New insights into friction, adhesion, and wear from in situ methods based on atomic force microscopy (AFM) and on transmission electron microscopy (TEM) are presented. I will first present results using AFM to develop nanoscale insights into practical lubrication mechanisms. For zinc dialkyldithiophosphates (ZDDPs), we previously reported an AFM-based approach for visualizing the tribochemical formation of ZDDP anti-wear films in situ at the nanoscale on iron. New results show how tribofilm growth occurs on many other substrates, provided they have sufficient mechanical hardness. I will also describe the in situ growth of anti-wear tribofilms formed in an AFM with zirconia nanoparticles dispersed in oils, which also form tribofilms. These tribofilm growth mechanisms also permit using AFM to pattern surfaces with nanoscale features. Finally, I will discuss asperity-on-asperity sliding experiments conducted using a nanoindentor inside a TEM. Forming and separating contacts without sliding revealed small adhesion forces; sliding during retraction resulted in a nearly 20 times increase in adhesion. We attribute this surprising sliding-dependent adhesion to the removal of passivating terminal species from the surfaces, followed by re-adsorption of these species after separating the surfaces. This shows how adhesion can potentially be tuned by controlled sliding.
Dr. Robert Carpick is the John Henry Towne Professor of Mechanical Engineering and Applied Mechanics, University of Pennsylvania. He studies nanotribology, nanomechanics, and scanning probes. He is a recipient of the ASME Newkirk Award, a R&D 100 award, and a NSF CAREER Award. He is a Fellow of the American Society of Mechanical Engineers, the American Physical Society, the Materials Research Society, the AVS, and the Society of Tribologists and Lubrication Engineers. He holds 6 patents and has authored over 190 peer-reviewed publications. Previously, he was a faculty member at the University of Wisconsin-Madison. He received his B.Sc. (University of Toronto, 1991) and his Ph.D. (University of California at Berkeley, 1997) in Physics, and was a postdoctoral researcher at Sandia National Laboratory. He served as Department Chair from 2011-2019, and currently serves as the Director of Diversity, Equity, and Inclusion for his department.