Wear Reduction and Surface Layer Formation by a ZDDP Additive

The results of wear tests and surface analyses confirmed that the wear behavior in the presence of ZDDP is controlled by chemically reacted surface films. Surface analysis by Auger microprobe showed that the reacted layer is composed of iron sulfide and zinc phosphate and that it extends to several hundred Å below the contact surface. It was found that the effectiveness of ZDDP in wear reduction was dependent on the test conditions. Under low loads and temperatures, it decreased wear by two orders of magnitude. However, at higher loads and temperatures, wear was increased by one order of magnitude. Scanning electron microscopy confirmed that at low loads, ZDDP changes the wear mechanism from plowing to a mild form of surface deformation, whereas at high loads plowing is changed to delamination of the surface layer. Surface analysis by Auger showed that higher loads and temperatures had resulted in reduction of sulfur content in the surface films. This reduction is attributed to changes in the reaction kinetics at higher contact temperatures caused by high contact loads and elevated oil temperatures. Since sulfides are usually harder than phosphates, the higher wear rate by delamination is explained by the loss of film strength due to reduction of sulfide content.