Properties of contact materials AgNi20 with dispersion particles of MgO added
Stanisława Gacek Akademia Górniczo-Hutnicza, Wydział Metalurgii i Inżynierii Materiałowej, al. Mickiewicza 30, 30-059 Kraków
Annals 3 No. 8, 2003 pages 370-375
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abstract Silver-nickel contact materials are used in various overhead breaker switches. One of the requirements they should fulfil is their high resistance to wear resulting from oxidation and spalling phenomena occurring [1, 2]. This paper presents properties of contacts made either of a mixture of AgNi20 and nickel carbonyl, their particle being < 10 μm, and electrolytic silver powder, its particles being < 55 μm, or of a chemically produced silver powder of a particle < 25 μm. MgO was introduced in the silver powders, and its amount was 0.5; 1; and 2.4% of their volume. MgO was introduced using two methods: either by chemical plating of MgO particles on the surface of silver powder’s electrolytic particles (Fig. 1), or by a process of simultaneous precipitation of silver powders and chemical plating of MgO within the volume of particles of the fine-grained silver powders. The mixtures of powders were pressed (moulded) and sintered in hydrogen, at a temperature of 930oC during 30, 120, and 480 min period [3, 4]. The MgO addition caused a reduction in the intensity of dimension changes and prevented nickel particles from growing during their sintering [3, 4]. To test the stability of coupling in sinters, some samples were selected and additionally pressed under a pressure of 1500 MPa to reach a relative density level of 96 to 98%. The hardness of samples made of electrolytic silver powder was essentially lower than the hardness of contacts produced of fine-grained silver powder (Fig. 2). While testing this stability, a specific criterion was assumed; it was a mass loss in contacts: an A movable contact and a K fixed contact, determined upon the completion of three series of connections, i.e. after 10,000, 50,000, and 100,000 connections (Figs. 3, 4). As soon as each series of connections was performed, the voltage drop and a contact resistivity were measured (Figs. 6, 7). In addition, a number of stable lasting connections in individual phases of the entire testing cycle was determined (Tab. 1). The results obtained from the connection stability tests performed on the materials under investigation allowed for a conclusion that the most favourable functional/usable properties showed contacts made of the following AgNi20 samples: AgNi20 samples contained fine-grained silver powder that was concurrently precipitated with 2.0% MgO; the contacts’ relative density upon the completed pressing was 70%; they were sintered under a temperature of 930oC within a 120 min period; and at the end, they were pressed (moulded) again to the 98% level of the theoretical density. Key words: sintered AgNi, re-pressing, electrical contact