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Abstract
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.
ACKNOWLEDGEMENTS
The author thanks the following individuals for their continued assistance and support over the course of this study: the Southwest Research Institute Advisory Committee for Research for funding to complete this project; Dr. D. Marton from the University of Texas Health Science Center of San Antonio for performing the XPS analysis on the test samples; F. W. Gerhart, the metrology technicians, and the fabrication technicians from the Southwest Research Institute for assistance in running the engine tests, measuring the test parts for wear, and preparing the test samples; and Dr. P. M. Lee and M. L. Moneer, Jr., from the Southwest Research Institute Tribology Laboratory for the use of lab space.