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Functional additives, particularly extreme-pressure and antiwear additives, in formulated oil will compete to adsorb and form a protective layer in tribological contacts. The thickness of the layer is determined by the equilibrium between the formation and removal processes. In this article, the interactions between additives and base oil molecules and operating conditions influence friction and wear are studied. One polar (ester oil) and one non-polar (poly-a-olefin) commercial base oil blended with zinc dialkyl dithiophosphates were studied. The tribological performance was evaluated using a ball-on-disc test rig under mixed rolling-sliding conditions in the boundary lubrication regime. An adapted in situ interferometry technique was used to monitor the additive-derived reaction layer formation. The properties of the additive-derived reaction layers were studied using surface analysis techniques, X-ray photoelectron spectroscopy and atomic force microscopy. A thicker layer was formed when the additive is blended in the nonpolar oil. This observation suggests that base oil polarity determines the transport of additives to the surface, thereby controlling the maximum reaction layer thickness, friction and wear, as well as the morphology of the additive-derived reaction layer. However, the reaction layer chemical composition is not strongly influenced by the base oil polarity. Among the operating conditions, shear was identified as a fundamental parameter for the activation of additives on rubbing steel surfaces and the properties of the derived reaction layer.

Ball-on-disc tribo tests and atomic force microscopy (AFM) were used to analyze the effect of base oil polarity on the friction behaviour of steel-steel contacts lubricated with base oil + zinc dialkyldithiophosphate (ZDDP) solutions. Understanding the lubrication properties of the first chemisorbed layer of additives on work pieces yields important information for the optimization of lubrication in various solutions, in particular with regard to the type of additive and amount needed.To characterize the influence of oil polarity, two reference base oils (hexadecane -non polar and diethylenglycol -polar) were blended with different concentrations of ZDDP-C 4 , and the solutions were tested. A monolayer of base oil/additive solution is deposited on an ASI 52100 steel plate and is scanned on AFM contact mode under several rubbing time and applied load conditions. An AFM technique is developed to estimate microscopic values of friction coefficients showing how the oil polarity contributes to the differences in friction behaviour of the solution due to the addition of ZDDP.

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Functional additives, particularly extreme pressure and antiwear additives, in formulated oil will compete to adsorb and function in tribological contacts. A low-polarity commercial base oil, poly-α-olefin (PAO), blended with zinc dialkyl dithiophosphates (ZDDP) has been studied. The tribological performance was evaluated using a ball-on-disk test rig under mixed rolling-sliding conditions in the boundary lubrication regime at 90°C. An adapted in situ interferometry technique was used to monitor the additive-derived reaction layer formation. The thickness of the reaction layer evolves with rubbing until reaching a limiting thickness value of approximately 70  nm. The evolution of the topography and mechanical properties of the ZDDP-derived reaction layer with rubbing time were studied using Atomic Force Microscopy. A constant roughening and hardening of the additive-derived layer with rubbing time is observed and related to the different tribological performance of the layer at different rubbing times.