Сraig. Dental Materials

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 (24 k) 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, and is tough, ductile, malleable, and can be produced as foil or fine-drawn wire. Platinum has a hardness similar to 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 characteristicsand 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. In the past, palladium was less than half the price of platinum and, because it imparts many of the properties of platinum to dental alloys, was often used as a replacement for platinum. However, in 2000, the price of palladium was greater than platinum because of market shortages, and for economic reasons alone could no longer be substituted for platinum. 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 2 10% 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 goldbased alloys. Although many of the palladiumbased 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 occurs largely because of 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 (1766" 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 (0s) Because of its tremendous expense and extremely high melting point, osmium is not used in dental casting alloys.

BASE METALS

Several base metals are combined with noble metals to develop alloys with properties suitable for dental restorations. Base metals used in dental alloys include silver, copper, zinc, indium, tin, gallium, and nickel (see Table 15-1, Fig. 6-11,

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 761.7" 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 silverbased 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 goldand 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 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 orderedsolution 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 goldand 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 palladiumsilver 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 5.91g/cm3. Gallium is not used in its pure form in dentistry, but is used as a component of some goldand 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 goldand 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/cc. When used in small quantities in gold-based alloys, nickel whitens the alloy and increases its strength and hardness. Ni-based alloys have increased in usage over the past 1j years. A more extensive discussion of these alloys can be found in Chapter 16.

BINARY COMBINATIONS OF METALS

Although most noble casting alloys have three or more elements, the properties of certain binary alloys are important because these binary combinations constitute the majority of the mass of many noble alloys. An understanding of the physical and manipulative properties of these binary alloys is therefore useful in understanding the behavior of the more complex alloys. Among the noble alloys, six binary combinations of elements are important: Au-Cu, Pd-Cu, AuAg, Pd-Ag, Au-Pd, and Au-Pt. Phase diagrams for these binary systems are shown in Fig. 15-1. Phase diagrams are powerful tools for understanding the physical and manipulative properties of binary alloys. A review of the theory of phase diagrams can be found in Chapter 6.

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, or 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, E;; 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), much larger for the Au-Pt system (see Fig. 15-1,R 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 liquidussolidus line 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 solidsolution 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 where 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 secondphase 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 singleand 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 rnay be higher than 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 orderedsolution 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 Brine11 hardness increases from 28 to 85 (see Table 15-2). The 90:10 Au-Cu mixture is the con~positionused in US. gold coins. If the positions of the two elements become ordered (forining 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 nxay 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 goldbased 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 tempera-

ture. 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 AgAu 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 multiplephase 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 basemetal alloys can be found in Chapter 17.

Formulation of Noble Alloys The desired qualities of noble dental casting alloys de-

termine 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. 15-11, Solid-solution systems are desirable for the formulation of alloys because they are generally

easier to manufacture and manipulate, have a lower tendency to corrode than multiplephase 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 multiplephase 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 AUoy s

For many years the gold content of goldcontaining 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 l/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 jineness refer only to gold content, not noble-metal content.

TYPES AND COMPOSITION

The number of dental casting alloys has increased dramatically in recent years. Formerly, the ADA Specification No. 5 classified these alloys as Types I through IV, with the type of alloy depending on its content of gold and platinum group metals. In the old system, the content of

noble metals (Types I through IV) ranged from 83 to 75 wt%, respectively, and all alloys were gold based. This classification reflected the use of alloys in dentistry at that time. The current ADA specification also classifies alloys by composition, but in a much more inclusive manner, dividing alloys into three groups: (1) high-noble, with a noble metal content of 260 wt% and a gold content of 240%; (2) noble, with a noble metal content 225% (no stipulation for gold); and

(3) predominately base metal, with a noble metal content <25%.The newer specification therefore includes non-noble alloys as well as those with no gold but high palladium. Under the current classification, all of the older alloy "types" are considered high-noble alloys. It is important to keep in mind that the percentages used as boundaries in the new specification are somewhat arbitrary.

The current ADA Specification No. 5 ( I S 0 1562) also uses a Type I through IV classification system in addition to the compositional classification previously described. However, in the current specification, the type of alloy (I through IV) is determined by its yield strength and elongation (Table 15-4). Thus a high-noble alloy might be Type I or Type IV, depending on its mechanical properties. This situation is some-

what confusing, because in the old specification the alloy type was tied to its composition and virtually all alloys were gold-based. In the current system, each type of alloy is recommended for intraoral use based on the amount of stress the restoration is likely to receive. Type I alloys have high elongation and are therefore easily burnished, but can survive only in low-stress environments, such as inlays that experience no occlusal forces. Type IV alloys are to be used in clinical situations where very high stresses are involved, such as long-span, fixed, partial dentures.

Although the number of casting alloys is immense, it is possible to subdivide each ADA compositional group into several classes (Table 15-5). The predominately base-metal alloys are not shown, but will be discussed in Chapter 16.These classes are simply a convenient way of organizing the diverse strategies that have been used to formulate casting alloys. For each class of alloy shown in Table 15-5,there are many variations; the compositions shown are meant only to be representative. Note that both the wt% and at% compositions of the alloys are shown in Table 15-5 (see previous discussion in this chapter). For the sake of simplicity, the following discussion will be in terms of wt% composition.

Most of the alloys contain some zinc as a deoxidizer and either Ir or Ru as grain refiners. Some of these compositions are used for both full metal castings and porcelain-metal restorations.

There are three classes of high-noble alloys: the Au-Ag-Pt alloys; the Au-Cu-Ag-Pd alloys with a gold content of >70 wt% (Au-Cu-Ag-Pd-I in Table 15-5); and the Au-Cu-Ag-Pd alloys with a gold content of about 50% to 65% (Au-Cu-Ag- Pd-11). The Au-Ag-Pt alloys typically consist of 78 wt% gold with roughly equal amounts of silver and platinum. These alloys have been used as casting alloys and porcelain-metal alloys. The

Au-Cu-Ag-Pd-I alloys are typically 75 wt% gold with approximately 10 wt% each of silver and copper and 2 to 3 wt% palladium. These alloys are identical to the Type I11 alloys under the old ADA compositional classification. The Au-Cu-Ag- Pd-IIalloys typically have <60 wt%gold, with the silver content increased to accommodate the reduced gold content. Occasionally, these alloys will have slightly higher palladium and lower silver percentages.

There are four classes of noble alloys: the Au-Cu-Ag-Pd alloys (Au-Cu-Ag-Pd-111 in Table 15-5); Au-Ag-Pd-In alloys; Pd-Cu-Ga alloys; and