Amalgamation Process / Basic Setting Reaction

1. Amalgamation Process/ Basic Setting Reaction Low-copper alloys High-copper alloys admixed alloys single composition alloys Endanus Harijanto

2. BASIC COMPOSITION • A silver-mercury matrix containing filler particles of silver-tin • Filler (bricks) • Ag3Sn called gamma • can be in various shapes • irregular (lathe-cut), spherical,or a combination • Matrix • Ag2Hg3 called gamma 1 • cement • Sn8Hgcalled gamma 2 • voids

3. Low-copper Alloys (1) • When a powder (Ag3Sn, g) is triturated, the Ag and Sn in the outer portion of the particles dissolve into Hg (mercury). • Hg also diffuses into the alloy particles. • Solubility for Ag = 0.035 wt%, for Sn = 0.6 wt%

4. As the remaining mercury dissolves the alloy particles, g1 and g2 crystals grow. Low-copper Alloys (2) • When the solubility is exceeded, crystals of two binary metallic compounds precipitate into the mercury. • Ag2Hg3 compound (g1) precipitates first. • Sn7-8Hg compound (g2) precipitates later. trituration condensable, carvable

5. Unconsumed particles (smaller after being partly dissolved) are surrounded and bound together by solid g1 and g2 phases. Low-copper Alloys (3) • As the mercury disappears, the amalgam hardens. Particles become covered with newly formed crystals, mostly g1. trituration condensable, carvable harden, no longer workable

6. P = b and g Ag-Sn E = e (Cu3Sn) G1 = g1 (Ag2Hg3) G2 = g2 (Sn7-8Hg) Low-copper Alloys (4) • A typical low-copper amalgam is a composite in which the unconsumed particles are embedded in g1 and g2 phases.

7. Low-copper Alloys (5) • In summary, Alloy particles (b + g) + Hg  g1 + g2 + unconsumed alloy particles (b + g) • Physical properties • g-phase  strongest, g2 phase  weakest • Hardness: g > g1 >>> g2 • g2  poor corrosion resistance

8. High-Cu: Admixed Alloys (1) • Spherical silver-copper (Ag-Cu) eutectic alloy particles are added to lathe-cut low-copper amalgam alloy particles (Ag-Sn or g +b). • The final powder is composed of two kinds of particles.  “admixed” • Ag-Cu particles act as strong fillers, strengthening the amalgam matrix.

9. High-Cu: Admixed Alloys (2) • Silver-Copper eutectic alloy • 71.9 wt% Ag, 28.1 wt% Cu • alloy, eutectic,n any combination of metals the melting point of which is lower than that of any of the individual metals of which it consists. An alloy in which the components are mutually soluble in the solid state. A eutectic alloy has a nonhomogeneous grain structure and is therefore likely to be brittle and subject to tarnishing and corrosion. Temperature Eutectic composition

10. High-Cu: Admixed Alloys (3) • Ag dissolves into the Hg from the Ag-Cu alloy particles. • Both Ag and Sn dissolve into Hg from the Ag-Sn alloy particles. (same as in low-Cu alloy) • Sn in solution diffuses to the surface of Ag-Cu alloy particles and reacts with the Cu to form the h phase (Cu6Sn5) (therefore, the Sn7-8Hg or g2 is eliminated)

11. High-Cu: Admixed Alloys (4) • A layer of h forms around unconsumed Ag-Cu particles. • g1 phase is the matrix. • The final structure composes of the g phase, Ag-Cu particles, e particles, g1 matrix, and h reaction layers.

12. High-Cu: Admixed Alloys (5) • In summary: Alloy particles (b + g) + Ag-Cu eutectic + Hg  g1 + h + unconsumed alloy of both types of particles • g2 has been eliminated in this reaction, being replaced by h. • The effectiveness in eliminating g2 depends on % of copper-containing particles. (net copper concentration of > 12% in alloy powder)

13. High-Cu: Single-composition Alloys (1) • Each particle has the same chemical composition. • Major components: Ag-Cu-Sn

14. g1 h P High-Cu: Single-composition Alloys (2) • Phases found in each single-composition alloy particle are b (Ag-Sn), g (Ag3Sn), and e (Cu3Sn). • h crystals are found as meshes of rod crystals at the surfaces of alloy particles (P), as well as dispersed in the matrix.

15. High-Cu: Single-composition Alloys (3) • In summary: Alloy particles (Ag-Sn-Cu) + Hg  g1 + h + unconsumed alloy particles • Little or none of g2 phase can form.