This review summarizes recent advances in the area of tribology based on the outcome of a Lorentz Center workshop surveying various physical, chemical and mechanical phenomena across scales. Among ...the main themes discussed were those of rough surface representations, the breakdown of continuum theories at the nano- and microscales, as well as multiscale and multiphysics aspects for analytical and computational models relevant to applications spanning a variety of sectors, from automotive to biotribology and nanotechnology. Significant effort is still required to account for complementary nonlinear effects of plasticity, adhesion, friction, wear, lubrication and surface chemistry in tribological models. For each topic, we propose some research directions.
Display omitted
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.
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.
Recently Diamond-Like Carbon (DLC) coatings have attracted considerable attention due to their low friction, high hardness, good wear and corrosion resistance, and high thermal and chemical ...stability. Although considerable research has been conducted on the effect of molybdenum-based additives, e.g., molybdenum dithiocarbamate (MoDTC), on the boundary lubrication of DLC coatings, the wear mechanisms leading to the coating removal are not fully understood to date. Moreover, the impact of the degradation of the MoDTC-containing base oil on the tribological properties of DLC coatings has not been investigated. This study could help to elucidate the mechanisms by which the coating is worn out in presence of MoDTC-containing base oil.
In this work, the friction and wear performances of DLC coatings with different hydrogen contents and doping elements have been investigated in the presence of MoDTC-containing base oil. To enable the correlation between these changes and the modification of the contact surface chemistry, tribofilms generated during friction were investigated by SEM combined with Energy Dispersive X-ray Spectrometry (EDX) and X-ray Photoelectron Spectroscopy (XPS). A new mechanism explaining the severe wear experienced by DLC coatings when lubricated with a MoDTC-containing base oil is proposed.
The tribological performance of lubricants containing TiO
2
nanoparticles and Molybdenum DiThioCarbamate (MoDTC) was investigated using a reciprocating ball-on-flat tribometer in steel–steel ...contacts. Lubricants containing only MoDTC were used for comparison. The influences of the additive concentration (0.1 wt% and 0.5 wt%) and of the roughness of the counterparts (
R
a
from 10 to 200 nm) on the performance of the lubricant were studied. Improved friction modification and anti-wear properties were found when TiO
2
nanoparticles were blended with MoDTC compared to MoDTC alone, even at low concentration and with rough surfaces. XPS characterizations and FIB-TEM analyses of tribofilms were performed and suggested that the formation of MoS
2
from MoDTC is favored in the presence of TiO
2
nanoparticles. The results are discussed, taking into account the tribocatalytic properties of TiO
2
nanoparticles.
A large amount of research has been devoted to the effect of molybdenum dithiocarbamate (MoDTC) additives on the lubricating performances of carbon-based coatings, showing that a high wear rate is ...produced when the MoDTC is blended with the base oil. However, the mechanisms leading to the coating removal are not fully understood yet.
In this work, the friction and wear performances of an amorphous hydrogenated DLC coating doped with silicon and oxygen have been analysed when lubricated by MoDTC-containing oils. Tribological experiments have been conducted with DLC/steel and DLC/DLC contacts under boundary lubrication conditions using a ball-on-flat tribometer. To understand the wear mechanism, the chemical composition of the tribofilm formed on the steel ball counterpart was investigated by X-ray Photoelectron Spectroscopy (XPS). Transmission Electron Microscopy (TEM) coupled with Energy Dispersive X-Ray Spectroscopy (EDX). A new DLC wear model has been proposed and validated.
•MoC species are formed on steel ball in a steel/DLC contacts lubricated with MoDTC.•The presence of MoC is related to high wear of the DLC.•The wear mechanism is mainly due to chemical reaction between C (DLC) and Mo(MoDTC).
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.
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
In this study, the influence of temperature on the tribological properties of unreinforced or glass-fiber reinforced PA66 in contact with carbon steel under boundary lubrication with grease was ...studied when considering the temperature dependence of the mechanical properties on the sliding surface of PA66 and the tribochemical reaction to zinc carboxylate additives in grease. XPS and ToF-SIMS analyses revealed the formation of a carboxylate tribofilm on the steel surface and a zinc sulfide reactive film on the PA66 surface, which are related to the tribochemical reaction of the additives present in the grease applied. The formation of the tribofilm contributed to an improvement of the tribological properties, particularly at 80 °C.
Display omitted
•Influence of temperature on the tribological properties of PA66 in contact with steel under grease lubrication was studied.•Temperature dependence on the PA66 mechanical properties and tribochemical reaction of grease additives was discussed.•ToF-SIMS analyses revealed the formation of carboxylate tribofilm on the steel related to tribochemical reaction.•XPS analyses revealed the formation of zinc sulfide reactive film on the PA66 related to tribochemical reaction.•The formation of the tribofilm contributed to the improvement of the tribological properties, particularly at 80 °C.
The influence of various iron-containing compounds on ZDDP tribofilm formation was investigated. Friction tests were carried out under boundary lubrication using two sliding materials: steel and ...sapphire. Lubricants were composed of dispersions of zinc metaphosphate in base oil. The tribofilms were characterized using Raman spectroscopy. In the presence of steel, zinc metaphosphate depolymerizes to form a tribofilm mainly composed of zinc orthophosphate. No depolymerization of zinc metaphosphate was observed with sapphire. In subsequent tests with sapphire, iron oxide particles of different natures were added to the dispersion to simulate the iron oxide species present at the surface of the steel substrate. Only the addition of FeOOH (goethite) micro-particles in the dispersion leads to the formation of a depolymerized zinc-iron phosphate tribofilm.
•Influence of various iron-containing compounds on ZDDP tribofilm formation.•Evidence of depolymerization of zinc metaphosphate in the presence of steel substrate.•The absence of depolymerization in the presence of sapphire substrate.•Formation of a depolymerized Zn/Fe phosphate tribofilm in the presence of sapphire+goethite particles.
PDF
