October 12, 2018
Nanoscale Factors Controlling Friction and Lubrication: From 2D Materials to Engine Oil
New insights into friction and lubrication from atomic force microscopy (AFM) are presented. First, nanocontacts with 2-dimensional materials like graphene are discussed, where friction depends on the number of layers. An initial model attributing this to puckering  is now enhanced by molecular dynamics (MD) simulations showing a strong role of energy barriers due to interfacial pinning and commensurability . I will then discuss very recent results where AFM is used to develop new insights into practical lubrication mechanisms. We study zinc dialkyldithiophosphates (ZDDPs), which are highly effective anti-wear additive molecules used nearly universally in engine oils. We developed a novel AFM-based approach for visualizing and quantifying the formation of ZDDP anti-wear films in situ at the nanoscale. Film growth depends exponentially on temperature and stress, which can explain the known graded-structure of the films. Our findings provide new insights into the mechanisms of formation of ZDDP derived anti-wear films and the control of lubrication in automotive applications .
 C. Lee et al. Frictional Characteristics of Atomically-Thin Sheets. Science, 328, 76 (2010).
 S. Li et al. The Evolving Quality of Frictional Contact with Graphene. Nature 539, 541 (2016).
 N.N. Gosvami et al. Mechanisms of Antiwear Tribofilm Growth Revealed in situ by Single Asperity Sliding Contacts, Science, 348, 102 (2015).