Dental ceramics

Dental Ceramics
Dr. Kriti Trehan
MDS I
03/04/2018

Contents
 Introduction
 History of dental ceramics
 Structure
 Composition
 Properties
 Classification
 Metal-ceramic systems: Composition and Properties
 Components of metal-ceramic restoration
 Fabrication of metal-ceramic prosthesis
 Bonding mechanisms
 Strengthening of metal ceramic
 References

o 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.
o The American Ceramic Society had defined ceramics
as inorganic, non-metallic materials, which are :
crystalline in nature
Introduction
CERAMICS : Compounds of one or more metals with a
nonmetallic element, usually oxygen. They are formed of
chemical and biochemical stable substances that are strong,
hard, brittle, and inert non-conductors of
thermal and electrical energy.

compounds formed between metallic and
nonmetallic elements such as aluminum & oxygen
(alumina - Al2O3), calcium & oxygen (calcia -
CaO), silicon & nitrogen (nitride- Si3N4).
o Most ceramics are characterized by their:
Biocompatibility
Esthetic potential
Refractory nature
High hardness
Excellent wear resistance
Chemical inertness

 Ceramic-like tools have been used by humans since the
end of the Old Stone Age around 10,000 B.C. to support the
lifestyles and needs of fisher-hunter-gatherer civilizations.
 The first porcelain tooth material was patented in 1789 by
de Chemant, a French dentist in collaboration with
Duchateau, a French pharmacist.
 In 1808, Fonzi, an Italian dentist, invented a
“terrometallic” porcelain tooth held in place by a platinum
pin or frame.
HISTORY OF DENTAL CERAMICS

o Planteau, a French dentist, introduced porcelain teeth to
the United States in 1817, and Peale, an artist, developed
a baking process in Philadelphia for these teeth in 1822.
o Charles Land introduced one of the first ceramic
crowns to dentistry in 1903.
o Two of the most important breakthroughs responsible for
the long-standing superb esthetic performance and
clinical survival probabilities of metal-ceramic
restorations are described in the patents of Weinstein
and Weinstein (1962) and Weinstein et al. (1962).
o The first commercial porcelain was developed by VITA
Zahnfabrik in about 1963.

 1965 – Mc Lean & Hughes used glass- alumina
composite instead of feldspar porcelain resulting in
stronger restorations.
 Improvement in all ceramic systems developed by
controlled crystallization of a glass (Dicor) was
demonstrated by Adair and Grossman (1984).
 1989 – The concept of All-Ceramic post & core was
introduced using Dicor glass-ceramic initially, followed
by In-cream, IPS Empress and Zirconia ceramics.
 New generation of ceramics, including Cercon, Lava, In
Ceram Zirconia, IPS Empress2, and Procera All Ceram
were used for ceramic prostheses.

 Ceramics can appear as either crystalline or amorphous
solids (also called glasses). Thus, ceramics can be
broadly classified as non crystalline (Amorphous Solids
or glasses) and Crystalline ceramics.
 The mechanical and optical properties of dental ceramics
mainly depend on the nature and the amount of
crystalline phase present.
 More the crystalline phase better will be the mechanical
properties which in turn would alter the aesthetics.
Structure

 Conventional or feldspathic porcelains are usually
noncrystalline ceramics. These conventional porcelains
are very weak and brittle in nature leading to fracture
even under low stresses.
 Recent developments in the processing technology of
dental ceramics have led to the development of
crystalline porcelains with suitable fillers such as
alumina, zirconia and hydroxy apatite.

Non- Crystalline Ceramics
 These are a mixture of crystalline
minerals (feldspar, silica and
alumina) in an amorphous (non-
crystalline matrix of glass)
vitreous phase.
 The glass-forming matrix of
dental porcelains uses the basic
silicone oxygen (Si-O) network.
 Their structures are
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.

 The primary structural unit in all silicate structures is the
negatively charged siliconoxygen tetrahedron (SiO4) 4− .
 The SiO4 tetrahedra are linked by sharing their corners.They
are arranged as linked chains of tetrahedra, each of which
contains two oxygen atoms for every silicon atom.
 The atomic bonds in this glass structure have both a covalent
and ionic character thus making it stable.
 This stable structure, imparts some important qualities like
excellent thermal, optical and insulating characteristics,
inertness, translucency to the glass matrix.

 Alkali cations such as
potassium or sodium tend
to disrupt silicate chains
leading to lower sintering
temperatures and
increased coefficients of
thermal expansion.
 Molecules with one
oxygen atom (such as
Na2O, K2O,or CaO) are
useful in dental porcelain
as fluxes. They may also
act as opacifiers.
 Molecules that contain three oxygen atoms for every two other
atoms (such as Al2O3) are used as stabilizers.

Crystalline Ceramics
 Regular dental porcelain, being of a glassy nature is largely
non crystalline and exhibits only a short range order in
anatomic arrangement.
 The only true crystalline ceramic used in restorative dentistry
is Alumina(A1 2 O 3 ); which is one of the hardest and
probably the strongest oxides known. Crystalline ceramics
may have ionic or covalent bonds (Ionic crystals are
compounds of metals with oxygen. e.g.: Alumina.

 Ceramics are reinforced with crystalline inclusions
such as alumina and leucite into the glass matrix to
form crystal glass composites as a part of
strengthening the material and improving its fracture
resistance (dispersion strengthening).
 McLean and Hughes (1965) introduced the first
generation of reinforced porcelains for porcelain
jacket crowns, which are generally referred to as
“Aluminous porcelains” .

Glass Formation
 When silica melts, it produces an extremely viscous
liquid.
The solid formed is more to likely be a glass (Vitreous
Structure) called Fused Quartz.
This process of forming a glass is called ‘Vitrification’.
cooled rapidly

 Dental ceramics are mainly composed with crystalline
minerals and glass matrix.
Composition
Denture Tooth
Porcelain
Feldspathic
Porcelain
Aluminous
porcelain
Begins as a mixture of
powders of feldspar,
clay and quartz.
Used for ceramo-metal
restorations; begins as
mixture of powders of
potassium feldspar and
glass. It can also be used
for fabricating porcelain
veneers and inlays.
Used in pjs’s. It is
composed of mixture
similar to that of
feldspathic porcelain
with increased amounts
of aluminium oxide.

Feldspar :is responsible for forming the glass matrix .
Feldspar is a naturally occurring mineral and composed of
two alkali aluminum silicates such as potassium aluminum
silicate (K2O-Al2O3-6SiO2); also called as potash feldspar
or ortho clase and soda aluminum silicate (Na2O-Al2O3-
6SiO2); also called as soda feldspar or albite .
 It is the lowest melting compound and melts first on firing.
 Most of the currently available porcelains contain potash
feldspar as it imparts translucency to the fired restoration.

Role of feldspar :
Glass phase formation: During firing, the feldspar fuses
and forms a glassy phase that softens and flows slightly
allowing the porcelain powder particles to coalesce
together.
 The glassy phase forms a translucent glassy matrix
between the other components in the dense solid.
Leucite formation: Another important property of feldspar
is its tendency to form the crystalline mineral leucite
when melted, which is exploited to advantage in the
manufacture of porcelain suitable for metal bonding.

Silica: It is one of the most abundant elements on earth and
can exist in many different forms such as crystalline
materials like Quartz,Cristobalite, Tridymite and amorphous
materials like Fused quartz,Agate, Jasper and Onyx.
 Pure Quartz crystals (SiO 2 ) are used for manufacturing
dental porcelain.
 Quartz (crystalline silica) used in porcelain as a filler and
strengthening agent.

 Kaolin is a type of clay material which is usually obtained
from igneous rock containing alumina.
 Kaolin acts as a binder and increases the moldability of the
unfired porcelain.
 It also imparts opacity to the porcelain restoration so dental
porcelains are formulated with limited quantity of kaolin.

 Glass modifiers are used as fluxes and they also lower the
softening temperature and increase the fluidity .
 Color pigments or frits are added to provide the
characteristic shade.
 Stains :It is created bymixxing the metallic oxides with low
fusing glasses. Stains also permit surface characterization
and color modification for custom shade matching.
 Glazes :generally colorless low-fusing porcelains that
posses considerable fluidity at high temperature.They fill
small surface porosities and irregularities.

Ingredient Functions
Feldspar (naturally occurring
minerals composed of potash
[K2O], soda [Na2O], alumina
and silica).
It is the lowest fusing component, which melts first and
flows during firing, initiating these components into a
solid mass.
Silica (Quartz • Strengthens the fired porcelain restoration.
• Remains unchanged at the temperature normally used
in firing porcelain and thus contribute stability to the
mass during heating by providing framework for the
other ingredients.
Kaolin (Al2O3.2 SiO2. 2H2O -
Hydrated aluminosilicates)
• Used as a binder. • Increases moldability of the
unfired porcelain. • Imparts opacity to the finished
porcelain product.
Glass modifiers e.g. K, Na, or
Ca oxides or basic oxides
They interrupt the integrity of silica network and acts
as flux.
Color pigments or frits, e.g.
Fe/Ni oxide, Cu oxide, MgO,
TiO2, and Co oxide.
To provide appropriate shade to the restoration
Zr/Ce/Sn oxides, and
Uranium oxide
To develop the appropriate opacity.

 Dental ceramics exhibit excellent biocompatibility with
the oral soft tissues and are also chemically inert in oral
cavity.
 Dental ceramics possesses very good resistance to the
compressive stresses, however, they are very poor under
tensile and shear stresses .
 This imparts brittle nature to the ceramics and tend to
fracture under tensile stresses.
Properties

Static fatigue
Fatigue is chemically-enhanced, rate-dependent crack growth in
the presence of moisture and cyclic application of stresses.
Water enters incipient fissures
Breaks down cohesive bonds holding the crack walls together
Results in initiation of slow crack growth which progresses steadily
over time
Accelerating at higher stress levels and ultimately leading to failure.

 Delayed failure in glasses had been attributed to a stress
enhanced chemical reaction between glass and water this is
likely to occur primarily at the tips of the surface cracks.
 Structural Defects may arise in the form of micro-cracks of
sub-millimeter scale; during fabrication of ceramic
prostheses and also from application of masticatory forces
in the oral cavity.
 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.

Optical properties:
 Translucency is another critical property of dental
porcelains. Incisal porcelains >body >opaque porcelains.
 Dental porcelains are translucent because there are no free
electrons and can be colored by pigments such as metallic
oxides to match the shade of teeth. Since the outer layers of
a porcelain crown are translucent, the apparent color is
affected by reflectance from the inner opaque or core
porcelain.
 The thickness of the body porcelain layer determines the
color obtained with a given opaque porcelain.
 The colours of commercial premixed dental porcelains are
in the yellow to yellow-red range.

 Dental ceramics can be classified according to one or
more of the following parameters:
Classification of dental ceramics
Uses or
indications
a) anterior and posterior
crown
b) veneer
c) post and core
d) fixed dental prosthesis
e) ceramic stain
f) glaze
a) Ultralow fusing -<850o C
b) Low fusing -850-1100o C
c) Medium fusing-
1101-1300o C
d) High fusing - >1300o C
Firing
temperature

a) Casting
b) Sintering
c) Partial sintering and glass
infiltration
d) Slip casting and sintering
e) Hot-isostatic pressing,
f) CAD-CAM milling
g) Copy-milling
Processing
method
Principal
crystal phase
a) Silica glass
b) Leucite based feldspathic
porcelain
c) Leucite-based glass-
ceramic
d) Lithia disilicate–based
glass-ceramic
e) Aluminous porcelain
f) Alumina
g) Glass-infused alumina,
h) Glass-infused spinel
i) Glass-infused
alumina/zirconia,
j) Zirconia

Translucency
a) Opaque
b) Translucent
c) Transparent
a) Amorphous glass
b) Crystalline
c) Crystalline particles in a
glass matrix
Microstructure

 These materials can be formed into inlays, onlays,
veneers, crowns, and more complex fixed dental
prostheses (FDPs).
 Several of the core ceramics can be resin-bonded
micromechanically to tooth structure.
 Zirconia can be used for endodontic posts and implant
abutments but their primary applications are for crowns
and bridges.
 Zirconia is better suited for applications involving
posterior teeth or for elderly patients whose teeth have
lost much of their original translucency.
Applications of ceramics in dentistry

Type Primary Application Secondary Applications Contraindications
Feldspathic
porcelain
• Metal-ceramic
veneers
• Anterior laminate
veneers
•Single surface
inlays/Low stress sites
• High translucency
needed
•Inlays, onlays,
crowns, and bridges
(except as metal-
ceramic veneers)
Aluminous
porcelain
Core ceramic for
anterior crowns
Low-stress premolar
crowns
•Molar crowns
•Bridges
Leucite glass-
ceramic
•Anterior single-unit
crowns
•Anterior laminate
veneers
•Low-stress premolar
inlays and crowns
•High translucency
needed
•High-stress
situations
•Bridges
Lithium
disilicate glass-
ceramic
•Anterior and
premolar crowns
•Anterior three-unit
bridges Premolar
crowns
•Anterior laminate
veneers
•Posterior three-unit
bridges
•High-stress
posterior situations
• Bridges involving
molar teeth
General Indications and Contraindications for Use of Dental Ceramics

Benefits of metal-ceramic prostheses
 The most outstanding advantage of metal-ceramic
restorations is their resistance to fracture.
 With metal occlusal surfaces, the fracture rate in posterior
sites could be reduced further.
 Advantage of metal-ceramic restorations over total ceramic
restorations is that less tooth structure needs to be removed
to provide the proper bulk for the crown.
Metal-ceramic systems: Composition and
Properties

CERAMIC COMPOSITION
a) A silica (SiO2) network and potash feldspar
(K2O•Al2O3•6SiO2), soda feldspar (Na2O•Al2O•6SiO2), or
both.
b) Feldspathic porcelains contain, by weight, a variety of oxides
including a SiO2 matrix (52% to 65%), Al2O3 (11% to 20%),
K2O (10% to 15%), Na2O (4% to 15%), and certain additives,
including B2O3, CeO2, Li2O, TiO2, and Y2O3.
a) When potassium feldspar is mixed with various metal oxides
and fired to high temperatures, it 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 to coalesce.

 These ceramics are called porcelains because they contain a
glass matrix and one or more crystal phases.
 When feldspar is heated at temperatures between 1150 °C
and 1530 °C, 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 controls thermal expansion during the use of
porcelains for metal bonding.

 Silicate glass represents the matrix phase of feldspathic
porcelains. Silica (SiO2) can exist in four different forms:
i. Crystalline quartz
ii. Crystalline cristobalite
iii. Crystalline tridymite
iv. Noncrystalline fused silica
 The abrasiveness of the finished surface will depend on the
thickness of the veneer and the presence or absence of
crystalline particles.

 Veneering ceramics (“porcelains”) for metals have higher
expansion and contraction coefficients than the ceramics
used to veneer alumina or zirconia core ceramics. There
are four types of veneering ceramics. Feldspathic
porcelains include
I. Ultralow- and low-fusing ceramics (feldspar-based
porcelain, nepheline syenite−based porcelain, and
apatite-based porcelain);
II. Low-fusing specialty ceramics (shoulder porcelains and
wash-coat ceramics);
III. Ceramic stains
IV. Ceramic glazes (autoglaze and overglaze).

GLASS MODIFIERS
 Can be defined as elements that interfere with the integrity
of the SiO 2 (glass) network and alter their three
dimensional state. Their functions are:
 to decrease the softening point
 decrease the viscosity (flux action increasing the flow)
 The main purpose of a flux is principally to lower the
softening temperature of a glass by reducing the amount of
cross-linking betweent he oxygen and glass forming
elements. E.g. Alkali metal ions such as Na, K or Ca
(usually as carbonates).

 However, higher concentration of glass modifiers could
result in :
 Reduced chemical durability (resistance to attack by water,
acids and alkalis)
 Devitrification due to disruption of too many tetrahedral
networks(crystallization occurs when the modifiers act as
nucleating agents for the process of crystal growth)
 Manufacturers employ glass modifiers to produce dental
porcelains with different firing temperatures such as high
medium and low fusing ceramics

 Boric oxide fluxes (B2O3) can behave as a glass modifier
to decrease viscosity, to lower the softening temperature,
and to form its own glass network.
 Incorporation of an intermediate oxide such as alumina (Al
2 O 3 ) increase hardness and viscosity of glass.
 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 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.

 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 blended with the unpigmented
powdered frit to provide the proper hue and chroma.
 Examples of metallic oxides and their respective color
contributions to porcelain include :
 Chromium or chrome (Pink)-aluminia: these pigments are
stable upto 13500C, and are useful in eliminating the
greenish hue andgiving a warm tone to the porcelain.

 Iron oxide (black) or platinum (Grey): used for producing
enamels or grayer section of the dentin colors, and also
for an effect of translucency.
 Cobalt salts in the form of oxide (Blue): are useful in
developing of the enamel shades.
 Other pigments used may be :
o Titanium oxide –yellow brown,
o Manganese oxide- lavender
o Iron/nickel oxide-brown
o Copper oxide – green.

 Opacity may be achieved by the addition of cerium oxide,
zirconium oxide, titanium oxide, or tin oxide to alter the
softening point and viscosity.
 Medium- and high-fusing porcelains are used for the
production of denture teeth.
 The low-fusing and ultralow-fusing types are used as
veneering ceramics for crown and bridge construction.
 Some of the ultralow-fusing porcelains are used for
titanium and titanium alloys because of their low
contraction coefficients that closely match those of the
metals and because the low firing temperatures reduce the
risk for growth of the metal oxide.

 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
cause localized softening of the glass phase.
 A higher proportion of glass modifiers tend to reduce the
resistance of the applied glazes to leaching by oral fluids.

METAL COMPOSITION
 Single-unit crowns and bridges may be made from
metalceramic systems (combinations of metal substructure
and veneering ceramic.
 Many alloys are available to be veneered with low-fusing
and ultralow-fusing porcelains.
 The compositions of these high noble, noble,
predominantly base metal alloys control the castability,
bonding ability to porcelain, the esthetics of the metal-
ceramic restoration, and the magnitudes of stresses that
develop in the porcelains during cooling from the sintering
temperature.

Classification
According to noble metal content, metal ceramics are broadly
classified by the ADA (1984) into 3 major categories:
 High Noble alloys
 Noble alloys
 Base-metal alloys
Generally classified into two general categories (Anusavice
1996)
Alloys - Noble metal alloys
a) Gold - Platinum
b) Gold – Platinum - Silver
c) Gold - Palladium
d) Palladium - silver
e) High palladium

System - Base-metal alloys
a) Nickel - Chromium
b) Cobalt - Chromium
c) Other systems
Characteristic features and Advantages of Base Metal
Alloys positive features:
 Higher hardness and elastic modulus (stiffness) values
permit the fabrication of thinner copings (upto 0.1mm) and
thus its use in long span FPD’s.
 More sag - resistant at elevated temperatures.
 Substantial cost difference between base - metal and noble
metal alloys.

Limiting features :
 Higher solidification shrinkage requires special
compensatory procedures to obtain acceptable fitting.
 Potential for porcelain delimitation due to separation of
poorly adherent oxide layer from the metal substrate.
 Scrap cannot be used.
 Potential toxicity of Beryllium and allergic potential of
Nickel.
 Poor resistance to tarnish and corrosion of nickel
containing alloys.

Components of metal-ceramic
restoration

CAST METAL FOR METAL-CERAMIC PROSTHESES
 To bond to alloys suitable for the copings, porcelains must
have a sufficiently
o Low sintering temperature
o CTEs and CTCs that are closely matched to those of the
alloys.
 The gold alloys developed for porcelain bonding have
higher melting ranges than typical gold alloys for all-metal
prostheses; the higher melting ranges are necessary to
prevent sag, creep, or melting of the coping or framework
during the sintering and/or glazing of porcelain.
Fabrication of metal-ceramic
prostheses

 These gold alloys contain small amounts (about 1%) of
base metals such as iron, indium, and tin.
 Both the metal and the ceramic must have coefficients of
thermal expansion and contraction that are closely matched
such that the metal must have a slightly higher value to
avoid the development of undesirable residual tensile
stresses in the porcelain.

CAST METAL COPINGS AND FRAMEWORKS
 Copings and frameworks for metal-ceramic prostheses are
produced by:
a) Casting of molten metal
b) CAD-CAM machining
c) Electrolytic deposition techniques
d) Swaged metal processes.
 The most common method is the melting and casting of
specialized metals for the casting process, the relatively
high melting temperatures of most alloys can break down
gypsum-bonded investments at the casting temperatures,
so the more refractory phosphate-bonded investment
must be used.

 Oil from fingers and other sources such as air lines
represents a possible contaminant.
 The surface may be cleansed 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.
 Opaque porcelain is condensed on the oxidized surface at a
thickness of approximately 0.3 mm and is then fired to its
sintering temperature. Translucent porcelain is then
applied, and the tooth form is created.

Oxidizing
The base metals form a surface
oxide layer during the oxidation
treatment, and this surface oxide
is responsible for development
of a bond with porcelain. This
process is sometimes called
degassing.
Controlled oxide layer should
be created .
59

Methods of 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 binder and condensed
into the desired form.
The porcelain is usually built to shape using a liquid binder
to hold the particles together. This process of packing the
particles and removing the liquid is known as condensation.
Proper and thorough condensation is also crucial in
obtaining dense packing of the powder particles.
 This provides two benefits:
a) Lower firing shrinkage
b) Less porosity in the fired porcelain.
60

This packing, or condensation, may be achieved by:
Vibration:
Mild vibrations are used to densely pack the wet powder upon
the underlying matrix. The excess water comes to the surface and
is blotted with a tissue paper.
Spatulation:
A small spatula is used, to apply and smoothen the wet porcelain.
This action brings excess water to the surface where it is
removed.
Brush technique:
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.

Types of binders:
Distilled water: Is the most popular binder used in dentin and
enamel porcelain.
Propylene glycol: Used in alumina core build up.
Alcohol or formaldehyde based liquid for opaque / core build
up.
62

Building porcelain:
1. The powder is mixed on a glass slab.
2. The mix should not be overstored to avoid the incorporation
of large air bubbles.
3. High room temperature and dry atmosphere is to be avoided
as the powder can dry out rapidly due to which all spaces are
created in the powder bed.

Firing dental porcelain:
After the condensation and building of a crown it is fired to
high density and correct form. At this stage the green
porcelain is introduced into the hot zone of the furnace and
the firing starts, the glass particles soften at their contact
areas and fuse together. This is often referred to as sintering.
64

 The condensed porcelain mass is placed in front of or
below the muffle of a preheated furnace at approximately
650 °C for low-fusing porcelain.
 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.
 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.
 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.
 During vaccum firing ,porcelain 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.
66

 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.
67

 Advantages of vacuum-fired porcelain
◦ There is a general increase in the strength of the
porcelain, which probably is more significant in jacket
crowns than bonded veneers.
◦ The porcelain will have greater translucence.
◦ Porcelain for vacuum firing can have a finer and graded
particle size, thus increasing the wet strength of the
materials and making it less difficult to carve a built-up
mass.
◦ Shade is markedly affected by vacuum firing. The
lessened number of air spaces decreases the internal
reflective surfaces. Thus, with opacity reduced and
density increased, it becomes impossible to reproduce
precisely the shades made with air firing.

Classification of the Stages in Maturity:
Low Bisque:
The surface of the porcelain is very porous and will easily
absorb a water soluble die. At this stage the grains of
porcelain will have started to soften. Shrinkage will be
minimal and the fired body is extremely weak and friable.
Lack translucency and glaze.
Medium bisque:
The surface will still be slightly porous but the flow of the
glass grains will have increased. A definite shrinkage will
have taken place. Lacks translucency and high glaze.
High bisque:
The surface of the porcelain would be completely sealed
and presents a much smoother surface with a slight shine.
shrinkage is complete. Appears glazed.

Glazing
Porcelains are glazed to give a smooth and glossy surface,
enhance, esthetics and promote hygiene. The glazing should be
done only on a slightly roughened surface and never should be
applied on glazed surfaces.
Over glaze:
These are ceramic powders containing more amount of glass
modifiers thus lowering fusion temperature. It may be applied
to porcelain and then fired.
Self glaze:
All the constituents on the surface are melted to form a molten
mass about 25 μm thick. Thus the porcelain is said to be self
glazed.
71

 Autoglazed feldspathic porcelain is stronger than unglazed
porcelain. The glaze is effective in sealing surface flaws and
reducing stress concentrations.
 If the glaze is removed by grinding, the transverse strength
is reduced and if this surface is left in rough condition it can
cause increased wear of enamel.
72

Add on porcelains
The add on porcelains are made from similar materials to
glaze porcelain except for the addition of opacifiers and
coloring pigments.
These are sparingly used for simplest corrections like
correcting of tooth contour / contact points.
73

METAL-CERAMIC CROWNS AND BRIDGES BASED ON
SWAGED METAL FOIL LAMINATES
 The most widely used product of this type has been Captek
(Precious Chemicals Co., Inc., Altamonte Springs, FL), which is
an acronym for “capillary assisted technology.”
 The product is designed to fabricate the metal coping of a metal-
ceramic crown without the use of a melting and casting process.
 It is a laminated gold alloy foil sold as a metal strip.
 For bridges, the pontics are made typically from a palladium-
based alloy that is gold-coated. The technology is based on the
principle of capillary action to produce a gold-based composite
metal.

 Captek P and G metals can yield thin metal copings for
crowns or frameworks for metal-ceramic bridges. The
maximal span length recommended for Captek-porcelain
bridges is 18 mm, which allows space for up to two
pontics.

FABRICATION BY CAPTEK
Captek™ alloys are composed of two major components:
1. The first component, when heated, forms a microscopic
three-dimensional network of capillaries.
2. The second, when melted, flows to fill these capillaries.
This microscopic process works by the forces of capillary
attraction to produce a solid-metal composite alloy .
 The process begins with the preparation of a master
refractory die that replicates the prepared tooth.
 The die is heat-treated, and the margins are marked with a
red pencil.

 Captek adhesive is applied to the die to enhance adhesion
to the Captek metal and to enhance capillary action.
 After heat treatment, Captek P metal, a malleable Au-Pt-Pd
alloy, is adapted to the surface.
 This metal layer provides a three-dimensional capillary
network that will subsequently be filled with Captek G
metal (97.5% Au, 2.5% Ag by weight) to form an alloy with
a high gold content.
 After this composite material is burnished on the die and
the margins are trimmed, it is sintered in a porcelain
furnace.

 The metal copings and Pd-Ag pontics (if needed) are
then coated with a slurry of Au, Pt, and Pd powder
(Capbond) and liquid, resulting in a thin coating of gold
to enhance areas of Captek P that have been ground
during adjustment and to provide a gold color similar
to that of areas that have not been ground.
 The completed copings have a thickness of
approximately 0.25 mm. Thus, this method provides
thinner metal copings than those (0.50 mm) typically
produced by the cast-metal process.
 The metal surfaces are veneered with two thin coats of
an opaque porcelain and additional layers of
translucent porcelain.

 The atomic bonding of veneering ceramics to Captek copings
and frameworks is controlled by a special bonder layer, while
bonding of ceramics to cast metal is controlled by oxidizable
elements within the alloy surface.
 Another potential concern for the Captek system is the
difficulty of bonding the dissimilar metals in the coping and
the pontic surfaces.
 Clinical data for Captek crowns and bridges are very limited.
Thus, caution must be exercised in using this system for
crowns and bridges in high-stress areas.

BONDING MECHANISMS
Three mechanism have been described to explain the bond
between the ceramic veneer and the metal substructure.
1. Mechanical entrapment
2. Compressive forces
3. Chemical bonding
80

Mechanical entrapment:
This creates attachment by interlocking the ceramic into the
microabrasions on the surface of the metal coping which are
created by finishing the metal with non contaminating stones
/ discs and are abrasives.
Air abrasion appears to enhance the wettability, provide
mechanical interlocking.
The use of a bonding agent having platinum spheres 3-6 μm
in diameter can also increase the bond significantly.

Compressive forces:
These are developed by a properly designed coping and a
slightly higher coefficient of thermal expansion than the
porcelain veneered over it.
This slight difference will cause the porcelain to draw
towards the metal coping when the restoration cools after
firing.
Chemical bonding
It is indicated by the formation of an oxide layer on the
metal. The trace elements like tin, indium, gallium/iron
form oxides and bond to similar oxides in the opaque layer
of the porcelain.

 The principal deficiencies faced by ceramics are -
brittleness, low fracture toughness and low tensile strength.
Methods used to overcome the deficiencies fall into 2
general categories:
1. Method of strengthening brittle materials.
2. Method of designing components to minimize the stress
concentrations and tensile stresses.
Methods for strengthening ceramics

In the oral environment tensile stresses are usually created by
bending forces, and the maximum tensile stresses occur at the
surface of the restoration.
It is for this reason removal of the surface flaws can result in
the increased strength of the material. Smoothing and reducing
flaws is one of the reason for glazing of dental porcelain.
Strengthening of the brittle materials can be done in a 2 ways.
a) Development of residual compressive stresses within the
surface of the material.
b) Interruption of crack propagation through the material.
I. Method of strengthening materials:

1. Development of residual compressive stresses within
the surface of the material:
One widely used method of strengthening ceramics is the
introduction of residual compressive stresses.
Strength is gained by virtue of the fact that the residual
stresses developed must first be negated by the
developing tensile stresses before a net tensile stress
develops in the material.
THREE of the methods used in achieving this objective
are:

a. Ion exchange mechanism:
 This technique is called as chemical tempering and is the
most sophisticated and effective way of introducing residual
compressive stresses.
 In this procedure a sodium containing glass is placed in a
bath of molten potassium nitrate, potassium ions in the bath
exchange places with some of the sodium ions in the surface
of the glass particle.
 The potassium ion is about 35% larger than the sodium ion.
The squeezing of the potassium ion into place formerly
occupied by sodium ion creates large residual compressive
stresses in the surface of the glass. These residual stresses
produce a strengthening effect.
 This process is best used on the internal surface of the
crown, veneer/inlay as the surface is protected from
grinding and exposure to acids.

The technique is as follows:
a) Characterize the finished crown and adjust the
occlusion.
b) Place the crown into a mould of analytically pure
potassium nitrate powder which is in a small
porcelain crucible/ stainless steel container.
c) Place the container in a cool furnace and raise the
temperature slowly to 500°C
d) Hold the temperature at 500 C for 6 hours.
e) Remove the crown from the solution and allow it to
drain in the furnace. Remove the crown from the
furnace and cool to room temperature.

b. Thermal tempering:
 This is the most common method of strengthening glass.
 This creates residual surface compressive stresses by
rapidly cooling (quenching) 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 solidifying molten core as it shrinks,
creates residual tensile stresses in the core and residual
compressive stresses within the outer surface.

 For dental applications it is more effective to quench the
glass phase ceramics in silicone oil or other special liquids
than using air as it may not uniformly cool the surface.
c.Thermal compatibility method applies to porcelain
fused metals. The metal and porcelain should be
selected with slight mismatch in their thermal
contraction coefficient.
 Usually the difference of 0.5 × 10–6/°C in thermal
expansion between metals and porcelain .
 It causes the metal to contract slightly more than does
the ceramic during cooling after firing the porcelain
which results in development of residual compression
in the ceramic surface

2) Interruption of crack propagation-
a) Dispersion of crystalline phase –
 Crystalline reinforcement:
◦ A method of strengthening glasses and ceramics is to
reinforce them with a dispresed phase of different material
that is capable of hindering crack propagation through the
material.
◦ The crystalline phase with greater thermal expansion
coefficient than the matrix produces tangential compressive
stress (and radial tension) near the crystal matrix interface.
Such tangential stresses divert the crack around the particle.
92

◦ When a tough, crystalline material such as a alumina in
particulate form is added to a glass, the glass is
toughened and strengthened because the crack cannot
penetrate the alumina particles as easily as it can the
glass and this technique is applied in the development of
aluminous porcelains for PJCs.
◦ Another ceramic dental material that uses reinforcement
of a glass by a dispersed crystalline substance is Dicor
glass-ceramic.

94
b) Transformation toughening-
◦ A newer technique of strengthening glasses involves the
incorporation of a crystalline material that is capable of
undergoing a change in crystal structure when placed
under stress.
◦ The crystalline material usually used is termed partially
stabilized Zirconia (PZC).
◦ The energy required for the transformation of PSZ is
taken from the energy that allows the crack to propagate.
◦ One drawback of PSZ is an opacifying effect that may not
be aesthetic in most dental restorations.

 Tetragonal phase is not stable at room temperature and it
can transform to the monoclinic phase leading to a
corresponding volume increase.
 When sufficient stress develops in the tetragonal structure
and a crack in the area begins to propagate, the metastable
tetragonal crystals (grains) precipitates next to the crack tip
can transform to the stable monoclinic form.

The design should avoid exposure of ceramics to high tensile
stresses. It should also avoid stress concentration at sharp
angles or marked changes in thickness.
a) Minimizing tensile stresses:
When porcelain is fired onto a rigid material the shape of
the metal will influence the stresses set up in the porcelain.
If it is a full coverage crown 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.
Methods of designing components to minimize stress
concentrations and tensile stresses

For partial metal coverage the junction between the metal
and porcelain is therefore a potential site for high stress as
the area with only metal will have no balancing compressive
forces.
b.Reducing stress raisers
Stress raisers are discontinuities in ceramic structures in
brittle materials that cause stress concentration.
Abrupt changes in shape/ thickness in the ceramic contour
can act as stress raisers and make the restoration more prone
to failure.
Sharp line angles in preparation and small particle of
porcelain along internal margin of crown also causes tensile
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.
 These contact stresses can lead to the formation of the so-
called Hertzian cone cracks, which may lead to chipping
of the occlusal surface.

Benefits and drawbacks of metal-ceramic restorations
1. A properly made metal-ceramic crown is more
fracture resistant and durable than most all-ceramic
crowns and bridges.
2. A metal coping or framework provides an advantage
compared with zirconia-based ceramic prostheses
when endodontic access openings through crowns are
required.
3. 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.

Disadvantages of metal-ceramic prostheses
 Abrasive damage to opposing dentition
 Potential for fracture
 Excessive exposure to acidulated fluoride can enhance
chemical degradation of ceramic surface.
 Patient may be exposed to silaceous dust by inhalation
during grinding.
 The potential for metal allergy.

 Not the best esthetic choice for restoring a single
maxillary anterior tooth.
 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.

References
 Phillips science of dental materials –First South Asia edition
 Craig’s Restorative dental materials –13th edition.
 W. Patrick Naylor,Introduction to Metal – Ceramic
Technology – Second edition
 William J.O Brien, Dental materials and their selection- 3rd
edition.
 Kelly JR. Nishimura I. Campbell SD. Ceramics in dentistry:
Historical roots and current perspectives. J prosthet dent
1996:75 18-32.

 Kelly J. Dental ceramics:current thinking and trends.
Dent Clin N Am 2004(48):513-530.
 Babu PJ, Krishna R .AllDental Ceramics: Part I – An
Overview of Composition, Structure and Properties .
American Journal of Materials Engineering and
Technology, 2015(3)1: 13-18

Dental ceramics

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