DENTAL CERAMICS
CONTENTS
• HISTORICAL PERSPECTIVE
• CLASSIFICATION AND COMPOSITION
• STRUCTURE
• PORCELAIN CONDENSATION
• ...
During the stone age more than 10,000 years ago ceramics were important
materials and they have retrained their importance...
produced the first individual porcelain “terrometallic teeth”. In 1844 the
nephew of Stockton whohad introduced porcelain ...
Obtained by fluxing white China with “Chine stone” to produce a white
translucent stoneware.
According to firing temperatu...
Silica glass
Leucite based glass ceramic
Lithia based glass ceramic
According to processing method:
Sintering
Partial sint...
control fusion temperature, sintering temperature, thermal contraction
coefficient and solubility.
Feldspar :
They are mix...
Potassium, lithium, sodium and calcium oxide and boric acid are used as
fluxes by interrupting the integrity of the SiO4 n...
As the natural teeth possess a yellow white fluorescence, in the early days
the absence of this quality was noticed under ...
Boric oxide can act as a flux and also as a glass former.
Oxides like alumina may react either way, depending on other fac...
The condensed porcelain mass is placed in front of the muffle of a preheated
furnace (approximately 650 degrees.). This pe...
can be made. The fewer the firing cycles to which the restoration is exposed,
the higher will be the strength and better t...
Over glaze
Self glaze.
Porcelains may be characterized with stains and glazes to provide a more life
like appearance.
One ...
Chemical bonding across the porcelain-metal interface. Alloys that form
adherent oxides during degassing cycle also form a...
When dental porcelain is fired onto metal with a definite oxide (indium, tin
or zinc oxide) layer, the oxygen surface of t...
as the spacing between several atoms in the material. These stress
concentration areas at the tip of each surface flaw can...
Repeated firing can causeincrease in the leucite concentration of the
porcelain which can cause increased coefficient of t...
THERMAL TEMPERING:
This creates residual surface compressive stresses by rapidly cooling the
surface of the object while i...
ABRASIVENESS OF DENTAL CERAMICS
Abrasive wear mechanisms for ceramics and tooth enamel are
predominantly due to micro frac...
• Readjust occlusion periodically if needed.
19
FINISHING AND POLISHING OF PORCELAINS:
1. Contour with flexible diamond disks, diamond burs, heatless
stones or green ston...
PREDOMINANTLY BASE:
Nickel-chromium
Nickel-chromium-beryllium
Cobalt-0chromium
HIGH NOBLE ALLOYS:
Noble metal content >60%...
In-Sn bonding Rh castability
Alloys that have been to be are composed of Gold 45-55%
Pd 35-45%
With a small amount of gall...
Mo-added to decrease CTE
Be –improve castability and hardening
Alloys are harder than noble alloys but have low yield stre...
Hue: dominant color of an object, wavelength
Chroma: saturation
Value: lightness or darkness ….independent of hue
Metameri...
Metals-high tensile strength, toughness, hardness, resistance to abrasion,
fracture resistance, elasticity, ductility fati...
FLEXURAL STRENGTH (BENDING STRENGTH OR MODULOUS
OF RUPTURE):
Force per unit area at the point of fracture of a test specim...
THERMAL CONDUCTIVITY (cal.cm/cm2.sec.C):
Ability of a body to transfer energy
Enamel: 0.0022
Dentin: 0.0015
Porcelain: 0.0...
RECENT ADVANCES IN DENTAL CERAMICS:
METAL CERAMIC CROWNS BASED ON BURNISHED FOIL
COPINGS: THE CAPTEK SYSTEM
Malleable Capt...
Improved esthetics-chameleon effect
Minimal shrinkage
Good marginal fit
High flexural strength
Low thermal expansion
Minim...
DISADV:
Potential to fracture in posterior areas
Need to use resin cement
INFILTRATED CERAMICS (INCERAM):
Available as two...
CAD –CAM CERAMICS (PROCERA, CEREC, CELAY, DICOR
MGC):
It stands for Computer aided design/Computer aided manufacturing.
It...
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Dental ceramics/ rotary endodontic courses by indian dental academy

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Indian Dental Academy: will be one of the most relevant and exciting training center with best faculty and flexible training programs for dental professionals who wish to advance in their dental practice,Offers certified courses in Dental implants,Orthodontics,Endodontics,Cosmetic Dentistry, Prosthetic Dentistry, Periodontics and General Dentistry.

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Dental ceramics/ rotary endodontic courses by indian dental academy

  1. 1. DENTAL CERAMICS CONTENTS • HISTORICAL PERSPECTIVE • CLASSIFICATION AND COMPOSITION • STRUCTURE • PORCELAIN CONDENSATION • SINTERING • BONDING PORCELAIN TO METAL • METHODS OF STRENGTHENING CERAMICS • ABRASIVENESS OF DENTAL CERAMICS • FINISHING AND POLISHING OF PORCELAINS • ALLOYS FOR METAL CERAMIC RESTORATIONS • FACTORS AFFECTING COLOR • PROPERTIES • REVIEW OF LITERATURE • RECENT ADVANCES IN CERAMICS • SUMMARY AND CONCLUSION • BIBLIOGRAPHY 1
  2. 2. During the stone age more than 10,000 years ago ceramics were important materials and they have retrained their importance inn human societies ever since. Craftsmen of that age used rocks that could be shaped into tools and artifacts by a process called flaking in which stone chips could be fractured away from surfaces of hard, fine grained or amorphous rocks. In 700 B.C., the Etruscans made ivory teeth and bone teeth that were held in place by gold framework. Animal bone and ivory from hippo or elephant were used for many years thereafter. Porcelain was obtained in China by fluxing white china clay with “ chine stone” to produce a white translucent stone ware in about 1000 A.D. this material was much stronger than the earthen ware and stone ware. The formulation was however a closely guarded secret . The Germans discovered “white porcelain” in 1708 but it lackedthe translucency of the Chinese product. In what might be labeled as the first known case of industrial espionage a Jesuit father named D’entrecolles was able to gain the confidence of Chinese potters and learnt the secret in 1717. In 1774 a French apothecary named Alexis Duchateau noticed that glazed ceramic utensils that he used everyday for mixing and grinding his various chemical resisted staining and abrasion. It would asppear that these were the circumstances that gave birth to the idea of using porcelain as a dental restorative material. Later Nicholas Dubious de chemant of Paris in collaboration with Alexis Duchateau considerably improved the method of fabricating dentures. In 1803 Elias Wildman formulated amuch more translucent porcelain with shades much closer toantural teeth. The foundation for modern mass production of artificial teeth was laid by the Italian dentist Fonzi when he 2
  3. 3. produced the first individual porcelain “terrometallic teeth”. In 1844 the nephew of Stockton whohad introduced porcelain to U.S.A founded the S.S. White Company and this led to refinement of the design and mass production of porcelain teeth. In 1880 Ambler test improved the design of dental coke burning oven and dentistry went through various designs of using gas and finally electric furnace was introduced at the end of century. Two of the most important breakthroughs responsible for the long standing superb esthetic performance and clinical survivability of metal ceramic restorations are the patents of Weinstein and Weinstein(1962) na Weinstein et al (1962) which described the formulations of feldspathicporcelain that allowed systemic control of sintering temperature and ther4mal expansion coefficient. The first commercial porcelain was developed in 1965 by Vita Zahnfabrik. A significant improvement in fracture resistance was reported by Hughes in 1965 with the introduction of aluminous core porcelain. CLASSIFICATION AND COMPOSITION According to history: Earthenware: Fired at low temperature and is relatively porus. Stoneware: Appeared in china in about 100 B.C Porcelain: 3
  4. 4. Obtained by fluxing white China with “Chine stone” to produce a white translucent stoneware. According to firing temperature: High fusing : 1300 degree centigrade Medium fusing : 1101-1300 degree centigrade Low fusing : 850-1100 degree centigrade Ultra low fusing :< 850 degree centigrade According to use: Anterior Posterior Crowns Veneers Post and cores FPD Stain ceramic Glaze ceramic According to composition: Pure alumina Pure Zirconia 4
  5. 5. Silica glass Leucite based glass ceramic Lithia based glass ceramic According to processing method: Sintering Partial sintering Glass infiltration CAD-CAM Copy milling According to tranclucency Opaque Translucent Transparent COMPOSITION Ceramic: Inorganic compound with non metallic properties typically composed of metallic and non metallic elements. Dental ceramic: An inorganic compound with non metallic properties typically consisting of oxygen and one or more semi metallic elements that is formulated to produce the whole or part of a ceramic based prosthesis. Dental Porcelain: is a vitreous ceramic based on silica network and potash feldspar or soda feldspar. Pigments, opacifiers and glasses are added to 5
  6. 6. control fusion temperature, sintering temperature, thermal contraction coefficient and solubility. Feldspar : They are mixtures of potassium aluminium silicate K2O.Al2O3.6SiO2, and albite Na2O.Al2O3.6SiO2. When feldspar is melted at approximately 1250- 1500 degree Celsius, it fuses to become a glass with a free crystalline silica phase. The soda form tends to lower fusion temperature while potash form increases the viscosity of the molten glass. When feldspar is heated at temperatures between 1150 and 1530 degree centigrade it undergoes incongruent melting to form crystals of leucite which is a K-Al-Silicate mineral with a large coefficient of thermal expansion. Kaolin: Is hydrated aluminium silicate (Al2O3.2SiO2.2H2O) that acts as a binder to increase the moldability of the unfired porcelain porcelain. Because of its opaqueness it is present in only very small quantities if at all. Quartz: It is a high fusing material forms the framework around which the other ingredients flow. It prevents the slumping of the crown during the liquid phase. Alumina: Many European tooth manufacturers use alumina in place of silica to strengthen the teeth, especially around the pins. Fluxes: 6
  7. 7. Potassium, lithium, sodium and calcium oxide and boric acid are used as fluxes by interrupting the integrity of the SiO4 network, and lower the softening temperature of a glass by reducing the amount of cross linking between silica and oxygen. Diagram from seminar. The O: Si ratio in a glass is of greatest importance and increasing this ratio will cause reduced viscosity, lowered fusion temperature and increased thermal expansion. Coloring agents: The coloring pigments added to porcelain are known as color frit. These are prepared by fritting metallic oxides into the basic glass used in porcelain. Some of the common colors used are: Pink : Tin chromium or chroma alumina Yellow : Indium or praesmodyium Blue : Cobalt salt Green : Chromium oxide Grey : Iron oxide or platinum Opacifying agent: Dental porcelain materials having varying degrees of translucency can be manufactured by the addition of opaque materials. These are fine particles of metal oxide and have a significantly different refractive index. Their melting point is also higher than that of the matrix. Fluorescence: 7
  8. 8. As the natural teeth possess a yellow white fluorescence, in the early days the absence of this quality was noticed under violet light. The agent commonly used is the uranium salt, sodium di urinate. T his salt produces a strong greenish-yellow color. The ceramist should be aware of the radiation hazards of including uranium. STRUCTURE Dental porcelains contain a crystal phase and glass phase based on the silica structure. This structure is characterized by the Si-O tetrahedron in which a Si 4+ cation is positioned at the center of a tetrahedron with O- anions at each four corners. The structure is not close packed and it has both covalent and ionic characteristics. The silica tetrahedra are linked together by sharing their corners. Fused silica is a material whose high melting temperature is attributed to the 3 – D networks of covalent bonds between silica tetrahedra which form the basic structural units of the glass network. Fluxes are added to reduce the temperature required to sinter the porcelain powder particles together at low enough temperatures so as that the alloy to which it is fired does not melt or sustain sag deformation. Dental porcelains use the basic silicon –oxygen network as the glass forming matrix, but additional properties, such as low fusing temperature, high viscosity and resistance to devitrification are built in by the addition of other oxides to the glass forming Si-O 4 lattice. These oxides generally consist of potassium, calcium, Aluminium and boric oxides. Potassium, sodium and calcium oxides are used as glass modifiers that is they interrupt the integrity of the sio4 network and act as fluxes. Thje purpose of a flux is to lower the softening temperature of a glass by reducing the amount of cross linking between the oxygen and the glass forming elements. If too many tetrahedrons are disrupted then the glass may devitryfy. 8
  9. 9. Boric oxide can act as a flux and also as a glass former. Oxides like alumina may react either way, depending on other factors such as composition. Such oxides are called intermediates. PORCELAIN CONDENSATION Porcelain is supplied as a fine powder that is designed to be mixed with water or another vehicle and condensed to desired form. The particles are of a particular size distribution to produce the most densely packed porcelain when packed. This provides two benefits: Lower firing shrinkage Less porosity. The methods of condensation are: Vibration technique Spatulation technique Brush technique The surface tension of the water is the driving force behind condensation and the porcelain must never be allowed to dry out until condensation is complete. SINTERING OF PORCELAIN Diagrams and matter to be scanned The purpose of firing is simply to fuse the particles together, a process called sintering. 9
  10. 10. The condensed porcelain mass is placed in front of the muffle of a preheated furnace (approximately 650 degrees.). This permits the remaining water vapor to dissipate. After preheating for 5 min the porcelain is placed into the furnace and the firing cycle is initiated. If it is placed directly without preheating it will result in rapid production of steam thereby inducing voids or fracturing large sections of veneer. The progressive changes that occur during the firing of porcelain is shown in the figure. The white areas are powder particles and the area between are voids. As the temperature is raised the fused glass gradually flows to fill up the air spaces but the air becomes trapped in the form of bubbles because the fused mass is too viscous to allow all of it to escape. STAGES IN FIRING LOW BISQUE: The glass grains have softened and have started to flow. The fired article exhibits rigidity but it is very porous. The powder particles lack complete cohesion. A negligible amount of firing shrinkage occurs. MEDIUM BISQUE: The glass grains have flowed to the extent that the powder particles exhibit complete cohesion. The article is still porous and at this stage there is definite shrinkage. HIGH BISQUE: After the high bisque stage, the shrinkage is complete and the mass exhibits a smoother surface but the body does not appear glazed. The work can be removed from the furnace and cooled at any of these stages, so that additions 10
  11. 11. can be made. The fewer the firing cycles to which the restoration is exposed, the higher will be the strength and better the esthetics. Minimum of three firings are needed for fabrication of ceramometal restoration: Opaque Dentin and enamel Stain and glaze Porcelain shrinks 30-40 % during firing- oversize the buildup. Firing in air – entrapment of air causes formation of pores in porcelain(6.3 % voids-undesirable roughness and pits and also affects strength and optical properties) Porcelain for PFM are fired under vacuum thus as the furnace door closes the pressure is lowered to 0.1 atmosphere and the temp is raise until firing tempo is reached . th e vacuum is then released and the furnace pressure returns to 1 atm- Dense pore free porcelain. GLAZING After porcelain is cleaned stains required are applied and porcelain returned to furnace for final glaze f9irting. When the glazing temp is reaches a thin glassy film( glaze) is formed by viscous flow on the porcelain surface. Glazed porcelain is stronger than unglazed. Glaze is effective in reducing crack propagation. If glaze is removed by grinding transverse strength is reduced to half. Two types of glazes : 11
  12. 12. Over glaze Self glaze. Porcelains may be characterized with stains and glazes to provide a more life like appearance. One metjhjod of ensuring that the stains remain permanently isn by incoprporating the stains internally. Internal stasining and charecterizat5ion can produce a lifelike result, particularly when simulated enamel craze lines and other features are built into the porcelain rather than merely applied to the surface. The disadvantage is that the porcelain must be stripped away completely if the color or characterization is unsuitable. COOLING: Must be carried out gradually and uniformly. Too rapid – surface cracking and loss of strength Too slow- might induce formation of additional leucite. Increased the overall coefficient of thermal expansion cracking, crazing. Less is the no of firing higher is the strength and better the esthetics. Too many firing cycles – lifeless over translucent porcelain. BONDING PORCELAIN TO METAL The primary requirement for the success of a metal ceramic prosthesis is the development of a durable bond between the porcelain and the alloy. Theories of metal ceramic bonding have historically fallen into two groups: Mechanical interlocking between porcelain and metal 12
  13. 13. Chemical bonding across the porcelain-metal interface. Alloys that form adherent oxides during degassing cycle also form a good bond to porcelain whereas those alloys with poorly adherent oxides form poor bonds. Some palladium silver alloys form no external oxide at all but rather oxidize internally. It is for these alloys that mechanical bonding is needed. The nature of bond can be divided into three maincomponents: Mechanical Compressive Chemical Mechanical: It is dependent upon good wetting of the metal or metal oxide surface by porcelain. It is improved by a textured surface. A rough surface may enhance the bond resistance against induced shear stresses, especially for base metal alloys. Eg: air abrasion. Advantages: Enhances wettability Additive bond strength Increased surface area Compressive: Ceramo-metal systems are deliberately designed with a very small degree of mismatch in order to leave the porcelain in a state of compression. Chemical bonding: 13
  14. 14. When dental porcelain is fired onto metal with a definite oxide (indium, tin or zinc oxide) layer, the oxygen surface of the molten glass diffuses within the oxygen surface on the metal to reduce then no. of bridging oxygen and thus improves the screening of cations at the interface. If the glass is not saturated with the particular oxide, it dissolves the oxygen with metallic cations. The glass at the glass oxide interface then becomes saturated with oxide. This glass remains constant in composition and is in thermodynamic equilibrium with the oxide resulting in a balance of bond energies and a chemical bond. The critical requirement for maintaining saturation at the oxygen- glass interface is that the rate of solution of the oxide at the interface is higher than the rate of diffusion of the dissolved oxide away from the interface. Procedure: The metal is degassed by heating at 1000 degrees in vacuum for around 10 min and then slowly air cooled in normal atmosphere. This procedure will: 1. Degas the casting. 2. Induce age hardening of the alloy. 3. Base metal atoms will diffuse onto the surface to form an oxide film. Shear strengths of enamel porcelain bonds: Type of bond failures: METHODS OF STRENGTHENING CERAMICS Minimize the effect of stress raisers: Numerous minute scratches and other defects are present on the surface of these materials which behave as sharp notches whose tips may be as narrow 14
  15. 15. as the spacing between several atoms in the material. These stress concentration areas at the tip of each surface flaw can increase the localized stress to the theoretical strength of the material. When the induced mechanical stresses exceed the actual strength of the material, the bond at the notch tip breaks forming a crack. The design of the ceramic dental restoration should also avoid stress raisers in ceramic. Conditions that can cause stress concentration: 1. Abrupt changes in shape or thickness in the ceramic contour. 2. Sharp line angles. 3. creases or folds of the platinum foil or gold foil substrate. 4. small particle of porcelain along the internal porcelain margin 5. improper occlusal contacts As ceramics tend to hasve no mechanism for plastically deforming withoutfrac ture as dometals, cracks may propogate through a ceramic at low stress levels. Thus ceramics and glasses have tensile strengths much lower than their compressive strengths. DEVELOP REDIDUAL COMPRESSIVE STRESSES: The metal and porcelain should be selected with a slight mismatch in their thermal contraction coefficients so that the metalcontracts slightly more than the porcelain on cooling . this mismatch leaves the porcelain in residual compression and provides additional strength for the prosthesis. The same principles apply to ceramic prosthesis in which the thermal contraction coefficient of of core ceramic is slightly greater than that of the veneering ceramic. MINIMIZE THE NUMBER OF FIRING CYCLES: 15
  16. 16. Repeated firing can causeincrease in the leucite concentration of the porcelain which can cause increased coefficient of thermal contraction which if exceeds that of the metal can lead to immediate or delayed crack formation in the porcelain. MINIMIZE TENSILE STRESS THROUGH OPTIMAL DESIGN OF CERAMIC PROSTHESIS: 1. Avoid sharp line angles 2. avoid great amount of vertical overlapin anteriors. 3. avoid marked changes in thickness 4. use maximal occlusal thickness of the porcelain i.e., 2mm. 5. the metal should be in occlusal contact or the porclain should cove the occlusal surface so that the metal ceramic interface should be atleast 1.5 mm away from occlusal contact. 6. in FPD’s use greater connector height and broaden the radius of curvature on gingival portion of interproximal connector. 7. use the finest grit abrasive for grinding. ION EXCHANGE: If a sodium containing porcelain is kept in a water bath containing molten potassium nitrate potassium ions exchange places with some of the sodium ions. Since K ions are 35% bigger than Na ions, the squeezing of K ion into the place of Na creates very large residual compressive stresses. Increases of 100% in flexural strength have been observed by this technique. The depth of this compression zone however is less than 100o micrometers. 16
  17. 17. THERMAL TEMPERING: This creates residual surface compressive stresses by rapidly cooling the surface of the object while it is still hot and in the softened state. This induces a protective region of compressive stress within the surface. DISPERSION STRENGTHENING: This is reinforcement with a dispersed phase of a different material that is capable of hindering a crack from propagating through the material. Some of the crystals that can reinforce the glass phase of ceramics are: Leucite (K2O.Al2O3.4SiO2) Lithia disilicate(Li2O.2SiO2) Alumina(Al2O3) Magnesia alumina spinel (MgO.Al2O3) Zirconia(ZrO2) TRANSFORMATION TOUGHENING: Dental ceramics based on Zirconia crystals undergo transformation toughening that involves the transformation of ZrO2 from a tetragonal crystal phase to a monoclinic phase at the tips of cracks that are in regions of tensile stress. Tetragonal phase is not stable at room temp. So it has a tendency to trasnsform to monoclinic phase. This transformation is prevented by the addition of yttrium oxide or Y2O3. Yttria stabilized zirconia ceramic is also called CERAMIC STEEL. 17
  18. 18. ABRASIVENESS OF DENTAL CERAMICS Abrasive wear mechanisms for ceramics and tooth enamel are predominantly due to micro fracture which results from gouging, asperities, impact, and contact stresses that cause cracks or localized fracture. The fracture toughness of different asperites and enamel is as follows: Alumina:3.4-4 MPa. m1/2 Yttrium stabilized zirconia : 6-9 MPa.m1/2 Glass : 0.75 Mpa.m1/2 Enamel : 0.77 Mpa.m1/2 Jagger and Harrison (1994) reported that the amount of wear produced by both glazed and unglazed porcelain was similar but the wear produced by polished porcelain was substantially less. Exposure to carbonated beverages has been shown to significantly increase the amount of wear. Steps to minimize wear: • Ensure cuspid guided disclusion • Eliminate occlusal prematurities • Use metal in functional bruxing areas • If occlusion in ceramic, use ultralow fusing ceramics • Polish functional ceramic surfaces • Repolish ceramic surfaces periodically 18
  19. 19. • Readjust occlusion periodically if needed. 19
  20. 20. FINISHING AND POLISHING OF PORCELAINS: 1. Contour with flexible diamond disks, diamond burs, heatless stones or green stones (Silicone carbide) 2. Finish with white stones or abrasive impregnated rubber disks, cups or points. 3. Polish with fine impregnated rubber cups, and points or diamond paste applied with a brush 4. Apply an over glaze layer. Scenarios: • New prostheses • New prosthesis needing grinding • Old in situ prosthesis which has roughened • Old in situ prosthesis which requires occlusal correction. ALLOYS USED FOR METAL-CERAMIC RESTORATIONS: HIGH NOBLE: Gold-platinum-palladium Gold palladium-silver Gold-palladium NOBLE: Palladium-silver High palladium 20
  21. 21. PREDOMINANTLY BASE: Nickel-chromium Nickel-chromium-beryllium Cobalt-0chromium HIGH NOBLE ALLOYS: Noble metal content >60% with at least 40% gold. The choice of an alloy will depend on a no of factors: cost. Rigidity. Castability, ease of finishing and polishing, corrosion resistance, compatibility with a specific porcelain and personal preference. Au-Pt-Pd: Good corrosion resistance Pt and Pd increase the melting range. Zn, Sn, Fe are present to form oxides and ceramometal bond. Rhumium-grain refiner High stiffness but low sag resistance. Costly.. Are yellow..Better esthetics than white alloys. Au-Pd: Corrosion resistance. Have increased Pd content. Indium bonding. Rh grain refiner. Ruthenium—castability,.. Gallium decrease the fusion temperature strong stiffer and harder. Au-Pd-Ag Less of palladium good corrosion resistance 21
  22. 22. In-Sn bonding Rh castability Alloys that have been to be are composed of Gold 45-55% Pd 35-45% With a small amount of gallium indium and tin. ]Dis Adv: Cost and incompatibility with certain types of porcelains. Noble alloys Have noble metal content of atleast 25% Pd-Ag contains no gold… have high Ag content Have lowest Noble content In-Sn bonding Rh castability. Some ceramics used with high Ag alloys resulted in what was called the greening effect. High Pd Contain high Pd with 10-15 % Cu. In bonding Gallium casting temp, high strength, hardness low density. Have low sag resistance and form dark oxides. BASE METAL ALLOYS: Have < 25 % noble content. Ni-Cr: Cr provides tarnish and corrosion resistance 22
  23. 23. Mo-added to decrease CTE Be –improve castability and hardening Alloys are harder than noble alloys but have low yield strengths. Higher elastic modulus thinner copings And framework could result lower densities and higher casting temperature but Be is a carcinogen can pose toxicity problems . Co-Cr Cr provides tarnish corrosion resistance Mo-lower the CTE Rh- improves castability Stronger and harder than noble and Ni-Cr alloys Casting and soldering is more difficult than noble metal alloys Ti types Pure Ti and Ti-6Al-V may become imp for ceramo metal; restorations but present processing difficulties indicated by casting temperature of 1760 degree to 1860 degree and their ease of oxidation FACTORS AFFECTING THE COLOR OF CERAMICS: “A dark red that is yellower and less strong than cranberry, paler and slightly yellower than average garnet, bluer, less strong, and slightly lighter than pomegranate, and bluer and paler than average wine” The three dimensions of color: 23
  24. 24. Hue: dominant color of an object, wavelength Chroma: saturation Value: lightness or darkness ….independent of hue Metamerism: objects that appear to be color matched under one type of light appear different under another light source Fluorescence: the property of an object to emit light of different wavelength than the one incident upon it Eincident=Escattered+Ereflected+Eabsorbed+Etranmitteed+Efluoresced In the dental operatory or laboratory color matching is usually performed by the use of shade guide. Dentin is more opaque than enamel and will reflect light.. pale yellow in color Enamel……crystalline……. different refractory indices at the incisal region ….bluish white (thick) at cervical margin-yellow (thin. reflects color of underlying dentin) …..Translucence……DEPTH “Northern light from a blue sky during the middle portion of day that is slightly overcast” PROPERTIES Discussion of mechanical properties… IN VACUUM……HOLISTIC Restorative materials at our disposal: 24
  25. 25. Metals-high tensile strength, toughness, hardness, resistance to abrasion, fracture resistance, elasticity, ductility fatigue resistance Polymers-inferior in most of these properties….BRITTLE FRACTURE Composites-BRITTLE FRACTURE superb aesthetics Ceramics-No ductility, high compressive strength, low shear and tensile strengths excellent aesthetics COMPRESSIVE STRENGTH: Maximal stress required to fracture a structure under compression. Enamel:37,800 psi Dentin: 44,200 psi Porcelain : 25,000 psi Metalceramic alloys : yield strength of 65-80,000 psi TENSILE STRENGTH: Maximal stress required to fracture a structure under tension. Porcelain: 5,000 psi HARDNESS (KHN): Enamel: 343 Dentin: 68 Porcelain: 460 25
  26. 26. FLEXURAL STRENGTH (BENDING STRENGTH OR MODULOUS OF RUPTURE): Force per unit area at the point of fracture of a test specimen subjected to flexural loading. Feldspathic porcelain: 141 MPa Aluminous porcelain: 139 MPa IPS Empress2: 400 MPa Gold alloy: 350-600 MPa FRACTURE TOUGHNESS: Feldspathic porcelain: 0.9-1.5 MPa.m1/2 Aluminous porcelain: 2-2.9 Yttria stabilized zirconia: 9 Gold alloy: 20 Enamel: 0.7 IPS Empress2: 3.3 THERMAL COEFFICIENT OF EXPANSION:( mm/mm.K)*10-6 Change in unit length per unit rise in temperature Tooth : 11.4 Low fusing ceramic: 12.2-15.8 IPS Empress 2: 10.6 Ceramometal: ? 26
  27. 27. THERMAL CONDUCTIVITY (cal.cm/cm2.sec.C): Ability of a body to transfer energy Enamel: 0.0022 Dentin: 0.0015 Porcelain: 0.0030 THERMAL DIFFUSIVITY (cm2/sec): Enamel: 0.0042 Dentin: 0.0026 Porcelain: 0.64 “Effectiveness of a material in preventing heat transfer is directly dependent on its thickness and inversely dependent on its thermal diffusivity” MODULOUS OF ELASTICITY Porcelain: 69GPa Type IV gold alloy: 99.3 GPa Composite: 16.6Gpa Because of their moderately high m of elasticity porcelains and relatively low tensile strength porcelains can undergo very little elastic deformation (0.1%) before they rupture i.e., they are not flexible Table 19-11 page 600 anu 27
  28. 28. RECENT ADVANCES IN DENTAL CERAMICS: METAL CERAMIC CROWNS BASED ON BURNISHED FOIL COPINGS: THE CAPTEK SYSTEM Malleable Captek metal strips are burnished on a refractory die to fabricate the metal coping of a metal ceramic crown without the use of a melting and casting process. The finished metal coping may be described as a composite material consisting of a gold matrix reinforced with small particles of a Pt-Pd-Au alloy. The units are then veneered with two thin layers of opaque porcelain and other veneering porcelains. The Captek coping has a thickness of 0.25 mm which is half of the traditional cast metals thus providing additional space for vennering porcelain. Indications: crowns and FPDs CASTABLE GLASS CERAMICS : DICOR Dicor is a castable glass (55% tetraflurosilicic mica crystals) that is formed into an inlay, facial veneer or full crown restoration by a lost wax casting process. After the glass casting core or coping is recovered, it is covered by a protective embedment material and subjected to heat treatment that causes mica to grow within the glass matrix. This process is called ceramming. Then it is fit on dies, ground as necessary and coated with veneering porcelain. ADV: Ease of fabrication 28
  29. 29. Improved esthetics-chameleon effect Minimal shrinkage Good marginal fit High flexural strength Low thermal expansion Minimal abrasiveness of enamel DISADV: Limited use in low stress areas (Low tensile strength) Inability to be colored internally PRESSABLE GLASS CERAMICS (IPS EMPRESS): It is provided as core ingots that are heated and pressed until the ingot flows into a mold. It contains a higher proportion of leucite crystals that increase resistance to crack propagation. The hot pressing process occurs over a 45 min period at high temperature to produce the ceramic substructure. The crown form can be either stained and glazed or built up using a conventional layering technique. ADV: Lack of metal Translucent ceramic core High flexural strength Excellent fit Excellent esthetics 29
  30. 30. DISADV: Potential to fracture in posterior areas Need to use resin cement INFILTRATED CERAMICS (INCERAM): Available as two component system: Powder: alumina/spinell/zirconia Low viscosity glass A slurry of the powder is slip cast on a refractory die and heated in a furnace at 1120 degree centigrade for 10 hrs and then it is infiltrated with the low viscosity glass at 1100 degree centigrade for 4 hrs to eliminate porosity and to strengthen the slip cast core. ADV: Lack of metal substructure High flexural strength] Excellent fit DISADV: Opacity Special die material and high temperature oven is required Have abrasive properties. 30
  31. 31. CAD –CAM CERAMICS (PROCERA, CEREC, CELAY, DICOR MGC): It stands for Computer aided design/Computer aided manufacturing. It is supplied as ceramic ingots available in various shades. These are placed in a machinable apparatus to produce the desired contours. This machined restoration is checked for fit on the tooth. Occlusal adjustment is done followed by polishing, etching and bonding the restoration to the prepared tooth. ADV: Negligible porosity levels Freedom from making an impression Need for a single patient appointment (with CEREC system) Good patient acceptance DISADV: Need for costly equipment Lack of computer controlled processing support for occlusal adjustment Technique sensitive nature of surface imaging. 31

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