Wear Scar Evolution in Refrigeration Oil PAG and ZDDP-PAG Blend Oil Under EP Condition

This study tracks the evolution of wear scar under extreme pressure (EP) conditions, using zinc dialkyl dithiophosphates (ZDDP) additives mixed with polyalkylene glycol (PAG) refrigeration oil via a four-ball test method using an AISI 52100 hardened steel as a test ball. The experiments show that the extreme pressure performance of the PAG oil declined significantly when the load was raised to a critical value. However, ZDDP additives mixed with the PAG oil in appropriate ratios can improve the extreme pressure performance of the PAG oil. According to the experimental results, ZDDP additives effectively form tribofilm on the rubbing surface to improve extreme pressure performance during run in process. In particular, some ZDDP tribofilm was detected in the region outside of the rubbing area. This was caused by the effects of the scraped off tribofilm flowing out, then accumulating on the outside of the wear scar. In addition, the high temperature caused ZDDP tribofilm formation near the rubbing region.