Dental casting alloys /certified fixed orthodontic courses by Indian dental academy

DENTAL CASTING
ALLOYS

INDIAN DENTAL ACADEMY
Leader in Continuing Dental Education
www.indiandentalacademy.com

INTRODUCTION
In dentistry, metals represent one of the three
major classes of materials used for the reconstruction
of damaged or missing oral tissues. Although metals
are readily distinguished from ceramics and polymers.


The wide varieties of complex dental alloy
compositions consist of the following:
Dental amalgams containing the major elements
mercury, silver, tin, and copper.
Noble metal alloys in which the major elements
are some combination of gold, palladium, silver
and important secondary elements including
copper, platinum, tin, indium and gallium.
Base metal alloys with a major element of nickel,
cobalt, iron or titanium and many secondary
elements that are found in the alloy compositions.


HISTORY OF METALS IN DENTISTRY
Dentistry as a specialty is believed
to have begun about 3000 BC. Gold bands
and wires were used by the Phoenicians
after 2500 BC.
Modern dentistry began in 1728
when Fauchard published different
treatment modalities describing many types
of dental restorations, including a method
for the construction of artificial dentures
made from ivory. Gold shell crowns were
described by Mouton in 1746 but they were
not patented until in 1873 by Beers. In
1885 Logan patented porcelain fused to
platinum post replacing the unsatisfactory
wooden post previously used to build up
intra-radicular areas of teeth. In 1907 a
detached post crown was introduced which
was more easily adjustable.

Year Event
1907 Introduction of Lost-Wax Technique
1933 Replacement of Co-Cr for Gold in
Removable Partial Dentures
1950 Development of Resin Veneers for Gold Alloys
1959 Introduction of the Porcelain Fused-to-Metal
Technique
1968 Palladium-Based Alloys as Alternatives to Gold
Alloy
1971 Nickel-Based Alloys as Alternatives to Gold Alloys
1980s Introduction of All-Ceramic Technologies
1999 Gold Alloys as Alternatives to Palladium-Based
Alloys


1971 – THE GOLD STANDARD
The United States abandoned the gold
standard in 1971. Gold then became a commodity
freely traded on the open markets. As a result, the
price of gold increased steadily over the next nine
years. In response to the increasing price of gold,
new dental alloys were introduced through the
following changes:
In some alloys, gold was replaced with
palladium.
In other alloys, palladium eliminated gold
entirely.
Base metal alloys with nickel as the
major element eliminated the exclusive need for
noble metals.

KEY TERMS
Grain–A microscopic single crystal in the microstructure
of a metallic material.
Metal – An element whose atomic structure readily
loses electrons to form positively charged ions, and
which exhibits metallic bonding (through a spatial
extension of valence electrons), opacity, good light
reflectance from a polished surface and high electrical
and thermal conductivity.
Noble metal – which are highly resistant to oxidation
and dissolution in inorganic acids. Gold and platinum
group metals (Platinum, palladium, rhodium, ruthenium,
iridium and osmium).
Base metal – A metal that readily oxidizes or dissolves
to release ions.

Alloy – A crystalline substance with metallic
properties that is composed of two or more
chemical elements, at least one of which is
metal.
Solid solution (metallic) – A solid crystalline
phase containing two or more elements, at least
one of which is a metal, that are intimately
combined at the atomic level.
Liquidus temperature – Temperature at which an
alloy begins to freeze on cooling or at which the
metal is completely molten on heating.
Solidus temperature – Temperature at which an
alloy becomes solid on cooling or at which the
metal begins to melt on heating.

PERIODIC TABLE


Dimitri Ivanovich Mendeleyev
(1834-1934)


Of the 115 elements currently listed in most
recent versions of the periodic tables of the
elements, about 81 can be classified as metals.
(Additional elements that have been created with
nuclear reactors have short half-lives.) It is of
scientific interest that the metallic elements can be
grouped according to density, ductility, melting point
and nobility. This indicates that the properties of
metals are closely related to their valence electron
configuration. The groupings of pure metal elements
can be seen in the periodic chart of the elements.
Several metals of importance for dental alloys are
transition elements, in which the outermost electron
subshells are occupied before the interior subshells
are completely filled.

INTERATOMIC PRIMARY BONDS
The forces that hold atoms together
are called cohesive forces. These
interatomic bonds may be classified as
primary or secondary. The strength of these
bonds and their ability to reform after
breakage determine the physical properties
of a material. Primary atomic bonds may be
of three different types.
1. Ionic
2. Covalent
3. Metallic

1. IONIC BOND FORMATION
Characterized by electron
transfer from one element (positive)
to another (negative).


2. COVALENT BOND
FORMATION
Characterized by electron
sharing and very precise bond
orientations.


3. METALLIC BOND FORMATION
Since the outer-shell valence electrons can be
removed easily from atoms in metals, the nuclei containing
the balance of the bound electrons form positively charged
ionic cores. The unbound or free valence electrons form a
“cloud” or “gas”, resulting in electrostatic attraction between
the free electron cloud and the positively charged ionic
cores. Closed-shell repulsion from the outer electrons of the
ionic cores balances this attractive force at the equilibrium
interatomic spacing for the metal.


The free electrons act as conductors of both
thermal energy and electricity. They transfer energy
by moving readily from areas of higher energy to
those of lower energy, under the influence of either
a thermal gradient or an electrical field (potential
gradient). Metallic bonding is also responsible for
the luster or mirror-reflecting property, of polished
metals and their typical capability of undergoing
significant permanent deformation (associated with
the properties of ductility and malleability) at
sufficiently high mechanical stresses. These
characteristics are not found in ceramic and
polymeric materials in which the atomic bonding
occurs through a combination of the covalent and
ionic modes.www.indiandentalacademy.com

INTERATOMIC SECONDARY BONDS
In contrast with primary bonds, secondary bonds do
not share electrons. Instead, charge variations among
molecules or atomic groups induce polar forces that attract
the molecules.

VAN DER WAALS FORCES
Fluctuating dipole that binds inert gas molecules
together. The arrows show how the fields may fluctuate so
that the charges become momentarily positive and negative.


PHYSICAL PROPERTIES
Stress
When a force is applied to a material there is a
resistance in the material to the external force.
The force is distributed over an area and the
ratio of the force to the area
is called stress.
STRESS= F/A

Strain
The change in length or deformation per unit
length when a material is subjected to a stress
is defined as strain.

STRESS vs STRAIN CURVE
If one plots stress vs. strain on a graph, a stress-
strain curve will result. The properties of various dental
materials, such as alloys, can be compared by analysis
of their respective stress-strain curves.

P = Elastic modulus or
Proportional Limit

Y-X curve = Yield Strength

X = Ultimate Strength


STRENGTH
It is the maximal stress required to fracture a structure.
Types of Strength:
- Compressive
- Tensile
- Shearing


TOUGHNESS
It is defined as the energy required to fracture a
material. It is a property of the material which
describes how difficult the material would be to
break.

DUCTILITY
It is the ability of a material to withstand permanent
deformation under a tensile load without rupture.
A metal may be drawn readily into a wire and is
said to be ductile. Ductility is dependent on
tensile strength.

MALLEABILITY
It is the ability of the material to withstand rupture
under compression, as in hammering or rolling
into a sheet. It is not dependent on strength as is
ductility. www.indiandentalacademy.com

COEFFICIENT OF THERMAL EXPANSION
(Linear Coefficient Of Expansion )
Change in length per unit of original length of a material
when its temperature is raised 1 ° K


HARDNESS
In mineralogy the hardness is described on the
basis of the material to resist scratching. In
metallurgy and in most other fields, the amounts
of the resistance of indentation is taken as the
measure of hardness for the respective
material).
Brinell hardness number ( BHN )
Rockwell hardness number ( RHN )
Vickers hardness test (VHN )
Knoop hardness test ( KHN )


TARNISH AND CORROSION
High-noble alloys used in
dentistry are so stable chemically that
they do not undergo significant
corrosion in the oral environment; the
major components of these alloys are
gold, palladium and platinum. (Iridium,
osmium, rhodium and ruthenium are
also classified as noble metals.) Silver
is not considered noble by dental
standards, since it will react with air,
water and sulfur to form silver sulfide, a
dark discoloration product.
Gold resists chemical attack very
well. Thus it was natural that this most
noble metal was employed early in
modern dental history for the
construction of dental appliances.

TARNISH is observable as a surface discoloration on a metal, or
as a slight loss or alteration of the surface finish or luster. In the
oral environment, tarnish often occurs from the formation of hard
and soft deposits on the surface of the restoration. Calculus is the
principal hard deposit, and its color varies from light yellow to
brown. The soft deposits are plaques and films composed mainly
of microorganisms and mucin. Stain or discoloration arises from
pigment-producing bacteria, drugs containing such chemicals as
iron or mercury and adsorbed food debris.

CORROSION is not merely a surface deposit. It is a process in
which deterioration of a metal is caused by reaction with its
environment. Frequently, the rate of corrosion attack may actually
increase over time, especially with surfaces subjected to stress,
with intergranular impurities in the metal or with corrosion products
that do not completely cover the metal surface.
Sulfur is probably the most significant factor causing surface
tarnish on casting alloys that contain silver, although chloride has
also been identified as a contributor.

1. Chemical or Dry Corrosion

In this the metal reacts to form oxides, sulphides in the
absence of electrolytes

2. Electrochemical or Wet Corrosion


a. Galvanic corrosion b. Heterogeneous Surface Composition

Difference in potential

c. Stress Corrosion

Occurs due to fatigue or cyclic loading


d. Concentration Cell Corrosion or Crevice Corrosion

• Pitting type (Oxygen concentration cell)

• Cervical type (Electrolyte concentration cell)


PROTECTION AGAINST CORROSION
i. Passivation
ii. Increase noble metal content
iii. Polishing restorations
iv. Avoid dissimilar metal restorations

Certain metals readily form strong adherent oxide film on
their surface, which protects them from corrosion. Such a metal
is said to be passive. Chromium, titanium and aluminium are
examples of such metals. Since this film is passive to oxidative
chemical attack, their formation is called passivation.

 Chromium provides this corrosion resistance by forming a
very thin, adherent surface oxide that prevents the diffusion of
oxygen or other corroding species to the underlying bulk metal.If
more than 12% Cr is added to iron or cobalt, we get stainless
steel or cobalt chromium alloys, which are lightly corrosion
resistant and therefore suitable for dental use.

 Noble metals resist corrosion
because their electromotive force is
positive with regard to any of the
common reduction reactions found in
the oral environment. In order to
corrode a noble metal under such
conditions, an external current (over
potential) is required.

 At least half the atoms should be
noble metals (gold, platinum, and
palladium) to ensure against corrosion.
Palladium has been found to be
effective in reducing the susceptibility
to sulfide tarnishing for alloys
containing silver.

SOLIDIFICATION
AND
CRYSTALLIZATION OF
METALS


SOLIDIFICATION OF METALS

The temperature decreases steadily from point A to
point B' . An increase in temperature then occurs from point
B' to point B, at which time the temperature remains
constant until the time indicated at point C is reached.
Subsequently, the temperature of the metal decreases
steadily to room temperature.


The temperature Tf, as indicated by the straight or
“plateau” portion of the curve at BC, is the freezing point, or
solidification temperature of the pure metal. This is also the
melting point, or fusion temperature. During melting, the
temperature remains constant. During freezing or
solidification, heat is released as the metal changes from
the higher-energy liquid state to the lower-energy solid
state.
The initial cooling of the liquid
metal from Tf to point B' is termed
super cooling. During the super
cooling process, crystallization
begins for the pure metal. Once the
crystals begin to form, release of the
latent heat of fusion causes the
temperature to rise to Tf where it
remains until crystallization is
completed atwww.indiandentalacademy.com
point C.

CRYSTALLIZATION OF METALS
Characteristically, a
pure metal crystallizes from
nuclei in a pattern that often
resembles the branches of a
tree, yielding elongated
crystals that are called
Dendrites. In three
dimensions, their general
appearance is similar to that
of the two dimensional frost
crystals that form on a
window pane in the winters.

Extensions or elevated areas
(termed protuberances) form
spontaneously on the
advancing front of the
solidifying metal and grow into
regions of negative temperature
gradient. Secondary and
tertiary protuberances result in
a three dimensional dendritic
structure.


Although dental base metal casting alloys typically
solidify with a dendritic micro-structure, most nobel metal
casting alloys solidify with an Equiaxed polycrystalline
microstructure. The microstructural features in this
figure are called grains, and the term Equiaxed means
that the three dimensions of each grain are similar, in
contrast to the elongated morphology of the dendrites.


All modern noble metal alloys are fine
grained. Smaller the grain size of the metal, the
more ductile and stronger it is. It also produces a
more homogenous casting and improves the
tarnish resistance. A large grain size reduces the
strength and increases the brittleness of the
metal. Factors controlling the grain size are the
rate of cooling, shape of the mold, and
compositon of the alloy.


PHYSICAL PROPERTIES
AND EFFECT OF
NOBLE METALS AND
BASE METALS ON DENTAL
CASTING ALLOYS


NOBLE METALS
The noble metals have been the basis of inlays,
crowns and bridges because of their resistance to corrosion
in the oral cavity.
Gold, platinum, palladium, rhodium, ruthenium,
iridium, osmium, and silver are the eight noble metals.
However, in the oral cavity, silver is more reactive and
therefore is not considered as a noble metal.
Of the eight noble metals, four are of major
importance in dental casting alloys, i.e., gold, platinum,
palladium and silver. All four have a face-centered cubic
crystal structure and all are white coloured except for gold.


GOLD

Pure gold is a soft and
ductile metal with a yellow
“Gold” hue. It has a density
of 19.3 gms/cm3 , melting
point of 1063oC, boiling point
of 2970 oC and CTE of
14.2×10-6/°C. Gold has a
good luster and takes up a
high polish. It has good
chemical stability and does
not tarnish and corrode.


Gold content:
Traditionally the gold content of dental casting alloys
have been referred to in terms of:
• Karat
• Fineness
Karat:
It is the parts of pure gold in 24 parts of alloys.
For Eg: a) 24 Karat gold is pure gold
b) 22 Karat gold is 22 parts of pure gold
and remaining 2 parts of other metal.
The term Karat is rarely used to describe gold content in
current alloys.
Fineness:
Fineness of a gold alloy is the parts per thousand of
pure gold. Pure gold is 1000 fine. Thus, if ¾ of the gold
alloy is pure gold, it is said to be 750 fine.


Physical and mechanical properties of cast pure gold,
gold alloys, and condensed gold foil

Material Density Hardness (VHN/BHN) Tensile Elongation
(g/cm3) (kg/mm2) Strength (%)
(MPa)
Cast 24k gold 19.3 28(VHN) 105 30
Cast 22k gold -- 60(VHN) 240 22
Coin gold -- 85 (BHN) 395 30
Typical Au-based 15.6 135/195(VHN) 425/525 30/12
casting alloy
(70 wt% Au)*
Condensed gold 19.1 60 (VHN) 250 12.8
foil

* Values are for softened / hardened condition.


SILVER
It is sometimes described
as the “Whitest” of all metals. It
whitens the alloy, thus helping
to counteract the reddish colour
of copper. To a slight extent it
increases strength and
hardness. In large amounts
however, it reduces tarnish
resistance. It has the lowest
density 10.4gms/cm3 and
melting point of 961oC, boiling
point of 2216 oC among the four
precious metals used in dental
casting alloys. Its CTE is
19.7×10-6/oC , which is
comparatively high.www.indiandentalacademy.com

PLATINUM

It increases the strength and
corrosion resistance. It also
increases the melting point
and has a whitening effect on
the alloy. It helps to reduce
the grain size.It has the
highest density of 21.45
gms/cm3 , highest melting
point of 1769oC, boiling point
of 4530 oC and the lowest
CTE 8.9×10-6/oC among the
four precious metals used in
dental casting alloys.

PALLADIUM
It is similar to platinum in its
effect. It hardens as well as
whitens the alloy. It also raises
the fusion temperature and
provides tarnish resistance. It
is less expensive than
platinum, thus reducing cost of
alloy. It has a density of
12.02gms/cm3. Palladium has
a higher melting point of
1552oC, boiling point of 3980
o
C and lower CTE which is
11.8×10-6/oC, when compared
to gold.

IRIDIUM, RUTHENIUM
They help to decrease the grain size. They are added in
very small quantities (about 100 to 150 ppm). IRIDIUM
has a high melting point of 2454°C , boiling point of 5300
°C , density of 22.5gm/cm3 and CTE 6.8×10-6/oC.
RUTHENIUM has a melting point of 1966°C , boiling
point of 4500 °C , density of 12.44 gm/cm 3 and CTE
8.3×10-6/oC


BASE METALS
These are non-noble metals. They are invaluable
components of dental casting alloys because of their
influences on physical properties, control of the amount and
type of oxidation, or their strengthening effect. Such metals
are reactive with their environment, and are referred to as
‘base metals’. Some of the base metals can be used to
protect an alloy from corrosion (passivation). Although they
are frequently referred as non precious, the preferred term is
base metal.
Examples of base metals are chromium, cobalt,
nickel, iron, copper, manganese etc.


COBALT

Imparts hardness,
strength and rigidity to
the alloy . It has a high
melting point of
1495°C , boiling point
of 2900 °C , density of
8.85 gm/cm3 and CTE
13.8×10-6/oC


NICKEL
Cobalt and nickel are
interchangeable.It decreases
strength, hardness, modulus
of elasticity and fusion
temperature. It increased
ductility. Bio-incompatibility
due to nickel, which is the
most common metal to
cause Contact Dermatitis. It
has a melting point of
1453°C , boiling point of
2730 °C , density of 8.9
gm/cm3 and CTE 13.3×10-
/C
6 o www.indiandentalacademy.com

CHROMIUM
Its passivating effect
ensures corrosion resistance.
The chromium content is
directly proportional to tarnish
and corrosion resistance. It
reduces the melting point. Along
with other elements, it also acts
in solid solution hardening.
Thirty percent chromium is
considered the upper limit for
attaining maximum mechanical
properties. It has melting point
of 1875°C , boiling point of
2665 °C , density of 7.19
gm/cm3 and CTE 6.2×10-6/ oC

COPPER
It is the principal hardener. It
reduces the melting point and
density of gold. If present in
sufficient quantity, it gives the
alloy a reddish colour. It also
helps to age harden gold alloys.
In greater amounts it reduces
resistance to tarnish and
corrosion of the gold alloy.
Therefore, the maximum content
should NOT exceed 16%. It has
melting point of 1083°C , boiling
point of 2595 °C , density of
8.96 gm/cm³ and CTE 16.5
×10-6/°C . www.indiandentalacademy.com

ZINC
It acts as a scavenger for
oxygen. Without zinc the
silver in the alloy causes
absorption of oxygen
during melting. Later
during solidification, the
oxygen is rejected
producing gas porosities
in the casting. It has a
melting point of 420°C ,
boiling point of 906 °C ,
density of 7.133gm/cm3
and CTE 39.7×10-6/oC


MOLYBDENUM OR
TUNGSTEN

They are effective
hardeners. Molybdenum is
preferred as it reduces
ductility to a lesser extent
than tungsten.
Molybdenum refines grain
structure. It has melting
point of 2610°C , boiling
point of 5560 °C , density
of 10.22 gm/cm3 and CTE
4.9 ×10-6/oC

IRON,BERYLLIUM
They help to harden the metal ceramic gold - palladium
alloys, iron being the most effective. In addition, beryllium
reduces fusion temperature and refines grain structure . IRON
has melting point of 1527°C , boiling point of 3000 °C , density
of 7.87 gm/cm3 and CTE 12.3 ×10-6/oC .


GALLIUM

It is added to
compensate for the
decreased coefficient
of thermal expansion
that results when the
alloy is made silver
free. The elimination of
silver reduces the
tendency for green
stain at the margin of
the metal-porcelain
interface. www.indiandentalacademy.com

MANGANESE AND SILICON
Primarily oxide scavengers to prevent oxidation of
other elements during melting. They are also hardeners.
MANGANESE has melting point of 650°C , boiling point
of 1107 °C , density of 1.74 gm/cm 3 and CTE 25.2 ×10-
/ C , where as SILICON has melting point of 1410°C ,
6 o

boiling point of 2480 °C , density of 2.33 gm/cm 3 and CTE
7.3 ×10-6/oC .


CARBON:
Carbon content is most
critical. Small amounts may
have a pronounced effect on
strength, hardness and
ductility. Carbon forms
carbides with any of the
metallic constituents which is
an important factor in
strengthening the alloy.
However when in excess it
increases brittleness. Thus,
control of carbon content in the
alloy is important. It has
melting point of 3700°C ,
boiling point of 4830 °C ,
density of 2.22 gm/cm3 and
CTE 6 ×10-6/oC .

BORON

It is a deoxidizer
and hardener, but
reduces ductility.


ALLOYS


ALLOYS

The use of pure metals is quite limited in dentistry. To
optimize properties, most metals commonly used in engineering
and dental applications are mixtures of two or more metallic
elements or in some cases one or more metals and/or
nonmetals. They are generally prepared by fusion of the
elements above their melting points. A solid material formed by
combining a metal with one or more other metals or nonmetals
is called an alloy.
For example, a small amount of carbon is added to iron to
form steel. A certain amount of chromium is added to iron,
carbon, and other elements to form stainless steel, an alloy that
is highly resistant to corrosion. Chromium is also used to impart
corrosion resistance to nickel or cobalt alloys, which comprise
two of the major groups of base metal alloys used in dentistry.

At least four factors determine the extent of solid solubility of
metals; atom size, valence, chemical affinity and chemical
structure.

ATOM SIZE:
If the sizes of two metallic atoms differ by less than
approximately 15% (first noted by Hume-Rothery), they
possess a favorable size factor for solid solubility.

VALENCE:
Metals of the same valence and size are more likely to
form extensive solid solutions than are metals of different
valences.

CHEMICAL AFFINITY:
When two metals exhibit a high degree of chemical
affinity, they tend to form an intermetallic compound upon
solidification rather than a solid solution.

CRYSTAL STRUCTURE:
Only metals with the same type of crystal structure can
form a complete series of solid solutions.
The simplest alloy is a solid solution, in which atoms of
two metals are located in the same crystal structure such as
body-centered cubic (bcc), face-centered cubic (fcc) and
hexagonal close-packed (hcp).


EQUILIBRIUM PHASE
DIAGRAM FOR
ALLOYS


Equilibrium phase diagram are of central
importance to the metallurgy of alloys, since the
phases that are present in an alloy system for
different compositions and temperatures. Eg Single
phase [isomorphous], eutectic, peritectic and
intermetallic.
Phase diagrams are useful for understanding
the structure of dental alloys and can provide
microstructural predictions when some cast dental
alloys are subjected to heat treatment.
This concept equilibrium phase diagram was
introduced by using the table salt-water system

EQUILIBRIUM PHASE DIAGRAM FOR ALLOYS

Liquidus temperature
Solidus temperature
Liquidus
Solidus

Liquidus temperature – Temperature at
which an alloy begins to freeze on cooling
or at which the metal is completely molten
on heating.
Solidus temperature – Temperature at
which an alloy becomes solid on cooling
or at which the metal begins to melt on
heating.


This equilibrium phase diagram is for palladium 65% and silver 35%. When an
alloy composition is undergoing equilibrium solidification, the percentage of
the liquid and solid phases present at a given temp. can be calculated by the
lever rule.
Dashed line PO perpendicular to composition line is drawn.
A point on line PO corresponds temp. 1500°C, the alloy is clearly in liquid state
Point R - Temp is approx. 1400°C and first
solid is formed (crystal), but the composition
is different from 65% Palladium and 35%
Silver. To determine the composition of first
solid extend the tie line to point M. This when
projected to the base line gives the
composition of first solid which is 77% of
Palladium.
Point S - The alloy is midway through its
freezing range, and the composition of solid
and liquid may be determined by drawing tie
line Y-W. These points when projected to the
base line gives liquid composition of 57%
Palladium at point Y and solid composition of
71%Palladium at point W.
Point T – As the temp. reaches point T (solid
phase) the concentration is 65% Palladium.

CORING

In the coring process the last liquid to solidify
is metal with lower solidus temperature and
solidifies between the dendrites. Thus under rapid
freezing conditions, the alloy has a corde structure.
The core consists of the dendrites composed of
compositions with higher solidus temperature, and
the matrix is the portion of the micro-structure
between the dendrites that contains compositions
with lower solidus temperatures.


HOMOGENIZATION
For homogenization heat treatment, the cast alloy is
held at a temperature near its solidus to achieve the
maximum amount of diffusion without melting. (This process
required 6 hr. for the alloy). Little or no grain growth occurs
when a casting receives this type of heat treatment eg.
Annealing done mainly for wrought alloys . The ductility of
an alloy usually increases after homogenization heat
treatment . Gold alloys are heat treated by softening (solution
heat treat) or hardening (age hardening heat treat)


EUTECTIC ALLOYS
Many binary alloy systems do not exhibit complete solubility in
both the liquid and the solid states. The eutectic system is an
example of an alloy for which the component metals have limited
solid solubility. Two metals, A and B, which are completely
insoluble in each other in the solid state, provide the simplest
illustration of a eutectic alloy.
In this case, some grains are
composed solely of metal A
and the remaining grains are
composed of metal B. The
salt and water molecules
intermingle randomly in
solution, the result upon
freezing is a mixture of salt
crystals and ice crystals that
form independently of each
other. www.indiandentalacademy.com

SILVER-COPPER SYSTEM:
The phase diagram for this system is presented in where 3 phases are
found:
• A liquid phase (L)
• A silver-rich substitutional solid solution phase (α) containing a small amount
of copper atoms.
• A copper-rich substitutional solid solution phase (β) containing a small
amount of silver atoms. The α and β phases are sometimes referred to as
terminal solid solutions because of their locations at the left and right sides
of the phase diagram.
Boundary ABEGD is the solidus
and AED is the liquidus. The major
portion of diagram below 780°C is
composed of a two phase region.


Liquidus and solidus meet at point E. This composition (72% silver and
28% copper) is known as the eutectic composition or simply the
eutectic. The following characteristics of this special composition
should be noted.
The temperature at which the eutectic composition melts (779°C or
1435°F) is lower than the fusion temperature of silver or copper
(eutectic literally means “lowest melting”).
There is no solidification range for composition E.
The eutectic reaction is sometimes written schematically as follows.
Liquid α solid solution + β solid solution


PERITECTIC ALLOY

Liquid + β solid solution α solid solution


CLASSIFICATION OF DENTAL
CASTING ALLOYS


1. ALLOY TYPES BY FUNCTIONS:
In 1927, the Bureau of Standard established gold casting alloys, type
I to type IV according to dental function with hardness increasing from
type I to type IV.
Type I (Soft):
It is used for fabrication of small inlays, class III and class V
restorations which are not subjected to great stress . These alloys
are easily burnishable.
Type -II (Medium):
These are used for fabrication of inlays subjected to moderate stress,
thick 3/4 crowns, abutments, pontics, full crowns and soft saddles.
Type I and II are usually referred to as inlay gold.
Type -III (Hard):
It is used for fabrication of inlays subjected to high stress, thin 3/4
crowns, thin cast backing abutments, pontics, full crowns, denture
bases and short span FPDs . Type III alloys can be age hardened.
Type-IV (Extra hard):
It is used for fabrication of inlays subjected to high stress, denture
bases, bars and clasps, partial denture frameworks and long span
FPDs. These alloys can be age hardened by heat treatment.

Type III and Type IV gold alloys are generally called "Crown
and Bridge Alloys", although type IV alloy is used for high
stress applications such as RPD framework.
Later, in 1960, metal ceramic alloys were introduced and
removable partial denture alloys were added in this
classification.

Metal ceramic alloys (hard and extra hard):
It is suitable for veneering with dental porcelain, copings, thin
walled crowns, short span FPDs and long span FPDs. These
alloy vary greatly in composition and may be gold, palladium,
nickel or cobalt based.

Removable partial denture alloys :
It is used for removable partial denture frameworks. Now a
days, light weight, strong and less expensive nickel or cobalt
based have replaced type IV alloys .

2. ALLOY TYPES BY DESCRIPTION:
By description, these alloys are classified into

A) CROWN AND BRIDGE ALLOYS
This category of alloys include both noble and base metal
alloys that have been or potentially could be used in the
fabrication of full metal or partial veneers.
1. Noble metal alloys:
i) Gold based alloy - type III and type IV gold alloys ,
low gold alloys
ii) Non-gold based alloy-Silver -palladium alloy
2. Base metal alloys:
i) Nickel-based alloys
ii) Cobalt based alloys
3. Other alloys:
i) Copper-zinc with Indium and nickel
ii) Silver-indium with palladium

B) METAL CERAMIC ALLOY

1. Noble metal alloys for porcelain bonding:
i) Gold-platinum -palladium alloy
ii) Gold-palladium-silver alloy
iii) Gold-palladium alloy
iv) Palladium silver alloy
v) High palladium alloy

2. Base metal alloys for porcelain bonding:
i) Nickel -chromium alloy
ii) Cobalt-chromium alloy

C) REMOVABLE PARTIAL DENTURE ALLOY

Although type-IV noble metal alloy may be used,
majority of removable partial framework are
made from base metal alloys:

1. Cobalt-chromium alloy
2. Nickel-chromium alloy
3. Cobalt-chromium-nickel alloy
4. Silver-palladium alloy
5. Aluminum -bronze alloy

3.ALLOY TYPE BY NOBILITY
High noble, noble, and predominantly base metal.

Alloy Classification of the American Dental
Association (1984)
ALLOY TYPE TOTAL NOBLE METAL CONTENT
Contains > 40 wt% Au and > 60 wt
High noble metal % of the noble metal elements (Au
+ Ir + Os + Pd + Pt + Rh + Ru)

Noble metal Contains > 25 wt % of the noble
metal elements
Predominantly base metal Contains < 25 wt % of the noble
metal elements


Classification of alloys for All-Metal restorations, metal ceramic restorations, and
frameworks for removable partial dentures.
Alloy type All-metal Metal-ceramic Removable partial
dentures
High noble Au-Ag-Cu-Pd Au-Pt-Pd Au-Ag-Cu-Pd
Metal ceramic alloys Au-Pd-Ag (5-12wt% Ag)
Au-Pd-Ag (>12wt%Ag)
Au-Pd (no Ag)
Noble Ag-Pd-Au-Cu Pd-Au (no Ag) Ag-Pd-Au-Cu
Ag-Pd Pd-Au-Ag Ag-Pd
Metal-ceramic alloys Pd-Ag
Pd-Cu
Pd-Co
Pd-Ga-Ag
Base Metal Pure Ti Pure Ti Pure Ti
Ti-Al-V Ti-Al-V Ti-Al-V
Ni-Cr-Mo-Be Ni-Cr-Mo-Be Ni-Cr-Mo-Be
Ni-Cr-Mo Ni-Cr-Mo Ni-Cr-Mo
Co-Cr-Mo Co-Cr-Mo Co-Cr-Mo
Co-Cr-W Co-Cr-W
www.indiandentalacademy.com Co-Cr-W
Al bronze

4. ALLOY TYPE BY MAJOR ELEMENTS: Gold-based,
palladium-based, silver-based, nickel-based, cobalt-based
and titanium-based .

5. ALLOY TYPE BY PRINCIPAL THREE ELEMENTS: Such
as Au-Pd-Ag, Pd-Ag-Sn, Ni-Cr-Be, Co-Cr-Mo, Ti-Al-V and
Fe-Ni-Cr.

(If two metals are present, a binary alloy is formed; if
three or four metals are present, ternary and quaternary
alloys, respectively, are produced and so on.)

6. ALLOY TYPE BY DOMINANT PHASE SYSTEM: Single
phase [isomorphous], eutectic, peritectic and intermetallic.

DESIRABLE PROPERTIES OF DENTAL CASTING
ALLOYS
Biocompatibility
Ease of melting
Ease of casting
Ease of brazing (soldering)
Ease of polishing
Little solidification shrinkage
Minimal reactivity with the mold material
Good wear resistance
High strength
Excellent corrosion resistance
Porcelain Bonding

To achieve a sound chemical bond to
ceramic veneering materials, a substrate
metal must be able to form a thin, adherent
oxide, preferably one that is light in color so
that it does not interfere with the aesthetic
potential of the ceramic. The metal must have
a thermal expansion/contraction coefficient
that is closely matched to that of the
porcelain.

GOLD CASTING ALLOYS


Composition Range (weight percent) of traditional type I to IV alloys and
four metal -ceramic alloys
Main elements Au Cu Ag Pd Sn, In, Fe, Zn, Ga
Alloy type
I High noble (Au base) 83 6 10 0.5 Balance
II High noble (Au base) 77 7 14 1 Balance
III High noble (Au base) 75 9 11 3.5 Balance
III Noble (Au base) 46 8 39 6 Balance
III Noble (Ag base) 70 25 Balance

IV High noble (Au base) 56 14 25 4 Balance
IV Noble (Ag base) 15 14 45 25 Balance
Metal-ceramic High noble (Au base) 52 38 Balance

Metal-ceramic Noble (Pd base) 30 60 Balance

Metal-ceramic High noble (Au base) 88 1 7 (+4Pt) Balance

Metal-ceramic Noble (Pd base) 0-6 0-15
0- 74-88 Balance
www.indiandentalacademy.com 10

GOLD CASTING ALLOYS:

ADA specification No. 5 classify dental gold casting
alloys as:
1. High Gold Alloys Type I
Inlay Gold Alloy
Type II

Type III Crown & Bridge Alloy
Type IV

2. Low Gold Alloys

3. White Gold Alloys

HIGH GOLD ALLOY:
These alloys contain 70% by weight or more of gold
palladium and platinum. ADA specification No.5 divides this
into four depending upon mechanical properties.

Type I (Soft):-

They are weak, soft and highly ductile, useful only in
areas of low occlusal stress designed for simple inlays such
as used in class I, III & V cavities.
These alloys have a high ductility so they can be
burnished easily. Such a characteristic is important since
these alloys are designed to be used in conjunction with a
direct wax pattern technique. Since such a technique
occasionally results in margins that are less than ideal it is
necessary to use a metal that can be burnished. At present,
these are used very rarely.

PROPERTIES

1. Hardness VHN (50 – 90)
2. Tensile Strength Quite Low
276 MPa or 40,000 PSi
3. Yield Strength 180 MPa or 26,000 PSi
4. Linear Casting Shrinkage 1.56% (according to
Anusavice)
5. Elongation or ductility 46% - William O Brien
18% - Anusavice

COMPOSITION

Au Ag Cu Pt Pd Zn&Ga
83% 10% 6% - 0.5% balance


Type II (Medium):-
These are used for conventional inlay or onlay restorations
subject to moderate stress, thick three quarter crowns, pontics and
full crowns. These are harder and have good strength.
Ductility is almost same as that of type I alloy however,
yield strength is higher. Since burnishability is a function of ductility
and yield strength, greater effort is required to deform the alloy.
They are less yellow in color due to less gold.
Properties:
1. Hardness VHN (90-120)
2. Tensile Strength 345 MPa
3. Yield Strength 300 MPa
4. Linear Casting Shrinkage 1.37%
5. Elongation 40.5% - William O Brien
10% - Anusavice
Composition:
Au Ag Cu Pt
www.indiandentalacademy.com Pd Zn&Ga
77% 14% 7% - 1% balance

Type III (Hard):

Inlays subject to high stress and for crown and bridge in
contrast to type I and type II, this type can be age hardened.
The type III alloy, burnishing is less important than strength.

Properties:
1. Hardness(VHN) 120 – 150
5. Elongation or ductility 39.4% - William O Brien
5% - Anusavice

Composition:
75% www.indiandentalacademy.com
11% 9% - 3.5% balance

Type IV (Extra Hard):

These are used in areas of very high stress, crowns and
long span bridges. It has lowest gold content of all four type (Less
than 70%) but has the highest percentage of silver, copper,
platinium and Palladium. It is most responsive to heat treatment
and yield strength but lowers ductility.

Properties:
1. Hardness VHN (150-200)
5. Elongation or ductility 17% - William O Brien
3% - Anusavice

Composition:
56% 25% 14% -
www.indiandentalacademy.com 4% balance

Type Hardness Proportional Strength Ductility Corrosion
limit resistance

I

II INCREASES DECREASES

III

IV


HEAT TREATMENT OF GOLD ALLOYS:

Heat treatment of alloys is done in order to
alter its mechanical properties.
Gold alloys can be heat treated if it contains
sufficient amount of copper. Only type III and type
IV gold alloys can be heat-treated.

There are two types of heat treatment.

1. Softening Heat Treatment (Solution heat treatment)

2. Hardening Heat Treatment (Age hardening)

1. SOFTENING HEAT TEMPERATURE

Softening heat treatment increased ductility, but
reduces tensile strength, proportional limit, and hardness.
Indications:
It is indicated for appliances that are to be grounded,
shaped, or otherwise cold worked in or outside the mouth.
Method:
The casting is placed in an electric furnace for 10
minutes at a temperature of 700oC and then it is quenched in
water. During this period, all intermediate phases are
presumably changed to a disordered solid solution, and the
rapid quenching prevents ordering from occurring during
cooling.
Each alloy has its optimum temperature. The
manufacturer should specify the most favorable temperature
and time. www.indiandentalacademy.com

2. HARDENING HEAT TREATMENT

Hardening heat treatment increases strength,
proportional limit, and hardness, but decreases ductility. It is
the copper present in gold alloys, which helps in the age
hardening process.
Indications:
It is indicated for metallic partial dentures, saddles,
bridges and other similar structures. It is not employed for
smaller structures such as inlays.
Method:
It is done by “soaking” or ageing the casting at a
specific temperature for a definite time, usually 15 to 30
minutes. It is then water quenched. The aging temperature
depends on the alloy composition but is generally between
200°C and 450°C. During this period, the intermediate
phases are changed to an ordered solid solution.

The proper time and temperature for age
hardening an alloy are specified by the
manufacturer.
Ideally, before age hardening an alloy, it
should first be subjected to a softening heat
treatment to relieve all strain hardening and to start
the age hardening treatment when the alloy is in a
disordered solid solution. This allows better control
of the hardening process.


METAL CERAMIC ALLOYS


METAL CERAMIC ALLOYS4,8,11,15,16,27,31,32,41&43
The main function of metal-ceramic alloys is to reinforce
porcelain, thus increasing its resistance to fracture.
Requirements:
1.They should be able to bond with porcelain.
2.Its coefficient of thermal expansion should be compatible with that of
porcelain.
3.Its melting temperature should be higher than the porcelain firing
temperature. It should be able to resist creep or sag at these
temperatures.
4.It should not stain or discolor porcelain.

The alloys used for metal-ceramic purposes are grouped under two
categories:
i) Noble metal alloys
ii) Base metal alloys.
In case of noble metal alloys for porcelain bonding , addition of
1% base metals (iron, indium, tin etc.) increases porcelain-metal bond
strength, which is due to formation of an oxide film on its surface. It
also increases strength and proportional limit.

PROPERTIES
Modulus of elasticity:
The base metal alloys have a modulus of elasticity approximately twice
that of gold alloys. Thus it is suited for long span bridges. Similarly, thinner
castings are possible.
Hardness:
The hardness of base metal alloys ranges from 175 to 360 VHN. Thus,
they are generally harder than noble metal alloys. Thus, cutting, grinding and
polishing requires high speed and other equipment.
Ductility:
It ranges from 10 to 28% for base metal alloys. Noble metal alloys have
an elongation of 25 to 40%.
Density:
The density of base metal alloys are less, which is approximately 8.0
gms/cm3 as compared to 18.39 gms/cm3 for noble metal alloys.
Sag Resistance:
Base metal alloys resist creep better than gold alloy when heated to high
temperatures during firing.
Bond Strength: Varies according to composition.
Technique Sensitivity: Base metals are more technique sensitive than high
noble metal-ceramic alloys.

The Gold-Platinum-Palladium (Au-Pt-Pd) System:

This is one of the oldest metal ceramic alloy system. But these alloys are not
used widely today because they are very expensive.

Composition:
Gold – 75% to 88%
Palladium – Upto 11%
Platinum – Upto 8%
Silver – 5%
Trace elements like Indium, Iron and Tin for porcelain bonding.

Advantages Disadvantages
1. Excellent castability 1. High cost
2. Excellent porcelain bonding 2. Poor sag resistance so not suited for
3. Easy to adjust and finish long span fixed partial dentures.
4. High nobility level 3. Low hardness (Greater wear)
5. Excellent corrosion and tarnish 4. High density (fewer casting per
resistance. ounce)
6. Biocompatible
7. Some are yellow in color
8. Not “Technique Sensitive”
9. Burnishable www.indiandentalacademy.com

Gold-Palladium-Silver (Au-Pd-Ag) System:

These alloys were developed in an attempt to overcome the major limitations in
the gold-platinum-palladium system (mainly poor sag resistance, low hardness & high
cost)
Two variations on the basic combination of gold, palladium and silver were
created and are identified as either high-silver or low-silver group.

Composition (High Silver Group):

Gold – 39% to 53%
Silver – 12% to 22%
Palladium – 25% to 35%
trace amount of oxidizable elements are added for porcelain bonding.

1. Less expensive than Au-Pt-Pd alloys 1. High silver content creates potential
2. Improved rigidity and sag resistance. for porcelain discoloration.
3. High malleability. 2. High Cost.
3. High coefficient of thermal expansion.
4. Less Tarnish and corrosion resistant.

Composition (Low Silver Group):

Gold – 52% to 77%
Silver- 5% to 12%
Trace amounts of oxidizable elements for porcelain bonding.

1. Less expensive than the Au-Pt-Pd alloys 1. Silver creates potential for porcelain

discoloration (but less than high
silver group)
2. Improved sag resistance 2. High cost.
3. High noble metal content 3. High coefficient of thermal
expansion.
4. Tarnish and corrosive resistant


Gold-Palladium (Au-Pd) System:

This particular system was developed in an attempt to overcome the major
limitations in the Au-Pt-Pd system and Au-Pd-Ag system. Mainly-
-Porcelain discoloration.
-Too high coefficient of thermal expansion & contraction.

Composition:

Gold – 44% to 55%
Gallium – 5%
Indium & Tin – 8% to 12%
Indium, Gallium and Tin are the oxidizable elements responsible for porcelain
bonding.

1. Excellent castability 1. Not thermally compatible with high
expansion dental porcelain.
2. Good bond strength 2. High cost
3. Corrosion and tarnish resistance
4. Improved hardness
5. Improved strengthwww.indiandentalacademy.com
( sag resistance)
6. Lower density

Palladium-Silver (Pd-Ag) System8

This was the first gold free system to be introduced in the
United States (1974) that still contained a noble metal
(palladium). It was offered as an economical alternative to the
more expensive gold-platinum-silver and gold-palladium-silver
(gold based) alloy systems.

Composition: (available in two compo.)
1. Palladium – 55% to 60% Silver – 25% to 30%
Indium and Tin
2. Palladium – 50% to 55% Silver – 35% to 40%
Tin (Little or no Indium)

Trace elements of other oxidizable base elements are also
present. www.indiandentalacademy.com


1. Low Cost 1. Discoloration (yellow, brown or green) may
occur with some dental porcelains.
2. Low density 2. Some castibility problems reported (with
induction casting)
3. Good castibility (when torch 3. Pd and Ag prone to absorb gases.
casting) 4. Require regular purging of the porcelain
4. Good porcelain bonding, furnace.
5. Burnishability 5. May form internal oxides (yet porcelain
6. Low hardness bonding does not appear to be a problem)
7. Excellent sag resistance 6. Should not be cast in a carbon crucible.
8. Moderate nobility level 7. Non-carbon phosphate bonded investments
9. Good tarnish and corrosion recommended.
resistance. 8. High coefficient of thermal expansion.
10. Suitable for long-span fixed
partial dentures.


HIGH PALLADIUM SYSTEM8,11,31,32,41&43

Several types of high palladium systems were originally introduced (Tuccillo, 1987).
More popular composition groups contained cobalt and copper.

Composition (PALLADIUM-COBALT ALLOY):

Palladium – 78% to 88% Cobalt – 4% to 10%
(Some high palladium-cobalt alloys may contain 2% gold)
Trace amounts of oxidizable elements (such as gallium and indium) are added for
porcelain bonding.

1. Low cost 1. More compatible with higher expansion
2. Reportedly good sag resistance porcelains.
3. Low density means more casting 2. Are more prone to over-heating than
per ounce then gold based alloys. high Pd-Cu.
4.They Melt and cast easily 3. Produces a thick, dark oxide
5. Good polishability (Supposed 4. Colored oxide layer may cause bluing of the
to be similar to Au-Pd alloys) porcelain.
6. Reportedly easier to presolder 5. Prone to gas absorption
than Pd-Cu alloys. www.indiandentalacademy.com
6. Little information on long-term clinical
success.

COMPOSITION (PALLADIUM-COPPER ALLOYS)
Palladium – 70% to 80% Copper – 9% to 15%
Gold – 1% to 2% Platinum – 1%
Some, but not all, high palladium-copper alloys contain small quantities ( 1% to 2%)
of gold and/or platinum. Trace amounts of the oxidizable elements gallium, indium and
tin are added for porcelain bonding.
1. Good castability 1. Produces dark, thick oxides
2. Lower cost (than gold based alloys) 2. May discolor (gray) some dental
3. Low density means more castings porcelains.
Per ounce 3. Must visually evaluate oxide color to
4. Tarnish and corrosion resistance determine if proper adherent oxide was
5. Compatible with many dental formed.
Porcelains. 4. Should not be cast in carbon crucibles
6. Some are available in one unit ingots. (electric casting machines)
5. Prone to gaseous absorption.
6. Subject to thermal creep.

7. May not be suitable for long span fixed
partial denture prosthesis.
8. Little information on long term clinical
success.
9. Difficult to polish
10. Resoldering is a problem

BASE METAL ALLOYS


BASE METAL ALLOYS1,3,4,7,9,10,15,16,18,20,23&34
-Nickel based
-Cobalt based
Alloys in both systems contain chromium as the second largest
constituent.
A classification of base metal casting alloys
Co-Cr
Removable
Partial denture Co-Cr-Ni

Ni-Cr

Co-Cr-Mo
Base metal Surgical
Casting alloy Implant Co-Cr-Mo

No Be. (Class-I)
Ni-Cr
Fixed Be. Cont.(Class-II)
Partial denture Co-Cr (Class-III)

Nickel-chromium (Ni-Cr) System1,7

These metal-ceramic alloy offer such economy that they are
also used for complete crown and all metal fixed partial denture
prosthesis (Bertolotti, 1983).
The major constituents are nickel and chromium, with a
wide array of minor alloying elements.

The system contains two major groups:
-Beryllium free (class 1)
-Beryllium (class 2)

Of the two, Ni-Cr-Beryllium alloy are generally regarded as
possessing superior properties and have been more popular
(Tuccillo and Cascone,1984).

NICKEL-CHROMIUM BERYLLIUM FREE ALLOYS9,10,23

Composition:

Nickel – 62% to 77% Chromium – 11% to 22%
Boron , iron, molybdenum, Niobium or columbium and tantalum (trace elements).

1. Do not contain beryllium 1. Cannot use with Nickel sensitive patients.
2. Low cost 2. Cannot be etched. (Cr doesn’t dissolve
in acid)
3. Low density means more casting 3. May not cast as well as Ni-Cr-Be alloys
per ounce 4. Produces more oxide than Ni-Cr-Be
alloys.


NICKEL-CHROMIUM-BERYLLIUM ALLOY9,10,23

Composition:

Nickel – 62% to 82% Chromium – 11% to 20%
Beryllium – 2.0%
Numerous minor alloying elements include aluminum, carbon, gallium, iron,
manganese, molybdenum, silicon, titanium and /or vanadium are present.

1. Low cost 1. Cannot use with nickel sensitive patients
2. Low density, permits more 2. Beryllium exposure may be potentially
casting per ounce. harmful to technicians and patients.
3. High sag resistance 3. Proper melting and casting is a learned skill.
4. Can produce thin casting 4. bond failure more common in the oxide layer.
5. Poor thermal conductor 5. High hardness (May wear opposing teeth)
6. Can be etched to increase 6. Difficult to solder
retention 7. Ingots do not pool
8. Difficult to cut through cemented castings

DISADVANTAGES OF NICKEL-CHROMIUM ALLOYS:

Nickel may produce allergic reactions in some
individuals (contact dermatitis). It is also a potential
carcinogen.
Beryllium which is present in many base metal
alloys is a potentially toxic substance.21,23 Inhalation of
beryllium containing dust or fumes is the main route of
exposure. It causes a condition know as ‘berylliosis’. It
is characterized by flu-like symptoms and granulomas
of the lungs.
Adequate precautions must be taken while
working with base metal alloys. Fumes from melting
and dust from grinding beryllium-containing alloys
should be avoided. The work area should be well
ventilated. www.indiandentalacademy.com

Comparative properties of Ni / Cr alloys and type III casting gold
alloys for small cast restorations
Property (Units) Ni/Cr Type III gold Comments
alloy
Density (g/cm3) 8 15 More difficult to produce defect free casting for
Ni/Cr alloys.
Fusion temperature As high as Normally lower Ni/Cr alloys require electrical induction
1350°C than 1000°C furnace or oxyacetylene equipment.
Casting shrinkage (%) 2 1.4 Mostly compensated for by correct choice of
investment
Tensile strength (MPa) 600 540 Both adequate for the applications being
considered.
Proportional limit 230 290 Both high enough to prevent distortion for
(MPa) applications being considered; not that values
are lower than for partial denture alloys
Modulus of elasticity 220 85 Higher modulus of Ni/Cr is an advantage for
(GPa) large restoration e.g. bridges and for porcelain
bonded restoration.
Hardness (VHN) 300 150 Ni/Cr more difficult to polish but retains polish
during service
Ductility upto 30% 20 (as cast)
Relatively large values suggest that burnishing
(% elongation) 10 (hardened)
is possible; however, large proportional limit
www.indiandentalacademy.comsuggests higher forces would be require.
value

COBALT CHROMIUM ALLOYS4,6,15,16,22&25

Cobalt chromium alloys have been available since the 1920’s. They possess
high strength. Their excellent corrosion resistance especially at high temperatures
makes them useful for a number of applications.
These alloys are also known as ‘satellite’ because they maintained their shiny,
star-like appearance under different conditions.
They have bright lustrous, hard, strong and non-tarnishing qualities.
APPLICATIONS:
1. Denture base
2. Cast removable partial denture framework.
3. Surgical implants.
4. Car spark plugs and turbine blades.
COMPOSITION:
Cobalt - 55 to 65%
Chromium - 23 to 30%
Nickel - 0 to 20%
Molybdenum - 0 to 7%
Iron - 0 to 5%
Carbon - upto 0.4%
Tungsten, Manganese, Silicon and Platinum in traces.
According to A.D.A specification No. 14 a minimum of 85% by weight of
chromium, cobalt, and nickel is required. Thus the gold base corrosion resistant
alloys are excluded.

PROPERTIES
The Cobalt-Chromium alloys have replaced Type IV
gold alloys because of their lower cost and adequate
mechanical properties. Chromium is added for tarnish
resistance since chromium oxide forms an adherent and
resistant surface layer.
1.Physical Properties:
Density: The density is half that of gold alloys, so they are lighter in
weight.
8 to 9 gms/cm3.
Fusion temperature: The casting temperature of this alloy is
considerably higher than that of gold alloys. 1250oC to 1480oC.

A.D.A. specification No. 14 divides it into two types, based on
fusion temperature (which is defined as the liquidus temperature)
Type-I (High fusing) – liquidus temperature greater than
1300oC
Type-II (Low fusing) – liquidus temperature lower than 1300oC

2. Mechanical Properties:

Yield strength: It is higher than that of gold alloys. 710Mpa
(103,000psi).

Elongation: Their ductility is lower than that of gold alloys. Depending
on the composition, rate of cooling, and the fusion and mold
temperature employed, it ranges from 1 to 12%.
These alloys work harden very easily, so care must be taken while
adjusting the clasp arms of the partial denture.

Modulus of elasticity: They are twice as stiff as gold alloys
22.5×103Mpa. Thus, casting can be made more thinner, thus
decreasing the weight of the R.P.D. Adjustment of clasp is not easy.

Hardness: These alloys are 50% harder than gold alloys 432 VHN.
Thus, cutting, grinding and finishing is difficult.


3. Tarnish and corrosion resistance: Formation of a
layer of chromium oxide on the surface of these alloys prevents
tarnish and corrosion in the oral cavity.
Solutions of hypochlorite and other compounds that are
present in some denture-cleaning agents will cause corrosion in
such base metal alloys. Even the oxygenating denture cleansers
will stain such alloys. Therefore, these solutions should not be
used for cleaning cobalt-chromium base alloys.

4. Casting Shrinkage: The casting shrinkage is much
greater than that of gold alloys (2.3%), so limited use in crown &
bridge.
The high shrinkage is due to their high fusion temperature.

5. Porosity: As in gold alloys, porosity is due to shrinkage and
release of dissolved gases which is not true in case of Co-Cr
alloys. Porosity is affected by the composition of the alloys and its
manipulations. www.indiandentalacademy.com

Comparative properties of Co / Cr alloys and type IV casting gold alloys
for partial denture
Property (Units) Co/Cr Type IV gold Comments
alloy
Density (g/cm3) 8-9 15 More difficult to produce defect free
casting for Co/Cr alloys but denture
frameworks are lighter
Fusion temperature as high Normally lower Co/Cr alloys require electrical induction
as than 1000°C furnace or oxyacetylene equipment.
1500°C Can not use gypsum bonded
investments for Co/Cr alloys
Casting shrinkage 2.3 1.4 Mostly compensated for by correct
(%) choice of investment
Tensile strength 850 750 Both acceptable
(MPa)
Proportional limit 710 500 Both acceptable; can resist stresses
(MPa) without deformation
Modulus of 225 100 Co/Cr more rigid for equivalent thickness;
elasticity (GPa) advantage for connectors; disadvantage
for clasps
Hardness (Vickers) 432 250 Co/Cr more difficult to polish but retains
polish during service
Ductility (% 2 15 (as cast) Co/Cr
www.indiandentalacademy.com clasps may fractured if
elongation) 8 (hardened) adjustments are attempted.

Summary of base metal alloy properties
Property Ni-Cr without Ni-Cr with Be Co-Cr
Be
Strength (MPa) 255-550 480-830 415-550
Ultimate 550-900 760-1380 550-900
tensile
strength (MPa)
% elongation 5-35 3-25 1-12
Modulus of 13.8-20.7 x 104 17.2-20.7 x 104 17.2-22.5x104
elasticity
(MPa)
Vickers 175-350 300-350 300-500
hardness
Casting 1430-1570 1370-1480 1430-1590
temperature
(°C)

TITANIUM AND TITANIUM ALLOYS4,13,19,45,46&48

Titanium is called “material of choice” in dentistry. This is attributed to the oxide
formation property which forms basis for corrosion resistance and biocompatibility of
this material. The term 'titanium' is used for all types of pure and alloyed titanium.

Properties of titanium:

-Resistance to electrochemical degradation
-Begins biological response
-Relatively light weight
-Low density (4.5 g/cm3)
-Low modulus (100 GPa)
-High strength (yield strength = 170-480 MPa; ultimate
strength = 240-550 MPa)
-Passivity
-Low coefficient of thermal expansion (8.5 x 10–6/°C)
-Melting & boiling point of 1668°C & 3260°C

Uses:
Commercially pure titanium is used for dental implants, surface coatings, crowns,
partial dentures, complete dentures and orthodontic wires

Commercially Pure Titanium (CP Ti):
It is available in four grades (according to American Society for Testing and
Materials ASTM) which vary according to the oxygen (0.18-0.40 wt.%), iron (0.20-
0.50 wt%) and other impurities. It has got an alpha phase structure at room
temperature and converts to beta phase structure at 883°C which is stronger but
brittle.


TITANIUM ALLOYS

Alloying elements are added to stabilize alpha or the beta
phase by changing beta transformation temperature e.g. in
Ti-6Al-4V48, Aluminum is an alpha stabilizer whereas Vanadium
as well as copper and palladium are beta stabilizer. Alpha
titanium is weld able but difficult to work with at room
temperature. Beta titanium is malleable at room temperature and
is used in orthodontics, but is difficult to weld.
Pure titanium is used to cast crowns, partial denture, and
complete denture.


CAST TITANIUM:
Cast titanium has been used for more than 50 years, and it
has been recently that precision casting can be obtained from
it. The two most important factors in casting titanium based
materials are its high melting point (1668°C) and chemical
reactivity. Because of the high melting point, special melting
procedures, cooling cycles, mold materials, and casting
equipments are required to prevent metal contamination,
because it readily reacts with hydrogen, oxygen and nitrogen
at temperatures greater than 600°C. So casting is done in a
vacuum or inert gas atmosphere. The investment materials
such as phosphate bonded silica and phosphate investment
material with added trace metal are used. It has been shown
that magnesium based investment cause internal porosity in
casting.

Because of its low density, it is difficult to cast in centrifugal
casting machine. So advanced casting machine combining
centrifugal, vacuum, pressure and gravity casting with electric
arc melting technology have been developed.

Difficulties in casting Titanium :

-High melting point
-High reactivity
-Low casting efficiency
-Inadequate expansion of investment
-Casting porosity
-Difficulty in finishing
-Difficulty in welding
-Requires expensive equipments

REVIEW OF LITERATURE

Moffa JP, Guckes AD, Okawa MT and Lilly GE (1973)23 did an evaluation of
nonprecious alloys for use with porcelain veneers and provided quantitative
information about the levels of beryllium produced during the finishing and polishing
of cast base metal dental alloys with there harmful effects.

Shillingburg HT, Hobo S and Fisher DW (1977)39 Studied Preparation design and
margin distortion in porcelain-fused-to-metal restorations.
The results of this study suggested that thermal incompatibility stresses were
likely to cause margin distortion in metal ceramic crowns. However, subsequent
studies support other potential mechanisms, including the effect of excessive sand
blasting time and/or pressure.

Baran GR (1983)7 did an extensive study on metallurgy of sixteen commercially
available Ni-Cr alloys for fixed prosthodontics and compared their alloy
compositions, mechanical properties (yield strength, tensile strength, %elongation
and hardness number), microstructures and clinically relevant considerations for
the use of these alloys.


Carr A.B., Cai Z., Brantley W.A.(1993)11 did a study on new high
palladium casting alloys (generation 1&2). For the five high-palladium
alloys studied, the following conclusions were drawn:
1. An increase in the investment burn out temperature from 1400°F to
1500 °F had little effect on microstructure and hardness, but grain or
dendritic size was found to vary substantially.
2. Hot tears were more prevalent in the alloys when the higher burnout
temperature was used.
3. Heat treatment simulating porcelain firing cycles for these alloys
generally caused decrease in hardness.

Reisbick NH and Brantley WA (1995)36 conducted a study on
mechanical properties and microstructural variations for recasting low
gold alloys. They concluded that significant decrease in yield strength
and percentage elongation were observed for recasting these alloys but
not in tensile strength when the Type III gold alloys were recasted upto 3
times. Scanning electron microscope examination revealed that the
number of casting defects (principally porosity) increased with the
number of times the alloy was remelted.

Berzins DW, Sarkar NK et al (2000)8 did an in-vitro electrochemical
evaluation of high palladium alloys in relation to palladium allergy.
The high incidence of allergic reaction was associated with Pd-Cu
based alloys. The “Pd-skin” of these alloys when in contact with saliva
release some Pd++ ions (an allergen) which can trigger the cascade
of biological reaction involved in allergy and hypersensitivity. It is a
time dependent process.
In Pd alloys containing Ag, formation of Ag-Cl film on the alloy surface
is supposed to prevent Pd in coming in contact with oral fluids, having
a masking effect and thus avoiding allergy.

Tufekci E, Mitchell JC et al (2002)43 did a study on spectroscopy
measurements of elemental release from high palladium dental casting
alloys into a corrosion testing medium. A highly sensitive analytical
technique shows that the release of individual elements over a one
month period, suggesting that there may be low risk of biological
reaction with the Pd-Ga alloys than with the Pd-Cu-Ga alloys tested.


Ahmad SAH, Omar MB, Homa D. (2003)1 did an investigation of the
cytotoxic effects of commercially available dental casting alloys and
concluded the following:
1.The high noble alloy Bioherador N was significantly less cytotoxic than all
the base metal alloys tested in this study (Ni-Cr, Co-Cr, Cu-based)
2. The Ni-Cr alloy CB Soft was significantly more cytotoxic than all the Ni-Cr
and Co-Cr alloys tested. This could be related to the content of Cu, low
content of Cr and absence of Mo in its composition.
3. Cu based alloys Thermobond showed a more severe cytotoxic reaction
than all the other alloys.
O’Brien WJ (2004)29 Biomaterial Properties Database, University of Michigan:
http://www.lib.umich.edu/dentlib/Dental tables/.
This database provides an electronic reference to the following properties of
dental materials; strength between restorative materials and tooth structures,
BHN, coefficient of thermal friction, coefficient of thermal expansion (linear),
colours of dental shade guide, contact angles, creep, density, dynamic
modulus, elastic modulus, heat of fusion, KHN, melting temperatures and
ranges, %elongation, permanent deformities, proportional limit, shear
strength, tear energy, tear strength, ultimate compressive strength, VHN and
yield strength. www.indiandentalacademy.com

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