Friction and wear of copper-graphite composites made with Cu-coated and uncoated graphite powders

Copper-graphite composites of graphite contents of 8, 15, and 20 wt.% made by powder metallurgy route using either Cu-coated graphite powders or mixture of copper and graphite powders. The Cu-coating process of graphite powders was carried out using electroless coating technique. For comparison, parallel compacts of pure copper were made under the same consolidation processing applied for copper-graphite composites. The wear testing was carried out using a pin-on-ring tribometer. Cylindrical pins of 7.9 mm in diameter, and 12 ± 1 mm in length made from the investigated materials were rubbed against a rotating steel ring (SAE 1045) of surface hardness of 62 HRc, at a constant sliding velocity of 0.2 ms-1 The applied normal load varied from 50 to 500 N. Continuous wear data obtained during each test; the decrease in length of the tested pin, and the friction force with reference to the sliding distance. Extensive metallographic investigation was carried out using both optical and scanning electron microscopy, to study the structural changes at the surface and sub-surface of each tested pin. The removed particles from each tested sample (debris) were collected and examined with scanning electron microscopy, and X-ray diffractometer. It was found that the investigated materials showed three distinct wear rate regimes: low, mild, and severe. Moreover, composites made by Cu-coated and uncoated graphite have lower wear rates and friction coefficients than those made from pure copper. In addition, the Cu-coated graphite composites showed the lowest wear rates and could withstand higher loads upto 450N for 8 and 15 wt.% coated graphite, and 500 N in case of Cu-20% coated graphite, while the uncoated graphite composites could withstand loads up to 300 N for 8 wt.% containing graphite and 250 N for 15 and 20 wt.% graphite. The wear mechanisms of pure copper compacts at each wear rate regime were believed to be oxidation-delamination, delamination, and seizure wear mechanisms, whereas the involved wear mechanisms of either of composites were the same, and they were: oxidative-dominant, strain-induced delamination, and sub-surface delamination. © 2002 Elsevier Science B.V. All rights reserved.