Transitions from nanoscale to microscale dynamic friction mechanisms on polyethylene and silicon surfaces

The dynamic friction mechanisms of polyethylene and silicon were investigat ed for apparent contact pressures and contact areas in the ranges of 8 MPa- 18 GPa and 17 nm(2)-9500 mu m(2), respectively. Friction force measurements were obtained with a friction force microscope, scanning force microscope, and pin-on-disk tribometer. Silicon and diamond tips with a nominal radius of curvature between 100 nm and 1.2 mm were slid against low- and high-den sity polyethylene and Si(100) substrates under contact loads in the range o f 5 nN-0.27 N. The low friction coefficients obtained with all material sys tems at low contact pressures indicated that deformation at the sliding int erface was primarily elastic. Alternatively, the significantly higher frict ion coefficients at higher contact pressures suggested that plastic deforma tion was the principal mode of deformation. The high friction coefficients of polyethylene observed with large apparent contact areas are interpreted in terms of the microstructure evolution involving the rearrangement of cry stalline regions (lamellae) nearly parallel to the sliding direction, which reduces the surface resistance to plastic shearing. Such differences in th e friction behavior of polyethylene resulting from stress-induced microstru ctural changes were found to occur over a relatively large range of the app arent contact area. The friction behavior of silicon was strongly affected by the presence of a native oxide film. Results are presented to demonstrat e the effect of the scale of deformation at the contact interface on the dy namic friction behavior and the significance of contact parameters on the f riction measurements obtained with different instruments. (C) 2000 American Institute of Physics. [S0021-8979(00)06305-2].