Origin of the tribofilm from MoS2 nanoparticle oil additives: Dependence of oil film thickness on particle aggregation in rolling point contact

The lubrication effectiveness of MoS2 nanoparticles as an oil additive remains unclear, restricting its application in industry to reduce friction. The goal of this work was to explore the lubrication mechanism of MoS2 nanoparticles as an oil additive. In this study, the oil film thickness behaviors of MoS2 nanoparticles in poly-alpha olefin (PAO4) base oil, PAO4 with 3 wt% dispersant (polyisobutyleneamine succinimide, PIBS), and 0W20 engine oil were investigated using an elastohydrodynamic lubrication (EHL) testing machine. Following the EHL tests, the flow patterns around the contact area and the tribofilm covering rate on contact area were studied using optical microscopy to understand the lubrication mechanism. The results indicate that both the dispersant and nanoparticle aggregation significantly affected the oil film thickness. The expected oil film thickness increase in the case of 0.1 wt% MoS2 in PAO4 base oil was obtained, with an increase from 30 to 60 nm over 15 min at a velocity of 50 mm/s. Flow pattern analysis revealed the formation of particle aggregation on the rolling path when lubricated with 0.1 wt% MoS2, which is associated with a tribofilm coverage rate of 41.5% on the contact area. However, an oil film thickness increase and particle aggregation were not observed during the tests with 0.1 wt% MoS2 blended with 3 wt% PIBS as the dispersant in PAO4 base oil, and for 0.75 wt% MoS2 in 0W20 engine oil. The results suggest that nanoparticles responsible for tribofilm formation originated from aggregates, but not the well-dispersed nanoparticles in point contact. This understanding should aid the advancement of novel lubricant additive design.

Journal Bearing Surface Topography Design Based on Transient Lubrication Analysis

A transient mixed lubrication model is used to study the performance of a journal bearing subjected to impulse loading, considering mass conservation and the effects of asperities on flow and contact, to explore a novel journal bearing surface design methodology. The bearing surface features include an outlet pocket and axial lip for topographical design consideration. A data-driven approach for a steadily loaded bearing was first presented to illustrate the behavior of an indented pocket design at low and high loads, and Pareto optimization and sensitivity analysis methods were employed to analyze the data and provide insight to the design. The results show that the pocket location was the most influential parameter for the optimal bearing surface design for energy-efficient lubrication performance. For transient operation, a comprehensive parametric study was conducted, and the results reveal that, when compared to the results of the unmodified bearing, the bearing designed with the optimal outlet pocket can run at 9% lower mean friction while maintaining the baseline film thickness. The bearing with the lip feature shows a 20% minimum film thickness increase and 3% mean friction reduction. The design with outlet pocket and lip features combined can lead to 20% film thickness increase and 10% mean friction reduction.

Study of Micro-Morphology Influence on the EHL Analysis of Water-Based Ferrofluid

The elastohydrodynamic lubrication (EHL) study of water-based ferrofluid bearing with single sine-shaped peak, rectangular-shaped valley and V-shaped valley was carried out. The influence of the amplitude and the width on the pressure and film thickness was discussed. The results reveal that the pressure and film thickness change apparently with the change of micro-morphology; The amplitudes of the local pressure peak and the film thickness increase with the increase of amplitudes; The widths of the local pressure peak and the film thickness increase with the increase of width.