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