2.Metals and Alloys for Prosthodontics - Copy.ppt

Materials Science & Engineering Dept.
Research Experience for Undergraduates

INTRODUCTION
DEFINITION
METALLURGY -TYPES
GENERAL CHARACTERISTICS OF
METALS
STRUCTURE AND PROPERTIES OF
METALS
CONTENTS

 CRYSTALLINE STRUCTURE
 NUCLEI
 DENDRITES
 GRAIN
 GRAIN BOUNDARIES
 EQUIAXED GRAIN STRUCTURE
 LATTICE TYPES
 DISLOCATIONS
 SLIP PLANES

 QUENCHING
 REFINED GRAIN STRUCTURE
 SEEDING
COLD WORKING
 DUCTILITY
 MALLEABILITY
 COLD WORKING
 FIBROUS STRUCTURE
 WORK HARDENING
 RECRYSTALLIZATION TEMPERATURE
 GRAIN GROWTH
ANNEALING
 STRESS RELIEF ANNEALING

STRUCTURE AND PROPERTIES
OF ALLOYS
ALLOY
ALLOY SYSTEM
METALLOID
AMALGAM
BINARY
TERNARY
QUATERNARY

SOLID SOLUTION – PHASE
 SUBSTITUTIONAL SOLID SOLUTION
 INTERSTITIAL SOLID SOLUTION
 SOLUTION HARDENING
 (COOLING CURVE) TIME-TEMPERATURE CURVE
 CORED STRUCTURE
 CORING
 PHASE DIAGRAM
 LIQUIDUS LINE
 SOLIDUS LINE
 HOMOGENISATION
INTERMETALLIC COMPOUND
EUTECTIC FORMATION
PERITECTIC ALLOYS

CLASSIFICATION OF METALS
AND ALLOY SYSTEMS
DENTAL CASTING ALLOYS -
DESIRABLE PROPERTIES
AND USES

 NICKEL-CHROMIUM ALLOYS
 COBALT-CHROMIUM ALOYS (STELLITES)
TITANIUM AND TITANIUM ALLOYS
ALLOYS FOR METAL CERAMIC
RESTORATIONS
PARTIAL DENTURE ALLOYS

 WROUGHT ALLOYS
AND
GOLD ALLOYS
CARBON STEELS
STAINLESS STEEL
COBALT-CHROMIUM (ELGILOY) NICKEL ALLOYS
NICKEL-TITANIUM ALLOYS (NITINOL)
β-TITANIUM ALLOYS
GOLD ALLOYS
SILVER-PALLADIUM ALLOYS
DENTAL IMPLANT MATERIALS

BIOCOMPATIBILITY
CONCLUSION
REFERENCES

GPT 8 (2005) defines “METAL” as any
strong relatively ductile substance that
provides electropositive ions to a
corrosive environment and that can be
polished to a high lustre. Characterized
by metallic atomic bonding.

The metals handbook (1992) defines a “metal as
an opaque lustrous chemical substance that is a
good conductor of heat and electricity and,
when polished, is a good reflector of light”

In dentistry, metals represent one of the four
major classes of materials used for the
reconstruction of decayed, damaged or
missing teeth.

The science and art of the extraction of metals
from their ores together with the refinement of
these metals and their adaption to various uses.
METALLURGY

The extensive use of metals and their
combination during recent years has made
specialization in this field. This specialization
has resulted in the development of several
branches of metallurgy , some of which are
closely associated with chemistry, physics and
mechanics.

Understanding of metallurgy and the
characteristic behavior of various metals, or
combination of metals to form alloys, is
highly desirable in the study of restorative
materials for several reasons like :
There are numerous metals which are used in
various restorative operations.

A knowledge of the characteristic behavior
of metals is essential for an understanding of
the quality of the restoration fabricated from
metals.
The properties that the metal or alloy will
display are quiet reproducible and serve as
guide in the study of the many related issues
to the fabrication of dental restorations.

METALLURGY
 CHEMICAL
 PHYSICAL
 MECHANICAL

Chemical metallurgy deals principally with
the production and refinement of metals.
Sometimes it is described as “process”
metallurgy since it considers the processing
of ores for the production of metals.
CHEMICAL METALLURGY

PHYSICAL METALLURGY
Physical metallurgy is newer science and
deals with the structure of possible
alteration in structure as well as the
characteristic physical properties of metals.
In some respects physical metallurgy and
metallography are closely related.

Metallography is primarily the microscopic
examination of the internal structure of
metals. This metallographic examination
gives some indication of the physical
behavior which the metal can be expected to
exhibit.

MECHANICAL METALLURGY
It includes various processes in the fabrication
of a structure such as the casting, rolling or
drawing operations.
In restorative materials, physical metallurgy
combined with metallography and the
mechanical phase of metallurgy are of greatest
importance.

FERROUS METALLURGY
It is the metallurgy of iron and steel.
In dentistry it is important in connection with
the manufacture and use of steel instruments
and equipments as well as stainless steel
appliances.
FURTHER SUBDIVISIONS

NON-FERROUS METALLURGY
It is the metallurgy of all metal and alloys
other than iron and steel .
E.g. : Gold alloys, platinum alloys, Cr - Co or
stellite alloys, as well as bronze, aluminum,
and low fusing alloys etc.,

General characteristics of metals
A metal is any element that ionizes positively in
solution.
Metals have certain typical and characteristic
properties that tend to identify and distinguish
them from the nonmetallic elements, such as
lustre, opacity, density, thermal and electrical
conductivity.

Extreme ductility and malleability are often
desirable in metals used in dentistry and
these are found to predominate in pure
metals rather than in alloys.

METALS
Metals usually have crystalline structures in the solid
state.

In 1665, Robert Hooke (1635 - 1703) simulated
the characteristic shapes of crystals by stacking
musket balls in piles.
A SPACE LATTICE can be defined as
any arrangement of atoms in space such that
every atom is situated similarly to every other
atom. It is also called a crystal.

There are 14 possible lattice types or forms, but
many of the metals used in dentistry belong to the
cubic system arrangement.
 Simple cubic space lattice
 Single cells of cubic space lattice
 Simple cubic
 Face-centered cubic
 Body-centered cubic
 Models

Other simple lattice types of dental interest.
A) Rhombohedral b) Orthorhombic
c) Monoclinic d) Triclinic e) Tetragonal
f) Simple hexagonal
g) Close packed hexagonal h) rhombic.

When a molten metal or alloy is cooled, the
solidification process is one of crystallization
and is initiated at specific sites called nuclei. The
nuclei are formed from impurities within the
molten mass of metal.

Crystals grow as dendrites, which can be
described as three-dimensional, branched
network structures emanating from the central
nucleus

Crystal growth continue until all the material has
solidified and all the dendritic crystals are in
contact.

Each crystal is known as a grain and the area
between two grains in contact is the grain
boundary

After crystallization, the grains, have
approximately the same dimensions in each
direction, measured from the central nucleus.
They are not perfectly spherical or cubic
however, nor do they conform to any other
geometric shape. They are said to have an
equiaxed grain structure.

A change from an equiaxed structure to one in
which the grains have a more elongated, fibrous
structure can cause important changes in
mechanical properties.

The arrangement adopted by any one
crystal depends on specific factors such as
atomic radius and charge distributions on
the atoms. Although there is a tendency
towards a perfect crystal structure,
occasional defects occur.

Such defects are normally referred to as
dislocations and their occurrence has an
effect on the ductility of the metal or alloy.

When the material is placed under a sufficiently
high stress the dislocation is able to move
through the lattice until it reaches a grain
boundary. The plane along which the dislocation
moves is called a slip plane and the stress
required to initiate movement is the elastic limit.

Grain boundaries form a natural barrier to the
movement of dislocations. The concentration of
grain boundaries increases as the grain size
decreases. Metals with finer grain structure are
generally harder and have higher values of
elastic limit than those with coarser grain
structure. Hence it can be seen that material
properties can be controlled to some extent by
controlling the grain size.

A fine grain structure can be achieved by rapid
cooling of the molten metal or alloy following
casting. This process, often referred to as
quenching, ensures that many nuclei of
crystallization are formed, resulting in a large
number of relatively small grains.

Slow cooling causes relatively few nuclei to be
formed which results in a larger grain size.
Some metals and alloys are said to have a refined
grain structure. This is normally a fine grain
structure which is achieved by seeding the
molten metal with an additive metal which forms
nuclei crystallization.

For an applied tensile force the maximum degree
of extension is a measure the ductility of the
metal or alloy.
For an applied compressive force the maximum
degree of compression is a measure of
malleability.
These changes occur when the stress is greater
than the elastic limit and at relatively low
temperatures.
COLD WORKING

Such cold working not only produces a change
in microstructure, with dislocations becoming
concentrated at grain boundaries, but also a
change in grain shape.
The grains are no longer equiaxed but take up a
more fibrous.

Cold working is sometimes referred to as work
hardening due to the effect on mechanical
properties. When mechanical work is carried out
on a metal or alloy at a more elevated
temperature it is possible for the object to change
shape without any alteration in grain shape or
mechanical properties.

The temperature below which work hardening is
possible is termed the recrystallization
temperature.
If the material is maintained above the
recrystallization temperature for sufficient time,
diffusion of atoms across grain boundaries may
occur, leading to grain growth.
 It is clear that grain growth should be avoided if
the properties are not to be adversely affected.

It is process of heating a metal to reverse the
effects associated with cold working such as strain
hardening, low ductility and distorted grains.
In general it has 3 stages.
1) Recovery
2) Recrystallization
3) Grain growth.
Annealing

 Recovery : is considered the stage at
which the coldwork properties begin to
disappear before any significant visible
changes are observed under the
microscope.

 Recrystallization :
when a severely cold worked metal is
annealed, recrystallization occurs after the
recovery stage. The old grains disappear
completely and are placed by a new set of
strain free grains.

 Grain growth:
The crystallized structure has a certain
average grain size, depending on the
number of nuclei .The more severe the cold
working, the greater the number of such
nuclei. Thus, the grain size for completely
recrystallized material can range from
rather fine to fairly coarse.

 Cold working may cause the formation of
internal stresses within a metal object. If these
stresses are gradually relieved they may cause
distortion which could lead to loss of fit of, for
example, an orthodontic appliance.
 For certain metals and alloys the internal stresses
can be wholly or partly eliminated by using a
low temperature heat treatment referred to as
stress relief annealing.

 This heat treatment is carried out well below the
recrystallization temperature and has no
deleterious effect on mechanical properties since
the original grain structure is maintained.

ALLOYS
 An alloy is a mixture of two or more metals.
Mixtures of two metals are termed Binary alloys,
mixtures of three metals are Ternary alloys
similarly mixture of four metals is termed as
Quaternary alloys etc.
 The term alloy system refers to all possible
compositions of an alloy. For example the silver-
copper system refers to all alloys with
compositions ranging between 100% silver and
100% copper.

 In the molten state metals usually show
mutual solubility, one within another.
When the molten mixture is cooled to
below the melting point the following
things can occur.
 The component metals may remain soluble
in each other forming a solid solution.

 The solid solution may take one of three
forms. It may be a random solid solution
in which the component metal atoms
occupy random sites in a common crystal
lattice.

Another possibility is the formation of an
ordered solid solution in which component metal
atoms occupy specific sites within a common
crystal lattice.

The third type of solid solution is the interstitial
solid solution in which, for binary alloys, the
primary lattice sites are occupied by one metal
atom and the atoms of the second component do
not occupy lattice sites but lie within the
interstices of the lattice. This is normally found
where the atomic radius of one component is
much smaller than that of the other.

Solid solutions are generally harder, stronger and
have higher values of elastic limit than the pure
metals from which they are derived. This explains
why pure metals are rarely used.
The hardening effect, known as solution
hardening, is thought to be due to the fact that
atoms of different atomic radii within the same
lattice form a mechanical resistance to the
movement of dislocations along slip planes.

Metals and alloys are sometimes characterized
using cooling curves. The material is heated till
molten then allowed to cool and a plot of
temperature against time is recorded.
 Super cooling
 Heterogeneous Nucleation

 Each alloy grain can be envisaged as having a
concentration of gradient metals; the higher
melting metal being concentrated close to the
nucleus and the lower melting metal close to the
grain boundaries. The material is said to have a
cored structure.
 Such coring may influence corrosion resistance
since electrolytic cells may be set up on the
surface of the alloy between areas of different
alloy composition.

Since coring may markedly reduce the corrosion
resistance of some alloys, a heat treatment is
some times used to eliminate the cored structure.
Such a heat treatment is termed a homogenization
heat treatment.
This involves heating the alloy to a temperature
just below the solidus temperature for a few
minutes to allow diffusion of atoms and the
establishment of homogeneous structure. The
alloy is then normally quenched in order to
prevent grain growth from occurring.
E.g., Au-Ag system.

If the temperatures T1 and T2 are obtained over
a range of compositions for an alloy system and
their values plotted against percentage
composition, a useful graph emerges.

This is illustrated for a hypothetical solid solution alloy
of metals A and B. The melting points of the pure metals
are indicated by the temperatures TmA and TmB. The
upper and lower temperature limits of the crystallization
range, T1 and T2 are shown for four alloys ranging in
composition from 80% A – 20% B to 20% A - 80% B.

The phase diagram is completed by joining
together all the T1 points and all the T2 points,
together with the melting points of the pure
metals, TmA and TmB.
At temperatures in the region above the top line,
known as the liquidus line, the alloy is totally
liquid. At temperatures in the region below the
bottom line, known as the Solidus line, the alloy
is totally solid.

At temperatures in the region between the solidus
and liquidus lines the alloy consists of a mixture
of solid and liquid. The composition of the solid
and liquid phases at any temperature between T1
and T2 can be predicted with the aid of the phase
diagram.

When two metals are completely miscible in
liquid state, they are capable of forming any
alloy. When such a combination is cooled, one of
the three possibilities may take place :
a) Solid solution
b) Intermetallic compound
c) Eutectic formation

Intermetallic compounds
Chemicals with chemical affinity for each other
can form intermetallic compounds.
E.g., Ag3Sn can be formed between silver and
tin, which is an essential constituent of
DENTAL AMALGAM ALLOYS.

Eutectic mixture
They occur when the metals are miscible in the
liquid state but separate in the solid state. The
two metals will be precipitated as very fine
layers of one metal over the other one : such a
combination as is called an eutectic mixture.
E.g.,72 % silver, 28 % copper.

Phase diagram for a Binary system where there
Is complete solid insolubility.
 CEF
 CDEGF
 ‘E’

A material of this composition is called a
“Eutectic alloy”
 Important features :
 Hard and Brittle
 Lowest melting alloy of the system - solders
 Poor corrosion resistance
 Time-temperature curve for this alloy has a
“Horizontal plateau” (like that of a pure metal)

Peritectic alloys
Limited solubility of two metals can lead to a
transformation referred as “Peritectic transformation”
E.g., Ag-Sn
(Basis for the original Dental Amalgam alloy, is a
Peritectic system)
Invariant reaction occurs at particular temperature and
composition.

CLASSIFICATION
OF
METALS AND ALLOY SYSTEMS

Metals can be broadly classified according to
composition as
NOBLE METALS
The term noble identifies elements in terms of their
chemical stability i.e., they resist oxidation and are
impervious to acids.
Gold, Platinum, Palladium, Rhodium, Ruthenium,
Iridium, Osmium, and Silver are the eight noble
metals.
 In the oral cavity Silver is more reactive and
therefore is not considered as a noble metal.

PRECIOUS METALS
The term “precious” merely indicates whether a
metal has intrinsic value, the noble metals (all eight)
are also precious metals and are defined as such by
major metallurgical societies and the federal
government agencies like National institute of
science and technology.
 All noble metals are precious but all precious metals
are not noble.
 Silver is usually the major ingredient in most alloys
considered as precious.

SEMIPRECIOUS METALS
There is no accepted composition that differentiates
“precious from semiprecious” therefore, this term is
usually avoided.

BASE METALS
These are Ignoble elements. These remain
invaluable components of dental casting alloys
because of their influence on physical properties,
control of the amount and type of oxidation, or
for their strengthening effects.
e.g., Chromium, Cobalt, Nickel, Iron, Copper
etc.

The bureau of standards established gold casting alloys
type i through type iv according to function, with
increasing hardness from type i to iv (1927)

In 1984, ADA proposed a simple classification for
Dental casting alloys

Removable partial denture alloys
Although type IV noble metal alloys may be
used, majority of the removable partial frame
works are made from base metal alloys.
E.g., Cobalt-chromium,
Nickel-chromium.

DENTAL CASTING ALLOYS
The history of dental casting alloys has been
influenced by three major factors.
a) The technological changes of dental prosthesis.
b) Metallurgic advancements
c) Price changes of noble metals since1968.
In 1932, the dental materials group at national
bureau of standards surveyed the alloys being used
and roughly classified them type I-IV.

Uses
1) Fabrication of inlay, onlays
2) Fabrication of crowns, conventional all metal
bridges, metal-ceramic bridges, resin bonded
bridges.
3) Endodontic posts.
4) Removable partial denture frameworks.

Desirable properties
1) Biocompatibility.
2) Ease of melting.
3) Ease of casting, brazing and polishing.
4) Less solidification shrinkage.
5) Minimal reactivity with the mould material.
6) Good wear resistance.
7) High strength and sag resistance.
8) Excellent tarnish and corrosion resistance.

Nickel-chromium and Cobalt-chromium
Alloys
Dental applications:
1) Partial denture framework: Co-Cr, Ni-Cr
2) Porcelain - metal restorations: Co-Cr, Ni-Cr
3) Crowns and bridges: Ni-Cr

During the years since the Co-Cr casting alloys
became available for cast removable partial
denture constructions, they have continued to
increase in popularity.

Function of various alloying elements:
• Chromium is responsible for the tarnish resistance
and stainless properties of these alloys.
• When chromium content of alloy is over 30% , the
alloy is difficult to cast. With this percentage of
chromium, the alloy also forms a brittle phase, known
as sigma phase. Therefore cast base metal dental
alloys should not contain more than 28-29% of
chromium.

• Cobalt increases the elastic modulus, strength and
hardness of alloy more than does nickel.
• One of the effective ways of increasing their hardness is
by altering carbon content.
0.2% increase changes the properties such that
alloy would no longer be used in dentistry.
[Too brittle]
0.2% decrease will reduce yield and ultimate
tensile and yield strengths.

• Aluminum in nickel containing alloys increases the
ultimate tensile and yield strengths.
Microstructure
Microstructure of any substance is the basic
parameter that controls the properties. In other words,
a change in the physical properties of material is a
strong indication that there must have been some
alteration in its microstructure.

The microstructure of Co-Cr alloys in the cast
condition is in homogeneous, consisting of an
austenitic matrix composed of a solid solution of
cobalt and chromium in a cased dendritic
structure. The dendritic regions are cobalt-rich,
where as the interdendritic regions can be a
quaternary mixture.

Three main disadvantages in employing these alloys
(Co-Cr)
Clasps made of such alloys break in service; some
break after relatively short time.
Due to relatively high hardness and low elongation
properties of these alloys some minor but necessary
adjustments needed at the time of delivery are
difficult and also will consume the chair time of
dentist.
Due to their high degree of hardness, the teeth
contacting the metal becomes worn easily.

Morris 1975 stated that Co-Cr alloys are harder
than iron base alloys .
In 1979, he stated that heat treatment decreases
strength of the alloy compared to Au-Pd alloys.
J.C. Wataha et.al, 1992 stated that preparative
procedures such as steam sterilization,
irradiation, plasma treatment and acid treatment
altered the surface of alloys.

In 1974, A C Rowe stated that adding Tantalum (13%)
to a Co-Cr-Ni alloy the properties like ultimate tensile
strength, yield strength are increased by 12-13%.
Tantalum reduces dislocations, a well ordered
structure is formed. Tantalum is a stabilizer. Example
for stabilizers are carbon, molybdenum, tungsten.
Hamid Mohammad and Kamal asgar 1973, indicated
that a cobalt made from 40% Co, 30% Ni, 30% Cr
strengthened by precipitation of coherent Intermetallic
compounds of Tantalum.

They also have criteria to select an additional element
1) Corrosion resistance.
2) Resistance to oxidation during alloying.
3) Efficiency as a nucleating agent during solidification.
4) Efficiency as a solid solution hardener.
5) Fineness of precipitate.
6) Coherency.

Titanium And Titanium Alloys:
Titanium’s resistance to electrochemical
degradation; the benign biological response that it
elicits; its relatively low weight; and its low
density, low modulus, and high strength make
titanium based materials attractive for use in
dentistry.

Ti forms a very stable oxide layer with a thickness
on the order of angstroms and it repassivates in a
time on the order of nanoseconds. This oxide
formation is the basis for the corrosion resistance
and biocompatibility of Ti.
Commercially Pure Titanium (CpTi) is used for
fabricating dental implants, and more recently, as
crowns, partial and complete dentures, and
orthodontic wires.
Wrought alloys of Ti and V and of Ti and Mo are
used for orthodontic wires.

Commercially pure Ti is available in 4 grades which
vary according to the Oxygen (0.18-0.40 wt%) and iron
(0.2-0.5 wt%) contents.
At room temperature CpTi has a HCP crystal lattice,
which is denoted as the alpha phase. On beating, an
allotropic phase transformation occurs, at 883°c, a BCC
phase, which is denoted as the beta(β) phase forms.
A component with a predominantly beta phase is
stronger but more brittle than a component with an alpha
phase microstructure,

Titanium alloys
Pure titanium is of two types –
 Grade I
 Grade II.
Alloying elements are added to stabilize either the
α or β phase by changing β to α transportation
temperature.

For example, in Ti 6 Al-4V, aluminum is an α
stabilizer, which expands the α phase field by
increasing the (α + β) to β transformation temperature.
Vanadium, as well as copper and palladium are β
stabilizers, which expand the ‘β’ - phase field by
decreasing (α + β) transformation temperature.

Ti-6Al-4V
 Most widely used.
 At room temperature, Ti-6 Al-4V is a two phase α +β
alloy.
 At approximately 975 °C an allotropic phase
transformation takes place, transforming the
microstructure to a single phase BCC β alloy.
 Mostly used for surgical implants.

Based on attributes, extensive knowledge, and clinical
success of wrought Ti implants, interest was developed
in cast titanium for dental applications.
The two most important factors in casting Titanium
based materials are the high melting point and
chemical reactivity.
Cast Titanium:

Ti readily reacts with gaseous elements such as
hydrogen, oxygen and nitrogen particularly at high
temperatures. So any manipulation of Ti at elevated
temperatures must be performed in a well-controlled
vaccum, Without a well controlled vaccum, Ti surfaces
will be contaminated with an oxygen enriched and
hardened surface layer, which can be as thick as
100 µm.
surface layers of this thickness reduce strength and
ductility and promote cracking because of embrittling
effect of oxygen.

Because of the high affinity Titanium has for
hydrogen, oxygen and nitrogen, standard crucibles and
investment materials cannot be used.
Investment materials must have oxides that are more
stable than the very stable Ti oxide and must also be
able to withstand a temperature sufficient to melt
titanium. if this is not the case, then diffusion of
oxygen into the molten is likely to occur.

Investment materials such as phosphate bonded
silica and phosphate investment materials with
added trace elements achieve this goal. It has
been shown that with magnesium oxide-based
investments, internal porosity results.

 Because of the low density of titanium, it is difficult to
cast. In the last 10 to 15 yrs, advanced casting
techniques, which combine centrifugation, vaccum
pressure and gravity casing, and new investment
materials are used.
 Properties of Alloyed Titanium
1) Lower melting points compared to pure Ti, but same
as as Ni-Cr or Co-Cr alloys.
2) Mechanical properties of cast CPTi are similar to
those of type III and IV gold alloys.

Other alloys  Ti-15V, Ti-20Cu, Ti-30pd,
Ti-Co, Ti-Cu.
Disadvantages (for dental purpose)
a) High melting point.
b) High reactivity.
c) Low casting efficiency.
d) Inadequate expansion of investment.
e) Casting porosity.

f) Difficulty in finishing this metal.
g) Difficult to weld, solder.
h) Expensive equipment.

Aluminum Bronze alloy
Traditionally bronze is copper-rich copper tin.
Composition of ADA approved alloy of this group has
81-88% copper
7-11% wt aluminum
2-4% nickel
1-4% iron.
Disadvantage:
Copper reacts with sulfur to form copper sulfide,
which tarnishes the surface of this alloy.

METAL CERAMIC RESTORATIONS:
The chief objection to the use of dental porcelain
as a restorative material is its low tensile and shear
strength. This can be minimized by bonding
porcelain directly to a cast alloy substructure made
to fit the prepared tooth. If a strong bond is
attained between the porcelain veneers and the
metal, the porcelain veneer is reinforced.

The original metal ceramic alloys contained 88%
gold and were much too soft for stress-bearing
restorations. As there was no evidence of a
chemical bond between these alloys and dental
porcelain, then mechanical retention and undercuts
were used to prevent detachment of the ceramic
veneer. By adding less than 1% of oxide elements
such as iron, indium and tin to this high-gold
content alloy, the porcelain metal bond strength
was improved by three folds.

Classification of alloys
used for metal ceramic restorations:
High noble
 Au-Pt-Pd
 Au-Pd-Ag
 Au-Pd
Noble
 Pd-Au
 Pd-Au-Ag
 Pd-Ag
Base metal
 Pure Ti, Ni-Cr-Mo-Be, Ti-Al-V, Ni-Cr-Mo

Inspite of vastly different chemical compositions,
all alloys share at least three common features:
 They have the potential to bond to dental
porcelain.
 They possess co-efficient of thermal contraction
compatible with those of dental porcelains.
 Their solidus temperature is sufficiently high to
permit the application of low-fusing porcelains.

The following high noble alloys are used
Gold based metal ceramic alloys
These have a gold content ranging up to 88% with
varying amounts of Pd, Pt and small amounts of
base metals. Alloys of this type are restricted to
Three unit spans, anterior cantilever or crowns.

Gold-Palladium Silver alloys
The gold based alloys contain between 39% and 77%
gold up to 35% palladium, and silver levels as high as
22%.
The silver increases the thermal contraction co-efficient
but it also has a tendency to discolor some porcelains.

Gold-Palladium alloys
They have 44-55% of gold and 35-45% of Pd.
Used with porcelains having low co-efficient of
thermal contraction to avoid the development of
axial and circumferential tensile stresses in
porcelain during the cooling part of the porcelain
firing cycle.
More economical than high gold alloys.

NOBLE ALLOYS
These are Pd based alloys.
These alloys were introduced in late 197O’s
The disadvantage was they had a tendency to
discolor the porcelain during firing
This greenish-yellow discoloration, popularity
termed an “GREENING” is due to the silver
vapour that escapes from the surface of these
alloys during firing of the porcelain.

The silver vapour diffuses as ionic silver into the
porcelain, and is reduced form colloidal metallic
silver in the surface of porcelain.
Some of the high palladium alloys develop a layer of
dark oxide on their surface during cooling from
the degassing cycle, and this layer has proven
difficult to mask by the opaque porcelain.

Composition of Pd-Ag alloys fall within a narrow
range 53% to 61% palladium and 28% 40%
silver, Tin or indium or both are usually added to
increase alloy hardness and to promote oxide
formation for adequate bonding of porcelain.

Palladium-Copper alloys
 Comparable in cost to Pd-Ag alloys.
 Susceptible to creep deformation at elevated firing
temperatures, so attention is given when these alloys
are used for long span FPD’s with small connectors.
 Composition: 74-80% Palladium, 2-15% copper.
 Porcelain discoloration due to copper is not a major
problem.
 These have 1145 Mpa of yield strength and hardness
values equal to base metal alloys.
 These have a poor potential for burnishing when the
marginal areas are thin

Palladium-Cobalt alloys
 Comparable in cost to Pd-Ag alloys.
 Often advertised as gold free, nickel free, beryllium
free.
 These have a fine grain size to minimize hot tearing
during the solidification process.
 It is the most sag-resistance of all noble alloys.
 Composition: 78-88% of Pd and 4-10% of Co.

Palladium-Gallium-Silver and Pa-Gallium-Silver
Gold alloys
 These are most recent alloys.
 These have a slightly lighter colored oxide than
the Pd-Cu or Pd-Co alloys and they are thermally
compatible with lower expansion porcelains.
 Silver content is low (5%) and is inadequate to
cause porcelain greening.
 Are compatible with lower expansion porcelains
such as vita porcelain.

Physical properties of high noble and noble alloys:
 Should have a high melting range so that the metal
is solid well above the porcelain sintering
temperature to minimize distortion of casting
during porcelain application.
 Must have considerably low fusing temperature.
 Good corrosion resistance.
 High modulus of elasticity.

Base metal alloys
Compared with ADA certified type IV gold alloys.
Cobalt based alloys, Nickel based alloys, and Pure
titanium have the following advantages.
1) Low cost
2) Low density
3) Greater stiffness
4) Higher hardness
5) High resistance to tarnish and corrosion.

Composition
Co-Cr  53-67% of cobalt
25-32% of chromium
02-06 wt % molybdenum.
Ni-Cr  61-81 wt % Nickel
11-27% chromium
02-05 wt of molybdenum.
Chromium provides passivation and
corrosion resistance.

Properties:
1) Higher hardness and stiffness.
2) More sag resistant at elevated temperatures.
3) It is improbable than significant occlusal wear of
these alloys occur. Therefore, particular attention
must be directed toward perfecting occlusal
equilibration.
4) It deforms only less than 25 µm when porcelain is
fired over it.

Metals for partial denture alloy
These are classified as:
High noble
Au-Ag-Cu-Pd
Noble
Ag-Pd-Au-Cu
Ag-Pd
Base Metal
Pure Ti, Ti-Al-V,
Ni-Cr-Mo-Be, Ni-Cr-Mo, Co-Cr-Mo.

Properties required
 High tarnish - corrosion resistance
 Should be easily castable
 Good modulus of elasticity, which is a measure of
stiffness and rigidity. It helps in determining
thickness of various portions of framework.
 Should have high strength and hardness.
 Ductility should be higher which represents a
measure of amount of plastic deformation that a
denture framework can withstand before it
fractures.

WROUGHT BASE METAL AND GOLD ALLOYS:
 When a casting is plastically deformed in any
manner, it is called wrought metal.
 Wrought base metal alloys are used in dentistry,
mainly as wires for orthodontics and as clasp arms
for removable partial dentures.
The alloys include:
Stainless steel : iron-chromium-nickel alloy
Co-Cr-Ni
Ni-Ti
β- Titanium alloys.

CARBON STEELS:
Steels are iron based alloys that usually contain less
than 1.2% carbon.
The different classes of steels are based on three
possible lattice arrangements of iron.

STAINLESS STEEL
When 12-30% Cr is added to steel, the alloy is called as Stainless steel

Ferritic stainless steel:
 Often designated as American Iron and Steel
institute (AISI) series 400 stainless steels.
 Good corrosion resistance.
 Is not hardenable by heat treatment.
 Limited application in dentistry.

Martensitic stainless steel:
 Share the AISI 400 designation.
 Have high strength and hardness, so used for
surgical and cutting instruments.
 Poor corrosion resistance.

Austenitic stainless steel:
 Most corrosion resistant of all.
 AISI 302 is basic type, containing 18% or 8% Ni
and 0.15% carbon.
 Type 304 has 0.08% of carbon.
 Both are designated as 18-8 stainless steel
 Type 316L (0.03% carbon) is ordinarily employed
for implants.

Generally austenite stainless steel is preferable to
ferritic because of the following characteristics.
1) Greater ductility and ability to undergo cold work
without fracturing.
2) Substantial strengthening during cold working.
3) Greater ease of welding.
4) Ability to fairly readily overcome sensitization.
5) Less critical grain growth.
6) Comparative ease in forming.

CORROSION RESISTANCE:
 The 18-8 stainless steel may lose its resistance to
corrosion if it is heated between 400°C and 900°C.
 The reason for a decrease in corrosion is the
precipitation of chromium carbide at the grain
boundaries at high temperature. The small, rapidly
diffusing carbon atoms migrate to grain
boundaries from all parts of the crystal to combine
with the large, slowly diffusing chromium atoms
at the periphery of the grain, where energy is
highest.

When chromium combines with the carbon in this
manner, its passivating qualities are lost, and, as a
consequence, corrosion resistance of the steel is
reduced.
Because that portion of grain adjacent to grain
boundary is generally depleted to produce
chromium carbide, intergranular corrosion occurs,
and a partial disintegration of metal may result
with general weakening of structure.

STABILIZATION:
By adding Titanium (approximately 6 times of
carbon) precipitation of chromium carbide can be
inhibited for a short period at temperatures
ordinarily encountered in soldering procedures.

Soldering for stainless steel:
Silver solders are used as their soldering
temperature is low. These are alloys of Ag, Cu,
and Zn to which Sn, In may be added to lower
fusion temperature and improve solder ability.

COBALT-CHROMIUM-NICKEl ALLOYS:
Co-Cr-Ni alloys are used successfully in orthodontic
appliances.
These alloys were originally developed for use as
watch springs (Elgiloy).
COMPOSITION:
A representative composition by mass is Co-40%,
Cr-20%, Ni-15%, Mo-70%, Mn-2%, C-0.16%,
Be-0.04%, Fe-15. 8%.

PROPERTIES:
 Excellent resistance to tarnish & corrosion.
 Yield strength, hardness, tensile strength are
approximately equal to 18-8 stainless steel.
 Ductility is greater than 18-8 stainless steel.
 More responsive to low temperature heat
treatment.

NICKEL-TITANIUM ALLOYS
 Called as NITINOL
 It has a large working range because of low
stiffness in combination with moderately high
strength.

COMPOSITION
 Ni-Ti alloys used in dentistry contain
approximately 54% Ni, 44% Ti and 2% or less
cobalt.
 This alloy can exist in various crystallographic
forms. At high temperatures, a BCC lattice
austentite phase occurs, on cooling a CH
Martensitic phase occurs.

These characteristics of the austentite to
martensite phase transition results in two unique
features of potential clinical relevance :
 Shape memory and Super elasticity.
Memory effect is achieved by first establishing a
shape at temperatures near 482°C.

 If the appliance such as an orthodontic arch wire, is
then cooled and formed into a second shape and
heated through a lower transition temperature, the
wire will return into its original shape.
 Inducing the austentite to martensite transition by
stress can produce super elasticity, a phenomenon
that is employed with some nickel-titanium
orthodontic wires and some endodontic files.

β-Titanium alloys
Like stainless steel and Nitinol, pure titanium has
different crystallographic forms at high and low
temperatures.
At temperatures lower than 885° C, the hexagonal
close-packed (HCP) or α-crystal lattice is stable,
where as at higher temperature, the metal re-
arranges into a BCC or β-crystal lattice.
An alloy with the composition of Titanium-11%,
molybdenum-6%, Zirconium-4%, tin is produced
in wrought wire form for orthodontic applications.

Properties:
1) Low elastic modulus.
2) High ratio of yield strength to elastic modulus
produces orthodontic appliances that can sustain
large elastic activations.
3) Highly cold worked.
4) Excellent corrosion resistance and environmental
stability.

GOLD ALLOYS :
Gold wires are occasionally employed in the
construction of removable partial denture clasps
but used in fabricating orthodontic appliances, and
as retention pins for restorations.

COMPOSITION:
 Many gold wires resemble the type IV gold casting
alloys in composition, but typically they contain less
gold.
 Two types of gold wires are recognized in ADA.
Specification No.7 (1984).
Type I-High noble or noble metal alloys, they must
contain at least 75% of gold and platinum group
metals.
Type II-High noble or noble metal alloys, that must
contain at least 65% of some noble metals.

GENERAL EFFECTS OF THE CONSTITUENTS:
Pt-Pd ensure that wire does not melt or recrystallize
during soldering procedures.
Ensure a fine grain structure.
Cu -contributes to ability of alloy to age harden,
Ni - strengthener, reduces ductility.
Zn - scavenger.

MECHANICAL PROPERTIES OF NOBLE ALLOY
WIRES:
A wire of a given composition is generally superior
in mechanical properties to a casting of the same
composition.

Because:
 Casting contains unavoidable porosity, which has
a weakening effect.
 When cast ingot is drawn into wire, the small
pores and surface projections may be collapsed,
and welding may occur so that defects disappear.
Any defects of this type that are not eliminated
will weaken the wire.
 Because of Fibrous microstructure.

Silver- palladium alloys:
 White in color
 Predominantly silver in composition but have
substantial mounts of palladium, that provide nobility
and promote the silver resistance.
 May or may not have copper and a small amount of
gold.
Disadvantages
 Poor castability
 Greater potential for tarnish and corrosion.

DENTAL IMPLANT MATERIALS:
Most commonly, metals and alloys are used.
Initially surgical grade stainless steel and Co-Cr
alloys were used because of their acceptable
physical properties and relatively good corrosion
resistance and biocompatibility.

STAINLESS STEEL: (S-S)
 Surgical stainless steel is an iron-carbon (0.05%)
alloy with approximately 18% chromium to impart
corrosion resistance and 8% nickel to stabilize the
austentite structure.
 The alloy is most frequently used in a wrought and
heat-treated condition.
 It has increased strength and ductility; thus it is
resistant to fracture.

Co-Cr-Mo alloy
 These are most often used in a cast or cast and
annealed condition.
 Composition 63% of Co, 30% of Cr, 5% Mo
and small concentrations of C, Mn, Ni.
 Molybdenum serves to stabilize the structure, and
carbon as hardener.
 These have outstanding resistance to corrosion.
 These are least ductile.

Commercially pure Ti (CPTi) has become one of
the material of choice because of its predictable
interaction with the biological environment.
Titanium is a highly reactive material, it oxidizes
on contact with air or normal tissue fluids. This
reactivity is favourable for implant devices
because it minimizes biocorrosion.
An oxide layer 10 A° thick forms on the cut
surface of pure Ti within a millisecond. Thus, any
scratch or nick in the oxide coating is essentially
self healing.

Composition of alloyed Ti
Ti - 90wt%
Al- 6wt%
Va- 4wt%

Properties:
 High strength : weight ratio.
 Modulus of elasticity approximately one half of
that of stainless steel or Cr-Co alloys.
 Few titanium substructures are plasma-sprayed or
coated with a thin layer of calcium phosphate
ceramic.

The rationale for coating the implant with tricalcium
phosphate or hydroxyapatite, both rich in calcium
and phosphorus is to produce a bioactive surface
that promotes bone growth and induces a direct
bond between the implant and hard tissue.
The rationale for plasma sprayed surface is to
provide a roughened, biologically acceptable
surface for bone ingrowth to ensure anchorage in
the jaw.

OTHER METALS:
Gold, Palladium, Tantalum, Platinum and alloys of
these metals.
Recently Zirconium, Tungsten are used.

BIOCOMPATABILITY OF METALS:
Laboratory techniques performed with metals
may expose us occasionally or routinely to
excessively high concentrations of Beryllium
and Nickel dust and Beryllium vapour.

BERYLLIUM
Although the beryllium concentration in dental
alloys rarely exceeds 2 wt % the amount of
beryllium vapor released in to the breathing space
during melting of Ni-Cr-Be alloys may be
significant over an extended period.

The risk of Beryllium vapour exposure is greatest
for dental technicians during alloy melting
especially in the absence of an adequate exhaust
and filtration system.
High levels of Beryllium have been measured
during finishing and polishing when a local
exhaust system was not used. They were reduced
to levels considered safe when exhaust fan was
used.

Exposure of beryllium may result in acute and
chronic forms of Beryllium disease
BERYLLIOSIS.
CLINICAL FEATURES:
Symptoms range from coughing, chest pain and
general weakness to pulmonary dysfunction.
Contact dermatitis
Chemical pneumonitis

NICKEL:
It is a great concern to dental patients with a
known allergy to this element.
The cloud of controversy continues to hang
over the use of nickel in Dentistry.
Dermatitis resulting from contact with nickel
solutions was described as early as 1989.

Inhalation, ingestion and dermal contact of
nickel or nickel containing alloys are
common because nickel is found in
environmental sources such as air, soil and
food as well as in synthetic objects such as
coins, kitchen utensils, and jewelry.

Nickel allergy was determined by PATCH TEST
(Luis-Blanco- Dalmau JPD 1982: 48; 99-101)
described a standard patch test consisting of 5%
Nickel sulfate solution or 5% Nickel sulfate
solution on a petrolatum base, in centre portion
of a square Band-Aid of good quality.

Band-Aids in position
One Band-Aid is removed. Observe for ++
Both the Band-Aids are removed for comparison
Magnified erythema,papules,and vesicles,+++

The patch is applied on medial aspect of upper arm,
which was cleaned with a alcohol swab, this is left
in place for 48 hrs undisturbed. The patient is
instructed not to moisten the arm or remove the
patch during this time. A Band-Aid without any
reagent is placed adjacent to the first acts a control.
After 48 hrs, the control Band-aid is removed. The
second Band-Aid is removed and the skin is
cleaned using alcohol or acetone, tests are read
after 20 min.

Signs for recording degrees of patch test reactions are :
0 No reaction.
+ Erythema.
++  Erythema, papules.
+++ Erythema, papules, vesicles.
++++  Marked edema with vesicles.

 DIMETHYL GLYOXINE TEST:
 FEIGL and SHORE stated that few drops of 1%
alcohol solution of dimethyl glyoxime, few drops
of ammonium hydroxide added to a metallic
object, skin on solution will produce a strawberry
red insoluble salt in presence of nickel.

 LAMSTER (1987), showed 2 cases
demonstrating Loss of alveolar bone around Ni rich
nonprecious alloy and porcelain crown within 18
months of placement. Reason for this was thought
that the electrolysis of metal leading to corrosion
and bioviability of Nickel.

 TIMOTHY. K. JONES (1986) stated that
incidence of Ni hypersensitivity was more in
women (l0 times more than men). The reason was
attributed to increased contact with nickel plated
objects at home.

Dental implications of Nickel
hypersensitivity

 JOHN. C. WATAHA (1998) stated that transient
exposure of casting alloys to an acidic oral
environment is likely to significantly increase
elemental release from Ni alloys, but not from
high noble alloys.
 J.GEIS-GERS (1993) - From point of corrosion
resistance Beryllium free Ni-Cr-Mo alloys should
be preferred in clinical use.

Symptoms of sensitivity range from urticaria :
 Pruritis,
 Xerostomia,
 Eczema
 Vesicular eruptions.

 Release of Ni ions from dental alloys is high
enough to be clinically significant.
 If so, potential alteration in endocrine functions,
changes in vital functions such as blood pressure,
pulse, temperature may be expected.
 Ni containing alloys has been linked to decrease
in lymphocytes in human.

CONCLUSION
Great variety of alloys currently available
can lead to uncertainty in choosing an
optimal alloy for a given patient and
situation. In addition to working and
mechanical properties an important
consideration is given to corrosion
resistance.

So selection of a specific alloy should be
based on a balanced consideration of cost
and alloy properties relevant to a particular
use of material.

REFERENCES
1. Science of dental materials- Anusavice, 11th Edn.
2. Restorative dental materials - Craig ,10th Edn.
3. Dental biomaterials- E.C. Coombe.
4. Applied dental Materials - John F. Mc Cabe,7th Edn.
5. Dental materials, Properties & Manipulation -Robert
G. Craig et.al, 5th Edn.

Journal of Prosthet Dent. 2002; 87:94-98.
Journal of Prosthet Dent. 1988; 80: 691-698.

Dental materials. 1987; 3: 125-130.
Dental materials. 1993; 9: 177-181.
Intl Journal Prosthodont. 1991;4:152-158.
J Dent Research. 1990; 69: 67-68.
J Periodont. 1987; 58: 486-490.

2.Metals and Alloys for Prosthodontics - Copy.ppt

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