Ceramic 2

INTRODUCTION
A ceramic so white that it was comparable only to snow, so strong that vessels
needed walls only 2–3 mm thick and consequently light could shine through it. So
continuous was the internal structure that a dish, if lightly struck would ring like a
bell.
This is porcelain!
It could be said that the ceramic material known as porcelain holds a special
place in dentistry because, not withstanding the many advances made in composites
and glass–ionomers, it is still considered to produce aesthetically the most pleasing
result. Its colour, translucency and vitality cannot as yet be matched by any material
except other ceramics. Most ceramics for metal-ceramic restorations contain from 15
to 25 vol% leucite as their major crystalline phase, but changes in the leucite volume
fraction can occur during thermal treatment of dental porcelains (Mackert and Evans,
1991a,b). Leucite is a potassium alumino-silicate with a high thermal expansion
coefficient (Mackert et al, 1986a). Materials for all-ceramic restorations use a wider
variety of crystalline phases as reinforcing agents and contain up to 90% by volume of
crystalline phase. The nature, amount, and particle size distribution of the crystalline
phase directly influence the mechanical and optical properties of the material (Morena
et al, 1986b; Kon et al, 1994). The match between the refractive indices of the
crystalline phase and glassy matrix is a key factor for controlling the translucency of
the porcelain. Similarly, the match between the thermal expansion coefficients of the
crystalline phase and glassy matrix is critical in controlling residual thermal stresses
within the porcelain (Mackert, 1988). The first glass-ceramics were developed in the
late 1950s (Stookey, 1959). Glass-ceramics are polycrystalline solids prepared by the
controlled crystallization of glasses" (McMillan, 1979). The crystallization is
achieved when the glass is submitted to a heat treatment during which crystal
nucleation and growth are thermodynamically possible. Proper control of the
crystallization heat treatment is necessary to ensure the nucleation of a sufficient
number of crystals and their growth to an effective size. The dual nature of glass-
ceramic materials confers upon them the esthetic, mechanical, and chemical qualities
of ceramics as well as the ability to be cast and processed as glasses. These
characteristics are of great interest for dental applications. Machinability is another
property desirable for the maximum utility of glassceramics as dental materials. The
ability of glass-ceramics to be machined is closely related to the nature and particle
size of the crystalline phase that develops during the crystallization heat treatment
(Utsumi and Sakka, 1970). Machinable glass-ceramics for industrial as well as dental
applications often contain mica as a major crystalline phase.
Hot-pressed ceramics constitute another application of high technology to
dentistry. This process relies on the application of external pressure at elevated
temperatures to obtain sintering of the ceramic body. Hotpressed ceramics are also
called "heat-pressed" ceramics. Hot-pressing classically helps avoid large pores
caused by non-uniform mixing. It also prevents extensive grain growth or secondary
crystallization, considering the temperature at which sintering is obtained. The
mechanical properties of many ceramic systems are maximized with high density and
1

small grain size. Therefore, optimum properties can be obtained by hot-pressing
techniques (Kingery et al, 1976).
In spite of their excellent esthetic qualities and their good biological
compatibility, dental ceramics, like all ceramic materials, are brittle. They are
susceptible to fracture at the time of placement or during function.
2

HISTORICAL PERSPECTIVE
Ceramics are the earliest group of inorganic materials to be structurally
modified by man and his early history is principally traced through these materials.
The origin of glazing techniques is probably the most interesting advancement.
Ceramic objects have been constructed for thousands of years. The earlier
techniques usually consisted of shaping the item in clay or soil and then backing it to
fuse the particles together. The initial attempts resulted primarily in coarse and some
what porous products, such as goblets and other forms of pottery. Later developments
led to unite detailed stone ware items.
1774: Nicholas Dubais de Chemant, a surgeon dentist of Paris is credited with
making porcelain dentures.
1791: Dechemont – obtains both French and English patent for dental porcelain.
1792: John Woodforde – manufactured porcelain pastes
1808: Giuseppangelo Fonzi – An Italian dentist produced porcelain metal backed
artificial teeth.
1850: First commercial production of porcelain denture teeth by white.
1860: Introduction of tube tooth and pivot crown in England.
1889: Porcelain inlays and jacket crowns introduced platinum matrix for fusing
porcelain inlays and crowns developed by Land in USA.
1903: Dr. Hugh Avery – Introduced new porcelain inlay technique
1905: Electric porcelain furnace
1908: Dr. A. Eschneider – Baked porcelain jacket crown
1923: Casting of dental porcelain for inlays and crown by lost technique.
1925: Dr. Albert LE Gro’s used porcelain by high fusing method. Jan Adrianasen –
pioneered the technique of building up porcelain with a brush.
1940: Vacuum firing of dental porcelain
1942: Fluorescent dental porcelain introduced
1962: Gold alloy for porcelain bonding were used
1963: Development of dental aluminous porcelain by McLean and Hughes
1968: Use of photosensitive glass ceramic in dentistry by Macclloch
1974: Palladium silver alloy introduced for porcelain fused to metal
1976: Platinum bonded alumina crown was used by McLean and Seed.
1983: High expansion core material by O’Brien
1984: First commercial castable dental glass ceramic
The earliest glazing technique was a Summerian invention made famous about
4000 BC as a Egyptian blue faience. More than 10,000 years ago stone age people
also used ceramics.
As early as the second half of the eighteen century, Fauchard and others
attempted to use porcelain for dental applications. Their efforts, working in the
demanding and potentially destructive intraoral environment, were largely
unsuccessful.
Porcelain was, however, successfully used for dental prosthesis by the end of
the 1800s, when the technique to fire all porcelain jacket crowns on a platinum matrix
was first developed but it was not until the mid 1950s that a dental porcelain was
3

developed with a coefficient of thermal expansion similar to that of existing dental
casting alloys.
4

TERMINOLOGIES
Alumina core: A ceramic containing sufficient crystalline alumina (Al2O3) to achieve
adequate strength and opacity when used for the production of a core for ceramic
jacket crowns.
Aluminous porcelain: A ceramic composed of a glass matrix phase and 35 vol% of
more of Al2O3.
CAD-CAM ceramic: A machinable ceramic material formulated for the production
of inlays and crowns through the use of a computer aided design, computer aided
machining process.
Castable dental ceramic: A dental ceramic specially formulated to be cast using a
lost wax process.
Ceramic: A compound of metallic and nonmetallic elements.
Ceramic, dental: A compound of metals (such as aluminium, calcium, lithium,
magnesium, potassium, sodium, tin, titanium, and zirconium) and non metals (such as
silicon, boron, fluorine, and oxygen) that may be used as a single structural
component, such as when used in a CAD-CAM inlay, or as one of several layers that
are used in the fabrication of a ceramic based prosthesis. Dental ceramics are
formulated to provide one or more of the following properties, castability,
moldability, injectability, color, opacity, translucency, machinability, abrasion
resistance, strength and toughness.
Note: All porcelains and glass ceramics are ceramics, but not all ceramics are
porcelains or glass ceramics.
Ceramic jacket crown (CJC): An all ceramic crown without a supporting metal
substrate that is made from a ceramic with a substantial crystal content (> 50 vol%)
from which its higher strength and/or toughness is derived. These crowns are
distinguished from porcelain jacket crowns that are made with porcelain to produce an
aesthetic porcelain margin as an alternative to a metal margin on a metal ceramic
crown.
Sintering: The process of heating closely packed particles to achieve interparticle
bonding and sufficient diffusion to decrease the surface area or increase the density of
the structure. For products such as In-Ceram and In-Ceram Spinel, surface contact
sintering and minimal density change are required.
Spinel or Spinelle: A hard crystalline mineral (MgAl2O4) consisting of magnesium
and aluminium. Also, any of a group of mineral oxides of ferrous iron, magnesium,
manganese or zinc.
5

Stain: A mixture of one or more pigmented metal oxides and usually a low fusing
glass that when dispersed in an aqueous slurry or monomer medium, applied to the
surface of porcelain or other specialized ceramic dried or light cured and fired, will
modify the shade of the ceramic based restoration. One product is supplied in a light
curable binder. These stain product are also called surface colorants or
characterization porcelains.
Thermal compatibility: The desirable condition of low transient and residual tensile
stress in porcelain adjacent to a metal coping that is associated with a small difference
in the thermal contraction coefficients between the metal and the veneering
porcelains. The contraction coefficient of the metal should be slightly greater than that
of the porcelains so that residual axial and knoop compressive stresses are produced.
This condition will ensure the cooling of metal ceramic prostheses without immediate
crack formation or delayed fracture caused by residual tensile stresses in porcelain.
6

CLASSIFICATION OF CERAMICS:
1. By content :
- Regular feldspathic porcelain
- Aluminous porcelain
- Leucite reinforced porcelain
- Glass infiltrated alumina
- Glass infiltrated spinel
2. By use:
- Artificial teeth
- Core ceramic
- Veneer ceramic
3. By processing method :
- Sintering
- Casting
- Machining
4. By their firing temperature :
- High fusing : 1300 c
- Medium fusing : 1100-13000 c
- Low fusing : 850 – 1100 c
- Ultra low fusing : less than 850C
5. By method of firing ;
- Air fired
- Vaccum fired
- Diffusable gas
6. By their area of application :
- Core porcelain
- Body dentin porcelain
- Gingival dentin porcelain
- Incisal enamel
COMPOSITION OF CERAMICS:
1. Feldspar:
 When mixed with metal oxides and fired, it forms a glass phase that is able to
soften and flow slightly
 This softening of glass allows porcelain particles to coalesce together. This is
called sinteringsintering
 Seen in concentration of 75-85 %.
2. Kaolin / clay:
 It acts as the binder.
 When mixed with water , it forms a sticky mass which allows unfired
porcelain to be easily worked and molded.
 On heating it reacts with feldspar and gives rigidity.
 Its white in color and reduces translucency .so its added only in
concentration of 4-5 %.
7

3. Quartz:
 It imparts more strength, firmness and translucency.
 It gives stability of mass during heating by providing a frame work.13-14%
• GLAZES:
It decreases pores on the surface of fired porcelain.
• This increases strength by decreasing the crack propagation.
if glaze is removed by grinding, the transverse strength is half of glazed porcelain.
1. Self glazing:
 External glaze layer is not applied here.
 The completed restorations is subjected to glazing here.
2. Add on glazes:
 External glaze layer is applied here.
 They are uncolored glasses whose fusing temperature is lowered by the
addition of glass modifiers.
Disadvantages: Low chemical durability, difficulty to apply evenly, difficult to
get exact surface characteristics.
6. Colouring agents:
 These coloring pigments are produced by fusing metallic oxides together with
fine glass and feldspar -Ex : iron / nickel oxides- brown , copper oxides-
green, titanium oxide –yellowish brown, cobalt oxide – blue.
7. Opacifying agents:
a. Opacifying agents consists of a metal oxide ground to a very fine particle
size. ex :cerium oxide, titanium oxide, zirconium oxide –most popular.
8. Stains:
 These powder is mixed with water and the wet mix is applied with brush to
the surface of porcelain before glazing.
 Internal staining is preferred as it gives life like results and prevents direct
damage to stains by surrounding environment.
9. Glass former:
Glass formers are silica.
1. Crystalline quartz
2. Crystalline cristobalite
3. Crystalline tridymite
4. Non crystalline fused silica
The vitreous matrix is made of silicate glass. Silica forms sio4 ions with
oxygen and is thus highly charged and fills the space between 4 oxygen atoms.the
tetrahydra must permit sharing of oxygen atoms to permit the formation of sio4
groups thus resulting in polymerization and a three dimensional network.
10. Glass modifiers :
 Potassium oxide, Sodium oxide, Calcium oxide are used as glass modifiers
 They act as fluxes by lowering the softening temperature of a glass
8

 When sodium oxide is introduced, instead of bridging the atoms together, it
contributes a oxygen atom which act as a non bridging oxygen and as a result
a gap is produced in the sio4 network. So the silica tetrahydra thus obtained is
able to move more easily at lower temperature than the earlier network.
11. Intermediate oxides :
 Glass modifiers reduces the viscousity of porcelain.
 It needs a high viscosity as well as low firing temperature. This is done by
the addition of Aluminium oxide and boric oxides.
The composition of the ceramic generally corresponds to that of the glasses in table,
except for an increased alkali content. The addition of greater quantities of soda,
potash, and/or leucite is necessary to increase the thermal expansion to a level
compatible with the metal coping. The opaque porcelains also contain relatively large
amounts of metallic oxide opacifiers to conceal the underlying metal and to minimize
the thickness of the opaque layer.
The high contraction porcelains have a greater tendency to devitrify because
of their alkali content. They should not be subjected to repeated firing, because this
may increase the risk for cloudiness within the porcelain, as well as changes in the
thermal contraction behavior. Thus it is obvious that a proper matching of the
properties of the alloy and porcelain is imperative to success. Criteria and test
methods for determining metal porcelain compatibility have been suggested. Testing
methods are focused on the measurement of coefficients of thermal expansion and
contraction, thermal shock resistance, and the strength of the bond, which are
discussed later.
Conventional dental porcelain is a vitreous ceramic based on a silica (SiO2)
network and potash feldspar (K2O.Al2O3.6SiO2) or soda feldspar (Na2O.Al2O3.6SiO2)
or both. Pigments, opacifiers, and glasses are added to control the fusion temperature,
sintering temperature, thermal contraction coefficient, and solubility. The feldspars
used for dental porcelains are relatively pure and colorless. Thus pigments must be
added to produce the hues of natural teeth or the color appearance of tooth-colored
restorative materials that may exist in adjacent teeth.
Silica (SiO2) can exist in four different forms: crystalline quartz, crystalline
cristobalite, crystalline tridymite, and noncrystalline fused silica. Fused silica is a
material whose high-melting temperature is attributed to the three - dimensional
network of covalent bonds between silica tetrahedral, which are the basic structural
the temperature required to sinter the porcelain powder particles together at low
enough temperatures so that the allow to which it is fired does not melt or sustain sag
(flextural creep).
Glass Modifiers:
The sintering temperature of crystalline silica is too high for use in veneering
aesthetic layers bonded to metal substrates. At such temperatures the alloys would
melt. In addition, the thermal contraction coefficient of crystalline silica is too low
for these alloys. Bonds between the silica terahedra can be broken by the addition of
9

alkali metal ions such as sodium, potassium, and calcium. These ions are associated
with the oxygen atoms at the corners of the tetrachedra and interrupt the oxygen
silicon bonds. As a result, the three-dimensional silica network contains many linear
chains of silica tetrahedral that are able to move more easily at lower temperatures
than the atoms that are locked into the three-dimensional structures of silica
tetrahedral. This ease of movement is responsible for the increased fluidity
(decreased viscosity), lower softening temperature, and increased thermal expansion
conferred by glass modifiers. Too high a modifier concentration, however, reduces
the chemical durability (resistance to attack by water, acids, and alkalis) of the glass.
In addition, if too many tetrahedral are disrupted, the glass may crystallizer (devitrify)
during porcelain firing operations. Hence, a balance between a suitable melting range
and good chemical durability must be maintained.
Manufactures employs glass modifiers to produce dental porcelains with
different firing temperatures. Dental porcelains are classified according to their firing
temperatures. A typical classification is as follows:
High fusing 13000
C (23720
F)
Medium fusing 11010
- 13000
C ( 20130
- 20720
F)
Low fusing 8500
- 11000
C ( 15620
- 20120
F)
Ultra – low fusing < 8500
C (15620
F)
The medium – fusing and high – fusing types are used for the production of
denture teeth. The low – fusing ultraslow – fusing porcelains are used for crown and
bridge construction. Some of the ultraslow – fusing porcelains are used for titanium
and titanium alloys because of their low contraction coefficients that closely match
those of these metals and because the low firing temperatures reduce the risk for
growth of the metal oxide. However, some of these ultraslow – fusing porcelains
conation enough leucite to raise porcelains. The potential advantage of ultraslow-
fusing veneering ceramics are the reduction in sintering times, decrease in sag
deformation of FPD frameworks, less thermal degradation of ceramic firing ovens,
and less wear of opposing enamel surfaces.
Because commercial dental laboratories do not fabricate denture teeth for
complete denture or removable partial dentures, it has become more common to
classify crown and bridge porcelains as high – fusing (850 - 11000
C) and low – fusing
(> 8500
C). However, this change in classification has not been universally adopted
Thus, to avoid confusion, the sintering temperature range should be identified (at least
initial) in discussions between dentists and dental technicians so that the less –
abrasive benefit claimed for ultraslow – fusing porcelains that were used exclusively
between the 1960s and 1990s.
Because it ensures adequate chemical durability, self – glazing of porcelain is
preferred to an add – on glaze. A thin external layer of glassy material is formed
during a self – glass phase and settling of crystalline particles within the surface for an
applied glaze procedure contains more glass modifiers and thus has a lower firing
10

temperature. However, a higher proportion of glass modifiers tends to reduce the
resistance of the applied glazes to leaching by oral fluids.
Another important glass modifier is water, although it is not an intentional
addition to dental porcelain. The hydronium ion, H3O+,
can replace sodium or other
metal i8o0ns in a ceramic that contains glass modifiers. This fact accounts for the
phenomenon of “slow crack growth” of ceramics that are exposed to tensile stresses
and moist environments. It also may account for the occasional long-term failure of
porcelain restorations after several years of service.
Feldspathic Porcelains
Potassium and sodium feldspar are naturally occurring minerals composed
primarily of potash (K2O) and soda (Na2O), respectively. The also contain alumina
(Al2O3) , and silica (SiO2) components. Feldspars are used in the preparation of man
dental porcelains designed for metal-ceramic crowns and many other dental glasses
and ceramics. When potassium feldspar is mixed with various metal oxides and
fired to high temperatures, lit can form leucite and a glass phase that will soften and
flow slightly. The softening of this glass phase during porcelain firing allows the
porcelain powder particles coalesce is called liquid-phase sintering, a process
controlled by diffusion between particles at a temperature sufficiently high to form a
dense solid. The driving force for sintering is the decrease in energy caused by a
reduction in surface area. As explained in the key terms section, section, three dental
products (In-ceram Alumina, spinell, and Zirconia) are slightly sintered to produce
interconnected pore channels that are necessary for subsequent glass infiltration.
Another important property of feldspar is its tendency to form the crystalline
mineral leucite when melted. Leucite is a potassium-aluminum-silicate mineral with a
large coefficient of thermal expansion (20to25ppm/o
C) compared with feldspar
glasses (which have coefficients of thermal expansion less than 10ppm/o
C). When
feldspar is heated at temperatures between 1150 o
C and 1530 it undergoes incongruent
melting to form crystals of leucite in a liquid glass. Incongruent melting is the process
by which one material melts to form a liquid plus a different crystalline material. This
tendency of feldspar to form leucite during incongruent melting is used to advantage
in the manufacture of porcelains for metal bonding. Further information is provided in
the sintering of porcelain section.
Man dental glasses do not contain leucite as a raw material. Since feldspar is
not essential as a precursor to the formation of leucite, as described earlier, these
glasses are modified with additions of leucite to control their thermal contraction
coefficients.
Feldspathic porcelains contain a variety of oxide components, including SiO2
(52-62 wt% ) , AlOO (11-16 wt%),k2O(9-11 wt%), Na2 O (5-7 wt%), and certain
additives, including Li2 O and B2O3. These ceramics are called porcelains because
they contain a glass matrix and one more crystal phases. They cannot be classified as
glass – ceramics because crystal formation does not occur through controlled
nucleation and crystal formation and growth. there are four types of veneering
ceramics. These include (1) low-fusing ceramics (feldspar – based porcelain and
nepheline senate- based porcelain); (2) ultra low-fusing ceramics (porcelains and
11

glasses); (3) stains; and (4) glazes ( self – glaze and add – on glaze). The particle type
and size of crystal particles, if present, will greatly influence the potential abrasives of
the ceramic prosthesis.
The thermal expansion coefficients of some ultraslow – fusing ceramics
(sintering temperatures be below 8500
C) and low-fusing ceramics are listed. These
ultra low-fusing ceramics represent an exciting new fail of ceramic core and
veneering materials because of their microstructural features. The ontain either a
well-distributed dispersion of small crystal particles or few or no crystals, depending
on the whether the ceramic is to be used as a veneer or glaze. Initial results of wear
studies are promising in several cases relative to reduced enamel wear caused these
ceramics. These results are summarized in a later section of this chapter (see wear of
Enamel by ceramic Products and Other Restorative Materials).
Other Additives:
Other metallic oxides can be introduced, as indicated in Table 21-1. Boric oxide
(B2O3) behaves as a glass modifier, that is, it decreases viscosity, lowers the softening
temperature and forms its own glass network. Because boric oxide forms a separate
lattice interspersed with the silica lattice, it still interrupts the more rigid silica
network and lowers the softening point of the glass. Alumina is not considered a true
glass former by itself because of the dimensions of the ion and the oxygen/aluminum
ratio. Nevertheless, it can take part in the glass network to alter the softening point
and viscosity.
1. Pigmenting oxides are added to obtain the various shades needed to
simulate natural teeth. These coloring pigments are produced by fusing metallic
oxides together with fine glass and feldspar and then regrinding to a powder.
These powders are blended with the unpigmented powdered frit to provide the
proper hue and chrome. Examples of metallic oxides and their respective color
contributions to oxide (yellowish brown) manganese oxide (lavender), and oxide
(green) titanium oxide (yellowish brown), manganese oxide (lavender), and
cobalt oxide (blue). Opacity may be achieved by the addition of cerium oxide,
zirconium oxide, titanium oxide, or tin oxide.
2. Composition:
Dental porcelains are essentially mixture of fine particles of Feldspar and
quartz. However the general trend towards the use of less kaolin (clay) with an
increase in the feldspar content in order to improve translucency suggests that dental
porcelain should be more correctly described as glasses. The feldspar melts first to
provide a glossy matrix for the quartz. The quartz thus act as a filler to provide
strength. The quartz may be replaced by alumina (Al2O3) such a material is referred to
as alumonous porcelain
Low fusing dental porcelain:
Oxide Weight %
SiO2 69.36
B2O3 7.53
12

CaO 1.85
K2O 8.33
Na2O 4.81
Al2O3 8.11
13

Medium fusing dental porcelain:
Oxide Weight %
SiO2 64.20
B2O3 2.80
K2O 8.20
Na2O 1.90
Al2O3 19.00
Ci2O 2.1
MgO 0.5
P2O5 0.7
14

Composition of dental ceramics for fusing to high temperature alloys:
Compound
Biodent
opaque
BG 2 (%)
Ceramco
opaque 60
(%)
VMK
opaque
131 (%)
Biodent
dentin BD
27 (%)
Ceramco
dentin T 69
(%)
SiO2 52.0 55.0 52.4 56.9 62.2
Al2O3 13.55 11.65 15.15 11.80 13.40
CaO - - - 0.61 0.98
K2O 11.05 9.6 9.9 10.0 11.3
Na2O 5.28 4.75 6.58 5.42 5.37
TiO2 3.01 - 2.59 0.61 -
ZrO2 3.22 0.16 5.16 1.46 0.34
SnO2 6.4 15.0 4.9 - 0.50
Rb2O 0.09 0.04 0.08 0.10 0.06
BaO 1.09 - - 3.52 -
ZnO - 0.26 - - -
UO3 - - - - -
B2O3, CO2
and H2O
4.31 3.54 3.24 9.58 5.85
15

PROPERTIES
GENERAL PROPERTIES OF CERAMICS
1. PHYSICAL PROPERTIES OF PORCELAIN
Strength: Porcelain is a material having good strength. However it is brittle and
tends to fracture. The strength of dental porcelain is usually measured by terms of this
flexure strength or modulus of rupture.
Flexure strength: It is a combination of compressive, tensile, as well as shear
strength.
Ground – 11,000 PSI (75.8MPa)
Glazed – 20,465 PSI (141.1 MPa)
1) Compressive strength of porcelain is 48000 psi (321 MPa) tensile strength [5000
psi (35 MPa)]. Tensile strength is low because of the unavoidable surface defects
like porosities and microscopic cracks.
Shear strength: It is low and is due to the lack of ductility caused by the complex
structure of dental porcelain [6000 PSI (110 MPa)]
Inadequate firing weakens porcelain, the firing also decrease strength as more
of the core gets dissolved in the flexure.
2) Surface Hardness: Porcelain is much harder than natural teeth. KHN – 460
(enamel 343).
3) Wear resistance: Porcelain is more resistant to wear than natural teeth.
4) Thermal properties: Porcelain has low thermal conductivity, co-efficient of
thermal expansion is close to that of natural teeth 6.4 to 7.8 x 10-6
/OC
5) Specific gravity: The specific gravity of fired porcelain is usually less, because
of the presence of air voids. It varies from 2.2 to 2.3
6) Dimensional stability: Porcelain is dimensionally stable after firing.
7) Chemical stability: It is insoluble and impermeable to oral fluids. Also it is
resistant to most solvents. However contact with hydrofluoric acid causes
etching of the porcelain surface. A source of this is APF (acidulated phosphate
fluoride) and stannous fluoride; which are used as topical fluorides.
8) Esthetic properties: The esthetic qualities of porcelain are excellent. It is to
match adjacent tooth structure in translucency, color and intensity. In addition,
attempts have also been made to match the fluorescent property of natural teeth
when placed under ultraviolet light.
9) Biocompatibility: It is compatible with the oral tissues. The margins of finishing
line can be even extended to the gingival sulcus
10) Modulous of elasticity: Porcelain has a high modulous of elasticity [10 x 106
PSI (69 GPa)]
11) Optical properties: The colors of commercial premixed dental porcelains are in
the yellow to yellow red range. Usually supplied in blue, yellow, pink, orange,
brown and grey.
The modifiers are added to the opaque and body porcelain during
building of the crown.
16

Surface staining: Disadvantages of surface staining are a lowered durability as a
result of high solubility and reduction of translucency. Opaque porcelains have
very low translucency values to mask metal substructure surfaces. Body
porcelain translucency values range between 20% and 35%.
Incisal porcelains have the highest values of translucency and range
between 45% and 50%. Since dental enamel is fluorescent under ultraviolet
light, uranium oxide have been added to produce fluorescence with porcelain.
However because of the low but detectable radioactivity of uranium, newer
formulations contain rare earth oxides such as cerium oxide which produce
fluorescence.
2. BIOLOGICAL PROPERTIES :
 They have excellent biocompatibility.
3. CHEMICAL PROPERTIES :
 It resist attack by chemicals.
 They have to be roughened by etching with hydrofluoric acid or sand blasting
to improve the retention of a cement to the internal surface of the restoration
4. MECHANICAL PROPERTIES:
 Low tensile strength
 Exhibits little plastic deformation
 Have good compressive strength
a) Compressive strength : 50000psi
b) tensile strength : 5000psi
c) Shear strength:16000psi
d) Elastic modulus : 10 × 106
psi
e) Knoop hardness : 460
f) C T E :12 × 10 -6
psi
g) R.I : 1.52 – 1.54
5. THERMAL PROPERTIES:
 They have insulating capacity.
6. OPTICAL PROPERTIES:
 They have good optical properties
 They are translucent because of absence of free electrons.
Strength of porcelains:
Strength of porcelain is decreased by,
1. By the presence of stress concentration areas
2. Porosity, roughness , machine damage
3. Sharp line and point angles
17

4. Interface between bonded structures where elastic modulus of 2 components are
different. More brittle material should have less elastic modulus. So it can transfer
stress to one with high modulus of elasticity.
5. Interface between bonded structure where large difference in thermal coefficient.
Material should have lower coefficient of thermal expansion, so the other has
protective compressive stress.
6. Areas of sharp point contacts on brittle material. Rounding of opposing cusp is
done, so that occlusal contacts are large areas.
Methods of strengthening porcelain:
1. Method of strengthening brittle materials
2. Method of designing components to minimize the stress concentration and tensile
stress.
1. Method of strengthening brittle materials:
Done in 2 ways
1. Development of residual compressive stresses within the surface of the
material
2. Ion exchange mechanism :
This techniques is called chemical tempering. in this procedure, a sodium
containing glass is placed in a bath of molten potassium nitrate , potassium ions in the
bath exchanges place with some of the sodium ions in the surface of the glass article.
The potassium ions being around 35% larger than the sodium ions, squeezes in to the
place formerly occupied by the sodium ions this creates large compressive stresses in
the surface of the glass these residual stresses produce a strengthening effect.
3. Thermal tempering:
This is the most common form of strengthening. This creates residual surface
compressive stresses by rapidly cooling the surface of the object while it is hot and in
the softened state. This rapid cooling produces a skin of rigid glass surrounding a soft
molten core.as the molten core solidifies, it tends to shrink, but the outer skin remains
rigid. The pull of the molten solidifying core as it shrinks, creates residual tensile
stresses in the core and residual compressive stresses within the outer surface.
4. Disruption of crack propagation:
by 3 ways
1. Crack tip interactions
2. Crack tip shielding
3. Crack bridging
1. Crack tip interactions: these occur when obstacles in the microstructure act to
impede the crack motion. These obstacles are second phase particles and act to
deflect the crack out of the crack plane. The re orientation of the crack plane leads
to a reduction of the force being exerted on the crack in the area of deflection.
When the crack is deflected out of plane, the crack is no longer subjected to pure
tensile stresses and will involve some shear displacement thus increasing the
difficulty of crack propogation.
18

2. Crack tip shielding :
By 2 ways
a) Transformation toughening
b) Microcrack toughening
a) Transformation toughening:
This is most commonly associated with the presence of zirconia. Under
unrestrained conditions, zirconia undergoes a high to low temperature phase
transformation which involves 3-5% volume increase. In toughened ceramic, the high
temperature phase of zirconia is constrained at room temperature. Applied tensile
stresses were to advance the crack plane. In the area directly behind the crack tip, the
matrix constrains on zirconia are released, allowing low temperature transformation to
take place. The transformed phase occupies a greater volume in the bulk material
resulting in compressive forces that tend to counteract any advancing crack tip
stresses.
b) Micro crack toughening :
The high coefficient of thermal contraction and volume reduction associated
with the high to low temperature phase transformation of leucite crystals create a
condition which causes the leucite crystals to contract significantly more than the
glass matrix. Compressive forces are created in the glass matrix surrounding the
particles leading to micro cracking in the leucite phase. The residual compressive
stresses in the glass phase around the particles can counteract the tensile stresses
which drive the crack forward.
3. Crack tip bridging:
It occurs when a second phase act as a ligament to make it more difficult for
the crack faces to open. This is better understood by bonded fiber composites. The
fibers act as ligaments which makes it more difficult to open the crack at an applied
stress.
Methods of designing components to minimize stress concentrations and
tensile stress:
1. Minimizing tensile stresses:
In a full coverage metal restoration with porcelain, the metal being of higher
thermal expansion will contract faster than the porcelain as a result the metal is
placed in tension and the porcelain in compression. For partial metal coverage
restorations, the junction between the metal and the porcelain is a potential site for
high stress as, the area with only metal will have no balancing compressive forces.so
ideally full coverage restorations are preferred. Porcelain unsupported by metal is
more subjective to fracture.
Reducing stress raisers:
Stress raisers are discontinuities in ceramic structures and in brittle materials
that cause stress concentration. The design of ceramic restorations should avoid stress
raisers. Abrupt changes in shape/ thickness in the ceramic contour can act as stress
raisers and make the restoration more prone to failure. Notches caused the porcelain
19

due to the folds of the underlying platinum foil substrate also is a stress raiser. Sharp
line angles, large changes in the thickness of porcelain are factors leading to stress
concentration. Usually contact points should be avoided and contact areas should be
preferred to avoid localized stress areas.
I. Reinforcement of inner surface by a higher strength ceramic:
Ex. Aluminous core porcelain
Cerestore
Reinforced high alumina crown
II. Reinforcement of inner surface by metal bonding:
Ex. Platinum foil
Gold foil swaged gold coping of 0.90-0.14mm thickness (renaissance system)
Titanium (procera) e.g. Titanium coping
III. Porcelain fusid to metal restorations
Ex. Noble metal alloys – Gold containing alloys
Gold free alloys
Base metal alloys Nickel – chromium alloys
Cobalt – chromium alloys
(rarely used in ceramic bonding)
IV. Designing of restorations:
The design should be such that it should not be subjected to tensile stress. To
avoid stress concentration in porcelain, sharp angles should be avoided and the
porcelain should be of uniform thickness.
Tensile stresses can be avoided by having a favorable occlusion in porcelain
jacket crown. In porcelain fused to metal restoration, the metal should be strong and
ductile not allowing flexing. Contact of opposing tooth or teeth should be either on
porcelain or on metal, but not at the junction.
Fluorescence and Opalescence:
For clinicians who practice esthetic restorative dentistry, particularly in the
field of ceramics, fluorescence is an important physical property. Natural teeth are
fluorescent. In other words, they emit visible light when exposed to ultraviolet light.
Fluorescence adds to the vitality of a restoration and minimizes the metameric effect
between teeth and restorative materials. The components of porcelain consist of
agents that cause them to fluoresce; thus, they also will emit visible light when
exposed to ultraviolet light. It is important that all the basic components of the
porcelain, including the dentins, enamels, stains and even the glazing agents, are
fluorescent. Opalescence is the ability of a translucent material to appear blue in
reflected light and orange-yellow in transmitted light. Opalescence also contributes to
the vitality of a restoration.
MODE OF SUPPLY
Dental porcelains are available as fine powders to be used with liquid I (or
distilled water). The powders and liquid are mixed to form a plastic mass which is
shaped or moulded into a desired shape, it is then fired (or sintered) at a high
temperature in order to fuse the particles together to form a ceramic body which is
esthetically like a natural crown.
20

Porcelain is supplied as a kit containing:
1. Fine ceramic powders in different shades:
 Enamel
 Dentin
 Core
2. Special liquid/distilled water
3. Stains of colour pigments
4. Glaze
CERAMIC PROCESSING METHODS
The single unit crown may be a metal ceramic crown (also called a porcelain
fused-to-metal crown), a traditional aluminous porcelain crown based on a core of
aluminous porcelain, or the newer ceramic crowns based on a core of leucite
reinforced porcelain, injection or pressure molded leucite based ceramic, glass
ceramic, sintered aluminous porcelain, sintered aluminum oxide, or glass-ceramic
processed from cast glass. The types of restoration, with their variations, are discussed
in detail in succeeding sections.
The processing stages of the ceramic core for production of ceramic
prostheses are summarized in Table. These seven different processes represent the
main procedures that were available in 2003. the quality of the final ceramic
prosthesis is dependent on each stage of the fabrication process. Machining or
grinding of the core structure is of particular importance since flaws or minute cracks
can be introduced that can possibly be propagated to the point of fracture during
subsequent intraoral stressing cycles. The use of computer aided manufacture (CAM)
bprocesses are most likely to induce such damage, although the ceramics with higher
fracture toughness are less likely to exhibit such damage. It is possible that subsequent
sintering or veneering procedures can reduce the potential for propagation of cracks in
the prostheses while in service. However, insufficient data are available from clinical
studies of ceramics.
The processing procedures for these ceramics are as follows. The feldspathic
porcelain of traditional PFM restorations, some aluminous porcelains (Vitadur-N, Hi-
Ceram), and pure alumina ceramic (Procera AllCeram) are condensed by vibration or
dry-pressed (Procera) and sintered at high temperature. Pressable ceramics (e.g., IPS
Empress, IPS Empress 2, Finesse All-Ceramic, OPC, and OPC-3G), when heated and
subjected to hydrostatic pressure, flow into a mold and after removal and divesting are
then veneered. Cast and cerammed crowns, such as the obsolete product Dicor, are
made using the lost-wax technique. The molten glass is cast into a mold, heat-treated
to form a glass-ceramic, and colored with shading porcelain and surface stains.
For slip cast ceramics (I-Ceram, In-Ceram Spinell, and In-Ceram Zirconia), a
slurry of liquid and particles of alumina, magnesia-alumina silicate (spinel), or
zirconia is placed on a dry refractory die that draws out the water from the slurry. The
slip-cast deposit is sintered on this die, and then it is coated with a slurry of a glass
phase layer. During firing, the glass melts and infiltrates the porous ceramic core.
Translucent porcelain veneers are then fired onto the core to provide the final contour
and color.
21

For CAD-CAM processes, the ceramic block materials (Dicor MGC, Vita
Cerec Mk I, and Vita Cerec Mk II) are shaped into inlays or crowns using a CAD-
CAM system (Cerec). CAM refers to computer-aided milling or machining. This
process is sometimes referred to as a CAD-CAM process, where CIM refers to
computer-integrated machining or milling. These blocks can also be used in copy
milling devices (Celay) that mill or machine blocks into core shapes in a manner
similar to that for cutting a key from a key blank, that is, by tracing over a master die
of the shape to be produced out of the ceramic.
SINTERED PORCELAINS
Leucite-reinforced feldspathic porcelain
Optec HSP material (leneric/Pentron, Inc.) is a feldspathic porcelain
containing up to 45 vol% tetragonal leucite (Schmid et al, 1992; Piche et al, 1994;
Denry and Rosenstiel, 1995). The greater leucite content of Optec HSP porcelain
compared with conventional feldspathic porcelain for metal-ceramics leads to a higher
modulus of rupture and compressive strength (Vaidyanathan et al, 1989). The large
amount of leucite in the material contributes to a high thermal contraction coefficient
(Katz, 1989). In addition, the large thermal contraction mismatch between leucite (22
to 25 x 10"6/°C) and the glassy matrix (8 x 10~6/°C) results in the development of
tangential compressive stresses in the glass around the leucite crystals when cooled.
These stresses can act as crack deflectors and contribute to increase the resistance of
the weaker glassy phase to crack propagation. After heat treatment of Optec HSP for
one hour at temperatures ranging from 705 to 980°C, a second metastable phase
identified as sanidine (KAlSi3O8) forms at the expense of the glassy matrix
(Vaidyanathan et al, 1989). The crystallization of sanidine is associated with a
modification of the optical properties of the material from translucent to opaque.
However, sanidine does not appear when the porcelain is heated to 980°C, since
sanidine is metastable in the temperature range 500-925°C. The recipitation of
sanidine has been reported as well upon isothermal heat treatment of conventional
feldspathic porcelain for metal-ceramics (Mackert et al, 1986b; Mackert, 1988;
Barreiro et al, 1989). An isothermal timetemperature- transformation diagram that
makes it possible to predict the amount of leucite and sanidine in samples subjected to
different thermal histories has been established (Barreiro and Vicente, 1993).
Alumina-based porcelain
Aluminous core porcelain is a typical example of strengthening by dispersion
of a crystalline phase (McLean and Kedge, 1987). Alumina has a high modulus of
elasticity (350 GPa) and high fracture toughness (3.5 to 4 MPa.m05). Its dispersion in
a glassy matrix of similar thermal expansion coefficient leads to significant
strengthening of the core. The first aluminous core porcelains contained 40 to 50%
alumina by weight (McLean and Hughes, 1965). The core was baked on a platinum
foil and later veneered with matched-expansion porcelain. Hi-Ceram (Vident,
Baldwin Park, CA) is a
more recent development of this technique. Aluminous core porcelain is now baked
directly onto a refractory die (McLean et al , 1994).
22

Magnesia-based core porcelain
Magnesia core ceramic was developed as an experimental material in 1985
(O'Brien, 1985). Its high thermal expansion coefficient (14.5 x 10'6/°C) closely
matches that of body and incisal porcelains designed for bonding to metal (13.5 x
10"6/°C). The flexural strength of unglazed magnesia core ceramic is twice as high
(131 MPa) as that of conventional feldspathic porcelain (65 MPa). The core material
is made by reacting magnesia with a silica glass within the 1100-1150°C temperature
range. This treatment leads to the formation of forsterite (Mg2Si04) in various
amounts, depending on the holding time. The proposed strengthening mechanism is
the precipitation of fine forsterite crystals (O'Brien et al, 1993). The magnesia core
material can be significantly strengthened by glazing, thereby placing the surface
under residual compressive stresses that have to be overcome before fracture can
occur (Wagner et al, 1992).
Zirconia-based porcelain
Mirage II (Myron International, Kansas City, KS) is a conventional feldspathic
porcelain in which tetragonal zirconia fibers have been included. Zirconia undergoes a
crystallographic transformation from monoclinic to tetragonal at 1173°C. Partial
stabilization can be obtained by using various oxides such as CaO, MgO, Y2O3, and
CeO, which allows the high-temperature tetragonal phase to be retained at room
temperature. The transformation of partially stabilized tetragonal zirconia into the
stable monoclinic form can also occur under stress and is associated with a slight
particle volume increase. The result of this transformation is that compressive stresses
are established on the crack surface, thereby arresting its growth. This mechanism is
called transformation toughening. The addition of yttria-stabilized zirconia to a
conventional feldspathic porcelain has been shown to produce substantial
improvements in fracture toughness, strength, and thermal shock resistance (Morena
et al 1986a; Kon et al, 1990). However, other properties, such as translucency and
fusion temperature, can be adversely affected. The modulus of rupture of
commercially available zirconia-reinforced feldspathic dental porcelain (Mirage II) is
not significantly different from that of conventional feldspathic porcelain (Seghi et al,
1990b).
GLASS-CERAMICS
Mica-based
As described earlier, glass-ceramics are obtained by controlled devitrification
of glasses with a suitable composition including nucleating agents. Depending on the
composition of the glass, various crystalline phases can 7(2):134-143 (1996) Crit Rev
Oral Biol Med 137 nucleate and grow within the glass. The advantage of this process
is that the dental restorations can be cast by means of the lost-wax technique, thus
increasing the homogeneity of the final product compared with conventional sintered
feldspathic porcelains.Dicor (Dentsply Inc., York, PA) is a mica-based machinable
glass-ceramic. The machinability of Dicor glass-ceramic is made possible by the
23

presence of a tetrasilicic fluormica (KMg25Si4O10F2) as the major crystalline phase
(Grossman and Johnson, 1987). Micas are classified as layer-type silicates. Cleavage
planes are situated along the layers, and this specific crystal structure dictates the
mechanical properties of the mineral itself. Crack propagation is not likely to occur
across the mica crystals and is more probable along the cleavage planes of these
layered silicates (Daniels and Moore, 1975). In the glass-ceramic material, the mica
crystals are usually highly interlocked within the glassy matrix, achieving a "house of
cards" microstructure (Grossman, 1972). The interlocking of the crystals is a key
factor in the fracture resistance of the glass-ceramic, and their random orientation
makes fracture propagation equally difficult in all directions. After being cast, the
Dicor glass is converted into a glass-ceramic by means of a single-step heat treatment
with a six-hour dwell at 1070°C. This treatment facilitates controlled nucleation and
growth of the mica crystals.However, it is critical to re-invest the cast glass
restoration prior to the crystallization heat treatment, to prevent sagging or rounding
of the edges at high temperature. The match in the thermal expansion coefficients of
the glass and the investment is achieved by use of a leucite based gypsum-bonded
investment. The interaction of the glass-ceramic and the investment during the
crystallization heat treatment leads to the formation of calcium magnesium silicate at
the surface of the glass-ceramic (Denry and Rosenstiel, 1993). This crystalline phase
could be formed by decomposition of the mica into magnesium silicate that later
reacts with the gypsum-bonded investment. This surface layer, called the"ceram
layer", has been reported to decrease the strength of glass ceramic crowns
significantly (Campbell and Kelly, 1989; Kelly et al, 1989). The effects of alumina
and zirconia additions on the bending strength of Dicor glass-ceramic have been
investigated. It was found that alumina additions successfully increase the bending
strength of Dicor glass-ceramic, whereas zirconia additions had no effect (Tzengetal,
1993).
Hydroxyapatite-based
Cerapearl (Kyocera, San Diego, CA) is a castable glass ceramic in which the
main crystalline phase is oxyapatite, transformable into hydroxyapatite when exposed
to moisture (Hobo and Iwata, 1985).
Lithia-based:
Glass-ceramics can be obtained from a wide variety of compositions, leading
to a wide range of mechanical and optical properties, depending on the nature of the
crystalline phase nucleating and growing within the glass. Experimental glass-
ceramics in the system Li2O-Al2O3- CaO-SiO2 are currently the object of extensive
research work. The choice of adequate additives is critical in the development of
tougher and higher-strength glassceramics (Anusavice et al. 1994b). Differential
thermal analysis can be efficiently used to determine the heat treatment leading to the
maximum lithium disilicate crystal population in the shortest amount of time, thereby
optimizing the nucleation and crystallization heat treatment of this type of glass-
ceramic (Parsell and Anusavice, 1994).
Machinable ceramics
24

Cerec system:The evolution of CAD-CAM systems for the production of machined
inlays, onlays, and crowns led to the development of a new generation of machinable
porcelains.There are two popular systems available for machining all-ceramic
restorations. The best-known is the Cerec system (Siemens, Bensheim, Germany). It
has been marketed for several years, and two materials can be used with this system:
Vita Mark II (Vident, Baldwin Park, CA) and Dicor MGC (Dentsply International,
Inc., York, PA). Vita Mark II contains sanidine (KAlSi3O8) as a major crystalline
phase within a glassy matrix. As explained earlier, the presence of sanidine could
explain the lack of translucency of this material. Dicor MGC is a machinable glass-
ceramic similar to Dicor, with the exception that the material is cast and cerammed by
the manufacturer. Colorants have been added to match tooth color. The glass-ceramic
contains 70 vol% of the crystalline phase (Grossman, 1991). Manufacturer's control
over the processing of this material and the higher volume percent of the crystalline
phase could explain the improved mechanical properties of Dicor MGC compared
with conventional Dicor glass-ceramic. The use of adhesive resinbased cements has
been shown to improve the fracture resistance of all-ceramic crowns (Eden and
Kacicz, 1987; Grossman and Nelson, 1987). Other studies have shown that the overall
fracture resistance of Dicor MGC was independent of cement film thickness (Scherrer
et al., 1994). Presently, the main identified weakness of the Cerec system is the
marginal fit of the restorations (Anusavice, 1993).
Celay system:
The Celay system (Mikrona Technologie, Spreitenbach, Switzerland) uses a
copy-milling technique to manufacture ceramic inlays or onlays from resin analogs.
The Celay system is a mechanical device based on pantographic tracing of a resin
inlay or onlay fabricated directly onto the prepared tooth or onto the master die
(EidenbenzeU/., 1994). As with the Cerec system, the starting material is a ceramic
blank available in different shades. One ceramic material currently available for use
with the Celay system is Vita-Celay (Vident, Baldwin Park, CA). This material
contains sanidine as the major crystalline phase within a glassy matrix. Recently, ln-
Ceram pre-sintered slip-cast alumina blocks (Vident, Baldwin Park, CA) have been
machined with the Celay copy-milling system used to generate copings for crowns
and fixed partial dentures (McLaren and Sorensen, 1995). The alumina copings were
further infiltrated with glass following the conventional
ln-Ceram technique, resulting in a final marginal accuracy within 50 urn.
SLIP-CAST CERAMICS
Alumina-based (n-Ceram)
ln-Ceram (Vident, Baldwin Park, CA) is a slip-cast aluminous porcelain. The
alumina-based slip is applied to a gypsum refractory die designed to shrink during
firing.The alumina content of the slip is more than 90%, with a particle size between
0.5 and 3.5 micrometers. After being fired for four hours at 1100°C, the porous
alumina coping is shaped and infiltrated with a lanthanum-containing glass during a
second firing at 1150°C for four hours. After removal of the excess glass, the
restoration is veneered with matched expansion veneer porcelain (Probster and Diehl,
1992). This processing technique is unique in dentistry and leads to a high-strength
25

material due to the presence of densely packed alumina particles and the reduction of
porosity. Two modified porcelain compositions for the In Ceram technique have been
recently introduced. In-Ceram Spinell contains a magnesium spinel (MgAl2O4) as the
major crystalline phase with traces of alpha-alumina, which seems to improve the
translucency of the final restoration. The second material contains tetragonal zirconia
and alumina. A variety of alumina-glass dental composites can be prepared by the
glass-infiltration process. However, research has shown that the fracture toughness of
the composites is relatively insensitive to the volume fraction of alumina in the range
investigated (Wolfrtfll., 1993).
Hot-pressed, injection-molded ceramics
Leucite-based
IPS Empress (Ivoclar USA, Amherst, NY) is a leucite-containing porcelain.
Ceramic ingots are pressed at 1150°C (under a pressure of 0.3 to 0.4 MPa) into the
refractory mold made by the lost-wax technique. This temperature is held for 20
minutes in a specially designed automatic press furnace (Dong et al, 1992). The
ceramic ingots are available in different shades. They are produced by sintering at
1200°C and contain leucite crystals obtained by surface crystallization (Holand et al. ,
1995). The leucite crystals are further dispersed by the hot-pressing step. The final
microstructure of IPS Empress exhibits 40% by volume of tetragonal leucite. The
leucite crystals measure 1-5 um and are dispersed in a glassy matrix. Two finishing
techniques can be used with IPS Empress: a staining technique or a layering technique
involving the application of veneering porcelain. The two techniques lead to
comparable mean flexure strength values for the resulting porcelain composite (Luthy
et al, 1993). The thermal expansion coefficient of the IPS Empress material for the
veneering technique (14.9 x 10"6/°C) is lower than that of the material for the staining
technique (18 x 10~6/°C) to be compatible with the thermal expansion coefficient of
the veneering porcelain. The flexural strength of IPS Empress material was
significantly improved after additional
firings (Dong et al, 1992). The strength increase is attributed to a good dispersion of
the fine leucite crystals as well as the tangential compressive stresses arising from the
thermal contraction mismatch between the leucite crystals and the glassy matrix.
Spinel-based
Alceram (Innotek Dental Corp, Lakewood, CO) is a material for injection-
molded technology and contains a magnesium spinel (MgAl2O4) as the major
crystalline phase (McLean and Kedge, 1987). This system was initially introduced as
the "shrink-free" Cerestore system, which relied on the conversion of alumina and
magnesium oxide to a magnesium aluminate spinel. One of the recognized advantages
of this system was the excellent marginal fit of the restorations (Wohlwend et al,
1989).
CONDENSATION (COMPACTION)
The process of packing the powder particles together and removing excess
water is known as condensation. Proper condensation gives dense packing and reduce
the shrinkage of porcelain and minimize porosity in the fired porcelain.Condensation
procedure is followed in application of core, dentin and enamel porcelain either in
26

porcelain jacket crown or porcelain fused to metal.The porcelain powder is mixed
with distilled water or special liquid supplied by the manufacturer to form a thick
paste. Small portions of the paste are then applied to the platinum matrix in jacket
crown preparation over the die until the desired shape of the crown has been attained.
Excess water is removed by blotting with a linen cloth or similar absorbent
material.The remaining water serves as a binder for the powder so that the crown may
be properly shaped before firing. Powder consisting of a mixture of particle sizes
compact more easily than those with particles of one size only. This reduces the size
of the spaces between the particles and thus reduces firing shrinkage.A well
compacted crown not only reduces firing shrinkage but also shows a regular
contraction over its entire surface.
Methods of condensation:
1) Vibration: Mild vibration are used to densely pack the wet powder upon the
underlying matrix. The excess water comes to the surface and its is blotted with a
tissue paper.
2) Spatulation: A small spatula is used to apply and smoothen the wet porcelain.
This action brings excess water to the surface.
3) Wet brush technique: The mix should be creamy and capable of being transferred
in small increments to the platinum matrix with hair brush.
4) Ultrasonic: A ceramosonic condenser can induce supersonic vibration in
porcelain creates intimate inter relation between metal and opaque porcelain.
5) Gravitational:
6) Whipping: Any method may be used for condensation but care is taken not to
allow the porcelain to dry out completely as the porcelain powder is held together
due to surface tension of water.
Dry the wet structure in a warm atmosphere before placing into the hot
furnace. After condensation the compacted mass supported by the matrix or metal
coping should be placed on a fire tray and inserted into the muffle of the ceramic
furnace.
Porcelain Condensation
Porcelains for ceramic and metal - ceramic prostheses, as well as for other
applications, is supplied as a fine powder that is designed to be mixed with water or
anther vehicle and condensed into the desired form (see Fig 21-2). The powder
particles are of a particular size distribution to produce the most densely packed
porcelain when they are properly condensed. If the produce the densely packed
porcelain when they are properly condensed. If the particles are of the same size, the
density of packing would not be nearly as high. Thorough condensation is also crucial
in obtaining dense packing would not be nearly as high. Thorough condensation is
also crucial in obtaining deus packing of the powder particles. Dense packing of the
powder particles. Dense packing of the powder particles dense packing of the powder
particles. Dense packing of the powder particles provides two benefits: lower firing
shrinkage and porosity in the fires porcelain. This packing, or condensation, may be
achieved by various methods, including vibration, spatulation, and brush techniques.
27

The first method uses mild vibration to pack the wet powder densely on the
underlying framework. The excess water is blotted or wiped away with a clean tissue
or fine brush, and condensation occurs toward the blotted or bushed area, in the
second method a small spatula is used to apply and smooth the it is removed. The
second method a small spatula is used to apply and smooth the wet porcelain. The
smoothing action brings the excess water to the surface, where it is removed. The
third method employs the addition of dry porcelain powder to the surface to absurd
the water. A brcelain powder to the surface to accord the water places the dry powder.
A brush to the side opposite from an increment of wet porcelain places the dry
powder. As the water is drawn toward the dry powder, the wet particles are pulled
together. Whichever method is used, it is important to remember that the surface
tension of the water is the driving force for condensation, and the porcelain must
never be allowed to dry out until condensation is complete.
Condensed mass is gradually heated by first placing it in front of the muffle of a
preheated furnace and later inserting into the furnace.
1) Low bisque stage: The flux begins to melt and flow in between the porcelain
particles. The mass attains some rigidity but very little cohesion. At this stage the
material is porous and undergoes minimum of shrinkage. The porcelain do not
have translucency and glaze.
2) Medium bisque stage: Here the flux flows freely in between the particles the
material is still porous, but there is complete cohesion between the particles and
most of the shrinkage is complete. In this stage also there is lack of translucencey
and glaze.
3) High bisque stage: Here with shrinkage is completed. There is very little
porosity, the mass has attained complete rigidity and smoothness, the body does
not appear to be glazed. Most of the addition and alterations are carried out after
the porcelain has attained medium bisque stage.
Less the number of firing, higher is the strength and better the esthetics. Too
many firings give a life less, over translucent porcelain.
PORCELAIN FURNACE
The ordinary air fire porcelain furnace consists of a muffle, a pyrometer, a
thermocouple and in its most simple form a rheostat or variable transformer for
control of firing temperature and sophisticated automatic and programmable time and
temperature controller for the most modern furnaces.
The muffle is the heating unit providing necessary high temperature for baking
of porcelain. The heating element is a coiled wire of platinum and is embedded into
the refractory material of the muffle. The muffle is provided with a door for easy
access and to prevent fluctuation of temperature due to heat loss.
The pyrometer is a millivoltmeter calibrated to read in degree of temperature.
The thermocouple consists of platinum wire joined at one end with another wire made
of 90% platinum and 10% rhodium. The joint is placed inside the muffle, this is
known as hot junction of the thermocouple. The free ends of the thermocouple are
attached to the pyrometer outside the muffle. When heat is generated inside the
muffle, the dissimilar metals of the thermocouple at the hot junction generates and
28

electromotive force which deflects the needle of the pyrometer indicating the
calibrated temperature.
As the electromotive force varies with variation in temperature inside the
muffle, such variations can be measured as temperature on the pyrometer. The
temperature controller regulates the current fed to the heating element inside the
muffle thereby inducing increase or decrease in muffle temperature.
The main problem in air fired furnace is the opacity of the porcelain due to
porosity.
Cooling:
The cooling of dental porcelain is complex matter, particularly when the
porcelain is fused to metal a metallic substrates.
Multiple firings of metal ceramic restorations can cause the co-efficient of
thermal contraction the porcelain to increase and can actually make it more likely to
craze or craze because of tensile stress development.
Cooling must be carried out slowly and uniformly. If shrinkage is not uniform
it causes cracking and loss of strength. During cooling, subsurface submicroscopic
surface cracks occur. Because of the low thermal conductivity of porcelain, the
differential between the thermal dimensional change of the outside and inside can
introduce stresses which embrittle the porcelain.
Different methods and porcelain firings are:
2) Air firing
3) Pressure firing
4) Gas firing
5) Vacuum firing
1) Air firing:
Air inside a furnace is modulated to the same atmospheric pressure during this
procedure. There is more chances of air entrapment in porcelain. We will get more
porous, less translucent porcelain.
2) Pressure firing:
The air inside the furnace is subjected to a pressure equal to 10 atmosphere as
the porcelain reaches its maturation temperatures. This compresses the air inside
the porcelain mass and reduces the size of the air bubble.
3) Gas firing:
The air in the furnace is replaced by a diffusible inert gas like argon or
hydrogen which diffuses out through the maturing porcelain.
4) Vacuum firing:
Partial vacuum firing reduces air voids, so porosity is reduced, so better
translucent effect.
The air from the furnace is evacuated and this eliminating air from porous
spaces which collapses on itself. This is the best and widely used method.
29

LABORATORY MANUFACTURING PROCEDURE
In the production of porcelain tooth, the powder ingredients are weighted and
mixed with water containing starch, gum tragacanth, or other organic materials to
form a putty like mass that can be handled conveniently.
The molding technique varies with different manufacturers. Generally, the
split molds are made of bronze and may be separated so that one portion contains the
negative pattern for the lingual surface of 12 teeth and the other contains the negative
pattern for the labial surface or face of the teeth.
When the two piece molds are used, a thin layer of the enamel mix is placed in
the labial mold to provide the enamel color, and the body mix, which forms the bulk
of the tooth, is placed over this. Then a thin veneer of enamel mix is placed in the
incisal portion of the lingual mold and is also covered with body mix. When
combined, the two halves with the porcelain mixes form a tooth with contours and
coloration similar to natural teeth.
The technique employs a third portion, which also fits against the labial
surface for the purpose of accurately forming the enamel colored porcelain separately
before the body portion is added.
The technique for the three piece molds involves placing the dough like
enamel mix in the labial half first, pressing the third or blender mold into it and
heating the molds until the mix stiffness. They are then opened, the excess mix is
trimmed away and the body mix is added to all the second and larger lingual half of
the mold.
Small noble metal rings are embedded in the porcelain to provide a base for
the gold plated nickel pins used for the retention of the teeth in the denture base.
These rings are made of a metal or alloy with a high melting point and usually are
split to allow for the shrinkage of the surrounding porcelain during fusion. Before the
moulds are filled, the rings are placed over the tips of tapered points that extend into
the tooth from the lingual half of the mould.
After the moulds are filled by either method, they are placed in a press and
heated until the porcelain mix develops sufficient hardness to allow handling. Each
anterior tooth at this stage is approximately one fifth over size to allow for shrinkage.
After 3 stage of firing, the teeth have been cooled slowly to prevent crazing,
all that remains and the attachment of the pins. For this operation small bits of solder
are stamped to the ends of the gold clad pins and they are inserted, solder downward,
to contact the metal ringes at the base of the tapered openings in the lingual body of
the teeth. When heated, either in a furnace or electrically, the solder melts and joins
the pin firmly to the embedded rings.
CERAMIC PROSTHESES:
Aluminous Porcelain Crowns:
Another method of bonding porcelain to metal makes use of tin oxide coatings
on platinum foil. The objective of this technique is to improve the aesthetics by a
replacement of the thicker metal coping with a thin platinum foil, thus allowing more
room for porcelain. The method consists of bonding aluminous porcelain to platinum
30

foil copings. Attachment of the porcelain is secured by electroplating the platinum
foil with a thin layer of tin and then oxidizing it in a furnance to provide a continuous
film of tin oxide for porcelain bonding. The rationale is that the bonded foil will act
as an inner skin on the fit surface to reduce subsurface porosity and formation of
microcracks in the porcelain, thereby increasing the fracture resistance of crowns and
bridges. The clinical performance of these crowns has been excellent for anterior
teeth, but approximately 15% of these crowns fractured within 7 years after they were
cemented to molar teeth with a glass ionomer cement.
Based on a 1994 survey, metal-ceramic crowns and bridges were used for
approximately 90% of all fixed restorations. However, recent developments in
ceramic products with improved fracture resistance and excellent aesthetic capability
have led to a significant increase in the use of all-ceramic products. Ceramic crowns
and bridges have been in widespread use since the beginning of the twentieth century.
The ceramics employed in the conventional ceramic crown were high fusing
feldspathic porcelains. The relatively low strength of this type of porcelain prompted
McLean and Hughes (1965) to develop an alumina-reinforced porcelain core material
for the fabrication of ceramic crowns.
The alumina-reinforced crowns are generally regarded as providing slightly
better aesthetics for anterior teeth than are the metal-ceramic crowns that employ a
metal coping. However, the strength of the core porcelain used for alumina-
reinforced crowns is inadequate to warrant the use of these prostheses for posterior
teeth. In fact McLean reported a fracture rate of molar aluminous porcelain crowns of
approximately 15% after 5 years.
Castable and machinable Glass-Ceramics (Dicor and Dicor MGC)
When used for posterior crowns, ceramic crowns are most susceptible to
fracture. Shown in Figure 21-6 (see also the color plate) is the stress distribution
computed by finite element analysis in a 0.5mm-thick molar Dicor crown loaded on
the occlusal surface, just within the marginal ridge area. The maximum tensile stress
is located within the internal surface directly below the point of applied force and just
above the 50 m-thick layer of resin cement (see the arrow in fig. 21.6). this site
represents the critical flaw responsible for crack initiation under an applied intraoral
force. The location of initial crack formation was consistent with the location of
maximum tensile stress predicted by the finite element calculations as shown in figure
21.6. an SEM image of a fractured clinical crown of Dicor glass-ceramic is shown in
fig 21.8. because of the smaller forces exerted on anterior crowns, the risk for
fracture of anterior crowns is significantly less than that for posterior crowns.
The first commercially available castable ceramic material for dental use,
Dicor, was developed by Corning Glass works and marketed by Dentsply
international. Dicro is a castable glass that is formed into an inlay, facial veneer, or
full-crown restoration by a lost-was casting process similar to that employed for
metals. After the glass casting core or coping is recovered, the glass is sandblasted to
remove resideual casting investment and the sprues are gently cut away. The glass is
then covered by a protective "embedment" material and subjected to a heat treatment
that causes microscopic platelike crystals of crystalline material (mica) to grow within
31

the glass matrix. This crystal nucleation and crystal growth process is called
ceramming. Once the glass has been cerammed, it is fit on the prepared dies, ground
as necessary, and then coated with veneering porcelain (as shown in fig. 21.8) to
match the shape and appearance of adjacent teeth. Dicor glass-ceramic is capable of
producing surprisingly good aesthetics, perhaps because of the "chameleon" effect,
where part of the color of the restoration is picked up from the adjacent teeth as well
as from the tinted cements used for luting the restorations.
Dicor glass-ceramic contains about 55 vol% of tetrasilicic fluormica crystals.
The ceramming process results in increased strength and toughness, increased
resistance to abrasion, thermal shock resistance, chemical durability, and decreased
transluency. Dicor MGC is a higher quality product that is crystallized by the
manufacturer and provided as CAD-CAM blanks or ingots. The CAD-CAM ceramic
Dicor MGC contains 70 vol% of tetrasilicic fluormica platelets, which are
approximately 2m in diameter. The mechanical properties of Dicor MGC are similar
to those of Dicor glass-ceramaic, although it has less translucency (contrast ratio of
0.41 -0.44 versus 0.56, respectively).
Dicor has recently been discontinued presumably because of low tensile
strength and the need to color the prosthesis on the exterior region rather that within
the core region, which would more closely resemble a natural tooth. Although Dicor
is no longer sold, the principles for selection are useful when products of similar
mechanical and physical properties are being considered. The advantages of Dicor
glass-ceramic were ease of fabrication, improved aesthetics, minimal processing
shrinkage, good marginal fit, moderately high flexural strength, low thermal
expansion equal to that of tooth structure, and minimal abrasiveness to tooth enamel.
The disadvantages of Dicor glass-ceramic were its limited use in low-stress
areas and its inability to be colored internally. As designed, it was colored with a thin
outer layer of shading porcelain and surface stain to ieve acceptable aesthetics.
However, Dicor MGC ingots have been supplied in light and dark shades, making it
possible for technicians to build depth of color into the fabrication process.
Although both of the Dicor products were based on a glass-ceramic core that
was minimally abrasive to opposing tooth enamel, the required shaduing or veneering
porcelains were more abrasive. Aesthetically, Dicor crowns were more lifelike than
metal-ceramic crowns, which often exhibit a metal collar, a gray shadow
subginigivally, or poor translucency. The life expectancy of Dicor crowns in high-
stress areas is not as good as that of PEM crowns. Two veneering materials were
used to improve the color of Dicor crowns: Dicor Plus, which consisted a pigmented
feldspathic porcelain veneer, and Willi's Glass, a veneer of Vitadur N aluminous
porcelain.
Tooth preparation for glass-ceramic of this type is the same as that required
for metal-ceramic prostheses except that, for first and second molars a reduction of
2mm is recommended. Occlusal surfaces and incisal edges must be reduced a
minimum of 1.5mm. Axial surfaces should be reduced a minimum of 1.0mm. The
preparation should be either a shoulder with a rounded gingivoaxial line angle or a
heavy chamfer.
32

Pressable Glass-Ceramics:
A glass-ceramic is a material that is formed into the desired shape as a glass,
then subjected to a heat treatment to induce partial devitrification (i.e., loss of glassy
structure by crystallization of the glass). The crystalline particles, needles, or plates
formed during this ceramming process serve to interrupt the propagation of cracks in
the material when an intraoral force is applied, thereby causing increased strength and
toughness. The use of glass-ceramics in dentistry was first proposed by MacCulloch
in 1968. He used a continuous glass-molding process to produce denture teeth. He
also suggested that it should be possible to fabricate crowns and inlays by centrifugal
casting of molten glass.
Pressure molding is used to make small, intricate objects. This method uses a
piston to force a heated ceramic ingot through a heated tube into a molk, where the
ceramic form cools and hardens to the shape of the mold. When the object has
solidified, the refractory mold (investment) is broken apart and the ceramic piece is
removed. It is then debrided and either stained and glazed (certain inlays) or veneered
with one or more layers of a thermally compatible ceramic.
IPS Empress is a glass-ceramic provided as core ingots that are heated and
pressed until the ingot flows into a mold. It contains a higher concentration of leucite
crystals that increase the resistance to crack propagation (fracture). The hot-pressing
process occurs over a 45 min period at a high temperature to produce the ceramic
substructure. This crown form can be either stained and glazed or build up using a
conventional layering technique.
The advantages of this ceramic are its lack of metal, a translucent ceramic
core, a moderately high flexural strength (similar to that of Optimal Pressable
ceramic), excellent fit, and excellent aesthetics. The disadvantages are its potential to
fracture in posterior areas and the need to use a resin cement to bond the crown
micromechanically to tooth structure.
IPS Empress and IPS Empress2 are typical products representative of several
other leucite-reinforced and lithia disilicate-reinforced glass-ceramics, respectively.
Some properties of IPS Empress and IPS Empress2 glass-ceramic core materials are
listed in table. 21.6. IPS Empress is a leucite-containing glass-ceramic that contains
about 35 vol% of leucite (KAISI2O6) crystals, which increases the resistance to crack
propagation (fracture). The veneering ceramic also contains leucite crystals in a glass
matrix. After hot pressing, divesting, and separation of the ceramic units the sprue
segments, they are veneered with porcelain containing leucite crystals in a glass
matrix.
A cross-sectional illustration of an IPS Empress crown is illustrated in fig.
21.9. The IPS Empress2 is similar except that the core consists of lithia disilicate
crystals in a glass matrix and the veneering ceramic contains apatite crystals. The
very small apatite crystals cause light scattering in a way that resemble by the
structure and components of tooth enamel. The coefficient of expansion of the apatite
glass-ceramic veneering ceramic is 9.7 ppm/0
C, which is similar to that of IPS
Empress2 core ceramic (10.6 ppm/0
C). Obviously, this veneering ceramic should not
33

be used with the IPS Empress core ceramic that has a much higher expansion
coefficient (150 ppm/0
C).
The core microstructure of IPS Empress2 glass ceramic is quite different from
that of IPS Empress, as evidenced by the 70 vol% of elongated lithia disilicate
crystals in IPS Empress2. The primary crystal particles in IPS Empress2 are 0.5 to
4m in length. A smaller concentration of lithium orthophosphate crystals (Li2 Si2 O5)
approximately 0.1 to 0.3µm in diameter has also been reported (Holand et al., 2000).
The microstructural difference between IPS Empress and IPS empress2 results in a
slight decrease in translucency for IPS Cmpress2 (0.55) (Holland et al., 2000). As is
the case for most pressable glass-ceramics, the advantages of IPS empress and IPS
Empress2 glass-ceramic core materials are their potential for accurate fit, excellent
transluency and overall aesthetics, and a metal-free structure. Disadvantages are their
low to moderately high flexural strength and fracture toughness. These properties
limit their use to conservative designs in low to moderate stress environments. Shown
in fig. 21-10, 21-11 and 21-12 are three-unit glass-ceramic FPDs made from a lithia-
disilicate-based core material. The FPD shown in fig. 21-12 was made without a
veneering ceramic to enhance the fracture resistnce. A summary of important
properties is presented in Table 21-7 for a variety of dental ceramics. A list of
pressable ceramics and their veneering ceramics is summarized.
OPC and OPC 3G are two pressable ceramics that are similar in nature to IPS
Empress and IPS Empress2, respectively. OPC is a leucite-containing ceramic and
OPC 3G contains lithia disilicate crystals. The ultralow-fusing temperature of the
veneering porcelain suggests a low level of wear of opposing enamel. However,
insufficient clinical data are available to support this hypothesis.
In-Ceram Alumina, In-Ceram Spinell, and In-Ceram Zirconia
In-Ceram is supplied as one of three core ceramics: (1) In-Ceram spinell (2)
in-Ceram Alumina, and (3) in-Ceram Zirconia. A slurry of one of these materials is
slip-cast on a porous refractory die and heated in a furnace to produce a partially
sintered coping or framework. The partially sintered core is infiltrated with glass at
11000
C for 4 hr to eliminate porosity and to strengthen the slip-cast core. The initial
sintering process for the alumina core produces a minimal shrinkage because the
temperature and time are sufficient only to cause bonding between particles and to
produce a desired level of sintering. Thus the marginal adaptation and fit of this core
material should be adequate because little shrinkage occurs. The flexural strength
(modulus or rupture) values of the glass-infiltrated core materials are approximately
350 Mpa for in-Ceram spinell (ICS), 500 Mpa for In-Ceram Alumina (ICA)and 700
Mpa for In-Ceram Zirconia (ICZ) compared with strengths of 100 to 400 Mpa for
Dicor, Optec Pressable Ceramic, IPS Empress and IPS Empress2. Despite the
relatively high strength of these materials, failures can still occur in single crowns as
well as FPDs.
Because of the variation in strength, the primary indications for these core
ceramics vary as shown in Table 21-9. For example, ICS is indicated for use as
anterior single-unit inlays, onlays, crowns, and veneers, ICA is indicated for anterior
and posterior crowns and anterior three-unit FPDs. Because of its high level of
34

opacity, ICZ is not recommended for anterior prostheses. However, because of its
extremely high strength and fracture toughness, it can be used for posterior crowns
and posterior FPDs. As suggested in chapter 4, it is essential that the gingival
embrasure areas of ceramic FPD connectors be designed with a large radius of
curvature to minimize the stress-raiser effect in areas of moderate to high tensile
stress. The connectors also should be sufficiently thick to minimize stresses during
loading. For Empress and Empress2 ceramics used in molar areas, the connector
height should be at least 4mm.
3 tables: Page No. 687 to 689
Until in-Ceram was introduced, aluminous porcelain had not been used
successfully to produce FPDs because of low flexural strength and high sintering
shrinkage. Thus the principal indications for aluminous porcelain crowns were the
restoration of maxillary anterior crowns when aesthetics was important and their use
in patients with allergies to metals. Its advantages and disadvantages are summerized
in the following.
A schematic drawing of an In-Ceram crown is shown in Fig 21.13 The same
diagram can be used to illustrate crowns made with In-Ceram Spinell (ICS) and In-
Ceram Zirconia (ICZ), which will be discussed below. The three In-Ceram ceramics
are glass-infiltrated core materials used for single anterior crowns (all three products),
posterior crowns (In-Ceram Alumina and in-Ceram Zirconia), anterior three-unit
FPDs (In-Ceram Alumina), and three-unit posteriro bridges (In-Ceram Zirconia).
The most translucent of the three ceramics- In-Ceramics, In-Ceram Spinell,
was introduced as an alternative to in-Ceram Alumina. This ceramic has a lower
flexural strength, but its increased translucency provides improved aesthetics in
clinical situations in which the adjacent teeth or restorations are quite translucent.
The core of ICS is MgAl1O4 and that for ICZ is a mixture of Al2O3 and ZrO2. These
core ceramics are also infiltrated with glass, and they are fabricated in a manner
similar to that for ICA, although the firing temperatures and times may be different.
The final ICA core consists pf 70 wt% alumina infiltrated with 30 wt%
sodium lanthanum glass. The final ICS core consists of glass-infiltrated magnesium
spindl (MgAl2O4). ICZ contains approximately 30wt% zirconia and 70 wt% alumina.
The power-liquid slurry is slip cast onto a porous die that absorbs water from the
slurry, thereby densifying the agglomeration of particles onto the die. Steps for
fabricating in-Ceram prostheses are as follows: (1) prepare teeth with an occlusal
reduction of 1.5 to 2.0mm and a heavy circumferential chamfer (1.2mm), (2) make an
impression and pour two dies, (3) apply Al2O3 on a porous duplicate die, (4) heat at
1200
C for 2 hours to dry Al2O3 , (5) sinter the coping for 10 hours at 11200
C, (6)
apply a sodium lanthanum glass slurry mixture on the coping, (7) fire for 4 hours at
11200
C to allow infiltration of glass, (8) trim excess glass from the coping with
diamond burs, (9) build up the core with dentin and enamel porcelain, (10) fire in
the oven, grind in the anatomy and occlusion, finish, and glaze.
The advantages of ICA include a moderately high flexural strength and
fracture toughness, a metal-free structure, and an ability to be used successfully with
conventional luting agents (Type 1 cements). The collective advantages of the three
35

glass-infiltrated core materials are their lack of metal, relatively high flexural strength
and toughness, and ability to be successfully cemented using any cement.
In spite of this high flexural strength (429 Mpa), the Weibull modulus of ICS
is quite low (5.7), which is indicative of a large scatter in the distribution of strength
values relative to the probability of fracture. (Tinschert et al., 2000). Its marginal
adaptation may not be as good as that achieved with other ceramic products. In one
study the mean marginal discrepancies were 83 m for Procera All Ceram, 63 m for
IPS Empress, and 161m for In-Ceram Alumina. Other drawbacks of ICA include its
relatively high degree of opacity, inability to be etched, technique sensitivity, and the
relatively great amount of skilled labor required. These disadvantages apply also to
In-Ceram Zirconia. Compared with ICM, the opacities of ICA and ICZ core ceramics
are much greater.
Although these newer core ceramics have excellent fracture resistance
inproper design of the connector area of a FPD can significantly reduce the fracture
resistance and clinical survivability of the prosthesis. Shown in fig. 21-14 is the stress
distribution in a three-unit FPD, which shows relatively high principal tensile stress
(red area) at the tissue side of the interproximal connector when an occlusal load of
250 N is applied to the occlusal surface of the pontic.
In summary, In-Ceram Spinell (ICS) is a glass-infiltrated core ceramic that
offers greater translucency for crowns than either the ICA or ICZ core ceramics.
However, ICA has lower strength and toughness compared with ICA and ICZ. Thus
the use of ICS is limited to anterior inlays, onlyas, veneers, and anterior crowns.
Although ICZ is the strongest and toughest of the three core ceramics, its use is
limited to posterior crowns and FPDs because of its high level of core opacity. ICZ is
a much stronger and tougher material and has greater opacity than ICA.
Procera AllCeram:
The Procera AllCeram crown is composed of densely sintered, high-purity
aluminum oxide core combined with a compatible allCeram veneering porcelain.
This ceramic material contains 99.9% alumina, and its hardness is one of the highest
among the ceramics used in dentistry. Procera AllCeram can be used for anterior and
posterior crowns, veneers, onlays, and inlyas.
A unique feature of the Procera system is the ability of the Procera Scanner to
scan the surface of the prepared tooth and transmit the data to the milling unit to
produce an enlarged die through a CAD-CAM process. The core ceramic form is dry
pressed onto the die, and the core ceramic is then sintered and veneered. Thus the
usual 15%-20% shrinkage of the core ceramic during sintering will be compensated
by constructing an oversized ceramic pattern, which will shrink during sintering to the
desired size to accurately fit the prepared tooth.
CAD-CAM Ceramics:
As shown in the ceramic classification chart in all-ceramic cores can be
produced by processes of condensation and sintering, casting and ceramming,
hotpressing and sintering, sintering and glass infiltration, and CAD-CAM processing
for the Cerec DAB-CAM system the internal surface of inlays, onlays, or crowns is
ground with diamond disks or other instruments to the dimensions obtained from a
36

scanned inage of the preparation. for some systems, the external surface must be
ground manually, although some recent CAD-CAM systems are capable of forming
the external surface as well.
A milling operation within a Cerec CAD-CAM unit (Siemens
Aktiengesellschaft, Bensheim, Germany ). The ceramic lock is being ground by a
diamond-coated disk whose translational movements are guided by computer-
controlled input. A cerec CAD-CAM ceramic block is shown in Figure 21-16 before
milling, at an intermediate milling stage, and after completion of the milling operation
for an inlay. These ceramics are supplied as small blocks that can be gound into inlays
and veneers in a computer-driven CAN-CAM system. Vitablocs MK II are feldspathic
procelains that are used in the same way as is Dicor HGC (machinable glass-ceramic).
The disadvantages of CAD-CAM restorations include the need for costly equipment,
the lack of computer-controlled processing support for occlusal adjustment, and the
technique-sensitive nature of surface imaging required for the prepared teeth.
Advantages include negligible porosity levels in the CAD-CAM core ceramics, the
freedom from making an impression, reduced assistant time associated with
impression procedures, the need for only a single patient appointment (with the Cerec
system), and good patient acceptance. A list of CAD-CAM and copy-milled
ceramics is given in Table 21-10.
An advantage of CAD-CAM ceramics is that one can select a core ceramic
either for strength and fracture resistance, for low abrasiveness, or for translucency.
for example, the extensive wear of opposing enamel that occurs when it is opposed by
a feldspathic porcelain surface in the absence of posterior occlusion can be
minimized by selscting a core ceramic that is minimally abrasive to enamel.
Cercon and Lava Zirconia Core Ceramics:
The cercon Zirconia system (Destsply Ceramco, Burlington, NJ) consists of
the following procedures for production of zirconia-based prostheses. After preparing
the teeth (2.0mm incisal or occlusal reduction and 1.5 mm axial reduction), an
impression is made and sent tot he laboratory, where it is poured with a model
material. A wax pattern approximately 0.8 mm in thickness is made for each coping
on the holding appliance on the left side of the scanning and milling unit (Cercon
Brain). A presintered Zirconia blank is attached to the right side of the Brain unit.
(Cercon Brain). A presintered zirconia blank is attached to the right side of the Brain
unit. The blank has an attached barcode, which contains the enlargement factor and
other milling parameters for computer control or the milling procedure After the unit
is activated, parameters for computer control of the milling procedure After the unit is
activated, the pattern in scanned and the blank is rough-milled and fine-milled on
occlusal.
37

Ceramic Block Ceramic type
Ceramic
veneer
Indications Manufacturer
CerAdapt Highly sintered Al2O3 All cream Implant
superstructure
Nobel Biocare
Cercon Base Presintered ZrO2;
postsintered after
milling
Coercion
Cream S
Crowns and
FPDs
Dentsply
Ceramco
DC- Kristen Leucite-base Triceram Crowns DCS Dental
AG/Esprident
DC- Zirkon Presintered ZrO2 hot
isostatic
postcompaction
Vitadur D
Triceram
Crowns and
FPDs
DCS Dental
AG/Vita/Espride
nt
Denzir Presintered ZrO2; hot
isostatic
postcompaction
Empress2 Crowns and
FPDs
Decim, Ivoclar
LAVA Frame ZrO2; presintered and
postsntered
LAVA
Ceram
Crowns and
FPDs
3M ESPE
ProCad Leucite-based Malthechnik Veneers, inlays,
onlays, and
crowns
Ivoclar
Procera
AllCeram
Al2O3; presintered and
postsintered
AllCeram Crowns and
FPDs
Nobel Biocare
Synthoceram Al2O3; reinforced;
pressed and
postsintered
Sintagon Crowns Elephant
VitaBlocs Mark
II
Feldspathic porcelain
block
Maltechnik Veneers, inlays,
onlays, and
crowns
Vident
VitaBlocks
Alumina
Sintered Al2O3;
followed by glass
infiltration
Vitadur
Alpha
Crowns and
FPDs
Crowns and
FPDs Vident
VitaBlocs
Spinell
Sintered
MgO-Al2O3 spinel
followed by glass
infiltration
Vitadur
Alpha
Crowns Vident
VitaBlocs
Zirconia
Sintered Al2O3/ZrO2
followed by glass
infiltration
Vitadur
Alpha
Crowns and
FPDs
Vident
38

Zircogon ZrO2; presintered and
postsintered
Zircogon Crowns Elephant
And gingival aspects in an enlarged size to compensate for the 20% shrinkage
that will occur during subsequent sintering at 13600
C. the processing times for
milling are approximately 35 for a crown and 80 min for a four – unit fixed FPD. The
milled are approximately 35 min for a crown and 80 min for a four- unit fixed FPD.
The milled prosthesis is removed from the and the remaining extraneous extension are
removed. The zircon coping or framework is then placed in the Cercon furnace and
fired at 13500
C for approximately 6 hours to fully sinter the yttria-stabilized zirconia
core coping or framework. The sintering shrinkage is achieved uniformly and linearly
in three – dimensional space by the in targeted process of scanning, enlarging the
pattern design, controlled milling and sintering.
After any subsequent trimming with a water – cooled, hinge- speed diamond
bur the finished ceramic core framework is then veneered with a veneering ceramic
(Cercon Ceram S) and stain ceramic
All-ceramic prostheses represent the most aesthetically pleasing, but also the
most fracture-prone prostheses. However, with adequate tooth reduction, an excellent
quality impression, a skilled technician, and a ceramic with reasonably high flexure
strength (≥250 MPa) and fracture toughness (≥ 2.5Mpa.m1/2
), reasonably high success
rates can be achieved. The material that has the greatest potential fracture toughness
(9 MPa.m½
) and flexural strength that has the greatest potential fracture toughness (9
MPa.m½
) and flexural strength (>900 MPa) is pure tetragonal stabilized zirconia
(ZrO2). Tinschert et al (200lb) reported that the fracture resistance of three-unit
ceramic FPDs (1278 N) made of Cercon zirconia core ceramic (Dentspy Ceramco)
was more than twice as great as the values reported for In-Ceram Alumina (514 N)
and Empress2 (621 N). Shown in Figure is a comparison of the force required to
fracture three-unit FPDs cemented to dies with zinc phosphate cement. The zirconia
product (Cercon) would be expected to exhibit less fracture resistance in this case, but
clinical data are needed to confirm this hypotehsis.
To ensure maximum survival times, adequate occlusal tooth reduction is
essential for posterior teeth. Optimal clinical performance of some ceramic products
require a minimal occlusal reduction of 2 mm for molar tooth preparations. If the
ceramic will be supported by a material with high elastic modulus such as a ceramic
or metal post or an amalgam build-up, less occlusal reduction (1.5 mm) may be
possible without compromising the survivability of the crowns. For patients
exhibiting extreme brusism, either metal or metal-ceramic prostheses should be used.
METHODS OF STRENGTHENING CERAMICS:
Minimize the effect of stress Raisers
Why do dental ceramic prostheses fail to exhibit the strengths that we would
expect from the high bond forces between atoms? The answer is that numerous
minute scratches and other defects are present on the surfaces of these materials.
These surface flaws 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
39

tip of each surface flaw can increase the localized stress to the theoretical strength of
the material even though a relatively low average stress exists throughout the bulk of
the structure. When the induced mechanical stress exceeds the actual strength of the
material, the bonds at the notch tip break, forming a crack. This stress concentration
phenomenon explains how materials fail at stresses far below their theoretical
strength.
Stress raisers are discontinuities the ceramic and metal-ceramic structures and
in other brittle materials that cause a stress concentration in these areas. The design of
ceramic dental restorations should also avoid stress raisers in the ceramic. Abrupt
changes in shape or thickness in the ceramic contour can act as stress raisers and
make the restoration more prone to failure. Thus the incisal line angles on an anterior
tooth prepared for a ceramic crown should be well rounded.
In ceramic crowns, several conditions can cause stress concentration. Creases
or folds of the platinum foil or gold foil substrate that become embedded in the
porcelain leave notches that act as stress raisers. Sharp line angles in the preparation
also create areas of stress concentration in the restoration. Large changes in porcelain
thickness, a factor also determined by the tooth preparation, can create areas of stress
concentration.
A small particle of porcelain along the internal porcelain margin of a crown
also induces locally high tensile stresses. A stray particle that is fused within the
inner surface of a shoulder porcelain margin of a metal-ceramic crown can cause
localized tensile stress concentrations in porcelain when an occlusal force is applied to
the crown.
Even though a metal-ceramic restoration is generally stronger than most
ceramic crowns of the same size and shape, care must be taken to avoid subjecting the
porcelain in a PFM to loading the produces large localized stresses. If the occlusion is
not adjusted properly on a porcelain surface, contact points rather than contact areas
will greatly increase the localized stresses in the porcelain surface as well as within
the internal surface of the crown.
Fracture mechanics is a science that allows scientists to analyze the influence
of flaw/stress interactions on the probability of crack propagation through an elastic
brittle solid. The principles of linear elastic fracture mechanics were developed in the
1950s by Irwin (1957). This pioneering research on fracture phenomena was based
on earlier investigations by Griffth (1921) and Orowan (1944, 1949, 1955). Irwin
found that when a brittle material was subjected to tensile stresses, specific crack
shapes in certain locations were associated with greatly increased stress levels. He
also recognized the importance of determining the fracture toughness of these
materials as a measure of their ability to resist fracture. The fracture toughness (KIC)
of a material represents the resistance of a material to rapid crack propagation. In
contrast, the strength of a material depends primarily on the size of the initiating crack
that is present. The strength of dental ceramics and other restorative materials is
controlled by the size of the cracks or defects that are introduced during processing,
production and handling. In this chapter a description is given of the processing
40

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