Alloy in Prosthodontics

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WHAT IS METAL?

Metal is an element like gold or palladium which ionizes positively in solution.

  • Examples of noble metals:  gold, platinum, rhodium, ruthenium, iridium and osmium
  • Examples of base metals: titanium, nickel, copper, silver and zinc

 

WHAT IS ALLOY?

Alloy is a metallic material formed by the combination of 2 or more (major & minor) metals. In their molten state, metals dissolve to various degrees in one another, allowing them to form alloys in the solid state. In dentistry, alloys are used in inlays, long span bridges, partial denture framework etc.

 

Structure of Alloys

Alloys are crystalline in structure. This structure consists of crystals or grains abutting one another. The boundaries between the grains are referred to as grain boundaries. Size of grain determines the properties of alloy. The smaller the grains the better as more boundaries prevent dislocations in the structure.

 

General Requirements of Alloy

1) Biological

  • Non toxic & non allergic (biocompatibility)
  • Resistant to tarnish & corrosion

 

2) Functional

  • Satisfactory physical (aesthetics) and mechanical properties (yield strength)

 

3) Working

  • Easy to process and handle (ease of casting, soldering, burnish-ability)
  • Readily available, relatively inexpensive constituents

 

Desirable properties of casting alloys

  • Exhibit biocompatibility
  • Ease of melting
  • Ease of casting
  • Ease of Brazing and soldering
  • Ease of Polishing
  • Little solidification shrinkage

 

Dental applications of alloys

žCast Co/Cr alloys

  • RPD framework
  • Porcelain-metal restorations

ž

Cast Ni/Cr alloys

  • RPD framework
  • Crown and bridges
  • Porcelain-metal restorations

ž

Cast Ti and Ti alloys

  • Crowns & bridges
  • RPD framework
  • Implants

ž

Wrought Ti and Ti alloys

  • Implants
  • Crowns
  • Bridges

ž

Wrought S/S alloys

  • Endodontic instruments
  • Orthodontic wires and brackets
  • Preformed crowns

ž

Wrought Co/Cr/Ni alloys

  • Ortho wires and endo files

ž

Wrought Ni/Ti alloys

  • Ortho wires and endo files

 

žWrought beta Ti alloys( Ti/Mo)

  • Ortho wires

 


 

Classification of Dental Casting Alloys

 

Classification based on nobility by American Dental Association (1984)

A) High noble alloys:

More than 40 wt% gold + 60 wt% other noble metals (gold, iridium, osmium, platinum, rhodium)

 

B) Noble alloys:

More than 25 wt% noble metals, i.e. no limit for gold content

 

C) Predominantly base metal alloy:

More than 75 wt% base metal + less than 25 wt% noble metals

 

wt% = % by weight of

 

Classification based on mechanical properties

Type I : Soft

  • Yield stress: 140 MPa
  • Hardness: low
  • % Elongation (ductility): 18% minimum
  • Restoration: Inlay

 

Type II : Medium

  • Yield stress: 140-200 MPa
  • Hardness: medium
  • % Elongation (ductility): 18% minimum
  • Restoration: Onlay

 

Type III : Hard

  • Yield stress: 200-340 MPa
  • Hardness: high
  • % Elongation (ductility): 12% minimum
  • Restoration: Crown, short bridge

 

Type IV : Extra hard

  • Yield stress: 340-500 MPa
  • Hardness: extra high
  • % Elongation (ductility): 10% minimum
  • Restoration: Crown, long bridge, post & core, partial denture

 

NOTE:

* The heating temperature increases from type I to type IV

* The strength increases from type I to type IV

* The elongation (ductility) decreases from type I to type IV

* The yield strength (force per unit area required to deform the alloy) increases from type I to type IV

 


 

Base Metal Alloy

 

Types:

1) Nickel Chromium alloy

2) Cobalt Chromium alloy

3) Titanium and titanium alloys

 

Nickel Chromium alloy

– A substitute for Type III gold alloy

 

Composition:

Major elements 90% by weight

  1. Nickel: 70-80%
  2. Chromium: 12-20%

 

Minor elements 10% by weight

  1. Molybdenum: 3-6%
  2. Silicon and Manganese
  3. Aluminium: 2-6%
  4. Beryllium: 0.5%

 

 

Cobalt Chromium alloy

  • A substitute for Type IV gold alloy
  • Almost all RPD frameworks are done by Co/Cr alloys

 

Composition:

Major elements 90% by weight

  1. Cobalt: 35-65%
  2. Chromium: 28-30%
  3. Nickel: 0-30%

 

Minor elements 10% by weight

  1. Molybdenum: 3-6%
  2. Silicon and Manganese
  3. Carbon: 0.2%

 

Titanium alloys

Composition:

  1. Titanium alloy
  2. Chromium – 5-15%
  3. Nickel 5-15%
  4. Molybdenum 3%
  5. Silicon, Manganese, iron, carbon

 

 

Role of the major elements

Cobalt: Increases strength, hardness, modulus of elasticity

 

Nickel: Increases strength, hardness, modulus of elasticity and ductility

However, nickel is allergic. Affecting more on females than males like swelling or gingival discoloration.

 

Chromium: Tarnish & corrosion resistance increases by passive layer (an oxide layer that is thin, uniform, non porous, adherent and transparent.

 

Role of the minor elements

– To increase strength, hardness & decrease ductility

Molybdenum: Grain refiner

 

Carbon: 0.2% as discontinuous precipitate in the grain boundaries. It affects the hardness, strength and ductility. Too much carbon will make alloy brittle.

 

Aluminium: Increases tensile and yield strength. It reacts with nickel forming intermetallic compound which precipitates inside the solid solution alloy -> precipitation hardening

 

– To improve cast-ability

Silicone & Manganese: increase fluidity of molten alloy, act as deoxidizer

 

Beryllium: Decreases the melting temperature (note: beryllium vapor is carcinogenic and may lead to fibrosis of the lungs)

 

 

Biological Properties

Biocompatibility – All base metal alloys are biocompatible due to the presence of passive layer except base metals containing Nickel and Beryllium.

Also, due the presence of passive layer, all base metal alloys are resistant to tarnish & corrosion.

 

 

Physical Properties

Melting temperature – most base metal alloys melt at temperatures of 1400°C to 1500°C. The addition of 1% to 2% beryllium lowers the melting temperature of Ni-Cr alloys about 100°C.

 

 

Mechanical Properties

Yield strength – the yield strength gives an indication when a permanent deformation of a device or part of a device will occur.

It is believed that dental alloys should have yield strengths of at least 415 Mpa to withstand permanent deformation.

 

Hardness – is an indication of the ease of finishing the structure and its resistance to scratching in service.

 

Tensile strength – tensile strength of cast base-metal dental alloys is greater than 800 Mpa.

 

Elongation –increasing the nickel content with a corresponding reduction in cobalt generally increases the ductility and elongation.

 

Elastic modulus – the higher the elastic modulus, the more rigid structure can be expected. Elastic modulus of base metal alloys is approximately double of type IV cast dental gold alloys.

 

Fatigue – cobalt chromium alloys possess superior fatigue resistance when compared to titanium, gold alloys. Any procedures that result in increasing the porosity or carbide content of the alloy will reduce the fatigue resistance.

 

 

– end –

 

 

 

 

 

 

References:
pocketdentistry.com/21-alloys-used-in-dentistry/

http://www.slideshare.net/UDDent/dental-casting-alloys

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