Wear resistance of Cu–18 vol.% Nb (P/M) composites

Abstract

Friction and wear behavior of copper matrix composites reinforced by unidirectional Nb fibers were explored using both pin-on-ring and pin on disk set-ups. Composite specimens were manufactured from a Cu–18 vol.% Nb powder mixture. Through various mechanical processes and heat treatments, this powder mixture was converted to a ‘ribbon-shaped Nb’ fiber reinforced copper composite with final average fiber thickness of about 10 nm and aspect ratio of about 30. The friction and wear properties of this composite were evaluated against gray cast iron with the hardness of 92 HRB. The effects of normal load, sliding speed and fiber orientation on the friction and wear of this composite were investigated. Steady state wear was observed upon formation of a thin film on the wear surface for all the specimens. This film formed due to the material transfer from the disk to the composite surface due to low ductility of the copper matrix. The wear rate was proportional to the sliding distance and normal load for all specimens, while specimens with fibers normal to the sliding surface showed the lowest wear rate. Moreover, the wear rate appeared to be an exponential function of the sliding speed. This was related to temperature rise at the interface of sliding surfaces and recovery of Cu matrix and possible breakage of the Nb fibers. Metalographic examination of Cu–Nb micro-fiber reinforced composite showed that when exposed to temperature greater than 600 °C, fibers start to break and spherodized. This phenomenon significantly reduces the strength of Cu–Nb composite and increases its wear rate. Some insight into the mechanisms of wear was obtained using optical and scanning electron microscopy and energy dispersive spectrometry analysis of the surface.