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Tribology of MoS2 Nanoparticles in Ambient and Liquid Suspension (pp. 227-254) $100.00
Authors:  (S.K. Biswasa, R.R. Sahoo, Department of Mechanical Engineering, Indian Institute of Science, Bangalore, India, and others)
Abstract:
In this chapter we explore mechanistic understanding of lubrication by MoS2 particles, with a focus on when they are suspended in oil. The problem is of relevance as such lubrication, well controlled, can be of great benefit to tribology. To gain such an understanding the chapter starts with a study of the response of single monolithic and single agglomerate particles to tangential traction in the ambient and in an oil media. This fundamental insight is used to understand the tribological responses of these particles when sprayed dry on a rigid substrate and when the particles are suspended in an oil medium to lubricate a reciprocating steel against steel contact.
As dispersion of MoS2 in a liquid medium is a key factor in its tribology we first focus our studies on a monolithic particle (20 μm width and 1 μm height) and a fully agglomerated assembly (20 μm width; loosely bound 50 nm crystallites). Lateral force microscopy of the two types of particles in the ambient and in the oil allows us to record the distinction in traction and damage, between these two types of particles. Depending on the environment and the contact pressure we record varying extents of intercrystalline slip, plastic grooving, fragmentation and fracture in both types of particles. Such observation allows us to identify interlayer basal planar slip to be the principal effect of tangential traction on the monolith, providing a rationale for the observed low coefficient
of friction. The inter-planar shear is determined by weak Vander Waal forces. The agglomerate records a friction coefficient which is 5 to 7 times greater than that of the monolith, the deformation and fracture of such a particle under traction is principally controlled by the bulk isotropic mechanical properties of the particles. Continuum modeling of both modes of dissipation is invoked to support the experimental observations.
Known such mechanisms we perform ball on disc tribology experiments using particles suspended in oil over a range of contact pressures and sliding velocities. We note particle migration, particle stability in the contact region, oxidation potential and particle adhesion to the substrate as phenomena which play key roles in influencing the tribology of the lubricants. The agglomerates, on sliding generally migrate out of the contact region while the large monoliths fragment to very small size by ingestion of oil and reside at contact by adhesion to undergo internal slip to raise a homogeneous but non uniform transfer film which flows in plasticity to yield large contact areas which aid in giving low friction and protection to the substrate. Our attempt to reduce the particle size by introducing a dispersant in the lubricant yielded only a marginal improvement in friction.
In the final part of the chapter we examine the transfer film ex-situ using Micro-Raman Spectroscopy. The spectroscopy data confirmed the presence of crystalline MoS2 in the transfer film and supported the profilometric data that the film thickness increase from 30 nm to 100 nm in moving from dry to wet tribology. 


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Tribology of MoS2 Nanoparticles in Ambient and Liquid Suspension (pp. 227-254)