Dialkyl phosphonate with carboxylic acid as antiwear additives for ester-base lubricants

Ester-base oils are commonly used in environmentally adapted lubricants (EALs) because of their biodegradability. However, their antiwear performance is difficult to improve using the conventional lubricant additives developed for mineral-base oils. In this study, dialkyl phosphonates with various functional groups were designed as antiwear additives for ester-base oils. The antiwear performances of these phosphonates were evaluated using four-ball tests. The results suggest that dialkyl phosphonate with carboxylic acid (DAPA) has better antiwear performance in ester-base oils than that of other phosphonates and conventional phosphorus additives. Remarkably, DAPA exhibits better antiwear performance under severe test conditions in ester-base oils than in poly-alpha olefin. X-ray photoelectron spectroscopy, transmission electron microscopy, and Fourier transform infrared spectroscopy were used to study the chemical compositions of steel surfaces after friction and better understand the antiwear mechanism of DAPA, particularly in ester-base oils.