The quartz crystal microbalance is normally used to study thin films and adsorbates from the liquid phase. If point contacts are present instead of thin films (Fig. 1), quartz crystals behave differently than under layer loading. Firstly, the frequency shift is positive.Secondly, there is a stress concentration at the point of contact and consequently non-linear force-displacement relations. The easiest way to demonstrate and study these non-linearities is by means of an amplitude dependence of the frequency shift^.1 Some of the results can be understood as a gradual transition from stiction-friction to sliding-friction.

These approaches are being investigated in greater depth in a DFG project:

• Investigation of ^{third-harmonic} generation2 to quantify nonlinearities on the nanosecond scale.
• Study of nonlinearities on the millisecond scale using ^{intermodulation} spectroscopy3.
• Identification of the different contributions to the nonlinearities
  (micro-slip, heating, wear, rolling friction/sliding friction, ...).
• An investigation into whether "shake-down "⁴ (i.e. a decrease in energy dissipation over time) is also observed under oscillatory frictional loading.
• Establishment of this methodology to also characterize soft layers (e.g. bio-adsorbates) nonlinearly (Fig. 1).