Tribological investigations on β-lactam cephalosporin antibiotics as efficient ashless antiwear additives with low SAPS and their theoretical studies

Abstract

The antiwear behavior of β-lactam cephalosporin antibiotics such as cefixime, cefadroxil and cephalexin has been studied using the Four Ball lubricant tester in paraffin oil. The results have been compared with high sulphated ash, phosphorous and sulfur (SAPS) containing conventional zinc dibutyldithiophosphate (ZDDP). The tests have been performed using the optimized concentration of the additives (1% w/v) at various loads from 294, 392, 490, 588, 686 to 784 N for 30 min test duration and for various test durations from 15, 30, 45, 60, 75 and 90 min at 392 N load. Various tribological parameters such as mean wear scar diameter (MWD), friction coefficient (μ), mean wear volume (MWV), running-in, steady-state and overall wear rates show that all the studied antibiotics act as efficient antiwear additives. Among all the investigated antibiotics, cefixime shows excellent antiwear properties along with very high load bearing capacity. The surface topography of the worn surface has been studied by scanning electron microscopy (SEM) and atomic force microscopy (AFM). Moreover, the AFM and SEM micrographs of the wear scar in the presence of cefixime at 392 N applied load for 90 min test duration show a drastic decrease in surface roughness. Energy-dispersive X-ray (EDX) analysis of the worn surface under similar experimental conditions in the presence of the cefixime exhibits the presence of sulfur, nitrogen and oxygen on the steel surface, indicating adsorption of the additive on the rubbing surface, resulting in the formation of a strong tribofilm. Further, X-ray photoelectron spectroscopy (XPS) of tribofilm shows the presence of FeSO₄ and Fe₂O₃/Fe₃O₄ and adsorbed nitrogen in the form of NC– and/or an amide moiety. Quantum chemical calculations using density functional theory have been performed to investigate the structure-antiwear activity relationship of these antibiotics. The theoretical calculations explain very well the observed experimental results.