Accelerated Molecular Dynamics Study of the Role of Crystalline Defects in Friction of 2-Dimensional Materials

石墨烯和其他二维材料是具有原子尺度厚度的材料，这使得它们成为微型机电系统(MEMS)等小型设备的理想固体润滑剂。众所周知，石墨烯还具有一种被称为超润滑性的非凡性质，即两层材料之间的摩擦变得非常小。然而，这些超润滑油现象中的大多数都是在为非常小的薄片准备充分的实验室条件下观察到的。该奖项支持使用一种新的基于分子动力学的预测计算方法在原子尺度上研究二维材料的摩擦机制，该方法被称为超动力学。这种方法将使研究人员能够研究大的二维层，例如那些用化学气相沉积方法生长的层。本研究对超润滑性的进一步认识将为在宏观尺度上实现无摩擦滑动铺平道路。由于摩擦是能量耗散或浪费的主要原因之一，这项研究可能会加强运输、制造业和其他移动部件消耗大量能源的行业的可持续性和效率努力。在该项目中开发的所有计算机代码将免费提供给研究界。作为辛辛那提大学合作项目的一部分，该项目将为研究生提供培训，并为本科生提供研究机会。还将为大辛辛那提地区的当地高中组织一个基于计算机模拟的科学和工程教育外展计划。用化学气相沉积方法合成的石墨烯和其他二维材料的特点是具有多晶结构和缺陷，这被认为会对其力学和摩擦性能造成不利影响。在本项目中，将建立能够可靠地预测多颗粒石墨烯和其他二维材料(如二硫化钼和六方氮化硼)的力学和摩擦性能的理论和计算模型。一种名为HyperDynamic的新型原子模拟方法将被应用来克服传统方法的局限性，使模拟能够在接近实际实验的时空条件下进行。各种滑动物体和缺陷结构，包括晶界、堆积和表面台阶，将在几种化学环境和外部条件下进行研究，如温度、滑动速度、法向力。这种系统的研究将极大地加深我们对二维材料中潜在的原子级摩擦和能量耗散机制的理解，这对于开发高效的宏观器件是至关重要的。

Graphene and other 2-dimensional materials are materials that have thickness on the atomic scale, which makes them ideal solid lubricants for small length scale devices such as micro-electro-mechanical-systems (MEMS). Graphene is also known to possess an extraordinary property called superlubricity, where friction between two material layers becomes vanishingly small. Most of these superlubric phenomena, however, have been observed in well-prepared laboratory conditions for very small flakes. This award supports research on the mechanics of friction mechanisms of 2-dimensional materials at the atomic scale using a novel predictive computational method based on molecular dynamics called hyperdynamics. This method will allow the researchers to study large 2-dimensional layers such as those grown with the chemical vapor deposition method. An enhanced understanding about superlubricity achieved in this research will pave the way for realization of frictionless sliding on macroscopic scales. Since friction is one of the primary causes of energy dissipation or waste, the research will potentially enhance sustainability and efficiency efforts in transportation, manufacturing, and other sectors where moving parts consume a lot of energy. All computer codes developed in this project will be made freely available to the research community. The project will provide training to graduate students as well as research opportunities to undergraduate students as part of the University of Cincinnati's co-op program. An outreach program for science and engineering education based on computer simulations will also be organized for local high schools in the Greater Cincinnati area. Graphene synthesized by the chemical vapor deposition method and other 2-dimensional materials are characterized by a multi-grain structure with defects, which has been hypothesized to cause detrimental effects on their mechanical and frictional properties. In this project, theoretical and computational models which can reliably predict mechanical and frictional properties of multi-grain graphene and other 2-dimensional materials such as molybdenum disulfide and hexagonal boron nitride will be constructed. A novel atomistic simulation method called hyperdynamics will be applied to overcome the limitations of conventional methods enabling simulations under spatio-temporal conditions close to actual experiments. Various sliding objects and defected structures comprising grain boundaries, stacking, and surface steps will be investigated under several chemical environments and externally applied conditions, such as temperature, sliding velocity, normal force. This systematic research will greatly enhance our understanding of the underlying atomic-level frictional and energy dissipation mechanisms operative in 2-dimensional materials, which is critical in developing efficient macroscopic devices.