Transforming friction: unveiling sliding-induced phase transitions in CVD-grown WS2 monolayers under single-asperity sliding nanocontacts.

Transition metal dichalcogenides (TMDs) exhibit diverse properties across different phases, making them promising materials for various engineering applications. In the present work, we employed a comprehensive approach, combining experimental investigations and computational simulations to elucidate the remarkable tunable frictional characteristics of chemical vapor deposition (CVD) grown WS2 monolayers through the sliding-induced transitions between the 1H and 1T' phases. Our atomic force microscopy (AFM) measurements reveal a significant contrast in friction between the two phases, with the 1H phase displaying higher friction (∼52%) than the 1T' phase. Surprisingly, under repeated scanning at constant stress, the friction of the 1H phase decreases, eventually matching the lower friction values of the 1T' phase. It was observed that the phase transformation is irreversible and is strongly dependent on contact stresses and is accelerated as the contact stress is increased by increasing the applied normal load. Molecular dynamics (MD) simulations provide further insights into the phase transition mechanism, highlighting the role of localized lateral stress and strain induced by sliding an AFM tip on the 1H phase. The simulations confirm that sliding induced localized lateral strain plays a crucial role in the phase transition, ultimately resulting in a decrease in friction. Moreover, our simulations unveil an intriguing connection between friction, potential energy surfaces, and the localized lateral strain during the phase transformation process. Our findings not only offer insights into the tribological properties of TMD materials but also open new possibilities for tailoring their performance in various applications where reducing friction and wear is crucial.

Recent Advancements in Understanding of Growth and Properties of Antiwear Tribofilms Derived from Zinc Dialkyl Dithiophosphate Additives under Nanoscale Sliding Contacts.

Zinc dialkyl dithiophosphate (ZDDP) is a key antiwear additive in lubricants that forms robust phosphate glass-based tribofilms to mitigate wear on rubbing surfaces. The quest to unravel the enigma of these antiwear film formations on sliding surfaces has persisted as an enduring mystery, despite nearly a century of fervent research. This paper presents a comprehensive review of nanotribological investigations, centering on the tribochemical decomposition of ZDDP antiwear additives. The core of the Review explores investigations conducted through the in situ AFM-based technique, which has been used to unveil the underlying stress-assisted thermal activation (SATA) mechanism behind the formation of antiwear tribofilms on diverse surfaces. A thorough analysis is presented, encompassing governing factors, such as compression, shear, and temperature, that wield influence over the intricate process of tribofilm formation. This is substantiated by a spectrum of structural and chemical characterization-based inferences. Furthermore, atomic-scale computer simulation studies are discussed that provide profound insights into tribochemical reaction mechanisms and elucidate the details of chemical processes at atomic level.

Multi-Functional Applications of H-Glass Embedded with Stable Plasmonic Gold Nanoislands.

Metal nanoparticles (MNPs) are synthesized using various techniques on diverse substrates that significantly impact their properties. However, among the substrate materials investigated, the major challenge is the stability of MNPs due to their poor adhesion to the substrate. Herein, it is demonstrated how a newly developed H-glass can concurrently stabilize plasmonic gold nanoislands (GNIs) and offer multifunctional applications. The GNIs on the H-glass are synthesized using a simple yet, robust thermal dewetting process. The H-glass embedded with GNIs demonstrates versatility in its applications, such as i) acting as a room temperature chemiresistive gas sensor (70% response for NO2 gas); ii) serving as substrates for surface-enhanced Raman spectroscopy for the identifications of Nile blue (dye) and picric acid (explosive) analytes down to nanomolar concentrations with enhancement factors of 4.8 × 106 and 6.1 × 105 , respectively; and iii) functioning as a nonlinear optical saturable absorber with a saturation intensity of 18.36 × 1015 W m-2 at 600 nm, and the performance characteristics are on par with those of materials reported in the existing literature. This work establishes a facile strategy to develop advanced materials by depositing metal nanoislands on glass for various functional applications.

Superlubricity and Stress-Shielding of Graphene Enables Ultra Scratch-Resistant Glasses.

Glasses, when subjected to scratch loading, incur damages affecting their optical and mechanical integrity. Here, it is demonstrated that silica glasses protected with mechanically exfoliated few-layer graphene sheets can exhibit remarkable improvement in scratch resistance. To this extent, the friction and wear characteristics of silica glasses with exfoliated graphene using atomic force microscopy (AFM) are explored. The friction forces recorded during AFM scratch tests of the graphene-glass surfaces at multiple loads exhibit ∼98% reduction compared to that of the bare silica glass, with the friction coefficient falling in the superlubricity regime. This dramatic reduction in friction achieved by the graphene sheets results in significantly lower wear of the graphene-glass surfaces postscratching. Further investigations employing atomistic simulations reveal that the stress-shielding mechanism is due to the reduced deformation of graphene-glass surfaces, thereby curtailing the overall damage. Altogether, the present work provides a new fillip toward the development of glasses with enhanced scratch resistance exploiting two-dimensional coatings.

Nanoscale friction and wear behavior of a CVD-grown aged WS2 monolayer: the role of wrinkles and surface chemistry.

Friction reduction by transition metal dichalcogenide (TMD) monolayers is well documented; however, wrinkle formation on the surface of TMDs takes place due to strain relaxation over time and leads to the deterioration of the tribological properties at a small scale. Herein, we report the role of wrinkles on the wear behavior of a chemical vapor deposition (CVD) grown aged WS2 monolayer and the comparison with wrinkle-free regions. Atomic force microscopy (AFM) was utilized to perform load-dependent experiments, and we noticed that the wear initiated near wrinkles resulted in the disintegration of the monolayer. In contrast, in the wrinkle-free regions, wear occurred at significantly higher loads, similar to that of freshly grown WS2, although the coefficient of friction (COF) was increased due to the changes in surface chemistry as a result of aging, which was confirmed using X-ray photoelectron spectroscopy (XPS). In the presence of wrinkles, a ten-fold reduction in the load-carrying capacity was observed compared to the wrinkle-free regions. Molecular dynamics (MD) simulations were used to corroborate experimental findings, which demonstrate the role of wrinkles in the initiation of wear due to the stress concentration under sliding nanocontacts near the wrinkles. In addition, simulations help establish a relationship between the adsorbed chemical species on the surface and increased COF.

Benefit of coconut-based hair oil via hair porosity quantification.

OBJECTIVE: The present study is intended to characterize the surfactant damage suffered by the hair cortex in routine washing and the mechanistic effect of Coconut Based Hair Oils (CBHO) to mitigate the damage. METHODS: Surfactants which diffuse into the hair structure solubilize protein moieties, leading to an increase in porosity and internal surface area as well as the pore volume. The changes in hair pores occurring in the hair cortex are measured by nitrogen sorption method in line with the Brunauer-Emmett-Teller (BET) theory. Single fiber tensile parameters were measured using Diastron MTT 175. Color protection was measured quantitatively using spectrophotometer as well as visual rating by trained panelists. RESULTS: The pore surface area data clearly show the benefit of introducing coconut-based hair oils (CBHO) into the hair by preventing increase in hair porosity. A statistically significant decrease in break stress and toughness were observed and the same were reversed by the application of CBHO. A pronounced color protection effect was also recorded with the application of CBHO. CONCLUSION: The porosity reduction effect seen with the use of CBHO is attributed to the CBHO molecules blocking the diffusion pathways in the endocuticle and the matrix part of the cortical cells, limiting protein surfactant interaction resulting in reduced solubilization and loss. Since, the color molecules are likely to be much smaller than the protein moieties, a pronounced color protection effect suggests that the penetrated CBHO molecules form a dense diffusion barrier in the matrix, cell membrane complex (CMC) and the endocuticle regions of hair - which are the main diffusion pathways out of hair. The study confirms the damage repair potential of CBHO and that it works by increasing the hydrophobicity of hair - both on the hair surface and in the cortex.

OBJECTIF: La présente étude a pour but de caractériser les dommages causés par les surfactants au cortex pilaire lors des lavages de routine et l'effet mécanique des huiles capillaires à base de noix de coco (CBHO) pour atténuer ces dommages. MÉTHODES: Les tensioactifs qui diffusent dans la structure du cheveu solubilisent les fractions de protéines, ce qui entraîne une augmentation de la porosité et de la surface interne ainsi que du volume des pores. Les modifications des pores du cheveu survenant dans le cortex pilaire sont mesurées par la méthode de sorption de l'azote, conformément à la théorie de Brunauer-Emmett-Teller (BET). Les paramètres de traction de la fibre unique ont été mesurés à l'aide du Diastron MTT 175. La protection de la couleur a été mesurée quantitativement en utilisant un spectrophotomètre ainsi qu'une évaluation visuelle par des panélistes formés. LES RÉSULTATS: Les données sur la surface des pores montrent clairement l'avantage d'introduire des huiles capillaires à base de noix de coco (CBHO) dans les cheveux en empêchant l'augmentation de la porosité des cheveux. Une diminution statistiquement significative de la contrainte de rupture et de la résistance a été observée, qui a été inversée par l'application de CBHO. Un effet prononcé de protection de la couleur a également été enregistré avec l'application de CBHO. LA CONCLUSION: L'effet de réduction de la porosité observé avec l'utilisation de CBHO est attribué aux molécules de CBHO bloquant les voies de diffusion dans l'endocuticule et la partie matricielle des cellules corticales, limitant l'interaction protéine surfactant résultant en une solubilisation et une perte réduite. Étant donné que les molécules de couleur sont probablement beaucoup plus petites que les parties protéiques, un effet prononcé de protection de la couleur suggère que les molécules CBHO pénétrées forment une barrière de diffusion dense dans la matrice, le complexe de la membrane cellulaire (CMC) et les régions endocuticulaires des cheveux - qui sont les principales voies de diffusion hors des cheveux. L'étude confirme le potentiel de réparation des dommages du CBHO et le fait qu'il agit en augmentant l'hydrophobie des cheveux - à la fois sur la surface du cheveu et dans le cortex.

Looking through glass: Knowledge discovery from materials science literature using natural language processing.

Most of the knowledge in materials science literature is in the form of unstructured data such as text and images. Here, we present a framework employing natural language processing, which automates text and image comprehension and precision knowledge extraction from inorganic glasses' literature. The abstracts are automatically categorized using latent Dirichlet allocation (LDA) to classify and search semantically linked publications. Similarly, a comprehensive summary of images and plots is presented using the caption cluster plot (CCP), providing direct access to images buried in the papers. Finally, we combine the LDA and CCP with chemical elements to present an elemental map, a topical and image-wise distribution of elements occurring in the literature. Overall, the framework presented here can be a generic and powerful tool to extract and disseminate material-specific information on composition-structure-processing-property dataspaces, allowing insights into fundamental problems relevant to the materials science community and accelerated materials discovery.

Dynamic shear force microscopy of viscosity in nanometer-confined hexadecane layers.

Hexadecane exhibits pronounced molecular layering upon confinement to gaps of a few nanometer width which is discussed for its role in boundary lubrication. We have probed the mechanical properties of the confined layers with the help of an atomic force microscope, by quasi-static normal force measurements and by analyzing the lateral tip motion of a magnetically actuated torsional cantilever oscillation. The molecular layering is modeled by a oscillatory force curve and the tip approach is simulated assuming thermal equilibrium correlations in the liquid. The shear response of the confined layers reveals gradually increasing stiffness and viscous dissipation for a decreasing number of confined layers.

Impact of van der Waals interactions on single asperity friction.

Single asperity measurements on Si wafers with variable SiO(2) layer thickness, yet identical roughness, revealed the influence of van der Waals (vdW) interactions on friction: on thin (1 nm) SiO(2) layers, higher friction and jump-off forces were observed as compared to thick (150 nm) SiO(2) layers. The vdW interactions were additionally controlled by a set of silanized Si wafers, exhibiting the same trend. The experimental results demonstrate the influence of the subsurface material and are quantitatively described by combining calculations of interactions of the involved materials and the Derjaguin-Müller-Toporov model.

Quantized friction across ionic liquid thin films.

Ionic liquids - salts in the liquid state under ambient conditions - are of great interest as precision lubricants. Ionic liquids form layered structures at surfaces, yet it is not clear how this nano-structure relates to their lubrication properties. We measured the friction force between atomically smooth solid surfaces across ionic liquid films of controlled thickness in terms of the number of ion layers. Multiple friction-load regimes emerge, each corresponding to a different number of ion layers in the film. In contrast to molecular liquids, the friction coefficients differ for each layer due to their varying composition.

Monolayer to Bilayer Structural Transition in Confined Pyrrolidinium-Based Ionic Liquids.

Ionic liquids can be intricately nanostructured in the bulk and at interfaces resulting from a delicate interplay between interionic and surface forces. Here we report the structuring of a series of dialkylpyrrolidinium-based ionic liquids induced by confinement. The ionic liquids containing cations with shorter alkyl chain substituents form alternating cation-anion monolayer structures on confinement to a thin film, whereas a cation with a longer alkyl chain substituent leads to bilayer formation. The crossover from monolayer to bilayer structure occurs between chain lengths of n = 8 and 10 for these pyrrolidinium-based ionic liquids. The bilayer structure for n = 10 involves full interdigitation of the alkyl chains; this is in contrast with previous observations for imidazolium-based ionic liquids. The results are pertinent to these liquids' application as electrolytes, where the electrolyte is confined inside the pores of a nanoporous electrode, for example, in devices such as supercapacitors or batteries.

Ageing of a microscopic sliding gold contact at low temperatures.

Nanometer-scale friction measurements on a Au(111) surface have been performed at temperatures between 30 and 300 K by means of atomic force microscopy. Stable stick slip with atomic periodicity is observed at all temperatures, showing only weak dependence on temperature between 300 and 170 K. Below 170 K, friction increases with time and a distortion of the stick-slip characteristic is observed. Low friction and periodic stick slip can be reestablished by pulling the tip out of contact and subsequently restoring the contact. A comparison with molecular dynamics simulations indicates that plastic deformation within a growing gold junction leads to the observed frictional behavior at low temperatures. The regular stick slip with atomic periodicity observed at room temperature is the result of a dynamic equilibrium shape of the contact, as microscopic wear damage is observed to heal in the sliding contact.

Self-assembly in the electrical double layer of ionic liquids.

We have studied the structure of two ionic liquids confined between negatively charged mica sheets. Both liquids exhibit interfacial layering, however the repeat distance is dramatically different for the two liquids. Our results suggest a transition from alternating cation-anion monolayers to tail-to-tail cation bilayers when the length of the cation hydrocarbon chain is increased.

Molecular order and disorder in the frictional response of alkanethiol self-assembled monolayers.

Molecular processes in the frictional response of an alkanethiol monolayer, self-assembled on a Au(111) surface, are studied by means of high-resolution friction force microscopy in ultrahigh vacuum. With increasing load, three regimes are observed on defect-free domains of the monolayer: smooth sliding with negligible friction, regular molecular stick-slip motion with increasing friction, and the onset of wear in the monolayer. Molecular contrast in the lateral force is found for inequivalent molecules within the unit cell of the c(4 × 2) superstructure. Significant differences in the frictional response are found between defect-free domains and areas including a domain boundary. Friction increases by an order of magnitude on domain boundaries in connection with irregular stick-slip motion. This increased friction at domain boundaries is observed at loads below the onset of wear.

Switching atomic friction by electrochemical oxidation.

Friction between the sliding tip of an atomic force microscope and a gold surface changes dramatically upon electrochemical oxidation of the gold surface. Atomic-scale variations of the lateral force reveal details of the friction mechanisms. Stick-slip motion with atomic periodicity on perfect Au(111) terraces exhibits extremely low friction and almost no dependence on load. Significant friction is observed only above a load threshold at which wear of the surface is initiated. In contrast, irregular stick-slip motion and a linear increase of friction with load are observed on electrochemically oxidized surfaces. The observations are discussed with reference to the amorphous structure of the oxo-hydroxide surface and atomic place exchange mechanisms upon oxidation. Reversible, fast switching between the two states of friction has been achieved in both perchloric and sulfuric acid solutions.