Dental Ceramics

INTRODUCTION
 An ideal restorative material should be biocompatible and durable,
should maintain its surface quality and esthetic characteristics over
an extended period of time, preferably for the lifetime of the patient.
 Dentists have been searching for an ideal restorative material for more
than a century.
 Although direct restorative materials such as amalgam, composites
and restorative cements have been used with reasonably good success
during the past several decades, they are not usually feasible for
multiunit restorations.
Phillips’Science of Dental Materials, 12th edition 2

 Dental ceramics are attractive because of their biocompatibility, long
– term color stability, wear resistance and ability to be formed into
precise shapes.
 They can realistically duplicate teeth, to the extent that an individual
may find it difficult to differentiate.
 Dental ceramics are strong, durable, wear resistant, impervious to oral
fluids and absolutely biocompatible.
Phillips’Science of Dental Materials, 12th edition 3

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 The word Ceramic is derived from the Greek word “keramos”,
which literally means ‘burnt stuff’, but which has come to mean
more specifically a material produced by burning or firing.
 Ceramics are inorganic, nonmetallic materials composed of metallic
or semi – metallic oxides, phosphates, sulfates, or other nonorganic
compounds.
Phillips’Science of Dental Materials, 12th edition

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 Dental ceramics are materials that are part of systems designed with
the purpose of producing dental prostheses that in turn are used to
replace missing or damaged dental structures.
 Literature defines ceramics as inorganic, non – metallic materials
made by man by the heating of raw minerals at high temperatures.
Shenoy A, Shenoy N. Dental ceramics: An update. J Conserv Dent 2010;13(4):195-203

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 Sintering: Process of heating closely packed particles below their
melting temperature.
 Fixed Dental Prosthesis (FDP): An inlay, onlay, veneer, crown, or
bridge that is cemented to one or more teeth or dental implant
abutments. The term is most often used to describe a bridge
prosthesis.
 Fixed Partial Denture (FPD): A bridge that replaces one or more
missing teeth. However, fixed dental prosthesis (FDP) is the
universally preferred term.

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 Ceramic Stain: A fine glass powder containing one or more pigments
(colored metal oxides) that is applied superficially to a ceramic
restoration.
 Ceramic Glaze: Fine glass powder that can be fired on dental
porcelain to form a smooth, glassy surface.
 Metal – Ceramic Prosthesis: A partial crown, full crown, or multiple
– unit fixed dental prosthesis made from a metal substrate to which
dental porcelain is bonded for esthetic enhancement and functional
anatomy.

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 Compressive Stress: When a body is placed under a load that tends to
compress or shorten it, the internal resistance to such a load is called
compressive stress.
 Shear Stress: This type of stress tends to resist the sliding or twisting
of one portion of a body over another.
 Tensile Stress: Stress caused by a load that tends to stretch or
elongate a body.
 Poisson’s Ratio: Deformation of a material in directions
perpendicular to the direction of loading.

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 Modulus of Elasticity/Elastic Modulus: Stiffness of a material that is
calculated as the ratio of elastic stress to elastic strain.
 Coefficient of Thermal Expansion (CTE): A material property that is
indicative of the extent to which a material expands upon heating.
 CAD – CAM Ceramic: A partially or fully sintered ceramic blank
that is used to produce a dental core or veneer structure using a
computer – aided design (CAD) and computer – aided manufacturing
(CAM) process.
 Glass Ceramic: A ceramic that is formed to shape in the glassy state
and subsequently heat treated to partially or completely crystallize the
object. Phillips’Science of Dental Materials, 12th edition

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 Porcelain, opaque: Fine dental porcelain, provided either as a paste
or powder that is used to mask the color of a metal substructure for
fixed prostheses.
 Porcelain, body (also called dentin or gingival porcelain): A dental
porcelain used to create the anatomy and shade of a fixed prosthesis.
 Porcelain, incisal (also called enamel porcelain): Dental porcelain
used to create the anatomy and incisal portion of a fixed prosthesis.
These porcelains are generally more translucent than opaque and
gingival (body) porcelains.

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• First porcelain tooth material patented by de
Chemant, a French dentist in collaboration with
Duchateau, a French pharmacist and introduced in
England thereafter.
1789
• Fonzi, an Italian dentist, invented a
“terrometallic” porcelain tooth held in place by a
platinum pin or frame.
1808
• Ash developed an improved version of the
porcelain tooth in England.1837

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• Nephew of Stockton founded the S.S. White Company,
which became active in the further refinement of the
design and mass production of porcelain denture teeth.
1844
• Charles Land published in the Independent Practitioner,
a technique for preparing the tooth cavity for an inlay,
making a platinum foil matrix, and fabricating a
ceramic inlay.
1886 &
1887
• Charles Land introduced one of the first ceramic
crowns to dentistry.1903

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• Weinstein and Weinstein identified the formulations of
feldspathic porcelain that enabled the systematic control
of the sintering temperature and coefficient of thermal
expansion.
1962
• Weinstein et al. described the components that could be
used to produce alloys that bond chemically to and that
are thermally compatible with feldspathic porcelains.
1962
• First commercial porcelain developed by VITA
Zahnfabrik.1963

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• McLean and Hughes developed a Porcelain Jacket
Crown with an inner core of aluminous porcelain
containing 40–50% alumina crystals to block the
propagation of cracks.
1965
• Improvement in all ceramic systems developed by
controlled crystallization of a glass (Dicor)
demonstrated by Adair and Grossman.
1984
• Computer-assisted CEramic REConstruction
(CEREC) 1 unit was introduced.1985
Krishna JV, Kumar VS, Savadi RC. Evolution of metal-free ceramics. J Indian Prosthodont Soc 2009;9(2):70-75

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• First chair – side inlay was fabricated.1985
• Pressable glass-ceramic (IPS Empress), containing
approximately 34% leucite by volume, was
introduced.
Early 1990s
• More fracture resistant pressable glass-ceramic (IPS
Empress 2) containing approximately 70% lithia
disilicate crystals by volume was introduced.
Late 1990s

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• Improvements in software led to CEREC 2 system
by which partial and full crowns could be fabricated.1994
• CEREC 3 system was introduced by which a three –
unit bridge frame could be fabricated.2000
• Introduction of CEREC 3D in 2005 marked the three
– dimensional virtual display of the prepared tooth.2005

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Dental ceramics can be classified according to one or more of the
following parameters:
Uses or indications:
 Anterior and posterior crown
 Veneer
 Post and core
 Fixed dental prosthesis
 Ceramic stain
 Glaze.

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Principal crystal phase and/or matrix phase:
 Silica glass
 Leucite – based
 Feldspathic porcelain
 Leucite – based glass-ceramic
 Lithia disilicate – based glass-ceramic
 Aluminous porcelain
 Alumina
 Glass – infused alumina
 Glass – infused spinel
 Glass – infused alumina/zirconia and
 Zirconia. Phillips’Science of Dental Materials, 12th edition

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Processing method:
 Casting
 Sintering
 Partial sintering and glass infiltration
 Slip casting and sintering
 Hot – isostatic pressing
 CAD – CAM milling and
 Copy – milling.
 Translucency:
 Opaque
 Transparent
 Translucent.

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 Firing temperature:
S. No. Class Sintering Temperature
Range
Applications
1. High fusing >1300°C Denture teeth, fully sintered
alumina and zirconia core
ceramics.
2. Medium fusing 1101°C – 1300°C Denture teeth, presintered
zirconia.
3. Low fusing 850°C – 1100°C Crown and bridge veneer ceramic
4. Ultralow fusing <850°C Crown and bridge veneer ceramic

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 Microstructure:
 Amorphous glass
 Crystalline
 Crystalline particles in a glass matrix.
Fracture resistance:
 Low
 Medium
 High.
 Abrasiveness.

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Most current ceramics consist of two phases:
 ••Glassy phase — acts as the matrix.
 Crystalline phase — dispersed within the matrix and improves
strength and other properties of the porcelain, e.g. quartz, alumina,
spinel, zirconia, etc.
 Traditionally, porcelains were manufactured from a mineral called
feldspar.
 These porcelains are referred to as feldspathic porcelains.
Basic Dental Materials by John J Manappallil, 4th edition

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Basic constituents of dental ceramics include:
 Feldspar:
 Responsible for forming the glass matrix.
 Lowest fusing component, which melts first and flows during firing,
initiating these components into a solid mass.
 Naturally occurring mineral composed of two alkali aluminum
silicates such as potassium aluminum silicate (K2O-Al2O3-6SiO2);
also called as potash feldspar and soda aluminum silicate (Na2O-
Al2O3-6SiO2).
Babu PJ, Alla RK, Alluri VR, Datla SR, Konakachi A. Dental Ceramics: Part I – An Overview of Composition,
Structure and Properties. American Journal of Materials Engineering and Technology 2015;3(1):15-18

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 Silica (Quartz):
 Has high fusion temperature and remains same at the firing
temperature of the porcelain thus strengthening the restoration.
 Acts as filler in the porcelain restoration.
 Kaolin:
 Type of clay material which acts as a binder and increases the
moldability of unfired porcelain.
 Imparts opacity to the porcelain restoration.

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 Glass modifiers (e.g. K, Na or Ca oxides):
 Used as flux.
 Lower the fusion temperature and increase the flow of porcelain
during firing.
 Color pigments:
 Provide appropriate shade to the restoration.
 Opacifiers:
 Since pure feldspathic porcelain is quite colorless, opacifiers are
added to increase the opacity in order to simulate natural teeth.
 Oxides of zirconium, titanium and tin are commonly used opacifiers.

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 Dental ceramics are nonmetallic, inorganic structures, primarily
containing compounds of oxygen with one or more metallic or semi-
metallic elements (aluminum, boron, calcium, cerium, lithium,
magnesium, phosphorus, potassium, silicon, sodium, titanium and
zirconium).
 Many dental ceramics contain a:
 Crystal phase and
 Silicate glass matrix phase.

33Phillips’Science of Dental Materials, 12th edition
 Structure is characterized by chains of (SiO4)4- tetrahedra in which
Si4+ cations are positioned at the center of each tetrahedron with O-
anions at each of the four corners.
 Resulting structure is not close – packed and it exhibits both covalent
and ionic bonds.
Two – dimensional
amorphous structure

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 They are arranged as linked chains of tetrahedra, each of which
contains two oxygen atoms for every silicon atom.
 Primary structural unit in all silicate structures is the negatively
charged siliconoxygen tetrahedron (SiO4)4-.
 It is composed of a central silicon cation (Si4+ ) bonded covalently to
four oxygen anions located at the corners of a regular tetrahedron.
 Alkali cations such as potassium or sodium tend to disrupt silicate
chains and increase the thermal expansion of these glasses.

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 Ceramic refers to any product made from a nonmetallic inorganic
material usually processed by firing at a high temperature to achieve
desirable properties.
 More restrictive term porcelain refers to a specific compositional
range of ceramic materials originally made by mixing kaolin
(hydrated aluminosilicate), quartz (silica) and feldspar (potassium
and sodium aluminosilicates) and firing at high temperature.
 Dental ceramics for metal – ceramic restorations belong to this
compositional range and are commonly referred to as dental
porcelains.
Craig’s Restorative Dental Materials, 14th edition

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PROPERTIES OF
DENTAL CERAMICS

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 Dental ceramics exhibit excellent biocompatibility with the oral soft
tissues.
 They possess excellent esthetics.
 Dental ceramics possess very good resistance to compressive
stresses, however, they are very poor under tensile and shear stresses.
 This imparts brittle nature to ceramics and tend to fracture under
tensile stresses.

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 Surface hardness of ceramics is very high hence they can abrade the
opposing natural or artificial teeth.
 Ceramics are good thermal insulators and their coefficient of thermal
expansion is almost close to the natural tooth.
 Adhesion of ceramic restoration to the natural tooth also plays a
significant role in the durability of the restoration.

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 Fracture toughness is another important property of ceramics as it
measures the resistance to brittle fracture when a crack is present.
 Fracture toughness of conventional feldspathic porcelains is very
similar to that of soda lime glass (0.78 MPa).
 Leucite – reinforced ceramics exhibit slightly higher fracture
toughness values (1.2 MPa), followed by lithium disilicate –
reinforced ceramics (2.75 MPa).
 3Y – TZP ceramics have the highest fracture toughness of all –
ceramic materials (greater than 6.0 MPa).

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 Elastic constants of dental ceramics are needed in the calculations of
both flexural strength and fracture toughness.
 Poisson’s ratio ranges between 0.21 and 0.30 for dental ceramics.
 Modulus of elasticity is about 70 GPa for feldspathic porcelain, 110
GPa for lithium disilicate heat – pressed ceramics, 210 GPa for 3Y –
TZP ceramics and reaches 350 GPa for alumina – based ceramics.

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 Density of fully sintered feldspathic porcelain is around 2.45 g/cm3
and decreases as the amount of porosity increases.
 Density of ceramic materials also depends on the amount and nature
of crystalline phase present.
 A density greater than 98.7% of the theoretical density is required for
medical-grade 3Y – TZP ceramics.
 All currently used 3Y – TZP dental ceramics have a density that meets
this standard requirement.

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 Thermal properties of feldspathic porcelain include a conductivity of
0.0030 cal/s/cm2 (°C/cm), a diffusivity of 0.64 mm2/s, and a linear
coefficient of thermal expansion (CTE) of about 12.0 × 10 −6/°C
between 25° and 500°C.
 The CTE is about 10 × 10 −6/°C for aluminous ceramics and lithium
disilicate ceramics, 10.5 × 10 −6/°C for zirconia based ceramics (3Y –
TZP), and 14 to 18 × 10 −6/°C for leucite – reinforced ceramics.

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 Ceramics are more resistant to corrosion than plastics.
 Ceramics do not react readily with most liquids, gases, alkalis and
weak acids.
 They also remain stable over long time periods.
 They exhibit good to excellent strength and fracture toughness.
 Although ceramics are strong, temperature – resistant and resilient,
these materials are brittle and may fracture without warning when
flexed excessively or when quickly heated and cooled.

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 Chemical inertness is an important characteristic because it ensures
that the chemically stable surface of dental restorations does not
release potentially harmful elements.
 This also reduces the risk for surface roughening and increased
abrasiveness or increased susceptibility to bacterial adhesion over
time.
 Other important attributes of dental ceramics are their potential for
matching the appearance of natural teeth, their thermal insulating
properties (low thermal conductivity and low thermal diffusivity), and
their freedom from galvanic effects (low electrical conductivity).

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Optical Properties
 Porcelain, being mostly amorphous in structure, cannot completely
match the optical properties of crystalline enamel.
 As a result, ultraviolet (UV) and visible light rays are reflected,
refracted, and absorbed unevenly by the combination dentin/enamel,
compared with porcelain.
 As a consequence, restorations viewed from one incidence angle may
not appear the same as they do when viewed from a different
incidence angle.

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 Translucency is another critical property of dental ceramics.
 By design, opaque porcelains have very low translucency, allowing
them to efficiently mask metal substructure surfaces.
 Tin oxide (SnO2) and titanium oxide (TiO2) are important opacifying
oxides for opaque porcelains.

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 To mimic the optical properties of human enamel, opalescence is also
a desirable optical property.
 Opalescence is a form of light scattering and occurs when the size of
crystalline phase particles is equal to or shorter than the wavelength of
light.
 An opalescent glass appears reddish orange in transmitted light and
blue in reflected or scattered light.
 Both zirconium oxide and yttrium oxide have been shown to increase
opalescence in ceramics due to their light scattering effect.

49
 Dental enamel also exhibits fluorescence.
 Fluorescence is the emission of light by a substance that has absorbed
light.
 This characteristic is achieved in dental porcelains by adding rare
earth oxides (such as cerium oxide).

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 Many dental porcelain manufacturers buy feldspar as powder already
screened and cleaned from impurities to their specifications.
 Other raw materials used in the manufacture of dental porcelains are
various types of silica (SiO2) in the form of fine powder, alumina
(Al2O3), as well as alkali and alkaline earth carbonates as fluxes.
Feldspar powder

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 During the manufacturing process, the ground components are
carefully mixed together and heated to about 1200°C in large
crucibles.
 Feldspar melts incongruently at about 1150°C to form a glassy phase
with an amorphous structure, and a crystalline phase consisting of
leucite, a potassium aluminosilicate (KAlSi2O6).
 Mix of leucite and glassy phase is then cooled very rapidly
(quenched) in water that causes the mass to shatter in small fragments.
 The product obtained, called a frit, is ball milled to achieve proper
particle size distribution.

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 Coloring pigments in small quantities are added at this stage to
obtain the delicate shades necessary to mimic natural teeth.
 Tin, titanium and zirconium oxides are used as opacifiers.
 After the manufacturing process is completed, feldspathic dental
porcelain consists of a glassy (or amorphous) phase and leucite
(KAlSi2O6) as a crystalline phase.
 Glassy phase formed during the manufacturing process has properties
typical of glass, such low toughness and strength, and high
translucency.

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 The crystalline structure of leucite is tetragonal at room temperature.
 Leucite undergoes a reversible crystallographic phase transformation
at 625°C, temperature above which its structure becomes cubic.
Three-dimensional structure of
leucite (KAl-Si2O6).
Al, Aluminum; K, potassium;
O, oxygen; Si, silicon

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APPLICATIONS OF
CERAMICS
IN DENTISTRY

56
 Dental ceramic science and technology represent the fastest growing
areas of dental materials research and development.
 During the past two decades, numerous types of ceramics and
processing methods have been introduced.
 Some of these materials can be formed into inlays, onlays, veneers,
crowns, and more complex fixed dental prostheses (FDPs).

57
 They are used in single and multi unit metal – ceramic restorations.
 Ceramic brackets are used in orthodontics.
 Development of high-strength zirconia – based systems has made
possible the fabrication of dental implant abutments and FDPs.
 In addition, ceramics are still used to fabricate denture teeth.

59
 Metal – ceramic restorations consist of a cast metallic framework on
which at least two layers of ceramic are baked.
Cross section of a metal – ceramic crown
showing metal coping, opaque porcelain
layer, dentin, and enamel porcelain layers.

60
 The first layer applied is a thin opaque layer, consisting of porcelain
modified with opacifying oxides.
 Its role is to mask the dark gray appearance of the oxidized metal
framework to permit the achievement of adequate esthetics.
 This thin opaque layer also establishes the metal – ceramic bond.
 Next step is the buildup of dentin and enamel porcelains to obtain an
esthetic appearance similar to that of a natural tooth.

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 Dentin and enamel porcelain powders are mixed with modeling
liquid (mainly distilled water) to a creamy consistency and applied
on the opaque layer.
 Porcelain is then condensed by vibration and removal of excess water
is achieved with an absorbent tissue.
 After building up of the porcelain powders, metal – ceramic
restorations are slowly dried to allow for adequate water diffusion
and evaporation, and sintered under vacuum in a porcelain furnace to
eliminate pores.

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 The result is a dense, relatively pore – free porcelain.
 This decrease in porosity is noticeable by the associated increase in
translucency.
Optical micrograph of air-fired
porcelain, showing porosity.
Optical micrograph of vacuum-fired
porcelain showing minimal porosity

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Requirements for a Metal – Ceramic System
 Alloy must have a high melting temperature. The melting range
must be substantially higher (greater than 100°C) than the firing
temperature of the porcelain and solders used to join segments of an
FDP.
 Porcelain must have a low fusing temperature so that no distortion
of the framework takes place during sintering.
 Porcelain must wet the alloy readily when applied as a slurry to
prevent voids forming at the metal – ceramic interface. In general, the
contact angle should be 60 degrees or less.

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 A strong bond between the ceramic and metal is essential and is
achieved by chemical reaction of the opaque porcelain with metal
oxides on the surface of metal and by mechanical interlocking made
possible by roughening of the metal coping.
 CTEs of the porcelain and metal must be compatible so that the
porcelain never undergoes tensile stresses, which would lead to
cracking.
 Adequate stiffness and strength of the metal framework are
especially important for FDPs and posterior crowns.

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 High resistance to deformation at high temperature is essential. No
distortion should occur during firing of the porcelain, or the fit of the
restorations would be compromised.
 Adequate design of the restoration is critical. The preparation should
provide for adequate thickness of the metal coping, as well as enough
space for an adequate thickness of the porcelain to yield an esthetic
restoration.

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 Ceramics for metal – ceramic restorations must fulfill five
requirements:
 They must simulate the appearance of natural teeth,
 They must fuse at relatively low temperatures,
 They must have thermal expansion coefficients compatible with
alloys used for metal frameworks,
 They must age well in the oral environment, and
 They must have low abrasiveness.

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Ceramic Composition
 Conventional dental porcelain is a vitreous ceramic based on a silica
(SiO2) network and potash feldspar (K2O•Al2O3•6SiO2), soda feldspar
(Na2O•Al2O3•6SiO2), or both.
 Pigments, opacifiers, and glasses are added to control the fusion
temperature, sintering temperature, coefficient of thermal contraction,
and solubility.
 Pigments also produce the hues of natural teeth or color appearance of
tooth – colored restorative materials that may exist in adjacent teeth.

68
 Feldspathic porcelains contain, by weight, a variety of oxides
including a SiO2 matrix (52% to 65%), Al2 O3 (11% to 20%), K2O
(10% to 15%), Na2O (4% to 15%), and certain additives, including
B2O3, CeO2, Li2O, TiO2, and Y2O3.
 Feldspathic porcelains include:
 Ultralow – and low – fusing ceramics (feldspar-based porcelain).
 Low – fusing specialty ceramics.
 Ceramic stains and
 Ceramic glazes.

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 Silicate glass represents the matrix phase of feldspathic porcelains.
 Silica (SiO2) can exist in four different forms:
 Crystalline quartz,
 Crystalline cristobalite,
 Crystalline tridymite and
 Noncrystalline fused silica.
 Fused silica is a high – melting material whose melting temperature
is attributed to the three dimensional network of covalent bonds
between silica tetrahedra.
 Fluxes (low – fusing glasses) are often included to reduce the
temperature. Phillips’Science of Dental Materials, 12th edition

70
 Another important property of feldspar is its tendency to form
crystalline leucite (K2O•Al2O3•4SiO2) when it is melted.
 Leucite is a potassium-aluminum-silicate mineral with a high
coefficient of thermal expansion (20 to 25 × 10−6/K) compared with
feldspar glasses that have much lower coefficients of thermal
expansion (8.6 × 10−6/K).
 This tendency of feldspar to form leucite during melting controls
thermal expansion during the use of porcelains for metal bonding.

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 For metal – ceramic porcelains, specific concentrations of soda,
potash, and/or leucite are necessary to reduce the sintering
temperature and to increase the thermal expansion to a level
compatible with that of the metal coping.
 Opaque porcelains also contain relatively large amounts of metallic
oxide opacifiers to conceal the underlying metal and to minimize the
thickness of the opaque porcelain layer.
 Porcelains should not be subjected to nonessential repeated firings,
because this may lead to an increased risk of cloudiness within the
porcelains as well as potential changes in their coefficient of thermal
expansion and contraction.

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 Feldspathic porcelains have other qualities that make them well suited
for metal – ceramic restorations.
 They fuse at lower temperatures than do many other ceramic
materials, lessening the potential for distortion of the metal coping.
 This is made possible by the presence of alkali oxides (Na2O and
K2O) in the glassy matrix which are responsible for lowering the
fusing temperatures to the range 930° to 980°C.

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Metal Composition
 Single – unit crowns and bridges may be made from metal – ceramic
systems (combinations of metal substructure and ceramic).
 Compositions of the high noble, noble, predominantly base metal
alloys control the bonding ability to porcelain, esthetics of the metal –
ceramic restoration and the magnitudes of stresses that develop in the
porcelains during cooling.
 Coefficient of thermal expansion or contraction of the metal, must
match closely to that of the porcelain to be used.

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Metal Alloys
 High gold alloys typically contain a high percentage of gold, usually
in the range of 80% to 85%.
 Alloy is strengthened with the addition of 7% to 10% platinum, and
trace elements such as indium, zinc, and tin are added to provide an
oxide layer for predictable porcelain bonding.
Noble Metal Alloys Base Metal Alloys
High gold Nickel – chrome – beryllium
Low gold Chrome – cobalt
Palladium – silver Titanium
Sturdevant’s Art and Science of Operative Dentistry, 7th edition

76
 Low gold alloys have a gold content in the range of 50% to 55%.
 Palladium is present in the range of 35% to 40%, along with the trace
elements essential for bonding.
 Small amounts of silver may improve the wettability of the metal
coping with the opaque porcelain, although this has not been
scientifically established.
 Examples of this type of alloy include Olympia (J.F. Jelenko,
Armonk, NY), USC Ceramic Alloy (Leach and Dillon, N. Attleboro,
MA), and W2 and W3 (Williams Gold Co., Bufalo, NY).

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 Palladium – silver alloys are composed primarily of high
concentrations of palladium (60%) and silver (30%).
 With such high concentrations of silver, discoloration of the
porcelain is a consideration unless special precautions are taken
during firing.
 An example of a palladium – silver alloy is Silhouette 150 (Leach and
Dillon, N. Attleboro, MA).

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 Nickel – chromium alloy is the most common of the base metals and
contains about 65% nickel for strength, 20% chromium for passivity,
and 2% beryllium for castability and control of oxide formation.
 Commercial products include Rexillium III (Jeneric Pentron) and
Lite – Cast B (Williams Gold Co., Bufalo, NY).
 Beryllium helps to control oxide formation with base metal alloys.
 However, it has been associated with the development of berylliosis, a
serious occupational pulmonary condition, and laboratory technicians
are at risk.

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 Chromium – cobalt alloys are primarily marketed as
“biocompatible” base metal alloys because they are nickel and
beryllium free.
 Because they contain approximately twice as much cobalt as they do
chromium, it has been suggested that these metals should be
designated cobalt – chromium alloys.
 It is now possible to very accurately mill chrome – cobalt frameworks,
which removes one of the major disadvantages (poor fit) of these
alloys.

80
 Titanium alloys have become available recently.
 An issue encountered with titanium fused – to – metal is that ceramics
with very low COEs are required, and the esthetic results achieved
with these ceramics are poor.

81
Glass Modifiers
 Manufacturers employ glass modifiers to produce dental porcelains
with different firing temperatures.
 Boric oxide (B2O3) can behave as a glass modifier to decrease
viscosity, to lower the softening temperature, and to form 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.

82
 Pigmenting oxides are added to obtain the various shades needed to
simulate natural teeth.
 These coloring pigments are produced by fusing metallic oxides with
fine glass and feldspar and then regrinding to a powder.
 These powders are then blended with the unpigmented powdered frit
to provide the proper hue and chroma.
 Opacity may be achieved by the addition of cerium oxide, zirconium
oxide, titanium oxide, or tin oxide.

83
 To ensure adequate chemical durability, a self – glaze of porcelain is
preferred to an add – on glaze.
 A thin external layer of glassy material is formed during a self – glaze
firing procedure at a temperature and time that causes localized
softening of the glass phase.
 The add – on glaze slurry material that is applied to the porcelain
surface contains more glass modifiers and thus has a lower firing
temperature.

84
 Another important glass modifier is water, although it is not an
intentional addition to dental porcelain.
 Hydronium ion (H3O+) can replace sodium or other metal ions in a
ceramic that contains glass modifiers.
 This fact accounts for the phenomenon of “slow crack growth” of
ceramics exposed to tensile stresses and moist environments.
 It may also account for the occasional long – term failure of porcelain
restorations after several years of service.

87
FABRICATION OF
METAL – CERAMIC
PROSTHESES

88
Porcelain Condensation
 Porcelain for ceramic and metal – ceramic prostheses as well as for
other applications is supplied as a fine powder designed to be mixed
with water or another liquid and condensed into the desired form.
 Powder particles are of a particular size distribution to produce the
most densely packed porcelain when they are properly condensed.
 Proper and thorough condensation is also crucial in obtaining dense
packing of the powder particles.

89
 This packing, or condensation, may be achieved by various
methods, including the vibration, spatulation and brush
techniques.
 Vibration uses mild vibration to pack the wet powder densely on the
underlying framework.
 Excess water is blotted away with a clean tissue and condensation
occurs toward the blotted area.
 In spatulation, a small spatula is used to apply and smooth the wet
porcelain. This smoothing action brings the excess water to the
surface, where it is removed.

90
 Brush method employs the addition of dry porcelain powder to the
surface to absorb the water.
 The dry powder is placed by a brush to the side opposite from an
increment of wet porcelain.
 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
porcelain must not be allowed to dry out until condensation is
complete.

91
Sintering Procedure
 Thermochemical reactions between the porcelain powder components
are virtually completed during the original manufacturing process.
 Thus, the purpose of firing is to sinter the particles of powder
together properly for a specific time and temperature combination to
form the prosthesis.
 Condensed porcelain mass is placed in front of or below the muffle of
a preheated furnace at approximately 650°C for low – fusing
porcelain.

92
 This preheating procedure permits the remaining water to evaporate.
 After preheating for approximately 5 minutes, the porcelain is placed
into the furnace and the firing cycle is initiated.
 Placement of the condensed mass directly into even a moderately
warm furnace results in a rapid production of steam, thereby
introducing voids or fracturing large sections of the ceramic.
 At the initial firing temperature, voids are occupied by the atmosphere
of the furnace.

93
 As sintering of the particles begins, the porcelain particles bond at
their points of contact and the structure shrinks and densifies.
 As the temperature is raised, the sintered glass gradually flows to fill
the air spaces.
 However, air becomes trapped in the form of voids because the fused
mass is too viscous to allow all of the air to escape.
 An aid in the reduction of porosity in dental porcelain is vacuum
firing.

94
 When the porcelain is placed into the furnace, the powder particles
are packed together with air channels around them.
 As the air pressure inside the furnace is reduced to about one tenth
of atmospheric pressure by the vacuum pump, the air around the
particles is also reduced to this pressure.
 As the temperature rises, the particles sinter together, and closed pores
are formed within the porcelain mass.
 Air inside these pores is isolated from the furnace atmosphere.

95
 At a temperature about 55°C below the sintering temperature, the
vacuum is released and the pressure inside the furnace increases by a
factor of 10, from 0.1 to 1 atm.
 Because the pressure is increased by a factor of 10, the pores are
compressed to one tenth of their original size, and the total volume of
porosity is accordingly reduced.
 A few bubbles are present, but they are markedly smaller than those
obtained with the usual air – firing method.
 Complete sintering is accomplished when the structure achieves 100%
of its theoretical density.

96
Cooling
 Proper cooling of a porcelain prosthesis from its firing temperature to
room temperature is the subject of considerable importance.
 Catastrophic fracture of glass that has been subjected to sudden
changes in temperature is a familiar experience and lab technicians are
cautious about exposing dental porcelain to extremely rapid cooling
after firing.
 Multiple firings of a metal – ceramic prosthesis can make it more
likely to crack or craze because of tensile stress development.

97
 Cracks may not propagate directly in the metal, but they can progress
through the ceramic.
 With proper design and physical properties of the porcelain and
metal, the porcelain is protected by residual compressive stress so that
brittle fracture of the porcelain can be avoided or at least minimized.
 Although most metal – ceramic restorations involve cast metal
copings, several novel non – cast approaches (electrodeposition,
milling, swaging, and burnishing) for the fabrication of metal
substructures have been developed in recent years.

98
TECHNICALASPECTS OF
METAL – CERAMIC
PRODUCTS

99
 Copings and frameworks for metal – ceramic prostheses are
produced by casting of molten metal, CAD – CAM machining,
electrolytic deposition techniques, or swaged metal processes.
 Each casting should be carefully cleaned to ensure a strong bond to
the porcelain.
 Oil from fingers and other sources can act as a possible contaminant.

100
 Surface should be cleaned adequately by finishing with clean ceramic
– bonded stones or sintered diamonds, which are used exclusively for
finishing.
 Final sandblasting with high – purity alumina abrasive before
oxidation ensures that the porcelain will be bonded to a clean and
mechanically retentive surface.
 Frameworks for metal – ceramic bridges must be designed such that it
does not get deformed at porcelain sintering temperatures.

101
Creep or Sag
 Creep is defined as the time – dependent plastic strain of a solid
under a static load or constant stress.
 Creep can be reduced, if the metal has the proper composition.
 High – temperature creep or sag of some high noble and noble alloys
occurs when the temperature approaches 980°C.
 Once the alloy temperature decreases by 100°C or more, no further
creep deformation occurs.

102
Bonding Porcelain to Metal
 Primary requirement for the success of a metal – ceramic restoration
is the development of a durable bond between the porcelain and the
metal.
 For a metal – ceramic bond to be maintained over time, there should
be minimal residual tensile stresses in the porcelain after cooling
from the sintering temperature.
 An unfavorable stress distribution during the cooling process can
result in immediate or delayed cracking of the porcelain.

103
 Three factors control the durability of metal – ceramic bonding:
 Mechanical interlocking or interatomic bonding at the interface
between porcelain and the metal oxide;
 Interatomic bonding across the oxide – porcelain interface; and
 Type and magnitude of residual stress in the ceramic.
 Atomic or chemical bonding is primarily responsible for metal –
porcelain adherence.

104
 Oxidation behavior of the alloys largely determines their potential
for bonding with porcelain.
 Research into the nature of metal – porcelain adherence has indicated
that those alloys that form adherent oxides during the oxidation cycle
also form a good bond to porcelain.
 Alloys with poorly adherent oxides or poor wetting porcelain to the
oxide form poor bonds.
 Quality of the oxide and its adhesion to the metal substrate appear to
be the most important factors.

105
 For metal alloys that do not oxidize easily, this oxide layer is formed
during a special firing cycle prior to opaque porcelain application.
 For metal alloys that do oxidize easily, the oxide layer is formed
during wetting of the alloy by the porcelain and subsequent firing
cycle.
 Most common mechanical failure for metal – ceramic restorations is
debonding of the porcelain from the metal.

106
 From a practical standpoint, the surface roughness at the metal –
ceramic interface has a large effect on the quality of the metal –
ceramic bond.
 Airborne particle abrasion is routinely used on metal frameworks for
metal – ceramic restorations to produce a clean surface with
controlled roughness.
 During the firing cycle, the porcelain softens, its viscosity decreases,
and the porcelain first wets the metal surface before the interlocking
between porcelain and metal is created.

107
 Increased area of the rough metal surface also permits the formation
of a greater density of chemical bonds.
 Contact angle between the porcelain and metal is a measure of the
wetting and, to some extent, the quality of the bond that forms.
 Low contact angles indicate good wetting.
 However, rough surfaces can reduce adhesion if the porcelain does not
wet the surface and voids are present at the interface.

108
 There are three main mechanisms of porcelain bonding:
 Mechanical,
 Chemical, and
 Compression bonding.
 Mechanical bonding occurs due to the inherent microscopically
rough metal surface.
 Van der Waal forces play a role, as does the improved wetting that
occurs when the surface of the alloy is air – abraded with 50-micron
aluminum – oxide particles prior to applying the opaque porcelain.
 It has been estimated that such mechanical bonding constitutes only
about 10% of the total bond of the porcelain to the metal.

109
 Compression bonding occurs as a result of the slight thermal
coefficient of expansion mismatch that exists between the porcelain
and a compatible alloy.
 When a metal – ceramic restoration is taken from the muffle of a
porcelain oven and allowed to cool, the metal coping will cool first
and begin to shrink slightly.
 This will pull the overlying porcelain under compression.
 This compression stress contributes to the overall bond strength.

110
 Chemical bonding is the most important means of bonding porcelain
to metal.
 Opaque porcelains are specially formulated with tetravalent oxides
that will bond to oxides formed on the surface of the metal.
 Metallic oxides either form naturally or are induced from trace
elements, and ideally will form a monomolecular layer on the surface
of the alloy.
 Trace elements such as indium, zinc, and tin are added to the alloy to
provide the oxide required as well as refine the grain structure.

111
 Chemical bonding is much less predictable with the base metal
alloys.
 Problem with these alloys is with the oxide layer.
 Care must be taken to avoid the formation of too thick an oxide layer.
 While the bond failure may occur at either the interface between the
metal and the oxide or between the oxide and the opaque, it most
frequently occurs within the thick oxide layer itself.

112
Noble metal alloy copings.
Opaque porcelain applied
to noble metal alloy copings.
Finalized porcelain-fused-
to-metal crowns.

113
Biocompatibility
 Major elements of interest in terms of biocompatibility are nickel and
beryllium.
 It is estimated that approximately 22% of women and less than 10%
men are allergic to nickel.
 Major reason for difference in incidence between genders is thought
to be that many women have pierced ears and wear costume jewelry.
 Much of this jewelry is nickel based, and it is believed that this will
induce the allergy. Sturdevant’s Art and Science of Operative Dentistry, 7th edition

114
 Signs and symptoms of nickel allergy can be local or systemic.
 Often the presenting symptom is a rash or eczema on the arm or legs.
 Patients often do not associate such peripheral symptoms with an
intraoral restoration and hence may suffer for prolonged periods
before a diagnosis of nickel allergy is made.
 When use of a base metal alloy is contemplated, the patient should be
specifically asked if he or she has any allergy to nickel or reaction to
any metal.

115
 Beryllium is primarily a potential problem for the laboratory
technician where grindings containing beryllium can be the etiologic
agent for a number of respiratory ailments.
 Proper ventilation and the routine wearing of a face mask can
prevent such untoward events.
 Beryllium has also been shown to migrate toward the surface of
restorations, and in that location can also dissolve in oral fluids.
 Clinical significance of these findings is unknown at this time but is a
concern to some.

116
 A similar situation may occur with the high palladium alloys.
 In a number of proprietary studies palladium has been shown to
dissolve in oral fluids and be cytotoxic.
 This most frequently occurs when the combined concentration of gold
and silver is less than 25% of the total alloy.
 Again the clinical significance of this is unknown.

117
Economic Considerations
 Cost of a metal alloy needs to be considered.
 This can be even more important when the price of gold fluctuates
extensively.
 Metal cost of laboratory work has increased with time, which has a
huge effect on laboratory costs and subsequently raises the cost of
restorative services to patients.
 Proper manipulation of the alloy chosen is probably even more
important than which alloy is chosen.

118
Effect of Design
 Because ceramics are weak in tension and can withstand very little
strain before fracturing, the metal framework must be rigid to
minimize deformation of the porcelain.
 However, copings should be as thin as possible to allow space for the
porcelain to mask the metal framework without overcontouring the
porcelain.
 This consideration is especially true for alloys that appear gray.

119
 Labial margin of metal – ceramic prostheses is a critical area
regarding design because there is little porcelain thickness at the
margin to mask the appearance of the metal coping and to resist
fracture.
 Because of the difference in modulus of elasticity between porcelain
and metal, stresses occur at the interface when the restoration is
loaded.
 These stresses should be minimized by placing the metal – ceramic
junction at least 1.5 mm from centric occlusal contacts.

120
 Sharp line angles in the preparation will also create areas of stress
concentration in the restoration.
 A small particle of ceramic along the internal porcelain margin of a
crown can induce locally high tensile stresses during try – in or final
cementation.
 Furthermore, when grinding of this surface is required for adjustment
of fit, one should use the finest grit abrasive to reduce the probability
of forming microcracks and reduce the depth of microfissures
produced by the abrasive particles.

121
Glazes and Stain Ceramics
 Esthetics of porcelains for metal – ceramic and ceramic prostheses,
veneers, and denture teeth may be enhanced through the application
of stains and glazes to provide a more lifelike appearance and color
match to adjacent teeth or restorations.
 Fusing temperatures of glazes are reduced by the addition of glass
modifiers, typically alkali oxides, which reduce the chemical
durability of glazes.
 Stains are simply tinted glazes that are also exposed to the same
chemical durability problems.

122
 One method for ensuring that the applied characterizing stains will be
permanent is to use them internally.
 Internal staining and characterization can produce a lifelike result,
particularly when simulated enamel craze lines and other features are
built into the porcelain rather than merely applied on the surface.
 Disadvantage of internal staining and characterization is that the
porcelain must be stripped completely if the color or characterization
is unacceptable.

123
 After the porcelain restoration is cemented in the mouth, it is
common practice for the dentist to adjust the occlusion by grinding
the surface of the porcelain with a diamond bur.
 This procedure can weaken the porcelain if the glaze is removed and
the surface is left in a rough condition.
 This can cause increased wear of enamel.
 An acceptable solution is to polish the surface with Sof-Lex (3M,
Minneapolis, MN) finishing disks, a Shofu (Shofu, Kyoto, Japan)
porcelain laminate polishing kit, or other abrasive system.

124
 It is generally believed that glazing of feldspathic porcelain eliminates
surface flaws and produces a smoother surface.
 However, an optimal method of producing the smoothest surface in
the shortest time has not been established.
 Even though one polishes and glazes a porcelain surface, the surface
will slowly or markedly break down in the presence of solvents in
our everyday diets, which include citric acid, acetic acid and alkalis.
 Further degradation can occur during exposure of porcelain to
acidulated phosphate fluoride (APF) gel.

125
Failure and Repair of Metal–Ceramic Restorations
 Metal – ceramic restorations remain the most popular material
combination selected for crown and bridge applications and have a 10
– year success rate of about 95%.
 Majority of retreatments are due to biological failures, such as tooth
fracture, periodontal disease, and secondary caries.
 Prosthesis fracture and esthetic failures account for only 20% of
retreatment cases for single – unit restorations.

126
 In cases of failure, the prosthesis should be retrieved, metal surfaces
should be cleaned, and a new oxide layer should be formed on the
exposed area of metal prior to porcelain application and firing.
 However, this cannot be achieved intraorally, and removal of the
prosthesis is both unpleasant for the patient and time consuming.
 Thus a variety of techniques have been developed for porcelain
repair using resin composites.
 All of these techniques present the challenge of bonding chemically
dissimilar materials.

127
 When porcelain fragments are available and no functional loading is
exerted on the fracture site, silane coupling agents can be used to
achieve good adhesion between the composite and porcelain.
 However, metal alloys have no such bonding agent and this type of
repair is considered only temporary.
 Systems are available for coating the metal surface with silica
particles through airborne particle abrasion.
 Particles are embedded in the metal surface upon impact, then a silane
coupling agent can be applied.

128
 Alternatively, base metal alloys can be coated with tin followed by
the application of an acidic primer.
 Both methods achieve adequate bond strength and may delay the
eventual need for remaking the prosthesis.

129
BENEFITS AND
DRAWBACKS OF
METAL – CERAMIC
RESTORATIONS

130
Benefits
 A properly made metal – ceramic crown is more fracture resistant
and durable than most all – ceramic crowns and bridges.
 This technology is well established compared with technologies
required of the most recent all – ceramic products.
 Although the biocompatibility of some metals used for copings and
frameworks may be a concern for patients who have known allergies
to those metals, these situations are relatively rare.

131
 A metal coping provides an advantage compared with zirconia –
based ceramic prostheses when endodontic access openings through
crowns are required.
 Temporary repairs for ceramic fractures that extend to the metal
framework are possible without the need for intraoral sandblasting
treatment by using current resin bonding agents.
 All – ceramic crowns may be more susceptible to chipping fracture
and to bulk fracture in posterior sites.
 Properly designed metal – ceramic crowns are highly esthetic when
adequate tooth reduction principles are satisfied.

132
 Several clinical studies over the past 50 years confirm the high
overall survival percentages of metal – ceramic prostheses.
 One clinical study revealed that the fracture rate of metal – ceramic
crowns as well as bridges made from a high noble alloy was as low as
2.3% over 7.5 years of service.
 The most outstanding advantage of metal – ceramic restorations is
their resistance to fracture.
 Another potential advantage is that less tooth structure needs to be
removed to provide bulk for the crown, especially if thinner layered
noble metal is used.

133
Drawbacks
 One of the most frequently mentioned disadvantages is the potential
for metal allergy.
 Although metal – ceramic restorations have accounted for about 60%
to 70% of all fixed restorations, a metal – ceramic crown is not the
best esthetic choice for restoring a single maxillary anterior tooth.
 An all – ceramic crown offers a greater potential for success in
matching the appearance of the adjacent natural tooth, especially
when a relatively high degree of translucency is desired.

134
 A dark line at the facial margin of a metal – ceramic crown
associated with a metal collar or metal margin is a significant esthetic
concern when gingival recession occurs.
 This adverse esthetic effect can be minimized by designing the crown
with a ceramic margin or by using a very thin margin of metal
veneered with opaque porcelain.
 This ceramic margin should be polished and/or glazed to avoid a
rough surface at the margin.

135
 A metal – ceramic bond may fail in few possible locations.
 Knowing the location of failure provides considerable information on
the quality of the bond.
 Highest bond strength leads to failure within the porcelain when
tested.
 This is observed with some alloys that were properly prepared with
excellent wetting by the porcelain and is also called a cohesive
failure.

136
 Another possible cohesive failure is within the oxide layer.
Ceramic – ceramic bond failure
(cohesive)
Metal oxide – metal oxide bond
failure (cohesive)

137
 Failures occurring at the interface between metal and oxide layer are
called adhesive failures.
 Commonly observed with metal alloys that are resistant to forming
surface oxides, such as pure gold or platinum, and exhibit poor
bonding.
Metal – metal Oxide
bond failure (adhesive)

138
Metal – Ceramic Bond Failure
 Clinically, an ideal metal – ceramic bond failure would be cohesive in
nature (within porcelain).
 That is, the bond between metal and porcelain should be greater than
the cohesive strength of the porcelain.
 When additional stresses are applied to the restoration, the probability
of failure due to fatigue crack propagation might increase, explaining
the veneer chipping or fracture.
Sayed NM. Shear bond strength and failure mode between veneering ceramic and
metal cores after multiple firing cycles. Egypt Dent J 2015; 61(1): 659-666.

139
 Failures can also be operator related.
 Most dentists tend to underprepare teeth that are to be restored with
metal – ceramic crowns, and do not provide the laboratory technician
sufficient room for the metal substructure, opaque layer and body,
and incisal porcelain.
 Many use outdated cervical margin designs, and soft tissue
management is generally poor, resulting in a high incidence of
inadequate impressions.
 Many dentists opt to send their laboratory work overseas and accept
low – quality restorations in the name of cost reduction.

140
METHODS FOR
STRENGTHENING
CERAMICS

141
Minimizing the Effect of Stress Concentrations
 Numerous minute scratches and other flaws are present on the
surfaces of ceramics.
 These surface flaws behave as sharp slits.
 Under intraoral loading, tensile stresses that develop within the
ceramic structure are greatly increased and concentrated at the tips of
these flaws.

142
 This stress concentration geometry at the tip of each surface flaw can
increase the localized stress to extremely high levels.
 When the induced tensile stress exceeds the nominal strength of the
material structure, the bonds at the notch tip rupture, forming a crack.
 This stress concentration phenomenon explains how materials fail at
stresses far below their theoretical strength.
 However, there are other variables as well that affect the magnitude
of these stresses, including prosthesis design, load orientation, loading
rate, microstructure, and residual processing stresses.

143
 Stress raisers are discontinuities in ceramic structures and in other
brittle materials that cause a stress concentration in these areas.
 Design of ceramic dental restorations should be carefully planned
with sufficient bulk and a minimum of sharp angular changes to 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.

144
 As the crack propagates through the material, the stress concentration
is maintained at the crack tip unless the crack moves completely
through the material or until it meets another crack, pore, or
crystalline particle, which may reduce the localized stress.
 Removal of surface flaws or a reduction in their size and number can
produce a very large increase in strength.
 Reducing the depth of surface flaws in the surface of a ceramic is one
of the reasons that polishing and glazing of dental porcelain is so
important.

145
 Fracture resistance of ceramic prostheses can be increased through
one or more of the following seven options:
 Selecting stronger and tougher ceramics.
 Developing residual compressive stresses within the surface of the
material by thermal tempering.
 Developing residual compressive stress within interfacial regions of
weaker, less tough ceramic layers by properly matching coefficients of
thermal expansion and contraction.
 Adhesively bonding the ceramic crowns to the tooth structure.

146
 Reducing the tensile stress in the ceramic by appropriate selection of
stiffer supporting materials (greater elastic moduli).
 Minimizing the number of firing cycles for feldspathic porcelains.
 Designing the ceramic prosthesis with greater bulk and broader radii
of curvature for connectors in areas of potential tensile stress to
minimize stress concentrations and the magnitude of tensile stresses
that can develop during function.

147
 Even though a metal – ceramic restoration is generally more fracture
resistant than most ceramic crowns of the same size and shape, care
must be taken to avoid subjecting the porcelain in a metal – ceramic
prosthesis to loading that produces large localized stresses.
 If occlusion is not adjusted properly on a porcelain surface, contact
points will greatly increase the localized stresses in the porcelain
surface as well as within the internal surface of the crown.
 These contact stresses can lead to the formation of the so – called
Hertzian cone cracks, which may lead to chipping of the occlusal
surface.

148
Development of Residual Compressive Stresses
 Fabrication of metal – ceramic and all – ceramic prostheses usually
involves sintering the ceramic at high temperature.
 Process of cooling to room temperature offers the opportunity to take
advantage of mismatches in coefficients of thermal contraction of
materials in the ceramic structure.
 However, if the porcelain contracts more than the metal coping or
framework, tensile stresses develop that can cause cracking of the
ceramic.

149
Crack in metal – ceramic crown after
cooling of a three unit bridge
 To prevent fracture of a ceramic prosthesis, one must prevent tensile
stresses from occurring.
 If one could produce a significant amount of compressive stress in
the area of the ceramic structure, a greater level of tensile stress
would need to be developed during oral function for the prosthesis to
reach the tensile stress needed to cause fracture.

150
 One method of introducing residual compressive stresses within the
ceramic is to choose veneering ceramics whose thermal expansion or
contraction coefficient is slightly less than that of the core ceramic.
 Another procedure is to rapidly cool the prosthesis by cooling it on
the benchtop rather than in the furnace.

151
Minimizing the Number of Firing Cycles
 Purpose of porcelain firing procedures is to densely sinter the
particles of powder together and produce a relatively smooth, glassy
layer (glaze) on the surface.
 In some cases, a stain layer is applied for shade adjustment or for
characterization, such as stain lines or fine cracks.
 Several chemical reactions occur over time at porcelain firing
temperatures; of particular importance is increase in the
concentration of crystalline leucite in the porcelains.

152
 Changes in the leucite content caused by multiple firings can alter the
coefficient of thermal contraction of some porcelain products.
 Some porcelains undergo an increase in leucite crystals after multiple
firings, which will increase their coefficient of thermal expansion.
 If the expansion coefficient increases above that for the metal, the
expansion mismatch between the porcelain and the metal can produce
stresses during cooling, sufficient enough to cause immediate or
delayed crack formation in the porcelain.

153
Ion Exchange
 Ion exchange is an effective method of introducing residual
compressive stresses into the surface of a ceramic.
 Increases of 100% or more in flexural strength of feldspathic
porcelains have been achieved with several ion exchange products
containing a significant concentration of small sodium ions.
 This strengthening effect may be lost if the porcelain or glass –
ceramic surface is ground, worn, or eroded by long – term exposure
to certain inorganic acids.

154
Thermal Tempering
 Thermal tempering is used to strengthen glass in automobile
windows and windshields, sliding glass doors, and diving masks.
 Perhaps, the most common method for strengthening glasses is by
thermal tempering, which creates residual surface compressive
stresses by rapidly cooling the surface of the object while it is hot and
in the softened (molten) state.
 This rapid cooling produces a skin of rigid glass surrounding a soft
(molten) core.

155
 For dental applications, it is more effective to quench hot glass –
phase ceramics in silicone oil or other special liquids.
 This thermal tempering treatment induces a protective region of
compressive stress within the surface.
 However, this process is technique – sensitive, since large
counterbalancing tensile stresses may develop when excessive cooling
rates occur during the tempering process.

157
 All – ceramic FDPs are considered an established treatment
alternative to metal – ceramic FDPs in daily clinical practice.
 Main reason to use of the all – ceramics instead of metal – ceramics is
based on more favorable esthetics.
 All – ceramic materials mimic very naturally the optical properties
of teeth.
Sailer I, Makarov NA, Thoma DS, Zwahlen M, Pjetursson BE. All-ceramic or metal-ceramic tooth-supported fixed dental prostheses
(FDPs)? A systematic review of the survival and complication rates. Part I: Single crowns (SCs). Dent Mater. 2015;31(6):603-23.

158
 Ceramic crowns and bridges have been in widespread use since the
beginning of the twentieth century.
 Although initial materials had reasonably good success rate for a few
years, their limitations slowly but surely led to the development of
stronger and tougher ceramics that allowed for a broader range of
uses.
 Recent developments in ceramic products with improved fracture
resistance, advanced CAD – CAM technology, and excellent esthetic
capability have led to a significant increase in the use of all – ceramic
products.

159
 Metal – free restorations allow to preserve soft tissue color more
similar to the natural one than porcelain fused to metal restorations.
 Many ceramics, such as alumina, ceramic reinforced with lithium
disilicate, and polycrystalline ceramics like zirconia, have been
proposed for the construction of metal – free restorations.
 Luthy measured average load – bearing capacities of 518 N for
alumina restorations, 282 N for lithium disilicate restorations, and 755
N for zirconia restorations.
Manicone PF, Rossi Iommetti P, Raffaelli L. An overview of zirconia ceramics:
basic properties and clinical applications. J Dent. 2007;35(11):819-26.

160
 Materials for all – ceramic restorations use a wide variety of
crystalline phases as reinforcing agents and contain up to 99% by
volume of crystalline phase.
 Nature, amount, and particle size distribution of the crystalline phase
directly influence the mechanical, thermal and optical properties of
the ceramic material.
 Match between the refractive indices of the crystalline phase and
glassy matrix is an important factor for controlling the translucency
of porcelain and glass ceramics, and polycrystalline ceramics such as
zirconia.

161
Aluminous Porcelain
 Until the 1960s, high – fusing feldspathic porcelains had been used
to produce all – ceramic crowns.
 Relatively low strength of this type of porcelain prompted McLean
and Hughes in 1965 to develop an alumina – reinforced porcelain
material for fabrication of ceramic crowns providing better esthetics.
 However, the strength of the core porcelain used for alumina –
reinforced crowns was inadequate to warrant its use for posterior
teeth.

162
 First aluminous core ceramics contained 40% to 50% alumina by
weight, dispersed in a low – fusing glassy matrix.
 The core was baked on a platinum foil and later veneered with
matched expansion porcelain.
 Aluminous core porcelains have flexural strengths approximately
twice that of feldspathic porcelains.
 Alumina has a high modulus of elasticity and relatively high
fracture toughness, compared with feldspathic porcelain.

163
Glass – Ceramics
 A glass – ceramic is a material that is formed into the desired shape
as a glass and then subjected to a heat treatment to induce partial
devitrification – that is, loss of glassy structure by crystallization of
the glass.
 Crystalline particles, needles, or plates formed during this ceramming
process interrupted the propagation of cracks in the material when an
intraoral force was applied, thereby promoting increased strength
and toughness.

164
 The use of glass – ceramics in dentistry was first proposed by
MacCulloch in 1968.
 First commercially available castable ceramic material for dental use,
Dicor, was developed by Corning Glass Works and marketed by
Dentsply International.
 Dicor was a castable glass formed into an inlay, facial veneer, or full
– crown by a lost – wax casting process similar to that employed for
metals.

165
 Dicor glass – ceramic was capable of producing remarkably good
esthetics, perhaps because of the “chameleon” effect, in which part
of the color of the restoration was picked up from the adjacent teeth as
well as from the tinted cements used for luting the restorations.
 When used for posterior crowns, the Dicor glass – ceramic crowns
were more susceptible to fracture than anterior crowns.
Scanning electron microscopic image of a
fractured Dicor glass-ceramic crown.
Arrow indicates the site of critical flaw
responsible for crack initiation under intraoral
loading.

166
 Dicor contained 55% by volume of tetrasilicic fluormica
(KMg2.5Si4O10F2) and was the first castable glass used for dental
prosthetic applications.
 Besides its relatively low flexural strength (110 to 172 MPa) and low
fracture toughness (1.6 to 2.1 MPa), the original cast form was
colorless and prostheses had to be colored by the application of a thin
layer of shading porcelain.
 Subsequent products were provided as dark and light shades of
machineable glass – ceramic (MGC).

167
 Malament and Socransky (1999) reported survival probabilities for
acid – etched Dicor and nonetched Dicor restorations of 76% and
50%, respectively, after 14 years (P < 0.001).
 Non – etched (nonbonded) Dicor crowns exhibited a 2.2 times
greater risk of failure than acid – etched restorations (P < 0.01).
 Ceramic crown survival was greatest for incisor teeth and decreased
progressively to a maximum failure level for second molar crowns.
 Survival of acid – etched and resin – bonded Dicor crowns for
subjects 33 to 52 years of age was 62% at 14 years compared with
82% for those 52 years of age and older.

168
 More recently, glass – ceramics based on leucite, lithium disilicate,
and hydroxyapatite have been used.
 These ceramics are available as powders or as solid blocks that can be
machined through CAD – CAM processes or hot – pressed.
 Dicor and Dicor MGC glass – ceramics are no longer used in
dentistry.

169
Leucite – Based Ceramics
 Formed by the heat – pressing process.
 Heat pressing relies on the application of external pressure at high
temperature to sinter and shape the ceramic.
 Heat pressing promotes a good dispersion of the crystalline phase
within the glassy matrix.
 Mechanical properties of heat – pressed ceramics are therefore
maximized.

170
 First – generation heat – pressed ceramics contain tetragonal leucite
(KAlSi2O6 or K2O·Al2O33·4SiO2) as a reinforcing phase, in amounts
varying from 35% to 55% by volume.
 Heat – pressing temperatures for this system are between 1150° and
1180°C with a dwell at temperature of about 20 minutes.
 Final microstructure of these heat – pressed ceramics consists of
leucite crystals, 1 to 5 μm, dispersed in a glassy matrix.

171
 Flexural strength of these ceramics (120 MPa) is almost double than
that of conventional feldspathic porcelains.
 This increase in strength can be explained by the fact that these
ceramics possess a higher crystallinity and that the heat – pressing
process generates an excellent dispersion of these fine leucite
crystals.
 Main advantages of leucite – reinforced ceramics are their excellent
esthetics and translucency, whereas their limitations lie in their
modest mechanical properties restricting their use to anterior single
– unit restorations.

172
 Most well – known leucite – based products are IPS Empress
(Ivoclar Vivadent), Cerpress SL Pressable Ceramic System (Leach and
Dillon), and Finesse All – Ceramic System (DENTSPLY Ceramco).
 Contain 35% by volume of leucite (K2O•Al2O3•4SiO2) crystals.
 These glass – ceramics have relatively low flexural strength (up to
112 MPa) and fracture toughness (0.9 to 1.3 MPa), so they are not
recommended for molar crowns or bridges.
Leucite-reinforced glass ceramic crowns (IPS Empress)

173
Lithium Disilicate – Based Materials
 Second generation of heat – pressed ceramics contain lithium
disilicate (Li2Si2O5) as a major crystalline phase.
 Heat pressing takes place in the 910° to 920°C temperature range,
using the same equipment as for the leucite – based ceramics.
 The final microstructure consists of about 65% by volume of highly
interlocking prismatic lithium disilicate crystals (2 to 5 μm in length,
0.8 μm in diameter) dispersed in a glassy matrix.

174
 Its mean flexural strength is approximately 350 MPa compared with
the 112 MPa strength of leucite – based glass-ceramics.
 This strength and a fracture toughness of 3.3 MPa for lithia disilicate
– based glass – ceramics are generally sufficient for most anterior and
posterior crowns and for anterior three unit bridges.
 Although the ceramic fracture resistance is moderately high, veneered
prostheses have been reported to be susceptible to chipping, which
may require replacement or recontouring of the affected prostheses.

175
 IPS Empress 2 (Ivoclar Vivadent) and Optec OPC 3G (Pentron
Laboratory Technologies) contain approximately 65% to 70% by
volume of lithia disilicate (Li2O•2SiO2) as the principal crystal phase.
 Lithia disilicate materials used as glass – ceramics have a narrow
sintering range, which makes processing of ceramic prostheses very
technique sensitive.
 It is fairly translucent but somewhat more opaque than the leucite –
based glass – ceramic (Empress), but is a stronger ceramic than leucite
– based glass – ceramic.

176Phillips’Science of Dental Materials, 12th edition
Crack in crown of a three – unit
bridge made with a lithia disilicate −
based glass-ceramic core.
Fracture of the crown
shown on the left.

177
Infiltrated Ceramics
 Infiltrated ceramics are made through a process called slip – casting,
which involves the condensation of an aqueous porcelain slip on a
refractory die.
 This fired porous core is later glass infiltrated, a process by which
molten glass is drawn into the pores by capillary action at high
temperatures.
 Materials processed in this way exhibit less porosity, fewer defects
from processing, greater strength and higher toughness than
conventional feldspathic porcelains.
Shenoy A, Shenoy N. Dental ceramics: An update. J Conserv Dent 2010;13(4):195-203

178
Glass – Infiltrated Core Ceramics
 To minimize sintering shrinkage and ensure adequate fit of ceramic
prostheses, three glass – infiltrated core ceramic systems have been
developed:
 Based on partially sintered alumina,
 Based on a magnesia – alumina spinel (MgAl2O4), and
 With a zirconia – alumina core.
 Each of these partially sintered ceramics can be infiltrated with a
lanthanum glass without any significant dimensional change.

179
 VITA In – Ceram Alumina contains approximately 85% of alumina
by volume.
 The partially sintered framework is formed by a slip – casting process,
which produces dense packing of particles against a porous die.
 After firing at 1120°C for 10 hours or more, a partially sintered
structure is formed.
 This porous core ceramic framework is then infused with molten
lanthanum glass.

180
 Same type of process can also be applied to In – Ceram – Spinel
(ICS), which is a magnesia alumina spinel (MgAl2O4) core ceramic,
and In – Ceram Zirconia.
 After glass infiltration, In – Ceram Spinel ceramic is more
translucent than In – Ceram Alumina or In – Ceram Zirconia but its
mean strength is significantly lower (approximately 350 MPa versus
600 MPa).
 Mean flexure strength of In – Ceram Zirconia (about 620 MPa) is only
slightly greater than that of In – Ceram Alumina.

181
 In – Ceram Zirconia is not made from a pure zirconia core but rather
a combination, by weight, of approximately 62% alumina, 20%
zirconia, and 18% infiltrated glass.
 In its glass – infused form, it is indicated primarily for crown copings
and three – unit anterior and posterior frameworks.
 Because there is no shrinkage associated with this process, the
marginal adaptation is expected to be comparable to that of the hot –
pressing method.

182
Alumina Core Ceramic
 Procera AllCeram (Nobel Biocare) is an alumina core ceramic that is
indicated for anterior and posterior crowns.
 It is more translucent than In – Ceram Zirconia and it has
comparable strength (620 to 700 MPa).
 Sandblasting the surface with silica – coated alumina particles is
required to ensure sufficient resin bonding.

183
Zirconia
 Zirconia has been used as a biomaterial since the 1970s.
 It has been used in dentistry for crown and bridge applications since
2004.
 Zirconium dioxide (ZrO2), or zirconia, is a white crystalline oxide of
zirconium.
Full contour zirconia restoration

184
 Some physical properties of zirconia must be considered in order to
obtain a good aesthetic outcome.
 As a matter of fact, zirconia has not only a color similar to teeth but is
also opaque.
 This can be an advantage when a dischromic tooth or a metal post
must be covered, a zirconia core allows concealment of this
unfavorable aspect.
 Radiopacity of zirconia is very useful for monitoring marginal
adaptation through radiographic evaluation.
Manicone PF, Rossi Iommetti P, Raffaelli L. An overview of zirconia ceramics:
basic properties and clinical applications. J Dent. 2007;35(11):819-26.

185
 Zirconia exhibits unique mechanical and electrical properties that
make it extremely useful in heat insulators, oxygen sensors, and fuel
cells.
 Under atmospheric pressure, pure zirconia can exhibit three different
crystal structures.
 At temperatures greater than 2367°C, zirconia has a cubic structure.
 Between 1167°C and 2367°C, zirconia is tetragonal, and below
1167°C the structure is monoclinic.

186
 Tetragonal to monoclinic phase transition results in a 3% to 5%
volume increase, which produces cracks in bulk zirconia samples and
a reduction in strength and toughness.
 However, if one modifies the composition by doping with Mg, Ca, Sc,
Y, or Nd, the high – temperature tetragonal phase can be stabilized at
room temperature.
 In this way, the tetragonal to monoclinic phase transformation stresses
are avoided, microcracks are prevented, and the positive mechanical
properties of the tetragonal phase are preserved.

187
 Zirconia is a nonmetal with an extremely low thermal conductivity —
about 20% as high as that of alumina (Al2O3).
 It is chemically inert and highly corrosion resistant.
 Pure ZrO2 has a monoclinic crystal structure at room temperature
and transforms to tetragonal and cubic zirconia at elevated
temperatures.

188
 Stability of single – phase tetragonal zirconia is enhanced by highly
soluble trivalent stabilizers such as yttria, which induces vacancies, or
tetravalent stabilizers such as ceria, which are oversized or undersized
with respect to zirconium.
 Most common stabilizer for dental applications is yttria (Y2O3).
 The addition of 3 to 5 mol% of Y2O3 results in a stabilized core
ceramic referred to as yttria – stabilized zirconia or yttria – stabilized
tetragonal zirconia polycrystals (Y – TZP).

189
 Structural stabilization of zirconia by yttria results in a significant
proportion of metastable tetragonal phase.
 This metastable tetragonal phase strengthens and toughens the
structure by a localized transformation to the monoclinic phase when
tensile stresses develop at crack tips.
 The resulting volume expansion adjacent to the crack tips produces a
high local compressive stress around the crack tips, which increases
the localized fracture toughness and inhibits the potential for crack
propagation.

190
 This phenomenon of transformation toughening increases the
flexural and tensile fracture resistance of stabilized zirconia prostheses
and presumably the survival probabilities of zirconia – based
restorations.
 Fracture toughness of a 92.2% dense non – doped monoclinic
zirconia has been reported to be 2.06 MPa.
 An extrapolation to full density yields a value of 2.6 MPa.
 In comparison, the fracture toughness of tetragonal 3Y – TZP is
approximately 8 to 10.3 MPa.

191
 When pure ZrO2 is heated to a temperature between 1470°C and
2010°C and cooled, its crystal structure begins to change from a
tetragonal to a monoclinic phase at approximately 1150°C.
 During cooling to room temperature, a volume increase of several
percent occurs when it transforms from the tetragonal to monoclinic
crystal structure.
Tetragonal and monoclinic
unit cell structures

192
 By controlling the composition, particle size, and the temperature
versus time cycle, zirconia can be densified by sintering at a high
temperature.
 The tetragonal structure can be maintained as individual grains or
precipitates as it is cooled to room temperature.
 When sufficient stress develops in the tetragonal structure and a crack
in the area begins to propagate, the metastable tetragonal crystals
(grains) or precipitates next to the crack tip can transform to the stable
monoclinic form.

193
 In this process a 3% expansion by volume of the ZrO2 crystals or
precipitates occurs that places the crack under a state of compressive
stress and crack progression is arrested.
 Because of this strengthening and toughening mechanism, the yttria-
stabilized zirconia ceramic is sometimes referred to as ceramic steel.
Four veneered
Zirconia − based crowns

194
 Although the fracture resistance of all – zirconia crowns is
exceptionally high, the risk for catastrophic wear of opposing enamel
and dental restorations is one of the major potential challenges to the
effective, safe use of solid zirconia prostheses.
 Three other disadvantages of an all – zirconia crown are:
 Difficulty in adjusting occlusion when significant premature contacts
are present,
 Cutting difficulty, and
 Heat generated in removing defective crowns or when making an
endodontic access opening with diamond burs.

195
 Long – term performance of Y – TZP may be compromised by its
susceptibility to hydrothermal degradation.
 Although hydrothermal effects have generally been reported between
200°C and 400°C, longer exposure times at oral temperatures may
also degrade zirconia, resulting in increased surface roughness,
fragmented grains, and microcracks.
 Degradation process is initiated by a transformation of the surface to
the monoclinic phase, which spreads through the surface grains and
into adjacent grains by stresses that develop in this process.

196
 A relatively new zirconia restoration has been introduced in recent
years called monolithic zirconia or full contour zirconia.
 It is basically a core of zirconia milled into a full crown and then
either glazed or polished.
 Based on studies of layered zirconia, where core fracture is extremely
rare, these restorations may reasonably be expected to have high
survival rates.
 One major advantage of monolithic zirconia is that very conservative
tooth preparations can be used because of the strength of the
material.

197
 More translucent zirconia compositions have also been introduced.
 Increase in translucency is obtained either by decreasing the amount
of alumina present to 0.05 wt% or less, or by adding higher amounts
of yttria (up to 5.3 mol.%) to stabilize the cubic polymorph of zirconia
as a major crystalline phase.
 Cubic zirconia is more translucent due to its isotropic crystal
symmetry, compared with tetragonal zirconia, which is anisotropic
and birefringent.
 Retention of cubic zirconia at room temperature is also accompanied
with a substantial increase in grain size.

198
Zirconia – Toughened Alumina
 Zirconia – toughened alumina (ZTA) is composed, by weight, of
70% to 90% alumina and 10% to 20% zirconia.
 Similar to the toughening of Y – TZP, ZTA is toughened by a stress –
induced transformation mechanism.
 During this process, the number of zirconia particles increases and
this change induces compressive stress within the alumina structure.
 Result of this process is that the strength of alumina is doubled and
the toughness is increased two to four times.

199
Dispersion Strengthening and Toughening
 Reinforcement of ceramics with a dispersed phase of a different
material can prevent or inhibit propagation of cracks.
 This process is referred to as dispersion strengthening.
 Many current dental veneering ceramics have a glassy matrix that is
reinforced by a dispersed crystal phase.

200
 Glass matrices in dental ceramics have been strengthened and
toughened by a variety of dispersed crystalline phases including
leucite, lithia disilicate, alumina, and tetrasilicic fluormica.
 Almost all of the modern higher – strength ceramics derive their
improved fracture resistance from the crack – blocking ability of the
crystalline particles.
 Toughening depends on the crystal type, its size, its volume fraction,
the interparticle spacing, and its relative coefficient of thermal
expansion relative to the glass matrix.

201
 For example, the fracture toughness (KIc) of soda-lime-silica glass is
0.75 MPa.
 If one disperses approximately 34 vol% of leucite crystals in the
glass, Kic increases only to 1.3 MPa.
 However, by dispersing 70% by volume of interlinked lithia
disilicate crystals in the glass matrix, KIc increases to 3.3 MPa.
 In contrast to dispersion strengthening, dental ceramics based
primarily on zirconia crystals undergo transformation toughening
involving the conversion from a tetragonal crystal phase to a
monoclinic phase.

202
BONDING INDIRECT
ADHESIVE
RESTORATIONS

203
 Glass – matrix ceramic restorations must be etched internally with
~5% to ~10% hydroluoric acid (HF) for 20 to 180 seconds to create
retentive microporosities analogous to those created in enamel by
phosphoric acid etching.
 HF must be rinsed off thoroughly with running water.
 After rinsing of the HF and air – drying, a silane coupling agent is
applied on the etched glass matrix ceramics surface and air – dried.

204
 Silane acts as a primer and modifies the surface characteristics of
etched glass – matrix ceramics.
 Application of a silane solution on HF – etched surfaces increases
bond strengths up to 50% compared to HF alone, including bond
strengths obtained with feldspathic porcelain and lithium disilicate –
reinforced ceramic blocks.
 Adhesion of a resin luting cement to glass – matrix ceramics is
achieved through a combination of mechanical retention from HF
etching and chemical adhesion provided by the silane coupling agent
to achieve durable adhesion.

205
 Etched porcelain is an inorganic substrate, which silane makes more
receptive to organic materials, the adhesive system, and resin cement.
 With the introduction of universal one – bottle adhesives and new
silane solutions containing 10 – MDP, some manufacturers do not
recommend the application of a separate silane coupling agent to HF-
etched glass-matrix ceramic surface.
 Self – etching ceramic primer Monobond Etch & Prime (MEP,
Ivoclar Vivadent, Schaan, Liechtenstein) was launched as an
alternative for the surface treatment of silica – based ceramic.

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