Measurements of stiff-material compliance on the nanoscale using ultrasonic force microscopy

Ultrasonic farce microscopy (UFM) was introduced to probe nanoscale mechani cal properties of stiff materials. This was achieved by vibrating the sampl e far above the first resonance of the probing atomic force microscope cant ilever where the cantilever becomes dynamically rigid. By operating UFM at different set force values, it is possible to directly measure the absolute values of the tip-surface contact stiffness. From this an evaluation of su rface elastic properties can be carried out assuming a suitable solid-solid contact model. In this paper we present curves of stiffness as a function of the normal load in the range of 0-300 nN. The dependence of stiffness on the relative humidity has also been investigated. Materials with different elastic constants (such as sapphire lithium fluoride, and silicon) have be en successfully differentiated. Continuum mechanics models cannot however e xplain the dependence of stiffness on the normal force and on the relative humidity. In this high-frequency regime, it is likely that viscous forces m ight play an important role modifying the tip-surface interaction. Plastic deformation might also occur due to the high strain rates applied when ultr asonically vibrating the sample. Another possible cause of these discrepanc ies might be the presence of water in between the two bodies in contact org anizing in a solidlike way and partially sustaining the load.