Showing papers in "Tribology Letters in 2018"

Review of Viscosity Modifier Lubricant Additives

Abstract: This article reviews viscosity modifiers, additives that increase the viscosity of lubricating oils. Viscosity modifiers are high molecular weight polymers whose functionality is derived from their thickening efficiency, viscosity–temperature relationship, and shear stability. There are now many different additive chemistries and architectures available, all of which have advantages and disadvantages, and affect solution viscosity through different mechanisms. Understanding these mechanisms and how they impart additive function is critical to the development of new viscosity modifiers that enable lubricants to function more efficiently over a wide range of temperatures.

Elastic Contact Mechanics of Randomly Rough Surfaces: An Assessment of Advanced Asperity Models and Persson’s Theory

Abstract: In this work, we discuss important improvements of asperity models. Specifically, we assess the predictive capabilities of a recently developed multiasperity model, which differs from the original Greenwood and Williamson model by (i) including the coupling between the elastic fields generated by each contact spot, and (ii) taking into account the coalescence among the contact areas, occurring during the loading process. Interaction of the elastic field is captured by summing the contributions, which are analytically known, of the elastic displacements in a given point of the surface due to each Hertzian-like contact spot. The coalescence is instead considered by defining an equivalent contact spot in such a way to guarantee conservation of contact area during coalescence. To evaluate the accuracy of the model, a comparison with fully numerical ‘exact’ calculations and Persson’s contact mechanics theory of elastic rough surfaces is proposed. Results in terms of contact area versus load and separation versus load show that the three approaches give almost the same predictions, while traditional asperity models neglecting coalescence and elastic coupling between contact regions are unable to correctly capture the contact behavior. Finally, very good results are also obtained when dealing with the probability distribution of interfacial stresses and gaps.

Self-Lubricating PTFE-Based Composites with Black Phosphorus Nanosheets

Abstract: Black phosphorus (BP), a newly emerging two-dimensional material, has recently received considerable attention. Our recent work suggested that BP nanosheets exhibit extraordinary mechanical and lubrication properties. In the present work, the tribological properties of polyetheretherketone (PEEK)/polytetrafluoroethylene (PTFE) and carbon fiber (CF)/PTFE composites with BP nanosheets have been investigated. The morphologies and surface element distribution of the worn tracks of the tribopair surfaces were examined by different analytical techniques. The results show that the coefficients of friction (COFs) of both the PEEK/PTFE and CF/PTFE composites decreased dramatically after the addition of BP nanosheets, and the minimum COF of the composite was 0.04, which was a quarter of that of the PTFE composite without BP nanosheets. After BP nanosheets were added into the composites, the wear rate of the PTFE/PEEK composite decreased dramatically, while that of the CF/PTFE composite increased significantly with the increase in the filler concentration. The analysis of the lubrication mechanism of the PTFE composite with BP nanosheets suggested that BP nanosheets could be constantly supplied into the contact area and gradually formed a BP film composed of phosphorus oxide and phosphoric acid on the counterpart surface instead of the formation of PTFE transfer film. The formed BP transfer film promoted the friction reduction and the disappearance of the adhesive wear.

The Evolution of White Etching Cracks (WECs) in Rolling Contact Fatigue-Tested 100Cr6 Steel

Abstract: The formation of white etching cracks (WECs) in steel rolling element bearings can lead to the premature rolling contact fatigue (RCF) failure mode called white structure flaking. Driving mechanisms are still debated but are proposed to be combinations of mechanical, tribochemical and electrical effects. A number of studies have been conducted to record and map WECs in RCF-tested samples and bearings failed from the field. For the first time, this study uses serial sectioning metallography techniques on non-hydrogen charged test samples over a range of test durations to capture the evolution of WEC formation from their initiation to final flaking. Clear evidence for subsurface initiation at non-metallic inclusions was observed at the early stages of WEC formation, and with increasing test duration the propagation of these cracks from the subsurface region to the contact surface eventually causing flaking. In addition, an increase in the amount of associated microstructural changes adjacent to the cracks is observed, this being indicative of the crack being a prerequisite of the microstructural alteration.

The Influence of Cu/Fe Ratio on the Tribological Behavior of Brake Friction Materials

Abstract: Copper and iron are the major ingredients in friction materials, among which Fe often been served as friction reinforcement in the past. But in our recent study, the coefficients of friction (COF) ...

Direct Formation of Lubricious and Wear-Protective Carbon Films from Phosphorus- and Sulfur-Free Oil-Soluble Additives

Abstract: Extreme pressure (EP) lubricant additives form protective tribofilms at the site of contact using the heat and pressure of contact and relative motion. Common EP additives contain undesirable elements such as phosphorus and sulfur. A novel EP lubricant additive, which contains no phosphorus and sulfur, is presented for generating lubricious carbon films. The additive consists of a surface-active molecule with a metastable cycloalkane ring, which dissociates readily during tribological contact to form lubricious carbon films. Friction and wear performance of PAO4 with this additive under a range of loads and speeds were shown to be superior to that without the additive. Optical and scanning electron microscopy and Raman spectroscopy were used to analyze the tribofilms formed on post-test contact surfaces, providing direct evidence for the formation of carbon films. Quantitative kinetics for the carbon tribofilm formation was analyzed as a function of temperature and stress, from which the activation energy for carbon tribofilm formation was obtained.

A Novel Surface Texture Shape for Directional Friction Control

Abstract: An experimental study is presented to evaluate the influence of anisotropically shaped textures on the behaviour of sliding friction and sensitivity to sliding direction. The plate samples were textured with triangular sloped dimples using an ultrafast laser surface texturing technique. Reciprocating cylinder-on-plate tests were conducted with steel sliding pairs using mineral base oil as a lubricant to compare the tribological performance of reference non-textured specimen and dimpled samples. The dimples were designed with varying converging angles in the transverse y–z plane and top-view x–y plane. In this study, no dimple was fully covered in the contact area since the dimples size is much larger than the Hertzian line contact width. Stribeck style dynamic friction curves across boundary, mixed and hydrodynamic lubrication regimes were used to determine the benefit or antagonism of texturing. Observation of the directional friction effect of the anisotropic textures indicated that the converging shapes are beneficial for friction reduction, and the dimpled specimens have a lower friction coefficient particular under prevailing boundary lubrication conditions. It was also found that the real contact length variation rate is a major factor controlling the local friction response. The sloped bottoms of the textures produce effective converging wedge action to generate hydrodynamic pressure and contribute to the overall directional friction effects.

Friction-Induced Inflammation

Abstract: Friction-induced inflammation is a new hypothesis that implicates interfacial shear stress as a trigger for the production of pro-inflammatory signals, which in severe cases may lead to chronic inflammation and pain. If frictional shear stresses at the interface exceed physiological norms, a likely cellular response may involve the release of pro-inflammatory signals (e.g., proteins, cytokines, chemokines, and enzymes). Ocular lubricity and comfort are critically dependent on the stability of the tear film and the health of the corneal and conjunctival epithelium. The insertion of a contact lens may increase contact pressures and shear stresses on the epithelia, but these are difficult to measure in vivo. Epithelial cells are known to sense and respond to mechanical strains and deformations through a process called mechanotransduction. The effects of frictional shear stress on cellular responses were experimentally measured in vitro by sliding soft, spherical shell hydrogel probes against human corneal epithelial (hTCEpi) cell monolayers with two different normal forces: 500 and 1000 µN, giving ~ 30 and ~ 60 Pa shear stresses, respectively. Molecular biology assays (quantitative reverse transcription—polymerase chain reaction, RT-qPCR, and enzyme-linked immunosorbent assay, ELISA) followed friction experiments. Compared to control populations, RT-qPCR revealed frictional shear stresses of 60 Pa were sufficient to increase gene expression of the pro-inflammatory genes Interleukin-1β (IL-1β), Interleukin-6 (IL-6), Matrix Metalloproteinase 9 (MMP9), and pro-apoptotic genes DNA Damage-Inducible Transcript 3 (DDIT3) and FAS Cell Surface Death Receptor (FAS). An ELISA of growth media sampled after sliding revealed IL-6 cytokine release increased by 100% following shear stress experiments. These results suggest that frictional shear stress may be at least partly responsible for inducing inflammatory responses in corneal epithelial cells in vitro.

Effect of Tribo-Oxidation on Friction and Wear Behaviour of HVOF Sprayed WC–10Co–4Cr Coating

Abstract: The effect of tribo-chemical reactions under varying sliding speed and load conditions on the friction and abrasive wear response of high-velocity oxy-fuel-sprayed WC–10Co–4Cr coating was studied. The abrasive wear rate and friction coefficient decreased with the increase in sliding speed while friction coefficient displayed increasing trend with increase in load. The decrease in friction coefficient and wear rate was attributed to formation of tribo-oxides and surface films with good lubricating properties. Severity of abrasive wear increased with increasing load which was associated with transition in wear mechanisms from plastic deformation and fatigue to delamination cracking, intergranular fracture and splat fracture. Increase in friction coefficient with load irrespective of sliding speed was due to increasing contribution of fracture-assisted mechanical wear as compared to oxidative wear. The nature, composition and properties of tribo-films imparted crucial role to influencing friction and abrasive wear of WC–10Co–4Cr coating.

Shear Thinning and Hydrodynamic Friction of Viscosity Modifier-Containing Oils. Part I: Shear Thinning Behaviour

Abstract: Viscosity versus shear rate curves have been measured up to 107 s−1 for a range of VM solutions and fully formulated oils of known composition at several temperatures. This shows large differences in the shear thinning tendencies of different engine oil VMs. It has been found that viscosity versus shear rate data at different temperatures can be collapsed onto a single master curve using time–temperature superposition based on a shear rate shift factor. This enables shear thinning equations to be derived that are able to predict the viscosity of a given oil at any shear rate and temperature within the range originally tested. One of the tested lubricants does not show this time temperature superposition collapse. This fluid also exhibits extremely high viscosity index and shear thins more easily at high than at low temperature, unlike all the other solutions tested. This unusual response may originate from the presence on the VM molecules of two structurally and chemically different components. In a companion paper, the master shear thinning curves obtained in this paper are used to explore how VMs impact film thickness and friction in a steadily loaded, isothermal journal bearing [1].

Experimental Investigation on the Compatibility of Nanoparticles with Vegetable Oils for Nanofluid Minimum Quantity Lubrication Machining

Abstract: Several health and environmental related issues caused by the application of traditional cutting fluids in machining can be solved by implementing eco-friendly technologies such as minimum quantity lubrication (MQL). Moreover, nanofluid MQL has been proposed to enhance the cooling/lubricating properties of pure MQL and displays significantly good results for machinability. However, the mechanism on compatibility of nanoparticles with cutting fluids has not been explored. In this study, nanoparticles with different hardness and vegetable oils with different viscosity were selected for nanofluids preparation. The end milling experiments were carried out on 7050 material by applying MQL with particularly prepared nanofluids. The cutting force and surface roughness were measured corresponding to the machining performance. The compatibility of hardness of nanoparticles with viscosity of base fluids has been evaluated, and the mechanism has been analyzed by new-designed tribology tests. Results show that canola oil-based diamond nanofluids MQL exhibit the lowest cutting force and natural77 oil-based diamond nanofluids perform the lowest surface roughness with reduction of 10.71 and 14.92%, respectively, compared to dry machining condition. The research is novel and contributes to the machining of such materials at the industry level.

The Competing Effects of Counterface Peaks and Valleys on the Wear and Transfer of Ultra-Low Wear Alumina–PTFE

Abstract: The wear rates of tribological polymers are strongly influenced by counterface roughness through competing mechanisms such as polymer abrasion by counterface asperities and mechanical engagement of debris into the features of the rough surface. Previous studies have shown that an ultra-low wear (k ~ 10−7 mm3/Nm) alumina–PTFE solid lubricant loses wear resistance and the ability to form stable transfer films when the counterface roughness exceeds a critical magnitude or aligns with the sliding direction. In this paper, we aimed to test the independent effects of counterface peaks and valleys on polymer wear and transfer using this well-studied alumina–PTFE system. Wear tests were performed against 304 stainless steel counterfaces of systematically varied peak height and valley depth. Interrupted microscopy measurements were used to record the details of debris engagement, migration, aggregation, and removal. Preferential removal of the tallest peaks on the counterface helped stabilize the transfer films and dramatically reduced the transient wear volume of the polymer composite even on very rough surfaces. The results illustrate the independent and competing effects of counterface peaks, plateaus, and valleys on the wear and transfer of this ultra-low wear polymer composite. While increased peak height promoted primary material removal from the polymer composite and inhibited the formation of the transfer film, intersecting valleys and smooth plateaus helped nucleate and stabilize transfer films. On rough surfaces with tall peaks, the composite eventually achieved ultra-low wear by gradually removing the tallest asperities to achieve a more favorable topography for transfer film formation.

Stability of Metal Matrix Composite Pads During High-Speed Braking

Abstract: Metal matrix composites are now commonly used as braking pads for the train running over 250 km/h by virtue of a number of desirable properties. To develop a fundamental understanding of the stability of metallic composites at high-speed braking, four typical composite materials, with different Cu and Fe contents, were subjected to a series of high-speed emergency braking at a simulative running speed of 380 km/h and a braking inertia of 27 kg/m−2 and a normal pressure of 1.27 MPa in this paper. The results showed that the sample with higher Cu content displayed a fade COF and deteriorated wear, but the one with higher Fe content could maintain a stable COF and low wear rate. The tribological behaviour is associated with the relative rate of generation and consumption of the tribo-oxide film. For the sample with higher Cu content, the generation rate of tribo-oxide film was less than the consumption rate, and the COF fading and wear deterioration with the increasing braking times were attributed to the reduction in resistance to deform or to shear the asperities, which was thought to be caused by the degradation of near-surface layer due to the removal of protective tribo-oxide film. In contrast, for the sample with higher Fe content, the generation rate was approximately equal to the consumption rate, and a well-established tribo-oxide film on the surface was responsible for the stable friction level and low wear rates.

Tribological Characteristics of Aqueous Graphene Oxide, Graphitic Carbon Nitride, and Their Mixed Suspensions

Abstract: The tribological performance of graphene oxide (GO), graphitic carbon nitride (g-C3N4), and their mixed (g-C3N4/GO) aqueous suspensions was investigated The 006 wt% GO, 006 wt% g-C3N4, and 006 wt% 1:1 g-C3N4/GO suspensions reduced the coefficient of friction (COF) by 37, 26 and 37% and wear mark radius by 191, 160 and 196%, respectively, in comparison with water Pure g-C3N4 and GO suspensions showed unstable lubrication in the tests with relatively high loads and speeds, while the g-C3N4/GO mixed suspension had superior tribological performance in all tested conditions This is because in the mixed suspension g-C3N4 agglomerates became smaller, and GO nanosheets exhibited fewer wrinkles and less stacking, which enabled the formation of a layer of tribo-composite film As a result, the friction, wear and tribo-corrosion were reduced during sliding

The Role of the Counterbody’s Oxide on the Wear Behavior of HSS and Hi-Cr

Abstract: The wear behavior of two high-speed steels and a high-chromium cast iron with cryogenic treatments was evaluated using a pin-on-disc configuration. An ASTM A36 steel disc, oxidized previously in a furnace at 950 °C, was used as a counterbody with the goal of developing an accurate representation of the industrial calamine formed on the surface of steels under industrial conditions. This is a new perspective to evaluate the real contribution of the normal load, sliding velocity and heat treatments on the wear phenomena of hot rolls. Optical microscopy, scanning electron microscopy and X-ray diffraction were used to correlate the wear rate, friction coefficient and wear mechanisms to the microstructure, hardness and types of oxides that formed on the counterbody. Experimental results showed a complex relation between the normal load, the sliding velocity and the interaction with counterbody, which establishes the wear rates and friction coefficients.

Thermal desorption analysis of hydrogen in non-hydrogen-charged rolling contact fatigue-tested 100Cr6 steel

Abstract: Hydrogen diffusion during rolling contact fatigue (RCF) is considered a potential root cause or accelerator of white etching cracks (WECs) in wind turbine gearbox bearing steels. Hydrogen entry into the bearing steel during operation is thought to occur either through the contact surface itself or through cracks that breach the contact surface, in both cases by the decomposition of lubricant through catalytic reactions and/or tribochemical reactions of water. Thermal desorption analysis (TDA) using two experimental set-ups has been used to measure the hydrogen concentration in non-hydrogen-charged bearings over increasing RCF test durations for the first time. TDA on both instruments revealed that hydrogen diffused into the rolling elements, increasing concentrations being measured for longer test durations, with numerous WECs having formed. On the other hand, across all test durations, negligible concentrations of hydrogen were measured in the raceways, and correspondingly no WECs formed. Evidence for a relationship between hydrogen concentration and either the formation or the acceleration of WECs is shown in the rollers, as WECs increased in number and severity with increasing test duration. It is assumed that hydrogen diffusion occurred at wear-induced nascent surfaces or areas of heterogeneous/patchy tribofilm, since most WECs did not breach the contact surface, and those that did only had very small crack volumes for entry of lubricant to have occurred.

Theoretical and Finite Element Analysis of Static Friction Between Multi-Scale Rough Surfaces

Abstract: The current work considers the multi-scale nature of roughness in a new model that predicts the static friction coefficient. This work is based upon a previous rough surface contact model, which used stacked elastic–plastic 3-D sinusoids to model the asperities at multiple scales of roughness. A deterministic model of a three-dimensional deformable rough surface pressed against a rigid flat surface is also carried out using the finite element method (FEM). The accuracy of the deterministic FEM model is also considered. At the beginning of contact, which is surface-point contact, the asperities or peaks are isolated, sharp, and the contact areas consist of an inadequate number of elements and sources of error. In this range of contact, the results are not presented as real or accurate. As the normal load increases, the number of the contact elements become larger, and thus, the results become more accurate. That is, the deterministic FEM results are most accurate at high loads. Spectral interpolation is used to smooth the geometry in between the original measured nodes. The effects of normal load and plasticity index on static friction are then analyzed. The results predicted by the theoretical model are also compared to other existing rough surface friction contact models and the FEM results. They are in a good qualitative agreement, especially for higher loads and higher plasticity indices. The FEM model also has significant error, but it is more accurate at higher loads where the proposed multi-scale static friction model and FEM model are in better agreement.

Lubricity of High Water Content Aqueous Gels

Abstract: Aqueous gels such as biopolymer gels, mucus, and high water content hydrogels are often qualitatively described as lubricious. In hydrogels, mesh size, ξ, has been found to be a controlling parameter in friction coefficient. In the tribology of aqueous gels, we suggest that the Weissenberg number (Wi) is a useful parameter to define different regimes, and following the original formulations in rheology, Wi is given by the polymer relaxation time (ηξ3/kBT) multiplied by the shear rate due to fluid shear through a single mesh (V/ξ): Wi = ηVξ2/kBT. At sliding speeds below a Weissenberg number of approximately 0.1, Wi < 0.1, the friction coefficient is velocity-independent and scales with mesh size to the − 1 power, µ ∝ ξ−1. De Gennes’ scaling concepts for elastic modulus, E, give a dependence on polymer mesh size to the − 3 power, E ∝ ξ−3, and following Hertzian contact analysis, the contact area is found to depend on the mesh size squared, A ∝ ξ2. Combining these concepts, the shear stress, τ, and therefore the lubricity of aqueous gels, is predicted to be highly dependent on the mesh size, τ ∝ ξ−3. Studies aimed at elucidating the fundamental mechanism of lubricity in biopolymer gels, mucus, and hydrogels have wrestled with comparisons across mesh size, which can be extremely difficult to accurately quantify. Using scaling concepts relating polymer mesh size to water content reveals that shear stress decreases rapidly with increasing water content, and plots of shear stress as a function of swollen water content are suggested as a useful method to compare aqueous gels of unknown mesh size. As a lower bound, these data are compared against estimates of fluid shear stress for free and bound water flowing through a mesh size estimated by the water content of the gels. The results indicate that the strong dependence on lubricity is likely due to a synergistic combination of a low viscosity solvent (water) coupled to a system that has a decreasing friction coefficient, modulus, and the resulting contact pressure with increasing water content. Although the permeability, K, of aqueous gels increases dramatically with water content (and mesh size), K ≅ ξ2/η, the stronger decrease of the elastic modulus and subsequent decrease in contact pressure due to an increase in the contact length, predicts that the draining time under contact, t, actually increases strongly with increasing water content and mesh size, t ∝ ξ2. Consistent with the finding of extremely high water content aqueous gels on the surfaces of biological tissues, these high water content gels are predicted to be optimal for lubrication as they are both highly lubricious and robust at resisting draining and sustaining hydration.

An In Situ Method for Simultaneous Friction Measurements and Imaging of Interfacial Tribochemical Film Growth in Lubricated Contacts

Abstract: Tribological investigations of macroscopic lubricated sliding contacts are critical for a wide range of industrial applications including automotive engines, gears, bearings, and any other contacting surfaces in relative motion. However, the inability of existing techniques to access buried sliding interfaces with high spatial resolution inhibits the development of fundamental insights into the tribological processes at play. Here we demonstrate a novel and general in situ method, based on atomic force microscopy (AFM), in which micrometer-scale spherical probes are attached to a standard microfabricated AFM cantilever which is then slid over a substrate while immersed in a liquid lubricant. In this case, steel colloidal probes and steel substrates were used, and the contact was immersed in a commercial polyalphaolefin oil with zinc dialkyl dithiophosphate (ZDDP) additive at both room temperature and 100 °C, but the method can be used for a broad range of material combinations, lubricants, and temperatures. We demonstrate that the in situ measurements of friction force and the morphological evolution of the tribochemical films on the substrate can be simultaneously achieved with nanometer-level spatial resolution. In addition, we demonstrate that the sliding zone is readily accessible for further characterization with higher spatial resolution using standard AFM probes with nanometer-scale tip radii. Ex situ characterization of the micrometer-scale probe and the sample is also feasible, which is demonstrated by acquiring high-resolution AFM topographic imaging of the final state of the probe.

Effect of Steel Counterface on the Dry Sliding Behaviour of a Cu-Based Metal Matrix Composite

Abstract: Pin-on-disc testing was used to investigate the friction and wear behaviour of a Cu-based metal matrix composite dry sliding against three different martensitic steels. The tests were carried out at two contact pressures (0.5 and 1 MPa) and two sliding velocities (1.57 and 7 m/s), and the results were explained by considering the characteristics of the friction layers formed on the pin and disc surfaces during sliding. At 7 m/s, pin and disc wear was very mild in every condition, because the high flash temperatures achieved during sliding induced intense oxidation of the disc asperities, irrespective of the steel disc compositions. At 1.57 m/s, the steel composition played an important role. When using a heat-treated steel and a conventional martensitic stainless steel, pin and disc wear was by ‘low-sliding speed tribo-oxidation’, regarded as mild wear. However, when using a martensitic stainless steel with a very high Cr-content and a very low C-content, i.e. by a very high oxidation resistance, pin and disc wear was by adhesion/delamination at 0.5 MPa, and thus severe in nature. The results presented herewith clearly suggest the importance of selecting suitable steel counterfaces in the optimization of the tribological systems tribological involving Cu-based metal matrix composites as a mating material.

Insight into the Viscous and Adhesive Contributions to Hydrogel Friction

Abstract: Investigation of the mechanisms underlying hydrogel lubrication is pivotal in understanding the complexity of biolubrication. In this work, the frictional characteristics of poly(acrylamide) hydrogels with varying composition have been studied over a wide range of sliding velocities and normal loads by colloidal probe lateral force microscopy. The results show that the friction force between the hydrogel and the colloid increases with velocity at sliding velocities above a transition value ( $${V^*}$$ ), while the friction force at slower sliding velocities depends on the composition, and it can either increase or decrease with velocity. Based on the viscoelastic behavior of hydrogels, we model hydrogel friction as the combination of viscous dissipation and the energy dissipated through the rupture of the transient adhesive bridges across the interface. The model parameters depend on relaxation characteristics of the confined polymer network at the interface and on the (bulk) viscoelastic behavior of the hydrogel and are sensitive to the compressive stress. We observe a collapse of the experimental data (at different loads and velocities and for hydrogels with different compositions) in a non-monotonic master curve with a minimum friction force at the transition velocity. Furthermore, a simple relation for the transition velocity $${V^*}$$ is derived from theory, thereby demonstrating the competing effect of both the adhesive and the viscous contributions to friction, which helps to reconcile discrepancies between previous studies of hydrogel friction.

Synergistic Effects of Carbon Nanotube/Nano-MoS 2 Hybrid on Tribological Performance of Polyimide Nanocomposite Films

Abstract: Modified carbon nanotube/nano-MoS2 (CMS) hybrid as a self-lubricating and anti-wear nanofiller was prepared through chemical compounding and then incorporated into polyimide (PI) matrix to yield CMS/PI composite by in situ polymerization. For comparison, carbon nanotube (CNT), nano-MoS2 as well as the mixture of them (CNT-MoS2) were also incorporated into PI matrix separately to evaluate the superior performance of CMS hybrid. Morphology, mechanical capacity and tribological behavior of the as-prepared nanocomposites were investigated, and the discrepancies on the above-mentioned properties caused by the different structures between CMS hybrid and CNT-MoS2 mixture were discussed in detail. With only 0.5 wt% addition of CMS, the friction coefficient and wear rate of CMS/PI composite decreased by 31 and 84%, respectively, compared to virgin PI. The results showed that the combinational structure of CMS hybrid, as CNT-coated few-layer MoS2 nanosheet, which took the advantage of both CNT and nano-MoS2, contributed to the synergistic effect on the tribological properties.

Friction Mechanisms by Carboxylic Acids in Aqueous Lubricants

Abstract: Carboxylic acids are well known for their friction-reducing abilities driven by the formation of low-shear-strength films on the steel surface. However, understanding of the adsorption mechanisms especially in polar solvents is yet not well explored. In this work, atomic force microscopy was used to visualise the adsorption behaviour of various carboxylic acids in both polar and less-polar solvents. The work was continued with a tribological study of the lubricants additivated with carboxylic acids in a laboratory scale ball-on-disc tribometer. During this study, the effect of concentration and carboxylic acid chain length was studied in polar media (water-based lubricants) and compared with commonly used synthetic non-polar lubricant (poly-α-olefin, PAO). It was observed that for both polar and less-polar lubricants, surface coverage of carboxylic acids increased with increasing length of hydrocarbon tail. In less-polar lubricants, carboxylic acids adsorbed to the surface by spreading on it evenly, whereas in polar lubricants, very dense multi-layered formation was promoted. Friction reduction achieved with the use of carboxylic acids in the non-polar lubricant was not as efficient as in the case of the polar lubricant. This was associated with the more pronounced multilayer formation of carboxylic acids in the polar lubricants, facilitating higher friction reduction as compared to the adsorption of carboxylic acids in a dense monolayer form seen in the less-polar lubricant.

Isotope- and Thickness-Dependent Friction of Water Layers Intercalated Between Graphene and Mica

Abstract: The lubricating properties of water have been discussed extensively for millennia. Water films can exhibit wearless high friction in the form of cold ice, or act as lubricants in skating and skiing when a liquid. At the fundamental level, friction is the result of a balance between the rate of energy generation by phonon excitation during sliding and drainage of the energy from the interface by coupling with bulk atoms. Using atomic force microscopy, we found that when H2O intercalates between graphene and mica, it increases the friction between the tip and the substrate, dependent on the thickness of the water and graphene layers, while the magnitude of the increase in friction was reduced by D2O intercalation. With the help of first-principles density functional theory calculations, we explain this unexpected behavior by the increased spectral range of the vibration modes of graphene caused by water, and by better overlap of the graphene vibration modes with mica phonons, which favors more efficient energy dissipation. The larger increase in friction with H2O versus D2O shows that the high-frequency vibration modes of the water molecules play a very important role in the transfer of the vibrational energy of the graphene to the phonon bath of the substrate.

A New Method for the Measurement of Real Area of Contact by the Adhesive Transfer of Thin Au film

Abstract: In this study, a new experimental method is proposed to measure the real area of contact between a ceramic sphere and an Al surface based on the adhesive transfer of the Au film and the scanning electron microscope (SEM) in the back-scattered mode. A thin film of Au is sputtered on the ceramic sphere before the indentation with the Al surface. The success of this method relies on the fundamental assumption that the adhesive transfer of Au only occurs everywhere inside the contact area. A thin polymer (PMMA) film is deposited between gold film and the ceramic surface to further reduce adhesive strength. After indentation, the interfaces of the ceramic sphere and Al surface are observed by SEM. Experimental evidence that the adhesive transfer of the Au film occurs inside the contact area is given. The entire contact regions on the ceramic sphere and the Al surface are captured in the second electron and back-scattered images with a magnification of 220× (resolution: 432 nm, i.e., distance between neighboring pixels). The contact area can be identified based on both the distributions of the ceramic and Au on the ceramic sphere and Al surface, respectively. The back-scattered images with the magnifications of 5000× and 10,000× (resolution: 20 and 4 nm) are captured at four different locations along the radial direction (starting from the contact center), respectively. The real area of contact decreases from the center to the contact edge.

On the Modeling of Adhesive Wear with Consideration of Loading Sequence

Abstract: Established models for predicting adhesive wear in sliding tribo-systems are based on the assumption that applied load remains constant However, in many applications, load rarely remains constant—changing from low-to-high and high-to-low, or always fluctuating Hence, the applicability of the existing wear models for prediction purposes becomes questionable In the present work, an attempt is made to develop a wear model using the principles of thermodynamics to address this drawback The proposed wear model establishes a relationship between the so-called degradation coefficient, load-dependent friction force, and the contact temperature Efficacy of the proposed wear model is demonstrated by considering four cases of published data with variable loading sequence as well as additional series of ball-on-disk experiments conducted as a part of this study to validate the theory A detailed explanation is provided to illustrate how a constant degradation coefficient value could predict the cumulative wear volume in applications dealing with variable loading while the Archard wear coefficient could not

Investigating Corrosion Performance and Corrosive Wear Behavior of Sol–gel/MAO-Coated Mg Alloy

Abstract: In this study, a hydroxyapatite composite coating was prepared by a sol–gel technique on the micro-arc oxidation (MAO)-coated AZ31 Mg alloy to seal the micro-pores. The composite coating achieved a larger hardness value and two times thickness more than pure MAO coating. The corrosion and wear resistance of the sol–gel/MAO coating in simulated body fluid were investigated compared to MAO coating. It was found that the composite coating presented a positive corrosion potential and a lower corrosion current density than MAO coating. The sol–gel/MAO composite coating could provide more effective barrier against corrosive ions than single MAO coating for AZ31 alloy. In the wear tests, a ball-on-disk tribometer was used to study the effect of loads on the wear properties of the coatings at 37 °C. The wear resistance of sol–gel/MAO composite coatings was apparently superior to MAO coating. The wear mechanisms of abrasion and adhesion in composite coatings are investigated. Finally, two physical models for the corrosion and sliding wear mechanisms of sol–gel/MAO composite coatings are proposed, respectively.

Measuring Evolution of Transfer Film–Substrate Interface Using Low Wear Alumina PTFE

Abstract: Polymeric solid lubricants lay down their own wear debris onto hard metallic counterfaces to form a protective transfer film which reduces friction and wear effectively without lubrication Adhesive shear strength at the hidden interface between the film and substrate determines the film persistence and correlates with system wear qualitatively Previous studies showed that an ultralow wear (k ~ 10− 7 mm3/Nm) alumina-PTFE solid lubricant forms an extremely adherent and complete transfer film, and strong chemical bonds between wear debris and counterface perpetuate the film–substrate adhesion very early in the sliding In this paper, we aimed to test the permanence of such adhesion by removing pre-developed transfer films using sliding rubber contact and measuring the topographical evolution of the interface throughout the course of a standard wear test using the well-studied alumina-PTFE system The results unexpectedly showed continuous wear of the counterface across the wear track, and counterface wear rate decreased proportionally from 3 × 10− 7 to 3 × 10− 8 mm3/Nm with increased film area fraction and sliding distance A proposed rule-of-mixtures wear model coincided closely with the experimental results and strongly suggested a coupled mechanism of adhesive and fatigue wear of the counterface The upper limit of the interfacial counterface fatigue wear rate was predicted to be 3 × 10− 8 mm3/Nm

Molecular Dynamics Simulation Study of Mechanical Effects of Lubrication on a Nanoscale Contact Process

Abstract: Using molecular dynamics simulation, we study the effect of a lubricant on indentation and scratching of a Fe surface By comparing a dry reference case with two lubricated contacts—differing in the adsorption strength of the lubricant—the effects of the lubricant can be identified We find that after an initial phase, in which the lubricant is squeezed out of the contact zone, the contact between the indenter and the substrate is essentially dry The number of lubricant molecules confined in the tip-substrate gap increases with the lubricant adsorption energy Trapped lubricant broadens the tip area active in the scratching process—mainly on the flanks of the groove—compared to a dry reference case This leads to a slight increase in chip height and volume, and also contributes to the scratching forces

Experimental and Numerical Study of Micropitting Initiation in Real Rough Surfaces in a Micro-elastohydrodynamic Lubrication Regime

Abstract: Micropitting is a form of surface fatigue damage that happens at the surface roughness scale in lubricated contacts in commonly used machine elements, such as gears and bearings. It occurs where the specific film thickness (ratio of smooth surface film thickness to composite surface roughness) is sufficiently low for the contacts to operate in the mixed lubrication regime, where the load is in part carried by direct asperity contacts. Micropitting is currently seen as a greater issue for gear designers than is regular pitting fatigue failure as the latter can be avoided by control of steel cleanliness. This paper describes the results of both theoretical and experimental studies of the onset of micropitting in test disks operated in the mixed lubrication regime. A series of twin disk mixed-lubrication experiments were performed in order to examine the evolution of micropitting damage during repeated cyclic loading of surface roughness asperities as they pass through the contact. Representative measurements of the surfaces used in the experimental work were then evaluated using a numerical model which combines a transient line contact micro-elastohydrodynamic lubrication (micro-EHL) simulation with a calculation of elastic sub-surface stresses. This model generated time-history of stresses within a block of material as it passes through the contact, based on the instantaneous surface contact pressure and traction at each point in the computing mesh at each timestep. This stress time-history was then used within a shear-strain-based fatigue model to calculate the cumulative damage experienced by the surface due to the loading sequence experienced during the experiments. The proposed micro-EHL model results and the experimental study were shown to agree well in terms of predicting the number of loading cycles that are required for the initial micropitting to occur.