Mechanics of Deformation under Traction and Friction of a Micrometric Monolithic MoS₂ Particle in Comparison with those of an Agglomerate of Nanometric MoS₂ Particles

Tribology of small inorganic nanoparticles in suspension in a liquid lubricant is often impaired because these particles agglomerate even when organic dispersants are used. In this paper we use lateral force microscopy to study the deformation mechanism and dissipation under traction of two extreme configurations (1) a large MoS₂ particle (~20 μm width) of about 1 μm height and (2) an agglomerate (~20 μm width), constituting 50 nm MoS₂ crystallites, of about 1 μm height. The agglomerate records a friction coefficient which is about 5-7 times that of monolithic particle. The paper examines the mechanisms of material removal for both the particles using continuum modeling and microscopy and infers that while the agglomerate response to traction can be accounted for by the bulk mechanical properties of the material, intralayer and interlayer basal planar slips determine the friction and wear of monolithic particles. The results provide a rationale for selection of layered particles, for suspension in liquid lubricants.