Nanomechanical Property Evaluation by using Ultrasonic Atomic Force Microscope

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For a quantitative evaluation of nano-scale elasticity, ultrasonic atomic force microscopy(UAFM)and related methods measure the contact stiffness(or force gradient)between the tip and sample surface. In these methods the key parameter is the contact radius, since the contact stiffness is changed not only by the elasticity of sample but also by the contact radius. However, the contact radius, is very uncertain and it makes the precision of measurements questionable. In this work we propose a novel in-situ method to estimate the tip shape and the contact radius at the nano-scale contact of the tip and sample. Because the measured resonance frequency sometimes does not depend so sensitively on the contact as expected from the parabolic tip model, we introduced a more general model of an axial symmetric body and derived an equation for the contact stiffness. Then the parameters in the model are unambiguously determined from a contact force dependence of the cantilever resonance frequency. … More We verified that this method is able to provide an accurate prediction of the cantilever thickness, the tip shape, and the effective elasticity of soft and rigid samples.As applications we evaluated machining damage and lattice defects. Graphite is an important material for use as a solid lubricant that works even at high temperature. Generally, the reason for the lubricity is that carbon layers easily side against each other due to the layered structure with weak interlayer interaction. However, the atomic nature of the interlayer interaction is still not fully unederstood. To improve this understanding, we applied UAFM to highly oriented pyrolytic graphite and observed edge dislocations accompanied by extra half-planes. From observation of the dislocation behavior under different loads, we found that the dislocation moved laterally by 20 nm as the load increased by 80 nN, and it returned to the original position as the load decreased. To explain this result, we proposed a model for the lateral displacement of the dislocation, which includes a spring and pinning point. This finding of the large lateral displacement confirms the extraordinarily easy sliding between carbon layers, it is relevant to the performance as a solid lubricant. It may also be relevant to the significant material transport in graphite intercalation compounds. Less