Graphite-graphite electrical contact under dynamic mechanical loading

Due to its electrical conductivity, thermal conductivity, with wear or abrasion studies stems from the fact that wear oxidation resistance and wear resistance, graphite is used or abrasion involves one element sliding against another, as an electrical contact material, particularly in sliding so that different points in a contact are not subjected to conditions, as encountered by brushes for electric motors dynamic stress in an in-phase manner. In contrast, dynamic and other devices and by sliding electrical contacts for compression does not involve sliding, so that each point in trams and other electric vehicles [1–8]. To further improve a contact is subjected to dynamic compression in an the conductivity, copper impregnated graphite may be used in-phase manner, i.e. all points experience the maximum [9,10]. Because of this application, the quality of graphite– compressive stress in a cycle simultaneously and all points graphite electrical contacts over time under dynamic experience the minimum stress in a cycle simultaneously. mechanical loading is of interest. Relevant questions As a result, correlation is possible between the effect (say concern how elastic and plastic deformations at the contact the contact electrical resistance at the contact) and the interface (particularly at the asperities) affect the quality of dynamic stress during dynamic loading. This correlation the electrical contact under mechanically loaded and allows identification of the point in a stress cycle at which unloaded conditions, and how these effects depend on the certain effect occurs, and moreover allows distinction stress amplitude and the number of loading cycles. between reversible effects (such as elastic deformation, Wear or abrasion involves subjecting each point of a which vanish upon unloading) and irreversible effects surface to dynamic shear. Studies of wear or abrasion are (such as plastic deformation, which remain upon unloadcommonly conducted by monitoring the effect over an area ing). Therefore, by studying the effect of dynamic comrather than that at a fixed point. For example, in wear pression rather than dynamic shear, this paper provides testing using the pin-on-disk configuration, the tip of the new information on the dynamic mechanical behavior of pin is continuously moved against the surface of the disk, contacts, i.e. the behavior of contacts under dynamic so that different points on the disk are subjected to stress at loading. different times and the effect of dynamic shear and the The technique used in this work for studying the stress variation within a cycle of dynamic shear at a dynamic mechanical behavior of contacts is contact electriparticular point of the surface are not monitored. Even if cal resistance measurement during dynamic compression the effect of wear or abrasion is monitored in real time, say below the yield stress. It involves simultaneous electrical by measuring the contact electrical resistance at the sliding and mechanical measurements. The technique requires that contact between the pin and the disk, the monitoring does the elements in contact are electrically conducting. The not allow correlation of the effect (say the electrical contact resistance of the interface between contacting resistance) at a point with the dynamic stress at the point elements can be conveniently measured by using the within a stress cycle. (The dynamic stress is to be elements as electrical leads — two for passing current and distinguished from the stress amplitude.) This difficulty two for voltage measurement (i.e. the four-probe method), as provided by two elements (beams) that overlap at 908 (Fig. 1). The volume resistance of each lead was negligible *Corresponding author. Tel.: 11-716-645-2593; fax: 11-716compared to the contact resistance of the junction, so the 645-3875. E-mail address: ddlchung@acsu.buffalo.edu (D.D.L. Chung). measured resistance (i.e. voltage divided by current) was