Abstract
Advances in nano-lubricant additives are vital to the pursuit of energy efficiency and sustainable development. Carbon dots (CDs) have been widely investigated in the domain of lubricant additives owing to their extraordinary tribological properties, in particular, their friction-reducing and anti-wear properties. Metal-doped CDs are a new type of CDs, and their friction-reducing and anti-wear properties are attracting increasing attention. Therefore, a series of CDs doped with various divalent metal ions have been successfully synthesized via one-pot pyrolysis. The tribological properties of the synthesized CDs as water-based lubricant additives are in the following order: Zn-CDs > Cu-CDs ≫ Mg-CDs > Fe-CDs > U-CDs. Specifically, adding 1.0 wt% of Zn-CDs into water-based lubricant results in 62.5% friction and 81.8% wear reduction. Meanwhile, the load-carrying capacity of the water-based lubricant increases from 120 N to at least 500 N. Zn-CDs as an additive have long service life. Additionally, anion-tuned Zn-CDs fabricated via anion exchange exhibit promise as lubricant additives for poly(ethylene glycol). Based on the results of wear scar surface analyses, it is discovered that tribochemical films, primarily composed of iron oxides, nitrides, metal carbonates, zinc oxides, zinc carbonates, organic compounds, and embedded carbon cores, formed on the rubbing surfaces with a thickness of approximately 270 nm when Zn-CDs are used as additives. This film combined with the “ball-bearing” and third-particle effects of Zn-CDs contributed to excellent lubrication performance.
Highlights
In recent years, sustainable development has become a typically discussed theme worldwide and has increasingly attracted people’s attention toward protecting the environment and energy saving, resulting in numerous advanced techniques being exploited
The HR-transmission electron microscopy (TEM) images (insets of Figs. 2(a)–2(e)) show that all the Carbon dots (CDs) were almost amorphous in structure, which is the characteristic of CDs synthesized via the bottom-up strategy
S2–S5 in the Electronic Supplementary Material (ESM), the HR elemental X-ray photoelectron spectroscopy (XPS) spectra of the Fe-CDs, Cu-CDs, Mg-CDs, and Zn-CDs can be deconvoluted into one or more peaks belonging to specific elemental species, verifying that the CDs were doped with various metal ions in the carbon cores and capped by oxygen-containing groups and [AMIm][Br] molecules on their surfaces
Summary
Sustainable development has become a typically discussed theme worldwide and has increasingly attracted people’s attention toward protecting the environment and energy saving, resulting in numerous advanced techniques being exploited. Adjusting and controlling the surface functional groups of CDs has been considered as a primary approach for enhancing their tribological performance Both theoretical and experimental results confirmed that doping the carbon cores of CDs with other elements is another effective approach for developing novel properties. A large number of studies have shown that the most efficient approach for enhancing the lubrication capacity of water-based lubricants, the antiwear and friction-reducing performances, is by introducing the additional additives [54,55,56] Owing to their abundant hydrophilic groups and facile preparation methods, IL-functionalized CDs (CD-ILs) have exhibited great potential as a type of commercial water-based lubricant additive [38, 40, 57, 58]. The possible lubrication mechanism of metal-doped CDs was revealed using various surface analysis techniques, such as scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), and cross-section transmission electron microscopy (TEM)
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