In-situ Atomic-Scale Observation on Interface Formation and Friction

在机械加工过程中，具有相对横向运动的两个固体表面之间的摩擦是一个重要的考虑因素。微电子机械系统和磁存储设备的不断小型化提高了摩擦的重要性，因为它在减少微电子机械系统的寿命和稳定性方面发挥着巨大的作用，并增加了其能耗和排放。基于连续介质力学模型的传统摩擦规律已不足以描述纳米尺度接触的摩擦行为，其中离散原子的位置和动力学对局部滑动过程起着重要作用。本研究将开发一种新的研究原子摩擦的方法，利用原位高分辨率透射电子显微镜来观察金属接触之间的原子尺度摩擦过程。研究结果将为金属接触之间原子摩擦的起源提供新的见解，并提供可用于改进微机电设备的设计和可靠性的知识，从而使美国工业和经济受益。该计划将把研究和教育结合起来，培训具有不同人口背景的研究生和本科生，特别是女性和其他代表性不足群体的成员，并为他们提供在国家实验室获得经验的机会。纳米摩擦学研究可以直接影响微机电系统（MEMS）和磁存储设备的性能和可靠性。这些设备的快速小型化提高了人们对理解和控制原子摩擦的需求。原子摩擦主要是通过原子力显微镜（AFM）和基于计算机的模拟来研究的，其中AFM尖端和衬底表面之间的反运动通常以“粘滑”的方式进行，伴随着与衬底晶格周期相称的锯齿状摩擦力。这么经典的？，打滑？在各种表面上观察到了摩擦行为。相比之下，在原子不相称的表面之间偶尔会发现以连续滑动和恒定微小摩擦力为特征的超润滑。然而，引起这种非常低的摩擦的原子机制仍然存在广泛的争论。阻碍理解控制摩擦的潜在机制的一个关键特征是原子粗糙接触界面的复杂性，主要由纳米级的凹凸不平组成。因此，要实现对原子摩擦的机械理解，需要关于单轴滑动接触的实时界面结构的信息，不幸的是，这超出了大多数现有实验方法的能力，包括基于原子力显微镜的技术，这是研究纳米/原子尺度摩擦最广泛应用的实验方法。本研究通过原子尺度的原位观察和摩擦力测量，揭示了界面结构对摩擦性能的显著影响。该研究有望显著推进对原子摩擦行为的基本理解，并为改进MEMS器件的设计和可靠性提供重要指导。原位透射电镜实验有望为直接观察原子摩擦过程开辟一条新的途径，也将丰富原子尺度上的摩擦学和摩擦理论。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Friction between two solid surfaces in contact with relative lateral motion is an important consideration in mechanical machine processes. The ongoing miniaturization of micro electro-mechanical systems and magnetic storage devices has elevated the importance of friction, because it plays an outsized role in reduction of the lifetime and stability of the micro electro-mechanical system and increases its energy-consumption and emissions. The conventional friction laws based on continuum mechanic models are no longer adequate to describe the frictional behaviors of nanometer-sized contacts in which the position and dynamics of discrete atoms play important roles in determining local sliding process. This research will develop a new methodology for studying atomic friction by in situ high resolution transmission electron microscopy to observe the atomic-scale frictional processes between metallic contacts. The research findings will provide new insights to the origin of atomic friction between metallic contacts, and provide knowledge that can be applied for improving the design and reliability of micro electro-mechanical devices, benefiting U.S. industries and the economy. The PI will integrate research and education, training graduate and undergraduate students with diverse demographic backgrounds, particularly female and other members of underrepresented groups, and providing opportunities for them to gain experience in national laboratories.Nanotribology studies can lead to insights that directly impact the performance and reliability of microelectro-mechanical systems (MEMS) and magnetic storage devices. The rapid miniaturization of these devices raises an increasing demand to understand and control atomic friction. Atomic friction has mainly been studied by atomic-force microscopy (AFM) and computer-based simulations, where counter motion between the AFM tip and the substrate surface is generally found to proceed in a "stick-slip" manner accompanied by sawtooth-like friction forces commensurate with the period of substrate lattice. Such a classic ?stick-slip? friction behavior has been observed on various surfaces. By contrast, super-lubricity featured by continuous sliding and constantly trivial friction forces has occasionally been found between atomically incommensurate surfaces. Yet, the atomistic mechanisms giving rise to this very low friction are still under extensive debate. A critical characteristic impeding understanding of the underlying mechanisms governing friction is the complex nature of the atomically rough contacting interface that mostly consists of nanoscopic asperities. Achieving a mechanistic understanding of atomic friction thus requires information on real-time interfacial structure of single-asperity sliding contacts, which is unfortunately beyond the capability of most existing experimental methods, including AFM-based techniques which is the most widely applied experimental method for studying nano-/atomic-scale friction. This research is to perform concurrent in-situ atomic-scale observation and friction force measurements during countermotion between nanosized single-asperity contacts and reveal the significant impact of the interfacial structure on frictional performances. The research is expected to significantly advance the fundamental understanding of atomic frictional behaviors, and provide important guidelines for improving the design and reliability of MEMS devices. The novel experiments on in-situ transmission electron microscope is expected to open a new avenue for direct observation of atomistic friction processes, and also will enrich tribology and friction theories at atomic scale.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.