Influence of biodiesel and water dilution on ZDDP antiwear performance: An in-situ AFM study combined with macroscopic experiments

When compression-ignition (CI) engines are fueled by biodiesel, the engine oil is prone to the dilution of biodiesel and water due to the incomplete combustion, resulted by a higher boiling point than traditional petrodiesel. Dilution affects the performance of widely used oil additives, e.g., zinc dialkyldithiophosphate (ZDDP). In this study, its influence on tribological performance of ZDDP was investigated using two different scale tests: ball-on-disc friction tests of 1000 m sliding distance at 75 °C under boundary lubrication regime at macroscopic scale and atomic force microscopy (AFM) experiments with the in situ measurement on tribofilms formation at microscopic scale. Three main compositions of biodiesels: methyl oleate, methyl laurate and methyl palmitate were blended with ZDDP containing polyalphaolefin (PAO) base oil, separately. Two water-diluted lubricants, namely oil-in-water (OW) and water-in-oil (WO) emulsions were also prepared. Results of ball-on-disc tests show that adding these fatty acid methyl esters (FAMEs) to ZDDP gave rise to improved lubricity and hence reduction in both friction and wear. Despite no obvious difference in friction, OW had high volumes of wear while WO produced the wear track of high surface roughness. Results of AFM tests revealed that both FAMEs and water hindered solid-like ZDDP tribofilm formation and the lowest growth rate was observed in WO emulsion. Young's modulus of all measured tribofilms were in a range of 50–150 GPa and the film formed in the presence of methyl palmitate showed significantly improved stiffness.