High Speed Atomic Force Microscopy Techniques for the Efficient Study of Nanotribology

As mechanical devices scale down to micro/nano length scales, it is crucial to understand friction and wear at the nanoscale (nanotribology) especially at technically relevant sliding velocities. Accordingly, three novel techniques have been developed to study nanotribology, leveraging recent advances in high speed AFM. The first method utilizes high line-scanning rates coupled with sinusoidal scanning along the AFM fast scan axis, enabling rapid friction measurements as a function of velocity up to 20 mm/sec. The second method rapidly acquires friction versus force curves through disabling the feedback loop during scanning and relating the resulting lateral data with the correspondingly varying normal loads. The third and most widely applicable technique rapidly creates a map of friction-force curves based on a sequence of high speed images each with incrementally lower loads. As a result, ‘images’ of the coefficient of friction, friction at zero load, and/or load for zero friction (typically adhesive) can be uniquely determined for heterogeneous surfaces. This work includes measurements on mica, nanocrystalline diamond, and Au/SiO2 micro-fabricated structures, and is applicable for wear of sliding or rolling components in MEMS, biological implants, contact lenses, data storage devices, etc. The sinusoidal scanning technique allows friction force measurements in two dimensions to be acquired faster than any system currently on the market. The high scan velocity friction properties of mica have been characterized, and viscous damping forces between the cantilever and substrate dominate in agreement with the thermally-activated Eyring and Tomlinson models. Friction force curves are also extracted at any scan velocity along the line scan, allowing less experimental time to acquire such a broad range of equivalent friction data. Friction force curves collected with a disabled vertical feedback loop allow for the rapid characterization of substrates with either low or high varying topographies. The theory has been demonstrated on a silica characterization grating, allowing the coefficient of friction, friction at zero applied force, and pull-off force to be extracted. Finally, an array of friction force curves was acquired on a SiO2/thiol substrate at a scanning velocity approaching 3 mm/s. The coefficient of friction and friction at zero applied force were determined for the SiO2 phase and the thiol phases, and were equivalent to the coefficients acquired at normal scan rates, approximately 300 times slower. Not limited to high scan velocities, the importance of this approach is that friction can be mapped for specimens with defects, topographic features, and/or phase differences at the micro- and nano- scale