TERMINOLOGY Alloy : two or more elements, at least one of

which is metal, and all of which are mutually soluble in the molten state.

Classification of alloysAccording to Use :1)Alloys for all metal restorations" (e.g. inlays,

posts, crowns & bridges) 2) Alloys for metal ceramics restorations

According to Yield Strength and percentage Elongation (ADA Classification)

Type I Soft Type II Medium Type III Hard Type IV Extra hard

According to Nobility (ADA 1984) A. High noble metal alloys Contains greater or equal 40 wt% Au and

greater or equal 60 wt% of noble metalsB. Noble metal alloys Contains greater or equal 25 wt% of noble

metalsC. Predominantly base metal Contains less than 25 wt% of noble metals

General Requirements of Casting Alloys All cast metals in dentistry have some basic

common requirements1) Resistance to tarnish and corrosion in the

mouth. 2) Sufficiently strong for the intended purpose.3) Biocompatible (non-toxic and non-allergic). 4)Easy to melt, cast, cut and grind (easy to

fabricate). 5)Flow well and duplicate fine details during

casting. 6) Minimal shrinkage on cooling after casting. 7) Easy to solder.

…

ALLOYS FOR ALL METAL RESTORATIONS . The early alloys were mostly gold alloys..

Currently, the use of these alloys are slowly declining because of the:

1)Increased esthetic awareness has reduced the trend for metal display.

2)Increasing popularity of all ceramic and metal ceramic restorations.

3)Reducing popularity of resin and composite as veneering material.

Classification . Type I Soft Small inlays, class III and class V cavities which are not

subjected to great stress. They are easily burnished. These being highly malleable and ductile have self-sealing margins like gold foil filling.

Type II Medium Inlays subject to moderate stress,. They are less resistance to

tarnish and corrosion than type I. Type III Hardonlays, crown and bridges, situations where there may be

great stresses involved.. Type IV Extra Hard onlays subjected to very high stresses,

partial denture frameworks and long span bridges. These are less resistant to tarnish and corrosion than type I,II and III alloys.

GOLD CONTENT Traditionally, gold content of dental casting alloys was called:1)Karat2)Fineness. Karat It refers to the parts of pure gold present in 24 parts of alloy,

e.g. - 24 karat gold is pure gold. - 22 karat gold is 22 parts pure gold and 2 parts of other

metal. " Fineness Fineness of a gold alloy is the parts per thousand of pure

gold. Pure gold is 1000 fine.

PROPERTIES OF GOLD ALLOYS Color :modern gold alloys can vary from gold to white.

It depends on the amount of whitening elements (silver, etc.) present in the alloy.

Melting Range :Ranges between 920 to 960°C. It indicates the type of investment required and

the type of heating source needed to melt the alloy.

Density :, lower density more number of cast

restorations per unit weight,. Gold alloys are lighter than pure gold. Higher density alloys cast better than lower

density alloys.

Hardness :The hardness indicates the ease with which these alloys

can be, ground or polished. Gold alloys are generally more user friendly than the base

metal alloys which are extremely hard.

Modulus of Elasticity Tarnish and Corrosion Resistance. Noble metals are less reactive. Elongationindicates the ductility of the alloy.. Type I alloys are easily

burnished. Alloys with low elongation are very brittleBiocompatibility Gold alloys are relatively biocompatible

Casting Shrinkage All alloys shrink when they change from liquid to

solid. The casting shrinkage in gold alloys is less when compared to base metal alloys.

. Shrinkage affects the fit of the restoration.

Therefore, it must be controlled and compensated for in the casting technique.

GoldGold(Au) (Au) CopperCopper(Cu) (Cu)

SilverSilver(Ag) (Ag)

Corrosion resistanceCorrosion resistanceHardnessHardnessCounteract orange color of copperCounteract orange color of copper

Effects of Alloys ComponentsEffects of Alloys Components

Palladium Palladium (Pd) (Pd) Platinum Platinum (Pt) (Pt)

((66))Zinc Zinc (Zn) (Zn)

Increase strength and hardnessIncrease strength and hardnessIncrease strengthIncrease strengthPrevent oxidation during melting Prevent oxidation during melting (O(O22 getter) getter)

The Minor Additions Indium, Tin and Iron : They help to increase the ceramic alloys bondCalcium :It is added to decreased CTE

METAL CERAMIC ALLOYS Metal ceramic alloys are those alloys that are

compatible with porcelain and capable of bonding to it.

Several types of alloys are used. All have coefficient of thermal expansion (CTE) values which match that of porcelain..

lithium carbonate to porcelain

EVOLUTION OF METAL-CERAMIC ALLOYS

In early days before porcelain-metal bonding was clearly understood, porcelain had to be retained by mechanical means with the help of undercuts.

Later, it was discovered that adding 1% of base metals like iron, tin, indium, etc. induced chemical bonding by the formation of an oxide layer.

REQUIREMENTS OF ALLOYS FOR PORCELAIN BONDING

In addition to the general requirements of alloys mentioned earlier, metal ceramic alloys have some special requirements in order to be compatible with porcelain veneering.

1)It's melting temperature should be higher than the porcelain firing temperature.

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

3) It should not stain or discolor porcelain.

TYPES (CLASSIFICATION) OF METAL CERAMIC ALLOYSAlloys for metal ceramics restorations may be

categorized as: 1)High noble (commonly referred to as gold

alloys)a) Gold-palladium-platinum alloysb) Gold-palladium-silver alloys c) Gold-palladium alloys

2)Noble (commonly referred to as palladium alloys)

Palladium-silver alloys Palladium-gallium-silver alloys Palladium-gold alloys Palladium-gold-silver alloys Palladium-copper alloys Palladium-cobalt alloys 3)Base metal Nickel-chromium alloys Nickel-chromium-beryllium alloys Cobalt-chromium alloys Pure titanium Titanium-aluminum-vanadium

THE HIGH NOBLE (GOLD BASED) METAL-CERAMIC ALLOYS

As mentioned earlier, the high noble alloys contain more than 40 wt.% gold and are therefore also referred to as gold alloys or gold based alloys

Common Features of High Noble (Gold Based) Alloys Cost. Color the color can range from white to gold

depending on the gold content. Melting range Porcelain is fired at a

temperature of900 to 960°C. Thus obviously these alloys must have melting temperatures much higher than the temperatures at which porcelain is fired.

Density (depending on the gold content). Because of the high gold and noble metal

content, these alloys have a high density.

Castability the high density of these alloys make them easy to cast

Biocompatibility high noble alloys have had a good and safe track record. They are not known to cause any problems in the mouth.

Soldering Gold based alloys are quite easy to solder

Tarnish and corrosion Because of their high noble metal content, these alloys are extremely stable in the oral environment

TYPES The following three will be briefly described. Gold-palladium-platinum alloysGold-palladium-silver alloys Gold-palladium alloys

Gold-Palladium-Platinum Alloys Composition Gold ( 80 to 88 wt%)Palladium (5 to 11 wt%)Platinum (6 to 8 wt%)Silver (0 to 4.9 wt%)Base metals ( balance around 1%)

Gold-Palladium-Silver Alloys Composition Gold (39 to 77 wt%)Palladium (10 to 44 wt%)Silver (9 to 22 wt%)Base metal (balance around 1%)The silver has a tendency to discolor some

porcelains.

Gold-Palladium Alloys CompositionGold ( 44 to 55 wt%)Palladium (35 to 45 wt%0Base metals ( balance around 1%)The absence of silver eliminates the

discoloration problem.

THE NOBLE (PALLADIUM BASED) METAL-CERAMIC ALLOYS

. Currently, the noble metal-ceramic alloys are mostly palladium based.

.

. Their properties were between that of the high noble alloys and the base metal alloys.

TYPES

Palladium-silver alloys Palladium-copper –gallium alloys Palladium- gallium

Common Features of Palladium Based (Noble) Alloys

Cost their cost range between that of the gold alloys and the base metal alloys.

Color they are white in color.Density they are less denser than the gold

alloys. The density ranges from 10.5 to 11.5 gm/cm",

Castability these alloys have a lower density than the gold alloys and so do not cast as well. However, they are better than the base metal-ceramic alloys in this regard.

. Melting range a typical melting range is

1155 to 1304°C. Porcelain bonding Like the gold alloys,

base metals like tin, indium, etc are added to assist porcelain bonding.

Tarnish and corrosion Because of their high noble metal content, these alloys are extremely stable in the oral environment.

Biological considerations These alloys are very safe and biocompatible

1.PAllADIUM -SILVER AllOYS Palladium is added to provide nobility and

resistance to tarnish. They are white in color. . They have a significantly lower density than

gold alloy. Their popularity has declined a little because of

the greening problem.( greenish-yellow discoloration)

2.Palladium-Copper-Gallium Alloys These are relatively new alloys..Composition Palladium (74 to 80 wt%)Copper (5 to 10 wt%)Gallium (4 to 9 wt%)Gold (1 to 2 wt%)Base metal (around 1 wt%)

Esthetics Copper does cause a slight discoloration of the porcelain, but is not a major problem.

Black oxide layer. Care should be taken to mask this completely with opaquer..

Castability These alloys are more technique sensitive. Slight errors can lead to faulty castings.

3.Palladium-Gallium Alloys Composition Palladium 75 wt%Gallium 6 wt%Gold 6 wt%Base metal around 1 wt%.

BASE METAL ALLOYS FOR METAL CERAMIC RESTORATIONS

Developing countries have shown a preference for base metal-ceramic alloys. This is because the economic concerns

Nickel-chromium (nickel based) alloysCobalt-chromium (cobalt based) alloy

NICKEL-CHROMIUM ALLOYS Basic elements:Nickel 61 to 81 wt%Chrome 11 to 27 wt%Molybdenum 2 to 9 wt%

General Features and of Nickel Based Alloys Cost they are the cheapest of the casting

alloys. Color They are white in color. Melting range a typical melting range is

1155 to 1304°C. The melting range of these alloys like the gold ceramic alloys are high.

Density ranges from 7.8 to 8.4 gm/cm-'. They have just half the density of the gold alloys making them much lighter. One can get more castings per gram compared to the gold alloys.

Castability they are extremely technique sensitive. One reason may be their lower density compared to the gold alloys.

Hardness and workability. They tend to be much harder than the high noble metal ceramic alloys.

. Yield strength ranges from 310 to 828 MPa. These alloys are stronger than the gold and palladium based alloys.

Porcelain bonding These alloys form an adequate oxide layer which is essential for successful porcelain bonding. However…

Sag resistance These materials are far more stable at porcelain firing temperatures than the gold based alloys. They have a higher sag resistance.

COBALT CHROMIUM ALLOYS Cobalt-chromium alloys have been available

since the 1920's. high strength.. These alloys are also known as 'stellite'

because of their shiny, star-like appearance..

Composition Cobalt 35 to 65%Chromium 23 to 30 %Nickel 0 to 20 %Molybdenum 0 to 7 %Iron 0 to 5 %Carbon up to 0.4 %

APPLICATIONS 1)Denture base 2)Cast removable partial denture framework3)Crowns and bridges4)Bar connectors

Advantages of base metal alloys1) Lighter in weight.2) Better mechanical properties3) Less expensive than gold alloys.

Disadvantages 1) More technique sensitive.2) Complexity in production of dental

appliance.3) High fusing temperatures.4) Extremely hard, so required special

equipment for finishing.5) The high hardness can cause excessive

wear of restorations and natural teeth contacting the restorations.

TITANIUM AND ITS ALLOYS 1)excellent biocompatibility, 2)light weight,3) good strength ability to passivate.

Uses in Dentistry 1)Metal ceramic restorations 2)Dental implants 3)Partial denture frames 4)Complete denture bases 5) Bar connectors (In dentistry it is especially useful as an

alternative alloy to those who are allergic to nickel