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
Recent engine design and emission trends have led to the commercial use of Atmospheric Plasma Spray (APS) coatings for cylinder liner applications like the TiO
2
APS coating. It was shown in our ...previous work that this type of coating showed better friction results compared to steel lubricated with MoDTC. To further investigate this feature, a parametric study was carried out involving the effect of MoDTC concentration, test temperature, Hertzian contact pressure and the change of counterpart materials from steel balls to ceramic balls (Al
2
O
3
and ZrO
2
). Ball-on-flat tribotests were carried out on a reciprocating (ball-on-flat) tribometer lubricated with base oil containing MoDTC. Results show that for all the test conditions used including the concentration of MoDTC, test temperature and the contact pressure, lower friction and wear is observed for the TiO
2
APS coating compared to reference steel. To explain the low friction behavior, tribofilm compositions were investigated and it was observed that MoS
2
is always formed in the case of TiO
2
APS with no oxysulphide species. For the reference steel, MoO
x
S
y
species are mainly detected in the tribofilms. XPS analyses performed on TiO
2
APS flats when the counterpart material was changed from steel balls to ceramic balls suggested the formation of MoS
2
(Mo in +
iv
oxidation state) and Mo-C (Mo in +
iv
or +
ii
oxidation state) species with a negligible amount of MoO
3
(Mo in +
vi
oxidation state). It was also shown that a significant amount of molybdenum atoms inside the tribofilm, originating from MoDTC (Mo in +
v
oxidation state) were reduced in the tribological contact. A mechanism for the decomposition of MoDTC on the basis of tribocatalytic behaviour hypothesized in our previous work was proposed and discussed.
Tribocatalytic behaviour of TiO
2
APS coatings in presence of MoDTC additive under boundary lubrication conditions - decomposition mechanism
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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 present a wide range of tribological behavior, including friction coefficients in ultra-high vacuum below 0.02. The engine oil environment which provides similar favourable air free conditions might lead to such low friction levels.
In this work, the friction and wear properties of DLC coatings in boundary lubrication conditions have been investigated as a function of the hydrogen content in the carbon coating. Their interaction with ZDDP which is the exclusive antiwear agent in most automotive lubrication blends and friction-modifier additive MoDTC has been studied. Hydrogenated DLC coatings can be better lubricated in the presence of the friction-modifier additive MoDTC through the formation of MoS
2 solid lubricant material than can non-hydrogenated DLC. In contrast, the antiwear additive ZDDP does not significantly affect the wear behavior of DLC coatings. The good tribological performances of the DLC coatings suggest that they can contribute to reduce friction and wear in the engine, and so permit the significant decrease of additive concentration.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SBCE, SBJE, UPUK
Amide molybdate has been recently introduced as a friction modifier for tribological applications. Combined with zinc dithiophosphate (ZDDP) and fatty amines, it provides an ultralow friction ...coefficient. The ultimate product of Mo compound transformations in tribological contact, due to frictional heating and shearing, as well as chemical interactions with oil additives, is molybdenum sulfide (MoS
2
). Understanding the decomposition of amide molybdate leading to MoS
2
is of primary importance to the optimization of the design of lubricant formulations. This study focuses on the investigation by Raman spectroscopy of amide molybdate decomposition intermediates. Raman spectra of tribofilms, obtained after friction tests under different temperatures and pressures, revealed the formation of an amorphous MoS
3
intermediate coexisting with MoS
2
. However, under severe conditions, the tribofilms are mostly composed of MoS
2
.
In order to investigate the decomposition mechanism of amide molybdate, two series of experimental friction tests have been performed, varying the temperature and pressure.
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MoDTC molecular additives reduce friction in boundary lubricated steel-steel contacts through formation of MoS2 sheets. The chemical pathway from MoDTC to MoS2 is investigated for optimizing MoS2 ...formation. Our experiments show that a MoDTC molecule containing sulfur only in its thiocarbamate ligands forms MoS2 sheets during friction, demonstrating that the presence of peripheral thiocarbamate ligands can be sufficient to provide the required sulfur source. This molecule is not only competitive with MoDTC containing sulfur in the core of the molecule but also more efficient at high contact pressures. Mechanistic investigations on the chemical transformation of MoDTC to MoS2 reveal that thermal activation alone is not sufficient thus suggesting that pressure and/or shear are necessary for MoS2 generation in this system.
•MoDTC with sulfur only in thiocarbamate ligands forms MoS2 in lubricated contacts.•This MoDTC outperforms “classical” ones at high contact pressures.•MoDTC transformation to MoS2 cannot be accounted solely by thermal activation.•Pressure and shear are leading factor in the MoS2 generation.
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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 calculations. In particular, we show that the work of separation and shear strength of iron dramatically decrease by increasing the phosphorus concentration at the interface. These results suggest that the functionality of phosphites as friction modifiers may be related to the amount of elemental phosphorus that they can release at the tribological interface.
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Among diamond-like carbon (DLC) coatings, hydrogenated amorphous carbon (a-C:H) films are of special interest, since some of them may exhibit coefficient of friction in the millirange, known as ...“superlow friction”. This paper will review some key requirements for achieving this specific friction regime.
At first, the formation of a carbonaceous transfer film on the steel counterface seems necessary for friction reduction. Oxide layers apparently slow down this build-up, as a-C:H surface reacts preferentially with metallic iron.
Secondly, the formation and evolution of the transfer film on the sliding counterface seems to control the frictional behavior. By performing tribological experiments under different controlled environment, it is possible to have some clues on the evolution of the transfer film and on its relation with the friction level observed. Phenomena of loss and restoration of superlow friction are for instance very helpful.
Finally, all hydrogenated amorphous carbon films do not lead to superlow friction under vacuum. The role of hydrogen content is known to be critical, but it appears to be strongly dependent on deposition process. Some mechanical properties of the films can also be correlated with friction reduction, like viscoplasticity. All these results suggest that surface rheological properties are of paramount importance in the achievement of superlow friction with a-C:H films.
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