Dental ceramics

dental ceramic
Structure
Composition
Properties
Classification
Metal-ceramic systems:

1. Dental Ceramics Dr. Kriti Trehan MDS I 03/04/2018

2. Contents Introduction History of dental ceramics Structure Composition Properties Classification Metal-ceramic systems: Composition and Properties Components of metal-ceramic restoration Fabrication of metal-ceramic prosthesis Bonding mechanisms Strengthening of metal ceramic References

3. The word Ceramic is derived from the Greek word “ keramos ”, which literally means ‘burnt stuff’, but which has come to mean more specifically a material produced by burning or firing. The American Ceramic Society had defined ceramics as inorganic, non-metallic materials, which are : crystalline in nature Introduction CERAMICS : Compounds of one or more metals with a nonmetallic element, usually oxygen. They are formed of chemical and biochemical stable substances that are strong, hard, brittle, and inert non-conductors of thermal and electrical energy.

4. compounds formed between metallic and nonmetallic elements such as aluminum & oxygen (alumina - Al2O3), calcium & oxygen ( calcia - CaO ), silicon & nitrogen (nitride- Si3N4). Most ceramics are characterized by their: Biocompatibility Esthetic potential Refractory nature High hardness Excellent wear resistance Chemical inertness

5. Ceramic-like tools have been used by humans since the end of the Old Stone Age around 10,000 B.C. to support the lifestyles and needs of fisher-hunter-gatherer civilizations. The first porcelain tooth material was patented in 1789 by de Chemant , a French dentist in collaboration with Duchateau , a French pharmacist. In 1808, Fonzi , an Italian dentist, invented a “ terrometallic ” porcelain tooth held in place by a platinum pin or frame. HISTORY OF DENTAL CERAMICS

6. Planteau , a French dentist, introduced porcelain teeth to the United States in 1817, and Peale , an artist, developed a baking process in Philadelphia for these teeth in 1822. Charles Land introduced one of the first ceramic crowns to dentistry in 1903. Two of the most important breakthroughs responsible for the long-standing superb esthetic performance and clinical survival probabilities of metal-ceramic restorations are described in the patents of Weinstein and Weinstein (1962) and Weinstein et al. (1962 ). The first commercial porcelain was developed by VITA Zahnfabrik in about 1963.

7. 1965 – Mc Lean & Hughes used glass- alumina composite instead of feldspar porcelain resulting in stronger restorations. Improvement in all ceramic systems developed by controlled crystallization of a glass ( Dicor ) was demonstrated by Adair and Grossman (1984 ) . 1989 – The concept of All-Ceramic post & core was introduced using Dicor glass-ceramic initially, followed by In-cream, IPS Empress and Zirconia ceramics. New generation of ceramics, including Cercon , Lava, In Ceram Zirconia , IPS Empress2, and Procera All Ceram were used for ceramic prostheses.

8. Ceramics can appear as either crystalline or amorphous solids (also called glasses). Thus, ceramics can be broadly classified as non crystalline (Amorphous Solids or glasses) and Crystalline ceramics. The mechanical and optical properties of dental ceramics mainly depend on the nature and the amount of crystalline phase present. More the crystalline phase better will be the mechanical properties which in turn would alter the aesthetics. Structure

9. Conventional or feldspathic porcelains are usually noncrystalline ceramics. These conventional porcelains are very weak and brittle in nature leading to fracture even under low stresses. Recent developments in the processing technology of dental ceramics have led to the development of crystalline porcelains with suitable fillers such as alumina, zirconia and hydroxy apatite.

10. Non- Crystalline Ceramics These are a mixture of crystalline minerals (feldspar, silica and alumina) in an amorphous (non-crystalline matrix of glass) vitreous phase. The glass-forming matrix of dental porcelains uses the basic silicone oxygen (Si-O) network. Their structures are characterized by chains of (SiO4) 4− tetrahedra in which Si 4+ cations are positioned at the center of each tetrahedron with O − anions at each of the four corners.

11. The primary structural unit in all silicate structures is the negatively charged siliconoxygen tetrahedron (SiO4) 4− . The SiO4 tetrahedra are linked by sharing their corners.They are arranged as linked chains of tetrahedra , each of which contains two oxygen atoms for every silicon atom. The atomic bonds in this glass structure have both a covalent and ionic character thus making it stable. This stable structure, imparts some important qualities like excellent thermal, optical and insulating characteristics, inertness, translucency to the glass matrix.

12. Alkali cations such as potassium or sodium tend to disrupt silicate chains leading to lower sintering temperatures and increased coefficients of thermal expansion. Molecules with one oxygen atom (such as Na 2 O, K 2 O,or CaO ) are useful in dental porcelain as fluxes. They may also act as opacifiers . Molecules that contain three oxygen atoms for every two other atoms (such as Al2O3) are used as stabilizers.

13. Crystalline Ceramics Regular dental porcelain, being of a glassy nature is largely non crystalline and exhibits only a short range order in anatomic arrangement. The only true crystalline ceramic used in restorative dentistry is Alumina(A1 2 O 3 ); which is one of the hardest and probably the strongest oxides known. Crystalline ceramics may have ionic or covalent bonds (Ionic crystals are compounds of metals with oxygen. e.g.: Alumina.

14. Ceramics are reinforced with crystalline inclusions such as alumina and leucite into the glass matrix to form crystal glass composites as a part of strengthening the material and improving its fracture resistance (dispersion strengthening). McLean and Hughes (1965) introduced the first generation of reinforced porcelains for porcelain jacket crowns, which are generally referred to as “Aluminous porcelains” .

15. Glass Formation When silica melts, it produces an extremely viscous liquid. The solid formed is more to likely be a glass (Vitreous Structure) called Fused Quartz. This process of forming a glass is called ‘ Vitrification ’. cooled rapidly

16. Dental ceramics are mainly composed with crystalline minerals and glass matrix . Composition Denture Tooth Porcelain Feldspathic Porcelain Aluminous porcelain Begins as a mixture of powders of feldspar, clay and quartz. Used for ceramo -metal restorations; begins as mixture of powders of potassium feldspar and glass. It can also be used for fabricating porcelain veneers and inlays. Used in pjs’s . It is composed of m ixture similar to t hat of feldspathic p orcelain with i ncreased amounts of aluminium oxide.

17. Feldspar :is responsible for forming the glass matrix . Feldspar is a naturally occurring mineral and composed of two alkali aluminum silicates such as potassium aluminum silicate (K2O-Al2O3-6SiO2); also called as potash feldspar or ortho clase and soda aluminum silicate (Na2O-Al2O3-6SiO2); also called as soda feldspar or albite . It is the lowest melting compound and melts first on firing. Most of the currently available porcelains contain potash feldspar as it imparts translucency to the fired restoration.

18. Role of feldspar : Glass phase formation : During firing, the feldspar fuses and forms a glassy phase that softens and flows slightly allowing the porcelain powder particles to coalesce together. The glassy phase forms a translucent glassy matrix between the other components in the dense solid. Leucite formation: Another important property of feldspar is its tendency to form the crystalline mineral leucite when melted, which is exploited to advantage in the manufacture of porcelain suitable for metal bonding.

19. Silica : It is one of the most abundant elements on earth and can exist in many different forms such as crystalline materials like Quartz,Cristobalite , Tridymite and amorphous materials like Fused quartz,Agate , Jasper and Onyx. Pure Quartz crystals ( SiO 2 ) are used for manufacturing dental porcelain. Quartz (crystalline silica) used in porcelain as a filler and strengthening agent.

20. Kaolin is a type of clay material which is usually obtained from igneous rock containing alumina. Kaolin acts as a binder and increases the moldability of the unfired porcelain. It also imparts opacity to the porcelain restoration so dental porcelains are formulated with limited quantity of kaolin.

21. Glass modifiers are used as fluxes and they also lower the softening temperature and increase the fluidity . Color pigments or frits are added to provide the characteristic shade. Stains : It is created bymixxing the metallic oxides with low fusing glasses. Stains also permit surface characterization and color modification for custom shade matching. Glazes : generally colorless low-fusing porcelains that posses considerable fluidity at high temperature.They fill small surface porosities and irregularities.

22. Ingredient Functions Feldspar (naturally occurring minerals composed of potash [K2O], soda [Na2O], alumina and silica). It is the lowest fusing component, which melts first and flows during firing, initiating these components into a solid mass. Silica (Quartz • Strengthens the fired porcelain restoration. • Remains unchanged at the temperature normally used in firing porcelain and thus contribute stability to the mass during heating by providing framework for the other ingredients. Kaolin (Al2O3.2 SiO2. 2H2O - Hydrated aluminosilicates) • Used as a binder. • Increases moldability of the unfired porcelain. • Imparts opacity to the finished porcelain product. Glass modifiers e.g. K, Na, or Ca oxides or basic oxides They interrupt the integrity of silica network and acts as flux. Color pigments or frits, e.g. Fe/Ni oxide, Cu oxide, MgO , TiO2, and Co oxide. To provide appropriate shade to the restoration Zr / Ce / Sn oxides, and Uranium oxide To develop the appropriate opacity.

23. Dental ceramics exhibit excellent biocompatibility with the oral soft tissues and are also chemically inert in oral cavity. Dental ceramics possesses very good resistance to the compressive stresses, however, they are very poor under tensile and shear stresses . This imparts brittle nature to the ceramics and tend to fracture under tensile stresses. Properties

24. Static fatigue Fatigue is chemically-enhanced, rate-dependent crack growth in the presence of moisture and cyclic application of stresses. Water enters incipient fissures Breaks down cohesive bonds holding the crack walls together Results in initiation of slow crack growth which progresses steadily over time Accelerating at higher stress levels and ultimately leading to failure.

25. Delayed failure in glasses had been attributed to a stress enhanced chemical reaction between glass and water this is likely to occur primarily at the tips of the surface cracks. Structural Defects may arise in the form of micro-cracks of sub-millimeter scale; during fabrication of ceramic prostheses and also from application of masticatory forces in the oral cavity. Surface hardness of ceramics is very high hence they can abrade the opposing natural or artificial teeth. Ceramics are good thermal insulators and their coefficient of thermal expansion is almost close to the natural tooth.

26. Optical properties: Translucency is another critical property of dental porcelains. Incisal porcelains >body >opaque porcelains. Dental porcelains are translucent because there are no free electrons and can be colored by pigments such as metallic oxides to match the shade of teeth. Since the outer layers of a porcelain crown are translucent, the apparent color is affected by reflectance from the inner opaque or core porcelain. The thickness of the body porcelain layer determines the color obtained with a given opaque porcelain. The colours of commercial premixed dental porcelains are in the yellow to yellow-red range.

27. Dental ceramics can be classified according to one or more of the following parameters: Classification of dental ceramics Uses or indications anterior and posterior crown veneer post and core fixed dental prosthesis ceramic stain glaze U ltralow fusing -<850 o C L ow fusing -850-1100 o C Medium fusing- 1101-1300 o C High fusing - >1300 o C Firing temperature

28. Casting Sintering P artial sintering and glass infiltration S lip casting and sintering Hot- isostatic pressing, CAD-CAM milling Copy-milling Processing method Principal crystal phase Silica glass Leucite based feldspathic porcelain Leucite -based glass-ceramic Lithia disilicate –based glass-ceramic Aluminous porcelain Alumina Glass-infused alumina, Glass-infused spinel Glass-infused alumina/ zirconia , Zirconia

29. Translucency Opaque Translucent Transparent Amorphous glass Crystalline Crystalline particles in a glass matrix Microstructure

30. These materials can be formed into inlays, onlays , veneers, crowns, and more complex fixed dental prostheses (FDPs). Several of the core ceramics can be resin-bonded micromechanically to tooth structure. Zirconia can be used for endodontic posts and implant abutments but their primary applications are for crowns and bridges. Zirconia is better suited for applications involving posterior teeth or for elderly patients whose teeth have lost much of their original translucency. Applications of ceramics in dentistry

31. Type Primary Application Secondary Applications Contraindications Feldspathic porcelain Metal-ceramic veneers Anterior laminate veneers Single surface inlays/Low stress sites High translucency needed Inlays, onlays , crowns, and bridges (except as metal-ceramic veneers) Aluminous porcelain Core ceramic for anterior crowns Low-stress premolar crowns Molar crowns Bridges Leucite glass-ceramic Anterior single-unit crowns Anterior laminate veneers Low-stress premolar inlays and crowns High translucency needed High-stress situations Bridges Lithium disilicate glass-ceramic Anterior and premolar crowns Anterior three-unit bridges Premolar crowns Anterior laminate veneers Posterior three-unit bridges High-stress posterior situations Bridges involving molar teeth General Indications and Contraindications for Use of Dental Ceramics

32. Benefits of metal-ceramic prostheses The most outstanding advantage of metal-ceramic restorations is their resistance to fracture. With metal occlusal surfaces, the fracture rate in posterior sites could be reduced further. Advantage of metal-ceramic restorations over total ceramic restorations is that less tooth structure needs to be removed to provide the proper bulk for the crown. Metal-ceramic systems: Composition and Properties

33. CERAMIC COMPOSITION A silica (SiO2) network and potash feldspar (K 2 O•Al 2 O 3 •6SiO 2 ), soda feldspar (Na 2 O•Al 2 O•6SiO 2 ), or both. Feldspathic porcelains contain, by weight, a variety of oxides including a SiO2 matrix (52% to 65%), Al2O3 (11% to 20%), K2O (10% to 15%), Na2O (4% to 15%), and certain additives, including B2O3, CeO2, Li2O, TiO2, and Y2O3. When potassium feldspar is mixed with various metal oxides and fired to high temperatures, it can form leucite and a glass phase that will soften and flow slightly. The softening of this glass phase during porcelain firing allows the porcelain powder particles to coalesce.

34. These ceramics are called porcelains because they contain a glass matrix and one or more crystal phases. When feldspar is heated at temperatures between 1150 °C and 1530 °C, it undergoes incongruent melting to form crystals of leucite in a liquid glass. Incongruent melting is the process by which one material melts to form a liquid plus a different crystalline material. This tendency of feldspar to form leucite during incongruent melting controls thermal expansion during the use of porcelains for metal bonding.

35. Silicate glass represents the matrix phase of feldspathic porcelains. Silica (SiO2) can exist in four different forms: Crystalline quartz Crystalline cristobalite Crystalline tridymite Noncrystalline fused silica The abrasiveness of the finished surface will depend on the thickness of the veneer and the presence or absence of crystalline particles.

36. Veneering ceramics (“porcelains”) for metals have higher expansion and contraction coefficients than the ceramics used to veneer alumina or zirconia core ceramics. There are four types of veneering ceramics. Feldspathic porcelains include Ultralow- and low-fusing ceramics (feldspar-based porcelain, nepheline syenite −based porcelain, and apatite-based porcelain); Low-fusing specialty ceramics (shoulder porcelains and wash-coat ceramics); Ceramic stains Ceramic glazes ( autoglaze and overglaze ).

37. GLASS MODIFIERS Can be defined as elements that interfere with the integrity of the SiO 2 (glass) network and alter their three dimensional state. Their functions are: to decrease the softening point decrease the viscosity (flux action increasing the flow) The main purpose of a flux is principally to lower the softening temperature of a glass by reducing the amount of cross-linking betweent he oxygen and glass forming elements. E.g. Alkali metal ions such as Na, K or Ca (usually as carbonates).

38. However, higher concentration of glass modifiers could result in : Reduced chemical durability (resistance to attack by water, acids and alkalis) Devitrification due to disruption of too many tetrahedral networks(crystallization occurs when the modifiers act as nucleating agents for the process of crystal growth) Manufacturers employ glass modifiers to produce dental porcelains with different firing temperatures such as high medium and low fusing ceramics

39. Boric oxide fluxes (B2O3) can behave as a glass modifier to decrease viscosity, to lower the softening temperature, and to form its own glass network. Incorporation of an intermediate oxide such as alumina (Al 2 O 3 ) increase hardness and viscosity of glass. Another important glass modifier is water, although it is not an intentional addition to dental porcelain. The hydronium ion, H3O+ , can replace sodium or other metal ions in a ceramic that contains glass modifiers. This fact accounts for the phenomenon of “slow crack growth” of ceramics exposed to tensile stresses and moist environments.

40. Pigmenting oxides are added to obtain the various shades needed to simulate natural teeth. These coloring pigments are produced by fusing metallic oxides with fine glass and feldspar and then regrinding to a powder. These powders are blended with the unpigmented powdered frit to provide the proper hue and chroma . Examples of metallic oxides and their respective color contributions to porcelain include : Chromium or chrome (Pink)- aluminia : these pigments are stable upto 1350 C, and are useful in eliminating the greenish hue andgiving a warm tone to the porcelain.

41. Iron oxide (black) or platinum (Grey): used for producing enamels or grayer section of the dentin colors , and also for an effect of translucency. Cobalt salts in the form of oxide (Blue): are useful in developing of the enamel shades. Other pigments used may be : Titanium oxide –yellow brown, Manganese oxide- lavender Iron/nickel oxide-brown Copper oxide – green.

42. Opacity may be achieved by the addition of cerium oxide, zirconium oxide, titanium oxide, or tin oxide to alter the softening point and viscosity. Medium- and high-fusing porcelains are used for the production of denture teeth. The low-fusing and ultralow-fusing types are used as veneering ceramics for crown and bridge construction. Some of the ultralow-fusing porcelains are used for titanium and titanium alloys because of their low contraction coefficients that closely match those of the metals and because the low firing temperatures reduce the risk for growth of the metal oxide.

43. To ensure adequate chemical durability, a self-glaze of porcelain is preferred to an add-on glaze. A thin external layer of glassy material is formed during a self-glaze firing procedure at a temperature and time that cause localized softening of the glass phase. A higher proportion of glass modifiers tend to reduce the resistance of the applied glazes to leaching by oral fluids.

44. METAL COMPOSITION Single-unit crowns and bridges may be made from metalceramic systems (combinations of metal substructure and veneering ceramic. Many alloys are available to be veneered with low-fusing and ultralow-fusing porcelains. The compositions of these high noble, noble, predominantly base metal alloys control the castability , bonding ability to porcelain, the esthetics of the metal-ceramic restoration, and the magnitudes of stresses that develop in the porcelains during cooling from the sintering temperature.

45. Classification According to noble metal content, metal ceramics are broadly classified by the ADA (1984) into 3 major categories: High Noble alloys Noble alloys Base-metal alloys  Generally classified into two general categories ( Anusavice 1996) Alloys - Noble metal alloys Gold - Platinum Gold – Platinum - Silver Gold - Palladium Palladium - silver High palladium

46. System - Base-metal alloys Nickel - Chromium Cobalt - Chromium Other systems Characteristic features and Advantages of Base Metal Alloys positive features: Higher hardness and elastic modulus (stiffness) values permit the fabrication of thinner copings ( upto 0.1mm) and thus its use in long span FPD’s. More sag - resistant at elevated temperatures. Substantial cost difference between base - metal and noble metal alloys.

47. Limiting features : Higher solidification shrinkage requires special compensatory procedures to obtain acceptable fitting. Potential for porcelain delimitation due to separation of poorly adherent oxide layer from the metal substrate. Scrap cannot be used. Potential toxicity of Beryllium and allergic potential of Nickel. Poor resistance to tarnish and corrosion of nickel containing alloys.

48. Components of metal-ceramic restoration

49. CAST METAL FOR METAL-CERAMIC PROSTHESES To bond to alloys suitable for the copings, porcelains must have a sufficiently Low sintering temperature CTEs and CTCs that are closely matched to those of the alloys. The gold alloys developed for porcelain bonding have higher melting ranges than typical gold alloys for all-metal prostheses; the higher melting ranges are necessary to prevent sag, creep, or melting of the coping or framework during the sintering and/or glazing of porcelain. Fabrication of metal-ceramic prostheses

50. These gold alloys contain small amounts (about 1%) of base metals such as iron, indium, and tin. Both the metal and the ceramic must have coefficients of thermal expansion and contraction that are closely matched such that the metal must have a slightly higher value to avoid the development of undesirable residual tensile stresses in the porcelain.

51. CAST METAL COPINGS AND FRAMEWORKS Copings and frameworks for metal-ceramic prostheses are produced by: Casting of molten metal CAD-CAM machining Electrolytic deposition techniques Swaged metal processes. The most common method is the melting and casting of specialized metals for the casting process, the relatively high melting temperatures of most alloys can break down gypsum-bonded investments at the casting temperatures, so the more refractory phosphate-bonded investment must be used.

52. Oil from fingers and other sources such as air lines represents a possible contaminant. The surface may be cleansed adequately by finishing with clean ceramic-bonded stones or sintered diamonds, which are used exclusively for finishing. Final sandblasting with high-purity alumina abrasive before oxidation ensures that the porcelain will be bonded to a clean and mechanically retentive surface. Opaque porcelain is condensed on the oxidized surface at a thickness of approximately 0.3 mm and is then fired to its sintering temperature. Translucent porcelain is then applied, and the tooth form is created.

53. Oxidizing The base metals form a surface oxide layer during the oxidation treatment, and this surface oxide is responsible for development of a bond with porcelain. This process is sometimes called degassing. Controlled oxide layer should be created . 59

54. Methods of condensation : Porcelain for ceramic and metal-ceramic prostheses as well as for other applications is supplied as a fine powder designed to be mixed with water or binder and condensed into the desired form. The porcelain is usually built to shape using a liquid binder to hold the particles together. This process of packing the particles and removing the liquid is known as condensation. Proper and thorough condensation is also crucial in obtaining dense packing of the powder particles. This provides two benefits: Lower firing shrinkage Less porosity in the fired porcelain. 60

55. This packing, or condensation, may be achieved by: Vibration: Mild vibrations are used to densely pack the wet powder upon the underlying matrix. The excess water comes to the surface and is blotted with a tissue paper. Spatulation : A small spatula is used, to apply and smoothen the wet porcelain. This action brings excess water to the surface where it is removed. Brush technique: The dry powder is placed by a brush to the side opposite from an increment of wet porcelain. As the water is drawn toward the dry powder, the wet particles are pulled together.

56. Types of binders: Distilled water : Is the most popular binder used in dentin and enamel porcelain. Propylene glycol : Used in alumina core build up. Alcohol or formaldehyde based liquid for opaque / core build up. 62

57. Building porcelain: The powder is mixed on a glass slab . The mix should not be overstored to avoid the incorporation of large air bubbles . High room temperature and dry atmosphere is to be avoided as the powder can dry out rapidly due to which all spaces are created in the powder bed.

58. Firing dental porcelain: After the condensation and building of a crown it is fired to high density and correct form. At this stage the green porcelain is introduced into the hot zone of the furnace and the firing starts, the glass particles soften at their contact areas and fuse together. This is often referred to as sintering. 64

59. The condensed porcelain mass is placed in front of or below the muffle of a preheated furnace at approximately 650 °C for low-fusing porcelain. This preheating procedure permits the remaining water to evaporate. After preheating for approximately 5 minutes, the porcelain is placed into the furnace and the firing cycle is initiated. As sintering of the particles begins, the porcelain particles bond at their points of contact and the structure shrinks and densifies .

60. As the temperature is raised, the sintered glass gradually flows to fill the air spaces. Air becomes trapped in the form of voids because the fused mass is too viscous to allow all of the air to escape. An aid in the reduction of porosity in dental porcelain is vacuum firing. During vaccum firing ,porcelain powder particles are packed together with air channels around them. As the air pressure inside the furnace is reduced to about one tenth of atmospheric pressure by the vacuum pump, the air around the particles is also reduced to this pressure. As the temperature rises, the particles sinter together, and closed pores are formed within the porcelain mass. 66

61. At a temperature about 55 °C below the sintering temperature, the vacuum is released and the pressure inside the furnace increases by a factor of 10, from 0.1 to 1 atm. Because the pressure is increased by a factor of 10, the pores are compressed to one tenth of their original size, and the total volume of porosity is accordingly reduced. 67

62. Advantages of vacuum-fired porcelain There is a general increase in the strength of the porcelain, which probably is more significant in jacket crowns than bonded veneers. The porcelain will have greater translucence. Porcelain for vacuum firing can have a finer and graded particle size, thus increasing the wet strength of the materials and making it less difficult to carve a built-up mass. Shade is markedly affected by vacuum firing. The lessened number of air spaces decreases the internal reflective surfaces. Thus, with opacity reduced and density increased, it becomes impossible to reproduce precisely the shades made with air firing.

63. 69

64. Classification of the S tages in Maturity: Low Bisque : The surface of the porcelain is very porous and will easily absorb a water soluble die. At this stage the grains of porcelain will have started to soften. Shrinkage will be minimal and the fired body is extremely weak and friable. Lack translucency and glaze. Medium bisque: The surface will still be slightly porous but the flow of the glass grains will have increased. A definite shrinkage will have taken place. Lacks translucency and high glaze. High bisque: The surface of the porcelain would be completely sealed and presents a much smoother surface with a slight shine. shrinkage is complete. Appears glazed.

65. Glazing Porcelains are glazed to give a smooth and glossy surface, enhance, esthetics and promote hygiene. The glazing should be done only on a slightly roughened surface and never should be applied on glazed surfaces. Over glaze: These are ceramic powders containing more amount of glass modifiers thus lowering fusion temperature. It may be applied to porcelain and then fired. Self glaze: All the constituents on the surface are melted to form a molten mass about 25 μ m thick. Thus the porcelain is said to be self glazed. 71

66. Autoglazed feldspathic porcelain is stronger than unglazed porcelain. The glaze is effective in sealing surface flaws and reducing stress concentrations. If the glaze is removed by grinding, the transverse strength is reduced and if this surface is left in rough condition it can cause increased wear of enamel. 72

67. Add on porcelains The add on porcelains are made from similar materials to glaze porcelain except for the addition of opacifiers and coloring pigments. These are sparingly used for simplest corrections like correcting of tooth contour / contact points. 73

68. METAL-CERAMIC CROWNS AND BRIDGES BASED ON SWAGED METAL FOIL LAMINATES The most widely used product of this type has been Captek (Precious Chemicals Co., Inc., Altamonte Springs, FL), which is an acronym for “capillary assisted technology.” The product is designed to fabricate the metal coping of a metal-ceramic crown without the use of a melting and casting process. It is a laminated gold alloy foil sold as a metal strip. For bridges, the pontics are made typically from a palladium-based alloy that is gold-coated. The technology is based on the principle of capillary action to produce a gold-based composite metal.

69. Captek P and G metals can yield thin metal copings for crowns or frameworks for metal-ceramic bridges. The maximal span length recommended for Captek -porcelain bridges is 18 mm, which allows space for up to two pontics .

70. FABRICATION BY CAPTEK Captek ™ alloys are composed of two major components: The first component, when heated, forms a microscopic three-dimensional network of capillaries. The second, when melted, flows to fill these capillaries. This microscopic process works by the forces of capillary attraction to produce a solid-metal composite alloy . The process begins with the preparation of a master refractory die that replicates the prepared tooth. The die is heat-treated, and the margins are marked with a red pencil.

71. Captek adhesive is applied to the die to enhance adhesion to the Captek metal and to enhance capillary action. After heat treatment, Captek P metal, a malleable Au-Pt-Pd alloy, is adapted to the surface. This metal layer provides a three-dimensional capillary network that will subsequently be filled with Captek G metal (97.5% Au, 2.5% Ag by weight) to form an alloy with a high gold content. After this composite material is burnished on the die and the margins are trimmed, it is sintered in a porcelain furnace.

72. The metal copings and Pd-Ag pontics (if needed) are then coated with a slurry of Au, Pt, and Pd powder ( Capbond ) and liquid, resulting in a thin coating of gold to enhance areas of Captek P that have been ground during adjustment and to provide a gold color similar to that of areas that have not been ground. The completed copings have a thickness of approximately 0.25 mm. Thus, this method provides thinner metal copings than those (0.50 mm) typically produced by the cast-metal process. The metal surfaces are veneered with two thin coats of an opaque porcelain and additional layers of translucent porcelain.

73. The atomic bonding of veneering ceramics to Captek copings and frameworks is controlled by a special bonder layer, while bonding of ceramics to cast metal is controlled by oxidizable elements within the alloy surface. Another potential concern for the Captek system is the difficulty of bonding the dissimilar metals in the coping and the pontic surfaces. Clinical data for Captek crowns and bridges are very limited. Thus, caution must be exercised in using this system for crowns and bridges in high-stress areas.

74. BONDING MECHANISMS Three mechanism have been described to explain the bond between the ceramic veneer and the metal substructure. Mechanical entrapment Compressive forces Chemical bonding 80

75. Mechanical entrapment: This creates attachment by interlocking the ceramic into the microabrasions on the surface of the metal coping which are created by finishing the metal with non contaminating stones / discs and are abrasives. Air abrasion appears to enhance the wettability , provide mechanical interlocking. The use of a bonding agent having platinum spheres 3-6 μ m in diameter can also increase the bond significantly.

76. Compressive forces: These are developed by a properly designed coping and a slightly higher coefficient of thermal expansion than the porcelain veneered over it. This slight difference will cause the porcelain to draw towards the metal coping when the restoration cools after firing. Chemical bonding It is indicated by the formation of an oxide layer on the metal. The trace elements like tin, indium, gallium/iron form oxides and bond to similar oxides in the opaque layer of the porcelain.

77. The principal deficiencies faced by ceramics are - brittleness, low fracture toughness and low tensile strength. Methods used to overcome the deficiencies fall into 2 general categories: Method of strengthening brittle materials. Method of designing components to minimize the stress concentrations and tensile stresses. Methods for strengthening ceramics

78. In the oral environment tensile stresses are usually created by bending forces, and the maximum tensile stresses occur at the surface of the restoration. It is for this reason removal of the surface flaws can result in the increased strength of the material. Smoothing and reducing flaws is one of the reason for glazing of dental porcelain. Strengthening of the brittle materials can be done in a 2 ways. Development of residual compressive stresses within the surface of the material. Interruption of crack propagation through the material. Method of strengthening materials :

79. Development of residual compressive stresses within the surface of the material: One widely used method of strengthening ceramics is the introduction of residual compressive stresses. Strength is gained by virtue of the fact that the residual stresses developed must first be negated by the developing tensile stresses before a net tensile stress develops in the material. THREE of the methods used in achieving this objective are:

80. Ion exchange mechanism : This technique is called as chemical tempering and is the most sophisticated and effective way of introducing residual compressive stresses. In this procedure a sodium containing glass is placed in a bath of molten potassium nitrate, potassium ions in the bath exchange places with some of the sodium ions in the surface of the glass particle. The potassium ion is about 35% larger than the sodium ion. The squeezing of the potassium ion into place formerly occupied by sodium ion creates large residual compressive stresses in the surface of the glass. These residual stresses produce a strengthening effect. This process is best used on the internal surface of the crown, veneer/inlay as the surface is protected from grinding and exposure to acids.

81. The technique is as follows: Characterize the finished crown and adjust the occlusion. Place the crown into a mould of analytically pure potassium nitrate powder which is in a small porcelain crucible/ stainless steel container. Place the container in a cool furnace and raise the temperature slowly to 500°C Hold the temperature at 500 C for 6 hours. Remove the crown from the solution and allow it to drain in the furnace. Remove the crown from the furnace and cool to room temperature.

82. b. Thermal tempering: This is the most common method of strengthening glass. This creates residual surface compressive stresses by rapidly cooling (quenching) the surface of the object while it is hot and in the softened state. This rapid cooling produces a skin of rigid glass surrounding a soft molten core. As the molten core solidifies, it tends to shrink, but the outer skin remains rigid. The pull of the solidifying molten core as it shrinks, creates residual tensile stresses in the core and residual compressive stresses within the outer surface.

83. For dental applications it is more effective to quench the glass phase ceramics in silicone oil or other special liquids than using air as it may not uniformly cool the surface. c.Thermal compatibility method applies to porcelain fused metals. The metal and porcelain should be selected with slight mismatch in their thermal contraction coefficient. Usually the difference of 0.5 × 10–6/°C in thermal expansion between metals and porcelain . It causes the metal to contract slightly more than does the ceramic during cooling after firing the porcelain which results in development of residual compression in the ceramic surface

84. 2) Interruption of crack propagation- Dispersion of crystalline phase – Crystalline reinforcement: A method of strengthening glasses and ceramics is to reinforce them with a dispresed phase of different material that is capable of hindering crack propagation through the material. The crystalline phase with greater thermal expansion coefficient than the matrix produces tangential compressive stress (and radial tension) near the crystal matrix interface. Such tangential stresses divert the crack around the particle. 92

85. When a tough, crystalline material such as a alumina in particulate form is added to a glass, the glass is toughened and strengthened because the crack cannot penetrate the alumina particles as easily as it can the glass and this technique is applied in the development of aluminous porcelains for PJCs. Another ceramic dental material that uses reinforcement of a glass by a dispersed crystalline substance is Dicor glass-ceramic.

86. 94 Transformation toughening- A newer technique of strengthening glasses involves the incorporation of a crystalline material that is capable of undergoing a change in crystal structure when placed under stress. The crystalline material usually used is termed partially stabilized Zirconia (PZC). The energy required for the transformation of PSZ is taken from the energy that allows the crack to propagate. One drawback of PSZ is an opacifying effect that may not be aesthetic in most dental restorations.

87. Tetragonal phase is not stable at room temperature and it can transform to the monoclinic phase leading to a corresponding volume increase. When sufficient stress develops in the tetragonal structure and a crack in the area begins to propagate, the metastable tetragonal crystals (grains) precipitates next to the crack tip can transform to the stable monoclinic form.

88. The design should avoid exposure of ceramics to high tensile stresses. It should also avoid stress concentration at sharp angles or marked changes in thickness . Minimizing tensile stresses: When porcelain is fired onto a rigid material the shape of the metal will influence the stresses set up in the porcelain. If it is a full coverage crown the metal being of higher thermal expansion will contract faster than the porcelain, as a result the metal is placed in tension and the porcelain in compression. Methods of designing components to minimize stress concentrations and tensile stresses

89. For partial metal coverage the junction between the metal and porcelain is therefore a potential site for high stress as the area with only metal will have no balancing compressive forces. b. Reducing stress raisers Stress raisers are discontinuities in ceramic structures in brittle materials that cause stress concentration. Abrupt changes in shape/ thickness in the ceramic contour can act as stress raisers and make the restoration more prone to failure. Sharp line angles in preparation and small particle of porcelain along internal margin of crown also causes tensile stresses.

90. If the occlusion is not adjusted properly on a porcelain surface, contact points rather than contact areas will greatly increase the localized stresses in the porcelain surface as well as within the internal surface of the crown. These contact stresses can lead to the formation of the so-called Hertzian cone cracks, which may lead to chipping of the occlusal surface.

91. Classification of cermo / metal failure has been made by O’Brien(1977) and is related to interface formed at fracture. Metal Ceramic Restorations – Common Failures: Types of Metal/ Porcelain Failure Failure Reason 1. Fracture during bisque bake. Improper condensation Improper moisture control Poor frame work design Incompatible metal-porcelain 2. Bubbles Too many firings Air entrapment during building of restoration Improper moisture control Poor metal preparation Poor casting technique 3. Clinical fracture Poor framework

92. Benefits and drawbacks of metal-ceramic restorations A properly made metal-ceramic crown is more fracture resistant and durable than most all-ceramic crowns and bridges. A metal coping or framework provides an advantage compared with zirconia -based ceramic prostheses when endodontic access openings through crowns are required. Temporary repairs for ceramic fractures that extend to the metal framework are possible without the need for intraoral sandblasting treatment by using current resin bonding agents.

93. Disadvantages of metal-ceramic prostheses Abrasive damage to opposing dentition Potential for fracture Excessive exposure to acidulated fluoride can enhance chemical degradation of ceramic surface. Patient may be exposed to silaceous dust by inhalation during grinding. The potential for metal allergy.

94. Not the best esthetic choice for restoring a single maxillary anterior tooth. A dark line at the facial margin of a metal-ceramic crown associated with a metal collar or metal margin is a significant esthetic concern when gingival recession occurs .

95. References Phillips science of dental materials –First South Asia edition Craig’s Restorative dental materials –13th edition. W. Patrick Naylor,Introduction to Metal – Ceramic Technology – Second edition William J.O Brien, Dental materials and their selection- 3 rd edition. Kelly JR. Nishimura I. Campbell SD. Ceramics in dentistry: Historical roots and current perspectives. J prosthet dent 1996:75 18-32.

96. Kelly J. Dental ceramics:current thinking and trends. Dent Clin N Am 2004(48):513-530. Babu PJ, Krishna R .All Dental Ceramics: Part I – An Overview of Composition, Structure and Properties . American Journal of Materials Engineering and Technology, 2015(3)1: 13-18