Gas Phase Lubrication Research Articles

Gas phase lubrication study with an organic friction modifier

AbstractFriction modifier additives play a crucial role in controlling friction and wear of lubricated tribological systems. Model experiments in a controllable atmosphere performed by integrating a tribometer into a system of in situ surface analytical methods in vacuum can give insights into the additives functionality. In this work, thin, well‐defined layers of an organic friction modifier (OFM) are adsorbed onto an iron oxide surface by means of an effusion cell immediately before measuring friction and wear. The results show that contrary to the assumption that homogeneous layers are formed, this OFM accumulates in droplets on the surface. Droplet number and radius increase with evaporation time. In friction tests, the smallest friction values are found for a low coverage of droplets. For larger droplets, friction increases due to a capillary neck of additive that forms between the sliding surfaces and is dragged along during the friction test.

Tribochemical Mechanisms of Trimethyl and Triethyl Phosphite on Oxidized Iron in Ultrahigh Vacuum

The surface chemistry of model lubricant additives, trimethyl phosphite (TMPi) and triethyl phosphite (TEPi), is studied on oxidized iron in ultrahigh vacuum (UHV) and the results compared with the gas-phase lubrication of TEPi on oxidized iron in a UHV tribometer. Oxide films are grown on a polycrystalline iron substrate and characterized by X-ray photoelectron spectroscopy (XPS) and assigned to the formation of an Fe3O4 film. Measurements of the friction coefficient and contact resistance of the oxide films show that the oxide film remains intact while rubbing in UHV at a normal load of 0.29 N. It is found that phosphite esters adsorb on the oxide by electron donation to the phosphorus atom, with a binding energy that increases in the order tributyl phosphite > TMPi > TEPi, and correlates well with the location of the vacant lowest-occupied molecular orbital (LUMO) energy. The phosphite esters decompose via sequential P‒O bond scission to form adsorbed alkoxy species, which then react on the surface either by hydrogen addition to form the corresponding alcohol, or by hydrogen abstraction to yield an aldehyde. XPS studies of the surface shows that essentially no carbon remains after the TMPi has reacted, while a small amount of carbon is present when TEPi has decomposed. On heating, the phosphite esters convert to phosphate species. Gas-phase lubrication experiments in the presence of 1 × 10−7 Torr of gas-phase TEPi reveal that the friction coefficient is significantly reduced, where the friction reduction is found to increase with increasing reaction temperature. The friction reduction correlates well with the proportion of phosphate product formed in the film and indicates that the formation of phosphate tribofilm is primarily responsible for reducing friction. However, friction is also reduced for a reaction at ~ 300 K, lower than the temperature at which the adsorbed phosphite ester reacts, suggesting that its rate of decomposition could be accelerated by interfacial shear.

Gas phase lubrication on diamond-like carbon film: Tribochemical reactions under isobutane condition

Abstract Diamond-like carbon (DLC) film, with a host of intriguing properties, has emerged as a unique tribological material to be used in many fields. However, incomplete knowledge about the crucial parts, which control the frictional behavior under different gases, limits the further use of them. Here, we show that the tribological performance of hydrogen-free DLC film changed over a wide range by introducing hydrocarbon molecules into the frictional system. And the friction coefficient increased while the wear decreased with the increasing of isobutane amount. It was that the adsorbates which dissociate from isobutane molecules during tribochemical process on sliding interfaces caused the increasing trend of friction coefficient and provided a layer to prevent the sliding surface from wear.

A multi-technique approach to the characterization of iron phosphide tribofilm

Tribofilms, formed by tribochemical reactions between steel surface and trimethylphosphite in gas phase, have been characterized by various structural and analytical techniques including transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and energy dispersive spectroscopy (EDS). Focus ion beam technique has been used to prepare cross-sections of the tribofilm for further TEM and EDS analyses. Cross-sectional views of tribofilms obtained by gas phase lubrication show that tribofilm has an inhomogeneous thickness ranging from 10 to 50nm and is amorphous. Complementary XPS and EDS analyses confirm the formation of various iron phosphides as the major component of the tribofilms, with a preferential nucleation of Fe3P stoichiometry at the interface between steel and tribofilm.

Friction of Diamond in the Presence of Water Vapor and Hydrogen Gas. Coupling Gas-Phase Lubrication and First-Principles Studies

Nanocrystalline diamond (NCD) has attracted much attention in recent years because of improvements in growth methodologies that have provided increases in both film thickness and growth rate, while...

Shear-induced boundary film formation from dialkyl sulfides on copper

The friction of a copper surface, which is exposed to diethyl disulfide (DEDS), dimethyl disulfide (DMDS) or dimethyl trisulfide (DMTS) while rubbing by a tungsten carbide pin, are explored in an ultrahigh vacuum tribometer to investigate the effect of the sulfur-to-carbon stoichiometry on gas-phase lubrication. Surface analyses by Auger spectroscopy of the wear scars after rubbing reveal that the amount of sulfur increases with sulfur content of the gas-phase lubricant, as anticipated. It is found that the friction reduction depends on the normal load, where the friction coefficient tends to decrease more at lower loads, and that the load at which the most friction-reducing tribofilm is formed depends on the stoichiometry of the gas-phase lubricant. DEDS (with a sulfur to carbon ratio of 0.5) only reduces the friction coefficient to a minimum value of ∼0.28 at a normal load of 0.25 N, for DMDS (S/C = 1.0) friction is reduced to ∼0.28 for loads below ∼0.7 N, and DMTS (S/C = 1.5) reduces friction to this value for all loads tested.

Experimental Simulation of Tribochemical Reactions Between Borates Esters and Steel Surface

In this study, tribochemical reactions of borate ester additives on steel surfaces were simulated experimentally by gas phase lubrication. Trimethylborate (TMB) was employed as a model molecule of the borate chemical function. TMB was introduced under gas phase up to 5 hPa in a new tribometer dedicated to gas phase lubrication. Friction tests were conducted at room temperature. In order to identify the nature of the generated tribofilm, X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) analyses were carried out inside and outside the track of the tested samples. The results showed a reduction of the friction coefficient down to 0.2 by increasing the TMB gas pressure. This reduction in friction was induced by the formation of a tribofilm of iron oxide partially digested by a borate glass network. The reduction in friction was attributed to the formation of the borate glass. The digestion of abrasive iron oxides by the borate glass was purported to be responsible for the reduction in the wear. The tribochemical reaction of TMB on steel surfaces is discussed in relation to the hard and soft acid base (HSAB) principle.

Gas-Phase Lubrication of ta-C by Glycerol and Hydrogen Peroxide. Experimental and Computer Modeling

Tetrahedrally coordinated hydrogen-free amorphous diamond-like carbon coating (denoted as ta-C) presents ultralow friction under boundary lubrication conditions at 80 °C in presence of OH-containing molecules. To understand the mechanism of ultralow friction, we performed gas-phase lubrication experiments followed by time-of-flight secondary ion mass spectrometry (ToF-SIMS) analyses and this using two simple molecules: deuterated glycerol and hydrogen peroxide. The experiments were complemented by computer simulations using the ReaxFF reactive force field. These simulations suggest a ta-C surface rich in sp^2 carbon with some reactive sp^1 carbon atoms, in agreement with previous energy filtered transmission electron microscopy (EFTEM) results. Sliding simulations show that the carbon surface atoms react with glycerol and hydrogen peroxide to form OH-termination. Moreover, the hydroxylation is then followed by the chemical dissociation of some of the glycerol molecules leading to the formation of water. This is in agreement with the secondary ion mass spectrometry (SIMS) analyses and mass spectrometer results obtained with gas-phase lubrication experiments with the same molecules. Both experimental and computer simulations strongly suggest that the hydroxylation of the carbon surface is at the origin of ultralow friction together with the formation of water-rich film in the sliding interface.

Surface and Tribological Chemistry of Water and Carbon Dioxide on Copper Surfaces

The tribological chemistry of carbon dioxide and water vapor is studied on copper surfaces at high pressures, with a view to understand the gas-phase lubrication of copper–copper sliding contacts. The adsorption and film formation properties are studied on vapor-deposited copper films in an ultrahigh vacuum chamber using a quartz crystal microbalance. The nature of the reactively formed film is studied after reaction by ex situ X-ray photoelectron spectroscopy (XPS). Carbon dioxide adsorbs reversibly on copper, while water vapor adsorbs both reversibly and irreversibly, where XPS reveals that the irreversibly formed film consists of a mixture of cuprous oxide/hydroxide. Measuring the thickness of the cuprous oxide/hydroxide film as a function of water vapor pressure and temperature reveals that its thickness varies between about 6 and 14 A and is proportional to the total amount of water adsorbed on the surface. This results in a cuprous oxide/hydroxide film that is thinner at higher temperatures. Measurements of the friction coefficient as a function of temperature and pressure in the presence of water vapor shows that it correlates with film thickness, reaching a limiting value of ~0.35 for thicker films.

Tribochemistry of tetrahedral hydrogen‐free amorphous carbon coatings in the presence of OH‐containing lubricants

AbstractDiamond‐like carbon (DLC) films are gaining attention for its use in a wide range of tribological applications because of their low friction coefficient and high wear resistance. Recently, ultra‐low friction or superlubricity as low as 0.006 has been observed between hydrogen‐free DLC films lubricated with oil containing a glycerol mono‐oleate (GMO) additive. Consequently, there are many efforts to understand their detailed tribological behaviour. In this study, first, a characterisation of a pristine hydrogen‐free tetrahedral amorphous DLC coating (denoted as ta‐C) is presented. The technique used for ta‐C characterisation is the energy‐filtered transmission electron microscopy studied on a focus ion beam cross section of the coating. Then, to simulate the action of GMO, which is used as an additive in engine oils, and to understand its mechanism of action in boundary lubrication conditions, simple and shorter molecules such as pure glycerol and hydrogen peroxide are used in friction tests. These two molecules and GMO have the same alcohol chemical function. Friction tests in the presence of pure glycerol and hydrogen peroxide are presented. Moreover, to understand the properties of these two molecules, the overtone of GMO and their reaction mechanism in boundary lubrication, liquid phase lubrication was simulated by gas phase lubrication. Results show that very low friction coefficients are obtained with no apparent wear. Finally, the mechanisms of ultra‐low friction are investigated by the wetting method and the X‐ray photoelectron spectroscopy (XPS) technique. These two techniques are used to identify the physical (wetting method) and chemical (XPS) changes occurring at the ta‐C surface after friction. Copyright © 2007 John Wiley & Sons, Ltd.

Experimental simulation of phosphites additives tribochemical reactions by gas phase lubrication

Tribochemical reactions of phosphites additives on steel surface have been simulated by gas phase lubrication. Trimethylphosphite (TMPi), P(OCH3)3, has been used as model molecule for phosphites additives. It has been introduced under gas phase up to 5 hPa in a new tribometer dedicated to gas phase lubrication. Friction tests have been carried out at ambient temperature and 100°C. Chemical analyses by X-ray photoelectron spectroscopy and by Auger electron spectroscopy have been conducted inside and outside of the track. Two kinds of analysis have been carried out: ex situ and in situ surface analyses after tribological test. Indeed, a new environmentally controlled tribometer allows friction test then accurate analyses without air exposure of the formed tribofilm. Tribotests conducted under TMPi gas phase show a reduction of friction coefficient until 0˙2 instead of 1˙4 under high vacuum. Jointly, formation of tribofilm has been confirmed by optical microscopy and ex situ chemical analysis. Comparison between analyses performed inside and outside of the wear scar indicates that the friction induces the formation of phosphide compound that could reduce friction. Moreover analyses show the formation of methoxy group (CH3O) and carbonate originally from the decomposition of TMPi under friction into H2 and CO. In situ analyses clearly show the importance to investigate an uncontaminated tribofilm in order to obtain a better characterisation of it and then a better comprehension of the tribochemical mechanisms.

Contribution of gas phase lubrication in understanding tribochemistry of organosulphur compounds

Organosulphur compounds have been long known to be effective antiwear (AW) and extreme pressure (EP) additives in a wide range of metalworking applications. Several interpretations to explain the properties of these compounds have been proposed, but the mechanisms and surface reaction products are still largely unknown. To understand the tribochemistry of these additives, especially thiols and polysulphides, a new device called 'environmentally controlled tribometer' was developed. It permits to simulate the action of the additives using gases having the same chemical function. After friction experiments, surface analyses were carried out on the tribofilm in order to understand the tribochemical mechanism. The evolution of the friction coefficient versus partial pressure of gaseous additive and sample temperature allowed clear differentiation of organosulphur compounds reactivity. The studies showed that the lubricating properties of the additives depend both on the quantity of sulphur and on the carbon chain length. The results for thiols series showed an increasing of EP and AW efficiency with increasing carbon chain length. The comparison between the liquid phase lubrication and the gas phase lubrication results validates this methodology.

Modelling tribochemical reactions of additives by gas-phase lubrication ∗

In the context of boundary lubrication, we are interested in interactions between lubricant additives as well as in the role of nascent surfaces formed during friction. Tribochemical reactions are very complex and their study requires simplified approaches. We have developed an analytical ultrahigh vacuum tribotester with the possibility of introducing low-molecular-weight molecules during (or before) a friction test, in order to simulate heavy lubricant components by means of their chemical functional group. Mixtures of gases can also be studied with this apparatus. In situ AES/XPS (depth profiling) is available at the end of the friction test. This permits the characterisation of the two friction counterfaces to be performed accurately. Results show that friction under gaseous feeds can be well related to lubricated tests, both in friction behaviour and in wear track and debris morphologies. The evolution of friction in the presence of pressures of selected gases (O2, hexane, hexene and hexanol) shows clear differentiation of the different cases as a function of chemical group. Finally we show that the method is well adapted to model some aspects of boundary lubrication. It can be used as a powerful tool to predict and understand mechanisms of action of individual additives as well as interactions between additives in oil formulations.

Tribochemistry in the analytical UHV tribometer

The knowledge of tribochemical reactions in boundary lubrication in the presence of lubricant additives is fundamental in order to improve chemical engineering of new molecules, multifunction compounds and to predict interactions between these additives. Experiments with in lubro, ex situ or post-mortem analyses are difficult and even dangerous to exploit because of numerous artefacts. We introduce two special approaches to simulate boundary lubrication tribochemistry. Basically, these approaches use a dedicated analytical UHV tribotester allowing in situ AES/XPS analysis of both counterfaces and introduction of partial pressure of gases during friction. The first model experiment is UHV friction on previously formed tribofilms from a lubricated test. The second experiment is gas phase lubrication with ethyl phosphate. The results reveal some fundamental aspects of the origin of friction-reduction by transfer of MoS 2 single sheets. The reaction of friction-activated nascent surfaces with gases is also emphasised.

Atmospheric effects of friction, friction noise and wear with silicon and diamond. Part II. SEM tribometry of silicon in vacuum and hydrogen

Scanning electron microscope (SEM) tribometric data on polycrystalline silicon (poly-Si) vs. poly-Si, Si(100) vs. Si(100) and Si(111) vs. Si(111) interfaces, obtained in \(\sim 1 \times 10^{ - 5}\) Torr and in 0.2 Torr partial pressure of hydrogen gas (\(P_{{\text{H}}_{\text{2}} }\)) from room temperature to 850°C, were performed under standard and much slower thermal ramping rates. The friction data were analyzed per the methodology described in part I of this paper series. The results indicate a highly beneficial friction- and wear-reducing regime within a relatively narrow thermal region. This desirable region coincides with some chemisorption of excited species of molecular hydrogen just before the mass thermal desorption of surface hydrides. These data represent the tribochemical equivalent of a method routinely used in electronics, whereby deep electron traps (dangling Si bonds) are passivated by baking in molecular hydrogen. The \(P_{{\text{H}}_{\text{2}} }\) also exerts a moderating influence on the size of the friction noise at all test temperatures. However, the general level of friction beyond the beneficial thermal region is high. In parallel, the general wear rate of Si representative of the entire range of standard thermal ramping in both atmospheric environments is in the extremely high 10-12m3/(N m) range. Operating strictly in the beneficial, low-friction thermal regime resulted in a several orders-of-magnitude reduction in the wear rate over those measured under standard thermal ramping conditions. Although the results confirm previous findings that Si is not a good material of construction for miniaturized moving mechanical assemblies (e.g., microbearings and gears), there seems to be some limited possibility of gas-phase lubrication of Si micromechanisms with rarefied hydrogen at surface temperatures between 100 and 300°C.

The kinetics of lubricant penetration action during machining

AbstractA new model of cutting fluid penetration in the interface zone during metal cutting is proposed. This model includes three stages relating to processes in a single capillary: 1. liquid phase entry, 2. ‘micro‐droplet explosion’, and 3. filling the capillary by gas phase lubrication. Calculations of model parameters have been done for two different liquids: water and castor oil. The correlation between model data and known experimental results is also given.

Adsorption and friction in the UHV tribometer

Some aspects of the effect of adsorption on friction have beenexperimentally investigated by using a multi-technique AES/XPSUHV tribotester. Actually, testing in-vacuo gives the opportunityto clean, to create and to characterise the surface. Moreover,AES microanalysis can give insight in tribochemical reactions.Preliminary data are presented in several fields of tribology,including gas-phase lubrication, MoS2 superlubricityand lubricated wear with lubricant additives. New insight intothe understanding of the processes are made possible because in-vacuo tribology permits the environmental factors to be perfectlycontrolled.

The Kinetics of Gas-Phase Lubrication in the Orthogonal Machining of an Aluminium Alloy

Tetrachloromethane (carbon tetrachloride) has been used as a model gas-phase lubricant to investigate the kinetics of lubrication in the low-speed orthogonal machining of a precipitation-hardened aluminium alloy, in a controlled low-pressure environment. The dependence of cutting force on lubricant gas pressure, feed (that is depth of cut in these experiments), cutting speed and interfacial temperature was investigated. The results are not consistent with previous theories of vapour-phase lubrication in which transport of the lubricant molecules is assumed to control the reaction rate. A new model is proposed involving activated chemisorption of the lubricant at the tool-chip interface. The predictions of this model are in good agreement with the experimental observations and an activation energy of 18.4 kJ/mol has been derived for the relevant reaction.