DENTAL CASTING ALLOYS

INTRODUCTION

For casting dental restorations and for the fabrication of

wire and other structures, it is necessary to combine various

metals to produce alloys with adequate properties, for dental

applications. These alloys are produced largely from gold

combined with other noble metals and certain base metals. The

alloys are compounded to produce properties most acceptable for

their intended dental applications, such as simple inlays, bridges,

removable cast restorations, solders, (or) wrought wire forms.

The noble metals are those elements with a good metallic

surface that retain their surfaces in dry air.

The six metals of the platinum group they are, platinum,

iridium, radium, osmium and ruthemium; along with gold they are

called noble metals.

HISTORICAL PERSPECTIVE ON DENTAL CASTING

ALLOYS

The history of dental casting alloys has been influenced by

3 major factors:

1

1. The technologic changes of dental prosthesis.

2. Metallurgic advancements; and

3. Price changes of the noble metals since 1968.

Taggart’s presentation to the New York odontological

group in 1907 on the fabrication of cast inlay restorations often

has been acknowledged as the first reported application of the lost

wax technique in dentistry. The inlay technique described by

Taggart was an instant success. It soon led to the casting of

complex inlays such as onlays, crowns, FPD, and RPD frame

works. Because pure gold did not have the physical properties

required of these dental restorations, existing jewellery alloys

were quickly adopted. These gold alloys were further strengthened

with copper, silver (or) palladium.

In 1932, the dental materials group at the National Bureau

of Standards surveyed the alloys being used and roughly classified

them as Type 1, 2, 3 and 4.

In the following years, several patents were issued for

alloys containing palladium as a substitute for platinum.

2

By 1948, the composition of Dental Noble metal alloys for

cast metal restorations had become rather diverse, with these

formulations, the tarnishing tendency of the original alloys

apparently had disappeared. It is now known that in gold alloys,

palladium is added to counter act the tarnish potential of silver.

The base metal removable partial denture alloys were

introduced in the 1930s. Since that time, both Nickel-chromium

and cobalt-chromium formulations have become increasingly

popular compared with conventional Type IV gold alloys, which

previously were the predominant metals used for such prosthesis.

The obvious advantages of base metal alloys are their lighter

weight, increased mechanical properties and reduced costs.

Likewise, by 1978, the price of gold was climbing so

rapidly the attention focused on the noble metal alloys. To reduce

the precious metal content yet retain the advantages of the noble

metals for dental use.

PROPERTIES OF NOBLE METAL ALLOYS

Since these metals have a wide range of properties and are

widely used in dentistry, it is worth while to describe some of

their properties.

3

a. GOLD

1. Pure gold is a soft, malleable, ductile metal that does

not oxidize under atmospheric conditions and is

attacked by only a few of the most powerful

oxidizing agents.

2. It has a rich yellow colour with a strong metallic

luster.

3. Although it is the most ductile and malleable of all

metals, it ranks much lower in strength.

The pure metal fuses at 106°C, which is only 20° below the

melting point of copper (1083°C).

4. Small amounts of impurities have a pronounced

effect on the mechanical properties of gold and its

alloys.

The presence of less than 0.2% lead will cause gold to be

extremely brittle.

Mercury in small quantities also has a harmful effect on its

properties.

5. Gold is nearly as soft as lead, with the result that in

dental alloys, coins, and articles of jewellery it must

4

be alloyed with copper, silver, platinum and other

metals to develop the necessary hardness, durability

and elasticity.

6. The specific gravity of pure gold is between 19.30

and 19.33, making it one of the heavy metal.

7. Air (or) water at any temperature does not affect (or)

tarnish gold.

8. Gold is not soluble in sulfuric, nitric (or)

hydrochloric acids.

PALLADIUM

1. Palladium is not used in the pure state in dentistry, but it is

used in many dental alloys, combined with either gold (or)

silver.

It is cheaper than platinum and since it imparts many of the

properties of platinum to dental alloys, it is often used as a

replacement for platinum.

2. Palladium is a white metal some what darker than platinum.

5

3. Its specific gravity is 11.4 (or) about half that of platinum

and a little more than half that of gold.

4. It is a malleable and ductile metal with a melting point of

1555°C, which is the lowest of the platinum group of

metals.

5. The metals has the quality of absorbing (or) occluding large

quantities of hydrogen gar when heated. This can be an

undesirable quality when alloys containing palladium are

heated with an improperly adjusted gas air torch.

IRIDIUM, RUTHENIUM, AND RHODIUM

1. Small amounts of iridium are some times present in dental

alloys, either as impurities combined with platinum (or) as

additions to modify the properties.

2. As little as 0.005% (50 ppm) is effective in refining the

grain size of cast gold alloys.

Ruthenium produces a similar effect.

3. Iridium is a hard metal that is quite brittle, white with a

high sp. Gravity of 22.42 and an exceptionally high

melting point estimated to be about 2440'C.

6

SILVER

1. Silver is malleable and ductile, white, the best-known

conductor of heat and electricity, and stronger and harder

than gold but softer than copper.

2. It melts at 960.5'C, which is below the melting point of both

gold and copper.

3. It is unaltered in clean, dry air at any temperature but

combines with sulfur, chlorine and phosphorous, (or)

vapors containing these elements (or) their compounds.

Foods containing sulfur compounds cause severe tarnish on

silver.

4. Pure silver is seldom employed in dental restorations

because of the black sulfide formation on the metal in the

mouth, although it is used extensively for small additions to

many gold alloys.

Addition of small amounts of palladium to silver containing

alloys prevents the rapid corrosion of such alloys in the oral

environment.

7

KARAT AND FINENESS OF GOLD

For many years the gold content of gold alloys has been

described on the basis of karat, (or) in terms of fineness, rather

than by weight %.

The term karat refers to the parts of pure gold in 24 parts of

an alloy.

For example, 24-karat gold is pure gold, where as 22-karat

gold is an alloy containing 22 parts pure gold and 2 parts of other

metals.

Fineness describes gold alloys by the number of parts per

1000 of gold. For example, pure gold has a fineness of 1000, and

650 fine alloy has a gold content of 65%. Thus the fineness rating

is to timer the gold % in an alloy.

Fineness is considered as more practical term than karat.

The terms karat and fineness are rarely used to describe the

gold content of current alloys.

8

CLASSIFICATION OF DENTAL CASTING ALLOYS (OR)

DENTAL GOLD ALLOYS

According to ADA specification No.5 these casting alloys

are described simply as:

Type I

Type II

Type III and

Type IV

Type I (Soft): These alloys are limited to use in inlays that are

subject only to slight stress during mastication.

This would include inlays for the gingival and

interproximal areas of a 91 tooth and for certain occlusal inlays of

such design (or) location that they are not subjected to severe

stress applications.

Alloys of this type often are useful for inlays prepared by

the direct technique, which requires the finishing operation to be

completed on the tooth with relatively simple hand instruments.

Type II (Medium): These medium alloys can be used for all types

of cast inlays and onlays.

9

Type III (Hard): These alloys are most acceptable for crowns,

thin 3/4th crowns, and anterior and posterior bridge abutments,

which should not be cast from the softer and weaker Type I and

Type II alloys.

Type IV (Extra hard): These alloys are designed to have

sufficient strength and adequate properties for cast removable

partial dentures with clasps, precision cast fixed bridges and ¾

crowns, which are not subjected to hard working (or) burnishing

operations.

Composition:

The composition of the gold casting alloys that meet the

requirements of ADA Sp. No. 5 are given in the Table below:

TYPE GOLD PLATINUM PALLADIUMI 81-83% - 0.2-4.5%II 76-78% - 1-3%III 73-77% - 2-4%IV 71-74% 0-1% 2-5%

It is apparent from the Table that there is some reduction in

gold content when a comparison is made between Type I and IV

alloys.

10

An increase in copper content is observed as the gold

content is decreased.

An increase in the zinc content also occurs in Type IV

alloys.

Platinum is rarely added to Type 1 gold alloys, but a small

amount of palladium is always added to all 4 types.

PROPERTIES OF GOLD CASTING ALLOYS

Type

Vickers hardness number (Kg/mm2)

Softened

Yield strength, (0.1% Offset) MPa Elongation

Minimum Maximum Softened minimum

Hardened minimum

Softened minimum

Hardened minimum

(%)

I 50 90 None None 18 None

II 90 120 140 None 12 None

III 120 150 200 None 12 None

IV 150 None 340 500 10 2

From the above table it may be seen that as the hardness

increases from Type I to Type IV, the yield strength and tensile

strength values are also increased, and the elongation generally

decreased.

11

Since the yield strength represents in general the resistance

to permanent deformation under stress, it can be seen that alloys

with increased hardness values offer an increased resistance to

permanent bending (or) deformation.

Soft alloys have a higher degree of elongation and a

relatively greater quality of ductility than the alloys of higher

hardness values.

FUSION TEMPERATURES OF DENTAL GOLD CASTING

ALLOYS

METAL (or) ALLOYTYPICAL FUSION

TEMPERATURE (°C)

Type I 1005-1070

Type II 900-970

Type III 875-1000

Type IV 875-1000

From the above table it is observed that the fusion

temperature of the 4 types of alloys decreases from Type I to Type

IV.

The fusion temperatures are important factors in choosing

the type of investment to be used.

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Alloys having fusion temperatures higher than about

1100°C should not be cast into calcium sulfate bonded

investment.

Numerous classification systems have been proposed to

categorize the wide variety of commercial gold-based and

palladium based alloys.

In 1984 the ADA proposed a simple classification of dental casting

alloys.

ALLOY TYPE TOTAL NOBLE METAL CONTENT

High noble metal

Contains 40 wt% AV and 60% wt of the noble metal

elements (Au+Ir+OS+Pt+Rh+Ru).

Noble metal Contains 25% of the noble metal elements.

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

Many manufacturers have adopted this classification to

simplify the communication between dentists and dental

laboratory technologists.

13

Some insurance companies use it as well to determine the

cost of crown and bridge treatment.

CLASSIFICATION OF ALLOYS FOR ALL METAL

RESTORATIONS METAL, CERAMIC RESTORATIONS AND

FRAME WORKS FOR REMOVABLE PARTIAL DENTURES

Alloy Type All-Metal Metal-Ceramic

Removable Partial

Dentures

High nobleAu-Ag-Cu-Pd metal ceramic

alloys

Au-Pt-PdAu-Pd-Ag (5-12 wt% Ag)Au-Pd-Ag

(>wt 12% Ag)Au-Pd (no Ag)

Au-Ag-Cu-Pd

Noble

Ag-Pd-Au-CuAg-Pd

Metal ceramic alloys

Pd-Au (no Ag)Pd-Au-Ag

Pd-AgPd-CuPd-Co

Pd-Ga-Ag

Ag-Pd-Au-CuAg-Pd

In this we have all the 4 types of alloys, described earlier

with both high noble and noble metal alloys.

Heat treatment of high noble and noble metal alloys:

Gold alloys can be significantly hardened if the alloy

contains a sufficient amount of copper. Types I and E alloys

usually do not harden, (or) harden to a lesser degree than do the

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types HI and IV alloys. The actual mechanism of hardening is

probably the result of several different solid - solid

transformations.

Alloys that can be hardened can of course, also be softened.

In metallurgical terminology the softening heat treatment is

referred to as solution heat treatment. The hardening heat

treatment is termed age hardening.

Softening heat treatment:

The casting is placed in an electric furnace for 10 minutes

at a temperature of 700°C (129°F) and then it is quenched in

water.

The tensile strength, hardness and proportional limit are

reduced by such a treatment but the ductility is increased.

The softening heat treatment is indicated for structures that

are to be ground, shaped (or) otherwise cold worked, either in or

out of the mouth.

15

Hardening heat treatment:

The age hardening of the dental alloys can be accomplished

'in several ways. One of the most practical hardening treatments

is by "Soaking" (or) aging the casting at a specific temperature for

a definite time, usually 15 to 30 minutes, before it is water

quenched. The aging temperature depends on the alloy

composition but is generally between 200°C and 450°C.

Ideally, before the alloy is given an age-hardening

treatment, it should be subjected to a softening heat treatment to

relieve all strain hardening, if it is present and to start the

hardening treatment with the alloy as a disordered solid solution.

The hardening heat treatment is indicated for metallic

partial dentures, bridges, and other similar structures.

Casting Shrinkage:

Most metals and alloys, including gold and noble metal

alloys, shrink when they change from the liquid to the solid state.

The values for the casting shrinkage differ for the various

alloys presumably because of differences in their composition. It

has been shown, for example, that platinum, palladium and copper

16

all are effective in reducing the casting shrinkage of the alloy. It

is of interest that the value for the casting shrinkage of pure gold

closely approaches that of its maximal linear thermal contraction.

Alloy Casting shrinkage (%)

Type I, Gold base 1.56%

Type II, Gold base 1.37%

Type III, Gold base 1.42%

Ni-Cr-Mo 2.3%

Silver Palladium alloys:

These alloys are white and predominantly silver 'm

composition but have substantial amounts of palladium (at least

25/o) that provide nobility and promote the silver tarnish

resistance. They may (or) may not contain copper and a small

amount of gold.

The copper (Cu) free Ag-Pd alloys may contain 70% to 72%

silver and 25% palladium and may have physical properties of a

Type III gold alloy.

Other silver-based might contain roughly 60% silver 25%

palladium and as much as 15% or more of copper and may have

properties more like a Type IV gold alloy. Despite early reports

17

of poor castability, the Ag-Pd alloys can produce acceptable

castings.

Because of the increasing interest in aesthetics by dental

patients, a decreased use of all metal restorations has occurred

during the past decade. The use of metal ceramic restorations in

posterior sites has increased relative to the use of all metal crowns

and onlays.

HIGH NOBLE ALLOYS FOR METAL CERAMIC

RESTORATIONS

The original metal ceramic alloys contained 88% gold and

were much to soft for stress bearing restorations such as FPD.

Because there was no evidence of a chemical bond between these

alloys and dental porcelain, mechanical retention and undercuts

were used to prevent detachment of the ceramic veneer.

Using the stress bond test, it was found that the bond

strength of the porcelain to this type of alloy was less than the

cohesive strength of the porcelain itself. This mean that if the

failure occurred in the metal-ceramic restoration, it would most

probably arise at the porcelain metal interface.

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By adding less than 1% of oxide forming elements such as

iron, idium, and tin to this high gold content alloy, the porcelain

metal bond strength was improved by a factor of 3. Iron also

increase the proportional limit and strength of the alloy.

This 1% addition of base metals to the gold, palladium and

platinum alloy was all that was necessary to produce a slight

oxide filum on the surface of the substructure to achieve a

porcelain-metal bond strength level that surpassed the cohesive

strength of porcelain itself.

The high noble alloys for metal ceramic restorations are:

A. Gold-Platinum - Palladium Alloys:

These alloys have a gold content ranging up to 88% with

varying amounts of palladium, platinum and small amounts of

base metals.

Some of these alloys are yellow in colour. Alloys of this

type are susceptible to sag deformation and FPD's should be

restricted to 3-unit spans, anterior cantilevers (or) crowns.

B. Gold-Palladium-Silver Alloys:

These gold based alloys contain between 39% and 77% upto

35% palladium and silver levels as high as 22%.

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The silver increases thermal contraction coefficient but it

also has a tendency to discolor some porcelains.

C. Gold-Palladium Alloys:

A gold content ranging from 44% to 55% and a palladium

level of 35% to 45% is present in these metal ceramic alloys,

which have remained popular despite their relatively high cost.

The lack of silver results in a decreased thermal contraction

coefficient and the freedom from silver discolouration of

porcelain.

Alloys of this type must be used with porcelains that have

low coefficients on to avoid the development of axial and

circumferential of thermal contraction to avoid the development

of axial and circumferential tensile stresses in Porcelain during

the cooling part of the porcelain firing cycle.

NOBLE ALLOYS FOR METAL CERAMIC RESTORATIONS

A. Palladium Based Alloys

1. Palladium - Silver alloys.

2. Palladium Copper alloys:

3. Palladium - Cobalt alloys:

4. Palladium - Gallium - Silver and Palladium - Gallium –

Silver – Gold alloys:

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A. Palladium based alloys

Noble palladium based alloys offer a compromise between

the high noble gold alloys and the predominantly base metal

alloys. The price per ounce of a palladium alloy is generally one

half to one third that of a gold alloys. The density of a palladium

based alloy is midway between that of base metal and of high

noble alloys.

1) Palladium - Silver Alloys:

Pd-Ag alloys were introduced widely in the late 1970s. This

alloy type was introduced to the U.S. market in 1974 as the first

gold free noble metal available for metal ceramic restorations.

Pd-Ag alloys enjoyed wide spread popularity for a few

years after they were introduced, but their popularity has declined

some what in recent years because of their tendency to discolor

porcelain during firing. One theory that has been proposed for

this greenish yellow discoloration, popularly termed "Selling" is

that the silver vapor escapes from the surface of these alloys

during firing of the porcelain, diffuses as ionic silver into the

porcelain and is reduced to form colloidal metallic silver in the

surface layer of porcelain.

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The compositions of Pd-Ag alloys fall within a narrow

range: 53% to 61% palladium and 28% to 40% silver.

Tin (or) Indium or both are usually added to increase alloy

hardness and to promote oxide formation for adequate bonding of

porcelain. In some of these alloys, the formation of an internal

oxide rather than an external oxide has been reported.

Other palladium alloys contain 75% to 90% palladium and

no silver and were developed to eliminate the greening problem

some of the high palladium alloys develop a layer of dark oxide

on their surface during cooling from the degassing cycle, and this

oxide layer has proven difficult to mask by the opaque porcelain.

Other high palladium alloys such as the Pd-Ga-Ag-Au type

seem not to be plagued by this problem.

The replacement of gold by palladium raises the melting

range but lower the contraction coefficient of an alloy. Increases

the silver content tends to lower the melting range and raises the

contraction coefficient.

Because of their high silver contents compared with the

gold based alloys, the silver discoloration effect is most severe for

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these alloys. Gold metal conditioners or ceramic coating agents

may minimize this effect.

The low specific gravity of these alloys (10.7-11.1)

combined with their low intrinsic cost makes these alloys

attractive as economical alternatives to the gold based alloys.

2) PALLADIUM-COPPER ALLOYS:

This alloy is comparable in cost to the Pd-Ag alloys.

Because of their low melting range of approximately

1170°C to 1190°C, these alloys are expected to be susceptible to

creep deformation (Sag) at elevated firing temperatures. Thus, one

should exercise caution in using these alloys for long-span FPDs

with relatively small connectors.

As is true for some Pd-Ag alloys, several of these products

contain 2% gold.

These alloys contain between 74-80% palladium and 9-15%

copper.

Porcelain discolouration due to copper is possible but does

not appear to be a major problem.

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One should be aware of the potential effect on aesthetics of

the dark brown (or) black oxide formed during oxidation and

subsequent porcelain firing cycles. Care should be taken, to mask

this oxide completely with opaque porcelain and to eliminate the

unaesthetic dark band that develops at metals porcelain junctions.

The Pd-Cu alloy have yield strengths upto 1145MPa.

Elongation values of 5% to 11% and hardness values as

high as some base metal alloys.

Thus, these alloys would appear to have a poor potential for

burnishing except when the marginal areas are relatively thin.

Although thermal incompatibility is not considered to be a

major concern, distortion of ultra thin metal copings (0.1mm) has

been occasionally reported.

3) PALLADIUM-COBALT ALLOYS:

This alloy group is comparable in cost to the Pd-Ag and Pd-

Cu alloys. They are often advertised as gold free, nickel-free,

beryllium-free, and silver-free alloys. The reference to nickel and

beryllium indicates that these alloys, as is true with the other

noble metals, are generally considered biocompatible.

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Like many noble metals, these alloys have a fine grain size

to minimize hot tearing during the solidification process.

This Pd-Co group is the most sag resistant of all noble

metal alloys.

The noble metal content (based on palladium) ranges from

78% to 88%.

The cobalt content ranges between 4 and 10wt% over

commercial alloy contains 8% gallium.

An example of typical properties of a Pd-Co alloy is as follows:

Hardness 250DPH

Yield strength 586MPa

Elongation 20% and

Modulus of elasticity 85.2Gpa

Although these alloys are silver-free, discolouration of

porcelain can still result because of the presence of cobalt. Any

way this is not considered a significant problem. Failure of the

technician to completely mask out the dark metal oxide color with

opaque porcelain is a more common cause of unacceptable

aesthetic results.

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4) PALLADIUM-GALLIUM-SILVER

AND PALLADIUM-GALLIUM-SILVER-GOLD ALLOYS

These alloys are the most recent of the noble metals. This

group of alloys was introduced because they tend to have a

slightly lighter coloured oxide than Pd-Cu (or) Pd-Co alloys and

they are thermally compatible with lower expansion porcelains.

The oxide that is required for bonding to porcelain is

relatively dark, but it is somewhat lighter than those of the Pd-Cu

and Pd-Co alloys. The silver content is relatively low (508 wt%)

and is usually inadequate to cause porcelain “greening”.

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TECHNICAL DATA CHART FROM DEGUSSA

HIGH GOLD CONTAINING CROWN AND BRIDGE ALLOYS

Alloy Type Colour Au Vicker’s hardness

Degulor A 1, soft Deep yellow 87.5 55

Degulor B 2, medium Yellow 75.7 95

Degulor C 3, hard Yellow 74.0 145

Degulor Mo

4, extra hard Yellow 65.5 195

SILVER PALLADIUM CROWN AND BRIDGE ALLOYS

Alloy Type Colour Au Pd Vickers hardness

Palliag MJ

4, extra hard White 55.0 20.9 150

HIGH GOLD CONTAINING ALLOYS FOR CERAMICS

Alloy Type Colour Au Vicker’s hardness

Degulent G Extra hard Yellow 86 150

Biobond III Extra hard Bright yellow 82.6 160

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PALLADIUM-BASE ALLOYS FOR CERAMICS

Alloy Type Colour Au Vicker’s hardness

Bond-on 4 Extra hard White 79.7 260

These Pd-Ga-Ag alloys generally tend to have a relatively

low thermal contraction coefficient and would be expected to be

more compatible with lower expansion porcelains such as vita

porcelains.

BASE METAL ALLOYS FOR DENTAL CASTINGS

The pressures of economics, as well as a search for

improved mechanical properties, have led to the development of

base metal alloys for the construction of dental prosthesis devices.

Composition:

The principal elements present in cast base metals for

partial dentures are chromium, cobalt and nickel, which together

make up approximately 90% of the most alloys used for dental

restorations. Representation compositions for typical dental

casting alloys are listed in the table.

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Composition of cast base metal alloys used in dentistry:

Ingredients Alloys (% of weight)

Vitallium Toconium Jelenko LG Nobilium

Chromium

Cobalt

Nickel

Molybdenum

Aluminium

Iron

Carbon

Beryllium

Silicon

Manganese

Gallium

30.0

Balance

-

5.0

-

1.0

0.5

-

0.6

0.5

-

17.0

-

Balance

5.0

5.0

0.5

0.1

1.0

0.5

5.0

-

27.0

Balance

13.0

4.0

-

1.0

0.2

-

0.6

0.7

-

30.0

Balance

-

5.0

-

-

0.35

-

0.35

-

0.05

On close examination of the table, one can observe the

following points:

1. Chromium is the only major metal that exists in all alloys of

the type. Cobalt is present in all alloys except Ticonium,

whereas nickel is absent in vitallium and nobelium.

2. The total wt of chromium, cobalt and nickel in these alloys

is over 90% yet, their effect on the physical properties of

these alloys are controlled by the presence of minor

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alloying elements such as carbon, molybdenum, beryllium,

tungsten and aluminium.

Effect of each alloy constituents

Chromium: Chromium content is responsible for the tarnish

resistance and stainless properties of these alloys. When the

chromium content of an alloy is over 30% of the alloy is more

difficult to cast. It also forms a brittle phase, known as the zigma

phase. Therefore dental alloys of these types should not contain

more than 28% or 29% chromium.

Cobalt and nickel: are somewhat interchangeable to a certain

percentage cobalt increases the elastic modulus, strength and

hardness of the alloy more than nickel does. Nickel may increase

ductility.

Carbon content: The hardness of cobalt base alloys is increased

by the increased content of carbon. A change in the carbon

content in this order of 0.2% in these alloys changes the properties

to such an extent that the alloy would no longer be usable in

dentistry.

Molybdenum: The presence of 3% to 6% molybdenum contributes

to the strength of the alloy.

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Aluminium: Aluminium in nickel-containing alloys forms a

compound of nickel and aluminium (Ni 3-Al). This compound

increases the ultimate tensile and yield strength.

Berylium: Addition of 1% beryllium to nickel-base alloy reduces

the fusion range of the alloy by about 100°C. It also aids in solid

solution hardening. It improves the casting characteristic are

possibly participate in porcelain bonding.

Silicon and Manganese: are added to increase and castability of

these alloys. They are present primarily on oxide to prevent

oxidation of other element during melting. The presence of

nitrogen which cannot be controlled unless the castings are made

in a controlled atmosphere as in vacuum or argon, also contributes

to the brittle qualities of these cast alloys. When the nitrogen

content of the final alloy is more than 0.1%, the castings lose

some of their ductility. Since the minor ingredients of carbon,

nitrogen and oxygen effectively influence the properties of the

final formulated and designed in such a way as to maximize the

rigidity of the prosthesis.

The obvious approach would be to increase the thickness of

the metal substructure, since doubling the thickness increases the

rigidity in bending by a factor of 8. However, the maximum

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thickness of the overall restoration is limited externally by

occlusion and proper anatomical contour internally by the desire

to retain as much tooth structure as possible. (Esthetics requires a

minimal thickness of overlying porcelain that results in severe

limits as to the maximum thickness of the metal).

An examination of the mechanical properties of base metal

alloys and a gold alloy shows that in general the base metal alloys

have a modulus of elasticity approximately twice that of

previously used gold alloys. Since elastic modulus is a measure of

the stiffness of rigidity of materials, this property would enhance

the application of base metal alloys for long-span bridges where

flexure, is a major cause of failure. Given an equal thickness of

precious metal alloy and base metal alloy, the base metal alloy

bridge would flex only half as much as the precious alloy material

under the same occlusal forces. In a similar manner, the higher

modulus of elasticity may be utilized to permit thinner castings.

The Vicker’s hardness of base metal alloys may range from

approximately 175 to 360DPH. Although certain of the base metal

alloys may approach the hardness of noble metal alloy

(approximately 160DPH), the majority of these alloys are

considerably harder. Clinically, it is improbable that significant

32

occlusal wear of the alloy will occur. Therefore, particular

attention must be directed toward perfecting occlusal

equilibration. The removal of defective clinical units is also more

difficult than with noble metal alloys, since the high hardness

results in rapid wear of carbide burs and diamond points.

The durability, as measured by the percentage elongation,

of base metal alloys ranges between approximately to 10 and 28

percent. Noble metal alloys have an elongation of approximately 5

to 10 percent.

The density of base metal alloys is approximately

8.0gm/cm3, as compared with 18.39gm/cm3 for comparable noble

metal alloys. Since casting alloys are purchased on a weight basis,

a lower density is indirectly reflected to the purchaser, who

receives more than twice the volume of material for each unit

weight acquired. Also, the intrinsic value of the component

elements in base metal alloys is significantly les than that of

comparable noble metal alloys. Thus, on the basis of their lower

density and the low intrinsic value of the component metals, the

cost differential between base metal and noble metal alloys can be

substantial.

33

When porcelain is first fired to a metal substructure, the

alloy is subjected to considerable temperature variations and

stresses induced by the shrinkage of the overlying porcelain. Sag

resistance is the property that has been used to describe the ability

of an alloy to resist the permanent deformation of creep induced

by thermal stresses. It is particularly important in long-span

bridges where the porcelain firing temperature may cause the

unsupported structure to deform permanently. Under controlled

conditions, it has been found that a base metal alloy will deform

less than 0.001 inch, while a noble metal alloy will deform 0.009

inch. It is likely that the higher fusion temperature common to

base metal alloys is a factor that contributes to the superior sag

resistance properties of these alloys.

The question of metal ceramic compatibility is basic to the

selection of an alloy system for this type of restoration. Two

requirements are implicit. The metal must not interact with the

ceramic in such a way to discolour the porcelain at the interface or

marginal regions. Moreover, the metal ceramic system must form

a stable bond at the interface that can withstand normal stresses in

the mouth.

34

Alloys for complete metal restorations are cast into calcium

surface bonded investment molars then the alloys have been

melted with gas-air blow porhes. The cast base metal alloys

cannot be melted with the conventional blowtorch uses for gold

alloys, and so it has been necessary to develop special electric

melting facilities or less commonly to melt the alloys which or

oxygen-oxetylene torch.

Electrical somers of melting are often used to advantage,

such as carbon areas, argon arcs, high frequency induction, or

silicon-confide resistance furnaces. In some insurances,

sophisticated electronic equipment is used to control the

temperature, casting time, and similar variables in order to

regulate the gain formation and confide precipitation. Less

commonly oxygen-nityfine torch is used to melt the alloys. The

confurizing section of the oxygen-acetylene flame caudd carbon to

the alloy. The extra carbon changes not only the microstructure

but also the mechanical properties. (In general, hardness and yield

strength increases whereas ductility decreases).

Therefore, when melting the alloy with an oxygen-acetylen

torch, the proportion of the two gases, the length of the flame and

the distance of the torch tip from the alloy should be standardized.

35

Carbon crucibles and carbon-containing investments should be

avoided.

Casting shrinkage compensation

Because of the high fusion temperature, the casting

shrinkage of the base metal alloys is greater than that of the fold

casting alloys. It is in the order of 2.3%, which requires that the

mold be expanded more than when the dental gold alloys are cast.

(Approximately 2% for Ni-Cr alloys). Thermal expansion

represents the principal method of mold expansion for

compensation of the alloy shrinkage. The use of special phosphate

of silicote-bonded investments permit adequate thermal expansion

of the molds when they are probably located and one can produce

castings that display the proper fit and adequate compensation

have yield strengths of at least 450MPa (60,000 lb/inch 2) to

withstand permanent deformation when used as partial denture

clasps.

Tensile strength studies have indicated that the ultimate

tensile strength of the cast base metal alloys is less influenced by

variations in test conditions than some other properties such as

elongation are.

36

Elongation: The percentage elongation of the an alloy is

important as an indication of the relative brittleness or ductility

that the restoration will exhibit. (There are many occasions

therefore when it can be considered to be an important property

for comparison of alloys for removable partial denture

appliances). The combined effect of elongation and ultimate

tensile strength is an indication of toughness of any material.

Partial denture claps cast of alloys with a high elongation and

tensile strength do not fracture in service as often as those with

low elongation, because of their toughness.

The percentage elongation is one of the properties that is

critical to test accurately and to control properly during test

preparatio. A very small amount of microporosity that may exist

in the test specimen will alter the elongation considerably,

whereas its effect on yield strength, elastic modulus and tensile

strength is rather limited. One can therefore assume that practical

castings may exhibit similar variations in elongation from one.

Casting to another. To some degree this is borne out in

practice, with some castings from the same product showing a

greater tendency toward brittleness than others. This observation

37

indicates that the control of the melting and casting variables is of

extreme importance if reproducible results are to be obtained.

Although nickel and cobalt are interchangeable in cobalt-

nickel-chromium alloys, in general increasing the nickel content

with a corresponding reduction in cobalt will increase the ductility

and elongation. Jelenko LG, a cobalt-chromium alloy with some

nickel and with rigid control of molybdenum and carbon, has a

high elongation without much decrease in strength.

Elastic modulus: the higher the value, the more rigid the structure

can be expected to be, provided the dimensions of the casting are

the same in both instances. (There are those within the profession

who recommend the use of a well-designed, rigid appliance on the

basis that it gives the proper distribution of forces on the

supporting tissues when in service. With a greater elastic moduls

it is possible to design the restorations with slightly reduced

dimensions. (It is a well established fact that the elastic modulus

of the cast metal alloys is at least twice that of the dental gold

alloys).

The cast cobalt-chromium dental alloys show comparable

values for elastic modulus of about 228GN/m2 (33x106lb/inch 2),

whereas nickel-chromium alloys possess an elastic modulus of

38

about 198MPa (27x106lb/inch2), which is approximately double

the value of 90MPa (13x106lb/inch2) for type IV cast metal gold

alloys.

MICROSTRUCTURE OF CAST BASE METAL ALLOYS

The microstructure of any substance is the basic parameter

that controls the properties. Cobalt-chromium or nickel-chromium

alloys microstructure changes by a slight alteration of

manipulative conditions. The microstructure of cobalt-chromium

alloys in these condition consists of an elastomeric matrix

composed of a solid solution of cobalt and chromium in a cored

dendritic structure. Many elements present in cast base metal

alloys, such as chromium, cobalt and molybdenum, are carbide

forming elements. Depending on the composition of a cast base

metal alloy and its manipulative condition, it may form more or

less of any given type of carbide.

Further more, the arrangements of these carbides may also

vary depending on the manipulative condition. The effect of the

microstructure on physical properties a commercial cobalt-

chromium alloy is illustrated in Figure. A one cane say that the

carbides are continuous along the grain boundaries. Such a

structure is obtained when the metal is cast as soon as it is

39

completely melted. In this condition the cast alloy possesses low

elongation values with a good and clean surface. Carbides that are

spherical and discontinuous like islands are shown in figure B.

Such a structure can be obtained if the alloy is heated about 100°C

about its normal melting temperature, and this results in casting

with good elongation values but with a very poor surface. The

surface is so poor that the casting cannot be used in dentistry.

Dark eutectoid areas which are lamellar in nature, are shown in

figure C. Such a structure is responsible for very low elongation

values but a good and clean casting.

Investment materials and casting operations:

The manner of casting relatively 1 stage partial denture

appliances differs in some details from the casting of simple

restorations such as single inlays or crowns, though in principle

the two operations are similar. In cast partial denture construction

a suitable cast of refractory material or investment serves as the

structure on which the wax pattern is formed, when the wax

pattern is completed on the refractory cast, both the wax pattern

and the cast are then invested in a appropriate investment

material. If gold alloys are to be used, the conventional gypsum-

bonded silica investment is acceptable, but only one cast nickel

40

chromium alloy (ticonium) has a sufficiently low melting

temperature to be cast into a specially formulated gypsum-silica

type of investment. For the higher melting base metals it is

necessary to use-------.

When casting any of the base metals in to molar designed to

accommodate the higher melting temperature of these onlays,

certain problems may be encountered that are less common when

casting lower melting alloys. One problem is that of trapping

gases in the mold during the casting process. To have sufficient

strength and resistance to thermal shock, some investments for the

cast base metal alloys lack sufficient porosity for the rapid escape

of gases from the mold cavity when the hot metal intern.

As a result, the gases may be trapped in the mold cavity and

produce voids and casting defects. Numerous methods have been

prepared to overcome such defects, such no venting is the surface

of the mold to permit rapid elimination of gases. The skillful

spruing at and venting of the mold, combined with complete

elimination of the wax residue and adequate heating of the metal,

tend to reduce this type of defective casting.

The melting of base metal alloys must be carefully

controlled to avoid inverse damage to the alloy during the melting

41

and casting process. Oxidation of the ingredient metals and

carbide or nitrite formation at the high temperature required to

melt these alloys demand procine control of the melting and

casting operation. Regardless of the method employed to melt the

alloy, it is possible to cause severe damage to the properties of the

casting if proper melting practices are not observed.

Other applications of cast base metal alloys:

It has been demonstrated that cast cobalt-chromium alloys

serve a useful purpose in appliances other than removable partial

denture restorations. In the surgical repair of bone fractures,

alloys of this type are used for bone plates, crown, various

fracture appliances and splints. Metallic obturation and oral

implants for a variety of purposes are formed from cast base metal

alloys. The use of the cobalt-chromium alloys for surgical

purposes in wall established, and these

-------------------------------------- periods of time without harmful

reactions. This favourable response of the tissues probably is

attributable is the low solubility and electrogalvanic action of the

alloy used, with the result that the metal is inert and produce no

inflammatory responses. The product known as surgical vitallium

is used extensively for this purposes. Cast titanium and its alloys

42

have recently been introduced as surgical implant materials

because of their excellent tissue compatibility.

Potential health hazards of nickel and beryllium

It is widely recognized beryllium is potentially toxic under

uncontrolled conditions. Lab-technicians may be exposed

occasionally or routinely to excessively high concentration of

beryllium and nickel dust and beryllium vapours.

The exposure to beryllium may result in acute and chronic

forms of beryllium disease (Physiologic). The responses may vary

from contact dermatitis to severe chemical pneumanitic which can

be fetal. However, the diagnosis of chronic beryllium disease is

difficult, since it often exhibits symptoms range from coughing,

chest pain and general weakness to pulmonary drifection and

requires the establishment of beryllium exposure.

The occupational health and safety administration specifies

that exposure to beryllium dust in an air should be limited to a

particular beryllium concentration of 2mg/m 3 of air for a time

weighed, 8 hours day. In laboratory and clinical situations in a

grinding of beryllium containing alloys is of performed, adequate

43

local exhaust ventilation safeguards should be employed, since all

forms of beryllium are toxic and the body cannot ------- beryllium.

It is also believed that beryllium localization (i.e.

movement of Be ions to the surface) enriches surface is the point

that beryllium makes upto 30% of the composition of the surface

layer. Beryllium release from the surface is enhanced by presence.

POTENTIAL HAZARDS OF NICKEL TO PATIENTS

In certain non-dental industraial applications and

subexperimental conditions, nickel and its compounds have been

implicated as potential carcinogens and as sensitizing agents.

Of great concern in dental patients is the intra oral exposure

to nickel, especially for patients with known allergy to this

elements, Dermatitis resulting from contact with nickel solutions

was reported as early as 1989. the incidence of allergic sensitivity

to nickel has been found to be 5 to 10 times higher for females

than for males with 5% to 8% of females showing sensitivity.

Many cases of respiratory organ concerns have been

documented in studies of workers involved in the plating,

refining, grinding and polishing of nickel and nickel alloys.

Because of the concerns over the carcinogenic potential of nickel,

44

the National institutes for occupational safety and health has

recommended a standard to limit employee exposure to inorganic

nickel in the work place to 15mg/m 3. It appears that the potential

carcinogenic risks of nickel are less likely to affect dental

patients.

To minimize exposing of metallic dust to patients and

dentists during intra oral metal grinding operations, a high-speed

evacuation system should be used. Patients should be informed of

the potential allergic effects of nickel exposures an a thorough

medical history should be taken to try to determine the patient

may be allergic to nickel.

Intra oral tissues are more resistant to symptoms of

sensitivity. However intra oral exposure to allergies can be

manifested in locations remote from dental restorations. The

systems of the sensitivity range from urticania, pruritis,

xerostomia, eczema or vesicular eruptions.

Etching of base metal alloys:

When it was first introduced micromechanical retentions of

etched metal resin-bonded retainers (Maryland bridges) was

obtained by electronically etching the base metal alloys. More

45

recently, chemical etchants have been marketed also less

expensive and more convenient for etching the metal substraction.

The intagets surface of the resin are to be binded to etched enamel

are treated with acid gels or liquids for a short period of time.

However reports on the comparative bond strength between

electrolytically etched and chemically etched surface on

conflicting. More study is needed to determine the relative value

of chemical etchants substitutes for conventional electrolytic

etching.

METAL CERAMIC ALLOYS

General Features:

The chief objection of the use of dental porcelain as a

restorative material is its low tensile and shear strength. Although

porcelain can resist compressive stresses with reasonable success,

substructure design does not permit shapes in which compressive

stress is the principal force.

A method by which this disadvantage can be minimized is

to fuse the porcelain directly to a cast alloy substructure made to

fit the prepared tooth. If a strong bond is attained between the

46

porcelain veneer and the metal, the porcelain veneer is reinforced.

Thus, brittle fracture can be avoided, or at least minimized.

The original metal ceramic alloy contained 88 percent gold

and were much too soft for stress-bearing restorations such as

fixed partial dentures. Since there was no evidence of a chemical

bond between these alloys and dental porcelain, mechanical

retention and undercuts were used to prevent detachments of the

ceramic veneer. By adding less than 1 percent oxide forming

elements such as iron, indium and tin to this high gold content

alloy, the porcelain-metal bond strength was improved by a factor

of 3 iron also increases the proportional limit and strength of the

alloy.

This 1 percent addition of base metals to the gold,

palladium, and platinum alloy was all that was necessary to

produce a slight oxide film on the surface of the substructure to

achieve a porcelain metal bond strength. This new type of alloy

with small amounts of base metals added, became the standard for

the metal ceramic restoration. In response to economic pressures,

other gold-and palladium-base metal ceramic alloys emerged. In

time, base metals were also developed for this same purpose.

47

Properties:

The clinical success of a metal ceramic restoration is

dependent in large measures on the ability of the underlying alloy

substructure to resist the potentially destructure masticatory

stresses. (Therefore it is imperative that the metal ceramic

restoration be use of variable casting conditions. Therefore, these

alloys are generally considered to be technique sensitive. One

reason for this sensitivity is that almost all elements in these

alloys such as chromium, silicon, molybdenum, cobalt and nickel

react with carbon to form carbides depending on the mold and

alloy-casting temperature, cooling rate, and other technical

variables, carbides of any one of these elements may form. The

formation of different carbides naturally changes the properties of

the alloys. As a result, careful control of manipulative variables in

the casting operations is necessary.

ANSI/ADA Specificaiton No. 14

According to this specification, the total weight of

chromium, cobalt and nickel should be not less than 85% or no

less than 20% chromium. Alloys having other compositions may

also be accepted by the ADA provides that the alloys comply

satisfactory with requirements for toxicity hypersensitivity and

48

corrosion. Composition to the nearest 0.5% shall be marked on the

package plus the presence and percentage of hazardous elements

and precautioning recommendations for processing the materials.

The specification also recommend minimum values for

elongation, yield strength and elastic modulus.

An important feature of this specification is that it has more

a standardized method of testing available, which has in turn,

made possible comparisons of results from one investigation to

another.

49

CONTENTS

Introduction

Historical Perspective on Dental Casting Alloys

Properties of Noble Metal Alloys

Classification of Dental Casting Alloys

Alloys for All Metal & Resin Veneer Restorations

High Noble Alloys for Metal Ceramic Restorations

Base metal Alloys for Dental Castings

Composition of Base metal Alloys for Small castings

Effect of Alloy Constituents

Handling Hazards and Precautions

Summary & Conclusion

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