NOBLE METAL ALLOYS

INTRODUCTION

For casting of dental restorations, 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 to produce properties most acceptable for their intended

dental applications such as inlays, onlays, bridges, removable cast restorations

etc.

The noble metals are those elements with a good metallic surfaces that

retain their surfaces in dry air.

The 6 metals of the platinum group are platinum, palladium, iridium,

rhodium, osmium, ruthenium, and along with gold, are called noble metals.

The history of dental casting alloys has been influenced by three major

factors:

1) The technologic changes of dental prosthesis.

2) Metallurgic advancements.

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 Jaggart was an instant success.

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It soon led to the casting of complex inlays such as onlays, crowns, fixed

partial dentures and removable partial denture frameworks.

Because pure gold did not have the physical properties required or these

dental restorations, existing jewellery alloys were quickly adopted.

These gold alloys were further strengthened with copper, silver, or platinum.

In 1932, the dental materials group at the National Bureau of Standards

surveyed, the alloys being used and roughly classified them as:

Type I – Soft-VHN-50 to 90.

Type II – Medium VHN of 90 to 120.

Type III – Hard VHN of 120 to 150.

Type IV – Extra Hard VHN 150.

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.

By 1978, the price of gold was climbing so rapidly that attention focused on

the noble metal alloys – to reduce the precious metal content, yet retain the

advantages of the noble metals for dental use.

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Desirable properties of casting alloys:

Cast metals used in dental laboratories must exhibit the following

properties:

1) It should be biocompatible.

2) Easy to melt.

3) Easy to carry out casting, brazing (soldering) and polishing.

4) Little solidification shrinkage.

5) Minimal reactivity with the mold material.

6) Good wear resistance.

7) High strength and sag resistance (metal ceramic alloys).

8) Excellent tarnish and corrosion resistance.

Classification of Dental Casting Alloys

According to ADA specification No.5, the castings alloys can be

classified as:

1) Type I (Soft) small inlays, easily burnished and for restorations subject

to very slight stress such as inlays.

2) Type II (Medium) Inlays subject to moderate stress including onlays,

thick ¾ crowns, thin cast backings, abutments, pontics and full crowns.

3) Type III (Hard) Inlays subject to high stress including thin ¾ crowns,

thin cast backings, abutments, pontics, full crowns denture bases and

short span fixed partial dentures.

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4) Type IV (Extra hard) Inlays subjected to very high stresses including

denture base bars and clasps, partial dentures, frameworks and long-span

fixed dentures, full crowns are often made for this type high stresses

such as endodontic posts & core.

The development of modern direct tooth coloured filling materials has

almost eliminated the use of type I- II alloys. Types I and II alloys are often

refined to as “Inlay alloys. Types III and IV are generally called “Crown &

bridge” alloys.

In 1984, the ADA proposed a simple classification for dental casting alloys:

Alloy Type Total Noble Metal Content1) High noble metal.

2) Noble metal.

3) Predominantly base metal.

Contains 40wt%Au+60wt% of noble metal elements (Au+IS+Os+Pd+Rh+Ru).

Contains 25wt% of the noble metal elements.

Contains <25wt% of the noble metal elements.

According to Marzouk

Type IType II

1) Class I – Gold and platinum group based alloys Type IIIType IV

2) Class II Low gold alloys (gold content <50%).

3) Class III Non-gold palladium based alloys.

4) Class IV Nickel-chromium based alloys.

5) Castable moldable ceramics.

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Other Classifications:

1. Yellow golds Yellow coloured

Gold content > 60%

2. Low gold / economy gold.

Usually yellow coloured.

But with gold content 60%

3. White Golds White coloured

Gold content > 50%

CLASSIFICATION OF ALLOYS FOR ALL-METAL RESTORATION, METAL CERAMIC RESTORATIONS, AND FRAMEWORKS FOR REMOVABLE PARTIAL DENTRUES

Alloy Type All-Metal Metal-Ceramic Removable Partial Denture

1) High Noble Au-Ag-Cu-PdMetal-Ceramic alloys

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

Au-Ag-Cu-Pd

2) Noble Ag-Pd-CuAg-PdMetal-Ceramic alloys

Pd-Au (Zo Ag)Pd-Au-AgPd-AuPd-CuPd-CoPd-Ga-Ag

Ag-Pd-Au-CuAg-Pd

Note : The principle reasons that alloys for all-metal restorations cannot be

used for metal-ceramic restorations are that: 1) the alloys may not form thin,

stable oxide layers to promote bonding to porcelain, 2) their melting range may

be too low to resist sag deformation or melting at porcelain firing temperatures

their thermal contraction co-efficients may not be close rough to those of

commercial porcelains.

Ingredients of noble metal alloys : The most important element in dental

gold alloys are gold, copper, silver, platinum metals and zinc).

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Gold

1) Gold is primarily responsible for Deformability (ductility)

Ranks lowest in strength.

Characteristic yellow colour with a strong metallic luster.

Density (sp.gravity) 19.3g/cm3.

Tarnish resistance.

Fusion temperature – 1063°C.

Not soluble in sulphuric, nitric or hydrochloric acids.

Lowest density necessitates more force in centrifugal casting to

compensate for lower wt/vol.

However, lower density will allow more restorations per unit wt which

can be economical to some extent lowest density necessitates more force in

centrifugal castings to compensate for the lower wt/vol. High density enables

case of castings. Alloys for dental use should be atleast 16K resistance.

2) Platinum : May be added to:

1. Strengthen the alloy.

2. Raise the fusion point (1755).

3. Import rigidity, nobility, hardness.

4. Whiten the alloy.

5. Sp gravity 21.37

6. Also malleable and ductile.

Its coefficient of exp is close to that of porcelain to prevent buckling of

the metal or fracture of porcelain during changes in temperature.

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3) Palladium Serves the same functions but is much less expensive than

platinum.

Disadvantages – Palladium hydrogen gas because gold containing palladium

may be more porous when cast SP.Gr 11.4 melting point – 1555°C.

4) Iridium, Ruthenium and Rhodium

Trace amounts of these metals are added as “Grain Refiners” Melting

point 960, 5°C below the melting point and both gold and copper as little

as 0.005% is sufficient to refine the grain size. Grain refiners produce smaller

grains.

Fine grained alloys have smaller grains compared to coarse-grained

alloys which relatively larger grains.

Fine-grained alloys are generally more stronger and more ductile than

coarse grained alloys.

Indium – can also act as a savenger for the alloy during cast procedure.

Can also serve to increase the tarnish and corrosion.

5) Silver also contributes to hardness and strength of the alloy. Although it

mimics gold in its deformability effect, it adversely affect the mobility.

In other words it lowers tarnish resistance.

- Food containing sulphur compounds cause severe

tarnish on silver.

- Silver serves to balance the red colour given by

copper.

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- Adding small amounts of palladium to silver

containing alloys prevents the rapid corrosion of such alloys in the

oral environment. Also occludes appreciable quantities of O2 in the

molten state. Porosities or rough casting surface can be prevented by

adding 5-10% Cu.

6) Copper : Contributes strength and hardness, but decreases the mobility

of the alloy, i.e. it decreases the tarnish and corrosion resistance because

the maximum content should not exceed 16%.

1) Gives the alloy and reddish appearance.

2) Lowers, the fusion temperature.

Zinc : (Present only in low percentages, around 0.5%) acts as a deoxidizer and

reduces the oxygen content. (Because O2 released during solidification results

in porosity).

Karat and Fineness

Karat is the traditional unit expressing gold content in an alloy. Karat refers

to the parts of pure-gold in 24 parts of an alloy.

- Pure gold is 24 karat.

- 22K gold is an alloy containing 22 parts pure gold

in and 2 parts of other metals.

Fineness Fineness is the percentage of gold multiplied by 10 because a 24

karat alloy would have a fineness of 100 x 10, or 1000 and a 12-karat alloy

would be 500 fine (500f).

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Heat Treatment of Gold Alloys

Gold alloys can be significantly hardened if the alloy contains a

sufficient amount of copper (at least 8 wt/copper).

Heat treatment is a process of healing a metal to improve its properties.

An alloy can be subjected to – Hardening heat treatment also “age

hardening”, softening heat treatment also referred to as “solution heat

treatment”.

“Softening Heat Treatment

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

700°C (1292°F) and then 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.

Softening heat treatment reduces - Tensile strength

- Proportional limit

- Hardness

but improves – Ductility.

Softening heat treatment is indicated for, structure that are to be ground,

shaped or otherwise cold worked.

Hardening heat treatment – can be accomplished in many ways. One of

the most practical hardening treatments is by “soaking” or aging the casting at

a specific temperature for a definite time usually 15-30 minutes before it is

water quenched.

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The aging temperature on the alloy composition but is generally between

200°C (400°F) and 450°C (840°F).

Age hardening improves- Strength (yield).

- Proportional limit.

- Hardness.

Age hardening is indicated for metallic partial dentures, saddle bridges and

other similar structures.

For small structures such as inlays, a hardening heat treatment is not

usually employed.

Ideally before the alloy is given an age- hardening heat treatment, it should

be subjected, 1) to a softening heat treatment for 2 reasons: to reline all strain

hardening, if it is present, 2) to start the hardening treatment with the alloy as a

disordered solid solution.

Casting Shrinkage:

Most metals and alloys including gold and the noble metal alloys shrink

when they change from the liquid to the solid state.

The shrinkage occurs in 3 stages:

1) The thermal contraction of the liquid metal between the temperature to

which it is heated and the liquidus temperature.

2) The contraction of the metal from liquid to solid shape.

3) The thermal contraction of the solid metal that occurs down to room

temperature.

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The values for casting shrinkage differ for the various alloys presumably

because of difference in their composition.

It has been shown that platinum, palladium and copper all are effective in

reducing the casting shrinkage of an alloy.

The following table shows the linear solidification shrinkage of casting alloys:

Alloy

Type I, gold base 1.56

Type II, gold base 1.37

Type III, gold base 1.42

Base metals 2.3

(Ni-Cr-Mo-Be-Co-Cr-Mo)

Composition of Traditional I to IV alloys

Type Au Cu Ag Pl Sn, In, Fe, Zn, Ga

I

II

III

IV

83%

77

75

69

6

7

9

10

10

14

11

12.5

0.5

1

3.5

3.5

Balance

Balance

Balance

Balance

Silver-Palladium Alloys

These alloys are white in colour.

Main component is silver, but small amounts of palladium (attract 25%) is

also present (Palladium added to provide mobility and tarnish resistance).

Copper may or may not be present and a small amount of gold.

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Copper-free silver-palladium alloys may contain 70-72% silver and 25%

palladium and may have physical properties of type III gold alloys.

Other silver-based alloys might contain roughly 60% silver, 25% palladium

and as much as 15% or more of copper. These alloys have properties triangular

to Type IV gold alloy.

Disadvantages : Greater potential for tarnish and corrosion.

High Noble Alloys for Metal-Ceramic Restoration:

The original metal-ceramic alloys contained 85% gold and were much

too soft for stress-bearing restorations such as fixed partial dentures.

There was no evidence of a chemical bond between these alloys and

dentinal porcelain.

Therefore 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.

So, stress was concentrated at Porcelain-metal interface. By adding less than

1% of oxide-forming elements such as iron, indium and tin to this high-gold

content alloy, the porcelain metal bond strength was improved.

(Iron also increases the proportional limit, and strength of the alloy).

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

platinum alloy produced a slight oxide film on the surface of the substructure to

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achieve a porcelain-metal bond strength level that surpassed the cohesive

strength of the porcelain itself.

This new type of alloy, with small amounts of base metals added

became the standard for the metal-ceramic restoration.

In spite of vastly different composition, these alloys share at least three

common features:

1) They have the potential to bond to dental porcelain.

2) They possess coefficients of thermal contraction compatible with those

of dental porcelains.

3) Their solidus temperature is sufficiently high to permit the application of

low-fusing porcelains.

The coefficient of thermal expansion tends to have a reciprocal

relationship with the melting point of alloys.

The higher the melting temperature of a metal, the lower its CTE.

The high noble alloys for metal-ceramic restorations are:

1) Gold-platinum-palladium alloys:

Gold content ranges upto 88%.

Varying amounts of palladium, platinum and small amounts of B metals.

Colour – yellow

Disadvantages : Susceptible to sag deformation and FPD should be

restricted to 3-unit spans, amount cantilever or crowns.

2) Gold-palladium-silver alloys:

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Gold content ranges between 39% - 77% gold.

Palladium – 35%.

Silver – less than 22%.

Silver increases the thermal contraction coefficient.

Disadvantages : Silver present in this alloy can discolour same porcelains.

3) Gold-Palladium alloys

Gold content ranges for 44% to 55%

Palladium 35% to 45%.

These alloys have remained popular despite of their relatively high cost.

Disadvantage : The lack of silver results in freedom from silver distry.

The lack of silver results in a decreased coefficient thermal content

because these alloys must be used only with porcelains that have low CT

contraction to avoid the development of initial circumferential tensile stresses

in deny cooling point of ---------.

Noble Alloys for Metal-Ceramic Resins

According to ADA classification of 1984, noble alloys must contain at

least 25wt% of the noble metals but do not necessarily contain any gold.

Noble palladium-based alloys after a compromise between the high-noble

gold alloys and predominantly base metal alloys.

Also, the price / ounce of a palladium alloy is generally one half to one third

that of a gold alloy.

Density is midway between that of base metal and high noble alloys.

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Palladium-based alloys have a workability angular to gold and scrap value.

1) Palladium-silver alloys:

These alloys were introduced widely in the late 1970s (to overcome some

disadvantages of early B.M. alloys e.g. : castability, porcelain bonding and

workability problems).

In the recent years the popularity of these alloys has declined because of the

disadvantages with these alloys is it discolours porcelain during firing.

This discolour, is usually a greenish-yellow discolor and is popularity

termed as “Greening”. Greening occurs mainly because of the presence of

silver Silver vapor escapes for the surface of these alloys during firing of the

porcelain.

Diffuses as ionic silver into the porcelain.

Finally reduced to colloidal metallic silver in the surface layer of

porcelain.

Not all porcelains are susceptible to silver discolour because some do

not contain the necessary elements to reduce the ionic silver.

To eliminate the greening problem, palladium alloys with no silver were

developed.

These alloys contain 75% to 90% palladium.

Some of these high palladium alloys develop a layer of dark oxide on their

surface during cooling from the degassing cycle.

This oxide layer has proven difficult to mark by the opaque porcelain.

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Other high palladium alloys such as the Pd-Ga-Ag-Au type do not seem to

be plaqued.

This alloy type was introduced to the U.S. market in 1974 as the 1 st free

noble metal available for metal ceramic restorations.

Composition :

Palladium – 53% to 61%.

Silver – 28% to 40%.

Small amounts of tin + indium or both are added to promote oxide

formation for adequate bonding of porcelain.

In some alloys, the formation of an internal oxide layer rather than an ext.

oxide layer has been reported.

Increasing the silver content tends to a lower the metal range and raises the

contraction coefficient of an alloy.

Because of the high silver content, silver discolouration effect is most

severe for these alloys.

However, gold metal conditioners or ceramic coating agents may minimize

this effect.

The low specific gravity of these alloys (10% to 11%) combined with their

low intrinsic cost makes these alloys attractive as economical alternatives to the

gold-based alloys.

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Palladium-copper Alloys

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

of this type are recent introductions to the dental market, little clinical

information is available on their long-term clinical success.

Disadvantages because of their low melting range of approx 1170°C to

1190°C, these alloys are susceptible to creep deformation (Sag) at elevated

firing temperatures.

Thus, one should exercise caution in using these alloys for long-span

fixed partial dentures with relatively small connectors.

Composition:

Palladium – 74% and 80%.

Copper – 9% to 15%.

Some may contain as little as 2% gold – this small amount of gold

serves no useful purpose.

Porcelain discolouration due to copper is possible but does not appear to be

a major problem.

There has also been some concern recently over the potential cytotoxic

effect of copper released intra-orally from certain Pd-Cu alloys.

One should also be aware of the potential effect on aesthetics of the dark

brown or black oxide formed during oxidation and subsequent firing cycles.

Care should be taken by the technician to mask the oxide completely with

opaque porcelain and to eliminate the unaesthetic dark bond that develops at

metal-porcelain junctions.

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It is also important that technician ensure that a brown rather than a black

oxide is formed on the metal surface during oxidation treatment otherwise,

poor adherence to porcelain may result.

Some of these alloys are technique sensitive with respect to casting.

In some instances the molten alloy should have a thin oxide firm appearing

on its surface at the casting temperature.

Some instructions specify that heating be maintained for an additional 7

seconds beyond the point at which a rolling motion of the alloys is observed.

Because of the lack of a specifically defined melt appearance, there may

be a tendency to overheat the alloy to eliminate this film.

This error would cause significant changes in the properties of the alloy

and a decrease in metal-porcelain bond strength.

Underheating of the molten alloy is also possible because of the difficulty in

judging the proper melt appearance this could result in incomplete castings

or rounded margins.

Because alloys have a poor potential burnishing, except when the marginal

areas are relatively thin.

Although thermal incompatibility is not considered to be a major distortion

of ultrathin metal copings (0.1mm) has been occasionally reported. The exact

cause of this effect is not known. It could be :

- Because of metal-porcelain incompatibility stresses.

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- High creep rate of these alloys at temperatures near

the glass transitions temperature of porcelain.

- Relaxation of elastic stresses due to solidification,

grinding and sandblasting.

This distortion of single unit / multiple unit castings could be overcome

by using thicker copings / connectors or changing the band of porcelain or by

using a different metal-ceramic system with more acceptable overall properties.

Palladium-Cobalt Alloys

This alloy group is comparable in cost to Pd-Ag and Pd-Cu alloys.

They are often advertised as gold-free, nickel-free, beryllium free and silver

free alloys.

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

tearing during the solidification process.

This Pd-Cu group is the most sag resistant of all noble metal alloys.

Composition :

- Palladium – 78% to 88%.

- Cobalt – 4% to 10wt%.

One commercial alloy contains 8% gallium.

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

Hardness – 250DPH.

Yield strength – 586 MPa.

Elongation – 20%.

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Modulus of elasticity 85.2 GPa

Although these alloys are silver free, discoloration of porcelain can still

result because of the presence of cobalt. But this is not a significant problem.

Failure of the technician to completely mask out the dark metal oxide colour

with opaque porcelain is a more common cause of unacceptable aesthetic

results.

No metal casting agents are required to mask the oxide colour or to promote

adherence to porcelain.

Like the Pd-Ag and Pd-Cu alloys the Pd-Co alloys generally tends to have a

relatively high thermal contraction coefficient. Hence would be more

compatible with high expansion porcelains.

Palladium-Gallium-Silver and Palladium-Gallium-Silver-Gold alloys

These alloys are the most recent of the noble metals.

This groups of alloys was introduced because they tend to have a slightly

lighter coloured oxide than the Pd-Cu or Pd-Co alloys.

The silver content is relatively low (5-8wt%) and is inadequate to cause

porcelain greening.

Since they have a low coefficient of thermal contraction they would be more

compatible with lower expansion porcelains (e.g. vita porcelains).

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Conclusion

Numerous types of casting alloys have been used for the restoration of

teeth while gold-based alloys have played the major role for many years their

dominance has been challenged now by base metal system. The main reason is

that base metals are less expensive as compared to gold alloys. Unfortunately

these substitutes have been less than ideal in numerous way and one of greatest

questions about them is their biological compatibility. However, further

investigation need to be conducted to evaluate their biocompatibility.

However, the performance of any restoration is related to multiple

factors for ex: the design of the appliance, the skill and accuracy with which it

has been fabricated and the properties of the materials used.

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