Tribochemical Reactions of Thioketones: A First-Principle Molecular Dynamics Study

Abstract

The extreme conditions in between sliding surfaces promote chemical reactions known as tribochemical reactions. There are two types of tribochemical reaction in systems with lubricant inserted in between the surface layers. The first type is the reaction among the lubricant bulk molecules and the second are reactions between the lubricant molecule and the surfaces. A class of tribochemical reactions which promote lubrication is known as functional lubrication. In this dissertation, a systematic study of tribochemical reactions of Me(C=S)X molecules (X is -H, -CH3, -Cl, -F, -CF3 and -NH2) in between the layers of MgO are explored using the FPMD. The objective is to understand how various functional groups affect both types of tribochemical reactions. Each type of tribochemical reaction required separate simulation models to decrease computational expenses. The results of simulations in which systems of bulk Me(C=S)X molecules were compressed revealed that the first pressure-induced reaction was the dimerization through the formation of C-S bond between two monomer molecules. The pressures at which this reaction occurred was dependent on the functional group –X. The effect of the substituent on the dimerization pressure was established using the Energy Decomposition Analysis. Compression also resulted in oligomerization reaction through the formation of S-S bond. The tribochemical reactions between Me(C=S)X molecules and surfaces were studied by compressing Me(C=S)X molecules inserted in between two layers of MgO. The results of simulations indicated that Me(C=S)X molecules form C-O or S-O bonds with the surface. The formed C-O and S-O bonds destabilized the cubic structure of MgO through the dissociation of Mg-O bonds. Simulations of sliding surfaces were performed with Me(C=S)X inserted in between the MgO layers. The results of the simulations indicated that most tribochemical reactions of Me(C=S)X molecules inhibited sliding. Me(C=S)H and Me(C=S)Me molecules required additional larger scale simulation to determine their functional lubrication ability.