Milling effect on Cu-graphite metal matrix composite prepared by powder metallurgy route

Abstract

Copper–graphite metal matrix composites possess the properties of copper, i.e. excellent thermal and electrical conductivities, and properties of graphite, i.e. solid lubricating and small thermal expansion coefficient. Copper matrix containing graphite are widely used as brushes, and bearing materials in many applications due to the excellent thermal and electrical conductivities, and the favorable self-lubricating performance. Copper-graphite with low percentages of graphite is also used for slip rings, switches, relays, connectors, plugs and low voltage d.c. machines with very high current densities. The addition of solid lubricant particles into a metal matrix improves not only the anti-friction properties, but also wear and friction properties. In the present investigation, attempts have been made for the fabrication of Cu-graphite MMC by conventional powder metallurgy route and effect of milling on the fabrication of MMCs. Copper-graphite MMCs were fabricated by mixing 1, 3, 5, and 10 vol. % of graphite powder into copper powder. The composite powder mixtures were then cold compacted by uni-axial pressing and sintered in tubular furnace using argon gas. In another set of experiments, Cu-1 vol. % graphite and Cu- 5 vol. % graphite powder mixtures were milled separately in a planetary mill for 8 h and then MMCs were fabricated by conventional powder metallurgy route using the composite powder mixtures. Different sintering parameters like compaction pressure, sintering time and temperature were varied to study the effect of these variables on the mechanical properties and microstructures of the MMCs. The MMCs were characterised by x-ray diffraction (XRD) and scanning electron microscopy (SEM). Different mechanical properties like bulk hardness, compressive strength, transverse rupture strength and wear study were conducted. It has been found that addition of graphite into copper does not result in much improvement on hardness due to the soft nature of graphite. However, 90 % of theoretical density has been achieved for the fabricated MMCs. A Vickers hardness value of 65 has been obtained for the same composite when it is fabricated by conventional sintering at 900C for 1 hour. The maximum flexural strength of 275 MPa has been achieved for 5 vol. % composite sintered at 900C for 1 hour. It has been found that wear depth decreases with increase in graphite content due to the lubricating properties of graphite. The maximum compressive strength of 897 MPa is obtained for Cu- 1 vol. % graphite. It is also found that compressive strength decreases with further addition of graphite. It has been noticed that milling of initial powder mixtures homogenize and reduces size of particles and eventually results in improvement of mechanical properties.