An XPS Study on the Composition of Zinc Dialkyl Dithiophosphate Tribofilms and Their Effect on Camshaft Lobe Wear

In this study, various zinc dialkyl dithiophosphate (ZDDP)-derived tribofilms were analyzed with X-ray photoelectron spectroscopy (XPS) to determine the relationship between tribofilm chemical composition and wear severity. Samples were generated on camshaft lobes from engine tests conducted at different oil temperatures, and wear depths were measured with a surface profilometer. Tribofilm specimens were analyzed by XPS to assess changes in film chemistry as a function of wear severity. Tribofilms present on all camshaft lobes were found to contain polyphosphate glass of various chain lengths. Long-chain polyphosphates were observed on low wear tracks, whereas short-chain polyphosphates were found on more severe wear tracks. Variations in polyphosphate chain length are also apparent at different wear depths along a low wear track. The presence of long-chain polyphosphates on low wear tracks was indicative of their superior antiwear properties, whereas the occurrence of short-chain polyphosphates on more pronounced wear tracks was indicative of their inferior antiwear properties.