Friction and wear characteristics of powder metallurgy copper-graphite brushes at high sliding speeds

Abstract

Friction experiments using several commercial powder metallurgy copper-graphite brushes against an AISI 4340 steel rotor were conducted at sliding velocities ranging from 20 to 235 m s −1. The measured wear rates ranged from a minimum of 4.3 × 10 −5 cm km −1 at a sliding velocity of 100 m s −1 for a brush with high graphite content to a maximum of 8.4 × 10 −3 cm km −1 at a sliding velocity of 230 m s −1 for a brush with high metal content. The coefficients of friction ranged from a minimum of about 0.08 to a maximum of about 0.47 and were greatly affected by the presence of oxide layers at the sliding interface. Almost all the brushes exhibited some degree of edge breaking. The velocity at which edge breaking occurred was dependent on the powder grain size. Brushes with a large grain size seem to exhibit edge breaking at a lower speed than brushes with a fine grain size. High interface temperatures which occur at high sliding speeds result in melting of the lead-tin binders used in most powder copper-graphite brushes.