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1. DENTAL CERAMICS
CONTENTS
• HISTORICAL PERSPECTIVE
• CLASSIFICATION AND COMPOSITION
• STRUCTURE
• PORCELAIN CONDENSATION
• SINTERING
• BONDING PORCELAIN TO METAL
• METHODS OF STRENGTHENING CERAMICS
• ABRASIVENESS OF DENTAL CERAMICS
• FINISHING AND POLISHING OF PORCELAINS
• ALLOYS FOR METAL CERAMIC RESTORATIONS
• FACTORS AFFECTING COLOR
• PROPERTIES
• REVIEW OF LITERATURE
• RECENT ADVANCES IN CERAMICS
• SUMMARY AND CONCLUSION
• BIBLIOGRAPHY
1

2. During the stone age more than 10,000 years ago ceramics were important
materials and they have retrained their importance inn human societies ever
since. Craftsmen of that age used rocks that could be shaped into tools and
artifacts by a process called flaking in which stone chips could be fractured
away from surfaces of hard, fine grained or amorphous rocks. In 700 B.C.,
the Etruscans made ivory teeth and bone teeth that were held in place by
gold framework. Animal bone and ivory from hippo or elephant were used
for many years thereafter.
Porcelain was obtained in China by fluxing white china clay with “ chine
stone” to produce a white translucent stone ware in about 1000 A.D. this
material was much stronger than the earthen ware and stone ware. The
formulation was however a closely guarded secret .
The Germans discovered “white porcelain” in 1708 but it lackedthe
translucency of the Chinese product. In what might be labeled as the first
known case of industrial espionage a Jesuit father named D’entrecolles was
able to gain the confidence of Chinese potters and learnt the secret in 1717.
In 1774 a French apothecary named Alexis Duchateau noticed that glazed
ceramic utensils that he used everyday for mixing and grinding his various
chemical resisted staining and abrasion. It would asppear that these were the
circumstances that gave birth to the idea of using porcelain as a dental
restorative material. Later Nicholas Dubious de chemant of Paris in
collaboration with Alexis Duchateau considerably improved the method of
fabricating dentures.
In 1803 Elias Wildman formulated amuch more translucent porcelain with
shades much closer toantural teeth. The foundation for modern mass
production of artificial teeth was laid by the Italian dentist Fonzi when he
2

3. produced the first individual porcelain “terrometallic teeth”. In 1844 the
nephew of Stockton whohad introduced porcelain to U.S.A founded the S.S.
White Company and this led to refinement of the design and mass
production of porcelain teeth.
In 1880 Ambler test improved the design of dental coke burning oven and
dentistry went through various designs of using gas and finally electric
furnace was introduced at the end of century.
Two of the most important breakthroughs responsible for the long standing
superb esthetic performance and clinical survivability of metal ceramic
restorations are the patents of Weinstein and Weinstein(1962) na Weinstein
et al (1962) which described the formulations of feldspathicporcelain that
allowed systemic control of sintering temperature and ther4mal expansion
coefficient. The first commercial porcelain was developed in 1965 by Vita
Zahnfabrik.
A significant improvement in fracture resistance was reported by Hughes in
1965 with the introduction of aluminous core porcelain.
CLASSIFICATION AND COMPOSITION
According to history:
Earthenware:
Fired at low temperature and is relatively porus.
Stoneware:
Appeared in china in about 100 B.C
Porcelain:
3

4. Obtained by fluxing white China with “Chine stone” to produce a white
translucent stoneware.
According to firing temperature:
High fusing : 1300 degree centigrade
Medium fusing : 1101-1300 degree
centigrade
Low fusing : 850-1100 degree
centigrade
Ultra low fusing :< 850 degree centigrade
According to use:
Anterior
Posterior
Crowns
Veneers
Post and cores
FPD
Stain ceramic
Glaze ceramic
According to composition:
Pure alumina
Pure Zirconia
4

5. Silica glass
Leucite based glass ceramic
Lithia based glass ceramic
According to processing method:
Sintering
Partial sintering
Glass infiltration
CAD-CAM
Copy milling
According to tranclucency
Opaque
Translucent
Transparent
COMPOSITION
Ceramic: Inorganic compound with non metallic properties typically
composed of metallic and non metallic elements.
Dental ceramic: An inorganic compound with non metallic properties
typically consisting of oxygen and one or more semi metallic elements that
is formulated to produce the whole or part of a ceramic based prosthesis.
Dental Porcelain: is a vitreous ceramic based on silica network and potash
feldspar or soda feldspar. Pigments, opacifiers and glasses are added to
5

6. control fusion temperature, sintering temperature, thermal contraction
coefficient and solubility.
Feldspar :
They are mixtures of potassium aluminium silicate K2O.Al2O3.6SiO2, and
albite Na2O.Al2O3.6SiO2. When feldspar is melted at approximately 1250-
1500 degree Celsius, it fuses to become a glass with a free crystalline silica
phase. The soda form tends to lower fusion temperature while potash form
increases the viscosity of the molten glass. When feldspar is heated at
temperatures between 1150 and 1530 degree centigrade it undergoes
incongruent melting to form crystals of leucite which is a K-Al-Silicate
mineral with a large coefficient of thermal expansion.
Kaolin:
Is hydrated aluminium silicate (Al2O3.2SiO2.2H2O) that acts as a binder to
increase the moldability of the unfired porcelain porcelain. Because of its
opaqueness it is present in only very small quantities if at all.
Quartz:
It is a high fusing material forms the framework around which the other
ingredients flow. It prevents the slumping of the crown during the liquid
phase.
Alumina:
Many European tooth manufacturers use alumina in place of silica to
strengthen the teeth, especially around the pins.
Fluxes:
6

7. Potassium, lithium, sodium and calcium oxide and boric acid are used as
fluxes by interrupting the integrity of the SiO4 network, and lower the
softening temperature of a glass by reducing the amount of cross linking
between silica and oxygen.
Diagram from seminar.
The O: Si ratio in a glass is of greatest importance and increasing this ratio
will cause reduced viscosity, lowered fusion temperature and increased
thermal expansion.
Coloring agents:
The coloring pigments added to porcelain are known as color frit. These are
prepared by fritting metallic oxides into the basic glass used in porcelain.
Some of the common colors used are:
Pink : Tin chromium or chroma alumina
Yellow : Indium or praesmodyium
Blue : Cobalt salt
Green : Chromium oxide
Grey : Iron oxide or platinum
Opacifying agent:
Dental porcelain materials having varying degrees of translucency can be
manufactured by the addition of opaque materials. These are fine particles of
metal oxide and have a significantly different refractive index. Their melting
point is also higher than that of the matrix.
Fluorescence:
7

8. As the natural teeth possess a yellow white fluorescence, in the early days
the absence of this quality was noticed under violet light. The agent
commonly used is the uranium salt, sodium di urinate. T his salt produces a
strong greenish-yellow color. The ceramist should be aware of the radiation
hazards of including uranium.
STRUCTURE
Dental porcelains contain a crystal phase and glass phase based on the silica
structure. This structure is characterized by the Si-O tetrahedron in which a
Si 4+ cation is positioned at the center of a tetrahedron with O- anions at
each four corners. The structure is not close packed and it has both covalent
and ionic characteristics. The silica tetrahedra are linked together by sharing
their corners. Fused silica is a material whose high melting temperature is
attributed to the 3 – D networks of covalent bonds between silica tetrahedra
which form the basic structural units of the glass network. Fluxes are added
to reduce the temperature required to sinter the porcelain powder particles
together at low enough temperatures so as that the alloy to which it is fired
does not melt or sustain sag deformation.
Dental porcelains use the basic silicon –oxygen network as the glass forming
matrix, but additional properties, such as low fusing temperature, high
viscosity and resistance to devitrification are built in by the addition of other
oxides to the glass forming Si-O 4 lattice. These oxides generally consist of
potassium, calcium, Aluminium and boric oxides.
Potassium, sodium and calcium oxides are used as glass modifiers that is
they interrupt the integrity of the sio4 network and act as fluxes. Thje
purpose of a flux is to lower the softening temperature of a glass by reducing
the amount of cross linking between the oxygen and the glass forming
elements. If too many tetrahedrons are disrupted then the glass may
devitryfy.
8

9. Boric oxide can act as a flux and also as a glass former.
Oxides like alumina may react either way, depending on other factors such
as composition. Such oxides are called intermediates.
PORCELAIN CONDENSATION
Porcelain is supplied as a fine powder that is designed to be mixed with
water or another vehicle and condensed to desired form. The particles are of
a particular size distribution to produce the most densely packed porcelain
when packed. This provides two benefits:
Lower firing shrinkage
Less porosity.
The methods of condensation are:
Vibration technique
Spatulation technique
Brush technique
The surface tension of the water is the driving force behind condensation
and the porcelain must never be allowed to dry out until condensation is
complete.
SINTERING OF PORCELAIN
Diagrams and matter to be scanned
The purpose of firing is simply to fuse the particles together, a process
called sintering.
9

10. The condensed porcelain mass is placed in front of the muffle of a preheated
furnace (approximately 650 degrees.). This permits the remaining water
vapor to dissipate. After preheating for 5 min the porcelain is placed into the
furnace and the firing cycle is initiated. If it is placed directly without
preheating it will result in rapid production of steam thereby inducing voids
or fracturing large sections of veneer.
The progressive changes that occur during the firing of porcelain is shown in
the figure. The white areas are powder particles and the area between are
voids. As the temperature is raised the fused glass gradually flows to fill up
the air spaces but the air becomes trapped in the form of bubbles because the
fused mass is too viscous to allow all of it to escape.
STAGES IN FIRING
LOW BISQUE:
The glass grains have softened and have started to flow. The fired article
exhibits rigidity but it is very porous. The powder particles lack complete
cohesion. A negligible amount of firing shrinkage occurs.
MEDIUM BISQUE:
The glass grains have flowed to the extent that the powder particles exhibit
complete cohesion. The article is still porous and at this stage there is
definite shrinkage.
HIGH BISQUE:
After the high bisque stage, the shrinkage is complete and the mass exhibits
a smoother surface but the body does not appear glazed. The work can be
removed from the furnace and cooled at any of these stages, so that additions
10

11. can be made. The fewer the firing cycles to which the restoration is exposed,
the higher will be the strength and better the esthetics.
Minimum of three firings are needed for fabrication of ceramometal
restoration:
Opaque
Dentin and enamel
Stain and glaze
Porcelain shrinks 30-40 % during firing- oversize the buildup.
Firing in air – entrapment of air causes formation of pores in porcelain(6.3
% voids-undesirable roughness and pits and also affects strength and optical
properties)
Porcelain for PFM are fired under vacuum thus as the furnace door closes
the pressure is lowered to 0.1 atmosphere and the temp is raise until firing
tempo is reached . th e vacuum is then released and the furnace pressure
returns to 1 atm- Dense pore free porcelain.
GLAZING
After porcelain is cleaned stains required are applied and porcelain returned
to furnace for final glaze f9irting. When the glazing temp is reaches a thin
glassy film( glaze) is formed by viscous flow on the porcelain surface.
Glazed porcelain is stronger than unglazed.
Glaze is effective in reducing crack propagation. If glaze is removed by
grinding transverse strength is reduced to half.
Two types of glazes :
11

12. Over glaze
Self glaze.
Porcelains may be characterized with stains and glazes to provide a more life
like appearance.
One metjhjod of ensuring that the stains remain permanently isn by
incoprporating the stains internally. Internal stasining and charecterizat5ion
can produce a lifelike result, particularly when simulated enamel craze lines
and other features are built into the porcelain rather than merely applied to
the surface. The disadvantage is that the porcelain must be stripped away
completely if the color or characterization is unsuitable.
COOLING:
Must be carried out gradually and uniformly.
Too rapid – surface cracking and loss of strength
Too slow- might induce formation of additional leucite. Increased the
overall coefficient of thermal expansion cracking, crazing.
Less is the no of firing higher is the strength and better the esthetics. Too
many firing cycles – lifeless over translucent porcelain.
BONDING PORCELAIN TO METAL
The primary requirement for the success of a metal ceramic prosthesis is the
development of a durable bond between the porcelain and the alloy.
Theories of metal ceramic bonding have historically fallen into two groups:
Mechanical interlocking between porcelain and metal
12

13. Chemical bonding across the porcelain-metal interface. Alloys that form
adherent oxides during degassing cycle also form a good bond to porcelain
whereas those alloys with poorly adherent oxides form poor bonds. Some
palladium silver alloys form no external oxide at all but rather oxidize
internally. It is for these alloys that mechanical bonding is needed.
The nature of bond can be divided into three maincomponents:
Mechanical
Compressive
Chemical
Mechanical:
It is dependent upon good wetting of the metal or metal oxide surface by
porcelain. It is improved by a textured surface. A rough surface may
enhance the bond resistance against induced shear stresses, especially for
base metal alloys. Eg: air abrasion. Advantages:
Enhances wettability
Additive bond strength
Increased surface area
Compressive:
Ceramo-metal systems are deliberately designed with a very small degree of
mismatch in order to leave the porcelain in a state of compression.
Chemical bonding:
13

14. When dental porcelain is fired onto metal with a definite oxide (indium, tin
or zinc oxide) layer, the oxygen surface of the molten glass diffuses within
the oxygen surface on the metal to reduce then no. of bridging oxygen and
thus improves the screening of cations at the interface. If the glass is not
saturated with the particular oxide, it dissolves the oxygen with metallic
cations. The glass at the glass oxide interface then becomes saturated with
oxide. This glass remains constant in composition and is in thermodynamic
equilibrium with the oxide resulting in a balance of bond energies and a
chemical bond. The critical requirement for maintaining saturation at the
oxygen- glass interface is that the rate of solution of the oxide at the
interface is higher than the rate of diffusion of the dissolved oxide away
from the interface.
Procedure:
The metal is degassed by heating at 1000 degrees in vacuum for around 10
min and then slowly air cooled in normal atmosphere. This procedure will:
1. Degas the casting.
2. Induce age hardening of the alloy.
3. Base metal atoms will diffuse onto the surface to form an oxide film.
Shear strengths of enamel porcelain bonds:
Type of bond failures:
METHODS OF STRENGTHENING CERAMICS
Minimize the effect of stress raisers:
Numerous minute scratches and other defects are present on the surface of
these materials which behave as sharp notches whose tips may be as narrow
14

15. as the spacing between several atoms in the material. These stress
concentration areas at the tip of each surface flaw can increase the localized
stress to the theoretical strength of the material. When the induced
mechanical stresses exceed the actual strength of the material, the bond at
the notch tip breaks forming a crack. The design of the ceramic dental
restoration should also avoid stress raisers in ceramic. Conditions that can
cause stress concentration:
1. Abrupt changes in shape or thickness in the ceramic contour.
2. Sharp line angles.
3. creases or folds of the platinum foil or gold foil substrate.
4. small particle of porcelain along the internal porcelain margin
5. improper occlusal contacts
As ceramics tend to hasve no mechanism for plastically deforming
withoutfrac ture as dometals, cracks may propogate through a ceramic at low
stress levels. Thus ceramics and glasses have tensile strengths much lower
than their compressive strengths.
DEVELOP REDIDUAL COMPRESSIVE STRESSES:
The metal and porcelain should be selected with a slight mismatch in their
thermal contraction coefficients so that the metalcontracts slightly more than
the porcelain on cooling . this mismatch leaves the porcelain in residual
compression and provides additional strength for the prosthesis. The same
principles apply to ceramic prosthesis in which the thermal contraction
coefficient of of core ceramic is slightly greater than that of the veneering
ceramic.
MINIMIZE THE NUMBER OF FIRING CYCLES:
15

16. Repeated firing can causeincrease in the leucite concentration of the
porcelain which can cause increased coefficient of thermal contraction
which if exceeds that of the metal can lead to immediate or delayed crack
formation in the porcelain.
MINIMIZE TENSILE STRESS THROUGH OPTIMAL DESIGN OF
CERAMIC PROSTHESIS:
1. Avoid sharp line angles
2. avoid great amount of vertical overlapin anteriors.
3. avoid marked changes in thickness
4. use maximal occlusal thickness of the porcelain i.e., 2mm.
5. the metal should be in occlusal contact or the porclain should cove
the occlusal surface so that the metal ceramic interface should be
atleast 1.5 mm away from occlusal contact.
6. in FPD’s use greater connector height and broaden the radius of
curvature on gingival portion of interproximal connector.
7. use the finest grit abrasive for grinding.
ION EXCHANGE:
If a sodium containing porcelain is kept in a water bath containing molten
potassium nitrate potassium ions exchange places with some of the sodium
ions. Since K ions are 35% bigger than Na ions, the squeezing of K ion into
the place of Na creates very large residual compressive stresses. Increases of
100% in flexural strength have been observed by this technique. The depth
of this compression zone however is less than 100o micrometers.
16

17. THERMAL TEMPERING:
This creates residual surface compressive stresses by rapidly cooling the
surface of the object while it is still hot and in the softened state. This
induces a protective region of compressive stress within the surface.
DISPERSION STRENGTHENING:
This is reinforcement with a dispersed phase of a different material that is
capable of hindering a crack from propagating through the material. Some of
the crystals that can reinforce the glass phase of ceramics are:
Leucite (K2O.Al2O3.4SiO2)
Lithia disilicate(Li2O.2SiO2)
Alumina(Al2O3)
Magnesia alumina spinel (MgO.Al2O3)
Zirconia(ZrO2)
TRANSFORMATION TOUGHENING:
Dental ceramics based on Zirconia crystals undergo transformation
toughening that involves the transformation of ZrO2 from a tetragonal
crystal phase to a monoclinic phase at the tips of cracks that are in regions of
tensile stress.
Tetragonal phase is not stable at room temp.
So it has a tendency to trasnsform to monoclinic phase.
This transformation is prevented by the addition of yttrium oxide or Y2O3.
Yttria stabilized zirconia ceramic is also called CERAMIC STEEL.
17

18. ABRASIVENESS OF DENTAL CERAMICS
Abrasive wear mechanisms for ceramics and tooth enamel are
predominantly due to micro fracture which results from gouging, asperities,
impact, and contact stresses that cause cracks or localized fracture.
The fracture toughness of different asperites and enamel is as follows:
Alumina:3.4-4 MPa. m1/2
Yttrium stabilized zirconia : 6-9 MPa.m1/2
Glass : 0.75 Mpa.m1/2
Enamel : 0.77 Mpa.m1/2
Jagger and Harrison (1994) reported that the amount of wear produced by
both glazed and unglazed porcelain was similar but the wear produced by
polished porcelain was substantially less.
Exposure to carbonated beverages has been shown to significantly increase
the amount of wear.
Steps to minimize wear:
• Ensure cuspid guided disclusion
• Eliminate occlusal prematurities
• Use metal in functional bruxing areas
• If occlusion in ceramic, use ultralow fusing ceramics
• Polish functional ceramic surfaces
• Repolish ceramic surfaces periodically
18

19. • Readjust occlusion periodically if needed.
19

20. FINISHING AND POLISHING OF PORCELAINS:
1. Contour with flexible diamond disks, diamond burs, heatless
stones or green stones (Silicone carbide)
2. Finish with white stones or abrasive impregnated rubber disks,
cups or points.
3. Polish with fine impregnated rubber cups, and points or
diamond paste applied with a brush
4. Apply an over glaze layer.
Scenarios:
• New prostheses
• New prosthesis needing grinding
• Old in situ prosthesis which has roughened
• Old in situ prosthesis which requires occlusal correction.
ALLOYS USED FOR METAL-CERAMIC RESTORATIONS:
HIGH NOBLE:
Gold-platinum-palladium
Gold palladium-silver
Gold-palladium
NOBLE:
Palladium-silver
High palladium
20

21. PREDOMINANTLY BASE:
Nickel-chromium
Nickel-chromium-beryllium
Cobalt-0chromium
HIGH NOBLE ALLOYS:
Noble metal content >60% with at least 40% gold.
The choice of an alloy will depend on a no of factors: cost. Rigidity.
Castability, ease of finishing and polishing, corrosion resistance,
compatibility with a specific porcelain and personal preference.
Au-Pt-Pd:
Good corrosion resistance Pt and Pd increase the melting range. Zn, Sn, Fe
are present to form oxides and ceramometal bond.
Rhumium-grain refiner
High stiffness but low sag resistance.
Costly.. Are yellow..Better esthetics than white alloys.
Au-Pd:
Corrosion resistance. Have increased Pd content. Indium bonding. Rh grain
refiner. Ruthenium—castability,.. Gallium decrease the fusion temperature
strong stiffer and harder.
Au-Pd-Ag
Less of palladium good corrosion resistance
21

22. In-Sn bonding Rh castability
Alloys that have been to be are composed of Gold 45-55%
Pd 35-45%
With a small amount of gallium indium and tin.
]Dis Adv:
Cost and incompatibility with certain types of porcelains.
Noble alloys
Have noble metal content of atleast 25%
Pd-Ag contains no gold… have high Ag content Have lowest Noble content
In-Sn bonding Rh castability.
Some ceramics used with high Ag alloys resulted in what was called the
greening effect.
High Pd
Contain high Pd with 10-15 % Cu.
In bonding Gallium casting temp, high strength, hardness low density. Have
low sag resistance and form dark oxides.
BASE METAL ALLOYS:
Have < 25 % noble content.
Ni-Cr:
Cr provides tarnish and corrosion resistance
22

23. Mo-added to decrease CTE
Be –improve castability and hardening
Alloys are harder than noble alloys but have low yield strengths.
Higher elastic modulus thinner copings And framework could result lower
densities and higher casting temperature but Be is a carcinogen can pose
toxicity problems .
Co-Cr
Cr provides tarnish corrosion resistance
Mo-lower the CTE
Rh- improves castability
Stronger and harder than noble and Ni-Cr alloys
Casting and soldering is more difficult than noble metal alloys
Ti types
Pure Ti and Ti-6Al-V may become imp for ceramo metal; restorations but
present processing difficulties indicated by casting temperature of 1760
degree to 1860 degree and their ease of oxidation
FACTORS AFFECTING THE COLOR OF CERAMICS:
“A dark red that is yellower and less strong than cranberry, paler and
slightly yellower than average garnet, bluer, less strong, and slightly lighter
than pomegranate, and bluer and paler than average wine”
The three dimensions of color:
23

24. Hue: dominant color of an object, wavelength
Chroma: saturation
Value: lightness or darkness ….independent of hue
Metamerism: objects that appear to be color matched under one type of light
appear different under another light source
Fluorescence: the property of an object to emit light of different wavelength
than the one incident upon it
Eincident=Escattered+Ereflected+Eabsorbed+Etranmitteed+Efluoresced
In the dental operatory or laboratory color matching is usually performed by
the use of shade guide.
Dentin is more opaque than enamel and will reflect light.. pale yellow in
color
Enamel……crystalline……. different refractory indices at the incisal region
….bluish white (thick) at cervical margin-yellow (thin. reflects color of
underlying dentin)
…..Translucence……DEPTH
“Northern light from a blue sky during the middle portion of day that is
slightly overcast”
PROPERTIES
Discussion of mechanical properties… IN VACUUM……HOLISTIC
Restorative materials at our disposal:
24

25. Metals-high tensile strength, toughness, hardness, resistance to abrasion,
fracture resistance, elasticity, ductility fatigue resistance
Polymers-inferior in most of these properties….BRITTLE FRACTURE
Composites-BRITTLE FRACTURE superb aesthetics
Ceramics-No ductility, high compressive strength, low shear and tensile
strengths excellent aesthetics
COMPRESSIVE STRENGTH:
Maximal stress required to fracture a structure under compression.
Enamel:37,800 psi
Dentin: 44,200 psi
Porcelain : 25,000 psi
Metalceramic alloys : yield strength of 65-80,000 psi
TENSILE STRENGTH:
Maximal stress required to fracture a structure under tension.
Porcelain: 5,000 psi
HARDNESS (KHN):
Enamel: 343
Dentin: 68
Porcelain: 460
25

26. FLEXURAL STRENGTH (BENDING STRENGTH OR MODULOUS
OF RUPTURE):
Force per unit area at the point of fracture of a test specimen subjected to
flexural loading.
Feldspathic porcelain: 141 MPa
Aluminous porcelain: 139 MPa
IPS Empress2: 400 MPa
Gold alloy: 350-600 MPa
FRACTURE TOUGHNESS:
Feldspathic porcelain: 0.9-1.5 MPa.m1/2
Aluminous porcelain: 2-2.9
Yttria stabilized zirconia: 9
Gold alloy: 20
Enamel: 0.7
IPS Empress2: 3.3
THERMAL COEFFICIENT OF EXPANSION:( mm/mm.K)*10-6
Change in unit length per unit rise in temperature
Tooth : 11.4
Low fusing ceramic: 12.2-15.8
IPS Empress 2: 10.6
Ceramometal: ?
26

27. THERMAL CONDUCTIVITY (cal.cm/cm2.sec.C):
Ability of a body to transfer energy
Enamel: 0.0022
Dentin: 0.0015
Porcelain: 0.0030
THERMAL DIFFUSIVITY (cm2/sec):
Enamel: 0.0042
Dentin: 0.0026
Porcelain: 0.64
“Effectiveness of a material in preventing heat transfer is directly dependent
on its thickness and inversely dependent on its thermal diffusivity”
MODULOUS OF ELASTICITY
Porcelain: 69GPa
Type IV gold alloy: 99.3 GPa
Composite: 16.6Gpa
Because of their moderately high m of elasticity porcelains and relatively
low tensile strength porcelains can undergo very little elastic deformation
(0.1%) before they rupture i.e., they are not flexible
Table 19-11 page 600 anu
27

28. RECENT ADVANCES IN DENTAL CERAMICS:
METAL CERAMIC CROWNS BASED ON BURNISHED FOIL
COPINGS: THE CAPTEK SYSTEM
Malleable Captek metal strips are burnished on a refractory die to fabricate
the metal coping of a metal ceramic crown without the use of a melting and
casting process.
The finished metal coping may be described as a composite material
consisting of a gold matrix reinforced with small particles of a Pt-Pd-Au
alloy.
The units are then veneered with two thin layers of opaque porcelain and
other veneering porcelains. The Captek coping has a thickness of 0.25 mm
which is half of the traditional cast metals thus providing additional space
for vennering porcelain.
Indications: crowns and FPDs
CASTABLE GLASS CERAMICS : DICOR
Dicor is a castable glass (55% tetraflurosilicic mica crystals) that is formed
into an inlay, facial veneer or full crown restoration by a lost wax casting
process. After the glass casting core or coping is recovered, it is covered by
a protective embedment material and subjected to heat treatment that causes
mica to grow within the glass matrix. This process is called ceramming.
Then it is fit on dies, ground as necessary and coated with veneering
porcelain.
ADV:
Ease of fabrication
28

29. Improved esthetics-chameleon effect
Minimal shrinkage
Good marginal fit
High flexural strength
Low thermal expansion
Minimal abrasiveness of enamel
DISADV:
Limited use in low stress areas (Low tensile strength)
Inability to be colored internally
PRESSABLE GLASS CERAMICS (IPS EMPRESS):
It is provided as core ingots that are heated and pressed until the ingot flows
into a mold. It contains a higher proportion of leucite crystals that increase
resistance to crack propagation. The hot pressing process occurs over a 45
min period at high temperature to produce the ceramic substructure. The
crown form can be either stained and glazed or built up using a conventional
layering technique.
ADV:
Lack of metal
Translucent ceramic core
High flexural strength
Excellent fit
Excellent esthetics
29

30. 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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31. CAD –CAM CERAMICS (PROCERA, CEREC, CELAY, DICOR
MGC):
It stands for Computer aided design/Computer aided manufacturing.
It is supplied as ceramic ingots available in various shades. These are placed
in a machinable apparatus to produce the desired contours. This machined
restoration is checked for fit on the tooth. Occlusal adjustment is done
followed by polishing, etching and bonding the restoration to the prepared
tooth.
ADV:
Negligible porosity levels
Freedom from making an impression
Need for a single patient appointment (with CEREC system)
Good patient acceptance
DISADV:
Need for costly equipment
Lack of computer controlled processing support for occlusal adjustment
Technique sensitive nature of surface imaging.
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