Сraig. Dental Materials

tin is replaced by 5% indium, whereas another product contains less than 1% palladium. Adding this small amount of palladium enhances the mechanical properties and corrosion resistance. The replacement of silver by an equal amount of copper produces a copper-tin compound (Cu,Sn).

In general, larger (>30%) or smaller (126%) quantities of tin in the alloy are detrimental to the final properties of the amalgam. The reason for this unfavorable shift in properties is generally considered related to the fact that the amount of Ag,Sn is reduced as the percentage of tin is altered beyond the indicated limits. This is the basis for the rather narrow limits of the alloy compositions of current products with acceptable properties.

Silver-tin amalgam alloys compounded to produce largely Ag,Sn react favorably with mercury to produce only slight dimensional setting changes when properly manipulated. The strength of the amalgam mass is greater from the Ag,Sn compound than from an excess of tin. In addition, the setting time is shortened by increasing silver content. Creep resistance is also superior when an alloy of Ag,Sn is used rather than one with higher tin content.

AMALGAMATION PROC

LOW-COPPER ALLOYS

The amalgam alloy is intimately mixed with liquid mercury to wet the surface of the particles so the reaction between liquid mercury and alloy can proceed at a reasonable rate. This mixing is called trituration. During this process, mercury diffuses into they phase of the alloy particles and begins to react with the silver and tin portions of the particles, forming various compounds, predominantly silver-mercury and tin-mercury compounds, which depend on the exact composition of the alloy. The silver-mercury compound is Ag2Hg, and is known as the gamma one (y,) phase, and the tin-mercury compound is Sn,,Hg and is known as the gamma two (yJ. phase. However, the silver-tin, silver-mercury, and tinmercury phases are not pure. For example,

Ag,Sn always contains some copper and occasionally small amounts of zinc. The Ag2Hg, dissolves small amounts (1% to 3%) of tin and Cu&, (q'), and, similarly, Cu6Sn, could dissolve various elements present. Therefore y, y,, and y2 are better descriptive terms of these three phases formed in dental amalgam than are the pure compounds.

While crystals of the y, and y2phases are being formed, the amalgam is relatively soft and easily condensable and carvable. As time progresses, more crystals of y, and y, are formed; the amalgam becomes harder and stronger, and is no longer condensable or carvable. The lapse of time between the end of the trituration and when the amalgam hardens and is no longer workable is called working time.

The amount of liquid mercury used to amalgamate the alloy particles is not sufficient to react with the particles completely. Therefore the set mass of amalgam contains unreacted particles. About 27% of the original Ag,Sn compound remains as unreacted particles. A simplified reaction of a low-copper amalgam alloy with mercury can be summarized in the following manner:

The dominating phase in a well-condensed, low-copper dental amalgam is the Ag2Hg, (y,) phase, which is about 54% to 56% by volume. The percentages of the y and y2phases are 27% to 35% and 11% to 13%,respectively.

HIGH-COPPER ALLOYS

The main difference between the lowand high-copper amalgam alloys is not merely the percentage of copper but the effect that the higher copper content has on the amalgam reaction. The copper in these alloys is in either the silver-copper eutectic or E (Cu,Sn) form. The proper amount of copper causes most, if not all, of the y2 phase to be eliminated within a few hours after its formation, or prevents its formation entirely. The y2 phase in amalgam is the weakest and is the most susceptible to corrosion; therefore

restorations using amalgam made with insufficient copper have a shorter period of serviceability, whereas high-copper amalgams tend to have superior physical and mechanical properties.

Reaction of Mercury in an Admixed High-Copper Amalgam Alloy During trituration, mercury diffuses into the amalgam particles and dissolves. The solubility of mercury in silver, tin, and copper differs considerably. Whereas 1 mg of mercury dissolves in copper, 10 mg can dissolve in silver and 170 mg in tin, all at the same temperature. Therefore particles composed mainly of silver and tin dissolve almost all the mercury, whereas very little mercury is dissolved by the silver-copper eutectic particles. The mercury dissolved in the silver-tin particles reacts as in low-copper alloys and forms the y, and y2 phases, leaving some silver-tin particles unreacted. In a relatively short time, however, the newly formed y2 phase (Sn,,Hg) around the silver-tin particles reacts with silvercopper particles, forming Cu6Sn,, the eta prime (q') phase of the copper-tin system, along with some of the y, phase (Ag2Hg3)around the silvercopper particles. The amalgamation reaction may be simplified as follows:

The initial reaction is the same as for lowcopper dental amalgam,

y (Ag3Sn)+Ag-Cu (eutectic) + Hg -+

Y (Ag3Sn)+ yl (Ag2Hg3)+ y2 (Sn7-8Hg)+ excess

Ag-Cu (eutectic) unreacted

and the secondary, slow solid-state reaction is

y2 (Sn,,Hg) +Ag-Cu (eutectic) -+

q' (Cu6Sn5)+y, (Ag,Hg3) +Ag-Cu (eutectic) excess

Reaction of Mercury in a Unicompositional Alloy In unicompositional alloys, too, the difference in solubility of mercury in tin, silver, and copper plays an important role. Because the solubility of mercury in tin is 170 times more than in copper and 17 times more than in silver, much more mercury dissolves and reacts

with tin than with copper or silver.Thus tin in the periphery of the particle is depleted by the formation of the y, phase, whereas the percentage of copper increases as a result of the limited reaction with mercury. As a result, particles of unicompositional alloys in the very early stages of setting are surrounded by y, and y2 phases, whereas the periphery of a unicompositional alloy becomes an alloy of silver and copper. As with the admixed type of alloy, the y, phase reacts with the silver-copper phase, forming Cu6Sn5(q? and more Ag,Hg3 (y,).

The difference in the elimination of the y, phase in an admixed and unicompositional alloy is that, in the admixed type, the y, forms around the silver-tin particles and is eliminated around the silver-copper particles. In unicompositional alloys the particles at the beginning of the reaction function like silver-tin particles of the admixed type, providing proper working time and ease of manipulation. Later, the same particles function like the silver-copper particles of the admixed type, eliminating the y2 phase.

The unicompositional particle is composed of a very fine distribution of Ag3Sn (y) and CusSn The overall simplified reaction with Hg is

Thus the reaction of mercury with either the high-copper admixed or the unicompositional alloys results in a final reaction, with Cu6Sn5(q') being produced rather than Sn,,Hg (y,).

In some high-copper alloys, there may be residual y, of less than 1%.Note that there is no definitive proof that the y, phase ever forms, even temporarily. By the time electron microprobe analyses can be performed, the reaction will have reached equilibrium, and the final reaction products of q' and yl will have already formed.

MICROSTRUCTURE O F AMALGAM

In dental applications the amount of liquid mercury used to amalgamate with the alloy particles is less than that required to complete the reaction. Thus the set amalgam mass consists of

unreacted particles surrounded by a matrix of the reaction products. The reaction is principally a surface reaction, and the matrix bonds the unreacted particles together. The initial diff~~sionand reaction of mercury and alloy are relatively rapid, and the mass changes rapidly from a plastic consistency to a hard mass. Completion of the reaction may take several days to several weeks, which is reflected by the change in mechanical properties over this time.

The microstructures of set amalgam of the low-copper, lathe-cut, and spherical types are shown in Fig 11-3.The outlines of the unreacted alloy particles (y) are visible (A). The y, and y, phases in the matrix are identified by the letters

Fig. 11-3 Microstructure of set dental amalgam, etched with iodine etch. A, Lathe-cut particles: A, unreacted original particle, y; B, y,; C, y,; D, void.

B, Spherical alloy particles: A, original particle; B, y,; C, y,; D, void.

(From Allen FC, Asgar K, Peyton FA: J Dent Res 44:1002, 1965.)

Fig. 11-4 Microstructure of high-copper admixed

(A) and spherical unicompositional(B, C) alloys. A, A is an unreacted portion of y; B is the yl phase;

C is the reaction zone around the Ag-Cu eutectic particle; D is an unreacted portion of an Ag-Cu particle.

C, A is an unreacted portion of a spherical unicompositional Ag-Sn-Cu particle; B is the yl phase; C is the reaction zone around an original particle.

B and C, respectively. Voids in each of the two samples are identified by the letter D. After the completion of the solid-state reaction in the highcopper admixed and unicompositional alloys, the microstructures show no y, phase (Fig. 11-41,

Important properties for dental amalgam include dimensional changes, compressive strength, creep, and corrosion resistance. These properties may be explained in part by the composition, microstructure, and manipulation of the amalgam.

ANSIIADA SPECIFICATION NO. 1

FOR AMALGAM ALLOY

ANSI/ADA Specification No. 1 for amalgam alloy contains requirements that help significantly control the qualities of dental amalgam. The specification lists three physical properties as a measure of amalgam quality: creep, compressive strength, and dimensional change. When a cylindrical specimen is 7 days old, a 36-MPa stress is applied in a 37' C environment. Creep is measured between 1 and 4 hours of stressing. The maximum allowable creep is 3%. The minimum allowable compressive strength 1 hour after setting, when a cylindrical specimen is compressed at a rate of 0.25 mm/minute, is 80 MPa. The dimensional change between 5 minutes and 24 hours must fall within the range of 520 pm/cm.

PHYSICAL AND MECHANICAL PROPERTIES

Compressive Strength Resistance to compression forces is the most favorable strength characteristic of amalgam. Because amalgam is strongest in compression and much weaker in tension and shear, the prepared cavity design should maximize the compression forces in service and minimize tension or shear forces. The early-compressive strengths (after 1 hour of setting) for several lowand high-copper alloys are listed in Table 11-2.The percent mercury used in preparing the samples is also listed; the lathe-cut

alloy requires the greatest amount of mercury, and the unicompositional alloy the least. Notice that amalgams are viscoelastic and the compressive strength is a function of the rate of loading. In general, the higher the rate of loading, the higher the compressive strength, although some

studies have shown that com~ressivestrength.

'

m.ly decrease at \el? high \tram rate5 .I\ A result. nhen comparing the con1pressn.e strength ot amalgam samples, it is imperative that they be tested at the same rate of loading.

When subjected to a rapid application of stress either in tension or in compression, a dental amalgam does not exhibit significant deformation or elongation and, as a result, functions as a brittle material. Therefore a sudden application of excessive forces to amalgam tends to fracture the amalgam restoration.

The high-copper unicompositional materials have the highest early-compressive strengths of more than 250 MPa at 1 hour. The compressive strength at 1 hour was lowest for lathe-cut alloy (45 MPa), followed by one of the low-copper spherical alloys (88 MPa), and then two lowcopper spherical alloys and the high-copper admixed alloy (118 to 141 MPa). These data indicate that only some of the older lathe-cut alloys would not meet the requirement for compressive strength at 1 hour of ANSI/ADA Specification No. 1. High values for early-compressive strength are an advantage for an amalgam, because they reduce the possibility of fracture by prematurely high contact stresses from the patient before the final strength is reached. The compressive strengths at 7 days and the final strengths are again highest for the high-copper unicompositional alloys, with only modest differences in the other alloys.

Tensile Strength The tensile strengths of various amalgams after 15 minutes and 7 days are listed in Table 11-3. The tensile strengths at 7 days for both non-y, and y,-containing alloys are about the same. The tensile strengths are only a fraction of their compressive strengths; therefore cavity designs should be constructed to reduce tensile stresses resulting from biting forces.

The tensile strengths at 15 minutes for the

high-copper unicompositional alloys are 75% to 175% higher than for the other alloys. However, no correlation exists between the tensile strengths at 15minutes and 7 days. The high early tensile strengths of the high-copper unicompositional alloys are important, because they resist fracture by premature biting stresses better than other amalgams.

These values are sometimes referred to as the modulus of rupture. Because amalgams are brittle materials, they can withstand little deformation during transverse strength testing. The main factors related to the high values of deformation are (1) the slow rates of load application, ( 2 ) high creep of the specific amalgam, and (3) higher temperature of testing. Thus, high copper amalgams with low creep should be supported by bases with high moduli to minimize deformation and transverse failure.

Strengthof Various Phases The relative strengths of the different amalgam phases are important. By studying the initiation and propagation of a crack in a set amalgam, the relative

strength of the different phases can be observed. Fig. 11-5 shows the propagation of a crack in a dental amalgam specimen. It is possible to view the crack initiation and propagation of an amalgam specimen under a conventional metallographical microscope with a strain viewer. The propagation of the crack can be halted and the specimen etched to identify the various phases. Results of such studies have led to the following ranking, from strongest to weakest, of the different phases of a set low-copper amalgam: Ag,Sn (y), the silver-mercury phase (y,), the tinmercury phase (y,), and the voids.

Silver-mercuryand tin-mercury act as a matrix to hold the unreacted amalgam alloy together. When relatively smaller amounts of silvermercury and tin-mercury phases form, up to a certain minimum required for bonding the unreacted particles, a set amalgam is stronger. When a higher percentage of mercury is left in the final mass, it reacts with more of the amalgam alloy, producing larger amounts of silver-mercury and tin-mercury phases and leaving relatively smaller amounts of unreacted particles. The result is a weaker mass. Therefore the effect of various manipulative conditions can be explained in this

manner. In highcopper amalgams, there is preferential crack propagation through the y,phase and around copper-containing particles.

Elastic Modulus When the elastic modulus is determined at low rates of loading, such as 0.025 to 0.125 mm/min, values in the range of 11 to 20 GPa are obtained. High-copper alloys tend to be stiffer than low-copper alloys. If the rate of loading is increased so the viscoelastic property does not significantly influence the elastic modulus, values of approximately 62 GPa have been obtained.

Creep The viscoelastic properties of amalgam are also reflected by the creep or permanent deformation under static loads. Under a continued application of force in compression, an amalgam shows a continued deformation, even after the mass has completely set. Amalgam has no tendency for work hardening or for resisting deformation more effectively after the mass has been deformed, as may be experienced with the cast gold alloys.

Values for creep are determined in an instrument similar to that shown in Fig. 11-6. A cylindrical sample is placed in the position indicated by the arrow 7 days after preparation. A static stress of 36 MPa is applied by the spring. The change in length of the sample is determined at 37 + 0.3" C by a calibrated differential trans-

former, the output of which is recorded on a chart. The change in length between 1 hour and 4 hours after placing the static stress is used to calculate the percentage creep.

Creep values for various amalgams are listed in Table 11-2. The highest value of 6.3% was found for the low-copper cut alloy, and the lowest values (0.05% to 0.09%) were determined for the high-copper unicompositional spherical alloys. The high-copper admixed alloy and one of the low-copper alloys had slightly higher creep values of 0.45% to 0.50%, and the remaining two low-copper spherical alloys had values of 1.3% to 1.5%.

Multiple regression analyses of creep data have shown that the most influential variables are volume percentage of the q' phase, grain size of the y, phase, volume percentage of the y and E phases, number of very small q' crystals (less than 1.5 pm)/mm, and weight percentage of mercury. All of these values except weight percentage of mercury correlate negatively with creep. When yl has a concentration of tin greater than 1%, creep is controlled more by the distribution of tin and tin-mercury intergranular precipitates than by grain size. After aging at oral temperature for 6 months, amalgam exhibits a decrease in creep. This decrease in creep is related to Pl formation and not to changes in either y,grain size or composition.

A direct relationship exists between y,content

Fig. 11-6 An instrument for measuring creep of amalgam. Arrow points to specimen.

and a high incidence of inarginal fracture of amalgam restorations. In addition, there is a general relationship between low static creep values and low marginal fracture in clinical service, which may be explained by the fact that the time to rupture under a constant load is inversely proportional to creep rate. Amalgams having higher compressive strengths at 7 days, determined at slow rates of loading, have demonstrated better marginal integrity. In general, amalgams having low values of creep and high 7-day compressive strength at slow rates of loading have better clinical performance.

Note that the low creep values of high-copper amalgams increase the brittleness of the amalgam

and decrease the relief of stresses at contact areas under load. As a result, a high-modulus base under a high-copper amalgam is essential to minimize deformation and the development of tensile stresses at the amalgam-cement base interface.

Dimensional Change The dimensional change during the setting of aiwalgatn is one of its most characteristic properties, Modern amalgams mixed with mechanical amalgamators usually have negative dimensional changes. The initial contraction after a short time (the first 20 minutes) is believed to be associated with the solution of mercury in the alloy particles. After this period an expansion occurs, although the total change remains negative, which is believed to be a result of the reaction of mercury with silver and tin and the formation of the intermetallic compounds. The dimensions become nearly constant after 6 to 8 hours, and thus the values after 24 hours are final values. The only exception to this statement is the excessive delayed dimensional change resulting from contamination of a zinc-containing alloy with water during trituration or condensation.

The dimensional change may be determined with an instrument such as the one shown in Fig. 11-7. The amalgam specimens identified by the arrows are placed in position 5 minutes after setting, and the probe is placed on top of them. The probe is mechanically attached to a differential transformer, and the electrical output is used to determine expansion or contraction. The change in length can be determined continuously, although ANSI/ADA Specification No. 1 requires only the value at 24 hours.

The dimensional changes in micrometers per centimeter for the various alloys are listed in Table 11-3. The largest dimensional change of -19.7 pm/cm occurred with the low-copper, lathe-cut alloy, and the lowest change of -1.9 pm/cm was for the high-copper admixed alloy. The remainder of the alloys had values ranging from -8.8 to -14.8 pm/cm. All the amalgams meet the requirements of ANSI/ADA Specification No. 1 o f f 20 pmlcm. Notice that the ranking of the dimensional change does not correlate

Fig. 11-7An instrument for measuring dimensional change of amalgam. Arrows point to amalgam s~ecimens.

with any of the other mechanical properties. The dimensional change is susceptible to influence from various manipulative factors, especially final mercury content. Higher mercury content results in less shrinkage but also in lower mechanical strength.

Some question remains concerning the significance of dimensional change with respect to clinical success. The belief was that if amalgam expanded during hardening, leakage around the margins of restorations would be eliminated. With current alloys and proper techniques of trituration, however, most alloys show some shrinkage. Evidently the detrimental effect of shrinkage occurs when the amalgam mass shrinks more than 50 ym. ANSVADA Specification No. 1 for dental amalgam allows up to

20 ym/cm shrinkage, and no correlation of clinical success with the magnitude of the shrinkage determined in the laboratory has been shown. Furthermore, the expansion of an amalgam mass may seem to have a beneficial effect for onesurface restorations such as Class 1 and 5, but offers hardly any advantage when Class 2 and 6 restorations are considered. The expanded amalgam around the cervical areas of Class 2 and 6 restorations would have to pull away from the preparation, and this may have as undesirable an effect as the shrinking of amalgams for onesurface restorations.

Corrosion In general, corrosion is the progressive destruction of a metal by chemical or electrochemical reaction with its environment. Excessive corrosion can lead to increased porosity, reduced marginal integrity, loss of strength, and the release of metallic products into the oral environment.

The following compounds have been identified on dental amalgams in patients: SnO, SnO,, Sn4(OH)&l2, Cu,O, CuCl, . 3Cu(OHj2, CuC1, CuSCN, and AgSCN.

Because of their different chemical compositions, the different phases of an amalgam have different corrosion potentials. Electrochemical measurements on pure phases have shown that the Ag,Hg3 (y,) phase has the highest corrosion resistance, followed by Ag3Sn (y), Ag3Cu2,Cu3Sn (el, Cu6Sn5(q?, and Sn,.,Hg (y,). However, the order of corrosion resistance assigned is true only if these phases are pure and they are not in the pure state in dental amalgam.

The presence of small amounts of tin, silver, and copper that may dissolve in various amalgam phases has a great influence on their corrosion resistance. The yl phase has a composition close to Ag,Hg3 with 1% to 3% of dissolved tin. The higher the tin concentration of Ag,Hg3 (y,), the lower its corrosion resistance. In general, the tin content of the y, phase is higher for low-copper alloys than for high-copper alloys. The presence of a relatively high percentage of tin in lowcopper alloys reduces the corrosion resistance of their y, phase so it is lower than their y phase. This is not true for high-copper alloys. The av-

erage depth of corrosion for most amalgam alloys is 100 to 500 pm.

In the low-copper amalgam system, the most corrodible phase is the Sn,,Hg or y, phase. Although a relatively small portion (11%to 13%) of the amalgam mass consists of the y, phase, in time and in an oral environment the structure of such an amalgam will contain a higher percentage of corroded phase. On the other hand, neither the y nor the y, phase is corroded as easily. Studies have shown that corrosion of the y, phase occurs throughout the restoration, because it is a network structure. Corrosion results in the formation of tin oxychloride from the tin in the y, and also liberates mercury, as shown in the following equation:

The reaction of the liberated mercury with unreacted y can produce additional y, and y,. It is proposed that the dissolution of the tin oxide or tin chloride and the production of additional y, and y, result in porosity and lower strength.

The high-copper admixed and unicompositional alloys do not have any y, phase in the final set mass. The Cu6Snj or q f phase formed with high-copper alloys is not an interconnected phase such as the y, phase, and it has better corrosion resistance. However, y' is the least corrosion-resistant phase in high-copper amalgams; and a corrosion product, CuC1,. 3Cu(OH),, has been associated with storage of amalgams in synthetic saliva, as shown below.

Phosphate buffer solutions inhibit the corrosion process; thus saliva may provide some protection of dental amalgams from corrosion.

A study of amalgams that had been in service for 2 to 25 years revealed that the bulk elemental compositions were similar to newly prepared amalgams, except for the presence of a small amount of chloride and other contaminants. The compositions of the phases were also similar to

new amalgams, except for internal amalgamation of the y particles. The distribution of phases in the clinically aged amalgams, however, differed from that of new amalgams. The low-copper amalgams had decreased amounts of y, y,, and y, and increased p, and tin-chloride. High-copper admixed amalgams had decreased y, , increased Dl, and enlarged reaction rings of y, and 7 ' . There was also evidence of a conversion of y, to p, and

Y2 to rlf.

Note that the processes of corrosion and wear are frequently coupled and that wear can lower the corrosion potential and increase the corrosion rate by an order of magnitude.

Fig. 11-8compares an amalgam restoration on the distal portion of a tooth prepared from a low-copper spherical alloy with one on the mesial portion prepared from high-copper admixed alloy. The restorations have been in service for 3 years, and the higher marginal fracture, presumably resulting from the corrosion of the y, phase of the low-copper amalgam, is readily apparent.

Surface tarnish of low-copper amalgams is more associated with y than y,, whereas in highcopper amalgams surface tarnish is related to the copper-rich phases, y' and silver-copper eutectic.

PROPERTIES O F MERCURY

ANSI/ADA Specification No. 6 for dental mercury requires that mercury have a clean reflecting surface that is free from surface film when agitated in air. It should have no visible evidence of surface contamination and contain less than 0.02% nonvolatile residue. Mercury that complies with the requirements of the United States Pharmacopoeia also meets requirements for purity in ANSI/ADA Specification No. 6. Mercury amalgamates with small amounts of many metals and is contaminated by sulfur gases in the atmosphere, which combine with mercury to form sulfides. Small quantities of these foreign materials in mercury destroy its bright, mirror-like surface and can be readily detected by visual inspection.

Mercury, which has a freezing point of