History of Metal Development in Dentistry

In the seventh century B.C. the Etruscans used ivory and bone supported by gold frameworks as tooth forms. In the 1800s, over 2500 years later, metal restorations were made by compacting aluminum, amalgam, gold, lead, platinum, and silver in tooth cavities. Although prostheses and restorations have been made from metals for centuries, precision casting technology was not available until the 20th century. The concepts of producing gold castings using the lost-wax process and a melting torch were introduced in 1903 at a meeting of the Society of Stomatology of Paris. In 1907 Taggert introduced a method for producing precise cast metal crown and bridge forms. Electricity-driven furnaces and casting machines became available in the early 1900s and casting technology advanced rapidly soon thereafter. However, the successful casting of titanium, one of the most biocompatible metals available for dentistry, was not achieved until the 1970s because of its reactivity with oxygen and the technique sensitivity of this metal. Since then, several technological advances such as CAD-CAM technology, electroforming, laser sintering, and laser welding have been introduced to facilitate the processing of metals for dental applications.

In dentistry, metals, ceramics, polymers, and composites are the four major material groups used for dental prostheses and restorations. It is not easy to define the word metal because of the wide variation in properties and characteristics of metallic materials. The Metals Handbook (1992) defined a metal as “an opaque lustrous chemical substance that is a good conductor of heat and electricity and, when polished, is a good reflector of light.” From the standpoint of dental restorations, this definition leaves a lot to be desired because dentists are concerned about plaque adhesion to metal surfaces, the ability to form bonds to porcelain, the ability to join components of a metal or metal-ceramic prosthesis accurately to ensure proper fit on prepared teeth, wear of opposing teeth, and the biocompatibility aspects of the component metallic elements.

Metals and Alloys

Pure metals have properties that can be markedly different when alloyed with other metals or nonmetals. For example, the element iron alloyed with relatively small amounts of carbon produces much stronger and harder metals called steels, which can be used for high-stress applications. When chromium is alloyed with iron and carbon, the corrosion resistance of this alloy, called stainless-steel, is greatly enhanced because of the formation of an adherent oxide of chromium (Cr_2-O₃). Certain carbon steels are electroplated with chromium to produce corrosion-resistant instruments.

Since an alloy is a material with metallic properties consisting of two or more chemical elements at least one of which is a metal, the choice of elements depends on which properties are required for specific clinical conditions. Characteristics such as castability, ability to be soldered (or brazed), solidification shrinkage, expansion coefficient, corrosion resistance, biocompatibility, and color are important for a variety of clinical applications,

Gallium and mercury, elements commonly used in dental alloys, are liquid at body temperature but when alloyed, they become a component of the solid alloy at oral temperatures. With the exception of pure gold foil, commercially pure titanium, and endodontic silver points, metals used for dental restorations, implants, partial denture frameworks, orthodontic wires, and endodontic instruments are alloys. All pure metals and alloys used as restorative materials in dentistry are crystalline solids when used to produce their functional prosthesis or restoration forms. Because the metals are crystalline, the microstructural changes that occur during processing or heat treatment control desired properties for dental applications.

Compared with the properties of ceramics, composites, and polymers, the unique characteristics of metal alloys are described qualitatively and quantitatively by properties and characteristics such as brittleness, color, ductility, electrical and thermal conductivity, hardness, luster, malleability, melting temperature, specific gravity, solderability, wear resistance, and weldability. In addition, the ability of most alloys to make a ringing sound when struck by a hard object can provide evidence that they are metals. Thus, no single property or characteristic defines a metal completely.

The wide range of dental alloy compositions is described briefly in this chapter and they are discussed further in Chapter 16. Dental casting alloys are associated with the following groups:

CP titanium, which is classified in four different grades, can technically be considered as an alloy because small percentages of other impurity elements are allowed as specified by a standard that has been established by the American Society for Testing and Materials (ASTM) for each grade. Grade 2 titanium is used widely in aerospace, marine, and medical applications because of its ease of forming, its moderately high strength (equal to or greater than 275 MPa), and its excellent corrosion resistance. Although it is classified as commercially pure, grade 2 CP-Ti may contain up to 0.10% C, 0.30% Fe, 0.015% H, 0.03% N, 0.25% O, and 0.40% of other elements.

Two nonwhite metals in the periodic table of the elements (Table 5-1) are gold and copper, both of which are important components of cast dental alloys. Alloys with a relatively high gold content are yellow in hue and those containing significant concentrations of copper are reddish-yellow in hue. Most cast dental alloys, which are based on either cobalt, nickel, palladium, or silver, are “white” or silver in appearance, although these alloys may exhibit slight differences in hue and chroma. There is one exception, however. In the late 1970s a palladium-indium (Pd-In) alloy was introduced that exhibited a yellow hue. This color was attributed to an optical effect involving the intermetallic compound Pd-In. Argenco Y+ (Argen Corporation, San Diego, CA) is an example of such an alloy. Another alloy of this type, Castell (Jensen Dental, North Haven, CT) is marketed as a yellow type III alloy. This alloy contains 20% Au, 20% Pd, 17% In, 38% Ag, 17% In, 4% Zn, 1% Cu and other minor elements. Because it contains more than 25% by weight of noble metals, it would be classified as a noble alloy. Alloys of this type exhibit a pale yellow to gold color derived from the interaction of two white metals, palladium and indium. Alloying additions are included to improve castability, ductility, strength, hardness, tarnish resistance, and corrosion resistance. These alloys have recently gained in popularity because of the high price of gold. The tarnish resistance of the alloy is highly dependent on the composition and quality of the casting. Any porosity or contamination will result in rapid discoloration of the alloy. Once lab technicians learn how to use the alloy correctly, it can have great success clinically.

A clean, polished, metallic surface exhibits a luster that cannot be duplicated by other types of solids. Most metals emit a metallic sound (“ring”) when they are struck by another metal, although some silica compounds can also emit a similar sound. A unique characteristic of metals is that they are generally excellent thermal and electrical conductors. Compared with ceramics, polymers, and composites, metals have greater fracture toughness (K_Ic)—that is, the ability to absorb energy and inhibit crack extension under an applied tensile stress. Fracture toughness is a measure of the resistance of a material to crack propagation when a microcrack exists in its structure. For example, the fracture toughness (K_Ic) of metals ranges from 25 to 60 MPa·m^1/2 compared with a range of 0.75 to 12 MPa·m^1/2 for dental ceramics. The latter value was reported for Ce-TZP/Al₂O₃, which contained 10 mol% CeO₂ as a stabilizer. Although higher K_Ic values have been reported for similar ceramics, these values were derived from the indentation crack method, which is known to be unreliable, especially when residual stresses are present.

Generally metal alloys are stronger and more dense than nonmetallic structures. Most metals are also far more ductile and malleable than nonmetals, which are generally brittle. Three elements of dental alloys—iron, nickel, and cobalt—can be magnetic, but they can also be produced in a nonmagnetic state. Although many metals are resistant to chemical attack in air at room temperature, some metals require alloying elements to resist tarnish and corrosion in the oral environment and to optimize their passivity when they are in contact with dissimilar metals in the mouth. As mentioned above, chromium is required as an alloying element in iron-nickel-cobalt alloys to provide passivation of the alloy through the formation of a thin surface layer of chromium oxide.

Noble metals [gold (Au), iridium (Ir), osmium (Os), palladium (Pd), platinum (Pt), rhodium (Rh), and ruthenium (Ru)] are highly resistant to corrosion and oxidation and do not require alloying elements for protection in corrosive environments. Pure noble metals must be alloyed to provide sufficient resistance to deformation and fracture when they are used for cast implants, endodontic posts and cores, and crown and bridge restorations. Small additions of oxidizable elements—such as iron, tin, and indium—are added to high noble (HN) and noble (N) alloys used for metal-ceramic prostheses to promote bonding between the ceramic veneer and the metal oxide on the metal surface.

Of the 118 elements currently listed in the periodic table, about 88 (74.6%) can be classified as metals. No element having an atomic number higher than 92 occurs naturally in large quantities. It is of scientific interest that the metallic elements can be grouped according to density, ductility, melting point, and nobility. This indicates that the properties of metals are closely related to their valence electron configuration. The groups of pure metal elements can be seen in Figure 5-1, the periodic chart of the elements. All elements in the same group—for example, the alkali metals in group 1—have the same number of outer electrons, which results in similar chemical properties. The halogens within group 17 also have similar properties. For example, when halogen atoms react, they gain an electron to form negatively charged ions. Each ion has the same electron configuration as the other noble gases in the same period. Thus, the ions formed are more chemically stable than the elements from which they were formed.

The elements in groups 3 to 12 comprise the transition metals. These elements have variable valences and are sometimes used as catalysts. Nonmetals are listed in groups 14 to 16. The lanthanide series of elements (58 to 71) are called the rare earth elements. The actinide series of elements (90 to 103) participate in or are produced by nuclear reactions. Several metals of importance for dental alloys are transition elements (typically 21 to 80, although groups 89 to 112 are also included), in which the outermost electron shells are occupied before the interior shells are filled.

All metal elements have loosely bound outer electrons around the neutral atom. If two different metals form the electrodes of a galvanic cell, the more easily oxidized metal becomes the anode and supplies electrons to an external circuit. The formation of positive ions (cations) from metals in solution is a consequence of the ionic cores that are associated with metallic bonding.

The properties of the pure elements do not change abruptly from metallic to nonmetallic as viewed from the left to the right side of the periodic chart in Figure 5-1. The boundary between metals and nonmetals is indistinct, and the elements near the boundary exhibit characteristics of both metals and nonmetals. The elements boron, carbon, and silicon are often combined with metals to form commercially important engineering materials. Silicon is termed a semiconductor because its electrical conductivity is intermediate between that of a metal and that of an insulator. However, in dentistry the most common casting alloys used for dental appliances and prostheses are based on a majority of one or more of the following metallic elements: cobalt (Co), gold (Au), iron (Fe), nickel (Ni), palladium (Pd), silver (Ag), and titanium (Ti).

Metallic Bonding

Metals have the ability to conduct heat and electricity. These properties are associated with the mobility of its free valence electrons. Since the outer valence electrons can be removed easily from metal atoms, the nuclei containing the balance of the bound electrons form positively charged ionic cores. In addition to covalent bonds (primarily elements in groups II to VII) and ionic bonds, atoms in solid metals are held together primarily by metallic bonding, which is based on a “cloud,” “gas,” or “sea” of free electrons. Because the cloud of electrons is shared among many atoms, the metallic bonds are nondirectional. Also, the difference in electronegativity between atoms is small and the valence electron orbitals are spherical, so electron sharing between atoms is minimal. Hence, the valence electrons in the cloud tend to be shared among many metal atoms rather than just between adjacent atoms. In this regard the metal atoms are ionic in nature. However, compared with metallic bonding, ionic and covalent bonding is typically associated with valence electrons localized near their central atoms.

Free electrons act as conductors of both thermal energy and electricity. They transfer energy by moving readily from bands of higher energy to those of lower energy under the influence of either a thermal gradient or the potential gradient of an electrical field. Metallic bonding is also responsible for the luster of polished metals and their typical ability of undergoing significant permanent deformation (associated with the properties of ductility and malleability) under mechanical stresses above their elastic limits or yield points. These characteristics are not typical of ceramics and polymers, whose atoms and molecules are bonded by covalent and ionic mechanisms.

Pure metals, in common with other chemical elements, can be identified by their specific melting and boiling points and by their basic physical and chemical properties. Some of these properties for metals of dental interest are listed in Table 5-1. Pure metals have limited uses in dental and engineering applications, because they are too soft and some may corrode excessively. To optimize properties, most metals used in engineering and dental applications are either mixtures of two or more metallic elements or mixtures of one or more metals and nonmetallic elements. These alloys are generally prepared by fusion of the elements above their melting points. For example, a small amount of carbon is added to iron to form steel. A certain amount of chromium is added to iron, carbon, and other elements to form stainless steel, an alloy that is highly resistant to corrosion. As previously noted, chromium is also used to impart corrosion resistance to nickel or cobalt alloys, which comprise two of the major groups of base metal alloys used in dentistry. Chromium provides this corrosion resistance by forming a very thin, adherent surface oxide (Cr₂O₃) that prevents the diffusion of oxygen or other corroding species to the underlying metal. Although pure gold is also highly resistant to corrosion, copper is added to gold for many dental alloys to increase their strength and resistance to permanent deformation. In this chapter, the term metal is used to describe alloys as well as pure metals.

Classification of Alloys

Cast dental alloys can be classified according to the following five categories: (1) use (all-metal inlays, crowns and bridges, metal-ceramic prostheses, posts and cores, removable partial dentures, and implants); (2) major elements (gold-based, palladium-based, silver-based, nickel-based, cobalt-based, and titanium-based); (3) nobility (high noble, noble, and predominantly base metal); (4) three principal elements (such as Au-Pd-Ag, Pd-Ag-Sn, Ni-Cr-Be, Co-Cr-Mo, Ti-Al-V, and Fe-Ni-Cr); and (5) dominant phase system (single phase, eutectic, peritectic, and intermetallic types). To specify the type of a particular alloy for dental applications, we can use a simple classification system such as the revised classification that was proposed by the American Dental Association in 2003 (high noble, HN; titanium and titanium alloys; noble, N; and predominantly base metal, PB), or we can be much more specific by listing the concentration of the most abundant and/or the most important elements contained in the alloy. The IdentAlloy system also provides certificates for high noble alloys (HN), noble alloys (N), titanium (TI), predominantly base alloys (PB), and cobalt-base alloys (cobalt base PB).

If two metals are present during solidification of a molten metal, a binary alloy is produced; if three or four metals are present, ternary and quaternary alloys, respectively, are produced. As the number of metals increases beyond two, the alloy structure becomes more complex. For simplicity’s sake, only binary alloys are described in this chapter.

The simplest alloy is a solid solution, in which atoms of two metals are mutually completely soluble and dispersed in specific arrays within the same crystal structure, such as face-centered cubic (FCC) as shown in Figure 5-1, body-centered cubic (BCC), and hexagonal close-packed (HCP). When observed with an optical microscope, the individual grains and the microstructures overall of solid solution alloys may resemble those of pure metals. The structure will appear to be entirely homogeneous, since only one phase is formed during solidification. The temperature-versus-composition graphs for such alloys are sometimes referred to as “isomorphous phase diagrams.” Most gold alloys used in clinical dentistry are predominantly solid solutions, although they usually contain more than two metals.

When two metals are not completely soluble in each other, the solid state is a mixture of two or more phases. Important examples are the eutectic alloys and peritectic alloys, which are discussed in the following sections. Alloys can have intermediate phases, which have a range of compositions different from the solid solutions formed by the nearly pure metals. In some alloy systems, intermetallic compounds with a fixed composition can also be formed.

Effects of Alloy Elements on Properties of High Noble and Noble Metal Alloys

Pure gold lacks sufficient strength and stiffness for any dental application except as direct filling gold (also known as gold foil). Alloys with high gold contents have been used in dentistry for centuries. Gold is inert in the oral environment, extremely malleable, and does not cause excessive wear of tooth structure. Cast gold prostheses have proven to be the most durable compared with all other materials used for indirect restorations.

A high gold concentration provides a warm, esthetically attractive hue, superb tarnish and corrosion resistance, excellent ductility, minimal abrasiveness, and superb wear resistance for its alloys. Gold is useful to raise the thermal expansion coefficient (TEC) of palladium alloys. Palladium has a higher melting point than gold and a higher modulus of elasticity. The thermal expansion coefficient (TEC) of palladium is too low for it to be used with most commercial porcelains, so elements such as silver are needed to raise the TEC of palladium alloys. On the other hand, palladium is used to lower the TEC of gold-based PFM alloys.

Palladium whitens gold alloys markedly. It raises the melting range of gold alloys as well as their elastic modulus, strength, and hardness. Palladium lowers the density of gold alloys. Small amounts of palladium improve the tarnish and corrosion resistance of Au-Ag-Cu crown and bridge alloys, especially of alloys containing less than 68% Au. Gold and palladium are completely soluble in one another and they are common components in many noble PFM alloys because they offset each other’s limitations. Gallium is used primarily in Pd-based PFM alloys. Gallium strengthens these alloys and decreases their melting range.

Platinum (Pt) is used primarily in yellow-gold PFM alloys to increase the melting range, hardness, strength, and elastic modulus. Like palladium (Pd), it also decreases the TEC of gold alloys. Additions of Pt affect the properties of gold alloys to a lesser extent compared with Pd. However, Pt has less of an effect on changes in the color of alloys with high gold content.

Silver is added to Au-Ag-Cu casting alloys to offset the reddish hue contributed by Cu. However, Ag-rich Au-Ag-Cu alloys tend to have a slightly greenish hue. In Pd-based PFM alloys, Ag is used primarily to raise the thermal expansion coefficient (TEC). Silver decreases the melting range of both Pd and Au alloys. It also tends to improve the flow of casting alloys and solders. However, Ag has been reported to cause a greenish-yellow discoloration of some dental porcelains, although this phenomenon of “greening” does not appear to be a problem with current porcelains.

Copper strengthens and reddens Au-Ag-Cu crown and bridge alloys. However, it is not used in alloys with high gold contents because, like Ag, it also tends to discolor porcelain. It is added to Pd-based PFM alloys to increase their TECs. However, higher Cu contents produce dark-colored oxide layers that may adversely affect the esthetics of metal-ceramic (PFM) restorations. Copper does not seem to cause porcelain discoloration if it is alloyed primarily with Pd. Cobalt has been used as an alternative to Cu in Pd-based PFM alloys. However, like Cu, it also forms dark-colored oxides.

Zinc is added to crown and bridge alloys as an oxygen scavenger, thereby reducing gas porosity in castings. For PFM alloys, zinc can be added also to strengthen and harden the alloys and/or to increase the TEC. It also decreases the melting range.

Indium (In) is used in some Au-Ag-Cu casting alloys to improve their castability. In Au- and Pd-based alloys, it strengthens and hardens the alloys, increases their TECs, and decreases their melting temperature range. It also contributes to the formation of a bonding oxide in PFM alloys. Tin also contributes to the formation of a bonding oxide and it strengthens and hardens Au- and Pd-based PFM alloys. It also decreases the melting range of Au-based and Pd-based alloys and increases the TECs of these alloys.

Iron is used primarily to strengthen Au-Pt alloys for PFM applications. Like Sn and In, it also forms a bonding oxide.

Three noble elements that are used to refine the grain structure of alloys are iridium (Ir), rhenium (Re), and ruthenium (Ru). Grain refinement restricts the growth of grains during solidification. Smaller grains block dislocation movement from grain to grain, resulting in increased yield strength.

Solidification and Microstructure of Cast Dental Alloys

If a pure metal is melted and cooled to room temperature, a graph of its temperature-versus-time behavior during cooling would look similar to that in Figure 5-2