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Silver impregnated Graphite (SIG) contacts are used in Metal to Carbon Relays, which are used in railway signaling on Indian Railways for operating various signaling functions viz. signals / points / track circuits etc. During the course of operation, various films viz. carbide / impurities etc. get developed on these contacts, which influence its contact resistance. EDX analysis confirmed the existence of carbide film on SIG surface, which introduced a tunnel resistance during mating of contacts. Consequently, the electrical current stops flowing and the relays fail. This causes disruption to rail operations and adversely affects the train safety & punctuality. The philosophy of development of surface films on these contacts and their effect on the contact resistance, including the relationship of film resistance with various contact parameters, has been deliberated and suggestions to minimize their effect have been discussed. Silver Impregnated Graphite contacts used in metal to carbon relays combine the best properties of conducting silver and refractory graphite carbon. The distinctive features of the silver-carbon material are its hard fusing characteristics, better conduction of heat and electricity, easy brazing etc. The electro – graphite for these SIG contacts is manufactured using the Press-Sinter-Infiltrate (PSI) process of powder metallurgy, which provides the highest density material. Silver impregnation is done to achieve high level of electrical conductivity. The SIG contacts are made from graphite blocks, by several cutting, machining & shaping processes. Impregnation is carried out by soaking the graphite pieces in silver nitrate solution under high vacuum for adequate time and then washing the pieces gently by triple distilled water to remove any sticking silver nitrate from the surface. Then the silver nitrate is reduced into silver through the process of ‘firing’ and then electroplated on backside for soldering. Finally, three-stage cleaning of SIG contacts is done to remove any water soluble impurities remaining after electroplating. The final product, as shown in figure 1, shall contain 55% to 60% by weight of silver (without considering the silver plated portion) and the distribution of silver throughout the volume of contact, as seen in micrographs, shall be uniform and free from defects viz. seams, threads, or silver globules [1]. The specific resistance of impregnated graphite shall not exceed 2×10-4 ohms-cm, while the compressive strength of contact shall not be less than 700 kg/cm2 [1]. The hardness of contact shall not be less than 22 VPN. The water-soluble impurities in the finished contacts shall not exceed 0.25% [1]. Fig. 1. Silver Impregnated Graphite Contacts The Silver Impregnated Graphite contacts are fixed on springs [2] as shown in figure 2. Fig. 2. SIG Contact fixed on contact spring SURFACE FILM ON SIG CONTACTS Even in absolutely clean metallic surfaces, thin films having low conductivity, semiconductor properties or even isolating characteristics develop on the surface [3], as shown in figure 3. These films are formed on the surface of materials due to presence of dirt / debris / impurities on the surface or due to oxidation / sulphation / corrosion etc. There are two types of films Thin films & Thick Films. Fig. 3 Metal surfaces covered with films Thin Films [3] are formed from the chemisorbed oxygen atoms on the metal surfaces. They are up to 20 Angstrom thick and are easily fractured mechanically. They conduct electricity by tunnel effect. Thick Films [3] are actually tarnish films; When clean metal surfaces are exposed to atmosphere, the metal combines with the atmospheric oxygen / sulphur dioxide / carbon dioxide etc. and forms metal oxide / sulphide / carbide films. Their average thickness is 100 Angstrom and are practically insulating. The conductivity through thick films is by fritting. The thickness of the layers and the speed of growth are dependent on the contact material, ambient atmosphere, temperature and time [3]. The films could also be from contamination from atmosphere or manufacturing impurities present on the surface. To study the film formation phenomenon on the surface of SIG contacts, surface analysis was done by Scanning Electron Microscopy (SEM) [4] and the chemical constituents of this film were ascertained using Energy Dispersive X-ray Spectroscopy (EDX)[7]. For this, used SIG samples (from two different metal to carbon relays, which were declared failed in the field due to High Contact Resistance), were analyzed by Scanning Electron Microscope [Model: JEOL JSM 6380 LA]. For comparison purpose, SEM & EDX of fresh unused SIG samples were also done. The results of SEM & EDX analysis [5] were as shown in figure 4, 5 and 6.

Impact of Surface Film on Electrical Contact Resistance of Silver Impregnated Graphite Contacts