(245c) Effect of Surface Topography and Superhydrophobicity On Friction

To investigate polymer-on-polymer friction and the role of surface roughness on polymer tribology, microtribometry testing was performed on various substrate films with functionalized 6 mm borosilicate lenses as the probe.  Three hydrocarbon films, which vary in their topography and surface properties, were assembled onto the probe and substrate surfaces: a smooth solid-like octadecyltrichlorosilane monolayer (ML), a polymethylene film (PM) that contains nanoscale asperities, and a superhydrophobic polymethylene film (SH) that contains microscale features.  The tribological tests were performed at a 98 mN load and a speed of 0.1 mm/s.  The results show that the rougher topography of the polymer films dramatically increases frictional forces.  The topographical effect is compounded when both the probe and substrate contain polymer films, as frictional forces were significantly increased when the probe surface was covered with a polymer film.  The results also highlight the lubricating properties of C18 monolayer films, as the COF was reduced to ~0.10 whenever a C18 surface was either on the substrate or probe.  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.  For the C18 ML, PM, and SH probes, the testing environment had no influence on the frictional results, as the coefficients of friction are nearly identical in water and in air for both polymer substrates and suggests that, at the loads and speeds tested, hydrophobic interactions do not provide any special lubrication properties.  The control probe, in contrast, showed decreases in frictional forces for the polymer substrates in the water environment.  This decrease was attributed to a chemisorbed water layer on the hydrophilic probe which lowered the shear and decreased the adhesion between the probe and substrate.