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
Cast Co/Cr alloys
- RPD framework
- Porcelain-metal restorations
Cast Ni/Cr alloys
- RPD framework
- Crown and bridges
- Porcelain-metal restorations
Cast Ti and Ti alloys
- Crowns & bridges
- RPD framework
- Implants
Wrought Ti and Ti alloys
- Implants
- Crowns
- Bridges
Wrought S/S alloys
- Endodontic instruments
- Orthodontic wires and brackets
- Preformed crowns
Wrought Co/Cr/Ni alloys
- Ortho wires and endo files
Wrought Ni/Ti alloys
- Ortho wires and endo files
Wrought 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
- Nickel: 70-80%
- Chromium: 12-20%
Minor elements 10% by weight
- Molybdenum: 3-6%
- Silicon and Manganese
- Aluminium: 2-6%
- 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
- Cobalt: 35-65%
- Chromium: 28-30%
- Nickel: 0-30%
Minor elements 10% by weight
- Molybdenum: 3-6%
- Silicon and Manganese
- Carbon: 0.2%
Titanium alloys
Composition:
- Titanium alloy
- Chromium – 5-15%
- Nickel 5-15%
- Molybdenum 3%
- 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 –
My Dental Technology Notes 
Good to see such a nice blog postsBuy Dental Products Online
LikeLike