For dental restorations, various elements are com- bined to produce alloys with adequate properties for dental applications because none of the elements by themselves have properties that are suitable. These alloys may be used for dental restorations as cast alloys or may be manipulated into wire or other wrought forms. The metallic elements that make up
dental alloys can be divided into two major groups, the noble metals and the base metals.
Noble Metals
Noble metals are elements with a good metallic sur- face that retain their surface in dry air. They react easily with sulfur to form sulfides, but their resis- tance to oxidation, tarnish, and corrosion during heating, casting, soldering, and use in the mouth is very good. The noble metals are gold, platinum, palladium, iridium, rhodium, osmium, and ruthe- nium (Table 10.6). These metals can be subdivided into two groups. The metals of the first group, con- sisting of ruthenium, rhodium, and palladium, have atomic weights of approximately 100 and densities of 12 to 13 g/cm3. The metals of the second group, TABLE 10.6 Properties of Elements in Dental Casting Alloys
Element Symbol
Atomic
Number Atomic Mass Density (g/cm3)
Melting Temperature
(°C) Color Comments
NOBLE
Ruthenium Ru 44 101.07 12.48 2310.0 White Grain refiner,
hard
Rhodium Rh 45 102.91 12.41 1966.0 Silver-white Grain refiner,
soft, ductile
Palladium Pd 46 106.42 12.02 1554.0 White Hard, malleable,
ductile
Osmium Os 76 190.20 22.61 3045.0 Bluish-white Not used in
dentistry
Iridium Ir 77 192.22 22.65 2410.0 Silver-white Grain refiner,
very hard
Platinum Pt 78 195.08 21.45 1772.0 Bluish-white Tough, ductile,
malleable
Gold Au 79 196.97 19.32 1064.4 Yellow Ductile,
malleable, soft, conductive BASE
Nickel Ni 28 58.69 8.91 1453.0 White Hard
Copper Cu 29 63.55 8.92 1083.4 Reddish Malleable,
ductile, conductive
Zinc Zn 30 65.39 7.14 419.6 Bluish-white Soft, brittle,
oxidizes
Gallium Ga 31 69.72 5.91 29.8 Grayish-white Low melting
Silver Ag 47 107.87 10.49 961.9 Soft, malleable,
ductile, conductive
Tin Sn 50 118.71 7.29 232.0 White Soft
Indium In 49 114.82 7.31 156.6 Gray-white Soft
TABLE 10.7 Physical and Mechanical Properties of Cast Pure Gold and Gold Alloys Material
Density (g/cm3)
Hardness (VHN/BHN) (kg/mm2)
Tensile Strength
(MPa) Elongation (%)
Cast 24k gold 19.3 28 (VHN) 105 30
Cast 22k gold — 60 (VHN) 240 22
Coin gold — 85 (BHN) 395 30
Typical Au-based casting
alloy (70 wt% Au)a 15.6 135/195 (VHN) 425/525 30/12
aValues are for softened/hardened condition.
BHN, Brinell hardness number; VHN, Vickers hardness number.
Modified from Rule RW. A further report on physical properties and clinical values of platinum-centered gold foil as compared to pure gold filling materi- als. J Am Dent Assoc. 1937;24:583–595.
consisting of osmium, iridium, platinum, and gold, have atomic weights of about 190 and densities of 19 to 23 g/cm3. The melting points of members of each group decrease with increasing atomic weight. Thus ruthenium melts at 2310°C, rhodium at 1966°C, and palladium at 1554°C. In the second group the melting points range from 3045°C for osmium to 1064°C for gold. The melting point and density are important properties when composing alloys, because they affect the casting process, which affects the overall accuracy and quality of the final product. The noble metals, together with silver, are sometimes called precious metals. The term precious comes from the relatively high cost of these metals and their trading on the commodities market. Some metallurgists consider silver a noble metal, but it is not considered a noble metal in dentistry because it corrodes considerably in the oral cavity. Thus the terms noble and precious are not synonymous in dentistry.
GOLD (AU)
Pure gold is a soft, malleable, ductile metal that has a rich yellow color with a strong metallic luster.
Although pure gold is the most ductile and mal- leable of all metals, it is relatively low in strength.
The density of gold depends somewhat on the condi- tion of the metal, whether it is cast, rolled, or drawn into wire. Small amounts of impurities have a pro- nounced effect on the mechanical properties of gold and its alloys. The presence of less than 0.2% lead causes gold to be extremely brittle. Mercury in small quantities also has a harmful effect. Therefore scrap of other dental alloys should not be mixed with gold used for dental restorations.
Air or water at any temperature does not affect or tarnish gold. Gold is not soluble in sulfuric, nitric, or hydrochloric acids. However, it readily dissolves in combinations of nitric and hydrochloric acids (aqua regia, 18 vol% nitric and 82 vol% hydrochloric acids) to form the trichloride of gold (AuCl3). It is also
dissolved by a few other chemicals, such as potas- sium cyanide and solutions of bromine or chlorine.
Because gold is nearly as soft as lead, it must be alloyed with copper, silver, platinum, and other metals to develop the hardness, durability, and elasticity necessary in dental alloys, coins, and jew- elry (Table 10.7). Through appropriate refining and purification, gold with an extremely high degree of purity may be produced. Gold can be work hard- ened to improve its physical properties. Without the improvement, cast gold would lack sufficient strength and hardness.
CARAT AND FINENESS OF GOLD-BASED ALLOYS For many years the gold content of gold- containing alloys has been described on the basis of the carat, or in terms of fineness, rather than by weight percentage. The term carat refers only to the gold content of the alloy; a carat represents a one- twenty-fourth part of the whole. Thus 24 carat indi- cates pure gold. The carat of an alloy is designated by a small letter k, for example, 18k or 22k gold.
The use of the term carat to designate the gold content of dental alloy is less common now. It is not unusual to find the weight percentage of gold listed or to have the alloy described in terms of fineness.
Fineness also refers only to the gold content, and represents the number of parts of gold in each 1000 parts of alloy. Thus 24k gold is the same as 100% gold or 1000 fineness (i.e., 1000 fine). An 18k gold would be designated as 750 fine, or, when the decimal sys- tem is used, it would be 0.750 fine; this indicates that 750/1000 of the total is gold. A comparison of the carat, fineness, and weight percentage of gold is given in Table 10.8. Both the whole number and the decimal system are in common use, especially for noble dental solders. The fineness system is some- what less relevant today because of the introduction of alloys that are not gold based. It is important to emphasize that the terms carat and fineness refer only to gold content, not noble-metal content.
PLATINUM (Pt)
Platinum is a bluish-white metal; is tough, ductile, and malleable; and can be produced as foil or fine- drawn wire. Platinum has hardness similar to that of copper. Pure platinum has numerous applica- tions in dentistry because of its high fusing point and resistance to oral conditions and elevated temperatures.
Platinum increases the hardness and elastic quali- ties of gold, and some dental casting alloys and wires contain quantities of platinum up to 8% combined with other metals. Platinum tends to lighten the color of yellow gold-based alloys.
PALLADIUM (Pd)
Palladium is a white metal somewhat darker than platinum. Its density is a little more than half that of platinum and gold.
Palladium is not used in the pure state in dentistry, but is used extensively in dental alloys. Palladium can be combined with gold, silver, copper, cobalt, tin, indium, or gallium for dental alloys. Alloys are read- ily formed between gold and palladium, and palla- dium quantities of as low as 5% by weight have a pronounced effect on whitening yellow gold-based alloys. Palladium-gold alloys with a palladium con- tent of 10% or more by weight are white. Alloys of pal- ladium and the other elements previously mentioned are available as substitutes for yellow-gold alloys, and the mechanical properties of the palladium- based alloys may be as good as or better than many traditional gold-based alloys. Although many of the
palladium-based alloys are white, some, such as palladium-indium-silver alloys, are yellow.
IRIDIUM (IR), RUTHENIUM (RU), AND RHODIUM (RH)
Iridium and ruthenium are used in small amounts in dental alloys as grain refiners to keep the grain size small. A small grain size is desirable because it improves the mechanical properties and uniformity of properties within an alloy. As little as 0.005% (50 ppm) of iridium is effective in reducing the grain size.
Ruthenium has a similar effect. The grain-refining properties of these elements are largely due to their extremely high melting points. Iridium melts at 2410°C and ruthenium at 2310°C. Thus these elements do not melt during the casting of the alloy and serve as nucle- ating centers for the melt as it cools, resulting in a fine- grained alloy.
Rhodium also has a high melting point (1966°C) and has been used in alloys with platinum to form wire for thermocouples. These thermocouples help measure the temperature in porcelain furnaces used to make dental restorations.
Base Metals
Several base metals are combined with noble metals to develop alloys with properties suitable for den- tal restorations. Base metals used in dental alloys include silver, copper, zinc, indium, tin, gallium, and nickel (see Table 10.6).
SILVER (Ag)
Silver is a malleable, ductile white metal. It is the best- known conductor of heat and electricity and is stron- ger and harder than gold but softer than copper. At 961.9°C, the melting point of silver is below the melt- ing points of both copper and gold. It is unaltered in clean, dry air at any temperature, but combines with sulfur, chlorine, phosphorus, and vapors containing these elements or their compounds. Foods contain- ing sulfur compounds cause severe tarnish on silver, and for this reason silver is not considered a noble metal in dentistry.
Pure silver is not used in dental restorations because of the black sulfide that forms on the metal in the mouth. Adding small amounts of palladium to silver-containing alloys prevents the rapid corrosion of such alloys in the oral environment.
Silver forms a series of solid solutions with pal- ladium (see Fig. 10.5D) and gold (see Fig. 10.5C), and is therefore common in gold- and palladium-based dental alloys. In gold-based alloys, silver is effective in neutralizing the reddish color of alloys containing appreciable quantities of copper. Silver also hardens the gold-based alloys via a solid-solution harden- ing mechanism. In palladium-based alloys, silver is TABLE 10.8 Comparison of Carat, Fineness, and
Weight Percentage of Gold in Gold Alloys
Carat
Amount of Gold by Carats
Weight (%) of Gold
Fineness Parts/1000 Decimal
24 24
24
100.0 1000.00 1.000
22 22
24
91.7 916.66 0.916
20 20
24
83.3 833.33 0.833
18 18
24
75.0 750.00 0.750
16 16
24
66.7 666.66 0.666
14 14
24
58.3 583.33 0.583
9 9
24
37.5 374.99 0.375
important in developing the white color of the alloy.
Although silver is soluble in palladium, the addition of other elements to these alloys, such as copper or indium, may cause the formation of multiple phases and increased corrosion.
COPPER (Cu)
Copper is a malleable and ductile metal with high thermal and electrical conductivity and a characteris- tic red color. When added to gold-based alloys, cop- per imparts a reddish color to the gold and hardens A
Composition (wt.% Au)
25 AuCu3
396
Cu
Temperature (C)
50 AuCu
424
884 1064
1083
75 Au
900
600
300
1200 25 50 75
L S
Composition (at.% Au)
B
1554 1600
1200
800
400
25 PdCu3
500
Cu
Temperature (C)
50 PdCu
600 1083
75 Pd
25 50 75
L S
at.% Pd
1600
1200
25 50
L S 962
D
75
800
400
1555
25
Ag 50 75 Pd
at.% Pd
E
1064 1600
1200
25 50
L S 1554
75
800
400
25
Pd 50 75 Au
at.% Au
L
975 1250
1772
PtAu3 1064
F
2000
1500
25 50
S
75
1000
500
25
Au 50 75 Pt
at.% Pt 40
962
C
25 Ag
Temperature (C)
50
1064
75 Au
900
600
300
1200 25 50 75
L S
at.% Au AgAu
FIG. 10.5 Phase diagrams for binary combinations. (A) Copper (Cu) and gold (Au); (B) copper and palladium (Pd);
(C) silver and gold; (D) silver and palladium; (E) palladium and gold; and (F) gold and platinum (Pt). Atomic percent- ages are shown along the bottom of each graph; weight percentages are shown along the top. L, Liquidus; S, solidus.
(Modified from Hansen M. Constitution of Binary Alloys. New York: McGraw Hill; 1958.)
the alloy via a solid-solution or ordered-solution mechanism. The presence of copper in gold-based alloys in quantities between approximately 40% and 88% by weight allows the formation of an ordered phase. Copper is also commonly used in palla- dium-based alloys, where it can be used to reduce the melting point and strengthen the alloy through solid-solution hardening and formation of ordered phases when Cu is between 15 and 55 wt%. The ratio of silver and copper must be carefully balanced in gold- and palladium-based alloys, because silver and copper are not miscible. Copper is also a common component of most hard dental solders.
ZINC (Zn)
Zinc is a blue-white metal with a tendency to tar- nish in moist air. In its pure form, it is a soft, brittle metal with low strength. When heated in air, zinc oxidizes readily to form a white oxide of relatively low density. This oxidizing property is exploited in dental alloys. Although zinc may be present in quantities of only 1% to 2% by weight, it acts as a scavenger of oxygen when the alloy is melted. Thus zinc is referred to as a deoxidizing agent. Because of its low density, the resulting zinc oxide lags behind the denser molten mass during casting and is there- fore excluded from the casting. If too much zinc is present, it will markedly increase the brittleness of the alloy.
INDIUM (In)
Indium is a soft, gray-white metal with a low melt- ing point of 156.6°C. Indium is not tarnished by air or water. It is used in some gold-based alloys as a replace- ment for zinc and is a common minor component of some noble ceramic dental alloys. Recently, indium has been used in greater amounts (up to 30% by weight) to impart a yellow color to palladium-silver alloys.
TIN (Sn)
Tin is a lustrous, soft, white metal that is not subject to tarnish in normal air. Some gold-based alloys con- tain limited quantities of tin, usually less than 5% by weight. Tin is also an ingredient in gold-based dental solders. It combines with platinum and palladium to produce a hardening effect, but also increases brittleness.
GALLIUM (Ga)
Gallium is a grayish metal that is stable in dry air but tarnishes in moist air. It has a very low melting point of 29.8°C and a density of only 5.91 g/cm3. Gallium is not used in its pure form in dentistry, but is used as a component of some gold- and palladium-based dental alloys, especially ceramic alloys. The oxides of gallium are important to the bonding of the ceramic to the metal.
NICKEL (Ni)
Nickel has limited application in gold- and palladium- based dental alloys, but is a common component in base-metal dental alloys. Nickel has a melting point of 1453°C and a density of 8.91 g/cm3. When used in small quantities in gold-based alloys, nickel whitens the alloy and increases its strength and hardness.