Effect of Copper Content on Compressive Strength and Microstructure of Dental Amalgams

The main goal of this research is to investigate the effect of copper on compressive strength of dental amalgam. For this purpose amalgam capsules with two different content of copper were used. Cylindrical samples with diameter to height ratio 1 to 2, were prepared via molding method. To evaluate the role of copper element on compressive strength, compression test was done at different strain rates i.e. 0.02, 0.2, 0.4 and 2 min –1. The results and microscopic evaluations showed that an increase in copper content in amalgam caused to eliminate gamma2 phase and leaded to promote com-pressive strength.


Ag Sn Ag Cu eutectic Hg Ag Hg
Cu Sn Ag Sn unreacted Ag Cu unreacted Although there are a lot of papers in the field of physiccal and chemical properties of dental amalgam [6][7][8][9][10][11][12][13][14], only a few consider the mechanical behavior of this material.

Materials
Two series of alloy powders and mercury were prepared.According to manufacturer claim the type of both alloy powders was lathe-cut and spherical.SEM micrograph and analysis of this powder are shown in Figures 1 and 2. The details of used materials are summarized in Table 1.Just for simplicity the materials were coded.The main goal of this research is to find out the effect of copper on compressive strength of dental amalgams.

Sample Preparation
The alloy powders and mercury were mixed mechanically at different ratio.Table 2 illustrates the mixing condition   for making amalgam.After mixing and before amalgamation, the standard samples were made using molding method.The heights of the samples were varied from 11 to 13 mm.Mold and prepared specimens are shown in Figure 3.

Microscopic Evaluation
To study the role of copper element on amalgam microstructure, the surface of some samples were polished to achieve a surface roughness of about 5 μm.Then the polished surfaces of the samples were investigated using both optical and SEM microscopes.

Mechanical Test
Compressive test were carried out with a Zwick tensile machine at different strain rates 0.02, 0.2, 0.4 and 2 min -1 .
Microhardness measurement carried out with a Vickers indentor under a load of 1 Kg and 500 g.

Results and Discussion
The microstructure of the A2 sample shows a continuous Ag-Hg matrix that simultaneously is formed with Cu 6 Sn 5 phase and embeds η phase and Sn-Ag alloy particles.Also Ag-Cu eutectic particles are surrounded with η phase.In this sample in addition to η layers, a small amount of η crystals are found (Figures 4-6).Figures 7-9 show microstructures of A1 samples.Comparison of both microstructure (i.e.A1 & A2) shows gam-ma2 phase amount decreases and eta phase amount increases as copper content add.
Since gamma1 phase plays like a barrier against crack propagate, thus fracture of amalgam alloy should be under control of copper content.Figures 10 and 11 illustrate the dependency of fracture stress of amalgam alloy on copper content.However residual Ag 3 Sn particles in amalgam increase, amalgam strength is also increased.This is important because the matrix phase is the weaker and more corrosion prone than the residual alloy particles gamma phase is stable in mouth periphery.Also pure gamma1 phase is stable but existence gamma1 in amalgam have a small quantity tin that is the most corrosion prone phase in mouth, so gamma2 phase have low corrosion resistant and low compressive strength and can cause to crack propagation in amalgam.Furthermore interface between gamma phase and matrix is very magnitude.If interface strength is not coefficient can be caused crack nucleation and growth.Also Cu 6 Sn 5 phase increases the strength of high copper amalgams.In amalgam with lower       copper content (A1), alloy powder consists of more lathe-cut particles than spherical particles.So particles total area and also required mercury content for diffusion in particles increase, on the other hand mercury diffusion causes to contraction, therefore contraction for this amalgam decreases.On the other hand γ 1 and γ 2 phases formation result in expansion.These changes neutralize one another in a good amalgam, but usually its expansion is more than contraction.It induces porosity that is a location for stress concentration and decreases compressive force, thus spherical particles not only harden rather stronger in compression than lathecut particles and Figure 12 Confirms this claim.Whereas compressive strength is a function of strain rate, samples were tested in 0.02, 0.2, 0.4 and 2 min -1 strain rates.The required force versus strain rate for A1 and A2 samples are shown in Figure 13.It can be observed a decrease in force with increasing strain rate for both samples.Also can be seen that A1 sample has less fracture force than A2 sample.Deviation of displacement and elasticity modulus versus strain rate as a function of copper content in turn are shown in Figures 14 and 15.Results of microhardness measurements (Table 3) are in agreement with results of compression test.

Figure 4 .
Figure 4.The microstructure of the A2 sample.

Figure 9 .Figure 10 .
Figure 9. Optical micrograph taken from the polished surface of A1 sample.

Figure 11 .
Figure 11.SEM micrograph of fracture surface of A2 sample.

Figure 13 .
Figure 13.Variation of force versus strain rate of A2 and A1 sample.

Figure 14 .
Figure 14.Variation of displacement versus strain rate as a function of Cu.