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.
