Noble Dental Alloys and Solders

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 malleable of all metals, it ranks much lower in strength. The density of gold depends somewhat on the condition of the metal, whether it is cast, rolled, or drawn into wire. Small amounts of impurities have a pronounced 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, such as technique alloy or other base-metal alloys, including amalgam, should not be mixed with gold used for dental restorations. The addition of calcium to pure gold improves the mechanical properties of gold foil restorations (see subsequent discussion).

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 potassium 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 jewelry (Table 15-2). Through appropriate refining and purification, gold with an extremely high degree of purity may be produced. Such highly refined ingots of pure gold (99.99% by weight) serve as the starting material for gold foil.

Gold foil is formed by a process known as gold beating. High-purity gold is first passed through a series of rollers and then annealed until the gold is in a ribbon about 0.0025 mm thick, which is comparable to the thickness of tissue paper. The ribbon is cut into small pieces, and each piece is placed between two sheets of paper, which are then placed one over the other to form a packet. The packet, which may contain 200 to 250 pieces of the small gold ribbons, is then beaten by a hammer until the desired thickness of gold is obtained, usually 0.00064 mm. The foil is then carefully weighed and annealed. Purity in the process of manufacturing gold foil is critical to maintaining its cohesive properties.

If uncontaminated, gold foil is cohesive; that is, it can be welded together at room temperature. This cohesive property has been exploited in the use of gold foil as a dental restorative material. If manipulated properly, small pieces of foil can be inserted and condensed into cavity preparations in teeth, providing a restoration with considerable longevity. As Table 15-2 shows, the tensile strength and hardness of condensed gold foil is more than twice that of pure (24k) cast gold. The reduced elongation of foil compared with cast gold is evidence of the considerable work hardening that the condensation process has accomplished (see Chapter 6, wrought alloys). It is this improvement of physical properties by work hardening that makes gold acceptable as a restoration in some areas of the mouth; without the improvement, cast gold would lack sufficient strength and hardness. The use of gold foil restorations has declined in recent years because of the time and skill required to properly place these materials and the development of more esthetic (tooth-colored) restorative materials for areas where foil restorations were placed.

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 a hardness similar to that of copper. Pure platinum has numerous applications in dentistry because of its high fusing point and resistance to oral conditions and elevated temperatures. Platinum foil serves as a matrix for the construction of fused porcelain restorations because it does not oxidize at high temperatures. Platinum foil also has a higher melting point than porcelain and has a coefficient of expansion sufficiently close to that of porcelain to prevent buckling of the metal or fracture of the porcelain during changes in temperature (see Chapter 18). Platinum has been used for pins and posts in crown and bridge restorations, and alloys may be cast or soldered to the posts without damage.

Platinum adds greatly to the hardness and elastic qualities of gold, and some dental casting alloys and wires contain quantities of platinum up to 8% combined with other metals. Platinum is a major component of alloys used for precision attachments in complex crown and bridge restorations because these alloys have excellent wear characteristics and high melting ranges. The high melting range is necessary because other gold alloys must be cast to these attachments without causing distortion of the attachment. 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 has the quality of absorbing or occluding large quantities of hydrogen gas when heated. This can be an undesirable quality when alloys containing palladium are heated with an improperly adjusted gas-air torch.

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 readily formed between gold and palladium, and palladium quantities of as low as 5% by weight have a pronounced effect on whitening yellow gold-based alloys. Palladium-gold alloys with a palladium content of 10% or more by weight are white. Alloys of palladium 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 grainrefining 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 nucleating 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.

Osmium (Os)

Because of its tremendous expense and extremely high melting point, osmium is not used in dental casting alloys.

Silver (Ag)

Silver is a malleable, ductile white metal. It is the best-known conductor of heat and electricity, and is stronger and harder than gold but softer than copper. At 961.9° C, the melting point of silver is below the melting 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 containing sulfur compounds cause severe tarnish on silver, and for this reason silver is not considered a noble metal in dentistry. Pure silver captures appreciable quantities of oxygen in the molten state, which makes it difficult to cast because the gas is evolved during solidification. As a result, small pits, porosity, and a rough casting surface develop. This tendency is reduced when 5% to 10% by weight of copper is added to the silver, for which reason castings are made of the alloy rather than the pure metal. This combination of elements is also used in silver-based dental solders to prevent pitting during soldering.

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. The electroforming of silver of high purity is readily accomplished and represents a popular method of forming metal dies.

Silver forms a series of solid solutions with palladium (see Fig. 15-1, D) and gold (see Fig. 15-1, C), 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 solidsolution hardening mechanism (see Chapter 6). More recent evidence indicates that a fine lamellar coherent precipitate of an Ag-rich phase at the grain boundaries may contribute to hardening as well. In palladium-based alloys, silver is 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 characteristic red color. Copper forms a series of solid solutions with both gold (Fig. 15-1, A) and palladium (Fig. 15-1, B), and is therefore an important component of noble dental alloys. When added to gold-based alloys, copper imparts a reddish color to the gold and hardens 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 causes allows the formation of an ordered phase. Copper is also commonly used in palladium-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 tarnish 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 therefore 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 melting point of 156.6° C. Indium is not tarnished by air or water. It is used in some gold-based alloys as a replacement 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 contain 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 non-noble 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. A more extensive discussion of these alloys can be found in Chapter 16.

Alloy Composition and Temperature

In each phase diagram in Fig. 15-1, the horizontal axis represents the composition of the binary alloy. For example, in Fig. 15-1, A, the horizontal axis represents a series of binary alloys of gold and copper ranging in composition from 0% gold (or 100% copper) to 100% gold. The composition can be given in atomic percent (at%) or weight percent (wt%). Weight percent compositions give the relative mass of each element in the alloy, whereas atomic percentages give the relative numbers of atoms in the alloys. It is a simple calculation to convert weight percentages to atomic percentages, and vice versa. Note that for the binary alloys shown in Fig. 15-1, the atomic percent composition is shown along the bottom of the phase diagram, whereas the weight percent composition is shown along the top. The atomic and weight percent compositions of the binary alloys can differ considerably. For example, for the Au-Cu system shown in Fig. 15-1, A, an alloy that is 50% gold by weight is only 25% gold by atoms. For other systems, like the Au-Pt system in Fig. 15-1, F, there is little difference between atomic and weight percentages. The difference between atomic and weight percentages depends on the differences in the atomic masses of the elements involved. The bigger the difference in atomic mass, the bigger the difference between the atomic and weight percentages in the binary phase diagram. Because it more convenient to use masses in the manufacture of alloys, the most common method to report composition is by weight percentages. However, the physical and biological properties of alloys relate best to atomic percentages. It is therefore important to keep the difference between atomic and weight percent in mind when selecting and using noble dental casting alloys. Alloys that appear high in gold by weight percentage may in reality contain far fewer gold atoms than might be thought.

Other aspects of the phase diagrams that deserve attention are the liquidus and solidus lines. The y-axes in Fig. 15-1 show temperature. If the temperature is above the liquidus line (marked L), the alloy will be completely molten. If the temperature is below the solidus line (marked S), the alloy will be solid. If the temperature lies between the liquidus and solidus lines, the alloy will be partially molten. Note that the distance between the liquidus and solidus lines varies among systems in Fig. 15-1. For example, the temperature difference between these lines is small for the Ag-Au system (see Fig. 15-1, C), is much larger for the Au-Pt system (see Fig. 15-1, F), and varies considerably with composition for the Au-Cu system (see Fig. 15-1, A). From a manipulative standpoint, it is desirable to have a narrow liquidus-solidus range, because one would like to keep the alloy in the liquid state for as short a time as possible before casting. While in the liquid state, the alloy is susceptible to significant oxidation and contamination. If the liquidus-solidus range is broad, the alloy will remain at least partially molten for a longer period after it is cast. The temperature of the liquidus line is also important, and varies considerably among alloys and with composition. For example the liquidus line of the Au-Ag system ranges from 962° to 1064° C (see Fig. 15-1, C) but the liquidus line of the Au-Pd system ranges from 1064° to 1554° C (see Fig. 15-1, E). It is often desirable to have an alloy with a liquidus line at lower temperatures; the method of heating is easier, fewer side reactions occur, and shrinkage is generally less of a problem (see Chapter 17, Casting and Soldering Procedures).

Phase Structure of Noble Alloys

The area below the solidus lines in Fig. 15-1 is also important to the behavior of the alloy. If this area contains no boundaries, then the binary system is a series of solid solutions. This means that the two elements are completely soluble in one another at all temperatures and compositions. The Ag-Pd system (see Fig. 15-1, D) and Pd-Au system (see Fig. 15-1, E) are examples of solid-solution systems. If the area below the solidus line contains dashed lines, then an ordered solution is present within the dashed lines. An ordered solution occurs when the two elements in the alloy assume specific and regular positions in the crystal lattice of the alloy (for a discussion on metallic crystal lattices, see Chapter 6). This situation differs from a solid solution in which the positions of the elements in the crystal lattice are random. Examples of systems containing ordered solutions are the Au-Cu system (see Fig. 15-1, A) the Pd-Cu system (see Fig. 15-1, B), and the Au-Ag system (see Fig. 15-1, C). Note that the ordered solutions occur over a limited range of compositions because the ratios between the elements must be correct to support the regular positions in the crystal lattices. If the area below the solidus line contains a solid line, it indicates the existence of a second phase. A second phase is an area with a composition distinctly different from the first phase. In the Au-Pt system (see Fig. 15-1, F) a second phase forms between 20 and 90 at% platinum. If the temperature is below the phase boundary line within these compositions, two phases exist in the alloy. The presence of a second phase is important because it significantly changes the corrosion properties of an alloy. Fig. 15-2 shows electron micrographs of single- and multiple-phase alloys. The single-phase alloy has little visible microstructure because its composition is more or less homogeneous. In the multiple-phase alloy, areas that have distinct compositions are clearly visible. These areas correspond to the different phases under the solidus in a phase diagram. Because the different phases may interact electrochemically, the corrosion of multiple-phase alloys may be higher than for a single-phase alloy.

Hardening of Noble Alloys

The use of pure cast gold is not practical for dental restorations because cast gold lacks sufficient strength and hardness. Solid-solution and ordered-solution hardening are two common ways of strengthening noble dental alloys sufficiently for use in the mouth. By mixing two elements in the crystal lattice randomly (forming a solid solution), the force needed to distort the lattice may be significantly increased. For example, adding just 10% by weight of copper to gold, the tensile strength increases from 105 to 395 MPa and the Brinell hardness increases from 28 to 85 (see Table 15-2). The 90:10 Au-Cu mixture is the composition used in U.S. gold coins. If the positions of the two elements become ordered (forming an ordered solution), the properties of the alloy are improved further (see Table 15-2). For a typical gold-based casting alloy, the formation of an ordered solution may increase yield strength by 50%, tensile strength by 25%, and hardness by at least 10%. It is important to note that the elongation of an alloy is reduced by the formation of the ordered solution. For the typical gold-based alloy, the percentage elongation will decrease from 30% to about 12%.

The formation of ordered solutions has been commonly used to strengthen cast dental restorations, particularly in gold-based alloys. As shown in Fig. 15-1, A, the Au-Cu system supports ordered solutions between about 20 and 70 at% gold. However, the manipulation of the alloy during casting will determine if the ordered solution will form. If Au-Cu containing about 50 at% gold is heated to the molten state and then cooled slowly, the mass will solidify at about 880° C as a solid solution. As the mass cools slowly to 424° C, the ordered solution will then form and will remain present at room temperature. However, if the mass is cooled rapidly to room temperature after the initial solidification, the ordered solution will not form because there is insufficient time for the mass to reorganize. Thus the alloy will be trapped in a nonequilibrium state of a solid solution and will be softer, weaker, and have greater elongation. The conversion between the ordered solution and solid solution is reversible in the solid state. By heating an alloy in either condition above 424° C (but below the solidus), the state of the alloy can be selected by picking the cooling rate. Rapid cooling will preserve the solid solution and the soft condition, whereas slow cooling will allow the formation of the ordered solution and the hardened condition. In alloys of gold and copper with other elements, Au-Cu ordered solutions are still possible as long as the ratio of copper to gold is greater than 30:70 (at%). As shown in Fig. 15-2, the formation of ordered solutions is possible in other noble alloy systems, such as Pd-Cu and Au-Pt. The ordered solution of the Ag-Au system exists but cannot be used in practice because the transition temperature is too low (almost body temperature).

The formation of a second phase has also been used to harden dental alloys, but this method is not commonly used for noble dental alloys. The dispersion of the second phase is very important to the effectiveness of the hardening. Recent evidence indicates that some Au-based alloys may contain Ag-rich coherent precipitates that, along with ordered solutions, contribute to alloy hardening. The advantages of the hardening must be balanced against the liabilities of the increased corrosion often seen with multiple-phase systems. It should be noted that, unlike the ordered solutions, the formation of second phases are not usually easily controlled by heat treatments and may not be reversible in the solid state. In fact, heat treatment commonly causes a deterioration of properties with these systems. Further discussion of cast and wrought base-metal alloys can be found in Chapter 16.

Formulation of Noble Alloys

The desired qualities of noble dental casting alloys determine the selection of elements that will be used to formulate the alloys. The ideal noble casting alloy should have (1) a low melting range and narrow solidus-liquidus temperature range, (2) adequate strength, hardness, and elongation, (3) a low tendency to corrode in the oral environment, and (4) low cost, among other properties. Traditionally, the noble elements gold and palladium have generally been the foundation to which other elements are added to formulate dental casting alloys. Gold and palladium are preferable to other noble elements because they have relatively low melting points, low corrosion, and form solid solutions with other alloy elements, such as copper or silver (see Fig. 15-1). Solidsolution systems are desirable for the formulation of alloys because they are generally easier to manufacture and manipulate, have a lower tendency to corrode than multiple phase systems, and provide increased strength through solid-solution or ordered-solution hardening. Furthermore, the systems shown in Fig. 15-1 generally have narrow liquidus-solidus ranges. Thus, it is not surprising that combinations of these elements have been extensively used in the formulation of noble dental casting alloys.

The Au-Pt alloys were initially developed out of fear that palladium posed a biological hazard. This fear persisted in spite of a lack of evidence that any hazard existed, other than a low frequency of allergic sensitivity. More recently, palladium-free alloys (Au-Pt and some other systems) have been popular because of the tremendous increase in the cost of palladium. Often, the palladium-free alloys have the disadvantages of high cost and limited compositional flexibility. As shown in Fig. 15-1, F, the addition of more than 20 at% platinum to gold forms a multiple-phase alloy. Thus these alloy systems generally have platinum concentrations below 20 at%. Because the Au-Pt systems may not be hard enough for oral applications, the addition of Zn as a dispersed phase-hardener has been used. Thus the corrosion of these alloys may be higher than the Au-Pd compositions they replaced. Furthermore, the cost of platinum is significantly higher than that of gold, and at least as expensive as palladium.

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 1/24 part of the whole. Thus, 24 carat indicates 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). The fineness represents a precise measure of the gold content of the alloy and is often the preferred measurement when an exact value is to be listed. An 18k gold would be designated as 750 fine, or, when the decimal system 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 15-3. Both the whole number and the decimal system are in common use, especially for noble dental solders. The fineness system is somewhat 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.

Melting Range

Dental casting alloys do not have melting points, but rather melting ranges, because they are combinations of elements rather than pure elements. The magnitude of the solidus-liquidus melting range is important to the manipulation of the alloys (see Fig. 15-1, Table 15-6). The solidus-liquidus range should be narrow to avoid having the alloy in a molten state for extended times during casting. If the alloy spends a long time in the partially molten state during casting, there is increased opportunity for the formation of oxides and contamination. Most of the alloys in Table 15-6 have solidus-liquidus ranges of 70° C or less. The Au-Ag-Pt, Pd-Cu-Ga, and Ag-Pd alloys have wider ranges, which makes them more difficult to cast without problems.

The liquidus temperature of the alloys determines the burnout temperature, type of investment, and type of heat source that must be used during casting. In general, the burnout temperature must be about 500° C below the liquidus temperature (for details see Chapter 17 on casting). For the Au-Cu-Ag-Pd-I alloys, therefore, a burnout temperature of about 450° to 475° C should be used. If the burnout temperature approaches 700° C, a gypsum-bonded investment cannot be used because the calcium sulfate will decompose and embrittle the alloys. At temperatures near 700° C or greater, a phosphate-bonded investment is used. As shown in Table 15-6, a gypsum-bonded investment may be used with the Au-Cu-Ag-Pd-I, II, and III and the Au-Ag-Pd-In alloys, but a phosphate-bonded investment is advisable for the other alloys. The gas-air torch will adequately heat alloys with liquidus temperatures below 1100° C. Above this temperature, a gas-oxygen torch or electrical induction method must be used. Again from Table 15-6, a gas-air torch would be acceptable only for the Au-Cu-Ag-Pd-I, II, and III and the Au-Ag-Pd-In alloys.

The composition of the alloys determines the liquidus temperatures. If the alloy contains a significant amount of an element that has a high melting point, it is likely to have a high liquidus. Thus alloys that contain significant amounts of palladium or platinum, both of which have high melting points (see Table 15-1), will have high liquidus temperatures. In Table 15-6, these alloys include the Pd-Cu-Ga, Ag-Pd, and Au-Ag-Pt alloys.

The solidus temperature is important to soldering and formation of ordered phases, because during both of these operations, the shape of the alloys is to be retained. Therefore during soldering or hardening-softening, the alloy may be heated only to the solidus before melting occurs. In practice, it is desirable to limit heating to 50° C below the solidus to avoid local melting or distortion of the casting.

Density

Density is important during the acceleration of the molten alloy into the mold during casting. Alloys with high densities will generally accelerate faster and tend to form complete castings more easily. Among the alloys shown in Table 15-6, all have densities sufficient for convenient casting. Lower densities (7 to 8 g/cm³) seen in the predominately base-metal alloys sometimes present problems in this regard. Alloys in Table 15-6 with high densities generally contain higher amounts of denser elements such as gold or platinum. Thus the Au-Ag-Pt alloys and Au-Cu-Ag-Pd-I alloys are among the densest of the casting alloys.

Strength

Strength of alloys can be measured by either the yield strength or tensile strength. Although tensile strength represents the maximum strength of the alloy, the yield strength is more useful in dental applications because it is the stress at which permanent deformation of the alloys occurs (see Chapter 4). Because permanent deformation of dental castings is generally undesirable, the yield strength is a reasonable practical maximum strength for dental applications. The yield strengths for the different classes of alloys are shown in Table 15-6. Where applicable, the hard and soft conditions, resulting from the formation of ordered solutions, are shown. For several alloys, such as Au-Cu-Ag-Pd-I, II, and III, the formation of the ordered phase increases the yield strength significantly. For example, the yield strength of the Au-Cu-Ag-Pd-II alloys increases from 350 to 600 MPa with the formation of an ordered phase. For other alloys, such as the Au-Ag-Pt and Ag-Pd alloys, the increase in yield strength is more modest in the hardened condition. The Pd-Cu-Ga alloys do not support the formation of ordered phase because the ratio of palladium and copper are not in the correct range for ordered phase formation (see Table 15-5 and Fig. 15-1, B).

The yield strengths of these alloys range from 320 to 1145 MPa (hard condition). The strongest alloy is Pd-Cu-Ga, with a yield strength of 1145 MPa. The other alloys range in strength from 320 to 600 MPa. These latter yield strengths are adequate for dental applications and are generally in the same range as those for the base-metal alloys, which range from 495 to 600 MPa. The effect of solid-solution hardening by the addition of copper and silver to the gold or palladium base is significant for these alloys. Pure cast gold has a tensile strength of 105 MPa (see Table 15-2). With the addition of 10 wt% copper (coin gold), solid-solution hardening increases the tensile strength to 395 MPa. With the further addition of 10 wt% silver and 3 wt% palladium (Au-Cu-Ag-Pd-I), the tensile strength increases to about 450 MPa and 550 MPa in the hard condition.

Hardness

Hardness is a good indicator of the ability of an alloy to resist local permanent deformation under occlusal load. Although the relationships are complex, hardness is related to yield strength and gives some indication of the difficulty in polishing the alloy. Alloys with high hardness will usually have high yield strengths and are more difficult to polish. As Table 15-6 shows, the values for hardness generally parallel those for yield strength. In the hard condition, the hardness of these alloys ranges from 155 kg/mm² for the Ag-Pd alloys to 425 kg/mm² for the Pd-Cu-Ga alloys. More typically, the hardness of the noble casting alloys is around 200 kg/mm². The Ag-Pd alloys are particularly soft because of the high concentration of silver, which is a soft metal. The Pd-Cu-Ga alloys are particularly hard because of the high concentration of Pd, which is a hard metal. The hardness of most noble casting alloys is less than that of enamel (343 kg/mm²), and typically less than that of base-metal alloys. If the hardness of an alloy is greater than enamel, it may wear the enamel of the teeth opposing the restoration.

Elongation

Elongation is a measure of the ductility of the alloy. For crown and bridge applications, a low value of elongation for an alloy is generally not a big concern, because permanent deformation of the alloys is generally not desirable. However, the elongation will indicate if the alloy can be burnished. Alloys with high elongation can be burnished without fracture. Elongation is sensitive to the presence or absence of an ordered phase, as shown in Table 15-6. In the hardened condition, the elongation will drop significantly. For example, for the Au-Cu-Ag-Pd-II alloys, the elongation is 30% in the soft condition versus only 10% in the hard condition. In the soft condition, the elongation of noble dental casting alloys ranges from 8% to 30%. These alloys are substantially more ductile than the base-metal alloys, which have elongation on the order of 1% to 2% (see Chapter 16).

Biocompatibility

The biocompatibility of noble dental alloys is equally important as other physical or chemical properties. A detailed discussion about the principles of biocompatibility can be found in Chapter 5, but a few general principles will be mentioned here. The biocompatibility of noble dental alloys is primarily related to elemental release from these alloys (i.e., their corrosion). Thus any toxic, allergic, or other adverse biological response is primarily influenced by elements released from these alloys into the oral cavity. The biological response is also influenced significantly by exactly which elements are released, their concentrations, and duration of exposure to oral tissues. For example, the short-term (more than 1 to 2 days) release of zinc may not be significant biologically, but longer-term (more than 2 to 3 years) release might have more significant effects. Similarly, equivalent amounts (in moles) of zinc, copper, or silver will have quite different biological effects, because each of the elements is unique in its interactions with tissues.

Unfortunately, there is currently no way of completely assessing the biocompatibility of noble alloys (or any other material), because the effects of elemental release on tissues are not completely understood. However, in general, several principles apply to alloy biocompatibility. The elemental release from noble alloys is not proportional to alloy composition, but rather is influenced by the numbers and types of phases in the alloy microstructure and the composition of the phases. In general, multiple-phase alloys release more mass than single-phase alloys. Some elements, such as copper, zinc, silver, cadmium, and nickel, are inherently more prone to be released from dental alloys than others, such as gold, palladium, platinum, and indium. Alloys with high noble metal content generally release less mass than alloys with little or no noble metal content. However, the only reliable way to assess elemental release is by direct measurement, because there are exceptions to each of the generalizations just mentioned. Similarly, it is difficult to predict, even knowing the elemental release from an alloy, what the biological response to the alloy will be. Thus the only reliable way is to measure the biological response directly, either in vitro, in animals, or in humans (see Chapter 5). It is important to also remember that combinations of alloys used in the mouth may alter their corrosion and biocompatibility.

The Identalloy program was developed in an effort to make dentists and patients more aware of the composition of dental alloys that are used. Under this program, each alloy has a certificate (Fig. 15-3) that lists the complete composition of the alloy, its manufacturer, name, and the ADA compositional classification (high-noble, noble, or predominately base metal). When the dental prosthesis is delivered by the laboratory to the dental office, a certificate is placed in the patient’s chart. In this manner, all parties know the exact composition of the material used. This information can be invaluable later if there are problems with the restoration; for example, if the patient develops an allergic reaction. This information is also useful when planning additional restorations that may contact the existing restoration, or if some modification (such as occlusal adjustment or contouring) becomes necessary.

Basis of Selecting Solders

Certain principles must be observed in selecting solders, regardless of the application. The ideal solder includes qualities such as (1) ease of flow at relatively low temperature, (2) sufficient fluidity to freely flow when melted, (3) strength compatible with that of the structure being soldered, (4) acceptable color to give an inconspicuous joint, (5) resistance to tarnish and corrosion, and (6) resistance to pitting during heating and application. No single, dental, gold-based solder has all of these qualities; therefore manufacturers provide solders that cover a range of fineness and a number of solders that have special qualities. Manufacturers will generally provide detailed information about which solder should be used with each of their alloys. The properties of solders are significantly influenced by the method used during the soldering operation. Thus, a recommended procedure must be faithfully followed to obtain the maximum quality from a product.

Composition

Gold-based solders for dental use are primarily alloys of gold, silver, and copper, with small quantities of tin, zinc, and sometimes phosphorus included to modify the fusion temperature and flow qualities. The typical composition and resulting fusion temperature values of a variety of gold solders are given in Table 15-9. The compositions of different solders vary considerably from one another. For example, the gold content may vary from 45 to 81 wt%, silver from 8 to 30 wt%, and copper from 7 to 20 wt%, with little variation in the tin or zinc content. Most solders have a copper/gold ratio to support the formation of a Au-Cu ordered phase. The fusion temperature is lower for alloys that have reduced gold content, but the reduction is not as great as sometimes believed. For example, the difference in fusion temperature between solders 1 and 4 is only 69° C when the gold content is reduced by approximately 16%.

In the past, solders were commonly referred to by a carat number. The number did not describe the actual carat of the solder, but rather the carat of the gold alloy on which the solder was to be used. This permitted a wide range of compositions, or actual carat values, to be used for solders described as a specific carat, because the solder was intended for use with an alloy of a definite carat. For example, solders that varied in gold content from 58.5 to 65 wt% might be described as 18k because they were to be used on an casting made of an 18k alloy. The gold content of an 18k alloy should be 75 wt% gold, but the solders contained only 58.5 to 65 wt% gold. This system led to much confusion. In recent years, the degree of fineness has been used to describe the various solders, such as those values designated in Tables 15-9 and 15-10. This designation is appropriate because it is specific, and when the fineness is changed, the actual carat change is indicated.

Easy-Flowing and Free-Flowing Qualities

Dental gold-based solders are frequently described by terms such as easy-flowing or free-flowing. Although these terms are sometimes interchanged, they refer to two different qualities in the solder alloys. An easy-flowing solder is one that has a relatively low fusion temperature; the lower the temperature of fusion, the easier to melt and form the joint. However, the difference in fusion temperatures of high- and low-fineness gold solder is approximately 56° C, so differences in flow caused by a lower fusion temperature are small among these solders (see Table 15-9).

The fusion temperature of the solder must be below that of the alloys being soldered, or the joined pieces will melt during the operation. Previous discussion of casting alloys indicated that the lowest solidus values for the casting alloys are for the Au-Cu-Ag-Pd-II and -III alloys and the Au-Ag-Pd-In alloys, ranging from 865° to 875° C (see Table 15-6). For wrought alloys, the lowest solidus values ranged from 875° to 935° C, although most of the wrought alloys have substantially higher solidus temperatures (see Table 15-8). As Table 15-9 indicates, most of the solders have fusion temperatures below the solidus temperatures for the casting or wrought alloys. Thus solders are available for even the lowest fusing alloys. In general, the fusion temperature of the solder should be at least 56° C below that of the parts being joined to prevent distortion.

The free-flowing quality refers specifically to the ability of the solder to spread and flow freely over the surfaces of the parts begin joined. This quality is closely related to the surface tension of the melted solder. The surface tension (and free-flowing qualities) control the capillary action, which causes the melted solder to penetrate into fine openings between the parts being assembled. In general, lower-fineness solders are more fluid in the molten state than higher-fineness solders because of their lower gold content and the presence of small quantities of alloying metals such as zinc and tin. For this reason, the lower-fineness solders are preferred when joining parts because the solder flows promptly and freely into the available spaces. Solders that spread slowly are often described as being sticky, and they resist spreading even when properly heated. As a result, they tend to penetrate or “burn through” the part being soldered if the solder is forced to spread by overheating. Obviously, the easy- and free-flowing qualities work together, because the lower-fineness gold solders have the lowest melting range and greatest freedom of flow.

Mechanical Properties

The values in Table 15-10 illustrate typical mechanical properties for dental solders. There are a wide range of strengths and hardness available. As stated previously, it is most desirable to use a solder with a strength similar to that of the parts being joined. All solders in Table 15-10 are subject to hardening by ordered-phase formation except solder 1. Solder 1 does not form an ordered phase because the gold/copper ratio is not appropriate (see Table 15-9). A considerable increase in strength and hardness can be obtained by slow-cooling the solder and allowing the ordered phase to form.

The percentages of elongation in noble solders are notably less than those of many noble casting alloys (see Tables 15-10 and 15-6). Even in the soft condition, the elongation ranges only from 3% to 9% for all of the solders except solder 1. In the hard condition the elongation is most often 1%, which is quite brittle. In general, it is more desirable in most appliances to obtain an improved hardness and proportional limit that accompany the ordered-phase formation than to sacrifice these properties for a small increase in elongation.

Color and Tarnish Resistance

The color of dental noble solders varies from deep yellow to light yellow and white in much the same manner as casting alloys. Although the ability to match the color of the appliances is an important quality of a solder, producing an inconspicuous solder joint is not difficult in practice. The location of the solder junction on the restoration and the total amount of solder applied are clearly factors that influence color matching.

Because of their gold content, the solders of high fineness are assumed to be more resistant to tarnish, discoloration, and corrosion in the mouth than the lower-fineness solders. However, there is little evidence or data to support such claims, because no good tests are available for tarnish and discoloration of such structures. High-fineness solders are often recommended to prevent tarnish in service. However, the qualities of increased fluidity and mechanical properties associated with the lower fineness (0.650 or less) may outweigh any increased tendency of those solders to tarnish in service. Actually, the lower-fineness solders are used extensively for the assembly of dental appliances without serious tendencies to discolor.

Biocompatibility

Little information exists about the biocompatibility of noble dental solders. However, the same principles govern the biocompatibility of solders as govern alloys (see previous discussion, this chapter). In vitro evidence indicates that the biological properties of solders may be completely different when the solder is tested by itself versus when it is tested in combination with a substrate alloy. Most solders appear to release less mass and have better in vitro biocompatibility when used on an appropriate substrate metal, but a few will be more cytotoxic on the substrate. The character of the substrate alloy also plays a role, as do the surface area of the solder, any pitting present, and other factors. Generally, these results indicate that the biological liabilities of any solder should be assessed with the solder-substrate combination versus the solder by itself.

Pitted Solder Joints

Hard solders have a tendency to be pitted after the soldering operation. In general, pitting results from improper heating of the solder, although some compositions of solder may be more susceptible to pitting than others. When solders of typical fineness are used, a pitted solder can often be associated with either excessive heating during the fusion of the solder or with improper fluxing during heating.

If the solder is heated to too high a temperature or for prolonged periods, the lower-melting point tin and zinc in the solder can boil or oxidize and form pits or porosities as the solder solidifies. These pits often become apparent only during finishing and polishing operations. If the solder is underheated and the flux is applied in excess or improperly melted, it may be trapped in the melted solder and form pits that are uncovered during polishing. To avoid pitting from these causes, the solder should be heated promptly to the fusion temperature, and heating should be stopped as soon as the solder has flowed into position.

Silver Solder

Hard solders composed of silver-based alloys are used extensively in some industries, but their application in dentistry has been limited. These solders are also commonly known as silver solders. Silver-based solders are used when a low-fusing point solder is needed for soldering onto stainless steel or other base-metal alloys. Orthodontic appliances are commonly soldered using silver-based solders. In general, the resistance of silver solders to tarnish is not as good as gold-based solders, but the strengths of the two types of solders are comparable.

The silver-based solders are composed of silver (10 to 80 wt%), copper (15 to 30 wt%), and zinc (4 to 35 wt%), with some products containing small percentages of cadmium, tin, or phosphorus to further modify the fusion temperature. The formation of the silver-copper eutectic is responsible for the low melting range and higher corrosion rate found in the silver-based solders. The liquidus temperatures for these solders range from 620° to 700° C, which is slightly below those of gold-based solders. This difference is important in the soldering of stainless steel.