Zirconia workpiece having layers with different yttria and alumina compositions

Abstract

To provide a zirconia workpiece for dentistry cutting in which a special sintering method such as HIP treatment is not required, and the color tone reproducibility similar to natural teeth and high strength can be imparted to a zirconia perfect sintered body.SOLUTION: A zirconia workpiece for dentistry cutting is configured as follows: layers LA and LB contained in sections A and B to 40% of a face-to-face distance from two parallel planes satisfy following expressions (1) and (2); the contrast ratios CA and CB in the specimen thickness of 1 mm of the sintered bodies SA and SB at 1550°C of test pieces being cut out of the layers LA and LB satisfy the following expression (3); and the change direction of the yttria concentration, the alumina concentration and the contrast ratio comprises two unchanged intermediate layer toward a laminating direction between the layers LA and LB. The expression (1) of the yttria concentration: layer LA-layer LB>1.0 mol%. The expression (2) of the alumina concentration: 0.0 wt%<layer LB-layer LA<0.3 wt%. The expression (3) of the contrast ratio: 0.71≤CA / CB≤0.86.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a zirconia workpiece for dental cutting. [Background technology]

[0002] In recent years, the technology of producing prosthetic devices by cutting using dental CAD / CAM systems has rapidly spread. This has made it possible to easily produce prosthetic devices by processing workpieces made of ceramic materials such as zirconia, alumina, and lithium disilicate, and resin materials such as acrylic resin and hybrid resin.

[0003] In particular, zirconia has been used clinically in a variety of cases due to its high strength. On the other hand, fully sintered zirconia (hereinafter referred to as fully sintered zirconia) is so hard that it cannot be machined using dental CAD / CAM systems. For this reason, zirconia cutting objects for dental machining are not fully sintered, but are pre-fired at a low firing temperature and adjusted to a hardness that allows machining.

[0004] A typical zirconia cutting object for dental cutting is produced by molding zirconia powder by press molding or the like, and then calcining at 800 to 1200°C.

[0005] The properties of the zirconia workpiece for dental cutting, that is, the properties of the fully sintered zirconia body, are affected by the properties of the zirconia powder used.

[0006] Patent Document 1 discloses a zirconia completely sintered body produced from a zirconia cutting body for dental cutting using zirconia powder containing 3 mol% yttrium with a reduced amount of alumina. The sintered body has improved translucency while maintaining high strength, and is therefore used in clinical applications such as long span bridges with 4 or more units and full crowns in the molar region. However, the translucency of the sintered body is insufficient, making it difficult to apply the sintered body to cases requiring high aesthetics, such as the anterior teeth.

[0007] Patent Document 2 discloses a zirconia completely sintered body produced from a zirconia cutting body for dental cutting using zirconia powder containing 4 to 6.5 mol% yttrium. The sintered body has high translucency, and has been used in cases where high aesthetics are required, such as anterior teeth. However, the translucency of the sintered body is still lower than that of lithium disilicate materials, etc., making it difficult to reproduce the color tone of the enamel layer of natural teeth.

[0008] Patent Document 3 discloses a fully sintered zirconia body using zirconia powder containing 2 to 7 mol% yttrium. The sintered body has high translucency similar to that of porcelain and lithium disilicate materials, and is therefore applicable not only to anterior teeth but also to cases such as inlays, onlays, and veneers. However, the sintered body requires hot isostatic pressing (HIP) treatment, making it difficult to manufacture in a general dental laboratory.

[0009] Patent Document 4 discloses a zirconia sintered body having layers with different yttria and light blocking agent concentrations. The zirconia cut body contains a layer with high translucency and a layer with low translucency, making it possible to create a prosthetic device in which translucency increases from the cervical to the incisal edge, making it possible to reproduce a color similar to that of natural teeth. However, since the difference in yttria concentration between layers of the zirconia sintered body is within 1 mol%, It has been difficult to achieve both strength and translucency, because attempting to increase the translucency of the incisal edge results in a decrease in the strength of the cervical area, and vice versa. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] Patent No. 5608976 [Patent Document 2] International Publication No. 2015-199018 [Patent Document 3] Patent No. 5396691 [Patent Document 4] International Publication No. 2016-1004724 Summary of the Invention [Problem to be solved by the invention]

[0011] The object of the present invention is to provide a zirconia workpiece for dental cutting that can impart color reproducibility similar to that of natural teeth and high strength to a fully sintered zirconia body without requiring a special sintering method such as HIP treatment. [Means for solving the problem]

[0012] The present inventors have studied a zirconia cutting object for dental cutting that can impart color reproducibility and high strength similar to that of natural teeth to a fully sintered zirconia body without requiring a special sintering method such as HIP treatment. As a result, they have found that it is particularly important to impart two layers in which the yttria concentration and the alumina concentration have a specific relationship to a fully sintered zirconia body for dental cutting in order to impart color reproducibility and high strength similar to that of natural teeth to a fully sintered zirconia body. The details of the present invention are described below.

[0013] The present invention relates to a zirconia workpiece for dental cutting, which contains yttria and alumina, and in which, between two opposing parallel flat surfaces, a section from one flat surface to 40% of the inter-surface distance is defined as section A, and a section from the other flat surface to 40% of the inter-surface distance is defined as section B, a layer LA included in section A and a layer LB included in section B satisfy the following formulas (1) and (2), A test piece cut out from the layer LA was sintered at 1550°C to obtain a sintered body SA, and a test piece cut out from the layer LB was sintered at 1550°C to obtain a sintered body SB. The contrast ratio of the sintered body SA at a sample thickness of 1 mm was defined as a contrast ratio CA, and the contrast ratio of the sintered body SB at a sample thickness of 1 mm was defined as a contrast ratio CB. The contrast ratios CA and CB satisfied the following formula (3), The present invention relates to a zirconia workpiece for dental cutting, which includes at least two intermediate layers between the layer LA and the layer LB, in which the directions of change in yttria concentration, alumina concentration, and contrast ratio do not change along the stacking direction. Yttria concentration in layer LA - Yttria concentration in layer LB > 1.0 mol% (1) 0.0wt%<alumina concentration in layer LB-alumina concentration in layer LA<0.3wt% (2) 0.71≦(contrast ratio CA) / (contrast ratio CB)≦0.86...(3)

[0014] In the present invention, it is preferable that any layer contains 3 mol % to 7 mol % of yttria as a stabilizer.

[0015] In the present invention, the contrast ratio CA is preferably 0.70 or less.

[0016] In the present invention, it is preferable that the contrast ratio CB is 0.74 or more.

[0017] In the present invention, the zirconia workpiece for dental cutting has both end faces in the lamination direction constituted by parallel flat surfaces, and it is preferable that both end faces in the lamination direction constitute the two opposing parallel flat surfaces.

[0018] In the present invention, the layer LA and the layer LB are preferably parallel to the two opposing parallel planes. Effect of the Invention

[0019] The zirconia cutting object for dental cutting of the present invention can impart color reproducibility similar to that of natural teeth and high strength to a zirconia sintered body. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] The constituent features of the present invention will now be described in detail. The present invention is a zirconia workpiece for dental cutting containing yttria and alumina, characterized in that it has layer A and layer B in which the yttria concentration and the alumina concentration have a predetermined relationship. Specifically, the zirconia workpiece for dental cutting contains yttria and alumina, and between two opposing parallel planes, the section from one plane to 40% of the inter-plane distance is defined as section A, and the section from the other plane to 40% of the inter-plane distance is defined as section B. Section A contains layer LA, and section B contains layer LB. The zirconia cutting body for dental cutting of the present invention is preferably configured such that both end faces in the lamination direction are parallel flat surfaces. In this case, both end faces in the lamination direction form two parallel flat surfaces facing each other. In this case, it is preferable that the layer LA and the layer LB are parallel to the two parallel flat surfaces facing each other. In the case where both end faces in the stacking direction are not formed by parallel planes, the two parallel planes facing each other in the present invention can be parallel imaginary planes at any position between the two outermost points in the stacking direction. In this case, it is preferable that each imaginary plane is a plane including at least a part of each of both end faces in the stacking direction. Alternatively, each imaginary plane can be entirely present in the zirconia workpiece for dental cutting. In the present invention, the inter-plane distance refers to the distance between the center of gravity of one of two opposing parallel planes and the intersection point between the straight line that passes through the center of gravity and is perpendicular to the one plane and the other plane. In the present invention, when a section includes a layer, it is sufficient that the section includes a part of the layer, preferably includes 50% or more of the volume of the layer, and most preferably includes the entire layer.

[0021] In the present invention, the yttria concentration and the alumina concentration in the layers LA and LB satisfy the following formulas (1) and (2). The yttria concentration of a layer in the present invention refers to the concentration of all yttrium (and yttrium compounds) contained in the layer when converted into oxides, and the alumina concentration of a layer in the present invention refers to the concentration of all aluminum (and aluminum compounds) contained in the layer when converted into oxides. Yttria concentration in layer LA - Yttria concentration in layer LB > 1.0 mol% (1) 0.0wt%<alumina concentration in layer LB-alumina concentration in layer LA≦0.3wt% (2) Furthermore, it is more preferable that the following formulas (5) and (6) are satisfied. Yttria concentration in layer LA – Yttria concentration in layer LB ≥ 2.5 mol% (5) 0.08wt%<alumina concentration in layer LB-alumina concentration in layer LA≦0.2wt% (6) When the yttria concentration in layer LA minus the yttria concentration in layer LB is 1.0 mol% or less, if the yttria concentration is increased to increase the light transmittance of layer LA, the yttria concentration in layer LB will inevitably be relatively high, and sufficient strength cannot be imparted to layer LB, which requires high strength. If the yttria concentration is decreased to impart high strength to layer LB, the yttria concentration in layer LA will inevitably be relatively low, and sufficient light transmittance cannot be imparted to layer LA, which requires high light transmittance. In other words, it becomes difficult to achieve both strength and light transmittance. If the alumina concentration of the layer LA is equal to or higher than the alumina concentration of the layer LB, the change in translucency in the zirconia sintered body becomes too small, making it difficult to impart color reproducibility similar to that of natural teeth to the zirconia sintered body. Also, if the alumina concentration of the layer LB-the alumina concentration of the layer LA≧0.3wt%, the change in translucency in the zirconia sintered body becomes too large, making it difficult to impart color reproducibility similar to that of natural teeth to the zirconia sintered body.

[0022] In the present invention, the zirconia workpiece for dental cutting preferably satisfies the following formula (3): SA is a sintered body obtained by sintering a test piece cut from the layer LA at 1550°C (specifically, firing temperature: 1550°C, holding time: 2 hours, heating rate: 5°C / min), and SB is a sintered body obtained by sintering a test piece cut from the layer LB at 1550°C (specifically, firing temperature: 1550°C, holding time: 2 hours, heating rate: 5°C / min). CA is the contrast ratio of the sintered body SA at a sample thickness of 1 mm, and CB is the contrast ratio of the sintered body SB at a sample thickness of 1 mm. 0.71≦(contrast ratio CA) / (contrast ratio CB)≦0.86...(3) Furthermore, it is more preferable that the following formula (6) is satisfied. 0.76≦(contrast ratio CA) / (contrast ratio CB)≦0.83 (6) If the contrast ratio CA / CB is less than 0.71, the change in translucency in the zirconia sintered body becomes too large, and color reproduction similar to that of natural teeth may not be obtained, which is not preferable. If the contrast ratio CA / CB is greater than 0.86, the change in translucency in the zirconia sintered body becomes too small, and color reproduction similar to that of natural teeth may not be obtained, which is not preferable. If the thickness of the layers LA and LB is not sufficient to prepare the 1 mm test specimen, the test specimen may be prepared from a zirconia cutting object for dental cutting of the same composition having a sufficient thickness.

[0023] The zirconia cutting body for dental cutting in the present invention includes at least two intermediate layers between the layer LA and the layer LB, in which the change directions of the yttria concentration, the alumina concentration, and the contrast ratio do not change in the stacking direction, and preferably includes three or more intermediate layers. If there is one or less intermediate layers, the change in translucency and color tone becomes too rapid, and aesthetics may be impaired.

[0024] The zirconia workpiece for dental cutting in the present invention preferably contains 3 mol% to 7 mol% yttria as a stabilizer in each layer, and more preferably 3.5 mol% to 6.5 mol%. If the yttria content is less than 3.0 mol%, the translucency of the zirconia sintered body becomes too low, which is not preferable. If the yttria content is higher than 7.0 mol%, the translucency increases, but the strength decreases, which is not preferable. The zirconia workpiece for dental cutting in the present invention may contain 3 mol% to 7 mol% yttria as a stabilizer in each layer from the layer LA to the layer LB.

[0025] The zirconia cutting object for dental cutting in the present invention may contain yttrium compounds that are not dissolved in solid solution.Specifically, the yttrium compounds used in the present invention are preferably water-soluble compounds consisting of yttrium oxide and / or yttrium halogen compounds, nitrates, sulfates, or organic acid salts.Specific examples of water-soluble yttrium compounds include yttrium chloride, yttrium nitrate, yttrium acetate, yttrium carboxylate, yttrium sulfate, and yttrium carbonate.

[0026] Among water-soluble yttrium compounds, organic acid salts of yttrium compounds are particularly preferred from the viewpoint of low decomposition temperature and little contamination of the sintering furnace. Specific examples include yttrium acetate and yttrium carbonate. The decomposition temperature of organic acid salts is lower than that of inorganic salts such as halogen compounds, nitrates, and sulfates. If the decomposition temperature is high, pores are left in the sintering process, making it difficult to impart sufficient translucency and strength to the fully sintered zirconia body.

[0027] In the present invention, the contrast ratio CA is preferably 0.70 or less, and more preferably 0.68 or less. If the contrast ratio CA is more than 0.70, the translucency of the zirconia sintered body is low, which makes it difficult to reproduce the enamel layer of natural teeth, which is not preferable.

[0028] The alumina concentration of the zirconia cutting object for dental cutting in the present invention is preferably 0.01wt% or more. If the alumina content is lower than 0.01wt%, the zirconia completely sintered body cannot be sintered sufficiently, and it tends not to be possible to impart sufficient strength or translucency.

[0029] In the present invention, the contrast ratio CB is preferably 0.74 or more, and more preferably 0.80 or more. If the contrast ratio CB is less than 0.74, the translucency of the zirconia sintered body is high, which makes it difficult to reproduce the cervical part of a natural tooth, which is not preferable.

[0030] The zirconia powder used in the manufacture of the zirconia cutting object for dental cutting of the present invention can be used without any restrictions as long as it is made from a known zirconia powder. Specifically, the zirconia powder used in the manufacture of the zirconia cutting object for dental cutting of the present invention is preferably made by hydrolysis. More specifically, the method is a method in which a solution in which a zirconium salt and an yttrium compound are mixed and dissolved is heated to carry out a hydrolysis reaction, and the resulting sol is dried and fired to prepare zirconia powder, which is then pulverized and granulated. In addition, by mixing an aluminum compound before this pulverization process, a zirconia powder containing alumina is prepared.

[0031] The primary particle size of the zirconia powder used in the manufacture of the zirconia cutting body for dental cutting of the present invention is preferably 1 to 500 nm. If the primary particle size is less than 1 nm, the translucency of the zirconia sintered body is improved, but it tends to be difficult to impart sufficient strength to the zirconia sintered body. On the other hand, if the primary particle size is 500 nm or more, it tends to be difficult to impart sufficient strength to the zirconia sintered body.

[0032] The zirconia cutting object for dental cutting in the present invention preferably contains a coloring material. Specifically, iron oxide for imparting a yellow color and erbium for imparting a red color can be mentioned. In addition to these coloring materials, coloring materials containing elements such as cobalt, manganese, and chromium can be used in combination to adjust the color tone without any problem. By including a coloring material in the present invention, it is possible to easily color the object to a tooth color.

[0033] The relative density of the zirconia sintered body obtained by sintering the zirconia cutting body for dental cutting in the present invention at 1550°C is preferably 98% or more of the theoretical density. The relative density is calculated by measuring density / theoretical density. If the relative density is less than 98%, the strength and translucency tend to decrease.

[0034] The crystalline phase of the zirconia cutting object for dental cutting in the present invention is preferably tetragonal and / or cubic. If the crystalline phase is monoclinic, it is not preferable because sufficient translucency cannot be imparted after complete sintering of the zirconia.

[0035] The manufacturing method of the zirconia cutting object for dental cutting in the present invention is not particularly limited, and any known manufacturing method can be used without any problem. Specifically, it is preferable to mold zirconia powder by press molding. Furthermore, it is more preferable to press zirconia powders with different colors and compositions in multiple stages to form a multi-layered mold.

[0036] The zirconia workpiece for dental cutting in the present invention is preferably one which has been isostatically pressed by cold isostatic pressing (CIP treatment) after press molding.

[0037] The maximum load pressure of the CIP treatment in the present invention is preferably 50 MPa or more. If the maximum load pressure is less than 50 MPa, it may not be possible to impart sufficient translucency and strength to the zirconia sintered body.

[0038] The holding time at the maximum load pressure in the CIP treatment in the present invention is not particularly limited, but is usually preferably 0 to 150 seconds, and more preferably 0 to 60 seconds.

[0039] The time required for the series of steps is not particularly limited, but is usually preferably 30 seconds to 10 minutes, and more preferably 3 to 7 minutes. If the time is too short, the molded body may be destroyed, and if it is too long, production efficiency is reduced, which is not preferable.

[0040] The pre-sintering temperature of the zirconia cutting object for dental cutting in the present invention is preferably 800 to 1200°C. If the pre-sintering temperature is less than 800°C, the Vickers hardness and / or bending strength will be too low, and chipping or fracture will tend to occur during cutting. On the other hand, if the pre-sintering temperature is 1200°C or higher, the Vickers hardness and / or bending strength will be too high, and the milling bur of the cutting machine will be worn out rapidly, and the running cost will tend to be high.

[0041] The zirconia object for dental cutting of the present invention can be obtained by such a production method. The obtained zirconia object for dental cutting is cut to a desired size, cut, and surface polished as necessary.

[0042] The method for completely sintering the zirconia cutting body for dental cutting of the present invention is not particularly limited, but a simple and preferred method is firing at normal pressure. The firing temperature is not particularly limited, but is preferably 1450 to 1600°C, and more preferably 1500 to 1600°C. The retention time at the firing temperature is not particularly limited, but is preferably 1 minute to 12 hours, and more preferably 2 to 4 hours. The heating rate is not particularly limited, but is preferably 1 to 400°C / min, and more preferably 3 to 100°C / h.

[0043] There is no particular restriction on the type of prosthetic device to be cut using the zirconia workpiece for dental cutting of the present invention, and there is no problem with any prosthetic device such as an inlay, onlay, veneer, crown, bridge, etc. Therefore, there is no particular restriction on the shape of the zirconia workpiece for dental cutting from which a prosthetic device is cut, and any shape of zirconia workpiece for dental cutting can be used, such as a block shape corresponding to an inlay, onlay, veneer, crown, etc., or a disk shape corresponding to a bridge. EXAMPLES

[0044] The present invention will be described in more detail and specifically below with reference to examples, but the present invention is not limited thereto.

[0045] Table 1 shows the yttria content, alumina content and iron oxide content in the zirconia powder used.

[0046] [Table 1]

[0047] Preparation of zirconia powder for forming the intermediate layer For each Example, the zirconia powder for forming the first layer and the zirconia powder for forming the fifth layer were mixed together to prepare the zirconia powder for forming the intermediate layer, as shown below.

[0048] [Table 2]

[0049] Preparation of zirconia workpiece A predetermined amount of powder for forming the first layer was filled into a mold (φ100 mm), and the top surface was smoothed by grinding. Next, the powder for forming the second layer was filled on the powder for forming the first layer, and the top surface was smoothed by grinding. This operation was repeated until the nth layer was formed, and then press molding (surface pressure: 50 MPa) was performed to obtain a molded body with n layers of zirconia powder laminated (total thickness 14.5 mm, first layer thickness 4.3 mm, nth layer thickness 5.1 mm, each intermediate layer thickness = 5.1 mm / number of intermediate layers). Furthermore, the molded body was subjected to CIP treatment (maximum load pressure: 200 MPa, load pressure after release: 0 MPa, holding time: 1 minute). After that, it was calcined in an electric furnace (1000 ° C, 30 minutes) to produce a zirconia presintered body.

[0050] Estimation of yttrium content The test specimens for evaluating the amount of yttrium were prepared by cutting each zirconia workpiece into a round plate (φ14mm × 1.6mm). The mole fraction of yttrium (yttrium oxide: Y2O3) contained in each test specimen was measured using a fluorescent X-ray analyzer (manufactured by Rigaku Corporation).

[0051] Alumina content evaluation Test specimens for evaluating the amount of alumina were prepared by cutting each zirconia workpiece into a round plate (φ14mm×1.6mm). The weight fraction of alumina contained in each test specimen was measured using a fluorescent X-ray analyzer (manufactured by Rigaku Corporation).

[0052] Translucency evaluation The test specimens for evaluating translucency were prepared by cutting each zirconia dental cutting workpiece into a round plate shape (φ14mm×1.6mm). Each test specimen was completely sintered in a firing furnace (firing temperature: 1550°C, heating rate: 5°C / min, holding time: 2 hours). After that, the thickness of each test specimen (1.0mm) was adjusted with a surface grinder. The translucency was evaluated by measuring the contrast ratio. The contrast ratio was measured using a spectrophotometer (Konica Minolta). The Y value when a white board was placed under each test specimen and color measurement was performed was defined as Yw, and the Y value when a black board was placed under each test specimen and color measurement was performed was defined as Yb. The contrast ratio was calculated from the following formula. The closer the contrast ratio is to 0, the more transparent the material is; the closer the contrast ratio is to 1, the more opaque the material is. Contrast ratio = (Yb / Yw) (Formula)

[0053] Esthetic evaluation Each zirconia workpiece for dental cutting was machined into the shape of a left upper central incisor using a processing machine (Roland, DWX51D). It was then sintered at 1550°C for 2 hours using a zirconia sintering furnace (Shofu, Esthemat Sinter) to obtain a crown specimen. The aesthetics of the crown specimen was evaluated visually as follows. ◎: Fully applicable to cases where high aesthetics are required ○: Can be used in cases where high aesthetics are required △: Not suitable for cases where high aesthetics are required ×: Not applicable to cases where high aesthetics are required

[0054] Tables 3 and 4 show the results of the property tests of the zirconia workpieces for dental cutting produced in the examples and comparative examples.

[0055] [Table 3]

[0056] [Table 4]

[0057] Examples 1 to 13 Yttria concentration in layer LA - Yttria concentration in layer LB > 1.0 mol% (1) 0.0wt%<alumina concentration in layer LB-alumina concentration in layer LA<0.3wt% (2) 0.71≦(contrast ratio CA) / (contrast ratio CB)≦0.86...(3) Since it satisfies these requirements, it has an appearance similar to natural teeth and can be used in cases where high aesthetics are required.

[0058] Comparative Example 1 Alumina concentration in layer A - Alumina concentration in layer B = 0wt% Therefore, it was confirmed that the (contrast ratio A) / (contrast ratio B) is large and it is not suitable for cases where high aesthetics are required.

[0059] It was confirmed that Comparative Example 2 is not suitable for cases requiring high aesthetics because the (contrast ratio A) / (contrast ratio B) is too small, resulting in too large a change in translucency within the crown.

[0060] Comparative Example 3 Yttria concentration in layer A - Yttria concentration in layer B = 1.0 mol% Therefore, it was confirmed that this method cannot be applied to cases where (contrast ratio A) / (contrast ratio B) is large and high aesthetics are required.

[0061] Comparative Example 4 Alumina concentration in layer A - Alumina concentration in layer B = -0.35wt% Therefore, it was confirmed that the (contrast ratio A) / (contrast ratio B) is small and that it is not suitable for cases requiring high aesthetics.

[0062] It was confirmed that Comparative Examples 5 and 6, in which the number of intermediate layers was less than 2, exhibited drastic changes in translucency and color tone within the crown and were therefore not applicable to cases requiring high aesthetics. [Industrial Applicability]

[0063] INDUSTRIAL APPLICABILITY The present invention relates to a zirconia cutting body for dental cutting, which can impart color reproducibility similar to that of natural teeth and high strength to a fully sintered zirconia body, and is a technology that can be used in the dental field.

Claims

1. A zirconia workpiece for dental cutting, which contains yttria and alumina, and between two opposing parallel flat surfaces, a section from one flat surface to 40% of the inter-surface distance is defined as section A, and a section from the other flat surface to 40% of the inter-surface distance is defined as section B. A layer LA included in section A and a layer LB included in section B satisfy the following formulas (1) and (2), A test piece cut out from the layer LA was sintered at 1550° C. to form a sintered body SA, and a test piece cut out from the layer LB was sintered at 1550° C. to form a sintered body SB. The contrast ratio of the sintered body SA at a sample thickness of 1 mm was defined as a contrast ratio CA, and the contrast ratio of the sintered body SB at a sample thickness of 1 mm was defined as a contrast ratio CB. The contrast ratios CA and CB satisfied the following formula (3), A zirconia workpiece for dental cutting includes at least two intermediate layers between the layer LA and the layer LB, which do not change the yttria concentration, alumina concentration, and contrast ratio between adjacent layers in the stacking direction from an increase to a decrease, and do not change the yttria concentration, alumina concentration, and contrast ratio between adjacent layers in the stacking direction from a decrease to an increase. Yttria concentration in layer LA−Yttria concentration in layer LB>1.0 mol% (1) 0.0 wt%<alumina concentration of layer LB−alumina concentration of layer LA<0.3 wt% (2) 0.71≦(contrast ratio CA) / (contrast ratio CB)≦0.86 (3)

2. 2. The zirconia workpiece for dental cutting according to claim 1, wherein all layers contain 3 mol % to 7 mol % yttria as a stabilizer.

3. 3. The zirconia workpiece for dental cutting according to claim 1, characterized in that the contrast ratio CA is 0.70 or less.

4. 4. The zirconia workpiece for dental cutting according to claim 1, wherein the contrast ratio CB is 0.74 or more.

5. The zirconia workpiece for dental cutting has both end faces in the lamination direction formed of parallel flat surfaces, 5. The zirconia workpiece for dental cutting according to claim 1, wherein both end faces in the lamination direction constitute the two opposing parallel flat surfaces.

6. 6. The zirconia workpiece for dental cutting according to claim 5, wherein the layer LA and the layer LB are parallel to the two opposing parallel planes.

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