Ceramic frameworks and methods of manufacture thereof

Technical field

This invention relates to ceramic frameworks and methods of manufacture-thereof.

Background

In the fields of medical engineering and dental engineering, ceramic materials are used to generate ceramic frameworks. A problem in using such ceramics is the amount of contraction, during sintering, in size (shrinkage) of the ceramic material, which can be up to about 20%. Such dimensional changes cannot be tolerated in certain of these industries, for example, dental shaped restorations such as bridgework require that distances between columns or height of the contact points to the surrounding natural teeth be of a size that is within micrometer range.

A method used to address the problem of shrinkage of ceramic frameworks is fully sintering ceramic blocks prior to machining them to produce the final shape of the ceramic framework. Milling of fully sintered ceramic blocks with the aid of diamond tools has become a generally accepted practice. However, this method is highly expensive because the fully sintered ceramic block is extremely hard. When ceramic blocks made out of, for example, aluminum oxide or zirconium oxide are sintered and then milled, the diamond tools wear out very quickly, resulting in unacceptable geometry tolerances for the resulting milled ceramic framework because the geometry of the tool, e.g., the diameter of the tool, changes during machining. Further, at points of the ceramic framework, for example at crown edges, material eruptions or micro-tears arise. Due to the hardness of the fully sintered ceramic block, chipping or flaking also occurs during the milling process.

To address the problem of wearing of the milling tool, another method used to produce ceramic frameworks involves pre-sintering ceramic material at a stage known as "a green body". Pre-sintering is performed at a temperature of about 800 ⁰ C, to produce a resulting pre-sintered ceramic block, followed by milling and fully sintering to form the ceramic framework. This method addresses the problem of wearing of the milling tool because the pre-sintered ceramic blocks are not as hard as fully sintered ceramic blocks. However, this method does not solve the problem of shrinkage between the green body size and the ceramic framework size. Because some shrinkage of the ceramic material occurs at

800 ⁰ C and because batches of ceramic material vary in shrinkage rates, a technician is required to continuously measure the pre-sintered ceramic block prior to milling.

Alternatively, the problem of shrinkage of ceramic frameworks has been addressed by making a green body having an enlarged size compared to the desired final size of the fully sintered ceramic framework, then milling the enlarged green body, and fully sintering the larger green body to form the ceramic framework. Using a green body having an enlarged size addresses the issue of shrinkage in size of the fully sintered ceramic framework. However, the green body is brittle and chips and hairline cracks form throughout during milling.

There is a need for a cost effective and more reliable method of manufacturing ceramic frameworks.

Summary

In one aspect, the invention provides a method of making a ceramic framework, the method involving: forming ceramic material into a green body, and soft-sintering the green body at a temperature range of about 270 ⁰ C to about 380 ⁰ C to form a soft-sintered body, such that shrinkage from the green body size to the soft-sintered body size is reduced or substantially eliminated; and milling the soft-sintered body into the shape of the ceramic framework, and sintering the resulting milled soft-sintered body to a final density at a temperature of at least about 900 ⁰ C to form the ceramic framework.

In a related embodiment of the method, the milling is manual. In an alternative embodiment of the method, the milling involves preparing a computer program having dimensional specifications for the ceramic framework, inputting the program into a computer-directed milling machine, for computer aided design/computer aided manufacturing (CAD/CAM) of the ceramic framework and milling the soft-sintered body according to the CAD/CAM. In another embodiment of this method, the green body is soft- sintered for a time ranging from about 100 minutes to about 300 minutes, at a temperature range of about 27O ⁰ C to about 38O ⁰ C to form the soft-sintered body.

In another aspect, the invention provides a method of making a ceramic framework, the method involving: forming ceramic material into a green body, and soft-sintering the green body at a temperature range of about 100 ⁰ C to about 380 ⁰ C to form a soft-sintered

body, such that shrinkage from the green body size to the soft-sintered body size is reduced or substantially eliminated; preparing a computer program having dimensional specifications for the ceramic framework, inputting the program into a computer-directed milling machine, for computer aided design/computer aided manufacturing (CAD/CAM) of the ceramic framework and milling the soft-sintered body according to the CAD/CAM; and milling the soft-sintered body according to the CAD/CAM into the shape of the ceramic framework, and sintering the resulting milled soft-sintered body to a final density at a temperature of at least about 900 ⁰ C to form the ceramic framework. In another embodiment of this method, the green body is soft-sintered for a time ranging from about 10 minutes to about 300 minutes, at a temperature range of about 100 ⁰ C to about 380 ⁰ C to form the soft-sintered body.

In certain embodiments of these methods, the ceramic material is at least one compound selected from the group of: unstabilized zirconium oxide; zirconium oxide partially stabilized with at least one selected from the group of yttrium oxide, calcium oxide, cerium oxide and magnesium oxide; and a mixture thereof. In another related embodiment of these methods, the ceramic material further includes a transition metal, for example, a transition metal oxide of group IV-VI; an aluminum compound; a silicon compound; iron oxide; manganese oxide; nickel oxide; iron sulfate; manganese sulfate; nickel sulfate; nickel acetate; iron acetate; and manganese acetate. In yet another related embodiment of these methods, the ceramic material further includes at least one compound selected from the group of: a phosphate, a silicate, a sulfate, a carbide, a suicide, a nitride, an oxynitride, or a boride.

In certain embodiments of these methods, the ceramic material is essentially or substantially free of agglomerates. In another embodiment of these methods, the ceramic material further includes at least one composition selected from the group of a pigment, a plastic agent, and at least one binder. Exemplary binders include, for example, a polyvinyl alcohol, a wax, an acrylic resin, and a dextrin.

In another embodiment of these methods, forming the green body further includes at least one process selected from the group of pressing, extrusion, slip casting, gel casting, die casting, and injection molding. In a related embodiment of these methods, pressing involves at least one process selected from the group of cold isostatic pressing, hot isostatic pressing and uniaxial pressing.

In certain embodiments of these methods, the ceramic framework includes a dental restoration, for example, an orthodontic appliance, a bridge, a space maintainer, a tooth replacement appliance, a splint, a crown, a partial crown, a denture, a post, a tooth, a jacket, an inlay, an onlay, a facing, a veneer, an implant, an abutment, a cylinder, and a connector.

In certain embodiments of these methods, prior to sintering to final density, the milled soft-sintered body retains the milled shape without the use of a mold. In a related embodiment of these methods, during sintering to final density, the milled soft-sintered body retains the milled shape without the use of a mold.

In another embodiment of these methods, the milled soft-sintered body is sintered for a time ranging from about less than about one second to about 360 minutes, at a temperature of at least about 900 ⁰ C to form the ceramic framework. In another embodiment of these methods, sintering the milled soft-sintered body is performed at a temperature range selected from at least one of the group of about 1000 ⁰ C to about 1600 ⁰ C, about 1200 ⁰ C to about 1700 ⁰ C, and about 1300 ⁰ C to about 1600 ⁰ C.

In another embodiment of these methods, the soft-sintered body has an extent of linear shrinkage in the range of about 0.001% to about 0.06% compared to the size of the green body. In a related embodiment, the shrinkage is substantially isotropic. In an alternative embodiment, the shrinkage is substantially anisotropic.

In another aspect, the invention provides a ceramic framework formed by the above methods. In another aspect, the invention provides a soft-sintered body for use in the manufacture of a ceramic framework, the soft-sintered body produced by a process involving: forming ceramic material into a green body, and soft-sintering the green body at a temperature range of about 27O ⁰ C to about 38O ⁰ C to form the soft-sintered body, such that shrinkage from the size of the green body to the size of the soft-sintered body is reduced or substantially eliminated.

In yet another aspect, the invention provides a milled soft-sintered body for use in the manufacture of a ceramic framework, the milled soft-sintered body produced by a process involving: forming ceramic material into a green body, and soft-sintering the green body at a temperature range of about 27O ⁰ C to about 38O ⁰ C to form a soft-sintered body, such that shrinkage from the size of the green body to the size of the soft-sintered body is reduced or

substantially eliminated; and milling the soft-sintered body to produce the shape of the ceramic framework.

In yet another aspect, the invention provides a milled soft-sintered body for use in the manufacture of a dental restoration, the milled soft-sintered body produced by a process involving: forming ceramic material into a green body, and soft-sintering the green body at a temperature range of about 27O ⁰ C to about 380 ⁰ C to form a soft-sintered body, such that shrinkage from the size of the green body to the size of the soft-sintered body is eliminated or substantially reduced; and milling the soft-sintered body to produce the shape of the dental restoration.

Brief Description of the Drawings

Figure 1 is a flow chart of various steps in embodiments of processes provided herein for manufacturing a ceramic framework.

Detailed Description

The present invention provides materials and methods for manufacturing ceramic frameworks. The present invention in one embodiment provides a method of making a ceramic framework, the method including: forming ceramic material into a green body, and soft-sintering the green body at a temperature range of about 270 ⁰ C to about 380 ⁰ C to form a soft-sintered body, in which shrinkage from the green body size to the soft-sintered body size is reduced or substantially eliminated; and milling the soft-sintered body into the shape of the ceramic framework, and sintering the resulting milled soft-sintered body to a final density at a temperature of at least about 900 ⁰ C to form the ceramic framework. In certain embodiments, the milling is manual. In another embodiment, the milling involves preparing a computer program having dimensional specifications for the ceramic framework, inputting the program into a computer-directed milling machine, for computer aided design/computer aided manufacturing (CAD/CAM) of the ceramic framework and milling the soft-sintered body according to the CAD/CAM.

In another embodiment, the invention provides a method of making a ceramic framework, the method including: forming ceramic material into a green body, and soft- sintering the green body at a temperature range of about 100 ⁰ C to about 38O ⁰ C to form a soft-

sintered body, in which shrinkage from the green body size to the soft-sintered body size is reduced or substantially eliminated; preparing a computer program having dimensional specifications for the ceramic framework, inputting the program into a computer-directed milling machine, for computer aided design/computer aided manufacturing (CAD/CAM) of the ceramic framework and milling the soft-sintered body according to the CAD/CAM; and milling the soft-sintered body according to the CAD/CAM into the shape of the ceramic framework, and sintering the resulting milled soft-sintered body to a final density at a temperature of at least about 900 ⁰ C to form the ceramic framework.

A method of making dental restorations using computer aided design/computer aided manufacture (CAD/CAM) is shown in Panzera (U.S. patent number 6,354,836, issued March 12, 2002). In this method, a ceramic precursor powder is combined with a binder agent and pressed into a block, forming a green body. The green body is pre-sintered at a temperate range of 1225°C to 1350 ⁰ C to obtain a bisque density less than 85% of the final density. The pre-sintered blocks are milled to a desired shape, which is intentionally enlarged or oversized compared to the desired final size, to account for anticipated shrinkage during final sintering. Final sintering at a temperature range of 1400 ⁰ C to 1600 ⁰ C results in the ceramic blocks having the final density.

A method of making an article of ceramic material that includes making a model having a contour, and positioning the model and a blank within a copying machine, is shown in Andersson (U.S. patent number 5,192,472, issued March 9, 1993). The copying machine senses the model and mills the blank such that the shape of the blank corresponds to the shape of model. Ceramic material is applied to enlarge the milled blank, and the milled blank is compacted with ceramic material, and is exposed to at least one sintering procedure such that the milled blank compacted with ceramic material is subjected to linear shrinkage until the size of the milled blank compacted with ceramic material corresponds to the size of the model. Pre-sintering is carried out at a temperature range of 800 ⁰ C to 1300 ⁰ C and final sintering is carried out at a temperature range of 1100 ⁰ C to 1600 ⁰ C.

A method of making a ceramic-formed body is shown in Katsuo et al. (Japanese patent application number H08-235783, published March 3, 1998). The method involves combining a ceramic powder with each of a thermoplastic organic binder and a plasticizer, and processing these compositions to form a green body. The green body is heat-treated at a

temperature that is lower than the decomposition/evaporation temperature of the thermoplastic organic binder, and higher than the thermoplastic temperature of the thermoplastic organic binder containing the plasticizer, forming a heat-treated body. Katsuo shows a temperature range for the heat treatment of 50 ⁰ C to 250 ⁰ C. The heat-treated body is then sintered at 1100 ⁰ C to produce the ceramic-formed body.

A method of making an artificial tooth crown shown in Oden (U.S. patent number 5,106,303, issued April 21, 1992) has steps of forming a negative reproduction of a tooth cavity, copy milling a core from the negative reproduction including a surface abutting the cavity and an external surface, and applying a veneer to at least part of the external surface of the core. The copy milled surface of the core is enlarged compared to the size of the tooth crown to account for shrinkage of the core during sintering, and the core is sintered at 1600 ⁰ C.

A nonporous ceramic bracket made of compressed zirconium oxide particles that are partially stabilized by a transition metal oxide, for bonding to the face of a tooth, is shown in Sadoun et al. (U.S. patent number 5,011,403, issued April 30, 1991; re-examination certificate issued October 31, 1995). The compressed zirconium oxide particles, partially stabilized by a transition metal oxide, are sintered at a temperature of 1300 ⁰ C to 1600 ⁰ C to obtain the ceramic bracket.

A process for making a ceramic dental prosthesis (Hintersehr, U.S. patent number 5,702,650, issued December 20, 1997) has the steps of: shaping an unfinished piece made out of 92.1 to 93.5 wt. % zirconium oxide, 4.5 to 5.5 wt. % yttrium oxide, 1.8 to 2.2. wt. % hafnium oxide, up to 0.2 wt. % of any other oxides; and reworking the unfinished piece to form a dental prosthesis by means of a rotating tool of metal-bonded diamond grains with speeds of revolution of 10,000 to 50,000 revolutions per minute. A first movement of the tool towards the piece has speeds of 0.1 to 0.5 millimeters per minute, and a second movement of the tool perpendicular to the first movement of 0.3 to 3.0 centimeters per second, and rotational speed along the circumference of the tool of 0.5 to 9.0 m/sec.

The terms below shall have the following meanings herein and in the claims, unless otherwise required by the context.

The phrase, "green body" refers to unsintered ceramic material that has been formed into a desired shape, for example, a square block, a rectangular block, a sphere, or a cylinder.

The green body can be formed by at least one of the following processes: pressing, extrusion, slip casting, gel casting, die casting, and injection molding, which procedures are well known in the art of ceramics (Rice, Ceramic Fabrication Technology, 2003, Marcel Dekker; and King, Ceramic Technology and Processing, 2002, William Andrew Inc., incorporated by reference herein in their entireties.

The term "sintering" refers to a process for making objects from ceramic material by heating the material to a temperature below its melting point until particles of the material adhere to each other. The term further includes without limitation each of the processes of sintering in a vacuum, sintering under normal atmospheric pressure, and sintering under increased pressure, for example, in connection with overpressure sintering of hot isostatic compaction, or alternatively, hot pressing. Pure oxide material can be sintered in air, and some ceramic materials are preferably sintered in an inert or in a controlled atmosphere.

In general, the temperature at which the particles of the ceramic material start to adhere to each other is the sintering temperature, which depends on the composition of the ceramic material. Sintering temperatures are, for example, at least about 900 ⁰ C, at least about 1000 ⁰ C, at least about 1300 ⁰ C, or at least about 1700 ⁰ C. An exemplary furnace for ceramic sintering is a DT-31-SBL-9912 furnace, manufactured by Deltech Inc., Denver, CO.

The length of time of the process of sintering is a function both of the composition of the ceramic material, and the sintering temperature. For example, sintering times may range from about less than 1.0 second to about 360 minutes.

During the sintering process, the ceramic material contracts in size such that the ceramic framework has a linear shrinkage compared to the initial dimensions of the green body and the soft-sintered body. Linear shrinkage is measured by the formula:

Linear Shrinkage = 100% X (Lgreen body - L _ce ramic framework) / Lgreen body

where L is one of the dimensions of the block of ceramic material. This formula is used to determine the percent of linear shrinkage that occurs during full sintering, and allows the ordinarily skilled artisan to appropriately adjust upwardly the size of the green body, i.e., enlarge, to account for shrinkage during the full sintering process.

The phrase "soft-sintered" refers to a process in which a green body is partially sintered at a temperature, such that the size of the green body remains substantially stable, i.e., the green body does not substantially shrink in size during the heating process, producing a soft-sintered body. For example, linear shrinkage of the soft-sintered body compared to the green body is about 0.001%, about 0.005%, about 0.01%, or about 0.06%.

In general, the soft-sintering temperature is a function of the composition of the ceramic material. Soft-sintering temperatures are, for example, at least about 100 ⁰ C, at least about 150 ⁰ C, at least about 300 ⁰ C, or at least about 38O ⁰ C. A soft-sintered body is more porous and consequently not as hard as the fully sintered ceramic framework, providing for milling of the soft-sintered body to a desired shape, with less wear on the milling tool. Properties of the soft-sintered body further include, for example, high strength and uniform density.

The soft-sintering time is a function both of the composition of the ceramic material, and the soft-sintering temperature. For example, soft-sintering times may range from about 100 minutes to about 300 minutes.

The phrases, "computer aided design/computer aided manufacturing" or "CAD/CAM" refer to a process that generally has two-step, a first step that is computer aided design (CAD) to draw a shape of the ceramic framework and to define the tool path, and a second step that is computer aided manufacturing (CAM) to convert the tool path into codes that is understood by the computer associated with a controller machine. CAD/CAM programs are commercially available, for example, from: Advanced Engineering Solutions, 146 Phelps Street, Marlborough, MA 01752; and, Delcam pic, Small Heath Business Park, Birmingham BlO OHJ, UK.

The shape of the ceramic framework made by a CAD/CAM process is in some embodiments determined by data received from scanning a reference object, for example, a tooth to be restored. The soft-sintered body is milled according to the CAD/CAM to produce the shape of the ceramic framework.

The phrase, "ceramic material" refers to at least one inorganic non-metallic material, which is formed into a ceramic product by the action of heat. In certain embodiments, the ceramic material is a composition such as an oxide compound, for example alumina or zirconia. In other embodiments, the ceramic material is a non-oxide compound, for example

a carbide, a boride, a nitride, a suicide, a phosphate, a sulfate, a silicate, an oxynitride. In other embodiments, the ceramic material is a composite, e.g., a combination of at least one oxide and at least one non-oxide. In certain embodiments, the ceramic material includes substantially uniform particles, dispersed and essentially free of agglomerates, such that the ceramic material sinters predictably and isotropically without appreciable distortion.

The ceramic material may be a substantially pure composition, for example, a compound such as zirconium oxide or aluminum oxide, or any suitable oxide that can be sintered to a high strength. Alternatively, the ceramic material may be a mixture of compositions, for example, a mixture of zirconium oxide and aluminum oxide. In certain embodiments, the ceramic material includes at least one composition selected from a pigment, a plastic agent, or at least one binder.

In certain embodiments, for example for, a ceramic material that includes a zirconium compound, a "stabilizing agent" is added to the ceramic material. Zirconium undergoes a phase change between room temperature and the sintering temperature. To prevent the phase change, in certain embodiments the ceramic material is stabilized with at least one compound selected from the group of yttrium oxide, calcium oxide, or magnesium oxide. Other examples of stabilizing agents that are used with zirconium in the ceramic material are: a transition metal oxide of group IV-VI; an aluminum compound; a silicon compound; iron oxide; manganese oxide; nickel oxide; iron sulfate; manganese sulfate; nickel sulfate; nickel acetate; iron acetate; and manganese acetate. Examples of ceramic material and stabilizing agents are shown in Oden et al. (U.S. patent number 5,080,589, issued January 14, 1992), Oden et al. (U.S. patent number 5,106,303, issued April 21, 1992), Sadoun et al. (U.S. patent number 5,011,403, issued April 30, 1991; re-examination certificate issued October 31, 1995), and Panzera (U.S. patent number 6,354,836, issued March 12, 2002), which are incorporated herein by reference.

The term "binder" refers to a compound that is included with ceramic material so that the green body retains its shape when ceramic material is formed to make the green body. Examples of binders include a polyvinyl alcohol, a wax, an acrylic resin, a dextrin, a polyethylene glycol, and a tetra-ethoxy-silane (TEOS).

The invention is not intended to be limited to the stated exemplary ceramic materials, plasticizers, or binders, and any suitable ceramic material, plasticizer, or binder may be used herein to achieve the desired result.

Figure 1 provides a flow chart showing exemplary steps in the process provided herein. In one embodiment of the method provided herein of manufacturing a ceramic framework, ceramic material is formed into a green body using any known forming method, including pressing, uniaxial or isostatic, extrusion, slip casting, gel casting and injection molding. Cold isostatic pressing is associated with one of the highest degrees of homogeneity attainable in green body density. Uniaxial pressing is also applicable for pressing certain shapes such as short cylinders. The green bodies are formed into a desired shape and configuration that will render a ceramic framework, for example, square blocks, rectangular blocks, spheres, or cylinders.

Binders such as cellulose, polyvinyl alcohol, polyethylene glycol, wax, TEOS, and the like may be mixed with ceramic powders to retain the shape of the green bodies during and after forming.

In accordance with the process herein, after the ceramic material has been formed into a green body, the green body is soft-sintered to provide a soft-sintered body. The soft- sintering process involves heating the green body to a temperature at which partial sintering occurs, to achieve a soft-sintered body that is more porous and not as hard as a fully sintered ceramic framework. Additional properties of the soft-sintered body include, for example, high strength and uniform density.

The soft-sintered body is produced, for example, by placing in a resin bag a suitable amount of a ceramic material, for example, a suitable ceramic material includes zirconia obtained from Hitachi Metals America, Ltd., Purchase, NY or aluminum oxide obtained from Reynolds Metal Company, Bauxite, AK or calcined aluminia A3000FL from Alcoa Industrial Ceramics, Pittsburgh, PA. The ceramic material is, for example, isostatically pressed in a suitable pressing device, e.g., AIP3-12C cold isostatic pressing device from AIP Inc., Columbus, OH, and compressed into a block at ambient temperature under pressure, such as, for example, under 50,000 psi. Thereafter, the block is heated in a furnace, such as for example, a Deltech furnace available from Deltech Inc. to a temperature ranging from about

100 ⁰ C to about 380 ⁰ C for a time ranging from about 100 minutes to about 300 minutes to form the soft-sintered body.

The soft-sintered body is then milled, for example, the soft-sintered body is milled manually into a desired shape of the ceramic framework to be produced. In an alternative embodiment, the soft-sintered body is milled according to the CAD/CAM program into a desired shape. In various embodiments, the ceramic framework is, for example, a dental restoration, i.e., an orthodontic appliance, a bridge, a space a maintainer, a tooth replacement appliance, a splint, a crown, a partial crown, a denture, a post, a tooth, a jacket, an inlay, an onlay, a facing, a veneer, an implant, an abutment, a cylinder, and a connector. In other embodiments, the ceramic framework is, for example, a replacement of a lost substance or tissue in the human body, such as an artificial cartilage or bone.

In embodiments in which the milling is performed according to a CAD/CAM program, the shape of the ceramic framework is determined from data received by scanning the reference object, for example, a tooth to be restored. A suitable CAD/CAM device is manufactured by CAD/CAM Ventures, Irving, TX, that includes a contact digitizer and a machining unit equipped with carbide tools.

The soft-sintered body to be milled is produced in a size that is greater than that of the desired final product, i.e., is oversized or enlarged, to allow for shrinkage that occurs when the soft-sintered body is fully sintered. Depending on the ceramic material used, the linear dimensions of the soft-sintered body have a size that is about five percent (5%) to about twenty-five percent (25%) larger than the size of the ceramic framework, based on the linear shrinkage of the soft-sintered body.

The soft-sintered body is then fully sintered to a final density at a temperature-time cycle specific for the material used, e.g., for alumina, at about 1600 ⁰ C for about four hours.

A problem to be solved by the methods of the present invention herein is producing ceramic frameworks using a method that causes less wear on the milling tools, and results in longer life of the milling tools. The present invention addresses this problem by milling prior to fully sintering the ceramic material. In the present invention, the green body is soft- sintered to produce a soft-sintered body that is more porous and not as hard as the fully sintered ceramic framework, yet has high strength and uniform density. The soft-sintered body can be milled more easily than the fully sintered ceramic framework, resulting in less

wear on the milling tools. As such, the same milling tools can continue to be used over a longer period of time before there is a need for replacements.

In certain embodiments, the soft-sintering occurs at a temperature range of about 270 ⁰ C to about 38O ⁰ C. In other embodiments, the soft-sintering occurs at a temperature range of about 100 ⁰ C to about 380 ⁰ C. Soft-sintering within these temperatures is advantageous because shrinkage of the ceramic material is substantially eliminated after soft-sintering in these temperature ranges, and the soft-sintered body is stronger and more resistant to cracking or hairline fracturing than the green body, while having capacity to be milled to the desired shaped compared to milling the green body.

At temperatures substantially greater than 380 ⁰ C, sintering causes shrinkage of the ceramic material. For example, at about 45O ⁰ C, or about 500 ⁰ C, or about 600 ⁰ C, a certain amount of shrinkage of the ceramic material was observed, as shown in the examples below. Because batches of ceramic material vary in shrinkage rates, a worker generally continuously measures the pre-sintered ceramic block prior to milling, which inefficient and costly process is a disadvantage of use of these higher temperatures.

The invention having been fully described, it is further illustrated by the following examples and claims, which are illustrative and are not meant to be further limiting. Those skilled in the art will recognize or be able to ascertain using no more than routine experimentation, numerous equivalents to the specific procedures described herein. Such equivalents are within the scope of the present invention and claims. The contents of all references, including issued patents and published patent applications, cited throughout this application are hereby incorporated by reference.

EXAMPLES

Example 1: Forming soft-sintered bodies

Ceramic material including TZ3Y Zirconia (three mole percent ytrria partially- stabilized) was obtained from TOSOH corporation, Japan. The binder, an acrylic acid, was added to the ceramic material to help maintain the shape of the ceramic material during and after forming the green body.

The ceramic material and binder were inserted into a resin bag and isostatically pressed using a cold isostatic pressing device (NPA systems) under 100-200 MPa into a block at ambient temperature to form a green body. The green body was cut from large uniaxially pressed plates of one inch in thickness into blocks having dimensions of 7.5cm x 2.5cm x 2.0cm.

The green bodies were soft-sintered at each of various temperatures, as shown below in Table 1, to measure extent as a percentage of shrinkage, observed after soft-sintering of the green body, i.e., soft-sintered body, in comparison to the size of the unsintered green body. An unsintered green body was used as a control.

Table 1: Measurement of shrinkage of green bodies after soft-sintering at various temperatures

The percent of linear shrinkage of the soft-sintered bodies was measured using the formula:

Linear Shrinkage = 100% X (Lgreen body - Lsoft sintered body) / Lgreen body

where L is one dimension of the block of ceramic material.

The data in Table 1 show that within the temperature range of 200 ⁰ C to 350 ⁰ C, shrinkage was substantially eliminated for the dimensions along the X and Z axes of the soft- sintered body, compared to these dimensions of the green body. Along the Y-axis a small amount of shrinkage (0.06% and 0.15%) was observed at 25O ⁰ C and 350 ⁰ C respectively. The

data further show that soft-sintering at 500 ⁰ C produces a small amount of shrinkage (from

0.01% to 0.18%) from the dimensions of the green body to the dimensions of the soft- sintered body.