dental alloys presentation
TRANSCRIPT
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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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