Effect of Roughness on the Microscale Friction of Hydrocarbon Films

To investigate polymer friction and the role of surface roughness on polymer tribology, microtribometry testing was performed using hydrocarbon films ranging in roughness from a molecularly smooth monolayer to extremely rough polymethylene coatings that were prepared on both the substrate (a flat silicon wafer) and the probe (a 6 mm borosilicate lens). The results show that the rough topography of the polymer films dramatically increases frictional forces when both the probe and substrate contain rough films, but if one surface contains a hydrophobic, smooth monolayer, friction values are greatly reduced. In addition, tribometry tests were performed with the substrates submerged under water to examine the tribometric influence of the testing medium and to determine whether hydrophobic interactions are relevant on the microscale. At the loads and speeds tested, hydrophobic interactions did not significantly affect the frictional properties between the coated probe and substrate in water. The hydrophilic control probe, in contrast, showed decreased frictional forces for the polymer substrates in the water environment, consistent with an adsorbed, lubricating water layer.