The Origin of Adhesive Nanoscopic Friction

The invention of the Atomic Force Microscope in 1982 [2] has allowed us to probe topography in the nanometer scale. At this scale, otherwise considered flat surfaces are rough
and the observed macroscopic friction becomes a sum of many single asperity interactions. However, even 37 years after its invention we do not understand the mechanism
by which friction dissipates energy in these non-destructive single asperity interactions.
Within this project we have investigated the use of Lateral Force Microscopy (LFM) to
measure friction forces on a single asperity. We have succeeded in calibrating both the
normal and lateral stiffness of our system. We Furthermore, investigated the method’s
ability to measure adhesion forces and estimate the radius of the tip apex through deconvolution. Evaluation of the tip also allows us to measure the wear of a single asperity
system. We present a novel theory which describes friction as an energy dissipation
through delocalized phonons. As phonons are periodic excitations of the crystalline
structure, their numbers are limited in nano-scale structures which contain a limited
number of atoms. To test the theory, we produced islands through Focused Ion Beam
(FIB) milling and performed local friction measurements through LFM. We have not
found a friction dependence on the size of our nano-patterns. However, we have found a
correlation between the friction and the effective stiffness of the tip and surface interface.
Furthermore, we were able to use the developed LFM method to measure single asperity
friction in different environments, opening a new path of nanoscale friction and wear
research.