Graphene Failure under MPa: Nanowear of Step Edges Initiated by Interfacial Mechanochemical Reactions.

The low wear resistance of macroscale graphene coatings does not match the ultrahigh mechanical strength and chemical inertness of the graphene layer itself; however, the wear mechanism responsible for this issue at low mechanical stress is still unclear. Here, we demonstrate that the susceptibility of the graphene monolayer to wear at its atomic step edges is governed by the mechanochemistry of frictional interfaces. The mechanochemical reactions activated by chemically active SiO2 microspheres result in atomic attrition rather than mechanical damage such as surface fracture and folding by chemically inert diamond tools. Correspondingly, the threshold contact stress for graphene edge wear decreases more than 30 times to the MPa level, and mechanochemical wear can be described well with the mechanically assisted Arrhenius-type kinetic model, i.e., exponential dependence of the removal rate on the contact stress. These findings provide a strategy for improving the antiwear of graphene-based materials by reducing the mechanochemical interactions at tribological interfaces.

Charge Redistribution in Mechanochemical Reactions for Solid Interfaces.

Mechanochemical strategies are widely used in various fields, ranging from friction and wear to mechanosynthesis, yet how the mechanical stress activates the chemical reactions at the electronic level is still open. We used first-principles density functional theory to study the rule of the stress-modified electronic states in transmitting mechanical energy to trigger chemical responses for different mechanochemical systems. The electron density redistribution among initial, transition, and final configurations is defined to correlate the energy evolution during reactions. We found that stress-induced changes in electron density redistribution are linearly related to activation energy and reaction energy, indicating the transition from mechanical work to chemical reactivity. The correlation coefficient is defined as the term "interface reactivity coefficient" to evaluate the susceptibility of chemical reactivity to mechanical action for material interfaces. The study may shed light on the electronic mechanism of the mechanochemical reactions behind the fundamental model as well as the mechanochemical phenomena.

Fabrication of Conical Microstructure Array for Stable Droplet Generation Over Wide Flow Rate Range.

Droplet generators with the ability to resist flow fluctuations are of importance for microfluidic chip analysis systems. However, obtaining stably desired-size droplets is still a bugbear since even slight fluctuations can cause polydisperse droplets. In this study, a high-performance droplet generator is achieved with a functional conical array housed in the junction of the channels. The conical microstructures are fabricated through the selective etching of the scratched silicon nitride/silicon (Si3N4/Si) substrate in potassium hydroxide (KOH) etchant, where the combination of lateral and normal material removal contributes to the structure formation. It is found that the key role of the conical microstructures is to regulate the flow rate of the continuous phase, which allows droplet generation to turn to the necking phase and enables droplets to shed more easily. It is also noted that the droplet generator with such a conical array can produce monodisperse droplets in wide-range flow, providing new insights for high-quality device design.

Nanoscale Wear Triggered by Stress-Driven Electron Transfer.

Wear of sliding contacts causes device failure and energy costs; however, the microscopic principle in activating wear of the interfaces under stress is still open. Here, the typical nanoscale wear, in the case of silicon against silicon dioxide, is investigated by single-asperity wear experiments and density functional theory calculations. The tests demonstrate that the wear rate of silicon in ambient air increases exponentially with stress and does not obey classical Archard's law. Series calculations of atomistic wear reactions generally reveal that the mechanical stress linearly drives the electron transfer to activate the sequential formation and rupture of interfacial bonds in the atomistic wear process. The atomistic wear model is thus resolved by combining the present stress-driven electron transfer model with Maxwell-Boltzmann statistics. This work may advance electronic insights into the law of nanoscale wear for understanding and controlling wear and manufacturing of material surfaces.

Charge Density Evolution Governing Interfacial Friction.

It is well-known that the electron nature of a solid in contact plays a predominant role in determining the many properties of the contact systems, but the general rules of electron coupling that govern interfacial friction remain an open issue for the surface/interface community. Here, density functional theory calculations were used to investigate the physical origins of friction of solid interfaces. It was found that interfacial friction can be inherently traced back to the electronic barrier to the change in the contact configuration of the joints in slip due to the resistance of energy level rearrangement leading to electron transfer, which applies for various interface types ranging from van der Waals, metallic, and ionic to covalent joints. The variation of the electron density accompanying contact conformation changes along the sliding pathways is defined to track the frictional energy dissipation process occurring in slip. The results demonstrate that the frictional energy landscapes evolve synchronously with responding charge density evolution along sliding pathways, yielding an explicitly linear dependence of frictional dissipation on electronic evolution. The correlation coefficient enables us to interpret the fundamental concept of shear strength. The present charge evolution model thereby provides insights into the classic hypothesis that the friction force scales with the real contact area. This may shed light on the intrinsic origin of friction at the electronic level, opening the way to the rational design of nanomechanical devices as well as the understanding of the natural faults.

Understanding and Preventing Lubrication Failure at the Carbon Atomic Steps.

Two-dimensional (2D) lamellar materials are normally capable of rendering super-low friction, wear protection, and adhesion reduction in nanoscale due to their ultralow shear strength between two basal plane surfaces. However, high friction at step edges prevents the 2D materials from achieving super-low friction in macroscale applications and eventually leads to failure of lubrication performance. Here, taking graphene as an example, the authors report that not all step edges are detrimental. The armchair (AC) step edges are found to have only a minor topographic effect on friction, while the zigzag (ZZ) edges cause friction two orders of magnitude larger than the basal plane. The AC step edge is less reactive and thus more durable. However, the ZZ structure prevails when step edges are produced mechanically, for example, through mechanical exfoliation or grinding of graphite. The authors found a way to make the high-friction ZZ edge superlubricious by reconstructing the (6,6) hexagon structure to the (5,7) azulene-like structure through thermal annealing in an inert gas environment. This will facilitate the realization of graphene-based superlubricity over a wide range of industrial applications in which avoiding the involvement of step edges is difficult.

Temperature-Dependent Friction-Induced Surface Amorphization Mechanism of Crystal Silicon.

Friction-induced surface amorphization of silicon is one of the most important surface wear and damage forms, changing the material properties and harming the reliability of silicon-based devices. However, knowledge regarding the amorphization mechanisms as well as the effects of temperature is still insufficient, because the experimental measurements of the crystal-amorphous interface structures and evolutions are extremely difficult. In this work, we aim to fully reveal the temperature dependence of silicon amorphization behaviors and relevant mechanisms by using reactive molecular dynamics simulations. We first show that the degree of amorphization is suppressed by the increasing temperature, contrary to our initial expectations. Then, we further revealed that the observed silicon amorphization behaviors are attributed to two independent processes: One is a thermoactivated and shear-driven amorphization process where the theoretical amorphization rate shows an interesting valley-like temperature dependence because of the competition between the increased thermal activation effect and the reduction of shear stress, and another one is a thermoactivated recrystallization process which shows a monotonically increasing trend with temperature. Thus, the observed reduction of amorphization with temperature is mainly due to the recrystallization effect. Additionally, analytical models are proposed in this work to describe both the amorphization and the recrystallization processes. Overall, the present findings provide deep insights into the temperature-dependent amorphization and recrystallization processes of silicon, benefiting the further development of silicon-based devices and technologies.

Inverse Relationship between Thickness and Wear of Fluorinated Graphene: "Thinner Is Better".

Atomically thin two-dimensional (2D) materials are excellent candidates for utilization as a solid lubricant or additive at all length scales from macro-scale mechanical devices to micro/nano-electromechanical systems (MEMS/NEMS). In such applications, wear resistance of ultrathin 2D materials is critical for sustained lubrication performance. Here, we investigated the wear of fluorinated graphene (FG) nanosheets deposited on silicon surfaces using atomic force microscopy (AFM) and discovered that the wear resistance of FG improves as the FG thickness decreases from 4.2 to 0.8 nm (corresponding to seven layers to single layer) and the surface energy of the substrate underneath the FG nanosheets increases. On the basis of density function theory (DFT) calculations, the negative correlation of wear resistance to FG thickness and the positive correlation to substrate surface energy could be explained with the degree of interfacial charge transfer between FG and substrate which affects the strength of FG adhesion to the substrate.

Suppression of mid-spatial-frequency waviness by a universal random tree-shaped path in robotic bonnet polishing.

Mid-spatial-frequency (MSF) errors seriously damages the imaging performance of optical components. Path pattern is an important factor that affects the generation of MSF waviness in polishing. This paper proposes a versatile pseudo random tree-shaped path (RTSP) generation method imitating the growth rules of tree branch in nature, which can efficiently generate continuous, uniformly distributed and multi-directional paths on a specified surface. Furthermore, the effectiveness of RTSP to suppress MSF waviness is verified by numerical simulation and experimental results. Finally, the RTSP is applied to the polishing of Ti-6Al-4V titanium alloy mirror. Results indicate that grinding marks have been significantly removed, while no obvious MSF waviness is introduced.

Definition of Atomic-Scale Contact: What Dominates the Atomic-Scale Friction Behaviors?

The definition of atomic-scale contact is a very ambiguous issue owing to the discrete atomic arrangement, which hinders the development of contact theory and nano-tribological techniques. In this work, we studied the atomic-scale contact area and their correlations with friction force based on three distinct contact definitions (interatomic distance, force, and interfacial chemical bonds) by performing large-scale atomistic simulations on a typical ball-on-disk contact model. In the simulations, the measured contact areas defined by interatomic distance, force, and interfacial chemical bonds (referred as to Adist, Aforce, and Abond, respectively) are not equivalent at all, while we interestingly clarify that only Adist is consistent with the one calculated by continuum Hertz contact mechanics, and moreover, only Abond is proportional to the friction force indicating that Abond is the dominant one for determining materials' frictional behaviors. The above fundamental insights into the atomic-scale contact problems are useful to deeply understand the origins of tribological phenomena and contribute to the further prediction of atomic-scale friction.

A New Strategy for Disc Cutter Wear Status Perception Using Vibration Detection and Machine Learning.

Carrying out status monitoring and fault-diagnosis research on cutter-wear status is of great significance for real-time understanding of the health status of Tunnel Boring Machine (TBM) equipment and reducing downtime losses. In this work, we proposed a new method to diagnose the abnormal wear state of the disc cutter by using brain-like artificial intelligence to process and analyze the vibration signal in the dynamic contact between the disc cutter and the rock. This method is mainly aimed at realizing the diagnosis and identification of the abnormal wear state of the cutter, and is not aimed at the accurate measurement of the wear amount. The author believes that when the TBM is operating at full power, the cutting forces are very high and the rock is successively broken, resulting in a complex circumstance, which is inconvenient to vibration signal acquisition and transmission. If only a small thrust is applied, to make the cutters just contact with the rock (less penetration), then the cutters will run more smoothly and suffer less environmental interference, which would be beneficial to apply the method proposed in this paper to detect the state of the cutters. A specific example was to use the frequency-domain characteristics of the periodic vibration waveform during the contact between the cutter and the granite to identify the wear status (including normal wear state, wear failure state, angled wear failure state) of the disc cutter through the artificial neural network, and the diagnosis accuracy rate is 90%.

Interfacial Capillary-Force-Driven Self-Assembly of Monolayer Colloidal Crystals for Supersensitive Plasmonic Sensors.

Colloidal lithography technology based on monolayer colloidal crystals (MCCs) is considered as an outstanding candidate for fabricating large-area patterned functional nanostructures and devices. Although many efforts have been devoted to achieve various novel applicatons, the quality of MCCs, a key factor for the controllability and reproducibility of the patterned nanostructures, is often overlooked. In this work, an interfacial capillary-force-driven self-assembly strategy (ICFDS) is designed to realize a high-quality and highly-ordered hexagonal monolayer MCCs array by resorting the capillary effect of the interfacial water film at substrate surface as well as controlling the zeta potential of the polystyrene particles. Compared with the conventional self-assembly method, this approach can realize the reself-assembly process on the substrate surface with few colloidal aggregates, vacancy, and crystal boundary defects. Furthermore, various typical large-scale nanostructure arrays are achieved by combining reactive ion etching, metal-assisted chemical etching, and so forth. Specifically, benefiting from the as-fabricated high-quality 2D hexagonal colloidal crystals, the surface plasmon resonance (SPR) sensors achieve an excellent refractive index sensitivity value of 3497 nm RIU-1 , which is competent for detecting bovine serum albumin with an ultralow concentration of 10-8 m. This work opens a window to prepare high-quality MCCs for more potential applications.

Nanoasperity Adhesion of the Silicon Surface in Humid Air: The Roles of Surface Chemistry and Oxidized Layer Structures.

The interfacial adhesion between silicon oxide surfaces is normally believed to be governed by the surface chemistry of the topmost surface affecting the water contact angle and hydrogen bonding interactions. In the case of a silicon wafer, the physical structure of the native oxide at the surface can vary drastically depending on the aging process; thus, not only the surface chemistry but also the history of surface treatment can also have a profound impact on nanoasperity adhesion. This study reports the effect of aging conditions (ambient air, liquid water, and liquid ethanol) on the nanoasperity adhesion behaviors of a silicon surface. When the silicon surface is kept in liquid alcohol, the surface remains hydrophobic, and adhesion in ambient air can be explained with the capillary effect of the liquid meniscus condensed around the annulus of the nanoasperity contact. When the silicon surface is oxidized in ambient air, the surface gradually becomes hydrophilic, and the strongly hydrogen-bonded water network of adsorbed water plays a dominant role in the nanoasperity interfacial adhesion force. When the silicon surface is aged in liquid water, the interfacial adhesion force measured in ambient air is significantly larger than the value predicted from the theoretical model based on the water contact angle and the hydrogen bonding interaction at the topmost surface. This is because the surface layer oxidized in liquid water is gel-like and thus can swell upon uptake of water from the humid air. To fully encompass all these behaviors, a solid-adsorbate-solid model predicting the adhesion force is developed by introducing a fitting parameter ß, which can be adjusted depending on the adsorbed water structure and the swelling capacity of the oxidized surface layer.

Rapid identification of ultrathin amorphous damage on monocrystalline silicon surface.

Amorphous silicon (a-Si) is a type of common surface damages during the ultra-precision machining of monocrystalline Si. However, it is difficult to identify the amorphous damage of several nanometers by traditional detection methods, which severely hinders the performance improvement of Si-based products. In this study, ultrathin a-Si was found to act as a mask against etching in HF/HNO3 mixtures, resulting in the formation of protrusive hillocks. Reciprocating sliding on an atomic force microscope (AFM) was employed to simulate a material removal event in surface manufacturing. The effects of normal load, etching time and etchant concentration on selective etching were investigated to optimize the parameters for amorphous damage detection. The mechanisms for selective etching were further addressed based on high-resolution transmission electron microscopy (HRTEM) detection and comparative etching of different surface structures. Further analysis demonstrated that a lower dangling bond density of a-Si could result in the reduction of the dissolution rate, while deformed Si lattices, including stacking faults, dislocations and microcracks, could facilitate rapid selective etching. By the proposed selective etching, ultrathin amorphous damage and its spatial distributions can be rapidly identified with high resolution and low destruction. This study sheds new light on achieving a high-quality Si surface in ultra-precision machining.

Force-Induced Transition of π-π Stacking in a Single Polystyrene Chain.

Although π-π interactions have been studied for several decades, the quantification of the strength of π-π interactions in a macromolecule remains a big challenge. Herein, we utilize single-molecule atomic force microscopy and steered molecular dynamics simulations to study the π-π interactions in polystyrene (PS). It is found that in high vacuum, the single-chain mechanics of PS differs largely from that of polyethylene (PE). Accordingly, the strength of intrachain π-π interactions in PS is estimated to be 0.7 kcal/(mol stack), which is much lower than that in a small-molecule system (benzene dimer, 2-3 kcal/(mol stack)). Further study shows that in high vacuum, there are two types of π-π stacking in the single PS chain, i.e., the every-other-moiety (E) type and the adjacent-moiety (A) type. Upon force stretching, a transition from E-type to A-type π-π stacking can be observed.

Temperature-Dependent Mechanochemical Wear of Silicon in Water: The Role of Si-OH Surfacial Groups.

Mechanochemical wear has attracted much attention due to its critical role in micro/nanodevice applications, reliable microscopy, and ultraprecision manufacturing. As a process of stress-associated chemical reactions, mechanochemical wear strongly depends on temperature; however, the impact mechanism is not fully understood at any length scale. Here, we reported different water-temperature dependence of mechanochemical wear on two typical single crystal silicon (Si) surfaces, involving oxide-covered Si partially terminated with Si-OH groups and oxide-free Si fully terminated with Si-H groups. As the water temperature increased from 10 to 80 °C, the mechanochemical wear of the oxide-covered Si underwent a process from no obvious surface damage to significant material removal but that occurring at all temperatures decreased gradually on the oxide-free Si surface. The opposite temperature-dependence was found to have a strong relation to the growth or degeneration of the Si-OH surfacial groups. The mechanochemical wear on the both Si surfaces decreased with the Si-OH coverage rising, which facilitated the growth of strongly hydrogen-bonded ordered water and then suppressed the chemical reaction between the sliding interfaces. These results can provide new insight into the mechanism of the surrounding temperature affecting the reliable micro/nanodevices, manufacturing, and microscopy.

Dependence of water adsorption on the surface structure of silicon wafers aged under different environmental conditions.

Most materials exposed to ambient air can adsorb water molecules and the adsorption capability strongly depends on the surface property. The water contact angle has been widely used as a measure for surface wettability; however, a question can still be asked whether the water contact angle can be used as an adequate sole predictor for water adsorption on the surface in humid air. In this paper, HF-etched silicon wafers were aged (oxidized) under different environmental conditions at room temperature to grow surface layers with varying water contact angles from ∼0° (fully hydrophilic) to ∼83° (highly hydrophobic), and water adsorption as a function of relative humidity (RH) was studied on such surfaces. The thickness and structure of the adsorbed water layer were found to depend on not only the surface wettability on each surface, but also the history of surface oxidation conditions. In particular, the silicon wafer surface oxidized in liquid water uptakes significantly more water from humid air than the fully-hydroxylated native oxide surface (SiOx/OH), even though its water contact angle is higher than that on the SiOx/OH surface. This could be attributed to the formation of a gel-like structure during oxidation in liquid water.

New model to explain tooth wear with implications for microwear formation and diet reconstruction.

Paleoanthropologists and vertebrate paleontologists have for decades debated the etiology of tooth wear and its implications for understanding the diets of human ancestors and other extinct mammals. The debate has recently taken a twist, calling into question the efficacy of dental microwear to reveal diet. Some argue that endogenous abrasives in plants (opal phytoliths) are too soft to abrade enamel, and that tooth wear is caused principally by exogenous quartz grit on food. If so, variation in microwear among fossil species may relate more to habitat than diet. This has important implications for paleobiologists because microwear is a common proxy for diets of fossil species. Here we reexamine the notion that particles softer than enamel (e.g., silica phytoliths) do not wear teeth. We scored human enamel using a microfabrication instrument fitted with soft particles (aluminum and brass spheres) and an atomic force microscope (AFM) fitted with silica particles under fixed normal loads, sliding speeds, and spans. Resulting damage was measured by AFM, and morphology and composition of debris were determined by scanning electron microscopy with energy-dispersive X-ray spectroscopy. Enamel chips removed from the surface demonstrate that softer particles produce wear under conditions mimicking chewing. Previous models posited that such particles rub enamel and create ridges alongside indentations without tissue removal. We propose that although these models hold for deformable metal surfaces, enamel works differently. Hydroxyapatite crystallites are "glued" together by proteins, and tissue removal requires only that contact pressure be sufficient to break the bonds holding enamel together.

What Governs Friction of Silicon Oxide in Humid Environment: Contact Area between Solids, Water Meniscus around the Contact, or Water Layer Structure?

In order to understand the interfacial parameters governing the friction force (Ft) between silicon oxide surfaces in humid environment, the sliding speed (v) and relative humidity (RH) dependences of Ft were measured for a silica sphere (1 µm radius) sliding on a silicon oxide (SiOx) surface, using atomic force microscopy (AFM), and analyzed with a mathematical model describing interfacial contacts under a dynamic condition. Generally, Ft decreases logarithmically with increasing v to a cutoff value below which its dependence on interfacial chemistry and sliding condition is relatively weak. Above the cutoff value, the logarithmic v dependence could be divided into two regimes: (i) when RH is lower than 50%, Ft is a function of both v and RH; (ii) in contrast, at RH ≥ 50%, Ft is a function of v only, but not RH. These complicated v and RH dependences were hypothesized to originate from the structure of the water layer adsorbed on the surface and the water meniscus around the annulus of the contact area. This hypothesis was tested by analyzing Ft as a function of the water meniscus area (Am) and volume (Vm) estimated from a thermally activated water-bridge formation model. Surprisingly, it was found that Ft varies linearly with Vm and correlates poorly with Am at RH < 50%; and then its Vm dependence becomes weaker as RH increases above 50%. Comparing the friction data with the attenuated total reflection infrared (ATR-IR) spectroscopy analysis result of the adsorbed water layer, it appeared that the solidlike water layer structure formed on the silica surface plays a critical role in friction at RH < 50% and its contribution diminishes at RH ≥ 50%. These findings give a deeper insight into the role of water condensation in friction of the silicon oxide single asperity contact under ambient conditions.

Effect of gamma irradiation on the wear behavior of human tooth dentin.

OBJECTIVES: The objective of this study was to evaluate the effect of gamma irradiation on the wear behavior of human tooth dentin in terms of possible alterations in crystallinity, grain size, and composition. MATERIALS AND METHODS: Human premolars (n = 19) were collected to obtain the perpendicular or parallel to the direction of the dentin tubule specimens. Each specimen was subjected to 60 Gy of gamma irradiation, in daily increments of 2 Gy. The nanoscratch tests were conducted. The scratch traces were observed via scanning electron microscope (SEM) and surface profilometer. X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) were used to investigate the alteration of crystallography and chemical composition of dentin after irradiation. The change of surface microhardness (SMH) was also evaluated. RESULTS: The nanoscratch results showed that the friction coefficient of dentin after irradiation became higher, and the depths and widths of scratch were greater than that of dentin before irradiation. Additionally, irradiation decreased the crystallinity of dentin and induced the formation of bigger crystals. The carbonate/mineral ratio was increased. Furthermore, a significant reduction in microhardness after irradiation was observed. The main damage mechanisms consisted of the formation of delamination and crack in both the specimens cut perpendicular and parallel to tubule dentin after irradiation. CONCLUSION: Irradiation affected directly the wear behavior of tooth dentin, accompanied by the alterations in crystallography, chemical composition, and surface microhardness of dentin. CLINICAL RELEVANCE: This would help extend understanding the influence of irradiation on dentin and provide suggestions for selecting more suitable materials for irradiated tooth.