The tribological behavior of lubricants, prepared with a mineral base oil, lauryl alcohol, and different concentrations of coumarin, were examined using a four-ball tester under constant and variable friction velocity conditions. At constant friction velocity, the maximum non-seizure load (PB) increased from 304 N to 392 N at a coumarin concentration of 0.5 wt.%. Lubricants with 0.7 wt.% coumarin exhibited optimum lubricating properties, and the maximum reductions in friction coefficient (FC) and wear scar diameter (WSD) were 20.0% and 11.88%, respectively. Further investigation of the tribological mechanism implied that the ester group in the coumarin molecule established a connection with the surface atom, resulting in the formation of a tribofilm, which further restricted the adhesion wear regime. Additionally, under variable friction velocity conditions, increasing the coumarin concentration had an obvious effect on the mixed lubrication (ML) and elasto-hydrodynamic lubrication (EHL) regions, but not on other lubrication regions. Moreover, a mathematical model was proposed to show the relationship between FC and friction velocity. Importantly, the present work clarifies the effect of friction velocity on the tribological behavior of coumarin, and also supports the use of coumarin as a novel additive in mineral oils.
Tribological performance of coumarin in copper rolling oil was studied by four-ball tests and cold rolling tests. Then a systemic investigation on surface topography and surface chemistry of rolled strips was performed by field emission scanning electron microscope (FE-SEM), laser scanning confocal microscopy (LSCM), x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS) and Raman spectroscopy (Raman). Results showed that coumarin was a kind of potential lubrication additive. The average friction coefficient (AFC) and wear scar diameter (WSD) were reduced by 30.2% and 30.1%, respectively, when the optimum concentration of coumarin (2.0 wt%) was added in copper rolling oil. Meanwhile, as the formation of tribofilm during rolling process, the nonuniform deformation and adhesive wear were effectively inhibited. Thinner copper strips with fine surface quality were obtained. The surface average roughness (Sa) of rolled copper strips was reduced by up to 68.8%. Surface characterization showed that the carbonyl group in coumarin molecule connected to Cu atoms with its molecular plane vertical or tilted to the metal surface. Besides, thermal analysis disclosed that this tribofilm had negligible surface residuals, especially when coumarin concentration was lower than 3.0 wt%.
Cinnamyl alcohol (CA) was added to the rolling oil of copper foils as a novel corrosion inhibitor, and its anticorrosion performance and mechanism were studied using potentiodynamic polarization and electrochemical impedance spectroscopy. Microstructure and chemical composition of the copper electrode surface were analyzed by scanning electron microscopy, energy‐dispersive spectroscopy, and X‐ray photoelectron spectroscopy. The results showed that CA acted as a mixed‐type inhibitor, and the maximum inhibition efficiency of 87.7% was achieved at 2.4 mM. Moreover, a protective film was formed on the copper surface, which attributed to the C–OH groups in the CA molecule. The absorption of CA on the copper surface was physisorption, which conformed to Langmuir adsorption isotherm, with a standard adsorption free energy of −11.38 kJ/mol. In the presence of CA, some flaky corrosion products changed into granular corrosion products. The synergistic effect of the granular corrosion products (copper chloride hydroxide) and the CA film further decreased the corrosion rate.
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