Chemical Composition and Microstructure
The general classification of dental amalgam alloys is presented in Fig 12-1 and Table 12-1. Contemporary amalgams are mainly classified as high-copper amalgams and have existed since the 1960s. In an attempt to circumvent the problems with mercury, a radically new composition was introduced, replacing mercury with a galliumindium-tin liquid. To understand the significance of these materials and how the solid-state reactions in amalgams has made them superior clinical materials, it is first necessary to look at the so-called traditional amalgams.
|Traditional||Lathe cut||Aristaloy (Goldsmith & Revere)|
|High copper||Lathe cut (single composition)||Epoque 80|
|High copper||Spheric (single composition)||Tytin (Kerr/Sybron)|
|High copper||Admixed (traditional + Ag-Cu eutectic)||Dispersalloy (Dentsply Caulk)|
Traditional amalgam alloys
Until the 1960s, the chemical composition and microstructure of available amalgam alloys were essentially the same as those of the most successful systems investigated by G.V. Black.1 Traditional alloys were delivered to the dentist as filings, which were lathe cut from a cast ingot. Milling and sifting produced the ultimate particlesize distribution, as well as the final form of the amalgam alloy particles. Figure 12-2 illustrates a typical traditional lathe-cut alloy.
The commercial alloy evolved into a blend of different particle sizes to optimize packaging efficiency. The particles in a commercial lathe-cut alloy might range in length from 60 to 120 µm, in width from 10 to 70 µm, and in thickness from 10 to 35 µm. The particle size has become still smaller (less than 30 µm) due to the introduction of spheric alloys. By weight, the traditional alloys contain 66% to 73% of silver and 25% to 29% of tin, with about 6% of copper, up to 2% of zinc, and up to 3% of mercury.
The structures of these traditional alloys are phase mixtures of the gamma (γ) phase of the silver-tin system (Ag3Sn) and the epsilon (η′) phase of the copper-tin system (Cu3Sn). It has been shown that Ag3Sn produces the best physical properties when alloys of the silver-tin system react with mercury.2 Some of these traditional alloys are still available, but they represent a minor component of the overall amalgam market.
Spheric alloys were introduced on the market during the 1960s. These alloys lowered the necessary mercury-alloy ratios and dramatically reduced condensation pressures. Their particle shape is created by means of an atomizing process whereby a spray of tiny drops is allowed to solidify in an inert gaseous (ie, argon) or liquid (ie, water) environment. Although all alloys produced in this way are classified as spheric, their particle shape might be irregular (Fig 12-3). The maximum particle size in a spheric alloy powder is generally 40 to 50 µm or less, although there usually is a particle size distribution.
High-copper blended and single-composition amalgam alloys
During the late 1960s, alloys with a significantly different chemical composition were introduced on the market. All of these alloys could be characterized by their higher copper content. A list of current high-copper alloy products is presented in Table 12-1.
The first alloy of this type (Dispersalloy)3 was a mechanical mixture of a traditional lathe-cut alloy with a spheric alloy in a ratio of 2:1 (Fig 12-4). The chemical composition of the spheric particle was 72% silver and 28% copper by weight, which corresponds to the eutectic composition of the silver-copper system. The overall composition of this alloy contained approximately 13% copper by weight, more than twice the maximum amount permitted in the American Dental Association’s (ADA’s) specifications for amalgam alloy at that time. Amalgams made from this alloy, however, were clinically superior to traditional amalgams with respect to marginal integrity,4 and consequently, other manufacturers developed similar compositions, some with a copper content greater than that found in traditional amalgam. At present, the copper content varies up to approximately 30% by weight in some commercial amalgam alloys (Table 12-2).
The structures of several high-copper alloys are similar to that of Dispersalloy. They can be classified as blended alloys in which the traditional and high-copper phases are mechanically blended. Other alloys are produced by melting together all components of a high-copper system and creating a single-composition spheric or lathe-cut alloy, rather than a mechanical mixture of two distinct powders. Depending on the number of components involved, these systems are also referred to as ternary, quaternary, or single-composition alloys.
In an effort to improve clinical handling properties, some amalgam alloy manufacturers supply admixed alloys. The chemical compositions and physical forms of the basic powders (lathe cut or spheric) in these alloys are varied. This system further differs from those using Dispersalloy in that both blended components represent copper-enriched alloys. It is important to stress that all of these copper-enriched alloys contain more than 10% copper by weight in the form of either the silver-copper eutectic or the copper-tin system. Several typical classifications and compositions of high-copper systems are presented in Table 12-2. The dynamics of the current marketplace preclude a comprehensive listing, as a great number of alloys appear and disappear worldwide in response to local demand.
TL = traditional lathe cut; TS = traditional spheric; HCS = high-copper spheric; HCAd = high-copper admixed; HCL = high-copper lathe cut; GA = alloy for gallium amalgam.
Although amalgam alloys containing many other metals have been proposed or investigated on an experimental basis, at present only palladium, selenium, and indium have been used as commercial additives. Because of economic reasons, the alloys intended for mercury amalgamation and containing palladium feature a relatively low (less than 1%) concentration. Selenium has been added in an attempt to improve biocompatibility,5 and indium has been admixed in large concentrations (10% by weight) in metallic form to high-copper amalgam to reduce the mercury vapor released during mastication.6,7
The melting temperature of gallium can be suppressed below room temperature with the addition of appropriate amounts of indium and tin, resulting in gallium alloys. This liquid can then be triturated with a silver-tin-copper alloy powder (spheric) in the same fashion as dental amalgam. Significant amounts of palladium are added to the alloy powder in current commercial compositions to improve corrosion properties. A current composition marketed in Japan, known as Gallium alloy GF (Tokuriki Honten), comes in powder form and contains the following elements (by weight): silver, 50%; tin, 25.7%; copper, 15%; palladium, 9%; and traces, 0.3%. It is also available as a liquid containing gallium, 65%; indium, 18.95%; tin, 16%; and traces, 0.5%.
Setting Reactions and Microstructure
The amalgamation reaction of the traditional alloy with mercury (known as trituration) as well as its microstructure after setting are described on the basis of a reaction of Ag3Sn (γ) with mercury. Copper and/or zinc are not usually taken into account, but their presence has important effects. During hardening, new reaction products with mercury are formed at the cost of the original alloy particles. The main reaction products formed are the gamma 1 (γ1) (silver-mercury) and gamma 2 (γ2) (tin-mercury) phases. Formation of a network is completed before all the original reactant is consumed. This amalgamation reaction can be symbolized as follows:
Ag3Sn + Hg → Ag2Hg3 + Sn7Hg + Ag3Sn
γ + Hg → γ1 + γ2 + γ (remnant)
After the amalgamation reaction is complete, the remnants of the high-melting-point silver-tin particles are embedded in a matrix of reaction products with mercury (Fig 12-5). In most traditional amalgams, the γ1 and γ2 phases form a continuous network. The formation of such an interconnecting structure is extremely important, because the γ2 phase is prone to corrosion and should be considered the weak link in many traditional dental amalgams. The copper contained in the original alloy will react with tin during trituration to form the eta prime (η′) phase Cu6Sn5. The presence of copper has long been associated with improving the physical properties of amalgam, particularly its flow or deformation under static load. This effect is magnified in high-copper amalgams. The presence of zinc appears to extend the working time and, hence, the plasticity of the traditional amalgam.
All high-copper amalgams are characterized by the γ2 phase being either absent or substantially reduced, because tin preferentially reacts with copper rather than with mercury, preventing the formation of the tin-mercury reaction product. During amalgamation of blended alloys, Cu6Sn5 is created from copper and tin. The same process occurs in traditional amalgams, but to a lesser extent, because the copper concentration is less. Because most of the reactive copper is present in the silver-copper spheres, the Cu6Sn5 phase is formed at the surface of these particles, creating a reaction zone that is easily identified in the microstructure (Fig 12-6). The mechanism to form Cu6Sn5 can be described by:
6Cu + 5Sn → Cu6Sn5
2Cu3Sn + 3Sn?→ Cu6Sn5
because the source of copper is the η′ phase in single-composition alloys. It is obvious that in dental amalgam alloys where equivalent amounts of copper and Cu3Sn are present, both reactions may be equally important in the formation of Cu6Sn5. It should be stressed that some high-copper amalgams may initially contain the γ2 phase if the mercury content is higher than a certain critical percentage. In these amalgams, the elimination of γ2 may occur over a substantial period because the reactions are diffusion controlled. In general, in lathe-cut and blended alloys, the mercury-alloy ratios are greater than or equal to 1.0, whereas in spheric alloy systems, the mercury-alloy ratios are less than 1.0 and may be as low as 0.7. As mentioned earlier, the γ2 phase is considered the weak link in a traditional low-copper amalgam. However, because the γ2 phase is absent in high-copper amalgams, attention should be focused on the least resistant phase in the multiphase structure associated with these amalgams. Preferential corrosion of the Cu6Sn5 phase reportedly has been shown to be significant both in vivo8 and in vitro.9
Evidence has been presented for the presence of an additional tin-mercury phase, delta 2 (δ2),10 at the grain boundaries of the resulting γ1 network. This phase results from the lower tin concentration in the last of the mercury to solidify. Its location at grain boundaries makes this phase important for determining the structure-sensitive properties of amalgam. Since copper and tin will preferentially combine in dental amalgam, the higher copper concentrations will also reduce the formation of δ2.
As mentioned earlier, admixing of indium has lowered the amount of mercury vapor released from amalgam. This phenomenon has also been verified recently for amalgams prepared from a mercury-indium liquid in which the indium concentration was as high as 30%.11 It is possible that through solid solution, indium may increase the stability of the γ1 phase (Sarkar NK, personal communication, 1994).
The structure of gallium amalgams has been interpreted in terms of a reaction zone of CuGa2 and PdGa5 surrounding the unreacted alloy particles, which are held together by a matrix of Ag9In4 containing islands of Ag9Ga3 and beta (β) tin. The structure of set gallium amalgam (GF alloy) is shown in Fig 12-7.
The physical properties of dental amalgam are usually compared with those specified in the American National Standards Institute/American Dental Association (ANSI/ ADA) specifications for dental amalgam. These properties are (1) 1-hour compressive strength, (2) creep, or resistance to static load, and (3) dimensional change. The ANSI/ADA limits are (1) 1-hour compressive strength of at least 80 MPa (11,000 psi), (2) dental creep of no more than 3%, and (3) dimensional change of ± 20 µm/cm. The corresponding properties of several commercial alloys are presented in Table 12-3. The rationale for these properties is that high early strength is important to withstand dental finishing procedures and occlusal stresses. Low creep is desirable for maintaining marginal integrity, and dimensional change must be controlled to prevent excessive marginal leakage.
Continual reaction occurs as a function of time. The 24-hour compressive strengths shown in Table 12-3 are adequate for most occlusal loadings. If the bite force is assumed to be 750 N (170 lb) and the contact area 2 mm2, the compressive stress offered to the amalgam would be on the order of 380 MPa (55,000 psi). As can be seen in Table 12-3, this value is similar to the compressive strengths of most set amalgams. Little additional hardening occurs beyond 24 hours, although additional phase changes are possible.
TL = traditional lathe cut; TS = traditional spheric; HCS = high-copper spheric; HCL = high-copper lathe cut; HCB = high-copper blend; GA = alloy for gallium amalgam.
The amount of residual mercury is very important in the determination of mechanical properties. In general, the compressive strength will decrease 1% with each 1% increase in mercury above 60%. Low mercury-alloy ratios after condensation are therefore desired. In addition to the effects of residual mercury, compressive strength will also decrease 1% with each 1% of porosity. Adequate condensation of amalgam is, therefore, mandatory in achieving maximum strength.
It should also be emphasized that amalgam, in both traditional and high-copper compositions, is a brittle material. Generally, the tensile strength of a brittle material is much less than its corresponding value in compression. For amalgam, the tensile strength values are about one-seventh of the compressive strength values (see Table 12-3). This means tensile failure is much more likely to occur than compressive failure. Tensile failure is particularly apt to occur in the margins where the amalgam may be unsupported or the mercury concentration is higher due to the condensation process. Obviously, the last bit to condense will have the higher mercury concentration because mercury expression occurs as the amalgam is packed. Because of the higher mercury concentration at the margin, this area may contain greater amounts of the δ2 phase, contributing to weakness in this region. Tensile failure may also occur at the isthmus of mesioocclusodistal restorations with too little bulk at the step.
Creep and flow are both deformations produced by constant load. The creep of amalgam is important because amalgam at oral temperatures is at 0.9 Tm, where Tm is the melting temperature. At these temperatures, atomic diffusion occurs easily, and deformation under static load is possible. As seen in Table 12-3, the creep of high-copper amalgam is at least an order of magnitude lower than the upper limit of 3% for traditional amalgams. This lower creep has been associated with the presence of Cu6Sn5 in the γ1 network and the decreased amount of available tin.12 The lower creep of high-copper amalgams may also be related to the absence of the δ2 phase. Furthermore, the lower creep of high-copper amalgam has been suggested as a possible reason for its demonstrably better marginal integrity.
The wear of amalgams is approximately the same magnitude as that of tooth enamel. The wear resistance of amalgams exceeds that of most posterior composite restorative materials; therefore, amalgams are much more likely than most composite restorative materials to maintain occlusal contacts.
The physical properties of gallium amalgam are intermediate compared with traditional and high-copper amalgams (see Table 12-3).