Sintering method for dental zirconia calcined bodies

Abstract

To provide technologies to improve translucency for manufacturing a zirconia sintered body with high translucency without using special equipment.SOLUTION: Provided is a method for sintering a dental zirconia sintered body, characterized by satisfying the following a) to c): a) the average rate of temperature increase in the section from room temperature to 1300°C is arbitrary; b) the average rate of temperature increase in the section from 1300°C to 1400°C is 3°C / min or less; and c) the average rate of temperature increase in the section from 1400°C to the maximum temperature of the sintering process is 2.5°C / min or less.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a sintering method for dental zirconia green bodies.

Background Art

[0002] In recent years, technology for producing prosthetic devices by cutting using dental CAD / CAM systems has been rapidly spreading. As a result, it has become 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 high strength and is clinically applied in various cases. On the other hand, sintered zirconia that can be used in the oral cavity (hereinafter referred to as zirconia sintered body) has very high hardness and thus cannot be machined using a dental CAD / CAM system. Therefore, zirconia workpieces for dental cutting are fired at a low firing temperature without final firing and adjusted to a hardness that allows cutting (dental zirconia green bodies).

[0004] When zirconia was first applied as a dental material, although it had high strength, its translucency was lower than that of natural teeth, and it was mainly used as coping or a frame. In recent years, zirconia with improved translucency (high-translucency zirconia) has been developed by increasing the concentration of stabilizers contained in zirconia, and its applications have been expanded from molar teeth to full crowns of anterior teeth.

[0005] However, even in high-translucency zirconia, the translucency is insufficient to reproduce the enamel of natural teeth. Therefore, in cases where aesthetics are particularly required, restorations with translucency similar to natural teeth are produced by building ceramic materials on zirconia. In this situation, there is a desire to obtain restorations with translucency that more closely resembles natural teeth using full-count zirconia, and the development of zirconia with superior translucency is needed.

[0006] However, even if the concentration of the stabilizer is increased to obtain transparency exceeding that of highly transparent zirconia, further higher transparency cannot be obtained. This is thought to be because the higher the concentration of the stabilizer, the more the zirconia grain growth is promoted, and the more closed pores tend to increase.

[0007] Suppressing such closed pores requires special manufacturing methods such as HIP (Hot Isostatic Pressing). Therefore, when manufacturing highly translucent zirconia with higher stabilizer concentrations, special equipment such as HIP equipment is required.

[0008] Zirconia prosthetics in dentistry are manufactured by machining a calcined dental zirconia body, sintering the workpiece, and then adjusting it. Since these processes are mainly carried out in dental laboratories, using specialized equipment such as HIP (High-Intensity Pulsation) devices would require each dental laboratory to install the equipment, which is not a practical solution.

[0009] From a manufacturing efficiency standpoint, it is desirable to minimize the time required for zirconia sintering. For example, if a dental laboratory operates for 8 hours, and the sintering time for zirconia is 8 hours or less, then zirconia can be sintered twice a day. However, if the sintering time exceeds 8 hours, zirconia can only be sintered once a day.

[0010] Patent Document 1 discloses a method for producing a dental zirconia calcined body using zirconia powder containing 3 mol% yttrium with reduced alumina content, and a zirconia sintered body made from this calcined body. This sintered body maintains high strength while improving translucency, and has therefore been clinically applied to long-span bridges of 4 units or more and full crowns for posterior teeth. However, even with this sintered body, the translucency is insufficient, making it difficult to apply to cases where high aesthetics are required, such as in the anterior teeth.

[0011] Patent Document 2 discloses a fully sintered zirconia body using zirconia powder containing 2-7 mol% yttrium. Because this sintered body has high translucency similar to that of porcelain and lithium disilicate materials, it can be applied not only to anterior teeth but also to cases involving inlays, onlays, and veneers. However, since this sintered body requires hot isostatic pressing (HIP) treatment, it has been difficult for general dental laboratories to manufacture. [Prior art documents] [Patent Documents]

[0012] [Patent Document 1] Japanese Patent Publication No. 2010-150063 [Patent Document 2] Japanese Patent Publication No. 2008-222450 [Overview of the project] [Problems that the invention aims to solve]

[0013] There was a need for technology to improve the transparency of zirconia sintered bodies, enabling the manufacture of highly transparent bodies without the use of special equipment. [Means for solving the problem]

[0014] The inventors investigated a sintering method for dental zirconia calcined bodies that can improve translucency to impart high translucency similar to natural tooth enamel to the zirconia sintered body without using special equipment, and that does not require long sintering times.

[0015] The present invention relates to a method for sintering a dental zirconia calcined body containing 5.5 ml to 7.0 ml of a stabilizer, and satisfies the following a) to c). a) The average heating rate in the range from room temperature to 1300°C is 9°C / min or higher. b) The average heating rate in the range from 1300°C to 1400°C is between 0.6°C / min and 3°C / min. c) The average heating rate in the section from 1400°C to the maximum temperature of this sintering process is 0.5°C / min or more and 2.5°C / min or less. In the present invention, the average heating rate in step a) can be 15°C / min or more and 60°C / min or less. In the present invention, the average heating rate in step b) can be set to 1°C / min or more and 2°C / min or less. In the present invention, the average heating rate in step c) can be set to 1°C / min or more and 1.5°C / min or less. In this invention, the stabilizer can be yttria. [Effects of the Invention]

[0016] The present invention provides a sintering method for dental zirconia calcined bodies that improves light transmission, thereby enabling the production of zirconia sintered bodies with high light transmission without the use of special equipment or the need for long sintering times. [Modes for carrying out the invention]

[0017] The present invention relates to a method for sintering a dental zirconia calcined body containing 5.5 ml to 7.0 ml of a stabilizer, and satisfies the following a) to c). a) The average heating rate in the range from room temperature to 1300°C is 9°C / min or higher. b) The average heating rate in the range from 1300 °C to 1400 °C is 0.6 °C / min or more and 3 °C / min or less. c) The average heating rate in the range from 1400 °C to the maximum temperature of the sintering process is 0.5 °C / min or more and 2.5 °C / min or less.

[0018] In the sintering method of the present invention, the average heating rate in the range from room temperature to 1300 °C in step a) is 9 °C / min or more, preferably 15 °C / min to 60 °C / min. When the average heating rate is less than 9 °C / min, the time required for the entire sintering process becomes long and the production efficiency decreases. If it is 15 °C / min or more, even when steps b) and c) are within the preferred ranges, the time required for the entire sintering process can be made 480 minutes or less, which is particularly preferable. When the average heating rate exceeds 60 °C / min, it is not preferable because it may impose a burden on the sintering furnace or temporarily exceed the end temperature of step a).

[0019] In the sintering method of the present invention, the average heating rate in the range from 1300 °C to 1400 °C in step b) is 0.6 °C / min or more and 3 °C / min or less, preferably 1 °C / min to 2 °C / min. When the average heating rate exceeds 3 °C / min, the translucency of the zirconia sintered body tends to decrease. When the average heating rate is less than 0.6 °C / min, the time required for the entire sintering process becomes long and the production efficiency decreases. Since the time required for the entire sintering process can be shortened while improving the translucency, it is preferably 1 °C / min to 2 °C / min. In the dental industry, an average heating rate of generally 3 °C / min or more and 10 °C / min or less is often used.

[0020] In the sintering method of the present invention, the average heating rate in the section from 1400°C to the maximum temperature of the main sintering process is 0.5°C / min or more and 2.5°C / min or less, preferably 1°C / min or more and 1.5°C / min or less. When the heating rate exceeds 2.5°C / min, the average heating rate in the section from 1400°C to the maximum temperature of the main sintering process may become higher than the average heating rate in the section from 1300°C to 1400°C in step b). In this case, the light transmittance of the zirconia sintered body tends to decrease. When the average heating rate is less than 0.5°C / min, the time required for the entire sintering process becomes longer and the production efficiency decreases. Since the light transmittance can be improved while shortening the time required for the entire sintering process, it is preferably 1°C / min or more and 1.5°C / min or less. In the dental industry, an average heating rate of generally 3°C / min or more and 10°C / min or less is often used.

[0021] There is no particular limitation on the type of stabilizer in the present invention. Examples include yttria, ceria, calcia, indium oxide, etc., but yttria, which is used in many dental zirconias, is preferred.

[0022] The concentration of the stabilizer in the present invention is preferably in the range of 5.5 mol% to 7.0 mol% in terms of oxide conversion, and particularly preferably 6.3 mol% to 6.7 mol%. When the stabilizer concentration is less than 5.5 mol% or exceeds 7.0 mol%, almost no effect of the sintering method described in the present invention can be obtained. In the present invention, the remainder other than the components specified as the components constituting the dental zirconia green body of the present invention can be composed of zirconia (ZrO2). Further, zirconia (ZrO2) can be 93 mol% to 94.5 mol% in terms of oxide conversion.

[0023] There is no limitation on the state of the stabilizer in the dental zirconia green body. Specifically, it may be dissolved in zirconia, or may exist as a crystal or amorphous separate from zirconia as a compound.

[0024] A preferred method for adding a stabilizer to a dental zirconia calcined body is one that allows for the uniform addition of a specified amount to the dental zirconia calcined body. For example, a method may be used in which the stabilizer is added when manufacturing zirconia particles, or a method may be used in which the dental zirconia calcined body is immersed in a solution containing the stabilizer.

[0025] When using a method in which a dental zirconia calcined body is immersed in a solution containing a stabilizer, the solvent of the stabilizer solution can be any, but water, alcohol, organic solvents, etc., can be used. Water or ethanol, or a mixture thereof, is particularly preferred because it is readily available and easy to handle.

[0026] The method for preparing the solution containing the stabilizer is not particularly limited; any method of preparation is acceptable as long as the stabilizer is dissolved in the solvent.

[0027] There are no particular restrictions on the specific atmosphere in which the stabilizer solution is impregnated into the dental zirconia calcined body; atmospheric pressure, reduced pressure, or pressurized pressure are all acceptable. From the viewpoint of shortening manufacturing time, placing the surrounding environment under reduced pressure or pressurized pressure is a preferred method as it promotes the penetration of the stabilizer solution. Furthermore, repeating the operation of returning to atmospheric pressure after a reduced pressure operation (reduced pressure / atmospheric pressure operation) or returning to atmospheric pressure after a pressurized operation (pressurized / atmospheric pressure operation) multiple times is effective in shortening the time required for the stabilizer solution to penetrate the space inside the dental zirconia calcined body that communicates with the outside of the dental zirconia calcined body.

[0028] The immersion time for dental zirconia calcined bodies in a solution containing a stabilizer is not determined by a single factor, but depends on the relative density and size of the dental zirconia calcined bodies, the degree of penetration of the stabilizer solution, and the immersion method, and can be adjusted as appropriate. For example, immersion is typically 1 to 120 hours, immersion under reduced pressure is typically 0.5 to 12 hours, and contact under pressure is typically 0.2 to 6 hours.

[0029] The primary particle size of the zirconia powder used in the production of the dental zirconia calcined body of the present invention is preferably 1 to 500 nm. When the primary particle size is less than 1 nm, although the light transmittance of the zirconia sintered body is improved, it tends to be difficult to impart sufficient strength. On the other hand, when the primary particle size is 500 nm or more, it tends to be difficult to impart sufficient strength to the zirconia sintered body.

[0030] The dental zirconia calcined body in this invention preferably contains a coloring agent. Specifically, an inorganic coloring agent is preferred. Specifically, iron oxide, erbium, cobalt, manganese, chromium, and rare earth elements are preferred. Iron oxide is added to impart a yellow color, and erbium is added to impart a red color. In addition to these coloring agents, it is preferable to use elements such as cobalt, manganese, and chromium in combination for color adjustment. It is preferable that the present invention is colored to the color of teeth by containing a coloring agent.

[0031] The dental zirconia calcined body in the present invention may contain a sintering aid. Specifically, it is preferable to contain 0.01 to 0.3% by weight of alumina for the purpose of improving sinterability and suppressing low-temperature degradation. If the amount of alumina is lower than 0.01% by weight, it is difficult to obtain sufficient properties even after final firing, and it tends not to be possible to impart sufficient strength and translucency. On the other hand, if the amount of alumina exceeds 0.3% by weight, although the strength of the zirconia sintered body improves, it tends not to be possible to impart sufficient translucency.

[0032] In the present invention, the relative density of the zirconia sintered body obtained by firing a dental zirconia calcined body at 1450°C to 1600°C is preferably 98% or more of the theoretical density. The relative density is determined by the ratio of the measured density to the theoretical density. If the relative density is 98% or less, the strength and light transmittance tend to decrease.

[0033] In the present invention, the crystalline phase of the dental zirconia calcined body is preferably tetragonal and / or cubic. If the crystalline phase is monoclinic, it is undesirable because it is difficult to impart sufficient translucency even after the final calcination.

[0034] The method for manufacturing the dental zirconia calcined body in this invention is not particularly limited, and any known manufacturing method can be used without any problems. Specifically, it is preferable to form the zirconia powder by press molding. Furthermore, it is even more preferable to form a multi-layered body by press molding zirconia powders of different colors and compositions in multiple stages.

[0035] In the present invention, the dental zirconia calcined body is preferably one that has been press-formed and then subjected to isotropic pressure by cold isostatic pressing (CIP treatment).

[0036] In the present invention, the maximum load pressure for the CIP treatment is preferably 50 MPa or higher. If the maximum load pressure is less than 50 MPa, it may not be possible to impart sufficient light transmission and strength to the zirconia sintered body.

[0037] In this invention, the calcination temperature of the dental zirconia calcined body is preferably 800 to 1200°C. If the calcination temperature is below 800°C, the Vickers hardness and / or flexural strength become too low, making chipping and fracture more likely during machining. On the other hand, if the calcination temperature is above 1200°C, the Vickers hardness and / or flexural strength become too high, leading to excessive wear of the milling bur of the cutting machine and a tendency for higher running costs.

[0038] In this way, a zirconia sintered body is obtained by the manufacturing method of the present invention. The obtained zirconia sintered body is subjected to morphological modification, color adjustment, and surface polishing as necessary.

[0039] There are no particular restrictions on the type of prosthetic device manufactured using the manufacturing method of the present invention; any prosthetic device such as inlays, onlays, veneers, crowns, or bridges can be used without any problems. Therefore, there are no particular restrictions on the shape of the dental zirconia calcined body used to manufacture the prosthetic device by machining; any shape of dental zirconia calcined body can be used, such as a block shape for inlays, onlays, veneers, or crowns, or a disc shape for bridges. [Examples]

[0040] The present invention will be described in more detail and specifically below with reference to examples, but the present invention is not limited thereto.

[0041] [Fabrication of zirconia workpieces (calcined dental zirconia bodies)] (Cut object 1) Zirconia powder containing 6.5 moles of solid-solution yttria (with 0.05 wt% alumina) was filled into a mold (φ100 mm) and press-molded (surface pressure: 50 MPa) to obtain a molded body. 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 workpiece.

[0042] (Cut object 2) A zirconia workpiece was prepared in the same manner as workpiece 1, except that zirconia powder containing 5.5 moles of solid-solution yttria (containing 0.05 wt% alumina) was used.

[0043] (Cut object 3) A zirconia workpiece was prepared in the same manner as workpiece 1, except that zirconia powder containing 7.0 moles of solid-solution yttria (containing 0.05 wt% alumina) was used.

[0044] (To be cut 4) A zirconia workpiece was prepared in the same manner as workpiece 1, except that zirconia powder containing 6.0 moles of solid-solution yttria (containing 0.05 wt% alumina) was used.

[0045] (Cut object 5) A zirconia workpiece was prepared in the same manner as workpiece 1, except that zirconia powder containing 6.3 moles of solid-solution yttria (containing 0.05 wt% alumina) was used.

[0046] (To be cut 6) A zirconia workpiece was prepared in the same manner as workpiece 1, except that zirconia powder containing 6.7 moles of solid-solution yttria (containing 0.05 wt% alumina) was used.

[0047] (To be cut 7) A zirconia workpiece was prepared in the same manner as workpiece 1, except that zirconia powder containing 5.0 moles of solid-solution yttria (containing 0.05 wt% alumina) was used.

[0048] (Cut object 8) A zirconia workpiece was prepared in the same manner as workpiece 1, except that zirconia powder containing 7.5 moles of solid-solution yttria (containing 0.05 wt% alumina) was used.

[0049] Yttria content (mol%) measurement: Test specimens for evaluating yttria content were prepared by machining each zirconia workpiece into a circular plate shape (φ14mm × 1.6mm). The amount of yttria on the top and bottom surfaces of each specimen was measured using an X-ray fluorescence analyzer (manufactured by Rigaku Corporation), and the average value of the top and bottom surfaces was defined as the yttria content. The yttria content (mol%) is expressed in terms of oxide equivalent.

[0050] [Sintering conditions] Each zirconia workpiece was machined to a predetermined shape, and then fired in a firing furnace according to the sintering schedule shown in Tables 1-4 to produce a sintered zirconia body.

[0051] [Table 1] [Table 2] [Table 3] [Table 4]

[0052] [Evaluation of light transmittance] Test specimens for evaluating light transmittance were prepared by machining each zirconia workpiece into a round plate shape (φ14mm × 1.6mm). Each specimen was sintered in a firing furnace. After that, the thickness of each specimen (1.0mm) was adjusted using a surface grinder. Light transmittance was evaluated by measuring the contrast ratio. The contrast ratio was measured using a spectrophotometer (Konica Minolta). The Y value when measuring with a white board placed beneath each specimen was defined as Yw, and the Y value when measuring with a black board placed beneath the specimen was defined as Yb. The contrast ratio was calculated using 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) Furthermore, the degree of improvement in translucency for each sintering schedule was calculated using the same dental zirconia calcined body. Transparency improvement = (Contrast ratio of test specimens sintered according to sintering schedule 9) (Contrast ratio of test specimens sintered using one of sintering schedules 1-19) When the degree of light transmission improvement is 1.15 or higher, the sintering schedule exhibits particularly high light transmission improvement performance. If the degree of light transmission improvement is 1.10 or higher and less than 1.15, the sintering schedule has high light transmission improvement performance. If the degree of light transmission improvement is between 1.05 and less than 1.10, the sintering schedule exhibits moderately high light transmission improvement performance. If the degree of light transmission improvement is less than 1.05, the light transmission improvement performance of that sintering schedule is low or nonexistent. The effect of the present invention was determined to be recognized when the degree of improvement in light transmittance was 1.05 or higher.

[0053] Example 1: A test specimen prepared from the workpiece 1 was sintered according to sintering schedule 1. Example 2: A test specimen made from the workpiece 1 was sintered according to sintering schedule 2. Example 3: A test specimen made from the workpiece 1 was sintered according to sintering schedule 3. Example 4: A test specimen made from the workpiece 1 was sintered according to sintering schedule 4. Example 5: A test specimen made from the workpiece 1 was sintered according to sintering schedule 5. Example 6: A test specimen prepared from the workpiece 2 was sintered according to sintering schedule 1. Example 7: A test specimen prepared from the workpiece 3 was sintered according to sintering schedule 1. Example 8: A test specimen prepared from the workpiece 4 was sintered according to sintering schedule 1. Example 9: A test specimen prepared from the workpiece 5 was sintered according to sintering schedule 1. Example 10: A test specimen prepared from the workpiece 6 was sintered according to sintering schedule 1. Example 11: A test specimen made from the workpiece 1 was sintered according to sintering schedule 10. Example 12: A test specimen prepared from the workpiece 1 was sintered according to sintering schedule 11. Example 13: A test specimen prepared from the workpiece 1 was sintered according to sintering schedule 13. Example 14: A test specimen made from the workpiece 1 was sintered according to sintering schedule 15. Example 15: A test specimen made from the workpiece 1 was sintered according to sintering schedule 16. Example 16: A test specimen made from the workpiece 1 was sintered according to sintering schedule 19. Example 17: A test specimen made from the workpiece 1 was sintered according to sintering schedule 20. Comparative Example 1: A test specimen prepared from the workpiece 1 was sintered according to sintering schedule 9. Comparative Example 2: A test specimen prepared from workpiece 1 was sintered according to sintering schedule 6. Comparative Example 3: A test specimen made from workpiece 1 was sintered according to sintering schedule 7. Comparative Example 4: A test specimen prepared from workpiece 1 was sintered according to sintering schedule 8. Comparative Example 5: A test specimen prepared from the workpiece 2 was sintered according to sintering schedule 9. Comparative Example 6: A test specimen prepared from the workpiece 3 was sintered according to sintering schedule 9. Comparative Example 7: A test specimen prepared from the workpiece 7 was sintered according to sintering schedule 1. Comparative Example 8: A test specimen prepared from the workpiece 8 was sintered according to sintering schedule 1. Comparative Example 9: A test specimen prepared from the workpiece 7 was sintered according to sintering schedule 9. Comparative Example 10: A test specimen prepared from the workpiece 8 was sintered according to sintering schedule 9. Comparative Example 11: A test specimen prepared from the workpiece 4 was sintered according to sintering schedule 9. Comparative Example 12: A test specimen prepared from the workpiece 5 was sintered according to sintering schedule 9. Comparative Example 13: A test specimen prepared from the workpiece 6 was sintered according to sintering schedule 9. Comparative Example 14: A test specimen prepared from the workpiece 1 was sintered according to sintering schedule 12. Comparative Example 15: A test specimen prepared from the workpiece 1 was sintered according to sintering schedule 14. Comparative Example 16: A test specimen prepared from the workpiece 1 was sintered according to sintering schedule 17. Comparative Example 17: A test specimen prepared from the workpiece 1 was sintered according to sintering schedule 18.

[0054] Tables 5 and 6 show the results of the characteristic tests performed on the dental zirconia calcined bodies produced in the examples and comparative examples.

[0055] [Table 5]

[0056] [Table 6]

[0057] In Examples 1 to 17, in the sintering of dental zirconia calcined bodies containing 5.5 ml to 7.0 ml of stabilizer, a) The average heating rate in the range from room temperature to 1300°C is 9°C / min or more. b) The average heating rate in the range from 1300°C to 1400°C is between 0.6°C / min and 3°C / min. c) The average heating rate in the section from 1400°C to the maximum temperature of this sintering process is 0.5°C / min or more and 2.5°C / min or less. Therefore, it showed a significant improvement in light transmittance, and the sintering time was within 8 hours.

[0058] In Comparative Examples 1 to 17, due to conditions other than those described above, there was little or no improvement in light transmittance, and / or the sintering time exceeded 8 hours. [Industrial applicability]

[0059] This invention relates to a method for sintering dental zirconia calcined bodies and is a technology applicable in the dental field.

Claims

1. A method for sintering a dental zirconia calcined body containing 5.5 mol% to 7.0 mol% of a stabilizer, wherein the method satisfies the following a) to c). a) The average heating rate in the range from room temperature to 1300°C is 9°C / min or higher. b) The average heating rate in the range from 1300°C to 1400°C is between 0.6°C / min and 3°C / min. c) The average heating rate in the section from 1400°C to the maximum temperature of this sintering process is 0.5°C / min or more and 2.5°C / min or less.

2. The method for sintering a dental zirconia calcined body according to claim 1, characterized in that the average heating rate in step a) is 15°C / min or more and 60°C / min or less.

3. The method for sintering a dental zirconia calcined body according to claim 1, characterized in that the average heating rate in step b) is 1°C / min or more and 2°C / min or less.

4. The method for sintering a dental zirconia calcined body according to claim 2, characterized in that the average heating rate in step b) is 1°C / min or more and 2°C / min or less.

5. The method for sintering a dental zirconia calcined body according to claim 1, characterized in that the average heating rate in step c) is 1°C / min or more and 1.5°C / min or less.

6. The method for sintering a dental zirconia calcined body according to claim 2, characterized in that the average heating rate in step c) is 1°C / min or more and 1.5°C / min or less.

7. The method for sintering a dental zirconia calcined body according to claim 3, characterized in that the average heating rate in step c) is 1°C / min or more and 1.5°C / min or less.

8. The method for sintering a dental zirconia calcined body according to claim 4, characterized in that the average heating rate in step c) is 1°C / min or more and 1.5°C / min or less.

9. A method for sintering a dental zirconia calcined body according to any one of claims 1 to 8, characterized in that the stabilizer is yttria.

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