WO2008144388A1 - Multilayer ceramic system for biomedical applications and method for fabricating the same - Google Patents

Abstract

This invention provides multilayered ceramic bodies and systems, fabricated using tape casting and reaction bonding technologies, which exhibit greater structural reliability as compared to ceramic compositions prepared by conventional methods. These multilayered ceramic bodies and systems can be used to form any type of biomedical prosthesis and are particularly useful in dental restorations. The invention also provides methods for making the described multilayered ceramic bodies and systems.

Description

MULTILAYER CERAMIC SYSTEM FOR BIOMEDICAL APPLICATIONS AND METHOD FOR FABRICATING THE SAME

CROSS REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit under 35 U.S.C. 119(e) of United States provisional patent application serial number 60/938,292, filed on May 16, 2007, the contents

of which is herein incorporated by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0002] Funding for the present invention was provided in part by the Government of

the United States by virtue of NIH/NIDCR Grant No. DE06672 by the National Institutes of Health. As such, the United States government may have certain rights in this invention.

FIELD OF THE INVENTION

[0003] The invention generally relates to multilayer ceramic compositions having increased structural reliability in comparison to multilayer ceramic compositions prepared by conventional methods, particularly to multilayer ceramic compositions useful for biomedical

restorative applications, and most particularly, to multilayer ceramic dental restorations fabricated using tape casting and reaction bonding technologies.

BACKGROUND OF THE INVENTION

[0004] Ceramics are inorganic materials generally defined as compounds formed from metallic and nonmetallic elements; i.e. aluminum and oxygen which form alumina, AI₂O₃. These compounds often exhibit properties not readily achieved by other materials, such as hardness, durability, resistance to wear and corrosion, and thermal and electrical

insulation. As a result of these diverse properties, ceramics are useful in a wide variety of applications, from the manufacture of everyday objects, such as dinnerware and tiles to use in electronics, information technology, and biomedical prostheses. However, several disadvantages exist, that complicate work with ceramics. First, there is the nature of the bonding between the atoms of these compounds, which are generally held together by a combination of covalent and ionic bonds. In materials formed by a combination of different types of bonds, electrons are not free to shift positions. As a result, the material is brittle. In other words, ceramics do not bend, they break. A further disadvantage is the unpredictable shrinkage of ceramics in going from the unfired state to the fired state. Ceramic products are typically formed from pastes or slurries of refractory materials, such as alumina, mixed with glass particles, organic solvents, solutes, and binders. The organics are volatized at substantially lower temperatures than the firing or processing temperatures of the ceramic bodies or substrates. Solvents typically evaporate at temperature below a 100⁰C and solutes evaporate at temperatures below 450⁰C. The loss of material leaves pores is the fired ceramic. See Background section of U.S. Patent 5,769,917. These pores act as stress concentrators further decreasing the tensile strength of the ceramic. The above and further general information about ceramics can be found at the web site of The American Ceramic Society (www.ceramics.org).

[0005] Those in the dental industry have worked consistently to overcome the disadvantages of working with ceramics. Porcelain restorations have been a fundamental component of dental care for many years. Reports dating from the seventeenth century recount the first successful attempts of a porcelain tooth replacement (Duchateau and Dubois de Chemant, Paris).

[0006] At the beginning of the nineteenth century, Charles Henry Land developed the porcelain jacket crown, based on a feldspathic composition, which is still used today in a slightly modified form. Fifty years later, reinforcement of the dental jacket crown with aluminum oxide was achieved as a result of the work of McLean and Hughes.

[0007] The use of ceramics in the production of dental prostheses continued to increase. However, critical flaws produced during processing reduced the structural reliability of these products. Although ceramics are durable and resistant to wear, brittleness renders them unsuitable to long-term use in withstanding the pressures of normal mastication.

[0008] However, ceramic materials have proven to be somewhat reliable in the fabrication of single unit dental restorations. See U.S. Pat. No. 4,798,536 to Katz and U.S. Pat. No. 5,653,791 to Panzera et al., which disclose ceramic compositions having leucite. The strength of the disclosed materials is approximately 170 MPa, which is much higher than that of conventional porcelain, which exhibits strengths of about 70 MPa. Nevertheless, the strength and/or toughness values of the ceramic materials may not be adequate for the fabrication of multiple unit restorations.

[0009] As a result of the requirement to provide patients with excellent, aesthetic and biocompatible prosthetic dental restorations, the search for ways to fabricate all-ceramic multi-unit bridges offering long-term stability in posterior applications has been hindered by the limitations of glass ceramics, infiltrated ceramics, and computer-aided design/computer- aided manufacturing systems (CAD/CAM systems).

[0010] Fabrication of ceramic dental crowns is challenging because exceptional skills of a technician are required to minimize stress concentrations and optimize marginal adaptation (fit). In addition, ceramic crowns must be translucent and resistant to fracture even in clinical situations where inadequate thickness precludes optimal design. Natural translucency is needed to achieve an appearance similar to that of human teeth. The core of ceramic restorations has been fabricated from feldspathic porcelain, aluminous porcelain, lithia-based ceramic, glass infiltrated magnesia aluminate spinel, glass-infiltrated alumina, glass-infiltrated zirconia, and mica-based glass-ceramics. However, poor resistance to fracture has been a limiting factor in their use, especially for long-span or multi-unit ceramic restorations (fixed partial dentures).

[0011] Due to the unique shape and size of dental restorations, fabrication techniques often involve manual blending of powders, liquids, dispersants which may not provide the optimal homogeneity in the mixture. In addition, these slurry preparation methods incorporate internal and surface defects such as porosities in the dental prostheses structure that lead to their ultimate failure. Properties such as esthetics, biocompatibility, fatigue resistance, fracture resistance are desired from multi unit or single unit prostheses.

[0012] All-ceramic tooth restorations are considered inert with respect to oral stability and biocompatibility. The accumulation of plaque is comparable to that on the natural tooth. Due to the low thermal conductivity of the ceramic, (unlike metal-supported units), sensitivity to temperature variation is no longer expected.

[0013] The main concern centers on adequate long-term strength under functional stress in the specified range of indications. From the clinical point-of view, it is not the initial strength of the ceramic material itself that is of prime importance, but the time that the permanent restoration will last. In the case of ceramics containing glass, the type of cementation, adhesive bonding or conventional, is usually a decisive factor. The type of cementation has a considerable effect on the stresses acting on the entire tooth preparation/restoration system.

[0014] The lack of long-term strength (subcritical crack growth, fatigue, stress corrosion) of the ceramic systems containing glass already on the market, as compared to the masticatory forces occurring in the mouth, is problematical. There is more noticeable loss of strength with current systems due to the effect of oral moisture and subcritical crack growth.

[0015] Prostheses formed with a combination of metals and ceramics dominated dental restoration during the past 35 years because of high survival rates, improved porcelain- metal bonding, and lower-fusing porcelains. The introduction of a "shrink-free" all-ceramic crown system (Cerestore, Coors Biomedical, Lakewood, Colorado) and a castable glass- ceramic crown system (Dicor Dentsply/York Division, York, PA) in the 1980s provided additional flexibility for achieving aesthetic results. These advanced ceramics with their innovative processing methods stimulated a renewed interest in all-ceramic prostheses (Sozio et al. J. Prosthet. Dent. 49(2):182-187 1983). However, both of these products have been discontinued because of their limited survival times.

[0016] The evolution of more fracture -resistant dental ceramics and the unacceptably high percentages of clinical fractures associated with these products raised concerns on the structural reliability of these products compared to the traditional metal- ceramic prostheses.

[0017] Further materials developments, which concentrated on the inadequate fracture resistance of the shell ceramics, were based on increasing the crystalline content, and incorporated leucite, mica, hydroxyapatite, or glass-infiltrated mixed (e.g. aluminum/ magnesium/zirconium) oxides. Casting, pressing, and grinding techniques are all used to create morphology with these materials.

[0018] Pure polycrystalline oxide ceramics, sold under the trademark Procera®, have only been in clinical use for about 10 years.

[0019] A glass-infiltrated ceramic system is manufactured by Vita and sold under the trademark InCeram®. The core consists of partially sintered alumina, magnesia-alumina silicate, and zirconia with infiltration of glass into the partially sintered network. A dispersion of alumina particles in water called a slip is painted onto a gypsum die. Water, flowing under capillary pressure into the gypsum die, compacts the alumina particles against the die. In this process, the compacted alumina particles are partially sintered together to form necks between contacting particles. This porous, partially sintered alumina, spinel, or zirconia is then infiltrated with a low-viscosity glass to yield a ceramic having high density and strength (Kelly et al. J. Prosthet. Dent. 75:18-32 1996).

[0020] The marginal fit of alumina crowns and fixed partial dentures constructed with InCeram® is indistinguishable from that of metal-ceramic units, with marginal openings of 24μm for individual crowns and 58μm for abutment crowns of fixed partial dentures (Sorensen et al. J. Dent. Res. 69:279 1990). Tensile strength values reported for the alumina core material are three or four times greater than those of previously developed dental ceramics (Seghi et al. Int. J. Prosthodont. 8:239-246 1995). No failures were recorded in one previous study with these crowns over periods of 4-35 months (Prόbster L. Int. J. Prosthodont. 6(3):259-263 1993). However, high failure rates have been reported after the first year of clinical trials with fixed partial dentures constructed with resin-bonded InCeram® (Durr et al. J. Dent. Res. 72:217 1993).

[0021] Based on fractographic observations of failed clinical fixed partial dentures constructed with InCeram® alumina, Kelly et al. concluded that fracture originated at the connectors, often internally, at the interface between the core ceramic and veneering porcelain (Kelly et al. J. Dent. Res. 74:1253-1258 1995). Observations of fractured fixed partial dentures, along with stress calculations and failure analysis of the failed core ceramic connectors, suggest that the veneering porcelain was primarily responsible for the fractures and that strengthening of the core material may not further improve the load-bearing ability of the connector for this ceramic system (Kelly et al. 1995). However, Thompson et al. have suggested that failure of all-ceramic crowns will initially occur along the inner core ceramic surface (Thompson et al. J. Dent. Res. 73(12): 1824-1832 1994). Therefore, depending on the loading conditions, preparation technique, and fabrication precision, failures can occur at different locations.

[0022] A heat-pressed glass ceramic system is manufactured by Ivoclar, AG Schaan, Liechtenstein and sold under the trademark IPS-Empress 2®. Problems of fit associated with traditional feldspathic porcelains are related to shrinkage that results from the sintering process. Pressable ceramics, manufactured using IPS-Empress 2®, demonstrate improved fit by transfer molding or pressing the ceramic into a mold at high temperatures via viscous flow. In this manner, the only dimensional change occurs during cooling and can be controlled with a refractory investment having the appropriate expansion during hot pressing. This core ceramic primarily includes a glass matrix and crystalline lithia disilicate used for opacity and to strengthen the ceramic.

[0023] The strength of ceramic, manufactured using IPS-Empress 2®, has been reported to improve as a result of controlled processing procedures (Hόland et al. J. Biomed. Mater. Res. 53(4):297-303 2000). However, clinical trials involving this ceramic revealed a significant percentage (25%) of fixed partial denture (FPD) failures after one year of service (Pospiech et al. J. Dent. Res. 78:445 2000 and Taskonak 2000).

[0024] In recent years, there has been a marked acceleration in the development of computer-aided design/computer-aided manufactured (CAD/CAM) laboratory systems as a result of the greatly increased performance of personal computers and software. An all ceramic system, manufactured by 3M ESPE Dental Products and sold under the trademark Lava®, is based on a CAD/CAM process for the fabrication of all-ceramic crowns and bridges for anterior and posterior applications. The ceramic framework consists of zirconia veneered with a porcelain overlay. The frameworks are fabricated using CAD/CAM manufacturing techniques (scanning, computer-aided design, computer-aided manufacturing) for pre-sintered zirconia blanks. Sintering of the milled frameworks in a special high- temperature furnace, the size of which has been increased to compensate for the shrinkage during sintering, leads to high-strength restorations. Following the fabrication of the framework, esthetic appearance of natural teeth is achieved by using a veneering porcelain. Traditional slurry preparation and sintering techniques are used for veneering. Even though these frameworks, constructed using Lava®, exhibit a high flexural strength (634 MPa) and a high fracture toughness (5.5 MPa-m^1/2), cracks within the veneer ceramic can progress to chipping failures of the prostheses. In this case, interfacial delamination can occur between the zirconia core and the glass veneer layers (Taskonak B. The Effects of Residual Stress, Viscoelastic, and Thermodynamic Parameters on Apparent Fracture Toughness of Dental Bilayer Ceramic Composites. PhD Dissertation 2004; University of Florida, Gainesville, FL). The fabrication of the framework requires a CAD/CAM system that is not cost effective for an average dental laboratory. Thus, CAD/CAM systems have not become popular.

[0025] Pressed ceramics (IPS-Empress 2®) have been used successfully for anterior crown applications for more than 10 years. Crowns, constructed with InCeram®, have also been used with long-term success for anterior tooth applications. However, bridges and fixed partial dentures (constructed with InCeram®) in posterior applications had a significant amount of clinical failures. In addition, a 50% failure rate of fixed partial dentures (constructed with IPS-Empress 2®) in a two year period has been reported in recent literature (Taskonak et al. J. Dent. Res. 85(3):277-281 2006). In view of the past success of porcelain fused to metal, any new all-ceramic system must be comparable. A minimum survival rate of 85 % after 10 years in situ is required, even for posterior teeth.

[0026] The survivability of full-ceramic prostheses is limited by several factors including the flaw distribution, prosthesis geometry, loading conditions, the environment, and processing damage. Currently, only about 10-15% of all fixed prostheses are fully ceramic because of their limited fracture resistance, potential abrasivity, and variations in marginal integrity (Piddock et al. J. Dent. 18:227-235 1990; Suarez et al. Int. J. Prosthodont. 17(l):35-38 2004). These concerns have led to the development of a variety of new dental ceramic restorative materials and techniques.

[0027] The use of dental ceramics for fixed prostheses has increased because of enhanced aesthetic potential (translucency) compared with metal ceramics. Despite the excellent translucency compared with traditional metal-ceramic systems, ceramic systems still have limited long-term fracture resistance, especially when they are used in posterior areas or for fixed partial dentures (FPDs). Previous studies revealed that metal-ceramic FPDs have a high survival rate of 98%, 90%, and 85% at 5, 10, and 15 years respectively (Suarez et al. Int. J. Prosthodont. 17(l):35-38 2004). Current clinical studies continue to demonstrate that current all-ceramic systems are not indicated for posterior and long-span restorations as an alternative to metal ceramic restorations because of their limited resistance to fracture. Recent clinical studies reported high failure rates (50% in a 2 year period) for all-ceramic FPDs compared to metal-ceramic FPDs especially when they are used in the posterior region (Taskonak B. Two Year Clinical Evaluation of Lithia Disilicate-Based Glass Ceramic Fixed Partial Dentures, Masters Thesis, 2000, Marmara University, Istanbul, Turkey and Olsson et al., 2004).

[0028] A solution to this problem is the use of multilayered ceramic systems for improved damage tolerance. Layered composites exhibit significantly better mechanical performance than monolithic composites. An example of the benefit of layered structures is supplied by nature. Fracture processes in many biological composites containing a high percentage of ceramic have been compared with those that occur in monolithic ceramics of the same composition to determine mechanisms that improve strength and fracture toughness. [0029] The hierarchical structure of shells, e.g., abalones, is formed by aragonite layers approximately lOμm in thickness. Because of this particular microarchitecture, abalone shells have approximately 10 times higher bending strength and toughness than aragonite single crystals (Laraia et al. J. Am. Ceram. Soc. 72(11):2177-2179 1989).

[0030] Hill studied the cross-lamellar structure in the Strombus gigas conch shell. Hill stressed specimens in a four-point bending fixture and determined that the shell exhibited a combination of toughening mechanisms such as crack bridging, microcracking, fiber pullout, microstructurally-induced crack arrest, and crack branching. These mechanisms can increase the toughness of the shell by orders of magnitude compared with that for single- crystal aragonite. (Hill, T.J. Quantitative Fracture Analysis of a Biological Ceramic Composite, PhD Dissertation 2001, University of Florida, Gainesville, FL).

[0031] Toughening mechanisms, as observed in these biological systems such as crack deviation and crack blunting, can be incorporated into current, high-strength engineering materials to create tougher ceramic composites. The multilayered structure of the instant invention was designed to imitate these toughening mechanisms.

[0032] There are many known processing techniques for fabrication of multilayer composites. Tape casting of metal/metal oxide powder compacts offers advantages over other techniques. Thin sheets of ceramic materials with precise, uniform thickness and smooth surfaces for use in fabricating laminates can be formed using the tape-casting technique (Mistier et al. Tape Casting of Ceramics, in Ceramic Processing Before Firing, Edited by G. Y. Onoda Jr. and LX. Hench, John Wiley & Sons, New York, 1978). The physical characteristics and microstructure of tapes to be used in laminates can be precisely controlled. For instance, interlay er tapes of a homogeneous mixture of the powders to be placed between the alternating layers can be formed less than 30 μm in thickness by tape casting. The interlayers can distribute residual stress across the interface region and can be used to alter the morphology of the interface regions (Clupper, D. C. Tape Cast Bioactive Metal Ceramic Laminates for Structural Applications, PhD Dissertation 1999, University of Florida, Gainesville, FL).

[0033] High-strength, fine-grained, and very low shrinkage alumina ceramics are fabricated by the reaction bonding of aluminum oxide (RBAO). These reaction-bonded alumina ceramics are useful for forming dental prostheses. In this technique, Al/ AI2O3 powder compacts are heat-treated in air to oxidize the Al metal into nanosized AI2O3 crystallites which bond with the originally added AI2O3 particles and then cosinter to a high density. In addition, the same method can be used to fabricate other ceramic types using their metal/metal oxide powder compacts such as Ti/TiO₂. However, tape casting of metal/metal oxide powder compacts is a recent technology and never been used for fabricating dental ceramics. The present invention employs a route for tape casting of RBAO by hydrophobization of the starting powder and preparing a suitable tape cast suspension.

[0034] Because of their material characteristics, frameworks based on tape-cast metal/metal oxide powder compacts are able to surmount all the limitations of the dental ceramic fabricating methods. Zirconium oxide (zirconia), with its excellent strength and biocompatibility, is a framework material of choice. This type of framework can be fabricated by tape casting metal/metal oxide powder. The enormous strength and natural esthetics of the framework mean that less tooth structure is removed during preparation. Traditional cementation techniques, as used in luting porcelain fused to metal restorations, are possible.

[0035] U.S. Pat. No. 6,648,645 uses tape casting technique to fabricate dental restorations. However, this patent does not employ a reaction bonding technique and the shrinkage problem is compensated by partially sintering adapted tapes on a die (Kim et al. J. Am. Ceram Soc. 82(11):3167-72 1999). Consequently, partial sintering results in a porous final structure where glass infiltration is required and fracture resistance is compromised.

[0036] She et al. (J. Am. Ceram. Soc. 81(5):1374-1376 1998) demonstrated that the process for reaction bonding of aluminum oxide (RBAO) can be used to fabricate three-layer alumina-based composites (mullite/alumina/mullite) with improved damage resistance. She et al. used a die-pressing technique to fabricate the layered composites. Layers produced by die-pressing are usually limited to a thickness of 3mm or greater.

[0037] Despite the enormous progress in recent years, there remains a need in the art for multilayer ceramic systems with maximized structural reliability.

SUMMARY OF THE INVENTION

[0038] This invention provides multilayered ceramic bodies and systems, fabricated using tape casting and reaction bonding technologies, which exhibit greater structural reliability as compared to ceramic compositions prepared by conventional methods. These multilayered ceramic bodies and systems can be used to form any type of biomedical prosthesis and are particularly useful in dental restorations and material, including, but not limited to, orthodontic appliances, bridges, space maintainers, tooth replacement appliances, splints, crowns, partial crowns, dentures, posts, teeth, jackets, inlays, onlays, facing, veneers, facets, implants, abutements, cylinders, and connectors.

[0039] A ceramic body, according to the invention, includes a plurality of layers formed by tape casting and bonded together by reaction bonding. The outer layers have a smaller thermal expansion coefficient than inner layers. Therefore, residual compressive stresses that increase fracture resistance can be generated in the outer layers during cooling from the sintering temperature. The resulting ceramic body can be used to form the core of a biomedical prosthesis or dental restoration. [0040] The ceramic layers can be fabricated from any suitable ceramic material. The ceramic body of the invention can include outer layers of mullite and inner layers of alumina-zirconia or alumina-titania.

[0041] The physical characteristics and microstructure of tapes to be used in laminates can be precisely controlled. Thickness of each of the individual layers is very important for obtaining the increased structural reliability of the ceramic systems. For instance, interlay er tapes of a homogeneous mixture of the powders to be placed between the alternating layers can be formed less than 0.03 mm in thickness by tape casting. In accordance with the invention, thickness of one of the layers is from 0.01 to 3mm. Furthermore, control of the thickness of the outer layers will also lead to increased strength by limiting the effective initial crack size that can be introduced. It is preferred that the thickness of one of the outer layers ranges from 0.01 to 0.15mm. Most preferably, thickness of any individual layer is not greater than 3mm.

[0042] The invention also encompasses a ceramic system that includes the ceramic body as a core and further includes a plurality of tape-casted, reaction bonded layers of veneering ceramic and a layer of glaze. The ceramic system forms a prosthesis, most preferably, a fixed partial denture (FPD) such as a crown or a bridge.

[0043] The invention also provides methods for making the described multilayered ceramic bodies and systems.

[0044] The invention encompasses a method of forming a ceramic body. The method includes preparing a plurality of layers using a tape casting technique. The next step is arranging the layers with a smaller thermal expansion coefficient outside a layer with a greater thermal expansion coefficient. The next step is bonding the plurality of layers together using a reaction bonding technique. [0045] The invention additionally encompasses a method of making a ceramic system for forming a biomedical prosthesis. The first step is providing a tape-casted ceramic material layer. The next step is providing a tape-casted veneering material layer. The next step is bonding the tape-casted ceramic material layer to the tape-casted veneering material layer by reaction bonding. The next step is coating the bonded layers with a layer of glaze.

[0046] The invention also provides ceramic bodies, systems, prostheses, and fixed partial dentures made by the described methods.

[0047] It is an objective of the invention to provide a multilayered ceramic body, system, and prosthesis with increased fracture toughness and flexure strength compared to those made by conventional methods.

[0048] It is another objective of the invention to provide a multilayered ceramic body and system having more interfaces introduced into the multilayered material to increase the toughness through a crack-deflection mechanism along the interfaces.

[0049] As a result of increased toughness due to the increased amount of interfaces in the multilayered material of the invention as compared to the amount of interfaces found in conventional systems, the system of the invention can be used for both posterior and anterior dental applications. Therefore, it is yet another objective of the invention to provide a reliable all-ceramic system designed for use in all posterior dental application as well as anterior dental applications.

[0050] It is an objective of the invention to provide a system that uses supragingival preparations such that less tooth structure is removed, as compared with systems using porcelain fused to metal restorations.

[0051] It is a further objective of the present invention to provide a system having several toughening mechanisms such as, crack stopping, crack deviation, interfacial delamination, and residual stress strengthening. As a result, this system is expected to have a prolonged clinical life time of 3-5 times more than the systems available currently.

[0052] The literature describes other ceramic - specific parameters, such as fracture toughness and Weibull modulus. The Weibull modulus indicates the distribution of strength values. A high Weibull modulus (> 10) reflects a close distribution and is therefore advantageous, especially if the strength is low. Therefore, it is an additional objective of the invention to provide a system with a Weibull modulus greater than 10 (>10).

[0053] Not the least consideration, a good accuracy of fit is also a determining factor for clinical success. The system of the present invention can achieve the accuracy at the crown margin of 50 μm - 100 μm. Thus, another objective of the invention is to provide a patient with a well-fitted prosthesis.

[0054] These requirements can now be achieved using tape cast ceramics - coupled with accurate knowledge of the reaction bonded zirconia and alumina ceramics. The system of the present invention provides the laboratory, dentist, and patient with a durable, aesthetic

all-ceramic, dental restoration.

[0055] Other objectives and advantages of this invention will become apparent from the following description taken in conjunction with the accompanying drawings, wherein are set forth, by way of illustration and example, certain embodiments of this invention. The drawings constitute a part of this specification and include exemplary embodiments of the present invention and illustrate various objects and features thereof. BRIEF DESCRIPTION OF THE DRAWINGS

[0056] A more complete understanding of the present invention may be obtained by references to the accompanying drawings, when considered in conjunction with the subsequent detailed description.

[0057] FIG. 1 is a schematic side view of a mullite/alumina-zirconia/veneer ceramic system according to the invention.

[0058] FIG. 2 is a schematic side view of a mullite/alumina-titania/veneer ceramic system according to the invention.

DEFINTIONS/ABBREVIATIONS

[0059] The following list defines terms, phrases, and abbreviations used throughout the specification. Although the terms, phrases, and abbreviations are listed in the singular tense, this list is intended to encompass all grammatical forms. Preferred embodiments are exemplary only and not intended to limit the scope of the invention.

[0060] As used herein, the term "prosthesis" refers to an artificial device to replace or augment a missing or impaired part of the body (M erriam- Webster's Medical Dictionary 2002).

[0061] As used herein, the term "biomedical restoration" refers to a returning to a normal or healthy condition or to an object that returns to a normal or healthy condition, such as a prosthesis (Merriam-Webster's Medical Dictionary 2002). The multilayer ceramic bodies and systems of the invention can be used to fabricate any type of prosthesis.

[0062] As used herein, the term "dental restoration" refers to the work, process, or result of replacing or restoring teeth or oral tissues (Stedman's Medical Dictionary 2002; Random House Unabridged Dictionary 2006). The multilayer ceramic bodies and systems of the invention can be used to fabricate dental restorations and material, including, but not limited to, orthodontic appliances, bridges, space maintainers, tooth replacement appliances, splints, crowns, partial crowns, dentures, posts, teeth, jackets, inlays, onlays, facing, veneers, facets, implants, abutements, cylinders, and connectors.

[0063] As used herein, the abbreviation "FPD" refers to a fixed partial denture, a tooth replacement that is fixed permanently in the mouth and is supported by natural teeth, such as a crown or bridge (32teethonline.com).

[0064] As used herein, the term "crown" refers to a fixed partial denture that covers the entire tooth like a cap (32teethonline.com).

[0065] As used herein, the term "bridge" refers to a prosthetic appliance that replaces missing teeth by attaching to the adjacent, remaining teeth.

[0066] As used herein, the term "ceramic" refers to any of various hard, brittle, heat-and-corrosion-resistant materials typically made of metallic elements combined with oxygen, carbon, nitrogen, or sulfur (The American Heritage Science Dictionary 2002).

[0067] As used herein, the term "porcelain" refers to a hard, white, translucent ceramic made by firing (at a low temperature) a ceramic material, such as clay, and then glazing (at a high temperature) with fusible materials (The American Heritage Dictionary 2006).

[0068] As used herein, the term "composite" refers to a complex material in which two or more distinct, structurally complementary substances combine to produce structural or functional properties not present in any individual component (The American Heritage Dictionary 2006).

[0069] As used herein, the term "firing" refers to the baking of ceramics or glass (The Random House Unabridged Dictionary 2006). [0070] As used herein, the term "sintering" refers to the process of forming a coherent mass of individual particles, such as ceramic particles, by heating without melting (The American Heritage Dictionary 2006).

[0071] As used herein, the term "thermal expansion coefficient" refers to the fractional change in length divided by the change in temperature: i.e. a measure of a materials tendency to expand when heated.

[0072] As used herein, the term "mullite" refers to a colorless, mineral clay including crystalline aluminum silicate that is able to withstand corrosion and very high temperatures. It can be produced artificially during various sintering and firing processes and often used as a refractory material (The Random House Unabridged Dictionary 2006).

[0073] As used herein, the term "lamination" refers to arrangement of thin layers and bonding them together (The Random House Unabridged Dictionary 2006).

[0074] As used herein, the term "tape casting" refers to a process for preparing thin sheets of material. A slurry, generally including ceramic powder, solvents, plasticizers, and binders, is cast onto a flat, moving carrier surface. The slurry passes beneath a knife edge as the carrier surface advances along a supporting table. The solvents evaporate to leave a relatively dense, flexible sheet that may be stored on rolls or stripped from the carrier in a continuous process (web site of the Dynamic Ceramic Company).

[0075] As used herein, the term "reaction bonding" refers to a process for bonding aluminum oxide. In this process, AI/AI₂O₃ powder compacts are heat treated in air to oxidize the Al metal into nanosized AI₂O₃ crystallites that sinter and bond to the initial AI₂O₃ particles (Claussen et al. J. Eur. Ceram. Soc. 5(l):29-35 1989). The expansion created during oxidation of the aluminum particles overcomes the shrinkage of alumina during sintering. DETAILED DESCRIPTION OF THE INVENTION

[0076] The present invention utilizes a tape casting procedure using variable particle sizes in the precursor powder composition. The amount of plasticizer, binder, solvent, and dispersant used is altered for the tape casting procedure to obtain the highest solids loading and ductility for these ceramics. Achievement of high solids loading is useful for reducing shrinkage of the produced tapes.

[0077] Reaction bonding of aluminum oxide (RBAO) has been proposed as a method for fabricating high strength, fine grained, and low-shrinkage alumina ceramics. In this technique AI/AI2O3 powder compacts are heat-treated in air to oxidize the Al metal into nanosized AI2O3 crystallites that sinter and bond to the initial AI2O3 particles. In most cases, ZrO₂ is added to the powders to develop a fine-grained microstructure and improve the mechanical properties. Titania has different thermal expansion coefficients than alumina. By introducing titania and zirconia to inner layers, compressive stresses are produced at the interfaces of the multi-layered ceramics. One can toughen ceramics through the mechanism of crack deflection in which desired residual stresses are controlled through the selection of thermal expansion coefficient differences between layers. The RBAO technique has been successfully applied to produce low-shrinkage mullite and alumina-mullite composites by incorporating SiC into the initial AI/AI2O3 powder mixtures (Wu et al. J. Am. Ceram. Soc. 74(10):2460-2463 1991). During heat treatment, SiC is oxidized to SiO₂, which reacts with AI2O3 to form mullite (3 AI2O3 -2SiO₂). By adjusting the initial powder particle size and shape so that an excess of Al₂θ3 remains in the final product various alumina-mullite composites can be obtained. Carbon atoms from oxidized SiC molecules form CO₂ molecules during the oxidation process.

[0078] Mullite/alumina plus zirconia/alumina and mullite/alumina plus titania/alumina systems were selected for the following reasons: (1) Alumina and mullite can be sintered to high density under the same sintering conditions (Wu et al. J. Am. Ceram. Soc. 76(4):970-980 1993).

(2) Alumina and mullite are structurally and thermodynamically stable when in contact at elevated temperatures. This condition permits these two oxides to form laminates without any intermediate phases.

(3) Mullite has a lower thermal expansion coefficient than alumina (Richerson DW Modern Ceramic Engineering, Marcel Dekker, New York 1992). Therefore, residual compressive stresses that increase fracture resistance can be generated in the mullite outer layers during cooling from the sintering temperature.

(4) RBAO precursor powder compositions with ZrO2 and TiO2 particles is used to develop a fine-grained microstructure and improve the mechanical properties of inner layers. Additionally, an increase in strength and toughness of the material is affected by a phase transformation mechanism in zirconia.

(5) Precursor powders are graded by blending fractions of different particle size in order to fill gaps between coarse particles in tape-cast ceramics. Typically, a ratio of 10/1 to 20/1 is necessary between two or three fractions of particles, in order to maximize packing (McGeary R.K. J. Am. Ceram. Soc. 44(10):513-522 1961). Different particle sizes and shapes are obtained by attrition milling.

[0079] The following multilayer ceramic composites are utilized in the present invention:

(1) Multilayer dental ceramic with a single alumina / zirconia inner-layer.

(2) Multilayer dental ceramic with a single alumina / titania inner-layer.

[0080] The starting materials and the initial compositions of the composites are given in Table 1. The alumina-containing mullite (AM) powder mixtures for the outer layer is attrition milled in acetone for 7 h at 700 rpm using 3 mm 3 Y-TZP balls as the milling medium. The zirconia containing alumina (ZA) powder mixtures for the inner-layer is milled with 0.8 mm 3 Y-TZP balls in ethanol for 3 h at 1200 rpm. The different milling parameters are derived from the use of pre-existing powders that are milled in small and large attrition mills. After drying in air at room temperature for about 48h, the ZA powders is sieved through a 200-μm screen. Commercially ready RBAO precursor powders are obtained from TUHH Technologie, GMBH, Hamburg, Germany.

Table 1. Initial powder compositions and expected final phase compositions of multilayer ceramic outer and inner layers.

[0081] By blending fractions of different particle size, gaps between coarse particles are filled. As a result of grading, the packing factor (defined as vol% occupied by solids) is initially increased, and then decreases when the ratio of coarse fraction to fine fraction changes from 0 to 1. Properly graded blends can have porosity of less than 10% even before sintering starts (McGeary, 1961). A previous study reported packing density of mixed spheres of different size (McGeary, 1961) (Table 2).

Table 2. Packing density of mixed spheres of different size (McGeary, 1961)

[0082] The first multilayer dental ceramic core system includes mullite, zirconia- containing alumina and glass layers (Fig. 1). Each flexible layer of material for forming complex shapes is prepared by tape casting (scalpel blade technique). Each tape-cast layer includes precursor powders for reaction bonding. A mullite powder with a mean particle diameter of lμm is used (TUHH Technologie, GMBH, Hamburg, Germany). A mixture of methyl ethyl ketone and ethyl alcohol with a 66:34 volume ratio is used as the solvent. A commercial dispersant is used for the formulation of tape casting slips, i.e., a phosphate ester (Emphos PS21-A, Witco Chemical Co., New York, NY).

[0083] An optimum dispersion is obtained by adding 0.5 wt% dispersant with respect to the mullite powder. Polyvinyl butyral is used as the binder and dibuthyl phathalate as a plasticizer. The amounts of the binder and plasticizer are varied systematically according to the selected weight ratio of binder to binder plus plasticizer, b/(b+pl), where b and pi present the weight of binder and plasticizer, respectively. A mixture of mullite powder, solvent, and dispersant is ball mixed in a polyethylene jar for 4 h. Subsequently, specific amounts of binder and plasticizer are added to achieve weight ratios of mullite powder to the organic additives (binder + plasticizer), (a / a + p₂), where a and p₂ are the weight of mullite powder and binder plus plasticizer, respectively. The mass will then be ball-milled for an additional 20 h. After air drying, the final tape thickness is about 0.15 mm. [0084] An alumina-zirconia precursor powder for reaction bonding with a particle size of 1 μm is used to produce a reinforcing phase. Zirconia is a phase-transforming material that absorbs fracture energy by undergoing a stress-induced phase transformation (Evans, A.G. J. Am. Ceram. Soc. 73(2):182-206 1990). A standard binder/plasticizer combination is used from previously reported tape-casting formulations (Plucknett et al. J. Am. Ceram. iSoc.77(8):2145-2153 1994). Alumina-zirconia tapes are tape-cast using the same steps as those used for mullite tapes.

[0085] The packing density of precursor powders in tapes, P, is calculated by using the equation:

P=(D_g^yD_1I1 (1)

where D_g, R₃, and D_th are the green density of tapes, the weight fraction of precursor powder in tapes, and the theoretical density of alumina respectively (Kim et al., 1999).

[0086] Tape-cast green body specimens are reaction-bonded and sintered in a box furnace in air using the following heating cycles: (1) heat at l°C/min to 45O⁰C, 2 h hold; (2) heat at 0.5⁰C /min to 68O⁰C, 2 h hold; (3) heat at I⁰C /min to HOO⁰C, 2 h hold; (4) heat at I⁰C /min to 115O⁰C, 15 h hold; and (5) heat at 1O⁰C /min to 155O⁰C, 1.5 h hold. AU specimens are cooled at 1O⁰C /min to room temperature. Thus, the outer layers are in a state of residual compression and the inner layers are in a state of residual tension in the plane of the plate because the outer layers have a smaller thermal expansion coefficient than the inner layers (Chen et al. J. Am. Ceram. Soc. 76(5): 1258- 1264 1993).

[0087] The second type of multilayer composite includes an alumina-titanium inner layer (AT) (Fig. 2). The outer-layer remains the same as that used in the first multilayer composites. For inner-layers, powder mixtures of Al, with additional AI2O3 are attrition milled for 7 h together with TiO₂ in acetone using 3 Y-TZP milling balls. After milling, the same procedures are applied to this multilayer ceramic as were used for the first multilayer ceramic. The firing temperatures and firing schedule will remain the same.

[0088] A commercial veneering ceramic (IPS Eris veneering ceramic, Ivoclar AG, Schaan, Liechtenstein) is used to produce the outer layers of the multilayered dental ceramic system. The particle size of the porcelain powder is measured using transmission electron microscopy. IPS Eris veneering ceramic has a glass matrix and a crystalline phase. Its crystalline phase consists of fluorapatite crystals. Various weight ratios of plasticizer, binder, solvent, and dispersant are used to obtain flexible tape-cast veneering ceramics (Fig. 2).

[0089] A preferred embodiment of the ceramic system of the invention is exemplified in FIG. 1. This schematic side view shows a five layer laminate composite, including a core and two layers of veneering ceramic. The core includes an alumina-zirconia inner layer enclosed within two outer layers of mullite.

[0090] Another preferred embodiment of the ceramic system of the invention is exemplified in FIG. 2. This schematic side view shows a six layer laminate composite, including a core and three layers of veneering ceramic. The core includes an alumina-titania inner layer enclosed within two outer layers of mullite. The alumina-titanium layer is mixed with a polyvinyl butyral binder.

[0091] Crown fabrication is established using the following steps: The green laminates are burnished to the prepared tooth model for intimate adaptation and the margins are trimmed and finished. In this preferred embodiment, each layer of tape-cast ceramics and veneering that are used for fabricating a dental crown is 0.15 mm in thickness. Including the glaze layer (0.1mm), the final thickness of 1.0 mm is obtained for prostheses by layering each tape-cast ceramics and adapting them to the die. The firing procedure will take place after layering and adapting.

[0092] The resulting crown, fabricated using tape casting and reaction bonding technologies, exhibits greater structural reliability as compared to other ceramic crowns prepared by conventional methods.

[0093] All patents and publications mentioned in this specification are indicative of the levels of those skilled in the art to which the invention pertains. All patents and publications are herein incorporated by reference to the same extent as if each individual publication was specifically and individually indicated to be incorporated by reference. It is to be understood that while a certain form of the invention is illustrated, it is not intended to be limited to the specific form or arrangement herein described and shown. It will be apparent to those skilled in the art that various changes may be made without departing from the scope of the invention and the invention is not to be considered limited to what is shown and described in the specification. One skilled in the art will readily appreciate that the present invention is well adapted to carry out the objectives and obtain the ends and advantages mentioned, as well as those inherent therein. The ceramic bodies, ceramic systems, methods, procedures, and techniques described herein are presently representative of the preferred embodiments, are intended to be exemplary and are not intended as limitations on the scope. Changes therein and other uses will occur to those skilled in the art which are encompassed within the spirit of the invention and are defined by the scope of appended claims. Although the invention has been described in connection with specific, preferred embodiments, it should be understood that the invention as claimed should not be unduly limited to such specific embodiments. Indeed various modifications of the described modes for carrying out the invention which are obvious to those skilled in the art are intended to be within the scope of the following claims.

Claims

THE CLAIMSWhat is claimed is:

1. A ceramic body comprising a plurality of layers formed by tape casting and bonded together by reaction bonding, wherein outer layers of said ceramic body have a smaller thermal expansion coefficient than an inner layer.

2. The ceramic body of claim 1, wherein said outer layers comprise mullite.

3. The ceramic body of claim 1 , wherein said inner layer comprises alumina-zirconia or alumina-titania.

4. The ceramic body of claim 1, wherein a thickness of one of said layers is from 0.01 to 3mm.

5. The ceramic body of claim 1, wherein a thickness of one of said outer layers is from 0.01 to 0.15mm.

6. A multilayer ceramic system for forming a prosthesis comprising the ceramic body of claim 1, a plurality of tape-casted, reaction-bonded layers including veneering ceramic and a layer of glaze.

7. The multilayer ceramic system of claim 6, wherein a thickness of one of said layers is from 0.01 to 3mm and a thickness of said layer of glaze is from 0.01 to 0.15mm.

8. The multilayer ceramic system of claim 6, wherein a total thickness is not greater than 3mm.

9. The multilayer ceramic system of claim 6, wherein said prosthesis formed is a fixed partial denture (FPD).

10. The multilayer ceramic system of claim 9, wherein said fixed partial denture is a crown or a bridge.

11. A method for forming a multilayer ceramic body comprising: preparing a plurality of layers using a tape casting technique; arranging layers with a smaller thermal expansion coefficient outside of a layer with a greater thermal expansion coefficient; and bonding said plurality of layers together using a reaction bonding technique.

12. The method of forming a multilayer ceramic body as forth in claim 11 , wherein said outer layers comprise mullite.

13. The method of forming a multilayer ceramic body as forth in claim 11, wherein said inner layer comprises alumina-zirconia or alumina-titania.

14. The method of forming a multilayer ceramic body as set forth in claim 11 , wherein a thickness of one of said layers is from 0.01 to 3mm.

15. The method of forming a multilayer ceramic body as set forth in claim 11, wherein a thickness of one of said outer layers is from 0.01 to 0.15mm.

16. A multilayer ceramic body formed by the method of claim 11.

17. A multilayer ceramic body formed by the method of claim 14.

18. A multilayer ceramic body formed by the method of claim 15.

19. A method for making a multilayer ceramic system for forming a prosthesis comprising: providing a tape-casted ceramic material layer; providing a tape-casted veneering material layer; bonding said tape-casted ceramic material layer to said tape-casted veneering material layer by reaction bonding; and coating bonded layers with a layer of glaze.

20. The method for making a multilayer ceramic system as set forth in claim 19, wherein a thickness of one of said layers is from 0.01 to 3mm and a thickness of said layer of glaze is from 0.01 to 0.15mm.

21. The method as set forth in claim 19, wherein a total thickness is not greater than 3mm.

22. The method as set forth in claim 19, wherein said prosthesis formed is a fixed partial denture (FPD).

23. The method as set forth in claim 22, wherein said fixed partial denture (FPD) is a crown or a bridge.

24. A multilayer ceramic system made by the method of claim 19.

25. A multilayer ceramic system made by the method of claim 20.

26. A multilayer ceramic system made by the method of claim 21.

27. A prosthesis formed by the method of claim 19.

28. A prosthesis formed by the method of claim 20.

29. A prosthesis formed by the method of claim 21.

30. A fixed partial denture (FPD) formed by the method of claim 19.

31. A fixed partial denture (FPD) formed by the method of claim 20.

32. A fixed partial denture (FPD) formed by the method of claim 21.

33. A ceramic system comprising: a first mullite layer; an alumina-zirconia layer deposed on said first mullite layer; a second mullite layer desposed on said alumina-zirconia layer; a first veneering ceramic layer deposed on said second mullite layer; and a second veneering ceramic layer deposed on said first veneering ceramic layer.

34. A ceramic system comprising: a first mullite layer; an alumina-titania layer deposed on said first mullite layer; a second mullite layer desposed on said alumina-titania layer; a first veneering ceramic layer deposed on said second mullite layer; a second veneering ceramic layer deposed on said first veneering ceramic layer; and a third veneering ceramic layer deposed on said second veneering ceramic layer.