Mechanism of triboactivity of Schiff bases: Experimental and molecular dynamics simulations studies

Schiff bases derived from o-tolidine through its condensation with salicylaldehyde (OH-BT), naphthaldehyde (H-NT) and 2-hydroxynaphthaldehyde (OH-NT) have been synthesized and characterized by FT-IR and H-1 NMR spectroscopic techniques. The synthesized Schiff bases are environmentally safe from tribological aspect as these do not contain undesirable elements like sulfur, phosphorus, and halogens which are known to affect the efficiency of catalytic converters. The triboactivity of these compounds has been studied in paraffin oil (PO) using four-ball tester (FBT) at an optimized concentration of 0.25% w/v under ASTM D4172 and D5183 test conditions. The results have been compared with that of a reference additive, zinc dialkyl dithiophosphate (ZDDP), under similar conditions. Based on tribological data, mean wear scar diameter (MWD), mean wear volume (MWV), coefficient of friction (COF), load carrying ability and wear rate, the following order has emerged for tribological behavior of the investigated additives: OH-BT>OH-NT>H-NT>ZDDP. Morphology and composition of wear scar have been studied by Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM) with Energy Dispersive X-ray analysis (EDX) and relate very well with the observed tribological data. The EDX analysis of the wear scar lubricated with synthesized additives provides evidence for the presence of nitrogen and oxygen. To ascertain the active involvement of the imine group towards tribological behavior, the reaction of the most active compound OH-BT was carried out with thioglycolic acid. Reduced anti-wear performance of the reaction product (OH-BTS) may be directly associated with the absence of the imine group (-CH=N-). Molecular dynamics (MD) simulations have been used to study the mechanism of adsorption of the studied Schiff bases in paraffin oil on the iron slab. The order of adsorption energies of the Schiff base additive molecules has been found to agree well with the experimentally observed data. The nature of orientation or configuration of molecules after adsorption has been presented. The findings of the MD simulations have been used to propose the mechanism for tribological behavior of the investigated Schiff base molecules and product OH-BTS obtained by cyclisation of OH-BT with thioglycolic acid. The mechanism proposed from MD simulation studies points to the combined effect of heteroatoms and planarity of the additive molecule after adsorption. Accordingly, molecules with tribological properties of a high order may be formulated. (C) 2019 Elsevier B.V. All rights reserved.