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Dental Alloys

Justin Jones, BSc.

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Basics

Metals & alloys are crystalline solids

Alloys consist of two or moremutually soluble metals when molten

Grain size is the size of the crystalsin the solid state

Small grain size gives bettermaterials

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Some Science

Small grain sizecan be achievedby:

Rapid cooling Addition of a grain

refiner (iridium)

Upon cooling: the grain refiner solidifies first,forming nucleation sites from which the crystals(grains) grow

Giving smaller grains with a more uniform size

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Material Requirements

Yield strength: stress (force) required tocause permanent strain (change of shape) > 300 MPa for oral use

Hardness: resistance to wear Enamel = 350 VHN (Vickers) Dentin = 60 VHN

Corrosion: resistance to chemical attack Thermal conductivity Adhesion: to tissue / other materials Chemical: corrosion Biological: biocompatibilty

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Classification

Traditional classification:1. Gold-based alloys

2. Metal-ceramic alloys

3. Alloys for removable partial dentures

New classification:

1. High-nobel alloys2. Nobel alloys

3. Base metal alloys

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Traditional Classification ofGold-based Alloys

Types I-IV based on compositionType Au (%) Ag (%) Cu (%) Pd/Pt

(%)

Use Yield Str.(MPa)

Elongatio

n (%)

I Soft 85 11 3 0 Low stress - inlays 340 10

Gold & silver: ductility, corrosion resistance

Copper: hardness

Platinum: strength, hardness, corrosion

Zinc: prevents oxidation, improved casting

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Metal-Ceramic Alloysfor Porcelain Bonding

Used with ceramic component

Different to gold alloys: Higher melting temp (more Pd) for ceramic application

Little/no Ag discolouration

Little/no Cu firing

Several types: High-gold (75%)

Low-gold (45%)

Silver-copper

Silver-palladium

Cobalt-based

Nickel-based

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Alloys for RemovablePartial Dentures

Types used: Type IV gold-based alloy Chromium-based alloys Cobalt-based alloys

Advantages of Co/Cr-based alloys over the Type IV gold: Stronger Harder Less dense cheaper More rigid make thinner castings

However: High casting temp Too hard finishing and polishing Low ductility chairside adjustment

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New Classification

Todays system is based on composition

Type Composition

High-noble Gold content > 40 wt%and

Noble metal content > 60 wt%

Noble Noble metal content > 25 wt%

Primarily base-metal Noble metal content < 25 wt%

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High-Noble Alloys

Gold > 40%, Nobel > 60%

Include:1. Gold-platinum (85% Au, 12% Pd)

Full-cast and metal-ceramic applications

2. Gold-palladium (52% Au, 38% Pd) Full-cast and metal-ceramic applications

3. Gold-copper-silver-palladium Low melting temp

Ag turns porcelain greenFull-cast only

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Noble Alloys

Noble metal > 25%

Diverse group of materials

Generally speaking:

High strength, hardness & ductility Yellow or white (white due to Pd)

Variable resistance to corrosion

Used for full-cast and metal ceramic

applications With the exception of gold-copper-silver-

palladium alloys

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Base-Metal Alloys

Noble metal < 25%But usually contain little or none Usually:

Nickle-chromium or

Colbalt-chromium alloys

Applications: Full-cast, metal-ceramic and RPDs

Advantages: Harder

Stronger

High elastic modulus can be made thinner

Cheaper

Disadvantages:

Difficult to cast high temperatures

Shrinkage (2.3%) must be compensated

Finishing & polishing difficult

Corrosion

Allergy

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