The effect of oleic acid, oleyl alcohol and oleyl amine based organic friction modifiers (OFM), ester and ethoxylated fatty ester based di-block polymeric friction modifiers (PFM) and Zinc dialkyldithiophosphate (ZDDP) on the boundary lubrication of steel surfaces was studied using a ball-on-disc tribometer equipped with optical interferometry. Frictional performance of oil blends containing the OFM/PFM, with and without ZDDP, was investigated at a slide roll ratio (SRR) of 100% and 130 °C for a rubbing duration of 24 h. Surface topography measurements using 3D surface profilometer and scanning electron microscopy (SEM) were used to quantify the anti-wear performance. Comparison of optical interferograms and friction data showed that formation of a thicker tribofilm does not always translate to effective friction reduction. Our experiments showed that PFMs generally provide better friction reduction than OFMs with or without ZDDP by effectively eliminating the boundary regime. This was attributed to the ability of PFMs to form a tenacious low shear strength film on the metal surface due to its polymeric nature. In contrast, both the friction modifiers had a negative effect on the anti-wear performance of ZDDP. The observed tribological response is attributed to either preferential adsorption of the friction modifier over ZDDP or ZDDP decomposition products. This notion was further corroborated by results obtained from energy-dispersive X-ray (EDX) spectroscopy and X-ray photoelectron spectroscopy (XPS) with depth profile analysis, which showed lower concentration of ZDDP derived products for the formulation containing both the friction modifier and ZDDP.
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