On Dental Alloys in the Market (1)

During and after the War, noble metals such as gold and platinum were used few amount on account of controll of the government and economical poverty of the people. So, the investigation on substitutes for gold alloy has widely developed and various kinds of substitutive dental alloys have been produced.In this report, the results on chemical analysis, micro-structure and Vickers hardness of these substitutes are shown and brief discussion on these alloys is stated from the standpoint of metallography.The results on silver alloys are shown in Table 1. The defect of silver alloys is to be tarnished by sulpher ion. It is well known that addition of palladium and gold prevent this, hewever most of alloys in the market increase their resistance to corrosion by addition of cadmium, zinc and tin. Alloys which contain about 80 % silver are not sufficient to prevent tarnishing, but have comparatively good ductility and high melting point, so they are directed for use as cast crown, swaged plate, etc. Alloys contain 60-70 % silver have good corrosion resistance and are used for cast inlay. Ag-Cu alloys show age-hardening characteristic by quenching and tempering, however maker of these alloys do not, direct to utilize this phenomena. It is doubtful to use silver alloy which contains 27.63 % copper for clasp and bur. For this alloy has eutectic structure and is known as brazing solder.Cu-Zn-Ni, Cu-Sn-Ni and Cu-Zn alloys are found in the market. (Table 2) They are used for only casting. Cu-Ni-Zn alloy contains more zinc than usual nickel silver to lower the melting temperature. These alloys have some defects, for instance, comparatively high melting point, somewhat poor resistance to corrosion and evaporation of zinc during melting. The evaporation of zink increase the melting point and decrease the hardness and corrosion-resistance. These alloys have dendritic structure when they are cast by investment process. (Photo. 3-5)The results on Sn-Sb alloy are shown in Table 3. This alloy has very low melting temperature, so care must be taken to prevent the growth of primary phase. To get fine structure, it is necessary to cool down the mould to room temperature. (Photo. 6-8)As wrought alloys, Ni-Cr, 18-8 stainless steel and Co-Cr alloys are in the market and chemical compositions of these alloys are shown in Table 4, 5 and 6 respectively. Ni-Cr alloy sheets are widely used in this country for crown. Co-Cr alloy is succeedingly used for precise casting. The effort to use this alloy for wrought wire produced the composition of Co-Cr-Fe-Ni system shown in Table 6. A part of cobalt is replaced by nickel and iron to increase their workability. In ferrous alloys, carbon which is contained in material as impurity, forms carbide by heat-treatment and intergranular corrosion takes place for the reason of this carbide. So that, care must be paid to the heat treatment.