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Atomically thin, mechanically flexible, memory‐functional, and power‐generating crystals play a crucial role in the technological advancement of portable devices. However, the adoption of these crystals in such technologies is sometimes impeded by expensive and laborious synthesis methods, as well as the need for large‐scale, mechanically stable, and air‐stable materials. Here, an instant‐in‐air liquid metal printing process utilizing liquid bismuth (Bi) is presented, forming naturally occurring, air‐stable, atomically thin, mechanically flexible nanogenerators and ferroelectric oxides. Despite the centrosymmetric nature of the monoclinic P21/c system of achieved α‐Bi2O3‐δ the high kinetics of liquid metal synthesis leads to the formation of vacancies that disrupt the symmetry which is confirmed by density functional theory (DFT) calculations. The polarization switching is measured and utilized for ferroelectric nanopatterning. The exceptional attributes of these atomically thin multifunctional stable oxides, including piezoelectricity, mechanical flexibility, and polarizability, present significant opportunities for developing nano‐components that can be seamlessly integrated into a wide range of devices. [ABSTRACT FROM AUTHOR]

In applications such as flexible electronic devices, graphene, and other 2D materials are frequently in contact with stretchable polymeric substrates. The interface between 2D materials and polymers is dominated by weak van der Waals forces and can eventually degrade due to the frequent dynamic mechanical loads that these devices experience. This can lead to significant local delamination and shear fracture of the 2D materials. Using the polydimethylsiloxane (PDMS) encapsulation method, it is shown that the damage in graphene is significantly mitigated when it is capped during dynamic loading. To track the spread of damage in both encapsulated and nonencapsulated graphene, in situ, cyclic loading is performed. The fundamental process driving this substantial reduction in damage propagation in the 2D lattice is explained by the conventional shear lag model. It is also observed that softer PDMS substrate and capping layer completely mitigate the fatigue damage in graphene for 100 cycles at 10% applied fatigue strain. [ABSTRACT FROM AUTHOR]

Piezoelectric materials convert between mechanical and electrical energy and are a basis for self-powered electronics. Current piezoelectrics exhibit either large charge (d33) or voltage (g33) coefficients but not both simultaneously, and yet the maximum energy density for energy harvesting is determined by the transduction coefficient: d33*g33. In prior piezoelectrics, an increase in polarization usually accompanies a dramatic rise in the dielectric constant, resulting in trade off between d33 and g33. This recognition led us to a design concept: increase polarization through Jahn-Teller lattice distortion and reduce the dielectric constant using a highly confined 0D molecular architecture. With this in mind, we sought to insert a quasi-spherical cation into a Jahn-Teller distorted lattice, increasing the mechanical response for a large piezoelectric coefficient. We implemented this concept by developing EDABCO-CuCl4 (EDABCO = N-ethyl-1,4-diazoniabicyclo[2.2.2]octonium), a molecular piezoelectric with a d33 of 165 pm/V and g33 of ~2110 × 10−3 V m N−1, one that achieved thusly a combined transduction coefficient of 348 × 10−12 m3 J−1. This enables piezoelectric energy harvesting in EDABCO-CuCl4@PVDF (polyvinylidene fluoride) composite film with a peak power density of 43 µW/cm2 (at 50 kPa), the highest value reported for mechanical energy harvesters based on heavy-metal-free molecular piezoelectric. Here, the authors utilise a combination of quasi-spherical theory and Jahn-Teller distortion to enhance the piezoelectric response of molecular metal halides, and the resulting piezoelectric energy harvesters exhibit superior power densities to the best-reported molecular hybrid energy harvesters. [ABSTRACT FROM AUTHOR]

Due to the strong in-plane but weak out-of-plane bonding, it is relatively easy to separate nanosheets of two-dimensional (2D) materials from their respective bulk crystals. This exfoliation of 2D materials can yield large 2D nanosheets, hundreds of micrometers wide, that can be as thin as one or a few atomic layers thick. However, the underlying physical mechanisms unique to each exfoliation technique can produce a wide distribution of defects, yields, functionalization, lateral sizes, and thicknesses, which can be appropriate for specific end applications. The five most commonly used exfoliation techniques include micromechanical cleavage, ultrasonication, shear exfoliation, ball milling, and electrochemical exfoliation. In this review, we present an overview of the field of 2D material exfoliation and the underlying physical mechanisms with emphasis on progress over the last decade. The beneficial characteristics and shortcomings of each exfoliation process are discussed in the context of their functional properties to guide the selection of the best technique for a given application. Furthermore, an analysis of standard applications of exfoliated 2D nanosheets is presented including their use in energy storage, electronics, lubrication, composite, and structural applications. By providing detailed insight into the underlying exfoliation mechanisms along with the advantages and disadvantages of each technique, this review intends to guide the reader toward the appropriate batch-scale exfoliation techniques for a wide variety of industrial applications. [ABSTRACT FROM AUTHOR]

In the present study, a relatively novel non-destructive testing (NDT) method called the coplanar capacitive sensing technique was applied in order to detect different sizes of rebars in a reinforced concrete (RC) structure. This technique effectively detects changes in the dielectric properties during scanning in various sections of concrete with and without rebars. Numerical simulations were carried out by three-dimensional (3D) finite element modelling (FEM) in COMSOL Multiphysics software to analyse the impact of the presence of rebars on the electric field generated by the coplanar capacitive probe. In addition, the effect of the presence of a surface defect on the rebar embedded in the concrete slab was demonstrated by the same software for the first time. Experiments were performed on a concrete slab containing rebars, and were compared with FEM results. The results showed that there is a good qualitative agreement between the numerical simulations and experimental results. [ABSTRACT FROM AUTHOR]

Molybdenum disulfide coatings have been employed as lubricants for spacecraft since the 1950s but continue to face major engineering challenges including performance in both terrestrial air and deep space vacuum environments and service lifetimes on the order of decades without maintenance. Co‐deposition of MoS2 with additive compounds provide enhancements in some circumstances but a lubricant which can perform in all space‐facing environments with long lifetimes remains an ongoing problem. Herein, it is demonstrated the multi‐environment adaptable performance of a novel MoS2 + tantalum lubricant coating, which excels as a lubricant in both terrestrial and space environments while the benchmark space‐qualified commercial MoS2 lubricants do not. It is noted that the 10% tantalum additive exhibits preferential oxidation in air to preserve the lubricating ability of MoS2 while forming phases of TaS2, which aid in the exceptional lubrication of MoS2 in ultra‐high vacuum. Additionally, completely different tribofilms of small particles and compact sheets are noted for air and vacuum environments, respectively, which allows for adaptable lubricating mechanisms from a single coating depending on the environment. This novel coating sets the benchmark as the first demonstrated instance of a fully versatile space lubricant which offers high‐performance in both terrestrial and deep space environments. [ABSTRACT FROM AUTHOR]

One of the most common methods for performing non-destructive testing (NDT) of the steel tank floors in aboveground storage tanks is dc magnetic flux leakage (MFL). This test method gives an estimate of the defect depth and width based on the MFL signal strength and its peak location, respectively. A key limitation is that the signal strength depends on not only the defect depth and width but also various other factors including a defect’s wall profile and sensor lift-off. Moreover, in a practical MFL test, the sensor lift-off changes due to surface roughness and uneven steel plate surface in tank floors. We present an ac MFL system to increase the accuracy of defect characterization by: 1) distinguishing between two common defect shapes located on the far side of the steel plate used in a typical above storage tank (AST) floor: lake-shaped and rectangular defects and 2) developing a sensor lift-off compensation scheme based on ac signal phase. Simulation results show that skewness of the ac MFL can be used to distinguish between far-side lake-shaped and rectangular defects. AC MFL signal phase is shown to be a suitable compensator for the dependence of signal strength on unintended variations in sensor lift-off. The simulation results have been validated through experiments. [ABSTRACT FROM AUTHOR]

The coplanar capacitive sensing method is considered a relatively novel electromagnetic technique in NDT. The size of the electrodes and the separation distance between them significantly affect the penetration depth, electric field strength, and sensitivity of the measurement. To achieve different penetration depths and electric field strengths, separate sensors with various sizes $/$ separations can be employed which is time-consuming. In order to reduce the costs and time, a multi-electrode sensor is designed and introduced in this paper. This type of sensor uses coplanar electrodes to generate the multiple fringing electric field to inspect the specimen. In this sensor, it is possible to have multiple driving and sensing electrodes at the same time, which provides a variety of different sizes and separation distances leading to various depths of penetration and electric field strengths. In the framework of this paper, the principle of the multi-electrode coplanar capacitive probe was explained. A 3D Finite Element Modelling (FEM) was employed to simulate and illustrate the electric field distribution from a multi-electrode coplanar capacitive sensor with different arrangements of driving and sensing electrodes, and how this field may be altered by changing the arrangement. The multi-electrode probe was manufactured and several sets of experiments were conducted under various conditions. The Measurement Sensitivity Distribution (MSD) was applied to characterise and interpret the variable output from the physical scan of the specimen. The case study on an acrylic sample comprised of defects with different depths demonstrated the feasibility of defect discrimination using a multi-electrode capacitive sensor. [ABSTRACT FROM AUTHOR]

This work combines experimental atomic force microscopy (AFM) and density functional theory (DFT) simulations to study oxidized-metal (oxidized copper & titanium) and 2D-material (graphene & MoS2) interfaces. Combining AFM and DFT allowed identifying the interfacial interaction and established a correlation between tribological behavior, interfacial charge distribution, and variations in the potential energy profile with sliding along the metal/2D-materials interfaces. The TiO2 (rutile) and CuO (cupric oxide) metal oxides were mostly found to chemisorb along the interface with the 2D-materials. Both the metal-oxide counter-surfaces (TiO2 and CuO) exhibited higher friction force and adhesion on graphene than on MoS2. The CuO surface was inferred to be copper rich based on comparison with DFT simulations. The interfacial electronic charge distribution and relative energy change were identified to strongly influence sliding and adhesive behavior between oxidized-metal/2D-material contacts when considering only electronic effects in the DFT simulations. More homogenous interfacial charge distribution/sharing and lower surface energy variation, as found on the MoS2 surfaces, were identified to lower friction and adhesion. Non-electronic effects not captured by simulations were found to likely dominate interfacial shear strength measurements experimentally. Therefore, MoS2 should be used in interfacial applications involving TiO2 and copper-rich CuO surfaces requiring lower adhesion and friction. [ABSTRACT FROM AUTHOR]

Many of the best-performing perovskite photovoltaic devices make use of 2D/3D interfaces, which improve efficiency and stability – but it remains unclear how the conversion of 3D-to-2D perovskite occurs and how these interfaces are assembled. Here, we use in situ Grazing-Incidence Wide-Angle X-Ray Scattering to resolve 2D/3D interface formation during spin-coating. We observe progressive dimensional reduction from 3D to n = 3 → 2 → 1 when we expose (MAPbBr3)0.05(FAPbI3)0.95 perovskites to vinylbenzylammonium ligand cations. Density functional theory simulations suggest ligands incorporate sequentially into the 3D lattice, driven by phenyl ring stacking, progressively bisecting the 3D perovskite into lower-dimensional fragments to form stable interfaces. Slowing the 2D/3D transformation with higher concentrations of antisolvent yields thinner 2D layers formed conformally onto 3D grains, improving carrier extraction and device efficiency (20% 3D-only, 22% 2D/3D). Controlling this progressive dimensional reduction has potential to further improve the performance of 2D/3D perovskite photovoltaics. Many best-performing perovskite photovoltaics use 2D/3D interfaces to improve efficiency and stability, yet the mechanism of interface assembly is unclear. Here, Proppe et al. use in-situ GIWAXS to resolve this transformation, observing progressive dimensional reduction from 3D to 2D perovskites. [ABSTRACT FROM AUTHOR]

It was recently demonstrated that a coplanar capacitive sensor could be applied to the evaluation of materials without the disadvantages associated with the other techniques. This technique effectively detects changes in the dielectric properties of the materials due to, for instance, imperfections or variations in the internal structure, by moving a set of simple electrodes on the surface of the specimen. An AC voltage is applied to one or more electrodes and signals are detected by others. This is a promising inspection method for imaging the interior structure of the numerous materials, without the necessity to be in contact with the surface of the sample. In this paper, finite element (FE) modeling was employed to simulate the electric field distribution from a coplanar capacitive sensor and the way it interacts with a nonconducting sample. Physical experiments with a prototype capacitive sensor were also performed on a Plexiglas sample with subsurface defects, to assess the imaging performance of the sensor. A good qualitative agreement was observed between the numerical simulation and experimental result. [ABSTRACT FROM AUTHOR]

Lithium metal is regarded as the "Holy Grail" of anode materials due to its low electrochemical potential and high theoretical capacity. Unfortunately, its unstable solid electrolyte interphase (SEI) leads to low Coulombic efficiency (CE) and serious safety issues. Herein, a hybrid nanoscale polymeric protective film with tunable composition and improved stiffness is developed by incorporating aluminum crosslinkers into the polymer chains. The Li plating/stripping process is regulated through the protective coating and the dendrite growth is effectively suppressed. Promisingly, the protected Li can deliver stable performance for more than 350 h with a cycling capacity of 2 mAh cm−2 without a notable increase in overpotential. Moreover, a stable charge/discharge cycling in Li–O2 batteries with the protected Li can be maintained for more than 600 h. This work provides guidance on the rational design of electrode interfaces and opens up new opportunities for the fabrication of next‐generation energy storage systems. [ABSTRACT FROM AUTHOR]

Sputter‐deposited molybdenum disulfide coatings are one of the most common lubricants for extreme environments. However, their performance predictability remains limited by the complexity of van der Waals wear and shear mechanics in bulk materials resulting in unexpected premature failure. In the present study, two nanostructured MoS2 coatings of similar macroscopic properties are shown to exhibit entirely different wear and shear mechanics due to their nanostructure. Friction force microscopy with steel‐beaded cantilevers is used to measure the per‐cycle evolution of friction, wear, and topography in situ over the lubricant lifetime under an inert nitrogen environment. Molecular dynamics simulations confirm the subsurface structural failure mechanisms of the coatings under shear stress, and atomic force microscope phase imaging and Raman spectroscopy are used to identify tribofilm formation mechanics. The nanocrystal–amorphous composite structure shows improved wear resistance but at the cost of limited stress relaxation which creates high‐stress failure and fracture‐dominated wear. The purely nanocrystalline coating exhibits lower shear resistance but consistent stress relaxation by van der Waals cleavage and triple junction fracture which results in higher wear rates with predictable abrasion‐dominated failure. The contrast in nanoscale performance of the coatings allows for the lubricant nanostructure to be tuned for ideal applications for extreme environments. [ABSTRACT FROM AUTHOR]

Photothermal excitation of the cantilever for use in subsurface imaging with atomic force microscopy was compared against traditional piezoelectric excitation. Photothermal excitation alleviates issues commonly found in traditional piezoelectrics such as spurious resonances by producing clean resonance peaks through direct cantilever excitation. A calibration specimen consisting of a 3 × 3 array of holes ranging from 200 to 30 nm etched into silicon and covered by graphite was used to compare these two drive mechanisms. Photothermal excitation exhibited a signal-to-noise ratio as high as four times when compared to piezoelectric excitation, utilizing higher eigenmodes for subsurface imaging. The cleaner and sharper resonance peaks obtained using photothermal excitation revealed all subsurface holes down to 30 nm through 135 nm of graphite. In addition, we demonstrated the ability of using photothermal excitation to detect the contact quality variation and evolution at graphite–polymer interfaces, which is critical in graphene-based nanocomposites, flexible electronics, and functional coatings. [ABSTRACT FROM AUTHOR]

One of the most common methods for performing non-destructive testing in steel tank floors is DC magnetic flux leakage (MFL). The magnetic dipole method is the most widely used mathematical technique to predict the MFL from defects in such structures. However, due to the complexity of an exact analytical description of an MFL system, researchers often make coarse approximations for the profile of the magnetic surface charge density $\sigma _{m}$ , orientation of the magnetic field H, and variation of relative permeability $\mu _{r}$. In this paper, the validity of these approximations is evaluated for 2-D rectangular defects in a steel plate, by comparing model predications with finite element results. The primary sources of deviation between the approximate solutions and true MFL profiles were found to be caused by assumptions that 1) $\sigma _{m}$ on the specimen surface adjacent to a flaw is zero. This assumption is equivalent to treating the orientation of H to be parallel to the specimen surface, even at locations in close proximity to a flaw and 2) local variation in permeability around the defect can be ignored. This approximation was found to cause an underestimation of $\sigma _{m}$ and, consequently, the predicted MFL. In contrast, approximating $\sigma _{m}$ to be zero at the bottom of a flaw, and approximating uniform distribution for $\sigma _{\mathbf {m}}$ on the vertical defect sides of a slot defect was found to generate only minor errors in an estimate of flux leakage. [ABSTRACT FROM AUTHOR]

A method for calibrating the dynamic torsional spring constant of cantilevers by directly measuring the thermally driven motion of the cantilever with an interferometer is presented. Random errors in calibration were made negligible (<1%) by averaging over multiple measurements. The errors in accuracy of ±5% or ±10% for both of the cantilevers calibrated in this study were limited only by the accuracy of the laser Doppler vibrometer (LDV) used to measure thermal fluctuations. This is a significant improvement over commonly used methods that result in large and untraceable errors resulting from assumptions made about the cantilever geometry, material properties, and/or hydrodynamic physics of the surroundings. Subsequently, the static torsional spring constant is determined from its dynamic counterpart after careful LDV measurements of the torsional mode shape, backed by finite element analysis simulations. A meticulously calibrated cantilever is used in a friction force microscopy experiment that measures the friction difference and interfacial shear strength (ISS) between graphene and a silicon dioxide AFM probe. Accurate calibration can resolve discrepancies between different experimental methods, which have contributed to a large scatter in the reported friction and ISS values in the literature to date. [ABSTRACT FROM AUTHOR]

Extracellular forces transmitted through the cytoskeleton can deformthe cell nucleus. Large nuclear deformations increase the risk of disrupting the integrity of the nuclear envelope and causing DNA damage. The mechanical stability of the nucleus defines its capability to maintain nuclear shape by minimizing nuclear deformation and allowing strain to be minimized when deformed. Understanding the deformation and recovery behavior of the nucleus requires characterization of nuclear viscoelastic properties. Here, we quantified the decoupled viscoelastic parameters of the cell membrane, cytoskeleton, and the nucleus. The results indicate that the cytoskeleton enhances nuclear mechanical stability by lowering the effective deformability of the nucleus while maintaining nuclear sensitivity to mechanical stimuli. Additionally, the cytoskeleton decreases the strain energy release rate of the nucleus and might thus prevent shape change-induced structural damage to chromatin. [ABSTRACT FROM AUTHOR]

Extracellular forces transmitted through the cytoskeleton can deform the cell nucleus. Large nuclear deformation increases the risk of disrupting the nuclear envelope's integrity and causing DNA damage. Mechanical stability of the nucleus defines its capability of maintaining nuclear shape by minimizing nuclear deformation and recovering strain when deformed. Understanding the deformation and recovery behavior of the nucleus requires characterization of nuclear viscoelastic properties. Here, we quantified the decoupled viscoelastic parameters of the cell membrane, cytoskeleton, and the nucleus. The results indicate that the cytoskeleton enhances nuclear mechanical stability by lowering the effective deformability of the nucleus while maintaining nuclear sensitivity to mechanical stimuli. Additionally, the cytoskeleton decreases the strain energy release rate of the nucleus and might thus prevent shape change-induced structural damage to chromatin. [ABSTRACT FROM AUTHOR]

The relative merits of three different periodic permanent magnet (PPM) configurations are investigated for the generation of SH0 waves with a Lorentz-force electromagnetic acoustic transducer (EMAT) on a 60-mm diameter aluminum pipe. Such transducers are often used for checking the integrity of pipelines. It was found that minimizing lift-off by using magnets machined to the same curvature as the pipe increased the magnitude of the generated wave by over 70%, compared to the use of a PPM with a single planar surface. A less expensive and more versatile alternative is to use a PPM for which the two lines of magnets are angled with respect to each other to realize closer average proximity to the pipe surface. Numerical simulations and experiments indicated that the amplitude of a SH0 wave with this magnet configuration would still be approximately 60% higher than the reference case of a PPM with a single planar surface. [ABSTRACT FROM AUTHOR]

We investigated electrically conductive nanocomposites made of thermoplastic vulcanizates (TPVs) and multiwalled carbon nanotubes (CNTs) that exhibit highly enhanced stretchability, low electrical percolation threshold, and improved electromechanical durability after cyclic loading. The TPV/CNT nanocomposites were fabricated by compounding pre-vulcanized rubber (PVR) fine particles with a maleic anhydride grafted polyethylene (MA-g-PE)/CNT compound. Our microstructural and morphological investigations showed that using PVR particles, rather than their more common virgin elastomer counterparts, locked the carbon nanotubes in the MA-g-PE phase. This guaranteed the formation of a segregated structure. Furthermore, it was confirmed that the chemical bonding forms between the PVR particles and the MA-g-PE matrix produced an excellent interfacial adhesion between the two phases. This engineered structure increased the TPV/CNT nanocomposites’ stretchability by 300%. Meanwhile their electrical percolation threshold was decreased by ∼50%, when compared with their MA-g-PE/CNT counterparts. Interestingly, the cyclic electromechanical properties were also improved, suggesting the nanocomposites’ great potential for flexible and stretchable electromechanical applications. The mechanisms linking the microstructure and their consequent characteristics were also discussed. Such property combinations can be extremely beneficial in flexible electronics, soft robotics, and health monitoring devices. [ABSTRACT FROM AUTHOR]

The tribological behavior of dry lubricants depends on their mechanical and physicochemical environment, making it difficult to predict in practice. Discrete Element Method-based modeling has been one successful approach to provide valuable insight into the tribology of dry lubricated contacts. However, it requires well-defined interactions between discrete elements, in particular between those simulating different materials. Measuring the properties governing those interactions, such as the work of adhesion ( W), is therefore critical. The present work describes a method for measuring the W between AISI440C steel and MoS2-based coatings used in spacecraft. Using Atomic Force Microscopy local asperity and adhesion measurements, the W between steel microbeads and MoS2 coatings is determined at different stages in its wear life. The distributions of W values in the worn coatings and pristine coatings agree well with earlier Time-of-Flight Secondary Ion Mass Spectroscopy studies on the physicochemistry of the samples, as well as contact angle measurements. Additional measurements between the same materials on a ball bearing from a real life-test unit of a spacecraft instrument also show a similar W distribution, suggesting that the approach used here provides relevant data for use in numerical simulations. [ABSTRACT FROM AUTHOR]

High Performance Space Lubrication of MoS2 with Tantalum (Adv. Funct. Keywords: lubricant; molybdenum disulfide; space; tantalum; ultra-high vacuum EN lubricant molybdenum disulfide space tantalum ultra-high vacuum 1 1 1 05/17/22 20220513 NES 220513 B Space Lubricants b In article number 2110429, Aurélien Saulot, Tobin Filleter, Guillaume Colas, and co-workers engineer a high-performance space lubricant for the extreme environments faced by spacecrafts. Lubricant, molybdenum disulfide, space, tantalum, ultra-high vacuum. [Extracted from the article]

Atomically thin films, such as graphene, graphene oxide, hexagonal-boron nitride (h-BN), and molybdenum disulfide (MoS2), have attracted intensive studies to explore their properties and potential applications as next generation materials due to their outstanding mechanical, electrical, thermal, and optical properties. The study of the mechanical behavior of this class of materials is in particular interesting as it not only physically determines the potential application fields where these materials can be utilized but also has revealed unique mechanical size effects and phenomena. Researchers have been studying the mechanical properties such as elastic modulus, strength, friction, and fracture behavior of atomically thin films for over a decade now. Here, we review recent results of the mechanical characterization and understanding of this class of materials. [ABSTRACT FROM AUTHOR]

The production of carbon nanotube (CNT) yarns possessing high strength and toughness remains a major challenge due to the intrinsically weak interactions between 'bare' CNTs. To this end, nanomechanical shear experiments between functionalized bundles of CNTs are combined with multiscale simulations to reveal the mechanistic and quantitative role of nanotube surface functionalization on CNT-CNT interactions. Notably, the in situ chemical vapor deposition (CVD) functionalization of CNT bundles by poly(methyl methacrylate) (PMMA)-like oligomers is found to enhance the shear strength of bundle junctions by about an order of magnitude compared with 'bare' van der Waals interactions between pristine CNTs. Through multiscale simulations, the enhancement of the shear strength can be attributed to an interlocking mechanism of polymer chains in the bundles, dominated by van der Waals interactions, and stretching and alignment of chains during shearing. Unlike covalent bonds, such synergistic weak interactions can re-form upon failure, resulting in strong, yet robust fibers. This work establishes the significance of engineered weak interactions with appropriate structural distribution to design CNT yarns with high strength and toughness, similar to the design paradigm found in many biological materials. [ABSTRACT FROM AUTHOR]

We review recent friction measurements on ordered superstructures performed by atomic force microscopy. In particular, we consider ultrathin KBr films on NaCl(001) and Cu(001) surfaces, single and bilayer graphene on SiC(0001), and the herringbone reconstruction of Au(111). Atomically resolved friction images of these systems show periodic features spanning across several unit cells. Although the physical mechanisms responsible for the formation of these superstructures are quite different, the experimental results can be interpreted within the same phenomenological framework. A comparison between experiments and modeling shows that, in the cases of KBr films on NaCl(001) and of graphene films, the tip-surface interaction is well described by a potential with the periodicity of the substrate which is modulated or, respectively, superimposed with a potential with the symmetry of the superstructure. [ABSTRACT FROM AUTHOR]

Atomically flat and clean metal surfaces exhibit a regime of ultra-low friction at low normal loads. Atomic force microscopy, performed in ultra-high vacuum on Cu(100) and Au(111) surfaces, reveals a clear stick-slip modulation in the lateral force but almost zero dissipation. Significant friction is observed only for higher loads (∼4–6 nN above the pull-off force) together with the onset of wear. We discuss the minor role of thermal activation in the low friction regime and suggest that a compliant metallic neck between tip and surface is formed which brings upon the low, load-independent shear stress. [ABSTRACT FROM AUTHOR]

The periodic permanent magnet electromagnetic acoustic transducer (PPM EMAT) is a sensor that can generate and receive shear horizontal (SH) waves without direct contact with the inspected medium using the Lorentz mechanism. However, the PPM EMAT experiences high signal variance on ferromagnetic steel under specific conditions, such as a change in signal amplitude when the sensor is moved in the direction of SH wave propagation. Magnetostriction effects are hypothesized to be the cause of these anomalous behaviors; the objective of this paper is to determine the relative strengths of the magnetostriction and Lorentz wave generation mechanisms for this type of EMAT on steel. This goal is accomplished through the use of a second EMAT, which induces only magnetostriction (MS-EMAT), to calibrate a novel semi-empirical magnetostriction model. It is found that magnetostriction effects reduce the amplitude of the SH wave generated by this particular PPM EMAT transmitter by an average of 29% over a range of input currents. It is also determined that magnetostriction is significant only in the investigated PPM EMAT transmitter, not the receiver. In terms of practical application, it is shown that the MS-EMAT is less sensitive to changes in the static and dynamic fields than PPM EMATs at specific operating points; this makes the MS-EMAT a viable alternative for nondestructive evaluation despite lower amplitudes. [ABSTRACT FROM AUTHOR]

Lubricating space mechanisms are a challenge. Lubrication must be sustained in different environments, for a very long period of time, and without any maintenance possible. This study focuses on the self-lubricating composite used in the double transfer lubrication of ball bearing. Ball/races contacts are lubricated via the transfer of materials from the cage that is made of the composite. A dedicated tribometer has been designed for the study. A specificity of the tribometer is to not fully constrain the composite sample but to let it move, as the cage would do in the bearing. Four composites (PTFE, MoS2, glass or mineral fibers) where tested in ultrahigh vacuum and humid air environments. Transfer was achieved with morphologies and composition similar to what is observed on real bearings. Adhesion measurements performed on composite materials before and after friction allowed one to explain the differences in tribological behaviors observed (transfer quality and contact instabilities). Beyond strengthening the composites, fibers are shown to be critical in trapping mechanically and chemically the transferred material to lubricate and prevent instabilities. Equilibrium between internal cohesion of transferred material, and adhesion to counterparts must be satisfied. Mass spectrometry showed that water appears also critical in the establishment of stable transfer film, even in vacuum. [ABSTRACT FROM AUTHOR]

Molybdenum disulfide is one of the most common lubricant coatings for space systems but it displays enormous susceptibility to environmental conditions making it hard to predict performance throughout the entire lifetime. The majority of mechanisms for space operate in low Earth orbit where temperatures typically reach 120 °C along with exposure to highly reactive atomic oxygen which can be detrimental to lubricant performance. In the present study, a MoS2 lubricant coating is tested using friction force microscopy under different environmental conditions including air and dry nitrogen environments with temperatures ranging from 25 °C to 120 °C. The increased temperature was found to be beneficial for friction behaviour in air up to 100 °C as ambient humidity is removed from the contact, but higher temperatures become detrimental as increased reactivity leads to oxidation. These competing effects resulted in a minimum coefficient of friction at 110 °C in the air environment. The high temperature also increases the wear of the coatings as the intrinsic shear strength decreases with thermal energy which in turn disrupts tribofilm formation leading to increased friction. The run-in duration and magnitude are both found to decrease with temperature as the energy barrier to optimal reconfiguration is reduced. Finally, contextualization of the present findings for mechanisms operating in low earth orbit is discussed. [ABSTRACT FROM AUTHOR]

In this work, the tribological behavior of ultrathin MoS2 is investigated to understand the independent roles of water and oxidation. Water adsorption is identified as the primary interfacial mechanism for both SiO2/pristine‐MoS2 and SiO2/graphene interfaces, however, tribological behavior of pristine‐MoS2 is observed to be more sensitive to presence of water due to stronger MoS2–water interaction. Comparison of pristine‐MoS2 and oxidized‐MoS2 reveals that the oxidation of MoS2 significantly increases its friction and sensitivity to water by playing a more detrimental role. The specific effect of oxygen on friction via chemical interactions is studied in isolation through density functional theory simulations of a tip sliding on MoS2 basal planes and over edges before and after oxidation. The maximum change in energy, or energy barrier correlating with friction, as the tip moves across the surface, increases after oxidation by up to 66% for the basal plane and by 25% at the edge. Charge density analysis suggests that the more localized and nonuniform interfacial charge distribution on oxygen‐rich surfaces, as compared to pristine surfaces, leads to higher resistance to sliding. This confirms that oxygen presence alone increases friction and when coupled with the presence of water, both effects are additive in increasing friction. [ABSTRACT FROM AUTHOR]