S. Niederberger et al., Transitions from nanoscale to microscale dynamic friction mechanisms on polyethylene and silicon surfaces, J APPL PHYS, 87(6), 2000, pp. 3143-3150
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].