Imaging the chemistry of transfer films in the AES/XPS analytical UHV tribotester

Abstract

Abstract Increasing demands in the mechanisms of lubricant additives and thin films have resulted in the development of new tribometers coupled with surface analysis techniques. In this paper, is presented the so-called AES/XPS analytical tribotester. In situ surface analysis inside and outside wear scars, just after the friction test are possible thanks to a tribometer directly installed in the UHV analytical chamber. In previous works, the spatial resolution of the analysis did not allow the wear scars to be analysed by XPS specifically. Recently, we have improved the performance of our analytical tribotester by installing a new electron spectrometer which now permits both micro-spot XPS and XPS imaging to be performed in the wear scars of a pin-on-flat tribometer. Moreover, the good spatial resolution of Auger analysis (about 0.5 micrometer) and XPS (about 60 micrometers) allows the tribochemical reactions to be studied accurately in a classical Hertzian sphere-on-plane contact. For studying transfer material distributions, line-scan AES spectra as well as AES and XPS mappings are available with the possibility of chemical bond specific imaging. In the state of our knowledge, this new machine has unsurpassed possibilities to study fundamentals of tribochemistry. In this work, we report the first data generated by our instrument, concerning the tribochemical reactions of lubricant additives. Molybdenum dithiophosphate (Modtp) is known to possess good friction-reducing properties which are due to M0S2 formation, a wellknown lamellar solid lubricant. Thanks to AES mapping and XPS, the presence of molybdenum sulphide is evidenced in the film transferred onto the pin during friction.