To achieve atomic-scale removal of Y2O3, the mechanochemical removal mechanism of Y2O3, modulated by UV/ O3 treatment, was systematically analyzed using atomic force microscopy. UV/O3-induced color centers are crucial for triggering mechanochemical removal at the Y2O3/Al2O3 interface. Color centers, created by electron excitation of Y atoms under UV light, facilitate the formation of Y-O-Al interfacial bonds under mechanical stress, which enables the atomic removal of the Y2O3 substrate while maintaining subsurface integrity and unchanged chemical composition and phase structure. Importantly, the formation of color centers is reversible, which makes it challenging for the Y2O3 surface to achieve nanoscale mechanochemical wear when mechanical chemical activity decreases. This work presents a novel, reversible, and non-destructive approach for atomicscale precision chemical mechanical polishing applications.

From soap-covered dishes to freshly cleaned floors, surfactants can make surfaces slippery; yet, the underlying mechanism remains poorly understood. Here, we identify the molecular origin behind this ubiquitous phenomenon using macroscopic tribology and surface molecular spectroscopy. We demonstrate that reducing friction through surfactants hinges on charge complementarity: surfactants of opposite charge to the solid surface reduce friction even at extreme contact pressures, whereas like-charged or neutral surfactants are ineffective. Oppositely charged surfactants self-assemble into dense and robust molecular brushes, creating a persistent lubrication beyond the limits of conventional mechanisms. This charge-mediated approach offers a universal and scalable framework for friction control across length scales without significant surface modification.

Diamond-like carbon (DLC) coatings are renowned for their low friction and wear resistance, which are crucial for various tribological applications. However, various wear issues still arise in practical applications of DLC. There is a pressing need to thoroughly understand and address the wear issues of DLC for long-term stable engineering applications. The properties of DLC are related to its composition and structure, among which the sp3 content is arguably the main factor affecting its performance. Here, based on the two-step reaction theory proposed in our previous work [Adv. Sci.,2021,8:2170010], this study employs molecular dynamics simulations to investigate the influence mechanism of sp3 content on interfacial bonding energy barrier and atom removal energy barrier in the friction process. The results indicate that the interfacial bonding energy barrier increases linearly with the sp3 content because of the reduction of active atoms on surface, while the atomic removal energy barrier seems independent to sp3 content due to the little change in the average number of broken bonds per removed atom. This work explores the correlation between the hybridization ratio of carbon atoms and the wear resistance of DLC coatings, and further elucidates the influence of the composition and structural characteristics of DLC on its wear resistance performance.

Plasma Electrochemical Polishing (PEP) is an advanced surface treatment technology with the potential to achieve high-precision surfaces with complex geometries. The key challenge lies in balancing the unique effects of plasma activation and electrochemical corrosion, which jointly act on the material surface during the material removal process. Due to the lack of dynamic studies on the PEP process, there are discrepancies in the understanding of its mechanisms. First, based on dynamic monitoring of the PEP process and analysis of the material surface composition, the removal mechanism between plasma and electrochemical effects during the PEP process of GH3044 alloy was elucidated. Material removal is primarily driven by electrochemical dissolution. Concurrently, the plasma activates common substances (H2O and -COOH) into high-energy species (& sdot;OH, H2O2, O3, and -COOH*). The preferential generation of reactive oxygen species leads to the formation of an oxide layer, thereby reducing the overall removal rate. Secondary-generated reactive complexes, such as -COOH*, further promote material removal. Through optimization, the surface roughness RMS in the local region can be significantly reduced to below 1.0 nm when a voltage of 250 V is applied. This study aims to elucidate the interaction between plasma and electrochemical effects, laying the foundation for achieving high-precision surface manufacturing with complex features.

In the pre-sliding friction regime, interfaces partially stick and partially slip. The pre-slip is thought to be locally initiated at regions of the interface where the ratio of shear stress to normal stress exceeds a critical value. The displacements involved in pre-slip can be limited to the nanoscale, especially for stiff interfaces. Furthermore, little is known experimentally about the interplay between surface topography, pre-sliding behavior and wear. In this work, we introduce a pre-sliding tribometer that enables the study of how the pre-sliding friction at various types of ball-on-flat interfaces evolves as a function of wear. Polytetrafluoroethylene-on-silicon (PTFE-on-Si) pre-sliding measurements covering interfacial displacements up to 50 nm, conducted with the new instrument, show good agreement with Mindlin theory predictions, without adjustable parameters.

Wear causes surfaces to be irreversibly damaged, thereby incurring significant economic cost, for instance in the semiconductor industry. Much progress has been made in describing wear at single asperity interfaces between silicon based materials (Si, SiOx, Si3N4), translating the fundamental understanding of wear into wear predictions and control over wear. Yet, predicting and controlling wear at industrially relevant multi-asperity interfaces remains a challenge, especially when considering the wear of the harder material subjected to repeated, nanometric scale displacement. We studied pre-sliding Si3N4-on-Si wear using the atomic force microscopy topography difference method and showed that the harder Si3N4 wears through either atomic attrition or ductile removal enhanced by subsurface damage, depending on the magnitude of the local Si3N4-on-Si contact pressure. Our methods and results bridge fundamental insight into wear based on nanoscale studies to industrial applications.

We provide experimental observations of the nucleation and growth of water capillary bridges in nanometer gaps between a laterally moving atomic force microscope probe and a smooth silicon wafer. We find rising nucleation rates with increasing lateral velocity and a smaller separation gap. The interplay between nucleation rate and lateral velocity is attributed to the entrainment of water molecules into the gap by the combination of lateral motion and collisions of the water molecules with the surfaces of the interface. The capillary volume of the full-grown water bridge increases with the distance between the two surfaces and can be limited by lateral shearing at high velocities. Our experimental results demonstrate a novel method to study in situ how water diffusion and transport impact dynamic interfaces at the nanoscale, ultimately leading to friction and adhesion forces at the macroscale.

The friction behavior of rough polycrystalline diamond (PCD) surfaces is important in many applications and devices that are required to operate in various harsh environments and it can be argued that a thorough understanding of the friction behavior is essential to the application performance. However, the interplay between electrochemical corrosion, capillary adhesion and friction behavior of PCD in multi-asperity contacts is still poorly understood. In this work, we quantify the interfacial capillary adhesion at contact interfaces and its effect on the friction response, between a colloidal microsphere and rough PCD films before and after electrochemical corrosion, for water-immersed, low RH, and humid air conditions. For these multi-asperity contacts, we demonstrate how electrochemical corrosion influences the surface hydrophilicity of the PCD surfaces, and how capillary adhesion due to water condensation contributes to the friction force. We estimate the capillary forces from both the microscopic lateral force experiments and elastoplastic boundary element method (BEM) contact calculations. The combined results indicate strongly that the observed increase in friction force on electrochemically-corroded PCD surfaces is governed by enhanced capillary adhesion at the contact interface, as affected by surface hydrophilicity and environmental humidity.

HEX-P is a probe-class mission concept that will combine high angular resolution (⪅ 5 ′′ at 6 keV) x-ray imaging and broad energy sensitivity (0.2 − 80 keV) to enable revolutionary new insights into black holes, neutron stars, and other extreme environments powering the high energy universe. HEX-P prioritizes broad band imagery and high resolution simultaneously, providing a wealth of information not possible with any other planned or operating observatory. HEX-P achieves its breakthrough performance by combining technologies developed by experienced partners: high resolution low energy imagery with silicon segmented mirrors provided by the Goddard Space Flight Center (GSFC, Greenbelt, MD); state of the art high energy imagery from nickel shell mirror technology developed by Media Lario (Bosisio Parini, Italy) and the National Institute for Astrophysics (INAF, Merate, Italy) through a contribution from the Italian Space Agency (ASI, Rome, Italy); high speed, high resolution Depleted P-Channel Field Effect Transistor (DEPFET) detectors through a contribution from the Max Planck Institute for Extraterrestrial Physics (MPE, Garching, Germany); photon counting high energy detectors from the NuSTAR team at the California Institute of Technology (Caltech, Pasadena CA); and a spacecraft and payload structure with a 20 m deployable boom developed by Northrop Grumman (Falls Church, VA).

Mechanochemical reactions at the sliding interface between a single-crystalline silicon (Si) wafer and a silica (SiO 2 ) microsphere were studied in three environmental conditions: humid air, potassium chloride (KCl) solution, and KCl solution with an applied voltage. Compared to that from humid air, mechanochemical material removal from the silicon surface increased substantially in the KCl-immersed condition, and further increased when electrochemistry was introduced into the tribological system. By measuring the load dependence of the material removal rate and analyzing the results using a mechanically assisted Arrhenius-type kinetic model, the activation energy ( E a ) and the mechanical energy ( E m ), by which this energy is reduced by mechanical activation, were compared qualitatively under different environmental conditions. In the KCl-immersed condition, mechanochemistry may decrease the required effective energy of reactions ( E eff = E a − E m ) and promote material removal mainly through improved catalysis of the mechanochemical reactions facilitated by greater availability of water molecules compared to the humid air condition. Thus, the effectiveness of the mechanochemistry is improved. In the electrochemical condition, electrochemically-accelerated oxidation of the silicon surface was confirmed by the X-ray photoelectron spectroscopy (XPS) characterization. The results strongly suggest that electrochemistry further stimulates mechanochemical reactions primarily by increasing the initial energy state of the surface via the facilitated formation of interfacial bonding bridges, i.e., a surface oxidation/hydroxylation process.

Polycrystalline diamond (PCD) deposited as a thin film is an attractive material, both technologically and from a scientific viewpoint, due to its unique combination of properties. However, because many applications require the PCD to have a high quality surface finish, efficient and cost-effective polishing has become a critical and limiting step in advancing the more widespread use of PCD. The most widely-used processes for the polishing of PCD make use of synergies that can be achieved through applying a combination of chemical and mechanical inputs. This paper reviews the current state-of-the-art of such processes, which are mainly represented by chemical mechanical polishing (CMP) technology, for the polishing of PCD. An in-depth and informative literature survey is presented of the effects of the PCD characteristics and process-dependent factors such as polishing slurry composition, polishing pad/plate material, and polishing parameters, on the polishing/material removal rate and surface quality. Particular attention is given to the underlying mechanisms governing the material removal during polishing, which are complex and vary depending on the process, and are still unclear. Three main routes to material removal during the polishing of diamond are identified and summarized based on experimental results, chemical characterizations and computational simulations: interfacial mechanochemical removal, chemically-stimulated mechanical removal, and mechanochemical transformation of diamond. Finally, more recently developed polishing methods that make use of ultraviolet and plasma irradiations are introduced, and the limitations of existing research and future research directions are discussed.

We calculate the maximal adhesive force that results from the capillary bridge at the tip-on-substrate interface. This adhesive force consists of three contributions; the capillary force (Fc), the tension force (Ft) and the van der Waals force (Fv), Fad ⃑⃑ ⃑⃑ ⃑ = Fc ⃑⃑⃑⃑ ⃑+ Ft ⃑⃑⃑ + Fv ⃑⃑⃑⃑ . The capillary force (Eq. (S1)) results from the pressure difference inside and outside the meniscus known as the Laplace pressure (PLaplace), which then acts on the circular projected area within which the meniscus wets the tip (A = πR), as illustrated in inset of Fig. 1(b):

Replication package of the paper "Measuring multi-asperity wear with nanoscale precision". Includes: Figures, raw data and scripts to obtain the figures.

In this work, high resolution integrated AFM–EC/SECM was used to reveal the spatially heterogeneous electroactivity of microcrystalline diamond (MCD) and nanocrystalline diamond (NCD) surfaces. During electrochemical corrosion, NCD surfaces undergo a stronger corrosion reaction than MCD because of the higher amount of sp 2 hybridized carbon. In-situ EC-AFM imaging shows no significant change in surface morphology, while corroded surfaces become more hydrophilic due to the oxidation reactions that occur in the outermost layer. On non-corroded MCD and NCD surfaces, intercrystallite boundaries exhibit stronger localized (electro)chemical reactivity than crystallites. However, after electrochemical corrosion, both MCD and NCD surfaces become thermodynamically stabilized by corrosion products that passivate the surface and inhibit further corrosion. In this way, the (electro)chemical reactivity of the intercrystallite boundaries is reduced to a greater extent by electrochemical corrosion than the (electro)chemical reactivity of the crystallites due to the more intense electrochemical oxidation reactions taking place at these boundaries. After corrosion, this results in a comparatively greater (electro)chemical reactivity on the crystallites than at the boundaries. This behavior suggests the following order of (electro)chemical reactivity: sp 2 > sp 3 > oxidized/passivated structures.

The influence of corrosion upon the nanoscale topography and friction response of a hydrogenated amorphous carbon film (a-C:H) was investigated. Electrochemical atomic force microscopy was used to characterise topographical changes to the coating at two oxidative potentials. Corrosion of the coating at 1.5 V (corrosion rate 0.5 nm h(-1)) resulted in no changes to the nanoscale topography; whereas corrosion at 2.5 V (corrosion rate 26.4 nm h(-1)) caused the root mean square roughness of the a-C:H film topography to decrease, but the local fine-scale irregularity or 'jaggedness' of the surface to increase. X-ray photoelectron spectroscopy revealed that corrosion at both potentials oxidised the a-C:H surface to form alcohol, carbonyl and carboxyl groups. Lateral force microscopy and adhesion force measurements showed that both the friction force and surface adhesion of the coating increased upon corrosion. The outcome was attributed to the surface oxidation that had occurred at both oxidative potentials, resulting in several potential mechanisms including increased attractive intermolecular interactions and capillary forces. The highest friction coefficient was observed for the a-C:H film corroded at 2.5 V, and identified as a consequence of the jagged surface topography promoting an interlocking friction mechanism. (C) 2021 The Authors. Published by Elsevier Ltd.

Friction originates at the area of real contact which depends on the (changing) surface topography. Observing and measuring the area of real contact at multi-asperity interfaces is difficult, making it challenging to quantitatively study the interplay between the frictional force and surface topography. In this paper, we systematically manipulate surface topography and use a fluorescence microscopy-based contact visualization technique to reveal this interplay. We demonstrate good agreement between elastoplastic boundary element method contact calculations and experimental visualization of the area of real contact. While the area of real contact and thus contact pressure could be varied by a factor of 4 through control of the surface topography, this had only a modest effect on the coefficient of friction (CoF). We do find a small but systematic increase in the proportionality constant between frictional force and normal force (CoF) with decreasing surface roughness. The observation that smoother surfaces have a greater CoF is due to capillary adhesion between the two surfaces. We quantitatively model this behavior using a simple capillary adhesion model without adjustable parameters. Our results provide quantitative insights into the interplay between contact mechanics, friction, and capillary adhesion. A predictive understanding of this interplay is essential to demanding applications such as precision positioning.

The running-in wear of a multi-asperity silicon carbide sphere-on-silicon flat interface is investigated at the micro- and nanoscale in relation to the friction behaviour of an unlubricated macroscale tribological system sliding in a unidirectional mode. Experiments and contact simulations indicated that the macroscale friction behaviour during running-in was governed by the wear behaviour of roughness asperities on the sphere and their influence on the interfacial contact pressure. Specific ploughing tracks on the flat corresponded to individual asperities on the sphere which, when worn-off, led to lower, more stable friction behaviour and mild wear at an atomic attrition-like rate. It was also found that single asperity contact simulations are unable to reliably predict multi-asperity friction and wear behaviour for this system.

The dependence of the sliding mode (repeated vs. non-repeated reciprocated sliding) on the friction and wear behavior of ball-on-flat, brittle non-metallic interfaces in ambient air conditions is evaluated. Repeated sliding promotes the formation of a third body (compressed wear particles) that stabilizes the friction. Non-repeated sliding shows reduced evidence of third body formation, and instead a steady increase in friction. The proposed mechanism driving the non-repeated friction behavior is attributed to a gradual reduction in the ball surface roughness, leading to an increased area of real contact and greater capillary bridge forming across non-contact regions of the interface.

It is postulated that frictional energy due to contact lens rubbing against corneal tissue correlates positively with cell damage; where the damage is due to a fatigue mechanism (repeated stressing). Efforts were made to develop a relatively rapid in-vitro method capable of exploring this postulate. Measurements of the dynamic coefficient of friction (DCoF) between corneal epithelium and contact lenses, associated frictional forces, frictional energy, and corresponding cell damage were made using SkinEthic (Lyon, France) human corneal epithelial (HCE) constructs and commercially available contact lenses. Five silicone hydrogels (SiHs) and two polyhydroxyethlymethacrylate (p-HEMA) lens types were employed. Frictional forces were measured while the lens was rubbed against a construct that was moistened using a tear-like fluid. The exposed constructs were stained, imaged, and processed using a custom Matlab code. The range of DCoF values observed here extended from about 0.04 to 0.07. The frictional energy varied from about 0.03 mJ to 0.08 mJ. The results indicated a moderate correlation (Pearson's R = 0.79, P = 3.4%) between the frictional energy and cell damage. The authors believe that these results support the notion that cell damage can be caused by fatigue. Future efforts should explore how cell damage relates to a potentially more relevant metric, power density.