Engineering Physics Colloquium
Wednesday, January 22
106 Engineering Research Building
Speaker: Dr. Tevis Jacobs, University of Pennsylvania
"Microfabricated Expandable Sensor Networks for Intelligent Structures"
Abstract: The atomic-scale mechanics, chemistry, and physics that govern the adhesion and wear of surfaces in contact are not well understood. Yet accurate description and prediction of such contact phenomena is critically important in advanced nanoscale applications, including micro-/nano-electromechanical systems (e.g., actuators, switches), nanomanufacturing processes (e.g., dip-pen nanolithography), and microscopy applications (e.g., nanoscale mapping of mechanical properties). In this work, nanoscale adhesion and wear tests were performed inside of a transmission electron microscope (TEM) using a commercial nanoindenter that was modified to enable contact between a flat diamond punch and the sharp nanoscale tip of an atomic force microscope (AFM) probe. This setup enables in situ interrogation of a contact interface while controlling the displacement of the two bodies and measuring normal forces with sub-nanonewton resolution. Quantitative data were extracted using custom analysis routines to resolve the geometry of the contacting bodies, adhesive forces, and volumes removed due to wear, all with unprecedented resolution. In the first part of the talk, TEM adhesion tests of carbon-based coatings on diamond performed using this setup will be discussed. Sub-nanonewton force resolution was paired with Angstrom-scale measurements of asperity geometry. Combined with complementary molecular dynamics simulations, these results revealed an order-of-magnitude eduction in apparent work of adhesion as tip roughness increased from atomic-scale corrugation to a root-meansquare value of 1 nm. These results demonstrate the strong effect of sub-nanoscale topography on adhesion, and highlight a key limitation of conventional approaches for measuring the work of adhesion. In the second part of the talk, in situ wear tests of silicon tips sliding on diamond at low applied loads reveal that wear occurs by atomic attrition: gradual material removal at the atomic scale. The process can be accurately described using stress-assisted chemical reaction kinetics. The activation parameters extracted from this approach are physically reasonable, and constitute the first direct validation of the atomic attrition process. This framework can be generalized to understand and potentially predict wear in many materials undergoing atomic attrition, and suggests strategies for rationally testing and choosing materials for improved wear resistance.
Biography: Dr. Tevis Jacobs received his B.Sc. at the University of Pennsylvania with a double major in Mechanical Engineering and Materials Science and Engineering. He went on to receive an M.Phil. in Computer Modeling of Materials from the University of Cambridge, and an M.Sc. in Materials Science and Engineering from Stanford University. During his undergraduate and Masters’ studies, he conducted experimental and modeling investigations into mechanical failure in a variety of materials including bulk metallic glasses, superalloys for aerospace applications, and advanced dielectric materials for semiconductor devices. He then worked for two years in Research and Development at Animas Corporation, a Johnson & Johnson company. Dr. Jacobs obtained his Ph.D. from the University of Pennsylvania, working in the group of Prof. Robert Carpick. His doctoral research focused on the investigation of the fundamentals of nanoscale adhesion and sliding wear using quantitative in situ transmission electron microscopy. He is currently a Post-Doctoral Researcher in the group of Prof. Carpick, investigating failure mechanisms in disordered materials. He received a Graduate Student Gold Award from the Materials Research Society’s Graduate Student Award competition, and is the recipient of the Dorothy M. and Earl S. Hoffman Scholarship Award from the American Vacuum Society.