IMX Talks - Seeing the Hidden Interface: Revealing Nanoscale Mechanisms of Contact, Adhesion, and Wear by in situ Experiments

As technology scales shrink, tribology plays an increasingly dominant role. This can be problematic (e.g., high friction and wear in micro/nano devices), or advantageous (e.g., using adhesion to drive nanostructure formation). The biggest challenge in exploring and exploiting these issues is that the interface between two materials is hidden from view. Recent advances in in situ methods have enabled tribological mechanisms at previously inaccessible interfaces to be studied with unprecedented resolution. I will discuss new science revealed by in situ experimental methods to develop physically-based insights into tribological processes.

First, I will discuss the use of crystalline metal oxide nanoparticles to prevent damage in harsh environment mechanical applications. Metal oxides powders typically require temperatures >1000° C to coalesce into dense solids. Remarkably, metal oxide nanocrystals, including ZrO₂ and TiO₂, dispersed in lubricants can sinter at room temperature due to tribological stresses (compression and frictional shear) in a process known as tribosintering. In this process, the nanoparticles forming solid, surface-bound films that we call tribocoatings. We show for the first time that such tribocoatings prevent wear and other common tribological failure mechanisms under a wide range of harsh conditions.

Second, new insights into nanoscale adhesion and wear are achieved using in situ transmission electron microscopy (TEM) wear tests. A strong, reversible, sliding-history dependence of adhesion between silicon nanoasperities occurs, attributed to shear-induced removal of adsorbates. I will also preview new results applying the technique to study contact between two-dimensional materials including MoS₂.

Bio: Robert Carpick is the John Henry Towne Professor of Mechanical Engineering and Applied Mechanics at the University of Pennsylvania. He studies nanotribology, nanomechanics, scanning probes, and mechanochemistry. He is a recipient of the AVS Nanotechnology Recognition Award, the American Society of Mechanical Engineers (ASME) Newkirk Award, a R&D 100 award, and a NSF CAREER Award. He is a Fellow of the ASME, the American Physical Society, the Materials Research Society, the AVS, and the Society of Tribologists and Lubrication Engineers. He holds 10 patents and has authored over 200 peer-reviewed publications. Before joining UPenn in 2007, he was a faculty member at the University of Wisconsin-Madison. He received his B.Sc. (U. Toronto, 1991) and his Ph.D. (U. California at Berkeley, 1997) in Physics, and was a postdoctoral researcher at Sandia National Laboratory. He served as Department Chair from 2011-2019, and since 2020 serves as the Director of Diversity, Equity, and Inclusion and now Director of Broader Impact for his Department.