Collaborative Research: Temperature-Dependence of Atomic-Scale Friction

减少工程系统中的摩擦可以大大减少全球能源消耗和有害的环境排放。润滑剂、工程表面和机械设计的改进带来了重大进展，但在考虑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操作或设计用于管理温度升高的应用相关。摩擦的温度依赖性在诸如卫星的航空航天系统中也很重要，卫星具有数千个暴露于从几百摄氏度到接近绝对零度的温度范围内的移动接触部件，但是一旦部署在太空中就不能维修，因此必须不能发生故障。因此，理解、预测和控制摩擦力随温度的变化至关重要。这项研究的重点是机制，确定纳米尺度的单一微凸体的摩擦温度依赖性。这项工作最终有助于更深入地理解和更精确和预测的方法来设计可靠的节能系统。该项目还将通过各种活动产生影响，包括通过参与以女性工程师为重点的项目，以及大学生和高中教师参与研究，开发基于摩擦的学习模块。这项研究的智力价值在于推进对单一粗糙体摩擦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，以便模拟和实验中的条件得到最佳匹配，从而可以直接比较和验证结果，最大限度地提高所获得的理解。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性地联系起来。这种独特的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;使用新型热探针，使接触温度快速变化;通过使用加速模拟和超快原子力显微镜扫描，以相同的滑动速度进行建模和模拟。研究将在三种不同的环境中进行，以隔离不同的温度依赖性的贡献：超高真空环境，在水蒸气的存在下，在碳氢化合物蒸气的存在下。通过这种全面的方法，可以明确地建立界面摩擦的温度依赖性的基本机制。

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. This 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 this 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.