Failure analysis of a railway copper contact strip

Abstract

The present work investigated the causes of premature wear of the contact strips of a railway line working under voltage of 1.500 Vcc, current of 1120 A, normal force of 70 N and presence of graphite in the wire/strip interface. In all investigated regions (without apparent wear, moderate wear and severe wear), the presence of cracks in the hardened tribo-surface of the Cu-strip – which is caused either by work hardening or thermal cycling is usually followed by material detachment and production of hard abrasive debris. The presence of hard abrasive particles (such as SiO 2 and Al 2O 3,) and wear debris (Cu 2O and hardened Cu) promotes a regime of severe abrasion. The debris showed preferentially a flake-like morphology, being composed of graphite and highly deformed copper, suggesting the dominant action of mechanical wear mechanisms. The presence of some raindrop-like debris featuring an as-cast microstructure confirmed the occurrence of incipient fusion on the copper strip tribo-interface, possibly caused by electrical discharge (electrical induced wear). The results indicated that the wear mechanism of the Cu strip is divided into different stages. First, there is a mixed wear regime (adhesive and abrasive wear) of the graphite layer associated with lubricated adhesive wear of the Cu strip. After, there is the occurrence of a mixed wear regime between the strip/wire tribo surfaces, with simultaneous action of unlubricated adhesive wear, third body abrasive wear and electrical induced wear (local fusion). Finally, once the graphite has been completely consumed, the wear grooves are parallel to the sliding direction and the centre of the strip shows a much more severe wear rate.