Disruption of Plastic Flow in Metals by Adsorbed Organic Monolayers

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NON-TECHNICAL SUMMARYThis project will support research leading to new knowledge for structural metals that will improve their durability, processing and re-use. By doing so, it will advance the science base of critically important materials and manufacturing processes, and provide technological benefits for industry sectors as diverse as energy systems, ground transportation and aerospace. The project will explore a recently uncovered phenomenon pertaining to metals - a large change in their fracture and plastic flow behavior arising from application of even nanoscale organic films onto their surface. It will study the changes produced in surface properties of the metals (e.g., surface and interface energy, surface stress, strength) by the films. Based on this study of the properties, and their relation to film attributes, e.g., chemistry, chain length, adsorption, it will identify the factors that control the flow and fracture phenomenon. The approach will combine advanced experimental techniques such as high-speed imaging and image analysis and nanoscale metrology, with atomistic modeling of interactions between organic molecule structure and metal surface attributes. The findings will impinge upon areas as diverse as materials processing, environmentally-assisted cracking and wear – areas, where synergistic effects of mechanical loading and chemistry often play a key role. Controllability of the mechanochemical phenomenon by tailoring of the organic film chemistry will enable enhancements in material removal (e.g., cutting, comminution) and surface deformation (e.g., friction-stir processing) processes for metals. The research integrates several disciplines including materials engineering, surface science and metrology. The multi-disciplinary approach will also contribute to broadening the participation of underrepresented minorities in research via involvement of summer students from the National Technical Institute for the Deaf, foster multi-disciplinary scientific collaborations, and positively impact engineering education. TECHNICAL SUMMARYThe proposed research will advance our understanding of chemical effects in surface plasticity for metals through a fundamental study of a recently uncovered mechanochemical effect - disruption of plastic flow and surface embrittlement by adsorbed organic monolayers. By integrating a surface molecular probe (Self-Assembled Monolayers (SAMs)) and high-resolution in situ deformation analysis, with atomistic and continuum modeling of monolayer attributes and materials behavior, the research will address two closely-related hypotheses on the effect: 1) an adsorbed monolayer is sufficient to disrupt surface plastic flow and induce a local ductile-to-brittle transition; and 2) energetics of the monolayer-metal interface (surface stress vs. interface energy) controls the surface flow and the flow-fracture transition. The hypotheses exploration is guided by four objectives: 1) Utilize molecular self-assembly (SAMs) to anchor various monolayers onto metal surfaces, and characterize their attributes. The principal thermodynamic parameters of the surface, namely surface/interface energy and surface stress, will be varied via molecule chemistry and chain length; 2) Analyze the plastic-deformation response of the metal and associated flow dynamics, with and without the organic films, under controlled mechanical loading (e.g., simple shear, uniaxial tension) of specimens with high surface area-to-volume; 3) Develop a model for explaining the mechanochemical effect due to adsorbed monolayers – from changes in nanoscale surface properties to mesoscale transitions in plastic flow; and 4) Integrate the experiments and modeling to understand how adsorbed films influence surface plasticity in metals, and what monolayer attributes control the mechanochemical effect. The study will be conducted specifically with commercially pure aluminum and iron, selected for their diversity in structure and deformation response, experimental suitability and technological interest. The findings will be of value for areas as diverse as materials processing, environmentally-assisted cracking and wear, wherein synergistic effects of mechanical loading and chemistry often play a key role. The education and outreach activities involves undergraduate students in creating a video gallery of plastic flow/fracture phenomena for structural metals and processing; a modest focus on entrepreneurship in graduate study; and involvement of summer students from the National Technical Institute for the Deaf.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.