No abstract

Overbased detergents are well known in the tribology field as anti-wear additives. In boundary lubrication, they generate a quite thick tribofilm on rubbing surfaces. They were studied by coupling XPS and AES depth profiles with XANES and ToF-SIMS analyses. Under friction, we show by ToF-SIMS analysis that detergent molecules are split into smaller structural units. Moreover, ionic bonds do not resist high pressure and shearing, and sulfur disappears from the contact zone. The overbased calcium carbonate core finally collapses and crystallizes to give a good anti-wear film between rubbing surfaces.

Tribochemical reactions of phosphites additives on steel surface have been simulated by gas phase lubrication. Trimethylphosphite (TMPi), P(OCH 3 ) 3 , has been used as model molecule for phosphites additives. It has been introduced under gas phase up to 5 hPa in a new tribometer dedicated to gas phase lubrication. Friction tests have been carried out at ambient temperature and 100uC. Chemical analyses by X-ray photoelectron spectroscopy and by Auger electron spectroscopy have been conducted inside and outside of the track. Two kinds of analysis have been carried out: ex situ and in situ surface analyses after tribological test. Indeed, a new environmentally controlled tribometer allows friction test then accurate analyses without air exposure of the formed tribofilm. Tribotests conducted under TMPi gas phase show a reduction of friction coefficient until 0?2 instead of 1?4 under high vacuum. Jointly, formation of tribofilm has been confirmed by optical microscopy and ex situ chemical analysis. Comparison between analyses performed inside and outside of the wear scar indicates that the friction induces the formation of phosphide compound that could reduce friction. Moreover analyses show the formation of methoxy group (CH 3 O) and carbonate originally from the decomposition of TMPi under friction into H 2 and CO. In situ analyses clearly show the importance to investigate an uncontaminated tribofilm in order to obtain a better characterisation of it and then a better comprehension of the tribochemical mechanisms.