Showing papers in "Tribology Letters in 2020"

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.

Triboelectrochemistry: Influence of Applied Electrical Potentials on Friction and Wear of Lubricated Contacts

Abstract: Research on the effects of applied electrical potential on friction and wear, a topic sometimes termed “Triboelectrochemistry”, has been reviewed. Historically, most such research has focussed on aqueous lubricants, whose relatively high electrical conductivities enable use of three-electrode electrochemical kinetic techniques, in which the electrode potential at a single electrode | fluid interface is controlled relative to a suitable reference electrode. This has led to identification of several different mechanisms by which applied electrode potentials can influence friction and wear. Of these, the most practically important are: (i) promotion of adsorption/desorption of polar additives on tribological surfaces by controlling the latters’ surface charges; (ii) stimulation or suppression of redox reactions involving either oxygen or lubricant additives at tribological surfaces. In recent years, there has been growing interest in the effects of applied electrical potentials on rubbing contacts lubricated by non-aqueous lubricants, such as ester- and hydrocarbon-based oils. Two different approaches have been used to study this. In one, a DC potential difference in the mV to V range is applied directly across a thin film, lubricated contact to form a pair of electrode | fluid interfaces. This has been found to promote some additive reactions and to influence friction and wear. However, little systematic exploration has been reported of the underlying processes and generally the electrode potentials at the interfaces have not been well defined. The second approach is to increase the conductivity of non-aqueous lubricants by adding secondary electrolytes and/or using micro/nanoscale electrodes, to enable the use of three-electrode electrochemical methods at single metal | fluid interfaces, with reference and counter electrodes. A recent development has been the introduction of ionic liquids as both base fluids and lubricant additives. These have relatively high electrical conductivities, allowing control of applied electrode potentials of individual metal | fluid interfaces, again with reference and counter electrodes. The broadening use of “green”, aqueous-based lubricants also enlarges the possible future scope of applied electrode potentials in tribology. From research to date, there would appear to be considerable opportunities for using applied electrical potentials both to promote desirable and to supress unwanted lubricant interactions with rubbing surfaces, thereby improving the tribological performance of lubricated machine components.

Prediction of Nanoscale Friction for Two-Dimensional Materials Using a Machine Learning Approach

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.

Effect of Mating Material and Graphitization on Wear of a-C:H Coating in Boundary Base Oil Lubrication

Abstract: Hydrogenated amorphous carbon (a-C:H) coating exhibits different wear behaviors depending on its counterpart material in boundary lubricated sliding contact. In previous works, tribological behaviors of a-C:H coating were investigated against steel, chromium, and germanium counterpart materials. The specific wear rate of a-C:H coating was found to decrease with the ability of its counterpart material to react with or dissolve carbon. The present study investigated how graphitization of a-C:H coating's top layers and interactions of the counterpart material with carbon influence wear behaviors of a-C:H coating in boundary lubrication. Results show that a-C:H coating shows graphitization of its top layers regardless the counterpart material. Correlation with differences in wear behaviors of the a-C:H coating leads to the conclusion that graphitization will induce high wear of a-C:H coating only when there are also atomic interactions between the DLC coating and its counterpart material.

Tribological Investigations of Nano and Micro-sized Graphite Particles as an Additive in Lithium-Based Grease

Abstract: Graphite is a well-known solid lubricant (SL) additive widely used both as a standalone SL and also as additive in lubricating oils, greases, composites, etc. Size of the additive particles especially nano-particles (NPs) is a major attribute to the performance properties of a composite, oil, grease, etc. This paper highlights the influence of graphite particles of four sizes viz. 50 nm, 450 nm, 4 microns and 10 microns incorporated in the grease in identical amount (4 wt. %) on the anti-friction (AF), anti-wear (AW) and extreme-pressure (EP) performance. The results indicated that all sizes proved beneficial for all the selected properties. Higher the size of particles, lower was the improvement in performance. The particles were most effective as anti-friction additive (AFA) followed by anti-wear additive (AWA) and then extreme-pressure additive (EPA). The NPs exhibited highest improvement as AFA (57%), AWA (41%) and EPA (25%). Raman Spectroscopy proved the formation of exfoliated graphitic layer on the worn surface of balls. Furthermore, SEM micrograph with elemental mapping and XPS spectroscopy analysis proved supportive in comprehending the mechanisms responsible for improved tribo-performance.

Ionic Liquids as Additives in Water-Based Lubricants: From Surface Adsorption to Tribofilm Formation

Abstract: Ionic liquids (ILs) are potential lubricant additives that can potentially perform simultaneously as friction modifiers and anti-wear agents. In addition, they possess good thermal stability, they are non-flammable, they have high polarity with negligible volatility, etc. These characteristics make them also ideal for polar lubricants, like water-based fluids. In this work, the friction and wear mechanisms of stainless steel 316L tested in water-based lubricants containing three different ionic liquids, i.e. Tributylmethylphosphonium dimethylphosphate, (2-hydroxyethyl) trimethylammonium dimethylphosphate and 1-butyl-1-methylpyrrolidinium tris(pentafluoroethyl)trifluorophosphate, have been investigated and compared with a reference water-based lubricant containing dodecanoic acid (Lauric acid, C12) as a well-known organic friction modifier. All lubricants formulated with the three ionic liquids showed frictional values lower than the water-based lubricant alone, but higher than the lubricant formulated with C12. A detailed surface adsorption study using Quartz Crystal Microbalance with Impedance measurements (QCM-I) revealed differences in the adsorption kinetics, strength of the adsorption bonds to the metallic surface and also different viscoelastic properties of the adsorbed layers for all the different additives. In the case of one of the ionic liquids (1-butyl-1-methylpyrrolidinium tris(pentafluoroethyl)trifluorophosphate), a tribofilm is formed after some sliding cycles and a significant friction to values lower than that of C12 is observed. A detailed surface and sub-surface investigation of the structure and the chemistry of the wear tracks using SEM/FIB, S(T)EM, and XPS showed that an oxide-rich tribolayer built in the wear track was the cause for the decrease in both wear rate and coefficient of friction. The other ionic liquids were not able to create a tribofilm on the surface of the steel and therefore friction and wear values were higher.

Tribofilm Formation, Friction and Wear-Reducing Properties of Some Phosphorus-Containing Antiwear Additives

Abstract: The film-forming, friction and wear properties of a range of model and commercial ashless P and P/S antiwear additives have been studied. A method has been developed for removing the tribofilms formed by such additives in order to effectively quantify mild wear. In general the P/S additives studied formed thinner tribofilms but gave lower wear than the S-free P ones. In extended wear tests, three P/S additives gave wear as low, or lower, than a primary zinc dialkyldithiophosphate (ZDDP). For almost all lubricants tested the wear rate measured in short tests was considerably higher than that in long tests due to the greater contribution of running-in wear in the former. This highlights the importance of basing antiwear additive choice on reasonably long tests, where running-in becomes only a small component of the wear measured. It has been found that for both P and P/S ashless additives the addition of oil-soluble metal compounds based on Ti and Ca boosts tribofilm formation and can lead to very thick films, comparable to those formed by ZDDP. However, this thick film formation tends to be accompanied by an increase in mixed friction and also does not appear to reduce wear but may even increase it.

Reply to the Comment on “A Scratch-Guide Model for the Motion of a Curling rock”

Abstract: Several issues were raised in the Comment on “A scratch-guide model for the motion of a curling rock”. A reply to these comments is provided.

Paradoxical Filler Size Effect on Composite Wear: Filler–Matrix Interaction and Its Tribochemical Consequences

Abstract: The addition of 0.2–5% nanoscale (40–80 nm) α-phase or microscale (40 μm) γ-phase Al2O3 particles in PTFE effectively reduce the matrix wear rate by 99.99%, whereas microscale (> 0.5 μm) α-Al2O3 or nanoscale (40–80 nm) γ-Al2O3 only reduce PTFE wear by ~ 90% under identical loading, dispersion and testing conditions. This paradoxical material system best illustrates the complexity of tribology and the importance of filler–matrix interactions at small scales. We studied the independent effect of the Al2O3/PTFE interface area and alumina structure by systematically varying the particle size over two orders of magnitude for both α- and γ-Al2O3/PTFE composites. Detailed characterizations of filler size, surface area and tribofilm’s chemical composition were conducted. The results found: (1) DLS median particle sizes conformed reasonably to vendor reported values and percentages of microscale filler aggregates correlated weakly with wear rates, (2) electron microscopy of the as-worn composite surface suggested a strong relation between the characteristic size of ‘unreinforced’ polymer domain and composite wear rate, (3) third bodies (i.e., transfer films, debris) played an important role in counterface abrasion, (4) wear rate correlated strongly with filler’s specific surface area and ultralow wear was only maintained ~ 0.3–10 m2/g nominal specific filler–matrix area values, (5) ultralow wear coincided with perfluorinated carboxylic salt rich tribofilms which supported a previously proposed wear reduction mechanism that mechanochemically degraded PTFE chelate with alumina and cause crosslinked and wear-resistant tribofilms, (6) tribofilm Al-F bond signal increased with filler surface area and high wear coincided with excessive tribofilm Al-F signal for γ-Al2O3/PTFE systems. Based on these results and literature hypothesis, we proposed that (1) the 1 μm α-Al2O3 provided the least filler–matrix interface and largest unreinforced polymer domain in PTFE, which lead to the least crosslinked and compartmentalized tribofilms; (2) in γ-alumina filled composites, Al-F bond forms as a product of mechanochemically degraded PTFE but also blocks chelation between the degraded PTFE and alumina fillers, (3) the 20 nm γ-Al2O3 provided the most filler–matrix interface which leads to excessive aluminum fluoride that blocked the filler–matrix chelation, prevented the tribofilm crosslinking and lead to high wear rates. This hypothesis was additionally supported by small molecule experiments in this study. However, this study provides no direct insight into how sensitive the filler–matrix tribochemical interaction is to filler phase or aggregate strength (strong, weak or fully dense).

Plastic Deformation of Rough Metallic Surfaces

Abstract: The contact between rough metallic bodies almost always involves plastic flow in the area of real contact. We performed indentation experiments on sandblasted aluminum surfaces to explore the plastic deformation of asperities and modeled the contact mechanics using the boundary element method, combined with a simple numerical procedure to take into account the plastic flow. The theory can quantitatively describe the modification of the roughness by the plastic flow. Since the long-wavelength roughness determines the fluid leakage of metallic seals in most cases, we predict that the leakage can be estimated based on the elastoplastic contact mechanics model employed here.

Effects of Thickness and Particle Size on Tribological Properties of Graphene as Lubricant Additive

Abstract: Increasing studies have demonstrated excellent tribological properties of graphene as lubricant additive. Thus, extended explanations of the relationship between microstructure and tribological properties are needed. This paper reported the effects of thickness and particle size of graphene on its frictional performance, using a ball-on-plate tribotester under reciprocating condition. Graphene with few-layer (G2) or multi-layer (G10) structure was added to PAO4 as lubricant additive. The particle of G10 was further separated through centrifugation into two types, large size G10 (G10L) and small size G10 (G10S). G10S exhibits excellent reduction in friction (37%) and wear (47%) relative to the neat base oil, indicating the superior tribological properties of graphene with more layers and smaller particle size. Wear scar analysis shows that the surface lubricated with G10 is flatter and glossier than that of G2. It can be hypothesized that the interlaminar shear slip is more likely to occur in multi-layer graphene, leading to better friction reduction and wear resistance performance. Graphene with smaller particle size is not only less prone to structural defects and wrinkles, but also more easily adsorbed onto the sliding surface to form a lubricating film, providing enhanced wear and frictional performance. The present work suggests that the thickness and size may have direct influence on the tribological properties of graphene, providing theoretical and experimental guidance for the application of graphene as lubricant additive.

Importance of Hydration and Surface Structure for Friction of Acrylamide Hydrogels

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.

Friction Reduction of Hydrogenated Graphene by Strain Engineering

Abstract: The pursuit of superlow friction in graphene systems has been a persistent target during the past decade. However, the friction is exhibited a remarkable increase for the chemically modified graphene with hydrogen element. To overcome this problem, both biaxial strain and uniaxial strains are applied on hydrogenated graphene to study the effect of strain on the interlayer friction between a rigid flake and a spring-supported hydrogenated graphene substrate by molecular dynamics simulations. Our simulation results indicate that with the increase of the hydrogenation coverage, the atomic-level roughness of hydrogenated graphene is found to increase, which eventually results in the increasing friction. During the stretching process, the atomic-level roughness of hydrogenated graphene is gradually reduced, which can be used to interpret the mechanism of the reduction of friction induced by strain. Such strain-induced reductions are robust over a wide range of commensurability, loads, and sizes. Moreover, it is demonstrated that the superlow friction also can be realized through the formation of incommensurate interface, the low rigidity of substrate, and the extended size of flake in sliding direction. These findings provide not only a fundamental understanding for the evolution of friction on hydrogenated graphene, but also an important insight for improving the tribological behaviors of nanodevices based on functionalized graphene.

The Tribological Mechanism of Cerium Oxide Nanoparticles as Lubricant Additive of Poly-Alpha Olefin

Abstract: Oleylamine (hereinafter referred to as OM)-modified CeO2 nanoparticles were synthesized by a one-pot pyrolysis method. The tribological properties of the as-prepared CeO2 nanoparticles as the lubricant additive in poly-alpha olefin (PAO) were investigated with a four-ball machine, and their lubricating mechanism was discussed in relation to worn surface analyses by SEM, EDS, and XPS. Findings indicate that these nanoparticles exhibit good dispersibility as well as excellent anti-wear ability in PAO. This is because OM-modified CeO2 nanoparticles can catalyze the oxidation of metallic Fe to form ferrite oxide-containing tribo-film. Under the condition of ASTM D2266-2001, the same lowest WDS was obtained at the concentration of 0.2 wt% and 1.8 wt%. When the concentration of CeO2 is 0.2 wt%, a compact catalytic oxidation tribo-film is formed, which has more outstanding long-term anti-wear ability. When 1.8 wt% CeO2 is added, the tribo-film formed is the combination of catalytic oxidation film and ceria deposition film, which has more significant bearing capacity.

Experiment to Investigate the Relationship Between the Third-Body Layer and the Occurrence of Squeals in Dry Sliding Contact

Abstract: Braking is an energy dissipation mechanism used to restrict the movement of vehicles. Friction brakes may induce vibrations and noise. These effects constitute a major shortcoming related to the functioning of friction braking systems. Known as brake squeal, this phenomenon involves unstable vibrations induced by coupling modes between components in frictional contact leading to large amplitude vibrations. Despite significant progress in experimental techniques and numerical modeling, the origin of squeal occurrence remains misunderstood and is still a matter of debate. It is, however, commonly admitted that squeal is affected by many different factors on both micro and macro scales. In addition, a close correlation between wear and squeal occurrence in braking system has been reported. This study examines linking the change in the third-body layer with the occurrence of squeals in sliding dry contact. A simplified customized test rig was used with a transparent glass disc and an artificial alumina third-body. Results show that squeal occurrence is strongly linked to the densification and redistribution of the third-body, as well as internal flows in the interface.

An EBSD Investigation on the Evolution of the Surface Microstructure of D2 Wheel Steel During Rolling Contact Fatigue

Abstract: In this work, the relationship between rolling contact fatigue (RCF) failure and the microstructure of D2 wheel steel was studied using a GPM-30 fatigue tester under oil lubrication conditions. The microstructural evolution during the RCF process can be divided into three stages: In the first stage, the misorientation of the proeutectoid ferrite is 2°–10°, the ferrite phase in pearlite is less than 2°, and the dislocation density is low. In the second stage, with the increase in cycles, the misorientation of the proeutectoid ferrite increases to more than 10°, and the ferrite phase in pearlite increases to 2°–10°. In the third stage, the misorientation of the ferrite phase in pearlite increases to more than 10°, the ferrite phase is divided into fine grains, and the dislocation density is high. RCF cracks are formed in the third stage. Crack initiation is ascribed to the refinement of the surface ferrite phase and proeutectoid ferrite and the increase in dislocation density. RCF cracks are initiated and propagate primarily at the interface of pearlite/proeutectoid ferrite and in proeutectoid ferrite.

Tribological Performance of Non-halogenated Phosphonium Ionic Liquids as Additives to Polypropylene and Lithium-Complex Greases

Abstract: Four non-halogenated ionic liquids (ILs) with trihexyl(tetradecyl)phosphonium cation are tested as lubricant additives to polypropylene (PP) and lithium-complex (LiX) greases. In pin-on-disk tests at elevated temperatures, the addition of an IL with bis(oxalato)borate ([BOB]) anion reduces wear by up to 50% when compared to the neat LiX base grease; an IL with bis(mandelato)borate ([BMB]) anion reduces friction by up to 60% for both PP and LiX. Elemental analysis reveals that oxygen-rich tribofilms help to reduce wear in case of [BOB], while the friction reduction observed for [BMB] is likely caused by adsorption processes. We find that temperature has a pronounced effect on additive expression, yet additive concentration is of minor importance under continuous sliding conditions. In contrast, rolling-sliding experiments at 90 °C show that the traction performance of LiX grease is dependent on additive concentration, revealing a reduction in traction by up to 30 and 40% for [BMB]- and [BOB]-containing ILs at concentrations of 10 wt%. Finally, an IL with dicyanamide anion reduces friction and increases wear in pin-on-disk tests at room temperature, while an IL with bis-2,4,4-(trimethylpentyl)phosphinate anion increases wear, showing only limited potential as grease additives. Overall, this work demonstrates the ability of non-halogenated ILs to significantly extend grease performance limits.

On the Origin of Frictional Energy Dissipation

Abstract: The origin of the friction between sliding bodies establishes an outstanding scientific problem. In this article, we demonstrate that the energy loss in each microscopic slip event between the bodies readily follows from the dephasing of phonons that are generated in the slip process. The dephasing mechanism directly links the typical timescales of the lattice vibrations with those of the experienced energy ‘dissipation’ and manifests itself as if the slip-induced motion were close to critically damped.

Correlation Between the Adsorption and the Nanotribological Performance of Fatty Acid-Based Organic Friction Modifiers on Stainless Steel

Abstract: Surface adsorption of amphiphilic molecules is a vital mechanism of boundary lubrication on stainless steel surfaces. The self-assembly of four fatty acid-based organic friction modifiers in two alkane solvents and their adsorption onto stainless steel surfaces was investigated using Dynamic Light Scattering and Quartz Crystal Balance with Dissipation, respectively. These properties were related to the friction force between a sharp tip and the steel surface measured using Lateral Force Microscopy. The molecular structures of the organic friction modifiers were chosen in order to study the effects of unsaturation and number of alkyl chains as well as the composition of the polar head groups on their assembly in solution, adsorption, and nanotribological behavior. Sorbitan monooleate and dioleate adsorb as monolayers with their alkyl chains either in the upright or tilted configuration, depending on their concentration. If large supramolecular structures were present in the solvent, i.e., for sorbitan monolaurate and glycerol monooleate, micelle adsorption and rearrangement on the surface and multilayer formation took place, respectively. A correlation between the adsorption rate constant and the coefficient of friction of the organic friction modifiers was revealed in these studies, with the coefficient of friction decreasing with an increase in the adsorption rate.

High-Temperature Friction and Wear Features of Nickel-Based Single Crystal Superalloy

Abstract: The nickel-based single crystal (NBSC) superalloy has been widely used in aero engines, gas turbines, and other power plants due to its excellent high-temperature performance. During the working process, wear often occurs on the contact surface of the NBSC superalloy, which will promote crack nucleation and reduce its working life. In this work, the dry-sliding friction and wear behaviors of NBSC superalloy with crystallographic orientation at different temperature and loading conditions are investigated by using a ball-on-disc tribometer. Results show that both the friction coefficient and wear rate of the NBSC superalloy are high at room temperature, and both them are significantly reduced as the temperature rises to 600 ℃ and 700 ℃. Analysis of the worn morphologies and cross-section SEM micrographs shows that a compacted glaze-layer is formed at high temperatures like 600 ℃ and 700 ℃ during the early stage of wear process, acting like a lubricant between the superalloy and rubbing ball. Hence, the change in wear mechanism caused by temperature leads to significant change of wear behaviors. In contrast to temperature, the magnitude of load has less effect on the wear mechanism.

Effect of Porosity on the Friction Properties of Porous Polyimide Impregnated with Poly-α-Olefin in Different Lubrication Regimes

Abstract: Porous polyimide materials (PPIs) having different porosity with a constant pore size were fabricated to further understand the effect of porosity on the friction properties in the boundary lubrication, mixed lubrication and elastohydrodynamic lubrication (EHL) regime, respectively. The tribological behaviors of non-porous polyimide (PI0) immersed in poly-α-olefin oil bath and PPIs impregnated with poly-α-olefin were firstly compared, and the effects of load and porosity on the friction performances were investigated. Results show that as the speed increases, the PPIs impregnated with oil exhibit the same friction coefficient (COF) change trend as PI0 immersed in poly-α-olefin oil, which is consistent with the stribeck curve, but the COFs in whole velocity range are higher, and the EHL arrived at a higher speed. The EHL critical speed of PPIs impregnated with oil, as well as the COFs during whole lubricating regime are closely related to the load and the porosity of the material. It mainly depends on two opposite effects of porosity on the friction properties. On one hand, PPIs with higher porosity will release more oil from pores under compression to form a lubricating film, on the other hand, higher porosity will lead to higher contact pressure under the same load, and provide larger contact areas as a result of the increase in the elastic collapse area, which will aggravate the solid–solid direct rubbing and increase the COFs.

A Comparison of Airborne Particles Generated from Disk Brake Contacts: Induction Versus Frictional Heating

Abstract: Volatile emissions of vehicle brakes relate to the high temperature of the brake friction pair. However, as a passive parameter of braking applications, temperature is usually studied together with other parameters such as sliding speed and load. Heating tests that increase the friction pair temperature with an induction heater instead of friction are proposed in this study to imitate the rise in temperature in friction tests. Non-friction airborne particles produced solely by the high temperature in heating tests were studied in comparison with friction tests. The results confirmed the existence of non-friction airborne particles and they can represent about 4.5% of the total airborne particles in friction tests. The high-temperature behaviour as well as the composition of the non-friction airborne particles is also presented.

Effect of Particle Size on Tribological Properties of Rubber/Steel Seal Pairs Under Contaminated Water Lubrication Conditions

Abstract: Abrasive wear is a common failure phenomenon that often limits the service life of seals. In this study, the abrasive wear test of acrylonitrile–butadiene rubber against steel was developed under conditions of lubrication contaminated with abrasives. The influence of particle size on coefficient of friction (COF) evolution, particle activities, wear morphologies, and damage mechanisms were discussed in detail. Results showed that the presence of abrasive particles dispersed in lubricated medium clearly deteriorates the tribological performance of the seal pairs. Two threshold values (approximately 7.5 and 75 μm) related to the “particle size effect” were obtained. And on this basis, the damage mechanism can be divided into three types. Several damage characteristics presented on respective worn surface, such as, cutting effect, rolling effect, the pinning effect of the “abrasive group,” an alternated feature with “ridge–valley–ridge,” or in the form of furrows and craters. In addition, the particles in the medium destroyed the lubricating film between the rubbing pairs and increased the COF and material removal capacity of tribo-pairs. Furthermore, the wear mode that clearly describes the wearing behavior of the seal pairs under abrasive-contained water lubrication conditions was identified. The results of this study enhanced our understanding of the wear degradation of rubber seal in all types of contaminated lubrication conditions.

Slurry Erosion Study on Nitrogen-Alloyed Austenitic Stainless Steel and Weld Beads

Abstract: Slurry erosion study on nitrogen-alloyed austenitic steel (NA-ASS) 23-8-N has been conducted. Solution annealing of base metal at two different temperatures 950 °C and 1150 °C dissolved the carbides precipitations and improved its strength and impact toughness. Bead-on-plate welds were made using gas metal arc welding process and these were solidified with 5–10% ferrite. Solution annealing results in homogenous microstructure with fine residual carbides and higher erosion resistance. Higher impact toughness and strength of 1150 °C solution annealed samples resulted in higher strain hardening with a minor loss of hardness. Solution treatment enhanced the erosion resistance of the base metal. Cumulative weight loss of bead-on-plate weld was relatively higher than that of the base material due to lower hardness and compositional difference. The weight loss from the surface due to erosive action was minimum for 1150 °C at 90° impingement angle. Wear and material loss occurred through micro-cutting and ploughing.

Relating Tribological Performance and Tribofilm Formation to the Adsorption Strength of Surface-Active Precursors

Abstract: Mechanochemical reactions induced by external stress provide a unique approach for in situ synthesis of carbon tribofilms that can improve friction and wear performance. In this work, we studied how tribofilm formation and tribological performance might be related to the adsorption strength of three additives in polyalphaolefin (PAO4) as base oil, viz., cyclopropanecarboxylic acid (CPCa), cyclopropanemethanol (CPMA), and 1-cyclopropylethanol (CPEA) as characterized by two different surface-active groups –COOH and –OH. Tribo-testing results reveal that addition of 2.5 wt% CPCa to PAO4 gave the lowest friction coefficient and wear volume. FTIR and Raman analysis demonstrate substantial tribofilm formation only in the case when CPCa was used as the oil additive, not CPMA or CPEA, in spite of the fact that all three additives contain the same metastable cyclopropane ring. Thermogravimetric analysis and molecular dynamics simulations indicate the stronger adsorption of CPCa on the iron oxide surface compared with CPMA and CPEA. Weak adsorption of the latter molecules results in their desorption from the surface due to flash heating during tribotesting before they have the chance to participate in mechanochemical reactions required for tribofilm formation. The stronger binding of CPCa to the steel surface is a necessary condition for this type of surface mechanochemistry and appears critical to the efficient formation of carbon-containing tribofilms under our tribo-testing conditions.

Tribological Performance of Cu–rGO–MoS 2 Nanocomposites Under Dry Sliding

Abstract: Nanostructured MoS2 grown on reduced graphene oxide (rGO–MoS2) is demonstrated as a lubricating reinforcement material for copper matrix composite. The Cu–rGO–MoS2 nanocomposites having variable dosages of rGO–MoS2 (0.5 to 2.0 wt%) are prepared via a combinational approach of powder metallurgy and then spark plasma sintering at 700 °C. The XRD and Raman analyses suggested the preparation of rGO–MoS2 hybrid, whereas HRTEM images revealed the thorough distribution of MoS2 nanosheets over the rGO. The tribological properties of Cu–rGO–MoS2 nanocomposites were evaluated against the EN 31 steel ball under the variable loads (4–10 N). The coefficient of friction was found to decrease with increasing of rGO–MoS2 content. The Cu–rGO–MoS2 nanocomposite with a 2.0 wt% of rGO–MoS2 hybrid exhibited the lowest and stable coefficient of friction (μ = 0.2) among all the nanocomposites. The high mechanical strength and low shearing properties driven by the lamellar structure of rGO–MoS2 furnished the self-lubricating properties to Cu–rGO–MoS2 nanocomposites. A combination of adhesion, oxidation, abrasion, and delamination of materials are revealed as major events for the wear mechanisms. These wear events are governed by the dosage of rGO–MoS2 reinforcement in the Cu–rGO–MoS2 nanocomposites and applied load. The results indicate that rGO–MoS2 has the potential to be used as a solid lubricant in the metal matrix composites.

Effects of Nano-SiO2 Addition in Drilling Fluid on Wear Behavior of 2Cr13 Steel Casing

Abstract: This work studied the effects of nano-SiO2 addition in drilling fluid on the wear behavior of casing. The disc specimens and pin specimens were made of 2Cr13 steel and G105 steel, respectively, and the water-based drilling fluids added with different amount of nano-SiO2 were tested, based on which, the wear rate, surface morphology, surface profile, and composition of the wear product were analyzed. Results showed that, the main wear mechanism of 2Cr13 steel casing in the drilling fluid is abrasive–corrosive wear. Adding nano-SiO2 to the drilling fluid can significantly reduce the casing wear. As the nano-SiO2 concentration increases, the wear rate and mean friction coefficient both increase first and then decrease. Drilling fluid shows best lubrication performance when the nano-SiO2 concentration is 2%. The shielding effect of nano-SiO2 can slow down the vicious cycle of “oxidation–destruction–reoxidation” during the wear process. Microhardness of tribofilms increases when adding more nano-SiO2 into drilling fluid, which increases the wear resistance of disc surfaces. However, the excessive addition of nano-SiO2 on a 2% concentration basis can lead to the decrease in bonding strength between tribofilms and matrix, leading to large delamination on the disc surface and therefore reducing the shielding effect of nano-SiO2.

A Modified Wear Model Considering Contact Temperature for Spur Gears in Mixed Elastohydrodynamic Lubrication

Abstract: In this study, a modified wear model considering contact temperature for spur gears in mixed elastohydrodynamic lubrication (EHL) is proposed. The contact temperature consists of bulk temperature and flash temperature. The bulk temperature is determined by the thermal network model, whereas the flash temperature is estimated through the published method. The bulk temperature, which was rarely included in the previous works, substantially has a considerable influence on the tooth wear in mixed EHL. It is also found that the lower contact temperature contributes to the reduction of gear wear depth. Furthermore, the effects of gear basic geometrical parameter and operating parameter on wear depth are investigated. The results show that the wear depth decreases with the increased tooth width, module, pressure angle and rotational velocity but increases with the surface roughness and torque. It indicates that wear resistance of tooth surfaces can be enhanced by optimising the design parameters of gear drives.

Surface Gel Layers Reduce Shear Stress and Damage of Corneal Epithelial Cells

Abstract: Soft contact lenses are medical devices made from aqueous polymeric gels that are worn on the eye to correct refractive errors. These devices interrupt the natural contact pairing between the cornea and the eyelid and create two interfaces comprised of a synthetic material and the epithelia—contact lens surfaces versus (1) the cornea and (2) the eyelid conjunctiva. The cellular responses to friction and shear stress are thought to contribute to contact lens discomfort. This study performs direct contact shear experiments using in vitro biotribological experiments using a microtribometer equipped with a hydrogel membrane probe. Sections from commercial contact lenses are held in place on a spherically capped membrane probe during reciprocating sliding experiments against confluent monolayers of living human telomerase-immortalized corneal epithelial cells (hTCEpi). The contact lenses were loaded against the cell monolayers to physiological contact pressures between 400 and 1300 Pa under an applied load of 200 µN. The reciprocating distance was 3 mm, at a sliding speed of 1 mm/s, and the maximum duration of sliding was 1000 cycles. Five commercially available lenses (somofilcon A, stenfilcon A, etafilcon A, verofilcon A, and delefilcon A) were used to evaluate the cell layer responses to aqueous gels of differing composition, surface modulus, and lubricity. Cell damage was measured via propidium iodide staining and in situ fluorescence microscopy. The shear stresses varied from 16 ± 2 Pa (delefilcon A and verofilcon A) to 86 ± 12 Pa (stenfilcon A), and cell damage increased with increasing shear stress and increasing sliding duration. The two lens materials that have high water content surface gel layers (delefilcon A and verofilcon A) showed distinctly lower measures of cell damage as compared to the other lenses. Surface gel layers with a large polymer mesh size and high water content are shown to be an effective approach to lower the contact pressure, lower the friction coefficient, and thereby lower the shear stress and cell damage.

Fluid Leakage in Metallic Seals

Abstract: Metallic seals are crucial machine elements in many important applications, e.g., in ultrahigh vacuum systems. Due to the high elastic modulus of metals, and the surface roughness which exists on all solid surfaces, if no plastic deformation would occur one expects in most cases large fluid flow channels between the contacting metallic bodies, and large fluid leakage. However, in most applications plastic deformation occurs, at least at the asperity level, which allows the surfaces to approach each other to such an extent that fluid leakage often can be neglected. In this study, we present an experimental set-up for studying the fluid leakage in metallic seals. We study the water leakage between a steel sphere and a steel body (seat) with a conical surface. The experimental results are found to be in good quantitative agreement with a (fitting-parameter-free) theoretical model. The theory predicts that the plastic deformations reduce the leak-rate by a factor $$\approx 8$$ .