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 Tribology LettersJournal Prestige (SJR): 1.204 Citation Impact (citeScore): 2Number of Followers: 7      Hybrid journal (It can contain Open Access articles) ISSN (Print) 1573-2711 - ISSN (Online) 1023-8883 Published by Springer-Verlag  [2626 journals]
• Synergy of Viscosity Wedge and Squeeze Under Zero Entrainment Velocity in
EHL Contacts
• Abstract: Abstract In an elastohydrodynamic lubricated (EHL) contact under Zero Entrainment Velocity (ZEV) condition, surfaces cannot be separated by hydrodynamic lift. In this work, two other phenomena responsible for a film thickness build-up in ZEV contacts are studied using a numerical model. First, the thermal effect called “viscosity wedge” is investigated in steady-state conditions. Second, the “squeeze” effect is described in an environment where dynamic (time dependent) loads are considered. Then, both the viscosity wedge and squeeze effects are considered together. For each one of the two mechanisms, a characteristic time is considered. The ratio of these two times allows the identification of a dominant effect. Depending on this ratio, a prediction is attempted using semi-analytical models describing each effect. For an ideal set of parameters, it is shown that the combination of squeeze and viscosity wedge in EHL contact under ZEV allows for an enhanced performance.
PubDate: 2020-06-23

• CdSe-Based Quantum Dots as In Situ Pressure and Temperature Non-intrusive
Sensors in Elastohydrodynamic Contacts
• Abstract: Abstract We present a new technique designed for in situ measurement of pressure and temperature in lubricating films. An innovative methodology has been developed, based on the photoluminescence properties of non-intrusive CdSe-based nanosize sensors (quantum dots). The sensitivity to pressure and temperature of these sensors dispersed in a carrier fluid was established through calibrations performed in diamond anvil cells. Elastohydrodynamic (EHD) contacts of different combinations of contacting solids (glass-steel, glass-Si3N4, sapphire-steel and sapphire-Si3N4) and submitted to various operating conditions were studied through in situ experiments and numerical simulations. Isothermal experiments were performed first: both experimental central pressures and pressure profiles were obtained, with a very good agreement with the values predicted by the numerical model. A series of non-isothermal experiments were then carried out to perform temperature measurements. Temperature rises in the central zone of EHD contacts involving various material pairs were measured and compared to predictions, leading to a very satisfying agreement. Overall, the deviation between measurements and predictions remained smaller than the uncertainty of the measurement method. Therefore, these findings proved the potential of the methodology to probe in situ pressure and temperature in EHD contacts. Comparative performance with competing techniques was examined in terms of intrusiveness, level of reliability, spatial resolution, accuracy and complexity. As this work is a pioneering development, the technique may be improved in the near future, opening an avenue for even more accurate or faster measurements for example, and eventually offering a better understanding of the mechanisms at work in this type of lubricated interface.
PubDate: 2020-06-22

• Potential-Controlled Boundary Lubrication Using MoS 2 Additives in Diethyl
Succinate
• Abstract: The active control of friction in oil-based lubricants was realized in the present study with the use of MoS2 particle additives and the application of an electric field. By modifying the surface charging state of the MoS2 particles, the dependence of potential-controlled boundary lubrication behavior on the electrical properties of the particles was demonstrated. For a diethyl succinate lubricant containing negatively charged MoS2 particles, the coefficient of friction (COF) was reduced by 60–70% when a positive potential was applied to a lower friction pair of copper plates. After modification with poly(diallyldimethylammonium chloride), the particles were positively charged, and the COF was reduced with the application of a negative potential. The mechanisms underlying the potential control of the COF were investigated by observing the distributions of the particle additives and characterizing the tribofilms formed at different potentials. Most of the charged particles were locally concentrated near the opposite pole, and this was reversed when the electric field changed. For locally high concentrations of MoS2 particles, a MoS2/MoOx tribofilm with a thickness of 100–500 nm and a loose structure formed on the lower friction pair, which significantly decreased the shear force during the friction process. Graphical
PubDate: 2020-06-22

• A Numerical Study on Thermal Elastohydrodynamic Lubrication of Coated
Polymers
• Abstract: Abstract The application of polymers in power-transmitting machine elements, e.g., gears, is limited by moderate thermo-mechanical properties and the detrimental accumulation of contact heat, even with external lubrication. Hence, polymer rolling–sliding elements are often prone to thermo-mechanical overload or abrasive wear. Diamond-like carbon (DLC) coatings are well known from steel applications for enhancing wear resistance and reducing friction. Since preliminary results indicate promising results for such coatings for polymers as well, their influence on the behavior of lubricated polymer contacts is investigated by numerical simulation. For polymer–steel contacts, the mechanical and thermophysical properties of coating and polymer are varied. The contact geometry is dominated by a local conformity, in which most of the deformation is related to the polymer. The DLC coatings affect film thickness and hydrodynamic pressure only little even for untypical high coating thicknesses. In contrast, the contact temperature decreases already for very thin coatings due to enhanced heat removal. Hence, DLC coatings can act as a thermal barrier protecting the polymer from detrimental heat and protecting the polymer from abrasive wear.
PubDate: 2020-06-01

• Improving the Lubrication of Silicon Surfaces Using Ionic Liquids as Oil
Additives: The Effect of Sulfur-Based Functional Groups
• Abstract: The performance of micro/nanoelectromechanical systems (MEMS/NEMS) relies on efficient lubrication. In the present work, new sulfur-based organic salts were tested as additives in a polyethylene glycol to lubricate silicon surfaces used in the manufacture of MEMS/NEMS. Seven salts were tested: 1-butylsulfonic-3-methylimidazolium triflate [(C4SO3H)MIM][TfO], thiamine triflate [Thiamine][TfO]2, 1-ethyl-3-methylimidazolium camphorsulfonate [C2MIM][CSA] [isomers (R) and (S)], 1,3-dimethylpiridinium methylsulfate [C1-3pic][MeSO4], methylimidazolium methanesulfonate [HMIM][MeSO3], and tetramethylguanidine methanesulfonate [TMG][MeSO3]. A nanotribometer was used to determine the friction coefficients using steel spheres as counter bodies. Excellent tribological properties were achieved with the additives containing the anions [MeSO4]− and [MeSO3]−. The films formed on the Si substrates were studied by FTIR, ellipsometry and AFM. A mixed lubrication mechanism was proposed where additive adsorption avoids contact between sliding surfaces. Graphical Sulfur-based organic salts as additives in the base oil PEG200 significantly improve the lubrication of silicon surfaces used in MEMS/NEMS
PubDate: 2020-05-30

• Study of Surface Roughness on Friction in Rolling/Sliding Contacts:
Ball-on-Disc Versus Twin-Disc
• Abstract: Abstract Although the efficiency of a gear pair is currently high, a better understanding of surface/lubricant contribution on efficiency is critical. Electrified drivelines will, for example, impose higher speed and alternate loading, and it is expected that these new conditions will, to a greater extent, rely on the surface/lubricant characteristics. Phenomena taking place in the gear contact is often measured using ball-on-disc and twin-disc tribometers. In this study, these two test set-ups were compared in order to assess differences in the behaviour of surface/lubricant interactions. Results showed that ball-on-disc and twin-disc set-ups reflect the same friction trends. However, the friction results differed by a factor of roughly two, even though the tribometers were set-up to run at the same contact pressure. The wear mechanisms also differed: micropits occurred on discs used in the twin-disc set-up, whereas normal or no wear was found on the ball-on-disc specimens. The contact conditions for the two test set-ups were also analysed using a numerical model. The comparison of these two machines may aid gear designers in selecting the proper experimental set-up for their purpose.
PubDate: 2020-05-28

• Correction to: The Possibility of Both Low Friction and Low Leakage by
Surface Texture of Mechanical Seals in Blood
• Abstract: The original version of this article unfortunately contained a mistake. The affiliation information was incorrect for the co-authors Shigeru Tazawa, Tsuyoshi Urano, Shinji Kobayashi.
PubDate: 2020-05-20

• Polymers Tribology Exposed: Eliminating Transfer Film Effects to Clarify
Ultralow Wear of PTFE
• Abstract: PTFE composite wear rates are known to vary by 1000 × depending on the size and strength of their nanofiller aggregates. While these effects have been attributed to variations in subsurface reinforcement, debris regulation, transfer films, and filler abrasivity, the chain of causation has proven difficult to test. This study aimed to clarify these causal relationships by eliminating confounding transfer film effects on wear reduction. We conducted indexed reciprocation experiments and tracked the interfacial development for PTFE filled with 5 wt% nano-alumina aggregates of varying strength (weak, strong, or a fully dense control). Weak aggregates were broken down most by processing, created the fewest abrasions, and produced the largest wear debris (~ 10 μm). Strong aggregates were largely retained following processing, produced the densest abrasions, and radically reduced debris size (< 100 nm). Despite these key interfacial differences, the composites produced comparable wear rates (2–7 × 10–5 mm3/Nm). The results provide the first direct evidence of the following: (1) even weak nanoparticle aggregates can be extremely abrasive; (2) ultralow wear rates (10–7 mm3/Nm) require transfer film stability; (3) the wear-reducing effects of unstable transfer films and loose debris are negligible; and (4) fillers directly reduce debris size even without a protective transfer film. The results suggest that successful fillers reduce debris size directly, that small debris nucleates a stable transfer, that stable transfer films reduce transfer wear rates, and that interfacial stability provides the time for needed for tribochemical reinforcement. Graphical
PubDate: 2020-05-16

• Numerical Simulation of Solid Particle Erosion of Epoxy by Overlapping
Angular Particle Impacts
• Abstract: Abstract The solid particle erosion of polymers occurs in a wide variety of industries and has been extensively studied experimentally. However, numerical models capable of accurately simulating the associated material removal mechanisms and predicting erosion rate do not yet exist. In this paper, a coupled smoothed particle hydrodynamics (SPH)/finite element (FE) model was developed to simulate the erosion of an epoxy by successive overlapping impacts of angular 22 µm, 97 µm and 152 µm silicon carbide particles at various angles of attack. The epoxy was modeled using a strain-rate-dependent elastic–plastic material model that fails at a critical plastic strain. It was found that, once the critical plastic strain was calibrated using a single experiment, the numerical model could predict both the length of the incubation period and the steady-state erosion rate to within 6% and 11%, respectively, of the measured values. It was found that fundamental material removal mechanisms such as cutting, ploughing and the accumulation of plastic deformation due to multiple overlapping impacts were all successfully simulated. Overall, it has been demonstrated that numerical models can be used to investigate the effect of influential parameters on the solid particle erosion of a polymer. This may have important implications for the development of effective methods to improve the erosion resistance of polymers.
PubDate: 2020-04-30

• The Possibility of Both Low Friction and Low Leakage by Surface Texture of
Mechanical Seals in Blood
• Abstract: Abstract Mechanical seal is installed in EVAHEART®, one of left ventricular assist device, which supports the role of original human heart. Low friction and low leakage are both required for mechanical seal for ventricular assist device to realize longer battery lifetime and less maintenance. In this study, several types of surface texture by combination of laser-induced periodic surface structure (LIPSS) and μm-scaled undulation were fabricated as surface texture on sealing surface of SiC-made seal rung. Friction and sealing properties were investigated by test apparatus specialized for mechanical seal for ventricular assist device. As a result, surface texture which has parallel LIPSS and μm-scaled undulation shows decreasing friction coefficient in blood. Furthermore, decreasing leakage rate was detected simultaneously. On sealing surface, native and thin protein film which consists of uniformly distributed albumin and fibrinogen was made. Therefore, possibility for low friction and low leakage was suggested and control of protein adsorption on sealing surface of mechanical seal was found to be crucial to realize low friction and low leakage.
PubDate: 2020-04-29

• Importance of Hydration and Surface Structure for Friction of Acrylamide
Hydrogels
• Abstract: Abstract To understand the dissipative mechanisms in soft hydrogel lubrication, polyacrylamide (PAAm) hydrogels with two distinct surface structures were examined under various contact conditions. The characteristic speed-dependent friction of the self-mated, crosslinked hydrogel surfaces could be explained by hydrodynamic shearing of a thin water layer between two rather impermeable bodies. On the other hand, the frictional response of brushy hydrogel surfaces is dependent on the contact conditions and the level of surface hydration. In a migrating contact, brushy hydrogels showed low, speed-independent friction (µ ~ 0.01) likely due to a thick layer of shearing liquid trapped within the sparse surface network. In stationary contact, however, brushy hydrogel surfaces can partially exude water from the near-surface region over time, as shown by time-resolved Fourier-transform infrared (FTIR) spectroscopy. This is assumed to be reflected in a friction increase over time. Interfacial shearing appears to shorten the characteristic exudation times compared to those observed under static loading. Once fluid has been exuded, brushy surfaces were shown to reach similar friction values as their crosslinked analogs. The results thus indicate that the dominating dissipation mechanism during sliding at low contact pressures is shearing of the interfacial liquid film, rather than poro-elastic dissipation within the bulk. Maintenance of surface hydration is therefore crucial, in order to take advantage of the low friction of such systems.
PubDate: 2020-04-23

• Nanoscale Friction of Hydrophilic and Hydrophobic Self-Assembled
Monolayers in Water
• Abstract: Abstract Self-assembled monolayers (SAMs) can reduce friction in boundary lubricated contacts by providing a low shear strength interface for sliding. However, the nanoscale mechanisms underlying low friction on SAMs are still not fully understood, especially in liquid environments in which hydrophobility or hydrophilicity affects friction. To understand this effect, friction of SAMs in water was measured using atomic force microscope experiments and molecular dynamics simulations, where hydrophilicity or hydrophobicity was determined by the terminal group of the alkanethiols. The friction on hydrophilic SAMs was larger than that on hydrophobic SAMs in both experiments and simulations, but this trend could not be explained by the strength of the adhesive force between the tip and the SAMs. Instead, analysis of the contributions of the water and SAMs to the total friction force revealed that the difference between the hydrophobic and hydrophilic SAMs could be explained by interactions between the tip and water during sliding. The much larger tip-water force on hydrophilic SAMs was attributed to a dense layer of water that was displaced during sliding as well as hydrogen bonds that formed between the water molecules and hydrophilic SAMs and were then broken by the tip as it slid, leading to higher friction force.
PubDate: 2020-04-23

• Sulfurized Methyl Esters of Soya Fatty Acids: Synthesis and
Characterization
• Abstract: Abstract Soy-based fatty acid methyl ester disulfide (FAME-S2) was synthesized in good yield by oxidation of polymercaptanized soybean oil fatty acid methyl ester (PM-FAME). The chemical structure of FAME-S2 is of interest because of its potential as a biobased multi-functional additive in lubricant formulations. Neat FAME-S2 and their blends (1–10% w/w) in polyalphaolefin (PAO-6) and high oleic sunflower oil (HOSuO) were characterized for its chemical, physical and tribological properties. Blends of FAME-S2 in HOSuO relative to similar blends of PM-FAME displayed higher oxidation onset temperature (> 15 °C) that remained constant with increasing concentration. Evaluation of FAME-S2 as an extreme pressure (EP) additive on a 4-ball tribometer showed increasing weld point (WP) with increasing concentration to a maximum of 220 and 180 kgf in HOSuO and PAO-6, respectively, at 10% w/w concentration. The results indicate that FAME-S2 has both anti-oxidant and EP properties and can be applied as a multi-functional biobased additive in lubricant formulation. This work demonstrates an encouraging progress towards the development of effective biobased lubricant additives.
PubDate: 2020-04-16

• The Effect of Working Parameters upon Elastohydrodynamic Film Thickness
• Abstract: Abstract There are a number of widely used machine components, such as rolling element bearings, gears and cams, which operate in the lubrication regime known as Elastohydrodynamics (EHD), where lubricant film thickness is governed by hydrodynamic action of convergent geometry, elastic deformation between non-conformal contacting surfaces, and the increase of lubricant viscosity with pressure. Variable loading conditions occur not only in all the machine components mentioned above, but also in natural joints such as hip or knee joints of humans or many vertebrates. Experimental studies of the behaviour of EHD films under variable loading are scarce and to authors’ knowledge systematic studies of the evolution of lubricant film thickness in EHD contacts subjected to forced harmonic variation of load are even less common. The aim of the present study is to explore the effect of load amplitude on the EHD film behaviour. This is done in alternating cycles with the load varying about a fixed, preset value at various amplitudes. Experimental results are compared with a simple theoretical analysis based on the speed of change of contact’s dimensions, a semi-analytical solution which includes both speed variation and squeeze effect, and finally with a full numerical solution.
PubDate: 2020-04-16

• Study of Dry Wear Behavior and Resistance in Samples of a Horizontally
Solidified and T6/Heat-Treated Automotive AlSiMg Alloy
• Abstract: Abstract To investigate dry wear behavior in an aluminum-based automotive alloy, transient horizontal solidification experiments using a water-cooled directional solidification device have been performed with the Al7Si0.3Mg alloy (wt%). Samples of the as-cast ingot at positions (P) 2, 4, 6, 40, and 80 mm from the cooled interface were subjected to precipitation hardening heat treatment (T6-type). The heat treatment has been applied under the following conditions: solution treatment for 3 h at 520 ± 2 °C, followed by quenching in warm water (70 ± 2 °C), aging for 3 h at 155 ± 2 °C and air-cooling. Dry wear tests were performed on both the as-cast and heat-treated samples. The wear tests were carried out by rotary-fixed ball wear machine means. The analyzed parameters were solidification growth and cooling rates (VL and TR), secondary dendritic spacings (λ2), and wear volume and rate (WV and WR). An interrelation between these parameters has been conducted and experimental mathematical equations have been proposed to characterize the WV and WR dependence on P, VL, TR and λ2. The T6-heat treatment has affected the length of the as-cast dendritic scale, increasing the λ2 values as well as the wear features of the investigated automotive alloy. Finer and coarser dendritic microstructures inherited better wear resistance for the as-cast and heat-treated samples, respectively. An evaluation by occupied area fraction (%IRAF) of interdendritic regions on wear resistance in the as-cast and heat-treated samples has been performed. It has been observed higher and lower IRAF values in the as-cast and heat-treated samples, respectively.
PubDate: 2020-04-13

• Investigations on the Thermocapillary Migration of Liquid Lubricants at
Different Interfaces
• Abstract: Thermocapillary migration describes an interfacial phenomenon that liquids can spontaneously move from the warm to the cold regions on nonuniformly heated solids. However, it is unknown how liquids react at various interfaces subjected to a thermal gradient. In this study, migration of liquid lubricants at the free surface, the plate/plate interface, and the sphere/plate interface is investigated. The results show that liquid lubricants can easily migrate at the free surface and the plate/plate interface, and the velocity at the free surface is much faster than that at the plate/plate interface. Interestingly, liquid lubricants maintain at the sphere/plate interface for a long time, and a continuous loss of a thin liquid film is observed at the cold side on the plate. The factors that influence the migration performances at these interfaces are examined. Numerical simulations are performed to reveal the mechanism and differences among them. Graphical
PubDate: 2020-04-10

• Influence of Microstructure and Mechanical Properties of Hot-work Tool
Steel on Wear Resistance Subjected to High-stress Wear Conditions
• Abstract: Abstract The aim of this study was to evaluate the effect of dissolution of the small carbides residual from annealing and earlier processing, on the mechanical and wear properties of hot-work tool steel. Recommended as well as extreme austenitization temperatures (950 °C, 1030 °C and 1150 °C) with subsequent tempering were used aiming at same hardness level of specimens of same material. This allows correlation in wear resistance variation to the microstructural elements and variations in other mechanical properties of the investigated steel. M23C6 and MC are still present at the Taus = 950 °C, which are being dissolved with higher austenitization temperature. Optimal combination of mechanical properties are obtained at recommended austenitization. Specimens subjected to lowest austenitization showed the worst abrasive wear resistance.
PubDate: 2020-04-09

• Prediction of Nanoscale Friction for Two-Dimensional Materials Using a
Machine Learning Approach
• Abstract: Abstract Several two-dimensional (2D) materials such as graphene, molybdenum disulfide, or boron nitride are emerging as alternatives for lubrication additives to control friction and wear at the interface. On the other hand, the initiative to accelerate materials discovery through data-driven computational methods has identified numerous novel topologies and families of 2D materials that can potentially be designed as low-friction additives. Hence, generating a structure–property (friction) correlations for 2D material-based additives that present a large variation in atomic composition is the next big challenge. Herein, we present a machine learning (ML) method using the Bayesian modeling and transfer learning approach to predict the maximum energy barrier (MEB) of the potential surface energy (correlated to intrinsic friction) of ten different 2D materials that were previously unexplored for their tribological properties. The descriptors (or properties) required to train the ML model with high accuracy are identified by taking into account the established physical models for dissipation in 2D materials. As a result, a difference of less than 8% in MEB values as predicted via the ML model presented here and the PES profiles generated using molecular dynamics simulations, for a select few 2D materials, was obtained. The model also enabled the identification of material properties that present the highest sensitivity to the corrugated potential, hence enabling the development of design routes for the synthesis of 2D materials with optimal tribological properties.
PubDate: 2020-04-08

• Effect of Tip Roundness on the Nanoindentation of Fe Crystals
• Abstract: Abstract Indentation tips are never atomically sharp, but rounded at their end. We use atomistic simulation to study the effect of tip roundness for the particular case of a cube-corner pyramidal indenter by comparing the results of a spherical, a sharp cube-corner, and a rounded cube-corner tip during indention into bcc Fe. We find that as soon as the tip has indented so deeply that the spherical geometry does not hold any longer, strong deviations between the dislocation plasticity behavior show up. The rounded cube-corner tip produces less dislocations and a smaller plastic zone than the spherical indenter, when indented to the same depth. The results are better comparable, however, when the same displaced volume is considered. Finally, the dislocation nucleation mode is affected by the geometry, changing from homogeneous to heterogeneous nucleation as the tip changes from rounded to sharp. The cube-corner tips are found to produce more twinning and delay the formation of prismatic loops. For a penetration depth beyond the radius of the rounded cube-corner tip, atomic sharp pyramidal tips produce similar quantitative (hardness, dislocation density) and qualitative (pileup, dislocation arrangement) results compared to its rounded counterpart. Our results will prove important for understanding the differences between spherical indenter tips, as they are often used in simulation, and pyramidal tips, as they are used in experiment.
PubDate: 2020-04-07

• Thermal-Controlled Frictional Behaviour of Nanopatterned Self-assembled
Monolayers as Triboactive Surfaces
• Abstract: Abstract Friction is an important limitation of energy efficiency performances of MEMS/NEMS but is, in the same time, a great opportunity for harvesting energy by designing optimized tribo-electric nano-Generators (TENG). Thus, frictional behaviour can be accurately controlled in real time by using thermally sensitive periodic patterned self-assembled monolayers of n-octadecyltrichlorosilane (OTS) grafted on MEMS surfaces. Nanopatterns are currently used in order to limit the wear rate without modifying the frictional behaviour. In this work, patterns have been created by micro-contact printing ($$\upmu \hbox {CP}$$) using a polydimethylsiloxane (PDMS) stamp displaying a trapezoidal profile. Hence, pattern periodicity can be continuously changed—and then optimized from discontinuous to pseudo-continuous—by applying a controlled normal load on the soft PDMS stamp. A multiscale tribological study has been carried out on these nanopatterns by using both single-asperity and multi-asperity nanotribometers. Lateral force microscopy (LFM) provides the individual frictional behaviour of each pattern’s component, whereas the multi-asperity nanotribometer rather gives the emerging frictional behaviour induced by the patterning according to temperature. As a macroscopic crucial parameter while designing TENG’s devices, this macroscopic behaviour has to be carefully optimized for each practical applications at the molecular scale. Thus, the microscale frictional behaviour can be precisely optimized by the pattern’s periodicity, whereas the macroscopic one can be accurately controlled with values of friction coefficient ranging from 0.12 to 0.04 by varying the contact temperature. In addition, any inertial effects observed in the thermal-controlled frictional behaviour of nanopatterns can be drastically reduced using infra-red emission as thermal source.
PubDate: 2020-04-07

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