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

Abstract: Tribology – the study of interacting surfaces in relative motion and the resulting phenomena of adhesion, friction, lubrication, and wear – is critical across a wide range of industries and technologies, and is highly consequential for for global energy consumption. Moreover, at small scales 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 normally 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 tribological 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. Here, the nanoparticles form solid, surface-bound films that we call tribocoatings. Despite being thinner than 100 nm, tribocoatings prevent wear and other common tribological failure modes 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, that is recoverable by readsorption. I will also preview new results applying the technique to study contact between two-dimensional materials including MoS₂.

Brief Biographical Sketch: 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 has authored over 220 peer-reviewed publications and holds 10 issued patents. 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.

Host: Dr. Lei Li