DENTAL METAL ALLOY (DENTAL MATERIALS)

DENTAL METAL ALLOY
ALLOY
It is a metal containing two or more elements, at least one of which is metal, and all of which are mutually soluble in the molten state.
Noble metals: They are materials resist corrosion in the mouth. (gold, platinum, palladium, silver, rhodium, ruthenium, iridium, osmium).
Precious metals: This term indicates the intrinsic value of the metal. The eight noble metals are also precious metal, but all precious metals are not noble.
1- GOLD: Pure gold is soft, ductile, yellow hue. The density is 19.3 gm/cm3, melting point is 1063°C, good chemical stability, not corrode and not tarnish.
2- SILVER: Whitest metal, its density is 10.4 gm/cm3, melting point is 961°C.
3- PALLADIUM: Its density is 12.02 gm/cm3, melting point is 1552°C.
4- PLATINUM: its density is 21.65 gm/cm3, melting point is 1769°C.
Base metals: These are not noble metals, (chromium, cobalt, nickel, copper,…..etc).
They are important components of dental casting alloy because:
a- Their influence on physical properties.
b- Control of the amount and type of oxidation.
c- Their strengthening effect.

CLASSIFICATION
I- ACCORDING TO YIELD STRENGTH

• Type I: soft.
• Type II: medium.
• Type III: hard.
• Type IV: extra-hard.

II- ACCORDING TO THE NUMBER OF ALLOY PRESENTS

1- Binary (2 elements).
2- Ternary (3 elements).
3- Quaternary (4 elements).

III- ACCORDING TO USE

1- Alloys for all metal; metal with resin veneer restorations.
2- Alloys for metal ceramic restorations.
3- Alloy for removable dentures.

GENERAL REQUIREMENTS OF CASTING ALLOY
1- It should not tarnish and corrode in the mouth.
2- It should strong.
3- Biocompatible (non-toxic, non-allergic).
4- It should be easy to fabricate (melt, cast, cut, and grind).
5- It should flow well, and duplicate fine details during casting.
6- It should have minimal shrinkage on cooling after casting.
7- It should easy to solder.

Alloys for all metal restorations
1- Gold alloys (composed of gold, copper, silver, platinum, palladium, and other additives).
2- Silver-palladium alloys.
3- Nickel-chromium alloys.
4- Cobalt-chromium alloys.

Alloys for metal ceramic restorations
1- Gold-palladium-platinum alloys.
2- Palladium-silver alloys.
3- Nickel-chromium alloys.
HOW DOES PORCELAIN BOND TO THE ALLOY?
Ceramic adheres to metal primarily by chemical bond. A covalent bond is established by sharing 02 in the elements in the porcelain and the metal alloy.
These elements include silicon dioxide (Si02 in the porcelain and oxidizing elements such as silicon, indium, and iridium in the metal alloy.
Alloys for removable dentures
ADDITIONAL REQUIREMENTS
8- It should be light weight.
9- It should have high stiffness (to make the framework thin).
10- It should have good fatigue resistance.
11- It should not react to commercial denture cleanser.
12- Economical consideration.
Alloy used:
1- Cobalt-chromium alloy
COMPOSITION
a- Cobalt (to give hardness, strength, rigidity).
b- Chromium (to ensure corrosion resistance by passivating effect).
c- Nickel (to decrease fusion temperature and increase ductility).
d- Molybdenum or tungsten (to increase hardness).
e- Iron and copper (to increase hardness).
f- Manganese and silicon (to prevent oxidation).
g- Boron (to increase hardness and deoxidizer).
h- Carbon (to strengthen the alloy).

2- Nickel-chromium alloy
COMPOSITION
a- Nickel.
b- Chromium.
c- Molybdenum.
d- Other minor additions like aluminum, iron, silicon, copper, manganese, tin.