Metals and alloys

METALS AND ALLOY
DR. RITESH SHIWAKOTI
MScD PROSTHODONTICS

 Ductility: the ability of a material to withstand permanent
deformation under a tensile load without rupture; ability of a
material to be plastically strained in tension.
 Malleable : capable of being extended or shaped with a
hammer or with the pressure of rollers.
 Vickers hardness number: (VHN) a measure of hardness
obtained with a diamond pyramidal indenter with a square
base and an angle of 136. VHNis proportional to the ratio of
the applied load to the area of the indentation. Devised in the
1920’s by engineers at Vickers, Ltd. in the United Kingdom

 Modulus of elasticity : in metallurgy, the coefficient found by
dividing the unit stress, at any point up to the proportional
limit, by its corresponding unit of elongation (tension) or
strain. A ratio of stress to strain. As the modulus of elasticity
rises, the material becomes more rigid
 Tensile stress : the internal induced force that resists the
elongation of a material in a direction parallel to the direction
of the stresses
 Proportional limit : that unit of stress beyond which
deformation is no longer proportional to the applied

 Corrosion resistance :A material's ability to resist deterioration
caused by exposure to an environment .
 Elongation at break also known as fracture strain, is the ratio
between changed length and initial length after breakage of
the test specimen. It expresses the capability of a material to
resist changes of shape without crack formation
 Sag resistance: the resistance to flow of a one-inch plug of
uncured sealant after being placed on a vertical plane.
 yield strength : the strength at which a small amount of
permanent (plastic) strain occurs, usually 0.1% or 0.2%, and
most frequently measured in MPa or psi

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.

 An opaque lustrous chemical
substance
that is a good conductor of heat and
electricity and, when polished, is a
good reflector of light.
- The Metals Handbook
(1992)

 An alloy is a substance with metallic
properties that consists of two or more
chemical elements, at least one of
which is a metal.
The Metals Handbook (1992)

PROPERTIES
 Exhibits a luster
 Good thermal and electrical
conductors
 High fracture toughness (KIc)
 Ductile
 Malleable

 Highly resistant to corrosion and
oxidation (Noble metals e.g. gold,
iridium, osmium, palladium, platinum,
rhodium, and ruthenium)
 Most metals have a “white”
appearance (e.g., aluminium, silver,
nickel, palladium, tin, and zinc.) Non
white metal e.g. Gold and Copper

CLASSIFICATION
HIGH MELTING
HEAVY METALS
LIGHT METALS
NOBLE METALS
( HEAVY METALS)
BRITTLE DUCTILE
www.bpc.edu

ALLOYS
 A crystalline substance with metallic
properties that is composed of two or
more chemical elements, at least one
of which is a metal.
 Pure metals are rarely used in
dentistry because they are weaker
than they are when mixed with other
metals.
APPLIED DENTAL MATERIALS –McCABBES AND WALLS

CLASSIFICATION
ADA CLASSIFICATION OF THE DENTAL
CASTING ALLOY:
• High noble alloys “precious metals”
◦ at least 60% noble. 40% of which is gold. The
remaining 40% is base metal
 Noble alloys(semiprecious)
◦ at least 25% noble (no gold requirements). 75%
base metal
 Base metal alloys
◦ Less than 25% noble

National Bureau of Standards
 Type I (Soft, Vickers hardness number
[HV] between 50 and 90)
 Type II (Medium, HV between 90 and
120)
 Type III(Hard, HV between 120 and
150)and
 Type IV (Extra Hard, HV ≥150).

ALLOY TYPES BY
DESCRIPTION
A) CROWN AND
BRIDGE ALLOYS
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
. 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
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

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 Co-Cr-W

ALLOY TYPE BY MAJOR ELEMENT: Gold-based, palladium-based,
silver-based, nickel-based, cobalt-based and titanium-based
.
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.)
ALLOY TYPE BY DOMINANT PHASE SYSTEM: Single
phase [isomorphous], eutectic, peritectic and intermetallic.

HISTORY AND ITS
PRESPECTIVE
THE LOST-WAX PROCESS(1905)
 "investment casting", "precision casting",
or cire perdue in French
 Presented by Taggart
 First used in dentistry for Inlay fabrication
 A duplicate metal structure is cast from wax
model or a copy of the wax model
 Examples: Taggart cast inlay (1907)
: Cave of the Treasure in Israel
http://en.wikipedia.org/wiki/Lost-wax_casting

 Use of Gold made it more expensive
 Performance compromised
 Not good aesthetics

Co-Cr replacement to Gold
(1933)
 Used as base metal for removable partial
denture
 Lighter in weight
 Greater stiffness(elastic modulus)
 Cheaper than gold alloy
PHILLIPS DENTAL MATERIALS

Porcelain-Fused-to-Metal
Process (1959)
 platinum and palladium were added into
gold alloy
 Lowered the coefficient of thermal
expansion
 Increased biocompatibility with porcelain
 Thermally compatible metal-porcelain was
made
PHILLIPS DENTAL MATERIALS

The Gold Standard (1971)
 Response to the increasing price of
gold introduced following changes:
1. Gold was replaced with palladium.
2. Palladium eliminated gold entirely.
3. Base metal alloys with nickel as the
major element eliminated the
exclusive need for noble metals
The United States abandoned the gold
standard in 1971.

The Medical and Dental
Devices Act (1976)
 Dental industry became under control of the
FDA
 Dental alloy for prosthesis was classified as
passive implants
 Manufactures were required to have a
quality system
-Organizations engaged in preparing standards for dental
materials ( George Corbley Paffenbarger )

Desirable Property of Dental
casting alloy
 Biocompatible : The material must
tolerate oral fluids and not release any
harmful products into the oral
environment.
 Should have good corrosion
resistance
 Good tarnish resistance

 Non-allergic
 Good aesthetics
 Must have adequate thermal
properties to tolerate
 Melting range should be low enough
to form smooth surface with the mould
walls
 Economic and easily available

NOBLE METALS
 Periodic table of the element shows
eight noble metals gold, the platinum
group metals (platinum, palladium,
rhodium, ruthenium, iridium, osmium),
and silver.

GOLD
 Oldest dental restorative materials
 Used since 4000 years
 Phoenician used gold wire to bind
teeth
 Etruscan and then roman used gold
strip to make fixed bridges
 Atomic number is 79
 Au – symbol

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

OCCURANCE
 The metal occurs often in free
elemental (native) form, as nuggets or
grains in rocks, in veins between two
rocks and in alluvial deposits. Less
commonly, it occurs in minerals as
gold compounds, such as
with tellurium as calaverite, sylvanite
and krennerite

PROPERTIES
 Dense
 Soft
 Ductile
 Most malleable : 1 gram can be
beaten into 1 square meter
 Lustre
 Most corrosion resistant

USES
 Dental wires to support mobile tooth
by Romans , Greeks , Arabs (5th
century BC to 11th century AD)
 Reduce and fix fractured mandible
during world war
 Filling teeth with gold foils
 Inlay and onlay
 Crown and bridges

GOLD CASTING ALLOYS:
ADA specification No. 5 classify dental gold casting
alloys as:
1. High Gold Alloys Type I
Type II
Type III
Type IV
2.Low Gold Alloys
3. White Gold Alloys
INLAY GOLD ALLOY
CROWN AND BRIDGE ALLOY

CASTING GOLD ALLOYS
TYPE 1 GOLD ALLOY
Soft
Strength – 50 to 90 HV
Casting subjects to very slight strength

COMPOSITION
Gold (Au) 85%
Silver (Ag) 11%
Copper (Cu) 03%
Platinum/ palladium (Pt/Pd) XX
Zinc (Zn) 1%

PROPERTY
Hardness (VHN) 50–90
Modulus of elasticity (GPa) 80
Tensile strength (MPa) 250
Proportional limit (MPa) 120
Elongation at break (%) 35
Melting range (ºC) 950–1100

USES
Inlay ( class 1, III , V cavities)
Electroforming
Gold foil for direct restoration
Telescopic crown

TYPE II
COMPOSITION
Gold (Au) 75%
Silver (Ag) 12%
Copper (Cu) 10%
Platinum/ palladium (Pt/Pd) 02
Zinc (Zn) 1%

PROPERTY
Hardness (VHN) 90-120
Elongation at break % 25
Melting range (ºC) 900-980

USES
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.

TYPE III
COMPOSITION
Gold (Au) 70%
Silver (Ag) 14%
Copper (Cu) 10%
Platinum/ palladium (Pt/Pd) 05
Zinc (Zn) 1%

PROPERTY
Hardness (VHN) 120 -160

USES
Inlays subject to high stress and for
crown and bridge in contrast to type I
and type II

TYPE IV
COMPOSITION
Gold (Au) 65%
Silver (Ag) 09%
Copper (Cu) 15%
Platinum/ palladium (Pt/Pd) 10%
Zinc (Zn) 1%

PROPERTY
Hardness (VHN) 150-230

USES
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.

Comparative properties of
casting gold alloys
Hardness , Proportional limit , Strength
Type IV > Type III > Type II > Type I
Ductility and Corrosion resistance
Type I > Type II > Type III > Type IV

LOW GOLD CONTENT ALLOY
SILVER PALLADIUM ALLOY
Contains no gold
25% palladium
Contains small quantity of copper, zinc,
indium
Low density that affects the castability
Low ductility,
less corrosion resistance

Property Ag/Pd Gold (type
3)
Hardness (VHN) 120–220 120–
160
Modulus of elasticity (GPa) 80–95 85
Proportional limit (MPa) 250 290
Elongation at break (%) 3–25 15–
25
Melting range (ºC) 900–1100 900–
1000
Density (g cm−3) 11–12 15–16

METAL CERAMIC ALLOYS
The main function of metal-ceramic
alloys is to reinforce porcelain, thus
increasing its resistance to fracture.

REQUIREMENT
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
4.It should not stain or discolor
porcelain

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.

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.

Composition (Low Silver Group):
Gold – 52% to 77%
Silver- 5% to 12%
Trace amounts of oxidizable elements for porcelain
bonding.

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.

Palladium-Silver (Pd-Ag) System
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.

BASE METAL ALLOYS
-Nickel based
-Cobalt based
Alloys in both systems contain chromium as the second largest
constituent.
A classification of base metal casting alloys
Base metal
Casting alloy
Removable
Partial denture
Co-Cr
Co-Cr-Ni
Ni-Cr
Co-Cr-Mo
Surgical
Implant
Ni-Cr
Fixed
Partial denture

Cobalt-chromium alloys
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.

The chemical composition of these
alloys specified in the ISO Standard
for Dental Base Metal Casting is as
follows:
 Cobalt main constituent
 Chromium no less than 25%
 Molybdenum no less than 4%
 Cobalt + nickel + chromium no less
than 85%

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

 The main purpose of the chromium is
to further harden the alloy by solution
hardening and also to impart corrosion
resistance.
 Silicon – Increases ductlity
 Molybdenum and beryllium refine the
grain structure and improve the
behaviour of base metal alloys during
casting

 Carbon – controls the brittleness and
ductility
 These alloys are also known as
‘satellite’ because they maintained
their shiny, star-like appearance under
different conditions

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.

PHYSICAL PROPERTY
lighter in weight – lesser density
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) – fusing temperature greater
than 1300oC
Type-II (Low fusing)– fusing temperature lower
than 1300oC

Mechanical Properties:
Yield strength: It is higher than that of gold alloys.
710Mpa (103,000psi).
Elongation: Ductlity is low 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
Hardness: 432 VHN.
Thus, cutting, grinding and finishing is difficult.

Modulus of elasticity: 22.5103Mpa. Due to high modulus
of elasticity casting can be made more thinner, thus
decreasing the weight of the R.P.D. Adjustment of clasp
is not easy.
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.

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.
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.

APPLICATIONS:
1. Denture base
2.Cast removable partial denture framework.
3. Surgical implants.
4. Car spark plugs and turbine blades.

Nickel-chromium (Ni-Cr) System
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

The chemical composition of these alloys
specified in the ISO Standard for Dental
Base Metal Casting Alloys:
Nickel main constituent
Chromium no less than 20%
Molybdenum no less than 4%
Beryllium no more than 2%
Nickel + cobalt + chromium no less than 85%

NICKEL-CHROMIUM
BERYLLIUM FREE
ALLOYS
Composition:
Nickel – 62% to 77% Chromium –
11% to 22%
Boron, molybdenum, Niobium,
columbium and tantalum (trace
elements).

Advantages
1. Do not contain beryllium which is
harmful to technician and patient
2. Low cost
3. Low density means more casting
per ounce

Disadvantages
1.Cannot use with Nickel sensitive patients.
2.Cannot be etched. (Cr doesn’t dissolve
in acid)
3. May not cast as well as Ni-Cr-Be alloys
4.Produces more oxide than Ni-Cr-Be
alloys.

NICKEL-CHROMIUM-BERYLLIUM
ALLOY
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

Advantages
1. Low cost
2. Low density, permits more
casting per ounce.
3. High sag resistance
4. Can produce thin casting
5. Poor thermal conductor
6. Can be etched to increase
retention

Disadvantages
1.Cannot use with nickel sensitive patients
2.Beryllium exposure can harmful to technicians and
patients.
3. Proper melting and casting is a learned skill.
4. Bond failure more common in the oxide layer.
5. High hardness (May wear opposing teeth)
6. Difficult to solder
8. Difficult to cut through cemented castings

TITANIUM
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.

PROPERTY
-Resistance to electrochemical degradation
- 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
106/°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

STEEL
 Steel is an alloy of iron and carbon in which
the carbon content is less than 2%.
 Carbon content makes it brittle
 One phase consists of a very dilute solid
solution of carbon in iron (up to 0.02% C),
called ferrite. The other phase is a specific
compound of iron and carbon with formula
Fe3C, called cementite. The mixture of
ferrite and cementite is termed pearlite

 Eutectic refers to the behaviour of an alloy
of two mutually insoluble metals during
crystallization.
 Alloys with greater concentrations of carbon
are called hypereutectoid alloys and those
with smaller carbon contents, hypoeutectoid
alloys.
 The hypereutectoid alloys contain relatively
greater amounts of cementite while the
hypoeutectoid alloys contain greater
amounts of ferrite.
 Cementite is a very hard, brittle material
whilst ferrite is softer and more ductile

SATINLESS STEEL
 In addition to iron and carbon the stainless
steels contain chromium which improves
corrosion resistance. This is achieved by
the passivating effect in which the
chromium exposed at the surface of the
alloy is readily oxidized to form a tenacious
surface film of chromic oxide.
 Nickel is also present in many stainless
steels. It contributes towards corrosion
resistance and helps to strengthen the alloy.

 sufficient quantities of these two metals are
incorporated, the austenitic structure
remains even at room temperature.
 One of the most commonly used stainless
steels contains 18% chromium and 8%
nickel (termed 18/8 stainless steel).
 18/8 stainless steels are used in
applications where heat hardening is not
necessary, for example, for noncutting
instruments, wires and occasionally as
denture bases

 When smaller quantities of chromium and
nickel are incorporated into steel it is
possible to produce an alloy which has
adequate corrosion resistance but which
can be hardened by heat treatment., 12%
chromium and little or no nickel. is capable
of forming a martensitic stainless steel.
 This type of alloy is commonly used to
construct cutting instruments and probes.

Stainless steel denture bases
 Formed from very thin pressed/rolled
sheets of wrought stainless steel.
 The wrought stainless steel sheets have
high values of modulus of elasticity and
proportional limit. This enables sufficient
rigidity to be achieved with a very thin sheet
of material.
 Conducts heat through metallic plates thus
ensuring the patient retains normal reflex
reaction to hot and cold

1. THE JOURNAL OF PROSTHETIC DENTISTRY VOLUME 94
NUMBER 1
2. Glossary of prosthodontic terms – 2005
3. Phillips science of dental materials 11th edition – Aunacavice
4. The Use of Gold in Dentistry-J. A. Donaldson
5. Gold Alloys, Uses and Performance-Helmut Knosp
6. APPLIED DENTAL MATERIALS –McCABBES AND WALLS
7. Internet

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