The requirements in automotive lubrication impose more complex lubricant formulation. For environmental issues, it is important to reduce additives containing sulfur and phosphorous compounds. We ...propose to revisit the lubrication of steel by stearic acid with a new approach combining experimental and molecular simulation techniques. The adsorption mechanism of stearic acid on iron oxide surface is investigated by ultra-accelerated quantum chemistry molecular dynamics simulations. Adsorption experiments are performed followed by in-situ XPS and ex-situ PMIRRAS analyses. The adsorption mechanism occurred through the acid group mainly by chemisorption. Friction behavior of stearic acid with iron oxide is studied. Results suggest that the friction process favors the formation of carboxylate function. The low friction behavior seems to be related to the generation of a SAM strongly bonded to the surface.
•We simulate the generation of an adsorbed layer of stearic acid on iron oxide surface by ultra-accelerated quantum chemistry molecular dynamics (UA-QCMD) calculations.•In situ adsorption followed by XPS analysis is performed to validate the model. In addition, PM-IRRAS analysis is also performed.•The adsorption of stearic acid under self-assembled monolayer (SAM) on iron oxide surface occurred through the acid group, mainly by chemisorptions.•Low friction coefficient is reported in presence of stearic acid, which is related to the generation of a SAM strongly bonded to the iron oxide surface.•Both UA-QCMD and PM-IRRAS confirmed that the formation of carboxylate is promoted by friction.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SBCE, SBJE, UPUK
The lubrication mechanism of iron-based materials by C18 fatty acids was investigated using experimental studies and computer simulations. In part I of this work S. Loehlé et al., Trib. Int. ...82(2015)218–227, the adsorption of fatty acids on iron oxide and the conditions to generate a strongly bonded Self-Assembled Monolayer (SAM) were studied. In this second part, the influence of several parameters on this adsorption mechanism of C18 fatty acids on steel-based surfaces was investigated. Among them, the effect of the degree of unsaturation in the alkyl chain, as well as the impact of the density of molecules adsorbed on the surface and the influence of the substrate composition (type of iron oxide/hydroxide), were examined. Then, the tribological behavior of adsorbed SAM was studied. It was confirmed that unsaturation in the alkyl chain leads to steric effects that inhibit the formation of well-organized monolayers and increase friction. The importance of the substrate composition on the adsorption mechanism of fatty acids on steel-based surfaces was also highlighted. Simulation results provide new insight into the in situ behavior of the molecules inside the contact, adding to the understanding of their tribological behavior.
•We simulate the generation of an adsorbed layer of C18 fatty acids on iron-based surfaces.•In situ adsorption followed by XPS analysis is performed to validate the model.•When the degree of oxidation or hydroxylation increases, the reactivity between C18 fatty acids and the surface decreases.•Steric effects induced by unsaturation prevent the formation of a well-arranged monolayer.•Tribotests and molecular dynamic simulation show that stearic acid has the best tribological behavior on iron oxide surface.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SBCE, SBJE, UPUK
Sulfur and phosphorus are key elements for the functionality of lubricant additives used in extreme pressure applications, such as synchronizer systems in cars. To understand their mechanism of ...action we combine first principles calculations and gas phase lubrication experiments. The surface spectroscopy analysis performed
in situ
after the tribological test indicates that iron sulfide (phosphide) is formed by rubbing steel-on-steel in the presence of organo-sulfur (-phosphorus) molecules. We, thus, study the effects of elemental sulfur and phosphorus on the interfacial properties of iron by spin-polarized density functional theory calculations. The results show that both the elements are very effective in reducing the adhesion and shear strength of iron. Sulfur is predicted to be more effective than phosphorus, especially at high pressure. Gas phase lubrication experiments confirm these results, indicating that the friction coefficient of iron-sulphide is lower than that of iron-phosphide and both S and P dramatically reduce the friction of steel-on-steel. These results indicate that the release of elemental sulfur and phosphorus may be the key mechanism to controlling the tribological properties of the metal interface and elucidate that the underling microscopic phenomenon is metal passivation.
Sulfur reduces the adhesion and shear strength of iron more effectively than phosphorus. The surface chemistry, well described by first principles calculations, impacts macroscale tribological properties.
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IJS, KILJ, NUK, UL, UM, UPUK
The reaction of trimethyl-phosphite, TMPi, with a clean Fe(110) surface has been investigated by
ab initio
calculations. The most stable configurations and energies are identified for both molecular ...and dissociative adsorption. The calculated reaction energies indicate that dissociation is energetically more favorable than molecular adsorption and we provide a description of the dissociation path and the associated energy barrier.
In situ
XPS analysis of adsorbed TMPi on metallic iron confirmed molecular chemisorption and dissociation at high temperature. These results shed light on the mechanism of phosphorus release from organophosphites at the iron surface, which is important for the functionality of these phosphorus-based additives, included in lubricants for automotive applications.
The reaction of trimethyl-phosphite, TMPi, with a clean Fe(110) surface has been investigated by
ab initio
calculations.
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IJS, KILJ, NUK, UL, UM, UPUK
Molybdenum dithiocarbamates (MoDTCs) are lubricant additives very efficient in reducing the friction of steel, and they are used in a number of industrial applications. The functionality of these ...additives is ruled by the chemical interactions occurring at the buried sliding interface, which are of key importance for the improvement of the lubrication performance. Yet, these tribochemical processes are very difficult to monitor in real time. Ab initio molecular dynamics simulations are the ideal tool to shed light on such a complicated reactivity. In this work, we perform ab initio simulations, both in static and tribological conditions, to understand the effect of surface oxidation on the tribochemical reactivity of MoDTC, and we find that when the surfaces are covered by oxygen, the first dissociative steps of the additives are significantly hindered. Our preliminary tribological tests on oxidized steel discs support these results. Bare metallic surfaces are necessary for a stable adsorption of the additives, their quick decomposition, and the formation of a durable MoS2 tribolayer. This work demonstrates the importance of the catalytic role of the substrate and confirms the full capability of the computational protocol in the pursuit of materials and compounds more efficient in reducing friction.
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•Oxidation of the steel discs limits the lubricating performance of MoDTC additives.•No catalytic effect of the metallic substrate because of poor adsorption of MoDTC.•Activation of MoDTC occurs only after removal of the oxide layer.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPUK, ZAGLJ, ZRSKP
We apply first principle calculations to investigate the effects of the hydrocarbon chain length in additive molecules under boundary lubrication conditions. In these conditions, occurring in ...high‐pressure applications, the thickness of the oil film becomes extremely thin, and the additive molecules remain confined between the two solid surfaces in contact. We consider two types of organophosphorous additives having the same phosphite group but hydrocarbon chains of different lengths. By comparing the molecular behavior under uniaxial stress applied, we elucidate the atomistic mechanisms that control the molecular capacity of maintaining an interfacial spacing under compression and the load‐induced molecular dissociation. This insight is relevant not only for a rational design of lubricant additives but also to provide understanding on the activation mechanisms of tribochemical reactions.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Context. Determining the source regions of meteorites is one of the major goals of current research in planetary science. Whereas asteroid observations are currently unable to pinpoint the source ...regions of most meteorite classes, observations of meteors with camera networks and the subsequent recovery of the meteorite may help make progress on this question. The main caveat of such an approach, however, is that the recovery rate of meteorite falls is low (<20%), implying that the meteoritic analogues of at least 80% of the observed falls remain unknown. Aims. Spectroscopic observations of incoming bolides may have the potential to mitigate this problem by classifying the incoming meteoritic material. Methods. To probe the use of spectroscopy to determine the meteoritic analogues of incoming bolides, we collected emission spectra in the visible range (320–880 nm) of five meteorite types (H, L, LL, CM, and eucrite) acquired in atmospheric entry-like conditions in a plasma wind tunnel at the Institute of Space Systems (IRS) at the University of Stuttgart (Germany). A detailed spectral analysis including a systematic line identification and mass ratio determinations (Mg/Fe, Na/Fe) was subsequently performed on all spectra. Results. It appears that spectroscopy, via a simple line identification, allows us to distinguish the three main meteorite classes (chondrites, achondrites and irons) but it does not have the potential to distinguish for example an H chondrite from a CM chondrite. Conclusions. The source location within the main belt of the different meteorite classes (H, L, LL, CM, CI, etc.) should continue to be investigated via fireball observation networks. Spectroscopy of incoming bolides only marginally helps precisely classify the incoming material (iron meteorites only). To reach a statistically significant sample of recovered meteorites along with accurate orbits (>100) within a reasonable time frame (10–20 years), the optimal solution may be the spatial extension of existing fireball observation networks.
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FMFMET, NUK, UL, UM, UPUK
Experimental meteors are produced by exposing samples of meteoritic material to an air plasma flow in an arcjet driven plasma wind tunnel facility, simulating the aerothermal conditions of an entry ...into the Earth’s atmosphere. A plenoptic camera is used to record sequences of light field images during the tests, allowing for the first time to derive the transient evolution of the ablating and melting surface in three dimensions. Results are presented for samples of various meteorites, which show the potential of this technique for the volumetric analysis of the complex interaction between an extraterrestrial body and the upper atmosphere. Data allow to derive recession rates, heats of ablation and a shape factor, which has been redefined to meet the recorded information. Recession is found to be non-linear, with different rates for different meteorite types, with mean rates between 0.28 and 0.7mm/s. Heats of ablation are not constant, but decrease during the experiment, with mean values between 1.7 and 10.1MJ/kg. A fairly linear correlation is found between the materials’ iron content and both the recession rate and the heat of ablation. Shape factors decrease with time and reach a plateau after about 3s.
•A plenoptic camera was used for transient measurements of the 3D shape of meteoroids in ground tests.•Data allow in-situ assessment of shape transformation due to melting and ablation.•Results permit to derive relevant parameters such as recession rate, heat of ablation and shape factor.•Heat of ablation decreases during the experiment with mean values between 1.7 and 10.1MJ/kg.•A correlation between the iron content and both recession rates and heats of ablation is found.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPUK, ZAGLJ, ZRSKP
•First successful presentation of PS for molecular oxygen in experiment and simulation.•Line-by-line code description for Schumann-Runge absorption PS.•New capability for high resolution analysis of ...molecular oxygen using PS.•High pressure, high temperature molecular oxygen characterization possibility.
Polarization spectroscopy measurements of O2B3Σu−←X3Σg−Schumann-Runge absorption are presented. The measurements are carried out in the resonator of a microwave-powered plasma torch operated with O2 at atmospheric pressure. A line-by-line spectral simulation code is developed in order to validate the measured spectra at the expected translational, rotational, and vibrational temperatures Ttrans=Trot=3000 K and Tvib=7000 K. The results show very good agreement with the spectral features of the measured O2polarization spectra. The range of probed vibrational and rotational transitions confirms the feasibility of polarization spectroscopy as a quantitative diagnostic tool for various applications.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPUK, ZAGLJ, ZRSKP
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