Study On The Electric Contact Properties Of The Silver Based Graded Composites

In this paper, different kinds of silver based composites, including the graded and homogenous composites, which are used as the electric contact materials under middle and high current condition, are prepared by the powder metallurgy(PM) technique and the tape casting(TP) technique for comparing. The test results show that the FGM materials prepared via the TP have better graded component distribution than the PM. With the same chemical composition of the contact surface for both FGM and homogenous materials, it differs significantly in the performances of anti-welding, welding resistance and other properties, which makes it possible to prepare the desired electric contact materials through different configuration of FGM. The FGM prepared by TP exhibits high erosion rate mainly duo to its poor mechanical properties. For the FGM prepared by PM and homogenous materials, determining which one have superior performances depends on the surface composition and graded structure. The inner metal oxides were discovered on the surface for some couples contact materials among all the electric tested FGMs. The stirring effect of molten silver poor generated by the arc as well as the nonuniform strength applied on the surface give rise to this phenomena. The second metal oxide appeared on the surface must be well wetted by the molten silver or it could be hard for molten silver to bring it upward. And this is also the reason why these inner metal oxides were conglomerating on the molten silver layer instead of the metal oxide layer. These finer particles effectively protect the molten silver from splattering and thus play an important role on the good welding resistance properties of the FGM. To be specific, the element Sn was found in the AgZnO of which is the contact surface of Ag/SnO₂/ZnO(FGM) while the element Ni was found in the AgSnO₂ surface for Ag/SnO₂/NiO. The SnO₂ and NiO particles play absolutely different role for the FGMs. Because SnO₂ particle behaves better than the ZnO and NiO on protecting silver from spattering, so the Ag/SnO₂/ZnO shows better welding resistance than the AgZnO while it is contrary for the Ag/SnO₂/NiO. The material transfer was found for the Ag/SnO₂/In₂O₃ graded material. After arc erosion, the In element was disappeared from the molten silver layer and at the same time the metal oxide layer has accumulated a certain amount of In₂O₃. That's to say, the distribution of In₂O₃ has been regrouped. Due to the poor wettability of In₂O₃ in the molten Ag, a large volume of In₂O₃ were excluded from the molten silver and accumulated on the metal oxide layer. The In₂O₃ particles remained were easily getting decomposed and finally evaporated under the high temperature created by the arc due to its poor thermal stability. From the overall point of view, the electric contact materials designed from the idea of FGM have showed many attracting properties, and have surpassed the commonly used contact materials in some parameters. Further research work should be done to investigate into the complicated phenomena and its mechanism, such as the material transfer during switching test, the impacting of graded structure on FGM's properties and so on.