Tribological Studies of Some Quinoline Derivatives and Their Synergistic Interaction with Phosphate Ester

Abstract

Antiwear properties of metal and sulfur-free synergistic formulations containing quinoline derivatives (QDs)—4-N-(7-chloroquinolin-4-yl)-1-N,1-N-diethylpentane-1,4-diamine (QD-1), 4-N-(6-methoxyquinolin-8-yl)pentane-1,4-diamine (QD-2) and 1-N,1-N-diethyl-4-N-(6-methoxy quinolin-8-yl)pentane-1,4-diamine (QD-3)—with triphenyl phosphate (PE) in polyethylene glycol (PEG) at a very low concentration, 0.25% w/v of each, have been studied on a four-ball tester using ASTM D4172 and D5183 standards. Phosphorus content in the formulations was significantly reduced compared to the frequently used antiwear additive zinc dialkyldithiophosphate (ZDDP) and it was much lower than the allowed concentration suggested by various norms. The synergistic admixtures, SQD-2 and SQD-3, significantly improved the antiwear properties of base lube compared to the ZDDP/PE/QDs alone but the behavior of SQD-1 is comparable to that of ZDDP. Minimum values of mean wear scar diameter and average coefficient of friction were found to be 0.458 mm and 0.034, respectively, for SQD-3, the admixture with the maximum antiwear efficiency. However, these values were 0.538 mm and 0.083, respectively, for ZDDP, and for base oil the values were 1.185 mm and 0.126, respectively. The load-carrying capacity was found to be much higher for the synergistic formulations than for ZDDP.Atomic force microscopy (AFM) and scanning electron microscopy (SEM) micrographs of the wear track support the tribological behavior of the above additives. The presence of nitrogen, oxygen, and phosphorus in energy-dispersive X-ray (EDX) spectra of the worn surface lubricated with SQDs indicates the adsorption of additive on the surface. The magnificent tribological behavior of SQDs may be attributed to the tribochemical film formed on the steel surface invoking phosphorus–nitrogen synergy. Quantum chemical calculations (density functional theory) for interactions of QDs with a steel surface agree well with the experimental observations.