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Functional Nanostructured Materials (including low-D carbon)
Superlubricity of Polyalkylene Glycol Aqueous Solutions Enabled by Ultrathin Layered Double Hydroxide Nanosheets Hongdong Wang, Yuhong Liu, Wenrui Liu, Yanmin Liu, Kunpeng Wang, Jinjin Li, Tianbao Ma, Osman Levent Eryilmaz, Yijun Shi, Ali Erdemir, and Jianbin Luo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b03014 • Publication Date (Web): 14 May 2019 Downloaded from http://pubs.acs.org on May 14, 2019
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ACS Applied Materials & Interfaces
Superlubricity of Polyalkylene Glycol Aqueous Solutions Enabled by Ultrathin Layered Double Hydroxide Nanosheets Hongdong Wang, †,
‡, §
Yuhong Liu, †,* Wenrui Liu, † Yanmin Liu, † Kunpeng Wang, † Jinjin Li, †
Tianbao Ma,† Osman Levent Eryilmaz,‡ Yijun Shi,§ Ali Erdemir,‡,* Jianbin Luo†,* †State
Key Laboratory of Tribology, Tsinghua University, Beijing 100084, China
‡Applied Materials Division, Argonne National Laboratory, Argonne, Illinois 60439, United States §Division
of Machine Elements, Luleå University of Technology, Luleå 97187, Sweden
KEYWORDS: Superlubricity, Layered Double Hydroxide, Nano-additive, Polyalkylene Glycol, Water-based, Tribological Properties
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ABSTRACT: It was previously proved that the existence of a large amount of hydrogen ions in the water-based lubricants can easily lead to a superlubric state; however, it was also shown that these hydrogen ions could cause severe corrosion. As part of a large family of attractive clays, layered double hydroxides (LDHs) possess excellent tribological properties in water-based lubrication systems. In the present work, two different kinds of LDHs are dispersed in polyalkylene glycol (PAG) aqueous solutions, in two distinct forms: ultrathin nanosheets (ULDH-NS) of ca. 60 nm wide and ca. 1 nm thick (single or double layer), and nanoparticles (LDH-NP) of ca. 19.73 nm wide and ca. 8.68 nm thick. We find that the addition of ULDH-NS greatly shortens (as much as 85%) the running-in period prior to reaching the superlubricity regime, and increases the ultimate load-bearing capacity by about 4 times. As compared to the fluid film thickness of the lubricating PAG solution, their ultrathin longitudinal dimension will not impair or influence the fluid film coverage in the contact zone. The analysis of sliding solid surfaces and AFM microscale friction test demonstrate that the adsorption of ULDH-NS enables the sliding solid surfaces to be polished and protected because of their relatively weak interlayer interaction and increased adhesion effect. Owing to the superior tribological properties as lubricant additives, ultrathin LDH nanosheets hold great potential for enabling liquid superlubricity in industrial applications in the future.
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ACS Applied Materials & Interfaces
INTRODUCTION In our highly industrialized society, unnecessary friction and wear losses adversely impact efficiency and reliability of all moving mechanical components, collectively costing about 5-7 % gross national products in most industrialized nations.1,2 Accordingly, in recent years, some of the most attractive research topics in tribology has been directed toward the minimization of friction coefficient (COF) as much as possible; i.e., below 0.01 which is referred to as a regime of superlubricity that was first studied back in early 1990s.3 Since then, the topic of superlubricity has attracted increased interest from various research groups with the hope that it can help reduce energy and material losses due to friction and wear in mechanical systems. Thanks to these concerted efforts, during the past two decades, significant strides have been made towards not only better understanding of the theoretical foundations of superlubricity but also realizing it experimentally across the scales.4,5 At present, superlubricity has been achieved with a range of solid and liquid lubricants. Some of the well-known examples for solid lubricants include: molybdenum disulfide,6-8 a range of carbon materials (graphite,9 carbon nanotubes,10 graphene nanoscroll with nanodiamond particles11) and a few other two-dimensional nanomaterials.12,13 In most of these cases, superlubricity was controlled by the structural nature of the specific materials or depended on a special atmosphere or gaseous/liquid species to provide such ultra-low friction values in the nano to micro scales. Some of the prime examples of superlubricity involving the liquid lubricants include polymeric brushes,14-16 and those that are very common in bio-lubrication area (i.e., 3
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brasenia schreberi,17,18 red microalga19 and joint lubrication20-22). They mainly depended on hydration, electric double layer effect, hydrogen bond network and fluid synergy to realize more stable superlubricity in the macro scale, but a certain running-in period was usually required. It was also reported that an ultralow COF (
