Tribological Performance of EP Lubricants with Phosphorus-Based Additives

Lubricants containing additives that protect mechanical components against extreme pressure by reducing friction and wear are known as extreme pressure (EP) lubricants. In the current study, phosphorus-based EP lubricants with different additives (amine phosphate and phosphate ester) were tested in a steel ball-on-disc assembly under different EP conditions. The phosphate ester–steel interaction resulted in significantly higher wear and marginally lower friction than the amine phosphate–steel interaction. The tribological performance (especially wear) depended on the contact conditions. The tribofilm that formed on the steel surface with both EP lubricants consisted of organic compounds, oxides, and phosphates. The greater formation of the wear-resistant iron phosphate for the amine phosphate–steel interaction resulted in lower wear. The friction and wear performance for both EP lubricants depended upon surface roughness parameters along with the compounds that formed in the tribofilm.