Morphological and structural features affecting the friction properties of carbon materials for brake pads

Six carbon materials, the widespread solid lubricant used in several applications, were selected for this study. The morphological and structural features were investigated with the purpose to understand how materials intrinsic properties affect the friction behaviour, using a pin-on-disc test applied to carbon powders against steel at different values of load and speed. The influence of test variables and material properties was studied using a chemometrical approach. Results suggest that spherical particles have a major lubricant effect thanks to an easier roll of the particles at the interface. Also, the dimension plays an important role, since its decrease results in a friction reduction, as a consequence of a higher particle density at the interface. The formation of a stable tribofilm at the interface, inducing a friction reduction, is directly proportional to the material density and to the stacking order along the graphite "c" axis of a single crystallite. Finally, a linear correlation between load and friction is observed, where the intercept of the straight line appears affected by the morphological features of the particles (aspect ratio and roundness).