Jacobs, Tevis

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  • Publication
    Imaging and Understanding Atomic-Scale Adhesion and Wear: Quantitative investigations Using in situ TEM
    (2013-01-01) Jacobs, Tevis
    The underlying physics governing tribological interactions - adhesion, friction, lubrication, and wear - are poorly understood. Significant progress has been enabled by nanoscale studies using the atomic force microscope (AFM). However, AFM lacks direct access to the contact geometry and structure. In this thesis, nanoscale adhesion and wear tests were performed inside of a transmission electron microscope (TEM), enabling real-time in situ interrogation of the contact in vacuum. Quantitative data was extracted using custom analysis routines to resolve tip shape, volume changes, and adhesive forces with unprecedented resolution. From in situ adhesion tests, a novel method was developed to extract the work of adhesion (0.66±0.14 J/m^2) and range of adhesion (0.25±0.06 nm) between silicon and diamond. The latter quantity has not previously been measured experimentally. TEM adhesion tests and complementary atomistic simulations reveal an order-of-magnitude reduction in apparent work of adhesion as tip roughness increased from atomic-scale to a root-mean-square value of 1 nm. Using an existing analytical model, an empirically derived roughness-independent adhesion parameter was extracted. In situ wear tests of silicon on diamond at low load revealed the mechanism of wear to be consistent with atom-by-atom processes. The rate of atomic removal varied exponentially with average normal stress, consistent with stress-mediated chemical reaction kinetics. This yields a physically reasonable activation energy (0.85±0.06 eV), and activation volume (6.7±0.3 Å). This framework can be generalized to understand and potentially predict wear in many materials undergoing atom-by-atom removal. Together, these investigations advance the scientific understanding of nanoscale adhesion and wear and help bridge the gap between experiments and atomistic simulations. Three examples are demonstrated where nanometer-scale trends can be predicted using continuum approaches: nanoscale adhesive forces can be calculated using an interaction potential; apparent work of adhesion depends on nanoscale root-mean-square roughness; and the rate of atomic-scale wear reactions is determined by the average normal contact stress. These examples, while only demonstrated in the specific systems studied, suggest strategies and future research directions for understanding, predicting, and controlling tribological phenomena.