Thierry Le Mogne

The two-layer structure of Zndtp tribofilms: Part I: AES, XPS and XANES analyses

The nature and properties of polyphosphate glasses formed in the antiwear action of zinc dithiophosphate (Zndtp) is investigated. Special attention is paid to the advantage of coupling three surface analytical techniques on the same Zndtp tribofilm: Auger (AES), XPS and XANES spectroscopies. The data show the two-layer structure of the Zndtp film and permit a clear identification of the chemical composition of each layer: a polymer-like zinc long chain polyphosphate overlying a mixed transition metal short chain phosphate. A Chemical Hardness model is found to predict the formation of such a layered tribofilm. Moreover, a tribochemical reaction between the zinc polyphosphate and the iron oxides species is proposed on the basis of the Hard and Soft Acids and Bases (HSAB) principle. This reaction explains the anti-abrasive mechanism of Zndtp and also predicts a depolymerisation of the long chain zinc polyphosphate glass, in very good agreement with AES/XPS/XANES analytical data. The role of residual sulphur in the lubricant is also explained and the model is in agreement with the formation of metal sulphides embedded in the short chain phosphate matrix. This first paper serves as a basis for a detailed study of the mechanical properties of each film.

Transfer films and friction under boundary lubrication

The role of transfer phenomena in the mechanisms of friction reduction by organic molybdenum compounds is studied with the aid of ultrahigh vacuum (UHV) analytical tribometry. Additives used are zinc dithiophosphate (Zndtp), molybdenum dithiophosphate (Modtp), molybdenum dithiocarbamate (Modtc) and Modtc/Zndtp combinations. Experiments involve UHV friction tests on tribofilms formed previously and in situ surface analyses by Auger electron spectroscopy (AES) and imaging. In the presence of Modtc, friction reduction was found to be associated with the transfer of highly dispersed MoS2 as isolated sheets, from the tribofilm to the slider. In the presence of Modtp (or the mixture Modtc/Zndtp), a two-step tribochemical reaction is generally observed. First, phosphate from the film is transferred to the oxide on the pin and the friction coefficient is about 0.3, after an induction period of a few cycles, pure MoS2 single sheets are transferred to the pin and the phosphate is eliminated as wear debris from the contact zone. Friction is then at its lowest value (0.02). The chemistry of the transfer phenomena is modeled using the hard and soft acid and base (HSAB) principle as described by Pearson. The overall data suggest that friction behavior under boundary lubrication with additives may be directly related to molecular scale transfer mechanisms in general.

Tribochemical interactions between Zndtp, Modtc and calcium borate

Tribochemical interactions between antiwear zinc dithiophosphate (Zndtp), friction modifier molybdenum dithiocarbamate (Modtc) and overbased detergent calcium borate (OCB) lubricant additives have been investigated. Friction tests were performed in mild wear conditions under boundary lubrication, in order to enhance tribochemical surface effects. The nature of tribofilms formed was studied by coupling high‐resolution TEM on wear fragments and inside‐wear‐scar, micro‐spot XPS in the same location of the wear track (so‐called dual analysis). The performance of the Modtc/Zndtp mixture is mainly due to the generation of MoS2 single sheets and the digestion of MoO3 in the zinc polyphosphate glass formed. The final result of the tribochemical reaction is a two‐phase tribofilm composed of (i) non‐oriented MoS2 sheets (friction modifier) embedded in a carbon‐rich phase and (ii) a mixed Zn/Mo polyphosphate glass (antiwear). The Modtc/OCB mixture has a similar antiwear mechanism except that the oxide is not completely eliminated, due to the softer action of borate anion compared with phosphate one. Compared to the data obtained with binary combinations (Modtc/Zndtp, Modtc/OCB and Zndtp/OCB), we show here that the ternary system Modtc/Zndtp/OCB provides both a low wear rate and an ultralow friction value, while adding detergent and anti‐corrosive properties to the formulation. Our analytical data indicate that the synergistic effect can be attributed to an outstanding nanostructure of the tribofilm formed. It is composed of a single‐phase material containing perfectly oriented MoS2 single sheets embedded in a calcium and zinc borophosphate glass. The ternary system produces a smart material in the interface, because both functions (antiwear and friction reduction) are correlated. Compared to phosphate alone, the mechanism by which MoS2 sheets have been oriented in the borophosphate could be related to aligned molecules of the glassy polymer in the direction of sliding.

Experimental investigation of friction on low-temperature ice

Abstract Using a unique technique, we formed a thin layer of ice on steel surface at temperature as low as 123 K in an ultra-high vacuum. We investigated the frictional behavior of such a thin ice layer in the same UHV using a pin-on-disk friction tester. The sample surfaces were monitored in situ using X-ray Photoelectron Spectroscopy (XPS). Testing data showed that the friction coefficient was a linear function of temperature. This study indicates that the sliding motion is lubricated by a frictionally melted liquid layer.

Experimental and Molecular Dynamics Simulations of Tribochemical Reactions with ZDDP: Zinc Phosphate–Iron Oxide Reaction

Zinc phosphate glass is considered to be the main constituent of tribofilms generated under boundary lubrication with zinc dialkyldithiophosphate (ZDDP), a well-known antiwear additive. The reaction occurring during friction between zinc phosphate glasses and steel native iron oxide layer is investigated by both an experimental approach and by Molecular Dynamics simulations (MD). The importance of this “tribochemical” reaction in the general ZDDP antiwear process is discussed.

Modelling Tribochemical Reactions of Additives by Gas-Phase Lubrication

In the context of boundary lubrication, we are interested in interactions between lubricant additives as well as in the role of nascent surfaces formed during friction. Tribochemical reactions are very complex and their study requires simplified approaches. We have developed an analytical ultrahigh vacuum tribotester with the possibility of introducing low-molecular-weight molecules during (or before) a friction test, in order to simulate heavy lubricant components by means of their chemical functional group. Mixtures of gases can also be studied with this apparatus. In situ AES/XPS (depth profiling) is available at the end of the friction test. This permits the characterisation of the two friction counterfaces to be performed accurately. Results show that friction under gaseous feeds can be well related to lubricated tests, both in friction behaviour and in wear track and debris morphologies. The evolution of friction in the presence of pressures of selected gases (O2, hexane, hexene and hexanol) shows clear differentiation of the different cases as a function of chemical group. Finally we show that the method is well adapted to model some aspects of boundary lubrication. It can be used as a powerful tool to predict and understand mechanisms of action of individual additives as well as interactions between additives in oil formulations.

Experimental simulation of phosphites additives tribochemical reactions by gas phase lubrication

Abstract 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 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 (CH3O) and carbonate originally from the decomposition of TMPi under friction into H2 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.

A novel experimental analysis of the rheology of ZDDP tribofilms

In this study, the rheology and durability of tribofilm has been investigated. A ZDDP tribofilm (6×8 mm) has been partially removed by Ar+ ion etching (5keV), using an aluminum mask with a comb shape. A striped sample was obtained with a lateral alternation of etched zones and ZDDP tribofilm areas, respectively (about 1 mm width). On this striped sample, a friction test was performed using a reciprocating pin-on-flat friction machine. Tests were carried out in different lubricant environments at ambient temperature with the maximum contact presure of 0.5 GPa. The friction signal and the Electrical Contact Resistance (ECR) were plotted as a function of position and number of cycles. The results can be explained by a mechanical process in the tribofilm zone (its removal) and chemical reaction in the etched zone.

Antispalling Effect of WS2 Nanoparticles on the Lubrication of Automotive Gearboxes

ABSTRACT The effect of WS2 nanoparticles used as lubricant additives in the lubrication of automotive gearboxes has been studied. The results suggest that nanoparticles can be used to increase the life span of the mechanical parts of gears. Chemical analyses and observations made after transmission tests have shown that nanoparticles are able to go inside cracks and may have a sealing effect, preventing spalling and further failure of the material. Moreover, the addition of nanoparticles in fully formulated oil reinforces the antispalling properties of the lubricant.

Alternative to ZDDP Additive: Study of Zinc Dialkylphosphate Lubricant Properties

The friction and wear behavior of ZP additive is investigated as an alternative to ZDDP (poisonous for catalyst-based emission control systems). The effect of temperature (25°C and 100°C), of load (200 N and 320 N) and the wear kinetics (5 min and 60 min experiments) were studied. Physico-chemical characterization of the wear tracks were performed by XPS. A protection against wear even at ambient temperature was found. This feature was attributed to the presence of zinc polyphosphate in the tribofilm.Copyright