Nanomechanical Property Evaluation by using Ultrasonic Atomic Force Microscope

为了定量评价纳米尺度的弹性，超声原子力显微镜（UAFM）和相关方法测量尖端与样品表面之间的接触刚度（或力梯度）。在这些方法中，关键参数是接触半径，因为接触刚度不仅受试样弹性的影响，而且受接触半径的影响。然而，接触半径是非常不确定的，这使得测量的精度受到质疑。在这项工作中，我们提出了一种新的原位方法来估计尖端形状和纳米尺度接触时的接触半径。由于测量的共振频率有时不像抛物线尖端模型那样敏感地依赖于接触，我们引入了轴对称体的更一般的模型，并推导了接触刚度方程。然后根据悬臂梁谐振频率与接触力的关系明确确定模型中的参数。结果表明，该方法能够准确预测悬臂梁的厚度、尖端形状以及软、刚性试样的有效弹性。作为应用，我们评估了加工损伤和晶格缺陷。石墨是一种重要的固体润滑剂，即使在高温下也能起作用。一般来说，产生润滑性的原因是由于层状结构，层间相互作用弱，碳层容易相互侧向。然而，层间相互作用的原子性质仍然没有被完全理解。为了提高这一认识，我们将UAFM应用于高取向热解石墨，并观察到边缘位错伴随着额外的半平面。通过观察不同载荷作用下的位错行为，我们发现，当载荷增加80 nN时，位错横向移动20 nm，当载荷减小时，位错回到原来的位置。为了解释这一结果，我们提出了一个模型的横向位移的错位，其中包括一个弹簧和钉点。这一巨大横向位移的发现证实了碳层之间非常容易滑动，这与作为固体润滑剂的性能有关。这也可能与石墨插层化合物中显著的物质输运有关。少

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