Dental casting alloys

Historically, precious, semiprecious,and nonprecious

classification which refers to the electrochemical character of the alloys:High noble, noble and base metal casting alloys.

Recent classification of dental alloys according to their biocompatibility:High noble alloyTitanium and titanium alloysNoble alloy Base metal alloys.

classification

Requirements of a dental casting alloys Biocompatibility Tarnish and corrosion resistance Strength Thermal characteristics The melting range of the alloy is important. The criteria

are: • The melting range must be low enough so that the alloy

can cast with the available equipment. • The melting range must be high enough so that the alloy

does not melt or distort during the porcelain baking cycles. • The solidus temperature must be high enough from the

porcelain baking temperature to allow a joining operation before the porcelain is applied.

•Porcelain adhesion is achieved on the noble metals through the formation of a chemical bond.•Color: The yellow color of the alloy surface plays an important role in developing the restorations esthetics.•Easy casting and finishing.

Effects of the Various ConstituentsGold: increases resistance to tarnish and corrosion; increases ductility and malleability

Copper: principal hardener; is necessary for heat treatment (when added in a concentration >12 wt%)

Silver: main purpose is to modify the red color produced by gold and copper; reduces melting temperature; increases ductility and malleability.

Platinum: raises melting temperature; increases tensile strength; decreases the coefficient of thermal expansion; reduces tarnish and corrosion

Palladium: raises melting temperature; increases hardness; acts to absorb hydrogen gas which can cause porosity in the casting; prevents tarnish and corrosion; has a very strong whitening effect on gold alloys even when used at alow concentration (i.e., 5 wt%)

Zinc: acts as a scavenger and prevents oxidation of the other metals during the manufacturing process; increases fluidity and decreases surface tension, which improves castability

Iridium: acts as a grain refiner; this increases tensile strength and percentage elongation (i.e., ductility); it also increases, to a much lesser degree (if at all), hardness and yield strength

Metal-ceramic Alloys for Porcelain BondingThese alloys differed from the gold-based alloys used for full-cast restorations in the following ways:

1. they had higher melting temperatures (due to greater palladium content) to accommodate the temperatures required for porcelain application2. they contained little, if any, silver which could cause the porcelain to turn green3. they contained little, if any, copper because it could cause the alloy to sag during porcelain firing

High-Noble Alloys•By definition, these alloys must have a noble metal content of at least 60 wt%, of which at least 40 wt% is gold.• They usually contain a small amount of tin, indium, and/or iron which provides for oxide layer formation. This increases the alloy’s bond strength to porcelain by a factor of three. •The disadvantages of these alloys include their lower rigidity and poor sag resistance during porcelain firing.

1-Au-Pt-Pd.–As previously noted, these were the–first casting alloys formulated to bond with dental porcelain. –Although these alloys have excellent corrosion resistance, they are susceptible to some dimensional changes during the porcelain firing cycles.–not recommended for multiple-unit FPD restorations.

High-Noble Alloys

2-Au-Pd-Ag,–These were the first lower-gold content alternative alloys to be widely used in the 1970s.–Platinum was eliminated from the alloy compositions, and the gold content was reduced to about 50%, with corresponding increases in the amounts of palladium and silver. –Although these alloys have excellent mechanical properties and porcelain adherence, green discoloration (resulting from diffusion of silver atoms into the porcelain) –Possible reasons for this effect may be the high sodium concentration of the porcelain or the relative sizes of the metal ions in the porcelain. –The discolored region can be ground away

High-Noble Alloys

Green discoloration has apparently been eliminated in some porcelain compositions by substituting potassium ions for sodium ions; the larger potassium ions impede the diffusion of silver into the porcelain.

3-Gold-platinum alloys:

–Initially developed as an alternative to palladium alloys.–these can be used for full-cast as well as metal-ceramic restorations. –Because they are subject to sag, they should be limited to short-span fixed partial dentures. –They often contain silver or zinc as a hardener.A typical composition is:1. Gold 85%2. Platinum 12%3. Zinc 1%4. Silver (in some brands)

High-Noble Alloys

4-Gold-palladium AlloysLike gold-platinum alloys, these can be used for full-cast or metal-ceramic restorations. They usually contain indium, tin, or gallium to promote oxide layer formation for better bonding to porcelain. A typical composition is:1. Gold 52%2. Palladium 38%3. Indium 8.5%4. Silver (in some brands)

High-Noble Alloys

Titanium and titanium alloys.

Advantages– excellent biocompatibility and corrosion resistance which is

due to the presence of a thin, adherent, passivating surface layer of TiO.– The low density (4.5 g/cm3) of titanium, compared to gold

(lighter and potentially less expensive restorations*).

Disadvantages: Special casting machines must be used that provide either a

vacuum environment or an argon atmosphere. laboratory cost of fabricating cast restorations from titanium

alloys may be high.

Noble Metal Alloys. The noble-metal alloyshave a minimum of 25% by weight of noble metal, with no requirement for gold percentage.As a general rule, however, they have relatively high strength, hardness, and ductility. They may be yellow or white in color. When white, it is due to their palladium content.

1-Palladium-copper-gallium Alloys–Introduced in 1982, these are extremely rigid and are used for full-cast and metal-ceramic restorations.-appear to have castability and castingaccuracy comparable to the high noble metal alloys–They contain some copper, so laboratories have to use them carefully to avoidsagging of the casting during porcelain firing. ---Gallium is added to reduce the temperature at which the alloy is completely melted; this enhances bonding of porcelain to the metal and increases strength.–Susceptibility to corrosion varies widely depending on the alloy’s specific composition.

Noble Metal Alloys.

2-Pd-Ga.–Their mechanical properties are generally more similar to those of Pd-Ag alloys rather than the Pd-Cu-Ga alloys. Compared to the Pd-Ga alloys, porcelain adherence is superior for the Pd-Cu-Ga alloys . –The Pd-Ga-Co alloy has a particularly dark oxide that is more difficult to mask with dental porcelain. This alloy has not yet achieved widespread clinical acceptance.–Multi-orifice torches are required to fuse the high-palladium alloys, and the use of ceramic crucibles dedicated to individual alloys is recommended." –Carbon-containing investments should not be used, because the incorporation of very small amounts of carbon in these alloys degrades the bond strength with porcelain.

Noble Metal Alloys.

2-Palladium-silver and Silver-palladiumIntroduced in the 1970s, these alloys are used for full-cast and metal-ceramic restorations. Their composition varies widely, ranging from those with a majority of silver to those with a majority of palladium. Generally, the higher-silver alloys exhibit more corrosion.. On the positiveside, the Pa-Ag and Ag-Pa alloys generally have low sag tendency, high rigidity,and are easy to solder.

Noble Metal Alloys.

A typical composition for a Palladium-silver alloy is:1. Palladium 61%2. Silver 24%3. Tin (in some formulations)A typical composition for a Silver-palladium alloy is:1. Silver 66%2. Palladium 23%3. Gold (in some formulations)

Noble Metal Alloys.

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