To explore the mechanism of dental amalgam reaction, a basic contact reaction in Hg-Ag system, simulated model experiments of $Ag_3Sn$ and high copper single compositioned alloy (Ag 60 WT%, Sn 27WT%, Cu 13WT%), and the solution kinetics of $Ag_3Sn$ were studied in this research.
In Hg-Ag contact reaction, the solid-liquid diffusion couple technique was employed to determine the interdiffusion coefficient of the gamma ($Ag_2Hg_3$) phase. Diffusion coefficient was calculated with the aid of an equation by wagner. The composition range of the gamma phase was determined to be between 55.3 AT% and 57.5 AT% by the electron probe micro analysis, and values for the average interdiffusion coefficient of the gamma phase were found to be $Dav(㎠/sec)=3.18 × 10^{-5} exp(-32539 (Jmol)/RT)$ in the temperature range 40 and 115℃. The amalgamation reaction between silver and liquid mercury proceeded with the formation of the gamma, epsilon, and a solid solution of Ag-Hg. The epsilon phase was only observed as a small globular shape in the relatively high temperature and long annealing time. But the growth of gamma phase followed the parabolic rate law. Low values of the frequency factor and activation energy of this diffusion indicate the possibility of the short circuit diffusion such as grain boundary diffusion in this couple. The suggested unknown phase which was claimed to exist by Suprinick was not found in this research and reaction products were formed according to the Hansen's phase diagram.
The simulated experiment for dental amalgam reaction was conducted by the rotating the same compositioned and heat treated rod in constant volume of liquid Hg (a simulation of the trituration) and evaporating the remained liquid in the rod with the aid of high vacuum pump after removing the rod in Hg vessel. (a simulation of saturation in the liquid phase with the dissolved elements due to diffusion of Hg into the powder). The results of in situ observation in SEM provide the evidence for the new amalgamation reaction mechanism in conventional and high copper alloyed system. The reaction mechanism by the Okabe could not explained the existence of tin amalgam in the excess Hg content system. But suggested mechanism in this research provide the reasons for their existance. The detailed suggested mechanism for the high copper single compositioned alloy were as follows.
1. Selective dissolution of the $Ag_3Sn$ phase into liquid Hg during the trituration.
2. Crumbled-off of the $Cu_3Sn$ phase into the liquid phase.
3. Supersaturation of the liquid phase due to the Hg diffusion into the alloy powders.
4. Tin gettering to $Cu_3Sn$ phase and heterogeneous nucleation of $Cu_6Sn_5$ on the seed of $Cu_3Sn$ and the growth.
5. Homogeneous nucleation of $Ag_2Hg_3$ and growth.
In the case of excess Hg was added to the powders, the available tin getter of crumbled-off $Cu_3Sn$ after trituration was limited in numbers because of continued Hg attack to $Ag_3Sn$ phase and liberation of Sn from this attack. So tin amalgam($Sn_{7-8}Hg$) phase precipitate to relieve the supersaturation of Sn in liquid phase.
The rate limiting step in the solution of $Ag_3Sn$ phase in liquid Hg was known to the mass transport in the liquid with the result of hydrodynamic solution study in rotating rod method.
