Zhao Deng, Rachel J. Cannara
Lamellar materials such as graphite, MoS2, and graphene typically have very low friction coefficients and thus excellent tribological performance. Because of that, this category of materials has been the subject of ongoing interests. Of particular interest are those mechanical and interfacial properties at nanoscale that enable their use technologically as components and lubricants in MEMS. This category of materials also concurrently offers a unique platform for studying the fundamental origins of friction and interlayer interactions.
While frictional forces always oppose motion, in this study friction is observed to increase with decreasing load on aged lamellar materials using atomic force microscopy. This results in a negative nanoscale coefficient of friction. The magnitude of the friction coefficient increases with tip-sample adhesion, which we control by varying the sample aging time. Adhesion increases as a result of increased surface oxygen content, which we characterize using x-ray photoemission spectroscopy (XPS). We demonstrate that the negative coefficient is a mechanical effect that behaves consistently with a change and deformability of the top layer with applied load. We further show that the magnitude of this negative friction coefficient is determined by the ratio of the work of adhesion to the exfoliation energy.