Silver and Silver Alloys

Abstract:

Silver and silver alloys are most often associated with beautiful tableware, jewelry and coins. However, more than half or world silver demands are from growing industrial requirements. It is difficult to imagine development in electronics without silver and silver alloys.
Silver alloys intended for industrial application should be generally characterized by high electrical conductivityas well as high mechanical and functional properties, stable also at elevated temperature.

Silver is another unique member of the metals family - the "whitest" of all metals. In its pure form this moon-colored metal is highly lustrous, and can be polished to a mirror finish.

Silver was known and used by primitive man. The ancient Hebrews called it by a name meaning pale. The Greeks knew it by a name meaning shining. American Indians called it "tears of the moon". The chemical symbol for silver, Ag, comes from its Latin name, argentinum.

Like gold, silver is considered a precious metal, and is extremely malleable and ductile. It is harder than gold, but softer than copper. Silver can be hammered into sheets so thin that it would take 100,000 of them to make a stack an inch high. Silver has a specific gravity of 10.5, and a melting point of 1760°F (960°C) - almost 200°F below that of gold.

The highest purity of silver commercially attainable is 99.95% (nominally considered 100%), but because of its softness and susceptibility to damage, silver is rarely used in pure form.

Many silver alloy compositions are known. Some of these are used for jewelry and flatware, while others are used in brazing compositions and as electrical conductors.

Pure silver is very soft and ductile but can be hardened by alloying. Copper is the favorite hardener and normally is employed in the production of sterling silver, which must contain a minimum of 92.5% silver, and also in the production of coin silver. Silver-copper eutectic and modifications containing other elements such as zinc, tin, cadmium, phosphorus, or lithium are widely used for brazing purposes, where strong joints having relatively good corrosion resistance are required.

Where higher strengths at elevated temperature are required, silver-copper-palladium alloys and other silver-palladium alloys are suitable. The addition of a small amount of silver to copper raises the recrystallizing temperature without adverse effect upon the electrical conductivity.

Silver may be alloyed with gold or palladium in any ratio, producing soft and ductile alloys; certain of these intermediate alloys are useful for electrical contacts, where resistance to sulfide formation must be achieved.

Silver has proved to be a useful component for high-duty bearings in aircraft engines, where it may be overlaid with a thin layer of lead and finally with a minute coating of indium. Specially developed alloys of silver with tin, plus small percentages of copper and zinc in the form of moderately fine powder, can be mixed with mercury to yield a mass which is plastic for a time and then hardens, developing relatively high strength despite the fact that it contains about 50% mercury. This material was developed specifically for dental use and is generally known as amalgam, although the term amalgam actually includes all the alloys of mercury with other metals.

Among these various silver-containing alloys, sterling silver must contain at least 92.5% pure fine silver. The balance of this alloy may be some other metal, but typically includes a substantial percentage of copper. The presence of copper tends to increase the hardness of the resulting alloy.

The use of the term "sterling" has a historical derivation. In the 12th century, five towns in eastern Germany banded together to form the Hanseatic League, an entity which engaged in substantial commerce with England. In payment for English cattle and grain, the League used its own currency, silver coins called "Easterlings" The English soon learned that these coins were extremely dependable, and it is believed that, under the reign of Henry II, the Easterling was used as a basis for standardizing English coinage. As time went on, the name was shortened to "Sterling", which is still in use today, referring to the English monetary system, and to a particular alloy of silver.

The word "sterling" represents the best known and most respected quality marking in use today. It signifies that the article so stamped is made from silver with a silver content of at least 92.5% by weight, with no stipulation about the remaining 7.5%. The reason for the relatively "low" silver content in sterling is a practical one: finer grades of silver are too soft for everyday use. Alloying elements are needed to increase strength and hardness. Since copper provides the best combination of wear qualities, it is the most common alloying element used by jewelers and silversmiths.

Silver has a known affinity for oxygen, which affinity increases with temperature. When exposed to air, molten silver will absorb about twenty-two times its volume of oxygen. Like silver, copper also has a great affinity for oxygen, typically forming copper oxide. This may be of the cupric or cuprous variety, or both. Hence, unless air is excluded during the casting process, the cast article may be porous and characterized by the presence of internal voids. Thus, in melting sterling silver and other silver-copper alloys, care must be taken to prevent oxidation.

Copper oxide, also known as fire scale, is typically a darkened portion which blemishes the cast article. Such fire scale is not limited to the surface of the cast article, as in the case of conventional tarnishes, but may penetrate the article to some depth. In some cases, such fire scale may not be removed by buffing and polishing. Moreover, the opportunity for the creation of fire scale exists when the alloy is initially formed as shot, when such shot is melted and recast to form the desired article, and subsequently if the cast article is thereafter annealed. In each of these cases, the alloy is heated, and, given the opportunity, may form fire scale.

Silver alloys are normally supplied soft- for easy working. If desired, the alloys can be supplied in various tempers, by reducing (working) the alloy without annealing it. Though virtually all sterling silvers consist of the same alloy of copper and silver, their properties are greatly affected by working and by heat treatment, such as annealing and quenching.

An improved sterling silver alloy composition, exhibiting the desirable properties of reduced fire scale, reduced porosity and reduced grain size, consists essentially of the following parts by weight: about 92.5% silver, about 0.5% copper, about 4.25% zinc, about 0.02% indium, about 0.48% tin, about 1.25% of a boron-copper alloy containing about 2% boron and about 98% copper, and about 1% of a silicon-copper alloy containing about 10% silicon and about 90% copper.

Silver and silver alloys are most often associated with beautiful tableware, jewelry and coins. However, more than half or world silver demands are from growing industrial requirements. It is difficult to imagine development in electronics without silver and silver alloys.

Silver alloys intended for industrial application should be generally characterized by high electrical conductivity (as pure silver) as well as high mechanical and functional properties, stable also at elevated temperature. Silver alloys designed for producing electrical contacts should also have high corrosion and erosion resistance, high temperature resistance, ability to extinct of electric arcs and it mustn’t weld during work.

Alloy additives are used to meet these requirements, particularly such which cause precipitation hardening or dispersion hardening. These mechanisms are well known in platinum alloys and copper alloys. Since it is important that the electrical conductivity decreasing has to remain small, the amount of alloy additive is limited.

Chosen alloy additives should slow down dislocation movement and grain boundaries migration, causing improvement in mechanical properties. The presence of particles in grain boundaries slows down grain growth at elevated temperature, which contributes to the stabilization of properties. The increase of mechanical properties depends on particle size and dispersity. Another useful feature should be fine structure, providing good functional properties.

Among the variety of precious metallic materials used in dental applications like gold, palladium and silver, there are significant differences in terms of price, elasticity modulus, strength, coefficient of thermal expansion, density, hardness and melting temperature. As far as the cost is concerned, the silver alloys can be regarded as a cheaper choice, relatively to other precious materials.

Two groups of cast silver alloys were used for the restoration of the analogous had the following compositions (per weight): Ag - 80% , Sn – 19%, Cu – 1% (Silver Pratalloy Degussa-Hüls), designated as Ag; and Ag - 58.5% - Pd 27.4%, Cu – 10.5, Au – 2%, Zn 1.5%, Ir 0.1% (Silver Palladium Palliag M Degussa-Hüls), designated as Ag-Pd.

Several silver based alloys have been developed to improve tarnish resistance in multi-layer stacks. These alloys traditionally have had 80-95% silver and employed gold or platinum group metals as alloying elements to stabilize the properties of the silver when exposed to moisture or mildly acidic environments. New, lower cost alloys have been developed that represent a favorable balance between cost and performance. These new alloys tend to be more complex than the standard binary or ternary alloys currently in use, but they can be produced using readily available production equipment.

Silver and some of its alloys have been employed for many years as reflectors in thick or thin film applications. In thick film applications such as paints, they were applied to the back side of the substrate and normally laminated into an assembly. In thin films, silver has been used on both the front and back side of substrates and has been employed as a mirroring material for IR, laser and visual light applications.

In all applications, pure silver thin films required protective layers on top, or in some cases, below to prevent degradation of the film. Silver also requires edgewise protection to inhibit corrosion at the edges in the film that slowly creep into the working surface area of the film.

Historically, gold, palladium or platinum have been added as a means of adding nobility to the silver. This has worked for many applications, but the added cost of gold or platinum group metals can increase the intrinsic raw material component of the cost by an order of magnitude or more. Therefore any improvements made would have to give due consideration to metal costs.

Due to the low density and the high palladium- and silver content Ag-Pd-Au alloys are of especially good value. They have, however, a tighter processing tolerance than the high gold and gold reduced metal alloys.

This group contains alloys with differing mechanical and physical properties. The alloys can be soldered without problems and can be used for the casting-on technique. When using plaster based investment compounds care has to be taken not to exceed a preheating temperature of 700°C in order to prevent an embrittlement of the alloy by the uptake of sulfur. Pd-based alloys absorb carbon in the melted condition. Therefore graphite based investment compounds should not be used. Also, these alloys should only be melted in ceramic or vitrified carbon crucibles to prevent embrittlement of the alloy during frequent remelting.

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