Andre Schirmeisen, Nanomechanics Group, CeNTech (Center for Nanotechnology), University of Münster
Friction processes at the nanoscale are the focus of numerous research projects, yet a comprehensive picture is still lacking. In this talk I will present recent nanoscale friction experiments with an ultrahigh vacuum atomic force microscope, where the sample temperature can be varied. Two aspects of nanoscale friction are highlighted: First, I will focus on the statistical analysis of discrete tip jump events (i.e., the "stick-slip" phenomenon) and secondly the influence of the sample temperature will be discussed.
Friction measurements on graphite (HOPG) show a discontinuous behaviour of the friction forces with unit cell periodicity, i.e. atomic scale "stick-slip" friction is observed. This effect is caused by individual jumps of the tip between the hollow sites of the atomic surface structure. We investigated the statistical distribution of the lateral forces, which are necessary to induce the tip jumps between the equilibrium sites. From our analysis we can extract a corresponding jump histogram, showing the relative occurrence of each jump force. These experimental histograms give direct insight into the fundamental processes governing atomic friction. We find that the histograms are in good agreement with the thermally activated Tomlinson model.
In the second part the dependence of friction on the sample temperature is discussed. On graphite the overall friction increases monotonically when lowering the temperature. It will be shown that this behaviour can also be understood in the framework of the thermally activated Tomlinson model: The temperature influences the probability of the tip to jump between adjacent potential minima, in effect causing friction to decrease with increasing temperature. In contrast, friction measurements on a silicon surface reveal a rather complex behaviour of the temperature dependence, with a characteristic friction maximum at 100 K. This friction enhancement effect results in profound consequences for tribological properties of silicon contacts in the area of MEMS/NEMS technology, where friction and wear are an important issue for the technical application of these devices.