Volker Scherer

On the nanoscale measurement of friction using atomic-force microscope cantilever torsional resonances

We studied friction and stick-slip phenomena on bare and lubricated silicon samples by measuring the torsional contact resonances of atomic force microscope cantilevers. A piezoelectric transducer placed below the sample generates in-plane sample surface vibrations which excite torsional vibrations of the cantilever. The resonance frequencies of the vibrating beam depend on the tip-sample forces. At low lateral surface amplitudes the cantilever behaves like a linear oscillator with viscous damping. Above a critical surface amplitude, typically 0.2 nm, the amplitude maximum of the resonance curves does not increase any more and the shape of the resonance curves changes, indicating the onset of sliding friction. The critical amplitude increases with increasing static cantilever load. For a bare silicon sample it is higher than for the lubricated silicon. Microslip known from macroscopic contacts causes energy dissipation in the atomic force microscope tip-contact before sliding friction sets in.

Imaging of flexural and torsional resonance modes of atomic force microscopy cantilevers using optical interferometry

Commercial rectangular atomic force microscope cantilever beams made of silicon were set into vibration, using a piezoelectric ultrasonic transducer coupled to the chip of a cantilever. The transducer was excited with continuous rf in the frequency range of 100 kHz to 3 MHz. The vibrations were monitored using an optical Michelson heterodyne-interferometer allowing the surface of the cantilever under examination to be scanned with a lateral resolution of several μm. A number of free torsional and flexural vibration modes of the beams were imaged quantitatively. Comparison of the experimental resonance frequencies and the amplitude and phase distribution of the modes to theoretical models showed that asymmetries in the beam strongly influence the vibrational behavior of the beam. The consequences for quantitative local stiffness measurements are discussed.

Atomic Force Microscopy with Lateral Modulation

Friction is present in our every day lives. There are unwanted phenomena such as energy loss in the relative motion of contacting surfaces (e.g., gears, bearings or sealings). For example, the friction loss in a modern internal combustion engine is approximately 10%, in terms of indicated power at full load. In terms of fuel consumption, a savings of up to 26% has been calculated for the hypothetical case of a ‘frictionless’ engine. It is believed that potential savings in fuel consumption could be 7–9% in real engines [1]. Other parts such as sliding members would not move at all if friction were not taken care of. Estimates claim a loss of up to 1.6% of the gross national product in developed countries, that is 116 billion US-Dollars for the year 1995 in the US, due to inappropriate friction management [2]. On the other hand, friction is essential to an incredible number of processes provided by nature and civilization. If friction were not present, there would be no controlled blood flow; also, we would have difficulties in slowing down vehicles. Although the phenomenon friction does affect our being and doing at least as much as gravity or electricity does, we have not yet understood its origin. Friction has been intensely investigated in macroscopic length scales, at both, low and high velocities. Just recently, much of the tribology research has been devoted to the micrometer level and below [3, 4]. It was at the end of the 1960s when Bowden and Tabor at the Cavendish Laboratory of the University of Cambridge studied friction and tribology of two bodies contacting each other on an area as small as a few micrometers squared [5].