Studies of antiwear film on 52100 steel using synchrotron light-based techniques

摘要

Zinc diakyl dithio phosphate (ZDDP) has been the most successful anti-wear additive used in commercial automobile engine oils since its introduction to the industry over 50 years ago. Though unrivalled in its achieved properties, its poisoning of catalytic converter has forced the search of cost-effective alternatives even in vain. Anti-wear chemistry of ZDDP is the critical knowledge for achieving that goal. Studying the anti-wear chemistry of ZDDP in situ is difficult and in general the spectroscopic study of the tribofilm has been accepted as a feasible approach to understanding the anti-wear mechanism of ZDDPrnAntiwear films were generated from formulations of zinc dialkyldithiophosphate (ZDDP) and various dispersants in base oil on 52100 steel in a pin-on-disk configuration within a boundary lubrication regime. X-ray absorption near edge structure (XANES) spectroscopy at the S, P, O, N, B, and Zn absorption edges and synchrotron light-based X-ray photoelectron spectroscopy (SR-XPS) were used to investigate the interactions of ZDDP and dispersants and to identify the chemical species formed in the antiwear films. Coupled with the XANES results, SR-XPS withrnvariable photon energy has been used to non-destructively elucidate the depth compositions and chemical variations from ～1nm to ～4 nm into the antiwear films. The elemental ratio of P/ Zn and "bridging" oxygen /"non-bridging" oxygen decrease as a function of distance from the top surface of the film suggesting a decrease of the polyphosphate chain-length . A 3-D polyphosphate structure and/or a mixture of zinc polyphosphate with organic phosphate with a P/Zn ratio larger than 2 has been identified in the topmost layer .This 3-D polyphosphate feature has also been confirmed by O (1s) SR-XPS. In addition, spectroscopic evidence of the existence of thermally-induced ZDDP intermediates in the tribofilm has been obtained by comparing SR-XPS on and off the wear-track.rnThese new insights into the chemical characteristics of the antiwear films will be correlated with the wear performance as measured using the wear scar width on the pin and coefficient of friction. This talk will show that XANES spectroscopy and SR-XPS are very valuable tools in exploring the chemistry of antiwear films.