Dental Alloys

Type I

Soft

Type II

Medium

Type III

Hard

Type IV

Extra hard

Restoration associated with Type I

inlay

Restoration associated with Type II

Onlay

Restoration associated with Type III

Crown or short bridge

Restoration associated with Type IV

Crown, long bridge, post and core, partial denture

Yield stress of type I

140 MPa

Yield stress of type II

140-200 MPa

Yield stress of type III

200-340 MPa

Yield stress of type IV

340-500 MPa

% elongation of type I and type II

18%

% elongation of type III

12%

% elongation of type IV

10%

Requirements for dental alloys

-High fusion temperature to withstand porcelain firing

-Thermal coefficient of expansion greater than porcelain firing

-Promote durable metal/porcelain bond

-High mechanical properties

-Biocompatible

-Tarnish resistance

-Usually nickel free

Noble metal alloys

-Gold-based & Palladium-based noble alloys are

corrosion resistants.

-Do not form stable oxides

Base metal alloys

-Base metals oxidize to form a chromium oxide surface layer

-Offer higher strength and hardness

What is the significance of the chromium oxide layer formed in base metal alloys?

Blocks the diffusion of oxygen and prevents corrosion

What are the main elements of base metal alloys?

Nickel and cobalt

What are the noble metals?

Gold, platinum, and palladium

What % was the original PFM alloy nobel metal?

98%

First low-gold alternative what made up of what metals?

Gold, palladium, and silver

How did silver improve the first low-gold allow?

Improves castability

High-noble metal alloy

Made up of 60% noble metals and at least 40% is gold

Noble metal alloy

25% noble metals with no gold requirement

Noble metal alloys available

-Palladium and gold

-Palladium, copper, and gallium

-Palladium and gallium

Base metal alloys available

-Ni-Cr

-Be 2%

-Ni

-Co-Cr

Ni-Cr base metal alloy

-Not as burnishable

-High modulus of elasticity

-Ideal for long-span FDPs

Be 2% base metal alloy

Lower melting point improves castability

Ni base metal alloy

TOXIC and related to lung/nasal cancers

Co-Cr base metal alloy

-No potential health probems

-Highest modulus of elasticity

-Finishing is difficult

-High level of hardness

-Becoming popular for PFM and FMC through CADCAM

What marginal adpatation is considered acceptable?

Less than 120 micrometers

Four theories of porcelain/metal attachment mechanism?

-Chemical bonding

-Van der waals forces

-Compression bonding

-Mechanical retention

What is the secondary bonding through Van der walls forces theory of porcelain/metal attachment?

The attraction between charged atoms that are in intimate contact yet do not actually exchange electrons; the better wetting of the metal surface with the porcelain liquid, the greater the van der walls forces

What is the mechanical retention theory of porcelain/metal attachment?

Metal casting contains many micro irregularities that porcelain flows into when fired but it does not necessarily NEED a roughened surface to bond

How is aluminum oxide used in mechanical retention?

Air abrasion enhances surface irregularities while increase the overall surface area available for bonding

Dental porcelain is strongest under ______ and weakest under _____

compression; tension

If the thermal expansion of the metal substructure is greater than that of the porcelain placed over it then what should happen to the porcelain?

It should compress during cooling

When cooling a restoration what contracts more? Porcelain or metal?

Metal contracts more

What does the difference in contraction rates between metal and porcelain create?

Tensile forces on the metal and compressive forces in the porcelain

Without a wraparound effect in a full PFM, there is (more/less) likelihood this compression bonding will develop

Less

What are the two hypotheses of the chemical bonding theory of porcelain/metal attachment?

-The oxide layer is bonded to the metal on one side and to the ither the porcelain

-Metal oxides are dissolved by the opaque porcelain layer

How thick should the opaque layer of porcelain be in porcelain/metal attachment?

0.3mm