The Formation of Tribofilms of MoS2 Nanotubes on Steel and DLC-Coated Surfaces

Solid-lubricant nanoparticles as additives in oil provide good tribological properties based on the physical lubrication mechanisms in the contact. For this reason, they are strong candidates for use in the lubrication of diamond-like carbon (DLC) coatings, which only poorly interact with the traditional, chemically based additives. In this study, we focused on how a tribofilm formed from MoS₂ nanotubes is related to the tribological properties of these nanotubes, and then, we analysed such a tribofilm on steel and DLC-coated surfaces using scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy and Auger electron spectroscopy. We demonstrated that when using oil containing MoS₂ nanoparticles, the formation of a tribofilm is a key factor in decreasing the friction for the steel and DLC-coated contacts. The major difference between the steel and the DLC contacts is the extent to which the MoS₂-based tribofilm covers the surface, which is 20 % in the case of the DLC/DLC contacts, but almost 40 % in the case of the steel/steel contacts. Moreover, the MoS₂-based tribofilm was found to be more oxidized on the DLC surface than on the steel surface. Nevertheless, we found that the chemical and functional properties of the MoS₂-based tribofilm are very similar, or even the same, for both the steel and DLC-coated surfaces. No direct evidence of any chemical reactions between the MoS₂ and the steel or DLC coating was observed.     

An Investigation on the Reduced Ability of IF-MoS2 Nanoparticles to Reduce Friction and Wear in the Presence of Dispersants

Inorganic fullerene-like molybdenum disulfide (IF-MoS₂) nanoparticles are known to exhibit great friction and wear-reducing abilities in severe boundary lubrication regimes, when added to a base oil alone. Their use in fully formulated lubricants was investigated in this study, and the tribological benefits attributed to the IF-MoS₂ nanoparticles were found to be lost in the presence of dispersants. Various experimental techniques were used on three reference oils (base oil containing only IF-MoS_2, only dispersants and both IF-MoS₂ and dispersants) in order to understand the effect of succinimide-based dispersants on the three phases needed for effective nanoparticle-based lubrication, namely (1) the passing of the nanoparticles through the contact (2) the exfoliation of the IF-MoS₂ inside the contact and (3) the adhesion of the released MoS₂ platelets on the friction surfaces. The dispersants were shown to improve the dispersion of the nanoparticles in the oil by reducing their agglomeration, but prevented the adhesion of a low-friction MoS₂ tribofilm on the steel surfaces. In-situ contact visualization revealed that the well-dispersed nanoparticles passed through the contact and exfoliated nanoparticles were observed after tribological testing. These results imply that nanoparticle dispersion itself does not seem to be an issue concerning nanoparticle effectiveness, even though the reduced agglomerate size and inertia may have affected nanoparticle flow near the contact, as well as entrapment and exfoliation conditions inside the contact. The use of succinimide-based dispersants may, however, have affected the tribochemistry of the contact, by an excessive adsorption on the steel surfaces and/or by encapsulating the released MoS₂ platelets, preventing tribofilm adhesion. A balance was finally found between nanoparticle dispersion and friction reduction, but for very low dispersant concentrations and after a running-in period. The role of succinimide-based dispersants and their effect on nanoparticle lubrication were discussed in the light of these results.     

Tribological behaviour of MoS2 and inorganic fullerene-like WS2 nanoparticles under boundary and mixed lubrication regimes

Abstract

For several years different types of nanoparticles have been considered and studied as potential friction modifying lubricant additives. Some nanoparticles can reduce the friction coefficient by 30–70%, depending on the base oil and the experimental conditions. In the present study, an experimental analysis on tribological properties of inorganic fullerene-like metal dichalcogenides was performed in comparison with MoS₂ 2H layered structures. Tribological tests were carried out on a pin on disc tribometer in ambient air. Several contact conditions are analysed in order to realise boundary and mixed lubrication regimes. The experimental study was performed on a mineral base oil, and particle concentration effects were analysed. Antifriction properties were evaluated by measuring the friction coefficient and are presented as generalised Stribeck diagrams. Inorganic fullerene-like WS₂ and MoS₂ nanoparticles present interesting friction reduction properties when tested in boundary and mixed lubrication.     

Friction and adhesion in boundary lubrication measured by microtribometers

The measuring and modelling of friction are critically important for the motion control in nanopositioning, particularly when bearings are employed to cover the wide working distances. Since the positioning system usually operates at very low speed to achieve fine positioning, the boundary lubrication is the dominant regime. A detailed characterization of the friction of boundary lubrication formed by Poly–α–Olefin (PAO) with and without surfactant and a suspension of MoS₂ in base oil has been performed in reciprocating sliding tests by steel/steel point contacts, and correlated with adhesion measurements by silicon/silicon point contacts. A microtribometer based on laser interferometers and glass springs, which can resolve 100 nN force in a speed range of 1–1000 μm/s was employed to detect the minute changes in forces. We find that a simple linear function instead of a logarithmic function is possible to describe the relationship between the friction force and operating speed for all the lubricants tested, though the gradients are quite different and under the influence of normal load. Comparing to PAO+surfactant and MoS₂ suspension, PAO shows a much higher load-dependent coefficient of friction. This result is further confirmed by the repulsion force measurements, which shows a higher increase of contact pressure with the increase of normal load for PAO.     

The effects of contact configuration and coating morphology on the tribological behaviour of tetrahedral amorphous diamond-like carbon (ta-C DLC) coatings under boundary lubrication

ABSTRACT

Tribological studies were carried out with tetrahedral amorphous diamond-like carbon (ta-C DLC) coatings, varying in thickness and roughness, using two different contact configurations lubricated with seven types of hydraulic oils. Tribopair of cast iron and ta-C coated steel were tested in both non-conformal and conformal, unidirectional sliding contacts. The friction and wear results were mainly affected by the thickness of the coating in the non-conformal contact and the surface roughness of the coating in the conformal contact. Tests done with mineral base oil containing rust inhibitor in the non-conformal contact and with Polyalphaolefins and synthetic ester base oils in the conformal contact resulted in the lowest friction while that with mineral base oil containing zinc resulted in high friction and counterface wear. The results highlight the interdependence of contact configuration, lubricant chemistry, coating’s surface morphology and coating’s thickness in determining the tribological behaviour of ta-C coatings under boundary lubrication.     

Influence of surface roughness on friction and scuffing behaviour of cast iron under sparse lubrication

Friction and scuffing behaviour of grey cast iron as influenced by the surface roughness under sparse lubrication conditions is studied. The studies are carried out on a three-shoe-on-disc machine under the conditions of parallel sliding. The experiments were conducted at three oil supply rates of 0.10, 0.22 and 0.36 μg/cm² per contact obtained through a mist oiling system and with specimens of cast iron shoes of different roughness values, Ra, between 0.04 and 2.0 μm against a 0.55% C steel disc. In a step load procedure, the friction torque at the end of each load step and the scuffing load are the major parameters measured. Results of friction and scuffing behaviour as a function of roughness and oil supply rates have been discussed.     

The mechanism of lubrication by molybdenum disulphide dispersed in oil and the effect of a zinc dialkyldithiophosphate additive

The effectiveness of the lubrication of hard smooth steel surfaces by molybdenum disulphide (MoS₂) dispersed in a mineral oil, both with and without a zinc dialkyldithiophosphate (ZDDP) additive, has been investigated using a four-ball extreme pressure lubricant test machine.Each oil, additive-oil or MoS₂ dispersion, exhibited a wear scar diameter-load curve with an abrupt transition from mild to severe wear at a characteristic load. When ZDDP was present in the oil, MoS₂ was effective at pre-transition but not post-transition loads, whereas in the absence of ZDDP the converse applied, with MoS₂ only beneficial above the transition load.These results are explained in terms of the adherence (or not) of MoS₂ particles to the films physically adsorbed on the steel surfaces at pre-transition loads. At the transition load the surface temperature reaches the desorption (or decomposition) temperature of the particular oil or additive film and MoS₂ then adheres to exposed metal, unless the latter chemically reacts with oil constituents to form a chemisorbed film.     

Effect of Graphite on Friction and Wear Characteristics of Molybdenum Dithiocarbamate

The effects of graphite powder on the friction and wear characteristics of molybdenum dithiocarbamate MoDTC were studied in reciprocated sliding contact using a ball-on-plate type tester. The oil used was squalane, a pure hydrocarbon. The addition of MoDTC alone to the oil showed a high coefficient of friction at an early stage of a rubbing test, since the extent of the formation of a surface film containing MoS₂ was insufficient to decrease the coefficient of friction. The high friction at the early stage brought about noticeable wear. On the other hand, the addition of graphite powder together with succinimide-type dispersant to the oil containing MoDTC considerably improved the friction and wear performance not only at the early stage but also at the steady stage. The added graphite powder seems to cover the part of rubbing surfaces without a film containing MoS₂ to reduce the friction and wear, not only at the steady stage after the running-in process, but also in the running-in process or at the early stage.     

Measurements of friction in strip drawing under thin film lubrication

Strip drawing is used to investigate the friction behaviour under thin film lubrication in metal forming with plastic deformation. Friction coefficients are measured under a wide range of tribological conditions. The surface roughness is measured on an interferometric profilometer. The results show that the friction coefficient decreases with increasing oil film thickness h_w, as estimated using a formula appropriate for smooth tool and workpiece. Measurements of the surface topography show that change in friction is associated with a change in contact ratio between the tool and strip. The effect of strip reduction, strip roughness and die roughness on the friction coefficient is also investigated.     

In Situ Raman Observations of the Formation of MoDTC-Derived Tribofilms at Steel/Steel Contact Under Boundary Lubrication

ABSTRACT

In this study, the time-dependent formation process of molybdenum dithiocarbamate (MoDTC)-derived tribofilms at steel/steel contact under boundary lubrication was investigated by using an in situ Raman tribometer. Especially, we focused on the effects of zinc dialkyldithiophosphate (ZDDP) concentration in MoDTC solution on MoDTC tribofilm formation process. A laboratory-built in situ Raman tribometer was used to evaluate friction and the formation process of MoDTC-derived tribofilms. All our results clearly suggest that there is an optimum ZDDP concentration in MoDTC solution for promoting the formation of MoS₂ tribofilms on the sliding surfaces, and there is also a threshold value for the formation rate of MoS₂ on the sliding surface for achieving low friction under lubrication with MoDTC-containing lubricants.     

Multi-environmental lubrication performance and lubrication mechanism of MoS2/Sb2O3/C composite films

MoS₂–Sb₂O₃–C composite films exhibit adaptive behavior, where surface chemistry changes with environment to maintain the good friction and wear characteristics. In previous work on nanocomposite coatings grown by PVD, this type of material was called a “chameleon” coating. Coatings used in this report were applied by burnishing mixed powders of MoS₂, Sb₂O₃ and graphite. The solid lubricant MoS₂ and graphite were selected to lubricate over a wide and complementary range including vacuum, dry air and humid air. Sb₂O₃ was used as a dopant because it acts synergistically with MoS₂, improving friction and wear properties. The MoS₂–Sb₂O₃–C composite films showed lower friction and longer wear life than either single component MoS₂ or C film in humid air. Very or even super low friction and long wear-life were observed in dry nitrogen and vacuum. The excellent tribological performance was verified and repeated in cycles between humid air and dry nitrogen. The formation of tribo-films at rubbing contacts was studied to identify the lubricating chemistry and microstructure, which varied with environmental conditions. Micro-Raman spectroscopy and Auger electron spectroscopy (AES) were used to determine surface chemistry, while scanning electron microscopy and transmission electron microscopy were used for microstructural analysis. The tribological improvement and lubrication mechanism of MoS₂–Sb₂O₃–C composite films were caused by enrichment of the active lubricant at the contact surface, alignment of the crystal orientation of the lubricant grains, and enrichment of the non lubricant materials below the surface. Sb₂O₃, which is not lubricious, was covered by the active lubricants (MoS₂ – dry, C – humid air). Clearly, the dynamics of friction during environmental cycling cleaned some Sb₂O₃ particles of one lubricant and coated it with the active lubricant for the specific environment. Mechanisms of lubrication and the role of the different materials will be discussed.     

Tribological behaviour of steel‐alumina sliding pairs under boundary lubrication conditions

The tribological behaviour of oil‐lubricated steel‐alumina sliding pairs was investigated using a ball‐on‐disc tribometer at room temperature. Commercial bearing balls of 10 mm diameter were mated to 99.7% Al₂O₃ discs, and additive‐free mineral oil was fed into the contact area. The sliding speed and the applied normal load were varied, and the initial surface roughness of the Al₂O₃ disc was altered using different polishing and grinding procedures. The results showed that the surface roughness of the ceramic discs dominated the tribological behaviour under the given experimental conditions. The sliding speed as well as the normal load showed less effect on the friction behaviour, but the amount of wear depended strongly on the normal load. From the results it was concluded that improvement of the surface roughness and optimised surface machining of the ceramic material can be essential for improving the tribological performance for boundary‐lubricated steel‐ceramic sliding pairs.     

Various MoS2-, WS2- and C-Based Micro- and Nanoparticles in Boundary Lubrication

Various solid lubricant particles have been experimentally evaluated as possible additives to oils. However, information in terms of a direct comparison of their tribological properties is still missing. In this study, we have compared the tribological properties of seven different solid lubricant micro- and nanoparticles as additives in polyalphaolefin (PAO) oil: MoS₂ nanotubes, MoS₂ platelets (2 and 10 μm), WS₂ nanotubes, WS₂ fullerene-like nanoparticles, graphite platelets (20 μm) and multi-walled carbon nanotubes. The experiments were performed in the boundary lubrication regime under a contact pressure of 1 GPa (Hertz, max) using a ball-on-disc tribotester. In general, the particles significantly decreased the friction and wear compared to the base PAO oil. We found that it was the material of the particles that largely determined their tribological performance. The effect of the size of the particles was much less important, and the morphology (shape) of the particles had little or no influence. We have also investigated the effect of ultrasonication during suspension preparation on particle damage and found that the solid lubricant particles were not notably affected, except the MoS₂ and WS₂ nanotubes, which became somewhat shorter.     

The Antiwear Action of Zinc Di-n-Butyl Phosphate

The dynamics of MoS₂ particles in a mineral oil dispersion are studied in the same manner as reported in Part I for graphite dispersions. A Hertzian contact consisting of a steel ball in contact with a glass disk is lubricated with MoS² dispersions and observed by optical microscopy at various. slide/roll conditions. In general, the behavior of MoS₂ and graphite are similar. That is, the solids lend to enter the contact and form a film on the contacting surfaces whenever a rolling component of motion is used, but solid particles seldom enter the contact during pure sliding. MoS₂ has more pronounced plastic flow behavior than graphite. However, the polished steel ball is more readily scratched by MoS₂ than by graphite. Under the conditions of these studies, lower friction and wear are observed with pure oil rather than with the dispersions. However, under other conditions (such as different contact geometry or rougher surfaces), the solid-lubricant dispersions might be beneficial.     

Influence of different surface modification technologies on friction of conformal tribopair in mixed and boundary lubrication regimes

Moving machine assemblies are generally designed to operate in full film lubrication regime to ensure high efficiency and durability of components. However, it is not always possible to ensure this owing to changes in operating conditions such as load, speed, and temperature. The overall frictional losses in machines are dependent on the operating lubrication regimes (boundary, mixed or full-film). The present work is thus aimed at investigating the role of different surface modification technologies on friction of a sliding bearing/roller tribopair both in boundary and mixed lubrication regimes. A special test rig comprising of two bearings was built for the experimental studies. Tribological tests were conducted in a wide speed range to enable studies in boundary and mixed lubrication regimes. The influence of application of different surface modification technologies on both the sliding bearing and the roller surfaces on friction has been studied. The rollers used in these studies were provided with five different coatings (hard DLCs and a soft self-lubricating coating). Additionally, two uncoated rollers having different surface roughness were also studied. Uncoated bearings were used in all tribopairs except two. These two bearings were coated with DLC and phosphate coatings respectively and uncoated rollers were the mating counterparts. Friction measurements were made on the new as well as the previously run-in surfaces. It was found that the rollers with self-lubricating coating resulted in lowest boundary friction closely followed by the rollers with the hardest DLC coatings. The DLC coating applied on to the bearing showed lower boundary friction after running-in. Mixed friction has been found to be mainly dependent on the surface topography characteristics of both the original and the run-in surfaces of bearings and rollers. The harder DLC coatings and the phosphated bearing showed the lowest mixed friction due to an efficient running-in of the bearing surface.     

MoS2 single sheet lubrication by molybdenum dithiocarbamate

The mechanisms by which Modtc reduces friction in the centirange under boundary lubrication have been investigated using analytical tribometry. First, the SRV friction test was coupled with energy-filtering TEM on wear fragments and spatially-resolved XPS inside the wear scars. Second, we performed UHV friction tests on Modtc tribofilms previously created on a large area. The overall data demonstrate that the mechanisms of friction-reduction by Modtc is attributed to the effect of sliding between single layers of MoS₂ only, and not to intra-sliding in MoS₂ 3-D crystal. Highly-dispersed MoS₂ sheets are present in a carbon matrix in the tribofilm material. The growth of the 2-D MoS₂ single sheets is thought to be formed by degradation of the Modtc molecule by electron transfer mechanisms activated by the friction process. The lubrication of the uncoated, stationary counterface is attributed to successive transfer of individual sheets towards the friction surface. Practically, in these conditions only a few per cent of dispersed MoS₂ is sufficient to lubricate at the same level as pure MoS₂.     

Theoretical investigation of transient elastohydrodynamic lubrication with newtonian solid‐liquid lubricants

This paper reports a theoretical investigation of transient elastohydrodynamic lubrication of a line contact. A time‐dependent Reynolds equation and elasticity equations for compressible solid‐liquid lubricants were solved using finite volume and multigrid techniques. The lubricants used were mineral oils mixed with very small solid particles, MoS₂ and PTFE, which can be treated as Newtonian fluids. The two surfaces were initially at rest and in contact. The transient oil film pressure and oil film thickness were calculated numerically. This simulation showed the significant effects of solid particles on the lubrication characteristics.     

Grooved surface texturing by electrical discharge machining (EDM) under different lubrication regimes

The aim of the present research work was to investigate the effectiveness of grooved surface texturing with a rhombic geometry under different lubrication regimes. Tribological investigation under unidirectional sliding was focused on the effect of texturing parameters including pattern area density on the coefficient of friction under different lubrication regimes, achieved by varying sliding speed and lubricant viscosity. Grooved patterns with different textured area densities were produced on steel samples by electrical discharge machining. Results of this investigation showed that under boundary lubrication, textures resist sliding thus resulting in increased friction. The largest improvement of friction reduction was observed under hydrodynamic lubrication, for low‐viscosity oil when using the textured disc with 21% pattern area density. The reduction of the coefficient of friction if compared with the untextured surface was of approximately 24%. Examination of the sliding surfaces has not shown any quantifiable wear for the contact conditions studied.     

Tribological properties of molybdenum disulphide nanoparticles in soybean oil

Abstract

The tribological properties of soybean oil (SO) with different molybdenum disulfide (MoS₂) additives (hollow nanosphere, nanoplatelet and microplatelet) were investigated. MoS₂ hollow nanospheres remarkably improved the tribological properties of SO. SO with MoS₂ hollow nanospheres decreased abrasive plowing and changed the main wear pattern on the steel friction surfaces into chemical corrosion. The MoS₂ hollow nanospheres easily entered the contact region than the other MoS₂ particles to lubricate the friction pair because of its good dispersibility in SO. The tribochemical reactions among MoS₂ hollow nanospheres, SO and friction material produced a lubricating film composed of MoO₃, Fe₂O₃, carbon containing compounds. Thus, the MoS₂ hollow nanospheres have potential lubricating applications with SO. By contrast, MoS₂ nanoplatelet and microplatelets had lesser effects on the lubricating effect of SO. The MoS₂ nanoplatelets, even with its smaller size and more active chemical properties, had more difficulty in entering into the contact region because of its low dispersibility in the base oil.     

Base fluid parameters for elastohydrodynamic lubrication and friction calculations and their influence on lubrication capability

In this study, base fluid parameters for elastohydrodynamic lubrication (EHL) and friction analyses have been determined experimentally. The viscosity at atmospheric pressure, η₀, the pressure‐viscosity index, α, and the EHL friction coefficient, γ, are important parameters in EHL theory and they are crucial in the selection of efficient lubricants for different applications. This investigation focuses on three important lubrication mechanisms: the capability of forming a separating lubricant film, the friction generated in a lubricated contact, and the height of pressure peaks, such as the outlet pressure spike and pressure ripple caused by surface roughness. The influence of different lubricant parameters on these three mechanisms is discussed. The value of α is measured in a Couette high‐pressure viscometer, and the value of γ is obtained from a jumping‐ball device. Other parameters discussed are temperature‐viscosity coefficient, β, bulk modulus, B₀, thermal conductivity at atmospheric pressure, λ₀, and heat capacity unit volume, ρc_p0. A comparison between traditional mineral base oils and environmentally adapted oil based on rapeseed oil and synthetic esters contributes to the further understanding of the performance of these new materials in lubrication applications. It is shown that rapeseed oil and synthetic esters have good lubricating properties and are, in most cases, better than mineral oils.