Effect of graphite powder on sliding performance of sintered polyimide composites at various normal loads and sliding velocities

Polymer composites with graphite additives are often used to control sliding properties. The lubricating action of graphite, however, strongly depends on the working conditions of normal load and sliding velocity. Sintered polyimide with 15 wt.-% graphite powders is worn under 50 to 200 N normal loads and 0.3 to 1.2 m/s sliding velocities in line contact with a steel counterface. The tribological properties of graphite are unfavourable under low sliding velocity and high normal loads, causing high friction and wear rates. The transfer film then mainly consists of rough graphite flakes, and large wear debris particles indicate overload, as the loading capacity of graphite is limited. Graphite powders reduce coefficients of friction from 0.45 to 0.12 under medium to high sliding velocities and normal loads. The transfer films are rendered smooth by mixing polyimide and graphite fillers. Stabilisation in friction and wear as a function of normal load and sliding velocity (pv-value) coincides with levelling of the maximum polymer surface temperature of 115 degrees C. The favourable action for graphite under sliding, i.e. mechanical shear, and the improved thermal conductivity of the polymer bulk, control the sliding mechanisms of polymer composites under certain sliding conditions.