metals and alloys

METALS AND ALLOYDR. 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

HEAVY METALS

LIGHT METALS

NOBLE METALS

( HEAVY METALS)

DUCTILEBRITTLE

HIGH MELTING

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

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

Al bronze

Classification of alloys for All-Metal restorations, metal ceramic restorations, and

frameworks for removable partial dentures.

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 threeor 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

PRESPECTIVETHE 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 resistanceAPPLIED DENTAL MATERIALS –McCABBES AND WALLS

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 FinenessKarat:It is the parts of pure gold in 24 parts of alloys.

For Eg: a) 24 Karat gold is pure goldb) 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

The main function of metal-ceramic

alloys is to reinforce porcelain, thus

increasing its resistance to fracture.

METAL CERAMIC ALLOYS

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 alloysystem. But these alloys are not used widelytoday 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 toovercome the major limitations in the Au-Pt-Pdsystem 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 elementsresponsible for porcelain bonding.

Palladium-Silver (Pd-Ag) System

This was the first gold free system to be introduced in theUnited States (1974) that still contained a noble metal(palladium). It was offered as an economical alternativeto 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 arealso 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-MoSurgical

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 Platinumin 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 mustbe taken while adjusting the clasp arms of the partialdenture

Hardness: 432 VHN.

Thus, cutting, grinding and finishing is difficult.

Modulus of elasticity: 22.5103Mpa. 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 ofchromium oxide on the surface of these alloys preventstarnish and corrosion in the oral cavity.

Solutions of hypochlorite and other compounds that arepresent in some denture-cleaning agents will causecorrosion in such base metal alloys.

Even the oxygenating denture cleansers will stain suchalloys.

Therefore, these solutions should not be used for cleaningcobalt-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

ALLOYSComposition:

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 elementsinclude aluminum, carbon, gallium, iron,manganese, molybdenum, silicon, titaniumand /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

Disadvantages1.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, hypoeutectoidalloys.

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 noncuttinginstruments, 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

metals and alloys

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