Evaluation of morphology and deposits on worn polyimide/graphite composite surfaces by contact-mode AFM

Abstract

Sintered polyimide/graphite composite cylinders were worn in a macroscale line-on-plate contact at different temperatures, revealing instable and relatively high coefficients of friction at 23 < T < 100 °C, decreasing coefficients of friction at 100 < T < 180 °C and constant values for coefficients of friction at 180 < T < 260 °C. The tendencies correspond with a transition from low wear rates at 23 < T < 100 °C into high and stable wear rates at 180 < T < 260 °C. These observations suggest that the lubricating effect of graphite does not prevail at all temperatures. Contact-mode atomic force microscopy of the worn surfaces was used for getting insight in the sliding processes at a micro- to nanoscale. The friction signals provide insight in the local distribution of the composite phases over the surface. Selection criterions for surface patterns representing optical interferences or shear banding patterns, were based on fast Fourier transform images. At low temperatures, severe wear grooves and shear bands within the graphite phase indicate the abrasiveness of graphite additives together with inhomogeneous graphitic transfer onto the polyimide surface areas. At intermediate temperatures, a very thin and discontinuous graphite film forms while the imide zones show a finely structured surface with homogeneous mixture of polyimide grains and graphitic particles. At high temperatures, the surfaces are relatively smooth and homogeneous with a thin graphite film covering the entire surface. The relation between coefficients of friction and surface morphology is most significantly quantified in the evolution of roughness parameters determined from AFM scans.