Collaborative Research: Temperature Dependence of Atomic Scale Friction

减少工程系统的摩擦可以大大减少全球的能源消耗和有害的环境排放。润滑油、工程表面和机械设计方面的改进已经取得了重大进展，但当考虑到极高或极低温度下的摩擦时，出现了一系列新的挑战。高温摩擦与许多在高温环境中运行或设计用于控制温升的应用有关。摩擦的温度依赖性在卫星等航空航天系统中也很重要，卫星拥有数千个运动的、接触的部件，暴露在从几百摄氏度到接近绝对零度的温度范围内，但一旦部署到太空中就无法维修，因此一定不能失败。因此，理解、预测和控制摩擦作为温度的函数是至关重要的。提出的研究重点是确定纳米尺度单颗粒摩擦的温度依赖性的机制。这项工作最终有助于更深入的理解和更精确的预测方法，以设计可靠、节能的系统。该项目还将通过参与以工程领域女性为重点的项目，以及让本科生和高中教师参与研究，开发基于摩擦的学习模块等活动，从外展的角度产生影响。这项研究的智力价值在于推进了对单一凸起摩擦的温度依赖性的基本理解。将进行原子模拟和原子力显微镜实验，其中使用最先进的方法，使模拟和实验中的条件最佳匹配，允许结果直接比较和验证，最大限度地获得理解。这种紧密耦合的方法将使接触的原子结构、力学、动力学和热行为与摩擦力和相应的能量耗散确定地联系在一起。提出的独特协作方法的主要特点是：集成了先进的变温原子力显微镜测量和最佳匹配系统的原子模拟；使用新型热探头，可以快速改变触点的温度；通过使用加速模拟和超快原子力显微镜扫描，在相同滑动速度下进行建模和仿真。研究将在三种不同的环境中进行，以分离出不同的温度依赖贡献：超高真空环境、存在水蒸气的环境和存在碳氢化合物蒸汽的环境。通过这种综合的方法，可以明确地建立控制界面摩擦的温度依赖性的潜在机制。

Reducing friction in engineered systems can substantially reduce worldwide energy consumption and detrimental environmental emissions. Improvements in lubricants, engineered surfaces, and mechanical design have led to significant progress, but a new set of challenges emerges when considering friction at very high or very low temperatures. High temperature friction is relevant to many applications that either operate in elevated temperature environments or are designed to manage temperature rise. The temperature dependence of friction is also important in aerospace systems such as satellites, which possess thousands of moving, contacting parts exposed to temperatures ranging from a few hundred degrees Celsius down to near absolute zero, but cannot be serviced once deployed in space and so must not fail. Understanding, predicting, and controlling friction as a function of temperature is therefore critical. The proposed research is focused on mechanisms that determine the temperature dependence of friction for nanoscale single asperities. This work can ultimately contribute to a deeper understanding and more precise and predictive approach to designing reliable, energy-efficient systems. The project will also have impact from an outreach perspective through activities including development of friction-based learning modules disseminated through participation in programs focused on women in engineering, and involvement of undergraduates and high school teachers in the research.The intellectual merit of this research lies in advancing the fundamental understanding of the temperature dependence of friction for single asperities. Atomistic simulations and atomic force microscopy experiments will be conducted, where state-of-the-art methods are used so that the conditions in the simulations and experiments are optimally matched, allowing results to be directly compared and validated, maximizing the understanding gained. This tightly-coupled approach will enable the atomic structure, mechanics, dynamics, and thermal behavior of the contact to be deterministically linked with friction forces and the corresponding energy dissipation. Key features of the proposed unique collaborative approach are: integration of advanced variable-temperature atomic force microscope measurements and atomistic simulations of optimally-matched systems; use of novel thermal probes that enable rapid variation the temperature of the contact; and modeling and simulation at the same sliding speeds through the use of accelerated simulations and ultrafast atomic force microscope scanning. Studies will be performed in three different environments to isolate distinct temperature-dependent contributions: ultra-high vacuum environment, in the presence of water vapor, and in the presence of hydrocarbon vapors. With this comprehensive approach, the underlying mechanisms governing the temperature dependence of interfacial friction can be definitively established.