INTERSTRUCFRICNANO · Investigating the Effect of Interface Structure on Friction at the Nanoscale

Investigating the Effect of Interface Structure on Friction at the Nanoscale

The physical phenomenon of friction is the main source of energy dissipation in a number of technical applications and industrial processes. Despite this fact, which holds significant economic importance, a complete understanding of fundamental physical principles governing frictional processes is still lacking. Considering that an ability to predict and control macroscopic friction depends on accurate investigations of friction at the nanometer scale, the research area of nanotribology –the science of friction, lubrication and wear at the nanoscale– has been established about 25 years ago. The main scientific tool that spearheaded developments in this field, the friction force microscope (FFM), provided researchers with a great deal of insight regarding frictional properties of nanoscale “single-asperity contacts” on different substrates as a function of various experimental parameters. Despite this success, many open questions remain regarding friction at the nanoscale, due to inherent limitations of the single-asperity FFM technique in terms of contact area and poorly characterized interface structures. Being motivated by recent developments in the field, it is proposed in this project to investigate the frictional properties of nano islands using commercially available atomic force microscopes. The main objective of this particular research project supported by the Marie Curie Actions of the FP7 program can be summarized as contributing experimentally to an understanding of the role of interface structure on friction at the nanoscale, with implications for friction prediction and control at larger length scales. Within this context, the work performed in Prof. Baykara’s research group has delivered scientifically interesting results. In particular, the following progress has been achieved since the beginning of the project: 1) Gold nanoparticles (AuNPs) of various size have been obtained on graphite substrates, by a combination of thermal evaporation under high vacuum conditions and post-deposition annealing. 2) Nanoparticle morphology and distribution have been successfully characterized using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). 3) The AuNP/graphite material system has been structurally and nanotribologically characterized via FFM measurements under ambient conditions. In particular, nanoscale friction measurements performed on AuNPs trapped between graphite steps revealed a “2/3” power law dependence of friction on normal load. 4) By way of nano-manipulation experiments conducted via AFM, it has been observed that AuNPs on graphite slide in a “structurally lubric” fashion, in quantitative agreement with the theory of structural lubricity. This result constitutes the first observation of structurally lubric sliding under ambient conditions at mesoscale interfaces. 5) A collaboration with theoretical scientists revealed the potential reasons behind the observed phenomenon of structurally lubric sliding under ambient conditions. Specifically, it has been shown that the atomically flat gold-graphite interface imposes energy barriers of several eV to incoming contaminant molecules under ambient conditions, thus keeping the interface molecularly clean and resulting in the conservation of structural lubricity under ambient conditions. 6) To answer the question of whether structural lubricity under ambient conditions is limited to the use of gold nanoparticles, experiments have been extended to platinum nanoparticles (PtNPs). In particular, PtNPs on graphite have been obtained by a combination of e-beam evaporation and post-deposition annealing, and their morphology/atomic structure have been characterized by SEM and TEM, respectively. Nano-manipulation experiments conducted via AFM have revealed structurally lubric sliding for PtNPs as well, although with a higher magnitude of friction forces. 7) Finally, to contribute further to an understanding of the influence of interface structure on friction at the nanoscale, oxidation of PtNPs has been achieved via oxygen plasma treatment, and confirmed by X-ray photoelectron spectroscopy (XPS) experiments. Nano-manipulation experiments are currently being conducted on such nanoparticles and the related efforts will continue after the conclusion of the CIG project. Points (1) to (6) mentioned above have been discussed in detail in the two journal publications listed below: - Cihan, E., İpek, S., Durgun, E., Baykara, M.Z., Structural Lubricity under Ambient Conditions, Nature Communications 7, 12055 (2016). - Cihan, E., Özoğul, A., Baykara, M.Z., Structure and Nanotribology of Thermally Deposited Gold Nanoparticles on Graphite, Applied Surface Science 354, 429 (2015). The scientific outcomes of the project are rather far-reaching and have important implications for the understanding of the physical mechanisms governing friction on the nanometer scale. In particular, the observation of the occurrence of structurally lubric sliding under ambient conditions between gold and graphite surfaces is a “first” in the field of nanotribology. This discovery has the potential to pave the way towards the development of revolutionary lubrication schemes for next-generation micro- and nano-mechanical systems as well as the realization of mechanical actuation with minimal loss of energy at such small length scales. Thus, the results of the project become not only important for the field of nanotribology, but also for nanoscience and nanotechnology research in general. Finally, the project has directly resulted in measurable societal impact, as Prof. Baykara has already delivered multiple presentations about his research activities not only at conferences and workshops, but also in high schools and science fairs for young people interested in a scientific career. Within this context, the heavy involvement of undergraduate students in his research laboratory is also noteworthy. Most recently, Prof. Baykara has been chosen as one of the “Ten Outstanding Young Persons of the World” by Junior Chamber International (JCI), based partially on his work performed via the CIG project. Research Group Website (includes project information): http://spm.bilkent.edu.tr Contact: Mehmet Z. Baykara Assistant Professor Department of Mechanical Engineering Scanning Probe Microscopy (SPM) Research Group Bilkent University 06800 Ankara, Turkey Tel: +90 (312) 290-3428 E-mail: mehmet.baykara@bilkent.edu.tr Web: spm.bilkent.edu.tr

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