CPM Seminar

Friction in Full View

Laurence Marks

Department of Materials Science
Northwestern University

Friction is a pervasive phenomenon, which is estimated to represent an economic cost of approximately $100 billion/year in the U.S. alone, and an estimated 1.6% of developed countries GDP. Although the importance of friction, and reducing it, was recognized as far back as the construction of the pyramids, we still have only a very fragmented basic understanding of the atomic mechanisms. One of the fundamental problems has been the lack of probes to determine what is taking place at the atomic scale at sliding contacts.

One aspect of friction which has seen surprisingly little attention is a description of the basic processes in terms of interfacial dislocation sliding; without need for adjustable parameters it turns out that one can approximately predict [1,2] both friction coefficients as well as a number of nanoscale phenomena which have been observed experimentally. Beyond just theory we have recently been able to perform sliding contact experiments using a scanning probe tip inside a transmission electron microscope [3]. This allows one to image using the full range of electron microscopy tools what is taking place around the contact region. Results to date include the direct observation of a nanoscale graphitic transfer layer when sliding a tungsten tip on graphite [4], as well as electron energy loss data indicating the formation of graphitic material in-situ during sliding on very-low friction amorphous carbon films [5]. Some other observations are less expected, for instance the observation of graphitic wear debris during sliding on graphite with a size which is consistent with an analytic model of sliding via misfit dislocations [1,2]; the size of the debris is consistent with the stand-off distance for misfit dislocations. Other phenomena have also been observed, for instance liquid-like deformation of gold films, similar to the well-known surface diffusion dominated liquid-like growth of thin films [6]; this may partially explain some phenomena in solid-solid lubrication, as well as recrystallization [7]. These and more recent results will be described.

[1] Merkle, A.P. and L.D. Marks, Comment on “friction between incommensurate crystals”. Philosophical Magazine Letters, 2007. 87(8): p. 527-532.
[2] Merkle, A.P. and L.D. Marks, A predictive analytical friction model from basic theories of interfaces, contacts and dislocations. Tribology Letters, 2007. 26(1): p. 73-84.
[3] Marks, L.D., O. Warren, A. Minor, and A. Merkle, Nanotribology in Full View. Materials Research Bulletin, 2008. 33(12): p. 1168-1173.
[4] Merkle, A.P. and L.D. Marks, Friction in full view. Applied Physics Letters, 2007. 90(6): p. 064101.
[5] Merkle, A.P., Erdemir, A., Eryilmaz, O.L., Johnson, J.A., Marks, L.D., In situ TEM studies of tribo-induced bonding modifications in near-frictionless carbon films. Carbon 48(3), 587-591 (2010).
[6] Merkle, A.P. and L.D. Marks, Liquid-like tribology of gold studied by in situ TEM. Wear, 2008. 265: p. 1864-1869.
[7] Liao, Y., EswaraMoorthy, S.K., & Marks, L.D., Direct observation of tribological recrystallization. Philos. Mag. Lett. 90(3), p. 219-223 (2010).