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Dental Casting Alloys Lecture II
Presented by
Mohammed Moustafa Shalaby lecturer of fixed prosthodontics
Available Alloy Systems for Metal-Ceramic Restorations
High Noble Metal Alloys
Noble Metal Alloys
Predominantly Base Metal Alloys
High Noble Metal Alloys
Noble elements 60% by weight; at least 40% is gold
There are three systems in this class:
Gold-platinum-palladium
Gold-palladium-silver
Gold-palladium
High Noble Metal Alloys
Gold-platinum-palladium
84-86% Gold
Pt & Pd → ↑ Melting range
↓ CTE
↑ Noble content → ↑ Corrosion resistance
Fe → ↑ Proportional limit
↑ Strength
Tin & indium → Oxides for porcelain bonding
High Noble Metal Alloys
Gold-platinum-palladium
↑ Castability
Easy to adjust & finish
Burnishable
Biocompatible
High Noble Metal Alloys
Gold-platinum-palladium
Disadvantages:
↓ melting range → ↑ risk for sagging
↓ modulus of elasticity (thick sections)
↑ cost
High Noble Metal Alloys
Gold-palladium-silver
Two types:
Au-Pd-Ag (low silver content) : 5 - 11.99% silver
Au-Pd-Ag (high silver content) : 12 - 22% silver
High Noble Metal Alloys
Gold-palladium-silver ↑ Tarnish & corrosion resistance ↑ melting range ↑ Rigidity & sag resistance Less expensive than Au-Pt-Pd Tin & indium → oxides for porcelain bonding Ruthenium → ↑ castability Rhenium → Grain refiner
High Noble Metal Alloys
Gold-palladium-silver Its main disadvantage is the greening of silver Substituting sodium with potassium ions Metal coating agents: Pure gold film Ceramic conditioner Non greening porcelain
↑ Cost ↑ CTE
High Noble Metal Alloys
Gold-palladium
Good adherence to porcelain without greening
Tin & indium → oxides for porcelain bonding
Gallium →↓ Fusion temperature
Ruthenium → ↑ Castability
Rhenium → Grain refiner
High Noble Metal Alloys
Gold-palladium
↑ Mechanical properties
↑ sag resistance
↑ Tarnish & corrosion resistance
↑ Cost
↓ CTE
Noble Metal Alloys
25% by weight of noble metal, with no gold requirement
There are three alloy systems in this class:
Palladium-silver
Palladium-copper-gallium
Palladium-gallium
Noble Metal Alloys
Palladium-silver
Tin & indium → oxides for porcelain bonding
Ruthenium → ↑ castability
Noble Metal Alloys
Palladium-silver
↓ Cost
Good castability
Good porcelain bonding
Burnishability
↑ Sag resistance
↑ Tarnish & corrosion resistance
Long span bridges
Noble Metal Alloys
Palladium-silver
Discoloration
↑ CTE
Noble Metal Alloys
Palladium-copper-gallium
Gallium → Control casting temperature
Indium → Bonding with porcelain
↑ Tarnish & corrosion resistance
↑ Strength
↓ Cost
Noble Metal Alloys
Palladium-copper-gallium
↓ Sag resistance
Dark oxides
May discolor porcelain
↑↑ Hardness
Carbon-containing investments
Palladium allergy
Noble Metal Alloys
Palladium-gallium
↓ Hardness than Pa-Cu-Ga
↓ Porcelain discoloration than Pa-Cu-Ga
↓ Adherence to porcelain
Base Metal Alloys
Less than 25%, by weight of noble metal with no requirements for gold
They include:
Nickel-chromium
Cobalt-chromium
Titanium
Base Metal Alloys
Nickel-chromium
Nickel → ↑ Hardness
↑ Strength
↑ Modulus of elasticity
Chromium → Tarnish & corrosion resistance
Above 18% → ↓ Castability & brittle phases
Base Metal Alloys
Nickel-chromium
Beryllium → ↑ Castability & strength
↓ Oxides
Allow electrolytic etching of the alloy
↓ Melting range
Aluminum & titanium →↑ Strength
Molybdenum → ↓ CTE
Base Metal Alloys
Nickel-chromium
↓ cost
↑ Modulus of elasticity (thin sections+ long span bridges)
Nickel allergy
Beryllium → Carcinogenic
Base Metal Alloys
Cobalt-chromium
Cobalt → ↑ Hardness
↑ Strength
↑ Modulus of elasticity
More than nickel so the alloys are stronger & harder
Chromium → Tarnish & corrosion resistance
Molybdenum → ↓ CTE
No carbon
Base Metal Alloys
Cobalt-chromium
Highest modulus of elasticity
↑ Strength
↑ Hardness
Very difficult finishing
Difficult casting & soldering
Base Metal Alloys
Titanium:
Commercially pure titanium (cp Ti)
At room temperature, it exists as α-phase (closed packed hexagonal) , while heating α-phase above 883°c → β-phase (body centered cubic)
Titanium alloys (Ti-6Al-4V) most common
α-phase → Soft & ductile
β-phase → Stronger & harder
Aluminum → Alfa stabilizer
Vanadium → Beta stabilizer
α & β phases at room temperature
Base Metal Alloys
Titanium
Advantages:
Excellent biocompatibility
Corrosion resistance
Low density
Less expensive material
High strength.
Base Metal Alloys
Challenges of casting Titanium:
↑ Melting point (≈1700°C)
↓ Density (4.5 g/cm³)
↑ High chemical reactivity at high temperatures
↓ CTE
Casting machines:
Vacuum/Argon
Centrifugal casting machine
Base Metal Alloys
Investment: Replacing silica with magnesia and alumina Bonding to porcelain: At 750°C → Thin oxide layer (Good adherence) At 1000°C→ Thick oxide layer (Poor adherence)
Enhancement of titanium-ceramic bond: Porcelain firing at argon atmosphere Changing titanium surface (Nitridation- Cr plating- gold sputter
coating Low fusing porcelain with compatible CTE
Base metal alloy Au alloy type IV Points of comparison
Ti alloy Co-Cr / Ni-Cr
- Very difficult & very sensitive tech. as the metal is highly reactive at high temperature.
- Need special casting machine, investment material, & atmosphere.
- Tech. sensitive due to difficulties in controlling carbon % during melting of the alloy. Excessive carbide formation will lead to brittleness of the alloy
- Tech. insensitive - No need for
special atmosphere due to nobility of the alloy
Casting Technique
Ti.6Al.4V 1650°c 1300-1500°c 850-950°c. Melting Range
4.5 gm/cm³
7 -8 gm/cm³
15.2- 16.6 gm/cm³ Density
Phosphate bonded investment with Al2O3 & MgO.
- Carbon free phosphate bonded investment.
- Silicate bonded investment with venting.
Gypsum bonded investment.
Type of investing material
Base metal alloy Au alloy type IV Points of comparison
Ti alloy Co-Cr / Ni-Cr
- Electric melting - Infrared melting
- Oxygen- acetylene flame.
- Electric melting (induction)
Gas- air torch Melting Method
- Vacuum/Argon - Centrifugal casting machine
Centrifugal casting machine
Air- pressure casting machine
Casting Machine
Copper Ceramics Graphite Crucible former
Slow bench cooling
Slow bench cooling Quenching Cooling
Difficult but can retain their polished surface due to ↑ hardness.
Easy, due to lower hardness but can not maintain ↑ surface polish
Finishing & Polishing
You
Thank