M. I. De Barros Bouchet

On the possible role of triboplasma in friction and wear of diamond-like carbon films in hydrogen-containing environments

Hydrogen-free diamond-like carbon (DLC) films (both amorphous (a-C) and tetrahedral amorphous carbon (ta-C)) suffer high friction and severe wear losses when tested in inert and/or high vacuum environments. However, they provide anomalous superlow friction and wear coefficients in the presence of hydrogen gas, water vapour and alcohol molecules in the test environment. In this paper, we used such films in a systematic study to further confirm that hydrogen indeed plays an important role in their friction and wear behaviours. To study the effect of hydrogen, we conducted sliding tests in a hydrogen-containing test chamber and analysed the chemistry of their sliding contact surfaces using a time-of-flight secondary ion mass spectrometer. Clearly, the sliding contact regions of the carbon films became very rich in hydrogen after tribological tests in the hydrogen-containing chamber. In an attempt to understand the fundamental tribochemical mechanisms involved, we performed additional tests on these DLC films using a highly instrumented tribometer that permitted us the visualization of triboplasmas generating at or in the vicinity of the sliding surfaces. In this test system, we confirmed the formation of a triboplasma inside the contact area of the DLC films as evidenced by the characteristic UV radiation. Based on these observations, we believe that the formation of such triboplasmas within the contact zones of these DLC films may have triggered unique tribochemical reactions between hydrogen and carbon atoms on their sliding surfaces and thus resulted in very low friction and wear during tests in hydrogen-containing environments.

Superlubricity mechanism of diamond-like carbon with glycerol. Coupling of experimental and simulation studies

We report a unique tribological system that produces superlubricity under boundary lubrication conditions with extremely little wear. This system is a thin coating of hydrogen-free amorphous Diamond-Like-Carbon (denoted as ta-C) at 353 K in a ta-C/ta-C friction pair lubricated with pure glycerol. To understand the mechanism of friction vanishing we performed ToF-SIMS experiments using deuterated glycerol and 13C glycerol. This was complemented by first-principles-based computer simulations using the ReaxFF reactive force field to create an atomistic model of ta-C. These simulations show that DLC with the experimental density of 3.24 g/cc leads to an atomistic structure consisting of a 3D percolating network of tetrahedral (sp3) carbons accounting for 71.5% of the total, in excellent agreement with the 70% deduced from our Auger spectroscopy and XANES experiments. The simulations show that the remaining carbons (with sp2 and sp1 character) attach in short chains of length 1 to 7. In sliding simulations including glycerol molecules, the surface atoms react readily to form a very smooth carbon surface containing OH-terminated groups. This agrees with our SIMS experiments. The simulations find that the OH atoms are mostly bound to surface sp1 atoms leading to very flexible elastic response to sliding. Both simulations and experiments suggest that the origin of the superlubricity arises from the formation of this OH-terminated surface.

Superlow friction of ta-C lubricated by glycerol: An electron energy loss spectroscopy study

Energy-filtering transmission electron microscopy (EFTEM) analysis coupled with the technique of samples preparation, focused ion beam, was used to study physical, chemical, and mechanical properties of diamond-like carbon coatings (DLCs). Two different coatings (ta-C and a-C:H) were investigated, presenting different tribological behaviors in a boundary lubrication regime with glycerol. Electron energy loss spectroscopy appears to be a very powerful technique to characterize such DLC coatings. Special attention was paid to the maximum energy of the plasmon peak, which was used to evaluate some physical and mechanical properties of DLC coatings (density, sp3∕sp2 ratio, hardness). For ta-C superlubric coating, EFTEM results show a rearrangement of the DLC bulk structure under the friction process. Typically, the transformation of sp3 carbon into sp2 carbon was clearly observed and permits a self-adaptation of the coating, allowing it to support shearing without any delamination in spite of important compre...

Improved mixed and boundary lubrication with glycerol-diamond technology

Abstract The 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 extensively studied as ultralow friction films to protect the surfaces of ductile metals for space applications are expected to fulfil this part. The main purpose of this work is to investigate the friction and wear properties of glycerol lubricated DLC coatings under boundary lubrication conditions. The DLC material consists of tetrahedral hydrogen free amorphous diamond-like carbon (denoted as ta-C) as shown by the time of flight secondary ion mass spectroscopy (ToF-SIMS) analyses and the nanoindentation measurements. The friction coefficient below 0·.01, called superlubricity, and no measurable wear were obtained by sliding the ta-C/ta-C friction pair in the presence of pure glycerol as a lubricant at 353 K. The mechanism by which glycerol is able to reduce the friction in the millirange was revealed by ToF SIMS analyses inside and outside wear scars formed by friction experiments using deuterated glycerol and 13C glycerol.

A comparative study on the functionality of S- and P-based lubricant additives by combined first principles and experimental analysis

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.

Trimethyl-phosphite dissociative adsorption on iron by combined first-principle calculations and XPS experiments

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.

MoDTC friction modifier additive degradation: Correlation between tribological performance and chemical changes

Due to the complexity of the processes, the degradation mechanisms of molybdenum dithiocarbamate (MoDTC)-containing oil are still not fully understood. In order to get a better understanding of how a MoDTC additive works at the molecular level, correlation between its chemical behaviour in a bulk oil during thermo-oxidative degradation and its ability to reduce friction has been investigated. The combination of High-Performance Liquid Chromatography (HPLC), Fourier Transform Infrared Spectroscopy (FT-IR) and Mass Spectroscopy (MS) techniques has provided much detailed information about the complex chemistry involved in the degradation process. Finally, the relationship between MoDTC additive depletion and its effectiveness in decreasing friction has been studied and a hypothesis on the chemical pathway followed by MoDTC during a thermo-oxidative degradation process has been proposed.

TOWARDS SUPERLUBRICITY UNDER BOUNDARY LUBRICATION

Fuel economy and reduction of harmless elements in lubricant are becoming crucial in the automotive industry. An approach to respond these requirements in engine components is the potential use of low friction coatings exposed to specific boundary lubrication conditions. Superlubricity is a new research field in tribology, dealing with very low friction values, typically below 0.01, and this even in dry or vacuum conditions. It is to be noticed that any friction coefficient below 0.001 is hardly measurable with the equipment at hand. Superlow friction was already experimentally observed only in ultrahigh vacuum and inert gas environment, with pure molybdenite (MoS2 ) coatings [1] and in presence of some hydrogenated DLC coatings [2]. Under boundary lubrication, we show here that the coupling of hydrogen-free carbon coatings and selected organic lubricant additives permits to reach friction values approaching superlubricity and also a wearless behavior.Copyright

Superlubricity of Steel Surfaces in Presence of Polyhydric Alcohols

Anomalous low friction of hydrogen-free tetrahedral hybridized carbon (ta-C) coated surfaces lubricated by pure glycerol was observed at 80°C. In the presence of glycerol, the friction coefficient is below 0.01 at steady state, corresponding to so-called superlubricity regime. This new mechanism of superlow friction is attributed to easy glide on tribo-formed OH-terminated surfaces. In addition to the formation of OH-terminated surfaces but at a lower temperature, we show here some evidence that superlow friction of polyhydric alcohols could also be associated with tribo-induced degradation of glycerol, producing a nanometer-thick film containing organic acids and water. Second, we show novel outstanding superlubricity of steel surfaces directly lubricated by a solution of myo-inositol in glycerol at ambient temperature (25°C). For the first time, under boundary lubrication at high contact pressure, friction of steel is below 0.01 in the absence of any long chain polar molecules. Mechanism is still unknown but could be associated with friction-induced dissociation of glycerol and interaction with steel surface.Copyright

Experimental Modelisation of Organosulphurs Tribochemistry by Gas Phase Lubrication

Organosulphurs compounds have been used for many years as Extreme Pressure (EP) and Anti Wear (AW) additives in a wide range of metal working applications. These additives act in severe conditions by building up a reaction film (tribofilm) on the metallic surfaces. Several interpretations have been proposed to explain the (EP) and the (AW) properties of these compounds, but the detailed mechanisms and surface reactions products are still largely unknown. To better understand the tribochemistry of these compounds, we developed a new device in our laboratory called “Environmentally Controlled Tribometer”. This apparatus permits to study the interaction of one or several additives on different metallic surfaces using a gas having the same chemical function as the additive. After friction experiments, in-situ surface analyses (XPS and AES) were carried out on the tribofilm, to clearly identify the reactions products in order to understand the tribochemical mechanism. In this paper, we simulate the metal working lubrication of steel by the gas phase lubrication. The results show a clear difference in tribological behaviour and surface products between triboreactivity on nascent and oxidized surfaces. These observations gave us new information to better understand the tribochemical reactions of these additives. The comparison between the liquid phase lubrication and the gas phase lubrication results validates this methodology.Copyright