Structural Ordering of Molybdenum Disulfide Studied via Reactive Molecular Dynamics Simulations

Abstract

Molybdenum disulfide (MoS) is a well-known and effective lubricant that provides extremely low values of coefficient of friction. It is known that the sliding process may induce structural transformations of amorphous or disordered MoS to the crystalline phase with basal planes oriented parallel to the sliding direction, which is optimal for reducing friction. However, the key reaction parameters and conditions promoting this structural transformation are still largely unknown. We investigate, by employing reactive molecular dynamics simulations, the formation of MoS layers from an amorphous phase as a function of temperature, initial sample density, and sliding velocity. We show that the formation of ordered crystalline structures can be explained in the framework of classical nucleation theory as it predicts the conditions for their nucleation and growth. These results may have important implications in the fields of coating and thin-film deposition, tribology, and in all technological applications where a fast and effective structural transition to an ordered phase is needed.

Limiting Domain Size of MoS2: Effects of Stoichiometry and Oxygen

“…The rate of nucleation is expected to exhibit an Arrhenius dependence and is therefore a function of the nucleation energy barrier and the temperature. 58 As expected, given the high temperature conditions of the simulations here, nucleation occurred rapidly at the start of the simulation for all cases. Therefore, it can be assumed that the limiting domain size was determined by the domain growth process.…”

“…55 Crystallization of the middle layer for each model was simulated using the same procedure as reported previously. 55,58 First, amorphous material was created by heating a single layer of crystalline MoS 2 to 5300 K and then rapidly quenching to room temperature. The system was then equilibrated at room temperature, and a pressure of 50 GPa was applied in the z-direction using a Parrinello-Rahman barostat.…”

Tribochemistry: A Review of Reactive Molecular Dynamics Simulations

“…The scope of the review is limited to simulations with empirical potentials (i.e., ab initio calculations are not included) that capture the formation and breaking of chemical bonds driven by shear force. The review excludes studies that do not consider sliding, although reactive molecular dynamics simulations have been used to model tribologically relevant reactions driven thermally and/or via normal force (see, for example, [51][52][53][54][55][56]). Furthermore, although the empirical model used for each study discussed in this review is mentioned, the different potentials are not compared.…”

On the Microstructural, Mechanical and Tribological Properties of Mo-Se-C Coatings and Their Potential for Friction Reduction against Rubber

“…It is well-known that lower relative humidity (RH) decreases the shear strength of the TMD materials, which increases their lubricity [ 36 ]. For the coatings with a higher degree of amorphous structure (e.g., MoSeC51 and MoSeC60), the additional thermal energy inputted to the contact probably facilitated the formation and reorientation of lubricious MoSe 2 tribolayers, parallel to the sliding direction, between the coating and the steel ball surfaces [ 37 ]. …”