Ceramic

Ceramics, glasses, and glass ceramics include a broad range of inorganic/nonmetallic substances. Ceramics are ideal biomaterials owing to their biocompatibility, bio-functionality and inert nature, the only disadvantage being their brittleness. They are inert in nature because they exist in an oxidized state and are corrosion resistant. These materials are used to repair or replace skeletal hard connective tissues and dental hard tissues.

Application of Bio-Ceramic materials

• Alumina

1. Orthopedic load-bearing applications
2. Coatings for tissue ingrowth in dental and maxillofacial prostheses
3. Alveolar ridge augmentation
4. Crowns and bridges
5. Dental implants

• Hydroxyapatite

1. Periodontal pocket obliteration
2. Dental implants
3. Alveolar ridge augmentation
4. Maxillofacial reconstruction

• Zirconia

1. Joint replacement
2. Crowns and bridges (indirect restorations)

• Calcium phosphate salts

1. Bone repair and augmentation
2. Surface coating on metals
3. Periodontal pocket obliteration
4. Temporary bone space fillers

• Bioactive glasses

1. Bone replacement
2. Coating for chemical bonding with metals
3. Dental implants
4. Periodontal pocket obliteration
5. Alveolar ridge augmentation
6. Maxillofacial reconstruction
7. Percutaneous access devices

• Porcelain

1. Inlays, onlays, crowns, and bridges (indirect restorations)

General classification of Ceramics

Traditional ceramics

Traditional ceramics include clay products, silicate glass, and cement.

Advanced ceramics

Advanced ceramics consist of carbides (SiC), pure oxides (Al2O3), nitrides (Si3N4), nonsilicate glasses, and many others.

Dental Ceramics

• Dental ceramics are nonmetallic, inorganic materials primarily containing compounds of one or more metallic or nonmetallic oxides, borides, carbides, and nitrides, as well as complex mixtures of these materials.
• The elements whose oxides are most commonly present are silicon, aluminum, magnesium, phosphorus, potassium, sodium, calcium, lithium, titanium, and zirconium.
• Dental ceramics are composed essentially of glasses, porcelains, glass ceramics or highly crystalline structures. Their atoms and molecules have a regular periodic arrangement and may exhibit ionic or covalent bonding. They contain a crystal phase and a glass (non-crystalline) phase based on the silica structure, which is characterized by Si-O tetrahedron (tetrahedron with Si4+ cation in the center and O− anions at the four corners). The SiO4 tetrahedra are linked together by sharing their corners. These materials are strong in compression but weak in tension.

Bridge from dental porcelain

Difference between Ceramics and Porcelains

Ceramic is a more generalized term for any product made from a nonmetallic inorganic material processed by fi ring at high temperatures. On the other hand, Porcelain is a restrictive term used for the mixture of kaolin, quartz, and feldspar which, when fired at high temperatures, gives a glassy, translucent finish and is less porous than ordinary ceramic. This dental porcelain is commonly used for ceramometal restorations. Hence, the term dental porcelain is used for metal–ceramic restorations, while dental ceramics is used for metal-free all-ceramic restorations.

Ceramic is a crystalline material; porcelain refers to a mixture of glass and ceramics, while glass is a non-crystalline material. However, dentistry refers to all three materials as dental ceramics.

Basic procedure in Firing of Ceramic material

This involves the process of sintering—the conversion of individual particles of silica (clay) held together with water, which acts as a binder to a single coherent solid. This fusion usually takes place at very high temperatures. Problems that may arise during this sintering are many. The evaporation of the binder causes the ceramic to shrink considerably. Moreover, the gases evolved during the process (such as water vapor and CO) create voids, which may result in crack formation and fracture. Early ceramic craftsmen overcame this problem by beating the clay before molding to get rid of the air. This process is referred to as wedging. Another method to prevent voids and cracks is to increase the firing temperature very gradually, allowing the diffusion of the steam and gases slowly, rather than bursting out and causing cracks. Most of the modern ceramics are fired under vacuum to reduce the entrapment of air.

The major component of most dental ceramics is SiO2. Silica can exist in four different forms: the three crystalline forms are 1.quartz, 2.cristobalite, 3.tridymite and 4.non-crystalline fused silica.

When ceramic material is fi red at high temperatures, the solid ceramic slowly starts to melt into a viscous liquid. This change in viscosity with temperature is called glass transition. This property enables the powder particles to partially melt, flow slightly, and fuse together at the firing temperature.

Advantages & Disadvantages of Dental Ceramics

Advantages

Dental ceramics are different from the other dental restorative materials due to their unique properties. These esthetic restorative materials remain stable for a long period of time, have excellent biocompatibility, are resistant to corrosion, and do not interact with most liquids, alkalis, acids, and gases within the oral environment. Once glazed, it provides a very smooth surface, thereby increasing the fracture resistance. Ceramics surpass all other materials in their ability to mimic natural tooth structure in color and translucency. They have very good compressive strength.

Disadvantages

Although ceramics have high compressive strength, they lack tensile strength and hence are brittle. Since they are very hard, they tend to abrade the opposing enamel during occlusal contacts. During the cooling stage of the fired ceramic, microcracks may develop on the ceramic surface, increasing its surface roughness. This also results in decreased strength due to stresses accumulating within these cracks.

Classification of Dental Ceramics

Dental ceramics can be classified in many ways depending on the following:

Composition

• Feldspathic porcelain
• leucite reinforced porcelain
• Aluminous porcelain,
• Alumina,
• Glass infiltrated alumina
• Glass infiltrated spinel,
• Glass infiltrated zirconia, and
• Glass ceramic

Fusion temperature

• High fusing(1315°C–1370°C)
• Medium fusing(1090°C–1260°C)
• Low fusing(870°C–1065°C) and
• Ultra-low fusing(<850°C)

Microstructural phases

• Glass based (mainly silica)
• Glass based with fillers (usually crystalline fillers such as leucite)
• Crystalline based with glass fillers (mainly alumina, spinel, zirconia)
• Polycrystalline solids (alumina, zirconia)

Substructure

• Metal
• Alumina
• Zirconia

Layering ceramic

Core, body/dentin, enamel, incisal, tints, glaze, etc.

Indications

• Ceramics for ceramometal crowns and fixed partial dentures;
• All-ceramic crowns,
• Inlays,
• Onlays,
• Veneers
• Ceramic denture teeth

Composition

The earliest dental porcelains were made from kaolin, feldspar, and quartz (triaxial porcelain composition). Kaolin was added as a binder to increase the ability to bind the ceramic powder. But since kaolin (hydrated aluminosilicate— Al2O2.SiO2.2H2O) was very opaque, the earliest porcelains lacked translucency. The next generation of ceramics hence did not include kaolin. These dental porcelains are called feldspathic porcelains, with quartz being the crystalline phase. Feldspars are naturally occurring mixtures of soda and potash aluminosilicates (Na2O.Al2O3.6SiO2 and K2O.Al2O3.6SiO2, respectively). Depending on the concentration of both soda and potash, the properties of feldspar vary, as soda tends to lower the fusion temperature, while potash increases the viscosity of the molten glass.

Feldspathic VM9 Porcelain Crowns -side view

Basic Oxides

• Silica(SiO2) is the principal glass forming oxide. It remains unchanged during the firing process and is present as a fine crystalline phase in the glass matrix of the melted feldspar. It is present in most dental ceramics up to 60%. More the

silica content, more is the melt fluidity. Increasing the silica content increases the melting temperature, acid resistance, and hardness and decreases expansion. It also increases gloss and devitrification.

• Alumina(Al2O3) is the hardest and strongest of the oxides used in ceramics. It combines well with silica to give body and stability. It helps in building strong chemical links between silica and the fluxes, thereby preventing crystallization, and gives body and chemical stability to the glaze. Addition of alumina increases melting temperature and hardness, improves tensile strength

and resistance to chemical attack, decreases expansion, and prevents devitrification.

Additional Oxides

• Lithium oxide (Li2O) is the lightest, smallest, and most reactive of the oxides. When added in small amounts by weight it diffuses into the surrounding matrix owing to its small ionic radius and acts as a powerful auxiliary alkaline flux with thermal expansion–lowering effects. It acts as a melter at low temperatures, similar to sodium and boron. It also gives intense color to the ceramic.
• Magnesium oxide (MgO) acts much like a flux as a refractory at lower temperatures. It is used as a matting agent and to increase opacity. Along with alumina, it prevents devitrification. It also lowers expansion and increases resistance to crazing.
• Zinc oxide (ZnO) in small amounts exhibits refractory properties and helps in achieving glossy and brilliant surfaces. In larger amounts, it causes opacity.
• Strontium oxide (SrO) has matting and crystallizing properties similar to barium. Although it has a high melting temperature, it is effective with other fluxes at lower temperatures.

Fluxes

• Boric oxide (B2O3) is a powerful flux added to glasses to lower their softening temperature. Its addition helps in forming good interfacial zones to inhibit crazing.
• Potash (K2O) and soda (Na2O) are modifying oxides that have similar properties. K2O, a heavy oxide, contributes to a brilliant glossy glaze and a longer firing range. It generally contributes to higher melt viscosity while Na2O tends to lower

viscosity. In higher amounts, both increase the co-efficient of thermal expansion and cause crazing. Soda also decreases tensile strength and elasticity compared to other bases.

• Calcium oxide (CaO) is the most commonly used flux in medium and high temperature ceramics. When added along with potash and soda, hardness, stability, and expansion properties of ceramics are greatly improved. CaO also makes the glaze scratch and acid resistant.

Opacifiers

• Zirconia/zirconium dioxide (ZrO2) is the most commonly used opacifier, giving a glassy white color in amounts >5%. It is also a highly refractory material.
• Tin oxide (SnO2), a very white powder, is twice as effective as zirconia in amounts 5%–15%. The mechanism of opacity depends on the suspension of the white tin oxide particles in the molten glass. Tin effectively transforms transparent glaze to white.
• Added in small amounts (0.1%), titanium oxide (TiO2) intensifies and stabilizes colors and opacity. It also modifies existing colors from metals such as Cr, Mn, Fe, Co, Ni, Cu. It can also act as a flux in combination with other fluxes. Cerium oxide (CeO2) is used as an opacifier instead of tin oxide in some porcelain. It gives yellow color in combination with titanium.

Pigments

• Naturally occurring porcelains have a greenish hue. To overcome this color and to give the dental ceramics lifelike enamel and dentin colors, various pigments are added. Iron oxides are the most common pigments used in dental ceramics.
• Ferric oxide (Fe2O3), the most naturally occurring iron oxide, imparts amber to yellow color when added up to 4%, tan when added up to 6%, and brown color when added more than 9%.
• Manganese dioxide (MnO2) produces black, brown, and purple colors in less than 5%.
• Copper oxide produces Green colour.
• Titanium oxide produces Yellowish brown.
• Cobalt oxide produces Blue.
• Zirconium/tin/titanium oxides produces Opacity.
• Uranium oxide/cerium oxide produces Fluorescence.

Ceramics for Metal-Ceramic Restorations

Metal-Ceramic restorations consist of a noble metal framework over which ceramic material is veneered. A veneering ceramic is fired onto the metal substructure to produce an esthetically acceptable restoration. The ceramic veneer is done in a minimum of two layers, the first being the opaque layer, which masks the dark metal and provides the metal–ceramic bond. The consecutive layers buildup the ceramic veneer to the full shape and form of the tooth.

Requirements of Metal-Ceramic System

• The alloy used as a metal substructure must have a substantially higher temperature than the firing temperature of the ceramic(>100°C).
• The fusing temperature of the ceramic should be lower than that of the metal or ceramic copings that are used for all-ceramic

restorations, so that the metal or ceramic coping does not sag when heated during subsequent firings.

• The ceramic materials must wet the metal thoroughly with the contact angle less than 60° to prevent void formation at the interface.
• The metal surface should be roughened and oxidized in order to achieve a good bond with the ceramic.
• During firing of the ceramic, the metal and the ceramic should expand and contract proportionately to prevent the ceramic from cracking. That is, the coefficient of thermal expansion (CTE) of ceramic should match that of the metal.
• Stiffer the metal core, lesser its deflection and strain. Higher stiffness in the alloy reduces stresses in the ceramic.
• The metal should have higher sag resistance, especially when fabricating long span bridges to prevent distortion during ceramic firing.

Ceramic Resources
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