Buzio Renato, Gerbi Andrea, Bernini Cristina, Repetto Luca, Silva Andrea, Vanossi Andrea
CNR-SPIN, C.so F.M. Perrone 24, Genova 16152, Italy.
Dipartimento di Fisica, Università degli Studi di Genova, Via Dodecaneso 33, Genova 16146, Italy.
ACS Appl Nano Mater. 2023 Jun 15;6(13):11443-11454. doi: 10.1021/acsanm.3c01477. eCollection 2023 Jul 14.
Solution-processed few-layer graphene flakes, dispensed to rotating and sliding contacts via liquid dispersions, are gaining increasing attention as friction modifiers to achieve low friction and wear at technologically relevant interfaces. Vanishing friction states, i.e., superlubricity, have been documented for nearly-ideal nanoscale contacts lubricated by individual graphene flakes. However, there is no clear understanding if superlubricity might persist for larger and morphologically disordered contacts, as those typically obtained by incorporating wet-transferred solution-processed flakes into realistic microscale contact junctions. In this study, we address the friction performance of solution-processed graphene flakes by means of colloidal probe atomic force microscopy. We use a state-of-the-art additive-free aqueous dispersion to coat micrometric silica beads, which are then sled under ambient conditions against prototypical material substrates, namely, graphite and the transition metal dichalcogenides (TMDs) MoS and WS. High resolution microscopy proves that the random assembly of the wet-transferred flakes over the silica probes results into an inhomogeneous coating, formed by graphene patches that control contact mechanics through tens-of-nanometers tall protrusions. Atomic-scale friction force spectroscopy reveals that dissipation proceeds via stick-slip instabilities. Load-controlled transitions from dissipative stick-slip to superlubric continuous sliding may occur for the graphene-graphite homojunctions, whereas single- and multiple-slips dissipative dynamics characterizes the graphene-TMD heterojunctions. Systematic numerical simulations demonstrate that the thermally activated single-asperity Prandtl-Tomlinson model comprehensively describes friction experiments involving different graphene-coated colloidal probes, material substrates, and sliding regimes. Our work establishes experimental procedures and key concepts that enable mesoscale superlubricity by wet-transferred liquid-processed graphene flakes. Together with the rise of scalable material printing techniques, our findings support the use of such nanomaterials to approach superlubricity in micro electromechanical systems.
通过液体分散体分配到旋转和滑动接触处的溶液处理少层石墨烯薄片，作为摩擦改性剂在技术相关界面实现低摩擦和磨损正受到越来越多的关注。对于由单个石墨烯薄片润滑的近理想纳米级接触，已记录到消失摩擦状态，即超润滑性。然而，对于更大且形态无序的接触（如通过将湿转移溶液处理薄片纳入实际微尺度接触结通常获得的接触）超润滑性是否会持续，尚无明确认识。在本研究中，我们通过胶体探针原子力显微镜研究溶液处理石墨烯薄片的摩擦性能。我们使用最先进的无添加剂水分散体涂覆微米级二氧化硅珠，然后在环境条件下将其在原型材料基底（即石墨和过渡金属二硫属化物（TMDs）MoS和WS）上进行滑动。高分辨率显微镜证明，湿转移薄片在二氧化硅探针上的随机组装导致形成不均匀涂层，该涂层由石墨烯片组成，这些片通过几十纳米高的凸起控制接触力学。原子尺度摩擦力谱显示，耗散通过粘滑不稳定性进行。对于石墨烯 - 石墨同质结，可能会发生从耗散粘滑到超润滑连续滑动的负载控制转变，而单滑和多滑耗散动力学则表征石墨烯 - TMD异质结。系统的数值模拟表明，热激活的单峰普朗特 - 汤姆林森模型全面描述了涉及不同石墨烯涂覆胶体探针、材料基底和滑动状态的摩擦实验。我们的工作建立了通过湿转移液体处理石墨烯薄片实现中尺度超润滑性的实验程序和关键概念。连同可扩展材料印刷技术的兴起，我们的发现支持使用此类纳米材料在微机电系统中实现超润滑性。
