We present a detailed study of the influence of ultrasonic surface acoustic
waves (SAWs) on point-contact friction. Lateral force microscopy (LFM) and
multimode scanning acoustic force microscopy (SAFM) were used to measure a
nd to distinguish between the influence of in-plane and vertical surface os
cillation components on the cantilever's torsion and bending. The experimen
ts show that friction can locally be suppressed by Rayleigh-type SAWs. Thro
ugh the mapping of crossed standing wave fields, the wave amplitude depende
nce of the friction is visualized within microscopic areas without changing
other experimental conditions. Above a certain wave amplitude threshold, f
riction vanishes completely. We found that the friction reduction effect is
caused by the vertical oscillation components of the SAW. Purely in-plane
polarized Love waves do not give rise to a significant friction reduction e
ffect. Thus, we conclude that the mechanical diode effect, i.e., the effect
ive shift of the cantilever off of the oscillating surface, is responsible
for the SAW-induced lubrication. This explanation is supported by vertical
and lateral SAFM measurements: in areas with completely vanishing friction,
low frequency vertical cantilever oscillations are still observable, where
as lateral (torsional) cantilever oscillations are no longer excited. Addit
ionally, at very high Rayleigh wave amplitudes an effect of lateral force r
ectification was observed. It results in a scan direction-independent appea
rance of the LFM traces. (C) 2001 American Institute of Physics.