A Computational Chemistry Study on Friction of h-MoS₂. Part II. Friction Anisotropy

Abstract

In this work, the friction anisotropy of hexagonal MoS(2) (a well-known lamellar compound) was theoretically investigated. A molecular dynamics method was adopted to study the dynamical friction of two-layered MoS(2) sheets at atomistic level. Rotational disorder was depicted by rotating one layer and was changed from 0° to 60°, in 5° intervals. The superimposed structures with misfit angle of 0° and 60° are commensurate, and others are incommensurate. Friction dynamics was simulated by applying an external pressure and a sliding speed to the model. During friction simulation, the incommensurate structures showed extremely low friction due to cancellation of the atomic force in the sliding direction, leading to smooth motion. On the other hand, in commensurate situations, all the atoms in the sliding part were overcoming the atoms in counterpart at the same time while the atomic forces were acted in the same direction, leading to 100 times larger friction than incommensurate situation. Thus, lubrication by MoS(2) strongly depended on its interlayer contacts in the atomic scale. According to part I of this paper [Onodera, T., et al. J. Phys. Chem. B 2009, 113, 16526-16536], interlayer sliding was source of friction reduction by MoS(2) and was originally derived by its material property (interlayer Coulombic interaction). In addition to this interlayer sliding, the rotational disorder was also important to achieve low friction state.

Advances in nonequilibrium molecular dynamics simulations of lubricants and additives

“…Examples of this include CarParrinello MD (CP-MD) [287][288][289][290][291] [214,[292][293][294][295], which have given important insights into the reactivity of common friction modifier (molybdenum disulphide, molybdenum dithiocarbamate) and anti-wear (zinc dialkyldithiophosphate) additives. Ab initio NEMD simulations have also been performed on additive systems subjected to confinement and shear.…”

A methodology for Raman characterisation of MoDTC tribofilms and its application in investigating the influence of surface chemistry on friction performance of MoDTC lubricants

“…MoDTC reduces friction by degradation of the molecule to form discrete MoS 2 sheets of about 10 nm -20 nm in diameter and 1-2 nm thick [3]. The presence of MoS 2 in the rubbing contact greatly reduces friction due to interlayer sliding of MoS 2 sheets between the sliding pair and only a few sheets are necessary for low friction to be achieved [4]. In a steel/steel sliding contact, the friction coefficient achieved in the presence of MoDTC can be as low as µ=0.04 at ambient conditions [5] and µ=0.02 in a vacuum environment [3].…”

Graphite and Hexagonal Boron-Nitride have the Same Interlayer Distance. Why?

“…2−8 In recent years, the most prominent member of the family of layered materials has been graphene, 9−12 which serves as a building block for few-layered graphene and graphite as well as for single-and multi-walled carbon nanotubes. 13 Here, each layer is an atomically thin hexagonal sheet of sp 2 bonded carbon atoms, where the unpaired p z electrons on each atomic site join to form a collective π system, turning the material into a semimetal. The main factors expected to dominate graphene interlayer binding are electrostatic interactions, dispersive interactions, and Pauli repulsions between the electron densities of each layer.…”