Balasubramaniam Vengudusamy

Interaction Between Selected MoS2 Nanoparticles and ZDDP Tribofilms

Nanoparticles based on transition metal dichalcogenides (TMD) are considered to hold great promise as boundary lubricating additive/material for improving friction and wear of engineering functional surfaces. However, TMD nanoparticles cannot provide a comprehensive surface protection against oxidation, corrosion or sludge control. Therefore, the current lubricant developments may still have to depend on conventional additives such as zinc dialkyl dithiophosphate (ZDDP), and it is essential to understand the interaction of nanoparticles with such additives in order to explore how these nanoparticles could be commercially employed in fully formulated lubricants. This paper examines the tribological properties of three different nanoparticles: inorganic fullerene-like MoS2, rhenium-doped MoS2 and MoS2 nanotubes in steel and steel with preformed ZDDP tribofilm surfaces using a pin-on-disc-type tribometer under reciprocating sliding conditions. The resulting tribofilms have been evaluated using scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy, transmission electron microscopy and atomic force microscopy. The results show that although the nanoparticles are able to reduce friction in all cases, the resulting tribofilm composition and morphology, and their lubricating mechanisms are significantly different. The MoS2 nanoparticles and nanotubes show good synergism with ZDDP, and tribofilms formed from nanoparticles exhibit improved friction and wear properties compared to that typically formed from ZDDP.

Film Forming Behavior of Greases Under Starved and Fully Flooded EHL Conditions

ABSTRACT Improving knowledge on the film forming behavior of greases is essential to be able to develop efficient greases. This article examines how operating conditions (e.g., temperature, lubrication condition [fully flooded/starved]) and base oil viscosity influence the film forming properties of greases by comparing the behavior of two lithium-based greases and their respective base oils in rolling point contact. It is found that the onset and degree of starvation is controlled by speed (u) × viscosity (ν)/load (W) factor (uν/W) and temperature and that low uν/W values promote entrainment of thickener into contact. Thus, grease with low base oil viscosity shows significant thickener entrainment in the low speed region compared to the one with high base oil viscosity, which leads to the formation of thickener-rich viscous material during extended running with the low base oil viscosity grease. The results suggest that the shape of the film thickness versus speed curve is viscosity and uν/W range dependent. Furthermore, for the test conditions used in this study, grease-lubricated contacts appear to shift from the initial fully flooded condition to starved condition over a prolonged running of 2 h. The results from this study concur with those reported in the literature that fully flooded oil elastohydrodynamic lubrication (EHL) theory or film thickness cannot be directly applied or taken as a guideline in grease-lubricated contacts.

Influence of Surface Roughness on the Tribological Behavior of Gear Oils in Steel–Steel Contacts: Part II—Mixed Friction Properties

Hypoid gears are known to be commonly used in rear-wheel-drive automobiles and they typically operate under high sliding and high contact pressure conditions. Providing efficient lubrication and necessary surface finish to such gear contacts is important in order to achieve a significant gain in energy and torque efficiencies. This requires knowledge of the behavior of gear oils under different operating conditions, especially their abilities to work under high contact pressure and sliding conditions. In light of this and in view of optimizing friction in transmission systems, this article explores the influence of surface roughness on friction in a mixed lubrication regime. In addition to friction–speed measurements and Stribeck curves described in a companion article (Vengudusamy, et al. (1)), this article explores the friction response of tribofilm-generated surfaces with base oil and highlights the significance of such experiments.