Metal Alloys

Thermal, Electrical Conduction, Ductilibility/Malleability of Metal Alloys

• Conducts both heat and electricity

• Lusters due to inability od electromagnetic radiation to penetrate

• Ductile and Malleable due to to non-directional bonding and atomic nuclei being physically moved

Elastic Limit of Metal Alloys

= Proportional limit = yield strength → indicates onset of plastic deformation

• Strain hardening increases work hardening and thus raises elastic limit

Define Metallic Bonding

Large interatomic forces are created by the sharing of electrons in a delocalized manner to form non-directional bonding

What are the major noble metals used in dentistry

Gold, Platinum an Palladium

• Precious metal resistant to tarnish

• Silver is not noble, prone to tarnishing

Explain Cast Metal Alloys

Formed into desired shapes by casting in a liquid state or by physically shaping (wrought)

• CTE should match or be slightly higher than the material of the veneer porcelain

• If CTE is matched appropriately, cooling should not produce fractures

New current ADA Casting Alloy classification

High Noble (HN)

Noble (N)

Predominantly Base Metal (PB)

Base Metal Alloys

High Noble (HN) > must be 40% or more Au and 60% or more total noble metal

• TARNISH RESISTANT, corrosion resistant

Noble (N) → must be more or equal to 35% noble

• Most common, most common is Ag-Pd (gold substitute) to save money

Predominantly Base Metal (PB) → less than 25% noble

Base Metal Alloys → more than 75% base metal and less than 25% noble

• Low cost, oxide formation for bonding porcelain, better for thin casting, high hardness, low ductilibility, low reactivity to oxygen

• Passivated with addition of Al, Cr and Ti

Characteristics of HN and N alloys for metal-ceramic prostheses

• Can bond to ceramics

• CTE matches to ceramic

• High melting point for use with low-fusing porcelains

Characteristics of metals

• Gold (Au)

• Copper (Cu)

• Silver (Ag)

• Palladium (Pd)

• Platinum (Pt)

• Zinc (Zn)

• Gold (Au): corrosion resistant

• Copper (Cu): Higher strength, lower MT, gets tarnished with >16%wt

  • Responsible for yellow color in yellow gold

• Silver (Ag): Higher strength, counteracts reddish tint of copper; not truly a noble metal and tarnishes

• Palladium (Pd) and Platinum (Pt): Increases MT, lowers CTE, increases hardness

  • Responsible for white color of white gold

• Zinc (Zn): Oxygen scavenger

Thermal mismatch stresses, what are they and what causes them

Metal and porcelain must have matched LCTE to avoid fractures upon cooling and multiple porcelain applications

• No thermal stress issues

• Large mismatch can cause fracture, small mismatch compresses porcelain and increases strength

• Metal is sandblasted so it mechanically lock into porcelain

• PFM vs ceramic preparations are different

Where do errors come from in the fit of indirect cast restorations

The key issue is error propagation due to multiple “hands on” manufacturing steps

• Dimensional errors in the impression can worsen as each step is completed

Describe the process of passivation

Oxide layer formed by some elements in base allows which inhibits further oxidation → aluminum, chromium or titanium

Wrought alloys

Rolling or forging an alloy through physical force at levels above yield strength 

• Facilitated by dislocation movement which increases elastic limit

• Displays better properties than cast alloys

• Most metal alloys used in dentistry, implants

Define Strain Hardening, cold/hot working, hardening and hardness

• Describe the strain-stress curve after work hardening

• Strain Hardening: the process of strengthening a metal through deformation due to increasing difficulty for dislocation motion

  • Done through stamping, drawing, rolling, pounding, bending

• Hardening: Increasing the elastic limit of a material

• Hardness: Resistance of a material to penetration by an indenter or stylus

• Curve: stronger but more brittle, ductility decreased

• Hot/Cold- working: work hardening at high or low temp respectively

Re- crystallization

After cold working, the grain of the metals are very disordered, at ½ the MT, re-crystallization occurs and re-precipitates crystals which makes the metal soft and ductile again

• Re-crystallization time decreases as the amount of cold working increases

Examples of uses of wrought alloys

Hand instruments: 316L Stainless Steel (S), low carbon, hardened during manufacturing

Clasps for RPDs: made of high noble alloys, clasps must not be too stiff or hard but must be durable

Endo files: SS or NiTi; Nitinol shape memory alloy

Ortho wires and bands: SS, Co-Cr-Ni, NiTi

Composition of Steel, composition of stainless steel and 316L

• Steel = Iron + Carbon

• Stainless steel = Steel + Chromium

• 316L = Stainless Steel + 16% Cr + low Chromium

Metal Alloys related to Implants

Almost all implants currently are made of commercially pure titanium (cp-Ti) and special instruments of the same material must be used when placing them as well as cleaning them or they may be scratched and left with iron residue from 316L instruments