How is Tribological Performance Aided by Doped Carbon Quantum Dots?

By Surbhi JainReviewed by Susha Cheriyedath, M.Sc.Jun 28 2022

In an article recently published in the journal Applied Surface Science, researchers discussed the utility of carbon quantum dots doped with silver as a lubricating oil to improve tribological performance at various temperatures.

Study: Carbon quantum dots doped with silver as lubricating oil additive for enhancing tribological performance at various temperatures. Image Credit: Tayfun Ruzgar/Shutterstock.com

Background

One of the scientific community's top concerns has been energy conservation. An important tactic for energy saving is the optimization and improvement of tribological processes, such as reducing friction and wear. The best method now available for reducing friction and wear, prolonging the useful life of mechanical systems, and lowering production costs is the use of lubricants. Nano-additives can successfully shield the friction pair against severe wear. Carbon quantum dots (CQDs) have created a brand-new category of carbon nanomaterial and are gaining popularity.

As a tactic, metal and metal oxides are frequently used to improve the mechanical and thermal properties of composite lubricants. Ag component has been demonstrated to be a promising functional choice for anti-wear and lubricating effects among various metal components. The CQDs' thermal and lubrication properties can be improved by doping a silver component. Ionic liquids (ILs) exhibit a wide range of beneficial physiochemical traits. ILs have recently gained acceptance as a stabilizing agent and medium for the synthesis and modification of carbon-based nanomaterials.

As an effective, environmentally friendly, and dispersion-promoting nano-additive, functional IL-modified CQDs have a lot of promise.

About the Study

In this study, the authors used a simple hydrothermal approach to create CQDs that were doped with silver (Ag-CQDs). The dispersion stability of the Ag-CQDs additive in poly alpha olefin (PAO) was enhanced by combining ultrasound vibration with IL. Through four-ball tests and a ball-on-disk reciprocating setup, the tribological performance of the CQDs and Ag-CQDs as additives was assessed over a broad temperature range.

The team developed the first functional Ag-CQDs chemical as an oil additive from ethylenediamine, critical acid, and silver acetate utilizing a simple hydrothermal synthesis process. The produced Ag-CQDs additive was given greater dispersion stability in the PAO base oil by using 1-octyl-3-methylimidazolium hexafluorophosphate and ultrasonic treatment. For minimizing wear and friction under various frictional situations, Ag-CQD oil suspensions were applied to metal/metal or ceramic/metal tribo-pairs.

The researchers ascertained the structure, chemical make-up, and morphology of the Ag-CQDs compound as well as the states of the worn surface through friction tests, and various characterizations. Based on the characterization analysis and the results of the friction test, a plausible hypothesis for the lubricating mechanism of the Ag-CQDs addition was made.

Observations

The Raman characteristic peaks for the worn surface that was lubricated by PAO with the CQDs/IL additive emerged at about 1322 cm^-1 and 1587 cm^-1. Iron oxides could be responsible for the Fe2p signal that appeared at 711.3 eV. At 476.4 eV, the Cr³⁺ in Cr₂O₃ was thought to be responsible for the Cr2p-intensive peak. Three chemically shifted peaks at 286.0 eV, 284.5 eV, and 288.5 eV were identified in the deconvoluted C1s spectra. Three distinct peaks at 530.6, 529.3, and 531.8 eV could be identified in the O1s spectrum.

When the additive concentration was 0.05 wt.%, the least temperature variation was visible. At 200 °C and 300 °C, the PAO oil showed severe wear morphologies with significant wear scar diameters. The worn track on the alloy plate showed a maximum reduced width and depth. At 25 °C and 100 °C, respectively, the PAO base oil had a low coefficient of friction (COF) values of 0.125 and 0.190.

The base oil had the lowest load-carrying ability of all the test samples with a maximum non-seizure load (P_B value) of roughly 490N. The wear scar diameter (WSD) values of the CQDs and Ag-CQDs dispersions were decreased by 29.8% and 35.3%, respectively, in comparison to that of PAO. With COFs of 0.076 and 0.067, respectively, the CQDs dispersion and Ag-CQDs dispersion had COFs that were 34.1% and 42.2% lower than the base oil.

The findings of the four-ball test showed that the CQD and Ag-CQD additives could successfully lower the base oil's COF and the WSD of lower balls. The Ag-CQDs additive could result in 42.2% reduced COF and 35.3% lower WSD than the base oil at a concentration of 0.05 wt.%. The CQDs and Ag-CQDs additives could significantly reduce friction and wear when the ambient temperature was over 200 °C.

In comparison to CQDs, Ag-CQDs demonstrated superior lubricating performance. The production of a tribofilm made up of Ag-CQDs, carbonates, multicomponent oxides, and nitrides as a result of tribochemical reactions were credited as the lubricating mechanism. This film effectively protected the rubbing pair and maintained low friction and wear.

Conclusions

In conclusion, this study elucidated the development of Ag-CQDs compound as a lubricating oil additive by using a hydrothermal process. The addition of IL with ultrasound vibration increased the dispersion stability in PAO base oil. The CQDs and Ag-CQDs could be made using a simple hydrothermal approach, and under the combination of IL modification and ultrasonic treatment, the prepared particles having narrow size distribution were well distributed in PAO.

Under four-ball testing, the lubricating capabilities of PAO oil could be greatly improved by both the Ag-CQDs and CQDs additions. The Ag-CQDs and CQDs additives might reach the damaged area to realize mending during friction because of their ultrasmall sizes.

Because of the addition of the Ag component, which improved the carrying capacity for the lubricants, the Ag-CQDs additive outperformed the CQDs additive in terms of reducing friction. At relatively low temperatures in the reciprocal sliding setup, the COFs of the CQD and Ag-CQD dispersions were marginally lower than those of the base oil. In comparison to the CQDs dispersion and the base oil, the Ag-CQDs dispersion showed lower COFs and a smaller wear volume as the temperature increased.

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