P), germanium (Ge), and rare earth elements, and preferably at least one selected from the group consisting of aluminum, silicon, nickel (Ni), cobalt (Co), manganese (Mn), iron (Fe), cadmium (Cd), vanadium (V), cerium (Ce), praseodymium (Pr), neodymium (Nd), europium (Eu), holmium (Ho), erbium (Er), and thulium (Tm).

The orange element may be one or more elements selected from the group consisting of aluminum, iron, cobalt, cerium, praseodymium, and erbium, with at least one of cerium and praseodymium being preferred.

Examples of yellow elements include one or more elements selected from the group consisting of silicon, iron, vanadium, and praseodymium, with at least one of vanadium and praseodymium being preferred.

Examples of red elements include cadmium and/or cerium, with cerium being preferred.

Examples of purple elements include one or more selected from the group consisting of aluminum, cobalt, manganese, iron, and neodymium, with neodymium being preferred.

The green element may be one or more elements selected from the group consisting of nickel, vanadium, praseodymium and thulium, with nickel being preferred.

The yellow-green element may be one or more elements selected from the group consisting of aluminum, silicon, nickel, cobalt, iron, vanadium, praseodymium, and thulium, with at least one of vanadium and thulium being preferred.

The blue-green element may be one or more elements selected from the group consisting of nickel, cobalt, vanadium, praseodymium, neodymium, and thulium, with at least one of cobalt and vanadium being preferred.

Examples of blue elements include at least one of cobalt and neodymium, with cobalt being preferred.

An example of the gray or light gray element is aluminum. The opaque zirconia part may contain unavoidable impurities such as hafnia, but it is preferable that it does not contain zinc (Zn) and chromium (Cr), and the contents of zinc and chromium are each less than 0.1 mass%, or even 0.05 mass% or less, and are preferably below the detection limit in general composition analysis (for example, 0.005 mass% or less). The total content of zinc and chromium is the content converted into oxide, which is the ratio of the total mass of zinc converted into zinc oxide (ZnO) and chromium converted into chromium oxide (Cr ₂ O ₃ ) to the mass of the opaque zirconia part.

The opaque zirconia portion preferably has a monoclinic ratio of 10% or less, more preferably 6.5% or less, even more preferably 2% or less, and particularly preferably 1% or less. The monoclinic ratio is a value calculated from the XRD pattern of the surface of the sintered body by the following formula.
Monoclinic rate (%) = [I _m (111) + I _m (11-1)] × 100
/[I _m (111)+I _m (11-1)+I _t (111)+I _c (111)]

In the above formula, I _m (111) is the area intensity of the (111) plane of monoclinic zirconia, I _m (11-1) is the area intensity of the (11-1) plane of monoclinic zirconia, I _t (111) is the area intensity of the (111) plane of tetragonal zirconia, and I _c (111) is the area intensity of the (111) plane of cubic zirconia. The XRD pattern of the sintered body can be measured, for example, using a general XRD device (for example, device name: RINT-UltimaIII, manufactured by Rigaku Corporation) under the following conditions.
Radiation source: CuKα radiation (λ=0.15418nm)
Measurement mode: Continuous scan
Scan speed: 4°/min
Step width: 0.02°
Measurement range: 2θ=26° to 33°

The peak intensity of each surface can be determined by analyzing the obtained XRD pattern using commercially available analysis software (e.g., PDXL2 (Version 2.6.1.2), manufactured by Rigaku Corporation).

The average crystal grain size of the zirconia contained in the opaque zirconia portion is preferably 3.0 μm or less, and more preferably 2.5 μm or less. Particularly preferred average crystal grain sizes include 0.3 μm to 2.5 μm, and even 0.5 μm to 1.3 μm.

The zirconia sintered body of this embodiment can be used not only for applications in which known zirconia sintered bodies can be used, but also as various components such as components for portable electronic devices, decorative components, and jewelry.

Next, we will explain the manufacturing method of the zirconia sintered body of this embodiment.

The zirconia sintered body of this embodiment can be manufactured by a manufacturing method including a sintering step in which a primary molded body made of either the raw material powder of the transparent zirconia part or the raw material powder of the opaque zirconia part is laminated with a molded body made of the other raw material powder to form a secondary molded body.

The molded body to be subjected to the sintering process is a secondary molded body (hereinafter simply referred to as "secondary molded body") in which a primary molded body (hereinafter simply referred to as "primary molded body") made of either the raw material powder for the transparent zirconia part (hereinafter also referred to as "transparent raw material") or the raw material powder for the opaque zirconia part (hereinafter also referred to as "opaque raw material") is laminated with a molded body made of the other raw material powder.

The primary molded body is either a molded body made of a transparent raw material (hereinafter also referred to as a "transparent molded body") or a molded body made of an opaque raw material (hereinafter also referred to as an "opaque molded body"). The secondary molded body is a molded body in which the primary molded body and a molded body made of the other raw material powder are stacked, and is a molded body made of a transparent molded body and an opaque molded body. In a preferred embodiment, the secondary molded body is a molded body in which a transparent molded body and an opaque molded body are joined together. Note that "primary" and "secondary" in the molded body are terms used for convenience to indicate a stacked state, and do not indicate top and bottom or order.

The shapes of the primary and secondary compacts are arbitrary, and may be the same as the desired sintered body, taking into account shrinkage caused by sintering. Furthermore, in the secondary compact, the primary compact may have at least one of a convex and concave structure, and the compact made of the other raw material powder may have a laminated structure covering the structure.

The primary and secondary molded bodies can be manufactured by any method. An example of a molding method is to first fill a mold with one of the raw material powders, either transparent or opaque, and then stack the other raw material powder on the one of the raw material powders, and then mold it to obtain a primary and secondary molded body at the same time. Other examples include a molding method in which one of the raw material powders is first filled into a mold and molded to obtain a primary molded body, and then the other raw material powder on top of it is stacked on the primary molded body to mold it into a secondary molded body, and a molding method in which one of the raw material powders is filled into a mold and molded to obtain a primary molded body, and then the primary molded body is placed in a mold for the secondary molded body, and the other raw material powder is stacked on the primary molded body to mold it into a secondary molded body.

The molding method for the primary molded body and the secondary molded body can be a known molding method, and examples of the molding method include one or more molding methods selected from the group consisting of uniaxial pressure molding, cold isostatic pressing (CIP) processing, casting molding, sheet molding, and injection molding, and preferably one or more molding methods selected from the group consisting of uniaxial pressure molding, CIP processing, and injection molding.

When the molding method is uniaxial pressure molding, examples of the uniaxial pressure conditions include 20 MPa or more and 70 MPa or less, when the molding method is CIP treatment, examples of the CIP treatment conditions include 150 MPa or more and 250 MPa or less, when the molding method is injection molding, examples of the injection molding conditions include 50 MPa or more and 150 MPa or less, and even 70 MPa or more and 130 MPa or less.

The transparent raw material is a precursor of the transparent zirconia portion, while the opaque raw material is a precursor of the opaque zirconia portion. The transparent raw material and the opaque raw material are usually powders with different compositions, and these raw material powders only need to have compositions that become a transparent zirconia sintered body and an opaque zirconia sintered body, respectively, when sintered.

The transparent raw material and the opaque raw material are preferably mixed powders containing a zirconia source, a stabilizer source, and a titania source, respectively, and more preferably mixed powders containing a stabilizer-containing zirconia source and a titania source, respectively. Furthermore, the transparent raw material and the opaque raw material may contain at least one of the coloring element sources.

The zirconia source is zirconia or a precursor thereof, and is preferably zirconia in a calcined state from a zirconia sol, more preferably zirconia in a calcined state from a zirconia sol obtained by at least one of a hydrothermal synthesis method and a hydrolysis method, and even more preferably zirconia in a calcined state from a zirconia sol obtained by a hydrolysis method.

The stabilizer source is a stabilizer or a precursor thereof, and can be exemplified by one or more selected from the group consisting of oxides, chlorides, and hydroxides containing a stabilizing element. When the stabilizer is yttria, the stabilizer source (yttria source) can be exemplified by one or more selected from the group consisting of yttria, yttrium chloride, and yttrium hydroxide, and at least one of yttria and yttrium chloride is preferable. When the stabilizer is calcia, the stabilizer source (calcia source) can be exemplified by at least one selected from the group consisting of calcia, calcium chloride, and calcium hydroxide, and at least one of calcia and calcium chloride is preferable. When the stabilizer is magnesia, the stabilizer source (magnesia source) can be exemplified by at least one selected from the group consisting of magnesia, magnesium chloride, and magnesium hydroxide, and at least one of magnesia and magnesium chloride is preferable.

The titania source is titania or a precursor thereof, and may be one or more selected from the group consisting of titania, titanium chloride, titanium hydroxide, and titanium tetraisopropoxide. At least one of titania and titanium chloride is preferred, and titania is more preferred. A more preferred titania source is titania powder having a purity of 99.9% or more, a BET specific surface area of 10 m ² /g to 100 m ² /g, an average crystallite size of 30 nm or less, and an average secondary particle size of 500 nm or less, and further preferably titania powder obtained by at least one of a sulfuric acid method and a gas-phase pyrolysis method. The titania source preferably has a larger BET specific surface area than the zirconia source and the stabilizer-containing zirconia source.

The stabilizer-containing zirconia source is zirconia in which a stabilizer is dissolved, and can be exemplified by one or more selected from the group consisting of yttria-containing zirconia, calcia-containing zirconia, and magnesia-containing zirconia, with yttria-containing zirconia being preferred. A preferred yttria-containing zirconia source is yttria-containing zirconia powder having a purity of 99.9% or more, a BET specific surface area of 5 m ² /g to 20 m ² /g, an average crystallite size of 10 nm to 50 nm, and an average secondary particle size of 100 nm to 500 nm, and further preferably yttria-containing zirconia powder obtained by at least one of a hydrothermal synthesis method and a hydrolysis method.

The coloring element source is contained to give the zirconia portion a desired color. The coloring element source is a compound containing a coloring element, and examples of the coloring element source include one or more selected from the group consisting of oxides, hydroxides, oxyhydroxides, chlorides, sulfides, acetates, nitrates, and sulfates of the coloring element, and is preferably one or more selected from the group consisting of oxides, hydroxides, and oxyhydroxides of the coloring element. When the coloring element is cobalt, the coloring element source (cobalt source) is a compound containing cobalt (Co), and examples thereof include at least one selected _{from the} group consisting of tricobalt tetroxide ( _Co3O4 ), cobalt (III) oxide ( _Co2O3 ), cobalt (II) oxide (CoO), cobalt oxyhydroxide (CoOOH), cobalt hydroxide (Co(OH) ₂ ), cobalt nitrate (Co( _NO3 ) ₂ ), cobalt _chloride ( _CoCl2 ) and cobalt sulfate ( _CoSO4 ) _, and is preferably at least one selected from the group consisting of _Co3O4 , _Co2O3 , CoO and CoOOH.

When the coloring element is iron, the coloring element source (iron source) is a compound containing iron (Fe), and examples thereof include at least one selected from _the group consisting of iron oxide (III, _II ) ( _Fe3O4 ), iron oxide (III) ( _Fe2O3 ), iron oxide (II) (FeO), iron oxyhydroxide (FeOOH), iron hydroxide (FeOH), iron nitrate (Fe( _NO3 ) ₂ ), iron chloride ( _FeCl ) and iron sulfate ( _FeSO4 ) _, and is preferably at least one selected from the group consisting of _Fe3O4 , _Fe2O3 , FeO and FeOOH.

When the coloring element is aluminum, the coloring element source (aluminum source) is a compound containing aluminum (Al), and examples thereof include at least one selected from the group consisting of alumina (Al ₂ O ₃ ), aluminum hydroxide (Al(OH) ₃ ), aluminum chloride (AlCl ₃ ), aluminum isopropoxide (C ₉ H ₂₁ O ₃ Al), and aluminum nitrate (Al(NO ₃ ) ₃ ), and at least one of alumina and aluminum hydroxide is preferred.

When the coloring element is nickel, the coloring element source (nickel source) is a compound containing nickel (Ni), and examples thereof include at least one selected from the group consisting of nickel oxide (II) (NiO), nickel oxide (III) (Ni ₂ O ₃ ), nickel carbonate (NiCO ₃ ), nickel chloride (NiCl ₂ ), nickel sulfate (NiSO ₄ ), and nickel sulfide (NiS), and at least one of NiO and Ni ₂ O ₃ is preferable.

When the coloring element is manganese, the coloring element source (manganese source) is a compound containing manganese (Mn), and examples thereof include at least one selected from _the group consisting of manganese oxide (II) (MnO), manganese oxide (III) ( _MnO2 ), manganese oxide (VII) _{( Mn2O7} ), manganese tetraoxide ( _Mn3O4 ), manganese sulfate ( _MnSO4 ), manganese chloride ( _MnCl2 ), and manganese nitrate (Mn( _NO3 ) ₂ ), and at least one selected from the group consisting _of MnO, _MnO2 , _Mn2O7 , and _{Mn3O4 is} preferred.

When the coloring element is silicon, the coloring element source (silicon source) is a compound containing silicon (Si), and examples thereof include at least one selected from the group consisting of silicon monoxide (SiO), silicon dioxide (SiO ₂ ), silicon carbide (SiC), silicon nitride (Si ₃ N ₄ ), and silicon tetrachloride (SiCl ₄ ), and is preferably at least one of SiO and SiO ₂ .

When the coloring element is cadmium, the coloring element source (cadmium source) is a compound containing cadmium (Cd), and examples thereof include at least one selected from the group consisting of cadmium oxide (CdO), cadmium hydroxide (Cd(OH) ₂ ), cadmium sulfide (CdS), cadmium chloride (CdCl ₂ ), and cadmium seranide (CdSe), and is preferably at least one of CdO and (Cd(OH) ₂ ).

When the coloring element is vanadium, the coloring element source (vanadium source) is a compound containing vanadium (V), and examples thereof include at least one selected from _the group consisting of vanadium oxide (VO), vanadium ( _{IV) oxide (VO2), vanadium (III) oxide (V2O3 ) ,} vanadium (V) oxide ( _V2O5 ), vanadium carbide (VC), vanadium nitride (VN), _vanadium chloride ( _VCl3 ), and vanadium bromide ( _{VBr2 )} , and is preferably at least one selected from the group consisting of _VO2 , _V2O3 , and _V2O5 .

When the coloring element is cerium, the coloring element source (cerium source) is a compound containing cerium (Ce), and examples thereof include at least one selected from _the group consisting of cerium (IV) oxide ( _CeO2 ), cerium ( _III ) oxide ( _Ce2O3 ), cerium hydroxide (Ce(OH) ₃ ), cerium sulfide ( _Ce2S3 ), _cerium chloride ( _CeCl4 ), _cerium sulfate (Ce(SO4) ₂ ), and cerium nitrate (Ce( _NO3 ) ₃ ), and is preferably at least one selected from the group consisting of _CeO2 , _Ce2O3 , and Ce(OH) ₃ .

When the coloring element is praseodymium, the coloring element source (praseodymium source) is a compound containing praseodymium (Pr), and examples thereof include at least one selected from _the group consisting of praseodymium(IV) oxide ( _PrO2 ), praseodymium( _III ) oxide ( _Pr2O3 ), hexapraseodymium oxide ( _Pr6O11 ), _praseodymium chloride ( _PrCl3 ), praseodymium sulfide ( _Pr2S3 ), and praseodymium sulfate ( _Pr2 ( _SO4 ) ₃ ), and at least one selected from _the group consisting _{of PrO2} , _Pr2O3 , and _Pr6O11 is preferred.

When the coloring element is neodymium (Nd), the coloring element source (neodymium source) is a compound containing neodymium (Nd), and examples thereof include at least one selected from the group consisting of neodymium oxide (Nd ₂ O ₃ ), neodymium hydroxide (Nd(OH) ₃ ), neodymium nitrate (Nd(NO ₃ ) ₃ ), and neodymium chloride (NdCl ₂ ), and is preferably at least one of Nd ₂ O ₃ and Nd(OH) ₃ .

When the coloring element is europium, the coloring element source (europium source) is a compound containing europium (Eu), and examples thereof include at least one selected from _the group consisting of europium oxide (EuO), europium(II) oxide ( _Eu2O3 ), europium hydroxide (Eu(OH) ₂ ), europium sulfide (EuS), and europium nitrate (Eu( _NO3 ) ₃ ), and at least one selected from the group consisting of EuO, _Eu2O3 , _and Eu(OH) ₂ is preferred.

When the coloring element is holmium, the coloring element source (holmium source) is a compound containing holmium (Ho), and examples thereof include at least one selected from _the group consisting of holmium(III) oxide ( _Ho2O3 ), holmium hydroxide ( _Ho2 (OH) ₃ ), holmium nitrate (Ho( _NO3 ) ₃ ), and holmium chloride ( _{HoCl3 )} , and at least one of _Ho2O3 and _Ho2 (OH) ₃ is preferable.

When the coloring element is erbium, the coloring element source (erbium source) is a compound containing erbium (Er), and examples thereof include at least one selected from _the group consisting of erbium(III) oxide ( _Er2O3 ), erbium hydroxide ( _Er2 (OH) ₃ ), erbium nitrate (Er( _NO3 ) ₃ ), and erbium _chloride ( _ErCl3 ), and is preferably at least one of _Er2O3 and _Er2 (OH) ₃ .

When the coloring element is thulium, the coloring element source (thulium source) is a compound containing thulium (Tm), and examples thereof include at least one selected from the group consisting of thulium (III) oxide (Tm ₂ O ₃ ), thulium hydroxide (Tm(OH) ₂ ), thulium nitrate (Tm(NO ₃ ) ₃ ), and thulium chloride (TmCl ₃ ), and is preferably at least one of Tm ₂ O ₃ and Tm(OH) ₃ .

The source of the coloring element may be a complex oxide, for example, cobalt aluminate (CoAlO ₄ ).

The content of the stabilizer in the transparent raw material is preferably an amount that stabilizes the zirconia in a cubic fluorite structure by sintering. For example, when the stabilizer is yttria, the yttria content is 6 mol% to 12 mol%, preferably 7 mol% to 12 mol%, more preferably 8 mol% to 11 mol%, and even more preferably 8 mol% to 10 mol%.

The titania content of the transparent raw material is 3 mol% or more and 20 mol% or less, preferably 5 mol% or more and 15 mol% or less, and more preferably 8 mol% or more and 12 mol% or less.

The content of the stabilizer in the opaque raw material is preferably an amount that stabilizes the zirconia in a tetragonal fluorite structure by sintering. For example, when the stabilizer is yttria, the yttria content is 2 mol% to 6 mol%, preferably 2 mol% to 4 mol%, and more preferably 2.5 mol% to 3.5 mol%.

The titania content of the opaque raw material is 1 mol% or more and 7 mol% or less, preferably 1.5 mol% or more and 6 mol% or less.

The content of the coloring element source in the opaque raw material may be any content depending on the desired color tone, and can be exemplified as 0.01% by mass to 50% by mass, preferably 0.1% by mass to 20% by mass, more preferably 0.3% by mass to 10% by mass. For example, the coloring element source may be, for example, 0.1% by mass to 5% by mass of _cobalt calculated as _Co3O4 relative to the mass of the opaque raw material, 0.1% by mass to 5% by mass of iron calculated as _Fe2O3 relative to the mass of _the opaque raw material, 0.1% by mass to 10% by mass of nickel calculated as NiO relative to the mass of the opaque raw material, and 0.1% by mass to ₄₀ % by mass of aluminum calculated as _Al2O3 relative to the mass of the opaque raw material.

In order to improve the fluidity of the raw material powder, at least one of the transparent raw material and the opaque raw material may contain an organic binder. When an organic binder is contained, the content of the organic binder in each raw material powder can be, for example, 25% by volume or more and 65% by volume or less, or even 35% by volume or more and 60% by volume or less.

The organic binder may be any known one used in molding ceramic powder, and may, for example, include at least one selected from the group consisting of acrylic resin, wax, and plasticizer. In this embodiment, the acrylic resin may be a polymer containing at least one of an acrylic acid ester residue unit or a methacrylic acid ester residue unit.

The transparent raw material and the opaque raw material are preferably in a state in which raw materials such as the zirconia source are uniformly mixed. The raw materials may be mixed by any method, and may be at least one of dry mixing and wet mixing, and wet mixing is preferable. Examples of preferable wet mixing include mixing by at least one of a ball mill and a stirring mill, and mixing by a ball mill using zirconia balls with a diameter of 1.0 mm to 10.0 mm is preferable. When a coloring element is contained, it is more preferable to mix the coloring element source and then mix the zirconia source and the coloring element source.

When the raw material powder contains an organic binder, any mixing method can be used as long as the raw material powder and the organic binder can be mixed uniformly. Examples of mixing methods include heat kneading and wet mixing.

It is preferable that the difference in linear shrinkage between the transparent raw material and the opaque raw material (hereinafter also simply referred to as "linear shrinkage difference") is 5.0% or less. Normally, the thermal shrinkage behavior of transparent raw materials and opaque raw materials is significantly different from that of transparent raw materials or opaque raw materials, so defects are likely to occur when sintered together, and the yield is likely to be significantly reduced. In contrast, in this embodiment, since the linear shrinkage difference is 5.0% or less, defects are unlikely to occur during sintering, and the yield is likely to be high. It is more preferable that the linear shrinkage difference is 4.0% or less, and even more preferable that it is 3.5% or less. Since the linear shrinkage of raw material powders with different compositions is difficult to match, the linear shrinkage difference is 0% or more, and even 0.1% or more.

In this embodiment, the "linear shrinkage rate" is one of the indicators of thermal shrinkage behavior, and can be calculated by subjecting a rectangular sample (hereinafter also referred to as a "rectangular sample") measuring 30 mm in width, 3 mm in thickness, and 40 mm in length to a baking process, and using the following formula from the values before and after the baking process.

S=(S _W +S _T +S _L )/3
Where:
S _w =100×{(L _w2 −L _w1 )/L _w1 }
S _T =100×{(L _T2 - L _T1 )/L _T1 }
S _L =100×{(L _L2 - _{L L1} )/L _L1 }
In addition, S _W is the linear shrinkage rate of width (%), L _w1 is the width (mm) of the rectangular parallelepiped sample before the firing treatment, L _w2 is the width (mm) of the rectangular parallelepiped sample after the firing treatment, S _T is the linear shrinkage rate of thickness (%), L _T1 is the thickness (mm) of the rectangular parallelepiped sample before the firing treatment, L _T2 is the thickness (mm) of the rectangular parallelepiped sample after the firing treatment, S _L is the linear shrinkage rate of length (%), L _L1 is the length (mm) of the rectangular parallelepiped sample before the firing treatment, and L _L2 is the length (mm) of the rectangular parallelepiped sample after the firing treatment.

The firing process for measuring the linear shrinkage rate in this embodiment includes firing in air using a firing program with a heating rate of 100°C/h, a holding temperature of either 1300°C, 1400°C or 1500°C, a holding time of 1 minute, and a cooling rate of 200°C/h.

In this embodiment, it is preferable that any one of the linear shrinkage difference in the firing treatment at a holding temperature of 1300° C. (hereinafter also referred to as “ΔS _W(1300) ”), the linear shrinkage difference in the firing treatment at a holding temperature of 1400° C. (hereinafter also referred to as “ΔS _W(1400) ”), and the linear shrinkage difference in the firing treatment at a holding temperature of 1500° C. (hereinafter also referred to as “ΔS _W(1500) ”) satisfies the above-mentioned linear shrinkage difference, but it is more preferable that at least the maximum value of ΔS _W(1300) , ΔS _W(1400) , and ΔS _W(1500) (hereinafter also referred to as “ΔS _W(MAX) ”) satisfies the above-mentioned value, and it is more preferable that at least ΔS _W(1400) satisfies the above-mentioned value.

A preferred ΔSW ₍₁₃₀₀₎ is 0% or more, preferably 0.1% or more, and 3.0% or less, preferably 2.0% or less, more preferably 1.0% or less. A preferred ΔSW ₍₁₄₀₀₎ is 0% or more, preferably 0.5% or more, and 5.0% or less, preferably 4.0% or less, more preferably 3.5% or less. A preferred ΔSW ₍₁₅₀₀₎ is 0% or more, preferably 0.1% or more, and 3.0% or less, preferably 2.0% or less, more preferably 1.0% or less. It is more preferred that the difference between ΔSW ₍₁₃₀₀₎ and ΔSW ₍₁₅₀₀₎ is 0% or more and 1.0% or less, and even more preferably 0.1% or more and 0.5% or less.

In the sintering process, the secondary compact is sintered to obtain a zirconia sintered body in which the transparent zirconia portion and the opaque zirconia portion are bonded together.

The sintering temperature for sintering the secondary molded body is preferably above 1100°C, more preferably 1200°C or higher, and even more preferably 1250°C or higher. The sintering temperature may be any temperature that can be applied with a general sintering device, and may be 1700°C or lower, or even 1600°C or lower.

Any sintering method can be applied to the sintering process, and examples of such methods include one or more selected from the group consisting of atmospheric sintering, microwave firing, and hot isostatic pressing (hereinafter also referred to as "HIP treatment"). In order to suppress the occurrence of defects at the interface between the transparent zirconia portion and the opaque zirconia portion, the sintering in the sintering process preferably includes at least HIP treatment, and more preferably atmospheric sintering and HIP treatment. In this embodiment, "atmospheric sintering" refers to a method of sintering without applying external pressure to the sintered object during sintering.

A preferred sintering method is atmospheric sintering at 1300°C to 1400°C, followed by HIP treatment at 1450°C to 1550°C.

The conditions for atmospheric sintering other than the sintering temperature are arbitrary, but examples include a sintering atmosphere being either an oxidizing atmosphere or an air atmosphere, preferably an air atmosphere, and a sintering time being 30 minutes or more and 5 hours or less, preferably 1 hour or more and 3 hours or less.

The conditions of the HIP treatment other than the HIP treatment temperature are arbitrary, but examples include using an inert gas, preferably at least one of nitrogen and argon, as the pressure medium, the HIP pressure being 50 MPa or more and 200 MPa or less, and the HIP treatment time being 0.5 hours or more and 10 hours or less. The HIP treatment atmosphere is preferably other than an oxidizing atmosphere, more preferably at least one of a reducing atmosphere and an inert atmosphere, and even more preferably a reducing atmosphere.

In the HIP process, it is preferable to place the sample in a reducing atmosphere during HIP sintering, and it is preferable to place the sample in a container made of a reducing material. Usually, reducing materials such as carbon are used for the components such as the heating element of the HIP processing device. Therefore, even if an inert gas is used as the pressure medium, the atmosphere during HIP processing tends to be unstable, ranging from an inert atmosphere to a weakly reducing atmosphere. However, by placing the sample in a reducing atmosphere during HIP sintering, it becomes easier to obtain the zirconia sintered body of this embodiment in a stable manner. The method of controlling the atmosphere during HIP processing, particularly the atmosphere near the sample during HIP processing, is arbitrary, but it is simple to place the sample in a container made of a reducing material. The atmosphere near the sample can be stabilized by selecting the material of the container in which the sample is placed during HIP processing. For example, by placing the sample in a container made of oxide ceramics such as alumina, zirconia, or mullite, the sample can be placed in an inert atmosphere during HIP sintering. On the other hand, by placing the sample in a container made of a reducing material such as carbon, the sample can be placed in a reducing atmosphere during HIP processing.

In the sintering process, it is preferable to perform an annealing treatment after sintering. This increases the linear transmittance of the transparent zirconia part. The conditions for the annealing treatment are arbitrary, but examples include an oxygen atmosphere, a treatment temperature of 850°C to 950°C, and a treatment time of 0.5 to 2 hours.

The manufacturing method of this embodiment may include a processing step of processing the zirconia sintered body into any shape. Processing can impart aesthetics that are more suited to the intended use, such as exposing the transparent zirconia portion and the opaque zirconia portion on the same surface, making the surface smoother, or making minor modifications to the shape.

Any processing method can be used, and examples of the processing method include one or more methods selected from the group consisting of lathe processing, surface grinding, R grinding, and NC processing (numerical control machining). In order to enhance the gloss, examples of polishing methods include at least one of barrel polishing and R polishing.

The zirconia sintered body of the present disclosure will be specifically described below with reference to examples and comparative examples. However, the present disclosure is not limited to the following examples.

(Linear transmittance)
A disk-shaped sintered body with a thickness of 1 mm and a diameter of 25 mm was prepared, and both surfaces of the disk were mirror-polished to a surface roughness of Ra = 0.02 μm or less to be used as a measurement sample. The linear transmittance was measured using a haze meter (device name: NDH5000, manufactured by Nippon Denshoku Industries Co., Ltd.) with a D65 light source as the incident light and a spot diameter of 15 mm.

(color tone measurement)
The color tone was measured in accordance with JIS Z 8722 using a color difference meter (apparatus name: Spectrophotometer SD 3000, manufactured by Nippon Denshoku Industries Co., Ltd.) under conditions of a D65 light source and a viewing angle of 10°.

(Biaxial bending strength)
A disk-shaped sintered body having a thickness of 1 mm and a diameter of 25 mm was prepared, and both surfaces of the disk were mirror-polished to a surface roughness of Ra = 0.02 μm or less, and this was used as a measurement sample. The measurement sample was used to measure the biaxial bending strength in accordance with the biaxial bending strength measurement specified in ISO / DIS6872. In the biaxial bending strength measurement, the support diameter was set to 22 mm. A zirconia ball with a ball diameter of 9.5 mm was used as the support.

Example 1
(transparent raw material)
A 10 mol% yttria-containing zirconia powder having a BET specific surface area of 5.3 m ² /g and a titania powder having a BET specific surface area of 20 m ² /g were mixed in an ethanol solvent by ball milling with a zirconia ball having a diameter of 10 mm. The ball mill mixing was performed by crushing the titania powder in an ethanol solvent and further mixing the yttria-containing zirconia powder. The mixed powder was dried in the air to obtain a 10 mol% yttria and 9.1 mol% titania-containing zirconia powder, which was used as a transparent raw material. Titanium (Ti) in the titania contained in the transparent raw material corresponds to 6.8 mass% as a mass ratio of titanium converted to TiO ₂ relative to the mass of the transparent raw material.

(opaque raw material)
Alumina powder, nickel oxide powder, cobalt aluminate powder and 3 mol % yttria-containing zirconia powder were wet mixed in an ethanol solvent using a ball mill with zirconia balls to obtain a mixed powder containing 0.25 mass % alumina, 9.1 mass % nickel oxide and 0.2 mass % cobalt aluminate, with the remainder being 3 mol % yttria-containing zirconia. The mixed powder was dried at 110°C in air and then sieved to obtain an opaque raw material (yellow-green raw material).

(Molded body)
A transparent raw material was filled into a circular plate-shaped primary mold having a diameter of 25 mm and an uneven shape, and a primary molded body consisting of a circular plate-shaped transparent molded body having a thickness of 2 mm and a diameter of 25 mm was obtained by uniaxially pressing the primary molded body with a diameter of 50 mm so that the convex shape of the primary molded body was the upper surface. An opaque raw material was filled so that the entire upper surface of the primary molded body was covered. After that, the primary molded body was uniaxially pressed at a pressure of 50 MPa to obtain a secondary molded body in which a transparent molded body and an opaque molded body were laminated, and then the secondary molded body was subjected to CIP treatment at a pressure of 200 MPa. As a result, a circular plate-shaped secondary molded body having a thickness of 3.5 mm and a diameter of 50 mm was obtained. FIG. 2 is a schematic diagram showing a cross section of a secondary molded body (200), and shows a secondary molded body in which a transparent molded body (201) is placed in a secondary mold (210), and the exposed surface of the primary molded body is covered with an opaque molded body (202), forming a laminate of a transparent molded body and an opaque molded body.

(Zirconia sintered body)
The obtained secondary compact was sintered at atmospheric pressure in air at a heating rate of 100°C/h, a sintering temperature of 1350°C, and a sintering time of 2 hours, and then HIP-treated at a temperature of 1500°C, a pressure of 150 MPa, and a holding time of 1 hour. After HIP-treatment, the sample was annealed in air at 900°C for 8 hours to obtain a sintered body. In the HIP-treatment, argon gas with a purity of 99.9% was used as a pressure medium, and the sample was placed in a carbon container with a lid.

The annealed sintered body was cut and polished until a transparent zirconia portion and an opaque zirconia portion were exposed on both sides. This resulted in a disk-shaped zirconia sintered body having a thickness of 1.0 mm and including a transparent zirconia portion and an opaque zirconia portion, with the transparent zirconia portion and the opaque zirconia portion exposed on the same surface, which was the zirconia sintered body of this example.

Figure 3 is a schematic diagram showing the appearance of the front (300a) and cross section (300b) of the zirconia sintered body of this embodiment after cutting and polishing. As shown in the front view (300a), the zirconia sintered body of this embodiment has a structure in which the opaque zirconia part (302) is arranged to surround the transparent zirconia part (301), and the transparent zirconia part (301) and the opaque zirconia part (302) have a disk shape exposed on the same surface. Also, as shown in the cross section (300b), the zirconia sintered body of this embodiment has a transparent zirconia part that is continuous in the thickness direction. Due to the transparency of the transparent zirconia part, when the zirconia sintered body of this embodiment is placed, it functions like a window material, and the design of the background can be visually recognized.

Example 2
(transparent raw material)
A transparent raw material was obtained in the same manner as in Example 1.

(opaque raw material)
A praseodymium oxide powder and a zirconia powder containing 3 mol% yttria were wet-mixed in an ethanol solvent using a ball mill with zirconia balls to obtain a mixed powder containing 1.0 mass% praseodymium oxide and the remainder being zirconia containing 3 mol% yttria. The mixed powder was dried at 110°C in air and then sieved to obtain an opaque raw material (yellow raw material).

(Molded body)
A disk-shaped secondary molded product having a thickness of 3.5 mm and a diameter of 50 mm was obtained in the same manner as in Example 1, except that a yellow raw material was used instead of an opaque raw material.

(Zirconia sintered body)
The obtained secondary compact was subjected to atmospheric sintering, HIP treatment, and annealing treatment to obtain a sintered body in the same manner as in Example 1. The sintered body after the annealing treatment was cut and polished in the same manner as in Example 1 to obtain a disk-shaped zirconia sintered body, which was used as the zirconia sintered body of this example.

Example 3
(transparent raw material)
A transparent raw material was obtained in the same manner as in Example 1.

(opaque raw material)
Alumina powder, iron oxyhydroxide powder, and 3 mol% yttria-containing zirconia powder were wet-mixed in an ethanol solvent using a ball mill with zirconia balls to obtain a mixed powder containing 0.05 mass% alumina, 0.15 mass% iron oxyhydroxide, and the remainder being 3 mol% yttria-containing zirconia. The mixed powder was dried at 110°C in air and then sieved to obtain an opaque raw material (purple raw material).

(Molded body)
A disk-shaped secondary molded product having a thickness of 3.5 mm and a diameter of 50 mm was obtained in the same manner as in Example 1, except that the opaque raw material was replaced with a purple raw material.

(Zirconia sintered body)
The obtained secondary compact was subjected to atmospheric sintering, HIP treatment, and annealing treatment to obtain a sintered body in the same manner as in Example 1. The sintered body after the annealing treatment was cut and polished in the same manner as in Example 1 to obtain a disk-shaped zirconia sintered body, which was used as the zirconia sintered body of this example.

Example 4
(transparent raw material)
A transparent raw material was obtained in the same manner as in Example 1.

(opaque raw material)
Alumina powder, cobalt oxide powder, iron oxyhydroxide powder and 3.2 mol % erbia-containing zirconia powder were wet-mixed in an ethanol solvent using a ball mill using zirconia balls to obtain a mixed powder containing 10 mass % alumina, 0.016 mass % cobalt and 0.19 mass % iron oxyhydroxide, with the remainder being 3.2 mol % erbia-containing zirconia. The mixed powder was dried at 110°C in air, and then sieved to obtain an opaque raw material (orange raw material).

(Molded body)
A disk-shaped secondary molded product having a thickness of 3.5 mm and a diameter of 50 mm was obtained in the same manner as in Example 1, except that the opaque raw material was replaced with an orange raw material.

(Zirconia sintered body)
The obtained secondary compact was subjected to atmospheric sintering, HIP treatment, and annealing treatment to obtain a sintered body in the same manner as in Example 1. The sintered body after the annealing treatment was cut and polished in the same manner as in Example 1 to obtain a disk-shaped zirconia sintered body, which was used as the zirconia sintered body of this example.

Example 5
The zirconia sintered body of this example was obtained in the same manner as in Example 1, except that a mixed powder containing 4.0 mass% praseodymium oxide and the remainder being 3 mol% yttria-containing zirconia was used as the opaque raw material by wet mixing a praseodymium oxide powder and a zirconia powder containing 3 mol% yttria in an ethanol solvent in a ball mill using zirconia balls.

Example 6
The zirconia sintered body of this example was obtained in the same manner as in Example 1, except that a mixed powder containing 2.0 mass% praseodymium oxide and the remainder being 3 mol% yttria-containing zirconia obtained by wet mixing a praseodymium oxide powder and a zirconia powder containing 3 mol% yttria in an ethanol solvent in a ball mill using zirconia balls was used as the opaque raw material.

Example 7
(transparent raw material)
A zirconia powder containing 10 mol% yttria and 9.1 mol% titania was obtained in the same manner as in Example 1. The titanium (Ti) in the titania contained in the mixed powder was equivalent to 6.8 mass% of titanium converted to _TiO2 relative to the mass of the mixed powder. The powder was mixed with an organic binder containing an acrylic resin to form a composition, which was used as the transparent raw material of this example.

(opaque raw material)
In the same manner as in Example 1, alumina powder, nickel oxide, cobalt aluminate powder, and 3 mol% yttria-containing zirconia powder were wet mixed in an ethanol solvent in a ball mill using zirconia balls to obtain a mixed powder containing 0.25 mass% alumina, 9.1 mass% nickel oxide, 0.2 mass% cobalt aluminate, and the remainder being 3 mol% yttria-containing zirconia. The mixed powder was mixed with an organic binder containing an acrylic resin to form a composition, which was used as the opaque raw material of this example.

(Molded body)
A transparent raw material was injection molded into a primary mold to obtain a primary molded body consisting of a disc-shaped transparent molded body having an uneven shape and a thickness of 2 mm and a diameter of 25 mm. The obtained primary molded body was placed in a disc-shaped secondary mold having a diameter of 50 mm so that the uneven shape of the primary molded body was on the upper surface. An opaque raw material was injection molded onto the primary molded body so that the entire upper surface (exposed surface) of the primary molded body was covered, and a secondary molded body having a thickness of 3.5 mm and a diameter of 50 mm, in which a transparent molded body and an opaque molded body were laminated, was obtained.

(Zirconia sintered body)
Except for using the obtained secondary molded body, the same methods as in Example 1 were used for atmospheric sintering, HIP treatment, annealing treatment and processing to obtain the zirconia sintered body of this example.

The zirconia sintered body in this example was a disk-shaped sintered body with a thickness of 1 mm, with the transparent zirconia portion and the opaque zirconia portion exposed on the same surface.

Comparative Example 1
According to JP 2013-14471 A, a ceramic bonded body was obtained as follows.

(Black zirconia sintered body)
Black zirconia powder (product name: TZ-Black, manufactured by Tosoh Corporation) was uniaxially pressurized at a pressure of 50 MPa, and then subjected to CIP treatment at a pressure of 200 MPa to obtain a plate-shaped molded body having a length of 30 mm and a width of 40 mm. The obtained molded body was sintered under normal pressure, and then the obtained zirconia sintered body was machined to obtain a frame-shaped black zirconia sintered body having a rectangular hollow portion having a thickness of 1.15 mm, a length of 14 mm, and a width of 22 mm, and having a thickness of 1.15 mm, a length of 28 mm, and a width of 36 mm.

(Transparent zirconia sintered body)
High-purity titania powder was added to 10 mol% yttria-containing zirconia powder (product name: TZ-10YS, manufactured by Tosoh Corporation) at 10 mol% relative to the zirconia, and the mixture was mixed in an ethanol solvent using a ball mill with zirconia balls of 10 mm diameter for 72 hours, and then dried to obtain a raw material powder. The raw material powder was uniaxially press molded at a pressure of 50 MPa, and then subjected to CIP treatment at a pressure of 200 MPa to obtain a plate-shaped molded body with a thickness of 2 mm, length of 50 mm, and width of 40 mm.

The obtained plate-shaped compact was sintered under atmospheric pressure in air at a heating rate of 100°C/h, a sintering temperature of 1350°C, and a sintering time of 2 hours to obtain an atmospheric sintered body. Next, this atmospheric sintered body was HIP-treated at a temperature of 1650°C, a pressure of 150 MPa, and a holding time of 1 hour. For the HIP treatment, argon gas with a purity of 99.9% was used as the pressure medium, and the sample was placed in a carbon container with a lid. After HIP sintering, it was annealed in air at 1000°C for 1 hour to obtain a transparent zirconia sintered body.

(Ceramics joint)
A ceramic bonded body was obtained by placing a transparent zirconia sintered body inside the black zirconia sintered body and performing HIP treatment at a pressure of 150 MPa, a holding temperature of 1200°C, and a holding time of 1 hour. The HIP treatment was performed at a holding temperature of 1200°C, a pressure of 150 MPa, and a holding time of 1 hour, using argon (Ar) gas with a purity of 99.9% as the pressure medium, and the sample was placed in an alumina container. After the HIP treatment, the HIP sintered body obtained at 1000°C for 1 hour in air was annealed to obtain the zirconia bonded body of this comparative example.

The results of the biaxial bending strength measurements for Examples 1 to 6 are shown in the table below, along with the results for Comparative Example 1.

The zirconia sintered bodies of Examples 1 to 6 had biaxial bending strengths exceeding 400 MPa, whereas the zirconia joined body of Comparative Example 1 had a biaxial bending strength of less than 250 MPa. In addition, when the source of fracture of the zirconia joined body of Comparative Example 1 was confirmed, it was confirmed that the fracture occurred at the portion where the black zirconia sintered body and the translucent zirconia sintered body were in contact with each other.
Synthesis Example 1 (Transparent zirconia sintered body)
A primary molded body was obtained in the same manner as in Example 1. A transparent zirconia sintered body was produced in the same manner as in Example 1, except that the obtained primary molded body was used instead of the secondary molded body.

The obtained zirconia sintered body exhibited transparency similar to that of the transparent zirconia portion of the zirconia sintered body of Example 1, with an in-line transmittance of 69% and an average crystal grain size of 25 μm.

Synthesis Example 1 (yellow-green zirconia sintered body)
A primary molded body was obtained in the same manner as in Example 1, except that the raw material powder for the opaque zirconia part obtained in Example 1 was used instead of the raw material powder for the transparent zirconia part. A yellow-green zirconia sintered body was produced in the same manner as in Example 1, except that the primary molded body obtained was used instead of the secondary molded body.

The obtained zirconia sintered body exhibited a color tone similar to that of the opaque zirconia part of the zirconia sintered body of Example 1, and had a linear transmittance of 0% (below the detection limit). In addition, in the L ^* a ^* b ^* color system, L ^* was 45.85, a ^* was −14.14, and b ^* was 13.94.

Synthesis Example 2 (Yellow Zirconia Sintered Body)
A primary molded body was obtained in the same manner as in Example 1, except that the raw material powder for the opaque zirconia part obtained in Example 2 was used instead of the raw material powder for the transparent zirconia part. A yellow zirconia sintered body was produced in the same manner as in Example 1, except that the obtained primary molded body was used instead of the secondary molded body.

The obtained zirconia sintered body exhibited a color tone similar to that of the opaque zirconia part of the zirconia sintered body of Example 2, and had a linear transmittance of 0% (below the detection limit). In addition, in the L ^* a ^* b ^* color system, L ^* was 55.69, a ^* was 28.22, and b ^* was 66.94.

Synthesis Example 3 (Purple zirconia sintered body)
A primary molded body was obtained in the same manner as in Example 1, except that the raw material powder for the opaque zirconia part obtained in Example 3 was used instead of the raw material powder for the transparent zirconia part. A purple zirconia sintered body was produced in the same manner as in Example 1, except that the obtained primary molded body was used instead of the secondary molded body.

The obtained zirconia sintered body exhibited a color tone similar to that of the opaque zirconia part of the zirconia sintered body of Example 3, and had a linear transmittance of 0% (below the detection limit). In addition, in the L ^* a ^* b ^* color system, L ^* was 58.87, a ^* was 13.83, and b ^* was −19.63.

Synthesis Example 4 (Orange Zirconia Sintered Body)
A primary molded body was obtained in the same manner as in Example 1, except that the raw material powder for the opaque zirconia part obtained in Example 4 was used instead of the raw material powder for the transparent zirconia part. An orange zirconia sintered body was produced in the same manner as in Example 1, except that the obtained primary molded body was used instead of the secondary molded body.

The obtained zirconia sintered body exhibited a color tone similar to that of the opaque zirconia part of the zirconia sintered body of Example 4, and had a linear transmittance of 0% (below the detection limit). In addition, in the L ^* a ^* b ^* color system, L ^* was 44.47, a ^* was 14.37, and b ^* was 19.22.

Synthesis Example 5 (Orange Zirconia Sintered Body)
A primary molded body was obtained in the same manner as in Example 1, except that the raw material powder for the opaque zirconia part obtained in Example 5 was used instead of the raw material powder for the transparent zirconia part. An orange zirconia sintered body was produced in the same manner as in Example 1, except that the obtained primary molded body was used instead of the secondary molded body.

The obtained zirconia sintered body exhibited a color tone similar to that of the opaque zirconia part of the zirconia sintered body of Example 5, and had a linear transmittance of 0% (below the detection limit). In addition, in the L ^* a ^* b ^* color system, L ^* was 47.11, a ^* was 36.37, and b ^* was 73.63.

Synthesis Example 6 (Orange Zirconia Sintered Body)
A primary molded body was obtained in the same manner as in Example 1, except that the raw material powder for the opaque zirconia part obtained in Example 6 was used instead of the raw material powder for the transparent zirconia part. An orange zirconia sintered body was produced in the same manner as in Example 1, except that the obtained primary molded body was used instead of the secondary molded body.

The obtained zirconia sintered body exhibited a color tone similar to that of the opaque zirconia part of the zirconia sintered body of Example 7, and had a linear transmittance of 0% (below the detection limit). In addition, in the L ^* a ^* b ^* color system, L ^* was 53.58, a ^* was 34.90, and b ^* was 78.47.

100a, 100b, 200a, 200b: Zirconia sintered body 101, 301: Transparent zirconia portion 102, 302: Opaque zirconia portion 110a, 110b, 110c: Support 120: Load 200: Secondary molded body 201: Primary molded body (transparent molded body)
202: Opaque molded body 210: Secondary mold 300a, 300b: Zirconia sintered body 301: Transparent zirconia part 302: Opaque zirconia part

Claims (16)

A zirconia sintered body comprising: a transparent zirconia portion having a linear transmittance of 50% or more when measured with a sample thickness of 1 mm and a D65 light source, the transparent zirconia portion containing zirconia having a cubic fluorite structure; and an opaque zirconia portion having a linear transmittance of less than 5% when measured with a sample thickness of 1 mm ^and a D65 light source, the opaque zirconia portion comprising zirconia having a tetragonal fluorite structure, the zirconia sintered body having a biaxial bending strength of 300 MPa or more, the opaque zirconia portion being a medium-light zirconia sintered body having an L ^* of more than 30 and not more than 60 in the L ^*a* b ^* color system . The zirconia sintered body according to claim 1, in which the transparent zirconia portion and the opaque zirconia portion are on the same plane. The zirconia sintered body according to claim 1 or 2 has a structure in which either the transparent zirconia portion or the opaque zirconia portion is arranged to surround the other. The zirconia sintered body according to any one of claims 1 to 3, wherein the in-line transmittance of the opaque zirconia portion is less than 2%. The opaque zirconia sintered body comprises at least one selected from the group consisting of aluminum (Al), silicon (Si), nickel (Ni), cobalt (Co), manganese (Mn), iron (Fe), cadmium (Cd), vanadium (V), cerium (Ce), praseodymium (Pr), neodymium (Nd), europium (Eu), holmium (Ho), erbium (Er) and thulium (Tm). The zirconia sintered body according to any one of claims 1 to 4, further comprising at least one selected from the group consisting of aluminum (Al), silicon (Si), nickel (Ni), cobalt (Co), manganese (Mn), iron (Fe), cadmium (Cd), vanadium (V), cerium (Ce), praseodymium (Pr), neodymium (Nd), europium (Eu), holmium (Ho), erbium (Er) and thulium (Tm). The zirconia sintered body according to any one of claims 1 to 5, wherein the transparent zirconia portion contains zirconia containing a stabilizer and titania. The zirconia sintered body according to claim 6, wherein the stabilizer is at least one selected from the group consisting of yttria, calcia, and magnesia. The zirconia sintered body according to claim 6 or 7, in which the stabilizer of the transparent zirconia portion is yttria, and the yttria content is 6 mol% or more and 12 mol% or less. The zirconia sintered body according to any one of claims 6 to 8, wherein the stabilizer of the opaque zirconia portion is yttria, and the yttria content is 2 mol% or more and less than 6 mol%. The zirconia sintered body according to any one of claims 1 to 9, wherein the opaque zirconia portion contains a coloring element. The zirconia sintered body according to claim 10, wherein the coloring element is at least one selected from the group consisting of transition metal elements, alkali metal elements, alkaline earth metal elements, aluminum, silicon, boron, phosphorus, germanium, and rare earth elements. The zirconia sintered body according to any one of claims 1 to 11, characterized in that the biaxial bending strength is 350 MPa or more. The method for producing the zirconia sintered body according to any one of claims 1 to 12, characterized in that it includes a sintering step of sintering a secondary molded body in which a primary molded body made of either the raw material powder of the transparent zirconia part or the raw material powder of the opaque zirconia part is laminated with a molded body made of the other raw material powder. The method for producing a zirconia sintered body according to claim 13, wherein the raw material powder for the transparent zirconia portion is a mixed powder containing a stabilizer-containing zirconia source and a titania source. The method for producing a zirconia sintered body according to claim 13 or 14, wherein the sintering includes at least a HIP treatment. The manufacturing method according to any one of claims 13 to 15, in which sintering is performed by atmospheric sintering at 1300°C to 1400°C, followed by HIP treatment at 1450°C to 1550°C.