Graphite lubrication mechanisms under high mechanical load
In this study, the fundamental wear mechanisms of graphite were studied in sliding friction under high mechanical load. For these investigations, graphite coatings of various thickness were applied with an airbrush spray gun onto substrates with two roughness levels. When tested in a microtribometer, as film thickness was increased from 0.2 to 17 , the coefficient of friction increased 3.6-fold, due to higher ploughing effects; the higher thickness, however, resulted in an extended lifetime of the coating and a lower counterbody wear. As the pressure was elevated up to 1 GPa a decrease in friction was observed with a coefficient of friction as low as 0.12. The performance could be additionally improved in terms of friction, wear, and lifetime by increasing the substrate roughness by a factor of seven. An increased lifetime up to 7.6-fold was achieved due to the formation of a stable, 30-150 nm thick carbon layer in the tribocontact. During all experiments graphite is transferred onto the counterbody and transported out of the wear track during the first ten friction cycles. The observed carbon layer consists of compressed graphite material in the friction track that was transported from the sides, thus filling and covering the trenches of the rough substrate and thereby reducing contact pressure.