Tribological Properties of TiO₂@CTAB Core–Shell Nanopowders as Lubricating Additives

Abstract

In this experimental investigation, a core–shell-structured nano-lubricating additive was synthesized utilizing the dielectric barrier discharge plasma (DBDP)-assisted ball grinding technique for a duration of 5 h. The microstructural analysis of the nano-TiO2 powder was performed employing advanced methods such as X-ray diffraction (XRD) and thermogravimetry–differential scanning calorimetry (TG-DSC). The initial particle size of TiO2 was refined from 1 μm to a range of 150–200 nm, resulting in a remarkable increase in lattice distortion rate by 88.2% and an oil affinity enhancement of 200%. Through the introduction of CTAB’s oil-compatible group onto the surface of nano-TiO2 particles, a modified layer with a thickness of 21 nm possessing superior thermal stability and an activation energy (E a) of 600 kJ/mol was successfully produced. Molecular dynamics simulations were conducted to elucidate the mechanism underlying the surface modification of nano-TiO2 powder facilitated by DBDP-assisted ball grinding, thereby revealing the pivotal role of electrostatic forces in the organic modification of the TiO2 surface. It was found that electrostatic forces dominantly govern the cetyl trimethyl ammonium bromide (CTAB)–TiO2 composite interface model, contributing to 70% of the total energy with a maximum energy proportion of −187.84 kcal/mol. To evaluate the lubrication performance of the composite oil samples under boundary lubrication conditions, comprehensive assessments were carried out using the four-ball method and reciprocating friction experiments. The results demonstrated noteworthy enhancements in viscosity index, dynamic viscosity, and oil film thickness within the composite oil samples. Particularly, the composite oil containing 0.5 wt % TiO2@CTAB exhibited outstanding extreme pressure resistance, manifesting a significant reduction of 41.7% in the average friction coefficient, a considerable increase of 25.8% in wear spot diameter, and a substantial elevation of 66.9% in maximum nonseizure load. Compared to the base oil, the incorporation of 0.5 wt % TiO2@CTAB led to a notable increment of 34.7% in oil film thickness, 6.7% in dynamic viscosity, and 9% in viscosity index. In the tribological experiment simulating marine diesel engines, the friction coefficient witnessed a remarkable reduction by 65.8%, accompanied by a substantial decrease of 54.1% in wear rate. This noteworthy improvement in boundary lubrication conditions of the friction pair effectively mitigated friction and wear. For comprehensive characterization of the wear marks, energy-dispersive spectrometry (EDS) and X-ray photoelectron spectroscopy (XPS) techniques were employed to analyze the physical structure and chemical composition. The implementation of TiO2@CTAB nano-lubricating additives resulted in nanobearing and deposition effects, leading to a reduction in contact area and surface roughness, thereby facilitating the restoration of the friction pairs. These findings possess significant implications for extending the service life of die...