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

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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.