Tribological studies of some SAPS-free Schiff bases derived from 4-aminoantipyrine and aromatic aldehydes and their synergistic interaction with borate ester

Abstract

Tribological performance of sulfur, phosphorous and metal-free Schiff bases of 4-aminoantipyrine with benzaldehyde (AAPB), salicylaldehyde (AAPS), p-chlorobenzaldehyde (AAPC) and p-methoxybenzaldehyde (AAPM) and their synergistic formulations with borate ester (BE) in paraffin oil has been evaluated using a four-ball tester at optimized concentrations of Schiff bases (1% w/v) and their synergistic formulations with BE (0.5% w/v for each) by varying loads for 30 min duration and varying test durations at 392 N load. Synergistic formulations effectively enhance antiwear properties of oil and possess high load carrying capacity in comparison to the conventional zinc dibutyldithiophosphate (ZDDP)/BE/Schiff bases alone. The best efficiency is shown by AAPM, followed by AAPC, AAPS and then AAPB. The same order is observed in their corresponding synergistic formulations. AFM and SEM micrographs of the wear scar lubricated with synergistic formulations show a drastic decrease in surface roughness in comparison to borate ester/ZDDP alone. EDX analysis of the worn surface in the presence of AAPM + BE exhibits nitrogen, boron and oxygen indicating adsorption of the additive on the surface. XPS of the tribofilm shows B₂O₃, BN, Fe₂O₃, Fe₃O₄ and adsorbed carbon in the form of –C–C– and –C(O)O– moieties. Quantum chemical calculations (DFT) for interactions of Schiff bases with surfaces show good agreement with experimental results.