Investigation On Oxidation Mechanism Of Ag-Sn Alloy And High Temperature Plastic Deformation Behaviour Of Ag-SnO₂ Materials

Electrical contact is one of the key parts of the electric apparatus, which function is making and breaking current. Ag-CdO contact materials have excellent resistance of electrical arc and welding and low electric contact resistance, which are widely used in many types of low-voltage apparatus from several to thousands of voltages and once considered as universal contacts. However, the steam of Cd metal is poisonous, which pollutes the environment and damages the human health. Consequently, many countries have developing the materials to substitute for the Ag-CdO materials. Ag-SnO₂ contact materials have excellent electric arc resistance and good welding resistance. Nowadays Ag-SnO₂ electrical contact materials are the optimum candidate for replacing the toxic Ag-CdO electrical contact materials. Lots of work has been done on the processing technic, application of micro-additive and microstructure variation of Ag-SnO₂ during the action of electric arc. However, there is a little report on the oxidation mechanism of Ag-Sn alloy and how to resolve the problem on bad process ability of Ag-SnO₂ materials. So the investigation on the oxidation mechanism of Ag-Sn alloy and high temperature plastic deformation behaviour of Ag-SnO₂ materials has an important theoretical significance and use value.Oxidation kinetic and microstructure of Ag-Sn alloy, Ag-Sn-Cu-Bi -Ni alloy and Ag-Sn-M (M=Sb, La, In) alloy powders were investigated and the effect of oxidation temperature, oxygen partial pressure and alloy composition on the oxidation behavior of these alloy were discussed in the present paper. High temperature deformation behaviors of Ag-SnO₂-Sb₂O₃ materials were studied through hot pressing experiment and the processing maps were plotted. In addition, Ag-SnO₂-Sb₂O₃ contact materials were prepared firstly using by internal oxidation+powders hot extrusion technology, which is compared with the Ag-SnO₂-Sb₂O₃ contact materials fabricated by internal oxidation + hot forging technology. The effect of processing technic on microstructure and properties of materials is discussed.The results obtained are as follows:(1) According to the investigation on the oxidation kinetics of Ag-2.99at.%Sn alloy, the relationship between the solubility of oxygen in silver ( N_O^S ) and the oxygen partial pressure ( P_O2 ) , temperature (T) is firstly deduced, which is as follow: In different temperature and oxygen partial pressure, the solubility of oxygen in silver can be easily calculated according to the above equation. In addition, at different oxygen pressure, the relationship between the oxidation rate and temperature is as follow:At different temperature, the relationship between the oxidation rate and oxygen partial pressure is as follow:(2) The oxidation front (interface between the oxidation zone and the oxidation-free zone) is flat like a line and the oxide particles uniformly and dispersedly distribute on the silver matrix after the Ag-2.99at.%Sn alloy internal oxidation. At the low temperature (600-700℃) oxidation case, the oxide particles mainly precipitate on the grain-boundaries; at the high temperature (800-850℃) oxidation case, the oxide particles mainly precipitate inside the grain. The higher oxygen partial pressure, the finer oxide particle size. While the oxidation front is undulation after the Ag-4.76at.%Sn alloy oxidation. The oxide particles mainly precipitate on the grain-boundaries when the Ag-4.76at.%Sn alloy is oxidized in air. When the oxygen partial pressure is 0.3MPa and 0.9MPa, many oxide bands are observed in the internal oxidation layer. The oxide particles size and inter-particle spacing become larger with the distance from surface.(3) The internal oxidation of Ag-6.46Sn-1.3Cu-0.27Bi-0.17Ni (wt.%) alloy is a process controlled by reaction diffusion. It mainly include: oxygen adsorption and decomposition on the alloy surface; nucleation of oxide particle and formation of oxide bands; diffusion of copper and nucleation of CuO. The internal oxidation of the cold rolled alloy and the recrystallization annealing alloy is different. The former oxidation front is flat and there are many oxide bands in the internal oxidation front. The oxide particle size and hardness become larger with the oxidation depth increasing. But the latter oxidation front is zigzag and the oxide particle size, hardness and the fracture stress of oxide particles are larger than that of the former. Because cold rolling increase the density of dislocation in alloy and the diffused channel of oxygen, the former oxidation rate (5.69×cm²/sec) is larger than the later oxidation rate (2.39×cm²/sec) .(4) Element La and In is more effective than element Sb for improving the oxidation character of Ag-Sn alloy powders. Oxygen atoms diffuse from surface to inner and part of solute atoms (Sn and Sb) diffuse from inner to surface during the oxidation of Ag-Sn-Sb alloy powders. Oxygen atoms diffuse from surface to inner and solute atoms (Sn and In) form oxides in situ during the oxidation of Ag-Sn-In alloy powders. La₂O₃ forms in situ during the oxidation of Ag-Sn-La alloy powders. When the La content is 0.44wt.% in the Ag-Sn-La alloy powders, Sn atoms will diffuse from inner to surface and react with the oxygen and form oxides. When the La content is 1.28wt.% and 3.4wt.%, all the Sn atoms will form oxide in situ.(5) With the increasing of strain rate, the stresses of Ag-SnO₂-Sb₂O₃ materials compressed in high temperature are increased. In low strain rate (0.01s^-1and0.1s^-1), the flow curves are smooth, but in high strain rate(1s^-1 and 10s^-1), those are fluctuate. According to the relative material parameters obtained from the compression experiment, the deformation constitutive equations of Ag-SnO₂-Sb₂O₃ materials describing the relationship of yield stress peak value, strain rate and temperature are given as follows:(6) Based on the principle of thermal processing map, high temperature compressed deformation datas were used to obtain the processing maps of Ag-6.92SnO₂-3.69Sb₂O₃ and Ag-9.12SnO₂-1.46Sb₂O₃ materials under the true strain of 0.03, 0.2, 0.4, 0.6 and 0.9. The adiabatic shear bands, flow localization, wedge crack, particle breakage and voids formed at oxide particle were observed on some damage samples. Ag-6.92SnO₂-3.69Sb₂O₃ materials processing temperature range is 720℃-840℃and strain rate range is 0.01s^-1-0.1s^-1. Ag-9.12SnO₂-1.46Sb₂O₃ materials processing temperature range is 790℃-845℃and strain rate range is 0.01 s^-1-0.18s^-1.(7) Ag-6.32SnO₂-3.69Sb₂O₃ and Ag-9.12SnO₂-1.46Sb₂O₃ materials are prepared by internal oxidation+powder hot extrusion technology. Their relative density is 99.55% and 99.89%, hardness is 93HB and 85HB, electrical conductivity is 71IACS % and 69IACS % . At the same time, Ag-6.32SnO₂-3.69Sb₂O₃ and Ag-9.12SnO₂-1.46Sb₂O₃ materials are prepared by internal oxidation+hot forging technology. Their relative density is 98.89% and 99.35%, hardness is 84.3HB and 74.8HB, electrical conductivity is 61IACS% and 66IACS%. Because the hot forging technology retains the intact shape of powders, but the powder hot extrusion technology can not only destroy the intact shape but also get streamline structure. So the microstructure and properties of Ag-SnO₂-Sb₂O₃ materials fabricated by internal oxidation+ powder hot extrusion technology is better than that by internal oxidation+ hot forging technology.