In a zirconia molded body or a zirconia pre-sintered body of the present invention, it is preferable that at least a part of the stabilizer be not dissolved in zirconia in the form of a solid solution. A part of stabilizer not being dissolved in zirconia in the form of a solid solution can be confirmed, for example, by XRD patterns. A stabilizer not dissolved in zirconia in the form of a solid solution is present in the zirconia molded body or zirconia pre-sintered body when a peak attributed to the stabilizer is confirmed in an XRD pattern of the zirconia pre-sintered body. Basically, an XRD pattern does not show a peak attributed to the stabilizer when the stabilizer is fully dissolved in the form of a solid solution. However, depending on conditions such as the crystal state of the stabilizer, the absence of a stabilizer peak in an XRD pattern does not necessarily mean that the stabilizer is dissolved in zirconia in the form of a solid solution. The stabilizer can be thought as having almost fully or basically fully dissolved in zirconia in the form of a solid solution when the main crystal system of zirconia is a tetragonal and/or cubic crystal system and when the XRD pattern does not show a peak attributed to the stabilizer. In a zirconia molded body or a zirconia pre-sintered body of the present invention, it is not necessarily required that the stabilizer be fully dissolved in zirconia in the form of a solid solution. As used herein, “stabilizer being dissolved in the form of a solid solution” means that, for example, elements (atoms) present in the stabilizer are dissolved in zirconia in the form of a solid solution.

In a zirconia molded body or a zirconia pre-sintered body of the present invention, the percentage presence f_yof yttria not dissolved in zirconia in the form of a solid solution (hereinafter, also referred to as “undissolved yttria”) can be calculated using the formula (3) below. The percentage presence f_yof undissolved yttria is preferably greater than 0%, more preferably 1% or greater, even more preferably 2% or greater, particularly preferably 3% or greater. The percentage presence f_yof undissolved yttria may be, for example, at most 15%. Preferably, the upper limit of the percentage presence f_yof undissolved yttria is dependent on the content of yttria in the zirconia molded body or zirconia pre-sintered body. The percentage presence f_ymay be at most 7% when the yttria content is 3 mol % or more and less than 4.5 mol %. The percentage presence f_ymay be at most 11% when the yttria content is 4.5 mol % or more and less than 5.8 mol %. The percentage presence f_ymay be at most 15% when the yttria content is 5.8 mol % to 7.5 mol %.

In a zirconia molded body or a zirconia pre-sintered body of the present invention, the percentage presence f_yis preferably 0.5% or greater, more preferably 1.0% or greater, even more preferably 2.0% or greater when the yttria content is 3 mol % or more and less than 4.5 mol %. The percentage presence f_yis preferably 1% or greater, more preferably 2% or greater, even more preferably 3% or greater when the yttria content is 4.5 mol % or more and less than 5.8 mol %. The percentage presence f_yis preferably 2% or greater, more preferably 3% or greater, even more preferably 4% or greater when the yttria content is 5.8 mol % to 7.5 mol %.

[

Math
.

1

]



f
y

(
%
)

=

I
y

(
111
)

I
y

(
111
)

+

I
m

(
111
)

+

I
m

(

11
-
1

)

+

I
t

(
111
)

+

I
c

(
111
)

×
100

(
3
)

In formula (3), I_y(111) represents the peak intensity of the (111) plane of yttria near 20=29° in an XRD pattern by CuKα radiation, I_m(111) and I_m(11-1) represent the peak intensities of the (111) plane and (11-1) plane, respectively, of the monoclinic system of zirconia, I_t(111) represents the peak intensity of the tetragonal (111) plane of zirconia, and I_c(111) represents the peak intensity of the (111) plane of the cubic crystal system of zirconia.

By substituting I_y(111) for other peaks, the formula (3) can be used to calculate the percentage presence of an undissolved fraction of a stabilizer other than yttria.

Preferably, the crystal system of zirconia in a zirconia molded body or a zirconia pre-sintered body of the present invention is mainly a monoclinic system. In the present invention, “crystal system being mainly a monoclinic system” means that the fraction f_mof the monoclinic system of zirconia calculated from formula (4) below is at least 50% of the total amount of all the crystal systems (monoclinic, tetragonal, and cubic) of zirconia. In a zirconia molded body or a zirconia pre-sintered body of the present invention, the fraction f_mof the monoclinic system of zirconia is preferably at least 55%, more preferably at least 60%, even more preferably at least 70%, yet more preferably at least 75%, particularly preferably at least 80%, still more preferably at least 85%, most preferably at least 90% of the total amount of the monoclinic, tetragonal, and cubic crystal systems. The fraction f_mof the monoclinic system can be calculated from formula (4) below, using peaks in an X-ray diffraction (XRD) pattern by CuKα radiation. The main crystal system in the zirconia molded body or zirconia pre-sintered body possibly contributes to raising shrinkage temperature and reducing firing time.

In a zirconia molded body or a zirconia pre-sintered body of the present invention, the peaks of tetragonal and cubic crystal systems may be essentially undetectable. That is, the fraction f_mof the monoclinic system may be 100%.

[

Math
.

2

]



f
m

(
%
)

=

I
m

(
111
)

+

I
m

(

11
-
1

)

I
m

(
111
)

+

I
m

(

11
-
1

)

+

I
t

(
111
)

+

I
c

(
111
)

×
100

(
4
)

In formula (4), I_m(111) and I_m(11-1) represent the peak intensities of the (111) plane and (11-1) plane, respectively, of the monoclinic system of zirconia, I_t(111) represents the peak intensity of the tetragonal (111) plane of zirconia, and I_c(111) represents the peak intensity of the (111) plane of the cubic crystal system of zirconia.

In a zirconia molded body or a zirconia pre-sintered body of the present invention, f_m/f_yis preferably 20 to 200, more preferably 25 to 100, even more preferably 30 to 60 when the yttria content is 3 mol % or more and less than 4.5 mol %. When the yttria content is 4.5 mol % or more and less than 5.8 mol %, f_m/f_yis preferably 5 to 45, more preferably 10 to 40, even more preferably 15 to 35. When the yttria content is 5.8 mol % to 7.5 mol %, f_m/f_yis preferably 2 to 40, more preferably 5 to 35, even more preferably 10 to 30.

A zirconia molded body or a zirconia pre-sintered body of the present invention may optionally comprise an additive. Examples of the additive include binders, colorants (including pigments and complex pigments), fluorescent agents, alumina (Al₂O₃), titanium oxide (TiO₂), and silica (SiO₂). The additive may be used alone, or two or more thereof may be used as a mixture.

Examples of the binders include polyvinyl alcohol, methyl cellulose, carboxymethyl cellulose, acrylic binders, wax binders (e.g., paraffin wax), polyvinyl butyral, polymethyl methacrylate, ethyl cellulose, polyethylene, polypropylene, an ethylene-vinyl acetate copolymer, polystyrene, atactic polypropylene, methacrylic resin, and stearic acid.

Examples of the pigments include oxides of at least one element selected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni, Zn, Y, Zr, Sn, Sb, Bi, Ce, Pr, Sm, Eu, Gd, Tb, and Er (specifically, for example, NiO, Cr₂O₃), preferably oxides of at least one element selected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni, Zn, Y, Zr, Sn, Sb, Bi, Ce, Pr, Sm, Eu, Gd, and Tb, more preferably oxides of at least one element selected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni, Zn, Y, Zr, Sn, Sb, Bi, Ce, Sm, Eu, Gd, and Tb. A zirconia molded body or a zirconia pre-sintered body of the present invention may be a zirconia molded body or a zirconia pre-sintered body that does not contain erbium oxide (Er₂O₃). Examples of the complex pigments include composite oxides, for example, such as (Zr,V)O₂, Fe(Fe,Cr)₂O₄, (Ni,Co,Fe)(Fe,Cr)₂O₄ZrSiO₄, and (Co,Zn)Al₂O₄. Examples of the fluorescent agents include Y₂SiO₅:Ce, Y₂SiO₅:Tb, (Y,Gd,Eu)BO₃, Y₂O₃:Eu, YAG:Ce, ZnGa₂O₄:Zn, and BaMgAl₁₀O₁₇:Eu.

The method used to produce a zirconia molded body in the present invention is not particularly limited. An example is a method that includes a step of pressing a mixed powder of zirconia and the stabilizer into a zirconia molded body under a pressure of 175 MPa or higher. By being pressed under this pressure, the mixed powder can form a zirconia molded body (and, in turn, a zirconia sintered body) having an increased bulk density, irrespective of the thickness. As used herein, “pressure of 175 MPa or higher” means the maximum pressure of press forming.

The method used to produce a zirconia pre-sintered body in the present invention is not particularly limited. In an example method, a zirconia molded body formed from a raw material powder containing zirconia particles and a stabilizer is fired (pre-sintered) at a temperature that does not sinter the zirconia particles. The zirconia molded body is as described above. An example of a method of producing a zirconia pre-sintered body of the present invention is described below. First, a raw material powder of zirconia molded body is produced. A powder of monoclinic zirconia is mixed with a stabilizer powder (for example, an yttria powder) to prepare a mixture having the desired stabilizer (for example, yttria) content. The mixture is added into water to make a slurry, and the slurry is pulverized and mixed wet with a ball mill until the particles reach the desired particle size. After pulverization, the slurry is dried to granulate, using a spray drier. The resultant powder is then fired to make a powder (primary powder) at a temperature (for example, 800 to 1,200° C.) that does not sinter the zirconia particles. A pigment may be added to the primary powder. The primary powder is then added into water to make a slurry, and the slurry is pulverized and mixed wet with a ball mill until the particles reach the desired particle size. After optionally adding an additive, such as a binder, to the pulverized slurry, the slurry is dried with a spray drier to obtain a mixed powder (secondary powder). The secondary powder is filled into a predetermined die, and, after leveling the top surface and installing an upper die, pressure is applied with a uniaxial pressing machine to form the secondary powder into a zirconia molded body. The pressure applied to press the mixed powder is preferably 175 MPa or higher. The zirconia molded body obtained may or may not be subjected to cold isostatic pressing (CIP).

The zirconia pre-sintered body may have a multilayer structure. When producing a multilayer zirconia pre-sintered body, the primary powder used in the method of production of a zirconia molded body may be divided into at least two (preferably four) separate portions to provide a multilayer structure in the zirconia molded body.

The zirconia molded body obtained in this fashion is pre-sintered to obtain a zirconia pre-sintered body. The pre-sintering temperature is, for example, preferably 800° C. or more, more preferably 900° C. or more, even more preferably 950° C. or more. For increased dimensional accuracy, the pre-sintering temperature is, for example, preferably 1,200° C. or less, more preferably 1,150° C. or less, even more preferably 1,100° C. or less. These pre-sintering temperatures are preferred because dissolution of the stabilizer does not seem to proceed in these temperature ranges.

The zirconia pre-sintered body used in the present invention may be a commercially available product. Examples of such commercially available products include Noritake KATANA® zirconia (Model: disc UTML, disc STML, disc ML, disc HT, disc LT; all manufactured by Kuraray Noritake Dental Inc.). In a method of production of a zirconia sintered body of the present invention, it is preferable that the commercially available zirconia pre-sintered body be used after being milled into a predetermined shape of a dental product.

In a method of production of a zirconia sintered body of the present invention, the rate of temperature increase in the temperature increasing step before shrinkage start temperature may be constant, or may be progressive with variable rates, provided that it is not detrimental to the effects of the present invention.

In view of providing a zirconia sintered body having superior lightness, translucency, and saturation, and enabling the composite oxides in the zirconia molded body or zirconia pre-sintered body to better develop color, the temperature increasing step in a method of production of a zirconia sintered body of the present invention may be followed by a step of further increasing temperature (second temperature-increasing step), after the temperature has reached the shrinkage end temperature. The second temperature-increasing step distinguishes itself from the temperature increasing step (first temperature-increasing step) in that the second temperature-increasing step is a temperature increasing step after zirconia has finished shrinking. The rate of temperature increase in the second temperature-increasing step is, for example, 10° C./min or more. In view of enabling further reduction of a processing time, the rate of temperature increase in the second temperature-increasing step is preferably 11° C./min or more, more preferably 12° C./min or more, even more preferably 13° C./min or more. In view of providing a zirconia sintered body having superior saturation and enabling the composite oxides in the zirconia molded body or zirconia pre-sintered body to better develop color, the rate of temperature increase in the second temperature-increasing step is, for example, 299° C./min or less, preferably 270° C./min or less, more preferably 250° C./min or less, even more preferably 200° C./min or less.

Retention Step

In view of facilitating development of translucency by grain growth, the heating step in a method of production of a zirconia sintered body of the present invention preferably includes a retention step of retaining the highest firing temperature of the temperature increasing step, following the temperature increasing step. Preferably, the highest firing temperature is retained for at most 30 minutes. In view of enabling further reduction of a processing time and providing a zirconia sintered body having excellent strength, the retention time is more preferably 1 to 30 minutes, even more preferably 10 to 20 minutes. The highest firing temperature is preferably 1,400 to 1,650° C. In view of providing a zirconia sintered body having superior lightness, translucency, and saturation, and enabling the composite oxides in the zirconia molded body or zirconia pre-sintered body to better develop color, the highest firing temperature is more preferably 1,450° C. or higher, even more preferably 1,500° C. or higher, particularly preferably 1,520° C. or higher. In view of enabling further reduction of a processing time and providing a zirconia sintered body having superior lightness, translucency, and saturation, and enabling the composite oxides in the zirconia molded body or zirconia pre-sintered body to better develop color, the highest firing temperature is more preferably 1,630° C. or less, even more preferably 1,620° C. or less, particularly preferably 1,610° C. or less.

In view of enabling further reduction of a processing time, the total firing time from the start of temperature increase in the temperature increasing step to the end of the retention period at the highest firing temperature in a method of production of a zirconia sintered body of the present invention is preferably at most 100 minutes, more preferably at most 80 minutes, even more preferably at most 50 minutes. The lower limit of the firing time is not limited, and the firing time is typically at least 10 minutes, provided that it is not detrimental to the effects of the present invention.

Cooling Step

Preferably, a method of production of a zirconia sintered body of the present invention comprises a step of cooling a zirconia sintered body after the zirconia sintered body is retained at the highest firing temperature for the predetermined time period. In the cooling step, the rate of temperature decrease is preferably 40° C./min or more, more preferably 45° C./min or more, even more preferably 50° C./min or more. The method of temperature decrease may be any of cooling by intake air from outside, water cooling, air cooling, gradual cooling, and letting the zirconia sintered body stand to cool, or a combination of these. The temperature reached at the end of the cooling step depends on factors such as the type and capabilities of the furnace, and may be 1,000° C., 900° C., or 800° C.

A zirconia sintered body obtained by a method of production of the present invention has a color difference ΔE*ab of preferably 2.7 or less, more preferably 2.0 or less, even more preferably 1.6 or less, particularly preferably 0.8 or less because such a zirconia sintered body is suited as a dental product. Preferably, the color difference ΔE*ab is measured against the chromaticity of a zirconia sintered body of when it is fired in a normal fashion (a total firing time: 6 to 12 hours). Chromaticity can be evaluated using known methods. The color difference ΔE*ab is, for example, a color difference ΔE* (ΔE*ab) determined from the following formula (5) for a sample zirconia sintered body (L₁*, a₁*, b₁*) fired in a normal fashion, and a sample zirconia sintered body (L₂*, a₂*, b₂*) subjected to short firing (a total firing time: 15 to 30 minutes) by measuring color with a dental colorimeter (a 7-band LED illuminant Crystaleye, manufactured by Olympus Corporation) using the lightness index L* and color coordinates a*, b* of the CIE 1976 L*a*b* color space.

ΔE*={(L₁*−L₂*)²+(a₁*−a₂*)²+(b₁*−b₂*)²}^1/2 (5)

The difference between the lightness index L* of a zirconia sintered body obtained by a method of production of the present invention and the lightness index L* of a zirconia sintered body fired in a normal fashion (a total firing time: 6 to 12 hours) is preferably 2.0 or less, more preferably 1.5 or less, even more preferably 1.0 or less because a zirconia sintered body having such a difference is suited as a dental product. The L*, a*, and b* of a zirconia sintered body obtained by a method of production of the present invention can be selected and set according to where the zirconia sintered body is intended for, for example, such as the cervical region or the incisal edge.

The present invention encompasses embodiments combining the foregoing features, provided that such combinations made in various forms within the technical idea of the present invention can produce the effects of the present invention. In the present specification, the upper limits and lower limits of numeric ranges (for example, (rates of temperature increase, rates of temperature decrease, firing times, temperatures, rate ratios, and contents of components (for example, a stabilizer) can be combined appropriately.

EXAMPLES

The following describes the present invention in greater detail by way of Examples. It should be noted, however, that the present invention is in no way limited by the following Examples, and various changes may be made by a person with ordinary skill in the art within the technical idea of the present invention.

Method of Determination of Shrinkage Start Temperature and Shrinkage End Temperature

In the Examples and Comparative Examples below, the shrinkage start temperature was determined by measuring the lowest firing temperature at which a prepared cuboidal sample block of unfired zirconia (1 cm×2 cm×1 cm) had a shrinkage rate of more than 0% when the sample block was fired with a furnace for 10 minutes at a firing temperature higher than the start temperature of the temperature increasing step. The shrinkage end temperature was determined by measuring the lowest temperature at which the difference between the shrinkage rate (%) at a firing temperature T (° C.) and the shrinkage rate (%) at a firing temperature T+50° C. was 0% when the temperature range of each zone of the shrinkage temperature region was 50° C.

Example 1

An HTML zirconia disc work (Noritake KATANA® zirconia, manufactured by Kuraray Noritake Dental Inc.) was milled into the shape shown in FIG. 1 ((a), plan view; (b), front view) to prepare a zirconia pre-sintered body sample, using a DWX52DC (manufactured by Roland D. G). The sample was fired with a furnace (F-2N, manufactured by SK Medical Electronics Co., Ltd.) under the firing schedule conditions shown in Table 1 to produce a zirconia sintered body (prosthesis).

TABLE 1

Rate of temperature

increase (° C./min)
Zone (° C.)
Retention time (min)

50
0 to 1000
0

42
1000 to 1050
0

13
1050 to 1100
0

7
1100 to 1150
0

6
1150 to 1200
0

5
1200 to 1250
0

21
1250 to 1300
0

16
1300 to 1350
0

50
1350 to 1400
0

50
1400 to 1450
0

50
1450 to 1500
0

50
1500 to 1515
0

0
1515
30

−45
1515 to 800
0

In Example 1, the shrinkage start temperature and shrinkage end temperature were determined to be 1,000° C. and 1,400° C., respectively, according to the above method. The rate of temperature increase in each zone of the temperature region from the shrinkage start temperature to the shrinkage end temperature was set according to the method including the Steps 1 to 5 described above. In Step 1, the temperature region was divided into 50° C. temperature ranges, and the shrinkage rates in response to firing at the temperatures at the boundaries of each zone (temperature range) were measured. Table 2 shows the shrinkage rate difference of each zone calculated in Step 2, and the coefficient of shrinkage of each zone calculated in Step 3. In Step 4, the acceptable rate of temperature increase in each zone was calculated with the maximum rate of temperature increase of furnace set at 50° C./min. In Step 5, the rate of temperature increase was set to be 1.0 times the acceptable rate of temperature increase in each zone.

TABLE 2

Zonal shrinkage rate
Zonal coefficient of

Zone (° C.)
difference (%)
shrinkage

1000 to 1050
0.6
1.2

1050 to 1100
2.0
4.0

1100 to 1150
3.6
7.2

1150 to 1200
4.2
8.4

1200 to 1250
5.1
10.2

1250 to 1300
1.2
2.4

1300 to 1350
1.6
3.2

1350 to 1400
0.5
1.0

Example 2

A zirconia sintered body (prosthesis) was produced in the same manner as in Example 1, except that a Noritake KATANA® zirconia HT (a zirconia pre-sintered body manufactured by Kuraray Noritake Dental Inc.) was used as a zirconia disc work, and the firing schedule was changed as shown in Table 3. Table 4 shows the shrinkage rate difference of each zone calculated in Step 2, and the coefficient of shrinkage of each zone calculated in Step 3. In Step 5, the rate of temperature increase was set to be 1.0 times the acceptable rate of temperature increase in each zone.

TABLE 3