Friction between titanium spheres and an artificially structured silicon surface was measured with a friction force microscope. Two spheres with radii of 2.3 lm and 7.9 lm were firmly glued to the tip of the microscope cantilever. A periodic stripe pattern with a groove depth of 26 nm and systematically increasing groove width from 500 nm to 3500 nm was fabricated from a silicon wafer with a focused ion beam. The sphere substrate friction coefficient shows a strong enhancement at a certain groove periodicity, which is related to geometrical interlocking of the two surfaces. This shows that careful modification of the surface roughness can help to control the tribological behavior of mesoscale contacts.

The kinetic signature of the β-relaxation of poly(methyl methacrylate) (PMMA) is investigated by friction force microscopy. The variation in friction force was measured as a function of scan velocity, temperature (300 K-410 K), and applied load using both sharp and blunt probe tips. The friction data show distinct maxima, which can be ascribed to the β-relaxation of PMMA. The contact area was varied over the ranges of approximately 20 to 70 nm(2) and 12,000 to 43,000 nm(2) through the use of probe tips with radii of approximately 15, 18, 1350, and 2650 nm. Kinetic analysis shows that the apparent activation energy of the β-relaxation decreases with the tip radius. Accompanying finite element simulations indicate that for the sharp tips a substantial subvolume of the polymer underneath the tip exceeds the yield stress of PMMA. This suggests that for small contact sizes and high stresses the activation barrier of the β-process decreases through the activation of the α-process by material yielding.