Limitations on use of ZDDP as an antiwear additive in boundary lubrication

Wear tests on rubbing elements under various contact pressures, oil temperatures and surface roughness were performed to elucidate the limitation conditions on antiwear performance of a typical zinc dialkyl dithiophosphate (ZDDP) added to a paraffin base oil. The analysis results indicate that the antiwear performance of ZDDP depends on the rates of recovery and growth of effective surface films. If these rates exceed that of scraping the films off the rubbing surfaces, antiwear performance is evident. However, the rates of recovery and growth of surface films are influenced by some essential factors, including the oil temperature, contact pressure and the surface roughness and hardness of rubbing elements. Under some suitable conditions, ZDDPs show favorable antiwear performance, but under some severe conditions, such as high pressure (over 90 MPa), high temperature (over 200 °C) with an insufficient concentration of ZDDP and the rubbing of a rough and harder surface on a softer surface, the antiwear performance of ZDDPs is negligible.