Lubricating Film Forming Properties of Ionic Nitrogen-doped Carbon Dots at Low Additive Levels

GUO Yang; LIU Xiaoling; WANG Haocheng; SUN Xiaowei; KONG Xinru

doi:10.16490/j.cnki.issn.1001-3660.2025.13.005

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Surface Technology ›› 2025, Vol. 54 ›› Issue (13) : 51-61. DOI: 10.16490/j.cnki.issn.1001-3660.2025.13.005

Friction, Wear and Lubrication

Lubricating Film Forming Properties of Ionic Nitrogen-doped Carbon Dots at Low Additive Levels

GUO Yang, LIU Xiaoling^*, WANG Haocheng, SUN Xiaowei, KONG Xinru

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Abstract

Lubrication is an effective way to alleviate friction and wear, and the development of high-performance lubrication additives is one of the effective ways to improve the lubrication performance of base oils. With the development of nanotechnology, nanomaterials have received wide attention in the field of lubrication due to their unique physicochemical properties. Carbon dots (CD_S), as a new type of carbon nanomaterials, have good application prospects in the lubrication field due to their small size, tunable surface functional groups, environmental protection, corrosion inhibition and strong interfacial adsorption ability. Most of the research on CD_S focuses on the field of friction and wear, but the research for the field of lubrication has not been carried out yet. In addition, low additive level of [NCD_S][DEHP] can improve the friction and wear performance of base oils, but whether it can improve the lubricating film forming properties needs to be further explored. Therefore, the work aims to investigate the lubricating film forming properties of [NCD_S][DEHP] at low additive levels.
Low additive levels (0.002 5%, 0.005%, 0.01%) of [NCD_S][DEHP] were added to PAO4 base oil and then subject to ultrasonic treatment to enable [NCD_S][DEHP] to be dispersed uniformly in PAO4. Static settling tests showed that [NCD_S][DEHP] could be dispersed in the base oil for at least 30 days. The viscosity of the four lubricants was tested with a rotational rheometer, and the results showed that the addition of [NCD_S][DEHP] had minimal effect on the lubricant viscosity. The film thickness was measured with a point contact ball-on-disc lubricant film tester and the surface morphology of the specimens and chemical elements were analyzed by AFM and EDS.
Under fully flooded conditions, it could be seen by observing the film thickness curves of the four lubricants that [NCD_S][DEHP] had minimal effect on the film forming properties. However, under the limited lubricant supply conditions, by comparing the contact zone in the optical interferograms of the oil film with the variation of entrainment velocity and the corresponding film thickness curves of the four lubricants, it was found that low additions of [NCD_S][DEHP] could increase the film thickness of lubricants and improve the starvation degree of lubricant in the contact zone, thus improving the lubricating film forming properties of the base oil. In particular, the lubricating film forming properties were optimum when the addition of [NCD_S][DEHP] was 0.005%. Based on the optimal mass fraction, the lubricating film forming properties of [NCD_S][DEHP] were investigated by changing oil supply level. From the optical interferograms and the degree of oil film collapse in the center of the contact zone, it was found that [NCD_S][DEHP] could reduce the width of the oil starvation in the contact zone and improve the effect of oil starvation most obviously at a low oil supply amount (5 μL). With the increase of oil supply amount, the lubrication enhancement effect of [NCD_S][DEHP] was weakened.
[NCD_S][DEHP] consists of NCD cations and DEHP anions, which will be adsorbed on the steel surface under the electrostatic effect. At the same time, DEHP anions promote the adsorption of NCD cations and generate a layer of adsorption film, which promotes the lubricant to be entrainment into the contact zone, changes the oil distribution and oil film shape in the contact zone, alleviates the degree of oil starvation, and improves the lubrication film forming ability.

Key words

ionic nitrogen-doped carbon dots / additive / film forming properties / anions and cations / lubrication mechanism

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GUO Yang, LIU Xiaoling, WANG Haocheng, SUN Xiaowei, KONG Xinru. Lubricating Film Forming Properties of Ionic Nitrogen-doped Carbon Dots at Low Additive Levels[J]. Surface Technology. 2025, 54(13): 51-61 https://doi.org/10.16490/j.cnki.issn.1001-3660.2025.13.005

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