Journal of Oral Rehabilitation, 1979, Volume 6, pages 1-8
Copper in dental amalgams
N . K. S A R K A R
School of Dentistry, Louisiana State University, New Orleans
Summary
The relationship between microstructure and physical and electro-chemical properties of copper-containing dental amalgams has been discussed in an attempt to understand the role of copper in currently used dental amalgams. Introduction
Copper has been traditionally added to silver-tin dental alloys to increase the compressive strength and to reduce the fiow and setting contraction of resultant amalgams (Phillips, 1973). Unless a small amount of copper is added, uniform comminution of silver-tin alloy becomes difficult (Phillips, 1973). It has been reported that the beneficial role of copper is restricted to the solubility range of copper in the silver-tin alloy, beyond which adverse effects such as strength loss, uncontrollable expansion and increase in flow are noted (Phillips, 1973). However, alloys have been produced in which 15 to 20 wt % copper has been included with the hope that such an alloy would develop as antiseptic quality in the restoration due to the presence of copper and copper oxides (Peyton, 1968). These alloys have not proved popular because of a greater tendency to tarnish. With the reported clinical success of Dispersalloy (Innes & Youdelis, 1963; Duperon, Neville & Kasloff, 1971; Mahler et al., 1970; Mathewson, Retzlaff & Porter, 1974; Mahler, Terkla & Van Eysden, 1973) a high copper containing alloy (12-0 wt%), there has been an increasing interest in high copper silver-tin amalgams in recent times. Attempts have been made to characterize the metallurgical changes associated with the introduction of copper in amalgam alloys. Attempts have also been made to study the effects of these changes on various properties and clinical performance. Despite these efforts, the role of copper in dental amalgams is not clearly defined. The present paper is a collection, collation and interpretation of the available information on copper in dental amalgams in an attempt to understand its role in currently used amalgams. The microstructure of copper-containing silver-tin alloys
Metallurgically, the principal component of conventional dental amalgam alloys that comply with American Dental Association (ADA) Specification No. 1 is a silver-tin (Ag3Sn) phase referred to as the y-phase (Gayler, 1937a). Copper is allowed up to Correspondence: Dr N. K. Sarkar, Department of Biomaterials, School of Dentistry, Louisiana State University, New Orleans, U.S.A.
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6-0 wt% in the alloy (American Dental Association, 1974). Gayler (1937b) assumed that copper atoms replace silver atoms up to 4 to 5 atomic per cent in such a way that the ideal stoichiometric formula of Ag3Sn (y) is maintained. Later it was observed that the addition of copper leads to the formation of a distinct separate phase(s). From metallographic observation, this phase was hypothesized to be Cu3Sn (e) by Crowell (1954). From microprobe analysis, Johnson, Asgar & Peyton (1969) identified the copper-tin phase as the CueSns {rj^) of the Cu-Sn system. In a more recent study, Mahler, Ady & Van Eysden (1975) identified the copper-tin intermetallic to be Cu3Sn (e). How do we explain this disagreement between various authors as to the nature of the Cu-Sn intermetallics ? The microstructure of an alloy under equilibrium conditions can be determined from the phase diagram (Lyman, 1973). The composition in such diagrams defines specifically the number of phases present at a certain temperature. Since the commercial dental amalgam alloys can have a wide composition range even within ADA specification limits (American Dental Association, 1974) and are subject to varying conditions of heat treatment (which is seldom the equilibrium condition), it is quite likely that these alloys can have myriad microstructures containing Ag3Sn (y), Ag5Sn (/S), Cu3Sn (e), CueSns {rj^), and Sn (of the eutectic). This has been demonstrated by Jensen (1972) in his X-ray diffraction work on the ternary silver-tincopper system (Sn—25 to 30 wt%, Cu—0 to 4 wt% and Ag—Bal), under equilibrium and nonequilibrium conditions. Further work in this area confirms the above general conclusion (Vrijhoef & Driessens, 1974a). From these studies, it is apparent that copper in conventional dental amalgam alloys may occur as distinct phases such as Cu3Sn (e), CueSns {-q^); or it maybe associated with Ag3Sn (y) Ag Sn (^), or Sn in the form of solid solution. The copper concentration in Ag3Sn may vary between 1-0 and 3-1 % depending on the amount of copper in the alloy (Jensen, 1972; Gubbels, Vrijhoef & Driessens, 1974; Mahler et al, 1975). The ;8-Ag5Sn contains less copper in solid solution that is present in the y-phase (Gubbels et al, 1974). The amount of copper in solid solution with Sn of the Sn-rich eutectic is less than 1-0 wt% (Lyman, 1973). Amalgamation reaction of copper-containing silver-tin alloys What happens to copper when the alloy is amalgamated? Two Cu-Sn intermetallics, Cu6Sn5 and CusSn, have been identified in the set amalgam microstructure (Johnson et al, 1969; Mahler et al, 1975; Vrijhoef & Driessens, 1974a; Jensen & Andersen, 1972; Sarkar & Greener, 1972). These Cu-Sn intermetallics have their origin in the alloy as well as in the amalgamation reaction. It has been mentioned before that both Cu3Sn and CueSns may be present in the original alloy. It has been observed by Johnson et al (1969) that CueSns in the alloy remains practically unattacked by mercury during amalgamation and shows up in the set amalgam microstructure. Alloys containing copper have been shown to precipitate CueSns during setting (Vrijhoef & Driessens, 1973). This precipitation involves the following reaction: when the alloy containing copper is triturated with mercury, copper associated Ag3Sn, Ag5Sn,Sn (eutectic) and Cu3Sn is released. When setting starts as a result of supersaturation and nucleation and growth, a part of copper (available in Hg) is incorporated in Ag2Hg3, a part in Sn7Hg and the rest precipitates as CueSns (Mahler et al, 1975). The formation of Cu-Hg (Allan, Asgar & Peyton, 1956), though unlikely, cannot be discounted and may occur at isolated areas away from tin. Thus from a metallurgical standpoint, the substitution of copper for silver may reduce the amount of tin-mercury (yz) by the formation
Copper in dental amalgams
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Manipulative and physical properties of copper-containing conventional dental amalgams The effect of copper on the manipulative and other physical and chemical properties can be rationalized in the following way: the reported uncontrollable expansion and the increase in amalgamation rate may be related to the presence of more of Ag5Sn-(^) phase in the alloy resulting from the substitution of copper for silver (Gruber, Skinner & Greener, 1967; Gayler, 1937b). Again the physical and chemical properties of a multiphase structure are governed by the properties of the individual phases. It is expected therefore that the nature and amount of Cu-Sn phase will contribute to the properties of of the composite amalgam. The brittle nature of Cu-Sn intermetallics (Westbrook, 1967) perhaps explains the claim that alloying with copper makes comminution easier. Since these intermetallics are also characterized by high compressive strength, the presence of Cu-Sn results in the strengthening of the amalgam. However, besides this direct strengthening effect, the presence of copper may have additional strengthening effects through solid solution strengthening of y, ^, yi, and y^. The observed reduction in flow and creep can be explained by the reduction of yi through the substitution of Copper for silver. The silver-mercury phase has been held responsible for creep (Vrijhoef & Driessens, 1974b; Espevic & Sorensen, 1975). Again, the presence of strong, hard Cu-Sn intermetallics dispersed in the matrix may also reduce the flow and creep by providing obstacles to the movement of dislocations. Furthermore, flow as is measured under existing ADA Specification No. 1 is more of a measure of the setting rate, and reduction in flow may be related to the increase in reaction kinetics of amalgamation of high copper amalgams. It should be mentioned here that the various properties discussed above are dependent on many other factors (e.g heat treatment, surface treatment, particle size, etc.) besides copper, and these other variables are assumed constant in the context ofthe above discussion. Effect of copper on tarnish resistance of amalgams The main problem with the high copper containing amalgams in the past has been the lack of optimum tarnish resistance (Peyton, 1968). How can it be related to the copper content of these alloys? Unfortunately, nothing is known about the microstructure of these amalgams used in the past. The available information indicates that the copper concentration of these alloys varied from 15-0 to 20-0% whereas the tin content was maintained between 25-0 and 30-0%. An alloy within the above composition limits is expected to contain a combination of the same phases present in commercial conventional alloys, viz., y, |8, e, rj^ and Sn. However, the presence or the relative amount of various phases will be dependent on the composition and heat treatment. With the amount of tin constant and with the substitution of copper for silver, the amount of y will decrease, and the amount of Cu-Sn (Cu6Sn5 + Cu3Sn) will increase. Again the increased amount of copper may bring the phase field to a point where ^8 becomes predominant over y phase. On the basis of the information available today, it may safely be assumed that amalgamation of such an alloy results in (1) a matrix containing mainly Ag2Hg3 (yi), (2) possible elimination or reduction in the amount of SnvHg (y2) phase, and (3) the formation of CueSns. Besides these amalgamation reaction products, a combination of several partially reacted phases, viz., y, ^8, e, 77I from the original alloy will be present in the microstructure. On the basis of this anticipated microstructure, the reported decreased tarnish resistance of high copper silver-tin amalgams of the past may be explained as follows: in a saline aqueous environment, general dulling or tarnishing may result from the formation of tin oxides on yi, yz.
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rp-, whereas y and e remain unaffected (Sarkar & Greener, 1975b). In a sulphide containing environment, yi, 72, and Cu-Sn (e + r]^) phases probably remain unaffected whereas mild attack may take place on the y-phase (Sarkar, Fuys & Stanford, 1976). Furthermore, j8-phase is more prone to sulphide attack than y-phase. Since the high copper silver-tin amalgam may contain more ^ than that found in conventional amalgams, it is expected that these amalgams will be more susceptible to tarnishing or dulling than the conventional amalgams. Again the presence of Cu-Hg, if any, will also impair the sulphide tarnish resistance (Sarkar et al, 1976). It should be emphasized, that sulphide tarnish is not a universal phenomenon since the sulphur concentration in the oral environment varies considerably from individual to individual. However, tarnishing or dulling due to the formation of Sn-oxides is universal even though y2 is not attacked by sulphide. Nonetheless, the presence of y2 is undesirable because of its detrimental effects on the margins (Jorgensen, 1965). The addition of copper, though, impairs the tarnish resistance but eliminates or reduces the tin-mercury (y2) phase. This has a positive contribution in providing superior marginal integrity and improved clinical performance as will be evidenced from an examination of two high copper amalgams which have created an enormous research interest in recent years. High copper silver-tin amalgams Admixed copper amalgam. One of these amalgams is the admixed copper amalgam. These amalgams are fabricated by mixing commercial copper-amalgams with conventional silver-tin amalgams. Details of fabrication of this amalgam have been reported elsewhere (Sarkar & Greener, 1972). The ratio of copper amalgam to silver-tin amalgam varies from practitioner to practitioner and these amalgam restorations have been found to possess improved marginal integrity over conventional amalgams (Granath & Hackansson-Holma, 1961; Giroux, 1970; Sabott, Cooley & Greener, 1975). Laboratory studies have shown that these amalgams have low fiow and creep (Acharya et al, 1973; Osborne et al, 1974). The diametral tensile strength values of these amalgams are, however, lower than those of conventional amalgams. The setting rate is quite fast as has been evidenced from the early strength data. X-ray diffraction data on one admixed copper amalgam revealed the absence of y2 phase and the presence of y, yi, and Cu-Sn phases (Sarkar & Greener, 1972). Isolated areas containing copper have also been identified in the microstructure (Marshall, Finkelstein & Greener, 1975). During trituration, copper in copper-amalgam (Cu-Hg) reacts with tin available from the solution of silver-tin alloy in mercury. However, the commercial copperamalgams contain as high as 70% mercury (Skinner & Phillips, 1960) and this mercury is available to the silver-tin alloy which is already mixed with mercury. This additional mercury, it is expected, may lead to an increase in the yi content with a corresponding reduction in the amount of the silver-tin (y) phase. The silver-mercury (yi) phase has been held responsible for the creep of dental amalgams (Espevic & Sorensen, 1975). Thus there is an apparent paradox in the low creep of these amalgams in view of their high yi content. This can be explained the following ways: (1) hard strong copper-tin intermetallics dispersed throughout the matrix (yi), change the creep deformation characteristics of the matrix phase, (2) as has been mentioned before, the nature, as well as the amount of the individual phases determine the ultimate physical properties of the multiphasic structure. The adverse effect of the increase in yi content is perhaps balanced by an increase in the Cu-Sn phase content. The Cu-Sn intermetallics prob-
Copper in dental amalgams
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ably act the same way as strong silver-tin intermetallics in reducing the creep by providing barriers to the motion of dislocation. However, it appears that the loss in strength produced by an increase in the yi phase and a reduction in the y phase could not be compensated by an increase in the Cu-Sn phase. The reduction in tensile strength is a manifestation of the presence of Cu-Sn phases which are inherently brittle. Because ofthe absence ofthe y^ phase, the chloride-corrosion behaviour of this amalgam is improved (Sarkar & Greener, 1975c) which is corroborated by superior marginal integrity of clinical restorations. However, these amalgams are prone to tarnish which has been demonstrated in the laboratory investigation (Greener & Sarkar, 1971) as well as in chnical situations (Sabott et al., 1975). Dispersalloy. Dispersalloy is a mschanical mixture of a conventional silver-tin alloy and silver-copper eutectic particles (Innes & Youdelis, 1963). The amalgamation reaction of Dispersalloy is very similar to the admixed copper amalgam described above. However, in the case of Dispersalloy, the reaction involves one additional step as shown in Table 1. This additional step involves the amalgamation of ^-Cu in Ag-Cu eutectic during trituration to form Cu-Hg. The rest of the reaction involves the precipitation of Cu-Sn intermetallics and the formation of Ag2Hg3 phases. Whether
Table 1. Amalgamation reaction of admixed copper amalgam and dispersalloy Admixed copper amalgam AgsSn (y) + Cu-Hg+Hg-> AgaHgs (yi) + Cu-Sn + unreacted (AgsSn + CuHg) (SnvHg++CuHg) Dispersalloy Stage I: Ag3Sn(y) +Ag-Cu + Hg->Ag2Hg3(yi) + Sn7Hg(y2) + Cu-Hg+unreacted (AgaSn + Ag-Cu) Stage n. SnvHg (y2) + Cu-Hg-»Cu-Sn Final reaction Ag3Sn (y) + Ag-Cu + Hg->-Ag2Hg3 (yi) + Cu-Sn + unreacted (Ag3Sn+Ag-Cu)
this amalgam contains any y2 or not depends on the surface oxidation of the Ag-Cu eutectic (Sarkar & Greener, 1975b) the degree of which determines the amount of copper that can go into solution with mercury. It is only in solution with mercury that copper can effectively unite with tin to prevent the formation of the tin-mercury phase. The surface oxidation of /S-Cu also perhaps determines the stoichiometric nature of the copper-tin intermetallics formed as well as the relative amount of Cu3Sn and CueSns. The effect of ageing of Dispersalloy on the corrosion behaviour of its amalgams has been studied by Sarkar & Greener (1975 a, b, c). It has been demonstrated that the chloride corrosion behaviour of this amalgam is impaired by the storage induced oxidation of the silver-copper eutectic. Recently it has been shown that oxidation of Ag-Cu eutectic in Dispersall'oy results in amalgams with high porosity, presence of y2 and reduced strength (Darvell, 1976). The set amalgam contains unreacted particles of Ag-Cu which are prone to sulphide tarnish (Greener & Sarkar, 1971). This has been confirmed in laboratory tests although no supporting clinical data are available. The low creep and low tensile strength (Mahler et al., 1970) of this amalgam can be related to the same metallurgical factors that have been discussed in relation to the admixed copper amalgams.
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However, the great asset of this amalgam is the virtual elimination of the y2 phase or its gradual disappearance through ageing (Asgar, 1971; Mahler, 1971; Mahler et al, 1975). This is believed to be responsible for the superior marginal integrity of these restorations. Laboratory studies have indicated that the interfacial corrosion resistance of this amalgam is superior to that of conventional amalgams (Marek, Hochman & Butler, 1973; Sarkar et al, 1976). The interfacial corrosion of the tinmercury phase is significant and important in that it destroys the margin and creates an atmosphere for decalcification and secondary decay (Jorgensen, 1969; Kurosaki & Fusayama, 1973). In this respect the development of Dispersalloy is indeed a significant step in the history of dental amalgam research. This is refiected in the current enormous research interest in high copper amalgams and in the introduction of several high copper alloys in the market. Tytin (S. S. White, Philadelphia, U.S.A.). This is one of these new generation of alloys containing 60-0% silver, 27-0% tin and 13-0% copper (Asgar, 1974). Microprobe analysis of this alloy (Mahler, Adey & Van Eysden, 1976) indicated the presence of minute dispersion of e, y and 8 with the latter two phases containing a small amount of copper in solution. The amalgam produced from this alloy is y2-free and is characterized by the presence of yi, and rj'^. The -q^ phase is assumed to be a reaction product of Cu3Sn and tin that is released from the dissolution of y and /3 in mercury. The high early strength of this amalgam is perhaps indicative of a faster reaction kinetics resulting from the presence of copper. The high compressive strength and low creep can be ascribed to the reduction in yi, and increase in Cu-Sn phase. The chloride corrosion resistance of this amalgam is improved because of the elimination of the y2 phase. Mild attack on the unreacted particles of this amalgam by sulphide has been observed in laboratory studies (Sarkar et al, 1976). As far as is known no clinical data on this amalgam have been published. Sybraloy (Kerr, Michigan, U.S.A.). Sybraloy with a nominal composition of 40% silver, 30% copper and 30% tin is a radical departure in composition from conventional commercial alloys. Such an alloy should contain y, ^, e and 77I, perhaps with the amount of the last three phases higher than in a conventional alloy or Tytin. The absence of the y2 phase has been indicated by X-ray diffraction measurements (Marek & Hochman, 1976). Amalgamation reaction products consist of yi and r]'^. The ehmination of the y2 phase, the reduction of yi, and the increase in Cu-Sn intermetallics resulted in high compressive strength and low creep. However, the elimination of the y2 phase did not significantly improve the crevice corrosion resistance of this amalgam (Marek & Hochman, 1976). Laboratory studies have indicated that the sulphide tarnish resistance of this amalgam is impaired by the addition of copper (Sarkar et al, 1976). Other high-copper alloys. Several other brands of high copper silver-tin alloys have also become available recently. They include (i) Aristaloy CR (Baker Dental, N.J.), (ii) Cupralloy (Weber Consumable Products, N.Y.), (iii) Indiloy (Shofu, Japan), (iv) Micro II (L. D. Caulk, U.S.A.), (v) Optalloy II (L. D. Caulk, U.S.A.), (vi) Amalcap non-y2 (Ivoclar, Liechtenstein), (vii) ANA 70 non-y2 (AB Nordiska Affineriet, Sweden), (viii) Luxalloy (Degussa, W. Germany), (ix) Spheriphase (Southern Dental, Austraha), (x) Phasalloy (Phasalloy, U.S.A.). Amalgams prepared from most of these alloys have been claimed to possess high compressive strength, low creep and virtually no y2 phase (Eames & McNamara, 1976; Jorgensen, 1976). However, details about the metallurgical nature of these alloys (or amalgams) have not been reported yet. No information is available currently concerning the corrosion or tarnish resistance of
Copper in dental amalgams
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these amalgams. It has been claimed that the presence of Indium in Indiloy renders copper, silver and tin chemically passive and thereby improves the tarnish resistance of amalgams prepared from Indiloy. Conclusion The beneficial and adverse effects of copper on the physical and electrochemical properties of dental amalgams originate in the microstructures of the alloy and the amalgam. The relationship between microstructure and properties are reversible and structures may be designed to develop specific properties. Thus the research in amalgam alloy development must take into account not only the overall chemical composition of alloy but the phase relationships and the kinetics of reaction with mercury involving dissolution, diffusion and precipitation of intermetallic compounds. References AcHARYA, A., SARKAR, N.K., MARKER, B . D . & GREENER, E.H. (1973) Some physical properties of an admixed high copper amalgam. Journal of Dental Research, 52,187. ALLAN, F . C , ASGAR, K . &PEYTON, F . A . (1956) Microstructure of dental amalgam. Journal of Dental Research, 44, 1002. AMERICAN DENTAL ASSOCIATION (1974) Guide to Dental Materials and Devices, 7th edn. Chicago, Illinois. ASGAR, K . (1971) Behavior of copper in dispersed amalgam alloy. IADR Program and Abstracts of Papers, No. 15. ASGAR, K . (1974) Amalgam alloy with a single composition behavior similar to Dispersalloy. IADR Program and Abstracts of Papers, No. 23. CROWELL, W.S. (1954) The metallography of dental amalgam alloys. Journal of Dental Research, 33, 592. DARVELL, B.W. (1976) Strength of Dispersalloy amalgam. British Dental Journal, 141, 273. DUPERON, D.F., NEVILLE, M . D . & KASLOFF, Z . (1971) Clinical evaluation of corrosion resistance of conventional alloy, spherical particle alloy and dispersion phase alloy. Journal of Prosthetic Dentistry, 25, 650. EAMES, W.B. & MCNAMARA, J.F. (1976) Eight high-copper amalgam alloys and six conventional alloys compared. Operative Dentistry, 1, 98. ESPEVIC, S. & SORENSEN, S.E. (1975) Creep of dental amalgam. Scandinavian Journal of Dental Research, 83, 245. GAYLER, M . L . V . (1937a) The constitution of the alloys of silver, tin and mercury. Journal of the Institute of Metals, 60, 379. GAYLER, M.L.V. (1937b) Dental amalgams. Journal ofthe Institute of Metals, 60, 407. GiROUX, P.P. (1970) A clinical study of copper alloy and its resistance to marginal breakdown. M.S. Thesis, Indiana University, Indianapolis. GRANATH, L.E. & HACKANSSON-HOLMA, B . (1961) The occurrence of certain defects in copper amalgam restorations in the primary dentition. Odontologisk revy, 12, 272. GREENER, E.H. & SARKAR, N . K . (1971) Corrosion of amalgams in chloride and sulfide solution. IADR Progam and Abstracts of Papers, No. 17. GRUBER, R.G., SKINNER, E . W . & GREENER, E.H. (1967) Some physical properties of silver-tin amalgams. Journal of Dental Research, 46, 497. GuBBELS, G.H.M., VRIJHOEF, M.M.A. & DRIESSENS, F . C . M . (1974) A short communication on the concentration of copper in silver-tin phases of dental amalgam alloy. Journal of Oral Rehabilitation, 1, 371. INNES, D . B . K . & YOUDELIS, W . V . (1963) Dispersion strengthened amalgams. Journal of the Canadian Dental Association, 29, 587. JENSEN, S.J. (1972) Copper-tin phases in dental silver amalgam alloy. Scandinavian Journal of Dental Research, ^Q, 158.
S.J. & ANDERSEN, P. (1972) Copper-tin phases in dental silver amalgam. Scandinavian Journal of Dental Research, 80, 349. JOHNSON, L.N., ASGAR, K . & PEYTON, F.A. (1969) Microanalysis of copper-tin phases in dental amalgam. Journal of Dental Research, 48, 872. JENSEN,
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•
^,^^
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v
JoRGENSEN, K.D. (1965) The mechanism of marginal fracture of amalgam fillings. Acta odontologica scandinavica, 23, 347. JoRGENSEN, K.D. (1969) Die kantenfractur occlusaler amalgamfullungen. Zahnarztiiehe Welt, 78, 711. JoRGENSEN, K.D. (1976) Recent developments in alloys for dental amalgams: their properties and proper use. International Dental Jotirnal, 26, 369. KuROSAKi, N. & FUSAYAMA, T . (1973) Penetration of elements from amalgam into dentin. Journal of Dental Research, 52, 309. LYMAN, T . ed. (1973) Metals Handbook, 8th edn. American Society for Metals, Metals Park, Ohio. MAHLER, D.B. (1971) Microprobe analysis of a dispersant-amalgam, IADR Program and Abstracts of Papers, No. 14. MAHLER, D.B., ADEY, J.D. & VAN EYSDEN, J. (1975) Ouantitative microprobe analysis of amalgam. Journal of Dental Research, 54, 218. MAHLER, D.B., ADEY, J.D. & VAN EYSDEN, J. (1976) Microprobe analysis of a high copper amalgam alloy. IADR Program and Abstracts of Papers, No. 886. MAHLER, D.B., TERKLA, L.G. & VAN EYSDEN, J. (1973) Marginal fracture of amalgam restorations. Journal of Dental Research, 52, 823. MAHLER, D.B., TERKLA, L.G., VAN EYSDEN, J. & REISBICK, M.H. (1970) Marginal fracture vs. mechanical properties of amalgam. Journal of Dental Research, 49, 1452. MAREK, M . , HOCHMAN, R . F . & BUTLER, M . F . (1973) Crevice corrosion in dental amalgam restorations. IADR Program and Abstracts of Papers, No. 194. MAREK, M . & HOCHMAN, R.F. (1976) Corrosion properties of a low silver, high copper dental amalgam. IADR Program and Abstracts of Papers, No. 881. MARSHALL, G.W., FINKELSTEIN, G.F. & GREENER, E.H. (1975) Microstructures of several copper-rich dental amalgams. IADR Program and Abstracts of Papers, No. 550. MATHEWSON, R.J., RETZLAFF, A.E. & PORTER, D.R. (1974) Marginal fracture of amalgam in deciduous teeth: a two year report. Journal of the American Dental Association, 88,134. OSBORNE, J., PHILLIPS, R.W., NORMAN, R . D . & SWARTZ, M.L. (1974) Static creep of certain commercial alloys. Journal of the American Dental Association, 89, 620. PEYTON, F.A. (1968) Restorative Dental Materials, 3rd edn, p. 374. C.V. Mosby Company, Saint Louis. PHILLIPS, R.W. (1973) Skinner's Science of Dental Materials, 7th edn, p. 308. W.B. Saunders Company, Philadelphia. SABOTT, D.G., CooLEY, R.W. & GREENER, E.H. (1975) A clinical evaluation of high copper admixed amalgam alloy. IADR Program and Abstracts of Papers, No. 551. SARKAR, N.K. & GREENER, E.H. (1972) Absence of the 72 phase in amalgams with high copper concentrations. Journal of Dental Research, 51, 1511. SARKAR, N.K. & GREENER, E.H. (1975a) Electrochemistry of the saline corrosion of conventional dental amalgams. Journal of Oral Rehabilitation, 2, 49. SARKAR, N.K. & GREENER, E.H. (1975b) The effect of ageing of Dispersalloy on the anodic behavior of its amalgams. Biomaterials, Medical Devices and Artificial Organs, 3, 429. SARKAR, N.K. & GREENER, E.H. (1975c) Electrochemical properties of copper and gold containing dental amalgams. Journal of Oral Rehabilitation, 2, 157. SARKAR, N.K., LEONARD, R . , FUYS, R.A., JR & STANFORD, J.W. (1976) Surface and interface corrosion of dental amalgams. IADR Program and Abstracts of Papers, No. 892. SARKAR, N.K., FUYS, R.A., JR & STANFORD, J.W. (1976) The effects of copper on the sulfide tarnish resistance of dental amalgams. IADR Program and Abstracts of Papers, No. 895. SKINNER, E.W. & PHILLIPS, R.W. (1960) The Science of Dental Materials, 5th edn, p. 409. W.B. Saunders Company, Philadelphia. VRIJHOEF, M.M.A. & DRIESSENS, F . C . M . (1973) X-ray diffraction analysis of CueSns formation during setting of dental amalgam. Journal of Dental Research, 52, 841. VRIJHOEF, M.M.A. & DRIESSENS, F.C.M. (1974a) Investigation of the phase composition of six dental amalgams by X-ray diffraction. Journal of Biomedical Materials Research, 8, 443. VRIJHOEF, M . M . A . & DRIESSENS, F.C.M. (1974b) The creep of dental amalgam—a factor determining the loss of an amalgam filling and its surrounding structme.Biorheology, 11, 191. WESTBROOK, J.H. (1967) Intermetallic Compounds. John Wiley & Sons, New York.
Manuscript accepted 5 July 1977
Copper in dental amalgams
N . K. S A R K A R
School of Dentistry, Louisiana State University, New Orleans
Summary
The relationship between microstructure and physical and electro-chemical properties of copper-containing dental amalgams has been discussed in an attempt to understand the role of copper in currently used dental amalgams. Introduction
Copper has been traditionally added to silver-tin dental alloys to increase the compressive strength and to reduce the fiow and setting contraction of resultant amalgams (Phillips, 1973). Unless a small amount of copper is added, uniform comminution of silver-tin alloy becomes difficult (Phillips, 1973). It has been reported that the beneficial role of copper is restricted to the solubility range of copper in the silver-tin alloy, beyond which adverse effects such as strength loss, uncontrollable expansion and increase in flow are noted (Phillips, 1973). However, alloys have been produced in which 15 to 20 wt % copper has been included with the hope that such an alloy would develop as antiseptic quality in the restoration due to the presence of copper and copper oxides (Peyton, 1968). These alloys have not proved popular because of a greater tendency to tarnish. With the reported clinical success of Dispersalloy (Innes & Youdelis, 1963; Duperon, Neville & Kasloff, 1971; Mahler et al., 1970; Mathewson, Retzlaff & Porter, 1974; Mahler, Terkla & Van Eysden, 1973) a high copper containing alloy (12-0 wt%), there has been an increasing interest in high copper silver-tin amalgams in recent times. Attempts have been made to characterize the metallurgical changes associated with the introduction of copper in amalgam alloys. Attempts have also been made to study the effects of these changes on various properties and clinical performance. Despite these efforts, the role of copper in dental amalgams is not clearly defined. The present paper is a collection, collation and interpretation of the available information on copper in dental amalgams in an attempt to understand its role in currently used amalgams. The microstructure of copper-containing silver-tin alloys
Metallurgically, the principal component of conventional dental amalgam alloys that comply with American Dental Association (ADA) Specification No. 1 is a silver-tin (Ag3Sn) phase referred to as the y-phase (Gayler, 1937a). Copper is allowed up to Correspondence: Dr N. K. Sarkar, Department of Biomaterials, School of Dentistry, Louisiana State University, New Orleans, U.S.A.
0305-182X/79/0100-0001 $02.00
© 1979 Blackwell Scientific Publications
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6-0 wt% in the alloy (American Dental Association, 1974). Gayler (1937b) assumed that copper atoms replace silver atoms up to 4 to 5 atomic per cent in such a way that the ideal stoichiometric formula of Ag3Sn (y) is maintained. Later it was observed that the addition of copper leads to the formation of a distinct separate phase(s). From metallographic observation, this phase was hypothesized to be Cu3Sn (e) by Crowell (1954). From microprobe analysis, Johnson, Asgar & Peyton (1969) identified the copper-tin phase as the CueSns {rj^) of the Cu-Sn system. In a more recent study, Mahler, Ady & Van Eysden (1975) identified the copper-tin intermetallic to be Cu3Sn (e). How do we explain this disagreement between various authors as to the nature of the Cu-Sn intermetallics ? The microstructure of an alloy under equilibrium conditions can be determined from the phase diagram (Lyman, 1973). The composition in such diagrams defines specifically the number of phases present at a certain temperature. Since the commercial dental amalgam alloys can have a wide composition range even within ADA specification limits (American Dental Association, 1974) and are subject to varying conditions of heat treatment (which is seldom the equilibrium condition), it is quite likely that these alloys can have myriad microstructures containing Ag3Sn (y), Ag5Sn (/S), Cu3Sn (e), CueSns {rj^), and Sn (of the eutectic). This has been demonstrated by Jensen (1972) in his X-ray diffraction work on the ternary silver-tincopper system (Sn—25 to 30 wt%, Cu—0 to 4 wt% and Ag—Bal), under equilibrium and nonequilibrium conditions. Further work in this area confirms the above general conclusion (Vrijhoef & Driessens, 1974a). From these studies, it is apparent that copper in conventional dental amalgam alloys may occur as distinct phases such as Cu3Sn (e), CueSns {-q^); or it maybe associated with Ag3Sn (y) Ag Sn (^), or Sn in the form of solid solution. The copper concentration in Ag3Sn may vary between 1-0 and 3-1 % depending on the amount of copper in the alloy (Jensen, 1972; Gubbels, Vrijhoef & Driessens, 1974; Mahler et al, 1975). The ;8-Ag5Sn contains less copper in solid solution that is present in the y-phase (Gubbels et al, 1974). The amount of copper in solid solution with Sn of the Sn-rich eutectic is less than 1-0 wt% (Lyman, 1973). Amalgamation reaction of copper-containing silver-tin alloys What happens to copper when the alloy is amalgamated? Two Cu-Sn intermetallics, Cu6Sn5 and CusSn, have been identified in the set amalgam microstructure (Johnson et al, 1969; Mahler et al, 1975; Vrijhoef & Driessens, 1974a; Jensen & Andersen, 1972; Sarkar & Greener, 1972). These Cu-Sn intermetallics have their origin in the alloy as well as in the amalgamation reaction. It has been mentioned before that both Cu3Sn and CueSns may be present in the original alloy. It has been observed by Johnson et al (1969) that CueSns in the alloy remains practically unattacked by mercury during amalgamation and shows up in the set amalgam microstructure. Alloys containing copper have been shown to precipitate CueSns during setting (Vrijhoef & Driessens, 1973). This precipitation involves the following reaction: when the alloy containing copper is triturated with mercury, copper associated Ag3Sn, Ag5Sn,Sn (eutectic) and Cu3Sn is released. When setting starts as a result of supersaturation and nucleation and growth, a part of copper (available in Hg) is incorporated in Ag2Hg3, a part in Sn7Hg and the rest precipitates as CueSns (Mahler et al, 1975). The formation of Cu-Hg (Allan, Asgar & Peyton, 1956), though unlikely, cannot be discounted and may occur at isolated areas away from tin. Thus from a metallurgical standpoint, the substitution of copper for silver may reduce the amount of tin-mercury (yz) by the formation
Copper in dental amalgams
3
Manipulative and physical properties of copper-containing conventional dental amalgams The effect of copper on the manipulative and other physical and chemical properties can be rationalized in the following way: the reported uncontrollable expansion and the increase in amalgamation rate may be related to the presence of more of Ag5Sn-(^) phase in the alloy resulting from the substitution of copper for silver (Gruber, Skinner & Greener, 1967; Gayler, 1937b). Again the physical and chemical properties of a multiphase structure are governed by the properties of the individual phases. It is expected therefore that the nature and amount of Cu-Sn phase will contribute to the properties of of the composite amalgam. The brittle nature of Cu-Sn intermetallics (Westbrook, 1967) perhaps explains the claim that alloying with copper makes comminution easier. Since these intermetallics are also characterized by high compressive strength, the presence of Cu-Sn results in the strengthening of the amalgam. However, besides this direct strengthening effect, the presence of copper may have additional strengthening effects through solid solution strengthening of y, ^, yi, and y^. The observed reduction in flow and creep can be explained by the reduction of yi through the substitution of Copper for silver. The silver-mercury phase has been held responsible for creep (Vrijhoef & Driessens, 1974b; Espevic & Sorensen, 1975). Again, the presence of strong, hard Cu-Sn intermetallics dispersed in the matrix may also reduce the flow and creep by providing obstacles to the movement of dislocations. Furthermore, flow as is measured under existing ADA Specification No. 1 is more of a measure of the setting rate, and reduction in flow may be related to the increase in reaction kinetics of amalgamation of high copper amalgams. It should be mentioned here that the various properties discussed above are dependent on many other factors (e.g heat treatment, surface treatment, particle size, etc.) besides copper, and these other variables are assumed constant in the context ofthe above discussion. Effect of copper on tarnish resistance of amalgams The main problem with the high copper containing amalgams in the past has been the lack of optimum tarnish resistance (Peyton, 1968). How can it be related to the copper content of these alloys? Unfortunately, nothing is known about the microstructure of these amalgams used in the past. The available information indicates that the copper concentration of these alloys varied from 15-0 to 20-0% whereas the tin content was maintained between 25-0 and 30-0%. An alloy within the above composition limits is expected to contain a combination of the same phases present in commercial conventional alloys, viz., y, |8, e, rj^ and Sn. However, the presence or the relative amount of various phases will be dependent on the composition and heat treatment. With the amount of tin constant and with the substitution of copper for silver, the amount of y will decrease, and the amount of Cu-Sn (Cu6Sn5 + Cu3Sn) will increase. Again the increased amount of copper may bring the phase field to a point where ^8 becomes predominant over y phase. On the basis of the information available today, it may safely be assumed that amalgamation of such an alloy results in (1) a matrix containing mainly Ag2Hg3 (yi), (2) possible elimination or reduction in the amount of SnvHg (y2) phase, and (3) the formation of CueSns. Besides these amalgamation reaction products, a combination of several partially reacted phases, viz., y, ^8, e, 77I from the original alloy will be present in the microstructure. On the basis of this anticipated microstructure, the reported decreased tarnish resistance of high copper silver-tin amalgams of the past may be explained as follows: in a saline aqueous environment, general dulling or tarnishing may result from the formation of tin oxides on yi, yz.
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N. K. Sarkar
rp-, whereas y and e remain unaffected (Sarkar & Greener, 1975b). In a sulphide containing environment, yi, 72, and Cu-Sn (e + r]^) phases probably remain unaffected whereas mild attack may take place on the y-phase (Sarkar, Fuys & Stanford, 1976). Furthermore, j8-phase is more prone to sulphide attack than y-phase. Since the high copper silver-tin amalgam may contain more ^ than that found in conventional amalgams, it is expected that these amalgams will be more susceptible to tarnishing or dulling than the conventional amalgams. Again the presence of Cu-Hg, if any, will also impair the sulphide tarnish resistance (Sarkar et al, 1976). It should be emphasized, that sulphide tarnish is not a universal phenomenon since the sulphur concentration in the oral environment varies considerably from individual to individual. However, tarnishing or dulling due to the formation of Sn-oxides is universal even though y2 is not attacked by sulphide. Nonetheless, the presence of y2 is undesirable because of its detrimental effects on the margins (Jorgensen, 1965). The addition of copper, though, impairs the tarnish resistance but eliminates or reduces the tin-mercury (y2) phase. This has a positive contribution in providing superior marginal integrity and improved clinical performance as will be evidenced from an examination of two high copper amalgams which have created an enormous research interest in recent years. High copper silver-tin amalgams Admixed copper amalgam. One of these amalgams is the admixed copper amalgam. These amalgams are fabricated by mixing commercial copper-amalgams with conventional silver-tin amalgams. Details of fabrication of this amalgam have been reported elsewhere (Sarkar & Greener, 1972). The ratio of copper amalgam to silver-tin amalgam varies from practitioner to practitioner and these amalgam restorations have been found to possess improved marginal integrity over conventional amalgams (Granath & Hackansson-Holma, 1961; Giroux, 1970; Sabott, Cooley & Greener, 1975). Laboratory studies have shown that these amalgams have low fiow and creep (Acharya et al, 1973; Osborne et al, 1974). The diametral tensile strength values of these amalgams are, however, lower than those of conventional amalgams. The setting rate is quite fast as has been evidenced from the early strength data. X-ray diffraction data on one admixed copper amalgam revealed the absence of y2 phase and the presence of y, yi, and Cu-Sn phases (Sarkar & Greener, 1972). Isolated areas containing copper have also been identified in the microstructure (Marshall, Finkelstein & Greener, 1975). During trituration, copper in copper-amalgam (Cu-Hg) reacts with tin available from the solution of silver-tin alloy in mercury. However, the commercial copperamalgams contain as high as 70% mercury (Skinner & Phillips, 1960) and this mercury is available to the silver-tin alloy which is already mixed with mercury. This additional mercury, it is expected, may lead to an increase in the yi content with a corresponding reduction in the amount of the silver-tin (y) phase. The silver-mercury (yi) phase has been held responsible for the creep of dental amalgams (Espevic & Sorensen, 1975). Thus there is an apparent paradox in the low creep of these amalgams in view of their high yi content. This can be explained the following ways: (1) hard strong copper-tin intermetallics dispersed throughout the matrix (yi), change the creep deformation characteristics of the matrix phase, (2) as has been mentioned before, the nature, as well as the amount of the individual phases determine the ultimate physical properties of the multiphasic structure. The adverse effect of the increase in yi content is perhaps balanced by an increase in the Cu-Sn phase content. The Cu-Sn intermetallics prob-
Copper in dental amalgams
5
ably act the same way as strong silver-tin intermetallics in reducing the creep by providing barriers to the motion of dislocation. However, it appears that the loss in strength produced by an increase in the yi phase and a reduction in the y phase could not be compensated by an increase in the Cu-Sn phase. The reduction in tensile strength is a manifestation of the presence of Cu-Sn phases which are inherently brittle. Because ofthe absence ofthe y^ phase, the chloride-corrosion behaviour of this amalgam is improved (Sarkar & Greener, 1975c) which is corroborated by superior marginal integrity of clinical restorations. However, these amalgams are prone to tarnish which has been demonstrated in the laboratory investigation (Greener & Sarkar, 1971) as well as in chnical situations (Sabott et al., 1975). Dispersalloy. Dispersalloy is a mschanical mixture of a conventional silver-tin alloy and silver-copper eutectic particles (Innes & Youdelis, 1963). The amalgamation reaction of Dispersalloy is very similar to the admixed copper amalgam described above. However, in the case of Dispersalloy, the reaction involves one additional step as shown in Table 1. This additional step involves the amalgamation of ^-Cu in Ag-Cu eutectic during trituration to form Cu-Hg. The rest of the reaction involves the precipitation of Cu-Sn intermetallics and the formation of Ag2Hg3 phases. Whether
Table 1. Amalgamation reaction of admixed copper amalgam and dispersalloy Admixed copper amalgam AgsSn (y) + Cu-Hg+Hg-> AgaHgs (yi) + Cu-Sn + unreacted (AgsSn + CuHg) (SnvHg++CuHg) Dispersalloy Stage I: Ag3Sn(y) +Ag-Cu + Hg->Ag2Hg3(yi) + Sn7Hg(y2) + Cu-Hg+unreacted (AgaSn + Ag-Cu) Stage n. SnvHg (y2) + Cu-Hg-»Cu-Sn Final reaction Ag3Sn (y) + Ag-Cu + Hg->-Ag2Hg3 (yi) + Cu-Sn + unreacted (Ag3Sn+Ag-Cu)
this amalgam contains any y2 or not depends on the surface oxidation of the Ag-Cu eutectic (Sarkar & Greener, 1975b) the degree of which determines the amount of copper that can go into solution with mercury. It is only in solution with mercury that copper can effectively unite with tin to prevent the formation of the tin-mercury phase. The surface oxidation of /S-Cu also perhaps determines the stoichiometric nature of the copper-tin intermetallics formed as well as the relative amount of Cu3Sn and CueSns. The effect of ageing of Dispersalloy on the corrosion behaviour of its amalgams has been studied by Sarkar & Greener (1975 a, b, c). It has been demonstrated that the chloride corrosion behaviour of this amalgam is impaired by the storage induced oxidation of the silver-copper eutectic. Recently it has been shown that oxidation of Ag-Cu eutectic in Dispersall'oy results in amalgams with high porosity, presence of y2 and reduced strength (Darvell, 1976). The set amalgam contains unreacted particles of Ag-Cu which are prone to sulphide tarnish (Greener & Sarkar, 1971). This has been confirmed in laboratory tests although no supporting clinical data are available. The low creep and low tensile strength (Mahler et al., 1970) of this amalgam can be related to the same metallurgical factors that have been discussed in relation to the admixed copper amalgams.
6
N. K. Sarkar
However, the great asset of this amalgam is the virtual elimination of the y2 phase or its gradual disappearance through ageing (Asgar, 1971; Mahler, 1971; Mahler et al, 1975). This is believed to be responsible for the superior marginal integrity of these restorations. Laboratory studies have indicated that the interfacial corrosion resistance of this amalgam is superior to that of conventional amalgams (Marek, Hochman & Butler, 1973; Sarkar et al, 1976). The interfacial corrosion of the tinmercury phase is significant and important in that it destroys the margin and creates an atmosphere for decalcification and secondary decay (Jorgensen, 1969; Kurosaki & Fusayama, 1973). In this respect the development of Dispersalloy is indeed a significant step in the history of dental amalgam research. This is refiected in the current enormous research interest in high copper amalgams and in the introduction of several high copper alloys in the market. Tytin (S. S. White, Philadelphia, U.S.A.). This is one of these new generation of alloys containing 60-0% silver, 27-0% tin and 13-0% copper (Asgar, 1974). Microprobe analysis of this alloy (Mahler, Adey & Van Eysden, 1976) indicated the presence of minute dispersion of e, y and 8 with the latter two phases containing a small amount of copper in solution. The amalgam produced from this alloy is y2-free and is characterized by the presence of yi, and rj'^. The -q^ phase is assumed to be a reaction product of Cu3Sn and tin that is released from the dissolution of y and /3 in mercury. The high early strength of this amalgam is perhaps indicative of a faster reaction kinetics resulting from the presence of copper. The high compressive strength and low creep can be ascribed to the reduction in yi, and increase in Cu-Sn phase. The chloride corrosion resistance of this amalgam is improved because of the elimination of the y2 phase. Mild attack on the unreacted particles of this amalgam by sulphide has been observed in laboratory studies (Sarkar et al, 1976). As far as is known no clinical data on this amalgam have been published. Sybraloy (Kerr, Michigan, U.S.A.). Sybraloy with a nominal composition of 40% silver, 30% copper and 30% tin is a radical departure in composition from conventional commercial alloys. Such an alloy should contain y, ^, e and 77I, perhaps with the amount of the last three phases higher than in a conventional alloy or Tytin. The absence of the y2 phase has been indicated by X-ray diffraction measurements (Marek & Hochman, 1976). Amalgamation reaction products consist of yi and r]'^. The ehmination of the y2 phase, the reduction of yi, and the increase in Cu-Sn intermetallics resulted in high compressive strength and low creep. However, the elimination of the y2 phase did not significantly improve the crevice corrosion resistance of this amalgam (Marek & Hochman, 1976). Laboratory studies have indicated that the sulphide tarnish resistance of this amalgam is impaired by the addition of copper (Sarkar et al, 1976). Other high-copper alloys. Several other brands of high copper silver-tin alloys have also become available recently. They include (i) Aristaloy CR (Baker Dental, N.J.), (ii) Cupralloy (Weber Consumable Products, N.Y.), (iii) Indiloy (Shofu, Japan), (iv) Micro II (L. D. Caulk, U.S.A.), (v) Optalloy II (L. D. Caulk, U.S.A.), (vi) Amalcap non-y2 (Ivoclar, Liechtenstein), (vii) ANA 70 non-y2 (AB Nordiska Affineriet, Sweden), (viii) Luxalloy (Degussa, W. Germany), (ix) Spheriphase (Southern Dental, Austraha), (x) Phasalloy (Phasalloy, U.S.A.). Amalgams prepared from most of these alloys have been claimed to possess high compressive strength, low creep and virtually no y2 phase (Eames & McNamara, 1976; Jorgensen, 1976). However, details about the metallurgical nature of these alloys (or amalgams) have not been reported yet. No information is available currently concerning the corrosion or tarnish resistance of
Copper in dental amalgams
7
these amalgams. It has been claimed that the presence of Indium in Indiloy renders copper, silver and tin chemically passive and thereby improves the tarnish resistance of amalgams prepared from Indiloy. Conclusion The beneficial and adverse effects of copper on the physical and electrochemical properties of dental amalgams originate in the microstructures of the alloy and the amalgam. The relationship between microstructure and properties are reversible and structures may be designed to develop specific properties. Thus the research in amalgam alloy development must take into account not only the overall chemical composition of alloy but the phase relationships and the kinetics of reaction with mercury involving dissolution, diffusion and precipitation of intermetallic compounds. References AcHARYA, A., SARKAR, N.K., MARKER, B . D . & GREENER, E.H. (1973) Some physical properties of an admixed high copper amalgam. Journal of Dental Research, 52,187. ALLAN, F . C , ASGAR, K . &PEYTON, F . A . (1956) Microstructure of dental amalgam. Journal of Dental Research, 44, 1002. AMERICAN DENTAL ASSOCIATION (1974) Guide to Dental Materials and Devices, 7th edn. Chicago, Illinois. ASGAR, K . (1971) Behavior of copper in dispersed amalgam alloy. IADR Program and Abstracts of Papers, No. 15. ASGAR, K . (1974) Amalgam alloy with a single composition behavior similar to Dispersalloy. IADR Program and Abstracts of Papers, No. 23. CROWELL, W.S. (1954) The metallography of dental amalgam alloys. Journal of Dental Research, 33, 592. DARVELL, B.W. (1976) Strength of Dispersalloy amalgam. British Dental Journal, 141, 273. DUPERON, D.F., NEVILLE, M . D . & KASLOFF, Z . (1971) Clinical evaluation of corrosion resistance of conventional alloy, spherical particle alloy and dispersion phase alloy. Journal of Prosthetic Dentistry, 25, 650. EAMES, W.B. & MCNAMARA, J.F. (1976) Eight high-copper amalgam alloys and six conventional alloys compared. Operative Dentistry, 1, 98. ESPEVIC, S. & SORENSEN, S.E. (1975) Creep of dental amalgam. Scandinavian Journal of Dental Research, 83, 245. GAYLER, M . L . V . (1937a) The constitution of the alloys of silver, tin and mercury. Journal of the Institute of Metals, 60, 379. GAYLER, M.L.V. (1937b) Dental amalgams. Journal ofthe Institute of Metals, 60, 407. GiROUX, P.P. (1970) A clinical study of copper alloy and its resistance to marginal breakdown. M.S. Thesis, Indiana University, Indianapolis. GRANATH, L.E. & HACKANSSON-HOLMA, B . (1961) The occurrence of certain defects in copper amalgam restorations in the primary dentition. Odontologisk revy, 12, 272. GREENER, E.H. & SARKAR, N . K . (1971) Corrosion of amalgams in chloride and sulfide solution. IADR Progam and Abstracts of Papers, No. 17. GRUBER, R.G., SKINNER, E . W . & GREENER, E.H. (1967) Some physical properties of silver-tin amalgams. Journal of Dental Research, 46, 497. GuBBELS, G.H.M., VRIJHOEF, M.M.A. & DRIESSENS, F . C . M . (1974) A short communication on the concentration of copper in silver-tin phases of dental amalgam alloy. Journal of Oral Rehabilitation, 1, 371. INNES, D . B . K . & YOUDELIS, W . V . (1963) Dispersion strengthened amalgams. Journal of the Canadian Dental Association, 29, 587. JENSEN, S.J. (1972) Copper-tin phases in dental silver amalgam alloy. Scandinavian Journal of Dental Research, ^Q, 158.
S.J. & ANDERSEN, P. (1972) Copper-tin phases in dental silver amalgam. Scandinavian Journal of Dental Research, 80, 349. JOHNSON, L.N., ASGAR, K . & PEYTON, F.A. (1969) Microanalysis of copper-tin phases in dental amalgam. Journal of Dental Research, 48, 872. JENSEN,
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•
^,^^
, .
;
v
JoRGENSEN, K.D. (1965) The mechanism of marginal fracture of amalgam fillings. Acta odontologica scandinavica, 23, 347. JoRGENSEN, K.D. (1969) Die kantenfractur occlusaler amalgamfullungen. Zahnarztiiehe Welt, 78, 711. JoRGENSEN, K.D. (1976) Recent developments in alloys for dental amalgams: their properties and proper use. International Dental Jotirnal, 26, 369. KuROSAKi, N. & FUSAYAMA, T . (1973) Penetration of elements from amalgam into dentin. Journal of Dental Research, 52, 309. LYMAN, T . ed. (1973) Metals Handbook, 8th edn. American Society for Metals, Metals Park, Ohio. MAHLER, D.B. (1971) Microprobe analysis of a dispersant-amalgam, IADR Program and Abstracts of Papers, No. 14. MAHLER, D.B., ADEY, J.D. & VAN EYSDEN, J. (1975) Ouantitative microprobe analysis of amalgam. Journal of Dental Research, 54, 218. MAHLER, D.B., ADEY, J.D. & VAN EYSDEN, J. (1976) Microprobe analysis of a high copper amalgam alloy. IADR Program and Abstracts of Papers, No. 886. MAHLER, D.B., TERKLA, L.G. & VAN EYSDEN, J. (1973) Marginal fracture of amalgam restorations. Journal of Dental Research, 52, 823. MAHLER, D.B., TERKLA, L.G., VAN EYSDEN, J. & REISBICK, M.H. (1970) Marginal fracture vs. mechanical properties of amalgam. Journal of Dental Research, 49, 1452. MAREK, M . , HOCHMAN, R . F . & BUTLER, M . F . (1973) Crevice corrosion in dental amalgam restorations. IADR Program and Abstracts of Papers, No. 194. MAREK, M . & HOCHMAN, R.F. (1976) Corrosion properties of a low silver, high copper dental amalgam. IADR Program and Abstracts of Papers, No. 881. MARSHALL, G.W., FINKELSTEIN, G.F. & GREENER, E.H. (1975) Microstructures of several copper-rich dental amalgams. IADR Program and Abstracts of Papers, No. 550. MATHEWSON, R.J., RETZLAFF, A.E. & PORTER, D.R. (1974) Marginal fracture of amalgam in deciduous teeth: a two year report. Journal of the American Dental Association, 88,134. OSBORNE, J., PHILLIPS, R.W., NORMAN, R . D . & SWARTZ, M.L. (1974) Static creep of certain commercial alloys. Journal of the American Dental Association, 89, 620. PEYTON, F.A. (1968) Restorative Dental Materials, 3rd edn, p. 374. C.V. Mosby Company, Saint Louis. PHILLIPS, R.W. (1973) Skinner's Science of Dental Materials, 7th edn, p. 308. W.B. Saunders Company, Philadelphia. SABOTT, D.G., CooLEY, R.W. & GREENER, E.H. (1975) A clinical evaluation of high copper admixed amalgam alloy. IADR Program and Abstracts of Papers, No. 551. SARKAR, N.K. & GREENER, E.H. (1972) Absence of the 72 phase in amalgams with high copper concentrations. Journal of Dental Research, 51, 1511. SARKAR, N.K. & GREENER, E.H. (1975a) Electrochemistry of the saline corrosion of conventional dental amalgams. Journal of Oral Rehabilitation, 2, 49. SARKAR, N.K. & GREENER, E.H. (1975b) The effect of ageing of Dispersalloy on the anodic behavior of its amalgams. Biomaterials, Medical Devices and Artificial Organs, 3, 429. SARKAR, N.K. & GREENER, E.H. (1975c) Electrochemical properties of copper and gold containing dental amalgams. Journal of Oral Rehabilitation, 2, 157. SARKAR, N.K., LEONARD, R . , FUYS, R.A., JR & STANFORD, J.W. (1976) Surface and interface corrosion of dental amalgams. IADR Program and Abstracts of Papers, No. 892. SARKAR, N.K., FUYS, R.A., JR & STANFORD, J.W. (1976) The effects of copper on the sulfide tarnish resistance of dental amalgams. IADR Program and Abstracts of Papers, No. 895. SKINNER, E.W. & PHILLIPS, R.W. (1960) The Science of Dental Materials, 5th edn, p. 409. W.B. Saunders Company, Philadelphia. VRIJHOEF, M.M.A. & DRIESSENS, F . C . M . (1973) X-ray diffraction analysis of CueSns formation during setting of dental amalgam. Journal of Dental Research, 52, 841. VRIJHOEF, M.M.A. & DRIESSENS, F.C.M. (1974a) Investigation of the phase composition of six dental amalgams by X-ray diffraction. Journal of Biomedical Materials Research, 8, 443. VRIJHOEF, M . M . A . & DRIESSENS, F.C.M. (1974b) The creep of dental amalgam—a factor determining the loss of an amalgam filling and its surrounding structme.Biorheology, 11, 191. WESTBROOK, J.H. (1967) Intermetallic Compounds. John Wiley & Sons, New York.
Manuscript accepted 5 July 1977
