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

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

produced the first individual porcelain “terrometallic teeth”. In 1844 the

nephew of Stockton whohad introduced porcelain to U.S.A founded the S.S.

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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:

Obtained by fluxing white China with “Chine stone” to produce a white

translucent stoneware.

3

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

Silica glass

4

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

control fusion temperature, sintering temperature, thermal contraction

coefficient and solubility.

5

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:

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.

6

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:

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

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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.

Boric oxide can act as a flux and also as a glass former.

8

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.

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

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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

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:

10

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 :

Over glaze

Self glaze.

Porcelains may be characterized with stains and glazes to provide a more life

like appearance.

11

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

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.

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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:

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

13

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

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:

14

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:

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.

15

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.

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.

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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.

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.

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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

Readjust occlusion periodically if needed.

18

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

19

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

20

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

21

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:

22

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:

23

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

24

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: ?

25

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

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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

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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

28

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.

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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.

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