Effects of polarity on the friction and wear of collector shoes deployed in a metro traction system

The effects of current density and polarity on the tribology characteristics of the contact between an aluminium/stainless steel composite third rail and a copper-impregnated carbon collector shoe are studied using a modified pin-on-disc apparatus. In particular, friction coefficient, temperature rise and wear volume loss of the friction pair are measured. The chemical constituents of worn surfaces are investigated using energy dispersive X-ray spectroscopy and X-ray diffraction techniques. The results show that with increasing current density the temperature of the contact area increases, the contact surfaces oxidize, the tangential force and friction coefficient decrease, and the wear volume loss of the friction pair increases. Oxidation degree and the wear volume losses of collector shoes with a positive polarity are larger than that of collector shoes with a negative polarity. Water molecules adsorbed by the sliding contact surface of the friction pair decompose into hydroxyl and hydrogen ions under the influence of the electric field; subsequently, the hydroxyl ions migrate to the anode where they generate oxygen. The anodic oxidation reaction at high temperature weakens the bond between the grains in the surface layer of the positive friction pair, and aggravates the wear of material, which in turn changes the friction and wear characteristics of the friction pair.