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Tribochemical reactions of phosphites additives on steel surface have been simulated by gas phase lubrication. Trimethylphosphite (TMPi), P(OCH3)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 100°C. 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 situand in situsurface 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 situchemical 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 (CH3O) and carbonate originally from the decomposition of TMPi under friction into H2and CO. In situanalyses 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.

The fuel economy and reduction of harmful elements in lubricants are becoming important issues in the automotive industry. An approach to respond to these requirements is the potential use of low friction coatings in engine components exposed to boundary lubrication conditions. Diamond-like carbon (DLC) coatings extensively studied as ultralow friction films to protect the surfaces of ductile metals for space applications are expected to fulfil this part. The main purpose of this work is to investigate the friction and wear properties of glycerol lubricated DLC coatings under boundary lubrication conditions. The DLC material consists of tetrahedral hydrogen free amorphous diamond-like carbon (denoted as ta-C) as shown by the time of flight secondary ion mass spectroscopy (ToF-SIMS) analyses and the nanoindentation measurements. The friction coefficient below 0&middot.01, called superlubricity, and no measurable wear were obtained by sliding the ta-C/ta-C friction pair in the presence of pure glycerol as a lubricant at 353 K. The mechanism by which glycerol is able to reduce the friction in the millirange was revealed by ToF SIMS analyses inside and outside wear scars formed by friction experiments using deuterated glycerol and 13C glycerol.

Abstract This paper presents a unique tribological system that is able to produce no measurable wear of material combination and that reduces friction markedly in the ultralow regime under boundary lubrication. Ultralow friction (0.03) was obtained by sliding hydrogen-free Diamond-Like-Carbon ta-C against ta-C lubricated with Poly-alpha Olefin base oil containing Glycerol Mono-Oleate (GMO) additive. The origin of ultralow friction in these conditions has been investigated by surface analysis techniques. Results are in agreement with the formation of a OH-terminated carbon surface. This new surface chemistry might be formed by the tribochemical reaction of alcohol function groups with the friction-activated ta-C atoms. The origin of low friction could be due to the very low-energy interaction between OH-terminated surfaces.