Microstructure and mechanical property study of 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. 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 and spark plasma sintering (SPS) techniques. Copper-graphite MMCs were fabricated by mixing 1, 3, 5, and 10 vol. % of graphite powder into copper powder followed by conventional powder metallurgy route. The composite powder mixture were cold compacted by uni-axial press and then sintered in tubular furnace using argon gas. In another set of experiments, Cu-1 vol. % graphite and Cu- 5 vol. % MMCs were fabricated by spark plasma sintering technique at 700C under vacuum for 5 minutes. The MMCs were characterised by x-ray diffraction (XRD) and scanning electron microscopy (SEM). Different mechanical properties like density, bulk hardness and wear study were also conducted. XRD spectra show the presence of Cu, graphite and Cu2O peaks which shows that no interaction between Cu and graphite takes place during fabrication. The presence of a weak peak of Cu2O proves that slight oxidation of Cu takes place during conventional sintering of MMCs. However, no peak of Cu2O is visible for SPS as it was conducted under vacuum. It has been found that addition of graphite into copper does not result in much improvement in hardness due to the soft nature of graphite. However, 90 and 97 % of theoretical density have been obtained for conventional sintered and SPS samples respectively. Maximum Vickers hardness value of around 100 has been achieved for Cu-1 vol. % graphite MMC when it is fabricated by SPS. However, a hardness value of 65 has been obtained for the same composite when it is fabricated by conventional sintering at 900C for 1 hour. To study the effect of milling, Cu-1vol. % graphite and Cu-5 vol. % graphite powder mixture were milled for various time periods and then sintered. It has been found that hardness increases with milling. The micrographs of Cu-graphite reflect the clean interface and good compatibility between matrix and reinforcement. It has also been found that graphite particles are uniformly distributed into copper matrix. From wear study, it is concluded that the wear resistance of the composite increases with increase in graphite content due to the lubricating properties of graphite. It has also been found that wear depth decreases with increase in graphite content. SPS sintered samples show higher wear resistance than conventional sintered samples. It has also been found that compressive strength increases with addition of graphite and maximum up to 3 vol. % of graphite. With further addition of graphite there is a decrease in compressive strength due to increase in brittle nature of composites.