Hosted by: Myung-Geun Han

The atomic-scale mechanisms that govern the adhesion, mechanical deformation, and bonding 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 scanning probe microscopy (e.g., nanoscale mapping of mechanical and functional properties), micro-/nano-electromechanical systems (e.g., actuators, switches), and nanomanufacturing processes (e.g., scanning probe lithography). In this talk, I will discuss contact and sliding tests on nanoscale tips of silicon and other technologically relevant materials. These tests were performed inside of a transmission electron microscope (TEM), enabling in situ interrogation of a contact interface while controlling the displacement of the 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 sliding 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 reduction in apparent work of adhesion as tip roughness increased from atomic-scale corrugation to a root-mean-square 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 sliding 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