The influence of synthetic caprylic methyl diethanolamine phosphate ester (abbreviated as MDEACP) on biodegradability and tribological properties of 400SN mineral base oil was studied. The biodegradability of the neat base oil and the oil doped with MDEACP was determined on a biodegradation tester. The tribological properties of the neat base oil and the oil doped with MDEACP were evaluated on a four-ball tester. Moreover, the worn surfaces were investigated by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The results revealed that MDEACP significantly promoted the biodegradation of the mineral base oil. The improvement in biodegradability was attributed to the enhanced growth and quantity of microbes by MDEACP. Furthermore, MDEACP enhanced the anti-wear properties, the friction-reducing properties, and the extreme pressure properties of the base oil. It was mainly attributed to the formation of the complex boundary lubrication film resulted from the adsorption and tribochemical reactions of MDEACP on the friction surface.
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