Collaborative Research: Determining the Physical Mechanisms of Atomic Stick -Slip Friction by Closing the Gap between Experiments and Atomistic Simulations

这个合作研究项目的目标是了解控制纳米级接触滑动的原子级摩擦机制，特别关注被称为“原子粘滑摩擦”的普遍现象。分子动力学（MD）模拟和原子力显微镜（AFM）实验，尽可能接近匹配摩擦学条件将允许两者的结果进行直接比较。目前，将AFM实验与包括分子动力学（MD）在内的动态原子模拟进行比较的能力是有限的。人们无法在AFM实验中看到原子的位置和速度，这激发了MD研究。然而，MD结果不能直接与AFM的结果进行比较，因为传统的模拟必须以比AFM实验快几个数量级的速度运行。同样，许多MD和AFM研究在其他重要条件如材料、载荷和针尖尺寸方面也有差异。在这项工作中，速度将通过同时使用由主要研究人员合作开发的专门用于原子尺度摩擦研究的新方法来匹配。贵金属的使用将确保界面被良好定义并可靠地建模。其他关键参数，包括刚度，尖端尺寸和形状，环境，晶体表面和滑动方向取向，也匹配。如果成功，这将使接触的原子结构、力学和动力学与相应的摩擦力和能量耗散直接联系起来。具体而言，通过缩小模拟和实验之间的差距，详细的结果和机制，只有在原子模拟可解决的可以通过实验验证，实验观察到的现象可以解释的参考模拟，都可以形成可靠的预测模型描述纳米级摩擦滑动的基础。这将提供一个深刻的和可靠的理解，单一的粗糙摩擦，这是一个重要的一步，充分了解的行为集合的粗糙，遇到的大规模接触？纳米摩擦学研究的一个长期目标。从技术的角度来看，这项工作可以有助于合理设计涉及接触，滑动表面的纳米机械设备所需的知识基础。从教育的角度来看，通过两位主要研究者之间的合作，将产生重大影响。这包括基于AFM的多用途演示模块的开发，本科生和高中生的参与，组织和提供纳米摩擦学研究生短期课程，并积极参与国际合作网络社区。

The objective of this collaborative research project is to understand the atomic-level mechanisms of friction that control the sliding of nanoscale contacts, with a particular focus on the prevalent phenomenon known as "atomic stick-slip friction". Molecular dynamics (MD) simulations and atomic force microscopy (AFM) experiments that match tribological conditions as closely as possible will allow results from both to be directly compared. The ability to compare AFM experiments with dynamic atomistic simulations, including molecular dynamics (MD), is limited at present. One is unable see the positions and velocities of the atoms in AFM experiments, which motivates MD studies. However, MD results cannot be directly compared with those from AFM because conventional simulations must be run at speeds several orders of magnitude faster rates than AFM experiments. As well, many MD and AFM studies differ in other important conditions such as materials, load, and tip size. In this work, speeds will be matched through the concurrent use of new methods being developed collaboratively by the principal investigators specifically for atomic-scale friction studies. The use of noble metals will ensure that the interface is well-defined and reliably modeled. Other critical parameters, including stiffness, tip size and shape, environment, and crystal surface and sliding direction orientation, are also matched. If successful, this will enable the atomic structure, mechanics, and dynamics of the contact to be directly linked with the corresponding friction forces and energy dissipation. Specifically, by closing the gaps between simulation and experiment, the detailed results and mechanisms resolvable only in atomistic simulations can be validated by experiments, phenomena observed experimentally can be explained by reference to the simulations, and both can form the basis for reliable predictive models describing nanoscale frictional sliding. This will provide a deep and reliable understanding of single asperity friction, which is an important step toward fully understanding the behavior of collections of asperities that one encounters in larger-scale contacts ? a longstanding goal for nanotribology research. From the technological perspective, the work can contribute to the knowledge base needed for the rational design of nanomechanical devices that involve contacting, sliding surfaces. From an educational perspective, there will be significant impact through collaborative efforts between the two principal investigators. This includes development of a multi-purpose demonstration module based on AFM, involvement of undergraduates and high school students, organizing and delivering a short course on nanotribology to graduate students, and active participation in international collaborative cyber-network communities.