In a recent article published in the journal Applied Surface Sciencesresearchers have discussed the usefulness of silver-doped carbon quantum dots as lubricating oil to improve tribological performance at different temperatures.
Study: Silver-doped carbon quantum dots as a lubricating oil additive to improve tribological performance at different temperatures. Image Credit: Tayfun Ruzgar/Shutterstock.com
One of the main concerns of the scientific community is the conservation of energy. An important tactic for saving energy is optimizing and improving tribological processes, such as reducing friction and wear. The best method currently available to reduce friction and wear, extend the useful life of mechanical systems and reduce production costs is the use of lubricants. Nano-additives can successfully protect the friction pair from severe wear. Carbon quantum dots (CQDs) have created a whole new class of carbon nanomaterials and are growing in popularity.
As a tactic, metals and metal oxides are frequently used to improve the mechanical and thermal properties of composite lubricants. The Ag component has been shown to be a promising functional choice for anti-wear and lubricating effects among various metallic components. The thermal and lubricating properties of CQDs can be improved by doping a silver component. Ionic liquids (IL) exhibit a wide range of beneficial physiochemical traits. ILs have recently been accepted as a stabilizer and carrier for the synthesis and modification of carbon-based nanomaterials.
As an effective, environmentally friendly and dispersion-promoting nano-additive, IL-modified functional CQDs show great promise.
About the study
In this study, the authors used a simple hydrothermal approach to create silver-doped CQDs (Ag-CQDs). The dispersion stability of Ag-CQD additive in poly alpha olefin (PAO) was improved by combining ultrasonic vibrations with IL. Using four-ball tests and an alternative ball-on-disk configuration, the tribological performance of CQDs and Ag-CQDs as additives was evaluated over a wide temperature range.
The team developed the first functional Ag-CQD chemical as an oil additive from ethylenediamine, critical acid and silver acetate using a simple hydrothermal synthesis process. The produced Ag-CQDs additive achieved higher dispersion stability in PAO base oil by using 1-octyl-3-methylimidazolium hexafluorophosphate and ultrasonic treatment. To minimize wear and friction in various friction situations, Ag-CQD oil suspensions were applied to metal/metal or ceramic/metal tribo-pairs.
The researchers determined the structure, chemical composition and morphology of the Ag-CQD compound as well as the states of the worn surface through friction tests and various characterizations. Based on the characterization analysis and friction test results, a plausible hypothesis for the lubricating mechanism of Ag-CQD addition was made.
The characteristic Raman peaks for the worn surface which was lubricated by PAO with the additive CQDs/IL appeared at approximately 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 spike. Three chemically shifted peaks at 286.0 eV, 284.5 eV and 288.5 eV were identified in the deconvolved 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% by weight, the least temperature variation was visible. At 200°C and 300°C, the PAO oil showed severe wear morphologies with large wear scar diameters. The worn track on the alloy plate had a reduced maximum width and depth. At 25°C and 100°C, respectively, the PAO base oil had a low coefficient of friction (COF) of 0.125 and 0.190.
The base oil had the lowest load carrying capacity of all test samples with a maximum non-seizing load (PB value) of about 490N. The wear scar diameter (WSD) values of CQD and Ag-CQD dispersions decreased by 29.8% and 35.3%, respectively, compared to those of PAO. With COFs of 0.076 and 0.067, respectively, the CQD dispersion and the Ag-CQD dispersion had COFs 34.1% and 42.2% lower than those of the base oil.
The results of the four-ball test showed that the CQD and Ag-CQD additives could successfully reduce the COF of the base oil and the WSD of the lower balls. The Ag-CQD additive could result in a 42.2% reduction in COF and a 35.3% reduction in WSD compared to base oil at a concentration of 0.05% by weight. CQDs and Ag-CQDs additives could greatly reduce friction and wear when the ambient temperature was above 200°C.
Compared to CQDs, Ag-CQDs demonstrated superior lubrication performance. The production of a tribofilm composed of Ag-CQD, carbonates, multicomponent oxides and nitrides as a result of tribochemical reactions has been credited as a lubricating mechanism. This film effectively protected the friction pair and kept friction and wear low.
In conclusion, this study elucidated the development of Ag-CQDs compound as a lubricating oil additive using a hydrothermal process. Addition of IL with ultrasonic vibration increased dispersion stability in PAO base oil. CQDs and Ag-CQDs could be fabricated using a simple hydrothermal approach, and under the combination of IL modification and ultrasonic treatment, the prepared particles having a narrow size distribution were well distributed in the PAO.
In a four-ball test, the lubricating capabilities of PAO oil could be significantly improved by the additions of Ag-CQD and CQD. Ag-CQD and CQD additives can reach the damaged area to achieve repair during rubbing due to their ultra-small sizes.
Due to the addition of the Ag component, which improved the carrying capacity of the lubricants, the Ag-CQDs additive outperformed the CQDs additive in reducing friction. At relatively low temperatures in the reciprocal slip configuration, the COFs of the CQD and Ag-CQD dispersions were slightly lower than those of the base oil. Compared to CQD dispersion and base oil, Ag-CQD dispersion showed lower COFs and lower wear volume as temperature increased.
Wang, J., Li, X., Deng, Y., et al. Silver-doped carbon quantum dots as a lubricating oil additive to improve tribological performance at different temperatures. Applied Surface Sciences 154029 (2022). https://www.sciencedirect.com/science/article/abs/pii/S0169433222015690