TRANSLUCENT ZIRCONIA SINTERED BODY, METHOD FOR MANUFACTURING SAME, AND USE THEREOF

Abstract

Provided is a zirconia sintered body having both high translucency and high strength. The zirconia sintered body includes crystal grains that include a cubic domain and a tetragonal domain, wherein a stabilizer and lanthanum is dissolved as a solid solution therein. The sintered body can be obtained by a manufacturing method including: a mixing step of obtaining a mixed powder by mixing a zirconia source, a stabilizer source, and a lanthanum source; a molding step of obtaining a green body by molding the obtained mixed powder; a sintering step of obtaining a sintered body by sintering the obtained green body at a sintering temperature of 1650° C. or higher; and a temperature lowering step of lowering the temperature from the sintering temperature to 1000° C. at a temperature lowering rate exceeding 1° C./min.

Claims

1. A zirconia sintered body comprising a crystal grain having a cubic domain and a tetragonal domain, a stabilizer and lanthanum being dissolved as a solid solution in the zirconia sintered body.

2. The zirconia sintered body according to claim 1, wherein an average crystallite size calculated from the full-width at half maximum of 2θ=30±2° in a powder X-ray diffraction pattern using CuKα as a radiation source is 255 nm or less.

3. The zirconia sintered body according to claim 1, wherein an average crystallite size calculated from the full-width at half maximum of 2θ=30±2° in a powder X-ray diffraction pattern using CuKα as a radiation source is 100 nm or less.

4. The zirconia sintered body according to claim 1, wherein a lanthanum content is 1 mol % or greater but 10 mol % or less.

5. The zirconia sintered body according to claim 1, wherein the stabilizer is at least one type selected from the group consisting of yttria, scandia, calcia, magnesia, and ceria.

6. The zirconia sintered body according to claim 1, wherein bending strength is 500 MPa or greater.

7. The zirconia sintered body according to claim 1, wherein a total light transmittance using illuminant D65 as a radiation source is 45% or greater when a sample thickness is 1 mm.

8. A method of manufacturing the zirconia sintered body described in claim 1, the method comprising: a mixing step of obtaining a mixed powder by mixing a zirconia source, a stabilizer source, and a lanthanum source; a molding step of obtaining a green body by molding the obtained mixed powder; a sintering step of obtaining a sintered body by sintering the obtained green body at a sintering temperature of 1650° C. or higher; and a temperature lowering step of lowering the temperature from the sintering temperature to 1000° C. at a temperature lowering rate exceeding 1° C./min.

9. The manufacturing method according to claim 8, wherein the sintering step comprises a primary sintering of obtaining a primary sintered body by sintering at 1000° C. or higher but lower than 1650° C., and a secondary sintering of sintering the primary sintered body at 1650° C. or higher.

10. A dental component comprising the zirconia sintered body described in claim 1.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0081] FIG. 1 is the Rietveld analysis result of the XRD pattern of the zirconia sintered body of Example 1.

[0082] FIGS. 2A to 2D are the TEM observation images of the zirconia sintered body of Example 1 (scale in the views are 100 nm), where A) is the light view image, B) is the element mapping of yttrium, C) is the element mapping of zirconium, and D) is the element mapping of lanthanum.

[0083] FIG. 3 is the SEM observation image of the zirconia sintered body of Example 1 (scale in the view is 50 μm).

[0084] FIG. 4 is the appearance of the zirconia sintered body of Example 1.

[0085] FIG. 5 is the spectrum obtained by goniophotometer of the zirconia sintered body of Example 1.

[0086] FIG. 6 is the UV-vis spectrum of the zirconia sintered body of Example 1, where a) is the total light transmittance and b) is the in-line transmittance.

[0087] FIG. 7 is the XRD patterns of the zirconia sintered body of Example 1 before and after the hydrothermal degradation test, where a) is prior to the hydrothermal degradation test and b) is after the hydrothermal degradation test.

[0088] FIG. 8 is the Raman spectrum of the zirconia sintered body of Example 11, where a) shows the fracture surface and b) shows the surface.

[0089] FIG. 9 is the XRD pattern of the zirconia sintered body of Comparative Example 3.

[0090] FIG. 10 is the XRD pattern of the zirconia sintered body of Comparative Example 4.

[0091] FIG. 11 is the Rietveld analysis result of the XRD pattern of the zirconia sintered body of Example 35.

[0092] FIGS. 12A to 12D are the TEM observation images of the zirconia sintered body of Example 35 (scale in the figures are 100 nm), where A) is the light view image, B) is the element mapping of yttrium, C) is the element mapping of zirconium, and D) is the element mapping of lanthanum.

[0093] FIGS. 13A to 13D are the TEM observation images of the zirconia sintered body of Example 38 (scale in the figures are 100 nm), where A) is the light view image, B) is the element mapping of yttrium, C) is the element mapping of zirconium, and D) is the element mapping of lanthanum.

EXAMPLES

[0094] The present invention will be described specifically with reference to examples and comparative examples hereinafter. However, the present invention is not limited to the examples.

Measurement of Density

[0095] The measured density of a sintered body sample was determined through an underwater weight measurement conducted by the Archimedes method.

Measurement of Average Crystal Grain Size

[0096] After the sintered body sample was surface-ground, mirror-polishing was performed by using 9 μm, 6 μm, and 1 μm diamond abrasive grains in this order. The polished surface was maintained at 1400° C. for 1 hour and thermally etched, and then the polished surface was observed by SEM. From the obtained SEM observation image, the average crystal grain size was determined by a planimetric method.

Identification of Crystal Structure

[0097] By subjecting the XRD pattern obtained by the XRD measurement of the sintered body sample to identification analysis, crystal structure of each sintered body sample was identified, and the presence of impurity layer was confirmed. The XRD measurement was performed for the sintered body sample which had undergone mirror-polishing by using an ordinary powder X-ray diffraction instrument (instrument name: Ultima III, manufactured by Rigaku Corporation). The XRD measurement conditions were as follows.

Radiation source: CuKα ray (λ=0.15418 nm)
Measurement mode: step scan
Scanning condition: 0.04°/sec
Divergence slit: 0.5 deg
Scattering slit: 0.5 deg
Receiving slit: 0.3 mm
Measurement time: 1.0 sec
Measurement range: 2θ=20° to 80°

[0098] For the identification analysis of the XRD pattern, an XRD analysis software (trade name: JADE 7, manufactured by MID) was used.

Measurement of Average Crystallite Size

[0099] The average crystallite size of the sintered body sample was determined using the Scherrer equation for the range of 2θ=27° to 30° of the XRD pattern obtained by the same measurement method that was conducted for the identification of crystal phase.

D=K×λ/((β−B)×cosθ)

[0100] In the equation above, D is the average crystallite size (nm), K is the Scherrer constant (1.0), λ is the wavelength of CuKα (0.15418 nm), β is the FWHM (°), B is the instrument constant (0.1177°), and θ is the diffraction angle (°) of the main peak.

[0101] Note that the main peak used the peak assigned to the (111) plane of the cubic phase and the peak assigned to the (111) plane of the tetragonal phase of the zirconia, which were overlapped, as a single peak.

[0102] Furthermore, the FWHM was determined using the Integral Analysis for Windows (Version 6.0), manufactured by Rigaku Corporation.

Rietveld Analysis

[0103] By subjecting the obtained XRD pattern by Rietveld analysis by the same measurement method that was conducted for the identification of crystal structure, the lattice constant, the crystallite size, and the proportion of each crystal structure of cubic phase and tetragonal phase in the sintered body sample were determined. A general purpose program (Rietan-2000) was used for the Rietveld analysis.

[0104] From the obtained lattice constant, the Y.sub.2O.sub.3 concentration in the tetragonal phase was determined based on the following equations.

YO.sub.1.5=(1.0223−cf/af)/0.001319

Y.sub.2O.sub.3=100×YO.sub.1.5/(200−YO.sub.1.5)

[0105] In the equations described above, YO.sub.1.5 is the yttria concentration, cf and af are respectively the lattice constant of the c-axis and the lattice constant of the a-axis of the tetragonal fluorite structure determined by the Rietveld analysis.

Measurement of Transmittance

[0106] The total light transmittance (hereinafter, referred to as “TT”), defraction transmittance (hereinafter, referred to as “DF”), and in-line transmittance (hereinafter, referred to as “PT”) of the sample were measured by the method in accordance with the method of JIS K321-1. The optical transmittance was measured by irradiating the measurement sample with the standard light D65 and by detecting the light flux transmitted through the measurement sample using an integrating sphere. An ordinary haze meter (instrument name: Haze Meter NDH 2000, manufactured by Nippon Denshoku Industries Co., Ltd.) was used for the measurement.

[0107] For the measurement sample, a disk-like green body having a diameter of 16 mm and a thickness of 1.0 mm was used. Prior to the measurement, the both surfaces of the measurement sample were mirror-polished until the surface roughness Ra became 0.02 μm or less.

Measurement of Wavelength Dependency of Transmittance

[0108] As the wavelength dependency of transmittance of the sintered body sample, the spectral total transmittance (hereinafter, referred to as “S-TT”) and the spectral in-line transmittance (hereinafter, referred to as “S-PT”) were measured by UV-Vis. The measurement conditions were as follows.

Light source: Deuterium lamp and halogen lamp
Measurement wavelength: 200 to 800 nm
Measurement step: 1 nm

[0109] For the UV-Vis measurement, an ordinary double beam spectrophotometer (instrument name: V-650, manufactured by JASCO Corporation) was used.

[0110] For the measurement sample, a disk-like green body having a diameter of 16 mm and a thickness of 1.0 mm was used. Prior to the measurement, the both surfaces of the measurement sample were mirror-polished until the surface roughness Ra became 0.02 μm or less.

Measurement of Angular Distribution of Transmitted Light

[0111] The angular distribution of transmitted light was measured using goniophotometer (instrument name: GP-200, manufactured by Murakami Color Research Laboratory Co., Ltd.). For the measurement sample, a disk-like green body having a diameter of 16 mm and a thickness of 1.0 mm was used. Prior to the measurement, the both surfaces of the measurement sample were mirror-polished until the surface roughness Ra became 0.02 μm or less.

Observation of Element Distribution

[0112] The element distribution in the crystal grain was measured by TEM observation. Prior to the measurement, the sample was processed into a thin piece by a focused ion beam (FIB). After the processing, the thin piece was subjected to ion milling finishing and carbon deposition to form a measurement sample. The TEM observation was performed by using an ordinary TEM (instrument name: EM-2000FX, manufactured by JEOL Ltd.) at an acceleration voltage of 200 kV.

Measurement of Biaxial Bending Strength

[0113] The biaxial bending strength of the sample was measured by biaxial bending strength measurement in accordance with ISO/DIS 6872. The measurement was performed for the measurement sample in which the both surfaces were mirror-polished and which had a thickness of 1 mm.

Measurement of Three-Point Bending Strength

[0114] The three-point bending strength of the sample was measured by a method in accordance with JIS R 1601 “Testing method for flexural strength of fine ceramics”. For the measurement sample, a sample which was mirror-polished until the surface roughness Ra became 0.02 μm or less was used. Furthermore, five measurements of the strength were performed for one sample, and the average value thereof was used as the three-point bending strength.

Measurement of Fracture Toughness

[0115] The fracture toughness of the sample was measured by the IF method and the SEPB method in accordance with JIS R 1607. For the measurement sample, a sample which was mirror-polished until the surface roughness Ra became 0.02 μm or less was used. Five measurements were performed for one sample, and the average value thereof was used as the fracture toughness of the sample. The measurement conditions in the IF method were as follows.

Indentation load: 5 kgf
Elastic modulus of sintered body: 205 GPa

[0116] The fracture toughness obtained by the IF method was recorded as K.sub.IC(IF) and the fracture toughness obtained by the SEPB method was recorded as K.sub.IC(SEPB).

Hydrothermal Degradation Test

[0117] The sintered body sample was treated in a hot water atmosphere, and degradation evaluation was performed. Pure water and the sintered body sample were placed in a pressure-resistant container made of stainless steel and maintained at 140° C. for 24 hours to perform the hydrothermal degradation test. After the maintenance, the collected sintered body sample was subjected to XRD measurement. The proportion of the XRD peak assigned to the monoclinic phase contained in the obtained XRD pattern was determined based on the following equation, and the volume fraction of the monoclinic phase in the sintered body sample (hereinafter, also referred to as “monoclinic phase fraction”) was determined.

X=(Im(111)+Im(1101))/(Im(111)+Im(11-1)+It9111)+Ic(111))

[0118] Note that X is the monoclinic phase fraction of the sample, Im(111) is the XRD peak intensity assigned to the (111) plane of the monoclinic phase, Im(11-1) is the XRD peak intensity assigned to the (11-1) plane of the monoclinic phase, It(111) is the XRD peak intensity assigned to the (111) plane of the tetragonal, and Ic(111) is the XRD peak intensity assigned to the (111) plane of the cubic.

Measurement of Thermal Conductivity

[0119] The thermal conductivity of the sintered body sample was measured by the laser flash method. For the measurement, a laser flash method thermal constant measurement system (instrument name: TC-1200RH, manufactured by Advance Riko, Inc.) was used.

Synthesis Example (Synthesis of La.sub.2Zr.sub.2O.sub.7 Powder)

[0120] An La.sub.2Zr.sub.2O.sub.7 powder was synthesized by a solid-phase method. That is, a mixed powder was obtained by mixing zirconium oxide (trade name: TZ-0Y, manufactured by Tosoh Corporation) and lanthanum oxide (purity: 99.99%; manufactured by Wako Pure Chemical Industries, Ltd.). The mixing was performed by wet mixing in an ethanol solvent using a ball mill with zirconia balls having a diameter of 10 mm.

[0121] The mixed powder after the mixing was dried or calcined to obtain a calcined powder. As the calcining condition, heat treatment was performed in atmosphere at 1100° C. for 10 hours. The obtained calcined powder was wet mixed in the same conditions as those of the mixing described above and then dried. The powder after the drying was sintered in the air at 1400° C. for 2 hours to obtain a white powder, and this was used as the La.sub.2Zr.sub.2O powder (hereinafter, also referred to as “LZO powder”).

[0122] By XRD measurement, it was confirmed that the obtained white powder was an La.sub.2Zr.sub.2O.sub.7 monophase.

EXAMPLE 1

[0123] The LZO powder was added to a 3 mol % yttria-containing zirconia powder having a BET specific surface area of 7 m.sup.2/g (trade name: TZ-3YS, manufactured by Tosoh Corporation) in a manner that the weight ratio of the LZO powder to the zirconia powder was 20 wt. %, and mixed to obtain a mixed powder. The mixing was performed by wet mixing in an ethanol solvent for 120 hours using a ball mill with zirconia balls having a diameter of 10 mm. The obtained mixed powder was dried in the air at 80° C. to form a source powder.

[0124] The source powder was molded by uniaxial pressing by mold pressing to obtain a pregreen body. The pressure of the uniaxial pressing was 50 MPa. The obtained pregreen body was subjected to cold isostatic pressing (hereinafter, referred to as “CIP”) treatment to obtain a cylindrical green body having a diameter of 20 mm and a thickness of approximately 3 mm. The pressure of the CIP treatment was 200 MPa.

[0125] The green body was subjected to primary sintering in the air, at a temperature elevation rate of 100° C./h, a sintering temperature of 1450° C., and for a sintering time of 2 hours to obtain a primary sintered body.

[0126] The obtained primary sintered body was placed in a container that was made of zirconia and that had a lid, and subjected to HIP treatment to obtain an HIP-treated body. The HIP-treated body was used as the zirconia sintered body of the present example. The HIP treatment conditions were as follows: in a 99.9% argon gas atmosphere as a pressure medium, temperature elevation rate of 600° C./h, HIP temperature of 1750° C., HIP pressure of 150 MPa, and maintaining time for 1 hour.

[0127] After the HIP treatment, an HIP-treated body was obtained by lowering the temperature from the sintering temperature to room temperature. Note that the temperature lowering rate from the HIP temperature to 1000° C. was 83° C./min.

[0128] The obtained HIP-treated body was subjected to heat treatment in the air at 1000° C. for 1 hour to obtain a colorless translucent sintered body.

[0129] The Rietveld analysis result of the zirconia sintered body of the present example is shown in FIG. 1, the TEM observation images are shown in FIGS. 2A to 2D, and the SEM observation image is shown in FIG. 3. By the XRD pattern of FIG. 1, it was confirmed that the zirconia sintered body of the present example did not contain lanthanum oxide and the like. Furthermore, Table 1 shows the composition analysis results of the crystal grain inner portion and the crystal grain boundary by SEM-EDS. Note that the analysis by the SEM-EDS was performed without subjecting the sintered body to thermal etching treatment. From the Table 1, it was confirmed that the inner portion of the crystal grain and the crystal boundary of the sintered body of the present example did not have different compositions and the sintered body of the present example was a uniform sintered body since the average compositions of the inner portion of the crystal grain and the portion close to the crystal boundary were similar.

TABLE-US-00001 TABLE 1 Concentration in inner Concentration at crystal portion of crystal grain grain boundary Detected element (mol %) (mol %) Oxygen (O) 60.3 60.1 Yttrium (Y) 2.2 2.7 Zirconium (Zr) 34.9 34.8 Lanthanum (La) 2.7 2.4

[0130] Furthermore, from FIG. 2A, the cubic domain and the tetragonal domain were confirmed. The domain was approximately 50 nm while the average crystal grain size was 88.3 μm, and it was confirmed that the domain was smaller than the crystal grain size. By this, it was confirmed that the zirconia sintered body of the present example contained the tetragonal domain and the cubic domain in the crystal grain.

[0131] Note that, by the Rietveld analysis, it was confirmed that 48.4 wt. % was the cubic phase and 51.6 wt. % was the tetragonal phase in the zirconia sintered body of the present example, the lattice constant of the cubic phase was a=0.51872 nm, the lattice constant of the tetragonal phase was af=0.50975 nm and cf=0.51917 nm, the crystallite size of the cubic phase was 21 nm, and the crystallite size of the tetragonal phase was 32 nm. Note that, in the Rietveld analysis, the reliability factor was Rwp=20% and S=1.28. The Y.sub.2O.sub.3 concentration of the tetragonal phase determined from the lattice constant was 1.48 mol %.

[0132] Furthermore, from the element mapping by the TEM observation, it was confirmed that the region where lanthanum was present and the region where lanthanum was almost absent (FIG. 2D). From the Rietveld analysis results and the element mapping, it was conceived that the region where lanthanum was present was the region of the cubic phase where the lanthanum was dissolved as a solid solution, and on the other hand, the region where lanthanum was absent was the region of the tetragonal phase. By this, it was confirmed that, in the sintered body of the present example, the lanthanum concentration was higher in the cubic domain than the tetragonal domain.

[0133] The sizes of the cubic domain and the tetragonal domain obtained by the TEM observation were the same as the average crystallite size and as each of the crystallite sizes of the tetragonal phase and the cubic phase obtained by the Rietveld analysis. From these results, it was confirmed that, in the sintered body of the present example, the cubic domain was the crystallite of cubic phase and that the tetragonal domain was the crystallite of tetragonal phase.

[0134] FIG. 4 shows the general image of the zirconia sintered body of the present example, FIG. 5 shows the spectrum obtained by goniophotometer, and FIG. 6 shows the UV-Vis spectrum. From FIG. 4, the line on the back surface was observed through the zirconia sintered body of the present example, and thus it was confirmed that the zirconia sintered body of the present invention had translucency. Furthermore, although the defraction transmittance (DF) of the zirconia sintered body of the present example was 24.28%, from FIGS. 5 and 6, it was confirmed that the most of it was the defraction transmittance at angles close to the in-line transmitted light, and has high transmittance in the in-line direction. Furthermore, it was also confirmed that high translucency was achieved in the wavelength range of visible light of 300 nm to 800 nm. By this, it was confirmed that even higher transparency was achieved by the zirconia sintered body of the present invention.

[0135] Furthermore, the monoclinic phase fraction after the hydrothermal degradation test was 0%, and it was confirmed that the zirconia sintered body of the present example was less likely to deteriorate. FIG. 7 shows the XRD pattern after the hydrothermal degradation test.

[0136] The evaluation results of the zirconia sintered body of the present example are shown in Table 2.

EXAMPLE 2

[0137] A zirconia sintered body of the present example was obtained by the same method as in Example 1 except for changing the HIP treatment temperature to 1700° C. The evaluation results of the zirconia sintered body of the present example are shown in Table 2.

EXAMPLE 3

[0138] A zirconia sintered body of the present example was obtained by the same method as in Example 1 except for changing the HIP treatment temperature to 1800° C. The evaluation results of the zirconia sintered body of the present example are shown in Table 2.

EXAMPLE 4

[0139] A zirconia sintered body of the present example was obtained by the same method as in Example 1 except for changing the HIP treatment pressure to 54 MPa. The evaluation results of the zirconia sintered body of the present example are shown in Table 2.

EXAMPLE 5

[0140] A zirconia sintered body of the present example was obtained by the same method as in Example 1 except for changing the HIP treatment time to 15 minutes. The evaluation results of the zirconia sintered body of the present example are shown in Table 2.

EXAMPLE 6

[0141] A zirconia sintered body of the present example was obtained by the same method as in Example 1 except for changing the primary sintering temperature to 1425° C. The evaluation results of the zirconia sintered body of the present example are shown in Table 2.

EXAMPLE 7

[0142] A zirconia sintered body of the present example was obtained by the same method as in Example 1 except for adding the LZO powder to the zirconia powder in a manner that the weight ratio of the LZO powder was 15 wt. %. The average crystal grain size of the zirconia sintered body of the present example was 82.1 μm. The evaluation results are shown in Table 2.

EXAMPLE 8

[0143] A zirconia sintered body of the present example was obtained by the same method as in Example 1 except for adding the LZO powder to the zirconia powder in a manner that the weight ratio of the LZO powder was 17.5 wt. % and changing the HIP treatment temperature to 1700° C. The average crystal grain size of the zirconia sintered body of the present example was 48.2 μm. The evaluation results are shown in Table 2.

EXAMPLE 9

[0144] A zirconia sintered body of the present example was obtained by the same method as in Example 1 except for adding the LZO powder to the zirconia powder in a manner that the weight ratio of the LZO powder was 17.5 wt. %, changing the primary sintering temperature to 1500° C., and changing the HIP treatment pressure to 15 MPa. The evaluation results of the zirconia sintered body of the present example are shown in Table 2.

EXAMPLE 10

[0145] A zirconia sintered body of the present example was obtained by the same method as in Example 1 except for adding the LZO powder to the zirconia powder in a manner that the weight ratio of the LZO powder was 25 wt. % and changing the HIP treatment temperature to 1700° C. The average crystal grain size of the zirconia sintered body of the present example was 45.6 μm. The evaluation results are shown in Table 2.

EXAMPLE 11

[0146] A zirconia sintered body of the present example was obtained by the same method as in Example 1 except for adding the LZO powder to the zirconia powder in a manner that the weight ratio of the LZO powder was 17.5 wt. % and changing the HIP treatment temperature to 1725° C. The average crystal grain size of the zirconia sintered body of the present example was 61.2 μm. The evaluation results are shown in Table 2.

[0147] Furthermore, Raman analysis was performed for the polished surface and the fracture surface of the test sample after the biaxial bending strength evaluation. The Raman analysis was performed by an ordinary microscopic Raman instrument (instrument name: NRS-5100, manufactured by JASCO Corporation) using a measurement laser wavelength of 532 nm. The obtained Raman spectrum is shown in FIG. 8.

[0148] From the Raman spectrum of the polishing surface of FIG. 8, peaks other than the peaks assigned to the tetragonal phase and the cubic phase were not observed in the zirconia sintered body of the present example. On the other hand, in addition to the peaks assigned to the tetragonal phase and the cubic phase, peaks assigned to monoclinic phase (550 cm.sup.−1, 500 cm.sup.−1, 470 cm.sup.−1, 380 cm.sup.−1, 190 cm.sup.−1, and 180 cm.sup.−1) were observed in the fracture surface.

[0149] By this, it was confirmed that higher strength was achieved in the zirconia sintered body of the present example since the tetragonal phase was subjected to transition into the monoclinic phase in the bending test.

EXAMPLE 12

[0150] A zirconia sintered body of the present example was obtained by the same method as in Example 1 except for adding the LZO powder to the zirconia powder in a manner that the weight ratio of the LZO powder was 25 wt. % and changing the HIP treatment temperature to 1725° C. The average crystal grain size of the zirconia sintered body of the present example was 67.2 μm. The evaluation results are shown in Table 2.

EXAMPLE 13

[0151] A zirconia sintered body of the present example was obtained by the same method as in Example 1 except for using a 3 mol % yttria-containing zirconia powder having a specific surface area of 14 m.sup.2/g (trade name: TZ-3Y, manufactured by Tosoh Corporation) as a zirconia powder, using an La.sub.2O.sub.3 powder (purity: 99.99%, manufactured by Wako Pure Chemical Industries, Ltd.) in place of the LZO powder, and adding the La.sub.2O.sub.3 powder to the zirconia powder in a manner that the weight ratio of the La.sub.2O.sub.3 powder was 10 wt. %. The average crystal grain size of the zirconia sintered body of the present example was 46.9 μm. The evaluation results are shown in Table 2.

EXAMPLE 14

[0152] A zirconia sintered body of the present example was obtained by the same method as in Example 1 except for using an La.sub.2O.sub.3 powder (purity: 99.99%, manufactured by Wako Pure Chemical Industries, Ltd.) in place of the LZO powder, adding the La.sub.2O.sub.3 powder to the zirconia powder in a manner that the weight ratio of the La.sub.2O.sub.3 powder was 10 wt. %, and changing the HIP treatment time to 15 minutes. The average crystal grain size was 33.0 μm. The evaluation results are shown in Table 2.

EXAMPLE 15

[0153] A zirconia sintered body of the present example was obtained by the same method as in Example 1 except for using an La.sub.2O.sub.3 powder (purity: 99.99%, manufactured by Wako Pure Chemical Industries, Ltd.) in place of the LZO powder, adding the La.sub.2O.sub.3 powder to the zirconia powder in a manner that the weight ratio of the La.sub.2O.sub.3 powder was 7.5 wt. %, and changing the HIP treatment temperature to 1725° C. The average crystal grain size of the zirconia sintered body of the present example was 85.3 gm. The evaluation results are shown in Table 2.

EXAMPLE 16

[0154] A zirconia sintered body of the present example was obtained by the same method as in Example 1 except for using an La.sub.2O.sub.3 powder (purity: 99.99%, manufactured by Wako Pure Chemical Industries, Ltd.) in place of the LZO powder, adding the La.sub.2O.sub.3 powder to the zirconia powder in a manner that the weight ratio of the La.sub.2O.sub.3 powder was 7.5 wt. %, and changing the HIP treatment temperature to 1700° C. The average crystal grain size of the zirconia sintered body of the present example was 61.3 μm. The evaluation results are shown in Table 2.

EXAMPLE 17

[0155] A zirconia sintered body of the present example was obtained by the same method as in Example 1 except for using an La.sub.2O.sub.3 powder (purity: 99.99%, manufactured by Wako Pure Chemical Industries, Ltd.) in place of the LZO powder, adding the La.sub.2O.sub.3 powder to the zirconia powder in a manner that the weight ratio of the La.sub.2O.sub.3 powder was 10 wt. %, and changing the temperature lowering rate after the HIP treatment to 80° C./min. The average crystal grain size of the zirconia sintered body of the present example was 80.2 μm. The evaluation results are shown in Table 2.

EXAMPLE 18

[0156] A zirconia sintered body of the present example was obtained by the same method as in Example 1 except for using an La.sub.2O.sub.3 powder (purity: 99.99%, manufactured by Wako Pure Chemical Industries, Ltd.) in place of the LZO powder, adding the La.sub.2O.sub.3 powder to the zirconia powder in a manner that the weight ratio of the La.sub.2O.sub.3 powder was 10 wt. %, and changing the temperature lowering rate after the HIP treatment to 20° C./min. The average crystal grain size of the zirconia sintered body of the present example was 40.2 μm, and the three-point bending strength was 827 MPa. The evaluation results are shown in Table 2.

EXAMPLE 19

[0157] A zirconia sintered body of the present example was obtained by the same method as in Example 1 except for using an La.sub.2O.sub.3 powder (purity: 99.99%, manufactured by Wako Pure Chemical Industries, Ltd.) in place of the LZO powder, adding the La.sub.2O.sub.3 powder to the zirconia powder in a manner that the weight ratio of the La.sub.2O.sub.3 powder was 10 wt. %, and changing the temperature lowering rate after the HIP treatment to 10° C./min. The average crystal grain size of the zirconia sintered body of the present example was 88.5 μm. The evaluation results are shown in Table 2.

TABLE-US-00002 TABLE 2 Sintered Average Biaxial Fracture Composition body crystallite bending toughness Transmittance (mol %) density size strength K.sub.IC (IF) (%) ZrO.sub.2 Y.sub.2O.sub.3 La.sub.2O.sub.3 (g/cm.sup.3) (nm) (MPa) (MPa .Math. m.sup.0.5) TT DF PT Example 1 92.69 2.57 4.73 6.074 18.8 1042 1.96 61.77 24.28 37.49 Example 2 92.69 2.57 4.73 6.079 22.5 592 2.11 61.00 55.59 4.41 Example 3 92.69 2.57 4.73 6.087 21.5 504 2.18 62.29 32.74 29.55 Example 4 92.89 2.57 4.73 6.113 17.5 554 2.23 60.92 33.33 27.59 Example 5 92.69 2.57 4.73 6.087 23.2 1295 2.10 63.26 59.77 3.49 Example 6 92.69 2.57 4.73 6.095 23.3 905 2.03 50.74 48.06 2.68 Example 7 93.83 2.69 3.49 6.108 21.8 832 2.24 64.64 15.20 49.44 Example 8 93.26 2.63 4.11 6.074 21.1 1051 2.12 64.64 44.06 20.58 Example 9 93.26 2.63 4.11 6.082 21.8 1053 2.28 60.53 29.20 31.33 Example 10 91.52 2.46 6.03 6.050 20.6 592 1.72 65.47 37.35 28.12 Example 11 93.26 2.63 4.11 6.078 20.4 1301 2.12 62.75 35.14 27.61 Example 12 91.52 2.46 6.03 6.099 16.8 916 1.82 62.96 35.96 27.00 Example 13 92.99 2.88 4.13 6.078 20.0 1051 2.12 56.94 45.72 11.22 Example 14 92.99 2.88 4.13 6.090 28.2 675 2.10 68.37 51.03 17.34 Example 15 94.04 2.91 3.05 6.086 25.7 967 2.36 65.61 13.03 52.58 Example 16 94.04 2.91 3.05 6.103 29.8 954 2.23 67.45 14.92 52.53 Example 17 92.99 2.88 4.13 6.052 29.7 949 1.83 62.07 26.12 35.95 Example 18 92.99 2.88 4.13 6.081 18.9 899 2.31 57.50 32.60 24.90 Example 19 92.99 2.88 4.13 6.089 19.0 601 2.42 51.56 26.49 25.07

EXAMPLE 20

[0158] The La.sub.2O.sub.3 powder was added to a 4 mol % yttria-containing zirconia powder having a BET specific surface area of 7 m.sup.2/g (trade name: TZ-4YS, manufactured by Tosoh Corporation) in a manner that the weight ratio of the La.sub.2O.sub.3 powder to the zirconia powder was 5 wt. %, and mixed to obtain a mixed powder. The mixing was performed by wet mixing in an ethanol solvent for 120 hours using a ball mill with zirconia balls having a diameter of 10 mm. The obtained mixed powder was dried in the air at 80° C. to form a source powder.

[0159] The source powder was molded by uniaxial pressing by mold pressing to obtain a premolded body. The pressure of the uniaxial pressing was 50 MPa. The obtained premolded body was subjected to cold isostatic pressing (hereinafter, referred to as “CIP”) treatment to obtain a cylindrical green body having a diameter of 20 mm and a thickness of approximately 3 mm. The pressure of the CIP treatment was 200 MPa.

[0160] The green body was subjected to primary sintering in the atmosphere, at a temperature elevation rate of 100° C./h, a sintering temperature of 1450° C., and for a sintering time of 2 hours to obtain a primary sintered body.

[0161] The obtained primary sintered body was placed in a container that was made of zirconia and that had a lid, and subjected to HIP treatment to obtain an HIP-treated body. The HIP-treated body was used as the zirconia sintered body of the present example. The HIP treatment conditions were as follows: in a 99.9% argon gas atmosphere as a pressure medium, temperature elevation rate of 600° C./h, HIP temperature of 1650° C., HIP pressure of 150 MPa, and maintaining time for 1 hour.

[0162] After the HIP treatment, an HIP-treated body was obtained by lowering the temperature from the sintering temperature to room temperature. Note that the temperature lowering rate from the HIP temperature to 1000° C. was 83° C./min.

[0163] The obtained HIP-treated body was subjected to heat treatment in the air at 1000° C. for 1 hour to obtain a colorless translucent sintered body. The evaluation results of the zirconia sintered body of the present example are shown in Table 3.

EXAMPLE 21

[0164] A zirconia sintered body of the present example was obtained by the same method as in Example 20 except for using a 5 mol % yttria-containing zirconia powder having a BET specific surface area of 7 m.sup.2/g (trade name: TZ-SYS, manufactured by Tosoh Corporation) as a zirconia powder of the source material, changing the weight ratio of the La.sub.2O.sub.3 powder to 10 wt. %, and changing the HIP treatment temperature to 1750° C. The evaluation results of the zirconia sintered body of the present example are shown in Table 3.

EXAMPLE 22

[0165] A 3 mol % yttria-containing zirconia powder having a BET specific surface area of 7 m.sup.2/g (trade name: TZ-3YS, manufactured by Tosoh Corporation) and a 4 mol % yttria-containing zirconia powder having a BET specific surface area of 7 m.sup.2/g (trade name: TZ-4YS, manufactured by Tosoh Corporation) as the zirconia powder of the source material were weighed out in a manner that the amount of yttria relative to the amount of zirconia was 3.3 mol %, a La.sub.2O.sub.3 powder was added to the zirconia powder in a manner that the weight ratio of the La.sub.2O.sub.3 powder was 10 wt. %, and these were mixed to obtain a mixed powder. A zirconia sintered body of the present example was obtained by the same method as in Example 20 except for using the mixed powder and changing the HIP treatment temperature to 1750° C. The evaluation results of the zirconia sintered body of the present example are shown in Table 3.

EXAMPLE 23

[0166] The La.sub.2O.sub.3 powder was added to a 3 mol % yttria-containing zirconia powder having a BET specific surface area of 7 m.sup.2/g (trade name: TZ-3YS, manufactured by Tosoh Corporation) as the zirconia powder of the source material in a manner that the weight ratio of the La.sub.2O.sub.3 powder to the zirconia powder was 7.5 wt. %.

[0167] A zirconia sintered body of the present example was obtained by the same method as in Example 20 except for using this mixed powder and changing the HIP treatment temperature to 1750° C. The evaluation results of the zirconia sintered body of the present example are shown in Table 3.

TABLE-US-00003 TABLE 3 Sintered Average Biaxial Fracture Composition body crystallite bending toughness Transmittance (mol %) density size strength K.sub.IC (IF) (%) ZrO.sub.2 Y.sub.2O.sub.3 La.sub.2O.sub.3 (g/cm.sup.3) (nm) (MPa) (MPa .Math. m.sup.0.5) TT DF PT Example 20 94.07 3.92 2.02 6.067 254.1 623 2.20 57.49 56.89 1.10 Example 21 91.02 4.79 4.19 6.039 241.3 560 1.34 70.01 18.47 51.54 Example 22 92.70 3.16 4.14 6.092 120.4 642 2.47 70.78 58.26 12.52 Example 23 94.04 2.91 3.05 6.106 105.7 617 2.42 65.16 50.89 14.27

EXAMPLE 24

[0168] A 2.45 mol % yttria-containing zirconia powder was obtained by mixing a 3 mol % yttria-containing zirconia powder having a BET specific surface area of 7 m.sup.2/g (trade name: TZ-3YS, manufactured by Tosoh Corporation) and a 2 mol % yttria-containing zirconia powder having a BET specific surface area of 16 m.sup.2/g (trade name: TZ-2Y, manufactured by Tosoh Corporation) as the zirconia powder of the source material. A sintered body of the present example was produced by the same method as in Example 1 except for adding the La.sub.2O.sub.3 powder to the powder in a manner that the weight ratio of the La.sub.2O.sub.3 powder to the powder was 10.5 wt. %. The average crystal grain size of the zirconia sintered body of the present example was 36.9 μm. The evaluation results are shown in Table 4.

EXAMPLE 25

[0169] A 2.5 mol % yttria-containing zirconia powder was obtained by mixing the zirconia powder in the same manner as in Example 24. A sintered body of the present example was produced by the same method as in Example 1 except for using the obtained zirconia powder and changing the weight ratio of the La.sub.2O.sub.3 powder to 10 wt. %. The average crystal grain size of the zirconia sintered body of the present example was 54.4 μm. The evaluation results are shown in Table 4.

EXAMPLE 26

[0170] A 2.6 mol % yttria-containing zirconia powder was obtained by mixing the zirconia powder in the same manner as in Example 24. A sintered body of the present example was produced by the same method as in Example 1 except for using the obtained zirconia powder and changing the weight ratio of the La.sub.2O.sub.3 powder to 11 wt. %. The average crystal grain size of the zirconia sintered body of the present example was 42.6 μm. The evaluation results are shown in Table 4.

EXAMPLE 27

[0171] A 2.8 mol % yttria-containing zirconia powder was obtained by mixing the zirconia powder in the same manner as in Example 24. A sintered body of the present example was produced by the same method as in Example 1 except for using the obtained zirconia powder and changing the weight ratio of the La.sub.2O.sub.3 powder to 10 wt. %. The average crystal grain size of the zirconia sintered body of the present example was 46.3 μm. The evaluation results are shown in Table 4.

EXAMPLE 28

[0172] A 2.8 mol % yttria-containing zirconia powder was obtained by mixing the zirconia powder in the same manner as in Example 24. A sintered body of the present example was produced by the same method as in Example 1 except for using the obtained zirconia powder and changing the weight ratio of the La.sub.2O.sub.3 powder to 9.2 wt. %. The average crystal grain size of the zirconia sintered body of the present example was 45.2 μm. The evaluation results are shown in Table 4.

TABLE-US-00004 TABLE 4 Sintered Average Biaxial Fracture Composition body crystallite bending toughness Transmittance (mol %) density size strength K.sub.IC (IF) (%) ZrO.sub.2 Y.sub.2O.sub.3 La.sub.2O.sub.3 (g/cm.sup.3) (nm) (MPa) (MPa .Math. m.sup.0.5) TT DF PT Example 24 93.32 2.34 4.34 6.090 19.96 1065 2.57 54.58 47.93 6.65 Example 25 93.49 2.40 4.11 6.099 30.95 962 2.56 62.82 23.59 39.23 Example 26 92.96 2.48 4.56 6.086 13.74 1207 2.43 57.98 48.34 9.64 Example 27 93.19 2.68 4.13 6.094 21.24 1098 2.29 65.54 37.11 27.43 Example 28 93.53 2.69 3.78 6.092 24.28 1102 2.32 64.56 37.21 27.35

EXAMPLE 29

[0173] The La.sub.2O.sub.3 powder was added to a 3 mol % yttria-containing zirconia powder having a BET specific surface area of 7 m.sup.2/g (trade name: TZ-3YS, manufactured by Tosoh Corporation) as the zirconia powder of the source material in a manner that the weight ratio of the La.sub.2O.sub.3 powder to the zirconia powder was 10 wt. %. Relative to the total weight of the zirconia powder and the La.sub.2O.sub.3 powder, 500 ppm by weight of a CaO powder (manufactured by Wako Pure Chemical Industries, Ltd., 99.9%) was added to obtain a mixed powder. A zirconia sintered body of the present example was obtained by the same method as in Example 20 except for using the mixed powder and changing the HIP treatment temperature to 1750° C. As a result of the XRD measurement, the peak of the crystal phase of the sintered body was only the zirconia peak, and it was confirmed that other crystal phases except the zirconia, such as CaO, were not contained. As a result, it was confirmed that the CaO functioned as a stabilizer similar to the Y.sub.2O.sub.3. The obtained sintered body was a colorless translucent sintered body. The composition of the zirconia sintered body of the present example included 92.88 mol % of ZrO.sub.2, 2.88 mol % of Y.sub.2O.sub.3, 0.12 mol % of CaO, and 4.13 mol % of La.sub.2O.sub.3. The average crystal grain size of the zirconia sintered body of the present example was 21.3 μm. The evaluation results of the zirconia sintered body of the present example are shown in Table 5.

EXAMPLE 30

[0174] A zirconia sintered body of the present example was obtained by the same method as in Example 29 except for using a MgO powder (trade name: 500A, manufactured by Ube Material Industries, Ltd.) in place of the CaO powder.

[0175] As a result of the XRD measurement, the peak of the crystal phase of the sintered body was only the zirconia peak, and it was confirmed that other crystal phases except the zirconia, such as MgO, were not contained. As a result, it was confirmed that the MgO functioned as a stabilizer similar to the Y.sub.2O.sub.3. The obtained sintered body was a colorless translucent sintered body. The composition of the zirconia sintered body of the present example included 92.83 mol % of ZrO.sub.2, 2.88 mol % of Y.sub.2O.sub.3, 0.17 mol % of MgO, and 4.13 mol % of La.sub.2O.sub.3. The average crystal grain size of the zirconia sintered body of the present example was 24.7 μm. The evaluation results are shown in Table 5.

EXAMPLE 31

[0176] The La.sub.2O.sub.3 powder was added to a 3 mol % yttria-containing zirconia powder having a BET specific surface area of 7 m.sup.2/g (trade name: TZ-3YS, manufactured by Tosoh Corporation) as the zirconia powder of the source material in a manner that the weight ratio of the La.sub.2O.sub.3 powder to the zirconia powder was 10 wt. %. Relative to the total weight of the zirconia powder and the La.sub.2O.sub.3 powder, 1000 ppm by weight of a γ-alumina powder having a BET specific surface area of 200 m.sup.2/g (trade name: TM-300D, manufactured by Taimei Chemicals Co., Ltd.) was added to obtain a mixed powder. A zirconia sintered body of the present example was obtained by the same method as in Example 1 except for using this mixed powder. The obtained sintered body was a colorless translucent sintered body.

[0177] The average crystal grain size of the zirconia sintered body of the present example was 52.1 μm. The three-point bending strength was 856 MPa. The evaluation results are shown in Table 5.

EXAMPLE 32

[0178] A zirconia sintered body of the present example was obtained by the same method as in Example 31 except for adding 250 ppm by weight of an α-alumina powder having a BET specific surface area of 6.7 m.sup.2/g (trade name: AKP-30, manufactured by Sumitomo Chemical Co., Ltd.) as the alumina powder. The obtained sintered body was a colorless translucent sintered body. The average crystal grain size of the zirconia sintered body of the present example was 78.5 μm. The three-point bending strength was 842 MPa. The evaluation results are shown in Table 5.

EXAMPLE 33

[0179] A zirconia sintered body of the present example was obtained by the same method as in Example 31 except for adding 500 ppm by weight of an α-alumina powder having a BET specific surface area of 6.7 m.sup.2/g (trade name: AKP-30, manufactured by Sumitomo Chemical Co., Ltd.) as the alumina powder. The obtained sintered body was a colorless translucent sintered body. The average crystal grain size of the zirconia sintered body of the present example was 78.5 μm. The three-point bending strength was 844 MPa. The evaluation results are shown in Table 5.

TABLE-US-00005 TABLE 5 Sintered Average Biaxial Fracture Composition Al.sub.2O.sub.3 body crystallite bending toughness Transmittance (mol %) (ppm by density size strength K.sub.IC (IF) (%) ZrO.sub.2 Y.sub.2O.sub.3 La.sub.2O.sub.3 weight) (g/cm.sup.3) (nm) (MPa) (MPa .Math. m.sup.0.5) TT DF PT Example 29 92.99 3.00*.sup.1 4.13 0 6.002 123.4 1473 2.07 69.55 62.14 7.41 Example 30 92.83 3.04*.sup.2 4.13 0 6.006 65.72 1260 2.15 68.86 55.91 12.95 Example 31 92.99 2.88 4.13 1000 6.082 31.86 1391 2.20 63.13 36.26 26.87 Example 32 92.99 2.88 4.13 250 6.089 23.98 1038 2.04 63.84 24.90 28.94 Example 33 92.99 2.88 4.13 500 6.088 23.59 1087 2.50 66.23 23.55 42.68 *.sup.1The Y.sub.2O.sub.3 concentration of Example 29 was the total of 2.88 mol % of Y.sub.2O.sub.3 and 0.12 mol % of CaO *.sup.2The Y.sub.2O.sub.3 concentration of Example 30 was the total of 2.88 mol % of Y.sub.2O.sub.3 and 0.17 mol % of MgO

[0180] From Table 5, it was confirmed that a sintered body having both translucency and strength was obtained wherein total light transmittance was 68% or greater and biaxial bending strength was 1200 MPa even when CaO or MgO was used as the stabilizer. Furthermore, it was confirmed that a sintered body having both translucency and strength can be obtained even when alumina was contained.

EXAMPLE 34

[0181] The La.sub.2O.sub.3 powder was added to a 3 mol % yttria-containing zirconia powder having a BET specific surface area of 7 m.sup.2/g (trade name: TZ-3YS, manufactured by Tosoh Corporation) as the zirconia powder of the source material in a manner that the weight ratio of the La.sub.2O.sub.3 powder to the zirconia powder was 10 wt. %. The source material powder was molded by uniaxial pressing by mold pressing to obtain a premolded body. The pressure of the uniaxial pressing was 50 MPa. The obtained premolded body was subjected to CIP treatment to obtain a cylindrical molded body having a diameter of 20 mm and a thickness of approximately 3 mm. The pressure of the CIP treatment was 200 MPa.

[0182] The green body was subjected to pressureless sintering in the air, at a temperature elevation rate of 100° C./h, a sintering temperature of 1775° C., and for a sintering time of 1 hour to obtain a zirconia sintered body of the present example. The average temperature lowering rate from the sintering temperature to 1000° C. was 16.7° C./min. The average crystal grain size of the zirconia sintered body of the present example was 12.1 μm. The evaluation results are shown in Table 6.

TABLE-US-00006 TABLE 6 Sintered Average Biaxial Fracture Composition body crystallite bending toughness Transmittance (mol %) density size strength K.sub.IC (IF) (%) ZrO.sub.2 Y.sub.2O.sub.3 La.sub.2O.sub.3 (g/cm.sup.3) (nm) (MPa) (MPa .Math. m.sup.0.5) TT DF PT Example 34 92.99 2.88 4.13 6.098 18.80 1054 2.88 51.30 50.75 0.55

[0183] From Table 6, the sintered body of Example 34 had a biaxial bending strength of 1000 MPa or greater and a total light transmittance of 50% or greater. As a result, it was confirmed that a sintered body having both translucency and strength can be obtained by a one-step sintering method.

EXAMPLE 35

[0184] A zirconia sintered body of the present example was obtained by the same method as in Example 1 except for using an La.sub.2O.sub.3 powder (purity: 99.99%, manufactured by Wako Pure Chemical Industries, Ltd.) in place of the LZO powder, adding the La.sub.2O.sub.3 powder to the zirconia powder in a manner that the weight ratio of the La.sub.2O.sub.3 powder was 10 wt. %, and changing the temperature lowering rate after the HIP treatment to 80° C./min. The results are shown in Table 7.

[0185] The Rietveld analysis result of the zirconia sintered body of the present example is shown in FIG. 11, and the TEM observation images are shown in FIGS. 12A to 12D. By the XRD pattern of FIG. 11, it was confirmed that the zirconia sintered body of the present example did not contain lanthanum oxide and the like.

[0186] Furthermore, from FIG. 12A, the cubic domain and the tetragonal domain of approximately 50 nm were confirmed. The domain was approximately 50 nm while the average crystal grain size was 55.8 μm, and it was confirmed that the domain was smaller than the crystal grain size. By this, it was confirmed that the zirconia sintered body of the present example contained the tetragonal domain and the cubic domain in the crystal grain.

[0187] Note that, by the Rietveld analysis, it was confirmed that 68.5 wt. % was the cubic phase and 31.5 wt. % was the tetragonal phase in the zirconia sintered body of the present example, the lattice constant of the cubic phase was a=0.51836 nm, the lattice constant of the tetragonal phase was af=0.51096 nm and cf=0.52067 nm, the crystallite size of the cubic phase was 36 nm, and the crystallite size of the tetragonal phase was 32 nm. Note that, in the Rietveld analysis, the reliability factor was Rwp=18% and S=1.49. The Y.sub.2O.sub.3 concentration of the tetragonal phase determined from the lattice constant was 1.27 mol %.

[0188] The three-point bending strength was 609 MPa, the fracture toughness K.sub.IC(SEPB) was 2.74 MPa.Math.m.sup.0.5, and the thermal conductivity was 1.81 W/mK.

EXAMPLE 36

[0189] A sintered body of the present example was produced by the same method as in Example 35 except for changing the temperature lowering rate from the HIP temperature to 1000° C. to 40° C./min. The three-point bending strength was 893 MPa, and the fracture toughness K.sub.IC(SEPB) was 2.74 MPa.Math.m.sup.0.5.

EXAMPLE 37

[0190] A sintered body of the present example was produced by the same method as in Example 35 except for changing the temperature lowering rate from the HIP temperature to 1000° C. to 30° C./min. The three-point bending strength was 1016 MPa, and the fracture toughness K.sub.IC(SEPB) was 2.93 MPa.Math.m.sup.0.5.

EXAMPLE 38

[0191] A sintered body of the present example was produced by the same method as in Example 35 except for changing the temperature lowering rate from the HIP temperature to 1000° C. to 20° C./min. The TEM observation images of the present example were shown in FIGS. 13A to 13D. As a result of the XRD measurement, it was found that the zirconia sintered body of the present example did not contain lanthanum oxide and the like.

[0192] Furthermore, from FIG. 13A, the cubic domain and the tetragonal domain of approximately 50 nm were confirmed. The domain was approximately 50 nm while the average crystal grain size was 77.9 μm, it was confirmed that the domain was smaller than the crystal grain size. By this, it was confirmed that the zirconia sintered body of the present example contained the tetragonal domain and the cubic domain in the crystal grain.

[0193] Note that, by the Rietveld analysis, it was confirmed that 58.0 wt. % was the cubic phase and 42.0 wt. % was the tetragonal phase in the zirconia sintered body of the present example, the lattice constant of the cubic phase was a=0.51718 nm, the lattice constant of the tetragonal phase was af=0.51082 nm and cf=0.52028 nm, the crystallite size of the cubic phase was 28 nm, and the crystallite size of the tetragonal phase was 35 nm. Note that, in the Rietveld analysis, the reliability factor was Rwp=18% and S=1.40. The Y.sub.2O.sub.3 concentration of the tetragonal phase determined from the lattice constant was 1.46 mol %. The three-point bending strength was 895 MPa, and the fracture toughness K.sub.IC(SEPB) was 3.32 MPa.Math.m.sup.0.5.

[0194] The results of Examples 35 and 38 are shown in Table 7.

TABLE-US-00007 TABLE 7 Fracture Three- Temperature Crystal phase toughness point lowering (wt. %) K.sub.IC bending rate Tetragonal Cubic (SEPB) strength (° C./min) phase phase (MPa .Math. m.sup.0.5) (MPa) Example 35 80 31.5 68.5 2.74 609 Example 38 20 42.0 58.0 3.32 895

[0195] From Table 7, it was confirmed that the tetragonal domain was increased as the temperature lowering rate was slower. Furthermore, along with this, the fracture toughness and the three-point bending strength were made higher. As a result, it was confirmed that the mechanical strength tends to be enhanced by lowering the temperature lowering rate.

Comparative Example 1

[0196] The 3 mol % yttria-containing zirconia powder having a BET specific surface area of 7 m.sup.2/g (trade name: 3YS, manufactured by Tosoh Corporation) was used as the source material powder of the present comparative example.

[0197] The source material powder was molded by uniaxial pressing by mold pressing to obtain a premolded body. By performing the CIP treatment, a cylindrical green body having a diameter of 20 mm and a thickness of approximately 3 mm was obtained. The pressure of the CIP was 200 MPa.

[0198] The green body was subjected to primary sintering in the air, at a temperature elevation rate of 100° C./hr, a sintering temperature of 1450° C., and for a sintering time of 2 hours to obtain a primary sintered body.

[0199] The obtained primary sintered body was placed in an alumina container having a lid and subjected to HIP treatment. The HIP treatment conditions were as follows: in a 99.9% argon gas atmosphere as a pressure medium, temperature elevation rate of 600° C./hr, HIP temperature of 1750° C., HIP pressure of 150 MPa, and maintaining time for 1 hour.

[0200] After the HIP treatment, the HIP treated body was cooled at a temperature lowering rate from the HIP temperature to 1000° C. of 83° C./min.

[0201] The obtained HIP-treated body was heat-treated in the air at 1000° C. for 1 hour to obtain a zirconia sintered body of the present comparative example. The average crystal grain size of the obtained zirconia sintered body was 1.80 μm. The evaluation results of the obtained zirconia sintered body are shown in Table 8. The biaxial bending strength of the zirconia sintered body of the present comparative example exhibited high strength exceeding 1 GPa; however, the total light transmittance was 39.00% and the zirconia sintered body had significantly low translucency.

Comparative Example 2

[0202] A zirconia sintered body of the present comparative example was obtained by the same method as in Comparative Example 1 except for changing the 8 mol % yttria-containing zirconia powder having a BET specific surface area of 7 m.sup.2/g (trade name: 8YS, manufactured by Tosoh Corporation) to the source material powder of the present comparative example.

[0203] The average crystal grain size of the obtained zirconia sintered body was 52.9 μm. The evaluation results of the obtained zirconia sintered body are shown in Table 8. The total light transmittance of the zirconia sintered body of the present comparative example was 62.00%, and the zirconia sintered body had high translucency. However, the biaxial bending strength was 253 MPa, and it was confirmed that the sintered body had significantly low strength.

Comparative Example 3

[0204] A sintered body was produced by the same conditions as in Comparative Example 1 except for using a 3 mol % yttria-containing zirconia powder having a BET specific surface area of 7 m.sup.2/g (trade name: TZ-3YS, manufactured by Tosoh Corporation), adding a LZO powder to the zirconia powder in a manner that the weight ratio of the LZO powder to the yttria-containing zirconia powder was 20 wt. %, and changing the temperature lowering rate in the HIP treatment to 1° C./min.

[0205] The evaluation results of the zirconia sintered body of the present comparative example are shown in Table 8, and the XRD pattern is shown in FIG. 9. From FIG. 9, it was confirmed that the sintered body of the present comparative example is a zirconia sintered body containing monoclinic phases. Furthermore, the total light transmittance was 44% or less, and the translucency was significantly low.

Comparative Example 4

[0206] A zirconia sintered body of the present comparative example was produced by the same conditions as in Comparative Example 1 except for using a zirconia powder having a BET specific surface area of 14 m.sup.2/g (trade name: 0Y, manufactured by Tosoh Corporation) and adding a La.sub.2O.sub.3 powder (purity: 99.99%, manufactured by Wako Pure Chemical Industries, Ltd.) to the zirconia powder in a manner that the weight ratio of the La.sub.2O.sub.3 powder to the yttria-containing zirconia powder was 10 wt. %. Note that the zirconia powder did not contain a stabilizer.

[0207] The evaluation results of the zirconia sintered body of the present comparative example are shown in Table 8, and the XRD pattern is shown in FIG. 10. The obtained zirconia sintered body was a sintered body that did not have translucency. Furthermore, from the XRD pattern, it was confirmed that the zirconia sintered body of the present comparative example was a mixed phase of monoclinic phases and La.sub.2Zr.sub.2O.sub.7. Furthermore, the zirconia sintered body of the present comparative example did not have a main peak, and the average crystallite size thereof was not determined.

Comparative Example 5

[0208] A zirconia sintered body of the present comparative example was obtained by the same method as in Example 1 except for using 10 wt. % of an ytterbium oxide powder in place of 20 wt. % of the LZO powder and using a 3 mol % yttria-containing zirconia powder having a BET specific surface area of 7 m.sup.2/g (trade name: TZ-3YS, manufactured by Tosoh Corporation). The results are shown in Table 8. As a result of the XRD measurement, a peak of only zirconia cubic phase was observed in the zirconia sintered body of the present comparative example. As a result, it was confirmed that the zirconia sintered body in which ytterbium, which is a lanthanoid element, was dissolved as a solid solution did not have crystal grains having the cubic domain and the tetragonal domain.

TABLE-US-00008 TABLE 8 Sintered Average Biaxial Fracture Composition body crystallite bending toughness Transmittance (mol %) density size strength K.sub.IC (IF) (%) ZrO.sub.2 Y.sub.2O.sub.3 La.sub.2O.sub.3 (g/cm.sup.3) (nm) (MPa) (MPa .Math. m.sup.0.5) TT DF PT Comparative 97.00 3.00 0.00 6.076 260.0 1286  4.61 39.00 38.86 0.14 Example 1 Comparative 92.00 8.00 0.00 5.980 — 253 1.80 62.00 24.70 37.30 Example 2 Comparative 92.99 2.88 4.13 5.971 14.81 692 2.38 43.59 40.40 3.19 Example 3 Comparative 95.97 0.00 4.03 5.634 —*.sup.1 — — — — — Example 4 Comparative 93.99 2.56 3.45*.sup.2 6.320 258.0 274 1.86 37.8  37.7  0.1 Example 5 *.sup.1In the table, “—” indicates “unmeasured” *.sup.2The La.sub.2O.sub.3 content in Comparative Example 5 is the Yb.sub.2O.sub.3 content

Example 39

Production of Compound

[0209] The La.sub.2O.sub.3 powder was mixed to a 3 mol % yttria-containing zirconia powder having a BET specific surface area of 7 m.sup.2/g (trade name: 3YS, manufactured by Tosoh Corporation) in a manner that the weight ratio of the La.sub.2O.sub.3 powder was 10 wt. %, and then wet-mixed in the same manner as in Example 1 to obtain a mixed powder. The mixed powder, a wax, and an organic binder containing a plasticizer and a thermoplastic resin were mixed to obtain a zirconia compound. Injection molding and production of sintered body

[0210] The obtained zirconia compound was molded by injection molding to form a plate-like green body having a length of 70 mm, a width of 30 mm, and a thickness of 2 mm. After the organic binder was removed by heating in the air at 450° C., and then sintering was performed in the air at 1450° C. for 2 hours to obtain a primary sintered body. The obtained primary sintered body was placed in a container that was made of zirconia and that had a lid, and subjected to HIP treatment to obtain an HIP-treated body. The HIP-treated body was used as the zirconia sintered body of the present example. The HIP treatment conditions were as follows: in a 99.9% argon gas atmosphere as a pressure medium, temperature elevation rate of 600° C./h, HIP temperature of 1750° C., HIP pressure of 150 MPa, and maintaining time for 1 hour. After the HIP treatment, an HIP-treated body was obtained by lowering the temperature from the sintering temperature to room temperature. Note that the temperature lowering rate from the HIP temperature to 1000° C. was 83° C./min.

[0211] The HIP-treated body obtained was subjected to heat treatment in the air at 1000° C. for 1 hour to obtain a colorless translucent sintered body. The obtained sintered body was a zirconia sintered body in which the lanthanum and the yttria were dissolved as a solid solution, and the composition thereof included 92.99 mol % of ZrO.sub.2, 2.88 mol % of Y.sub.2O.sub.3, and 4.13 mol % of La.sub.2O.sub.3. The average crystal grain size was 54.5 μm. The results are shown in Table 9.

EXAMPLE 40

[0212] A sintered body was obtained by the same method as in Example 39 except for changing the primary sintering temperature to 1475° C. The composition of the obtained sintered body included 92.99 mol % of ZrO.sub.2, 2.88 mol % of Y.sub.2O.sub.3, and 4.13 mol % of La.sub.2O.sub.3. The results are shown in Table 9.

EXAMPLE 41

[0213] A sintered body was obtained by the same method as in Example 39 except for changing the primary sintering temperature to 1475° C. and changing the temperature lowering rate from the HIP temperature to 1000° C. to 20° C./min. The composition of the obtained sintered body included 92.99 mol % of ZrO.sub.2, 2.88 mol % of Y.sub.2O.sub.3, and 4.13 mol % of La.sub.2O.sub.3. The average crystal grain size was 35.5 μm. The results are shown in Table 9.

TABLE-US-00009 TABLE 9 Biaxial Fracture Sintered Average bending toughness Transmittance body density crystallite size strength K.sub.IC (IF) (%) (g/cm.sup.3) (nm) (MPa) (MPa .Math. m.sup.0.5) TT DF PT Example 39 6.100 23.8 841 1.99 66.18 46.40 19.78 Example 40 6.086 32.2 689 2.31 66.01 35.81 30.20 Example 41 6.091 19.2 943 2.40 56.33 19.19 37.14

[0214] From these results, it was confirmed that the zirconia sintered body that was equivalent to a zirconia sintered body obtained by press molding was obtained even when injection molding was performed.

EXAMPLE 42

[0215] An orthodontic bracket (length 3.6 mm×width 3.3 mm×height 2.5 mm) formed from lanthanum-dissolved zirconia sintered body was produced by performing the molding, degreasing, sintering, and HIP-treatment in the same manner as in Example 39 except for changing the form of the green body to an orthodontic bracket shape.

EXAMPLE 43

[0216] An orthodontic bracket formed from a lanthanum-dissolved zirconia sintered body was produced by the same method as in Example 42 except for changing the temperature lowering rate from the HIP temperature to 1000° C. to 30° C./min.

EXAMPLE 44

[0217] An orthodontic bracket formed from a lanthanum-dissolved zirconia sintered body was produced by the same method as in Example 42 except for changing the temperature lowering rate from the HIP temperature to 1000° C. to 20° C./min.

EXAMPLE 45

[0218] An orthodontic bracket formed from a lanthanum-dissolved zirconia sintered body was produced by the same method as in Example 42 except for changing the primary sintering temperature to 1475° C. and the temperature lowering rate from the HIP temperature to 1000° C. to 20° C./min.

EXAMPLE 46

[0219] An orthodontic bracket formed from a lanthanum-dissolved zirconia sintered body was produced by the same method as in Example 42 except for changing the primary sintering temperature to 1475° C., placing the primary sintered body in an unused alumina container during the HIP treatment, and no heat treatment was performed after the HIP treatment. The obtained orthodontic bracket had translucency.

Measurement Example 1 (Torque Strength Test)

[0220] The torque strengths of the orthodontic brackets obtained in Examples 42 to 46 were measured. The orthodontic bracket was fixed on a base as a sample, and a stainless steel wire (0.019×0.025 inches) was passed through a slot portion of the sample to fix the sample. The surface of the slot portion of the sample was in a condition after the HIP treatment. The base was rotated, and the torque strength at the time when the bracket was broken was measured as a torque strength of the sample. Three measurements were performed for each sample, and the average value thereof was used as the torque strength of the sample. The torque strength of the measurement result is shown in Table 10. Furthermore, the torque strength of the orthodontic bracket (length 4.4 mm×width 3.7 mm×height 3.0 mm) formed from translucent alumina used as the orthodontic bracket is also shown in Table 10.

TABLE-US-00010 TABLE 10 Torque strength (kgf .Math. cm) Example 42 0.41 Example 43 0.51 Example 44 0.52 Example 45 0.59 Example 46 0.63 Translucent alumina 0.50

[0221] It was confirmed that the torque strength of the zirconia sintered body of the present invention was equivalent to the torque strength of a commercially available orthodontic bracket formed from translucent alumina. A larger orthodontic bracket has a higher torque strength. On the other hand, the orthodontic bracket of the examples had the same degree of torque strength although the orthodontic bracket is smaller than a translucent alumina bracket. That is, the zirconia sintered body of the present invention can make the orthodontic bracket even smaller, thereby making the orthodontic bracket not noticeable compared to conventional orthodontic brackets having translucency. Thus, the zirconia sintered body can be used as an orthodontic bracket having excellent aesthetic quality.

Measurement Example 2 (Torque Strength Test)

[0222] The orthodontic brackets obtained in Examples 42 and 45 were measured for torque strength in the same manner as in Measurement Example 1 except for using samples in which the surface of the slot portion of the orthodontic bracket was mirror-polished. The results are shown in Table 11.

TABLE-US-00011 TABLE 11 Torque strength (kgf .Math. cm) Measurement Example 1 Measurement Example 2 Example 42 0.41 0.63 Example 45 0.52 0.81

[0223] From the results described above, it was confirmed that the torque strength is enhanced by polishing the slot portion. It was confirmed that the orthodontic bracket of Example 42 had a higher torque strength compared to an orthodontic bracket formed from translucent alumina due to the surface polishing, thereby having practical strength.

Measurement Example 3 (Plasma Etching Test)

[0224] Using a reactive plasma etching machine (machine name: DEM-451, manufactured by Anelva), plasma-resistant characteristics of the sample were evaluated. That is, each sample was irradiated with plasma under the following conditions to measure the depth of etching and the etching rate.

Plasma intensity: 300 W
Irradiation duration: 4 hours
Reaction gas: CF.sub.4 25.2 sccm [0225] O.sub.2 6.3 sccm [0226] Ar 126 sccm

[0227] As the measurement samples, the zirconia sintered bodies of Examples 1 and 13 were used. Furthermore, quartz glass that has been used as a current semiconductor manufacturing device was used as a comparative sample. Before the measurement, the surface of each sample was subjected to mirror-polishing until the surface roughness became 0.02 μm or less.

[0228] After the plasma etching test, the center line average roughness (Ra), maximum height (Ry), and ten-point average height (Rz) of the etched surface of the sample were measured by the method in accordance with JIS B 0601-1994. A laser microscope (instrument name: VK-9500NK-9510, manufactured by Keyence Corporation) was used for the measurement. The results are shown in Table 12.

TABLE-US-00012 TABLE 12 Depth of etching Etching rate Ra Ry Rz Sample (μm) (nm/min) (μm) (μm) (μm) Example 1 2.25 9.38 0.04 0.38 0.36 Example 13 1.66 6.92 0.03 0.47 0.47 Quartz glass 16.5 68.5 0.07 3.29 3.19

[0229] Ra is the degree of unevenness relative to the average height of the etched surface after the etching, and a greater value indicates more unevenness in the etched surface. Ry indicates the difference between the part on which the etching proceeded the most and the part on which the etching proceeded the least on the etched surface after the etching. A greater value of Ry indicates that a deep etching proceeded locally. Rz indicates the average depth of the unevenness of the etched surface. Thus, a greater value of Rz indicates that the unevenness on the entire etched surface is deeper.

[0230] As described above, it was confirmed that the sintered body of the present invention had higher plasma-resistant characteristics compared to those of quartz glass.

INDUSTRIAL APPLICABILITY

[0231] The zirconia sintered body of the present invention has both high translucency and high strength. Thus, the zirconia sintered body can be used in dental prosthetic materials or dental components such as components for orthodontics that require aesthetic quality. Furthermore, since the zirconia sintered body of the present invention has excellent design characteristics, the zirconia sintered body can be used as decorative members of timepieces and jewelries as well as plasma-resistant members of components for semiconductor manufacturing devices.

[0232] All of the content of the specifications, scopes of patent claims, abstracts, and drawings of Japanese Patent Application No. 2015-005981 filed on Jan. 15, 2015 and Japanese Patent Application No. 2015-233643 filed on Nov. 30, 2015 is cited here and incorporated as a disclosure of the specification of the present invention.

REFERENCE SIGNS LIST

[0233] ⊚: Peak assigned to monoclinic phase zirconia

[0234] *: XRD peak assigned to La.sub.2Zr.sub.2O.sub.7