Friction and wear of periodically micro-patterned polypropylene in dry sliding

The influence of well-defined periodic micro-scale pillars on the friction and wear behavior of polypropylene (PP) surfaces was examined in dry sliding against roughened steel surfaces. The steel surfaces were linearly ground, allowing monitoring of the difference in friction when samples passed over the grooves and ridges of the steel surface in perpendicular and parallel directions. Micro-scale patterns with different surface coverage (SC) on the PP surface were produced by preparation of micro-depressions on aluminum foil and subsequent use of the foils as mold inserts in injection molding. The effects of the patterning on friction and wear of the PP surface were evaluated as a function of surface coverage of the micro-pillar features. The friction behavior in sliding of the patterned PP against steel surfaces was affected by the micro patterns. The coefficient of friction (COF) was slightly lower for the less densely patterned surfaces than the unpatterned PP. The COF increased with further SC. Patterning reduced the noise of the COF curves of the micro-patterned surfaces. Wear of the PP surface was strongly affected by the percentage of surface covered by the micro bumps. Scanning electron microscopy studies of the contact surfaces revealed that the transfer layer between PP and the steel surface evolved at different rates for perpendicular and parallel passing direction.