Influence of surface morphology on the tribological properties of silver-graphite overlays

Silver-graphite overlays of 10-20 mum in thickness were deposited on mirror polished bronze substrates at 250-300degreesC in an electron cyclotron resonance enhanced hollow cathode sputtering system. A cylindrically shaped target consisting of a pile of m silver and n graphite rings. (m = 7 and n = 3) of 5 mm in thickness, 80 mm in inner diameter and 90 mm in outer diameter was used. The electron probe microanalysis showed a graded distribution of carbon in the film as function of depth (rich in the surface). The surface morphology of the films analyzed by atomic force microscopy showed decreasing roughness with increasing distance between the target and the sample holder. Tribological properties measured with a ball-on-disk tribometer were found to be dependent on the surface morphology. Minimum values of the coefficient of friction and wear rate were obtained for intermediate roughness in dry sliding. Tribological performance of the silver/graphite overlays were improved compared to that of the substrate material decreasing the wear rate to 4-27% and the coefficient of friction to 54-65%. (C) 2002 Elsevier Science Ltd. All rights reserved.