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Tribochemistry of Phosphorus Additives: Experiments and First-Principles Calculations

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Tribochemistry of Phosphorus Additives: Experiments and First-Principles Calculations RSC Advances View Article Online PAPER View Journal | View Issue Tribochemistry of phosphorus additives: experiments and first-principles calculations† Cite this: RSC Adv.,2015,5,49270 M. I. De Barros-Bouchet,*a M. C. Righi,*b D. Philippon,a S. Mambingo-Doumbe,c T. Le-Mogne,a J. M. Martina and A. Bouffetc Organophosphorus compounds are common additives included in liquid lubricants for many applications, in particular automotive applications. Typically, organic phosphites function as friction-modifiers whereas phosphates as anti-wear additives. While the antiwear action of phosphates is now well understood, the mechanism by which phosphites reduce friction is still not clear. Here we study the tribochemistry of both phosphites and phosphates using gas phase lubrication (GPL) and elucidate the microscopic mechanisms that lead to the better frictional properties of phosphites. In particular, by in situ spectroscopic analysis we show that the friction reduction is connected to the presence of iron phosphide, which is formed by tribochemical reactions involving phosphites. The functionality of elemental phosphorus in reducing the friction of iron-based interfaces is elucidated by first principle Received 13th January 2015 calculations. In particular, we show that the work of separation and shear strength of iron dramatically Accepted 21st May 2015 decrease by increasing the phosphorus concentration at the interface. These results suggest that the DOI: 10.1039/c5ra00721f functionality of phosphites as friction modifiers may be related to the amount of elemental phosphorus www.rsc.org/advances that they can release at the tribological interface. I. Introduction a previous publication,19 Philippon et al. evidenced clearly the formation of iron phosphide species from trimethyl-phosphite Organophosphorus compounds are important extreme pres- (TMPi) molecules and they detailed the reaction mechanism of sure (EP) and anti-wear (AW) additives used in different TMPi on an iron surface. They also investigated the role of the industrial elds, especially automotive applications.1,2 While freshly nascent surfaces obtained by friction in UHV on the Published on 22 May 2015. Downloaded by RSC Internal 18/08/2015 16:05:46. phosphate ester additives were extensively investigated during decomposition of the TMPi molecules using gas phase lubrication the last few decades,3–9 few studies have been devoted to phos- (GPL) approach thanks to an environmentally controlled analyt- phite compounds despite their increasing use in engine oils,10–16 ical tribometer (ECAT).20 The in situ chemical analysis clearly gave especially for gearbox components. Among the wide variety of some evidence of the chemical dissociation of TMPi on iron phosphite additives used to improve the oil friction reduction surfaces under friction leading to the formation of iron phos- performances, the most common ones are triallyl-phosphite, phide compound which was fully characterized by focused ion tributyl-phosphite, dilauryl-phosphite, dioleyl-phosphite and beam/high resolution transmission microscopy (FIB/HRTEM).21 triphenyl-phosphite. The chemical structure of these additives GPL is not a new concept in lubrication technology, but it is a greatly inuences the nature of the formed tribolm and its well-known way to lubricate systems subject to high tempera- tribological performances. Riga et al.16 showed that shorter tures, as those present in cutting and working metal processes for phosphite chains give better anti-wear performances. This example.22,23 It has been shown that GPL is able to simulate the seems to be related to the formation of a tribolm containing boundary lubrication regime by bringing directly the molecules 17 24,25 iron phosphide that was identi ed by Barcro as Fe2P on on the surface in contact, thus avoiding the use of solvents for rubbed surfaces lubricated by tricresyl-phosphate (TCP). The cleaning the rubbed surfaces prior to spectroscopic analyses and lubrication mechanism of TCP was attributed to a polishing therefore reducing the risk of their contamination and/or dena- action via the formation of an iron/iron phosphide eutectic.18 In turation. This is a huge advantage compared to standard liquid phase lubrication coupled with ex situ post mortem surface analyses. Using this approach, the effect of active nascent metallic aLaboratoire de Tribologie et Dynamique des Syst`emes, Ecole Centrale de Lyon, Universit´e de Lyon, 69134 Ecully, France. E-mail: [email protected] surfaces on decomposition of adsorbed molecules was high- 20,26,27 bCNR Istituto di Nanoscienze Centro S3, Dipartimento di Fisica, Universita' di Modena lighted and the importance of dangling bonds passivation to e Reggio Emilia, 41100 Modena, Italy. E-mail: [email protected] achieve low friction in carbon-based materials was evidenced.28,29 cTOTAL, Centre de Recherche de Solaize, 69360 Solaize, France Here, we performed GPL experiments coupled with in situ † Electronic supplementary information (ESI) available. See DOI: surface analyses to investigate the tribochemistry of different 10.1039/c5ra00721f 49270 | RSC Adv.,2015,5,49270–49279 This journal is © The Royal Society of Chemistry 2015 View Article Online Paper RSC Advances phosphorus-containing organic additives. We consider trimethyl under an environmentally controlled gas phase (named Envi- phosphate (TMPa), trimethyl phosphite (TMPi) and dimethyl ronmentally Controlled Tribometer, ECT) and a second UHV phosphite (DMPi), respectively. The latter exists in two different chamber dedicated to surface analyses (XPS and Auger spec- chemical equilibrium states, namely the phosphite and the troscopies). The ECAT is schematically presented in Fig. 1. A phosphonate. These molecules present the same chemical reciprocating pin-on-at machine with a movable hemispherical groups as commercial organo-phosphorus additives, but have pin being connected to a brushless motor is located inside ECT. shorter hydrocarbon chains. This allowed us to use them in the The temperature of the friction contact can be controlled from gas phase, without losing any information on tribological effects. 100 C to 600 C thanks to the at holder equipped with a À The main functionality of the hydrocarbons groups is, in fact, to nitrogen cooling and a heating system. Both normal and solubilize or micellize the additive molecules into the base oil. tangential forces are accurately measured thanks to optical We found that phosphites reduce friction more efficiently than sensors. The ECT is able to measure both super-low friction phosphates and their friction reduction property is directly (milli-range) and very high (above unity) friction coefficients. A related to the tribo-induced formation of iron phosphide. The pure gas or a gas mixture can be introduced inside the ECT by measured friction coefficient decreases, in fact, as the amount of using 3 micro-leak valves and their pressure can be continuously 9 iron phosphide observed aer the tribological experiment varied from 10À hPa to 2000 hPa (twice the atmospheric pres- increases, TMPi presenting the lowest friction coefficient. sure), thanks to a membrane vacuum gauge. The tribometer is To shed light into these experimental observations, we per- computer-controlled and the normal load is automatically formed rst principles calculations of the interfacial properties adjusted when the gas pressure inside the ECT is changed. The of adhesion and shear strength. The knowledge of these pin-on-at contact can be optically observed sideways by a low intrinsic properties of materials in contact is important to level SIT camera. The ECT is connected to the analysis chamber interpret their tribological behavior, in particular to provide a by a third intermediate UHV chamber allowing the transfer of microscopic understanding on the functionality of lubricant the rubbed surfaces without any exposure to air. additives. In boundary lubrication conditions, where hydrody- Heavy molecular weight compounds like triphenyl-phosphite namic lubrication is no longer effective, the friction reduction and tricresyl-phosphate are used as additives in commercial and wear protection is provided by the tribolm formed by the lubricants because of the solubility in oil provided by the presence chemical additives reaction on the sliding surfaces. Indeed, of long carbon chains in the molecule. However, it is difficult to these compounds react with the surfaces and modify their evaporate these heavy molecular weight additives in a vacuum. chemical composition, with consequent modications of their Therefore smaller molecules, having the same chemical function, adhesion and resistance to sliding. First-principle calculations were used as phosphite/phosphate model molecules for GPL, based on density functional theory are able to provide an namely TMPi, DMPi and TMPa having a purity of 99+% (>99%), accurate description of the surface chemistry and its effects on provided by the Aldrich Chemical Co. These molecules were further the surface–surface interactions, such accuracy is typically puried by freeze pumping thaw cycles before being introduced unattainable by classical force elds based on empirical into the ECT. Table 1 provides the chemical formula, molecular parameterizations of the atomic interactions. However, the use weights and the saturating vapor pressures (SVP) at room Published on 22 May 2015. Downloaded by RSC Internal 18/08/2015 16:05:46. of rst-principle calculations in tribochemistry has been tradi- temperature for these three compounds. The
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