Frictional properties of molecularly-thin organic coatings: Discovering the fundamental mechanisms of friction

Erin Flater
Physics Department
Luther College

Tribology, the study of friction, wear, and lubrication, has grown out of a longstanding relationship between scientific curiosity and technological need. Friction is a common yet complex phenomenon that is not understood on a fundamental level. Atomic force microscopy (AFM) is a versatile tool for studying the frictional properties of nanoscale single asperity contacts. Quantitative AFM measurements are used to study the friction properties of organic monolayer coatings.

Systematic studies octadecyltrichlorosilane (OTS) monolayers and silicon using both uncoated and OTS-coated silicon AFM tips and surfaces show that f riction is reduced by the presence of the OTS coating, and the overall shape of the friction vs. load plot strikingly depends on whether or not the substrate is coated with OTS, regardless of tip material. Uncoated substrates exhibit the common sublinear dependence of friction on load, while coated substrates exhibit an unusual superlinear dependence. These results can be explained qualitatively by invoking molecular plowing as a significant contribution the frictional behavior of OTS. As well, direct in-situ comparison of two intrinsic OTS structural phases of otherwise identical molecules on the substrate show that the lower packing density phase exhibits higher friction, decisively observed in single, uninterrupted images on the same monolayer for the first time. The lateral stiffness of the two OTS structural phases are indistinguishable, which implies that the packing density directly affects the interface's intrinsic resistance to shear as opposed to simply modifying the stiffness of the monolayer.

Direct comparisons of results from different nanotribological techniques have been rare, but they are important for providing insight into frictional mechanisms over different regimes of size, pressure, and velocity. Such studies promise to bridge the technical and scientific areas of this expanding field of study. For these reasons, a collaboration between the speaker and her students at Luther College , and Prof. Borovsky and his students at St. Olaf College is anticipated. In this way, AFM friction measurements can be compared to measurements performed on Prof. Borovsky's QCM/nanoindenter system using identical interfaces on each system. This collaboration will focus on the velocity dependence of friction of organic monolayers, since AFM and QCM probe friction in different velocity regimes. Identical interfaces will allow us to isolate the dependence of friction on velocity from contributions to friction from other tribological variables.