C. Minfray

A Computational Chemistry Study on Friction of h-MoS2. Part I. Mechanism of Single Sheet Lubrication

In this work, we theoretically investigated the friction mechanism of hexagonal MoS(2) (a well-known lamellar compound) using a computational chemistry method. First, we determined several parameters for molecular dynamics simulations via accurate quantum chemistry calculations and MoS(2) and MoS(2-x)O(x) structures were successfully reproduced. We also show that the simulated Raman spectrum and peak shift on X-ray diffraction patterns were in good agreement with those of experiment. The atomic interactions between MoS(2) sheets were studied by using a hybrid quantum chemical/classical molecular dynamics method. We found that the predominant interaction between two sulfur layers in different MoS(2) sheets was Coulombic repulsion, which directly affects the MoS(2) lubrication. MoS(2) sheets adsorbed on a nascent iron substrate reduced friction further due to much larger Coulombic repulsive interactions. Friction for the oxygen-containing MoS(2) sheets was influenced by not only the Coulomb repulsive interaction but also the atomic-scale roughness of the MoS(2)/MoS(2) sliding interface.

A multi-technique approach of tribofilm characterisation

The characterization of tribofilms has already been studied by various surface analysis tools for many years. The analytical techniques usually performed on this kind of samples are X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES) and X-ray absorption near-edge structure (XANES). XPS is interesting for chemical environment information. AES has a good spatial resolution (approx. 0.5 μm) permitting AES mapping or making AES linescans. With XANES, the polyphosphate chain length can be characterized in case of ZDDP tribofilm. AES and XPS depth profiles can also be performed in order to follow the composition of the tribofilm with depth. Therefore, coupling XPS, AES and XANES is interesting to clarify the different layer composition of the tribofilm. In this work, AES, XPS, secondary ion mass spectroscopy (SIMS) and transmission electron microscopy (TEM) are carried out on tribofilm made by ZDDP additives. By a SIMS analysis, information on the elemental distribution in the surface near layers can be obtained with high lateral and high depth resolution. The use of a time-of-flight (ToF) analyser allows to obtain information on all elements quasi-simultaneously resulting in a very high sensitivity. Our aim is to investigate the tribofilm in its depth and the interface tribofilm/substrate by using a multi-technique approach on the same ZDDP tribofilm. First, depth profiles (by AES XPS and SIMS) were carried out. Secondly, a transverse section made by focused ion beam (FIB) was evaluated by TEM. All results are discussed in order to give a better understanding of the tribofilm composition.

A Computational Chemistry Study on Friction of h-MoS2. Part II. Friction Anisotropy

In this work, the friction anisotropy of hexagonal MoS(2) (a well-known lamellar compound) was theoretically investigated. A molecular dynamics method was adopted to study the dynamical friction of two-layered MoS(2) sheets at atomistic level. Rotational disorder was depicted by rotating one layer and was changed from 0° to 60°, in 5° intervals. The superimposed structures with misfit angle of 0° and 60° are commensurate, and others are incommensurate. Friction dynamics was simulated by applying an external pressure and a sliding speed to the model. During friction simulation, the incommensurate structures showed extremely low friction due to cancellation of the atomic force in the sliding direction, leading to smooth motion. On the other hand, in commensurate situations, all the atoms in the sliding part were overcoming the atoms in counterpart at the same time while the atomic forces were acted in the same direction, leading to 100 times larger friction than incommensurate situation. Thus, lubrication by MoS(2) strongly depended on its interlayer contacts in the atomic scale. According to part I of this paper [Onodera, T., et al. J. Phys. Chem. B 2009, 113, 16526-16536], interlayer sliding was source of friction reduction by MoS(2) and was originally derived by its material property (interlayer Coulombic interaction). In addition to this interlayer sliding, the rotational disorder was also important to achieve low friction state.

Chemistry of ZDDP Tribofilm by ToF-SIMS

Time-of-flight secondary ion mass spectrometry (ToF-SIMS) (static and dynamic modes) has been applied to chemical characterization of zinc dithiophosphate additives tribofilm. Main result concerns insights in the spatial distribution of phosphate and sulphide species in the whole tribofilm thickness, at a sub-micrometer scale. The disappearance of oxide layer at the interface between steel and the tribofilm is also noticed.

The origin of anti-wear chemistry of ZDDP

Molecular Dynamics has been used to simulate the anti-wear chemistry of zinc dialkyl dithiophosphate (ZDDP). The model simulates the digestion of abrasive particles into the zinc polyphosphate glass. The main result is that the driving force for the tribochemical reaction is not temperature but entropy due to mechanical mixing at the atomic scale.

Formation Mechanism of a Low Friction ZDDP Tribofilm on Iron Oxide

In a previous study, an iron oxide layer (mixture of Fe 3 O 4 and FeO) formed by water-vapor treatment on a steel plate resulted in an anomalous low friction (μ < 0.06) when slid against a steel cylinder in a lubricant containing ZDDP. This value is about a half of a steel/steel friction contact under the same condition. The formation of a tribofilm containing zinc and sulfur has been identified on the iron oxide. In this study, the formation mechanism of such tribofilm is discussed. The adsorption characteristics of ZDDP on the iron oxide and steel are investigated experimentally. The adsorption layer on the iron oxide analyzed by XPS is rich in zinc but contains almost no sulfur, while the one on steel includes phosphorous and oxygen, and probably phosphates. The adsorption plays a significant role in increasing the zinc content in the tribofilm on the iron oxide, while other mechanisms such as mechanical mixing are necessary to incorporate sulfur in the tribofilm.

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.

Zinc phosphate chain length study under high hydrostatic pressure by Raman spectroscopy

The aim of this study is to combine a diamond anvil cell with in-situ Raman spectroscopy to simulate and analyze the effect of pure pressure on the length of phosphate chains in an antiwear film formed in a tribological contact. In-situ Raman spectra of Zn2P2O7 glass, α-Zn3(PO4)2, and γ−Zn2P2O7 crystals submitted to high hydrostatic pressure up to 20 GPa were recorded. Evolution of Raman spectra as a function of pressure was studied in the characteristic high frequency range of PO4 tetrahedra molecular resonance (650−1300 cm−1). When exposed to high pressure, the structure of the sample becomes less ordered. Phase transitions in α-Zn3(PO4)2 structure are observed during compression from ambient pressure to 3 GPa. The length of the phosphate chains is conserved up to 20 GPa when samples are subjected to hydrostatic pressure.The aim of this study is to combine a diamond anvil cell with in-situ Raman spectroscopy to simulate and analyze the effect of pure pressure on the length of phosphate chains in an antiwear film formed in a tribological contact. In-situ Raman spectra of Zn2P2O7 glass, α-Zn3(PO4)2, and γ−Zn2P2O7 crystals submitted to high hydrostatic pressure up to 20 GPa were recorded. Evolution of Raman spectra as a function of pressure was studied in the characteristic high frequency range of PO4 tetrahedra molecular resonance (650−1300 cm−1). When exposed to high pressure, the structure of the sample becomes less ordered. Phase transitions in α-Zn3(PO4)2 structure are observed during compression from ambient pressure to 3 GPa. The length of the phosphate chains is conserved up to 20 GPa when samples are subjected to hydrostatic pressure.

Antiwear Chemistry of ZDDP: Coupling Classical MD and Tight-Binding Quantum Chemical MD Methods (TB-QCMD)

Zinc dialkyl dithiophosphate (ZDDP) is an antiwear additive for steel surfaces currently used in most of engine oils. The mechanism by which the additive is active is based on tribochemical reactions. These reactions occur in the contact zone under the combined effects of pressure and shear. These reactions are predictable on the basis of the HSAB principle (or Chemical Hardness model). We show here that computer simulations can describe the reactions much more accurately than the HSAB principle thanks to the use of a hybrid technique, coupling classical MD and tight-binding quantum chemical MD. In this study, we focused on one of the basic tribochemical reactions of ZDDP: the ability of zinc polyphosphate to react with abrasive metal oxides nanoparticles under pressure and shear. Results show that the driving forces for the reaction are mainly the increases of molecular shearing and entropy, besides temperature. We also studied the case of other metal oxide particles that emanate from elements of addition in steel compositions. We show that manganese and chromium oxides are eliminated in the same way as iron oxides, being in agreement with experimental data obtained by X-ray microanalysis and FIB-TEM characterizations. Eventually, we investigated the case of Al/Si alloys and showed that alumina particles can hardly be digested by zinc phosphate, at the opposite of silica particles. This explains very well why ZDDP is not a good antiwear additive for aluminium alloys and why the presence of silicon grain in the alloy is favourable.

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.