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Showing papers in "Tribology Letters in 2014"


Journal ArticleDOI
TL;DR: In this article, a kinetic model of sand blasting is presented, which gives surface topographies and surface roughness power spectra in good agreement with experiments, showing that most natural surfaces and surfaces of engineering interest are self affine fractal over a wide range of length scales.
Abstract: Most natural surfaces and surfaces of engineering interest, e.g., polished or sand blasted surfaces, are self affine fractal over a wide range of length scales, with the fractal dimension \(D_\mathrm{f} = 2.15 \pm 0.15\). We give several examples which illustrate this and a simple argument, based on surface fragility, for why the fractal dimension usually is \(<\)2.3. A kinetic model of sand blasting is presented, which gives surface topographies and surface roughness power spectra in good agreement with experiments.

233 citations


Journal ArticleDOI
TL;DR: In this paper, multilayer graphene (MLG) was used as a bentone lubricating grease additive to improve the loadbearing capacities and thermal stability of bentone lube, which is strongly dependent on the formation of a versatile boundary lubricating film.
Abstract: A continuing desire exists to explore graphene as a lubricant additive and increase the performance of oil/grease products in efforts to acquire a fundamental knowledge of its tribology. As compared to graphite and ionic liquid, multilayer graphene (MLG) as a bentone lubricating grease additive not only provides lower friction and better wear resistance, but also greatly improves the load-bearing capacities and thermal stability of bentone lubricating grease. These benefits are strongly dependent on the formation of a versatile boundary lubricating film, which is provided by the laminated structure and good adsorption action of MLG on the rubbing surfaces, as well as good dispersion of MLG in grease.

113 citations


Journal ArticleDOI
TL;DR: In this paper, the authors review the origins of this debate, which primarily concerns a divergence of views between researchers using high pressure, high shear rate viscometry and those concerned with the measurement and analysis of elastohydrodynamic friction; the former advocate a Carreau-based shear stress/strain rate model while the latter generally favour an Eyring-based one.
Abstract: There is currently considerable debate concerning the most appropriate rheological model to describe the behaviour of lubricant films in rolling–sliding, elastohydrodynamic contacts. This is an important issue since an accurate model is required to predict friction in such contacts. This paper reviews the origins of this debate, which primarily concerns a divergence of views between researchers using high pressure, high shear rate viscometry and those concerned with the measurement and analysis of elastohydrodynamic friction; the former advocate a Carreau-based shear stress/strain rate model while the latter generally favour an Eyring-based one. The crucial importance of accounting for shear heating effects in analysing both viscometric and friction data is discussed. The main criticisms levied by advocates of a Carreau-based model against Eyring’s model are discussed in some detail. Finally, the ability of both types of rheological model to fit elastohydrodynamic friction measurements for a quite simple, well-defined base fluid is tested, using previously measured pressure–viscosity behaviour for the fluid. Both models appear to fit the experimental data over a wide temperature range quite well, though fit of the Eyring model appears slightly closer than that of the Carreau–Yasuda model. Friction data from a wider range of well-defined fluid types are needed to identify categorically the most appropriate model to describe elastohydrodynamic friction.

113 citations


Journal ArticleDOI
TL;DR: In this paper, the sapphire/AISI 316L contact has been lubricated with water, with the protic ionic liquid (PIL) bis (2-hydroxyethylammonium) succinate (MSu), and with a 1-wt% solution of MSu in water.
Abstract: The sapphire/AISI 316L contact has been lubricated with water, with the protic ionic liquid (PIL) bis (2-hydroxyethylammonium) succinate (MSu), and with a 1 wt% solution of MSu in water. Neat water evaporates after the running-in period to give a transition to dry contact. Neat MSu reduces the running-in period, reducing the friction coefficient to 0.09, an 88 % lower than water. Water + 1 wt% MSu reduces the running-in period, and after water evaporation, a PIL boundary film is formed, with an ultralow minimum friction coefficient of 0.0001, and a mean friction value of 0.02, a 97 % lower than that of water, and a 78 % lower than that of neat MSu. Surface interactions have been studied by 3D profilometry, FTIR, SEM-EDX, and XPS analysis.

102 citations


Journal ArticleDOI
TL;DR: In this article, the authors explore the distinction between the self-mated Gemini interface and hydrogels sliding against hard impermeable countersurfaces and show that the Gemini interface exhibits very low friction μ < 0.06, with little dependence on sliding speed or time in contact.
Abstract: The sliding interfaces found in the body—within the eyes, the digestive system, and the articulating joints, for example—are soft and permeable yet extremely robust, possessing low friction. The common elements among these systems are hydrophilic biopolymer networks that provide physical surfaces, elasticity, and fluid permeability. Stiff, impermeable probes are traditionally used to assess the frictional properties of most surfaces, including soft, permeable materials. However, both sides of physiological articulating interfaces are soft and hydrated. Measuring the friction response on just one-half of the cornea–eyelid interface or the cartilage–cartilage interface using a stiff, impermeable probe may not reproduce physiological lubrication. Here, we present lubricity measurements of the interface between two soft, hydrated, and permeable hydrogels. We explore the distinctions between the self-mated “Gemini” hydrogel interface and hydrogels sliding against hard impermeable countersurfaces. A rigid impermeable probe sliding against a soft permeable hydrogel exhibits strong frictional dependence on sliding speed, and a hydrogel probe sliding against flat glass shows a strong friction dependence on time in contact. The twin Gemini interface shows very low friction μ < 0.06, with little dependence on sliding speed or time in contact. This consistently low-friction Gemini interface emulates the physiological condition of two like permeable surfaces in contact, providing excellent lubricity.

99 citations


Journal ArticleDOI
TL;DR: In this paper, the authors compared the tribological properties of seven different solid lubricant micro-and nanoparticles as additives in polyalphaolefin (PAO) oil, and found that the particles significantly decreased the friction and wear compared to the base PAO oil.
Abstract: 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.

95 citations


Journal ArticleDOI
TL;DR: In this article, the formation of a soft amorphous carbon (a-C) layer with increased sp2 content was demonstrated, which grows faster than an a-C tribolayer found on self-mated diamond sliding under similar conditions.
Abstract: Wear in self-mated tetrahedral amorphous carbon (ta-C) films is studied by molecular dynamics and near-edge X-ray absorption fine structure spectroscopy. Both theory and experiment demonstrate the formation of a soft amorphous carbon (a-C) layer with increased sp2 content, which grows faster than an a-C tribolayer found on self-mated diamond sliding under similar conditions. The faster $$\hbox{sp}^{3} \rightarrow\,\hbox{ sp}^{2}$$ transition in ta-C is explained by easy breaking of prestressed bonds in a finite, nanoscale ta-C region, whereas diamond amorphization occurs at an atomically sharp interface. A detailed analysis of the underlying rehybridization mechanism reveals that the $$\hbox{sp}^{3}\, \rightarrow\hbox{ sp}^{2}$$ transition is triggered by plasticity in the adjacent a-C. Rehybridization therefore occurs in a region that has not yet experienced plastic yield. The resulting soft a-C tribolayer is interpreted as a precursor to the experimentally observed wear.

93 citations


Journal ArticleDOI
TL;DR: In this article, the microstructure of snakes was analyzed using scanning electron microscopy, atomic force microscope and confocal laser scanning microscopy to investigate the role of the stiffness of underlying layers on the frictional anisotropy.
Abstract: Since the ventral body side of snakes is in almost continuous contact with the substrate during undulating locomotion, their skin is presumably adapted to generate high friction for propulsion and low friction to slide along the substrate In this study, the microstructure of ventral scales was analyzed using scanning electron microscopy, atomic force microscope and confocal laser scanning microscopy Dynamic friction was investigated by a microtribometer The ventral scales demonstrated anisotropic frictional properties To analyze the role of the stiffness of underlying layers on the frictional anisotropy, two different types of scale cushioning (hard and soft) were tested To estimate frictional forces of the skin surface on rough substrates, additional measurements with a rough surface were performed Frictional anisotropy for both types of scale cushioning and rough surfaces was revealed However, for both types of surface roughness, the anisotropy was stronger expressed in the soft-cushioned sample This effect could be caused by (1) the stronger interaction of the microstructure with the substrate in soft-cushioned samples due to larger real contact area with the substrate and (2) the composite character of the skin of this snake species with embedded, highly ordered fiber-like structures, which may cause anisotropy in material properties

83 citations


Journal ArticleDOI
TL;DR: In this article, the wear and friction behavior of PTFE and nanocomposites were evaluated under a broad range of vacuum environments from 760 to 4 9 10 -6 Torr.
Abstract: Polytetrafluoroethylene (PTFE) is widely regarded as an excellent candidate for solid lubrication in vacuum. However, it is often precluded from many prac- tical applications due to its intrinsically high wear rate. Over the past decade, it has been discovered that small loading fractions of alumina nanofillers can increase the wear resistance of PTFE by three to four orders of mag- nitude. This dramatic increase in wear resistance has in turn prompted numerous tribological studies to examine the robustness of this performance. In this study, the wear and friction behavior of unfilled PTFE and PTFE and alumina nanocomposites were evaluated under a broad range of vacuum environments from 760 to 4 9 10 -6 Torr. The nanocomposites of PTFE/alumina showed a dramatic increase in wear of over two orders of magnitude at the highest vacuum conditions. There appears to be an optimal vacuum environment around 1-10 Torr, in which these samples achieved the lowest wear rates of approximately 2.5 9 10 -7 mm 3 /(Nm).

82 citations


Journal ArticleDOI
TL;DR: In this paper, the effects of water and oxygen on ambient Molybdenum disulfide (MoS2) friction were investigated. And the experimental findings were used to develop a qualitative model for the effects on MoS2 friction, which was used to explain transients, hysteretic effects, oxidation effects, and effects of physically bound water.
Abstract: Molybdenum disulfide (MoS2), a lamellar solid lubricant, is used extensively in space applications due to its exceptional performance in vacuum and inert environ- ments. The friction and wear of MoS2, however, increase in the presence of atmospheric contaminants, such as water. Despite numerous studies of the moisture-sensitive friction response of MoS2 over the decades, important fundamental questions remain unanswered. Two leading hypotheses suggest that water affects friction by causing the MoS2 to oxidize or by physically bonding to edge sites, and thereby disrupting easy lamellar shear. This paper presents a parametric study to (1) isolate the effects of water and oxygen on ambient MoS2 friction, (2) identify the effect of water and oxygen on MoS2 oxidation, and (3) distinguish between the effects of water diffusion and surface oxida- tion on the frictional response of MoS2 coatings. The experimental findings were used to develop a qualitative model for the effects of environment on MoS2 friction; the model is used to explain transients, hysteretic effects, oxidation effects, and effects of physically bound water.

81 citations


Journal ArticleDOI
TL;DR: In this paper, a series of pin-on-disk wear tests and computational fluid dynamics analysis are conducted to investigate the influence of distribution and geometry of the textures under various operating conditions, which suggest that surface texturing suitably interacts with the material properties of Babbitt metal favorably improving its tribological performance.
Abstract: Aiming at improving the tribological performances of sliding bearings in mixed or starved lubrication regime, textures in square and linear radiating arrays are ablated on the surface of Babbitt alloy disks by laser radiation. Series of pin-on-disk wear tests and computational fluid dynamics (CFD) analysis are conducted to investigate the influence of distribution and geometry of the textures under various operating conditions. Results suggest that surface texturing suitably interacts with the material properties of Babbitt metal favorably improving its tribological performance. Friction coefficients of the disks with textures arrayed in square are generally much lower and more stable as compared to their counterparts with textures arrayed in linear radiation. Also, textures arrayed in a square with an area density of 8.6 % allow the lowest friction coefficient, as low as 0.015, to be achievable. Theoretical analysis sheds the light that proper texture arrangements tend to generate favorable distribution of micro-hydrodynamic pressure to improve the tribological performance of Babbitt alloy significantly.

Journal ArticleDOI
TL;DR: In this paper, the authors derived the pressure dependence of the relative contact area and the mean interfacial separation between solids with self-affine surface roughness by combining dimensional analysis with numerical simulations and presented the results in a compact form.
Abstract: The description of elastic, nonadhesive contacts between solids with self-affine surface roughness seems to necessitate knowledge of a large number of parameters. However, few parameters suffice to determine many important interfacial properties as we show by combining dimensional analysis with numerical simulations. This insight is used to deduce the pressure dependence of the relative contact area and the mean interfacial separation $$\Updelta \bar{u}$$ and to present the results in a compact form. Given a proper unit choice for pressure p, i.e., effective modulus E * times the root mean square gradient $$\bar{g}$$ , the relative contact area mainly depends on p but barely on the Hurst exponent H even at large p. When using the root mean square height $$\bar{h}$$ as unit of length, $$\Updelta \bar{u}$$ additionally depends on the ratio of the height spectrum cutoffs at short and long wavelengths. In the fractal limit, where that ratio is zero, solely the roughness at short wavelengths is relevant for $$\Updelta \bar{u}$$ . This limit, however, should not be relevant for practical applications. Our work contains a brief summary of the employed numerical method Green’s function molecular dynamics including an illustration of how to systematically overcome numerical shortcomings through appropriate finite-size, fractal, and discretization corrections. Additionally, we outline the derivation of Persson theory in dimensionless units. Persson theory compares well to the numerical reference data.

Journal ArticleDOI
TL;DR: In this article, 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.
Abstract: 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-MoS2, 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.

Journal ArticleDOI
TL;DR: In this paper, it is shown that applying a thin diamond-like carbon (DLC) coating to metal surfaces creates an insulating effect that due to the increased liquid lubricant film temperature at the center of the contact, locally reduces lubricant viscosity and thus friction.
Abstract: Reducing friction is of utmost importance to improve efficiency and lifetime of many products used in our daily lives. Thin hard coatings like diamond-like carbon (DLC) have been shown to reduce friction in full-film-lubricated contacts. In this work, it is shown that contrarily to common belief, the friction reduction stems mainly from a thermal phenomenon and not only a chemical/surface interaction one. It is shown that a few micrometer-thin DLC coating can significantly influence the thermal behavior in a lubricated mechanical system. The presented simulations, validated by experiments, show that applying a thin DLC coating to metal surfaces creates an insulating effect that due to the increased liquid lubricant film temperature at the center of the contact, locally reduces lubricant viscosity and thus friction. The results of the investigation show that the addition of thin insulating layers could lead to substantial performance increases in many applications. On a component level, the contact friction coefficient in some common machine components like gears, rolling element bearings, and cam followers can potentially be reduced by more than 40 %. This will most likely open up the way to new families of coatings with a focus on thermal properties that may be both cheaper and more suitable in certain applications than DLC coatings.

Journal ArticleDOI
TL;DR: In this article, the size, morphology and phase structure of as-prepared Cu nanoparticle were analyzed by means of X-ray diffraction and transmission electron microscopy, and the tribological properties of the as-synthesized Cu nanoparticles as an additive in distilled water were investigated with a four-ball machine and the morphology and elemental composition of worn steel surfaces were examined using Xray photoelectron spectroscopy and scanning electron microscope equipped with an energy-dispersive spectrometer attachment.
Abstract: Cu nanoparticle surface-capped by methoxylpolyethyleneglycol xanthate was synthesized using in situ surface-modification technique. The size, morphology and phase structure of as-prepared Cu nanoparticle were analyzed by means of X-ray diffraction and transmission electron microscopy. The tribological properties of as-synthesized Cu nanoparticle as an additive in distilled water were investigated with a four-ball machine, and the morphology and elemental composition of worn steel surfaces were examined using X-ray photoelectron spectroscopy and scanning electron microscope equipped with an energy-dispersive spectrometer attachment. Results show that as-synthesized Cu nanoparticle as a water-based lubricant additive is able to significantly improve the tribological properties and load-carrying capacity of distilled water, which is ascribed to the deposition of Cu nanoparticles on steel sliding surfaces giving rise to a protective and lubricious Cu layer thereon. In the meantime, they may also tribochemically react with rubbing steel surfaces to generate a boundary lubricating film consisting of Cu, FeS and FeSO4 on the rubbed steel surface, which helps to result in greatly improved tribological properties of distilled water, thereby reducing friction and wear of the steel–steel pair.

Journal ArticleDOI
TL;DR: In this paper, the authors revisited the lubrication mechanism of fatty acids with a new approach combining experimental and computational chemistry studies, and the lubricating properties of single and mixtures of stearic, oleic and linoleic acids in a synthetic poly-Alpha-Olefin base oil (PAO4) on iron oxide surface are investigated under mixed boundary regime with temperatures from 50°C up to 150°C.
Abstract: In this paper, the lubrication mechanism of fatty acids is revisited with a new approach combining experimental and computational chemistry studies. The lubricating properties of single and mixtures of stearic, oleic and linoleic acids in a synthetic Poly-Alpha-Olefin base oil (PAO4) on iron oxide surface are investigated under mixed boundary regime with temperatures from 50 °C up to 150 °C. Low friction coefficient (about 0.055) with no visible wear is reported in the presence of single stearic acid at high temperature. This lubricating behavior is inhibited in the presence of unsaturated fatty acids highlighting an anti-synergic effect of a saturated/unsaturated mixture, especially at 150 °C. To understand the anti-synergic effect and the adsorption mechanism of these molecules, molecular dynamic (MD) and quantum chemistry simulations are performed to evaluate their diffusion coefficient in PAO4 and their adsorption mechanism on iron oxide at different temperatures. MD simulation results show a faster diffusion toward the surface for unsaturated fatty acids than for saturated fatty acid at all the studied temperatures. This means that unsaturated molecules arrive and mainly adsorb before stearic acid on the surface leading to a tribological behavior of the mixture characteristic of the unsaturated molecule. Computational chemistry suggests that all fatty acids (saturated and unsaturated) adsorption mechanism is due to the chemisorption of the carboxylic group on iron oxide surface with no desorption up to 150 °C.

Journal ArticleDOI
TL;DR: In this article, the effect of WS2 fullerene-like nanoparticles in PAO base oil when adding a ZDDP additive was studied at 100°C in the boundary lubrication regime.
Abstract: The effect in the tribological performance of WS2 fullerene-like nanoparticles in PAO base oil when adding a ZDDP additive was studied at 100 °C in the boundary lubrication regime. The tribological properties of the dispersion surpass those obtained without one of the two additives. The friction modifier properties of the particles are improved in the presence of ZDDP, while the anti-wear properties of the ZDDP are increased when the particles are added to the dispersion. The composition of the formed tribofilm was investigated. Results show that a 50–60 nm tribofilm is formed on the steel surface composed by WS2 mixed on the ZDDP chemical tribofilm. A WS2-rich layer is observed at the top of the tribofilm. A correlation between the chemical composition of the tribofilm and the tribological properties of the “PAO + WS2 + ZDDP” dispersion was made. Synergy between the two additives was proven.

Journal ArticleDOI
L. Wang1, Q. Y. Zhang1, X. X. Li1, X. H. Cui1, S. Q. Wang1 
TL;DR: In this article, the authors investigated the wear behavior of Ti-6Al-4V andTi-6.5Al-3.5Mo-1.5Zr-0.3Si alloys against AISI 52100 steel under a load of 50-250-N at 25-600°C and found that a severe-to-mild wear transition occurred with an increase in temperature.
Abstract: Dry sliding wear behaviors of Ti–6Al–4V and Ti–6.5Al–3.5Mo–1.5Zr–0.3Si alloys (code-named TC4 and TC11, respectively) against AISI 52100 steel under a load of 50–250 N at 25–600 °C were systematically investigated. For two titanium alloys, a severe-to-mild wear transition occurred with an increase in temperature. The critical transition temperatures of TC4 and TC11 alloys were 400 and 300 °C, respectively. Below the critical temperature, titanium alloys showed poor wear performance. As the temperature surpassed the critical temperature, the extremely low wear rates demonstrated excellent elevated-temperature wear performance of titanium alloys in the titanium alloy/steel tribo-system. The wear transition was characterized with the appearance of continuous, hard tribo-layer containing more oxides, especially Fe2O3, which showed a pronounced wear-reduced role. Adhesive and abrasive wear predominated in the severe wear regime; oxidative mild wear prevailed in the mild wear regime. Adhesive wear, abrasive wear and oxidative mild wear cooperated at the critical transition temperatures.

Journal ArticleDOI
TL;DR: In this article, the authors demonstrate the preparation of stable suspensions of single-walled carbon nano-horns (SWCNHs) in engine oil and show their tribological behavior at different temperatures.
Abstract: The work illustrates the preparation of stable suspensions of single-walled carbon nano-horns (SWCNHs) in engine oil and shows their tribological behavior at different temperatures. The suspension stability was verified by dynamic light scattering, and the viscosity of suspensions was measured. Stribeck test was used to evaluate the tribological performances of nano-lubricants in the 25–80 °C temperature range. The coefficient of friction was reduced with the addition of SWCNHs at all temperatures and concentrations. In particular, at T = 25 °C, the coefficient friction was reduced up to about 12 % in boundary regime even with a very low SWCNH concentration (0.01 vol%). Wear tests were carried out to verify results obtained from Stribeck characterization. A decrease in mean wear rate was observed at each temperature, between 25 and 30 %. In particular, at 25 °C, a decrease of about 30 % in wear rate was estimated. Friction coefficient decrease was also observed during all wear tests, probably due to the ball-bearing effect played by SWCNHs spherical aggregates. Therefore, SWCNHs revealed the potential to be promising candidates as additives, to develop a new class of lubricants that are suitable and effective in different operating environments.

Journal ArticleDOI
TL;DR: In this paper, a tribotracks after friction test was investigated with Raman Spectroscopy, scanning electron microscopy (SEM) and 3D optical profilometry to understand the action mechanism.
Abstract: Lithium stearate soap and layered MoS2 nanoparticles encapsulated in lithium stearate soap are prepared in the laboratory, and their lubricating properties are compared with respect to the particle and particle concentration. The tribotracks after friction test was investigated with Raman Spectroscopy, scanning electron microscopy (SEM) and 3D optical profilometry to understand the action mechanism. The status of the soap particles on a tribotrack changes with time, contact pressure and sliding speed. At low pressure and speed, individual solid undeformed soap particle stand proud of the surface and the topography shows marginal difference with sliding time. In these conditions, no frictional difference between the performance of grease with and without the nanoparticles is observed. Increasing the contact pressure and temperature (low speed and high speed) has a dramatic effect as the soap particles melt and the liquid soap flows over the track releasing the hitherto encapsulated nanoparticles. Consequently, the soap smears the track like a liquid, and the nanoparticles now come directly into the interface and are sheared to generate a low-friction tribofilm. At high particle concentration, the sliding time required for melting of the soap and release of MoS2 is reduced, and the tribofilm is more substantial and uniform consisting of smeared MoS2 and carboxylate soap as observed by SEM and 3D optical profilometry. A change in the Raman Spectra is observed with particle concentration, and this is related to morphology and microstructure of the tribofilm generated.

Journal ArticleDOI
TL;DR: In this paper, the effect of ionic liquids (ILs) as additives for synthetic ionic liquid lubricants was investigated by using a ball-on-disc-type tribometer under boundary lubrication conditions.
Abstract: The assessment of ionic liquids (ILs) as lubricants in several tribological systems has shown their ability to provide remarkable reduced friction and protection against wear, whether they are used as additives or in the neat form. However, their corrosion and limited solubility in non-polar hydrocarbon oils represent the bottleneck-limiting factors for the use of ILs as lubricants. Therefore, in order to tackle these problems, mixtures of alkylborane–imidazole complexes with one halogen-free IL as additive were used in this study. The knowledge of the additive–surface interactions and hence the understanding of tribological properties are an important issue for lubricant formulations and were also investigated in this work. Thus, combination effects between two ionic liquid additives, a halogenated and a halogen-free one, were evaluated by a ball-on-disc-type tribometer under boundary lubrication conditions. Effective friction reduction and anti-wear properties have been demonstrated in tribological investigations when adding between 0.7 and 3.4 wt% of the halogen-free IL into base fluid composed of alkylborane–imidazole complexes. X-ray photoelectron spectroscopy analyses of the steel specimens were conducted to study the correlation between tribological properties and chemical surface composition of the boundary films formed on the rubbing surface. This work suggests potential applications for using halogen-free ILs as additives for synthetic ionic liquid lubricants.

Journal ArticleDOI
TL;DR: In this paper, the classic problem of an elastic solid with a two-dimensional wavy surface squeezed against an elastic flat half-space from infinitesimal to full contact is revisited.
Abstract: We revisit the classic problem of an elastic solid with a two-dimensional wavy surface squeezed against an elastic flat half-space from infinitesimal to full contact. Through extensive numerical calculations and analytic derivations, we discover previously overlooked transition regimes. These are seen in particular in the evolution with applied load of the contact area and perimeter, the mean pressure and the probability density of contact pressure. These transitions are correlated with the contact area shape, which is affected by long range elastic interactions. Our analysis has implications for general random rough surfaces, as similar local transitions occur continuously at detached areas or coalescing contact zones. We show that the probability density of null contact pressures is nonzero at full contact. This might suggest revisiting the conditions necessary for applying Persson’s model at partial contacts and guide the comparisons with numerical simulations. We also address the evaluation of the contact perimeter for discrete geometries and the applicability of Westergaard’s solution for three-dimensional geometries.

Journal ArticleDOI
TL;DR: In this article, reversible thermodynamic theory is employed to study the mechanical degradation of lubricating grease undergoing shearing action, and the degradation rate is linearly related to the entropy generation and that it can be used for estimation of the mechanically degraded grease life.
Abstract: Irreversible thermodynamic theory is employed to study the mechanical degradation of lubricating grease undergoing shearing action. A correlation between the mechanical degradation of grease and entropy generation is established, and the results are verified experimentally using a rheometer, a journal bearing test rig, and a modified grease worker machine. It is shown that the degradation rate is linearly related to the entropy generation and that it can be used for estimation of the mechanically degraded grease life.

Journal ArticleDOI
TL;DR: In this article, the conductivities and tribological properties of the prepared lubricating greases were investigated in detail using scanning electron microscopy and X-ray photoelectron spectroscopy.
Abstract: Ionic liquid (IL) lubricating greases were prepared using 1-hexyl-3-methylimidazolium tetrafluoroborate and 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl) amide as base oil and polytetrafluoroethylene (PTFE) as thickener, respectively. Three kinds of lithium greases were also prepared using lithium ILs ([Li(PAG)]X) as base oil and PTFE as thickener. 1-Ethyl-3-methyl imidazolium hexafluorophosphate as an additive was added to the PAG grease, which was prepared using polyalkylene glycol monobutyl ether (PAG) as base oil and PTFE as thickener. The conductivities and tribological properties of the prepared lubricating greases were investigated in detail. Scanning electron microscopy and X-ray photoelectron spectroscopy were employed to explore the friction and wear mechanism. The results showed that the IL and lithium lubricating greases have conductivities and excellent tribological properties. Especially, IL greases have the highest conductivity. The excellent tribological properties are attributed to the formation of boundary films consisting of both tribo-chemical reaction films and physical absorption films, while high conductivities are attributed to the intrinsic electric fields of the ILs.

Journal ArticleDOI
TL;DR: In this article, the sliding friction anisotropy at the nano-, micro-, and macroscales with respect to surface asperity orientation was investigated and the mechanisms behind this phenomenon were explored.
Abstract: The work reported in this paper aims at understanding sliding friction anisotropy at the nano-, micro-, and macroscales with respect to surface asperity orientation and exploring the mechanisms behind this phenomenon. Experiments were conducted by probing surfaces with grooves parallel or perpendicular to the direction of relative motion. Continuum mechanics analyses with the FEM and a semi-analytical static friction model and the atomic molecular dynamics simulation were performed for the mechanism exploration. Friction anisotropy was understood from the differences in contact area, surface stiffness, stiction length, and energy barrier from the continuum mechanics prospective and from that in the stick–slip phenomena at the atomic level.

Journal ArticleDOI
TL;DR: In this article, the effect of graphene (PG), the ionic liquid (IL) and PG modified by mechanical blend with the IL 1-octyl-3methylimidazolium tetrafluorobotate (IL+PG) on the tribological performance of epoxy resin (ER) was determined.
Abstract: In the present study, we have determined the effect of graphene (PG), the ionic liquid (IL) and PG modified by mechanical blend with the IL 1-octyl-3-methylimidazolium tetrafluorobotate (IL + PG) on the tribological performance of epoxy resin (ER). IL + PG stable suspensions have been added to an epoxy resin (ER) matrix to obtain the new nanocomposite (ER + IL + PG), and its tribological performance has been compared with that of neat epoxy resin and with the nanocomposites containing PG (ER + PG) or IL (ER + IL). While neat ER presents a high dynamic friction coefficient of up to 0.31 and a severe wear with a specific wear rate of 8.1 × 10−4 mm3 N−1m−1, the new nanocomposites show negligible surface damage, as determined by surface roughness and profilometry. All nanocomposites show low friction coefficients and negligible wear. The maximum friction reduction, up to a 70 %, is obtained for ER + PG. Results are discussed upon the basis of TEM microscopy, SEM microscopy and EDX analysis, differential scanning calorimetry, thermogravimetric analysis and dynamic mechanical analysis. Addition of IL or IL + PG has a plasticizing effect on ER, while addition of PG increases the thermal stability and stiffness of ER. PG shifts the storage modulus onset and in the loss modulus and tan δ maximum peaks to higher temperatures, while a shift to lower values is observed with addition of IL or IL + PG.

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TL;DR: In this article, the authors investigated the sliding tribological properties of TiAl matrix self-lubricating composites (TMSC) with the addition of Ag and Ti3SiC2 (TAT) and found that TMSC with TAT exhibited lower friction coefficients (0.32-0.43) and less wear rates (1.23-4.13-n−1m−1) in the wide temperature range of 25-800°C.
Abstract: Dry sliding tribological properties of TiAl matrix self-lubricating composites (TMSC) containing Ag, Ti3SiC2, Ag and Ti3SiC2 were investigated from 25 to 800 °C under ball-on-disk test conditions against Si3N4 counterface under the same conditions of 10 N-0.234 m/s. The results indicated that the tribological properties were strongly dependent on the lubricant additives. TMSC with the addition of Ag and Ti3SiC2 (TAT) exhibited the lower friction coefficients (0.32–0.43) and less wear rates (1.23–4.13 × 10−4 mm3 N−1m−1) in the wide temperature range of 25–800 °C. The excellent tribological properties of TAT over the wide temperature range were attributed to the synergetic effect of Ag and Ti3SiC2 lubricants, silver diffusion forming a rich-silver smooth tribo-film on the frictional surface of TAT at low and moderate temperatures from 25 to 400 °C, while Ti3SiC2 oxidation reaction forming rich-oxide tribo-film on the worn surface of TAT at higher temperatures of 600 and 800 °C.

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TL;DR: In this article, the authors review specific and real needs from EHL and show that data and models describing the viscosity-pressure dependence are already available and how they can properly be used.
Abstract: That classical elastohydrodynamic lubrication (EHL) is not a quantitative field can be illustrated by its failure to provide a consistent and rigorous definition of the viscosity-pressure coefficient. Indeed, if the pressure dependence of viscosity cannot be accurately described, then the viscosity-pressure coefficient cannot be defined. Classical EHL has employed fictional narratives to justify the pressure dependences that have been utilized. In this context, the purpose of this perspective article is to review specific and real needs from EHL and to show that data and models describing the viscosity-pressure dependence are already available and how they can properly be used. The final aim is to encourage researchers to change their philosophy of classical EHL to a quantitative approach, in which every hypothesis and every result, whether experimental or numerical, would be justified on the basis of acceptable physics.

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TL;DR: In this article, a nearly faithful reproduction of da Vinci's apparatus for measuring friction based on his notebook illustrations was constructed and tested on roughly cut and brusquely squared samples of dry wood.
Abstract: Leonardo da Vinci (1452–1519) is universally regarded as a brilliant polymath, designer, astronomer, artist, philosopher, and a visionary engineer of the Renaissance era. Interestingly, due to the delayed discovery of several caches of his notebook pages (as late as the 1960s), his immense contribution to the field of tribology has only recently surfaced. From these salvaged documents, da Vinci’s three notable observations that preceded the development of the laws of friction were uncovered: (1) Friction is independent of apparent contact area, (2) the resistance of friction is directly proportional to applied load, and (3) friction has a consistent value of µ = 0.25. In this work, we have attempted to construct a nearly faithful recreation of Leonardo da Vinci’s apparatus for measuring friction based on his notebook illustrations and investigate the conditions under which Leonardo da Vinci’s experiments produced his findings. Our experiments, performed roughly 500 years later, reproduced Leonardo da Vinci’s findings of friction coefficients with wood of µ = 0.25, but only under conditions of roughly cut and brusquely squared samples of dry wood that were handled and sullied by hand in a fashion typical of wood working but inconsistent with the modern laboratory practice. Thus, our interpretation of Leonardo da Vinci’s findings is that these first tribological studies were actually performed on roughly cut and unpolished samples that had been handled extensively prior to and during testing; Such a procedure of sample preparation is entirely reasonable for the time period and suggests an active, dusty, and dynamic laboratory environment.

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TL;DR: In this paper, a detailed transient analysis of piston design for motorsport applications is presented, which integrates piston dynamics, thermo-elastic distortion and transient elastohydrodynamics, and is verified using noninvasive ultrasonic-assisted lubricant film thickness measurement from a fired engine under normal operating conditions.
Abstract: Advanced piston technology for motorsport applications is driven through development of lightweight pistons with preferentially compliant short partial skirts. The preferential compliance is achieved through structural stiffening, such that a greater entrainment wedge is achieved at the skirt’s bottom edge through thermo-elastic deformation, whilst better conforming contact geometry at the top of the skirt. In practice, the combination of some of these conditions is intended to improve the load-carrying capacity and reduce friction. The approach is fundamental to the underlying ethos of race and high-performance engine technology. Contact loads of the order of 5 kN and contact kinematics in the range 0–35 m/s result in harsh transient tribological conditions. Therefore, piston design requires detailed transient analysis, which integrates piston dynamics, thermo-elastic distortion and transient elastohydrodynamics. The paper provides such a detailed analysis as well as verification of the same using non-invasive ultrasonic-assisted lubricant film thickness measurement from a fired engine under normal operating conditions, an approach not hitherto reported in literature. Good agreement is noted between measured film thickness and predictions.