1. dental ceramics

8/3/2019 1. Dental Ceramics

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Dental Ceramicsdr . Kirti sharma


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Introduction to porcelain Definition of ceramics Pottery Feldspar

Fluxes

Silica Aluminum Oxide

Quartz Kaolinite Greenware Sintering Fusing

Earthenware Stoneware Porcelain Dental feldspathic porcelain
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The basics from pottery to

porcelain


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Porcelain

Marco polo, 13thcentury

porcelino (italian)-cowrie orvenus shell

Meaning little pig

Shells thinness, translucency,

hardness.


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To quote a tenth century European reflecting onthe porcelain he encountered on his journeythrough China:

"A ceramic so white that it was comparable onlyto snow, so strong that vessels needed walls only2-3 mm thick and consequently light could shinethrough it. So continuous was the internal

structure that a dish, if lightly struck would ring likea bell.

This is porcelain!"


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covers variousmaterials

hard,

brittle,

non metallic,

heat-resistantand

corrosion-resistant.

Derived from:

the Greek word

keramos meaning

potters clay/ burntstuff.


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Shaping and then firing a nonmetallicmineral, such as clay, at a high

temperature.

The non metallic minerals:

aluminum oxide (alumina) andsilicone dioxide (silica).


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Refractory

non-meltable

skeletal structuresintered (fused) particles of a metallicoxide (aluminum oxide).

Glassinfiltrated between the sinteredrefractory particles.
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Pottery was the first, and still is theforemost ceramic.

Pottery is made from clay, andcontains both of these components.


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HISTORICAL

PERSPECTIVE


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The first ceramics were createdsometime before 5500 BC in the form

of earthenware pottery. CLAY (with water->too sticky to

handle)

SAND and GROUND SEASHELLS Kiln

Firing/sintering


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Gases-VOIDSin CLAYfracture

during FIRING

BEATING ( WEDGING ) and raising thetemperature very slowly.


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Before firing, the "ceramic" body is in a very fragilegreen state, and at this stage is called greenware.

In its green state, the body has not yet actuallybeen converted into a ceramic.

A fragile pile of microscopic rocks.

When totally dry, the greenware/unfired bodyplaced into a kiln for a low temperature firingknown as a biscuit bake. During this low fire process,little if any feldspathic glass is produced.

Greenware


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mass of individual particles loosely held togetherby a

water binderCalledSintering

coherent solid

the points at which the individual particles arein contact fuse at sufficiently high temperatures


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Sintering appears to happen not so much becauseof melting, but because of diffusion of the rapidlymoving atoms between the neighboring refractory

particles.

Diffusion is accelerated at elevated temperatures.


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Once biscuit fired-> ceramic easily handled; notyet fully fired; most of the feldspar still in crystallinestate.

Apply glaze coat over it ( highly fluxed silica) andfired to a higher temperature.

Glaze and feldspar melt to form a glass.

This second firing is called glaze firing or fusing

firing.


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UP-DRAUGHT KILN

EARLY KILNS---900 C---pottery fired at thistemperature is known as EARTHENWARE.

(low temperature firing, porous, opaque, unsuitablefor storing liquids)

higher kiln temperatures---impervious pottery---STONEWARE.


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Europeans produced->stoneware.could be made to lookwhite, but in a thickness that it wasinvariably opaque.

Chinese produced-> porcelain.whiteand made in such thin sections that itappeared translucent.

In1717, the secret was leaked from Chinaby a Jesuit missionary, FatherdEntercolles-kaolin, silica and feldspar.


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Clay bodies :

earthenware,

stoneware and

domestic

porcelain.

Glass

Alumina
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Dental porcelain is a further subdivisionof domestic porcelain. It is impossible

to understand dental porcelains andtheir associated cores without firstunderstanding the art and science ofceramics, and this begins at thepotters wheel.


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Clay is a specialized form of mud.

Clay requires three specific constituents toqualify as a good ceramic medium

feldspar,

quartzand

kaolinite.


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Water

- reduces the friction between the clay particles

- lends the clay plasticity so that it can easily beformed into shape by hand


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Plasticity Minerals eg.ball clay or bentonite

Increase the surface area available toretain water

(NOTE: Manufacturers of dental porcelain frits addsugar and starch to their porcelain powders for thesame reason.)


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Porcelain is defined as a:

fine kind of earthernware having a translucent body

and a transparent glaze. Blending of clay with other common minerals such as

feldspar, flint (silica) and firing them at hightemperatures produced translucency and strength.

Ceramic materials containing these additionalimportant ingredients were given the namePorcelain.


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Dental porcelain belongs to one class of ceramics

Consists of a glass and a crystalline phase

( glass-crystal composite)

Other ceramics are composed entirely ofcrystalline oxides that are sintered together,sometimes under high pressure.


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Dental porcelain:

Kaolin omitted; feldspar translucency.

Hence considered as a feldspathic glass with crystalline

inclusions of silica.


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The dental application of porcelain dates from1774,when a French apothecary named Alexis

Duchateau considered the possibility ofreplacing his ivory dentures with porcelain


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The use of porcelain in dentistry was first mentionedby Pierre Fauchard


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1789Fused porcelain was introduced formanufacture of teeth.

By 1820Porcelain denture teeth wereintroduced, which replaced ivory/ natural

denture teeth in U.S.A.. 1837John Murphy of London introduced the

plantium foil technique which made possible thedevelopment of present day method ofporcelain inlay construction.


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1838Elias Wildman formulated translucentporcelain.

1887Dr C. H. Land of Detroit developed the first

all-porcelain jacket crown (PJC) using the PlatinumFoil Matrix technique.


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1889Dr. Charles H. Land patented the PlatinumFoil Matrix technique for PJC.

1903E. B. Spaulding developed gingival shoulderporcelain for the PJC.


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1930Frederick Gardner of the Steuben division ofCorning Glass Works developed the LostWax orcire-perdue method (Taggarts method of 1907 )of forming three dimensional glass articles.

1962 - incorporation of a high proportion of leucitecrystals into the feldspathic porcelain composition.

1965Mc Lean & Hughes used glass- aluminacomposite instead of feldspar porcelain resulting instronger restorations.


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1970 Southan first demonstrated the applicationof ion exchange strengthening to dental porcelain.

1983 - First dental CAD/CAM prototype waspresented in France.


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1997IPS Empress Cosmo Ingot (Ivoclar) ,a glass-ceramic material that can be heatpressed directly onto zirconia posts (eg;Cosmopost) was introduced .

1999IPS SIGN (Ivoclar AG), a feldspar-free fluorapatite glass ceramic system for

use in metal-ceramics was presented.


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1985 Hobo & Kyocera (Biocream group )developed a castable glass-ceramicwhich melts at 14600C and flows like molten

glass.

1986The first generation CEREC 1(Siemens) CAD/CAM system wasintroduced.

1988Michael Sadoun first introduced In-ceram, a glass-infiltrated aluminousporcelain.


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1994The second generation CEREC 2(Siemens/Sirona) CAD/CAM System waspresented.

Late 1990sIPS Empress 2, a second generationpressable ceramic made from lithium-disilicateframe work with an apatite layered ceramicwas introduced.


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CLASSIFICATION OF

CERAMIC MATERIALS


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According to application (By R.W. Phillips, 1982, Skinners

8th edition)

For porcelain teeth ( denture)

For Ceramo-metal restorations (Metal-Ceramic Systems)

For All-ceramic restorations (All-Ceramic System)


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Classes of Dental Ceramics for Fixed Prosthetics (By K.J.

Anusavice, 1996, Phillips 10Th edition).

There are several categories of dental ceramics: conventional

leucite-containing porcelain, leucite-enriched porcelain that may

contain leucite, glass-ceramic, specialized core ceramic (alumina,

glass-infiltrated alumina, magnesia, and spinel), and CAD-CAMceramics.


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Dental ceramics can be classified By type : feldspathic porcelain, leucite-

reinforced porcelain, aluminous porcelain,alumina, glass-infiltrated alumina, glass-

infiltrated spinel and glass-ceramic By use : denture teeth, metal-ceramics,

veneers, inlays, crowns and anterior bridges By processing methods : sintering, casting or

machining

By substructure material : cast metal, swagedmetal, glass-ceramic, CAD-CAM porcelain orsintered ceramic core.


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Methods of fabricating ceramic restorations include :

condensation and sintering, pressure moulding and sintering,

casting and ceramming, slip casting, sintering and glass-infiltration, milling by computer control.

Dental porcelains are classified according to the firingtemperatures as:

High fusing 1300C (2372F)

Medium fusing 11011300C (20132072 F)

Low fusing 8501100C (19622012F)

Ultra-low fusing


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The medium-fusing and high-fusing types are used for the

production of denture teeth.

The low-fusing and ultralow fusing porcelains are used for crown

and bridge construction.

Some of the ultra-low fusing porcelains are used for titanium and

titanium alloys

(low-contraction coefficients that closely match those of the metalsreduced risk for growth of the metal oxide).


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However, some of these ultra-low fusing porcelainscontain enough leucite to raise their thermalcontraction coefficients as high as conventional

low-fusing porcelains.


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According To Use

METALCERAMIC SYSTEMS :

1) Cast metal systems : eg: Vita Metall Keramik (VMK 95)

2) Non- Cast Metal Systems (Foil Crown Systems)

ALL CERAMIC SYSTEMS : Classified according to

method of fabrication (Marc Rosenblum & Alan Schulman A

Review of All-Ceramic Restorations. JADA Mar1997).

Cl ifi d di t th d f f b i ti


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1)Conventional Powder Slurry Ceramics : using condensing& sintering.

(a) Alumina reinforced Porcelain e.g. : Hi-Ceram

(b) Magnesia reinforced Porcelain e.g.: Magnesia cores

(c) Leucite reinforced (High strength porcelain)

e.g. : Optec HSP

(d) Zirconia whiskerfibre reinforced e.g.:MirageII(Myron Int)

(e) Low fusing ceramics (LFC): (i) Hydrothermal LFC

e.g.: Duceram LFC :

(ii) Finesse (Ceramco Inc)

Classified according to method of fabrication(Marc Rosenblum & Alan Schulman A Review ofAll-Ceramic Restorations. JADA Mar1997).


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2) Castable Ceramics : Using casting & ceramming

1) Flouromicas e.g: Dicor

2) Apatite based Glass-Ceramics e.g Cera Pearl

3) Other Glass-Ceramics e.g: Lithia based, Calcium phosphate

based


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3) Machinable Ceramics : Milling machining by mechanical

digital control

A)Analogous Systems (Pantograph systemscopying methods)

1)Copy milling / grinding techniques : a) Mechanical

e.g. : Celay

b)Automatic

e.g:Ceramatic II. DCP

2)Erosive techniques : a) Sono-erosion e.g: DFE, Erosonic

b) Spark-erosion e.g: DFE, Procera

B)Digital systems (CAD / CAM):

1) Direct e.g: Cerec 1 & Cerec 2

2) Indirect e.g : Cicero, Denti CAD, Automill, DCS-President


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4) Pressable Ceramics : by pressure molding & sintering

1) Shrink-Free Alumina Reinforced Ceramic

(Injection Molded)E.g. : Cerestore / Alceram

2) Leucite Reinforced Ceramic (Heat Transfer

Molded)E.g.: IPS Empress, IPS Empress 2, Optec OPC.

5) Infiltrated Ceramics : by slip-casting, sintering &

glass infiltration

1) Alumina based e.g: In-Ceram Alumina

2) Spinel based e.g: In-Ceram Spinel

3) Zirconia based e.g.: In-Ceram Zirconia


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According to microstructure:

1) Non-Crystalline Ceramics e.g.: Feldspathic porcelain

2) Crystalline Ceramics e.g.: Aluminous porcelain,

Glass- Ceramics

According to application:

1) Core porcelain

2) Body porcelain

3) Enamel porcelain

According to method of firing:

1) Air fired (i.e, at atmospheric pressure)

2) Vacuum fired (i.e, below atmospheric pressure)


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Feldspar

Quartz

Kaolin


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Feldspar :

naturally occurring crystalline rocks.

Devitrification (cooled very slowly)

vitrification formation of glass

devitrification formation of crystals

really just naturally devitrified glass


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There are twelve naturally occurring types offeldspar (and numerous combinations). Theirformulas are all similar :


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Al2O3 in this form does not affect the

translucency. Si also exists in 2 forms:

- as a part of feldspar (glass former )

- as crystalline quartz scattered throughout the

glass


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Feldspars are naturally occurring substances, sothe

ratio between the potash (K2O) and the soda

(Na2O)will vary somewhat.

This affects the properties of the feldspar:

soda tends to lower the fusion temperature

potash increases the viscosity of the molten glass.


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During the firing of porcelain there is always the

danger of excessive pyroplastic flow, which mayresult in rounding of the edges and loss of toothform.

It is important that the correct amount of potash ispresent to prevent this.


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The Na2O, K2O and CaO in the aboveformulas are used as fluxes.

fluxes cause crystalline structures to melt atlower temperatures than would otherwise bepossible, a bit like water melts a cube of sugarat room temperature.

Without fluxes present, none of the otherconstituents in the ceramic body would be ableto melt at normally attainable temperatures.

Feldspars melt at about 1150 degrees C.
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Feldspars melt at about 1150 degrees C.

Incongruent melting (Anusavice) (1150-1530 C)

Liquid glass phase + CrystallineLeucite

(KAlSi2O6/K2O.Al2O3.4SiO2)

softens and flows slightlyallowing the porcelainpowder particles to coalescetogether

forms a translucentglassy matrix between theother components in thedense solid.

exploited toadvantage in themanufacture ofporcelain suitable

for metal bonding.


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Leucite

High coefficient ofthermal expansion

and elasticity.

It affects the opticalproperties, strengthand hardness of theporcelain.

Leucite containingporcelains tend towear opposing

tooth structure by


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Pure form called kaolin (china clay)

KaolinChinese term for high ridge.place where

it was first found.

Chemical formula :Al2O3 2SiO2 2H2O (hydrated aluminium silicate)

Crystalline structure.


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Crystalline slabs stacked one on top of the other


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Weak hydroxyl bondswith adjacent gibbsite

layer

Strong bond to

silica layer via

shared oxygen

atoms


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Crystalline structurevertical stack of hexagonal

plates.

Can support Compression but not shear.

(Compression)

(shear)


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The flux from the feldspar prevents the free silicato form stable crystals.

Instead, an amorphous glass is formed.

Feldspar

Flux

Glass from kaolin.


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The leftover gibbsite layer which has lost itshydrogen atom becomes refractory crystallinealuminum oxide, also known as alumina.

Not all of the kaolinite will melt and quite a bit ofthe original kaolinite remains behind as plate-like

crystalline inclusions in the glass gel matrix.

Kaolinitecontd


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Therefore, when the clay body melts at hightemperature, it consists of the followingconstituents:

Feldspathic glass formed from the melting of thefeldspar

Glass from the kaolinite--- the debonded silicalayer from the melted kaolinite

Refractory Alumina crystals---the debonded

gibbsite layer from the melted kaolinite Refractory kaolinite particles in the form of flat

plates Refractory Quartz particles


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Glass Phase


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All glass formulations have three things in common:

Glass formers

fluxes and

stabilizers (also known as modifiers).


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The most common glass former issilicone dioxide (SiO2), also known as silica

It is also the glass former found in mostfeldspars.

Feldspars account for the glassy phase in

dental porcelain.

Silica is also the basis of the glazes


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Oxides Of

Boron (small amounts,5-15%...tough and

heatresistant glass)

Phosphorous Antimony Arsenic Germanium

Selenium.


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Chemical formula of SiO2

Forms tetrahedral crystalline structures

The tetrahedrons are bonded together via sharedoxygen atoms at each apex of the tetrahedron.

This describes crystalline silica.

Difference between glasses and crystals.


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Silicone


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6-sided quartz crystalsSiO4 lattice network

Red-O2

Blue-silicone

Highly directional covalent bonds. Hence, an orderlylattice representative of quartz crystals results.


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crystalsglasses


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It has a very high fusion temperature (> 16850 C).

provides a frameworkfor other ingredients andhelps retain the shape during firing.

filler - to provide strength.


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The quartz may be replaced by alumina(Al2O3)aluminous porcelain

Alumina much stronger and rigid than quartz.


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Increased amounts of oxides devitrificationclouding.

Functions:a) reduce the softening temperature

b) increase the thermal expansion

c) reduce the viscosity


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Make the glass strong and water resistant.

Dissolve as the glass melts, so they do not add

opacity to the glass.

Examples: Calcium oxide most common

Aluminium oxide strength also

Boron oxide

Lead oxide


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Not an intentional addition.

important glass modifier.

The hydronium ion, H3O can replace Na+ or

other metal ions in a ceramic that contains glassmodifiers.

- slow crack growth of ceramics exposed totensile

stresses and stored in moist environment.


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Pure silica (quartz)melted cooled relativelyquicklypure fused silica glass formed

Very high melting temperature.

Properties superior to any of the other forms ofglass.

Insoluble in water because it lacks alkaline fluxmolecules.

Most heat resistant of all glasses and can sustain

temperatures of 1200 degrees centigrade.


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Increase the hardness and viscosity.

Prevent the slump of porcelains during firing.

Eg. Aluminium oxide usually added to the melt inthe form of its hydroxide.


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Flux + glass former

Critical value < 12%

Boron Anomaly:

< 12% BO4 tetrahedra forms a twin lattice withSiO4more stable glass produced.

>12% BO3 triangles formed less stable glass.


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Diagram showing the formation of alumino-silicate glass.


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Glass formers: SiO269.4 wt%

B2O3.7.5 wt%

Glass modifiers: CaO.1.9 wt%Na2O4.8 wt%

K2O.8.3 wt%

Intermediate oxides: Al2O3.8.1%


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Li2Oas an additional flux.

- Risks.increased pyroplastic flow

increased devitrification

MgO

Phosphorous pentoxide

- opalescence

-sometimes glass forming.


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Syenite:

- igneous, granular rock

- high in feldspar

- little or no quartz.

Tried as a replacement for feldspar.

Not used anymore because of increased

pyroplasticity


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Manufacturers fluxed porcelain with limited degree offusion and pyrochemical reaction.

So, a part of feldspar remains un-dissolved

Differences in the refractive indices final glass frit mayappear opalescent or exhibit grey-blue translucency similarto incisal enamel.

Major color problem slight greenish hue (all glasses)


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Reduce the greenish hue inherent in all glasses.

Heat resistant pigments used (metallic oxides).


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Pink chromium tin or chrome alumina--warmtone.

Yellow indium or praesodymiummost stable;

ivory shade.

Grey iron oxide or platinum grey enamel shades

addition to the

greyer sections of

dentin color.


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oxides of iron.brown color

copper oxide green color

titanium oxide yellowish brown color

cobalt oxideblue color (some enamel

shades)

Manganese oxide lavendercolor


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Metal oxides

Zirconium (most popular), cerium, titanium andtin oxides

Ground to a very fine particle size < 5m

Prevent speckled appearance.

Why add them??Porcelain is translucent.

Need to stimulate the underlying dentin.


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Size and volume distribution of the particlesvarious wavelengths of light scattered differently bythe particles.

Difference in the refractive indices between theglass and the opacifier.


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Natural teeth lamps in dance halls fluoresce abluish-white color simulated in porcelain too.

Uranium salt highly radioactive hence banned.

Rare earth oxides samarium }

Spinels } not as

effective

as uranium.


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Newer porcelain materials recently introduced arehighly fluorescent and are described as opticalbrighteners

E.g : Luminaries (Vita, Bad Sackingen, Germany)contain naturally occurring fluorescent agents thatare non-toxic.


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Why is a Self glaze preferred??

Increases the chemical durability ( increasedresistance to corrosion, acid attack, etc.)


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Made from the same material as the glaze;colouring agents and opacifiers added

Used sparingly simple corrections of toothcontour

or contact points.


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1) SilicaFiller

2) Kaolin (China clay)Binder

3) FeldsparBasic glassformer

4) Nepheline Syenite &

Leucite

5)WaterImportant glass

modifier

6) FluxesGlass modifiers

7) Color pigments

8) Opacifying agents

9)Stains and colourmodifiers

10) Fluorescent agents11) Glazes and Add-on

porcelain

12) Alumina

13) Alternative Additivesto Porcelain


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MANUFACTURE

&

DISPENSING


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Manufacture of porcelain:

Pyrochemical reactions :

water of crystallization is lost

the flux reacts with the outer layers of the grains of silica (filler),

kaolin (binder) and feldspar (basic glass former) and partly combines

them together.

The feldspar fuses and further intermingles with kaolin & quartz.


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The resultant frit is a brittle structure

Readily ground to a fine powder form

Powder particles form a very viscous liquid when

re-fired flows together and particlescoalesce/sintered.


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Various methods of fabrication:


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Condensing and Sintering,

Pressure molding & Sintering,

Casting & Ceramming,

Slip casting, Sintering & Glass infiltration

Milling (Machining) by mechanical and digital systems.


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Fabrication of a conventional porcelain restoration is basically

composed of the following stages:

CondensationSinteringGlazingCooling.


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CONDENSATION OF DENTAL

PORCELAIN


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Shrinkage after sintering dependent on porosityof the powder bed after condensation andshaping of the tooth form.

Porosity governed by:

a) condensation technique

b) original packing density of the powder.


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Size of the particles

Shape of the particles


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Size of the particles:

Irregular particle system

larger voids filledwith

smaller particles.

Gap-grading system 3 sizes of particles used.

(suitable for vacuum firing)

Obtained by


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2) Shape of the particles

Rounded particles by dry grinding pack better than

angular particles obtained by wet ground powder.


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Mild vibration to pack the wet powderdensely on the underlying framework .

The excess water is blotted or wiped awaywith clean tissue or brush ,and

condensation occurs.


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A small spatula is used to apply and smooththe wet porcelain .

Smoothening action brings the excesswater to the surface , where it is removed .


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Employs the addition of dry porcelainpowder to the surface to absorb the water.

The dry powder is placed by a brush to theside opposite from an increment of wetporcelain .

As the water is drawn towards the drypowder , the wet particles are pulledtogether.


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A working model / die of the prepared tooth is used for

condensation of porcelain. A matrix is used to support the


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unfired porcelain both during condensation and firing.

(For inlay / onlay restorations, ceramics are fired on refractory

dies)

Restoration E.g. Matrix used

All-ceramic PJC Platinum foiladapted on the die to form a matrix

Metal-ceramic PFM Metal coping of suitable design and alloy type


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Mixing: Dry porcelain powder is mixed with the binder on a glass

slab using bone or nylon spatula (or glass mixing rod) into a thick

creamy mix, which can be carried in small increments with an

instrument or brush.

The instruments used can vary from fine bladed spatulas and carving

points to fine sable hair brushes of varying thickness.


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Sable hair brushes more efficient:

A wet brush maintains the moisture content inporcelain while the metal spatulas cause morerapid drying out

Apply enamel colours or stains without changinginstruments.


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Greater control over applying small increments ofporcelain

Greater detail in surface characterization

Greater delicacy in blending of enamel veneers.


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Instruments


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The build-up brush: (sable hair no. 6)

Most versatile


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Foldable and a removable cap ensures that moisture can bemaintained for several hours. This maintains the flame-shape

and density also


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Fine tipforpreciseanddelicateadditions


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Soft andflexible

Cervico-incisal

direction Direct,

realign orhighlightthe axial

morphology


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High densityand maximumwaterretention.

Blend a very

small additionto the build upby surfacetension withoutdehydration

Occlusaladditions andgrooving.


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Opaque manipulation various sizes


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Fine:

Emphasize fineembrasure space withsurface colorant.

Extra fine:

Very short and very rigid.

To highlight the inneraspects of occlusal pits.


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Flat, fine, square point.

Lateral segmentation

Create illusion of enamel cracks.


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Brush for enamel crack to stain a defined area.


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Fabricating porcelain margins

Cleaning occlusal surfaces

Stroke always in the direction of the fissures.


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Gross axial outlines, all labial and incisal cutbacks carvedin overlapping sweeping strokes.


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Trenches in powder-irrigation and wetting. Creamy mix


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Porcelain build up brush at 45 angle between theglass plate and the mix. The length of its penetrationcontrols the size of the bead to be picked up.


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Reservoir of distilled water underneath the glass slab.

Maintains a certain degree of moisture essential fordental porcelain processing.


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Condensation- unfired porcelain particles packedtogether.

Translucency directly related to residual aircontent.

Maximum packing density:

increased translucency.low firing shrinkage.


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Increased porosity due to:

Insufficient saturation of the mix Over stirring of powder in the liquid binder

Addition of too large amounts at once

Dehydration and frequent remoistening

Addition to an already dried zone


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Paper tissue

Initial support to control axial direction ofbuild up.

Absorb excess moisture.


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Moisture control through build up sequencing

Large additions- increased air entrapment


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Build up started at cervix.

Used as a base.

Additions proceedalongside and betweenthe lobes, finally filling inthe whole surface.


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The construction must proceed to a different area through aprogression of proximal and occlusal ridges until anothertrilobed support is created by continuity..draws moisture

and allows the lobes to be developed against one anotheruntil they reach the other aspect of the crown.


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Moisture control and working time


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Distilled water- ideal binder(leaves no contaminating residues onburnout)

Prolongation of working time:distilled water + modeling fluid { plasticity also ed }

( 8 parts) (1 part)

On completion of all constructions, water must stillconstitute 30% to 40 % of the porcelain bodyweight.


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Pre-heating the furnace

The green porcelain is placed into the hot zone of the furnace and the

firing cycle is initiated.

Glaze: At the end of high bisque stage, if the porcelain is held in the

furnace for a greater length of time, the surface porcelain would undergo

pyroplastic flow, i.e. the matte surface would disappear and a smooth

shiny surface would result (self-glaze; depth of 100um) .


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Condensed porcelain mass is placed in front of orbelow the muffle.

Permits the remaining water vapor to dissipate .

Placement of the condensed mass directly into

even a moderately warm furnace

rapid production of steam

voids or fracture of large sections of the porcelainmass .


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Firing 3 stages of maturity:

a) low- bisque

b) medium- bisque

c) high - bisque

The common expression used for describing the surface

appearance of un-glazed porcelain is bisque or biscuit since

this gives a fairly accurate picture of its surface texture.


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Surface very porous

Grains start to soften and lenseat their contact point

Shrinkage minimal

Air spaces are irregular

Fired body is extremely weakand friable

Opaque


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There will always besome porosity in theporcelain, with small

voids being exposed atthe surface.

To avoid this, thesurface is glazed toproduce a smooth, shinyand impervious outerlayer.


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Internal staining and application of characterized stains:-

Advantage- The effect is permanent and can produce lifelike

results e.g: simulated enamel craze lines.

Disadvantage : Porcelain must be stripped completely if the

colour or characterization is unsuitable.

Advantages over air-fired porcelains:


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Improved esthetics

Better handling properties

Reduced porosity

Diffusible Gas-Firing Process :- This is an alternative

technique for producing high densities in dental porcelain in

which a diffusible gas (He, H2, or steam) is substituted for theordinary furnace atmosphere.

Ceramic Furnaces :

Horizontal Muffle e.g. : Vita-Caccumat S


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De Try Biodent Systomat,

Unitek Ultra-Mat Furnace,

Rapid Cycle furnace (Doxc Euromat).


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Vertically mountedmuffle

Cylindrical better

heat distribution thanhorizontal muffles.

2 muffles :a) pre-heatingb) vacuum firing.


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Fully automatic


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Term does not mean quickfiring;

Heat is brought to the

porcelain and not theporcelain to the heat.

Work to be fired inserted

from the top of thefurnace assists viewing.

No moving parts.

In Vacuumfired porcelain


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Air is removed from the interstitial spaces before sealing of the

surface occurs.

( not all the air is removed ). The residual air becomes sphere-shaped under the influence of

surface tension and increased furnace temperature.

When air at normal atmosphere pressure is once again allowed to

enter the furnace muffle

Hydraulically compresses internal bubbles.

Relatively dense pore-free porcelain


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Limitation of vacuum firing:

Large bubbles trapped due to poor condensation

technique cannot be reduced in size to any

significant degree and can be seen as blistering of

the material.


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PROPERTIES OF DENTAL CERAMICS

Desirable Properties


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Good esthetic qualities

High hardness

High compressive strength

Good chemical durability

Excellent biocompatibility


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Brittleness

Low fracture toughness

Low tensile strength susceptible to fracture duringplacement, mastication and trauma

Color Stability


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Ceramicsmost stable tooth colored materials:

a) Metallic oxides (colorants) dont undergo anychanges after firing.

b) Smooth glossy surface resists exogenous stains.


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Brittlefracture


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fracture

Plasticdeformation


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Natural tooth - 343 KHN

Porcelain 460KHN

Hence, it causes wearing of natural tooth andmetalrestorations.

(particularly if porcelain is not glazed properly).


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Porcelain has a coefficient of thermal expansion ,slightly less than that of the tooth structure .

It does not exhibit micro leakage and iscomparable to a cemented metal restoration .

It also does not imbibe or synergize water.


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Volumetric shrinkage 3545 %

Linear shrinkage 1114 %

Minimized by:a) Using lesser binder

b) Proper condensation

c) Buildup of restoration 1/3rd larger than

original sized) Firing in successive stages.


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Generally resistant to degradation in the oralenvironment

Susceptible to :

a) mechanical degradation by brittle fracture

(chipping)

b) chemical degradation by fluoride attack


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Strength


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Theoretical strength is dependent upon thesiliconeoxygen bond.

The practical strength is 10 to 1000 times less thanthe nominal strengths.

Attributed to the phenomenon of stressconcentration around surface flaws (microcracks).


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Stress applied

The stress on the crownconcentrates the strain atthe pre-existing internalmicrocracks causing oneof them to fracture.

Pre-existinginternalmicrocracksformed due to

tensilestresses duringcooling.


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The condensation, melting and sintering process.

The high contact angle of ceramics on metal.

Differences in the coefficient of thermal expansion

between alloy or core and veneers.

Grinding and abrasion.

Tensile stresses during manufacture , function and trauma.


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Methods of Strengthening The Ceramics


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The critical strain of dental ceramic is low.

(the materials can withstand a deformation ofapproximately 0.1% before fracture )

So, cracks propagate at low average stress levels.(ceramics and glasses have tensile strengths thatare much lower than their compressive strengths).


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oral environment

bending forces

Tensile stresses (cause fracture)

maximum at the surface of a prosthesis


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According to K. J. Anusavice

(Phillips Science of Dental Materials, 11th edition)


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I. Development of residual compressive stresses

Ion exchange (Chemical tempering)

Thermal tempering

Thermal compatibility (Thermal expansion coefficient

mismatch)

II. Interruption of crack propagation

Dispersion of crystalline phase Transformation toughening

III. Others


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Minimize the effect of stress raisers

Minimize the number of firing cycles

Minimize the tensile stresses through

optimal design of ceramic prostheses.


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1. Ion exchange

Na+ containing glass article placed in a bath ofmolten KNO3

K+ and Na+ exchange.

K + 35% larger than Na+ squeeze into the place

formerly occupied by Na+ very large residualcompressive stresses.


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E.g. GC Tufcoat (GC Corp)potassium-richslurry applied on the ceramic surface; whenheated to 4500C for 30 minutes ion exchange.


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2. Thermal tempering

Most common method.

Rapidly cool (quench) the surface of an object while hot and

in the molten state.

skin of rigid glass surrounding a soft (molten) core.

as the core solidifies

tends to shrink


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Outer skin remains rigid

tensile stresses in core and compressivestresses in the outer surface.


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Stress raisers are discontinuities that cause astress concentration:

a) Creases or folds of the Platinum foil substrate thatbecome embedded in the porcelain and leavebehind notches (stress raisers).

b) Sharp line angles in the tooth preparation.


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c) Large change in porcelain thickness (determinedby

tooth preparation)

d) Small particles of porcelain along the internalporcelain margin of a crown


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Improperly adjusted occlusion

contact points (rather than contact areas)

localized stresses within the external and internalsurface of the porcelain crown


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Several firings

Changes in the leucite content

(high expansion crystal)

cTE altered

Expansion mismatch between porcelain andmetal

Stresses in porcelain during cooling crack


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Conventional feldspathic porcelain not to be usedas core ceramic in posterior areas large occlusalforces.

To reduce tensile stresses on the cemented surfaceat the occlusal region use maximum occlusalthickness possible ( typically 2 mm)

Use of MC crownsmetal coping minimizes

porcelain flexure.


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Interruption of crack propagation


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Dispersed

particles ascrack stoppers

Crystals of high strength and elasticity dispersed in theglass matrix, interfere with crack propagation.


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Both the dispersed particles(crystals) and the glassphase should have similar cTE

strength of the glass-crystal composite will

in proportion to the crystalline phase.

Finer the particle size a) strength

b) opacity


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Types of reinforcing crystals:

1) Quartz

2) Aluminaa) calcined alumina alpha type

b) fused alumina

c) alumina whiskers

McLean and Hughes high strength aluminouscore

porcelain


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Coarser grains strength probably due toincreased

notch-effect created at thegrain

boundaries of the crystals.

Influence of alumina crystal concentration on

strength: about 40 wt%

at this concentration, the glass can flow aroundand


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To reduce the opacity, sinter the aluminacrystals to the glass matrix rather than havingfree crystals.

The glass used with alumina:

a) viscosity

b) transition temperature

c) finer powder size

To obtain easy flowof the glass grainsaround the aluminacrystals at lowtemperature


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METHOD OF STRENGTHENING CERAMICS

According to John w. McLean (Science &Art of Dental Ceramics-

Op.Dent.1991:16:149-156)


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1) Enameling of metals :

a) Metal-ceramic restoration

2) Dispersion strengthening

a) Aluminous porcelain

b) Slip-cast alumina ceramics (In-Ceram)

c) Non-shrink ceramics (Cerestore).

3) Crystallization of glasses - Dicor, Dicor plus4) Chemical toughening - Ion exchange

5) Bonding to foils - Platinum foil, swaged copings techniques.


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Modifications (manipulation and tailoring) of theporcelain microstructure.

Conventional feldspathic porcelains are mainlyglassy.

Newer toughened materials crystalline phasepresent in the glassy phase.


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3 mechanisms:

1) Crack-tip interactions

2) Crack-tip shielding

3) Crack bridging.


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Obstacles act to impede the crack motion.

Reorientation of the crack plane.

Crack no longer subjected to just pure tensilestresses; some shear displacement is also involved.

Overall deflection manifested as roughness of thefinal fractured surface.


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For materials with similar surface flaws, strengthand fracture toughness are directlyproportional.

Fracture resistance resistance of a material torapid crack propagation.

Strength

depends mainly on the size of theinitiating crack.


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Crackbridging

Crack tipinteraction

MicrocracktougheningTransformation

toughening


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Microcrack toughening Transformation toughening


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Transformation toughening

Microcrack toughening


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Associated with the presence of zirconia.

Zr at 1173 C

MONOCLINIC TETRAGONAL

low temperature phase High temperaturephase

High volume low volume


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Addition of oxides like:

Calcium oxide

Magnesium Oxide Helps retain the

Yttrium Oxide tetragonal phase at Cerium Oxide room temperature.

This is called partial stablilization


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Stress

Partially stabilized Stable monoclinic Zr

Tetragonal Zr (3-5% in volume)

compressive stresses

established on the

crack surface.

its growth is arrested


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Tetragonal phase Monoclinic phase


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Seen in multiphase materials having differences inthermal expansion or elastic modulus.

These materials contain residual stresses act toshield the crack.

E.g. Leucite reinforced porcelains.


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Leucite high coefficient of thermal contraction

volume reduction associated with phase

transformation.

So, leucite contracts much more than the glass matrix.

compressive forces in glass matrix surrounding leucite crystals

microcracking in the leucite phase.

these residual compressive forces in the glass matrix preventthe crack propagation.


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The second phase acts as a ligament to make itmore difficult for the crack faces to open.

Best e.g. bonded fiber composites ( fibres actas ligaments).

This mechanism is important in large grainalumina (A1

2O

3) and possibly whisker reinforced

ceramics. E.g.: Hi-Ceram (core), Vitadur-N(core), Mirage II (Fiber).


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It is the Increase of fracture resistance with crackextension.

Is a desirable mechanical property because moreenergy is needed to propagate a microscopiccrack.

J Dent Res 81(8): 547-551, 2002

H. Fischer, W. Rentzch, and R. Marx


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Ceramics do not withstand tensile stresses.

Tensile stresses force crack extension.

In ceramic materials, 3 types of crack extensionscan occur:

a) sub-critical

b) stable

c) unstable


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Occurs below a critical value K10 called thecrack-tip toughness.

Is the reason for the well known time dependentstrength decrease of ceramic materials.

When the load reaches K10 the crackpropagates stably and finally unstably until thecomponent fails.

This point of failure is characterized by the

critical

stress intensity factor called fracturetoughness

KIc


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Most important.Friction at the border of thecrack tip, which produces the so-calledbridging effect

Other causes: (all energy-consuming effects)

a) crack branching.

b) phase transformation effects (characteristicof

zirconia ceramics).

i i i f

Causes of R-curve behaviorcontd


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This property is more dominant for larger cracksthan smaller cracks( friction at the border of thecrack tip increases with the increase in cracksize).

This study showed that the R-curve behavior ispronounced for the high strength materials e.g.

In-ceram alumina and especially Empress 2.


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Silane coating of an etched glass surface toincrease its surface affinity to polymers.

Si crystals in glassmatrix ofporcelain

Resin

Silanes/coupling

agentEthoxy/Chloro/Aminogroups

Vinylgroup

Wh b d th t ti t th t th?


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Why bond the restoration to the tooth?

The reinforced cores ( e.g. aluminous or glass ceramic)

microcrack formation on the internal surface of the

restoration.

The bonding technique turns the tooth structure itself intoa sort of unbreakable core.

Henceminor cracks on the internal surface will notcause

catastrophic fracture.


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Chemical adhesion of the resin to the etched porcelain is


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Chemical adhesion of the resin to the etched porcelain isgenerally done by the dentist when inserting the restoration.

This is done by the application of silane to the preparedporcelain

M lti sided molec le


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Multi-sided molecule.

One side bonds to the silica in the porcelain.

The other side bonds to the acrylic bondingagents.

In combination with the mechanical bonding, thismakes for a strong bond


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Methyl chlorosilanes


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Bond formedbetween the

resingbonding agentand the glassin theporcelain.

Rocatec system


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Rocatec system

SiO2 abrasive particle (50-m diameter)

roughens the substrate surface and increases the Sicontent

silanes bond the resin to this surface effectively.


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FELDSPATHIC PORCELAINS

High- , medium- , and low- fusing

Crystalline particles and amorphous matrix


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Crystalline particles and amorphous matrix(heterogeneous microstructure unlike the LFP)

Use- manufacture of denture teeth.


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Anterior teeth - By mechanical interlocking made

with projecting metal pins that becomesurrounded with the denture base resin during

processing.

Posterior teethmolded with diatoric spaces into

which the denture base resin may flow.

Porcelain denture teeth compared to acrylic resindenture teeth

Advantages

1 More esthetic or natural


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1. More esthetic or natural

2. More resistant to wear & distortion.

3. Denture can be rebased

4. Biocompatible

5. Dimensional & colour stability

Disadvantages of porcelain teeth


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1. Brittleness.

2. Clicking sound on contact

3. Cant be easily polished after grinding.

4. Higher density ( increased weight of teeth )

5. No bonding to acrylic base (Mechanical attachment).

6 Mismatch in coefficient of thermal expansion


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6. Mismatch in coefficient of thermal expansionstresses in acrylic denture base.

7. Require greater inter-ridge distance cannot beground thin in the ridge-lap area withoutdestroying the diatoric channels (retentive part).

Historically any porcelain fired below the


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Historically- any porcelain fired below themelting point of pure gold ( 1,064.4C)

Traditionally porcelain materials:

low-fusing 850 to 1060 C

medium-fusing 1,090 to 1,260C

high- fusing.. 1315 to 1,380C


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Refers to ceramics, which generally fuse at

temperatures lower than metal alloys (8501100C).

Low fusion temperature


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Low fusion temperature

Chemically similar, but microstructurally differentfrom the high-fusing porcelains.

Relatively higher proportion of glass modifiers(oxides of Na+ & K+ that readily react with SiO2 &Al2O3 at high temperatures to produce an

amorphous glass).


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Unlike HFP, the components of a LFP are nearly

completely dissolved when cooled shows anearly homogeneous microstructure of glass.


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Need for low-fusing porcelains?


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Attempts to use gold rather than platinum foil as a

burnishable matrix created a need for low-fusingporcelains for inlay work in the late 1800s.

Southan - preference for lower firing schedules

largely influenced by the instability of pigments athigher temperatures.

cTE mismatch (porcelain- slightly lesser cTE->


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cTE mismatch. (porcelain slightly lesser cTE >during cooling below the strain point it shrinksless than the metal and is placed into slightcompression at the interface)

Provides one method for improving the degreeof control of oxide formation and interaction atthe interface when using the base metal alloys.


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Uses of Low fusing porcelains


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Mainly ceramo-metal restorations which require ceramics withlower fusing temperature than the metal alloy.

Low fusing porcelains containing certain insoluble oxides can beused to alter the colour and the degree of opacity to produce toothlike shades (stains, overglazes).

Compatibility with a variety of popular metals includingtitani m allo ing a greater range of allo choice


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Co pa b y a a e y o popu a e a s c ud gtitanium, allowing a greater range of alloy choice.

Low abrasive wear against natural enamel, hence it can bea prime indication for use against natural dentition.

Highly polishable in the mouth, eliminating the need forglazing procedures after intraoral adjustments.

Opaque porcelain


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Opaque porcelain

Body porcelain

Stains and glazes

Aluminous porcelain (LFP + alumina )

Low fusing glass with insoluble oxides (TiO2, ZrO2)


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Low fusing glass with insoluble oxides (TiO , ZrO )

Mature at slightly higher temperatures than the

overlying body porcelains to minimize thedispersion within the body porcelains duringrepeated firings and thereby lose their opaquequalities.

a) incisal (or enamel ) shades- no colorant oxides


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a) incisal (or enamel ) shades no colorant oxides

b) gingival (or dentin ) shades- small amounts of

colorants

c) modifiers- larger amounts of colorants ranging

across the color spectrum and

including white and gray.


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Lower proportion of alumina and silica than


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Lower proportion of alumina and silica thanbody porcelains.

High amounts of oxides fluidity at high

temperatures

Create a glassy veneer and superficialcharacterization

Balanced for nearly equal cTE with body andopaque porcelains.

Commercially available low fusingceramics


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Vita Alpha ( Vita Zahnafabrik, Bad Sackingen Germany)

Vita Omega (Vita Zahnafabrik, bad Sackingen Germany)

Procera (Nobelpharma Gothenstein)

Empress ( Ivoclar, schaan, Liechtenstein)

Finesse (Ceramco, Addleston, Weybridge, U.K.)

Golden Gate system

Other low fusing ceramic systems


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Golden Gate system

Ceramco 3

Vita Omega 900 (Vita)

Mirage P (Chameleon Dent products, Kansas )

Ducera Gold(Degussal, Flanan, Germany)

Creation (Klema Dental Products, Melningen, Austria)

Ti-Ceram (Nobelpharma)

Duceram LFC

Hydrothermal ceramics(Hydrothermal ceramic for PFM crowns:Quintessence international vol 27, # 8/ 1996)


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Are basically low fusing porcelains containing hydroxylgroups in the glass matrix.

Reduced melting, softening and sintering temperatures

Exhibit an increase in thermal expansion and mechanicalstrength without a compromise in their chemical solubility.

The hydroxyl addition is called a palstified layer


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y y p y

OH- added to the porcelain structure through exposure towater or water vapours (hence the term hydrothermal)

Increases chemical resistance

generates a smoother surface profile

possesses the unique capacity of healing surface flawsthrough the ion exchange process.

HC + high noble alloy with low M.P


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HC high noble alloy with low M.P

a single alloy can be used for all types of

dental restorations and reconstructions.


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Hydrothermal ceramics can be formulated as two types :

A single phase porcelain

E.g: Duceram LFC(Degussa Dental, South Plainfield,NJ)

A leucite containing two phase material

E.g: Duceragold

(Degussa Dental, South Plainfield, NJ)

.


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In hydrothermal ceramics


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y

at the surface layer

ionic exchange between alkali and hydroxylgroups

hydrothermal layer(1m thick in vivo and 3m in vitro)

seals the surface microcracks

thi i i h i ti f ff t f


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this ionic exchange is suggestive of an effect ofhealing the surface flaws

an increase in strength

Advantages of hydrothermal ceramics overconventional porcelains

Lower fusion temperature (680 7000 C)


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Lower fusion temperature (680-7000 C)

Increased coefficient of thermal expansion

Minimal abrasion of opposing dentition

Greater toughness and durability

Stronger bond to the deep gold coloured Degunormalloy(Degussa Dental, S. Plainfield, NJ).

Low fusing hydrothermal ceramic

Duceram LFC


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Low fusing hydrothermal ceramic

Composed of an amorphous glass containing hydroxylions.

Has a non crystalline structure (No leucite crystals)

Lower hardness than feldspathic porcelain (due to absence

of leucite crystals)

less abrasion of opposing natural tooth structure.


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Second layer- Over the base layer, a veneer ofDuceram LFC is applied using powder slurry


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Duceram LFC is applied using powder slurrytechnique

baked at relatively low temperature (660C)

Supplied in different shades

No special equipment needed

Greater density

Advantages of Duceram LFC over feldspathicporcelain:


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Higher flexural strength

Greater fracture resistance

Lower abrasion than feldspathic porcelain (wear rate equal to that ofnatural teeth)

Surface resistant to chemical attack by fluoride containing agents.

Highly polishable, not requiring re-glazing during adjustment.

Duceram LFC and Duceragold do not containlarge leucite crystals and hence maintain a stable


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large leucite crystals and hence maintain a stablecTE over several firings.

Cannot be directly sintered on the metallicsubstructure because of the low coefficient of


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substructure because of the low coefficient ofexpansion.

Thus, an inner lining of conventional high-fusingceramic is required on the metal substructure

because of the low coefficient of expansion.

Golden Gate System:

(Hydrothermal ceramic for PFM crowns: Quintessence international


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vol 27, # 8/ 1996)

Golden-yellow alloy with high gold content

(Degunorm, Degussa) + a hydrothermal low fusing ceramic(Duceragold, Ducera) is together called Golden gate system

Based on the idea of being able to veneer a universal, lowfusing alloy that is gold-coloured.

Duceragold (Ducera Dental)

Leucite-containing


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g

Hydrothermal glass-ceramic system

Fusion temperature of approximately 8000C.

The leucite crystals are highly dispersed required highthermal expansion of the ceramic.

Hydrothermal ceramic hence high resistance to hydrolysis,chemical attack without the addition of fluxes and also has highflexural strength.

Type IV high Gold (deep yellow):73% Au


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- 73% Au

- 9% Pt

- 9.2% Ag.

Palladium-free

Extra hard

Low melting range (9000C-9900C).

Advantages of Golden Gate System:

) Ch i id i li h bl i h bb h l


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1) Chairside ceramics polishable with rubber wheelsno glazing required

- Low sintering point the thermal effect of the mechanical polishingcauses a local sintering of the boundary layer.

- Their highly polished surface is comparable to that of a highly flame-polished ceramic

Hence, an accurately smooth occlusally adjusted surfacecan be obtained without additional flame polishing (unlike


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conventional ceramics)

occlusal changes because of glazing during the last firing

process avoided

more accurate occlusal adjustment

2) Smoother surfaces with very little plaque adhesion

3) Esthetic appearance :

Advantages of Golden Gate Systemcontd.


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) pp

Translucence ideal (the leucite content is low and highlydispersed).

Opalescent and fluorescent appearance due to their lowsintering temperature.

Available in different shades for the reproduction of life-like

appearance in porcelain.

Gold-background of the high gold content alloy(Degunorm)


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(Degunorm)

- improves the esthetics of the restorations.

- neutralises the dark, sub-gingival margins of white

dental alloys among anterior teeth

No tendency to discolor.


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4) Ability to have both veneered and unveneered units in



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one casting.

5) Fitting accuracy is as precise as that of conventional procedures.

6) The low melting range of the high gold content alloy allows the use ofplaster or gypsum bonded investment (better fitting casting and low riskinvolved in devesting).


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Surface areas of hydrothermal Duceragold ceramic veneers and accurately fittingmargins on the die. Veneered crown, tooth 36; ceramic-veneered inlay, tooth 37.

7) A Pre-manufactured attachment (multi CON1System) made completely of Degunorm metal


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y ) p y gstructure is available that can be fastened to fixedand removable parts without soldering, thusreducing the number of different types of alloysused in a patients mouth.


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Support for the attachments on

ceramic veneered crowns 22 and 23,made from Degunorm/Duceragold

Activated counter die


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Ceramic-veneered crowns, teeth 34, 35 and 36, made from Degunorm andDuceragold; complete crown, tooth 37, made from Degunorm. All are shown afterdying of the plaque ( 18 months post insertion)


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Ceramic veneered crowns, teeth 45 and 46, made from Degunorm and Duceragold;complete crown, tooth 47, made from Degunorm


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Treatment of the buccal surface of a tooth (ceramic-veneered crowns, teeth 25and 26; inlay tooth 24; complete crown, tooth 27; Degunorm and Duceragold), 12months post insertion.

8) A self-healing hydroxyl layer (hydrothermal



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8) A self healing hydroxyl layer (hydrothermalproperty) increases flexural strength by upto 30%.

9) The Golden Gate System is multi-indicative for:-inlays and onlays,- crown milling work-all types bridge construction, for fixed or removable

cast dentures (in combination with attachments) aswell as superstructures.

Metal copings


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Veneered porcelain

Metal -ceramicRestorations

Strength and accuracy of metal with esthetics ofporcelain


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0.3mm thick metal for noble metal and 0.2 mm

thick for base metal on the facial side

Opaque porcelain veneer 0.3 mm thick

Body porcelain 1 mm thick.

Cast (wax pattern, cast, finished, heattreated/oxidized, opaque and veneer


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/ p qporcelains

applied) most commonly used.

Non- Cast

- sintering

- machining

- swaging

- burnishing

Casting- Metal alloy substructure is cast using a phosphate-bondedinvestment.


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Heat degassing treatment - to produce a surface oxide layer andensure a clean meta surface for bonding.

Finishing- Ceramic bonded stones or sintered diamonds are used forfurther cleaning and surface finishing.

Sandblasting - Final sandblasting with high-purity alumina abrasiveensures that the porcelain is bonded to a clean and mechanicallyretentive surface.

Condensation of procelain

Until the mid 20th century, gold and amalgamwere virtually the only materials available for the


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restoration and replacement of posterior teeth.

Porcelain jacket crowns were available for frontteeth, but they did not fit very well, and they wereprone to easy fracture.

In 1962 that all changed when Dr. AbrahamWeinstein patented the first gold based alloy upon


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which porcelain could be baked.

The metal substructure reinforced the porcelainand gave it the durability and the strength to resistfracture in the mouth.

It made it possible for the first time to replacemissing teeth with natural looking tooth coloured


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fixed bridgework.

In addition, due to the accuracy of the lost waxtechnique, the appliances could fit the toothpreparations exactly.

Porcelain will not chemically bond with gold by itself.


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trace elements in the composition of the alloy

oxide layeron its surface

bonds the porcelain to the metal.

The three oxide-forming elements are :

iron, indium and tin.

Porcelain is, itself, made of metal oxides. Thus it will bindwith the oxides on the surface of the gold framework.

Metal oxides formed on the surface of the casting

i ith th l i


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mix with the porcelain

colour, reflective properties and translucency affected.

Thus the porcelain must be formulated to overcome theseeffects.

Porcelain melts at high temperatures (between 850C and1350C depending on the type of porcelain used).


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It is applied as wet powder over the metal framework, andbaked, orfired in order to fuse the powder particles

together.

the metal substructure must resist sagging and deformationwhile being held at this high temperature for several hourswhile the porcelain is fused over it.

Otherwise, the casting will not fit the teeth in the mouth.

The metal is opaque and generally has a gold or graycolour. Porcelain must be translucent, or it fails the tests ofaesthetics.


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aesthetics.

There must be a mechanism to mask" the underlying metal

framework, or the finished appliance will have a gray castand not look real.

The index of thermal expansion of the metal must be nearlyidentical to that of the porcelain.


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Otherwise, the porcelain will simply shatter off of the

framework as it cools after being fired.

Ideally, porcelain should be under slight compression in thefinal restoration.


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Select an alloy/porcelain combination in which the alloycontracts slightly more than the porcelain on cooling to room

temperature

compression of porcelain--> crackpropagation.

All porcelains used to veneer metallic substructurescontain leucite crystals.


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propagation of cracks in the porcelain veneer

cTE

Buccal gingival margin is removed on the die.

- done to allow a butt porcelain margin so that no metal will

show in the final crown


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show in the final crown

The cast metal coping is placed back on the die .


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Next, a thin layer of opaque porcelain powder (frit)is layered over the metal in order to mask theunderlying darkness Otherwise the finished crown


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underlying darkness. Otherwise, the finished crownwould always show a gray caste.

Applied in a minimum of 2 layers.

- first thin layer wetting layer,

- subsequent layers fill in the irregularities andmask the metal.


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Ultra-PAKE Opaquing porcelain system (Ceramco, Inc):

utilizes Enhanced Ultra - Escent Crystals


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sprinkled over the thinnest paint

inner scattering of light

improve the overall vitality and fluorescence of restoration.

The crystals create a light-refractive opaque.


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25% thinner than standard opaque layers.

This system inhibits greening, prevents graymargin lines and saves time with premixedpastes.

Available in wide-range of premixed modifierpastes.

Biopaque (Detrey, Dentsply)paste system to be applied directly on the metallicsubstructure.


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substructure.

consists of 2 pastes for light and dark shades.

a) base paste (first layer)

covered by powdered dispersing crystals

first bake.

b) coloured opaque paste(second layer) available in thinnerconsistency and 8 different shades compared to the basepaste.

Fine translucent crystal powder is sprinkled and fired at thesame temperature as the first opaque layer.

Diffusion of a gas depends on:

- temperature

- time


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time

- distance

TemperatureFiring time

Distance to the surface

Greater mass of gaswill escape

Since it is crucial to avoid bubble in the opaque, use diffusionlaw to the maximum advantage while firing this layer.


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Dangerous to alter the recommended firing cycle properfusion is dependent on both the peak temperature and rateof firing.

So, decrease the distance through which the air must diffuseby firing 2 thin layers of opaque rather than one thick layer.

Although an extra step, this initial application of opaquehelps minimize the voids at the critical metal-ceramicinterface.


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Base paste application Crystal application

First firing


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Shaded paste application Modifier application


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Step 1. Thin the opaque mixture and apply a wash coat, workingit into the bonder.

Step 1: Thin the opaque mixture and apply a wash coat, workingit into the bonder.

Opaque Application


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Step 2. Check and remove any opaque on the inside of the coping and thenfire on the opaque firing cycle.

Step 2: Check and remove any opaque on the inside of the coping and thenfire on the opaque firing cycle.


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Step 3. Process in the furnace. Follow the porcelain manufacturersopaque firing cycles.Step 3: process in the furnace. Follow the manufacturers opaque firing cycles.


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Step 4. The second layer is slightly thicker. Remove any opaque on the inside ofthe coping and fire.

Step 4: The second layer is slightly thicker. Remove any opaque on theinside of the coping and fire.


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The coping, along with its "green" porcelain is removed from the die andplaced in a vacuum kiln and fired at about 1700 degrees F


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The green porcelain shrinks during its firing, so a second layer ofporcelain frit is layered over the first bake.


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After rebuilding the correct contours, the crown is replaced in thevacuum kiln for its second and final firing.


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METAL CERAMIC BOND

Ideal metallurgical properties

Structural design

Surface design and finishing


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Surface design and finishing

High modulus of elasticity: lesser flexion ( stresses) High yield strength: (resistance to permanent deformation)

Fine grain structure:


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g

- mechanical stability of the marginal area.

- corrosion resistance

- hardness

Sag resistance

Castability: accurately fitting castings

Bond potential:

- alloy should allow good wetting- thermally compatible with the veneer material.

Deflection directly proportional to L3

inversely proportional to T4


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As thick as possible occlusogingivally in theinterproximal region, especially for long spanbridges, without impinging on the embrasurespace.

Metal reaching the occlusal surface;adequate embrasure space; optimum rigidityby maximizing Occluso-Gingival thickness.


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Adequate thickness, but at the expense ofembrasure space.

Adequate embrasure space, but risks ofdeflection due to decreased thickness.

DCNA : 21,

1. dental ceramics

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