Dental Casting Alloys

1

INTRODUCTION

In dentistry, metals represent one of the three major classes of materials used for

the reconstruction of damaged or missing oral tissues. Although metals are readily

distinguished from ceramics and polymers.

The wide varieties of complex dental alloy compositions consist of the following:

1. Dental amalgams containing the major elements mercury, silver, tin, and

copper.

2. Noble metal alloys in which the major elements are some combination of

gold, palladium, silver and important secondary elements including copper,

platinum, tin, indium and gallium.

3. Base metal alloys with a major element of nickel, cobalt, iron or titanium and

many secondary elements that are found in the alloy compositions.

HISTORY OF METALS IN DENTISTRY

Dentistry as a specialty is believed to have begun about 3000 BC. Gold bands

and wires were used by the Phoenicians after 2500 BC.

Modern dentistry began in 1728 when Fauchard published different treatment

modalities describing many types of dental restorations, including a method for the

construction of artificial dentures made from ivory. Gold shell crowns were described

by Mouton in 1746 but they were not patented until in 1873 by Beers. In 1885

Logan patented porcelain fused to platinum post replacing the unsatisfactory wooden

post previously used to build up intra-radicular areas of teeth. In 1907 a detached post

crown was introduced which was more easily adjustable.

2

Year Event

1907 Introduction of Lost-Wax Technique

1933 Replacement of Co-Cr for Gold in Removable Partial Dentures

1950 Development of Resin Veneers for Gold Alloys

1959 Introduction of the Porcelain Fused-to-Metal Technique

1968 Palladium-Based Alloys as Alternatives to Gold Alloy

1971 Nickel-Based Alloys as Alternatives to Gold Alloys

1980s Introduction of All-Ceramic Technologies

1999 Gold Alloys as Alternatives to Palladium-Based Alloys

1971 – The Gold Standard:

The United States abandoned the gold standard in 1971. Gold then became a

commodity freely traded on the open markets. As a result, the price of gold increased

steadily over the next nine years. In response to the increasing price of gold, new

dental alloys were introduced through the following changes:

1. In some alloys, gold was replaced with palladium.

2. In other alloys, palladium eliminated gold entirely.

3. Base metal alloys with nickel as the major element eliminated the exclusive

need for noble metals.

3

KEY TERMS

Grain – A microscopic single crystal in the microstructure of a metallic material.

Metal – An element or alloy whose atomic structure readily loses electrons to form

positively charged ions, and which exhibits metallic bonding (through a spatial

extension of valence electrons), opacity, good light reflectance from a polished

surface and high electrical and thermal conductivity.

Noble metal – which are highly resistant to oxidation and dissolution in inorganic

acids. Gold and platinum group metals (Platinum, palladium, rhodium, ruthenium,

iridium and osmium).

Base metal – A metal that readily oxidizes or dissolves to release ions.

Alloy – A crystalline substance with metallic properties that is composed of two or

more

chemical elements, at least one of which is metal.

Solid solution (metallic) – A solid crystalline phase containing two or more

elements, at least one of which is a metal, that are intimately combined at the atomic

level.

Liquidus temperature – Temperature at which an alloy begins to freeze on cooling

or at which the metal is completely molten on heating.

Solidus temperature – Temperature at which an alloy becomes solid on cooling or at

which the metal begins to melt on heating.

4

PERIODIC TABLE

Of the 115 elements currently listed in most recent versions of the periodic

tables of the elements, about 81 can be classified as metals. (Additional elements that

have been created with nuclear reactors have short half-lives.) It is of scientific

interest that the metallic elements can be grouped according to density, ductility,

melting point and nobility. This indicates that the properties of metals are closely

related to their valence electron configuration. The groupings of pure metal elements

can be seen in the periodic chart of the elements. Several metals of importance for

dental alloys are transition elements, in which the outermost electron subshells are

occupied before the interior subshells are completely filled.

INTERATOMIC PRIMARY BONDS:

The forces that hold atoms together are called cohesive forces. These

interatomic bonds may be classified as primary or secondary. The strength of these

bonds and their ability to reform after breakage determine the physical properties of a

material. Primary atomic bonds may be of three different types.

1. Ionic

2. Covalent

3. Metallic

1. IONIC BOND FORMATION:-

Characterized by electron transfer from one element (positive) to another

(negative).

2. COVALENT BOND FORMATION:-

Characterized by electron sharing and very precise bond orientations.

3. METALLIC BOND FORMATION:-

Since the outer-shell valence electrons can be removed easily from atoms in

metals, the nuclei containing the balance of the bound electrons form positively

charged ionic cores. The unbound or free valence electrons form a “cloud” or “gas”,

resulting in electrostatic attraction between the free electron cloud and the positively

charged ionic cores. Closed-shell repulsion from the outer electrons of the ionic cores

balances this attractive force at the equilibrium interatomic spacing for the metal.

5

The free electrons act as conductors of both thermal energy and electricity.

They transfer energy by moving readily from areas of higher energy to those of lower

energy, under the influence of either a thermal gradient or an electrical field (potential

gradient). Metallic bonding is also responsible for the luster or mirror-reflecting

property, of polished metals and their typical capability of undergoing significant

permanent deformation (associated with the properties of ductility and malleability) at

sufficiently high mechanical stresses. These characteristics are not found in ceramic

and polymeric materials in which the atomic bonding occurs through a combination of

the covalent and ionic modes.

INTERATOMIC SECONDARY BONDS:

In contrast with primary bonds, secondary bonds do not share electrons.

Instead, charge variations among molecules or atomic groups induce polar forces that

attract the molecules.

VAN DER WAALS FORCES:

Fluctuating dipole that binds inert gas molecules together. The arrows show

how the fields may fluctuate so that the charges become momentarily positive and

negative.

PHYSICAL PROPERTIES

Stress

When a force is applied to a material there is a resistance in the material to the

external force. The force is distributed over an area and the ratio of the force to the

area is called stress.

STESS= F/A

Strain

The change in length or deformation per unit length when a material is

subjected to a force is defined as strain.

6

Stress vs. Strain Curve

If one plots stress vs. strain on a graph, a stress-strain curve will result. The

properties of various dental materials, such as alloys, can be compared by analysis of

their respective stress-strain curves.

P = Elastic modulus

or Proportional Limit

Y-X curve = Yield Strength

X = Ultimate Strength

Strength

It is the maximal stress required to fracture a structure.

Types of Strength:

- Compressive

- Tensile

- Shearing

Toughness

It is defined as the energy

required to fracture a material. It is a property of the material which describes how

difficult the material would be to break.

7

Ductility

It is the ability of a material to withstand permanent deformation under a

tensile load without rupture. A metal may be drawn readily into a wire and is said to

be ductile. Ductility is dependent on tensile strength.

Malleability

It is the ability of the material to withstand rupture under compression, as in

hammering or rolling into a sheet. It is not dependent on strength as is ductility.

Hardness

In mineralogy the hardness is described on the basis of the material to resist

scratching. In metallurgy and in most other fields, the amounts of the resistance of

indentation is taken as the measure of hardness for the respective material).

• Brinell hardness number ( BHN )

• Rockwell hardness number ( RHN )

• Vickers hardness test (VHN )

• Knoop hardness test ( KHN )

Coefficient Of Thermal Expansion (Linear Coefficient Of Expansion )

Change in length per unit of original length of a material when its temperature

is raised 1 °

8

TARNISH AND CORROSION

High-noble alloys used in dentistry are so stable chemically that they do not

undergo significant corrosion in the oral environment; the major components of these

alloys are gold, palladium and platinum. (Iridium, osmium, rhodium and ruthenium

are also classified as noble metals.) Silver is not considered noble by dental standards,

since it will react with air, water and sulfur to form silver sulfide, a dark discoloration

product.

Gold resists chemical attack very well. Thus it was natural that this most noble

metal was employed early in modern dental history for the construction of dental

appliances.

Tarnish is observable as a surface discoloration on a metal, or as a slight loss or

alteration of the surface finish or luster. In the oral environment, tarnish often occurs

from the formation of hard and soft deposits on the surface of the restoration.

Calculus is the principal hard deposit, and its color varies from light yellow to brown.

The soft deposits are plaques and films composed mainly of microorganisms and

mucin. Stain or discoloration arises from pigment-producing bacteria, drugs

containing such chemicals as iron or mercury and adsorbed food debris.

Corrosion is not merely a surface deposit. It is a process in which deterioration of a

metal is caused by reaction with its environment. Frequently, the rate of corrosion

attack may actually increase over time, especially with surfaces subjected to stress,

with intergranular impurities in the metal or with corrosion products that do not

completely cover the metal surface.

Sulfur is probably the most significant factor causing surface tarnish on casting

alloys that contain silver, although chloride has also been identified as a contributor.

1. Chemical or Dry Corrosion

2. Electrochemical or Wet Corrosion

1 Galvanic corrosion

2 Heterogeneous Surface Composition

9

3 Stress Corrosion

4 Concentration Cell Corrosion or Crevice Corrosion

• Pitting type

• Cervical type

Protection Against Corrosion

i. Passivation

ii. Increase noble metal content

iii. Polishing restorations

iv. Avoid dissimilar metal restorations

Certain metals readily form strong adherent oxide film on their surface, which

protects them from corrosion. Such a metal is said to be passive. Chromium, titanium

and aluminium are examples of such metals. Since this film is passive to oxidative

chemical attack, their formation is called passivation.

If more than 12% Cr is added to iron or cobalt, we get stainless steel or cobalt

chromium alloys, which are lightly corrosion resistant and therefore suitable for

dental use.

• Noble metals resist corrosion because their electromotive force is positive with

regard to any of the common reduction reactions found in the oral

environment. In order to corrode a noble metal under such conditions, an

external current (over potential) is required.

• At least half the atoms should be noble metals (gold, platinum, and palladium)

to ensure against corrosion. Palladium has been found to be effective in

reducing the susceptibility to sulfide tarnishing for alloys containing silver.

• Chromium provides this corrosion resistance by forming a very thin, adherent

surface oxide that prevents the diffusion of oxygen or other corroding species

to the underlying bulk metal.

10

SOLIDIFICATION OF METALS

The temperature decreases steadily from point A to point B. An increase in

temperature then occurs from point B to point B, at which time the temperature

remains constant until the time indicated at point C is reached. Subsequently, the

temperature of the metal decreases steadily to room temperature.

The temperature Tf, as indicated by the straight or “plateau” portion of the

curve at BC, is the freezing point, or solidification temperature of the pure metal. This

is also the melting point, or fusion temperature. During melting, the temperature

remains constant. During freezing or solidification, heat is released as the metal

changes from the higher-energy liquid state to the lower-energy solid state.

The initial cooling of the liquid metal from Tf to point B is termed super

cooling. During the super cooling process, crystallization begins for the pure metal.

Once the crystals begin to form, release of the latent heat of fusion causes the

temperature to rise to Tf where it remains until crystallization is completed at point

C.

CRYSTALLIZATION OF METALS

Characteristically, a pure metal crystallizes from nuclei in a pattern that often

resembles the branches of a tree, yielding elongated crystals that are called Dendrites.

In three dimensions, their general appearance is similar to that of the two dimensional

frost crystals that form on a window pane in the winters.

Extensions or elevated areas (termed protuberances) form spontaneously on

the advancing front of the solidifying metal and grow into regions of negative

temperature gradient. Secondary and tertiary protuberances result in a three

dimensional dendritic structure.

Although dental base metal casting alloys typically solidify with a dendritic

micro-structure, most nobel metal casting alloys solidify with an Equiaxed

polycrystalline microstructure. The microstructural features in this figure are called

grains, and the term Equiaxed means that the three dimensions of each grain are

similar, in contrast to the elongated morphology of the dendrites.

11

NOBLE METALS

The noble metals have been the basis of inlays, crowns and bridges because of

their resistance to corrosion in the oral cavity.

Gold, platinum, palladium, rhodium, ruthenium, iridium, osmium, and silver are

the eight noble metals. However, in the oral cavity, silver is more reactive and

therefore is not considered as a noble metal.

Of the eight noble metals, four are of major importance in dental casting alloys,

i.e., gold, platinum, palladium and silver. All four have a face-centered cubic crystal

structure and all are white coloured except for gold.

Gold

Pure gold is a soft and ductile metal with a yellow “Gold” hue. It has a density

of 19.3 gms/cm3 , melting point of 1063oC, boiling point of 2970 oC and CTE of

14.2×10-6/°C. Gold has a good luster and takes up a high polish. It has good chemical

stability and does not tarnish and corrode.

Gold content:

Traditionally the gold content of dental casting alloys have been referred to in

terms of:

1. Karat

2. 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.

12

Silver

It is sometimes described as the “Whitest” of all metals. It has the lowest

density 10.4gms/cm3 and melting point of 961oC, boiling point of 2216 oC among the

four precious metals used in dental casting alloys. Its CTE is 19.7×10-6/oC , which is

comparatively high.

Palladium

It has a density of 12.02gms/cm3. Palladium has a higher melting point of

1552oC, boiling point of 3980 oC and lower CTE which is 11.8×10-6/oC, when

compared to gold.

Platinum

It has the highest density of 21.45 gms/cm3 , highest melting point of 1769oC,

boiling point of 4530 oC and the lowest CTE 8.9×10-6/oC among the four precious

metals used in dental casting alloys.

Carbon:

Carbon content is most critical. Small amounts may have a pronounced effect

on strength, hardness and ductility. Carbon forms carbides with any of the metallic

constituents which is an important factor in strengthening the alloy. However when in

excess it increases brittleness. Thus, control of carbon content in the alloy is

important.

Boron:

Deoxidizer and hardner, but reduces ductility.

Copper:

It is the principal hardner. It reduces the melting point and density of gold. If

present in sufficient quantity, it gives the alloy a reddish colour. It also helps to age

harden gold alloys. In greater amounts it reduces resistance to tarnish and corrosion of

the gold alloy. Therefore, the maximum content should not exceed 16%.

13

Silver:

It whitens the alloy, thus helping to counteract the reddish colour of copper.

To a slight extent it increases strength and hardness. In large amounts however, it

reduces tarnish resistance.

Platinum:

It increases the strength and corrosion resistance. It also increases the melting

point and has a whitening effect on the alloy. It helps to reduce the grain size.

Palladium:

It is similar to platinum in its effect. It hardens as well as whitens the alloy. It

also raises the fusion temperature and provides tarnish resistance. It is less expensive

than platinum, thus reducing cost of alloy.

Zinc:

It acts as a scavenger for oxygen. Without zinc the silver in the alloy causes

absorption of oxygen during melting. Later during solidification, the oxygen is

rejected producing gas porosities in the casting.

Indium, Tin and Iron:

They help to harden the metal ceramic gold - palladium alloys, iron being the

most effective.

Gallium:

It is added to compensate for the decreased coefficient of thermal expansion

that results when the alloy is made silver free. The elimination of silver reduces the

tendency for green stain at the margin of the metal-porcelain interface.

Iridium, Ruthenium, Rhenium:

They help to decrease the grain size. They are added in very small quantities

(about 100 to 150 ppm).

14

Crystals in alloys:

All modern noble metal alloys are fine grained. Smaller the grain size of the

metal, the more ductile and stronger it is. It also produces a more homogenous casting

and improves the tarnish resistance. A large grain size reduces the strength and

increases the brittleness of the metal. Factors controlling the grain size are the rate of

cooling, shape of the mold, and composition of the alloy.

NOBLE METALS

The noble metals have been the basis of inlays, crowns and bridges because of

their resistance to corrosion in the oral cavity.

Gold, platinum, palladium, rhodium, ruthenium, iridium, osmium, and silver are

the eight noble metals. However, in the oral cavity, silver is more reactive and

therefore is not considered as a noble metal.

Of the eight noble metals, four are of major importance in dental casting alloys,

i.e., gold, platinum, palladium and silver. All four have a face-centered cubic crystal

structure and all are white coloured except for gold.

Gold

Pure gold is a soft and ductile metal with a yellow “Gold” hue. It has a density

of 19.3 gms/cm3 , melting point of 1063oC, boiling point of 2970 oC and CTE of

14.2×10-6/°C. Gold has a good luster and takes up a high polish. It has good chemical

stability and does not tarnish and corrode.

Gold content:

Traditionally the gold content of dental casting alloys have been referred to in

terms of:

3. Karat

4. Fineness

Karat:

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

15

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.

Silver

It is sometimes described as the “Whitest” of all metals. It has the lowest

density 10.4gms/cm3 and melting point of 961oC, boiling point of 2216 oC among the

four precious metals used in dental casting alloys. Its CTE is 19.7×10-6/oC , which is

comparatively high.

Palladium

It has a density of 12.02gms/cm3. Palladium has a higher melting point of

1552oC, boiling point of 3980 oC and lower CTE which is 11.8×10-6/oC, when

compared to gold.

Platinum

It has the highest density of 21.45 gms/cm3 , highest melting point of 1769oC,

boiling point of 4530 oC and the lowest CTE 8.9×10-6/oC among the four precious

metals used in dental casting alloys.

Carbon:

Carbon content is most critical. Small amounts may have a pronounced effect

on strength, hardness and ductility. Carbon forms carbides with any of the metallic

constituents which is an important factor in strengthening the alloy. However when in

excess it increases brittleness. Thus, control of carbon content in the alloy is

important.

Boron:

Deoxidizer and hardner, but reduces ductility.

16

Copper:

It is the principal hardner. It reduces the melting point and density of gold. If

present in sufficient quantity, it gives the alloy a reddish colour. It also helps to age

harden gold alloys. In greater amounts it reduces resistance to tarnish and corrosion of

the gold alloy. Therefore, the maximum content should not exceed 16%.

Silver:

It whitens the alloy, thus helping to counteract the reddish colour of copper.

To a slight extent it increases strength and hardness. In large amounts however, it

reduces tarnish resistance.

Platinum:

It increases the strength and corrosion resistance. It also increases the melting

point and has a whitening effect on the alloy. It helps to reduce the grain size.

Palladium:

It is similar to platinum in its effect. It hardens as well as whitens the alloy. It

also raises the fusion temperature and provides tarnish resistance. It is less expensive

than platinum, thus reducing cost of alloy.

Iron, Copper, Beryllium

They are hardeners. In addition, beryllium reduces fusion temperature and

refines grain structure . IRON has melting point of 1527°C , boiling point of 3000 °C

, density of 7.87 gm/cm3 and CTE 12.3 ×10-6/oC . where as COPPER has melting

point of 1083°C , boiling point of 2595 °C , density of 8.96 gm/cm3 and CTE 16.5

×10-6/oC .

Manganese and Silicon:

Primarily oxide scavengers to prevent oxidation of other elements during

melting. They are also hardeners. MANGANESE has melting point of 650°C ,

boiling point of 1107 °C , density of 1.74 gm/cm3 and CTE 25.2 ×10-6/oC , where as

17

SILICON has melting point of 1410°C , boiling point of 2480 °C , density of 2.33

gm/cm3 and CTE 7.3 ×10-6/oC .

ALLOYS

The use of pure metals is quite limited in dentistry. To optimize properties,

most metals commonly used in engineering and dental applications are mixtures of

two or more metallic elements or in some cases one or more metals and/or nonmetals.

They are generally prepared by fusion of the elements above their melting points. A

solid material formed by combining a metal with one or more other metals or

nonmetals is called an alloy.

For example, a small amount of carbon is added to iron to form steel. A

certain amount of chromium is added to iron, carbon, and other elements to form

stainless steel, an alloy that is highly resistant to corrosion. Noted chromium is also

used to impart corrosion resistance to nickel or cobalt alloys, which comprise two of

the major groups of base metal alloys used in dentistry.

At least four factors determine the extent of solid solubility of metals; atom

size, valence, chemical affinity and chemical structure.

Atom Size:

If the sizes of two metallic atoms differ by less than approximately 15% (first

noted by Hume-Rothery), they possess a favorable size factor for solid solubility.

Valence:

Metals of the same valence and size are more likely to form extensive solid

solutions than are metals of different valences.

Chemical Affinity:

When two metals exhibit a high degree of chemical affinity, they tend to form

an intermetallic compound upon solidification rather than a solid solution.

Crystal structure:

Only metals with the same type of crystal structure can form a complete series

of solid solutions.

18

The simplest alloy is a solid solution, in which atoms of two metals are located

in the same crystal structure such as face-centered cubic (fcc), body-centered cubic

(bcc) and hexagonal close-packed (hcp).

Liquidus and Solidus temperature

Liquidus temperature – Temperature at which an alloy begins to freeze on

cooling or at which the metal is completely molten on heating.

Solidus temperature – Temperature at which an alloy becomes solid on

cooling or at which the metal begins to melt on heating.

Coring

In the coring process the last liquid to solidify is metal with lower solidus

temperature and solidifies between the dendrites. Thus under rapid freezing

conditions, the alloy has a core structure. The core consists of the dendrites

composed of compositions with higher solidus temperature, and the matrix is the

portion of the micro-structure between the dendrites that contains compositions with

lower solidus temperatures.

Homogenization

For homogenization heat treatment, the cast alloy is held at a temperature near

its solidus to achieve the maximum amount of diffusion without melting. (This

process required 6 hr. for the alloy). Little or no grain growth occurs when a casting

receives this type of heat treatment eg. Annealing done mainly for wrought alloys .

The ductility of an alloy usually increases after homogenization heat treatment . Gold

alloys are heat treated by softening (solution heat treat) or hardening (age hardening

heat treat)

Eutectic alloys:

Many binary alloy systems do not exhibit complete solubility in both the liquid

and the solid states. The eutectic system is an example of an alloy for which the

component metals have limited solid solubility. Two metals, A and B, which are

completely insoluble in each other in the solid state, provide the simplest illustration

of a eutectic alloy. In this case, some grains are composed solely of metal A and the

remaining grains are composed of metal B. The salt and water molecules intermingle

19

randomly in solution, the result upon freezing is a mixture of salt crystals and ice

crystals that form independently of each other.

Silver-copper system

The phase diagram for this system is presented in where 3 phases are found:

1. A liquid phase (L)

2. A silver-rich substitutional solid solution phase (α) containing a small amount

of copper atoms.

3. A copper-rich substitutional solid solution phase (β) containing a small

amount of silver atoms. The α and β phases are sometimes referred to as

terminal solid solutions because of their locations at the left and right sides of

the phase diagram.

This composition (72% silver and 28% copper) is known as the eutectic

composition or simply the eutectic. The following characteristics of this special

composition should be noted.

1. The temperature at which the eutectic composition melts (779oC or

1435oF) is lower than the fusion temperature of silver or copper (eutectic

literally means “lowest melting”).

2. There is no solidification range for composition E.

3. The eutectic reaction is sometimes written schematically as follows.

Liquid α solid solution + β solid solution

20

DENTAL CASTING ALLOYS

CLASSIFICATION:

1. Alloy types by functions:

In 1927, the Bureau of Standard established gold casting alloys, type I to type

IV according to dental function with hardness increasing from type I to type IV.

Type I (Soft) :

It is used for fabrication of small inlays, class III and class V restorations

which are not subjected to great stress . These alloys are easily burnishable.

Type -II (Medium):

These are used for fabrication of inlays subjected to moderate stress, thick 3/4

crowns, abutments, pontics, full crowns and soft saddles.

Type I and II are usually referred to as inlay gold.

Type -III (Hard):

It is used for fabrication of inlays subjected to high stress, thin 3/4 crowns,

thin cast backing abutments, pontics, full crowns, denture bases and short strength

FPDs . Type III alloys can be age hardened.

Type-IV (Extra hard):

It is used for fabrication of inlays subjected to high stress, denture bases, bars

and clasps, partial denture frameworks and long span FPDs. These alloys can be age

hardened by heat treatment.

Type III and Type IV gold alloys are generally called "Crown and Bridge

Alloys", although type IV alloy is used for high stress applications such as RPD

framework.

Later, in 1960, metal ceramic alloys were introduced and removable partial

denture alloys were added in this classification.

21

Metal ceramic alloys (hard and extra hard):

It is suitable for veneering with dental porcelain, copings, thin walled crowns,

short span FPDs and long span FPDs. These alloy vary greatly in composition and

may be gold, palladium, nickel or cobalt based.

Removable partial denture alloys :

It is used for removable partial denture frameworks. Now a days, light weight, strong

and less expensive nickel or cobalt based have replaced type IV alloys .

2. Alloy types by description:

By description, these alloys are classified into;

A) Crown and bridge alloys:

This category of alloys include both noble and base metal alloys that have

been or potentially could be used in the fabrication of full metal or partial veneers.

1. Noble metal alloys:

i) Gold based alloy - type III and type IV gold alloys , low gold alloys

ii) Non-gold based alloy-Silver -palladium alloy

2. Base metal alloys:

i) Nickel-based alloys

ii) Cobalt based alloys

3. Other alloys:

i) Copper-zinc with Indium and nickel

ii) Silver-indium with palladium

B) Metal ceramic alloy:

1. Noble metal alloys for porcelain bonding:

i) Gold-platinum -palladium alloy

ii) Gold-palladium-silver alloy

iii) Gold-palladium alloy

iv) Palladium silver alloy

v) High palladium alloy

2. Base metal alloys for porcelain bonding:

i) Nickel -chromium alloy

ii) Cobalt-chromium alloy

22

C) Removable partial denture alloy:

Although type-IV noble metal alloy may be used, majority of removable

partial framework are made from base metal alloys:

1. Cobalt-chromium alloy

2. Nickel-chromium alloy

3. Cobalt-chromium-nickel alloy

4. Silver-palladium alloy

5. Aluminum -bronze alloy

3. Alloy type by nobility:

High noble, noble, and predominantly base metal.

Alloy Classification of the American Dental Association (1984)

Alloy type Total noble metal content

High noble metal Contains > 40 wt% Au and > 60 wt% of the noble

metal elements (Au + Ir + Os + Pd + Pt + Rh + Ru)

Noble metal Contains > 25 wt % of the noble metal elements

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

23

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

Al bronze

4. Alloy Type By Major Elements:

Gold-based, palladium-based, silver-based, nickel-based, cobalt-based and

titanium-based .

5. Alloy Type By Principal Three Elements:

Such as Au-Pd-Ag, Pd-Ag-Sn, Ni-Cr-Be, Co-Cr-Mo, Ti-Al-V and Fe-Ni-Cr.

6. Alloy Type By Dominant Phase System:

Single phase [isomorphous], eutectic, peritectic and intermetallic.

24

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.

DESIRABLE PROPERTIES OF DENTAL CASTING ALLOYS

1. Biocompatibility

2. Ease of melting

3. Ease of casting

4. Ease of brazing (soldering)

5. Ease of polishing

6. Little solidification shrinkage

7. Minimal reactivity with the mold material

8. Good wear resistance

9. High strength

10. Excellent corrosion resistance

11. Porcelain Bonding

To achieve a sound chemical bond to ceramic veneering materials, a substrate

metal must be able to form a thin, adherent oxide, preferably one that is light in color

so that it does not interfere with the aesthetic potential of the ceramic. The metal must

have a thermal expansion/contraction coefficient that is closely matched to that of the

porcelain.

25

Gold alloys:

ADA specification No.5 classify dental gold casting alloys as:

1. High Gold Alloys Type I

Inlay Gold

Type II

Type III

Crown & Bridge Alloy

Type IV

2. Low Gold Alloys

3. White Gold Alloys

High gold alloy:

These alloys contain 70% by weight or more of gold palladium and platinium.

ADA specification No.5 divides this into four depending upon mechanical properties.

Type I (Soft)

They are weak, soft and highly ductile, useful only in areas of low occlusal

stress designed for simple inlays such as used in class I, III & V cavities.

These alloys have a high ductility so they can be burnished easily. Such a

characteristic is important since these alloys are designed to be used in conjunction

with a direct wax pattern technique. Since such a technique occasionally results in

margins that are less than ideal it is necessary to use a metal that can be burnished. At

present, these are used very rarely.

Properties:-

1. Hardness VHN (50 – 90)

2. Tensile Strength Quite Low

276 MPa or 40,000 PSi

3. Yield Strength 180 MPa or 26,000 PSi

26

4. Linear Casting Shrinkage 1.56% (according to Anusavice)

5. Elongation or ductility 46% - William O Brien

18% - Anusavice

Composition:

Au Ag Cu Pt Pd Zn & Ga

83% 10% 6% - 0.5% balance

Type II (Medium)

These are used for conventional inlay or onlay restorations subject to moderate

stress, thick three quarter crowns, pontics and full crowns. These are harder and have

good strength.

Ductility is almost same as that of type I alloy however, yield strength is

higher. Since burnishability is a function of ductility and yield strength, greater effort

is required to deform the alloy. They are less yellow in color due to less gold.

Properties:

1. Hardness VHN (90-120)

2. Tensile Strength 345 MPa

3. Yield Strength 300 MPa

4. Linear Casting Shrinkage 1.37%

5. Elongation 40.5% - William O Brien

10% - Anusavice

Composition:-

Au Ag Cu Pt Pd Zn &Ga

77% 14% 7% - 1% balance

27

Type III (Hard):

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

type II, this type can be age hardened. The type III alloy, burnishing is less important

than strength.

Properties:

1. Hardness VHN (120 – 150)




5. Elongation or ductility 39.4% - William O Brien

5% - Anusavice

Composition:


75% 11% 9% - 3.5% balance

Type IV (Extra Hard):

These are used in areas of very high stress, crowns and long span bridges. It has

lowest gold content of all four type (Less than 70%) but has the highest percentage of

silver, copper, platinum and Palladium. It is most responsive to heat treatment and

yield strength but lowers ductility.

Properties:

1. Hardness VHN (150-200)




5. Elongation or ductility 17% - William O Brien

3% - Anusavice

28

Composition:


56% 25% 14% - 4% balance

Type Hardness Proportional limit Strength Ductility Corrosion

resistance

I

II INCREASES DECREASES

III

IV

Composition Range (weight percent) of traditional type I to IV alloys and four

metal -ceramic alloys

Alloy type Main elements Au Cu Ag Pd Sn, In, Fe, Zn, Ga

I High noble (Au base) 83 6 10 0.5 Balance

II High noble (Au base) 77 7 14 1 Balance

III High noble (Au base) 75 9 11 3.5 Balance

III Noble (Au base) 46 8 39 6 Balance

III Noble (Ag base) 70 25 Balance

IV High noble (Au base) 56 14 25 4 Balance

IV Noble (Ag base) 15 14 45 25 Balance

Metal-ceramic High noble (Au base) 52 38 Balance

Metal-ceramic Noble (Pd base) 30 60 Balance

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

Metal-ceramic Noble (Pd base) 0-6 0-15 0-10 74-88 Balance

Linear solidification shrinkage of casting alloys

Alloy Casting shrinkage (%)

Type I, gold base 1.56

Type II, gold base 1.37

Type III, gold base 1.42

Ni-Cr-Mo-Be 2.3

Co-Cr-Mo 2.3

29

Heat treatment of gold alloys:

Heat treatment of alloys is done in order to alter its mechanical properties.

Gold alloys can be heat treated if it contains sufficient amount of copper.

Only type III and type IV gold alloys can be heat-treated.

There are two types of heat treatment.

1. Softening Heat Treatment (Solution heat treatment)

2. Hardening Heat Treatment (Age hardening)

Softening Heat Temperature:

Softening heat treatment increased ductility, but reduces tensile strength,

proportional limit, and hardness.

Indications: It is indicated for appliances that are to be grounded, shaped, or

otherwise cold worked in or outside the mouth.

Method:

The casting is placed in an electric furnace for 10 minutes at a temperature of

700oC and then it is quenched in water. During this period, all intermediate phases are

presumably changed to a disordered solid solution, and the rapid quenching prevents

ordering from occurring during cooling.

Each alloy has its optimum temperature. The manufacturer should specify the

most favorable temperature and time.

Hardening Heat Treatment:

Hardening heat treatment increases strength, proportional limit, and hardness,

but decreases ductility. It is the copper present in gold alloys, which helps in the age

hardening process.

Indications:

It is indicated for metallic partial dentures, saddles, bridges and other similar

structures. It is not employed for smaller structures such as inlays.

30

Method:

It is done by “soaking” or ageing the casting at a specific temperature for a

definite time, usually 15 to 30 minutes. It is then water quenched or cooled slowly.

The aging temperature depends on the alloy composition but is generally between

200oC and 450oC. During this period, the intermediate phases are changed to an

ordered solid solution.

The proper time and temperature for age hardening an alloy are specified by the

manufacturer.

Ideally, before age hardening an alloy, it should first be subjected to a

softening heat treatment to relieve all strain hardening & to start the hardening

treatment when the alloy is in a disordered solid solution. This allows better control of

the hardening process.

31

METAL CERAMIC ALLOYS

The main function of metal-ceramic alloys is to reinforce porcelain, thus

increasing its resistance to fracture.

Requirements:

1. They should be able to bond with porcelain.

2. Its coefficient of thermal expansion should be compatible with that of

porcelain.

3. Its melting temperature should be higher than the porcelain firing temperature.

It should be able to resist creep or sag at these temperatures.

4. It should not stain or discolor porcelain.

The alloys used for metal-ceramic purposes are grouped under two categories:

i) Noble metal alloys

ii) Base metal alloys.

In case of noble metal alloys for porcelain bonding , addition of 1% base metals

(iron, indium, tin etc.) increases porcelain-metal bond strength, which is due to

formation of an oxide film on its surface. It also increases strength and proportional

limit.

Properties:

Modules of elasticity:

The base metal alloys have a modulus of elasticity approximately twice that of

gold alloys. Thus it is suited for long span bridges. Similarly, thinner castings are

possible.

Hardness:

The hardness of base metal alloys ranges from 175 to 360 VHN. Thus, they are

generally harder than noble metal alloys. Thus, cutting, grinding and polishing

requires high;- speed and other equipment.

32

Ductility:

It ranges from 10 to 28% for base metal alloys. Noble metal alloys have an

elongation of 5 to 10%.

Density:

The density of base metal alloys are less, which is approximately 8.0 gms/cm3 as

compared to 18.39 gms/cm3 for noble metal alloys.

Sag Resistance:

Base metal alloys resist creep better than gold alloy when heated to high

temperatures during firing.

Bond Strength: Varies according to composition.

Technique Sensitivity: Base metals are more technique sensitive than high noble

metal-ceramic alloys.

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

This is one of the oldest metal ceramic alloy system. But these alloys are not

used widely today because they are very expensive.

Composition:

Gold – 75% to 88%

Palladium – Upto 11%

Platinum – Upto 8%

Silver – 5%

Trace elements like Indium, Iron and Tin for porcelain bonding.

33

Advantages Disadvantages

1. Excellent castability 1. High cost

2. Excellent porcelain bonding 2. Poor sag resistance so not suited for

3. Easy to adjust and finish long span fixed partial dentures.

4. High nobility level 3. Low hardness (Greater wear)

5. Excellent corrosion and tarnish 4. High density (fewer casting per

resistance. ounce)

6. Bio compatible

7. Some are yellow in color

8. Not “Technique Sensitive”

9. Burnish able

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.

Poor sag resistance, low hardness & high cost, two variations on the basic

combination of gold, palladium and silver were created and are identified as the either

the high-silver.

Composition (High Silver Group):

Gold – 39% to 53%

Silver – 12% to 22%

Palladium – 25% to 35%

Force amounts of oxidizable elements are added for porcelain bonding.


1. Less expensive than Au-Pt-Pd alloys 1. High silver content creates potential

2. Improved rigidity and sag resistance. for porcelain discoloration.

3. High mobility. 2. High Cost.

3. High coefficient of thermal expansion.

4. Tarnish and corrosion resistant.

Composition (Low Silver Group):

Gold – 52% to 77%

Silver- 5% to 12%

34


Trace amounts of oxidizable elements for porcelain bonding.


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

discoloration (but less than high

silver group)

2. Improved sag resistance 2. High cost.

3. High noble metal content 3. High coefficient of thermal

expansion.

4. Tarnish and corrosive resistant

Gold-Palladium (Au-Pd) System:

This particular system was developed in an attempt to overcome the major

limitations in the gold-platinum-silver system and Pd-Ag alloys.

-Porcelain discoloration.

-Too high a coefficient of thermal expansion & contraction.

Composition:

Gold – 44% to 55%

Gallium – 5%


Indium & Tin – 8% to 12%

Indium, Gallium and Tin are the oxidizable elements responsible for porcelain

bonding.


1. Excellent castability 1. Not thermally compatible with high

expansion dental porcelain.

2. Good bond strength 2. High cost

3. Corrosion and tarnish resistance

4. Improved hardness

5. Improved strength ( sag resistance)

6. Lower density

35

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) metals.

Composition:

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.

1. Low Cost 1. Discoloration (yellow, brown or green) may

occur with some dental porcelains.

2. Low density 2. Some castibility problems reported (with

induction casing)

3. Good castibility (when torch 3. Pd and Ag prone to absorb gases.

casting) 4. Require regular purging of the porcelain

4. Good porcelain bonding, furnace.

5. Burnishability 5. May form internal oxides (yet porcelain

6. Low hardness bonding does not appear to be a problem)

7. Excellent sag resistance 6. Should not be cast in a carbon crucible.

8. Moderate nobility level 7. Non-carbon phosphate bonded investments

9. Good tarnish and corrosion recommended.

resistance. 8. High coefficient of thermal expansion.

10. Suitable for long-span fired

partial dentures.

High Palladium System:

Several types of high palladium were originally introduced (Tuccillo, 1987).

More popular composition, group containing cobalt and the other containing copper.

Copper appears to be more popular.

36

Composition (Palladium-Cobalt) alloy:

Palladium – 78% to 88% Cobalt – 4% to 10%

(Some high palladium-cobalt alloys may contain 2% gold)

Trace amounts of oxidizable elements (such as gallium and indium) are added for

porcelain bonding.


1. Low cost 1. More compatible with higher expansion

2. Reportedly good sag resistance porcelains.

3. Low density means more casting 2. Are more prone to over-heating than

per ounce (then gold based alloys) high Pd-Cu.

4. Some melt and cast easily 3. Produces a thick, dark oxide

5. Good polishability (Supposed 4. Colored oxide layer may cause bluing of to

be similar to Au-Pd alloys) porcelain.

6. Reportedly easier to presolder 5. Prone to gas absorption

than Pd-Cu alloys. 6. Little information on long-term clinical

success.

Composition (Palladium-Copper alloys):

Palladium – 70% to 80% Copper – 9% to 15%

Gold – 1% to 2% Platinum – 1%

Some, but not all, high palladium-copper alloys contain small quantities ( 1% to

2%) of gold and/or platinum. Trace amounts of the oxidizable elements gallium,

indium and tin are added for porcelain bonding.


1. Good castability 1. Produces dark, thick oxides

2. Lower cost (than gold based alloys) 2. May discolor (gray) some dental 3. 3.

3. Low density means more casings porcelains.

Per ounce 3. Must visually evaluate oxide color to

4. Tarnish and corrosion resistance determine if proper adherent oxide was

5. Compatible with many dental formed.

37

Porcelains. 4. Should not be cast in carbon crucibles

6. Some are available in 1- dwt ingots. (electric casting machines)

5. Prone to gaseous absorption.

6. Subject to thermal creep (marginal

bonding)

7. May not be suitable for long span fixed

partial dentures.

8. Little information on long term clinical

success.

9. May be difficult to polish

10. Resoldering may be a problem

11. High hardness.

38

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

Nickel-chromium (Ni-Cr) System:

These metal-ceramic alloy offer such economy that they are also used for

complete crown and all metal fixed partial dentures (Bertolotti, 1984).

The major constituents are nickel and chromium, with a wide array of minor

alloying elements.

The system contains two major groups, those that contain

- Beryllium

- Beryllium free

Co-Cr Removable Co-Cr-Ni Partial denture Ni-Cr

Base metal Surgical Co-Cr-Mo Casting alloy Implant Ni-Cr-Co Be Cont. (Class-II Fixed Ni-Cr Partial denture No Be (Class-I) Co-Cr

(Class-III)

39

Of the two, Ni-Cr-Beryllium alloy are generally regarded as possessing superior

properties and have been more popular (Tuccillo and Cascone,1983).

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.


1. Low cost 1. Cannot use with nickel sensitive patients

2. Low density, permits more 2. Beryllium exposure may be potentially

casing per ounce. harmful to technicians and patients.

3. High sag resistance 3. Proper melting and casting is a learned skill.

4. Can produce thin casting 4. bond failure more common in the oxide layer.

5. Poor thermal conductor 5. High hardness (May wear opposing teeth)

6. Can be etched. 6. Difficult to solder

7. Ingots do not pool

8. Difficult to cut through cemented castings

Nickel-Chromium Beryllium free alloys:

Composition:

Nickel – 62% to 77% Chromium – 11% to 22%

Boron (some), iron, molybdenum, Niobium (or columbium) and/or tantalum.


1. Do not contain beryllium 1. Cannot use with Nickel sensitive patients.

2. Low cost 2. Cannot be etched.

3. Low density means more casting 3. May not cast as well as Ni-Cr-Be alloys

per ounce (Cr doesn’t dissolve in acids)

4. Produces more oxide than Ni-Cr-Be

40

alloys.

Comparative properties of Ni / Cr alloys and type III casting gold alloys for

small cast restorations

Property

(Units)

Ni/Cr Type III gold

alloy

Comments

Density (g/cm3) 8 15 More difficult to produce defect free casting for

Ni/Cr alloys.

Fusion

temperature

as high as

1350°C

Normally lower

than 1000°C

Ni/Cr alloys require electrical induction furnace

or oxyacetylene equipment.

Casting

shrinkage (%)

2 1.4 Mostly compensated for by correct choice of

investment

Tensile strength

(MPa)

600 540 Both adequate for the applications being

considered.

Proportional

limit (MPa)

230 290 Both high enough to prevent distortion for

applications being considered; not that values

are lower than for partial denture alloys

Modulus of

elasticity (GPa)

220 85 Higher modulus of Ni/Cr is an advantage for

large restoration e.g. bridges and for porcelain

bonded restoration.

Hardness

(Vickers)

300 150 Ni/Cr more difficult to polish but retains polish

during service

Ductility (%

elongation)

upto 30% 20 (as cast)

10 (hardened)

Relatively large values suggest that burnishing is

possible; however, large proportional limit value

suggests higher forces would be require.

RECYCLING NOBLE METAL CASTING ALLOY:

The alloy scrap should be recycled because of the high value of the precious

metals. It can be collected and sent back to the manufacturer of it can be recast. These

alloys are stable so it can be recast two or three times without much change in its

composition. However, the more volatile base metals like zinc, indium, tin and iron

may be lost. To compensate for this equal amount of new alloy should be added to the

scrap during recasting. They should be carefully cleaned before reuse.

41

Alloys of different types and manufacturers should not be mixed as it may change its

composition and properties.

COBALT CHROMIUM ALLOYS:

Cobalt chromium alloys have been available since the 1920’s. They possess

high strength. Their excellent corrosion resistance especially at high temperatures

makes them useful for a number of applications.

These alloys are also known as ‘satellite’ because they maintained their shiny,

star-like appearance under different conditions.

They have bright lustrous, hard, strong and non-tarnishing qualities.

APPLICATIONS:

1. Denture base

2. Cast removable partial denture framework.

3. Surgical implants.

4. Car spark plugs and turbine blades.

COMPOSITION:

Cobalt - 55 to 65%

Chromium - 23 to 30%

Nickel - 0 to 20%

Molybdenum - 0 to 7%

Iron - 0 to 5%

Carbon - upto 0.4%

Tungsten, Manganese, Silicon and Platinum in traces.

According to A.D.A specification No. 14 a minimum of 85% by weight of

chromium, cobalt, and nickel is required. Thus the iron base corrosion resistant alloys

are excluded.

PROPERTIES:

The Cobalt-Chromium alloys have replaced Type IV gold alloys because of

their lower cost and adequate mechanical properties. Chromium is added for tarnish

resistance since chromium oxide forms an adherent and resistant surface layer.

42

1. Physical Properties:

Density: The density is half that of gold alloys, so they are lighter in weight.

8 to9gms/cm2.

Fusion temperature: The casting temperature of this alloy is considerably higher

than that of gold alloys. 1250oC to 1480oC.

A.D.A. specification No. 14 divides it into two types, based on fusion

temperature, which is defined as the liquidus temperature.

Type-I (High fusing) – liquidus temperature greater than 1300oC

Type-II (Low fusing) – liquidus temperature not greater than 1300oC

2. Mechanical Properties:

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

Elongation: Their ductility is lower than that of gold alloys. Depending on the

composition, rate of cooling, and the fusion and mold temperature employed, it ranges

from 1 to 12%.

These alloys work harden very easily, so care must be taken while adjusting the

clasp arms of the partial denture.

Modulus of elasticity: They are twice as stiff as gold alloys. Thus, casting can be

made more thinner, thus decreasing the weight of the R.P.D. Adjustment of clasp is

not easy. 225×103Mpa.

Hardness: These alloys are 50% harder than gold alloys. Thus, cutting, grinding and

finishing is difficult. 432 VHN.

3. Tarnish and corrosion resistance: Formation of a layer of chromium oxide on the

surface of these alloys prevents tarnish and corrosion in the oral cavity. This is called

‘passive effect’.

43

Solutions of hypochlorite and other containing 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 hould

not be used for cleaning chromium base alloys.

4. Casting Shrinkage: The casting shrinkage is much greater than that of gold alloys,

so limited use in crown & bridge. 2.3%

The high shrinkage is due to their high fusion temperature.

5. Porosity: As in gold alloys, porosity is due to shrinkage and release of

dissolved gases. Porosity is affected by the composition of the alloys and its

manipulations.

44

Comparative properties of Co / Cr alloys and type IV casting gold alloys for

partial denture

Property (Units) Co/Cr Type IV gold

alloy

Comments

Density (g/cm3) 8 15 More difficult to produce defect

free casting for Co/Cr alloys but

denture frameworks are lighter

Fusion temperature as high as

1500°C

Normally

lower than

1000°C

Co/Cr alloys require electrical

induction furnace or oxyacetylene

equipment.

Can not use gypsum bonded

investments for Co/Cr alloys

Casting shrinkage

(%)

2.3 1.4 Mostly compensated for by correct

choice of investment

Tensile strength

(MPa)

850 750 Both acceptable

Proportional limit

(MPa)

700 500 Both acceptable; can resist stresses

without deformation

Modulus of

elasticity (GPa)

220 100 Co/Cr more rigid for equivalent

thickness; advantage for

connectors; disadvantage for

clasps

Hardness (Vickers) 420 250 Co/Cr more difficult to polish but

retains polish during service

Ductility (%

elongation)

2 15 (as cast)

8 (hardened)

Co/Cr clasps may fractured if

adjustments are attempted.

45

Summary of base metal alloy properties

Property Ni-Cr without Be Ni-Cr with Be Co-Cr

Strength (MPa) 255-550 480-830 415-550

Ultimate tensile strength

(MPa)

550-900 760-1380 550-900

% elongation 5-35 3-25 3-10

Modulus of elasticity

(MPa)

13.8-207 x 104 17.2-20.7 x 104 17.2-20.7x104

Vickers hardness 175-350 300-350 300-500

Casting temperature (°C) 1430-1570 1370-1480 1430-1590

TECHNICAL CONSIDERATIONS:

The high casting temperature prevents the use of gypsum bonded investments.

Phosphate-bonded or silica-bonded investments are used during the casting of these

alloys.

A slow burnout is done at a temperature of 732oC to 982oC. It is done two hours

after investing.

The high fusion temperature also prevents the use of gas-air torches for melting

these alloys. Oxygen-acetylene torches are usually employed. Electrical sources of

melting such as carbon arcs, argon arcs, high frequency induction, or silicon-carbide

resistance-furnaces, may also be used.

These alloys are difficult to cut, grind, or finish. Special hard, high-speed finishing

tools are necessary.

DISADVANTAGES OF ETCHING BASE METAL ALLOYS:

Etching of the base metal alloys is done to improve the retention of resin-

bonded retainers (“Maryland Bridge”).

Nickel may produce allergic reactions in some individuals. It is also a potential

carcinogen.

46

Beryllium which is present in many base metal alloys is a potentially toxic

substance. Inhalation of beryllium containing dust or fumes is the main route of

exposure. It causes a condition know as ‘berylliosis’. It is characterized by flu-like

symptoms and granulomas of the lungs.

Adequate precautions must be taken while working with base metal alloys.

Fumes from melting and dust from grinding beryllium-containing alloys should be

avoided. The work area should be well ventilated.

47

TITANIUM AND TITANIUM ALLOYS:

Titanium is called “material of choice” in dentistry. This is attributed to the

oxide formation property which forms basis for corrosion resistance and

biocompatibility of this material. The term 'titanium' is used for all types of pure and

alloyed titanium.

Properties of titanium:

1. Resistance to electrochemical degradation

2. Begins biological response

3. Relatively light weight

4. Low density (4.5 g/cm3)

5. Low modulus (100 GPa)

6. High strength (yield strength = 170-480 MPa; ultimate strength = 240-550

MPa)

7. Passivity

8. Low coefficient of thermal expansion (8.5 x 10–6/°C)

9. Melting & boiling point of 1668°C & 3260°C

Uses:

Commercially pure titanium is used for dental implants, surface coatings,

crowns, partial dentures, complete dentures and orthodontic wires.

Commercially Pure Titanium (CP Ti):

It is available in four grades (according to American Society for Testing and

Materials ASTM) which vary according to the oxygen (0.18-0.40 wt.%), iron (0.20-

0.50 wt%) and other impurities. It has got an alpha phase structure at room

temperature and converts to beta phase structure at 883°C which is stronger but

brittle.

Titanium alloys:

Alloying elements are added to stabilize alpha or the beta phase by changing

beta transformation temperature e.g. in Ti-6Al-4V, Aluminum is an alpha stabilizer

whereas vanadium as well as copper and palladium are beta stabilizer. Alpha titanium

48

is weld able but difficult to work with at room temperature. Beta titanium is malleable

at room temperature and is used in orthodontics, but is difficult to weld.

Pure titanium is used to cast crowns, partial denture, and complete denture.

Cast titanium:

Cast titanium has been used for more than 50 years, and it has been recently

that precision casting can be obtained from it. The two most important factors in

casting titanium based materials are its high melting point (1668°C) and chemical

reactivity. Because of the high melting point, special melting procedures, cooling

cycles, mold materials, and casting equipments are required to prevent metal

contamination, because it readily reacts with hydrogen, oxygen and nitrogen at

temperatures greater than 600°C. So casting is done in a vacuum or inert gas

atmosphere. The investment materials such as phosphate bonded silica and phosphate

investment material with added trace metal are used. It has been shown that

magnesium based investment cause internal porosity in casting.

Because of its low density, it is difficult to cast in centrifugal casting machine.

So advanced casting machine combining centrifugal, vacuum, pressure and gravity

casting with electric arc melting technology have been developed.

Difficulties in casting Titanium :

1. High melting point

2. High reactivity

3. Low casting efficiency

4. Inadequate expansion of investment

5. Casting porosity

6. Difficulty in finishing

7. Difficulty in welding

8. Requires expensive equipments

49

REVIEW OF LITERATURE

Anusavice KJ, Okabe T, Galloway SE, Hoyt DJ, and Morse PK: Flexure test

evaluation of presoldered base metal alloys. J Prosthet Dent 54:507, 1985.

Wide variability in the strength of brazed joints in NI-Cr-Mo-Be and NI-Cr-Mo

alloys was reported. The strength of the brazed joint ranged from 20% to 90% of that

of a solid bar of the same metals and was not affected by gap widths of 0.25 or 0.51

mm.

Anusavice KJ, and Shafagh I: Inert gas presoldering of nickel-chromium alloys. J

Prosthet Dent 55: 3137, 1986.

An argon gas environment did not improve the strength of presoldered joint

strength of nickel-chromium-molybdenum-beryllium alloys. Most of the fractures

appeared to originate within the solder filler alloy. Entrapped flux particles and gases

were the most likely cause of these failures.

Baran GR: The metallurgy of Ni-Cr alloys for fixed prosthodontics. J Prosthet Dent

50: 639, 1983.

A Classic article that contains an extensive presentation of alloy compositions,

mechanical properties, microstructures and clinically relevant considerations for the

use of these alloys.

Moffa JP, Guckes AD, Okawa MT and Lilly GE: An evaluation of nonprecious

alloys for use with porcelain veneers. Part II. Industrial safety and biocompatibility. J.

Prosthet Dent 30:432, 1973.

This article provides quantitative information about the levels of beryllium

produced during the finishing and polishing of cast base metal dental alloys.

Monday JL and Asgar K: Tensile strength comparison of presoldered and

postsoldered joints J Prosthet Dent 55:23, 1986.

No significant differences in the tensile strength of presoldered and

postsoldered joints were found when the same technique was used. Torch soldering

yielded significantly stronger joints than the vacuum oven technique employed.

50

Rasmussen EJ, Goodkind RJ, and Gerberich WW: An investigation of tensile

strength of dental solder joints. J. Prosthet Dent 41: 418, 1979.

Higher strengths were reported for Type III gold alloy as gap distance was

increased, but that trend was not noted for a gold palladium alloy. These and other

observations are partially explained in terms of the competing effects of yield

strength, wettability and voids at the various gap distances.

Shillingburg HT, Hobo S and Fisher DW: Preparation design and margin distortion

in porcelain-fused-to-metal restorations. J. Prosthet Dent, 29: 276, 1973.

The results of this study suggested that thermal incompatibility stresses were

likely to cause margin distortion in metal ceramic crowns. However, subsequent

studies support other potential mechanisms, including the effect of excessive sand

blasting time and/or pressure.

Zavanelli R.A., Henriques G.E.P., Ferreira I., Rollo J.M.: Corrosion-

fatigue life of commercially pure titanium and Ti-6Al-4V alloys in different

storage environments. J Prosthet Dent. 84: 274-279, 2000 Ti-6Al-4V:

At room temperature, Ti-6Al-4V is a two phase (α + β alloy) and transformation takes

place at 975°C. The mechanical properties of this alloy is dictated by amount, size,

shape and morphology of alpha phase.

Allowing of Titanium with palladium and copper , lowers its melting

temperature to 1350°C. Binary and ternary based alloys have been cast. Ti-13Cu-

4.5Ni have been used to cast crowns and partial dentures.

The use of Ti-6Al-4V, Ti-15V, Ti-20Cu, Ti-30Pd, Ti-Co and Ti-Cu for casting

is still under research.

51

REFERENCE

1. Anusavice K.J.: Phillips science of dental materials, 10th ed. W.B.

saunders Co.:111-555,1996

2. Asgar K., Techow B.O., Jacobson J.M.: A new alloy for partial

dentures. J Prosthet Dent. 23 (1): 36-43, 1970.

3. Aydin A.K.: Evaluation of finishing and polishing techniques on

surface roughness of chromium -cobalt castings. J Prosthet Dent. 65

(6): 763-767, 1991.

4. Bezzon O.L., de Mattos M.G.C., Ribeiro R.F., Rollo J.M.: Effect of

beryllium on the castability and resistance of ceramometal bonds in

nickel - chromium alloys. J Prosthet Dent. 80: 570-574, 1998.

5. Bezzon O.L., Ribeiro R.F., Rollo J.M., Crosara S.: Castability and

resistance of ceremometal bonding in Ni-Cr and Ni-Cr-Be alloys. J

Prosthet Dent. 85: 299-304, 2001.

6. Carr A.B., Cai Z., Brantley W.A.: New high-palladium casting

alloys: Part 2. Effects of heat treatment and burnout temperature. Int J

Prosthodont. 6: 233-241, 1993

7. Caul H.J., Clabaugh W.S., Susa M.E.: Procedure for determination of

noble metal content of dental gold alloys J Am Dent Assoc. 61: 439-

443, 1960.

8. Chan D., Guillory V., Blackman R.: The effects of sprue design on

the roughness and porosity of titanium castings. J Prosthet Dent. 78:

400-404, 1997.

9. Civjan S., Huget E.F., Dvivedi N., Cosner H.J.: Further studies on

gold alloys used in fabrication of porcelain-fused -to-metal

restorations. J Am Dent Assoc. 90: 659-665, 1974.

10. Craig R.G., Powers J.M.: Restorative Dental Materials, 11th ed. C.V.

Mosby Co.:329-626, 2002.

11. Craig G.R., O'Brien W.J., Powers J.M.: Dental materials- properties

and manipulation. 4th ed., C.V. Mosby Co.: 114-272, 1987.

12. Elliott R.W.: The effects of heat on gold partial denture castings. J

Prosthet Dent. 13 (4): 688-698, 1963.

52

13. Federation Dentaire Internationale Technical Report No. 34.:

Alternative casting alloys for fixed prosthodontics. J Prosthet Dent. 40:

54-55, 1990.

14. Hamanaka H., Doi H., Yoneyama T., Okuno O.: Dental casting of

titanium and Ni-Ti alloys by a new casting machine. J Dent Res. 68

(11): 1529-1533, 1989.

15. Hesby D.A., Kobes P, Garver D.G, Pelleu CB.: Physical properties of

repeatedly used non precious metal alloy. J Prosthet Dent. 44: 291-299,

1980.

16. Hinman RW, LyndeTA, Pelleu GB Jr. and Gaugler RW: Factors affecting

airborne beryllium in dental spaces. J Prosthet Dent 33: 210, 1975.

17. Iwama C.Y., Preston J.D.: Cobalt-chromium-titanium alloy for

removable partial dentures. Int J Prosthodont.10: 309-317, 1997.

18. Morris HF: Properties of cobalt-chromium metal ceramic alloys after heat

treatment. J. Prosthet Dent 62: 426, 1989.

19. Morris H.F.: Veterans administration cooperative studies project No.

147/242. Part VII: The mechanical properties of metal ceramic alloys

as cast and after simulated porcelain firing. J Prosthet Dent. 61: 160-

169, 1989.

20. O'Brien W.J.: Dental Materials: Properties and selection, Ist ed.

Quintessence Publishing Co.: 303-467, 1989.

21. O'Brien W.J.: Practical application of current casting research. J

Prosthet Dent. 10 (3): 558-560, 1960.

22. Oruc S., Tulunoglu Y.: Fit of titanium and a base metal alloy metal-

ceramic crown. Int J Prosthodont. 83: 314-318, 2000.

23. Papazoglou E, Brantley WA, Carr AB and Johnston WM: Porcelain

adherence to high-palladium alloys. J. Prosthet Dent 70: 386, 1993.

24. Papazoglou E., Brantley W.A., Mitchell J.C., Cai Z., Carr A.B.:

New high- palladium casting alloys: studies of the interface with

porcelain. Int J Prosthodont. 9: 315-322, 1996.

25. Presswood R.G., Skjonsby H.S., Hopkins G., Presswood T.L,

Pendleton M.: A base metal alloy for ceramo-metal restorations. J

Prosthet Dent.44 (6): 624-629, 1980.

53

26. Rudd K.D., Morrow R.M., Bange A.A.: Accurate casts. J Prosthet

Dent. 21 (5): 545-554, 1969.

27. Rudd R.W., Rudd K.D.: A review of 246 errors possible during

fabrication of a RPD. J Prosthet Dent. 86 (3): 251-288, 2001.

28. Schmidt A.H.: Repairing chrome-cobalt castings. J Prosthet Dent. 5

(3): 385-387, 1955.

29. Syverud M., Dahi J.E., Hero H., Morisbak E.: Corrosion and

biocompatibility testing of palladium alloy castings. Dent Mat. 17: 7-

13: 2001.

30. Tamura K.: Essentials of dental technology, Ist ed. Quintessence

Publishing Co.: 159-289, 1987.

31. Valderrama S., Roekel N.V., Andersson M., Goodacre C.J., Munoz

C.A.: A comparison of the marginal and internal adaptation of titanium

and gold-platinum-palladium metal ceramic crowns. Int J Prosthodont.

8 (1): 29-37, 1995.

32. Vallittu P.K., Luotio K.: effect of cobalt-chromium alloy surface

casting on resistance to deflection fatigue and surface hardness of

titanium. Int J Prosthodont. 9 (6): 525-531, 1996.

33. Wakabayashi N.: Thickness and accuracy of superplastic Ti-6AI-4V

Alloy Denture Frameworks. Int J Prosthodont. 9 (6): 520-526, 1996.

34. Wataha J.C.: Biocompatibility of dental casting alloys: A review. J

Prosthet Dent. 83: 223-234, 2000.

35. Zavanelli R.A., Henriques G.E.P., Ferreira I., Rollo J.M.:

Corrosion-fatigue life of commercially pure titanium and Ti-6Al-4V

alloys in different storage environments. J Prosthet Dent. 84: 274-279,

2000

Documents

Dental Casting Alloys