Noble metals and base metal alloys/ dental implant courses by Indian dental academy

NOBLE METALS AND BASE METAL ALLOYS 1. Introduction.

2. Desirable properties.

3. Classification.

4. Ingredients.

5. Alloys and its description.

Noble

metal

and

Base

metal

alloys,

properties, advantages and dis-advantages.

1

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 surface that retain their surfaces in dry air.

The six metals of platinum group are: platinum, palladium, iridium, rhodium, osmium, ruthenium and along with gold are 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.

2

This technique was an instant success, which soon led to casting of complex inlays such as onlays, crowns, fixed partial dentures and removable partial denture frame works.

As pure gold did not have the physical properties required of these dental restorations existing jewelery alloys were quickly adopted.

These gold alloys were further strength end 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 as: Type I – Soft – VHN 50 to 90.

Type II – Medium – VHN 90 to 120.

Type III – Hard – VHN 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 not known that in gold alloys, palladium is added to counter act the tarnish potential of silver.

In the late 1950’s a breakthrough occurred in dental technology that was to influence significantly the fabrication of dental restorations.

This was the successful veneering of a metal substructure with dental porcelain.

It was found that adding both platinum and palladium to gold would lower the alloys coefficient of thermal contraction sufficiently to 3

ensure physical compatibility between the porcelain veneer and metal substructure.

The melting range of the alloy was also raised significantly to permit firing of the porcelain onto the gold-based alloy at 1040 o C without deforming the metal substructure.

The base metal removable partial denture alloys were introduced in 1930’s. By then both Ni-Cr and Co-Cr formulations had become popular compared to conventional Type-IV gold.

The success of base metal alloys for constructing rpd framework led to its application to fabricate other types of restorations.

Likewise by 1978 the price of gold was climbing so rapidly that attention focused on the noble metal alloys.

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 carryout 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: 1. Type-I (Soft): For restorations subject to very slight stress such as inlays.

2. Type-II (Medium): For restorations subject to moderate stress such as onlays.

3. Type-III (Hard): For high-stress situations, including onlays, crowns, thick veneer crowns and short-span fixed partial dentures.

4

4. Type-IV (Extra hard): For extremely high stress states, such as endodontic posts and cores, thin veneer crowns, long span fixed partial dentures and removable partial dentures.

The development of modern direct tooth coloured filling materials has almost eliminated the use of type I – II alloys. Type I and II are often refined to as inlay alloys. Type III and IV are generally called Crown and Bridge alloys.

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

Alloy Type

Total noble metal content

High Noble (HN)

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

Noble metal (N)

Contains ≥ 25 wt% of the noble metal elements.

Predominantly base metal (PB)

Contains < 25 wt% of the noble metal elements.

ACCORDING TO MARZOUK: 1. Class-I: Gold and platinum group based alloys.

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.

OTHER CLASSIFICATIONS: 1. Yellow gold → Yellow coloured. → Gold content > 60%.

2. Low gold

→ Usually yellow coloured.

Economy gold → But with gold content 60%.

3. White gold → White coloured. → Gold content > 50%.

Classification of alloys for all metal restoration. restorations and frameworks for RPD.

Metal ceramic

5

Alloy type 1. High Noble

All metal Au-Ag-Cu-Pd Metal ceramic alloys.

2. Noble

Ag-Pd-Cu Ag-Pd Metal ceramic alloys.

Metal-Ceramic Au-Pt-Pd Au-Pd-Ag (5-12 wt% Ag) Au-Pd-Ag (>12 wt% Ag) Au-Pd (Zn Ag) Pd-Au (Zn Ag) Pd-Au-Ag Pd-Au Pd-Cu Pd-Co Pd-Ga-Ag

RPD Au-Ag-Cu-Pd

Ag-Pd-Au-Cu Ag-Pd

Note: The principle reasons that alloys for all metal restorations cannot be used for metal ceramic restorations is 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.

3.

Their thermal contraction coefficients may not be close enough to those of commercial porcelains.

INGREDIENTS OF NOBLE METAL ALLOYS: The most important element in dental gold alloys is gold, copper, silver, platinum metals and zinc.

1. Gold: Gold is primarily responsible for deformability (ductility).  Ranks lowest in strength.  Characteristic yellow colour with a strong metallic luster.  Density (Sp gravity) → 19.3 g/cm3.  Tarnish resistance.  Fusion temperature – 1063o C.  Not soluble in sulphuric, nitric or HCl acids.

6

Lowest density necessitates more force in centrifugal casting to compensate for lower weight / volume.

However, lower density will allow more restorations per unit weight, which can be economical. To some extent high density enables case of casting. Alloys for dental use should be at least 16K resistance.

2. Platinum: May be added to 1. Strengthen the alloy.

2. Raise the fusion point (1755).

3. Import rigidity, nobility and hardness.

4. Whiten the alloy.

5. Specific gravity-- 21.37.

6. Also malleable and ductile.

Its coefficient of expansion is close to that of porcelain to prevent buckling of the metal or fracture of porcelain during changes in temperature.

3. Palladium: Serves the same functions but is much less expensive than platinum.

Dis-advantage: Palladium hydrogen gas released containing palladium may be more porous when cast.

because

gold

Sp Gr – 11.4, MP – 1555o C. 4. Iridium, Ruthonium and Rhodium: Trace amounts of these metals are added as “Grain Refiners” melting point 960o C .5o C below the MP of both gold and copper. As little as 0.005% is sufficient to refuse the grain size. Grain refiners produce smaller grains.

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

Indium can also act as a scavenger for the alloy during casting procedure. Also serve to increase the tarnish and corrosion.

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5. Silver: It contributes to the strength and hardness of the alloy. Although it mimics gold in its deformability effect, it adversely affects the malleability or it lowers tarnish resistance.

Food containing suplhur compounds, cause severe tarnish on silver.

Silver serves to balance the red colour given by copper.

Adding small amounts of palladium to silver containing alloys prevents the rapid corrosion of such alloys in the oral environment.

Also includes appreciable quantities of O 2 in the molten state porosities or rough casting surface can be prevented by adding 5â&#x20AC;&#x201C; 10% copper.

6. Copper: Contributes strength and hardness, but decreases the malleability of the alloy, i.e., it decreases the tarnish and corrosion resistance because the more content should not exceed 16%.

Gives the alloy

reddish appearance. Lowers the fusion temperature.

Zinc: (Present only in low percentages, around 0.5%) Acts as a deoxidizer and reduces the oxygen content (because O 2 released during solidification results in porosity).

Karat and fineness: Karat is the traditional unit expressing gold content in al alloy.

Karat refers to parts of pure gold in 24 parts of an alloy. Pure gold is 24 karat.

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

8

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 (500 f).

Heat treatment of gold alloys: Gold alloys can be significantly hardened if the alloy contains sufficient amount of copper (at least 8 wt% copper).

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

An alloy can be subjected to;  Hardening heat treatment hardening.  Softening heat treatment / solution heat treatment.

SOFTENING HEAT TREATMENT: Casting is placed in an electric furnace for 10 minutes at a temperature of 700o C (1292o 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 is to be grooved, shaped or otherwise cold worked, either in or out of the mouth.

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HARDENING HEAT TREATMENT: The age hardening 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.

The aging temperature depends on the alloy composition but is generally between 200o C (400o F) and 450o C (840o 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 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.

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 three 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.

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3.

The thermal contraction of the solid metal that occurs down to room temperature.

The values for casting shrinkage differ for the various alloys

presumably because of difference in 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

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

Base Metals

Co-Cr-Mo

2.3

Composition of traditional I to IV alloys.

Type

Au

Cu

Ag

Pd

Su,In,Fc,Zn,Ga

I

83%

6

10

0.5

Balance

II

77%

7

14

1

Balance

III

75%

9

11

3.5

Balance

IV

69%

10

12.5

3.5

Balance

Silver palladium alloys:

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These alloys are white in colour.

Main component is silver, but small amounts of palladium (approximately 25%) are also present.

Palladium is added to provide

nobility and tarnish resistance.

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

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 similar to type-IV gold alloy.

Dis-advantage: Greater potential for tarnish and corrosion.

HIGH NOBLE ALLOYS FOR METAL CERAMIC RESTORATIONS: The original metal-ceramic alloys contained 88% 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 dental porcelain.

Therefore mechanical retention and undercuts were used to prevent detachment of the ceramic veneer.

Using the â&#x20AC;&#x153;stress bond testâ&#x20AC;?, 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 metals 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 12

was improved. (Iron also increased the proportional limit and strength of the alloy).

Because of 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 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 metal 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.

This solids temperature is sufficiently high to permit the application of low fusing porcelain.

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 is its coefficient of thermal expansion.

The high noble alloys for metal-ceramic restorations are: 1. GOLD-PLATINUM-PALLADIUM ALLOYS:  Gold content ranges up to 88%.  Varying amounts of palladium, platinum  And, small amount of base metals  Colour – yellow.

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Dis-advantage: Susceptible to sag deformation and FPD should be restricted to three unit spans, anterior cantilevers or crowns.

2. GOLD-PALLADIUM-SILVER ALLOYS:  Gold content ranges from 39% - 77% gold.  Palladium – 35%  Silver less than 22%.  Silver increases the thermal contraction coefficient.

Dis-advantage: Silver present in this alloy can discolour some porcelain.

3. GOLD-PALLADIUM ALLOYS:  Gold content ranges from 44% to 55%.  Palladium – 35 to 45% These alloys have remained popular despite of their relatively high cost.

Dis-advantage: The lack of silver results in a decreased thermal contraction coefficient and the freedom from silver discolouration of porcelain.

NOBLE ALLOYS FOR METAL CERAMIC RESTORATIONS: According to ADA classification of 1984, noble alloys must contain at least 25 wt% of the noble metals but do not necessarily contain any gold.

Noble palladium based alloys offer a compromise between the high noble gold alloys and predominantly base metal alloys.

Also the rice per ounce of palladium alloy is generally one half to one third that of gold alloy.

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

Palladium based alloys have workability similar to gold and scarp value.

PALLADIUM-SILVER ALLOYS: Introduced

widely

in

the

late

1970â&#x20AC;&#x2122;s

to

overcome

some

disadvantages of early base metal alloys. eg.Castability, porcelain bonding and workability problems In the recent years the popularity of these alloys has declined because of the dis-advantages with these alloys as it discolors porcelain during firing.

This discolouration is usually a greenish-yellow and is popularly termed as â&#x20AC;&#x153;Greeningâ&#x20AC;?. Greening occurs mainly because of the presence of silver.

Silver vapor escapes during firing

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 discolouration because some do not contain the necessary elements to reduce the ionic silver.

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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 mask by the opaque porcelain.

Other high palladium alloys such as the Pd-Ga-Ag-Au type do not seem to be plagued by this problem.

This alloy type was introduced to the U.S. market in 1974 as the first free noble metal available for metal ceramic restorations.

Composition:  Palladium – 53% to 61%.  Silver – 28% to 40%.

Small amounts of tin and indium or both are added to promote oxide layer formation for adequate bonding of porcelain.

In some alloys, the formation of an internal oxide layer has been reported.

Increasing the silver content tends to lower the melting range and raises the contraction coefficient of an alloy.

Because of high silver content, silver discolouration effect is most severe for these alloys.

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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.

PALLADIUM-COPPER ALLOYS: This type of alloy 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.

DIS-ADVANTAGES: Because of their low melting range of approximately 1170 o C to 1190o 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 denture with relatively small connectors.

Composition:  Palladium – 74 to 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.

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There has also been some concern recently over the potential cytotoxic effect of copper released intraorally 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.

It is also important that technician ensure that a brown rather than 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 film appearing on its surface at the casting temperature.

Some instructions specify that heating to 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.

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Under heating of the molten alloy is also possible because of this difficulty and could result in incomplete castings or rounded margins.

Because alloys have a poor potential burnishing except when the marginal areas relatively thin.

Although thermal incompatibility is not considered to be a major concern distortion of ultra thin metal copings (0.1 mm) has been occasionally seen.

The exact cause of this effect is not known. It could be because of metal porcelain incompatibility stresses.

High creep rate of these alloys at temperature near the glass transitions temperature of porcelain. Relaxation of elastic stresses due to solidification, grinding and sand blasting.

The 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.

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

Composition:  Palladium – 78% to 88%.  Cobalt – 4% to 10-wt%.

One commercial alloy contains 8% gallium.

An example of typical properties of a Pd-Cu alloy is as follows:  Hardness – 250 DPH.  Yield strength – 586 MPa.  Elongation – 20%.  Modulus of elasticity – 85.2 GPa. Although these alloys are silver free, discolouration 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 porcelain.

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PALLADIUM-GALLIUM-SILVER AND Pd-Ga-Ag 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 the Pd-Cu or Pd-Co alloys.

The silver content is relatively low (5-8 wt%) 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).

BASE

METAL

ALLOYS:(for cast metal and metal ceramic

restorations) Base metal alloys became popular after 1980’s due to its cost factor (cheaper to Type-IV).

Other

factors

like

lower

density,

greater

stiffness,

higher

hardness, comparable chemical resistance to tarnish and corrosion also made it popular.

Most widely used are the Ni-Cr and the Co-Cr alloys.

These alloys may also contain one or more of the following elements; Aluminum, beryllium, boron, carbon, cobalt, copper, cerium, gallium, iron, manganese, niobium, silicon, tin, titanium and zirconium.

Ni – Cr

Co – Cr

Nickel: 61 – 81 wt%

Cobalt: 53 to 67 wt%

Chromium: 11 – 27 wt%

Chromium: 25 to 32 wt%

Molybdenum: 2 – 5 wt%

Molybdenum: 2 – 6 wt% 21

As base metal alloys melt at elevated temperatures the use of phosphate or silicate bonded investments is indicated.

Functions of the alloying elements: Chromium:  Responsible for tarnish and corrosion resistance.  Its passivation effect, ensure corrosion resistance.  Reduces the melting point.  Also acts in solid solution hardening.  30% is the maximum limit to attaining maximum mechanical properties.

Cobalt:  Imparts hardness, strength and rigidity to the alloy.  It has a high melting point.

Nickel:  It increases ductility.  Ni and Co are interchangeable.  Decreases strength, hardness, MOE and fusion temperature.

Molybdenum / Tungsten:  Contributes to the strength of the alloy.  Mo reduces ductility to a lesser extend than tungsten.  Mo refines grain structure.

Fe / Cu / Be: 22

Increases hardness – Fe,Cu. Be reduces fusion temperature and refines grain structure.

Manganese / Silicon:  Primarily oxide scavengers to prevent oxidation of other elements during melting.  Are also hardeners.

Boron:  Deoxidize and hardener.  But reduces ductility.

Carbon: Most critical content Small amounts may have a pronounced effect of strength, hardness and ductility.

It forms carbides with any of the metallic constituents, which strengthens the alloy.

When in excess it increases brittleness.

Properties: Biocompatibility: The allergenic effects of nickel on dental patients and the potential toxic effects of nickel and beryllium are laboratory technicians continue to cause concern within the dental profession.

Dermatological and systemic effects, which may result from patient and personal exposure to cobalt alloys.

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Dentists and administrators of dental laboratory should be encouraged to inform employers (tech) regarding the need to avoid inhalation exposure to dusts from alloys.

Ni-Cr, Ni-Cr-Be and Co-Cr: Base metal alloys generally have higher hardness and elastic modulus (stiffness) values and are more say resistant at elevated temperature but they may be more difficult to cast and presolder than Au-Pd or Pd-Ag alloys.

Base metal are more technique sensitive than well-established noble metal alloys.

Another potential dis-advantages of nickel-based or cobalt-based alloys is their potential for porcelain delamination due to separation of a poorly adherent oxide layer from the metal substrate.

The creep resistance of nickel-based alloys at porcelain firing temperature is considered to be far superior to the resistance of gold based and platinum based alloys under the same conditions.

The tarnish and corrosion resistance of base metal alloys containing Ni is of principal concern because of the allergic potential of Ni and its compounds.

Amalgams are expected to sustain less galvanic attack when they are placed in contact with Ni-Cr or Co-Cr alloys than with gold alloys.

Some research indicates that certain characteristics of the oxide layer that forms on these alloys during pre-oxidation and porcelain

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firing cycles may adversely affect the bond strength of metal ceramic restorations.

Some alloys require a “light” sand blasting procedure after the initial oxide is formed during the pre-oxidation cycle.

The elastic modulus of base metal alloys is as much as two times greater than the values for some of the more popular noble metal alloys.

Some clinicians have proposed that the coping thickness in veneered areas can be reduced from 0.3 mm recommended for noble metal alloys, to a uniform thickness of 0.1 mm.

In general, the high hardness and high strength of base metal alloys contribute to certain difficulties in clinical practice. Grinding and polishing of these require chair-side time.

Mechanical properties: Base metal alloys have a modulus of elasticity approximately twice that of previously used gold alloys.

Because elasticity modulus is a measure of the stiffness or 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.

The VHN – 175 – 360 DPH Ductility of base metal alloys ranges between approximately 10% and 28%. Noble metal alloys have an elongation of approximately 5% to 10%.

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The density of base metal alloy is 8.0 g / cm 3 as compared with 18.4 g/cm3 for comparable high noble metal, metal ceramic alloys.

Sag resistance is the property that has been used to describe the ability of an alloy to resist the permanent deformation or creep induced by thermal stresses.

Base metal alloy deforms less than 25 Âľm, whereas a noble metal deforms 225 Âľm.

Base metal is compatible with porcelain. It must not react with the ceramic in such a way as to visibly discolour the porcelain at the interface or marginal regions and the metal-ceramic system must form a stable bond at the interface that can withstand normal stresses, which occur in the mouth.

The base metal alloys are much more technique sensitive than their high noble metal alloy counterparts.

A high degree of satisfaction with most of these alloys in terms of the casting, finishing and application of porcelain veneers.

The successful use of the base metal alloys requires some modification of the laboratory procedures, but it appears to provide clinically acceptable and durable restorations.

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COMPARISON OF GOLD ALLOYS AND BASE METAL ALLOYS PROPERTIES

BASE METAL

GOLD ALLOYS

TENSILE STRENGTH

MORE (1,142 MPa)

LESS (490 MPa)

YIELD STRENGTH

MORE (591 MPa)

LESS (400 MPa)

MORE (207x103 MPa)

LESS (55 x 103 MPa)

DUCTILITY

MORE (23.9%)

LESS (9.1%)

VICKERS HARDNESS

MORE (293 DPH)

LESS (161 DPH)

DENSITY

LESS (81.9 / cm3)

MORE (18.39/cm3)

PORCELAIN BOND

LOW (97.9 MPa)

HIGH (111 MPa)

HIGH (1300o C)

LOW (900o C)

CASTING SHRINKAGE

HIGH (2.25%)

LOW (1.25 – 1.65%)

HEAT TREATMENT

COMPLICATED

SIMPLE

TARNISH &

CHROMIUM OXIDE

CORROSION

LAYER

PRICE

LOW

HIGH

SAG RESISTANCE

LOW (< 25 µm)

HIGH (225 µm)

MORE

LESS

PHOSPHATE/SILICATE

GYPSUM BONDED

GOOD

EXCELLENT

MODULUS OF ELASTICITY

MELTING TEMPERATURE

TECHNIQUE SENSITIVE CASTING INVESTMENT BIOCOMPATIBILITY

NOBLE METAL

PHYSICAL PROPERTIES OF SOME MODERN NOBLE METAL DENTAL ALLOYS Alloy

Main

Melting

Density

Yield

Hardness

type

elements

range (o C)

(g/cm3)

strength

(VHN) 27

(MPa) I

HIGH NOBLE

943-960

16.6

103

80

II

HIGH NOBLE

924-960

15.9

186

101

III

HIGH NOBLE

924-960

15.5

207

121

NOBLE

834-916

12.8

241

138

Ag-Pd NOBLE

1021-1099

10.6

262

143

HIGH NOBLE

921-943

15.2

275

149

HIGH NOBLE

871-932

13.6

372

186

NOBLE

930-1021

11.3

434

180

HIGH NOBLE

1271-1304

13.5

572

220

NOBLE

1232-1304

10.7

462

189

HIGH NOBLE

1149-1177

18.3

450

182

NOBLE

1155-1302

476-685

235-270

III

METAL CERAMIC

10.611.5

REFERENCES: 1. Science of Dental Materials by Anusavice.

2. Notes in Dental Materials by K.V.SubbaRao.

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