Table of Contents
- 1 The Metals Used in Dental Casting Alloys
The Metals Used in Dental Casting Alloys
This series represents a mini course in dental alloys for the beginner, and persons seriously interested in gaining a basic working knowledge of dental alloys are advised to take the time to start at the beginning.
If all five pages are read in order, the reader will gain a good understanding of just what dental alloys really are, their internal crystalline structures, how they differ from each other and how different alloys are utilized in various applications.
Soft, malleable and yellow colored with a low melting point. Looks great, but by itself it lacks sufficient strength to stand up to the forces generated in the oral cavity.
Gold is a noble metal and does not corrode or tarnish in the mouth. The softer alloys are “burnishable”, meaning that the margins can be rubbed with a blunt instrument to seal them and increase marginal adaptation. It is also very kind to the opposing dentition, and will not wear down opposing teeth.
Its native thermal expansion is too high to be used by itself as a base upon which to build a porcelain superstructure. If porcelain were bonded directly to a gold understructure, it would “shiver” and break off the substructure during cooling. This characteristic, however, can be modified by alloying it with other metals.
Finally, since Gold is so inert, it cannot by itself chemically bond to porcelain.
Almost the opposite of gold. It is hard, very strong, white and has a high melting point. It has a very high modulus of elasticity which means it is not very ductile.
Palladium is also a noble metal which means that it resists corrosion and tarnish.
It’s native thermal expansion is very low and by itself cannot be used with porcelain because porcelain would “craze” (the opposite of shivering) and break off the substructure during cooling.
Even relatively small amounts of palladium will whiten gold dramatically. When added to a gold alloy, it will raise the melting range, raise the modulus of elasticity, and improve strength and hardness.
Small amounts of palladium dramatically improve the tarnish and corrosion resistance of gold-silver-copper crown and bridge alloys. It is an essential component for preventing tarnish and corrosion in Au-Ag-Cu alloys with gold content below 68% by weight.
Palladium and gold are completely soluble in one another, both as liquids in the molten state, and as solids in the finished alloy.
Palladium and gold are found together in so many dental alloys because they compliment each other. Unfortunately, the correct combination of gold and palladium sufficient to produce, say, the correct coefficient of expansion will not necessarily produce an alloy that meets the other necessary characteristics such as modulus, color or stiffness that a lab or manufacturer may need to produce a correct product. Hence, it is necessary to balance the formula with other metals as well.
Platinum is used as an alternative to palladium in order to maintain a yellow color in the final alloy. It raises the melting range, increases the hardness, strength, and modulus, and lowers the thermal expansion of the alloy. It is less effective than palladium in producing these effects, but it is able to alter these characteristics with less impact on the golden color of the finished product.
In PFM alloys, silver is used principally to raise the thermal expansion of the alloy in order to balance the low thermal expansion of Palladium.
Silver also lowers the melting range of both gold and palladium and adds fluidity to the melt improving its casting properties.
In gold-silver-copper alloys used for all-gold restorations, the silver compensates for the reddish color imparted by the copper. It also acts along with the copper to increase the strength and hardness of the alloy.
The major problem with silver in PFM formulations is that the silver can impart a greenish tint to the finished porcelain. This danger is offset by the very dramatic effect the silver can have on the modulus of expansion, and by the fact that modern porcelains are now formulated to resist this greening effect.
In crown and bridge alloys (all-gold), copper’s major job is hardening and strengthening the alloy. It also imparts a reddish color, which may be an advantage, but can be offset by adding silver.
In PFM alloys, it is used mostly to increase the modulus of thermal expansion, and is responsible for the dark oxide layer characteristic of palladium-copper-gallium alloys.
Unfortunately, copper, like silver can cause discoloration of the overlying porcelain, however this effect is seldom seen when there is a very high percentage of palladium in the mix. Thus copper is seldom used in high noble PFM alloys (these alloys have lots of gold and little palladium.
Zinc is used in crown and bridge alloys primarily as an oxygen scavenger. Zinc readily combines with oxygen that may have dissolved in alloy when it was in a molten state. This prevents the oxygen from forming gas porosity in the casting.
In PFM formulations, zinc also lowers the melting range, increases strength and hardness, and raises the thermal expansion.
In crown and bridge alloys such as gold-silver-copper, indium is added to improve the fluidity of the melt thus improving castability.
In PFM alloys, it strengthens and hardens both gold and palladium, and raises the thermal expansion of both. Indium also lowers the melting range of both gold and palladium.
Indium contributes to the formation of the porcelain bonding oxide layer.
Tin is added to an alloy to increase the strength and hardness of both palladium and gold. It also lowers the melting range and raises the thermal expansion.
Like indium, tin also contributes to the formation of the porcelain bonding oxide layer.
Gallium is used almost exclusively in palladium based PFM alloys. Gallium can be a potent strengthener, and it lowers the melting range of palladium.
Iron is used almost exclusively in gold-platinum based PFM alloys. It is used as a strengthener.
Iron also contributes to the formation of the porcelain bonding oxide layer.
Copper is sometimes used as a substitute for copper in palladium based PFM alloys.
Mostly, it is used along with nickel to formulate alloys for partial denture frameworks.
These three elements are used in very small concentrations as grain refiners.
Alloys have better characteristics if the grain structures are small (see the discussion of small grain size on page 2.) The addition of small amounts of any of these three elements helps to produce small grain size when the alloy is cooling from the molten state.
The theory behind this is as follows: These elements are have a fairly high melting point and tend to be the first to form crystals in the molten matrix. Their low concentration allows their atoms to “atomize” and distribute themselves more or less evenly throughout the melt. As the grains of these elements form, they remain very small due to their low concentration throughout the solution. Since they crystallize first, these tiny grains form the nucleus around which the other elements begin to form their larger grains. The even distribution of grain formation throughout the solution limits the size of the larger grains as well.