Structural friction anisotropy on the nanometer scale

The ability to understand and control friction on an atomic scale is becoming increasingly important, not only considering the increasingly small scale of mechanical systems that are being developed, but also in respect of furthering the fundamental understanding of friction.
In this thesis, the friction anisotropy at the atomic level was investigated.
This investigation demanded special requirements from the experimental setup, and accordingly, in section 2, a detailed description of a newly developed scanning probe microscope incorporating new electronics and a significantly developed ultra-high vacuum system is given.
In particular, with this newly developed microscope, it is possible to use a specially designed sample holder which rotates the sample in situ, enabling the measurement of friction forces along arbitrary directions of the sample surface.
Measurements on NaCl(100), a well known surface in the field of nanotribology, were compared with Prandtl-Tomlinson simulations. Beside the anisotropy investigations, some newly discovered features along the [100] and [110] directions are presented. Three main conclusions can be drawn from these results: the tip path is influencing the average friction force, friction is reduced by 27\% on one ionic species (whether it is Na or Cl is depending on the tip), and the tip asymmetry is leading to a shift of forward and backward friction force maps along the slow scan direction.
In previous studies, the tip-sample interaction in the Prandtl-Tomlinson model was well described by a sinusoidal potential. This potential, however, fails to sufficiently describe the present results. New simulations were conducted and are presented, which are based on an ab initio calculated potential using density functional theory, and reproduce the main features of the experimental results well.
Investigations on the organic surface of a benzylammonium crystal have shown that the molecular orientation is influencing friction and producing a friction contrast on a molecular scale. While the experimental results clearly show that the corrugation potential is influenced by the molecular orientation, adequate simulations reproducing this phenomena require a potential which includes the relaxation of the surface and tip in contact.
In addition, anisotropy measurements show a strong increase of friction along the [100] orientation.
Friction measurements on patterned pristine and hydrogenated graphene initially revealed a contrast between these two surfaces which are initially covered by a contamination layer. In the course of continuous scanning, a mechanical cleaning occurs. The stability of the contamination layer under mechanical treatment is related to the extent of hydrogenation of the subjacent graphene, the hydrogenated regions require a more intense treatment for cleaning.
It is found that on the cleaned surface, friction reduces to approximately a quarter of its value, and, after this mechanical treatment, the friction contrast between graphene and hydrogenated graphene completely disappears.
It is concluded therefore that despite the strong effect of the hydrogenation to the electronic properties of graphene, it is not degrading its properties as a lubricant.