Nanomechanical Property Evaluation by using Ultrasonic Atomic Force Microscope

为了定量评估纳米级的弹性，超声原子力显微镜(UAFM)和相关方法测量针尖和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件弹性的影响，还受接触半径的影响。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估计针尖形状和针尖与样品纳米级接触处的接触半径的方法。由于测量的共振频率有时并不像抛物线尖端模型所期望的那样敏感地依赖于接触，我们引入了一个更一般的轴对称体模型，并推导了接触刚度的方程。然后根据悬臂梁共振频率与接触力的关系明确地确定模型中的参数。…此外，我们还验证了该方法能够提供对悬臂梁厚度、尖端形状和软硬样品的有效弹性的准确预测。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，即使在高温下也能使用。通常情况下，润滑性的原因是由于层状结构，层间相互作用较弱，碳层容易相互对立。然而，层间相互作用的原子本质仍然没有完全被理解。为了更好地理解这一点，我们将UAFM应用于高取向热解石墨，观察到伴随着额外的半平面的边缘位错。通过对不同载荷下位错行为的观察发现，载荷增加80nN时，位错横向移动20 nm，载荷减小时，位错恢复到原来的位置。为了解释这一结果，我们提出了一个包括弹簧和钉扎点的位错横向位移模型。这种大的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。这也可能与石墨层间化合物中重要的物质传输有关。较少

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